Protein aggregation and impaired clearance constitute central pathological hallmarks of numerous neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, and frontotemporal dementia. Misfolded proteins, both extracellular and intracellular, disrupt proteostasis and contribute to synaptic dysfunction, neuroinflammation, and neuronal loss. Within the central nervous system, glial cells serve as the primary mediators of pathological protein clearance. Microglia detect, phagocytose, and degrade aggregated proteins. However, under chronic pathological stress, microglial function turns from clearing pathological proteins to paradoxically facilitating their spread. Astrocytes, which provide metabolic and homeostatic support for neurons, also participate in the clearance of aggregates through phagocytosis and glymphatic drainage. Critically, crosstalk between astrocytes and microglia enhances the clearance: microglia can recruit and process aggregates internalized by astrocytes, while astrocyte-derived signals modulate the microglial phagocytic activity. Aging, genetic risk factors, or sustained neuroinflammation may reduce the efficiency of glial removal of aggregates and accelerate disease progression. Understanding the cellular and molecular underpinnings of microglial and astrocytic clearance pathways is essential for developing therapies aimed at restoring proteostasis and mitigating neurodegeneration.
Abstract Neurodegenerative diseases such as Alzheimer’s disease (AD), frontotemporal dementia (FTD), and Parkinson’s disease (PD) are characterized by the accumulation of intrinsically disordered proteins (IDPs), including amyloid-β (Aβ), tau, and α-synuclein (αSyn). These proteins disrupt neuronal homeostasis through toxic aggregation, impaired proteostasis, and progressive neurodegeneration, in part via dysfunction of the autophagy-lysosomal pathway (ALP). A central feature of this impairment is defective vacuolar-type (H + )-ATPase (V-ATPase) function, leading to lysosomal deacidification and reduced degradative capacity. Pathological IDP species converge on V-ATPase through direct interactions with its subunits or by perturbing its assembly, while also exerting protein-specific effects, including lysosomal membrane destabilization, impaired hydrolase activity, and disrupted endolysosomal trafficking. These combined mechanisms create a reciprocal relationship between lysosomal dysfunction and IDP accumulation, in which impaired lysosomal degradation further promotes the persistence and aggregation of pathological IDP species. This review synthesizes current insights into the molecular interplay between IDPs and lysosomal acidification across AD, FTD, and PD, and highlights how these processes contribute to disease progression. We further discuss emerging therapeutic strategies aimed at restoring lysosomal function. These findings underscore lysosomal dysfunction as a common feature of IDP-driven neurodegeneration and a potential therapeutic target across these disorders.
Transient receptor potential vanilloid 1 (TRPV1) is increasingly recognized as an important regulator of microglial response during neuroinflammation, a key pathological feature of various neurodegenerative diseases such as Alzheimer’s disease. Capsaicin, the primary pungent component in chili peppers, executes its anti-inflammatory properties as a TRPV1 agonist. Although the mechanism of TRPV1 channel activation and signal transduction by capsaicin has been extensively studied, the protein–protein interaction-based mechanisms by which TRPV1 modulates microglial inflammatory responses in the context of neurodegeneration remains unclear. We used a lipopolysaccharide-induced acute neuroinflammation model to examine the effects of TRPV1 activation and pharmacological blockade of TRPV1 channel gating on microglial inflammatory responses. Proximity labeling-based proteomics, bioluminescence resonance energy transfer, and NanoLuc Binary Technology were used to identify and characterize TRPV1-interacting proteins. Signal sequence receptor subunit 4 (SSR4) knockdown and forced interaction between TRPV1 and SSR4 were performed to assess the functional contribution of the TRPV1–SSR4 interaction. Structural modeling, BRET-based domain mapping, and colocalization analyses were further used to examine the molecular and subcellular features of SSR4 recruitment by TRPV1. In the neuroinflammatory model, the TRPV1 agonist capsaicin attenuated pro-inflammatory cytokine production, neuronal apoptosis, microgliosis, and microglial soma enlargement in a TRPV1-dependent manner. Notably, pharmacological blockade of TRPV1 channel gating by capsazepine did not abolish the anti-inflammatory action of capsaicin, suggesting an alternative mechanism of action beyond the canonical ligand-gated Ca2⁺ influx. Screening of TRPV1 prey proteins and validation strategy identified SSR4 as a capsaicin-responsive, TRPV1-associated protein. SSR4 knockdown in microglia significantly diminished the suppression of inflammatory responses caused by TRPV1 activation. Conversely, forced interaction between TRPV1 and SSR4 partially mimicked the anti-inflammatory effects of TRPV1 activation. Further structural and cellular analyses suggested that the transmembrane region of TRPV1 supports SSR4 recruitment. Our findings suggest that capsaicin suppresses microglial inflammation mediated by the TRPV1–SSR4 interaction, providing new insights into the chemical biology basis of TRPV1 interoceptive signaling and potential applications of natural TRPV1 agonists for modifying therapy of neuroinflammation-associated neurodegeneration.
Autophagy-lysosomal dysfunction contributes to lipid droplet (LD) accumulation, which drives pathological progression in Alzheimer's disease (AD). Exercise is reported to restore autophagy-lysosomal function and attenuate cognitive impairment in AD, but its regulation of LD deposition remains unknown. 5×FAD transgenic mice underwent a three-month exercise intervention. Cognitive and emotional behaviors were assessed using the Morris water maze, novel object recognition, and open field test. LD deposition was evaluated by oil red O and BODIPY staining. Proteomic sequencing was performed to identify proteins affected by exercise. Pathological changes were examined by immunofluorescence, western blotting, and transmission electron microscopy. Intracerebroventricular injection of AAV was used to overexpress or knock down microglial TMEM9 to investigate the mechanisms by which physical exercise affects behavior and pathology in AD. In vitro, BV2 cells were cultured and incubated with oligomeric amyloid-β (oAβ), with or without oleic acid to induce LD formation. Chloroquine and rapamycin, which regulate autophagy-lysosomal function, were used to examine the effects of LDs on mitochondrial integrity under glucose deprivation conditions. Lentiviral or plasmid transfection was used to explore the key role of TMEM9 in regulating lipophagy, and the interaction between TMEM9 and ATP6AP1 was detected by co-immunoprecipitation. Clinically, plasma TMEM9 and ATP6AP1 levels in AD patients were measured by ELISA, and receiver operating characteristic (ROC) analysis was performed to evaluate diagnostic performance. Exercise mitigated cerebral LD deposition through enhanced lipophagy, preserving mitochondrial integrity and attenuating cognitive decline. Mechanistically, exercise suppressed microglial TMEM9 expression, restoring LD degradation. Tmem9 downregulation facilitated LKB1 lysosomal translocation, activating AMPK signaling. In vitro, oleic acid-induced LDs preserved mitochondrial membrane potential and increased ATP production in BV2 microglia under glucose deprivation and oAβ exposure. Tmem9 overexpression exacerbated lipophagy impairment and mitochondrial dysfunction, whereas Tmem9 knockdown enhanced lipophagy and cytoprotection. Aβ clearance was negatively regulated by TMEM9. Clinically, AD patients exhibited characteristic Aβ deposition and glucose hypometabolism, with plasma TMEM9 levels strongly correlating with p-tau217 progression. Plasma TMEM9 and the TMEM9/ATP6AP1 ratio emerged as potential AD diagnostic biomarkers (AUC=0.791 and 0.853, respectively). These findings establish exercise-induced TMEM9 inhibition as a novel therapeutic strategy targeting lipidostasis to combat AD progression.
