ER, Mitochondria, and ISR Regulation by mt‐HSP70 and ATF5 upon Procollagen Misfolding in Osteoblasts

Abstract

Cellular response to protein misfolding underlies multiple diseases. Collagens are the most abundant vertebrate proteins, yet little is known about cellular response to misfolding of their procollagen precursors. Osteoblasts (OBs)—the cells that make bone—produce so much procollagen that it accounts for up to 40% of mRNAs in the cell, which is why bone bears the brunt of mutations causing procollagen misfolding in osteogenesis imperfecta (OI). The present study of a G610C mouse model of OI by multiple transcriptomic techniques provides first solid clues to how OBs respond to misfolded procollagen accumulation in the endoplasmic reticulum (ER) and how this response affects OB function. Surprisingly, misfolded procollagen escapes the quality control in the ER lumen and indirectly triggers the integrated stress response (ISR) through other cell compartments. In G610C OBs, the ISR is regulated by mitochondrial HSP70 (mt‐HSP70) and ATF5 instead of their BIP and ATF4 paralogues, which normally activate and regulate ISR to secretory protein misfolding in the ER. The involvement of mt‐HSP70 and ATF5 together with other transcriptomic findings suggest that mitochondria might initiate the ISR upon disruption of ER‐mitochondria connections or might respond to the ISR activated by a yet unknown sensor.

ER procollagen storage defect without coupled unfolded protein response drives precocious arthritis

“…Moreover, the downregulation of genes involved in translation could be related to an attempt to alleviate the protein folding challenge in the ER, though the lack of apparent UPR/ISR activation makes it unclear how this response is initiated. Leiken and co-workers have described a non-canonical response, 61,62 but as noted above that response is not observed in our transcription data.…”

“…We looked for the non-canonical stress response to procollagen-I misfolding first described by Mirigian et al . (Gorrell et al, 2022; Mirigian et al, 2016), but we did not detect any significant upregulation of genes associated with this response ( Fig. 7 ).…”

“…60 We next sought to determine if chondrocytes were instead raising any other organized transcriptional stress response when expressing Gly1170Ser procollagen. We looked for the non-canonical stress response to procollagen-I misfolding first described by Mirigian et al, 61,62 but we did not detect any significant upregulation of genes associated with this response (Fig. 7).…”

“…7). We also scrutinized activation of the integrated stress response (ISR) 61,63 but observed no robust activation of this pathway for the clinically relevant heterozygote.…”

“…In the heterozygous line, Xchromosome deactivation was eroded, as is a common occurrence in iPSC lines, [90][91][92] so hits caused by differential expression of X-linked genes were disregarded. All the gene sets considered were either previously published, 35,53,[61][62][63] or curated from previously described gene ontologies.…”