Vascular Smooth Muscle Cell Subpopulations and Neointimal Formation in Mouse Models of Elastin Insufficiency

Objective: Using a mouse model of Eln (elastin) insufficiency that spontaneously develops neointima in the ascending aorta, we sought to understand the origin and phenotypic heterogeneity of smooth muscle cells (SMCs) contributing to intimal hyperplasia. We were also interested in exploring how vascular cells adapt to the absence of Eln. Approach and Results: We used single-cell sequencing together with lineage-specific cell labeling to identify neointimal cell populations in a noninjury, genetic model of neointimal formation. Inactivating Eln production in vascular SMCs results in rapid intimal hyperplasia around breaks in the ascending aorta’s internal elastic lamina. Using lineage-specific Cre drivers to both lineage mark and inactivate Eln expression in the secondary heart field and neural crest aortic SMCs, we found that cells with a secondary heart field lineage are significant contributors to neointima formation. We also identified a small population of secondary heart field-derived SMCs underneath and adjacent to the internal elastic lamina. Within the neointima of SMC-Eln knockout mice, 2 unique SMC populations were identified that are transcriptionally different from other SMCs. While these cells had a distinct gene signature, they expressed several genes identified in other studies of neointimal lesions, suggesting that some mechanisms underlying neointima formation in Eln insufficiency are shared with adult vessel injury models. Conclusions: These results highlight the unique developmental origin and transcriptional signature of cells contributing to neointima in the ascending aorta. Our findings also show that the absence of Eln, or changes in elastic fiber integrity, influences the SMC biological niche in ways that lead to altered cell phenotypes.

Strong as a Hippo’s Heart: Biomechanical Hippo Signaling During Zebrafish Cardiac Development

The heart is comprised of multiple tissues that contribute to its physiological functions. During development, the growth of myocardium and endocardium is coupled and morphogenetic processes within these separate tissue layers are integrated. Here, we discuss the roles of mechanosensitive Hippo signaling in growth and morphogenesis of the zebrafish heart. Hippo signaling is involved in defining numbers of cardiac progenitor cells derived from the secondary heart field, in restricting the growth of the epicardium, and in guiding trabeculation and outflow tract formation. Recent work also shows that myocardial chamber dimensions serve as a blueprint for Hippo signaling-dependent growth of the endocardium. Evidently, Hippo pathway components act at the crossroads of various signaling pathways involved in embryonic zebrafish heart development. Elucidating how biomechanical Hippo signaling guides heart morphogenesis has direct implications for our understanding of cardiac physiology and pathophysiology.

STAG2 cohesin is essential for heart morphogenesis

The distinct functions of cohesin complexes carrying STAG1 or STAG2 need to be unraveled. STAG2 is commonly mutated in cancer and germline mutations have been identified in cohesinopathy patients. To better understand the underlying pathogenic mechanisms, we here report the consequence of Stag2 ablation in mice. STAG2 is largely dispensable in adults and its tissue-wide inactivation does not lead to tumors but reduces fitness and affects both hematopoiesis and intestinal homeostasis. STAG2 is also dispensable for murine embryonic fibroblasts in vitro. In contrast, null embryos die by mid gestation showing global developmental delay and heart defects. Histopathological analysis and RNA-sequencing unveiled that STAG2 is required both for proliferation and regulation of cardiac transcriptional programs and in its absence, secondary heart field progenitors fail to enter the heart tube. These results provide compelling evidence on cell- and tissue-specific roles of the two cohesin complexes and how their dysfunction contributes to disease.

Abstract 312: Molecular Characterization and Prospective Isolation of Human Secondary Heart Field Progenitors From Pluripotent Stem Cells

The secondary heart field (SHF) progenitors ultimately contributes to diverse cardiovascular cell types through the formation of an early, multipotent heart progenitor pool and are marked by expression of ISL1, a LIM-homeodomain transcription factor. Human SHF can be derived from human pluripotent stem cells but their characterization has been limited due to the inefficiency of the differentiation protocols and lack of a proper reporter or surface marker based purification system. Using genetic tools and antibiotic selection we were able to purify ISL1+ cells for global gene expression analysis to identify key pathways that SHF identity. Genetic and small molecule based manipulation of these pathways alter ISL1 induction in differentiating cultures. Further proteomic analysis of enriched ISL1+ cells identified a hit surface marker that enables prospective isolation of ISL1+ secondary heart field progenitor cells with more than 90% purity. Purified SHF cells were multipotent and differentiate into pacemaker cells, endothelial and smooth muscle cells as well as mature beating cardiomyocytes. Finally transplantation of hPSC-derived purified SHF progenitors using this surface marker, restored myocardial function and regenerated infarcted area in mice myocardial infarction model.