alpha-, beta- or gamma-chain-specific RNA interference of laminin assembly in Drosophila Kc167 cells

Serpent/dGATAb regulates Laminin B1 and Laminin B2 expression during Drosophila embryogenesis

Transcriptional regulation of Laminin expression during embryogenesis is a key step required for proper ECM assembly. We show, that in Drosophila the Laminin B1 and Laminin B2 genes share expression patterns in mesodermal cells as well as in endodermal and ectodermal gut primordia, yolk and amnioserosa. In the absence of the GATA transcription factor Serpent, the spatial extend of Laminin reporter gene expression was strongly limited, indicating that Laminin expression in many tissues depends on Serpent activity. We demonstrate a direct binding of Serpent to the intronic enhancers of Laminin B1 and Laminin B2. In addition, ectopically expressed Serpent activated enhancer elements of Laminin B1 and Laminin B2. Our results reveal Serpent as an important regulator of Laminin expression across tissues.

Laminin β1a controls distinct steps during the establishment of digestive organ laterality

Visceral organs, including the liver and pancreas, adopt asymmetric positions to ensure proper function. Yet the molecular and cellular mechanisms controlling organ laterality are not well understood. We identified a mutation affecting zebrafish laminin β1a (lamb1a) that disrupts left-right asymmetry of the liver and pancreas. In these mutants, the liver spans the midline and the ventral pancreatic bud remains split into bilateral structures. We show that lamb1a regulates asymmetric left-right gene expression in the lateral plate mesoderm (LPM). In particular, lamb1a functions in Kupffer's vesicle (KV), a ciliated organ analogous to the mouse node, to control the length and function of the KV cilia. Later during gut-looping stages, dynamic expression of Lamb1a is required for the bilayered organization and asymmetric migration of the LPM. Loss of Lamb1a function also results in aberrant protrusion of LPM cells into the gut. Collectively, our results provide cellular and molecular mechanisms by which extracellular matrix proteins regulate left-right organ morphogenesis.

Epistatic dissection of laminin-receptor interactions in dystrophic zebrafish muscle

Laminins form essential components of the basement membrane and are integral to forming and maintaining muscle integrity. Mutations in the human Laminin-alpha2 (LAMA2) gene result in the most common form of congenital muscular dystrophy, MDC1A. We have previously identified a zebrafish model of MDC1A called candyfloss (caf), carrying a loss-of-function mutation in the zebrafish lama2 gene. In the skeletal muscle, laminins connect the muscle cell to the extracellular matrix (ECM) by binding either dystroglycan or integrins at the cell membrane. Through epistasis experiments, we have established that both adhesion systems individually contribute to the maintenance of fibre adhesions and exhibit muscle detachment phenotypes. However, larval zebrafish in which both adhesion systems are simultaneously genetically inactivated possess a catastrophic failure of muscle attachment that is far greater than a simple addition of individual phenotypes would predict. We provide evidence that this is due to other crucial laminins present in addition to Lama2, which aid muscle cell attachments and integrity. We have found that lama1 is important for maintaining attachments, whereas lama4 is localized and up-regulated in damaged fibres, which appears to contribute to fibre survival. Importantly, our results show that endogenous secretion of laminins from the surrounding tissues has the potential to reinforce fibre attachments and strengthen laminin-ECM attachments. Collectively these findings provide a better understanding of the cellular pathology of MDC1A and help in designing effective therapies.

Drosophila laminins act as key regulators of basement membrane assembly and morphogenesis

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 alpha chains, one beta and one Gamma, 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 beta 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.

C-terminal-binding protein directly activates and represses Wnt transcriptional targets in Drosophila

Regulation of Wnt transcriptional targets is thought to occur by a transcriptional switch. In the absence of Wnt signaling, sequence-specific DNA-binding proteins of the TCF family repress Wnt target genes. Upon Wnt stimulation, stabilized beta-catenin binds to TCFs, converting them into transcriptional activators. C-terminal-binding protein (CtBP) is a transcriptional corepressor that has been reported to inhibit Wnt signaling by binding to TCFs or by preventing beta-catenin from binding to TCF. Here, we show that CtBP is also required for the activation of some Wnt targets in Drosophila. CtBP is recruited to Wnt-regulated enhancers in a Wnt-dependent manner, where it augments Armadillo (the fly beta-catenin) transcriptional activation. We also found that CtBP is required for repression of a subset of Wnt targets in the absence of Wnt stimulation, but in a manner distinct from previously reported mechanisms. CtBP binds to Wnt-regulated enhancers in a TCF-independent manner and represses target genes in parallel with TCF. Our data indicate dual roles for CtBP as a gene-specific activator and repressor of Wnt target gene transcription.

The role of the laminin beta subunit in laminin heterotrimer assembly and basement membrane function and development in C. elegans

Laminins are components of basement membranes that are required for morphogenesis, organizing cell adhesions and cell signaling. Studies have suggested that laminins function as alpha(x) beta(y) gamma(z) heterotrimers in vivo. In C. elegans, there is only one laminin beta gene, suggesting that it is required for all laminin functions. Our analysis is consistent with the role of the laminin beta as a subunit of laminin heterotrimers; the same cells express the laminin alpha, beta, and gamma subunits, the laminin beta subunit localizes to all basement membranes throughout development, and secretion of the beta subunit requires an alpha subunit. RNAi inhibition of the beta subunit gene or of the other subunit genes causes an embryonic lethality phenotype. Furthermore, a distinctive set of phenotypes is caused by both viable laminin alpha and beta partial loss-of-function mutations. These results show developmental roles for the laminin beta subunit, and they provide further genetic evidence for the importance of heterotrimer assembly in vivo.

Mammalian and Drosophila cells adhere to the laminin alpha4 LG4 domain through syndecans, but not glypicans

We have previously shown that the LG4 (laminin G-like) domain of the laminin alpha4 chain is responsible for the significantly higher affinity of the alpha4 chain to heparin than found for other alpha chains [Yamaguchi, Yamashita, Mori, Okazaki, Nomizu, Beck and Kitagawa (2000) J. Biol. Chem. 275, 29458-29465]; four basic residues were identified to be essential for this activity [Yamashita, Beck and Kitagawa (2004) J. Mol. Biol. 335, 1145-1149]. By creating GST (glutathione S-transferase)-fused LG1, LG2, LG4 and LG5 proteins, we found that only LG4 is active for the adhesion of human HT1080 cells, human umbilical vein endothelial cells and Drosophila haemocytes Kc167 with a half-saturating concentration of 20 microg/ml. Adhesion was counteracted by treatment of the cells with heparin, heparan sulphate and heparitinase I. Upon mutating the four basic residues essential for heparin binding within LG4, the adhesion activity was abolished. Pull-down experiments using glutathione beads/GST-fusion proteins indicate a direct interaction of LG4 with syndecan-4, which might be the major receptor for cell adhesion. Neither the release of glypican-1 by treating human cells with phosphatidylinositol-specific phospholipase C nor targeted knockdown of dally or dally-like protein impaired the cell-adhesion activity. As the LG4-LG5 domain of the alpha4 chain is cleaved in vivo from the main body of laminin-8 (alpha4beta1gamma1), we suggest that the heparan sulphate proteoglycan-binding activity of LG4 is significant in modulating the signalling of Wnt, Decapentaplegic and fibroblast growth factors.

Drosophila hemolectin gene is expressed in embryonic and larval hemocytes and its knock down causes bleeding defects

We have previously identified and characterized Drosophila hemolectin (Hml) in the conditioned medium of a Drosophila cell line. The deduced amino acids sequence contains a number of domains conserved in human von Willebrand factor, coagulation factor V/VIII, and complement factors. To characterize Hml localization and function, we have established two transgenic lines (hml-GAL4 and UAS-hmlRNAi). By crossing hml-GAL4 with UAS-eGFP, we observed that Hml is specifically expressed in embryonic and in larval hemocytes. We determined that Hml is expressed in a subpopulation of plasmatocytes and crystal cells but not in lamellocytes. hml RNAi larvae are viable and do not display obvious developmental defects under normal conditions. However, they show a bleeding defect upon injury. We confirmed that the failure to seal a wound is not due to a defect in melanin production or in phenoloxidase activity. The expression of antimicrobial peptides was not significantly affected on hml RNAi adults. Altogether, our data strongly suggest that Hml is involved in hemostasis and/or coagulation in Drosophila larvae.

Drosophila MCM10 interacts with members of the prereplication complex and is required for proper chromosome condensation

Mcm10 is required for the initiation of DNA replication in Saccharomyces cerevisiae. We have cloned MCM10 from Drosophila melanogaster and show that it complements a ScMCM10 null mutant. Moreover, Mcm10 interacts with key members of the prereplication complex: Mcm2, Dup (Cdt1), and Orc2. Interactions were also detected between Mcm10 and itself, Cdc45, and Hp1. RNAi depletion of Orc2 and Mcm10 in KC cells results in loss of DNA content. Furthermore, depletion of Mcm10, Cdc45, Mcm2, Mcm5, and Orc2, respectively, results in aberrant chromosome condensation. The condensation defects observed resemble previously published reports for Orc2, Orc5, and Mcm4 mutants. Our results strengthen and extend the argument that the processes of chromatin condensation and DNA replication are linked.