Phosphorylation of the presynaptic protein B-50 (GAP-43) is increased during electrically induced long-term potentiation

GAP43 Located on Corticostriatal Terminals Restrains Novelty-Induced Hyperactivity in Mice

Growth-associated protein of 43 kDa (GAP43) is a key cytoskeleton-associated component of the presynaptic terminal that facilitates neuroplasticity. Downregulation of GAP43 expression has been associated to various psychiatric conditions in humans and evokes hippocampus-dependent memory impairments in mice. Despite the extensive studies conducted on hippocampal GAP43 in past decades, however, very little is known about its roles in modulating the excitatory versus inhibitory balance in other brain regions. We recently generated conditional knock-out mice in which the Gap43 gene was selectively inactivated in either telencephalic glutamatergic neurons (Gap43fl/fl ;Nex1Cre mice, hereafter Glu-GAP43-/- mice) or forebrain GABAergic neurons (Gap43fl/fl ;Dlx5/6Cre mice, hereafter GABA-GAP43-/- mice). Here, we show that Glu-GAP43-/- but not GABA-GAP43-/- mice of either sex show a striking hyperactive phenotype when exposed to a novel environment. This behavioral alteration of Glu-GAP43-/- mice was linked to a selective activation of dorsal-striatum neurons, as well as to an enhanced corticostriatal glutamatergic transmission and an abrogation of corticostriatal endocannabinoid-mediated long-term depression. In line with these observations, GAP43 was abundantly expressed in corticostriatal glutamatergic terminals of wild-type mice. The novelty-induced hyperactive phenotype of Glu-GAP43-/- mice was abrogated by chemogenetically inhibiting corticostriatal afferences with a Gi-coupled "designer receptor exclusively activated by designer drugs" (DREADDs), thus further supporting that novelty-induced activity is controlled by GAP43 at corticostriatal excitatory projections. Taken together, these findings show an unprecedented regulatory role of GAP43 in the corticostriatal circuitry and provide a new mouse model with a delimited neuronal-circuit alteration for studying novelty-induced hyperactivity, a phenotypic shortfall that occurs in diverse psychiatric diseases.

Impaired Cognitive Function and Altered Hippocampal Synaptic Plasticity in Mice Lacking Dermatan Sulfotransferase Chst14/D4st1

Chondroitin sulfate (CS) and dermatan sulfate (DS) proteoglycans (PGs) are major extracellular matrix (ECM) components of the central nervous system (CNS). A large body of evidence has shown that CSPGs/DSPGs play critical roles in neuronal growth, axon guidance, and plasticity in the developing and mature CNS. It has been proposed that these PGs exert their function through specific interaction of CS/DS chains with its binding partners in a manner that depends on the sulfation patterns of CS/DS. It has been reported that dermatan 4-O-sulfotransferase-1 (Chst14/D4st1) specific for DS, but not chondroitin 4-O-sulfotransferase-1 (Chst11/C4st1) specific for CS, regulates proliferation and neurogenesis of neural stem cells (NSCs), indicating that CS and DS play distinct roles in the self-renewal and differentiation of NSCs. However, it remains unknown whether specific sulfation profiles of DS has any effect on CNS plasticity. In the present study, Chst14/D4st1-deficient (Chst14^−/−) mice was employed to investigate the involvement of DS in synaptic plasticity. First, behavior study using Morris Water Maze (MWM) showed that the spatial learning and memory of Chst14^−/− mice was impaired when compared to their wild type (WT) littermates. Corroborating the behavior result, long-term potentiation (LTP) at the hippocampal CA3-CA1 connection was reduced in Chst14^−/− mice compared to the WT mice. Finally, the protein levels of N-Methyl-D-aspartate (NMDA) receptor, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, postsynaptic density 95 (PSD95), growth associated protein 43 (GAP-43), synaptophysin (SYN) and N-ethylmaleimide sensitive factor (NSF) which are important in synaptic plasticity were examined and Chst14/D4st1 deficiency was shown to significantly reduce the expression of these proteins in the hippocampus. Further studies revealed that Akt/mammalian target rapamycin (mTOR) pathway proteins, including protein kinase B (p-Akt), p-mTOR and p-S6, were significantly lower in Chst14^−/− mice, which might contribute to the decreased protein expression. Together, this study reveals that specific sulfation of DS is critical in synaptic plasticity of the hippocampus and learning and memory, which might be associated with the changes in the expression of glutamate receptors and other synaptic proteins though Akt/mTOR pathway.

Increased expression of GAP43 in interneurons in a rat model of experimental polymicrogyria

To investigate seizure susceptibility in polymicrogyria, the seizure threshold and growth-associated protein GAP43 expression were analyzed in a rat experimental model of polymicrogyria induced by intracerebral injection of ibotenate. A total of 72 neonates from 9 pregnant rats were used. Intraperitoneal pentylenetetrazole injection did not induce any seizure activity in the control rats, although it elicited seizures of variable severity in the polymicrogyria rats. Fluoro-Jade B-positive degenerating interneurons were found in the polymicrogyria brains; however, no such neurons were detected in the control brains. In the polymicrogyria rats, the GAP43 expression was significantly and widely distributed in the brain, and the percentage of parvalbumin-positive interneurons in the GAP43-positive cells was significantly higher than that observed in the nonphosphorylated neurofilament-positive pyramidal cells. We conclude that the relatively selective vulnerability of inhibitory interneurons constitutes the basis for the decreased seizure threshold observed in this model of polymicrogyria.

Naps in school can enhance the duration of declarative memories learned by adolescents

Sleep helps the consolidation of declarative memories in the laboratory, but the pro-mnemonic effect of daytime naps in schools is yet to be fully characterized. While a few studies indicate that sleep can indeed benefit school learning, it remains unclear how best to use it. Here we set out to evaluate the influence of daytime naps on the duration of declarative memories learned in school by students of 10–15 years old. A total of 584 students from 6th grade were investigated. Students within a regular classroom were exposed to a 15-min lecture on new declarative contents, absent from the standard curriculum for this age group. The students were then randomly sorted into nap and non-nap groups. Students in the nap group were conducted to a quiet room with mats, received sleep masks and were invited to sleep. At the same time, students in the non-nap group attended regular school classes given by their usual teacher (Experiment I), or English classes given by another experimenter (Experiment II). These 2 versions of the study differed in a number of ways. In Experiment I (n = 371), students were pre-tested on lecture-related contents before the lecture, were invited to nap for up to 2 h, and after 1, 2, or 5 days received surprise tests with similar content but different wording and question order. In Experiment II (n = 213), students were invited to nap for up to 50 min (duration of a regular class); surprise tests were applied immediately after the lecture, and repeated after 5, 30, or 110 days. Experiment I showed a significant ~10% gain in test scores for both nap and non-nap groups 1 day after learning, in comparison with pre-test scores. This gain was sustained in the nap group after 2 and 5 days, but in the non-nap group it decayed completely after 5 days. In Experiment II, the nap group showed significantly higher scores than the non-nap group at all times tested, thus precluding specific conclusions. The results suggest that sleep can be used to enhance the duration of memory contents learned in school.

In vivo single branch axotomy induces GAP-43-dependent sprouting and synaptic remodeling in cerebellar cortex

Plasticity in the central nervous system in response to injury is a complex process involving axonal remodeling regulated by specific molecular pathways. Here, we dissected the role of growth-associated protein 43 (GAP-43; also known as neuromodulin and B-50) in axonal structural plasticity by using, as a model, climbing fibers. Single axonal branches were dissected by laser axotomy, avoiding collateral damage to the adjacent dendrite and the formation of a persistent glial scar. Despite the very small denervated area, the injured axons consistently reshape the connectivity with surrounding neurons. At the same time, adult climbing fibers react by sprouting new branches through the intact surroundings. Newly formed branches presented varicosities, suggesting that new axons were more than just exploratory sprouts. Correlative light and electron microscopy reveals that the sprouted branch contains large numbers of vesicles, with varicosities in the close vicinity of Purkinje dendrites. By using an RNA interference approach, we found that downregulating GAP-43 causes a significant increase in the turnover of presynaptic boutons. In addition, silencing hampers the generation of reactive sprouts. Our findings show the requirement of GAP-43 in sustaining synaptic stability and promoting the initiation of axonal regrowth.

Structural plasticity of climbing fibers and the growth-associated protein GAP-43

Structural plasticity occurs physiologically or after brain damage to adapt or re-establish proper synaptic connections. This capacity depends on several intrinsic and extrinsic determinants that differ between neuron types. We reviewed the significant endogenous regenerative potential of the neurons of the inferior olive (IO) in the adult rodent brain and the structural remodeling of the terminal arbor of their axons, the climbing fiber (CF), under various experimental conditions, focusing on the growth-associated protein GAP-43. CFs undergo remarkable collateral sprouting in the presence of denervated Purkinje cells (PCs) that are available for new innervation. In addition, severed olivo-cerebellar axons regenerate across the white matter through a graft of embryonic Schwann cells. In contrast, CFs undergo a regressive modification when their target is deleted. In vivo knockdown of GAP-43 in olivary neurons, leads to the atrophy of their CFs and a reduction in the ability to sprout toward surrounding denervated PCs. These findings demonstrate that GAP-43 is essential for promoting denervation-induced sprouting and maintaining normal CF architecture.

Anterior thalamic nuclei lesions in rats disrupt markers of neural plasticity in distal limbic brain regions

In two related experiments, neurotoxic lesions were placed in the anterior thalamic nuclei of adult rats. The rats were then trained on behavioral tasks, immediately followed by the immunohistochemical measurement of molecules linked to neural plasticity. These measurements were made in limbic sites including the retrosplenial cortex, the hippocampal formation, and parahippocampal areas. In Experiment 1, rats with unilateral anterior thalamic lesions explored either novel or familiar objects prior to analysis of the immediate-early gene zif268. The lesions reduced zif268 activity in the granular retrosplenial cortex and postsubiculum. Exploring novel objects resulted in local changes of hippocampal zif268, but this change was not moderated by anterior thalamic lesions. In Experiment 2, rats that had received either bilateral anterior thalamic lesions or control surgeries were exposed to novel room cues while running in the arms of a radial maze. In addition to zif268, measurements of c-AMP response element binding protein (CREB), phosphorylated CREB (pCREB), and growth associated protein43 (GAP-43) were made. As before, anterior thalamic lesions reduced zif268 in retrosplenial cortex and postsubiculum, but there were also reductions of pCREB in granular retrosplenial cortex. Again, the hippocampus did not show lesion-induced changes in zif268, but there were differential effects on CREB and pCREB consistent with reduced levels of hippocampal CREB phosphorylation following anterior thalamic damage. No changes in GAP-43 were detected. The results not only point to changes in several limbic sites (retrosplenial cortex and hippocampus) following anterior thalamic damage, but also indicate that these changes include decreased levels of pCREB. As pCREB is required for neuronal plasticity, partly because of its regulation of immediate early-gene expression, the present findings reinforce the concept of an 'extended hippocampal system' in which hippocampal function is dependent on distal sites such as the anterior thalamic nuclei.

Neural functions of calcineurin in synaptic plasticity and memory

Major brain functions depend on neuronal processes that favor the plasticity of neuronal circuits while at the same time maintaining their stability. The mechanisms that regulate brain plasticity are complex and engage multiple cascades of molecular components that modulate synaptic efficacy. Protein kinases (PKs) and phosphatases (PPs) are among the most important of these components that act as positive and negative regulators of neuronal signaling and plasticity, respectively. In these cascades, the PP protein phosphatase 2B or calcineurin (CaN) is of particular interest because it is the only Ca(2+)-activated PP in the brain and a major regulator of key proteins essential for synaptic transmission and neuronal excitability. This review describes the primary properties of CaN and illustrates its functions and modes of action by focusing on several representative targets, in particular glutamate receptors, striatal enriched protein phosphatase (STEP), and neuromodulin (GAP43), and their functional significance for synaptic plasticity and memory.

Ectopic growth of hippocampal mossy fibers in a mutated GAP-43 transgenic mouse with impaired spatial memory retention

In a previous study, it was shown that transgenic mice, designated G-NonP, forget the location of a water maze hidden platform when tested 7 days after the last training day (Holahan and Routtenberg (2008) Hippocampus 18:1099-1102). The memory loss in G-NonP mice might be related to altered hippocampal architecture suggested by the fact that in the rat, 7 days after water maze training, there is discernible mossy fiber (MF) growth (Holahan et al. (2006) Hippocampus 16:560-570; Rekart et al. (2007) Learn Mem 14:416-421). In the present report, we studied the distribution of the MF system within the hippocampus of naïve, untrained, G-NonP mouse. In WT mice, the MF projection was restricted to the stratum lucidum of CA3 with no detectable MF innervation in distal stratum oriens (dSO). In G-NonP mice, in contrast, there was an ectopic projection terminating in the CA3 dSO. Unexpectedly, there was nearly a complete loss of immunostaining for the axonal marker Tau1 in the G-NonP transgenic mice in the MF terminal fields indicating that transgenesis itself leads to off-target consequences (Routtenberg (1996) Trends Neurosci 19:471-472). Because transgenic mice overexpressing nonmutated, wild type GAP-43 do not show this ectopic growth (Rekart et al., in press) and the G-NonP mice overexpress a mutated form of GAP-43 precluding its phosphorylation by protein kinase C (PKC), the possibility exists that permanently dephosphorylated GAP-43 disrupts normal axonal fasciculation which gives rise to the ectopic growth into dSO.

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

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

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.

Proteomics analysis of the expression of neurogranin in murine neuroblastoma (Neuro-2a) cells reveals its involvement for cell differentiation

Neurogranin (Ng) is a neural-specific, calmodulin (CaM)-binding protein that is phosphorylated by protein kinase C (PKC). Although its biochemical property has been well characterized, the physiological function of Ng needs to be elucidated. In the present study, we performed proteomics analysis of the induced compositional changes due to the expression of Ng in murine neuroblastoma (Neuro-2a) cells using isotope coded affinity tags (ICAT) combined with 2-dimensional liquid chromatography/tandem mass spectrometry (2D-LC/MS/MS). We found that 40% of identified proteins were down-regulated and most of these proteins are microtubule components and associated proteins that mediated neurite outgrowth. Western blot experiments confirmed the expression of α-tubulin and microtubule- associated protein 1B (MAP 1B) was dramatically reduced in Neuro-2a-Ng cells compared to control. Cell morphology of Neuro-2a-Ng showed far less neurites than the control. Serum deprivation induced the extension of only one or two long neurites per cell in Neuro-2a-Ng, contrasting to the extension of multiple neurites per control cell. Ng may be linked to neurite formation by affecting expression of several microtubule related proteins. Furthermore, the PKC activator (PMA) induced an enhanced ERK1/2 activity in the cells that expressed Ng. The mutation of Ng at S36A caused sustained increase of ERK1/2 activity, whereas the ERK1/2 activity in mutation at I33Q showed no difference compared to wild type Ng, suggesting the phosphorylation of Ng but not the CaM /Ng interaction plays an important role in ERK activation. Ng may be involved in neuronal growth and differentiation via PKC and ERK1/2 signaling pathways.

Involvement of protein kinase C-epsilon in activity-dependent potentiation of large dense-core vesicle exocytosis in chromaffin cells

Neurotransmitter release is modulated in an activity-dependent manner. We showed previously that repetitive stimulation of nicotinic acetylcholine receptor (nAChR) induced activity-dependent potentiation (ADP) of large dense-core vesicle (LDCV) exocytosis in chromaffin cells. Here we report that protein kinase C (PKC)-epsilon is critically involved in ADP. Stimulation of nAChR induced activation of PKC-epsilon, and inhibition of PKC-epsilon by expression of the dominant-negative mutant of PKC-epsilon (DN-PKC-epsilon) or short interfering (siRNA) against PKC-epsilon abolished ADP via decreasing the frequency and quantal size of fused vesicles without affecting basal exocytosis, suggesting that PKC-epsilon is specifically involved in ADP. Electron microscopy revealed that inhibition of PKC-epsilon disrupts activity-induced vesicle translocation required for ADP. We also suggest the involvement of myristoylated alanine-rich C kinase substrate (MARCKS), which is known as a downstream target of PKC-epsilon, in ADP of LDCV exocytosis. The level of phospho-MARCKS correlated with the time course of ADP and was reduced by transfection with DN-PKC-epsilon. Actin filament disassembly induced by MARCKS phosphorylation was also significantly blocked by transfection of DN-PKC-epsilon. Furthermore, knockdown of MARCKS by siRNA resulted in inhibition of ADP and reduction of the number of fused vesicles. Together, we provide evidence that ADP of LDCV exocytosis is regulated by PKC-epsilon and its downstream target MARCKS via modulating vesicle translocation.

GAP-43 heterozygous mice show delayed barrel patterning, differentiation of radial glia, and downregulation of GAP-43

GAP-43 heterozygous (HZ) mice exhibit abnormal thalamocortical pathfinding, fasciculation, and terminal arborization at postnatal day 7 (P7). Here we tested whether these defects are correlated with delayed development of HZ cortical patterns. We assessed the rate of barrel segregation and radial glia differentiation in wild-type (WT) and HZ cortices. Since GAP-43 is involved in some forms of neural plasticity, we also compared the duration of the critical period for lesion-induced plasticity in both genotypes. Cytochrome oxidase histochemistry revealed a delay of approximately 1 day in barrel pattern formation in GAP-43 HZ mice. GAP-43 WT barrels showed complete segregation between P2-P3, while HZ barrels did not reach the same level of segregation until P3-P4. We found a similar delay in the transformation of radial glia from monopolar to multipolar phenotypes, from P5 in WT to P7 in HZ cortex. Radial glial cells represent many of the neuronal progenitors in developing cortex and aid in cell migration. Thus, the delay in radial glial differentiation may contribute to the delay in HZ barrel segregation. Interestingly, we found no change in the extent of the critical period for HZ cortical responsiveness to early peripheral damage or in the time course of the cortical response. As expected, GAP-43 expression in HZ cortex is significantly reduced early in development. However, HZ GAP-43 expression remains at maximum levels after P9, when it is normally downregulated. As a result, HZ GAP-43 expression is near-normal by P26, by which time near-normal barrel dimensions have been restored. Our findings indicate that GAP-43 deficiency leads to early delays in barrel development and suggest that these failures are followed by homeostatic responses, including prolonged GAP-43 expression. These compensatory mechanisms may rescue normal cortical reorganization in neonates and near-normal barrel morphology and GAP-43 expression in adulthood.

Leptin increases axonal growth cone size in developing mouse cortical neurons by convergent signals inactivating glycogen synthase kinase-3beta

We examined the effects of the adipose hormone leptin on the development of mouse cortical neurons. Treatment of neonatal and adult mice with intraperitoneal leptin (5 mg/kg) induced extracellular signal-regulated kinase (ERK) 1/2 phosphorylation in pyriform and entorhinal cortex neurons. Stimulation of cultured embryonic cortical neurons with leptin evoked Janus kinase 2 and ERK1/2 phosphorylation and activated the downstream effector 90-kDa ribosomal protein S6 kinase. Moreover, leptin elicited the phosphorylation of the phosphatidylinositol 3-kinase effector Akt and evoked Ser-9 phosphorylation of glycogen synthase kinase-3beta (GSK3beta), an event inactivating this kinase. Leptin-mediated GSK3beta phosphorylation was prevented by the MEK/ERK inhibitor PD98059, the phosphatidylinositol 3-kinase inhibitor LY294002, or the protein kinase C inhibitor GF109203X. Exposure of cortical neurons to leptin also induced Ser-41 phosphorylation of the neuronal growth-associated protein GAP-43, an effect prevented by LY294002 and GF109203X but not by PD98059. Ser-41-GAP-43 phosphorylation is usually high in expanding axonal growth cones. Neurons exposed to 100 ng/ml leptin for 72 h displayed reduced rate of growth cone collapse, a shift of growth cone size distribution toward higher values, and a 4-fold increase in mean growth cone surface area compared with control cultures. The leptin-induced growth cone spreading was hampered in cortical neurons from Lepr(db/db) mice lacking functional leptin receptors; it was associated with localized Ser-9-GSK3beta phosphorylation and mimicked by the GSK3beta inhibitor SB216763. At concentrations preventing GSK3beta phosphorylation, PD98059, LY294002, or GF109203X reversed the leptin-induced growth cone surface enlargement. We concluded that the leptin-mediated regulation of growth cone morphogenesis in cortical neurons relies on upstream regulators of GSK3beta activity.

Plasticity-related regulation of the hippocampal proteome

Plasticity of glutamatergic synapses is considered to be a pivotal mechanism underlying the ability of the CNS to re-configure its neural circuits. A large number of studies have focused on investigating how individual proteins, biochemical pathways and structural processes alter both the induction and maintenance of synaptic plasticity. However, it is likely that synaptic plasticity involves temporally and spatially coordinated regulation of multiple protein complexes within the activated neural circuit. By using a global proteomics-based approach we have now been able to reveal that highly diverse protein classes exhibit altered expression in response to both the activation of glutamate receptors and the induction of long-term potentiation (LTP) of glutamatergic synaptic strength in the hippocampus; a brain area where plastic synaptic modification is believed to be key to cognitive processes, such as spatial learning. Of the 2946 resolvable protein spots detected in this study, 79 (2.7%) were significantly altered in abundance in response to 100 microM glutamate application (all P < 0.05). The majority (56 out of 79) of these changes were due to the activation of the N-methyl-d-aspartate (NMDA) subtype of glutamate receptor. Likewise, the induction of LTP was associated with an altered abundance of 2.4% of the detectable proteome during the early (10 min) phase and 1.7% during the late (4 h) phase of its development. Observed changes in temporal and protein class-specific patterns of expression depict a widespread shift from metabolic to structural protein alteration as the plasticity process matures.

Expression of protein kinase C-substrate mRNAs in the basal ganglia of adult and infant macaque monkeys

We performed in situ hybridization histochemistry on the monkey basal ganglia to investigate the mRNA localization of three protein kinase C substrates (GAP-43, MARCKS, and neurogranin), of which expression plays a role in structural changes in neurites and synapses. Weak hybridization signals for GAP-43 mRNA and intense signals for both MARCKS and neurogranin mRNAs were observed in the adult neostriatum. All three of the mRNAs were expressed in both substance P-positive direct pathway neurons and enkephalin-positive indirect pathway neurons. In the nucleus accumbens, the hybridization signals for the three mRNAs were weaker than those in the neostriatum. Double-label in situ hybridization histochemistry in the neostriatum revealed that GAP-43 and neurogranin mRNAs were expressed in a subset of MARCKS-positive neurons. While intense hybridization signals for MARCKS mRNA were observed in all of the other basal ganglia regions such as the globus pallidus, substantia innominata, subthalamic nucleus, and substantia nigra, intense signals for GAP-43 mRNA were restricted to the substantia innominata and substantia nigra pars compacta. No signal for neurogranin mRNA was observed in the basal ganglia regions outside the neostriatum and the nucleus accumbens. These results indicate that the protein kinase C substrates are abundant in some specific connections in cortico-basal ganglia circuits. Developmental analysis showed that the expression level in the putamen and nucleus accumbens, but not in the caudate nucleus, was higher in the infant than in the adult, suggesting that synaptic maturation in the caudate nucleus occurs earlier than that in the putamen and nucleus accumbens.

Developmental changes in the expression of growth-associated protein-43 mRNA in the monkey thalamus: northern blot and in situ hybridization studies

The expression of growth-associated protein-43 has been related to axonal elongation and synaptic sprouting. Using the Northern blot analysis, we investigated the developmental changes of growth-associated protein-43 mRNA in the thalamus of macaque monkeys. The amount of growth-associated protein-43 mRNA was high at embryonic day 125, and decreased at postnatal day 1. It increased again at postnatal day 8, reached its peak value at postnatal days 50-70, and then decreased gradually until postnatal year 1. We previously reported that the amount of growth-associated protein-43 mRNA in the cerebral cortex decreased roughly exponentially during perinatal and postnatal periods and that it approached the asymptote by postnatal day 70 [Oishi T, Higo N, Umino Y, Matsuda K, Hayashi M (1998) Development of GAP-43 mRNA in the macaque cerebral cortex. Dev Brain Res 109:87-97]. The present findings may indicate that extensive synaptic growth of thalamic neurons continues even after that of cortical neurons has finished. We then performed in situ hybridization to investigate whether the expression level of growth-associated protein-43 mRNA was different among various thalamic nuclei. In the infant thalamus (postnatal days 70-90), moderate to intense expression of growth-associated protein-43 mRNA was detected in all thalamic nuclei. Quantitative analysis in the infant thalamus indicated that the expression levels were different between the nuclear groups that are defined by the origin of their afferents. The expression in the first order nuclei, which receive their primary afferent fibers from ascending pathways [Guillery RW (1995) Anatomical evidence concerning the role of the thalamus in corticocortical communication: a brief review. J Anat 187 (Pt 3):583-592], was significantly higher than that in the higher order nuclei. While moderate expression was also detected in the adult dorsal thalamus, the expression in the first order nuclei was almost the same as that in the higher order nuclei. Thus, the in situ hybridization experiments indicated that the transient postnatal increase in the amount of growth-associated protein-43 mRNA, which was shown by the Northern blot analysis, was mainly attributed to enhanced expression in the first order nuclei during the postnatal period. This may be a molecular basis for environmentally induced modification of thalamocortical synapses.

Effects of hypothyroidism induced by perinatal exposure to PTU on rat behavior and synaptic gene expression

Hypothyroidism in the rat induced by perinatal exposure to propylthiouracil (PTU) is a useful animal model to study molecular changes underlying neurobehavioral defects associated with this condition. Understanding the developmental alterations in gene expression related to the neurobehavioral dysfunction should help to identify molecular markers for developmental neurotoxicity at an early stage of development. In the present study, we evaluate the effects of PTU on the expression of a set of genes implicated in neural network formation or synaptic function at a minimal dose of PTU causing behavioral alteration. Various doses of PTU were administered to dams from late pregnancy to the lactation period and the expression of selected genes in the hippocampus and the cerebral cortex of offspring was examined by quantitative RT-PCR. Behavioral performance of PTU-treated rats was also assessed. PTU-treated rats showed increased motor activity and impairment of E-maze learning at weaning and after maturation. At doses causing such behavioral alteration, expression of GAP-43 and M1 mRNAs was changed during neuronal network formation, suggesting that levels of these factors during development are important for accurate postnatal development and function.

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.

Reduced basal CAMKII levels in hippocampal CA1 region: Possible cause of stress-induced impairment of LTP in chronically stressed rats

Chronic psychosocial stress markedly reduces the expression of high-frequency stimulation (HFS)-evoked early long-term potentiation (LTP) in the CA1 region of the hippocampus of anesthetized rats. Immunoblotting was performed to determine changes in molecular levels of key signaling proteins that might be responsible for this inhibitory effect. Western blot analysis of the CA1 region demonstrates that chronic psychosocial stress decreases basal levels of calcium calmodulin kinase II (CaMKII), phosphorylated (P)-CaMKII, calmodulin, and protein kinase C (PKCgamma) while markedly increasing protein phosphatase 2B (calcineurin) levels. The decrease of basal levels of P-CaMKII may be triggered primarily by excessive dephosphorylation resulting from enhanced basal levels of calcineurin. The decline in the basal levels of the upstream molecules, PKCgamma and calmodulin may be a consequence of the diminished basal P-CaMKII levels. Analysis of signaling molecules in CA1 region of chronically stressed rat subjected to HFS in vivo showed only one difference compared to similarly stimulated control rats; no increase in P-CaMKII levels. Our results suggest that decreased P-CaMKII levels may be primarily responsible for the stress-induced reduction in LTP expression.

Cell type- and region-specific expression of protein kinase C-substrate mRNAs in the cerebellum of the macaque monkey

We performed nonradioactive in situ hybridization histochemistry in the monkey cerebellum to investigate the localization of protein kinase C-substrate (growth-associated protein-43 [GAP-43], myristoylated alanine-rich C-kinase substrate [MARCKS], and neurogranin) mRNAs. Hybridization signals for GAP-43 mRNA were observed in the molecular and granule cell layers of both infant and adult cerebellar cortices. Signals for MARCKS mRNA were observed in the molecular, Purkinje cell, and granule cell layers of both infant and adult cortices. Moreover, both GAP-43 and MARCKS mRNAs were expressed in the external granule cell layer of the infant cortex. In the adult cerebellar vermis, signals for both GAP-43 and MARCKS mRNAs were more intense in lobules I, IX, and X than in the remaining lobules. In the adult hemisphere, both mRNAs were more intense in the flocculus and the dorsal paraflocculus than in other lobules. Such lobule-specific expressions were not prominent in the infant cerebellar cortex. Signals for neurogranin, a postsynaptic substrate for protein kinase C, were weak or not detectable in any regions of either the infant or adult cerebellar cortex. The prominent signals for MARCKS mRNA were observed in the deep cerebellar nuclei, but signals for both GAP-43 and neurogranin mRNAs were weak or not detectable. The prominent signals for both GAP-43 and MARCKS mRNAs were observed in the inferior olive, but signals for neurogranin were weak or not detectable. The cell type- and region-specific expression of GAP-43 and MARCKS mRNAs in the cerebellum may be related to functional specialization regarding plasticity in each type of cell and each region of the cerebellum.

Cavernous nerve injury elicits GAP-43 mRNA expression but not regeneration of injured pelvic ganglion neurons

Recovery of erectile dysfunction after cavernous nerve injury takes a long period. To elucidate this mechanism, unilateral cavernous nerve of male rat was cut, and the expression level of a nerve regeneration marker, the growth associated protein-43 (GAP-43) mRNA was evaluated by in situ hybridization and RT-PCR. While GAP-43 mRNA expression was transiently increased in the injured neurons of the major pelvic ganglion (MPG) at 7 days after nerve injury, continuous increase of GAP-43 mRNA was observed in the contralateral MPG from 7 days to 6 months after the nerve injury. Histochemical double-labeling studies for either neuronal NOS (nNOS) or tyrosine hydroxylase (TH) and the GAP-43 mRNA expression demonstrated that in injured MPG the transient up-regulation of GAP-43 mRNA was mainly seen in nNOS negative and/or TH positive neurons, suggesting non-parasympathetic post-ganglionic neurons, and also demonstrated that in contralateral MPG GAP-43 mRNA positive neurons were gradually increased in nNOS positive but TH negative neurons, suggesting parasympathetic post-ganglionic neurons. When a retrograde tracer Fluorogold (FG) was injected into the penile crus 7 days before histological experiments, FG-positive neurons were, if any, hardly seen in nNOS-positive neurons of the injured MPG for at least 6 months, whereas numerous FG-positive cells were seen in nNOS-positive neurons of the contralateral MPG. These results suggest that post-ganglionic projecting neurons of the intact side, which express increased GAP-43 mRNA, would be most likely to contribute to the recovery of the erectile function after unilateral cavernous nerve injury possibly by a plastic change such as nerve sprouting.

Delayed and bilateral changes of GAP-43/B-50 phosphorylation after circling training during a critical period in rat striatum

During the critical period of activity-dependent plasticity in rat striatum (30-37 days after birth) physiological circling behavior induces delayed modifications in GAP-43/B-50 phosphorylation by PKC. Postexercise, ipsi- and contralateral striatum to the circling direction show a similar temporal pattern of GAP-43/B-50 phosphorylation, with an initial decrease followed by a subsequent increase. However, there is a lag between initiation of the phosphorylation response in this asymmetrical task which does not occur when animals are subjected to exercise under conditions of symmetrical motor activity.

Abnormal thalamocortical pathfinding and terminal arbors lead to enlarged barrels in neonatal GAP-43 heterozygous mice

GAP-43 has been implicated in axonal pathfinding and sprouting, synaptic plasticity, and neurotransmitter release. However, its effect on cortical development in vivo is poorly understood. We have previously shown that GAP-43 knockout (-/-) mice fail to develop whisker-related barrels or an ordered whisker map in the cortex. Here we used cytochrome oxidase (CO) histochemistry to demonstrate that GAP-43 heterozygous (+/-) mice develop larger than normal barrels at postnatal day 7 (P7), despite normal body and brain weight. Using serotonin transporter (5HT-T) histochemistry to label thalamocortical afferents (TCAs), we found no obvious abnormalities in other somatosensory areas or primary visual cortex of GAP-43 (+/-) mice. However, TCA projections to (+/-) primary auditory cortex were not as clearly defined. To clarify the mechanism underlying the large-barrel phenotype, we used lipophilic (DiI) axon labeling. We found evidence for multiple pathfinding abnormalities among GAP-43 (+/-) TCAs. These axons show increased fasciculation within the internal capsule, as well as abnormal turning and branching in the subcortical white matter. These pathfinding errors most likely reflect failures of signal recognition and/or transduction by ingrowing TCAs. In addition, many DiI-labeled (+/-) TCAs exhibit widespread, sparsely branched terminal arbors in layer IV, reflecting the large-barrel phenotype. They also resemble those found in rat barrel cortex deprived of whisker inputs from birth, suggesting a failure of activity-dependent synaptogenesis and/or synaptic stabilization in (+/-) cortex. Our findings suggest that reduced GAP-43 expression can alter the fine-tuning of a cortical map through a combination of pathfinding and synaptic plasticity mechanisms.

Changes in protein kinase C (PKC) activity, isozyme translocation, and GAP-43 phosphorylation in the rat hippocampal formation after a single-trial contextual fear conditioning paradigm

The hippocampus plays an important role in spatial learning and memory. However, the biochemical alterations that subserve this function remain to be fully elucidated. In this study, rats were subjected to a single-trial contextual fear conditioning (CFC) paradigm; the activation of different protein kinase C (PKC) subtypes and the levels and phosphorylation of the plasticity-associated protein GAP-43 were assayed in the hippocampus at varying times after training. We observed a rapid activation of hippocampal PKC (15 min through 24 h), with differential translocation of the PKC isotypes studied. At early times after CFC (15-90 min), PKCalpha and PKCgamma translocated to the membrane, while PKCbetaII and PKCepsilon moved more transiently (15 to 30 min) to the cytosol. These PKC isotypes returned to the membrane at later time points after CFC. Correlating with these changes in PKC translocation and activity, there was an early decrease in GAP-43 phosphorylation followed by a more sustained increase from 1.5-72 h. GAP-43 protein levels were also increased after 3 h, and these levels remained elevated for at least 72 h. These changes in PKC and GAP-43 were specific to the CFC trained animals and no changes were seen in animals exposed to the same stimuli in a non-associative fashion. Comparison of translocation of different PKC isotypes with the changes in GAP-43 phosphorylation suggested that PKCbetaII and PKCepsilon may mediate both the early changes in the phosphorylation of this protein and the increases in GAP-43 expression at later times after CFC.

Corticothalamic cells in layers 5 and 6 of primary and secondary sensory cortex express GAP-43 mRNA in the adult rat

The expression of a presynaptic phosphoprotein, growth-associated protein (GAP)-43, is associated with synaptogenesis during development and synaptic remodeling in the adult. This study examined GAP-43 mRNA expression and distribution in primary and secondary areas of visual, auditory, and somatosensory cortex of the adult rat, by in situ hybridization with a digoxigenin-coupled mRNA probe, focusing particularly on the corticothalamic cells in layers 5 and 6. In the six cortical areas studied, GAP-43 mRNA was expressed predominantly in layers 5 and 6 and was greater in secondary than primary areas. There were densely labeled cells in layers 5 and 6 of all areas, which showed a restricted sublaminar distribution in primary areas and more even distribution in secondary areas. Combining retrograde transport of rhodamine beads with in situ hybridization in visual and auditory cortex showed that corticothalamic cells in layers 5 and 6 express GAP-43 mRNA. There are more of these GAP-43 mRNA positive corticothalamic cells in layer 5 of secondary areas than in primary areas. The evidence suggests that in the adult rat, plasticity related to GAP-43 is present in primary and secondary sensory cortex and more so in secondary areas.

Impairment of long-term potentiation and spatial memory in leptin receptor-deficient rodents

Leptin is well known to be involved in the control of feeding, reproduction and neuroendocrine functions through its action on the hypothalamus. However, leptin receptors are found in brain regions other than the hypothalamus (including the hippocampus and cerebral cortex) suggesting extrahypothalamic functions. We investigated hippocampal long-term potentiation (LTP) and long-term depression (LTD), and the spatial-memory function in two leptin receptor-deficient rodents (Zucker rats and db/db mice). In brain slices, the CA1 hippocampal region of both strains showed impairments of LTP and LTD; leptin (10(-12) M) did not improve these impairments in either strain. These strains also showed lower basal levels of Ca(2+)/calmodulin-dependent protein kinase II activity in the CA1 region than the respective controls, and the levels did not respond to tetanic stimulation. These strains also showed impaired spatial memory in the Morris water-maze test (i.e. longer swim-path lengths during training sessions and less frequent crossings of the platform's original location in the probe test. From these results we suggest that the leptin receptor-deficient animals show impaired LTP in CA1 and poor spatial memory due, at least in part, to a deficiency in leptin receptors in the hippocampus.

Nuclear transcription factors in the hippocampus

In the mammalian hippocampus, there is a trisynaptic loop that has been often referred to in studies on learning and memory mechanisms and their physiological correlate, the long-term potentiation (LTP). The three sets of synapses are formed by the fibers of perforant pathway terminating on granule cells and by the mossy fibers and Schaeffer collaterals making connections with the pyramidal cells. Each of the three types of synapses can develop LTP. LTP is accompanied by changes in gene expression and it is the nuclear transcription, involving specific transcription factors, that is the starting point for the series of biological amplifications and consolidations both necessary for such sustained changes. The transcription factors are proteins that control gene expression, development and functional formation in every eukaryotic cell. Two categories of transcription factors have been defined to date: general factors that comprise at least 20 proteins to form multiple preinitiation complex at the TATA box (TATA rich sequence) or regulatory factors that bind to promoter or enhancer regions at specific sites on the DNA close to, or distant from, the TATA box. Transcription factors have been divided into five different major classes according to unique protein motifs. These include basic domain, zinc-finger, helix-turn-helix, beta-Scaffold factors with minor groove contacts and other transcription factors not specifically classified. Much evidence has been accumulating in favor of the participation of several transcription factors in the consolidation of memory in the mammalian hippocampus following a spatial memory task. It is, therefore, of great importance that the involvement of transcription factors in de novo protein synthesis relevant to the synaptic mechanisms that mediate the formation of long-term memory should be summarized and discussed. No specific correlation between transduction of extracellular signals and expression of nuclear transcription factors, however, has been demonstrated to date.

A point mutant of GAP-43 induces enhanced short-term and long-term hippocampal plasticity

The growth-associated protein GAP-43 (or neuromodulin or B-50) plays a critical role during development in mechanisms of axonal growth and formation of synaptic networks. At later times, GAP-43 has also been implicated in the regulation of synaptic transmission and properties of plasticity such as long-term potentiation. In a molecular approach, we have analyzed transgenic mice overexpressing different mutated forms of GAP-43 or deficient in GAP-43 to investigate the role of the molecule in short-term and long-term plasticity. We report that overexpression of a mutated form of GAP-43 that mimics constitutively phosphorylated GAP-43 results in an enhancement of long-term potentiation in CA1 hippocampal slices. This effect is specific, because LTP was affected neither in transgenic mice overexpressing mutated forms of non-phosphorylatable GAP-43 nor in GAP-43 deficient mice. The increased LTP observed in transgenic mice expressing a constitutively phosphorylated GAP-43 was associated with an increased paired-pulse facilitation as well as an increased summation of responses during high frequency bursts. These results indicate that, while GAP-43 is not necessary for LTP induction, its phosphorylation may regulate presynaptic properties, thereby affecting synaptic plasticity and the induction of LTP.

GAP-43 is critical for normal development of the serotonergic innervation in forebrain

Serotonergic (5-HT) axons from the raphe nuclei are among the earliest afferents to innervate the developing forebrain. The present study examined whether GAP-43, a growth-associated protein expressed on growing 5-HT axons, is necessary for normal 5-HT axonal outgrowth and terminal arborization during the perinatal period. We found a nearly complete failure of 5-HT immunoreactive axons to innervate the cortex and hippocampus in GAP-43-null (GAP43-/-) mice. Abnormal ingrowth of 5-HT axons was apparent on postnatal day 0 (P0); quantitative analysis of P7 brains revealed significant reductions in the density of 5-HT axons in the cortex and hippocampus of GAP43-/- mice relative to wild-type (WT) controls. In contrast, 5-HT axon density was normal in the striatum, septum, and amygdala and dramatically higher than normal in the thalamus of GAP43-/- mice. Concentrations of serotonin and its metabolite, 5-hydroxyindolacetic acid, and norepinephrine were decreased markedly in the anterior and posterior cerebrum but increased in the brainstem of GAP43-/- mice. Cell loss could not account for these abnormalities, because unbiased stereological analysis showed no significant difference in the number of 5-HT dorsal raphe neurons in P7 GAP43-/- versus WT mice. The aberrant 5-HT innervation pattern persisted at P21, indicating a long-term alteration of 5-HT projections to forebrain in the absence of GAP-43. In heterozygotes, the density and morphology of 5-HT axons was intermediate between WT and homozygous GAP43-/- mice. These results suggest that GAP-43 is a key regulator in normal pathfinding and arborization of 5-HT axons during early brain development.

PKC activation rescues LTP from NMDA receptor blockade

It has been proposed that a critical step in long-term potentiation (LTP) expression is the activation of presynaptic protein kinase C (PKC) after activation of postsynaptic NMDA receptors. A prediction from this "synaptic dialogue" hypothesis (Routtenberg, Trends Neurosci 1999;22:255-256) is that the well-known blockade of LTP by NMDA receptor antagonists would be rescued by direct activation of PKC. To test this prediction we recorded extracellular EPSPs in the molecular layer of the dentate gyrus (DG) in the intact, anesthetized mouse after stimulation of the perforant path. Three experimental series were performed in which tetanization was applied after continuous infusion of 1) vehicle, 2) NMDA receptor antagonist dl-2-amino-5-phosphonovaleric acid (APV) (2.5+/-1.0 nmol), or 3) both APV and then PKC activator 4-beta-phorbol-12,13-dibutyrate (PDBu, 9.0+/-1.0 pmol). LTP was reliably induced in the first series (124+/-5%, N = 6; 2.5 h after the tetanus), suppressed by APV in the second series (95+/-18%, N = 4), and restored in the third series (121+/-13%, N = 5). Decreased paired-pulse facilitation, an index of presynaptic involvement in LTP expression, was observed after tetanization in the first and third series, but not in the second series. Blockade of LTP by NMDA receptor antagonists that can be overridden by presynaptic activation of PKC is thus consistent with the proposed hypothesis. As LTP is rescued after NMDA receptor blockade in transgenic mice overexpressing growth-associated presynaptic protein GAP-43, we suppose that this protein is one of the presynaptic targets of PKC activation.

Protein phosphatase-mediated regulation of protein kinase C during long-term depression in the adult hippocampus in vivo

The neural substrates of learning and memory are thought to involve use-dependent long-term changes in synaptic function, including long-term depression (LTD) of synaptic strength. One biochemical event hypothesized to contribute to the maintenance and expression of LTD is decreased protein phosphorylation, caused by a decrease in protein kinase activity and/or an increase in protein phosphatase activity. We tested whether the activity of protein kinase C (PKC) decreases after the induction of LTD in area CA1 of the adult hippocampus in vivo, and then investigated the mechanism responsible for the LTD-associated alteration in PKC activity. We found that LTD was associated with a significant decrease in both autonomous and cofactor-dependent PKC activity. The decrease in PKC activity was prevented by NMDA receptor blockade and was not accompanied by a decrease in the level of either PKCalpha, beta, gamma, or zeta. Western blot analysis with phosphospecific antibodies revealed that phosphorylation of Ser-657 on the catalytic domain of PKCalpha (Ser-660 on PKCbetaII) was decreased significantly after the induction of LTD, and that this dephosphorylation was prevented by the protein phosphatase inhibitor okadaic acid. The decrease in autonomous and cofactor-dependent PKC activity likewise was prevented by okadaic acid. These findings suggest that LTD in the adult hippocampus in vivo involves a decrease in PKC activity that is mediated, at least in part, by dephosphorylation of the catalytic domain of PKC by protein phosphatases activated after LTD-inducing stimulation. Our findings are consistent with the idea that protein dephosphorylation contributes to the expression of LTD.

Concentration-dependent effects of protein phosphatase (PP) inhibitors implicate PP1 and PP2A in different stages of memory formation

Numerous studies have demonstrated roles for protein phosphorylation and for specific kinases in memory formation; however, a role for specific protein phosphatases has not been established. Previous studies using pharmacobehavioral methods to implicate protein phosphatase activity in memory formation have been unable to discriminate between protein phosphatases 1 (PP1) and 2A (PP2A), as available cell-permeable agents generally inhibit both enzyme classes. To address this difficulty the present study exploited differences in the potency of the selective phosphatase inhibitor, okadaic acid, toward PP1 and PP2A. Within the context of a temporally precise animal model of memory, developed using the day-old chick (Gallus domesticus), acute administration of various concentrations of okadaic acid was found to disrupt two temporally distinct stages of memory formation. When administered bilaterally into an area of the chick brain implicated in memory formation, concentrations of okadaic acid known to selectively inhibit PP2A in vitro disrupted memory from 50 min posttraining. Higher concentrations, reported to inhibit both PP2A and PP1 in vitro, produced significant retention deficits from 20 min posttraining. Identical temporally specific effects were also obtained by varying the concentration and time of administration of calyculin A, a phosphatase inhibitor with equal potency toward both enzyme classes. Hence, different phosphatase enzymes may contribute to different stages of the enzymatic cascade believed to underlie memory formation.

Long term depression in the CA1 field is associated with a transient decrease in pre- and postsynaptic PKC substrate phosphorylation

Induction of homosynaptic long term depression (LTD) in the CA1 field of the hippocampus is thought to require activation of N-methyl-d-aspartate receptors, an elevation of postsynaptic Ca(2+) levels, and a subsequent increase in phosphatase activity. To investigate the spatial and temporal changes in protein phosphatase activity following LTD induction, we determined the in situ phosphorylation state of a pre- (GAP-43/B-50) and postsynaptic (RC3) protein kinase C substrate during N-methyl-d-aspartate receptor-dependent LTD in the CA1 field of rat hippocampal slices. We show that LTD is associated with a transient (<30 min) and D-AP5-sensitive reduction in GAP-43/B-50 and RC3 phosphorylation and that LTD is prevented by the phosphatase inhibitors okadaic acid and cyclosporin A. Our data provide strong evidence for a transient increase in pre- and postsynaptic phosphatase activity during LTD. Since the in situ phosphorylation of the calmodulin-binding proteins GAP-43/B-50 and RC3 changes during both LTD and long term potentiation, these proteins may form part of the link between the Ca(2+) signal and Ca(2+)/calmodulin-dependent processes implicated in long term potentiation and LTD.

Activation of pre- and postsynaptic protein kinase C during tetraethylammonium-induced long-term potentiation in the CA1 field of the hippocampus

Tetraethylammonium (TEA) induces a form of long-term potentiation (LTP) that is independent on N-methyl-D-aspartate (NMDA) receptor activation (LTP(K)). LTP(K) may be a suitable chemical model to study molecular mechanisms underlying LTP. We monitored the phosphorylation state of two identified neural-specific protein kinase C (PKC) substrates (the presynaptic protein GAP-43/B-50 and postsynaptic protein RC3) after different chemical depolarisations. TEA induced a long-lasting increase in synaptic efficacy in the CA1 field of the hippocampus and increased the phosphorylation of both GAP-43/B-50 and RC3 (51 and 56.1%, respectively). These effects were blocked by the voltage-dependent calcium channel antagonist nifedipine, but not by the NMDA receptor antagonist AP5. These data show that in LTP(K) the in situ phosphorylation of pre-and postsynaptic PKC substrates is increased, indicating that NMDA receptor-dependent and NMDA receptor-independent LTP share common Ca(2+)-dependent expression mechanisms, including activation of pre- and postsynaptic PKC.

Hyperammonemia impairs NMDA receptor-dependent long-term potentiation in the CA1 of rat hippocampus in vitro

Hyperammonemia is considered the main factor responsible for the neurological and cognitive alterations found in hepatic encephalopathy and in patients with congenital deficiencies of the urea cycle enzymes. The underlying mechanisms remain unclear. Chronic moderate hyperammonemia reduces nitric oxide-induced activation of soluble guanylate cyclase and glutamate-induced formation of cGMP. NMDA receptor-associated transduction pathways, including activation of soluble guanylate cyclase, are involved in the induction of long-term potentiation (LTP), a phenomenon that is considered to be the molecular basis for some forms of memory and learning. Using an animal model we show that chronic hyperammonemia significantly reduces the degree of long-term potentiation induced in the CA1 of hippocampus slices (200% increase in control and 50% increase in slices of hyperammonemic animals). Also, addition of 1 mM ammonia impaired the maintenance of non-decremental LTP. The LTP impairment could be involved in the intellectual impairment present in chronic hepatocerebral disorders associated with hyperammonemia.

Involvement of hippocampal PKCbetaI isoform in the early phase of memory formation of an inhibitory avoidance learning

Several evidences demonstrate that protein kinase C (PKC) is involved in hippocampal long-term potentiation (LTP) and in different forms of learning, including inhibitory avoidance training in rats. Here, we evaluated the levels of conventional PKC isozymes (alpha, betaI, betaII, gamma) in synaptic plasma membrane (SPM) fractions isolated from hippocampus of rats subjected to a one-trial inhibitory avoidance paradigm. At 0, 30 and 120 min after training, there was a significant increase in the total amount of PKCbetaI. Densitometric analysis of the immunoblots showed an increase of 142+/-11% at 0 min, 193+/-16% at 30 min and 156+/-6% at 120 min after training relative to shocked control values. No changes were found in PKCbetaI levels in SPM fractions of the shocked animals relative to naive control values. No training-specific increments in the levels of PKCalpha, betaII and gamma were observed at any time point tested. However, an increase in PKCgamma levels was found in trained and shocked animals sacrificed 120 min after each experimental procedure. In addition, bilateral microinjections of a fairly selective inhibitor of PKCbetaI isozyme into the CA1 of the dorsal hippocampus produced amnesia when given 10 min before training, or 50, 110, but not 170 min, after training. Thus, the present findings demonstrate the participation of PKCbetaI in the early synaptic events responsible for the acquisition and consolidation of an inhibitory avoidance learning, and suggest a putative role of this presynaptic isozyme on the enhanced PKC-dependent B-50/GAP-43 phosphorylation previously detected by us during this associative learning.

Decreased protein phosphatase 2A activity in hippocampal long-term potentiation

Using autophosphorylated Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) as substrate, we now find that long-term potentian (LTP) induction and maintenance are also associated with a significant decrease in calyculin A-sensitive protein phosphatase (protein phosphatase 2A) activity, without changes in Mg2+-dependent protein phosphatase (protein phosphatase 2C) activity. This decrease in protein phosphatase 2A activity was prevented when LTP induction was inhibited by treatment with calmidazolium or D-2-amino-5-phosphonopentanoic acid. In addition, the application of high-frequency stimulation to 32P-labeled hippocampal slices resulted in increases in the phosphorylation of a 55-kDa protein immunoprecipitated with anti-phosphatase 2A antibodies. Use of a specific antibody revealed that the 55-kDa protein is the B'alpha subunit of protein phosphatase 2A. Following purification of brain protein phosphatase 2A, the B'alpha subunit was phosphorylated by CaM kinase II, an event that led to the reduction of protein phosphatase 2A activity. These results suggest that the decreased activity in protein phosphatase 2A following LTP induction contributes to the maintenance of constitutively active CaM kinase II and to the long-lasting increase in phosphorylation of synaptic components implicated in LTP.

Transient translocation of protein kinase Cgamma in hippocampal long-term potentiation depends on activation of metabotropic glutamate receptors

Protein kinase C has been implicated in long-term regulation of cellular functions including induction and maintenance of hippocampal long-term potentiation. In the present study the time-course of long-term potentiation-induced translocation of Ca(2+)-dependent protein kinase C isoenzymes (PKCalpha/beta and PKCgamma) was investigated. Quantitative immunoblot analysis was used to measure translocation of these isoenzymes between cytosolic, membrane-associated and membrane-inserted fraction at 5, 15 and 60 min after induction of long-term potentiation in the dentate gyrus in vivo. To investigate the involvement of metabotropic glutamate receptors in protein kinase C regulation during long-term potentiation induction, additional animals were treated before tetanization with (R,S)-alpha-methyl-4-carboxyphenylglycine, an antagonist of metabotropic glutamate receptors. Brief tetanic stimulation of the perforant path resulted in a 100-150% increase in the population spike amplitude in response to test stimuli 5, 15 or 60 min after stimulation in both untreated and (R,S)-alpha-methyl-4-carboxyphenylglycine-treated animals. Only those rats showing clear potentiation were selected for further biochemical analysis of the potentiated dentate gyrus. Five minutes after high-frequency stimulation the subcellular distribution of all studied protein kinase C isoenzymes was unchanged compared with controls. PKC-gamma translocated into the cytosol 15 min after tetanization and this redistribution was blocked by (R,S)-alpha-methyl-4-carboxyphenylgly-cine pretreatment. By contrast, PKC alpha/beta levels increased in the cytosolic fraction only 60 min after tetanization, but in a (R,S)-alpha-methyl-4-carboxyphenylglycine-independent manner. In an additional set of experiments it was shown that (R,S)-alpha-methyl-4-carboxyphenylglycine alone applied intraventricularly had no effect on the subcellular distribution of the studied isoenzymes. The data suggest that PKCalpha/beta and PKCgamma are activated during different post-tetanic phases and metabotropic glutamate receptor activation might be essential for tetanus-induced translocation of postsynaptic PKCgamma only.

Age-dependent differences in glutamate-induced phosphorylation systems in rat hippocampal slices

Glutamate receptor induced changes in the activity of different phosphorylation systems were measured in hippocampal slices from 12- and 56-day-old rats, by determining the endogenous phosphorylation of 2.5% perchloric acid (PCA) soluble proteins. We identified among these proteins an 85, 80 kDa and the tau protein as specific substrates for protein kinase A (PKA), MARCKS, and neurogranin as specific substrates for protein kinase C (PKC), and prostaglandin-D-synthase as substrate for casein kinase II (CKII). In addition, a 35 kDa protein was phosphorylated by calcium/calmodulin dependent kinase II and protein kinase C and a 21 kDa protein was a substrate for all investigated kinases. The basal endogenous phosphorylation of 2.5% PCA soluble proteins changed during development qualitatively and quantitatively. Thus, the phosphorylation degree of nearly all proteins declines during maturation. Activation of mGluR induced an increased phosphorylation of PKA, PKC, and CKII substrates in hippocampal slices from 12-day-old rats, but in slices of 56-day-old rats only PKA and to a lower extent PKC substrates were affected. In contrast, stimulation of NMDA receptors led to an enhancement of CKII and PKA dependent phosphorylation only in slices of young animals, whereas the endogenous phosphorylation of some proteins in adult slices was actually decreased. These data showing developmental changes in the coupling of metabotropic and ionotropic glutamate receptors to different phosphorylation systems are discussed in the light of altered physiological properties of the mature hippocampus.

Modulation of protein kinases and protein phosphatases by reactive oxygen species: implications for hippocampal synaptic plasticity

1. Reactive oxygen species are known for their role in neurotoxicity. However, recent studies indicate that reactive oxygen species also play a role in cell function under physiological conditions. 2. Both superoxide and hydrogen peroxide alter the activity of various protein kinases and protein phosphatases, some of which are involved in hippocampal synaptic plasticity. Specifically, the activity of protein kinase C, extracellular-regulated kinase 2, and a protein tyrosine kinase(s) is increased in the presence of these reactive oxygen species, whereas the activity of protein phosphatases 2A and 2B, and a protein tyrosine phosphatase(s) is decreased. 3. Protein kinase C, extracellular-regulated kinase 2, and protein tyrosine kinases critically participate in the induction and/or early expression of long-term potentiation at glutamatergic synapses in hippocampus. Protein phosphatases 2A and 2B participate in the induction and/or early expression of long-term depression at these synapses. 4. Treatment of hippocampal slices with scavengers of either superoxide or hydrogen peroxide prevents the full expression of long-term potentiation. Long-term potentiation in hippocampus also is attenuated in transgenic mice that overexpress Cu/Zn superoxide dismutase. 5. The link between reactive oxygen species and long-term potentiation may be the activating effect on protein kinases. The inhibiting effect of reactive oxygen species on protein phosphatases may also contribute to long-term potentiation. 6. The authors hypothesize that reactive oxygen species play a critical role in hippocampal long-term potentiation by favoring the activation of a protein kinase over a protein phosphatase signaling cascade.

Activity-dependent phosphorylation of SNAP-25 in hippocampal organotypic cultures

Synaptosomal-associated protein of 25 kDa (SNAP-25) is thought to play a key role in vesicle exocytosis and in the control of transmitter release. However, the precise mechanisms of action as well as the regulation of SNAP-25 remain unclear. Here we show by immunoprecipitation that activation of protein kinase C (PKC) by phorbol esters results in an increase in SNAP-25 phosphorylation. In addition, immunochemical analysis of two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels shows that SNAP-25 focuses as three or four distinct spots in the expected range of molecular weight and isoelectric point. Changing the phosphorylation level of the protein by incubating the slices in the presence of either a PKC agonist (phorbol 12,13-dibutyrate) or antagonist (chelerythrine) modified the distribution of SNAP-25 among these spots. Phorbol 12,13-dibutyrate increased the intensity of the spots with higher molecular weight and lower isoelectric point, whereas chelerythrine produced the opposite effect. This effect was specific for regulators of PKC, as agonists of other kinases did not produce similar changes. Induction of long-term potentiation, a property involved in learning mechanisms, and production of seizures with a GABA(A) receptor antagonist also increased the intensity of the spots with higher molecular weight and lower isoelectric point. This effect was prevented by the PKC inhibitor chelerythrine. We conclude that SNAP-25 can be phosphorylated in situ by PKC in an activity-dependent manner.

Interactions between neurogranin and calmodulin in vivo

Neurogranin is a neural-specific, calmodulin (CaM)-binding protein that is phosphorylated by protein kinase C (PKC) within its IQ domain at serine 36. Since CaM binds to neurogranin through the IQ domain, PKC phosphorylation and CaM binding are mutually exclusive. Consequently, we hypothesize that neurogranin may function to concentrate CaM at specific sites in neurons and release free CaM in response to increased Ca2+ and PKC activation. However, it has not been established that neurogranin interacts with CaM in vivo. In this study, we examined this question using yeast two-hybrid methodology. We also searched for additional proteins that might interact with neurogranin by screening brain cDNA libraries. Our data illustrate that CaM binds to neurogranin in vivo and that CaM is the only neurogranin-interacting protein isolated from brain cDNA libraries. Single amino acid mutagenesis indicated that residues within the IQ domain are important for CaM binding to neurogranin in vivo. The Ile-33 --> Gln point mutant completely inhibited and Arg-38 --> Gln and Ser-36 --> Asp point mutants reduced neurogranin/CaM interactions. These data demonstrate that CaM is the major protein that interacts with neurogranin in vivo and support the hypothesis that phosphorylation of neurogranin at Ser-36 regulates its binding to CaM.

Decreased GAP-43/B-50 phosphorylation in striatal synaptic plasma membranes after circling motor behavior during development

We evaluated the in vitro phosphorylation of the presynaptic substrate of protein kinase C (PKC), GAP-43/B-50 and the PKC activity in the striatum of rats submitted to a circling training (CT) test during postnatal development. Motor activity at 30 days of age, but not at other ages, produced a unilateral reduction (-29.5%; p<0.001) in the level of GAP-43/B-50 endogenous phosphorylation in the contralateral striatum with respect to the ipsilateral side, while non-trained control animals did not show asymmetric differences. Compared to controls, the contralateral striatum of trained animals also showed a significant reduction (-29.3%; p<0. 001) in the incorporation of 32P-phosphate into GAP-43. This decreased in vitro GAP-43 phosphorylation was seen at 30 min, but not immediately after circling motor behavior. This contralateral change in GAP-43 phosphorylation correlated with the running speed developed by the animals [(r=0.9443, p=0.0046, n=6, relative to control group) and (r=0.8813, p=0.0203, n=6, with respect to the ipsilateral side of the exercised animals)]. On the contrary, GAP-43/B-50 immunoblots did not show changes in the amount of this phosphoprotein among the different experimental groups. Back phosphorylation assays, performed in the presence of bovine purified PKC, increased the level of GAP-43/B-50 phosphorylation in the striatum contralateral to the sense of turning [(+22%; p<0.05, with respect to ipsilateral side of the same trained group) and (+21%; p<0.05, relative to control group)]. Taken together, these results demonstrate that the activity developed in the CT test induces a reduction in the phosphorylation state of GAP-43/B-50 in the specific site for PKC. We conclude that general markers of activity-dependent neuronal plasticity are also altered in the same period that long-lasting changes in striatal neuroreceptors are triggered by circling motor behavior.