Lysosomal acidification dysfunction in microglia: an emerging pathogenic mechanism of neuroinflammation and neurodegeneration

Abstract

Microglia are the resident innate immune cells in the brain with a major role in orchestrating immune responses. They also provide a frontline of host defense in the central nervous system (CNS) through their active phagocytic capability. Being a professional phagocyte, microglia participate in phagocytic and autophagic clearance of cellular waste and debris as well as toxic protein aggregates, which relies on optimal lysosomal acidification and function. Defective microglial lysosomal acidification leads to impaired phagocytic and autophagic functions which result in the perpetuation of neuroinflammation and progression of neurodegeneration. Reacidification of impaired lysosomes in microglia has been shown to reverse neurodegenerative pathology in Alzheimer's disease. In this review, we summarize key factors and mechanisms contributing to lysosomal acidification impairment and the associated phagocytic and autophagic dysfunction in microglia, and how these defects contribute to neuroinflammation and neurodegeneration. We further discuss techniques to monitor lysosomal pH and therapeutic agents that can reacidify impaired lysosomes in microglia under disease conditions. Finally, we propose future directions to investigate the role of microglial lysosomal acidification in lysosome-mitochondria crosstalk and in neuron-glia interaction for more comprehensive understanding of its broader CNS physiological and pathological implications.

Keywords: Acidic nanoparticles; Autophagy; Cytokines; Lysosomal acidification; Neurodegenerative diseases; Neuroinflammation; Phagocytosis; Toxic protein aggregates.

Fig. 1

Implications of microglial lysosomal acidification dysfunction in neuroinflammation and neurodegeneration. a In healthy microglia, lysosomes with sufficiently acidic lumen can fuse with autophagosomes or phagosomes to form functional autolysosomes for efficient degradation of both intracellular and extracellular cargoes (left). Under stimulated or diseased conditions, lysosomes with poorly acidic lumen (impaired lysosomal acidification) will have inefficient fusion with autophagosomes or phagosomes, or even no fusion, leading to reduced phagocytic and autophagic functions (right). b In the context of neuroinflammation, stimuli-activated microglia with impaired lysosomal acidification express and secrete more cytokines to perpetuate neuroinflammation. Through releasing more inflammatory cytokines, these dysfunctional microglia recruit and activate immune cells and participate in detrimental crosstalk with astrocytes to propagate inflammatory response. c In the context of neurodegeneration, increased inflammatory cytokine secretion by dysfunctional microglia with lysosomal acidification defect contributes to neuronal death via mechanisms such as necroptosis. In addition, these impaired microglia have reduced phagocytic and autophagic capabilities in the clearance of toxic protein aggregates, damaged organelles such as mitochondria, and myelin debris, as well as in synaptic pruning, leading to eventual neuronal death and neurodegeneration. The figure was created with BioRender.com

Fig. 2

Factors affecting microglial lysosomal acidification and associated phagocytic and autophagic function. a Presenilin-1 (PS1) deficient in phosphorylation at Ser367 (S367) leads to reduced ATP6V0a1 levels, impairing lysosomal acidification. Furthermore, PS1 S367A reduces binding to Annexin A2 and decreases VAMP8 binding to autophagosomal Syntaxin 17, thereby preventing autophagosome-lysosome fusion, leading to autophagic inhibition. b Cytokine stimulations induce differential effects of lysosomal acidification alterations in microglia. c Lipids such as myelin debris and TREM2 mediated lipid accumulation impair microglial lysosomal acidification through different mechanisms and decrease lipid catabolic activity. d Purinergic receptors signaling regulate lysosomal acidification. Extracellular ATP activate P2X7R, leading to influx of Ca²⁺, accumulation of autophagosomes, elevation of lysosomal pH as well as increased cytokine release via inflammasome. On the other hand, inhibition of P2X4R by TNP-ATP results in increased cytokine release, while activation of P2X4R by ivermectin improves lysosomal acidification and promotes microglial function. e Accumulation of Aβ protein has been shown to lead to defective lysosomal acidification, potentially due to efflux of TFEB out of nucleus. Similarly, a decrease in PKA activity along with decreased ClC-7 chloride transporter function leads to lysosomal pH elevation, resulting in reduced degradation of Aβ, further driving neurodegeneration. The figure was created with BioRender.com

Fig. 3

An overview of lysosome-targeting therapeutic agents developed to reacidify impaired lysosomes under diseased conditions. a Impaired lysosomal acidification results in lysosomal membrane permeabilization, leading to the release of cathepsins into cytosol that impair microglial function. b Lysosome reacidifying agents can restore lysosomal function to counteract undesired downstream consequences including LMP. Current lysosome-targeting therapeutic strategies have focused on using c small-molecule modulators such as C381 and EN6 to activate V-ATPase activity, d small-molecule activator of TFEB (PF-11) to increase lysosomal protein transcription and expression as well as promote lysosomal acidification, e mTOR inhibitors such as OSI-027 and PP242 to lower lysosomal pH, and f lysosome-targeting acidic nanoparticles to directly acidify the lysosome lumen. The figure was created with BioRender.com