Antisense GAP-43 inhibits the evoked release of dopamine from PC12 cells

Alterations in mossy fiber physiology and GAP-43 expression and function in transgenic mice overexpressing HuD

HuD is a neuronal RNA-binding protein associated with the stabilization of mRNAs for GAP-43 and other neuronal proteins that are important for nervous system development and learning and memory mechanisms. To better understand the function of this protein, we generated transgenic mice expressing human HuD (HuD-Tg) in adult forebrain neurons. We have previously shown that expression of HuD in adult dentate granule cells results in an abnormal accumulation of GAP-43 mRNA via posttranscriptional mechanisms. Here we show that this mRNA accumulation leads to the ectopic expression of GAP-43 protein in mossy fibers. Electrophysiological analyses of the mossy fiber to CA3 synapse of HuD-Tg mice revealed increases in paired-pulse facilitation (PPF) at short interpulse intervals and no change in long-term potentiation (LTP). Presynaptic calcium transients at the same synapses exhibited faster time constants of decay, suggesting a decrease in the endogenous Ca(2+) buffer capacity of mossy fiber terminals of HuD-Tg mice. Under resting conditions, GAP-43 binds very tightly to calmodulin sequestering it and then releasing it upon PKC-dependent phosphorylation. Therefore, subsequent studies examined the extent of GAP-43 phosphorylation and its association to calmodulin. We found that despite the increased GAP-43 expression in HuD-Tg mice, the levels of PKC-phosphorylated GAP-43 were decreased in these animals. Furthermore, in agreement with the increased proportion of nonphosphorylated GAP-43, HuD-Tg mice showed increased binding of calmodulin to this protein. These results suggest that a significant amount of calmodulin may be trapped in an inactive state, unable to bind free calcium, and activate downstream signaling pathways. In conclusion, we propose that an unregulated expression of HuD disrupts mossy fiber physiology in adult mice in part by altering the expression and phosphorylation of GAP-43 and the amount of free calmodulin available at the synaptic terminal.

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.

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.

Gene targeting of presynaptic proteins in synaptic plasticity and memory: across the great divide

The past few decades have seen an explosion in our understanding of the molecular basis of learning and memory. The majority of these studies in mammals focused on post-synaptic signal transduction cascades involved in post-synaptic long-lasting plasticity. Until recently, relatively little work examined the role of presynaptic proteins in learning and memory in complex systems. The synaptic cleft figuratively represents a "great divide" between our knowledge of post- versus presynaptic involvement in learning and memory. While great strides have been made in our understanding of presynaptic proteins, we know very little of how presynaptically expressed forms of short- and long-term plasticity participate in information processing and storage. The paucity of cognitive behavioral research in the area of presynaptic proteins, however, is in stark contrast to the plethora of information concerning presynaptic protein involvement in neurotransmitter release, in modulation of release, and in both short- and long-term forms of presynaptic plasticity. It is now of great interest to begin to link the extensive literature on presynaptic proteins and presynaptic plasticity to cognitive behavior. In the future there is great promise with these approaches for identifying new targets in the treatment of cognitive disorders. This review article briefly surveys current knowledge on the role of presynaptic proteins in learning and memory in mammals and suggests future directions in learning and memory research on the presynaptic rim of the "great divide."

Hippocampal-dependent memory is impaired in heterozygous GAP-43 knockout mice

Cajal proposed that the rearrangement and growth of neurites and synaptic terminals is a substrate for the formation and storage of long-term memories. Proteins that regulate this learning-dependent growth are therefore likely to be "core determinants" (Sanes and Lichtman, Nat Neurosci 1999; 2:597-604) of such information storage processes. Although the growth-associated, protein kinase C (PKC) substrate GAP-43 has been oft-implicated in synaptic plasticity and memory, it has never been demonstrated that a reduction in the level of this protein has a deleterious effect on memory, because most homozygotes die perinatally. In this report, we observe significant memory impairments in heterozygous GAP-43 knockout mice with GAP-43 levels reduced by one-half. Impaired memory for a context was demonstrated in contextual fear conditioning. Importantly, no significant impairments in cued conditioning or on tests of nociceptive or auditory perception were observed in the heterozygous knockout, indicating that the observed impairments were unlikely related to performance or acquisition factors and are the result of reduced GAP-43 levels in the hippocampus. The present results, taken together with the prior demonstration of enhanced memory in transgenic mice overexpressing GAP-43, provide strong evidence for a pivotal role of hippocampal GAP-43 in the bidirectional regulation of mnemonic processing.

Fetal Alcohol Exposure Alters GAP-43 Phosphorylation and Protein Kinase C Responses to Contextual Fear Conditioning in the Hippocampus of Adult Rat Offspring

BACKGROUND

The growth- and plasticity-associated protein GAP-43 plays a significant role in the establishment and remodeling of neuronal connections. We have previously shown that GAP-43 levels, protein kinase C (PKC) activity, and GAP-43 phosphorylation increase during contextual fear conditioning and that fetal alcohol exposure (FAE) decreases PKC activity and GAP-43 phosphorylation in the hippocampus of adult offspring. Drawing on these observations, we hypothesized that FAE manifests its cognitive impairment by disrupting PKC activation and membrane translocation, thereby decreasing GAP-43 phosphorylation and function.

METHODS

Three groups of pregnant rat dams (FAE and two control diet groups) were placed on different diet regimens. Offspring from each of these groups were placed into each of four test groups, a contextual fear conditioned (CFC) group, a naïve unhandled group, and two nonlearning stress control groups. Hippocampi were dissected, homogenized, and used to prepare a cytosolic and a membrane fraction. These fractions were probed for total GAP-43, PKC-phosphorylated GAP-43, and several PKC subtypes. PKC activity also was measured in total homogenates.

RESULTS

Compared with both control diet groups, FAE animals showed a deficit in the activation of PKC in the hippocampus at 24 hr but not at 1.5 hr after CFC. Likewise, we found that the amount of GAP-43 and its phosphorylation were decreased 24 hr after CFC in FAE rats but not at early times after training. Analysis of the translocation of various PKC isoforms revealed that FAE animals had decreased levels of membrane-bound PKC beta2 and PKC epsilon 24 hr after CFC.

CONCLUSIONS

Considering the role of PKC activation and GAP-43 phosphorylation in synaptic plasticity, our results suggest that deficient translocation of PKC beta2 and PKC epsilon in the hippocampus may mediate the electrophysiological and behavioral deficits observed in fetal alcohol exposed animals.

From membrane phospholipid defects to altered neurotransmission: is arachidonic acid a nexus in the pathophysiology of schizophrenia?

Schizophrenia (SZ) is a devastating neuropsychiatric disorder affecting 1% of the general population, and is characterized by symptoms such as delusions, hallucinations, and blunted affect. While many ideas regarding SZ pathogenesis have been put forth, the majority of research has focused on neurotransmitter function, particularly in relation to altered dopamine activity. However, treatments based on this paradigm have met with only modest success, and current medications fail to alleviate symptoms in 30-60% of patients. An alternative idea postulated a quarter of a century ago by Feldberg (Psychol. Med. 6 (1976) 359) and Horrobin (Lancet 1 (1977) 936) involves the theory that SZ is associated in part with phospholipid/fatty acid abnormalities. Since then, it has been repeatedly shown that in both central and peripheral tissue, SZ patients demonstrate increased phospholipid breakdown and decreased levels of various polyunsaturated fatty acids (PUFAs), particularly arachidonic acid (AA). Given the diverse physiological function of membrane phospholipids and PUFAs, an elucidation of their role in SZ pathophysiology may provide novel strategies in the treatment of this disorder. The purpose of this review is to summarize the relevant data on membrane phospholipid/PUFA defects in SZ, the physiological consequence of altered AA signaling, and how they relate to the neurobiological manifestations of SZ and therapeutic outcome.

Inhibition of neurite outgrowth by reduced level of NDRG4 protein in antisense transfected PC12 cells

NDRG4, a member of the new NDRG gene family, was originally cloned as a gene that was expressed predominantly in the early postnatal rat brain. To determine whether the NDRG4 protein contributes to differentiation of neural cells, the effect of lowering the cellular NDRG4 protein level on the nerve growth factor (NGF)-induced neurite formations and transcription factor activations in PC12 cells was examined. An antisense construct of rat NDRG4 cDNA was made and transfected to PC12 cells, which constitutively express a basal level of the NDRG4 protein. Of the stably transfected antisense cell clones that expressed exogenous NDRG4 antisense RNA, six clones showed reduced levels of the NDRG4 protein, but unexpectedly two clones showed quite higher levels of NDRG4 protein than the control cells. The clones having decreased levels of the NDRG4 protein extended shorter neurites than control cells in response to NGF or dibutyryl cAMP. In contrast, the NDRG4 protein-highly expressing clones did not show suppressed neurite outgrowth induced by NGF. NGF-mediated activation of the transcription factor AP-1 was found to be suppressed in the NDRG4 protein-diminished clone and enhanced in the NDRG4 protein-upregulated clone as compared with those in the control cells. These results suggest that NDRG4 plays a role in neurite outgrowth and has an influence on an NGF-stimulated AP-1 activation by an undefined mechanism in PC12 cells.

Synaptophysin and GAP-43 mRNA levels in the hippocampus of subjects with schizophrenia

Synaptophysin and growth associated protein-43 (GAP-43) are synaptic proteins colocalized to the presynaptic terminal, and involved in regulating transmitter release and synaptic plasticity. Recent studies have proposed an alteration in the number of synapses in the brains of individuals with schizophrenia. As a corollary, we hypothesized that there may be an alteration in the level of mRNAs that code for synaptic proteins in brains of patients with schizophrenia. Using in situ hybridization, we investigated the levels of synaptophysin and GAP-43 mRNA in the medial temporal lobe of 10 normal subjects, 11 subjects with schizophrenia and 10 psychiatric control subjects. Synaptophysin mRNA levels were significantly reduced in several hippocampal subfields in both the schizophrenic and psychiatric control groups. GAP-43 mRNA levels were not significantly reduced in either group. The implications of these findings are discussed in relation to neuroleptic treatment and the pathophysiology of mental illness.

Effects of S-nitroso-cysteine on proteins that regulate exocytosis in PC12 cells: inhibitory effects on translocation of synaptophysin and ADP-ribosylation of GTP-binding proteins

S-Nitroso-cysteine (SNC) inhibits Ca2+-induced noradrenaline (NA) release from PC12 cells. Since SNC stimulated Ca2+ mobilization from intracellular Ca2+ pools and SNC-induced inhibition of NA release was not washed-out, SNC may modify exocytosis-related proteins that overcome Ca2+ mobilization. In the present study, we investigated the effects of SNC on exocytosis-related proteins in PC12 cells. Ionomycin stimulated NA release and increased the immunoreactivity of synaptophysin in the cytosol fraction. A 25-kDa synaptosome-associated protein (SNAP-25), which localizes to plasma membranes and vesicles, increased in the cytosol fraction after stimulation. The increases in these proteins by ionomycin were inhibited in PC12 cells treated with 0.6 mM SNC. Synaptobrevin and synapsin-1 in the cytosol fraction, and syntaxin and 43 kDa growth-associated protein in the membrane fraction were not affected by ionomycin or SNC. Incubation of each protein with SNC did not affect antibody immunoreactivity. [32P]ADP-ribosylation of GTP-binding proteins (Gi/Go) by pertussis toxin, but not Gs by cholera toxin, was inhibited in SNC-treated PC12 cells and by co-addition of SNC to the assay mixture. These findings suggest that 1) SNC inhibits translocation of vesicles containing synaptophysin and SNAP-25, and 2) SNC reacts with cysteine residues in Gi/Go, causing inhibition of ADP-ribosylation by pertussis toxin.

Depletion of a fatty acid-binding protein impairs neurite outgrowth in PC12 cells

Epithelial fatty acid-binding protein (E-FABP) is up-regulated in rat dorsal root ganglia after sciatic nerve crush and in differentiating neurons during development. The present study investigates the role of E-FABP during nerve growth factor (NGF)-mediated neurite outgrowth in PC12 cells. Undifferentiated PC12 cells express low levels of E-FABP, while NGF triggers a 6- and 8-fold induction of E-FABP mRNA and protein, respectively. Up-regulation of E-FABP mRNA occurs as early as 24 h after NGF treatment and remains highly expressed over the course of several days, corresponding to NGF-mediated neurite outgrowth. Withdrawal of NGF leads to down-regulation of E-FABP mRNA and retraction of neurites. Immunofluorescence microscopy reveals E-FABP immunoreactivity in the perinuclear cytoplasm, neurites and growth cones of NGF-differentiated cells. To examine the role of E-FABP during neurite outgrowth, PC12 cells were transfected with a constitutive antisense E-FABP vector to create the E-FABP-deficient line PC12-AS. By morphometric analysis, PC12-AS cells treated for 2, 4, and 7 days with NGF exhibited significantly decreased neurite expression relative to control (mock-transfected) cells. Taken together, these data indicate that E-FABP is important in normal NGF-mediated neurite outgrowth in PC12 cells, a finding that is consistent with a potential role in axonal development and regeneration.

cis-acting regulatory elements in the GAP-43 mRNA 3'-untranslated region can function in trans to suppress endogenous GAP-43 gene expression

The expression of the GAP-43 gene is controlled partly by changes in the stability of its mRNA, a process that is mediated by the interaction of specific sequences in the 3'-untranslated region (3'UTR) with neuronal-specific RNA-binding proteins. Limiting amounts of these trans-acting factors are available in the cell, thus we proposed that overexpression of the GAP-43 3'UTR could affect the levels of the endogenous mRNA via competitive binding to specific RNA-binding proteins. In this study, we show that chronic expression of GAP-43 3'UTR sequences in PC12 cells causes the depletion of the endogenous mRNA and consequent reduction of GAP-43 protein levels. The levels of the mRNAs for c-fos, the amyloid precursor protein (APP) and the microtubule associated protein tau, all three containing similar 3'UTR sequences, were not affected by the treatment. These results thus suggest that the effect of excess GAP-43 3'UTR is specific for its corresponding mRNA. We also used an HSV (herpes simplex virus)-1 vector and a mammalian expression vector with an inducible promoter to acutely express a 10 to 50 fold excess of 3'UTR sequences. Under these conditions, we found that transient expression of the GAP-43 3'UTR was effective in inhibiting both GAP-43 gene expression and neurite outgrowth in nerve growth factor (NGF)-treated PC12 cells and in primary neuronal cultures. These results underscore the role of 3'UTR sequences in the control of GAP-43 gene expression and suggest that overexpression of specific 3'UTR sequences could be used as a potential tool for probing the function of other post-transcriptionally-regulated proteins during neuronal differentiation.

Inhibition of noradrenaline release from PC12 cells by the long-term treatment with cholera toxin

Guanine nucleotide-binding (G) proteins are required for intracellular vesicular transport and endocytosis. In this study, we investigated the effects of short-term (2 h) and long-term (24 h) treatment with cholera toxin (CTX), which ADP-ribosylates proteins having arginine residues such as the alpha subunit of Gs (G(s alpha)), on exocytosis from the neurosecretory rat pheochromocytoma PC 12 cell line. Short-term treatment with CTX stimulated the accumulation of cyclic AMP, and synergistically enhanced both extracellular Ca2+-dependent [3H]noradrenaline (NA) releases (induced by high K+ and ATP) and Ca2+-independent release (induced by mastoparan, a peptide in wasp venom). Long-term treatment with CTX for 24h inhibited Ca2+-dependent and -independent stimulated [3H]NA release. The inhibitory effect of long-term CTX treatment was not derived from a cyclic AMP-dependent system, because (1) H-89, an inhibitor of protein kinase A, had no effect on the inhibition induced by CTX, (2) the long-term treatment with forskolin did not show an inhibitory effect. [32P]ADP-ribosylation of G(s alpha) and its immunoreactivity with anti-G(s alpha) antiserum in the crude membrane fraction was inhibited in the long-term CTX-treated cells, but not in the long-term forskolin-treated cells. [32P]ADP-ribosylation of G(s alpha) in the membrane fraction of short-term CTX-treated cells was approximately 90% of the level in the control cells. These findings suggest that CTX stimulates [3H]NA release via a cyclic AMP-dependent system in the short-term, and that long-term CTX treatment inhibited its release, maybe via ADP-ribosylation of CTX-sensitive proteins such as G(s alpha).

Anti-B-50 (GAP-43) antibodies decrease exocytosis of glutamate in permeated synaptosomes

The involvement of the protein kinase C substrate, B-50 (GAP-43), in the release of glutamate from small clear-cored vesicles in streptolysin-O-permeated synaptosomes was studied by using anti-B-50 antibodies. Glutamate release was induced from endogenous as well as 3H-labelled pools in a [Ca(2+)]-dependent manner. This Ca(2+)-induced release was partially ATP dependent and blocked by the light-chain fragment of tetanus toxin, demonstrating its vesicular nature. Comparison of the effects of anti-B-50 antibodies on glutamate and noradrenaline release from permeated synaptosomes revealed two major differences. Firstly, Ca(2+)-induced glutamate release was decreased only partially by anti-B-50 antibodies, whereas Ca(2+)-induced noradrenaline release was inhibited almost completely. Secondly, anti-B-50 antibodies significantly reduced basal glutamate release, but did not affect basal noradrenaline release. In view of the differences in exocytotic mechanisms of small clear-cored vesicles and large dense-cored vesicles, these data indicate that B-50 is important in the regulation of exocytosis of both types of neurotransmitters, probably at stages of vesicle recycling and/or vesicle recruitment, rather than in the Ca(2+)-induced fusion step.

The neuronal growth-associated protein GAP-43 interacts with rabaptin-5 and participates in endocytosis

Structural plasticity of nerve cells is a requirement for activity-dependent changes in the brain. The growth-associated protein GAP-43 is thought to be one determinant of such plasticity, although the molecular mechanism by which it mediates dynamic structural alterations at the synapse is not known. GAP-43 is bound by calmodulin when Ca2+ levels are low, and releases the calmodulin when Ca2+ levels rise, suggesting that calmodulin may act as a negative regulator of GAP-43 during periods of low activity in the neurons. To identify the function of GAP-43 during activity-dependent increases in Ca2+ levels, when it is not bound to calmodulin, we sought proteins with which GAP-43 interacts in the presence of Ca2+. We show here that rabaptin-5, an effector of the small GTPase Rab5 that mediates membrane fusion in endocytosis, is one such protein. We demonstrate that GAP-43 regulates endocytosis and synaptic vesicle recycling. Modulation of endocytosis by GAP-43, in association with rabaptin-5, may constitute a common molecular mechanism by which GAP-43 regulates membrane dynamics during its known roles in activity-dependent neurotransmitter release and neurite outgrowth.

The regulation of neurotransmitter secretion by protein kinase C

The effect of protein kinase C (PKC) on the release of neurotransmitters from a number preparations, including sympathetic nerve endings, brain slices, synaptosomes, and neuronally derived cell lines, is considered. A comparison is drawn between effects of activation of PKC on neurotransmitter release from small synaptic vesicles and large dense-cored vesicles. The enhancement of neurotransmitter release is discussed in relation to the effect of PKC on: 1. Rearrangement of the F-actin-based cytoskeleton, including the possible role of MARCKS in this process, to allow access of large dense-cored vesicles to release sites on the plasma membrane. 2. Phosphorylation of key components in the SNAP/SNARE complex associated with the docking and fusion of vesicles at site of secretion. 3. Ion channel activity, particularly Ca2+ channels.

Axons containing the growth associated protein GAP-43 specifically target rat corticotrophs following adrenalectomy

An extensive network of nerve fibers immunoreactive for the neuronal growth associated protein GAP-43 (GAP-43-IR) is present within the anterior pituitary (AP) of the rat, and the density of these fibers has been reported to increase 4 days after adrenalectomy (ADX). In the present study, we employed confocal dual-label immunofluorescence microscopy to determine whether GAP-43-IR fibers are specifically associated with corticotrophs at various intervals after ADX. A dramatic increase in the density of GAP-43-IR was apparent 4 days after ADX, and this increase was sustained at 7 and 14 days post-ADX. The percentage of corticotrophs in apparent contact with GAP-43-IR axons was 87% at 4 days after ADX and 92% at 14 days. In addition, fewer than 15% of GAP-43-IR terminals were associated with cells other than corticotrophs in either group. This highly specific targeting of corticotrophs during a period in which these cells are undergoing both hypertrophy and hyperplasia indicates that axonal sprouting is occurring in response to ADX. While the less intense GAP-43-IR in the AP of intact rats precluded precise quantitative analysis, the majority of corticotrophs also appeared to be selectively innervated in these animals. The observations that GAP-43-IR axons selectively contact corticotrophs, and that both the specificity and thoroughness of innervation are maintained by targeted growth of GAP-43-IR axons following ADX, strongly suggest that these fibers are of functional significance.

Protein kinase Cdelta mediates ethanol-induced up-regulation of L-type calcium channels

Brief ethanol exposure inhibits L-type, voltage-gated calcium channels in neural cells, whereas chronic exposure increases the number of functional channels. In PC12 cells, this adaptive response is mediated by protein kinase C (PKC), but the PKC isozyme responsible is unknown. Since chronic ethanol exposure increases expression of PKCdelta and PKCepsilon, we investigated the role these isozymes play in up-regulation of L-type channels by ethanol. Incubation with the PKC inhibitor GF 109203X or expression of a PKCdelta fragment that inhibits phorbol ester-induced PKCdelta translocation largely prevented ethanol-induced increases in dihydropyridine binding and K+-stimulated 45Ca2+ uptake. A corresponding PKCepsilon fragment had no effect on this response. These findings indicate that PKCdelta mediates up-regulation of L-type channels by ethanol. Remaining responses to ethanol in cells expressing the PKCdelta fragment were not inhibited by GF 109203X, indicating that PKCdelta-independent mechanisms also contribute. PKCdelta overexpression increased binding sites for dihydropyridine and L-channel antagonists, but did not increase K+-stimulated 45Ca2+ uptake, possibly because of homeostatic responses that maintain base-line levels of channel function. Since L-type channels modulate drinking behavior and contribute to neuronal hyperexcitability during alcohol withdrawal, these findings suggest an important role for PKCdelta in alcohol consumption and dependence.

Modulation of rat rotational behavior by direct gene transfer of constitutively active protein kinase C into nigrostriatal neurons

The modulation of motor behavior by protein kinase C (PKC) signaling pathways in nigrostriatal neurons was examined by using a genetic intervention approach. Herpes simplex virus type 1 (HSV-1) vectors that encode a catalytic domain of rat PKCbetaII (PkcDelta) were developed. PkcDelta exhibited a constitutively active protein kinase activity with a substrate specificity similar to that of rat brain PKC. As demonstrated in cultured sympathetic neurons, PkcDelta caused a long-lasting, activation-dependent increase in neurotransmitter release. In the rat brain, microinjection of HSV-1 vectors that contain the tyrosine hydroxylase promoter targeted expression to dopaminergic nigrostriatal neurons. Expression of pkcDelta in a small percentage of nigrostriatal neurons (approximately 0.1-2%) was sufficient to produce a long-term (>/=1 month) change in apomorphine-induced rotational behavior. Nigrostriatal neurons were the only catecholaminergic neurons that contained PkcDelta, and the amount of rotational behavior was correlated with the number of affected nigrostriatal neurons. The change in apomorphine-induced rotational behavior was blocked by a dopamine receptor antagonist (fluphenazine). D2-like dopamine receptor density was increased in those regions of the striatum innervated by the affected nigrostriatal neurons. Therefore, this strategy enabled the demonstration that a PKC pathway or PKC pathways in nigrostriatal neurons modulate apomorphine-induced rotational behavior, and altered dopaminergic transmission from nigrostriatal neurons appears to be the affected neuronal physiology responsible for the change in rotational behavior.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

S100A1 regulates neurite organization, tubulin levels, and proliferation in PC12 cells

As a first step in determining what cellular processes are regulated by the calcium-modulated protein S100A1 isoform in neurons, the effects of ablated S100A1 expression on neurite organization and microtubule/tubulin levels in PC12 cells were examined. A mammalian expression vector containing the rat S100A1 cDNA in the antisense orientation with respect to a cytomegalovirus promoter was constructed and transfected into PC12 cells. Indirect immunofluorescence microscopy confirmed decreased S100A1 protein levels in all three stable transfectants (pAntisense clones) that expressed exogenous S100A1 antisense mRNA. In response to nerve growth factor, pAntisense clones extended significantly more neurites than control cells (4.01 +/- 0.16 versus 2.93 +/- 0.16 neurites/cell). This increase in neurite number was accompanied by an increase in total alpha-tubulin levels in untreated (4.0 +/- 0.6 versus 1.76 +/- 0.4 ng of alpha-tubulin/mg of total protein) and nerve growth factor-treated pAntisense clones (4.15 +/- 0.4 versus 2. 04 +/- 0.5 ng of alpha-tubulin/mg of total protein) when compared with control cells. At high cell densities, pAntisense clones exhibited a significant decrease in anchorage-dependent growth. In soft agar, pAntisense clones formed significantly more colonies (153 +/- 8%) than control cells (116 +/- 5%). However, the pAntisense soft agar colonies were significantly smaller than those observed in control cells (40.6 +/- 3.0 versus 59.5 +/- 1.2 micron). These data suggest that cell density inhibits both anchorage-independent and -dependent growth of pAntisense clones. In summary, ablation of S100A1 expression in PC12 cells results in increased tubulin levels, altered neurite organization, and decreased cell growth. Thus, S100A1 may directly link the cytoskeleton and calcium signal transduction pathways to cell proliferation.

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.

RC3/neurogranin, a postsynaptic calpacitin for setting the response threshold to calcium influxes

In this review, we attempt to cover the descriptive, biochemical and molecular biological work that has contributed to our current knowledge about RC3/neurogranin function and its role in dendritic spine development, long-term potentiation, long-term depression, learning, and memory. Based on the data reviewed here, we propose that RC3, GAP-43, and the small cerebellum-enriched peptide, PEP-19, belong to a protein family that we have named the calpacitins. Membership in this family is based on sequence homology and, we believe, a common biochemical function. We propose a model wherein RC3 and GAP-43 regulate calmodulin availability in dendritic spines and axons, respectively, and calmodulin regulates their ability to amplify the mobilization of Ca2+ in response to metabotropic glutamate receptor stimulation. PEP-19 may serve a similar function in the cerebellum, although biochemical characterization of this molecule has lagged behind that of RC3 and GAP-43. We suggest that these molecules release CaM rapidly in response to large influxes of Ca2+ and slowly in response to small increases. This nonlinear response is analogous to the behavior of a capacitor, hence the name calpacitin. Since CaM regulates the ability of RC3 to amplify the effects of metabotropic glutamate receptor agonists, this activity must, necessarily, exhibit nonlinear kinetics as well. The capacitance of the system is regulated by phosphorylation by protein kinase C, which abrogates interactions between calmodulin and RC3 or GAP-43. We further propose that the ratio of phosphorylated to unphosphorylated RC3 determines the sliding LTP/LTD threshold in concept with Ca2+/ calmodulin-dependent kinase II. Finally, we suggest that the close association between RC3 and a subset of mitochondria serves to couple energy production with the synthetic events that accompany dendritic spine development and remodeling.

Antisense strategies in neurobiology

The use of antisense oligodeoxynucleotides, targeted to the transcripts encoding biologically active proteins in the nervous system, provides a novel and highly selective means to further our understanding of the function of these proteins. Recent studies of these agents also suggest the possibility of their being used therapeutically for a variety of diseases involving neuronal tissue. In this paper we review studies showing the in vitro and in vivo effects of antisense oligodeoxynucleotides as they relate to neurobiological functions. Particular attention is paid to the behavioral and biochemical effects of antisense oligodeoxynucleotides directed to the various subtypes of receptors for the neurotransmitter dopamine. An example is also provided showing the effects of a plasmid vector expressing an antisense RNA targeted to the calmodulin mRNAs in the PC12 pheochromocytoma cell line. The advantages of antisense oligodeoxynucleotides over traditional pharmacological treatments are assessed, and the advantages of using vectors encoding antisense RNA over the use of antisense oligodeoxynucleotides are also considered. We also describe the criteria that should be used in designing antisense oligodeoxynucleotides and several controls that should be employed to assure their specificity of action.

The growth-associated protein GAP-43 is specifically expressed in tyrosine hydroxylase-positive cells of the rat retina

In the adult retina, the growth-associated protein GAP-43 is exclusively present in three distinct sublaminae of the inner plexiform layer. During postnatal development, it is transiently expressed in the optic nerve fibers. No conclusions about the GAP-43 expressing cells can be derived from immunohistochemical stainings because GAP-43 protein is rapidly transported into the distal neuronal processes. We have combined immunohistochemistry to study the protein expression of GAP-43 and non-radioactive in situ hybridization to study the cellular expression of GAP-43 in the rat retina. We have found that in the mature retina GAP-43 mRNA is present only in retinal ganglion cells and in a small subset of cells of the inner nuclear layer. During postnatal development, no cells besides retinal ganglion cells and a subpopulation of cells in the inner nuclear layer express GAP-43 mRNA. Double staining experiments with tyrosine hydroxylase (TH) immunohistochemistry and GAP-43 in situ hybridization showed that GAP-43 expressing cells in the inner nuclear layer are immunoreactive for TH. They are most probably dopaminergic amacrine cells. Our results show that GAP-43 expression in the retina is restricted to very few cell types. They suggest that TH-positive cells (probably dopaminergic amacrine cells) retain a higher degree of structural plasticity in the adult retina.

Ca2+-dependent interaction of the growth-associated protein GAP-43 with the synaptic core complex

The synaptic vesicle exocytosis occurs by a highly regulated mechanism: syntaxin and 25 kDa synaptosome-associated protein (SNAP-25) are assembled with vesicle-associated membrane protein (VAMP) to form a synaptic core complex and then synaptotagmin participates as a Ca2+ sensor in the final step of membrane fusion. The 43 kDa growth-associated protein GAP-43 is a nerve-specific protein that is predominantly localized in the axonal growth cones and presynaptic terminal membrane. In the present study we have examined a possible interaction of GAP-43 with components involved in the exocytosis. GAP-43 was found to interact with syntaxin, SNAP-25 and VAMP in rat brain tissues and nerve growth factor-dependently differentiated PC12 cells, but not in undifferentiated PC12 cells. GAP-43 also interacted with synaptotagmin and calmodulin. These interactions of GAP-43 could be detected only when chemical cross-linking of proteins was performed before they were solubilized from the membranes with detergents, in contrast with the interaction of the synaptic core complex, which was detected without cross-linking. Experiments in vitro showed that the interaction of GAP-43 with these proteins occurred Ca2+-dependently; its maximum binding with the core complex was observed at 100 microM Ca2+, whereas that of syntaxin with synaptotagmin was at 200 microM Ca2+. These values of Ca2+ concentration are close to that required for the Ca2+-dependent release of neurotransmitters. Furthermore we observed that the interaction in vitro of GAP-43 with the synaptic core complex was coupled with protein kinase C-mediated phosphorylation of GAP-43. Taken together, our results suggest a novel function of GAP-43 that is involved in the Ca2+-dependent fusion of synaptic vesicles.

Regulated binding of the protein kinase C substrate GAP-43 to the V0/C2 region of protein kinase C-delta

The interaction between protein kinase C-delta and its neuronal substrate, GAP-43, was studied. Two forms of protein kinase C-delta were isolated from COS cells and characterized by differences in gel mobility, GAP-43 binding, and specific GAP-43 and histone kinase activities. A slow migrating, low specific activity form of protein kinase C-delta bound directly to immobilized GAP-43. Binding was abolished in the presence of EGTA, suggesting Ca2+ dependence of the interaction. The free catalytic domain of protein kinase C-delta did not bind GAP-43, suggesting the existence of a binding site in the regulatory domain. Glutathione S-transferase-protein kinase C-delta regulatory domain fusion proteins were generated and tested for binding to GAP-43. The V0/C2-like amino-terminal domain was defined as the GAP-43-binding site. GAP-43 binding to this region is inhibited by EGTA and regulated at Ca2+ levels between 10(-7) and 10(-6) M. The interaction between protein kinase C-delta and GAP-43 was studied in intact cells by coexpression of the two proteins in human embryonic kidney cells followed by immunoprecipitation. Complex formation occurred only after treatment of the cells with the Ca2+ ionophore ionomycin, indicating that elevation of intracellular Ca2+ is required for interaction in vivo. It is concluded that protein kinase C-delta interacts with GAP-43 through the V0/C2-like domain, outside the catalytic site, and that this interaction is modulated by intracellular Ca2+.

Post-transcriptional regulation of the GAP-43 gene by specific sequences in the 3' untranslated region of the mRNA

We have shown previously that GAP-43 gene expression during neuronal differentiation is controlled by selective changes in mRNA stability. This process was found to depend on highly conserved sequences in the 3' untranslated region (3' UTR) of the mRNA. To map the sequences in the GAP-43 3' UTR that mediate this post-transcriptional event, we generated specific 3' UTR deletion mutants and chimeras with the beta-globin gene and measured their half-lives in transfected PC12 cells. Our results indicate that there are two distinct instability-conferring elements localized at the 5' and 3' ends of the GAP-43 3' UTR. Of these destabilizing elements, only the one at the 3' end is required for the stabilization of the mRNA in response to treatment with the phorbol ester TPA. This 3' UTR element consists of highly conserved uridine-rich sequences and contains specific recognition sites for two neural-specific GAP-43 mRNA-binding proteins. Analysis of the levels of mRNA and protein derived from various 3' UTR deletion mutants indicated that all mutants were translated effectively and that differences in gene expression in response to TPA were attributable to changes in GAP-43 mRNA stability. In addition, the phorbol ester was found to affect the binding of specific RNA-binding proteins to the 3' UTR of the GAP-43 mRNA. Given that, like the GAP-43 mRNA, its degradation machinery and the GAP-43 mRNA-binding proteins are expressed primarily in neural cells, we propose that these factors may be involved in the post-transcriptional regulation of GAP-43 gene expression during neuronal differentiation.

Protein kinase C in synaptic plasticity: changes in the in situ phosphorylation state of identified pre- and postsynaptic substrates

1. Long-term potentiation and its counterpart long-term depression are two forms of activity dependent synaptic plasticity, in which protein kinases and protein phosphatases are essential. 2. B-50/GAP-43 and RC3/neurogranin are two defined neuronal PKC substrates with different synaptic localization. B-50/GAP-43 is a presynaptic protein and RC3/neurogranin is only found at the postsynaptic site. Measuring their phosphorylation state in hippocampal slices, allows us to simultaneously monitor changes in pre- and postsynaptic PKC mediated phosphorylation. 3. Induction of LTP in the CA1 field of the hippocampus is accompanied with an increase in the in situ phosphorylation of both B-50/GAP-43 and RC3/neurogranin, during narrow, partially overlapping, time windows. 4. Pharmacological data show that mGluR stimulation results in an increase in the in situ phosphorylation of B-50/GAP-43 and RC3/neurogranin.

GAP-43: an intrinsic determinant of neuronal development and plasticity

Several lines of investigation have helped clarify the role of GAP-43 (FI, B-50 or neuromodulin) in regulating the growth state of axon terminals. In transgenic mice, overexpression of GAP-43 leads to the spontaneous formation of new synapses and enhanced sprouting after injury. Null mutation of the GAP-43 gene disrupts axonal pathfinding and is generally lethal shortly after birth. Manipulations of GAP-43 expression likewise have profound effects on neurite outgrowth for cells in culture. GAP-43 appears to be involved in transducing intra- and extracellular signals to regulate cytoskeletal organization in the nerve ending. Phosphorylation by protein kinase C is particularly significant in this regard, and is linked with both nerve-terminal sprouting and long-term potentiation. In the brains of humans and other primates, high levels of GAP-43 persist in neocortical association areas and in the limbic system throughout life, where the protein might play an important role in mediating experience-dependent plasticity.

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.

Age-dependent neuronal and synaptic degeneration in mice transgenic for the C terminus of the amyloid precursor protein

The molecular basis for the degeneration of neurons and the deposition of amyloid in plaques and in the cerebrovasculature in Alzheimer's disease (AD) is incompletely understood. We have proposed that one molecule common to these abnormal processes is a fragment of the Alzheimer amyloid precursor protein (APP) comprising the C-terminal 100 amino acids of this molecule (APP-C100). We tested this hypothesis by creating transgenic mice expressing APP-C100 in the brain. We report here that aging (18-28 month) APP-C100 transgenic mice exhibit profound degeneration of neurons and synapses in Ammon's horn and the dentate gyrus of the hippocampal formation. Of the 106 transgenic mice between 8 and 28 months of age that were examined, all of those older than 18 months displayed severe hippocampal degeneration. The numerous degenerating axonal profiles contained increased numbers of neurofilaments, whorls of membrane, and accumulations of debris resembling secondary lysosomes near the cell body. The dendrites of degenerating granule and pyramidal cells contained disorganized, wavy microtubules. Cerebral blood vessels had thickened refractile basal laminae, and microglia laden with debris lay adjacent to larger venous vessels. Mice transgenic for Flag-APP-C100 (in which the hydrophilic Flag tag was fused to the N terminus of APP-C100) showed a similar degree of neurodegeneration in the hippocampal formation as early as 12 months of age. The 45 control mice displayed only occasional necrotic cells and no extensive cell degeneration in the same brain regions. These findings show that APP-C100 is capable of causing some of the neuropathological features of AD.

Levels of the growth-associated protein GAP-43 are selectively increased in association cortices in schizophrenia

The pathophysiology of schizophrenia may involve perturbations of synaptic organization during development. The presence of cytoarchitectural abnormalities that may reflect such perturbations in the brains of patients with this disorder has been well-documented. Yet the mechanistic basis for these features of the disorder is still unknown. We hypothesized that altered regulation of the neuronal growth-associated protein GAP-43, a membrane phosphoprotein found at high levels in the developing brain, may play a role in the alterations in brain structure and function observed in schizophrenia. In the mature human brain, GAP-43 remains enriched primarily in association cortices and in the hippocampus, and it has been suggested that this protein marks circuits involved in the acquisition, processing, and/or storage of new information. Because these processes are known to be altered in schizophrenia, we proposed that GAP-43 levels might be altered in this disorder. Quantitative immunoblots revealed that the expression of GAP-43 is increased preferentially in the visual association and frontal cortices of schizophrenic patients, and that these changes are not present in other neuropsychiatric conditions requiring similar treatments. Examination of the levels of additional markers in the brain revealed that the levels of the synaptic vesicle protein synaptophysin are reduced in the same areas, but that the abundance of the astrocytic marker of neurodegeneration, the glial fibrillary acidic protein, is unchanged. In situ hybridization histochemistry was used to show that the laminar pattern of GAP-43 expression appears unaltered in schizophrenia. We propose that schizophrenia is associated with a perturbed organization of synaptic connections in distinct cortical associative areas of the human brain, and that increased levels of GAP-43 are one manifestation of this dysfunctional organization.

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.

Growth-associated protein-43 (GAP-43) facilitates peptide hormone secretion in mouse anterior pituitary AtT-20 cells

The neuronal growth-associated protein (GAP)-43 (neuromodulin, B-50, F1), which is concentrated in the growth cones of elongating axons during neuronal development and in nerve terminals in restricted regions of the adult nervous system, has been implicated in the release of neurotransmitter. To study the role of GAP-43 in evoked secretion, we transfected mouse anterior pituitary AtT-20 cells with the rat GAP-43 cDNA and derived stably transfected cell lines. Depolarization-mediated beta-endorphin secretion was greatly enhanced in the GAP-43-expressing AtT-20 cells without a significant change in Ca2+ influx; in contrast, expression of GAP-43 did not alter corticotropin-releasing factor-evoked hormone secretion. The transfected cells also displayed a flattened morphology and extended processes when plated on laminin-coated substrates. These results suggest that AtT-20 cells are a useful model system for further investigations on the precise biological function(s) of GAP-43.

Alterations in GAP-43-immunoreactive innervation in the aging rat pituitary

The levels and distribution of the growth-associated protein, GAP-43, were examined in the pituitary glands of young and aging Sprague-Dawley rats, using immunohistochemical techniques on tissue sections and Western blot analyses. GAP-43-immunoreactive innervation was observed in sections in the intermediate and neural lobes of animals aged 8-15 months, while in the oldest rats studied (17 months), stained fibers were observed mainly in the neural, but not the intermediate lobe. Western blots revealed reduced levels of GAP-43 in samples from 15 month old animals, as compared to 12 month old rats, in the neurointermediate lobes. There was no immunoreactivity for GAP-43 in the anterior lobes in the tissue sections or in the blots in any of the glands examined. A diminished level of GAP-43 in pituitary innervation in aged animals suggests a reduced ability for nerve terminals to undergo 'plastic' changes in their relationship to target endocrine cells. Since GAP-43 has also been suggested to modulate neurotransmitter release, a reduction in the protein in aging nerve terminals may diminish availability of transmitters at presynaptic sites.

The use of the human neuroblastoma SH-SY5Y to study the effect of second messengers on noradrenaline release

1. Recent data suggesting that the human neuroblastoma SH-SY5Y is a suitable cell line in which to study the effect of second messengers on NA release are discussed in the context of current views on exocytosis. 2. Release of NA is evoked by depolarization, as well as activation of muscarinic (M3) and bradykinin (B2) receptors in SH-SY5Y cells which have not been differentiated by the addition of growth factors. 3. Evoked release is enhanced by activation of protein kinase C. 4. Activation of protein kinase C decreases the changes in intracellular calcium evoked by carbachol, bradykinin and 100 mM K+. 5. SH-SY5Y express N-type and L-type voltage sensitive Ca2+ channels. L-Type Ca(2+)-channels are coupled to NA release under conditions of weak depolarization. However with strong depolarization (100 mM K+) both L-type and N-type channels are involved. 6. Muscarinic- and neuropeptide Y receptors are coupled to the inhibition of Ca2+ channel activity.

GAP-43 mRNA suppression by the ribozyme in PC12 cells and inhibition of evoked dopamine release

We constructed a ribozyme designed to cleave the GAP-43 mRNA and which contained a bacteriophage T7 transcription terminator at its 3' site to maintain stability. The ribozyme was overexpressed in PC12 cells by pEF-BOS, a powerful mammalian expression vector. Consequently, PC12 cells overexpressing the GAP-43 ribozyme revealed a drastic decrease in the levels of GAP-43 mRNA expression, and the evoked dopamine release was significantly suppressed in these cell lines. These results support the previous observations that GAP-43 is associated with Ca-dependent dopamine release in PC12 cells, and the ribozyme expression system used in the present study was demonstrated to be useful for suppression of the functions of specific mRNAs and exploration of specific gene products.

TGF-beta s and cAMP regulate GAP-43 expression in Schwann cells and reveal the association of this protein with the trans-Golgi network

We have shown previously that growth-associated protein 43 (GAP-43) is expressed by rat Schwann cells and is restricted to non-myelin-forming Schwann cells in vivo. Here we examined the regulation of GAP-43 using agents that are known to control Schwann cell differentiation in vitro. GAP-43 protein and mRNA levels are decreased by forskolin and other agents that elevate intracellular cAMP (and promote expression of the myelinating Schwann cell phenotype). We also found that expression of GAP-43 protein but not mRNA is down-regulated by transforming growth factor betas (TGF-beta s). Moreover, TGF-beta treatment of Schwann cells results in cell clumping, process retraction and disappearance of GAP-43 from the plasma membrane, revealing that GAP-43 is associated with the Golgi apparatus. This association was confirmed by partial overlap of GAP-43 with the trans-Golgi network marker (23c) and the disruption of the Golgi with brefeldin A or monensin leading to altered GAP-43 distribution. Golgi-associated GAP-43 appeared to have the same molecular weight as the plasma membrane-associated GAP-43. Thus these results show that GAP-43 expression in Schwann cells is subject to regulation by both extracellular and intracellular signalling molecules and that Schwann cell GAP-43 is often associated with the Golgi apparatus.

Temporal differences in the phosphorylation state of pre- and postsynaptic protein kinase C substrates B-50/GAP-43 and neurogranin during long-term potentiation

The phosphorylation state of two identified neuralspecific protein kinase C substrates (the presynaptic protein B-50 and the postsynaptic protein neurogranin) was monitored after the induction of long term potentiation in the CA1 field of rat hippocampus slices by quantitative immunoprecipitation following 32Pi labeling in the recording chamber. B-50 phosphorylation was increased from 10 to 60 min, but no longer at 90 min after long term potentiation had been induced, neurogranin phosphorylation only at 60 min. Increased phosphorylation was not found when long term potentiation was blocked with the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate, when only low frequency stimulation was applied or tetanic stimulation failed to induce long term-potentiation. Our data show that both B-50 and neurogranin phosphorylation are increased following the induction of long term potentiation, thus providing strong evidence for pre- and postsynaptic protein kinase C activation during narrow, partially overlapping, time windows after the induction of long term potentiation.

Changes in protein kinase C and its presynaptic substrate B-50/GAP-43 after intrauterine exposure to methylazoxy-methanol, a treatment inducing cortical and hippocampal damage and cognitive deficit in rats

The involvement of protein kinase C (PKC)-dependent processes in adaptive and plastic changes underlying neuronal plasticity was tested in an in vivo animal model characterized by targeted cellular ablation of cortical and hippocampal neurons, cognitive impairment and lack of induction of long-term potentiation. [3H]Phorbol ester binding performed on brain slices revealed a 67.4 and 35.0% increase in membrane-bound protein kinase C in the cortex and hippocampus respectively of rats treated with methylazoxy-methanol acetate compared with saline-treated control rats, and there was no modification in the expression of mRNAs of different protein kinase C isozymes. In situ phosphorylation experiments performed with 32Pi-labelled synaptosomes from the affected areas demonstrated that the phosphorylation of the nervous tissue-specific presynaptic membrane-associated protein kinase C substrate B-50/GAP-43 was increased by 51.4 and 44.8% in cortex and hippocampus respectively. Western blot analysis of protein kinase C in synaptosomal cytosol and membrane fractions prepared from cortex and hippocampus showed an increased proportion of protein kinase C in the membrane compartment in treated animals, but no change in the total synaptosomal protein kinase C activity. Our data are consistent with increased activity of presynaptic protein kinase C and predict a sustained increase in glutamate release in methylazoxy-methanol-treated rats.

Plasticity in cultured carotid body chemoreceptors: environmental modulation of GAP-43 and neurofilament

In this study we use dissociated cell cultures of the rat carotid body to investigate the adaptive capabilities of endogenous oxygen chemoreceptors, following chronic stimulation by various environmental factors. These oxygen chemoreceptors are catecholamine-containing glomus cells, which derive from the neural crest and resemble adrenal medullary chromaffin cells. Using double-label immunofluorescence, we found that chronic exposure of carotid body cultures to hypoxia (2% to 10% oxygen) caused a significant fraction of tyrosine hydroxylase-positive (TH+) glomus cells to acquire detectable immunoreactivity for growth-associated protein GAP-43. The effect was dose-dependent and peaked around an oxygen tension of 6%, where approximately 30% of glomus cells were GAP-43 positive. Treatment with agents that elevate intracellular cyclic adenosine monophosphate (cAMP) (i.e., dibutyryl cAMP or forskolin) also markedly stimulated GAP-43 expression. Since hypoxia is known to increase cAMP levels in glomus cells, it is possible that the effect of hypoxia on GAP-43 expression was mediated, at least in part, by a cAMP-dependent pathway. Unlike hypoxia, however, cAMP analogs also stimulated neurofilament (NF 68 or NF 160 kD) expression and neurite outgrowth in glomus cells, and these properties were enhanced by retinoic acid. Nerve growth factor, which promotes neuronal differentiation in related crest-derived endocrine cells, and dibutyryl cGMP were ineffective. Thus, it appears that postnatal glomus cells are plastic and can express neuronal traits in vitro. However, since hypoxia stimulated GAP-43 expression, without promoting neurite outgrowth, it appears that the two processes can be uncoupled. We suggest that stimulation of GAP-43 by hypoxia may be important for other physiological processes, e.g., enhancing neurotransmitter release or sensitization of G-protein-coupled receptor transduction.

GAP-43 and its relation to autonomic and sensory neurons in sciatic nerve and gastrocnemius muscle in the rat

The presence of the growth-associated protein, GAP-43, in rat sciatic nerve and gastrocnemius muscle was studied, using indirect immunofluorescence, in lumbar sympathectomized and sham-sympathectomized rats. To study fast axonal transport and accumulation of immunogenic GAP-43, the sciatic nerves were crush operated, 6 h before perfusion fixation. In sections of normal, crushed sciatic nerve GAP-43-like immunoreactivity (LI) rapidly accumulated, on both sides of the crushes, in medium and thin sized axons. In double immunostaining experiments, GAP-43-LI was mainly colocalized with tyrosine hydroxylase (TH)-LI, or neuropeptide Y (NPY)-LI, markers of sympathetic nerves. In some axons, GAP-43-LI was colocalized with Substance P (SP)-LI. In perivascular nerve terminals in the gastrocnemic muscle, strong GAP-43-immunofluorescence was observed, in most cases colocalized with TH-LI, but in some terminals with SP-LI. Three days after lumbar sympathectomy (removal of the L1-L4 sympathetic ganglia bilaterally), TH-LI and NPY-LI positive axons in the sciatic nerve were reduced in number by more than 90%. GAP-43-LI positive axons were reduced by about 50%. In the gastrocnemic muscle, some GAP-43-LI positive terminals, but very few TH-LI positive nerve fibres, were found around blood vessels. No further changes were seen 8 days after sympathectomy. SP-LI in axons in the sciatic nerve and in perivascular nerve terminals of the gastrocnemic muscle, did not change after sympathectomy; most of these axons also contained GAP-43-LI. S-100-LI was present periaxonally in the sciatic nerves, but it did not colocalize with GAP-43, indicating that Schwann cells contained no GAP-43-LI in these experiments. The results show that, in normal adult rats, GAP-43-LI is mainly present in sympathetic and sensory nerve fibers in sciatic nerve and in perivascular nerve terminals. The peptide is axonally transported, mainly in sensory and adrenergic axons.

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.

Inhibition of protein kinase C- and casein kinase II-mediated phosphorylation of GAP-43 by S100 beta

The effect of the glial-derived protein, S100 beta, on the in vitro phosphorylation of the growth-associated protein GAP-43 was investigated. S100 beta inhibited in a dose dependent manner the phosphorylation of GAP-43 by protein kinase C (PKC) or by casein kinase II (CKII). S100 beta appeared to slow down the rate and the degree to which GAP-43 can be phosphorylated by either kinase. The specificity of the inhibition was demonstrated by the observation that the phosphorylation of two other CKII substrates, casein and a selective peptide substrate, was not inhibited by S100 beta. The marked inhibitory effect of S100 beta required the presence of calcium in the phosphorylation reactions. In addition, S100 beta inhibition of GAP-43 phosphorylation was seen with GAP-43 purified under a variety of conditions that alter acylation, suggesting that the acylation state of GAP-43 does not affect the ability of S100 beta to modulate CKII- or PKC-mediated phosphorylation of GAP-43.