Developmental changes in B-50 (GAP-43) in primary cultures of cerebral cortex: B-50 immunolocalization, axonal elongation rate and growth cone morphology

Urokinase-type plasminogen activator (uPA) regulates the expression and function of growth-associated protein 43 (GAP-43) in the synapse

Growth-associated protein 43 (GAP-43) plays a central role in the formation of presynaptic terminals, synaptic plasticity, and axonal growth and regeneration. During development, GAP-43 is found in axonal extensions of most neurons. In contrast, in the mature brain, its expression is restricted to a few presynaptic terminals and scattered axonal growth cones. Urokinase-type plasminogen activator (uPA) is a serine proteinase that, upon binding to its receptor (uPAR), catalyzes the conversion of plasminogen into plasmin and activates signaling pathways that promote cell migration, proliferation, and survival. In the developing brain, uPA induces neuritogenesis and neuronal migration. In contrast, the expression and function of uPA in the mature brain are poorly understood. However, recent evidence reveals that different forms of injury induce release of uPA and expression of uPAR in neurons and that uPA/uPAR binding triggers axonal growth and synapse formation. Here we show that binding of uPA to uPAR induces not only the mobilization of GAP-43 from the axonal shaft to the presynaptic terminal but also its activation in the axonal bouton by PKC-induced calcium-dependent phosphorylation at Ser-41 (pGAP-43). We found that this effect requires open presynaptic N-methyl-d-aspartate receptors but not plasmin generation. Furthermore, our work reveals that, following its activation by uPA/uPAR binding, pGAP-43 colocalizes with presynaptic vesicles and triggers their mobilization to the synaptic release site. Together, these data reveal a novel role of uPA as an activator of the synaptic vesicle cycle in cerebral cortical neurons via its ability to induce presynaptic recruitment and activation of GAP-43.

Histological analysis of single peripheral nerve fiber in acute nerve elongation process

To observe the histological alterations of single nerve fiber structures after nerve elongation by employing a rabbit peroneal nerve stretching model. 14 rabbits weighing mean 3. 0 kg (2.02-3.31 kg.) were used in the experiment. Two rabbits were used as control when only a sham operation was done (group one, 0% stretch). Acute stretching of the peroneal nerves to elongate them by 10% was done in 6 rabbits (group two, 10% elongation) and by 20% (group three, 20% elongation) in another 6 rabbits. All animals were evaluated by tissue staining technology in a teased-fiber study. The internodal lengths were measured, and nodes of Ranvier and Schmidt-Lanterman notch were observed. The nerve fiber length was increased after stretching. The mean internodal length was 1208.31 microm in group one, 1347.26 microm in group two, and 1411.35 microm in group three. Compared with the control group, mean internodal length was elongated by 11.50% in group two and 16.80% in group three. The difference was statistically significant. The node of Ranvier and Schmidt-Lanterman notch was wider in both group two and group three. Rupture of nerve fiber at the node of Ranvier was observed in group three. The peroneal nerve in rabbits can adapt to mild stretching by internodal length elongation. Elongation by 20% will cause structural rupture and therefore is the limit for nerve elongation.

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.

Effects of docosahexaenoic acid on the survival and neurite outgrowth of rat cortical neurons in primary cultures

Effects of docosahexaenoic acid (DHA) on survival and neurite outgrowth were investigated in primary cultures of rat cortical neurons. Cell cultures were prepared from cortex on embryonic day 18 (E-18) for treatment with a series of DHA concentrations (12.5, 25, 50, 75, 100 and 200 microM). Docosahexaenoic acid (25-50 microM) significantly enhanced neuronal viability, but lower concentration of DHA (12.5 microM) did not show an obvious effect. In contrast, higher concentrations of DHA (100-200 microM) exerted the significant opposite effects by decreasing neuronal viability. Furthermore, treatment with 25 microM DHA significantly prevented the neurons from death after different culture days in vitro (DIV). Moreover, measurements from the cultures exposed to 25 microM DHA immediately after plating showed significant increases in the percentage of cells with neurites, the mean number of neurite branches, the total neuritic length per cell and the length of the longest neurite in each cell after 24 and 48 h in vitro (HIV). The DHA-treated neurons had greater growth-associated protein-43 (GAP-43) immunoactivity and higher phosphatidylserine (PS) and phosphatidylethanolamine (PE) contents, but lower phosphatidylcholine (PC) content than control neurons. The significant increased DHA contents were also observed in both PE and PS in the treated neurons. These findings suggest that optimal DHA (25 microM) may have positive effects on the survival and the neurite outgrowth of the cultured fetal rat cortical neurons, and the effects probably are related to DHA-stimulating neuron-specific protein synthesis and its enhancing the discrete phospholipid (PL) content through enrichment of DHA in the PL species.

Neuron degeneration induced by verapamil and attenuated by EGb761

Calcium channel blockers are used as neuroprotective agents, as glutamate antagonists. However, it has been found that calcium channel blockers may compromise neuronal survival after long-term exposure. To explore the mechanisms of the toxicity of calcium channel blockers on neurons, we studied the morphological characteristics and biochemical changes of cultured cortical neurons treated with verapamil, a calcium channel blocker. We now report that cerebral cortical cultures exposed to verapamil for 48 h undergo neuronal degeneration in both concentration-dependent and time-dependent fashion, possibly partially through the activation of apoptosis. On the other hand, it was found that Ginkgo biloba extract (EGb761) attenuated verapamil-induced neuronal injury, suggesting the possibility of using verapamil combined with EGb761 clinically. Furthermore, both B-50 immunoactivity (BIA) and the concentration of intracellular calcium in single neurons ([Ca2+]i) decreased after a 48-h exposure to verapamil, suggesting that the mechanisms of verapamil-induced degeneration may be associated with the disruption of intracellular calcium homeostasis and the inhibition of normal axonal elongation.

Differentiative effects of dopamine on striatal neurons involve stimulation of the cAMP/PKA pathway

The neurotransmitter dopamine (DA) stimulates neurite outgrowth and growth cone formation in cultures of embryonic rat striatum through activation of D1 but not D2 receptors. We show here that neurite outgrowth could be stimulated to a similar extent by elevating cellular cAMP levels. Second, the neuritotrophic effect of DA was completely abolished by inhibiting adenylate cyclase or protein kinase A (PKA) but not protein kinase C (PKC). Third, double staining of cultures with antibodies against growth-associated protein-43 (GAP-43) and the phosphorylated form of the cAMP response element binding protein (pCREB) showed that pCREB was nearly exclusively associated with GAP-43-positive, i.e., actively growing, neurons. Again, this effect depended on D1 receptor and PKA activation. Although cross-talk with other signaling pathways needs to be studied further, we conclude that DA promotes the differentiation of striatal neurons via stimulation of D1 receptors and the cAMP/PKA signal transduction pathway.

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

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

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

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

Activation of dopaminergic D1 receptors promotes morphogenesis of developing striatal neurons

The early dopaminergic input from the midbrain may play an important role in the development of the basal ganglia. We therefore investigated whether and how dopamine affects the morphogenesis of striatal target neurons. Dissociated cell cultures of embryonic day 17 rat striatum were raised for seven days. Cells were then incubated with dopamine or various receptor-specific ligands for 1 h. At various times after termination of the treatment, cells were immunostained for growth-associated protein-43. Morphological parameters including numbers of growth cones, length of neurites, number of bifurcations, and neuronal soma size were assessed by means of a computer-based morphometric device. Treatment with dopamine in low concentrations as well as with the D1-like receptor agonist SKF 38393 increased the numbers of growth cones and neurite length and arborization. The morphogenetic effect took several hours to evolve and remained stable for at least 24 h. It could be blocked by the D1-like receptor antagonist SCH 23390 or by cycloheximide but not by pretreatment of the cultures with tetrodotoxin. The D2-like receptor agonist quinpirole had no effect on the morphological parameters and did not contribute to that of SKF 38393. Dopamine and SKF 38393 but not quinpirole also induced an increase in the number of neurons immunoreactive for Fos-like proteins. However, this effect was restricted to growth-associated protein-43-negative neurons. This is the first observation of a positive regulatory effect of D1-like receptors on neuronal morphogenesis. We conclude that the changes reflect true differentiation rather than short-term modulation of cellular properties and that c-fos induction is not an obligatory step in the transduction pathway coupling D1-like receptors to neurite outgrowth. Our results suggest that the differentiation of embryonic striatal neurons is promoted by the dopaminergic nigrostriatal projection through D1-like receptors.

Development and activity-dependent expression of neuronal marker proteins in organotypic cultures of rat visual cortex

We are interested in activity-dependent mechanisms which govern the structural and functional maturation of neurons in the visual cortex. We have asked whether the expression of neuronal markers microtubule-associated proteins tau, MAP-2, synaptophysin (p38), and the growth-associated protein GAP-43 are dependent on cortical afferents or spontaneous activity. As a model system we have employed organotypic monocultures of rat visual cortex (OTCs, isolated from subcortical structures) in comparison with visual cortex in vivo (innervated by thalamic and other afferents) at different postnatal ages. We know from previous work that the OTCs, like the cortex in vivo, display a high rate of spontaneously generated action potentials. Therefore, as a third objective, we have analysed OTCs grown as monocultures under chronic blockade of spontaneous action potentials. Protein expression was detected by protein blots and/or immunohistochemistry. The proteins examined in this study are expressed in OTCs, even when grown under activity blockade. However, the pattern of expression differs from the cortex in vivo. Tau is expressed much weaker in OTCs than in cortex in vivo. The expression of the major band of about 50 kDa increases over time in vivo and in OTCs. Smaller isoforms of tau are dramatically downregulated, and larger (adult) isoforms do not appear within 35 days in vitro (DIV). Under activity blockade the expression of tau reaches a maximum by 21 DIV and decreases dramatically, so that the protein is hardly detectable by 47 DIV. MAP-2-immunoreactive proteins are localized in somata and dendrites, but also persist in axons. The expression in OTCs of p38 and GAP-43 correlates well with the expression observed in vivo. Synaptophysin (p38) occurs with a similar time course and amount in OTCs as in cortex in vivo. Synaptic boutons appear in all layers, and specialized terminal elements have been observed. Activity blockade slightly affects the p38 expression, although the late postnatal decline in p38 immunoreactivity observed on protein blots from cortex in vivo and in normal OTCs appears more accentuated in activity-blocked OTCs. The GAP-43 expression is prominent from birth onwards in vivo and in OTCs. However, in normal OTCs GAP-43 is not declining as it is in vivo, although it is downregulated in activity-blocked OTCs. As a major finding we report that neuronal markers which are normally expressed in immature neurons and axons during the period of differentiation and structural plasticity are continuously expressed in OTCs, suggesting that a monocultured cortex retains the ability for growth and structural changes longer than the cortex in vivo.

Expression levels of B-50/GAP-43 in PC12 cells are decisive for the complexity of their neurites and growth cones

To study the role of the protein B-50/GAP-43 in NGF-induced neurite outgrowth, a number of stable PC12 subclones with either very low or considerably enhanced expression levels of the protein were selected. Cell bodies of subclones with suppressed B-50 expression (-B2, -B5, or -B12) possessed a relative small spherical shape and, on NGF-treatment for 7 d, developed processes that were virtually devoid of branches and that mostly bore short or blunt-ended growth cones. Cells of subclones with overexpression of B-50 (+B3, +B4, or +B11), on NGF treatment, acquired a flattened, spiky appearance with highly branched neurites possessing extended and complex growth cones. Confocal microscopy with immunofluorescence for B-50 and F-actin revealed that in neurites and growth cones of the B-50-deficient subclone -B2, no detectable B-50 and reduced amounts of filamentous F-actin were present, whereas in overexpressing +B3 cells, cell membranes, neurites, and complex growth cones were intensively stained for B-50 and exhibited numerous spikes, in which B-50 was strikingly colocalized with F-actin. These data suggest that, under normal conditions of neuritogenesis, the expression level of B-50 in PC12 cells is decisive for the complexity of neurites and growth cones.

Reversal of alcohol's effects on neurite extension and on neuronal GAP43/B50, N-myc, and c-myc protein levels by retinoic acid

Alcohol teratogenesis may be due in part to inhibition of neuronal differentiation by ethanol. We showed previously that alcohol decreased neuronal differentiation (neurite extension) and increased N-myc and c-myc neuronal protein levels. Since Growth-Associated Protein 43 (GAP43/B50) levels must increase for neurons to differentiate, alcohol may decrease GAP43/B50. Alcohol dose-dependently (0-0.5%) decreased GAP43/B50 protein levels by up to 92% in immature LA-N-5 cells. Five nM retinoic acid alone induced differentiation and increased GAP43/B50 levels to 230% of control. These retinoic acid-induced increases in GAP43/B50 and neurite outgrowth, and decreases in N-myc and c-myc, were reversed dose-dependently by alcohol (0-0.5%). Conversely, the adverse effects of 0.25% alcohol on neurite extension, GAP43/B50, N-myc, and c-myc were prevented by 15 and 45 nM retinoic acid. These results suggest that inhibition of neuronal differentiation by alcohol and prevention of such effects by retinoic acid are related to changes in GAP43/B50, N-myc and c-myc.

Effects of electrical stimulation on GAP-43 expression in mouse sensory neurons

Effects of electrical activity on GAP-43 expression were tested in mouse dorsal root ganglion (DRG) neurons subjected to electrical stimulation in culture. Patterned electrical stimulation was provided through extracellular electrodes placed in multicompartment cell culture chambers. Stimulation was delivered at 10 Hz, in 0.5 s bursts every 2 s for up to 3 days. Expression of GAP-43 was assessed by immunocytochemistry, two ELISA methods, and Northern blot analysis within three experimental protocols: (1) prior to synaptogenesis, (2) after synaptogenesis with spinal cord neurons, and (3) within the context of activity-dependent synaptic competition, in which synapses from active and inactive DRG neurons converge on the same postsynaptic neurons. None of the stimulation treatments produced a measurable change in GAP-43 or RNA message for the protein, although this electrical stimulus induces persistent changes in synaptic strength, and alters neurite outgrowth in these cultures. The decline in GAP-43 levels between 1 and 3 weeks in culture, which has been reported in other studies, was readily detectable by our measurements. We conclude that regulation of GAP-43 expression is not required for activity-dependent regulation of growth cone motility, synaptogenesis and synapse elimination, or changes in synaptic strength. Instead, post-translational modification, such as phosphorylation, may be the primary means of regulating any GAP-43 functions associated with these activity-dependent processes.

Immunolocalization of B-50 (GAP-43) in the mouse olfactory bulb: predominant presence in preterminal axons

Because the growth-associated protein B-50 (GAP-43) has been implicated in neurite outgrowth as well as in synaptic plasticity, we studied its light and electron microscopical distribution in the mouse olfactory bulb, an area of the nervous system which exhibits a high degree of synaptic plasticity. Immunofluorescent staining with monospecific affinity-purified anti-B-50 antibodies revealed that B-50 is most abundantly expressed in the olfactory nerve fibre layer and the granule cell layer neuropil, while little staining was observed in the external plexiform layer and in cell bodies. B-50 is absent from dendrites and myelinated axons as indicated by double labelling with monoclonal antibodies against microtubule-associated protein 2 and the large neurofilament protein, respectively. Using post-embedding immunogold labelling on ultrathin Lowicryl sections, B-50 was found to be highly concentrated in presumed growth cones in the olfactory nerve fibre layer and in thin unmyelinated axons and presynaptic terminals in the granule cell layer neuropil. Near background immunolabelling was seen in perikarya, dendrites and myelinated axons. In view of the implication of B-50 in plasticity-related phenomena, its abundance in the thin unmyelinated preterminal axons suggests that these are potential sites of extrasynaptic plasticity.

Developmental changes in B-50 (GAP-43) in primary cultures of cerebral cortex: content and phosphorylation of B-50

The content and phosphorylation of the neuronal growth-associated protein B-50 (GAP-43) were studied in cultured neocortex as a function of normal development and development in the presence of tetrodotoxin (TTX), a blocker of bioelectric activity (BEA). The observations were correlated with previous morphological findings on neurite outgrowth and B-50 immunolocalization in the same cultures. In control cultures, the concentration of B-50 reached a maximum at 7 days in vitro (DIV) and decreased thereafter, whereas the concentration of neuron specific enolase (NSE), which was used as a neuronal reference marker, rose till 28 DIV and leveled off towards 42 DIV. The degree of basal phosphorylation of B-50 (relative to that of total protein) decreased after the first week in vitro. Stimulation of B-50 phosphorylation by phorbol ester also decreased with age in vitro, indicating that changes in B-50 phosphorylation were mainly due to changes in protein kinase C (PKC) activity. The chronic presence of TTX led to a reduced content of B-50 and NSE after 14 DIV. The basal phosphorylation of B-50 was neither affected by acute nor chronic TTX treatment. However, upon stimulation of PKC with phorbol esters, some alterations of B-50 phosphorylation were revealed in cultures grown in TTX. These biochemical observations are in line with the absence of effects of TTX on neurite outgrowth during the first 2 weeks in culture, and later effects of TTX on neuronal survival. The developmental changes in B-50 concentration and phosphorylation largely correlate with previous morphological observations on axonal outgrowth and growth cone shape in the same cultures. We suggest that B-50 phosphorylation plays an important role in transducing extracellular signals into directed neurite outgrowth.