Compartment-Dependent Degradation of Mutant Huntingtin Accounts for Its Preferential Accumulation in Neuronal Processes.

UNLABELLED: In neurodegenerative diseases caused by misfolded proteins, including Huntington's disease (HD), the neuronal processes and terminals are particularly prone to the accumulation of misfolded proteins, leading to axonal and synaptic dysfunction. This compartment-dependent accumulation can result from either the altered transport of misfolded proteins or impaired protein degradation. Mutant huntingtin (mHtt), the HD protein, is known to affect intracellular transport and can be degraded by the proteasome and autophagy, but how mHtt accumulates in the neuronal processes, an early pathological event in the brains of HD patients, still remains unclear. Using an "optical pulse-chase" assay that can quantify protein degradation in specific subcellular regions, we found that neuronal mHtt is removed faster in the cell body than in neurites. Furthermore, mHtt is cleared more rapidly in astrocytes than in neurons. The ubiquitin-proteasome system plays a much bigger role than autophagy in degrading soluble mHtt via K48 ubiquitination in both the cytoplasm and processes of neurons and astrocytes. By injecting adenoviral vectors expressing mHtt into the mouse brain, we confirmed that mHtt is removed more slowly in neurites than in the cytoplasm of the cell body of neurons. Our findings provide evidence for the cell type- and compartment-dependent degradation of mHtt and explain why mHtt preferentially accumulates and aggregates in the neuropils of vulnerable neurons. In addition, our findings suggest that enhancing proteasomal activity could be an effective way to reduce the preferential accumulation of soluble mHtt in neuronal processes. SIGNIFICANCE STATEMENT: The clearance of misfolded proteins is key to preventing neurodegeneration in Huntington's disease, but how mutant huntingtin (mHtt) accumulates differentially in different cell types and subcellular regions remains unclear. We found mHtt is cleared slowly in neuronal processes compared with the cytoplasm and is cleared more efficiently in astrocytes than in neurons. Moreover, this compartment-dependent degradation of soluble mHtt is mediated primarily by the ubiquitin-proteasome system rather than autophagy. Our findings imply that enhancing proteasome activity could be an efficient way to clear soluble misfolded proteins in the neuronal processes.