Analysis of Hsp70 and Hsp90 function in Huntington´s disease: A potential therapeutic target
Zusammenfassung der Projektergebnisse
Inclusion bodies of misfolded and aggregated mutant huntingtin (mhtt) are a neuropathological hallmark of Huntington disease (HD). Molecular chaperones colocalize to inclusion bodies and are neuroprotective in HD animal models if overexpressed. First, I asked in this study if molecular chaperones have direct effects on mhtt misfolding and aggregation. Using size exclusion chromatography and atomic force microscopy, I found that mhtt fragments misfold and assemble into soluble oligomeric species with a broad size distribution and that the molecular chaperone Hsp70 associated with these mhtt oligomers in an Hsp40- and ATP-dependent manner. This chaperone-mhtt interaction inhibited mhtt aggregate formation coincident with suppression of caspase 3 activity in PC12 cells. These results suggest that molecular chaperones recognize mhtt misfolding and improve cell function and viability. However, it is unclear why the overall protective mechanism of endogenous chaperones is insufficient in preventing IB formation of mHtt and neurodegeneration in HD. The chaperone network is clearly overwhelmed by chronic expression of mhtt in HD. Therefore, in the second part of this study I analyzed the impact of chronic oligomerization/aggregation of misfolded mhtt on necessary cell-physiological “housekeeping” folding functions of Hsp70 and Hsp90. Strikingly, mhtt oligomers inhibit chaperone assisted post- and co-translational folding, steroid hormone receptor- and heat shock response (HSR)- activation. Although molecular chaperones recognize highly reactive misfolded proteins and provide a first cellular defense mechanism for cells, these results indicate that as soon as the stochiometric ratio of mHtt oligomers exceed the cell specific chaperone capacity, their housekeeping functions become impaired. This fellowship study provided new insights of how molecular chaperones deal with the burden of misfolded mHtt proteins. The collaboration of different chaperone systems are needed to induced the first cellular defense mechanism to avoid unspecific interaction of unwanted conformers of mHtt. However, this study also show that once the protein misfolding load exceed a cell specific threshold, the chaperone capacity will be insufficient, and will inhibit chaperone housekeeping function that contribute to pathogenesis. This different view of the interplay of mHtt oligomers and molecular chaperones open new entry points for the development of therapies against HD.
Projektbezogene Publikationen (Auswahl)
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Hsp70 and Hsp40 functionally interact with soluble mutant huntingtin oligomers in a classic ATP-dependent reaction cycle. J Biol. Chem. 2010 Dec, 3; 285 (49):38183-9 3
Lotz, G.P., Legleiter, J.,Aron, R., Mitchell, E.J., Huang, S.Y., Ng, C.P., Glabe, C., Thompson, L. M., Muchowski, P.J.
