Chicken growth-associated protein GAP-43 is tightly bound to the actin-rich neuronal membrane skeleton

Selective inhibition of cannabinoid CB1 receptor-evoked signalling by the interacting protein GAP43

Cannabinoids exert pleiotropic effects on the brain by engaging the cannabinoid CB1 receptor (CB1R), a presynaptic metabotropic receptor that regulates key neuronal functions in a highly context-dependent manner. We have previously shown that CB1R interacts with growth-associated protein of 43 kDa (GAP43) and that this interaction inhibits CB1R function on hippocampal excitatory synaptic transmission, thereby impairing the therapeutic effect of cannabinoids on epileptic seizures in vivo. However, the underlying molecular features of this interaction remain unexplored. Here, we conducted mechanistic experiments on HEK293T cells co-expressing CB1R and GAP43 and show that GAP43 modulates CB1R signalling in a strikingly selective manner. Specifically, GAP43 did not affect the archetypical agonist-evoked (i) CB1R/Gi/o protein-coupled signalling pathways, such as cAMP/PKA and ERK, or (ii) CB1R internalization and intracellular trafficking. In contrast, GAP43 blocked an alternative agonist-evoked CB1R-mediated activation of the cytoskeleton-associated ROCK signalling pathway, which relied on the GAP43-mediated impairment of CB1R/Gq/11 protein coupling. GAP43 also abrogated CB1R-mediated ROCK activation in mouse hippocampal neurons, and this process led in turn to a blockade of cannabinoid-evoked neurite collapse. An NMR-based characterization of the CB1R-GAP43 interaction supported that GAP43 binds directly and specifically through multiple amino acid stretches to the C-terminal domain of the receptor. Taken together, our findings unveil a CB1R-Gq/11-ROCK signalling axis that is selectively impaired by GAP43 and may ultimately control neurite outgrowth.

A crosslinking analysis of GAP-43 interactions with other proteins in differentiated N1E-115 cells

It has been suggested that GAP-43 (growth-associated protein) binds to various proteins in growing neurons as part of its mechanism of action. To test this hypothesis in vivo, differentiated N1E-115 neuroblastoma cells were labeled with [³⁵S]-amino acids and were treated with a cleavable crosslinking reagent. The cells were lysed in detergent and the lysates were centrifuged at 100,000 × g to isolate crosslinked complexes. Following cleavage of the crosslinks and analysis by two-dimensional gel electrophoresis, it was found that the crosslinker increased the level of various proteins, and particularly actin, in this pellet fraction. However, GAP-43 was not present, suggesting that GAP-43 was not extensively crosslinked to proteins of the cytoskeleton and membrane skeleton and did not sediment with them. GAP-43 also did not sediment with the membrane skeleton following nonionic detergent lysis. Calmodulin, but not actin or other proposed interaction partners, co-immunoprecipitated with GAP-43 from the 100,000 × g supernatant following crosslinker addition to cells or cell lysates. Faint spots at 34 kDa and 60 kDa were also present. Additional GAP-43 was recovered from GAP-43 immunoprecipitation supernatants with anti-calmodulin but not with anti-actin. The results suggest that GAP-43 is not present in complexes with actin or other membrane skeletal or cytoskeletal proteins in these cells, but it is nevertheless possible that a small fraction of the total GAP-43 may interact with other proteins.

Association of Gap-43 (neuromodulin) with microtubule-associated protein MAP-2 in neuronal cells

Gap-43 (B-50, neuromodulin) is a presynaptic protein implicated in axonal growth, neuronal differentiation, plasticity, and regeneration. Its activities are regulated by its dynamic interactions with various neuronal proteins, including actin and brain spectrin. Recently we have shown that Gap-43 co-localizes with an axonal protein DPYSL-3 in primary cortical neurons. In the present study we provide evidence that Gap-43 co-localizes and potentially interacts with microtubule-associated protein MAP-2 in adult and fetal rat brain, as well as in primary neuronal cultures. Our studies suggest that this interaction may be developmentally regulated.

Co-localization and interaction of DPYSL3 and GAP43 in primary cortical neurons

Dihydropyrimidinase-like 3 (DPYSL3) and GAP43 are both involved in neurite outgrowth, a crucial process for the differentiation of neurons. The present study shows for the first time that DPYSL3 co-localizes with GAP43 in primary cortical neurons. Further co-immunoprecipitation and overlay assay showed the ability of both recombinant and endogenous DPYSL3 to bind GAP43, indicating a specific interaction between DPYSL3 and GAP43 in primary cortical neurons.

Early profiles of axonal growth and astroglial response after spinal cord hemisection and implantation of Schwann cell-seeded guidance channels in adult rats

We previously demonstrated that transplantation of Schwann cell-seeded channels promoted the regrowth of injured axons in the adult spinal cord. It is not clear, however, whether injured axons recapitulate the developmental scenarios to accomplish regeneration. In the present study, we investigated the early events associated with axonal regrowth after spinal cord hemisection at the eighth thoracic level and implantation of a Schwann cell-seeded minichannel in adult rats. Animals were sacrificed at postoperative days (PO) 2, 4, 7, and 14. Anterograde tracing with fluoro-ruby showed that regenerating axons grew into the graft prior to PO2 and reached the distal end of the channel at PO7. These axons expressed both embryonic neural cell adhesion molecule (E-NCAM) and growth associated protein-43 (GAP-43). Although the expression of E-NCAM decreased by PO7, that of GAP-43 remained high throughout the first 2 weeks after implantation. A close relation of vimentin-positive astroglia to the growing axons in the host tissue suggested a contact-mediated role of these cells in axon guidance. Aggregation of glial fibrillary acidic protein (GFAP)-positive astrocytes together with the increased expression of chondroitin sulfate proteoglycans (CSPGs) starting at PO7 appeared to inhibit axonal growth at the host-graft interface. Thus, adult regenerating axons and astroglia do express developmentally related molecules that may facilitate axonal growth into a permissive graft at the early phase of injury and regeneration. These results suggest that molecules and astroglia essential to development are both important in influencing axonal regrowth in the adult spinal cord.

Temporal and spatial distribution of growth-associated molecules and astroglial cells in the rat corticospinal tract during development

To understand better the role of growth-promoting and -inhibiting molecules in the development of the corticospinal tract (CST), temporospatial expression of embryonic neural cell adhesion molecule (E-NCAM), growth-associated protein-43 (GAP-43), and chondroitin sulfate proteoglycan (CSPG) was studied in developing rats. Transverse sections of the seventh cervical (C7), seventh thoracic (T7), and fourth lumbar (L4) segments were examined at postnatal days (P) 2, 6, 10, 14, and 28. The highest E-NCAM immunoreactivity appeared at the C7 level on P2 and shifted caudally to the T7 on P6 and L4 on P10, which correlated closely with the time course of CST development. The peak expression of GAP-43 emerged at C7 on P2 and shifted to the T7 and L4 levels at a relatively lagging pace compared with that of E-NCAM. Conversely, a transient reduction in CSPG immunoreactivity was found within the CST at the C7 level on P2, T7 level on P6, and L4 level on P10, corresponding well with the arrival of CST-leading axons at these levels. Interestingly, higher levels of CSPG were found to surround the growing CST, suggesting a repulsive environment that channels the growth of CST. Moreover, a transition from immature to mature astrocytes in a rostrocaudal direction during CST development was evidenced by anti-vimentin and anti-glial fibrillary acidic protein (GFAP) immunostaining, suggesting a guidance role of immature astroglia in axonal outgrowth. Our study thus demonstrated dynamic changes of multiple growth-related molecules and astroglial environment that contribute to postnatal development of the CST.

Utilization of an HSV-based amplicon vector encoding the axonal marker hPLAP to follow neurite outgrowth in cultured DRG neurons

Delivery of genes into DRG neurons by viral vectors is a powerful tool for the study of axonal outgrowth. In order to achieve efficient transfer of growth-related genes and simultaneously label neuronal processes, we have utilized the HSV-based amplicon vector system. A bicistronic expression cassette encoding the growth associated protein-43 (GAP-43) and the axonal marker human placental alkaline phosphatase (hPLAP) reporter gene under translation control of an internal ribosomal entry site was cloned into the HGCX amplicon vector. This hPLAP reporter enabled efficient labeling of neurites in both dissociated adult DRG neurons and embryonic DRG explants. Using this reporter, the effect of GAP-43 on neurite outgrowth in transduced DRG neurons could be demonstrated. HSV-based amplicon vectors can contribute to the study of axonal growth and guidance in cultured neurons.

Down-regulation of GAP-43 During Oligodendrocyte Development and Lack of Expression by Astrocytes In Vivo: Implications for Macroglial Differentiation

The discovery of molecular markers which are selectively expressed during the development of specific classes of rat central nervous system macroglia has greatly advanced our understanding of how these cells are related. In particular, it has been shown in tissue culture that oligodendrocytes and some astrocytes (type-2) may be derived from a common progenitor cell (O-2A progenitor). However, the existence of type-2 astrocytes in vivo has yet to be unequivocally established. Recently, it has been reported that the neural-specific growth-associated protein-43 (GAP-43, otherwise known as B-50, F1, pp46 and neuromodulin) may be expressed by cells of the O-2A lineage in vitro. We set out to examine the cellular specificity of GAP-43 in O-2A progenitors and their descendants in vitro and in vivo. Using a polyclonal antiserum against a GAP-43 fusion protein we have shown the presence of immunoreactive GAP-43 in the membranes of bipotential O-2A glial progenitor cells and type-2 astrocytes by Western blotting and immunocytochemistry of cells in culture. In contrast to previous studies, double labelling with mature oligodendrocyte markers showed that GAP-43 is down-regulated during oligodendrocyte differentiation in vitro. Immunohistochemical staining of sections of developing rat brain demonstrated the same developmental regulation of GAP-43, suggesting that oligodendrocytes only express GAP-43 at immature stages. In addition, normal and reactive astrocytes in tissue sections were not labelled with GAP-43.

Noradrenergic, but not Adrenergic Chromaffin Cells in the Adrenal Gland Express Neuromodulin (GAP-43)

Neuroendocrine chromaffin cells of the adrenal gland express certain molecular markers either transiently during development or permanently. In the present study, the expression of neuromodulin (GAP-43), a neuronal protein often associated with neurite outgrowth, was examined in adult adrenals. Neuromodulin was detected by Western blot analysis in extracts of both rat adrenals and cultured bovine chromaffin cells, and was localized in situ in a subpopulation of chromaffin cells, as well as in nerve fibres and Schwann cells. The use of anti-tyrosine hydroxylase or anti-phenylethanolamine N-methyltransferase antibodies in combination with anti-neuromodulin antibodies in double immunofluorescent labelling of cryostat sections of rat glands demonstrated that neuromodulin is expressed by noradrenergic, and not by adrenergic chromaffin cells. The results provide further evidence that neuromodulin is not limited to neurons; it is also expressed in a subpopulation of neuroendocrine chromaffin cells. Neuromodulin may play a role in the development of the adrenal medulla or in the specific regulation of noradrenalin secretion from chromaffin cells.

Isolation and characterization of detergent-resistant microdomains responsive to NCAM-mediated signaling from growth cones

It is still largely unclear how cell adhesion molecule (CAM)-mediated signaling evokes responses from the growth cone cytoskeleton. Here we used TX-114 extraction of growth cones followed by equilibrium gradient centrifugation to isolate subfractions of detergent-resistant microdomains (DRMs) that could be structurally and functionally distinguished on the basis of localization and activation of components of CAM-mediated signaling pathways. DRMs enriched in cholesterol, caveolin, NCAM140, GPI-linked NCAM120, fyn, and GAP-43, all conventional markers of microdomains or rafts, were located in areas 2 and 3 of the gradient. Coimmunoprecipitation of specific components of CAM signaling pathways by GAP-43 then identified distinct subpopulations of DRMs. GAP-43 from area 2 DRMs coprecipitated GPI-linked NCAM120 and was inactive, i.e., PKC phosphorylation had not been stimulated. In contrast the GAP-43 from area 3 DRMs coprecipitated both transmembrane NCAM140 and caveolin and was active, i.e., highly phosphorylated by PKC. A different subset of DRMs from both area 2 and area 3 contained fyn that could not be coprecipitated with GAP-43 antibodies. In this case area 2 DRMs contained activated fyn that was phosphorylated on Y415. In contrast area 3 DRMs contained inactive fyn. Hence fyn and GAP-43, both targets of NCAM signaling, are located in distinct populations of DRMs, and their activated forms are reciprocally distributed on the gradient. A detergent-resistant membrane fraction recovered from area 4 was enriched in NCAM140, phosphorylated GAP-43, and actin, but not cholesterol, caveolin, or fyn. Immunoelectron microscopy revealed that phosphorylated GAP-43 was localized where the membranes and F-actin interacted. Our results provide evidence for NCAM-mediated signaling in DRMs and suggest that the DRMs responsible for fyn and PKC/GAP-43-mediated NCAM signaling are structurally distinct and differentially distributed in growth cones.

bFGF stimulates GAP-43 phosphorylation at ser41 and modifies its intracellular localization in cultured hippocampal neurons

Cultured hippocampal neurons have been used to study GAP-43 phosphorylation and subcellular distribution. By immunofluorescence, GAP-43 was found associated with adherent membrane patches that remained attached to the substratum after in situ permeabilization with Nonidet-NP40. This association increases during neuronal development and is stabilized by the actin cytoskeleton. Basic fibroblast growth factor (bFGF) promotes GAP-43 translocation from the cytosol to adherent membrane patches and, at the same time, stimulates GAP-43 phosphorylation, mainly at the protein kinase C (PKC) site (Ser41). Inhibition of PKC prevented bFGF-stimulated GAP-43 phosphorylation and translocation, while activation by phorbol esters mimicked bFGF effects, suggesting that phosphorylation at Ser41 regulates GAP-43 subcellular localization. Using biochemical fractionation and phosphorylation analysis, it was found that Ser41 phosphorylation was highest in cytoskeleton-associated GAP-43 and lowest in membrane-associated GAP-43. It is proposed that GAP-43 is continuously cycling between intracellular compartments depending on its phosphorylation state and could be taking part in initial adhesive complexes assembled during growth cone advance.

Rhombomere origin plays a role in the specificity of cranial motor axon projections in the chick

Guidance of cranial motor axons to their targets conforms to a segmental plan in the chick embryo. Trigeminal motor neurons lie within rhombomeres 2 and 3 and project via an exit point in rhombomere 2 to innervate the first branchial arch. Facial motor neurons lie within rhombomeres 4 and 5 and grow out via an exit point in rhombomere 4 to innervate the second branchial arch. We have investigated the axial level-specific matching of motor neurons and branchial arches using donor to host transplantation in avian embryos. Previous work has shown that rostrocaudal reversal of a single hindbrain segment (rhombomere 3) leads to misprojection of a contingent of trigeminal axons via the facial nerve exit point. Using the same experimental manipulation in chick embryos and quail-chick chimaeras, we have analysed the pathways of these aberrant projections. We have found that in the majority of embryos analysed from stage 19 to 31, trigeminal axons from the transplanted rhombomere projected towards second branchial arch muscles, in addition to their normal first arch muscle targets. However, from stage 32 to 36, aberrant projections to second arch-derived muscles were detected only in a small minority of embryos. These experiments show that trigeminal motor neurons show a lack of specificity in their early projection into the periphery but that inappropriate projections may be later eliminated. This suggests that segmental mechanisms intrinsic to the hindbrain specify motor neurons with respect to their eventual innervation pattern.

Hypoxia/ischemia induces dephosphorylation of rat brain neuromodulin/GAP-43 in vivo

The in vivo state of phosphorylation and the modification of two Cys residues of neuromodulin/ GAP-43 (Nm) were analyzed by electrospray ionization-mass spectrometry (ES-MS). The protein was purified from rat brain with homogenization buffer containing 1% Nonidet P-40, protease inhibitors, protein phosphatase inhibitors, and sulfhydryl reagent, 4-vinylpyridine. Nm was purified by HPLC and ion-exchange chromatography, and the various fractions were identified by ES-MS as unphosphorylated and mono-, di-, tri-, and tetraphosphorylated species. All of these Nm species contained 2 mol of added 4-vinylpyridine per mol of Nm, suggesting that the two Cys residues are in the reduced form in the brain. In vivo, the majority of Nm is in the phosphorylated form (approximately 80%), of which the levels of the mono- and diphospho forms are higher than those of the tri- and tetraphospho species. Four in vivo phosphorylation sites, Ser41, Thr95, Ser142, and Thr172, were identified by amino acid sequencing and tandem ES-MS of the peptides derived from Lys-C endoproteinase digestion. Among these sites, only Ser41 is a known target of PKC, whereas the kinases responsible for the phosphorylation of the other three novel sites are unknown. Hypoxia/ischemia caused a preferential dephosphorylation of Ser41 and Thr172, whereas Thr95 is the least susceptible to dephosphorylation.

Neurite outgrowth stimulated by neural cell adhesion molecules requires growth-associated protein-43 (GAP-43) function and is associated with GAP-43 phosphorylation in growth cones

The mechanisms whereby cell adhesion molecules (CAMs) promote axonal growth and synaptic plasticity are poorly understood. Here we show that the neurite outgrowth stimulated by NCAM-mediated fibroblast growth factor (FGF) receptor activation in cerebellar granule cells is associated with increased GAP-43 phosphorylation on serine-41. In contrast, neither NCAM nor FGF was able to stimulate neurite outgrowth in similar neurons from mice in which the GAP-43 gene had been deleted by homologous recombination. Integrin-mediated neurite outgrowth was unaffected by GAP-43 deletion. Both neurite outgrowth and rapid phosphorylation of GAP-43 in isolated growth cones required the first three Ig domains of a NCAM-Fc chimera and were stimulated maximally at 5 micrograms/ml (approximately 50 nM). Likewise, GAP-43 phosphorylation in isolated growth cones also was stimulated by an L1-Fc chimera. Both neurite outgrowth and NCAM-stimulated GAP-43 phosphorylation were inhibited by antibodies to the FGF receptor and a diacylglycerol lipase inhibitor (RHC80267) that blocks the production of arachidonic acid in response to activation of the FGF receptor. Direct activation of the FGF receptor and the arachidonic acid cascade with either basic FGF or melittin also resulted in increased GAP-43 phosphorylation. These data suggest that the stimulation of neurite outgrowth by NCAM requires GAP-43 function and that GAP-43 phosphorylation in isolated growth cones occurs via an FGF receptor-dependent increase in arachidonic acid.

Distribution of phosphorylated GAP-43 (neuromodulin) in growth cones directly reflects growth cone behavior

Phosphorylation of GAP-43 (neuromodulin) by protein kinase C (PKC) occurs at a single site, serine41. In vivo, phosphorylation is induced after initiation of axonogenesis and is confined to distal axons and growth cones. Within individual growth cones, phosphorylation is nonuniformly distributed. Here, we have used high-resolution video-enhanced microscopy of cultured dorsal root ganglia neurons together with immunocytochemistry with a monoclonal antibody that recognizes PKC-phosphorylated GAP-43 to correlate the distribution of phosphorylated GAP-43 with growth cone behavior. In "quiescent," nontranslocating growth cones, phosphorylated GAP-43 was confined to the proximal neurite and the central organelle-rich region, and was low in organelle-poor lamellae. However, levels in lamellae were elevated when they became motile. Conversely, levels of phosphorylated GAP-43 were low in either lamellae that were actively retracting or in the central organelle-rich region and proximal neurite of growth cones that had totally collapsed. The results suggest a mechanism whereby phosphorylation of GAP-43 by PKC, potentially in response to extracellular signals, could direct the functional behavior of the growth cone.

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.

Intrinsic neuronal determinants locally regulate extrasynaptic and synaptic growth at the adult neuromuscular junction

Long-term functional plasticity in the nervous system can involve structural changes in terminal arborization and synaptic connections. To determine whether the differential expression of intrinsic neuronal determinants affects structural plasticity, we produced and analyzed transgenic mice overexpressing the cytosolic proteins cortical cytoskeleton-associated protein 23 (CAP-23) and growth-associated protein 43 (GAP-43) in adult neurons. Like GAP-43, CAP-23 was downregulated in mouse motor nerves and neuromuscular junctions during the second postnatal week and reexpressed during regeneration. In transgenic mice, the expression of either protein in adult motoneurons induced spontaneous and greatly potentiated stimulus-induced nerve sprouting at the neuromuscular junction. This sprouting had transgene-specific features, with CAP-23 inducing longer, but less numerous sprouts than GAP-43. Crossing of the transgenic mice led to dramatic potentiation of the sprout-inducing activities of GAP-43 and CAP-23, indicating that these related proteins have complementary and synergistic activities. In addition to ultraterminal sprouting, substantial growth of synaptic structures was induced. Experiments with pre- and postsynaptic toxins revealed that in the presence of GAP-43 or CAP-23, sprouting was stimulated by a mechanism that responds to reduced transmitter release and may be independent of postsynaptic activation. These results demonstrate the importance of intrinsic determinants in structural plasticity and provide an experimental approach to study its role in nervous system function.

GAP-43: an intrinsic determinant of neuronal development and plasticity

Several lines of investigation have helped clarify the role of GAP-43 (FI, B-50 or neuromodulin) in regulating the growth state of axon terminals. In transgenic mice, overexpression of GAP-43 leads to the spontaneous formation of new synapses and enhanced sprouting after injury. Null mutation of the GAP-43 gene disrupts axonal pathfinding and is generally lethal shortly after birth. Manipulations of GAP-43 expression likewise have profound effects on neurite outgrowth for cells in culture. GAP-43 appears to be involved in transducing intra- and extracellular signals to regulate cytoskeletal organization in the nerve ending. Phosphorylation by protein kinase C is particularly significant in this regard, and is linked with both nerve-terminal sprouting and long-term potentiation. In the brains of humans and other primates, high levels of GAP-43 persist in neocortical association areas and in the limbic system throughout life, where the protein might play an important role in mediating experience-dependent plasticity.

Identification of NAP-22 and GAP-43 (neuromodulin) as major protein components in a Triton insoluble low density fraction of rat brain

NAP-22 is a membrane-localized brain enriched acidic protein having a Ca(2+)-dependent calmodulin binding activity. Further fractionation of the NAP-22 containing membrane showed the localization of NAP-22 in a Triton insoluble fraction of low density. Besides NAP-22, this fraction was found to contain GAP-43 (neuromodulin), trimeric G proteins, and some GPI-anchored proteins such as Thy-1 and N-CAM-120. Presence of some protein tyrosine kinases, such as src and fyn, was also shown.

Analysis of the role of calmodulin binding and sequestration in neuromodulin (GAP-43) function

We demonstrated previously that forced expression of the neuronal phosphoprotein neuromodulin (also known as GAP-43, F1, B-50, and p57) in mouse anterior pituitary AtT-20 cells enhances depolarization-mediated secretion and alters cellular morphology. Here we analyze the role of calmodulin binding by neuromodulin in these responses. In cells expressing wild-type neuromodulin, a complex with calmodulin that is sensitive to intracellular calcium and phosphorylation is localized to the plasma membrane. Transfection of several mutant forms of neuromodulin shows that the effects of this protein on secretion are dependent on both calmodulin binding and association with the plasma membrane. In contrast, the morphological changes depend only on membrane association. Thus, the multitude of effects of neuromodulin noted in previous studies may result from divergent properties of this protein.

Growth-associated protein-43 (GAP-43) facilitates peptide hormone secretion in mouse anterior pituitary AtT-20 cells

The neuronal growth-associated protein (GAP)-43 (neuromodulin, B-50, F1), which is concentrated in the growth cones of elongating axons during neuronal development and in nerve terminals in restricted regions of the adult nervous system, has been implicated in the release of neurotransmitter. To study the role of GAP-43 in evoked secretion, we transfected mouse anterior pituitary AtT-20 cells with the rat GAP-43 cDNA and derived stably transfected cell lines. Depolarization-mediated beta-endorphin secretion was greatly enhanced in the GAP-43-expressing AtT-20 cells without a significant change in Ca2+ influx; in contrast, expression of GAP-43 did not alter corticotropin-releasing factor-evoked hormone secretion. The transfected cells also displayed a flattened morphology and extended processes when plated on laminin-coated substrates. These results suggest that AtT-20 cells are a useful model system for further investigations on the precise biological function(s) of GAP-43.

Mutagenesis of ser41 to ala inhibits the association of GAP-43 with the membrane skeleton of GAP-43-deficient PC12B cells: effects on cell adhesion and the composition of neurite cytoskeleton and membrane

To investigate the molecular basis for GAP-43 function in axon outgrowth, we produced a mutant, GAP-43 (Ala41), whose interaction with calmodulin in vitro was unaffected by increasing Ca2+ concentrations, and stably transfected it into GAP-43-deficient PC12B cells. Several lines that expressed wild-type or mutant protein at levels that resembled endogenous GAP-43 expression in PC12 controls were subcloned and characterized. GAP-43 (Ala41) was significantly more extractable with Nonidet P-40 and less tightly associated with the membrane skeleton than the wild-type protein. Furthermore, GAP-43 (Ala41) expression by PC12B cells profoundly affected their phenotype: First, observation of living cells using video-enhanced microscopy revealed irregular plasma membranes with numerous blebs and protrusions and neurites that appeared thin and varicose. Second, both the cells' ability to remain attached to laminin substrates and the amount of alpha 1 beta 1 integrin expressed on the cell surface was significantly decreased. Finally, peripherin transport, which is abnormal in PC12B cells, could be rescued by transfection of wild-type GAP-43 but not the GAP-43 (Ala41) mutant. The phenotypic abnormalities resemble other cell types in which membrane skeleton/plasma membrane interactions have been functionally decoupled, and our results are consistent with the notion that these interactions may be abnormal in GAP-43 (Ala41)-expressing PC12B cells, either as a direct consequence of the mutation or arising secondarily to the altered availability of calmodulin in the growing neurite.

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

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

Absence of persistent spreading, branching, and adhesion in GAP-43-depleted growth cones

The growth-associated protein GAP-43 is a major protein kinase C substrate of growth cones and developing nerve terminals. In the growth cone, it accumulates near the plasma membrane, where it associates with the cortical cytoskeleton and membranes. The role of GAP-43 in neurite outgrowth is not yet clear, but recent findings suggest that it may be a crucial competence factor in this process. To define the role of GAP-43 in growth cone activity, we have analyzed neurite outgrowth and growth cone activity in primary sensory neurons depleted of GAP-43 by a specific antisense oligonucleotide procedure. Under optimal culture conditions, but in the absence of GAP-43, growth cones adhered poorly, displayed highly dynamic but unstable lamellar extensions, and were strikingly devoid of local f-actin concentrations. Upon stimulation, they failed to produce NGF-induced spreading or insulin-like growth factor-1-induced branching, whereas growth factor-induced phosphotyrosine immunoreactivity and acceleration of neurite elongation were not impaired. Unlike their GAP-43-expressing counterparts, they readily retracted when exposed to inhibitory central nervous system myelin-derived liposomes. Frequency and extent of induced retraction were attenuated by NGF. Our results indicate that GAP-43 can promote f-actin accumulation, evoked morphogenic activity, and resistance to retraction of the growth cone, suggesting that it may promote regulated neurite outgrowth during development and regeneration.

Light-microscopic study of phosphoprotein B-50 in myopathies

The growth-associated protein B-50, also termed GAP-43, is a membrane-bound phosphoprotein that is expressed in neurons. It is particularly abundant during periods of axonal outgrowth in development and regeneration of the central and peripheral nervous system. In this paper we study the expression of B-50 in inflammatory and dystrophic myopathies. To investigate the state of regeneration, N-CAM and vimentin serial sections were performed, because N-CAM and cytoskeletal protein vimentin are excellent markers for regenerating muscle. Light-microscopic evaluation showed that muscle fiber regeneration in myopathies corresponds closely to B-50 immunoreactivity in satellite cells, myoblasts, myotubes and small regenerating myocytes in cytoplasmatic distribution. In normal muscle and in biopsies of neurogenic muscular atrophy, however, no light-microscopically demonstrable B-50 staining was found. B-50 in muscles apparently plays a role in the growth morphology of regenerating myocytes, and the phosphoprotein B-50 can no longer be regarded as a neuron-specific molecule.

In vivo induction of the growth associated protein GAP43/B-50 in rat astrocytes following transient middle cerebral artery occlusion

Immunohistochemistry was used to investigate the induction of growth-associated protein GAP43/B-50 in the astrocytes of rat cerebrum in vivo following ischemic injury produced by 30 min of transient middle cerebral artery occlusion. Three days after operation, GAP43 immunoreactivity first appeared in some astrocytic populations surrounding the infarcted lesion. Induction of GAP43 in those astrocytes persisted for up to 14 days and disappeared at 30 days postoperation. Double-immunofluorescence staining confirmed that the GAP43-immunoreactive astrocytes examined were all positive for glial fibrillary acidic protein. Our present data suggest that certain astrocytes could be induced to synthesize GAP43 in vivo in response to an ischemic insult in adult rats.

Injury induced expression of growth-associated protein-43 in adult mouse retinal ganglion cells in vitro

In optic fibers, as in most axons of the central nervous system, the axonal growth-associated protein, GAP-43, is abundant during development but absent in adults. Since optic fibers can be induced to regenerate in culture, we examined whether this was associated with an increased expression of GAP-43 in adult mouse optic fibers that were regenerating from organotypic retinal explants on to laminin substrates. We found that simply placing adult mouse retina in culture under serum-free conditions was sufficient to induce GAP-43, which was detectable after about four to five days in vitro, coincident with the initiation of neurite outgrowth. In explants taken from animals in which the optic nerve was crushed in the orbit eight days prior to culturing, GAP-43 was observed within one day, as was neurite outgrowth. This priming effect was also seen in vivo as an increased level of GAP-43 reactivity in retinal ganglion cells and optic fibers in histological sections taken eight days after nerve crush. Reactivity in the adult fibers in culture was comparable to that observed in optic neurites growing from embryonic retinal explants and could be maintained for at least four weeks in culture. In the adult neurites, especially with longer times in culture, GAP-43 tended to be concentrated into varicosities that were often found in terminal-like arbors that formed in culture. Placing adult retina in culture under serum-free conditions in sufficient to induce re-expression of GAP-43 for an indefinite period of time. This suggests that GAP-43 expression and the propensity for growth in vivo may be repressed by a factor that is absent in vitro.

Developmental changes in GAP-43 expression in primary cultures of rat cerebellar granule cells

GAP-43 is a growth-associated protein that has been implicated in the developmental outgrowth of axons. We have examined the profile of GAP-43 levels in rat cerebellar granule cells during their development in vitro. During the first 1-2 days after plating, the majority of cells expressed neurites and after 8 days a complex neuronal network had developed. In situ hybridization studies showed that GAP-43 mRNA levels rapidly increased to peak at 1-2 days and gradually returned to initial values after 7-8 days. Analysis of GAP-43 protein levels followed a similar transient profile. Initially, granule cell perikarya and structures associated with neuritogenesis all displayed GAP-43 immunoreactivity. In older cultures, perikaryal labelling was lost after 10 days whilst process staining decreased more gradually. During the first 48 hours detailed analysis of GAP-43 mRNA revealed two populations of granule cells. It was suggested that cells with significant label originated from the external germinal layer which displays much GAP-43 mRNA in cerebellar sections. Cells with little or no GAP-43, however, probably originated from the internal granular layer since this region displayed no specific labelling. Granule cells within clumps expressed more GAP-43 mRNA compared to isolated cells perhaps indicating cell-cell regulation of expression. These results describe the transient rise in GAP-43 protein and mRNA levels expressed by developing cerebellar granule cell neurons in vitro and provide further evidence for the role GAP-43 plays during neuritogenesis.

ADP-ribosylation of brain neuronal proteins is altered by in vitro and in vivo exposure to inorganic mercury

ADP-ribosylation is an essential process in the metabolism of brain neuronal proteins, including the regulation of assembly and disassembly of biological polymers. Here, we examine the effect of HgCl2 exposure on the ADP-ribosylation of tubulin and actin, both cytoskeletal proteins also found in neurons, and B-50/43-kDa growth-associated protein (B-50/GAP-43), a neuronal tissue-specific phosphoprotein. In rats we demonstrate, with both in vitro and in vivo experiments, that HgCl2 markedly inhibits the ADP-ribosylation of tubulin and actin. This is direct quantitative evidence that HgCl2, a toxic xenobiotic, alters specific neurochemical reactions involved in maintaining brain neuron structure.

Inverse patterns of myelination and GAP-43 expression in the adult CNS: neurite growth inhibitors as regulators of neuronal plasticity?

In the central nervous system (CNS) myelin is present not only in white matter, but also in varying amounts in many gray matter areas. In addition to the function of electrical insulation of axons, myelin and oligodendrocytes contain molecules that are powerful inhibitors of neurite growth. Nevertheless plastic changes involving sprouting of nerve terminals occur in several brain regions of adult animals after partial lesions. In this study we have tried to correlate the plastic potential of CNS regions with the degree of their myelination. The expression of the growth-associated protein GAP-43 was used as an indicator of the potential for plastic changes, and a histological myelin stain was used to assess myelination. We have found that myelination and GAP-43 expression have strikingly inverse expression patterns in the majority of CNS gray matter areas. Densely myelinated regions, that is, most brainstem nuclei, the tegmentum, and the inferior colliculus, are low in GAP-43. In contrast, unmyelinated or lightly myelinated areas, such as the substantia gelatinosa of the spinal cord, the nucleus of the solitary tract, or the septum, express high levels of GAP-43. Areas known to show lesion-induced sprouting are typically high in GAP-43 and only lightly myelinated. During postnatal development the myelination pattern precedes the GAP-43 pattern, a sequence that is consistent with a role of myelin and the associated neurite growth inhibitors in modifying GAP-43 expression. Our results support the hypothesis that myelin-associated neurite growth inhibitors are involved in regulating the stability of neural connections.

The ontogeny of GAP-43 (neuromodulin) mRNA in postnatal rat brain: evidence for a sex dimorphism

GAP-43 is a membrane-bound protein selectively concentrated in axonal growth cones during brain development and implicated in axonal outgrowth and elongation. A sex dimorphism in the number of synapses in certain regions of the adult rat brain has been attributed to differences in gonadal steroid hormone action during early postnatal life. The results of recent studies have demonstrated that gonadal steroids modulate GAP-43 mRNA in regions of the postnatal and adult brain where steroid hormone receptors are concentrated. Since gonadal steroids influence the development of the sexually undifferentiated brain during the first few weeks of postnatal life, the present study investigated the ontogeny of GAP-43 mRNA in the male and female rat brain between postnatal days 1 and 25. On postnatal days 1, 3, 6, 12, 18, and 25, brains were collected from male and female postnates and frozen, and 16 microns cryostat sections were processed and hybridized with a 35S-labeled antisense riboprobe complementary to GAP-43 mRNA. Evaluation of film autoradiograms demonstrated a widespread distribution of GAP-43 mRNA in postnatal brain regions, including the cerebral cortex; bed nucleus of the stria terminalis; and medial preoptic area, ventromedial nucleus, and arcuate nucleus of the hypothalamus. Densitometric measurements revealed that GAP-43 mRNA was transiently elevated during early postnatal life, with a subsequent decrease during brain maturation, although the pattern of change varied among the brain regions investigated. In addition, the level of GAP-43 hybridization signal was significantly higher in the male cortex, bed nucleus, and medial preoptic nucleus, but not the ventromedial and arcuate nuclei, than in postnatal females. Analysis of slide autoradiograms demonstrated that the change in GAP-43 mRNA during postnatal development was due to changes at the cellular level. The present results indicate that expression of GAP-43 mRNA is transiently elevated and sexually dimorphic in certain regions of the early postnatal rat brain. The results further suggest that the differential expression of GAP-43 in the male and female postnatal brain may be related to sex differences in neuronal outgrowth and connectivity resulting in a dimorphism in the pattern of adult neuronal circuitry.

Novel inhibitory action of tunicamycin homologues suggests a role for dynamic protein fatty acylation in growth cone-mediated neurite extension

In neuronal growth cones, the advancing tips of elongating axons and dendrites, specific protein substrates appear to undergo cycles of posttranslational modification by covalent attachment and removal of long-chain fatty acids. We show here that ongoing fatty acylation can be inhibited selectively by long-chain homologues of the antibiotic tunicamycin, a known inhibitor of N-linked glycosylation. Tunicamycin directly inhibits transfer of palmitate to protein in a cell-free system, indicating that tunicamycin inhibition of protein palmitoylation reflects an action of the drug separate from its previously established effects on glycosylation. Tunicamycin treatment of differentiated PC12 cells or dissociated rat sensory neurons, under conditions in which protein palmitoylation is inhibited, produces a prompt cessation of neurite elongation and induces a collapse of neuronal growth cones. These growth cone responses are rapidly reversed by washout of the antibiotic, even in the absence of protein synthesis, or by addition of serum. Two additional lines of evidence suggest that the effects of tunicamycin on growth cones arise from its ability to inhibit protein long-chain acylation, rather than its previously established effects on protein glycosylation and synthesis. (a) The abilities of different tunicamycin homologues to induce growth cone collapse very systematically with the length of the fatty acyl side-chain of tunicamycin, in a manner predicted and observed for the inhibition of protein palmitoylation. Homologues with fatty acyl moieties shorter than palmitic acid (16 hydrocarbons), including potent inhibitors of glycosylation, are poor inhibitors of growth cone function. (b) The tunicamycin-induced impairment of growth cone function can be reversed by the addition of excess exogenous fatty acid, which reverses the inhibition of protein palmitoylation but has no effect on the inhibition of protein glycosylation. These results suggest an important role for dynamic protein acylation in growth cone-mediated extension of neuronal processes.

Spontaneous morphological changes by overexpression of the growth-associated protein B-50/GAP-43 in a PC12 cell line

In order to study the direct effects of B-50 on neural cell morphology, rat B-50 cDNA was transfected into a PC12 cell line (PC-B2) exhibiting neurite outgrowth independent of the expression of endogenous B-50. The morphological changes were visualized by confocal scanning laser microscopy using fluorescence labelling for B-50 and for F-actin. The transfected cells exhibited filopodia and/or blebs on the plasma membrane, containing most of the B-50 immunoreactivity. No spontaneous neurite outgrowth was observed. Following NGF treatment transfected and nontransfected PC-B2 cells extended F-actin positive filopodia and neurites with a striking colocalisation of B-50 and F-actin. Our data show that the presence of B-50 can influence cell surface morphology independent of the presence of NGF. The colocalisation of B-50 and F-actin in the filopodial protrusions but not in the blebs might be indicative for a role of B-50 in actin polymerization and depolymerization.

Partial denervation of the medial gastrocnemius muscle results in growth-associated protein-43 immunoreactivity in sprouting axons and Schwann cells

Regeneration in the mammalian peripheral nervous system following nerve injury is associated with the upregulation of a developmentally regulated phosphoprotein, growth-associated protein-43 (GAP-43), in the injured neurons. We have examined whether uninjured adult neurons also express GAP-43 when they sprout. The model system investigated has been the sprouting induced in the terminal axons of intact motor neurons by a partial muscle denervation. Partial denervation of the medial gastrocnemius muscle in adult rats was produced by resecting the terminal nerve supply to the anterolateral quadrant of the muscle. Three zones could be identified in the motor endplate region of the muscle after such a denervation using protein gene product (PGP) 9.5, calcitonin gene-related peptide and silver staining as axonal markers and S-100 to identify Schwann cells: a normally innervated zone, a totally denervated zone and a border or intermediate zone between the two which contained axons at the endplates with nodal and terminal sprouts. The endplates in the normally innervated zone were GAP-43 negative. In the denervated zone, Schwann cells were GAP-43 positive and had a distinctive appearance with a lack of any normal endplate organization. Endplates in the intermediate zone were GAP-43 immunoreactive. In approximately half, the GAP-43 immunoreactivity was axonal-like, identical to PGP 9.5 in an adjacent section; in the remainder it was Schwann cell-like, identical to S-100 staining. Partial muscle denervation results, therefore, in the appearance of GAP-43 both in axons and Schwann cells in the endplates bordering the denervated zone. The presence of GAP-43 in these cells may contribute to their capacity to sprout.

Actin-based cytoskeleton in growth cone activity

A growing tip of neurite, the neuronal growth cone, is a highly motile and adhesive form of cytoarchitecture. The growth cone plays vital roles for navigation, elongation and maintenance of neurites. One major constituent of growth cones, regulated by the intracellular Ca2+ signal, is the actin-based cytoskeleton. In this article, I have summarized four types of Ca(2+)-dependent regulation of the actin-based cytoskeleton in growth cones: gelsolin-actin, myosin II-actin microfilament, myosin II-actin, and Ca(2+)-sensitive alpha-actinin-actin systems. The four examples of Ca(2+)-dependent regulation described here may be involved in growth cone motility. The actin-based membrane skeleton forming a meshwork beneath the cytoplasmic surface of the growth cone membrane is also important for adhesion of growth cones to recognize cue molecules. The actin-based membrane skeleton participates in this recognition process and the adhesion-dependent signal transduction in association with receptors for cell adhesion molecules.

Depletion of 43-kD growth-associated protein in primary sensory neurons leads to diminished formation and spreading of growth cones

The 43-kD growth-associated protein (GAP-43) is a major protein kinase C (PKC) substrate of growing axons, and of developing nerve terminals and glial cells. It is a highly hydrophilic protein associated with the cortical cytoskeleton and membranes. In neurons it is rapidly transported from the cell body to growth cones and nerve terminals, where it accumulates. To define the role of GAP-43 in neurite outgrowth, we analyzed neurite regeneration in cultured dorsal root ganglia (DRG) neurons that had been depleted of GAP-43 with any of three nonoverlapping antisense oligonucleotides. The GAP-43 depletion procedure was specific for this protein and an antisense oligonucleotide to the related PKC substrate MARCKS did not detectably affect GAP-43 immunoreactivity. We report that neurite outgrowth and morphology depended on the levels of GAP-43 in the neurons in a substrate-specific manner. When grown on a laminin substratum, GAP-43-depleted neurons extended longer, thinner and less branched neurites with strikingly smaller growth cones than their GAP-43-expressing counterparts. In contrast, suppression of GAP-43 expression prevented growth cone and neurite formation when DRG neurons were plated on poly-L-ornithine. These findings indicate that GAP-43 plays an important role in growth cone formation and neurite outgrowth. It may be involved in the potentiation of growth cone responses to external signals affecting process formation and guidance.

Increase in neurite formation and acetylcholine release by transfection of growth-associated protein-43 cDNA into NG108-15 cells

We previously reported that growth-associated protein-43 (GAP-43) could be involved in the maintenance of elongated neurites and that a decline in protein kinase C activity may be involved in accumulation of GAP-43. In the present study, to clarify the functional significance of GAP-43 for neurite maintenance and acetylcholine (ACh) release, we prepared NG-G11 cells by transfection of GAP-43 cDNA into NG108-15 cells. NG-G11 cells expressed GAP-43 mRNA at levels approximately twice that in nontransfected or vector-transfected cells under control conditions and after treatment with dibutyryl cyclic AMP (diBu-cAMP) or 12-O-tetradecanoylphorbol 13-acetate (TPA) plus diBu-cAMP. Neurite outgrowth after addition of diBu-cAMP was greater in NG-G11 than in control cells. In NG-G11 cells, neurites elongated by treatment with diBu-cAMP for 72 h were maintained after removal of the drug. Treatment with TPA plus diBu-cAMP for 24 h induced neurite outgrowth in NG-G11 cells, although control cells required 72 h. Depolarization by 50 mM KCl induced ACh release in both NG-G11 and control cells treated with diBu-cAMP or TPA/diBu-cAMP. Although removal of the drugs following diBu-cAMP treatment reversed ACh release to nontreated levels in control cells, a high-K(+)-induced level of ACh release remained in NG-G11 cells after removal of diBu-cAMP. ACh release induced by TPA plus diBu-cAMP for 24 h was further enhanced after removal of the drugs in NG-G11 cells, but it was not seen in control cells. These results suggest that levels of GAP-43 mRNA are correlated with neurite maintenance and the level of ACh release. Thus, GAP-43 may be involved in neuronal differentiation in NG108-15 cells.

Glycation of brain actin in experimental diabetes

Actin is a neuronal protein involved in axonal transport and nerve regeneration, both of which are known to be impaired in experimental diabetes. To determine if actin is subject to glycation, we rendered rats diabetic by injection of streptozotocin. Two or 6 weeks later brains were removed and a preparation of cytoskeletal proteins was analyzed by two-dimensional polyacrylamide gel electrophoresis. Brains from diabetic animals contained an extra polypeptide that migrated close to actin and reacted with monoclonal antibody C4 against actin. It was also found in a preparation of soluble synaptic proteins from diabetic rat brain, indicating that it was at least partly neuronal in origin. This polypeptide could be produced by incubation of cytoskeletal proteins from brains of nondiabetic rats with glucose-6-phosphate in vitro. The appearance of this glycated actin in diabetic animals was prevented by administration of insulin for a period of 6 weeks. We could not detect any effect of glycation in vitro on the ability of muscle G-actin to form F-actin filaments and its significance for the function of actin remains to be determined. The finding that glycation of platelet-derived actin from diabetic patients was significantly increased implies that the abnormality may also occur in clinical diabetes.

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.

How is protein kinase C activated in CNS

Protein kinase C (PKC) enzyme family consists of the Ca(2+)-dependent and -independent subgroups of phospholipid/diacylglycerol (DAG)-stimulated serine/threonine protein kinases. These enzymes exhibit distinct cellular and subcellular localizations in CNS and subtle differences in their biochemical characteristics and substrate specificities. It is believed that each of these isoenzymes respond differently to different input signals. However, detailed mechanism for the functioning of these enzymes in vivo is largely unknown; this is in part due to the absence of specific activator, inhibitor, or substrate for each of these enzymes. Recent advances in biochemical, biophysical, and molecular characterizations have defined certain structural features important to confer the stimulatory responses of these enzymes to Ca2+, DAG or phorbol ester, and Zn2+; other features important for the binding of anionic phospholipids, Ca2+/phospholipid complexes, and cis-unsaturated fatty acids have not yet been characterized. Activation of PKC requires the increase in [Ca2+]i and DAG and/or cis-unsaturated fatty acids. Ca2+ promotes the interactions of the Ca(2+)-dependent subgroup of PKCs with membrane phosphatidylserine (PS) and the enzymes become partially active when simultaneously associated with phosphatidylinositol 4,5-bisphosphate or fully active when DAG is available. Free fatty acids such as arachidonic acid, generated by the activation of phospholipase A2, could synergize with DAG to activate the enzyme maximally. The Ca(2+)-independent subgroup of PKCs also become active when associated with PS at elevated level of DAG. Sustained activation of PKCs leads to the conversion of these enzymes into membrane-inserted and membrane protein-associated forms, which may be responsible for certain long-term neural responses. Activation of PKC results in the phosphorylation of cellular proteins; among them, several calmodulin (CaM)-binding proteins are the prominent substrates of these kinases. Phosphorylation of these proteins by PKC favors the release of CaM, which is required for the Ca2+/CaM-dependent enzymes. Thus, activation of PKCs can lead to diverse cellular responses through such amplification steps. Future studies should be directed at the elucidation of the activation of each PKC isoform in vivo to correlate with the physiological responses.

Distribution of GAP-43 (B50/F1) mRNA in the adult rat brain by in situ hybridization using an alkaline phosphatase labeled probe

GAP-43 (B-50,F1, pp46) is a calmodulin binding protein which is specific to the nervous system and also a substrate for the protein kinase C. Furthermore an enrichment of this protein in the growth cone and developmental brain indicate that this protein is related to nerve development, regeneration, and outgrowth. While its level dramatically decreases after the completion of synaptogenesis, the protein is still to some extent continuously expressed in certain regions of the mature brain. In order to clarify GAP-43 localization in mature normal rats, we investigated the distribution of GAP-43 mRNA in the rat central nervous system by using a non-radioisotopic in situ hybridization histochemistry. This method demonstrated GAP-43 mRNA expressing cells with high resolution. GAP-43 mRNA was more abundant in the forebrain than in the lower brainstem. Intense hybridization signal was observed in the mitral cells of olfactory bulb, cerebral cortex, CA3 region of hippocampus, diagonal band, substantia nigra, raphe nuclei, locus coeruleus, and dorsal motor nucleus of vagus. Weak to moderate hybridization signals were also widely expressed in thalamus, hypothalamus, and midbrain. Moreover, most noradrenergic, adrenergic, serotonergic, histaminergic, and caudal part of dopaminergic cells exhibited an intense GAP-43 mRNA signal. Thus, GAP-43 mRNA is abundantly expressed under normal conditions in the brain and may play an important physiological role particularly in the forebrain and in monoaminergic neurons supporting the findings that GAP-43 could be implicated in plasticity and monoamine release.

Reactive astrocytes in the kainic acid-damage hippocampus have the phenotypic features of type-2 astrocytes

Kainic acid treatment, a model of temporal lobe epilepsy, induces Ammon's horn sclerosis characterized by degeneration of CA3 pyramidal neurons and reactive gliosis. We now report that in kainic acid treated rats, reactive astrocytes in the hippocampus are A2B5 immunopositive and express GAP-43 immunoreactivity. A2B5 is a cell surface ganglioside selectively expressed in the glial O-2A lineage (oligodendrocytes and type-2 astrocytes in vitro). Since A2B5-positive cells were also GFAP immunoreactive, our observation suggest that hippocampal-reactive astrocytes in the epileptic process are type-2 astrocytes. GAP-43 is a membrane-associated phosphoprotein involved in neurite outgrowth. In vitro analysis showed that the glial O-2A lineage may express this phosphoprotein. In this study, we found that GAP-43 was coexpressed in astrocytes with A2B5 suggesting that in vivo as in vitro type-2 astrocytes express GAP-43.

Opioid peptides activate phospholipase D and protein kinase C-epsilon in chicken embryo neuron cultures

The mu-opioid peptide morphiceptin stimulated a Ca(2+)-independent protein kinase C (PKC-epsilon) that is expressed both in embryonic day 6 chicken telencephalon and in derived neuronal cultures. This activation was seen as a 2-fold increase in the activity and level of cytosolic PKC-epsilon and as a transient increase in membrane-associated PKC-epsilon following morphiceptin treatment. Morphiceptin did not activate phospholipase C-mediated phosphatidylinositol hydrolysis but did transiently activate (2- to 3-fold) phospholipase D (PLD), as measured by phosphatidylethanol formation in neuron cultures derived from embryonic day 6 or day 7 cerebral hemispheres. This PLD activation could provide an alternative source of diacylglycerol for the activation of PKC-epsilon and was naloxone-reversible and at least partially blocked by the tyrosine kinase inhibitor herbimycin A. Addition of phorbol 12-myristate 13-acetate stimulated both PLD and PKC-epsilon activities to a greater extent than opioids. The phorbol ester and insulin stimulation of PLD was also blocked by herbimycin. Both morphiceptin (in a naloxone-reversible manner) and phorbol ester increased phosphorylation of similar cytosolic proteins in intact cells, demonstrating a functional role for the PKC-epsilon activation by opioids. This is evidence that opioid receptors are transiently coupled to tyrosine kinase, PLD and PKC-epsilon activation and, by implication, to neuronal cell growth during brain morphogenesis.

Antisense GAP-43 inhibits the evoked release of dopamine from PC12 cells

To investigate the role of the neuronal growth-associated protein GAP-43 (neuromodulin, B-50, F1, P-57) in neurotransmitter release, we transfected PC12 cells with a recombinant expression vector coding for antisense human GAP-43 cRNA. Two stable transfectants, designated AS1 and AS2, were selected that had integrated the recombinant sequence and expressed antisense GAP-43 RNA. Immunoblot analysis of proteins from AS1 and AS2 cells indicated that the level of GAP-43 in these cell lines was reduced. In the presence of extracellular calcium, a depolarizing concentration of K+ (56 mM) evoked dopamine release from control cells, but not from AS1 and AS2 cells. Similarly, the calcium ionophore A23187 evoked dopamine release from control cells, but was ineffective in stimulating dopamine release from AS1 and AS2 cells. The antisense transfectants, as well as the control cells, contained appreciable quantities of dopamine and secretory granules with a normal appearance. Because the expression of antisense GAP-43 RNA in PC12 cells leads to a decrease in GAP-43 expression and to the loss of evoked dopamine release, these results provide evidence of a role for GAP-43 in calcium-dependent neurotransmitter release.

Phosphorylation-site mutagenesis of the growth-associated protein GAP-43 modulates its effects on cell spreading and morphology

The 43-kD growth-associated protein (GAP-43) is a major protein kinase C (PKC) substrate of axonal growth cones, developing nerve terminals, regenerating axons, and adult central nervous system areas associated with plasticity. It is a cytosolic protein associated with the cortical cytoskeleton and the plasmalemma. Membrane association of GAP-43 is mediated by palmitoylation at Cys3Cys4. In vitro and in vivo, phosphorylation by PKC exclusively involves Ser41 of mammalian GAP-43 (corresponding to Ser42 in the chick protein). To identify aspects of GAP-43 function, we analyzed the actions of wild-type, membrane-association, and phosphorylation-site mutants of GAP-43 in nonneuronal cell lines. The GAP-43 constructs were introduced in L6 and COS-7 cells by transient transfection. Like the endogenous protein in neurons and their growth cones, GAP-43 in nonneuronal cells associated with the cell periphery. GAP-43 accumulated in the pseudopods of spreading cells and appeared to interact with cortical actin-containing filaments. Spreading L6 cells expressing high levels of recombinant protein displayed a characteristic F-actin labeling pattern consisting of prominent radial arrays of peripheral actin filaments. GAP-43 had dramatic effects on local surface morphology. Characteristic features of GAP-43-expressing cells were irregular cell outlines with prominent and numerous filopodia. The effects of GAP-43 on cell morphology required association with the cell membrane, since GAP-43(Ala3Ala4), a mutant that failed to associate with the cell cortex, had no morphogenetic activity. Two GAP-43 phosphorylation mutants (Ser42 to Ala42 preventing and Ser42 to Asp42 mimicking phosphorylation by PKC) modulated the effects of GAP-43 in opposite ways. Cells expressing GAP-43(Asp42) spread extensively and displayed large and irregular membranous extensions with little filopodia, whereas GAP-43(Ala42) produced small, poorly spreading cells with numerous short filopodia. Therefore, GAP-43 influences cell surface behavior and phosphorylation modulates its activity. The presence of GAP-43 in growing axons and developing nerve termini may affect the behavior of their actin-containing cortical cytoskeleton in a regulatable manner.

Expression of GAP-43 in normal and regenerating nerves in the frog

A polyclonal antiserum to chicken, growth-associated protein-43 (GAP-43), raised in rabbit, was shown to recognize a molecule with similar properties to GAP-43 in frogs. Using this antiserum, GAP-43 immunoreactivity was shown to be present throughout the brain and white matter of the spinal cord of larval frogs, but became restricted to specific regions in the adult frog central nervous system. In the peripheral nervous system, GAP-43 was present in normal tadpole and adult axons. After cutting the adult sciatic nerve, GAP-43 slowly disappeared from axons in the distal stump, but appeared in Schwann cells and other (uncharacterized) cells. The constitutive expression of GAP-43 in the adult frog sciatic nerve may be related to the phenomenon of remodelling of motor end-plates, which is known to occur throughout life in frogs.