Spatially and temporally regulated modification of the receptor-like protein tyrosine phosphatase zeta/beta isoforms with keratan sulphate in the developing chick brain

Evidence for multiple, developmentally regulated isoforms of Ptprq on hair cells of the inner ear

Ptprq is a receptor-like inositol lipid phosphatase associated with the shaft connectors of hair bundles. Three lines of evidence suggest Ptprq is a chondroitin sulfate proteoglycan: (1) chondroitinase ABC treatment causes a loss of the ruthenium-red reactive, electron-dense particles associated with shaft connectors, (2) chondroitinase ABC causes an increase in the electrophoretic mobility of Ptprq, and (3) hair bundles in the developing inner ear of wild-type mice, but not those of Ptprq(-/-) mice, react with monoclonal antibody (mAb) 473-HD, an IgM that recognizes the dermatan-sulfate-dependent epitope DSD1. Two lines of evidence indicate that there may be multiple isoforms of Ptprq expressed in hair bundles. First, although Ptprq is expressed throughout the lifetime of most hair cells, hair bundles in the mouse and chick inner ear only express the DSD1 epitope transiently during development. Second, mAb H10, a novel mAb that recognizes an epitope common to several avian inner-ear proteins including Ptprq, only stains mature hair bundles in the extrastriolar regions of the vestibular maculae. MAb H10 does not stain mature hair bundles in the striolar regions of the maculae or in the basilar papilla, nor does it stain immature hair bundles in any organ. Three distinct, developmentally regulated isoforms of Ptprq may therefore be expressed on hair bundles of the chick inner ear. Hair bundles in the mature chick ear that do not express the H10 epitope have longer shaft connectors than those that do, indicating the presence or absence of the H10 epitope on Ptprq may modulate the spacing of stereocilia.

A role for receptor protein tyrosine phosphatase lambda in midbrain development

The mid-hindbrain boundary (MHB) harbors an important organizing center for the adjacent brain regions. Here, we present evidence that the receptor protein tyrosine phosphatase lambda (RPTPlambda) is part of the complex molecular network that maintains and shapes the MHB region. RPTPlambda is expressed in a tight band of cells in the caudal midbrain, anterior to the transverse ring of Wnt1 expression. Forced expression of RPTPlambda across the mid-hindbrain region repressed expression of Wnt1, whereas RNA interference-mediated knock-down of RPTPlambda resulted in expansion and distortion of the Wnt1 domain. When ectopically expressed in the mesencephalon, RPTPlambda specifically inhibited the induction of Wnt1 expression after subsequent stimulation with Fgf8. Reduced Wnt1 expression after RPTPlambda transfection correlated with a decrease in Ras- mitogen-activated protein kinase activity at the MHB. We further show that in the embryonic midbrain, RPTPlambda can bind to beta-catenin, a central component of the canonical Wnt signaling pathway. Overexpression of RPTPlambda suppressed the activity of a beta-catenin responsive promoter in the midbrain and reduced progenitor cell proliferation. Cotransfection of Wnt1 or of a stabilized form of beta-catenin together with RPTPlambda partially rescued the RPTPlambda-mediated proliferation defect. Together, these data suggest that RPTPlambda may play a dual role in the control of midbrain development: as a negative modulator of Fgf8-induced Wnt1 expression at the MHB, which may help to confine the Wnt1 domain to it characteristic tight ring at the MHB; and as an inhibitor of canonical Wnt signaling through interaction with and presumably sequestration of beta-catenin.

Heterogeneity of the chondroitin sulfate portion of phosphacan/6B4 proteoglycan regulates its binding affinity for pleiotrophin/heparin binding growth-associated molecule

PTP zeta is a receptor-type protein-tyrosine phosphatase that is synthesized as a chondroitin sulfate proteoglycan and uses pleiotrophin as a ligand. The chondroitin sulfate portion of this receptor is essential for high affinity binding to pleiotrophin. Here, we purified phosphacan, which corresponds to the extracellular domain of PTP zeta, from postnatal day 7 (P7) and P12 rat cerebral cortex (PG-P7 and PG-P12, respectively) and from P20 rat whole brain (PG-P20). The chondroitin sulfate of these preparations displayed immunologically and compositionally different structures. In particular, only PG-P20 reacted with the monoclonal antibody MO-225, which recognizes chondroitin sulfate containing the GlcA(2S)beta 1-3GalNAc(6S) disaccharide unit (D unit). Analysis of the chondroitinase digestion products revealed that GlcA beta 1-3GalNAc(4S) disaccharide unit (A unit) was the major component in these preparations and that PG-P20 contained 1.3% D unit, which was not detected in PG-P7 and PG-P12. Interaction analysis using a surface plasmon resonance biosensor indicated that PG-P20 had approximately 5-fold stronger affinity for pleiotrophin (dissociation constant (KD) = 0.14 nM) than PG-P7 and PG-P12, although all these preparations showed similar low affinity binding to pleiotrophin after chondroitinase ABC digestion (KD = 1.4 approximately 1.6 nM). We also found that shark cartilage chondroitin sulfate D containing approximately 20% D unit bound to pleiotrophin with moderate affinity (KD = 2.7 nM), whereas whale cartilage chondroitin sulfate A showed no binding to this growth factor. These results suggest that variation of chondroitin sulfate plays important roles in the regulation of signal transduction in the brain.

Receptor protein tyrosine phosphatases in nervous system development

Receptor protein tyrosine phosphatases (RPTPs) are key regulators of neuronal morphogenesis in a variety of different vertebrate and invertebrate systems, yet the mechanisms by which these proteins regulate central nervous system development are poorly understood. In the past few years, studies have begun to outline possible models for RPTP function by demonstrating in vivo roles for RPTPs in axon outgrowth, guidance, and synaptogenesis. In addition, the crystal structures of several RPTPs have been solved, numerous downstream effectors of RPTP signaling have been identified, and a small number of RPTP ligands have been described. In this review, we focus on how RPTPs transduce signals from the extracellular environment to the cytoplasm, using a detailed comparative analysis of the different RPTP subfamilies. Focusing on the roles RPTPs play in the development of the central nervous system, we discuss how the elucidation of RPTP crystal structures, the biochemical analysis of phosphatase enzyme catalysis, and the characterization of complex signal transduction cascades downstream of RPTPs have generated testable models of RPTP structure and function.

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.