The carbonic anhydrase domain of receptor tyrosine phosphatase beta is a functional ligand for the axonal cell recognition molecule contactin

Caspr2, a new member of the neurexin superfamily, is localized at the juxtaparanodes of myelinated axons and associates with K+ channels

Rapid conduction in myelinated axons depends on the generation of specialized subcellular domains to which different sets of ion channels are localized. Here, we describe the identification of Caspr2, a mammalian homolog of Drosophila Neurexin IV (Nrx-IV), and show that this neurexin-like protein and the closely related molecule Caspr/Paranodin demarcate distinct subdomains in myelinated axons. While contactin-associated protein (Caspr) is present at the paranodal junctions, Caspr2 is precisely colocalized with Shaker-like K+ channels in the juxtaparanodal region. We further show that Caspr2 specifically associates with Kv1.1, Kv1.2, and their Kvbeta2 subunit. This association involves the C-terminal sequence of Caspr2, which contains a putative PDZ binding site. These results suggest a role for Caspr family members in the local differentiation of the axon into distinct functional subdomains.

Protein tyrosine phosphatase alpha (PTPalpha) and contactin form a novel neuronal receptor complex linked to the intracellular tyrosine kinase fyn

Glycosyl phosphatidylinositol (GPI)-linked receptors and receptor protein tyrosine phosphatases (RPTPs), both play key roles in nervous system development, although the molecular mechanisms are largely unknown. Despite lacking a transmembrane domain, GPI receptors can recruit intracellular src family tyrosine kinases to receptor complexes. Few ligands for the extracellular regions of RPTPs are known, relegating most to the status of orphan receptors. We demonstrate that PTPalpha, an RPTP that dephosphorylates and activates src family kinases, forms a novel membrane-spanning complex with the neuronal GPI-anchored receptor contactin. PTPalpha and contactin associate in a lateral (cis) complex mediated through the extracellular region of PTPalpha. This complex is stable to isolation from brain lysates or transfected cells through immunoprecipitation and to antibody-induced coclustering of PTPalpha and contactin within cells. This is the first demonstration of a receptor PTP in a cis configuration with another cell surface receptor, suggesting an additional mode for regulation of a PTP. The transmembrane and catalytic nature of PTPalpha indicate that it likely forms the transducing element of the complex, and we postulate that the role of contactin is to assemble a phosphorylation-competent system at the cell surface, conferring a dynamic signal transduction capability to the recognition element.

Ataxia and abnormal cerebellar microorganization in mice with ablated contactin gene expression

Axon guidance and target recognition depend on neuronal cell surface receptors that recognize and elicit selective growth cone responses to guidance cues in the environment. Contactin, a cell adhesion/recognition molecule of the immunoglobulin gene superfamily, regulates axon growth and fasciculation in vitro, but its role in vivo is unknown. To assess its function in the developing nervous system, we have ablated contactin gene expression in mice. Contactin-/- mutants displayed a severe ataxic phenotype consistent with defects in the cerebellum and survived only until postnatal day 18. Analysis of the contactin-/- mutant cerebellum revealed defects in granule cell axon guidance and in dendritic projections from granule and Golgi cells. These results demonstrate that contactin controls axonal and dendritic interactions of cerebellar interneurons and contributes to cerebellar microorganization.

Regional distribution and cell type-specific expression of the mouse F3 axonal glycoprotein: a developmental study

The expression of the mouse axonal adhesive glycoprotein F3 and of its mRNA was studied on sections of mouse cerebellar cortex, cerebral cortex, hippocampus, and olfactory bulb from postnatal days 0 (P0) to 30 (P30). In cerebellar cortex, a differential expression of F3 in granule versus Purkinje neurons was observed. F3 was highly expressed during migration of and initial axonal growth from cerebellar granule cells. The molecule was then downregulated on cell bodies and remained expressed, although at low levels, on their axonal extensions. On Purkinje cells, F3 was strongly expressed on cell bodies and processes at the beginning of the second postnatal week; by P16 it was restricted to neurites of Purkinje cells subpopulations. In the cerebral cortex, the molecule was highly expressed on migrating neurons at P0; by P16, it was found essentially within the neuropil with a diffuse pattern. In the hippocampal formation, where F3 was expressed on both pyramidal and granule neurons, a clear shift from the cell bodies to neurite extensions was observed on P3. In the olfactory pathway, F3 was expressed mainly on olfactory nerve fibers, mitral cells, and the synaptic glomeruli from P0 to P3, with a sharp decline from P11 to P16. As a whole, the data show that F3 protein expression is regulated at the regional, cellular, and subcellular levels and suggest that, in different regions, it can be proposed as a reliable neuronal differentiation marker.

The receptor tyrosine phosphatase CRYPalpha promotes intraretinal axon growth

Retinal ganglion cell axons grow towards the optic fissure in close contact with the basal membrane, an excellent growth substratum. One of the ligands of receptor tyrosine phosphatase CRYPalpha is located on the retinal and tectal basal membranes. To analyze the role of this RPTP and its ligand in intraretinal growth and guidance of ganglion cell axons, we disrupted ligand- receptor interactions on the retinal basal membrane in culture. Antibodies against CRYPalpha strongly reduced retinal axon growth on the basal membrane, and induced a dramatic change in morphology of retinal growth cones, reducing the size of growth cone lamellipodia. A similar effect was observed by blocking the ligand with a CRYPalpha ectodomain fusion protein. These effects did not occur, or were much reduced, when axons were grown either on laminin-1, on matrigel or on basal membranes with glial endfeet removed. This indicates that a ligand for CRYPalpha is located on glial endfeet. These results show for the first time in vertebrates that the interaction of a receptor tyrosine phosphatase with its ligand is crucial not only for promotion of retinal axon growth but also for maintenance of retinal growth cone lamellipodia on basal membranes.

Compartmentation of Fyn kinase with glycosylphosphatidylinositol-anchored molecules in oligodendrocytes facilitates kinase activation during myelination

In many cell types, glycosylphosphatidylinositol (GPI)-anchored proteins are sequestered in detergent-resistant membrane rafts. These are plasma membrane microdomains enriched in glycosphingolipids and cholesterol and are suggested to be platforms for cell signaling. Concomitant with the synthesis of myelin glycosphingolipids, maturing oligodendrocytes progressively associate GPI-anchored proteins, including the adhesion molecules NCAM 120 and F3, in rafts. Here we show that these microdomains include Fyn and Lyn kinases. Both kinases are maximally active in myelin prepared from young animals, correlating with early stages of myelination. In the rafts, Fyn kinase is tightly associated with NCAM 120 and F3. In contrast, in oligodendrocyte progenitor cells lacking rafts or in raft-free membrane domains of more mature cells, F3 does not associate with Fyn. The addition of anti-F3 antibodies to oligodendrocytes results in stimulation of Fyn kinase specifically in rafts. Compartmentation of oligodendrocyte GPI-anchored proteins in rafts is thus a prerequisite for association with Fyn, permitting kinase activation. Interaction of oligodendrocyte F3 with axonal ligands such as L1 and ensuing kinase activation may play a crucial role in initiating myelination.

LacSwitch inducible mammalian expression system in mouse Swiss 3T3 fibroblasts

Swiss 3T3 fibroblasts were transfected with the provided plasmids of LacSwitch Inducible Mammalian Expression System (Stratagene). Stable transfectants were selected, expanded and characterised. At first, the production of CAT in these cell lines could be induced by IPTG treatment, but the inducibility was lost after a few months in culture in a reproducible manner. Further analysis revealed that the transfectants did not lose the cat gene nor the lac repressor protein. As a result, we conclude that LacSwitch Inducible Mammalian Expression System needs further modification for use in Swiss 3T3 fibroblasts.

NrCAM, cerebellar granule cell receptor for the neuronal adhesion molecule F3, displays an actin-dependent mobility in growth cones

The neuronal adhesion glycoprotein F3 is a multifunctional molecule of the immunoglobulin superfamily that displays heterophilic binding activities. In the present study, NrCAM was identified as the functional receptor mediating the inhibitory effect of F3 on axonal elongation from cerebellar granule cells. F3Fc-conjugated microspheres binding to neuronal growth cones resulted from heterophilic interaction with NrCAM but not with L1. Time-lapse video-microscopy indicated that F3Fc beads bind at the leading edge and move retrogradely to reach the base of the growth cone within a lapse of 30–60 seconds. Such velocity (5.7 microm/minute) is consistent with a coupling between F3 receptors and the retrograde flow of actin filaments. When actin filaments were disrupted by cytochalasin B, the F3Fc beads remained immobile at the leading edge. The retrograde mobility appeared to be dependent on NrCAM clustering since it was induced upon binding with cross-linked but not dimeric F3Fc chimera. These data indicate that F3 may control growth cone motility by modulating the linkage of its receptor, NrCAM, to the cytoskeleton. They provide further insights into the mechanisms by which GPI-anchored adhesion molecules may exert an inhibitory effect on axonal elongation.

Protein tyrosine phosphatase zeta/RPTPbeta interacts with PSD-95/SAP90 family

PTPzeta/RPTPbeta is a proteoglycan-type receptor-like protein tyrosine phosphatase specifically expressed in the brain. Although several ligands of PTPzeta have been identified, proteins interacting with the intracellular region of PTPzeta are still unknown. We performed yeast two-hybrid screening using the intracellular region of PTPzeta as a bait, and found that the C-terminal sequence of PTPzeta binds to the PSD-95/SAP90 family through the second PDZ domain. Immunohistochemical analysis revealed that PTPzeta and PSD-95/SAP90 are similarly distributed in the dendrites of pyramidal neurons of the hippocampus and neocortex. Furthermore, subcellular fractionation experiments indicated that PTPzeta is concentrated in the postsynaptic density fraction. These results suggested that PTPzeta is involved in the regulation of synaptic function as postsynaptic macromolecular complexes with PSD-95/SAP90.

Retinotectal ligands for the receptor tyrosine phosphatase CRYPalpha

The cell adhesion molecule-like tyrosine phosphatase CRYPalpha is localized on retinal axons and their growth cones. We present evidence that two isoforms of this type IIa phosphatase, CRYPalpha1 and CRYPalpha2, have extracellular ligands along the developing retinotectal pathway. Using alkaline phosphatase fusion proteins containing the CRYPalpha1 ectodomain, we detect a prominent ligand on basement membranes of the early retina, optic stalk, and chiasm. A second ligand is observed in the endfeet region of radial processes in the developing stratum opticum, the site of initial retinal axon invasion. This latter ligand binds CRYPalpha2 preferentially. Further ligand interactions are detected for both CRYPalpha protein isoforms in retinorecipient tectal laminae and on retinal fibers themselves. CRYPalpha thus has cell- and matrix-associated ligands along the entire retinotectal projection. Moreover, these ligands appear to be heterotypic and interact with CRYPalpha through both its immunoglobulin and fibronectin type III regions. The anteroposterior levels of the ligands are relatively uniform within the retina and tectum, suggesting that the CRYPalpha protein within retinal axons does not directly recognise topographically graded guidance cues. We propose that CRYPalpha may have a permissive role in promoting retinal axon growth across the eye and tectum and that its functions are modulated temporally and spatially by isoform-specific interactions with cell- and matrix-associated ligands.

Functional interactions of the immunoglobulin superfamily member F11 are differentially regulated by the extracellular matrix proteins tenascin-R and tenascin-C

The axon-associated protein F11 is a GPI-anchored member of the immunoglobulin superfamily that promotes axon outgrowth and that shows a complex binding pattern toward multiple cell surface and extracellular matrix proteins including tenascin-R and tenascin-C. In this study, we demonstrate that tenascin-R and tenascin-C differentially modulate cell adhesion and neurite outgrowth of tectal cells on F11. While soluble tenascin-R increases the number of attached cells and the percentage of cells with neurites on immobilized F11, tenascin-C stimulates cell attachment to a similar extent but decreases neurite outgrowth. The cellular receptor interacting with F11 has been previously identified as NrCAM; however, in the presence of tenascin-R or tenascin-C cell attachment and neurite extension are independent of NrCAM. Antibody perturbation experiments indicate that beta(1) integrins instead of NrCAM function as receptor for neurite outgrowth of tectal cells on an F11.TN-R complex. Cellular binding assays support the possibility that the interaction of F11 to NrCAM is blocked in the presence of tenascin-R and tenascin-C. Furthermore, a sandwich binding assay demonstrates that tenascin-R and tenascin-C are able to form larger molecular complexes and to link F11 polypeptides by forming a molecular bridge. These results suggest that the molecular interactions of F11 might be regulated by the presence of tenascin-R and tenascin-C.

The glial scar and central nervous system repair

Damage to the central nervous system (CNS) results in a glial reaction, leading eventually to the formation of a glial scar. In this environment, axon regeneration fails, and remyelination may also be unsuccessful. The glial reaction to injury recruits microglia, oligodendrocyte precursors, meningeal cells, astrocytes and stem cells. Damaged CNS also contains oligodendrocytes and myelin debris. Most of these cell types produce molecules that have been shown to be inhibitory to axon regeneration. Oligodendrocytes produce NI250, myelin-associated glycoprotein (MAG), and tenascin-R, oligodendrocyte precursors produce NG2 DSD-1/phosphacan and versican, astrocytes produce tenascin, brevican, and neurocan, and can be stimulated to produce NG2, meningeal cells produce NG2 and other proteoglycans, and activated microglia produce free radicals, nitric oxide, and arachidonic acid derivatives. Many of these molecules must participate in rendering the damaged CNS inhibitory for axon regeneration. Demyelinated plaques in multiple sclerosis consists mostly of scar-type astrocytes and naked axons. The extent to which the astrocytosis is responsible for blocking remyelination is not established, but astrocytes inhibit the migration of both oligodendrocyte precursors and Schwann cells which must restrict their access to demyelinated axons.

Protein-tyrosine phosphatases in development

One of the most important mechanisms of eukaryotic signalling is protein phosphorylation on tyrosine residues, which plays a pivotal role in development by regulating cell proliferation, differentiation and migration. Cellular phosphotyrosine (P.Tyr) levels are regulated by the antagonistic activities of the protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs). We have good insight into the function of PTKs at the molecular level and into the role of PTK-mediated signalling in development. Intuitively, PTPs and PTKs are equally important in development. Over the past decade, much emphasis has been placed on elucidation of the function of PTPs, which has led to good insights into the mechanism of PTP-mediated dephosphorylation. Although still relatively little is known about the role of PTPs in cell signalling and development, evidence is now emerging that several PTPs are crucial for proper development. Here I will introduce PTP-mediated signalling and discuss recent findings regarding the function of PTPs in development.

The supporting-cell antigen: a receptor-like protein tyrosine phosphatase expressed in the sensory epithelia of the avian inner ear

After noise- or drug-induced hair-cell loss, the sensory epithelia of the avian inner ear can regenerate new hair cells. Few molecular markers are available for the supporting-cell precursors of the hair cells that regenerate, and little is known about the signaling mechanisms underlying this regenerative response. Hybridoma methodology was used to obtain a monoclonal antibody (mAb) that stains the apical surface of supporting cells in the sensory epithelia of the inner ear. The mAb recognizes the supporting-cell antigen (SCA), a protein that is also found on the apical surfaces of retinal Müller cells, renal tubule cells, and intestinal brush border cells. Expression screening and molecular cloning reveal that the SCA is a novel receptor-like protein tyrosine phosphatase (RPTP), sharing similarity with human density-enhanced phosphatase, an RPTP thought to have a role in the density-dependent arrest of cell growth. In response to hair-cell damage induced by noise in vivo or hair-cell loss caused by ototoxic drug treatment in vitro, some supporting cells show a dramatic decrease in SCA expression levels on their apical surface. This decrease occurs before supporting cells are known to first enter S-phase after trauma, indicating that it may be a primary rather than a secondary response to injury. These results indicate that the SCA is a signaling molecule that may influence the potential of nonsensory supporting cells to either proliferate or differentiate into hair cells.

Mice deficient for tenascin-R display alterations of the extracellular matrix and decreased axonal conduction velocities in the CNS

Tenascin-R (TN-R), an extracellular matrix glycoprotein of the CNS, localizes to nodes of Ranvier and perineuronal nets and interacts in vitro with other extracellular matrix components and recognition molecules of the immunoglobulin superfamily. To characterize the functional roles of TN-R in vivo, we have generated mice deficient for TN-R by homologous recombination using embryonic stem cells. TN-R-deficient mice are viable and fertile. The anatomy of all major brain areas and the formation and structure of myelin appear normal. However, immunostaining for the chondroitin sulfate proteoglycan phosphacan, a high-affinity ligand for TN-R, is weak and diffuse in the mutant when compared with wild-type mice. Compound action potential recordings from optic nerves of mutant mice show a significant decrease in conduction velocity as compared with controls. However, at nodes of Ranvier there is no apparent change in expression and distribution of Na+ channels, which are thought to bind to TN-R via their beta2 subunit. The distribution of carbohydrate epitopes of perineuronal nets recognized by the lectin Wisteria floribunda or antibodies to the HNK-1 carbohydrate on somata and dendrites of cortical and hippocampal interneurons is abnormal. These observations indicate an essential role for TN-R in the formation of perineuronal nets and in normal conduction velocity of optic nerve.

DSD-1-proteoglycan is the mouse homolog of phosphacan and displays opposing effects on neurite outgrowth dependent on neuronal lineage

DSD-1-PG is a chondroitin sulfate proteoglycan (CSPG) expressed by glial cells that can promote neurite outgrowth from rat embryonic mesencephalic (E14) and hippocampal (E18) neurons, an activity that is associated with the CS glycosaminoglycans (GAGs). Further characterization of DSD-1-PG has included sequencing of peptides from the core protein and the cloning of the corresponding cDNA using polyclonal antisera against DSD-1-PG to screen phage expression libraries. On the basis of these studies we have identified DSD-1-PG as the mouse homolog of phosphacan, a neural rat CSPG. Monoclonal antibodies 3H1 and 3F8 against carbohydrate residues on rat phosphacan recognize these epitopes on DSD-1-PG. The epitopes of the antibodies, L2/HNK-1 and L5/Lewis-X, which have been implicated in functional interactions, are also found on DSD-1-PG. Although DSD-1-PG has previously been shown to promote neurite outgrowth, its upregulation after stab wounding of the CNS and its localization in regions that are considered boundaries to axonal extension suggested that it may also have inhibitory functions. Neonatal dorsal root ganglion (DRG) explants grown on a rich supportive substrate (laminin) with and without DSD-1-PG were strikingly inhibited by the proteoglycan. The inhibitory effects of DSD-1-PG on the DRG explants were not relieved by removal of the CS GAGs, indicating that this activity is associated with the core glycoprotein. The neurite outgrowth from embryonic hippocampal neurons on laminin was not affected by the addition of DSD-1-PG. This indicates that DSD-1-PG/mouse phosphacan can have opposing effects on the process of neurite outgrowth dependent on neuronal lineage.

Isolation and characterization of CA XIV, a novel membrane-bound carbonic anhydrase from mouse kidney

Carbonic anhydrase (CA) is involved in various physiological processes such as acid-base balance and transport of carbon dioxide and ions. In this study, we have succeeded in the isolation of a novel CA from the mouse kidney by use of the signal sequence trap method. It is a 337-amino acid polypeptide with a calculated molecular mass of 37.5 kDa, consisting of a putative amino-terminal signal sequence, a CA domain, a transmembrane domain, and a short hydrophilic carboxyl terminus, which we designated CA XIV. The CA domain of CA XIV is highly homologous with those of known CAs, especially extracellular CAs including CA XII, IX, VI, and IV. The expression study of an epitope-tagged protein has suggested that CA XIV is located on the plasma membrane. When expressed in COS-7 cells, CA XIV exhibits CA activity that is predominantly associated with the membrane fraction. By Northern blot analysis, the gene expression of CA XIV is most abundant in the kidney and heart, followed by the skeletal muscle, brain, lung, and liver. In situ hybridization has revealed that, in the kidney, the gene is expressed intensely in the proximal convoluted tubule, which is the major segment for bicarbonate reabsorption and also in the outer border of the inner stripe of the outer medulla. In conclusion, we have cloned a functional cDNA encoding a novel membrane-bound CA. This study will bring new insights into our understanding of carbon dioxide metabolism and acid-base balance.

Nr-CAM promotes neurite outgrowth from peripheral ganglia by a mechanism involving axonin-1 as a neuronal receptor

Nr-CAM is a neuronal cell adhesion molecule (CAM) belonging to the immunoglobulin superfamily that has been implicated as a ligand for another CAM, axonin-1, in guidance of commissural axons across the floor plate in the spinal cord. Nr-CAM also serves as a neuronal receptor for several other cell surface molecules, but its role as a ligand in neurite outgrowth is poorly understood. We studied this problem using a chimeric Fc-fusion protein of the extracellular region of Nr-CAM (Nr-Fc) and investigated potential neuronal receptors in the developing peripheral nervous system. A recombinant Nr-CAM-Fc fusion protein, containing all six Ig domains and the first two fibronectin type III repeats of the extracellular region of Nr-CAM, retains cellular and molecular binding activities of the native protein. Injection of Nr-Fc into the central canal of the developing chick spinal cord in ovo resulted in guidance errors for commissural axons in the vicinity of the floor plate. This effect is similar to that resulting from treatment with antibodies against axonin-1, confirming that axonin-1/Nr-CAM interactions are important for guidance of commissural axons through a spatially and temporally restricted Nr-CAM positive domain in the ventral spinal cord. When tested as a substrate, Nr-Fc induced robust neurite outgrowth from dorsal root ganglion and sympathetic ganglion neurons, but it was not effective for tectal and forebrain neurons. The peripheral but not the central neurons expressed high levels of axonin-1 both in vitro and in vivo. Moreover, antibodies against axonin-1 inhibited Nr-Fc-induced neurite outgrowth, indicating that axonin-1 is a neuronal receptor for Nr-CAM on these peripheral ganglion neurons. The results demonstrate a role for Nr-CAM as a ligand in axon growth by a mechanism involving axonin-1 as a neuronal receptor and suggest that dynamic changes in Nr-CAM expression can modulate axonal growth and guidance during development.

Zinc(II) complexes of tripodal peptides mimicking the zinc(II)-coordination structure of carbonic anhydrase

Two new tripodal peptide ligands with histidine side chains have been synthesized and were shown to form stable zinc(II) complexes. Their NMR and mass spectra indicate a structure that is analogous to the active center of carbonic anhydrase. Both the ligands and the zinc complexes were titrated potentiometrically in order to obtain the pKa values for the coordinated water of the zinc complexes; due to the low solubility of the complexes only estimates could be obtained.

A receptor-like protein-tyrosine phosphatase PTPzeta/RPTPbeta binds a heparin-binding growth factor midkine. Involvement of arginine 78 of midkine in the high affinity binding to PTPzeta

Midkine is a 13-kDa heparin-binding growth factor with 45% sequence identity to pleiotrophin. Pleiotrophin has been demonstrated to bind to protein-tyrosine phosphatase zeta (PTPzeta) with high affinity. In this study, we examined the binding of midkine to PTPzeta by solid-phase binding assay. Midkine and pleiotrophin binding to PTPzeta were equally inhibited by soluble pleiotrophin and also by some specific glycosaminoglycans. For both bindings, Scatchard analysis revealed low (3.0 nM) and high (0.58 nM) affinity binding sites. These results suggested that PTPzeta is a common receptor for midkine and pleiotrophin. Midkine is structurally divided into the N- and C-terminal halves, and the latter exhibited full activity for PTPzeta binding and neuronal migration induction. The C-terminal half contains two heparin-binding sites consisting of clusters of basic amino acids, Clusters I and II. A mutation at Arg78 in Cluster I resulted in loss of the high affinity binding and reduced neuronal migration-inducing activity, while mutations at Lys83 and Lys84 in Cluster II showed almost no effect on either activity. Chondroitinase ABC-treated PTPzeta exhibited similar low affinity binding both to the native midkine and midkine mutants at Arg78. These results suggested that Arg78 in midkine plays an essential role in high affinity binding to PTPzeta by interacting with the chondroitin sulfate portion of this receptor.

The interaction between F3 immunoglobulin domains and protein tyrosine phosphatases zeta/beta triggers bidirectional signalling between neurons and glial cells

F3, a mouse glycosyl-phosphatidylinositol anchored molecule of the immunoglobulin superfamily, is known to influence axonal growth and fasciculation via multiple interactions of its modular immunoglobulin-like domains. We prepared an Fc chimeric molecule (F3IgFc) to identify molecules interacting with these domains and characterize the functional impact of the interactions. We affinity-isolated tenascin-C and isoforms of the proteoglycan-type protein tyrosine phosphatases zeta/beta (PTPzeta/RPTPbeta) from extracts of developing mouse brain. We showed that both PTPzeta/RPTPbeta and tenascin-C can bind directly to F3, possibly in an exclusive manner, with the highest affinity for the F3-PTPzeta/RPTPbeta interaction. We observed a strong binding of F3IgFc-coated fluorospheres to astrocytes in neural primary cultures and to C6 astrocytoma cells, and demonstrated, in antibody perturbation experiments, that F3-Ig binding on astrocytes depends on its interaction with PTPzeta/RPTPbeta. We also found by confocal analysis that tenascin-C and PTPzeta/RPTPbeta were colocalized on astrocytes which suggests a complex interplay of interactions between PTPzeta/RPTPbeta, tenascin-C and F3. We showed that the interaction between PTPzeta/RPTPbeta and F3-Ig-like domains can trigger bidirectional signalling. C6 glia-expressed PTPzeta/RPTPbeta stimulated neurite outgrowth by cortical and cerebellar neurons, whereas preclustered F3IgFc specifically modified the distribution of phosphotyrosine labelling in these glial cells. Both effects could be prevented and/or mimicked by anti-F3 and anti-6B4PG antibodies. These results identify F3 and PTPzeta/RPTPbeta as potential mediators of a reciprocal exchange of information between glia and neurons.

Characterization of a novel rat brain glycosylphosphatidylinositol-anchored protein (Kilon), a member of the IgLON cell adhesion molecule family

In the central nervous system, many cell adhesion molecules are known to participate in the establishment and remodeling of the neural circuit. Some of the cell adhesion molecules are known to be anchored to the membrane by the glycosylphosphatidylinositol (GPI) inserted to their C termini, and many GPI-anchored proteins are known to be localized in a Triton-insoluble membrane fraction of low density or so-called "raft." In this study, we surveyed the GPI-anchored proteins in the Triton-insoluble low density fraction from 2-week-old rat brain by solubilization with phosphatidylinositol-specific phospholipase C. By Western blotting and partial peptide sequencing after the deglycosylation with peptide N-glycosidase F, the presence of Thy-1, F3/contactin, and T-cadherin was shown. In addition, one of the major proteins, having an apparent molecular mass of 36 kDa after the peptide N-glycosidase F digestion, was found to be a novel protein. The result of cDNA cloning showed that the protein is an immunoglobulin superfamily member with three C2 domains and has six putative glycosylation sites. Since this protein shows high sequence similarity to IgLON family members including LAMP, OBCAM, neurotrimin, CEPU-1, AvGP50, and GP55, we termed this protein Kilon (a kindred of IgLON). Kilon-specific monoclonal antibodies were produced, and Western blotting analysis showed that expression of Kilon is restricted to brain, and Kilon has an apparent molecular mass of 46 kDa in SDS-polyacrylamide gel electrophoresis in its expressed form. In brain, the expression of Kilon is already detected in E16 stage, and its level gradually increases during development. Kilon immunostaining was observed in the cerebral cortex and hippocampus, in which the strongly stained puncta were observed on dendrites and soma of pyramidal neurons.

Role for myelin-associated glycoprotein as a functional tenascin-R receptor

The expression of the immunoglobulin superfamily member myelin-associated glycoprotein (MAG) and the extracellular matrix glycoprotein tenascin-R (TN-R) by oligodendrocytes overlaps in time and space. The two molecules can be neurite outgrowth-inhibitory or -promoting depending on the neuronal cell type and the environment in which they are presented. Here we show that the two molecules directly bind to each other in vitro and that binding sites on TN-R localize to two domains, the fibrinogen domain and the epidermal growth factor-like repeat domain with the N-terminal cysteine-rich stretch. We further show by a functional assay, namely the repulsion of MAG-transfected Chinese hamster ovary cells (CHO) cells from a TN-R substrate, that MAG is part of the signalling pathway of TN-R for cell repulsion. When coated as a uniform substrate, MAG was inhibitory for neurite outgrowth of hippocampal and cerebellar neurons in vitro, when compared to poly-L-lysine, while TN-R enhanced neurite outgrowth. When added to MAG, TN-R neutralized the neurite outgrowth-inhibitory effects of MAG, presumably by blocking the neurite outgrowth-inhibitory domain of MAG.

Airway nerves and protein phosphatases

Much attention has focused on the important role played by phosphatases in the control of gene transcription, cell differentiation and memory regulation. It is also clear that phosphatases may regulate a number of biochemical pathways which can modulate cellular function. Of particular interest is the role of phosphatases in the control of neuronal function. Alterations in neuronal function may contributed to the heightened airways responsiveness observed in asthma to a number of physiological stimuli including distilled water, sulfur dioxide, metabisulfite, hypertonic saline, exercise, allergens, viruses and cold air. An understanding of the mechanisms which regulate the function of sensory nerves could have important clinical implications. In this review we will highlight a number of studies that have investigated the role of phosphatases in the regulation of airway nerve function.

Cell adhesion activity for murine carcinoma cells of a wheat germ 55-kDa protein with binding affinity for animal extracellular matrix proteins

A wheat germ 55-kDa protein was isolated by affinity chromatography with Matrigel immobilized on agarose, followed by preparative gel electrophoresis. This Matrigel-binding protein designated as WG-55 had an amino-terminal amino acid sequence which is identical to that of a putative mature form of wheat storage protein Gbl 1. WG-55 reacted with concanavalin A, indicating its glycoprotein nature as expected from the amino acid sequence of Gbl 1. As expected, similarly, WG-55 exhibited RGD-dependent cell adhesion activity for murine carcinoma cells. These data suggest that WG-55 or mature Gbl 1 protein may play a role in plant cell adhesion.

Cell signaling by protein tyrosine phosphorylation

The above data, and others not described herein, indicate the following: First, that phosphatases are not scavenger enzymes, simply there to remove the phosphate groups introduced by the kinases. They cannot be viewed simply as providing an 'off' switch in an 'on/off' kinase/phosphatase system. Kinases and phosphatases do not carry out one-way and opposing reactions. The same enzyme, depending on where it localizes within the cell, or the molecule with which it might interact, can serve either as a positive or negative determinant in defining cell behavior. In many instances, it can act synergistically with the kinases to enhance the phosphorylation reaction. Second, the factors that determine whether a phosphatase would enhance or oppose a kinase reaction would seem to depend less on its state of activity than on its subcellular localization. This would suggest that if one wanted to call upon it to control transformation, one should try to tamper with its localization segments or whatever binding proteins it might be attached to--rather than with its catalytic domains. Displacement of these enzymes from where they are meant to bind would seem a more promising approach than trying to modulate their catalytic activity. Finally, their architectural features are so basically different from those of the kinases, with receptor tyrosine phosphatases displaying all the structural characteristics of cell adhesion molecules, that they must also have a mission of their own in cell development, survival and death, quite apart from that of the kinases.

Evolutionarily conserved, "acatalytic" carbonic anhydrase-related protein XI contains a sequence motif present in the neuropeptide sauvagine: the human CA-RP XI gene (CA11) is embedded between the secretor gene cluster and the DBP gene at 19q13.3

Conserved amino acid motifs are found in numerous expressed genes. Proteins and peptides with functional relationships may be identified using probes designed to hybridize with these motifs. An oligonucleotide probe was prepared to match the sequence of the expected active region of a frog corticotropin-releasing factor-like peptide sauvagine and used to screen a sheep brain cDNA library. A novel 1331-bp cDNA encoding a putative 328-residue protein with a theoretical mass of 36 kDa was identified. The presence of a strong signal sequence indicates that it is a secreted protein. The amino- and carboxy-terminal regions are characterized by several potential phosphorylation sites and binding motifs, suggesting a role in intracellular signal transduction. Although the protein possesses a 7-residue sequence identical to that found in sauvagine, its overall primary structure most closely resembles those of the alpha-carbonic anhydrases (alpha-CAs). Moreover, the detection of the human and mouse orthologues in the EST databases, together with an evolutionary analysis, indicates that the protein represents a new member of the alpha-CA gene family, which we designate carbonic anhydrase-related protein XI (CA-RP XI), encoded by CA11 (human) and Car11 (mouse, rat). The human CA11 gene appears to be located between the secretor type alpha(1,2)-fucosyltransferase gene cluster (FUT1-FUT2-FUT2P) and the D-site binding protein gene (DBP) on chromosome 19q13.3. Despite potentially inactivating changes in the active-site residues, CA-RP XI is evolving very slowly in mammals, a property indicative of an important function, which has also been observed in the two other "acatalytic" CA isoforms, CA-RP VIII and CA-RP X, whose functions are unknown.

Protein tyrosine phosphatase-eta expression is upregulated by the PKA-dependent and is downregulated by the PKC-dependent pathways in thyroid cells

We have recently reported the isolation of a rat cDNA encoding a receptor-type tyrosine phosphatase, which appears to be a marker of thyroid differentiation. To elucidate the molecular mechanisms underlying r-PTPeta expression in normal thyroid cells both in vitro and in vivo, we investigated the regulation of r-PTPeta expression in cultured thyrocytes (the rat cell line PC Cl 3) and in an animal model of TSH-dependent thyroid goitrogenesis. In vitro studies showed that mRNA expression of r-PTPeta in thyroid cells is induced in a time- and dose-dependent manner by the activation of growth- and differentiation-linked PKA pathways (TSH and forskolin), whereas it is down-regulated by the activation of the proliferative dedifferentiating PKC-dependent transduction pathway (TPA). However, the regulation of r-PTPeta expression by TSH and TPA, respectively, is observed only in normal thyroid cells, but is lost in transformed thyroid cells. In vivo studies with thiouracil-fed rats demonstrated that increased serum levels of TSH up-regulated r-PTPeta mRNA expression in parallel with the stimulation of thyroid growth and function. The reduction of blood TSH levels due to iodide refeeding to goitrous rats determined a marked down-regulation of r-PTPeta expression, in parallel with involution of thyroid hyperplasia. Taken together these results demonstrate that the phosphatase r-PTPeta is regulated by the two main thyroid regulatory pathways and suggest that it may play an important role in the growth and differentiation of thyroid cells.

Cytokine-induced transcription of protein-tyrosine-phosphatases in human astrocytoma cells

Interleukin-1 (IL-1) and Tumor Necrosis Factor-a (TNFalpha) are potent mediators of inflammatory reactions in the brain. Although much is known about the effects of IL-1 on expression of secretory proteins, few studies have addressed the question of a selective, IL-1-dependent expression of genes involved in neuromodulatory effects of inflammation. Protein-tyrosine-phosphatases (PTP's) have been shown to regulate signal transduction and adhesion processes in the developing nervous system. They are candidates for inflammation-induced neuromodulation. Therefore, we investigated if IL-1 regulates expression of PTP's. We applied a DNA-fingerprinting method based on the PCR-amplification of conserved domains of gene families and observed IL-1-dependent induction of two PTP's, cytoplasmic PTPvarepsilon and receptor-PTPgamma, RPTPgamma, in human U373-MG astrocytoma cells. Using Northern blot analysis, we confirmed this result and also show that in addition to IL-1, TNFalpha but not IL-6 induces the transcription of cytoplasmic PTPvarepsilon and RPTPgamma in human astrocytoma cells. Given the important role for PTP's in neuromodulatory aspects such as axonal guidance and neurite outgrowth, cytokine-induced induction of PTP's may play an important pathenogenic role in the development of chronic inflammatory diseases in the brain.

Neurexin IV, caspr and paranodin--novel members of the neurexin family: encounters of axons and glia

Axonal insulation is of key importance for the proper propagation of action potentials. In Drosophila and other invertebrates, it has recently been demonstrated that septate junctions play an essential role in axonal insulation or blood-brain-barrier formation. Neurexin IV, a molecular component of Drosophila septate junctions, has been shown to be essential for axonal insulation in the PNS in embryos and larvae. Interestingly, a vertebrate homolog of Neurexin IV, caspr--also named paranodin--has been shown to localize to septate-like junctional structures. These vertebrate junctions are localized to the paranodal region of the nodes of Ranvier, between axons and Schwann cells. Caspr/paranodin might play an important role in barrier formation, and link neuronal membrane components with the axonal cytoskeletal network.

Mouse models of myelin diseases

Dys- and demyelination are the common endpoints of several inherited diseases of glial cells, which elaborate myelin and which maintain the myelin sheath very much like an "external" cellular organelle. Whereas some of the genes that are affected by mutations appear to be glial-specific, other genes are expressed in many cell types but their defect is restricted to oligodendrocytes or Schwann cells. Many of the disease genes and their encoded proteins have been studied with the help of mouse models, and a number of different molecular pathomechanisms have emerged which have been summarized in Figure 8. Some of the new concepts in the field, which have been addressed in this review, have only emerged because similar pathomechanisms were discovered for different myelin proteins. Mouse models have clearly helped to address both, the molecular pathology of myelin diseases and the normal function of myelin genes, but as discussed in this review, these questions turned out to be very different. Despite the progress in understanding the role of the abundant myelin proteins, there also remain a number of open questions that concern, among other things, the initial axon-glia recognition, the assembly process of the myelin sheath, and the long-term interaction of axons with their myelinating glia. Finally, animal models of human neurological diseases should not be restricted to the study of pathology, but they should also contribute to the development of experimental treatments. It is encouraging that a few attempts have been made.

Dissection of complex molecular interactions of neurofascin with axonin-1, F11, and tenascin-R, which promote attachment and neurite formation of tectal cells

Neurofascin is a member of the L1 subgroup of the Ig superfamily that promotes axon outgrowth by interactions with neuronal NgCAM-related cell adhesion molecule (NrCAM). We used a combination of cellular binding assays and neurite outgrowth experiments to investigate mechanisms that might modulate the interactions of neurofascin. In addition to NrCAM, we here demonstrate that neurofascin also binds to the extracellular matrix glycoprotein tenascin-R (TN-R) and to the Ig superfamily members axonin-1 and F11. Isoforms of neurofascin that are generated by alternative splicing show different preferences in ligand binding. While interactions of neurofascin with F11 are only slightly modulated, binding to axonin-1 and TN-R is strongly regulated by alternatively spliced stretches located in the NH2-terminal half, and by the proline-alanine-threonine-rich segment. In vitro neurite outgrowth and cell attachment assays on a neurofascin-Fc substrate reveal a shift of cellular receptor usage from NrCAM to axonin-1, F11, and at least one additional protein in the presence of TN-R, presumably due to competition of the neurofascin- NrCAM interaction. Thereby, F11 binds to TN-R of the neurofascin/TN-R complex, but not to neurofascin, whereas axonin-1 is not able to bind directly to the neurofascin/TN-R complex as shown by competition binding assays. In conclusion, these investigations indicate that the molecular interactions of neurofascin are regulated at different levels, including alternative splicing and by the presence of interacting proteins.

Gathering STYX: phosphatase-like form predicts functions for unique protein-interaction domains

The effects of tyrosine phosphorylation are manifested and regulated through protein domains that bind to specific phosphotyrosine motifs. STYX is a unique modular domain found within proteins implicated in mediating the effects of tyrosine phosphorylation in vivo. Individual STYX domains are not catalytically active; however, they resemble protein tyrosine phosphatase (PTP) domains and, like PTPs, contain core sequences that recognize phosphorylated substrates. Thus, the STYX domain adds to the repertoire of modular domains that can mediate intracellular signaling in response to protein phosphorylation.

Characterization and chromosomal localization of PTP-NP-2, a new isoform of protein tyrosine phosphatase-like receptor, expressed on synaptic boutons

Recently, there have been several reports describing the cloning and characterization of the novel family of protein tyrosine phosphatase-like receptor molecules (known as IA-2 and PTP-NP/PTP-IAR/IA-2beta/phogrin), which may act as autoantigens in diabetes. Here, we report the molecular characterization and chromosomal localization of a new isoform of this family in brain termed PTP-NP-2 (for PTP-NP tyrosine phosphatase isoform), and its function in rat primary hippocampal neurons. PTP-NP-2 has 48% identity to IA-2. The principal difference between PTP-NP-2 and PTP-NP is a 17-amino-acid insert near the N-terminus of PTP-NP that is absent in PTP-NP-2. Genomic DNA analysis indicates that the 17-amino-acid insert is coded by a separate exon, suggesting that both IA-2beta and PTP-NP-2 are isoforms arising by alternate splicing of the same gene. Reverse transcriptase-PCR revealed that both isoforms are present in human SH-SY5Y neuroblastoma cells. PTP-NP-2 mRNA expression is highly restricted, with a 5.5-kb specific transcript in human fetal and adult brain and 5.5 and 3. 8 kb in human adult pancreas. SH-SY5Y neuroblastoma and U87-MG glioblastoma cells showed specific transcripts of 5.5 and 3.8<HSP SP = "0.25">kb, respectively, indicating the existence of several isoforms of this molecule in the nervous system. The human gene encoding PTP-NP-2 was assigned to human chromosome 7q22-qter using Southern blot analysis of genomic DNAs from rodent/human somatic hybrid cell lines. Confocal microscopy analyses of rat primary hippocampal neurons revealed that PTP-NP-2 is abundantly expressed on synaptic boutons in primary neurons. Wild-type PTP-NP-2 showed no measurable tyrosine phosphatase activity using an in-vitro pNPP assay. Examination of the PTP-NP-2 catalytic consensus sequence revealed that this sequence differed from the typical tyrosine phosphatase-domain consensus sequence by an alanine to aspartate change (amino acid 930). Mutation of aspartate 930 to alanine produced a catalytically active enzyme, suggesting that native PTP-NP and its isoform PTP-NP-2 are catalytically inactive receptor protein tyrosine phosphatase homologues. Taken together, these results indicate that the tyrosine phosphatase PTP-NP-2 is a new isoform of PTP-NP tyrosine phosphatase, is expressed on synaptic boutons and may participate in the regulation of synaptic bouton endocytosis.

Regulated expression of the cell adhesion glycoprotein F3 in adult hypothalamic magnocellular neurons

F3, a glycoprotein of the immunoglobulin superfamily implicated in axonal growth, occurs in oxytocin (OT)-secreting and vasopressin (AVP)-secreting neurons of the adult hypothalamo-neurohypophysial system (HNS) whose axons undergo morphological changes in response to stimulation. Immunocytochemistry and immunoblot analysis showed that during basal conditions of HNS secretion, there are higher levels of this glycosylphosphatidyl inositol-anchored protein in the neurohypophysis, where their axons terminate, than in the hypothalamic nuclei containing their somata. Physiological stimulation (lactation, osmotic challenge) reversed this pattern and resulted in upregulation of F3 expression, paralleling that of OT and AVP under these conditions. In situ hybridization revealed that F3 expression in the hypothalamus is restricted to its magnocellular neurons and demonstrated a more than threefold increase in F3 mRNA levels in response to stimulation. Confocal and electron microscopy localized F3 in secretory granules in all neuronal compartments, a localization confirmed by detection of F3 immunoreactivity in granule-enriched fractions obtained by sucrose density gradient fractionation of rat neurohypophyses. F3 was not visible on any cell surface in the magnocellular nuclei. In contrast, in the neurohypophysis, it was present not only in secretory granules but also on the surface of axon terminals and glia and in extracellular spaces. Taken together, our observations reveal that the cell adhesion glycoprotein F3 is colocalized with neurohypophysial peptides in secretory granules. It follows, therefore, the regulated pathway of secretion in HNS neurons to be released by exocytosis at their axon terminals in the neurohypophysis, where it may intervene in activity-dependent structural axonal plasticity.

Involvement of receptor-like protein tyrosine phosphatase zeta/RPTPbeta and its ligand pleiotrophin/heparin-binding growth-associated molecule (HB-GAM) in neuronal migration

Pleiotrophin/heparin-binding growth-associated molecule (HB-GAM) is a specific ligand of protein tyrosine phosphatase zeta (PTPzeta)/receptor-like protein tyrosine phosphatase beta (RPTPbeta) expressed in the brain as a chondroitin sulfate proteoglycan. Pleiotrophin and PTPzeta isoforms are localized along the radial glial fibers, a scaffold for neuronal migration, suggesting that these molecules are involved in migratory processes of neurons during brain development. In this study, we examined the roles of pleiotrophin-PTPzeta interaction in the neuronal migration using cell migration assay systems with glass fibers and Boyden chambers. Pleiotrophin and poly-L-lysine coated on the substratums stimulated cell migration of cortical neurons, while laminin, fibronectin, and tenascin exerted almost no effect. Pleiotrophin-induced and poly-L-lysine-induced neuronal migrations showed significant differences in sensitivity to various molecules and reagents. Polyclonal antibodies against the extracellular domain of PTPzeta, PTPzeta-S, an extracellular secreted form of PTPzeta, and sodium vanadate, a protein tyrosine phosphatase inhibitor, added into the culture medium strongly suppressed specifically the pleiotrophin-induced neuronal migration. Furthermore, chondroitin sulfate C but not chondroitin sulfate A inhibited pleiotrophin-induced neuronal migration, in good accordance with our previous findings that chondroitin sulfate constitutes a part of the pleiotrophin-binding site of PTPzeta, and PTPzeta-pleiotrophin binding is inhibited by chondroitin sulfate C but not by chondroitin sulfate A. Immunocytochemical analysis indicated that the transmembrane forms of PTPzeta are expressed on the migrating neurons especially at the lamellipodia along the leading processes. These results suggest that PTPzeta is involved in the neuronal migration as a neuronal receptor of pleiotrophin distributed along radial glial fibers.

A neurite outgrowth-inhibitory proteoglycan expressed during development is similar to that isolated from adult brain after isomorphic injury

The expression of proteoglycans (PGs) in the mammalian central nervous system (CNS) appears to be strictly regulated both during development and after damage to the mammalian CNS. Recently, we have isolated from membranes of injured adult brain a neurite outgrowth-inhibitory proteoglycan (IMP), the activity of which could be specifically counteracted by a monoclonal antibody (mAB) against the PG. We described in this report the characterization of perinatal membrane proteoglycan (PMP), a heparan-sulfate/chondroitin-sulfate-containing PG expressed during brain development. Its maximal expression was observed around postnatal day 3, decreasing strongly in normal adult tissue. This PG was purified and characterized using mABs generated against IMP. The comparison of PMP and IMP properties indicates that the two PGs are highly related and share expression patterns, biochemical characteristics, and the ability to inhibit neurite initiation in culture. However, IMP and PMP displayed a distinct effect on neurite elongation, which may be explained by their differences in glycosilation pattern. The data presented in this report support the idea that proteoglycans expressed during CNS development are re-expressed following injury.

Human carbonic anhydrase XII: cDNA cloning, expression, and chromosomal localization of a carbonic anhydrase gene that is overexpressed in some renal cell cancers

We report the cloning and characterization of a tumor-associated carbonic anhydrase (CA) that was identified in a human renal cell carcinoma (RCC) by serological expression screening with autologous antibodies. The cDNA sequence predicts a 354-amino acid polypeptide with a molecular mass of 39,448 Da that has features of a type I membrane protein. The predicted sequence includes a 29-amino acid signal sequence, a 261-amino acid CA domain, an additional short extracellular segment, a 26-amino acid hydrophobic transmembrane domain, and a hydrophilic C-terminal cytoplasmic tail of 29 amino acids that contains two potential phosphorylation sites. The extracellular CA domain shows 30-42% homology with known human CAs, contains all three Zn-binding histidine residues found in active CAs, and contains two potential sites for asparagine glycosylation. When expressed in COS cells, the cDNA produced a 43- to 44-kDa protein in membranes that had around one-sixth the CA activity of membranes from COS cells transfected with the same vector expressing bovine CA IV. We have designated this human protein CA XII. Northern blot analysis of normal tissues demonstrated a 4.5-kb transcript only in kidney and intestine. However, in 10% of patients with RCC, the CA XII transcript was expressed at much higher levels in the RCC than in surrounding normal kidney tissue. The CA XII gene was mapped by using fluorescence in situ hybridization to 15q22. CA XII is the second catalytically active membrane CA reported to be overexpressed in certain cancers. Its relationship to oncogenesis and its potential as a clinically useful tumor marker clearly merit further investigation.

Protein tyrosine phosphatases and neural development

During neural development, cells interact dynamically with each other and with the extracellular matrix, using cell signaling to control differentiation, axonogenesis, and survival. Enzymes that regulate protein tyrosine phosphorylation often lie at the core of such cell signaling. Protein tyrosine phosphatases (PTPases) are recognized as being of central importance here, and a growing family of PTPases are now known to be expressed in embryonic neurons and glia. Both receptor-like and cytoplasmic enzymes have been identified. The receptor family includes immunoglobulin superfamily members that influence cell-cell adhesion, proteoglycans that control neurite growth, and enzymes in Drosophila that regulate axon guidance and target cell recognition. Cytoplasmic PTPases are implicated in nerve cell commitment and potentially in the regulation of cell survival. This review outlines what we currently know about PTPases in the nervous system and presents concepts concerning their possible modes of action.

Molecular characterization of the human carbonic anhydrase-related protein (HCA-RP VIII)

The very evolutionarily conserved human carbonic anhydrase-related polypeptide (CA-RP VIII) lacks the carbon-dioxide hydration-activity, characteristic of the enzymatically active carbonic anhydrases. We have expressed HCA-RP VIII as a glutathione-S-transferase fusion protein (GST-HCA-RP VIII). The purified HCA-RP VIII showed a substantially higher apparent molecular weight by gel-filtration compared to the molecular weight calculated from the amino acid sequence, indicating a larger than expected Stoke's radius. Like other studied CA's, the protein unfolds through two transitions at increasing concentrations of guanidine hydrochloride. The far-UV CD spectra of HCA-RP VIII indicates a secondary structure similar to that of the catalytically active HCA II. The very high sequence identity between human and mouse CA-RP VIII (98%), might indicate that the function of the protein involves binding of another protein. However, an attempt to use the GST-HCA-RP VIII fusion protein to affinity purify a ligand was unsuccessful.

Neurons as well as astrocytes express proteoglycan-type protein tyrosine phosphatase zeta/RPTPbeta: analysis of mice in which the PTPzeta/RPTPbeta gene was replaced with the LacZ gene

PTPzeta/RPTPbeta is a receptor-like protein tyrosine phosphatase expressed as a chondroitin sulfate proteoglycan. We generated mice in which the PTPzeta gene was replaced by the LacZ gene by gene targeting. Analysis of heterozygous PTPzeta-targeted mice allowed us to identify PTPzeta-producing cells during development by examining expression of the LacZ gene. LacZ expression was detected only in the central nervous system throughout development from embryonic day 8.5. In the postnatal period, subsets of neurons and astrocytes in the brain, including pyramidal cells and astrocytes in the hippocampus, expressed LacZ. Primary cultures of cells from the cerebral cortex of embryonic day 16 mice also indicated that both neurons and astrocytes were positive for LacZ. These results indicated that neurons and astrocytes express PTPzeta.

Identification and characterization of RPTP rho, a novel RPTP mu/kappa-like receptor protein tyrosine phosphatase whose expression is restricted to the central nervous system

We describe the cloning, chromosomal localization and characterization of RPTPrho, a new member of the RPTPmu/kappa phosphatase subfamily. Receptor tyrosine phosphatases in this subfamily are comprised of a MAM domain near the N-terminal, an immunoglobulin-like domain, four fibronectin type III repeats, a single transmembrane domain, and a large juxtamembrane segment followed by two intracellular phosphatase domains. An alternatively spliced mini-exon was identified in the extracellular segment of RPTPrho, between the fourth fibronectin type III repeat and the transmembrane domain. The RPTPrho gene was mapped to human chromosome 20 and mouse chromosome 2. Northern blot analysis demonstrated that RPTPrho expression was restricted to the central nervous system, and in situ hybridization studies showed that the RPTPrho transcript was distributed throughout the murine brain and spinal cord. Exceptionally high levels of the transcript were present in the cortex and olfactory bulbs during perinatal development, but were down-regulated during postnatal week two. The motifs found in the extracellular segment of type II receptor protein tyrosine phosphatases are commonly found in neural cell adhesion molecules, suggesting that RPTPrho may be involved in both signal transduction and cellular adhesion in the central nervous system.

The Croonian Lecture 1997. The phosphorylation of proteins on tyrosine: its role in cell growth and disease

The reversible phosphorylation of tyrosines in proteins plays a key role in regulating many different processes in eukaryotic organisms, such as growth control, cell cycle control, differentiation cell shape and movement, gene transcription, synaptic transmission, and insulin action. Phosphorylation of proteins is brought about by enzymes called protein-tyrosine kinases that add phosphate to specific tyrosines in target proteins; phosphate is removed from phosphorylated tyrosines by enzymes called protein-tyrosine phosphatases. Phosphorylated tyrosines are recognized by specialized binding domains on other proteins, and such interactions are used to initiate intracellular signaling pathways. Currently, more than 95 protein-tyrosine kinases and more than 55 protein-tyrosine phosphatase genes are known in Homo sapiens. Aberrant tyrosine phosphorylation is a hallmark of many types of cancer and other human diseases. Drugs are being developed that antagonize the responsible protein-tyrosine kinases and phosphatases in order to combat these diseases.

Two receptor tyrosine phosphatases of the LAR family are expressed in the developing leech by specific central neurons as well as select peripheral neurons, muscles, and other cells

Receptor protein tyrosine phosphatases (rPTPs) are thought to play a crucial role in neuronal development, particularly in pathfinding by growing processes. We have cloned and sequenced two Hirudo medicinalis rPTPs that are homologous to the Drosophila and vertebrate rPTPs of the Leukocyte common antigen-related (LAR) subfamily. These Hirudo rPTPs, HmLAR1 and HmLAR2, are products of different, homologous genes, both containing two tandem intracellular phosphatase domains and ectodomains with three tandem Ig domains and different numbers of tandem fibronectin type III (FIII) domains. They are expressed in distinct patterns during embryogenesis. HmLAR1 mRNA is expressed by a subset of central and peripheral neurons and by several peripheral muscular structures, whereas HmLAR2 mRNA is expressed by a different subset of central neurons and by the peripheral, neuron-like Comb cells. HmLAR1 and HmLAR2 proteins are located on the neurites of central neurons. In addition, HmLAR2 is expressed on the cell body, processes, and growth cones of the Comb cells. Because of their CAM-like ectodomains and homology to proteins known to be involved in pathfinding and because they are expressed by different subsets of neurons, we hypothesize that HmLAR1 and HmLAR2 participate in navigational decisions that distinguish the sets of neurons that express them. Furthermore, we hypothesize that HmLAR2 is also involved in setting up the highly regular array of parallel processes established by the Comb cells. Lastly, we propose that the HmLAR1 ectodomain on peripheral muscle cells plays a role in target recognition via interactions with neuronal receptors, which might include HmLAR1 or HmLAR2.

Multi-ligand interactions with receptor-like protein tyrosine phosphatase beta: implications for intercellular signaling

Receptor-like protein tyrosine phosphatase beta (RPTP beta) shows structural and functional similarity to cell adhesion molecules (CAMs). It binds to several neuronal CAMs and extracellular matrix (ECM) proteins that combine to form cell-recognition complexes. Here, the authors discuss the implications of such complexes for intercellular signaling, and the regulation of RPTP activity by cell-cell and cell-ECM contact.

Immunohistochemistry of carbonic anhydrase isozyme IX (MN/CA IX) in human gut reveals polarized expression in the epithelial cells with the highest proliferative capacity

MN/CA IX is a recently discovered member of the carbonic anhydrase (CA) gene family that has been identified in the plasma membranes of certain tumor and epithelial cells and found to promote cell proliferation when transfected into NIH3T3 cells. This study presents localization of MN/CA IX in human gut and compares its distribution to those of CA I, II, and IV, which are known to be expressed in the intestinal epithelium. The specificity of the monoclonal antibody for MN/CA IX was confirmed by Western blots and immunostaining of COS-7 cells transfected with MN/CA IX cDNA. Immunohistochemical stainings of human gut revealed prominent polarized staining for MN/CA IX in the basolateral surfaces of the enterocytes of duodenum and jejunum, the reaction being most intense in the crypts. A moderate reaction was also seen in the crypts of ileal mucosa, whereas the staining became generally weaker in the large intestine. The results indicate isozyme-specific regulation of MN/CA IX expression along the cranial-caudal axis of the human gut and place the protein at the sites of rapid cell proliferation. The unique localization of MN/CA IX on the basolateral surfaces of proliferating crypt enterocytes suggests that it might serve as a ligand or a receptor for another protein that regulates intercellular communication or cell proliferation. Furthermore, MN/CA IX has a completely conserved active site domain of CAs suggesting that it could also participate in carbon dioxide/bicarbonate homeostasis.

High affinity binding and overlapping localization of neurocan and phosphacan/protein-tyrosine phosphatase-zeta/beta with tenascin-R, amphoterin, and the heparin-binding growth-associated molecule

We have studied the interactions of the nervous tissue-specific chondroitin sulfate proteoglycans neurocan and phosphacan with the extracellular matrix protein tenascin-R and two heparin-binding proteins, amphoterin and the heparin-binding growth-associated molecule (HB-GAM), using a radioligand binding assay. Both proteoglycans show saturable, high affinity binding to tenascin-R with apparent dissociation constants in the 2-7 nM range. Binding is reversible, inhibited in the presence of unlabeled proteoglycan, and increased by approximately 60% following chondroitinase treatment of the proteoglycans, indicating that the interactions are mediated via the core (glyco)proteins rather than by the glycosaminoglycan chains, which may in fact partially shield the binding sites. In contrast to their interactions with tenascin-C, in which binding was decreased by approximately 75% in the absence of calcium, binding of phosphacan to tenascin-R was not affected by the absence of divalent cations in the binding buffer, although there was a small but significant decrease in the binding of neurocan. Neurocan and phosphacan are also high affinity ligands of amphoterin and HB-GAM (Kd = 0.3-8 nM), two heparin-binding proteins that are developmentally regulated in brain and functionally involved in neurite outgrowth. The chondroitin sulfate chains on neurocan and phosphacan account for at least 80% of their binding to amphoterin and HB-GAM. The presence of amphoterin also produces a 5-fold increase in phosphacan binding to the neural cell adhesion molecule contactin. Immunocytochemical studies showed an overlapping localization of the proteoglycans and their ligands in the embryonic and postnatal brain, retina, and spinal cord. These studies have therefore revealed differences in the interactions of neurocan and phosphacan with the two major members of the tenascin family of extracellular matrix proteins, and also suggest that chondroitin sulfate proteoglycans play an important role in the binding and/or presentation of differentiation factors in the developing central nervous system.

Functional cooperation of beta1-integrins and members of the Ig superfamily in neurite outgrowth induction

Neurite outgrowth is a central aspect of the ontogenetic formation of neural networks and is regulated by distinct groups of cell surface molecules. One protein involved in neurite elongation and fasciculation is the neural Ig superfamily member F11/contactin. We have shown previously that F11 promotes neurite extension of chick tectal neurons by interaction with the tectal receptor NrCAM, a member of the L1 subgroup of the Ig superfamily. By contrast, it does not induce outgrowth of retinal neurons despite the fact that these cells also express NrCAM, suggesting that in retinal cells the F11-NrCAM interaction alone is not sufficient to induce neurite extension. In this report we present a novel image analysis procedure to quantify neurite outgrowth and use it to demonstrate that F11 enhances the fibronectin-induced outgrowth response of embryonic retinal neurons. We reveal that NrCAM is the neuronal receptor mediating the enhanced outgrowth of retinal neurons, whereas the related F11-binding molecule NgCAM is not involved. Furthermore, we provide evidence that a beta1-integrin may represent the fibronectin-dependent receptor that cooperates indirectly with the F11-NrCAM pathway. Our results support the concept of a combinatorial labeling of cells in nervous system histogenesis by different classes of cell surface proteins, in particular by integrins and molecules of the Ig superfamily.

Receptor-like protein-tyrosine phosphatase alpha specifically inhibits insulin-increased prolactin gene expression

A physiologically relevant response to insulin, stimulation of prolactin promoter activity in GH4 pituitary cells, was used as an assay to study the specificity of protein-tyrosine phosphatase function. Receptor-like protein-tyrosine phosphatase alpha (RPTPalpha) blocks the effect of insulin to increase prolactin gene expression but potentiates the effects of epidermal growth factor and cAMP on prolactin promoter activity. RPTPalpha was the only protein-tyrosine phosphatase tested that did this. Thus, the effect of RPTPalpha on prolactin-chloramphenicol acetyltransferase (CAT) promoter activity is specific by two criteria. A number of potential RPTPalpha targets were ruled out by finding (a) that they are not affected or (b) that they are not on the pathway to insulin-increased prolactin-CAT activity. The negative effect of RPTPalpha on insulin activation of the prolactin promoter is not due to reduced phosphorylation or kinase activity of the insulin receptor or to reduced phosphorylation of insulin receptor substrate-1 or Shc. Inhibitor studies suggest that insulin-increased prolactin gene expression is mediated by a Ras-like GTPase but is not mitogen-activated protein kinase dependent. Experiments with inhibitors of phosphatidylinositol 3-kinase suggest that insulin-increased prolactin-CAT expression is phosphatidylinositol 3-kinase-independent. These results suggest that RPTPalpha may be a physiological regulator of insulin action.