Highlights of glycosylation and adhesion related genes involved in myogenesis

BACKGROUND

Myogenesis is initiated by myoblast differentiation and fusion to form myotubes and muscle fibres. A population of myoblasts, known as satellite cells, is responsible for post-natal growth of muscle and for its regeneration. This differentiation requires many changes in cell behaviour and its surrounding environment. These modifications are tightly regulated over time and can be characterized through the study of changes in gene expression associated with this process. During the initial myogenesis steps, using the myoblast cell line C2C12 as a model, Janot et al. (2009) showed significant variations in expression for genes involved in pathways of glycolipid synthesis. In this study we used murine satellite cells (MSC) and their ability to differentiate into myotubes or early fat storage cells to select glycosylation related genes whose variation of expression is myogenesis specific.

RESULTS

The comparison of variant genes in both MSC differentiation pathways identified 67 genes associated with myogenesis. Comparison with data obtained for C2C12 revealed that only 14 genes had similar expression profiles in both cell types and that 17 genes were specifically regulated in MSC. Results were validated statistically by without a priori clustering. Classification according to protein function encoded by these 31 genes showed that the main regulated cellular processes during this differentiation were (i) remodeling of the extracellular matrix, particularly, sulfated structures, (ii) down-regulation of O-mannosyl glycan biosynthesis, and (iii) an increase in adhesion protein expression. A functional study was performed on Itga11 and Chst5 encoding two highly up-regulated proteins. The inactivation of Chst5 by specific shRNA delayed the fusion of MSC. By contrast, the inactivation of Itga11 by specific shRNA dramatically decreased the fusion ability of MSC. This result was confirmed by neutralization of Itga11 product by specific antibodies.

CONCLUSIONS

Our screening method detected 31 genes specific for myogenic differentiation out of the 383 genes studied. According to their function, interaction networks of the products of these selected genes converged to cell fusion. Functional studies on Itga11 and Chst5 demonstrated the robustness of this screening.

Docosahexaenoic acid decreases TNF-α-induced lectin-like oxidized low-density lipoprotein receptor-1 expression in THP-1 cells

Docosahexaenoic acid (DHA) prevents atherosclerosis and may decrease monocyte/macrophage activation by tumor necrosis factor (TNF)-α. Here, we sought to determine the protective effects of DHA against TNF-α-induced stimulation of lectin-like oxidized low-density lipoprotein (LDL) receptor-1 (LOX-1) expression, which is associated with atherosclerosis. Using reverse transcription polymerase chain reaction, we found that TNF-α induced the expression of LOX-1 (OLR1), NADPH oxidase 2 (Nox2), p47phox (NCF1), very late antigen-4 (ITGA4), and lymphocyte function-associated antigen (ITGAL) genes. Additionally, DHA attenuated TNF-α-induced acetylated (Ac)-LDL uptake and reactive oxygen species (ROS) production, as measured using fluorescently labeled LDL and H2DCFDA, respectively, and reduced the expression levels of these genes. Moreover, the PI3 kinase inhibitor LY294002 blocked these effects of DHA. These results indicated that DHA inhibited several events associated with redox regulation in a PI3K-dependent manner, thereby mediating the expression of LOX-1 in monocytes/macrophages.

Epidermal Th22 and Tc17 cells form a localized disease memory in clinically healed psoriasis

Psoriasis is a common and chronic inflammatory skin disease in which T cells play a key role. Effective treatment heals the skin without scarring, but typically psoriasis recurs in previously affected areas. A pathogenic memory within the skin has been proposed, but the nature of such site-specific disease memory is unknown. Tissue-resident memory T (TRM) cells have been ascribed a role in immunity after resolved viral skin infections. Because of their localization in the epidermal compartment of the skin, TRM may contribute to tissue pathology during psoriasis. In this study, we investigated whether resolved psoriasis lesions contain TRM cells with the ability to maintain and potentially drive recurrent disease. Three common and effective therapies, narrowband-UVB treatment and long-term biologic treatment systemically inhibiting TNF-α or IL-12/23 signaling were studied. Epidermal T cells were highly activated in psoriasis and a high proportion of CD8 T cells expressed TRM markers. In resolved psoriasis, a population of cutaneous lymphocyte-associated Ag, CCR6, CD103, and IL-23R expressing epidermal CD8 T cells was highly enriched. Epidermal CD8 T cells expressing the TRM marker CD103 responded to ex vivo stimulation with IL-17A production and epidermal CD4 T cells responded with IL-22 production after as long as 6 y of TNF-α inhibition. Our data suggest that epidermal TRM cells are retained in resolved psoriasis and that these cells are capable of producing cytokines with a critical role in psoriasis pathogenesis. We provide a potential mechanism for a site-specific T cell-driven disease memory in psoriasis.

[Characteristics of migration of adipose tissue derived mesenchymal stromal cells after co-cultivation with activated monocytes in vitro]

Mesenchymal stromal cells (MSC) are considered to be promising tool of regenerative medicine. Migration of MSC toward damaged inflammatory site is essential for physiological tissue reparation. Therefore we studied modifications of migratory features of adipose tissue derived MSC (AT-MSC) after co-cultivation with activated monocytes derived from THP-1 cell line. As a result, we have observed an increased migration rate of AT-MSC in vitro in the absence of chemoattractant gradient as well as toward the gradient of PDGF BB (platelet-derived growth factor BB), which is well known chemoattractant for the cells of mesenchymal origin. Furthermore, the rate of directional AT-MSC migration through fibronectin was also increased. We have established that signaling from PDGFRβ which is activated through binding of integrin receptors with extracellular matrix may be possible way to stimulate cellular migration under simulated inflammatory conditions.

Natural genetic variation of integrin alpha L (Itgal) modulates ischemic brain injury in stroke

During ischemic stroke, occlusion of the cerebrovasculature causes neuronal cell death (infarction), but naturally occurring genetic factors modulating infarction have been difficult to identify in human populations. In a surgically induced mouse model of ischemic stroke, we have previously mapped Civq1 to distal chromosome 7 as a quantitative trait locus determining infarct volume. In this study, genome-wide association mapping using 32 inbred mouse strains and an additional linkage scan for infarct volume confirmed that the size of the infarct is determined by ancestral alleles of the causative gene(s). The genetically isolated Civq1 locus in reciprocal recombinant congenic mice refined the critical interval and demonstrated that infarct size is determined by both vascular (collateral vessel anatomy) and non-vascular (neuroprotection) effects. Through the use of interval-specific SNP haplotype analysis, we further refined the Civq1 locus and identified integrin alpha L (Itgal) as one of the causative genes for Civq1. Itgal is the only gene that exhibits both strain-specific amino acid substitutions and expression differences. Coding SNPs, a 5-bp insertion in exon 30b, and increased mRNA and protein expression of a splice variant of the gene (Itgal-003, ENSMUST00000120857), all segregate with infarct volume. Mice lacking Itgal show increased neuronal cell death in both ex vivo brain slice and in vivo focal cerebral ischemia. Our data demonstrate that sequence variation in Itgal modulates ischemic brain injury, and that infarct volume is determined by both vascular and non-vascular mechanisms.

Genetic influences on craving for alcohol

INTRODUCTION

Craving is being considered for inclusion in the Diagnostic and Statistical Manual (DSM) DSM-5. However, little is known of its genetic underpinnings - specifically, whether genetic influences on craving are distinct from those influencing DSM-IV alcohol dependence.

METHOD

Analyses were conducted in a sample of unrelated adults ascertained for alcohol dependence (N=3976). Factor analysis was performed to examine how alcohol craving loaded with the existing DSM-IV alcohol dependence criteria. For genetic analyses, we first examined whether genes in the dopamine pathway, including dopamine receptor genes (DRD1, DRD2, DRD3, DRD4) and the dopamine transporter gene (SLC6A3), which have been implicated in neurobiological studies of craving, as well as alpha-synuclein (SNCA), which has been previously found to be associated with craving, were associated with alcohol craving in this sample. Second, in an effort to identify novel genetic variants associated with craving, we conducted a genomewide association study (GWAS). For variants that were implicated in the primary analysis of craving, we conducted additional comparisons - to determine if these variants were uniquely associated with alcohol craving as compared with alcohol dependence. We contrasted our results to those obtained for DSM-IV alcohol dependence, and also compared alcohol dependent individuals without craving to non-dependent individuals who also did not crave alcohol.

RESULTS

Twenty-one percent of the full sample reported craving alcohol. Of those reporting craving, 97.3% met criteria for DSM-IV alcohol dependence with 48% endorsing all 7 dependence criteria. Factor analysis found a high factor loading (0.89) for alcohol craving. When examining genes in the dopamine pathway, single nucleotide polymorphisms (SNPs) in DRD3 and SNCA were associated with craving (p<0.05). There was evidence for association of these SNPs with DSM-IV alcohol dependence (p<0.05) but less evidence for dependence without craving (p>0.05), suggesting that the association was due in part to craving. In the GWAS, the greatest evidence of association with craving was for a SNP in the integrin alpha D (ITGAD) gene on chromosome 7 (rs2454908; p=1.8×10(-6)). The corresponding p-value for this SNP with DSM-IV alcohol dependence was similar (p=4.0×10(-5)) but was far less with dependence without craving (p=0.02), again suggesting the association was due to alcohol craving. Adjusting for dependence severity (number of endorsed criteria) attenuated p-values but did not eliminate association.

CONCLUSIONS

Craving is frequently reported by those who report multiple other alcohol dependence symptoms. We found that genes providing evidence of association with craving were also associated with alcohol dependence; however, these same SNPs were not associated with alcohol dependence in the absence of alcohol craving. These results suggest that there may be unique genetic factors affecting craving among those with alcohol dependence.

The integrin-ligand interaction regulates adhesion and migration through a molecular clutch

Adhesive and migratory behavior can be cell type, integrin, and substrate dependent. We have compared integrin and substrate differences using three integrin receptors: α5β1, α6β1, and αLβ2 expressed in a common cell type, CHO.B2 cells, which lack integrin α subunits, as well as in different cell types that express one or more of these integrins. We find that CHO.B2 cells expressing either α6β1 or αLβ2 integrins migrate and protrude faster and are more directionally persistent on laminin or ICAM-1, respectively, than CHO.B2 cells expressing α5β1 on fibronectin. Despite rapid adhesion maturation and the presence of large adhesions in both the α6β1- and αLβ2-expressing cells, they display robust tyrosine phosphorylation. In addition, whereas myosin II regulates adhesion maturation and turnover, protrusion rates, and polarity in cells migrating on fibronectin, surprisingly, it does not have comparable effects in cells expressing α6β1 or αLβ2. This apparent difference in the integration of myosin II activity, adhesion, and migration arises from alterations in the ligand-integrin-actin linkage (molecular clutch). The elongated adhesions in the protrusions of the α6β1-expressing cells on laminin or the αLβ2-expressing cells on ICAM-1 display a novel, rapid retrograde flux of integrin; this was largely absent in the large adhesions in protrusions of α5β1-expressing cells on fibronectin. Furthermore, the force these adhesions exert on the substrate in protrusive regions is reduced compared to similar regions in α5-expressing cells, and the adhesion strength is reduced. This suggests that intracellular forces are not efficiently transferred from actomyosin to the substratum due to altered adhesion strength, that is, avidity, affinity, or the ligand-integrin-actin interaction. Finally, we show that the migration of fast migrating leukocytes on fibronectin or ICAM-1 is also largely independent of myosin II; however, their adhesions are small and do not show retrograde fluxing suggesting other intrinsic factors determine their migration differences.

The newcomer in the integrin family: integrin α9 in biology and cancer

Integrins are heterodimeric transmembrane receptors regulating cell-cell and cell-extracellular matrix interactions. Of the 24 integrin heterodimers identified in humans, α9β1 integrin is one of the least studied. α9, together with α4, comprise a more recent evolutionary sub-family of integrins that is only found in vertebrates. Since α9 was thought to have similar functions as α4, due to many shared ligands, it was a rather overlooked integrin until recently, when its importance for survival after birth was highlighted upon investigation of the α9 knockout mouse. α9β1 is expressed on a wide variety of cell types, interacts with many ligands for example fibronectin, tenascin-C and ADAM12, and has been shown to have important functions in processes such as cell adhesion and migration, lung development, lymphatic and venous valve development, and in wound healing. This has sparked an interest to investigate α9β1-mediated signaling and its regulation. This review gives an overview of the recent progress in α9β1-mediated biological and pathological processes, and discusses its potential as a target for cancer diagnosis and therapy.

Phenotypic and functional properties of Helios+ regulatory T cells

Helios, an Ikaros family transcription factor, is preferentially expressed at the mRNA and protein level in regulatory T cells. Helios expression previously appeared to be restricted to thymic-derived Treg. Consistent with recent data, we show here that Helios expression is inducible in vitro under certain conditions. To understand phenotypic and functional differences between Helios⁺ and Helios⁻ Treg, we profiled cell-surface markers of FoxP3⁺ Treg using unmanipulated splenocytes. We found that CD103 and GITR are expressed at high levels on a subset of Helios⁺ Treg and that a Helios⁺ Treg population could be significantly enriched by FACS sorting using these two markers. Quantitative real-time PCR (qPCR) analysis revealed increased TGF-β message in Helios⁺ Treg, consistent with the possibility that this population possesses enhanced regulatory potential. In tumor-bearing mice, we found that Helios⁺ Treg were relatively over-represented in the tumor-mass, and BrdU studies showed that, in vivo, Helios⁺ Treg proliferated more than Helios⁻ Treg. We hypothesized that Helios-enriched Treg might exert increased suppressive effects. Using in vitro suppression assays, we show that Treg function correlates with the absolute number of Helios⁺ cells in culture. Taken together, these data show that Helios⁺ Treg represent a functional subset with associated CD103 and GITR expression.

Respiratory syncytial virus interferon antagonist NS1 protein suppresses and skews the human T lymphocyte response

We recently demonstrated that the respiratory syncytial virus (RSV) NS1 protein, an antagonist of host type I interferon (IFN-I) production and signaling, has a suppressive effect on the maturation of human dendritic cells (DC) that was only partly dependent on released IFN-I. Here we investigated whether NS1 affects the ability of DC to activate CD8+ and CD4+ T cells. Human DC were infected with RSV deletion mutants lacking the NS1 and/or NS2 genes and assayed for the ability to activate autologous T cells in vitro, which were analyzed by multi-color flow cytometry. Deletion of the NS1, but not NS2, protein resulted in three major effects: (i) an increased activation and proliferation of CD8+ T cells that express CD103, a tissue homing integrin that directs CD8+ T cells to mucosal epithelial cells of the respiratory tract and triggers cytolytic activity; (ii) an increased activation and proliferation of Th17 cells, which have recently been shown to have anti-viral effects and also indirectly attract neutrophils; and (iii) decreased activation of IL-4-producing CD4+ T cells--which are associated with enhanced RSV disease--and reduced proliferation of total CD4+ T cells. Except for total CD4+ T cell proliferation, none of the T cell effects appeared to be due to increased IFN-I signaling. In the infected DC, deletion of the NS1 and NS2 genes strongly up-regulated the expression of cytokines and other molecules involved in DC maturation. This was partly IFN-I-independent, and thus might account for the T cell effects. Taken together, these data demonstrate that the NS1 protein suppresses proliferation and activation of two of the protective cell populations (CD103+ CD8+ T cells and Th17 cells), and promotes proliferation and activation of Th2 cells that can enhance RSV disease.

Memory T cells persisting within the brain after local infection show functional adaptations to their tissue of residence

The brain is not routinely surveyed by lymphocytes and is defined as an immuno-privileged site. However, viral infection of the brain results in the infiltration and long-term persistence of pathogen-specific CD8(+) T cells. These cells survive without replenishment from the circulation and are referred to as resident memory T cells (Trm). Brain Trm selectively express the integrin CD103, the expression of which is dependent on antigen recognition within the tissue. After clearance of virus, CD8(+) T cells persist in tight clusters, presumably at prior infection hot spots. Antigen persistence is not a prerequisite for T-cell retention, as suggested by the failure to detect viral genomes in the T-cell clusters. Furthermore, we show that an intracranial dendritic cell immunization regimen, which allows the transient introduction of antigen, also results in the generation of memory T cells that persist long term in the brain. Brain Trm die rapidly on isolation from the tissue and fail to undergo recall expansion after adoptive transfer into the bloodstream of antigen-challenged recipients. These ex vivo defects imply a dependency on the local milieu for function and survival. Cumulatively, this work shows that Trm are a specialized population of memory T cells that can be deposited in tissues previously thought to be beyond routine immune surveillance.

[Expression and significance of integrins subunits in laryngeal squamous cell carcinoma]

OBJECTIVE

This study was to investigate the expression and significance of Integrins subunits in laryngeal squamous cell carcinoma (LSCC).

METHOD

The expression of Integrins subunits was detected by cDNA microarray in 4 cases of primary LSCC tissues and corresponding adjacent normal tissues. Semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) were used to identify the different expression of Integrins subunits in 24 cases of primary LSCC tissues and corresponding adjacent normal tissues.

RESULT

A cDNA microarray analysis revealed significant changes in the expression of Integrins subunits, with IntegrinalphaV, Integrinbeta8 being up-regulated and Integrinalpha8 being down-regulated. The result of RT-PCR was consistent with that of cDNA microarray. The mRNA levels of IntegrinalphaV and Integrinbeta8 were significantly higher in LSCC tissues than that in corresponding adjacent normal tissues (1.0131 +/- 0.4780 vs 0.7591 +/- 0.4678 for IntegrinalphaV, P<0.05, 1.7362 +/- 1.3849 vs 1.2267 +/- 0.9363 for Integrinbeta8, P<0.05). The mRNA levels of Integrinalpha8 were significantly lower in LSCC tissues than that in corresponding adjacent normal tissues (0.2646 +/- 0.2622 vs 0.5457 +/- 0.3827, P<0.05).

CONCLUSION

The expression of IntegrinalphaV, Integrinbeta8, Integrinalpha8 were significantly up-regulated or down-regulated in laryngeal squamous cell carcinoma, which may relate to tumorigenesis and development of laryngeal squamous cell carcinoma.

Ataxin-3 plays a role in mouse myogenic differentiation through regulation of integrin subunit levels

Background

During myogenesis several transcription factors and regulators of protein synthesis and assembly are rapidly degraded by the ubiquitin-proteasome system (UPS). Given the potential role of the deubiquitinating enzyme (DUB) ataxin-3 in the UPS, and the high expression of the murine ataxin-3 homolog in muscle during embryogenesis, we sought to define its role in muscle differentiation.

Methodology/Principal Findings

Using immunofluorescence analysis, we found murine ataxin-3 (mATX3) to be highly expressed in the differentiated myotome of E9.5 mouse embryos. C2C12 myoblasts depleted of mATX3 by RNA interference exhibited a round morphology, cell misalignment, and a delay in differentiation following myogenesis induction. Interestingly, these cells showed a down-regulation of α5 and α7 integrin subunit levels both by immunoblotting and immunofluorescence. Mouse ATX3 was found to interact with α5 integrin subunit and to stabilize this protein by repressing its degradation through the UPS. Proteomic analysis of mATX3-depleted C2C12 cells revealed alteration of the levels of several proteins related to integrin signaling.

Conclusions

Ataxin-3 is important for myogenesis through regulation of integrin subunit levels.

MMP-9, uPAR and cathepsin B silencing downregulate integrins in human glioma xenograft cells in vitro and in vivo in nude mice

BACKGROUND

Involvement of MMP-9, uPAR and cathepsin B in adhesion, migration, invasion, proliferation, metastasis and tumor growth has been well established. In the present study, MMP-9, uPAR and cathepsin B genes were downregulated in glioma xenograft cells using shRNA plasmid constructs and we evaluated the involvement of integrins and changes in their adhesion, migration and invasive potential.

METHODOLOGY/PRINCIPAL FINDINGS

MMP-9, uPAR and cathepsin B single shRNA plasmid constructs were used to downregulate these molecules in xenograft cells. We also used MMP-9/uPAR and MMP-9/cathepsin B bicistronic constructs to evaluate the cumulative effects. MMP-9, uPAR and cathepsin B downregulation significantly inhibits xenograft cell adhesion to several extracellular matrix proteins. Treatment with MMP-9, uPAR and cathepsin B shRNA of xenografts led to the downregulation of several alpha and beta integrins. In all the assays, we noticed more prominent effects with the bicistronic plasmid constructs when compared to the single plasmid shRNA constructs. FACS analysis demonstrated the expression of alphaVbeta3, alpha6beta1 and alpha9beta1 integrins in xenograft cells. Treatment with bicistronic constructs reduced alphaVbeta3, alpha6beta1 and alpha9beta1 integrin expressions in xenograft injected nude mice. Migration and invasion were also inhibited by MMP-9, uPAR and cathepsin B shRNA treatments as assessed by spheroid migration, wound healing, and Matrigel invasion assays. As expected, bicistronic constructs further inhibited the adhesion, migration and invasive potential of the xenograft cells as compared to individual treatments.

CONCLUSIONS/SIGNIFICANCE

Downregulation of MMP-9, uPAR and cathespin B alone and in combination inhibits adhesion, migration and invasive potential of glioma xenografts by downregulating integrins and associated signaling molecules. Considering the existence of integrin inhibitor-resistant cancer cells, our study provides a novel and effective approach to inhibiting integrins by downregulating MMP-9, uPAR and cathepsin B in the treatment of glioma.

Quadruplex structures of muscle gene promoter sequences enhance in vivo MyoD-dependent gene expression

Gene promoters are enriched in guanine clusters that potentially fold into quadruplex structures. Such quadruplexes were implicated in the regulation of gene expression, plausibly by interacting with transcription factors. We showed previously that homodimers of the myogenic transcription factor MyoD bound in vitro most tightly bimolecular quadruplexes of promoter sequences of muscle-specific genes. By contrast, MyoD-E47 heterodimers formed tighter complexes with d(CANNTG) E-box motifs that govern muscle gene expression. Here, we show that DNA quadruplexes enhance in vivo MyoD and E-box-driven expression of a firefly luciferase (FL) reporter gene. HEK293 cells were transfected with FL expressing p4RTK-FL vector alone or together with MyoD expressing pEMSV-MyoD plasmid, with quadruplexes of α7 integrin or sarcomeric mitochondrial creatine kinase (sMtCK) muscle gene promoters or with a combination thereof. Whereas MyoD elevated by ∼10-fold the levels of FL mRNA and protein, the DNA quadruplexes by themselves did not affect FL expression. However, together with MyoD, quadruplex DNA increased by ∼35-fold the amounts of FL mRNA and protein. Without affecting its expression, DNA quadruplexes bound MyoD in the cells. Based on these results, we propose models for the regulation of muscle gene transcription by direct interaction of MyoD with promoter quadruplex structures.

Expression of Th2-skewed pathology mediators in monocyte-derived type 2 of dendritic cells (DC2)

The information conveyed from dendritic cells (DCs) to naïve CD4(+) T cells has crucial influence on their differentiation toward effector T cells. In an effort to identify DC-derived molecules directly contributing to T cell differentiation, we searched for molecules distinctively expressed between two DC subtypes, which were differentiated from peripheral monocytes by cultivation with GM-CSF (for DC1) or IL-3 (for DC2) in the presence of IL-4 and had the ability to induce naïve T cells to differentiate into Th1 or Th2 cells, respectively. As the first step to address this issue, we subtracted DC1 transcripts from those of DC2 and compiled the gene profile dominantly expressed in DC2, whose products are known to reside in other than the nucleus. Intriguingly, many of them were molecules involved in Th2-skewed disease pathologies, such as FN1, ITGAE, GPNMB, PLAUR, FPRL2, LILRB4, SERPINE1, ALOX15, TBXAS1, NCF2, CCL3, IL1RN, SPARC, and STAB1, suggesting that DCs function not only as antigen presenting cells but also as producers of Th2 pathology specific milieus leading to disease deteriorations. We also found that expressions of CYP27A1, PPAP2B, RSAD2, and ABCC3 were up-regulated in DC2, implying their significant function in Th2-deviated states. The identification of differentially expressed genes between DC subtypes provides new insights into their functions and our comparative gene expression profile will be highly useful for the identification of DC-derived key molecules for T cell differentiation.

Migfilin, a molecular switch in regulation of integrin activation

The linkage of heterodimeric (alpha/beta) integrin receptors with their extracellular matrix ligands and intracellular actin cytoskeleton is a fundamental step for controlling cell adhesion and migration. Binding of the actin-linking protein, talin, to integrin beta cytoplasmic tails (CTs) induces high affinity ligand binding (integrin activation), whereas binding of another actin-linking protein, filamin, to the integrin beta CTs negatively regulates this process by blocking the talin-integrin interaction. Here we show structurally that migfilin, a novel cytoskeletal adaptor highly enriched in the integrin adhesion sites, strongly interacts with the same region in filamin where integrin beta CTs bind. We further demonstrate that the migfilin interaction dissociates filamin from integrin and promotes the talin/integrin binding and integrin activation. Migfilin thus acts as a molecular switch to disconnect filamin from integrin for regulating integrin activation and dynamics of extracellular matrix-actin linkage.

Genetically determined proteolytic cleavage modulates alpha7beta1 integrin function

The dystrophin-glycoprotein complex and the alpha7beta1 integrin are trans-sarcolemmal linkage systems that connect and transduce contractile forces between muscle fibers and the extracellular matrix. alpha7beta1 is the major laminin binding integrin in skeletal muscle. Different functional variants of this integrin are generated by alternative splicing and post-translational modifications such as glycosylation and ADP-ribosylation. Here we report a species-specific difference in alpha7 chains that results from an intra-peptide proteolytic cleavage, by a serine protease, at the 603RRQ605 site. Site-directed mutagenesis of RRQ to GRQ prevents this cleavage. This RRQ sequence in the alpha7 integrin chain is highly conserved among vertebrates but it is absent in mice. Protein structure modeling indicates this cleavage site is located in an open region between the beta-propeller and thigh domains of the alpha7 chain. Compared with the non-cleavable alpha7 chain, the cleaved form enhances cell adhesion and spreading on laminin. Cleavage of the alpha7 chain is elevated upon myogenic differentiation, and this cleavage may be mediated by urokinase-type plasminogen activator. These results suggest proteolytic cleavage is a novel mechanism that regulates alpha7 integrin functions in skeletal muscle, and that the generation of such cleavage sites is another evolutionary mechanism for expanding and modifying protein functions.

Expression of integrin alpha10 is induced in malignant melanoma

Recently, integrin alpha10 was described as a collagen type II-binding integrin expressed mainly in chondrocytes. However, by array studies we detected integrin alpha10 also to be upregulated in malignant melanoma compared to primary melanocytes. Subsequent analysis of melanoma cell lines and melanoma tumor samples confirmed this finding. Further, we demonstrated that expression of integrin alpha10 is controlled by AP-2 and Ets-1, two transcription factors known to be involved in melanoma development and progression. To investigate the functional relevance of integrin alpha10, expression was downregulated via stable antisense transfection. Proliferation assays and colony forming assays revealed no differences comparing antisense integrin alpha10 cell clones with control and wild type melanoma cells, respectively. However, antisense integrin alpha10 cell clones and Mel Im cells treated with an inhibitory antibody against integrin alpha10 showed a reduced migratory potential. In summary, these data indicate that AP-2 and Ets-1 regulated expression of integrin alpha10 plays a role in migration of malignant melanoma cells.

Multiple alpha subunits of integrin are involved in cell-mediated responses of the Manduca immune system

The cell-mediated responses of the insect innate immune system-phagocytosis, nodulation, encapsulation-involve multiple cell adhesion molecules of hemocyte surfaces. A hemocyte-specific (HS) integrin and a member of the immunoglobulin (Ig) superfamily (neuroglian) are involved in the encapsulation response of hemocytes in Manduca sexta. In addition, two new integrin alpha (alpha) subunits have been found on these hemocytes. The alpha2 subunit is mainly expressed in epidermis and Malphigian tubules, whereas the alpha3 subunit is primarily expressed on hemocytes and fat body cells. Of the three known alpha subunits, the alpha1 subunit found in HS integrin is the predominant subunit of hemocytes. Cell adhesion assays indicate that alpha2 belongs to the integrin family with RGD-binding motifs, confirming the phylogenetic analysis of alpha subunits based on the amino-acid sequence alignment of different alpha subunits. Double-stranded RNAs (dsRNAs) targeting each of these three integrin alpha subunits not only specifically decreased transcript expression of each alpha subunit in hemocytes, but also abolished the cell-mediated encapsulation response of hemocytes to foreign surfaces. The individual alpha subunits of M. sexta integrins, like their integrin counterparts in mammalian immune systems, have critical, individual roles in cell-substrate and cell-cell interactions during immune responses.

Modulation of ligand binding by alternative splicing of the alphaPS2 integrin subunit

The Drosophila alphaPS2 integrin subunit is found in two isoforms. alphaPS2C contains 25 residues not found in alphaPS2m8, encoded by the alternative eighth exon. Previously, it was shown that cells expressing alphaPS2C spread more effectively than alphaPS2m8 cells on fragments of the ECM protein Tiggrin, and that alphaPS2C-containing integrins are relatively insensitive to depletion of Ca(2+). Using a ligand mimetic probe for Tiggrin affinity (TWOW-1), we show that the affinity of alphaPS2CbetaPS for this ligand is much higher than that of alphaPS2m8betaPS. However, the two isoforms become more similar in the presence of activating levels of Mn(2+). Modeling indicates that the exon 8-encoded residues replace the third beta strand of the third blade of the alpha subunit beta-propeller structure, and generate an exaggerated loop between this and the fourth strand. alphaPS2 subunits with the extra loop structure but with an m8-like third strand, or subunits with a C-like strand but an m8-like short loop, both fail to show alphaPS2C-like affinity for TWOW-1. Surprisingly, a single C > m8-like change at the third strand-loop transition point is sufficient to make alphaPS2C require Ca(2+) for function, despite the absence of any known cation binding site in this region. These data indicate that alternative splicing in integrin alpha subunit extracellular domains may affect ligand affinity via relatively subtle alterations in integrin conformation. These results may have relevance for vertebrate alpha6 and alpha7, which are alternatively spliced at the same site.

Placental defects in alpha7 integrin null mice

The alpha7beta1 integrin is a heterodimeric transmembrane receptor that links laminin in the extracellular matrix to the cell cytoskeleton. Loss of the alpha7 integrin chain results in partial embryonic lethality. We have previously shown that alpha7 integrin null embryos exhibit vascular smooth muscle cell defects that result in cerebral vascular hemorrhaging. Since the placenta is highly vascularized, we hypothesized that placental vascular defects in alpha7 integrin null embryos may contribute to the partial embryonic lethality. Placentae from embryonic day (ED) 9.5 and 13.5 alpha7 integrin knockout embryos showed structural defects including infiltration of the spongiotrophoblast layer into the placental labyrinth, a reduction in the placental labyrinth and loss of distinct placental layers. Embryos and placentae that lacked the alpha7 integrin weighed less compared to wild-type controls. Blood vessels within the placental labyrinth of alpha7 integrin null embryos exhibited fewer differentiated vascular smooth muscle cells compared to wild-type. Loss of the alpha7 integrin resulted in altered extracellular matrix deposition and reduced expression of alpha5 integrin. Together our results confirm a role for the alpha7beta1 integrin in placental vascular development and demonstrate for the first time that loss of the alpha7 integrin results in placental defects.

Reversion mutations in patients with leukocyte adhesion deficiency type-1 (LAD-1)

Leukocyte adhesion deficiency type-1 (LAD-1) is an autosomal recessive immunodeficiency caused by mutations in the beta2 integrin, CD18, that impair CD11/CD18 heterodimer surface expression and/or function. Absence of functional CD11/CD18 integrins on leukocytes, particularly neutrophils, leads to their incapacity to adhere to the endothelium and migrate to sites of infection. We studied 3 LAD-1 patients with markedly diminished neutrophil CD18 expression, each of whom had a small population of lymphocytes with normal CD18 expression (CD18(+)). These CD18(+) lymphocytes were predominantly cytotoxic T cells, with a memory/effector phenotype. Microsatellite analyses proved patient origin of these cells. Sequencing of T-cell subsets showed that in each patient one CD18 allele had undergone further mutation. Interestingly, all 3 patients were young adults with inflammatory bowel disease. Somatic reversions of inherited mutations in primary T-cell immunodeficiencies are typically associated with milder clinical phenotypes. We hypothesize that these somatic revertant CD18(+) cytotoxic T lymphocytes (CTLs) may have altered immune regulation. The discovery of 3 cases of reversion mutations in LAD-1 at one center suggests that this may be a relatively common event in this rare disease.

Loss of the alpha7 integrin promotes extracellular signal-regulated kinase activation and altered vascular remodeling

Vascular smooth muscle cell (VSMC) proliferation and migration are underlying factors in the development and progression of cardiovascular disease. Studies have shown that altered expression of vascular integrins and extracellular matrix proteins may contribute to the vascular remodeling observed after arterial injury and during disease. We have recently shown that loss of the alpha7beta1 integrin results in VSMC hyperplasia. To investigate the cellular mechanisms underlying this phenotype, we have examined changes in cell signaling pathways associated with VSMC proliferation. Several studies have demonstrated the mitogen-activated protein kinase signaling pathway is activated in response to vascular injury and disease. In this study, we show that loss of the alpha7 integrin in VSMCs results in activation of the extracellular signal-regulated kinase and translocation of the activated kinase to the nucleus. Forced expression of the alpha7 integrin or use of the mitogen-activated protein kinase kinase 1 inhibitor U0126 in alpha7 integrin-deficient VSMCs suppressed extracellular signal-regulated kinase activation and restored the differentiated phenotype to alpha7 integrin-null cells in a manner dependent on Ras signaling. Alpha7 integrin-null mice displayed profound vascular remodeling in response to injury with pronounced neointimal formation and reduced vascular compliance. These findings demonstrate that the alpha7beta1 integrin negatively regulates extracellular signal-regulated kinase activation and suggests an important role for this integrin as part of a signaling complex regulating VSMC phenotype switching.