Dynamic rearrangement of the spectrin membrane skeleton during the generation of epithelial polarity in Drosophila

The origin of epithelial cell polarity during development is a fundamental problem in cell biology, Central to this process is the establishment of asymmetric membrane domains that will ultimately form the apical and basolateral surfaces. The spectrin-based membrane skeleton has long been thought to participate in the generation of this asymmetry. Drosophila melanogaster contains two known beta-spectrin isoforms: a conventional beta-spectrin chain, and the novel isoform beta(Heavy)-spectrin. These two proteins are restricted to the basolateral and apical membrane domains, respectively. To assay for the emergence of membrane asymmetry, we have characterized the distribution of these two beta-spectrins during the formation of the primary epithelium in the fly embryo. Our results show that the syncytial embryo contains a maternally established apical membrane skeleton containing beta(Heavy)-spectrin into which the basolateral beta-spectrin membrane skeleton is added. We have called this process basolateral interpolation. Although basolateral membrane skeleton addition begins during cellularization, it does not become fully established until the formation of a mature zonula adherens at mid to late gastrulation, The behavior of beta-spectrin is consistent with a primary role in establishing and/or maintaining the basolateral domain while the behavior of beta(Heavy)-spectrin suggests that its primary role is associated with a specialized DE-cadherin complex associated with the furrow canals and with the maturation of the zonula adherens, Thus, the apical spectrin membrane skeleton appears to play a distinct rather than analogous role to the basolateral spectrin membrane skeleton, during the emergence of cell polarity. We find that there are several parallels between our observations and previous studies on the establishment of primary epithelial polarity in vertebrates, suggesting that basolateral interpolation of the membrane skeleton may be a common mechanism in many organisms.