Basal and Stress-Induced Network Activity in the Adrenal Medulla In Vivo

The adrenal medulla plays a critical role in mammalian homeostasis and the stress response. It is populated by clustered chromaffin cells that secrete epinephrine or norepinephrine along with peptides into the bloodstream affecting distant target organs. Despite been heavily studied, the central control of adrenal medulla and in-situ spatiotemporal responsiveness remains poorly understood. For this work, we continuously monitored the electrical activity of individual adrenomedullary chromaffin cells in the living anesthetized rat using multielectrode arrays. We measured the chromaffin cell activity under basal and physiological stress conditions and characterized the functional micro-architecture of the adrenal medulla. Under basal conditions, chromaffin cells fired action potentials with frequencies between ~0.2 and 4 Hz. Activity was almost completely driven by sympathetic inputs coming through the splanchnic nerve. Chromaffin cells were organized into independent local networks in which cells fired in a specific order, with latencies from hundreds of microseconds to a few milliseconds. Electrical stimulation of the splanchnic nerve evoked almost exactly the same spatiotemporal firing patterns that occurred spontaneously. Hypoglycemic stress, induced by insulin administration resulted in increased activity of a subset of the chromaffin cells. In contrast, respiratory arrest induced by lethal anesthesia resulted in an increase in the activity of virtually all chromaffin cells before cessation of all activity. These results suggest a stressor-specific activation of adrenomedullary chromaffin cell networks and revealed a surprisingly complex electrical organization that likely reflects the dynamic nature of the adrenal medulla's neuroendocrine output during basal conditions and during different types of physiological stress.

Actions of Rab27B-GTPase on mammalian central excitatory synaptic transmission

Members of the Rab3 gene family are considered central to membrane trafficking of synaptic vesicles at mammalian central excitatory synapses. Recent evidence, however, indicates that the Rab27B-GTPase, which is highly homologous to the Rab3 family, is also enriched on SV membranes and co-localize with Rab3A and Synaptotagmin at presynaptic terminals. While functional roles of Rab3A have been well-established, little functional information exists on the role of Rab27B in synaptic transmission. Here we report on functional effects of Rab27B at SC-CA1 and DG-MF hippocampal synapses. The data establish distinct functional actions of Rab27B and demonstrate functions of Rab27B that differ between SC-CA1 and DG-MF synapses. Rab27B knockout reduced frequency facilitation compared to wild-type (WT) controls at the DG/MF-CA3 synaptic region, while increasing facilitation at the SC-CA1 synaptic region. Remarkably, Rab27B KO resulted in a complete elimination of LTP at the MF-CA3 synapse with no effect at the SC-CA1 synapse. These actions are similar to those previously reported for Rab3A KO. Specificity of action on LTP to Rab27B was confirmed as LTP was rescued in response to lentiviral infection and expression of human Rab27B, but not to GFP, in the DG in the Rab27B KO mice. Notably, the effect of Rab27B KO on MF-CA3 LTP occurred in spite of continued expression of Rab3A in the Rab27B KO. Overall, the results provide a novel perspective in suggesting that Rab27B and Rab3A act synergistically, perhaps via sequential effector recruitment or signaling for presynaptic LTP expression in this hippocampal synaptic region.

The ubiquitin-proteasome system functionally links neuronal Tomosyn-1 to dendritic morphology

Altering the expression of Tomosyn-1 (Tomo-1), a soluble, R-SNARE domain-containing protein, significantly affects behavior in mice, Drosophila, and Caenorhabditis elegans Yet, the mechanisms that modulate Tomo-1 expression and its regulatory activity remain poorly defined. Here, we found that Tomo-1 expression levels influence postsynaptic spine density. Tomo-1 overexpression increased dendritic spine density, whereas Tomo-1 knockdown (KD) decreased spine density. These findings identified a novel action of Tomo-1 on dendritic spines, which is unique because it occurs independently of Tomo-1's C-terminal R-SNARE domain. We also demonstrated that the ubiquitin-proteasome system (UPS), which is known to influence synaptic strength, dynamically regulates Tomo-1 protein levels. Immunoprecipitated and affinity-purified Tomo-1 from cultured rat hippocampal neurons was ubiquitinated, and the levels of ubiquitinated Tomo-1 dramatically increased upon pharmacological proteasome blockade. Moreover, Tomo-1 ubiquitination appeared to be mediated through an interaction with the E3 ubiquitin ligase HRD1, as immunoprecipitation of Tomo-1 from neurons co-precipitated HRD1, and this interaction increases upon proteasome inhibition. Further, in vitro reactions indicated direct, HRD1 concentration-dependent Tomo-1 ubiquitination. We also noted that the UPS regulates both Tomo-1 expression and functional output, as HRD1 KD in hippocampal neurons increased Tomo-1 protein level and dendritic spine density. Notably, the effect of HRD1 KD on spine density was mitigated by additional KD of Tomo-1, indicating a direct HRD1/Tomo-1 effector relationship. In summary, our results indicate that the UPS is likely to participate in tuning synaptic efficacy and spine dynamics by precise regulation of neuronal Tomo-1 levels.

Pharmacological correction of obesity-induced autophagy arrest using calcium channel blockers

Autophagy deregulation during obesity contributes to the pathogenesis of diverse metabolic disorders. However, without understanding the molecular mechanism of obesity interference in autophagy, development of therapeutic strategies for correcting such defects in obese individuals is challenging. Here we show that chronic increase of cytosolic calcium concentration in hepatocytes upon obesity and lipotoxicity attenuates autophagic flux by preventing the fusion between autophagosomes and lysosomes. As a pharmacological approach to restore cytosolic calcium homeostasis in vivo, we administered the clinically approved calcium channel blocker verapamil to obese mice. Such treatment successfully increases autophagosome-lysosome fusion in liver, preventing accumulation of protein inclusions and lipid droplets and suppressing inflammation and insulin resistance. As calcium channel blockers have been safely used in clinics for the treatment of hypertension for more than thirty years, our results suggest they may be a safe therapeutic option for restoring autophagic flux and treating metabolic pathologies in obese patients.

Distinct actions of Rab3 and Rab27 GTPases on late stages of exocytosis of insulin

Rab GTPases associated with insulin-containing secretory granules (SGs) are key in targeting, docking and assembly of molecular complexes governing pancreatic β-cell exocytosis. Four Rab3 isoforms along with Rab27A are associated with insulin granules, yet elucidation of the distinct roles of these Rab families on exocytosis remains unclear. To define specific actions of these Rab families we employ Rab3GAP and/or EPI64A GTPase-activating protein overexpression in β-cells from wild-type or Ashen mice to selectively transit the entire Rab3 family or Rab27A to a GDP-bound state. Ashen mice carry a spontaneous mutation that eliminates Rab27A expression. Using membrane capacitance measurements we find that GTP/GDP nucleotide cycling of Rab27A is essential for generation of the functionally defined immediately releasable pool (IRP) and central to regulating the size of the readily releasable pool (RRP). By comparison, nucleotide cycling of Rab3 GTPases, but not of Rab27A, is essential for a kinetically rapid filling of the RRP with SGs. Aside from these distinct functions, Rab3 and Rab27A GTPases demonstrate considerable functional overlap in building the readily releasable granule pool. Hence, while Rab3 and Rab27A cooperate to generate release-ready SGs in β-cells, they also direct unique kinetic and functional properties of the exocytotic pathway.

Differential interaction of tomosyn with syntaxin and SNAP25 depends on domains in the WD40 β-propeller core and determines its inhibitory activity

Neuronal exocytosis depends on efficient formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes and is regulated by tomosyn, a SNARE-binding protein. To gain new information about tomosyn's activity, we characterized its mobility and organization on the plasma membrane (PM) in relation to other SNARE proteins and inhibition of exocytosis. By using direct stochastic optical reconstruction microscopy (dSTORM), we found tomosyn to be organized in small clusters adjacent to syntaxin clusters. In addition, we show that tomosyn is present in both syntaxin-tomosyn complexes and syntaxin-SNAP25-tomosyn complexes. Tomosyn mutants that lack residues 537-578 or 897-917 from its β-propeller core diffused faster on the PM and exhibited reduced binding to SNAP25, suggesting that these mutants shift the equilibrium between tomosyn-syntaxin-SNAP25 complexes on the PM to tomosyn-syntaxin complexes. As these deletion mutants impose less inhibition on exocytosis, we suggest that tomosyn inhibition is mediated via tomosyn-syntaxin-SNAP25 complexes and not tomosyn-syntaxin complexes. These findings characterize, for the first time, tomosyn's dynamics at the PM and its relation to its inhibition of exocytosis.

Efficient transfection of dissociated mouse chromaffin cells using small-volume electroporation

We have developed an improved procedure for isolating and transfecting a chromaffin cell-enriched population of primary cells from adult mouse adrenal glands. Significantly, the parameters of a novel electroporation transfection technique were optimized to achieve an average transfection efficiency of 45 % on the small number of cells derived from the mouse glands. Such transfection efficiency was previously unachievable with the electroporation protocols conventionally used with bovine chromaffin cells, even with use of large cell numbers. Our small scale technique now makes feasible the use of genetically homogenous inbred mouse models for investigations on the exocytotic pathway without the time, expense, and cellular changes associated with viral approaches. High fidelity co-expression of multiple plasmids in individual cells is a further advantage of the procedure. To assess whether the biophysical characteristics of mouse adrenal chromaffin cells were altered by this process, we examined structural integrity using immunocytochemistry and functional response to stimuli using calcium imaging, amperometry, and whole-cell capacitance and current clamp recordings. We conclude these parameters are minimally affected. Finally, we demonstrate that high transfection efficiency makes possible the use of primary mouse adrenal chromaffin cells, rather than a cell line, in human growth hormone secretion assays for high throughput evaluation of secretion.

A new role for the dynamin GTPase in the regulation of fusion pore expansion

The role of dynamin GTPase activity in controlling fusion pore expansion and postfusion granule membrane topology was investigated. The experiments show that, in addition to playing a role in endocytosis, GTPase activity of dynamin regulates the rapidity of fusion pore expansion from tens of milliseconds to seconds after fusion.

Dynamin is a master regulator of membrane fission in endocytosis. However, a function for dynamin immediately upon fusion has also been suspected from a variety of experiments that measured release of granule contents. The role of dynamin guanosine triphosphate hydrolase (GTPase) activity in controlling fusion pore expansion and postfusion granule membrane topology was investigated using polarization optics and total internal reflection fluorescence microscopy (pTIRFM) and amperometry. A dynamin-1 (Dyn1) mutant with increased GTPase activity resulted in transient deformations consistent with rapid fusion pore widening after exocytosis; a Dyn1 mutant with decreased activity slowed fusion pore widening by stabilizing postfusion granule membrane deformations. The experiments indicate that, in addition to its role in endocytosis, GTPase activity of dynamin regulates the rapidity of fusion pore expansion from tens of milliseconds to seconds after fusion. These findings expand the membrane-sculpting repertoire of dynamin to include the regulation of immediate postfusion events in exocytosis that control the rate of release of soluble granule contents.

Structural and functional analysis of tomosyn identifies domains important in exocytotic regulation

Tomosyn is a 130-kDa cytosolic R-SNARE protein that associates with Q-SNAREs and reduces exocytotic activity. Two paralogous genes, tomosyn-1 and -2, occur in mammals and produce seven different isoforms via alternative splicing. Here, we map the structural differences between the yeast homologue of m-tomosyn-1, Sro7, and tomosyn genes/isoforms to identify domains critical to the regulation of exocytotic activity to tomosyn that are outside the soluble N-ethylmaleimide-sensitive attachment receptor motif. Homology modeling of m-tomosyn-1 based on the known structure of yeast Sro7 revealed a highly conserved functional conformation but with tomosyn containing three additional loop domains that emanate from a β-propeller core. Notably, deletion of loops 1 and 3 eliminates tomosyn inhibitory activity on secretion without altering its soluble N-ethylmaleimide-sensitive attachment receptor pairing with syntaxin1A. By comparison, deletion of loop 2, which contains the hypervariable splice region, did not reduce the ability of tomosyn to inhibit regulated secretion. However, exon variation within the hypervariable splice region resulted in significant differences in protein accumulation of tomosyn-2 isoforms. Functional analysis of s-tomosyn-1, m-tomosyn-1, m-tomosyn-2, and xb-tomosyn-2 demonstrated that they exert similar inhibitory effects on elevated K(+)-induced secretion in PC12 cells, although m-tomosyn-2 was novel in strongly augmenting basal secretion. Finally, we report that m-tomosyn-1 is a target substrate for SUMO 2/3 conjugation and that mutation of this small ubiquitin-related modifier target site (Lys-730) enhances m-tomosyn-1 inhibition of secretion without altering interaction with syntaxin1A. Together these results suggest that multiple domains outside the R-SNARE of tomosyn are critical to the efficacy of inhibition by tomosyn on exocytotic secretion.

Tomosyn expression pattern in the mouse hippocampus suggests both presynaptic and postsynaptic functions

The protein tomosyn decreases synaptic transmission and release probability of vesicles, and is essential for modulating synaptic transmission in neurons. In this study, we provide a detailed description of the expression and localization patterns of tomosyn1 and tomosyn2 in the subareas of the mouse hippocampus. Using confocal and two-photon high-resolution microscopy we demonstrate that tomosyn colocalizes with several pre- and postsynaptic markers and is found mainly in glutamatergic synapses. Specifically, we show that tomosyn1 is differentially distributed in the mouse hippocampus and concentrated mainly in the hilus and mossy fibers. Surprisingly, we found that tomosyn2 is expressed in the subiculum, CA1 and CA2 pyramidal cell bodies, dendrites and spines, and colocalizes with PSD95, suggesting a postsynaptic role. These results suggest that in addition to the well-characterized presynaptic function of tomosyn in neurotransmitter release, tomosyn2 might have a postsynaptic function, and place tomosyn as a more general regulator of synaptic transmission and plasticity.

Mapping dynamic protein interactions to insulin secretory granule behavior with TIRF-FRET

Biological processes are governed by extensive networks of dynamic molecular interactions. Yet, establishing a spatial and temporal map of these interactions and their direct relationship to specific cell functions has remained a challenge. Here, we implement sensitized emission Förster resonance energy transfer (FRET) stoichiometry under total internal reflection fluorescence (TIRF) microscopy. We demonstrate through quantitative analysis and modeling that evanescent fields must be precisely matched between FRET excitation wavelengths to isolate dynamic interactions between bimolecular FRET pairs that are not entirely membrane-delimited. We then use TIRF-FRET to monitor the behavior of individual insulin-containing secretory granules at the plasma membrane of living cells, while simultaneously tracking the dynamic interaction between the GTPase Rab27A and its effector Slp4A, on those same granules. Notably, insulin granules that underwent exocytosis demonstrated a specific increase in Rab27A-GTP/Slp4A FRET in the 5 s before membrane fusion, which coincided temporally with an increase in granule displacement and mobility. These results demonstrate an initial spatiotemporal mapping of a dynamic protein-protein interaction on individual secretory granules that is linked to a specific granule behavior in living cells.

Kinetics of Rab27a-dependent actions on vesicle docking and priming in pancreatic beta-cells

The small GTPase Rab27a, along with the isoforms of Rab3, is present on insulin secretory granules and has been implicated in regulation of Ca(2+)-triggered exocytosis. We have used membrane capacitance measurements to define the role of Rab27a in regulating the size and refilling of distinct pools of insulin granules by comparison of evoked secretory responses from Rab27a-null ashen and strain-matched wild-type control pancreatic beta-cells. We find that ashen beta-cells display a kinetic defect in refilling of readily releasable and immediately releasable vesicle pools (RRP and IRP, respectively) in response to depolarization-evoked Ca(2+) influx. The deficit in IRP refilling was not observed in the presence of stimulatory glucose concentrations (16.7 mm), though incomplete refilling of the RRP persisted. Comparatively, beta-cells from Rab3a(-/-) mice exhibited complete refilling of the IRP and RRP, demonstrating that Rab27a and Rab3a exert distinct roles in the insulin granule secretory pathway. Further, depletion of the RRP in ashen beta-cells was twofold faster than that of control beta-cells. These deficits in refilling and exocytotic rate in ashen beta-cells were absent when cAMP-regulatory pathways were activated. Elevated cAMP increased the RRP pool size, and complete refilling of the RRP occurred in ashen beta-cells; responses were comparable to wild-type controls. These effects of cAMP were largely eliminated by Rp-cAMP inhibition of PKA, indicating that PKA acts on vesicle priming downstream or via pathways independent of Rab27a. In summary, Rab27a exerts dual roles in glucose-mediated insulin granule exocytosis, facilitating refilling of releasable granule pools while also limiting the rate of release from these pools.

SNARE-catalyzed fusion events are regulated by Syntaxin1A-lipid interactions

Membrane fusion is a process that intimately involves both proteins and lipids. Although the SNARE proteins, which ultimately overcome the energy barrier for fusion, have been extensively studied, regulation of the energy barrier itself, determined by specific membrane lipids, has been largely overlooked. Our findings reveal a novel function for SNARE proteins in reducing the energy barrier for fusion, by directly binding and sequestering fusogenic lipids to sites of fusion. We demonstrate a specific interaction between Syntaxin1A and the fusogenic lipid phosphatidic acid, in addition to multiple polyphosphoinositide lipids, and define a polybasic juxtamembrane region within Syntaxin1A as its lipid-binding domain. In PC-12 cells, Syntaxin1A mutations that progressively reduced lipid binding resulted in a progressive reduction in evoked secretion. Moreover, amperometric analysis of fusion events driven by a lipid-binding-deficient Syntaxin1A mutant (5RK/A) demonstrated alterations in fusion pore dynamics, suggestive of an energetic defect in secretion. Overexpression of the phosphatidic acid-generating enzyme, phospholipase D1, completely rescued the secretory defect seen with the 5RK/A mutant. Moreover, knockdown of phospholipase D1 activity drastically reduced control secretion, while leaving 5RK/A-mediated secretion relatively unaffected. Altogether, these data suggest that Syntaxin1A-lipid interactions are a critical determinant of the energetics of SNARE-catalyzed fusion events.

Multiple functional domains are involved in tomosyn regulation of exocytosis

Tomosyn is a cytoplasmic protein that was shown to bind to Syntaxin1 and SNAP-25 through an R-SNARE domain, forming a complex that is almost identical in structure to the neuronal SNARE complex. Tomosyn inhibits exocytosis in various cell types and these effects were attributed to direct competition between tomosyn's SNARE domain and Synaptobrevin/VAMP. In the present study, we investigated the contribution of different domains of tomosyn to its activity. We show that a tomosyn mutant that lacks the entire SNARE domain is a potent inhibitor of vesicle priming, similar to the full-length tomosyn. The SNARE domain of tomosyn failed to inhibit exocytosis, indicating that this domain is not required for the inhibition. In contrast, over-expression of a N-terminally truncated mutant did not lead to inhibition of exocytosis although this mutant still bound to Syntaxin. Our results indicate that tomosyn can inhibit exocytosis independently of its SNARE interaction with Syntaxin and that the integrity of the WD40-domain is crucial for tomosyn's inhibitory function. Furthermore, we demonstrate that the entire N-terminal region of tomosyn, the WD40-repeats and the linker, is required for tomosyn's inhibitory effect.

Receptor-mediated regulation of tomosyn-syntaxin 1A interactions in bovine adrenal chromaffin cells

Tomosyn, a soluble R-SNARE protein identified as a binding partner of the Q-SNARE syntaxin 1A, is thought to be critical in setting the level of fusion-competent SNARE complexes for neurosecretion. To date, there has been no direct evaluation of the dynamics in which tomosyn transits through tomosyn-SNARE complexes or of the extent to which tomosyn-SNARE complexes are regulated by secretory demand. Here, we employed biochemical and optical approaches to characterize the dynamic properties of tomosyn-syntaxin 1A complexes in live adrenal chromaffin cells. We demonstrate that secretagogue stimulation results in the rapid translocation of tomosyn from the cytosol to plasma membrane regions and that this translocation is associated with an increase in the tomosyn-syntaxin 1A interaction, including increased cycling of tomosyn into tomosyn-SNARE complexes. The secretagogue-induced interaction was strongly reduced by pharmacological inhibition of the Rho-associated coiled-coil forming kinase, a result consistent with findings demonstrating secretagogue-induced activation of RhoA. Stimulation of chromaffin cells with lysophosphatidic acid, a nonsecretory stimulus that strongly activates RhoA, resulted in effects on tomosyn similar to that of application of the secretagogue. In PC-12 cells overexpressing tomosyn, secretagogue stimulation in the presence of lysophosphatidic acid resulted in reduced evoked secretory responses, an effect that was eliminated upon inhibition of Rho-associated coiled-coil forming kinase. Moreover, this effect required an intact interaction between tomosyn and syntaxin 1A. Thus, modulation of the tomosyn-syntaxin 1A interaction in response to secretagogue activation is an important mechanism allowing for dynamic regulation of the secretory response.

Munc18c interaction with syntaxin 4 monomers and SNARE complex intermediates in GLUT4 vesicle trafficking

In the process of insulin-stimulated GLUT4 vesicle exocytosis, Munc18c has been proposed to control SNARE complex formation by inactivating syntaxin 4 in a self-associated conformation. Using in vivo fluorescence resonance energy transfer in 3T3L1 adipocytes, co-immunoprecipitation, and in vitro binding assays, we provide data to indicate that Munc18c also associates with nearly equal affinity to a mutant of syntaxin 4 in a constitutively open (unfolded) state (L173A/E174A; LE). To bind to the open conformation of syntaxin 4, we found that Munc18c requires an interaction with the N terminus of syntaxin 4, which resembles Sly1 interaction with the N terminus of ER/Golgi syntaxins. However, both N and C termini of syntaxin 4 are required for Munc18c binding, since a mutation in the syntaxin 4 SNARE domain (I241A) reduces the interaction, irrespective of syntaxin 4 conformation. Using an optical reporter for syntaxin 4-SNARE pairings in vivo, we demonstrate that Munc18c blocks recruitment of SNAP23 to wild type syntaxin 4 yet associates with syntaxin 4LE-SNAP23 Q-SNARE complexes. Fluorescent imaging of GLUT4 vesicles in 3T3L1 adipocytes revealed that syntaxin 4LE expressed with Munc18c bypasses the requirement of insulin for GLUT4 vesicle plasma membrane docking. This effect was attenuated by reducing the Munc18c-syntaxin 4LE interaction with the I241A mutation, indicating that Munc18c facilitates vesicle docking. Therefore, in contradiction to previous models, our data indicates that the conformational "opening" of syntaxin 4 rather than the dissociation of Munc18c is the critical event required for GLUT4 vesicle docking.

Progesterone regulation of catecholamine secretion from chromaffin cells

Stress stimulates the adrenal medulla to rapidly secrete catecholamines (CAs), and the adrenal cortex to release progesterone (PROG), which may locally regulate stress-induced CA release. We used bovine chromaffin cells to investigate the effects of PROG on CA secretion. PROG dose-dependently inhibited CA secretion induced by nicotinic acetylcholine receptor (nAChR) agonist 1,1-dimethyl-4-phenlypiperazinium iodide (DMPP) up to 77%. Pre-incubation with PROG up to 1 h increased this inhibition. 3alpha,5alpha-Tetrahydroprogesterone (3alpha,5alpha-THP) and dexamethasone were less potent inhibitors. Patch-clamp techniques revealed that PROG co-applied with DMPP inhibited peak DMPP-induced current up to 68% and with 3 min pre-incubation inhibited both peak and integrated current up to approximately 95%. Monitoring of FURA-2 showed that PROG similarly inhibited parallel changes in intracellular-free Ca(++) concentration. PROG also inhibited CA secretion elicited by elevated K(+) (38%), and, in single cells, suppressed Ca(++) current evoked by step depolarization, inhibiting amplitude by 15%, and reducing the time constant of current decay during depolarization by 57%. In contrast to the immediate inhibition of nicotinic current, inhibition of Ca(++) current became statistically significant only after 1 min exposure to PROG. PROG did not inhibit secretion stimulated by high Ca(++) perfusion of permeabilized cells. These data suggest that PROG inhibits CA secretion from chromaffin cells predominantly by rapidly inhibiting nAChRs, and by gradually enhancing the inactivation of voltage-dependent Ca(++) channels (VDCCs), but not by affecting secretory processes downstream of Ca(++) influx. This study supports a role for adrenocortical PROG in the regulation of CA secretion during stress.

Taurine replacement attenuates hyperalgesia and abnormal calcium signaling in sensory neurons of STZ-D rats

The etiology of painful diabetic neuropathy is poorly understood, but may result from neuronal hyperexcitability secondary to alterations of Ca2+ signaling in sensory neurons. The naturally occurring amino acid taurine functions as an osmolyte, antioxidant, Ca2+ modulator, inhibitory neurotransmitter, and analgesic such that its depletion in diabetes may predispose one to neuronal hyperexcitability and pain. This study reports the effects of taurine replacement on hyperalgesia and sensory neuron Ca2+ homeostasis in streptozotocin-diabetic (STZ-D) rats. Nondiabetic and STZ-D rats were treated with a 2% taurine-supplemented diet for 6-12 wk. Thermal hyperalgesia and mechanical allodynia were determined by measuring hindpaw withdrawal latency to radiant heat and the withdrawal threshold to the von Frey anesthesiometer. Intracellular Ca2+ signaling was explored in neurons from L4-L6 dorsal root ganglia (DRG), using fura 2 fluorescence. Taurine replacement of diabetic rats attenuated deficits of nerve conduction and prevented reductions of mechanical and thermal withdrawal threshold and latency, respectively. In small DRG sensory neurons from diabetic rats, recovery of intracellular Ca2+ concentration ([Ca2+]i) in response to KCl was slowed and 73% corrected by taurine. The amplitudes of caffeine and ATP-induced [Ca2+]i transients were decreased by 47 and 27% (P < 0.05), respectively, in diabetic rat DRG sensory neurons and corrected by 74 and 93% (P < 0.05), respectively, by taurine replacement. These data indicate that taurine is important in the regulation of neuronal Ca2+ signaling and that taurine deficiency may predispose one to nerve hyperexcitability and pain, complicating diabetes.

Fluorescence Resonance Energy Transfer Reports Properties of Syntaxin1A Interaction with Munc18-1 in Vivo

Syntaxin1A, a neural-specific N-ethylmaleimide-sensitive factor attachment protein receptor protein essential to neurotransmitter release, in isolation forms a closed conformation with an N-terminal alpha-helix bundle folded upon the SNARE motif (H3 domain), thereby limiting interaction of the H3 domain with cognate SNAREs. Munc18-1, a neural-specific member of the Sec1/Munc18 protein family, binds to syntaxin1A, stabilizing this closed conformation. We used fluorescence resonance energy transfer (FRET) to characterize the Munc18-1/syntaxin1A interaction in intact cells. Enhanced cyan fluorescent protein-Munc18-1 and a citrine variant of enhanced yellow fluorescent protein-syntaxin1A, or mutants of these proteins, were expressed as donor and acceptor pairs in human embryonic kidney HEK293-S3 and adrenal chromaffin cells. Apparent FRET efficiency was measured using two independent approaches with complementary results that unambiguously verified FRET and provided a spatial map of FRET efficiency. In addition, enhanced cyan fluorescent protein-Munc18-1 and a citrine variant of enhanced yellow fluorescent protein-syntaxin1A colocalized with a Golgi marker and exhibited FRET at early expression times, whereas a strong plasma membrane colocalization, with similar FRET values, was apparent at later times. Trafficking of syntaxin1A to the plasma membrane was dependent on the presence of Munc18-1. Both syntaxin1A(L165A/E166A), a constitutively open conformation mutant, and syntaxin1A(I233A), an H3 domain point mutant, demonstrated apparent FRET efficiency that was reduced approximately 70% from control. In contrast, the H3 domain mutant syntaxin1A(I209A) had no effect. By using phosphomimetic mutants of Munc18-1, we also established that Ser-313, a Munc18-1 protein kinase C phosphorylation site, and Thr-574, a cyclin-dependent kinase 5 phosphorylation site, regulate Munc18-1/syntaxin1A interaction in HEK293-S3 and chromaffin cells. We conclude that FRET imaging in living cells may allow correlated regulation of Munc18-1/syntaxin1A interactions to Ca(2+)-regulated secretory events.

Ghrelin stimulates neurogenesis in the dorsal motor nucleus of the vagus

Ghrelin, a gastric peptide hormone, has been reported to regulate growth hormone secretion and energy homeostasis. Here we show that ghrelin promotes neural proliferation in vivo and in vitro in the rat dorsal motor nucleus of the vagus (DMNV). Ghrelin receptor mRNA and immunoreactivity were detected in tissues from DMNV. Systemic administration of ghrelin (130 nmol kg(-1)) significantly increased 5-bromo-2'-deoxyuridine (BrdU) incorporation in the DMNV in adult rats with cervical vagotomy (BrdU positive cells; from 27 +/- 4 to 69 +/- 14 n = 5, P < 0.05). In vitro, exposure of cultured DMNV neurones to ghrelin significantly increased the percentage of BrdU incorporation into cells in both dose-dependent (10(-9) -10(-6)m), and time-dependent (6 h to 48 h) manners. Ghrelin significantly increased voltage-activated calcium currents in isolated single DMNV neurones from a mean maximal change of 141 +/- 26 pA to 227 +/- 37 pA. Upon removal of ghrelin, calcium currents slowly returned to baseline. Blocking L-type calcium channels by diltiazem (10 microm) significantly attenuated ghrelin-mediated increments in BrdU incorporation (n = 5, P < 0.05). Ghrelin acts directly on DMNV neurones to stimulate neurogenesis.

Regulation of syntaxin1A-munc18 complex for SNARE pairing in HEK293 cells

The formation and dissolution of SNARE protein complexes is essential for Ca(2+)-triggered fusion of neurotransmitter-filled vesicles at the presynaptic membrane. Among the pre-synaptic SNARE proteins, the activation of the Q-SNARE syntaxin1A is a critical event for SNARE complex formation. Activation requires syntaxin1A to transit from a munc18-bound non-interacting state to one competent for SNARE binding. The molecular mechanisms that regulate this transition remain unclear. The propensity of syntaxin1A to promote voltage-dependent steady-state inactivation of N-type Ca(2+) channels and accelerate their entry into inactivation was used in a heterologous cell expression system to elucidate regulation of syntaxin1A protein-protein interactions. We report that coexpression of munc18 eliminated the promoting effect of syntaxin1A on inactivation. This effect of munc18 was completely disrupted by coexpression of munc13-1, but not munc13-2 or munc13-3. Also, since expression of munc13-1 with syntaxin1A resulted in an inactivation phenotype identical to that of munc18 with syntaxin1A, the action of munc13-1 on the munc18-syntaxin1A complex was functionally unique and did not result from competitive binding interactions. Furthermore, munc13 expressed with syntaxin1A and munc18 promoted redistribution of a cytosolic SNAP25 mutant to the membrane, a result indicative of syntaxin1A-SNAP25 SNARE pairing. These data demonstrate an important role of munc13 to control the protein-protein interactions of syntaxin1A in vivo, and support munc13 as critical to dissociating syntaxin1A-munc18 complexes and making syntaxin1A available for SNARE interactions.

Calcium negatively modulates calmodulin interaction with IQGAP1

IQGAP1 regulates cytoskeletal dynamics through interactions with the Rho family GTPases Rac1 and Cdc42, F-actin, and beta-catenin. Calmodulin interaction with IQ motifs of IQGAP1 negatively influences these IQGAP1 interactions. Although, calmodulin interacts with IQGAP1 in the absence of Ca(2+) and was suggested to exhibit reduced binding when Ca(2+) bound, recent reports show substantially greater binding when Ca(2+) is present. Binding evaluations have primarily relied on IQGAP1 interaction with calmodulin conjugated to Sepharose 4B. In this study we evaluated the Ca(2+)-dependence of calmodulin interaction with native IQGAP1 using a series of independent biochemical approaches. We found the apparent binding of calmodulin to IQGAP1 was Ca(2+)-independent, being between 5- and 20-fold greater in the absence than in the presence of Ca(2+). In addition, calmodulin interaction with IQGAP1 was negatively regulated by buffer [Ca(2+)] (IC(50)=3.4x10(-7)M). Regulation was specific to Ca(2+), as Ba(2+) was approximately 400-fold less effective than Ca(2+) at modulating the interaction. Moreover, testing of calmodulin mutants demonstrated that apocalmodulin tightly binds IQGAP1 and that the N- and C-terminal pair of EF hands are important for Ca(2+) sensitivity. These data indicate that calmodulin may disassemble from IQGAP1 to facilitate IQGAP1 interaction with effectors of cytoskeletal reorganization during conditions of cell activation that promote increased cytosolic [Ca(2+)].

Mechanism of alpha-latrotoxin action at nerve endings of neurohypophysis

The neurotoxin alpha-latrotoxin elicits spontaneous exocytosis of neurotransmitter from neurons and peptide hormones from endocrine cells. While the mechanism of action is not fully understood, both Ca(2+)-dependent and Ca(2+)-independent pathways participate in the facilitation of release, with the relative contribution of the pathways differing among neuronal and endocrine cell types. Here, we investigate the actions of alpha-latrotoxin on neuroendocrine nerve endings that emanate from central nervous system neurons and, therefore, are unique in that they possess properties of central nerve endings and endocrine cells. Using intracellular [Ca(2+)] measurements both calcium-independent receptors for latrotoxin (CIRL or latrophilin) and neurexin 1 alpha receptors were found to be functionally present. Interaction of alpha-latrotoxin with these receptors stimulated secretion of vasopressin and oxytocin neuropeptide. The secretory response was entirely dependent upon toxin-mediated extracellular Ca(2+) influx, although alpha-latrotoxin also consistently triggered mobilization of Ca(2+) from an intracellular store. The mobilization of intracellular Ca(2+) relied on alpha-latrotoxin-mediated Na(+) influx and was blocked by the protonophore FCCP, thereby implicating mitochondria as the Ca(2+) store being mobilized. Using the whole cell recording configuration of the patch clamp, we report that alpha-latrotoxin interaction with the CIRL receptor on these nerve endings resulted in ionic pore formation, generating unitary inward current steps of 20 pA and a channel conductance of approximately 220 pS in Ca(2+)-free saline. Thus, alpha-latrotoxin stimulates Ca(2+)-dependent exocytosis in neurohypophysial nerve endings through receptor interaction and insertion of Ca(2+) permeable membrane pores. While alpha-latrotoxin mobilizes intracellular Ca(2+) stores the elevation in [Ca(2+)] reached is insufficient to trigger measurable exocytosis.

Ca(2+) and frequency dependence of exocytosis in isolated somata of magnocellular supraoptic neurones of the rat hypothalamus

In addition to action potential-evoked exocytotic release at neurohypophysial nerve terminals, the neurohormones arginine vasopressin (aVP) and oxytocin (OT) undergo Ca(2+)-dependent somatodendritic release within the supraoptic and paraventricular hypothalamic nuclei. However, the cellular and molecular mechanisms that underlie this release have not been elucidated. In the present study, the whole-cell patch-clamp technique was utilized in combination with high-time-resolved measurements of membrane capacitance (C(m)) and microfluorometric measurements of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) to examine the Ca(2+) and stimulus dependence of exocytosis in the somata of magnocellular neurosecretory cells (MNCs) isolated from rat supraoptic nucleus (SON). Single depolarizing steps (> or =20 ms) that evoked high-voltage-activated (HVA) Ca(2+) currents (I(Ca)) and elevations in intracellular Ca(2+) concentration were accompanied by an increase in C(m) in a majority (40/47) of SON neurones. The C(m) responses were composed of an initial Ca(2+)-independent, transient component and a subsequent, sustained phase of increased C(m) (termed DeltaC(m)) mediated by an influx of Ca(2+), and increased with corresponding prolongation of depolarizing step durations (20-200 ms). From this relationship we estimated the rate of vesicular release to be 1533 vesicles s(-1). Delivery of neurone-derived action potential waveforms (APWs) as stimulus templates elicited I(Ca) and also induced a DeltaC(m), provided APWs were applied in trains of greater than 13 Hz. A train of APWs modelled after the bursting pattern recorded from an OT-containing neurone during the milk ejection reflex was effective in supporting an exocytotic DeltaC(m) in isolated MNCs, indicating that the somata of SON neurones respond to physiological patterns of neuronal activity with Ca(2+)-dependent exocytotic activity.

Facilitation of Ca(2+)-dependent exocytosis by Rac1-GTPase in bovine chromaffin cells

Rho family GTPases are primary mediators of cytoskeletal reorganization, although they have also been reported to regulate cell secretion. Yet, the extent to which Rho family GTPases are activated by secretory stimuli in neural and neuroendocrine cells remains unknown. In bovine adrenal chromaffin cells, we found Rac1, but not Cdc42, to be rapidly and selectively activated by secretory stimuli using an assay selective for the activated GTPases. To examine effects of activated Rac1 on secretion, constitutively active mutants of Rac1 (Rac1-V12, Rac1-L61) were transiently expressed in adrenal chromaffin cells. These mutants facilitated secretory responses elicited from populations of intact and digitonin-permeabilized cells as well as from cells under whole cell patch clamp. A dominant negative Rac1 mutant (Rac1-N17) produced no effect on secretion. Expression of RhoGDI, a negative regulator of Rac1, inhibited secretory responses while overexpression of effectors of Rac1, notably, p21-activated kinase (Pak1) and actin depolymerization factor (ADF) promoted evoked secretion. In addition, expression of effector domain mutants of Rac1-V12 that exhibit reduced activation of the cytoskeletal regulators Pak1 and Partner of Rac1 (POR1) resulted in a loss of Rac1-V12-mediated enhancement of evoked secretion. These findings suggest that Rac1, in part, functions to modulate secretion through actions on the cytoskeleton. Consistent with this hypothesis, the actin modifying drugs phalloidin and jasplakinolide enhanced secretion, while latrunculin-A inhibited secretion and eliminated the secretory effects of Rac1-V12. In summary, Rac1 was activated by secretory stimuli and modulated the secretory pathway downstream of Ca2+ influx, partly through regulation of cytoskeletal organization.

Synergistic effects of Munc18a and X11 proteins on amyloid precursor protein metabolism

X11 proteins have been shown to modulate metabolism of the amyloid precursor protein (APP) and to reduce the secretion of beta-amyloid peptides (Abeta) that are associated with Alzheimer's disease. Whereas X11alpha interacts with APP via its phosphotyrosine-binding domain, recent reports indicate that additional regulatory interactions involve the N terminus of X11. Here we report that the syntaxin-1a-binding protein Munc18a, which interacts with the Munc18a-interacting domain (MID) at the N terminus of X11, strongly regulates the actions of X11 on APP metabolism. When co-expressed with X11alpha, Munc18a potentiated the retention of APP and suppression of Abeta secretion by X11alpha. As a result, the constitutive release of Abeta40 was nearly abolished. Experiments using N terminus deletion mutants of X11alpha/beta and the MID-deficient X11gamma revealed that the majority of the regulatory effect by Munc18a occurred independent of a direct interaction of Munc18a with X11, although the presence of X11 was required. Munc18a expression induced a small increase in beta-secretase activity, whereas it also intensified the reduction in Abeta40 secretion by X11alpha. These data indicate that Munc18a in concert with X11 acts to suppress gamma-secretase processing. We conclude that Munc18a acts through direct and indirect interactions with X11 proteins and powerfully regulates APP metabolism and Abeta secretion.

A pleckstrin homology domain specific for phosphatidylinositol 4, 5-bisphosphate (PtdIns-4,5-P2) and fused to green fluorescent protein identifies plasma membrane PtdIns-4,5-P2 as being important in exocytosis

Kinetically distinct steps can be distinguished in the secretory response from neuroendocrine cells with slow ATP-dependent priming steps preceding the triggering of exocytosis by Ca(2+). One of these priming steps involves the maintenance of phosphatidylinositol 4, 5-bisphosphate (PtdIns-4,5-P(2)) through lipid kinases and is responsible for at least 70% of the ATP-dependent secretion observed in digitonin-permeabilized chromaffin cells. PtdIns-4,5-P(2) is usually thought to reside on the plasma membrane. However, because phosphatidylinositol 4-kinase is an integral chromaffin granule membrane protein, PtdIns-4,5-P(2) important in exocytosis may reside on the chromaffin granule membrane. In the present study we have investigated the localization of PtdIns-4,5-P(2) that is involved in exocytosis by transiently expressing in chromaffin cells a pleckstrin homology (PH) domain that specifically binds PtdIns-4, 5-P(2) and is fused to green fluorescent protein (GFP). The PH-GFP protein predominantly associated with the plasma membrane in chromaffin cells without any detectable association with chromaffin granules. Rhodamine-neomycin, which also binds to PtdIns-4,5-P(2), showed a similar subcellular localization. The transiently expressed PH-GFP inhibited exocytosis as measured by both biochemical and electrophysiological techniques. The results indicate that the inhibition was at a step after Ca(2+) entry and suggest that plasma membrane PtdIns-4,5-P(2) is important for exocytosis. Expression of PH-GFP also reduced calcium currents, raising the possibility that PtdIns-4,5-P(2) in some manner alters calcium channel function in chromaffin cells.

Calcium-independent receptor for alpha-latrotoxin and neurexin 1alpha [corrected] facilitate toxin-induced channel formation: evidence that channel formation results from tethering of toxin to membrane

alpha-Latrotoxin binding to the calcium-independent receptor for alpha-latrotoxin (CIRL-1), a putative G-protein-coupled receptor, stimulates secretion from chromaffin and PC12 cells. Using patch clamp techniques and microspectrofluorimetry, we demonstrate that the interaction of alpha-latrotoxin with CIRL-1 produces a high conductance channel that permits increases in cytosolic Ca(2+). alpha-Latrotoxin interaction with CIRL-1 transiently expressed in bovine chromaffin cells produced a 400-pS channel, which rarely closed under Ca(2+)-free conditions. The major effect of overexpressing CIRL-1 was to greatly increase the sensitivity of chromaffin cells to channel formation by alpha-latrotoxin. alpha-Latrotoxin interaction with CIRL-1 transiently overexpressed in non-neuronal human embryonic kidney 293 (HEK293) cells produced channels that were nearly identical with those observed in chromaffin cells. Channel currents were reduced by millimolar Ca(2+). At alpha-latrotoxin concentrations below 500 pM, channel formation occurred many seconds after binding of toxin to CIRL-1 indicating distinct steps in channel formation. In all cases there was a rapid, sequential addition of channels once the first channel appeared. An analysis of CIRL-1 mutants indicated that channel formation in HEK293 cells is unlikely to be transduced by a G-protein-dependent mechanism. alpha-Latrotoxin interaction with a fusion construct composed of the extracellular domain of CIRL-1 anchored to the membrane by the transmembrane domain of vesicular stomatitis virus glycoprotein, and with neurexin 1alpha, an alpha-latrotoxin receptor structurally unrelated to CIRL-1, produced channels virtually identical with those observed with wild-type CIRL-1. We propose that alpha-latrotoxin receptors recruit toxin to facilitate its insertion across the membrane and that alpha-latrotoxin itself controls the conductance properties of the channels it produces.

Mitochondria regulate the Ca(2+)-exocytosis relationship of bovine adrenal chromaffin cells

The present study expands the contemporary view of mitochondria as important participants in cellular Ca(2+) dynamics and provides evidence that mitochondria regulate the supply of release-competent secretory granules. Using pharmacological probes to inhibit mitochondrial Ca(2+) import, the ability of mitochondria to modulate secretory activity in single, patch-clamped bovine chromaffin cells was examined by simultaneously monitoring rapid changes in membrane surface area (DeltaC(m)) and cytosolic Ca(2+) levels ([Ca(2+)](c)). Repetitive step depolarizations or action potential waveforms were found to raise the [Ca(2+)](c) of chromaffin cells into the 1 microM to tens of micromolar range. Inhibiting mitochondria by treatment with carbonyl cyanide p-(trifuoro-methoxy)phenylhydrazone, antimycin-oligomycin, or ruthenium red revealed that mitochondria are a prominent component for the clearance of Ca(2+) that entered via voltage-activated Ca(2+) channels. Disruption of cellular Ca(2+) homeostasis by poisoning mitochondria enhanced the secretory responsiveness of chromaffin cells by increasing the amplitude of the transient rise and the time course of recovery to baseline of the evoked Delta[Ca(2+)](c). The enhancement of the secretory response was represented by significant deviation of the Ca(2+)-exocytosis relationship from a standard relationship that equates Ca(2+) influx and DeltaC(m). Thus, mitochondria would play a critical role in the control of secretory activity in chromaffin cells that undergo tonic or repetitive depolarizing activity, likely by limiting the Ca(2+)-dependent activation of specific proteins that recruit or prime secretory granules for exocytosis.

Stimulation of oscillatory uterine contraction by the PCB mixture Aroclor 1242 may involve increased [Ca2+]i through voltage-operated calcium channels

Polychlorinated biphenyls (PCBs) are persistent environmental pollutants associated with spontaneous abortion and shortened gestation length in women and animals. In previous studies, we showed that PCB mixtures and noncoplanar ortho-substituted PCB congeners increased contractions in pregnant rat uterus. In the present study, we hypothesized that the PCB mixture Aroclor 1242 (A1242) stimulates oscillatory uterine contraction in pregnant uterus by increasing intracellular calcium concentration ([Ca2+]i). Pretreatment of uterine strips with ryanodine or thapsigargin, to deplete specific intracellular calcium stores, did not prevent the increased frequency of oscillatory contraction due to 50 microM A1242, whereas thapsigargin effectively blocked carbachol-induced stimulation of uterine contraction. However, 100 microM A1242 was unable to increase contraction in the absence of extracellular calcium or in the presence of the voltage-operated L-type calcium channel blocker nifedipine. A1242 (100 microM) was observed to partially depolarize the cell membrane of myometrial cells from pregnant rats, as measured with a potential-sensitive carbocyanine dye. Changes of [Ca2+]i were monitored in single myometrial cells loaded with the fluorescent calcium-sensitive probe fura-2. Cells exposed to 100 microM A1242 showed a delayed and sustained increase of [Ca2+]i, and this increase was completely blocked in the absence of extracellular calcium or the presence of nifedipine. Therefore, the data suggest that depolarization of the cell membrane by A1242 enabled myometrial cells to increase [Ca2+]i through activation of voltage-operated calcium channels, and the increased [Ca2+]i consequently stimulated contraction of uterine smooth muscle.

Regulation of exocytosis by cyclin-dependent kinase 5 via phosphorylation of Munc18

Munc18a, a mammalian neuronal homologue of Saccharomyces cerevisiae Sec1p protein, is essential for secretion, likely as a result of its high affinity interaction with the target SNARE protein syntaxin 1a (where SNARE is derived from SNAP receptor (the soluble N-ethylmaleimide-sensitive fusion protein)). However, this interaction inhibits vesicle SNARE interactions with syntaxin that are required for secretory vesicles to achieve competency for membrane fusion. As such, regulation of the interaction between Munc18a and syntaxin 1a may provide an important mechanism controlling secretory responsiveness. Cyclin-dependent kinase 5 (Cdk5), a member of the Cdc2 family of cell division kinases, co-purifies with Munc18a from rat brain, interacts directly with Munc18a in vitro, and utilizes Munc18a as a substrate for phosphorylation. We have now demonstrated that Cdk5 is capable of phosphorylating Munc18a in vitro within a preformed Munc18a.syntaxin 1a heterodimer complex and that this results in the disassembly of the complex. Using site-directed mutagenesis, the Cdk5 phosphorylation site on Munc18a was identified as Thr574. Stimulation of secretion from neuroendocrine cells produced a corresponding rapid translocation of cytosolic Cdk5 to a particulate fraction and an increase of Cdk5 kinase activity. Inhibition of Cdk5 with olomoucine decreased evoked norepinephrine secretion from chromaffin cells, an effect not observed with the inactive analogue iso-olomoucine. The effects of olomoucine were independent of calcium influx as evidenced by secretory inhibition in permeabilized chromaffin cells and in cells under whole-cell voltage clamp. Furthermore, transfection and expression in chromaffin cells of a neural specific Cdk5 activator, p25, led to a strong increase in nicotinic agonist-induced secretory responses. Our data suggest a model whereby Cdk5 acts to regulate Munc18a interaction with syntaxin 1a and thereby modulates the level of vesicle SNARE interaction with syntaxin 1a and secretory responsiveness.

Effects of depolarization evoked Na+ influx on intracellular Na+ concentration at neurosecretory nerve endings

Electrophysiological measurements of voltage-dependent Na+ influx using patch-clamp methodology were combined with optical monitoring of the free intracellular Na+ concentration in isolated rat neurohypophysial nerve endings to determine the relationship between Na+ influx generated by repetitive stimulation and change in [Na+]i. Application of step depolarizations under voltage-clamp-evoked tetrodotoxin-sensitive inward currents that were dependent upon extracellular Na+ and that exhibited rapid activation and inactivation properties. These characteristics substantiated the evoked current as a voltage-dependent Na+ current. Application of stimulus trains consisting of step depolarizations that mimick in frequency and duration those of action potentials were found to result in increases in [Na+]i. The induced change in [Na+]i was found to be related to the frequency and period of stimulation. Changes in [Na+]i were greatest at frequencies of 40 Hz and gave maximal changes with 30 s of continuous stimulation of approximately 2.4 mM. Sodium influx expressed as a molar quantity resulted in a nearly directly proportional increase in [Na+]i during the initial period of stimulation at low Na+ loads. When expressed as a charge density (pC/microm2) Na+ influx was found to increase with smaller diameter nerve endings as did the rate of change in [Na+]i in response to applied repetitive step depolarizations. Repetitive step depolarizations which simulate impulse activity that invade neuroendocrine nerve endings in vivo in response to physiological demand for hormone secretion resulted in an increased [Na+]i. It is postulated that this increased [Na+]i may provide a modulatory influence on the secretory response indirectly via alteration of intracellular calcium regulation or, perhaps, via a direct action on the secretory mechanism.

Optical measurement of stimulus-evoked membrane dynamics in single pancreatic acinar cells

Stimulation of pancreatic acinar cells induces the release of digestive enzymes via the exocytotic fusion of zymogen granules and activates postfusion granule membrane retrieval and receptor cycling. In the present study, changes in membrane surface area of rat single pancreatic acinar cells were monitored by cell membrane capacitance (Cm) measurements and by the membrane fluorescent dye FM1-43. When measured with the Cm method, agonist treatment evoked a graded, transient increase in acinar cell surface area averaging 3. 5%. In contrast, a 13% increase in surface area was estimated using FM1-43, corresponding to the fusion of 48 zymogen granules at a rate of 0.5 s-1. After removal of FM1-43 from the surface-accessible membrane, a residual fluorescence signal was shown by confocal microscopy to be localized in endosome-like structures and confined to the apical regions of acinar cells. The development of an optical method for monitoring the membrane turnover of single acinar cells, in combination with measurements of Cm changes, reveals coincidence of exocytotic and endocytotic activity in acinar cells after hormonal stimulation.

A Ca2+-independent receptor for alpha-latrotoxin, CIRL, mediates effects on secretion via multiple mechanisms

alpha-Latrotoxin (alpha-Ltx), a component of black widow spider venom, stimulates secretion from nerve terminals and from PC12 cells. In this study we examine the effects of expression of a newly cloned Ca2+-independent receptor for alpha-Ltx (CIRL) on secretion from bovine chromaffin cells. We first characterized the effect of alpha-Ltx on secretion from untransfected cells. alpha-Ltx, by binding in a Ca2+-independent manner to an endogenous receptor, causes subsequent Ca2+-dependent secretion from intact cells. The stimulation of secretion is correlated with Ca2+ influx caused by the toxin. In permeabilized cells in which the Ca2+ concentration is regulated by buffer, alpha-Ltx also enhances Ca2+-dependent secretion, indicating a direct role of the endogenous receptor in the secretory pathway. Expression of CIRL increased the sensitivity of intact and permeabilized cells to the effects of alpha-Ltx, demonstrating that this protein is functional in coupling to secretion. Importantly, in the absence of alpha-Ltx, the expression of CIRL specifically inhibited the ATP-dependent component of secretion in permeabilized cells without affecting the ATP-independent secretion. This suggests that this receptor modulates the normal function of the regulated secretory pathway and that alpha-Ltx may act by reversing the inhibitory effects of the receptor.

Regulation of Munc-18/syntaxin 1A interaction by cyclin-dependent kinase 5 in nerve endings

The Munc-18-syntaxin 1A complex has been postulated to act as a negative control on the regulated exocytotic process because its formation blocks the interaction of syntaxin with vesicle SNARE proteins. However, the formation of this complex is simultaneously essential for the final stages of secretion as evidenced by the necessity of Munc-18's homologues in Saccharomyces cerevisiae (Sec1p), Drosophila (ROP), and Caenorhabditis elegans (Unc-18) for proper secretion in these organisms. As such, any event that regulates the interaction of these two proteins is important for the control of secretion. One candidate for such regulation is cyclin-dependent kinase 5 (Cdk5), a member of the Cdc2 family of cell division cycle kinases that has recently been copurified with Munc-18 from rat brain. The present study shows that Cdk5 bound to its neural specific activator p35 not only binds to Munc-18 but utilizes it as a substrate for phosphorylation. Furthermore, it is demonstrated that Munc-18 that has been phosphorylated by Cdk5 has a significantly reduced affinity for syntaxin 1A. Finally, it is shown that Cdk5 can also bind to syntaxin 1A and that a complex of Cdk5, p35, Munc-18, and syntaxin 1A can be fashioned in the absence of ATP and promptly disassembled upon the addition of ATP. These results suggest a model in which p35-activated Cdk5 becomes localized to the Munc-18-syntaxin 1A complex by its affinity for both proteins so that it may phosphorylate Munc-18 and thus permit the positive interaction of syntaxin 1A with upstream protein effectors of the secretory mechanism.

Neuropeptide Y2 receptors on nerve endings from the rat neurohypophysis regulate vasopressin and oxytocin release

Neuropeptide Y and peptide YY are important central and peripheral modulators of cardiovascular and neuroendocrine functions, that act through multiple receptor subtypes, Y1 through Y5. A neuropeptide Y-binding site of the Y2 type was characterized by ligand-binding studies in isolated nerve terminals from the rat neurohypophysis. Functionally, neuropeptide Y and peptide YY dose-dependently triggered arginine 8-vasopressin and oxytocin release from perfused isolated terminals, and potentiated the arginine-8-vasopressin release induced by depolarization. Osmotic stimulation by salt loading of rats for two and seven days caused a more than three-fold increase in the neuropeptide Y content of the nerve endings. However, the Y2 receptor expression and arginine-8-vasopressin content declined, showing that the neuropeptide Y system is dynamic and suggesting that it plays a physiological role in salt and water homeostasis. Two sets of observations suggest the arginine-8-vasopressin release by neuropeptide Y may not be explained by neuropeptide Y effects on intracellular Ca2+. First, absence of Ca2+ from the perfusion medium did not affect the arginine-8-vasopressin release, and secondly neuropeptide Y did not change intraterminal Ca2+ concentrations. Pretreatment with pertussis toxin blocked arginine-8-vasopressin secretion by neuropeptide Y, suggesting activation of Gi or Go heterotrimeric G-proteins are required for secretion. It is concluded, that the nerve endings of the neurohypophysis contain a complete neuropeptide Y system with ligand and receptors. Neuropeptide Y may act in an autocrine fashion via activation of Y2 neuropeptide Y receptors to stimulate the release of vasopressin and oxytocin via a Gi/Go dependent secretory mechanism.

Kappa-opioid receptor activation modulates Ca2+ currents and secretion in isolated neuroendocrine nerve terminals

Whole-cell patch-clamp recordings were performed together with time-resolved measurements of membrane capacitance (Cm) in nerve terminals acutely dissociated from neurohypophysis of adult rats to investigate modulation of Ca2+ currents and secretion by activation of opioid receptors. Bath superfusion of the kappa-opioid agonists U69,593 (0.3-1 microM), dynorphin A (1 microM), or U50,488H (1-3 microM) reversibly suppressed the peak amplitude of Ca2+ currents 32. 7 +/- 2.7% (in 41 of 56 terminals), 37.4 +/- 5.3% (in 5 of 8 terminals), and 33.5 +/- 8.1% (in 5 of 10 terminals), respectively. In contrast, tests in 11 terminals revealed no effect of the mu-opioid agonist [D-Pen2,5]-enkephalin (1-3 microM; n = 7) or of the delta-agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (1 microM; n = 4) on Ca2+ currents. Three components of high-threshold current were distinguished on the basis of their sensitivity to blockade by omega-conotoxin GVIA, nicardipine, and omega-conotoxin MVIIC: N-, L-, and P/Q-type current, respectively. Administration of U69,593 inhibited N-type current in these nerve terminals on average 32%, whereas L-type current was reduced 64%, and P/Q-type current was inhibited 28%. Monitoring of changes in Cm in response to brief depolarizing steps revealed that the kappa-opioid-induced reductions in N-, L-, or P/Q-type currents were accompanied by attenuations in two kinetically distinct components of Ca2+-dependent exocytotic release. These data provide strong evidence of a functional linkage between blockade of Ca2+ influx through voltage-dependent Ca2+ channels and inhibitory modulation of release by presynaptic opioid receptors in mammalian central nerve endings.

Role of Q-type Ca2+ channels in vasopressin secretion from neurohypophysial terminals of the rat

1. The nerve endings of rat neurohypophyses were acutely dissociated and a combination of pharmacological, biophysical and biochemical techniques was used to determine which classes of Ca2+ channels on these central nervous system (CNS) terminals contribute functionally to arginine vasopressin (AVP) and oxytocin (OT) secretion. 2. Purified neurohypophysial plasma membranes not only had a single high-affinity binding site for the N-channel-specific omega-conopeptide MVIIA, but also a distinct high-affinity site for another omega-conopeptide (MVIIC), which affects both N- and P/Q-channels. 3. Neurohypophysial terminals exhibited, besides L- and N-type currents, another component of the Ca2+ current that was only blocked by low concentrations of MVIIC or by high concentrations of omega-AgaIVA, a P/Q-channel-selective spider toxin. 4. This Ca2+ current component had pharmacological and biophysical properties similar to those described for the fast-inactivating form of the P/Q-channel class, suggesting that in the neurohypophysial terminals this current is mediated by a 'Q'-type channel. 5. Pharmacological additivity studies showed that this Q-component contributed to rises in intraterminal Ca2+ concentration ([Ca2+]i) in only half of the terminals tested. 6. Furthermore, the non-L- and non-N-component of Ca(2+)-dependent AVP release, but not OT release, was effectively abolished by the same blockers of Q-type current. 7. Thus Q-channels are present on a subset of the neurohypophysial terminals where, in combination with N- and L-channels, they control AVP but not OT peptide neurosecretion.

Regulation of secretory granule recruitment and exocytosis at rat neurohypophysial nerve endings

1. Time-resolved cell membrane capacitance (Cm) measurements were used in combination with fura-2 microfluorometry under whole-cell patch clamp recording to investigate the kinetics and Ca2+ sensitivity of exocytotic granule fusion evoked by depolarizing stimuli at single, isolated nerve endings of the rat neurohypophysis. 2. Single step depolarizations or trains of depolarizing pulses evoked voltage-dependent, inward Ca2+ currents (ICa) and induced both Ca(2+)-dependent and Ca(2+)-independent changes in Cm. Three distinct Cm responses were observed and were differentiated by their kinetics and Ca2+ sensitivity: a non-exocytotic transient (delta Cm,t) and an exocytotic Cm 'jump' (delta Cm,J) and a slower, often latent, exocytotic Cm rise (delta Cm,s) that outlasted the depolarizing pulse stimulus. 3. The delta Cm,t was characterized by a rapid, transient component observed in 70% of nerve endings and a voltage-activation relationship that preceded that of the ICa. The amplitude and kinetics of the delta Cm,t were unaffected by ICa block by Cd2+, Ca2+ load reduction, or alterations in intracellular Ca2+ buffering. 4. In contrast to the delta Cm,t, both the delta Cm,J and delta Cm,s were Ca2+ dependent as evidenced by their sensitivity to Cd2+ block of ICa, intraterminal application of 10 mM BAPTA and reduced [Ca2+]o or replacement of Ca2+ as the charge carrier with Ba2+. 5. The delta Cm,J was proportional to depolarization-evoked Ca2+ influx with initial exocytotic rate of approximately 350 granule fusions s-1. The amplitude of the delta Cm,J rose exponentially (tau = 40 ms) and approached an asymptote (15.5 fF) with longer duration depolarizations indicating the fusion from and depletion of an immediately releasable pool (IRP) estimated at nineteen docked and primed secretory granules. 6. The delta Cm,s was induced by the application of repetitive long duration pulses and defined as the exocytosis of secretory granules from a readily releasable granule pool (RRP). The delta Cm,s response occurred only after exceeding a [Ca2+]i threshold value and rose thereafter in proportion to Ca2+ influx with a mean initial secretory rate of 36 granule fusions s-1. The mean latency for delta Cm,s activation was 850 ms following the initiation of the step depolarizations. The delta Cm,s response magnitude, reflecting the size of the RRP, was dependent on the resting [Ca2+]i and the nerve ending size, and was depletable using repetitive depolarizations of long duration. 7. Recruitment into and release from the RRP and IRP were differentially sensitive to changes in intraterminal Ca2+ buffering conditions. For example, introduction of 5 mM EGTA was shown to have no effect on the evoked IRP but significantly reduced the RRP. In comparison, diminishment of the endogenous Ca2+ buffering capacity of nerve endings by treatment with the mitochondrial Ca2+ uniporter blocker Ruthenium Red (10 microM) potentiated the RRP size but had no significant effect on the IRP size. 8. The present study indicates that the Ca(2+)-dependent recruitment of and release from functionally distinct pools of peptide-containing secretory granules in combination with the [Ca2+]i regulatory properties of neurohypophysial nerve endings may explain both the depletion of peptide release under prolonged stimulus and the potentiation of peptide release observed to occur during recurrent phasic action potential activity in this system.

Arachidonic acid regulation of vasopressin release and intracellular Ca2+ in neurohypophysial nerve endings

The effects of arachidonic acid (AA) and arachidonic acid metabolites on vasopressin secretion and on intracellular free calcium concentration ([Ca2+]i) from both intact and streptolysin-O permeabilized isolated nerve endings of the rat neurohypophysis were studied. Arachidonic acid induced a dose-dependent increase in resting vasopressin (AVP) secretion in both intact and streptolysin-O permeabilized nerve endings. Although AA also dose-dependently induced an increase in [Ca2+]i in intact nerve endings, the AA-induced secretory response was largely independent of an increase in [Ca2+]i. Secretory responses in intact nerve endings showed AA-induced secretion to be sustained and that AA-induced vasopressin secretion occurs via exocytosis. Arachidonic acid also dose-dependently potentiated K+-depolarization evoked vasopressin release. The potentiation of secretion occurred despite an AA-induced reduction in K+-evoked Ca2+ influx. In addition, AA reinitiated secretion following a decline in the Ca2+-dependent exocytotic secretory response suggesting a separate secretory mechanism from Ca2+-induced secretion. Inhibition of the metabolic pathways for AA suggested that AA itself mediates the secretory effects and that AA is likely subject to rapid metabolism by lipoxygenases.

Characterization and distribution of SNARE proteins at neuroendocrine nerve endings

Substantial evidence now exists to support a defined complex of interacting proteins, comprised of soluble, vesicle and plasma membrane components, as the core of a general membrane fusion mechanism. Specializations to the general secretory model occur based on cell-specific differences in Ca2+ regulation, secretory organelle types and secretory dynamics. The variation in secretory properties may also result, in part, from isoform diversity and selective-pairing of the molecular components of the core complex. The present report attempts to identify the SNARE proteins found in isolated peptidergic nerve endings of the rat neurohypophysis. The results demonstrate the presence of synaptosomal-associated protein of 25 kD, syntaxin and synaptobrevin as membrane-associated proteins in these nerve endings. Furthermore, we have utilized sucrose density gradient subcellular fractionation and immunoprecipitation protocols to investigate the synaptobrevin isotypes present on secretory granules and to probe using electrophysiological methods their functional relationship to secretion. Secretory granules were found to contain only the synaptobrevin 2 isoform, although the nerve endings themselves were found to possess in addition, synaptobrevin 1 and the closely related protein cellubrevin. Analysis of the secretory characteristics of single nerve endings using membrane capacitance measurements together with Botulinum B toxin dialysis demonstrated the critical importance of synaptobrevin 2 to both the rapid exocytotic release and a slower secretory process, that perhaps includes secretory granule recruitment and priming, in these peptidergic nerve endings.

An NMDA receptor on isolated secretory nerve endings

While the presence of post-synaptic NMDA receptors in the CNS is well-established, the present study addressed the question of whether NMDA receptors may also be present on secretory nerve endings. Using microspectrofluorometry of fura-2 loaded isolated neurohypophysial nerve endings of the rat, we found that both glutamate (EC50 = 50 microM) and NMDA (EC50 = 30 microM) induced a rapid rise in (Ca2+]i. These responses were glycine-dependent and abolished by 1 mM Mg2+, 1 microM dizocilpine, and removal of extracellular Ca2+. Responses were not significantly affected by treatment with Ca2+ channel blockers or 10 microM CNQX.

Dde chlorinated insecticide 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane (p,p'-DDD) increases intracellular calcium in rat myometrial smooth muscle cells

A previous report from this laboratory showed that two DDT isomers, o,p'-DDT and p,p'-DDD, increased the frequency of spontaneous oscillatory contractions to a similar extent in isolated rat uterine segments. Because regulation of intracellular calcium is fundamental for the development of oscillatory contractions, the present study examined the effects of p,p'-DDD on intracellular free calcium concentration ([Ca2+]i) in individual rat myometrial smooth muscle cells loaded with the fluorescent Ca2+ indicator fura 2. In the presence of extracellular calcium, 50 and 100 microM p,p'-DDD significantly increased peak [Ca2+]i 586 and 921%, respectively, over basal [Ca2+]i. No significant effect was observed with 10 microM p,p'-DDD. In the absence of extracellular calcium, the response to 100 microM p,p'-DDD was significantly attenuated, with cells averaging a 108% increase in peak [Ca2+]i over basal levels, presumably through Ca2+ release from intracellular stores. Nifedipine and cadmium chloride, blockers of voltage-dependent calcium channels, inhibited 100 microM p,p'-DDD-stimulated increases in [Ca2+]i such that peak [Ca2+]i was increased 250% and 259%, respectively. Because of the prominent inhibition observed with the voltage-dependent calcium channel blockers, the effect of p,p'-DDD on membrane depolarization was examined using a cationic fluorescent indicator of membrane potential, [diS-C2(5)]. A concentration of 50 microM p,p'-DDD depolarized the cells by 35% of maximum during treatment with p,p'-DDD. The data demonstrate that p,p'-DDD increased [Ca2+]i in rat myometrial smooth muscle cells in a concentration-related manner, and that this increase was largely dependent on influx of extracellular calcium through dihydropyridine-sensitive, voltage-dependent calcium channels. The data further show that p,p'-DDD depolarized the plasma membrane, providing a possible mechanism for activation of voltage-dependent calcium channels. Additionally, another calcium source, perhaps an intracellular pool, contributes significantly less to the rise of [Ca2+]i. Whether p,p'-DDD initiates the calcium response by direct actions on the plasma membrane or by other means remains to be determined.

Glutamate receptor agonists modulate [Ca2+]i in isolated rat melanotropes

Although glutamate is the predominant excitatory amino acid in the vertebrate central nervous system (CNS) where it affects a variety of physiological processes and pathophysiological states, the role that glutamate receptors may play outside the CNS has not been clearly established. In the present study, the effects of N-methyl-D-aspartate (NMDA), alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA) kainate, and metabotropic glutamate receptor agonists and antagonists were investigated on neuroendocrine melanotropes of the rat pars intermedia using single-cell dual-wavelength microfluorometry and the Ca(2+)-sensitive probe, fura-2, to monitor changes in [Ca2+]i. Glutamate induced a rapid, concentration-dependent rise in [Ca2+]i with an EC50 of 24 microM that was Mg(2+)-sensitive and dependent on the presence of extracellular Ca2+. NMDA increased [Ca2+]i in a glycine-dependent manner with an EC50 of 83 microM that was blocked by 1 microM MK-801 and 1 mM Mg2+. The non-NMDA receptor agonists kainate, AMPA, and quisqualate increased [Ca2+]i with an EC50 of 124, 5 and 8 microM, respectively. Responses to kainic acid were blocked by 10 microM CNQX and were shown to be sensitive to Mg2+ and dihydropyridine. AMPA stimulation was the most potent, and glutamate stimulation was the most efficacious at mediating increases in [Ca2+]i. The metabotropic receptor-specific agonist, trans-ACPD, failed to induce a change in [Ca2+]i. The glutamate-induced Ca2+ influx was about half of that elicited by a 50 mM K(+)-induced membrane depolarization and activation of voltage-sensitive Ca2+ channels. These results demonstrate the presence of glutamate receptors on rat melanotropes and suggest that glutamate receptors in the intermediate lobe of the pituitary may provide the excitatory counterbalance to the well-described secretoinhibiting input via dopamine and gamma-aminobutyric acid receptors.

Thrombin-induced inhibition of potassium currents in human retinal glial (Müller) cells

1. Glial cells are known to play a role in regulating the microenvironment of the nervous system. While earlier considerations of glial function assumed a passive, static physiology for these cells, this is not likely to be the case. In this study, we begin to examine how the physiology of Müller glial cells changes in response to molecules in the microenvironment. 2. Perforated-path recordings and intracellular calcium measurements were performed on human retinal Müller cells in vitro. 3. Analysis of whole-cell currents revealed that the human Müller glial cells have an inwardly rectifying K+ current (IK(IR) which is active near the resting membrane potential. This IK(IR) is significantly inhibited when the Müller cell is exposed to thrombin, a molecule that is likely to enter the retina with a breakdown of the blood-retinal barrier and may be endogenous to the nervous system. 4. A variety of experiments point to a role for Ca2+ as a second messenger mediating the inhibitory effect of thrombin on the IK(IR) of Müller cells. Specifically, thrombin evokes an increase in intracellular [Ca2+] in the Müller cells; the Ca2+ chelator BAPTA blocks the effects of thrombin on both the inhibition of IK(IR) and the rise in intracellular [Ca2+]; exposure to ionomycin, a calcium ionophore, induces a reduction in the IK(IR) of Müller cells. 5. A thrombin- induced inhibition in the IK(IR) of Müller cells is likely to have significant functional consequences for the retina since these ion channels are involved in K+ homeostasis. 6. Our experiments support the idea that the physiology of Müller glial cells is dynamic and can be markedly affected by molecules in the microenvironment.

Lindane inhibition of gap junctional communication in myometrial myocytes is partially dependent on phosphoinositide-generated second messengers

The ability of environmental contaminants to modulate gap junctional communication between uterine smooth muscle cells is generally unknown, despite recognition that myometrial gap junctions may play a role in synchronizing uterine contractions during parturition. The present study tested the hypothesis that the organochlorine pesticide lindane (gamma-hexachlorocyclohexane) inhibits gap junctional communication in myometrial myocytes due to the release of phosphoinositide-dependent second messengers. The effect on gap junctional communication by lindane was tested in cultured rat myometrial smooth muscle cells by monitoring transfer of the fluorescent dye Lucifer yellow. A rapid, concentration-dependent, but reversible inhibition of dye transfer was noted with 4-min exposures, and inhibition was complete with 10 microM lindane. Lindane also stimulated the production of the Ca(2+)-releasing species inositol 1,4,5-trisphosphate which peaked at 5 min (100 pmol/mg protein) and remained elevated after a 15-min exposure. To examine the possible inhibitory role of Ca2+ on gap junctions, the Ca2+ ionophore 4-br-A23187 was used. Although A23187 also inhibited gap junctional communication, inhibition was not complete even at concentrations that appeared cytotoxic (70% inhibition at 2 microM A23187). Cells were then loaded with the Ca2+ chelator BAPTA-AM, which blocked the lindane-induced rise in calcium, and dye transfer experiments with lindane were repeated in Ca(2+)-free medium. Inhibition of dye transfer was still complete under these conditions, showing that increased intracellular calcium was not required for lindane-induced inhibition of gap junctional communication. Subsequently, 10 microM lindane was shown to produce a sustained increase in protein kinase C (PKC) activity (31, 17, and 15 pmol of PKC peptide phosphorylated/min/mg protein for 2-, 5-, and 10-min exposures, respectively). Known activators of PKC, 12-O-tetradecanoylphorbol 13-acetate (TPA) and 1,2-dioctanoyl-sn-glycerol, abolished gap junctional communication at nanomolar concentrations. Although use of the PKC inhibitor staurosporine failed to reverse lindane's inhibitory action, depletion of PKC activity through prolonged exposure to TPA partially reversed lindane's effect. This suggests that PKC activation potentiates but does not solely mediate lindane's inhibitory action on gap junctional communication.