Perk Is Dispensable for Smooth Muscle Cell Phenotype Switching in Atherosclerosis

BACKGROUND: Smooth muscle cell (SMC) derived cells form the bulk of cells in atherosclerotic lesions and modulate lesion stability and cardiovascular disease outcomes. Unfolded protein response (UPR) markers, thin fibrous caps, and inflammation correlate with human lesion instability and rupture. In mice, UPR drives macrophage and endothelial apoptosis and inflammation, but its impact on lesion stability through SMC modulation is debated. The UPR protein Perk (protein kinase RNA-like ER kinase) was recently shown to drive SMC modulation in vivo, suggesting that depletion of SMC Perk may regulate lesion stability. METHODS: hypercholesterolemic mice. Lesions were scored for features of lesion stability and analyzed for differential expression at the single-cell level. Perk knockdown in primary murine SMCs was used to study Perk's effect on SMC modulation in vitro. UPR marker expression in human carotid lesions was assessed for UPR markers through scRNA-seq, bulk RNA-seq, and Xenium spatial transcriptomics. RESULTS: SMC Perk deletion in adult atherogenic mice did not affect weight gain or serum cholesterol levels. Lesions from Perk knockout mice resembled Perk WT counterparts with similar progression, lesion stability features, and cell populations. Scoring of UPR activity and differential expression analysis found little UPR activity in SMC and smooth muscle-derived cell populations. Perk was not required for in vitro SMC modulation in atherogenic conditions. No correlation was found between UPR markers and lesion stability or symptomatic clinical presentation in human carotid lesions, and UPR markers were expressed primarily in infiltrating leukocytes rather than in SMCs and stromal cells. CONCLUSIONS: SMC Perk UPR does not play a significant role in atherosclerotic SMC modulation, disease progression, or features of lesion stability in mice. Similarly, expression of markers of the Perk UPR pathway in humans does not correlate with human carotid lesion stability or clinical presentation.