Structural Insight into the Interaction between Platelet Integrin αIIbβ3 and Cytoskeletal Protein Skelemin

Cell Adhesion by Integrins

Integrins are heterodimeric cell surface receptors ensuring the mechanical connection between cells and the extracellular matrix. In addition to the anchorage of cells to the extracellular matrix, these receptors have critical functions in intracellular signaling, but are also taking center stage in many physiological and pathological conditions. In this review, we provide some historical, structural, and physiological notes so that the diverse functions of these receptors can be appreciated and put into the context of the emerging field of mechanobiology. We propose that the exciting journey of the exploration of these receptors will continue for at least another new generation of researchers.

Skelemin association with αIIbβ3 integrin: a structural model

Over the last two decades, our knowledge concerning intracellular events that regulate integrin’s affinity to their soluble ligands has significantly improved. However, the mechanism of adhesion-induced integrin clustering and development of focal complexes, which could further mature to form focal adhesions, still remains under-investigated. Here we present a structural model of tandem IgC2 domains of skelemin in complex with the cytoplasmic tails of integrin α_IIbβ₃. The model of tertiary assembly is generated based upon NMR data and illuminates a potential link between the essential cell adhesion receptors and myosin filaments. This connection may serve as a basis for generating the mechanical forces necessary for cell migration and remodeling.

Skelemin in integrin α(IIb)β(3) mediated cell spreading

Skelemin, a myosin-associated protein in skeletal muscle, has been demonstrated to interact with integrin α(IIb)β(3) in nonmuscle cells during initial stages of cell spreading. The significance of this interaction and the role of skelemin in integrin signaling and cytoskeletal reorganization were investigated in this study. We established a series of Chinese hamster ovary cell lines expressing wild-type or mutant α(IIb)β(3) receptors in which skelemin binding residues at the membrane proximal region of integrin tails were mutated to alanine. Most cells displayed unimpaired adhesive capacity and spreading on immobilized fibrinogen at the early stages of cell spreading. In addition, they formed normal focal adhesions and stress fibers with no indication of impaired cell spreading. R995A/R997A/L1000A, H722A, and K716A exhibited the greatest cell spreading, which was associated with enhanced p-Src activation but was independent of FAK activation. Transfection of the cells with GFP-skelemin, containing only the C2 integrin binding domain, caused wild-type cells to round up, but had no effect on R995A/R997A/L1000A, H722A, and K716A cell spreading. Furthermore, the protrusions of the leading edge of K716A cells showed strong colocalization of talin with α(IIb)β(3) which was associated with a loss in skelemin binding. Thus, we propose that during early stages of cell spreading, skelemin exerts contractile force on cell spreading and modulates the attachment of cytoskeletal proteins and Src to integrin clusters.

Myofibrillogenesis regulator 1 induces hypertrophy by promoting sarcomere organization in neonatal rat cardiomyocytes

Human myofibrillogenesis regulator 1, a novel 17-kDa protein, is closely involved in cardiac hypertrophy. We studied the molecular mechanism that links MR-1 to hypertrophic response. Hypertrophic hallmarks such as cell size and [³H]-leucine incorporation were significantly increased when MR-1 was transfected into cardiomyocytes for 48 h. However, sarcomere organization was promoted when MR-1 was transfected for 8 h. The finding that cardiac hypertrophy was induced long after increase of sarcomere organization indicates that the promoted sarcomere organization may be one of the crucial factors causing hypertrophy. Furthermore, when MR-1 was transfected into cardiomyocytes, the nuclear localization of myomesin-1 was shifted to the cytoplasm. Transfection with small ubiquitin-like modifier-1 (SUMO-1) mimicked the effect of MR-1 inducing translocation of myomesin-1. However, transfection with SUMO-1 in MR-1-silenced cardiomyocytes failed to induce translocation and sarcomere organization, even though SUMO-1 expression was at the same level. Overexpression of MR-1 may induce cardiomyocyte hypertrophy via myomesin-1-mediated sarcomere organization.

Tyrosine phosphorylation as a conformational switch: a case study of integrin β3 cytoplasmic tail

Reversible protein phosphorylation is vital for many fundamental cellular processes. The actual impact of adding and removing phosphate group(s) is 3-fold: changes in the local/global geometry, alterations in the electrostatic potential and, as the result of both, modified protein-target interactions. Here we present a comprehensive structural investigation of the effects of phosphorylation on the conformational as well as functional states of a crucial cell surface receptor, α(IIb)β(3) integrin. We have analyzed phosphorylated (Tyr(747) and Tyr(759)) β(3) integrin cytoplasmic tail (CT) primarily by NMR, and our data demonstrate that under both aqueous and membrane-mimetic conditions, phosphorylation causes substantial conformational rearrangements. These changes originate from novel ionic interactions and revised phospholipid binding. Under aqueous conditions, the critical Tyr(747) phosphorylation prevents β(3)CT from binding to its heterodimer partner α(IIb)CT, thus likely maintaining an activated state of the receptor. This conclusion was tested in vivo and confirmed by integrin-dependent endothelial cells adhesion assay. Under membrane-mimetic conditions, phosphorylation results in a modified membrane embedding characterized by significant changes in the secondary structure pattern and the overall fold of β(3)CT. Collectively these data provide unique molecular insights into multiple regulatory roles of phosphorylation.

TRAM (Transcriptome Mapper): database-driven creation and analysis of transcriptome maps from multiple sources

Background

Several tools have been developed to perform global gene expression profile data analysis, to search for specific chromosomal regions whose features meet defined criteria as well as to study neighbouring gene expression. However, most of these tools are tailored for a specific use in a particular context (e.g. they are species-specific, or limited to a particular data format) and they typically accept only gene lists as input.

Results

TRAM (Transcriptome Mapper) is a new general tool that allows the simple generation and analysis of quantitative transcriptome maps, starting from any source listing gene expression values for a given gene set (e.g. expression microarrays), implemented as a relational database. It includes a parser able to assign univocal and updated gene symbols to gene identifiers from different data sources. Moreover, TRAM is able to perform intra-sample and inter-sample data normalization, including an original variant of quantile normalization (scaled quantile), useful to normalize data from platforms with highly different numbers of investigated genes. When in 'Map' mode, the software generates a quantitative representation of the transcriptome of a sample (or of a pool of samples) and identifies if segments of defined lengths are over/under-expressed compared to the desired threshold. When in 'Cluster' mode, the software searches for a set of over/under-expressed consecutive genes. Statistical significance for all results is calculated with respect to genes localized on the same chromosome or to all genome genes. Transcriptome maps, showing differential expression between two sample groups, relative to two different biological conditions, may be easily generated. We present the results of a biological model test, based on a meta-analysis comparison between a sample pool of human CD34+ hematopoietic progenitor cells and a sample pool of megakaryocytic cells. Biologically relevant chromosomal segments and gene clusters with differential expression during the differentiation toward megakaryocyte were identified.

Conclusions

TRAM is designed to create, and statistically analyze, quantitative transcriptome maps, based on gene expression data from multiple sources. The release includes FileMaker Pro database management runtime application and it is freely available at http://apollo11.isto.unibo.it/software/, along with preconfigured implementations for mapping of human, mouse and zebrafish transcriptomes.

Mechanisms that regulate adaptor binding to beta-integrin cytoplasmic tails

Cells recognize and respond to their extracellular environment through transmembrane receptors such as integrins, which physically connect the extracellular matrix to the cytoskeleton. Integrins provide the basis for the assembly of intracellular signaling platforms that link to the cytoskeleton and influence nearly every aspect of cell physiology; however, integrins possess no enzymatic or actin-binding activity of their own and thus rely on adaptor molecules, which bind to the short cytoplasmic tails of integrins, to mediate and regulate these functions. Many adaptors compete for relatively few binding sites on integrin tails, so regulatory mechanisms have evolved to reversibly control the spatial and temporal binding of specific adaptors. This Commentary discusses the adaptor proteins that bind directly to the tails of beta integrins and, using talin, tensin, filamin, 14-3-3 and integrin-linked kinase (ILK) as examples, describes the ways in which their binding is regulated.

Adhesion-induced unclasping of cytoplasmic tails of integrin alpha(IIb)beta3

Integrin alpha(IIb)beta(3) plays a pivotal role in hemostasis and thrombosis by mediating adhesive interactions of platelets. Binding of alpha(IIb)beta(3) to its physiological ligands, immobilized fibrinogen and fibrin, induces outside-in signaling in platelets, leading to their adhesion and spreading even without prior stimulation by agonists. Implicit in these phenomena is a requirement for the linkage between integrins' cytoplasmic tails and intracellular proteins. However, the nature of the initiating signal has not been established. In this study, we examined whether binding of alpha(IIb)beta(3) to immobilized fibrin(ogen), per se, triggers interaction of the integrin with cytoplasmic proteins. Using the integrin-binding skelemin fragment as a marker of exposure of residues involved in the clasp between alpha(IIb) and beta(3) cytoplasmic tails, we showed that its binding site in the membrane-proximal beta(3) 715-730 segment is cryptic and becomes exposed as a result of binding of isolated alpha(IIb)beta(3) to immobilized ligands. Furthermore, the skelemin-like protein present in platelets and CHO cells does not associate with alpha(IIb)beta(3) in resting platelets or suspended alpha(IIb)beta(3)-expressing CHO cells but is recruited to integrin during cell adhesion. In addition, not only beta(3) but also the membrane-proximal 989-1000 segment of the alpha(IIb) cytoplasmic tail binds the skelemin fragment. Finally, the same residues, alpha(IIb) Val(990), alpha(IIb) Arg(995), and beta(3) His(722), involved in the formation of the clasp between the tails are also required for skelemin binding. These studies suggest that ligation of alpha(IIb)beta(3) by immobilized ligands during platelet adhesion induces a transmembrane conformation change in the integrin, resulting in unclasping of the complex between the membrane-proximal parts of cytoplasmic tails, thereby unmasking residues involved in binding the skelemin-like protein. Thus, the junction between alpha(IIb) and beta(3) cytoplasmic tails may contain the critical structural information for the initiation of outside-in signaling.

Group IVA cytosolic phospholipase A2 (cPLA2alpha) and integrin alphaIIbbeta3 reinforce each other's functions during alphaIIbbeta3 signaling in platelets

Group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) catalyzes release of arachidonic acid from glycerophospholipids, leading to thromboxane A(2) (TxA(2)) production. Some platelet agonists stimulate cPLA(2)alpha, but others require fibrinogen binding to alphaIIbbeta3 to elicit TxA(2). Therefore, relationships between cPLA(2)alpha and alphaIIbbeta3 were examined. cPLA(2)alpha and a cPLA(2)alpha binding partner, vimentin, coimmunoprecipitated with alphaIIbbeta3 from platelets, independent of fibrinogen binding. Studies with purified proteins and with recombinant proteins expressed in CHO cells determined that the interaction between cPLA(2)alpha and alphaIIbbeta3 was indirect and was dependent on the alphaIIb and beta3 cytoplasmic tails. Fibrinogen binding to alphaIIbbeta3 caused an increase in integrin-associated cPLA(2)alpha activity in normal platelets, but not in cPLA(2)alpha-deficient mouse platelets or in human platelets treated with pyrrophenone, a cPLA(2)alpha inhibitor. cPLA(2)alpha activation downstream of alphaIIbbeta3 had functional consequences for platelets in that it was required for fibrinogen-dependent recruitment of activated protein kinase Cbeta to the alphaIIbbeta3 complex and for platelet spreading. Thus, cPLA(2)alpha and alphaIIbbeta3 interact to reinforce each other's functions during alphaIIbbeta3 signaling. This provides a plausible explanation for the role of alphaIIbbeta3 in TxA(2) formation and in the defective hemostatic function of mouse or human platelets deficient in cPLA(2)alpha.

Nuclear localization of Myomesin-1: possible functions

Myomesin-I (also known as Skelemin) is a approximately 185 kDa protein, which is highly expressed in striated muscle. It contains the prototypic class-I (type-III fibronectin) and class-II (C2-immunoglobulin) motifs. Previous studies have shown the presence of Myomesin-I at the M-line of the sarcomere, where it is thought to interact with thick filament constituents. As reported previously, Myomesin-I was localized to the M-line in the adult cardiac myocytes (adult-myocytes). However, we found that Myomesin-I was also present exclusively in the nucleus of myocytes isolated from new born pups (neonatal-myocytes). In addition, the ectopically expressed Myomesin-I was primarily targeted to the nucleus, similar to the neonatal myocytes. Further investigations revealed that the nuclear-targeting signals were present within the N-terminal 256 residues. A strong consensus sequence for sumoylation is present within the N-terminal 256 residues and is implicated in the shuttling of Myomesin-I between nucleus and cytoplasm. Gene array analysis showed that the presence of Myomesin-I in the nucleus led to the differential expression of more than 42 genes. These studies show a novel and previously unknown localization and function for Myomesin-I.

New aspects of obscurin in human striated muscles

Obscurin is a giant protein (700-800 kDa) present in both skeletal muscles and myocardium. According to animal studies, obscurin interacts with myofibrillar Z-discs during early muscle development, but is translocalised to be predominantly associated with the M-bands in mature muscles. The proposed function for obscurin is in the assembly and organisation of myosin into regular A-bands during formation of new sarcomeres. In the present study, the precise localisation of obscurin in developing and mature normal human striated muscle is presented for the first time. We show that obscurin surrounded myofibrils at the M-band level in both developing and mature human skeletal and heart muscles, which is partly at variance with that observed in animals. At maturity, obscurin also formed links between the peripheral myofibrils and the sarcolemma, and was a distinct component of the neuromuscular junctions. Obscurin should therefore be regarded as an additional component of the extrasarcomeric cytoskeleton. To test this function of obscurin, biopsies from subjects with exercise-induced delayed onset muscle soreness (DOMS) were examined. In these subjects, myofibrillar alterations related to sarcomerogenesis are observed. Our immunohistochemical analysis revealed that obscurin was never lacking in myofibrillar alterations, but was either preserved at the M-band level or diffusely spread over the sarcomeres. As myosin was absent in such areas but later reincorporated in the newly formed sarcomeres, our results support that obscurin also might play an important role in the formation and maintenance of A-bands.