A novel beta 1 integrin isoform produced by alternative splicing: unique expression in cardiac and skeletal muscle

The Effect of Mechanical Stretch on Myotube Growth Suppression by Colon-26 Tumor-Derived Factors

Preclinical models and in vitro experiments have provided valuable insight into the regulation of cancer-induced muscle wasting. Colon-26 (C26) tumor cells induce cachexia in mice, and conditioned media (CM) from these cells promotes myotube atrophy and catabolic signaling. While mechanical stimuli can prevent some effects of tumor-derived factors on myotubes, the impact of mechanical signaling on tumor-derived factor regulation of myosin heavy chain (MyHC) expression is not well understood. Therefore, we examined the effects of stretch-induced mechanical signaling on C2C12 myotube growth and MyHC expression after C26 CM exposure. C26 CM was administered to myotubes on day 5 of differentiation for 48 h. During the last 4 or 24 h of C26 CM exposure, 5% static uniaxial stretch was administered. C26 CM suppressed myotube growth and MyHC protein and mRNA expression. Stretch for 24 h increased myotube size and prevented the C26 CM suppression of MyHC-Fast protein expression. Stretch did not change suppressed MyHC mRNA expression. Stretch for 24 h reduced Atrogin-1/MAFbx, MuRF-1, and LC3B II/I ratio and increased integrin β1D protein expression and the myogenin-to-MyoD protein ratio. Stretch in the last 4 h of CM increased ERK1/2 phosphorylation but did not alter the CM induction of STAT3 or p38 phosphorylation. These results provide evidence that in myotubes pre-incubated with CM, the induction of mechanical signaling can still provide a growth stimulus and preserve MyHC-Fast protein expression independent of changes in mRNA expression.

Evidence for skeletal muscle fiber type-specific expressions of mechanosensors

Mechanosensors govern muscle tissue integrity and constitute a subcellular structure known as costameres. Costameres physically link the muscle extracellular matrix to contractile and signaling 'hubs' inside muscle fibers mainly via integrins and are localized beneath sarcolemmas of muscle fibers. Costameres are the main mechanosensors converting mechanical cues into biological events. However, the fiber type-specific costamere architecture in muscles is unexplored. We hypothesized that fiber types differ in the expression of genes coding for costamere components. By coupling laser microdissection to a multiplex tandem qPCR approach, we demonstrate that type 1 and type 2 fibers indeed show substantial differences in their mechanosensor complexes. We confirmed these data by fiber type population-specific protein analysis and confocal microscopy-based localization studies. We further show that knockdown of the costamere gene integrin-linked kinase (Ilk) in muscle precursor cells results in significantly increased slow-myosin-coding Myh7 gene, while the fast-myosin-coding genes Myh1, Myh2, and Myh4 are downregulated. In parallel, protein synthesis-enhancing signaling molecules (p-mTORSer2448, p < 0.05; p-P70S6KThr389, tendency with p < 0.1) were reduced upon Ilk knockdown. However, overexpression of slow type-inducing NFATc1 in muscle precursor cells did not change Ilk or other costamere gene expressions. In addition, we demonstrate fiber type-specific costamere gene regulation upon mechanical loading and unloading conditions. Our data imply that costamere genes, such as Ilk, are involved in the control of muscle fiber characteristics. Further, they identify costameres as muscle fiber type-specific loading management 'hubs' and may explain adaptation differences of muscle fiber types to mechanical (un)loading.

Integrin signaling: linking mechanical stimulation to skeletal muscle hypertrophy

The α₇β₁-integrin is a transmembrane adhesion protein that connects laminin in the extracellular matrix (ECM) with actin in skeletal muscle fibers. The α₇β₁-integrin is highly expressed in skeletal muscle and is concentrated at costameres and myotendious junctions, providing the opportunity to transmit longitudinal and lateral forces across the membrane. Studies have demonstrated that α₇-integrin subunit mRNA and protein are upregulated following eccentric contractions as a mechanism to reinforce load-bearing structures and resist injury with repeated bouts of exercise. It has been hypothesized for many years that the integrin can also promote protein turnover in a manner that can promote beneficial adaptations with resistance exercise training, including hypertrophy. This review provides basic information about integrin structure and activation and then explores its potential to serve as a critical mechanosensor and activator of muscle protein synthesis and growth. Overall, the hypothesis is proposed that the α₇β₁-integrin can contribute to mechanical-load induced skeletal muscle growth via an mammalian target of rapamycin complex 1-independent mechanism.

β1D integrin splice variant stabilizes integrin dynamics and reduces integrin signaling by limiting paxillin recruitment

Heterodimeric integrin receptors control cell adhesion, migration and extracellular matrix assembly. While the α integrin subunit determines extracellular ligand specificity, the β integrin chain binds to an acidic residue of the ligand, and cytoplasmic adapter protein families such as talins, kindlins and paxillin, to form mechanosensing cell matrix adhesions. Alternative splicing of the β1 integrin cytoplasmic tail creates ubiquitously expressed β1A, and the heart and skeletal muscle-specific β1D form. To study the physiological difference between these forms, we developed fluorescent β1 integrins and analyzed their dynamics, localization, and cytoplasmic adapter recruitment and effects on cell proliferation. On fibronectin, GFP-tagged β1A integrin showed dynamic exchange in peripheral focal adhesions, and long, central fibrillar adhesions. In contrast, GFP-β1D integrins exchanged slowly, forming immobile and short central adhesions. While adhesion recruitment of GFP-β1A integrin was sensitive to C-terminal tail mutagenesis, GFP-β1D integrin was recruited independently of the distal NPXY motif. In addition, a P786A mutation in the proximal, talin-binding NPXY⁷⁸³ motif switched β1D to a highly dynamic integrin. In contrast, the inverse A786P mutation in β1A integrin interfered with paxillin recruitment and proliferation. Thus, differential β1 integrin splicing controls integrin-dependent adhesion signaling, to adapt to the specific physiological needs of differentiated muscle cells.

Co-regulated gene expression of splicing factors as drivers of cancer progression

Splicing factors (SFs) act in dynamic macromolecular complexes to modulate RNA processing. To understand the complex role of SFs in cancer progression, we performed a systemic analysis of the co-regulation of SFs using primary tumor RNA sequencing data. Co-regulated SFs were associated with aggressive breast cancer phenotypes and enhanced metastasis formation, resulting in the classification of Enhancer- (21 genes) and Suppressor-SFs (64 genes). High Enhancer-SF levels were related to distinct splicing patterns and expression of known oncogenic pathways such as respiratory electron transport, DNA damage and cell cycle regulation. Importantly, largely identical SF co-regulation was observed in almost all major cancer types, including lung, pancreas and prostate cancer. In conclusion, we identified cancer-associated co-regulated expression of SFs that are associated with aggressive phenotypes. This study increases the global understanding of the role of the spliceosome in cancer progression and also contributes to the development of strategies to cure cancer patients.

The talin–integrin interface under mechanical stress

Interactions and force resistance of different β-integrin–talin complexes were analysed in a set of steered molecular dynamics simulations.

Integrins protect cardiomyocytes from ischemia/reperfusion injury

Ischemic damage is recognized to cause cardiomyocyte (CM) death and myocardial dysfunction, but the role of cell-matrix interactions and integrins in this process has not been extensively studied. Expression of α7β1D integrin, the dominant integrin in normal adult CMs, increases during ischemia/reperfusion (I/R), while deficiency of β1 integrins increases ischemic damage. We hypothesized that the forced overexpression of integrins on the CM would offer protection from I/R injury. Tg mice with CM-specific overexpression of integrin α7β1D exposed to I/R had a substantial reduction in infarct size compared with that of α5β1D-overexpressing mice and WT littermate controls. Using isolated CMs, we found that α7β1D preserved mitochondrial membrane potential during hypoxia/reoxygenation (H/R) injury via inhibition of mitochondrial Ca2+ overload but did not alter H/R effects on oxidative stress. Therefore, we assessed Ca2+ handling proteins in the CM and found that β1D integrin colocalized with ryanodine receptor 2 (RyR2) in CM T-tubules, complexed with RyR2 in human and rat heart, and specifically bound to RyR2 amino acids 165-175. Integrins stabilized the RyR2 interdomain interaction, and this stabilization required integrin receptor binding to its ECM ligand. These data suggest that α7β1D integrin modifies Ca2+ regulatory pathways and offers a means to protect the myocardium from ischemic injury.

Regulation of cardiac melusin gene expression by hypertrophic stimuli in the rat

AIM

Melusin is an integrin β1-interacting protein proposed to act as a biomechanical sensor in the heart. We characterized mechanisms and signalling pathways regulating cardiac melusin expression.

METHODS

Infusion of arginine(8) -vasopressin (AVP) in Sprague-Dawley (SD) rats, spontaneously hypertensive rats (SHR) and double transgenic rats (dTGR) harbouring both human angiotensinogen and renin genes as well as infusion of angiotensin II (Ang II) in SD rats were used. The effect of direct left ventricular (LV) wall stretch was analysed by using isolated perfused rat heart preparation. For the cell culture studies, mouse atrial HL-1 cell line and neonatal rat ventricular myocytes (NRVMs) were used.

RESULTS

Left atrial melusin mRNA levels increased already after 30 min of AVP infusion. Ang II caused significant upregulation of left atrial melusin mRNA (2.1-fold at 6 h, P < 0.05) and protein (1.9-fold at 72 h, P < 0.05) levels. In contrast, LV melusin mRNA levels remained unchanged in response to both infusions, as well as to aortic banding-induced pressure overload. Direct LV wall stress or late-stage hypertensive heart disease did not modify LV melusin gene expression either. Interestingly, in atrial HL-1 cells, cyclic stretching increased melusin mRNA levels. Stretching and treatments with hypertrophic agonists increased melusin mRNA and protein levels in NRVMs, endothelin-1 being the most potent. PD98059, an extracellular signal-regulated protein kinase 1/2 inhibitor, markedly attenuated the endothelin-1-induced upregulation of melusin gene expression in NRVMs.

CONCLUSION

Multiple hypertrophic stimuli regulate melusin expression predominately in the atria, which may represent a necessary initial step in early adaptive remodelling processes.

Semaphorin7A and its receptors: pleiotropic regulators of immune cell function, bone homeostasis, and neural development

Semaphorins form a large, evolutionary conserved family of cellular guidance signals. The semaphorin family contains several secreted and transmembrane proteins, but only one GPI-anchored member, Semaphorin7A (Sema7A). Although originally identified in immune cells, as CDw108, Sema7A displays widespread expression outside the immune system. It is therefore not surprising that accumulating evidence supports roles for this protein in a wide variety of biological processes in different organ systems and in disease. Well-characterized biological effects of Sema7A include those during bone and immune cell regulation, neuron migration and neurite growth. These effects are mediated by two receptors, plexinC1 and integrins. However, most of what is known today about Sema7A signaling concerns Sema7A-integrin interactions. Here, we review our current knowledge of Sema7A function and signaling in different organ systems, highlighting commonalities between the cellular effects and signaling pathways activated by Sema7A in different cell types. Furthermore, we discuss a potential role for Sema7A in disease and provide directions for further research.

Expression of muscle-specific integrins in masseter muscle fibers during malocclusion disease

Integrins are heterodimeric cell surface membrane proteins linking the extracellular matrix to actin. α7B integrin is detected in proliferating and adult myofibers, whereas α7A plays a role in regenerating muscle fibers with a minor function in mature muscle fibers. The expression levels of β1A appear to be very low, whereas β1D appears to be the predominant integrin form in mature muscle. Considering the important features of masseter muscle we have studied integrin expression in masseter muscle specimens of surgical patients with posterior right crossbite and comparing them to left side masseter muscle specimens. Our results showed that the expression of integrins was significantly lower in the crossbite side muscle. Furthermore, the most important finding is that β1A is clearly detectable in adult masseter muscle. This behavior could be due to the particular composition of masseter, since it contains hybrid fibers showing the capacity to modify the contractile properties to optimize the energy efficiency or the action of the muscle during contraction. Moreover, masseter is characterized by a high turnover of muscle fibers producing a regeneration process. This may indicate a longer time to heal, justifying the loss of β1D and the consequential increase of β1A. Thus, our data provide the first suggestion that integrins in masseter muscle play a key role regulating the functional activity of muscle and allowing the optimization of contractile forces.

The extracellular matrix dimension of skeletal muscle development

Cells anchor to substrates by binding to extracellular matrix (ECM). In addition to this anchoring function however, cell-ECM binding is a mechanism for cells to sense their surroundings and to communicate and coordinate behaviour amongst themselves. Several ECM molecules and their receptors play essential roles in muscle development and maintenance. Defects in these proteins are responsible for some of the most severe muscle dystrophies at every stage of life from neonates to adults. However, recent studies have also revealed a role of cell-ECM interactions at much earlier stages of development as skeletal muscle forms. Here we review which ECM molecules are present during the early phases of myogenesis, how myogenic cells interact with the ECM that surrounds them and the potential consequences of those interactions. We conclude that cell-ECM interactions play significant roles during all stages of skeletal muscle development in the embryo and suggest that this "extracellular matrix dimension" should be added to our conceptual network of factors contributing to skeletal myogenesis.

Focal adhesion kinase signaling regulates the expression of caveolin 3 and beta1 integrin, genes essential for normal myoblast fusion

An essential phase of skeletal myogenesis is the fusion of mononucleated myoblasts to form multinucleated myotubes. Many cell adhesion proteins, including integrins, have been shown to be important for myoblast fusion in vertebrates, but the mechanisms by which these proteins regulate cell fusion remain mostly unknown. Here, we focused on the role of focal adhesion kinase (FAK), an important nonreceptor protein tyrosine kinase involved in integrin signaling, as a potential mediator by which integrins may regulate myoblast fusion. To test this hypothesis in vivo, we generated mice in which the Fak gene was disrupted specifically in muscle stem cells ("satellite cells") and we found that this resulted in impaired myotube formation during muscle regeneration after injury. To examine the role of FAK in the fusion of myogenic cells, we examined the expression of FAK and the effects of FAK deletion on the differentiation of myoblasts in vitro. Differentiation of mouse primary myoblasts was accompanied by a rapid and transient increase of phosphorylated FAK. To investigate the requirement of FAK in myoblast fusion, we used two loss-of-function approaches (a dominant-negative inhibitor of FAK and FAK small interfering RNA [siRNA]). Inhibition of FAK resulted in markedly impaired fusion but did not inhibit other biochemical measures of myogenic differentiation, suggesting a specific role of FAK in the morphological changes of cell fusion as part of the differentiation program. To examine the mechanisms by which FAK may be regulating fusion, we used microarray analysis to identify the genes that failed to be normally regulated in cells that were fusion defective due to FAK inhibition. Several genes that have been implicated in myoblast fusion were aberrantly regulated during differentiation when FAK was inhibited. Intriguingly, the normal increases in the transcript of caveolin 3 as well as an integrin subunit, the beta1D isoform, were suppressed by FAK inhibition. We confirmed this also at the protein level and show that direct inhibition of beta1D subunit expression by siRNA inhibited myotube formation with a prominent effect on secondary fusion. These data suggest that FAK regulation of profusion genes, including caveolin 3 and the beta1D integrin subunit, is essential for morphological muscle differentiation.

Subunit 3 of the COP9 signalosome is poised to facilitate communication between the extracellular matrix and the nucleus through the muscle-specific beta1D integrin

Yeast two-hybrid analysis (Fields and Song, 1989, Nature, 340:245-246) was used to screen a human heart library to isolate proteins interacting with the adult muscle-specific beta1D integrin but not with beta1A integrin. In addition to previously identified interactions (RACK 1(Liliental and Chang, 1998, Journal of Biological Chemistry, 273:2379-2383) and alpha-actinin (Otey et al., 1990, Journal of Cell Biology, 111:721-729), the authors isolated several novel candidates. These include subunit 3 (CSN3/Sgn3) of the COP9 signalosome complex, cyclins D1, D2, and D3, RanBPM, and a recently identified protein COG8/DOR1. These protein interactions were specific for beta1D integrin, as no binding to beta1A integrin cytoplasmic domain was measurable by two-hybrid analysis. This paper presents the initial characterization of the interaction of CSN3 with beta1D integrin, the localization of CSN3 and the other COP9 signalosome subunits in embryonic and adult cardiac myocytes and their response to muscle cell differentiation.

Extended interaction of beta1 integrin subunit-deficient cells (GD25) with surfaces modified with fibronectin-derived peptides: Culture optimization, adhesion and cytokine panel studies

The modification of biomaterials with extracellular matrix-mimicking factors is a common technique used to influence the cellular response through integrin-mediated signaling. The inherent limitations of antibody-inhibition studies necessitate the use of complementary methods to block integrin function to confirm cell-surface interaction. In this study, we employed a beta1 integrin-deficient cell line, GD25, to investigate the role of beta1 subunit in cell adhesion and subsequent cytokine (granulocyte macrophage colony stimulating factor; interleukin (IL)-1alpha; IL-1beta; IL-6; monocyte chemoattractant protein-1; regulated upon activation, normal T-cell expressed, and secreted; tumor necrosis factor-alpha) release kinetics in the presence of tissue culture polystyrene (TCPS) and semi-interpenetrating polymer networks (sIPN) modified with fibronectin (FN)-mimic peptides (RGD, PHSRN). Culture conditions (i.e. seeding density, medium, serum supplementation) were optimized for long-term observation. Differences in cell adhesion, cell viability and cytokine release behavior were dependent on the presence of the beta1 integrin subunit, FN, sIPN cast method and peptide identity. By comparing two complementary techniques for assaying integrin function, we observed both similarities (i.e. decreased adhesion to FN-absorbed TCPS and increased IL-1beta release at 96h) and differences (i.e. no difference in adhesion or IL-1beta release in the presence of different sIPN surfaces) when the function of the beta1 subunit was blocked in cell adhesion and signaling in the presence of biomaterials.

The impact of alternative splicing in vivo: mouse models show the way

Alternative splicing is widely believed to have a major impact on almost all biological processes since it increases proteome complexity and thereby controls protein function. Recently, gene targeting in mice has been used to create in vivo models to study the regulation and consequences of alternative splicing. The evidence accumulated so far argues for a nonredundant, highly specific role of individual splicing factors in mammalian development, and furthermore, demonstrates the importance of distinct protein isoforms in vivo. In this review, we will compare phenotypes of mouse models for alternative splicing to crystallize common themes and to put them into perspective with the available in vitro data.

Effect of cyclic stretch on β1D-integrin expression and activation of FAK and RhoA

Integrins play a pivotal role in proliferation, differentiation, and survival in skeletal and cardiac myocytes. The β1D-isoform of the β1-integrin is specifically expressed in striated skeletal muscle. However, little is known about the role and the mechanisms by which the splice variant β1D-integrin regulates myogenesis and mechanotransduction. We observed that cyclic mechanical stretch increases β1D-integrin protein levels and activates the downstream cytoskeletal signaling proteins focal adhesion kinase (FAK) and RhoA. Elimination of native β1D-integrin expression by RNA interference in immature developing myoblasts abolished stretch-induced increases in FAK phosphorylation and further downregulated RhoA activity. Blocking of β1D-integrin expression prevented myocellular fusion to form multinucleated mature myotubes. Restoration of human β1D-integrin expression in β1D-integrin-deficient cells partially restored myotube formation. The onset of myofusion also requires the generation of nitric oxide (NO). The release of NO affects cytoskeletal proteins by mediating RhoA activity and protein degradation. Our previous study demonstrated that stretch-induced NO positively modulates mechanical properties of differentiating skeletal myocytes. We found a significant decrease in NO production and apparent elastic modulus in β1D-integrin-deficient cells, suggesting signaling interactions between β1D-integrin and neuronal NO synthase to mediate mechanotransduction and myogenesis in skeletal myocytes. These results suggest that, in addition to regulating differentiation, the β1D-integrin isoform plays a critical role in the response of skeletal myoblasts to cyclic stretch by activating the downstream components of FAK and RhoA activity and affecting NO release.

Impact of acute and enduring volume overload on mechanotransduction and cytoskeletal integrity of canine left ventricular myocardium

It is poorly understood how mechanical stimuli influence in vivo myocardial remodeling during chronic hemodynamic overload. Combined quantitation of ventricular mechanics and expression of key proteins involved in mechanotransduction can improve fundamental understanding. Adult anesthetized dogs ( n = 20) were studied at sinus rhythm (SR) and 0, 3, 10, and 35 days of complete atrioventricular block (AVB). Serial left ventricular (LV) myofiber mechanics were measured. Repeated LV biopsies were analyzed for mRNA and/or protein expression of β1D-integrin, melusin, Akt, GSK3β, muscle LIM protein (MLP), four-and-a-half LIM protein 2 (fhl2), desmin, and calpain. Upon AVB, increased ejection strain (0.29 ± 0.01 vs. 0.13 ± 0.02, SR) and end-diastolic stress (4.8 ± 1.1 vs. 2.7 ± 0.4 kPa) dominated mechanical changes. Brain natriuretic peptide plasma levels were correspondingly high (33 ± 4 vs. 19 ± 1 pg/ml, SR). β1D-Integrin protein expression increased chronically after AVB. Melusin was temporarily overexpressed (+33 ± 9%, 3 days AVB vs. SR), followed by elevated ratios of phosphorylated (P)-Akt to Akt and P-GSK3β to GSK3β (+26 ± 6% and +30 ± 8% at 10 days AVB vs. SR). These changes corresponded to peak hypertrophic growth at 3 to 10 days. MLP increased gradually to maxima at chronic AVB (+36 ± 7%). In contrast, fhl2 (−22 ± 3%, 3 days) and desmin (−30 ± 9%, 10 days AVB) transiently declined but recovered at chronic AVB. Calpain protein expression remained unaltered. In conclusion, volume overload after AVB causes a transient compromise of cytoskeletal integrity based, at least partly, on transcriptional downregulation. Subsequent cytoskeletal reorganization coincides with the upregulation of melusin, P-Akt, P-GSK3β, and MLP, indicating a strong drive to compensated hypertrophy.

Talin2 is induced during striated muscle differentiation and is targeted to stable adhesion complexes in mature muscle

The cytoskeletal protein talin serves as an essential link between integrins and the actin cytoskeleton in several similar, but functionally distinct, adhesion complexes, including focal adhesions, costameres, and intercalated disks. Vertebrates contain two talin genes, TLN1 and TLN2, but the different roles of Talin1 and Talin2 in cell adhesion are unclear. In this report we have analyzed Talin1 and Talin2 in striated muscle. Using isoform-specific antibodies, we found that Talin2 is highly expressed in mature striated muscle. Using mouse C2C12 cells and primary human skeletal muscle myoblasts as models of muscle differentiation, we show that Talin1 is expressed in undifferentiated myoblasts and that Talin2 expression is upregulated during muscle differentiation at both the mRNA and protein levels. We have also identified regulatory sequences that may be responsible for the differential expression of Talin1 and Talin2. Using GFP-tagged Talin1 and Talin2 constructs, we found that GFP-Talin1 targets to focal adhesions while GFP-Talin2 targets to abnormally large adhesions in myoblasts. We also found that ectopic expression of Talin2 in myoblasts, which do not contain appreciable levels of Talin2, dysregulates the actin cytoskeleton. Finally we demonstrate that Talin2, but not Talin1, localizes to costameres and intercalated disks, which are stable adhesions required for the assembly of mature striated muscle. Our results suggest that Talin1 is the primary link between integrins and actin in dynamic focal adhesions in undifferentiated, motile cells, but that Talin2 may serve as the link between integrins and the sarcomeric cytoskeletonin stable adhesion complexes in mature striated muscle.

Human gingival fibroblast integrin subunit expression on titanium implant surfaces

BACKGROUND

Implant surface characteristics have been shown to modify cell behavior and regulate integrin expression. Integrin expression and resultant integrin-mediated cellular activity are essential components of tissue healing and homeostasis. Although both osseous and soft tissue healing around dental implants are critical to clinical success, there is limited information available on the effect of implant surfaces on integrin expression in soft tissues. Therefore, the aim of this study was to examine integrin expression for gingival fibroblasts on titanium surfaces and the influence of titanium surface roughness on integrin expression and cell morphology.

METHODS

Human gingival fibroblasts were cultured on smooth (polished) and rough (sand-blasted acid-etched) titanium surfaces and a cell culture plastic (control) surface. To analyze integrin expression, total RNA was isolated from experimental and control cells, and levels of integrin subunit mRNA were assessed by reverse transcription-polymerase chain reaction (RT-PCR) using primers specific for the alpha2, alpha4, alpha5, alpha(v), and beta1 integrin subunits and aldolase (internal control). PCR products were analyzed by polyacrylamide gel electrophoresis (PAGE), confirmed via DNA sequencing, and quantified using computer-assisted densitometry. The expression of the integrin subunits was analyzed at the protein level using flow cytometry, as well as fluorescence and confocal laser microscopy. Cell morphology was evaluated using scanning electron microscopy (SEM).

RESULTS

Our experiments demonstrated cellular expression of the alpha2, alpha4, alpha5, alpha(v), and beta1 integrin subunits at both mRNA and protein levels on all surfaces. In addition, the alpha4 and beta1 mRNA levels were significantly increased on smooth titanium relative to plastic surfaces (P <.05) with intermediate mRNA levels found on the rough titanium surfaces. The smooth titanium surfaces exhibited a flat monolayer of cells, while rough titanium surfaces showed cells orienting themselves along surface irregularities.

CONCLUSIONS

These results demonstrate the presence of multiple integrin subunits in human gingival fibroblasts grown in contact with titanium implant surfaces and that titanium surface roughness alters cellular morphology but appears to have limited effects on integrin expression. This study provides insight into the complicated cellular and molecular events occurring at the implant surface that may be critical to optimizing the soft tissue interactions with the soft tissue-implant interface.

Transgenic expression of {alpha}7{beta}1 integrin maintains muscle integrity, increases regenerative capacity, promotes hypertrophy, and reduces cardiomyopathy in dystrophic mice

We previously reported that enhanced expression of the alpha7beta1 integrin ameliorates the development of muscular dystrophy and extends longevity in alpha7BX2-mdx/utr(-/-) transgenic mice (Burkin DJ, Wallace GQ, Nicol KJ, Kaufman DJ, Kaufman SJ: Enhanced expression of the alpha7beta1 integrin reduces muscular dystrophy and restores viability in dystrophic mice. We now report on the mechanism by which these mice were rescued by the integrin. As a result of increased integrin in alpha7BX2-mdx/utr(-/-) mice the structural integrity of the myotendinous and neuromuscular junctions are maintained. A twofold increase in satellite cells in alpha7BX2-mdx/utr(-/-) skeletal muscle was detected by immunofluorescence using the satellite cell marker c-met. These cells enhanced the regenerative capacity of muscle in the transgenic animals as determined by fusion of BrdUrd-labeled cells into muscle fibers. Increased integrin also leads to hypertrophy. Finally, transgenic expression of alpha7BX2 integrin chain in skeletal muscle secondarily reduces the development of cardiomyopathy, the ultimate cause of death in these animals. We believe this multiplicity of responses to increased alpha7beta1 integrin collectively inhibits the development of muscle disease and increases longevity in these mice.

Increased syndecan expression following myocardial infarction indicates a role in cardiac remodeling

Finsen, Alexandra Vanessa, Per Reidar Woldbaek, Jian Li, Jiaping Wu, Torstein Lyberg, Theis Tonnessen, and Geir Christensen. Increased syndecan expression following myocardial infarction indicates a role in cardiac remodeling. Physiol Genomics 16: 301-308, 2004. First published November 18, 2003; 10.1152/physi-olgenomics. 00144.2002.—The purpose of this study was to identify essential genes involved in myocardial growth and remodeling following myocardial infarction (MI). Left ventricular noninfarcted tissues from six mice subjected to MI under general anesthesia and from six sham-operated mice were obtained 1 wk after primary surgery and analyzed by means of cDNA filter arrays. Out of a total of 1,176 genes, 641 were consistently expressed, twenty-three were upregulated and thirteen downregulated. Five genes were only expressed following MI. Syndecan-3, a transmembranous heparan sulfate proteoglycan, was found to be upregulated together with a transcriptional activator of syndecans, Wilms tumor protein 1 (WT-1). Northern blotting demonstrated a significant upregulation of syndecan-1, -2, -3, and -4, WT-1, fibronectin, and basic fibroblast growth factor (FGF) receptor 1. Furthermore, Western blot analysis showed statistically significant increases in protein levels for syndecan-3 and -4. In conclusion, we have identified a subset of genes with increased expression in noninfarcted left ventricular tissue following MI, including syndecans 1–4, WT-1, fibronectin, collagen 6A, and FGF receptor 1. Since the syndecans link the cytoskeleton to the extracellular matrix and function as required coreceptors for FGF, we suggest a role for the syndecans in cardiac remodeling following MI.

β1 integrin mediation of myogenic differentiation: implications for satellite cell differentiation

Myoblast-extracellular matrix interactions mediated by integrin receptors have been shown to play a pivotal role in skeletal muscle development. In the current study, the avian genetic muscle weakness, low score normal (LSN), which exhibits modified myotube and sarcomere structure and a reduction in beta1 integrin expression during satellite cell differentiation, was used as a model system to further investigate the role of beta1 integrins in myogenic differentiation. During normal satellite cell differentiation, the beta1 integrin was localized at areas of pseudopodial activity and points of cell contact. In contrast, in LSN satellite cell cultures the beta1 integrin was predominantly observed in clusters not associated with points of cell to cell contact. The LSN satellite cells underwent apoptosis at the same time normal satellite cells were fusing to form multinucleated myotubes. During muscle development, the beta1 integrin is regulated by alternative splicing from a beta1A to beta1D form. During both embryonic and posthatch pectoralis major muscle development, regulation of the beta1A and beta1D variants was altered in the LSN. Normal and LSN myogenic satellite cell cultures did not express the beta1D integrin, but the LSN satellite cells expressed higher concentrations of the beta1A integrin than did normal satellite cells. Taken together, these data further demonstrate the importance of the beta1 integrin subunit in the regulation of cell-extracellular matrix signal transduction pathways during muscle growth and differentiation.

The beta1 cytoplasmic domain regulates the laminin-binding specificity of the alpha7X1 integrin

During muscle development, the laminin-specific alpha7 integrin is alternatively spliced in the putative ligand-binding domain to yield either the alpha7X1 or the alpha7X2 variant. The relative level of alpha7X1 and alpha7X2 is developmentally regulated. Similarly, the partner beta1 integrin cytoplasmic domain is converted from the beta1A to the beta1D splice variant. To determine whether beta1D modulates the activity of the alpha7 receptor, cells were transfected with alpha7X1 and beta1D cDNA. alpha7X1 coupled with beta1A failed to adhere to laminin-1, whereas cotransfectants expressing alpha7X1 and beta1D showed strong adhesion. Interestingly, alpha7X1 complexed with beta1A and beta1D displayed the same level of poor adhesion to laminin-2/4 or strong adhesion to laminin-10/11. These findings indicate that alpha7 function is regulated not only by X1/X2 in its extracellular domain but also by beta1 cytoplasmic splice variants. It is likely that expression of beta1D alters alpha7X1 binding to laminin isoforms by a process related to ligand affinity modulation. Functional regulation of alpha7beta1 by developmentally regulated splicing events may be important during myogenic differentiation and repair because the integrin mediates adhesion, motility, and cell survival.

Knock-in of integrin beta 1D affects primary but not secondary myogenesis in mice

Integrins are extracellular matrix receptors composed of alpha and beta subunits involved in cell adhesion, migration and signal transduction. The beta1 subunit has two isoforms, beta 1A ubiquitously expressed and beta 1D restricted to striated muscle. They are not functionally equivalent. Replacement of beta 1A by beta 1D (beta 1D knock-in) in the mouse leads to midgestation lethality on a 50% Ola/50% FVB background [Baudoin, C., Goumans, M. J., Mummery, C. and Sonnenberg, A. (1998). Genes Dev. 12, 1202-1216]. We crossed the beta 1D knock-in line into a less penetrant genetic background. This led to an attenuation of the midgestation lethality and revealed a second period of lethality around birth. Midgestation death was apparently not caused by failure in cell migration, but rather by abnormal placentation. The beta 1D knock-in embryos that survived midgestation developed until birth, but exhibited severely reduced skeletal muscle mass. Quantification of myotube numbers showed that substitution of beta 1A with beta 1D impairs primary myogenesis with no direct effect on secondary myogenesis. Furthermore, long-term primary myotube survival was affected in beta 1D knock-in embryos. Finally, overexpression of beta 1D in C2C12 cells impaired myotube formation while overexpression of beta 1A primarily affected myotube maturation. Together these results demonstrate for the first time distinct roles for beta1 integrins in primary versus secondary myogenesis and that the beta 1A and beta 1D variants are not functionally equivalent in this process.

Expression patterns of focal adhesion associated proteins in the developing retina

Adhesive interactions between integrin receptors and the extracellular matrix (ECM) are intimately involved in regulating development of a variety of tissues within the organism. In the present study, we have investigated the relationships between beta(1) integrin receptors and focal adhesion associated proteins during eye development. We used specific antibodies to examine the distribution of beta(1) integrin ECM receptors and the cytoplasmic focal adhesion associated proteins, talin, vinculin, and paxillin in the developing Xenopus retina. Immunoblot analysis confirmed antibody specificity and indicated that beta(1) integrins, talin, vinculin, and paxillin were expressed in developing retina and in the retinal-derived Xenopus XR1 glial cell line. Triple-labeling immunocytochemistry revealed that talin, vinculin, paxillin, and phosphotyrosine proteins colocalized with beta(1) integrins at focal adhesions located at the termini of F-actin filaments in XR1 cells. In the retina, these focal adhesion proteins exhibited developmentally regulated expression patterns during eye morphogenesis. In the embryonic retina, immunoreactivities for focal adhesion proteins were expressed in neuroepithelial cells, and immunoreactivity was especially strong at the interface between the optic vesicle and overlying ectoderm. At later stages, these proteins were expressed throughout all retinal layers with higher levels of expression observed in the plexiform layers, optic fiber layer, and in the region of the inner and outer limiting membrane. Strong immunoreactivities for beta(1) integrin, paxillin, and phosphotyrosine were expressed in the radially oriented Müller glial cells at later stages of development. These results suggest that focal adhesion-associated proteins are involved in integrin-mediated adhesion and signaling and are likely to be essential in regulating retinal morphogenesis.

Expression of alpha7beta1 integrin splicing variants during skeletal muscle regeneration

Integrin alpha7beta1 is a laminin receptor, both subunits of which have alternatively spliced, developmentally regulated variants. In skeletal muscle beta1 has two major splice variants of the intracellular domain (beta1A and beta1D). alpha7X1 and alpha7X2 represent variants of the alpha7 ectodomain, whereas alpha7A and alpha7B are variants of the intracellular domain. Previously we showed that during early regeneration after transection injury of muscle alpha7 integrin mediates dynamic adhesion of myofibers along their lateral aspects to the extracellular matrix. Stable attachment of myofibers to the extracellular matrix occurs during the third week after injury, when new myotendinous junctions develop at the ends of the regenerating myofibers. Now we have analyzed the relative expression of beta1A/beta1D and alpha7A/alpha7B and alpha7X1/alpha7X2 isoforms during regeneration for 2 to 56 days after transection of rat soleus muscle using reverse transcriptase-polymerase chain reaction and immunohistochemistry. During early regeneration beta1A was the predominant isoform in both the muscle and scar tissue. Expression of muscle-specific beta1D was detected in regenerating myofibers from day 4 onwards, ie, when myogenic mitotic activity began to decrease, and it became more abundant with the progression of regeneration. alpha7B isoform predominated on day 2. Thereafter, the relative expression of alpha7A transcripts increased until day 7 with the concomitant appearance of alpha7A immunoreactivity on regenerating myofibers. Finally, alpha7B again became the predominant variant in highly regenerated myofibers. Similarly as in the controls, alpha7X1 and alpha7X2 isoforms were both expressed throughout the regeneration with a peak in alpha7X1 expression on day 4 coinciding with the dynamic adhesion stage. The results suggest that during regeneration of skeletal muscle the splicing of beta1 and alpha7 integrin subunits is regulated according to functional requirements. alpha7A and alpha7X1 appear to have a specific role during the dynamic phase of adhesion, whereas alpha7B, alpha7X2, and beta1D predominate during stable adhesion.

A lethal perinatal cardiac phenotype resulting from altered integrin function in cardiomyocytes

BACKGROUND

Integrins are heterodimeric receptors that couple the extracellular matrix to intracellular signaling pathways and the cyoskeleton. Integrins are strain transducers and candidates for modulators or effectors of cardiac hypertrophy.

METHODS

To begin to probe this function, we have transgenically expressed a chimeric protein that alters integrin function in cardiomyocytes. The transgene (Tac-beta(1D)) consists of the biologically inert extracellular and transmembrane domain of the interleukin-2 receptor alpha subunit (Tac) fused to the cytoplasmic tail of the human beta(1D) integrin driven by the cardiac alpha-myosin heavy chain promoter. Transgene expression results in a severe, usually fatal, perinatal cardiac phenotype, characterized by initial electrocardiographic abnormalities followed by extensive myocyte loss, macrophage infiltration, and replacement fibrosis.

RESULTS

Expression of Tac-beta(1D) resulted in displacement of endogenous beta(1D) integrin from Z-lines and T-tubules, decreased expression of endogenous beta(1D), and disrupted the fibronectin pericellular matrix. These results are consistent with an essential role for beta(1) integrins in maintenance of cardiomyocyte viability and interaction with extracellular matrix.

CONCLUSION

The appearance of conduction abnormalities before morphologic changes suggests that integrins are important in the development or maintenance of the conducting system of the heart.

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.

The extracellular matrix of muscle--implications for manipulation of the craniofacial musculature

Successful adaptation of craniofacial skeletal muscle is dependent upon the connective tissue component of the muscle. This is exemplified by procedures such as distraction histo/osteogenesis. The mechanisms underlying remodelling of intramuscular connective tissue are complex and multifactorial and involve extracellular matrix (ECM) molecules, receptors for the ECM (integrins) and enzymes that remodel the ECM (MMPs). This review discusses the current state of knowledge and clinical implications of connective tissue biology as applied to craniofacial skeletal muscle.

Integrins and the myocardium

Extracellular matrix provides a structural, chemical, and mechanical substrate that is essential in cardiac development, growth, and responses to pathophysiological signals. Transmembrane receptors termed integrins provide a dynamic interaction of environmental cues and intracellular events. Integrins orchestrate multiple functions in the intact organism including organogenesis, regulation of gene expression, cell proliferation, differentiation, migration, and death. They are expressed in all cellular components of the cardiovascular system, including the vasculature, blood, cardiac myocytes and nonmuscle cardiac cells. The focus of this review will be on the role of integrins in the myocardium. We will provide background on integrin structure and function, discuss how the expression of integrins is critical to the form and function of the developing and postnatal myocardium, and review the known data on integrins as signaling molecules in the heart. Finally, we will offer insights to the future research directions into this important family of extracellular matrix receptors in the myocardium.

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.

Type XIII collagen: a novel cell adhesion component present in a range of cell-matrix adhesions and in the intercalated discs between cardiac muscle cells

Recent analysis of type XIII collagen surprisingly showed that it is anchored to the plasma membranes of cultured cells via a transmembrane segment near its amino terminus. Here we demonstrate that type XIII collagen is concentrated in cultured skin fibroblasts and several other human mesenchymal cell lines in the focal adhesions at the ends of actin stress fibers, co-localizing with the known focal adhesion components talin and vinculin. This co-occurrence was also observed in rapidly forming adhesive structures of spreading and moving fibroblasts and in disrupting focal adhesions following microinjection of the Rho-inhibitor C3 transferase into the cells, suggesting that type XIII collagen is an integral focal adhesion component. Moreover, it appears to have an adhesion-related function since cell-surface expression of type XIII collagen in cells with weak basic adhesiveness resulted in improved cell adhesion on selected culture substrata. In tissues type XIII collagen was found in a range of integrin-mediated adherens junctions including the myotendinous junctions and costameres of skeletal muscle as well as many cell-basement membrane interfaces. Some cell-cell adhesions were found to contain type XIII collagen, most notably the intercalated discs in the heart. Taken together, the results strongly suggest that type XIII collagen has a cell adhesion-associated function in a wide array of cell-matrix junctions.

Striated muscle-specific beta(1D)-integrin and FAK are involved in cardiac myocyte hypertrophic response pathway

Alterations in the extracellular matrix occur during the cardiac hypertrophic process. Because integrins mediate cell-matrix adhesion and beta(1D)-integrin (beta1D) is expressed exclusively in cardiac and skeletal muscle, we hypothesized that beta1D and focal adhesion kinase (FAK), a proximal integrin-signaling molecule, are involved in cardiac growth. With the use of cultured ventricular myocytes and myocardial tissue, we found the following: 1) beta1D protein expression was upregulated perinatally; 2) alpha(1)-adrenergic stimulation of cardiac myocytes increased beta1D protein levels 350% and altered its cellular distribution; 3) adenovirally mediated overexpression of beta1D stimulated cellular reorganization, increased cell size by 250%, and induced molecular markers of the hypertrophic response; and 4) overexpression of free beta1D cytoplasmic domains inhibited alpha(1)-adrenergic cellular organization and atrial natriuretic factor (ANF) expression. Additionally, FAK was linked to the hypertrophic response as follows: 1) coimmunoprecipitation of beta1D and FAK was detected; 2) FAK overexpression induced ANF-luciferase; 3) rapid and sustained phosphorylation of FAK was induced by alpha(1)-adrenergic stimulation; and 4) blunting of the alpha(1)-adrenergically modulated hypertrophic response was caused by FAK mutants, which alter Grb2 or Src binding, as well as by FAK-related nonkinase, a dominant interfering FAK mutant. We conclude that beta1D and FAK are both components of the hypertrophic response pathway of cardiac myocytes.

Integrins as receptors for laminins

Laminins are a family of trimeric glycoproteins present in the extracellular matrix and the major constituents of basement membranes. Integrins are alpha beta transmembrane receptors that play critical roles in both cell-matrix and cell-cell adhesion. Several members of the integrin family, including alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, alpha 6 beta 1, alpha 7 beta 1 and alpha 6 beta 4 heterodimers serve as laminin receptors on a variety of cell types. This review summarizes recent advances in understanding the involvement of individual integrins in cell interactions with laminins and the roles of laminin-binding integrins in adhesion-mediated events in vertebrates, including embryonic development, cell migration and tumor cell invasiveness, cell proliferation and differentiation, as well as basement membrane assembly. We discuss the regulation of integrin function via alternative splicing of cytoplasmic domains of alpha and beta subunits of the integrin receptors for laminins and present examples of functional collaboration between laminin-binding integrins and non-integrin laminin receptors. Advances in our understanding of the laminin-binding integrins continue to demonstrate the essential roles these receptors play in maintaining cell polarity and tissue architecture.

Distribution of the integrin beta 1 subunit on radial cells in the embryonic and adult avian retina

The distribution of the beta1 integrin subunit was investigated in the developing and adult chick retina at the light and electron microscopic levels, using two different monoclonal antibodies. Western blotting revealed a single band with a molecular weight of approximately 130 kDa in the retina and in a number of other tissues, indicating the specificity of the antibodies. In the retina, immunoreactivity was detected on radial cells spanning the entire width between the pigment epithelium and the vitreal border. These cells were undifferentiated neuroepithelial cells at early stages and radial Müller glial cells at later stages of development. At all stages, the beta1 subunit was concentrated at the vitreal border of the retina around the inner limiting membrane. Mechanical isolation of the inner limiting membrane, as well as immunoelectron microscopy, demonstrated that this immunoreactivity was due to a concentration of the beta1 subunit in the endfeet of neuroepithelial and Müller glial cells. Injection of collagenase into the vitreous of live embryos, a procedure that selectively removes the inner limiting membrane, but does not proteolytically degrade the integrin protein, resulted in a redistribution of the integrin immunoreactivity, demonstrating that the integrity of the basal lamina is required for the maintenance of the concentration of the beta1 subunit in the endfeet. These results suggest a role for the beta1 subunit-containing integrin heterodimers in the adhesion of neuroepithelial and Müller glial cells to extracellular matrix components of the inner limiting membrane, possibly stabilizing the radial morphology of these cells.

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.

beta1B integrin subunit contains a double lysine motif that can cause accumulation within the endoplasmic reticulum

Human epidermal keratinocytes are one of the few cell types that express the beta1B splice variant of the beta1 integrin subunit. Although in transfection experiments beta1B acts as a dominant negative inhibitor of cell adhesion, we found that beta1B was expressed at very low levels in keratinocytes, both in vivo and in culture, and had a predominantly cytoplasmic distribution, concentrated within the endoplasmic reticulum. To examine why beta1B accumulated in the cytoplasm, we prepared chimeras between CD8alpha and the beta1A and beta1B integrin cytoplasmic domains. In transfected HeLa cells, both constructs reached the cell surface but the rate of maturation of the beta1B chimera was considerably retarded relative to beta1A. The beta1B cytoplasmic domain contains two lysine residues that resemble the double lysine motif characteristic of many proteins that are resident within the endoplasmic reticulum. Mutation of each lysine individually to serine had no effect on CD8beta1B maturation, but when both residues were mutated the rate of CD8beta1B maturation increased to that of CD8beta1A. Further analysis of beta1B function in keratinocytes must, therefore, take into account the low abundance of the isoform relative to beta1A and the potential for beta1B to accumulate in the endoplasmic reticulum.

The role of extracellular and cytoplasmic splice domains of alpha7-integrin in cell adhesion and migration on laminins

The major laminin-binding integrin of skeletal, smooth, and heart muscle is alpha7beta1-integrin, which is structurally related to alpha6beta1. It occurs in three cytoplasmic splice variants (alpha7A, -B, and -C) and two extracellular forms (X1 and X2) which are developmentally regulated and differentially expressed in skeletal muscle. Previously, we have shown that ectopic expression of the alpha7beta-integrin splice variant in nonmotile HEK293 cells specifically induced cell locomotion on laminin-1 but not on fibronectin. To investigate the specificity and the mechanism of the alpha7-mediated cell motility, we expressed the three alpha7-chain cytoplasmic splice variants, as well as alpha6A- and alpha6B-integrin subunits in HEK293 cells. Here we show that all three alpha7 splice variants (containing the X2 domain), as well as alpha6A and alpha6B, promote cell attachment and stimulate cell motility on laminin-1 and its E8 fragment. Deletion of the cytoplasmic domain (excluding the GFFKR consensus sequence) from alpha7B resulted in a loss of the motility-enhancing effect. On laminin-2/4 (merosin), the predominant isoform in mature skeletal muscle, only alpha7-expressing cells showed enhanced motility, whereas cells transfected with alpha6A and alpha6B neither attached nor migrated on laminin-2. Adhesion of alpha7-expressing cells to both laminin-1 and laminin-2 was specifically inhibited by a new monoclonal antibody (6A11) specific for alpha7. Expression of the two extracellular splice variants alpha7X1 and alpha7X2 in HEK293 cells conferred different motilities on laminin isoforms: Whereas alpha7X2B promoted cell migration on both laminin-1 and laminin-2, alpha7X1B supported motility only on laminin-2 and not on laminin-1, although both X1 and X2 splice variants revealed similar adhesion rates to laminin-1 and -2. Fluorescence-activated cell sorter analysis revealed a dramatic reduction of surface expression of alpha6-integrin subunits after alpha7A or -B transfection; also, surface expression of alpha1-, alpha3-, and alpha5-integrins was significantly reduced. These results demonstrate selective responses of alpha6- and alpha7-integrins and of the alpha7 splice variants to laminin-1 and -2 and indicate differential roles in laminin-controlled cell adhesion and migration.

Impaired axonal regeneration in alpha7 integrin-deficient mice

The interplay between growing axons and the extracellular substrate is pivotal for directing axonal outgrowth during development and regeneration. Here we show an important role for the neuronal cell adhesion molecule alpha7beta1 integrin during peripheral nerve regeneration. Axotomy led to a strong increase of this integrin on regenerating motor and sensory neurons, but not on the normally nonregenerating CNS neurons. alpha7 and beta1 subunits were present on the axons and their growth cones in the regenerating facial nerve. Transgenic deletion of the alpha7 subunit caused a significant reduction of axonal elongation. The associated delay in the reinnervation of the whiskerpad, a peripheral target of the facial motor neurons, points to an important role for this integrin in the successful execution of axonal regeneration.

Expression of integrin subunit beta1B in integrin beta1-deficient GD25 cells does not interfere with alphaVbeta3 functions

We have expressed the beta1B integrin subunit in beta1-deficient GD25 cells to examine beta1B functions without the interference of endogenous beta1A expression. As previously reported [Retta et al., 1998, Mol. Biol. Cell 9, 715-731], the beta1B integrins did not mediate cell adhesion under normal culture conditions, while the presence of 0.3 mM Mn(2+) allowed beta1B integrins to support adhesion. Mn(2+), as well as the small soluble peptide GRGDS, induced a beta1B conformation, which was recognized by the mAb 9EG7, a marker for active or ligand-bound integrins. beta1B integrins were found to localize to a subset of focal contacts in a ligand-independent manner on fibronectin, but not on vitronectin. However, clustering of beta1B did not induce tyrosine phosphorylation of FAK, p130(Cas), or paxillin, as studied by beta1B-mediated adhesion, to fibronectin in the presence of Mn(2+) or to anti-beta1 antibody in DMEM. Induction of ligand-occupied conformation by the GRGDS peptide during the adhesion to anti-beta1 antibody also failed to trigger FAK phosphorylation. Stimulation of tyrosine phosphorylation on FAK, p130(Cas), and paxillin by adhesion via integrin alphaVbeta3 to fibronectin or vitronectin was not disturbed in GD25-beta1B cells compared to the untransfected GD25 cells, nor were any negative effects of beta1B observed on alphaVbeta3-mediated cell attachment, spreading, and actin organization, or on the cell proliferation rate. These results show that the reported negative effects of beta1B on adhesive events do not apply to alphaVbeta3-dependent interactions and suggest that they may specifically act on beta1 integrins.

Integrin signaling's potential for mediating gene expression in hypertrophying skeletal muscle

Overloaded skeletal muscle undergoes dramatic shifts in gene expression, which alter both the phenotype and mass. Molecular biology techniques employing both in vivo and in vitro hypertrophy models have demonstrated that mechanical forces can alter skeletal muscle gene regulation. This review's purpose is to support integrin-mediated signaling as a candidate for mechanical load-induced hypertrophy. Research quantifying components of the integrin-signaling pathway in overloaded skeletal muscle have been integrated with knowledge regarding integrins role during development and cardiac hypertrophy, with the hope of demonstrating the pathway's importance. The role of integrin signaling as an integrator of mechanical forces and growth factor signaling during hypertrophy is discussed. Specific components of integrin signaling, including focal adhesion kinase and low-molecular-weight GTPase Rho are mentioned as downstream targets of this signaling pathway. There is a need for additional mechanistic studies capable of providing a stronger linkage between integrin-mediated signaling and skeletal muscle hypertrophy; however, there appears to be abundant justification for this type of research.

Integrin subunits (beta)1C-1 and (beta)1C-2 expressed in GD25T cells are retained and degraded intracellularly rather than localised to the cell surface

We have previously identified the integrin (beta)1C-2 and characterised the distribution of (beta)1C-1 and (beta)1C-2 transcripts in various cell lines and normal cells. In this study we have investigated the expression of the two (beta)1C-variants in integrin (beta)1 deficient mouse GD25T cells. After stable transfection of the GD25T cells with cDNAs coding for (beta)1A, (beta)1C-1 and (beta)1C-2, the cell surface expression of the (beta)1C-1 and (beta)1C-2 variants was found to be very low while the (beta)1A variant was expressed at high levels. Northern blot analysis showed that the level of (beta)1-transcript in the (beta)1C-1 and (beta)1C-2 clones was equal or higher than in the (beta)1A clones. Metabolic labelling and deglycosylation by endoglycosidase H treatment clearly demonstrated that the majority of the (beta)1C-1 and (beta)1C-2 chains did not become maturely glycosylated, nor did they dimerize with (alpha) subunits. After 20 hours of chase, the labelled (beta)1C-1 and (beta)1C-2 chains had been gradually degraded, whereas immature (beta)1A was converted into the maturely glycosylated form during the same period of time. Immunostaining showed intracellular (beta)1 localisation in the (beta)1C-1 and (beta)1C-2 expressing clones, while in the (beta)1A expressing clones the (beta)1 chains were mainly localised to focal adhesion sites and along fibronectin fibres. Taken together, we have shown that expression of both integrin (beta)1C-1 and (beta)1C-2 in GD25T cells result in very low cell surface expression compared with the normal (beta)1A isoform. Instead, both (beta)1C-1 and (beta)1C-2 chains remain in the endoplasmic reticulum until they are intracellularly degraded.

Induction of cell scattering by expression of beta1 integrins in beta1-deficient epithelial cells requires activation of members of the rho family of GTPases and downregulation of cadherin and catenin function

Adhesion receptors, which connect cells to each other and to the surrounding extracellular matrix (ECM), play a crucial role in the control of tissue structure and of morphogenesis. In this work, we have studied how intercellular adhesion molecules and beta1 integrins influence each other using two different beta1-null cell lines, epithelial GE11 and fibroblast-like GD25 cells. Expression of beta1A or the cytoplasmic splice variant beta1D, induced the disruption of intercellular adherens junctions and cell scattering in both GE11 and GD25 cells. In GE11 cells, the morphological change correlated with the redistribution of zonula occluden (ZO)-1 from tight junctions to adherens junctions at high cell confluency. In addition, the expression of beta1 integrins caused a dramatic reorganization of the actin cytoskeleton and of focal contacts. Interaction of beta1 integrins with their respective ligands was required for a complete morphological transition towards the spindle-shaped fibroblast-like phenotype. The expression of an interleukin-2 receptor (IL2R)-beta1A chimera and its incorporation into focal adhesions also induced the disruption of cadherin-based adhesions and the reorganization of ECM-cell contacts, but failed to promote cell migration on fibronectin, in contrast to full-length beta1A. This indicates that the disruption of cell-cell adhesion is not simply the consequence of the stimulated cell migration. Expression of beta1 integrins in GE11 cells resulted in a decrease in cadherin and alpha-catenin protein levels accompanied by their redistribution from the cytoskeleton-associated fraction to the detergent-soluble fraction. Regulation of alpha-catenin protein levels by beta1 integrins is likely to play a role in the morphological transition, since overexpression of alpha-catenin in GE11 cells before beta1 prevented the disruption of intercellular adhesions and cell scattering. In addition, using biochemical activity assays for Rho-like GTPases, we show that the expression of beta1A, beta1D, or IL2R-beta1A in GE11 or GD25 cells triggers activation of both RhoA and Rac1, but not of Cdc42. Moreover, dominant negative Rac1 (N17Rac1) inhibited the disruption of cell-cell adhesions when expressed before beta1. However, all three GTPases might be involved in the morphological transition, since expression of either N19RhoA, N17Rac1, or N17Cdc42 reversed cell scattering and partially restored cadherin-based adhesions in GE11-beta1A cells. Our results indicate that beta1 integrins regulate the polarity and motility of epithelial cells by the induction of intracellular molecular events involving a downregulation of alpha-catenin function and the activation of the Rho-like G proteins Rac1 and RhoA.

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

We have determined the structure and the exon size pattern of the human integrin alpha7 subunit gene (ITGA7), which has been shown to be affected in a form of congenital myopathy. The gene is composed of at least 27 exons spanning a region of about 22.5 kb. The sequence of all exon/intron boundaries was determined and conforms to the GT/AG splicing consensus. We investigated the different splicing forms previously described in human and rodents. The major cytoplasmic variants alpha7A and alpha7B, which are developmentally regulated and tissue specific, were identified in human tissues, as well as the extracellular isoforms X1 and X2. The recently described D variant was detected in adult tissues by RT-PCR but not the C variant. We localized ITGA7 on chromosome 12q13 by high-resolution radiation hybrid mapping between D12S312 and D12S90 and identified a new CA-repeat microsatellite in intron 1.

Integrins: alternative splicing as a mechanism to regulate ligand binding and integrin signaling events

Integrins are a family of transmembrane proteins composed of heterodimers of alpha and beta subunits. With their extracellular domain they bind extracellular matrix proteins or other cell surface molecules, and their cytoplasmic domain binds to cytoskeletal and signaling proteins. Thus, they are in an ideal position to transfer information from the extracellular environment to the interior of the cell and vice versa. For several integrin subunits, alternative splicing of mRNA leads to variations in the sequence of both extracellular and cytoplasmic domains. Many integrin splice variants have specific expression patterns, but for some time, functional differences between these variants were not evident. Recent experiments using transfected cell lines and gene targeting of specific splice variants have contributed significantly to our understanding of the function of these splice variants. The results indicate that alternative splicing is a mechanism to subtly regulate the ligand binding and signaling activity of integrins.

Secondary reduction of alpha7B integrin in laminin alpha2 deficient congenital muscular dystrophy supports an additional transmembrane link in skeletal muscle

The integrins are a large family of heterodimeric transmembrane cellular receptors which mediate the association between the extracellular matrix (ECM) and cytoskeletal proteins. The alpha7beta1 integrin is a major laminin binding integrin in skeletal and cardiac muscle and is thought to be involved in myogenic differentiation and migration processes. The main binding partners of the alpha7 integrin are laminin-1 (alpha1-beta1-gamma1), laminin-2 (alpha2-beta1-gamma1) and laminin-4 (alpha2-beta2-gamma1). Targeted deletion of the gene for the alpha7 integrin subunit (ITGA7) in mice leads to a novel form of muscular dystrophy. In the present study we have investigated the expression of two alternative splice variants, the alpha7B and beta1D integrin subunits, in normal human skeletal muscle, as well as in various forms of muscular dystrophy. In normal human skeletal muscle the expression of the alpha7 integrin subunit appeared to be developmentally regulated: it was first detected at 2 years of age. In contrast, the beta1D integrin could be detected in immature and mature muscle in the sarcolemma of normal fetal skeletal muscle at 18 weeks gestation. The expression of alpha7B integrin was significantly reduced at the sarcolemma in six patients with laminin alpha2 chain deficient congenital muscular dystrophy (CMD) (age >2 years). However, this reduction was not correlated with the amount of laminin alpha2 chain expressed. In contrast, the expression of the laminin alpha2 chain was not altered in the skeletal muscle of the alpha7 knock-out mice. These data argue in favor that there is not a tight correlation between the expression of the alpha7 integrin subunit and that of the laminin alpha2 chain in either human or murine dystrophic muscle. Interestingly, in dystrophinopathies (Duchenne and Becker muscular dystrophy; DMD/BMD) expression of alpha7B was upregulated irrespective of the level of dystrophin expression as shown by a strong sarcolemmal staining pattern even in young boys (age <2 years). The expression of the beta1D integrin subunit was not altered in any of our patients with different types of muscular dystrophy. In contrast, sarcolemmal expression of beta1D integrin was significantly reduced in the alpha7 integrin knock-out mice, whereas the expression of the components of the DGC was not altered. The secondary loss of alpha7B in laminin alpha2 chain deficiency defines a biochemical change in the composition of the plasma membrane resulting from a primary protein deficiency in the basal lamina. These findings, in addition to the occurrence of a muscular dystrophy in alpha7 deficient mice, implies that the alpha7B integrin is an important laminin receptor within the plasma membrane which plays a significant role in skeletal muscle function and stability.

The muscle-specific laminin receptor alpha7 beta1 integrin negatively regulates alpha5 beta1 fibronectin receptor function

alpha7 beta1 is the major integrin complex expressed in differentiated muscle cells where it functions as a laminin receptor. In this work we have expressed the alpha7 integrin subunit in CHO cells to investigate the functional properties of this receptor. After transfection with alpha7 CHO cells acquired the ability to adhere and spread on laminin 1 consistent with the laminin receptor activity of the alpha7 beta1. alpha7 transfectants, however, showed a 70% reduction in the ability to adhere to fibronectin and were unable to assemble a fibronectin matrix. The degree of reduction was inversely related to the level of alpha7 expression. To define the mechanisms underlying this adhesive defect we analyzed surface expression and functional properties of the alpha5 beta1 fibronectin receptor. Although cell surface expression of alpha5 beta1 was reduced by a factor of 20-25% in alpha7 transfectants compared to control untransfected cells, this slight reduction was not sufficient to explain the dramatic reduction in cell adhesion (70%) and matrix assembly (close to 100%). Binding studies showed that the affinity of 125I-fibronectin for its surface receptor was decreased by 50% in alpha7 transfectants, indicating that the alpha5 beta1 integrin is partially inactivated in these cells. Inactivation can be reversed by Mn2+, a cation known to increase integrin affinity for their ligands. In fact, incubation of cells with Mn2+ restored fibronectin binding affinity, adhesion to fibronectin, and assembly of fibronectin matrix in alpha7 transfectants. These data indicate that alpha7 expression leads to the functional down regulation of alpha5beta1 integrin by decreasing ligand binding affinity and surface expression. In conclusion, the data reported establish the existence of a negative cooperativity between alpha7 and alpha5 integrins that may be important in determining functional regulation of integrins during myogenic differentiation.