Autophagy is a regulated lysosomal degradation process that involves autophagosome formation and transport. Although recent evidence indicates that basal levels of autophagy protect against neurodegeneration, the exact mechanism whereby this occurs is not known. By using conditional knockout mutant mice, we report that neuronal autophagy is particularly important for the maintenance of local homeostasis of axon terminals and protection against axonal degeneration. We show that specific ablation of an essential autophagy gene, Atg7 , in Purkinje cells initially causes cell-autonomous, progressive dystrophy (manifested by axonal swellings) and degeneration of the axon terminals. Consistent with suppression of autophagy, no autophagosomes are observed in these dystrophic swellings, which is in contrast to accumulation of autophagosomes in the axonal dystrophic swellings under pathological conditions. Axonal dystrophy of mutant Purkinje cells proceeds with little sign of dendritic or spine atrophy, indicating that axon terminals are much more vulnerable to autophagy impairment than dendrites. This early pathological event in the axons is followed by cell-autonomous Purkinje cell death and mouse behavioral deficits. Furthermore, ultrastructural analyses of mutant Purkinje cells reveal an accumulation of aberrant membrane structures in the axonal dystrophic swellings. Finally, we observe double-membrane vacuole-like structures in wild-type Purkinje cell axons, whereas these structures are abolished in mutant Purkinje cell axons. Thus, we conclude that the autophagy protein Atg7 is required for membrane trafficking and turnover in the axons. Our study implicates impairment of axonal autophagy as a possible mechanism for axonopathy associated with neurodegeneration.

Autophagy is a highly regulated cellular mechanism for the bulk degradation of cytoplasmic contents. It has been implicated in a variety of physiological and pathological conditions relevant to neurological diseases. However, the regulation of autophagy in neurons and its role in neuronal and axonal pathology are not yet understood. Using transgenic mice producing green fluorescent protein-tagged autophagic marker microtubule-associated protein light chain 3 (GFP–LC3), we provide molecular evidence for the induction of autophagy in axonal dystrophy and degeneration in Purkinje cells of theLurchermice, a model for excitotoxic neurodegeneration. We show that the excitotoxic insult ofLurchermutation triggers an early response of Purkinje cells involving accumulation of GFP–LC3-labeled autophagosomes in axonal dystrophic swellings (a hallmark of CNS axonopathy). In brain, LC3 interacts with high affinity with the microtubule-associated protein 1B (MAP1B). We show that MAP1B binds to LC3 of both cytosolic form (LC3I) and lipidated form (LC3II). Moreover, in cell culture, overexpression of MAP1B results in reduced LC3II levels and number of GFP–LC3-labeled autophagosomes; phosphorylated MAP1B is associated with GFP–LC3-labeled autophagosomes. Furthermore, in brain, phosphorylated MAP1B accumulates in axonal dystrophic swellings of degenerating Purkinje cells and binds to LC3 at increased level. Therefore, the MAP1B–LC3 interaction may participate in regulation of LC3-associated autophagosomes in neurons, in particular at axons, under normal and pathogenic conditions. We propose that induction of autophagy serves as an early stress response in axonal dystrophy and may participate in the remodeling of axon structures.