Signal transduction from the extracellular matrix: the integrin-tyrosine kinase connection

C-H stretch for probing kinetics of self-assembly into macromolecular chiral structures at interfaces by chiral sum frequency generation spectroscopy

Self-assembly of molecules into chiral macromolecular and supramolecular structures at interfaces is important in various fields, such as biomedicine, polymer sciences, material sciences, and supramolecular chemistry. However, probing the kinetics at interfaces remains challenging because it requires a real-time method that has selectivity to both interface and chirality. Here, we introduce an in situ approach of using the C-H stretch as a vibrational probe detected by chiral sum frequency generation spectroscopy (cSFG). We showed that the C-H stretch cSFG signals of an amphiphilic peptide (LK7β) can reveal the kinetics of its self-assembly into chiral β-sheet structures at the air-water interface. The cSFG experiments in conjunction with measurements of surface pressure allow us to propose a mechanism of the self-assembly process, which involves an immediate adsorption of disordered structures followed by a lag phase before the self-assembly into chiral antiparallel β-sheet structures. Our method of using the C-H stretch signals implies a general application of cSFG to study the self-assembly of bioactive, simple organic, and polymeric molecules into chiral macromolecular and supramolecular structures at interfaces, which will be useful in tackling problems, such as protein aggregation, rational design of functional materials, and fabrication of molecular devices.

Scaffold mediated regulation of MAPK signaling and cytoskeletal dynamics: a perspective

Cell migration is critical for many physiological processes and is often misregulated in developmental disorders and pathological conditions including cancer and neurodegeneration. MAPK signaling and the Rho family of proteins are known regulators of cell migration that exert their influence on cellular cytoskeleton during cell adhesion and migration. Here we review data supporting the view that localized ERK signaling mediated through recently identified scaffold proteins may regulate cell migration.

Colocalization of c-Src (pp60src) and bone morphogenetic protein 2/4 expression during mandibular distraction osteogenesis: in vivo evidence of their role within an integrin-mediated mechanotransduction pathway

Craniofacial distraction osteogenesis (DO) is an evolving reconstructive technique with expanding applications for the treatment of bony deficiencies of the facial skeleton. Mechanical force has been known to play a fundamental role in modulating sustained osteogenic response and therefore is believed to function as a critical regulator of DO. We hypothesize that key clustering components of an integrin-mediated signaling pathway, including c-Src (pp60), are necessary for mediating the response to mechanical force. The specific aim of this study is to demonstrate up-regulation of a key focal adhesion molecule, c-Src, selectively in new bone formation subject to the mechanical forces of distraction and to demonstrate a lack of that same up-regulation in new bone formation associated with simple fracture healing. An additional specific aim is to demonstrate colocalization of c-Src expression and bone morphogenetic protein (BMP 2/4) expression during mandibular DO. Using a rat model of mandibular DO, c-Src and BMP 2/4 expression were evaluated in critical size defects, subcritical size defects, and mandibles undergoing gradual distraction. Osseous regeneration was observed in the course of gradual distraction; this process was associated with increased expression of c-Src. Furthermore, the presence of BMP 2/4 closely approximated c-Src expression spatially and temporally, suggesting a link between cytoplasmic focal adhesion activation and the resultant nuclear regulation of osteogenic protein expression. In significant contradistinction, minimal c-Src expression was found in the subcritical-sized defects where the fractures healed secondarily but where no gradual distraction was performed. Instead, the new bone formation inherent in the secondarily healed subcritical-sized defects demonstrated expected BMP 2/4 expression but was devoid of an up-regulation of c-Src. Finally, as expected, minimal expression of both c-Src and BMP was found in fibrous nonunion specimens. C-src expression was observed during gradual distraction; furthermore, minimal c-Src expression was visualized during subacute and critical-size defect fracture healing. C-Src expression also closely approximated BMP expression during DO. These findings that c-Src expression is found primarily only during conditions of cyclic distraction forces strongly implicates that mechanical force during gradual distraction is associated with c-Src expression. These results provide in vivo support for previous in vitro evidence that mechanical force profoundly influences osseous regeneration during distraction osteogenesis by means of a c-Src dependent mechanotransduction pathway, resulting in increased expression of osteogenic proteins, including BMP 2/4.

Increased expression of apoptotic markers in melanoma

Extensive labelling for the apoptotic markers calcium channel receptor P2X(7) and caspase-3 and telomerase activity was co-localized at a similar intensity in areas affected by superficial spreading melanoma obtained from 80 patients. Labelling for each of these markers also extended 2 microm from the melanoma into the keratinocyte layer of the adjacent normal epidermis. Conversely, the calcium-regulating receptors P2X(1-3) and P2Y(2) (found in normal but not neoplastic skin) were fully de-expressed within 2 microm of the melanoma but fully expressed beyond that distance. The cell adhesion protein E-cadherin (also only present in normal skin) was progressively de-expressed from a point 2 microm from the melanoma until full de-expression within the lesion. These results show that telomerase-induced proliferation and defensive apoptosis are co-localized and simultaneous processes in melanoma tissue. Melanoma cell proliferation appears to overwhelm the apoptotic defence, perhaps due to the anti-apoptotic effects of telomerase. In addition, keratinocyte regulation of the epidermis and dermis is severely compromised by the loss of E-cadherin and P2X(1-3) and P2Y(2) receptors, resulting in a lesion that is aggressive and malignant.

Expression, regulation, and function of P2X(4) purinergic receptor in human cervical epithelial cells

Micromolar concentrations of ATP stimulate biphasic change in transepithelial conductance across CaSki cultures on filters, an acute transient increase (phase I response; triggered by P2Y(2) receptor and mediated by calcium mobilization-dependent cell volume decrease) followed by a slower decrease in permeability (phase II response). Phase II response is mediated by augmented calcium influx and protein kinase C-dependent increase in tight junctional resistance. The objective of the study was to determine the role of P2X(4) receptor as a mediator of phase II response. Human cervical epithelial cells express P2X(4) receptor mRNA (1.4-, 2.2-, and 4.4-kb isoforms by Northern blot analysis) and P2X(4) protein. Depletion of vitamin A reversibly downregulated P2X(4) receptor mRNA and protein and ATP-induced calcium influx. Depletion of vitamin A abrogated phase II response, and the effect could be partially reversed only with retinoic acid receptor (RAR)-selective retinoids but not retinoid X receptor (RXR) agonists. Depletion of vitamin A also abrogated protein kinase C increase in tight junctional resistance, and the effect could not be reversed with retinoids. Depletion of vitamin A also abrogated phase I increase in permeability and reversibly downregulated P2Y(2) receptor mRNA and ATP-induced calcium mobilization. However, in contrast to phase II response, both RAR and RXR agonists could fully reverse those effects. These results suggest that phase II response is mediated by a P2X(4) receptor mechanism.

Tenascin, E-cadherin and P2X calcium channel receptor expression is increased during rat blastocyst implantation

The calcium-activated cell-adhesion proteins tenascin, E-cadherin and the purinergic (P2X) calcium channel receptors are expressed in an identical spatial and temporal pattern in uterine epithelium in the rat during implantation. On Day 1 of pregnancy (estrous), a diffuse cytoplasmic and specific basement membrane label for each of the proteins was observed throughout the uterine epithelium. On Day 3 of pregnancy, a specific and prominent lateral plasma membrane label for each protein was seen. At the time of implantation on Day 6, an additional and significant increase in the label for each was observed on the apical epithelium. At this time, the label for tenascin in the apical epithelium was increased 2.1-fold (p < 0.0004), that of E-cadherin was increased 2.5-fold (p < 0.0001) and the P2X receptor label was increased 2.0-fold (p < 0.0001). These observations suggest a major role for the calcium-activated adhesion proteins tenascin and E-cadherin in attachment and implantation, with ionic calcium for protein activation possibly provided by the P2X calcium channels. These events occur along the entire length of the uterine epithelium in preparation for blastocyst adhesion.

Application of passive stretch and its implications for muscle fibers

To increase range of motion, physical therapists frequently use passive stretch as a means of gaining increased excursion around a joint. In addition to clinical studies showing effectiveness, thereby supporting evidence-based practice, the basic sciences can provide an explanation how a technique might work once a technique is known to be effective. The goal of this article is to review the potential cellular events that may occur when muscle fibers are stretched passively. A biomechanical example of passive stretch applied to the ankle is used to provide a means to discuss passive stretch at the cellular and molecular levels. The implications of passive stretch on muscle fibers and the related connective tissue are discussed with respect to tissue biomechanics. Emphasis is placed on structures that are potentially involved in the sensing and signal transduction of stretch, and the mechanisms that may result in myofibrillogenesis are explored.

Distributional changes of purinergic receptor subtypes (P2X 1-7) in uterine epithelial cells during early pregnancy

Expression of each of the purinergic receptor subtypes (P2X7) was studied by immunohistochemical localization in the apical, lateral and basal plasma membranes of rat uterine epithelial cells during early pregnancy to the time of implantation on Day 6. Labelling for each P2X subtype was seen in the apical, lateral and basal compartments on Days 1 and 3, except for P2X2 which was only observed in the basement membrane. The P2X5 signal was similar in temporal and spatial expression to the other subtypes, but with a greatly reduced intensity. At the time of implantation on Day 6, this pattern altered dramatically. Apical expression markedly increased for most subtypes while the lateral and basal signals were markedly reduced. The exceptions to this pattern were P2X2, which displayed both a strong basal and apical label, and P2X4 which became de-expressed in all areas. We propose that the changing spatial and temporal expression of the P2X receptors is a significant factor in the regulation of events during early pregnancy. They are expressed in the same location as remodelling. apoptosis, and protein activation events prior to implantation on Day 6. These observations suggest an up-regulation of calcium-mediated events, including cytoskeletal alterations, a decrease in luminal pH and transmembrane molecule activation.

Renewal of photoreceptor outer segments and their phagocytosis by the retinal pigment epithelium

The discovery of disc protein renewal in rod outer segments, in 1960s, was followed by the observation that old discs were ingested by the retinal pigment epithelium. This process occurs in both rods and cones and is crucial for their survival. Photoreceptors completely degenerate in the Royal College of Surgeons mutant rat, whose pigment epithelium cannot ingest old discs. The complete renewal process includes the following sequential steps involving both photoreceptor and pigment epithelium activity: new disc assembly and old disc shedding by photoreceptor cells; recognition and binding to pigment epithelium membranes; then ingestion, digestion, and segregation of residual bodies in pigment epithelium cytoplasm. Regulating factors are involved at each step. While disc assembly is mostly genetically controlled, disc shedding and the subsequent pigment epithelium phagocytosis appear regulated by environmental factors (light and temperature). Disc shedding is rhythmically controlled by an eye intrinsic circadian oscillator using endogenous dopamine and melatonin as light and dark signal, respectively. Of special interest is the regulation of phagocytosis by multiple receptors, including specific phagocytosis receptors and receptors for neuroactive substances released from the neuroretina. The candidates for phagocytosis receptors are presented, but it is acknowledged that they are not completely known. The main neuromodulators are adenosine, dopamine, glutamate, serotonin, and melatonin. Although the transduction mechanisms are not fully understood, attention was brought to cyclic AMP, phosphoinositides, and calcium. The chapter points to the multiplicity of regulating factors and the complexity of their intermingling modes of action. Promising areas for future research still exist in this field.

Kinetics, Ca2+ dependence, and biophysical properties of integrin-mediated mechanical modulation of transmitter release from frog motor nerve terminals

Neurotransmitter release from frog motor nerve terminals is strongly modulated by change in muscle length. Over the physiological range, there is an approximately 10% increase in spontaneous and evoked release per 1% muscle stretch. Because many muscle fibers do not receive suprathreshold synaptic inputs at rest length, this stretch-induced enhancement of release constitutes a strong peripheral amplifier of the spinal stretch reflex. The stretch modulation of release is inhibited by peptides that block integrin binding of natural ligands. The modulation varies linearly with length, with a delay of no more than approximately 1-2 msec and is maintained constant at the new length. Moreover, the stretch modulation persists in a zero Ca2+ Ringer and, hence, is not dependent on Ca2+ influx through stretch activated channels. Eliminating transmembrane Ca2+ gradients and buffering intraterminal Ca2+ to approximately normal resting levels does not eliminate the modulation, suggesting that it is not the result of release of Ca2+ from internal stores. Finally, changes in temperature have no detectable effect on the kinetics of stretch-induced changes in endplate potential (EPP) amplitude or miniature EPP (mEPP) frequency. We conclude, therefore, that stretch does not act via second messenger pathways or a chemical modification of molecules involved in the release pathway. Instead, there is direct mechanical modulation of release. We postulate that tension on integrins in the presynaptic membrane is transduced mechanically into changes in the position or conformation of one or more molecules involved in neurotransmitter release, altering sensitivity to Ca2+ or the equilibrium for a critical reaction leading to vesicle fusion.

Actin plays a role in both changes in cell shape and gene-expression associated with Schwann cell myelination

Schwann cell (SC) differentiation into a myelinating cell requires concurrent interactions with basal lamina and an axon destined for myelination. As SCs differentiate, they undergo progressive morphological changes and initiate myelin-specific gene expression. We find that disrupting actin polymerization with cytochalasin D (CD) inhibits myelination of SC/neuron co-cultures. Basal lamina is present, neurons are healthy, and the inhibition is reversible. Electron microscopic analysis reveals that actin plays a role at two stages of SC differentiation. At 0.75-1.0 microg/ml CD, SCs do not differentiate and appear as "rounded" cells in contact with axons. This morphology is consistent with disruption of actin filaments and cell shape changes. However, at 0.25 microg/ml CD, SCs partially differentiate; they elongate and segregate axons but generally fail to form one-to-one relationships and spiral around the axon. In situ hybridizations reveal that SCs in CD-treated cultures do not express mRNAs encoding the myelin-specific proteins 2',3'-cyclic nucleotide phosphodiesterase (CNP), myelin-associated glycoprotein (MAG), and P0. Our results suggest that at the lower CD dose, SCs commence differentiation as evidenced by changes in cell shape but are unable to elaborate myelin lamellae because of a lack of myelin-specific mRNAs. We propose that F-actin influences myelin-specific gene expression in SCs.

Integrin-mediated calcium signaling and regulation of cell adhesion by intracellular calcium

Integrins are ubiquitous trans-membrane adhesion molecules that mediate the interaction of cells with the extracellular matrix (ECM). Integrins link cells to the ECM by interacting with the cell cytoskeleton. In cases such as leukocyte binding, integrins mediate cell-cell interactions and cell-ECM interactions. Recent research indicates that integrins also function as signal transduction receptors, triggering a number of intracellular signaling pathways that regulate cell behavior and development. A number of integrins are known to stimulate changes in intracellular calcium levels, resulting in integrin activation. Although changes in intracellular calcium regulate a vast number of cellular functions, this review will discuss the stimulation of calcium signaling by integrins and the role of intracellular calcium in the regulation of integrin-mediated adhesion.

Engagement of L-selectin impairs the actin polymerizing capacity of beta 2-integrins on neutrophils

A sequential activation of L-selectin and beta 2-integrins on neutrophils is crucial for the rolling, adherence and subsequent migration of these cells on the endothelium. However, little is known about a possible interplay between these adhesion receptors in the final regulation of cell motility. The results presented here show that sulfatides themselves (here used as tools to activate L-selectins), have no major effect on the cellular content of filamentous actin (F-actin), but cause a time-related decrease in the beta 2-integrin-induced formation of F-actin. This effect of sulfatides was abolished in cells lacking L-selectin as a result of pretreatment with chymotrypsin. A similar sulfatide-induced activation of L-selectin also caused a pronounced and time-related decrease of a subsequent chemotactic peptide-induced F-actin response. The effect of sulfatides on both beta 2-integrin- and chemotactic peptide-induced F-actin were abolished if L-selectin were blocked by preincubating the cells with specific antibodies to L-selectin. These effects of L-selectin engagement on cellular F-actin content were neither abolished by blocking the cytosolic free Ca2+ signal with bis-(2-amino-5-methylphenoxy)ethane-N,N,N',N'-tetraaceticacid tetraacetoxymethyly ester (MAPT/AM) nor by blocking a cAMP-induced activation of protein kinase A by pretreating the cells with adenosine-3′,5′-cyclic monophos-phorothioate (Rp-cAMPS). Instead we found that L-selectin engagement impaired an early beta 2-integrin-induced tyrosine kinase activation, an event shown to be necessary for a normal beta 2-integrin-mediated F-actin response. The present demonstration of a negative feed-back function of L-selectin on beta 2-integrin-induced modulations of the actin cytoskeleton, suggests that the relative distribution and/or density of the respective L-selectin and beta 2-integrin ligands on endothelial cells might be important factors in determining the final site of firm adhesion and extravasation of neutrophils.

Mechanisms of integrin-mediated calcium signaling in MDCK cells: regulation of adhesion by IP3- and store-independent calcium influx

Peptides containing Arg-Gly-Asp (RGD) immobilized on beads bind to integrins and trigger biphasic, transient increases in intracellular free Ca2+ ([Ca2+]i) in Madin-Darby canine kidney epithelial cells. The [Ca2+]i increase participates in feedback regulation of integrin-mediated adhesion in these cells. We examined influx pathways and inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ store release as possible sources of the [Ca2+]i rise. The RGD-induced [Ca2+]i response requires external Ca2+ (threshold approximately 150 microM), and its magnitude is proportional to extracellular calcium. RGD-induced transients were attenuated by Ca2+ channel inhibitors (Ni2+ and carboxy-amidotriazole) or by plasma membrane depolarization, indicating that Ca2+ influx contributes to the response. Loading cells with heparin reduced the size of RGD-induced [Ca2+]i transients, indicating that IP3-mediated release of Ca2+ from stores may also contribute to the RGD response. Depletion of Ca2+ stores with thapsigargin activated Ni(2+)-sensitive Ca2+ influx that might also be expected to occur after IP3-mediated depletion of stored Ca2-. However, RGD elicited a Ni(2+)-sensitive Ca2+ influx even after pretreatment with thapsigargin, indicating that Ca2+ influx is controlled by a mechanism independent of IP3-mediated store depletion. We conclude that RGD-induced [Ca2+]i transients in Madin-Darby canine kidney cells result primarily from the combination of two distinct mechanisms: 1) IP3-mediated release of intracellular stores, and 2) activation of a Ca2+ influx pathway regulated independently of IP3 and Ca2+ store release. Because Ni2+ and carboxy-amidotriazole inhibited adhesion, whereas store depletion with thapsigargin had little effect, we suggest that the Ca2+ influx mechanism is most important for feedback regulation of integrin-mediated adhesion by increased [Ca2+]i.

Stimulation of beta1 integrins on fibroblasts induces PDGF independent tyrosine phosphorylation of PDGF beta-receptors

We report that integrin-mediated signaling induces a rapid and transient tyrosine phosphorylation of platelet-derived growth factor (PDGF) beta-receptors in human diploid foreskin AG 1518 fibroblasts. A transient tyrosine phosphorylation of PDGF beta-receptors was evident one and two hours after cells had been plated on collagen type I and fibronectin, as well as on immobilized anti-integrin subunit IgG, but not on poly-L-lysine. In contrast EGF or PDGF alpha-receptors were not phosphorylated on tyrosine residues under these conditions. Tyrosine phosphorylation of PDGF beta-receptors induced by plating on collagen type I was inhibited by cytochalasin D and herbimycin A, unaffected by cycloheximide and enhanced by orthovanadate. Furthermore, a transient phosphorylation of PDGF beta-receptors occurred when AG 518 fibroblasts were cultured in three-dimensional collagen lattices or exposed to external strain exerted through centrifugation. The latter effect was evident already after two minutes. Clustering of cell surface beta1 integrins led to PDGF beta-receptor phosphorylation both in suspended and firmly attached AG 1518 fibroblasts. Plating of cells on collagen type I, fibronectin, and anti-beta1-integrin IgG resulted in the formation of PDGF beta-receptor aggregates as detected by immunofluorescence. Suramin or anti-PDGF-BB IgG had no effect on the plating-induced tyrosine phosphorylation of PDGF beta-receptors. PDGF-B chain mRNA, or protein, were not detected in AG 1518 fibroblasts. Our data suggest that a ligand-independent PDGF beta-receptor activation during cell adhesion and early phases of cell spreading is involved in integrin-mediated signaling in fibroblasts, and constitutes parts of a mechanism for cells to respond during the dynamic phases of externally applied tension as well as fibroblast-mediated tension during cell adhesion and collagen gel contraction.

Integrins and modulation of transmitter release from motor nerve terminals by stretch

The stretch of a frog muscle within the physiological range can more than double the spontaneous and evoked release of neurotransmitter from its motor nerve terminals. Here, stretch enhancement of release was suppressed by peptides containing the sequence arginine-glycine-aspartic acid (RGD), which blocks integrin binding. Integrin antibodies also inhibited the enhancement obtained by stretching. Stretch enhancement depended on intraterminal calcium derived both from external calcium and from internal stores. Muscle stretch thus might enhance the release of neurotransmitters either by elevating internal calcium concentrations or by increasing the sensitivity of transmitter release to calcium in the nerve terminal.

Protein tyrosine phosphorylation in human platelets induced by interaction between glycoprotein Ib and von Willebrand factor

The interaction between von Willebrand factor (vWF) and glycoprotein Ib (GPIb) induced by ristocetin or botrocetin resulted in associated platelet aggregation, protein tyrosine phosphorylation (PTP) of a 64 kDa protein, as detected by a monoclonal antibody against phosphotyrosine (PY-20), and intracellular Ca2+ elevation that is largely dependent upon Ca2+ influx in human platelets. It is of interest that 75-80, 97 and 125 kDa proteins which are strongly tyrosine-phosphorylated in platelet activation induced by thrombin and other agonists were not detected. Neither vWF nor a coaggregating agent (ristocetin or botrocetin) alone induced aggregation, [Ca2+]i elevation or the 64 kDa PTP. NMC-4, an antibody which inhibits both ristocetin- or botrocetin-induced vWF binding to GPIb, abolished the appearance of the 64 kDa PTP as well as other responses, suggesting that it is specifically induced by the GPIb-vWF interaction. Aspirin, or ONO-3708, a competitive inhibitor of thromboxane A2, did not modify the 64 kDa PTP, while [Ca2+]i elevation was moderately suppressed. Depletion of extracellular Ca2+ or RGD peptides suppressed neither the 64 kDa PTP nor aggregation. H-7, a protein kinase C inhibitor, did not inhibit the 64 kDa PTP, while staurosporine, a potent protein kinase inhibitor, inhibited the 64 kDa PTP and Ca2+ influx, but not aggregation, in a dose-dependent manner. It is suggested that the 64 kDa PTP is associated with platelet aggregation induced by the interaction between GPIb and vWF.

Identification of a large complex containing the integrin alpha 6 beta 1 laminin receptor in neural retinal cells

Integrin alpha 6 beta 1 is a laminin receptor involved in adhesion and neurite extension of retinal neurons on laminin. The present study was carried out to identify potential interactions between the alpha 6 beta 1 receptor and cellular proteins that may be involved in integrin signaling and function. For this purpose we have used a biochemical approach based on the solubilization of retinal neurons cultured on laminin with nonionic detergents, followed by centrifugation on sucrose velocity gradients. Analysis of the distribution of the alpha 6 and beta 1 integrin subunits in the gradients showed that they migrate as a large complex after extraction of cells with octylglucoside, but not after Triton X-100 extraction. Cytoskeletal proteins known to localize in adhesion plaques did not comigrate with alpha 6 beta 1 in octylglucoside gradients, while a set of polypeptides whose tyrosine phosphorylation was enhanced by culture on laminin colocalized with alpha 6 beta 1 on the gradients after octylglucoside solubilization. Culture of retinal neurons on bovine serum albumin, a nonadhesive substratum, partially affected the gradient distribution of the receptor after octylglucoside extraction. Furthermore, analysis of the gradient distribution of two alternatively spliced isoforms of the alpha 6 subunit, alpha 6-cytoA and alpha 6-cytoB, showed that the effect of non-adhesion on the sedimentation properties of the two integrin alpha 6 isoforms was more dramatic for alpha 6-cytoB than alpha 6-cytoA. These differences in the sedimentation behaviour indicate distinct biochemical properties of the two alpha 6 isoforms that, together with previous observations on their differential distribution in the developing retina, may reflect functional specificities.

Receptors for ADP on human blood platelets

It is well established that ADP causes aggregation of human blood platelets, and indeed it was the first aggregating agent to be studied, but the ways in which platelets respond to ADP are still relatively obscure. Although it is apparent that increases in intracellular Ca2+ concentrations are of major importance in activating platelets, it is not clearly understood how ADP causes these increases and what other signal transduction mechanisms it uses. It is not even clear whether ADP causes its effects by interacting with only one receptor, or whether multiple receptors for ADP exist on platelets. In this review, Susanna Hourani and David Hall examine some of the conflicting evidence in this field, and draw some tentative conclusions about the number and nature of receptors for ADP on human platelets.

Cytological and immunocytochemical approaches to the study of corneal endothelial wound repair

The vertebrate corneal endothelium represents a unique model system for investigating many cellular aspects of wound repair within an organized tissue in situ. The tissue exists as a cell monolayer that resides upon its own natural basement membrane that can be prepared as a flat mount to observe the entire cell population. Thus, it readily avails itself to many cytological and immunocytochemical methods at both the light microscopic and ultrastructural levels. In addition, the tissue is easily explanted into organ culture where further investigations can be carried out. These techniques have enabled investigators to use many approaches to explore function and changes in response to injury. In vivo, the endothelium acts as a transport tissue to actively pump Na+ and bicarbonate ions from the corneal stroma into the aqueous humor to control corneal transparency. Physiological findings indicate that fluid diffuses back into the stroma, across the endothelium, and thus hydration is said to be controlled by a pump-leak mechanism. Ultrastructural investigations, some employing horseradish peroxidase and lanthanum, have established the morphological basis for this mechanism as apical focal junctions that are not the classical tight junctions and do not constitute a complete zona occludens. Along with these apical focal junctions are gap junctions that appear identical to their counterparts in other cell types. Cytochemical studies localized both Na+K(+)-ATPase and carbonic anhydrase, the main pump enzymes associated with corneal hydration, to the lateral plasma membranes. Corneal endothelial cells of noninjured tissue do not traverse the cell cycle and are considered to be in the "Go" phase of the cell cycle as determined by microfluorometric analysis with DNA binding dyes such as auramin O and pararosaniline-Feulgen. However, injury can initiate cell cycle transverse and histochemical and cytological methods have been used to understand the tissue's response. Classical histochemical studies revealed that increased staining was observed for metabolic (NADase and NADPase) and lysosomal enzymes in cells bordering the wound area. The use of radiolabelled agents has further lead to an understanding of the endothelial wound response. Autoradiographic analyses of 3H-actinomycin D incorporation indicated that injury initiates changes in chromatin leading to increased binding levels of the drug in cells surrounding the wound. This change suggests that those cells undergo heightened macromolecular synthesis and this was confirmed by examining 3H-uridine and 3H-thymidine incorporation. The major mechanism involved in corneal endothelial repair is cell migration. Cytochemical and immunocytochemical investigations have allowed investigators an opportunity to gain some insight into changes that occur during this cellular process.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of extracellular matrix components on cell locomotion

The extracellular matrix (ecm), which is composed of collagens, glycoproteins, and proteoglycans, has emerged as an important regulator of cell locomotion. This review describes some of the mechanisms by which the ecm may regulate locomotion, focusing primarily on cell extension and lamellae formation. Ecm-receptor interactions form an important part of cell recognition of ecm. Such interactions can result in altered cell adhesion, signal transduction, and cytoskeletal organization, all of which impact on cell locomotion. It is important to note that although the effects of single ecm components have been studied, generally, the cell is likely to perceive ecm in vivo as a macromolecular complex. It will fall to future work to define how complexes of ecm regulate cell behavior. Because of our own particular research bias, we focus on reviewing the role of fibronectin, integrins, chondroitin sulfate, hyaluronan, and hyaluronan receptors in the regulation of cell locomotion and examine their effect on adhesion, signal transduction, and cytoskeletal integrity. Cytoskeleton assembly mechanisms, particularly those that might be regulated by the ecm, are also described. These events are summarized in a working model of ecm-promoted locomotion.

ADP-stimulated fibrinogen binding is necessary for some of the inositol phospholipid changes found in ADP-stimulated platelets

ADP-stimulation of washed human platelets suspended in Tyrode/albumin solution containing Ca2+ (2 mM) and fibrinogen (0.4 mg/ml) causes extensive, reversible aggregation without appreciable secretion of granule contents. Under these conditions ADP (10 microM) stimulation decreased the amounts of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) and phosphatidylinositol 4-phosphate (PtdInsP) at 10 s. Omitting fibrinogen from the suspending medium or blocking fibrinogen binding to the platelets using Arg-Gly-Asp-Ser (RGDS, 0.23 mM) inhibited these decreases in PtdInsP2 and PtdInsP. In contrast, ADP-induced decreases in PtdInsP2 and increases in PtdInsP at 60 s compared to 10 s were not affected by RGDS or the absence of fibrinogen. In platelets prelabelled with [3H]glycerol and [32P]phosphate, changes in labelling of the inositol phospholipids paralleled the changes in amount. The ADP-induced changes in phosphatidic acid (PtdOH) at 10 s were unaffected by RGDS; this finding supported previous reports that phospholipase C was not the cause of the early decreases in PtdInsP2 and PtdInsP. These results indicate that the early decreases in PtdInsP2 and PtdInsP at 10 s are dependent on fibrinogen binding to the platelets and occur after fibrinogen binding which is activated by ADP stimulation. It is proposed that the fibrinogen-dependent changes in PtdInsP2 and PtdInsP may have a feedback role augmenting platelet aggregation or other responses of platelets that might occur after fibrinogen binding, possibly due to effects on actin polymerisation.

Evidence that activation of platelet calpain is induced as a consequence of binding of adhesive ligand to the integrin, glycoprotein IIb-IIIa

Calpain (a Ca(2+)-dependent protease) is present in many cell types. Because it is present in the cytosol, the potential exists that it may regulate critical intracellular events by inducing crucial proteolytic cleavages. However, the concentrations of Ca2+ required to activate calpain are higher than those attained in the cytoplasm of most cells. Thus, the physiological importance of calpain and the mechanisms involved in its activation have remained elusive. In this study, we show that calpain rapidly moved to a peripheral location upon the addition of an agonist to suspensions of platelets, but it remained unactivated. We provide three lines of evidence that calpain was subsequently activated by a mechanism that required the binding of an adhesive ligand to the major platelet integrin, glycoprotein (GP) IIb-IIIa: calpain activation was prevented by RGDS, a tetrapeptide that inhibits the binding of adhesive ligand to GP IIb-IIIa; it was also prevented by monoclonal antibodies that inhibit adhesive ligand binding to GP IIb-IIIa; and its activation was markedly reduced in platelets from patients whose platelets have greatly reduced levels of functional GP IIb-IIIa. Thus, in platelets, binding of the extracellular domain of GP IIb-IIIa to its adhesive ligand can initiate a transmembrane signal that activates intracellular calpain. Because calpain is present in focal contacts of adherent cells, the interaction of integrins with adhesive ligands in the extracellular matrix may regulate activation of calpain in other cell types as well.