4-Aminopyridine stimulates B-50 (GAP-43) phosphorylation in rat synaptosomes

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.

Voltage-dependent ebselen and diorganochalcogenides inhibition of 45Ca2+ influx into brain synaptosomes

By mediating the Ca(2+) influx, Ca(2+) channels play a central role in neurotransmission. Chemical agents that potentially interfere with Ca(2+) homeostasis are potential toxic agents. In the present investigation, changes in Ca(2+) influx into synaptosomes by organic forms of selenium and tellurium were examined under nondepolarizing and depolarizing conditions induced by high KCl concentration (135 mM) or by 4-aminopyridine (4-AP). Under nondepolarizing conditions, ebselen (400 micro M) increased Ca(2+) influx; diphenyl ditelluride (40-400 micro M) decreased Ca(2+) in all concentrations tested; and diphenyl diselenide decreased Ca(2+) influx at 40 and 100 micro M, but had no effect at 400 micro M. In the presence of KCl as depolarizing agent, ebselen and diphenyl ditelluride decreased Ca(2+) influx in a linear fashion. In contrast, diphenyl diselenide did not modify Ca(2+) influx into isolated nerve terminals. In the presence of 4-AP (3 mM) as depolarizing agent, ebselen (400 micro M) caused a significant increase, whereas diphenyl diselenide and diphenyl ditelluride inhibited Ca(2+) influx into synaptosomes. The results can be explained by the fact that the mechanism through which 4-AP and high K(+) induced elevation of intracellular Ca(2+) is not exactly coincident. The mechanism by which diphenyl ditelluride and ebselen interact with Ca(2+) channel is unknown, but may be related to reactivity with critical sulfhydryl groups in the protein complex. The results of the present study indicate that the effects of organochalcogenides were rather complex depending on the condition and the depolarizing agent used.

Paired pulse facilitation is turned into paired pulse depression in hippocampal slices after epilepsy induced by 4-aminopyridine in vivo

Modifications in synaptic plasticity seem to play a key role in the origin and persistence of epilepsy. 4-Aminopyridine (4-AP) induces intense and long lasting epileptic seizures and neurodegeneration when applied into the hippocampus in vivo, effects that seem to be mediated by overactivation of glutamate receptors due to the enhancement of glutamate release from nerve endings. We have studied presynaptic modifications of CA1 responses, using the paired pulse paradigm, in hippocampal slices obtained from 4-AP-treated rats killed during epileptic activity (ex vivo). The paired pulse facilitation (PPF) observed in control slices with interstimulus intervals of 10-30 ms was changed into paired pulse depression (PPD) after 100 microM 4-AP added in vitro. A strikingly similar change was observed in the ex vivo slices even though 4-AP was no longer present in the tissue. We conclude that the facilitation of glutamate release induced by 4-AP becomes chronic after a transient exposure to the drug. This suggests that the facilitated neurotransmitter release induced by 4-AP triggers a more permanent plastic change that may be responsible for the persistence of epilepsy.

Characterization of neocortical and hippocampal synaptosomes from temporal lobe epilepsy patients

To investigate epilepsy-associated changes in the presynaptic terminal, we isolated and characterized synaptosomes from biopsies resected during surgical treatment of drug-resistant temporal lobe epilepsy (TLE) patients. Our main findings are: (1) The yield of synaptosomal protein from biopsies of epilepsy patients was about 25% of that from rat brain. Synaptosomal preparations were essentially free of glial contaminations. (2) Synaptosomes from TLE patients and naive rat brain, quickly responded to K(+)-depolarization with a 70% increase in intrasynaptosomal Ca(2+) ([Ca(2+)](i)), and a 40% increase in B-50/GAP-43 phosphorylation. (3) Neocortical and hippocampal synaptosomes from TLE patients contained 20-50% of the glutamate and gamma-aminobutyric acid (GABA) contents of rat cortical synaptosomes. (4) Although the absolute amount of glutamate and GABA released under basal conditions from neocortical synaptosomes of TLE patients was lower than from rat synaptosomes, basal release expressed as percentage of total content was higher (16.4% and 17.3%, respectively) than in rat (11.5% and 9. 9%, respectively). (5) Depolarization-induced glutamate and GABA release from neocortical synaptosomes from TLE patients was smaller than from rat synaptosomes (3.9% and 13.0% vs. 21.9% and 25.0%, respectively). (6) Analysis of breakdown of glial fibrillary acid protein (GFAP) indicates that resection time (anoxic period during the operation) is a critical parameter for the quality of the synaptosomes. We conclude that highly pure and viable synaptosomes can be isolated even from highly sclerotic human epileptic tissue. Our data show that in studies on human synaptosomes it is of critical importance to distinguish methodological (i.e., resection time) from pathology-related abnormalities.

Effects of calcium channel antagonists on the phosphorylation of major protein kinase C substrates in the rat hippocampus

K(+)-induced depolarization of rat hippocampal slices resulted in significant increases in the phosphorylation state of myristoylated, alanine-rich C kinase substrate (MARCKS; also known as 87K, pp80) and neuromodulin [also known as growth associated protein 43 (GAP43), B50, F1] as determined by back-phosphorylation using protein kinase C. The effect of organic and inorganic Ca2+ antagonists on the phosphorylation of these major protein kinase C substrates in the rat hippocampus was studied to determine whether Ca2+ influx through L- or N-type voltage-sensitive Ca2+ channels was required for the phosphorylation changes observed. The depolarization-induced changes appeared to be dependent on extracellular Ca2+, based on evidence indicating that the chelation of extracellular Ca2+ with ethylene glycol-bis (beta-amino-ethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) inhibited these changes. In addition, pretreatment of the slices with 500 microM Cd2+, but not 300 nM nimodipine, 10 microM omega-conotoxin GVIA or 10 microM MK-801, blocked the K(+)-induced change in phosphorylation. These results suggest that K(+)-induced changes in the phosphorylation of MARCKS and neuromodulin are mediated by Ca(2+)-dependent mechanisms other than, or in addition to, those sensitive to the organic Ca2+ channel antagonists employed.

Effects of 4-aminopyridine on protein phosphorylation in heat-blocked peripheral nerve

The topical application of 4-aminopyridine (4-AP) reverses within 1-3 min the partial conduction block that results from heating 5-mm segments of rat posterior tibial, peroneal and sural nerves to 45 degrees C for several minutes. Nerves previously blocked in vitro or in vivo were incubated in vitro in the presence of [gamma-32P]ATP. The rate of phosphorylation of soluble nerve proteins that had entered the media was determined. Labeled proteins were separated electrophoretically and autoradiograms prepared. We found that 5 mM 4-AP increases the phosphorylation rate in heat-blocked nerve by approximately 50-fold. The process is calcium-dependent and is heat-labile. Soluble proteins with a molecular weight in the 53-55 kDa range are preferentially but not exclusively phosphorylated in the presence of 4-AP at levels effective in restoring conduction. The results suggest that the potassium channel blocker 4-AP may restore conduction by inducing changes in channel proteins.

Presynaptic mechanism of action of 4-aminopyridine: changes in intracellular free Ca2+ concentration and its relationship to B-50 (GAP-43) phosphorylation

Recently we have shown that 4-aminopyridine (4-AP), a drug known to enhance transmitter release, stimulates the phosphorylation of the protein kinase C substrate B-50 (GAP-43) in rat brain synaptosomes and that this effect is dependent on the presence of extracellular Ca2+. Hence, we were interested in the relationship between changes induced by 4-AP in the intracellular free Ca2+ concentration ([Ca2+]i) and B-50 phosphorylation in synaptosomes. 4-AP (100 microM) elevates the [Ca2+]i (as determined with fura-2) to approximately the same extent as depolarization with 30 mM K+ (from an initial resting level of 240 nM to approximately 480 nM after treatment). However, the underlying mechanisms appear to be different: In the presence of 4-AP, depolarization with K+ still evoked an increase in [Ca2+]i, which was additive to the elevation caused by 4-AP. Several Ca2+ channel antagonists (CdCl2, LaCl3, and diphenylhydantoin) inhibited the increase in B-50 phosphorylation by 4-AP. It is interesting that the increase in [Ca2+]i and the increase in B-50 phosphorylation by 4-AP were attenuated by tetrodotoxin, a finding pointing to a possible involvement of Na+ channels in this action. These results suggest that 4-AP (indirectly) stimulates both Ca2+ influx and B-50 phosphorylation through voltage-dependent channels by a mechanism dependent on Na+ channel activity.

The role of protein kinase C and its neuronal substrates dephosphin, B-50, and MARCKS in neurotransmitter release

This article focuses on the role of protein phosphorylation, especially that mediated by protein kinase C (PKC), in neurotransmitter release. In the first part of the article, the evidence linking PKC activation to neurotransmitter release is evaluated. Neurotransmitter release can be elicited in at least two manners that may involve distinct mechanisms: Evoked release is stimulated by calcium influx following chemical or electrical depolarization, whereas enhanced release is stimulated by direct application of phorbol ester or fatty acid activators of PKC. A markedly distinct sensitivity of the two pathways to PKC inhibitors or to PKC downregulation suggests that only enhanced release is directly PKC-mediated. In the second part of the article, a framework is provided for understanding the complex and apparently contrasting effects of PKC inhibitors. A model is proposed whereby the site of interaction of a PKC inhibitor with the enzyme dictates the apparent potency of the inhibitor, since the multiple activators also interact with these distinct sites on the enzyme. Appropriate PKC inhibitors can now be selected on the basis of both the PKC activator used and the site of inhibitor interaction with PKC. In the third part of the article, the known nerve terminal substrates of PKC are examined. Only four have been identified, tyrosine hydroxylase, MARCKS, B-50, and dephosphin, and the latter two may be associated with neurotransmitter release. Phosphorylation of the first three of these proteins by PKC accompanies release. B-50 may be associated with evoked release since antibodies delivered into permeabilized synaptosomes block evoked, but not enhanced release. Dephosphin and its PKC phosphorylation may also be associated with evoked release, but in a unique manner. Dephosphin is a phosphoprotein concentrated in nerve terminals, which, upon stimulation of release, is rapidly dephosphorylated by a calcium-stimulated phosphatase (possibly calcineurin [CN]). Upon termination of the rise in intracellular calcium, dephosphin is phosphorylated by PKC. A priming model of neurotransmitter release is proposed where PKC-mediated phosphorylation of such a protein is an obligatory step that primes the release apparatus, in preparation for a calcium influx signal. Protein dephosphorylation may therefore be as important as protein phosphorylation in neurotransmitter release.

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.