Structure, genetic localization, and identification of the cardiac and skeletal muscle transcripts of the human integrin alpha7 gene (ITGA7)

A redox-dependent thiol-switch and a Ca2+ binding site within the hinge region hierarchically depend on each other in α7β1 integrin regulation

Integrin-mediated cell contacts with the extracellular matrix (ECM) are essential for cellular adhesion, force transmission, and migration. Several effectors, such as divalent cations and redox-active compounds, regulate ligand binding activities of integrins and influence their cellular functions. To study the role of the Ca²⁺ binding site within the hinge region of the integrin α7 subunit, we genetically abrogated it in the α7hiΔCa mutant. This mutant folded correctly, associated with the β1 subunit and was exposed on the cell surface, but showed reduced ligand binding and weaker cell adhesion to laminin-111. Thus, it resembles the α7hiΔSS mutant, in which the redox-regulated pair of cysteines, closeby to the Ca²⁺ binding site within the hinge, was abrogated. Comparing both mutants in adhesion strength and cell migration revealed that both Ca²⁺ complexation and redox-regulation within the hinge interdepend on each other. Moreover, protein-chemical analyses of soluble integrin ectodomains containing the same α7 hinge mutations suggest that integrin activation via the subunit α hinge is primed by the formation of the cysteine pair-based crosslinkage. Then, this allows Ca²⁺ complexation within the hinge, which is another essential step for integrin activation and ligand binding. Thus, the α hinge is an allosteric integrin regulation site, in which both effectors, Ca²⁺ and redox-active compounds, synergistically and hierarchically induce far-ranging conformational changes, such as the extension of the integrin ectodomain, resulting in integrin activation of ECM ligand binding and altered integrin-mediated cell functions.

Integrin α7 high expression correlates with deteriorative tumor features and worse overall survival, and its knockdown inhibits cell proliferation and invasion but increases apoptosis in breast cancer

Background

This study aimed to investigate the correlation of integrin α7 (ITGA7) expression with clinical/pathological characteristics and overall survival (OS), and its knockdown on inhibiting cell activities in breast cancer.

Methods

A total of 191 breast cancer patients underwent surgery were retrospectively reviewed, and ITGA7 expression in tumor tissues was determined by immunofluorescence and real‐time quantitative polymerase chain reaction. Patients’ clinical/pathological data were recorded, and OS was calculated. In vitro, control shRNA and ITGA7 shRNA plasmids were transfected into MCF7 cells to evaluate the influence of ITGA7 knockdown on cell proliferation, apoptosis, and invasion.

Results

Ninety‐two (48.2%) patients presented with ITGA7 high expression, and 99 patients (51.8%) presented with ITGA7 low expression. ITGA7 expression was positively correlated with T stage, tumor‐node metastasis (TNM) stage, and pathological grade. Kaplan‐Meier curves showed that ITGA7 high expression was associated with shorter OS, and multivariate Cox's proportional hazards regression displayed that ITGA7 high expression was an independent predictive factor for poor OS. Moreover, in vitro experiments disclosed that cell proliferation (by Cell Counting Kit‐8 assay) and cell invasion (by Matrigel invasion assay) were reduced, while cell apoptosis rate (by Annexin V/propidium iodide assay) was enhanced by ITGA7 knockdown in MCF‐7 cells.

Conclusion

Integrin α7 high expression correlates with increased T stage, TNM stage, and pathological grade as well as worse OS, and its knockdown enhances cell apoptosis but inhibits cell proliferation and invasion in breast cancer.

Cell-matrix interactions in muscle disease

The extracellular matrix (ECM) provides a solid scaffold and signals to cells through ECM receptors. The cell-matrix interactions are crucial for normal biological processes and when disrupted they may lead to pathological processes. In particular, the biological importance of ECM-cell membrane-cytoskeleton interactions in skeletal muscle is accentuated by the number of inherited muscle diseases caused by mutations in proteins conferring these interactions. In this review we introduce laminins, collagens, dystroglycan, integrins, dystrophin and sarcoglycans. Mutations in corresponding genes cause various forms of muscular dystrophy. The muscle disorders are presented as well as advances toward the development of treatment.

β1D chain increases α7β1 integrin and laminin and protects against sarcolemmal damage in mdx mice

The dystrophin-glycoprotein complex connects myofibers with extracellular matrix laminin. In Duchenne muscular dystrophy, this linkage system is absent and the integrity of muscle fibers is compromised. One potential therapy for addressing muscular dystrophy is to augment the amount of α7β1 integrin, the major laminin-binding integrin in skeletal muscle. Whereas transgenic over-expression of α7 chain may alleviate development of muscular dystrophy and extend the lifespan of severely dystrophic mdx/utrn(-/-) mice, further enhancing levels of α7 chain provided little additional membrane integrin and negligible additional improvement in mdx mice. We demonstrate here that normal levels of β1 chain limit formation of integrin heterodimer and that increasing β1D chain in mdx mice results in more functional integrin at the sarcolemma, more matrix laminin and decreased damage of muscle fibers. Moreover, increasing the amount of β1D chain in vitro enhances transcription of α7 integrin and α2 laminin genes and the amounts of these proteins. Thus manipulation of β1D integrin expression offers a novel approach to enhance integrin-mediated therapy for muscular dystrophy.

Def-6, a guanine nucleotide exchange factor for Rac1, interacts with the skeletal muscle integrin chain alpha7A and influences myoblast differentiation

Integrin alpha7beta1 is the major laminin binding integrin receptor of muscle cells. The alpha7 chain occurs in several splice isoforms, of which alpha7A and alpha7B differ in their intracellular domains only. The fact that the expression of alpha7A and alpha7B is tightly regulated during skeletal muscle development suggests different and distinct roles for both isoforms. However, so far, functional properties and interacting proteins were described for the alpha7B chain only. Using a yeast two-hybrid screen, we have found that Def-6, a guanine nucleotide exchange factor for Rac1, binds to the intracellular domain of the alpha7A subunit. The specificity of the Def-6-alpha7A interaction has been shown by direct yeast two-hybrid binding assays and coprecipitation experiments. This is the first description of an alpha7A-specific and -exclusive interaction, because Def-6 did not bind to any other tested integrin cytoplasmic domain. Interestingly, the binding of Def-6 to alpha7A was abolished, when cells were cotransfected with an Src-related kinase, which is known to phosphorylate Def-6 and stimulate its exchange activity. We found expression of Def-6 was not only restricted to T-lymphocytes as described thus far but in a more widespread manner, including different muscle tissues. In cells, Def-6 is seen in newly forming cell protrusions and focal adhesions, and its localization partially overlaps with the alpha7A integrin receptor. C2C12 myoblasts overexpressing Def-6 show a delay of Rac1 inactivation during myogenic differentiation and abnormal myotube formation. Thus, our data suggest a role for Def-6 in the fine regulation of Rac1 during myogenesis with the integrin alpha7A chain guiding this regulation in a spatio-temporal manner.

Diagnostic immunohistochemistry in neuromuscular disorders

Most neuromuscular disorders display only non-specific myopathological features in routine histological preparations. However, a number of proteins, including sarcolemmal, sarcomeric, and nuclear proteins as well as enzymes with defects responsible for neuromuscular disorders, have been identified during the past two decades, allowing a more specific and firm diagnosis of muscle diseases. Identification of protein defects relies predominantly on immunohistochemical preparations and on Western blot analysis. While immunohistochemistry is very useful in identifying abnormal expression of primary protein abnormalities in recessive conditions, it is less helpful in detecting primary defects in dominantly inherited disorders. Abnormal immunohistochemical expression patterns can be confirmed by Western blot analysis which may also be informative in dominant disorders, although its role has yet to be established. Besides identification of specific protein defects, immunohistochemistry is also helpful in the differentiation of inflammatory myopathies by subtyping cellular infiltrates and demonstrating up-regulation of subtle immunological parameters such as cell adhesion molecules. The role of immunohistochemistry in denervating disorders, however, remains controversial in the absence of a reliable marker of muscle fibre denervation. Nevertheless, as well as the diagnostic value of immunocytochemical analysis it may also widen understanding of muscle fibre pathology as well as help in the development of therapeutic strategies.

Role for the α7β1 integrin in vascular development and integrity

The alpha7beta1 integrin is a laminin receptor that has been implicated in muscle disease and the development of neuromuscular and myotendinous junctions. Studies have shown the alpha7beta1 integrin is also expressed in nonskeletal muscle tissues. To identify the expression pattern of the alpha7 integrin in these tissues during embryonic development, alpha7 integrin chain knockout mice were generated by a LacZ knockin strategy. In these mice, expression from the alpha7 promoter is reported by beta-galactosidase. From embryonic day (ED) 11.5 to ED14.5, beta-galactosidase was detected in the developing central and peripheral nervous systems and vasculature. The loss of the alpha7 integrin gene resulted in partial embryonic lethality. Several alpha7 null embryos were identified with cerebrovascular hemorrhages and showed reduced vascular smooth muscle cells and cerebral vascularization. The alpha7 null mice that survived to birth exhibited vascular smooth muscle defects, including hyperplasia and hypertrophy. In addition, altered expression of alpha5 and alpha6B integrin chains was detected in the cerebral arteries of alpha7 null mice, which may contribute to the vascular phenotype. Our results demonstrate for the first time that the alpha7beta1 integrin is important for the recruitment or survival of cerebral vascular smooth muscle cells and that this integrin plays an important role in vascular development and integrity.

The LIM-only proteins FHL2 and FHL3 interact with alpha- and beta-subunits of the muscle alpha7beta1 integrin receptor

FHL1, FHL2, and FHL3 are members of the four and one-half LIM domain protein subclass that are expressed in striated muscles. Here we show that FHL2 and FHL3 are novel alpha(7)beta(1) integrin-interacting proteins. They bind both the alpha- and the beta-subunit as well as different splice isoforms. The minimal binding sites for FHL2 and FHL3 on beta(1A)-chain overlap, whereas on alpha(7A) and alpha(7B) subunits they are situated adjacent. Determining the binding sites for integrins on FHL2 or FHL3 revealed that the suprastructure of the whole molecule is important for these associations, rather than any single LIM domain. Immunofluorescence studies with cells expressing full-length FHL proteins or their deletion mutants showed that FHL2 and FHL3 but not FHL1 colocalize with integrins at cell adhesion sites. Further, their recruitment to the membrane results from binding to either the alpha- or the beta-chain of the integrin receptor. The association of FHL2 or FHL3 with integrin receptors neither influences attachment of cells to different substrates nor changes their migration capacity. However, in cardiac and skeletal muscles, FHL2 and FHL3, respectively, are colocalized with alpha(7)beta(1) integrin receptor at the periphery of Z-discs, suggesting a role in mechanical stabilization of muscle cells.

Collagen-binding I domain integrins--what do they do?

Collagens are the most abundant proteins in the mammalian body and it is well recognized that collagens fulfill an important structural role in the extracellular matrix in a number of tissues. Inactivation of the collagen alpha 1(I) gene in mice results in embryonic lethality and collagen mutations in humans cause defects leading to disease. Integrins constitute a major group of receptors for extracellular matrix components, including collagens. Currently four collagen-binding I domain-containing integrins are known, namely alpha 1 beta 1, alpha 2 beta 1, alpha 10 beta 1 and alpha 11 beta 1. Unlike the undisputed role of collagens as structural elements, the biological importance of integrin mediated cell-collagen interactions is far from clear. This is in part due to the limited information available on the most recent additions of the integrin family, alpha 10 beta 1 and alpha 11 beta 1. Future studies using gene inactivation of individual and multiple integrin genes will allow testing of the hypothesis that collagen-binding integrins have redundant functions but will also shed light on their importance in pathological conditions. In this review we will describe what is currently known about the collagen-binding integrins and discuss their biological functions.

Integrin alpha 7 beta 1 in muscular dystrophy/myopathy of unknown etiology

To investigate the role of integrin alpha 7 in muscle pathology, we used a "candidate gene" approach in a large cohort of muscular dystrophy/myopathy patients. Antibodies against the intracellular domain of the integrin alpha 7A and alpha 7B were used to stain muscle biopsies from 210 patients with muscular dystrophy/myopathy of unknown etiology. Levels of alpha 7A and alpha 7B integrin were found to be decreased in 35 of 210 patients (approximately 17%). In six of these patients no integrin alpha 7B was detected. Screening for alpha 7B mutation in 30 of 35 patients detected only one integrin alpha 7 missense mutation (the mutation on the second allele was not found) in a patient presenting with a congenital muscular dystrophy-like phenotype. No integrin alpha 7 gene mutations were identified in all of the other patients showing integrin alpha 7 deficiency. In the process of mutation analysis, we identified a novel integrin alpha 7 isoform presenting 72-bp deletion. This isoform results from a partial deletion of exon 21 due to the use of a cryptic splice site generated by a G to A missense mutation at nucleotide position 2644 in integrin alpha 7 cDNA. This spliced isoform is present in about 12% of the chromosomes studied. We conclude that secondary integrin alpha 7 deficiency is rather common in muscular dystrophy/myopathy of unknown etiology, emphasizing the multiple mechanisms that may modulate integrin function and stability.

Enhanced expression of the alpha 7 beta 1 integrin reduces muscular dystrophy and restores viability in dystrophic mice

Muscle fibers attach to laminin in the basal lamina using two distinct mechanisms: the dystrophin glycoprotein complex and the alpha 7 beta 1 integrin. Defects in these linkage systems result in Duchenne muscular dystrophy (DMD), alpha 2 laminin congenital muscular dystrophy, sarcoglycan-related muscular dystrophy, and alpha 7 integrin congenital muscular dystrophy. Therefore, the molecular continuity between the extracellular matrix and cell cytoskeleton is essential for the structural and functional integrity of skeletal muscle. To test whether the alpha 7 beta 1 integrin can compensate for the absence of dystrophin, we expressed the rat alpha 7 chain in mdx/utr(-/-) mice that lack both dystrophin and utrophin. These mice develop a severe muscular dystrophy highly akin to that in DMD, and they also die prematurely. Using the muscle creatine kinase promoter, expression of the alpha 7BX2 integrin chain was increased 2.0-2.3-fold in mdx/utr(-/-) mice. Concomitant with the increase in the alpha 7 chain, its heterodimeric partner, beta 1D, was also increased in the transgenic animals. Transgenic expression of the alpha 7BX2 chain in the mdx/utr(-/-) mice extended their longevity by threefold, reduced kyphosis and the development of muscle disease, and maintained mobility and the structure of the neuromuscular junction. Thus, bolstering alpha 7 beta 1 integrin-mediated association of muscle cells with the extracellular matrix alleviates many of the symptoms of disease observed in mdx/utr(-/-) mice and compensates for the absence of the dystrophin- and utrophin-mediated linkage systems. This suggests that enhanced expression of the alpha 7 beta 1 integrin may provide a novel approach to treat DMD and other muscle diseases that arise due to defects in the dystrophin glycoprotein complex. A video that contrasts kyphosis, gait, joint contractures, and mobility in mdx/utr(-/-) and alpha 7BX2-mdx/utr(-/-) mice can be accessed at http://www.jcb.org/cgi/content/full/152/6/1207.

Transfection of MCF-7 Carcinoma Cells with Human Integrin α7 cDNA Promotes Adhesion to Laminin

The laminin-binding alpha7beta1 integrin receptor is highly expressed by skeletal and cardiac muscles, and has been suggested to be a crucial molecule during myogenic cell migration and differentiation. Absence of integrin alpha7 subunit contributes to a form of muscular dystrophy in integrin alpha7 null mice, whereas specific mutations in the alpha7 gene are associated in humans with congenital myopathy. To examine in more detail the potential role of integrin alpha7 in human-related muscular disorders, we cloned alpha7 cDNA by RT-PCR from human skeletal muscle mRNA and then expressed the full-length human integrin alpha7 cDNA by transfection in several cell lines including MCF-7, COS-7, and NIH3T3 cells. The isolated cDNA corresponds to the human alpha7X2B alternative splice form. Expression of human alpha7 was further confirmed by transfection of chimeric human/mouse alpha7 cDNA constructs. To demonstrate the functionality of expressed human alpha7, adhesion experiments with transfected MCF-7 cells have confirmed the specific binding of human alpha7 to laminin. In addition, mouse polyclonal and monoclonal antibodies were generated against the extracellular domain of human alpha7 and used to analyze by flow cytometry MCF-7 and NIH3T3 cells transfected with the full-length of human alpha7 cDNA. These results show for the first time the exogenous expression of functional full-length human alpha7 cDNA, as well as the development of monoclonal antibodies against the human alpha7 extracellular domain. Antibodies developed will be useful for further analysis of human disorders involving alpha7 dysfunction and facilitate isolation of muscle stem cells (satellite cells) and thereby expand the opportunities for genetically modified transplantation treatment of human disease.