Receptor protein tyrosine phosphatases regulate neural development and axon guidance

Analysis of Receptor-Type Protein Tyrosine Phosphatase Extracellular Regions with Insights from AlphaFold

The receptor-type protein tyrosine phosphatases (RPTPs) are involved in a wide variety of physiological functions which are mediated via their diverse extracellular regions. They play key roles in cell-cell contacts, bind various ligands and are regulated by dimerization and other processes. Depending on the subgroup, they have been described as everything from 'rigid rods' to 'floppy tentacles'. Here, we review current experimental structural knowledge on the extracellular region of RPTPs and draw on AlphaFold structural predictions to provide further insights into structure and function of these cellular signalling molecules, which are often mutated in disease and are recognised as drug targets. In agreement with experimental data, AlphaFold predicted structures for extracellular regions of R1, and R2B subgroup RPTPs have an extended conformation, whereas R2B RPTPs are twisted, reflecting their high flexibility. For the R3 PTPs, AlphaFold predicts that members of this subgroup adopt an extended conformation while others are twisted, and that certain members, such as CD148, have one or more large, disordered loop regions in place of fibronectin type 3 domains suggested by sequence analysis.

A novel binding pocket in the D2 domain of protein tyrosine phosphatase mu (PTPmu) guides AI screen to identify small molecules that modulate tumour cell adhesion, growth and migration

Approximately 40% of people will get cancer in their lifetime in the US, and 20% are predicted to die from the condition when it is invasive and metastatic. Targeted screening for drugs that interact with proteins that drive cancer cell growth and migration can lead to new therapies. We screened molecular libraries with the AtomNet® AI-based drug design tool to identify compounds predicted to interact with the cytoplasmic domain of protein tyrosine phosphatase mu. Protein tyrosine phosphatase mu (PTPmu) is proteolytically downregulated in cancers such as glioblastoma generating fragments that stimulate cell survival and migration. Aberrant nuclear localization of PTPmu intracellular fragments drives cancer progression, so we targeted a predicted drug-binding site between the two cytoplasmic phosphatase domains we termed a D2 binding pocket. The function of the D2 domain is controversial with various proposed regulatory functions, making the D2 domain an attractive target for the development of allosteric drugs. Seventy-five of the best-scoring and chemically diverse computational hits predicted to interact with the D2 binding pocket were screened for effects on tumour cell motility and growth in 3D culture as well as in a direct assay for PTPmu-dependent adhesion. We identified two high-priority hits that inhibited the migration and glioma cell sphere formation of multiple glioma tumour cell lines as well as aggregation. We also identified one activator of PTPmu-dependent aggregation, which was able to stimulate cell migration. We propose that the PTPmu D2 binding pocket represents a novel regulatory site and that inhibitors targeting this region may have therapeutic potential for treating cancer.

Role of protein tyrosine phosphatase receptor type M in epithelial ovarian cancer progression

BACKGROUND

Epithelial ovarian cancer (EOC) is often diagnosed at advanced stages with low survival rates. Protein tyrosine phosphatase receptor type M (PTPRM) is involved in cancer development and progression; however, its role in EOC remains unclear. In this study,we aimed to detect PTPRM expression in ovarian epithelial tumors, analyze its relationship with the clinicopathological features and survival prognosis of patients with EOC, and provide a theoretical basis for new targets for EOC treatment. Fifty-seven patients with EOC treated at our hospital between January 2012-January 2014 were included; along with 18 borderline and 30 benign epithelial ovarian tumors and 15 normal ovarian and uterine tube tissue samples from patients surgically treated at our hospital during the same period. PTPRM expression was immunohistochemically detected, and we analyzed its relationship with clinicopathological features and prognosis. Associations between PTPRM expression and survival prognosis of patients with EOC were analyzed using the Gene Expression Profiling Interactive Analysis (GEPIA) and Kaplan-Meier Plotter databases.

RESULTS

PTPRM had the highest expression rates in normal ovarian and uterine tube tissues, followed by benign and borderline epithelial ovarian tumors; the lowest positive expression rate was observed in EOC tumors. PTPRM expression differed significantly among groups (P < 0.05). The positive PTPRM expression rate significantly decreased with age, progressing clinical stage, and tumor recurrence, and the larger the mass diameter, the higher the positive PTPRM expression rate. PTPRM expression was significantly lower in ovarian cancer compared with that in normal tissues in the GEPIA database (P < 0.05). The overall survival (OS) and disease-free survival(DFS) rates were higher in the PTPRM high-expression group, with statistically significant (P < 0.05) and insignificant (P > 0.05) differences, respectively. The OS rate of the high-expression group compared with the low-expression group in the Kaplan-Meier Plotter database was higher, although without statistical significance (P > 0.05), and progression-free survival(PFS) was higher with statistical significance (P < 0.05).

CONCLUSION

PTPRM expression was low in patients with EOC, and the PTPRM positive-expression rate significantly decreased with progressing stages of EOC and tumor recurrence, suggesting that PTPRM acts as a tumor suppressor in EOC progression. Negative PTPRM expression may predict poor clinical outcomes in patients with EOC.

Artificial Intelligence-Based Computational Screening and Functional Assays Identify Candidate Small Molecule Antagonists of PTPmu-Dependent Adhesion

PTPmu (PTPµ) is a member of the receptor protein tyrosine phosphatase IIb family that participates in cell-cell adhesion and signaling. PTPmu is proteolytically downregulated in glioblastoma (glioma), and the resulting extracellular and intracellular fragments are believed to stimulate cancer cell growth and/or migration. Therefore, drugs targeting these fragments may have therapeutic potential. Here, we used the AtomNet^® platform, the first deep learning neural network for drug design and discovery, to screen a molecular library of several million compounds and identified 76 candidates predicted to interact with a groove between the MAM and Ig extracellular domains required for PTPmu-mediated cell adhesion. These candidates were screened in two cell-based assays: PTPmu-dependent aggregation of Sf9 cells and a tumor growth assay where glioma cells grow in three-dimensional spheres. Four compounds inhibited PTPmu-mediated aggregation of Sf9 cells, six compounds inhibited glioma sphere formation/growth, while two priority compounds were effective in both assays. The stronger of these two compounds inhibited PTPmu aggregation in Sf9 cells and inhibited glioma sphere formation down to 25 micromolar. Additionally, this compound was able to inhibit the aggregation of beads coated with an extracellular fragment of PTPmu, directly demonstrating an interaction. This compound presents an interesting starting point for the development of PTPmu-targeting agents for treating cancer including glioblastoma.

Cytoneme-like protrusion formation induced by LAR is promoted by receptor dimerization

Actin-based protrusions called cytonemes are reported to function in cell communication by supporting events such as morphogen gradient establishment and pattern formation. Despite the crucial roles of cytonemes in cell signaling, the molecular mechanism for cytoneme establishment remains elusive. In this study, we showed that the leukocyte common antigen-related (LAR) receptor protein tyrosine phosphatase plays an important role in cytoneme-like protrusion formation. Overexpression of LAR in HEK293T cells induced the formation of actin-based protrusions, some of which exceeded 200 µm in length and displayed a complex morphology with branches. Upon focusing on the regulation of LAR dimerization or clustering and the resulting regulatory effects on LAR phosphatase activity, we found that longer and more branched protrusions were formed when LAR dimerization was artificially induced and when heparan sulfate was applied. Interestingly, although the truncated form of LAR lacking phosphatase-related domains promoted protrusion formation, the phosphatase-inactive forms did not show clear changes, suggesting that LAR dimerization triggers the formation of cytoneme-like protrusions in a phosphatase-independent manner. Our results thus emphasize the importance of LAR and its dimerization in cell signaling.

This article has an associated First Person interview with the first author of the paper.

Summary: We showed that the formation of cytoneme-like protrusions, which function in cell signaling, is induced by LAR and clarified that it is LAR dimerization which promotes protrusion formation.

Protein Tyrosine Phosphatase Receptor Type D Regulates Neuropathic Pain After Nerve Injury via the STING-IFN-I Pathway

Neuropathic pain is usually caused by injury or dysfunction of the somatosensory system, and medicine is a common way of treatment. Currently, there are still no satisfactory drugs, like opioids and lidocaine, which carry a high risk of addiction. Protein tyrosine phosphatase receptor type D (PTPRD) is a known therapeutic target in addiction pathways and small molecule inhibitors targeting it, such as 7-butoxy illudalic acid analog (7-BIA), have recently been developed to tackle addition. PTPRD is also upregulated in the dorsal root ganglion (DRG) in a rat model of neuropathic pain, but is not yet clear whether PTPRD contributes to the development of neuropathic pain. Here, we established a chronic constriction injury (CCI) and evaluated PTPRD expression and its association with neuropathic pain. PTPRD expression was found to gradually increase after CCI in DRGs, and its expression was concomitant with the progressive development of hypersensitivity as assessed by both mechanical and thermal stimuli. Both PTPRD knockdown and administration of PTPRD inhibitor 7-BIA alleviated CCI-induced neuropathic pain while upregulating STING and IFN-α in the DRG. Treatment with H-151, a STING inhibitor, abolished the analgesic effects of PTPRD knockdown. Taken together, our study suggests that increased levels of PTPRD in the DRG following CCI are involved in the development of neuropathic pain via the STING-IFN-I pathway. 7-BIA, a small molecule inhibitor of PTPRD with anti-addiction effects, may represent a novel and safe therapeutic strategy for the clinical management of neuropathic pain without the risk of addiction.

The Zn2+ and Ca2+ -binding S100B and S100A1 proteins: beyond the myths

The S100 genes encode a conserved group of 21 vertebrate-specific EF-hand calcium-binding proteins. Since their discovery in 1965, S100 proteins have remained enigmatic in terms of their cellular functions. In this review, we summarize the calcium- and zinc-binding properties of the dimeric S100B and S100A1 proteins and highlight data that shed new light on the extracellular and intracellular regulation and functions of S100B. We point out that S100B and S100A1 homodimers are not functionally interchangeable and that in a S100A1/S100B heterodimer, S100A1 acts as a negative regulator for the ability of S100B to bind Zn²⁺ . The Ca²⁺ and Zn²⁺ -dependent interactions of S100B with a wide array of proteins form the basis of its activities and have led to the derivation of some initial rules for S100B recognition of protein targets. However, recent findings have strongly suggested that these rules need to be revisited. Here, we describe a new consensus S100B binding motif present in intracellular and extracellular vertebrate-specific proteins and propose a new model for stable interactions of S100B dimers with full-length target proteins. A chaperone-associated function for intracellular S100B in adaptive cellular stress responses is also discussed. This review may help guide future studies on the functions of S100 proteins in general.

FDR-Corrected Sparse Canonical Correlation Analysis With Applications to Imaging Genomics

Reducing the number of false discoveries is presently one of the most pressing issues in the life sciences. It is of especially great importance for many applications in neuroimaging and genomics, where data sets are typically high-dimensional, which means that the number of explanatory variables exceeds the sample size. The false discovery rate (FDR) is a criterion that can be employed to address that issue. Thus it has gained great popularity as a tool for testing multiple hypotheses. Canonical correlation analysis (CCA) is a statistical technique that is used to make sense of the cross-correlation of two sets of measurements collected on the same set of samples (e.g., brain imaging and genomic data for the same mental illness patients), and sparse CCA extends the classical method to high-dimensional settings. Here, we propose a way of applying the FDR concept to sparse CCA, and a method to control the FDR. The proposed FDR correction directly influences the sparsity of the solution, adapting it to the unknown true sparsity level. Theoretical derivation as well as simulation studies show that our procedure indeed keeps the FDR of the canonical vectors below a user-specified target level. We apply the proposed method to an imaging genomics data set from the Philadelphia Neurodevelopmental Cohort. Our results link the brain connectivity profiles derived from brain activity during an emotion identification task, as measured by functional magnetic resonance imaging, to the corresponding subjects' genomic data.

Two receptor tyrosine phosphatases dictate the depth of axonal stabilizing layer in the visual system

Formation of a functional neuronal network requires not only precise target recognition, but also stabilization of axonal contacts within their appropriate synaptic layers. Little is known about the molecular mechanisms underlying the stabilization of axonal connections after reaching their specifically targeted layers. Here, we show that two receptor protein tyrosine phosphatases (RPTPs), LAR and Ptp69D, act redundantly in photoreceptor afferents to stabilize axonal connections to the specific layers of the Drosophila visual system. Surprisingly, by combining loss-of-function and genetic rescue experiments, we found that the depth of the final layer of stable termination relied primarily on the cumulative amount of LAR and Ptp69D cytoplasmic activity, while specific features of their ectodomains contribute to the choice between two synaptic layers, M3 and M6, in the medulla. These data demonstrate how the combination of overlapping downstream but diversified upstream properties of two RPTPs can shape layer-specific wiring.

Protein Tyrosine Phosphatase δ Mediates the Sema3A-Induced Cortical Basal Dendritic Arborization through the Activation of Fyn Tyrosine Kinase

Leukocyte common antigen-related (LAR) class protein tyrosine phosphatases (PTPs) are critical for axonal guidance; however, their relation to specific guidance cues is poorly defined. We here show that PTP-3, a LAR homolog in Caenorhabditis elegans, is involved in axon guidance regulated by Semaphorin-2A-signaling. PTPδ, one of the vertebrate LAR class PTPs, participates in the Semaphorin-3A (Sema3A)-induced growth cone collapse response of primary cultured dorsal root ganglion neurons from Mus musculus embryos. In vivo, however, the contribution of PTPδ in Sema3A-regualted axon guidance was minimal. Instead, PTPδ played a major role in Sema3A-dependent cortical dendritic growth. Ptpδ^-/- and Sema3a^-/- mutant mice exhibited poor arborization of basal dendrites of cortical layer V neurons. This phenotype was observed in both male and female mutants. The double-heterozygous mutants, Ptpδ^+/-; Sema3a^+/-, also showed a similar phenotype, indicating the genetic interaction. In Ptpδ^-/- brains, Fyn and Src kinases were hyperphosphorylated at their C-terminal Tyr527 residues. Sema3A-stimulation induced dephosphorylation of Tyr527 in the dendrites of wild-type cortical neurons but not of Ptpδ^-/- Arborization of cortical basal dendrites was reduced in Fyn^-/- as well as in Ptpδ^+/-; Fyn^+/- double-heterozygous mutants. Collectively, PTPδ mediates Sema3A-signaling through the activation of Fyn by C-terminal dephosphorylation.SIGNIFICANCE STATEMENT The relation of leukocyte common antigen-related (LAR) class protein tyrosine phosphatases (PTPs) and specific axon guidance cues is poorly defined. We show that PTP-3, a LAR homolog in Caenorhabditis elegans, participates in Sema2A-regulated axon guidance. PTPδ, a member of vertebrate LAR class PTPs, is involved in Sema3A-regulated cortical dendritic growth. In Sema3A signaling, PTPδ activates Fyn and Src kinases by dephosphorylating their C-terminal Tyr residues. This is the first evidence showing that LAR class PTPs participate in Semaphorin signaling in vivo.

Meta-analysis of genome-wide SNP- and pathway-based associations for facets of neuroticism

Neuroticism is a heritable personality trait that is comprised of distinct sub-factors, or facets. Sub-factors of neuroticism are linked to different emotional states or psychiatric symptoms and studying the genetic variants associated with these facets may help reveal the biological mechanisms underlying psychiatric disorders. In the present study, a meta-analysis of genome-wide association studies for six facets of neuroticism was performed in 5584 participants from three cohorts. Additionally, a Gene Set Enrichment Analysis was conducted to find biological pathways associated with each facet. Six neuroticism facets (N1: anxiety, N2: angry hostility, N3: depression, N4: self-consciousness, N5: impulsivity and N6: vulnerability) were assessed using the Korean version of the Revised NEO Personality Inventory. In the single-nucleotide polymorphism-based analysis, results showed genome-wide significance for N2 within the MIR548H3 gene (rs1360001, P=4.14 × 10^-9). Notable genes with suggestive associations (P<1.0 × 10^-6) were ITPR1 for N1, WNT7A for N2, FGF10 and FHIT for N3, DDR1 for N4, VGLL4 for N5 and PTPRD for N6. In the pathway-based analysis, the axon guidance pathway was identified to be associated with multiple facets of neuroticism (N2, N4 and N6). The focal adhesion and extracellular matrix receptor interaction pathways were significantly associated with N2 and N3. Our findings revealed genetic influences and biological pathways that are associated with facets of neuroticism.

Investigating the effect of different transducer stiffness values on the contactin complex detachment by steered molecular dynamics

This study investigated the adhesion behavior of Contactin4 (CNTN4), a member of Immunoglobulin Super Family (Ig-SF) of cell adhesion molecules. Contactin4 plays a crucial role in the formation, maintenance, and plasticity of neuronal networks. Contactin in its complex configuration with protein tyrosine phosphatase gamma (PTPRG) was selected for simulation. By utilizing Steered Molecular Dynamics (SMD), the uniaxial force was applied to induce unbinding of the complex, and the force-induced detachment of complex components was probed. Three sets of simulations with three values of transducer stiffness and five pulling speeds were designed. Our results showed the dependence of unbinding force on both accessible parameters of pulling speed and spring stiffness. By increasing the stiffness value and pulling speed the rupture force increased. Accordingly, the dissociation rates due to the Bell's theory based on rupture forces and loading rates were calculated.

Chondroitin sulfates and their binding molecules in the central nervous system

Chondroitin sulfate (CS) is the most abundant glycosaminoglycan (GAG) in the central nervous system (CNS) matrix. Its sulfation and epimerization patterns give rise to different forms of CS, which enables it to interact specifically and with a significant affinity with various signalling molecules in the matrix including growth factors, receptors and guidance molecules. These interactions control numerous biological and pathological processes, during development and in adulthood. In this review, we describe the specific interactions of different families of proteins involved in various physiological and cognitive mechanisms with CSs in CNS matrix. A better understanding of these interactions could promote a development of inhibitors to treat neurodegenerative diseases.

Receptor Tyrosine Kinases and Phosphatases in Neuronal Wiring: Insights From Drosophila

Tyrosine phosphorylation is at the crossroads of many signaling pathways. Brain wiring is not an exception, and several receptor tyrosine kinases (RTKs) and tyrosine receptor phosphates (RPTPs) have been involved in this process. Considerable work has been done on RTKs, and for many of them, detailed molecular mechanisms and functions in several systems have been characterized. In contrast, RPTPs have been studied considerably less and little is known about their ligands and substrates. In both families, we find redundancy between different members to accomplish particular wiring patterns. Strikingly, some RTKs and RPTPs have lost their catalytic activity during evolution, but not their importance in biological processes. In this regard, we have to keep in mind that these proteins have multiple domains and some of their functions are independent of tyrosine phosphorylation/dephosphorylation. Since RTKs and RPTPs are enzymes involved not only in early stages of axon and dendrite pathfinding but also in synapse formation and physiology, they have a potential as drug targets. Drosophila has been a key model organism in the search of a better understanding of brain wiring, and its sophisticated toolbox is very suitable for studying the function of genes with pleiotropic functions such as RTKs and RPTPs, from wiring to synaptic formation and function. In these review, we mainly cover findings from this model organism and complement them with discoveries in vertebrate systems.

ORTI: An Open-Access Repository of Transcriptional Interactions for Interrogating Mammalian Gene Expression Data

Transcription factors (TFs) play a fundamental role in coordinating biological processes in response to stimuli. Consequently, we often seek to determine the key TFs and their regulated target genes (TGs) amidst gene expression data. This requires a knowledge-base of TF-TG interactions, which would enable us to determine the topology of the transcriptional network and predict novel regulatory interactions. To address this, we generated an Open-access Repository of Transcriptional Interactions, ORTI, by integrating available TF-TG interaction databases. These databases rely on different types of experimental evidence, including low-throughput assays, high-throughput screens, and bioinformatics predictions. We have subsequently categorised TF-TG interactions in ORTI according to the quality of this evidence. To demonstrate its capabilities, we applied ORTI to gene expression data and identified modulated TFs using an enrichment analysis. Combining this with pairwise TF-TG interactions enabled us to visualise temporal regulation of a transcriptional network. Additionally, ORTI enables the prediction of novel TF-TG interactions, based on how well candidate genes co-express with known TGs of the target TF. By filtering out known TF-TG interactions that are unlikely to occur within the experimental context, this analysis predicts context-specific TF-TG interactions. We show that this can be applied to experimental designs of varying complexities. In conclusion, ORTI is a rich and publicly available database of experimentally validated mammalian transcriptional interactions which is accompanied with tools that can identify and predict transcriptional interactions, serving as a useful resource for unravelling the topology of transcriptional networks.

Structural Basis for Interactions Between Contactin Family Members and Protein-tyrosine Phosphatase Receptor Type G in Neural Tissues

Protein-tyrosine phosphatase receptor type G (RPTPγ/PTPRG) interacts in vitro with contactin-3-6 (CNTN3-6), a group of glycophosphatidylinositol-anchored cell adhesion molecules involved in the wiring of the nervous system. In addition to PTPRG, CNTNs associate with multiple transmembrane proteins and signal inside the cell via cis-binding partners to alleviate the absence of an intracellular region. Here, we use comprehensive biochemical and structural analyses to demonstrate that PTPRG·CNTN3-6 complexes share similar binding affinities and a conserved arrangement. Furthermore, as a first step to identifying PTPRG·CNTN complexes in vivo, we found that PTPRG and CNTN3 associate in the outer segments of mouse rod photoreceptor cells. In particular, PTPRG and CNTN3 form cis-complexes at the surface of photoreceptors yet interact in trans when expressed on the surfaces of apposing cells. Further structural analyses suggest that all CNTN ectodomains adopt a bent conformation and might lie parallel to the cell surface to accommodate these cis and trans binding modes. Taken together, these studies identify a PTPRG·CNTN complex in vivo and provide novel insights into PTPRG- and CNTN-mediated signaling.

Catalytic and substrate promiscuity: distinct multiple chemistries catalysed by the phosphatase domain of receptor protein tyrosine phosphatase

The presence of latent activities in enzymes is posited to underlie the natural evolution of new catalytic functions. However, the prevalence and extent of such substrate and catalytic ambiguity in evolved enzymes is difficult to address experimentally given the order-of-magnitude difference in the activities for native and, sometimes, promiscuous substrate/s. Further, such latent functions are of special interest when the activities concerned do not fall into the domain of substrate promiscuity. In the present study, we show a special case of such latent enzyme activity by demonstrating the presence of two mechanistically distinct reactions catalysed by the catalytic domain of receptor protein tyrosine phosphatase isoform δ (PTPRδ). The primary catalytic activity involves the hydrolysis of a phosphomonoester bond (C─O─P) with high catalytic efficiency, whereas the secondary activity is the hydrolysis of a glycosidic bond (C─O─C) with poorer catalytic efficiency. This enzyme also displays substrate promiscuity by hydrolysing diester bonds while being highly discriminative for its monoester substrates. To confirm these activities, we also demonstrated their presence on the catalytic domain of protein tyrosine phosphatase Ω (PTPRΩ), a homologue of PTPRδ. Studies on the rate, metal-ion dependence, pH dependence and inhibition of the respective activities showed that they are markedly different. This is the first study that demonstrates a novel sugar hydrolase and diesterase activity for the phosphatase domain (PD) of PTPRδ and PTPRΩ. This work has significant implications for both understanding the evolution of enzymatic activity and the possible physiological role of this new chemistry. Our findings suggest that the genome might harbour a wealth of such alternative latent enzyme activities in the same protein domain that renders our knowledge of metabolic networks incomplete.

Chondroitin sulfate proteoglycans: Key modulators in the developing and pathologic central nervous system

Chondroitin Sulfate Proteoglycans (CSPGs) are a major component of the extracellular matrix in the central nervous system (CNS) and play critical role in the development and pathophysiology of the brain and spinal cord. Developmentally, CSPGs provide guidance cues for growth cones and contribute to the formation of neuronal boundaries in the developing CNS. Their presence in perineuronal nets plays a crucial role in the maturation of synapses and closure of critical periods by limiting synaptic plasticity. Following injury to the CNS, CSPGs are dramatically upregulated by reactive glia which form a glial scar around the lesion site. Increased level of CSPGs is a hallmark of all CNS injuries and has been shown to limit axonal plasticity, regeneration, remyelination, and conduction after injury. Additionally, CSPGs create a non-permissive milieu for cell replacement activities by limiting cell migration, survival and differentiation. Mounting evidence is currently shedding light on the potential benefits of manipulating CSPGs in combination with other therapeutic strategies to promote spinal cord repair and regeneration. Moreover, the recent discovery of multiple receptors for CSPGs provides new therapeutic targets for targeted interventions in blocking the inhibitory properties of CSPGs following injury. Here, we will provide an in depth discussion on the impact of CSPGs in normal and pathological CNS. We will also review the recent preclinical therapies that have been developed to target CSPGs in the injured CNS.

Identification of receptor-type protein tyrosine phosphatase μ as a new marker for osteocytes

Osteocytes are the predominant cells in bone, where they form a cellular network and display important functions in bone homeostasis, phosphate metabolism and mechanical transduction. Several proteins strongly expressed by osteocytes are involved in these processes, e.g., sclerostin, DMP-1, PHEX, FGF23 and MEPE, while others are upregulated during differentiation of osteoblasts into osteocytes, e.g., osteocalcin and E11. The receptor-type protein tyrosine phosphatase µ (RPTPμ) has been described to be expressed in cells which display a cellular network, e.g., endothelial and neuronal cells, and is implied in mechanotransduction. In a capillary outgrowth assay using metatarsals derived from RPTPμ-knock-out/LacZ knock-in mice, we observed that the capillary structures grown out of the metatarsals were stained blue, as expected. Surprisingly, cells within the metatarsal bone tissue were positive for LacZ activity as well, indicating that RPTPμ is also expressed by osteocytes. Subsequent histochemical analysis showed that within bone, RPTPμ is expressed exclusively in early-stage osteocytes. Analysis of bone marrow cell cultures revealed that osteocytes are present in the nodules and an enzymatic assay enabled the quantification of the amount of osteocytes. No apparent bone phenotype was observed when tibiae of RPTPμ-knock-out/LacZ knock-in mice were analyzed by μCT at several time points during aging, although a significant reduction in cortical bone was observed in RPTPμ-knock-out/LacZ knock-in mice at 20 weeks. Changes in trabecular bone were more subtle. Our data show that RPTPμ is a new marker for osteocytes.

Genetic markers of white matter integrity in schizophrenia revealed by parallel ICA

It is becoming a consensus that white matter integrity is compromised in schizophrenia (SZ), however the underlying genetics remains elusive. Evidence suggests a polygenic basis of the disorder, which involves various genetic variants with modest individual effect sizes. In this work, we used a multivariate approach, parallel independent component analysis (P-ICA), to explore the genetic underpinnings of white matter abnormalities in SZ. A pre-filtering step was first applied to locate 6527 single nucleotide polymorphisms (SNPs) discriminating patients from controls with a nominal uncorrected p-value of 0.01. These potential susceptibility loci were then investigated for associations with fractional anisotropy (FA) images in a cohort consisting of 73 SZ patients and 87 healthy controls (HC). A significant correlation (r = −0.37, p = 1.25 × 10⁻⁶) was identified between one genetic factor and one FA component after controlling for scanning site, ethnicity, age, and sex. The identified FA-SNP association remained stable in a 10-fold validation. A 5000-run permutation test yielded a p-value of 2.00 × 10⁻⁴. The FA component reflected decreased white matter integrity in the forceps major for SZ patients. The SNP component was overrepresented in genes whose products are involved in corpus callosum morphology (e.g., CNTNAP2, NPAS3, and NFIB) as well as canonical pathways of synaptic long term depression and protein kinase A signaling. Taken together, our finding delineates a part of genetic architecture underlying SZ-related FA reduction, emphasizing the important role of genetic variants involved in neural development.

RPTPs in axons, synapses and neurology

Receptor-like protein tyrosine phosphatases represent a large protein family related to cell adhesion molecules, with diverse roles throughout neural development in vertebrates and invertebrates. This review focuses on their roles in axon growth, guidance and repair, as well as more recent findings demonstrating their key roles in pre-synaptic and post-synaptic maturation and function. These enzymes have been linked to memory and neuropsychiatric defects in loss-of-function rodent models, highlighting their potential as future drug targets.

Regulation of development and cancer by the R2B subfamily of RPTPs and the implications of proteolysis

The initial cloning of receptor protein tyrosine phosphatases (RPTPs) was met with excitement because of their hypothesized function in counterbalancing receptor tyrosine kinase signaling. In recent years, members of a subfamily of RPTPs with homophilic cell-cell adhesion capabilities, known as the R2B subfamily, have been shown to have functions beyond that of counteracting tyrosine kinase activity, by independently influencing cell signaling in their own right and by regulating cell adhesion. The R2B subfamily is composed of four members: PTPmu (PTPRM), PTPrho (PTPRT), PTPkappa (PTPRK), and PCP-2 (PTPRU). The effects of this small subfamily of RPTPs is far reaching, influencing several developmental processes and cancer. In fact, R2B RPTPs are predicted to be tumor suppressors and are among the most frequently mutated protein tyrosine phosphatases (PTPs) in cancer. Confounding these conclusions are more recent studies suggesting that proteolysis of the full-length R2B RPTPs result in oncogenic extracellular and intracellular protein fragments. This review discusses the current knowledge of the role of R2B RPTPs in development and cancer, with special detail given to the mechanisms and implications that proteolysis has on R2B RPTP function. We also touch upon the concept of exploiting R2B proteolysis to develop cancer imaging tools, and consider the effects of R2B proteolysis on axon guidance, perineural invasion and collective cell migration.

Developmental co-expression and functional redundancy of tyrosine phosphatases with neurotrophin receptors in developing sensory neurons

Receptor-type protein tyrosine phosphatases (RPTPs) have been implicated as direct or indirect regulators of neurotrophin receptors (TRKs). It remains less clear if and how such RPTPs might regulate TRK proteins in vivo during development. Here we present a comparative expression profile of RPTP genes and Trk genes during early stages of murine, dorsal root ganglion maturation. We find little if any specific, temporal mRNA co-regulation between individual RPTP and Ntrk genes between E12.5 and E14.5. Moreover, a double fluorescent in-situ hybridization and immunofluorescence study of seven Rptp genes with Ntrks revealed widespread co-expression of RPTPs in individual neurons, but no tight correlation with Trk expression profiles. No Rptp is expressed in 100% of Ntrk1-expressing neurons, whereas at least 6 RPTPs are expressed in 100% of Ntrk2- and Ntrk3-expressing neurons. An exception is Ptpro, which showed very selective expression. Short hairpin RNA suppression of Ptprf, Ptprs or Ptpro in primary, E13.5 DRG neurons did not alter TRK signalling. We therefore propose that TRK signalling may not be simply dependent on rate-limiting regulation by individual RPTP subtypes during sensory neuron development. Instead, TRK signalling has the potential to be buffered by concurrent inputs from several RPTPs in individual neurons.

Inactivation of LAR family phosphatase genes Ptprs and Ptprf causes craniofacial malformations resembling Pierre-Robin sequence

Leukocyte antigen related (LAR) family receptor protein tyrosine phosphatases (RPTPs) regulate the fine balance between tyrosine phosphorylation and dephosphorylation that is crucial for cell signaling during development and tissue homeostasis. Here we show that LAR RPTPs are required for normal development of the mandibular and maxillary regions. Approximately half of the mouse embryos lacking both Ptprs (RPTPσ) and Ptprf (LAR) exhibit micrognathia (small lower jaw), cleft palate and microglossia/glossoptosis (small and deep tongue), a phenotype closely resembling Pierre-Robin sequence in humans. We show that jaw bone and cartilage patterning occurs aberrantly in LAR family phosphatase-deficient embryos and that the mandibular arch harbors a marked decrease in cell proliferation. Analysis of signal transduction in embryonic tissues and mouse embryonic fibroblast cultures identifies an increase in Bmp-Smad signaling and an abrogation of canonical Wnt signaling associated with loss of the LAR family phosphatases. A reactivation of β-catenin signaling by chemical inhibition of GSK3β successfully resensitizes LAR family phosphatase-deficient cells to Wnt induction, indicating that RPTPs are necessary for normal Wnt/β-catenin pathway activation. Together these results identify LAR RPTPs as important regulators of craniofacial morphogenesis and provide insight into the etiology of Pierre-Robin sequence.

Contactin 4, -5 and -6 differentially regulate neuritogenesis while they display identical PTPRG binding sites

The neural cell-adhesion molecules contactin 4, contactin 5 and contactin 6 are involved in brain development, and disruptions in contactin genes may confer increased risk for autism spectrum disorders (ASD). We describe a co-culture of rat cortical neurons and HEK293 cells overexpressing and delivering the secreted forms of rat contactin 4-6. We quantified their effects on the length and branching of neurites. Contactin 4-6 effects were different depending on the contactin member and duration of co-culture. At 4 days in culture, contactin 4 and -6 increased the length of neurites, while contactin 5 increased the number of roots. Up to 8 days in culture, contactin 6 progressively increased the length of neurites while contactin 5 was more efficient on neurite branching. We studied the molecular sites of interaction between human contactin 4, -5 or -6 and the human Protein Tyrosine Phosphatase Receptor Gamma (PTPRG), a contactin partner, by modeling their 3D structures. As compared to contactin 4, we observed differences in the Ig2 and Ig3 domains of contactin 5 and -6 with the appearance of an omega loop that could adopt three distinct conformations. However, interactive residues between human contactin 4-6 and PTPRG were strictly conserved. We did not observe any differences in PTPRG binding on contactin 5 and -6 either. Our data suggest that the differential contactin effects on neurite outgrowth do not result from distinct interactions with PTPRG. A better understanding of the contactin cellular properties should help elucidate their roles in ASD.

Tumour suppressor function of protein tyrosine phosphatase receptor-T

It has long been thought that PTPs (protein tyrosine phosphatases) normally function as tumour suppressors. Recent high-throughput mutational analysis identified loss-of-function mutations in six PTPs in human colon cancers, providing critical cancer genetics evidence that PTPs can act as tumour suppressor genes. PTPRT (protein tyrosine phosphatase receptor-T), a member of the family of type IIB receptor-like PTPs, is the most frequently mutated PTP among them. Consistent with the notion that PTPRT is a tumour suppressor, PTPRT knockout mice are hypersensitive to AOM (azoxymethane)-induced colon cancer. The present review focuses on the physiological and pathological functions of PTPRT as well as the cellular pathways regulated by this phosphatase.

Should I stay or should I go? Shedding of RPTPs in cancer cells switches signals from stabilizing cell-cell adhesion to driving cell migration

Dissolution of cell-cell adhesive contacts and increased cell-extracellular matrix adhesion are hallmarks of the migratory and invasive phenotype of cancer cells. These changes are facilitated by growth factor binding to receptor protein tyrosine kinases (RTKs). In normal cells, cell-cell adhesion molecules (CAMs), including some receptor protein tyrosine phosphatases (RPTPs), antagonize RTK signaling by promoting adhesion over migration. In cancer, RTK signaling is constitutive due to mutated or amplified RTKs, which leads to growth factor independence, or autonomy. An alternative route for a tumor cell to achieve autonomy is to inactivate cell-cell CAMs such as RPTPs. RPTPs directly mediate cell adhesion and regulate both cadherin-dependent adhesion and signaling. In addition, RPTPs antagonize RTK signaling by dephosphorylating molecules activated following ligand binding. Both RPTPs and cadherins are downregulated in tumor cells by cleavage at the cell surface. This results in shedding of the extracellular, adhesive segment and displacement of the intracellular segment, altering its subcellular localization and access to substrates or binding partners. In this commentary we discuss the signals that are altered following RPTP and cadherin cleavage to promote cell migration. Tumor cells both step on the gas (RTKs) and disconnect the brakes (RPTPs and cadherins) during their invasive and metastatic journey.

The cytoplasmic adaptor protein Caskin mediates Lar signal transduction during Drosophila motor axon guidance

The multiprotein complexes that receive and transmit axon pathfinding cues during development are essential to circuit generation. Here, we identify and characterize the Drosophila sterile α-motif (SAM) domain-containing protein Caskin, which shares homology with vertebrate Caskin, a CASK [calcium/calmodulin-(CaM)-activated serine-threonine kinase]-interacting protein. Drosophila caskin (ckn) is necessary for embryonic motor axon pathfinding and interacts genetically and physically with the leukocyte common antigen-related (Lar) receptor protein tyrosine phosphatase. In vivo and in vitro analyses of a panel of ckn loss-of-function alleles indicate that the N-terminal SAM domain of Ckn mediates its interaction with Lar. Like Caskin, Liprin-α is a neuronal adaptor protein that interacts with Lar via a SAM domain-mediated interaction. We present evidence that Lar does not bind Caskin and Liprin-α concurrently, suggesting they may assemble functionally distinct signaling complexes on Lar. Furthermore, a vertebrate Caskin homolog interacts with LAR family members, arguing that the role of ckn in Lar signal transduction is evolutionarily conserved. Last, we characterize several ckn mutants that retain Lar binding yet display guidance defects, implying the existence of additional Ckn binding partners. Indeed, we identify the SH2/SH3 adaptor protein Dock as a second Caskin-binding protein and find that Caskin binds Lar and Dock through distinct domains. Furthermore, whereas ckn has a nonredundant function in Lar-dependent signaling during motor axon targeting, ckn and dock have overlapping roles in axon outgrowth in the CNS. Together, these studies identify caskin as a neuronal adaptor protein required for axon growth and guidance.

The C. elegans nck-1 gene encodes two isoforms and is required for neuronal guidance

The NCK adaptor proteins are composed entirely of SH3 and SH2 domains and serve as protein interaction bridges for several receptors during signal transduction events. Here we report the molecular and genetic analysis of the Caenorhabditis elegans nck-1 gene. C. elegans nck-1 encodes two isoforms: NCK-1A and a shorter isoform that lacks the first SH3 domain, NCK-1B. C. elegans nck-1 mutants exhibit defects in axon guidance and neuronal cell position, as well as defects in the excretory canal cell, gonad, and male mating. NCK-1 is broadly expressed in neurons and epithelial cells with NCK-1B being the most abundant isoform. NCK-1A and NCK-1B share a similar expression pattern in parts of the nervous system, but also have independent expression patterns in other tissues. Interestingly, NCK-1B is localized to the nuclei of many cells. Genetic rescue experiments show that NCK-1 functions cell autonomously and, in general, either NCK-1A or NCK-1B is sufficient to function in axon guidance. However, there appears to be specific roles for each isoform, for example NCK-1B is required for HSN cell migration while NCK-1A is required for efficient male mating. Genetic epistasis experiments show that NCK-1 functions redundantly with the LAR Receptor Tyrosine Phosphatase, PTP-3, and the Netrin receptor UNC-40.

Tyrosine phosphatase PTPRD suppresses colon cancer cell migration in coordination with CD44

PTPRD is a receptor-type tyrosine-protein phosphatase. Recent analyses of comprehensive mutations and copy numbers have revealed that PTPRD is frequently mutated and homozygously deleted in various types of cancer, including glioblastoma, melanoma, breast and colon cancer. However, the molecular functions of PTPRD in cancer progression have yet to be elucidated. Herein, PTPRD suppressed colon cancer cell migration and was required for appropriate cell-cell adhesion. In addition, PTPRD regulated cell migration in cooperation with β-catenin/TCF signaling and its target CD44. Furthermore, expression levels of PTPRD were down-regulated in highly invasive cancers and were significantly correlated with patient survival. Our findings suggest that PTPRD is required for colon cancer invasion and progression.

Receptor tyrosine phosphatase PTPγ is a regulator of spinal cord neurogenesis

During spinal cord development the proliferation, migration and survival of neural progenitors and precursors is tightly controlled, generating the fine spatial organisation of the cord. In order to understand better the control of these processes, we have examined the function of an orphan receptor protein tyrosine phosphatase (RPTP) PTPγ, in the developing chick spinal cord. Widespread expression of PTPγ occurs post-embryonic day 3 in the early cord and is consistent with a potential role in either neurogenesis or neuronal maturation. Using gain-of-function and loss-of-function approaches in ovo, we show that PTPγ perturbation significantly reduces progenitor proliferation rates and neuronal precursor numbers, resulting in hypoplasia of the neuroepithelium. PTPγ gain-of-function causes widespread suppression of Wnt/β-catenin-driven TCF signalling. One potential target of PTPγ may therefore be β-catenin itself, since PTPγ can dephosphorylate it in vitro, but alternative targets are also likely. PTPγ loss-of-function is not sufficient to alter TCF signalling. Instead, loss-of-function leads to increased apoptosis and defective cell–cell adhesion in progenitors and precursors. Furthermore, motor neuron precursor migration is specifically defective. PTPγ therefore regulates neurogenesis during a window of spinal cord development, with molecular targets most likely related to Wnt/β-catenin signalling, cell survival and cell adhesion.

Cancer cells cut homophilic cell adhesion molecules and run

The term contact inhibition (CI) encompasses the cellular changes that result in cessation of cell migration and of proliferation due to signals transduced when one cell comes into physical contact with another cell. Cancer cells, however, do not contact inhibit. A molecular understanding of the loss of CI in cancer cells is important for understanding tumor progression. In this Perspective, we propose that the loss of CI observed in cancer cells is the result of extracellular proteolysis of transmembrane cell-cell cell adhesion molecules (CAM) in the tumor microenvironment. Proteolysis of homophilic cell-cell CAMs results in a shed extracellular fragment and released cytoplasmic fragment(s) that disrupts adhesion and induces signals that promote proliferation and/or migration. The importance of this observation in tumor progression is supported by the presence of the shed extracellular fragments of homophilic cell-cell CAMs in serum and tumor tissue of cancer patients suggesting that instead of acting as tumor suppressors, the shed CAM extracellular and cytoplasmic fragments actually function as oncogenes. The study of cell-cell CAM cleavage will provide important and novel means of diagnosing, imaging, and treating tumor progression.

The receptor protein tyrosine phosphatase HmLAR1 is up-regulated in the CNS of the adult medicinal leech following injury and is required for neuronal sprouting and regeneration

LAR-like receptor protein tyrosine phosphatases (RPTPs), which are abundantly expressed in the nervous systems of most if not all bilaterian animals thus far examined, have been implicated in regulating a variety of critical neuronal processes. These include neuronal pathfinding, adhesion and synaptogenesis during development and, in adult mammals, neuronal regeneration. Here we explored a possible role of a LAR-like RPTP (HmLAR1) in response to mechanical trauma in the adult nervous system of the medicinal leech. In situ hybridization and QPCR analyses of HmLAR1 expression in individual segmental ganglia revealed a significant up-regulation in receptor expression following CNS injury, both in situ and following a period in vitro. Furthermore, we observed up-regulation in the expression of the leech homologue of the Abelson tyrosine kinase, a putative signaling partner to LAR receptors, but not among other tyrosine kinases. The effects on neuronal regeneration were assayed by comparing growth across a nerve crush by projections of individual dorsal P neurons (P(D)) following single-cell injection of interfering RNAs against the receptor or control RNAs. Receptor RNAi led to a significant reduction in HmLAR1 expression by the injected cells and resulted in a significant decrease in sprouting and regenerative growth at the crush site relative to controls. These studies extend the role of the HmLARs from leech neuronal development to adult neuronal regeneration and provide a platform to investigate neuronal regeneration and gene regulation at the single cell level.

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.

Binding of alphaII spectrin to 14-3-3beta is involved in NCAM-dependent neurite outgrowth

Members of the 14-3-3 protein family have been implicated in neuronal migration, synaptic plasticity and learning. Using affinity chromatography followed by mass spectrometry analysis, we show here that the cytoskeletal protein alphaII spectrin is a novel ligand of 14-3-3beta. We found that 14-3-3beta interacts with alphaII spectrin via the mode 2 14-3-3 binding motif RLIQS(1302)HP. Binding required phosphorylation of Ser(1302) by casein kinase II and was enhanced in the presence of calmodulin. Co-immunoprecipitation of alphaII spectrin and 14-3-3beta with the neural cell adhesion molecule NCAM suggested that the 14-3-3-spectrin-interaction affects NCAM function. Indeed, disruption of the 14-3-3beta/alphaII spectrin interaction by mutating Ser(1302) to Ala enhanced NCAM-dependent neurite outgrowth. Our results indicate that the phosphorylation-dependent interaction between 14-3-3beta and alphaII spectrin acts as a switch between positive and negative regulation of neurite outgrowth stimulated by NCAM, representing a novel and acute mechanism preventing uncontrolled elongation of neuronal processes.

Expression levels of a LAR-like receptor protein tyrosine phosphatase correlate with neuronal branching and arbor density in the medicinal leech

LAR-like receptor protein tyrosine phosphatases (RPTPs), which are reported to be highly expressed in the nervous systems of most bilaterian animals, have been implicated in the regulation of critical developmental processes, such as neuronal pathfinding, cell adhesion and synaptogenesis. Here we report that two LAR-like RPTPs in the medicinal leech, HmLAR1 and HmLAR2, play roles in regulating the size and density of neuronal arbors within the developing nervous system and in the body wall. Employing single-cell RNAi knockdown and transgene expression techniques, we demonstrate that the expression level of HmLAR1 is directly correlated with the density of an identified neuron's arborization. Knocking down HmLAR1 mRNA levels in the mechanosensory pressure (P) neurons produces a reduced central arbor and a smaller arbor in the peripheral body wall, with fewer terminal branches. By contrast, overexpression of this receptor in a P cell leads to extensive neuronal sprouting, including many supernumerary neurites and terminal branches as well as, in some instances, the normal monopolar morphology of the P cell becoming multipolar. We also report that induced neuronal sprouting requires the expression of the receptor's membrane tethered ectodomain, including the NH(2)-Ig domains, but not of the intracellular phosphatase domains of the receptor. Interestingly, sprouting could be elicited upon ectopic expression of HmLAR1 and the related RPTP, HmLAR2 in the P and other neurons, including those that do not normally express either RPTP, suggesting that the substrates involved in HmLAR-induced sprouting are present in most neurons irrespective of whether they normally express these LAR-like RPTPs. Our data are consistent with the hypothesis that the receptors' ectodomains promote an adhesive interaction that enhances the maintenance of new processes.

Stimulation of N-cadherin-dependent neurite outgrowth by small molecule peptide mimetic agonists of the N-cadherin HAV motif

N-cadherin is a cell adhesion molecule that promotes axon outgrowth and synapse formation during the development of the central nervous system. In addition, N-cadherin promotes glial cell adhesion and myelination of axons. Therefore, stimulating N-cadherin function with N-cadherin agonists could be used therapeutically to promote regeneration of the nervous system and remyelination after injury or disease. In the extracellular domain of N-cadherin, the amino acid sequence HAV is required for N-cadherin-mediated adhesion and neurite outgrowth. The ADH-1 cyclic peptide, derived from the N-cadherin HAV site, is an effective antagonist of N-cadherin-mediated neurite outgrowth and is currently being tested in clinical trials for cancer chemotherapy. Of interest, a dimeric version of this cyclic peptide, N-Ac-CHAVDINGHAVDIC-NH(2), functions as an N-cadherin agonist. This dimeric peptide agonist and the peptide antagonist ADH-1 both have limitations as drugs due to their metabolic instability and lack of oral delivery. To address this issue Adherex Technologies Inc. generated a small molecule library of peptidomimetics to the HAV region of N-cadherin, which would be more amenable to therapeutic use. We screened the Adherex library for compounds that altered neurite outgrowth and identified eight N-cadherin agonists that stimulated N-cadherin-dependent neurite outgrowth. Five of these agonists also stimulated retinal cell migration. These small molecule agonists may be effective reagents for promoting axon growth and remyelination after injury or disease.

Distinct PTPmu-associated signaling molecules differentially regulate neurite outgrowth on E-, N-, and R-cadherin

Classical cadherins play distinct roles in axon growth and guidance in the visual system, however, the signaling pathways they activate remain unclear. Growth cones on each cadherin substrate have a unique morphology suggesting that distinct signals are activated by neurite outgrowth on E-, N-, and R-cadherin. We previously demonstrated that receptor protein tyrosine phosphatase-mu (PTPmu) is required for E- and N-cadherin-dependent neurite outgrowth. In this manuscript, we demonstrate that PTPmu regulates R-cadherin-mediated neurite outgrowth. Furthermore, we evaluated whether known PTPmu-associated signaling proteins, Rac1, Cdc42, IQGAP1 and PKCdelta, regulate neurite outgrowth mediated by these cadherins. While Rac1 activity is required for neurite outgrowth on all three cadherins Cdc42/IQGAP1 are required only for N- and R-cadherin-mediated neurite outgrowth. In addition, we determined that PKC activity is required for E- and R-cadherin-mediated, but not N-cadherin-mediated neurite outgrowth. In summary, distinct PTPmicro-associated signaling proteins are required to promote neurite outgrowth on cadherins.

PTPmu suppresses glioma cell migration and dispersal

The cell-surface receptor protein tyrosine phosphatase mu (PTPmu) is a homophilic cell adhesion molecule expressed in CNS neurons and glia. Glioblastomas (GBMs) are the highest grade of primary brain tumors with astrocytic similarity and are characterized by marked dispersal of tumor cells. PTPmu expression was examined in human GBM, low-grade astrocytoma, and normal brain tissue. These studies revealed a striking loss of PTPmu protein expression in highly dispersive GBMs compared to less dispersive low-grade astrocytomas and normal brain. We hypothesized that PTPmu contributes to contact inhibition of glial cell migration by transducing signals in response to cell adhesion. Therefore, loss of PTPmu may contribute to the extensive dispersal of GBMs. The migration of brain tumor cells was assessed in vitro using a scratch wound assay. Parental U-87 MG cells express PTPmu and exhibited limited migration. However, short-hairpin RNA (shRNA)-mediated knockdown of PTPmu induced a morphological change and increased migration. Next, a brain slice assay replicating the three-dimensional environment of the brain was used. To assess migration, labeled U-87 MG glioma cells were injected into adult rat brain slices, and their movement was followed over time. Parental U-87 MG cells demonstrated limited dispersal in this assay. However, PTPmu shRNA induced migration and dispersal of U-87 MG cells in the brain slice. Finally, in a mouse xenograft model of intracranially injected U-87 MG cells, PTPmu shRNA induced morphological heterogeneity in these xenografts. Together, these data suggest that loss of PTPmu in human GBMs contributes to tumor cell migration and dispersal, implicating loss of PTPmu in glioma progression.

Proteolytic cleavage of protein tyrosine phosphatase mu regulates glioblastoma cell migration

Glioblastoma multiforme (GBM), the most common malignant primary brain tumor, represents a significant disease burden. GBM tumor cells disperse extensively throughout the brain parenchyma, and the need for tumor-specific drug targets and pharmacologic agents to inhibit cell migration and dispersal is great. The receptor protein tyrosine phosphatase mu (PTPmu) is a homophilic cell adhesion molecule. The full-length form of PTPmu is down-regulated in human glioblastoma. In this article, overexpression of full-length PTPmu is shown to suppress migration and survival of glioblastoma cells. Additionally, proteolytic cleavage is shown to be the mechanism of PTPmu down-regulation in glioblastoma cells. Proteolysis of PTPmu generates a series of proteolytic fragments, including a soluble catalytic intracellular domain fragment that translocates to the nucleus. Only proteolyzed PTPmu fragments are detected in human glioblastomas. Short hairpin RNA-mediated down-regulation of PTPmu fragments decreases glioblastoma cell migration and survival. A peptide inhibitor of PTPmu function blocks fragment-induced glioblastoma cell migration, which may prove to be of therapeutic value in GBM treatment. These data suggest that loss of cell surface PTPmu by proteolysis generates catalytically active PTPmu fragments that contribute to migration and survival of glioblastoma cells.

Identification of genes expressed as a result of lindane exposure in Oreochromis niloticus using differential display

In order to assess the effect of lindane exposure on gene expression in tilapia (Oreochromis niloticus), twenty male fish were individually weighted and injected intraperitoneally with a single dose of lindane (19.09 mg/kg bw) using corn oil as a carrier vehicle, while a second group of twenty male fish (controls) was only injected with the carrier vehicle. Groups of four fish each were then sacrificed at 3, 6, 12, 18 and 24h after treatment application and total RNA was extracted from liver tissue. The differential display (DD) technique was then used to identify differentially expressed cDNA fragments between treatment and control fish. A total of fifty cDNA fragments were isolated and sequenced, from which only four showed homology with genes previously described in other fish species, namely the immunoglobulin heavy chain (IgH), coagulation factor V (FV), casein kinase 2 alpha (CK2a), and the receptor protein-tyrosine-like phosphatase (RPT-LP). The expression of such genes was confirmed using quantitative real time-polymerase chain reaction (QRT-PCR). Results showed that lindane exposure triggered the differential expression of these genes during the first 6, 18 and 24h subsequent to treatment application, suggesting that lindane exposure can trigger a rapid immune system response in tilapias.

The trip of the tip: understanding the growth cone machinery

The central component in the road trip of axon guidance is the growth cone, a dynamic structure that is located at the tip of the growing axon. During its journey, the growth cone comprises both 'vehicle' and 'navigator'. Whereas the 'vehicle' maintains growth cone movement and contains the cytoskeletal structural elements of its framework, a motor to move forward and a mechanism to provide traction on the 'road', the 'navigator' aspect guides this system with spatial bias to translate environmental signals into directional movement. The understanding of the functions and regulation of the vehicle and navigator provides new insights into the cell biology of growth cone guidance.

Tumor-derived extracellular mutations of PTPRT /PTPrho are defective in cell adhesion

Receptor protein tyrosine phosphatase T (PTPRT/PTPrho) is frequently mutated in human cancers including colon, lung, gastric, and skin cancers. More than half of the identified tumor-derived mutations are located in the extracellular part of PTPrho. However, the functional significance of those extracellular domain mutations remains to be defined. Here we report that the extracellular domain of PTPrho mediates homophilic cell-cell aggregation. This homophilic interaction is very specific because PTPrho does not interact with its closest homologue, PTPmu, in a cell aggregation assay. We further showed that all five tumor-derived mutations located in the NH(2)-terminal MAM and immunoglobulin domains impair, to varying extents, their ability to form cell aggregates, indicating that those mutations are loss-of-function mutations. Our results suggest that PTPrho may play an important role in cell-cell adhesion and that mutational inactivation of this phosphatase could promote tumor migration and metastasis.

The receptor phosphatase HmLAR2 collaborates with focal adhesion proteins in filopodial tips to control growth cone morphology

Receptor protein tyrosine phosphatases (RPTPs) have been shown to play key roles in regulating axon guidance and synaptogenesis. HmLAR2, one of two closely related LAR-like RPTPs in the embryonic leech, is expressed in a few central neurons and in a unique segmentally-iterated peripheral cell, the comb cell (CC). Here we show that tagged HmLAR2-EGFP has a punctate pattern of expression in the growth cones of the CC, particularly at the tips of extending filopodia. Moreover, although expression of the wild-type EGFP-tagged receptor does not affect CC growth cone morphology, expression of a putative dominant-negative mutant of the receptor, CS-HmLAR2, leads to the enlargement of the growth cones, a shortening of filopodia, and errors in cellular tiling. RNAi of several candidate substrate signaling proteins, Lena (leech Ena/Vasp), beta-integrin and paxillin, but not beta-catenin, phenocopies particular aspects of the effects of HmLAR2 RNAi. For paxillin, which co-localizes with HmLAR2 at growth cone puncta, knock-down led to a reduction in the number of such puncta. Together, our data suggests that HmLAR2 regulates the morphology of the growth cone by controlling F-actin polymerization and focal adhesion complexes.

E-cadherin promotes retinal ganglion cell neurite outgrowth in a protein tyrosine phosphatase-mu-dependent manner

During development of the visual system, retinal ganglion cells (RGCs) require cell-cell adhesion molecules and extracellular matrix proteins for axon growth. In this study, we demonstrate that the classical cadherin, E-cadherin, is expressed in RGCs from E6 to E12 and promotes neurite outgrowth from all regions of the chick retina at E6, E8 and E10. E-cadherin is also expressed in the optic tectum. E-cadherin adhesion blocking antibodies specifically inhibit neurite outgrowth on an E-cadherin substrate. The receptor-type protein tyrosine phosphatase, PTPmu, associates with E-cadherin. In this manuscript, we demonstrate that antisense-mediated down-regulation of PTPmu, overexpression of catalytically inactive PTPmu and perturbation of endogenous PTPmu using a specific PTPmu inhibitor peptide results in a substantial reduction in neurite outgrowth on E-cadherin. Taken together, these findings demonstrate that E-cadherin is an important adhesion molecule for chick RGC neurite outgrowth and suggest that PTPmu expression and catalytic activity are required for outgrowth on an E-cadherin substrate.

Rho GTPases regulate PTPmu-mediated nasal neurite outgrowth and temporal repulsion of retinal ganglion cell neurons

Members of the receptor protein tyrosine phosphatase (RPTP) subfamily of cell adhesion molecules (CAMs) mediate neurite outgrowth and growth cone repulsion. PTPmu is a growth permissive substrate for nasal retinal ganglion cell (RGC) neurites and a growth inhibitory substrate for temporal RGCs. In this manuscript, we demonstrate that the distinct PTPmu-dependent phenotypes of nasal outgrowth and temporal repulsion are regulated by Rho GTPases. The role of Rho GTPases in the regulation of nasal outgrowth and temporal repulsion was tested by utilizing dominant negative and constitutively active forms of Rac1, RhoA and Cdc42 in Bonhoeffer stripe assays. Nasal neurite outgrowth on PTPmu was blocked by Cdc42-DN. Temporal repulsion to a PTPmu substrate was substantially reduced by addition of Cdc42-DN. The molecule that regulates the switch between permissive versus repulsive responses to PTPmu is Rac1 for temporal neurons. Inhibition of Rac1 is required for repulsion of temporal neurons. Interestingly, adding Rac1-CA to temporal RGC neurons converted PTPmu-dependent repulsion to a permissive response. In addition, adding exogenous Rac1-DN to nasal neurons induced a phenotype switch from a permissive to repulsive response to PTPmu. Together these data suggest that Cdc42 activity is required for both permissive and repulsive responses to PTPmu. However, the key to PTPmu-dependent repulsion is inhibition of Rac1 activity in temporal RGC neurons.