Evidence of Swim secretion and association with extracellular matrix in the Drosophila embryo

Secreted wingless-interacting protein (Swim) is the Drosophila ortholog gene of the mammalian Tubulointerstitial Nephritis Antigen Like 1 (TINAGL1), known also as lipocalin-7 (LCN7), or adrenocortical zonation factor 1 (AZ-1). Swim and TINAGL1 proteins share a significant homology, including the somatomedin B and the predictive inactive C1 cysteine peptidase domains. In mammals, both TINAGL1 and its closely related homolog TINAG have been identified in basement membranes, where they may function as modulators of integrin-mediated adhesion. In Drosophila, Swim was initially identified in the eggshell matrix and subsequently was detected in the culture medium of S2 cells. Further biochemical analysis indicated that Swim binds to wingless (wg) in a lipid-dependent manner. This observation together with RNAiknockdown studies suggested that Swim is an essential cofactor of wg-signalling. However, recent elegant genetic studies ruled out the possibility that Swim is required alone to facilitate wgsignalling in Drosophila, because flies without Swim are viable and fertile. Here, we use the UAS/Gal4 expression system together with confocal imaging to analyze the in vivo localization of a chimeric Swim-GFP in the developing Drosophila embryo. Our data fully support the notion that Swim is an extracellular matrix component that upon ectopic expression is secreted and preferentially associates with the basement membranes of various organs and with the specialized tendon matrix at the muscle attachment sites (MAS). Interestingly, the accumulation of Swim at the MAS does not require integrins. In conclusion, Swim is an extracellular matrix component, and it is possible that Swim exhibits overlapping functions in concert with other undefined components.