Partial rescue of epithelial phenotype in integrin beta4 null mice by a keratin-5 promoter driven human integrin beta4 transgene

β1 integrin is a crucial regulator of pancreatic β-cell expansion

Development of the endocrine compartment of the pancreas, as represented by the islets of Langerhans, occurs through a series of highly regulated events encompassing branching of the pancreatic epithelium, delamination and differentiation of islet progenitors from ductal domains, followed by expansion and three-dimensional organization into islet clusters. Cellular interactions with the extracellular matrix (ECM) mediated by receptors of the integrin family are postulated to regulate key functions in these processes. Yet, specific events regulated by these receptors in the developing pancreas remain unknown. Here, we show that ablation of the β1 integrin gene in developing pancreatic β-cells reduces their ability to expand during embryonic life, during the first week of postnatal life, and thereafter. Mice lacking β1 integrin in insulin-producing cells exhibit a dramatic reduction of the number of β-cells to only ∼18% of wild-type levels. Despite the significant reduction in β-cell mass, these mutant mice are not diabetic. A thorough phenotypic analysis of β-cells lacking β1 integrin revealed a normal expression repertoire of β-cell markers, normal architectural organization within islet clusters, and a normal ultrastructure. Global gene expression analysis revealed that ablation of this ECM receptor in β-cells inhibits the expression of genes regulating cell cycle progression. Collectively, our results demonstrate that β1 integrin receptors function as crucial positive regulators of β-cell expansion.

Epithelial responses to lung injury: role of the extracellular matrix

The key role of extracellular matrices in alveolar epithelial cell (AEC) biology is highlighted by the phenotypes of primary AECs cultured on a soft laminin gel contrasted with that on a stiff, fibronectin matrix. On laminin, AECs maintain an epithelial phenotype, and progenitor cells within this population proliferate. In contrast, on fibronectin, AECs rapidly lose surfactant expression and spread extensively, changes that depend on activation of latent TGF-β1 by engagement of fibronectin-binding integrins. The progenitor subpopulation responding to TGF-β1 undergoes epithelial mesenchymal transition (EMT). Although it remains uncertain to what degree EMT contributes directly to collagen 1 production, signaling pathways critical to EMT are important for repair and fibrosis, implying that EMT is part of the general program of lung repair. EMT reprogramming requires not only Smad signaling but also pY654-β-catenin. Generation of pY654-β-catenin requires assembly of complexes of the integrin α3β1, E-cadherin, and TGF-β1 receptors, and such assembly is a function of cell-cell and cell-matrix contacts. Sequestration of α3β1 or E-cadherin in such contacts prevents complex assembly, TGF-β1 induced pY654-β-catenin generation and EMT. Disruption of these contacts is a signal for the cells to initiate repair. Critical remaining questions center around better definition of direct versus indirect effects of EMT on collagen deposition and the nature of AEC progenitors differentiating during fibrogenesis. Elucidation of specific inhibitors of EMT should further test the question of whether the process is important to fibrosis in vivo and a viable therapeutic target.

Animal models of epidermolysis bullosa

For more than 2 decades, animal models have been used to clarify the pathogenic mechanisms of human diseases and develop new therapeutics for these diseases. Several therapies for human diseases have become available through trials using animal models. Epidermolysis bullosa (EB) is one of the most severe inherited skin disorders, whose effective treatments have not been fully available. EB is characterized by abnormalities of the proteins that consist of the dermoepidermal junction. EB has been classified into three major subtypes according to the level of skin cleavage: EB simplex, junctional EB, and dystrophic EB. To date, 13 genes have been shown to cause EB phenotype. After the discovery of the causative genes responsible for each EB subtype, many researchers have tried to develop EB animal models by genetically manipulating the corresponding genes.

Laminin-511 is an epithelial message promoting dermal papilla development and function during early hair morphogenesis

Hair morphogenesis takes place through reciprocal epithelial and mesenchymal signaling; however, the mechanisms controlling signal exchange are poorly understood. Laminins are extracellular proteins that play critical roles in adhesion and signaling. Here we demonstrate the mechanism of how laminin-511 controls hair morphogenesis. Dermal papilla (DP) from laminin-511 mutants showed developmental defects by E16.5, including a failure to maintain expression of the key morphogen noggin. This maintenance was critical as exogenous introduction of noggin or sonic hedgehog (Shh) produced downstream from noggin was sufficient to restore hair follicle development in lama5(-/-) (laminin-511-null) skin. Hair development required the beta1 integrin binding but not the heparin binding domain of laminin-511. Previous studies demonstrated that Shh signaling requires primary cilia, microtubule-based signaling organelles. Laminin-511 mutant DP showed decreased length and structure of primary cilia in vitro and in vivo. Laminin-511, but not laminin-111, restored primary cilia formation in lama5(-/-) mesenchyme and triggered noggin expression in an Shh- and PDGF-dependent manner. Inhibition of laminin-511 receptor beta1 integrin disrupted DP primary cilia formation as well as hair development. These studies show that epithelial-derived laminin-511 is a critical early signal that directs ciliary function and DP maintenance as a requirement for hair follicle downgrowth.

Reduced expression of the epithelial adhesion ligand laminin 5 in the skin causes intradermal tissue separation

Laminin 5, the major keratinocyte adhesion ligand, is found in the lamina lucida subregion of the epidermal basement membrane of the skin, where it colocalizes with the anchoring filaments. Mutations in the genes encoding laminin 5 cause junctional epidermolysis bullosa, an inherited skin blistering disease characterized by abnormal hemidesmosomes and cleavage of the lamina lucida leading to epidermal detachment. In this work we describe the genetic basis of a new subtype of lethal inherited epidermolysis bullosa associated with reduced skin reactivity to laminin 5, presence of mature hemidesmosomes, and intradermal cleavage of the skin. The epidermolysis bullosa patients were heterozygous for a nonsense mutation (Q896X) and a splice site mutation (764-10T-->G) in the gene (LAMC2) for the gamma2 chain of laminin 5. The nonsense mutation causes accelerated decay of the corresponding mRNA, while the splice site mutation results in maturation of a cryptic wild-type gamma2 mRNA leading to reduced expression of wild-type laminin 5. In vitro studies using the probands' keratinocytes showed that secretion of reduced amounts of functional laminin 5 in the patient, although permitting formation of hemidesmosomes, fail to restore efficient cell adhesion. Our results provide the first evidence that laminin 5 contributes to the firm adhesion of the epithelial basement membrane to the underlying stroma. They also show that a low expression level of laminin 5 induces assembly of mature hemidesmosomes in vivo but fails to assure a stable cohesion of the dermal-epidermal junction.

Epithelial development and differentiation in the mammary gland is not dependent on alpha 3 or alpha 6 integrin subunits

In the mammary gland, both laminin and integrins have been shown to be required for normal ductal morphogenesis during development in vivo, and for functional differentiation in culture models. Major integrin receptors for laminins in the mammary gland are alpha 3 beta 1, alpha 6 beta 1, and alpha 6 beta 4. However, the specific subunits that contribute to laminin-mediated mammary cell function and development have not been identified. In this study, we use a genetic approach to test the hypothesis that laminin-binding integrins are required for the function of the mammary gland in vivo. Rudiments of embryonic mammary gland were shown to develop in the absence of these integrin subunits. Postnatal development of the mammary gland was studied in integrin null tissue that had been transplanted into the mammary fat pads of syngeneic hosts. In mammary epithelium lacking alpha 6 integrin, the beta 4 subunit was not apparent and hemidesmosome formation was only rudimentary. However, despite this deficiency, normal ductal morphogenesis and branching of the mammary gland occurred and myoepithelial cells were distributed normally with respect to luminal cells. Mammary alveoli devoid of alpha 3 or alpha 6 integrin formed in pregnancy and were histologically and functionally identical to those in wild-type mammary gland. The tissue underwent full morphological differentiation, and the epithelial cells retained the ability to synthesize beta-casein. This work demonstrates that mammary tissue genetically lacking major laminin-binding integrin receptors is still able to develop and function.

Cutaneous gene transfer and therapy: the present and the future

The easy accessibility of the skin as a therapeutic target provides an exciting potential for this organ for the development of gene therapy protocols for cutaneous diseases and a variety of metabolic disorders. Thus far, full phenotypic reversion of a diseased phenotype has been achieved in vivo for junctional epidermolysis bullosa and X-linked or lamellar ichthyosis and in vitro for xeroderma pigmentosum. These recessive skin diseases are characterized by skin blistering, abnormalities in epidermal differentiation and increased development of skin cancers, respectively. Corrective gene delivery at both molecular and functional levels was achieved by transduction of cultured skin cells using retroviral vectors carrying the specific curative cDNA. These positive results should prompt clinical trials based on transplantation of artificial epithelia reconstructed ex vivo using genetically modified keratinocytes. Promising results have also been obtained in phenotypic reversion of cells isolated from patients suffering from a number of metabolic diseases such as gyrate atrophy, familial hypercholesterolemia or phenylketonuria. In these diseases transplantation of autologous artificial epithelia expressing the transgenes of interest or direct transfer of the DNA to the skin represents a potential therapeutic approach for the systemic delivery of active molecules. Successful cutaneous gene therapy trials, however, require development of protocols for efficient gene transfer to epidermal stem cells, and information about the host immune response to the recombinant polypeptides produced by the implanted keratinocytes. The availability of spontaneous animal models for genodermatoses will validate the gene therapy approach in preclinical trials.

alpha3beta1 and alpha6beta4 integrin receptors for laminin-5 are not essential for epidermal morphogenesis and homeostasis during skin development

Continuous regeneration and homeostasis of the stratified epidermis requires coordinated regulation of cell proliferation, cell differentiation, and cell survival. Integrin-mediated cell adhesion to the extracellular matrix has important roles in regulating each of these processes. Integrins alpha3beta1 and alpha6beta4 are both receptors on epidermal keratinocytes for the basement membrane protein laminin-5, the major ligand for epidermal adhesion in mature skin. Ablation in mice of either alpha3beta1 or alpha6beta4, through null mutation of the gene encoding the alpha3, alpha6, or beta4 integrin subunit, results in epidermal blistering of varying severity. Our previous studies showed that, despite blistering, differentiation and stratification of the epidermis appeared essentially normal in mice that lacked either alpha3beta1 or alpha6beta4. However, these studies did not definitively address the specific developmental importance of each integrin, since they may have overlapping and/or compensatory functions. Given the individual importance of alpha3beta1 or alpha6beta4 in maintaining the dermo-epidermal junction in mature skin, we sought to determine the importance of these integrins for embryonic skin development and epidermal morphogenesis. In the current study, we analyzed skin development in mutant embryos that completely lack both integrins alpha3beta1 and alpha6beta4. Although alpha3beta1/alpha6beta4-deficient embryos displayed epidermal blistering by stage E15.5 of development, they also retained regions of extensive epidermal adhesion to the basement membrane through stage E16.5, indicating alternative adhesion mechanisms. Apoptosis was induced in detached epidermis of alpha3beta1/alpha6beta4-deficient embryos, exemplifying vividly the importance of epithelial attachment to the basement membrane for cell survival. However, apoptotic cells were completely absent from attached epidermis of alpha3beta1/alpha6beta4-deficient embryos, showing that epithelial adhesion that occurred independently of alpha3beta1 and alpha6beta4 also protected cells from apoptosis. Remarkably, in the absence of the known laminin-5 binding integrins (alpha3beta1, alpha6beta4, and alpha6beta1), keratinocytes retained the capacity to proliferate in the epidermis, and epidermal stratification and skin morphogenesis appeared normal prior to blister formation. These findings show that while alpha3beta1 and alpha6beta4 are both required for integrity of the dermo-epidermal junction, neither one is essential for epidermal morphogenesis during skin development.