Overexpression of the neural growth-associated protein GAP-43 induces nerve sprouting in the adult nervous system of transgenic mice

Overexpression of GAP-43 modifies the distribution of the receptors for myelin-associated growth-inhibitory proteins in injured Purkinje axons

Abstract Neurons with enhanced intrinsic growth capabilities can elongate their axons into non-permissive territories, but the mechanisms that enable the outgrowing processes to overcome environmental inhibition are largely unknown. To address this issue, we examined adult mouse Purkinje cells that overexpress the axonal growth-associated protein GAP-43. After injury, these neurons exhibit sprouting along the intracortical neuritic course and at the severed stump in the white matter. To determine whether GAP-43-overexpressing Purkinje cells are responsive to extrinsic inhibitory cues, we investigated the content and subcellular localization of major receptors for myelin-associated inhibitory proteins, PlexinB1 and the Nogo receptor (NgR) with the related co-receptors LINGO-1 and p75. Expression of these molecules, estimated by measuring perikaryal immunostaining intensity and Western blot, was not different in wild-type or transgenic mice, and it was not overtly modified after axotomy. Following injury, however, the content of PlexinB1 was significantly reduced in GAP-43-overexpressing neurites. Furthermore, in the same axons the distribution of both PlexinB1 and NgR was altered, being inverse to that of GAP-43. Labelling for the two receptors was conspicuously reduced on the axonal surface and it was almost undetectable in the outgrowing sprouts, which showed strong GAP-43 immunoreactivity. These observations indicate that although GAP-43 overexpression does not modify the expression of receptors for myelin-associated inhibitory factors, it interferes with their subcellular localization and exposure on the neuritic membrane. Therefore, GAP-43 promotes axon growth by multiple synergistic mechanisms that potentiate the intrinsic motility of the elongating processes, while reducing their sensitivity to environmental inhibition.

Functional whole-genome analysis identifies Polo-like kinase 2 and poliovirus receptor as essential for neuronal differentiation upstream of the negative regulator alphaB-crystallin

This study aimed at identifying transcriptional changes associated to neuronal differentiation induced by six distinct stimuli using whole-genome microarray hybridization analysis. Bioinformatics analyses revealed the clustering of these six stimuli into two categories, suggesting separate gene/pathway dependence. Treatment with specific inhibitors demonstrated the requirement of both Janus kinase and microtubule-associated protein kinase activation to trigger differentiation with nerve growth factor (NGF) and dibutyryl cAMP. Conversely, activation of protein kinase A, phosphatidylinositol-3-kinase alpha, and mammalian target of rapamycin, although required for dibutyryl cAMP-induced differentiation, exerted a negative feedback on NGF-induced differentiation. We identified Polo-like kinase 2 (Plk2) and poliovirus receptor (PVR) as indispensable for NGF-driven neuronal differentiation and alphaB-crystallin (Cryab) as an inhibitor of this process. Silencing of Plk2 or PVR blocked NGF-triggered differentiation and Cryab down-regulation, while silencing of Cryab enhanced NGF-induced differentiation. Our results position both Plk2 and PVR upstream of the negative regulator Cryab in the pathway(s) leading to neuronal differentiation triggered by NGF.

Wnt5a mediates nerve growth factor-dependent axonal branching and growth in developing sympathetic neurons

Nerve growth factor (NGF) is a potent survival and axon growth factor for neuronal populations in the peripheral nervous system. Although the mechanisms by which target-derived NGF influences survival of innervating neurons have been extensively investigated, its regulation of axonal growth and target innervation are just being elucidated. Here, we identify Wnt5a, a member of the Wnt family of secreted growth factors, as a key downstream effector of NGF in mediating axonal branching and growth in developing sympathetic neurons. Wnt5a is robustly expressed in sympathetic neurons when their axons are innervating NGF-expressing targets. NGF:TrkA signaling enhances neuronal expression of Wnt5a. Wnt5a rapidly induces axon branching while it has a long-term effect on promoting axon extension. Loss of Wnt5a function revealed that it is necessary for NGF-dependent axonal branching and growth, but not survival, in vitro. Furthermore, Wnt5a(-/-) mice display reduced innervation of NGF-expressing target tissues, and a subsequent increase in neuronal apoptosis, in vivo. Wnt5a functions in developing sympathetic neurons by locally activating protein kinase C in axons. Together, our findings define a novel regulatory pathway in which Wnt5a, expressed in sympathetic neurons in response to target-derived NGF, regulates innervation of peripheral targets.

NFAT-3 is a transcriptional repressor of the growth-associated protein 43 during neuronal maturation

Transcription is essential for neurite and axon outgrowth during development. Recent work points to the involvement of nuclear factor of activated T cells (NFAT) in the regulation of genes important for axon growth and guidance. However, NFAT has not been reported to directly control the transcription of axon outgrowth-related genes. To identify transcriptional targets, we performed an in silico promoter analysis and found a putative NFAT site within the GAP-43 promoter. Using in vitro and in vivo experiments, we demonstrated that NFAT-3 regulates GAP-43, but unexpectedly, does not promote but represses the expression of GAP-43 in neurons and in the developing brain. Specifically, in neuron-like PC-12 cells and in cultured cortical neurons, the overexpression of NFAT-3 represses GAP-43 activation mediated by neurotrophin signaling. Using chromatin immunoprecipitation assays, we also show that prior to neurotrophin activation, endogenous NFAT-3 occupies the GAP-43 promoter in PC-12 cells, in cultured neurons, and in the mouse brain. Finally, we observe that NFAT-3 is required to repress the physiological expression of GAP-43 and other pro-axon outgrowth genes in specific developmental windows in the mouse brain. Taken together, our data reveal an unexpected role for NFAT-3 as a direct transcriptional repressor of GAP-43 expression and suggest a more general role for NFAT-3 in the control of the neuronal outgrowth program.

Regulation of GAP-43 at serine 41 acts as a switch to modulate both intrinsic and extrinsic behaviors of growing neurons, via altered membrane distribution

GAP-43 is the major neuronal substrate of protein kinase C (PKC). Its phosphorylation status dictates the severity of pathfinding errors by GAP-43 (+/-) growth cones in vivo, as well as its modulation of actin dynamics in vitro. These experiments show that stably overexpressing cDNAs mutant at its single PKC phosphorylation site at serine41 in retinoic acid treated SH-Sy5Y neuroblastoma cells regulates intrinsic and extrinsic behaviors of growing neurons. Intrinsically, only Wt and pseudophosphorylated GAP-43Ser41Asp precipitated with F-actin and potentiated F-actin - regulated filopodia formation. GAP-43Ser41Asp inhibited neurite outgrowth whereas only unphosphorylatable GAP-43Ser41Ala precipitated neurotubulin, potentiated neurotubulin accumulation in neurites and increased outgrowth. When PI3-kinase was inhibited GAP-43Ser41Asp-mediated filopodia formation was inhibited whereas GAP-43Ser41Ala-mediated neurite extension was potentiated. Extrinsically, only Wt and GAP-43Ser41Asp potentiated both homotypic adhesion and neurite outgrowth on NCAM-expressing monolayers and promoted NCAM stability. With respect to the underlying mechanism, more F-actin and NCAM colocalized with Wt and GAP-43Ser41Asp in detergent resistant membranes (DRMs) isolated from live cells and GAP-43Ser41Asp-mediated functions were insensitive to cholesterol depletion. In contrast, GAP-43Ser41Ala-mediated functions were sensitive to cholesterol depletion. Neither GAP-43Ser41Asp nor GAP-43Ser41Ala was able to protect against growth cone collapse mediated by PIP2 inhibitors. The results show that modification of GAP-43 at its PKC phosphorylation site directs its distribution to different membrane microdomains that have distinct roles in the regulation of intrinsic and extrinsic behaviors in growing neurons.

Conservation and expression of IQ-domain-containing calpacitin gene products (neuromodulin/GAP-43, neurogranin/RC3) in the adult and developing oscine song control system

Songbirds are appreciated for the insights they provide into regulated neural plasticity. Here, we describe the comparative analysis and brain expression of two gene sequences encoding probable regulators of synaptic plasticity in songbirds: neuromodulin (GAP-43) and neurogranin (RC3). Both are members of the calpacitin family and share a distinctive conserved core domain that mediates interactions between calcium, calmodulin, and protein kinase C signaling pathways. Comparative sequence analysis is consistent with known phylogenetic relationships, with songbirds most closely related to chicken and progressively more distant from mammals and fish. The C-terminus of neurogranin is different in birds and mammals, and antibodies to the protein reveal high expression in adult zebra finches in cerebellar Purkinje cells, which has not been observed in other species. RNAs for both proteins are generally abundant in the telencephalon yet markedly reduced in certain nuclei of the song control system in adult canaries and zebra finches: neuromodulin RNA is very low in RA and HVC (relative to the surrounding pallial areas), whereas neurogranin RNA is conspicuously low in Area X (relative to surrounding striatum). In both cases, this selective downregulation develops in the zebra finch during the juvenile song learning period, 25-45 days after hatching. These results suggest molecular parallels to the robust stability of the adult avian song control circuit.

The yellow fluorescent protein (YFP-H) mouse reveals neuroprotection as a novel mechanism underlying chondroitinase ABC-mediated repair after spinal cord injury

Chondroitinase ABC (ChABC) represents a promising therapeutic strategy for the treatment of spinal cord injury due to its potent effects on restoring function to spinal-injured adult mammals. However, there is limited mechanistic insight as to the underlying effects of ChABC treatment, where the effects are mediated, and which signaling pathways are involved in ChABC-mediated repair. Here we use a transgenic (YFP-H) mouse to demonstrate that cortical layer V projection neurons undergo severe atrophy 4 weeks after thoracic dorsal column injury and that ChABC is neuroprotective for these neurons after ICV infusion. ChABC also prevented cell atrophy after localized delivery to the spinal cord, suggesting a possible retrograde neuroprotective effect mediated at the injury site. Furthermore, neuroprotection of corticospinal cell somata coincided with increased axonal sprouting in the spinal cord. In addition, Western blot analysis of a number of kinases important in survival and growth signaling revealed a significant increase in phosphorylated ERK1 at the spinal injury site after in vivo ChABC treatment, indicating that activated ERK may play a role in downstream repair processes after ChABC treatment. Total forms of PKC and AKT were also elevated, indicating that modification of the glial scar by ChABC promotes long-lasting signaling changes at the lesion site. Thus, using the YFP-H mouse as a novel tool to study degenerative changes and repair after spinal cord injury we demonstrate, for the first time, that ChABC treatment regulates multiple signaling cascades at the injury site and exerts protective effects on axotomized corticospinal projection neurons.

Neonatal exposure to PFOS and PFOA in mice results in changes in proteins which are important for neuronal growth and synaptogenesis in the developing brain

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) belong to the family of perfluorinated compounds. They are used in industrial and consumer applications, e.g., clothing fabrics, carpets, and food packaging. PFOS and PFOA are present in the environment and are found in dust and human milk, which implies that newborns and toddlers can be directly exposed to these agents during brain development. Recently, we reported that PFOS and PFOA can cause neurobehavioral defects and changes in the cholinergic system, in the adult animal, when given directly to neonatal mice, and thereby showing similarities with other investigated persistent organic pollutants, such as dichloro-diphenyl-trichloroethan, polychlorinated biphenyls, and polybrominated diphenyl ethers (PBDEs). In recent studies, we have also seen that highly brominated PBDEs can affect the levels of proteins that are important for neuronal growth and synaptogenesis in the neonatal mouse brain. The present study shows that a single oral dose of either 21 micromol PFOS or PFOA/kg body weight (11.3 or 8.70 mg), given directly to the neonatal mice on postnatal day 10, significantly increased the levels of CaMKII, GAP-43, and synaptophysin in the hippocampus of the neonatal mouse. Both compounds significantly increased the levels of synaptophysin and tau in cerebral cortex, and PFOA also increased the levels of tau in hippocampus. These proteins are important for normal brain development, and altered levels of these proteins during a critical period of the brain growth spurts could be one of the mechanisms behind earlier reported behavioral defects.

The protein kinase C phosphorylation site on GAP-43 differentially regulates information storage

Protein kinase C (PKC) is known to regulate phosphorylation of substrates such as MARCKS, GAP-43, and the NMDA receptor, all of which have been linked to synaptic plasticity underlying information storage processes. Here we report on three transgenic mice isoforms differentiated both by mutation of the PKC site on GAP-43 as well as by their performance in three learning situations: (1) a radial arm maze task, which evaluates spatial memory and its retention, (2) fear conditioning which assesses contextual memory, and (3) the water maze which also evaluates spatial memory and its retention. The present results show, for the first time to our knowledge, that the phosphorylation state of a single site on an identified brain growth- and plasticity-associated protein differentially regulates performance of three different memory-associated tasks.

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

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

LEDGFp52 controls rat retinal ganglion cell neurite growth in culture and regulates specific neuronal growth-associated genes and protein production

We investigated the regulation of primary neurite growth and expression of specific growth-associated genes by lens epithelium-derived growth factor (LEDGF) in rat retinal ganglion cells (RGCs). A pAd-LEDGFp52 adenovirus vector and a siRNA-LEDGFp52 eucaryotic expression vector were transfected into cultured RGCs. Transfection with pAd-LEDGFp52 significantly increased the number of neurites and their lengths compared with untransfected control RGCs. The expression of growth associated protein 43 (GAP43), microtubule-associated protein 2 (MAP2), and low-molecular-weight neurofilament (NF-L) genes and proteins were also significantly up-regulated. In contrast, the introduction of siRNA-LEDGFp52 significantly decreased the number and length of neurites, and significantly down-regulated the expression GAP43, NF-L and MAP2 genes and proteins compared with controls. Our findings suggest that LEDGFp52 might act as a dendritic arborization gene as well as an axonal elongation gene in RGCs and that it might be beneficial to the functional recovery of regenerating RGCs.

Identification of Phox2b-regulated genes by expression profiling of cranial motoneuron precursors

Background

Branchiomotor neurons comprise an important class of cranial motor neurons that innervate the branchial-arch-derived muscles of the face, jaw and neck. They arise in the ventralmost progenitor domain of the rhombencephalon characterized by expression of the homeodomain transcription factors Nkx2.2 and Phox2b. Phox2b in particular plays a key role in the specification of branchiomotor neurons. In its absence, generic neuronal differentiation is defective in the progenitor domain and no branchiomotor neurons are produced. Conversely, ectopic expression of Phox2b in spinal regions of the neural tube promotes cell cycle exit and neuronal differentiation and, at the same time, induces genes and an axonal phenotype characteristic for branchiomotor neurons. How Phox2b exerts its pleiotropic functions, both as a proneural gene and a neuronal subtype determinant, has remained unknown.

Results

To gain further insights into the genetic program downstream of Phox2b, we searched for novel Phox2b-regulated genes by cDNA microarray analysis of facial branchiomotor neuron precursors from heterozygous and homozygous Phox2b mutant embryos. We selected for functional studies the genes encoding the axonal growth promoter Gap43, the Wnt antagonist Sfrp1 and the transcriptional regulator Sox13, which were not previously suspected to play roles downstream of Phox2b and whose expression was affected by Phox2b misexpression in the spinal cord. While Gap43 did not produce an obvious phenotype when overexpressed in the neural tube, Sfrp1 induced the interneuron marker Lhx1,5 and Sox13 inhibited neuronal differentiation. We then tested whether Sfrp1 and Sox13, which are down-regulated by Phox2b in the facial neuron precursors, would antagonize some aspects of Phox2b activity. Co-expression of Sfrp1 prevented Phox2b from repressing Lhx1,5 and alleviated the commissural axonal phenotype. When expressed together with Sox13, Phox2b was still able to promote cell cycle exit and neuronal differentiation, but the cells failed to relocate to the mantle layer and to extinguish the neural stem cell marker Sox2.

Conclusion

Our results suggest novel roles for Sfrp1 and Sox13 in neuronal subtype specification and generic neuronal differentiation, respectively, and indicate that down-regulation of Sfrp1 and Sox13 are essential aspects of the genetic program controlled by Phox2b in cranial motoneurons.

Structural reorganization of the dentate gyrus following entorhinal denervation: species differences between rat and mouse

Deafferentation of the dentate gyrus by unilateral entorhinal cortex lesion or unilateral perforant pathway transection is a classical model to study the response of the central nervous system (CNS) to denervation. This model has been extensively characterized in the rat to clarify mechanisms underlying denervation-induced gliosis, transneuronal degeneration of denervated neurons, and collateral sprouting of surviving axons. As a result, candidate molecules have been identified which could regulate these changes, but a causal link between these molecules and the postlesional changes has not yet been demonstrated. To this end, mutant mice are currently studied by many groups. A tacit assumption is that data from the rat can be generalized to the mouse, and fundamental species differences in hippocampal architecture and the fiber systems involved in sprouting are often ignored. In this review, we will (1) provide an overview of some of the basics and technical aspects of the entorhinal denervation model, (2) identify anatomical species differences between rats and mice and will point out their relevance for the axonal reorganization process, (3) describe glial and local inflammatory changes, (4) consider transneuronal changes of denervated dentate neurons and the potential role of reactive glia in this context, and (5) summarize the differences in the reorganization of the dentate gyrus between the two species. Finally, we will discuss the use of the entorhinal denervation model in mutant mice.

Purple sweet potato color repairs d-galactose-induced spatial learning and memory impairment by regulating the expression of synaptic proteins

Purple sweet potato color (PSPC), a class of naturally occurring anthocyanins used to color food (E163), has been reported to possess a variety of biological activities, including anti-oxidant, anti-tumor, and anti-inflammatory. The effect of PSPC on the spatial learning and memory of mice treated with d-galactose (d-gal) was evaluated by the Morris water maze; d-gal-treated mice had decreased performance compared with mice in the vehicle and PSPC groups, while the PSPC+d-gal group showed significantly shortened escape latency to platform, increased swimming speed, more target quadrant search time and more platform crossings as compared with the d-gal group. Brain functions, such as memory formation and recovery of function after injury, depend on proper regulation of the expression levels of the pre- and post-synaptic proteins. We investigated the expression of four pre-synaptic proteins (growth-associated protein-43, synapsin-I, synaptophysin, and synaptotagmin) and two post-synaptic proteins (post-synaptic density protein-95 and Ca(2+)/calmodulin-dependent protein kinase II) in the hippocampus and cerebral cortex, respectively, in response to different treatments. Western blotting analysis showed that there were significant decreases in the expression of these representative synaptic proteins in the hippocampus and cerebral cortex of d-gal-treated mice. Interestingly, these decreased expression levels of synaptic proteins could be reversed by PSPC. The levels of expression of these representative synaptic proteins in mice treated with PSPC alone were not significantly different from those in untreated mice. The results of this study suggested that memory impairment and synaptic protein loss in d-gal-treated mice may be improved by treatment with PSPC.

Long-term monitoring of post-stroke plasticity after transient cerebral ischemia in mice using in vivo and ex vivo diffusion tensor MRI

WE USED A MURINE MODEL OF TRANSIENT FOCAL CEREBRAL ISCHEMIA TO STUDY: 1) in vivo DTI long-term temporal evolution of the apparent diffusion coefficient (ADC) and diffusion fractional anisotropy (FA) at days 4, 10, 15 and 21 after stroke 2) ex vivo distribution of a plasticity-related protein (GAP-43) and its relationship with the ex vivo DTI characteristics of the striato-thalamic pathway (21 days). All animals recovered motor function. In vivo ADC within the infarct was significantly increased after stroke. In the stroke group, GAP-43 expression and FA values were significantly higher in the ipsilateral (IL) striatum and contralateral (CL) hippocampus compared to the shams. DTI tractography showed fiber trajectories connecting the CL striatum to the stroke region, where increased GAP43 and FA were observed and fiber tracts from the CL striatum terminating in the IL hippocampus.Our data demonstrate that DTI changes parallel histological remodeling and recovery of function.

Cognitive outcome following brain injury and treatment with an inhibitor of Nogo-A in association with an attenuated downregulation of hippocampal growth-associated protein-43 expression

OBJECT

Central nervous system axons regenerate poorly after traumatic brain injury (TBI), partly due to inhibitors such as the protein Nogo-A present in myelin. The authors evaluated the efficacy of anti-Nogo-A monoclonal antibody (mAb) 7B12 administration on the neurobehavioral and cognitive outcome of rats following lateral fluid-percussion brain injury, characterized the penetration of the 7B12 or control antibodies into target brain regions, and evaluated the effects of Nogo-A inhibition on hemispheric tissue loss and sprouting of uninjured motor tracts in the cervical cord. To elucidate a potential molecular response to Nogo-A inhibition, we evaluated the effects of 7B12 on hippocampal GAP-43 expression.

METHODS

Beginning 24 hours after lateral fluid-percussion brain injury or sham injury in rats, the mAb 7B12 or control antibody was infused intracerebroventricularly over 14 days, and behavior was assessed over 4 weeks.

RESULTS

Immunoreactivity for 7B12 or immunoglobulin G was detected in widespread brain regions at 1 and 3 weeks postinjury. The brain-injured animals treated with 7B12 showed improvement in cognitive function (p < 0.05) at 4 weeks but no improvement in neurological motor function from 1 to 4 weeks postinjury compared with brain-injured, vehicle-treated controls. The enhanced cognitive function following inhibition of Nogo-A was correlated with an attenuated postinjury downregulation of hippocampal GAP-43 expression (p < 0.05).

CONCLUSIONS

Increased GAP-43 expression may be a novel molecular mechanism of the enhanced cognitive recovery mediated by Nogo-A inhibition after TBI in rats.

Co-localization and interaction of DPYSL3 and GAP43 in primary cortical neurons

Dihydropyrimidinase-like 3 (DPYSL3) and GAP43 are both involved in neurite outgrowth, a crucial process for the differentiation of neurons. The present study shows for the first time that DPYSL3 co-localizes with GAP43 in primary cortical neurons. Further co-immunoprecipitation and overlay assay showed the ability of both recombinant and endogenous DPYSL3 to bind GAP43, indicating a specific interaction between DPYSL3 and GAP43 in primary cortical neurons.

Ursolic acid ameliorates cognition deficits and attenuates oxidative damage in the brain of senescent mice induced by D-galactose

Ursolic acid (UA), a pentracyclic triterpene, is reported to have an antioxidant activity. Here we assessed the protective effect of UA against the d-galactose (D-gal)-induced neurotoxicity. We found that UA markedly reversed the D-gal induced learning and memory impairment by behavioral tests. The following antioxidant defense enzymes were measured: superoxide dismutases (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione reductase (GR). The content of the lipid peroxidation product malondialdehyde (MDA) was also analyzed. Our results indicated that the neuroprotective effect of UA against D-gal induced neurotoxicity might be caused, at least in part, by the increase in the activity of antioxidant enzymes with a reduction in lipid peroxidation. And UA also inhibited the activation of caspase-3 induced by D-gal. Furthermore, we found that UA significantly increased the level of growth-associated protein GAP43 in the brain of D-gal-treated mice. These results suggest that the pharmacological action of UA may offer a novel therapeutic strategy for the treatment of age-related conditions.

VEGF reduces astrogliosis and preserves neuromuscular junctions in ALS transgenic mice

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease resulting from motor neuron loss in the spinal cord and brain stem. In the present study, we found that systemic administration of recombinant vascular endothelial growth factor (VEGF) significantly diminished astrogliosis and increased the number of neuromuscular junctions in a Cu/Zn superoxide dismutase (SOD1) transgenic mouse model of ALS. Our results thus demonstrate a novel regulatory role of VEGF on astrocytes and are suggestive of protective effects of VEGF both in the peripheral and central nervous system in the SOD1 transgenic mouse model. These findings warrant further evaluation of the mechanism(s) of regulatory effects of VEGF on neuronal and non-neuronal cells, and the relation of these events to motor neuron degeneration and the onset and progression of ALS.

Expression of protein kinase-C substrate mRNA in the motor cortex of adult and infant macaque monkeys

To understand the molecular and cellular bases of plasticity in the primate motor cortex, we investigated the expression of three protein kinase-C (PKC) substrates: GAP-43, myristoylated alanine-rich C-kinase substrate (MARCKS), and neurogranin, which are key molecules regulating synaptic plasticity. Prominent signals for the three mRNAs were primarily observed in pyramidal cells. Large pyramidal cells in layer V, from which the descending motor tract originates, contained weaker hybridization signals for GAP-43 and neurogranin mRNAs than did the smaller pyramidal cells. We also performed double-label in situ hybridization showing that GAP-43 and neurogranin mRNAs were expressed in a subset of MARCKS-positive neurons. Quantitative analysis showed that the expression was different between the layers: layer VI contained the strongest and layer II the weakest signals for all three mRNAs. The expression levels of GAP-43 and MARCKS mRNA in layer V were higher than in layer III, while the expression level of neurogranin mRNA in layer V was almost the same as in layer III. Developmental analysis from the newborn to adult indicated that the expression levels of the three mRNAs were higher in the infant motor cortex than in the adult. The expression of both GAP-43 and neurogranin mRNAs transiently increased over several months postnatally. The present study showed that the expression of the three PKC substrates was specific to cell types, cortical layers, and postnatal developmental stage. The specific expression may reflect functional specialization for plasticity in the motor cortex of both infants and adults.

Dlx transcription factors promote migration through repression of axon and dendrite growth

In the mouse telencephalon, Dlx homeobox transcription factors are essential for the tangential migration of subpallial-derived GABAergic interneurons to neocortex. However, the mechanisms underlying this process are poorly understood. Here, we demonstrate that Dlx1/2 has a central role in restraining neurite growth of subpallial-derived immature interneurons at a stage when they migrate tangentially to cortex. In Dlx1-/-;Dlx2-/- mutants, neurite length is increased and cells fail to migrate. In Dlx1-/-;Dlx2+/- mutants, while the tangential migration of immature interneurons appears normal, they develop dendritic and axonal processes with increased length and decreased branching, and have deficits in their neocortical laminar positions. Thus, Dlx1/2 is required for coordinating programs of neurite maturation and migration. In this regard, we provide genetic evidence that in immature interneurons Dlx1/2 repression of the p21-activated serine/threonine kinase PAK3, a downstream effector of the Rho family of GTPases, is critical in restraining neurite growth and promoting tangential migration.

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.

Increased thalamocortical synaptic response and decreased layer IV innervation in GAP-43 knockout mice

The growth-associated protein, GAP-43, is an axonally localized neuronal protein with high expression in the developing brain and in regenerating neurites. Mice that lack GAP-43 (GAP-43 -/-) fail to form a whisker-related barrel map. In this study, we use GAP-43 -/- mice to examine GAP-43 synaptic function in the context of thalamocortical synapse development and cortical barrel map formation. Examination of thalamocortical synaptic currents in an acute brain slice preparation and in autaptic thalamic neurons reveals that GAP-43 -/- synapses have larger alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate receptor (AMPAR)-mediated currents than controls despite similar AMPAR function and normal probability of vesicular release. Interestingly, GAP-43 -/- synapses are less sensitive to blockade by a competitive glutamate receptor antagonist, suggesting higher levels of neurotransmitter in the cleft during synaptic transmission. Field excitatory postsynaptic potentials (EPSPs) from GAP-43 -/- thalamocortical synapses reveal a reduced fiber response, and anatomical analysis shows reduced thalamic innervation of barrel cortex in GAP-43 -/- mice. Despite this fact synaptic responses in the field EPSPs are similar in GAP-43 -/- mice and wild-type littermate controls, and the ratio of AMPAR-mediated to N-methyl-d-aspartate receptor (NMDAR)-mediated currents (AMPAR:NMDAR ratio) is larger than normal. This suggests that GAP-43 -/- mice form fewer thalamocortical synapses in layer IV because of decreased anatomical innervation of the cortex, but the remaining contacts are individually stronger possibly due to increased neurotransmitter concentration in the synaptic cleft. Together, these results indicate that in addition to its well known role in axonal pathfinding GAP-43 plays a functional role in regulating neurotransmitter release.

Coordinated expression of HuD and GAP-43 in hippocampal dentate granule cells during developmental and adult plasticity

Previous work from our laboratory demonstrated that the RNA-binding protein HuD binds to and stabilizes the GAP-43 mRNA. In this study, we characterized the expression of HuD and GAP-43 mRNA in the hippocampus during two forms of neuronal plasticity. During post-natal development, maximal expression of both molecules was found at P5 and their levels steadily decreased thereafter. At P5, HuD was also present in the subventricular zone, where it co-localized with doublecortin. In the adult hippocampus, the basal levels of HuD and GAP-43 were lower than during development but were significantly increased in the dentate gyrus after seizures. The function of HuD in GAP-43 gene expression was confirmed using HuD-KO mice, in which the GAP-43 mRNA was significantly less stable than in wild type mice. Altogether, these results demonstrate that HuD plays a role in the post-transcriptional control of GAP-43 mRNA in dentate granule cells during developmental and adult plasticity.

GAP-43 gene expression regulates information storage

Previous reports have shown that overexpression of the growth- and plasticity-associated protein GAP-43 improves memory. However, the relation between the levels of this protein to memory enhancement remains unknown. Here, we studied this issue in transgenic mice (G-Phos) overexpressing native, chick GAP-43. These G-Phos mice could be divided at the behavioral level into "spatial bright" and "spatial dull" groups based on their performance on two hidden platform water maze tasks. G-Phos dull mice showed both acquisition and retention deficits on the fixed hidden platform task, but were able to learn a visible platform task. G-Phos bright mice showed memory enhancement relative to wild type on the more difficult movable hidden platform spatial memory task. In the hippocampus, the G-Phos dull group showed a 50% greater transgenic GAP-43 protein level and a twofold elevated transgenic GAP-43 mRNA level than that measured in the G-Phos bright group. Unexpectedly, the dull group also showed an 80% reduction in hippocampal Tau1 staining. The high levels of GAP-43 seen here leading to memory impairment find its histochemical and behavioral parallel in the observation of Rekart et al. (Neuroscience 126: 579-584) who described elevated levels of GAP-43 protein in the hippocampus of Alzheimer's patients. The present data suggest that moderate overexpression of a phosphorylatable plasticity-related protein can enhance memory, while excessive overexpression may produce a "neuroplasticity burden" leading to degenerative and hypertrophic events culminating in memory dysfunction.

M-calpain-mediated cleavage of GAP-43 near Ser41 is negatively regulated by protein kinase C, calmodulin and calpain-inhibiting fragment GAP-43-3

Neuronal protein GAP-43 performs multiple functions in axon guidance, synaptic plasticity and regulation of neuronal death and survival. However, the molecular mechanisms of its action in these processes are poorly understood. We have shown that in axon terminals GAP-43 is a substrate for calcium-activated cysteine protease m-calpain, which participates in repulsion of axonal growth cones and induction of neuronal death. In pre-synaptic terminals in vivo, in synaptosomes, and in vitro, m-calpain cleaved GAP-43 in a small region near Ser41, on either side of this residue. In contrast, micro-calpain cleaved GAP-43 in vitro at several other sites, besides Ser41. Phosphorylation of Ser41 by protein kinase C or GAP-43 binding to calmodulin strongly suppressed GAP-43 proteolysis by m-calpain. A GAP-43 fragment, lacking about forty N-terminal residues (named GAP-43-3), was produced by m-calpain-mediated cleavage of GAP-43 and inhibited m-calpain, but not micro-calpain. This fragment prevented complete cleavage of intact GAP-43 by m-calpain as a negative feedback. GAP-43-3 also blocked m-calpain activity against casein, a model calpain substrate. This implies that GAP-43-3, which is present in axon terminals in high amount, can play important role in regulation of m-calpain activity in neurons. We suggest that GAP-43-3 and another (N-terminal) GAP-43 fragment produced by m-calpain participate in modulation of neuronal response to repulsive and apoptotic signals.

Engineered expression of polysialic acid enhances Purkinje cell axonal regeneration in L1/GAP-43 double transgenic mice

Purkinje axons in adult mammals are generally unable to regenerate after axotomy. Our recent work has shown that over-expression of growth related genes, GAP-43 and L1, in Purkinje cells increased their axonal outgrowth into a predegenerated peripheral nerve graft, but not into a fresh graft [Zhang et al., (2005) Proc. Natl Acad. Sci. USA, 102, 14883-14888]. In the current study we investigated whether engineered expression of growth permissive molecule polysialic acid (PSA) in the glial scar or on transplanted Schwann cells could overcome the inhibitory environment and promote Purkinje axonal regeneration. A stab wound was introduced in the cerebellum of the L1/GAP-43 transgenic mice and a lentiviral vector (LV) carrying the polysialyltransferase (PST) cDNA (LV/PST) was injected into the lesion site to transduce the cells in the glial scar. Regenerating Purkinje axons were examined by calbindin immunostaining. There was increased Purkinje axonal sprouting in the area expressing high-level PSA. However, Purkinje axons were unable to grow into the lesion cavity. In the second set of experiments when LV/PST transduced Schwann cells were transplanted into the lesion site, the number of Purkinje axons growing into the transplant was nine times more than that growing into Schwann cell transplant expressing GFP two months post operation. Our result suggests that transplanted Schwann cells engineered to express PSA support axonal regeneration better than naïve Schwann cells.

Lesion-Induced Axonal Sprouting in the Central Nervous System

Injury or neuronal death often come about as a result of brain disorders. Inasmuch as the damaged nerve cells are interconnected via projections to other regions of the brain, such lesions lead to axonal loss in distal target areas. The central nervous system responds to deafferentation by means of plastic remodeling processes, in particular by inducing outgrowth of new axon collaterals from surviving neurons (collateral sprouting). These sprouting processes result in a partial reinnervation, new circuitry, and functional changes within the deafferented brain regions. Lesioning of the entorhinal cortex is an established model system for studying the phenomenon of axonal sprouting. Using this model system, it could be shown that the sprouting process respects the pre-existing lamination pattern of the deafferented fascia dentata, i. e., it is layer-specific. A variety of different molecules are involved in regulating this reorganization process (extracellular matrix molecules, cell adhesion molecules, transcription factors, neurotrophic factors, growth-associated proteins). It is proposed here that molecules of the extracellular matrix define the boundaries of the laminae following entorhinal lesioning and in so doing limit the sprouting process to the deafferented zone. To illustrate the role of axonal sprouting in disease processes, special attention is given to its significance for neurodegenerative disorders, particularly Alzheimer's disease (AD), and temporal lobe epilepsy. Finally, we discuss both the beneficial as well as disadvantageous functional implications of axonal sprouting for the injured organism in question.

PACAP type I receptor transactivation is essential for IGF-1 receptor signalling and antiapoptotic activity in neurons

Insulin-like growth factor-1 (IGF-1) and pituitary adenylyl cyclase activating polypeptide (PACAP) are both potent neurotrophic and antiapoptotic factors, which exert their effects via phosphorylation cascades initiated by tyrosine kinase and G-protein-coupled receptors, respectively. Here, we have adapted a recently described phosphoproteomic approach to neuronal cultures to characterize the phosphoproteomes generated by these neurotrophic factors. Unexpectedly, IGF-1 and PACAP increased the phosphorylation state of a common set of proteins in neurons. Using PACAP type 1 receptor (PAC1R) null mice, we showed that IGF-1 transactivated PAC1Rs constitutively associated with IGF-1 receptors. This effect was mediated by Src family kinases, which induced PAC1R phosphorylation on tyrosine residues. PAC1R transactivation was responsible for a large fraction of the IGF-1-associated phosphoproteome and played a critical role in the antiapoptotic activity of IGF-1. Hence, in contrast to the general opinion that the trophic activity of IGF-1 is solely mediated by tyrosine kinase receptor-associated signalling, we show that it involves a more complex signalling network dependent on the PAC1 Gs-protein-coupled receptor in neurons.

Characterization of transcriptional regulation of neurogranin by nitric oxide and the role of neurogranin in SNP-induced cell death: implication of neurogranin in an increased neuronal susceptibility to oxidative stress

Neurogranin (Ng), a calmodulin (CaM)-binding protein kinase C (PKC) substrate, regulates the availability of Ca²⁺/CaM complex and modulates the homeostasis of intracellular calcium in neurons. Previous work showed Ng oxidation by NO donor induces increase in [Ca²⁺]_i. The current study demonstrated that the gene transcription of Ng could be up-regulated by various nitric oxide (NO) donors via a NO-soluble guanylyl cyclase (sGC)-mediated pathway. Furthermore, ectopic expression of neuronal nitric oxide synthase (nNOS) in human embryonic kidney 293 cells (HEK 293) exhibited a nNOS-concentration-dependent biphasic regulatory effect on Ng gene transcription. One of the NO donors, sodium nitroprusside (SNP), however, induced cell death of neuroblastoma Neuro-2a cells. The potency of SNP-induced cell death was shown to be higher in Neuro-2a cells expressing recombinant Ng, as compared with Neuro-2a control cells without Ng expression in cell viability and apoptosis assays. Single-cell fluorescence imaging and site-directed mutagenesis studies suggest that Ng promotes SNP-induced cell death through an amplification of calcium-mediated signaling, which requires the interaction between CaM and IQ motif of Ng. Increased neuronal susceptibility rendered by Ng in response to pathophysiological NO production is suggested to be involved in the selective vulnerability of neurons to oxidative insults in the CNS.

GAP-43 regulates NCAM-180-mediated neurite outgrowth

The neural cell adhesion molecule (NCAM), and the growth-associated protein (GAP-43), play pivotal roles in neuronal development and plasticity and possess interdependent functions. However, the mechanisms underlying the functional association of GAP-43 and NCAM have not been elucidated. In this study we show that (over)expression of GAP-43 in PC12E2 cells and hippocampal neurons strongly potentiates neurite extension, both in the absence and in the presence of homophilic NCAM binding. This potentiation is crucially dependent on the membrane association of GAP-43. We demonstrate that phosphorylation of GAP-43 by protein kinase C (PKC) as well as by casein kinase II (CKII) is important for the NCAM-induced neurite outgrowth. Moreover, our results indicate that in the presence of GAP-43, NCAM-induced neurite outgrowth requires functional association of NCAM-180/spectrin/GAP-43, whereas in the absence of GAP-43, the NCAM-140/non-receptor tyrosine kinase (Fyn)-associated signaling pathway is pivotal. Thus, expression of GAP-43 presumably acts as a functional switch for NCAM-180-induced signaling. This suggests that under physiological conditions, spatial and/or temporal changes of the localization of GAP-43 and NCAM on the cell membrane may determine the predominant signaling mechanism triggered by homophilic NCAM binding: NCAM-180/spectrin-mediated modulation of the actin cytoskeleton, NCAM-140-mediated activation of Fyn, or both.

Regulation of intrinsic neuronal properties for axon growth and regeneration

Regulation of neuritic growth is crucial for neural development, adaptation and repair. The intrinsic growth potential of nerve cells is determined by the activity of specific molecular sets, which sense environmental signals and sustain structural extension of neurites. The expression and function of these molecules are dynamically regulated by multiple mechanisms, which adjust the actual growth properties of each neuron population at different ontogenetic stages or in specific conditions. The neuronal potential for axon elongation and regeneration are restricted at the end of development by the concurrent action of several factors associated with the final maturation of neurons and of the surrounding tissue. In the adult, neuronal growth properties can be significantly modulated by injury, but they are also continuously tuned in everyday life to sustain physiological plasticity. Strict regulation of structural remodelling and neuritic elongation is thought to be required to maintain specific patterns of connectivity in the highly complex mammalian CNS. Accordingly, procedures that neutralize such mechanisms effectively boost axon growth in both intact and injured nervous system. Even in these conditions, however, aberrant connections are only formed in the presence of unusual external stimuli or experience. Therefore, growth regulatory mechanisms play an essentially permissive role by setting the responsiveness of neural circuits to environmental stimuli. The latter exert an instructive action and determine the actual shape of newly formed connections. In the light of this notion, efficient therapeutic interventions in the injured CNS should combine targeted manipulations of growth control mechanisms with task-specific training and rehabilitation paradigms.

Suppression of astroglial scar formation and enhanced axonal regeneration associated with functional recovery in a spinal cord injury rat model by the cell cycle inhibitor olomoucine

It is well established that axons of the adult mammalian CNS are capable of regrowing only a limited amount after injury. Astrocytes are believed to play a crucial role in the failure to regenerate, producing multiple inhibitory proteoglycans, such as chondroitin sulphate proteoglycans (CSPGs). After spinal cord injury (SCI), astrocytes become hypertrophic and proliferative and form a dense network of astroglial processes at the site of lesion constituting a physical and biochemical barrier. Down-regulations of astroglial proliferation and inhibitory CSPG production might facilitate axonal regeneration. Recent reports indicated that aberrant activation of cell cycle machinery contributed to overproliferation and apoptosis of cells in various insults. In the present study, we sought to determine whether a cell cycle inhibitior, olomoucine, would decrease neuronal cell death, limit astroglial proliferation and production of inhibitory CSPGs, and eventually enhance the functional compensation after SCI in rats. Our results showed that up-regulations of cell cycle components were closely associated with neuronal cell death and astroglial proliferation as well as the production of CSPGs after SCI. Meanwhile, administration of olomoucine, a selective cell cycle kinase (CDK) inhibitor, has remarkably reduced the up-regulated cell cycle proteins and then decreased neuronal cell death, astroglial proliferation, and accumulation of CSPGs. More importantly, the treatment with olomoucine has also increased expression of growth-associated proteins-43, reduced cavity formation, and improved functional deficits. We consider that suppressing astroglial cell cycle in acute SCIs is beneficial to axonal growth. In the future, therapeutic strategies can be designed to achieve efficient axonal regeneration and functional compensation after traumatic CNS injury.

Transplantation of preconditioned schwann cells in peripheral nerve grafts after contusion in the adult spinal cord. Improvement of recovery in a rat model

BACKGROUND

Recovery after injury to the peripheral nervous system is based on the pro-regenerative relationship between axons and the extracellular matrix, a relationship established by Schwann cells. As mechanical conditioning of Schwann cells has been shown to stimulate their regenerative behavior, we sought to determine whether transplantation of these cells to the central nervous system (i.e., the spinal cord), with its limited regenerative capacity after injury, would improve axonal regeneration and functional recovery.

METHODS

A moderate contusion injury of the spinal cord was created with a force-directed impactor in forty-eight adult Sprague-Dawley rats, and, at one week postinjury, the spinal cords were reexposed in all animals. In twenty-four of these animals, peripheral nerve grafts with Schwann cells that had been obtained from the sciatic nerves of donor animals, and had been either untreated or subjected to mechanical conditioning, were transplanted to the contused area of the cords following resection of the glial scar. Another group of animals was treated with glial scar excision only, and a fourth group had the contusion injury but neither glial excision nor transplantation. Scores according to the Basso, Beattie, Bresnahan (BBB) Locomotor Rating Scale were assigned preoperatively and weekly thereafter. Tract tracing of descending and ascending spinal cord tracts was performed at six weeks postoperatively for quantitative histological evaluation of axonal regeneration.

RESULTS

While the recovery following glial scar excision without peripheral nerve transplantation was significantly worse than the recovery in the other groups, both transplantation groups had significantly higher BBB scores than the controls (no transplantation) in the early postoperative period (p < 0.05). Moreover, histological analysis showed markedly increased axonal regeneration at the lesional sites in the animals treated with the mechanically conditioned grafts than in the other groups (p < 0.05).

CONCLUSIONS

Functional recovery after spinal cord contusion improved following glial scar excision with transplantation of Schwann cells in peripheral nerve grafts to the contusion areas. Although recovery did not differ significantly between the transplantation groups, only the preconditioned grafts led to axonal regeneration at and past the lesional site. These grafts may further enhance functional recovery as the descending tracts eventually reach their target end-organs.

Regeneration following spinal cord injury, from experimental models to humans: where are we?

Regeneration in the adult CNS following injury is extremely limited. Traumatic spinal cord injury causes a permanent neurological deficit followed by a very limited recovery due to failed regeneration attempts. In fact, it is now clear that the spinal cord intrinsically has the potential to regenerate, but cellular loss and the presence of an inhibitory environment strongly limit tissue regeneration and functional recovery. The molecular mechanisms responsible for failed regeneration are starting to be unveiled. This gain in knowledge led to the design of therapeutic strategies aimed to limit the tissue scar, to enhance the proregeneration versus the inhibitory environment, and to replace tissue loss, including the use of stem cells. They have been very successful in several animal models, although results are still controversial in humans. Nonetheless, novel experimental approaches hold great promise for use in humans.

Role of the neurogranin concentrated in spines in the induction of long-term potentiation

Synaptic plasticity in CA1 hippocampal neurons depends on Ca2+ elevation and the resulting activation of calmodulin-dependent enzymes. Induction of long-term depression (LTD) depends on calcineurin, whereas long-term potentiation (LTP) depends on Ca2+/calmodulin-dependent protein kinase II (CaMKII). The concentration of calmodulin in neurons is considerably less than the total concentration of the apocalmodulin-binding proteins neurogranin and GAP-43, resulting in a low level of free calmodulin in the resting state. Neurogranin is highly concentrated in dendritic spines. To elucidate the role of neurogranin in synaptic plasticity, we constructed a computational model with emphasis on the interaction of calmodulin with neurogranin, calcineurin, and CaMKII. The model shows how the Ca2+ transients that occur during LTD or LTP induction affect calmodulin and how the resulting activation of calcineurin and CaMKII affects AMPA receptor-mediated transmission. In the model, knockout of neurogranin strongly diminishes the LTP induced by a single 100 Hz, 1 s tetanus and slightly enhances LTD, in accord with experimental data. Our simulations show that exchange of calmodulin between a spine and its parent dendrite is limited. Therefore, inducing LTP with a short tetanus requires calmodulin stored in spines in the form of rapidly dissociating calmodulin-neurogranin complexes.

Characterization of the domains of zRICH, a protein induced during optic nerve regeneration in zebrafish

Teleost fish show a remarkable capability of nerve regeneration in their CNS, while injuries to axon fibers in the CNS of mammals result in degeneration and loss of function. Understanding this difference has biomedical consequences to humans. Both extrinsic factors from the neuronal environment and intrinsic neuronal factors seem to play a role in successful nerve regeneration. Among the intrinsic factors, a number of proteins termed axonal growth associated proteins (GAPs) are strongly induced during axon regeneration. RICH proteins are axonal GAPs that show homology to mammalian myelin marker proteins termed CNPases. Sequence analysis distinguishes three domains in these proteins. In this report, mutant versions of zebrafish RICH proteins were generated to study the roles of the domains of the protein at biochemical and cellular levels. The central CNPase homology domain was sufficient for catalytic activity. The amino terminal acidic domain causes the anomalous electrophoretic migration observed for RICH proteins. The small C-terminal domain bears an isoprenylation motif and is necessary for the interaction of zRICH with cellular membranes. At the cellular level, expression of wild-type zRICH protein in PC12 cells did not induce neurite generation. Additionally, neither the expression of wild-type zRICH nor the expression of mutant versions of the protein interfered with the levels of differentiation of PC12 cells induced by nerve growth factor, suggesting that, at least in this model of neuronal differentiation, zRICH proteins do not participate in the process of generation of neurites.

Chemorepellent axon guidance molecules in spinal cord injury

Regenerating axons stop growing when they reach the border of the glial-fibrotic scar, presumably because they encounter a potent molecular barrier inhibiting growth cone advance. Chemorepulsive axon guidance molecules provide a non-permissive environment restricting and channeling axon growth in the developing nervous system. These molecules could also act as growth-inhibitory molecules in the regenerating nervous system. The receptors for repulsive guidance cues are expressed in the mature nervous system, suggesting that adult neurons are sensitive to the activity of developmentally active repulsive proteins. In this review, we summarize recent observations on semaphorins, ephrins, and slits in the injured brain and spinal cord, providing evidence that these proteins are major players in inhibiting axonal regeneration and establishing the glial-fibrotic scar.

Growth-associated protein 43 in lesions and cerebrospinal fluid in multiple sclerosis

Axonal damage in multiple sclerosis (MS) is correlated to disease progression. Early axonal damage may be compensated for by regenerative processes. Growth-associated protein 43 (GAP-43) is a marker for axonal growth and synaptogenesis in various neurodegenerative diseases. We investigated the expression of GAP-43 in 48 MS grey and white matter lesions of different stages. Decreased GAP-43 expression was found in 74% of the white matter lesions, independent of the lesion stage. In 19 out of 35 white matter lesions, areas of increased GAP-43 expression were present immediately adjacent to the lesions. Increased or unaltered expression was observed in remyelinated lesions. GAP-43 was expressed in neurofilament-positive structures. GAP-43 expression appeared unchanged in grey matter lesions. Macrophages were present in the areas of changed GAP-43 expression. cerebrospinal fluid GAP-43 levels were negatively correlated with magnetic resonance imaging measures of whole-brain atrophy (r = -0.30). In conclusion, these results indicate that decreased GAP-43 immunopositivity reflects axonal damage in MS lesions, which may again be reflected in decreased cerebrospinal fluid levels. The increased levels of GAP-43 in remyelinated or nondemyelinated white matter close to MS lesions may reflect regenerative attempts by damaged axons.

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.

AAV-mediated expression of CNTF promotes long-term survival and regeneration of adult rat retinal ganglion cells

We compared the effects of intravitreal injection of bi-cistronic adeno-associated viral (AAV-2) vectors encoding enhanced green fluorescent protein (GFP) and either ciliary neurotrophic factor (CNTF), brain-derived neurotrophic factor (BDNF) or growth-associated protein-43 (GAP43) on adult retinal ganglion cell (RGC) survival and regeneration following (i) optic nerve (ON) crush or (ii) after ON cut and attachment of a peripheral nerve (PN). At 7 weeks after ON crush, quantification of betaIII-tubulin immunostaining revealed that, compared to AAV-GFP controls, RGC survival was not enhanced by AAV-GAP43-GFP but was increased in AAV-CNTF-GFP (mean RGCs/retina: 17 450+/-358 s.e.m.) and AAV-BDNF-GFP injected eyes (10 200+/-4064 RGCs/retina). Consistent with increased RGC viability in AAV-CNTF-GFP and AAV-BDNF-GFP injected eyes, these animals possessed many betaIII-tubulin- and GFP-positive fibres proximal to the ON crush. However, only in the AAV-CNTF-GFP group were regenerating RGC axons seen in distal ON (1135+/-367 axons/nerve, 0.5 mm post-crush), some reaching the optic chiasm. RGCs were immunoreactive for CNTF and quantitative RT-PCR revealed a substantial increase in CNTF mRNA expression in retinas transduced with AAV-CNTF-GFP. The combination of AAV-CNTF-GFP transduction of RGCs with autologous PN-ON transplantation resulted in even greater RGC survival and regeneration. At 7 weeks after PN transplantation there were 27 954 (+/-2833) surviving RGCs/retina, about 25% of the adult RGC population. Of these, 13 352 (+/-1868) RGCs/retina were retrogradely labelled after fluorogold injections into PN grafts. In summary, AAV-mediated expression of CNTF promotes long-term survival and regeneration of injured adult RGCs, effects that are substantially enhanced by combining gene and cell-based therapies/interventions.

PKCepsilon increases endothelin converting enzyme activity and reduces amyloid plaque pathology in transgenic mice

Deposition of plaques containing amyloid beta (Abeta) peptides is a neuropathological hallmark of Alzheimer's disease (AD). Here we demonstrate that neuronal overexpression of the epsilon isozyme of PKC decreases Abeta levels, plaque burden, and plaque-associated neuritic dystrophy and reactive astrocytosis in transgenic mice expressing familial AD-mutant forms of the human amyloid precursor protein (APP). Compared with APP singly transgenic mice, APP/PKCepsilon doubly transgenic mice had decreased Abeta levels but showed no evidence for altered cleavage of APP. Instead, PKCepsilon overexpression selectively increased the activity of endothelin-converting enzyme, which degrades Abeta. The activities of other Abeta-degrading enzymes, insulin degrading enzyme and neprilysin, were unchanged. These results indicate that increased neuronal PKCepsilon activity can promote Abeta clearance and reduce AD neuropathology through increased endothelin-converting enzyme activity.

Actions of neurotrophic factors and their signaling pathways in neuronal survival and axonal regeneration

Adult axons in the mammalian central nervous system do not elicit spontaneous regeneration after injury, although many affected neurons have survived the neurotrauma. However, axonal regeneration does occur under certain conditions. These conditions include: (a) modification of regrowth environment, such as supply of peripheral nerve bridges and transplantation of Schwann cells or olfactory ensheathing glia to the injury site; (b) application of neurotrophic factors at the cell soma and axon tips; (c) blockade of growth-inhibitory molecules such as Nogo-A, myelin-associated glycoprotein, and oligodendrocyte-myelin glycoprotein; (d) prevention of chondroitin-sulfate-proteoglycans-related scar tissue formation at the injury site using chondroitinase ABC; and (e) elevation of intrinsic growth potential of injured neurons via increasing intracellular cyclic adenosine monophosphate level. A large body of evidence suggests that these conditions achieve enhanced neuronal survival and axonal regeneration through sometimes overlapping and sometimes distinct signal transduction mechanisms, depending on the targeted neuronal populations and intervention circumstances. This article reviews the available information on signal transduction pathways underlying neurotrophic-factor-mediated neuronal survival and neurite outgrowth/axonal regeneration. Better understanding of signaling transduction is important in helping us develop practical therapeutic approaches for encouraging neuronal survival and axonal regeneration after traumatic injury in clinical context.