Abstract
The septate junction (SJ) provides an occluding function for epithelial tissues in invertebrate organisms. This ability to seal the paracellular route between cells allows internal tissues to create unique compartments for organ function and endows the epidermis with a barrier function to restrict the passage of pathogens. Over the past twenty-five years, numerous investigators have identified more than 30 proteins that are required for the formation or maintenance of the SJs in Drosophila melanogaster, and have determined many of the steps involved in the biogenesis of the junction. Along the way, it has become clear that SJ proteins are also required for a number of developmental events that occur throughout the life of the organism. Many of these developmental events occur prior to the formation of the occluding junction, suggesting that SJ proteins possess non-occluding functions. In this review, we will describe the composition of SJs, taking note of which proteins are core components of the junction versus resident or accessory proteins, and the steps involved in the biogenesis of the junction. We will then elaborate on the functions that core SJ proteins likely play outside of their role in forming the occluding junction and describe studies that provide some cell biological perspectives that are beginning to provide mechanistic understanding of how these proteins function in developmental contexts.
Highlights
The septate junction (SJ) serves as the occluding junction in epithelial tissues in invertebrate organisms [1]
Two molecularly distinct versions of the SJ are present in Drosophila that differ in their ultrastructural appearance and molecular composition: smooth SJs and pleated SJ. sSJs are present in the midgut and other endodermally-derived epithelia, and are comprised of the proteins Snakeskin, Mesh, Tetraspanin 2A, Fasciclin
Reduction of Nrg via RNAi results in failure to mount an encapsulation response [96], whereas Rac mutations result in reduced and mislocalized Cora expression that prevents encapsulation [97]. It is unclear if there are functions for SJ proteins that are independent of forming an occluding junction in this system, but investigating the steps involved in the formation of an encapsulation epithelium may serve as an excellent model to study SJ biogenesis
Summary
The septate junction (SJ) serves as the occluding junction in epithelial tissues in invertebrate organisms [1]. Two molecularly distinct versions of the SJ are present in Drosophila (and other invertebrate organisms) that differ in their ultrastructural appearance and molecular composition: smooth SJs (sSJs) and pleated SJ (pSJs). Studies in Drosophila have revealed a few dozen genes that function in the establishment or maintenance of SJs and have provided insights into the biogenesis of the junction during embryogenesis and in the follicular epithelium of the ovary. Mutations in many of the genes that encode core SJ proteins show defects in developmental and cellular processes that suggest that these proteins have functions that may be independent of their roles in providing an occluding junction. We will describe the known composition of SJs, the biogenesis of the SJ in embryonic ectodermal epithelia, and the functions that core SJ proteins likely play outside of their role in forming the occluding junction. We apologize to any colleagues for omissions of their work in this area as we endeavor to highlight non-occluding functions of SJ proteins and are not necessarily providing an exhaustive catalog of functions ascribed to SJ proteins
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