Septate junction proteins are required for egg elongation and border cell migration during oogenesis in Drosophila

Abstract

Protein components of the invertebrate occluding junction - known as the septate junction (SJ) - are required for morphogenetic developmental events during embryogenesis in Drosophila melanogaster. In order to determine whether SJ proteins are similarly required for morphogenesis during other developmental stages, we investigated the localization and requirement of four representative SJ proteins during oogenesis: Contactin, Macroglobulin complement-related, Neurexin IV, and Coracle. A number of morphogenetic processes occur during oogenesis, including egg elongation, formation of dorsal appendages, and border cell migration. We found that all four SJ proteins are expressed in the egg throughout oogenesis, with the highest and most sustained levels in the follicular epithelium (FE). In the FE, SJ proteins localize along the lateral membrane during early and mid-oogenesis, but become enriched in an apical-lateral domain (the presumptive SJ) by stage 10b. SJ protein relocalization requires the expression of other SJ proteins, as well as rab5 and rab11 in a manner similar to SJ biogenesis in the embryo. Knocking down the expression of these SJ proteins in follicle cells throughout oogenesis results in egg elongation defects and abnormal dorsal appendages. Similarly, reducing the expression of SJ genes in the border cell cluster results in border cell migration defects. Together, these results demonstrate an essential requirement for SJ genes in morphogenesis during oogenesis, and suggests that SJ proteins may have conserved functions in epithelial morphogenesis across developmental stages.

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“…While the phenotypes caused by Lac RNAi knockdown are mild, Lac is likely to be a true regulator of border cell migration. Recent work by Alhadyian et al found that four additional septate junction proteins, Macroglobulin complement-related (Mcr), Contactin, Neurexin-IV and Coracle, localize to border cells and are required for both border cell cluster migration and cohesion (Alhadyian et al 2021). Because border cells do not have mature septate junctions (which form the tight occluding junctions), septate junction proteins may instead regulate cluster polarity and/or adhesion during migration (Alhadyian et al 2021).…”

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

“…Core SJ proteins are localized all along the lateral membrane in stage 2–10 egg chambers, but are relocalized to an apical lateral location during stages 11 and 12. This relocalization of SJ proteins requires the presence of other SJ proteins, as RNAi against Nrx-IV or Mcr result in mislocalization of SJ proteins in stage 12 egg chambers [ 55 ]. Likewise, expression of a dominant negative Rab5 in the follicular epithelium or expression of rab11-RNAi results in mislocalized SJ proteins in stage 11/12 egg chambers.…”

“…A recent study has revealed roles for several SJ proteins in morphogenetic events that occur during oogenesis [ 55 ]. The ovaries in adult females are composed of approximately 20 ovarioles, each of which consists of linked chains of successively more developed egg chambers.…”

“…A number of important morphogenetic events occur during egg chamber development, including egg elongation, dorsal appendage formation, and border cell migration [ 89 , 94 ]. Using pan-follicle cell RNAi against Mcr , Nrx-IV , Cont , cora , sinu , and lac , Alhadyian and colleagues [ 55 ] demonstrated that these core SJ genes are required for the establishment or maintenance of an elongated egg shape (e.g., Figure 2 N). They went on to show defects in the morphology of the dorsal appendages in all but the sinu-RNAi egg chambers.…”

“…Given the timing of these events, it is unclear if the defects seen in these SJ-RNAi-induced egg chambers are due to a loss of the occluding junction, or if they result from an independent function of these proteins. RNAi against cora , Nrx-IV and Cont in border cells led to mild defects in border cell migration characterized by incomplete migration, with some border cell clusters failing to delaminate at the beginning of the process or disassociating during the migration [ 55 ]. In this case, the defective phenotypes occur prior to the formation of a mature occluding junction, arguing for an independent (although unknown) function for SJ proteins in this process.…”