Expanding the Junction: New Insights into Non-Occluding Roles for Septate Junction Proteins during Development

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.

Keywords: Drosophila; apical/basal polarity; dorsal closure; epithelia; morphogenesis; organogenesis; planar polarity; secretion; septate junction; wound healing.

Figure 1

Structure and biogenesis of SJs. (A) Diagram of a polarized epithelium, showing the location of the adherens junction (AJ) and the septate junction (SJ). Apical is to the top. Note that the septa do not completely encircle the cell but are discontinuous, generating an extended path from the apical to basal side of the epithelium. (B) Electron micrograph of the epidermis in a stage 17 wild type embryo showing the electron dense extracellular septa in the SJ. (C,D) Diagram of the anterior portion (including a salivary gland) of a stage 14 (C) and a stage 17 (D) wild type embryo that had been injected with a 10-kDa rhodamine-labeled dextran (pink). The labeled dextran can cross the salivary gland epithelium and enter the lumen in a stage 14 embryo, but not in a stage 17 embryo due to the formation of the SJ. (E–G) Confocal micrographs of hindguts from w¹¹¹⁸ stage 13–16 embryos stained with antibodies against the AJ protein E-cadherin (in magenta) and the SJ protein Mcr in cyan. (E’–G’) Inverted images of the Mcr channel from (E–G). Mcr localizes all along the lateral membrane in stage 13 embryos (arrows) and begins to be enriched at the SJ (asterisk) with some lateral expression (arrow) and punctate cytoplasmic expression in stage 14 (arrow in inset). By stage 16, Mcr is tightly localized to the region of the SJ (asterisks) and is clearly basal to the AJ protein E cadherin.

Figure 2

Embryonic phenotypes associated with SJ mutations. (A,B) Cuticle preparations of w¹¹¹⁸ (A) and cor⁴ (B) embryos showing defective dorsal closure (asterisk in (B)) and extra cuticle deposits in the region of the salivary gland (arrow in (B)). (C,D) Wheat germ agglutinin stained stage 17 wild type and Mcr¹ embryos showing the long and convoluted Mcr¹ trachea (arrows). (E,F) Confocal images of salivary glands from w¹¹¹⁸ and Nrx-IV⁴³⁰⁴ stage 16 embryos stained for ATPα to outline cells. The Nrx-IV gland is short and fat, with longer apical-basal dimensions (arrows). (G,H) Confocal sections of Z-series through salivary glands from stage 16 w¹¹¹⁸ (G) and Mcr¹ (H) embryos stained with antibodies against Uninflatable (green) and ATPα (red). DAPI is in blue. White line indicates cross section shown in (G’) and (H’) showing the morphology of the lumen. In w¹¹¹⁸ SGs there are 7.5 ± 0.3 nuclei around the lumen compared to 9.2 ± 1.1 in Mcr¹. (I,J) Confocal images of hindguts from Cont^ex956/+ and Cont^ex956 stage 16 embryos stained for Mcr and Crb. The Cont hindgut has much less curvature than the wild type gut (arrows). (K,L) Dorsal thoraces of a control adult (K) and adults expressing RNAi against sinu (L) in the dorsal wing disc using ap-GAL4. Note the misoriented bristles and overall smaller scutellum and the sinu-RNAi thorax (arrows), and the overall disorganization of the bristles in the dorsal thorax (boxed in region). (M,N) Brightfield photomicrographs of GR1-GAL4 (control) and GR1>lac-RNAi stage 14 egg chambers. Note that the lacRNAi egg chamber is substantially rounder than the control egg chamber.