wing blister, a new Drosophila laminin alpha chain required for cell adhesion and migration during embryonic and imaginal development

Extracellular matrix in development: insights from mechanisms conserved between invertebrates and vertebrates

The extracellular matrix (ECM) and its receptors make diverse contributions to development. The ECM comes in a variety of forms, including the more "standard" ECM that is internal to the animal and on the basal side of epithelial sheets, as well as the apical ECM, which is especially elaborated in the invertebrates to form the exoskeleton. ECM proteins accumulate adjacent to particular target tissues in the developing animal by a variety of mechanisms: local synthesis in the target tissue; local synthesis by migrating cells; and secretion from a distant source and capture by the target tissue. The diverse developmental functions of the ECM are discussed, including the generation of a road for cell migration, creation of morphogenetic checkpoints for differentiation, modulation of morphogen gradients, insulation of organs, gluing together cell layers, and providing structure for the organism.

Extracellular matrix and its receptors in Drosophila neural development

Extracellular matrix (ECM) and matrix receptors are intimately involved in most biological processes. The ECM plays fundamental developmental and physiological roles in health and disease, including processes underlying the development, maintenance, and regeneration of the nervous system. To understand the principles of ECM-mediated functions in the nervous system, genetic model organisms like Drosophila provide simple, malleable, and powerful experimental platforms. This article provides an overview of ECM proteins and receptors in Drosophila. It then focuses on their roles during three progressive phases of neural development: (1) neural progenitor proliferation, (2) axonal growth and pathfinding, and (3) synapse formation and function. Each section highlights known ECM and ECM-receptor components and recent studies done in mutant conditions to reveal their in vivo functions, all illustrating the enormous opportunities provided when merging work on the nervous system with systematic research into ECM-related gene functions.

PS integrins and laminins: key regulators of cell migration during Drosophila embryogenesis

During embryonic development, there are numerous cases where organ or tissue formation depends upon the migration of primordial cells. In the Drosophila embryo, the visceral mesoderm (vm) acts as a substrate for the migration of several cell populations of epithelial origin, including the endoderm, the trachea and the salivary glands. These migratory processes require both integrins and laminins. The current model is that αPS1βPS (PS1) and/or αPS3βPS (PS3) integrins are required in migrating cells, whereas αPS2βPS (PS2) integrin is required in the vm, where it performs an as yet unidentified function. Here, we show that PS1 integrins are also required for the migration over the vm of cells of mesodermal origin, the caudal visceral mesoderm (CVM). These results support a model in which PS1 might have evolved to acquire the migratory function of integrins, irrespective of the origin of the tissue. This integrin function is highly specific and its specificity resides mainly in the extracellular domain. In addition, we have identified the Laminin α1,2 trimer, as the key extracellular matrix (ECM) component regulating CVM migration. Furthermore, we show that, as it is the case in vertebrates, integrins, and specifically PS2, contributes to CVM movement by participating in the correct assembly of the ECM that serves as tracks for migration.

Moleskin is essential for the formation of the myotendinous junction in Drosophila

It is the precise connectivity between skeletal muscles and their corresponding tendon cells to form a functional myotendinous junction (MTJ) that allows for the force generation required for muscle contraction and organismal movement. The Drosophila MTJ is composed of secreted extracellular matrix (ECM) proteins deposited between integrin-mediated hemi-adherens junctions on the surface of muscle and tendon cells. In this paper, we have identified a novel, cytoplasmic role for the canonical nuclear import protein Moleskin (Msk) in Drosophila embryonic somatic muscle attachment. Msk protein is enriched at muscle attachment sites in late embryogenesis and msk mutant embryos exhibit a failure in muscle-tendon cell attachment. Although the muscle-tendon attachment sites are reduced in size, components of the integrin complexes and ECM proteins are properly localized in msk mutant embryos. However, msk mutants fail to localize phosphorylated focal adhesion kinase (pFAK) to the sites of muscle-tendon cell junctions. In addition, the tendon cell specific proteins Stripe (Sr) and activated mitogen-activated protein kinase (MAPK) are reduced in msk mutant embryos. Our rescue experiments demonstrate that Msk is required in the muscle cell, but not in the tendon cells. Moreover, muscle attachment defects due to loss of Msk are rescued by an activated form of MAPK or the secreted epidermal growth factor receptor (Egfr) ligand Vein. Taken together, these findings provide strong evidence that Msk signals non-autonomously through the Vein-Egfr signaling pathway for late tendon cell late differentiation and/or maintenance.

Loss of PTB or negative regulation of Notch mRNA reveals distinct zones of Notch and actin protein accumulation in Drosophila embryo

Polypyrimidine Tract Binding (PTB) protein is a regulator of mRNA processing and translation. Genetic screens and studies of wing and bristle development during the post-embryonic stages of Drosophila suggest that it is a negative regulator of the Notch pathway. How PTB regulates the Notch pathway is unknown. Our studies of Drosophila embryogenesis indicate that (1) the Notch mRNA is a potential target of PTB, (2) PTB and Notch functions in the dorso-lateral regions of the Drosophila embryo are linked to actin regulation but not their functions in the ventral region, and (3) the actin-related Notch activity in the dorso-lateral regions might require a Notch activity at or near the cell surface that is different from the nuclear Notch activity involved in cell fate specification in the ventral region. These data raise the possibility that the Drosophila embryo is divided into zones of different PTB and Notch activities based on whether or not they are linked to actin regulation. They also provide clues to the almost forgotten role of Notch in cell adhesion and reveal a role for the Notch pathway in cell fusions.

Born to run: creating the muscle fiber

From the muscles that control the blink of your eye to those that allow you to walk, the basic architecture of muscle is the same: muscles consist of bundles of the unit muscle cell, the muscle fiber. The unique morphology of the individual muscle fiber is dictated by the functional demands necessary to generate and withstand the forces of contraction, which in turn leads to movement. Contractile muscle fibers are elongated, syncytial cells, which interact with both the nervous and skeletal systems to govern body motion. In this review, we focus on three key cell-cell and cell-matrix contact processes, that are necessary to create this exquisitely specialized cell: cell fusion, cell elongation, and establishment of a myotendinous junction. We address these processes by highlighting recent findings from the Drosophila model system.

Characterization of the laminin gene family and evolution in zebrafish

Laminins are essential components of all basement membranes and are fundamental to tissue development and homeostasis. Humans possess at least 16 different heterotrimeric laminin complexes formed through different combinations of alpha, beta, and gamma chains. Individual chains appear to exhibit unique expression patterns, leading to the notion that overlap between expression domains governs the constitution of complexes found within particular tissues. However, the spatial and temporal expression of laminin genes has not been comprehensively analyzed in any vertebrate model to date. Here, we describe the tissue-specific expression patterns of all laminin genes in the zebrafish, throughout embryonic development and into the "post-juvenile" period, which is representative of the adult body form. In addition, we present phylogenetic and microsynteny analyses, which demonstrate that the majority of our zebrafish sequences are orthologous to human laminin genes. Together, these data represent a fundamental resource for the study of vertebrate laminins.

Hemocyte-secreted type IV collagen enhances BMP signaling to guide renal tubule morphogenesis in Drosophila

Details of the mechanisms that determine the shape and positioning of organs in the body cavity remain largely obscure. We show that stereotypic positioning of outgrowing Drosophila renal tubules depends on signaling in a subset of tubule cells and results from enhanced sensitivity to guidance signals by targeted matrix deposition. VEGF/PDGF ligands from the tubules attract hemocytes, which secrete components of the basement membrane to ensheath them. Collagen IV sensitizes tubule cells to localized BMP guidance cues. Signaling results in pathway activation in a subset of tubule cells that lead outgrowth through the body cavity. Failure of hemocyte migration, loss of collagen IV, or abrogation of BMP signaling results in tubule misrouting and defective organ shape and positioning. Such regulated interplay between cell-cell and cell-matrix interactions is likely to have wide relevance in organogenesis and congenital disease.

Connecting muscles to tendons: tendons and musculoskeletal development in flies and vertebrates

The formation of the musculoskeletal system represents an intricate process of tissue assembly involving heterotypic inductive interactions between tendons, muscles and cartilage. An essential component of all musculoskeletal systems is the anchoring of the force-generating muscles to the solid support of the organism: the skeleton in vertebrates and the exoskeleton in invertebrates. Here, we discuss recent findings that illuminate musculoskeletal assembly in the vertebrate embryo, findings that emphasize the reciprocal interactions between the forming tendons, muscle and cartilage tissues. We also compare these events with those of the corresponding system in the Drosophila embryo, highlighting distinct and common pathways that promote efficient locomotion while preserving the form of the organism.

Basement membranes in development and disease

Basement membranes (BMs) are specializations of the extracellular matrix that act as key mediators of development and disease. Their sheet like protein matrices typically serve to separate epithelial or endothelial cell layers from underlying mesenchymal tissues, providing both a biophysical support to overlying tissue as well as a hub to promote and regulate cell-cell and cell-protein interactions. In the latter context, the BM is increasingly being recognized as a mediator of growth factor interactions during development. In this review, we discuss recent findings regarding the structure of the BM and its roles in mediating the normal development of the embryo, and we examine congenital diseases affecting the BM which impact embryonic development and health in later life.

Increased apoptosis of myoblasts in Drosophila model for the Walker-Warburg syndrome

Walker-Warburg syndrome, a progressive muscular dystrophy, is a severe disease with various kinds of symptoms such as muscle weakness and occasional seizures. The genes of protein O-mannosyltransferases 1 and 2 (POMT1 and POMT2), fukutin, and fukutin-related protein are responsible for this syndrome. In our previous study, we cloned Drosophila orthologs of human POMT1 and POMT2 and identified their activity. However, the mechanism of onset of this syndrome is not well understood. Furthermore, little is known about the behavioral properties of the Drosophila POMT1 and POMT2 mutants, which are called rotated abdomen (rt) and twisted (tw), respectively. First, we performed various kinds of behavioral tests and described in detail the muscle structures by using these mutants. The mutant flies exhibited abnormalities in heavy exercises such as climbing or flight but not in light movements such as locomotion. Defective motor function in mutants appeared immediately after eclosion and was exaggerated with aging. Along with motor function, muscle ultrastructure in the tw mutant was altered, as seen in human patients. We demonstrated that expression of RNA interference (RNAi) for the rt gene and the tw mutant was almost completely lethal and semi-lethal, respectively. Flies expressing RNAi had reduced lifespans. These findings clearly demonstrate that Drosophila POMT mutants are models for human muscular dystrophy. We then observed a high density of myoblasts with an enhanced degree of apoptosis in the tw mutant, which completely lost enzymatic activity. In this paper, we propose a novel mechanism for the development of muscular dystrophy: POMT mutation causes high myoblast density and position derangement, which result in apoptosis, muscle disorganization, and muscle cell defects.

Tenectin is a novel alphaPS2betaPS integrin ligand required for wing morphogenesis and male genital looping in Drosophila

Morphogenesis of the adult structures of holometabolous insects is regulated by ecdysteroids and juvenile hormones and involves cell-cell interactions mediated in part by the cell surface integrin receptors and their extracellular matrix (ECM) ligands. These adhesion molecules and their regulation by hormones are not well characterized. We describe the gene structure of a newly described ECM molecule, tenectin, and demonstrate that it is a hormonally regulated ECM protein required for proper morphogenesis of the adult wing and male genitalia. Tenectin's function as a new ligand of the PS2 integrins is demonstrated by both genetic interactions in the fly and by cell spreading and cell adhesion assays in cultured cells. Its interaction with the PS2 integrins is dependent on RGD and RGD-like motifs. Tenectin's function in looping morphogenesis in the development of the male genitalia led to experiments that demonstrate a role for PS integrins in the execution of left-right asymmetry.

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.

Integrins during evolution: evolutionary trees and model organisms

The integrins form a large family of cell adhesion receptors. All multicellular animals express integrins, indicating that the family evolved relatively early in the history of metazoans, and homologous sequences of the component domains of integrin alpha and beta subunits are seen in prokaryotes. Some integrins, however, seem to be much younger. For example, the alphaI domain containing integrins, including collagen receptors and leukocyte integrins, have been found in chordates only. Here, we will discuss what conclusions can be drawn about integrin function by studying the evolutionary conservation of integrins. We will also look at how studying integrins in organisms such as the fruit fly and mouse has helped our understanding of integrin evolution-function relationships. As an illustration of this, we will summarize the current understanding of integrin involvement in skeletal muscle formation.

Fak56 functions downstream of integrin alphaPS3betanu and suppresses MAPK activation in neuromuscular junction growth

Background

Focal adhesion kinase (FAK) functions in cell migration and signaling through activation of the mitogen-activated protein kinase (MAPK) signaling cascade. Neuronal function of FAK has been suggested to control axonal branching; however, the underlying mechanism in this process is not clear.

Results

We have generated mutants for the Drosophila FAK gene, Fak56. Null Fak56 mutants display overgrowth of larval neuromuscular junctions (NMJs). Localization of phospho-FAK and rescue experiments suggest that Fak56 is required in presynapses to restrict NMJ growth. Genetic analyses imply that FAK mediates the signaling pathway of the integrin αPS3βν heterodimer and functions redundantly with Src. At NMJs, Fak56 downregulates ERK activity, as shown by diphospho-ERK accumulation in Fak56 mutants, and suppression of Fak56 mutant NMJ phenotypes by reducing ERK activity.

Conclusion

We conclude that Fak56 is required to restrict NMJ growth during NMJ development. Fak56 mediates an extracellular signal through the integrin receptor. Unlike its conventional role in activating MAPK/ERK, Fak56 suppresses ERK activation in this process. These results suggest that Fak56 mediates a specific neuronal signaling pathway distinct from that in other cellular processes.

Mutations in the Drosophila alphaPS2 integrin subunit uncover new features of adhesion site assembly

The Drosophila alphaPS2betaPS integrin is required for diverse development events, including muscle attachment. We characterized six unusual mutations in the alphaPS2 gene that cause a subset of the null phenotype. One mutation changes a residue in alphaPS2 that is equivalent to the residue in alphaV that contacts the arginine of RGD. This change severely reduced alphaPS2betaPS affinity for soluble ligand, abolished the ability of the integrin to recruit laminin to muscle attachment sites in the embryo and caused detachment of integrins and talin from the ECM. Three mutations that alter different parts of the alphaPS2 beta-propeller, plus a fourth that eliminated a late phase of alphaPS2 expression, all led to a strong decrease in alphaPS2betaPS at muscle ends, but, surprisingly, normal levels of talin were recruited. Thus, although talin recruitment requires alphaPS2betaPS, talin levels are not simply specified by the amount of integrin at the adhesive junction. These mutations caused detachment of talin and actin from integrins, suggesting that the integrin-talin link is weaker than the ECM-integrin link.

AlphaPS2 integrin-mediated muscle attachment in Drosophila requires the ECM protein Thrombospondin

During Drosophila embryogenesis, the attachment of somatic muscles to epidermal tendon cells requires heterodimeric PS-integrin proteins (alpha- and beta-subunits). The alpha-subunits are expressed complementarily, either tendon cell- or muscle-specific, whereas the beta-integrin subunit is expressed in both tissues. Mutations of beta-integrin cause a severe muscle detachment phenotype, whereas alpha-subunit mutations have weaker or only larval muscle detachment phenotypes. Furthermore, mutations of extracellular matrix (ECM) proteins known to act as integrin binding partners have comparatively weak effects only, suggesting the presence of additional integrin binding ECM proteins required for proper muscle attachment. Here, we report that mutations in the Drosophila gene thrombospondin (tsp) cause embryonic muscle detachment. tsp is specifically expressed in both developing and mature epidermal tendon cells. Its initial expression in segment border cells, the tendon precursors, is under the control of hedgehog-dependent signaling, whereas tsp expression in differentiated tendon cells depends on the transcription factor encoded by stripe. In the absence of tsp activity, no aspect of muscle pattern formation as well as the initial contact between muscle and tendon cells nor muscle-to-muscle attachments are affected. However, when muscle contractions occur during late embryogenesis, muscles detach from the tendon cells. The Tsp protein is localized to the tendon cell ECM where muscles attach. Genetic interaction studies indicate that Tsp specifically interacts with the alphaPS2 integrin and that this interaction is needed to withstand the forces of muscle contractions at the tendon cells.

Thrombospondin-mediated adhesion is essential for the formation of the myotendinous junction in Drosophila

Organogenesis of the somatic musculature in Drosophila is directed by the precise adhesion between migrating myotubes and their corresponding ectodermally derived tendon cells. Whereas the PS integrins mediate the adhesion between these two cell types, their extracellular matrix (ECM) ligands have been only partially characterized. We show that the ECM protein Thrombospondin (Tsp), produced by tendon cells, is essential for the formation of the integrin-mediated myotendinous junction. Tsp expression is induced by the tendon-specific transcription factor Stripe, and accumulates at the myotendinous junction following the association between the muscle and the tendon cell. In tsp mutant embryos, migrating somatic muscles fail to attach to tendon cells and often form hemiadherens junctions with their neighboring muscle cells, resulting in nonfunctional somatic musculature. Talin accumulation at the cytoplasmic faces of the muscles and tendons is greatly reduced, implicating Tsp as a potential integrin ligand. Consistently, purified Tsp C-terminal domain polypeptide mediates spreading of PS2 integrin-expressing S2 cells in a KGD- and PS2-integrin-dependent manner. We propose a model in which the myotendinous junction is formed by the specific association of Tsp with multiple muscle-specific PS2 integrin receptors and a subsequent consolidation of the junction by enhanced tendon-specific production of Tsp secreted into the junctional space.

Heart development in Drosophila

The Drosophila heart, also called the dorsal vessel, is an organ for hemolymph circulation that resembles the vertebrate heart at its transient linear tube stage. Dorsal vessel morphogenesis shares several similarities with early events of vertebrate heart development and has proven to be an insightful system for the study of cardiogenesis due to its relatively simple structure and the productive use of Drosophila genetic approaches. In this review, we summarize published findings on Drosophila heart development in terms of the regulators and genetic pathways required for cardiac cell specification and differentiation, and organ formation and function. Emerging genome-based strategies should further facilitate the use of Drosophila as an advantageous system in which to identify previously unknown genes and regulatory networks essential for normal cardiac development and function.

Tissue-specific laminin expression facilitates integrin-dependent association of the embryonic wing disc with the trachea in Drosophila

The interaction of heterologous tissues involves cell adhesion mediated by the extracellular matrix and its receptor integrins. The Drosophila wing disc is an ectodermal invagination that contacts specific tracheal branches at the basolateral cell surface. We show that an alpha subunit of laminin, encoded by wing blister (wb), is essential for the establishment of the interaction between the wing and trachea. During embryogenesis, wing disc cells present Wb at their basolateral surface and extend posteriorly, expanding their association to more posteriorly located tracheal branches. These migratory processes are impaired in the absence of the trachea, Wb, or integrins. Time-lapse and transmission electron microscopy analyses suggest that Wb facilitates integrin-dependent contact over a large surface and controls the cellular behavior of the wing cells, including their exploratory filopodial activity. Our data identify Wb laminin as an extracellular matrix ligand that is essential for integrin-dependent cellular migration in Drosophila.

Slit coordinates cardiac morphogenesis in Drosophila

Slit is a secreted guidance cue that conveys repellent or attractive signals from target and guidepost cells. In Drosophila, responsive cells express one or more of three Robo receptors. The cardial cells of the developing heart express both Slit and Robo2. This is the first report of coincident expression of a Robo and its ligand. In slit mutants, cardial cell alignment, polarization and uniform migration are disrupted. The heart phenotype of robo2 mutants is similar, with fewer migration defects. In the guidance of neuronal growth cones in Drosophila, there is a phenotypic interaction between slit and robo heterozygotes, and also with genes required for Robo signaling. In contrast, in the heart, slit has little or no phenotypic interaction with Robo-related genes, including Robo2, Nck2, and Disabled. However, there is a strong phenotypic interaction with Integrin genes and their ligands, including Laminin and Collagen, and intracellular messengers, including Talin and ILK. This indicates that Slit participates in adhesion or adhesion signaling during heart development.

The Drosophila melanogaster T-box genes midline and H15 are conserved regulators of heart development

The Drosophila melanogaster genes midline and H15 encode predicted T-box transcription factors homologous to vertebrate Tbx20 genes. All identified vertebrate Tbx20 genes are expressed in the embryonic heart and we find that both midline and H15 are expressed in the cardioblasts of the dorsal vessel, the insect organ equivalent to the vertebrate heart. The midline mRNA is first detected in dorsal mesoderm at embryonic stage 12 in the two progenitors per hemisegment that will divide to give rise to all six cardioblasts. Expression of H15 mRNA in the dorsal mesoderm is detected first in four to six cells per hemisegment at stage 13. The expression of midline and H15 in the dorsal vessel is dependent on Wingless signaling and the transcription factors tinman and pannier. We find that the selection of two midline-expressing cells from a pool of competent progenitors is dependent on Notch signaling. Embryos deleted for both midline and H15 have defects in the alignment of the cardioblasts and associated pericardial cells. Embryos null for midline have weaker and less penetrant phenotypes while embryos deficient for H15 have morphologically normal hearts, suggesting that the two genes are partially redundant in heart development. Despite the dorsal vessel defects, embryos mutant for both midline and H15 have normal numbers of cardioblasts, suggesting that cardiac cell fate specification is not disrupted. However, ectopic expression of midline in the dorsal mesoderm can lead to dramatic increases in the expression of cardiac markers, suggesting that midline and H15 participate in cardiac fate specification and may normally act redundantly with other cardiogenic factors. Conservation of Tbx20 expression and function in cardiac development lends further support for a common ancestral origin of the insect dorsal vessel and the vertebrate heart.

Dose-sensitive autosomal modifiers identify candidate genes for tissue autonomous and tissue nonautonomous regulation by the Drosophila nuclear zinc-finger protein, hindsight

The nuclear zinc-finger protein encoded by the hindsight (hnt) locus regulates several cellular processes in Drosophila epithelia, including the Jun N-terminal kinase (JNK) signaling pathway and actin polymerization. Defects in these molecular pathways may underlie the abnormal cellular interactions, loss of epithelial integrity, and apoptosis that occurs in hnt mutants, in turn causing failure of morphogenetic processes such as germ band retraction and dorsal closure in the embryo. To define the genetic pathways regulated by hnt, 124 deficiencies on the second and third chromosomes and 14 duplications on the second chromosome were assayed for dose-sensitive modification of a temperature-sensitive rough eye phenotype caused by the viable allele, hntpeb; 29 interacting regions were identified. Subsequently, 438 P-element-induced lethal mutations mapping to these regions and 12 candidate genes were tested for genetic interaction, leading to identification of 63 dominant modifier loci. A subset of the identified mutants also dominantly modify hnt308-induced embryonic lethality and thus represent general rather than tissue-specific interactors. General interactors include loci encoding transcription factors, actin-binding proteins, signal transduction proteins, and components of the extracellular matrix. Expression of several interactors was assessed in hnt mutant tissue. Five genes--apontic (apt), Delta (Dl), decapentaplegic (dpp), karst (kst), and puckered (puc)--are regulated tissue autonomously and, thus, may be direct transcriptional targets of HNT. Three of these genes--apt, Dl, and dpp--are also regulated nonautonomously in adjacent non-HNT-expressing tissues. The expression of several additional interactors--viking (vkg), Cg25, and laminin-alpha (LanA)-is affected only in a nonautonomous manner.

Laminin functions in tissue morphogenesis

Significant advances have been made in the application of genetics to probe the functions of basement membrane laminins. These studies have shown that different laminin subunits profoundly affect tissue morphogenesis, starting around the time of embryonic implantation and extending through organogenesis and into the postnatal period. Collectively they have revealed common functions that include the induction and maintenance of cell polarity, the establishment of barriers between tissue compartments, the organization of cells into tissues, and the protection of adherent cells from detachment-induced cell death, anoikis. Interpreted in light of what is known about laminin structure and self-assembly and binding activities, these advances have begun to provide insights into mechanisms of action. In this review we focus on the contributions of the laminins in invertebrate and vertebrate tissue morphogenesis.

Papillote and Piopio: Drosophila ZP-domain proteins required for cell adhesion to the apical extracellular matrix and microtubule organization

Adhesion between epithelial cells and extracellular substrates is normally mediated through basal adhesion complexes. However, some cells also possess comparable junctions on their apical surface. Here, we describe two new Drosophila proteins, Piopio and Papillote, that are required for the link between the apical epithelial surface and the overlying apical extracellular matrix (aECM). The two proteins share a zona pellucida (ZP) domain with mammalian aECM components, including the tectorins found in the vertebrate inner ear. Tagged versions of both proteins localized to the apical epithelial surface. Mutations in piopio, papillote and dumpy (another gene encoding a ZP-domain protein) cause defects in the innermost layer of the aECM and its detachment from the epidermis. Loss of Piopio, but not Papillote or Dumpy, causes the absence of specialized microtubule bundles from pupal wings, suggesting that Piopio plays a role in microtubule organization. Thus, ZP domain-containing proteins may have shared functions within the aECM, while also exhibiting specific interactions with the cytoskeleton.

Syndecan-dependent binding of Drosophila hemocytes to laminin alpha3/5 chain LG4-5 modules: potential role in sessile hemocyte islets formation

Heparin-column chromatography and elastase-digestion of medium from hemocyte Kc167 gave Drosophila laminin alpha3/5betagamma trimer, alpha3/5LG2-3 and alpha3/5LG4-5 modules with eluting NaCl concentrations of 450, 280 and 450 mM, respectively. Kc167 cells bound dish surface with alpha3/5betagamma trimer or alpha3/5LG4-5, but not with alpha3/5LG2-3 modules. Cell binding was counteracted by treating with heparin or heparan sulfate. RNA interference of syndecan in Kc167 cells impaired the binding, but that of dally or dally-like did not. Green fluorescent protein-expressing hemocytes also bound surface with alpha3/5betagamma trimer or alpha3/5LG4-5 module. Thus, syndecan-dependent binding of hemocytes to laminin may have a potential role in sessile hemocytes islets formation in T2-A8 segments of Drosophila.

Molecular and comparative genetics of mental retardation

Affecting 1-3% of the population, mental retardation (MR) poses significant challenges for clinicians and scientists. Understanding the biology of MR is complicated by the extraordinary heterogeneity of genetic MR disorders. Detailed analyses of >1000 Online Mendelian Inheritance in Man (OMIM) database entries and literature searches through September 2003 revealed 282 molecularly identified MR genes. We estimate that hundreds more MR genes remain to be identified. A novel test, in which we distributed unmapped MR disorders proportionately across the autosomes, failed to eliminate the well-known X-chromosome overrepresentation of MR genes and candidate genes. This evidence argues against ascertainment bias as the main cause of the skewed distribution. On the basis of a synthesis of clinical and laboratory data, we developed a biological functions classification scheme for MR genes. Metabolic pathways, signaling pathways, and transcription are the most common functions, but numerous other aspects of neuronal and glial biology are controlled by MR genes as well. Using protein sequence and domain-organization comparisons, we found a striking conservation of MR genes and genetic pathways across the approximately 700 million years that separate Homo sapiens and Drosophila melanogaster. Eighty-seven percent have one or more fruit fly homologs and 76% have at least one candidate functional ortholog. We propose that D. melanogaster can be used in a systematic manner to study MR and possibly to develop bioassays for therapeutic drug discovery. We selected 42 Drosophila orthologs as most likely to reveal molecular and cellular mechanisms of nervous system development or plasticity relevant to MR.

Assembly and tissue functions of early embryonic laminins and netrins

Vertebrate laminins and netrins share N-terminal domain structure, but appear to be only distantly related. Both families can be divided into different subfamilies on the basis of structural considerations. Recent observations suggest that specific laminin and netrin members have developmental functions that are highly conserved across species. Vertebrate laminin-1 (alpha1beta1gamma1) and laminin-10 (alpha5beta1gamma1), like the two Caenorhabditis elegans laminins, are embryonically expressed and are essential for basement membrane assembly. Basement membrane assembly is a cooperative process in which laminins polymerize through their LN domains and anchor to the cell surface through their G domains; this leads to cell signaling through integrins and dystroglycan (and possibly other receptors) recruited to the adherent laminin. Netrins may associate with this network through heterotypic LN domain interactions. Vertebrate netrin-1, like invertebrate UNC-6/netrins, is well known as an extracellular guidance cue that directs axon migration towards or away from the ventral midline. It also regulates cell adhesions and migrations, probably as a basement membrane component. Although sharing structural features, these two vertebrate protein families are quite distinct, having both retained members that mediate the ancestral developmental functions.

Active sites in the carboxyl-terminal region of the laminin alpha chain in Drosophila neuronal cell spreading

An established Drosophila neuronal cell line (BG2-c6) proved to be useful to analyze laminin-mediated cell spreading and signal transduction [Takagi et al. (2000) Biochem Biophys Res Commun 270:482-487]. Here, we report, in addition to the whole molecule, the truncated alpha chain of Drosophila laminin (containing the entire carboxyl-terminal globular domain) and two dodecapeptides corresponding to the cell-binding sites identified in the alpha1 chain of mouse laminin were also active to stimulate BG2-c6 cell spreading. Our previous study [Takagi et al. (1996) J Biol Chem 271:18074-18081] revealed that these recombinant protein and synthetic peptides promoted neurite outgrowth in the primary cell culture system prepared from Drosophila embryo. Therefore, the similar effects by these proteins and peptides suggest the presence of a common mechanism of laminin and neuronal cell interaction working in both primary and established cells. One of the two active peptides contains the sequence SIKVGV. Its murine counterpart carries the sequence SIKVAV by which the interaction of laminin and cells is mediated. Furthermore, laminin-dependent BG2-c6 cell spreading was inhibited by heparin. This observation suggests that cell surface glycoproteins participate in the interaction of laminin and BG2-c6 cells.

Identification of integrin beta subunit mutations that alter heterodimer function in situ

We conducted a genetic screen for mutations in myospheroid, the gene encoding the Drosophila betaPS integrin subunit, and identified point mutants in all of the structural domains of the protein. Surprisingly, we find that mutations in very strongly conserved residues will often allow sufficient integrin function to support the development of adult animals, including mutations in the ADMIDAS site and in a cytoplasmic NPXY motif. Many mutations in the I-like domain reduce integrin expression specifically when betaPS is combined with activating alphaPS2 cytoplasmic mutations, indicating that integrins in the extended conformation are unstable relative to the inactive, bent heterodimers. Interestingly, the screen has identified alleles that show gain-of-function characteristics in cell culture, but have negative effects on animal development or viability. This is illustrated by the allele mys(b58); available structural models suggest that the molecular lesion of mys(b58), V409>D, should promote the "open" conformation of the beta subunit I-like domain. This expectation is supported by the finding that alphaPS2betaPS (V409>D) promotes adhesion and spreading of S2 cells more effectively than does wild-type alphaPS2betaPS, even when betaPS is paired with alphaPS2 containing activating cytoplasmic mutations. Finally, comparisons with the sequence of human beta8 suggest that evolution has targeted the "mys(b58)" residue as a means of affecting integrin activity.

Novel Functions for Integrins in Epithelial Morphogenesis

Dorsal closure during Drosophila embryogenesis provides a valuable model for epithelial morphogenesis and wound healing. Previous studies have focused on two cell populations, the dorsal epidermis and the extraembryonic amnioserosa. Here, we demonstrate that there is an additional player, the large yolk cell. We find that integrins are expressed in the amnioserosa and yolk cell membrane and that they are required for three processes: (1) assembly of an intervening extracellular matrix, (2) attachment between these two cell layers, and (3) contraction of the amnioserosa cells. We also provide evidence for integrin-extracellular matrix interactions occurring between the lateral surfaces of the amnioserosa cell and the leading edge epidermis that effectively mediate cell-cell adhesion. Thus, dorsal closure shares mechanistic similarities with vertebrate epithelial morphogenetic events, including epiboly, that also employ an underlying substrate.

Molecular and Comparative Genetics of Mental Retardation

Affecting 1-3% of the population, mental retardation (MR) poses significant challenges for clinicians and scientists. Understanding the biology of MR is complicated by the extraordinary heterogeneity of genetic MR disorders. Detailed analyses of >1000 Online Mendelian Inheritance in Man (OMIM) database entries and literature searches through September 2003 revealed 282 molecularly identified MR genes. We estimate that hundreds more MR genes remain to be identified. A novel test, in which we distributed unmapped MR disorders proportionately across the autosomes, failed to eliminate the well-known X-chromosome overrepresentation of MR genes and candidate genes. This evidence argues against ascertainment bias as the main cause of the skewed distribution. On the basis of a synthesis of clinical and laboratory data, we developed a biological functions classification scheme for MR genes. Metabolic pathways, signaling pathways, and transcription are the most common functions, but numerous other aspects of neuronal and glial biology are controlled by MR genes as well. Using protein sequence and domain-organization comparisons, we found a striking conservation of MR genes and genetic pathways across the ∼700 million years that separate Homo sapiens and Drosophila melanogaster. Eighty-seven percent have one or more fruit fly homologs and 76% have at least one candidate functional ortholog. We propose that D. melanogaster can be used in a systematic manner to study MR and possibly to develop bioassays for therapeutic drug discovery. We selected 42 Drosophila orthologs as most likely to reveal molecular and cellular mechanisms of nervous system development or plasticity relevant to MR.

Egfr signalling defines a protective function for ommatidial orientation in the Drosophila eye

Ommatidial rotation in the Drosophila eye provides a striking example of the precision with which tissue patterning can be achieved. Ommatidia in the adult eye are aligned at right angles to the equator, with dorsal and ventral ommatidia pointing in opposite directions. This pattern is established during disc development, when clusters rotate through 90 degrees, a process dependent on planar cell polarity and rotation-specific factors such as Nemo and Scabrous. Here, we demonstrate a requirement for epidermal growth factor receptor (Egfr) signalling in rotation, further adding to the manifold actions of this pathway in eye development. Egfr is distinct from other rotation factors in that the initial process is unaffected, but orientation in the adult is greatly disrupted when signalling is abnormal. We propose that Egfr signalling acts in the third instar imaginal disc to 'lock' ommatidia in their final position, and that in its absence, ommatidial orientation becomes disrupted during the remodelling of the larval disc into an adult eye. This lock may be achieved by a change in the adhesive properties of the cells: cadherin-based adhesion is important for ommatidia to remain in their appropriate positions. In addition, we have evidence that there is an error-correction mechanism operating during pupal stages to reposition inappropriately orientated ommatidia. Our results suggest that initial patterning events are not sufficient to achieve the precise architecture of the fly eye, and highlight a novel requirement for error-correction, and for an Egfr-dependent protection function to prevent morphological disruption during tissue remodelling.

Laminin alpha subunits and their role in C. elegans development

Laminins are heterotrimeric (alpha/beta/gamma) glycoproteins that form a major polymer within basement membranes. Different alpha, beta and gamma subunits can assemble into various laminin isoforms that have different, but often overlapping, distributions and functions. In this study, we examine the contributions of the laminin alpha subunits to the development of C. elegans. There are two alpha, one beta and one gamma laminin subunit, suggesting two laminin isoforms that differ by their alpha subunit assemble in C. elegans. We find that near the end of gastrulation and before other basement membrane components are detected, the alpha subunits are secreted between primary tissue layers and become distributed in different patterns to the surfaces of cells. Mutations in either alpha subunit gene cause missing or disrupted extracellular matrix where the protein normally localizes. Cell-cell adhesions are abnormal: in some cases essential cell-cell adhesions are lacking, while in other cases, cells inappropriately adhere to and invade neighboring tissues. Using electron microscopy, we observe adhesion complexes at improper cell surfaces and disoriented cytoskeletal filaments. Cells throughout the animal show defective differentiation, proliferation or migration, suggesting a general disruption of cell-cell signaling. The results suggest a receptor-mediated process localizes each secreted laminin to exposed cell surfaces and that laminin is crucial for organizing extracellular matrix, receptor and intracellular proteins at those surfaces. We propose this supramolecular architecture regulates adhesions and signaling between adjacent tissues.

Laminin-10 is crucial for hair morphogenesis

The role of the extracellular matrix in cutaneous morphogenesis is poorly understood. Here, we describe the essential role of laminin-10 (alpha5beta1gamma1) in hair follicle development. Laminin-10 was present in the basement membrane of elongating hair germs, when other laminins were downregulated, suggesting a role for laminin-10 in hair development. Treatment of human scalp xenografts with antibodies to laminin-10, or its receptor beta1 integrin, produced alopecia. E16.5 Lama5 -/- mouse skin, lacking laminin-10, contained fewer hair germs compared with controls, and after transplantation, Lama5 -/- skin showed a failure of hair germ elongation followed by complete hair follicle regression. Lama5 -/- skin showed defective basement membrane assembly, without measurable increases in anoikis. Instead, Lama5 -/- skin showed decreased expression of early hair markers including sonic hedgehog and Gli1, implicating laminin-10 in developmental signaling. Intriguingly, treatment of Lama5 -/- skin with purified laminin-10 corrected basement membrane defects and restored hair follicle development. We conclude that laminin-10 is required for hair follicle development and report the first use of exogenous protein to correct a cutaneous developmental defect.

The role of laminin in embryonic cell polarization and tissue organization

Genetic analyses have revealed that members of the laminin glycoprotein family are required for basement membrane assembly and cell polarization, with subsequent effects on cell survival and tissue organization during metazoan embryogenesis. These functions depend upon the cooperation between laminin polymerization and cell anchorage mediated via interactions with beta1-integrins, dystroglycan, and other cell surface receptors.

Retraction of the Drosophila germ band requires cell-matrix interaction

Integrins and laminins are important mediators of cell-matrix interactions in both vertebrates and invertebrates. Here, we show that germ-band retraction in the Drosophila embryo, during which the tail end of the embryo retracts to its final posterior position, allows the investigation of cell spreading and lamellipodia formation in real time in vivo. We demonstrate that alpha1, 2 laminin and alphaPS3betaPS integrin are required for the spreading of a small group of cells of the amnioserosa epithelium over the tail end of the germ band. We further implicate a role for this spreading in the process of germ-band retraction.

Migrating mesoderm establish a uniform distribution of laminin in the developing grasshopper embryo

The basal lamina is composed of molecules which physically interact to form a network that serves as a migrational scaffold for many cell types. In the developing peripheral nervous system of the grasshopper, neuronal growth cones are intimately associated with the basal lamina as they migrate. Laminin is a major component of the basal lamina and is a potent promoter of neurite outgrowth in vitro. However, it is unclear what the source of laminin is or how the distribution of laminin within the basal lamina is established. To address this question, grasshopper laminin subunit genes were cloned. As expected, laminin was found within the basal lamina throughout the embryo, in particular in the limb bud, where its expression is coincident with the outgrowth and guidance of the Tibial (Til) pioneer neurons. Surprisingly, the synthesis of beta and gamma chains of laminin was restricted to migratory mesodermal cells, while in other nonmigratory tissues, such as epithelium and presumptive muscle, beta and gamma chains of laminin were not detected. In spite of this, laminin immunoreactivity in the basal lamina appears uniform and is available as a substrate for axonal outgrowth.

Pericardin, aDrosophilatype IV collagen-like protein is involved in the morphogenesis and maintenance of the heart epithelium during dorsal ectoderm closure

The steps that lead to the formation of a single primitive heart tube are highly conserved in vertebrate and invertebrate embryos. Concerted migration of the two lateral cardiogenic regions of the mesoderm and endoderm (or ectoderm in invertebrates) is required for their fusion at the midline of the embryo. Morphogenetic signals are involved in this process and the extracellular matrix has been proposed to serve as a link between the two layers of cells.

Pericardin (Prc), a novel Drosophila extracellular matrix protein is a good candidate to participate in heart tube formation. The protein has the hallmarks of a type IV collagen α-chain and is mainly expressed in the pericardial cells at the onset of dorsal closure. As dorsal closure progresses, Pericardin expression becomes concentrated at the basal surface of the cardioblasts and around the pericardial cells, in close proximity to the dorsal ectoderm. Pericardin is absent from the lumen of the dorsal vessel.Genetic evidence suggests that Prc promotes the proper migration and alignment of heart cells. Df(3)vin6 embryos, as well as embryos in which prc has been silenced via RNAi, exhibit similar and significant defects in the formation of the heart epithelium. In these embryos, the heart epithelium appears disorganized during its migration to the dorsal midline. By the end of embryonic development, cardial and pericardial cells are misaligned such that small clusters of both cell types appear in the heart; these clusters of cells are associated with holes in the walls of the heart. A prc transgene can partially rescue each of these phenotypes, suggesting that prc regulates these events. Our results support, for the first time, the function of a collagen-like protein in the coordinated migration of dorsal ectoderm and heart cells.

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

Drosophila laminin alphabetagamma trimer assembly in Kc167 cells was perturbed by chain-specific RNA interference (RNAi). The intracellular pool of alpha and gamma chains remained unchanged under beta-chain RNAi by lipofection of double-stranded RNA encoding a beta-chain partial sequence. This was also the case for the intracellular pool of alpha and beta chains under gamma-chain-specific RNAi. Nonetheless, the intracellular pool of beta and gamma chains increased markedly under alpha-chain-specific RNAi. Non-reducing SDS/PAGE revealed that some of the increased beta and gamma chains migrated as disulphide-linked betagamma dimers but that the rest migrated as monomers. Since the monomeric beta and gamma bands detected under alpha-chain RNAi were denser than the beta band under gamma-chain RNAi and the gamma band under beta-chain RNAi, respectively, beta and gamma also appeared to accumulate by forming betagamma dimers without the disulphide linkage. We suggest that interconversion of these betagamma dimers is crucial for the replaceable and selective assembly of the alpha chain for alphabetagamma trimer formation.

Laminins of the neuromuscular system

The mammalian neuromuscular system expresses seven laminin genes (alpha 1, alpha 2, alpha 4, alpha 5, beta 1, beta 2, and gamma 1), produces seven isoforms of the laminin trimer (laminins 1, 2, 4, 8, 9, 10, and 11), and distributes these trimers to at least seven distinct basal laminae (perineurial, endoneurial, terminal Schwann cell, myotendinous junction, synaptic cleft, synaptic fold, and extrajunctional muscle). The patterns of expression, assembly, and distribution are regulated during development, and primary and secondary changes in laminin expression occur in several neuromuscular genetic disorders. Functional studies using knockout and transgenic mice, and purified laminins and cell types, demonstrate that laminins are required components of basal laminae in the neuromuscular system. Collectively, laminins have both structural and signaling functions; individually, laminin isoforms have unique roles in regulating the behavior of nerve, muscle, and Schwann cell. Among them, laminin-2 (alpha 2 beta 1 gamma 1) plays an important structural role in supporting the muscle plasma membrane, laminin-4 regulates adhesion and differentiation of the myotendinous junction, and laminin-11 regulates nerve terminal differentiation and Schwann cell motility. Together, these observations reveal remarkable diversity in the formation and function of laminins and basal laminae, and suggest avenues for addressing some neuromuscular diseases.

An integrin chicken and egg problem: which comes first, the extracellular matrix or the cytoskeleton?

Integrins have the ability to organise macromolecular structures both inside and outside the cell. Analysis of integrin function in the developing embryos of worms and flies suggests that, although the extracellular matrix directs integrins to organise intracellular proteins, the cytoskeleton may have the first word.

Still more complexity in mammalian basement membranes

At the epithelial/mesenchymal interface of most tissues lies the basement membrane (BM). These thin sheets of highly specialized extracellular matrix vary in composition in a tissue-specific manner, and during development and repair. For about two decades it has been apparent that all BMs contain laminins, entactin-1/nidogen-1, Type IV collagen, and proteoglycans. However, within the past few years this complexity has increased as new components are described. The entactin/nidogen (E/N) family has expanded with the recent description of a new isoform, E/N-2/osteonidogen. Agrin and Type XVIII collagen have been reclassified as heparan sulfate proteoglycans (HSPGs), expanding the repertoire of HSPGs in the BM. The laminin family has become more diverse as new alpha-chains have been characterized, increasing the number of laminin isoforms. Interactions between BM components are now appreciated to be regulated through multiple, mostly domain-specific mechanisms. Understanding the functions of individual BM components and their assembly into macromolecular complexes is a considerable challenge that may increase as further BM and cell surface ligands are discovered for these proteins.

Cell-cell adhesion via the ECM: integrin genetics in fly and worm

Integrins are essential for the development of the two genetically tractable invertebrate model organisms, the nematode worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster. Just two integrins are present in C. elegans: one putative RGD binding integrin alphapat-2betapat-3, corresponding to Drosophila alphaPS2betaPS and vertebrate alpha5beta1, alphaVbeta1 and alpha8beta1, and one putative laminin binding integrin alphaina-1betapat-3, corresponding to Drosophila alphaPS1betaPS and vertebrate alpha3beta1, alpha6beta1 and alpha7beta1. In this review, the function of this minimal set of integrins during the development of these two invertebrates is compared. Despite the differences in bodyplan and developmental strategy, integrin adhesion to the extracellular matrix is required for similar processes: the formation of the link that translates muscle contraction into movement of the exoskeleton, cell migration, and morphogenetic interactions between epithelia. Other integrin functions, such as regulation of gene expression, have not yet been experimentally demonstrated in both organisms. Additional proteins have been characterised in each organism that are essential for integrin function, including extracellular matrix ligands and intracellular interacting proteins, but so far different proteins have been found in the two organisms. This in part represents the fact that the characterisation of the full set of interacting proteins is not complete in either system. However, in other cases different proteins appear to be used for similar functions in the two animals. The continued use of genetic approaches to identify proteins required for integrin function in these two model organisms should lead to the identification of the minimal set of conserved components that form integrin adhesive structures.

Inflated wings, tissue autolysis and early death in tissue inhibitor of metalloproteinases mutants of Drosophila

In vertebrates, tissue inhibitors of metalloproteinases (TIMPs) play key roles in extracellular matrix (ECM) homeostasis and growth control. Deletion of the recently cloned Timp gene of Drosophila results in a subviable phenotype. Adult flies display inflated wings similar to integrin mutants, suffer from a bloated gut and progressive dissolution of internal tissues, and die prematurely. Our results demonstrate that the Timp gene product controls selective aspects of ECM function in Drosophila, and suggest that it is involved in cell adhesion/cell signaling pathways. Hence, Drosophila Timp mutants may prove useful as a model system for a wide variety of pathological conditions related to ECM dysregulation.

Perlecan domain V of Drosophila melanogaster. Sequence, recombinant analysis and tissue expression

The C-terminal domain V of the basement membrane proteoglycan perlecan was previously shown to play a major role in extracellular matrix and cell interactions. A homologous sequence of 708 amino-acid residues from Drosophila has now been shown to be 33% identical to mouse perlecan domain V. It consists of three laminin G-type (LG) and epidermal growth factor-like (EG) modules but lacks the EG3 module and a link region found in mammalian perlecans. Recombinant production of Drosophila perlecan domain V in mammalian cells yielded a 100-kDa protein which was folded into a linear array of three globular LG domains. Unlike the mouse counterpart, domain V from Drosophila was not modified by glycosaminoglycans and endogenous proteolysis, due to the absence of the link region. It showed moderate affinities for heparin and sulfatides but did not bind to chick alpha-dystroglycan or to various mammalian basement membrane proteins. A single RGD sequence in LG3 of Drosophila domain V was also incapable of mediating cell adhesion. Production of a proteoglycan form of perlecan (approximately 450 kDa) in one Drosophila cell line could be demonstrated by immunoblotting with antibodies against Drosophila domain V. A strong expression was also found by in situ hybridization and immunohistology at various stages of embryonic development and expression was localized to several basement membrane zones. This indicates, as for mammalian species, a distinct role of perlecan during Drosophila development.

Form and function: the laminin family of heterotrimers

The laminins are a family of glycoproteins that provide an integral part of the structural scaffolding of basement membranes in almost every animal tissue. Each laminin is a heterotrimer assembled from alpha, beta, and gamma chain subunits, secreted and incorporated into cell-associated extracellular matrices. The laminins can self-assemble, bind to other matrix macromolecules, and have unique and shared cell interactions mediated by integrins, dystroglycan, and other receptors. Through these interactions, laminins critically contribute to cell differentiation, cell shape and movement, maintenance of tissue phenotypes, and promotion of tissue survival. Recent advances in the characterization of genetic disruptions in humans, mice, nematodes and flies have revealed developmental roles for the different laminin subunits in diverse cell types, affecting differentiation from blastocyst formation to the post-natal period. These genetic defects have challenged some of the previous concepts about basement membranes and have shed new light on the diversity and complexity of laminin functions as well as established the molecular basis of several human diseases.