A novel role for PTEN in the inhibition of neurite outgrowth by myelin-associated glycoprotein in cortical neurons

Bridging the gap of axonal regeneration in the central nervous system: A state of the art review on central axonal regeneration

Neuronal regeneration in the central nervous system (CNS) is an important field of research with relevance to all types of neuronal injuries, including neurodegenerative diseases. The glial scar is a result of the astrocyte response to CNS injury. It is made up of many components creating a complex environment in which astrocytes play various key roles. The glial scar is heterogeneous, diverse and its composition depends upon the injury type and location. The heterogeneity of the glial scar observed in different situations of CNS damage and the consequent implications for axon regeneration have not been reviewed in depth. The gap in this knowledge will be addressed in this review which will also focus on our current understanding of central axonal regeneration and the molecular mechanisms involved. The multifactorial context of CNS regeneration is discussed, and we review newly identified roles for components previously thought to solely play an inhibitory role in central regeneration: astrocytes and p75NTR and discuss their potential and relevance for deciding therapeutic interventions. The article ends with a comprehensive review of promising new therapeutic targets identified for axonal regeneration in CNS and a discussion of novel ways of looking at therapeutic interventions for several brain diseases and injuries.

Targeting chondroitinase ABC to axons enhances the ability of chondroitinase to promote neurite outgrowth and sprouting

BACKGROUND

There is currently no effective treatment for promoting regeneration of injured nerves in patients who have sustained injury to the central nervous system such as spinal cord injury. Chondroitinase ABC is an enzyme, which promotes neurite outgrowth and regeneration. It has shown considerable promise as a therapy for these conditions. The aim of the study is to determine if targeting chondroitinase ABC expression to the neuronal axon can further enhance its ability to promote axon outgrowth. Long-distance axon regeneration has not yet been achieved, and would be a significant step in attaining functional recovery following spinal cord injury.

METHODOLOGY/PRINCIPAL FINDINGS

To investigate this, neuronal cultures were transfected with constructs encoding axon-targeted chondroitinase, non-targeted chondroitinase or GFP, and the effects on neuron outgrowth and sprouting determined on substrates either permissive or inhibitory to neuron regeneration. The mechanisms underlying the observed effects were also explored. Targeting chondroitinase to the neuronal axon markedly enhances its ability to promote neurite outgrowth. The increase in neurite length is associated with an upregulation of β-integrin staining at the axonal cell surface. Staining for phosphofocal adhesion kinase, is also increased, indicating that the β-integrins are in an activated state. Expression of chondroitinase within the neurons also resulted in a decrease in expression of PTEN and RhoA, molecules which present a block to neurite outgrowth, thus identifying two of the pathways by which ChABC promotes neurite outgrowth.

CONCLUSIONS / SIGNIFICANCE

The novel finding that targeting ChABC to the axon significantly enhances its ability to promote neurite extension, suggests that this may be an effective way of promoting long-distance axon regeneration following spinal cord injury. It could also potentially improve its efficacy in the treatment of other pathologies, where it has been shown to promote recovery, such as myocardial infarction, stroke and Parkinson's disease.

Exploring Optic Nerve Axon Regeneration

Background:

Traumatic optic nerve injury is a leading cause of irreversible blindness across the world and causes progressive visual impairment attributed to the dysfunction and death of retinal ganglion cells (RGCs). To date, neither pharmacological nor surgical interventions are sufficient to halt or reverse the progress of visual loss. Axon regeneration is critical for functional recovery of vision following optic nerve injury. After optic nerve injury, RGC axons usually fail to regrow and die, leading to the death of the RGCs and subsequently inducing the functional loss of vision. However, the detailed molecular mechanisms underlying axon regeneration after optic nerve injury remain poorly understood.

Methods:

Research content related to the detailed molecular mechanisms underlying axon regeneration after optic nerve injury have been reviewed.

Results:

The present review provides an overview of regarding potential strategies for axonal regeneration of RGCs and optic nerve repair, focusing on the role of cytokines and their downstream signaling pathways involved in intrinsic growth program and the inhibitory environment together with axon guidance cues for correct axon guidance. A more complete understanding of the factors limiting axonal regeneration will provide a rational basis, which contributes to develop improved treatments for optic nerve regeneration. These findings are encouraging and open the possibility that clinically meaningful regeneration may become achievable in the future.

Conclusion:

Combination of treatments towards overcoming growth-inhibitory molecules and enhancing intrinsic growth capacity combined with correct guidance using axon guidance cues is crucial for developing promising therapies to promote axon regeneration and functional recovery after ON injury.

[Signaling and functions of G-protein-coupled receptor 3 in cerebellar granular neurons]

G-protein-coupled receptor 3 (GPR3) is a member of the class A rhodopsin-type GPCR family and is highly expressed in various neurons. A unique feature of GPR3 is its ability to constitutively activate the Gαs protein without the addition of ligands, which results in the elevation of the basal level of intracellular cAMP. During the development of the cerebellum, GPR3 expression is upregulated in cerebellar granular neurons (CGNs) and maintained thereafter. In our previous studies, we showed that the intrinsic expression of GPR3 in CGNs is highly associated with neurite outgrowth, neurite differentiation, and neuronal survival. Recently, we have focused on the possible signaling pathways associated with GPR3-mediated neurite outgrowth in CGNs. Interestingly, GPR3-mediated neurite outgrowth is mediated by not only PKA-dependent signaling pathways but also PI3K-mediated signaling pathways. Moreover, the Gβγ-mediated signaling pathway is involved in GPR3-mediated neurite outgrowth. These results suggested that neural expression of GPR3 stimulates multiple downstream signaling pathways, contributing to the maintenance of homeostasis in neurons. Further precise analyses of constitutively active GPCRs may help in unveiling novel neuronal functions.

miR-181d-5p promotes neurite outgrowth in PC12 Cells via PI3K/Akt pathway

INTRODUCTION

miRNAs dysregulate in spinal cord injury (SCI) and have been demonstrated to play a crucial role in neurite outgrowth. However, the underlying mechanism remains elusive. In this study, we constructed a mouse model of SCI, extracted RNA from injured spinal cord tissue for the use of microarray assay. miR-181d-5p which is one of the most significantly expressed miRNAs in miRNA-mRNA network, abundantly expressed in center system and highly conserved across different spices, was chosen for our further study.

AIMS

To demonstrate whether miR-181d-5p can promote neurite outgrowth in PC12 cells via PI3K/Akt signaling pathway, we performed function analysis of miR-181d-5p with LV-miR-181d-5p and LV-sh-GFP to infect PC12 cells.

RESULTS

Through microarray assay analysis, we totally found 262 significantly expressed miRNAs and 2973 target genes in SCI and observed that their expression dynamically changed postinjury. Here, we provided enough evidences that the overexpression of miR181d-5p significantly decreased the expression of PTEN, upregulated p-Akt expression, increased neurite outgrowth-related proteins (GAP-43 and NF-200) and synaptic vesicle-related proteins (Synapsin and PSD95), and then promoted neurite outgrowth in PC12 cells. Furthermore, we confirmed that miR-181d-5p could directly target to the 3'-UTR of PTEN mRNA through dual-luciferase report assay.

CONCLUSIONS

Our study supports that aberrant expression of miRNAs is involved in the pathogenesis of SCI, miR-181d-5p plays an important role in neurite growth in PC12 cells via PI3K/Akt signaling pathway and may be a candidate target for the treatment of SCI in the future.

Sialic Acid Is Required for Neuronal Inhibition by Soluble MAG but not for Membrane Bound MAG

Myelin-Associated Glycoprotein (MAG), a major inhibitor of axonal growth, is a member of the immunoglobulin (Ig) super-family. Importantly, MAG (also known as Siglec-4) is a member of the Siglec family of proteins (sialic acid-binding, immunoglobulin-like lectins), MAG binds to complex gangliosides, specifically GD1a and/or GT1b. Therefore, it has been proposed as neuronal receptors for MAG inhibitory effect of axonal growth. Previously, we showed that MAG binds sialic acid through domain 1 at Arg118 and is able to inhibit axonal growth through domain 5. We developed a neurite outgrowth (NOG) assay, in which both wild type MAG and mutated MAG (MAG Arg118) are expressed on cells. In addition we also developed a soluble form NOG in which we utilized soluble MAG-Fc and mutated MAG (Arg118-Fc). Only MAG-Fc is able to inhibit NOG, but not mutated MAG (Arg118)-Fc that has been mutated at its sialic acid binding site. However, both forms of membrane bound MAG- and MAG (Arg118)- expressing cells still inhibit NOG. Here, we review various results from different groups regarding MAG’s inhibition of axonal growth. Also, we propose a model in which the sialic acid binding is not necessary for the inhibition induced by the membrane form of MAG, but it is necessary for the soluble form of MAG. This finding highlights the importance of understanding the different mechanisms by which MAG inhibits NOG in both the soluble fragmented form and the membrane-bound form in myelin debris following CNS damage.

Prospects for mTOR-mediated functional repair after central nervous system trauma

Recent research has suggested that the growth of central nervous system (CNS) axons during development is mediated through the PI3K/Akt/mammalian target of rapamycin (mTOR) intracellular signalling axis and that suppression of activity in this pathway occurs during maturity as levels of the phosphatase and tensin homologue (PTEN) rise and inhibit PI3K activation of mTOR, accounting for the failure of axon regeneration in the injured adult CNS. This hypothesis is supported by findings confirming that suppression of PTEN in experimental adult animals promotes impressive axon regeneration in the injured visual and corticospinal motor systems. This review focuses on these recent developments, discussing the therapeutic potential of a mTOR-based treatment aimed at promoting functional recovery in CNS trauma patients, recognising that to fulfil this ambition, the new therapy should aim to promote not only axon regeneration but also remyelination of regenerated axons, neuronal survival and re-innervation of denervated targets through accurate axonal guidance and synaptogenesis, all with minimal adverse effects. The translational challenges presented by the implementation of this new axogenic therapy are also discussed.

Extrinsic and intrinsic regulation of axon regeneration at a crossroads

Repair of the injured spinal cord is a major challenge in medicine. The limited intrinsic regenerative response mounted by adult central nervous system (CNS) neurons is further hampered by astrogliosis, myelin debris and scar tissue that characterize the damaged CNS. Improved axon regeneration and recovery can be elicited by targeting extrinsic factors as well as by boosting neuron-intrinsic growth regulators. Our knowledge of the molecular basis of intrinsic and extrinsic regulators of regeneration has expanded rapidly, resulting in promising new targets to promote repair. Intriguingly certain neuron-intrinsic growth regulators are emerging as promising targets to both stimulate growth and relieve extrinsic inhibition of regeneration. This crossroads between the intrinsic and extrinsic aspects of spinal cord injury is a promising target for effective therapies for this unmet need.

The intracellular domain of L1CAM binds to casein kinase 2α and is neuroprotective via inhibition of the tumor suppressors PTEN and p53

Cell adhesion molecule L1 promotes neuritogenesis and neuronal survival through triggering MAPK pathways. Based on the findings that L1 is associated with casein kinase 2 (CK2), and that deficiency in PTEN promotes neuritogenesis in vitro and regeneration after trauma, we examined the functional relationship between L1 and PTEN. In parallel, we investigated the tumor suppressor p53, which also regulates neuritogenesis. Here, we report that the intracellular domain of L1 binds to the subunit CK2α, and that knockdown of L1 leads to CK2 dephosphorylation and an increase in PTEN and p53 levels. Overexpression of L1, but not the L1 mutants L1 (S1181N, E1184V), which reduced binding between L1 and CK2, reduced expression levels of PTEN and p53 proteins, and enhanced levels of phosphorylated CK2α and mammalian target of rapamycin, which is a downstream effector of PTEN and p53. Treatment of neurons with a CK2 inhibitor or transfection with CK2α siRNA increased levels of PTEN and p53, and inhibited neuritogenesis. The combined observations indicate that L1 downregulates expression of PTEN and p53 via direct binding to CK2α. We suggest that L1 stimulates neuritogenesis by activating CK2α leading to decreased levels of PTEN and p53 via a novel, L1-triggered and CK2α-mediated signal transduction pathway. L1CAM (L1 cell adhesion molecule) is implicated in neural functions through the cognate src/MAP kinase signaling pathway. We now describe a novel signaling platform operating via the alpha subunit of casein kinase 2 which binds to the intracellular domain of L1. Knockdown of L1CAM leads to increased levels of tumor suppressor PTEN (phosphatase and tensin homolog) and p53, known to inhibit neuritogenesis in vitro and recovery from trauma in vivo. By activating this enzyme, L1CAM adds to its beneficial functions by decreasing the levels of PTEN and p53.

Polysialic acid as an antigen for monoclonal antibody HIgM12 to treat multiple sclerosis and other neurodegenerative disorders

CNS regeneration is a desirable goal for diseases of brain and spinal cord. Current therapeutic strategies for the treatment of multiple sclerosis (MS) aim to eliminate detrimental effects of the immune system, so far without reversing disability or affecting long-term prognosis in patients. Approachable molecular targets that stimulate CNS repair are not part of the clinical praxis or have not been identified yet. The purpose of this study was to identify the molecular target of the human monoclonal antibody HIgM12. HIgM12 reverses motor deficits in chronically demyelinated mice, a model of MS. Here, we identified polysialic acid (PSA) attached to the neural cell adhesion molecule (NCAM) as the antigen for HIgM12 by using different NCAM knockout strains and through PSA removal from the NCAM protein core. Antibody binding to CNS tissue and primary cells, antibody-mediated cell adhesion, and neurite outgrowth on HIgM12-coated nitrocellulose was detected only in the presence of PSA as assessed by western blotting, immunoprecipitation, immunocytochemistry, and histochemistry. We conclude that HIgM12 mediates its in vivo and in vitro effects through binding to PSA and has the potential to be an effective therapy for MS and neurodegenerative diseases. The human antibody HIgM12 stimulates neurite outgrowth in vitro and promotes function in chronically demyelinated mice, a model of multiple sclerosis. The cellular antigen for HIgM12 was undetermined. Here, we identified polysialic acid attached to NCAM (neural cell adhesion molecule) as the cellular target for HIgM12. This includes glial fibrillary acidic protein (GFAP)-positive mouse astrocytes (GFAP, red; HIgM12, green; DAPI, blue) among other cell types of the central nervous system. These findings indicate a new strategy for the treatment of neuro-motor disorders including multiple sclerosis.

Hyperactivity of newborn Pten knock-out neurons results from increased excitatory synaptic drive

Developing neurons must regulate morphology, intrinsic excitability, and synaptogenesis to form neural circuits. When these processes go awry, disorders, including autism spectrum disorder (ASD) or epilepsy, may result. The phosphatase Pten is mutated in some patients having ASD and seizures, suggesting that its mutation disrupts neurological function in part through increasing neuronal activity. Supporting this idea, neuronal knock-out of Pten in mice can cause macrocephaly, behavioral changes similar to ASD, and seizures. However, the mechanisms through which excitability is enhanced following Pten depletion are unclear. Previous studies have separately shown that Pten-depleted neurons can drive seizures, receive elevated excitatory synaptic input, and have abnormal dendrites. We therefore tested the hypothesis that developing Pten-depleted neurons are hyperactive due to increased excitatory synaptogenesis using electrophysiology, calcium imaging, morphological analyses, and modeling. This was accomplished by coinjecting retroviruses to either "birthdate" or birthdate and knock-out Pten in granule neurons of the murine neonatal dentate gyrus. We found that Pten knock-out neurons, despite a rapid onset of hypertrophy, were more active in vivo. Pten knock-out neurons fired at more hyperpolarized membrane potentials, displayed greater peak spike rates, and were more sensitive to depolarizing synaptic input. The increased sensitivity of Pten knock-out neurons was due, in part, to a higher density of synapses located more proximal to the soma. We determined that increased synaptic drive was sufficient to drive hypertrophic Pten knock-out neurons beyond their altered action potential threshold. Thus, our work contributes a developmental mechanism for the increased activity of Pten-depleted neurons.

shRNA against PTEN promotes neurite outgrowth of cortical neurons and functional recovery in spinal cord contusion rats

AIM

To explore neurite growth/regeneration and spinal cord injury repair after PTEN silencing via lentivirus-mediated RNAi.

MATERIALS & METHODS

Cortical neurons were seeded on or adjacent to chondroitin sulfate proteoglycans. The length, number and crossing behavior of neurites were calculated. Lentivirus was locally injected into spinal cord contusion rats. The functional recovery and immunohistochemical staining were analyzed.

RESULTS

Neurites with PTEN silencing exhibited significant enhancements in elongation, initiation and crossing ability when they encountered chondroitin sulfate proteoglycans in vitro. In vivo PTEN silencing improved functional recovery significantly, and promoted axon and synapse formation, but not scar formation.

CONCLUSIONS

PTEN silencing may be promising for spinal cord injury repair.

PTEN depletion decreases disease severity and modestly prolongs survival in a mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is the second most common genetic cause of death in childhood. However, no effective treatment is available to halt disease progression. SMA is caused by mutations in the survival motor neuron 1 (SMN1) gene. We previously reported that PTEN depletion leads to an increase in survival of SMN-deficient motor neurons. Here, we aimed to establish the impact of PTEN modulation in an SMA mouse model in vivo. Initial experiments using intramuscular delivery of adeno-associated vector serotype 6 (AAV6) expressing shRNA against PTEN in an established mouse model of severe SMA (SMNΔ7) demonstrated the ability to ameliorate the severity of neuromuscular junction pathology. Subsequently, we developed self-complementary AAV9 expressing siPTEN (scAAV9-siPTEN) to allow evaluation of the effect of systemic suppression of PTEN on the disease course of SMA in vivo. Treatment with a single injection of scAAV9-siPTEN at postnatal day 1 resulted in a modest threefold extension of the lifespan of SMNΔ7 mice, increasing mean survival to 30 days, compared to 10 days in untreated mice. Our data revealed that systemic PTEN depletion is an important disease modifier in SMNΔ7 mice, and therapies aimed at lowering PTEN expression may therefore offer a potential therapeutic strategy for SMA.

PTEN inhibition enhances neurite outgrowth in human embryonic stem cell-derived neuronal progenitor cells

We investigated the role of PTEN (phosphatase and tensin homolog deleted on chromosome 10) during neurite outgrowth of human embryonic stem cell (hESC)-derived neuronal progenitors. PTEN inhibits phosphoinositide 3-kinase (PI3K)/Akt signaling, a common and central outgrowth and survival pathway downstream of neuronal growth factors. It is known that PTEN inhibition, by either polymorphic mutation or gene deletion, can lead to the development of tumorigenesis (Stambolic et al., ; Tamura et al., ). However, temporary inhibition of PTEN, through pharmacological manipulation, could regulate signaling events such as the PI3K/Akt signaling pathway, leading to enhanced recovery of central nervous system (CNS) injury and disease. We demonstrate that pharmacological inhibition of PTEN in hESC-derived neuronal progenitors significantly increased neurite outgrowth in vitro in a dose- and time-dependent manner. Our results indicate that inhibition of PTEN augments neurite outgrowth beyond that of traditional methods such as elevation of intracellular cyclic adenosine monophosphate (cAMP) levels, and depends on upregulation of the PI3K/Akt signaling pathway and its downstream effectors, such as mammalian target of rapamycin (mTOR). PTEN inhibition also rescued neurite outgrowth over an inhibitory substrate in vitro. These findings indicate a remarkable impact on hESC-derived neuronal progenitor plasticity through PTEN inhibition. Overall, these findings identify a novel therapeutic strategy for neurite outgrowth in CNS injury and disease.

Subcellular targeting and dynamic regulation of PTEN: implications for neuronal cells and neurological disorders

PTEN is a lipid and protein phosphatase that regulates a diverse range of cellular mechanisms. PTEN is mainly present in the cytosol and transiently associates with the plasma membrane to dephosphorylate PI(3,4,5)P3, thereby antagonizing the PI3-Kinase signaling pathway. Recently, PTEN has been shown to associate also with organelles such as the endoplasmic reticulum (ER), the mitochondria, or the nucleus, and to be secreted outside of the cell. In addition, PTEN dynamically localizes to specialized sub-cellular compartments such as the neuronal growth cone or dendritic spines. The diverse localizations of PTEN imply a tight temporal and spatial regulation, orchestrated by mechanisms such as posttranslational modifications, formation of distinct protein–protein interactions, or the activation/recruitment of PTEN downstream of external cues. The regulation of PTEN function is thus not only important at the enzymatic activity level, but is also associated to its spatial distribution. In this review we will summarize (i) recent findings that highlight mechanisms controlling PTEN movement and sub-cellular localization, and (ii) current understanding of how PTEN localization is achieved by mechanisms controlling posttranslational modification, by association with binding partners and by PTEN structural or activity requirements. Finally, we will discuss the possible roles of compartmentalized PTEN in developing and mature neurons in health and disease.

Loss of GABAergic cortical neurons underlies the neuropathology of Lafora disease

Background

Lafora disease is an autosomal recessive form of progressive myoclonic epilepsy caused by defects in the EPM2A and EPM2B genes. Primary symptoms of the pathology include seizures, ataxia, myoclonus, and progressive development of severe dementia. Lafora disease can be caused by defects in the EPM2A gene, which encodes the laforin protein phosphatase, or in the NHLRC1 gene (also called EPM2B) codifying the malin E3 ubiquitin ligase. Studies on cellular models showed that laforin and malin interact and operate as a functional complex apparently regulating cellular functions such as glycogen metabolism, cellular stress response, and the proteolytic processes. However, the pathogenesis and the molecular mechanism of the disease, which imply either laforin or malin are poorly understood. Thus, the aim of our study is to elucidate the molecular mechanism of the pathology by characterizing cerebral cortex neurodegeneration in the well accepted murine model of Lafora disease EPM2A-/- mouse.

Results

In this article, we want to asses the primary cause of the neurodegeneration in Lafora disease by studying GABAergic neurons in the cerebral cortex. We showed that the majority of Lafora bodies are specifically located in GABAergic neurons of the cerebral cortex of 3 months-old EPM2A-/- mice. Moreover, GABAergic neurons in the cerebral cortex of younger mice (1 month-old) are decreased in number and present altered neurotrophins and p75NTR signalling.

Conclusions

Here, we concluded that there is impairment in GABAergic neurons neurodevelopment in the cerebral cortex, which occurs prior to the formation of Lafora bodies in the cytoplasm. The dysregulation of cerebral cortex development may contribute to Lafora disease pathogenesis.

Differential role of PTEN phosphatase in chemotactic growth cone guidance

Negatively targeting the tumor suppressor and phosphoinositide phosphatase PTEN (phosphatase and tensin homologue) promotes axon regrowth after injury. How PTEN functions in axon guidance has remained unknown. Here we report the differential role of PTEN in chemotactic guidance of axonal growth cones. Down-regulating PTEN expression in Xenopus laevis spinal neurons selectively abolished growth cone chemorepulsion but permitted chemoattraction. These findings persisted during cAMP-dependent switching of turning behaviors. Live cell imaging using a GFP biosensor revealed rapid PTEN-dependent depression of phosphatidylinositol 3,4,5-trisphosphate levels in the growth cone induced by the repellent myelin-associated glycoprotein. Moreover, down-regulating PTEN expression blocked negative remodeling of β1-integrin adhesions triggered by myelin-associated glycoprotein, yet permitted integrin clustering by a positive chemotropic treatment. Thus, PTEN negatively regulates growth cone phosphatidylinositol 3,4,5-trisphosphate levels and mediates chemorepulsion, whereas chemoattraction is PTEN-independent. Regenerative therapies targeting PTEN may therefore suppress growth cone repulsion to soluble cues while permitting attractive guidance, an essential feature for re-forming functional neural circuits.

Atypical protein kinase C and Par3 are required for proteoglycan-induced axon growth inhibition

When the CNS is injured, damaged axons do not regenerate. This failure is due in part to the growth-inhibitory environment that forms at the injury site. Myelin-associated molecules, repulsive axon guidance molecules, and extracellular matrix molecules including chondroitin sulfate proteoglycans (CSPGs) found within the glial scar inhibit axon regeneration but the intracellular signaling mechanisms triggered by these diverse molecules remain largely unknown. Here we provide biochemical and functional evidence that atypical protein kinase C (PKCζ) and polarity (Par) complex proteins mediate axon growth inhibition. Treatment of postnatal rat neurons in vitro with the NG2 CSPG, a major component of the glial scar, activates PKCζ, and this activation is both necessary and sufficient to inhibit axonal growth. NG2 treatment also activates Cdc42, increases the association of Par6 with PKCζ, and leads to a Par3-dependent activation of Rac1. Transfection of neurons with kinase-dead forms of PKCζ, dominant-negative forms of Cdc42, or mutant forms of Par6 that do not bind to Cdc42 prevent NG2-induced growth inhibition. Similarly, transfection with either a phosphomutant Par3 (S824A) or dominant-negative Rac1 prevent inhibition, whereas expression of constitutively active Rac1 inhibits axon growth on control surfaces. These results suggest a model in which NG2 binding to neurons activates PKCζ and modifies Par complex function. They also identify the Par complex as a novel therapeutic target for promoting axon regeneration after CNS injury.

PTEN modulators: a patent review

INTRODUCTION

PTEN (phosphatase and tensin homolog deleted on chromosome 10) plays a pivotal role in controlling intracellular signaling for cell survival and proliferation by inhibiting the PI3K/Akt pathway, and its dysfunction is associated with several neoplastic diseases. PTEN is frequently found mutated in many pathological conditions highlighting its importance in normal physiological function. Unlike several cellular proteins which are activated by phosphorylation, PTEN is inactivated upon phosphorylation by specific kinases which phosphorylate serine and threonine residues in its C-terminal region. Therefore, development of therapeutic agents that specifically target kinases and kinase-domain-containing proteins affecting PTEN would lead to the treatment of PTEN-related diseases.

AREAS COVERED

With increasing evidence on the role of PTEN in many human diseases, the present review focuses on the clinical relevance of PTEN with a comprehensive list of currently identified modulators of PTEN, and proposes potentially novel molecular targets which could aid in the development of drug candidates for the treatment of PTEN-related diseases such as cardiovascular diseases, diabetes, obesity, cancer, autism, Parkinson's and Alzheimer's diseases.

EXPERT OPINION

This review describes several target sites that could help in the development of novel drug candidates to regulate or restore the normal physiological functions of PTEN and are essential in the treatment of human diseases where PTEN plays a pivotal role.

Promoting optic nerve regeneration

Vision is the most important sense for humans and it is irreversibly impaired by axonal damage of retinal ganglion cells (RGCs) in the optic nerve due to the lack of axonal regeneration. The failure of regeneration is partially attributable to factors located in the inhibitory environment of the forming glial scar and myelin as well as an insufficient intrinsic ability for axonal regrowth. Moreover, RGCs undergo apoptotic cell death after optic nerve injury, eliminating any chance for regeneration. In this review, we discuss the different aspects that cause regenerative failure in the optic nerve. Moreover, we describe discoveries of the last two decades demonstrating that under certain circumstances mature RGCs can be transformed into an active regenerative state allowing these neurons to survive axotomy and to regenerate axons in the injured optic nerve. In this context we focus on the role of the cytokines ciliary neutrophic factor (CNTF) and leukemia inhibitory factor (LIF), their receptors and the downstream signaling pathways. Furthermore, we discuss strategies to overcome inhibitory signaling induced by molecules associated with optic nerve myelin and the glial scar as well as the regenerative outcome after combinatorial treatments. These findings are encouraging and may open the possibility that clinically meaningful regeneration may become achievable one day in the future.

Role of mTOR in neuroprotection and axon regeneration after inflammatory stimulation

Mature retinal ganglion cells (RGCs) do not normally regenerate injured axons, but degenerate after axotomy. However, inflammatory stimulation (IS) enables RGCs to survive axotomy and regenerate axons in the injured optic nerve. Similar effects are achieved by the genetic deletion of phosphatase and tensin homolog (PTEN) and subsequent mammalian target of rapamycin (mTOR) activation. Here, we report that IS prevents the axotomy-induced decrease of mTOR activity in RGCs in a CNTF/LIF-dependent manner. Inactivation of mTOR significantly reduced the number of long axons regenerating in the optic nerve, but surprisingly, did not affect the initial switch of RGCs into the regenerative state, or the neuroprotective effects associated with IS. In vitro, inhibition of mTOR activity reduced regeneration on myelin or chondroitin sulfate proteoglycans (CSPGs), but not on a growth-permissive substrate. Thus, mTOR activity is not generally required for neuroprotection or switching mature neurons into an active regenerative state, but it is important for the maintenance of the axonal growth state and overcoming of inhibitory effects caused by myelin and CSPGs.