Microencephaly reduces the phosphorylation of the PKC substrate B-50/GAP43 in rat cortex and hippocampus

Mechanisms underlying epileptogenesis in cortical malformations

The presence of developmental cortical malformations is associated with epileptogenesis and other neurological disorders. In recent years, animal models specific to certain malformations have been developed to study the underlying epileptogenic mechanisms. Teratogens (chemical, thermal or radiation) applied during cortical neuroblast division and migration result in lissencephaly and focal cortical dysplasia. Animals with these malformations have a lowered seizure threshold as well as histopathologies typical of those found in human dysgenic brains. Alterations that may promote epileptogenesis have been identified in lissencephalic brains, such as increased numbers of bursting types of neurons, and abnormal connections between hippocampus, subcortical heterotopia, and neocortex. A distinct set of pathological properties is present in animal models of 4-layered microgyria, induced with cortical lesions made during late stages of cortical neuroblast migration. Hyperexcitability has been demonstrated in cortex adjacent to the microgyrus (paramicrogyral zone) in in vitro slice preparations. A number of observations suggest that cellular differentiation is delayed in microgyric brains. Other studies show increases in postsynaptic glutamate receptors and decreases in GABA(A) receptors in microgyric cortex. These alterations could promote epileptogenesis, depending on which cell types have the altered receptors. The microgyrus lacks thalamic afferents from sensory relay nuclei, that instead appear to project to the paramicrogyral region, thereby increasing excitatory connectivity within this epileptogenic zone. These studies have provided a necessary first step in understanding molecular and cellular mechanisms of epileptogenesis associated with cortical malformations.

CaMKII-dependent phosphorylation of NR2A and NR2B is decreased in animals characterized by hippocampal damage and impaired LTP

The calcium-calmodulin-dependent protein kinase II (CaMKII) subserves activity-dependent plasticity in central neurons. To examine in vivo the implication of CaMKII activity in synaptic plasticity, we used an animal model characterized by developmentally induced targeted neuronal ablation within the cortex and the hippocampus, and showing, at presynaptic level, molecular alterations leading to facilitation of glutamate release in hippocampal synapses (methylazoxymethanol-treated rats, MAM-rats). We report here that at the postsynaptic side, the activity of CaMKII is markedly decreased in MAM-rats when compared to controls, although the concentration of the enzyme in Post Synaptic Density (PSD) is not altered. This effect is confined to PSD-associated CaMKII, as enzyme activity tested in the soluble fraction is unchanged in MAM-rats. In addition, the decreased activity is not due to inhibition by autophosphorylation in specific sites within the calmodulin-binding domain, as preincubation with purified phosphatases 1 and 2A failed to restore CaMKII activity in PSD of MAM-rats. The CaMKII-dependent phosphorylation of NR2A/B subunits of NMDA receptor is lower in MAM-rats when compared to controls (51.77 +/- 7.39% of controls level), as revealed in back-phosphorylation experiments. In addition, a treatment able to restore long-term potentiation (LTP) in hippocampal slices from MAM-rats, e.g. exposure to D-serine, is able to restore CaMKII activity to the control value.

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

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

B-50/GAP-43 phosphorylation and PKC activity are increased in rat hippocampal synaptosomal membranes after an inhibitory avoidance training

Several lines of evidence indicate that protein kinase C (PKC) is involved in long-term potentiation (LTP) and in certain forms of learning. Recently, we found a learning-specific, time-dependent increase in [3H]phorbol dibutyrate binding to membrane-associated PKC in the hippocampus of rats subjected to an inhibitory avoidance task. Here we confirm and extend this observation, describing that a one trial inhibitory avoidance learning was associated with rapid and specific increases in B-50/GAP-43 phosphorylation in vitro and in PKC activity in hippocampal synaptosomal membranes. The increased phosphorylation of B-50/GAP-43, was seen at 30 min (+35% relative to naive or shocked control groups), but not at 10 or 60 min after training. This learning-associated increase in the phosphorylation of B-50/GAP-43 is mainly due to an increase in the activity of PKC. This is based on three different sets of data: 1) PKC activity increased by 24% in hippocampal synaptosomal membranes of rats sacrificed 30 min after training; 2) B-50/GAP-43 immunoblots revealed no changes in the amount of this protein among the different experimental groups; 3) phosphorylation assays, performed in the presence of bovine purified PKC or in the presence of the selective PKC inhibitor CGP 41231, exhibited no differences in B-50/GAP-43 phosphorylation between naive and trained animals. In conclusion, these results support the contention that hippocampal PKC participates in the early neural events of memory formation of an aversively-motivated learning task.

Selective in vitro blockade of neuroepithelial cells proliferation by methylazoxymethanol, a molecule capable of inducing long lasting functional impairments

In order to characterize the antiproliferative effect of methylazoxymethanol neuroepithelial cells derived from the rat striata primordia at embryonic day 14 have been exposed to graded doses of this compound. It was found that methylazoxymethanol application to striatal neuroblasts elicits a blockade of cell proliferation at a dose which does not interfere with cell survival. By using synchronized cells and short term exposures to this compound, we found that the antiproliferative effect of methylazoxymethanol is strikingly correlated to the number of cells actively dividing in culture, thus indicating that the cells targeted by methylazoxymethanol must be in an active mitotic phase. To test for the selectivity of action of Methylazoxymethanol for dividing neuroblasts either cultures composed of mature proliferating astrocytes or muscle cells have been subjected to the same treatment. It has been observed that astrocytes proliferation was not affected by the dose of methylazoxymethanol shown to be effective on neuroepithelial cells. Finally we demonstrated that methylazoxymethanol is able only transiently to interfere with smooth muscle cell division, further supporting its selectivity of action within the developing CNS.

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

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

Distribution of GAP-43 mRNA in the adult rat brain

Regional distribution of gene expression of the axonal growth-associated protein, GAP-43, was studied in adult rat brains by in situ hybridization autoradiography to determine the features of mature neuronal populations that synthesize GAP-43 protein. Such synthesis appears to correlate with axonal growth during maturation and regrowth after axotomy. In most adult neurons, the sharp decline in GAP-43 gene expression implies a reduced capacity for axonal growth. Neurons capable of extending axonal knobs in the absence of injury may indicate a "plasticity" underlying dynamic processes of interaction between neurons and their synaptic targets. Antisense and sense (control) riboprobes were used on serial sections in the three principal axes, and the magnitude of hybridization signal was examined to determine regional patterns. GAP-43 mRNA levels are pronounced in diverse neuronal groups including the locus coeruleus, raphé nn., dopaminergic nigral and ventral tegmental nn., mitral cells, hippocampal CA3, inferior olivary n., vagal motor n. and other parasympathetic preganglionic neurons, select thalamic midline and intralaminar nn., several specific nn. of the hypothalamus and basal forebrain, the granular layer of cerebellar cortex, the infragranular neocortex, and the granular olfactory paleocortex; there is a substantial range in the magnitude of expression. Regions revealing minimal signal include most thalamic sensory relay nuclei, the granule neurons of the olfactory bulb and dentate gyrus, and the caudate and putamen. Possible concomitants of GAP-43 expression include regulation of ion flux and neurotransmitter release. Those neurons with long, extensively dispersed and numerous synaptic connections display the strongest signals and may possess the greatest propensity for continuous growth and turnover of their axon terminals, in contrast to short-axon and specific projection neurons exhibiting minimal levels. These data may enable inferring which populations display normal or experimentally induced axonal growth.

The impaired long-term potentiation in the CA1 field of the hippocampus of cognitive deficient microencephalic rats is restored by D-serine

Rat embryos exposed on gestational day 15 to methyl-azoxymethanol acetate develop a microencephaly characterized primarily by a hypoplasia of the neocortex and CA fields of the hippocampus that in adulthood is associated with disturbances in learning. In brain slices prepared from microencephalic rats, we have examined the field excitatory postsynaptic potentials and population spike in the CA1 field of the hippocampus evoked by stimulation of the stratum radiatum. These parameters did not differ from those obtained in slices from control rats. High frequency stimulation of the stratum radiatum afferent fibres, which readily induced long-term potentiation of the field excitatory postsynaptic potentials and population spike in the CA1 field of the hippocampus of control rats, failed to induce long-term potentiation in that of microencephalic rats. High frequency stimulation of the perforant path readily elicited long-term potentiation in the dentate gyrus of both control and microencephalic rats. Picrotoxin had no apparent effect on field excitatory postsynaptic potentials and population spike in the CA1 field of the microencephalic rats, indicating that little GABAergic inhibition was present in slices from these rats. D-2-Amino-phosphonovalerate suppressed the field potentials in slices from microencephalic rats by more than 50%, suggesting that N-methyl-D-aspartate receptors contributed markedly to the synaptic responses evoked by single stimuli. D-Serine, but not picrotoxin, restored long-term potentiation in the CA1 field of the microencephalic rats. The D-serine effect was prevented by pretreating the slices with either 7-chloro-kynurenate or D-2-amino-phosphonovalerate. The failure to induce long-term potentiation, if also found in vivo, may be among the factors related to the learning deficits displayed by these rats.

Selective alteration in B-50/GAP-43 phosphorylation in brain areas of animals characterized by cognitive impairment

When methylazoxymethanol acetate is administered to pregnant rats at gestational day 19, the offspring are greatly impaired in the learning of a two-way active avoidance task and these behavioral changes are paralleled by a change in the phosphorylation of the protein B-50/GAP-43 in hippocampus but not in cortex. The expression of the protein is not altered, indicating that the phosphorylation of B-50 is a sensitive marker of alterations in synaptic plasticity associated with impairments of learning abilities in rats.

Cellular expression of somatostatin in MAM-induced microencephaly in the rat

Methylazoxymethanol acetate (MAM) is a mitotic inhibitor that has been used to selectively destroy neuroblasts at specific times during gestation. The administration of MAM results in a dose-dependent microencephaly. Following MAM treatment at 15 days of gestation, we have noted an increase in the level of SS immunoreactivity in the neocortex, as determined by radioimmunoassay. Northern blot analysis for preproSS mRNA revealed an increase in MAM-treated cortex. The cellular distribution of SS has been determined using in situ hybridization and immunocytochemistry. There was a 30% increase in the density of SS-immunoreactive neurons in the cortex of the MAM-treated animals. These data suggest that SS neurons in the cortex are spared following MAM treatment at GD 15.

Excitatory amino acid receptors in brains of rats with methylazoxymethanol-induced microencephaly

We used methylazoxymethanol-acetate (MAM), a potent alkylating agent, to produce microencephaly in offspring by injecting it into pregnant rats on day 15 of gestation. Binding activities of central excitatory amino acid receptors were examined in Triton-treated membranes prepared from brains of adult offspring with MAM-induced microencephaly (MAM rats). MAM rats exhibited approximately 40-50% reductions of the wet weights of the cerebral cortex, hippocampus and striatum compared to those in controls. In the cortex and hippocampus of MAM-rats, total bindings of [3H]glutamate (Glu) (which is sensitive to N-methyl-D-aspartate (NMDA) receptor), and strychnine-insensitive [3H]glycine (Gly) and (+)-5-[3H]methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801; a noncompetitive antagonist of NMDA receptor), were reduced to approximately 40% of those in controls. Similarly, in both regions of MAM rats, total bindings of [3H]kainate and DL-alpha-amino-3-[3H]hydroxy-5-methylisoxazole-4-propionic acid (an agonist of quisqualate receptors), were reduced to approximately 35-50% of those in controls. However, total bindings of these radioligands in the striatum of MAM rats were more than 65% of those in controls, despite the significant loss of striatum mass. However, specific bindings of radioligands in the striatum of MAM rats were elevated by more than 60% of those in controls, and Scatchard analysis revealed that elevations of [3H]Glu, [3H]Gly and [3H]MK-801 bindings were due to a significant increase in the densities of binding sites, with their affinities remaining unaltered. Spatial recognition ability examined by an 8-armed radial maze task was markedly impaired compared to those in controls. These results suggest that the proliferation of neurons bearing excitatory amino acid receptors (EAA) in the striatum is less affected by MAM treatment on day 15 of gestation than that in the cortex and hippocampus in spite of drastic weight loss in these brain regions. The significant reduction of EAA receptors in the cortex and hippocampus may be involved in the impairment of spatial memory observed in MAM-treated rats.

GAP-43 mRNA localization in the rat hippocampus CA3 field

Gene expression of the axonal growth-associated protein, GAP-43, has been studied in the adult rat brain by in situ hybridization histochemistry. This protein is synthesized at high levels in neuronal somata in immature and regenerating neurons, but after establishment of mature synaptic relations its synthesis generally declines sharply, thus providing a marker denoting propensity for exhibiting synaptic plasticity. Detailed examination of the distribution of mRNA for GAP-43 in rat hippocampus is selectively and robustly expressed in the pyramidal neurons of field CA3 and, to a lesser extent, the polymorph neurons of the hilus of the dentate gyrus. Additional hippocampal regions of moderate expression include the tenia tecta and the subicular and entorhinal fields, but CA1 and CA2 are strikingly lower in signal. The significance of this pattern of localization is considered in the context of the phosphorylation of GAP-43 and its role in influencing synaptic events underlying the establishment and maintenance of long-term potentiation and plasticity in the hippocampus.