The mutant mouse on the right shows the limb-clasping reflex, whereas the wild-type mouse on the left does not.
The threat of a sanitation strike reminds us of the importance of waste disposal. Autophagy and the ubiquitin–proteasome system comprise the cell's garbage disposal systems. Komatsu et al. and Hara et al. find neurodegeneration in mice with neuronal deficiency of genes necessary for autophagy in recent back–to–back articles in Nature.
The proteasome breaks down ubiquitinated proteins, but it may not recognize misfolded proteins and protein aggregates. In contrast to the specificity of the proteasome, the autophagosome engulfs entire cellular pockets, including the cytoplasm and organelles. Then it fuses with a lysosome, and its cellular contents are degraded and recycled. The autophagy genes that were disrupted in these studies produce proteins that maintain the autophagosome membrane.
Komatsu et al. generated conditional autophagy-related 7 (Atg7) knockouts by breeding mice with Atg7 flanked by cre recombination sites to mice with cre recombinase expression driven by the neuron-specific nestin promoter. Hara et al. generated conditional knockouts of Atg5 using a similar technique. The Atg7 and Atg5 conditional knockouts were viable at birth, but viability declined within 3–4 weeks.
Motor impairments developed in both mouse models within 4 weeks of birth. When lifted by their tails, normal mice extend their limbs, but the knockout mice flexed their limbs toward their bodies, similar to the clasping reflex in some mouse models of neurodegeneration. The rotarod tests coordination. Although wild-type mice walked across the rotating rod, the knockout mice barely held on. The Atg5 conditional knockouts also showed ataxic walking patterns and diminished stride length relative to wild-type mice.
Cellular staining confirmed neurodegeneration in the knockout mice. Knockout mice at least 56 days old had reduced numbers of Purkinje cells and cortical and hippocampal pyramidal neurons relative to wild-type mice. They had increased cell death in the granular layer of the cerebellum, as indicated by TUNEL staining. The Atg7 conditional knockouts showed a 5-fold increase in TUNEL-positive cells in the cortex and a 500-fold increase in the granular layer of the cerebellum.
The authors found disease-associated, ubiquitin-positive inclusion bodies in the knockout cortex, hippocampus, cerebellum and hypothalamus. The amount of polyubiquitinated proteins and their insolubility increased over time. Hara et al. localized the inclusion bodies to neurons identified by NeuN staining. Despite the autophagy impairment, Komatsu et al. found no changes in proteasome function in the Atg7 conditional knockout.
Together, these data indicate that autophagy is necessary for the disposal of neurotoxic protein aggregates. In later stages of neurodegenerative disorders, including Huntington disease, Alzheimer disease and Parkinson disease, protein aggregates may overwhelm endogenous autophagy, leading to neuronal cell death. Perhaps drugs that enhance autophagy can prolong the early stages of these diseases, when symptoms are fewer and protein aggregates are just beginning to form.