The role of muscle-tendon cell interaction during epithelial notum morphogenesis of Drosophila melanogaster

Abstract

Tissue-tissue interaction is essential to drive morphogenesis and contributing to the final shape of tissues and organs. The interaction between muscles and tendons during the establishment of the muscle-skeletal system is a great model to study this problem. During Drosophila melanogaster metamorphosis a group of cells of the dorsal thorax (notum) epithelium, specialized as tendon cells, attach to the developing Indirect Flight Muscles (IFMs). Likewise, epithelial cells anchor to the cuticle exoskeleton through apical projections. Both interactions enable the adaptation of notum epithelium to mechanical strain generated by muscle contraction, by modulating its mechanoresponse. However, scarce evidence exists about how muscle-tendon interaction contributes to the final shape of the notum. Thus, we hypothesized that the interaction between IFMs and tendon cells plays a role in notum epithelium morphogenesis. Geometric morphometric analysis of adult thorax shape shows that interfering with muscle development results in dorsal thorax deformation, however, the absence of muscles does not affect,collective-epithelial movement of the epithelium towards anterior during notum morphogenesis, suggesting that early cellular mechanisms such as cell division, rearrangements and cell delamination are not altered. Conversely, force distribution along epithelium plane changes in muscle depletion condition during notum morphogenesis, displaying anisotropic tendency in tendon-cell and midline domains. Further, impairing muscle-contraction does not affect adult thorax shape compared with wild-type conditions, indicating that muscle function as a structural support for thorax epithelium. On the other hand, the ability of notum epithelium to adapt to the mechanical strain during IFMs contraction becomes crucial to maintain the shape and integrity of the tissue. Notum epithelium lacking Chascon, a scaffold/adaptor protein involved in cytoskeleton organization upstream of Jbug/Filamin, displays epithelium deformations and impaired collective-epithelial movement during morphogenesis. Interestingly, IFMs ablation rescues backward epithelial movement associated with chascon knockdown condition, resembling wild-type phenotype, although it affects tissue-movement velocity and the ability of tendon cells to guide collective cell movement. Since notum epithelium anchors apically to the cuticle we tested whether Chascon is required for this interaction. We found that chascon knockdown in tendon cells results in epithelial detachment from the cuticle during muscles shortening stage, supporting the role of Chascon in cell adhesion and collective epithelial-cell movement. Additionally, we observed an increased anisotropy at tendon cell domains in absence of Chascon after muscle shortening, indicating the great unbalance in mechanical homeostasis after muscle pulling under this condition. Since muscle-tendon interaction is required for tendon cell differentiation in embryos we tested whether muscle was required for the expression of chascon and dumpy, a membrane protein responsible for exoskeleton-epithelium attachment, which along with Chascon is enriched in tendon cell domains during terminal differentiation. We found no significant differences in mRNA levels of chascon and dumpy, between animals lacking muscles versus wild type during muscle shortening, suggesting a muscle-independent alternative regulation of chascon and dumpy expression. Our results support the notion that Chascon is required for tension-adaptation response of notum epithelium during muscle-contraction, ensuring collective-epithelial cell movement through regulation of tendon-cell attachment to the cuticle. We suggest that Chascon, along with a multi-protein complex, regulate the mechano-response of tendon-cells during muscle contraction, by enabling collective-epithelial cell movement under mechanical load due to muscle development. Finally, these analyses will contribute to a better understanding of the role of tissue-tissue interaction in tissue morphogenesis and differentiation.