Summary The systemic amyloidoses are diverse disorders in which misfolded proteins are secreted by effector organs and deposited as proteotoxic aggregates at downstream tissues. Although well described clinically, the contribution of synthesizing organs to amyloid disease pathogenesis is unknown. Here, we utilize hereditary transthyretin amyloidosis (ATTR amyloidosis) induced pluripotent stem cells (iPSCs) to define the contribution of hepatocyte-like cells (HLCs) to the proteotoxicity of secreted transthyretin (TTR). To this end, we generated isogenic, patient-specific iPSCs expressing either amyloidogenic or wild-type TTR. We combined this tool with single-cell RNA sequencing to identify hepatic proteostasis factors correlating with destabilized TTR production in iPSC-derived HLCs. By generating an ATF6 inducible patient-specific iPSC line, we demonstrated that enhancing hepatic ER proteostasis preferentially reduces the secretion of amyloidogenic TTR. These data highlight the liver's capacity to chaperone misfolded TTR prior to deposition, and moreover suggest the potential for unfolded protein response modulating therapeutics in the treatment of diverse systemic amyloidoses.

In ATTR amyloidosis, transthyretin (TTR) protein is secreted from the liver and deposited as toxic aggregates at downstream target tissues. Despite recent advancements in treatments for ATTR amyloidosis, the mechanisms underlying misfolded TTR-mediated cellular damage remain elusive. In an effort to define early events of TTR-associated stress, we exposed neuronal (SH-SY5Y) and cardiac (AC16) cells to wild-type and destabilized TTR variants (TTRV122I and TTRL55P) and performed transcriptional (RNAseq) and epigenetic (ATACseq) profiling. We subsequently compared TTR-responsive signatures to cells exposed to destabilized antibody light chain protein associated with AL amyloidosis as well as ER stressors (thapsigargin, heat shock). In doing so, we observed overlapping, yet distinct cell type- and amyloidogenic protein-specific signatures, suggesting unique responses to each amyloidogenic variant. Moreover, we identified chromatin level changes in AC16 cells exposed to mutant TTR which resolved upon pre-incubation with kinetic stabilizer tafamidis. Collectively, these data provide insight into the mechanisms underlying amyloid-mediated cellular damage and provide a robust resource representing cellular responses to aggregation-prone proteins and ER stress.

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