Role of nonmuscle myosin IIB and N‐RAP in cell spreading and myofibril assembly in primary mouse cardiomyocytes

We investigated the role of nonmuscle myosin heavy chain (NMHC) IIB in cultured embryonic mouse cardiomyocytes by specific knockdown using RNA interference. NMHC IIB protein levels decreased 90% compared with mock‐transfected cells by 3 days post transfection. NMHC IIB knockdown resulted in a slow decrease in N‐RAP protein levels over 6 days with no change in N‐RAP transcript levels. N‐RAP is a scaffold for α‐actinin and actin assembly during myofibrillogenesis, and we quantitated myofibril accumulation by morphometric analysis of α‐actinin organization. Between 3 and 6 days, NMHC IIB knockdown was accompanied by the abolishment of cardiomyocyte spreading. During this period the rate of myofibril accumulation steadily decreased, correlating with the slowly decreasing levels of N‐RAP. Between 6 and 8 days NMHC IIB and N‐RAP protein levels recovered, and cardiomyocyte spreading and myofibril accumulation resumed. Inhibition of proteasome function using MG132 led to accumulation of excess N‐RAP, and the secondary decrease in N‐RAP that otherwise accompanied NMHC IIB knockdown was abolished. The results show that NMHC IIB knockdown led to decreased N‐RAP levels through proteasome‐mediated degradation. Furthermore, these proteins have distinct functional roles, with NMHC IIB playing a role in cardiomyocyte spreading and N‐RAP functioning in myofibril assembly. Cell Motil. Cytoskeleton 2008. Published 2008 Wiley‐Liss, Inc.