Regulation of the collagen IV network by the basement membrane protein perlecan is crucial for squamous epithelial cell morphogenesis and organ architecture.

All epithelia have their basal side in contact with a specialized extracellular matrix, the basement membrane (BM). During development, the BM contributes to the shaping of epithelial organs via its mechanical properties. These properties rely on two core components of the BM, collagen type IV and perlecan/HSPG2, which both interact with another core component, laminin, the initiator of BM assembly. While collagen type IV supplies the BM with rigidity to constrain the tissue, perlecan antagonizes this effect. Nevertheless, the number of organs that has been studied is still scarce, and given that epithelial tissues exhibit a wide array of shapes, their forms are bound to be regulated by distinct mechanisms. This is underscored by mounting evidence that BM composition and assembly/biogenesis is tissue-specific. Moreover, previous reports have essentially focused on the mechanical role of the BM in morphogenesis at the tissue scale, but not the cell scale. Here, we took advantage of the robust conservation of core BM proteins and the limited genetic redundancy of the Drosophila model system to address how this matrix shapes the wing imaginal disc, a complex organ comprising a squamous, a cuboidal and a columnar epithelium. With the use of a hypomorphic allele, we show that the depletion of Trol (Drosophila perlecan) affects the morphogenesis of the three epithelia, but particularly that of the squamous one. The planar surface of the squamous epithelium (SE) becomes extremely narrow, due to a function for Trol in the control of the squamous shape of its cells. Furthermore, we find that the lack of Trol impairs the biogenesis of the BM of the SE by modifying the structure of the collagen type IV lattice. Through atomic force microscopy and laser surgery, we demonstrate that Trol provides elasticity to the SE's BM, thereby regulating the mechanical properties of the SE. Moreover, we show that Trol acts via collagen type IV, since the global reduction in the trol mutant context of collagen type IV or the enzyme that cross-links its 7S -but not the enzyme that cross-links its NC1- domain substantially restores the morphogenesis of the SE. In addition, a stronger decrease in collagen type IV achieved by the overexpression of the matrix metalloprotease 2 exclusively in the BM of the SE, significantly rescues the organization of the two other epithelia. Our data thus sustain a model in which Trol counters the rigidity conveyed by collagen type IV to the BM of the SE, via the regulation of the NC1-dependant assembly of its scaffold, allowing the spreading of the squamous cells, spreading which is compulsory for the architecture of the whole organ.

Two different sources of Perlecan cooperate for its function in the basement membrane of the Drosophila wing imaginal disc.

BACKGROUND: The basement membrane (BM) provides mechanical shaping of tissues during morphogenesis. The Drosophila BM proteoglycan Perlecan is vital for this process in the wing imaginal disc. This function is thought to be fostered by the heparan sulfate chains attached to the domain I of vertebrate Perlecan. However, this domain is not present in Drosophila, and the source of Perlecan for the wing imaginal disc BM remains unclear. Here, we tackle these two issues. RESULTS: In silico analysis shows that Drosophila Perlecan holds a domain I. Moreover, by combining in situ hybridization of Perlecan mRNA and protein staining, together with tissue-specific Perlecan depletion, we find that there is an autonomous and a non-autonomous source for Perlecan deposition in the wing imaginal disc BM. We further show that both sources cooperate for correct distribution of Perlecan in the wing imaginal disc and morphogenesis of this tissue. CONCLUSIONS: These results show that Perlecan is fully conserved in Drosophila, providing a valuable in vivo model system to study its role in BM function. The existence of two different sources for Perlecan incorporation in the wing imaginal disc BM raises the possibility that inter-organ communication mediated at the level of the BM is involved in organogenesis.