Inhibition of noradrenaline release by antibodies to B-50 (GAP-43)

SVHRSP Alleviates Age-Related Cognitive Deficiency by Reducing Oxidative Stress and Neuroinflammation

BACKGROUND

Our previous studies have shown that scorpion venom heat-resistant synthesized peptide (SVHRSP) induces a significant extension in lifespan and improvements in age-related physiological functions in worms. However, the mechanism underlying the potential anti-aging effects of SVHRSP in mammals remains elusive.

METHODS

Following SVHRSP treatment in senescence-accelerated mouse resistant 1 (SAMR1) or senescence-accelerated mouse prone 8 (SAMP8) mice, behavioral tests were conducted and brain tissues were collected for morphological analysis, electrophysiology experiments, flow cytometry, and protein or gene expression. The human neuroblastoma cell line (SH-SY5Y) was subjected to H2O2 treatment in cell experiments, aiming to establish a cytotoxic model that mimics cellular senescence. This model was utilized to investigate the regulatory mechanisms underlying oxidative stress and neuroinflammation associated with age-related cognitive impairment mediated by SVHRSP.

RESULTS

SVHRSP significantly ameliorated age-related cognitive decline, enhanced long-term potentiation, restored synaptic loss, and upregulated the expression of synaptic proteins, therefore indicating an improvement in synaptic plasticity. Moreover, SVHRSP demonstrated a decline in senescent markers, including SA-β-gal enzyme activity, P16, P21, SIRT1, and cell cycle arrest. The underlying mechanisms involve an upregulation of antioxidant enzyme activity and a reduction in oxidative stress-induced damage. Furthermore, SVHRSP regulated the nucleoplasmic distribution of NRF2 through the SIRT1-P53 pathway. Further investigation indicated a reduction in the expression of proinflammatory factors in the brain after SVHRSP treatment. SVHRSP attenuated neuroinflammation by regulating the NF-κB nucleoplasmic distribution and inhibiting microglial and astrocytic activation through the SIRT1-NF-κB pathway. Additionally, SVHRSP significantly augmented Nissl body count while suppressing neuronal loss.

CONCLUSION

SVHRSP could remarkably improve cognitive deficiency by inhibiting oxidative stress and neuroinflammation, thus representing an effective strategy to improve brain health.

Elevated CSF GAP-43 is associated with accelerated tau accumulation and spread in Alzheimer's disease

In Alzheimer's disease, amyloid-beta (Aβ) triggers the trans-synaptic spread of tau pathology, and aberrant synaptic activity has been shown to promote tau spreading. Aβ induces aberrant synaptic activity, manifesting in increases in the presynaptic growth-associated protein 43 (GAP-43), which is closely involved in synaptic activity and plasticity. We therefore tested whether Aβ-related GAP-43 increases, as a marker of synaptic changes, drive tau spreading in 93 patients across the aging and Alzheimer's spectrum with available CSF GAP-43, amyloid-PET and longitudinal tau-PET assessments. We found that (1) higher GAP-43 was associated with faster Aβ-related tau accumulation, specifically in brain regions connected closest to subject-specific tau epicenters and (2) that higher GAP-43 strengthened the association between Aβ and connectivity-associated tau spread. This suggests that GAP-43-related synaptic changes are linked to faster Aβ-related tau spread across connected regions and that synapses could be key targets for preventing tau spreading in Alzheimer's disease.

GAP-43 and BASP1 in Axon Regeneration: Implications for the Treatment of Neurodegenerative Diseases

Growth-associated protein-43 (GAP-43) and brain acid-soluble protein 1 (BASP1) regulate actin dynamics and presynaptic vesicle cycling at axon terminals, thereby facilitating axonal growth, regeneration, and plasticity. These functions highly depend on changes in GAP-43 and BASP1 expression levels and post-translational modifications such as phosphorylation. Interestingly, examinations of GAP-43 and BASP1 in neurodegenerative diseases reveal alterations in their expression and phosphorylation profiles. This review provides an overview of the structural properties, regulations, and functions of GAP-43 and BASP1, highlighting their involvement in neural injury response and regeneration. By discussing GAP-43 and BASP1 in the context of neurodegenerative diseases, we also explore the therapeutic potential of modulating their activities to compensate for neuron loss in neurodegenerative diseases.

Role of Protein Kinase C in Bipolar Disorder: A Review of the Current Literature

Bipolar disorder (BD) is a major health problem. It causes significant morbidity and imposes a burden on the society. Available treatments help a substantial proportion of patients but are not beneficial for an estimated 40-50%. Thus, there is a great need to further our understanding the pathophysiology of BD to identify new therapeutic avenues. The preponderance of evidence pointed towards a role of protein kinase C (PKC) in BD. We reviewed the literature pertinent to the role of PKC in BD. We present recent advances from preclinical and clinical studies that further support the role of PKC. Moreover, we discuss the role of PKC on synaptogenesis and neuroplasticity in the context of BD. The recent development of animal models of BD, such as stimulant-treated and paradoxical sleep deprivation, and the ability to intervene pharmacologically provide further insights into the involvement of PKC in BD. In addition, the effect of PKC inhibitors, such as tamoxifen, in the resolution of manic symptoms in patients with BD further points in that direction. Furthermore, a wide variety of growth factors influence neurotransmission through several molecular pathways that involve downstream effects of PKC. Our current understanding identifies the PKC pathway as a potential therapeutic avenue for BD.

Dexamethasone-Induced Effects on B-50/GAP-43 Expression and Neurite Outgrowth in PC 12 Cells

Undifferentiated PC 12 cells contain detectable levels of the nervous-specific protein B-50/GAP-43. Upon treatment with NGF or change of culture medium, B-50/ GAP-43 levels remained unchanged during the first 12 hours while neuritogenesis starts. Both, B-50/GAP-43 levels and neurite outgrowth peak at 24 hours. These results suggest that in PC 12 cells the amount of B-50 already present is sufficient to support the start of NGF-induced neuritogenesis, presumably by translocation from cytosolic compartments to the membrane. Addition of DEX reversed the rise in B-50/GAP-43 levels induced by either the change of medium or by NGF. In contrast, neurite outgrowth was inhibited to a lesser extent, although after 36 hours of pretreatment with DEX neurite length was lower than control. NGF was capable of enhancing B-50/GAP-43 levels both in the presence and absence of DEX. This corroborates data from others, who concluded that DEX and NGF exert their effects through different mechanisms, e.g., transcription versus mRNA stabilization, respectively. The inhibitory effect of DEX under various conditions on both B-50 expression and neurite outgrowth in the normal PC 12 cell line demonstrates the tight coupling of these parameters that might be indicative of a threshold effect of B-50 levels on neurite outgrowth.

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.

Molecular mechanisms, biological actions, and neuropharmacology of the growth-associated protein GAP-43

GAP-43 is an intracellular growth-associated protein that appears to assist neuronal pathfinding and branching during development and regeneration, and may contribute to presynaptic membrane changes in the adult, leading to the phenomena of neurotransmitter release, endocytosis and synaptic vesicle recycling, long-term potentiation, spatial memory formation, and learning. GAP-43 becomes bound via palmitoylation and the presence of three basic residues to membranes of the early secretory pathway. It is then sorted onto vesicles at the late secretory pathway for fast axonal transport to the growth cone or presynaptic plasma membrane. The palmitate chains do not serve as permanent membrane anchors for GAP-43, because at steady-state most of the GAP-43 in a cell is membrane-bound but is not palmitoylated. Filopodial extension and branching take place when GAP-43 is phosphorylated at Ser-41 by protein kinase C, and this occurs following neurotrophin binding and the activation of numerous small GTPases. GAP-43 has been proposed to cluster the acidic phospholipid phosphatidylinositol 4,5-bisphosphate in plasma membrane rafts. Following GAP-43 phosphorylation, this phospholipid is released to promote local actin filament-membrane attachment. The phosphorylation also releases GAP-43 from calmodulin. The released GAP-43 may then act as a lateral stabilizer of actin filaments. N-terminal fragments of GAP-43, containing 10-20 amino acids, will activate heterotrimeric G proteins, direct GAP-43 to the membrane and lipid rafts, and cause the formation of filopodia, possibly by causing a change in membrane tension. This review will focus on new information regarding GAP-43, including its binding to membranes and its incorporation into lipid rafts, its mechanism of action, and how it affects and is affected by extracellular agents.

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.

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

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

Hippocampal synapsin I, growth-associated protein-43, and microtubule-associated protein-2 immunoreactivity in learned helplessness rats and antidepressant-treated rats

Learned helplessness rats are thought to be an animal model of depression. To study the role of synapse plasticity in depression, we examined the effects of learned helplessness and antidepressant treatments on synapsin I (a marker of presynaptic terminals), growth-associated protein-43 (GAP-43; a marker of growth cones), and microtubule-associated protein-2 (MAP-2; a marker of dendrites) in the hippocampus by immunolabeling. (1) Learned helplessness rats showed significant increases in the expression of synapsin I two days after the attainment of learned helplessness, and significant decreases in the protein expression eight days after the achievement of learned helplessness. Subchronic treatment of naïve rats with imipramine or fluvoxamine significantly decreased the expression of synapsin I. (2) Learned helplessness increased the expression of GAP-43 two days and eight days after learned helplessness training. Subchronic treatment of naïve rats with fluvoxamine but not imipramine showed a tendency to decrease the expression of synapsin I. (3) Learned helplessness rats showed increased expression of MAP-2 eight days after the attainment of learned helplessness. Naïve rats subchronically treated with imipramine showed a tendency toward increased expression of MAP-2, but those treated with fluvoxamine did not. These results indicate that the neuroplasticity-related proteins synapsin I, GAP-43, and MAP-2 may play a role in the pathophysiology of depression and the mechanisms of antidepressants.

Anomalous functional organization of barrel cortex in GAP-43 deficient mice

Growth associated protein 43 (GAP-43), found only in the nervous system, regulates the response of neurons to axon guidance signals. It is also critical for establishing normal somatotopy. Mice lacking GAP-43 (KO) show aberrant pathfinding by thalamocortical afferents, and do not form cortical whisker/barrels. GAP-43 heterozygous (HZ) mice show more subtle deficits--delayed barrel segregation and enlarged barrels at postnatal day 7. Here, we used cortical intrinsic signal imaging to characterize adult somatotopy in wildtype (WT), GAP-43 KO, and HZ mice. We found clear foci of activation in GAP-43 KO cortex in response to single-whisker stimulation. However, the KO spatial activation patterns showed severe anomalies, indicating a loss of functional somatotopy. In some cases, multiple foci were activated by single whiskers, while in other cases, the same cortical zone was activated by several whiskers. The results are consistent with our previous findings of aberrant pathfinding and clustering by thalamocortical afferent axons, and absence of barrel patterning. Our findings indicate that cortex acts to cluster afferents from a given whisker, even in the absence of normal topography. By contrast, single-whisker stimulation revealed normal adult topographic organization in WT and HZ mice. However, we found that functional representations of adult HZ barrels are larger than those found in WT mice. Since histological HZ barrels recover normal dimensions by postnatal day 26, the altered circuit function in GAP-43 HZ cortex could be a secondary consequence of the rescue of barrel dimensions.

Specific Proteolysis of Neuronal Protein GAP-43 by Calpain: Characterization, Regulation, and Physiological Role

The mechanism of specific proteolysis of the neuronal protein GAP-43 in axonal terminals has been investigated. In synaptic terminals in vivo and in synaptosomes in vitro GAP-43 is cleaved only at the single peptide bond formed by Ser41; this is within the main effector domain of GAP-43. Proteolysis at this site involves the cysteine calcium-dependent neutral protease calpain. The following experimental evidences support this conclusion: 1) calcium-dependent proteolysis of GAP-43 in synaptosomes is insensitive to selective inhibitor of micro-calpain (PD151746), but it is completely blocked by micro- and m-calpain inhibitor PD150606; 2) GAP-43 proteolysis in the calcium ionophore A23187-treated synaptosomes is activated by millimolar concentration of calcium ions; 3) the pattern of fragmentation of purified GAP-43 by m-calpain (but not by micro-calpain) is identical to that observed in synaptic terminals in vivo. GAP-43 phosphorylated at Ser41 by protein kinase C (PKC) is resistant to the cleavage by calpain. In addition, calmodulin binding to GAP-43 decreases the rate of calpain-mediated GAP-43 proteolysis. Our results indicate that m-calpain-mediated GAP-43 proteolysis regulated by PKC and calmodulin is of physiological relevance, particularly in axonal growth cone guidance. We suggest that the function of the N-terminal fragment of GAP-43 (residues 1-40) formed during cleavage by m-calpain consists in activation of neuronal heterotrimeric GTP-binding protein G(o); this results in growth cone turning in response to repulsive signals.

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.

Resting-state brain glucose utilization as measured by PET is directly related to regional synaptophysin levels: a study in baboons

It is classically recognized that regional cerebral glucose consumption (CMRglc), as measured by positron emission tomography (PET) and [18F]-2-fluorodeoxyglucose (FDG), is a precise index of the integrated local neuronal activity. However, despite extensive use of the FDG-PET method, the significance of the measured CMRglc has been little addressed so far. In the present study, we aimed for the first time to test whether resting-state CMRglc is directly related to synaptic density. To this end, we investigated in the baboon the relationships between CMR(glc) and the levels of synaptophysin (SY), a presynaptic protein classically used to assess synaptic density. CMR(glc), measured in vivo by FDG-PET at the resting-state, and SY levels, assessed postmortem by the Western blot technique, were quantified in seven brain areas of five baboons. By applying these two techniques to the same animals, we found significant positive correlations between CMR(glc) and SY levels, across all regions and all animals, as well as within individual baboons. These findings strongly support the hypothesis that resting-state CMR(glc) reflects integrated synaptic activity.

Two distinct types of corticothalamic EPSPs and their contribution to short-term synaptic plasticity

The lateral posterior nucleus (LPN) is innervated by two different morphological types of cortical terminals that originate from cortical layers V and VI. Here we describe two distinct types of excitatory postsynaptic potentials (EPSPs) that were recorded in the LPN after stimulation of corticothalamic fibers. These types of EPSPs differed in amplitude, latency, rise time, and response to increasing levels of stimulus intensity. The most frequently encountered EPSP, type I, displayed a longer latency and slower rise time than the less frequently encountered type II EPSP. Type I EPSPs also showed a graded increase in amplitude with increasing levels of stimulation, whereas type II EPSPs showed an all-or-none response. In response to repetitive stimulation (0.5-20 Hz), type I EPSPs displayed frequency-dependent facilitation, whereas type II EPSPs displayed frequency-dependent depression. Further details of these distinct forms of short-term synaptic plasticity were explored using paired-pulse stimuli. Pharmacology experiments revealed that both N-methyl-d-aspartate (NMDA) and non-NMDA glutamate receptors are involved in corticothalamic synaptic transmission in the LPN and contribute to both synaptic facilitation and depression. Taken together with the results of our previous anatomical studies, these results suggest that type I EPSPs arise from stimulation of layer VI afferents, whereas type II EPSPs arise from stimulation of layer V inputs. Moreover, type I and II EPSPs in the LPN may be functionally similar to corticogeniculate and retinogeniculate EPSPs, respectively.

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.

Regulation of GAP-43 expression by chronic desipramine treatment in rat cultured hippocampal cells

BACKGROUND

The importance of molecular and cellular changes in hippocampus in major depression and in the mechanism of action of antidepressants has become increasingly clear. Identification of novel targets for antidepressants in hippocampus is important to understanding their therapeutic effects.

METHODS

We used cDNA microarray to measure the expression patterns of multiple genes in primary cultured rat hippocampal cells. In situ hybridization and Northern and immunoblotting analysis were used to determine brain regional distribution and mRNA and protein levels of target genes.

RESULTS

After comparing hybridized signals between control and desipramine treated groups, we found that chronic treatment with desipramine increased the expression of six genes and decreased the expression of two genes. One of the upregulated genes is growth associated protein GAP-43. In situ hybridization revealed that desipramine increased GAP-43 gene expression in dentate gyrus but not other brain regions. Northern and immunoblotting analysis revealed that desipramine increased GAP-43 mRNA and protein levels. GAP-43 expression is also increased by another antidepressant, tranylcypromine, but not by lithium or haloperidol.

CONCLUSIONS

Because GAP-43 regulates growth of axons and modulates the formation of new connections, our findings suggest that desipramine may have an effect on neuronal plasticity in the central nervous system.

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.

On the Distribution of GAP-43 and its Relation to Serotonin in Adult Monkey and Cat Spinal Cord and Lower Brainstem

By use of a monoclonal antibody, the distribution of growth-associated protein (GAP)-43-like immunoreactivity (LI) has been studied in the spinal cord of adult grey monkeys (Macaca fascicularis) and adult cats by use of immunofluorescence and peroxidase - antiperoxidase techniques. The brainstem was also studied with in situ hybridization histochemistry. In both monkeys and cats, a dense innervation of GAP-43-immunoreactive (IR) fibres was seen in close apposition to large cell bodies and their processes in the motor nucleus of the ventral horn. Double-labelling experiments revealed a high degree of coexistence between GAP-43- and 5-hydroxytryptamine (5-HT, serotonin)-LI in the monkey motor nucleus, while in the cat no such colocalization could be verified. At the electron microscopic level, GAP-43 labelling was seen as a coating of vesicles and axolemma inside the terminals. In both monkey and cat, cell bodies expressing mRNA encoding GAP-43 were demonstrated in the medullary midline raphe nuclei. A similar location was also encountered for mRNA for aromatic l-amino acid decarboxylase, an enzyme found in both catecholamine- and serotonin-containing neurons. The present results suggest that GAP-43 is present in the 5-HT bulbospinal pathway of the monkey. In the cat, GAP-43 mRNA-expressing cell bodies were demonstrated in areas where descending 5-HT neurons are located, but no convincing colocalization of 5-HT- and GAP-43-LI was found at spinal cord levels, despite the existence of extensive fibre networks containing either of the two compounds. Possible explanations for this species discrepancy are discussed. The function of GAP-43 in nerve terminals impinging on the motoneurons is unknown. However, it may play a role in transmitter release and/or plasticity, since such roles have been proposed for this protein in other systems.

Noradrenergic, but not Adrenergic Chromaffin Cells in the Adrenal Gland Express Neuromodulin (GAP-43)

Neuroendocrine chromaffin cells of the adrenal gland express certain molecular markers either transiently during development or permanently. In the present study, the expression of neuromodulin (GAP-43), a neuronal protein often associated with neurite outgrowth, was examined in adult adrenals. Neuromodulin was detected by Western blot analysis in extracts of both rat adrenals and cultured bovine chromaffin cells, and was localized in situ in a subpopulation of chromaffin cells, as well as in nerve fibres and Schwann cells. The use of anti-tyrosine hydroxylase or anti-phenylethanolamine N-methyltransferase antibodies in combination with anti-neuromodulin antibodies in double immunofluorescent labelling of cryostat sections of rat glands demonstrated that neuromodulin is expressed by noradrenergic, and not by adrenergic chromaffin cells. The results provide further evidence that neuromodulin is not limited to neurons; it is also expressed in a subpopulation of neuroendocrine chromaffin cells. Neuromodulin may play a role in the development of the adrenal medulla or in the specific regulation of noradrenalin secretion from chromaffin cells.

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 and CaMKII dependent synaptic potentiation in cultured cerebral neurons

We have reported that the long-lasting potentiation of spontaneous excitatory postsynaptic currents (SEPSCs) was induced by a Mg(2+)-free treatment in cultured chick cerebral neurons and a factor(s) extracellularly released during the treatment could induce the potentiation by itself. In this paper, protein kinase C (PKC) and calcium/calmodulin-dependent protein kinase type II (CaMKII) but not protein kinase A (PKA) were reported to contribute to the potentiation mechanism during a step between the activation of the N-methyl-D-aspartate receptors by the Mg(2+)-free treatment and the secretion of the protein factor(s).

Synaptic vesicle proteins and neuronal plasticity in adrenergic neurons

The neurons in the superior cervical ganglion are active in plasticity and re-modelling in order to adapt to requirements. However, so far, only a few studies dealing with synaptic vesicle related proteins during adaptive processes have been published. In the present paper, changes in content and expression of the synaptic vesicle related proteins in the neurons after decentralization (cutting the cervical sympathetic trunk) or axotomy (cutting the internal and external carotid nerves) were studied. Immunofluorescence studies were carried out using antibodies and antisera against integral membrane proteins, vesicle associated proteins, NPY, and the enzymes TH and PNMT. For colocalization studies, the sections were simultaneously double labelled. Confocal laser scanning microscopy was used for colocalization studies as well as for semi-quantification analysis, using the computer software. Westen blot analysis, in situ 3'-end DNA labelling, and in situ hybridization were also employed. After decentralization of the ganglia several of the synaptic vesicle proteins (synaptotagmin I, synaptophysin, SNAP-25, CLC and GAP-43) were increased in the iris nerve terminal network, but with different time patterns, while TH-immunoreactivity had clearly decreased. In the ganglia, these proteins had decreased at 1 day after decentralization, probably due to degeneration of the pre-ganglionic nerve fibres and terminals. At later intervals, these proteins, except SNAP-25, had increased in the nerve fibre bundles and re-appeared in nerve fibres outlining the principal neurons.

Modulation of calcium-evoked [3H]noradrenaline release from permeabilized cerebrocortical synaptosomes by the MARCKS protein, calmodulin and the actin cytoskeleton

In order to examine intracellular modulation of CNS catecholamine release, cerebrocortical synaptosomes were prelabeled with [3H]noradrenaline and permeabilized with streptolysin-O in the absence or presence of Ca(2+). Plasma membrane permeabilization allowed efflux of cytosol and left a compartmentalized pool of [3H]noradrenaline intact, approximately 10% of which was released by addition of 10(-5) M Ca(2+). Addition of activators or inhibitors of protein kinase C, as well as inhibitors of Ca(2+)-calmodulin kinase II or calcineurin, failed to change Ca(2+)-induced noradrenaline release. Evoked release from permeabilized synaptosomes deficient in the vesicle-associated phosphoprotein synapsin I was also unchanged. In contrast, addition of a synthetic 'active domain' peptide from the myristoylated, alanine-rich C-kinase substrate (MARCKS) protein increased, while addition of calmodulin decreased Ca(2+)-induced release from the permeabilized synaptosomes, the latter effect being reversed by a peptide inhibitor of calcineurin. Moreover, addition of the actin-destabilizing agent DNase I, as well as antibodies to MARCKS, appeared to increase spontaneous, Ca(2+)-independent release from noradrenergic vesicles. These results indicate that the MARCKS protein may modulate release from permeabilized noradrenergic synaptosomes, possibly by modulating calmodulin levels and/or the actin cytoskeleton.

Decrease in phorbol ester-induced potentiation of noradrenaline release in synapsin I-deficient mice

Synapsin I is involved in regulating amino acid neurotransmitter release, but has a less clear role in noradrenergic nerve terminals. To better understand the role of synapsin I in the function of noradrenergic nerve terminals, we compared noradrenaline release in wild-type and synapsin I-deficient mice. No difference was found in the accumulation or in the Ca(2+)-independent release of [(3)H]noradrenaline in cerebrocortical synaptosomes from wild-type and synapsin I-deficient mice. Synaptosomes lacking synapsin I also displayed no gross alterations in either the time course or the Ca(2+)-dependency of [(3)H]noradrenaline release when stimulated by depolarizing secretagogues or ionophore treatment. In wild-type synaptosomes, activation of protein kinase C by phorbol ester treatment resulted in a Ca(2+)-dependent increase in [(3)H]noradrenaline release evoked by depolarizing secretagogues and ionophore treatment. The phorbol ester-mediated enhancement of [(3)H]noradrenaline release evoked by depolarizing secretagogues, but not by ionophore treatment, was greatly reduced in synapsin I-deficient synaptosomes. These results indicate that synapsin I plays a role in regulating noradrenaline release.

Entrapping of impermeant probes of different size into nonpermeabilized synaptosomes as a method to study presynaptic mechanisms

Small molecules present during brain tissue homogenization are known to be entrapped within subsequently isolated synaptosomes. We have revisited this technique in view of its systematic utilization to incorporate into nerve endings impermeant probes of large size. Rat neocortical synaptosomes were prepared in the absence or in the presence of each of the following compounds: 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), tetanus toxin (TeTx) or its light chain (TeTx-LC), pertussis toxin (PTx), anti-syntaxin, or anti-SNAP25 monoclonal antibodies. Release of endogenous GABA and glutamate was then evoked by high K+ depolarization. GABA and glutamate overflows were inhibited by entrapped BAPTA and in synaptosomes prepared by homogenization in the presence of varying concentrations of TeTx or TeTx-LC. When synaptobrevin cleavage in synaptosomes entrapped with TeTx was monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by western blotting, the extent of proteolysis was found to correspond quantitatively to that of release inhibition. GABA and glutamate overflows were increased by entrapped PTx; moreover, (-)-baclofen inhibited amino acid overflow more potently in standard than in PTx-containing synaptosomes. The overflows of GABA and glutamate were similarly decreased following incorporation of anti-syntaxin or anti-SNAP25 antibodies. Synaptosomal entrapping may be routinely used to internalize membrane-impermeant agents of different size in studies of presynaptic mechanisms.

Lithium and synaptic plasticity

Lithium, a small cation, has been used in the treatment of bipolar disorders since its introduction in the 1950s by John Cade. Extensive research on the mechanism of action of lithium has revealed several possible targets. For some time, the most widely accepted action of lithium was its inhibitory effect on the synthesis of inositol, resulting in depletion of inositol with profound effects on neuronal signal transduction pathways. However, several studies show that some effects of lithium are not mediated through inositol depletion. Recent findings demonstrate that lithium directly inhibits, in a non-competitive fashion, the activity of glycogen synthase kinase (GSK)-3beta, a serine/threonine kinase highly expressed in the central nervous system. Interestingly, inhibition of GSK-3beta has been shown to regulate neuronal plasticity by inducing axonal remodelling and increasing the levels of synaptic proteins. These findings raise the possibility for developing new therapeutic approaches for the treatment of bipolar disorders.

Bilirubin inhibits Ca2+-dependent release of norepinephrine from permeabilized nerve terminals

Although the well-known neurotoxic agent bilirubin can induce alterations in neuronal signaling, direct effects on neurotransmitter release have been difficult to demonstrate. In the present study we have used permeabilized nerve terminals (synaptosomes) from rat brain prelabeled with [3H]norepinephrine to examine the effects of bilirubin on transmitter release. Rat cerebrocortical synaptosomes were permeabilized with streptolysin-O (2 U/ml) in the absence or presence of bilirubin (10 microM-320 microM) and Ca2+ (100 microM), and the amount of radiolabeled transmitter released during 5 min to the medium was analysed. Low levels of bilirubin decreased Ca2+-evoked release in a dose-dependent manner, with half-maximal effect at approx 25 microM bilirubin. Higher levels of bilirubin (100-320 microM) increased [3H]norepinephrine efflux in the absence of Ca2+, suggesting that high bilirubin levels induced leakage of transmitter from vesicles. The nontoxic precursor biliverdin had no effect on Ca2+-dependent exocytosis. Our data indicate that bilirubin directly inhibits both exocytotic release and vesicular storage of brain catecholamines.

Potential role of calcineurin for brain ischemia and traumatic injury

Calcineurin belongs to the family of Ca2+/calmodulin-dependent protein phosphatase, protein phosphatase 2B. Calcineurin is the only protein phosphatase which is regulated by a second messenger, Ca2+. Furthermore, calcineurin is highly localized in the central nervous system, especially in those neurons vulnerable to ischemic and traumatic insults. For these reasons, calcineurin is considered to play important roles in neuron-specific functions. Recently, on the basis of the finding that FK506 and cyclosporin A serve as calcineurin-specific inhibitors, this enzyme has become the subject of much study. It is clear that calcineurin is involved in many neuronal (or non-neuronal) functions such as neurotransmitter release, regulation of receptor functions, signal transduction systems, neurite outgrowth, gene expression and neuronal cell death. In this review, we describe the calcineurin functions, functions of the substrates, and the pathogenesis of traumatic and ischemic insults, and we discuss the potential role of calcineurin. There are many similarities in traumatic and ischemic pathogenesis of the brain in which the release of excessive glutamate is followed by an intracellular Ca2+ increase. However, the intracellular cascade which leads to neuronal cell death after the release of excess Ca2+ is unclear. Although calcineurin is thought to be a key toxic enzyme on the basis of studies using immunosuppressants (FK506 or cyclosporin A), many of the functions of the substrates for calcineurin protect against neuronal cell death. We concluded that calcineurin is a bi-directional enzyme for neuronal cell death, having protective and toxic actions, and the balance of the bi-directional effects may be important in ischemic and traumatic pathogenesis.

Regulation of calcium-dependent [3H]noradrenaline release from rat cerebrocortical synaptosomes by protein kinase C and modulation of the actin cytoskeleton

The effects that active phorbol esters, staurosporine, and changes in actin dynamics, might have on Ca2+ -dependent exocytosis of [3H]-labelled noradrenaline, induced by either membrane-depolarizing agents or a Ca2+ ionophore, have been examined in isolated nerve terminals in vitro. Depolarization-induced openings of voltage-dependent Ca2+ channels with 30 mM KCl or 1 mM 4-aminopyridine induced limited exocytosis of [3H]noradrenaline, presumably from a readily releasable vesicle pool. Application of the Ca2+ ionophore calcimycin (10 microM) induced more extensive [3H]noradrenaline release, presumably from intracellular reserve vesicles. Stimulation of protein kinase C with phorbol 12-myristate,13-acetate increased release evoked by all secretagogues. Staurosporine (1 microM) had no effect on depolarization-induced release, but decreased ionophore-induced release and reversed all effects of the phorbol ester. When release was induced by depolarization, internalization of the actin-destabilizing agent DNAase I into the synaptosomes gave a slight increase in [3H]NA release and strongly increased the potentiating effect of the phorbol ester. In contrast, when release was induced by the Ca2+ ionophore, DNAase I had no effect, either in the absence or presence of phorbol ester. The results indicate that depolarization of noradrenergic rat synaptosomes induces Ca2+ -dependent release from a releasable pool of staurosporine-insensitive vesicles. Activation of protein kinase C increases this release by staurosporine-sensitive mechanisms, and destabilization of the actin cytoskeleton further increases this effect of protein kinase C. In contrast, ionophore-induced noradrenaline release originates from a pool of staurosporine-sensitive vesicles, and although activation of protein kinase C increases release from this pool, DNAase I has no effect and also does not change the effect of protein kinase C. The results support the existence of two functionally distinct pools of secretory vesicles in noradrenergic CNS nerve terminals, which are regulated in distinct ways by protein kinase C and the actin cytoskeleton.

Hypoxia/ischemia induces dephosphorylation of rat brain neuromodulin/GAP-43 in vivo

The in vivo state of phosphorylation and the modification of two Cys residues of neuromodulin/ GAP-43 (Nm) were analyzed by electrospray ionization-mass spectrometry (ES-MS). The protein was purified from rat brain with homogenization buffer containing 1% Nonidet P-40, protease inhibitors, protein phosphatase inhibitors, and sulfhydryl reagent, 4-vinylpyridine. Nm was purified by HPLC and ion-exchange chromatography, and the various fractions were identified by ES-MS as unphosphorylated and mono-, di-, tri-, and tetraphosphorylated species. All of these Nm species contained 2 mol of added 4-vinylpyridine per mol of Nm, suggesting that the two Cys residues are in the reduced form in the brain. In vivo, the majority of Nm is in the phosphorylated form (approximately 80%), of which the levels of the mono- and diphospho forms are higher than those of the tri- and tetraphospho species. Four in vivo phosphorylation sites, Ser41, Thr95, Ser142, and Thr172, were identified by amino acid sequencing and tandem ES-MS of the peptides derived from Lys-C endoproteinase digestion. Among these sites, only Ser41 is a known target of PKC, whereas the kinases responsible for the phosphorylation of the other three novel sites are unknown. Hypoxia/ischemia caused a preferential dephosphorylation of Ser41 and Thr172, whereas Thr95 is the least susceptible to dephosphorylation.

Alterations in the organization of the isocortical layer I in trisomy 22

The isocortical layer I of human fetal brains obtained from different cases of chromosomal abnormalities (trisomy 18, 21, 22) and controls without pathological disturbances were investigated histologically and immunohistochemically by using the antibodies SMI 311, SMI 35 and SMI 81 (SNAP 25) as well as antibodies against GAP 43 and calretinin. In cases of trisomy 22 the Cajal-Retzius cells in Nissl-sections and in SMI 311-immunopreparations do not reveal any alterations regarding their location or morphology. However, the axonal plexus, selectively labelled with SMI 35, normally located in layer Ib, is malpositioned in Ia. Likewise, SNAP 25- and GAP 43-immunoreactive structures, which were taken as signs of synaptogenesis, are displaced and appear in Ia instead of Ib. Cases of trisomy 18 and 21 show no changes within the organization of layer I. In trisomy 22 the isocortical layer I reveals malpositioned axonal plexus and a corresponding displacement of synaptic proteins. The possible significance of this alteration in the developmental process of the isocortex is discussed.

The protein kinase C (PKC) substrate GAP-43 is already expressed in neural precursor cells, colocalizes with PKCeta and binds calmodulin

Expression of the growth-associated protein of 43-kDa (GAP-43), which is described as a postmitotic, neuron-specific major protein kinase C (PKC) substrate, was investigated in the murine embryonic carcinoma cell line PCC7-Mz1 which develops into a brain-tissue-like pattern of neuronal, fibroblast-like and astroglial cells upon stimulation with all-trans retinoic acid (RA). GAP-43 expression was very low in stem cells, but increased on mRNA and protein level within the 12 h after differentiation was initiated. While the P1 promoter of the GAP-43 gene gave rise to a 1.6-kb mRNA and was already active at a very low level in PCC7-Mz1 stem cells, transcription of the P2 promoter, which resulted in a 1.4-kb mRNA, was completely blocked in stem cells but increased rapidly after RA treatment. Within the first 2 days of neural differentiation, GAP-43 was localized with the cytoplasmic membrane and the Golgi complex of proliferating neural precursor cells. Then, GAP-43 was translocated to the growth cones and neurites, and from day 6, when neurons began to acquire polarity, the protein was found in the axons. GAP-43 was never detected in the non-neuronal PCC7-Mz1 derivatives, i.e. in fibroblasts or glial cells. In the foetal rat brain (prenatal day F11), GAP-43 was expressed in the optic stalk, the lense plakode and in the postmitotic neurons of the marginal zone of the hindbrain. Moreover, in a layer between the ventricular and marginal zone of the hindbrain (F13) and forebrain (F15), GAP-43 was already expressed in mitotic neural precursor cells. In PCC7-Mz1 cultures, 2 days after addition of RA, GAP-43 became phosphorylated upon activation of PKC, and colocalized specifically with the novel PKC isoform eta. Phosphorylation of GAP-43 caused a disruption of its complex with calmodulin. These data demonstrate that GAP-43 is already a functional PKC substrate in prolific neuronal precursor cells, and may participate in neuronal cell lineage determination.

Excitatory synaptic transmission and its modulation by PKC is unchanged in the hippocampus of GAP-43-deficient mice

We compared excitatory synaptic transmission between hippocampal pyramidal cells in dissociated hippocampal cell cultures and in area CA3 of hippocampal slice cultures derived from wild-type mice and mice with a genetic deletion of the presynaptic growth associated protein GAP-43. The basal frequency and amplitude of action potential-dependent and -independent spontaneous excitatory postsynaptic currents were similar in both groups. The probability that any two CA3 pyramidal cells in wild-type or GAP-43 knockout (-/-) slice cultures were synaptically connected was assessed with paired recordings and was not different. Furthermore, unitary synaptic responses were similar in the two genotypes. Bath application of phorbol 12,13-diacetate (0.6-3 microM) elicited a comparable increase in the frequency of miniature excitatory synaptic currents in wild-type and GAP-43 (-/-) cultures. This effect was blocked by the protein kinase C inhibitor, bisindolylmaleimide I (1.2 microM). Finally, 3 microM phorbol 12,13-diacetate potentiated the amplitude of unitary synaptic currents to a comparable extent in wild-type and GAP-43 (-/-) slice cultures. We conclude that GAP-43 is not required for normal excitatory synaptic transmission or the potentiation of presynaptic glutamate release mediated by activation of protein kinase C in the hippocampus.

Substrate phosphorylation in the protein kinase Cgamma knockout mouse

The phosphorylation state of three identified neural-specific protein kinase C substrates (RC3, GAP-43/B-50, and MARCKS) was monitored in hippocampal slices of mice lacking the gamma-subtype of protein kinase C and wild-type controls by quantitative immunoprecipitation following 32Pi labeling. Depolarization with potassium, activation of glutamate receptors with glutamate, or direct stimulation of protein kinase C with a phorbol ester increased RC3 phosphorylation in wild-type animals but failed to affect RC3 phosphorylation in mice lacking the gamma-subtype of protein kinase C. Our results suggests the following biochemical pathway: activation of a postsynaptic (metabotropic) glutamate receptor stimulates the gamma-subtype of protein kinase C, which in turn phosphorylates RC3. The inability to increase RC3 phosphorylation in mice lacking the gamma-subtype of protein kinase C by membrane depolarization or glutamate receptor activation may contribute to the spatial learning deficits and impaired hippocampal LTP observed in these mice.

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.

Expression of protein kinase C gene family members is temporally and spatially regulated during neural development in vitro

We used primary cultures of rat hippocampal neurons and PCC7-Mz1 cells to correlate the expression of the protein kinase C (PKC) gene family with specific events during neural differentiation. Multipotent PCC7-Mz1 embryonic carcinoma stem cells develop into a tissue-like pattern of neuronal, fibroblast-like and astroglial cells by all-trans retinoic acid (RA) treatment. Western blot analyses demonstrate that PKCalpha, betaI, gamma, theta, mu, lambda, and zeta were constitutively expressed but the expression of PKCbetaII, delta, epsilon, and eta was up-regulated three days after addition of RA when cells mature morphologically. While the protein levels of the PKC isoforms betaII, delta and eta decreased after d6, when the major phenotypical alterations of the developing neurons were completed, PKCepsilon expression remained at a high level. Immunofluorescence studies demonstrated that PKCalpha, lambda and zeta were constantly expressed in stem cells and the arising cell types. PKCdelta was detected in all differentiated cell types, whereby PKCbetaII, gamma, epsilon, and zeta were solely found in the neuronal derivatives with PKCgamma predominantly located in the nuclei. PKCeta was weakly expressed at the Golgi complex of stem cells but expanded throughout the entire somata of all developing neurons. In contrast, PKCbetaII was abundant only in the somata of a minor fraction of all neurons (approximately 2.5%). Also, PKCepsilon was exclusively synthesized by a subpopulation of neurons (40+/-5%), where it was localized in the somata and in the axons. PKCzeta was persistently expressed in two forms, the full-length PKCzeta and the constitutively active, proteolytic product PKMzeta, reasoning that permanent PKCzeta activity is important for PCC7-Mz1 physiology. Fractionation of extracts from undifferentiated and differentiating PCC7-Mz1 cells revealed that the conventional cPKCalpha was partly and the cPKCbetaI and the novel nPKCs delta and epsilon were mainly membrane bound, implying that they were also in an active state. However, when using the PKC substrate MARCKS (myristoylated alanine-rich C kinase substrate) to monitor cellular PKC activity, we observed that activation of PKC by phorbol ester was required for complete MARCKS phosphorylation and its translocation from the membrane to the cytoplasm. Our data show that the cell type-specific expression, subcellular localization and activation of PKCs are regulated in an isoform-specific manner during neurogenesis suggesting that they are involved in the control of neural development and in particular in neuronal differentiation.

Paralemmin, a prenyl-palmitoyl-anchored phosphoprotein abundant in neurons and implicated in plasma membrane dynamics and cell process formation

We report the identification and initial characterization of paralemmin, a putative new morphoregulatory protein associated with the plasma membrane. Paralemmin is highly expressed in the brain but also less abundantly in many other tissues and cell types. cDNAs from chicken, human, and mouse predict acidic proteins of 42 kD that display a pattern of sequence cassettes with high inter-species conservation separated by poorly conserved linker sequences. Prenylation and palmitoylation of a COOH-terminal cluster of three cysteine residues confers hydrophobicity and membrane association to paralemmin. Paralemmin is also phosphorylated, and its mRNA is differentially spliced in a tissue-specific and developmentally regulated manner. Differential splicing, lipidation, and phosphorylation contribute to electrophoretic heterogeneity that results in an array of multiple bands on Western blots, most notably in brain. Paralemmin is associated with the cytoplasmic face of the plasma membranes of postsynaptic specializations, axonal and dendritic processes and perikarya, and also appears to be associated with an intracellular vesicle pool. It does not line the neuronal plasmalemma continuously but in clusters and patches. Its molecular and morphological properties are reminiscent of GAP-43, CAP-23, and MARCKS, proteins implicated in plasma membrane dynamics. Overexpression in several cell lines shows that paralemmin concentrates at sites of plasma membrane activity such as filopodia and microspikes, and induces cell expansion and process formation. The lipidation motif is essential for this morphogenic activity. We propose a function for paralemmin in the control of cell shape, e.g., through an involvement in membrane flow or in membrane-cytoskeleton interaction.

Enhanced neocortical neural sprouting, synaptogenesis, and behavioral recovery with D-amphetamine therapy after neocortical infarction in rats

BACKGROUND AND PURPOSE

D-Amphetamine administration increases behavioral recovery after various cortical lesions including cortical ablations, contusions, and focal ischemia in animals and after stroke in humans. The purpose of the present study was to test the enhanced behavioral recovery and increased expression of proteins involved in neurite growth and synaptogenesis in D-amphetamine-treated rats compared with vehicle-treated controls after a focal neocortical infarct.

METHODS

Unilateral neocortical ischemia was induced in male spontaneously hypertensive Wistar rats (n=8 per time point per group) by permanently occluding the distal middle cerebral artery and ipsilateral common carotid artery in 2 groups of rats: D-amphetamine treated (2 mg/kg IP injections) and vehicle treated (saline IP injections). To determine the spatial and temporal distribution of neurite growth and/or synaptogenesis, growth-associated protein (GAP-43), a protein expressed on axonal growth cones, and synaptophysin, a calcium-binding protein found on synaptic vesicles, were examined by immunohistochemical techniques, and both density and distribution of reaction product were measured. Since the resulting infarction included a portion of the forelimb neocortex, behavioral assessments of forelimb function using the foot-fault test of Hernandez and Schallert were performed on the same rats used for immunohistochemical studies during the period of drug action and 24 hours later. A Morris water maze and other indices of behavioral assays were also measured similarly. Recovery times were 3, 7, 14, 30, and 60 days postoperatively.

RESULTS

Both GAP-43 and synaptophysin proteins demonstrated statistically significant increases in density and distribution of immunoreaction product as determined by optical density measurements in the neocortex of the infarcted group treated with D-amphetamines compared with vehicle-treated infarcted controls. The GAP-43 was elevated to statistically significant levels in forelimb, hindlimb, and parietal neocortical regions ipsilateral to the infarction only at days 3, 7, and 14. By contrast, the synaptophysin demonstrated no statistically significant changes in expression at 3 or 7 days but demonstrated statistically significant increases at 14, 30, and 60 days in the forelimb, hindlimb, and parietal neocortical regions ipsilateral to the infarction as well as increased distribution in the contralateral parietal neocortex. Behavioral assessment of forelimb function indicated that improved recovery of forelimb placement on the side contralateral to the infarction was statistically significant in the D-amphetamine-treated group compared with the vehicle-treated group (P<0.025). Spatial memory, as measured with the Morris water maze, worsened in the vehicle-treated group compared with the D-amphetamine-treated group at 60 days (P<0.025).

CONCLUSIONS

These data support the occurrence of neurite growth followed by synaptogenesis in the neocortex in a pattern that corresponds both spatially and temporally with behavioral recovery that is accelerated by D-amphetamine treatment. While the specific mechanisms responsible for D-amphetamine-promoted expression of proteins involved in neurite growth and synaptogenesis and of enhanced behavioral recovery are not known, it is suggested that protein upregulation occurs as a result of functional activation of pathways able to remodel in response to active behavioral performance.

MgATP-dependent and MgATP-independent [3H]noradrenaline release from perforated synaptosomes both use N-ethylmaleimide-sensitive fusion protein

In streptolysin-O (SLO)-perforated rat brain cortical synaptosomes, Ca2+-induced [3H]noradrenaline (3H-NA) release began with a phase lasting about 1 min that did not depend on MgATP. Subsequent release became increasingly MgATP-dependent. The first phase involved release from previously "primed" synaptic vesicles. MgATP-dependent release, on the other hand, was release from unprimed vesicles that needed to be primed by ATP hydrolysis before they could be fused with the presynaptic membrane. Vesicle depriming was detected by observing that the initial release decreased when the synaptosomes were perforated and incubated for 2 min in the absence of MgATP before increasing Ca2+ to promote release. One millimolar N-ethylmaleimide (NEM) inhibited both MgATP-dependent and MgATP-independent release at all times of incubation (0.5-5 min), and inhibition by NEM was partially reversed at short (0.5 min) and longer (5 min) times by adding intact N-ethylmaleimide sensitive fusion protein (NSF) to the perforated synaptosomes. Polyclonal antibodies against the N-terminal domain of NSF produced dose-dependent inhibition of Ca2+-induced 3H-NA release. This inhibition occurred in both early and late release phases and was highly significant at early times if the perforated synaptosomes were preincubated for 2 min with anti-NSF. These results indicate participation of NSF both after vesicular fusion, probably for separation of SNARE proteins in v/t-SNARE complexes before endocytosis, and, surprisingly, after docking, possibly to maintain vesicles in a primed state and reverse depriming during regulated secretion.

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.

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.

Manipulation of gene expression in the mammalian nervous system: application in the study of neurite outgrowth and neuroregeneration-related proteins

A fundamental issue in neurobiology entails the study of the formation of neuronal connections and their potential to regenerate following injury. In recent years, an expanding number of gene families has been identified involved in different aspects of neurite outgrowth and regeneration. These include neurotrophic factors, cell-adhesion molecules, growth-associated proteins, cytoskeletal proteins and chemorepulsive proteins. Genetic manipulation technology (transgenic mice, knockout mice, viral vectors and antisense oligonucleotides) has been instrumental in defining the function of these neurite outgrowth-related proteins. The aim of this paper is to provide an overview of the above-mentioned four approaches to manipulate gene expression in vivo and to discuss the progress that has been made using this technology in helping to understand the molecular mechanisms that regulate neurite outgrowth. We will show that work with transgenic mice and knockout mice has contributed significantly to the dissection of the function of several proteins with a key role in neurite outgrowth and neuronal survival. Recently developed viral vectors for gene transfer in postmitotic neurons have opened up new avenues to analyze the function of a protein following local expression in naive adult rodents. The initial results with viral vector-based gene transfer provide a conceptual framework for further studies on genetic therapy of neuroregeneration and neurodegenerative diseases.

[3H]-noradrenaline secretion from rat cortex synaptosomes perforated with Staphylococcus aureus alpha-toxin

Rat cortex synaptosomes have been successfully perforated with high concentrations (> or = 400 U/ml) of Staphylococcus aureus alpha-toxin. The free Ca2+-concentration dependence of [3H]-noradrenaline release was similar to that observed for PC 12 and chromaffin cells. Release from the alpha-toxin perforated synaptosomes was not significantly inhibited by omega-conotoxin GVIA. Initially, Ca2+-dependent release was independent of MgATP (for 0.5 min), but became increasingly dependent on MgATP with time. Lactate dehydrogenase efflux from alpha-toxin-perforated synaptosomes was not different than efflux from control synaptosomes, and an antibody to N-ethylmaleimide-sensitive fusion protein did not enter the synaptosomes. [3H]-noradrenaline release was temperature and alpha-toxin-concentration dependent. Ca2-dependent release was more resistant to rundown from alpha-toxin- than from streptolysin-O-perforated synaptosomes. This preparation of perforated synaptosomes should be useful for studies of regulated exocytosis from nerve endings.

Age-related alteration of PKC, a key enzyme in memory processes: physiological and pathological examples

Brain aging is characterized by a progressive decline of the cognitive and memory functions. It is becoming increasingly clear that protein phosphorylation and, in particular, the activity of the calcium-phospholipid-dependent protein kinase C (PKC) may be one of the fundamental cellular changes associated with memory function. PKC is a multigene family of enzymes highly expressed in brain tissues. The activation of kinase C is coupled with its translocation from the cytosol to different intracellular sites and recent studies have demonstrated the key role played by several anchoring proteins in this mechanism. PKC-phosphorylating activity appears to be impaired during senescence at brain level in a strain-dependent fashion in rodents. Whereas the levels of the various isoforms do not show age-related alterations, the enzyme translocation upon phorbol-ester treatment is deficitary among all strains investigated. Anchoring proteins may contribute to this activation deficit. We discuss also modifications of the PKC system in Alzheimer's disease that may be related to pathological alterations in neurotransmission. A better insight of the different factors controlling brain-PKC activation may be important not only for elucidating the molecular basis of neuronal transmission, but also for identifying new approaches for correcting or even preventing age-dependent changes in brain function.