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The ubiquitin ligase UBR5 suppresses proteostasis collapse in immortal pluripotent stem cells and Huntington’s disease models

ABSTRACT: Pluripotent stem cells undergo unlimited self-renewal while maintaining their potential to differentiate into post-mitotic cells with an intact proteome, a capacity that demands a highly induced proteostasis network. As such, induced pluripotent stem cells (iPSCs) suppress the aggregation of polyQ-expanded huntingtin (HTT), the mutant protein underlying Huntington’s disease (HD). Here we show that proteasome activity determines HTT levels, preventing the accumulation of polyQ-expanded aggregates in iPSCs from HD patients (HD-iPSCs). iPSCs exhibit intrinsic high levels of UBR5, an E3 ubiquitin ligase that we find required for proteasomal degradation of both normal and mutant HTT. When UBR5 fails to monitor HTT proteostasis, the concomitant up-regulation of HTT expression particularly results in polyQ-expanded aggregation in HD-iPSCs. Moreover, UBR5 knockdown hastens protein aggregation and neurotoxicity in polyQ-expanded invertebrate models. Notably, ectopic expression of UBR5 is sufficient to induce polyubiquitination and degradation of mutant HTT, reducing polyQ-expanded aggregates in HD cell models. However, UBR5 is dispensable for the proteostasis of other aggregation-prone proteins linked with Machado-Joseph disease or amyotrophic lateral sclerosis in iPSCs. Besides its role in HTT regulation, we find that intrinsic high levels of UBR5 also determine the global proteostatic ability of iPSCs preventing aggresome formation, suggesting a role in the control of misfolded proteins ensued from normal metabolism. Thus, our findings indicate UBR5 as a central component of super-vigilant proteostasis of iPSCs with the potential to correct proteostatic deficiencies in HD.

INSTRUMENT(S): LTQ Orbitrap

ORGANISM(S): Homo Sapiens (human)

TISSUE(S): Cell Culture, Kidney

SUBMITTER: David Vilchez

LAB HEAD: David Vilchez

PROVIDER: PXD009803 | Pride | 2018-06-14

REPOSITORIES: Pride

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Project description:Protein aggregation is a hallmark of many neurodegenerative diseases. In order to cope with misfolding and aggregation, cells have evolved an elaborate network of molecular chaperones, composed of different families. But while chaperoning mechanisms for different families are well established, functional and regulatory diversification within chaperone families is still largely a mystery. Here we decided to explore chaperone functional diversity, through the lens of pathological aggregation. We revealed that different naturally-occurring isoforms of DNAJ chaperones showed differential effects on different types of aggregates. We performed a chaperone screen for modulators of two neurodegeneration-related aggregating proteins, the Huntington’s disease-related HTT-polyQ, and the ALS-related mutant FUS (mutFUS). The screen identified known modulators of HTT-polyQ aggregation, confirming the validity of our approach. Surprisingly, modulators of mutFUS aggregation were completely different than those of HTT-polyQ. Interestingly, different naturally-occurring isoforms of DNAJ chaperones had opposing effects on HTT-polyQ vs. mutFUS aggregation. We identified a complex of the full length (FL) DNAJB14 and DNAJB12 isoforms which substantially alleviated mutFUS aggregation, in an HSP70-dependent manner. Their naturally occurring short isoforms were unable to form the complex, nor to interact with HSP70, and lost their ability to reduce mutFUS aggregation. In contrast, the short isoform of DNAJB12 significantly alleviated HTT-polyQ aggregation, while DNAJB12-FL aggravated HTT-polyQ aggregation. Finally, we demonstrated that full-length DNAJB14 ameliorated mutFUS aggregation compared to DNAJB14-short in primary neurons. Together, our data unraveled distinct molecular properties required for aggregation protection in different neurodegenerative diseases, and revealed a new layer of complexity of the chaperone network elicited by naturally occurring J-protein isoforms, highlighting functional diversity among the DNAJ family.

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The ubiquitin ligase UBR5 suppresses proteostasis collapse in immortal pluripotent stem cells and Huntington’s disease models

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