Expression of JUN, KROX, and CREB transcription factors in goldfish and rat retinal ganglion cells following optic nerve lesion is related to axonal sprouting

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.

Diffuse traumatic axonal injury in the optic nerve does not elicit retinal ganglion cell loss

Much of the morbidity after traumatic brain injury (TBI) is associated with traumatic axonal injury (TAI). Although most TAI studies focus on corpus callosum white matter, the visual system has received increased interest. To assess visual system TAI, we developed a mouse model of optic nerve TAI. It is unknown, however, whether this TAI causes retinal ganglion cell (RGC) death. To address this issue, YFP (yellow fluorescent protein)-16 transgenic mice were subjected to mild TBI and followed from 2 to 28 days. Neither TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling)-positive or cleaved caspase-3-immunoreactive RGCs were observed from 2 to 28 days after TBI. Quantification of immunoreactivity of Brn3a, an RGC marker, demonstrated no RGC loss; parallel electron microscopic analysis confirmed RGC viability. Persistent RGC survival was also consistent with the finding of reorganization in the proximal axonal segments after TAI, wherein microglia/macrophages remained inactive. In contrast, activated microglia/macrophages closely enveloped the distal disconnected, degenerating axonal segments at 7 to 28 days after injury, thereby confirming that this model consistently evoked TAI followed by disconnection. Collectively, these data provide novel insight into the evolving pathobiology associated with TAI that will form a foundation for future studies exploring TAI therapy and its downstream consequences.

Neuronal c-Jun is required for successful axonal regeneration, but the effects of phosphorylation of its N-terminus are moderate

Although neural c-Jun is essential for successful peripheral nerve regeneration, the cellular basis of this effect and the impact of c-Jun activation are incompletely understood. In the current study, we explored the effects of neuron-selective c-Jun deletion, substitution of serine 63 and 73 phosphoacceptor sites with non-phosphorylatable alanine, and deletion of Jun N-terminal kinases 1, 2 and 3 in mouse facial nerve regeneration. Removal of the floxed c-jun gene in facial motoneurons using cre recombinase under control of a neuron-specific synapsin promoter (junΔS) abolished basal and injury-induced neuronal c-Jun immunoreactivity, as well as most of the molecular responses following facial axotomy. Absence of neuronal Jun reduced the speed of axonal regeneration following crush, and prevented most cut axons from reconnecting to their target, significantly reducing functional recovery. Despite blocking cell death, this was associated with a large number of shrunken neurons. Finally, junΔS mutants also had diminished astrocyte and microglial activation and T-cell influx, suggesting that these non-neuronal responses depend on the release of Jun-dependent signals from neighboring injured motoneurons. The effects of substituting serine 63 and 73 phosphoacceptor sites (junAA), or of global deletion of individual kinases responsible for N-terminal c-Jun phosphorylation were mild. junAA mutants showed decrease in neuronal cell size, a moderate reduction in post-axotomy CD44 levels and slightly increased astrogliosis. Deletion of Jun N-terminal kinase (JNK)1 or JNK3 showed delayed functional recovery; deletion of JNK3 also interfered with T-cell influx, and reduced CD44 levels. Deletion of JNK2 had no effect. Thus, neuronal c-Jun is needed in regeneration, but JNK phosphorylation of the N-terminus mostly appears to not be required for its function.

Molecular control of axon growth: insights from comparative gene profiling and high-throughput screening

Axon regeneration in the mammalian adult central nervous system (CNS) is limited by an intrinsically low capacity for axon growth in many CNS neurons. In contrast, embryonic, peripheral, and many nonmammalian neurons are capable of successful regeneration. Numerous studies have compared mammalian CNS neurons to their counterparts in regenerating systems in an effort to identify candidate genes that control regenerative ability. This review summarizes work using this comparative strategy and examines our current understanding of gene function in axon growth, highlighting the emergence of genome-wide expression profiling and high-throughput screening strategies to identify novel regulators of axon growth.

Mutations in zebrafish lrp2 result in adult-onset ocular pathogenesis that models myopia and other risk factors for glaucoma

The glaucomas comprise a genetically complex group of retinal neuropathies that typically occur late in life and are characterized by progressive pathology of the optic nerve head and degeneration of retinal ganglion cells. In addition to age and family history, other significant risk factors for glaucoma include elevated intraocular pressure (IOP) and myopia. The complexity of glaucoma has made it difficult to model in animals, but also challenging to identify responsible genes. We have used zebrafish to identify a genetically complex, recessive mutant that shows risk factors for glaucoma including adult onset severe myopia, elevated IOP, and progressive retinal ganglion cell pathology. Positional cloning and analysis of a non-complementing allele indicated that non-sense mutations in low density lipoprotein receptor-related protein 2 (lrp2) underlie the mutant phenotype. Lrp2, previously named Megalin, functions as an endocytic receptor for a wide-variety of bioactive molecules including Sonic hedgehog, Bone morphogenic protein 4, retinol-binding protein, vitamin D-binding protein, and apolipoprotein E, among others. Detailed phenotype analyses indicated that as lrp2 mutant fish age, many individuals—but not all—develop high IOP and severe myopia with obviously enlarged eye globes. This results in retinal stretch and prolonged stress to retinal ganglion cells, which ultimately show signs of pathogenesis. Our studies implicate altered Lrp2-mediated homeostasis as important for myopia and other risk factors for glaucoma in humans and establish a new genetic model for further study of phenotypes associated with this disease.

Complex genetic inheritance, including variable penetrance and severity, underlies many common eye diseases. In this study, we present analysis of a zebrafish mutant, bugeye, which shows complex inheritance of multiple ocular phenotypes that are known risk factors for glaucoma, including high myopia, elevated intraocular pressure, and up-regulation of stress-response genes in retinal ganglion cells. Molecular genetic analysis revealed that mutations in low density lipoprotein receptor-related protein 2 (lrp2) underlie the mutant phenotypes. Lrp2 is a large transmembrane protein expressed in epithelia of the eye. It facilitates transport and clearance of multiple secreted bioactive factors through receptor-mediated endocytosis. Glaucoma, a progressive blinding disorder, usually presents in adulthood and is characterized by optic nerve damage followed by ganglion cell death. In bugeye/lrp2 mutants, ganglion cell death was significantly elevated, but surprisingly moderate, and therefore they do not model this endpoint of glaucoma. As such, bugeye/lrp2 mutants should be considered valuable as a genetic model (A) for buphthalmia, myopia, and regulated eye growth; (B) for identifying genes and pathways that modify the observed ocular phenotypes; and (C) for studying the initiation of retinal ganglion cell pathology in the context of high myopia and elevated intraocular pressure.

Gene expression analysis of zebrafish retinal ganglion cells during optic nerve regeneration identifies KLF6a and KLF7a as important regulators of axon regeneration

Unlike mammals, teleost fish are able to mount an efficient and robust regenerative response following optic nerve injury. Although it is clear that changes in gene expression accompany axonal regeneration, the extent of this genomic response is not known. To identify genes involved in successful nerve regeneration, we analyzed gene expression in zebrafish retinal ganglion cells (RGCs) regenerating their axons following optic nerve injury. Microarray analysis of RNA isolated by laser capture microdissection from uninjured and 3-day post-optic nerve injured RGCs identified 347 up-regulated and 29 down-regulated genes. Quantitative RT-PCR and in situ hybridization were used to verify the change in expression of 19 genes in this set. Gene ontological analysis of the data set suggests regenerating neurons up-regulate genes associated with RGC development. However, not all regeneration-associated genes are expressed in differentiating RGCs indicating the regeneration is not simply a recapitulation of development. Knockdown of six highly induced regeneration-associated genes identified two, KLF6a and KLF7a, that together were necessary for robust RGC axon re-growth. These results implicate KLF6a and KLF7a as important mediators of optic nerve regeneration and suggest that not all induced genes are essential to mount a regenerative response.

Regeneration of retinal ganglion cell axons in organ culture is increased in rats with hereditary buphthalmos

This study used organ cultures to examine whether retinal ganglion cells (RGCs) retain their ability to regenerate axons in buphthalmos. A rat mutant with hereditary buphthalmos was used to (1) determine whether the extent of RGC loss corresponds to the severity and duration of elevated intraocular pressure (IOP), (2) examine whether RGCs exposed to an elevated IOP are able to regenerate their axons in a retina culture model, and (3) analyze the proteome of the regenerating retina in order to identify putative regeneration-associated proteins. Retrograde labeling of RGCs revealed a decrease in their numbers in the retinas of buphthalmic eyes that increased with age. Quantification of axons growing out of retinal explants taken at different stages of the disease demonstrated that buphthalmic RGCs possess a remarkable potential to regrow axons. As expected, immunohistochemistry and immunoblotting revealed that elevated IOP was associated with upregulation of certain known proteins, such as growth-associated protein 43, glial fibrillary acidic protein, and endothelin-1. In addition, two-dimensional polyacrylamide gel electrophoresis and mass spectrometry revealed several spots corresponding to proteins that were specifically regulated when buphthalmic RGCs were permitted to regrow their axons. Out of the proteins identified, heat-shock protein (HSP)-60 was constantly expressed during axonal growth at all stages of the disease. Antibodies against HSP-60 reduced axonal growth, indicating the involvement of this protein in regenerative axonal growth. These data are the first to show that diseased retinal neurons can grow their axons, and that HSP-60 supports neuritogenesis. This model may help to elucidate the fundamental mechanisms of optic neuropathy at stages preceding death caused by chronic injury, and aid in the development of neuroprotective strategies.

Regenerating optic axons restore topography after incomplete optic nerve injury

Following complete optic nerve injury in a lizard, Ctenophorus ornatus, retinal ganglion cell (RGC) axons regenerate but fail to restore retinotectal topography unless animals are trained on a visual task (Beazley et al. [ 1997] J Comp Neurol 370:105-120, [2003] J Neurotrauma 20:1263-1270). Here we show that incomplete injury, which leaves some RGC axons intact, restores normal topography. Strict RGC axon topography allowed us to preserve RGC axons on one side of the nerve (projecting to medial tectum) while lesioning those on the other side (projecting to lateral tectum). Topography and response properties for both RGC axon populations were assessed electrophysiologically. The majority of intact RGC axons retained appropriate topography in medial tectum and had normal, consistently brisk, reliable responses. Regenerate RGC axons fell into two classes: those that projected topographically to lateral tectum with responses that tended to habituate and those that lacked topography, responded weakly, and habituated rapidly. Axon tracing by localized retinal application of carbocyanine dyes supported the electrophysiological data. RGC soma counts were normal in both intact and axotomized RGC populations, contrasting with the 30% RGC loss after complete injury. Unlike incomplete optic nerve injury in mammals, where RGC axon regeneration fails and secondary cell death removes many intact RGC somata, lizards experience a "win-win" situation: intact RGC axons favorably influence the functional outcome for regenerating ones and RGCs do not succumb to either primary or secondary cell death.

Synaptogenesis regulates axotomy-induced activation of c-Jun-activator protein-1 transcription

The activator protein-1 (AP1) transcription complex remains active for long periods after axotomy, but its activity diminishes during target contact. This raises the possibility that the function of this complex is regulated by the synaptic connections. Using Aplysia californica, we found that crushing peripheral nerves in vivo enhanced AP1 binding in the sensory neurons that lasted for weeks and then declined as regeneration was completed. The AP1 complex in Aplysia is a c-Jun homodimer. Its activation, after axotomy, is mediated by Aplysia c-Jun-N-terminal kinase (apJNK), which enters the nucleus of sensory neurons and phosphorylates c-Jun at Ser-73 (p73-c-Jun). Active AP1 in the sensory neurons did not mediate apoptosis and was not involved in the appearance of the long-term hyperexcitability that develops in these cells after axotomy, and blocking the activation of apJNK in vitro did not influence neurite outgrowth. In contrast, the levels of activated apJNK and p73-c-Jun declined markedly when sensory neurons formed synapses with motor neuron L7 in vitro. Furthermore, inhibiting the pathway accelerated synaptogenesis between sensory neurons and L7. These data suggest that positive and negative modulation of the JNK-c-Jun-AP1 pathway functions in alerting the nucleus to the loss and gain of synapses, respectively.

Modulation of ERK1/2 and p38(MAPK) by lead in the cerebellum of Brazilian catfish Rhamdia quelen

Lead (Pb2+) is a neurotoxic trace metal, widespread in aquatic environment that can change physiologic, biochemical and behavioral parameters in diverse fish species. Chemical exposure may drive modulation of mitogen-activated protein kinases (MAPKs) that are a family of highly conserved enzymes which comprise ubiquitous groups of signaling proteins playing critical regulatory roles in cell physiology. Extracellular signal-regulated kinases (ERK1/2) and p38(MAPK) control complex programs such as gene expression, embryogenesis, cell differentiation, cell proliferation, cell death and synaptic plasticity. Little information is available about MAPKs in aquatic organisms and their modulation by trace metals. The aim of this work was to determine the modulation of ERK1/2 and p38(MAPK) phosphorylation by Pb2+ in vivo and in vitro, in cerebellar slices of the catfish, Rhamdia quelen. In the in vitro model, slices were incubated for 3 h with lead acetate (1-10 microM). In the in vivo studies, the animals were exposed for 2 days to lead acetate (1 mg L(-1)). ERK1/2 and p38(MAPK) (total and phosphorylated forms) were immunodetected in cerebellar slices by Western blotting. Pb2+ added in vitro at 5 and 10 microM increased significantly the phosphorylation of both MAPKs. The in vivo exposed animals also showed a significant increase of ERK1/2 and p38(MAPK) phosphorylation without changes in the total content of the enzymes. In conclusion, the present work indicates that it is possible to evaluate the ERK1/2 and p38(MAPK) activation in the central nervous system (CNS) of a freshwater fish largely distributed in South America. Moreover, Pb2+, an important environmental pollutant may activate in vitro and in vivo ERK1/2 and p38(MAPK) enzymes. These findings are important considering the functional and ecologic implications associated to Pb2+ exposure of a freshwater fish species, such as R. quelen, and the roles of ERK1/2 and p38(MAPK) in the control of brain development, neuroplasticity and cell death.

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.

The transcription factor c-jun is activated in retinal ganglion cells in experimental rat glaucoma

This study investigates the role of the MAP kinase pathway including c-jun, ATF-2 and JNK in glaucomatous eyes of rats and in optic nerve transection. Glaucoma was induced in one eye of 51 adult Wistar rats by laser treatment to the trabecular meshwork. Eighteen further rats underwent unilateral optic nerve transection. We studied the transcription factor c-jun, its activated form, phospho-c-jun, the transcription factor p-ATF-2, and the enzyme JNK by immunohistochemistry. The activation of p-c-jun was also investigated using western blot analysis. Treated and control eyes were compared in a masked way at multiple time points after injury. We found a statistically significant increase in immunolabelling for c-jun and phospho-c-jun in retinal ganglion cells (RGCs) from 1 day to 4 weeks after intraocular pressure (IOP) elevation. At 1 and 2 days after the laser treatment, a mean of 2.9+/-3.3 RGCsmm(-1) were positive for c-jun (n=12, p=0.005, t-test), increasing to a mean of 13.4+/-7.5 cells mm(-1) at 1 week (n=18, p=0.00005), and decreasing to 2.3+/-2.0 cells mm(-1) at 2 weeks (n=5, p=0.04) and 0.1+/-0.1 cells mm(-1) at 2 months. Few of the 47 control eyes had any labelling for c-jun or phospho-c-jun, while between 80 and 100% of elevated IOP eyes showed positivity during the first 2 weeks of experimental glaucoma. After optic nerve transection, c-jun and phospho-c-jun were also significantly activated at 1, 2 and 9 days (p<0.03, t-test). Western blot analysis demonstrated significantly increased phospho-c-jun amounts in both transected and glaucomatous eyes compared to control fellow eyes 1 week following treatment. JNK was not significantly activated in glaucoma or optic nerve transection. P-ATF-2 was not significantly activated in glaucoma, but was significantly increased 2 days after optic nerve transection. We conclude that the process leading to RGC death in experimental glaucoma and after optic nerve transection involves the activation of c-jun at the RGC layer. C-jun is activated more gradually in glaucoma then after optic nerve transection.

Bcl-2 enhances Ca(2+) signaling to support the intrinsic regenerative capacity of CNS axons

At a certain point in development, axons in the mammalian CNS undergo a profound loss of intrinsic growth capacity, which leads to poor regeneration after injury. Overexpression of Bcl-2 prevents this loss, but the molecular basis of this effect remains unclear. Here, we report that Bcl-2 supports axonal growth by enhancing intracellular Ca(2+) signaling and activating cAMP response element binding protein (CREB) and extracellular-regulated kinase (Erk), which stimulate the regenerative response and neuritogenesis. Expression of Bcl-2 decreases endoplasmic reticulum (ER) Ca(2+) uptake and storage, and thereby leads to a larger intracellular Ca(2+) response induced by Ca(2+) influx or axotomy in Bcl-2-expressing neurons than in control neurons. Bcl-x(L), an antiapoptotic member of the Bcl-2 family that does not affect ER Ca(2+) uptake, supports neuronal survival but cannot activate CREB and Erk or promote axon regeneration. These results suggest a novel role for ER Ca(2+) in the regulation of neuronal response to injury and define a dedicated signaling event through which Bcl-2 supports CNS regeneration.

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.

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.

Corticospinal neurons up-regulate a range of growth-associated genes following intracortical, but not spinal, axotomy

The failure of some CNS neurons to up-regulate growth-associated genes following axotomy may contribute to their failure to regenerate axons. We have studied gene expression in rat corticospinal neurons following either proximal (intracortical) or distal (spinal) axotomy. Corticospinal neurons were retrogradely labelled with cholera toxin subunit B prior to intracortical lesions or concomitantly with spinal lesions. Alternate sections of forebrain were immunoreacted for cholera toxin subunit B or processed for mRNA in situ hybridization for ATF3, c-jun, GAP-43, CAP-23, SCG10, L1, CHL1 or krox-24, each of which has been associated with axotomy or axon regeneration in other neurons. Seven days after intracortical axotomy, ATF3, c-jun, GAP-43, SCG10, L1 and CHL1, but not CAP-23 or krox-24, were up-regulated by layer V pyramidal neurons, including identified corticospinal neurons. The maximum distance between the lesion and the neuronal cell bodies that up-regulated genes varied between 300 and 500 microm. However, distal axotomy failed to elicit changes in gene expression in corticospinal neurons. No change in expression of any molecule was seen in the neocortex 1 or 7 days after corticospinal axotomy in the cervical spinal cord. The expression of GAP-43, CAP-23, L1, CHL1 and SCG10 was confirmed to be unaltered after this type of injury in identified retrogradely labelled corticospinal neurons. Thus, while corticospinal neuronal cell bodies fail to respond to spinal axotomy, these cells behave like regeneration-competent neurons, up-regulating a wide range of growth-associated molecules if axotomized within the cerebral cortex.

Early modifications in the expression of mitogen-activated protein kinase (MAPK/ERK), stress-activated kinases SAPK/JNK and p38, and their phosphorylated substrates following focal cerebral ischemia

Focal ischemia induced by middle cerebral artery occlusion (MCAO) to adult rats results in necrosis at the infarct core and activation of complex signal pathways for cell death and cell survival in the penumbra. Upstream from the cell death promoters and executioners are several kinases that, once activated by phosphorylation, may activate several transcription factor substrates involved in cell death and cell survival. In the present study we examined, by immunohistochemistry, the expression of phosphorylated (active) mitogen-activated protein kinase, extracellular signal-regulated kinase (MAPK/ERK), stress-activated protein kinase (SAPK), c-Jun N-terminal kinase (JNK) and p-38 kinase at early stages (1-4 h) following 1 h of MCAO in the rat. The expression of phosphorylation-dependent, active transcription substrates of these kinases, including cyclic AMP-responsive element-binding protein (CREB) Alk-1, ATF-2, c-Myc and c-Jun was examined at early stages following reperfusion. Increased nuclear phosphorylated SAPK/JNK (SAPK/JNK-P) and c-Jun-PSer63, and reduced CREB-P, occurred in the infarct core at 1 h following reperfusion, suggesting increased phosphorylated SAPK/JNK and c-JunSer63, together with decreased phospho-CREB associated with cell death in the infarct core. However, increased cytoplasmic expression of MAPK/ERK-P, SAPK/JNK-P, p38-P, CREB-P, Elk-1-P, c-Myc-P, ATF-2-P and c-Jun-P occurred in the region bordering the infarct core (penumbra) at 4 h following reperfusion. This indicates that different signals converge in the cytoplasm of neurons located at the borders of the infarct at 4 h following reperfusion, revealing the struggle of death promoters and life facilitators at the penumbra. Whether phosphorylated kinases and specific substrates participate in promoting cell death or survival in the penumbra probably depends on additional factors and on the interaction with other proteins.

The ontogeny of Krox-20 expression in brainstem and cerebellar neurons

The central expression of Krox-20, a C(2)H(2)-type zinc-finger transcription factor and immediate early gene, is primarily studied in the young embryo, where it contributes to rhombomere (r) r3 and r5 development. Data regarding the cellular localization and developmental regulation of Krox-20 protein expression in brainstem neurons are lacking. Our interest in brainstem development, coupled with findings from our lab and others that demonstrate a profound impact of a Krox-20 null mutation on brainstem-mediated behaviors, led us to investigate the spatiotemporal expression of Krox-20 protein in brainstem and cerebellar neurons to gain insight into potential cellular targets of the mutation. Understanding the cellular localization of Krox-20 is important in light of studies showing the impact of immediate early gene expression on neuronal function. Krox-20 immunohistochemistry experiments were conducted on animals at embryonic days (E) 17.0 and 18.5 and postnatal days (P) 0-1, 3-4, 7, 14, 22, and adulthood. Krox-20 expression is developmentally regulated in motoneurons, somatosensory-related neurons, Purkinje cells, and components of auditory circuitry. Neurons in the ventral cochlear nucleus and inferior colliculus show a sustained Krox-20 expression. Ultrastructural data demonstrate Krox-20 expression in somata and dendrites of central neurons. Our identification of Krox-20 expressing neurons provides us a better understanding of the behavioral consequences of the mutation. Furthermore, our results suggest that Krox-20 protein has a role in the maturation of particular brainstem and cerebellar neurons and fluctuations in Krox-20 protein expression coincide with the development of circuitry underlying brainstem-mediated behaviors.

Differential effects of immunophilin-ligands (FK506 and V-10,367) on survival and regeneration of rat retinal ganglion cells in vitro and after optic nerve crush in vivo

Immunophilins belong to the large family of peptidyl-prolyl-cis-trans-isomerases known to be involved in many cellular processes (e.g., protein trafficking and transcriptional regulation). Beside the widespread therapeutic use of ligands of immunophilins as immunosuppressants, it has been shown that some of these compounds such as FK506 and V-10,367 may mediate neuroprotection and improve axonal regeneration following damage to peripheral nerve fibers. Here, we have analyzed the effects of these two compounds on neurite outgrowth of retinal explants in vitro and on axonal regeneration of retinal ganglion cells, a population of central intrinsic neurons, ten days following optic nerve crush in vivo. FK506 enhanced neurite outgrowth/regrowth in vitro in a dose dependent manner up to 135% (control = 100%), while V-10,367 was more effective (up to 168%). In vivo, intravitreal V-10,367 and FK506 significantly reduced the number of dying retinal ganglion cells as demonstrated by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling. Local application of FK506 into the vitreous body, but not V-10,367, immediately provided after the optic nerve crush induced the elongation of regenerating fibers across the lesion site for around 1.2 mm. Our data provide evidence that the ligands of the FK506-binding proteins FK506 and V-10,367 protect (otherwise dying) retinal ganglion cells from optic nerve crush-induced cell death, promote neurite outgrowth in vitro and that locally applied FK506 enhances the sprouting of axotomized central intrinsic neurons such as retinal ganglion cells in vivo after optic nerve crush.

Regeneration of retinal axons in the lizard Gallotia galloti is not linked to generation of new retinal ganglion cells

Using anterograde tracing with HRP and antibodies (ABs) against neurofilaments, we show that regrowth of retinal ganglion cell (RGC) axons in the lizard Gallotia galloti commences only 2 months after optic nerve transection (ONS) and continues over at least 9 months. This is unusually long when compared to RGC axon regeneration in fish or amphibians. Following ONS, lizard RGCs up-regulate the immediate early gene C-JUN for 9 months or longer, indicating their reactive state. In keeping with the in vivo data, axon outgrowth from lizard retinal explants is increased above control levels from 6 weeks, reaches its maximum as late as 3 months, and remains elevated for at least 1 year after ONS. By means of BrdU incorporation assays and antiproliferating cell nuclear antigen immunohistochemistry, we show that the late axon outgrowth is not derived from new RGCs that might have arisen in reaction to ONS: no labeled cells were detected in lizard retinas at 0.5, 1, 1.5, 3, 6, and 12 months after ONS. Conversely, numbers of RGCs undergoing apoptosis were too low to be detectable in TUNEL assays at any time after ONS. These results demonstrate that retinal axon regeneration in G. galloti is due to axon regrowth from the resident population of RGCs, which remain in a reactive state over an extended time interval. Neurogenesis does not appear to be involved in RGC axon regrowth in G. galloti.

c-Jun mediates axotomy-induced dopamine neuron death in vivo

Expression of the transcription factor c-Jun is induced in neurons of the central nervous system (CNS) in response to injury. Mechanical transection of the nigrostriatal pathway at the medial forebrain bundle (MFB) results in the delayed retrograde degeneration of the dopamine neurons in the substantia nigra pars compacta (SNc) and induces protracted expression and phosphorylation of c-Jun. However, the role of c-Jun after axotomy of CNS neurons is unclear. Here, we show that adenovirus-mediated expression of a dominant negative form of c-Jun (Ad.c-JunDN) inhibited axotomy-induced dopamine neuron death and attenuated phosphorylation of c-Jun in nigral neurons. Ad.c-JunDN also delayed the degeneration of dopaminergic nigral axons in the striatum after MFB axotomy. Taken together, these findings suggest that activation of c-Jun mediates the loss of dopamine neurons after axotomy injury.

Transcriptional regulation of hemopoiesis

The regulation of blood cell formation, or hemopoiesis, is central to the replenishment of mature effector cells of innate and acquired immune responses. These cells fulfil specific roles in the host defense against invading pathogens, and in the maintenance of homeostasis. The development of hemopoietic cells is under stringent control from extracellular and intracellular stimuli that result in the activation of specific downstream signaling cascades. Ultimately, all signal transduction pathways converge at the level of gene expression where positive and negative modulators of transcription interact to delineate the pattern of gene expression and the overall cellular hemopoietic response. Transcription factors, therefore, represent a nodal point of hemopoietic control through the integration of the various signaling pathways and subsequent modulation of the transcriptional machinery. Transcription factors can act both positively and negatively to regulate the expression of a wide range of hemopoiesis-relevant genes including growth factors and their receptors, other transcription factors, as well as various molecules important for the function of developing cells. The expression of these genes is dependent on the complex interactions between transcription factors, co-regulatory molecules, and specific binding sequences on the DNA. Recent advances in various vertebrate and invertebrate systems emphasize the importance of transcription factors for hemopoiesis control and the evolutionary conservation of several of such mechanisms. In this review we outline some of the key issues frequently identified in studies of the transcriptional regulation of hemopoietic gene expression. In teleosts, we expect that the characterization of several of these transcription factors and their regulatory mechanisms will complement recent advances in a number of fish systems where identification of cytokine and other hemopoiesis-relevant factors are currently under investigation.

c-Jun regulation in rat neonatal motoneurons postaxotomy

Motoneurons respond to peripheral nerve transection by either regenerative or degenerative events depending on their state of maturation. Since the expression of c-Jun has been involved in the early signalling of the regenerative process that follows nerve transection in adults, we have investigated c-Jun on rat neonatal axotomized motoneurons during the period in which neuronal death is induced. Changes in levels of c-Jun protein and its mRNA were determined by means of quantitative immunocytochemistry and in situ hybridization. Three hours after nerve transection performed on postnatal day (P)3, c-Jun protein and mRNA is induced in axotomized spinal cord motoneurons, and high levels were reached between 1 and 10 days after. This response is associated with a detectable c-Jun activation by phosphorylation on serine 63. No changes were found in the levels of activating transcription factor -2. Most of dying motoneurons were not labelled by either a specific c-Jun antibody or a c-jun mRNA probe. However, dying motoneurons were specifically stained by a polyclonal anti c-Jun antibody, indicating that some c-Jun antibodies react with unknown epitopes, probably distinct from c-Jun p39, that are specifically associated with apoptosis.

Expression of netrin-1 and its receptors DCC and UNC-5H2 after axotomy and during regeneration of adult rat retinal ganglion cells

Netrins are a family of chemotropic factors that guide axon outgrowth during development; however, their function in the adult CNS remains to be established. We examined the expression of the netrin receptors DCC and UNC5H2 in adult rat retinal ganglion cells (RGCs) after grafting a peripheral nerve (PN) to the transected optic nerve and following optic nerve transection alone. In situ hybridization revealed that both Dcc and Unc5h2 mRNAs are expressed by normal adult RGCs. In addition, netrin-1 was found to be constitutively expressed by RGCs. Quantitative analysis using in situ hybridization demonstrated that both Dcc and Unc5h2 were down-regulated by RGCs following axotomy. In the presence of an attached PN graft, Dcc and Unc5h2 were similarly down-regulated in surviving RGCs regardless of their success in regenerating an axon. Northern blot analysis demonstrated expression of netrin-1 in both optic and sciatic nerve, and Western blot analysis revealed the presence of netrin protein in both nerves. Immunohistochemical analysis indicated that netrin protein was closely associated with glial cells in the optic nerve. These results suggest that netrin-1, DCC, and UNC5H2 may contribute to regulating the regenerative capacity of adult RGCs.

A purine-sensitive pathway regulates multiple genes involved in axon regeneration in goldfish retinal ganglion cells

In lower vertebrates, retinal ganglion cells (RGCs) can regenerate their axons and reestablish functional connections after optic nerve injury. We show here that in goldfish RGCs, the effects of several trophic factors converge on a purine-sensitive signaling mechanism that controls axonal outgrowth and the expression of multiple growth-associated proteins. In culture, goldfish RGCs regenerate their axons in response to two molecules secreted by optic nerve glia, axogenesis factor-1 (AF-1) and AF-2, along with ciliary neurotrophic factor. The purine analog 6-thioguanine (6-TG) blocked outgrowth induced by each of these factors. Previous studies in PC12 cells have shown that the effects of 6-TG on neurite outgrowth may be mediated via inhibition of a 47 kDa protein kinase. Growth factor-induced axogenesis in RGCs was accompanied by many of the molecular changes that characterize regenerative growth in vivo, e.g. , increased expression of GAP-43 and certain cell surface glycoproteins. 6-TG inhibited all of these changes but not those associated with axotomy per se, e.g., induction of jun family transcription factors, nor did it affect cell survival. Additional studies using RGCs from transgenic zebrafish showed that expression of Talpha-1 tubulin is likewise stimulated by AF-1 and blocked by 6-TG. The purine nucleoside inosine had effects opposite to those of 6-TG. Inosine stimulated outgrowth and the characteristic pattern of molecular changes in RGCs and competitively reversed the inhibitory effects of 6-TG. We conclude that axon regeneration and the underlying program of gene expression in goldfish RGCs are mediated via a common, purine-sensitive pathway.

Lithium and valproate differentially regulate brain regional expression of phosphorylated CREB and c-Fos

Previous studies in our laboratory have shown that the mood stabilizers, lithium and valproate (VPA), regulate the transcription factors, cyclic AMP responsive element binding protein (CREB), c-Fos and c-Jun, differentially in cultured human neuroblastoma SH-SY5Y cells. Here, we confirm these findings in rat brain and further study the brain-regional effects of these drugs using immunohistochemistry. We found that although chronic treatment with LiCl or VPA did not change the expression of c-Fos and c-Jun, acute treatment with either drugs increased c-Fos expression but not c-Jun expression in CA1 and CA3 regions of hippocampus. Chronic treatment with LiCl, but not VPA, decreased CREB phosphorylation in rat cerebral cortex and hippocampus. These results suggest that lithium and VPA may act on different pathways to bring about their long-term prophylactic effects on bipolar disorder (BD). The regulation of CREB phosphorylation may be relevant to lithium effect. VPA, which is also effective in BD, may be linked to other pathways.

c-jun Is dispensable for developmental cell death and axogenesis in the retina

Although a number of studies have implicated c-Jun in neuronal death and axonal regeneration, it is unknown whether Jun function is essential for either response. One approach to resolve this issue is to analyze knock-out mice. However, c-jun-null mice die at midgestation, precluding critical investigation. Therefore, a xenograft paradigm was used in which retinas from embryonic day 12.5 (E12.5) c-jun nullizygous or wild-type mice were transplanted onto the superior colliculus of newborn rats. The rats were allowed to develop, and the grafts were assayed at various times for cell death and axon growth. Histologically, grafts of both genotypes developed in identical manners and had morphological characteristics of retinas. A functional c-jun allele was not essential for axogenesis, because ganglion cells in retinal grafts from c-jun nullizygous mice developed axons that projected into the colliculus. Programmed cell death (PCD) was also evident in the age-appropriate regions of the retina in both wild-type and c-jun-null grafts. Furthermore, there were no discernible differences in the number or location of dying cells in the two genotypes. That c-jun was not essential for PCD was supported by two additional findings. First, a c-jun-lacZ reporter gene was expressed in many cells in developing and grafted retinas, although only a few of these cells were destined to die. Second, in E12.5 c-jun-null embryos there were normal levels of PCD in the trigeminal ganglion. Together, these data indicate that c-Jun is not essential for axon growth in the retina or for PCD in the retina and trigeminal ganglion.

Co-expression of c-Jun and ATF-2 characterizes the surviving retinal ganglion cells which maintain axonal connections after partial optic nerve injury

The expression of c-fos, c-jun, jun-b, jun-d, srf and pc4 mRNA was examined after partial optic nerve crush in the adult rat retina by in situ hybridization. Optic nerve injury led exclusively to the upregulation of c-jun, with cellular label indicative for c-jun mRNA in the retinal ganglion cell layer after two days, three days and one week post-injury. This expression pattern was in accordance with the appearance of c-Jun immunoreactivity in retinal flat mounts. Injection of an antisense but not a missense oligonucleotide against c-jun after partial crush resulted in a reduced number of connected retinal ganglion cells (RGCs) as shown by retrograde labeling. Prelabeling of RGCs with fluorogold before optic nerve section and subsequent antisense targeting against c-jun, however, led to a slightly higher number of surviving but axotomized RGCs. C-Jun antibody staining of retinal whole mounts pre- or postlabeled after crush by intracollicular administration of fluorogold showed strong c-Jun immunoreactivity in connected RGCs and also in a population of disconnected RGCs. Double labeling with an antibody directed against the transcription factor ATF-2 revealed strong co-expression of c-Jun and ATF-2 in connected RGCs but not in axotomized cells. Taken together these data indicate that both RGCs in continuity and those in discontinuity with the superior colliculus respond both equally to the noxious stimulus with c-Jun expression. Moreover, the co-expression of c-Jun with high levels of ATF-2 appears to be essential for either the continuity or survival of RGCs which remain connected with their target. In disconnected RGCs, however, low levels of ATF-2 and the co-expression of c-Jun may be related to cell death.

Cloning and characterization of zRICH, a 2',3'-cyclic-nucleotide 3'-phosphodiesterase induced during zebrafish optic nerve regeneration

We previously reported cloning of cDNAs encoding both components of a protein doublet induced during goldfish optic nerve regeneration. The predicted protein sequences showed significant homology with the mammalian 2',3'-cyclic-nucleotide 3'-phosphodiesterases (CNPases). CNPases are well-established markers of mammalian myelin; hence, the cDNAs were designated gRICH68 and gRICH70 (for goldfish Regeneration-Induced CNPase Homologues of 68 and 70 kDa). Homologous cDNAs have now been isolated from zebrafish encoding a highly related protein, which we have termed zRICH. RNase protection assays show that zRICH mRNA is induced significantly (fivefold) in optic nerve regenerating zebrafish retinas 7 days following nerve crush. Western blots show a single band in zebrafish brain and retina extracts, with immunoreactivity increasing three-fold in regenerating retinas 21 days postcrush. Immunohistochemical analysis indicated that this increase in zRICH protein expression is localized to the retinal ganglion cell layer in regenerating retina. We have characterized and evaluated the relevance of a conserved beta-ketoacyl synthase motif in zRICH to CNPase activity by means of site-directed mutagenesis. Two residues within the motif, H334 and T336, are critical for enzymatic activity. A cysteine residue within the motif, which corresponds to a critical residue for beta-ketoacyl synthase, does not appear to participate in the phosphodiesterase activity.

Cochlear ablation alters acoustically induced c-fos mRNA expression in the adult rat auditory brainstem

Expression of c-fos mRNA was studied in the adult rat brain following cochlear ablations by using in situ hybridization. In normal animals, expression was produced by acoustic stimulation and was found to be tonotopically distributed in many auditory nuclei. Following unilateral cochlear ablation, acoustically driven expression was eliminated or decreased in areas normally activated by the ablated ear, e.g., the ipsilateral dorsal and ventral cochlear nuclei, dorsal periolivary nuclei, and lateral nucleus of the trapezoid body and the contralateral medial and ventral nuclei of the trapezoid body, lateral lemniscal nuclei, and inferior colliculus. These deficits did not recover, even after long survivals up to 6 months. Results also indicated that neurons in the dorsal cochlear nucleus could be activated by contralateral stimulation in the absence of ipsilateral cochlear input and that the influence of the contralateral ear was tonotopically organized. Results also indicated that c-fos expression rose rapidly and persisted for up to 6 months in neurons in the rostral part of the contralateral medial nucleus of the trapezoid body following a cochlear ablation, even in the absence of acoustic stimulation. This response may reflect a release of constitutive excitatory inputs normally suppressed by missing afferent input or changes in homeostatic gene expression related to sensory deprivation. Instances of transient, surgery-dependent increases in c-fos mRNA expression in the absence of acoustic stimulation were observed in the superficial dorsal cochlear nucleus and the cochlear nerve root on the ablated side.

Unilateral injury to the adult rat optic nerve causes multiple cellular responses in the contralateral site

This study was undertaken to examine whether unilateral injury to one optic nerve (ON) elicits a response in the microglia, neuroglia and ganglion cells of the retina and ON of the contralateral site as well. Bilateral activation of the transcription factor c-jun could be immunohistochemically detected in the ganglion cell layer 2 days after crush and later. Microglial cells were detected with the activation-specific antibodies MUC 102 and OX-42. They showed an immediate and clear pattern of activation within the contralateral ON and retina, although this response was less pronounced than in the directly lesioned site. Astrocytes and Müller cells showed a typical up-regulation of glial fibrillary acidic protein in the lesioned retina and only focal but virtually no generalized up-regulation in the contralateral eye. Ganglion cells whose axons had been crushed responded with vigorous axonal growth after 2 days in culture, in addition to exhibiting in situ reactions. However, ganglion cells of the contralateral retina responded with a moderate regeneration, too. Growth was less pronounced than in the crushed retina but significantly better than in retinas on untreated animals. The results suggest that unilateral lesion of the optic nerve elicits a defined response in the major cell types of the contralateral retinofugal system. The findings suggest that it is advisable to maintain caution in the use of the contralateral optic nerve and retina as a control in experiments dealing with cellular processes of de- and regeneration.

Fetal spinal cord transplants and exogenous neurotrophic support enhance c-Jun expression in mature axotomized neurons after spinal cord injury

The responses of the central (CNS) and peripheral (PNS) nervous system to axotomy differ in a number of ways; these differences can be observed in both the cell body responses to injury and in the extent of regeneration that occurs in each system. The cell body responses to injury in the PNS involves the upregulation of genes that are not upregulated following comparable injuries to CNS neurons. The expression of particular genes following injury may be essential for regeneration to occur. In the present study, we have evaluated the hypothesis that expression of the inducible transcription factor c-Jun is associated with regrowth of axotomized CNS neurons. In these experiments, we compared c-Jun expression in axotomized brainstem neurons after thoracic spinal cord hemisection alone (a condition in which no regrowth occurs) and in groups of animals where hemisections were combined with treatments such as transplants of fetal spinal cord tissue and/or application of neurotrophic factors to the lesion site. The latter conditions enhance the capacity of the CNS for regrowth. We have demonstrated that hemisections alone do not upregulate expression of c-Jun, indicating that this particular cell body response is not a direct result of axotomy. However, c-Jun expression is upregulated in animals that received application of transplants and neurotrophins. Because these interventions also promote sprouting and regrowth of CNS axons after spinal cord lesions, we suggest that transplants and exogenous neurotrophic factor application activate a cell body response consistent with a role for c-Jun in axonal growth.

Nonobligate role of early or sustained expression of immediate-early gene proteins c-fos, c-jun, and Zif/268 in hippocampal mossy fiber sprouting

Axon sprouting in dentate granule cells is an important model of structural plasticity in the hippocampus. Although the process can be triggered by deafferentation, intense activation of glutamate receptors, and other convulsant stimuli, the specific molecular steps required to initiate and sustain mossy fiber (MF) reorganization are unknown. The cellular immediate early genes (IEGs) c-fos, c-jun, and zif/268 are major candidates for the initial steps of this plasticity, because they encode transcription factors that may trigger cascades of activity-dependent neuronal gene expression and are strongly induced in all experimental models of MF sprouting. The mutant mouse stargazer offers an important opportunity to test the specific role of IEGs, because it displays generalized nonconvulsive epilepsy and intense MF sprouting in the absence of regional cell injury. Here we report that stargazer mice show no detectable elevations in c-Fos, c-Jun, or Zif/268 immediate early gene proteins (IEGPs) before or during MF growth. Experimental results in stargazer, including (1) a strong IEGP response to kainate-induced convulsive seizures, (2) no IEGP response after prolongation of spike-wave synchronization, (3) no IEGP increase at the developmental onset of seizures or after prolonged seizure suppression, and (4) unaltered levels of the intracellular Ca2+-buffering proteins calbindin-D28k or parvalbumin, exclude the possibility that absence of an IEGP response in stargazer is either gene-linked or suppressed by known refractory mechanisms. These data demonstrate that increased levels of these IEGPs are not an obligatory step in MF-reactive sprouting and differentiate the early downstream molecular cascades of two major seizure types.

Axotomy-induced c-JUN expression in young medial septal neurons is regulated by nerve growth factor

In the present study we investigated the axotomy-induced expression of the proto-oncogene c-jun in young rat medial septal neurons and its regulation by nerve growth factor. First, medial septal neurons were retrogradely labelled by Fast Blue injection into the hippocampus at postnatal day 1 (P1). Rats of different developmental ages (P6, P9, P14, P21, P28 and P42) were then subjected to bilateral fimbria-fornix transection resulting in the axotomy of septohippocampal projection neurons. After the lesion, c-JUN immunoreactivity was observed in the nuclei of axotomized medial septal neurons of all stages examined, suggesting that c-JUN induction is an age-independent feature of axotomized medial septal neurons. Double immunolabelling for choline acetyltransferase and c-JUN or parvalbumin and c-JUN, respectively, revealed that both cholinergic and GABAergic septohippocampal projection neurons express c-JUN after axotomy. In addition, a co-localization of immunostaining for c-JUN and the neuropeptide galanin was found after lesion, as both proteins were induced in the same medial septal neurons following fimbria-fornix transection. Next, the regulation of c-JUN expression in axotomized medial septal neurons was studied in organotypic cultures of the medial septum. Axotomized medial septal neurons in culture did not express c-JUN in contrast to the in vivo situation. With the concept that nerve growth factor suppresses c-JUN expression, slice cultures of the medial septum were treated with antibodies against nerve growth factor. This treatment caused a dose-dependent increase in c-JUN-positive cells in these slice cultures. Simultaneous addition of nerve growth factor and antibodies against nerve growth factor resulted in the reversal of this effect. These data suggest an age-independent induction of c-JUN in axotomized medial septal neurons and its regulation by nerve growth factor.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

A sequence-specific splicing activator, tra2beta, is up-regulated in response to nerve injury

Tra2beta is the first mammalian protein which is proved to activate mRNA splicing in sequence-specific manner. Following hypoglossal nerve injury, the expression of Tra2beta mRNA was elevated in injured motoneurons transiently. The up-regulation of Tra2beta mRNA was observed from post-operative day 3 to 21. In addition to the nerve injury in PNS, a brain lesion in CNS also enhanced the expression of Tra2beta mRNA. The present study could be the first observation showing that an expression of the sequence-specific splicing activator is enhanced in neuronal cells in response to nerve injury, and indicates that Tra2beta may participate in the control of injury-specific splicing patterns in order to express molecules which are necessary for regeneration.

MAP kinase phosphatase 1 is expressed and enhanced by FK506 in surviving mamillary, but not degenerating nigral neurons following axotomy

The MAP kinase phosphatase 1 (MKP-1), a dual serine-threonine phosphatase, inactivates the MAP kinases ERK and JNK/SAPK which are involved in neuronal survival and neuronal cell death following injury and degenerative stimuli. We have studied by immunocytochemistry whether regulation of MKP-1 is part of the cell-body response following nerve fiber transection. The expression of MKP-1 was investigated in axotomized neurons of the corpus mamillaris (CMm) and substantia nigra pars compacta (SNC) following transection of the mamillo-thalamic tract (MT) and the medial forebrain bundle (MFB), respectively. In contrast to the surviving CMm neurons, the vast majority of SNC neurons undergoes cell death following axotomy. MKP-1 immunoreactivity which is absent in untreated adult rats, appeared in CMm neurons 24 h following MT transection, reached a maximum after 2 days and persisted in a substantial proportion of CMm neurons until 20 days, the end of observation period. In contradistinction, MKP-1 could not be detected in the SNC neurons. MKP-1 immunoreactivity was virtually restricted to the nuclei of neurons. Subcutaneous injection of the immunosuppressant FK506 that protects axotomized SNC neurons against neuronal cell death, enhanced the expression of MKP-1 in CMm, but failed to do so in SNC neurons. The selective expression of MKP-1 in CMm is the first finding on a different regulation of components in the stress kinase signal pathway in surviving vs. degenerating axotomized neurons.

Axonal injury and peripheral nerve grafting in the thalamus and cerebellum of the adult rat: upregulation of c-jun and correlation with regenerative potential

The protooncogene c-jun is highly expressed for long periods in axotomized PNS neurons. This may be related to their growth and regeneration. In contrast, axotomized CNS neurons show only a small and transient upregulation of c-jun. It has been suggested that there may be a correlation between this failure to maintain high levels of c-jun expression after axotomy and abortive CNS axonal regeneration. We have studied, by in situ hybridization and immunohistochemistry, the c-jun response after stab wound lesion, and after peripheral nerve grafting in the thalamus and cerebellum of the adult rat. A lesion elicits upregulation of c-jun in thalamic neurons ipsilateral to the lesion. This is most evident and prolonged in neurons such as those of the thalamic reticular nucleus, which have an established propensity to regenerate. After peripheral nerve grafting, the c-jun response in thalamic neurons is enhanced, mostly in neurons which have axons regenerating along the grafts. These neurons also upregulate growth-associated protein 43 (GAP-43). By comparison, injured Purkinje cells of the cerebellum which do not regenerate their axons along a graft, do not upregulate either c-jun or GAP-43, although they increase their expression of p75. Thus CNS neurons able to regenerate their axons along a peripheral nerve graft are those in which c-jun is induced after injury, and c-jun may play a critical role in the control of gene programs for axonal regeneration. Moreover, the observed differences in the ability of CNS neurons to regenerate their axons may relate to a difference in their intrinsic molecular response to axotomy.

Regenerative capacity of retinal ganglion cells in mammals

Retinal ganglion cells (RGCs) and their projections in the optic nerve offer a convenient model to study survival and regeneration of mammalian central nervous system (CNS) nerve cells following injury. Possible factors affecting the death of RGCs following axotomy and various approaches to rescue the axotomized RGCs are discussed. In addition, two main strategies currently used to enhance axonal regeneration of damaged RGCs are described. The first focuses on overcoming the unfavorable extrinsic CNS environment and the second concentrates on upregulating the intrinsic growth potential of RGCs. Thus, the failure or success of RGC axonal regrowth after injury depends on the complicated interplay between the extrinsic and intrinsic factors.

Long-term effects of axotomy on excitability and growth of isolated Aplysia sensory neurons in cell culture: potential role of cAMP

Crushing nerves, which contain the axons of central sensory neurons, in Aplysia causes the neurons to become hyperexcitable and to sprout new processes. Previous experiments that examined the effects of axonal injury on Aplysia sensory neurons have been performed in the intact animal or in the semi-intact CNS of Aplysia. It therefore has been unclear to what extent the long-term neuronal consequences of injury are due to intrinsic or extrinsic cellular signals. To determine whether injury-induced changes in Aplysia sensory neurons are due to intrinsic or extrinsic signals, we have developed an in vitro model of axonal injury. Isolated central sensory neurons grown for 2 days in cell culture were axotomized. Approximately 24 h after axotomy, sensory neurons exhibited a greater excitability-reflected, in part, as a significant reduction in spike accommodation-and greater neuritic outgrowth than did control (unaxotomized) neurons. Rp diastereoisomer of the cyclic adenosine 3',5'-monophosphorothiate (Rp-cAMPS), an inhibitor of protein kinase A, blocked both the reduction in accommodation and increased neuritic outgrowth induced by axotomy. Rp-cAMPS also blocked similar, albeit smaller, alterations observed in control sensory neurons during the 24-h period of our experiments. These results indicate that axonal injury elevates cAMP levels within Aplysia sensory neurons, and that this elevation is directly responsible, in part, for the previously described long-term electrophysiological and morphological changes induced in Aplysia sensory neurons by nerve crush. In addition, the results indicate that control sensory neurons in culture are also undergoing injury-related electrophysiological and structural changes, probably due to cellular processes triggered when the neurons are axotomized during cell culturing. Finally, the results provide support for the idea that the cellular processes activated within Aplysia sensory neurons by injury, and those activated during long-term behavioral sensitization, overlap significantly.

Transplants and neurotrophic factors prevent atrophy of mature CNS neurons after spinal cord injury

Axotomy of mature rubrospinal neurons leads to a substantial atrophy of these neurons within days after surgery. In addition, these neurons do not successfully regenerate following axotomy. The relationship of atrophy to regenerative failure is not clear, and the signals which regulate these events have not been identified. However, it is possible that the atrophy of these neurons plays a role in preventing regeneration. In the present study, we evaluated the hypothesis that interventions which have been shown to promote growth of axotomized CNS neurons are also capable of reversing the axotomy-induced atrophy. To test this hypothesis, adults rats received thoracic spinal cord hemisection alone or in combination with transplants of fetal spinal cord tissue and/or neurotrophic factor support. Our data indicate that application of either transplants or neurotrophic factors partially reverse the axotomy-induced atrophy in rubrospinal neurons, but that both interventions together reverse the atrophy completely. These results suggest that the same pathways that are activated to enhance growth of rubrospinal neurons after axotomy may also be involved in the maintenance of cell morphology.

c-Jun expression in adult rat dorsal root ganglion neurons: differential response after central or peripheral axotomy

The response of the mature central nervous system (CNS) to injury differs significantly from the response of the peripheral nervous system (PNS). Axotomized PNS neurons generally regenerate following injury, while CNS neurons do not. The mechanisms that are responsible for these differences are not completely known, but both intrinsic neuronal and extrinsic environmental influences are likely to contribute to regenerative success or failure. One intrinsic factor that may contribute to successful axonal regeneration is the induction of specific genes in the injured neurons. In the present study, we have evaluated the hypothesis that expression of the immediate early gene c-jun is involved in a successful regenerative response. We have compared c-Jun expression in dorsal root ganglion (DRG) neurons following central or peripheral axotomy. We prepared animals that received either a sciatic nerve (peripheral) lesion or a dorsal rhizotomy in combination with spinal cord hemisection (central lesion). In a third group of animals, several dorsal roots were placed into the hemisection site along with a fetal spinal cord transplant. This intervention has been demonstrated to promote regrowth of severed axons and provides a model to examine DRG neurons during regenerative growth after central lesion. Our results indicated that c-Jun was upregulated substantially in DRG neurons following a peripheral axotomy, but following a central axotomy, only 18% of the neurons expressed c-Jun. Following dorsal rhizotomy and transplantation, however, c-Jun expression was upregulated dramatically; under those experimental conditions, 63% of the DRG neurons were c-Jun-positive. These data indicate that c-Jun expression may be related to successful regenerative growth following both PNS and CNS lesions.

Expression of c-Jun protein in degenerating retinal ganglion cells after optic nerve lesion in the rat

Axonal lesions to the optic nerve (ON) induce c-Jun expression in retinal ganglion cells (RGCs) of the rat in vivo. Detailed investigations using retrograde tracers, and double labeling studies for c-Jun and regeneration-associated factors, such as the growth-associated protein GAP-43, have suggested that this upregulation of c-Jun is part of a cell body response in an abortive attempt of affected RGCs to survive and regenerate an axon. On the other hand, prolonged expression of c-Jun protein has in several paradigms of neurodegeneration been linked to the induction of apoptotic cell death. In the present study, we examined the time course and subcellular localization of c-Jun protein by immunocytochemistry on retinal sections after optic nerve crush and carried out double labeling for c-Jun protein and DNA strand breaks to detect apoptosis on the same sections. Several days after ON lesion, a subpopulation of RGCs was detected in which c-Jun protein was not confined to the nucleus, but also located in the cytoplasm. In addition, RGCs were seen that displayed morphological signs of apoptosis, DNA strand breaks, and c-Jun immunoreactivity at the same time. Therefore, c-Jun expression is not confined to intact or regenerating ganglion cells, but also occurs in cells that are destined to die. Our results suggest that the decision to undergo either fate depends on additional signaling events that modulate the transcriptional actions of c-Jun.

Intrinsic neuronal determinants that promote axonal sprouting and elongation

Nerve processes elongate, branch and form synaptic contacts in a highly regulated and specific manner. Long-distance axon elongation is restricted to the main phase of axon formation during development, but can be reinduced upon lesions in the adult (regeneration). It correlates with the expression of defined genes, including proteins involved in signalling (e.g. src, NCAM, integrins), transcription factors (e.g. c-jun) and structural proteins (e.g. actin and tubulin isoforms). Activation of an exon elongation program may require bcl-2. The formation and growth of local branches (sprouting) is controlled by mechanisms in the target region. In addition, the expression of growth-associated proteins such as GAP-43 and CAP-23 in neurons lowers the threshold for nerve sprouting and potentiates its vigour. Recent studies suggest that nerve sprouting and long-distance elongation depend on the expression of different intrinsic components in neurons. One implication of these findings is that the differential expression of genes facilitating local branching may affect structural plasticity in the intact adult nervous system.

Axotomized septal cholinergic neurons rescued by nerve growth factor or neurotrophin-4/5 fail to express the inducible transcription factor c-Jun

The inducible transcription factor c-Jun increases in neurons in response to axotomy by unknown mechanisms, and it has been postulated that c-Jun may regulate genes involved in promoting either degeneration or regeneration of axotomized neurons. In this report, we investigated the effect of daily or twice daily intraventricular administration of the neurotrophins nerve growth factor or neurotrophin-4/5 on the decrease in choline acetyltransferase expression and the increase in c-Jun expression in rat medial septum/diagonal band neurons three, seven and 14 days following unilateral, complete, fornix fimbria lesion. We also examined whether medial septum/diagonal band neurons might die by apoptosis within two weeks of fornix fimbria lesion using terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labelling. Our results show that both nerve growth factor and neurotrophin-4/5 maintain the phenotype of basal forebrain cholinergic neurons following axotomy. Furthermore, using double-labelling immunofluorescence, we found that while c-Jun was expressed in cholinergic neurons in control-treated rats seven days following fornix fimbria lesion, cholinergic neurons rescued by either nerve growth factor or neurotrophin-4/5 in neurotrophin-treated rats failed to express c-Jun. At no time-point (three, seven or 14 days post-axotomy) did any neurons in the medial septum/diagonal band stain positive for terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling, suggesting that medial septum/diagonal band neurons do not undergo apoptosis within the first two weeks following axotomy at the time-points observed by us. Therefore, these results show that both nerve growth factor and neurotrophin-4/5 rescue the phenotype of axotomized cholinergic neurons and that these rescued neurons fail to express c-Jun in response to axotomy. In addition, since neither nerve growth factor nor neurotrophin-4/5 induced c-Jun in medial septum/diagonal band cholinergic neurons, it seems unlikely that the neurotrophic effects of nerve growth factor and neurotrophin-4/5 on cholinergic neurons are mediated via c-Jun expression. Furthermore, since axotomy failed to increase terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labelling in septal neurons, it appears unlikely that c-Jun expression in these axotomized neurons is related to neuronal degeneration via apoptosis.

The c-Jun transcription factor--bipotential mediator of neuronal death, survival and regeneration

Axon interruption elicits a complex neuronal response that leaves neurons poised precariously between death and regeneration. The signals underlying this dichotomy are not fully understood. The transcription factor c-Jun is one of the earliest and most consistent markers for neurons that respond to nerve-fiber transection, and its expression can be related to both degeneration and survival including target re-innervation. In vitro experiments have demonstrated that expression of c-Jun can kill neonatal neurons but, in the adult nervous system, c-Jun might also be involved in neuroprotection and regeneration. The functional characteristics of c-Jun offer a model for the ability of a single molecule to serve as pivotal regulator for death or survival, not only in the response of the cell body to axonal lesions but also following neurodegenerative disorders. In this model, the fate of neurons is determined by a novel transcriptional network comprising c-Jun, ATF-2 (activating transcription factor-2) and JNKs (c-Jun N-terminal kinases).