Integrin signalling at a glance

Anabolic effects of IGF-1 signaling on the skeleton

This review focuses on the anabolic effects of IGF-1 signaling on the skeleton, emphasizing the requirement for IGF-1 signaling in normal bone formation and remodeling. We first discuss the genomic context, splicing variants, and species conservation of the IGF-1 locus. The modulation of IGF-1 action by growth hormone (GH) is then reviewed while also discussing the current model which takes into account the GH-independent actions of IGF-1. Next, the skeletal phenotypes of IGF-1-deficient animals are described in both embryonic and postnatal stages of development, which include severe dwarfism and an undermineralized skeleton. We then highlight two mechanisms by which IGF-1 exerts its anabolic action on the skeleton. Firstly, the role of IGF-1 signaling in the modulation of anabolic effects of parathyroid hormone (PTH) on bone will be discussed, presenting in vitro and in vivo studies that establish this concept and the proposed underlying molecular mechanisms involving Indian hedgehog (Ihh) and the ephrins. Secondly, the crosstalk of IGF-1 signaling with mechanosensing pathways will be discussed, beginning with the observation that animals subjected to skeletal unloading by hindlimb elevation are unable to mitigate cessation of bone growth despite infusion with IGF-1 and the failure of IGF-1 to activate its receptor in bone marrow stromal cell cultures from unloaded bone. Disrupted crosstalk between IGF-1 signaling and the integrin mechanotransduction pathways is discussed as one of the potential mechanisms for this IGF-1 resistance. Next, emerging paradigms on bone-muscle crosstalk are examined, focusing on the potential role of IGF-1 signaling in modulating such interactions. Finally, we present a future outlook on IGF research.

Integrins and small GTPases as modulators of phagocytosis

Phagocytosis is the mechanism whereby cells engulf large particles. This process has long been recognized as a critical component of the innate immune response, which constitutes the organism's defense against microorganisms. In addition, phagocytic internalization of apoptotic cells or cell fragments plays important roles in tissue homeostasis and remodeling. Phagocytosis requires target interactions with receptors on the plasma membrane of the phagocytic cell. Integrins have been identified as important mediators of particle clearance, in addition to their well-established roles in cell adhesion, migration and mechanotransduction. Indeed, these ubiquitously expressed proteins impart phagocytic capacity to epithelial, endothelial and mesenchymal cell types. The importance of integrins in particle internalization is emphasized by the ability of microbial and viral pathogens to exploit their signaling pathways to invade host cells, and by the wide variety of disorders that arise from abnormalities in integrin-dependent phagocytic uptake.

Lck tyrosine kinase mediates β1-integrin signalling to regulate Schwann cell migration and myelination

The interaction between laminin and β1-integrin on the surface of Schwann cells regulates Schwann cell proliferation, maturation and differentiation. However, the signalling mediators that fine-tune these outcomes are not fully elucidated. Here we show that lymphoid cell kinase is the crucial effector of β1-integrin signalling in Schwann cells. Lymphoid cell kinase is activated after laminin treatment of Schwann cells, while downregulation of β1-integrin with short interfering RNAs inhibits lymphoid cell kinase phosphorylation. Treatment of Schwann cells with a selective lymphoid cell kinase inhibitor reveals a pathway that involves paxillin and CrkII, which ultimately elevates Rac-GTP levels to induce radial lamellipodia formation. Inhibition of lymphoid cell kinase in Schwann cell-dorsal root ganglion cocultures and dorsal root ganglions from Lck(-/-) mice show a reduction of Schwann cell longitudinal migration, reduced myelin formation and internode length. Finally, Lck(-/-) mice exhibit delays in myelination, thinner myelin with abnormal g-ratios and aberrant myelin outfoldings. Our data implicate lymphoid cell kinase as a major regulator of cytoskeletal dynamics, migration and myelination in the peripheral nervous system.

Presenilin-1 regulates the constitutive turnover of the fibronectin matrix in endothelial cells

BACKGROUND

Presenilin-1 (PS1) is a transmembrane protein first discovered because of its association with familial Alzheimer's disease. Mice with null mutations in PS1 die shortly after birth exhibiting multiple CNS and non-CNS abnormalities. One of the most prominent features in the brains of PS1-/- embryos is a vascular dysgenesis that leads to multiple intracerebral hemorrhages. The molecular and cellular basis for the vascular dysgenesis in PS1-/- mice remains incompletely understood. Because the extracellular matrix plays key roles in vascular development we hypothesized that an abnormal extracellular matrix might be present in endothelial cells lacking PS1 and examined whether the lack of PS1 affects expression of fibronectin a component of the extracellular matrix known to be essential for vascular development.

RESULTS

We report that primary as well as continuously passaged PS1-/- endothelial cells contain more fibronectin than wild type cells and that the excess fibronectin in PS1-/- endothelial cells is incorporated into a fibrillar network. Supporting the in vivo relevance of this observation fibronectin expression was increased in microvascular preparations isolated from E14.5 to E18.5 PS1-/- embryonic brain. Reintroduction of PS1 into PS1-/- endothelial cells led to a progressive decrease in fibronectin levels showing that the increased fibronectin in PS1-/- endothelial cells was due to loss of PS1. Increases in fibronectin protein in PS1-/- endothelial cells could not be explained by increased levels of fibronectin RNA nor based on metabolic labeling studies by increased protein synthesis. Rather we show based on the rate of turnover of exogenously added biotinylated fibronectin that increased fibronectin in PS1-/- endothelial cells results from a slower degradation of the fibronectin fibrillar matrix on the cell surface.

CONCLUSIONS

These studies show that PS1 regulates the constitutive turnover of the fibronectin matrix in endothelial cells. These studies provide molecular clues that may help to explain the origin of the vascular dysgenesis that develops in PS1-/- embryonic mice.

Integrin/Fak/Src-mediated regulation of cell survival and anoikis in human intestinal epithelial crypt cells: selective engagement and roles of PI3-K isoform complexes

In human intestinal epithelial crypt (HIEC) cells, the PI3-K/Akt-1 pathway is crucial for the promotion of cell survival and suppression of anoikis. Class I PI3-K consists of a complex formed by a catalytic (C) and regulatory (R) subunit. Three R (p85α, β, and p55γ) and four C (p110α, β, γ and δ) isoforms are known. Herein, we analyzed the expression of PI3-K isoforms in HIEC cells and determined their roles in cell survival, as well as in the β1 integrin/Fak/Src-mediated suppression of anoikis. We report that: (1) the predominant PI3-K complexes expressed by HIEC cells are p110α/p85β and p110α/p55γ; (2) the inhibition and/or siRNA-mediated expression silencing of p110α, but not that of p110β, γ or δ, results in Akt-1 down-activation and consequent apoptosis; (3) the expression silencing of p85β or p55γ, but not that of p85α, likewise induces Akt-1 down-activation and apoptosis; however, the impact of a loss of p55γ on both Akt-1 activation and cell survival is significantly greater than that from the loss of p85β; and (4) both the p110α/p85β and p110α/p55γ complexes are engaged by β1 integrin/Fak/Src signaling; however, the engagement of p110α/p85β is primarily Src-dependent, whereas that of p110α/p55γ is primarily Fak-dependent (but Src-independent). Hence, HIEC cells selectively express PI3-K isoform complexes, translating into distinct roles in Akt-1 activation and cell survival, as well as in a selective engagement by Fak and/or Src within the context of β1 integrin/Fak/Src-mediated suppression of anoikis.

The β6-integrin-ERK/MAP kinase pathway contributes to chemo resistance in colon cancer

5-Fluorouracil (5-FU) is the most widely used chemo drug for the treatment of colon cancer. However, a sub-population of colon cancer patients do not respond to 5-FU and this treatment does not provide survival benefit due to chemo resistance. The mechanisms involved in 5-FU resistance are not fully understood and multiple factors have been involved in the sensitivity of cancer cells to 5-FU. We previously reported that β6-integrin plays an important role in invasion, metastasis and degradation of extracellular matrix of colon cancer. In this study, we investigated whether β6-integrin is associated with chemo resistance in colon cancer. We found that over-expression of β6-integrin protected SW480 and HT-29 colon cancer cells from 5-FU-induced growth inhibition and apoptosis, which were accompanied by changes in cytochrome C released from the mitochondria and activity of caspase-3 and caspase-9. Moreover, β6-integrin resulted in up-regulation of Bcl-2 and down-regulation of Bax. We also found that β6-integrin induced 5-FU resistance through the ERK/MAP kinase pathway and the β6-ERK2 direct binding. The results indicate β6-integrin might be a novel therapeutic target in colon cancer therapy.

Zasp regulates integrin activation

Integrins are heterodimeric adhesion receptors that link the extracellular matrix (ECM) to the cytoskeleton. Binding of the scaffold protein, talin, to the cytoplasmic tail of β-integrin causes a conformational change of the extracellular domains of the integrin heterodimer, thus allowing high-affinity binding of ECM ligands. This essential process is called integrin activation. Here we report that the Z-band alternatively spliced PDZ-motif-containing protein (Zasp) cooperates with talin to activate α5β1 integrins in mammalian tissue culture and αPS2βPS integrins in Drosophila. Zasp is a PDZ-LIM-domain-containing protein mutated in human cardiomyopathies previously thought to function primarily in assembly and maintenance of the muscle contractile machinery. Notably, Zasp is the first protein shown to co-activate α5β1 integrins with talin and appears to do so in a manner distinct from known αIIbβ3 integrin co-activators.

Cell adhesion: a FERM grasp of the tail sorts out integrins

As well as modulating integrin activation, a conserved NPxY motif in integrin cytoplasmic tails that binds the FERM-domain-containing proteins kindlin and sorting nexin 17 plays pivotal roles in integrin recycling and degradation.

Cyclic stretch-induced thrombin generation by rat vascular smooth muscle cells is mediated by the integrin αvβ3 pathway

AIMS

Vascular smooth muscle cell (VSMC) phenotypic modulation plays a pivotal role in atherothrombotic diseases. Thrombin generation at the surface of VSMCs and activation of integrin mechanotransduction pathways represent potential mechanisms. Here, we examine whether mechanical stretch increases thrombin generation on cultured rat aortic VSMCs.

METHODS AND RESULTS

The integrin α(v)β(3) antagonist peptide (cRGDPV) dose-dependently decreased thrombin generation without stretch. Static stretch (5%, 1 Hz) failed to modify the thrombin-forming capacity of VSMCs, whereas 10% cyclic stretch during 60 and 360 min enhanced integrin α(v)β(3) expression and thrombin generation at the surface of VSMCs by 30% without inducing apoptosis. Cyclic stretch also stimulated Src phosphorylation, cleavage of talin, and binding of prothrombin to VSMCs. Upregulation of α(v)β(3) expression, Src phosphorylation, and enhanced thrombin generation by cyclic stretch were abolished by cRGDPV and silencing RNA (siRNA) against α(v) as well as by selective inhibition of integrin α(v)β(3) inside-out signalling by a talin-siRNA. Complete abolition of stretch-induced VSMC-supported thrombin generation by the RGT peptide, which disrupts the interaction of Src with the β(3) cytoplasmic tail, demonstrates the link between outside-in pathways involving β(3)-Src interaction and thrombin activity dependent on inside-out signalling.

CONCLUSION

These data show that the contribution of cyclic stretch to VSMC-supported thrombin generation is driven by the integrin α(v)β(3) signalling pathway and suggest a role for pulsatility-induced intramural thrombin in VSMC-dependent vascular remodelling.

Nanomaterial scaffolds for stem cell proliferation and differentiation in tissue engineering

Tissue engineering is a clinically driven field and has emerged as a potential alternative to organ transplantation. The cornerstone of successful tissue engineering rests upon two essential elements: cells and scaffolds. Recently, it was found that stem cells have unique capabilities of self-renewal and multilineage differentiation to serve as a versatile cell source, while nanomaterials have lately emerged as promising candidates in producing scaffolds able to better mimic the nanostructure in natural extracellular matrix and to efficiently replace defective tissues. This article, therefore, reviews the key developments in tissue engineering, where the combination of stem cells and nanomaterial scaffolds has been utilized over the past several years. We consider the high potential, as well as the main issues related to the application of stem cells and nanomaterial scaffolds for a range of tissues including bone, cartilage, nerve, liver, eye etc. Promising in vitro results such as efficient attachment, proliferation and differentiation of stem cells have been compiled in a series of examples involving different nanomaterials. Furthermore, the merits of the marriage of stem cells and nanomaterial scaffolds are also demonstrated in vivo, providing early successes to support subsequent clinical investigations. This progress simultaneously drives mechanistic research into the mechanotransduction process responsible for the observations in order to optimize the process further. Current understanding is chiefly reported to involve the interaction of stem cells and the anchoring nanomaterial scaffolds by activating various signaling pathways. Substrate surface characteristics and scaffold bulk properties are also reported to influence not only short term stem cell adhesion, spreading and proliferation, but also longer term lineage differentiation, functionalization and viability. It is expected that the combination of stem cells and nanomaterials will develop into an important tool in tissue engineering for the innovative treatment of many diseases.

Negative regulators of integrin activity

Integrins are heterodimeric transmembrane adhesion receptors composed of α- and β-subunits. They are ubiquitously expressed and have key roles in a number of important biological processes, such as development, maintenance of tissue homeostasis and immunological responses. The activity of integrins, which indicates their affinity towards their ligands, is tightly regulated such that signals inside the cell cruicially regulate the switching between active and inactive states. An impaired ability to activate integrins is associated with many human diseases, including bleeding disorders and immune deficiencies, whereas inappropriate integrin activation has been linked to inflammatory disorders and cancer. In recent years, the molecular details of integrin 'inside-out' activation have been actively investigated. Binding of cytoplasmic proteins, such as talins and kindlins, to the cytoplasmic tail of β-integrins is widely accepted as being the crucial step in integrin activation. By contrast, much less is known with regard to the counteracting mechanism involved in switching integrins into an inactive conformation. In this Commentary, we aim to discuss the known mechanisms of integrin inactivation and the molecules involved.

Collagen/β1 integrin signaling up-regulates the ABCC1/MRP-1 transporter in an ERK/MAPK-dependent manner

Collagen/β1 integrin/extracellular signal-regulated kinase signaling up-regulates the expression and function of ABCC1 transporter. This suggests that its activation could represent an important pathway in cancer chemoresistance.

The mechanisms by which β1 integrins regulate chemoresistance of cancer cells are still poorly understood. In this study, we report that collagen/β1 integrin signaling inhibits doxorubicin-induced apoptosis of Jurkat and HSB2 leukemic T-cells by up-regulating the expression and function of the ATP-binding cassette C 1 (ABCC1) transporter, also known as multidrug resistance–associated protein 1. We find that collagen but not fibronectin reduces intracellular doxorubicin content and up-regulates the expression levels of ABCC1. Inhibition and knockdown studies show that up-regulation of ABCC1 is necessary for collagen-mediated reduction of intracellular doxorubicin content and collagen-mediated inhibition of doxorubicin-induced apoptosis. We also demonstrate that activation of the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase signaling pathway is involved in collagen-induced reduction of intracellular doxorubicin accumulation, collagen-induced up-regulation of ABCC1 expression levels, and collagen-mediated cell survival. Finally, collagen-mediated up-regulation of ABCC1 expression and function also requires actin polymerization. Taken together, our results indicate for the first time that collagen/β1 integrin/ERK signaling up-regulates the expression and function of ABCC1 and suggest that its activation could represent an important pathway in cancer chemoresistance. Thus simultaneous targeting of collagen/β1 integrin and ABCC1 may be more efficient in preventing drug resistance than targeting each pathway alone.

Detection and clinical significance of CD44v6 and integrin-β1 in pancreatic cancer patients using a triplex real-time RT-PCR assay

The cell adhesion molecules CD44v6 and integrin-β1 are associated with the progression and metastasis of cancer. A novel triplex real-time reverse transcription polymerase chain reaction (qRT-PCR) assay was developed to quantify CD44v6 and integrin-β1 gene expression in peripheral blood mononuclear cells from 30 pancreatic cancer (PC) patients and 12 healthy individuals. The standard curve of the triplex qRT-PCR was constructed by optimizing the reaction condition and the amplification efficiency was 102.5, 101.1, and 100.6 % for CD44v6, integrin-β1 and endogenous gene (β-actin) amplification. Nonspecific bands were not observed from the triplex qRT-PCR amplification and the detection limit of this assay was 100 copies. Expression levels of CD44v6 and integrin-β1 gene were significantly lower in healthy individuals than PC patients (P<0.05). CD44v6 and integrin-β1 gene expression were not associated with the sex, age, and tumor position in PC (P>0.05). CD44v6 gene expression was significantly associated with clinical stage, liver metastasis, and tumor size (P<0.05). Integrin-β1 gene expression was significantly associated with clinical stage and liver metastasis (P<0.05). This triplex qRT-PCR assay may provide a useful tool for diagnosis, prognosis, and therapeutic evaluation in PC.

Human apolipoprotein(a) kringle V inhibits ischemia-induced retinal neovascularization via suppression of fibronectin-mediated angiogenesis

Retinal neovascularization is observed in progression of diabetic retinopathy. New vessels grow into the vitreous cavity in proliferative diabetic retinopathy, resulting in traction retinal detachment and vitreous hemorrhage. To overcome the catastrophic visual loss due to these complications, efforts have been focused on the treatment of retinal neovascularization. In this study, we demonstrated the inhibitory effect of recombinant human apolipoprotein(a) kringle V (rhLK8) in an animal model of ischemia-induced retinal neovascularization. rhLK8 induced no definite toxicity on endothelial cells and retinal tissues at the therapeutic dosage. Interestingly, rhLK8 showed antiangiogenic effect, particularly on fibronectin-mediated migration of endothelial cells. Further experiments demonstrated high binding affinity of rhLK8 to α3β1 integrin, and suppression of it might be the mechanism of antiangiogenic effect of rhLK8. Furthermore, rhLK8 inhibited phosphorylation of focal adhesion kinase, resulting in suppression of activation of consequent p130CAS-Jun NH(2)-terminal kinase. Taken together, our data suggested the possible application of rhLK8 in the treatment of retinal neovascularization by suppression of fibronectin-mediated angiogenesis.

AT14A mediates the cell wall-plasma membrane-cytoskeleton continuum in Arabidopsis thaliana cells

AT14A has a small domain that has sequence similarities to integrins from animals. Integrins serve as a transmembrane linker between the extracellular matrix and the cytoskeleton, which play critical roles in a variety of biological processes. Because the function of AT14A is unknown, Arabidopsis thaliana AT14A, which is a transmembrane receptor for cell adhesion molecules and a middle member of the cell wall-plasma membrane-cytoskeleton continuum in plants, has been described. AT14A, co-expressed with green fluorescent protein (GFP), was found to localize mainly to the plasma membrane. The mutant Arabidopsis at14a-1 cells exhibit various phenotypes with cell shape, cell cluster size, thickness, and cellulose content of cell wall, the adhesion between cells, and the adhesion of plasma membrane to cell wall varied by plasmolysis. Using direct staining of filamentous actin and indirect immunofluorescence staining of microtubules, cortical actin filaments and microtubules arrays were significantly altered in cells, either where AT14A was absent or over-expressed. It is concluded that AT14A may be a substantial middle member of the cell wall-plasma membrane-cytoskeleton continuum and play an important role in the continuum by regulating cell wall and cortical cytoskeleton organization.

Directing epithelial to mesenchymal transition through engineered microenvironments displaying orthogonal adhesive and mechanical cues

Cell interactions with their extracellular matrix (ECM) microenvironments play a major role in directing cellular processes that can drive wound healing and tissue regeneration but, if uncontrolled, lead to pathological progression. One such process, epithelial to mesenchymal transition (EMT), if finely controlled could have significant potential in regenerative medicine approaches. Despite recent findings that highlight the influence of biochemical and mechanical properties of the ECM on EMT, it is still unclear how these two orthogonal cues act synergistically to control epithelial cell phenotype. Here, we cultured lung epithelial cells on combinations of different mutants of fibronectin's cell binding domain that preferentially engage specific integrins and substrates of varying stiffness. Our results suggest that while stiff substrates induce spontaneous EMT, this response can be overcome by with fragments of fibronectin that support α3 and α5 integrin engagement. Furthermore, we found that substrate-induced EMT correlates with transforming growth factor beta activation by resident epithelial cells and is dependent on Rho/ROCK signaling. Suppressing cell-contractility was sufficient to maintain an epithelial phenotype. Our results suggest that integrin-specific engagement of fibronectin adhesive domains and the mechanics of the ECM act synergistically to direct EMT.

Collagen I confers gamma radiation resistance

The effect of collagen on the response of somatomammotroph tumor cells (GH3) to gamma, radiation therapy was studied in vitro. After incubating confluent GH3 cell monolayers in a serum-free, maintaining medium, either with or without collagen, the monolayers were irradiated with 137Cs, gamma radiation. Collagen reduces cell mortality via ERK1/2 activation, abolishing gamma radiation, cell death, and promotes cell invasion when acting in synergy with collagen and in association with the, MAPK/ERK1/2 signaling pathway activation. The presence of collagen in somatomammotroph tumors, confers resistance to radiation.

Integrin β1 signals through Arg to regulate postnatal dendritic arborization, synapse density, and behavior

Integrins are heterodimeric extracellular matrix receptors that are essential for the proper development of the vertebrate nervous system. We report here that selective loss of integrin β1 in excitatory neurons leads to reductions in the size and complexity of hippocampal dendritic arbors, hippocampal synapse loss, impaired hippocampus-dependent learning, and exaggerated psychomotor sensitivity to cocaine in mice. Our biochemical and genetic experiments demonstrate that the intracellular tail of integrin β1 binds directly to Arg kinase and that this interaction stimulates activity of the Arg substrate p190RhoGAP, an inactivator of the RhoA GTPase. Moreover, genetic manipulations that reduce integrin β1 signaling through Arg recapitulate the integrin β1 knock-out phenotype in a gene dose-sensitive manner. Together, these results describe a novel integrin β1-Arg-p190RhoGAP pathway that regulates dendritic arbor size, promotes synapse maintenance, supports proper hippocampal function, and mitigates the behavioral consequences of cocaine exposure.

CD98 at the crossroads of adaptive immunity and cancer

Adaptive immunity, a vertebrate specialization, adds memory and exquisite specificity to the basic innate immune responses present in invertebrates while conserving metabolic resources. In adaptive immunity, antigenic challenge requires extremely rapid proliferation of rare antigen-specific lymphocytes to produce large, clonally expanded effector populations that neutralize pathogens. Rapid proliferation and resulting clonal expansion are dependent on CD98, a protein whose well-conserved orthologs appear restricted to vertebrates. Thus, CD98 supports lymphocyte clonal expansion to enable protective adaptive immunity, an advantage that could account for the presence of CD98 in vertebrates. CD98 supports lymphocyte clonal expansion by amplifying integrin signals that enable proliferation and prevent apoptosis. These integrin-dependent signals can also provoke cancer development and invasion, anchorage-independence and the rapid proliferation of tumor cells. CD98 is highly expressed in many cancers and contributes to formation of tumors in experimental models. Strikingly, vertebrates, which possess highly conserved CD98 proteins, CD98-binding integrins and adaptive immunity, also display propensity towards invasive and metastatic tumors. In this Commentary, we review the roles of CD98 in lymphocyte biology and cancer. We suggest that the CD98 amplification of integrin signaling in adaptive immunity provides survival benefits to vertebrates, which, in turn, bear the price of increased susceptibility to cancer.

Integrin β1 mediates vaccinia virus entry through activation of PI3K/Akt signaling

Vaccinia virus has a broad range of infectivity in many cell lines and animals. Although it is known that the vaccinia mature virus binds to cell surface glycosaminoglycans and extracellular matrix proteins, whether additional cellular receptors are required for virus entry remains unclear. Our previous studies showed that the vaccinia mature virus enters through lipid rafts, suggesting the involvement of raft-associated cellular proteins. Here we demonstrate that one lipid raft-associated protein, integrin β1, is important for vaccinia mature virus entry into HeLa cells. Vaccinia virus associates with integrin β1 in lipid rafts on the cell surface, and the knockdown of integrin β1 in HeLa cells reduces vaccinia mature virus entry. Additionally, vaccinia mature virus infection is reduced in a mouse cell line, GD25, that is deficient in integrin β1 expression. Vaccinia mature virus infection triggers the activation of phosphatidylinositol 3-kinase (PI3K)/Akt signaling, and the treatment of cells with inhibitors to block P13K activation reduces virus entry in an integrin β1-dependent manner, suggesting that integrin β1-mediates PI3K/Akt activation induced by vaccinia virus and that this signaling pathway is essential for virus endocytosis. The inhibition of integrin β1-mediated cell adhesion results in a reduction of vaccinia virus entry and the disruption of focal adhesion and PI3K/Akt activation. In summary, our results show that the binding of vaccinia mature virus to cells mimics the outside-in activation process of integrin functions to facilitate vaccinia virus entry into HeLa cells.

Talin contains a C-terminal calpain2 cleavage site important in focal adhesion dynamics

Talin is a large (∼2540 residues) dimeric adaptor protein that associates with the integrin family of cell adhesion molecules in cell-extracellular matrix junctions (focal adhesions; FAs), where it both activates integrins and couples them to the actin cytoskeleton. Calpain2-mediated cleavage of talin between the head and rod domains has previously been shown to be important in FA turnover. Here we identify an additional calpain2-cleavage site that removes the dimerisation domain from the C-terminus of the talin rod, and show that an E2492G mutation inhibits calpain cleavage at this site in vitro, and increases the steady state levels of talin1 in vivo. Expression of a GFP-tagged talin1 E2492G mutant in CHO.K1 cells inhibited FA turnover and the persistence of cell protrusion just as effectively as a L432G mutation that inhibits calpain cleavage between the talin head and rod domains. Moreover, incorporation of both mutations into a single talin molecule had an additive effect clearly demonstrating that calpain cleavage at both the N- and C-terminal regions of talin contribute to the regulation of FA dynamics. However, the N-terminal site was more sensitive to calpain cleavage suggesting that lower levels of calpain are required to liberate the talin head and rod fragments than are needed to clip off the C-terminal dimerisation domain. The talin head and rod liberated by calpain2 cleavage have recently been shown to play roles in an integrin activation cycle important in FA turnover and in FAK-dependent cell cycle progression respectively. The half-life of the talin head is tightly regulated by ubiquitination and we suggest that removal of the C-terminal dimerisation domain from the talin rod may provide a mechanism both for terminating the signalling function of the talin rod and indeed for inactivating full-length talin thereby promoting FA turnover at the rear of the cell.

Insulin receptor substrate protein 53kDa (IRSp53) is a negative regulator of myogenic differentiation

Fusion of mononucleated myoblasts to generate multinucleated myotubes is a critical step in skeletal muscle development. Filopodia, the actin cytoskeleton based membrane protrusions, have been observed early during myoblast fusion, indicating that they could play a direct role in myogenic differentiation. The control of filopodia formation in myoblasts remains poorly understood. Here we show that the expression of IRSp53 (Insulin Receptor Substrate protein 53kDa), a known regulator of filopodia formation, is down-regulated during differentiation of both mouse primary myoblasts and a mouse myoblast cell line C2C12. Over-expression of IRSp53 in C2C12 cells led to induction of filopodia and decrease in cell adhesion, concomitantly with inhibition of myogenic differentiation. In contrast, knocking down the IRSp53 expression in C2C12 cells led to a small but significant increase in myotube development. The decreased cell adhesion of C2C12 cells over-expressing IRSp53 is correlated with a reduction in the number of vinculin patches in these cells. Mutations in the conserved IMD domain (IRSp53 and MIM (missing in metastasis) homology domain) or SH3 domain of IRSp53 abolished the ability of this protein to inhibit myogenic differentiation and reduce cell adhesion. Over-expression of the IMD domain alone was sufficient to decrease the cell-extracellular matrix adhesion and to inhibit myogenesis in a manner dependent on its function in membrane shaping. Based on our data, we propose that IRSp53 is a negative regulator of myogenic differentiation which correlates with the observed down regulation of IRSp53 expression during myoblast differentiation to myotubes.

Kindlins, integrin activation and the regulation of talin recruitment to αIIbβ3

Talins and kindlins bind to the integrin β3 cytoplasmic tail and both are required for effective activation of integrin αIIbβ3 and resulting high-affinity ligand binding in platelets. However, binding of the talin head domain alone to β3 is sufficient to activate purified integrin αIIbβ3 in vitro. Since talin is localized to the cytoplasm of unstimulated platelets, its re-localization to the plasma membrane and to the integrin is required for activation. Here we explored the mechanism whereby kindlins function as integrin co-activators. To test whether kindlins regulate talin recruitment to plasma membranes and to αIIbβ3, full-length talin and kindlin recruitment to β3 was studied using a reconstructed CHO cell model system that recapitulates agonist-induced αIIbβ3 activation. Over-expression of kindlin-2, the endogenous kindlin isoform in CHO cells, promoted PAR1-mediated and talin-dependent ligand binding. In contrast, shRNA knockdown of kindlin-2 inhibited ligand binding. However, depletion of kindlin-2 by shRNA did not affect talin recruitment to the plasma membrane, as assessed by sub-cellular fractionation, and neither over-expression of kindlins nor depletion of kindlin-2 affected talin interaction with αIIbβ3 in living cells, as monitored by bimolecular fluorescence complementation. Furthermore, talin failed to promote kindlin-2 association with αIIbβ3 in CHO cells. In addition, purified talin and kindlin-3, the kindlin isoform expressed in platelets, failed to promote each other's binding to the β3 cytoplasmic tail in vitro. Thus, kindlins do not promote initial talin recruitment to αIIbβ3, suggesting that they co-activate integrin through a mechanism independent of recruitment.

Inhibition of integrin α2β1 ameliorates glomerular injury

Mesangial cells and podocytes express integrins α1β1 and α2β1, which are the two major collagen receptors that regulate multiple cellular functions, including extracellular matrix homeostasis. Integrin α1β1 protects from glomerular injury by negatively regulating collagen production, but the role of integrin α2β1 in renal injury is unclear. Here, we subjected wild-type and integrin α2-null mice to injury with adriamycin or partial renal ablation. In both of these models, integrin α2-null mice developed significantly less proteinuria and glomerulosclerosis. In addition, selective pharmacological inhibition of integrin α2β1 significantly reduced adriamycin-induced proteinuria, glomerular injury, and collagen deposition in wild-type mice. This inhibitor significantly reduced collagen synthesis in wild-type, but not integrin α2-null, mesangial cells in vitro, demonstrating that its effects are integrin α2β1-dependent. Taken together, these results indicate that integrin α2β1 contributes to glomerular injury by positively regulating collagen synthesis and suggest that its inhibition may be a promising strategy to reduce glomerular injury and proteinuria.

Role of integrins in regulating proteases to mediate extracellular matrix remodeling

The extracellular matrix (ECM) is an extracellular scaffold composed of complex mixtures of proteins that plays a pivotal role in tumor progression. ECM remodeling is crucial for tumor migration and invasion during the process of metastasis. ECM can be remodeled by several processes including synthesis, contraction and proteolytic degradation. In order to cross through the ECM barriers, malignant cells produce a spectrum of extracellular proteinases including matrix metalloproteinases (MMPs), serine proteases (mainly the urokinase plasminogen activator (uPA) system) and cysteine proteases to degrade ECM components. As major adhesion molecules to support cell attachment to ECM, integrins play critical roles in tumor progression by enhancing tumor cell survival, migration and invasion. Previous studies have shown that integrins can regulate the expression and activity of these proteases through different pathways. This review summarizes the roles of MMPs and uPA system in ECM remodeling and discusses the regulatory functions of integrins on these proteases in invasive tumors.

The role of cell-extracellular matrix interactions in glomerular injury

Glomerulosclerosis is characterized by excessive deposition of extracellular matrix within the glomeruli of the kidney, glomerular cell death, and subsequent loss of functional glomeruli. While in physiological situations the levels of extracellular matrix components are kept constant by a tight balance between formation and degradation, in the case of injury that results in fibrosis there is increased matrix deposition relative to its breakdown. Multiple factors control matrix synthesis and degradation, thus contributing to the development of glomerulosclerosis. This review focuses primarily on the role of cell-matrix interactions, which play a critical role in governing glomerular cell cues in both healthy and diseased kidneys. Cell-extracellular matrix interactions are made possible by various cellular receptors including integrins, discoidin domain receptors, and dystroglycan. Upon binding to a selective extracellular matrix protein, these receptors activate intracellular signaling pathways that can either downregulate or upregulate matrix synthesis and deposition. This, together with the observation that changes in the expression levels of matrix receptors have been documented in glomerular disease, clearly emphasizes the contribution of cell-matrix interactions in glomerular injury. Understanding the molecular mechanisms whereby extracellular matrix receptors regulate matrix homeostasis in the course of glomerular injury is therefore critical for devising more effective therapies to treat and ideally prevent glomerulosclerosis.

Involvement of host cell integrin α2 in Cryptosporidium parvum infection

Cryptosporidium parvum is an opportunistic pathogen in AIDS patients. It is an intracellular but extracytoplasmic parasite residing in a host cell-derived parasitophorous vacuole. It is still poorly understood how this parasite interacts with host cells. We observed that expression of the integrin α2 (ITGA2) gene in host cells was significantly upregulated upon C. parvum infection, and a higher level of ITGA2 protein was present in the parasite infection sites. The infection could be reduced by the treatment of antibodies against ITGA2 and integrin β1 (ITGB1) subunits, as well as by type I collagen (an integrin α2β1 ligand). We also generated stable knockdown of ITGA2 gene expression in HCT-8 cells and observed consistent reduction of parasite infection in these knockdown cells. Collectively, our evidence indicates that host cell ITGA2 might be involved in interacting with Cryptosporidium during infection, probably acting as part of the regulatory elements upstream of the reported recruiting and reorganization of F actin at the infection sites.

Varicose veins: role of mechanotransduction of venous hypertension

Varicose veins affect approximately one-third of the adult population and result in significant psychological, physical, and financial burden. Nevertheless, the molecular pathogenesis of varicose vein formation remains unidentified. Venous hypertension exerted on veins of the lower extremity is considered the principal factor in varicose vein formation. The role of mechanotransduction of the high venous pressure in the pathogenesis of varicose vein formation has not been adequately investigated despite a good progress in understanding the mechanomolecular mechanisms involved in transduction of high blood pressure in the arterial wall. Understanding the nature of the mechanical forces, the mechanosensors and mechanotransducers in the vein wall, and the downstream signaling pathways will provide new molecular targets for the prevention and treatment of varicose veins. This paper summarized the current understanding of mechano-molecular pathways involved in transduction of hemodynamic forces induced by blood pressure and tries to relate this information to setting of venous hypertension in varicose veins.

Functional differences between kindlin-1 and kindlin-2 in keratinocytes

Integrin-β1-null keratinocytes can adhere to fibronectin through integrin αvβ6, but form large peripheral focal adhesions and exhibit defective cell spreading. Here we report that, in addition to the reduced avidity of αvβ6 integrin binding to fibronectin, the inability of integrin β6 to efficiently bind and recruit kindlin-2 to focal adhesions directly contributes to these phenotypes. Kindlins regulate integrins through direct interactions with the integrin-β cytoplasmic tail and keratinocytes express kindlin-1 and kindlin-2. Notably, although both kindlins localize to focal adhesions in wild-type cells, only kindlin-1 localizes to the integrin-β6-rich adhesions of integrin-β1-null cells. Rescue of these cells with wild-type and chimeric integrin constructs revealed a correlation between kindlin-2 recruitment and cell spreading. Furthermore, despite the presence of kindlin-1, knockdown of kindlin-2 in wild-type keratinocytes impaired cell spreading. Our data reveal unexpected functional consequences of differences in the association of two homologous kindlin isoforms with two closely related integrins, and suggest that despite their similarities, different kindlins are likely to have unique functions.

Tetraspanins: Interactions and interplay with integrins

Tetraspanins are small transmembrane proteins present on the cell surface of almost every eukaryotic cell. Through binding with other transmembrane and intracellular proteins, they regulate diverse cellular processes ranging from cell adhesion and motility to synapse formation and tumor progression. Here, we provide a brief overview of molecular, cellular and clinical studies to illustrate how the multiple functions of this fascinating family of molecules stem from their interplay with multiple molecular partners. In particular, we emphasize the special relationship between tetraspanins and the cell adhesion molecules integrins in regulating cell physiology in health and disease.

Filopodia and adhesion in cancer cell motility

Slender bundled actin containing plasma membrane protrusions, called filopodia, are important for many essential cellular processes like cell adhesion, migration, angiogenesis and the formation of cell-cell contacts. In migrating cells, filopodia are the pioneers at the leading edge which probe the environment for cues. Integrins are cell surface adhesion receptors critically implicated in cell migration and they are transported actively to filopodia tips by an unconventional myosin, myosin-X. Integrin mediated adhesion stabilizes filopodia and promotes cell migration even though integrins are not essential for filopodia initiation. Myosin-X binds also PIP3 and this regulates its activation and localization to filopodia. Filopodia stimulate cell migration in many cell types and increased filopodia density has been described in cancer. Furthermore, several proteins implicated in filopodia formation, like fascin, are also relevant for cancer progression. To investigate this further, we performed a meta-analysis of the expression profiles of 10 filopodia-linked genes in human breast cancer. These data implicated that several different filopodia inducing genes may contribute in a collective manner to cancer progression and the high metastasis rates associated with basal-type breast carcinomas.

A conserved lipid-binding loop in the kindlin FERM F1 domain is required for kindlin-mediated αIIbβ3 integrin coactivation

The activation of heterodimeric integrin adhesion receptors from low to high affinity states occurs in response to intracellular signals that act on the short cytoplasmic tails of integrin β subunits. Binding of the talin FERM (four-point-one, ezrin, radixin, moesin) domain to the integrin β tail provides one key activation signal, but recent data indicate that the kindlin family of FERM domain proteins also play a central role. Kindlins directly bind integrin β subunit cytoplasmic domains at a site distinct from the talin-binding site, and target to focal adhesions in adherent cells. However, the mechanisms by which kindlins impact integrin activation remain largely unknown. A notable feature of kindlins is their similarity to the integrin-binding and activating talin FERM domain. Drawing on this similarity, here we report the identification of an unstructured insert in the kindlin F1 FERM domain, and provide evidence that a highly conserved polylysine motif in this loop supports binding to negatively charged phospholipid head groups. We further show that the F1 loop and its membrane-binding motif are required for kindlin-1 targeting to focal adhesions, and for the cooperation between kindlin-1 and -2 and the talin head in αIIbβ3 integrin activation, but not for kindlin binding to integrin β tails. These studies highlight the structural and functional similarities between kindlins and the talin head and indicate that as for talin, FERM domain interactions with acidic membrane phospholipids as well β-integrin tails contribute to the ability of kindlins to activate integrins.

Directing integrin-linked endocytosis of recombinant AAV enhances productive FAK-dependent transduction

Recombinant adeno-associated virus (rAAV) is a widely used gene therapy vector. Although a wide range of rAAV serotypes can effectively enter most cell types, their transduction efficiencies (i.e., transgene expression) can vary widely depending on the target cell type. Integrins play important roles as coreceptors for rAAV infection, however, it remains unclear how integrin-dependent and -independent mechanisms of rAAV endocytosis influence the efficiency of intracellular virus processing and ultimately transgene expression. In this study, we examined the contribution of integrin-mediated endocytosis to transduction of fibroblasts by rAAV2. Mn(++)-induced integrin activation significantly enhanced (~17-fold) the efficiency of rAAV2 transduction, without altering viral binding or endocytosis. rAAV2 subcellular localization studies demonstrated that Mn(++) promotes increased clustering of rAAV2 on integrins and recruitment of intracellular vinculin (an integrin effector) to sites of rAAV2 binding at the cell surface. Focal adhesion kinase (FAK), a downstream effector of integrin signals, was essential for rAAV2/integrin complex internalization and transduction. These findings support a model whereby integrin activation at the cell surface can redirect rAAV2 toward a FAK-dependent entry pathway that is more productive for cellular transduction. This pathway appears to be conserved for other rAAV serotypes that contain a capsid integrin-binding domain (AAV1 and AAV6).

β3 integrin interacts directly with GluA2 AMPA receptor subunit and regulates AMPA receptor expression in hippocampal neurons

The integrins are transmembrane receptors for ECM proteins, and they regulate various cellular functions in the central nervous system. In hippocampal neurons, the β3 integrin subtype is required for homeostatic synaptic scaling of AMPA receptors (AMPARs) induced by chronic activity deprivation. The surface level of β3 integrin in postsynaptic neurons directly correlates with synaptic strength and the abundance of synaptic GluA2 AMPAR subunit. Although these observations suggest a functional link between β3 integrin and AMPAR, little is known about the mechanistic basis for the connection. Here we investigate the nature of β3 integrin and AMPAR interaction underlying the β3 integrin-dependent control of synaptic AMPAR expression and thus synaptic strength. We show that β3 integrin and GluA2 subunit form a complex in mouse brain that involves the direct binding between their cytoplasmic domains. In contrast, β3 integrin associates with GluA1 AMPAR subunit only weakly, and, in a heterologous expression system, the interaction requires the coexpression of GluA2. Surprisingly, in hippocampal pyramidal neurons, expressing β3 integrin mutants with either increased or decreased affinity for extracellular ligands has no differential effects in elevating excitatory synaptic currents and surface GluA2 levels compared with WT β3 integrin. Our findings identify an integrin family member, β3, as a direct interactor of an AMPAR subunit and provide molecular insights into how this cell-adhesion protein regulates the composition of cell-surface AMPARs.

The Bro1 domain-containing Myopic/HDPTP coordinates with Rab4 to regulate cell adhesion and migration

Protein tyrosine phosphatases (PTPs) are a group of tightly regulated enzymes that coordinate with protein tyrosine kinases to control protein phosphorylation during various cellular processes. Using genetic analysis in Drosophila non-transmembrane PTPs, we identified one role that Myopic (Mop), the Drosophila homolog of the human His domain phosphotyrosine phosphatase (HDPTP), plays in cell adhesion. Depletion of Mop results in aberrant integrin distribution and border cell dissociation during Drosophila oogenesis. Interestingly, Mop phosphatase activity is not required for its role in maintaining border cell cluster integrity. We further identified Rab4 GTPase as a Mop interactor in a yeast two-hybrid screen. Expression of the Rab4 dominant negative mutant leads to border cell dissociation and suppresses Mop-induced wing-blade adhesion defects, suggesting a critical role of Rab4 in Mop-mediated signaling. In mammals, it has been shown that Rab4-dependent recycling of integrins is necessary for cell adhesion and migration. We found that human HDPTP regulates the spatial distribution of Rab4 and integrin trafficking. Depletion of HDPTP resulted in actin reorganization and increased cell motility. Together, our findings suggest an evolutionarily conserved function of HDPTP-Rab4 in the regulation of endocytic trafficking, cell adhesion and migration.

Dwarfism in mice lacking collagen-binding integrins α2β1 and α11β1 is caused by severely diminished IGF-1 levels

Mice with a combined deficiency in the α2β1 and α11β1 integrins lack the major receptors for collagen I. These mutants are born with inconspicuous differences in size but develop dwarfism within the first 4 weeks of life. Dwarfism correlates with shorter, less mineralized and functionally weaker bones that do not result from growth plate abnormalities or osteoblast dysfunction. Besides skeletal dwarfism, internal organs are correspondingly smaller, indicating proportional dwarfism and suggesting a systemic cause for the overall size reduction. In accordance with a critical role of insulin-like growth factor (IGF)-1 in growth control and bone mineralization, circulating IGF-1 levels in the sera of mice lacking either α2β1 or α11β1 or both integrins were sharply reduced by 39%, 64%, or 81% of normal levels, respectively. Low hepatic IGF-1 production resulted from diminished growth hormone-releasing hormone expression in the hypothalamus and, subsequently, reduced growth hormone expression in the pituitary glands of these mice. These findings point out a novel role of collagen-binding integrin receptors in the control of growth hormone/IGF-1-dependent biological activities. Thus, coupling hormone secretion to extracellular matrix signaling via integrins represents a novel concept in the control of endocrine homeostasis.

Affinity of talin-1 for the β3-integrin cytosolic domain is modulated by its phospholipid bilayer environment

Binding of the talin-1 FERM (4.1/ezrin/radixin/moesin) domain to the β3 cytosolic tail causes activation of the integrin αIIbβ3. The FERM domain also binds to acidic phospholipids. Although much is known about the interaction of talin-1 with integrins and lipids, the relative contribution of each interaction to integrin regulation and possible synergy between them remain to be clarified. Here, we examined the thermodynamic interplay between FERM domain binding to phospholipid bilayers and to its binding sites in the β3 tail. We found that although both the F0F1 and F2F3 subdomains of the talin-1 FERM domain bind acidic bilayers, the full-length FERM domain binds with an affinity similar to F2F3, indicating that F0F1 contributes little to the overall interaction. When free in solution, the β3 tail has weak affinity for the FERM domain. However, appending the tail to acidic phospholipids increased its affinity for the FERM domain by three orders of magnitude. Nonetheless, the affinity of the FERM for the appended tail was similar to its affinity for binding to bilayers alone. Thus, talin-1 binding to the β3 tail is a ternary interaction dominated by a favorable surface interaction with phospholipid bilayers and set by lipid composition. Nonetheless, interactions between the FERM domain, the β3 tail, and lipid bilayers are not optimized for a high-affinity synergistic interaction, even at the membrane surface. Instead, the interactions appear to be tuned in such a way that the equilibrium between inactive and active integrin conformations can be readily regulated.

The involvement of integrin β1 signaling in the migration and myofibroblastic differentiation of skin fibroblasts on anisotropic collagen-containing nanofibers

Utilization of nanofibrous matrices for skin wound repair holds great promise due to their morphological and dimensional similarity to native extracellular matrix (ECM). It becomes highly desired to understand how various nanofibrous matrices regulate skin cell behaviors and intracellular signaling pathways, important to tuning the functionality of tissue-engineered skin grafts and affecting the wound healing process. In this study, the phenotypic expressions of normal human dermal fibroblasts (NHDFs) on collagen-containing nanofibrous matrices with either isotropic (i.e., fibers collected randomly with no alignment) or anisotropic (i.e., fibers collected with alignment) fiber organizations were studied by immunostaining, migration assay and molecular analyses. Results showed that both nanofibrous matrices supported the attachment and growth of NHDFs similarly, while showing different cell morphology with distinct variation in focal adhesion formation and distribution. Anisotropic nanofibers significantly triggered the integrin β1 signaling pathway in NHDFs as evidenced by an increase of active integrin β1 (130 kD mature form) and phosphorylation of focal adhesion kinase (FAK) at Tyr-397. Anisotropic matrices also promoted the migration of NHDFs along the fibers, while neutralization of the integrin β1 activity abolished this promotion. Moreover, the fibroblast-to-myofibroblast differentiation was greatly enhanced for the NHDFs cultured on anisotropic nanofibrous matrices over a period of 48 h. Inhibition of cellular integrin β1 activity by neutralizing antibody eliminated this enhancement. These findings suggest the important role of integrin β1 signaling pathway in regulating the nanofiber-induced fibroblast phenotypic alteration and providing insightful understanding of the possible application of collagen-containing nanofibrous matrices for skin regeneration.

β1 integrin is required for anchorage-independent growth and invasion of tumor cells in a context dependent manner

Recent studies suggest that extracellular matrix (ECM) components within the tumor microenvironment can influence malignant progression, thus we investigated the influence of the ECM binding receptor β1 integrin, on the hallmark properties of tumorigenesis. Small interfering (si) or short hairpin (sh) RNA approaches were used to deplete β1 integrin in cancer cell lines. β1 integrin-depleted cells were then assessed for their growth and invasive capabilities using 2-dimensional (2D) or 3D culture conditions. Depletion of β1 integrin expression did not impact cell growth in 2D assay systems; however, β1 integrin and its ligand fibronectin were required for growth in 3D. β1 integrin-depleted cells also had reduced invasive capabilities, in part due to increased tissue inhibitor of metalloprotease (TIMP)-2 expression in conjunction with down-regulation of matrix metalloprotease (MMP)-9 levels in β1 integrin-depleted cells. Our results suggest that despite no apparent effect on 2D cell growth, fibronectin-β1 integrin signaling is a critical mediator of the 3D growth and invasive properties of tumor cells. These observations highlight the importance of investigating the role of adhesion molecules in the appropriate context and furthermore identify β1 integrin as a possible therapeutic target to inhibit the aggressive growth and invasion of tumor cells.

A helix heterodimer in a lipid bilayer: prediction of the structure of an integrin transmembrane domain via multiscale simulations

Dimerization of transmembrane (TM) α helices of membrane receptors plays a key role in signaling. We show that molecular dynamics simulations yield models of integrin TM helix heterodimers, which agree well with available NMR structures. We use a multiscale simulation approach, combining coarse-grained and subsequent atomistic simulation, to model the dimerization of wild-type (WT) and mutated sequences of the αIIb and β3 integrin TM helices. The WT helices formed a stable, right-handed dimer with the same helix-helix interface as in the published NMR structure (PDB: 2K9J). In contrast, the presence of disruptive mutations perturbed the interface between the helices, altering the conformational stability of the dimer. The αIIb/β3 interface was more flexible than that of, e.g., glycophorin A. This is suggestive of a role for alternative packing modes of the TM helices in transbilayer signaling.

Focal adhesion kinase signaling mediates acute renal injury induced by ischemia/reperfusion

Renal ischemia/reperfusion (I/R) injury is associated with cell matrix and focal adhesion remodeling. Focal adhesion kinase (FAK) is a nonreceptor protein tyrosine kinase that localizes at focal adhesions and regulates their turnover. Here, we investigated the role of FAK in renal I/R injury, using a novel conditional proximal tubule-specific fak-deletion mouse model. Tamoxifen treatment of FAK(loxP/loxP)//γGT-Cre-ER(T2) mice caused renal-specific fak recombination (FAK(ΔloxP/ΔloxP)) and reduction of FAK expression in proximal tubules. In FAK(ΔloxP/ΔloxP) mice compared with FAK(loxP/loxP) controls, unilateral renal ischemia followed by reperfusion resulted in less tubular damage with reduced tubular cell proliferation and lower expression of kidney injury molecule-1, which was independent from the postischemic inflammatory response. Oxidative stress is involved in the pathophysiology of I/R injury. Primary cultured mouse renal cells were used to study the role of FAK deficiency for oxidative stress in vitro. The conditional fak deletion did not affect cell survival after hydrogen peroxide-induced cellular stress, whereas it impaired the recovery of focal adhesions that were disrupted by hydrogen peroxide. This was associated with reduced c-Jun N-terminal kinase-dependent phosphorylation of paxillin at serine 178 in FAK-deficient cells, which is required for focal adhesion turnover. Our findings support a role for FAK as a novel factor in the initiation of c-Jun N-terminal kinase-mediated cellular stress response during renal I/R injury and suggest FAK as a target in renal injury protection.

Kindler syndrome: extension of FERMT1 mutational spectrum and natural history

Mutations in the FERMT1 gene (also known as KIND1), encoding the focal adhesion protein kindlin-1, underlie the Kindler syndrome (KS), an autosomal recessive skin disorder with an intriguing progressive phenotype comprising skin blistering, photosensitivity, progressive poikiloderma with extensive skin atrophy, and propensity to skin cancer. Herein we review the clinical and genetic data of 62 patients, and delineate the natural history of the disorder, for example, age at onset of symptoms, or risk of malignancy. Although most mutations are predicted to lead to premature termination of translation, and to loss of kindlin-1 function, significant clinical variability is observed among patients. There is an association of FERMT1 missense and in-frame deletion mutations with milder disease phenotypes, and later onset of complications. Nevertheless, the clinical variability is not fully explained by genotype-phenotype correlations. Environmental factors and yet unidentified modifiers may play a role. Better understanding of the molecular pathogenesis of KS should enable the development of prevention strategies for disease complications.

14-3-3 proteins and regulation of cytoskeleton

The proteins of the 14-3-3 family are universal adapters participating in multiple processes running in the cell. We describe the structure, isoform composition, and distribution of 14-3-3 proteins in different tissues. Different elements of 14-3-3 structure important for dimer formation and recognition of protein targets are analyzed in detail. Special attention is paid to analysis of posttranslational modifications playing important roles in regulation of 14-3-3 function. The data of the literature concerning participation of 14-3-3 in regulation of intercellular contacts and different elements of cytoskeleton formed by microfilaments are analyzed. We also describe participation of 14-3-3 in regulation of small G-proteins and protein kinases important for proper functioning of cytoskeleton. The data on the interaction of 14-3-3 with different components of microtubules are presented, and the probable role of 14-3-3 in developing of certain neurodegenerative diseases is discussed. The data of the literature concerning the role of 14-3-3 in formation and normal functioning of intermediate filaments are also reviewed. It is concluded that due to its adapter properties 14-3-3 plays an important role in cytoskeleton regulation. The cytoskeletal proteins that are abundant in the cell might compete with the other protein targets of 14-3-3 and therefore can indirectly regulate many intracellular processes that are dependent on 14-3-3.

Comparative studies of vertebrate Beta integrin genes and proteins: ancient genes in vertebrate evolution

Intregins are heterodimeric α- and β-subunit containing membrane receptor proteins which serve various cell adhesion roles in tissue repair, hemostasis, immune response, embryogenesis and metastasis. At least 18 α- (ITA or ITGA) and 8 β-integrin subunits (ITB or ITGB) are encoded on mammalian genomes. Comparative ITB amino acid sequences and protein structures and ITB gene locations were examined using data from several vertebrate genome projects. Vertebrate ITB genes usually contained 13-16 coding exons and encoded protein subunits with ~800 amino acids, whereas vertebrate ITB4 genes contained 36-39 coding exons and encoded larger proteins with ~1800 amino acids. The ITB sequences exhibited several conserved domains including signal peptide, extracellular β-integrin, β-tail domain and integrin β-cytoplasmic domains. Sequence alignments of the integrin β-cytoplasmic domains revealed highly conserved regions possibly for performing essential functions and its maintenance during vertebrate evolution. With the exception of the human ITB8 sequence, the other ITB sequences shared a predicted 19 residue α-helix for this region. Potential sites for regulating human ITB gene expression were identified which included CpG islands, transcription factor binding sites and microRNA binding sites within the 3'-UTR of human ITB genes. Phylogenetic analyses examined the relationships of vertebrate beta-integrin genes which were consistent with four major groups: 1: ITB1, ITB2, ITB7; 2: ITB3, ITB5, ITB6; 3: ITB4; and 4: ITB8 and a common evolutionary origin from an ancestral gene, prior to the appearance of fish during vertebrate evolution. The phylogenetic analyses revealed that ITB4 is the most likely primordial form of the vertebrate β integrin subunit encoding genes, that is the only β subunit expressed as a constituent of the sole integrin receptor 'α6β4' in the hemidesmosomes of unicellular organisms.