María D. Martín-Bermudo

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 α chains, one β and one Γ, 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 β 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.

Specificity of PS integrin function during embryogenesis resides in the alpha subunit extracellular domain.

We tested the ability of different integrin α subunits to substitute for each other during embryonic development. Two α subunits, which form heterodimers with the same βPS subunit, are expressed in complementary tissues in the Drosophila embryo, with αPS1 expressed in the epidermis and endoderm, and αPS2 expressed in the mesoderm. As a result the two integrin heterodimers are present on opposite surfaces at sites of interaction between the mesoderm and the other cell layers where they are required for normal development. Using the GAL4 system, we are able to rescue fully the embryonic lethality of an αPS2 null mutation with a UAS‐αPS2 transgene, but only partially with a UAS‐αPS1 gene, due to partial rescue of both muscle and midgut phenotypes. Similarly we are able to rescue the embryonic/first instar larval lethality of an αPS1 null mutation gene with UAS‐αPS1, but only partially with UAS‐αPS2. Each UAS‐α gene, when it contains the cytoplasmic domain from the other α subunit, maintains an equivalent ability to rescue its own mutation and cannot fully rescue a mutation in the other α. We conclude that the two α subunits are not equivalent and have distinct functions which reside in the extracellular domains.

ojoplano-mediated basal constriction is essential for optic cup morphogenesis

Although the vertebrate retina is a well-studied paradigm for organogenesis, the morphogenetic mechanisms that carve the architecture of the vertebrate optic cup remain largely unknown. Understanding how the hemispheric shape of an eye is formed requires addressing the fundamental problem of how individual cell behaviour is coordinated to direct epithelial morphogenesis. Here, we analyze the role of ojoplano (opo), an uncharacterized gene whose human ortholog is associated with orofacial clefting syndrome, in the morphogenesis of epithelial tissues. Most notably, when opo is mutated in medaka fish, optic cup folding is impaired. We characterize optic cup morphogenesis in vivo and determine at the cellular level how opo affects this process. opo encodes a developmentally regulated transmembrane protein that localizes to compartments of the secretory pathway and to basal end-feet of the neuroepithelial precursors. We show that Opo regulates the polarized localization of focal adhesion components to the basal cell surface. Furthermore, tissue-specific interference with integrin-adhesive function impairs optic cup folding, resembling the ocular phenotype observed in opo mutants. We propose a model of retinal morphogenesis whereby opo-mediated formation of focal contacts is required to transmit the mechanical tensions that drive the macroscopic folding of the vertebrate optic cup.

Integrin-ECM interactions regulate the changes in cell shape driving the morphogenesis of the Drosophila wing epithelium

During development, morphogenesis involves migration and changes in the shape of epithelial sheets, both of which require coordination of cell adhesion. Thus, while modulation of integrin-mediated adhesion to the ECM regulates epithelial motility, cell-cell adhesion via cadherins controls the remodelling of epithelial sheets. We have used the Drosophila wing epithelium to demonstrate that cell-ECM interactions mediated by integrins also regulate the changes in cell shape that underly epithelial morphogenesis. We show that integrins control the transitions from columnar to cuboidal cell shape underlying wing formation, and we demonstrate that eliminating the ECM has the same effect on cell shape as inhibiting integrin function. Furthermore, lack of integrin activity also induces detachment of the basal lamina and failure to assemble the basal matrix. Hence, we propose that integrins control epithelial cell shape by mediating adherence of these cells to the ECM. Finally, we show that the ECM has an instructive rather than a structural role, because inhibition of Raf reverses the cell shape changes caused by perturbing integrins.

The Ste20 kinase misshapen is essential for the invasive behaviour of ovarian epithelial cells in Drosophila

Stationary‐to‐migratory transitions of epithelial cells have a key role in development and tumour progression. Border cell migration is a powerful system in which to investigate this transition in living organisms. Here, we identify the Ste20‐like kinase misshapen (msn) as a novel regulator of border‐cell migration in Drosophila. Expression of msn in border cells is independent of the transcription factor slow border cells and of inputs from all pathways that are known to control border‐cell migration. The msn gene functions to modulate the levels and/or distribution of Drosophila E‐cadherin to promote the invasive migratory behaviour of border cells.

Integrins contribute to the establishment and maintenance of cell polarity in the follicular epithelium of the Drosophila ovary

The generation of epithelial cell polarity is a key process during development. Although the induction and orientation of cell polarity by cell-cell and cell-extracellular matrix (ECM) interactions is well established, the molecular mechanisms by which signals from the ECM control cell polarity in developing epithelial tissues remain poorly understood. Here, we have used the follicular epithelium of the Drosophila ovary to investigate the role that integrins, the main cell-ECM receptors, play in the establishment of apicobasal polarity. Mature follicle cells have an apical side facing the germ line and a basal side in contact with a basement membrane. Our results show that integrins - presumably via interactions with the basement membrane - play a reinforcing role in follicle cell polarization, as they are required to establish and/or maintain follicle cell membrane asymmetry only when contact with the germ line is prevented. We suggest that the primary cue for polarization of the follicular epithelium is contact with the germline cells. In addition, while interfering with apical and lateral polarization cues leads to apoptosis, we show here that inhibition of contact with the basement membrane mediated by integrins does not affect cell survival. Finally, we provide evidence to suggest that integrins are required to orientate epithelial polarity in vivo.

Myosin light-chain phosphatase regulates basal actomyosin oscillations during morphogenesis.

Contractile actomyosin networks generate forces that drive tissue morphogenesis. Actomyosin contractility is controlled primarily by reversible phosphorylation of the myosin-II regulatory light chain through the action of myosin kinases and phosphatases. While the role of myosin light-chain kinase in regulating contractility during morphogenesis has been largely characterized, there is surprisingly little information on myosin light-chain phosphatase (MLCP) function in this context. Here, we use live imaging of Drosophila follicle cells combined with mathematical modelling to demonstrate that the MLCP subunit flapwing (flw) is a key regulator of basal myosin oscillations and cell contractions underlying egg chamber elongation. Flw expression decreases specifically on the basal side of follicle cells at the onset of contraction and flw controls the initiation and periodicity of basal actomyosin oscillations. Contrary to previous reports, basal F-actin pulsates similarly to myosin. Finally, we propose a quantitative model in which periodic basal actomyosin oscillations arise in a cell-autonomous fashion from intrinsic properties of motor assemblies.

Scutoids are a geometrical solution to three-dimensional packing of epithelia

As animals develop, tissue bending contributes to shape the organs into complex three-dimensional structures. However, the architecture and packing of curved epithelia remains largely unknown. Here we show by means of mathematical modelling that cells in bent epithelia can undergo intercalations along the apico-basal axis. This phenomenon forces cells to have different neighbours in their basal and apical surfaces. As a consequence, epithelial cells adopt a novel shape that we term “scutoid”. The detailed analysis of diverse tissues confirms that generation of apico-basal intercalations between cells is a common feature during morphogenesis. Using biophysical arguments, we propose that scutoids make possible the minimization of the tissue energy and stabilize three-dimensional packing. Hence, we conclude that scutoids are one of natures solutions to achieve epithelial bending. Our findings pave the way to understand the three-dimensional organization of epithelial organs.Cell arrangement in the plane of epithelia is well studied, but its three-dimensional packing is largely unknown. Here the authors model curved epithelia and predict that cells adopt a geometrical shape they call “scutoid”, resulting in different apical and basal neighbours, and confirm the presence of scutoids in curved tissues.

DrosAfrica: Building an African biomedical research community using Drosophila

The impact that research has on shaping the future of societies is perhaps as significant as never before. One of the problems for most regions in Africa is poor quality and quantity of research-based education, as well as low level of funding. Hence, African researchers produce only around one percent of the worlds research. We believe that research with Drosophila melanogaster can contribute to changing that. As seen before in other places, Drosophila can be used as a powerful and cost-effective model system to scale-up and improve both academia and research output. The DrosAfrica project was founded to train and establish a connected community of researchers using Drosophila as a model system to investigate biomedical problems in Africa. Since founding, the project has trained eighty scientists from numerous African countries, and continues to grow. Here, we describe the DrosAfrica project, its conception and its mission. We also give detailed insights into DrosAfricas approaches to achieve its aims, as well as future perspectives, and opportunities beyond Africa.

Drosophila Embryonic Hemocytes Produce Laminins to Strengthen Migratory Response

Summary The most prominent developmental function attributed to the extracellular matrix (ECM) is cell migration. While cells in culture can produce ECM to migrate, the role of ECM in regulating developmental cell migration is classically viewed as an exogenous matrix presented to the moving cells. In contrast to this view, we show here that Drosophila embryonic hemocytes deposit their own laminins in streak-like structures to migrate efficiently throughout the embryo. With the help of transplantation experiments, live microscopy, and image quantification, we demonstrate that autocrine-produced laminin regulates hemocyte migration by controlling lamellipodia dynamics, stability, and persistence. Proper laminin deposition is regulated by the RabGTPase Rab8, which is highly expressed and required in hemocytes for lamellipodia dynamics and migration. Our results thus support a model in which, during embryogenesis, the Rab8-regulated autocrine deposition of laminin reinforces directional and effective migration by stabilizing cellular protrusions and strengthening otherwise transient adhesion states.