Differentiated markers in undifferentiated cells: Expression of smooth muscle contractile proteins in multipotent bone marrow mesenchymal stem cells

In studying the differentiation of stem cells along smooth muscle lineage, smooth muscle cell ( SMC ) contractile proteins serve as markers for the relative state of maturation. Yet, recent evidence suggests that some SMC markers are probably expressed in multipotent mesenchymal stem cells ( MSC s). Such a paradox necessitates investigations to re‐examine their role as differentiated markers in MSC s. We tried to detect the expression of four widely used SMC markers including α‐smooth muscle actin (α‐ SMA ), h1‐calponin, desmin and smooth muscle myosin heavy chain ( SM ‐ MHC ), as well as the other isoforms of calponin family in resting MSC s. Then we used three different conditions to initiate MSC s differentiation along SMC lineage, and examined the alternation of SMC markers expression at both the transcript level and protein level. Desmin and h1‐calponin are expressed in MSC s, in the presence or absence of SMC induction conditions. Moreover, MSC s are shown to express all known isoforms of calponin. Double‐staining reveals that h1‐calponin +/α‐ SMA – cells constitute the majority of resting MSC s. Under differentiated conditions, expression of SM ‐ MHC was initiated and expression of α‐ SMA was promoted. The expression of SM ‐ MHC and upregulation of α‐ SMA are relatively reliable indications of a mature smooth muscle phenotype in MSC s. Given that the cells are particularly rich in calponins expression, we postulate possible roles of these proteins in regulating cellular function by taking part in actin cytoskeleton and signaling. These findings imply that an extensive study of the cell physiology of MSC s should focus on the functional roles for these proteins, rather than simply regard them as differentiated markers.