Sm22α transcription occurs at the early onset of the cardiovascular system and the intron 1 is dispensable for its transcription in smooth muscle cells during mouse development.
SM22α, also known as SM22, has been widely used as a smooth muscle cell (SMC) marker and is known to be expressed in the embryonic heart. The intron 1 of Sm22 contains multiple important and evolutionarily conserved regulatory elements. To determine the role of the intron 1 in Sm22 transcriptional regulation and the function of SM22 during development, we generated a Sm22 knockout mouse by replacing the intron 1 and the translation initiation with a nuclear localized LacZ (nLacZ) reporter. The resulting Sm22 knockout mice (Sm22(-)/(-)) were viable and fertile without any apparent developmental defects. Using X-gal staining assay, we found that Sm22 transcription was detectable in the chorion formation region and in the heart field before formation of the heart tube at E7.5, namely much earlier than the looped heart stage where it had been previously reported. The expression of lacZ progressively expanded throughout the heart tube by E8.5. LacZ was transiently expressed in the heart and somites and then became restricted to the vascular and visceral SMC organs. These results indicate that SM22 is not required for mouse basal homeostatic function and that the intron 1 is dispensable for Sm22 transcription during development. Given the importance of vasculature in organogenesis and in diseases, this mouse line may be a valuable tool to trace the development and pathology of the cardiovascular system.