P57(Kip2) expression is enhanced during mid-cardiac murine development andis restricted to trabecular myocardium

During embryonic development the heart is required to grow in size and cell number, undergo complex morphologic alterations, and function to circulate the blood. Between embryonic d 10.5 (E10.5) and E11.5, cardiac myocytes un dergo rapid cell division, resulting in doubling of cardiac mass, while met abolic requirements are increased and contraction force is enhanced. Accele rated cardiomyocyte differentiation is accompanied by a significant increas e in trabeculation of ventricular myocardium. Many single gene mutations in the mouse result in a "thinned myocardium" and embryonic lethality between E10.5 and E13.5 secondary to heart failure. This is the case in the Splotc h mouse in which a mutation of the Pax3 gene results in neural crest and ca rdiac defects. Nevertheless, the molecular events governing these important developmental steps remain largely unknown. Here, we describe the use of s uppression subtractive hybridization to identify mRNA transcripts whose exp ression is enhanced during this critical period in normal hearts. These gen es encode functions related to maturation of the contractile apparatus, car diomyocyte differentiation, altered cellular metabolism, and transcriptiona l regulation. One of the genes that we identified, p57(Kip2), encodes a cyc lin-dependent kinase inhibitor of the p21 family. We show that p57(Kip2) is normally expressed in the inner trabecular layer of the developing heart. In Splotch embryos, expression of p57(Kip2) is expanded to encompass the en tire thickness of the myocardium. This result and further structural analys is suggests that the myocardial defect of Splotch embryos is associated wit h precocious cardiomyocyte differentiation.