Lysosomal Proteolysis Inhibition Selectively Disrupts Axonal Transport of Degradative Organelles and Causes an Alzheimer's-Like Axonal Dystrophy

Abstract

In the hallmark neuritic dystrophy of Alzheimer’s disease (AD), autophagic vacuoles containing incompletely digested proteins selectively accumulate in focal axonal swellings, reflecting defects in both axonal transport and autophagy. Here, we investigated the possibility that impaired lysosomal proteolysis could be a basis for both defects leading to neuritic dystrophy. In living primary mouse cortical neurons expressing fluorescence-tagged markers, LC3-positive autophagosomes forming in axons rapidly acquired the endo-lysosomal markers, Rab7 and LAMP1, and underwent exclusive retrograde movement. Proteolytic clearance of these transported autophagic vacuoles was initiated upon fusion with bi-directionally moving lysosomes that increase in number at more proximal axon levels and in the perikaryon. Disrupting lysosomal proteolysis by either inhibiting cathepsins directly or by suppressing lysosomal acidification slowed the axonal transport of autolysosomes, late endosomes and lysosomes and caused their selective accumulation within dystrophic axonal swellings. Mitochondria and other organelles lacking cathepsins moved normally under these conditions, indicating that the general functioning of the axonal transport system was preserved. Dystrophic swellings induced by lysosomal proteolysis inhibition resembled in composition those in several mouse models of AD and also acquired other AD-like features, including immunopositivity for ubiquitin, APP, and neurofilament protein hyperphosphorylation. Restoration of lysosomal proteolysis reversed the affected movements of proteolytic Rab7 vesicles, which in turn, largely cleared autophagic substrates and reversed the axonal dystrophy. These studies identify the AD-associated defects in neuronal lysosomal proteolysis as a possible basis for the selective transport abnormalities and highly characteristic pattern of neuritic dystrophy associated with AD.

Massive accumulation of luminal protease-deficient axonal lysosomes at Alzheimer’s disease amyloid plaques

“…9C). This model is supported by multiple studies that point to an active process of lysosome biogenesis within axons that begins with the merging of organelles derived from the endocytic and autophagic pathways followed by further maturation toward lysosomes that is coupled to their retrograde transport (44)(45)(46)(47)(48)(49). More specifically, there is a high level of constitutive autophagosome biogenesis that occurs within distal regions of axons (46,48,50,51).…”

Autophagosome dynamics in neurodegeneration at a glance

“…Live-cell imaging studies in cultured neurons reveal robust retrograde transport of autophagosomes from the axon tip towards the cell body (Lee et al, 2011;Maday et al, 2012). This transport is driven by the retrograde motor dynein (Katsumata et al, 2010;Lee et al, 2011;Maday et al, 2012) and its activator dynactin (Ikenaka et al, 2013) (see poster).…”

“…This transport is driven by the retrograde motor dynein (Katsumata et al, 2010;Lee et al, 2011;Maday et al, 2012) and its activator dynactin (Ikenaka et al, 2013) (see poster). Not surprisingly, dynein mutations in both fly and mouse models lead to impaired autophagic clearance of polyQ-htt (Ravikumar et al, 2005).…”

“…However, the unique dynamics of autophagosomes in neurons have only recently been studied using live-cell imaging, revealing the preferential formation of autophagosomes at the axon tip and their subsequent retrograde axonal transport towards the cell body (Lee et al, 2011;Maday et al, 2012). These studies provide a new model for studying neuronal autophagy and its potential defects during neurodegeneration.…”

Autophagy interaction with herpes simplex virus type-1 infection

“…115 Another hallmark of Alzheimer disease is the accumulation of autophagic vesicles filled with incompletely digested proteins in axons, suggesting abnormalities in axonal transport or a consequence of jammed vesicles and/or not enough space to transport. 116 Notably, neuronal communication is dependent upon the robustness of synaptic size and response, which is positively correlated with autophagy. In this specific context, autophagy's role in synaptic viability may also serve the underlying cause in promoting the development of neurodegenerative disorders.…”