No abstract

Autophagosome biogenesis and maturation in primary neurons is a constitutive process that is spatially and temporally regulated along the axon.

Axonal transport is essential for neuronal function, and many neurodevelopmental and neurodegenerative diseases result from mutations in the axonal transport machinery. Anterograde transport supplies distal axons with newly synthesized proteins and lipids, including synaptic components required to maintain presynaptic activity. Retrograde transport is required to maintain homeostasis by removing aging proteins and organelles from the distal axon for degradation and recycling of components. Retrograde axonal transport also plays a major role in neurotrophic and injury response signaling. This review provides an overview of the axonal transport pathway and discusses its role in neuronal function.

Summary Autophagy is an essential degradative pathway in neurons, yetlittle is known about the mechanisms driving this process in highly polarized cells. Here, we use dual-color live-cell imaging to investigate the neuron-specific mechanisms of constitutiveautophagosome biogenesis in primary DRG and hippocampal cultures. Under basal conditions autophagosomes are continuously generated in the axon tip. There is an ordered assembly of proteins recruited with stereotypical kinetics onto the developing autophagosome. Plasma- or mitochondrial-derived membraneswere not incorporated into nascent autophagosomes in the distal axon. Rather, autophagosomes are generated at DFCP1-positive subdomains of the endoplasmic reticulum, distinct from ER exit sites. Biogenesis events arehighly enriched distally; autophagosomes form infrequently in dendrites, the cell soma or mid-axon, consistent with a highly compartmentalized pathway for constitutive autophagy in primary neurons. This distal biogenesis may facilitate the degradation of damaged mitochondria and long-lived cytoplasmic proteins that reach the axon tip via slow axonal transport.

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.