Reprogramming of mature SMCs to vascular progenitor cells: Focus on Vascular Fibrosis

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ABSTRACT Excessive perivascular fibrosis results from the highly dynamic and dysregulated process of normal tissue repair and is defined by the excessive accumulation of extracellular matrix (ECM) material resulting in decreased vascular compliance and increased vessel stiffness. In contrast, a fibrotic response is critical to the formation and stability of a fibrous cap in the setting of atherosclerosis. While advances have been made in our understanding of fibrosis pathobiology, significant gaps remain. In particular, while mesenchymal cells with the potential to transition into activated myofibroblasts are believed to be key sources of excessive ECM deposition, their origin remain debated. Using cell-specific lineage tracing and RNA sequencing, our group made the paradigm-shifting discovery that a subpopulation of resident vascular stem cells residing within the vascular adventitia originate from mature vascular smooth muscle cells (SMC)(termed AdvSca1-SM cells). SMC reprogramming and AdvSca1-SM cell maintenance is dependent on induction and activity of the transcription factor, Klf4. In contrast, vascular injury-induced or AdvSca1-SM cell-specific genetic Klf4 depletion promote the transition to profibrotic myofibroblasts, which consequently serve as major contributors to perivascular fibrosis. However, the molecular mechanism whereby Klf4 dictates the maintenance and differentiation of AdvSca1-SM cells remains unclear. Leveraging a highly specific AdvSca1-SM cell reporter system and single-cell RNA- sequencing (scRNA-seq), we demonstrate the profibrotic differentiation trajectory of AdvSca1-SM cells. Differentiation is characterized by loss of Klf4 and its downstream effector PI16, a negative regulator of the epigenetic protein, histone deacetylase 1 (HDAC1), and the lncRNA, Meg3, but gain of expression of a profibrotic phenotype. Importantly, these changes were recapitulated in human fibrotic tissue, underlying the translational significance. Statin treatment was previously shown to reduce tissue fibrosis; however, the molecular mechanism is unknown. Querying Connectivity Map database with gene signatures of AdvSca1-SM cells and their profibrotic derivatives indicate that statins are potential candidates to antagonize the profibrotic transition of AdvSca1-SM cells. Finally, while inflammation and fibrosis drive the wound healing process in most adult mammals, scarless regenerative wound healing in adult rodents of the species Acomys has been reported. Our recent findings show a complete absence of fibrosis with rapid regeneration in two models of kidney injury in Acomys. Here, we propose a novel concept that a Klf4-Meg3 axis plays a central role in the maintenance of the stem cell phenotype; loss of Klf4 and/or Meg3 activates a signaling cascade facilitating the phenotypic transition of AdvSca1-SM (Aim One). Further, we propose that Acomys is resistant to pathological vascular remodeling and fibrosis in part due to activation of an AdvSca1-SM-specific regenerative gene expression signature (Aim Two). Finally, statin treatment will inhibit or reverse the profibrotic transition of AdvSca1-SM cells (Aim Three).