Dynamics of Basement Membrane Repair
The basement membrane (BM) is an ancient, highly conserved structure that lies under the basal surface of epithelial cells and wraps around organs such as muscles and nerves. It provides mechanical stiffness and modulates cellular communication by interacting with signaling molecules. Although much is known about BM structure, its dynamic aspects – the ability to expand, shrink, and be repaired – are poorly understood. This led Vanderbilt Basic Sciences investigator Andrea Page-McCaw and her laboratory to explore BM repair in larvae of Drosophila melanogaster. This model offered the ability to express each of the major BM components – laminin, collagen IV, perlecan, and nidogen – with a GFP label for easy visualization and the ability to inflict a sterile wound by pinching the cuticle of the larvae, which damages the underlying epidermis and BM while leaving the cuticle intact. The investigators found that both the epidermis and BM had closed the wound within 24 hours; however, the newly formed BM was thicker, more fibrous, and less well organized in structure than that observed in unwounded tissue. In other words, the BM had formed a scar. Although one might expect that BM components would be produced by the overlying epidermis, in Drosophila, both hemocytes and adipose tissue also contribute to BM formation. The investigators used RNAi technology to selectively knockdown GFP-labeled BM components from individual tissues. This approach demonstrated that laminin, collagen IV, and perlecan deposited during BM repair all come from adipose tissue, whereas nidogen derived from muscle. Further studies revealed that these were also the sources of the BM components in healthy, growing epidermis. Finally, the investigators used temperature-sensitive RNAi techniques to globally knockdown expression of each of the BM components in larvae several days before wounding. These results showed that no single component was required for epidermal wound closure. Similarly, neither laminin nor collagen IV deposition required the presence of any of the other components, although perlecan and nidogen deposition were dependent on the presence of collagen IV and laminin, respectively. This was the first in vivodemonstration of the dependence of nidogen deposition on laminin, and the first demonstration of collagen IV deposition that did not require laminin, which is usually deposited first during BM formation. The findings provide critical new insights into BM repair. Unfortunately, the rapid transformation of larvae into pupae soon after wound healing prevented further observations on the ultimate fate of the BM scars, which will require exploration in a different model system. The work is published in the journal Matrix Biology[W. Ramos-Lewis, et al. (2018) Matrix Biol., published online July 4, DOI: 10.1016/j.matbio.2018.07.004].