Regulation of a neuronal form of focal adhesion kinase by anandamide

FAK+ and PYK2/CAKbeta, two related tyrosine kinases highly expressed in the central nervous system: similarities and differences in the expression pattern

Focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2/cell adhesion kinase beta (PYK2/CAKbeta) are related, non-receptor, cytoplasmic tyrosine kinases, highly expressed in the central nervous system (CNS). In addition, FAK+ is a splice isoform of FAK containing a 3-amino acid insertion in the carboxy-terminal region. In rat hippocampal slices, FAK+ and PYK2/CAKbeta are differentially regulated by neurotransmitters and depolarization. We have studied the regional and cellular distribution of these kinases in adult rat brain and during development. Whereas PYK2/CAKbeta expression increased with postnatal age and was maximal in the adult, FAK+ levels were stable. PYK2/CAKbeta mRNAs, detected by in situ hybridization, were expressed at low levels in the embryonic brain, and became very abundant in the adult forebrain. Immunocytochemistry of the adult brain showed a widespread neuronal distribution of FAK+ and PYK2/CAKbeta immunoreactivities (ir). PYK2/CAKbeta appeared to be particularly abundant in the hippocampus. In hippocampal neurons in culture at early stages of development, FAK+ and PYK2/CAKbeta were enriched in the perikarya and growth cones. FAK+ extended to the periphery of the growth cones tips, whereas PYK2/CAKbeta appeared to be excluded from the lamellipodia. During the establishment of polarity, a proximal-distal gradient of increasing PYK2/CAKbeta-ir could be observed in the growing axon. In most older neurons, FAK+-ir was confined to the cell bodies, whereas PYK2/CAKbeta-ir was also present in the processes. In vitro and in vivo, a subpopulation of neurons displayed neurites with intense FAK+-ir. Thus, FAK+ and PYK2/CAKbeta are differentially regulated during development yet they are both abundantly expressed in the adult brain, with distinctive but overlapping distributions.

Signaling through focal adhesion kinase

Integrin receptor binding to extracellular matrix proteins generates intracellular signals via enhanced tyrosine phosphorylation events that are important for cell growth, survival, and migration. This review will focus on the functions of the focal adhesion kinase (FAK) protein-tyrosine kinase (PTK) and its role in linking integrin receptors to intracellular signaling pathways. FAK associates with several different signaling proteins such as Src-family PTKs, p130Cas, Shc, Grb2, PI 3-kinase, and paxillin. This enables FAK to function within a network of integrin-stimulated signaling pathways leading to the activation of targets such as the ERK and JNK/mitogen-activated protein kinase pathways. Focus will be placed on the structural domains and sites of FAK tyrosine phosphorylation important for FAK-mediated signaling events and how these sites are conserved in the FAK-related PTK, Pyk2. We will review what is known about FAK activation by integrin receptor-mediated events and also non-integrin stimuli. In addition, we discuss the emergence of a consensus FAK substrate phosphorylation sequence. Emphasis will also be placed on the role of FAK in generating cell survival signals and the cleavage of FAK during caspase-mediated apoptosis. An in-depth discussion will be presented of integrin-stimulated signaling events occurring in the FAK knockout fibroblasts (FAK-) and how these cells exhibit deficits in cell migration. FAK re-expression in the FAK- cells confirms the role of this PTK in the regulation of cell morphology and in promoting cell migration events. In addition, these results reinforce the potential role for FAK in promoting an invasive phenotype in human tumors.

FAK and PYK2/CAKbeta in the nervous system: a link between neuronal activity, plasticity and survival?

A major aim of neurobiology today is to improve understanding of the signaling pathways that couple rapid events, such as the action potential and neurotransmitter release, to long-lasting changes in synaptic strength and increased neuronal survival. These adaptations involve interactions of neurons with other cells and with the extracellular matrix. They use, in part, the same molecular machinery that controls adhesion, motility or survival in non-neuronal cells. This machinery includes two homologous non-receptor tyrosine kinases, FAK and PYK2/CAKbeta, and the associated SRC-family tyrosine kinases. Specific brain isoforms of FAK with distinct properties are regulated by neurotransmitters, whereas PYK2/CAKbeta is highly sensitive to depolarization. The multiplicity of the pathways that can be activated by these tyrosine kinases indicates their importance in signal transduction in the adult brain.

Cannabinoids and neuroprotection in global and focal cerebral ischemia and in neuronal cultures

Marijuana and related drugs (cannabinoids) have been proposed as treatments for a widening spectrum of medical disorders. R(+)-[2, 3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1, 4-benzoxazin-yl]-(1-naphthalenyl)methanone mesylate (R(+)-WIN 55212-2), a synthetic cannabinoid agonist, decreased hippocampal neuronal loss after transient global cerebral ischemia and reduced infarct volume after permanent focal cerebral ischemia induced by middle cerebral artery occlusion in rats. The less active enantiomer S(-)-WIN 55212-3 was ineffective, and the protective effect of R(+)-WIN 55212-2 was blocked by the specific central cannabinoid (CB1) cannabinoid receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide-hydrochloride. R(+)-WIN 55212-2 also protected cultured cerebral cortical neurons from in vitro hypoxia and glucose deprivation, but in contrast to the receptor-mediated neuroprotection observed in vivo, this in vitro effect was not stereoselective and was insensitive to CB1 and CB2 receptor antagonists. Cannabinoids may have therapeutic potential in disorders resulting from cerebral ischemia, including stroke, and may protect neurons from injury through a variety of mechanisms.

Overexpression of striatal enriched phosphatase (STEP) promotes the neurite outgrowth induced by a cAMP analogue in PC12 cells

A cytoplasmic protein tyrosine phosphatase (PTPase) designated as striatal enriched phosphatase with a molecular weight of 46 kDa (STEP46) is highly expressed in striatal neurons with dopamine D1-receptors. To examine the hypothesis that STEP46 is involved in the neuronal functions modulated by the cyclic adenosine 3', 5'-monophosphate (cAMP)-signaling system, we introduced the complementary DNA of STEP46 into the pheochromocytoma cell line PC12, which exhibits neuronal differentiation characterized by neurite outgrowth in response to cAMP and nerve growth factor stimulation, and we established subclonal cell lines that constitutively overexpress STEP46 protein with PTPase activity. The subclones expressing STEP46 showed increased neurite outgrowth during differentiation induced by a cAMP analogue (dibutyryl cAMP). The positive regulatory role of STEP46 in the cAMP-induced neuronal differentiation of PC12 cells indicates that STEP46 may play a role in neuronal processes modulated by the cAMP-signaling cascade as a PTPase.

Association of the dystroglycan complex isolated from bovine brain synaptosomes with proteins involved in signal transduction

Dystroglycan is a transmembrane heterodimeric complex of alpha and beta subunits that links the extracellular matrix to the cell cytoskeleton. It was originally identified in skeletal muscle, where it anchors dystrophin to the sarcolemma. Dystroglycan is also highly expressed in nonmuscle tissues, including brain. To investigate the molecular interactions of dystroglycan in the CNS, we fractionated a digitonin-soluble extract from bovine brain synaptosomes by laminin-affinity chromatography and characterized the protein components. The 120-kDa alpha-dystroglycan was the major 125I-laminin-labeled protein detected by overlay assay. This complex, in addition to beta-dystroglycan, was also found to contain Grb2 and focal adhesion kinase p125FAK (FAK). Anti-FAK antibodies co-immunoprecipitated Grb2 with FAK. However, no direct interaction between beta-dystroglycan and FAK was detected by co-precipitation assay. Grb2, an adaptor protein involved in signal transduction and cytoskeleton organization, has been shown to bind beta-dystroglycan. We isolated both FAK and Grb2 from synaptosomal extracts by chromatography on immobilized recombinant beta-dystroglycan. In the CNS, FAK phosphorylation has been linked to membrane depolarization and neurotransmitter receptor activation. At the synapses, the adaptor protein Grb2 may mediate FAK-beta-dystroglycan interaction, and it may play a role in transferring information between the dystroglycan complex and other signaling pathways.

Stage-specific excitation of cannabinoid receptor exhibits differential effects on mouse embryonic development

Anandamide (N-arachidonoylethanolamine), an arachidonic acid derivative, is an endogenous ligand for both the brain-type (CB1-R) and spleen-type (CB2-R) cannabinoid receptors. We have previously demonstrated that preimplantation mouse embryos express mRNA for these receptors and that the periimplantation uterus contains the highest level of anandamide yet discovered in a mammalian tissue. We further demonstrated that 2-cell mouse embryos exposed to low levels of anandamide (7 nM) or other known cannabinoid agonists in culture exhibit markedly compromised embryonic development to blastocysts and that this effect is mediated by CB1-R. In contrast, the present study demonstrates that blastocysts exposed in culture to the same low levels of cannabinoid agonists exhibited accelerated trophoblast differentiation with respect to fibronectin-binding activity and trophoblast outgrowth. Again, these effects resulted from activation of embryonic CB1-R. There was a differential concentration-dependent effect of cannabinoids on the trophoblast, with an observed inhibition of differentiation at higher doses. These results provide evidence for the first time that cannabinoid effects are differentially executed depending on the embryonic stage and cannabinoid levels in the environment. Since uterine anandamide levels are lowest at the sites of implantation and highest at the interimplantation sites, the new findings imply that site-specific levels of anandamide and/or other endogenous ligands in the uterus may regulate implantation spatially by promoting trophoblast differentiation at the sites of blastocyst implantation.

A sensitive and specific radiochromatographic assay of fatty acid amide hydrolase activity

A radiochromatographic method has been set up in order to determine fatty acid amide hydrolase (FAAH) activity, based on reversed-phase high-performance liquid chromatography and on-line scintillation counting. The reaction products were separated using a C18 column eluted with methanol-water-acetic acid and quantitated with an external standard. Baseline separation of the acid product from the substrate was completed in less than 4 min, with a detection limit of 2.5 fmol arachidonic acid at a signal to noise ratio of 4:1. The method enabled to determine the kinetic constants (i.e., apparent Km of 2.0 +/- 0.2 microM and Vmax of 800 +/- 75 pmol. min-1. mg protein-1 toward anandamide) and the substrate specificity of human brain FAAH, as well as the extent of enzyme inhibition by some anandamide congeners. The femtomole sensitivity and the accuracy of the method allow detection and characterization of the activity of FAAH in very minute tissue samples or in samples where the enzymatic activity is very low.

The benzoquinone ansamycin geldanamycin stimulates proteolytic degradation of focal adhesion kinase

FAK is a nonreceptor tyrosine kinase involved in adhesion-mediated signal transduction whose level of expression is related to the invasiveness of malignant tumors. In seeking strategies to downregulate FAK, we treated various cell lines in vitro with the benzoquinone ansamycin geldanamycin (GA) which was previously described as a tyrosine kinase inhibitor, but recently has been shown to exert its effects by interfering with the chaperone function of members of the hsp90 family of heat-shock proteins. We evaluated the effects of benzoquinone ansamycins on FAK steady-state protein level and FAK half-life in breast and prostate carcinoma, Ewing's sarcoma, and 3T3 fibroblasts. Our data demonstrate that GA stimulates the proteolytic degradation of FAK in all cell lines examined and markedly reduces the half-life of newly synthesized FAK protein without significantly altering the level of FAK mRNA. These data demonstrate FAK to be another tyrosine kinase sensitive to the destabilizing effects of benzoquinone ansamycins and further show that small molecule-mediated pharmacologic modulation of FAK protein level is a feasible approach to the interdiction of FAK function.

The peripheral cannabinoid receptor, Cb2, in retrovirally-induced leukemic transformation and normal hematopoiesis

Following retroviral insertional mutagenesis we recently identified the gene encoding the peripheral cannabinoid receptor (Cb2) near a common virus integration site (VIS), Evi11. In 13 out of 105 Cas-Br-M murine leukemia virus (MuLV) induced leukemias retroviral integrations occured either in the 5' or 3' part of the Cb2 gene. The Cb2 receptor protein is 44% homologous to the central cannabinoid receptor Cb1, which belongs to the superfamily of seven transmembrane (7TM) receptors. Cb1 is mainly expressed in brain, whereas Cb2 encodes the hematopoietic form. Besides the natural cannabinoids, delta9-tetrahydrocannabinol (delta9-THC) and cannabinol, and the many synthetic agonists that have been generated, e.g CP55,940 or WIN55,212-2, several endogenous ligands have recently been identified. These include the arachidonic acid derivatives anandamide and 2-arachidonylglycerol as well as the fatty acid palmitoylethanolamide. Although in the past many studies described growth inhibitory effects of cannabinoid agonists on the in vitro proliferation of hematopoietic cells, recent studies demonstrated that activation of Cb2 may have growth stimulatory effects on blood precursor cells. We demonstrated that many murine hematopoietic growth factor (HGF) dependent cell lines also require the presence of anandamide for optimal growth in serum free culture. Thus, the Cb2 receptor may be an important regulator of normal hematopoietic growth and development. These results strengthen our finding that Cb2 is a proto-oncogene and may implicate a growth advantage for leukemia cells that aberrantly express Cb2. Here we briefly review the mechanisms and application of retroviral insertional mutagenesis in leukemic transformation in mice and discuss the role of the peripheral cannabinoid receptor in leukemia development and normal hematopoiesis.

Endocannabinoids: endogenous cannabinoid receptor ligands with neuromodulatory action

The existence of an endogenous cannabinoid system was demonstrated conclusively with the discovery of endogenous brain constituents capable of activating the cannabinoid receptors functionally. These compounds are synthesized by neuronal cells and inactivated through re-uptake and enzymatic hydrolysis by both neurons and astrocytes. In analogy with the endorphins they can be referred to as endocannabinoids. Apart from the identification of their metabolic pathways, research carried out in the past six years has focused on the possible cellular and molecular targets for the actions of endocannabinoids. These studies have confirmed a similarity between the endocannabinoids and the psychoactive substance in marijuana, delta9(-)-tetrahydrocannabinol, and have suggested a role for endocannabinoids in the modulation of neurotransmitter action and release.

Anandamide hydrolysis by human cells in culture and brain

Anandamide (arachidonylethanolamide; AnNH) has important neuromodulatory and immunomodulatory activities. This lipid is rapidly taken up and hydrolyzed to arachidonate and ethanolamine in many organisms. As yet, AnNH inactivation has not been studied in humans. Here, a human brain fatty-acid amide hydrolase (FAAH) has been characterized as a single protein of 67 kDa with a pI of 7.6, showing apparent Km and Vmax values for AnNH of 2.0 +/- 0.2 microM and 800 +/- 75 pmol.min-1.mg of protein-1, respectively. The optimum pH and temperature for AnNH hydrolysis were 9.0 and 37 degreesC, respectively, and the activation energy of the reaction was 43.5 +/- 4.5 kJ.mol-1. Hydro(pero)xides derived from AnNH or its linoleoyl analogues by lipoxygenase action were competitive inhibitors of human brain FAAH, with apparent Ki values in the low micromolar range. One of these compounds, linoleoylethanolamide is the first natural inhibitor (Ki = 9.0 +/- 0.9 microM) of FAAH as yet discovered. An FAAH activity sharing several biochemical properties with the human brain enzyme was demonstrated in human neuroblastoma CHP100 and lymphoma U937 cells. Both cell lines have a high affinity transporter for AnNH, which had apparent Km and Vmax values for AnNH of 0.20 +/- 0.02 microM and 30 +/- 3 pmol.min-1.mg of protein-1 (CHP100 cells) and 0.13 +/- 0.01 microM and 140 +/- 15 pmol.min-1.mg of protein-1 (U937 cells), respectively. The AnNH carrier of both cell lines was activated up to 170% of the control by nitric oxide.

Expression and characterization of splice variants of PYK2, a focal adhesion kinase-related protein

Focal adhesion kinase and the recently identified proline-rich tyrosine kinase 2 (PYK2), also known as cell adhesion kinase β, related adhesion focal tyrosine kinase or calcium-dependent protein tyrosine kinase, define a new family of non-receptor protein tyrosine kinases. Activation of PYK2 has been implicated in multiple signaling events, including modulation of ion channels, T- and B-cell receptor signaling and cell death. Mechanisms underlying the functional diversity of PYK2 are unclear. Here, we provide evidence for two novel alternatively expressed isoforms of PYK2. One isoform, designated PYK2s (PYK2 splice form), appears to be a splice variant of PYK2 lacking 42 amino acids within the C-terminal domain. A second isoform, referred to as PRNK (PYK2-related non-kinase), appears to be specified by mRNAs that encode only part of the C-terminal domain of PYK2. Northern blot analysis indicates that the unspliced PYK2 is expressed at high levels in the brain and poorly expressed in the spleen, whereas PYK2s and PRNK are expressed in the spleen. In situ hybridization studies of rat brain demonstrate that the unspliced PYK2 is selectively expressed at high levels in hippocampus, cerebral cortex and olfactory bulb, whereas PYK2s and PRNK are expressed at low levels in all regions of rat brain examined. Immunofluorescence analysis of ectopically expressed PRNK protein shows that PRNK, in contrast to full-length PYK2, is localized to focal adhesions by sequences within the focal adhesion targeting domain. In addition, PYK2, but not PRNK, interacts with p130(cas)and Graf. These results imply that PRNK may selectively regulate PYK2 function in certain cells by binding to some but not all PYK2 binding partners, and the functional diversity mediated by PYK2 may be due in part to complex alternative splicing.

Dual actions of sphingosine-1-phosphate: extracellular through the Gi-coupled receptor Edg-1 and intracellular to regulate proliferation and survival

Sphingosine-1-phosphate (SPP), a bioactive lipid, acts both intracellularly and extracellularly to cause pleiotropic biological responses. Recently, we identified SPP as a ligand for the G protein-coupled receptor Edg-1 (Lee, M.-J., J.R. Van Brocklyn, S. Thangada, C.H. Liu, A.R. Hand, R. Menzeleev, S. Spiegel, and T. Hla. 1998. Science. 279:1552-1555). Edg-1 binds SPP with remarkable specificity as only sphinganine-1-phosphate displaced radiolabeled SPP, while other sphingolipids did not. Binding of SPP to Edg-1 resulted in inhibition of forskolin-stimulated cAMP accumulation, in a pertussis toxin-sensitive manner. In contrast, two well-characterized biological responses of SPP, mitogenesis and prevention of apoptosis, were clearly unrelated to binding to Edg-1 and correlated with intracellular uptake. SPP also stimulated signal transduction pathways, including calcium mobilization, activation of phospholipase D, and tyrosine phosphorylation of p125(FAK), independently of edg-1 expression. Moreover, DNA synthesis in Swiss 3T3 fibroblasts was significantly and specifically increased by microinjection of SPP. Finally, SPP suppresses apoptosis of HL-60 and pheochromocytoma PC12 cells, which do not have specific SPP binding or expression of Edg-1 mRNA. Conversely, sphinganine-1-phosphate, which binds to and signals via Edg-1, does not have any significant cytoprotective effect. Thus, SPP is a prototype for a novel class of lipid mediators that act both extracellularly as ligands for cell surface receptors and intracellularly as second messengers.

'Endocannabinoids' and other fatty acid derivatives with cannabimimetic properties: biochemistry and possible physiopathological relevance

The only endogenous substances isolated and characterised so far that are capable of mimicking the pharmacological actions of the active principle of marijuana, (-)-Delta9-tetrahydrocannabinol, are amides and esters of fatty acids. Some of these compounds, like anandamide (N-arachidonoylethanolamine) and 2-arachidonoylglycerol, act as true 'endogenous cannabinoids' by binding and functionally activating one or both cannabinoid receptor subtypes present on nervous and peripheral cell membranes. The metabolic pathways and molecular mode of actions of these metabolites, as well as their possible implication in physiopathological responses, are reviewed here.

Identification of a new Pyk2 isoform implicated in chemokine and antigen receptor signaling

Pyk2 is a protein tyrosine kinase that links G-protein-coupled receptors, inflammatory cytokines, and extracellular stimuli that elevate intracellular calcium concentration with activation of the mitogen-activated protein kinase pathways and regulation of ion channel functions. Here we describe the identification, cloning, and characterization of a new isoform of Pyk2 (Pyk2-H) that is generated by alternative RNA splicing. Pyk2-H is mainly expressed in hematopoietic cells including T-cells, B-cells, and natural killer cells. Engagement of T-cell or B-cell antigen receptors leads to rapid tyrosine phosphorylation of Pyk2-H. Pyk2-H is also activated in response to the chemokines RANTES and macrophage inflammatory protein-1beta in T cells. In addition, we show that glutathione S-transferase fusion proteins containing the carboxyl termini of Pyk2 and Pyk2-H bind to a different set of tyrosine-phosphorylated proteins in thymus lysates. Specific expression of Pyk2-H and its activation by antigens or chemokines in hematopoietic cells may contribute toward the generation of cell type-specific signals involved in host immune responses.

Differential regulation of FAK+ and PYK2/Cakbeta, two related tyrosine kinases, in rat hippocampal slices: effects of LPA, carbachol, depolarization and hyperosmolarity

FAK+, an isoform of focal adhesion kinase preferentially expressed in brain and PYK2/Cakbeta (proline-rich tyrosine kinase 2/cell adhesion kinasebeta) are two related cytoplasmic tyrosine kinases. They are candidates for coupling electrical activity and stimulation of neurotransmitter receptors to short and long-term changes in synaptic properties, cytoskeletal organization and gene expression in neurons. As the same set of stimuli appear capable of stimulating FAK and/or PYK2 in non-neuronal cells and in cell lines with neuronal characteristics, we investigated the selectivity of regulation of these two kinases in mature nervous tissue. Using rat hippocampal slices, we compared the regulation of FAK+ and PYK2 by stimuli known to be active on one or the other of these two kinases in other cell types: lysophosphatidic acid (LPA), carbachol, depolarization, and hyperosmolarity. Phosphorylation of FAK+ was markedly increased by carbachol and LPA. Carbachol effects occurred via activation of M1 muscarinic receptors and nicotinic receptors. The effects of carbachol and LPA were prevented by protein kinase C inhibitors, whereas 8-Br-cAMP attenuated the effects of carbachol but not of LPA. Tyrosine phosphorylation of PYK2 but not of FAK+ was very strongly enhanced by depolarization and hyperosmolarity. This study and our previous results show that FAK+ and PYK2 are regulated differentially in hippocampal slices: FAK+ is phosphorylated on tyrosine in response to stimulation of G protein-coupled receptors, whereas PYK2 is mainly sensitive to depolarization and hyperosmolarity. Thus, FAK+ and PYK2 may provide specific and separate links between activation of neurotransmitters receptors, depolarization and tyrosine phosphorylation in mature hippocampus.

Immunohistochemical distribution of cannabinoid CB1 receptors in the rat central nervous system

Immunohistochemical distribution of cannabinoid receptors in the adult rat brain was studied using specific purified antibodies against the amino-terminus of the CB1 receptor. Our results generally agree well with the previous studies using CB1 receptor autoradiography and messenger RNA in situ hybridization. However, because of its greater resolution, immunohistochemistry allowed identification of particular neuronal cells and fibers that possess cannabinoid receptors. CB1-like immunoreactivity was found in axons, cell bodies and dendrites, where it appeared as puncta in somata and processes. Both intensely and moderately or lightly stained neurons were observed. The intensely stained neurons were dispersed and only occur in cortical structures including hippocampal formation and olfactory bulb. Moderately or lightly stained neurons were found in caudate-putamen and amygdala. In the hippocampal formation only intensely stained neurons were observed. The cell bodies of pyramidal neurons in CA1 and CA3 fields appeared to be unstained but surrounded by a dense plexus of immunoreactive fibers. The granule cells in the dentate area were also immunonegative. Many intensely stained neurons were located at the base of the granule cell layer. CB1-like immunoreactive neurons and fibers were also found in the somatosensory, cingulate, perirhinal, entorhinal and piriform cortices, in claustrum, amygdaloid nuclei, nucleus accumbens and septum. Beaded immunoreactive fibers were detected in periaqueductal gray, nucleus tractus solitarius, spinal trigeminal tract and nucleus, dorsal horn and lamina X of the spinal cord. A triangular cap-like mass of immunoreactivity was found to surround the basal part of the Purkinje cell body in the cerebellum. Only small, lightly stained cells were found in the molecular layer in the cerebellum close to the Purkinje cell layer. The CB1 receptor is widely distributed in the forebrain and has a more restricted distribution in the hindbrain and the spinal cord. It appears to be expressed on cell bodies, dendrites and axons. According to the location and morphology, many, but not all, CB1-like immunoreactive neurons appear to be GABAergic. Therefore, cannabinoids and cannabinoid receptors may play a role in modulating GABAergic neurons.

Alternatively spliced focal adhesion kinase in rat brain with increased autophosphorylation activity

pp125 focal adhesion kinase (FAK), a cytoplasmic tyrosine kinase transducing signals initiated by integrin engagement and G protein-coupled receptors, is highly expressed in brain. FAK from brain had a higher molecular weight and an increased autophosphorylation activity, than from other tissues. In addition to a 9-base insertion in the 3'-coding region, which defines FAK+, rat striatal FAK mRNAs contained several additional short exons, coding for peptides of 28, 6, and 7 residues, respectively (termed boxes 28, 6, and 7), surrounding the autophosphorylated Tyr-397. In transfected COS 7 cells, the presence of boxes 6 and 7 conferred an increased overall tyrosine phosphorylation, a higher phosphorylation of Tyr-397 assessed with a phosphorylation state-specific antibody, and a more active autophosphorylation in immune precipitates. The presence of box 28 did not alter further these parameters. Two-dimensional phosphopeptide maps of hippocampal FAK were identical to those of FAK+6,7. The presence of the various exons did not alter the interaction of FAK with c-Src, n-Src, or Fyn. Thus, several splice isoforms of FAK are preferentially expressed in rat brain, some of which have an increased autophosphorylation activity, suggesting that FAK may have specific properties in neurons.

Paranodin, a glycoprotein of neuronal paranodal membranes

Ranvier nodes are flanked by paranodal regions, at the level of which oligodendrocytes or Schwann cells interact closely with axons. Paranodes play a critical role in the physiological properties of myelinated nerve fibers. Paranodin, a prominent 180 kDa transmembrane neuronal glycoprotein, was purified and cloned from adult rat brain, and found to be highly concentrated in axonal membranes at their junction with myelinating glial cells, in paranodes of central and peripheral nerve fibers. The large extracellular domain of paranodin is related to neurexins, and its short intracellular tail binds protein 4.1, a cytoskeleton-anchoring protein. Paranodin may be a critical component of the macromolecular complex involved in the tight interactions between axons and myelinating glial cells characteristic of the paranodal region.

Molecular characterization of human and mouse fatty acid amide hydrolases

Recently, we reported the isolation, cloning, and expression of a rat enzyme, fatty acid amide hydrolase (FAAH), that degrades bioactive fatty acid amides like oleamide and anandamide to their corresponding acids, thereby serving to terminate the signaling functions of these molecules. Here, we report the molecular characterization of both a mouse and a human FAAH and compare these enzymes to the rat FAAH. The enzymes are well conserved in primary structure, with the mouse and rat FAAHs sharing 91% amino acid identity and the human FAAH sharing 82% and 84% identity with the rat FAAH and mouse FAAH, respectively. In addition, the expressed human and rat FAAHs behave biochemically as membrane proteins of comparable molecular size and show similar, but distinguishable, enzymological properties. The identification of highly homologous FAAH proteins in rat, mouse, and human supports a general role for the fatty acid amides in mammalian biology.

Arg-Gly-Asp-Ser-selective adhesion and the stabilization of long-term potentiation: pharmacological studies and the characterization of a candidate matrix receptor

Peptides known to block the extracellular interactions of adhesion receptors belonging to a subclass of the integrin family were tested for their effects on the stabilization of long-term potentiation (LTP) in hippocampal slices. Theta burst stimulation delivered after infusions of Gly-Ala-Val-Ser-Thr-Ala (GAVSTA) resulted in a potentiation effect that decayed steadily over a period of 40 min; LTP elicited in the presence of inactive control peptides remained stable over this time period. GAVSTA had no detectible influence on baseline responses, induction processes, or the initial degree of potentiation. Infusions of integrin antagonists after application of theta bursts also resulted in the occurrence of a decremental form of LTP. Affinity chromatography was then used in an effort to identify targets of the structurally dissimilar integrin blockers that disrupt LTP stabilization. Both integrin antagonists Gly-Arg-Gly-Asp-Ser-Pro and GAVSTA eluted a major species of 55 kDa (synaptegrin-1) from GRGDSP-affinity columns that had been loaded with solubilized synaptic membranes; lesser concentrations of three polypeptides of approximately 20, 27, and 30 kDa were also collected. Synaptegrin-1 was labeled by antibodies to the RGDS-binding integrin alpha5beta1. In addition, the synaptegrin, as well as the 27 kDa, protein was found to copurify with pre- and postsynaptic markers during the isolation of forebrain synaptosomes. These results indicate that a matrix recognition event occurring several minutes after induction of LTP is a necessary step in the stabilization of potentiated synapses; they also identify an integrin-like matrix receptor of 55 kDa that may contribute to this event.

Focal adhesion kinase: at the crossroads of signal transduction

Morphogenetic processes during development, including cell migration, depend on signals from both the extracellular matrix (ECM) and soluble signaling factors. Extensive evidence has shown that the nonreceptor tyrosine kinase, focal adhesion kinase (FAK), is activated in response to both kind of signal. The most definitive evidence that FAK is directly downstream of signals initiated by the ECM comes from comparing the phenotypes of mice deficient for FAK and the ECM molecule, fibronectin: in both cases embryos die at about E8.5 and display almost identical severe vascular and other mesodermal defects. It is now clear that there are additional FAK-like proteins, indicating the existence of a FAK family. Furthermore, FAK is not located at adhesive sites in all cells where it is expressed. This, plus extensive data indicating that FAK becomes activated in response to several soluble signaling factors, suggests that the FAK family may be at the crossroads of multiple signaling pathways that affect cell and developmental processes.

Differential regulation of proline-rich tyrosine kinase 2/cell adhesion kinase beta (PYK2/CAKbeta) and pp125(FAK) by glutamate and depolarization in rat hippocampus

The mechanisms by which stimuli that raise cytosolic free Ca2+ concentrations in neurons can increase protein tyrosine phosphorylation are not known. Using rat hippocampal slices and cortical synaptosomes, we have examined the regulation of two highly related cytoplasmic tyrosine kinases, pp125 focal adhesion kinase (pp125(FAK)) and proline-rich tyrosine kinase 2/cell adhesion kinase beta (PYK2/CAKbeta). Membrane depolarization increased tyrosine phosphorylation of PYK2/CAKbeta and pp125(FAK). These effects were blocked by EGTA or by protein kinase C inhibitors (RO31-8220; GF109203X) and mimicked by ionomycin or phorbol 12-myristate 13-acetate, in the case of pp125(FAK), or their combination in the case of PYK2/CAKbeta. Glutamate and specific agonists of ionotropic (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate and N-methyl-D-aspartate) or metabotropic (trans-1-aminocyclopentane-1,3, -dicarboxylate) glutamate receptors stimulated the phosphorylation of pp125(FAK), but not of PYK2/CAKbeta. Glutamate effects were prevented by GF109203X. Thus, in hippocampal slices, tyrosine phosphorylation of pp125(FAK) and PYK2/CAKbeta are regulated differentially by pathways involving Ca2+ and protein kinase C. pp125(FAK) and PYK2/CAKbeta may provide specific links between neuronal activity, increases in cytosolic Ca2+ and protein tyrosine phosphorylation, which may be important for neuronal survival, and synaptic plasticity.