Developmental roles for Srf, cortical cytoskeleton and cell shape in epidermal spindle orientation.

During development, a polarized epidermal sheet undergoes stratification and differentiation to produce the skin barrier. Through mechanisms that are poorly understood, the process involves actin dynamics, spindle reorientation and Notch signalling. To elucidate how epidermal embryogenesis is governed, we conditionally targeted serum response factor (Srf), a transcription factor that is essential for epidermal differentiation. Unexpectedly, previously ascribed causative defects are not responsible for profoundly perturbed embryonic epidermis. Seeking the mechanism for this, we identified actins and their regulators that were downregulated after ablation. Without Srf, cells exhibit a diminished cortical network and in mitosis, they fail to round up, features we recapitulate with low-dose actin inhibitors in vivo and shRNA-knockdown in vitro. Altered concomitantly are phosphorylated ERM and cortical myosin-IIA, shown in vitro to establish a rigid cortical actomyosin network and elicit critical shape changes. We provide a link between these features and Srf loss, and we show that the process is physiologically relevant in skin, as reflected by defects in spindle orientation, asymmetric cell divisions, stratification and differentiation.