Spatial expression of myocardin protein in normal and disease states

Vascular smooth muscle cell (VSMC) dedifferentiation, a phenomenon found in virtually all vascular diseases, is characterized by a transcriptional switch from a contractile to a phenotypically modulated state. Myocardin (MYOCD) is a smooth muscle cell-restricted coactivator that is necessary and sufficient for the differentiation of VSMC through the transcriptional activation of SMC-restricted cytoskeletal and contractile genes. Despite 25 yr of research on MYOCD, the reliable expression of this protein continues to be poorly represented and understood. Accordingly, we generated a novel rat model carrying HA-tagged MYOCD to address pervasive disparities in the literature and elucidate MYOCD protein expression in vivo. Western blotting studies documented the highest MYOCD protein expression in aorta, bladder, and uterus, with low or undetectable expression in all other tissues of the rat, including heart. Predictive modeling supports the C-terminus of MYOCD to be most immunoreactive, where the HA tag and one other commercial immunogen reside. Of note, the latter immunogen was used to generate what appears to be the most trustworthy commercial antibody against MYOCD. In vitro transcription/translation and phosphatase treatment of ectopic and endogenous MYOCD protein reveal an intrinsically high molecular weight of MYOCD that has eluded prior reports. Importantly, we present the very first spatial expression profile of MYOCD protein in several mouse and rat tissues under baseline and vascular injury conditions. The results offer the SMC community new resources and insight into the reliable detection of MYOCD protein.NEW & NOTEWORTHY Endogenous MYOCD protein has eluded reliable detection due to pervasive commercial antibody failures, likely due to protein disorder. A CRISPR-generated 3xHA knock-in rat enabled the first in vivo spatial characterization of MYOCD, revealing its presence in SMC-rich tissues and, notably, within the microvasculature of SMC-poor organs. Phosphorylation largely accounts for the high molecular weight of MYOCD. Strikingly, MYOCD is detected in neointimal cells after vascular insult, suggesting persistence or reexpression during vascular remodeling.