Binding of the neuronal protein B-50, but not the metabolite B-60, to calmodulin

Specific Proteolysis of Neuronal Protein GAP-43 by Calpain: Characterization, Regulation, and Physiological Role

The mechanism of specific proteolysis of the neuronal protein GAP-43 in axonal terminals has been investigated. In synaptic terminals in vivo and in synaptosomes in vitro GAP-43 is cleaved only at the single peptide bond formed by Ser41; this is within the main effector domain of GAP-43. Proteolysis at this site involves the cysteine calcium-dependent neutral protease calpain. The following experimental evidences support this conclusion: 1) calcium-dependent proteolysis of GAP-43 in synaptosomes is insensitive to selective inhibitor of micro-calpain (PD151746), but it is completely blocked by micro- and m-calpain inhibitor PD150606; 2) GAP-43 proteolysis in the calcium ionophore A23187-treated synaptosomes is activated by millimolar concentration of calcium ions; 3) the pattern of fragmentation of purified GAP-43 by m-calpain (but not by micro-calpain) is identical to that observed in synaptic terminals in vivo. GAP-43 phosphorylated at Ser41 by protein kinase C (PKC) is resistant to the cleavage by calpain. In addition, calmodulin binding to GAP-43 decreases the rate of calpain-mediated GAP-43 proteolysis. Our results indicate that m-calpain-mediated GAP-43 proteolysis regulated by PKC and calmodulin is of physiological relevance, particularly in axonal growth cone guidance. We suggest that the function of the N-terminal fragment of GAP-43 (residues 1-40) formed during cleavage by m-calpain consists in activation of neuronal heterotrimeric GTP-binding protein G(o); this results in growth cone turning in response to repulsive signals.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Site-specific calcium-dependent proteolysis of neuronal protein GAP-43

GAP-43 is a presynaptic protein participating in signal transduction processes in nerve terminals. GAP-43 exists in neurons along with two truncated forms devoid of 4 and 40 N-terminal residues. In this report, we show that these forms of GAP-43 are proteolytic fragments derived from calcium-dependent cleavage of GAP-43 molecule at 5th and 41st residues. GAP-43 site-specific proteolysis in synaptosome and cytosol fractions proved to be dependent on the addition of millimolar amounts of calcium. This fact together with inhibition of GAP-43 proteolysis by calpain inhibitors as well as local composition of the cleavage sites indicates to the participation of calpain in this process. The proteolysis disturbs some properties characteristic for whole GAP-43 molecules, in particular, calmodulin binding and Ser-41 phosphorylation, when the cleavage occurs at 41st residue. Some other GAP-43 properties (G(o) protein activation and membrane attachment) are retained by separate fragments. Therefore, calcium controlled site-specific proteolysis of GAP-43 can be of great physiological significance.

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.

Substance P-, calcitonin gene-related peptide, growth-associated protein-43, and neurotrophin receptor-like immunoreactivity associated with unmyelinated axons in feline ventral roots and pia mater

The spinal pia mater receives a rich innervation of small sensory axons via the ventral roots. Other sensory axons enter the ventral roots but end blindly or turn abruptly in hairpin loop-like formations and continue in a distal direction. In the present study, the content of substance P (SP)-, calcitonin gene-related peptide (CGRP)-, growth-associated protein (GAP-43)-, and low-affinity neurotrophin receptor protein (p75NGFr)-like immunoreactivity (-LI) associated with these different types of sensory axons was assessed with light and electron microscopic immunohistochemical techniques. In addition, the binding of antibodies against synthetic peptides representing unique sequences of residues in the products of the trk and trkB protooncogenes was analyzed. These genes encode membrane spanning proteins, which have been shown to constitute specific high affinity binding sites for several members of the nerve growth factor family of neurotrophic factors. The results of the present study imply that the ventral root afferents comprise several different types of sensory axons, which all contain SP-, CGRP-, GAP-43-, and p75NGFr-like immunoreactivities. In addition, at least some of the presumed sensory fiber bundles in ventral roots and the pia mater were immunoreactive for the trkB gene product. Moreover, leptomeningeal cells and nonneuronal cells of the ventral roots were shown to bind antibodies to both the trk and trkB gene products. The ventral root afferents seem to share their immunohistochemical pattern with pain-transducing axons at some other locations, such as the tooth pulp. The contents of SP- and CGRP-LI in sensory axons that reach the central nervous system (CNS) through the ventral root indicate that ventral root afferents may be involved in sensory mechanisms, such as the ventral root pain reaction, as well as in the control of the pial blood vessels. The demonstration of GAP-43 and neurotrophin receptor-immunoreactivities associated with unmyelinated fibers in ventral roots and the pia mater is discussed in relation to previous reports on postnatal plasticity in these axonal populations.

Functional consequences of expression of the neuron-specific, protein kinase C substrate RC3 (neurogranin) in Xenopus oocytes

RC3 (neurogranin) is a neuron-specific substrate of protein kinase C (PKC) that accumulates predominantly in dendritic spines of forebrain neurons and undergoes long-term potentiation (LTP)-associated increases in PKC-phosphorylation in hippocampal slices. Here the hypothesis that RC3 functions by modulating the IP3/DAG second messenger pathway after its phosphorylation by DAG-activated PKC was tested by heterologous expression in Xenopus oocytes. Acetylcholine-evoked inward chloride (Cl-) currents, dependent on both IP3 release and intracellular calcium (Ca2+), were 2- to 3-fold higher in RC3-injected oocytes than in uninjected control oocytes. RC3-oocytes did not exhibit enhanced currents when preincubated with the protein kinase inhibitor H-7 or when a glycine residue was substituted for serine, the PKC phosphorylation site of RC3. Activation of endogenous oocyte PKC by phorbol esters generated inward Cl- currents in RC3 oocytes but not in control oocytes. RC3-dependent Cl- currents were also elicited by phorbol ester in Ca(2+)-free media. We propose that PKC-phosphorylated RC3 is capable of enhancing the mobilization of intracellular Ca2+ in Xenopus oocytes and, by inference, may play a role in Ca2+ homeostasis in dendrites of forebrain neurons.

Localization of the protein kinase C phosphorylation/calmodulin-binding substrate RC3 in dendritic spines of neostriatal neurons

The rodent protein RC3 is expressed mainly by forebrain neurons during postnatal development and maturity. RC3 and its bovine homolog neurogranin/B-50 immunoreactive C-kinase substrate (BICKS) contain overlapping sites for protein kinase C phosphorylation and calmodulin binding that resemble those of the presynaptic 43-kDa growth-associated protein (GAP-43). However, morphological evidence suggests that RC3 has a postsynaptic localization. To test this hypothesis, we used two polyclonal antisera against synthetic peptides corresponding to nonoverlapping sequences within RC3 and compared cellular distributions of their binding in neostriatum of adult rats by immunohistochemistry, Golgi impregnation/gold toning, and correlative light/electron microscopy. Somatic and punctate patterns of RC3 immunoreactivity were observed. Somatic RC3 was found in cyto- and nucleoplasmic compartments of all neuronal phenotypes (medium spiny, medium aspiny, and large aspiny cells). Punctate RC3 was found mostly in dendritic spines. In contrast to the 43-kDa growth-associated protein, RC3 was seen infrequently in axons. We conclude that RC3 accumulates postsynaptically in dendritic spines of neostriatal neurons. We propose that RC3 acts as a "third messenger" substrate of protein kinase C-mediated molecular cascades during synaptic development and remodeling.

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.

A rapid purification method for neurogranin, a brain specific calmodulin-binding protein kinase C substrate

A rapid purification method is reported for bovine brain neurogranin, a calmodulin-binding protein kinase C (PKC) substrate. This method takes advantage of the fact that the protein remains soluble in 2.5% perchloric acid (PCA) and that it binds to a calmodulin-Sepharose column in the absence of calcium: Other PKC substrate proteins that remain to be identified were also found to share these two properties, suggesting that a class of calmodulin-binding PKC substrates may exist in the brain.

B-50 (GAP-43): biochemistry and functional neurochemistry of a neuron-specific phosphoprotein

The biochemistry and functional neurochemistry of the synaptosomal plasma membrane phosphoprotein B-50 (GAP-43) are reviewed. The protein is putatively involved in seemingly diverse functions within the nervous system, including neuronal development and regeneration, synaptic plasticity, and formation of memory and other higher cognitive behaviors. There is a considerable amount of information concerning the spatial and temporal localization of B-50 (GAP-43) in adult, fetal, and regenerating nervous tissue but far less is known about the physical chemistry and biochemistry of the protein. Still less information is available about posttranslational modifications of B-50 (GAP-43) that may be the basis of neurochemical mechanisms that could subsequently permit a variety of physiological functions. Hence, consideration is given to several plausible roles for B-50 (GAP-43) in vivo, which are discussed in the context of the cellular localization of the protein, significant posttranslational enzymes, and regulatory proteins, including protein kinases, phosphoinositides, calmodulin, and proteases.

Differential effects of protein kinase inhibitors on pre-established long-term potentiation in rat hippocampal neurons in vitro

The possibility that a permanent protein kinase C (PKC) activity is necessary for the maintenance of long-term potentiation (LTP) was investigated in rat hippocampal slices. The action of the potent kinase inhibitors K-252a, K-252b and staurosporine on LTP of orthodromic population excitatory postsynaptic potentials (EPSPs) recorded from CA1 pyramidal cells was tested both during tetanization and after establishment of LTP. Confirming earlier studies, all inhibitors applied during tetanization at a concentration of 50 nM eliminate late LTP. Only staurosporine, but not K-252a or K-252b, blocked already established late LTP (i.e. late application). Normal synaptic transmission was influenced only weakly by staurosporine. Considering that all inhibitors have similar potencies against PKC and were all effective if applied during tetanization these data suggest that the late maintenance of LTP depends on a staurosporine/H7-sensitive process (or kinase) rather than permanent activation of PKC.

Corticotropin (ACTH) inhibits the specific proteolysis of the neuronal phosphoprotein B-50/GAP-43

The modulatory action of corticotropin (ACTH) on the specific proteolysis of B-50 to B-60 [B-50(41-226)] was investigated using a previously characterized synaptosomal membrane extract [ASP(57-82)] that was highly enriched in B-50, B-50 kinase activity and a B-50 protease. In common with the previously described inhibitory action on B-50 phosphorylation at Ser41, ACTH(1-24) inhibited B-50 proteolysis in a concentration and structurally dependent manner, while ACTH(1-10) and ACTH(11-24) or a combination of both peptides were ineffective. Structural similarities between segments of ACTH(1-24) and B-50(40-55) may explain the inhibitory action of the peptide on B-50 phosphorylation and proteolysis. In addition, calmodulin, which binds to the N-terminus of the dephosphorylated form of the protein, also protected B-50 from degradation.

Neurotransmitter release

Axon terminals release more than one physiologically active substance. Synaptic messengers may be stored in two different types of vesicles. Small electron-lucent vesicles mainly store classical low molecular weight transmitter substances and the larger electron-dense granules store and release proteins and peptides. Release of the two types of substances underlies different physiological control. Release of messenger molecules from axon terminals is triggered by influx of Ca2+ through voltage sensitive Ca2+ channels and a rise in cytosolic Ca2+ concentrations. Neither the immediate Ca2+ target(s) nor the molecular species involved in synaptic vesicle docking, fusion and retrieval are known. It is, however, likely that steps involved in the molecular cascade of transmitter release include liberation of vesicles from their association with the cytonet and phosphorylation by protein kinase C of proteins which have the ability to alter between membrane bound and cytoplasmic forms and thus facilitate or initiate the molecular interaction between synaptic vesicles and the plasma membrane.

Distribution of growth-associated protein, B-50 (GAP-43) in the mammalian enteric nervous system

The presence of the growth-associated protein, B-50 (also known as GAP-43) was investigated in the adult mammalian enteric nervous system. The small intestine of rat, ferret and human was examined by immunohistochemistry. Dense B-50-like immunoreactivity was localized in nerves throughout the wall of the rat, ferret and human small intestine, notably in the myenteric and submucous plexuses, where in the ferret ileum it co-localized with vasoactive intestinal polypeptide-immunoreactive fibre groups. Material with the biochemical and immunological characteristics of rat B-50 was extracted from the rat ileum. In-situ hybridization demonstrated that enteric neurons express B-50. These findings are consistent with a role for B-50 in the documented plasticity of the adult enteric nervous system.