Abstract 3142: Pik3c3 Safeguards Smooth Muscle Identity By Promoting Lysosome-mediated Yap1 Degradation

Background: Mature vascular smooth muscle cells (VSMCs) are quiescent and geared for maintaining vascular homeostasis by expressing a unique repertoire of contractile genes. However, key factors in controlling the contractile phenotype of VSMCs are poorly defined. Class III PI3K, PIK3C3 (Phosphatidylinositol 3-Kinase Catalytic Subunit Type 3) is a critical regulator of vesicular trafficking. Although PIK3C3 is known to play an essential role in cardiac homeostasis, its role in VSMCs remains elusive. Methods: To elucidate the role of Pik3c3 in regulating SMC fate and plasticity, we generated smooth muscle-specific Pik3c3 inducible knock out (KO) mice with SMC-lineage tracing. Proteomic mass spectrometry, next-generation RNA sequencing, SMC fate mapping coupled with single-cell RNA sequencing were applied to determine the phenotype and trajectories of SMCs after Pik3c3 ablation. Results: Specific ablation of Pik3c3 in SMCs results in severe gastrointestinal pseudo-obstruction and premature death. Analysis of aorta from the KO mice revealed dramatic vascular remodeling including dilatation, thickening of the vascular wall, spontaneous neointimal formation that was largely contributed by cells of SMC origin, and complete loss of contractility. Proteomics, bulk RNA-seq, and scRNA-seq studies of SMC-restricted Pik3c3 KO mouse aorta showed near complete loss of smooth muscle identity and the acquisition of an intermediate smooth muscle progenitor-like cell phenotype that underwent re-programming into multiple cell types including macrophages, T cells and osteoblasts. Mechanistically, we found that Pik3c3 deletion in VSMCs induces the accumulation of YAP1 (Yes associated protein 1) and its downstream target genes; this elevated YAP1 program appears to result from a blockade of autophagic lysosome-mediated degradation of YAP1, leading to phenotypic modulation of SMCs. Importantly, pharmacological inhibition of YAP by Verteporfin or genetic deletion of Yap1 rescues the vascular phenotype of Pik3c3 SM-specific KO mice. Conclusion: Our study reveals PIK3C3 as a master regulator safeguarding VSMC identity, and perturbation of the PIK3C3-YAP axis may be a novel mechanism underlying VSMC-driven vascular diseases in humans.