Abstract 1988: c-kit and the Histone Methyltransferase SMYD3 Regulate Growth and Differentiation of Cardiac Progenitor Cells
The adult heart contains a pool of c-kit-positive cardiac progenitor cells (CPCs). The objective of this work was to establish whether c-kit represents only a useful epitope for cell isolation or whether it controls the functional properties of CPCs. For this purpose, we studied initially CPCs from W/Wv mice which have a mutated non-functional c-kit receptor. With IGF-1 and SCF, the fraction of BrdU-labeled CPCs was nearly 50% lower in W/WV than in wild type (WT). Also, a 4-fold higher level of apoptosis was seen in W/WV-CPCs suggesting that c-kit plays a crucial role in CPC growth and survival. To determine whether these alterations in CPC behavior were dictated by changes in gene expression, the transcriptional profile of WT and W/WV hearts was determined by Affymetrix microarray. Antiapoptotic and prosurvival gene transcripts were decreased in W/WV. Similarly, mRNAs for genes favoring growth and differentiation were reduced in W/WV. Among these differentially expressed genes, we have elected to study the methyltransferase SMYD3 that promotes methylation of histone H3 at lysine 4 enhancing transcription. Putative targets of SMYD3 include genes that control cell proliferation and commitment suggesting that this enzyme favors CPC activation and the formation of transit amplifying myocytes. These cells have the unique property to replicate and simultaneously differentiate. By quantitative RT-PCR and Western blotting, the levels of SMYD3 transcript and protein were, respectively, 2.7-fold and 2.3 fold lower in W/WV-CPCs. Importantly, c-kit-positive bone marrow cells from W/WV mice showed a reduced level of SMYD3 mRNA and protein further confirming that c-kit controls the function of this methyltransferase. To establish whether these findings are applicable to the human heart, c-kit-positive human CPCs (hCPCs) were transfected with an expression plasmid carrying SMYD3. SMYD3-overexpressing hCPCs showed ~2-fold upregulation of telomerase and Nkx2.5 which regulate the transit amplifying state of differentiating hCPCs by promoting proliferation and commitment. In conclusion, the c-kit receptor through the modulation of SMYD3 favors CPC activation and cardiomyogenesis by upregulating telomerase and Nkx2.5.