Cardiac stem cells in the postnatal heart: lessons from development

Heart development in mammals is followed by a postnatal decline in cell proliferation and cell renewal from stem cell populations. A better understanding of the developmental changes in cardiac microenvironments occurring during heart maturation will be informative regarding the loss of adult regenerative potential. We reevaluate the adult heart’s mitotic potential and the reported adult cardiac stem cell populations, as these are two topics of ongoing debate. The heart’s early capacity for cell proliferation driven by progenitors and reciprocal signalling is demonstrated throughout development. The mature heart architecture and environment may be more restrictive on niches that can host progenitor cells. The engraftment issues observed in cardiac stem cell therapy trials using exogenous stem cells may indicate a lack of supporting stem cell niches, while tissue injury adds to a hostile microenvironment for transplanted cells. Engraftment may be improved by preconditioning the cultured stem cells and modulating the microenvironment to host these cells. These prospective areas of further research would benefit from a better understanding of cardiac progenitor interactions with their microenvironment throughout development and may lead to enhanced cardiac niche support for stem cell therapy engraftment

Lsh is essential for maintaining global DNA methylation levels in amphibia and fish and interacts directly with Dnmt1

Eukaryotic genomes are methylated at cytosine bases in the context of CpG dinucleotides, a pattern which is maintained through cell division by the DNA methyltransferase Dnmt1. Dramatic methylation losses are observed in plant and mouse cells lacking Lsh (lymphoid specific helicase), predominantly at repetitive sequences and gene promoters. However, the mechanism by which Lsh contributes to the maintenance of DNA methylation is unknown. Here we show that DNA methylation is lost in Lsh depleted frog and fish embryos, both of which exhibit developmental delay. Additionally, we show that both Lsh and Dnmt1 are associated with chromatin and that Lsh knockdown leads to a decreased Dnmt1-chromatin association. Coimmunoprecipitation experiments reveal that Lsh and Dnmt1 are found in the same protein complex, and pulldowns show this interaction is direct. Our data indicate that Lsh is usually diffuse in the nucleus but can be recruited to heterochromatin in a HP1α-dependent manner. These data together (a) show that the role of Lsh in DNA methylation is conserved in plants, amphibian, fish, and mice and (b) support a model in which Lsh contributes to Dnmt1 binding to chromatin, explaining how its loss can potentially lead to perturbations in DNA methylation maintenance

Tup1-Ssn6 and Swi-Snf remodelling activities influence long-range chromatin organization upstream of the yeast SUC2 gene

The traditional model for chromatin remodelling during transcription has focused upon the remodelling of nucleosomes at gene promoters. However, in this study, we have determined that Tup1-Ssn6 and Swi-Snf chromatin remodelling activities extend far upstream of the SUC2 gene promoter into the intergenic region of the Saccharomyces cerevisiae chromosome. We mapped the nucleosomal array over a 7.5 kb region that encompassed the SUC2 gene promoter and upstream region but was devoid of other transcriptionally active genes. Nucleosome positioning over this region was determined under conditions of glucose repression and derepression, and in snf2, ssn6 and snf2 ssn6 mutant strains. A map detailing remodelling events extending as much as 5 kb upstream of the SUC2 gene promoter underlines the roles of the Tup1-Ssn6 and Swi-Snf complexes in respectively organizing and disrupting nucleosome arrays. The gene specificity of these events suggests a role in gene regulation. We propose that long-range chromatin remodelling activities of Swi-Snf and Tup1-Ssn6 may ultimately influence whether the chromosomal state of the SUC2 gene is proficient for transcription. These data raise the possibility that remodelling of extensive chromatin domains may be a general property of the Swi-Snf and Tup1-Ssn6 complexes