Drosophilalaminins act as key regulators of basement membrane assembly and morphogenesis

Abstract

Laminins are heterotrimeric molecules found in all basement membranes. In mammals, they have been involved in diverse developmental processes, from gastrulation to tissue maintenance. The Drosophila genome encodes two laminin a chains, one b and one g, which form two distinct laminin trimers. So far, only mutations affecting one or other trimer have been analysed. In order to study embryonic development in the complete absence of laminins, we mutated the gene encoding the sole laminin b chain in Drosophila, LanB1, so that no trimers can be made. We show that LanB1 mutant embryos develop until the end of embryogenesis. Electron microscopy analysis of mutant embryos reveals that the basement membranes are absent and the remaining extracellular material appears disorganised and diffuse. Accordingly, abnormal accumulation of major basement membrane components, such as Collagen IV and Perlecan, is observed in mutant tissues. In addition, we show that elimination of LanB1 prevents the normal morphogenesis of most organs and tissues, including the gut, trachea, muscles and nervous system. In spite of the above structural roles for laminins, our results unravel novel functions in cell adhesion, migration and rearrangement. We propose that while an early function of laminins in gastrulation is not conserved in Drosophila and mammals, their function in basement membrane assembly and organogenesis seems to be maintained throughout evolution. can serve as PS2 integrin ligands (Graner et al., 1998). This is further supported by studies showing that lamininW is not recruited to muscle attachment sites in embryos carrying a mutation in the aPS2 subunit affecting the RGD binding site (Devenport et al., 2007).Only mutations in the a subunits have been described in Drosophila so far. Null mutations in the LanA gene result in embryonic lethality with defects in somatic muscles, dorsal vessel (heart) and endoderm (Henchcliffe et al., 1993;Yarnitzky and Volk, 1995). The a 3,5 chain is also required for proper localisation of anteroposterior markers in the oocyte and for normal pathfinding of pioneer axons in the brain (Deng and RuoholaBaker, 2000; Garcia-Alonso et al., 1996). In addition, hypomorphic mutants and trans-heteroallelic combinations of LanA give rise to adult escapers that have disorganised rhabdomeres and display vein defects and wing blistering (Henchcliffe et al., 1993). Mutations in wb have wing blisters and defects in the dorsal vessel, trachea, muscles and rhabdomeres . Mutations in the laminin b subunit (encoded by Laminin B1, LanB1) or g subunit (encoded by laminin B2, lanB2) have not been characterised.In this work, we have isolated null mutations in LanB1, allowing the generation of embryos lacking all laminin function. These embryos develop until the end of embryogenesis, suggesting that in Drosophila, as it is the case in nematodes and contrary to mice, laminins are not required for early embryonic morphogenetic events. Our analysis of LanB1 loss reveals that laminins are required for accumulation of major EC...

Genome wide identification ofTriboliumdorsoventral patterning genes

“…S5). Thus, in Tribolium, LanB1 is required earlier than in Drosophila for correct morphogenesis and the late embryonic knockdown phenotypes appear to be much stronger than the LanB1 mutant phenotypes of Drosophila, which show rather mild muscle defects (Urbano et al, 2009).…”

“…6N and Fig. 7K; Table S8, data not shown) (Dekkers et al, 2004;Montell and Goodman, 1989;Schneider and Baumgartner, 2008;Urbano et al, 2009). The interaction of laminin with surface receptors like integrins and dystroglycan plays an important role in gastrulation in many organisms, in particular in vertebrates (Ettensohn and Winkelbauer, 2004).…”

“…S5E-H). In Drosophila, the complete loss of LanB1 function leads to much milder defects during late organogenesis (Urbano et al, 2009). …”

Basement Membranes: Cell Scaffoldings and Signaling Platforms

“…The b-and g-subunits primarily play a structural role, mediating polymerization and nidogen-binding but also serve to modulate receptor-binding. , the first to be expressed in mammalian embryos, are essential for basement membrane assembly and early embryonic morphogenesis (Smyth et al 1999;Huang et al 2003;Urbano et al 2009). The laminin a1 subunit was found to be required for assembly of the extraembryonic Reichert's membrane whereas formation of the basement membrane of the embryonic plate required either the Lma1 or Lma5 subunit .…”

Laminin is required to orient epithelial polarity in theC. eleganspharynx

“…In Drosophila, similar to C. elegans, some laminin subunits are encoded by single genes; however, the respective Drosophila mutants do not exhibit defects in coordinating epithelial polarity (Urbano et al, 2009). Thus, laminin could function as a polarity signal in only certain epithelia, such as the pharynx, or might function redundantly with other signals such as other basement membrane proteins.…”