Thar's Tendons in Them Thar Valves!

摘要

In this issue of Circulation Research, Levay et al report an in vivo requirement for scleraxis (scx) during murine valvulogenesis and demonstrate its role in cell lineage differentiation and matrix distribution in remodeling valve structures. The scleraxis (Scx) gene encodes a basic helix-loop-helix transcription factor that is expressed in the progenitors and cells of all tendon tissues. Recent studies have identified the specific regulatory pathway in which fibroblast growth factor activates the mitogen-activated protein kinase (extracellular signal-regulated kinase 1/2) signaling cascade to promote expression of scx, which, in turn, induces expression of tenascin as being shared by developing valves and limb buds. In initial studies in chick, Lincoln et al found a restricted pattern of scx expression, suggesting that atrioven-tricular (AV). valve precursor cells diversify into leaflets (cartilage-like) or chordae tendineae (tendon-like) to produce both a leaflet and supporting apparatus, as opposed to semilunar valve precursor cells that exhibit both cartilage-and tendon-like characteristics and diversify into cusps with an internal supporting apparatus. However, hi the present study, Lincoln and colleagues identified high levels of scx expression in remodeling AV and semilunar valve structures from embryonic day (E)15.5 through postnatal stages; the biological significance of this apparent difference between chick and mouse is not known.