A Differentiation Transcription Factor Establishes Muscle-Specific Proteostasis in Caenorhabditis elegans

Abstract

Safeguarding the proteome is central to the health of the cell. In multi-cellular organisms, the composition of the proteome, and by extension, protein-folding requirements, varies between cells. In agreement, chaperone network composition differs between tissues. Here, we ask how chaperone expression is regulated in a cell type-specific manner and whether cellular differentiation affects chaperone expression. Our bioinformatics analyses show that the myogenic transcription factor HLH-1 (MyoD) can bind to the promoters of chaperone genes expressed or required for the folding of muscle proteins. To test this experimentally, we employed HLH-1 myogenic potential to genetically modulate cellular differentiation of Caenorhabditis elegans embryonic cells by ectopically expressing HLH-1 in all cells of the embryo and monitoring chaperone expression. We found that HLH-1-dependent myogenic conversion specifically induced the expression of putative HLH-1-regulated chaperones in differentiating muscle cells. Moreover, disrupting the putative HLH-1-binding sites on ubiquitously expressed daf-21(Hsp90) and muscle-enriched hsp-12.2(sHsp) promoters abolished their myogenic-dependent expression. Disrupting HLH-1 function in muscle cells reduced the expression of putative HLH-1-regulated chaperones and compromised muscle proteostasis during and after embryogenesis. In turn, we found that modulating the expression of muscle chaperones disrupted the folding and assembly of muscle proteins and thus, myogenesis. Moreover, muscle-specific over-expression of the DNAJB6 homolog DNJ-24, a limb-girdle muscular dystrophy-associated chaperone, disrupted the muscle chaperone network and exposed synthetic motility defects. We propose that cellular differentiation could establish a proteostasis network dedicated to the folding and maintenance of the muscle proteome. Such cell-specific proteostasis networks can explain the selective vulnerability that many diseases of protein misfolding exhibit even when the misfolded protein is ubiquitously expressed.

The landscape of molecular chaperones across human tissues reveals a layered architecture of core and variable chaperones

“…Variable chaperones, in contrast, might be upregulated by tissue-specific transcription factors. In support, the main muscle differentiation transcription factor, MYOD1 (HLH-1 in C. elegans), was shown to drive the expression of chaperones in C2C12 mouse myoblast and C. elegans muscles (19,60,61).…”

“…We further tested the functional conservation of muscle chaperones by comparing between human and the evolutionary distant multicellular organism Caenorhabditis elegans. We used a set of C. elegans chaperones that were enriched in muscle (19), and compared them to orthologous chaperones that were upregulated in human muscle (see Methods). We found that the two subsets overlapped significantly (p=0.0009, Fisher exact test, Fig.…”

“…We annotated the functionality of core chaperones by manual curation of their Uniprot entries (79) and the literature. Data of curated, muscle chaperones in C. elegans were obtained from (19). Data of orthologous chaperones in C. elegans were obtained via the OrthoList2 tool for comparative genomic analysis between C. elegans and humans (80), and each human chaperone were fitted with the C. elegnas gene(s) that was identified as orthologue by the maximal number of programs.…”

“…Whereas chaperone overexpression typically improves the folding capacity of the chaperone system, overexpression of specific chaperones was shown to disrupt folding (19)(20)(21)(22). For example, overexpression of the folding enzyme FKBP51 in a tau transgenic mouse model resulted in accumulation of tau and its toxic oligomers (19)(20)(21)(22). Likewise, chaperone downregulation or genetic aberration can cause diseases, such as mutation in the NEF co-chaperone BAG3 that leads to myopathy (23).…”

Bacterial Hsp90 mediates the degradation of aggregation-prone Hsp70-Hsp40 substrates preferentially by HslUV proteolysis

“…The artificial over-expression of Hsp90 in C. elegans embryos was recently shown to cause the massive degradation of muscle proteins (Bar-Lavan, Shemesh et al 2016), indicating that in the cytosol of stressed eukaryotic cells, Hsp90 might also mediate the specific degradation of misfolding proteins that escaped the action of other members of the chaperone network. It will now be interesting to address which are the particular protein partners in bacteria that mediate the interaction of Hsp90 and Hsp70 clients with the cellular proteases and further address the possibility that also in eukaryotes, Hsp90s could mediate the degradation by proteasome of Hsp70-clients that failed proper folding under stress or owing to mutations.…”