Fibroblast growth factor plays a critical role in SM22 alpha expression during Xenopus embryogenesis

Although smooth muscle cells (SMCs) are critical components of the circulat ory system, the regulatory mechanisms of SMC differentiation remain largely unknown. In the present study, we examined the mechanism of SMC differenti ation by using Xenopus laevis SM22 alpha (XSM22 alpha) as an SMC-specific m arker. XSM22 alpha cDNA contained;a 600-bp open reading frame, and the pred icted amino acid sequences were highly conserved in evolution. XSM22 alpha transcripts were first detected in heart anlage, head mesenchyme, and the d orsal side of the lateral plate mesoderm at the tail-bud stage, possibly re presenting the precursors of muscle lineage. At the tadpole stage, XSM22 al pha transcripts were restricted to the vascular and visceral SMCs. XSM22 al pha was strongly induced by basic fibroblast growth factor (FGF) in animal caps. Although expressions of Xenopus cardiac actin were not affected by th e expression of a dominant-negative FGF receptor, its injection dramaticall y suppressed the XSM22 alpha expression. These results suggest that XSM22 a lpha is a useful molecular marker for the SMC lineage in Xenopus and that F GF signaling plays an important role in the induction of XSM22 alpha and in the differentiation of SMCs.