Abstract P1181: Targeted Loss Of Sarcolemmal Membrane Associated Protein Isoform 3 (slmap3) In Cardiac Progenitors Stunts The Embryonic Growth Of Myocardium

Abstract

Sarcolemmal membrane associated proteins (SLMAPs) belong to the superfamily of tail anchored membrane proteins known to regulate variety of functions including vesicle transport. SLMAP3 is the largest isoform which is ubiquitously expressed and has been linked to Brugada syndrome and sodium channel activity. SLMAP3 is expressed during cardiogenesis, and we examined its role by nullifying its expression in mouse cardiac progenitors using the Nkx2.5-cre-lox system. Hearts from SLMAP3-knockout (KO) mouse embryos at different stages of cardiogenesis, e9.5, e12.5 and e16.5, were smaller (p<0.05) than wildtype (Wt) littermates. Histological analysis of heart wall measurements from the left ventricles revealed thinner walls in SLMAP3-KO from e9.5 (-25.16%, p<0.05), e12.5 (-29.36%, p<0.05) and e16.5 (-40.27%, p<0.05) compared to Wt although normal chamber formations was observed at these developmental stages. To evaluate the potential reasons for the early difference in cardiac growth, we examined myocardial cell numbers and size with markers of proliferation and hypertrophy. Immunofluorescent analysis of e12.5 hearts with proliferative markers (pH3, Ki67) was not significantly (-5%, p>0.05) altered in KO hearts while cardiomyocyte size differences assessed by immunofluorescence with anti-Troponin C, revealed that KO cardiomyocytes were significantly (-19.4%, p<0.05) smaller than Wt. However, the phospho-to total ratios of AKT1(1.45%, p>0.05) and MTOR1(-12%, p>0.05) were not significantly altered in KO hearts when compared to Wt. SLMAP3 has been implicated in Hippo signaling via the STRIPAK (striatin interacting phosphatase and kinase) complex to impact YAP/TAZ and cell growth. Western blot analysis of e12.5 Wt and KO hearts showed no change in phosphorylation of MST1/2(-6.64%, p>0.05) or YAP-Ser 127(-8.84%, p>0.05) ruling out involvement of Hippo signaling. Interestingly, the expression of the other SLMAP isoforms, SLMAP1 and SLMAP2 remained unaffected during cardiac development in SLMAP3 KO hearts. Thus, SLMAP expression is critical for normal development since the loss of SLMAP3 isoform leads to stunted growth of the embryonic heart. Supported by CIHR.