Selective expression of Jun proteins following axotomy and axonal transport block in peripheral nerves in the rat: evidence for a role in the regeneration process

The Jun-dependent axon regeneration gene program: Jun promotes regeneration over plasticity

The regeneration-associated gene (RAG) expression program is activated in injured peripheral neurons after axotomy and enables long-distance axon re-growth. Over 1000 genes are regulated, and many transcription factors are upregulated or activated as part of this response. However, a detailed picture of how RAG expression is regulated is lacking. In particular, the transcriptional targets and specific functions of the various transcription factors are unclear. Jun was the first-regeneration-associated transcription factor identified and the first shown to be functionally important. Here we fully define the role of Jun in the RAG expression program in regenerating facial motor neurons. At 1, 4 and 14 days after axotomy, Jun upregulates 11, 23 and 44% of the RAG program, respectively. Jun functions relevant to regeneration include cytoskeleton production, metabolic functions and cell activation, and the downregulation of neurotransmission machinery. In silico analysis of promoter regions of Jun targets identifies stronger over-representation of AP1-like sites than CRE-like sites, although CRE sites were also over-represented in regions flanking AP1 sites. Strikingly, in motor neurons lacking Jun, an alternative SRF-dependent gene expression program is initiated after axotomy. The promoters of these newly expressed genes exhibit over-representation of CRE sites in regions near to SRF target sites. This alternative gene expression program includes plasticity-associated transcription factors and leads to an aberrant early increase in synapse density on motor neurons. Jun thus has the important function in the early phase after axotomy of pushing the injured neuron away from a plasticity response and towards a regenerative phenotype.

Cellular reprogramming for successful CNS axon regeneration is driven by a temporally changing cast of transcription factors

In contrast to mammals, adult fish display a remarkable ability to fully regenerate central nervous system (CNS) axons, enabling functional recovery from CNS injury. Both fish and mammals normally undergo a developmental downregulation of axon growth activity as neurons mature. Fish are able to undergo damage-induced “reprogramming” through re-expression of genes necessary for axon growth and guidance, however, the gene regulatory mechanisms remain unknown. Here we present the first comprehensive analysis of gene regulatory reprogramming in zebrafish retinal ganglion cells at specific time points along the axon regeneration continuum from early growth to target re-innervation. Our analyses reveal a regeneration program characterized by sequential activation of stage-specific pathways, regulated by a temporally changing cast of transcription factors that bind to stably accessible DNA regulatory regions. Strikingly, we also find a discrete set of regulatory regions that change in accessibility, consistent with higher-order changes in chromatin organization that mark (1) the beginning of regenerative axon growth in the optic nerve, and (2) the re-establishment of synaptic connections in the brain. Together, these data provide valuable insight into the regulatory logic driving successful vertebrate CNS axon regeneration, revealing key gene regulatory candidates for therapeutic development.

Intrinsic Neuronal Stress Response Pathways in Injury and Disease

From injury to disease to aging, neurons, like all cells, may face various insults that can impact their function and survival. Although the consequences are substantially dictated by the type, context, and severity of insult, distressed neurons are far from passive. Activation of cellular stress responses aids in the preservation or restoration of nervous system function. However, stress responses themselves can further advance neuropathology and contribute significantly to neuronal dysfunction and neurodegeneration. Here we explore the recent advances in defining the cellular stress responses within neurodegenerative diseases and neuronal injury, and we emphasize axonal injury as a well-characterized model of neuronal insult. We highlight key findings and unanswered questions about neuronal stress response pathways, from the initial detection of cellular insults through the underlying mechanisms of the responses to their ultimate impact on the fates of distressed neurons.

Identification of the key genes associated with neuropathic pain

Neuropathic pain is a chronic pain state associated with multiple etiologies that results in considerable social and economic burden. The identification of key genes associated with neuropathic pain is important for the development of novel therapies. Therefore, the present study downloaded the gene expression profile GSE15041 from the Gene Expression Omnibus database. The unverified gene chip was removed and the microarray data was normalized following quality control. The limma package in R was used to screen the differentially expressed genes (DEGs), followed by Gene Ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. Furthermore, a protein‑protein interaction (PPI) network based on the identified DEGs was constructed to select hub proteins, and reverse transcription‑quantitative polymerase chain reaction was performed to detect the expression of these proteins in a mouse model of neuropathic pain. In total, 86 common DEGs were identified. DEGs were significantly enriched in ̔extracellular space̓ and KEGG pathway enrichment analysis demonstrated that the DEGs were significantly enriched in inflammatory diseases and the mitogen‑activated protein kinase signaling pathway. The PPI network consisted of 27 nodes (proteins) and 47 PPI edges (interactions). Interleukin (IL)‑6, transcription factor AP‑1 (c‑Jun) and urikinase‑type plasminogen activator (Plau) were identified as hub proteins and key genes in neuropathic pain. The mRNA expression of these hub proteins was significantly increased in the neuropathic pain model, compared with the sham group. IL‑6, c‑Jun, and Plau may be involved in development of neuropathic pain and further research investigating the exact role of these key genes is required.

Review : The Immediate-Early Gene Response and Neuronal Death and Regeneration

The induction of the cellular immediate-early gene (cIEG) response is the earliest known transcriptional con sequence of neuronal excitation. It is believed that the products of the cIEGs (such as c- fos and c- jun) serve to bring about persistent changes in neuronal phenotype by altering gene expression and modifying signal transduction pathways. This has led, on the one hand, to the burgeoning use of Fos immunohistochemistry as a surrogate form of neuronal activity mapping and, on the other hand, to a quest for the biochemical functions of individual cIEG products in the nervous system. Recent studies of neuronal death and regen eration have contributed substantially to our vision of the molecular and biological characteristics of the cIEG response. Indeed, they have challenged some of its long-held tenets. Therefore, we will use these results to illustrate our most contemporary view of the cIEG response in the nervous system. The Neuroscientist 1:68- 75,1995

■ REVIEW : Gene Expression in Nerve Regeneration

Injury of peripheral nerve in mammals leads to a complex but stereotypical pattern of histological events that comprise a highly reproducible sequence of degenerative reactions (Wallerian degeneration) succeeded by regenerative responses. These reactions are based on a corresponding sequence of cellular and mo lecular interactions that, in turn, reflect the differential expression of specific genes with functions in nerve degeneration and repair. We report on more than 60 genes and their products that show a specific pattern of regulation following peripheral nerve lesion. The group of regulated genes encoding, e.g., transcription factors, growth factors and their receptors, cytokines, neuropeptides, myelin proteins and lipid carriers, and cytoskeletal proteins as well as extracellular matrix and cell adhesion molecules. We describe and compare the distinct time-courses and cellular origin of expression and further discuss established or putative mo lecular interrelationships and functions with respect to the contribution of these genes/gene products to the molecular regeneration program of the PNS. NEUROSCIENTIST 3:112-122, 1997

Overexpression of ATF3 or the combination of ATF3, c-Jun, STAT3 and Smad1 promotes regeneration of the central axon branch of sensory neurons but without synergistic effects

Peripheral nerve injury results in the activation of a number of transcription factors (TFs) in injured neurons, some of which may be key regulators of the regeneration-associated gene (RAG) programme. Among known RAG TFs, ATF3, Smad1, STAT3 and c-Jun have all been linked to successful axonal regeneration and have known functional and physical interactions. We hypothesised that TF expression would promote regeneration of the central axon branch of DRG neurons in the absence of a peripheral nerve lesion and that simultaneous overexpression of multiple RAG TFs would lead to greater effects than delivery of a single TF. Using adeno-associated viral vectors, we overexpressed either the combination of ATF3, Smad1, STAT3 and c-Jun with farnesylated GFP (fGFP), ATF3 only with fGFP, or fGFP only, in DRG neurons and assessed axonal regeneration after dorsal root transection or dorsal column injury and functional improvement after dorsal root injury. ATF3 alone and the combination of TFs promoted faster regeneration in the injured dorsal root. Surprisingly, however, the combination did not perform better than ATF3 alone. Neither treatment was able to induce functional improvement on sensory tests after dorsal root injury or promote regeneration in a dorsal column injury model. The lack of synergistic effects among these factors indicates that while they do increase the speed of axon growth, there may be functional redundancy between these TFs. Because axon growth is considerably less than that seen after a conditioning lesion, it appears these TFs do not induce the full regeneration programme.

P75 and phosphorylated c-Jun are differentially regulated in spinal motoneurons following axotomy in rats

The neurotrophin receptor (p75) activates the c-Jun N-terminal kinase (JNK) pathway. Activation of JNK and its substrate c-Jun can cause apoptosis. Here we evaluate the role of p75 in spinal motoneurons by comparing immunoreactivity for p75 and phosphorylated c-Jun (p-c-Jun), the production of JNK activation in axotomized motoneurons in postnatal day (PN)1, PN7, PN14 and adult rats. Intensive p-c-Jun was induced in axotomized motoneurons in PN1 and PN7. In PN14, p-c-Jun expression was sharply reduced after the same injury. The decreased expression of p-c-Jun at this age coincided with a developmental switch of re-expression of p75 in axotomized cells. In adult animals, no p-c-Jun but intensive p75 was detected in axotomized motoneurons. These results indicate differential expression or turnover of phosphorylation of c-Jun and p75 in immature versus mature spinal motoneurons in response to axonal injury. The non-co-occurrence of p75 and p-c-Jun in injured motoneurons indicated that p75 may not activate JNK pathway, suggesting that the p75 may not be involved in cell death in axotomized motoneurons.

Retrograde response in axotomized motoneurons: nitric oxide as a key player in triggering reversion toward a dedifferentiated phenotype

The adult brain retains a considerable capacity to functionally reorganize its circuits, which mainly relies on the prevalence of three basic processes that confer plastic potential: synaptic plasticity, plastic changes in intrinsic excitability and, in certain central nervous system (CNS) regions, also neurogenesis. Experimental models of peripheral nerve injury have provided a useful paradigm for studying injury-induced mechanisms of central plasticity. In particular, axotomy of somatic motoneurons triggers a robust retrograde reaction in the CNS, characterized by the expression of plastic changes affecting motoneurons, their synaptic inputs and surrounding glia. Axotomized motoneurons undergo a reprograming of their gene expression and biosynthetic machineries which produce cell components required for axonal regrowth and lead them to resume a functionally dedifferentiated phenotype characterized by the removal of afferent synaptic contacts, atrophy of dendritic arbors and an enhanced somato-dendritic excitability. Although experimental research has provided valuable clues to unravel many basic aspects of this central response, we are still lacking detailed information on the cellular/molecular mechanisms underlying its expression. It becomes clear, however, that the state-switch must be orchestrated by motoneuron-derived signals produced under the direction of the re-activated growth program. Our group has identified the highly reactive gas nitric oxide (NO) as one of these signals, by providing robust evidence for its key role to induce synapse elimination and increases in intrinsic excitability following motor axon damage. We have elucidated operational principles of the NO-triggered downstream transduction pathways mediating each of these changes. Our findings further demonstrate that de novo NO synthesis is not only "necessary" but also "sufficient" to promote the expression of at least some of the features that reflect reversion toward a dedifferentiated state in axotomized adult motoneurons.

Hericium erinaceus (Bull.: Fr.) Pers., a medicinal mushroom, activates peripheral nerve regeneration

OBJECTIVE

To study the ability of aqueous extract of Hericium erinaceus mushroom in the treatment of nerve injury following peroneal nerve crush in Sprague-Dawley rats.

METHODS

Aqueous extract of Hericium erinaceus was given by daily oral administration following peroneal nerve crush injury in Sprague-Dawley rats. The expression of protein kinase B (Akt) and mitogen-activated protein kinase (MAPK) signaling pathways; and c-Jun and c-Fos genes were studied in dorsal root ganglia (DRG) whereas the activity of protein synthesis was assessed in peroneal nerves by immunohistochemical method.

RESULTS

Peripheral nerve injury leads to changes at the axonal site of injury and remotely located DRG containing cell bodies of sensory afferent neurons. Immunofluorescence studies showed that DRG neurons ipsilateral to the crush injury in rats of treated groups expressed higher immunoreactivities for Akt, MAPK, c-Jun and c-Fos as compared with negative control group (P <0.05). The intensity of nuclear ribonucleoprotein in the distal segments of crushed nerves of treated groups was significantly higher than in the negative control group (P <0.05).

CONCLUSION

H. erinaceus is capable of promoting peripheral nerve regeneration after injury. Potential signaling pathways include Akt, MAPK, c-Jun, and c-Fos, and protein synthesis have been shown to be involved in its action.

Induction of phosphorylated c-Jun in neonatal spinal motoneurons after axonal injury is coincident with both motoneuron death and regeneration

c-Jun activation has been implicated not only in neuronal degeneration, but also in survival and regeneration. Here, we investigated c-Jun activation in injured motoneurons by using a nerve crush model in neonatal rats. We identified two distinct subpopulations of motoneurons: about 60% underwent degeneration following injury whereas the remaining 40% survived and induced a regeneration response at 3 weeks post injury. However, all motoneurons examined expressed phosphorylated-c-Jun-immunoreactivity (p-c-Jun-IR) at the early stage of 3 days following injury. These results suggest that active c-Jun was induced in all neonatal motoneurons following nerve crush injury, regardless of whether they were destined to degenerate or undergo successful regeneration at a later stage. Our findings therefore support the hypothesis that active c-Jun is involved in both neuronal degeneration and regeneration.

Extreme thermal noxious stimuli induce pain responses in zebrafish larvae

Exposing tissues to extreme high or low temperature leads to burns. Burned animals sustain several types of damage, from the disruption of the tissue to degeneration of axons projecting through muscle and skin. Such damage causes pain due to both inflammation and axonal degeneration (neuropathic-like pain). Thus, the approach to cure and alleviate the symptoms of burns must be twofold: rebuilding the tissue that has been destroyed and alleviating the pain derived from the burns. While tissue regeneration techniques have been developed, less is known on the treatment of the induced pain. Thus, appropriate animal models are necessary for the development of the best treatment for pain induced in burned tissues. We have developed a methodology in the zebrafish aimed to produce a new animal model for the study of pain induced by burns. Here, we show that two events linked to the onset of burn-induced inflammation and neuropathic-like pain in mammals, degeneration of axons innervating the affected tissues and over-expression of specific genes in sensory tissues, are conserved from zebrafish to mammals.

Redistribution of Kv2.1 ion channels on spinal motoneurons following peripheral nerve injury

Pathophysiological responses to peripheral nerve injury include alterations in the activity, intrinsic membrane properties and excitability of spinal neurons. The intrinsic excitability of α-motoneurons is controlled in part by the expression, regulation, and distribution of membrane-bound ion channels. Ion channels, such as Kv2.1 and SK, which underlie delayed rectifier potassium currents and afterhyperpolarization respectively, are localized in high-density clusters at specific postsynaptic sites (Deardorff et al., 2013; Muennich and Fyffe, 2004). Previous work has indicated that Kv2.1 channel clustering and kinetics are regulated by a variety of stimuli including ischemia, hypoxia, neuromodulator action and increased activity. Regulation occurs via channel dephosphorylation leading to both declustering and alterations in channel kinetics, thus normalizing activity (Misonou et al., 2004; Misonou et al., 2005; Misonou et al., 2008; Mohapatra et al., 2009; Park et al., 2006). Here we demonstrate using immunohistochemistry that peripheral nerve injury is also sufficient to alter the surface distribution of Kv2.1 channels on motoneurons. The dynamic changes in channel localization include a rapid progressive decline in cluster size, beginning immediately after axotomy, and reaching maximum within one week. With reinnervation, the organization and size of Kv2.1 clusters do not fully recover. However, in the absence of reinnervation Kv2.1 cluster sizes fully recover. Moreover, unilateral peripheral nerve injury evokes parallel, but smaller effects bilaterally. These results suggest that homeostatic regulation of motoneuron Kv2.1 membrane distribution after axon injury is largely independent of axon reinnervation.

Specificity of peripheral nerve regeneration: interactions at the axon level

Peripheral nerves injuries result in paralysis, anesthesia and lack of autonomic control of the affected body areas. After injury, axons distal to the lesion are disconnected from the neuronal body and degenerate, leading to denervation of the peripheral organs. Wallerian degeneration creates a microenvironment distal to the injury site that supports axonal regrowth, while the neuron body changes in phenotype to promote axonal regeneration. The significance of axonal regeneration is to replace the degenerated distal nerve segment, and achieve reinnervation of target organs and restitution of their functions. However, axonal regeneration does not always allows for adequate functional recovery, so that after a peripheral nerve injury, patients do not recover normal motor control and fine sensibility. The lack of specificity of nerve regeneration, in terms of motor and sensory axons regrowth, pathfinding and target reinnervation, is one the main shortcomings for recovery. Key factors for successful axonal regeneration include the intrinsic changes that neurons suffer to switch their transmitter state to a pro-regenerative state and the environment that the axons find distal to the lesion site. The molecular mechanisms implicated in axonal regeneration and pathfinding after injury are complex, and take into account the cross-talk between axons and glial cells, neurotrophic factors, extracellular matrix molecules and their receptors. The aim of this review is to look at those interactions, trying to understand if some of these molecular factors are specific for motor and sensory neuron growth, and provide the basic knowledge for potential strategies to enhance and guide axonal regeneration and reinnervation of adequate target organs.

Multiple transcription factor families regulate axon growth and regeneration

Understanding axon regenerative failure remains a major goal in neuroscience, and reversing this failure remains a major goal for clinical neurology. Although an inhibitory central nervous system environment clearly plays a role, focus on molecular pathways within neurons has begun to yield fruitful insights. Initial steps forward investigated the receptors and signaling pathways immediately downstream of environmental cues, but recent work has also shed light on transcriptional control mechanisms that regulate intrinsic axon growth ability, presumably through whole cassettes of gene target regulation. Here we will discuss transcription factors that regulate neurite growth in vitro and in vivo, including p53, SnoN, E47, cAMP-responsive element binding protein (CREB), signal transducer and activator of transcription 3 (STAT3), nuclear factor of activated T cell (NFAT), c-Jun activating transcription factor 3 (ATF3), sex determining region Ybox containing gene 11 (Sox11), nuclear factor κ-light chain enhancer of activated B cells (NFκB), and Krüppel-like factors (KLFs). Revealing the similarities and differences among the functions of these transcription factors may further our understanding of the mechanisms of transcriptional regulation in axon growth and regeneration.

Co-localization of activating transcription factor 3 and phosphorylated c-Jun in axotomized facial motoneurons

Activating transcription factor 3 (ATF3) and c-Jun play key roles in either cell death or cell survival, depending on the cellular background. To evaluate the functional significance of ATF3/c-Jun in the peripheral nervous system, we examined neuronal cell death, activation of ATF3/c-Jun, and microglial responses in facial motor nuclei up to 24 weeks after an extracranial facial nerve axotomy in adult rats. Following the axotomy, neuronal survival rate was progressively but significantly reduced to 79.1% at 16 weeks post-lesion (wpl) and to 65.2% at 24 wpl. ATF3 and phosphorylated c-Jun (pc-Jun) were detected in the majority of ipsilateral facial motoneurons with normal size and morphology during the early stage of degeneration (1-2 wpl). Thereafter, the number of facial motoneurons decreased gradually, and both ATF3 and pc-Jun were identified in degenerating neurons only. ATF3 and pc-Jun were co-localized in most cases. Additionally, a large number of activated microglia, recognized by OX6 (rat MHC II marker) and ED1 (phagocytic marker), gathered in the ipsilateral facial motor nuclei. Importantly, numerous OX6- and ED1-positive, phagocytic microglia closely surrounded and ingested pc-Jun-positive, degenerating neurons. Taken together, our results indicate that long-lasting co-localization of ATF3 and pc-Jun in axotomized facial motoneurons may be related to degenerative cascades provoked by an extracranial facial nerve axotomy.

Roles of membrane trafficking in nerve repair and regeneration

Successful axonal repair following injury is critical for nerve regeneration and functional recovery. Nerve repair relies on three functionally distinct events involving membrane trafficking. First, axonally transported vesicles accumulate, while others are generated at the cut end to restore a selective barrier to the severed axon. Then, retrograde transport of vesicles along microtubules informs the cell body that damage has occurred in the distal axon. Finally, membrane addition to a newly formed growth cone, or to the axonal membrane is required to promote axonal re-growth and elongation. Yet, how these membrane trafficking events are regulated and what are the identities of the molecules and organelles involved remains largely unknown. Several potential factors have been recently identified. Members of the SNARE machinery appear to regulate fusion of vesicles in a calcium-dependent manner to promote axolemmal resealing. Retrograde transport of endosomes powered by the dynein-dynactin molecular motor complex represents a potential injury-signaling platform. Several classes of secretory and endocytic vesicles may coordinate axonal membrane extension and re-growth. Here we discuss recent advances in understanding the mechanisms of the membrane trafficking involved in nerve repair.

Activating transcription factor 3, a useful marker for regenerative response after nerve root injury

Activating transcription factor 3 (ATF3) is induced in various tissues in response to stress. In this experiment, ATF3 expression was studied in adult rats subjected either to a dorsal or ventral root avulsion (VRA; L4-6), or sciatic nerve transection (SNT). Post-operative survival times varied between 1.5 h and 3 weeks. In additional experiments an avulsed ventral root was directly replanted to the spinal cord. Dorsal root ganglias (DRGs) from humans exposed to traumatic dorsal root avulsions were also examined. After SNT ATF3 immunoreactivity (ATF3 IR) was detected in a few DRG neurons already 6 h after the lesion. After 24 h the number had clearly increased and still at 3 weeks DRG neurons remained labeled. In the ventral horn, ATF3 IR in motoneurons (MN) was first detected 24 h after the SNT, and still 3 weeks post-operatively lesioned MN showed ATF3 labeling. After a VRA many spinal MN showed ATF3 IR already after 3 h, and after 6 h all MN were labeled. At 3 weeks a majority of the lesioned MN had died, but all the remaining ones were labeled. When an avulsed ventral root was directly replanted, MN survived and were still labeled at 5 weeks. In DRG, a few neurons were labeled already at 1.5 h after a dorsal root avulsion. At 24 h the number had increased but still only a minority of the neurons were labeled. At 3 days the number of labeled neurons was reduced, and a further reduction was at hand at 7 days and 3 weeks. In parallel, in humans, 3 days after a traumatic dorsal root avulsion, only a few DRG neurons showed ATF3 IR. At 6 weeks no labeled neurons could be detected. These facts imply that ATF3 response to axotomy involves a distance-dependent mechanism. ATF3 also appears to be a useful and reliable neuronal marker of nerve lesions even in humans. In addition, ATF3 up-regulation in both motor and sensory neurons seems to be linked to regenerative competence.

Activation and function of immediate-early genes in the nervous system

Immediate-early genes have important roles in processes such as brain development, learning, and responses to drug abuse. Further, immediate-early genes play an essential role in cellular responses that contribute to long-term neuronal plasticity. Neuronal plasticity is a characteristic of the nervous system that is not limited to the first stages of brain development but persists in adulthood and seems to be an inherent feature of everyday brain function. The plasticity refers to the neuron's capability of showing short- or long-lasting phenotypic changes in response to different stimuli and cellular scenarios. In this review, we focus on the immediate-early genes encoding transcription factors (AP-1 and Egr) that are relevant for neuronal responses. Our current understanding of the mechanisms involved in the induction of the immediate-early genes is presented.

Pelvic Nerve Injury Causes a Rapid Decrease in Expression of Choline Acetyltransferase and Upregulation of c-Jun and ATF-3 in a Distinct Population of Sacral Preganglionic Neurons

Autonomic regulation of the urogenital organs is impaired by injuries sustained during pelvic surgery or compression of lumbosacral spinal nerves (e.g., cauda equina syndrome). To understand the impact of injury on both sympathetic and parasympathetic components of this nerve supply, we performed an experimental surgical and immunohistochemical study on adult male rats, where the structure of this complex part of the nervous system has been well defined. We performed unilateral transection of pelvic or hypogastric nerves and analyzed relevant regions of lumbar and sacral spinal cord, up to 4 weeks after injury. Expression of c-Jun, the neuronal injury marker activating transcription factor-3 (ATF-3), and choline acetyltransferase (ChAT) were examined. We found little evidence for chemical or structural changes in substantial numbers of functionally related but uninjured spinal neurons (e.g., in sacral preganglionic neurons after hypogastric nerve injury), failing to support the concept of compensatory events. The effects of injury were greatest in sacral cord, ipsilateral to pelvic nerve transection. Here, around half of all preganglionic neurons expressed c-Jun within 1 week of injury, and substantial ATF-3 expression also occurred, especially in neurons with complete loss of ChAT-immunoreactivity. There did not appear to be any death of retrogradely labeled neurons, in contrast to axotomy studies performed on other regions of spinal cord or sacral ventral root avulsion models. Each of the effects we observed occurred in only a subpopulation of preganglionic neurons at that spinal level, raising the possibility that distinct functional subgroups have different susceptibility to trauma-induced degeneration and potentially different regenerative abilities. Identification of the cellular basis of these differences may provide insights into organ-specific strategies for attenuating degeneration or promoting regeneration of these circuits after trauma.

Novel, continuous visual motion induces c-fos expression in the avian optokinetic nuclei and optic tectum

We studied the stimulus characteristics necessary for the expression of c-fos protein in optokinetic system neurons using immunocytochemistry. Using whole-field visual motion as a stimulus, we found substantial c-fos expression in the optic tectum (TeO), the nucleus of the basal optic root (nBOR) and the pretectal nucleus lentiformis mesencephali (LM); in all cases immunostaining was seen only on the side contralateral to the eye viewing whole-field unidirectional motion; the side of the brain contralateral to the eye wearing a diffuser showed no staining. In the nBOR and the LM, different regions showed a remarkable specificity of c-fos expression depending on the direction of visual motion stimulation. Neurons were stained primarily in regions known from previous electrophysiological recordings to be maximally responsive to that direction of motion; little staining was seen after motion orthogonal to the preferred motion direction. Novel, continuous visual motion stimuli, lasting more than 30 min, was required for maximal c-fos expression, suggesting that brief periods of unidirectional optic flow, as would be experienced during normal life, do not stimulate the expression of c-fos. The largest number of neurons was labeled when birds raised from hatching with one eye covered by a diffuser were exposed to full-field visual motion immediately after the diffuser was switched from one eye to the other, so that only the previously naive eye was visually stimulated. We conclude that the expression of c-fos in the optokinetic nuclei is linked to near peak firing rates on the one hand, and the novelty and duration of the visual signals, on the other, supporting the assumption that this expression is mainly related to stimulus contexts leading to neuronal plastic changes.

Nerve injury signaling

Although neurons within the peripheral nervous system (PNS) have a remarkable ability to repair themselves after injury, neurons within the central nervous system (CNS) do not spontaneously regenerate. This problem has remained recalcitrant despite a century of research on the reaction of axons to injury. The balance between inhibitory cues present in the environment and the intrinsic growth capacity of the injured neuron determines the extent of axonal regeneration following injury. The cell body of an injured neuron must receive accurate and timely information about the site and extent of axonal damage in order to increase its intrinsic growth capacity and successfully regenerate. One of the mechanisms contributing to this process is retrograde transport of injury signals. For example, molecules activated at the injury site convey information to the cell body leading to the expression of regeneration-associated genes and increased growth capacity of the neuron. Here we discuss recent studies that have begun to dissect the injury-signaling pathways involved in stimulating the intrinsic growth capacity of injured neurons.

Differences in c-jun and nNOS expression levels in motoneurons following different kinds of axonal injury in adult rats

In the peripheral nervous system (PNS), root avulsion causes motoneuron degeneration, but the majority of motoneurons can survive axotomy. In order to study the mechanism of motoneuron degeneration, we compared the expression patterns of c-jun and neuronal nitric oxide synthase (nNOS), the well-known molecular players in PNS regeneration and degeneration, among adult rats having undergone axotomy (Ax), avulsion (Av), or pre-axotomy plus secondary avulsion (Ax + Av) of the brachial plexus. Our results showed that the highest and longest-lasting c-jun activation occurred in Ax, which was much stronger than those in Av and Ax + Av. The time course and intensity of c-jun expression in Ax + Av were similar to those in Av except on day 1, while the pre-axotomy condition resulted in a transient up-regulation of c-jun to a level comparable to that in Ax. Axotomy alone did not induce nNOS expression in motoneurons. Pre-axotomy left-shifted the time course of nNOS induction in Ax + Av compared to that in Av. Motoneuron loss was not evident in Ax, while it was 70% in Av and more than 85% in Ax + Av at 8 weeks postinjury. The survival of motoneurons was positively correlated with c-jun induction, but not with nNOS expression in motoneurons. Moreover, c-jun induction was negatively correlated with nNOS induction in injured motoneurons. Our results indicate that functional crosstalk between c-jun and nNOS might play an important role in avulsion-induced motoneuron degeneration, while c-jun might act as a prerequisite survival factor and nNOS might act as a predictor for the onset of motoneuron degeneration.

Expression of the regeneration-associated protein SPRR1A in primary sensory neurons and spinal cord of the adult mouse following peripheral and central injury

Small proline-rich repeat protein 1A (SPRR1A) is expressed in dorsal root ganglion (DRG) neurons following peripheral nerve injury but it is not known whether SPRR1A is differentially expressed following injury to peripheral versus central DRG projections and a detailed characterization of expression in sensory neuron subpopulations and spinal cord has not been performed. Here we use immunocytochemical techniques to characterize SPRR1A expression following sciatic nerve, dorsal root, and dorsal column injury in adult mice. SPRR1A was not detected in naïve spinal cord, DRG, or peripheral nerves and there was minimal expression following injury to the centrally projecting branches of DRG neurons. However, following peripheral (sciatic) nerve injury, intense SPRR1A immunoreactivity was observed in the dorsal horn and motoneurons of the spinal cord, in L4/5 DRG neurons, and in the injured nerve. A time-course study comparing expression following sciatic nerve crush and transection revealed maximum SPRR1A levels at day 7 in both models. However, while SPRR1A was downregulated to baseline by 30 days postlesion following crush injury, it remained elevated 30 days after transection. Cell-size and double-labeling studies revealed that SPRR1A was expressed by DRG cells of all sizes and colocalized with classical markers of DRG subpopulations and their primary afferent terminals. High coexpression of SPRR1A with activating transcription factor-3 and growth-associated protein-43 was observed, indicating that it is expressed by injured and regenerating neurons. This study supports the hypothesis that SPRR1A is a regeneration-associated gene and that SPRR1A provides a valuable marker to assess the regenerative potential of injured neurons.

Ocular HSV-1 latency, reactivation and recurrent disease

Ocular infection with HSV-1 continues to be a serious clinical problem despite the availability of effective antivirals. Primary infection with HSV-1 can involve ocular and adenaxial sites and can manifest as blepharitis, conjunctivitis, or corneal epithelial keratitis. After initial ocular infection, HSV-1 can establish latent infection in the trigeminal ganglia for the lifetime of the host. During latency, the viral genome is retained in the neuron without producing viral proteins. However, abundant transcription occurs at the region encoding the latency-associated transcript, which may play significant roles in the maintenance of latency as well as neuronal reactivation. Many host and viral factors are involved in HSV-1 reactivation from latency. HSV-1 DNA is shed into tears and saliva of most adults, but in most cases this does not result in lesions. Recurrent disease occurs as HSV-1 is carried by anterograde transport to the original site of infection, or any other site innervated by the latently infected ganglia, and can reinfect the ocular tissues. Recurrent corneal disease can lead to corneal scarring, thinning, stromal opacity and neovascularization and, eventually, blindness. In spite of intensive antiviral and anti-inflammatory therapy, a significant percentage of patients do not respond to chemotherapy for herpetic necrotizing stromal keratitis. Therefore, the development of therapies that would reduce asymptomatic viral shedding and lower the risks of recurrent disease and transmission of the virus is key to decreasing the morbidity of ocular herpetic disease. This review will highlight basic HSV-1 virology, and will compare the animal models of latency, reactivation, and recurrent ocular disease to the current clinical data.

Axotomy-induced dopaminergic neurodegeneration is accompanied with c-Jun phosphorylation and activation transcription factor 3 expression

Accumulating evidence has shown that both phosphorylated c-Jun (pc-Jun) and activating transcription factor 3 (ATF3) were upregulated in a variety of tissue injuries and proposed to play an important role in cell death/survival. To elucidate the significance and functional role of these immediate-early genes during neuronal damage in the central nervous system, we examined temporal and spatial profiles of pc-Jun and ATF3 in dopaminergic neurons of the substantia nigra (SN) following transection of the medial forebrain bundle (MFB) in adult rats. Morphological characteristics of pc-Jun-positive dopaminergic neurons as well as microglial reaction in response to the axotomy-induced neurodegeneration were also investigated. Following MFB transection, both c-Jun phosphorylation and ATF3 were found in the nuclei of tyrosine hydroxylase-immunoreactive (TH-ir) neurons of the ipsilateral SN, but not in those of the contralateral SN. In the ipsilateral SN, the number of pc-Jun- and ATF3-positive nuclei was increased by 5-7 days post-lesion, and then progressively decreased probably due to the loss of neurons. Retrograde tracing with FluoroGold (FG) in hemi-axotomized rat brain demonstrated that none of the intact, unaxotomized (FG-ir) neurons was pc-Jun-positive, indicating phosphorylation of c-Jun occurs only in axotomized neurons. Concomitant co-localization of pc-Jun and ATF3 in the same TH-ir neuron was also demonstrated by triple immunofluorescence labeling. Many TH-ir neurons that underwent various steps of consecutive neurodegenerative changes retained pc-Jun in the condensed or fragmented nuclei. Moreover, numerous activated microglia, identified by both phagocytic (ED1) and MHC II (OX6) markers, closely apposed to these neurons throughout the entire neurodegenerative process, suggesting that they are actively phagocytosing dying neurons. Taken together, these results support the idea that pc-Jun and its putative dimeric partner ATF3 may be closely participating in axotomy-induced neurodegeneration.

ATF3 increases the intrinsic growth state of DRG neurons to enhance peripheral nerve regeneration

Peripheral axons of dorsal root ganglion (DRG) neurons, but not their central axons in the dorsal columns, regenerate after injury. However, if the neurons are conditioned by a peripheral nerve injury into an actively growing state, the rate of peripheral axonal growth is accelerated and the injured central axons begin to regenerate. The growth-promoting effects of conditioning injuries have two components, increased axonal growth and a reduced response to inhibitory myelin cues. We have examined which transcription factors activated by peripheral axonal injury may mediate the conditioning effect by regulating expression of effectors that increase the intrinsic growth state of the neurons. Activating transcription factor 3 (ATF3) is a prime candidate because it is induced in all injured DRG neurons after peripheral, but not central, axonal damage. To investigate if ATF3 promotes regeneration, we generated transgenic mice that constitutively express this transcription factor in non-injured adult DRG neurons. The rate of peripheral nerve regeneration was enhanced in the transgenic mice to an extent comparable to that produced by a preconditioning nerve injury. The expression of some growth-associated genes, such as SPRR1A, but not others like GAP-43, was increased in the non-injured neurons. ATF3 increased DRG neurite elongation when cultured on permissive substrates but did not overcome the inhibitory effects of myelin or promote central axonal regeneration in the spinal cord in vivo. We conclude that ATF3 contributes to nerve regeneration by increasing the intrinsic growth state of injured neurons.

The bright side of JNKs-Multitalented mediators in neuronal sprouting, brain development and nerve fiber regeneration

The c-Jun N-terminal kinases (JNKs) are important regulators of physiological and pathological processes in the central and peripheral nervous system. In general, JNKs are considered as mediators of neuronal degeneration in response to stress and injury. However, recent data have provided substantial evidence that JNKs are also essential for physiological and regenerative signalling in neurons. This review summarizes the importance of JNKs for neurite formation and outgrowth, brain development, dendritic architecture and regeneration of nerve fibers after injury. We discuss putative mechanisms which control the bipartite actions of individual JNK isoforms for neuronal death and repair after nerve fiber injury with a particular focus on the role of the transcription factor c-Jun.

The transcription factor ATF-3 promotes neurite outgrowth

Dorsal root ganglion (DRG) neurons regenerate after a peripheral nerve injury but not after injury to their axons in the spinal cord. A key question is which transcription factors drive the changes in gene expression that increase the intrinsic growth state of peripherally injured sensory neurons? A prime candidate is activating transcription factor-3 (ATF-3), a transcription factor that we find is induced in all DRG neurons after peripheral, but not central axonal injury. Moreover, we show in adult DRG neurons that a preconditioning peripheral, but not central axonal injury, increases their growth, correlating closely with the pattern of ATF-3 induction. Using viral vectors, we delivered ATF-3 to cultured adult DRG neurons and find that ATF-3 enhances neurite outgrowth. Furthermore, ATF-3 promotes long sparsely branched neurites. ATF-3 overexpression did not increase c-Jun expression. ATF-3 may contribute, therefore, to neurite outgrowth by orchestrating the gene expression responses in injured neurons.

Phosphorylation of c-Jun in avian and mammalian motoneurons in vivo during programmed cell death: an early reversible event in the apoptotic cascade

c-Jun is a transcription factor that is involved in various cellular events, including apoptotic cell death. For example, phosphorylation of c-Jun is one of the earliest biochemical changes detected in dying sympathetic neurons after NGF deprivation in vitro. However, currently, it is not known whether a similar molecular event is involved in the developmental programmed cell death (PCD) of neurons in vivo. We observed that only a subpopulation of motoneurons (MNs) exhibit c-Jun phosphorylation during the PCD period in chick [embryonic day 5 (E5)-E12] and mouse (E13-E18) embryos. Experimental perturbation of MN survival-promoting signals by limb bud removal (reduced signals) or by activity blockade (increased signals) in the chick embryo demonstrated that the presence of those signals is negatively correlated with the number of c-Jun-phosphorylated MNs. This suggests that insufficient survival signals (e.g., neurotrophic factors) may induce c-Jun phosphorylation of MNs in vivo. Consistent with the idea that c-Jun phosphorylation is a reversible event during normal PCD of MNs, we found that c-Jun phosphorylation was transiently observed in a subpopulation of mouse MNs rescued from PCD by deletion of the proapoptotic gene Bax. Inhibition of c-Jun signaling significantly reduced MN death in chick embryo, indicating that activation of c-Jun signaling is necessary for the PCD of MNs. Together, c-Jun phosphorylation appears to be required for the initiation of an early and reversible event in the intracellular PCD cascade in vivo after loss of survival-promoting signals such as neurotrophic factors.

Effects of electro-acupuncture on the expression of c-jun and c-fos in spared dorsal root ganglion and associated spinal laminae following removal of adjacent dorsal root ganglia in cats

This study evaluated the plastic changes of c-jun and c-fos in the right sixth lumbar dorsal root ganglion (L6 DRG), Rexed's lamina II in representative spinal segments L3, L5, and L6 and in the nucleus dorsalis (ND) at L3 segments after electro-acupuncture (EA) in cats subjected to removal of L1-L5 and L7-S2 DRG. Following dorsal root ganglionectomy, there was a significant increase in the density of c-jun immunoreactivity in the neurons and glia in spinal lamina II and in the ND; there was also marked elevation in the expression of c-fos in ND. In both cases there was no change in the c-jun and c-fos immunoreactivity in the DRG. After EA in the operated animals, there was an up-regulation in the expression of c-jun in the L6 DRG and the associated spinal lamina II; however, increased c-fos expression was detected only in the L6 DRG. Western blot and RT-PCR were also performed to quantitatively explore the mRNA and protein expression changes in the spinal dorsal horn and associated DRG. Following partial deafferentation, there was a significant increase in the protein level of both c-jun and c-fos in the dorsal horn, while, in both cases there was no change in c-jun and c-fos protein and mRNA in the DRG. After EA in the operated animals, both c-jun protein and its mRNA in the L6 DRG as well as the associated dorsal horn of L6 spinal segment were upregulated, but increased c-fos protein and its mRNA was observed only in the L6 DRG. These findings suggested that c-jun and c-fos might be related to the acupuncture promoted spinal cord plasticity as reported previously.

Activation of c-Jun and ATF-2 in primate motor cranial nerve nuclei is not associated with apoptosis following axotomy

Nerve transection induces complex changes in gene regulation and expression that can have profound phenotypic effects on the fate of axotomized neurons. The transcription factors c-Jun and ATF-2 (activating transcription factor-2) are components of a regulatory network that mediates survival, regeneration, and apoptosis following axotomy in rodents. The activation and function of c-Jun and ATF-2 after nerve injury have not been examined in primates. Using a novel model of cranial nerve injury in baboons, we have examined the temporality of c-Jun activation (phosphorylation) in cranial nerve (CN) III and CN VI neurons and ATF-2 activation in CN VI neurons at 2, 4, and 9 days post-injury by immunohistochemistry. Furthermore, we have addressed whether the activation of these factors is associated with apoptosis by the TUNEL assay. We report that activated c-Jun is present in CN III and CN VI neurons ipsilateral to axotomy at 2, 4, and 9 days post-injury, but not in neurons contralateral to injury. Additionally, CN VI neurons ipsilateral to injury at 4 and 9 days contain activated ATF-2. Furthermore, no evidence of TUNEL reactivity was observed in either nucleus, regardless of laterality, at any of the examined time points. These findings suggest that activation of both c-Jun and ATF-2 does not mediate apoptosis in axotomized primate CN III and CN VI neurons at time points up to 9 days. This report serves as a basic inquiry into the neuronal response to cranial nerve injury in primates.

Sunday Driver links axonal transport to damage signaling

Neurons transmit long-range biochemical signals between cell bodies and distant axonal sites or termini. To test the hypothesis that signaling molecules are hitchhikers on axonal vesicles, we focused on the c-Jun NH2-terminal kinase (JNK) scaffolding protein Sunday Driver (syd), which has been proposed to link the molecular motor protein kinesin-1 to axonal vesicles. We found that syd and JNK3 are present on vesicular structures in axons, are transported in both the anterograde and retrograde axonal transport pathways, and interact with kinesin-I and the dynactin complex. Nerve injury induces local activation of JNK, primarily within axons, and activated JNK and syd are then transported primarily retrogradely. In axons, syd and activated JNK colocalize with p150Glued, a subunit of the dynactin complex, and with dynein. Finally, we found that injury induces an enhanced interaction between syd and dynactin. Thus, a mobile axonal JNK-syd complex may generate a transport-dependent axonal damage surveillance system.

Early changes in gene expression in the dorsal root ganglia after transection of the sciatic nerve; effects of amphiregulin and PAI-1 on regeneration

To characterize the gene activity that may be required for neuronal survival and regeneration, we used the Affymetrix GeneChip Mu74A to screen 12000 genes and expressed sequence tag (EST) mRNA from L4 and L5 mouse dorsal root ganglia (DRG) 12 h and 24 h after sciatic nerve transection. At 12 h, we found 17 upregulated transcripts, and at 24 h, 49 that met our criteria of a significant 2-fold increase in expression. The alterations included a total of eight transcription factors and several genes associated with TGF-beta- and IL-6-mediated signaling. Two of the changes, amphiregulin and plasminogen activator inhibitor-1 (PAI-1), were confirmed by real-time quantitative PCR (QPCR). Addition of amphiregulin (20 ng/ml) to organ-cultured DRG stimulated axonal outgrowth while PAI-1 (20 nM) inhibited migration of Schwann cells from the ganglia.

Neuronal and astroglial alterations in the hippocampus of a mouse model for type 1 diabetes

The influence of diabetes mellitus on brain pathology is increasingly recognized. Previous contributions of our laboratory demonstrated in models of type 1 diabetes (nonobese diabetic and streptozotocin (STZ)-treated mice), a marked astrogliosis and neurogenesis deficit in hippocampus and increased expression of hypothalamic neuropeptides. In the present investigation, we further analyzed alterations of astroglia and neurons in the hippocampus of mice 1 month after STZ-induced diabetes. Results showed that these STZ-diabetic mice presented: (a) increased number of astrocytes positive for apolipoprotein-E (Apo-E), a marker of ongoing neuronal dysfunction; (b) abnormal expression of early gene products associated with neuronal activation, including a high number of Jun + neurons in CA1 and CA3 layers and dentate gyrus, and of Fos-expressing neurons in CA3 layer; (c) augmented activity of NADPH-diaphorase, linked to oxidative stress, in CA3 region. These data support the concept that uncontrolled diabetes leads to hippocampal pathology, which adjoin to changes in other brain structures such as hypothalamus and cerebral cortex.

Cell death or survival: Molecular and connectional conditions for olivocochlear neurons after axotomy

We aimed to determine whether rat olivocochlear neurons survive axotomy inflicted through cochlear ablation, or if they degenerate. To estimate their intrinsic potential for axonal regeneration, we investigated the expression of the transcription factor c-Jun and the growth-associated protein-43 (GAP43). Axonal tracing studies based on application of Fast Blue into the cochlea and calcitonin gene-related peptide immunostaining revealed that many, but not all, lateral olivocochlear neurons in the ipsilateral lateral superior olive degenerated upon cochleotomy. A decrease of their number was noticed 2 weeks after the lesion, and 2 months postoperative the population was reduced to approximately one quarter (27-29%) of its original size. No further reduction took place at longer survival times up to 1 year. Most or all shell neurons and medial olivocochlear neurons survived axotomy. Following cochleotomy, 56-60% of the lateral olivocochlear neurons in the ipsilateral lateral superior olive were found to co-express c-Jun and GAP43. Only a small number of shell and medial olivocochlear neurons up-regulated c-Jun expression, and only a small number of shell neurons expressed GAP43. Up-regulation of c-Jun and GAP43 in lateral olivocochlear neurons upon axotomy suggests that they have an intrinsic potential to regenerate after axotomy, but cell counts based on the markers Fast Blue and calcitonin gene-related peptide indicate that this potential cannot be exploited and degeneration is induced instead. The survival of one quarter of the axotomized lateral olivocochlear neurons and of all, or almost all, shell and medial olivocochlear neurons appeared to depend on connections of these cells to other regions than the cochlea by means of axon collaterals, which remained intact after cochleotomy.

Cell signaling and the genesis of neuropathic pain

Damage to the nervous system can cause neuropathic pain, which is in general poorly treated and involves mechanisms that are incompletely known. Currently available animal models for neuropathic pain mainly involve partial injury of peripheral nerves. Multiple inflammatory mediators released from damaged tissue not only acutely excite primary sensory neurons in the peripheral nervous system, producing ectopic discharge, but also lead to a sustained increase in their excitability. Hyperexcitability also develops in the central nervous system (for instance, in dorsal horn neurons), and both peripheral and spinal elements contribute to neuropathic pain, so that spontaneous pain may occur or normally innocuous stimuli may produce pain. Inflammatory mediators and aberrant neuronal activity activate several signaling pathways [including protein kinases A and C, calcium/calmodulin-dependent protein kinase, and mitogen-activated protein kinases (MAPKs)] in primary sensory and dorsal horn neurons that mediate the induction and maintenance of neuropathic pain through both posttranslational and transcriptional mechanisms. In particular, peripheral nerve lesions result in activation of MAPKs (p38, extracellular signal-regulated kinase, and c-Jun N-terminal kinase) in microglia or astrocytes in the spinal cord, or both, leading to the production of inflammatory mediators that sensitize dorsal horn neurons. Activity of dorsal horn neurons, in turn, enhances activation of spinal glia. This neuron-glia interaction involves positive feedback mechanisms and is likely to enhance and prolong neuropathic pain even in the absence of ongoing peripheral external stimulation or injury. The goal of this review is to present evidence for signaling cascades in these cell types that not only will deepen our understanding of the genesis of neuropathic pain but also may help to identify new targets for pharmacological intervention.

Neurodegenerative and physiological actions of c-Jun N-terminal kinases in the mammalian brain

The research in the field of AP-1 transcription factor expression, such as Jun or Fos proteins, in the brain was a milestone in neurosciences. The last years have provided growing insights into the upstream signal transduction which controls the expression and activation of these transcriptional regulators. In particular, the c-Jun N-terminal kinases (JNKs) were considered to confer degeneration by activation of c-Jun. Recent findings, however, demonstrate an essential physiological role of JNKs in the nervous system. Here we review the specific control and dual functions of JNK isoforms which are relevant for the development of the intact brain on the one hand, and which can confer dramatic neurodegenerative effects and microglial activation on the other hand.

Reparative mechanisms in the cerebellar cortex

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.

ATF3 enhances c-Jun-mediated neurite sprouting

The AP-1 transcription factor c-Jun is induced in axotomized neurons of the peripheral and central nervous systems, and in both cases upregulation of c-Jun expression has been correlated with axonal regeneration. More recently there has been interest in the c-Jun-related bZIP transcription factor, ATF3, and its function in neurons. ATF3 is also induced in nerve cells in response to axotomy and there is a correlation between increased ATF3 expression and upregulation of c-Jun in surviving neurons. Moreover, c-Jun is able to induce expression of ATF3. We investigated the effect of co-expressing c-Jun and ATF3 in two neuronal-like cell lines to model transcriptional events occurring in axotomized neurons undergoing regeneration. We show that expression of ATF3 with c-Jun significantly enhances c-Jun-mediated neurite sprouting, and that this phenotype is most likely mediated by a physical association of these two transcription factors. Our results suggest that a program of axonal regeneration is initiated when both c-Jun and ATF3 are upregulated in neurons in response to axotomy.

Developmentally regulated expression of c-Fos and c-Jun in the brainstem auditory nuclei ofGallus domesticus is modified by prenatal auditory enrichment

Recognition of mother's voice by human neonates and behavioral responses of birds and animals to sounds experienced prenatally emphasize the role of sensory inputs in auditory system development. Spontaneous and experience driven neural activity influence the neural circuits' refinement in developing brain. However, cellular mechanisms endowing plasticity for such structural refinement during critical developmental periods are less understood. Sensory stimulation induces fluctuating expression of transcription factors (TFs) of Fos, Jun, and Krox families in the related brain nuclei to activate genes to synthesize proteins such as those needed for cytoskeletal structures, ion channels, and regeneration. To understand the cellular mechanism of response to prenatal auditory stimulation, we studied the expression of c-Fos and c-Jun in brainstem auditory nuclei, nucleus magnocellularis, and nucleus laminaris of the domestic chick. The chick brainstems, five each of E8 (embryonic day 8), E12, E16, E20, and posthatch day 1 were processed for immunohistochemistry as well as Western blotting and quantified using image analysis systems. In controls, c-Fos and c-Jun expression in both the nuclei was developmentally up-regulated. Reduced c-Fos expression and increase in c-Jun was temporarily observed between E12-16. In the stimulated groups, c-Fos expression was elevated while c-Jun showed a reduction matched to controls. This diametrically opposing pattern of c-Fos and c-Jun expression in response to stimulation is indicative of cell survival. Thus the expression of TFs in the auditory nuclei shows a relationship beyond a simple stimulation-activity-expression. While developmental signals control the expression of TFs, extra sensory stimulation modulates their expression to possibly support neuronal survival and enhance synthesis of other proteins.

Homeobox gene expression in adult dorsal root ganglia during sciatic nerve regeneration: is regeneration a recapitulation of development?

After damage of the sciatic nerve, a regeneration process is initiated. Neurons in the dorsal root ganglion regrow their axons and functional connections. The molecular mechanisms of this neuronal regenerative process have remained elusive, but a relationship with developmental processes has been conceived. This chapter discusses the applicability of the developmental hypothesis of regeneration to the dorsal root ganglion; this hypothesis states that regeneration of dorsal root ganglion neurons is a recapitulation of development. We present data on changes in gene expression upon sciatic nerve damage, and the expression and function of homeobox genes. This class of transcription factors plays a role in neuronal development. Based on these data, it is concluded that the hypothesis does not hold for dorsal root ganglion neurons, and that regeneration-specific mechanisms exist. Cytokines and the associated Jak/STAT (janus kinase/signal transducer and activator of transcription) signal transduction pathway emerge as constituents of a regeneration-specific mechanism. This mechanism may be the basis of pharmacological strategies to stimulate regeneration.

Extrinsic regulation of injury/growth-related gene expression in the inferior olive of the adult rat

Successful axon regeneration relies on the capability of the lesioned neurons to up-regulate a specific set of injury/growth-associated genes. In the adult central nervous system, the strength of the cell body response is generally related to the distance of the injury site from the perikaryon, being stronger for proximal lesions. Nevertheless, inferior olive (IO) cells react to injury and regenerate their axons even after distal transections. To investigate the mechanisms that regulate the IO growth properties, we examined the expression of injury/growth markers (nitric oxide synthase, growth-associated protein 43 and c-Jun) after target deletion or axotomy performed at different sites along the olivocerebellar pathway. Both axon injury and target loss disclose two subsets of IO neurons distributed within precise subnuclei: one subset up-regulates all markers in all conditions, whereas the other shows a mild c-Jun expression but remains unresponsive even after a very proximal axotomy. These observations indicate that distinct subpopulations of IO cells respond to different regulatory strategies. Unresponsive neurons appear insensitive to environmental positive or negative cues, suggesting that they are intrinsically unable to set up a cellular reaction to injury. In contrast, cell body changes in reactive neurons are elicited after the removal of retrogradely transported target-derived inhibitory signals. Target loss also induces degeneration of IO cells, whose survival remains partially dependent on Purkinje targets in adulthood. Thus, the intrinsic regenerative potential of a functionally homogeneous population is regulated by multiple mechanisms, specific for distinct neuronal subsets.

Identification of regeneration-associated genes after central and peripheral nerve injury in the adult rat

BACKGROUND

It is well known that neurons of the peripheral nervous system have the capacity to regenerate a severed axon leading to functional recovery, whereas neurons of the central nervous system do not regenerate successfully after injury. The underlying molecular programs initiated by axotomized peripheral and central nervous system neurons are not yet fully understood.

RESULTS

To gain insight into the molecular mechanisms underlying the process of regeneration in the nervous system, differential display polymerase chain reaction has been used to identify differentially expressed genes following axotomy of peripheral and central nerve fibers. For this purpose, axotomy induced changes of regenerating facial nucleus neurons, and non-regenerating red nucleus and Clarke's nucleus neurons have been analyzed in an intra-animal side-to-side comparison. One hundred and thirty five gene fragments have been isolated, of which 69 correspond to known genes encoding for a number of different functional classes of proteins such as transcription factors, signaling molecules, homeobox-genes, receptors and proteins involved in metabolism. Sixty gene fragments correspond to genomic mouse sequences without known function. In situ-hybridization has been used to confirm differential expression and to analyze the cellular localization of these gene fragments. Twenty one genes (approximately 15%) have been demonstrated to be differentially expressed.

CONCLUSIONS

The detailed analysis of differentially expressed genes in different lesion paradigms provides new insights into the molecular mechanisms underlying the process of regeneration and may lead to the identification of genes which play key roles in functional repair of central nervous tissues.

Identification of reciprocally regulated gene modules in regenerating dorsal root ganglion neurons and activated peripheral or central nervous system glia

Differential gene expression in the rat after injury of dorsal root ganglion neurons in vivo, and simulation injury of Schwann cells and oligodendrocytes in vitro was analyzed using high-density cDNA microarrays. The analyses were carried out to study the genetic basis of peripheral nerve regeneration, and to compare gene regulation in glia of the central (oligodendrocyte) and peripheral (Schwann cell) nervous systems. The genes showing significant differential regulation in the three study groups represented all aspects of cellular metabolism. However, two unexpected observations were made. Firstly, a number of identical genes were differentially regulated in activated Schwann cells, activated oligodendrocytes and regenerating DRG neurons. Specifically, a group of 113 out of 210 genes that were down-regulated in Schwann cells upon lipopolysaccharide (LPS) treatment, were identical to genes up-regulated in the injured, regenerating DRG. Furthermore, a group of 53 out of 71 genes that were down-regulated in interferon gamma (IFN-gamma)/LPS-activated oligodendrocytes, were identical to genes up-regulated in the DRG neurons. Finally, 22 genes were common to these three groups, i.e., down-regulated in activated oligodendrocytes, down-regulated in activated Schwann cells, and up-regulated in regenerating DRG neurons. Secondly, a group of 16 cell-cycle and proliferation-related genes were up-regulated in the DRG following sciatic nerve crush, despite the absence of cells undergoing mitosis in the DRG, or any significant presence of apoptosis-related gene expression. Therefore, it appears that in these three cell types, large sets of genes are reciprocally regulated upon injury and/or activation. This suggests that the activation of the injury-related gene expression program in cell derivatives of the neuroectoderm involves, in part, highly conserved genetic elements.

Discovery of CEP-1347/KT-7515, an inhibitor of the JNK/SAPK pathway for the treatment of neurodegenerative diseases

Apoptosis has been proposed as a mechanism of cell death in Alzheimer's, Huntington's and Parkinson's diseases and the occurrence of apoptosis in these disorders suggests a common mechanism. Events such as oxidative stress, calcium toxicity, mitochondria defects, excitatory toxicity, and deficiency of survival factors are all postulated to play varying roles in the pathogenesis of the diseases. However, the transcription factor c-jun may play a role in the pathology and cell death processes that occur in Alzheimer's disease. Parkinson's disease (PD) is also a progressive disorder involving the specific degeneration and death of dopamine neurons in the nigrostriatal pathway. In Parkinson's disease, dopaminergic neurons in the substantia nigra are hypothesized to undergo cell death by apoptotic processes. The commonality of biochemical events and pathways leading to cell death in these diseases continues to be an area under intense investigation. The current therapy for PD and AD remains targeting replacement of lost transmitter, but the ultimate objective in neurodegenerative therapy is the functional restoration and/or cessation of progression of neuronal loss. This chapter will describe a novel approach for the treatment of neurodegenerative diseases through the development of kinase inhibitors that block the active cell death process at an early transcriptional independent step in the stress activated kinase cascade. In particular, preclinical data will be presented on the c-Jun Amino Kinase pathway inhibitor, CEP-1347/KT-7515, with respect to it's properties that make it a desirable clinical candidate for treatment of various neurodegenerative diseases.