Expression of GAP43 mRNA in normally developing and transplanted neurons from the rat ventral mesencephalon

Plasticity following Injury to the Adult Central Nervous System: Is Recapitulation of a Developmental State Worth Promoting?

The adult central nervous system (CNS) appears to initiate a transient increase in plasticity following injury, including increases in growth-related proteins and generation of new cells. Recent evidence is reviewed that the injured adult CNS exhibits events and patterns of gene expression that are also observed during development and during regeneration following damage to the mature peripheral nervous system (PNS). The growth of neurons during development or regeneration is correlated, in part, with a coordinated expression of growth-related proteins, such as growth-associated-protein-43 (GAP-43), microtubule-associated-protein-1B (MAP1B), and polysialylated-neural-cell-adhesion-molecule (PSA-NCAM). For each of these proteins, evidence is discussed regarding its specific role in neuronal development, signals that modify its expression, and reappearance following injury. The rate of adult hippocampal neurogenesis is also affected by numerous endogenous and exogenous factors including injury. The continuing study of developmental neurobiology will likely provide further gene and protein targets for increasing plasticity and regeneration in the mature adult CNS.

FGF-2 modulates expression and distribution of GAP-43 in frog retinal ganglion cells after optic nerve injury

Basic fibroblast growth factor (bFGF or FGF-2) has been implicated as a trophic factor that promotes survival and neurite outgrowth of neurons. We found previously that application of FGF-2 to the proximal stump of the injured axon increases retinal ganglion cell (RGC) survival. We determine here the effect of FGF-2 on expression of the axonal growth-associated phosphoprotein (GAP)-43 in retinal ganglion cells and tectum of Rana pipiens during regeneration of the optic nerve. In control retinas, GAP-43 protein was found in the optic fiber layer and in optic nerve; mRNA levels were low. After axotomy, mRNA levels increased sevenfold and GAP-43 protein was significantly increased. GAP-43 was localized in retinal axons and in a subset of RGC cell bodies and dendrites. This upregulation of GAP-43 was sustained through the period in which retinal axons reconnect with their target in the tectum. FGF-2 application to the injured nerve, but not to the eyeball, increased GAP-43 mRNA in the retina but decreased GAP-43 protein levels and decreased the number of immunopositive cell bodies. In the tectum, no treatment affected GAP-43 mRNA but FGF-2 application to the axotomized optic nerve increased GAP-43 protein in regenerating retinal projections. We conclude that FGF-2 upregulates the synthesis and alters the distribution of the axonal growth-promoting protein GAP-43, suggesting that it may enhance axonal regrowth.

Growth-associated protein-43 is increased in cerebrum of immature rats following induction of hydrocephalus

Hydrocephalus is associated with gradual progressive impairment and destruction of cerebral axons and neurons. Growth associated protein-43 appears to be permissive for neuro-axonal regeneration and synaptic remodeling. Hydrocephalus was induced in three-week-old rats by injection of kaolin into the cisterna magna. Compared to controls, cerebral growth-associated protein-43 messenger RNA was significantly up-regulated one week after kaolin injection and the overall cerebral growth-associated protein-43 protein level was significantly higher at four weeks when the ventricles were severely enlarged. One and three weeks after kaolin injection, growth-associated protein-43-like immunoreactivity was increased in periventricular axons, and also in the cerebral cortex at three weeks. Hydrocephalic rats that had been treated by shunting after one week, exhibited growth-associated protein-43 messenger RNA and protein levels intermediate between hydrocephalic rats and control rats. The increase in periventricular axon growth-associated protein-43, early in the course of experimental hydrocephalus, suggests that through early intervention there may be a chance for preventing or reversing the axonal injury. Cortical expression of growth associated protein-43 suggests that an alteration in synaptogenesis may also occur.

Choline availability modulates the expression of TGFbeta1 and cytoskeletal proteins in the hippocampus of developing rat brain

Choline availability influences long-term memory in concert with changes in the spatial organization and morphology of septal neurons, however little is known concerning the effects of choline on the hippocampus, a region of the brain also important for memory performance. Pregnant rats on gestational day 12 were fed a choline control (CT), choline supplemented (CS), or choline deficient (CD) diet for 6 days and fetal brain slices were prepared on embryonic day 18 (E18). The hippocampus in these brain slices was studied for the immunohistochemical localization of the growth-related proteins transforming growth factor beta type 1 (TGFbeta1) and GAP43, the cytoskeletal proteins vimentin and microtubule associated protein type 1 (MAP1), and the neuronal cell marker neuron specific enolase (NSE). In control hippocampus, there was weak expression of TGFbeta1 and vimentin proteins, but moderately intense expression of MAP1 protein. These proteins were not homogeneously distributed, but were preferentially localized to cells with large cell bodies located in the central (approximately CA1-CA3) region of the hippocampus, and to the filamentous processes of small cells in the fimbria region. Feeding a choline-supplemented diet decreased, whereas a choline-deficient diet increased the intensity of immunohistochemical labeling for these proteins in E18 hippocampus. GAP43 and NSE were localized to peripheral nervous tissue but not hippocampus, indicating that the maturation of axons and neurite outgrowth in embryonic hippocampus were unaffected by the availability of choline in the diet. These data suggest that the availability of choline affects the differentiation of specific regions of developing hippocampus.

Effect of lead on cytoskeletal proteins expressed in E14 mesencephalic primary cultures

Several lines of evidence indicated that Pb exposure in vivo and in vitro altered neurite morphology in central and peripheral neurons. The present report shows that neurite length in mesencephalic primary cultures, consisting of neurons and glia, was decreased by Pb exposure when serum factors, presumably essential for glial functions, were absent in the culture medium. We studied whether a serum factor might control the mechanisms involved in the uptake and accumulation of Pb and its effect on cytoskeleton proteins. The total amount of Pb taken up in cell cultures was measured by atomic absorption spectroscopy and appeared to be down-regulated by a non-albumin-like serum component. In presence of serum, Pb exposure failed to alter cytoskeletal proteins. Instead, in serum-free neurobasal medium, Pb uptake failed to reach saturation within 6 h. Western blot analysis showed that the tau, 280 kDa MAP-2b, 70 kDa MAP-2c and GAP-43 protein bands were decreased 24 h after a 3 h exposure to 3 or 6 microM Pb in absence of serum. However, if cultures were maintained in serum-containing media after a 3 h Pb exposure without serum, the immunoblots did not differ from those of controls. It can be inferred that a serum factor prevents cytoskeletal protein alterations by Pb. In serum free medium, Pb that is primarily scavenged by the metallothionein I/II isoforms present in glial cells, may bind to thiol residues of proteins involved in either oxidative stress response or transcriptional regulation of cytoskeletal proteins.

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.

6-hydroxydopamine induces the loss of the dopaminergic phenotype in substantia nigra neurons of the rat. A possible mechanism for restoration of the nigrostriatal circuit mediated by glial cell line-derived neurotrophic factor

Intraparenchymal injections of the neurotoxin 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle in rats destroys the dopaminergic neurons in the pars compacta of the substantia nigra. In other transmitter systems it has been found that axotomy or neurotoxin exposure produces an initial loss of neurotransmitter phenotype, with cell death occurring over a much slower time course. To determine whether this also occurs in dopamine neurons after 6-OHDA, two approaches were utilized. First, the effect of injections of 6-OHDA into the medial forebrain bundle on nigral dopaminergic neurons was studied using combined fluorogold and immunocytochemical labeling. Four weeks after the 6-OHDA injection, there was an 85% reduction in the number of tyrosine hydroxylase (TH)-immunoreactive cells on the lesioned side. In contrast, there was only a 50% reduction in the number of fluorogold-labeled cells on the lesioned side. Second, the time course of the rescue of dopaminergic neurons after 6-OHDA by glial cell line-derived neurotrophic factor (GDNF) was determined using TH immunocytochemistry. Greater numbers of dopamine neurons were rescued 9 weeks after GDNF, compared with counts made 5 weeks after GDNF. Taken together, these results suggest loss of dopaminergic phenotype is greater than cell loss following 6-OHDA injections, and that GDNF restores the phenotype of affected cells.

Cell surface molecules (NCAM and L1) in intrastriatal transplants of embryonic mesencephalon in rats

Cell surface molecules, NCAM and L1, reported to have a role in synaptogenesis, growth and fasciculation of the neurites in the brain, were traced in the embryonic nigral transplants in the host striatum of adult rats. Substantia nigra of five, 15 and 25 postnatal days were also examined for the same molecules. Tyrosine hydroxylase label was used as a marker to localize the nigral neurons and glial fibrillary acidic protein to detect if glial scar present. In the control as well as transplants large neurons had expressed tyrosine hydroxylase. By 15th postnatal day tyrosine hydroxylase neurons appeared mature and were scattered, suggesting a well-formed neuropil. NCAM and L1 reaction was seen as a peripheral rim in most of the cells on the fifth postnatal day. The reaction was mainly in relation to the large cells and more extensive on the 15th day. Thereafter on the 25th day, activity was negligible. Large neurons demonstrated strong reactivity for NCAM and L1 during early post-transplantation days. After 30 days only smaller cells were reactive, many of which could be identified as neurons. Strong reaction for these molecules was present only until 60 days, though faint reaction could be detected even on the 90th day. These observations indicate that the growth promoting molecules, the type seen in the neonatal period, can be detected normally only until the neurons mature. Prolonged expression of these molecules by the grafted neurons indicate delay in the maturation of these cells due to absence of adequate target sites for synaptic connections. Some of the smaller cells expressing these molecules after 30 days of transplantation could be astroglia, either proliferating or reactive.