Rescue of a lysosomal storage disorder caused by Grn loss of function with a brain penetrant progranulin biologic

Label-free quantitative shotgun analysis of bis(monoacylglycero)phosphate lipids

Interest in the role of bis(monoacylglycero)phosphate (BMP) lipids in lysosomal function has significantly grown in recent years. Emerging evidence highlights BMPs as critical players not only in Niemann-Pick disease type C (NPC) but also in other pathologies such as neurodegeneration, cardiovascular diseases, and cancers. However, the selective analysis of BMPs is significantly hindered by isomeric phosphatidylglycerol (PG) lipids. While this can be addressed by chromatographic separation, it poses a significant challenge for shotgun lipidomics approaches. Here, we present a shotgun lipidomics strategy to detect and separate BMPs from PGs using differential fragmentation of sodiated ions. This approach, including isotope correction, is integrated into an existing quantitative shotgun lipidomics workflow (Lipidyzer combined with Shotgun Lipidomics Assistant software) that simultaneously quantifies >1400 lipids. Validation using K-562 cell extracts demonstrated acceptable linearity, trueness, repeatability, and a limit of quantification of 0.12 µM, confirming robust analytical performance. Finally, characteristic accumulation of BMP lipids is shown in bone marrow-derived macrophages from NPC mice, demonstrating its applicability. Our method presents a quantitative, selective, rapid, and robust solution for shotgun-based BMP analysis without the need for extensive chromatographic separation or derivatization. The integration of BMP lipid detection into the Lipidyzer platform, alongside the recently launched iSODA data visualization tool, empowers chemists and biologists to gain deeper insights into BMP lipid biology.

Hepatic lipid remodeling in cold exposure uncovers direct regulation of bis(monoacylglycero)phosphate lipids by phospholipase A2 group XV

Cold exposure is a selective environmental stress that elicits a rapid metabolic shift to maintain energy homeostasis. In response to cold exposure, the liver rewires the metabolic state, shifting from glucose to lipid catabolism. By probing the liver lipids in cold exposure, we observed that the lysosomal bis(monoacylglycero)phosphate (BMP) lipids were rapidly increased during cold exposure. BMP lipid changes occurred independently of lysosomal abundance but were dependent on the lysosomal transcriptional regulator transcription factor EB (TFEB). Knockdown of Tfeb in hepatocytes decreased BMP lipid levels and led to cold intolerance in mice. We assessed TFEB-binding sites of lysosomal genes and determined that the phospholipase a2 group XV (PLA2G15) regulates BMP lipid catabolism. Decreasing Pla2g15 levels in mice increased BMP lipids, ablated the cold-induced rise in BMP lipids, and improved cold tolerance. Mutation of the catalytic site of PLA2G15 ablated the BMP lipid breakdown. Together, our studies uncover TFEB regulation of BMP lipids through PLA2G15 catabolism.

Development of novel prognostic protein signatures in ovarian cancer: Molecular structure and immune function of AQP5 protein and CTDP1 protein

Ovarian cancer is a serious gynecological malignancy, and early identification of prognostic markers is critical to improve clinical outcomes. Aquaporin 5 (AQP5) and cell cycle regulatory protein CTDP1 are considered as potential prognostic protein markers, but their specific molecular structure and immune function have not been thoroughly studied. The aim of this study was to investigate the molecular structure of AQP5 and CTDP1 and their immune function in ovarian cancer, to evaluate their potential as prognostic markers, and to provide new ideas for the diagnosis and efficacy monitoring of ovarian cancer. Cox regression analysis and LASSO regression were used to construct the risk prognosis model, and the preliminary verification was carried out. Finally, cell culture and high-resolution gene expression analysis were used to investigate the expression of AQP5 and CTDP1 and their roles in the immune microenvironment. The results showed that AQP5 and CTDP1 were highly expressed in ovarian cancer cell lines and were closely related to tumor immune invasion. The immunomicroenvironment analysis based on GRN showed that they showed a positive correlation with tumor-related immune cells. Through the constructed risk model, AQP5 and CTDP1 significantly affect patient prognosis, suggesting their potential as prognostic markers.

Exosome-Functionalized Self-Carrier Enzyme-Like/Drug With Triple Amplified Anti-Oxidative Stress for Synergistic Depression Therapy

Depression, a severe disorder affecting both physical and mental health, is commonly treated with first-line antidepressants, which often exhibit limited efficacy due to poor penetration of the blood-brain barrier (BBB) and significant side effects, thus requiring the exploitation of biocompatible and effective treatments. Recent studies suggest that depression is closely linked to an imbalance in oxidative stress and subsequent inflammatory responses. Antioxidant therapies and targeting oxidative stress in inflammatory depression are therefore emerging as promising strategies. In this study, an exosome-functionalized and geniposide (GEN) self-carried Prussian blue (PB) nanotherapeutic approach is fabricated to realize efficient BBB penetration for synergistic depression therapy. The porous PB carrier possesses multi-enzyme capabilities, which can effectively scavenge the accumulated ROS, protecting the slightly inflammatory acidic environment released GEN from oxidation, and the GEN subsequently works simultaneously with PB to activate the Nrf2-ARE pathway, enhancing the body's oxidative stress defense mechanisms synergistically. The triple-amplified anti-oxidant strategy of this nanomaterial is shown to mitigate microglial activation and the reduction in neuroplasticity, ultimately alleviating the pathological markers of inflammatory depression. Overall, the constructed nanomaterials underscore the therapeutic potential of anti-oxidative stress for synergistic removal of ROS and activation of the Nrf2-ARE pathway in the treatment of inflammatory depression.

Progranulin deficiency in the brain: the interplay between neuronal and non-neuronal cells

Heterozygous mutations in GRN gene lead to insufficient levels of the progranulin (PGRN) protein, resulting in frontotemporal dementia (FTD) with TAR DNA-binding protein 43 (TDP-43) inclusions, classified pathologically as frontotemporal lobar degeneration (FTLD-TDP). Homozygous GRN mutations are exceedingly rare and cause neuronal ceroid lipofuscinosis 11, a lysosomal storage disease with onset in young adulthood, or an FTD syndrome with late-onset manifestations. In this review, we highlight the broad spectrum of clinical phenotypes associated with PGRN deficiency, including primary progressive aphasia and behavioral variant of frontotemporal dementia. We explore these phenotypes alongside relevant rodent and in vitro human models, ranging from the induced pluripotent stem cell-derived neural progenitors, neurons, microglia, and astrocytes to genetically engineered heterotypic organoids containing both neurons and astrocytes. We summarize advantages and limitations of these models in recapitulating the main FTLD-GRN hallmarks, highlighting the role of non-cell-autonomous mechanisms in the formation of TDP-43 pathology, neuroinflammation, and neurodegeneration. Data obtained from patients' brain tissues and biofluids, in parallel with single-cell transcriptomics, demonstrate the complexity of interactions among the highly heterogeneous cellular clusters present in the brain, including neurons, astrocytes, microglia, oligodendroglia, endothelial cells, and pericytes. Emerging evidence has revealed that PGRN deficiency is associated with cell cluster-specific, often conserved, genetic and molecular phenotypes in the central nervous system. In this review, we focus on how these distinct cellular populations and their dysfunctional crosstalk contribute to neurodegeneration and neuroinflammation in FTD-GRN. Specifically, we characterize the phenotypes of lipid droplet-accumulating microglia and alterations of myelin lipid content resulting from lysosomal dysfunction caused by PGRN deficiency. Additionally, we consider how the deregulation of glia-neuron communication affects the exchange of organelles such as mitochondria, and the removal of excess toxic products such as protein aggregates, in PGRN-related neurodegeneration.

Isomerization of bis(monoacylglycero)phosphate by acyl migration

Bis(monoacylglycero)phosphates (BMPs) are biologically functional acidic lipids present in late endosomes and lysosomes. We recently reported that lysosomal phospholipase A2 (LPLA2, PLA2G15), the lysosomal enzyme mediating BMP catabolism, degrades BMP isomers with distinct substrate specificity. Specifically, sn-(3-oleoyl-2-hydroxy)-glycerol-1-phospho-sn-1'-(3'-oleoyl-2'-hydroxy)-glycerol (S,S-(3,3'-diC18:1)-BMP) is a significantly better substrate for LPLA2 than S,S-(2,2'-diC18:1)-BMP. S,S-(2,2'-diC18:1)-BMP is generally considered the only biologically relevant BMP isomer. We investigated the isomerization of S,S-(2,2'-diC18:1)-BMP to (S,S-(3,3'-diC18:1)-BMP) in vitro and in cells. Thin-layer chromatography was used to distinguish S,S-(3,3'-diC18:1)-BMP from S,S-(2,2'-diC18:1)-BMP. S,S-(2,2'-diC18:1)-BMP/1,2-di-O-(9Z-octadecenyl)-sn-glycero-3-phosphocholine liposomes were incubated at varying pH in the presence or absence of test substances. First, we studied bovine serum albumin, which is known to promote isomerization of 1-acyl-2-lysophosphatidylcholine. The formation of S,S-(3,3'-diC18:1)-BMP in the presence of albumin increased in a time-dependent and albumin concentration-dependent manner under neutral conditions and was dependent on pH and the molar ratio of S,S-(2,2'-diC18:1)-BMP in liposomes. Treatment of isomeric products generated during isomerization reaction with sn-1,3-specific lipase produced both oleic acid but also lyso-phosphatidylglycerol, indicating that the conversion of S,S-(2,2'-diC18:1)-BMP to S,S-(3,3'-diC18:1)-BMP is preceded via S,S-(2,3'-diC18:1)-BMP. S,S-(3,3'-diC18:1)-BMP formed was preferentially degraded by LPLA2 over the S,S-(2,2'-diC18:1)-BMP. Proteins such as HSP70 and human serum albumin and metal ions such as Fe3+ and Zn2+ acted as cofactors promoting the isomerization of S,S-(2,2'-diC18:1)-BMP under neutral conditions. At baseline, RAW 264.7 cells showed nonnegligible amounts of sn-1,3-specific lipase-sensitive BMPs. However, lipase-sensitive BMPs were increased by exposure to chloroquine or NH4Cl, suggesting that cells undergo S,S-(2,2'-diacyl)-BMP isomerization upon alkalinization of intracellular acidic compartments.

Mechanisms and functions of lysosomal lipid homeostasis

Lysosomes are the central degradative organelle of mammalian cells and have emerged as major intersections of cellular metabolite flux. Macromolecules derived from dietary and intracellular sources are delivered to the acidic lysosomal lumen where they are subjected to degradation by acid hydrolases. Lipids derived from lipoproteins, autophagy cargo, or autophagosomal membranes themselves constitute major lysosomal substrates. Dysregulation of lysosomal lipid processing, defective export of lipid catabolites, and lysosomal membrane permeabilization underly diseases ranging from neurodegeneration to metabolic syndromes and lysosomal storage disorders. Mammalian cells are equipped with sophisticated homeostatic control mechanisms that protect the lysosomal limiting membrane from excessive damage, prevent the spillage of luminal hydrolases into the cytoplasm, and preserve the lysosomal membrane composition in the face of constant fusion with heterotypic organelles such as endosomes and autophagosomes. In this review we discuss the molecular mechanisms that govern lysosomal lipid homeostasis and, thereby, lysosome function in health and disease.

Fc-engineered large molecules targeting blood-brain barrier transferrin receptor and CD98hc have distinct central nervous system and peripheral biodistribution

Blood brain barrier-crossing molecules targeting transferrin receptor (TfR) and CD98 heavy chain (CD98hc) are widely reported to promote enhanced brain delivery of therapeutics. Here, we provide a comprehensive and unbiased biodistribution characterization of TfR and CD98hc antibody transport vehicles (ATVTfR and ATVCD98hc) compared to control IgG. Mouse whole-body tissue clearing reveals distinct organ localization for each molecule. In the brain, ATVTfR and ATVCD98hc achieve enhanced exposure and parenchymal distribution even when brain exposures are matched between ATV and control IgG in bulk tissue. Using a combination of cell sorting and single-cell RNAseq, we reveal that control IgG is nearly absent from parenchymal cells and is distributed primarily to brain perivascular and leptomeningeal cells. In contrast, ATVTfR and ATVCD98hc exhibit broad and unique parenchymal cell-type distribution. Finally, we profile in detail brain region-specific biodistribution of ATVTfR in cynomolgus monkey brain and spinal cord. Taken together, this in-depth multiscale characterization will guide platform selection for therapeutic targets of interest.

Modulation of hepatic transcription factor EB activity during cold exposure uncovers direct regulation of bis(monoacylglycero)phosphate lipids by Pla2g15

Cold exposure is a selective environmental stress that elicits a rapid metabolic shift to maintain energy homeostasis. In response to cold exposure, the liver rewires the metabolic state shifting from glucose to lipid catabolism. By probing the liver lipids in cold exposure, we observed that the lysosomal bis(monoacylglycero)phosphate (BMP) lipids were rapidly increased during cold exposure. BMP lipid changes occurred independently of lysosomal abundance but were dependent on the lysosomal transcriptional regulator transcription factor EB (TFEB). Knockdown of TFEB in hepatocytes decreased BMP lipid levels and led to cold intolerance in mice. We assessed TFEB binding sites of lysosomal genes and determined that the phospholipase Pla2g15 regulates BMP lipid catabolism. Knockdown of Pla2g15 in mice increased BMP lipid levels, ablated the cold-induced rise, and improved cold tolerance. Knockout of Pla2g15 in mice and hepatocytes led to increased BMP lipid levels, that were decreased with re-expression of Pla2g15. Mutation of the catalytic site of Pla2g15 ablated the BMP lipid breakdown. Together, our studies uncover TFEB regulation of BMP lipids through Pla2g15 catabolism.

PLA2G15 is a Lysosomal BMP Hydrolase and its Targeting Ameliorates Lysosomal Disease

Lysosomes catabolize lipids and other biological molecules, a function essential for cellular and organismal homeostasis. Key to lipid catabolism in the lysosome is bis(monoacylglycero)phosphate (BMP), a major lipid constituent of intralysosomal vesicles and a stimulator of lipid-degrading enzymes. BMP levels are altered in a broad spectrum of human conditions, including neurodegenerative diseases. While a lysosomal BMP synthase was recently discovered, the enzymes that mediate BMP turnover has remained elusive. Here we show that the lysosomal phospholipase PLA2G15 is a physiological BMP hydrolase. We further demonstrate that BMP's resistance to hydrolysis in the lysosome is conferred by the combination of its unique sn2, sn2' esterification position and stereochemistry, as neither feature alone is sufficient to provide this resistance. Purified PLA2G15 catabolizes most BMP species derived from cell and tissue lysosomes under acidic conditions. Furthermore, PLA2G15 catalytic activity against synthesized BMP stereoisomers with primary esters was comparable to its canonical substrates challenging the long-held thought that BMP's unique stereochemistry is sufficient to confer resistance to acid phospholipases. Conversely, BMP with secondary esters and S,S stereoconfiguration is intrinsically stable in vitro and requires acyl migration for hydrolysis in lysosomes. Consistent with our biochemical data, PLA2G15-deficient cells and tissues accumulate multiple BMP species, a phenotype reversible by supplementing wildtype PLA2G15 but not its catalytically dead mutant. In addition, targeting PLA2G15 to increase BMP reverses the cholesterol phenotype in Niemann Pick Disease Type C (NPC1) patient fibroblasts and significantly ameliorates disease pathologies in NPC1-deficient mice leading to extended lifespan. Our findings establish the rules that govern the stability of BMP in the lysosome and identify PLA2G15 as a lysosomal BMP hydrolase and a potential target for therapeutic intervention in neurodegenerative diseases.

Neuronal Ceroid Lipofuscinosis-Concepts, Classification, and Avenues for Therapy

Neuronal ceroid lipofuscinosis (NCL) is a group of neurodegenerative lysosomal storage disorders characterized by excessive accumulation of lysosomal lipofuscin. Thirteen subtypes of NCL have been identified, each associated with distinct genes encoding various transmembrane proteins, secretory proteins, or lysosomal enzymes. Clinically, NCL manifests in infants through vision impairment, motor and cognitive dysfunctions, epilepsy, and premature death. The pathological complexity of NCL has hindered the development of effective clinical protocols. Current treatment modalities, including enzyme replacement therapy, pharmacological approaches, gene therapy, and stem cell therapy, have demonstrated limited efficacy. However, emerging evidence suggests a significant relationship between NCL and microglial cells, highlighting the potential of novel microglial cell replacement therapies. This review comprehensively examines the pathogenic genes associated with various NCL subtypes, elucidating their roles, clinical presentations, and corresponding mouse models. Especially, we thoroughly discuss the advances in the clinical study of potential therapeutics, which crucially calls for early diagnosis and treatment more than ever.

Molecular and cellular characteristics of cerebrovascular cell types and their contribution to neurodegenerative diseases

Many diseases and disorders of the nervous system suffer from a lack of adequate therapeutics to halt or slow disease progression, and to this day, no cure exists for any of the fatal neurodegenerative diseases. In part this is due to the incredible diversity of cell types that comprise the brain, knowledge gaps in understanding basic mechanisms of disease, as well as a lack of reliable strategies for delivering new therapeutic modalities to affected areas. With the advent of single cell genomics, it is now possible to interrogate the molecular characteristics of diverse cell populations and their alterations in diseased states. More recently, much attention has been devoted to cell populations that have historically been difficult to profile with bulk single cell technologies. In particular, cell types that comprise the cerebrovasculature have become increasingly better characterized in normal and neurodegenerative disease contexts. In this review, we describe the current understanding of cerebrovasculature structure, function, and cell type diversity and its role in the mechanisms underlying various neurodegenerative diseases. We focus on human and mouse cerebrovasculature studies and discuss both origins and consequences of cerebrovascular dysfunction, emphasizing known cell type-specific vulnerabilities in neuronal and cerebrovascular cell populations. Lastly, we highlight how novel insights into cerebrovascular biology have impacted the development of modern therapeutic approaches and discuss outstanding questions in the field.

Proteostasis and lysosomal repair deficits in transdifferentiated neurons of Alzheimer's disease

Aging is the most prominent risk factor for Alzheimer's disease (AD). However, the cellular mechanisms linking neuronal proteostasis decline to the characteristic aberrant protein deposits in AD brains remain elusive. Here, we develop transdifferentiated neurons (tNeurons) from human dermal fibroblasts as a neuronal model that retains aging hallmarks and exhibits AD-linked vulnerabilities. Remarkably, AD tNeurons accumulate proteotoxic deposits, including phospho-Tau and Aβ, resembling those in AD patient and APP mouse brains. Quantitative tNeuron proteomics identify aging and AD-linked deficits in proteostasis and organelle homeostasis, most notably in endosome-lysosomal components. Lysosomal deficits in aged tNeurons, including constitutive lysosomal damage and ESCRT-mediated lysosomal repair defects, are exacerbated in AD tNeurons and linked to inflammatory cytokine secretion and cell death. Supporting lysosomal deficits' centrality in AD, compounds ameliorating lysosomal function reduce Aβ deposits and cytokine secretion. Thus, the tNeuron model system reveals impaired lysosomal homeostasis as an early event of aging and AD.

Regulatory Mechanisms and Therapeutic Implications of Lysosomal Dysfunction in Alzheimer's Disease

Alzheimer's disease (AD) is characterized by the accumulation of amyloid-beta (Aβ) plaques, neurofibrillary tangles (NFTs) formed from hyperphosphorylated Tau, and widespread neuronal loss. The autophagy-lysosomal pathway plays a crucial role in maintaining cellular homeostasis by degrading and recycling of damaged organelles and aggregate amyloid proteins implicated in AD. Lysosomes are key effectors of autophagic process, responsible for the breakdown of a variety of damaged organelles and aggregate or dysfunctional proteins. This review examines the role of lysosomal dysfunction in AD pathophysiology, focusing on genetic factors, acidification abnormalities, and other contributing factors. We also explore the involvement of lysosomal dysfunction of microglia in AD pathology, and cover the role of lysosomal stress response (LSR) in cellular response to neuronal injury associated with AD. Furthermore, we discuss potential therapeutic strategies targeting lysosomal proteolysis pathway and addressing lysosomal dysfunction for AD treatment, including the pharmacologically activating lysosomal activity, regulating TFEB, and considering other emerging approaches.

Decreased lipidated ApoE-receptor interactions confer protection against pathogenicity of ApoE and its lipid cargoes in lysosomes

While apolipoprotein E (APOE) is the strongest genetic modifier for late-onset Alzheimer's disease (LOAD), the molecular mechanisms underlying isoform-dependent risk and the relevance of ApoE-associated lipids remain elusive. Here, we report that impaired low-density lipoprotein (LDL) receptor (LDLR) binding of lipidated ApoE2 (lipApoE2) avoids LDLR recycling defects observed with lipApoE3/E4 and decreases the uptake of cholesteryl esters (CEs), which are lipids linked to neurodegeneration. In human neurons, the addition of ApoE carrying polyunsaturated fatty acids (PUFAs)-CE revealed an allelic series (ApoE4 > ApoE3 > ApoE2) associated with lipofuscinosis, an age-related lysosomal pathology resulting from lipid peroxidation. Lipofuscin increased lysosomal accumulation of tau fibrils and was elevated in the APOE4 mouse brain with exacerbation by tau pathology. Intrahippocampal injection of PUFA-CE-lipApoE4 was sufficient to induce lipofuscinosis in wild-type mice. Finally, the protective Christchurch mutation also reduced LDLR binding and phenocopied ApoE2. Collectively, our data strongly suggest decreased lipApoE-LDLR interactions minimize LOAD risk by reducing the deleterious effects of endolysosomal targeting of ApoE and associated pathogenic lipids.

Granulins rescue inflammation, lysosome dysfunction, lipofuscin, and neuropathology in a mouse model of progranulin deficiency

Progranulin (PGRN) deficiency is linked to neurodegenerative diseases, including frontotemporal dementia (FTD), Alzheimer's disease, and Parkinson's disease. Proper PGRN levels are critical for brain health; however, the function of PGRN is unclear. PGRN is composed of 7.5 repeat domains, called granulins, and processed into granulins inside the lysosome. PGRN is beneficial for neuronal health, but the role of individual granulins is controversial and unclear. We find that the expression of single granulins broadly rescues disease pathology in Grn-/- mice. Adeno-associated virus (AAV)-mediated expression of human granulin-2/F, granulin-4/A, or PGRN in Grn-/- mouse brain ameliorates dysregulated lysosomal proteins and lipids, microgliosis, and lipofuscinosis. Mechanistically, granulins localize to lysosomes in Grn-/- mouse brains or fibroblasts. These data support the hypothesis that PGRN is a precursor to granulins, which share a beneficial function inside the lysosome to maintain lipid and protein homeostasis to prevent neurodegeneration. Thus, granulins are potential therapeutics to treat FTD-GRN and related diseases.

FMR1 Disorders: Basics of Biology and Therapeutics in Development

Fragile X Syndrome (FXS) presents with a constellation of phenotypes, including trouble regulating emotion and aggressive behaviors, disordered sleep, intellectual impairments, and atypical physical development. Genetic study of the X chromosome revealed that substantial repeat expansion of the 5' end of the gene fragile X messenger ribonucleoprotein 1 (FMR1) promoted DNA methylation and, consequently, silenced expression of FMR1. Further analysis proved that shorter repeat expansions in FMR1 also manifested in disease at later stages in life. Treatment and therapy options do exist, but they only manage symptoms. Up to now, no cure for FMR1 disorders exists. In this review, we aim to provide an overview of FMR1 biology and the latest research focused on developing therapeutic interventions that can potentially prevent and/or reverse FXS.

Myeloid cell-specific loss of NPC1 in mice recapitulates microgliosis and neurodegeneration in patients with Niemann-Pick type C disease

Niemann-Pick type C (NPC) disease is an inherited lysosomal storage disorder mainly driven by mutations in the NPC1 gene, causing lipid accumulation within late endosomes/lysosomes and resulting in progressive neurodegeneration. Although microglial activation precedes neuronal loss, it remains elusive whether loss of the membrane protein NPC1 in microglia actively contributes to NPC pathology. In a mouse model with depletion of NPC1 in myeloid cells, we report severe alterations in microglial lipidomic profiles, including the enrichment of bis(monoacylglycero)phosphate, increased cholesterol, and a decrease in cholesteryl esters. Lipid dyshomeostasis was associated with microglial hyperactivity, marked by an increase in translocator protein 18 kDa (TSPO). These hyperactive microglia initiated a pathological cascade resembling NPC-like phenotypes, including a shortened life span, motor impairments, astrogliosis, neuroaxonal pathology, and increased neurofilament light chain (NF-L), a neuronal injury biomarker. As observed in the mouse model, patients with NPC showed increased NF-L in the blood and microglial hyperactivity, as visualized by TSPO-PET imaging. Reduced TSPO expression in blood-derived macrophages of patients with NPC was measured after N-acetyl-l-leucine treatment, which has been recently shown to have beneficial effects in patients with NPC, suggesting that TSPO is a potential marker to monitor therapeutic interventions for NPC. Conclusively, these results demonstrate that myeloid dysfunction, driven by the loss of NPC1, contributes to NPC disease and should be further investigated for therapeutic targeting and disease monitoring.

PLD3 and PLD4 synthesize S,S-BMP, a key phospholipid enabling lipid degradation in lysosomes

Bis(monoacylglycero)phosphate (BMP) is an abundant lysosomal phospholipid required for degradation of lipids, particularly gangliosides. Alterations in BMP levels are associated with neurodegenerative diseases. Unlike typical glycerophospholipids, lysosomal BMP has two chiral glycerol carbons in the S (rather than the R) stereo-conformation, protecting it from lysosomal degradation. How this unusual and yet crucial S,S-stereochemistry is achieved is unknown. Here, we report that phospholipases D3 and D4 (PLD3 and PLD4) synthesize lysosomal S,S-BMP, with either enzyme catalyzing the critical glycerol stereo-inversion reaction in vitro. Deletion of PLD3 or PLD4 markedly reduced BMP levels in cells or in murine tissues where either enzyme is highly expressed (brain for PLD3; spleen for PLD4), leading to gangliosidosis and lysosomal abnormalities. PLD3 mutants associated with neurodegenerative diseases, including risk of Alzheimer's disease, diminished PLD3 catalytic activity. We conclude that PLD3/4 enzymes synthesize lysosomal S,S-BMP, a crucial lipid for maintaining brain health.

Functionally overlapping intra- and extralysosomal pathways promote bis(monoacylglycero)phosphate synthesis in mammalian cells

Bis(monoacylglycero)phosphate (BMP) is a major phospholipid constituent of intralumenal membranes in late endosomes/lysosomes, where it regulates the degradation and sorting of lipid cargo. Recent observations suggest that the Batten disease-associated protein CLN5 functions as lysosomal BMP synthase. Here, we show that transacylation reactions catalyzed by cytosolic and secreted enzymes enhance BMP synthesis independently of CLN5. The transacylases identified in this study are capable of acylating the precursor lipid phosphatidylglycerol (PG), generating acyl-PG, which is subsequently hydrolyzed to BMP. Extracellularly, acyl-PG and BMP are generated by endothelial lipase in cooperation with other serum enzymes of the pancreatic lipase family. The intracellular acylation of PG is catalyzed by several members of the cytosolic phospholipase A2 group IV (PLA2G4) family. Overexpression of secreted or cytosolic transacylases was sufficient to correct BMP deficiency in HEK293 cells lacking CLN5. Collectively, our observations suggest that functionally overlapping pathways promote BMP synthesis in mammalian cells.

Mechanisms driving epigenetic and transcriptional responses of microglia in a neurodegenerative lysosomal storage disorder model

Lysosomal dysfunction is causally linked to neurodegeneration in many lysosomal storage disorders (LSDs) and is associated with various age-related neurodegenerative diseases 1,2 , but there is limited understanding of the mechanisms by which altered lysosomal function leads to changes in gene expression that drive pathogenic cellular phenotypes. To investigate this question, we performed systematic imaging, transcriptomic, and epigenetic studies of major brain cell types in Sgsh null (KO) mice, a preclinical mouse model for Sanfilippo syndrome (Mucopolysaccharidosis Type IIIA, MPS-IIIA) 3,4 . MPS-IIIA is a neurodegenerative LSD caused by homozygous loss-of-function (LoF) mutations in SGSH which results in severe early-onset developmental, behavioral, and neurocognitive impairment 5-15 . Electron microscopy, immunohistochemistry, and single-nucleus RNA-sequencing analysis revealed microglia as the cell type exhibiting the most dramatic phenotypic alterations in Sgsh KO mice. Further temporal analysis of microglia gene expression showed dysregulation of genes associated with lysosomal function and immune signaling pathways beginning early in the course of the disease. Sgsh deficiency similarly resulted in increases in open chromatin and histone acetylation at thousands of putative microglia-specific enhancers associated with upregulated genes but had much less impact on the epigenetic landscapes of neurons or oligodendrocytes. We provide evidence for dominant and context-dependent roles of members of the MITF/TFE family as major drivers of microglia-specific epigenetic and transcriptional changes resulting from lysosomal stress that are dependent on collaborative interactions with PU.1/ETS and C/EBP transcription factors. Lastly, we show that features of the transcriptomic and epigenetic alterations observed in murine Sgsh deficiency are also observed in microglia derived from mouse models of age-related neurodegeneration and in human Alzheimer's disease patients, revealing common and disease-specific transcriptional mechanisms associated with disease-associated microglia phenotypes.

Adipose-derived stem cell transplantation enhances spinal cord regeneration by upregulating PGRN expression

This study aims to investigate the effect of adipose-derived stem cells (ADSCs) transplantation on progranulin (PGRN) expression and functional recovery in rats with spinal cord injury (SCI). ADSCs were isolated from the inguinal adipose tissue of rats. A SCI model was created, and ADSCs were injected into the injured area. Various techniques were used to assess the effects of ADSCs transplantation, including hematoxylin-eosin staining, Masson staining, immunofluorescence staining, electron microscopy, MRI, and motor function assessment. The potential mechanisms of ADSC transplantation were investigated using gene expression analysis and protein analysis. Finally, the safety of this therapy was evaluated through hematoxylin-eosin staining and indicators of liver and kidney damage in serum. PGRN expression increased in the injured spinal cord, and ADSCs transplantation further enhanced PGRN levels. The group that received ADSCs transplantation showed reduced inflammation, decreased scar formation, increased nerve regeneration, and faster recovery of bladder function. Importantly, motor function significantly improved in the ADSC transplantation group. ADSCs transplantation enhances functional regeneration in SCI by upregulating PGRN expression, reducing inflammation and scar formation, and promoting nerve regeneration and myelin repair. These findings suggest that ADSC transplantation is a potential therapy for SCI.

Mechanisms of autophagy-lysosome dysfunction in neurodegenerative diseases

Autophagy is a lysosome-based degradative process used to recycle obsolete cellular constituents and eliminate damaged organelles and aggregate-prone proteins. Their postmitotic nature and extremely polarized morphologies make neurons particularly vulnerable to disruptions caused by autophagy-lysosomal defects, especially as the brain ages. Consequently, mutations in genes regulating autophagy and lysosomal functions cause a wide range of neurodegenerative diseases. Here, we review the role of autophagy and lysosomes in neurodegenerative diseases such as Alzheimer disease, Parkinson disease and frontotemporal dementia. We also consider the strong impact of cellular ageing on lysosomes and autophagy as a tipping point for the late-age emergence of related neurodegenerative disorders. Many of these diseases have primary defects in autophagy, for example affecting autophagosome formation, and in lysosomal functions, especially pH regulation and calcium homeostasis. We have aimed to provide an integrative framework for understanding the central importance of autophagic-lysosomal function in neuronal health and disease.

Biochemical, Biomarker, and Behavioral Characterization of the GrnR493X Mouse Model of Frontotemporal Dementia

Heterozygous loss-of-function mutations in the progranulin gene (GRN) are a major cause of frontotemporal dementia due to progranulin haploinsufficiency; complete deficiency of progranulin causes neuronal ceroid lipofuscinosis. Several progranulin-deficient mouse models have been generated, including both knockout mice and knockin mice harboring a common patient mutation (R493X). However, the GrnR493X mouse model has not been characterized completely. Additionally, while homozygous GrnR493X and Grn knockout mice have been extensively studied, data from heterozygous mice is still limited. Here, we performed more in-depth characterization of heterozygous and homozygous GrnR493X knockin mice, which includes biochemical assessments, behavioral studies, and analysis of fluid biomarkers. In the brains of homozygous GrnR493X mice, we found increased phosphorylated TDP-43 along with increased expression of lysosomal genes, markers of microgliosis and astrogliosis, pro-inflammatory cytokines, and complement factors. Heterozygous GrnR493X mice did not have increased TDP-43 phosphorylation but did exhibit limited increases in lysosomal and inflammatory gene expression. Behavioral studies found social and emotional deficits in GrnR493X mice that mirror those observed in Grn knockout mouse models, as well as impairment in memory and executive function. Overall, the GrnR493X knockin mouse model closely phenocopies Grn knockout models. Lastly, in contrast to homozygous knockin mice, heterozygous GrnR493X mice do not have elevated levels of fluid biomarkers previously identified in humans, including neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP) in both plasma and CSF. These results may help to inform pre-clinical studies that use this Grn knockin mouse model and other Grn knockout models.

Targeting lysosomal quality control as a therapeutic strategy against aging and diseases

Previously, lysosomes were primarily referred to as the digestive organelles and recycling centers within cells. Recent discoveries have expanded the lysosomal functional scope and revealed their critical roles in nutrient sensing, epigenetic regulation, plasma membrane repair, lipid transport, ion homeostasis, and cellular stress response. Lysosomal dysfunction is also found to be associated with aging and several diseases. Therefore, function of macroautophagy, a lysosome-dependent intracellular degradation system, has been identified as one of the updated twelve hallmarks of aging. In this review, we begin by introducing the concept of lysosomal quality control (LQC), which is a cellular machinery that maintains the number, morphology, and function of lysosomes through different processes such as lysosomal biogenesis, reformation, fission, fusion, turnover, lysophagy, exocytosis, and membrane permeabilization and repair. Next, we summarize the results from studies reporting the association between LQC dysregulation and aging/various disorders. Subsequently, we explore the emerging therapeutic strategies that target distinct aspects of LQC for treating diseases and combatting aging. Lastly, we underscore the existing knowledge gap and propose potential avenues for future research.

High-throughput screening for small-molecule stabilizers of misfolded glucocerebrosidase in Gaucher disease and Parkinson's disease

Glucocerebrosidase (GCase) is implicated in both a rare, monogenic disorder (Gaucher disease, GD) and a common, multifactorial condition (Parkinson's disease, PD); hence, it is an urgent therapeutic target. To identify correctors of severe protein misfolding and trafficking obstruction manifested by the pathogenic L444P-variant of GCase, we developed a suite of quantitative, high-throughput, cell-based assays. First, we labeled GCase with a small proluminescent HiBiT peptide reporter tag, enabling quantitation of protein stabilization in cells while faithfully maintaining target biology. TALEN-based gene editing allowed for stable integration of a single HiBiT-GBA1 transgene into an intragenic safe-harbor locus in GBA1-knockout H4 (neuroglioma) cells. This GD cell model was amenable to lead discovery via titration-based quantitative high-throughput screening and lead optimization via structure-activity relationships. A primary screen of 10,779 compounds from the NCATS bioactive collections identified 140 stabilizers of HiBiT-GCase-L444P, including both pharmacological chaperones (ambroxol and noninhibitory chaperone NCGC326) and proteostasis regulators (panobinostat, trans-ISRIB, and pladienolide B). Two complementary high-content imaging-based assays were deployed to triage hits: The fluorescence-quenched substrate LysoFix-GBA captured functional lysosomal GCase activity, while an immunofluorescence assay featuring antibody hGCase-1/23 directly visualized GCase lysosomal translocation. NCGC326 was active in both secondary assays and completely reversed pathological glucosylsphingosine accumulation. Finally, we tested the concept of combination therapy by demonstrating synergistic actions of NCGC326 with proteostasis regulators in enhancing GCase-L444P levels. Looking forward, these physiologically relevant assays can facilitate the identification, pharmacological validation, and medicinal chemistry optimization of small molecules targeting GCase, ultimately leading to a viable therapeutic for GD and PD.

Intrinsic link between PGRN and Gba1 D409V mutation dosage in potentiating Gaucher disease

Gaucher disease (GD) is caused by biallelic GBA1/Gba1 mutations that encode defective glucocerebrosidase (GCase). Progranulin (PGRN, encoded by GRN/Grn) is a modifier of GCase, but the interplay between PGRN and GCase, specifically GBA1/Gba1 mutations, contributing to GD severity is unclear. Mouse models were developed with various dosages of Gba1 D409V mutation against the PGRN deficiency (Grn-/-) [Grn-/-;Gba1D409V/WT (PG9Vwt), Grn-/-;Gba1D409V/D409V (PG9V), Grn-/-;Gba1D409V/Null (PG9VN)]. Disease progression in those mouse models was characterized by biochemical, pathological, transcriptomic, and neurobehavioral analyses. Compared to PG9Vwt, Grn-/-;Gba1WT/Null and Grn-/- mice that had a higher level of GCase activity and undetectable pathologies, homozygous or hemizygous D409V in PG9V or PG9VN, respectively, resulted in profound inflammation and neurodegeneration. PG9VN mice exhibited much earlier onset, shorter life span, tissue fibrosis, and more severe phenotypes than PG9V mice. Glycosphingolipid accumulation, inflammatory responses, lysosomal-autophagy dysfunction, microgliosis, retinal gliosis, as well as α-Synuclein increases were much more pronounced in PG9VN mice. Neurodegeneration in PG9VN was characterized by activated microglial phagocytosis of impaired neurons and programmed cell death due to necrosis and, possibly, pyroptosis. Brain transcriptomic analyses revealed the intrinsic relationship between D409V dosage, and the degree of altered gene expression related to lysosome dysfunction, microgliosis, and neurodegeneration in GD, suggesting the disease severity is dependent on a GCase activity threshold related to Gba1 D409V dosage and loss of PGRN. These findings contribute to a deeper understanding of GD pathogenesis by elucidating additional underlying mechanisms of interplay between PGRN and Gba1 mutation dosage in modulating GCase function and disease severity in GD and GBA1-associated neurodegenerative diseases.

Progranulin Facilitates Corneal Repair Through Dual Mechanisms of Inflammation Suppression and Regeneration Promotion

The cornea serves as a vital protective barrier for the eye; however, it is prone to injury and damage that can disrupt corneal epithelium and nerves, triggering inflammation. Therefore, understanding the biological effects and molecular mechanisms involved in corneal wound healing and identifying drugs targeting these pathways is crucial for researchers in this field. This study aimed to investigate the therapeutic potential of progranulin (PGRN) in treating corneal injuries. Our findings demonstrated that PGRN significantly enhanced corneal wound repair by accelerating corneal re-epithelialization and re-innervation. In vitro experiments with cultured epithelial cells and trigeminal ganglion cells further revealed that PGRN stimulated corneal epithelial cell proliferation and promoted axon growth in trigeminal ganglion cells. Through RNA-sequencing (RNA-seq) analysis and other experimental techniques, we discovered that PGRN exerted its healing effects modulating Wnt signaling pathway, which played a critical role in repairing epithelial cells and promoting axon regeneration in trigeminal neurons. Importantly, our study highlighted the anti-inflammatory properties of PGRN by inhibiting the NF-κB signaling pathway, leading to decreased infiltration of macrophages. In conclusion, our findings underscored the potential of PGRN in facilitating corneal wound healing by promoting corneal epithelial cell proliferation, trigeminal ganglion cell axon regeneration, and suppressing ocular inflammation. These results suggest that PGRN could potentially expedite the healing process and improve visual outcomes in patients with corneal injuries.

Carboxy-terminal blockade of sortilin binding enhances progranulin gene therapy, a potential treatment for frontotemporal dementia

Frontotemporal dementia is commonly caused by loss-of-function mutations in the progranulin gene. Potential therapies for this disorder have entered clinical trials, including progranulin gene therapy and drugs that reduce progranulin interactions with sortilin. Both approaches ameliorate functional and pathological abnormalities in mouse models of progranulin insufficiency. Here we investigated whether modifying the progranulin carboxy terminus to block sortilin interactions would improve the efficacy of progranulin gene therapy. We compared the effects of treating progranulin-deficient mice with gene therapy vectors expressing progranulin with intact sortilin interactions, progranulin with the carboxy terminus blocked to reduce sortilin interactions, or GFP control. We found that expressing carboxy-terminally blocked progranulin generated higher levels of progranulin both at the injection site and in more distant regions. Carboxy-terminally blocked progranulin was also more effective at ameliorating microgliosis, microglial lipofuscinosis, and lipid abnormalities including ganglioside accumulation and loss of bis(monoacylglycero)phosphate lipids. Finally, only carboxy-terminally blocked progranulin reduced plasma neurofilament light chain, a biomarker of neurodegeneration, in progranulin-deficient mice. These results demonstrate that modifying the progranulin cargo to block sortilin interactions may be important for increasing the effectiveness of progranulin gene therapy.

One-sentence Summary

The effectiveness of progranulin gene therapy in models of FTD is improved by blocking the protein's carboxy terminus, which prevents sortilin binding.

Emerging trends in virus and virus-like particle gene therapy delivery to the brain

Recent advances in gene therapy and gene-editing techniques offer the very real potential for successful treatment of neurological diseases. However, drug delivery constraints continue to impede viable therapeutic interventions targeting the brain due to its anatomical complexity and highly restrictive microvasculature that is impervious to many molecules. Realizing the therapeutic potential of gene-based therapies requires robust encapsulation and safe and efficient delivery to the target cells. Although viral vectors have been widely used for targeted delivery of gene-based therapies, drawbacks such as host genome integration, prolonged expression, undesired off-target mutations, and immunogenicity have led to the development of alternative strategies. Engineered virus-like particles (eVLPs) are an emerging, promising platform that can be engineered to achieve neurotropism through pseudotyping. This review outlines strategies to improve eVLP neurotropism for therapeutic brain delivery of gene-editing agents.

Lysosomal TMEM106B interacts with galactosylceramidase to regulate myelin lipid metabolism

TMEM106B is an endolysosomal transmembrane protein not only associated with multiple neurological disorders including frontotemporal dementia, Alzheimer's disease, and hypomyelinating leukodystrophy but also potentially involved in COVID-19. Additionally, recent studies have identified amyloid fibrils of C-terminal TMEM106B in both aged healthy and neurodegenerative brains. However, so far little is known about physiological functions of TMEM106B in the endolysosome and how TMEM106B is involved in a wide range of human conditions at molecular levels. Here, we performed lipidomic analysis of the brain of TMEM106B-deficient mice. We found that TMEM106B deficiency significantly decreases levels of two major classes of myelin lipids, galactosylceramide and its sulfated derivative sulfatide. Subsequent co-immunoprecipitation assay showed that TMEM106B physically interacts with galactosylceramidase. We also found that galactosylceramidase activity was significantly increased in TMEM106B-deficient brains. Thus, our results suggest that TMEM106B interacts with galactosylceramidase to regulate myelin lipid metabolism and have implications for TMEM106B-associated diseases.

Sortilin is associated with progranulin deficiency and autism-like behaviors in valproic acid-induced autism rats

INTRODUCTION

Neuroinflammation and microglial activation-related dendritic injury contribute to the pathogenesis of Autism Spectrum Disorder (ASD). Previous studies show that Progranulin (PGRN) is a growth factor associated with inflammation and synaptic development, but the role of PGRN in autism and the mechanisms underlying changes in PGRN expression remain unclear.

AIMS

To investigate the impact of PGRN in autism, we stereotactically injected recombinant PGRN into the hippocampus of ASD model rats. Additionally, we explored the possibility that sortilin may be the factor behind the alterations in PGRN by utilizing SORT1 knockdown. Ultimately, we aimed to identify potential targets for the treatment of autism.

RESULTS

PGRN could alleviate inflammatory responses, protect neuronal dendritic spines, and ameliorate autism-like behaviors. Meanwhile, elevated expression of sortilin and decreased levels of PGRN were observed in both ASD patients and rats. Enhanced sortilin levels facilitated PGRN internalization into lysosomes. Notably, suppressing SORT1 expression amplified PGRN levels, lessened microglial activation, and mitigated inflammation, thereby alleviating autism-like behaviors.

CONCLUSION

Collectively, our findings highlight elevated sortilin levels in ASD rat brains, exacerbating dendrite impairment by affecting PGRN expression. PGRN supplementation and SORT1 knockdown hold potential as therapeutic strategies for ASD.

Gaucher disease provides a unique window into Parkinson disease pathogenesis

An exciting development in the field of neurodegeneration is the association between the rare monogenic disorder Gaucher disease and the common complex disorder Parkinson disease (PD). Gaucher disease is a lysosomal storage disorder resulting from an inherited deficiency of the enzyme glucocerebrosidase, encoded by GBA1, which hydrolyses the glycosphingolipids glucosylceramide and glucosylsphingosine. The observation of parkinsonism in a rare subgroup of individuals with Gaucher disease first directed attention to the role of glucocerebrosidase deficiency in the pathogenesis of PD. PD occurs more frequently in people heterozygous for Gaucher GBA1 mutations, and 3-25% of people with Parkinson disease carry a GBA1 variant. However, only a small percentage of individuals with GBA1 variants develop parkinsonism, suggesting that the penetrance is low. Despite over a decade of intense research in this field, including clinical and radiological evaluations, genetic studies and investigations using model systems, the mechanism underlying GBA1-PD is still being pursued. Insights from this association have emphasized the role of lysosomal pathways in parkinsonism. Furthermore, different therapeutic strategies considered or developed for Gaucher disease can now inform drug development for PD.

Neurological glycogen storage diseases and emerging therapeutics

Glycogen storage diseases (GSDs) comprise a group of inherited metabolic disorders characterized by defects in glycogen metabolism, leading to abnormal glycogen accumulation in multiple tissues, most notably affecting the liver, skeletal muscle, and heart. Recent findings have uncovered the importance of glycogen metabolism in the brain, sustaining a myriad of physiological functions and linking its perturbation to central nervous system (CNS) pathology. This link resulted in classification of neurological-GSDs (n-GSDs), a group of diseases with shared deficits in neurological glycogen metabolism. The n-GSD patients exhibit a spectrum of clinical presentations with common etiology while requiring tailored therapeutic approaches from the traditional GSDs. Recent research has elucidated the genetic and biochemical mechanisms and pathophysiological basis underlying different n-GSDs. Further, the last decade has witnessed some promising developments in novel therapeutic approaches, including enzyme replacement therapy (ERT), substrate reduction therapy (SRT), small molecule drugs, and gene therapy targeting key aspects of glycogen metabolism in specific n-GSDs. This preclinical progress has generated noticeable success in potentially modifying disease course and improving clinical outcomes in patients. Herein, we provide an overview of current perspectives on n-GSDs, emphasizing recent advances in understanding their molecular basis, therapeutic developments, underscore key challenges and the need to deepen our understanding of n-GSDs pathogenesis to develop better therapeutic strategies that could offer improved treatment and sustainable benefits to the patients.

Targeting the transferrin receptor to transport antisense oligonucleotides across the mammalian blood-brain barrier

Antisense oligonucleotides (ASOs) are promising therapeutics for treating various neurological disorders. However, ASOs are unable to readily cross the mammalian blood-brain barrier (BBB) and therefore need to be delivered intrathecally to the central nervous system (CNS). Here, we engineered a human transferrin receptor 1 (TfR1) binding molecule, the oligonucleotide transport vehicle (OTV), to transport a tool ASO across the BBB in human TfR knockin (TfRmu/hu KI) mice and nonhuman primates. Intravenous injection and systemic delivery of OTV to TfRmu/hu KI mice resulted in sustained knockdown of the ASO target RNA, Malat1, across multiple mouse CNS regions and cell types, including endothelial cells, neurons, astrocytes, microglia, and oligodendrocytes. In addition, systemic delivery of OTV enabled Malat1 RNA knockdown in mouse quadriceps and cardiac muscles, which are difficult to target with oligonucleotides alone. Systemically delivered OTV enabled a more uniform ASO biodistribution profile in the CNS of TfRmu/hu KI mice and greater knockdown of Malat1 RNA compared with a bivalent, high-affinity TfR antibody. In cynomolgus macaques, an OTV directed against MALAT1 displayed robust ASO delivery to the primate CNS and enabled more uniform biodistribution and RNA target knockdown compared with intrathecal dosing of the same unconjugated ASO. Our data support systemically delivered OTV as a potential platform for delivering therapeutic ASOs across the BBB.

An anti-sortilin affibody-peptide fusion inhibits sortilin-mediated progranulin degradation

Heterozygous loss-of-function mutations in the GRN gene are a common cause of frontotemporal dementia. Such mutations lead to decreased plasma and cerebrospinal fluid levels of progranulin (PGRN), a neurotrophic factor with lysosomal functions. Sortilin is a negative regulator of extracellular PGRN levels and has shown promise as a therapeutic target for frontotemporal dementia, enabling increased extracellular PGRN levels through inhibition of sortilin-mediated PGRN degradation. Here we report the development of a high-affinity sortilin-binding affibody-peptide fusion construct capable of increasing extracellular PGRN levels in vitro. By genetic fusion of a sortilin-binding affibody generated through phage display and a peptide derived from the progranulin C-terminus, an affinity protein (A3-PGRNC15*) with 185-pM affinity for sortilin was obtained. Treating PGRN-secreting and sortilin-expressing human glioblastoma U-251 cells with the fusion protein increased extracellular PGRN levels up to 2.5-fold, with an EC50 value of 1.3 nM. Our results introduce A3-PGRNC15* as a promising new agent with therapeutic potential for the treatment of frontotemporal dementia. Furthermore, the work highlights means to increase binding affinity through synergistic contribution from two orthogonal polypeptide units.

Sexually Dimorphic Response to Hepatic Injury in Newborn Suffering from Intrauterine Growth Restriction

Intrauterine growth restriction (IUGR), when a fetus does not grow as expected, is associated with a reduction in hepatic functionality and a higher risk for chronic liver disease in adulthood. Utilizing early developmental plasticity to reverse the outcome of poor fetal programming remains an unexplored area. Focusing on the biochemical profiles of neonates and previous transcriptome findings, piglets from the same fetus are selected as models for studying IUGR. The cellular landscape of the liver is created by scRNA-seq to reveal sex-dependent patterns in IUGR-induced hepatic injury. One week after birth, IUGR piglets experience hypoxic stress. IUGR females exhibit fibroblast-driven T cell conversion into an immune-adapted phenotype, which effectively alleviates inflammation and fosters hepatic regeneration. In contrast, males experience even more severe hepatic injury. Prolonged inflammation due to disrupted lipid metabolism hinders intercellular communication among non-immune cells, which ultimately impairs liver regeneration even into adulthood. Additionally, Apolipoprotein A4 (APOA4) is explored as a novel biomarker by reducing hepatic triglyceride deposition as a protective response against hypoxia in IUGR males. PPARα activation can mitigate hepatic damage and meanwhile restore over-expressed APOA4 to normal in IUGR males. The pioneering study offers valuable insights into the sexually dimorphic responses to hepatic injury during IUGR.

Benzoxazole-derivatives enhance progranulin expression and reverse the aberrant lysosomal proteome caused by GRN haploinsufficiency

Heterozygous loss-of-function mutations in the GRN gene are a major cause of hereditary frontotemporal dementia. The mechanisms linking frontotemporal dementia pathogenesis to progranulin deficiency are not well understood, and there is currently no treatment. Our strategy to prevent the onset and progression of frontotemporal dementia in patients with GRN mutations is to utilize small molecule positive regulators of GRN expression to boost progranulin levels from the remaining functional GRN allele, thus restoring progranulin levels back to normal within the brain. This work describes a series of blood-brain-barrier-penetrant small molecules which significantly increase progranulin protein levels in human cellular models, correct progranulin protein deficiency in Grn+/- mouse brains, and reverse lysosomal proteome aberrations, a phenotypic hallmark of frontotemporal dementia, more efficiently than the previously described small molecule suberoylanilide hydroxamic acid. These molecules will allow further elucidation of the cellular functions of progranulin and its role in frontotemporal dementia and will also serve as lead structures for further drug development.

CNS-wide repopulation by hematopoietic-derived microglia-like cells corrects progranulin deficiency in mice

Hematopoietic stem cell transplantation can deliver therapeutic proteins to the central nervous system (CNS) through transplant-derived microglia-like cells. However, current conditioning approaches result in low and slow engraftment of transplanted cells in the CNS. Here we optimized a brain conditioning regimen that leads to rapid, robust, and persistent microglia replacement without adverse effects on neurobehavior or hematopoiesis. This regimen combines busulfan myeloablation and six days of Colony-stimulating factor 1 receptor inhibitor PLX3397. Single-cell analyses revealed unappreciated heterogeneity of microglia-like cells with most cells expressing genes characteristic of homeostatic microglia, brain-border-associated macrophages, and unique markers. Cytokine analysis in the CNS showed transient inductions of myeloproliferative and chemoattractant cytokines that help repopulate the microglia niche. Bone marrow transplant of progranulin-deficient mice conditioned with busulfan and PLX3397 restored progranulin in the brain and eyes and normalized brain lipofuscin storage, proteostasis, and lipid metabolism. This study advances our understanding of CNS repopulation by hematopoietic-derived cells and demonstrates its therapeutic potential for treating progranulin-dependent neurodegeneration.

Meeting report of the fifth annual workshop on Principles and Techniques for Improving Preclinical to Clinical Translation in Alzheimer's Disease Research

The fifth annual workshop on Principles and Techniques for Improving Preclinical Translation of Alzheimer's Disease Research was held in May 2023 at The Jackson Laboratory in Bar Harbor, Maine, USA. The workshop was established in 2018 to address training gaps in preclinical translational studies for Alzheimer's disease (AD). In addition to providing fundamental knowledge and hands-on skills essential for executing rigorous in vivo studies that are designed to facilitate translation, each year the workshop aims to provide insight on state-of-the-field technological advances and new resources including novel animal models, publicly available datasets, novel biomarkers, and new medical imaging tracers. This innovative and comprehensive workshop continues to deliver training for the greater AD research community in order to provide investigators and trainees with the knowledge and skillsets essential for enabling improved preclinical to clinical translation and accelerate the process of advancing safe and effective therapeutic interventions for AD. HIGHLIGHTS: Translational research is not typically available as a course of study at academic institutions, yet there are fundamental skillsets and knowledge required to enable successful translation from preclinical experiments to clinical efficacy. It is important that there are resources and opportunities available to researchers planning preclinical translational experiments. Here we present proceedings from the fifth annual NIA-sponsored workshop focused on enabling improved preclinical to clinical translation for Alzheimer's disease research that includes didactic lectures on state-of-the-field approaches and hands-on practicums for acquiring essential translational laboratory techniques.

Role of Tau Protein in Neurodegenerative Diseases and Development of Its Targeted Drugs: A Literature Review

Tau protein is a microtubule-associated protein that is widely distributed in the central nervous system and maintains and regulates neuronal morphology and function. Tau protein aggregates abnormally and forms neurofibrillary tangles in neurodegenerative diseases, disrupting the structure and function of neurons and leading to neuronal death, which triggers the initiation and progression of neurological disorders. The aggregation of tau protein in neurodegenerative diseases is associated with post-translational modifications, which may affect the hydrophilicity, spatial conformation, and stability of tau protein, promoting tau protein aggregation and the formation of neurofibrillary tangles. Therefore, studying the role of tau protein in neurodegenerative diseases and the mechanism of aberrant aggregation is important for understanding the mechanism of neurodegenerative diseases and finding therapeutic approaches. This review describes the possible mechanisms by which tau protein promotes neurodegenerative diseases, the post-translational modifications of tau protein and associated influencing factors, and the current status of drug discovery and development related to tau protein, which may contribute to the development of new therapeutic approaches to alleviate or treat neurodegenerative diseases.

Peripheral expression of brain-penetrant progranulin rescues pathologies in mouse models of frontotemporal lobar degeneration

Progranulin (PGRN) haploinsufficiency is a major risk factor for frontotemporal lobar degeneration with TAR DNA-binding protein 43 (TDP-43) pathology (FTLD-GRN). Multiple therapeutic strategies are in clinical development to restore PGRN in the CNS, including gene therapy. However, a limitation of current gene therapy approaches aimed to alleviate FTLD-associated pathologies may be their inefficient brain exposure and biodistribution. We therefore developed an adeno-associated virus (AAV) targeting the liver (L) to achieve sustained peripheral expression of a transferrin receptor (TfR) binding, brain-penetrant (b) PGRN variant [AAV(L):bPGRN] in two mouse models of FTLD-GRN, namely, Grn knockout and GrnxTmem106b double knockout mice. This therapeutic strategy avoids potential safety and biodistribution issues of CNS-administered AAVs and maintains sustained concentrations of PGRN in the brain after a single dose. AAV(L):bPGRN treatment reduced several FTLD-GRN-associated pathologies including severe motor function deficits, aberrant TDP-43 phosphorylation, dysfunctional protein degradation, lipid metabolism, gliosis, and neurodegeneration in the brain. The potential translatability of our findings was tested in an in vitro model using cocultured human induced pluripotent stem cell (hiPSC)-derived microglia lacking PGRN and TMEM106B and wild-type hiPSC-derived neurons. As in mice, aberrant TDP-43, lysosomal dysfunction, and neuronal loss were ameliorated after treatment with exogenous TfR-binding protein transport vehicle fused to PGRN (PTV:PGRN). Together, our studies suggest that peripherally administered brain-penetrant PGRN replacement strategies ameliorate FTLD-GRN relevant phenotypes including TDP-43 pathology, neurodegeneration, and behavioral deficits. Our data provide preclinical proof of concept for the use of this AAV platform for treatment of FTLD-GRN and potentially other CNS disorders.

Frontotemporal dementia: from genetics to therapeutic approaches

INTRODUCTION

Frontotemporal dementia (FTD) includes a group of neurodegenerative diseases characterized clinically by behavioral disturbances and by neurodegeneration of brain anterior temporal and frontal lobes, leading to atrophy. Apart from symptomatic treatments, there is, at present, no disease-modifying cure for FTD.

AREAS COVERED

Three main mutations are known as causes of familial FTD, and large consortia have studied carriers of mutations, also in preclinical Phases. As genetic cases are the only ones in which the pathology can be predicted in life, compounds developed so far are directed toward specific proteins or mutations. Herein, recently approved clinical trials will be summarized, including molecules, mechanisms of action and pharmacological testing.

EXPERT OPINION

These studies are paving the way for the future. They will clarify whether single mutations should be addressed rather than common proteins depositing in the brain to move from genetic to sporadic FTD.

Targeting complement C3a receptor resolves mitochondrial hyperfusion and subretinal microglial activation in progranulin-deficient frontotemporal dementia

Mutations in progranulin ( GRN ) cause frontotemporal dementia ( GRN -FTD) due to deficiency of the pleiotropic protein progranulin. GRN -FTD exhibits diverse pathologies including lysosome dysfunction, lipofuscinosis, microgliosis, and neuroinflammation. Yet, how progranulin loss causes disease remains unresolved. Here, we report that non-invasive retinal imaging of GRN -FTD patients revealed deficits in photoreceptors and the retinal pigment epithelium (RPE) that correlate with cognitive decline. Likewise, Grn -/- mice exhibit early RPE dysfunction, microglial activation, and subsequent photoreceptor loss. Super-resolution live imaging and transcriptomic analyses identified RPE mitochondria as an early driver of retinal dysfunction. Loss of mitochondrial fission protein 1 (MTFP1) in Grn -/- RPE causes mitochondrial hyperfusion and bioenergetic defects, leading to NF-kB-mediated activation of complement C3a-C3a receptor signaling, which drives further mitochondrial hyperfusion and retinal inflammation. C3aR antagonism restores RPE mitochondrial integrity and limits subretinal microglial activation. Our study identifies a previously unrecognized mechanism by which progranulin modulates mitochondrial integrity and complement-mediated neuroinflammation.

Progranulin AAV gene therapy for frontotemporal dementia: translational studies and phase 1/2 trial interim results

GRN mutations cause progranulin haploinsufficiency, which eventually leads to frontotemporal dementia (FTD-GRN). PR006 is an investigational gene therapy delivering the granulin gene (GRN) using an adeno-associated virus serotype 9 (AAV9) vector. In non-clinical studies, PR006 transduced neurons derived from induced pluripotent stem cells of patients with FTD-GRN, resulted in progranulin expression and improvement of lipofuscin, lysosomal and neuroinflammation pathologies in Grn-knockout mice, and was well tolerated except for minimal, asymptomatic dorsal root ganglionopathy in non-human primates. We initiated a first-in-human phase 1/2 open-label trial. Here we report results of a pre-specified interim analysis triggered with the last treated patient of the low-dose cohort (n = 6) reaching the 12-month follow-up timepoint. We also include preliminary data from the mid-dose cohort (n = 7). Primary endpoints were safety, immunogenicity and change in progranulin levels in cerebrospinal fluid (CSF) and blood. Secondary endpoints were Clinical Dementia Rating (CDR) plus National Alzheimer's Disease Coordinating Center (NACC) Frontotemporal Lobar Degeneration (FTLD) rating scale and levels of neurofilament light chain (NfL). One-time administration of PR006 into the cisterna magna was generally safe and well tolerated. All patients developed treatment-emergent anti-AAV9 antibodies in the CSF, but none developed anti-progranulin antibodies. CSF pleocytosis was the most common PR006-related adverse event. Twelve serious adverse events occurred, mostly unrelated to PR006. Deep vein thrombosis developed in three patients. There was one death (unrelated) occurring 18 months after treatment. CSF progranulin increased after PR006 treatment in all patients; blood progranulin increased in most patients but only transiently. NfL levels transiently increased after PR006 treatment, likely reflecting dorsal root ganglia toxicity. Progression rates, based on the CDR scale, were within the broad ranges reported for patients with FTD. These data provide preliminary insights into the safety and bioactivity of PR006. Longer follow-up and additional studies are needed to confirm the safety and potential efficacy of PR006. ClinicalTrials.gov identifier: NCT04408625 .

Regulation of muscle hypertrophy through granulin: Relayed communication among mesenchymal progenitors, macrophages, and satellite cells

Skeletal muscles exert remarkable regenerative or adaptive capacities in response to injuries or mechanical loads. However, the cellular networks underlying muscle adaptation are poorly understood compared to those underlying muscle regeneration. We employed single-cell RNA sequencing to investigate the gene expression patterns and cellular networks activated in overloaded muscles and compared these results with those observed in regenerating muscles. The cellular composition of the 4-day overloaded muscle, when macrophage infiltration peaked, closely resembled that of the 10-day regenerating muscle. In addition to the mesenchymal progenitor-muscle satellite cell (MuSC) axis, interactome analyses or targeted depletion experiments revealed communications between mesenchymal progenitors-macrophages and macrophages-MuSCs. Furthermore, granulin, a macrophage-derived factor, inhibited MuSC differentiation, and Granulin-knockout mice exhibited blunted muscle hypertrophy due to the premature differentiation of overloaded MuSCs. These findings reveal the critical role of granulin through the relayed communications of mesenchymal progenitors, macrophages, and MuSCs in facilitating efficient muscle hypertrophy.

Epigenetic modulation through BET bromodomain inhibitors as a novel therapeutic strategy for progranulin-deficient frontotemporal dementia

Frontotemporal dementia (FTD) is a debilitating neurodegenerative disorder with currently no disease-modifying treatment options available. Mutations in GRN are one of the most common genetic causes of FTD, near ubiquitously resulting in progranulin (PGRN) haploinsufficiency. Small molecules that can restore PGRN protein to healthy levels in individuals bearing a heterozygous GRN mutation may thus have therapeutic value. Here, we show that epigenetic modulation through bromodomain and extra-terminal domain (BET) inhibitors (BETi) potently enhance PGRN protein levels, both intracellularly and secreted forms, in human central nervous system (CNS)-relevant cell types, including in microglia-like cells. In terms of potential for disease modification, we show BETi treatment effectively restores PGRN levels in neural cells with a GRN mutation known to cause PGRN haploinsufficiency and FTD. We demonstrate that BETi can rapidly and durably enhance PGRN in neural progenitor cells (NPCs) in a manner dependent upon BET protein expression, suggesting a gain-of-function mechanism. We further describe a CNS-optimized BETi chemotype that potently engages endogenous BRD4 and enhances PGRN expression in neuronal cells. Our results reveal a new epigenetic target for treating PGRN-deficient forms of FTD and provide mechanistic insight to aid in translating this discovery into therapeutics.

Lysosomal damage sensing and lysophagy initiation by SPG20-ITCH

Cells respond to lysosomal membrane permeabilization by membrane repair or selective macroautophagy of damaged lysosomes, termed lysophagy, but it is not fully understood how this decision is made. Here, we uncover a pathway in human cells that detects lipid bilayer perturbations in the limiting membrane of compromised lysosomes, which fail to be repaired, and then initiates ubiquitin-triggered lysophagy. We find that SPG20 binds the repair factor IST1 on damaged lysosomes and, importantly, integrates that with the detection of damage-associated lipid-packing defects of the lysosomal membrane. Detection occurs via sensory amphipathic helices in SPG20 before rupture of the membrane. If lipid-packing defects are extensive, such as during lipid peroxidation, SPG20 recruits and activates ITCH, which marks the damaged lysosome with lysine-63-linked ubiquitin chains to initiate lysophagy and thus triages the lysosome for destruction. With SPG20 being linked to neurodegeneration, these findings highlight the relevance of a coordinated lysosomal damage response for cellular homeostasis.

Progranulinopathy: A diverse realm of disorders linked to progranulin imbalances

Progranulin (PGRN), encoded by the GRN gene in humans, was originally isolated as a secreted growth factor that implicates in a multitude of processes ranging from regulation of tumorigenesis, inflammation to neural proliferation. Compelling evidence indicating that GRN mutation can lead to various common neuronal degenerative diseases and rare lysosomal storage diseases. These findings have unveiled a critical role for PGRN as a lysosomal protein in maintaining lysosomal function. The phenotypic spectrum of PGRN imbalance has expanded to encompass a broad spectrum of diseases, including autoimmune diseases, metabolic, musculoskeletal and cardiovascular diseases. These diseases collectively referred to as Progranulinopathy- a term encompasses the wide spectrum of disorders influenced by PGRN imbalance. Unlike its known extracellular function as a growth factor-like molecule associated with multiple membrane receptors, PGRN also serves as an intracellular co-chaperone engaged in the folding and traffic of its associated proteins, particularly the lysosomal hydrolases. This chaperone activity is required for PGRN to exert its diverse functions across a broad range of diseases, encompassing both the central nervous system and peripheral systems. In this comprehensive review, we present an update of the emerging role of PGRN in Progranulinopathy, with special focus on elucidating the intricate interplay between PGRN and a diverse array of proteins at various levels, ranging from extracellular fluids and intracellular components, as well as various pathophysiological processes involved. This review seeks to offer a comprehensive grasp of PGRN's diverse functions, aiming to unveil intricate mechanisms behind Progranulinopathy and open doors for future research endeavors.

PLD3 and PLD4 synthesize S,S-BMP, a key phospholipid enabling lipid degradation in lysosomes

Bis(monoacylglycero)phosphate (BMP) is an abundant lysosomal phospholipid required for degradation of lipids, in particular gangliosides. Alterations in BMP levels are associated with neurodegenerative diseases. Unlike typical glycerophospholipids, lysosomal BMP has two chiral glycerol carbons in the S (rather than the R) stereo-conformation, protecting it from lysosomal degradation. How this unusual and yet crucial S,S-stereochemistry is achieved is unknown. Here we report that phospholipases D3 and D4 (PLD3 and PLD4) synthesize lysosomal S,S-BMP, with either enzyme catalyzing the critical glycerol stereo-inversion reaction in vitro. Deletion of PLD3 or PLD4 markedly reduced BMP levels in cells or in murine tissues where either enzyme is highly expressed (brain for PLD3; spleen for PLD4), leading to gangliosidosis and lysosomal abnormalities. PLD3 mutants associated with neurodegenerative diseases, including Alzheimer's disease risk, diminished PLD3 catalytic activity. We conclude that PLD3/4 enzymes synthesize lysosomal S,S-BMP, a crucial lipid for maintaining brain health.