In addition, Hsp/c70s have disordered, C-terminal regions that engage in additional protein–protein interactions (PPIs). Hsp/c70s are composed of a nucleotide-binding domain (NBD), which has ATPase activity, and a substrate-binding domain (SBD), which contains the canonical binding cleft for misfolded proteins. The general biochemical mechanisms of Hsp70 action are well understood. Although Hsp70 is protective against ASyn pathogenicity in cell and animal models ( 17, 18, 19, 20, 21, 22), little is known about its protective mechanisms. Hsp70 polymorphisms are associated with PD ( 15) and in PD patients, stress-induced Hsp70 accumulates in a thwarted attempt to clear aggregated ASyn ( 16). The constitutive (Hsc70) and inducible (Hsp70) forms of the 70-KDa cytosolic heat shock molecular chaperones (Hsp/c70s) assist a wide variety of folding processes and provide broad protection against protein misfolding in the cell. Although central to disease, ASyn misfolding has been challenging to target therapeutically.Ī potential approach might be to target cellular chaperones that mitigate protein misfolding ( 14). The severity of disease correlates with the progressive spread of aggregated ASyn in patients ( 12), and misfolding is associated with toxicity in cell and animal models ( 13). Significant evidence indicates that misfolded ASyn seeding and spread throughout the brain underlie disease progression ( 10, 11). ASyn gene multiplications and mutations causing PD and LBD are associated with enhanced oligomer and membrane-associated fibril formation ( 5, 6, 7, 8, 9). The sequential misfolding of ASyn into oligomers and fibrils is central to the pathogenesis of the synucleinopathies. Aggregated ASyn inclusions are a hallmark of these diseases ( 1), and ASyn gene mutations or multiplications cause early onset PD or LBD ( 2, 3, 4). Neuropathological, biochemical, and genetic evidence strongly implicates α-Synuclein (ASyn) in the onset and progression of Parkinson’s disease (PD) and related synucleinopathies including Lewy body dementias (LBD), multiple systems atrophy, and Alzheimer’s disease. Additionally, these results raise the question of whether other misfolded proteins might also engage Hsp70 via the same noncanonical mechanism. Such approaches are likely to be more specific than targeting Hsp70’s canonical action. Together, these findings suggest that new chemical approaches will be required to target the Hsp70-ASyn interaction in synucleinopathies. Finally, we report a biological role for a similar mode of action in H4 neuroglioma cells.
Using truncations, mutations, and inhibitors, we confirm that Hsp70 interacts with ASyn via an as yet unidentified, noncanonical interaction site in the C-terminal domain. We use novel ASyn oligomerization assays to show that Hsp70 directly blocks ASyn oligomerization, an early event in ASyn misfolding. Here we report that this activity is due to a novel and unexpected mode of Hsp70 action, involving neither ATP nor the typical substrate-binding cleft. It is generally assumed that Hsp70 binds to ASyn using its canonical and promiscuous substrate-binding cleft to limit aggregation. Elevated levels of the stress-induced chaperone Hsp70 protect against ASyn misfolding and ASyn-driven neurodegeneration in cell and animal models, yet there is minimal mechanistic understanding of this important protective pathway. Overexpression and aggregation of α-synuclein (ASyn) are linked to the onset and pathology of Parkinson’s disease and related synucleinopathies.
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