The proper architecture of an outer body wall is fundamental to animals and plants. The insect cuticle provides exoskeletal stability and protection against wounding, desiccation, invading pathogens and other potential harms. In order to adapt to increasing body growth the insect cuticle is periodically shedded in a molting process. Molting and wounding requires organized cuticle rearrangement. It has been discussed that chitin microfibrils as a main component of the cuticle may undergo spontaneously self-assembly, but how exactly the complex architecture of the cuticle is arranged and reorganized during growth, molting and wound healing is not known.
We discovered a molecular mechanism of cuticle formation, maturation and protection in the Drosophila larval epidermal cuticle.
Upon cuticle synthesis chitin is secreted by the epidermal cells and forms to fibrils of various lengths at the apical cell surface. The chitin binding protein Obstructor-A (Obst-A) binds these nascent chitin-fibrils and forms a scaffold for their proper integration into the lamellar cuticle structure. The Obst-A scaffold is required to recruit and localize the deacetylases Serp and Verm for matrix assembly and maturation. Loss of obst-A leads to severe growth defects, impaired epidermal cuticle integrity and death before the molt to second instar larval stage. In summary, the molecular Obst-A core unit is essential to form, mature and keep an intact cuticle in insects.
For cuticle organization Chitinases process chitin-fibrils into fragments of different lengths. Surprisingly, our results indicate that Chitinases also play a role in maintaining structural and mechanical integrity of the cuticle. The Chitinase 2 is necessary for lamellar organization and for proper function of Obst-A at the apical cell surface.
All molecular mechanisms of cuticle formation are primarily concentrated at the apical cell surface in a so far completely uncharacterized region - the cuticle assembly zone. Our findings place the assembly zone in the center of cuticle organization. Due to protein sequence conservations this may hold true for insects and most chitinous invertebrates.
Finally, our data provide first evidence for a novel feedback mechanism how the cuticle assembly zone protein Obst-A regulates ecdysone signaling for larval molting and growth control.