Myelin suppresses axon regeneration by PIR-B/SHP-mediated inhibition of Trk activity

The effects and possible mechanisms of whole-body vibration on cognitive function: A narrative review

Whole-body vibration (WBV) is a physical stimulation method that transmits mechanical oscillations to the entire body through a vibration platform or device. Biokinetic and epidemiologic studies have shown that prolonged exposure to high-intensity WBV increases health risks, primarily to the lumbar spine and the nervous system connected to it. There is currently insufficient evidence to demonstrate a quantitative relationship between vibration exposure and risk of health effects. The positive effects of WBV on increasing muscle strength and improving balance and flexibility are well known, but its effects on cognitive function are more complex, with mixed findings, largely related to vibration conditions, including frequency, amplitude, and duration. Studies have shown that short-term low-frequency WBV may have a positive impact on cognitive function, demonstrates potential rehabilitation benefits in enhancing learning and memory, possibly by promoting neuromuscular coordination and enhancing neural plasticity. However, long term exposure to vibration may lead to chronic stress in nerve tissue, affecting nerve conduction efficiency and potentially interfering with neuroprotective mechanisms, thereby having a negative impact on cognitive ability, even causes symptoms such as cognitive decline, mental fatigue, decreased attention, and drowsiness. This literature review aimed to explore the effects of WBV on cognitive function and further to analyze the possible mechanisms. Based on the analysis of literatures, we came to the conclusion that the impact of WBV on cognitive function depends mainly on the frequency and duration of vibration, short-term low-frequency WBV may have a positive impact on cognitive function, while long term exposure to WBV may lead to cognitive decline, and the mechanisms may be involved in neuroinflammation, oxidative stress, synaptic plasticity, and neurotransmitter changes. This review may provide some theoretical foundations and guidance for the prevention and treatment of WBV induced cognitive impairment.

CD3ζ-Mediated Signaling Protects Retinal Ganglion Cells in Glutamate Excitotoxicity of the Retina

Excessive levels of glutamate activity could potentially damage and kill neurons. Glutamate excitotoxicity is thought to play a critical role in many CNS and retinal diseases. Accordingly, glutamate excitotoxicity has been used as a model to study neuronal diseases. Immune proteins, such as major histocompatibility complex (MHC) class I molecules and their receptors, play important roles in many neuronal diseases, while T-cell receptors (TCR) are the primary receptors of MHCI. We previously showed that a critical component of TCR, CD3ζ, is expressed by mouse retinal ganglion cells (RGCs). The mutation of CD3ζ or MHCI molecules compromises the development of RGC structure and function. In this study, we investigated whether CD3ζ-mediated molecular signaling regulates RGC death in glutamate excitotoxicity. We show that mutation of CD3ζ significantly increased RGC survival in NMDA-induced excitotoxicity. In addition, we found that several downstream molecules of TCR, including Src (proto-oncogene tyrosine-protein kinase) family kinases (SFKs) and spleen tyrosine kinase (Syk), are expressed by RGCs. Selective inhibition of an SFK member, Hck, or Syk members, Syk or Zap70, significantly increased RGC survival in NMDA-induced excitotoxicity. These results provide direct evidence to reveal the underlying molecular mechanisms that control RGC death under disease conditions.

Lhx2 promotes axon regeneration of adult retinal ganglion cells and rescues neurodegeneration in mouse models of glaucoma

The axons of retinal ganglion cells (RGCs) form the optic nerve, transmitting visual information from the eye to the brain. Damage or loss of RGCs and their axons is the leading cause of visual functional defects in traumatic injury and degenerative diseases such as glaucoma. However, there are no effective clinical treatments for nerve damage in these neurodegenerative diseases. Here, we report that LIM homeodomain transcription factor Lhx2 promotes RGC survival and axon regeneration in multiple animal models mimicking glaucoma disease. Furthermore, following N-methyl-D-aspartate (NMDA)-induced excitotoxicity damage of RGCs, Lhx2 mitigates the loss of visual signal transduction. Mechanistic analysis revealed that overexpression of Lhx2 supports axon regeneration by systematically regulating the transcription of regeneration-related genes and inhibiting transcription of Semaphorin 3C (Sema3C). Collectively, our studies identify a critical role of Lhx2 in promoting RGC survival and axon regeneration, providing a promising neural repair strategy for glaucomatous neurodegeneration.

Immune receptors and aging brain

Aging brings about a myriad of degenerative processes throughout the body. A decrease in cognitive abilities is one of the hallmark phenotypes of aging, underpinned by neuroinflammation and neurodegeneration occurring in the brain. This review focuses on the role of different immune receptors expressed in cells of the central and peripheral nervous systems. We will discuss how immune receptors in the brain act as sentinels and effectors of the age-dependent shift in ligand composition. Within this 'old-age-ligand soup,' some immune receptors contribute directly to excessive synaptic weakening from within the neuronal compartment, while others amplify the damaging inflammatory environment in the brain. Ultimately, chronic inflammation sets up a positive feedback loop that increases the impact of immune ligand-receptor interactions in the brain, leading to permanent synaptic and neuronal loss.

Brain-derived neurotrophic factor (BDNF) induces antagonistic action to Nogo signaling by the upregulation of lateral olfactory tract usher substance (LOTUS) expression

Neurons in the central nervous system (CNS) have limited capacity for axonal regeneration after trauma and neurological disorders due to an endogenous nonpermissive environment for axon regrowth in the CNS. Lateral olfactory tract usher substance (LOTUS) contributes to axonal tract formation in the developing brain and axonal regeneration in the adult brain as an endogenous Nogo receptor-1 (NgR1) antagonist. However, how LOTUS expression is regulated remains unclarified. This study examined molecular mechanism of regulation in LOTUS expression and found that brain-derived neurotrophic factor (BDNF) increased LOTUS expression in cultured hippocampal neurons. Exogenous application of BDNF increased LOTUS expression at both mRNA and protein levels in a dose-dependent manner. We also found that pharmacological inhibition with K252a and gene knockdown by siRNA of tropomyosin-related kinase B (TrkB), BDNF receptor suppressed BDNF-induced increase in LOTUS expression. Further pharmacological analysis of the TrkB signaling pathway revealed that BDNF increased LOTUS expression through mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) cascades, but not phospholipase C-γ (PLCγ) cascade. Additionally, treatment with c-AMP response element binding protein (CREB) inhibitor partially suppressed BDNF-induced LOTUS expression. Finally, neurite outgrowth assay in cultured hippocampal neurons revealed that BDNF treatment-induced antagonism for NgR1 by up-regulating LOTUS expression. These findings suggest that BDNF may acts as a positive regulator of LOTUS expression through the TrkB signaling, thereby inducing an antagonistic action for NgR1 function by up-regulating LOTUS expression. Also, BDNF may synergistically affect axon regrowth through the upregulation of LOTUS expression.

Review: Myelin clearance is critical for regeneration after peripheral nerve injury

Traumatic peripheral nerve injury occurs frequently and is a major clinical and public health problem that can lead to functional impairment and permanent disability. Despite the availability of modern diagnostic procedures and advanced microsurgical techniques, active recovery after peripheral nerve repair is often unsatisfactory. Peripheral nerve regeneration involves several critical events, including the recreation of the microenvironment and remyelination. Results from previous studies suggest that the peripheral nervous system (PNS) has a greater capacity for repair than the central nervous system. Thus, it will be important to understand myelin and myelination specifically in the PNS. This review provides an update on myelin biology and myelination in the PNS and discusses the mechanisms that promote myelin clearance after injury. The roles of Schwann cells and macrophages are considered at length, together with the possibility of exogenous intervention.

Involvement of Paired Immunoglobulin-like Receptor B in Diabetes-Associated Cognitive Dysfunction Through Modulation of Axon Outgrowth and Dendritic Remodeling

Type 2 diabetic patients have high risk of developing cognitive dysfunction, in which neural structural plasticity has played a pivotal role. Paired immunoglobulin-like receptor B (PirB), a receptor mainly expressed in neurons, acts as a critical inhibitor of neurite outgrowth and neural plasticity. However, the role of PirB in type 2 diabetes-associated cognitive dysfunction remains unknown. In this study, learning and memory impairment was observed in 24-week-old db/db mice by performing Morris water maze task, and the number of synapses along with the length of postsynaptic density by transmission electron microscopy were reduced in the hippocampus of db/db mice. Furthermore, PirB expression in the hippocampus of db/db mice was significantly upregulated using western blotting and immunofluorescence analysis. In cultured hippocampal neurons, high glucose treatment reduced the length of the longest neurite as well as axon initial segment (AIS), whereas silencing PirB expression rescued high glucose-induced neurite outgrowth inhibition, but not AIS. Additionally, cognitive deficits, dendrite morphology defects, and synapse-related proteins loss in db/db mice were alleviated when PirB knockdown was performed by adeno-associated virus injection. In conclusion, PirB is involved in diabetes-associated cognitive dysfunction through modulation of axon outgrowth and dendritic remodeling, providing a potential therapeutic target for diabetes-associated cognitive dysfunction.

The Regulatory Role of Reticulons in Neurodegeneration: Insights Underpinning Therapeutic Potential for Neurodegenerative Diseases

In the last few decades, cytoplasmic organellar dysfunction, such as that of the endoplasmic reticulum (ER), has created a new area of research interest towards the development of serious health maladies including neurodegenerative diseases. In this context, the extensively dispersed family of ER-localized proteins, i.e. reticulons (RTNs), is gaining interest because of its regulative control over neural regeneration. As most neurodegenerative diseases are pathologically manifested with the accretion of misfolded proteins with subsequent induction of ER stress, the regulatory role of RTNs in neural dysfunction cannot be ignored. With the limited information available in the literature, delineation of the functional connection between rising consequences of neurodegenerative diseases and RTNs need to be elucidated. In this review, we provide a broad overview on the recently revealed regulatory roles of reticulons in the pathophysiology of several health maladies, with special emphasis on neurodegeneration. Additionally, we have also recapitulated the decisive role of RTN4 in neurite regeneration and highlighted how neurodegeneration and proteinopathies are mechanistically linked with each other through specific RTN paralogues. With the recent findings advocating zebrafish Rtn4b (a mammalian Nogo-A homologue) downregulation following central nervous system (CNS) lesion, RTNs provides new insight into the CNS regeneration. However, there are controversies with respect to the role of Rtn4b in zebrafish CNS regeneration. Given these controversies, the connection between the unique regenerative capabilities of zebrafish CNS by distinct compensatory mechanisms and Rtn4b signalling pathway could shed light on the development of new therapeutic strategies against serious neurodegenerative diseases.

PirB functions as an intrinsic suppressor in hippocampal neural stem cells

Neural stem cells play pivotal roles during prenatal development and throughout life. Here, we report that Paired immunoglobulin-like receptor B (PirB) functions as a suppressor during brain neurogenesis in the adult mouse. PirB expression increased with age during development, and its deficiency promoted neural stem cell proliferation and differentiation in vivo and in vitro. Furthermore, we detected an increase in Type 1 neural stem cells in PirB-deficient mice compared to their wild-type littermates. PirB deficiency promoted stemness marker gene expression of Sox2 and KLF4 by activating Akt1 phosphorylation. These findings suggest that PirB inhibits the self-renewal and differentiation capacities of neural stem cells. Thus, PirB may have the potential to serve as a therapeutic target for treatment of reduced neurogenesis in adults due to aging or other pathological conditions.

Netrin-G1 Regulates Microglial Accumulation along Axons and Supports the Survival of Layer V Neurons in the Postnatal Mouse Brain

Microglia, the resident immune cells of the central nervous system, accumulate along subcerebral projection axons and support neuronal survival during the early postnatal period. It remains unknown how microglia follow an axon-specific distribution pattern to maintain neural circuits. Here, we investigated the mechanisms of microglial accumulation along subcerebral projection axons that were necessary for microglial accumulation in the internal capsule. Screening of molecules involved in this accumulation of microglia to axons of layer V cortical neurons identified netrin-G1, a member of the netrin family of axon guidance molecules with a glycosyl-phosphatidylinositol anchor. Deletion or knockdown of the netrin-G1 gene Ntng1 reduced microglial accumulation and caused loss of cortical neurons. Netrin-G1 ligand-Ngl1 knockout-mice-derived microglia showed reduced accumulation along the axons compared with wild-type microglia. Thus, microglia accumulate around the subcerebral projection axons via NGL1-netrin-G1 signaling and support neuronal survival. Our observations unveil bidirectional neurotrophic interactions between neurons and microglia.

Mechanisms and repair strategies for white matter degeneration in CNS injury and diseases

White matter degeneration is an important pathophysiological event of the central nervous system that is collectively characterized by demyelination, oligodendrocyte loss, axonal degeneration and parenchymal changes that can result in sensory, motor, autonomic and cognitive impairments. White matter degeneration can occur due to a variety of causes including trauma, neurotoxic exposure, insufficient blood flow, neuroinflammation, and developmental and inherited neuropathies. Regardless of the etiology, the degeneration processes share similar pathologic features. In recent years, a plethora of cellular and molecular mechanisms have been identified for axon and oligodendrocyte degeneration including oxidative damage, calcium overload, neuroinflammatory events, activation of proteases, depletion of adenosine triphosphate and energy supply. Extensive efforts have been also made to develop neuroprotective and neuroregenerative approaches for white matter repair. However, less progress has been achieved in this area mainly due to the complexity and multifactorial nature of the degeneration processes. Here, we will provide a timely review on the current understanding of the cellular and molecular mechanisms of white matter degeneration and will also discuss recent pharmacological and cellular therapeutic approaches for white matter protection as well as axonal regeneration, oligodendrogenesis and remyelination.

Leukocyte immunoglobulin-like receptor subfamily B: therapeutic targets in cancer

Inhibitory leukocyte immunoglobulin-like receptors (LILRBs 1–5) transduce signals via intracellular immunoreceptor tyrosine-based inhibitory motifs that recruit phosphatases to negatively regulate immune activation. The activation of LILRB signaling in immune cells may contribute to immune evasion. In addition, the expression and signaling of LILRBs in cancer cells especially in certain hematologic malignant cells directly support cancer development. Certain LILRBs thus have dual roles in cancer biology—as immune checkpoint molecules and tumor-supporting factors. Here, we review the expression, ligands, signaling, and functions of LILRBs, as well as therapeutic development targeting them. LILRBs may represent attractive targets for cancer treatment, and antagonizing LILRB signaling may prove to be effective anti-cancer strategies.

(H)Elping nerve growth factor: Elp1 inhibits TrkA's phosphatase to maintain retrograde signaling

Nerve growth factor (NGF) regulates many aspects of neuronal biology by retrogradely propagating signals along axons to the targets of those axons. How this occurs when axons contain a plethora of proteins that can silence those signals has long perplexed the neurotrophin field. In this issue of the JCI, Li et al. suggest an answer to this vexing problem, while exploring why the Elp1 gene that is mutated in familial dysautonomia (FD) causes peripheral neuropathy. They describe a distinctive function of Elp1 as a protein that is required to sustain NGF signaling by blocking the activity of its phosphatase that shuts off those signals. This finding helps explain the innervation deficits prominent in FD and reveals a unique role for Elp1 in the regulation of NGF-dependent TrkA activity.

Runx proteins mediate protective immunity against Leishmania donovani infection by promoting CD40 expression on dendritic cells

The level of CD40 expression on dendritic cells (DCs) plays a decisive role in disease protection during Leishmania donovani (LD) infection. However, current understanding of the molecular regulation of CD40 expression remains elusive. Using molecular, cellular and functional approaches, we identified a role for Runx1 and Runx3 transcription factors in the regulation of CD40 expression in DCs. In response to lipopolysaccharide (LPS), tumor necrosis factor alpha (TNFα) or antileishmanial drug sodium antimony gluconate (SAG), both Runx1 and Runx3 translocated to the nucleus, bound to the CD40 promoter and upregulated CD40 expression on DCs. These activities of Runx proteins were mediated by the upstream phosphatidylinositol 3-kinase (PI3K)-Akt pathway. Notably, LD infection attenuated LPS- or TNFα-induced CD40 expression in DCs by inhibiting PI3K-Akt-Runx axis via protein tyrosine phosphatase SHP-1. In contrast, CD40 expression induced by SAG was unaffected by LD infection, as SAG by blocking LD-induced SHP-1 activation potentiated PI3K-Akt signaling to drive Runx-mediated CD40 upregulation. Adoptive transfer experiments further showed that Runx1 and Runx3 play a pivotal role in eliciting antileishmanial immune response of SAG-treated DCs in vivo by promoting CD40-mediated type-1 T cell responses. Importantly, antimony-resistant LD suppressed SAG-induced CD40 upregulation on DCs by blocking the PI3K-Akt-Runx pathway through sustained SHP-1 activation. These findings unveil an immunoregulatory role for Runx proteins during LD infection.

Combinational Approach of Genetic SHP-1 Suppression and Voluntary Exercise Promotes Corticospinal Tract Sprouting and Motor Recovery Following Brain Injury

Background. Brain injury often causes severe motor dysfunction, leading to difficulties with living a self-reliant social life. Injured neural circuits must be reconstructed to restore functions, but the adult brain is limited in its ability to restore neuronal connections. The combination of molecular targeting, which enhances neural plasticity, and rehabilitative motor exercise is an important therapeutic approach to promote neuronal rewiring in the spared circuits and motor recovery. Objective. We tested whether genetic reduction of Src homology 2-containing phosphatase-1 (SHP-1), an inhibitor of brain-derived neurotrophic factor (BDNF)/tropomyosin receptor kinase B (TrkB) signaling, has synergistic effects with rehabilitative training to promote reorganization of motor circuits and functional recovery in a mouse model of brain injury. Methods. Rewiring of the corticospinal circuit was examined using neuronal tracers following unilateral cortical injury in control mice and in Shp-1 mutant mice subjected to voluntary exercise. Recovery of motor functions was assessed using motor behavior tests. Results. We found that rehabilitative exercise decreased SHP-1 and increased BDNF and TrkB expression in the contralesional motor cortex after the injury. Genetic reduction of SHP-1 and voluntary exercise significantly increased sprouting of corticospinal tract axons and enhanced motor recovery in the impaired forelimb. Conclusions. Our data demonstrate that combining voluntary exercise and SHP-1 suppression promotes motor recovery and neural circuit reorganization after brain injury.

Paired Immunoglobulin-like Receptor B Inhibition in Müller Cells Promotes Neurite Regeneration After Retinal Ganglion Cell Injury in vitro

In the central nervous system (CNS), three types of myelin-associated inhibitors (MAIs) have major inhibitory effects on nerve regeneration. They include Nogo-A, myelin-associated glycoprotein, and oligodendrocyte-myelin glycoprotein. MAIs possess two co-receptors, Nogo receptor (NgR) and paired immunoglobulin-like receptor B (PirB). Previous studies have confirmed that the inhibition of NgR only results in a modest increase in regeneration in the CNS; however, the inhibitory effects of PirB with regard to nerve regeneration after binding to MAIs remain controversial. In this study, we demonstrated that PirB is expressed in primary cultures of retinal ganglion cells (RGCs), and the inhibitory effects of the three MAIs on the growth of RGC neurites are not significantly decreased after direct PirB knockdown using adenovirus PirB shRNA. Interestingly, we found that retinal Müller cells expressed PirB and that its knockdown enhanced the regeneration of co-cultured RGC neurites. PirB knockdown also activated the JAK/Stat3 signaling pathway in Müller cells and upregulated ciliary neurotrophic factor levels. These findings indicate that PirB plays a novel role in retinal Müller cells and that its action in these cells may indirectly affect the growth of RGC neurites. The results also reveal that PirB in Müller cells affects RGC neurite regeneration. Our findings provide a novel basis for the use of PirB as a target molecule to promote nerve regeneration.

Strategies to Promote Long-Distance Optic Nerve Regeneration

Mammalian retinal ganglion cells (RGCs) in the central nervous system (CNS) often die after optic nerve injury and surviving RGCs fail to regenerate their axons, eventually resulting in irreversible vision loss. Manipulation of a diverse group of genes can significantly boost optic nerve regeneration of mature RGCs by reactivating developmental-like growth programs or suppressing growth inhibitory pathways. By injury of the vision pathway near their brain targets, a few studies have shown that regenerated RGC axons could form functional synapses with targeted neurons but exhibited poor neural conduction or partial functional recovery. Therefore, the functional restoration of eye-to-brain pathways remains a greatly challenging issue. Here, we review recent advances in long-distance optic nerve regeneration and the subsequent reconnecting to central targets. By summarizing our current strategies for promoting functional recovery, we hope to provide potential insights into future exploration in vision reformation after neural injuries.

Nogo-A/Pir-B/TrkB Signaling Pathway Activation Inhibits Neuronal Survival and Axonal Regeneration After Experimental Intracerebral Hemorrhage in Rats

Intracerebral hemorrhage (ICH) leads to widespread pathological lesions in the brain, especially impacting neuronal survival and axonal regeneration. This study aimed to elucidate whether the Nogo-A (a myelin-related protein)/paired immunoglobulin-like receptor B (Pir-B)/tropomyosin receptor kinase B (TrkB) pathway could exert a regulatory effect in ICH. An ICH model was first established in Sprague Dawley rats, followed by different administrations of vehicle, k252a, or NSC 87877. The Morris water maze test was performed to observe ICH-induced cognitive dysfunction in rats. Rats in the ICH + NSC 87877 group showed better cognitive performance compared with those injected with vehicle or k252a. Neurobehavioral scores were identical. By harvesting brain tissues at different time points after ICH, we detected the expression levels of Nogo-A and PirB with western blot and immunofluorescence and found that they were markedly upregulated at 48 h after ICH. TUNEL and Fluoro-Jade B staining showed that NSC 87877 treatment attenuated ICH-induced apoptosis and neuronal death, whereas k252a treatment aggravated these pathological changes. The expression levels of growth-associated protein 43 (GAP43) and neurofilament 200 (NF200) were higher in the ICH + NSC 87877 group compared with the ICH + vehicle group, but were lower in the ICH + k252a group. Finally, we confirmed the protective role of p-TrkB/TrkB in ICH by western blot. To sum up, our study identified the inhibitory role of the Nogo-A/PirB/TrkB pathway in ICH; however, p-TrkB/TrkB may serve as a potential target for secondary brain injury post-ICH.

Inhibition of neurite outgrowth using commercial myelin associated glycoprotein-Fc in neuro-2a cells

Myelin-associated glycoprotein (MAG) inhibits the growth of neurites from nerve cells. Extraction and purification of MAG require complex operations; therefore, we attempted to determine whether commercially available MAG-Fc can replace endogenous MAG for research purposes. Immunofluorescence using specific antibodies against MAG, Nogo receptor (NgR) and paired immunoglobulin-like receptor B (PirB) was used to determine whether MAG-Fc can be endocytosed by neuro-2a cells. In addition, neurite outgrowth of neuro-2a cells treated with different doses of MAG-Fc was evaluated. Enzyme linked immunosorbent assays were used to measure RhoA activity. Western blot assays were conducted to assess Rho-associated protein kinase (ROCK) phosphorylation. Neuro-2a cells expressed NgR and PirB, and MAG-Fc could be endocytosed by binding to NgR and PirB. This activated intracellular signaling pathways to increase RhoA activity and ROCK phosphorylation, ultimately inhibiting neurite outgrowth. These findings not only verify that MAG-Fc can inhibit the growth of neural neurites by activating RhoA signaling pathways, similarly to endogenous MAG, but also clearly demonstrate that commercial MAG-Fc is suitable for experimental studies of neurite outgrowth.

Regulation of Brain-Derived Neurotrophic Factor and Growth Factor Signaling Pathways by Tyrosine Phosphatase Shp2 in the Retina: A Brief Review

SH2 domain-containing tyrosine phosphatase-2 (PTPN11 or Shp2) is a ubiquitously expressed protein that plays a key regulatory role in cell proliferation, differentiation and growth factor (GF) signaling. This enzyme is well expressed in various retinal neurons and has emerged as an important player in regulating survival signaling networks in the neuronal tissues. The non-receptor phosphatase can translocate to lipid rafts in the membrane and has been implicated to regulate several signaling modules including PI3K/Akt, JAK-STAT and Mitogen Activated Protein Kinase (MAPK) pathways in a wide range of biochemical processes in healthy and diseased states. This review focuses on the roles of Shp2 phosphatase in regulating brain-derived neurotrophic factor (BDNF) neurotrophin signaling pathways and discusses its cross-talk with various GF and downstream signaling pathways in the retina.

PirB inhibits axonal outgrowth via the PI3K/Akt/mTOR signaling pathway

Accumulating data strongly suggests that leukocyte immunoglobulin like receptor B1 (PirB) inhibits axonal outgrowth. However, the underlying mechanisms remain unclear. In the present study, cortical neurons of newborn mice were cultured with Nogo‑66 (Nogo‑p4; 4 µmol/l; a PirB ligand) together with NEP1‑40 (Nogo inhibitory peptide) and/or anti‑PirB body (50 mg/ml). PirB mRNA and protein was higher in cultured neurons induced by Nogo‑66 compared with untreated cells. Neurite outgrowth assays demonstrated that the inhibitory effects of Nogo‑66 on axonal outgrowth were reversed by anti‑PirB body. Reverse transcription‑quantitative polymerase chain reaction and western blot assays demonstrated that anti‑PirB treatment led to reduced mRNA and protein expression of phosphoinositide 3‑kinase (PI3K), Akt serine/threonine kinase (Akt), mechanistic target of rapamycin kinase (mTOR), myosin IIA and cofilin, which are involved in axonal outgrowth. Furthermore, blockade of the PI3K/Akt/mTOR pathway using a PI3K inhibitor or an mTOR inhibitor diminished the stimulatory effect of anti‑PirB on axonal outgrowth, and the reduced effect of anti‑PirB on factors that were activation by anti‑PirB. In addition, blockade of PI3K/Akt/mTOR enhanced anti‑PirB‑induced gene and protein expression. These results revealed that PirB functions as a potential suppressor in axonal outgrowth via repressing PI3K/Akt/mTOR signaling pathway, and PirB/PI3K/Akt/mTOR may be a novel target for enhancing axonal outgrowth for developing rational therapeutic strategies.

Regulation of immune and neural function via leukocyte Ig-like receptors

Leukocyte Ig-like receptors (LILRs)/Ig-like transcripts (ILTs) are expressed on innate and adaptive immune cells and maintain immune homeostasis. LILRs consist of activating and inhibitory-type receptors that regulate adequate cellular functions. LILRs were firstly identified as MHC class I receptors, therefore expression and/or polymorphisms of LILRs are reported to associate with autoimmune disorders and transplant rejection; however, recent accumulating evidences have revealed that LILRs recognize with diverse ligands including bacteria and virus. In addition, inhibitory LILRB2 (ILT4) and murine relative paired Ig-like receptor (PIR)-B are expressed on neuron and is involved in the dysregulation of central nervous system via interaction with neuronal ligands including amyloid β-protein. In this review, we summarize recent discoveries on the functions of inhibitory MHC class I receptors, and discuss their regulatory roles in immune responses and neural functions.

The effects of immune protein CD3ζ development and degeneration of retinal neurons after optic nerve injury

Major histocompatibility complex (MHC) class I molecules and their receptors play fundamental roles in neuronal death during diseases. T-cell receptors (TCR) function as MHCI receptor on T-cells and both MHCI and a key component of TCR, CD3ζ, are expressed by mouse retinal ganglion cells (RGCs) and displaced amacrine cells. Mutation of these molecules compromises the development of RGCs. We investigated whether CD3ζ regulates the development and degeneration of amacrine cells after RGC death. Surprisingly, mutation of CD3ζ not only impairs the proper development of amacrine cells expressing CD3ζ but also those not expressing CD3ζ. In contrast to effects of MHCI and its receptor, PirB, on other neurons, mutation of CD3ζ has no effect on RGC death and starburst amacrine cells degeneration after optic nerve crush. Thus, unlike MHCI and PirB, CD3ζ regulates the development of RGCs and amacrine cells but not their degeneration after optic nerve crush.

Nogo-A in the visual system development and in ocular diseases

Nogo-A is a potent myelin-associated inhibitor for neuronal growth and plasticity in the central nervous system (CNS). Its effects are mediated by the activation of specific receptors that intracellularly control cytoskeleton rearrangements, protein synthesis and gene expression. Moreover, Nogo-A has been involved in the development of the visual system and in a variety of neurodegenerative diseases and injury processes that can alter its function. For example, Nogo-A was shown to influence optic nerve myelinogenesis, the formation and maturation of retinal axon projections, and retinal angiogenesis. In adult animals, the inactivation of Nogo-A exerted remarkable effects on visual plasticity. Relieving Nogo-A-induced inhibition increased axonal sprouting after optic nerve lesion and axonal rewiring in the visual cortex of intact adult mice. This review aims at presenting our current knowledge on the role of Nogo-A in the visual system and to discuss how its therapeutic targeting may promote visual improvement in ophthalmic diseases.

The Involvement of the Myelin-Associated Inhibitors and Their Receptors in CNS Plasticity and Injury

The limited capacity for the central nervous system (CNS) to repair itself was first described over 100 years ago by Spanish neuroscientist Ramon Y. Cajal. However, the exact mechanisms underlying this failure in neuronal regeneration remain unclear and, as such, no effective therapeutics yet exist. Numerous studies have attempted to elucidate the biochemical and molecular mechanisms that inhibit neuronal repair with increasing evidence suggesting that several inhibitory factors and repulsive guidance cues active during development actually persist into adulthood and may be contributing to the inhibition of repair. For example, in the injured adult CNS, there are various inhibitory factors that impede the outgrowth of neurites from damaged neurons. One of the most potent of these neurite outgrowth inhibitors is the group of proteins known as the myelin-associated inhibitors (MAIs), present mainly on the membranes of oligodendroglia. Several studies have shown that interfering with these proteins can have positive outcomes in CNS injury models by promoting neurite outgrowth and improving functional recovery. As such, the MAIs, their receptors, and downstream effectors are valid drug targets for the treatment of CNS injury. This review will discuss the current literature on MAIs in the context of CNS development, plasticity, and injury. Molecules that interfere with the MAIs and their receptors as potential candidates for the treatment of CNS injury will additionally be introduced in the context of preclinical and clinical trials.

Protein tyrosine phosphatase SHP-1 modulates osteoblast differentiation through direct association with and dephosphorylation of GSK3β

SHP-1, the Src homology-2 (SH2) domain-containing phosphatase 1, is a cytosolic protein-tyrosine phosphatase (PTP) predominantly expressed in hematopoietic-derived cells. Previous studies have focused on the involvement of SHP-1 in osteoclastogenesis. Using primary cultured mouse fetal calvaria-derived osteoblasts as a model, this study aims to investigate the effects of SHP-1 on differentiation and mineralization of osteoblasts and elucidate the signaling pathways responsible for these effects. We found that osteoblasts treated by osteogenic media showed significant increase in SHP-1 expression, which contributed to osteoblastic differentiation and mineralization. Using immunoprecipitation assay, we found that a direct association between SHP-1 and glycogen synthase kinase (GSK)-3β could be detected in differentiated osteoblasts and was significantly inhibited by SHP-1 inhibitor NSC87877. Inhibition of SHP-1 activated GSK3β, thereby leading to suppression of osteoblast differentiation and mineralization, which could be rescued by the inhibitor of GSK3β. In addition, we found that rosiglitazone (RSG) treatment led to significant decrease in SHP-1 expression. Overexpression of SHP-1 reversed RSG-induced GSK3β activation, thus rescuing the inhibitory effect of RSG on osteoblast differentiation and mineralization. These findings suggest that protein tyrosine phosphatase SHP-1 may act as a positive regulator of osteoblast differentiation through direct association with and dephosphorylation of GSK3β. Downregulation of SHP-1 may contribute to RSG-induced inhibition of mouse calvaria osteoblast differentiation by activating GSK3β-dependent pathway.

AAV-mediated transfer of RhoA shRNA and CNTF promotes retinal ganglion cell survival and axon regeneration

The aim of the present study was to determine whether adeno-associated viral vector (AAV) mediated transfer of ciliary neurotrophic factor (CNTF) and RhoA shRNA has additive effects on promoting the survival and axon regeneration of retinal ganglion cells (RGCs) after optic nerve crush (ONC). Silencing effects of AAV-RhoA shRNA were confirmed by examining neurite outgrowth in PC12 cells, and by quantifying RhoA expression levels with western blotting. Young adult Fischer rats received an intravitreal injection of (i) saline, (ii) AAV green fluorescent protein (GFP), (iii) AAV-CNTF, (iv) AAV-RhoA shRNA, or (v) a combination of both AAV-CNTF and AAV-RhoA shRNA. Two weeks later, the ON was completely crushed. Three weeks after ONC, RGC survival was estimated by counting βIII-tubulin-positive neurons in retinal whole mounts. Axon regeneration was evaluated by counting GAP-43-positive axons in the crushed ON. It was found that AAV-RhoA shRNA decreased RhoA expression levels and promoted neurite outgrowth in vitro. In the ONC model, AAV-RhoA shRNA by itself had only weak beneficial effects on RGC axon regeneration. However, when combined with AAV-CNTF, AAV-RhoA shRNA significantly improved the therapeutic effect of AAV-CNTF on axon regeneration by nearly two fold, even though there was no significant change in RGC viability. In sum, this combination of vectors increases the regenerative response and can lead to more successful therapeutic outcomes following neurotrauma.

TAT-PEP Enhanced Neurobehavioral Functional Recovery by Facilitating Axonal Regeneration and Corticospinal Tract Projection After Stroke

Paired immunoglobulin-like receptor B (PirB) has been identified as a new receptor for myelin-associated inhibitory (MAI) proteins, which may play important role in axonal regeneration and corticospinal tract (CST) projection associated with neurobehavioral function recovery after stroke. Here, we found that the expression of PirB was increased in the cortical penumbra from 1 to 28 days after transient focal cerebral ischemic reperfusion of rats. Then, transactivator of transcription-PirB extracellular peptide (TAT-PEP) was generated that might block the interactions between MAIs and PirB. The results showed that TAT-PEP displayed high affinity for MAIs and ameliorated their inhibitory effect on neurite growth. Furthermore, TAT-PEP can widely distribute in the penumbra after intraperitoneal injection. Then, we found that TAT-PEP enhanced neurite growth and alleviated growth cone collapse after oxygen glucose deprivation (OGD) injury. In addition, TAT-PEP promoted long-term neurobehavioral functional recovery through enhancing axonal regeneration and CST projection. Finally, the observations demonstrated that POSH/RhoA/growth-associated protein 43 (GAP43) as PirB-associated downstream signaling molecules played important role in neurobehavioral functional recovery after stroke. Moreover, the underlying mechanism associated with TAT-PEP-mediated promoting axonal regeneration and CST projection was by intervening in the expression of POSH, RhoA, and GAP43. These studies suggest that TAT-PEP may represent an attractive therapeutic strategy against stroke.

Electroacupuncture enhances rehabilitation through miR-181b targeting PirB after ischemic stroke

Recent studies have demonstrated microRNAs (miRNAs) and proteins are beneficial to axon regeneration, which may be involved in Electroacupuncture (EA) therapy against stroke. In this study, we aimed to determine the pivotal role of PirB in EA-produced rehabilitation against ischemic stroke; and to screen and investigate the potential miRNAs directly regulating PirB expression. The results showed EA treatment enhanced axon regeneration and new projections from the corticospinal tract at 28 d after cerebral ischemic reperfusion injury of rats. Then, we found EA decreased pirb mRNA and PirB protein expression in the penumbra within 28 days after reperfusion. The reduction of PirB expression facilitated neurite outgrowth after oxygen-glucose deprivation injury. The miRNA microarray showed the level of twenty kinds of miRNAs changed in the penumbra after EA administration. The bioinformatics study and luciferase assay verified miR-181b directly regulated pirb mRNA expression. EA increased miR-181b levels in the penumbras, and improved neurobehavioral function rehabilitation through miR-181b direct targeting of pirb mRNA to regulate the expression of PirB, RhoA and GAP43. In conclusion, we provide the first evidence that EA enhances rehabilitation against stroke by regulating epigenetic changes to directly act on its targets, such as the miR-181b/PirB/RhoA/GAP43 axis, which is a novel mechanism of EA therapy.

MHC-I and PirB Upregulation in the Central and Peripheral Nervous System following Sciatic Nerve Injury

Major histocompatibility complex class one (MHC-I) antigen-presenting molecules participate in central nervous system (CNS) synaptic plasticity, as does the paired immunoglobulin-like receptor B (PirB), an MHC-I ligand that can inhibit immune-cells and bind to myelin axon growth inhibitors. Based on the dual roles of both molecules in the immune and nervous systems, we evaluated their expression in the central and peripheral nervous system (PNS) following sciatic nerve injury in mice. Increased PirB and MHC-I protein and gene expression is present in the spinal cord one week after nerve transection, PirB being mostly expressed in the neuropile region. In the crushed nerve, MHC-I protein levels increased 2 weeks after lesion (wal) and progressively decreased over the next eight weeks. The same kinetics were observed for infiltrating cytotoxic T lymphocytes (CTLs) but not for PirB expression, which continuously increased. Both MHC-I and PirB were found in macrophages and Schwann cells but rarely in axons. Interestingly, at 8 wal, PirB was mainly restricted to the myelin sheath. Our findings reinforce the participation of MHC-I and PirB in CNS plasticity events. In contrast, opposing expression levels of these molecules were found in the PNS, so that MHC-I and PirB seem to be mostly implicated in antigen presentation to CTLs and axon myelination, respectively.

Soluble LILRA3 promotes neurite outgrowth and synapses formation through high affinity interaction with Nogo 66

Inhibitory proteins, particularly Nogo 66, a highly conserved 66 amino acid loop of Nogo A, play key roles in limiting the intrinsic capacity of the central nervous system to regenerate after injury. Ligation of surface Nogo receptors (NgRs) and/or leukocyte immunoglobulin like receptor B2 (LILRB2) and its mouse orthologue the paired-immunoglobulin-like receptor B (PIRB) by Nogo 66 transduces inhibitory signals that potently inhibit neurite outgrowth. Here we show that soluble leukocyte immunoglobulin-like receptor A3 (LILRA3) is a high affinity receptor for Nogo 66, suggesting that LILRA3 might be a competitive antagonist to these cell surface inhibitory receptors. Consistent with this, LILRA3 significantly reversed Nogo 66-mediated inhibition of neurite outgrowth and promoted synapse formation in primary cortical neurons via regulation of the MEK/ERK pathway. LILRA3 represents a new antagonist to Nogo 66-mediated inhibition of neurite outgrowth in the CNS, a function distinct from its immune-regulatory role in leukocytes. This report is also the first to demonstrate that a member of LILR family normally not expressed in rodents exerts functions on mouse neurons through the highly homologous Nogo 66 ligand.

Acetylation of NDPK-D Regulates Its Subcellular Localization and Cell Survival

Nucleoside diphosphate kinases (NDPK) are ubiquitous enzymes that catalyze the reversible phosphotransfer of γ-phosphates between di- and triphosphonucleosides. NDPK-D (Nm23-H4) is the only member of the NDPK family with a mitochondrial targeting sequence. Despite the high expression of NDPK-D in the developing central nervous system, its function remains to be determined. In this study, we show that NDPK-D knockdown induces apoptosis in neuroblastoma cells as well as in mouse cortex, suggesting that NDPK-D is required for neuronal survival. We identified NDPK-D as a binding partner of NAD⁺-dependent histone deacetylase, SIRT1, by yeast two-hybrid screening. NDPK-D co-localized with SIRT1, and the association of these molecules was confirmed by co-immunoprecipitation. Inhibition of SIRT1 increases the acetylation of NDPK-D. Overexpression of NDPK-D along with SIRT1, or mutation in the acetylated lysine residues in NDPK-D, increases its nuclear accumulation. Furthermore, the NDPK-D acetylation-mimic mutant increased apoptosis in N1E-115 cells. Our data demonstrate that acetylation regulates the shuttling of NDPK-D between nucleus and cytoplasm, and increased acetylation of NDPK-D causes apoptosis.

Distribution of paired immunoglobulin-like receptor B in the nervous system related to regeneration difficulties after unilateral lumbar spinal cord injury

Paired immunoglobulin-like receptor B (PirB) is a functional receptor of myelin-associated inhibitors for axonal regeneration and synaptic plasticity in the central nervous system, and thus suppresses nerve regeneration. The regulatory effect of PirB on injured nerves has received a lot of attention. To better understand nerve regeneration inability after spinal cord injury, this study aimed to investigate the distribution of PirB (via immunofluorescence) in the central nervous system and peripheral nervous system 10 days after injury. Immunoreactivity for PirB increased in the dorsal root ganglia, sciatic nerves, and spinal cord segments. In the dorsal root ganglia and sciatic nerves, PirB was mainly distributed along neuronal and axonal membranes. PirB was found to exhibit a diffuse, intricate distribution in the dorsal and ventral regions. Immunoreactivity for PirB was enhanced in some cortical neurons located in the bilateral precentral gyri. Overall, the findings suggest a pattern of PirB immunoreactivity in the nervous system after unilateral spinal transection injury, and also indicate that PirB may suppress repair after injury.

Axon regeneration impediment: the role of paired immunoglobulin-like receptor B

Regenerative capacity is weak after central nervous system injury because of the absence of an enhancing microenvironment and presence of an inhibitory microenvironment for neuronal and axonal repair. In addition to the Nogo receptor (NgR), the paired immunoglobulin-like receptor B (PirB) is a recently discovered coreceptor of Nogo, myelin-associated glycoprotein, and myelin oligodendrocyte glycoprotein. Concurrent blocking of NgR and PirB almost completely eliminates the inhibitory effect of myelin-associated inhibitory molecules on axonal regeneration. PirB participates in a key pathological process of the nervous system, specifically axonal regeneration inhibition. PirB is an inhibitory receptor similar to NgR, but their effects are not identical. This study summarizes the structure, distribution, relationship with common nervous system diseases, and known mechanisms of PirB, and concludes that PirB is also distributed in cells of the immune and hematopoietic systems. Further investigations are needed to determine if immunomodulation and blood cell migration involve inhibition of axonal regeneration.

Axon growth inhibition by RhoA/ROCK in the central nervous system

Rho kinase (ROCK) is a serine/threonine kinase and a downstream target of the small GTPase Rho. The RhoA/ROCK pathway is associated with various neuronal functions such as migration, dendrite development, and axonal extension. Evidence from animal studies reveals that RhoA/ROCK signaling is involved in various central nervous system (CNS) diseases, including optic nerve and spinal cord injuries, stroke, and neurodegenerative diseases. Given that RhoA/ROCK plays a critical role in the pathophysiology of CNS diseases, the development of therapeutic agents targeting this pathway is expected to contribute to the treatment of CNS diseases. The RhoA/ROCK pathway mediates the effects of myelin-associated axon growth inhibitors—Nogo, myelin-associated glycoprotein (MAG), oligodendrocyte-myelin glycoprotein (OMgp), and repulsive guidance molecule (RGM). Blocking RhoA/ROCK signaling can reverse the inhibitory effects of these molecules on axon outgrowth, and promotes axonal sprouting and functional recovery in animal models of CNS injury. To date, several RhoA/ROCK inhibitors have been under development or in clinical trials as therapeutic agents for neurological disorders. In this review, we focus on the RhoA/ROCK signaling pathway in neurological disorders. We also discuss the potential therapeutic approaches of RhoA/ROCK inhibitors for various neurological disorders.

The newly identified migration inhibitory protein regulates the radial migration in the developing neocortex

Neuronal migration is a crucial process in the organization of the developing cerebral cortex. Although a number of positive regulatory mechanisms of radial migration have been identified, negative cell-autonomous mechanisms have yet to be fully described. Here we report a newly identified Migration Inhibitory Protein (MINP, formerly known as 2900011O08Rik) that negatively regulates radial migration. MINP mRNA was specifically detected in the central and peripheral nervous system, and especially enriched in the cerebral cortex. MINP immunoreactivity co-localized with the neuronal marker Tuj1 and was detected in the cytoplasm of post-mitotic neurons. To elucidate the function of MINP in the developing brain, we performed in utero electroporation of MINP siRNA, MINP shRNA, or MINP-overexpressing vectors into mouse cortices and carried out in vivo migration assays. Whereas knockdown of MINP did not alter neuronal morphology, the radial migration was found accelerated by MINP knockdown, and reduced by MINP overexpression. This migration phenotype was also confirmed in vitro, indicating that MINP regulates neuronal migration in a cell-autonomous fashion. Furthermore, downregulation of MINP affected microtubule stability by interacting with tubulin that is a potential mechanism involved in the regulation of neuronal migration.

Myelin-associated inhibitors in axonal growth after CNS injury

There are multiple barriers to axonal growth after CNS injury. Myelin-associated inhibitors represent one group of barriers extrinsic to the injured neurons. Nogo, MAG and OMgp are three prototypical myelin inhibitors that signal through multiple neuronal receptors to exert growth inhibition. Targeting myelin inhibition alone modulates the compensatory sprouting of uninjured axons but the effect on the regeneration of injured axons is limited. Meanwhile, modulating sprouting, a naturally occurring repair mechanism, may be a more attainable therapeutic goal for promoting functional repair after CNS injury in the near term.

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.

PirB is a novel potential therapeutic target for enhancing axonal regeneration and synaptic plasticity following CNS injury in mammals

A major barrier to axonal regeneration in mammals is the unfavorable extracellular environment that develops following injury to the central nervous system (CNS). In particular, three myelin-associated inhibitory proteins (MAIs) - Nogo, myelin-associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein (OMgp) - are known to inhibit axonal regeneration and functional recovery. These MAIs share a common receptor, glycosylphosphatidylinositol-anchored Nogo receptor (NgR). However, paired immunoglobulin-like receptor B (PirB) - which was originally identified as a receptor for class I major histocompatibility complex (MHCI) in the immune system - is also expressed in neurones and plays a similarly inhibitory role in axonal regeneration and synaptic plasticity following CNS injury through its association with MAIs. Importantly, suppression of PirB activity through antibody antagonism or genetic means can partially relieve the inhibition of neurite outgrowth in vitro and in vivo. In this review, we present the molecular features, expression patterns and known signaling pathways of PirB, and we specifically focus on putative roles for PirB in the CNS and its potential as a target of molecular therapies for enhancing axonal regeneration and synaptic plasticity following CNS injury.

The biological functions and signaling mechanisms of the p75 neurotrophin receptor

The p75 neurotrophin receptor (p75(NTR)) regulates a wide range of cellular functions, including programmed cell death, axonal growth and degeneration, cell proliferation, myelination, and synaptic plasticity. The multiplicity of cellular functions governed by the receptor arises from the variety of ligands and co-receptors which associate with p75(NTR) and regulate its signaling. P75(NTR) promotes survival through interactions with Trk receptors, inhibits axonal regeneration via partnerships with Nogo receptor (Nogo-R) and Lingo-1, and promotes apoptosis through association with Sortilin. Signals downstream of these interactions are further modulated through regulated intramembrane proteolysis (RIP) of p75(NTR) and by interactions with numerous cytosolic partners. In this chapter, we discuss the intricate signaling mechanisms of p75(NTR), emphasizing how these signals are differentially regulated to mediate these diverse cellular functions.

Osteoprotegerin causes apoptosis of endothelial progenitor cells by induction of oxidative stress

OBJECTIVE

Elevated serum osteoprotegerin (OPG) levels represent an independent risk factor for atherosclerotic disease, although the underlying mechanism is not clear. The aim of this study was to investigate the association of serum OPG levels and circulating endothelial progenitor cell (EPC) numbers, and to explore the effect of OPG on EPC apoptosis and its underlying mechanisms.

METHODS

Flow cytometry was used to enumerate EPCs in the peripheral blood of 91 patients with systemic lupus erythematosus (SLE). Cultured EPCs, isolated from peripheral blood, were challenged with OPG, and apoptosis was evaluated by TUNEL staining. Expression of apoptosis-related proteins was measured by real-time quantitative polymerase chain reaction (qPCR) and Western blotting. Reactive oxygen species (ROS) were detected by flow cytometry, and the expression of NADPH oxidase (NOX) and MAP kinases (MAPK) was measured by qPCR and Western blotting.

RESULTS

The serum OPG level was independently associated with reduced numbers of EPCs in patients with SLE. In vitro treatment with OPG significantly induced apoptosis of EPCs; this effect was mediated by syndecan 4. OPG-induced apoptosis was abolished by the ROS scavenger N-acetylcysteine and the NOX inhibitor diphenyleniodonium. OPG increased ROS production through activation of NOX-2 and NOX-4 and triggered phosphorylation of ERK-1/2 and p38 MAPK. Quenching of ROS by knockdown of NOX-2 or NOX-4 transcripts inhibited phosphorylation of ERK-1/2 and p38 MAPK. Moreover, inhibitors of ERK-1/2 and p38 MAPK decreased ROS production and subsequent EPC apoptosis, indicating a feed-forward loop between NOX and MAPK to amplify ROS production related to apoptosis.

CONCLUSION

Elevated OPG levels increase apoptosis of EPCs by induction of oxidative stress.

Human LilrB2 is a β-amyloid receptor and its murine homolog PirB regulates synaptic plasticity in an Alzheimer's model

Soluble β-amyloid (Aβ) oligomers impair synaptic plasticity and cause synaptic loss associated with Alzheimer's disease (AD). We report that murine PirB (paired immunoglobulin-like receptor B) and its human ortholog LilrB2 (leukocyte immunoglobulin-like receptor B2), present in human brain, are receptors for Aβ oligomers, with nanomolar affinity. The first two extracellular immunoglobulin (Ig) domains of PirB and LilrB2 mediate this interaction, leading to enhanced cofilin signaling, also seen in human AD brains. In mice, the deleterious effect of Aβ oligomers on hippocampal long-term potentiation required PirB, and in a transgenic model of AD, PirB not only contributed to memory deficits present in adult mice, but also mediated loss of synaptic plasticity in juvenile visual cortex. These findings imply that LilrB2 contributes to human AD neuropathology and suggest therapeutic uses of blocking LilrB2 function.

Brimonidine promotes axon growth after optic nerve injury through Erk phosphorylation

It is well known that axons of the adult mammalian central nervous system have a very limited ability to regenerate after injury. Therefore, the neurodegenerative process of glaucoma results in irreversible functional deficits, such as blindness. Brimonidine (BMD) is an alpha2-adrenergic receptor agonist that is used commonly to lower intraocular pressure in glaucoma. Although it has been suggested that BMD has neuroprotective effects, the underlying mechanism remains unknown. In this study, we explored the molecular mechanism underlying the neuroprotective effect of BMD in an optic nerve injury (ONI) model. BMD treatment promoted optic nerve regeneration by inducing Erk1/2 phosphorylation after ONI. In addition, an Erk1/2 antagonist suppressed BMD-mediated axonal regeneration. A gene expression analysis revealed that the expression of the neurotrophin receptor gene p75 was increased and that the expression of the tropomyosin receptor kinase B (TrkB) gene was decreased after ONI. BMD treatment abrogated the changes in the expression of these genes. These results indicate that BMD promotes optic nerve regeneration via the activation of Erk1/2.

hnRNP A1 and secondary structure coordinate alternative splicing of Mag

Myelin-associated glycoprotein (MAG) is a major component of myelin in the vertebrate central nervous system. MAG is present in the periaxonal region of the myelin structure, where it interacts with neuronal proteins to inhibit axon outgrowth and protect neurons from degeneration. Two alternatively spliced isoforms of Mag mRNA have been identified. The mRNA encoding the shorter isoform, known as S-MAG, contains a termination codon in exon 12, while the mRNA encoding the longer isoform, known as L-MAG, skips exon 12 and produces a protein with a longer C-terminal region. L-MAG is required in the central nervous system. How inclusion of Mag exon 12 is regulated is not clear. In a previous study, we showed that heteronuclear ribonucleoprotein A1 (hnRNP A1) contributes to Mag exon 12 skipping. Here, we show that hnRNP A1 interacts with an element that overlaps the 5' splice site of Mag exon 12. The element has a reduced ability to interact with the U1 snRNP compared with a mutant that improves the splice site consensus. An evolutionarily conserved secondary structure is present surrounding the element. The structure modulates interaction with both hnRNP A1 and U1. Analysis of splice isoforms produced from a series of reporter constructs demonstrates that the hnRNP A1-binding site and the secondary structure both contribute to exclusion of Mag exon 12.

Suppression of SHP-1 promotes corticospinal tract sprouting and functional recovery after brain injury

Reorganization of spared neural network connections is one of the most important processes for restoring impaired function after brain injury. However, plasticity is quite limited in the adult brain due to the presence of inhibitory molecules and a lack of intrinsic neuronal signals for axonal growth. Src homology 2-containing phosphatase (SHP)-1 has been shown to have a role in axon growth inhibition. Here, we tested the hypothesis that SHP-1 negatively affects axonal reorganization. We observed that unilateral motor cortex injury led to increased expression and activity of SHP-1 in the contralesional cortex. In this model, corticospinal axons originating from the contralesional cortex sprouted into the denervated side of the cervical spinal cord after injury. We observed that the number of sprouting fibers was increased in SHP-1-deficient heterozygous viable motheaten (+/me^v) mice, which show reduced SHP-1 activity, and in wild-type mice treated with an SHP inhibitor. Motor function recovery of impaired forelimb was enhanced in +/me^v mice. Collectively, our results indicate that downregulation of SHP-1 activity promotes corticospinal tract sprouting and functional recovery after brain injury.

Suppression of the p75 receptor signal attenuates the effect of ephrin-B3 and promotes axonal regeneration of the injured optic nerve

The p75 neurotrophin receptor (p75NTR) is known to transduce the signal from some myelin-associated axon growth inhibitors, including Nogo and myelin-associated glycoprotein. As ephrin-B3, a member of the ephrin family, is also expressed in myelin and inhibits axon growth, the purpose of this study was to assess the possible involvement of p75NTR in ephrin-B3 signaling. Here, we report that p75NTR is required for the inhibitory effect of ephrin-B3 on neurite growth in vitro. While ephrin-B3 inhibited neurite elongation of embryonic cortical neurons, the neurons with p75NTR knockdown or with EphA4 knockdown were less sensitive to ephrin-B3. Although no direct interaction of p75NTR with ephrin-B3 was observed, Pep5, a peptide that specifically inhibits RhoA activation mediated by p75NTR, reduced the effect of ephrin-B3. Therefore, p75NTR functions as a signal transducer for ephrin-B3. Moreover, axonal regeneration in vivo was induced by Pep5 application after optic nerve crush injury in mice. Thus, Pep5 is a promising agent that contributes to axonal regeneration in the central nervous system.