Plasmin-mediated processing of protein tyrosine phosphatase receptor type Z in the mouse brain

Protein Tyrosine Phosphatase Receptor Zeta 1 as a Potential Target in Cancer Therapy and Diagnosis

Protein tyrosine phosphatase receptor zeta 1 (PTPRZ1) is a type V transmembrane tyrosine phosphatase that is highly expressed during embryonic development, while its expression during adulthood is limited. PTPRZ1 is highly detected in the central nervous system, affecting oligodendrocytes' survival and maturation. In gliomas, PTPRZ1 expression is significantly upregulated and is being studied as a potential cancer driver and as a target for therapy. PTPRZ1 expression is also increased in other cancer types, but there are no data on the potential functional significance of this finding. On the other hand, low PTPRZ1 expression seems to be related to a worse prognosis in some cancer types, suggesting that in some cases, it may act as a tumor-suppressor gene. These discrepancies may be due to our limited understanding of PTPRZ1 signaling and tumor microenvironments. In this review, we present evidence on the role of PTPRZ1 in angiogenesis and cancer and discuss the phenomenal differences among the different types of cancer, depending on the regulation of its tyrosine phosphatase activity or ligand binding. Clarifying the involved signaling pathways will lead to its efficient exploitation as a novel therapeutic target or as a biomarker, and the development of proper therapeutic approaches.

Proteolysis: a key post-translational modification regulating proteoglycans

Proteoglycans are composite molecules comprising a protein backbone, i.e., the core protein, with covalently attached glycosaminoglycan chains of distinct chemical types. Most proteoglycans are secreted or attached to the cell membrane. Their specialized structures, binding properties, and biophysical attributes underlie diverse biological roles, which include modulation of tissue mechanics, cell adhesion, and the sequestration and regulated release of morphogens, growth factors, and cytokines. As an irreversible post-translational modification, proteolysis has a profound impact on proteoglycan function, abundance, and localization. Proteolysis is required for molecular maturation of some proteoglycans, clearance of extracellular matrix proteoglycans during tissue remodeling, generation of bioactive fragments from proteoglycans, and ectodomain shedding of cell-surface proteoglycans. Genetic evidence shows that proteoglycan core protein proteolysis is essential for diverse morphogenetic events during embryonic development. In contrast, dysregulated proteoglycan proteolysis contributes to osteoarthritis, cardiovascular disorders, cancer, and inflammation. Proteolytic fragments of perlecan, versican, aggrecan, brevican, collagen XVIII, and other proteoglycans are associated with independent biological activities as so-called matrikines. Yet, proteoglycan proteolysis has been investigated to only a limited extent to date. Here, we review the actions of proteases on proteoglycans and illustrate their functional impact with several examples. We discuss the applications and limitations of strategies used to define cleavage sites in proteoglycans and explain how proteoglycanome-wide proteolytic mapping, which is desirable to fully understand the impact of proteolysis on proteoglycans, can be facilitated by integrating classical proteoglycan isolation methods with mass spectrometry-based proteomics.

Protein Tyrosine Phosphatase Receptor Type Z in Central Nervous System Disease

Gliomas are among the most common tumors of the central nervous system and include highly malignant subtypes, such as glioblastoma, which are associated with poor prognosis. Effective treatments are therefore urgently needed. Despite the recent advances in neuroimaging technologies, differentiating gliomas from other brain diseases such as multiple sclerosis remains challenging in some patients, and often requires invasive brain biopsy. Protein tyrosine phosphatase receptor type Z (PTPRZ) is a heavily glycosylated membrane protein that is highly expressed in the central nervous system. Several reports analyzing mouse tumor models suggest that PTPRZ may have potential as a therapeutic target for gliomas. A soluble cleaved form of PTPRZ (sPTPRZ) in the cerebrospinal fluid is markedly upregulated in glioma patients, making it another promising diagnostic biomarker. Intriguingly, PTPRZ is also involved in the process of remyelination in multiple sclerosis. Indeed, lowered PTPRZ glycosylation by deletion of the glycosyltransferase gene leads to reduced astrogliosis and enhanced remyelination in mouse models of demyelination. Here, we review the expression, molecular structure, and biological roles of PTPRZ. We also discuss glioma and demyelinating diseases, as well as the pathological role of PTPRZ and its application as a diagnostic marker and therapeutic target.

A head-to-toe dimerization has physiological relevance for ligand-induced inactivation of protein tyrosine receptor type Z

Protein-tyrosine phosphatase (PTPase) receptor type Z (PTPRZ) has two receptor isoforms, PTPRZ-A and -B, containing tandem intracellular PTP-D1 and -D2 domains, with only D1 being active. Pleiotrophin (PTN) binding to the extracellular PTPRZ region leads to inactivation of its PTPase activity, thereby facilitating oligodendrocyte precursor cell (OPC) differentiation and myelination in the central nervous system. However, the mechanisms responsible for PTN-induced PTPRZ inactivation remain unclear. We herein report that the crystal structure of the intracellular region of PTPRZ (PTPRZ-ICR) shows a "head-to-toe"-type dimer conformation, with D2 masking the catalytic site of D1. MS analyses revealed that PTPRZ-ICR proteins remain in monomer-dimer equilibrium in aqueous solution and that a substrate-derived inhibitory peptide or competitive inhibitor (SCB4380) specifically bind to the monomer form in a 1:1 ratio. A D2 deletion (ΔD2) or dimer interface mutation (DDKK) disrupted dimer formation, but SCB4380 binding was maintained. Similar to WT PTPRZ-B, monomer-biased PTPRZ-B-ΔD2 and PTPRZ-B-DDKK variants efficiently dephosphorylated p190RhoGAP at Tyr-1105 when co-expressed in BHK-21 cells. The catalytic activities of these variants were not suppressed by PTN treatment, but were inhibited by the cell-permeable PTPase inhibitor NAZ2329. Of note, the PTN treatment did not enhance OPC differentiation in primary cultured glial cells from ΔD2 or PTPase-inactive PTPRZ-B (CS) mutant knock-in mice. Our results thus indicate that PTN-induced PTPRZ inactivation results from dimer formation of the intracellular tandem PTP domains in a head-to-toe configuration, which is physiologically relevant to the control of OPC differentiation in vivo.

Mice deficient in protein tyrosine phosphatase receptor type Z (PTPRZ) show reduced responsivity to methamphetamine despite an enhanced response to novelty

Methamphetamine (METH), a commonly abused drug, elevates extracellular dopamine (DA) levels by inducing DA efflux through the DA transporter (DAT). Emerging evidence in rodent models suggests that locomotor responses to a novel inescapable open field may predict behavioral responses to abused drugs; METH produces more potent stimulant effects in high responders to novelty than in low responders. We herein found that mice deficient in protein tyrosine phosphatase receptor type Z (Ptprz-KO) exhibited an enhanced behavioral response to novelty; however, METH-induced hyperlocomotion was significantly lower in Ptprz-KO than in wild-type mice when METH was administered at a non-toxic dose of 1 mg per kg body weight (bdw). Single-cell RT-PCR revealed that the majority of midbrain DA neurons expressed PTPRZ. No histological alterations were observed in the mesolimbic or nigrostriatal dopaminergic pathways in Ptprz-KO brains; however, a significant decrease was noted in brain DA turnover, suggesting functional alterations. In vivo microdialysis experiments revealed that METH-evoked DA release in the nucleus accumbens was significantly lower in Ptprz-KO mice than in wild-type mice. Consistent with this result, Ptprz-KO mice showed significantly fewer cell surface DAT as well as weaker DA uptake activity in striatal synaptosomes prepared 1 hr after the administration of METH than wild-type mice, while no significant differences were observed in the two groups treated with saline. These results indicate that the high response phenotype of Ptprz-KO mice to novelty may not be simply attributed to hyper-dopaminergic activity, and that deficits in PTPRZ reduce the effects of METH by reducing DAT activity.

RIP at the Synapse and the Role of Intracellular Domains in Neurons

Regulated intramembrane proteolysis (RIP) occurs in a cell when transmembrane proteins are cleaved by intramembrane proteases such as secretases to generate soluble protein fragments in the extracellular environment and the cytosol. In the cytosol, these soluble intracellular domains (ICDs) have local functions near the site of cleavage or in many cases, translocate to the nucleus to modulate gene expression. While the mechanism of RIP is relatively well studied, the fate and function of ICDs for most substrate proteins remain poorly characterized. In neurons, RIP occurs in various subcellular compartments including at the synapse. In this review, we summarize current research on RIP in neurons, focusing specifically on synaptic proteins where the presence and function of the ICDs have been reported. We also briefly discuss activity-driven processing of RIP substrates at the synapse and the cellular machinery that support long-distance transport of ICDs from the synapse to the nucleus. Finally, we describe future challenges in this field of research in the context of understanding the contribution of ICDs in neuronal function.

Behavioral and neurological analyses of adult mice carrying null and distinct loss-of-receptor function mutations in protein tyrosine phosphatase receptor type Z (PTPRZ)

Protein tyrosine phosphatase receptor type Z (PTPRZ) is preferentially expressed in the central nervous system as two transmembrane receptor isoforms PTPRZ-A/B and one secretory isoform PTPRZ-S. Ptprz-knockout mice lacking the expression of all three isoforms show behavioral, learning, and neurological abnormalities, including increased exploratory activities to novelty, deficits in spatial and contextual learning, and reduced responses to methamphetamine, relative to wild-type mice. To investigate whether PTPRZ isoforms play distinct physiological roles, we herein performed behavioral studies on two knock-in mouse lines: One expresses the catalytically inactive Cys-1930 to Ser (CS) mutants of PTPRZ-A/B, while the other generated in the present study expresses catalytically active mutants of PTPRZ-A/B lacking the negative regulatory PTP-D2 domain and C-terminal PDZ-binding motif (ΔD2) instead of wild-type PTPRZ-A/-B. In contrast to Ptprz-knockout mice, neither increased responses to novelty in the open field nor memory impairments in the inhibitory-avoidance task were observed in Ptprz-CS or Ptprz-ΔD2 mice. However, the effects of methamphetamine on locomotor activity were significantly weaker in Ptprz-KO mice and CS mutant mice than in wild-type mice, but were normal in ΔD2 mutant mice. Furthermore, microdialysis experiments revealed that methamphetamine-evoked dopamine release in the nucleus accumbens was reduced in Ptprz-KO mice and CS mutant mice. These results suggest that the extracellular region of PTPRZ, including the secretory isoform, is crucial for behavioral responses to novelty and the formation of aversive memories, whereas the PTPase activities of PTPRZ receptor isoforms are involved in regulating the dopaminergic system.

The PTN-PTPRZ signal activates the AFAP1L2-dependent PI3K-AKT pathway for oligodendrocyte differentiation: Targeted inactivation of PTPRZ activity in mice

Protein tyrosine phosphatase receptor type Z (PTPRZ) maintains oligodendrocyte precursor cells (OPCs) in an undifferentiated state. The inhibition of PTPase by its ligand pleiotrophin (PTN) promotes OPC differentiation; however, the substrate molecules of PTPRZ involved in the differentiation have not yet been elucidated in detail. We herein demonstrated that the tyrosine phosphorylation of AFAP1L2, paxillin, ERBB4, GIT1, p190RhoGAP, and NYAP2 was enhanced in OPC-like OL1 cells by a treatment with PTN. AFAP1L2, an adaptor protein involved in the PI3K-AKT pathway, exhibited the strongest response to PTN. PTPRZ dephosphorylated AFAP1L2 at tyrosine residues in vitro and in HEK293T cells. In OL1 cells, the knockdown of AFAP1L2 or application of a PI3K inhibitor suppressed cell differentiation as well as the PTN-induced phosphorylation of AKT and mTOR. We generated a knock-in mouse harboring a catalytically inactive Cys to Ser (CS) mutation in the PTPase domain. The phosphorylation levels of AFAP1L2, AKT, and mTOR were higher, and the expression of oligodendrocyte markers, including myelin basic protein (MBP) and myelin regulatory factor (MYRF), was stronger in CS knock-in brains than in wild-type brains on postnatal day 10; however, these differences mostly disappeared in the adult stage. Adult CS knock-in mice exhibited earlier remyelination after cuprizone-induced demyelination through the accelerated differentiation of OPCs. These phenotypes in CS knock-in mice were similar to those in Ptprz-deficient mice. Therefore, we conclude that the PTN-PTPRZ signal stimulates OPC differentiation partly by enhancing the tyrosine phosphorylation of AFAP1L2 in order to activate the PI3K-AKT pathway.

GlcNAc6ST3 is a keratan sulfate sulfotransferase for the protein-tyrosine phosphatase PTPRZ in the adult brain

Keratan sulfate (KS) is a carbohydrate side chain covalently attached to extracellular proteoglycans. KS is composed of disaccharide units of 6-sulfated N-acetylglucosamine (GlcNAc) and galactose. We have previously shown that GlcNAc-6-O-sulfotransferase (GlcNAc6ST) 1 encoded by Chst2 is an enzyme necessary for the synthesis of GlcNAc-6-sulfated KS chains that are required for neuronal plasticity in the visual cortex of the mouse brain during the critical period, but not in adulthood. Here, we show that GlcNAc-6-sulfated KS recognized by the R-10G anti-KS antibody, of which the minimum epitope structure is Galß1-4GlcNAc(6S)ß1-3Galß1-4GlcNAc(6S), distributes diffusely in neuropils and presents densely in close proximity to the perineuronal region of the perineuronal net (PNN)-positive neurons in the adult visual cortex. Surprisingly, GlcNAc6ST3, which was discovered as an intestinal GlcNAc6ST encoded by Chst5, is a major brain KS sulfotransferase expressed in oligodendrocytes in adulthood. Moreover, we identified an isoform of the protein-tyrosine phosphatase PTPRZ as a R-10G-reactive KS proteoglycan. These results indicate that GlcNAc6ST3 may play a role in synthesis of a component of PNN in the adult brain, and that the KS-modified isoform of PTPRZ encoded by Ptprz1 could be an extracellular molecule associated with PNNs.

Keratan sulfate (KS)-proteoglycans and neuronal regulation in health and disease: the importance of KS-glycodynamics and interactive capability with neuroregulatory ligands

Compared to the other classes of glycosaminoglycans (GAGs), that is, chondroitin/dermatan sulfate, heparin/heparan sulfate and hyaluronan, keratan sulfate (KS), have the least known of its interactive properties. In the human body, the cornea and the brain are the two most abundant tissue sources of KS. Embryonic KS is synthesized as a linear poly-N-acetyllactosamine chain of d-galactose-GlcNAc repeat disaccharides which become progressively sulfated with development, sulfation of GlcNAc is more predominant than galactose. KS contains multi-sulfated high-charge density, monosulfated and non-sulfated poly-N-acetyllactosamine regions and thus is a heterogeneous molecule in terms of chain length and charge distribution. A recent proteomics study on corneal KS demonstrated its interactivity with members of the Slit-Robbo and Ephrin-Ephrin receptor families and proteins which regulate Rho GTPase signaling and actin polymerization/depolymerization in neural development and differentiation. KS decorates a number of peripheral nervous system/CNS proteoglycan (PG) core proteins. The astrocyte KS-PG abakan defines functional margins of the brain and is up-regulated following trauma. The chondroitin sulfate/KS PG aggrecan forms perineuronal nets which are dynamic neuroprotective structures with anti-oxidant properties and roles in neural differentiation, development and synaptic plasticity. Brain phosphacan a chondroitin sulfate, KS, HNK-1 PG have roles in neural development and repair. The intracellular microtubule and synaptic vesicle KS-PGs MAP1B and SV2 have roles in metabolite transport, storage, and export of neurotransmitters and cytoskeletal assembly. MAP1B has binding sites for tubulin and actin through which it promotes cytoskeletal development in growth cones and is highly expressed during neurite extension. The interactive capability of KS with neuroregulatory ligands indicate varied roles for KS-PGs in development and regenerative neural processes.

Protamine neutralizes chondroitin sulfate proteoglycan-mediated inhibition of oligodendrocyte differentiation

Chondroitin sulfate proteoglycans (CSPGs), which are enriched in demyelinating plaques in neurodegenerative diseases, such as multiple sclerosis (MS), impair remyelination by inhibiting the migration and differentiation of oligodendrocyte precursor cells (OPCs) in the central nervous system (CNS). We herein show that protamine (PRM, also known as a heparin antagonist) effectively neutralizes the inhibitory activities of CSPGs, thereby enhancing OPC differentiation and (re)myelination in mice. Cell-based assays using mouse OPC-like OL1 cells revealed that the PRM treatment exerted masking effects on extracellular CSPGs and improved oligodendrocyte differentiation on inhibitory CSPG-coated substrates. PRM also bound to the extracellular region of protein tyrosine phosphatase receptor type Z (PTPRZ), a membrane-spanning CSPG predominantly expressed in OPCs, and functioned as a ligand mimetic of PTPRZ, thereby suppressing its negative regulatory activity on oligodendrocyte differentiation. In primary cultures, the differentiation of OPCs from wild-type and Ptprz-deficient mice was equally enhanced by PRM. Moreover, the intranasal administration of PRM accelerated myelination in the developing mouse brain, and its intracerebroventricular administration stimulated remyelination after cuprizone-induced demyelination. These results indicate that PRM has CSPG-neutralizing activity which promotes oligodendrocyte differentiation under developmental and morbid conditions.

Identification of novel splicing variants of protein tyrosine phosphatase receptor type Z

Protein tyrosine phosphatase receptor type Z (PTPRZ, also known as PTPζ or RPTPβ) is preferentially expressed in the central nervous system (CNS). PTPRZ plays important roles during development and adulthood in CNS myelination, learning and memory. Three splicing isoforms for PTPRZ have been identified to date: two receptor type isoforms, PTPRZ-A and PTPRZ-B, and one secretory isoform, PTPRZ-S. We herein identified novel PTPRZ receptor sub-isoforms without a seven-amino acid sequence encoded by exon 16. This sequence forms a part of the helix-turn-helix segment called the 'wedge' structure, which is located at the N-terminal region in the membrane-proximal protein tyrosine phosphatase domain. In contrast to conventional receptor isoforms with uniform expression, the deleted isoforms were expressed in the brain, but not in the retina, indicating the tissue-specific splicing of exon 16. Biochemical analyses of PTPRZ intracellular regions revealed differences in the characteristics of the deleted form, namely, stronger binding activity to postsynaptic density protein 95 (PSD95) and greater enrichment in the postsynaptic density fraction than the full-length form. Furthermore, the exon 16-deleted form exhibited higher catalytic efficiency in vitro. These results suggest that sub-isoforms of PTPRZ have different functions because of variations in the wedge structure.

Targeting PTPRZ inhibits stem cell-like properties and tumorigenicity in glioblastoma cells

The R5 subfamily of receptor-type protein tyrosine phosphatases (RPTPs) comprises PTPRZ and PTPRG. A recent study on primary human glioblastomas suggested a close association between PTPRZ1 (human PTPRZ) expression and cancer stemness. However, the functional roles of PTPRZ activity in glioma stem cells have remained unclear. In the present study, we found that sphere-forming cells from the rat C6 and human U251 glioblastoma cell lines showed high expression levels of PTPRZ-B, the short receptor isoform of PTPRZ. Stable PTPRZ knockdown altered the expression levels of stem cell transcription factors such as SOX2, OLIG2, and POU3F2 and decreased the sphere-forming abilities of these cells. Suppressive effects on the cancer stem-like properties of the cells were also observed following the knockdown of PTPRG. Here, we identified NAZ2329, a cell-permeable small molecule that allosterically inhibits both PTPRZ and PTPRG. NAZ2329 reduced the expression of SOX2 in C6 and U251 cells and abrogated the sphere-forming abilities of these cells. Tumor growth in the C6 xenograft mouse model was significantly slower with the co-treatment of NAZ2329 with temozolomide, an alkylating agent, than with the individual treatments. These results indicate that pharmacological inhibition of R5 RPTPs is a promising strategy for the treatment of malignant gliomas.

Role of Chondroitin Sulfate (CS) Modification in the Regulation of Protein-tyrosine Phosphatase Receptor Type Z (PTPRZ) Activity: PLEIOTROPHIN-PTPRZ-A SIGNALING IS INVOLVED IN OLIGODENDROCYTE DIFFERENTIATION

Protein-tyrosine phosphatase receptor type Z (PTPRZ) is predominantly expressed in the developing brain as a CS proteoglycan. PTPRZ has long (PTPRZ-A) and short type (PTPRZ-B) receptor forms by alternative splicing. The extracellular CS moiety of PTPRZ is required for high-affinity binding to inhibitory ligands, such as pleiotrophin (PTN), midkine, and interleukin-34; however, its functional significance in regulating PTPRZ activity remains obscure. We herein found that protein expression of CS-modified PTPRZ-A began earlier, peaking at approximately postnatal days 5-10 (P5-P10), and then that of PTN peaked at P10 at the developmental stage corresponding to myelination onset in the mouse brain. Ptn-deficient mice consistently showed a later onset of the expression of myelin basic protein, a major component of the myelin sheath, than wild-type mice. Upon ligand application, PTPRZ-A/B in cultured oligodendrocyte precursor cells exhibited punctate localization on the cell surface instead of diffuse distribution, causing the inactivation of PTPRZ and oligodendrocyte differentiation. The same effect was observed with the removal of CS chains with chondroitinase ABC but not polyclonal antibodies against the extracellular domain of PTPRZ. These results indicate that the negatively charged CS moiety prevents PTPRZ from spontaneously clustering and that the positively charged ligand PTN induces PTPRZ clustering, potentially by neutralizing electrostatic repulsion between CS chains. Taken altogether, these data indicate that PTN-PTPRZ-A signaling controls the timing of oligodendrocyte precursor cell differentiation in vivo, in which the CS moiety of PTPRZ receptors maintains them in a monomeric active state until its ligand binding.

Inactivation of Protein Tyrosine Phosphatase Receptor Type Z by Pleiotrophin Promotes Remyelination through Activation of Differentiation of Oligodendrocyte Precursor Cells

Multiple sclerosis (MS) is a progressive neurological disorder associated with myelin destruction and neurodegeneration. Oligodendrocyte precursor cells (OPCs) present in demyelinated lesions gradually fail to differentiate properly, so remyelination becomes incomplete. Protein tyrosine phosphatase receptor type Z (PTPRZ), one of the most abundant protein tyrosine phosphatases expressed in OPCs, is known to suppress oligodendrocyte differentiation and maintain their precursor cell stage. In the present study, we examined the in vivo mechanisms for remyelination using a cuprizone-induced demyelination model. Ptprz-deficient and wild-type mice both exhibited severe demyelination and axonal damage in the corpus callosum after cuprizone feeding. The similar accumulation of OPCs was observed in the lesioned area in both mice; however, remyelination was significantly accelerated in Ptprz-deficient mice after the removal of cuprizone. After demyelination, the expression of pleiotrophin (PTN), an inhibitory ligand for PTPRZ, was transiently increased in mouse brains, particularly in the neurons involved, suggesting its role in promoting remyelination by inactivating PTPRZ activity. In support of this view, oligodendrocyte differentiation was augmented in a primary culture of oligodendrocyte-lineage cells from wild-type mice in response to PTN. In contrast, these cells from Ptprz-deficient mice showed higher oligodendrocyte differentiation without PTN and differentiation was not enhanced by its addition. We further demonstrated that PTN treatment increased the tyrosine phosphorylation of p190 RhoGAP, a PTPRZ substrate, using an established line of OPCs. Therefore, PTPRZ inactivation in OPCs by PTN, which is secreted from demyelinated axons, may be the mechanism responsible for oligodendrocyte differentiation during reparative remyelination in the CNS.

SIGNIFICANCE STATEMENT

Multiple sclerosis (MS) is an inflammatory disease of the CNS that destroys myelin, the insulation that surrounds axons. Associated damages to oligodendrocytes (the cells that produce myelin) and nerve fibers produce neurological disability. Most patients with MS have an initial relapsing-remitting course for 5-15 years. Remyelination during the early stages of the disease process has been documented; however, the molecular mechanism underlying remyelination has not been understood. Protein tyrosine phosphatase receptor type Z (PTPRZ) is a receptor-like protein tyrosine phosphatase preferentially expressed in the CNS. This study shows that pleiotrophin, an inhibitory ligand for PTPRZ, is transiently expressed and released from demyelinated neurons to inactivate PTPRZ in oligodendrocyte precursor cells present in the lesioned part, thereby allowing their differentiation for remyelination.

Protein tyrosine phosphatase receptor type z negatively regulates oligodendrocyte differentiation and myelination

Background

Fyn tyrosine kinase-mediated down-regulation of Rho activity through activation of p190RhoGAP is crucial for oligodendrocyte differentiation and myelination. Therefore, the loss of function of its counterpart protein tyrosine phosphatase (PTP) may enhance myelination during development and remyelination in demyelinating diseases. To test this hypothesis, we investigated whether Ptprz, a receptor-like PTP (RPTP) expressed abuntantly in oligodendrocyte lineage cells, is involved in this process, because we recently revealed that p190RhoGAP is a physiological substrate for Ptprz.

Methodology/Principal Findings

We found an early onset of the expression of myelin basic protein (MBP), a major protein of the myelin sheath, and early initiation of myelination in vivo during development of the Ptprz-deficient mouse, as compared with the wild-type. In addition, oligodendrocytes appeared earlier in primary cultures from Ptprz-deficient mice than wild-type mice. Furthermore, adult Ptprz-deficient mice were less susceptible to experimental autoimmune encephalomyelitis (EAE) induced by active immunization with myelin/oligodendrocyte glycoprotein (MOG) peptide than were wild-type mice. After EAE was induced, the tyrosine phosphorylation of p190RhoGAP increased significantly, and the EAE-induced loss of MBP was markedly suppressed in the white matter of the spinal cord in Ptprz-deficient mice. Here, the number of T-cells and macrophages/microglia infiltrating into the spinal cord did not differ between the two genotypes after MOG immunization. All these findings strongly support the validity of our hypothesis.

Conclusions/Significance

Ptprz plays a negative role in oligodendrocyte differentiation in early central nervous system (CNS) development and remyelination in demyelinating CNS diseases, through the dephosphorylation of substrates such as p190RhoGAP.

Receptor-type tyrosine phosphatase ligands: looking for the needle in the haystack

Reversible protein phosphorylation plays a pivotal role in intercellular communication. Together with protein tyrosine kinases, protein tyrosine phosphatases (PTPs) are involved in the regulation of key cellular processes by controlling the phosphorylation levels of diverse effectors. Among PTPs, receptor-like protein tyrosine phosphatases (RPTPs) are involved in important developmental processes, particularly in the formation of the nervous system. Until recently, few ligands had been identified for RPTPs, making it difficult to grasp the effects these receptors have on cellular processes, as well as the mechanisms through which their functions are mediated. However, several potential RPTP ligands have now been identified to provide us with unparalleled insights into RPTP function. In this review, we focus on the nature and biological outcomes of these extracellular interactions between RPTPs and their associated ligands.

Proteolytic processing of the protein tyrosine phosphatase α extracellular domain is mediated by ADAM17/TACE

The receptor protein tyrosine phosphatase alpha (PTPα) is involved in the regulation of tyrosine kinases like the Src kinase and the insulin receptor. As with other PTPs, its function is determined by alternative splicing, dimerisation, phosphorylation and proteolytical processing. PTPα is cleaved by calpain in its intracellular domain, which decreases its potential to dephosphorylate Src kinase. Here, we demonstrate that PTPα is also processed in the extracellular domain. Extracellular processing was exclusively found for a splice variant containing an extra nine amino acid insert three residues amino-terminal from the transmembrane domain. Processing was sensitive to the metalloprotease-inhibitor Batimastat, and CHO-M2 cells lacking a disintegrin and metalloproteinase 17 (ADAM17; tumor-necrosis-factor α converting enzyme) activity were not able to cleave PTPα. After transient overexpression of ADAM17 and PTPα in these cells, processing was restored, proving that ADAM17 is involved in this process. Further characterization of the consequences of processing revealed that dephosphorylation of the insulin receptor or activation of Src was not affected but focus formation was reduced. We conclude that extracellular proteolytic processing is a novel mechanism for PTPα regulation.

Proteins linked to spatial memory formation of CD1 mice in the multiple T-maze

In own previous work CD1 mice were tested in the Multiple T-maze (MTM), a robust land maze allowing determination of latency to reach the goal box with food reward and to evaluate correct decisions made on the way to the goal box. Herein, hippocampi of these animals were used for the current study with the aim to investigate differences in protein levels between trained and yoked mice and, moreover, to determine differences in protein levels between trained and yoked mice with and without memory formation in the MTM. Three training sessions were carried out for four training days each, followed by probe trials on Days 5 and 12. Good and no-performers in the MTM were separated based on means and median of latency to reach the goal box on probe trial Day 12. Six hours following the probe trial on Day 12, animals were sacrificed and hippocampi were taken. Proteins were extracted and run on two-dimensional gel electrophoresis, spots were quantified and differentially expressed proteins were identified by mass spectrometry using an ion trap. Levels of 17 proteins were significantly different in trained vs. yoked mice. Seven proteins were differentially expressed comparing trained vs. yoked mice from good and no-performers. A series of proteins were significantly correlated with latency and may link these proteins to spatial memory formation. Differential protein expression in trained vs. yoked mice and in good and no-performers may allow insight into spatial memory formation as well as represent tentative pharmacological targets.

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.

Tissue-type plasminogen activator induces plasmin-dependent proteolysis of intracellular neuronal nitric oxide synthase

BACKGROUND INFORMATION

Despite its pro-fibrinolytic activity, tPA (tissue plasminogen activator) is a serine protease known to influence a number of physiological and pathological functions in the central nervous system. Accordingly, tPA was reported to mediate some of its functions in the central nervous system through NMDA (N-methyl-D-aspartate) receptors, LRP (low-density lipoprotein receptor-related protein) or annexin II.

RESULTS

We provide here both in vitro and in vivo evidence that tPA could mediate proteolysis and subsequent delocalization of neuronal nitric oxide synthase, thereby reducing endogenous neuronal nitric oxide release. We also demonstrate that although this effect is independent of NMDA receptors, LRP signalling and calpain-mediated proteolysis, it is dependent on the ability of tPA to promote the conversion of plasminogen into plasmin.

CONCLUSION

Altogether, these results demonstrate a new function for tPA in the central nervous system, which most likely contributes to its pleiotropic functions.

A novel molecular diagnostic of glioblastomas: detection of an extracellular fragment of protein tyrosine phosphatase mu

We recently found that normal human brain and low-grade astrocytomas express the receptor protein tyrosine phosphatase mu (PTPmu) and that the more invasive astrocytomas, glioblastoma multiforme (GBM), downregulate full-length PTPmu expression. Loss of PTPmu expression in GBMs is due to proteolytic cleavage that generates an intracellular and potentially a cleaved and released extracellular fragment of PTPmicro. Here, we identify that a cleaved extracellular fragment containing the domains required for PTPmicro-mediated adhesion remains associated with GBM tumor tissue. We hypothesized that detection of this fragment would make an excellent diagnostic tool for the localization of tumor tissue within the brain. To this end, we generated a series of fluorescently tagged peptide probes that bind the PTPmu fragment. The peptide probes specifically recognize GBM cells in tissue sections of surgically resected human tumors. To test whether the peptide probes are able to detect GBM tumors in vivo, the PTPmu peptide probes were tested in both mouse flank and intracranial xenograft human glioblastoma tumor model systems. The glial tumors were molecularly labeled with the PTPmu peptide probes within minutes of tail vein injection using the Maestro FLEX In Vivo Imaging System. The label was stable for at least 3 hours. Together, these results indicate that peptide recognition of the PTPmu extracellular fragment provides a novel molecular diagnostic tool for detection of human glioblastomas. Such a tool has clear translational applications and may lead to improved surgical resections and prognosis for patients with this devastating disease.

Metalloproteinase- and gamma-secretase-mediated cleavage of protein-tyrosine phosphatase receptor type Z

Protein-tyrosine phosphatase receptor type Z (Ptprz) is preferentially expressed in the brain as a major chondroitin sulfate proteoglycan. Three splicing variants, two receptor isoforms and one secretory isoform, are known. Here, we show that the extracellular region of the receptor isoforms of Ptprz are cleaved by metalloproteinases, and subsequently the membrane-tethered fragment is cleaved by presenilin/gamma-secretase, releasing its intracellular region into the cytoplasm; of note, the intracellular fragment of Ptprz shows nuclear localization. Administration of GM6001, an inhibitor of metalloproteinases, to mice demonstrated the metalloproteinase-mediated cleavage of Ptprz under physiological conditions. Furthermore, we identified the cleavage sites in the extracellular juxtamembrane region of Ptprz by tumor necrosis factor-alpha converting enzyme and matrix metalloproteinase 9. This is the first evidence of the metalloproteinase-mediated processing of a receptor-like protein-tyrosine phosphatase in the central nervous system.