Abstract 15295: Myocardin Condensation Drives Cell Lineage Specification During Cardiogenesis

Abstract

During cardiovascular development, cells make switch-like decisions to activate the expression of new gene programs leading to lineage specification. The mechanisms underlying these decisive choices remain unclear. Myocardin is a cardiomyocyte and smooth muscle cell specific transcriptional coactivator, which is necessary and important for activating cardiovascular genes. We show that Myocardin activates lineage-specific gene programs by concentration-dependent and switch-like formation of nuclear condensates. By modeling the natural changes in Myocardin concentration during development, coupled with quantitative fluorescence microscopy, single-cell resolution reporter assays, and cellular differentiation assays, we demonstrate that condensate formation is directly linked to transcriptional activation and cardiovascular lineage specification. During cardiomyocyte and smooth muscle cell differentiation, the formation of Myocardin condensates precedes activation of cell identity genes and these condensates are present at sites of cell identity gene transcription. Myocardin condensates form, activate gene expression, and specify cell state at critical concentration thresholds, dependent upon the C-terminal disordered region of Myocardin. Disrupting condensate formation by manipulating the sequence of this region abolishes gene activation, which can be rescued by replacing this region with condensate-forming disordered regions from other unrelated proteins. Our work demonstrates that Myocardin condensation at critical concentrations enables switch-like changes in gene expression programs, providing new insights into cardiomyocyte and smooth muscle cell lineage specification during cardiovascular development.