Compartment-Specific Regulation of Autophagy in Primary Neurons

Abstract

Autophagy is an essential degradative pathway that maintains neuronal homeostasis and prevents axon degeneration. Initial observations suggest that autophagy is spatially regulated in neurons, but how autophagy is regulated in distinct neuronal compartments is unclear. Using live-cell imaging in mouse hippocampal neurons, we establish the compartment-specific mechanisms of constitutive autophagy under basal conditions, as well as in response to stress induced by nutrient deprivation. We find that at steady state, the cell soma contains populations of autophagosomes derived from distinct neuronal compartments and defined by differences in maturation state and dynamics. Axonal autophagosomes enter the soma and remain confined within the somatodendritic domain. This compartmentalization likely facilitates cargo degradation by enabling fusion with proteolytically active lysosomes enriched in the soma. In contrast, autophagosomes generated within the soma are less mobile and tend to cluster. Surprisingly, starvation did not induce autophagy in either the axonal or somatodendritic compartment. While starvation robustly decreased mTORC1 signaling in neurons, this decrease was not sufficient to activate autophagy. Furthermore, pharmacological inhibition of mammalian target of rapamycin with Torin1 also was not sufficient to markedly upregulate neuronal autophagy. These observations suggest that the primary physiological function of autophagy in neurons may not be to mobilize amino acids and other biosynthetic building blocks in response to starvation, in contrast to findings in other cell types. Rather, constitutive autophagy in neurons may function to maintain cellular homeostasis by balancing synthesis and degradation, especially within distal axonal processes far removed from the soma.

Autophagy-mediated Regulation of BACE1 Protein Trafficking and Degradation

“…3, A and B). Our data are consistent with a recent study that axon-generated autophagosomes enter the soma and then remain in the somatodendritic domain, where mature lysosomes are mainly located (33).…”

“…Microtubule-based long-distance axonal transport is essential for autophagic clearance because autophagosomes are predominantly generated in distal axons and mainly rely on retrograde transport to somatic lysosomes for degradation (23,29,(31)(32)(33). Defective autophagy has been implicated in AD pathogenesis (27,42,43).…”

“…Autophagosomes are predominantly generated in distal axons and then undergo predominant retrograde transport toward the soma (23,29,30,32,33). By recruiting LE-loaded dynein-Snapin (motor adaptor) transport complexes, nascent autophagosomes gain retrograde motility through fusion with LEs into amphisomes (32), which delivers autophagic cargoes to somatic lysosomes for clearance.…”

Intrinsic and Innate Defenses of Neurons: Détente with the Herpesviruses

“…Autophagy is a highly evolutionarily conserved pathway that, among other functions, serves to eliminate potentially harmful protein aggregates and damaged organelles (40,41). An emerging theme, however, is that neurons are atypical in their autophagy responses and exhibit unexpected phenotypes when autophagy is modulated (42). The postmitotic nature of neurons renders them highly dependent on autophagy to maintain their integrity.…”

“…Indeed, ablation of autophagy in neurons results in their death, and defects in the autophagy pathway are implicated in a variety of neurodegenerative disorders (43). Moreover, neuronal autophagy is highly compartmentalized and is insensitive to modulation by standard methods (e.g., starvation and rapamycin), and the autophagosomes form unusual clusters under conditions of virus infection and IFN treatment (42,44).…”