Brain-specific glycosylation enzyme GnT-IX maintains levels of protein tyrosine phosphatase receptor PTPRZ, thereby mediating glioma growth

Gliomas are the most prevalent primary tumor of the central nervous system. Despite advances in imaging technologies, neurosurgical techniques, and radiotherapy, a cure for high-grade glioma remains elusive. Several groups have reported that protein tyrosine phosphatase receptor type Z (PTPRZ) is highly expressed in glioblastoma, and that targeting PTPRZ attenuates tumor growth in mice. PTPRZ is modified with diverse glycan, including the PTPRZ-unique human natural killer-1 capped O-mannosyl core M2 glycans. However, the regulation and function of these unique glycans are unclear. Using CRISPR genome-editing technology, we first demonstrated that disruption of the PTPRZ gene in human glioma LN-229 cells resulted in profoundly reduced tumor growth in xenografted mice, confirming the potential of PTPRZ as a therapeutic target for glioma. Furthermore, multiple glycan analyses revealed that PTPRZ derived from glioma patients and from xenografted glioma expressed abundant levels of human natural killer-1-capped O-Man glycans via extrinsic signals. Finally, since deficiency of O-Man core M2 branching enzyme N-acetylglucosaminyltransferase IX (GnT-IX) was reported to reduce PTPRZ protein levels, we disrupted the GnT-IX gene in LN-229 cells and found a significant reduction of glioma growth both in vitro and in the xenograft model. These results suggest that the PTPR glycosylation enzyme GnT-IX may represent a promising therapeutic target for glioma.

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.

Pleiotrophin interacts with glycosaminoglycans in a highly flexible and adaptable manner

Pleiotrophin (PTN) is a potent mitogenic cytokine whose activities are controlled by its interactions with glycosaminoglycan (GAG). We examined the specificity of PTN for several types of GAG oligosaccharides. Our data indicate that the interaction of PTN with GAGs is dependent on the sulfation density of GAGs. Surprisingly, an acidic peptide also had similar interactions with PTN as GAGs. This shows that the interaction of PTN with anionic polymers is flexible and adaptable and that the charge density is the main determinant of the interaction. In addition, we show that PTN can compensate for the loss of its termini in interactions with heparin oligosaccharides, allowing it to maintain its affinity for GAGs in the absence of the termini. Taken together, these data provide valuable insight into the interactions of PTN with its proteoglycan receptors.

Pleiotrophin: Activity and mechanism

Pleiotrophin (PTN) is a potent mitogenic cytokine with a high affinity for the polysaccharide glycosaminoglycan (GAG). Although it is most strongly associated with neural development during embryogenesis and the neonatal period, its expression has also been linked to a plethora of other physiological events including cancer metastasis, angiogenesis, bone development, and inflammation. A considerable amount of research has been carried out to understand the mechanisms by which PTN regulates these events. In particular, PTN has now been shown to bind a diverse collection of receptors including many GAG-containing proteoglycans. These interactions lead to the activation of many intracellular kinases and, ultimately, activation and transformation of cells. Structural studies of PTN in complex with both GAG and domains from its non-proteoglycan receptors reveal a binding mechanism that relies on electrostatic interactions and points to PTN-induced receptor oligomerization as one of the possible ways PTN uses to control cellular functions.

Soluble protein tyrosine phosphatase receptor type Z (PTPRZ) in cerebrospinal fluid is a potential diagnostic marker for glioma

Background

High-grade glioma is the most pervasive and lethal of all brain malignancies. Despite advances in imaging technologies, discriminating between gliomas and other brain diseases such as multiple sclerosis (MS) often requires brain biopsy. Several reports show that protein tyrosine phosphatase receptor Z (PTPRZ) is highly expressed in glioblastoma, and we found that a soluble cleaved form of PTPRZ (sPTPRZ) was present in the cerebrospinal fluid (CSF). The aim of this study was to determine whether the sPTPRZ level in CSF has utility as a diagnostic marker for glioma.

Methods

Microarray datasets from normal brain tissue and brain tumors were obtained from the Gene Expression Omnibus. PTPRZ protein expression in clinical specimens was evaluated by immunohistochemistry. Semiquantitative western blotting was used to measure sPTPRZ levels in CSF samples from patients with glioma, schwannoma, MS, or nontumor disorders.

Results

Expression of PTPRZ mRNA and protein was markedly increased in glioblastoma, astrocytoma, oligodendroglioma, and schwannoma tissues compared with control brain tissue. sPTPRZ was present at significantly elevated levels in the CSF of patients with glioma (grades 1–4), but not in patients with schwannoma or MS, compared with the control samples. Receiver operating characteristic curve analysis showed that sPTPRZ in CSF could discriminate between glioma and MS patients (area under the curve 0.9676; P < .0001).

Conclusions

sPTPRZ in CSF is a promising diagnostic biomarker for glioma and could reduce the need for a surgical biopsy.

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.

Connecting Metainflammation and Neuroinflammation Through the PTN-MK-RPTPβ/ζ Axis: Relevance in Therapeutic Development

Inflammation is a common factor of pathologies such as obesity, type 2 diabetes or neurodegenerative diseases. Chronic inflammation is considered part of the pathogenic mechanisms of different disorders associated with aging. Interestingly, peripheral inflammation and the associated metabolic alterations not only facilitate insulin resistance and diabetes but also neurodegenerative disorders. Therefore, the identification of novel pathways, common to the development of these diseases, which modulate the immune response and signaling is key. It will provide highly relevant information to advance our knowledge of the multifactorial process of aging, and to establish new biomarkers and/or therapeutic targets to counteract the underlying chronic inflammatory processes. One novel pathway that regulates peripheral and central immune responses is triggered by the cytokines pleiotrophin (PTN) and midkine (MK), which bind its receptor, Receptor Protein Tyrosine Phosphatase (RPTP) β/ζ, and inactivate its phosphatase activity. In this review, we compile a growing body of knowledge suggesting that PTN and MK modulate the immune response and/or inflammation in different pathologies characterized by peripheral inflammation associated with insulin resistance, such as aging, and in central disorders characterized by overt neuroinflammation, such as neurodegenerative diseases and endotoxemia. Evidence strongly suggests that regulation of the PTN and MK signaling pathways may provide new therapeutic opportunities particularly in those neurological disorders characterized by increased PTN and/or MK cerebral levels and neuroinflammation. Importantly, we discuss existing therapeutics, and others being developed, that modulate these signaling pathways, and their potential use in pathologies characterized by overt neuroinflammation.

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.