Differential expression of GAP-43 mRNA in adult central cholinergic neuronal populations

A Shift from a Pivotal to Supporting Role for the Growth-Associated Protein (GAP-43) in the Coordination of Axonal Structural and Functional Plasticity

In a number of animal species, the growth-associated protein (GAP), GAP-43 (aka: F1, neuromodulin, B-50, G50, pp46), has been implicated in the regulation of presynaptic vesicular function and axonal growth and plasticity via its own biochemical properties and interactions with a number of other presynaptic proteins. Changes in the expression of GAP-43 mRNA or distribution of the protein coincide with axonal outgrowth as a consequence of neuronal damage and presynaptic rearrangement that would occur following instances of elevated patterned neural activity including memory formation and development. While functional enhancement in GAP-43 mRNA and/or protein activity has historically been hypothesized as a central mediator of axonal neuroplastic and regenerative responses in the central nervous system, it does not appear to be the crucial substrate sufficient for driving these responses. This review explores the historical discovery of GAP-43 (and associated monikers), its transcriptional, post-transcriptional and post-translational regulation and current understanding of protein interactions and regulation with respect to its role in axonal function. While GAP-43 itself appears to have moved from a pivotal to a supporting factor, there is no doubt that investigations into its functions have provided a clearer understanding of the biochemical underpinnings of axonal plasticity.

Memory impairment associated with a dysfunction of the hippocampal cholinergic system induced by prenatal and neonatal exposures to bisphenol-A

One of the most common chemicals that behaves as an endocrine disruptor is the compound 4,4'-isopronylidenediphenol, called bisphenol-A. In the previous study, we reported that exposure to bisphenol-A induced the abnormality of dopamine receptor functions in the mouse limbic area, resulting in a supersensitivity of drugs of abuse-induced pharmacological actions. The present study was undertaken to investigate whether prenatal and neonatal exposures to bisphenol-A could alter other behavioral abnormalities such as anxiogenic behavior, motor learning behavior, or memory. In the present study, adult female mice were chronically treated with bisphenol-A-admixed powder food from mating to weaning. All experiments were performed using male pups. Here we found that prenatal and neonatal exposures to bisphenol-A failed to induce anxiogenic effects and motor-learning impairment using the light-dark test, elevated plus maze test, and rota-rod test. On the other hand, we found that prenatal and neonatal exposures to bisphenol-A induced the memory impairment using the step-through passive avoidance test. Immunohistochemical study showed the dramatic reduction in choline acetyltransferase-like immunoreactivity, which is a marker of acetylcholine (ACh) production, in the hippocampus of mice prenatally and neonatally exposed to bisphenol-A. These results suggest that chronic exposures to bisphenol-A could induce the memory impairment associated with the reduction in ACh production in the hippocampus.

Effects of estrogens on choline-acetyltransferase immunoreactivity and GAP-43 mRNA in the forebrain of young and aging male rats

1. Previous work demonstrated that estradiol (E2) treatment prevented the abnormal response to stress and the reduction of glucocorticoid receptors (GR) in hippocampus from aging male rats. The mechanisms originating these effects were unknown. 2. In the present work, we investigated the E2 effects on the cholinergic, growth-associated protein (GAP-43) expressing neurons of the medial septum (MS) and vertical limb of diagonal band of Broca (VDB). These areas project to the hippocampus, and may be involved in the mentioned E2 effects in aging animals. Therefore, the response to E2 of choline-acetyltransferase (ChAT) in neurons and cell processes and GAP-43 mRNA as a marker of neurite outgrowth was studied in young and old male rats. 3. Young (3-4 months) and old (18-20 months) male Sprague-Dawley rats remained untreated or were implanted s.c. with a 14 mg pellet of E2 benzoate during 6 weeks. We used immoucytochemistry to determine ChAT and isotopic in situ hybridization to analyze GAP-43 mRNA expression. 4. Aging males showed a reduction in the number and length of ChAT-immunoreactive cell processes, but not in the number of positive neurons in MS and VDB. E2 reverted both parameters in old rats to levels of young animals. Regarding basal levels of GAP-43 mRNA, they were similar in old and young animals, but E2 treatment up-regulated GAP-43 mRNA expression in MS and VDB of old animals only. 5. Our data suggest that prolonged E2 treatment may affect hippocampal function of aging male rats by regulating in part the plasticity of cholinergic, GAP-43 expressing neurones of the basal forebrain. Without discarding a direct E2 effect on the limbic tissue, effects on the cholinergic system may have a pronounced impact on the neuroendocrine and stress responses of the aging hippocampus.

Cellular basis for progesterone neuroprotection in the injured spinal cord

Progesterone (PROG) exerts beneficial and neuroprotective effects in the injured central and peripheral nervous system. In the present work, we examine PROG effects on three measures of neuronal function under negative regulation (choline acetyltransferase [ChAT] and Na,K-ATPase) or stimulated (growth-associated protein [GAP-43]) after acute spinal cord transection injury in rats. As expected, spinal cord injury reduced ChAT immunostaining intensity of ventral horn neurons. A 3-day course of intensive PROG treatment of transected rats restored ChAT immunoreactivity, as assessed by frequency histograms that recorded shifts from predominantly light neuronal staining to medium, dark or intense staining typical of control rats. Transection also reduced the expression of the mRNA for the alpha3 catalytic and beta1 regulatory subunits of neuronal Na,K-ATPase, whereas PROG treatment restored both subunit mRNA to normal levels. Additionally, the upregulation observed for GAP-43 mRNA in ventral horn neurons in spinal cord-transected rats, was further enhanced by PROG administration. In no case did PROG modify ChAT immunoreactivity, Na,K-ATPase subunit mRNA or GAP-43 mRNA in control, sham-operated rats. Further, the PROG-mediated effects on these three markers were observed in large, presumably Lamina IX motoneurons, as well as in smaller neurons measuring approximately <500 micro2. Overall, the stimulatory effects of PROG on ChAT appears to replenish acetylcholine, with its stimulatory effects on Na,K-ATPase seems capable of restoring membrane potential, ion transport and nutrient uptake. PROG effects on GAP-43 also appear to accelerate reparative responses to injury. As the cellular basis for PROG neuroprotection becomes better understood it may prove of therapeutic benefit to spinal cord injury patients.

Up-regulation of growth-associated protein 43 mRNA in rat medial septum neurons axotomized by fimbria-fornix transection

Transection of septohippocampal fibres is widely used to study the response of CNS neurons to axotomy. Septohippocampal projection neurons survive axotomy and selectively up-regulate the transcription factor c-Jun. In the present study we investigated whether these cells concomitantly up-regulate the growth-associated protein-43 (GAP-43), a potential target gene of c-Jun implicated in axonal growth and regeneration. Using in situ hybridization histochemistry (ISHH) it was demonstrated that postlesional c-jun mRNA expression is accompanied by an increased expression of GAP-43 mRNA in the medial septum 3 days following fimbria-fornix transection (FFT). The increase reached a maximum at 7 days and gradually declined thereafter (17 days, 3 weeks). Retrograde prelabeling with Fluoro-Gold followed by axotomy and ISHH revealed that GAP-43 mRNA was up-regulated in septohippocampal projection neurons. Colocalization of GAP-43 mRNA and choline acetyltransferase protein showed that GAP-43 mRNA was expressed by cholinergic medial septal neurons after axotomy. Selective immunolesioning of the cholinergic component of the septohippocampal projection with 192 IgG-saporin followed by FFT demonstrated that GAP-43 mRNA was also synthesized by axotomized GABAergic neurons. These results demonstrate an up-regulation of GAP-43 mRNA in axotomized septohippocampal projection neurons independent of their transmitter phenotype which is closely correlated with c-Jun expression. Because the GAP-43 gene contains an AP-1 site, we hypothesize a c-Jun-driven up-regulation of GAP-43 in lesioned medial septal neurons that may contribute to their survival and regenerative potential following axotomy.

trkA, trkB, and trkC messenger RNA expression by bulbospinal cells of the rat

Previous research has shown that corticospinal as well as rubrospinal neurons express the high-affinity trkB and trkC receptors but not the high-affinity trkA receptor. To determine if bulbospinal neurons in other brainstem areas show the same pattern of trk receptor expression, bulbospinal cells were labelled via the injection of the retrograde tracer FluoroGold into the spinal cord. Brainstem sections were then processed for in situ hybridization using oligonucleotide probes to the trkA, trkB, and trkC receptors. The results indicated that, although trkA expression occurred in brainstem areas that contain bulbospinal neurons (e.g., the vestibular nuclei, and the pontine reticular formation), very few FluoroGold-labelled cells expressed the trkA receptor. In contrast, at least 90% of bulbospinal cells in each brainstem area examined expressed the trkB receptor. Quantitative analysis indicated differences in the level of trkB labelling between bulbospinal cells in different brainstem areas, with the highest levels seen in the locus coeruleus and magnocellular portion of the red nucleus, and the lowest levels seen in the medial and superior vestibular nuclei and the raphe obscurus. With the exception of the accessory trigeminal nucleus, over 84% of bulbospinal cells in each brainstem area also expressed the trkC receptor. TrkC receptor expression was greatest in the locus coeruleus and subcoeruleus and lowest in the accessory trigeminal nucleus, the raphe magnus, and the vestibular nuclei. Results indicate that, as with other descending pathways, virtually all bulbospinal pathways should be amenable to treatment with brain-derived neurotrophic factor, neurotrophin-4/5 or neurotrophin-3, but not nerve growth factor, following spinal cord damage.

Role of NGF in axotomy-induced c-Jun expression in medial septal cholinergic neurons

The extent of neuronal regeneration after axotomy largely depends on the survival capacity of the injured cell. It has been shown for a long time that nerve fiber transection results in retrograde changes in the parent neuronal cell body, and that these changes may eventually lead to neuronal degeneration. At present, little is known about the sequence of events initiated in a nerve cell body by the transection of its axonal process. In this report, we will focus on an interaction of nerve growth factor (NGF) with the transcription factor c-Jun in intact and axotomized septohippocampal projection neurons.

Septo-hippocampal cholinergic and neurotrophin markers in age-induced cognitive decline

Messenger RNA (mRNA) molecules encoding proteins related to the presynaptic cholinergic and neurotrophin systems were quantitated in the hippocampus and basal forebrain of Long-Evans rats with spatial learning ability assessed in the Morris water maze. The reverse transcriptase-polymerase chain reaction showed that the mRNAs for the low-affinity neurotrophin receptor (p75-NTR) and the growth-associated protein GAP-43 were decreased in level in the basal forebrain of aged-impaired rats. In the hippocampus of these aged-impaired rats, the mRNA for VGF, another neurotrophin-inducible gene, also was decreased. In situ hybridization histochemistry revealed that mRNAs for nerve growth factor (NGF) and brain-derived neurotrophic factor increased in level in the aged rat hippocampus; when age effects were removed, NGF mRNA level remained significantly correlated with maze performance. Enzyme-linked immunosorbent assay indicated that NGF protein was expressed at normal levels in the aged rat hippocampus. These mRNA and protein alterations may signify that a defect in neurotrophin signaling exists in the brains of aged Long-Evans rats, underlying reduced plasticity responses in the basal forebrain cholinergic system.

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.

Use of capillary electrophoresis with laser-induced fluorescence detection to assess messenger ribonucleic acid molecules amplified by the polymerase chain reaction: applications in the cloning of cells

Progressive and selective degeneration of specific classes of neurons occurs in the Alzheimer's disease (AD) brain. Differential vulnerability in this disease is evident even within supopulations that synthesize and release acetylcholine as a transmitter; i.e., basal forebrain cholinergic neurons degenerate but other classes of cholinergic neurons are relatively preserved. The basis for this selective vulnerability is unknown. Studies of differential neuronal vulnerability in AD would be facilitated if cell lines expressing neurotransmitter-specific phenotypes could be cloned from the brain. Capillary electrophoresis (CE) with laser-induced fluorescence (LIF) has been shown to be a sensitive method of detection and quantitation of the DNA products of the polymerase chain reaction (PCR). CE/LIF was combined with the PCR to detect phenotypic messenger RNA (mRNA) molecules, converted to cDNA using reverse transcriptase (RT), in cultures of virally immortalized brainstem progenitor cells produced during establishment of a cloning strategy. RT/PCR methods were developed for detection of the mRNAs for choline acetyltransferase (ChAT), the neuronal, constitutive isoform of nitric oxide synthase (c-NOS), and the growth-associated protein GAP-43, three genes known to be expressed in central cholinergic neurons. A "nondestructive" method of screening cultured cells for their expression of c-NOS was established using depolarization with medium containing 50 mM potassium ion. These approaches were first validated using cultured SN56 (cholinergic) and N1E-115 (c-NOS-positive) neuroblastoma cells, and with primary brainstem cultures. For the cloning of novel cell lines, progenitor cells were isolated from the embryonic day 13 fetal brainstem and were immortalized by transfection with a retroviral vector that confers a temperature-sensitive SV-40 transforming activity and neomycin resistance. Cell colonies surviving in G418-containing media were isolated and cloned by dilution. Clonal cultures were expanded by growth at 33 degrees C, differentiated by switching to a low-serum medium and growth at 39 degrees C, and screened for depolarization-induced accumulation of nitrite in the medium. The subset of putative c-NOS-positive clones (about 4%) were then screened for their expression of mRNAs using RT/PCR in combination with CE/LIF. This screening protocol proved to be powerful in the rapid isolation and phenotypic characterization of immortalized progenitor cells cloned from embryonic rat brainstem.

Estrogen increases GAP-43 (neuromodulin) mRNA in the preoptic area of aged rats

Estrogen has been shown to affect the growth, differentiation, and survival of brain neurons and to modulate processes involved in synapse formation and connectivity. These trophic effects are diminished with aging as secretion of estrogen declines. The growth associated protein GAP-43 is found concentrated in axonal growth cones and is implicated in neuronal growth and regeneration. Previous studies have established that expression of GAP-43 can be modulated by estrogen in the preoptic area of developing and adult rat brain. This study was undertaken to determine whether this estrogenic regulation of GAP-43 mRNA is retained in aged rat brain. Young (3 months) and aged (24 months) rats were ovariectomized to remove endogenous estrogen and GAP-43 mRNA in the preoptic area was evaluated using in situ hybridization to compare estrogen and vehicle treatments between age groups. The results demonstrate an age-related decline in GAP-43 mRNA hybridization signal that can be restored to levels comparable to that seen in young animals with estrogen treatment.

Striatal interneurones: chemical, physiological and morphological characterization

The neostriatum is the largest component of the basal ganglia, and the main recipient of afferents to the basal ganglia from the cerebral cortex and thalamus. Studies of the cellular organization of the neostriatum have focused upon the spiny projection neurones, which represent the vast majority of neurones, but the identity and functions of interneurones in this structure have remained enigmatic despite decades of study. Recently, the discovery of cytochemical markers that are specific for each of the major classes of striatal interneurones, and the combination of this with intracellular recording and staining, has revealed the identities of interneurones and some of their functional characteristics in a way that could not have been imagined by the classical morphologists. These methods also suggest some possible modes of action of interneurones in the neostriatal circuitry.

Identified cholinergic neurones in the adult rat brain are enriched in GAP-43 mRNA: a double in situ hybridisation study

The cellular expression of growth associated protein-43 mRNA by identified choline acetyl transferase mRNA positive cells was investigated in the mature rat brain using a combined radioactive and non-radioactive in situ hybridisation technique. Cellular sites of growth associated protein-43 mRNA were detected using a 35S-oligonucleotide while choline acetyl transferase mRNA positive neurones were identified using two alkaline phosphatase-labelled probes. In the cholinergic cells of the corpus striatum, basal forebrain and laterodorsal tegmental nucleus a specific growth associated protein-43 hybridisation signal (silver grains) was detected, demonstrating that these choline acetyl transferase mRNA positive cells are enriched in growth associated protein-43 gene transcripts. By contrast, the large cholinergic cells of the motor nucleus of the trigeminal nerve did not express growth associated protein-43 mRNA. Quantification of the growth associated protein-43 hybridisation signal expressed by identified choline acetyl transferase mRNA positive cells showed regional variations in the relative cellular abundance of this transcript; cholinergic cells in the laterodorsal tegmental nucleus and corpus striatum expressed the strongest cellular hybridisation signal. Mean cross-sectional somatic area measurements of these growth associated protein-43/cholinergic positive cells confirmed the identity of these neurones as belonging to the cholinergic phenotype. A strong 35S-growth associated protein-43 hybridisation signal was detected also in numerous other non-choline acetyl transferase mRNA positive nerve cells in other regions of the brain, although the chemical phenotypes of these neurones were not determined. Our data reveal that expression of the growth-associated protein GAP-43 is maintained in identified cholinergic neurones in the postnatal rat brain, suggesting that this protein may subserve important functions in cholinergic and other neurones of the adult mammalian brain.