Axonal regeneration in adult CNS neurons ? signaling molecules and pathways

NFIL3 and cAMP response element-binding protein form a transcriptional feedforward loop that controls neuronal regeneration-associated gene expression

Successful regeneration of damaged neurons depends on the coordinated expression of neuron-intrinsic genes. At present however, there is no comprehensive view of the transcriptional regulatory mechanisms underlying neuronal regeneration. We used high-content cellular screening to investigate the functional contribution of 62 transcription factors to regenerative neuron outgrowth. Ten transcription factors are identified that either increase or decrease neurite outgrowth. One of these, NFIL3, is specifically upregulated during successful regeneration in vivo. Paradoxically however, knockdown of NFIL3 and overexpression of dominant-negative NFIL3 both increase neurite outgrowth. Our data show that NFIL3, together with CREB, forms an incoherent feedforward transcriptional regulatory loop in which NFIL3 acts as a negative regulator of CREB-induced regeneration-associated genes.

Molluscan memory of injury: evolutionary insights into chronic pain and neurological disorders

Molluscan preparations have yielded seminal discoveries in neuroscience, but the experimental advantages of this group have not, until now, been complemented by adequate molecular or genomic information for comparisons to genetically defined model organisms in other phyla. The recent sequencing of the transcriptome and genome of Aplysia californica, however, will enable extensive comparative studies at the molecular level. Among other benefits, this will bring the power of individually identifiable and manipulable neurons to bear upon questions of cellular function for evolutionarily conserved genes associated with clinically important neural dysfunction. Because of the slower rate of gene evolution in this molluscan lineage, more homologs of genes associated with human disease are present in Aplysia than in leading model organisms from Arthropoda (Drosophila) or Nematoda (Caenorhabditis elegans). Research has hardly begun in molluscs on the cellular functions of gene products that in humans are associated with neurological diseases. On the other hand, much is known about molecular and cellular mechanisms of long-term neuronal plasticity. Persistent nociceptive sensitization of nociceptors in Aplysia displays many functional similarities to alterations in mammalian nociceptors associated with the clinical problem of chronic pain. Moreover, in Aplysia and mammals the same cell signaling pathways trigger persistent enhancement of excitability and synaptic transmission following noxious stimulation, and these highly conserved pathways are also used to induce memory traces in neural circuits of diverse species. This functional and molecular overlap in distantly related lineages and neuronal types supports the proposal that fundamental plasticity mechanisms important for memory, chronic pain, and other lasting alterations evolved from adaptive responses to peripheral injury in the earliest neurons. Molluscan preparations should become increasingly useful for comparative studies across phyla that can provide insight into cellular functions of clinically important genes.

Expression profile of rat hippocampal neurons treated with the neuroprotective compound 2,4-dinitrophenol: up-regulation of cAMP signaling genes

2,4-Dinitrophenol (DNP) is classically known as a mitochondrial uncoupler and, at high concentrations, is toxic to a variety of cells. However, it has recently been shown that, at subtoxic concentrations, DNP protects neurons against a variety of insults and promotes neuronal differentiation and neuritogenesis. The molecular and cellular mechanisms underlying the beneficial neuroactive properties of DNP are still largely unknown. We have now used DNA microarray analysis to investigate changes in gene expression in rat hippocampal neurons in culture treated with low micromolar concentrations of DNP. Under conditions that did not affect neuronal viability, high-energy phosphate levels or mitochondrial oxygen consumption, DNP induced up-regulation of 275 genes and down-regulation of 231 genes. Significantly, several up-regulated genes were linked to intracellular cAMP signaling, known to be involved in neurite outgrowth, synaptic plasticity, and neuronal survival. Differential expression of specific genes was validated by quantitative RT-PCR using independent samples. Results shed light on molecular mechanisms underlying neuroprotection by DNP and point to possible targets for development of novel therapeutics for neurodegenerative disorders.

Dramatic co-activation of WWOX/WOX1 with CREB and NF-kappaB in delayed loss of small dorsal root ganglion neurons upon sciatic nerve transection in rats

BACKGROUND

Tumor suppressor WOX1 (also named WWOX or FOR) is known to participate in neuronal apoptosis in vivo. Here, we investigated the functional role of WOX1 and transcription factors in the delayed loss of axotomized neurons in dorsal root ganglia (DRG) in rats.

METHODOLOGY/PRINCIPAL FINDINGS

Sciatic nerve transection in rats rapidly induced JNK1 activation and upregulation of mRNA and protein expression of WOX1 in the injured DRG neurons in 30 min. Accumulation of p-WOX1, p-JNK1, p-CREB, p-c-Jun, NF-kappaB and ATF3 in the nuclei of injured neurons took place within hours or the first week of injury. At the second month, dramatic nuclear accumulation of WOX1 with CREB (>65% neurons) and NF-kappaB (40-65%) occurred essentially in small DRG neurons, followed by apoptosis at later months. WOX1 physically interacted with CREB most strongly in the nuclei as determined by FRET analysis. Immunoelectron microscopy revealed the complex formation of p-WOX1 with p-CREB and p-c-Jun in vivo. WOX1 blocked the prosurvival CREB-, CRE-, and AP-1-mediated promoter activation in vitro. In contrast, WOX1 enhanced promoter activation governed by c-Jun, Elk-1 and NF-kappaB. WOX1 directly activated NF-kappaB-regulated promoter via its WW domains. Smad4 and p53 were not involved in the delayed loss of small DRG neurons.

CONCLUSIONS/SIGNIFICANCE

Rapid activation of JNK1 and WOX1 during the acute phase of injury is critical in determining neuronal survival or death, as both proteins functionally antagonize. In the chronic phase, concurrent activation of WOX1, CREB, and NF-kappaB occurs in small neurons just prior to apoptosis. Likely in vivo interactions are: 1) WOX1 inhibits the neuroprotective CREB, which leads to eventual neuronal death, and 2) WOX1 enhances NF-kappaB promoter activation (which turns to be proapoptotic). Evidently, WOX1 is the potential target for drug intervention in mitigating symptoms associated with neuronal injury.

Effect of hydrogen sulfide on cyclic AMP production in isolated bovine and porcine neural retinae

Hydrogen sulfide (H(2)S) has been reported to exert pharmacological effects on neural and non-neural tissues from several mammalian species. In the present study, we examined the role of the intracellular messenger, cyclic AMP in retinal response to H(2)S donors, sodium hydrosulfide (NaHS) and sodium sulfide (Na(2)S) in cows and pigs. Isolated bovine and porcine neural retinae were incubated in oxygenated Krebs buffer solution prior to exposure to varying concentrations of NaHS, Na(2)S or the diterpene activator of adenylate cyclase, forskolin. After incubation at different time intervals, tissue homogenates were prepared for cyclic AMP assay using a well established methodology. In isolated bovine and porcine retinae, the combination of both phosphodiesterase inhibitor, IBMX (2 mM) and forskolin (10 microM) produced a synergistic increase (P < 0.001) in cyclic AMP concentrations over basal levels. NaHS (10 nM-100 microM) produced a time-dependent increase in cyclic AMP concentrations over basal levels which reached a maximum at 20 min in both bovine and porcine retinae. At this time point, both NaHS and Na(2)S (10 nM-100 microM) caused a significant (P < 0.05) dose-dependent increase in cyclic AMP levels in bovine and porcine retinae. For instance, NaHS (100 nM) elicited a four-fold and three-fold increase in cyclic AMP concentrations in bovine and porcine retinae respectively whilst higher concentrations of Na(2)S (100 microM) produced a much lesser effect in both species. In bovine and porcine retinae, the effects caused by forskolin (10 microM) on cyclic AMP production were not potentiated by addition of low or high concentrations of both NaHS and Na(2)S. We conclude that H(2)S donors can increase cyclic AMP production in isolated neural retinae from cows and pigs. Bovine retina appears to be more sensitive to the stimulatory effect of H(2)S donors on cyclic nucleotide production than its porcine counterpart indicating that species differences exist in the magnitude of this response. Furthermore, effects produced by forskolin on cyclic AMP formation were not additive with those elicited by H(2)S donors suggesting that these agents may share a common mechanism in their action on the adenylyl cyclase pathway.

Maintaining the brain: insight into human neurodegeneration from Drosophila melanogaster mutants

The fruitfly Drosophila melanogaster has enabled significant advances in neurodegenerative disease research, notably in the identification of genes that are required to maintain the structural integrity of the brain, defined by recessive mutations that cause adult onset neurodegeneration. Here, we survey these genes in the fly and classify them according to five key cell biological processes. Over half of these genes have counterparts in mice or humans that are also associated with neurodegeneration. Fly genetics continues to be instrumental in the analysis of degenerative disease, with notable recent advances in our understanding of several inherited disorders, Parkinson's disease, and the central role of mitochondria in neuronal maintenance.

Translating nociceptor sensitivity: the role of axonal protein synthesis in nociceptor physiology

The increased sensitivity of peripheral pain-sensing neurons, or nociceptors, is a major cause of the sensation of pain that follows injury. This plasticity is thought to contribute to the maintenance of chronic pain states. Although we have a broad knowledge of the factors that stimulate changes in nociceptor sensitivity, the cellular mechanisms that underlie this plasticity are still poorly understood; however, they are likely to involve changes in gene expression required for the phenotypic and functional changes seen in nociceptive neurons after injury. While the regulation of gene expression at the transcriptional level has been studied extensively, the regulation of protein synthesis, which is also a tightly controlled process, has only recently received more attention. Despite the established role of protein synthesis in the plasticity of neuronal cell bodies and dendrites, little attention has been paid to the role of translation control in mature undamaged axons. In this regard, several recent studies have demonstrated that the control of protein synthesis within the axonal compartment is crucial for the normal function and regulation of sensitivity of nociceptors. Pathways and proteins regulating this process, such as the mammalian target of rapamycin signaling cascade and the fragile X mental retardation protein, have recently been identified. We review here recent evidence for the regulation of protein synthesis within a nociceptor's axonal compartment and its contribution to this neuron's plasticity. We believe that an increased understanding of this process will lead to the identification of novel targets for the treatment of chronic pain.

The non-apoptotic role of p53 in neuronal biology: enlightening the dark side of the moon

The transcription factor p53 protects neurons from transformation and DNA damage through the induction of cell-cycle arrest, DNA repair and apoptosis in a range of in vitro and in vivo conditions. Indeed, p53 has a crucial role in eliciting neuronal cell death during development and in adult organisms after exposure to a range of stressors and/or DNA damage. Nevertheless, accumulating evidence challenges this one-sided view of the role of p53 in the nervous system. Here, we discuss how-unexpectedly-p53 can regulate the proliferation and differentiation of neural progenitor cells independently of its role in apoptosis, and p53 post-translational modifications might promote neuronal maturation, as well as axon outgrowth and regeneration, following neuronal injury. We hope to encourage a more comprehensive view of the non-apoptotic functions of p53 during neural development, and to warn against oversimplifications regarding its role in neurons. In addition, we discuss how further insight into the p53-dependent modulation of these mechanisms is necessary to elucidate the decision-making processes between neuronal cell death and differentiation during development, and between neuronal degeneration and axonal regeneration after injury.

Exogenous CNTF stimulates axon regeneration of retinal ganglion cells partially via endogenous CNTF

Intravitreal injections of exogenous CNTF stimulate axon regeneration of RGCs in vivo. Nevertheless, controversy exists over the ability of exogenous CNTF to directly stimulate axon regeneration of mature RGCs. Here we demonstrate that CNTF potently stimulated axon outgrowth of mature RGCs in culture in a JAK/STAT3- and PI3K/AKT-signaling pathway-dependent fashion and stronger than oncomodulin. Additional cAMP elevation or inhibition of MAPK activity increased these effects. In vivo intravitreal injections of exogenous CNTF induced endogenous CNTF expression in astrocytes in a manner that depended on the MAPK/ERK-signaling pathway activation. Reduction of endogenous CNTF expression by MAPK/ERK pathway inhibitors or its absence in CNTF deficient mice markedly reduced the neurite growth-promoting effects of exogenous CNTF. These data demonstrate that CNTF is a potent axon growth-promoting factor for mature RGCs. However, exogenously applied CNTF stimulates RGCs in vivo partially indirectly via a mechanism that depends on astrocyte-derived CNTF.

Phosphodiesterase genes and antidepressant treatment response: a review

Depression results in a tremendous burden to individuals suffering from the disorder and to the global health economy. Available pharmacologic treatments for depression target monoamine levels and monoamine receptors. However, delayed onset of effect, partial or inadequate treatment response, and side-effects are significant limitations of current therapies. The search for a better understanding of mechanisms of depression and for new treatment targets has turned attention to intracellular mediators. Phosphodiesterases (PDEs) are enzymes that break down the intracellular second messenger mononucleotides cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Recent data from animal and human studies indicate that PDEs may play a role in depression and in related stress conditions. PDE genes have been linked directly to depression and to other genes associated with psychiatric disorders.

A p53-CBP/p300 transcription module is required for GAP-43 expression, axon outgrowth, and regeneration

Transcription regulates axon outgrowth and regeneration. However, to date, no transcription complexes have been shown to control axon outgrowth and regeneration by regulating axon growth genes. Here, we report that the tumor suppressor p53 and its acetyltransferases CBP/p300 form a transcriptional complex that regulates the axonal growth-associated protein 43, a well-characterized pro-axon outgrowth and regeneration protein. Acetylated p53 at K372-3-82 drives axon outgrowth, GAP-43 expression, and binds specific elements on the neuronal GAP-43 promoter in a chromatin environment through CBP/p300 signaling. Importantly, in an axon regeneration model, both CBP and p53 K372-3-82 are induced following axotomy in facial motor neurons, where p53 K372-3-82 occupancy of GAP-43 promoter is enhanced as shown by in vivo chromatin immunoprecipitation. Finally, by comparing wild-type and p53 null mice, we demonstrate that the p53/GAP-43 transcriptional module is specifically switched on during axon regeneration in vivo. These data contribute to the understanding of gene regulation in axon outgrowth and may suggest new molecular targets for axon regeneration.

Erythropoietin promotes axonal growth in a model of neuronal polarization

Erythropoietin (EPO) enhances neurogenesis, neuroprotection and regeneration. Here, we examined the effects of EPO on axonal and dendritic growth in a model of neuronal polarization. EPO did not effect survival or the polarized morphology of hippocampal neurons but its effect on neurite outgrowth depended upon the stage of polarization. When added to neurons in the process of establishing polarity (0-2 days in vitro (DIV)), it enhanced axonal and dendritic growth, while EPO added to early polarized cultures at 3-4 DIV promoted the growth of axons but not dendrites. EPO stimulated the phosphorylation of Akt at serine-473 and co-incubation of the Akt/PI-3 kinase pathway inhibitor LY294002 with EPO abolished its effects on Akt phosphorylation and axonal growth. However, while Leukemia Inhibitory Factor (LIF) similarly stimulated phosphorylation of Akt, it had no effect on axonal or dendritic growth, indicating that AKT phosphorylation is necessary but not sufficient for neurite outgrowth in hippocampal neurons.

Ndel1 promotes axon regeneration via intermediate filaments

Failure of axons to regenerate following acute or chronic neuronal injury is attributed to both the inhibitory glial environment and deficient intrinsic ability to re-grow. However, the underlying mechanisms of the latter remain unclear. In this study, we have investigated the role of the mammalian homologue of aspergillus nidulans NudE, Ndel1, emergently viewed as an integrator of the cytoskeleton, in axon regeneration. Ndel1 was synthesized de novo and upregulated in crushed and transected sciatic nerve axons, and, upon injury, was strongly associated with neuronal form of the intermediate filament (IF) Vimentin while dissociating from the mature neuronal IF (Neurofilament) light chain NF-L. Consistent with a role for Ndel1 in the conditioning lesion-induced neurite outgrowth of Dorsal Root Ganglion (DRG) neurons, the long lasting in vivo formation of the neuronal Ndel1/Vimentin complex was associated with robust axon regeneration. Furthermore, local silencing of Ndel1 in transected axons by siRNA severely reduced the extent of regeneration in vivo. Thus, Ndel1 promotes axonal regeneration; activating this endogenous repair mechanism may enhance neuroregeneration during acute and chronic axonal degeneration.

cAMP promotes neurite outgrowth and extension through protein kinase A but independently of Erk activation in cultured rat motoneurons

It is well established that cAMP counteracts myelin inhibition to permit axon regeneration in the central nervous system. On the other hand, the role of cAMP in axonal growth on permissive substrates remains controversial because the evidence available is contradictory. In view that elevation of cAMP represents an attractive therapeutic target to promote nerve regeneration in vivo, we investigated the effect of cAMP on neurite outgrowth and extension in motoneurons. We manipulated cAMP levels pharmacologically in cultured motoneurons and investigated targets downstream of cAMP of neurite outgrowth and extension on a permissive substrate. Reduction of cAMP by the adenylyl cyclase inhibitor SQ22536 inhibited, and elevation of cAMP by forskolin, dibutyryl cAMP, IBMX and rolipram increased outgrowth and extension of neurites. The cAMP-mediated effects occur via activation of protein kinase A (PKA) and were reduced by the inhibitors, H89 and Rp-cAMP. However, cAMP elevation did not lead to Erk activation that is an essential downstream component of neurotrophin signaling. These findings provide evidence for a key role of cAMP in promoting peripheral nerve regeneration after nerve injuries and indicate that this effect is unusual in not being mediated via Erk phosphorylation.

Protein kinase A modulates retinal ganglion cell growth during development

During development, retinal ganglion cells (RGCs) extend their axons toward their thalamic and mesencephalic targets. Their navigation is largely directed by guidance cues present in their environment. Since cAMP is an important second messenger that mediates the neural response to guidance molecules and its intracellular levels seem to decrease significantly following birth, we tested whether modulation of the cAMP/protein kinase A (PKA) pathway would affect the normal development of RGC axons. At postnatal day 1, hamsters received a unilateral intraocular injection of either 0.9% saline solution, 12 mM of the membrane-permeable cAMP analogue (dibutyryl cAMP; db-cAMP), or 10 microM of the PKA inhibitor KT5720. Intraocular elevation of cAMP significantly accelerated RGC axonal growth while inhibition of PKA activity decreased it. Moreover, when highly purified RGC cultures were treated with forskolin (an activator of adenylate cyclase) or cAMP analogues (db-cAMP and Sp-cAMP), neurite length, growth cone (GC) surface area and GC filopodia number were significantly increased. This indicates that intraocular elevation of cAMP acts directly on RGCs. Since these effects were prevented by PKA inhibitors, it demonstrates that cAMP also exerts its action via the PKA pathway. Taken together, these results suggest that the cAMP/PKA cascade is essential for the normal development of retinothalamic projections.

Leptin as a neuroprotective agent

Leptin is a hormone produced by adipocytes that regulates satiety (food uptake) and energy homeostasis by activating receptors expressed in neurons of the hypothalamus. Leptin receptors are also found in other brain regions such as the hippocampus and cerebral cortex, and have known roles in regulating neural development and neuroendocrine functions. Recent evidence indicates that leptin could be neuroprotective, enhancing neuronal survival both in vitro and in vivo. Intriguingly, administration of leptin protects against neuronal death in animal models of cerebral ischemic injury and hemiparkisonism. Activation of the Janus kinase (JAK)-signal transducers and activator of transcription (STAT), phosphatidylinositol (PI) 3-kinase and the extracellular signal regulated kinase (ERK) pathways are known downstream events of leptin receptor signaling, all of which are pro-survival and anti-apoptotic. The relative ease of leptin's accessibility to the brain by peripheral administration makes it a potential drug candidate in the development of therapeutics for brain injuries and neurodegeneration.

Analysis of gene expression during neurite outgrowth and regeneration

Background

The ability of a neuron to regenerate functional connections after injury is influenced by both its intrinsic state and also by extrinsic cues in its surroundings. Investigations of the transcriptional changes undergone by neurons during in vivo models of injury and regeneration have revealed many transcripts associated with these processes. Because of the complex milieu of interactions in vivo, these results include not only expression changes directly related to regenerative outgrowth and but also unrelated responses to surrounding cells and signals. In vitro models of neurite outgrowth provide a means to study the intrinsic transcriptional patterns of neurite outgrowth in the absence of extensive extrinsic cues from nearby cells and tissues.

Results

We have undertaken a genome-wide study of transcriptional activity in embryonic superior cervical ganglia (SCG) and dorsal root ganglia (DRG) during a time course of neurite outgrowth in vitro. Gene expression observed in these models likely includes both developmental gene expression patterns and regenerative responses to axotomy, which occurs as the result of tissue dissection. Comparison across both models revealed many genes with similar gene expression patterns during neurite outgrowth. These patterns were minimally affected by exposure to the potent inhibitory cue Semaphorin3A, indicating that this extrinsic cue does not exert major effects at the level of nuclear transcription. We also compared our data to several published studies of DRG and SCG gene expression in animal models of regeneration, and found the expression of a large number of genes in common between neurite outgrowth in vitro and regeneration in vivo.

Conclusion

Many gene expression changes undergone by SCG and DRG during in vitro outgrowth are shared between these two tissue types and in common with in vivo regeneration models. This suggests that the genes identified in this in vitro study may represent new candidates worthy of further study for potential roles in the therapeutic regrowth of neuronal connections.

Astrocyte-derived CNTF switches mature RGCs to a regenerative state following inflammatory stimulation

Retinal ganglion cells (RGCs) normally fail to regenerate injured axons and undergo apoptosis soon after injury. We have recently shown that lens injury (LI) or intravitreally applied zymosan allow RGCs to survive axotomy and regenerate axons in the injured optic nerve. Activated macrophages and oncomodulin have been suggested to be the principal mediators of this phenomenon. However, several lines of evidence show that macrophage-derived factors alone cannot account for all the beneficial effects of intraocular inflammation. We show here that LI or zymosan induce upregulation of ciliary neurotrophic factor (CNTF) in retinal astrocytes and release CNTF independent of macrophages and activate the transcription factor signal transducers and activators of transcription 3 (STAT3) in RGCs. Levels of CNTF expressed in retinal glia and STAT3 activation in RGC were correlated with the time course of RGCs switching to an active regenerative state. Intravitreal injections of antibodies against CNTF or a Janus-kinase inhibitor compromised the beneficial effects of LI, whereas an antiserum against oncomodulin was ineffective. Like the action of CNTF, the effects of LI were potentiated by drugs that increase intracellular cAMP levels, resulting in strong axon regeneration in vivo. These data indicate that astrocyte-derived CNTF is a major contributor to the neuroprotective and axon-growth-promoting effects of LI and zymosan. These findings could lead to the development of a therapeutic principle for promoting axon regeneration in the CNS as a whole.

CREB and cAMP response element-mediated gene expression in the ischemic brain

Cerebral ischemia triggers robust phosphorylation of cAMP response element-binding protein (CREB) and CRE-mediated gene expression in neurons. Glutamate receptor activation and subsequent calcium influx may activate CREB shortly after ischemia. CREB activation leads to expression of genes encoding neuroprotective molecules, such as the antiapoptotic protein Bcl-2, and contributes to survival of neurons after ischemic insult. Recent studies have suggested that CREB may be involved in acquisition of ischemic tolerance, a phenomenon that occurs after sublethal ischemic stress. CREB activation is also involved in the survival of newborn neurons in the dentate gyrus of the hippocampus after ischemia. Therefore, CREB-related therapeutics may be promising for brain protection and endogenous neurogenesis and could promote functional recovery in ischemic stroke patients. This minireview summarizes our current understanding for the role of CREB in regulating CRE-mediated gene expression during cerebral ischemia.

The length of hippocampal cholinergic fibers is reduced in the aging brain

Cholinergic deficits occur in the aged hippocampus and they are significant in Alzheimer's disease. Using stereological and biochemical approaches, we characterized the cholinergic septohippocampal pathway in old (24 months) and young adult (3 months) rats. The total length of choline acetyltransferase (ChAT)-positive fibers in the dorsal hippocampus was significantly decreased by 32% with aging (F((1,9))=20.94, p=0.0014), along with the levels of synaptophysin, a presynaptic marker. No significant changes were detected in ChAT activity or in the amounts of ChAT protein, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), tropomyosin related kinase receptor (Trk) A, TrkB, or p75 neurotrophin receptor (p75(NTR)) in the aged dorsal hippocampus. The number and size of ChAT-positive neurons and the levels of ChAT activity, NGF and BDNF were not statistically different in the septum of aged and young adult rats. This study suggests that substantial synaptic loss and cholinergic axonal degeneration occurs during aging and reinforces the importance of therapies that can protect axons and promote their growth in order to restore cholinergic neurotransmission.

Role of PKA in the anti-Thy-1 antibody-induced neurite outgrowth of dorsal root ganglionic neurons

Thy-1 is highly expressed in the mammalian nervous system. Our previous study showed that addition of anti-Thy-1 antibody to cultured dorsal root ganglionic (DRG) neurons promotes neurite outgrowth. In this study, we identified a novel signaling pathway mediating this event. Treatment with function-blocking anti-Thy-1 antibodies enhanced neurite outgrowth of DRG neurons in terms of total neurite length, longest neurite length, and total neurite branching points. To elucidate the possible signal transduction pathway involved, activation of kinases was evaluated by Western blotting. Transient phosphorylation of protein kinase A (PKA) and mitogen-activated kinase kinase (MEK) was induced after 15 min of anti-Thy-1 antibody treatment. Pretreatment with a PKA inhibitor (PKI) or an MEK inhibitor, PD98059, significantly decreased the neurite outgrowth response triggered by anti-Thy-1 antibody, indicating the involvement of both kinases. In addition, anti-Thy-1 antibody treatment also induced transient phosphorylation of cyclic AMP-response element-binding protein (CREB) and this effect was also blocked by a PKI or PD98059. Furthermore, the fact that PKI abolished anti-Thy-1 antibody-induced MEK phosphorylation showed that PKA acts upstream of the MEK-CREB cascade. In summary, the PKA-MEK-CREB pathway is a new pathway involved in the neurite outgrowth-promoting effect of anti-Thy-1 antibody.

Genetic Manipulation of Neural Stem Cells for Transplantation into the Injured Spinal Cord

The injured adult spinal cord is not conducive for neuronal regeneration and neurogenesis. Engrafted neural precursor cells (NPCs) differentiate largely into astroglia, with only a very small percentage becoming neurons (which might replace injured neurons) or oligodendroglia (which might improve injury induced demyelination of spared neurons). Several recent attempts have been made to enhanced neurogenesis or oligodendroglia differentiation of transplanted NPCs by genetic manipulation. These include exogenous expression of noggin, with the idea of antagonizing the astroglia differentiation promoting Bone Morphogenetic Proteins (BMPs). Direct attempts to enhance neurogenesis have also been made in transgenic over-expression of neurogenic basic helix-loop-helix transcription factors. These experiments resulted in some interesting observations, which we discuss here in the light of recent advances in development of cell-based engraftment therapy for spinal cord injuries.

Interleukin-6 promotes sprouting and functional recovery in lesioned organotypic hippocampal slice cultures

Interleukin (IL)-6 is a pro-inflammatory cytokine now widely recognized to contribute to the molecular events that follow CNS injury. Little is known, however, about its action on axonal sprouting and regeneration in the brain. We addressed this issue using the model of transection of Schaffer collaterals in mice organotypic hippocampal slice cultures. Transection of slice cultures was associated with a marked release of IL-6 that could be neutralized by an IL-6 blocking antibody. We monitored functional recovery across the lesion by recording synaptic responses using a multi-electrode array. We found that application of IL-6 antibodies to the cultures after lesioning significantly reduced functional recovery across the lesion. Furthermore, the level of expression of the 43-kDa growth-associated protein (GAP-43) was lower in slices treated with the IL-6 neutralizing antibody than in those treated with a control IgG. Conversely, addition of exogenous IL-6 to the culture medium resulted in a dose-dependent enhancement of functional recovery across the lesion and a higher level of expression of GAP-43. Co-culture of CA3 hemi-slices from thy1-YFP mice with CA1 hemi-slices from wild-type animals confirmed that IL-6-treated co-cultures exhibited an increased number of growing fluorescent fibres across the lesion site. Taken together these data indicate that IL-6 plays an important role in CNS repair mechanisms by promoting regrowth and axon regeneration.

TACE‐induced cleavage of NgR and p75 NTR in dorsal root ganglion cultures disinhibits outgrowth and promotes branching of neurites in the presence of inhibitory CNS myelin

After binding, central nervous system (CNS) myelin-derived axon growth inhibitory ligands, the Nogo-66 receptor (NgR), complexes with LINGO-1 and either the low-affinity neurotrophin receptor (p75(NTR)) or TROY to initiate growth cone collapse via a Rho-A inhibitory signaling pathway and/or Ca(2+)-dependent activation of epidermal growth factor receptor (EGFR) through an unknown signaling pathway. We have shown that axon growth through CNS myelin is disinhibited after neurotrophic factor administration by 1) initiating intramembranous proteolysis (RIP) of p75(NTR), leading to cleavage of the extracellular (p75(ECD)) and intracellular domains (p75(ICD)) by alpha- and gamma-secretase, respectively, thereby paralyzing inhibitory signaling; 2) shedding of soluble NgR(ECD), which acts as a competitive antagonist to NgR for binding of inhibitory ligands; and 3) antagonizing NgR/p75(NTR) clustering by competitive p75(ECD)/NgR interaction. Here, we report that TNF-alpha converting enzyme (TACE) (a disintegrin and metalloproteinase 17, ADAM17) induces disinhibition of FGF2-stimulated neurite outgrowth of dorsal root ganglion neurons (DRGN) cultured in the presence of a predetermined concentration of inhibitory CNS myelin-derived ligands. After addition of TACE (which has alpha-secretase activity) to mitotically arrested adult rat mixed DRG cultures, we demonstrate 1) NgR(ECD) shedding; 2) release of p75(ECD) and p75(ICD) by RIP of p75(NTR); 3) blockade of Rho-A activation; 4) reduced EGFR phosphorylation; and 5) increased FGF2-stimulated DRGN neurite outgrowth and branching in the presence of CNS myelin-derived inhibitory ligands. Thus, TACE-induced cleavage of NgR and RIP of p75(NTR) abrogates axon growth inhibitory signaling, thereby disinhibiting CNS axon/neurite growth.