A radioimmunoassay for the phosphoprotein B-50: distribution in rat brain

Neuromodulatory effects of repetitive transcranial magnetic stimulation on neural plasticity and motor functions in rats with an incomplete spinal cord injury: A preliminary study

We investigated the effects of intermittent theta-burst stimulation (iTBS) on locomotor function, motor plasticity, and axonal regeneration in an animal model of incomplete spinal cord injury (SCI). Aneurysm clips with different compression forces were applied extradurally around the spinal cord at T10. Motor plasticity was evaluated by examining the motor evoked potentials (MEPs). Long-term iTBS treatment was given at the post-SCI 5th week and continued for 2 weeks (5 consecutive days/week). Time-course changes in locomotor function and the axonal regeneration level were measured by the Basso Beattie Bresnahan (BBB) scale, and growth-associated protein (GAP)-43 expression was detected in brain and spinal cord tissues. iTBS-induced potentiation was reduced at post-1-week SCI lesion and had recovered by 4 weeks post-SCI lesion, except in the severe group. Multiple sessions of iTBS treatment enhanced the motor plasticity in all SCI rats. The locomotor function revealed no significant changes between pre- and post-iTBS treatment in SCI rats. The GAP-43 expression level in the spinal cord increased following 2 weeks of iTBS treatment compared to the sham-treatment group. This preclinical model may provide a translational platform to further investigate therapeutic mechanisms of transcranial magnetic stimulation and enhance the possibility of the potential use of TMS with the iTBS scheme for treating SCIs.

Growth associated protein (GAP-43): cloning and the development of a sensitive ELISA for neurological disorders

GAP-43 has been studied in the rodent and mammalian brain and shown to be present specifically in areas undergoing axonal elongation and synapse formation. GAP-43 was cloned using the baculovirus expression system and purified. A sandwich ELISA was developed using the recombinant GAP-43 as standard and validated. CSF GAP-43 levels were analysed in benign intracranial hypertension, movement disorders, multiple sclerosis, neuropathy, CNS infections, motor neuron disease, and headache (neurological controls). GAP-43 levels were low in all disorders analysed (in particular motor neuron disease; p=0.001, and movement disorders and multiple sclerosis; p<0.0001) compared to controls, aside from CNS infections. GAP-43 is preferentially reduced in the CSF of neurological disorders associated with neurodegeneration.

Growth-associated phosphoprotein expression is increased in the supragranular regions of the dentate gyrus following pilocarpine-induced seizures in rats

Neuroplasticity has been investigated considering the neuronal growth-associated phosphoprotein as a marker of neuronal adaptive capabilities. In the present work, studying the hippocampal reorganization observed in the epilepsy model induced by pilocarpine, we carried out quantitative western blotting associated with immunohistochemistry to determine the distribution of growth-associated phosphoprotein in the hippocampus of rats in acute, silent and chronic periods of this epilepsy model. The fibers and punctate elements from the inner molecular layer of the dentate gyrus were strongly immunostained in animals killed 5 h after status epilepticus, compared with the same region in control animals. Rats presenting partial seizures showed no alterations in the immunostaining pattern compared with saline-treated animals. The hippocampal dentate gyrus of animals during the seizure-free period and presenting spontaneous recurrent seizures was also characterized by strong growth-associated phosphoprotein immunostaining of fibers and punctate elements in the inner molecular layer, contrasting with the control group. As determined by western blotting analysis, growth-associated phosphoprotein levels increased following status epilepticus and remained elevated at the later time-points, both during the silent period and during the period of chronic recurring seizures. Pilocarpine-treated animals, which did not develop status epilepticus, showed no change in growth-associated phosphoprotein levels, indicating that status epilepticus is important to induce growth-associated phosphoprotein overexpression. The measurement of this overexpression could represent one of the early signals of hippocampal reorganization due to status epilepticus-induced damage.

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.

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.

Distribution of GAP-43 mRNA in the immature and adult cerebellum: a role for GAP-43 in cerebellar development and neuroplasticity

Expression of GAP-43 mRNA in the rat cerebellum and inferior olivary nucleus was examined at birth, during postnatal development and in the adult by both Northern and in situ hybridization. Northern blot analysis revealed that cerebellar GAP-43 mRNA expression increases from birth to postnatal day (PD) 7 and then declines to a lower level in the adult. At birth, in situ hybridization experiments showed intense labeling of GAP-43 mRNA in the premigratory, but not the germinal, zone of the cerebellar external granule cell layer. Localization of GAP-43 within the premigratory zone, a layer containing post-mitotic granule cells, indicates that granule cells begin expressing GAP-43 mRNA after final mitosis and during axonal outgrowth of the parallel fibers. The deep cerebellar nuclei and the inferior olive were also intensely labeled at birth. GAP-43 mRNA was localized in granule cells during their migration through the molecular layer of the developing cerebellum and after their arrival in the internal granule cell layer. By PD 21, the pattern of GAP-43 expression was similar to that observed in the adult; GAP-43 mRNA was localized to the internal granule layer and the inferior olive with minimal to no hybridization in the deep cerebellar nuclei and none in the molecular layer. Purkinje cells were devoid of GAP-43 mRNA throughout the postnatal and adult periods. In light of our observations, we propose that GAP-43 is a critical factor in granule cell differentiation/migration, as well as in the parallel and climbing fiber axonal outgrowth and synaptogenesis during development. Localization of GAP-43 mRNA within granule and inferior olivary cells of adult animals indicates that GAP-43 protein observed in the molecular layer is transported from these cells to their terminals in the molecular layer suggesting that GAP-43 is also an intrinsic presynaptic determinant in cerebellar neuroplasticity.

Upregulation of B50/GAP-43 protein mRNA in rat dorsal root ganglia during cisplatin intoxication

Expression of the growth-associated protein B50 (GAP-43) mRNA in dorsal root ganglia (DRG) of rats was studied by in situ hybridization. In response to treatment with the neurotoxic agent cisplatin, B50 mRNA expression was significantly enhanced following a cumulative cisplatin dose of 14 mg/kg. In the untreated age-matched control animals, only half of the ganglion cells exhibited expression of B50 mRNA (mean hybridization signal, 10 times background), whereas at a cumulative cisplatin dose of 14 mg cisplatin every neuron exhibited well above background expression (mean hybridization signal, 34 times background). Cotreatment with a neuroprotective ACTH4-9 analog known to prevent cisplatin neuropathy in rats did not affect the overall expression of B50 mRNA. However, in the subpopulation of large sensory neurons, B50 mRNA content was significantly higher in the group cotreated with the ACTH4-9 analog as compared with the saline-cotreated group after 14 mg/kg of cisplatin. We conclude that in analogy with the well-known upregulation of B50 mRNA following mechanical nerve lesions, treatment with the neurotoxic drug cisplatin also leads to an increase in B50 mRNA expression. This observation lends strength to the hypothesis that in neuropathies an imbalance between regenerative and degenerative mechanisms exists. The ability of the larger sensory neurons to retain an increased B50 mRNA expression better after cotreatment with the peptide than without may be related to stimulation of regenerative processes by this ACTH4-9 analog.

Cloning and promoter analysis of the human B-50/GAP-43 gene

We here report isolation of exon 1 and analysis of the human B-50 promoter. A human genomic lambda EMBL3 library was screened with a homologous PCR probe. Two independent clones were analyzed and partially sequenced: They contained up to 5 kb sequence upstream of the translation start site and approx 13 kb of intron 1 sequence. There was a high degree of homology between the rat and the human gene with 100% homology from -504 to -427, with respect to the translation start codon. However, relatively long GT and GA repeats as seen in the rat gene were absent. Various promoter-reporter constructs, containing 5.0 to 0.12 kb of the upstream region, were transfected into undifferentiated and neuroectodermally differentiated P19-EC. Two promoter activities were found. The minimal fragment with promoter activity still responsive to differentiation was the 0.22 kb construct, similar to rat promoter P2. We conclude that the human B-50 gene is expressed in a similar way to the rat B-50 gene, based on the presence of two transcripts, the high degree of homology between the rat and the human sequence, and the two promoter activities found in P19-EC cells.

Identification of two promoter regions in the rat B-50/GAP-43 gene

To determine cis-acting elements controlling the rat B-50/GAP-43 gene expression, the genomic DNA encoding exon 1 and the 5' flanking sequence was isolated. Sequence analysis of 1 kb 5' untranslated region (UTR) revealed the presence of a (GA)-repeat and a (GT)-repeat. The size of the (GA)-repeat varied due to both an instability of phage lambda lambda DNA in E. coli and genomic variation between rats. Transcription initiation sites were mapped in 8-day-old rat brain poly(A)+ mRNA. Primer extension indicated multiple transcription start sites at -159 and -339/-342 nt upstream of the translation start site; reverse transcriptase coupled PCR showed that the most 5' transcription start site is located between -465 and -440. Northern blotting demonstrated that approximately 90% of the B-50 mRNAs initiates at approximately -50. Promoter analysis by transient transfection assays in undifferentiated and retinoic acid-differentiated P19-EC cells revealed that the rat B-50 gene contains two promoters. P1 (located between -750 and -407) contains commonly observed promoter elements such as a TATA box and CCAAT boxes. P2 (located between -233 and -1) neither contains TATA boxes, CCAAT boxes nor consensus sequences of house-keeping gene promoters like GC-boxes. The activity of P1 is inhibited at neuroectodermal differentiation of P19-EC cells whereas the activity of P2 is stimulated. In 8 day old rat brain the majority of the B-50 mRNA transcripts are derived from P2. It is concluded that at this developmental stage P2 is the most important promoter.

Purification of B-50 by 2-mercaptoethanol extraction from rat brain synaptosomal plasma membranes

Several methods have been described previously for the purification of the nervous-tissue specific protein kinase C substrate B-50 (GAP-43). In this paper we present a new purification method for B-50 from rat brain which employs 2-mercaptoethanol to release the protein from isolated synaptosomal plasma membranes. Most likely, 2-mercaptoethanol reduces disulfide bonds involved in the linkage of B-50 to the membrane. After washing the membranes with 100 mM NaCl to detach loosely bound proteins, B-50 is the major protein (and the only protein kinase C substrate) released by 0.5% 2-mercaptoethanol treatment. Further purification to apparent homogeneity is achieved by affinity chromatography on calmodulin sepharose. B-50 binds to calmodulin in the absence of calcium and specifically elutes from the column with 3 mM calcium. The procedures described is simple, rapid and highly suitable for large scale purification of B-50 from rat brain.

B-50/GAP43 localization in polarized hippocampal neurons in vitro: an ultrastructural quantitative study

Hippocampal pyramidal neurons cultured in vitro gradually develop morphologically and biochemically distinct axons and dendrites, resulting in functional neuronal polarization [Dotti C. G. et al. (1988) J. Neurosci. 8, 1454-1468]. We have studied the distribution of the growth-associated protein B-50 in hippocampal neurons of the rat at stage 3 of development by means of light and electron microscopic immunocytochemistry. Hippocampal neurons grown for two to three days in vitro were aldehyde fixed and immunolabelled using polyclonal rabbit antibodies to B-50 and goat anti-rabbit immunoglobulins tagged with 1 nm gold particles. In order to permit visualization by both light and electron microscopy, the gold probes were silver intensified. Light microscopy demonstrated the absence of B-50 immunostaining in living neurons and the presence after permeabilization by fixation and subsequent treatment of the neurons with sodium borohydride, indicating that B-50 is located intracellularly. Both immunofluorescence and immunogold-silver labelling revealed that B-50 immunoreactivity outlined all neurites of the morphologically polarized neurons. For quantitative electron microscopy, six morphologically polarized neurons (developmental stage 3) were carefully selected from immunolabelled Epon-embedded neurons and processed completely to ultrathin sections. In this way the ultrastructural localization of B-50 has been studied in the cell body, the neurites and their growth cones. For each sectioned neuron, the relative distribution of the gold-silver deposits (representing B-50) over the plasma membrane of various cellular compartments was quantitated. B-50 is located at the plasma membrane of the neuronal cell body and all neurites including their growth cones. The density of B-50 on the plasma membrane of growth cones is not different from that of the neuritic shaft. In addition, B-50 is present on the cytosolic side of the membrane of small electron-lucent vesicles (average diameter 102.7 +/- 2.5 nm) resembling transport vesicles. These vesicles are present in the cell body and the neurites. A two-fold concentration is found in the central region of the growth cones, suggesting a role of these vesicles in axonal transport, membrane insertion and (or) recycling. Since, at the onset of neuronal polarization, B-50 is present at the plasma membrane in all compartments of the hippocampal neuron, we suggest that at this stage of development B-50 does not participate directly in the processes leading to morphological polarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical detection of the growth-associated protein B-50 by newly characterized monoclonal antibodies in human brain and muscle

The growth-associated protein B-50 also termed GAP-43, F1, pp46, P-57 and neuromodulin is a nervous tissue-specific protein kinase C (PKC) substrate that is considered to play a major role in neurite formation, regeneration, and neuroplasticity. We describe the isolation of seven mouse monoclonal antibodies (Mabs) directed against B-50. The Mabs are produced against the bovine B-50, selected by ELISA for cross-reactivity with its human counterpart, and evaluated on Western blots in comparison with the well-characterized affinity-purified rabbit polyclonal antibodies to rat-B-50. The Western blots show that the Mabs NM1, NM4, and NM6 recognize specifically the B-50 of bovine, human, and rat brain extract and the purified PKC phosphorylated and unphosphorylated rat B-50 isoforms. The Mabs NM2 and NM3 cross-react with bovine B-50 immunoreactive c-kinase substrate (BICKS), a protein sharing a 17 amino acid sequence homology with B-50. Two Mabs are useful for the detection of B-50 immunoreactivity in formalin-fixed human and rat brain tissues. In human specimen of the hippocampus, a characteristic neuropil distribution of B-50 is detected by the Mabs. In human muscle, Mabs reveal B-50 in nerve bundles and in axons at motor end plates. Thus, these Mabs are useful in investigating the function and localization of the B-50 protein.

Estrogenic regulation and sex dimorphism of growth-associated protein 43 kDa (GAP-43) messenger RNA in the rat

In addition to effects on brain protein synthesis, neurotransmitter release, and electrophysiology, estrogens alter neurite outgrowth and synaptogenesis. This study examined in the adult rat the effects of estrogen and sex on the expression of the GAP-43 gene; encoding a phosphoprotein structurally and physiologically linked to these two processes in the rat CNS. Ovariectomized (OVX) rats were injected with vehicle or estrogen, or male and female rats were either gonadectomized or left intact. Brains were dissected to obtain ventromedial hypothalamus (VMH), posterior hypothalamus (PH), or frontal cortex (CTX). Total RNA from these areas were extracted, and slot-blots of equal masses of total RNA were hybridized to 32P-labeled cDNAs for GAP-43 and beta-actin, and also to synthetic poly-dT. Resultant autoradiograms were scanned by laser densitometry, quantitated, and ratios of the gray scale generated by each probe were compared between experimental groups. GAP-43 mRNA expression, when compared to expression of either beta-actin mRNA or total poly(A)-containing RNA (poly(A) RNA), was higher in VMH and PH as compared to CTX. Estrogen treatment of OVX rats resulted in a 48-74% increase in GAP-43 mRNA levels in the VMH--in one experiment, this increase was noted after 2 h of estradiol treatment, and in another after 3 days of estradiol benzoate treatment; but PH and CTX were unaffected by either estrogen regimen. Conversely, ovariectomy of intact rats decreased GAP-43 mRNA expression by 45% in the VMH, but not in the CTX.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the growth-associated protein B-50/GAP-43 in neuronal plasticity

The neuronal phosphoprotein B-50/GAP-43 has been implicated in neuritogenesis during developmental stages of the nervous system and in regenerative processes and neuronal plasticity in the adult. The protein appears to be a member of a family of acidic substrates of protein kinase C (PKC) that bind calmodulin at low calcium concentrations. Two of these substrates, B-50 and neurogranin, share the primary sequence coding for the phospho- and calmodulin-binding sites and might exert similar functions in axonal and dendritic processes, respectively. In the adult brain, B-50 is exclusively located at the presynaptic membrane. During neuritogenesis in cell culture, the protein is translocated to the growth cones, i.e., into the filopodia. In view of many positive correlations between B-50 expression and neurite outgrowth and the specific localization of B-50, a role in growth cone function has been proposed. Its phosphorylation state may regulate the local intracellular free calmodulin and calcium concentrations or vice versa. Both views link the B-50 protein to processes of signal transduction and transmitter release.

An ELISA assay for GAP-43

An ELISA assay for the growth associated protein GAP-43 was developed to determine rapidly its relative abundance in neuronal tissue. The assay was performed with affinity-purified anti-GAP-43 antibody that detected a single band of Mr = 42,000-45,000 on Western blots of rat brain homogenates but no bands on blots of liver homogenates. GAP-43 was determined by ELISA assay in as little as 0.6 microgram protein of brain homogenate. The assay was highly reproducible; the standard error of the mean of sample to sample variation was less than 5%. When ELISA development time was held constant, the standard error of the mean of inter-assay variation was between 2 and 7%. Using this method, GAP-43 immunoreactivity was examined in developing rat brain. At post-natal day 1, GAP-43 immunoreactivity was 3-4 times greater than that observed in the adult, remained elevated for several weeks, and decreased by the end of the first month of life. These results are in accord with previous studies on the expression or synthesis of GAP-43 during neuronal development.

Ultrastructural localization of adrenocorticotrophic hormone and the phosphoprotein B-50/growth-associated protein 43 in freeze-substituted, Lowicryl HM20-embedded mesencephalic central gray substance of the rat

Previous studies have shown that the endogenous phosphorylation of the neuron-specific protein B-50 in isolated synaptic plasma membranes is inhibited by adrenocorticotrophic hormone(1-24). The aim of this study is to examine if there is a specific neuroanatomical interaction of adrenocorticotrophic hormone and B-50 in the mesencephalic central gray substance of the rat. With light microscopy, high B-50 immunoreactivity was detected throughout the mesencephalic central gray substance, overlapping with those areas where adrenocorticotrophic hormone-immunoreactive fibres were present. To study the ultrastructural localization of B-50 and adrenocorticotrophic hormone, we employed a method of immunogold labelling on ultrathin sections of freeze-substituted and Lowicryl HM20-embedded fixed brain tissue. This offered optimal morphological preservation together with high retention of antigenicity. At the electron microscopic level, adrenocorticotrophic hormone immunoreactivity was detected in dense-core secretory granules present in non-junctional regions of axoinal varicosities. This suggests a non-synaptic release of adrenocorticotrophic hormone from the axons. Using double immunolabelling techniques we showed that in adrenocorticotrophic hormone-innervated areas of the mesencephalic central gray substance B-50 immunoreactivity was present at plasma membranes of all unmyelinated axons and axonal varicosities and virtually absent in dendrites. The result on B-50 localization agrees well with previous studies in the hippocampus [Van Lookeren Campagne et al. 1990 J. Neurocytol. 19, 948-961] and in the pyramidal tract [Gorgels et al. 1989 J. Neurosci. 9, 3861-3869] of the rat and suggests that in the mature rat central nervous system, B-50 expression in axons is a general phenomenon. For the adrenocorticotrophic hormone-innervated areas, we discuss the proposal that non-synaptically released adrenocorticotrophic hormone modulates B-50 phosphorylation in axons and axon terminals.

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.

Synaptic receptors and intracellular signal transduction in the cerebellum

Glutamate receptor subtypes mediating excitatory synaptic neurotransmission in the cerebellar cortex are briefly reviewed from molecular biological, electrophysiological and pharmacological points of view. In particular, molecular biological findings of a novel family of AMPA-selective glutamate receptors are introduced, and the pharmacological and electrophysiological properties and the identity of cerebellar N-methyl-D-aspartate-sensitive receptors probably existing on Purkinje cells are discussed in comparison with well-established cerebral NMDA receptors. As possible intracellular mechanisms of the long-term depression of parallel fiber-Purkinje cell neurotransmission, the perspective of the roles of novel messengers, nitric oxide and arachidonic acid, is particularly commented based on recent information about cerebral long-term events. The specificity and possible independence of cerebellar excitatory amino acid receptors and linked intracellular second messengers are also suggested, taking the highly active guanylate cyclase system in Purkinje cells and other cerebellum-specific proteins into consideration.

Regulation of immunoreactive GAP-43 expression in rat cortical macroglia is cell type specific

Growth-associated protein 43 (GAP-43) is an abundant, intensely investigated membrane phosphoprotein of the nervous system (Benowitz, L.I., and A. Routtenberg. 1987. Trends Neurosci. 10:527-532; Skene, J. H. P. 1989. Annu. Rev. Neurosci. 12:127-156), with a hitherto unknown function. We have previously demonstrated that astrocytes, brain macroglial cells, contain GAP-43 (Steisslinger, H. W., V. J. Aloyo, and L. Vitković, 1987. Brain Res. 415:375-379; Vitković, L., H. W. Steisslinger, V. J. Aloyo, and M. Mersel. 1988. Proc. Natl. Acad. Sci. USA. 85:8296-8300; Vitković L., and M. Mersel. 1989. Metab. Brain Dis. 4:47-53). Results from double immunofluorescent labeling experiments presented here show that oligodendrocytes also contain GAP-43 immunoreactivity (GAP-43ir). Thus, all three macroglial cell types of the central nervous system (type I and type 2 astrocytes and oligodendrocytes) contain GAP-43. Whereas immunoreactive GAP-43 is expressed by progenitors of all macroglial cell types, the developmental regulation of its expression is cell type specific. Immunoreactive GAP-43 is downregulated in type 1 astrocytes, and constitutively expressed in both type 2 astrocytes and oligodendrocytes. These results may be relevant to potential function(s) of GAP-43.

Neuroplasticity in the olfactory system: differential effects of central and peripheral lesions of the primary olfactory pathway on the expression of B-50/GAP43 and the olfactory marker protein

The regeneration of the olfactory neuroepithelium following olfactory bulbectomy or peripheral deafferentation was studied with mRNA probes and antibodies for B-50/GAP43 and for olfactory marker protein (OMP). Two stages in the regeneration of the olfactory epithelium could be discerned with these reagents. The first stage occurs following either peripheral deafferentation of the olfactory epithelium with Triton X-100 (TX-100) or after bulbectomy and is characterized by the formation of a large population of immature olfactory receptor neurons. These newly formed neurons express B-50/GAP43, a phosphoprotein related to neuronal growth and plasticity. During the second stage of the regeneration process the newly formed olfactory neurons mature, as evidenced by a decrease in their expression of B-50/GAP43 and an increase in the expression of OMP. This stage is only manifested if the developing neurons have access to the target olfactory bulb. Formation of a full complement of OMP-expressing neurons occurs only after peripheral lesion with TX-100. In contrast, following bulbectomy the reconstituted olfactory epithelium lacks its normal target and is compromised in its ability to recover from nerve damage, as evidenced by the presence of a large number of B-50/GAP43-expressing neurons up to 3 months after the lesion and its failure to establish a full complement of OMP-expressing neurons. These results demonstrate that the olfactory epithelium is capable of replacing its sensory neurons independently of the presence of its target, the olfactory bulb. However, the differential patterns of expression of B-50/GAP43 and OMP at long times after peripheral lesion with TX-100 or bulbectomy illustrate the profound effect the olfactory bulb has on neuronal maturation in reconstituted olfactory neuroepithelium.

Regulation of free calmodulin levels by neuromodulin: neuron growth and regeneration

Neuromodulin is a neurospecific calmodulin binding protein that is implicated in neurite extension, axonal elongation and long-term potentiation. Yuechueng Liu and Daniel Storm propose that neuromodulin binds and concentrates calmodulin on growth cone membranes and that stimulation of protein kinase C releases high local concentrations of calmodulin in the growth cone. Interactions between released calmodulin and cytoskeleton proteins may affect the polymerization, crosslinking and membrane attachment of cytoskeleton polymers. This local 'softening' of the membrane may be an initial event in filopodia formation and extension.

Colchicine effect on B-50/GAP43 phosphoprotein localization in rat dorsal root ganglion explants

In rat embryonic dorsal root ganglion explants stimulated by nerve growth factor, the neuron-specific phosphoprotein B-50 (GAP43) is primarily localized in the distal portion of outgrowing neurites. Addition of colchicine leads to a decrease in total amount of B-50 and a marked redistribution in the neurons. The data underscore the role of axonal transport in the concentration of B-50 in growth cones of growing neurites.

Posttranslational membrane attachment and dynamic fatty acylation of a neuronal growth cone protein, GAP-43

Growth cones, the motile apparatus at the ends of elongating axons, are sites of extensive and dynamic membrane-cytoskeletal interaction and insertion of new membrane into the growing axon. One of the most abundant proteins in growth cone membranes is a protein designated GAP-43, whose synthesis increases dramatically in most neurons during periods of axon development or regeneration. We have begun to explore the role of GAP-43 in growth cone membrane functions by asking how the protein interacts with those membranes. Membrane-washing experiments indicate that mature GAP-43 is tightly bound to growth cone membranes, and partitioning of Triton X-114-solubilized GAP-43 between detergent-enriched and detergent-depleted phases indicates considerable hydrophobicity. The hydrophobic behavior of the protein is modulated by divalent cations, particularly zinc and calcium. In vivo labeling of GAP-43 in neonatal rat brain with [35S]methionine shows that GAP-43 is initially synthesized as a soluble protein that becomes attached to membranes posttranslationally. In tissue culture, both rat cerebral cortex cells and neuron-like PC12 cells actively incorporate [3H]palmitic acid into GAP-43. Isolated growth cones detached from their cell bodies also incorporate labeled fatty acid into GAP-43, suggesting active turnover of the fatty acid moieties on the mature protein. Hydrolysis of ester-like bonds with neutral hydroxylamine removes the bound fatty acid and exposes new thiol groups on GAP-43, suggesting that fatty acid is attached to the protein's only two cysteine residues, located in a short hydrophobic domain at the amino terminus. Modulation of the protein's hydrophobic behavior by divalent cations suggests that other domains, containing large numbers of negatively charged residues, might also contribute to GAP-43-membrane interactions. Our observations suggest a dynamic and reversible interaction of GAP-43 with growth cone membranes.

Determination of changes in the phosphorylation state of the neuron-specific protein kinase C substrate B-50 (GAP43) by quantitative immunoprecipitation

To determine changes in the degree of phosphorylation of the protein kinase C substrate B-50 in vivo, a quantitative immunoprecipitation assay for B-50 (GAP43, F1, pp46) was developed. B-50 was phosphorylated in intact hippocampal slices with 32Pi or in synaptosomal plasma membranes with [gamma-32P]ATP. Phosphorylated B-50 was immunoprecipitated from slice homogenates or synaptosomal plasma membranes using polyclonal anti-B-50 antiserum. Proteins in the immunoprecipitate were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the incorporation of 32P into B-50 was quantified by densitometric scanning of the autoradiogram. Only a single 48-kilodalton phosphoband was detectable in the immunoprecipitate, but this band was absent when preimmune serum was used. The B-50 immunoprecipitation assay was quantitative under the following condition chosen, as (1) recovery of purified 32P-labelled B-50 added to slice homogenates or synaptosomal plasma membranes was greater than 95%; and (2) modulation of B-50 phosphorylation in synaptosomal plasma membranes with adrenocorticotrophic hormone, polymyxin B, or purified protein kinase C in the presence of phorbol diester resulted in EC50 values identical to those obtained without immunoprecipitation. With this immunoprecipitation assay we found that treatment of hippocampal slices with 4 beta-phorbol 12,13-dibutyrate stimulated B-50 phosphorylation, whereas 4 alpha-phorbol 12,13-didecanoate was inactive. Thus, we conclude that the B-50 immunoprecipitation assay is suitable to monitor changes in B-50 phosphorylation in intact neuronal tissue.

Molecular transduction mechanisms in ACTH-induced grooming

Intraventricular administration of ACTH1-24 induces excessive grooming in the rat. Ethogram analysis shows that the peptide does not alter grooming behavior seen in a novel box, but that it prolongs the duration of the grooming bout. Extensive structure-activity studies have been performed which suggest that the active site lies in a region (5-13) of the ACTH molecule. Interestingly, the (1-24) sequence is fully active, whereas (1-10) and (11-24) alone or in combination are inactive, pointing to a specific stereoconformation necessary to induce grooming. However, despite the fact that there are ACTH-and/or alpha-MSH-containing peptidergic neurons, no conclusive evidence is available demonstrating stereospecific, saturable binding sites for these peptides in brain. The analysis of the neural substrate underlying ACTH-induced excessive grooming has been performed by means of electrolytic lesions of specific brain regions and by neuropharmacological manipulations. The data suggest that the periaqueductal gray is the primary target for ACTH and that the activity of neostriatum and accumbens, via a nigro-colliculus-periaqueductal gray pathway, modulates the display of excessive grooming. An important feature of the neural substrate is that it displays single-dose tolerance to the peptide during the first hours after the first peptide injection. It is suggested that the tolerance is a feature of an opioid receptor-containing component of the neural substrate. The molecular mechanism of action of ACTH is complex and may involve different transmembrane signal transduction systems. The peptide decreases the degree of phosphorylation of a neuron-specific, synaptic phosphoprotein B-50 by inhibition of protein kinase C. It is concluded that changes in the degree of phosphorylation of B-50 regulate the activity of the lipid kinase phosphatidylinositol 4-phosphate kinase. Therefore, the B-50 protein seems to be part of a negative feedback loop in the receptor-activated hydrolysis of phosphatidylinositol 4,5-bis-phosphate (PIP2). There is increasing evidence that the molecular mechanism by which ACTH brings about the grooming response involves a change in phosphorylation of B-50. Firstly, the structure-activity relationship of ACTH-induced excessive grooming is nearly identical to that obtained for ACTH-induced inhibition of protein kinase C.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical distribution of the protein kinase C substrate B-50 (GAP43) in developing rat pyramidal tract

The neuron-specific phosphoprotein B-50 is a major substrate of kinase C in fetal nerve growth cones, neonatal neural and synaptosomal plasma membranes. B-50 is identical to a growth-associated protein GAP43. Similarly, increases in B-50 occur during rat brain development, neuronal differentiation and axon regeneration. To document the relation between the expression of B-50 and the outgrowth of central axons, we studied B-50 in the developing pyramidal tract in rats at postnatal days 2, 7 and 90 (P2, P7 and P90), at the third cervical spinal segment C3, using affinity-purified antibodies to B-50. At P2 and P7, when outgrowth of pyramidal tract fibers is occurring, B-50 immunoreactivity (BIR) is intense in these fibers. BIR is reduced from P2 to P7 in the ascending fiber tracts of the cuneatus and the gracilis, which develop earlier. At P90 when most of the dorsal funiculus fibers have reached their targets and many are myelinated, BIR is dramatically reduced. In agreement, a 10-fold decrease in B-50 content was measured at P90, as compared to P7. Therefore, our results indicate that B-50 is only expressed relatively abundant in axons of the funiculus posterior during outgrowth. By inference, B-50 may be a differentiating marker to detect elongating fibers.

The kinase C substrate protein B-50 and axonal regeneration

As reported previously the prominent protein kinase C substrate protein B-50 is present in growth cones isolated from fetal rat brain and in outgrowing hippocampal neurites. These findings suggest that B-50 plays a role in axonal growth during development of the nervous system. In the present paper the fate of B-50 is investigated in regenerating rat sciatic nerve. Using affinity-purified anti-B-50 antibodies B-50 levels have been compared in crushed and contralateral intact nerves by means of immunoblotting and radioimmunoassay. B-50 levels in the crushed nerve increased 5.3-fold as compared to non-crushed controls. Furthermore, the cellular localization of B-50 has been assessed by immunohistochemistry. Virtually no B-50 immunoreactivity was seen in control nerves, but bright immunofluorescence appeared in regenerating sprouts. Our data are in line with current evidence from several laboratories that B-50 is a member of a small family of growth-associated proteins and support the hypothesis that B-50 is involved in axonal growth.

Nerve growth factor enhances the level of the protein kinase C substrate B-50 in pheochromocytoma PC12 cells

Exposure of PC12 cells to nerve growth factor results in arrest of cell growth and induction of differentiation to sympathetic neuron-like cells, bearing neurites. In this study we identify a 48 kDa PC12 phosphoprotein as the neuron-specific protein kinase C substrate B-50 (Mr 48 kDa; IEP 4.5) on basis of comigration with purified B-50, immunoreactivity and phosphopeptide mapping. B-50 is present in both undifferentiated and differentiated PC12 cells. Exposure of PC12 cells to nerve growth factor for two days results in a 2.5-fold increase in the amount of B-50 as measured by RIA. Indirect immunofluorescence microscopy reveals that B-50 is mainly localized at the cell membrane and in growth cones. Our data are in line with the hypothesis that B-50 plays a role in neurite outgrowth and indicate that PC12 cells provide a suitable model to study this hypothesis.