PIK3C3, a master regulator for smooth muscle identity

Phenotypic switching of vascular smooth muscle cells (VSMCs) from a contractile to a proliferative phenotype, plays a causal role in many human occlusive vascular diseases. To better understand key biological events occurring in human vascular diseases, we analyzed proteomic data from human atherosclerotic plaques and genomic data associated with human coronary artery disease. This unbiased analysis revealed that many genes involved in vesicle trafficking/fusion are over-represented. Previous studies have shown that the lipid kinase PIK3C3 is an essential regulator of vesicle trafficking/fusion. However, its functional role in VSMCs remains completely unknown. To examine the role of PIK3C3 in VSMCs, we generated inducible SM-specific Pik3c3 knockout (iSM KO) mice driven by Myh11-CreERT2 transgene. Unexpectedly, Pik3c3 iSM KO mice exhibited lethality 4 weeks after deletion of Pik3c3, due to a pseudo-obstructive intestine resulting from deletion of Pik3c3 in visceral SMCs in addition to VSMCs. The iSM Pik3c3 KO mice also exhibit dramatic remodeling of the vascular wall including thickening, aneurysmal dilation and spontaneous neointima. Proteomic analysis and bulk RNA- seq of Pik3c3-deficient aorta revealed loss of contractile proteins while increased expression of inflammation genes and targets of the Hippo-YAP1 pathway which has been shown to be critical for VSMC development and phenotypic modulation. Single cell RNA-seq revealed that Pik3c3-deficient aortic VSMCs almost completely lose their identity of contractile VSMCs while acquiring markers of inflammatory cells and mesenchymal stem cells. These exciting data suggest a previously undocumented role for PIK3C3 in maintaining SMC identity. Mechanistically, Pik3c3 inactivation induced YAP1 protein expression and silencing Yap1 largely restored a contractile phenotype in Pik3c3-deficient VSMCs. We hypothesize that PIK3C3 is a “master” regulator of the contractile phenotype of VSMC via regulating autophagosome-mediated degradation of YAP1. Three specific aims are proposed to test this hypothesis. To circumvent the early lethal visceral phenotype seen with Myh11- CreERT2 transgene, in Aim 1 we will employ a novel vascular-specific inducible Itga8-CreERT2 mouse to generate VSMC-specific Pik3c3 KO mice. Atherosclerosis will be induced using PCSK9 AAV and the effects of VSM- specific deletion of Pik3c3 on lesion formation will be evaluated. Wire injury-induced neointimal formation will be assessed as well by using this novel KO mouse model. Aim 2 will test that YAP1 is a critical mediator conferring the effects of Pik3c3 deficiency on VSMCs. YAP1 will be pharmacologically and genetically inactivated, and its effect on vascular remodeling and gene expression will be determined. Aim 3 will test that YAP1 protein accumulation induced by Pik3c3 deficiency is due to the impaired autophagic flux that attenuates autolysosome- mediated YAP1 degradation. Proposed studies will determine the role of PIK3C3 in autophagic flux in vivo and the role of ubiquitin and p62/SQSTM1 in PIK3C3-mediated degradation of YAP1 in human VSMCs in vitro. Completion of these studies will provide novel insights into the mechanism of controlling VSMC phenotype.