Intact Integrin Research Articles

Full-length αIIbβ3 cryo-EM structure reveals intact integrin initiate-activation intrinsic architecture

Integrin αIIbβ3 is the key receptor regulating platelet retraction and accumulation and a proven drug-target for antithrombotic therapies. Here we resolve the cryo-EM structures of the full-length αIIbβ3, which covers three distinct states along the activation pathway. Firstly, we obtain the αIIbβ3 structure at 3Å resolution in the inactive state, revealing the overall topology of the heterodimer with the transmembrane (TM) helices and the ligand-binding domain tucked in a specific angle proximity to the TM region. After the addition of a Mn2+ agonist, we resolve two coexisting structures representing two new states between inactive and active state. Our structures show conformational changes of the αIIbβ3 activating trajectory and a unique twisting of the integrin legs, which is required for platelets accumulation. Our structure provides direct structural evidence for how the lower legs are involved in full-length integrin activation mechanisms and offers a new strategy to target the αIIbβ3 lower leg.

Heterotropic roles of divalent cations in the establishment of allostery and affinity maturation of integrin αXβ2.

SUMMARYAllosteric activation and silencing of leukocyte β2-integrins transpire through cation-dependent structural changes, which mediate integrin biosynthesis and recycling, and are essential to designing leukocyte-specific drugs. Stepwise addition of Mg2+ reveals two mutually coupled events for the αXβ2 ligand-binding domain—the αX I-domain—corresponding to allostery establishment and affinity maturation. Electrostatic alterations in the Mg2+-binding site establish long-range couplings, leading to both pH− and Mg2+-occupancy-dependent biphasic stability change in the αX I-domain fold. The ligand-binding sensorgrams show composite affinity events for the αX I-domain accounting for the multiplicity of the αX I-domain conformational states existing in the solution. On cell surfaces, increasing Mg2+ concentration enhanced adhesiveness of αXβ2. This work highlights how intrinsically flexible pH− and cation-sensitive architecture endows a unique dynamic continuum to the αI-domain structure on the intact integrin, thereby revealing the importance of allostery establishment and affinity maturation in both extracellular and intracellular integrin events.

Rac1 promotes kidney collecting duct integrity by limiting actomyosin activity.

A polarized collecting duct (CD), formed from the branching ureteric bud (UB), is a prerequisite for an intact kidney. The small Rho GTPase Rac1 is critical for actin cytoskeletal regulation. We investigated the role of Rac1 in the kidney collecting system by selectively deleting it in mice at the initiation of UB development. The mice exhibited only a mild developmental phenotype; however, with aging, the CD developed a disruption of epithelial integrity and function. Despite intact integrin signaling, Rac1-null CD cells had profound adhesion and polarity abnormalities that were independent of the major downstream Rac1 effector, Pak1. These cells did however have a defect in the WAVE2-Arp2/3 actin nucleation and polymerization apparatus, resulting in actomyosin hyperactivity. The epithelial defects were reversible with direct myosin II inhibition. Furthermore, Rac1 controlled lateral membrane height and overall epithelial morphology by maintaining lateral F-actin and restricting actomyosin. Thus, Rac1 promotes CD epithelial integrity and morphology by restricting actomyosin via Arp2/3-dependent cytoskeletal branching.

Heterotropic Roles of Divalent Cations in the Establishment of Allostery and Affinity Maturation of Integrin αXβ2

SummaryAllosteric activation and silencing of leukointegrins transpire through highly complex structural changes between and within domains, mediating integrin affinity and bidirectional allostery relay. However, the landscape of these conformational changes, which is essential for elaborating integrin dynamics on cell surfaces and designing leukointegrin specific small molecules, has not yet been mapped. Herein we combine biophysical approaches and cell-based affinity assays to fine-tune the conformational dynamism and adhesiveness of a leukocyte integrin, αXβ2, using its physiological cofactor Mg2+. Stepwise addition of Mg2+ revealed two mutually coupled events for the αXβ2 ligand-binding domain—called the αX I-domain—corresponding to allostery establishment and affinity maturation. Electrostatic alterations in the statistically co-evolved Mg2+-binding site establish long-range couplings, leading to pH and Mg2+-occupancy dependent bimodal stability change in the αX I-domain fold. The ligand-binding sensorgrams revealed composite affinity events for the αX I-domain accounting for the multiplicity of conformational states existing in the solution. This bi-faceted transition appears to progressively amalgamate the adhesiveness and allostery within the structural interconversion zone. This work highlights how the intrinsically flexible architecture endows a dynamic continuum to the αI-domain structure on the intact integrin platform, thereby progressively regulating allostery establishment and affinity maturation. This dynamic characteristic appears as a specialization of the αX I-domain and is consistent with its rapid equilibration between open and closed conformations, facilitating rapid upregulation of leukocyte adhesiveness and activation.

Structural Analysis of Four Newly Identified Extracellular Membrane-Proximal Missense Mutations in Patients with Glanzmann Thrombasthenia

Background: Glanzmann thrombasthenia (GT), an autosomal recessive inherited platelet disorder, is a moderate to severe bleeding syndrome caused by the absence of platelet aggregation due to quantitative and/or qualitative deficiencies of the αIIbβ3 integrin. We recently identified 41 causative missense mutations of which 24 were novel in a large cohort of 76 GT families (Genoscope project). These mutations mainly localize to the headpiece region of the integrin that has been well studied but 4 mutations although extracellular were proximal to the plasma membrane. We therefore performed molecular modeling of these 4 mutations to obtain new insights into the structure of a poorly understood region of this unique receptor.Aim: To identify structures or conformations engaged in the stability of the integrin and which are important for maturation and expression.Results: Of the 4 novel selected mutations, 3 concerned the calf-2 domain of αIIb - Gly792Glu (G823E, nomenclature with leader sequence), Leu924Gln (L955Q) and Thr953Lys (T984K) and one the EGF-3 domain of β3 Gly540Asp (G566D). All of these mutations affected highly conserved amino acids and were predicted to be damaging by in silico analysis (SIFT, Polyphen). None influenced glycosylation or mRNA splicing. They were present either in a homozygous form (β3 G540D) or were heterozygous in association with an identified and proven null mutation. Three were associated with type I GT (<5% αIIbβ3), while the αIIbG792E mutation occurred in a patient with type II GT (with 10% residual αIIbβ3) whose much reduced but partial transport to the surface was confirmed following expression of the recombinant integrin in CHO cells (with pro-αIIb predominating in the cytoplasm). The structural implications of these amino acid substitutions was assessed using PyMol Molecular Graphics System version 1.3 (www.pymol.org) based on the crystallographic data of αIIbβ3 in the bent non-activated state (3fcs PDB file). Amino acids were visualized in the rotamer form showing side change orientations incorporated from the Dunbrack Backbone library with the maximum probability. We first determined that the αIIb calf-2 domain has a β barrel-like structure largely composed of hydrophobic amino acids whose side chains orientate towards the inner cavity. Interestingly, L924Q and T953K substitutions occur at or adjacent to a conserved motif consisting of five polar amino acids central to the β barrel protected from H2O molecules and involved in H-bond interactions. This particular motif, specific to calf-2, may introduce rigidity close to the membrane. Both L924Q and T953K disrupt the β barrel motif and promote flexibility. G792E is situated between the calf-1 and calf-2 domains in an unstructured connecting loop between two adjacent β sheets. Its replacement by the larger negatively charged Glu introduces steric encumbrance and results in an increase of the angle formed by the two calf domains, probably leading to the straightening of the second distal part of the long arm of αIIb. The β3 G540D substitution is found in the EGF3 domain of β3 that occurs at the axe of the cysteine-rich domain of the β3 arm, facing the αIIb calf-1 and calf-2 domains in the intact integrin. This substitution with the introduction of a charged and larger amino acid results in a weaker link between the two β sheets of EGF-3 and a loss of H-bonds. The result is an increased fragility within the β3 arm structure notably at the site of two stacked aromatic amino acids (H539 and W553) with a moving apart of the β sheets.Conclusions: We show that 4 novel missense mutations in the extracellular membrane-proximal domains of αIIb and β3 cause conformational changes in domains that control the overall structure of the newly formed integrin. They show how the structure of both domains is under tight quality control and that precisely defined conformations are indispensable for αIIbβ3 maturation. DisclosuresNo relevant conflicts of interest to declare.

The Structure of Integrin α1I Domain in Complex with a Collagen-mimetic Peptide

We have determined the structure of the human integrin α1I domain bound to a triple-helical collagen peptide. The structure of the α1I-peptide complex was investigated using data from NMR, small angle x-ray scattering, and size exclusion chromatography that were used to generate and validate a model of the complex using the data-driven docking program, HADDOCK (High Ambiguity Driven Biomolecular Docking). The structure revealed that the α1I domain undergoes a major conformational change upon binding of the collagen peptide. This involves a large movement in the C-terminal helix of the αI domain that has been suggested to be the mechanism by which signals are propagated in the intact integrin receptor. The structure suggests a basis for the different binding selectivity observed for the α1I and α2I domains. Mutational data identify residues that contribute to the conformational change observed. Furthermore, small angle x-ray scattering data suggest that at low collagen peptide concentrations the complex exists in equilibrium between a 1:1 and 2:1 α1I-peptide complex.

Two differential flows in a bioreactor promoted platelet generation from human pluripotent stem cell–derived megakaryocytes

Induced pluripotent stem cell (iPSC) technology enables us to investigate various potential iPSC-based therapies. Although the safety of iPSC derivation has not been completely validated, anucleate cells, such as platelets or erythrocytes, derived from iPSCs are promising targets. However, the efficiency of invitro platelet generation from megakaryocytes (MKs) under static culture conditions is lower than is seen invivo. In this study, we demonstrate the proof of concept by a two-dimensional flow culture system that enabled us to increase platelet yield from human embryonic stem cell or iPSC-derived MKs using a biomimetic artificial blood vessel system. The bioreactor was composed of biodegradable scaffolds with ordered arrays of pores made to mimic invivo bone marrow through salt leaching. Within the system, two flows in different directions in which the angle between the directions of flow is 60 degrees but not 90 degrees contributed to suitable pressure and shear stress applied to MKs to promote platelet generation. Generated platelets derived from human embryonic stem cells or human induced pluripotent stem cells through the bioreactor with a 60-degree angle revealed intact integrin αIIbβ3 activation after agonist stimulation. Collectively, our findings indicate that two flows in different directions of two-dimensional flow culture may be a feasible system for invitro generation of platelets from pluripotent stem cells (i.e., iPSC-derived MKs) in numbers sufficient for transfusion therapy.

Photosensitized Oxidative Stress to ARPE-19 Cells Decreases Protein Receptors that Mediate Photoreceptor Outer Segment Phagocytosis

To determine whether previously shown photodynamic (PD)-induced inhibition of specific photoreceptor outer segment (POS) phagocytosis by ARPE-19 cells is associated with reductions in receptor proteins mediating POS phagocytosis, and if PD treatment with merocyanine-540 (MC-540) produces additional effects leading to its inhibition of nonspecific phagocytosis. ARPE-19 cells preloaded with MC-540 or rose bengal (RB) were sublethally irradiated with green light. Phagocytosis of POS was measured by flow cytometry and POS receptor proteins (Mer tyrosine kinase receptor [MerTK] and integrin subunits αv and β5) and β-actin were quantified by Western blotting at 0.5 and 24 hours after irradiation, with comparison to samples from nonsensitized control cultures. The intact integrin heterodimer αvβ5 was quantified by immunoprecipitation followed by blotting. The distribution of N-cadherin, ZO-1, and F-actin was visualized by fluorescence microscopy. Mild PD stress mediated by both photosensitizers that elicits no significant morphologic changes produces transient and recoverable reductions in MerTK. The individual αv and β5 integrin subunits are also reduced but only partially recover. However, there is sufficient recovery to support full recovery of the functional heterodimer. Light stress mediated by MC-540 also reduced levels of actin, which is known to participate in the internalization of particles regardless of type. After PD treatment POS receptor protein abundance and phagocytosis show a coincident in time reduction then recovery suggesting that diminution in receptor proteins contributes to the phagocytic defect. The additional inhibition of nonspecific phagocytosis by MC-540-mediated stress may result from more widespread effects on cytosolic proteins. The data imply that phagocytosis receptors in RPE cells are sensitive to oxidative modification, raising the possibility that chronic oxidative stress in situ may reduce the efficiency of the RPE's role in photoreceptor turnover, thereby contributing to retinal degenerations.

NAD+ Biosynthesis Ameliorates a Zebrafish Model of Muscular Dystrophy

Author SummaryA variety of diseases, both inherited and acquired, affect muscle tissues in humans. Critical to muscle homeostasis is the anchoring of muscle fibers to their surrounding microenvironment through cell adhesion complexes that help to resist the repeated stress experienced during muscle contraction. Genetic mutations in these complexes weaken this mechanical attachment, making fibers more susceptible to damage and death. The resulting increased fiber degeneration can eventually lead to progressive muscle-wasting diseases, known collectively as muscular dystrophies. Although clinical trials are ongoing, there is presently no way to cure the loss of muscle structure and function associated with these diseases. We identified a novel cell adhesion pathway involving integrin alpha6 that promotes adhesion of muscle cells to their microenvironment. Here, we show that activation of this pathway not only significantly reduces muscle degeneration but also improves the swimming ability of dystrophic zebrafish. We explore the likely benefits and limitations of this pathway in treating symptoms of congenital muscular dystrophies. Our findings suggest that activation of this pathway (for example, by boosting levels of NAD+) has the potential to ameliorate loss of muscle structure and function in multiple muscular dystrophies.

Sorting nexin 17 prevents lysosomal degradation of β1 integrins by binding to the β1-integrin tail

Integrin functions are controlled by regulating their affinity for ligand, and by the efficient recycling of intact integrins through endosomes. Here we demonstrate that the Kindlin-binding site in the β1-integrin cytoplasmic domain serves as a molecular switch enabling the sequential binding of two FERM-domain-containing proteins in different cellular compartments. When β1 integrins are at the plasma membrane, Kindlins control ligand-binding affinity. However, when they are internalized, Kindlins dissociate from integrins and sorting nexin 17 (SNX17) is recruited to free β1-integrin tails in early endosomes to prevent β1-integrin degradation, leading to their recycling back to the cell surface. Our results identify SNX17 as a β1-integrin-tail-binding protein that interacts with the free Kindlin-binding site in endosomes to stabilize β1 integrins, resulting in their recycling to the cell surface where they can be reused.

Intact αIIbβ3 Integrin Is Extended after Activation as Measured by Solution X-ray Scattering and Electron Microscopy

Integrins are bidirectional signaling molecules on the cell surface that have fundamental roles in regulating cell behavior and contribute to cell migration and adhesion. Understanding of the mechanism of integrin signaling and activation has been advanced with truncated ectodomain preparations; however, the nature of conformational change in the full-length intact integrin molecule remains an active area of research. Here we used small angle x-ray scattering and electron microscopy to study detergent-solubilized, intact platelet integrin α(IIb)β(3). In the resting state, the intact α(IIb)β(3) adopted a compact, bent conformation. Upon activation with Mn(2+), the average integrin extension increased. Further activation by addition of ligand led to stabilization of the extended state and opening of the headpiece. The observed extension and conformational rearrangement upon activation are consistent with the extension and headpiece opening model of integrin activation.

Computational design of a β-peptide that targets transmembrane helices.

The design of β-peptide foldamers targeting the transmembrane (TM) domains of complex natural membrane proteins has been a formidable challenge. A series of β-peptides was designed to stably insert in TM orientations in phospholipid bilayers. Their secondary structures and orientation in the phospholipid bilayer was characterized using biophysical methods. Computational methods were then devised to design a β-peptide that targeted a TM helix of the integrin α(IIb)β(3). The designed peptide (β-CHAMP) interacts with the isolated target TM domain of the protein and activates the intact integrin in vitro.

An Engineered α1 Integrin-binding Collagenous Sequence

Collagen is an extracellular matrix structural component that can regulate cellular processes through its interaction with the integrins, α1β1, α2β1, α10β1, and α11β1. Collagen-like proteins have been identified in a number of bacterial species. Here, we used Scl2 from Streptococcus pyogenes serotype M28 strain MGAS6274 as a backbone for the introduction of discrete integrin-binding sequences. The introduced sequences GLPGER, GFPGER, or GFPGEN did not affect triple helix stability of the Scl (Streptococcal collagen-like) protein. Using ELISA and surface plasmon resonance, we determined that Scl2(GLPGER) and Scl2(GFPGER) bound to recombinant human α1 and α2 I-domains in a metal ion-dependent manner and without a requirement for hydroxyproline. We predicted a novel and selective integrin-binding sequence, GFPGEN, through the use of computer modeling and demonstrated that Scl2(GFPGEN) shows specificity toward the α1 I-domain and does not bind the α2 I-domain. Using C2C12 cells, we determined that intact integrins interact with the modified Scl2 proteins with the same selectivity as recombinant I-domains. These modified Scl2 proteins also acted as cell attachment substrates for fibroblast, endothelial, and smooth muscle cells. However, the modified Scl2 proteins were unable to aggregate platelets. These results indicate that Scl2 is a suitable backbone for the introduction of mammalian integrin-binding sequences, and these sequences may be manipulated to individually target α1β1 and α2β1.

Structural basis of integrin transmembrane activation

Integrins are cell adhesion receptors that transmit bidirectional signals across plasma membrane and are crucial for many biological functions. Recent structural studies of integrin transmembrane (TM) and cytoplasmic domains have shed light on their conformational changes during integrin activation. A structure of the resting state was solved based on Rosetta computational modeling and experimental data using intact integrins on mammalian cell surface. In this structure, the alpha(IIb) GXXXG motif and their beta(3) counterparts of the TM domains associate with ridge-in-groove packing, and the alpha(IIb) GFFKR motif and the beta(3) Lys-716 in the cytoplasmic segments play a critical role in the alpha/beta association. Comparing this structure with the NMR structures of the monomeric alpha(IIb) and beta(3) (represented as active conformations), the alpha subunit helix remains similar after dissociation whereas beta subunit helix is tilted by embedding additional 5-6 residues into the lipid bilayer. These conformational changes are critical for integrin activation and signaling across the plasma membrane. We thus propose a new model of integrin TM activation in which the recent NMR structure of the alpha(IIb)beta(3) TM/cytoplasmic complex represents an intermediate or transient state, and the electrostatic interaction in the cytoplasmic region is important for priming the initial alpha/beta association, but not absolutely necessary for the resting state.

Just a Drop of Blood: Exploration of Shear-Dependent Platelet Adhesion and Activation in Microfluidic Devices.

Interactions of platelets and their adhesion receptors with extracellular matrices are essential for hemostasis. Platelets experience different shear rates as they circulate through the vasculature. Conventional studies of platelets in shear flow are performed in simple flow chambers with relatively large volumes of cells and reagents, limiting testing when these are in short supply. Microfluidics technology should enable the concurrent study of multiple small volume samples across a wide range of shear rates, thereby allowing trends to emerge that might be difficult to detect otherwise. To achieve this goal, we fabricated PDMS microfluidic devices that permit testing of dynamic platelet adhesion over a 100-fold span of shear rates from a single 200μl blood sample. Alternate device design permits synchronous monitoring of platelet adhesion from two genetically distinct blood samples or treatment groups. We have used this technology to gain new insights into integrin αIIbβ3 function in mepacrine-labeled platelets under shear flow. In whole blood, the adhesion of wild-type mouse platelets to a fibrinogen-coated substrate was shear rate-dependent, similarly to human platelets. In contrast, adhesion of αIIbβ3-deficient (β3−/−) platelets was virtually absent above a shear rate of 100 s−1. To distinguish between requirements for the presence of an intact extracellular αIIbβ3 ligand binding domain versus an intact integrin activation process, we used mice with a Y/A mutation at residue 747 in the β3 cytoplasmic tail (β3Y747A), that selectively blocks talin interaction with β3, agonist-induced αIIbβ3 activation, and platelet thrombus formation in vivo. When compared to wild-type and β3−/− platelets, a normal αIIbβ3 extracellular domain on β3Y747A platelets partially rescued dynamic adhesion to fibrinogen by 50–80% at ≤130 s−1, but by only 25% at 250 s−1. Treatment of wild-type platelets with PGE1 to inhibit platelet activation similarly reduced adhesion to fibrinogen at higher shear rates. On fibrillar type I collagen, wild-type platelets formed an initial monolayer and progressively larger thrombi over time. In addition, platelets supported rolling and firm adhesion of granulocytes in a manner dependent on shear rate, platelet P-selectin and granulocyte PSGL1. In contrast, no platelet thrombus growth on collagen was observed with αIIbβ3-deficient or β3Y747A platelets, or with wild-type platelets treated with PGE1. Furthermore, even the initial adhesion of αIIbβ3-deficient and β3Y747A platelets to collagen rapidly declined at increasing shear rates (120%, 30% and 7% of wild-type platelets at 70, 1000 and 4000 s−1, respectively (p < 0.01)). Taken together, these studies establish that microfluidics provides an efficient and high-throughput platform to study mechanisms of dynamic platelet adhesion, activation and thrombus formation on extracellular matrices. Furthermore, they demonstrate a role for talin-dependent αIIbβ3 activation in all of these processes. This platform will be particularly useful under conditions where blood sample volumes or reagents are limiting, such as in neonates, and genetically-modified model organisms.

Conformational changes in human integrin αIIbβ3 after platelet activation, monitored by FRET

Integrin α IIb β 3, an abundant heterodimeric receptor at the surface of blood platelets, binds adhesive proteins after platelet activation and plays a primary role in haemostasis. In solution, it has been observed mainly in two conformations: the bent and the extended forms. Based on X-ray crystallography, electron microscopy and immunochemical observations of full-length integrin ectodomains and intact integrins, it has been agreed that unactivated integrins are in the bent conformation, both isolated in solution and in living cells. However, consensus is yet to emerge on the bent or extended conformation of activated integrins and on their mechanism of activation (the switchblade, the deadbolt and the S–S reduction models), which require further experimental tests at the cell level to become established facts. Here, we tested the proposed structural rearrangements undergone by integrin α IIb β 3 after cell activation, by using Förster-type fluorescence resonance energy transfer (FRET) and attached fluorescent labels to Fab fragments of monoclonal antibodies directed to the βA domain of the β 3 subunit (donor, Alexa488-P97 Fab) and to the Calf-2 domain of the α IIb subunit (acceptor, Cy3-M3 Fab or Cy3-M10 Fab). The FRET efficiencies observed after ADP or TRAP platelet activation changed less than 20% from the resting values, showing that the distance between the labeled Fab fragments changes only modestly after platelet activation by physiological agonists. This observation is consistent with a conformational model of the activated integrin in the cell less extended than in the switchblade model.

Mutation of a Conserved Asparagine in the I-like Domain Promotes Constitutively Active Integrins αLβ2 and αIIbβ3

The leukocyte beta2 integrins are heterodimeric adhesion receptors required for a functional immune system. Many leukocyte adhesion deficiency-1 (LAD-1) mutations disrupt the expression and function of beta2 integrins. Herein, we further characterized the LAD-1 mutation N329S in the beta2 inserted (I)-like domain. This mutation converted alphaLbeta2 from a resting into a high affinity conformer because alphaLbeta2N329S transfectants adhered avidly to ligand intercellular adhesion molecule (ICAM)-3 in the absence of additional activating agent. An extended open conformation is adopted by alphaLbeta2N329S because of its reactivity with the beta2 activation reporter monoclonal antibodies MEM148 and KIM127. A corresponding mutation in beta3 generated constitutively active alphaIIbbeta3 that adhered to fibrinogen. This Asn is conserved in all human beta subunits, and it resides before the last helix of the I-like domain, which is known to be important in activation signal propagation. By mutagenesis studies and review of existing integrin structures, we conjectured that this conserved Asn may have a primary role in shaping the I-like domain by stabilizing the conformation of the alpha7 helix and the beta6-alpha7 loop in the I-like domain.

The use of mild trypsinization conditions in the detachment of endothelial cells to promote subsequent endothelialization on synthetic surfaces

A necessary condition for endothelialization of small diameter grafts is rapid and firm adhesion of endothelial cells upon exposure to flow. To retain integrins on the cell surface, we assessed the effects of trypsin concentration, the duration of trypsin incubation, and trypsin neutralization methods on endothelial cell adhesion. Human umbilical vein endothelial cells which were detached using 0.025% trypsin for 5 min and seeded onto glass pretreated with fibronectin had close to 100% cell retention when shear stresses as high as 200 dyn/cm 2 were applied for 2 min. An equivalent level of cell retention was observed on fibronectin coated Teflon-AF™ for shear stresses up to 60 dyn/cm 2 applied for 4 h. Using 0.025% trypsin, initial cell spreading and cell surface α 5 β 1 integrins were increased relative to cells treated with 0.5% trypsin. After 1 h of attachment, focal adhesions formed when low trypsin concentrations were used but were less evident with high trypsin concentrations. These results showed that low trypsin concentrations produced faster spreading, a higher number of intact integrins, and rapid focal adhesion formation.

Development of an ESI-MS screening method for evaluating binding affinity between integrin fragments and RGD-based peptides

Electrospray ionization mass spectrometry (ESI-MS) is used to evaluate binding affinities between integrin fragments and RGD-based peptide ligands. The integrin fragment peptides synthesized and studied were selected based on crystallographic and literature data which highlighted the specific binding pockets for the RGD motif in intact integrin proteins. Relative binding constants were determined by fitting the data obtained from ESI-MS titration experiments to a quadratic equation based on a 1:1 association model. Frontal analysis capillary electrophoresis (FACE) was used as a complementary solution phase technique to measure the absolute binding constants for the same host–guest systems and the results from both techniques were compared. Gas phase collision activated dissociation threshold measurements in an ion trap mass spectrometer also were carried out on the complexes to further validate binding constant measurements. Better correlation was observed between the MS/MS and CE results, than for ESI-MS titration. The ESI-MS titration results showed some correlation; however, it is apparent that there are important methodological shortcomings which need to be addressed. Several factors related to the titration method, ionization efficiencies, enthalpy/entropy contributions during phase transfer, and solvent effects are discussed. This work introduces a first approach to development of a high throughput ESI-MS-based method suitable for screening potential peptide-based drug compounds that target integrins on cancerous cells. It also serves as a cautionary tale to those interested in performing similar types of analyses.

Contribution of postmortem changes of integrin, desmin and μ-calpain to variation in water holding capacity of pork

The purpose of this study was to examine the relationship between integrin, desmin, μ-calpain and water holding capacity in fresh pork. High levels of intact integrin at one day postmortem were negatively correlated with day 1 ( P < 0.05) and days 1–5 (cumulative) drip loss ( P < 0.05). High levels of intact integrin at five days postmortem were negatively correlated with days 1–7 (cumulative) purge loss ( P < 0.05). Intensity of intact desmin at one day postmortem was positively correlated with days 1–7 purge loss ( P < 0.01). There were positive correlations between intensity of intact desmin at day 7 and day 1 ( P < 0.01), days 1–5 drip loss ( P < 0.01) and days 1–7 purge loss ( P < 0.05). Autolysis of μ-calpain was associated with the degradation of desmin and drip or purge loss postmortem. Our results indicate that low levels of degradation of integrin and high levels of desmin degradation were associated with low drip loss values in fresh pork.