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.

Regulation of laminin 1-induced pancreatic beta-cell differentiation by alpha6 integrin and alpha-dystroglycan

BACKGROUND

The ability to manipulate the development of pancreatic insulin-producing beta cells has implications for the treatment of type 1 diabetes. Previously, we found that laminin-1, a basement membrane trimeric glycoprotein, promotes beta-cell differentiation. We have investigated the mechanism of this effect, using agents that block the receptors for laminin-1, alpha6 integrin, and alpha-dystroglycan (alpha-DG).

MATERIALS AND METHODS

Dissociated cells from 13.5-day postcoitum (dpc) fetal mouse pancreas were cultured for 4 days with laminin-1, with and without monoclonal antibodies and other agents known to block integrins or alpha-DG. Fetuses fixed in Bouin's solution or fetal pancreas cells fixed in 4% paraformaldehyde were processed for routine histology and for immunohistology to detect hormone expression and bromodeoxyuridine (BrdU) uptake.

RESULTS

Blocking the binding of laminin-1 to alpha6 integrin with a monoclonal antibody, GoH3, abolished cell proliferation (BrdU uptake) and doubled the number of beta cells. Inhibition of molecules involved in alpha6 integrin signaling (phosphotidylinositol 3-kinase, F-actin, or mitogen-activated protein kinase) had a similar effect. Nevertheless, beta cells appeared to develop normally in alpha6 integrin-deficient fetuses. Blocking the binding of laminin-1 to alpha-DG with a monoclonal antibody, IIH6, dramatically decreased the number of beta cells. Heparin, also known to inhibit laminin-1 binding to alpha-DG, had a similar effect. In the presence of heparin, the increase in beta cells in response to blocking alpha integrin with GoH3 was abolished.

CONCLUSIONS

These findings reveal an interplay between alpha6 integrin and alpha-DG to regulate laminin-1-induced beta-cell development. Laminin-I had a dominant effect via alpha-DG to promote cell survival and beta-cell differentiation, which was modestly inhibited by alpha6 signaling.

Integrin and ECM functions: roles in vertebrate development

The analysis of mutant mice is bringing novel insights on the role of extracellular matrix (ECM) and integrin receptors during a variety of physiological processes, including embryonic development. The requirement of these adhesion molecules in epithelial morphogenesis or histogenesis in organs such as kidneys and lungs, in limbs, and in the development of mesoderm and the nervous system has been unraveled by the study of single or compound mutants. Their role in tissue integrity has also been highlighted. Models have been produced that should prove very useful in defining the cellular mechanisms and the functions of integrins and ECM signaling cascades in vivo.

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.

Synergistic activities of alpha3 and alpha6 integrins are required during apical ectodermal ridge formation and organogenesis in the mouse

Integrins alpha6beta1 and alpha6beta4 are cell surface receptors for laminins. Integrin alpha6-null mice die at birth with severe skin blistering and defects in the cerebral cortex and in the retina. Integrin alpha3beta1 can associate with laminins and other ligands. Integrin alpha3-null mice also die at birth, with kidney and lung defects at late stages of development, and moderate skin blistering. To investigate possible overlapping functions between alpha3 and alpha6 integrins, we analyzed the phenotype of compound alpha3−/−/alpha6−/− mutant embryos. Double homozygous mutant embryos were growth-retarded and displayed several developmental defects not observed in the single mutant animals. First, limb abnormalities characterized by an absence of digit separation and the fusion of preskeletal elements were observed. Further analyses indicated a defect in the apical ectodermal ridge, an essential limb organizing center. In the double mutant, the ridge appeared flattened, and ridge cells did not show a columnar morphology. A strong reduction in ridge cell proliferation and alterations of the basal lamina underlying the ectoderm were observed. These results suggest that alpha3 and alpha6 integrins are required for the organization or compaction of presumptive apical ectodermal ridge cells into a distinct differentiated structure. Additional defects were present: an absence of neural tube closure, bilateral lung hypoplasia, and several abnormalities in the urogenital tract. Finally, an aggravation of brain and eye lamination defects was observed. The presence of novel phenotypes in double mutant embryos demonstrates the synergism between alpha3 and alpha6 integrins and their essential roles in multiple processes during embryogenesis.

Mouse primordial germ cells lacking beta1 integrins enter the germline but fail to migrate normally to the gonads

Primordial germ cells are the founder cells of the gametes. They are set aside at the initial stages of gastrulation in mammals, become embedded in the hind-gut endoderm, then actively migrate to the sites of gonad formation. The molecular basis of this migration is poorly understood. Here we sought to determine if members of the integrin family of cell surface receptors are required for primordial germ cell migration, as integrins have been implicated in the migration of several other motile cell types. We have established a line of mice which express green fluorescent protein in germline cells that has enabled us to efficiently purify primordial germ cells at different stages by flow cytometry. We have catalogued the spectrum of integrin subunit expression by primordial germ cells during and after migration, using flow cytometry, immunocytochemistry and RT-PCR. Through analysis of integrin beta1(−/−)-->wild-type chimeras, we show that embryonic cells lacking beta1 integrins can enter the germline. However, integrin beta1(−/−) primordial germ cells do not colonize the gonad efficiently. Embryos with targeted deletion of integrin subunit alpha3, alpha6, or alphaV show no major defects in primordial germ cell migration. These results demonstrate a role for beta1-containing integrins in the development of the germline, although an equivalent role for * integrin subunit(s) has yet to be established.

Essential role of alpha 6 integrins in cortical and retinal lamination

Extracellular matrix (ECM) is believed to play important roles in many aspects of nervous system development [1]. The laminins are ECM glycoproteins expressed in neural tissues and are potent stimulators of neurite outgrowth in vitro [1-3]. Genetic approaches using Drosophila and Caenorhabditis elegans have demonstrated a role for laminin and a laminin receptor in vivo in axon pathfinding and fasciculation, respectively [4,5]. In higher organisms, however, the role of laminins in the development of the nervous system is poorly understood. Integrins alpha 6 beta 1 and alpha 6 beta 4 are major laminin receptors. A role for the alpha 6 integrin in neurulation has been reported in amphibians [6]. We previously described mice lacking integrin alpha 6; these mice died at birth with severe skin blistering [7]. Detailed analyses of integrin alpha 6-/- mice reported here revealed abnormalities in the laminar organization of the developing cerebral cortex and retina. Ectopic neuroblastic outgrowths were found on the brain surface and in the vitreous body in the eye. Alterations of laminin deposition were found in mutant brains. Thus, this study provides evidence for an essential role of integrin-laminin interactions in the proper development of the nervous system. These observations are particularly significant given the recent report that human patients suffering from epidermolysis bullosa can carry mutations in ITGA6, the gene encoding the alpha 6 integrin chain [8,9].

Genetic analyses of integrin function in mice

Recent mutations of most integrin genes in the mouse have provided new exciting insights into the role of these integrins in cell-extracellular matrix interactions during development. The embryonic lethal phenotypes obtained by ablating integrins which are predominantly expressed in the mesenchyme confirmed the essential function of those integrins in morphogenesis. In contrast, null alleles for several epithelial integrins which bind components of basement membranes showed milder phenotypes, suggesting the presence of novel and unexpected redundant and compensatory mechanisms.

Absence of integrin alpha 6 leads to epidermolysis bullosa and neonatal death in mice

Cell-extracellular matrix interactions have important roles in many biological processes, including embryonic development, growth control and differentiation. Integrins are the principal receptors for extracellular matrix. They are composed of non-covalently associated alpha and beta chains. Integrin alpha 6 can associate with either beta 1 or beta 4 (refs 2,3). Both integrin complexes are receptors for laminins, major components of basement membranes. The distribution of alpha 6 (refs 4-10) as well as studies using function-blocking antibodies have suggested an essential role for this laminin receptor during embryogenesis, in processes such as endoderm migration or kidney tubule formation9. Here we report that, surprisingly, mice lacking the alpha 6 integrin chain develop to birth. However, they die at birth with severe blistering of the skin and other epithelia, a phenotype reminiscent of the human disorder epidermolysis bullosa. Hemidesmosomes are absent in mutant tissue. This absence is likely to result from the lack of alpha 6/beta 4, the only integrin in hemidesmosomes of stratified squamous and transitional epithelia. Mutations in the genes encoding integrin beta 4 and chains of laminin-5 have been implicated in junctional epidermolysis bullosa. Our study provides evidence that some forms of epidermolysis bullosa may originate from defects of the alpha 6 gene.