LAP2 Isoform Profile in Heart Ageing and in Cardiac Cell Proliferation and Differentiation: Input From...

Haploinsufficiency of Lap2 alpha (LAP2a), a nuclear partner of Lamins A/C, has been associated with cardiac disease in rare cases, but LAP2a function remains largely unknown. To investigate the functional role of LAP2a in cardiomyocytes, we generated clones of embryonic myocardium-derived H9C2 cells in which LAP2a expression was specifically reduced through gene editing of the LAP2a gene Tmpo by CRISPR-Cas9. Downregulation (+/-) and absence (-/-) of LAP2a expression led to a decreased proliferation capacity of cardiomyocytes in vitro. Upon differentiation, the expression of myocardial markers (alpha cardiac Actin 1/Actc1, cardiac Troponin T2/Tnnt2, Myosin-2/Myh2 and Myosin-7/Myh7) was higher in LAP2a -/- cells compared to LAP2a +/- or LAP2a +/+ cells, with consistently higher expression of their upstream regulator Mef2c in LAP2a-devoid cells. These results suggest that LAP2a promotes cardiomyocyte proliferation and negatively modulates cardiomyocyte differentiation, through mechanisms including Mef2c regulation. Accordingly, normal protein expression of LAP2a was downregulated upon cardiomyocyte differentiation, contrary to LAP2b and a LAP2b-related shorter isoform. The latter tended to increase upon differentiation in all cells, most significantly in the LAP2a -/- clone. In postnatal mouse hearts, LAP2a levels were higher in the right than in the left ventricle, and lowest in the septum. The LAP2a:LAP2b ratio was much lower in murine hearts than in H9C2 cells, and decreased significantly upon ageing, specifically in the left ventricle. Finally, our data show that expression of the nuclear envelope proteins LEMD2 and Lamin A might be influenced by LAP2a upon cardiac differentiation. Our results show that LAP2 expression is finely regulated upon cardiac differentiation in vitro and is dependent on age and heart compartment in vivo. They contribute to clarifying the potential impact of genetic LAP2a defects and their connection with heart disease, possibly including reduced cardiomyoblast proliferation, increased cardiomyocyte differentiation and altered nuclear envelope remodelling.