Advertisement
Trends in Pharmacological Sciences
This journal offers authors two options (open access or subscription) to publish research

Progranulin as a therapeutic target in neurodegenerative diseases

Open AccessPublished:January 15, 2022DOI:https://doi.org/10.1016/j.tips.2021.11.015

      Highlights

      • Progranulin (PGRN) is a secreted, immune regulatory protein produced by myeloid cells and some neurons that regulates lysosomal function, neuronal survival, and inflammation.
      • GRN loss-of-function mutations cause neuronal ceroid lipofuscinosis and frontotemporal dementia-GRN (FTD-GRN) in a dosage-dependent manner.
      • Mutations that reduce PGRN levels are also associated with higher risk for amyotrophic lateral sclerosis (ALS) and FTD caused by the hexanucleotide repeat expansion in the C9orf72 gene, Parkinson’s disease (PD), Alzheimer’s disease (AD), limbic-predominant age-related transactivation response DNA-binding protein 43 (TDP-43) encephalopathy, Gaucher disease, and autism.
      • PGRN overexpression is protective in animal models for AD, PD, FTD, ALS, stroke, Gaucher disease, and arthritis.
      • PGRN is being investigated as a therapeutic target for neurodegenerative diseases through multiple mechanisms of action.
      Progranulin (PGRN, encoded by the GRN gene) plays a key role in the development, survival, function, and maintenance of neurons and microglia in the mammalian brain. It regulates lysosomal biogenesis, inflammation, repair, stress response, and aging. GRN loss-of-function mutations cause neuronal ceroid lipofuscinosis or frontotemporal dementia-GRN (FTD-GRN) in a gene dosage-dependent manner. Mutations that reduce PGRN levels increase the risk for developing Alzheimer’s disease, Parkinson’s disease, and limbic-predominant age-related transactivation response DNA-binding protein 43 encephalopathy, as well as exacerbate the progression of amyotrophic lateral sclerosis (ALS) and FTD caused by the hexanucleotide repeat expansion in the C9orf72 gene. Elevating and/or restoring PGRN levels is an attractive therapeutic strategy and is being investigated for neurodegenerative diseases through multiple mechanisms of action.

      Keywords

      Progranulin deficiency is causative of neuronal ceroid lipofuscinosis and frontotemporal dementia

      PGRN (encoded by the GRN gene) is a conserved 593 amino acid, 88-kDa glycosylated, secreted protein composed of 7.5 cysteine-rich granulin domains that is expressed in myeloid cells and a subset of neurons in the central nervous system (CNS), as well as in myeloid, epithelial, and activated fibroblasts and in endothelial cells at the periphery [
      • Bateman A.
      • et al.
      A brief overview of progranulin in health and disease.
      ]. PGRN appears to act extracellularly, through the tyrosine kinase ephrin type-A receptor 2 (EphA2) [
      • Neill T.
      • et al.
      EphA2 is a functional receptor for the growth factor progranulin.
      ] and Notch signaling [
      • Cui Y.
      • et al.
      Progranulin: a conductor of receptors orchestra, a chaperone of lysosomal enzymes and a therapeutic target for multiple diseases.
      ], and functions as a neuronal survival and axonal growth factor [
      • Van Damme P.
      • et al.
      Progranulin functions as a neurotrophic factor to regulate neurite outgrowth and enhance neuronal survival.
      ,
      • Gass J.
      • et al.
      Progranulin: an emerging target for FTLD therapies.
      ] (Figure 1). PGRN also enters the lysosomes of cells [
      • Hu F.
      • et al.
      Sortilin-mediated endocytosis determines levels of the frontotemporal dementia protein, progranulin.
      ] through its interactions with prosaposin (PSAP) and its receptors, mannose 6-phosphate receptor (M6PR), and the low-density lipoprotein receptor-related protein 1 (LRP1) [
      • Zhou X.
      • et al.
      Prosaposin facilitates sortilin-independent lysosomal trafficking of progranulin.
      ]. In the lysosome, PGRN is thought to act as a chaperone for lysosomal enzymes, such as cathepsin D (CTSD), which plays a role in the degradation of misfolded proteins [
      • Butler V.J.
      • et al.
      Progranulin stimulates the in vitro maturation of pro-cathepsin D at acidic pH.
      ]. PGRN also enters the lysosomes following binding to the clearance receptor, sortilin, where it is likely being directed for degradation [
      • Hu F.
      • et al.
      Sortilin-mediated endocytosis determines levels of the frontotemporal dementia protein, progranulin.
      ].
      Figure 1
      Figure 1The roles of progranulin under healthy and deficient conditions.
      Progranulin (PGRN) under healthy conditions has roles in homeostasis, lysosomal function, neuroprotection, and microglial regulation. Mutations resulting in loss of function or single-nucleotide polymorphisms may cause a reduction in PGRN levels, with implications including transactivation response DNA-binding protein 43 (TDP-43) pathology, abnormal microglial activation, neuroinflammation, neuronal loss, synaptic pruning, and lysosomal dysfunction.
      Deficiency in GRN causes a range of neurodegenerative disorders, with an allele (see Glossary) dose-dependent pattern (Figure 2). Homozygous loss-of-function (LOF) GRN mutations result in the complete absence of the PGRN protein and cause neuronal ceroid lipofuscinosis (NCL) type 11 (CLN11) in a fully penetrant fashion [
      • Kamate M.
      • et al.
      Neuronal ceroid lipofuscinosis type-11 in an adolescent.
      ]. CLN11 is a lysosomal storage disorder with early adulthood onset, characterized by vision loss, dementia, and epilepsy, quickly progressing toward death [
      • Johnson T.B.
      • et al.
      Therapeutic landscape for Batten disease: current treatments and future prospects.
      ,
      • Warrier V.
      • et al.
      Genetic basis and phenotypic correlations of the neuronal ceroid lipofusinoses.
      ]. Heterozygous LOF GRN mutations result in a more than 50% reduction in PGRN levels, causing frontotemporal lobar degeneration (FTLD) with transactivation response DNA-binding protein 43 (TDP-43) accumulation [
      • Baker M.
      • et al.
      Mutations in progranulin cause tau-negative frontotemporal dementia linked to chromosome 17.
      ]. The ensuing clinical syndrome, frontotemporal dementia-GRN (FTD-GRN), is a rapidly progressing, fatal neurodegenerative disease, and the mutation is almost 100% penetrant by age 80 years [
      • Goldman J.S.
      • Van Deerlin V.M.
      Alzheimer's disease and frontotemporal dementia: the current state of genetics and genetic testing since the advent of next-generation sequencing.
      ]. A meta-analysis investigating sex differences in the prevalence of FTD gene mutations reported a 33% higher prevalence of females with FTD-GRN [
      • Curtis A.F.
      • et al.
      Sex differences in the prevalence of genetic mutations in FTD and ALS: A meta-analysis.
      ], suggesting that sex-related risk factors may moderate the expression of the disease phenotype. Animal models demonstrate steroid-dependent induction of Grn gene expression contributing to masculinization of the developing brain and active neurogenesis in the adult brain [
      • Suzuki M.
      • et al.
      Roles of progranulin in sexual differentiation of the developing brain and adult neurogenesis.
      ]. FTD-GRN is characterized by changes to personality, language, decision-making, behavior, and movement [
      • Cruts M.
      • et al.
      Null mutations in progranulin cause ubiquitin-positive frontotemporal dementia linked to chromosome 17q21.
      ]. Although the clinical and pathologic features of NCL and FTD-GRN are different, both diseases present with lysosomal dysfunction, lipofuscinosis, gliosis, neuroinflammation, and neurodegeneration [
      • Mackenzie I.R.A.
      The neuropathology and clinical phenotype of FTD with progranulin mutations.
      ,
      • Smith K.R.
      • et al.
      Strikingly different clinicopathological phenotypes determined by progranulin-mutation dosage.
      ]. Features of NCL, such as retinal lipofuscinosis, NCL-like storage material, and impaired lysosomal activity, have been reported in heterozygous GRN mutation carriers [
      • Ward M.E.
      • et al.
      Individuals with progranulin haploinsufficiency exhibit features of neuronal ceroid lipofuscinosis.
      ] (Box 1).
      Figure 2
      Figure 2Progranulin deficiency in neurodegenerative disease.
      Genetic deficiency from loss-of-function mutations and single-nucleotide polymorphisms may cause a reduction in progranulin levels. Progranulin deficiency has been implicated in multiple diseases, including neuronal ceroid lipofuscinosis (NCL), frontotemporal dementia (FTD), amyotrophic lateral sclerosis (ALS), Alzheimer’s disease (AD), Parkinson’s disease (PD), and limbic-predominant age-related transactivation response DNA-binding protein 43 encephalopathy (LATE). Identified receptors for progranulin include sortilin, M6P/PSAP, ephrin type-A receptor 2 (EphA2), low-density lipoprotein receptor-related protein 1 (LRP1), Notch, and epidermal growth factor receptor (EGFR). Abbreviations: LOF, loss of function; SNPs, single-nucleotide polymorphisms.
      Insights and future directions
      • Most risk genes for neurodegeneration are disease specific. In contrast, PGRN deficiency appears as a common feature of FTD, NCL, AD, PD, ALS, LATE, and normal brain aging. This commonality suggests that PGRN is a critical regulator of brain health. The fact that PGRN regulates lysosomal function argues for dysfunctional lysosomal activity as a possible common mechanism for degenerative brain disorders, and therapies that enhance or repair lysosomal function in the aged brain may have broad therapeutic utility.
      • FTD-GRN is caused by autosomal loss-of-function mutations in the GRN gene that lead to protein haploinsufficiency. Conceptually, FTD-GRN is a simple monogenic disease; namely, a single secreted protein is not produced in sufficient quantities due to genetic mutations, leading to an early-onset form of dementia. The apparent mechanistic simplicity of FTD-GRN makes this disease a good case study of targeted therapy for brain disorders. Specifically, drugs that normalize PGRN levels in FTD-GRN may help determine whether dementia caused by inherited mutations can be prevented or slowed by functionally correcting the genetic deficit in adults.
      • An exciting array of new disease-modifying therapies that aim to normalize PGRN deficiency are now underway in clinical trials for FTD-GRN. These approaches include monoclonal antibody therapy, gene therapy, a BBB-penetrant transport vehicle for protein replacement, and small molecule transcriptional activators. Therapeutic efficacy in FTD-GRN may expand the utility of such drugs for multiple additional neurodegenerative indications.
      • The mechanism of action of PGRN and its granulin peptide products is not completely understood. We have not identified all the relevant receptors, nor do we know what proportion of PGRN and granulin activities are mediated by cell membrane receptors versus in the lysosomes, and in neurons versus glia. We also do not yet understand why the brain is preferentially susceptible to PGRN deficiency. A better understanding of PGRN activities and mechanisms of action will lead to new insights into brain function in health and disease and may facilitate the development of new therapies for brain disorders and even for normal aging.
      In addition to these rare LOF GRN mutations, a more frequent haplotype at the GRN locus that modifies PGRN levels was identified by genome-wide association studies as a genetic determinant of Alzheimer’s disease (AD) [
      • Bellenguez C.
      • et al.
      Genetics of Alzheimer's disease: where we are, and where we are going.
      ], and limbic-predominant age-related TDP-43 encephalopathy (LATE) [
      • Nelson P.T.
      • et al.
      Limbic-predominant age-related TDP-43 encephalopathy (LATE): consensus working group report.
      ], as well as FTD and ALS caused by the hexanucleotide repeat expansion in the C9orf72 gene (C9-FTD and C9-ALS) [
      • van Blitterswijk M.
      • et al.
      Genetic modifiers in carriers of repeat expansions in the C9ORF72 gene.
      ]. This minor allele of PGRN, designated rs5848 single-nucleotide polymorphism (SNP), is present in 30% of the general population and causes a ~15% reduction in PGRN in plasma and cerebrospinal fluid per allele. Other common variants have been identified that affect the PGRN protein sequence or levels and result in earlier age of ALS onset and shorter survival time [
      • Sleegers K.
      • et al.
      Progranulin genetic variability contributes to amyotrophic lateral sclerosis.
      ]. The rs5848 minor allele was also shown to increase the rate of biological aging in the human brain in the absence of disease [
      • Rhinn H.
      • Abeliovich A.
      Differential aging analysis in human cerebral cortex identifies variants in TMEM106B and GRN that regulate aging phenotypes.
      ] and is associated with an increase in the amount of TDP-43 inclusion bodies on neuropathologic examinations [
      • Pao W.C.
      • et al.
      Hippocampal sclerosis in the elderly: genetic and pathologic findings, some mimicking Alzheimer disease clinically.
      ]. Novel multisignal risk variants rs2269906 and rs850738 were identified for Parkinson’s disease (PD) [
      • Nalls M.A.
      • et al.
      Identification of novel risk loci, causal insights, and heritable risk for Parkinson's disease: a meta-analysis of genome-wide association studies.
      ], while SNP genotyping has also identified an association between PGRN and Gaucher disease, another lysosomal storage disease with decreased levels of circulating PGRN [
      • Jian J.
      • et al.
      Association between progranulin and Gaucher disease.
      ]. Lower than normal levels of PGRN were reported in idiopathic PD [
      • Yao Y.-N.
      • et al.
      Reduced plasma progranulin levels are associated with the severity of Parkinson’s disease.
      ] and autism [
      • Al-Ayadhi L.Y.
      • Mostafa G.A.
      Low plasma progranulin levels in children with autism.
      ]. The neurotrophic, lysosomal enhancing, and anti-inflammatory properties of PGRN, and the common thread of PGRN deficiency underlying different neurodegenerative disorders, make it a uniquely attractive therapeutic target.

      Progressive loss of nerve cells, TDP-43 pathology, and neurodegenerative diseases

      Progressive loss of nerve cells is a common feature in neurodegenerative diseases, including FTD, ALS, PD, and AD, each with distinctive patterns of regional atrophy and abnormal accumulation of specific proteins that are considered the pathologic hallmarks of the disease. TDP-43 protein aggregates are found in all FTD with GRN mutations, in most cases of FTD and ALS with C9orf72 mutations, and in over half of all idiopathic FTD cases [
      • Mackenzie I.R.
      • et al.
      Heterogeneity of ubiquitin pathology in frontotemporal lobar degeneration: classification and relation to clinical phenotype.
      ]. Over 50 genetic mutations in TDP-43 have been linked to ALS, and TDP-43 is found in approximately 97% of all ALS [
      • Arai T.
      • et al.
      TDP-43 is a component of ubiquitin-positive tau-negative inclusions in frontotemporal lobar degeneration and amyotrophic lateral sclerosis.
      ] and is identified as the third predominant protein inclusion in AD [
      • Amador-Ortiz C.
      • et al.
      TDP-43 immunoreactivity in hippocampal sclerosis and Alzheimer's disease.
      ,
      • Higashi S.
      • et al.
      Concurrence of TDP-43, tau and alpha-synuclein pathology in brains of Alzheimer's disease and dementia with Lewy bodies.
      ]. TDP-43, encoded by the TARDBP gene, is a DNA-binding protein but is also an important RNA-binding protein and plays a role in mRNA splicing, nuclear transcription, and mRNA transport and stability, as well as in the formation of stress granules [
      • Arai T.
      • et al.
      TDP-43 is a component of ubiquitin-positive tau-negative inclusions in frontotemporal lobar degeneration and amyotrophic lateral sclerosis.
      ,
      • Gao J.
      • et al.
      Pathomechanisms of TDP-43 in neurodegeneration.
      ]. The formation of TDP-43 aggregates likely results in both gain and loss of function pathology, and its mislocalization is viewed as a major disease mechanism in multiple neurodegenerative diseases [
      • Nussbacher J.K.
      • et al.
      Disruption of RNA metabolism in neurological diseases and emerging therapeutic interventions.
      ].
      TDP-43 is thought to bind UG-rich domains of the 3′-untranslated regions of several thousand RNA transcripts in neuronal cells encoding different regulatory proteins, including sortilin [
      • Gao J.
      • et al.
      Pathomechanisms of TDP-43 in neurodegeneration.
      ] and SQSTM1 (i.e., p62), an ALS- and FTLD-associated protein involved in protein quality control. The TDP-43–SQTSM1 interaction is proposed to link the ubiquitin–proteasome system and autophagy [
      • Cohen-Kaplan V.
      • et al.
      The ubiquitin-proteasome system and autophagy: Coordinated and independent activities.
      ]. Disruption of the physical interaction between TDP-43 and SQSTM1 has been reported in patients with FTLD-TDP [
      • Tanji K.
      • et al.
      p62/sequestosome 1 binds to TDP-43 in brains with frontotemporal lobar degeneration with TDP-43 inclusions.
      ]. Overexpression of PGRN mRNA rescues motor neurons in a TDP-43 zebrafish model [
      • Laird A.S.
      • et al.
      Progranulin is neurotrophic in vivo and protects against a mutant TDP-43 induced axonopathy.
      ], protects axon fibers in the lateral spinal cord, and prolongs survival in a TDP-43 overexpression mouse model [
      • Beel S.
      • et al.
      Progranulin reduces insoluble TDP-43 levels, slows down axonal degeneration and prolongs survival in mutant TDP-43 mice.
      ]. Mice lacking the PGRN protein due to knockout of Grn (Grn–/–) demonstrate hypoactivity of important autophagy regulators and are more prone to pathologic TDP-43 accumulation [
      • Chang M.C.
      • et al.
      Progranulin deficiency causes impairment of autophagy and TDP-43 accumulation.
      ]. This argues for PGRN as a regulator of autophagy and provides a mechanism for lysosomal dysfunction, TDP-43 aggregation, and neurodegeneration observed with PGRN haploinsufficiency in FTD-GRN. PGRN deficiency moves microglia from a homeostatic state to a disease-activated, hyperinflammatory state. In vitro studies reveal that conditioned media from Grn−/− microglia can promote TDP-43 granule formation, nuclear pore defects, and cell death in excitatory neurons in part via the complement pathway [
      • Zhang J.
      • et al.
      Neurotoxic microglia promote TDP-43 proteinopathy in progranulin deficiency.
      ]. PGRN was shown to regulate caspase-dependent processing of TDP-43 [
      • Zhang Y.J.
      • et al.
      Progranulin mediates caspase-dependent cleavage of TAR DNA binding protein-43.
      ]. However, the mechanisms by which PGRN reduces insoluble TDP-43 levels and prevents TDP-43-mediated neurodegeneration are not well understood.

      Neuronal versus microglial PGRN, microglial activation, astrogliosis, and the autophagy–lysosome pathway

      PGRN is preferentially expressed by microglia and neurons in the CNS and is an important regulator of microglial cell recruitment and activation. Mouse models have been used to examine the role of microglial PGRN and neuronal PGRN by selective inhibition of PGRN expression in these cell types. The selective depletion of 50–70% of microglial-derived PGRN in mice, which still produced normal levels of neuron-derived PGRN, did not cause lipofuscinosis or increased inflammation, and there was no increased astrogliosis or microgliosis (except in the hippocampus) when compared with controls [
      • Petkau T.L.
      • et al.
      Selective depletion of microglial progranulin in mice is not sufficient to cause neuronal ceroid lipofuscinosis or neuroinflammation.
      ]. Restoring neuronal PGRN in aged Grn−/− mice using adeno-associated virus (AAV) vector expressing mouse PGRN with a neuronal promoter delivered into the medial prefrontal cortex corrected lysosomal dysfunction and reversed social dominance deficits assessed in the tube test assay [
      • Arrant A.E.
      • et al.
      Restoring neuronal progranulin reverses deficits in a mouse model of frontotemporal dementia.
      ]. Reducing microglial PGRN in a mouse model did not significantly change brain PGRN levels, but reduction of neuronal PGRN resulted in an approximately 50% decrease of PGRN in the frontal cortex and an approximately 25% decrease of PGRN in the hippocampus [
      • Arrant A.E.
      • et al.
      Progranulin gene therapy improves lysosomal dysfunction and microglial pathology associated with frontotemporal dementia and neuronal ceroid lipofuscinosis.
      ]. These findings argue for the importance of neuronally produced PGRN in the mouse. However, Grn mRNA is >50-fold enriched in microglia compared with neurons [
      • Lui H.
      • et al.
      Progranulin deficiency promotes circuit-specific synaptic pruning by microglia via complement activation.
      ], and depletion of PGRN results in severe microglial pathology, including impaired phagocytosis [
      • Minami S.S.
      • et al.
      Progranulin protects against amyloid beta deposition and toxicity in Alzheimer's disease mouse models.
      ], accumulation of myelin debris in microglial lysosomes [
      • Wu Y.
      • et al.
      Microglial lysosome dysfunction contributes to white matter pathology and TDP-43 proteinopathy in GRN-associated FTD.
      ], an exacerbated inflammatory response following acute injury [
      • Tanaka Y.
      • et al.
      Exacerbated inflammatory responses related to activated microglia after traumatic brain injury in progranulin-deficient mice.
      ], as well as excessive synaptic pruning and destruction by microglia via complement activation [
      • Lui H.
      • et al.
      Progranulin deficiency promotes circuit-specific synaptic pruning by microglia via complement activation.
      ,
      • Hickman S.
      • et al.
      Microglia in neurodegeneration.
      ]. Microglia from PGRN-deficient mice display overexpression of complement genes, including C1qA, C1qB, C1qC, and C3, and microglia activation genes, including CD68 and Trem2 [
      • Lui H.
      • et al.
      Progranulin deficiency promotes circuit-specific synaptic pruning by microglia via complement activation.
      ]. The classical complement cascade proteins, C1q and C3, were shown to regulate synapse elimination [
      • Stevens B.
      • et al.
      The classical complement cascade mediates CNS synapse elimination.
      ], and the excessive production of these proteins, which is followed by their deposition onto synapses, is associated with synapse elimination in PGRN-deficient mice [
      • Lui H.
      • et al.
      Progranulin deficiency promotes circuit-specific synaptic pruning by microglia via complement activation.
      ]. Overexpression of complement proteins has also been reported in the serum of patients with FTD-GRN [
      • Katzeff J.S.
      • et al.
      Altered serum protein levels in frontotemporal dementia and amyotrophic lateral sclerosis indicate calcium and immunity dysregulation.
      ], providing a link between PGRN deficiency, microglia hyperactivity, TDP-43 pathology, and synaptic and neuronal destruction in neurodegeneration [
      • Zhang J.
      • et al.
      Neurotoxic microglia promote TDP-43 proteinopathy in progranulin deficiency.
      ].
      Despite preferential expression of PGRN by microglia and neurons, astrocytes have also been implicated in the PGRN-mediated neuropathological phenotype. PGRN has been found in lysosome-associated membrane glycoprotein 1 (LAMP-1)-positive astrocyte vesicles, suggesting that astrocytes store or transport PGRN to the lysosome [
      • Almeida S.
      • et al.
      Progranulin, a glycoprotein deficient in frontotemporal dementia, is a novel substrate of several protein disulfide isomerase family proteins.
      ]. Exogenous PGRN attenuates a proinflammatory phenotype of astrocytes in culture [
      • Menzel L.
      • et al.
      Progranulin protects against exaggerated axonal injury and astrogliosis following traumatic brain injury.
      ]. Aged Grn−/− mice show prominent astrogliosis in addition to robust neuroinflammation, microgliosis, and lipofuscinosis [
      • Petkau T.L.
      • et al.
      Core neuropathological abnormalities in progranulin-deficient mice are penetrant on multiple genetic backgrounds.
      ,
      • Ghoshal N.
      • et al.
      Core features of frontotemporal dementia recapitulated in progranulin knockout mice.
      ,
      • Yin F.
      • et al.
      Exaggerated inflammation, impaired host defense, and neuropathology in progranulin-deficient mice.
      ]. Further, elevated glial fibrillary acidic protein (GFAP) concentrations, a marker of astrogliosis, have been observed in symptomatic GRN mutation carriers, but not in symptomatic carriers of other mutations causing FTD (i.e., C9orf72 or MAPT carriers) [
      • Heller C.
      • et al.
      Plasma glial fibrillary acidic protein is raised in progranulin-associated frontotemporal dementia.
      ]. While the mechanism remains unclear, evidence suggests astrogliosis may be in part a consequence of microglial pathology. Activated neuroinflammatory microglia, which are induced by PGRN deficiency [
      • Lui H.
      • et al.
      Progranulin deficiency promotes circuit-specific synaptic pruning by microglia via complement activation.
      ], have been shown to induce reactive astrocytes, termed A1 astrocytes, which upregulate classical complement cascade genes and are abundant in neurodegenerative diseases [
      • Liddelow S.A.
      • et al.
      Neurotoxic reactive astrocytes are induced by activated microglia.
      ]. These findings support the idea that PGRN deficiency mediates a progressive neuroinflammatory cascade culminating in neurodegeneration.
      PGRN plays a role in regulating lysosomal homeostasis and lipid metabolism, and dysfunctional lysosomes are a hallmark of many neurodegenerative diseases. Cells with PGRN deficiency display abnormal lysosomal morphology [
      • Ward M.E.
      • et al.
      Individuals with progranulin haploinsufficiency exhibit features of neuronal ceroid lipofuscinosis.
      ,
      • Evers B.M.
      • et al.
      Lipidomic and transcriptomic basis of lysosomal dysfunction in progranulin deficiency.
      ] and increase in lysosomal size [
      • Lui H.
      • et al.
      Progranulin deficiency promotes circuit-specific synaptic pruning by microglia via complement activation.
      ]. PGRN-deficient cells also display impaired lysosomal functions, including a decrease in the activity, but not the levels, of the aspartyl protease CTSD [
      • Butler V.J.
      • et al.
      Progranulin stimulates the in vitro maturation of pro-cathepsin D at acidic pH.
      ,
      • Valdez C.
      • et al.
      Progranulin-mediated deficiency of cathepsin D results in FTD and NCL-like phenotypes in neurons derived from FTD patients.
      ]. In fact, the levels of CTSD and other lysosomal proteins were reported to increase in FTD-GRN [
      • Huang M.
      • et al.
      Network analysis of the progranulin-deficient mouse brain proteome reveals pathogenic mechanisms shared in human frontotemporal dementia caused by GRN mutations.
      ,
      • Gotzl J.K.
      • et al.
      Common pathobiochemical hallmarks of progranulin-associated frontotemporal lobar degeneration and neuronal ceroid lipofuscinosis.
      ], possibly as a compensatory response to the reduced enzymatic activity. CTSD is important in the breakdown of disease-associated protein aggregates in neurodegenerative diseases [
      • Bunk J.
      • et al.
      Cathepsin D variants associated with neurodegenerative diseases show dysregulated functionality and modified alpha-synuclein degradation properties.
      ,
      • Bae E.-J.
      • et al.
      Haploinsufficiency of cathepsin D leads to lysosomal dysfunction and promotes cell-to-cell transmission of alpha-synuclein aggregates.
      ,
      • Cullen V.
      • et al.
      Cathepsin D expression level affects alpha-synuclein processing, aggregation, and toxicity in vivo.
      ], providing a link between PGRN deficiency and the accumulation of misfolded proteins in neurodegeneration. PGRN deficiency also leads to dysfunction in lipid homeostasis and metabolism in lysosomes, including the accumulation of polyunsaturated triacylglycerides, as well as a reduction of diacylglycerides and phosphatidylserine [
      • Evers B.M.
      • et al.
      Lipidomic and transcriptomic basis of lysosomal dysfunction in progranulin deficiency.
      ], supporting the idea that PGRN may play a broad role in regulating lysosomal autophagy and recycling of organelles.
      Transcriptomic and lipidomic analyses further revealed that PGRN deficiency led to abnormal expression of multiple lysosomal, immune-related, and lipid metabolic genes [
      • Evers B.M.
      • et al.
      Lipidomic and transcriptomic basis of lysosomal dysfunction in progranulin deficiency.
      ]. Likewise, a network analysis study supports the argument that insufficiency of PGRN, as well as granulins, causes neurodegeneration, in part via lysosomal dysfunction, defects in autophagy, and neuroinflammation [
      • Huang M.
      • et al.
      Network analysis of the progranulin-deficient mouse brain proteome reveals pathogenic mechanisms shared in human frontotemporal dementia caused by GRN mutations.
      ]. These findings suggest that increasing levels of PGRN may restore beneficial lysosomal activity and the lysosome’s ability to counteract protein aggregation and lipid accumulation during stress and neurodegeneration [
      • Marques A.R.A.
      • et al.
      Enzyme replacement therapy with recombinant pro-CTSD (cathepsin D) corrects defective proteolysis and autophagy in neuronal ceroid lipofuscinosis.
      ].
      While FTD is nearly 100% penetrant by age 80 years [
      • Goldman J.S.
      • Van Deerlin V.M.
      Alzheimer's disease and frontotemporal dementia: the current state of genetics and genetic testing since the advent of next-generation sequencing.
      ], the age of symptom onset is highly variable, ranging from 25 to 90 years [
      • Moore K.M.
      • et al.
      Age at symptom onset and death and disease duration in genetic frontotemporal dementia: an international retrospective cohort study.
      ]. Further, the PGRN protein is widely distributed in the brain, but FTD pathology seems restricted mainly to the frontal and temporal lobes. Taken together, this suggests that there are important modifiers of PGRN pathobiology through development and adulthood. TMEM106B is a recognized risk modifier for FTD-GRN, although there is still debate as to whether it is a gain or loss of TMEM106B function that is critical for modifying FTD risk [
      • Clayton E.L.
      • Isaacs A.M.
      Progranulin and TMEM106B: when two become wan.
      ]. PGRN has growth factor-like properties, and like other growth factors, extracellular protein–protein interactions can regulate PGRN function, as demonstrated in mice by knockouts of neutrophil elastase and proteinase-3 and slpi [
      • Kessenbrock K.
      • et al.
      Proteinase 3 and neutrophil elastase enhance inflammation in mice by inactivating antiinflammatory progranulin.
      ,
      • Zhu J.
      • et al.
      Conversion of proepithelin to epithelins: roles of SLPI and elastase in host defense and wound repair.
      ]. Less is known about the regulation of GRN expression in different cell types. Grn knock-in mice followed up from embryonic stages to mature adulthood showed different developmental expression patterns in neurons versus microglia. Grn expression increased with maturation in neurons, whereas microglial Grn expression was upregulated in response to injury [
      • Petkau T.L.
      • et al.
      Progranulin expression in the developing and adult murine brain.
      ]. Epigenetic regulation of GRN expression due to aberrant DNA methylation in the promoter region of GRN was demonstrated in lymphoblast cell lines derived from patients with FTLD and healthy individuals. DNA methyltransferase 3a (DNMT3a) was found to be upregulated in FTLD, and overexpression of DNMT3a reduces GRN promoter activity and expression [
      • Banzhaf-Strathmann J.
      • et al.
      Promoter DNA methylation regulates progranulin expression and is altered in FTLD.
      ]. Human-induced pluripotent stem cell-derived neurons with a GRN nonsense mutation responded to the aminoglycoside G418 combined with a premature termination codon enhancer with increased PGRN levels compared with G418 alone [
      • Frew J.
      • et al.
      Premature termination codon readthrough upregulates progranulin expression and improves lysosomal function in preclinical models of GRN deficiency.
      ].

      Granulins

      Full-length PGRN has neurotrophic and anti-inflammatory effects, but when PGRN is trafficked into the lysosome by the PSAP-dependent pathway, it is thought to be cleaved into seven individual, highly conserved, disulfide-bond-containing, 6-kDa granulin peptides, designated granulins A through G, and the 3.5-kDa paragranulin [
      • Mohan S.
      • et al.
      Processing of progranulin into granulins involves multiple lysosomal proteases and is affected in frontotemporal lobar degeneration.
      ,
      • Holler C.J.
      • et al.
      Intracellular proteolysis of progranulin generates stable, lysosomal granulins that are haploinsufficient in patients with frontotemporal dementia caused by GRN mutations.
      ]. While some functions of individual granulins have been identified, their full range of activity is not yet fully understood [
      • Bateman A.
      • Bennett H.P.
      The granulin gene family: from cancer to dementia.
      ]. Granulin E, as well as multigranulin domain peptides, was shown to enhance the conversion of pro-CTSD to mature CTSD and increase its activity, whereas granulin C did not [
      • Valdez C.
      • et al.
      Progranulin-mediated deficiency of cathepsin D results in FTD and NCL-like phenotypes in neurons derived from FTD patients.
      ,
      • Butler V.J.
      • et al.
      Multi-granulin domain peptides bind to pro-cathepsin D and stimulate its enzymatic activity more effectively than progranulin in vitro.
      ]. It has been proposed that the granulin peptides can localize to LAMP-1-positive lysosomes and may play an additional important function in lysosomal activity [
      • Holler C.J.
      • et al.
      Intracellular proteolysis of progranulin generates stable, lysosomal granulins that are haploinsufficient in patients with frontotemporal dementia caused by GRN mutations.
      ]. By contrast, granulins were shown to be detrimental in some experimental models. In a Caenorhabditis elegans model of TDP-43 proteinopathy, but not in Grn−/− mouse microglia [
      • Zhang J.
      • et al.
      Neurotoxic microglia promote TDP-43 proteinopathy in progranulin deficiency.
      ], complete loss of the pgrn-1 gene did not exacerbate TDP-43 toxicity, but pgrn-1 heterozygosity did. If granulins were coexpressed with TDP-43 in C. elegans, the toxicity of TDP-43 was amplified and the granulins increased TDP-43 levels via a post-translational mechanism [
      • Salazar D.A.
      • et al.
      The progranulin cleavage products, granulins, exacerbate TDP-43 toxicity and increase TDP-43 levels.
      ]. Granulin peptides can be produced as a result of stress and aging and can impair the expression and activity of lysosomal proteases [
      • Butler V.J.
      • et al.
      Age- and stress-associated C. elegans granulins impair lysosomal function and induce a compensatory HLH-30/TFEB transcriptional response.
      ]. Of note, regionally localized changes in the protease asparagine endopeptidase (AEP), which cleaves granulin F from PGRN, were observed post-mortem in brain regions with severe neurodegenerative change compared with regions with little degeneration in FTD-GRN cases [
      • Mohan S.
      • et al.
      Processing of progranulin into granulins involves multiple lysosomal proteases and is affected in frontotemporal lobar degeneration.
      ]. Like granulin F, granulin A also takes on a defined 3D structure in solution and has been shown to inhibit cancer cell growth via ENO1 [
      • Qiao G.
      • et al.
      Granulin A synergizes with cisplatin to inhibit the growth of human hepatocellular carcinoma.
      ]. Further work on individual granulin peptides would be helpful to reveal their role in GRN haploinsufficiency and neuroinflammation, and in the impairment of lysosomal function and enhancement of TDP-43 levels and toxicity.

      PGRN as a therapeutic target

      Given the importance of PGRN for lysosomal function, neuronal survival, and inflammation in the CNS, restoring PGRN to normal levels in patients with GRN mutations may be a beneficial therapeutic strategy [
      • Lee W.C.
      • et al.
      Targeted manipulation of the sortilin-progranulin axis rescues progranulin haploinsufficiency.
      ] (Figure 3). The sortilin receptor has been identified as the major regulator of PGRN levels in plasma and the brain [
      • Hu F.
      • et al.
      Sortilin-mediated endocytosis determines levels of the frontotemporal dementia protein, progranulin.
      ,
      • Carrasquillo M.M.
      • et al.
      Genome-wide screen identifies rs646776 near sortilin as a regulator of progranulin levels in human plasma.
      ]. The C-terminus of the PGRN protein can bind to the beta-propeller region of sortilin to control PGRN trafficking and lysosomal degradation [
      • Zheng Y.
      • et al.
      C-terminus of progranulin interacts with the beta-propeller region of sortilin to regulate progranulin trafficking.
      ]. Sortilin was not reported to be required for the subcellular localization or function of PGRN. In the absence of sortilin, PGRN can still traffic to the lysosome through M6PR and LRP1 in a PSAP-dependent manner [
      • Zhou X.
      • et al.
      Prosaposin facilitates sortilin-independent lysosomal trafficking of progranulin.
      ]. Moreover, PGRN does not require sortilin for its neuroprotective effects [
      • De Muynck L.
      • et al.
      The neurotrophic properties of progranulin depend on the granulin E domain but do not require sortilin binding.
      ]. Exogenous PGRN with a C-terminal tag that does not bind to sortilin was able to induce survival neurite outgrowth in Grn−/− neurons [
      • Gass J.
      • et al.
      Progranulin regulates neuronal outgrowth independent of sortilin.
      ]. Gene delivery of PGRN with a tag that prevents binding to sortilin was still able to reduce lysosomal pathology and microgliosis in Grn−/− mice [
      • Arrant A.E.
      • et al.
      Progranulin gene therapy improves lysosomal dysfunction and microglial pathology associated with frontotemporal dementia and neuronal ceroid lipofuscinosis.
      ]. Individuals who are haploinsufficient for sortilin (~5/100 000 of the population) were not reported to display any form of neurodegenerationi [
      • Carrasquillo M.M.
      • et al.
      Genome-wide screen identifies rs646776 near sortilin as a regulator of progranulin levels in human plasma.
      ]. Sortilin knockout mice, which have elevated PGRN levels, show no signs of lysosomal dysfunction, microgliosis, neuronal pathology, or neurodegeneration [
      • Hu F.
      • et al.
      Sortilin-mediated endocytosis determines levels of the frontotemporal dementia protein, progranulin.
      ], whereas mice that are deficient in M6PR [
      • Ludwig T.
      • et al.
      Targeted disruption of the mouse cation-dependent mannose 6-phosphate receptor results in partial missorting of multiple lysosomal enzymes.
      ], LRP1 [
      • Liu Q.
      • et al.
      Neuronal LRP1 knockout in adult mice leads to impaired brain lipid metabolism and progressive, age-dependent synapse loss and neurodegeneration.
      ], or PSAP [
      • Nicholson A.M.
      • et al.
      Prosaposin is a regulator of progranulin levels and oligomerization.
      ] do. These data support the idea that PGRN exerts its multiple functions independent of sortilin [
      • Cui Y.
      • et al.
      Progranulin: a conductor of receptors orchestra, a chaperone of lysosomal enzymes and a therapeutic target for multiple diseases.
      ]. Findings from an open-label Phase 2 clinical trial in patients with symptomatic FTD further support sortilin as an attractive drug target for increasing PGRN levels. In this trial, patients received AL001, an antibody that increases PGRN levels by blocking its interaction with sortilinii.
      Figure 3
      Figure 3Multiple therapeutic targets are under investigation to increase progranulin levels.
      AL001 antibody therapy blocks the sortilin degradation pathway of progranulin (PGRN) to increase PGRN levels. Protein replacement therapy uses technology to increase blood–brain barrier transport to get PGRN into the central nervous system (CNS). Gene therapy utilizes viral vectors injected into the cisterna magna to restore PGRN levels in the CNS.

      Concluding remarks and future perspectives

      Within the CNS, PGRN plays important roles in lysosomal health, neuronal survival and function, the autophagy–lysosomal pathway, astrogliosis, and neuroinflammation. PGRN interacts with multiple receptors to carry out these activities, but its actual mechanism of action and signaling cascades are poorly understood. Insights into the processing of PGRN into its granulin cleavage products in the lysosome via sortilin or PSAP are relatively recent, and much remains to be learned about the interplay between PGRN and granulins, as well as the contribution of granulins to nervous system homeostasis in health and disease. Unlike most risk genes, such as APP, SNCA, and SOD1, which are disease specific, PGRN deficiency appears as a common feature in a diverse group of neurodegenerative diseases including FTD, NCL, AD, PD, ALS, and LATE, as well as in normal aging, supporting the idea that GRN is a unique risk gene that sits atop the regulatory cascade controlling brain health (Figure 4). The fact that PGRN regulates lysosomal function indicates that lysosomal health may be a common requirement for brain health in both disease and normal aging and points to a unifying underlying mechanism of neurodegeneration. A better understanding of the role of PGRN in brain health could open novel therapeutic avenues for treating neurodegeneration.
      Figure 4
      Figure 4Proposed disease cascade by which PGRN deficiency leads to neurodegeneration.
      PGRN deficiency resulting from genetic mutations appears to be a common feature in a diverse group of neurodegenerative diseases (e.g., FTD, NCL, AD, PD, ALS, and LATE) as well as normal aging, suggesting that PGRN is a critical regulator of brain health. Links between PGRN deficiency and TDP-43 pathology, lysosomal dysfunction, complement activation, inflammation, astrogliosis, microglia hyperactivity, and synaptic and neuronal destruction further support the hypothesis that PGRN deficiency is a shared pathomechanism underlying neurodegeneration and cognitive decline. Abbreviations: PGRN, progranulin; TDP-43, transactivation response DNA-binding protein 43.
      PGRN deficiency is a common feature across several neurodegenerative diseases and argues for the identification of therapeutic targets that can increase PGRN levels to restore or normalize neuronal health. But such drug development strategies also raise questions. Although PGRN deficiency is associated with increased risk of autoimmune diseases, its deficiency primarily leads to brain disorders. How do the lysosomes and peripheral immune system compensate for PGRN deficiency? Understanding the apparent redundancy of PGRN in peripheral organs may also enable the development of replacement therapies for PGRN deficiency in the brain. Several additional, important questions remain to be answered regarding GRN expression and modulation of PGRN activities (see Outstanding questions).
      While ongoing basic research continues to tease apart PGRN biology, clinical research has identified four different strategies to increase levels of PGRN in FTD-GRN: blocking the degradation pathway of PGRN, gene therapy, protein replacement therapy, and small molecule histone deacetylase (HDAC) inhibitors that increase expression of PGRN [
      • Cenik B.
      • et al.
      Suberoylanilide hydroxamic acid (vorinostat) up-regulates progranulin transcription: rational therapeutic approach to frontotemporal dementia.
      ]. AL001 is a human monoclonal antibody that blocks the sortilin–PGRN interaction to prevent the degradation of PGRN, thereby increasing the half-life of PGRN and elevating the level of PGRN in the brain and serum from twofold to threefold. The Phase 1 study of AL001 (NCT03636204)iii is complete, and the Phase 2 study of AL001 (INFRONT-2; NCT03987295)iv to evaluate the safety of long-term dosing in patients with FTD-GRN is ongoing. Additionally, AL001 is currently being investigated in a pivotal Phase 3 study (INFRONT-3; NCT04374136)v to evaluate safety and efficacy in FTD-GRN. Another ongoing Phase 2 study (NCT05053035)vi is evaluating the safety, pharmacokinetics, and pharmacodynamics of AL001 in ALS-C9orf72.
      Gene therapy is an alternative therapeutic strategy under investigation in separate Phase 1/2 studies to evaluate the use of two different AAV vectors to raise PGRN levels. The PROCLAIM trial (NCT04408625)vii is testing the experimental therapy PR006 with the AAV9 vector, and the upliFT-D study (NCT04747431)viii is testing the experimental therapy PBFT02 with the AAV1 vector. These studies will also determine if using an AAV vector with DNA encoding the GRN gene injected into the cisterna magna is an option for increasing PGRN levels in the CNS. AAV serotypes expressing a GFP reporter tested in cell culture and in vivo via intracerebroventricular injection in neonatal mice appear to vary in their selectivity for neurons and astroglia, with AAV9 demonstrating preference for neuronal transduction [
      • Hammond S.L.
      • et al.
      Cellular selectivity of AAV serotypes for gene delivery in neurons and astrocytes by neonatal intracerebroventricular injection.
      ]. Whether AAV serotype transduction selectivity will follow similar patterns in adult humans via cisterna magna delivery remains to be seen. In addition, protein replacement therapy is being evaluated as a method to increase PGRN levels. Because the transport of large molecules across the blood–brain barrier (BBB) is a limiting factor, a protein transport method is being investigated to increase the penetrance of PGRN into the CNS [
      • Logan T.
      • et al.
      Rescue of a lysosomal storage disorder caused by Grn loss of function with a brain penetrant progranulin biologic.
      ]. The BBB protein transport vehicle fused to PGRN (DNL593) is currently in investigational new drug-enabling studiesix. Lastly, small molecule HDAC inhibitors that increase GRN transcription are being used to elevate PGRN levels in patients with FTD-GRNx. HDACs are key enzymes in the epigenetic regulation of gene expression. By removing acetyl groups from histone proteins in chromatin, HDACs compress the chromatin, and in this way, repress gene expression. HDAC inhibition appears to release chromatin compression and increases the expression of GRN, as well as other genes.
      In conclusion, the data support that PGRN deficiency is causative of NCL and FTD-GRN and is a risk factor for multiple other neurodegenerative diseases, including AD, PD, ALS, and LATE. The neurotrophic, anti-inflammatory roles of PGRN, along with its roles in lysosomal function, suggest that PGRN elevation may be a promising therapeutic approach to alter the course of multiple neurodegenerative diseases.
      How does PGRN promote neuronal survival and axonal growth in early development as well as in the aging brain?
      How does PGRN regulate the function of microglia in the brain?
      How much of PGRN activity is mediated by the full-length protein versus its 6-kDa granulin cleavage products?
      Do the granulins act both extracellularly and in the lysosome, and if yes, how?
      How does PGRN regulate the autophagy–lysosome pathway?
      Can the autophagy–lysosome pathway regulate PGRN levels and function?
      What is the relationship between PGRN, TMEM106B, and other risk factors?
      How does PGRN impact brain health and disease so broadly?
      What role does PGRN play in the peripheral immune system?
      What is the critical developmental period during which PGRN is essential?
      How does the central (and peripheral) nervous system compensate for PGRN deficiency until adulthood and throughout aging given the mid- to late-life onset of most neurodegenerative diseases?
      Will PGRN-elevating therapies still be effective if administered in adults?
      How tightly do the therapeutic levels of PGRN need to be regulated given that it mediates cell cycle progression, inflammation, wound healing, and tumorigenesis?

      Acknowledgments

      Editorial support and publication assistance were provided by SCIENT Healthcare Communications.

      Declaration of interests

      No interests are declared.

      Resources

      References

        • Bateman A.
        • et al.
        A brief overview of progranulin in health and disease.
        Methods Mol. Biol. 2018; 1806: 3-15
        • Neill T.
        • et al.
        EphA2 is a functional receptor for the growth factor progranulin.
        J. Cell Biol. 2016; 215: 687-703
        • Cui Y.
        • et al.
        Progranulin: a conductor of receptors orchestra, a chaperone of lysosomal enzymes and a therapeutic target for multiple diseases.
        Cytokine Growth Factor Rev. 2019; 45: 53-64
        • Van Damme P.
        • et al.
        Progranulin functions as a neurotrophic factor to regulate neurite outgrowth and enhance neuronal survival.
        J. Cell Biol. 2008; 181: 37-41
        • Gass J.
        • et al.
        Progranulin: an emerging target for FTLD therapies.
        Brain Res. 2012; 1462: 118-128
        • Hu F.
        • et al.
        Sortilin-mediated endocytosis determines levels of the frontotemporal dementia protein, progranulin.
        Neuron. 2010; 68: 654-667
        • Zhou X.
        • et al.
        Prosaposin facilitates sortilin-independent lysosomal trafficking of progranulin.
        J. Cell Biol. 2015; 210: 991-1002
        • Butler V.J.
        • et al.
        Progranulin stimulates the in vitro maturation of pro-cathepsin D at acidic pH.
        J. Mol. Biol. 2019; 431: 1038-1047
        • Kamate M.
        • et al.
        Neuronal ceroid lipofuscinosis type-11 in an adolescent.
        Brain and Development. 2019; 41: 542-545
        • Johnson T.B.
        • et al.
        Therapeutic landscape for Batten disease: current treatments and future prospects.
        Nat. Rev. Neurol. 2019; 15: 161-178
        • Warrier V.
        • et al.
        Genetic basis and phenotypic correlations of the neuronal ceroid lipofusinoses.
        Biochim. Biophys. Acta. 2013; 1832: 1827-1830
        • Baker M.
        • et al.
        Mutations in progranulin cause tau-negative frontotemporal dementia linked to chromosome 17.
        Nature. 2006; 442: 916-919
        • Goldman J.S.
        • Van Deerlin V.M.
        Alzheimer's disease and frontotemporal dementia: the current state of genetics and genetic testing since the advent of next-generation sequencing.
        Mol. Diagn. Ther. 2018; 22: 505-513
        • Curtis A.F.
        • et al.
        Sex differences in the prevalence of genetic mutations in FTD and ALS: A meta-analysis.
        Neurology. 2017; 89: 1633-1642
        • Suzuki M.
        • et al.
        Roles of progranulin in sexual differentiation of the developing brain and adult neurogenesis.
        J. Reprod. Dev. 2009; 55: 351-355
        • Cruts M.
        • et al.
        Null mutations in progranulin cause ubiquitin-positive frontotemporal dementia linked to chromosome 17q21.
        Nature. 2006; 442: 920-924
        • Mackenzie I.R.A.
        The neuropathology and clinical phenotype of FTD with progranulin mutations.
        Acta Neuropathol. 2007; 114: 49-54
        • Smith K.R.
        • et al.
        Strikingly different clinicopathological phenotypes determined by progranulin-mutation dosage.
        Am. J. Hum. Genet. 2012; 90: 1102-1107
        • Ward M.E.
        • et al.
        Individuals with progranulin haploinsufficiency exhibit features of neuronal ceroid lipofuscinosis.
        Sci. Transl. Med. 2017; 9eaah5642
        • Bellenguez C.
        • et al.
        Genetics of Alzheimer's disease: where we are, and where we are going.
        Curr. Opin. Neurobiol. 2020; 61: 40-48
        • Nelson P.T.
        • et al.
        Limbic-predominant age-related TDP-43 encephalopathy (LATE): consensus working group report.
        Brain. 2019; 142: 1503-1527
        • van Blitterswijk M.
        • et al.
        Genetic modifiers in carriers of repeat expansions in the C9ORF72 gene.
        Mol. Neurodegener. 2014; 9: 38
        • Sleegers K.
        • et al.
        Progranulin genetic variability contributes to amyotrophic lateral sclerosis.
        Neurology. 2008; 71: 253-259
        • Rhinn H.
        • Abeliovich A.
        Differential aging analysis in human cerebral cortex identifies variants in TMEM106B and GRN that regulate aging phenotypes.
        Cell Syst. 2017; 4: 404-415.e5
        • Pao W.C.
        • et al.
        Hippocampal sclerosis in the elderly: genetic and pathologic findings, some mimicking Alzheimer disease clinically.
        Alzheimer Dis. Assoc. Disord. 2011; 25: 364-368
        • Nalls M.A.
        • et al.
        Identification of novel risk loci, causal insights, and heritable risk for Parkinson's disease: a meta-analysis of genome-wide association studies.
        Lancet Neurol. 2019; 18: 1091-1102
        • Jian J.
        • et al.
        Association between progranulin and Gaucher disease.
        EBioMedicine. 2016; 11: 127-137
        • Yao Y.-N.
        • et al.
        Reduced plasma progranulin levels are associated with the severity of Parkinson’s disease.
        Neurosci. Lett. 2020; 725134873
        • Al-Ayadhi L.Y.
        • Mostafa G.A.
        Low plasma progranulin levels in children with autism.
        J. Neuroinflammation. 2011; 8: 111
        • Mackenzie I.R.
        • et al.
        Heterogeneity of ubiquitin pathology in frontotemporal lobar degeneration: classification and relation to clinical phenotype.
        Acta Neuropathol. 2006; 112: 539-549
        • Arai T.
        • et al.
        TDP-43 is a component of ubiquitin-positive tau-negative inclusions in frontotemporal lobar degeneration and amyotrophic lateral sclerosis.
        Biochem. Biophys. Res. Commun. 2006; 351: 602-611
        • Amador-Ortiz C.
        • et al.
        TDP-43 immunoreactivity in hippocampal sclerosis and Alzheimer's disease.
        Ann. Neurol. 2007; 61: 435-445
        • Higashi S.
        • et al.
        Concurrence of TDP-43, tau and alpha-synuclein pathology in brains of Alzheimer's disease and dementia with Lewy bodies.
        Brain Res. 2007; 1184: 284-294
        • Gao J.
        • et al.
        Pathomechanisms of TDP-43 in neurodegeneration.
        J. Neurochem. 2018; 146: 7-20https://doi.org/10.1111/jnc.14327
        • Nussbacher J.K.
        • et al.
        Disruption of RNA metabolism in neurological diseases and emerging therapeutic interventions.
        Neuron. 2019; 102: 294-320
        • Cohen-Kaplan V.
        • et al.
        The ubiquitin-proteasome system and autophagy: Coordinated and independent activities.
        Int. J. Biochem. Cell Biol. 2016; 79: 403-418
        • Tanji K.
        • et al.
        p62/sequestosome 1 binds to TDP-43 in brains with frontotemporal lobar degeneration with TDP-43 inclusions.
        J. Neurosci. Res. 2012; 90: 2034-2042
        • Laird A.S.
        • et al.
        Progranulin is neurotrophic in vivo and protects against a mutant TDP-43 induced axonopathy.
        PLoS One. 2010; 5e13368
        • Beel S.
        • et al.
        Progranulin reduces insoluble TDP-43 levels, slows down axonal degeneration and prolongs survival in mutant TDP-43 mice.
        Mol. Neurodegener. 2018; 13: 55
        • Chang M.C.
        • et al.
        Progranulin deficiency causes impairment of autophagy and TDP-43 accumulation.
        J. Exp. Med. 2017; 214: 2611-2628
        • Zhang J.
        • et al.
        Neurotoxic microglia promote TDP-43 proteinopathy in progranulin deficiency.
        Nature. 2020; 588: 459-465
        • Zhang Y.J.
        • et al.
        Progranulin mediates caspase-dependent cleavage of TAR DNA binding protein-43.
        J. Neurosci. 2007; 27: 10530-10534
        • Petkau T.L.
        • et al.
        Selective depletion of microglial progranulin in mice is not sufficient to cause neuronal ceroid lipofuscinosis or neuroinflammation.
        J. Neuroinflammation. 2017; 14: 225
        • Arrant A.E.
        • et al.
        Restoring neuronal progranulin reverses deficits in a mouse model of frontotemporal dementia.
        Brain. 2017; 140: 1447-1465
        • Arrant A.E.
        • et al.
        Progranulin gene therapy improves lysosomal dysfunction and microglial pathology associated with frontotemporal dementia and neuronal ceroid lipofuscinosis.
        J. Neurosci. 2018; 38: 2341-2358
        • Lui H.
        • et al.
        Progranulin deficiency promotes circuit-specific synaptic pruning by microglia via complement activation.
        Cell. 2016; 165: 921-935
        • Minami S.S.
        • et al.
        Progranulin protects against amyloid beta deposition and toxicity in Alzheimer's disease mouse models.
        Nat. Med. 2014; 20: 1157-1164
        • Wu Y.
        • et al.
        Microglial lysosome dysfunction contributes to white matter pathology and TDP-43 proteinopathy in GRN-associated FTD.
        Cell Rep. 2021; 36109581
        • Tanaka Y.
        • et al.
        Exacerbated inflammatory responses related to activated microglia after traumatic brain injury in progranulin-deficient mice.
        Neuroscience. 2013; 231: 49-60
        • Hickman S.
        • et al.
        Microglia in neurodegeneration.
        Nat. Neurosci. 2018; 21: 1359-1369
        • Stevens B.
        • et al.
        The classical complement cascade mediates CNS synapse elimination.
        Cell. 2007; 131: 1164-1178
        • Katzeff J.S.
        • et al.
        Altered serum protein levels in frontotemporal dementia and amyotrophic lateral sclerosis indicate calcium and immunity dysregulation.
        Sci. Rep. 2020; 10: 13741
        • Almeida S.
        • et al.
        Progranulin, a glycoprotein deficient in frontotemporal dementia, is a novel substrate of several protein disulfide isomerase family proteins.
        PLoS One. 2011; 6e26454
        • Menzel L.
        • et al.
        Progranulin protects against exaggerated axonal injury and astrogliosis following traumatic brain injury.
        Glia. 2017; 65: 278-292
        • Petkau T.L.
        • et al.
        Core neuropathological abnormalities in progranulin-deficient mice are penetrant on multiple genetic backgrounds.
        Neuroscience. 2016; 315: 175-195
        • Ghoshal N.
        • et al.
        Core features of frontotemporal dementia recapitulated in progranulin knockout mice.
        Neurobiol. Dis. 2012; 45: 395-408
        • Yin F.
        • et al.
        Exaggerated inflammation, impaired host defense, and neuropathology in progranulin-deficient mice.
        J. Exp. Med. 2010; 207: 117-128
        • Heller C.
        • et al.
        Plasma glial fibrillary acidic protein is raised in progranulin-associated frontotemporal dementia.
        J. Neurol. Neurosurg. Psychiatry. 2020; 91: 263-270
        • Liddelow S.A.
        • et al.
        Neurotoxic reactive astrocytes are induced by activated microglia.
        Nature. 2017; 541: 481-487
        • Evers B.M.
        • et al.
        Lipidomic and transcriptomic basis of lysosomal dysfunction in progranulin deficiency.
        Cell Rep. 2017; 20: 2565-2574
        • Valdez C.
        • et al.
        Progranulin-mediated deficiency of cathepsin D results in FTD and NCL-like phenotypes in neurons derived from FTD patients.
        Hum. Mol. Genet. 2017; 26: 4861-4872
        • Huang M.
        • et al.
        Network analysis of the progranulin-deficient mouse brain proteome reveals pathogenic mechanisms shared in human frontotemporal dementia caused by GRN mutations.
        Acta Neuropathol. Commun. 2020; 8: 163
        • Gotzl J.K.
        • et al.
        Common pathobiochemical hallmarks of progranulin-associated frontotemporal lobar degeneration and neuronal ceroid lipofuscinosis.
        Acta Neuropathol. 2014; 127: 845-860
        • Bunk J.
        • et al.
        Cathepsin D variants associated with neurodegenerative diseases show dysregulated functionality and modified alpha-synuclein degradation properties.
        Front. Cell Dev. Biol. 2021; 9581805
        • Bae E.-J.
        • et al.
        Haploinsufficiency of cathepsin D leads to lysosomal dysfunction and promotes cell-to-cell transmission of alpha-synuclein aggregates.
        Cell Death Dis. 2015; 6e1901
        • Cullen V.
        • et al.
        Cathepsin D expression level affects alpha-synuclein processing, aggregation, and toxicity in vivo.
        Mol. Brain. 2009; 2: 5
        • Marques A.R.A.
        • et al.
        Enzyme replacement therapy with recombinant pro-CTSD (cathepsin D) corrects defective proteolysis and autophagy in neuronal ceroid lipofuscinosis.
        Autophagy. 2020; 16: 811-825
        • Moore K.M.
        • et al.
        Age at symptom onset and death and disease duration in genetic frontotemporal dementia: an international retrospective cohort study.
        Lancet Neurol. 2020; 19: 145-156
        • Clayton E.L.
        • Isaacs A.M.
        Progranulin and TMEM106B: when two become wan.
        EMBO Rep. 2020; 21e51668
        • Kessenbrock K.
        • et al.
        Proteinase 3 and neutrophil elastase enhance inflammation in mice by inactivating antiinflammatory progranulin.
        J. Clin. Invest. 2008; 118: 2438-2447
        • Zhu J.
        • et al.
        Conversion of proepithelin to epithelins: roles of SLPI and elastase in host defense and wound repair.
        Cell. 2002; 111: 867-878
        • Petkau T.L.
        • et al.
        Progranulin expression in the developing and adult murine brain.
        J. Comp. Neurol. 2010; 518: 3931-3947
        • Banzhaf-Strathmann J.
        • et al.
        Promoter DNA methylation regulates progranulin expression and is altered in FTLD.
        Acta Neuropathol. Commun. 2013; 1: 16
        • Frew J.
        • et al.
        Premature termination codon readthrough upregulates progranulin expression and improves lysosomal function in preclinical models of GRN deficiency.
        Mol. Neurodegener. 2020; 15: 21
        • Mohan S.
        • et al.
        Processing of progranulin into granulins involves multiple lysosomal proteases and is affected in frontotemporal lobar degeneration.
        Mol. Neurodegener. 2021; 16: 51
        • Holler C.J.
        • et al.
        Intracellular proteolysis of progranulin generates stable, lysosomal granulins that are haploinsufficient in patients with frontotemporal dementia caused by GRN mutations.
        eNeuro. 2017; 4ENEURO.0100-17.2017
        • Bateman A.
        • Bennett H.P.
        The granulin gene family: from cancer to dementia.
        BioEssays. 2009; 31: 1245-1254
        • Butler V.J.
        • et al.
        Multi-granulin domain peptides bind to pro-cathepsin D and stimulate its enzymatic activity more effectively than progranulin in vitro.
        Biochemistry. 2019; 58: 2670-2674
        • Salazar D.A.
        • et al.
        The progranulin cleavage products, granulins, exacerbate TDP-43 toxicity and increase TDP-43 levels.
        J. Neurosci. 2015; 35: 9315-9328
        • Butler V.J.
        • et al.
        Age- and stress-associated C. elegans granulins impair lysosomal function and induce a compensatory HLH-30/TFEB transcriptional response.
        PLoS Genet. 2019; 15e1008295
        • Qiao G.
        • et al.
        Granulin A synergizes with cisplatin to inhibit the growth of human hepatocellular carcinoma.
        Int. J. Mol. Sci. 2018; 19: 3060
        • Lee W.C.
        • et al.
        Targeted manipulation of the sortilin-progranulin axis rescues progranulin haploinsufficiency.
        Hum. Mol. Genet. 2014; 23: 1467-1478
        • Carrasquillo M.M.
        • et al.
        Genome-wide screen identifies rs646776 near sortilin as a regulator of progranulin levels in human plasma.
        Am. J. Hum. Genet. 2010; 87: 890-897
        • Zheng Y.
        • et al.
        C-terminus of progranulin interacts with the beta-propeller region of sortilin to regulate progranulin trafficking.
        PLoS One. 2011; 6e21023https://doi.org/10.1371/journal.pone.0021023
        • De Muynck L.
        • et al.
        The neurotrophic properties of progranulin depend on the granulin E domain but do not require sortilin binding.
        Neurobiol. Aging. 2013; 34: 2541-2547
        • Gass J.
        • et al.
        Progranulin regulates neuronal outgrowth independent of sortilin.
        Mol. Neurodegener. 2012; 7: 33
        • Ludwig T.
        • et al.
        Targeted disruption of the mouse cation-dependent mannose 6-phosphate receptor results in partial missorting of multiple lysosomal enzymes.
        EMBO J. 1993; 12: 5225-5235
        • Liu Q.
        • et al.
        Neuronal LRP1 knockout in adult mice leads to impaired brain lipid metabolism and progressive, age-dependent synapse loss and neurodegeneration.
        J. Neurosci. 2010; 30: 17068-17078
        • Nicholson A.M.
        • et al.
        Prosaposin is a regulator of progranulin levels and oligomerization.
        Nat. Commun. 2016; 7: 11992
        • Cenik B.
        • et al.
        Suberoylanilide hydroxamic acid (vorinostat) up-regulates progranulin transcription: rational therapeutic approach to frontotemporal dementia.
        J. Biol. Chem. 2011; 286: 16101-16108
        • Hammond S.L.
        • et al.
        Cellular selectivity of AAV serotypes for gene delivery in neurons and astrocytes by neonatal intracerebroventricular injection.
        PLoS One. 2017; 12e0188830
        • Logan T.
        • et al.
        Rescue of a lysosomal storage disorder caused by Grn loss of function with a brain penetrant progranulin biologic.
        Cell. 2021; 184: 4651-4668.e25

      Glossary

      Allele
      a specific version of a gene.
      Autophagy
      process of clearing damaged cells and unwanted materials.
      Cisterna magna
      subarachnoid space between the cerebellum and the medulla oblongata.
      Frontotemporal dementia
      a clinical term for the spectrum of diseases that include changes in behavior, language, executive function, and motor symptoms.
      Frontotemporal lobar degeneration (FTLD)
      a pathology term referring to the neurodegeneration predominantly found in the frontal and temporal lobes.
      Gaucher disease
      a rare metabolic disease in which an enzyme to break down lipids is missing.
      Gliosis
      a reactive change in glial cells in response to damage that creates scarring in the CNS.
      Haplotype
      a group of alleles inherited from a single parent.
      Heterozygous loss of function (LOF)
      when the function of a specific gene is lost on one allele.
      Homozygous loss of function
      when the function of a specific gene is lost at both alleles due to both copies of an allele having mutation(s).
      Limbic-predominant age-related TDP-43 encephalopathy (LATE)
      a neurodegenerative condition affecting elderly adults that presents clinically with amnestic dementia and is characterized with TDP-43 proteinopathy in limbic structures.
      Lipofuscinosis
      abnormal accumulation of lipofuscin in the tissues.
      Neuronal ceroid lipofuscinosis (NCL) type 11 (CLN11)
      neuronal ceroid lipofuscinosis caused by homozygous loss of function of the granulin gene (GRN).
      Neurotrophic
      involved in the regulation of nervous tissues.
      Proteinopathy
      the pathogenic accumulation of proteins.
      Single-nucleotide polymorphism (SNP)
      the most common type of genetic mutations, in which there is a variation of a single base pair.
      Ubiquitin–proteasome system
      a system that regulates protein degradation and homeostasis.