Developmental regulation of CPG15 expression in Xenopus

Neurotrophic factor neuritin ameliorates streptozotocin-induced Alzheimer's disease-like impairment of memory, neuroinflammation, apoptotic factors and compensates hippocampal neuritin expression

Alzheimer's disease (AD) is the main cause of dementia in the elderly, and is becoming one of the most expensive and deadly diseases. Deficiency of neurotrophic factors signaling is an important cause of this disease. Therefore, we investigated whether neuritin as a neurotrophic factor can have a neuroprotective effect against streptozotocin (STZ)-induced rat model of AD. The animals were bilaterally injected with intra hippocampal-STZ (3 mg/kg). Different concentrations of neuritin (0.5, 1, 1.5 µg/rat) were administrated 15 min before STZ injection. After 14 days, the rats were evaluated for cognitive performance using novel object recognition (NOR), open field and Morris water maze (MWM) tests and then sacrificed for biochemical analysis (by real-time PCR and western blot examinations). The results demonstrated that the STZ- induced learning and memory impairments were significantly prevented by 1.5 µg neuritin. Moreover, the increased levels of inflammatory factors (NF-κb, TNF-α and IL-1β) and apoptotic parameters (cytochrome c and caspase‑3) in STZ- treated rats were also significantly decreased by neuritin. In addition, hippocampal neuritin gene expression was downregulated by STZ injection, which was reversed by intra hippocampal neuritin injection. In conclusion, the present study suggests that neuritin prevents cognitive defects in AD rat model and its expression level is associated with cognitive resilience.

Preparation of recombinant neuritin protein

Neuritin plays an important role in promoting nerve injury repair and maintaining synaptic plasticity, making it a potential therapeutic target for the treatment of nerve injury and neurodegenerative diseases. The present study aimed to obtain an active, unlabeled neuritin protein. Initially, a neuritin protein expression system with an enterokinase site was constructed in Escherichia coli. After optimizing induction conditions and screening for high expression, a neuritin recombinant protein with purity exceeding 85 % was obtained through Ni-affinity chromatography. Subsequently, unlabeled neuritin with a molecular weight of 11 kDa was obtained through the enzymatic cleavage of the His label using an enterokinase. Furthermore, a neuritin recombinant protein with purity exceeding 95 % was obtained using gel chromatography. Functional investigations revealed that neurite outgrowth of PC12 cells was stimulated by the isolated neuritin. This study establishes a method to obtain active and unlabeled neuritin protein, providing a foundation for subsequent research on its biological functions.

Follicular regulatory T cells produce neuritin to regulate B cells

Regulatory T cells prevent the emergence of autoantibodies and excessive IgE, but the precise mechanisms are unclear. Here, we show that BCL6-expressing Tregs, known as follicular regulatory T (Tfr) cells, produce abundant neuritin protein that targets B cells. Mice lacking Tfr cells or neuritin in Foxp3-expressing cells accumulated early plasma cells in germinal centers (GCs) and developed autoantibodies against histones and tissue-specific self-antigens. Upon immunization, these mice also produced increased plasma IgE and IgG1. We show that neuritin is taken up by B cells, causes phosphorylation of numerous proteins, and dampens IgE class switching. Neuritin reduced differentiation of mouse and human GC B cells into plasma cells, downregulated BLIMP-1, and upregulated BCL6. Administration of neuritin to Tfr-deficient mice prevented the accumulation of early plasma cells in GCs. Production of neuritin by Tfr cells emerges as a central mechanism to suppress B cell-driven autoimmunity and IgE-mediated allergies.

Neuritin-overexpressing transgenic mice demonstrate enhanced neuroregeneration capacity and improved spatial learning and memory recovery after ischemia-reperfusion injury

Acute ischemia-reperfusion (IR)-induced brain injury is further exacerbated by a series of slower secondary pathogenic events, including delayed apoptosis due to neurotrophic factor deficiency. Neuritin, a neurotrophic factor regulating nervous system development and plasticity, is a potential therapeutic target for treatment of IR injury. In this study, Neuritin-overexpressing transgenic (Tg) mice were produced by pronuclear injection and offspring with high overexpression used to generate a line with stable inheritance for testing the neuroprotective capacity of Neuritin against transient global ischemia (TGI). Compared to wild-type mice, transgenic mice demonstrated reduced degradation of the DNA repair factor poly [ADP-ribose] polymerase 1 (PARP 1) in the hippocampus, indicating decreased hippocampal apoptosis rate, and a greater number of surviving hippocampal neurons during the first week post-TGI. In addition, Tg mice showed increased expression of the regeneration markers NF-200, synaptophysin, and GAP-43, and improved recovery of spatial learning and memory. Our findings exhibited that the window of opportunity of neural recovery in Neuritin transgenic mice group had a tendency to move ahead after TGI, which indicated that Neuritin can be used as a potential new therapeutic strategy for improving the outcome of cerebral ischemia injury.

CPG15/Neuritin Mimics Experience in Selecting Excitatory Synapses for Stabilization by Facilitating PSD95 Recruitment

A key feature of brain plasticity is the experience-dependent selection of optimal connections· implemented by a set of activity-regulated genes that dynamically adjust synapse strength and number. The activity-regulated gene cpg15/neuritin has been previously implicated in stabilization and maturation of excitatory synapses. Here· we combine two-photon microscopy with genetic and sensory manipulations to dissect excitatory synapse formation in vivo and examine the role of activity and CPG15 in dendritic spine formation, PSD95 recruitment, and synapse stabilization. We find that neither visual experience nor CPG15 is required for spine formation. However, PSD95 recruitment to nascent spines and their subsequent stabilization requires both. Further, cell-autonomous CPG15 expression is sufficient to replace experience in facilitating PSD95 recruitment and spine stabilization. CPG15 directly interacts with α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors on immature dendritic spines, suggesting a signaling mode for this small extracellular molecule acting as an experience-dependent “selector” for spine stabilization and synapse maturation.

Experience plays a key role in formation and continuous optimization of brain circuits. Subramanian et al. show that the molecule CPG15/neuritin can replace experience in selecting which nascent contacts between neurons are retained, facilitating the recruitment of proteins that promote synapse maturation and stabilization.

Decreased cpg15 augments oxidative stress in sleep deprived mouse brain

Sleep deprivation (SD) has detrimental effects on the physiological function of the brain. However, the underlying mechanism remains elusive. In the present study, we investigated the expression of candidate plasticity-related gene 15 (cpg15), a neurotrophic gene, and its potential role in SD using a REM-SD mouse model. Immunofluorescent and Western blot analysis revealed that the expression of cpg15 protein decreased in the hippocampus, ventral group of the dorsal thalamus (VENT), and somatosensory area of cerebral cortex (SSP) after 24-72 h of REM-SD, and the oxidative stress in these brain regions was increased in parallel, as indicated by the ratio of glutathione (GSH) to its oxidative product (GSSG). Over-expression of cpg15 in thalamus, hippocampus, and cerebral cortex mediated by AAV reduced the oxidative stress in these regions, indicating that the decrease of cpg15 might be a cause that augments oxidative stress in the sleep deprived mouse brain. Collectively, the results imply that cpg15 may play a protective function in the SD-subjected mouse brain via an anti-oxidative function. To our knowledge, this is the first time to provide evidences in the role of cpg15 against SD-induced oxidative stress in the brain.

Production of polyclonal antibody against human Neuritin and its application of immunodetection

OBJECTIVE

To date, a commercial antibody to human Neuritin for immunoprecipitation is still limited. In this study, we aimed to develop a specific antibody for further research on the potential function of Neuritin.

METHODS AND RESULTS

By epitope prediction of recombinant human Neuritin, the active fragment of human Neuritin that could be used as an excellent immunogen. Soluble His-tagged Neuritin was expressed and purified from Pichia pastoris. Polyclonal antibody against Neuritin was obtained by immunizing Sprague-Dawley rats with purified recombinant human Neuritin. Affinity-purified polyclonal antibody against Neuritin was characterized with indirect enzyme-linked immunosorbent assay, immunoblotting, immunoprecipitation, and immunofluorescence. The results demonstrated that the polyclonal antibody against Neuritin had been prepared successfully. The prepared antibody bound to both exogenous and endogenous Neuritin. Importantly, the anti-Neuritin polyclonal antibody could be used in immunoprecipitation assays.

CONCLUSIONS

The prepared polyclonal antibody could be used in immunoprecipitation and provide researchers with a useful tool for further investigating the function and mechanism of Neuritin.

Functions and the related signaling pathways of the neurotrophic factor neuritin

Neuritin is a member of the neurotrophic factor family, which is activated by neural activity and neurotrophins, and promotes neurite growth and branching. It has shown to play an important role in neuronal plasticity and regeneration. It is also involved in other biological processes such as angiogenesis, tumorigenesis and immunomodulation. Thus far, however, the primary mechanisms of neuritin, including whether or not it acts through a receptor or which downstream signals might be activated following binding, are not fully understood. Recent evidence suggests that neuritin may be a potential therapeutic target in several neurodegenerative diseases. This review focuses on the recent advances in studies regarding the newly identified functions of neuritin and the signaling pathways related to these functions. We also discuss current hot topics and difficulties in neuritin research.

Neuritin 1 expression in human normal tissues and its association with various human cancers

OBJECTIVE(S)

Neuritin (Nrn1) is a glycophosphatidylinositol-linked protein that can be induced by neural activity in the central nervous system. However, its expression outside the nervous system and association with human cancers is unclear. This study investigated the expression of Nrn1 in human tissues as well as its association with human cancers.

MATERIALS AND METHODS

Nrn1 gene expression in human adult tissues was evaluated with the Clontech Multiple Tissue cDNA panel. Nrn1 protein in various tissues was detected by immunohistochemistry. Signal v.4.0 and TMHMM v.2.0 software were used to identify the signal peptide and transmembrane helix of Nrn1. The subcellular localization of Nrn1 in cultured SH-SY5Y cells was assessed by immunocytochemistry and western blotting. The expression of Nrn1 in human cancers were assessed using the online tools GEPIA.

RESULTS

Nrn1 mRNA was expressed in various tissues, compared to mRNA levels in the brain tissues, expression was high in the placenta, lungs, skeletal muscle, thymus, pancreas, liver and the heart tissues; lower levels were detected in the small intestine, ovary, spleen, and testes, but there was no detectable expression in the kidneys, colon, prostate or leukocytes. In SY5Y cells, Nrn1 was colocalized with caveolin 1 at the plasma membrane. Nrn1 was downregulated in Bladder Urothelial Carcinoma (BLCA); Breast invasive carcinoma (BRCA); Cervical squamous cell carcinoma and endocervical adenocarcinoma (CESC); Colon adenocarcinoma (COAD); Glioblastoma multiforme (GBM); Kidney Chromophobe (KIHC); Kidney renal papillary cell carcinoma (KIRP); Lower Grade GLioma (LGG); Rectum adenocarcinoma (READ); Uterine Corpus Endometrial Carcinoma (UCEC); Lung adenocarcinoma (LUA), Ovarian serous cystadenocarcinoma (OV) and upregulated in Lymphoid Neoplasm Diffuse Large B-cell Lymphoma (DLBC). A combination of the overall survival analysis of the 12 kinds of human tumors with Nrn1 downregulation revealed that patients with high levels of Nrn1 present a long term survival. But there is no significant effect on DLBC patients' survival.

CONCLUSION

Nrn1 is expressed in various human tissues including the nervous system, specifically in the lipid rafts of cell membranes. We also provided the strong evidence that Nrn1 is associated with 13 kinds of human cancers and could function as biomarkers and therapeutic targets for these cancers.

Recombinant hNeuritin Promotes Structural and Functional Recovery of Sciatic Nerve Injury in Rats

Neuritin is a new neurotropic factor implicated in nervous system development and plasticity. Studies have shown that Neuritin is upregulated in injured nerves, suggesting that it is involved in nerve repair. To test this hypothesis, we investigated whether recombinant human Neuritin could restore nerve structure and function in a rat model of sciatic nerve injury. Neuritin treatment had a dose-dependent effect on functional recovery 4 weeks after injury, as determined by the walking-track test. Similar trends were observed for gastrocnemius muscular strength and nerve conduction velocity. Additionally, sciatic nerve fiber density and organization as well as degree of remyelination were increased, while growth-associated protein 43 and neurofilament 200 expression was upregulated upon treatment with Neuritin. These findings demonstrate that Neuritin stimulates nerve regeneration and functional recovery and thus promotes the repair of injured sciatic nerves.

Neuritin can normalize neural deficits of Alzheimer's disease

Reductions in hippocampal neurite complexity and synaptic plasticity are believed to contribute to the progressive impairment in episodic memory and the mild cognitive decline that occur particularly in the early stages of Alzheimer's disease (AD). Despite the functional and therapeutic importance for patients with AD, intervention to rescue or normalize dendritic elaboration and synaptic plasticity is scarcely provided. Here we show that overexpression of neuritin, an activity-dependent protein, promoted neurite outgrowth and maturation of synapses in parallel with enhanced basal synaptic transmission in cultured hippocampal neurons. Importantly, exogenous application of recombinant neuritin fully restored dendritic complexity as well as spine density in hippocampal neurons prepared from Tg2576 mice, whereas it did not affect neurite branching of neurons from their wild-type littermates. We also showed that soluble recombinant neuritin, when chronically infused into the brains of Tg2576 mice, normalized synaptic plasticity in acute hippocampal slices, leading to intact long-term potentiation. By revealing the protective actions of soluble neuritin against AD-related neural defects, we provide a potential therapeutic approach for patients with AD.

Neuritin attenuates cognitive function impairments in tg2576 mouse model of Alzheimer's disease

Neuritin, also known as CPG15, is a neurotrophic factor that was initially discovered in a screen to identify genes involved in activity-dependent synaptic plasticity. Neuritin plays multiple roles in the process of neural development and synaptic plasticity, although its binding receptor(s) and downstream signaling effectors remain unclear. In this study, we found that the cortical and hippocampal expression of neuritin is reduced in the brains of Alzheimer's disease (AD) patients and demonstrated that viral-mediated expression of neuritin in the dentate gyrus of 13-month-old Tg2576 mice, an AD animal model, attenuated a deficit in learning and memory as assessed by a Morris water maze test. We also found that neuritin restored the reduction in dendritic spine density and the maturity of individual spines in primary hippocampal neuron cultures prepared from Tg2576 mice. It was also shown that viral-mediated expression of neuritin in the dentate gyrus of 7-week-old Sprague-Dawley rats increased neurogenesis in the hippocampus. Taken together, our results demonstrate that neuritin restores the reduction in dendritic spine density and the maturity of individual spines in primary hippocampal neurons from Tg2576 neurons, and also attenuates cognitive function deficits in Tg2576 mouse model of AD, suggesting that neuritin possesses a therapeutic potential for AD.

The Xenopus retinal ganglion cell as a model neuron to study the establishment of neuronal connectivity

Neurons receive inputs through their multiple branched dendrites and pass this information on to the next neuron via long axons, which branch within the target. The shape the neuron acquires is thus the key to its proper functioning in the neural circuit in which it participates. Both axons and dendrites grow in a directed fashion to their target partner neurons by responding to a large number of molecular cues in the milieu through which they extend. They then go through the process of synaptogenesis, first choosing a neuron on which to synapse, and then the appropriate subcellular location. How a neuron acquires its unique shape, establishes and modifies appropriate synaptic connectivity, and the molecular signals involved, are key questions in developmental neurobiology. Such questions of nervous system wiring are being pursued actively with a variety of different animal models and neuron types, each with its own unique advantages. Among these, the developing retinal ganglion cell (RGC) of the South African clawed frog, Xenopus laevis, has proven particularly fruitful for revealing the secrets of how axons and dendrites acquire their final morphology and connectivity. In this review, we describe how this system can be used to understand the multiple molecular events that instruct the incorporation of RGCs into the neural circuit that controls vision.

Activity-regulated genes as mediators of neural circuit plasticity

Modifications of neuronal circuits allow the brain to adapt and change with experience. This plasticity manifests during development and throughout life, and can be remarkably long lasting. Evidence has linked activity-regulated gene expression to the long-term structural and electrophysiological adaptations that take place during developmental critical periods, learning and memory, and alterations to sensory map representations in the adult. In all these cases, the cellular response to neuronal activity integrates multiple tightly coordinated mechanisms to precisely orchestrate long-lasting, functional and structural changes in brain circuits. Experience-dependent plasticity is triggered when neuronal excitation activates cellular signaling pathways from the synapse to the nucleus that initiate new programs of gene expression. The protein products of activity-regulated genes then work via a diverse array of cellular mechanisms to modify neuronal functional properties. Synaptic strengthening or weakening can reweight existing circuit connections, while structural changes including synapse addition and elimination create new connections. Posttranscriptional regulatory mechanisms, often also dependent on activity, further modulate activity-regulated gene transcript and protein function. Thus, activity-regulated genes implement varied forms of structural and functional plasticity to fine-tune brain circuit wiring.

Interaction of survival of motor neuron (SMN) and HuD proteins with mRNA cpg15 rescues motor neuron axonal deficits

Spinal muscular atrophy (SMA), caused by the deletion of the SMN1 gene, is the leading genetic cause of infant mortality. SMN protein is present at high levels in both axons and growth cones, and loss of its function disrupts axonal extension and pathfinding. SMN is known to associate with the RNA-binding protein hnRNP-R, and together they are responsible for the transport and/or local translation of β-actin mRNA in the growth cones of motor neurons. However, the full complement of SMN-interacting proteins in neurons remains unknown. Here we used mass spectrometry to identify HuD as a novel neuronal SMN-interacting partner. HuD is a neuron-specific RNA-binding protein that interacts with mRNAs, including candidate plasticity-related gene 15 (cpg15). We show that SMN and HuD form a complex in spinal motor axons, and that both interact with cpg15 mRNA in neurons. CPG15 is highly expressed in the developing ventral spinal cord and can promote motor axon branching and neuromuscular synapse formation, suggesting a crucial role in the development of motor axons and neuromuscular junctions. Cpg15 mRNA previously has been shown to localize into axonal processes. Here we show that SMN deficiency reduces cpg15 mRNA levels in neurons, and, more importantly, cpg15 overexpression partially rescues the SMN-deficiency phenotype in zebrafish. Our results provide insight into the function of SMN protein in axons and also identify potential targets for the study of mechanisms that lead to the SMA pathology and related neuromuscular diseases.

The function of activity-regulated genes in the nervous system

The mammalian brain is plastic in the sense that it shows a remarkable capacity for change throughout life. The contribution of neuronal activity to brain plasticity was first recognized in relation to critical periods of development, when manipulating the sensory environment was found to profoundly affect neuronal morphology and receptive field properties. Since then, a growing body of evidence has established that brain plasticity extends beyond development and is an inherent feature of adult brain function, spanning multiple domains, from learning and memory to adaptability of primary sensory maps. Here we discuss evolution of the current view that plasticity of the adult brain derives from dynamic tuning of transcriptional control mechanisms at the neuronal level, in response to external and internal stimuli. We then review the identification of "plasticity genes" regulated by changes in the levels of electrical activity, and how elucidating their cellular functions has revealed the intimate role transcriptional regulation plays in fundamental aspects of synaptic transmission and circuit plasticity that occur in the brain on an every day basis.

Cloning and Phylogenetic Analysis of NMDA Receptor Subunits NR1, NR2A and NR2B in Xenopus laevis Tadpoles

N-methyl-d-aspartate receptors (NMDARs) play an important role in many aspects of nervous system function such as synaptic plasticity and neuronal development. NMDARs are heteromers consisting of an obligate NR1 and most commonly one or two kinds of NR2 subunits. While the receptors have been well characterized in some vertebrate and invertebrate systems, information about NMDARs in Xenopus laevis brain is incomplete. Here we provide biochemical evidence that the NR1, NR2A and NR2B subunits of NMDARs are expressed in the central nervous system of X. laevis tadpoles. The NR1-4a/b splice variants appear to be the predominant isoforms while the NR1-3a/b variants appear to be expressed at low levels. We cloned the X. laevis NR2A and NR2B subunits and provide a detailed annotation of their functional domains in comparison with NR2A and NR2B proteins from 10 and 13 other species, respectively. Both NR2A and NR2B proteins are remarkably well conserved between species, consistent with the importance of NMDARs in nervous system function.

Androgen regulates neuritin mRNA levels in an in vivo model of steroid-enhanced peripheral nerve regeneration

Following crush injury to the facial nerve in Syrian hamsters, treatment with androgens enhances axonal regeneration rates and decreases time to recovery. It has been demonstrated in vitro that the ability of androgen to enhance neurite outgrowth in motoneurons is dependent on neuritin-a protein that is involved in the re-establisment of neuronal connectivity following traumatic damage to the central nervous system and that is under the control of several neurotrophic and neuroregenerative factors--and we have hypothesized that neuritin is a mediator of the ability of androgen to increase peripheral nerve regeneration rates in vivo. Testosterone treatment of facial nerve-axotomized hamsters resulted in an approximately 300% increase in neuritin mRNA levels 2 days post-injury. Simultaneous treatment with flutamide, an androgen receptor blocker that is known to prevent androgen enhancement of nerve regeneration, abolished the ability of testosterone to upregulate neuritin mRNA levels. In a corroborative in vitro experiment, the androgen dihydrotestosterone induced an approximately 100% increase in neuritin mRNA levels in motoneuron-neuroblastoma cells transfected with androgen receptors, but not in cells without androgen receptors. These data confirm that neuritin is under the control of androgens, and suggest that neuritin is an important effector of androgen in enhancing peripheral nerve regeneration following injury. Given that neuritin has now been shown to be involved in responses to both central and peripheral injuries, and appears to be a common effector molecule for several neurotrophic and neurotherapeutic agents, understanding the neuritin pathway is an important goal for the clinical management of traumatic nervous system injuries.

cpg15 and cpg15-2 constitute a family of activity-regulated ligands expressed differentially in the nervous system to promote neurite growth and neuronal survival

Many ligands that affect nervous system development are members of gene families that function together to coordinate the assembly of complex neural circuits. cpg15/neuritin encodes an extracellular ligand that promotes neurite growth, neuronal survival, and synaptic maturation. Here we identify cpg15-2 as the only paralogue of cpg15 in the mouse and human genome. Both genes are expressed predominantly in the nervous system, where their expression is regulated by activity. cpg15-2 expression increases by more than twofold in response to kainate-induced seizures and nearly fourfold in the visual cortex in response to 24 hours of light exposure following dark adaptation. cpg15 and cpg15-2 diverge in their spatial and temporal expression profiles. cpg15-2 mRNA is most abundant in the retina and the olfactory bulb, as opposed to the cerebral cortex and the hippocampus for cpg15. In the retina, they differ in their cell-type specificity. cpg15 is expressed in retinal ganglion cells, whereas cpg15-2 is predominantly in bipolar cells. Developmentally, onset of cpg15-2 expression is delayed compared with cpg15 expression. CPG15-2 is glycosylphosphatidylinositol (GPI) anchored to the cell membrane and, like CPG15, can be released in a soluble-secreted form, but with lower efficiency. CPG15 and CPG15-2 were found to form homodimers and heterodimers with each other. In hippocampal explants and dissociated cultures, CPG15 and CPG15-2 promote neurite growth and neuronal survival with similar efficacy. Our findings suggest that CPG15 and CPG15-2 perform similar cellular functions but may play distinct roles in vivo through their cell-type- and tissue-specific transcriptional regulation.

Neuritin (cpg15) enhances the differentiating effect of NGF on neuronal PC12 cells

Neuritin is a small, highly conserved GPI-anchored protein involved in neurite outgrowth. We have analyzed the involvement of neuritin in NGF-induced differentiation of PC12 cells by investigating the time-course of neuritin expression, the effects of its overexpression or silencing, and the possible mechanisms of its regulation and action. Real-time PCR analysis has shown that neuritin gene is upregulated by NGF in PC12 cells hours before neurite outgrowth becomes appreciable. PC12 cells transfected with a plasmid expressing neuritin display a significant increase in the response to NGF: 1) in the levels of SMI312 positive phosphorylated neurofilament proteins (markers for axonal processes) and tyrosine hydroxylase; 2) in the percentage of cells bearing neurites; as well as 3) in the average length of neurites when compared to control cells. On the contrary, neuritin silencing significantly reduces neurite outgrowth. These data suggest that neuritin is a modulator of NGF-induced neurite extension in PC12 cells. We also showed that neuritin potentiated the NGF-induced differentiation of PC12 cells without affecting TrkA or EGF receptor mRNAs expression. Moreover, the S-methylisothiourea (MIU), a potent inhibitor of inducible nitric oxide synthases, partially counteracts the NGF-mediated neuritin induction. These data suggest that NGF regulates neuritin expression in PC12 cells via the signaling pathway triggered by NO. This study reports the first evidence that neuritin plays a role in modulating neurite outgrowth during the progression of NGF-induced differentiation of PC12 cells. PC12 cells could be considered a valuable model to unravel the mechanism of action of neuritin on neurite outgrowth. (c) 2007 Wiley-Liss, Inc.

Motoneuron injury and repair: New perspectives on gonadal steroids as neurotherapeutics

In this review, we will summarize recent work from our laboratory on the role of gonadal steroids as neuroprotective agents in motoneuron viability following cell stress. Three motoneuron models will be discussed: developing axotomized hamster facial motoneurons (FMNs); adult axotomized mouse FMNs; and immortalized, cultured mouse spinal motoneurons subjected to heat shock. New work on two relevant motoneuron proteins, the survival of motor neuron protein, and neuritin or candidate plasticity-related gene 15, indicates differential steroid regulation of these two proteins after axotomy. The concept of gonadal steroids as cellular stress correction factors and the implications of this for acute neurological injury situations will be presented as well.

Identification of genes co-upregulated with Arc during BDNF-induced long-term potentiation in adult rat dentate gyrus in vivo

Brain-derived neurotrophic factor (BDNF) is a critical regulator of transcription-dependent adaptive neuronal responses, such as long-term potentiation (LTP). Brief infusion of BDNF into the dentate gyrus of adult anesthetized rats triggers stable LTP at medial perforant path-granule synapses that is transcription-dependent and requires induction of the immediate early gene Arc. Rather than acting alone, Arc is likely to be part of a larger BDNF-induced transcriptional program. Here, we used cDNA microarray expression profiling to search for genes co-upregulated with Arc 3 h after BDNF-LTP induction. Of nine cDNAs encoding for known genes and up-regulated more than four-fold, we selected five genes, Narp, neuritin, ADP-ribosylation factor-like protein-4 (ARL4L), TGF-beta-induced immediate early gene-1 (TIEG1) and CARP, for further validation. Real-time PCR confirmed robust up-regulation of these genes in an independent set of BDNF-LTP experiments, whereas infusion of the control protein cytochrome C had no effect. In situ hybridization histochemistry further revealed up-regulation of all five genes in somata of post-synaptic granule cells following both BDNF-LTP and high-frequency stimulation-induced LTP. While Arc synthesis is critical for local actin polymerization and stable LTP formation, several of the co-upregulated genes have known functions in excitatory synaptogenesis, axon guidance and glutamate receptor clustering. These results provide novel insight into gene expression responses underlying BDNF-induced synaptic consolidation in the adult brain in vivo.

Regulation of cpg15 expression during single whisker experience in the barrel cortex of adult mice

Regulation of gene transcription by neuronal activity is thought to be key to the translation of sensory experience into long-term changes in synaptic structure and function. Here we show that cpg15, a gene encoding an extracellular signaling molecule that promotes dendritic and axonal growth and synaptic maturation, is regulated in the somatosensory cortex by sensory experience capable of inducing cortical plasticity. Using in situ hybridization, we monitored cpg15 expression in 4-week-old mouse barrel cortex after trimming all whiskers except D1. We found that cpg15 expression is depressed in the deprived barrels and enhanced in the barrel column corresponding to the spared D1 whisker. Changes in cpg15 mRNA levels first appear in layer IV, peak 12 h after deprivation, and then decline rapidly. In layers II/III, changes in cpg15 expression appear later, peak at 24 h, and persist for days. Induction of cpg15 expression is significantly diminished in adolescent as well as adult CREB knockout mice. cpg15's spatio-temporal expression pattern and its regulation by CREB are consistent with a role in experience-dependent plasticity of cortical circuits. Our results suggest that local structural and/or synaptic changes may be a mechanism by which the adult cortex can adapt to peripheral manipulations.

Homer expression in the Xenopus tadpole nervous system

Homer proteins are integral components of the postsynaptic density and are thought to function in synaptogenesis and plasticity. In addition, overexpression of Homer in the developing Xenopus retinotectal system results in axonal pathfinding errors. Here we report that Xenopus contains the homer1 gene, expressed as the isoform, xhomer1b, which is highly homologous to the mammalian homer1b. The mammalian homer1 gene is expressed as three isoforms, the truncated or short form homer1a and the long forms homer1b and -1c. For Xenopus, we cloned three very similar variants of homer1b, identified as Xenopus xhomer1b.1, xhomer1b.2, and xhomer1b.3, which display up to 98% homology with each other and 90% similarity to mammalian homer1b. Furthermore, we demonstrate that Xenopus also contains a truncated form of the Homer1 protein, which could be induced by kainic acid injection and is likely homologous to the mammalian Homer1a. xHomer1b expression was unaffected by neuronal activity levels but was developmentally regulated. Within the brain, the spatial and temporal distributions of both Homer isoforms were similar in the neuropil and cell body regions. Homer1 was detected in motor axons. Differential distribution of the two isoforms was apparent: Homer1b immunoreactivity was prominent at junctions between soma and the ventricular surface; in the retina, the Mueller radial glia were immunoreactive for Homer1, but not Homer1b, suggesting the retinal glia contain only the Homer1a isoform. Homer1b expression in muscle was prominent throughout development and was aligned with the actin striations in skeletal muscle. The high level of conservation of the xhomer1 gene and the protein expression in the developing nervous system suggest that Homer1 expression may be important for normal neuronal circuit development.

Coordinated motor neuron axon growth and neuromuscular synaptogenesis are promoted by CPG15 in vivo

We have used in vivo time-lapse two-photon imaging of single motor neuron axons labeled with GFP combined with labeling of presynaptic vesicle clusters and postsynaptic acetylcholine receptors in Xenopus laevis tadpoles to determine the dynamic rearrangement of individual axon branches and synaptogenesis during motor axon arbor development. Control GFP-labeled axons are highly dynamic during the period when axon arbors are elaborating. Axon branches emerge from sites of synaptic vesicle clusters. These data indicate that motor neuron axon elaboration and synaptogenesis are concurrent and iterative. We tested the role of Candidate Plasticity Gene 15 (CPG15, also known as Neuritin), an activity-regulated gene that is expressed in the developing motor neurons in this process. CPG15 expression enhances the development of motor neuron axon arbors by promoting neuromuscular synaptogenesis and by increasing the addition of new axon branches.

Neuronal plasticity after spinal cord injury: identification of a gene cluster driving neurite outgrowth

Functional recovery after spinal cord injury (SCI) may result in part from axon outgrowth and related plasticity through coordinated changes at the molecular level. We employed microarray analysis to identify a subset of genes the expression patterns of which were temporally coregulated and correlated to functional recovery after SCI. Steady-state mRNA levels of this synchronously regulated gene cluster were depressed in both ventral and dorsal horn neurons within 24 h after injury, followed by strong re-induction during the following 2 wk, which paralleled functional recovery. The identified cluster includes neuritin, attractin, microtubule-associated protein 1a, and myelin oligodendrocyte protein genes. Transcriptional and protein regulation of this novel gene cluster was also evaluated in spinal cord tissue and in single neurons and was shown to play a role in axonal plasticity. Finally, in vitro transfection experiments in primary dorsal root ganglion cells showed that cluster members act synergistically to drive neurite outgrowth.

Regulation of cpg15 by signaling pathways that mediate synaptic plasticity

Transcriptional activation is a key link between neuronal activity and long-term synaptic plasticity. Little is known about genes responding to this activation whose products directly effect functional and structural changes at the synapse. cpg15 is an activity-regulated gene encoding a membrane-bound ligand that regulates dendritic and axonal arbor growth and synaptic maturation. We report that cpg15 is an immediate-early gene induced by Ca(2+) influx through NMDA receptors and L-type voltage-sensitive calcium channels. Activity-dependent cpg15 expression requires convergent activation of the CaM kinase and MAP kinase pathways. Although activation of PKA is not required for activity-dependent expression, cpg15 is induced by cAMP in active neurons. CREB binds the cpg15 promoter in vivo and partially regulates its activity-dependent expression. cpg15 is an effector gene that is a target for signal transduction pathways that mediate synaptic plasticity and thus may take part in an activity-regulated transcriptional program that directs long-term changes in synaptic connections.