Lamin B2 Levels Regulate Polyploidization of Cardiomyocyte Nuclei and Myocardial Regeneration.

Heart regeneration requires cardiomyocyte proliferation. It is thought that formation of polyploid nuclei establishes a barrier for cardiomyocyte proliferation, but the mechanisms are largely unknown. Here, we show that the nuclear lamina filament Lamin B2 (Lmnb2), whose expression decreases in mice after birth, is essential for nuclear envelope breakdown prior to progression to metaphase and subsequent division. Inactivating Lmnb2 decreased metaphase progression, which led to formation of polyploid cardiomyocyte nuclei in neonatal mice, which, in turn, decreased myocardial regeneration. Increasing Lmnb2 expression promoted cardiomyocyte M-phase progression and cytokinesis and improved indicators of myocardial regeneration in neonatal mice. Inactivating LMNB2 in human iPS cell-derived cardiomyocytes reduced karyokinesis and increased formation of polyploid nuclei. In primary cardiomyocytes from human infants with heart disease, modifying LMNB2 expression correspondingly altered metaphase progression and ploidy of daughter nuclei. In conclusion, Lmnb2 expression is essential for karyokinesis in mammalian cardiomyocytes and heart regeneration.

Control of cytokinesis by ß-adrenergic receptors indicates an approach for regulating cardiomyocyte endowment.

One million patients with congenital heart disease (CHD) live in the United States. They have a lifelong risk of developing heart failure. Current concepts do not sufficiently address mechanisms of heart failure development specifically for these patients. Here, analysis of heart tissue from an infant with tetralogy of Fallot with pulmonary stenosis (ToF/PS) labeled with isotope-tagged thymidine demonstrated that cardiomyocyte cytokinesis failure is increased in this common form of CHD. We used single-cell transcriptional profiling to discover that the underlying mechanism of cytokinesis failure is repression of the cytokinesis gene ECT2, downstream of ß-adrenergic receptors (ß-ARs). Inactivation of the ß-AR genes and administration of the ß-blocker propranolol increased cardiomyocyte division in neonatal mice, which increased the number of cardiomyocytes (endowment) and conferred benefit after myocardial infarction in adults. Propranolol enabled the division of ToF/PS cardiomyocytes in vitro. These results suggest that ß-blockers could be evaluated for increasing cardiomyocyte division in patients with ToF/PS and other types of CHD.

Functional and genetic characterization of the promoter region of apolipoprotein H (beta2-glycoprotein I).

This study characterized the human apolipoprotein H [APOH; beta(2)-glycoprotein I (beta(2)GPI)] promoter and its variants by in vitro functional experiments and investigated their relationship with human plasma beta(2)GPI levels. We examined the individual effects of 12 APOH promoter single nucleotide polymorphisms in the 5' flanking region of APOH (approximately 1.4 kb) on luciferase activity in COS-1 cells and HepG2 cells and their impact on plasma beta(2)GPI levels in 799 American White people, the DNA binding properties of the APOH promoter using an electrophoretic mobility shift assay in HepG2 cells, the effects of serial deletion analysis of the APOH 5' flanking region in COS-1 and HepG2 cells and cross-species conservation of the APOH promoter sequence. The variant alleles of three single nucleotide polymorphisms (-1219G>A, -643T>C and -32C>A) showed significantly lower luciferase expression (51, 40 and 37%, respectively) as compared with the wild-type allele. The electrophoretic mobility shift assay demonstrated that these three variants specifically bind with protein(s) from HepG2 cell nuclear extracts. Three-site haplotype analysis (-1219G>A, -643T>C and -32C>A) revealed one haplotype carrying -32A (allele frequency = 0.075) to be significantly associated with decreased plasma beta(2)GPI levels (P < 0.001). Deletion analysis localized the core APOH promoter to approximately 160 bp upstream of ATG codon with the presence of critical cis-acting elements between -166 and -65. Cross-species conservation analysis of the APOH promoters of seven species indicated that basic promoter elements are highly conserved across species. In conclusion, we have characterized the functional promoter of APOH and identified functional variants that affect the transcriptional activity of the APOH promoter.