The translational landscape of human vascular smooth muscle cells identifies novel short open reading frame-encoded peptide regulators for phenotype alteration

The plasticity of vascular smooth muscle cells (VSMCs) enables them to alter phenotypes under various physiological and pathological stimuli. The alteration of VSMC phenotype is a key step in vascular diseases, including atherosclerosis. Although the transcriptome shift during VSMC phenotype alteration has been intensively investigated, uncovering multiple key regulatory signalling pathways, the translatome dynamics in this cellular process, remain largely unknown. Here, we explored the genome-wide regulation at the translational level of human VSMCs during phenotype alteration.

Our study indicates that an increase in the capacity of translation, which is attributable to an increased quantity of translational machinery components, mainly controls alterations of VSMC phenotype at the level of translational regulation. In addition, SEPs could function as important regulators in the phenotype alteration of human VSMCs.

We generated nucleotide-resolution translatome and transcriptome data from human VSMCs undergoing phenotype alteration. Deep sequencing of ribosome-protected fragments (Ribo-seq) revealed alterations in protein synthesis independent of changes in messenger ribonucleicacid levels. Increased translational efficiency of many translational machinery components, including ribosomal proteins, eukaryotic translation elongation factors and initiation factors were observed during the phenotype alteration of VSMCs. In addition, hundreds of candidates for short open reading frame-encoded polypeptides (SEPs), a class of peptides containing 200 amino acids or less, were identified in a combined analysis of translatome and transcriptome data with a high positive rate in validating their coding capability. Three evolutionarily conserved SEPs were further detected endogenously by customized antibodies and suggested to participate in the pathogenesis of atherosclerosis by analysing the transcriptome and single cell RNA-seq data from patient atherosclerotic artery samples. Gain- and loss-of-function studies in human VSMCs and genetically engineered mice showed that these SEPs modulate the alteration of VSMC phenotype through different signalling pathways, including the mitogen-activated protein kinase pathway and p53 pathway.