The function of Ca(2+) channel subtypes in exocytotic secretion: new perspectives from synaptic and non-synaptic release

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.

Overexpressed Ca(v)beta3 inhibits N-type (Cav2.2) calcium channel currents through a hyperpolarizing shift of ultra-slow and closed-state inactivation

It has been shown that β auxiliary subunits increase current amplitude in voltage-dependent calcium channels. In this study, however, we found a novel inhibitory effect of β3 subunit on macroscopic Ba²⁺ currents through recombinant N- and R-type calcium channels expressed in Xenopus oocytes. Overexpressed β3 (12.5 ng/cell cRNA) significantly suppressed N- and R-type, but not L-type, calcium channel currents at “physiological” holding potentials (HPs) of −60 and −80 mV. At a HP of −80 mV, coinjection of various concentrations (0–12.5 ng) of the β3 with Ca_v2.2α₁ and α₂δ enhanced the maximum conductance of expressed channels at lower β3 concentrations but at higher concentrations (>2.5 ng/cell) caused a marked inhibition. The β3-induced current suppression was reversed at a HP of −120 mV, suggesting that the inhibition was voltage dependent. A high concentration of Ba²⁺ (40 mM) as a charge carrier also largely diminished the effect of β3 at −80 mV. Therefore, experimental conditions (HP, divalent cation concentration, and β3 subunit concentration) approaching normal physiological conditions were critical to elucidate the full extent of this novel β3 effect. Steady-state inactivation curves revealed that N-type channels exhibited “closed-state” inactivation without β3, and that β3 caused an ∼40-mV negative shift of the inactivation, producing a second component with an inactivation midpoint of approximately −85 mV. The inactivation of N-type channels in the presence of a high concentration (12.5 ng/cell) of β3 developed slowly and the time-dependent inactivation curve was best fit by the sum of two exponential functions with time constants of 14 s and 8.8 min at −80 mV. Similar “ultra-slow” inactivation was observed for N-type channels without β3. Thus, β3 can have a profound negative regulatory effect on N-type (and also R-type) calcium channels by causing a hyperpolarizing shift of the inactivation without affecting “ultra-slow” and “closed-state” inactivation properties.

R-type voltage-gated ca2+ channel interacts with synaptic proteins and recruits synaptotagmin to the plasma membrane of xenopus oocytes

It is well established that syntaxin 1A, synaptosomal-associated protein of 25 kDa (SNAP-25) and synaptotagmin either alone or in combination, modulate the kinetic properties of voltage-gated Ca(2+) channels Ca(v)1.2 (Lc-channel) Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type). The interaction interface was found to reside at the cytosolic II-III domain of the alpha1 subunit of the channels. In this study, we demonstrated a functional coupling of human neuronal Ca(v)2.3 (R-type channel) with syntaxin 1A, SNAP-25 and synaptotagmin in BAPTA injected Xenopus oocytes. The kinetic properties of Ca(v)2.3 assembled with syntaxin 1A, SNAP-25 or synaptotagmin individually differed from Ca(v)2.3 associated with binary complexes syntaxin 1A/SNAP-25, syntaxin 1A/synaptotagmin or SNAP-25/synaptotagmin. Co-expression of Ca(v)2.3 with syntaxin 1A, SNAP-25 and synaptotagmin together, produced a channel with distinctive kinetic properties analogous to excitosome multiprotein complex generated by Ca(v)1.2 and Ca(v)2.2. Exchanging the current-carrying ions altered the kinetics of channel/synaptic proteins interaction, indicating a tight crosstalk formed between the permeation pathway of Ca(v)2.3 and the fusion apparatus during membrane depolarization. This putative coupling could predict how the release site might be organized to allow a rapid communication between the channel and the release machinery. In vivo confocal imaging of oocytes revealed GFP-synaptotagmin at the plasma membrane when the channel was present, as opposed to random distribution in its absence, consistent with Ca(2+)-independent molecular link of synaptotagmin and the channel. Synaptotagmin was detected at the membrane also in oocytes co-expressing the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). Both imaging studies and protein-protein interactions in Xenopus oocytes show that channel linkage to synaptotagmin precedes Ca(2+) influx. Altogether, the R-type channel appears to associate with synaptic proteins to generate a multiprotein excitosome complex prior to Ca(2+)-entry. We propose that the distinct kinetics of the Ca(2+)-channel acquired by the close association with the vesicle and the t-SNAREs within the excitosome complex may be essential for depolarization evoked transmitter release.

Regulation of exocytosis by purinergic receptors in pancreatic duct epithelial cells

In epithelial cells, several intracellular signals regulate the secretion of large molecules such as mucin via exocytosis and the transport of ions through channels and transporters. Using carbon fiber amperometry, we previously reported that exocytosis of secretory granules in dog pancreatic duct epithelial cells (PDEC) can be stimulated by pharmacological activation of cAMP-dependent protein kinase (PKA) or protein kinase C (PKC), as well as by an increase of intracellular free Ca2+ concentration ([Ca2+]i). In this study, we examined whether exocytosis in these cells is modulated by activation of endogenous P2Y receptors, which increase cAMP and [Ca2+]i. Low concentrations of ATP (<10 microM) induced intracellular Ca2+ oscillation but no significant exocytosis. In contrast, 100 microM ATP induced a sustained [Ca2+]i rise and increased the exocytosis rate sevenfold. The contribution of Ca2+ or cAMP pathways to exocytosis was tested by using the Ca2+ chelator BAPTA or the PKA inhibitors H-89 or Rp-8-bromoadenosine 3',5'-cyclic monophosphorothioate. Removal of [Ca2+]i rise or inhibition of PKA each partially reduced exocytosis; when combined, they abolished exocytosis. In conclusion, ATP at concentrations >10 microM stimulates exocytosis from PDEC through both Ca2+ and cAMP pathways.

Promiscuous and reversible blocker of presynaptic calcium channels in frog and crayfish neuromuscular junctions from Phoneutria nigriventer spider venom

Peptide channel blockers found in venoms of many predators are useful pharmacological tools and potential therapeutic agents. The venom of the Brazilian spider Phoneutria nigriventer contains a fraction, omega-phonetoxin-IIA (omega-Ptx-IIA, 8360 MW), which blocks Ca2+ channels. At frog neuromuscular junctions (NMJ) bathed in normal Ca2+ (1.8 mM) saline, omega-Ptx IIA did not affect spontaneous transmitter release but reversibly reduced evoked transmitter release by 75 and 95% at 12 and 24 nM, respectively. In contrast, toxin effects were irreversible in low-Ca2+ (0.5 mM) saline. Ca2+ imaging in normal-Ca2+ saline showed that omega-Ptx-IIA partially blocked stimulus-dependent presynaptic Ca2+ signals, and the blockade was almost completely reversible. Increases in spontaneous release frequency induced by high extracellular K+ were blocked by omega-Ptx-IIA. Therefore omega-Ptx-IIA blocks N-type Ca2+ channels, which admit Ca2+ that triggers transmitter release at the frog NMJ. Additional evidence predicts that omega-Ptx-IIA binds to N-type Ca2+ channels at a different site from that of omega-Conotoxin-GVIA. omega-Ptx-IIA also gave a low-affinity partial blockade of transmitter release and presynaptic Ca2+ signals at crayfish NMJs where P-type channels are blocked by omega-agatoxin-IVA. The Ca2+-dependent reversibility and promiscuity of this toxin may make it highly useful experimentally and therapeutically.

Mechanisms of lateral inhibition in the olfactory bulb: efficiency and modulation of spike-evoked calcium influx into granule cells

Granule cells are axonless local interneurons that mediate lateral inhibitory interactions between the principal neurons of the olfactory bulb via dendrodendritic reciprocal synapses. This unusual arrangement may give rise to functional properties different from conventional lateral inhibition. Although granule cells spike, little is known about the role of the action potential with respect to their synaptic output. To investigate the signals that underlie dendritic release in these cells, two-photon microscopy in rat brain slices was used to image calcium transients in granule cell dendrites and spines. Action potentials evoked calcium transients throughout the dendrites, with amplitudes increasing with distance from soma and attaining a plateau level within the external plexiform layer, the zone of granule cell synaptic output. Transient amplitudes were, on average, equal in size in spines and adjacent dendrites. Surprisingly, both spine and dendritic amplitudes were strongly dependent on membrane potential, decreasing with depolarization and increasing with hyperpolarization from rest. Both the current-voltage relationship and the time course of inactivation were consistent with the known properties of T-type calcium channels, and the voltage dependence was blocked by application of the T-type calcium channel antagonists Ni2+ and mibefradil. In addition, mibefradil reduced action potential-mediated synaptic transmission from granule to mitral cells. The implication of a transiently inactivating calcium channel in synaptic release from granule cells suggests novel mechanisms for the regulation of lateral inhibition in the olfactory bulb.

Increased tyrosine phosphorylation of alpha(1C) subunits of L-type voltage-gated calcium channels and interactions among Src/Fyn, PSD-95 and alpha(1C) in rat hippocampus after transient brain ischemia

It has been reported that the Src family kinases-mediated tyrosine phosphorylation of alpha(1C) subunits of L-type voltage-gated calcium channels (L-VGCCs) potentiates the channel currents. In this study, we evaluated the alterations in the tyrosine phosphorylation level of alpha(1C) and in the interactions involving Src/Fyn, alpha(1C) and PSD-95 in the hippocampus after transient (15 min) brain ischemia followed by various times of reperfusion using immunoprecipitation and immunoblotting. Transient brain ischemia was induced by the method of four-vessel occlusion in Sprague-Dawley rats. The tyrosine phosphorylation level of alpha(1C) subunits elevated immediately after brain ischemia. The elevation in phosphorylation sustained for at least 6 h and peaked at 15 min of reperfusion. Transient brain ischemia and reperfusion also caused rapid and sustained increases in the interactions of Src and Fyn with alpha(1C) subunits. More interestingly, co-immunoprecipitation analysis showed that 15 min of reperfusion dramatically increased the interaction between PSD-95 and alpha(1C) and promoted the formation of alpha(1C)-PSD-95-Src complexes, for the first time. The protein levels of alpha(1C), Src, Fyn and PSD-95 showed no differences at all time points. These results suggest a novel mechanism involving the ischemia/reperfusion-induced recruitment of L-VGCCs, Src and Fyn to the PSD-95 signaling complex that facilitates the tyrosine phosphorylation of alpha(1C) subunits by Src family kinases and may contribute to the up-regulation of L-VGCCs activity in postischemic hippocampus.

Neurotrophin secretion: current facts and future prospects

The proteins of the mammalian neurotrophin family (nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5)) were originally identified as neuronal survival factors. During the last decade, evidence has accumulated implicating them (especially BDNF) in addition in the regulation of synaptic transmission and synaptogenesis in the CNS. However, a detailed understanding of the secretion of neurotrophins from neurons is required to delineate their role in regulating synaptic function. Some crucial questions that need to be addressed include the sites of neurotrophin secretion (i.e. axonal versus dendritic; synaptic versus extrasynaptic) and the neuronal and synaptic activity patterns that trigger the release of neurotrophins. In this article, we review the current knowledge in the field of neurotrophin secretion, focussing on activity-dependent synaptic release of BDNF. The modality and the site of neurotrophin secretion are dependent on the processing and subsequent targeting of the neurotrophin precursor molecules. Therefore, the available data regarding formation and trafficking of neurotrophins in the secreting neurons are critically reviewed. In addition, we discuss existing evidence that the characteristics of neurotrophin secretion are similar (but not identical) to those of other neuropeptides. Finally, since BDNF has been proposed to play a critical role as an intercellular synaptic messenger in long-term potentiation (LTP) in the hippocampus, we try to reconcile this possible role of BDNF in LTP with the recently described features of synaptic BDNF secretion.

N-type Ca2+ channel

Ca(2+) entry through voltage-dependent Ca(2+) channels (VDCCs) regulates various aspects of physiological function, including neurotransmitter release, regulation of cell membrane excitability, and control of gene expression. VDCCs are classified into several sub-types (L-, N-, P/Q-, R-, and T-types) based on electrophysiological and pharmacological properties. Each type of channels except the T-type is composed of at least four subunits, designated alpha(1), alpha(2), beta, and delta. During the past decade, a number of genes encoding these subunits have been cloned, and cDNA expression studies using heterologous expression systems have revealed the intricate nature of subunit interaction and many biophysical aspects of channel function. In recent years, an entirely new strategy has been introduced in attempts to clarify the physiological role of each of the VDCCs, and this has proven to be very useful in defining previously unknown in vivo functions of VDCCs. In this article, we briefly review the recent advances in our understanding of VDCCs with special emphasis on the N-type channel, which is mainly expressed in neural tissues and is the essential component of neurotransmitter release. We will mainly discuss the subunit composition, channel regulation by G proteins and exocytotic proteins, and the mouse phenotypes in which N-type channel subunits have been deleted by gene targeting technology.

Presynaptic quantal plasticity: Katz's original hypothesis revisited

Changes in the amplitudes of signals conveyed at synaptic contacts between neurons underlie many brain functions and pathologies. Here we review the possible determinants of the amplitude and plasticity of the elementary postsynaptic signal, the miniature. In the absence of a definite understanding of the molecular mechanism releasing transmitters, we investigated a possible alternative interpretation. Classically, both the quantal theory and the vesicle theory predict that the amount of transmitter producing a miniature is determined presynaptically prior to release and that rapid changes in miniature amplitude reflect essentially postsynaptic alterations. However, recent data indicates that short-term and long-lasting changes in miniature amplitude are in large part due to changes in the amount of transmitter in individual released packets that show no evidence of preformation. Current representations of transmitter release derive from basic properties of neuromuscular transmission and endocrine secretion. Reexamination of overlooked properties of these two systems indicate that the amplitude of miniatures may depend as much, if not more, on the Ca(2+) signals in the presynaptic terminal than on the number of postsynaptic receptors available or on vesicle's contents. Rapid recycling of transmitter and its possible adsorption at plasma and vesicle lumenal membrane surfaces suggest that exocytosis may reflect membrane traffic rather than actual transmitter release. This led us to reconsider the disregarded hypothesis introduced by Fatt and Katz (1952; J Physiol 117:109-128) that the excitability of the release site may account for the "quantal effect" in fast synaptic transmission. In this case, changes in excitability of release sites would contribute to the presynaptic quantal plasticity that is often recorded.

The magnocellular oxytocin system, the fount of maternity: adaptations in pregnancy

Oxytocin secretion from the posterior pituitary gland is increased during parturition, stimulated by the uterine contractions that forcefully expel the fetuses. Since oxytocin stimulates further contractions of the uterus, which is exquisitely sensitive to oxytocin at the end of pregnancy, a positive feedback loop is activated. The neural pathway that drives oxytocin neurons via a brainstem relay has been partially characterised, and involves A2 noradrenergic cells in the brainstem. Until close to term the responsiveness of oxytocin neurons is restrained by neuroactive steroid metabolites of progesterone that potentiate GABA inhibitory mechanisms. As parturition approaches, and this inhibition fades as progesterone secretion collapses, a central opioid inhibitory mechanism is activated that restrains the excitation of oxytocin cells by brainstem inputs. This opioid restraint is the predominant damper of oxytocin cells before and during parturition, limiting stimulation by extraneous stimuli, and perhaps facilitating optimal spacing of births and economical use of the store of oxytocin accumulated during pregnancy. During parturition, oxytocin cells increase their basal activity, and hence oxytocin secretion increases. In addition, the oxytocin cells discharge a burst of action potentials as each fetus passes through the birth canal. Each burst causes the secretion of a pulse of oxytocin, which sharply increases uterine tone; these bursts depend upon auto-stimulation by oxytocin released from the dendrites of the magnocellular neurons in the supraoptic and paraventricular nuclei. With the exception of the opioid mechanism that emerges to restrain oxytocin cell responsiveness, the behavior of oxytocin cells and their inputs in pregnancy and parturition is explicable from the effects of hormones of pregnancy (relaxin, estrogen, progesterone) on pre-existing mechanisms, leading through relative quiescence at term inter alia to net increase in oxytocin storage, and reduced auto-inhibition by nitric oxide generation. Cyto-architectonic changes in parturition, involving evident retraction of glial processes between oxytocin cells so they get closer together, are probably a response to oxytocin neuron activation rather than being essential for their patterns of firing in parturition.

Glycerotoxin from Glycera convoluta stimulates neurosecretion by up-regulating N-type Ca2+ channel activity

We report here the purification of glycerotoxin from the venom of Glycera convoluta, a novel 320 kDa protein capable of reversibly stimulating spontaneous and evoked neurotransmitter release at the frog neuromuscular junction. However, glycerotoxin is ineffective at the murine neuromuscular junction, which displays a different subtype of voltage- dependent Ca(2+) channels. By sequential and selective inhibition of various types of Ca(2+) channels, we found that glycerotoxin was acting via Ca(v)2.2 (N-type). In neuroendocrine cells, it elicits a robust, albeit transient, influx of Ca(2+) sensitive to the Ca(v)2.2 blockers omega-conotoxin GVIA and MVIIA. Moreover, glycerotoxin triggers a Ca(2+) transient in human embryonic kidney (HEK) cells over-expressing Ca(v)2.2 but not Ca(v)2.1 (P/Q-type). Whole-cell patch-clamp analysis of Ca(v)2.2 expressing HEK cells revealed an up-regulation of Ca(2+) currents due to a leftward shift of the activation peak upon glycerotoxin addition. A direct interaction between Ca(v)2.2 and this neurotoxin was revealed by co-immunoprecipitation experiments. Therefore, glycerotoxin is a unique addition to the arsenal of tools available to unravel the mechanism controlling Ca(2+)-regulated exocytosis via the specific activation of Ca(v)2.2.

Diversification of synaptic strength: presynaptic elements

Synapses are not static; their performance is modified adaptively in response to activity. Presynaptic mechanisms that affect the probability of transmitter release or the amount of transmitter that is released are important in synaptic diversification. Here, we address the diversity of presynaptic performance and its underlying mechanisms: how much of the variation can be accounted for by variation in synaptic morphology and how much by molecular differences? Significant progress has been made in defining presynaptic structural contributions to synaptic strength; by contrast, we know little about how presynaptic proteins produce normally observed functional differentiation, despite abundant information on presynaptic proteins and on the effects of their individual manipulation. Closing the gap between molecular and physiological synaptic diversification still represents a considerable challenge.