Introduction

Neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are a heterogeneous group of neurological disorders caused by progressive structural and functional degeneration of the central nervous system (CNS). These diseases are pathologically characterized by the deposition of misfolded protein aggregates in the brain, such as amyloid-β and tau in AD, α-synuclein (α-Syn) in PD and trans-activator response (TDR) DNA-binding protein 43 (TDP-43) in FTD and ALS patients1,2. While these proteinopathy hallmarks are classically associated with specific neurodegenerative disorders, emerging evidence reveals TDP-43 pathology in AD3,4 and PD5, demonstrating their broader involvement but different levels across multiple diseases. Aging is the major risk factor for neurodegenerative diseases. With the global increase in the aging population, such incurable diseases bring both economic burdens and mental sufferings to society and families6,7,8. Therefore, exploring the pathogenesis of neurodegenerative diseases and new treatment options has important scientific significance.

The brain is the second most lipid-rich organ in the body. Lipids function not only as fundamental molecules for the structural composition and energy reservoir, but also serve as mediators of a variety of cell signaling in the brain9. Lipid homeostasis is governed by finely regulated lipid metabolism. In most cells, lysosomes are the main organelles responsible for intracellular lipid metabolisms10. Abnormal lipid accumulation caused by lysosomal dysfunction is usually associated with various disease conditions collectively termed lysosomal lipid storage disorders or lipidosis11, a subcategory of lysosomal storage diseases (LSDs), often leading to progressive neurodegeneration when the CNS is affected12,13. Indeed, dysregulated lipid metabolism has been implicated in various neurodegenerative diseases, such as AD, PD and FTD14. Abnormal lipid accumulation within glia was first described by Alois Alzheimer in the original report15. Nevertheless, the molecular mechanisms underlying lipid accumulation and the contribution of abnormal lipid metabolism to neurodegeneration are still poorly understood and merit further investigation.

Genetic studies have shown that mutants or variants of GRN, a gene encoding a lysosomal protein progranulin (PGRN), are associated with various neurodegenerative diseases such as FTD, ALS, AD and PD16,17,18,19,20,21,22,23,24. Interestingly, PGRN has also been implicated in the metabolism of various lipids in the brain and lipid droplet formation in microglia25,26,27,28. It has been suggested that PGRN deficiency-mediated alteration of lysosomal function may lead to dysregulated lipid metabolism, subsequently inducing lipid droplet formation. Hence, PGRN may represent a central factor linking neurodegeneration and lipid dysregulation. Nevertheless, the exact molecular mechanisms underlying the lysosomal protein regulation of lipid metabolism and lipid droplet formation remain elusive.

In this review, we discuss the current knowledge of the role of lysosomal protein PGRN in neurodegenerative diseases, with a particular focus on PGRN as an emerging regulator of microglial and neuronal lipid metabolism. We also discuss various strategies for targeting PGRN as potential therapeutics for these neurodegenerative diseases.

Neuroinflammation and dysregulated lipid metabolism in neurodegenerative diseases

Neuroinflammation mediated by microglia represents a prominent pathological feature of aging and neurodegenerative diseases29,30. As resident myeloid cells in the CNS, microglia play a crucial role in maintaining brain homeostasis31,32. However, in response to aging and neurodegeneration, microglia can be activated, resulting in morphological and molecular changes and release of pro-inflammatory cytokines33,34,35. Recently, lipid droplets have been considered as dynamic organelles comprising neutral lipids (triacylglycerides and sterol esters) not only for lipid storage but also for mediators of inflammation in myeloid cells36. Under aging and neurodegenerative conditions, lipid droplets were found to be significantly increased in microglia26,37,38,39. This is probably due to excessive lipid uptake and biogenesis or/and impaired lipid metabolism and degradation. Lipid droplet-accumulating microglia show dysfunctional phagocytosis and secrete proinflammatory cytokines26. Moreover, disease-associated microglia and senescent microglia have been identified in aging or neurodegenerative conditions displaying a status associated with abnormal lipid profiles40,41,42,43. Lipid droplet-accumulating microglia, disease-associated microglia and senescent microglia show overlapping but not identical molecular signatures. Hence, dysregulated lipid metabolism in microglia has emerged as a hallmark of neuroinflammation and an important contributing factor to neurodegeneration.

Advances in lipidomic, metabolomic and transcriptomic technologies have contributed to uncovering details and the underlying mechanisms of dysregulated lipid metabolism in neurodegenerative diseases. For instance, lipidomic analysis of AD patients and animal models has revealed alterations of numerous lipids including glycerolipids, sphingolipids and cholesterol44,45,46. Altered lipid levels further influence disease pathogenesis through interacting with misfolded proteins, modulating neuroinflammation and oxidative stress or regulating myelin degeneration/regeneration14. Moreover, there is increasing interest in using lipids as biomarkers for neurodegenerative diseases47,48. Therefore, understanding how lipid metabolism is altered during neurodegeneration and how lipid dysregulation contributes to the pathogenesis of these diseases has become a central focus of the research.

Importantly, several genetic analyses have uncovered multiple genes involved in lipid metabolism as risk factors for neurodegenerative diseases49,50,51,52,53. These risk genes include APOE, TREM2, GRN, CLU, GBA1, LRRK2, ABCA7, etc. Hence, lipid metabolism dysregulation may not only be a consequence of neurodegeneration but also a causal factor for the development and progression of these diseases. Mechanisms underlying how these risk factors affect lipid metabolism and lead to neurodegeneration are currently subjects of intensive investigation. Targeting lipid metabolism and restoring lipid homeostasis holds potential for ameliorating or even curing these diseases.

Neurodegenerative diseases associated with PGRN deficiency

PGRN is a lysosomal glycoprotein mainly expressed in microglia and neurons in the CNS54. PGRN can be secreted into extracellular compartments and function as an immunomodulator to regulate microglial activation and neuroinflammation or as a neurotrophin to promote neuron survival and neurite growth55,56. PGRN can also be internalized into lysosomes57,58, thereby playing a crucial role in exerting lysosomal function and facilitating lysosomal homeostasis. Recent studies have shown that PGRN may maintain lysosomal acidification and participate in lysosomal transport of key lysosomal sphingolipid metabolism enzymes such as prosaposin (PSAP)59,60,61. Importantly, PGRN can also be cleaved into 7.5 individual granulin peptides (GRN A-G) in the lysosomes, which may function differently from the precursor PGRN62,63,64,65,66. The Amiee Kao lab has demonstrated that endolysosomal proteases cleave full-length PGRN into both multi- and single-granulin fragments67. Their research further revealed a novel role for asparagine endopeptidase (AEP) in PGRN processing, showing its unique ability to specifically liberate granulin F from the proprotein. In brain tissues of frontotemporal lobar degeneration with GRN mutations patients, degenerated regions exhibited both elevated AEP activity and enhanced PGRN cleavage into granulin F, whereas these changes were absent in unaffected areas68. Moreover, they found that in C. elegans, PGRN-1 undergoes proteolytic processing to generate granulin-1, -2 and -3, with granulin-3 exhibiting the most severe pathological manifestations, including enzymatic dysfunction, lysosomal shrinkage, and increased stress sensitivity. Notably, while granulin-3 overexpression rescues lysosomal morphology defects, it fails to reverse associated impairments in aspartyl protease activity, developmental processes, stress tolerance, or TDP-43-mediated toxicity67. Nevertheless, the mechanisms of the cleavage and exert role of these peptides remain to be fully dissected.

The associations of GRN gene with various neurodegenerative diseases have been assessed using genome-wide association studies20,21,22,24. Heterozygous mutation in GRN usually leads to frontotemporal lobar degeneration, followed by rapid development of FTD16 (Fig. 1). Most heterozygous GRN mutations introduce a premature stop codon, resulting in PGRN deficiency through nonsense mediated mRNA decay16. Whereas missense or nonsense mutations in some cases lead to loss of PGRN function through other mechanisms such as cytoplasmic missense, increased protein degradation or decreased secretion17,69. GRN mutations account for ~20% of familial FTD cases, and haploinsufficiency of PGRN almost always leads to FTD70. The GRN-FTD is neuropathologically characterized by TDP-43 deposition, microglia hyperplasia and cortical neuron loss54,65. Notably, homozygous loss-of-function mutations in the GRN gene lead to a subtype of LSDs, known as neuronal ceroid lipofuscinosis, neuropathologically characterized by intracellular accumulation of autofluorescent lipofuscin (Fig. 1). The main clinical symptoms of neuronal ceroid lipofuscinosis include retinitis pigmentosa, cerebellar ataxia, and cognitive impairment71. Interestingly, GRN-FTD and neuronal ceroid lipofuscinosis share some neuropathological features. For instance, lipofuscin deposition associated with lysosomal dysfunction was also found in the brain and retina of GRN-FTD patients72. Interestingly, while Grn−/− mice show neuropathological and behavioral phenotypes resembling FTD patients, Grn+/− mice are generally indistinguishable from the wild type control73,74. This is probably due to the requirement of different doses of PGRN to maintain its physiological integrity in humans and mice. Therefore, most animal experiments examining PGRN functions are based on Grn−/− mice.

Fig. 1: Association of GRN genotype and PGRN dosage with various neurodegenerative diseases.
figure 1

Haploinsufficiency of PGRN caused by heterozygous GRN mutations almost always leads to FTD, whereas homozygous mutation of GRN causes NCL. Additionally, variants in GRN are linked to a high risk of developing AD, PD and ALS. Under physiological conditions, PGRN is located in lysosomes and plays important roles in keeping microglia homeostatic, promoting neuronal survival, regulating lysosomal functions and maintaining lipid homeostasis. However, deficiency of PGRN often results in neuroinflammation, neuronal loss, lysosomal dysfunction and dysregulated lipid metabolism, leading to neurodegeneration. Notably, some GRN SNPs could exert a protective effect in cognitively healthy centenarians.

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The genetic association between GRN variants and AD pathology has been evaluated in autopsy and clinical cohort studies75,76. PGRN levels in the blood and CSF are correlated with the neurodegeneration and cognitive decline in AD patients77,78. AD patients with the frequent GRN variant rs5848 had an increased incidence of hippocampal sclerosis and TDP-43 deposits. Some studies reported that PGRN deficiency increased amyloid-β deposition in AD mouse models, and overexpressing PGRN both in vivo and in vitro could reduce the burden of amyloid plaques79,80,81. Whereas other studies observed that PGRN deficiency leads to a reduction in diffuse plaques82,83. Additionally, PGRN deficiency also accelerated tau deposition and phosphorylation in mice models expressing human tau proteins83,84,85. Hence, the exact molecular mechanisms of PGRN and AD remain to be elucidated.

A decrease in PGRN levels was also found in PD patients86. Genome-wide association studies of PD patients revealed GRN variants23 and a decrease in PGRN in the brain were strongly associated with an increased chance of developing PD87. Recent studies suggested that PGRN might participate in α-Syn degradation by accelerating the autophagy lysosome pathway88. Genome-wide association studies of ALS patients revealed several variants in GRN are associated with earlier onset and shorter survival19,20. Moreover, the level of PGRN in the cerebrospinal fluid of ALS patients was associated with smaller motor amplitudes89. Measurement of PGRN in the spinal cord of the ALS mouse model indicated that PGRN increased after the onset of degenerative symptoms, which may be regulated through microglia90.

Collectively, the aforementioned studies indicate that mutations or variants in GRN are closely associated with a broad spectrum of neurodegenerative diseases (Fig. 1). Notably, it has also been identified that single nucleotide polymorphisms (SNPs) in the GRN and TMEM106B genes demonstrate protective effects and contribute to healthy aging in centenarians91 (Fig. 1). While previous studies implicate a critical role of PGRN in maintaining lysosomal function and restricting neuroinflammation, further investigations on the exact functions of PGRN in microglia and neurons are crucial for elucidating its neuroprotective mechanisms under various neurodegenerative conditions.

PGRN as a regulator of lipid metabolism in neurodegenerative diseases

PGRN deficiency causes lipid dysregulation in the brain and microglia

The dose-dependent effect of PGRN level on lipofuscin deposition and the critical role of PGRN in lysosomal function prompted the hypothesis that PGRN may play an important role in lipid metabolism. A lipidomic study demonstrated altered levels of triacylglycerol, diacylglycerol, phosphatidylethanolamine and phosphatidylserine in PGRN-deficient human or mouse brains25. Transcriptome analysis of brains from Grn−/− mice showcased differential expression patterns of genes involved in lysosomes, immune regulation and lipid metabolism, supporting that GRN ablation leads to lysosomal dysfunction and lipid abnormalities25,92. Proteomic analysis of the whole brain of young (3-month) and old (19-month) mice revealed that PGRN deficiency led to decreased expression of lysosomal proteins involved in lipid metabolism in young mice, and this lysosomal dysregulation was exacerbated with age93.

To specify which cell types are most affected, a single-nucleus RNA-sequencing of Grn−/− mice showed that microglia were the first cell type to exhibit transcriptomic changes and transition from homeostatic state to disease state56,94. Other studies confirmed that PGRN dysfunction in microglia leads to metabolic changes of both glycerolipids and sphingolipids26,27,28,95. A correlation of Grn mutations and abnormal sphingolipid metabolism was also observed in C. elegans96.

A CRISPR-Cas9 screen identified Grn as a genetic modifier of lipid droplet formation in microglia. Therefore, lipid droplet accumulation significantly increased in hippocampal microglia of Grn−/− mice at 9-10 months of age, as BODIPY+ microglia in the hippocampus of Grn−/− mice with a twofold increase compared with wild type littermates26. Monocyte-derived microglia-like cells from GRN-FTD patients showed accumulation of lipid droplets with profound lysosome abnormalities95. The above studies thus convergently suggest that PGRN is crucial for maintaining cellular lipid metabolism and lipid homeostasis, including sphingolipids, phospholipids, gangliosides and cholesterol (Fig. 2).

Fig. 2: Role of PGRN in lipid metabolism.
figure 2

Left: The biogenesis of lipid droplets in the endoplasmic reticulum (ER) of microglia. Glycerol-3-phosphate (G3P, a product of glycolysis) is converted into lysophosphatidic acid (LPA) by glycerol-3-phosphate acyltransferase (GPAT). LPA is further acylated to form phosphatidic acid (PA) by acylglycerol phosphate acyltransferase (AGPAT). PA is then converted into diacylglycerol (DAG) by Mg2+-dependent PA phosphatase (PAP). DAG is finally converted into triacylglycerol (TAG) by diacylglycerol acyltransferase (DGAT). Cholesterol ester (CE) and sterol ester (SE) are derived from cholesterol by acetyl-coenzyme A acetyltransferase (ACAT). Nucleation of the neutral lipids including TAG and CE/SE and incorporation of related proteins such as perilipins are followed by budding and release of lipid droplets from the ER. How PGRN in the lysosomes regulates the lipid droplet formation remains unknown. Right: Regulation of ganglioside degradation in neuronal lysosomes by PGRN. PGRN enters the neuronal lysosomes via sortilin receptor or cation-independent mannose-6-phosphate receptor (Cl-M6PR)/low-density lipoprotein receptor-related protein 1 (LRP1) upon binding to PSAP. In lysosomes, PGRN directly or indirectly regulates the enzymes responsible for the metabolism of sphingolipids, such as GCase and HexA. Therefore, deficiency of PGRN leads to defects of these enzymes, resulting in gangliosidosis. GLB1: β-galactosidase; HexA/B: β-hexosaminidase A/B; NEU3/4: neuraminidase 3/4; GALC: galactosylceramidase; GCase: β-glucocerebrosidase; ASAH1: N-acylsphingosine amidohydrolase 1; LacCer: lactosylceremide; GlcCer: glucosylceramide; Cer: ceremide; Sph: sphingosine. Proteins or lipid molecules are written in black, enzymes are in green. Black arrows: chemical reaction; red arrows: regulation; dotted red arrows with question mark: speculative regulation; blue arrows: validated interaction; dotted blue arrows with question mark: putative interaction.

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PGRN regulates GCase activity and glucocerebroside degradation

β-glucocerebrosidase (GCase) is a lysosomal hydrolase encoded by the GBA gene, heterozygous variants of which are important genetic risk factors for PD and homozygous mutations of GBA are causes of Gaucher disease97,98,99. GCase catabolizes glucocerebrosides (also referred as glucosylceramides, GlcCer) into glucose and ceramide100,101. Loss of GCase activity leads to accumulation of GCase substrates such as GlcCer, which in turn impairs autophagic and lysosomal function, resulting in deposition of sphingolipids and α-synuclein in dopaminergic neurons and exacerbation of PD pathology102. Moreover, elevated levels of glucosylsphingosine (GlcSph) - the diacylation product of GlcCer - have been observed in FTD-GRN and shown to contribute to microglial dysfunction and neurotoxicity27. Abnormal regulation of sphingolipids by GBA mutations further promotes an increase in α-synuclein103.

PGRN regulates GCase activity by direct binding and facilitating the processing and trafficking of GCase to lysosomes104,105. Additionally, PGRN interacts with PSAP to regulate GCase activity106. The PGRN-PSAP complex enters the neuronal lysosome through binding to the lysosomal membrane receptor sortilin or cation-independent mannose-6-phosphate receptor/low-density lipoprotein receptor-related protein 157,58. In the lysosomes, PGRN binds to the cathepsin D proenzyme and mediates the maturation of cathepsin D (CTSD) to modulate its enzymatic activity107. Mature CTSD then cleaves PSAP into saposins (Sap A, B, C and D), which activate various hydrolases such as GCase. Therefore, the absence of PGRN not only alters cathepsin D activity, but also affects PSAP and GCase levels in an age-dependent manner108,109,110. GRN-FTD patients showed a reduced GCase activity in the frontal lobe. Loss of neuronal GCase in these GRN-FTD patients was caused by a decrease in mature GCase in each neuron. Moreover, GCase was the only affected sphingolipid metabolic enzyme. However, the brains of GRN-FTD patients did not accumulate GCase substrates such as GlcCer or GluSph probably due to residual GCase activity111. The similarly attenuated GCase activity and augmented substrates accumulation were also observed in the brains of aged Grn−/− mice, partly due to decreased level of GCase activator Bis (monoacylglycero) phosphate (BMP)27,112 (Fig. 2). Furthermore, PGRN deficiency leads to broad dysregulation of lysosomal proteins. The Kukar lab made the key discovery that PGRN loss results in abnormal accumulation of lysosomal enzymes cathepsin Z (CTSZ) and galectin-3 (LGALS3) - a phenotype that can be rescued by treatment with either full-length human PGRN (hPGRN) or specific granulin fragments (hGRN2/hGRN4)113. In complementary findings, the Farse/Walther lab observed moderate upregulation of multiple glycosphingolipid-degrading enzymes114.

Recent studies detected exacerbated Gba mutation-associated pathology upon PGRN deficiency in mice115. The double-mutated PG9V mice generated by breeding Grn−/− mice with Gba9v/9v mice exhibited more apparent inflammatory symptoms. In these mice, Grn ablation aggravated the GCase activity reduction and enhanced substrate accumulation115. In a mouse model of tauopathy, PGRN reduction increased tau and α-synuclein inclusions and worsened neurodegeneration via attenuating GCase activity and promoting GlcCer accumulation85. In addition, iPSC-derived cortical neurons with a heterozygous GRN mutation showed impaired processing of prosaposin to Sap C116. In comparison with wild type mice, a significant decrease in the ratio of Sap C to the prosaposin was observed in Grn−/− mice, along with a decrease in GCase activity and an increase of lipid accumulation and insoluble α-synuclein. Therefore, PGRN deficiency may contribute to neurodegeneration through impairment of GCase activity and accumulation of GlcCer (Fig. 2).

PGRN regulates BMP level and ganglioside catabolism

BMP lipids are unique phospholipids found in lysosomal membranes, which are implicated in lysosomal integrity and required for the degradation of lipids including gangliosides. Unlike classical lysosomal storage disorders (LSDs), frontotemporal dementia associated with PGRN deficiency exhibits significantly decreased BMP levels27,114. This phenotype stems from impaired lysosomal lipid homeostasis and reduced expression of BMP-synthesizing enzymes, rather than mutations in genes encoding BMP-metabolizing enzymes25,117. BMP are dysregulated under conditions such as lysosomal storage disorders, metabolic diseases and neurodegenerative diseases118. Gangliosides are a type of glycosphingolipid distributed on the cell membrane of neurons and have various biological functions. Gangliosides are composed of a ceramide lipid backbone and varying amounts of sialic acid residues119. There are several categories of gangliosides depending on their different carbohydrate components, such as GM1, GM2 and GM3. Among them, GM2 gangliosides are degraded as substrates in lysosomes by β-hexosaminidase A (HexA). HexA deficiency caused by HEXA gene (encoding the α subunit of HexA) mutations is the primary cause of Tay-Sachs disease120.

PGRN is essential for BMP homeostasis and Grn−/− mice showed a pronounced deficiency of BMP27. Targeted lipidomics and metabolomics of young (2 months) and old (13 months) Grn+/+ and Grn−/− mouse brains showed a decrease of various BMP species upon PGRN deficiency27. Furthermore, BMP is a positive regulator of GCase activity; therefore, BMP deficiency may also underlie reduced GCase activity in Grn−/− mice27. Although the amounts or activities of glycosphingolipid metabolic enzymes in PGRN-depleted HeLa cells remained unchanged, the level of lysosomal lipid BMP required for the catabolism of ganglioside was significantly reduced28. Nevertheless, how PGRN deficiency leads to reduced BMP remains to be delineated.

PGRN plays a crucial role in the catabolism of ganglioside. In GRN−/− HeLa cells, gangliosides accumulate, and this phenotype is rescued by exogenous PGRN supplementation28. PGRN likely regulates ganglioside catabolism by modulating BMP level27,28 (Fig. 2). BMP, a lysosomal membrane lipid, facilitates glycosphingolipid catabolism through the following mechanisms:1) Membrane charge modulation, BMP localizes to lysosomal vesicles (e.g., endosome-derived multivesicular bodies, MVBs, or lysosomal membrane microdomains), where its phosphate groups create a negative surface charge. 2) Enzyme recruitment and activation, positively charged acid hydrolases (e.g., β-glucocerebrosidase, α-galactosidase A) are electrostatically recruited to BMP-enriched regions. Saposin proteins (e.g., Saposin B) depend on BMP’s negative charge to form active complexes, enhancing sphingolipid degradation (e.g., Saposin C activates β-glucocerebrosidase). 3) Substrate presentation, BMP’s conical structure exposes glycosphingolipids from the membrane bilayer, increasing their accessibility to hydrolases120,121,122,123. Notably, GM2 gangliosides accumulation has been detected in Grn−/− mice28,124. Moreover, the granulin G and E domains promote the binding of PGRN to HexA, thereby mitigating GM2 deposition. Therefore, treatment with recombinant PGRN or Pcgin (an engineered PGRN derivative that contains the granulin E domain) effectively reduced the accumulation of GM2 in Grn−/− mice124. Similarly, gangliosides accumulation was observed in frontal lobe of FTD patients. However, the mechanism by which PGRN regulates gangliosides is not fully understood. Further research is needed to elucidate the complex details and functional significance in neural cells.

TMEM106B associates with PGRN to regulate lipid metabolism

Transmembrane protein 106B (TMEM106B) is a glycosylated type 2 transmembrane protein located in late endo-lysosomes and has been reported as a modifier of disease risk in FTD, particularly in GRN pathogenic variants carriers125,126,127,128,129. TMEM106B is characterized by its large C-terminal domain residing in the lysosomal lumen and the smaller N-terminal domain locating in the cytosol, thereby regulating lysosomal morphology and function130,131. Aggregates containing the C-terminal (amino acid residues 120-254) of TMEM106B have been detected in the brains of a variety of neurodegenerative diseases and neurologically normal aged individuals132,133,134,135.

Both genome-wide association studies and genetic studies on clinical cases suggested a correlation between TMEM106B and GRN mutations in FTD patients125,126. Some studies investigated the interaction between TMEM106B and PGRN to unravel their underlying mechanisms. Three independent studies reached similar conclusions that TMEM106B ablation exacerbated lysosomal abnormalities, gliosis, and neurodegenerative phenotypes caused by PGRN deficiency136,137,138. However, two other studies showed opposite results139,140. Among them, both TMEM106B ablation138 and overexpression140 led to lipofuscin accumulation. Besides, a partial reduction of TMEM106B failed to normalize the increased GCase activity in Grn haploinsufficiency mice but was able to suppress the HexA and glucuronidase activity independent of Grn genotype141. TMEM106B was found to interact with galactosylceramidase (GALC) and TMEM106B deficiency led to increased GALC activity and decreased levels of myelin lipids including galactosylceramide (GalCer) and its sulfated derivative sulfatide142. A recent study showed more severe lipid dysregulation in Grn−/−; Tmem106b−/− relative to Grn−/− alone143. Therefore, an appropriate level of TMEM106B is key to maintaining lysosomal function and lipid homeostasis. Despite the current lack of clear understanding of specific mechanisms, there is a close relationship between TMEM106B and PGRN deficiency in lipid metabolism (Fig. 2). This suggests that TMEM106B might serve as a promising regulator of lipid metabolism abnormalities caused by GRN mutation.

Other regulators and mediators of lipid metabolism associated with neurodegeneration

APOE4

Lipid metabolism disorders and the pathogenesis of AD are closely related49,144. Apolipoprotein E ε4 (APOE4) is the most significant risk factor for late-onset AD145,146, which can lead to amyloid-β and tau protein aggregation147,148,149. APOE acts as a lipid transporter and affects the lipid metabolism of various neural cells. Ablation of ApoE in mice leads to impaired cholesterol transport and clearance upon demyelinating lesion150,151. Compared to the apolipoprotein E ε3 (APOE3), the risk isoform APOE4 can cause dysfunction of microglia, astrocytes and oligodendrocytes, which may lead to impaired cholesterol metabolic homeostasis152,153. Indeed, differential effects of APOE3 and APOE4 on lipid metabolism were observed in aged brains or tau-mediated neurodegeneration154,155,156. Binding of APOE4 to sortilin receptors disrupts the uptake and conversion of polyunsaturated fatty acids157. Impaired lipid homeostasis induced by APOE4 in microglia leads to neuronal network abnormalities158. Recent studies showed that APOE4 regulates triglyceride saturation and lipid droplet size in astrocytes, indicating that APOE serves as a surface protein regulating the size and composition of lipid droplets159. Moreover, APOE4 is associated with damaging lipid droplets in the microglia of AD patients, with the most abundant lipid droplet-associated enzyme ACSL1+ microglia in APOE4/4 genotype AD patients39. Hence, targeting the regulation of lipid metabolism through APOE4 is a potential method for treating AD160,161. Importantly, PGRN and APOE appear to functionally interact in lipid metabolism. In ApoE/Grn double-knockout (DKO) mice, PGRN deficiency leads to significantly reduced plasma cholesterol and triglyceride levels while simultaneously increasing hepatic production of pro-inflammatory cytokines TNF-α and IL-1β162, suggesting a potential interplay between lipid regulation and inflammation.

TREM2

In late-onset AD cases, elevated plasma PGRN levels show a significant positive association with soluble triggering receptor expressed on myeloid cells 2 (sTREM2), with both biomarkers correlating with more advanced disease stages and greater cognitive impairment163, suggesting coordinated involvement of neuroinflammation and lysosomal dysfunction in disease progression.

TREM2 is an innate immune receptor mainly expressed by microglia in the CNS. An arginine to histidine substitution at 47 amino acid position (R47H) in TRME2 has been shown to increase the risk of AD164,165. TREM2 acts as a lipid-sensitive activating receptor that affects lipid metabolism in the CNS and participates in metabolism processes involving cholesterol, phospholipids and lipoproteins151,166,167,168. An in vitro study found that TREM2-R47H mutation in microglia exacerbated lipid accumulation169, while another in vivo study showed that TREM2-R47H mutation in microglia led to reduced lipid droplet aggregation in an AD mouse model37. Research on TREM2 as a therapeutic target for AD has shown that effective activation of TREM2 with specific antibodies can promote the clearance of amyloid plaques by microglia170,171, which provides a new approach to AD treatment.

β-galactosidase and hexosaminase

Depending on the substance accumulated, LSDs can be classified as sphingolipidoses, mucopolysaccharidoses, glycoproteinoses, etc13. When neurons are affected, LSDs are often accompanied by progressive neurodegeneration12. Among various LSDs, sphingolipidoses are a group of inherited diseases including GM1 gangliosidosis and GM2 gangliosidosis172. Mutations of GLB1 gene, which encodes β-galactosidase (GLB) disrupt the activity of GLB, leading to the impaired conversion of GM1 into GM2 and accumulation of GM1 and other glycoconjugates in the lysosomes of neurons. When accumulated to a certain extent, these lipids ultimately cause serious dysfunction of the nervous system173,174.

GM2 gangliosidosis, such as Tay-Sachs and Sandhoff diseases is caused by a deficiency of lysosomal β-hexosaminidases (HexA and HexB) or GM2 activator protein175. The lack of HexA/HexB prevents the degradation of GM2 to GM3, leading to the accumulation of GM2 in neurons. The most common β-hexosaminidase isozymes HexA and HexB contain two subunits encoded by the HEXA and HEXB genes. HexA contains heterodimer subunits encoded by HEXA and HEXB, whereas HexB contains homodimer subunits encoded by HEXB. HexA and HexB hydrolyze various substrates from the terminal residues of N-acetylglucosamine, thereby preventing the accumulation of gangliosides176. In addition, Tay-Sachs and Sandhoff diseases are usually associated with microglia expansion, astrocyte activation and neuroinflammation177. There is no approved therapy for GM2 gangliosidosis, although different treatment strategies have been studied175,178. A recent clinical trial treated Tay-Sachs children with adeno-associated virus (AAV) carrying HEXA and HEXB, which, to some extent, slowed down disease progression179. Recent studies also indicate that PGRN haploinsufficiency causes decreased β-hexosaminidase A (HexA) enzymatic activity, leading to defective ganglioside metabolism and contributing to gangliosidosis pathogenesis27.

Therapeutic effect restoring PGRN level in neurodegenerative diseases

Recombinant PGRN protein or PGRN derived peptides

Several strategies have been reported to explore the therapeutic potential of improving PGRN levels in FTD patients due to GRN mutations70,180. Due to the blood-brain barrier and attendant limited bioavailability of PGRN in the brain, it is necessary to use brain penetrating biologics to deliver PGRN to the brain in order to improve PGRN deficiency181,182. A brain penetrant progranulin biologic PTV: PGRN (protein transport vehicle: progranulin) has been reported to rescue FTD and LSD caused by Grn dysfunction27. The PTV vector contains an engineered Fc domain of antibody recognizing transferrin receptor (TfR). Upon fusion with the modified Fc domain, the permeability of PGRN into the CNS can be enhanced through its binding ability with the transferrin receptor. In vivo experiments showed that high concentrations of PTV: PGRN could be detected in all Grn−/− mouse brain tissues, which proved that PTV: PGRN could significantly enhance the brain’s ability to absorb PGRN. In addition, intraperitoneal injection of PTV: PGRNv2 (an engineered variant with stable C terminus) can rescue lipid abnormalities and pathological changes in the CNS of Grn−/−; TfRmu/hu mice, including gliosis, lipofuscin deposition, and nerve cell damage. Therefore, PTV: PGRN represents a potential biotherapeutics to treat GRN-FTD patients (Fig. 3). In addition, a PGRN-derived peptide, named ND7, containing Grn E domain, effectively ameliorated autophagy defects and disease phenotype of fibroblasts in PGRN-deficient Gaucher disease patients115,183. The Grn E domain interacts with Rab2, which is a critical molecule involved in autophagosome-lysosome fusion. ND7 penetrated the blood-brain barrier and effectively improved Gaucher disease expression and PD pathology in Gba9v/null and PG9V mice, demonstrating a potential therapeutic application for rare LSDs and common neurodegenerative disorders115.

Fig. 3: Therapeutic strategies for neurodegenerative diseases such as FTD by restoring PGRN level.
figure 3

Recombinant PGRN proteins, anti-sortilin antibody, AAV-mediated PGRN expression or small molecule modulators can increase PGRN level and restore lysosomal function and lipid homeostasis, thereby suppressing neuroinflammation and ameliorating neurodegeneration. Notably, anti-sortilin antibody can increase plasma and CSF PGRN levels but whether it can restore lysosomal function remain to be elucadated.

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Sortilin antibody

Several cellular transmembrane receptors are associated with neuronal uptake and intracellular degradation of PGRN protein; hence, regulating the activity of these receptors can effectively modulate PGRN levels. Sortilin is a major binding site for PGRN in neurons57. Sortilin-mediated endocytosis of PGRN significantly reduces steady-state PGRN levels, therefore, sortilin-deficient mice have increased PGRN levels in the brain and serum. This suggests that inhibition of sortilin represents a therapeutic strategy to increase the level of PGRN57,58. A human monoclonal antibody latozinemab was reported to block the interaction between PGRN and sortilin, thereby reducing the degradation of PGRN58. In a phase 1 clinical trial, a single infusion of latozinemab in healthy volunteers decreased sortilin in white blood cells and increased PGRN in both plasma and CSF184. Multiple doses of latozinemab could restore plasma and CSF PGRN to physiological levels in asymptomatic GRN mutation carriers185. Although the effect of latozinemab treatment on lipid metabolism requires further studies, this sortilin antibody holds promise to increase PGRN levels in the CNS and ameliorate the symptoms of GRN-FTD patients (Fig. 3). Notably, while latozinemab demonstrated a favorable safety profile, some participants experienced elevated cholesterol or triglyceride levels185. Furthermore, the underlying mechanisms of neuroprotection by the elevated CSF and plasma PGRN levels but reduced lysosomal PGRN in the neurons remain unknown.

Gene therapy

Manipulation at the genetic level can improve FTD caused by GRN haploinsufficiency by increasing intracellular PGRN levels in a long-term manner. In both preclinical research and clinical trials, it has been shown that using gene transduction vectors effectively improved intracellular PGRN in the brain, and showed significant therapeutic effects on different categories of FTD186. Adeno-associated viruses (AAVs) are the most commonly used vector for transducing genes to the CNS and have shown promising results for the treatment of brain disorders. Previous studies have found that injecting adeno associated virus (AAV) vectors expressing mouse PGRN into the medial prefrontal cortex of Grn+/− mice repaired neuronal PGRN deficiency, restored neuronal lysosomal function and normal animal behavior in aged Grn+/− mice187. In addition, PGRN overexpression mediated by lentivirus reduced plaque burden in the 5xFAD mouse model of AD, preventing spatial memory deficits and hippocampal neuronal loss79. The selective expression of PGRN in neural cells using AAV may be safer than widespread cellular transduction, as it can correct the microglial phenotype lacking PGRN in the motor cortex, thalamus and hippocampus. It is therefore conceivable that gene therapy to heighten PGRN levels has the potential to improve lipid metabolism in the CNS (Fig. 3). Indeed, an AAV-mediated PGRN gene therapy could restore GCase activity in Grn−/− mice111. A recent preclinical study and a clinical trial using AAV to deliver PGRN expression increased PGRN levels and restored lysosomal lipid metabolism including BMP levels in Grn−/− mice and GRN-FTD patients188, supporting further clinical development of this gene therapy in these patients. Another AAV-mediated expression of a brain-penetrant PGRN variant in liver could reach the CNS and ameliorate neuroinflammation and restore lipid metabolism in Grn−/− and Grn−/−; Tmem106b−/− mice143. Furthermore, AAV-mediated expression of human GRN A or GRN F both ameliorate dysregulated lysosomal lipids and lipofuscinosis, supporting the potential use of GRNs as therapeutics for diseases associated with PGRN deficiency and related lipid dysfunction113.

Small molecule modulators

Identification of compounds with the ability to heighten PGRN levels by small molecule screening is an effective strategy to identify potential PGRN therapeutic agents (Fig. 3). Trehalose, a lead compound that can upregulate endogenous PGRN, was screened from various novel activators of a human GRN promoter reporter189. The trehalose-treated group significantly upregulated PGRN levels in the brain tissues of PGRN heterozygous knockout mice. Screening revealed that nor-binaltorphimine dihydrochloride and dibutyryl-cAMP sodium salt were both effective phenotypic regulators of PGRN deficiency190. These two compounds not only enhance the lysosomal function of PGRN-deficient microglia but also normalize dysregulated cell cycle genes. A high-throughput drug screening using nematodes revealed two small molecules, rottlerin and rivastigmine, which might provide therapeutic applications for PGRN deficiency96,191. A functional cell-based assay has identified a novel compound ARKD-104 that can stimulate PGRN secretion from a microglial cell line192. Oral administration of ARKD-104 to cynomolgus monkeys led to a dose- and exposure-dependent increase in CSF PGRN levels192, suggesting therapeutic potential for PGRN-deficient disorders. Besides CNS permeability, the specificity and efficacy concerns of small molecules should be addressed.

Conclusions and perspectives

Lipid droplet accumulation in microglia reflects a pro-inflammatory state and functional impairment of the brain, suggesting that abnormal lipid metabolism may be involved in neurodegenerative diseases. However, the underlying mechanism of lipid droplet accumulation during brain aging, neuroinflammation and neurodegeneration is not fully understood. Lysosomes play a critical role in the degradation and recycling of lipids in the brain. Notably, loss-of-function mutations in the GRN gene lead to FTD probably due to impaired lysosomal function. In addition, variants of GRN are risk factors for a variety of other neurodegenerative diseases. Key molecules that regulate lysosomal lipid metabolism, including BMP, GCase and TMEM106B, are closely related to neurodegeneration caused by PGRN deficiency. The abnormal lysosomal lipid metabolism caused by PGRN deficiency may lead to susceptibility to neuroinflammation and subsequent neurodegeneration in FTD patients. The specific biological mechanisms still remain to be fully elucidated. Exploring and developing effective therapeutic avenues to regulate lipid metabolism and improve pathological symptoms in the CNS is challenging, but increasing the level of PGRN in the brain to restore lipid homeostasis is a potential approach. As PGRN represents a key regulator of lipid metabolism and homeostasis, a deeper understanding of the important role of lipid droplet accumulation and lysosomal lipid metabolism abnormalities caused by PGRN deficiency in neurodegeneration will help to explore new strategies for the treatment of related neurodegenerative diseases.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.