Molecular analysis of the function of the neuronal growth-associated protein GAP-43 by genetic intervention

Selective inhibition of cannabinoid CB1 receptor-evoked signalling by the interacting protein GAP43

Cannabinoids exert pleiotropic effects on the brain by engaging the cannabinoid CB1 receptor (CB1R), a presynaptic metabotropic receptor that regulates key neuronal functions in a highly context-dependent manner. We have previously shown that CB1R interacts with growth-associated protein of 43 kDa (GAP43) and that this interaction inhibits CB1R function on hippocampal excitatory synaptic transmission, thereby impairing the therapeutic effect of cannabinoids on epileptic seizures in vivo. However, the underlying molecular features of this interaction remain unexplored. Here, we conducted mechanistic experiments on HEK293T cells co-expressing CB1R and GAP43 and show that GAP43 modulates CB1R signalling in a strikingly selective manner. Specifically, GAP43 did not affect the archetypical agonist-evoked (i) CB1R/Gi/o protein-coupled signalling pathways, such as cAMP/PKA and ERK, or (ii) CB1R internalization and intracellular trafficking. In contrast, GAP43 blocked an alternative agonist-evoked CB1R-mediated activation of the cytoskeleton-associated ROCK signalling pathway, which relied on the GAP43-mediated impairment of CB1R/Gq/11 protein coupling. GAP43 also abrogated CB1R-mediated ROCK activation in mouse hippocampal neurons, and this process led in turn to a blockade of cannabinoid-evoked neurite collapse. An NMR-based characterization of the CB1R-GAP43 interaction supported that GAP43 binds directly and specifically through multiple amino acid stretches to the C-terminal domain of the receptor. Taken together, our findings unveil a CB1R-Gq/11-ROCK signalling axis that is selectively impaired by GAP43 and may ultimately control neurite outgrowth.

A Shift from a Pivotal to Supporting Role for the Growth-Associated Protein (GAP-43) in the Coordination of Axonal Structural and Functional Plasticity

In a number of animal species, the growth-associated protein (GAP), GAP-43 (aka: F1, neuromodulin, B-50, G50, pp46), has been implicated in the regulation of presynaptic vesicular function and axonal growth and plasticity via its own biochemical properties and interactions with a number of other presynaptic proteins. Changes in the expression of GAP-43 mRNA or distribution of the protein coincide with axonal outgrowth as a consequence of neuronal damage and presynaptic rearrangement that would occur following instances of elevated patterned neural activity including memory formation and development. While functional enhancement in GAP-43 mRNA and/or protein activity has historically been hypothesized as a central mediator of axonal neuroplastic and regenerative responses in the central nervous system, it does not appear to be the crucial substrate sufficient for driving these responses. This review explores the historical discovery of GAP-43 (and associated monikers), its transcriptional, post-transcriptional and post-translational regulation and current understanding of protein interactions and regulation with respect to its role in axonal function. While GAP-43 itself appears to have moved from a pivotal to a supporting factor, there is no doubt that investigations into its functions have provided a clearer understanding of the biochemical underpinnings of axonal plasticity.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Response of GAP-43 and p75 in human neuromas over time after traumatic injury

OBJECTIVE

GAP-43 and p75 are proteins that promote growth cone and neurite formation, elongation, and arborization in regenerating nerve axons. The objectives of this study were to determine whether GAP-43 and the low-affinity nerve growth factor receptor p75 are elevated in traumatic neuromas and whether there is a correlation between the relative amount of GAP-43 or p75 and demographic characteristics such as time elapsed between injury and repair.

METHODS

Traumatic neuromas from 21 randomly selected patients were studied, and the charts were reviewed. Specimens were collected at the time of nerve resection and grafting. Immunohistochemical analysis was performed on each sample and normal human nerve with antibodies to GAP-43 and p75. Western blot and computerized gel analyses were performed.

RESULTS

All neuroma specimens harvested within 13 months of injury exhibited markedly elevated GAP-43 levels compared with normal nerve. Specimens harvested at 14 months or more after injury showed precipitously lower GAP-43 levels, similar to or less than those of normal nerve. The correlation between the amount of intra-axonal GAP-43 and postinjury time interval was statistically significant, P = 0.0038. High GAP-43 levels were also correlated with transection injury, high postoperative sensory grade, and pain. p75 levels were elevated, without consistent variation in our population.

CONCLUSION

These preliminary data suggest that the expression of intra-axonal GAP-43 may vary over time after injury, remaining elevated for approximately the first year, then decreasing abruptly to normal or subnormal levels. These results correlate with clinical experience, indicating that peripheral nerves should be repaired relatively early if repair is indicated.

Neuron tolerance during hydrocephalus

Whether or not neuron death plays a major role in pathophysiology during hydrocephalus is not well known. The goals of this study were to determine if neural degeneration occurred during hydrocephalus, and to determine if neuron tolerance developed during this pathophysiologic procedure.Neural damage as visualized by a sensitive staining technique, silver impregnation, was observed in three experimental groups: (1) adult hydrocephalic rats induced by kaolin injection into the cisterna magna, (2) adult rats with chronic hydrocephalus for 10 weeks subjected to acute forebrain ischemia induced by four-vessel occlusion, and (3) adult rats without hydrocephalus subjected to acute forebrain ischemia. The magnitude of hydrocephalus was also evaluated during this time. In mild or moderate hydrocephalus, little cell death was found. In severe hydrocephalus, axon and neuropil degeneration was extensively distributed, but cell death was still rarely observed. Although some neuron degeneration was found after acute forebrain ischemia in hydrocephalic rats, the extensive cell death in cortical layers III and V, and in hippocampal areas CA1 and CA4 that is commonly observed in the ischemic brain without hydrocephalus, was not seen. This study suggests that neuron death was not a major pathological change in the brain during hydrocephalus, with cerebral ventricles being enlarged during the development of hydrocephalus. Less neuron death in hydrocephalic rats after acute forebrain ischemia suggests that neuronal tolerance to ischemia occurs during hydrocephalus.

Regulation of GAP-43 protein and mRNA in nigrostriatal dopaminergic neurons after the partial destruction of dopaminergic terminals with intrastriatal 6-hydroxydopamine

Changes in the level of GAP-43 and its mRNA in nigrostriatal dopaminergic neurons in an animal model of the presymptomatic period of Parkinson's disease were measured to find the characteristic features of GAP-43 in nigrostriatal dopaminergic neurons. Since the dopaminergic neurons possess a relatively large amount of GAP-43 protein and mRNA, the dopaminergic neurons must be endowed with specific functions related to those of GAP-43. In this study, dopaminergic axon terminals were partially destroyed by intrastriatal 6-hydroxydopamine (6-OHDA). Rats were decapitated 3, 14, and 56 days following treatment. Levels of GAP-43 and tyrosine hydroxylase (TH) in the striatum were detected by immunoblotting and quantified. The number of GAP-43 mRNA-positive neurons and that of TH mRNA-positive neurons in the substantia nigra pars compacta (SNc) were detected by in situ hybridization using alkaline phosphatase (ALP)-labeled probes. Levels of GAP-43 in the striatum showed no significant alteration during the period of the experiment, although levels of TH were gradually restored. The number of GAP-43 mRNA-positive neurons as well as that of TH mRNA-positive neurons in the SNc decreased. These results suggests that dopaminergic neurons restore their axon terminals with little change in GAP-43, and that transcription and/or stability of GAP-43 mRNA in the dopaminergic neurons are susceptible to the toxin, although the dopaminergic neurons can maintain the translational product in the terminals. This feature may be related with a degeneration of dopaminergic neurons in Parkinson's disease.

HSV-1-based vectors for gene therapy of neurological diseases and brain tumors: part II. Vector systems and applications

Many properties of HSV-1 are especially suitable for using this virus as a vector to treat diseases affecting the central nervous system (CNS), such as Parkinson's disease or malignant gliomas. These advantageous properties include natural neurotropism, high transduction efficiency, large transgene capacity, and the ability of entering a latent state in neurons. Selective oncolysis in combination with modulation of the immune response mediated by replication-conditional HSV-1 vectors appears to be a highly promising approach in the battle against malignant glioma. Helper virus-free HSV/AAV hybrid amplicon vectors have great promise in mediating long-term gene expression in the PNS and CNS for the treatment of various neurodegenerative disorders or chronic pain. Current research focuses on the design of HSV-1-derived vectors which are targeted to certain cell types and support transcriptionally regulatable transgene expression. Here, we review the recent developments on HSV-1-based vector systems and their applications in experimental and clinical gene therapy protocols.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

B-50/growth-associated protein-43, a marker of neural development in Xenopus laevis

To study the regulation and function of the growth-associated protein B-50/growth-associated protein-43 (mol. wt 43,000) in Xenopus laevis, B-50/growth-associated protein-43 complementary DNAs were isolated and characterized. The deduced amino acid sequence revealed potential functional domains of Xenopus B-50/growth-associated protein-43 that may be involved in G-protein interaction, membrane-binding, calmodulin-binding and protein kinase C phosphorylation. The expression of B-50/growth-associated protein-43 at the RNA and protein level during development was investigated using the Xenopus complementary DNA and the monoclonal B-50/growth-associated protein-43 antibody NM2. The antibody NM2 recognized the gene product on western blot and in whole-mount immunocytochemistry of Xenopus embryos. Moreover, visualization of the developmentally regulated appearance of B-50/growth-associated protein-43 immunoreactivity showed that this mode of detection may be used to monitor axonogenesis under various experimental conditions. In the adult Xenopus, XB-50/growth-associated protein-43 messenger RNA was shown to be expressed at high levels in brain, spinal cord and eye using northern blotting. The earliest expression detected on northern blot was at developmental stage 13 with poly(A) RNA. By whole-mount immunofluorescence, applying the confocal laser scanning microscope, the protein was first detected in embryos from stage 20, where it was expressed in the developing trigeminal ganglion. Also later in development the expression of the B-50/growth-associated protein-43 gene was restricted to the nervous system in Xenopus, as was previously found for the mouse. In conclusion, we find that XB-50/growth-associated protein-43 is a good marker to study the development of the nervous system in Xenopus laevis.

Analysis of the role of calmodulin binding and sequestration in neuromodulin (GAP-43) function

We demonstrated previously that forced expression of the neuronal phosphoprotein neuromodulin (also known as GAP-43, F1, B-50, and p57) in mouse anterior pituitary AtT-20 cells enhances depolarization-mediated secretion and alters cellular morphology. Here we analyze the role of calmodulin binding by neuromodulin in these responses. In cells expressing wild-type neuromodulin, a complex with calmodulin that is sensitive to intracellular calcium and phosphorylation is localized to the plasma membrane. Transfection of several mutant forms of neuromodulin shows that the effects of this protein on secretion are dependent on both calmodulin binding and association with the plasma membrane. In contrast, the morphological changes depend only on membrane association. Thus, the multitude of effects of neuromodulin noted in previous studies may result from divergent properties of this protein.

Distribution of GAP-43 mRNA in the immature and adult cerebellum: a role for GAP-43 in cerebellar development and neuroplasticity

Expression of GAP-43 mRNA in the rat cerebellum and inferior olivary nucleus was examined at birth, during postnatal development and in the adult by both Northern and in situ hybridization. Northern blot analysis revealed that cerebellar GAP-43 mRNA expression increases from birth to postnatal day (PD) 7 and then declines to a lower level in the adult. At birth, in situ hybridization experiments showed intense labeling of GAP-43 mRNA in the premigratory, but not the germinal, zone of the cerebellar external granule cell layer. Localization of GAP-43 within the premigratory zone, a layer containing post-mitotic granule cells, indicates that granule cells begin expressing GAP-43 mRNA after final mitosis and during axonal outgrowth of the parallel fibers. The deep cerebellar nuclei and the inferior olive were also intensely labeled at birth. GAP-43 mRNA was localized in granule cells during their migration through the molecular layer of the developing cerebellum and after their arrival in the internal granule cell layer. By PD 21, the pattern of GAP-43 expression was similar to that observed in the adult; GAP-43 mRNA was localized to the internal granule layer and the inferior olive with minimal to no hybridization in the deep cerebellar nuclei and none in the molecular layer. Purkinje cells were devoid of GAP-43 mRNA throughout the postnatal and adult periods. In light of our observations, we propose that GAP-43 is a critical factor in granule cell differentiation/migration, as well as in the parallel and climbing fiber axonal outgrowth and synaptogenesis during development. Localization of GAP-43 mRNA within granule and inferior olivary cells of adult animals indicates that GAP-43 protein observed in the molecular layer is transported from these cells to their terminals in the molecular layer suggesting that GAP-43 is also an intrinsic presynaptic determinant in cerebellar neuroplasticity.

Differential expression of p140trk, p75NGFR and growth-associated phosphoprotein-43 genes in nucleus basalis magnocellularis, thalamus and adjacent cortex following neocortical infarction and nerve growth factor treatment

A loss of target-derived neurotrophic factors is hypothesized to be one of the major determinants of central nervous system neuronal degeneration. In order to obtain further insight into early neuronal responses to injury, lesion-induced alterations in the expression of high- and low-affinity nerve growth factor receptors, as well as growth-associated phosphoprotein-43 genes in nucleus basalis magnocellularis, thalamic and neocortical neurons were studied. For this purpose, unilateral cortical devascularization operations were conducted on adult rats. Animals received i.c.v. infusions of vehicle or nerve growth factor (12 micrograms/day) and were killed at one, three, seven and 15 days post-lesion. In situ hybridization studies using 35S-labelled oligonucleotide probes for p75NGFR, p140trk and growth-associated phosphoprotein-43 messenger RNAs reveals that these genes were differentially regulated following the lesion. In the nucleus basalis magnocellularis ipsilateral to the lesion, p140trk gene expression significantly decreased on days 3 and 7, while p75NGFR messenger RNA initially increased on day 3 and decreased on days 7 and 15 after lesion. GAP-43 messenger RNA levels were significantly increased in the nucleus basalis magnocellularis on post-lesion days 3 and 7. Moreover, in contrast to p75NGFR or 140trk, growth-associated phosphoprotein-43 messenger RNA levels were significantly increased in pyramidal neurons located in the remaining cortex adjacent to the cortical lesion at all time points. In the lateral and ventroposterior nuclei of the thalamus, growth-associated phosphoprotein-43 messenger RNA level was slightly increased on days 1 and 3 and was dramatically decreased, significantly below the levels in sham-operated controls, on post-lesion days 7 and 15. During nerve growth factor application, the level of p140trk messenger RNA in the lesioned nucleus basalis magnocellularis returned to values observed in the contralateral nucleus basalis magnocellularis while p75NGFR messenger RNA was increased above values noted in all animals not treated with nerve growth factor. Nerve growth factor treatment did not affect the expression of growth-associated phosphoprotein-43 messenger RNA in any of the areas studied. p140trk messenger RNA was not up-regulated during the time that nerve growth factor was applied, as observed for p75NGFR, but only eight days after interrupting nerve growth factor treatment. Three cell types, nucleus basalis magnocellularis, cortical pyramidal and thalamic neurons, were probably affected in different ways by the devascularization with respect to lesion extent. Consequently, the remaining number of synaptic contacts in each of these brain areas is most likely different which may lead to a differential regulation of growth-associated phosphoprotein-43 messenger RNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical study of the differentiation process of the septal organ of Masera in developing rats

The septal organ of Masera is a small patch of olfactory epithelium located near the base of the nasal septum. Using the growth-associated protein B-50/GAP-43 as neuronal marker, we have studied the differentiation process of this organ from the olfactory sheet in embryonic and newborn rats. Results show that the septal organ first appeared at embryonic day 16. Even though it was included in the olfactory sheet, the presumptive septal organ could be distinguished by a higher density of B-50/GAP-43-positive neurons. Concomitantly to its morphological development, the septal organ progressively isolated from the main olfactory epithelium. This isolation resulted from the extension of a transitional area which progressively lost its typical features of olfactory epithelium to become a putative respiratory epithelium in late embryonic stages. Results strongly suggest that the septal organ should be a proper chemosensory system with its own time-course of development.

Rapid and stable gene expression in hippocampal slice cultures from a defective HSV-1 vector

Stable transfer of genetic information into neurons is a powerful strategy to elucidate specific mechanisms of neurophysiology and to develop therapies for neurological disorders. To evaluate the optimal parameters for efficient gene delivery of defective herpes simplex virus type one (HSV-1) vectors into a specific brain region, an HSV-1 vector expressing E. coli beta-galactosidase was used to infect organotypic cultures of hippocampal slices. beta-Galactosidase was expressed as early as 2 h after infection in a dose-dependent manner as measured on immunoblots, and reached a maximum level after approximately 35 h. Expression of the RNA and the antigen was still evident after the longest time sampled (11-12 days), whereas no beta-galactosidase was ever detected in cultured slices infected with a control virus lacking the reporter gene. Hippocampal cells expressing the reporter gene outlined the contour of the neuronal cell body layers in fields CA3 and dentate gyrus; such correspondence was less evident in field CA1. Anatomical, morphological, and immunohistochemical criteria also confirmed that the majority of these infected cells were neurons. beta-Galactosidase was also detected in the somata and processes of infected interneurons. Tests for synaptic pathology associated with virus infection showed no changes in pre- and postsynaptic markers.

Developmental changes in GAP-43 expression in primary cultures of rat cerebellar granule cells

GAP-43 is a growth-associated protein that has been implicated in the developmental outgrowth of axons. We have examined the profile of GAP-43 levels in rat cerebellar granule cells during their development in vitro. During the first 1-2 days after plating, the majority of cells expressed neurites and after 8 days a complex neuronal network had developed. In situ hybridization studies showed that GAP-43 mRNA levels rapidly increased to peak at 1-2 days and gradually returned to initial values after 7-8 days. Analysis of GAP-43 protein levels followed a similar transient profile. Initially, granule cell perikarya and structures associated with neuritogenesis all displayed GAP-43 immunoreactivity. In older cultures, perikaryal labelling was lost after 10 days whilst process staining decreased more gradually. During the first 48 hours detailed analysis of GAP-43 mRNA revealed two populations of granule cells. It was suggested that cells with significant label originated from the external germinal layer which displays much GAP-43 mRNA in cerebellar sections. Cells with little or no GAP-43, however, probably originated from the internal granular layer since this region displayed no specific labelling. Granule cells within clumps expressed more GAP-43 mRNA compared to isolated cells perhaps indicating cell-cell regulation of expression. These results describe the transient rise in GAP-43 protein and mRNA levels expressed by developing cerebellar granule cell neurons in vitro and provide further evidence for the role GAP-43 plays during neuritogenesis.

B-50/GAP-43 expression by the olfactory receptor cells and the neurons migrating from the olfactory placode in embryonic rats

B-50/GAP-43 is a growth-associated phosphoprotein that is commonly expressed in all developing neuronal systems. Using an immunocytochemistry approach, we have investigated the expression of this protein in the rat olfactory system during embryogenesis and neonatal development with a particular emphasis on the early developmental stages of the olfactory placode. Data show that already at embryonic day 12 (E12), a strong B-50/GAP-43 immunoreactivity was detected in few olfactory receptor cells well-recognizable by their positive short neuritic processes. The B-50/GAP-43 expression in the placodal epithelium thus appeared to coincide with the onset of neurite outgrowth. From E13 onwards, there was a rapid increase in the number of B-50/GAP-43-positive olfactory neurons and from E18, the protein was strongly expressed by nearly all neurons. In addition, results clearly demonstrate that as early as E13, B-50/GAP-43 was strongly expressed by many migrating cells which were seen leaving the pit epithelium in association with the first olfactory axons that penetrated the nasal mesenchyme. Many immunoreactive cells were also observed in the presumptive olfactory nerve layer. Experiments of double-labeling showed that B-50/GAP-43-immunostained migrating cells were also stained with anti-neuron-specific enolase (NSE). This confirms the neuronal nature of these early labeled migrating cells. The progressive disappearance of migrating neurons noted during the late stages of embryonic development is discussed in relation with their possible function in the early stages of development of the peripheral olfactory system.

Phosphorylation of neuromodulin in rat striatum after acute and repeated, intermittent amphetamine

Repeated, intermittent treatment of rats with amphetamine results in a sensitization of locomotor and stereotyped behaviors that is accompanied by an enhancement in stimulus-induced dopamine release. Increased phosphorylation of the neural specific calmodulin-binding protein, neuromodulin (GAP-43, B-50, F1) has been demonstrated in other forms of synaptic plasticity and plays a role in neurotransmitter release. To determine whether neuromodulin phosphorylation was altered during amphetamine sensitization, the in vivo phosphorylated state of neuromodulin was examined in rat striatum in a post hoc phosphorylation assay. Female, Holtzman rats received saline or 2.5 mg/kg amphetamine twice weekly for 5 weeks. One week after the last dose of amphetamine, rats were challenged with either 1 mg/kg or 2.5 mg/kg amphetamine or saline and the rats were sacrificed 30 min later. Purified synaptic plasma membranes were prepared in the presence of EGTA and okadaic acid to inhibit dephosphorylation, and were subsequently phosphorylated in the presence of purified protein kinase C and [gamma-32P]ATP. The protein kinase C-mediated post hoc phosphorylation of neuromodulin was significantly reduced in groups that received either acute or repeated amphetamine suggesting that neuromodulin in those groups contained more endogenous phosphate. The acute, challenge dose of amphetamine increased neuromodulin phosphorylation in the saline-treated controls but not in the repeated amphetamine-pretreated group. Anti-neuromodulin immunoblots showed no change in neuromodulin levels in any group. There was no significant change in protein kinase C activity in any treatment group. To further investigate the effect of acute amphetamine, the ability of amphetamine to alter neuromodulin phosphorylation in 32Pi-preincubated Percoll-purified rat striatal synaptosomes was examined. Amphetamine (10 microM) significantly increased phosphorylation of a 53 kDa band that migrated with authentic neuromodulin in the synaptosomes by 22% while 500 nM 12-O-tetradecanoylphorbol 13-acetate (TPA) increased neuromodulin phosphorylation by 45%. These data suggest that one injection of amphetamine can increase neuromodulin phosphorylation in rat striatum and that this increase is maintained for at least 1 week following a repeated, sensitizing regimen of amphetamine. Since sensitization can be induced with one dose of amphetamine, it is possible that enhanced neuromodulin phosphorylation could contribute to neurochemical events leading to enhanced release of dopamine and/or behavioral sensitization.

Posttranscriptional regulation of GAP-43 gene expression in PC12 cells through protein kinase C-dependent stabilization of the mRNA

We have previously shown that nerve growth factor (NGF) selectively stabilizes the GAP-43 mRNA in PC12 cells. To study the cellular mechanisms for this post-transcriptional control and to determine the contribution of mRNA stability to GAP-43 gene expression, we examined the effects of several agents that affect PC12 cell differentiation on the level of induction and rate of degradation of the GAP-43 mRNA. The NGF-mediated increase in GAP-43 mRNA levels and neurite outgrowth was mimicked by the phorbol ester TPA, but not by dibutyryl cAMP or the calcium ionophore A12783. Downregulation of protein kinase C (PKC) by high doses of phorbol esters or selective PKC inhibitors prevented the induction of this mRNA by NGF, suggesting that NGF and TPA act through a common PKC-dependent pathway. In mRNA decay studies, phorbol esters caused a selective 6-fold increase in the half-life of the GAP-43 mRNA, which accounts for most of the induction of this mRNA by TPA. The phorbol ester-induced stabilization of GAP-43 mRNA was blocked by the protein kinase inhibitor polymyxin B and was partially inhibited by dexamethasone, an agent that blocks GAP-43 expression and neuronal differentiation in PC12 cells. In contrast, the rates of degradation and the levels of the GAP-43 mRNA in control and TPA-treated cells were not affected by cycloheximide treatment. Thus, changes in GAP-43 mRNA turnover do not appear to require continuous protein synthesis. In conclusion, these data suggest that PKC activity regulates the levels of the GAP-43 mRNA in PC12 cells through a novel, translation-independent mRNA stabilization mechanism.