Identification of an ectodomain within the LAR protein tyrosine phosphatase receptor that binds homophilically and activates signalling pathways promoting neurite outgrowth

Inhibition of LAR attenuates neuroinflammation through RhoA/IRS-1/Akt signaling pathway after intracerebral hemorrhage in mice

Leukocyte common antigen-related phosphatase (LAR) is widely expressed in the central nervous system and is known to regulate a variety of processes including cell growth, differentiation, and inflammation. However, little is currently known about LAR signaling mediated neuroinflammation after intracerebral hemorrhage (ICH). The objective of this study was to investigate the role of LAR in ICH using autologous blood injection-induced ICH mouse model. Expression of endogenous proteins, brain edema and neurological function after ICH were evaluated. Extracellular LAR peptide (ELP), an inhibitor of LAR, was administered to ICH mice and outcomes were evaluated. LAR activating-CRISPR or IRS inhibitor NT-157 was administered to elucidate the mechanism. The results showed that expressions of LAR, its endogenous agonist chondroitin sulfate proteoglycans (CSPGs) including neurocan and brevican, and downstream factor RhoA increased after ICH. Administration of ELP reduced brain edema, improved neurological function, and decreased microglia activation after ICH. ELP decreased RhoA and phosphorylated serine-IRS1, increased phosphorylated tyrosine-IRS1 and p-Akt, and attenuated neuroinflammation after ICH, which was reversed by LAR activating-CRISPR or NT-157. In conclusion, this study demonstrated that LAR contributed to neuroinflammation after ICH via RhoA/IRS-1 pathway, and ELP may be a potential therapeutic strategy to attenuate LAR mediated neuroinflammation after ICH.

LAR Receptor Tyrosine Phosphatase Family in Healthy and Diseased Brain

Protein phosphatases are major regulators of signal transduction and they are involved in key cellular mechanisms such as proliferation, differentiation, and cell survival. Here we focus on one class of protein phosphatases, the type IIA Receptor-type Protein Tyrosine Phosphatases (RPTPs), or LAR-RPTP subfamily. In the last decade, LAR-RPTPs have been demonstrated to have great importance in neurobiology, from neurodevelopment to brain disorders. In vertebrates, the LAR-RPTP subfamily is composed of three members: PTPRF (LAR), PTPRD (PTPδ) and PTPRS (PTPσ), and all participate in several brain functions. In this review we describe the structure and proteolytic processing of the LAR-RPTP subfamily, their alternative splicing and enzymatic regulation. Also, we review the role of the LAR-RPTP subfamily in neural function such as dendrite and axon growth and guidance, synapse formation and differentiation, their participation in synaptic activity, and in brain development, discussing controversial findings and commenting on the most recent studies in the field. Finally, we discuss the clinical outcomes of LAR-RPTP mutations, which are associated with several brain disorders.

Interaction between a Novel Oligopeptide Fragment of the Human Neurotrophin Receptor TrkB Ectodomain D5 and the C-Terminal Fragment of Tetanus Neurotoxin

This article presents experimental evidence and computed molecular models of a potential interaction between receptor domain D5 of TrkB with the carboxyl-terminal domain of tetanus neurotoxin (Hc-TeNT). Computational simulations of a novel small cyclic oligopeptide are designed, synthesized, and tested for possible tetanus neurotoxin-D5 interaction. A hot spot of this protein-protein interaction is identified in analogy to the hitherto known crystal structures of the complex between neurotrophin and D5. Hc-TeNT activates the neurotrophin receptors, as well as its downstream signaling pathways, inducing neuroprotection in different stress cellular models. Based on these premises, we propose the Trk receptor family as potential proteic affinity receptors for TeNT. In vitro, Hc-TeNT binds to a synthetic TrkB-derived peptide and acts similar to an agonist ligand for TrkB, resulting in phosphorylation of the receptor. These properties are weakened by the mutagenesis of three residues of the predicted interaction region in Hc-TeNT. It also competes with Brain-derived neurotrophic factor, a native binder to human TrkB, for the binding to neural membranes, and for uptake in TrkB-positive vesicles. In addition, both molecules are located together in vivo at neuromuscular junctions and in motor neurons.

The Protein Tyrosine Phosphatase Receptor Delta Regulates Developmental Neurogenesis

PTPRD is a receptor protein tyrosine phosphatase that is genetically associated with neurodevelopmental disorders. Here, we asked whether Ptprd mutations cause aberrant neural development by perturbing neurogenesis in the murine cortex. We show that loss of Ptprd causes increases in neurogenic transit-amplifying intermediate progenitor cells and cortical neurons and perturbations in neuronal localization. These effects are intrinsic to neural precursor cells since acute Ptprd knockdown causes similar perturbations. PTPRD mediates these effects by dephosphorylating receptor tyrosine kinases, including TrkB and PDGFRβ, and loss of Ptprd causes the hyperactivation of TrkB and PDGFRβ and their downstream MEK-ERK signaling pathway in neural precursor cells. Moreover, inhibition of aberrant TrkB or MEK activation rescues the increased neurogenesis caused by knockdown or homozygous loss of Ptprd. These results suggest that PTPRD regulates receptor tyrosine kinases to ensure appropriate numbers of intermediate progenitor cells and neurons, suggesting a mechanism for its genetic association with neurodevelopmental disorders.

The Biology of Regeneration Failure and Success After Spinal Cord Injury

Since no approved therapies to restore mobility and sensation following spinal cord injury (SCI) currently exist, a better understanding of the cellular and molecular mechanisms following SCI that compromise regeneration or neuroplasticity is needed to develop new strategies to promote axonal regrowth and restore function. Physical trauma to the spinal cord results in vascular disruption that, in turn, causes blood-spinal cord barrier rupture leading to hemorrhage and ischemia, followed by rampant local cell death. As subsequent edema and inflammation occur, neuronal and glial necrosis and apoptosis spread well beyond the initial site of impact, ultimately resolving into a cavity surrounded by glial/fibrotic scarring. The glial scar, which stabilizes the spread of secondary injury, also acts as a chronic, physical, and chemo-entrapping barrier that prevents axonal regeneration. Understanding the formative events in glial scarring helps guide strategies towards the development of potential therapies to enhance axon regeneration and functional recovery at both acute and chronic stages following SCI. This review will also discuss the perineuronal net and how chondroitin sulfate proteoglycans (CSPGs) deposited in both the glial scar and net impede axonal outgrowth at the level of the growth cone. We will end the review with a summary of current CSPG-targeting strategies that help to foster axonal regeneration, neuroplasticity/sprouting, and functional recovery following SCI.

Intramyocardial angiogenetic stem cells and epicardial erythropoietin save the acute ischemic heart

Ischemic heart failure is the leading cause of mortality worldwide. An early boost of intracardiac regenerative key mechanisms and angiogenetic niche signaling in cardiac mesenchymal stem cells (MSCs) could improve myocardial infarction (MI) healing. Epicardial erythropoietin (EPO; 300 U kg^-1) was compared with intraperitoneal and intramyocardial EPO treatments after acute MI in rats (n=156). Real-time PCR and confocal microscopy revealed that epicardial EPO treatment enhanced levels of intracardiac regenerative key indicators (SDF-1, CXCR4, CD34, Bcl-2, cyclin D1, Cdc2 and MMP2), induced transforming growth factor β (TGF-β)/WNT signaling in intramyocardial MSC niches through the direct activation of AKT and upregulation of upstream signals FOS and Fzd7, and augmented intracardiac mesenchymal proliferation 24 h after MI. Cardiac catheterization and tissue analysis showed superior cardiac functions, beneficial remodeling and increased capillary density 6 weeks after MI. Concomitant fluorescence-activated cell sorting, co-cultures with neonatal cardiomyocytes, angiogenesis assays, ELISA, western blotting and RAMAN spectroscopy demonstrated that EPO could promote cardiomyogenic differentiation that was specific of tissue origin and enhance paracrine angiogenetic activity in cardiac CD45^-CD44⁺DDR2⁺ MSCs. Epicardial EPO delivery might be the optimal route for efficient upregulation of regenerative key signals after acute MI. Early EPO-mediated stimulation of mesenchymal proliferation, synergistic angiogenesis with cardiac MSCs and direct induction of TGF-β/WNT signaling in intramyocardial cardiac MSCs could initiate an accelerated healing process that enhances cardiac recovery.

Perturbing chondroitin sulfate proteoglycan signaling through LAR and PTPσ receptors promotes a beneficial inflammatory response following spinal cord injury

BACKGROUND

Traumatic spinal cord injury (SCI) results in upregulation of chondroitin sulfate proteoglycans (CSPGs) by reactive glia that impedes repair and regeneration in the spinal cord. Degradation of CSPGs is known to be beneficial in promoting endogenous repair mechanisms including axonal sprouting/regeneration, oligodendrocyte replacement, and remyelination, and is associated with improvements in functional outcomes after SCI. Recent evidence suggests that CSPGs may regulate secondary injury mechanisms by modulating neuroinflammation after SCI. To date, the role of CSPGs in SCI neuroinflammation remains largely unexplored. The recent discovery of CSPG-specific receptors, leukocyte common antigen-related (LAR) and protein tyrosine phosphatase-sigma (PTPσ), allows unraveling the cellular and molecular mechanisms of CSPGs in SCI. In the present study, we have employed parallel in vivo and in vitro approaches to dissect the role of CSPGs and their receptors LAR and PTPσ in modulating the inflammatory processes in the acute and subacute phases of SCI.

METHODS

In a clinically relevant model of compressive SCI in female Sprague Dawley rats, we targeted LAR and PTPσ by two intracellular functionally blocking peptides, termed ILP and ISP, respectively. We delivered ILP and ISP treatment intrathecally to the injured spinal cord in a sustainable manner by osmotic mini-pumps for various time-points post-SCI. We employed flow cytometry, Western blotting, and immunohistochemistry in rat SCI, as well as complementary in vitro studies in primary microglia cultures to address our questions.

RESULTS

We provide novel evidence that signifies a key immunomodulatory role for LAR and PTPσ receptors in SCI. We show that blocking LAR and PTPσ reduces the population of classically activated M1 microglia/macrophages, while promoting alternatively activated M2 microglia/macrophages and T regulatory cells. This shift was associated with a remarkable elevation in pro-regenerative immune mediators, interleukin-10 (IL-10), and Arginase-1. Our parallel in vitro studies in microglia identified that while CSPGs do not induce an M1 phenotype per se, they promote a pro-inflammatory phenotype. Interestingly, inhibiting LAR and PTPσ in M1 and M2 microglia positively modulates their inflammatory response in the presence of CSPGs, and harnesses their ability for phagocytosis and mobilization. Interestingly, our findings indicate that CSPGs regulate microglia, at least in part, through the activation of the Rho/ROCK pathway downstream of LAR and PTPσ.

CONCLUSIONS

We have unveiled a novel role for LAR and PTPσ in regulating neuroinflammation in traumatic SCI. Our findings provide new insights into the mechanisms by which manipulation of CSPG signaling can promote recovery from SCI. More importantly, this work introduces the potential of ILP/ISP as a viable strategy for modulating the immune response following SCI and other neuroinflammatory conditions of the central nervous system.

Mechanisms of axon regeneration: The significance of proteoglycans

BACKGROUND

Therapeutics specific to neural injury have long been anticipated but remain unavailable. Axons in the central nervous system do not readily regenerate after injury, leading to dysfunction of the nervous system. This failure of regeneration is due to both the low intrinsic capacity of axons for regeneration and the various inhibitors emerging upon injury. After many years of concerted efforts, however, these hurdles to axon regeneration have been partially overcome.

SCOPE OF REVIEW

This review summarizes the mechanisms regulating axon regeneration. We highlight proteoglycans, particularly because it has become increasingly clear that these proteins serve as critical regulators for axon regeneration.

MAJOR CONCLUSIONS

Studies on proteoglycans have revealed that glycans not only assist in the modulation of protein functions but also act as main players-e.g., as functional ligands mediating intracellular signaling through specific receptors on the cell surface. By regulating clustering of the receptors, glycans in the proteoglycan moiety, i.e., glycosaminoglycans, promote or inhibit axon regeneration. In addition, proteoglycans are involved in various types of neural plasticity, ranging from synaptic plasticity to experience-dependent plasticity.

GENERAL SIGNIFICANCE

Although studies on proteins have progressively facilitated our understanding of the nervous system, glycans constitute a new frontier for further research and development in this field. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Two PTP receptors mediate CSPG inhibition by convergent and divergent signaling pathways in neurons

Receptor protein tyrosine phosphatase σ (PTPσ) and its subfamily member LAR act as transmembrane receptors that mediate growth inhibition of chondroitin sulfate proteoglycans (CSPGs). Inhibition of either receptor increases axon growth into and beyond scar tissues after CNS injury. However, it is unclear why neurons express two similar CSPG receptors, nor whether they use the same or different intracellular pathways. We have now studied the signaling pathways of these two receptors using N2A cells and primary neurons derived from knockout mice. We demonstrate that both receptors share certain signaling pathways (RhoA, Akt and Erk), but also use distinct signals to mediate CSPG actions. Activation of PTPσ by CSPGs selectively inactivated CRMP2, APC, S6 kinase and CREB. By contrast LAR activation inactivated PKCζ, cofilin and LKB1. For the first time, we propose a model of the signaling pathways downstream of these two CSPG receptors. We also demonstrate that deleting both receptors exhibits additive enhancement of axon growth in adult neuronal cultures in vitro. Our findings elucidate the novel downstream pathways of CSPGs and suggest potential synergy of blocking their two PTP receptors.

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.

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.

Molecular mechanisms of scar-sourced axon growth inhibitors

Astrogliosis is a defense response of the CNS to minimize primary damage and to repair injured tissues, but it ultimately generates harmful effects by upregulating inhibitory molecules to suppress neuronal elongation and forming potent barriers to axon regeneration. Chondroitin sulfate proteoglycans (CSPGs) are highly expressed by reactive scars and are potent contributors to the non-permissive environment in mature CNS. Surmounting strong inhibition by CSPG-rich scar is an important therapeutic goal for achieving functional recovery after CNS injuries. Currently, enzymatic digestion of CSPGs with locally applied chondroitinase ABC is the main in vivo approach to overcome scar inhibition, but several disadvantages may prevent using this bacterial enzyme as a therapeutic option for patients. A better understanding of molecular mechanisms underlying CSPG function may facilitate development of new effective therapies to overcome scar-mediated inhibition. Previous studies support that CSPGs act by non-specifically hindering the binding of matrix molecules to their cell surface receptors through steric interactions, but two members of the leukocyte common antigen related (LAR) phosphatase subfamily, protein tyrosine phosphatase σ and LAR, are functional receptors that bind CSPGs with high affinity and mediate CSPG inhibition. CSPGs may also act by binding two receptors for myelin-associated growth inhibitors, Nogo receptors 1 and 3. Thus, CSPGs inhibit axon growth through multiple mechanisms, making them especially potent and difficult therapeutic targets. Identification of CSPG receptors is not only important for understanding the scar-mediated growth suppression, but also for developing novel and selective therapies to promote axon sprouting and/or regeneration after CNS injuries. This article is part of a Special Issue entitled SI: Spinal cord injury.

Mechanisms of axon regeneration and its inhibition: roles of sulfated glycans

Axons in the peripheral nervous system can regenerate after injury, whereas axons in the central nervous system (CNS) do not readily regenerate. Intrinsic regenerating capacity and emerging inhibitors could explain these contrasting phenotypes. Among the inhibitors, sulfated sugar chains including chondroitin sulfate and keratan sulfate have recently attracted attention, since these sugar chains strongly inhibit axon regeneration and also induce dystrophic endball formation, a hallmark of injured axons in the adult mammalian CNS. In addition, chondroitin sulfate is a negative regulator of synaptic plasticity. To overcome the inability of CNS axons to regenerate, a comprehensive understanding of both the positive and negative regulations of axon regeneration is required. These may include signaling waves from the injury site to the nucleus, intracellular signals for growth cone formation and axon regeneration, intracellular signals for the inhibition of axon regeneration, and extracellular inhibitory signals and their receptors. This review addresses these issues, with a focus on the roles of chondroitin sulfate and keratan sulfate.

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.

Regulation of acrosome reaction by Liprin α3, LAR and its ligands in mouse spermatozoa

Zona pellucida-based induction of acrosome reaction (AR) is a popular and well-accepted hypothesis. However, this hypothesis is being challenged in recent years and it has been proposed that the cumulus cells might be the site of AR. In our previous study, we reported the presence of a synaptic protein Liprin α3 on sperm acrosome, and proposed its role in AR. This study was designed to understand the role of Liprin α3 and its interacting proteins in regulation of AR. It is observed that the presence of anti-Liprin α3 antibody inhibits the process of AR. Colocalization experiments demonstrate the coexistence of leucocyte antigen related (LAR) protein, Rab-interacting molecule (RIM) and Liprin α3 on sperm acrosome thereby completing the identification of all the members of RIM/MUNC/Rab3A/liprinα complex required for membrane fusion. This study demonstrates the effect of LAR ligands such as Syndecans, Nidogens and LAR wedge domain peptide on AR. We could see an increase in AR in presence of these ligands. On the basis of these data, we speculate that in presence of ligands or wedge peptide, LAR undergoes dimerization leading to inhibition of phosphatase activity and increase in AR. The presence of one of the ligands Syndecan-1 on cumulus cells led us to hypothesize that it is Syndecan which induces AR in vivo and thus another site of AR could lie in cumulus.

From molecular to nanotechnology strategies for delivery of neurotrophins: emphasis on brain-derived neurotrophic factor (BDNF)

Neurodegenerative diseases represent a major public health problem, but beneficial clinical treatment with neurotrophic factors has not been established yet. The therapeutic use of neurotrophins has been restrained by their instability and rapid degradation in biological medium. A variety of strategies has been proposed for the administration of these leading therapeutic candidates, which are essential for the development, survival and function of human neurons. In this review, we describe the existing approaches for delivery of brain-derived neurotrophic factor (BDNF), which is the most abundant neurotrophin in the mammalian central nervous system (CNS). Biomimetic peptides of BDNF have emerged as a promising therapy against neurodegenerative disorders. Polymer-based carriers have provided sustained neurotrophin delivery, whereas lipid-based particles have contributed also to potentiation of the BDNF action. Nanotechnology offers new possibilities for the design of vehicles for neuroprotection and neuroregeneration. Recent developments in nanoscale carriers for encapsulation and transport of BDNF are highlighted.

Potential role of growth factors in the management of spinal cord injury

OBJECTIVE

To review central nervous system growth factors and their therapeutic potential and clinical translation into spinal cord injury (SCI), as well as the challenges that have been encountered during clinical development.

METHODS

A systemic review of the available current and historical literature regarding central nervous system growth factors and clinical trials regarding their use in spinal cord injury was conducted.

RESULTS

The effectiveness of administering growth factors as a potential therapeutic strategy for SCI has been tested with the use of brain-derived neurotrophic factor, glial cell-derived neurotrophic factor, neurotrophin 3, and neurotrophin-4/5. Delivery of growth factors to injured SC has been tested by numerous methods. Unfortunately, most of clinical trials at this time are uncontrolled and have questionable results because of lack of efficacy and/or unacceptable side effects.

CONCLUSIONS

There is promise in the use of specific growth factors therapeutically for SCI. However, more studies involving neuronal regeneration and functional recovery are needed, as well the development of delivery methods that allow sufficient quantity of growth factors while restricting their distribution to target sites.

Scar-mediated inhibition and CSPG receptors in the CNS

Severed axons in adult mammals do not regenerate appreciably after central nervous system (CNS) injury due to developmentally determined reductions in neuron-intrinsic growth capacity and extracellular environment for axon elongation. Chondroitin sulfate proteoglycans (CSPGs), which are generated by reactive scar tissues, are particularly potent contributors to the growth-limiting environment in mature CNS. Thus, surmounting the strong inhibition by CSPG-rich scar is an important therapeutic goal for achieving functional recovery after CNS injuries. As of now, the main in vivo approach to overcoming inhibition by CSPGs is enzymatic digestion with locally applied chondroitinase ABC (ChABC), but several disadvantages may prevent using this bacterial enzyme as a therapeutic option for patients. A better understanding of the molecular mechanisms underlying CSPG action is needed in order to develop more effective therapies to overcome CSPG-mediated inhibition of axon regeneration and/or sprouting. Because of their large size and dense negative charges, CSPGs were thought to act by non-specifically hindering the binding of matrix molecules to their cell surface receptors through steric interactions. Although this may be true, recent studies indicate that two members of the leukocyte common antigen related (LAR) phosphatase subfamily, protein tyrosine phosphatase σ (PTPσ) and LAR, are functional receptors that bind CSPGs with high affinity and mediate CSPG inhibitory effects. CSPGs also may act by binding to two receptors for myelin-associated growth inhibitors, Nogo receptors 1 and 3 (NgR1 and NgR3). If confirmed, it would suggest that CSPGs have multiple mechanisms by which they inhibit axon growth, making them especially potent and difficult therapeutic targets. Identification of CSPG receptors is not only important for understanding the scar-mediated growth suppression, but also for developing novel and selective therapies to promote axon sprouting and/or regeneration after CNS injuries, including spinal cord injury (SCI).

Brain-derived neurotrophic factor and neuropsychiatric disorders

Brain derived neurotrophic factor (BDNF) is the most prevalent growth factor in the central nervous system (CNS). It is essential for the development of the CNS and for neuronal plasticity. Because BDNF plays a crucial role in development and plasticity of the brain, it is widely implicated in psychiatric diseases. This review provides a summary of clinical and preclinical evidence for the involvement of this ubiquitous growth factor in major depressive disorder, schizophrenia, addiction, Rett syndrome, as well as other psychiatric and neurodevelopmental diseases. In addition, the review includes a discussion of the role of BDNF in the mechanism of action of pharmacological therapies currently used to treat these diseases, such antidepressants and antipsychotics. The review also covers a critique of experimental therapies such as BDNF mimetics and discusses the value of BDNF as a target for future drug development.

Leukocyte common antigen-related phosphatase is a functional receptor for chondroitin sulfate proteoglycan axon growth inhibitors

Chondroitin sulfate proteoglycans (CSPGs) are a family of extracellular matrix molecules with various functions in regulating tissue morphogenesis, cell division, and axon guidance. A number of CSPGs are highly upregulated by reactive glial scar tissues after injuries and form a strong barrier for axonal regeneration in the adult vertebrate CNS. Although CSPGs may negatively regulate axonal growth via binding and altering activity of other growth-regulating factors, the molecular mechanisms by which CSPGs restrict axonal elongation are not well understood. Here, we identified a novel receptor mechanism whereby CSPGs inhibit axonal growth via interactions with neuronal transmembrane leukocyte common antigen-related phosphatase (LAR). CSPGs bind LAR with high affinity in transfected COS-7 cells and coimmunoprecipitate with LAR expressed in various tissues including the brain and spinal cord. CSPG stimulation enhances activity of LAR phosphatase in vitro. Deletion of LAR in knock-out mice or blockade of LAR with sequence-selective peptides significantly overcomes neurite growth restrictions of CSPGs in neuronal cultures. Intracellularly, CSPG-LAR interaction mediates axonal growth inhibition of neurons partially via inactivating Akt and activating RhoA signals. Systemic treatments with LAR-targeting peptides in mice with thoracic spinal cord transection injuries induce significant axon growth of descending serotonergic fibers in the vicinity of the lesion and beyond in the caudal spinal cord and promote locomotor functional recovery. Identification of LAR as a novel CSPG functional receptor provides a therapeutic basis for enhancing axonal regeneration and functional recovery after CNS injuries in adult mammals.

Olfactory Receptor-Related Duplicons Mediate a Microdeletion at 11q13.2q13.4 Associated with a Syndromic Phenotype

By array-CGH, we identified a cryptic deletion of about 3.4 Mb involving the chromosomal region 11q13.2q13.4 in a child with speech and developmental delay. Highly homologous segmental duplications related to the well-known olfactory receptor (OR)-containing clusters at 8p and 4p are located at the breakpoints of the imbalance and may be involved in its occurrence. Although these structural features are known to promote recurrent chromosomal rearrangements and previous studies had included the 11q13.2q13.4 deletion region among those considered potentially more unstable, neither deletions nor duplications of this region had been reported until now. Among the deleted genes, SHANK2 might play a role in the phenotype of the patient since it encodes a postsynaptic scaffolding protein similar to SHANK3, whose haploinsufficiency is a well-known cause of severe speech delay and autistic-like behavior, and recently deletions and mutations of SHANK2 have been described in patients with an autistic spectrum disorder or mental retardation.

Novel folate binding protein-1 interactions in embryonic orofacial tissue

AIM

To identify proteins with which FolBp1 may interact within lipid rafts in tissue derived from embryonic orofacial tissue.

METHODS

A yeast two-hybrid screen of a cDNA library, derived from orofacial tissue from gestational day 11 to 13 mouse embryos, was conducted.

KEY FINDINGS

Using the full-length FolBp1 protein as bait, two proteins that bind FolBp1 were identified, Bat2d, and a fibronectin type III-containing domain protein. Results were confirmed by glutathione S-transferase pull-down assays.

SIGNIFICANCE

As a component of membrane lipid raft protein complexes, these binding proteins may represent "helper" or chaperone proteins that associate with FolBp1 in order to facilitate the transport of folate across the plasma membrane. The protein-protein interactions detected, while limited in number, may be critical in mediating the role of FolBp1 in folate transport, particularly in the developing embryo.

PrPs: Proteins with a purpose: Lessons from the zebrafish

The best-known attribute of the prion protein (PrP) is its tendency to misfold into a rogue isoform. Much less understood is how this misfolded isoform causes deadly brain illnesses. Neurodegeneration in prion disease is often seen as a consequence of abnormal PrP function yet, amazingly little is known about the normal, physiological role of PrP. In particular, the absence of obvious phenotypes in PrP knockout mice has prevented scientists from answering this important question. Using knockdown approaches, we previously produced clear PrP loss-of-function phenotypes in zebrafish embryos. Analysis of these phenotypes revealed that PrP can modulate E-cadherin-based cell-cell adhesion, thereby controlling essential morphogenetic cell movements in the early gastrula. Our data also showed that PrP itself can elicit homophilic cell-cell adhesion and trigger intracellular signaling via Src-related kinases. Importantly, these molecular functions of PrP are conserved from fish to mammals. Here we discuss the use of the zebrafish in prion biology and how it may advance our understanding of the roles of PrP in health and disease.

The NGL family of leucine-rich repeat-containing synaptic adhesion molecules

Cell adhesion molecules at neuronal synapses regulate diverse aspects of synaptic development, including axo-dendritic contact establishment, early synapse formation, and synaptic maturation. Recent studies have identified several synaptogenic adhesion molecules. The NGL (netrin-G ligand; LRRC4) family of synaptic cell adhesion molecules belongs to the superfamily of leucine-rich repeat (LRR) proteins. The three known members of the NGL family, NGL-1, NGL-2, and NGL-3, are mainly localized to the postsynaptic side of excitatory synapses, and interact with the presynaptic ligands, netrin-G1, netrin-G2, and LAR, respectively. NGLs interact with the abundant postsynaptic density (PSD) protein, PSD-95, and other postsynaptic proteins, including NMDA receptors. These interactions are thought to couple synaptic adhesion events to the assembly of synaptic proteins. In addition, NGL proteins regulate axonal outgrowth and lamina-specific dendritic segmentation, suggesting that the NGL-dependent adhesion system is important for the development of axons, dendrites, and synapses. Consistent with these functions, defects in NGLs and their ligands are associated with impaired learning and memory, hyperactivity, and an abnormal acoustic startle response in transgenic mice, and schizophrenia, bipolar disorder, and Rett syndrome in human patients.

A novel liver specific isoform of the rat LAR transcript is expressed as a truncated isoform encoded from a 5'UTR located within intron 11

BACKGROUND

The leukocyte common antigen related receptor (LAR) protein has been shown to modulate the signal transduction of a number of different growth factors, including insulin and insulin-like growth factor 1. Splice variants exhibit differing roles and are expressed according to tissue type and developmental stage.

RESULTS

Using 5'RACE, we identified a 5'UTR within intron 11 of the rat LAR gene. We demonstrated that this gives rise to a novel isoform of the LAR transcript encoded from the identified region within intron 11. By priming across the site from exon 11 to exon 15 we show that the novel 5'UTR is not represented in the full-length transcript and thus, it produces a truncated form of the LAR mRNA. We examined the tissue distribution of this novel isoform and found it to be exclusively expressed in liver. We additionally identified a liver specific 150 kDa band with western blotting which we propose may represent the protein product of the novel transcript. Luciferase assays showed the region immediately upstream of the 5'UTR to possesses considerable promoter activity and that this may be conferred by the presence of a number of putative binding sites for liver enriched transcription factors.

CONCLUSION

In summary, we describe a novel, liver specific, truncated isoform of the LAR transcript transcribed under the control of an intronic promoter, potentially representing a previously unidentified modulator of hepatic insulin signalling.

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.

Protein tyrosine phosphatases: regulatory mechanisms

Protein-tyrosine phosphatases are tightly controlled by various mechanisms, ranging from differential expression in specific cell types to restricted subcellular localization, limited proteolysis, post-translational modifications affecting intrinsic catalytic activity, ligand binding and dimerization. Here, we review the regulatory mechanisms found to control the classical protein-tyrosine phosphatases.

Presenilin/gamma-secretase-mediated cleavage regulates association of leukocyte-common antigen-related (LAR) receptor tyrosine phosphatase with beta-catenin

Leukocyte-common antigen-related (LAR) receptor tyrosine phosphatase regulates cell adhesion and formation of functional synapses and neuronal networks. Here we report that LAR is sequentially cleaved by alpha- and presenilin (PS)/gamma-secretases, which also affect signaling and/or degradation of type-I membrane proteins including the Alzheimer disease-related beta-amyloid precursor protein. Similar to the previously characterized PS/gamma-secretase substrates, inhibition of gamma-secretase activity resulted in elevated LAR C-terminal fragment (LAR-CTF) levels in stably LAR-overexpressing Chinese hamster ovary (CHO) cells, human neuroglioma cells, and mouse cortical neurons endogenously expressing LAR. Furthermore, LAR-CTF levels increased in cells lacking functional PS, indicating that gamma-secretase-mediated cleavage of LAR was PS-dependent. Inhibition of alpha-secretase activity by TAPI-1 treatment blocked LAR-CTF accumulation, demonstrating that prior ectodomain shedding was prerequisite for PS/gamma-secretase-mediated cleavage of LAR. Moreover, we identified the product of PS/gamma-secretase cleavage, LAR intracellular domain (LICD), both in vitro and in cells overexpressing full-length (FL) LAR or LAR-CTFs. LAR localizes to cadherin-beta-catenin-based cellular junctions. Assembly and disassembly of these junctions are regulated by tyrosine phosphorylation. We found that endogenous tyrosine-phosphorylated beta-catenin coimmunoprecipitated with LAR in CHO cells. However, when PS/gamma-secretase activity was inhibited, the association between LAR and beta-catenin significantly diminished. In addition to cell adhesion, beta-catenin is involved in transcriptional regulation. We observed that LICD significantly decreased transcription of cyclin D1, one of the beta-catenin target genes. Thus, our results show that PS/gamma-secretase-mediated cleavage of LAR controls LAR-beta-catenin interaction, suggesting an essential role for PS/gamma-secretase in the regulation of LAR signaling.

LAR protein tyrosine phosphatase receptor associates with TrkB and modulates neurotrophic signaling pathways

The identities of receptor protein tyrosine phosphatases (PTPs) that associate with Trk protein tyrosine kinase (PTK) receptors and modulate neurotrophic signaling are unknown. The leukocyte common antigen-related (LAR) receptor PTP is present in neurons expressing TrkB, and like TrkB is associated with caveolae and regulates survival and neurite outgrowth. We tested the hypothesis that LAR associates with TrkB and regulates neurotrophic signaling in embryonic hippocampal neurons. Coimmunoprecipitation and coimmunostaining demonstrated LAR interaction with TrkB that is increased by BDNF exposure. BDNF neurotrophic activity was reduced in LAR-/- and LAR siRNA-treated LAR+/+ neurons and was augmented in LAR-transfected neurons. In LAR-/- neurons, BDNF-induced activation of TrkB, Shc, AKT, ERK, and CREB was significantly decreased; while in LAR-transfected neurons, BDNF-induced CREB activation was augmented. Similarly, LAR+/+ neurons treated with LAR siRNA demonstrated decreased activation of Trk and AKT. LAR is known to activate the Src PTK by dephosphorylation of its negative regulatory domain and Src transactivates Trk. In LAR-/- neurons, or neurons treated with LAR siRNA, phosphorylation of the Src regulatory domain was increased (indicating Src inactivation), consistent with a role for Src in mediating LAR's ability to up-regulate neurotrophic signaling. Interactions between LAR, TrkB, and Src were further confirmed by the findings that Src coimmunoprecipitated with LAR, that the Src inhibitor PP2 blocked the ability of LAR to augment TrkB signaling, and that siRNA-induced depletion of Src decreased LAR interaction with TrkB. These studies demonstrate that receptor PTPs can associate with Trk complexes and promote neurotrophic signaling and point to receptor PTP-based strategies as a novel approach for modulating neurotrophin function.

Dimerization of protein tyrosine phosphatase sigma governs both ligand binding and isoform specificity

Signaling through receptor protein tyrosine phosphatases (RPTPs) can influence diverse processes, including axon development, lymphocyte activation, and cell motility. The molecular regulation of these enzymes, however, is still poorly understood. In particular, it is not known if, or how, the dimerization state of RPTPs is related to the binding of extracellular ligands. Protein tyrosine phosphatase sigma (PTPsigma) is an RPTP with major isoforms that differ in their complements of fibronectin type III domains and in their ligand-binding specificities. In this study, we show that PTPsigma forms homodimers in the cell, interacting at least in part through the transmembrane region. Using this knowledge, we provide the first evidence that PTPsigma ectodomains must be presented as dimers in order to bind heterophilic ligands. We also provide evidence of how alternative use of fibronectin type III domain complements in two major isoforms of PTPsigma can alter the ligand binding specificities of PTPsigma ectodomains. The data suggest that the alternative domains function largely to change the rotational conformations of the amino-terminal ligand binding sites of the ectodomain dimers, thus imparting novel ligand binding properties. These findings have important implications for our understanding of how heterophilic ligands interact with, and potentially regulate, RPTPs.

Protein tyrosine phosphatases: from genes, to function, to disease

The protein tyrosine phosphatase (PTP) superfamily of enzymes functions in a coordinated manner with protein tyrosine kinases to control signalling pathways that underlie a broad spectrum of fundamental physiological processes. In this review, I describe recent breakthroughs in our understanding of the role of the PTPs in the regulation of signal transduction and the aetiology of human disease.

N-cadherin is an in vivo substrate for protein tyrosine phosphatase sigma (PTPsigma) and participates in PTPsigma-mediated inhibition of axon growth

Protein tyrosine phosphatase sigma (PTPsigma) belongs to the LAR family of receptor tyrosine phosphatases and was previously shown to negatively regulate axon growth. The substrate for PTPsigma and the effector(s) mediating this inhibitory effect were unknown. Here we report the identification of N-cadherin as an in vivo substrate for PTPsigma. Using brain lysates from PTPsigma knockout mice, in combination with substrate trapping, we identified a hyper-tyrosine-phosphorylated protein of approximately 120 kDa in the knockout animals (relative to sibling controls), which was identified by mass spectrometry and immunoblotting as N-cadherin. beta-Catenin also precipitated in the complex and was also a substrate for PTPsigma. Dorsal root ganglion (DRG) neurons, which highly express endogenous N-cadherin and PTPsigma, exhibited a faster growth rate in the knockout mice than in the sibling controls when grown on laminin or N-cadherin substrata. However, when N-cadherin function was disrupted by an inhibitory peptide or lowering calcium concentrations, the differential growth rate between the knockout and sibling control mice was greatly diminished. These results suggest that the elevated tyrosine phosphorylation of N-cadherin in the PTPsigma(-/-) mice likely disrupted N-cadherin function, resulting in accelerated DRG nerve growth. We conclude that N-cadherin is a physiological substrate for PTPsigma and that N-cadherin (and likely beta-catenin) participates in PTPsigma-mediated inhibition of axon growth.

Cell surface nucleolin on developing muscle is a potential ligand for the axonal receptor protein tyrosine phosphatase-sigma

Reversible tyrosine phosphorylation, catalyzed by receptor tyrosine kinases and receptor tyrosine phosphatases, plays an essential part in cell signaling during axonal development. Receptor protein tyrosine phosphatase-sigma has been implicated in the growth, guidance and repair of retinal axons. This phosphatase has also been implicated in motor axon growth and innervation. Insect orthologs of receptor protein tyrosine phosphatase-sigma are also implicated in the recognition of muscle target cells. A potential extracellular ligand for vertebrate receptor protein tyrosine phosphatase-sigma has been previously localized in developing skeletal muscle. The identity of this muscle ligand is currently unknown, but it appears to be unrelated to the heparan sulfate ligands of receptor protein tyrosine phosphatase-sigma. In this study, we have used affinity chromatography and tandem MS to identify nucleolin as a binding partner for receptor protein tyrosine phosphatase-sigma in skeletal muscle tissue. Nucleolin, both from tissue lysates and in purified form, binds to receptor protein tyrosine phosphatase-sigma ectodomains. Its expression pattern also overlaps with that of the receptor protein tyrosine phosphatase-sigma-binding partner previously localized in muscle, and nucleolin can also be found in retinal basement membranes. We demonstrate that a significant amount of muscle-associated nucleolin is present on the cell surface of developing myotubes, and that two nucleolin-binding components, lactoferrin and the HB-19 peptide, can block the interaction of receptor protein tyrosine phosphatase-sigma ectodomains with muscle and retinal basement membranes in tissue sections. These data suggest that muscle cell surface-associated nucleolin represents at least part of the muscle binding site for axonal receptor protein tyrosine phosphatase-sigma and that nucleolin may also be a necessary component of basement membrane binding sites of receptor protein tyrosine phosphatase-sigma.

Downregulation of the LAR protein tyrosine phosphatase receptor is associated with increased dentate gyrus neurogenesis and an increased number of granule cell layer neurons

Growth factors stimulating neurogenesis act through protein tyrosine kinases which are counterbalanced by protein tyrosine phosphatases (PTPs); thus, downregulation of progenitor PTP function might provide a novel strategy for promoting neurogenesis. We tested the hypotheses that the leukocyte common antigen-related (LAR) PTP is present in adult dentate gyrus progenitors, and that its downregulation would promote neurogenesis. In adult mice, LAR immunostaining was present in Ki-67- and PCNA-positive subgranular zone cells. At 1 h post-BrdU administration, LAR-/- mice demonstrated an approximately 3-fold increase in BrdU- and PCNA-positive cells, indicating increased progenitor proliferation. At 1 day and 4 weeks following 6 days of BrdU administration, LAR-/- mice exhibited a significant increase in BrdU and NeuN colabeled cells consistent with increased neurogenesis. In association with increased neurogenesis in LAR-/- mice, stereological analysis revealed a significant 37% increase in the number of neurons present in the granule cell layer. In cultured progenitor clones derived from LAR+/+ mice, LAR immunostaining was present in PCNA- and BrdU-positive cells. Progenitor clones derived from adult LAR-/- hippocampus or LAR+/+ clones made LAR-deficient with LAR siRNA demonstrated increased proliferation and, under differentiation conditions, increased proportions of Tuj1- and MAP2-positive cells. These studies introduce LAR as the first PTP found to be expressed in dentate progenitors and point to inhibition of LAR as a potential strategy for promoting neurogenesis. These findings also provide a rare in vivo demonstration of an association between increased dentate neurogenesis and an expanded population of granule cell layer neurons.