Sall1 and Sall4 cooperatively interact with Myocd and SRF to promote cardiomyocyte proliferation by regulating CDK and cyclin genes.

Sall1 and Sall4 (Sall1/4), zinc-finger transcription factors, are expressed in the progenitors of the second heart field (SHF) and in cardiomyocytes during the early stages of mouse development. To understand the function of Sall1/4 in heart development, we generated heart-specific Sall1/4 functionally inhibited mice by forced expression of the truncated form of Sall4 (ΔSall4) in the heart. The ΔSall4-overexpression mice exhibited a hypoplastic right ventricle and outflow tract, both of which were derived from the SHF, and a thinner ventricular wall. We found that the numbers of proliferative SHF progenitors and cardiomyocytes were reduced in ΔSall4-overexpression mice. RNA-sequencing data showed that Sall1/4 act upstream of the cyclin-dependent kinase (CDK) and cyclin genes, and of key transcription factor genes for the development of compact cardiomyocytes, including myocardin (Myocd) and serum response factor (Srf). In addition, ChIP-sequencing and co-immunoprecipitation analyses revealed that Sall4 and Myocd form a transcriptional complex with SRF, and directly bind to the upstream regulatory regions of the CDK and cyclin genes (Cdk1 and Ccnb1). These results suggest that Sall1/4 are critical for the proliferation of cardiac cells via regulation of CDK and cyclin genes that interact with Myocd and SRF.