Faster kinetics of quantal catecholamine release in mouse chromaffin cells stimulated with acetylcholine, compared with other secretagogues

TRPC5 controls the adrenaline-mediated counter regulation of hypoglycemia

Hypoglycemia triggers autonomic and endocrine counter-regulatory responses to restore glucose homeostasis, a response that is impaired in patients with diabetes and its long-term complication hypoglycemia-associated autonomic failure (HAAF). We show that insulin-evoked hypoglycemia is severely aggravated in mice lacking the cation channel proteins TRPC1, TRPC4, TRPC5, and TRPC6, which cannot be explained by alterations in glucagon or glucocorticoid action. By using various TRPC compound knockout mouse lines, we pinpointed the failure in sympathetic counter-regulation to the lack of the TRPC5 channel subtype in adrenal chromaffin cells, which prevents proper adrenaline rise in blood plasma. Using electrophysiological analyses, we delineate a previously unknown signaling pathway in which stimulation of PAC1 or muscarinic receptors activates TRPC5 channels in a phospholipase-C-dependent manner to induce sustained adrenaline secretion as a crucial step in the sympathetic counter response to insulin-induced hypoglycemia. By comparing metabolites in the plasma, we identified reduced taurine levels after hypoglycemia induction as a commonality in TRPC5-deficient mice and HAAF patients.

Deletion of the α2δ-1 calcium channel subunit increases excitability of mouse chromaffin cells

High voltage-gated Ca2+ channels (HVCCs) shape the electrical activity and control hormone release in most endocrine cells. HVCCs are multi-subunit protein complexes formed by the pore-forming α1 and the auxiliary β, α2δ and γ subunits. Four genes code for the α2δ isoforms. At the mRNA level, mouse chromaffin cells (MCCs) express predominantly the CACNA2D1 gene coding for the α2δ-1 isoform. Here we show that α2δ-1 deletion led to ∼60% reduced HVCC Ca2+ influx with slower inactivation kinetics. Pharmacological dissection showed that HVCC composition remained similar in α2δ-1-/- MCCs compared to wild-type (WT), demonstrating that α2δ-1 exerts similar functional effects on all HVCC isoforms. Consistent with reduced HVCC Ca2+ influx, α2δ-1-/- MCCs showed reduced spontaneous electrical activity with action potentials (APs) having a shorter half-maximal duration caused by faster rising and decay slopes. However, the induced electrical activity showed opposite effects with α2δ-1-/- MCCs displaying significantly higher AP frequency in the tonic firing mode as well as an increase in the number of cells firing AP bursts compared to WT. This gain-of-function phenotype was caused by reduced functional activation of Ca2+-dependent K+ currents. Additionally, despite the reduced HVCC Ca2+ influx, the intracellular Ca2+ transients and vesicle exocytosis or endocytosis were unaltered in α2δ-1-/- MCCs compared to WT during sustained stimulation. In conclusion, our study shows that α2δ-1 genetic deletion reduces Ca2+ influx in cultured MCCs but leads to a paradoxical increase in catecholamine secretion due to increased excitability. KEY POINTS: Deletion of the α2δ-1 high voltage-gated Ca2+ channel (HVCC) subunit reduces mouse chromaffin cell (MCC) Ca2+ influx by ∼60% but causes a paradoxical increase in induced excitability. MCC intracellular Ca2+ transients are unaffected by the reduced HVCC Ca2+ influx. Deletion of α2δ-1 reduces the immediately releasable pool vesicle exocytosis but has no effect on catecholamine (CA) release in response to sustained stimuli. The increased electrical activity and CA release from MCCs might contribute to the previously reported cardiovascular phenotype of patients carrying α2δ-1 loss-of-function mutations.

A PACAP-activated network for secretion requires coordination of Ca2+ influx and Ca2+ mobilization

Chromaffin cells of the adrenal medulla transduce sympathetic nerve activity into stress hormone secretion. The two neurotransmitters principally responsible for coupling cell stimulation to secretion are acetylcholine and pituitary adenylate activating polypeptide (PACAP). In contrast to acetylcholine, PACAP evokes a persistent secretory response from chromaffin cells. However, the mechanisms by which PACAP acts are poorly understood. Here, it is shown that PACAP induces sustained increases in cytosolic Ca2+ which are disrupted when Ca2+ influx through L-type channels is blocked or internal Ca2+ stores are depleted. PACAP liberates stored Ca2+ via inositol trisphosphate receptors (IP3Rs) on the endoplasmic reticulum (ER), thereby functionally coupling Ca2+ mobilization to Ca2+ influx and supporting Ca2+-induced Ca2+-release. These Ca2+ influx and mobilization pathways are unified by an absolute dependence on phospholipase C epsilon (PLCε) activity. Thus, the persistent secretory response that is a defining feature of PACAP activity, in situ, is regulated by a signaling network that promotes sustained elevations in intracellular Ca2+ through multiple pathways.

PACAP and acetylcholine cause distinct Ca2+ signals and secretory responses in chromaffin cells

The adrenomedullary chromaffin cell transduces chemical messages into outputs that regulate end organ function throughout the periphery. At least two important neurotransmitters are released by innervating preganglionic neurons to stimulate exocytosis in the chromaffin cell-acetylcholine (ACh) and pituitary adenylate cyclase activating polypeptide (PACAP). Although PACAP is widely acknowledged as an important secretagogue in this system, the pathway coupling PACAP stimulation to chromaffin cell secretion is poorly understood. The goal of this study is to address this knowledge gap. Here, it is shown that PACAP activates a Gαs-coupled pathway that must signal through phospholipase C ε (PLCε) to drive Ca2+ entry and exocytosis. PACAP stimulation causes a complex pattern of Ca2+ signals in chromaffin cells, leading to a sustained secretory response that is kinetically distinct from the form stimulated by ACh. Exocytosis caused by PACAP is associated with slower release of peptide cargo than exocytosis stimulated by ACh. Importantly, only the secretory response to PACAP, not ACh, is eliminated in cells lacking PLCε expression. The data show that ACh and PACAP, acting through distinct signaling pathways, enable nuanced and variable secretory outputs from chromaffin cells.

Neurotransmitter Readily Escapes Detection at the Opposing Microelectrode Surface in Typical Amperometric Measurements of Exocytosis at Single Cells

For decades, carbon-fiber microelectrodes have been used in amperometric measurements of neurotransmitter release at a wide variety of cell types, providing a tremendous amount of valuable information on the mechanisms involved in dense-core vesicle fusion. The electroactive molecules that are released can be detected at the opposing microelectrode surface, allowing for precise quantification as well as detailed kinetic information on the stages of neurotransmitter release. However, it remains unclear how much of the catecholamine that is released into the artificial synapse escapes detection. This work examines two separate mechanisms by which released neurotransmitter goes undetected in a typical amperometric measurement. First, diffusional loss is assessed by monitoring exocytosis at single bovine chromaffin cells using carbon-fiber microelectrodes fabricated in a recessed (cavity) geometry. This creates a microsampling vial that minimizes diffusional loss of analyte prior to detection. More molecules were detected per exocytotic release event when using a recessed cavity sensor as compared to the conventional configuration. In addition, pharmacological inhibition of the norepinephrine transporter (NET), which serves to remove catecholamine from the extracellular space, increased both the size and the time course of individual amperometric events. Overall, this study characterizes distinct physical and biological mechanisms by which released neurotransmitter escapes detection at the opposing microelectrode surface, while also revealing an important role for the NET in "presynaptic" modulation of neurotransmitter release.

Acute reversible SERCA blockade facilitates or blocks exocytosis, respectively in mouse or bovine chromaffin cells

Pre-blockade of the sarco-endoplasmic reticulum (ER) calcium ATPase (SERCA) with irreversible thapsigargin depresses exocytosis in adrenal bovine chromaffin cells (BCCs). Distinct expression of voltage-dependent Ca2+-channel subtypes and of the Ca2+-induced Ca2+ release (CICR) mechanism in BCCs versus mouse chromaffin cells (MCCs) has been described. We present a parallel study on the effects of the acute SERCA blockade with reversible cyclopizonic acid (CPA), to repeated pulsing with acetylcholine (ACh) at short (15 s) and long intervals (60 s) at 37 °C, allowing the monitoring of the initial size of a ready-release vesicle pool (RRP) and its depletion and recovery in subsequent stimuli. We found (i) strong depression of exocytosis upon ACh pulsing at 15-s intervals and slower depression at 60-s intervals in both cell types; (ii) facilitation of exocytosis upon acute SERCA inhibition, with back to depression upon CPA washout in MCCs; (iii) blockade of exocytosis upon acute SERCA inhibition and pronounced rebound of exocytosis upon CPA washout in BCCs; (iv) basal [Ca2+]c elevation upon stimulation with ACh at short intervals (but not at long intervals) in both cell types; and (v) augmentation of basal [Ca2+]c and inhibition of peak [Ca2+]c amplitude upon CPA treatment in both cell types, with milder effects upon stimulation at 60-s intervals. These results are compatible with the view that while in MCCs the uptake of Ca2+ via SERCA contributes to the mitigation of physiological ACh triggered secretion, in BCCs the uptake of Ca2+ into the ER facilitates such responses likely potentiating a Ca2+-induced Ca2+ release mechanism. These drastic differences in the regulation of ACh-triggered secretion at 37 °C may help to understand different patterns of the regulation of exocytosis by the circulation of Ca2+ at a functional ER Ca2+ store.

Recent Progress in Quantitatively Monitoring Vesicular Neurotransmitter Release and Storage With Micro/Nanoelectrodes

Exocytosis is one of the essential steps for chemical signal transmission between neurons. In this process, vesicles dock and fuse with the plasma membrane and release the stored neurotransmitters through fusion pores into the extracellular space, and all of these steps are governed with various molecules, such as proteins, ions, and even lipids. Quantitatively monitoring vesicular neurotransmitter release in exocytosis and initial neurotransmitter storage in individual vesicles is significant for the study of chemical signal transmission of the central nervous system (CNS) and neurological diseases. Electrochemistry with micro/nanoelectrodes exhibits great spatial-temporal resolution and high sensitivity. It can be used to examine the exocytotic kinetics from the aspect of neurotransmitters and quantify the neurotransmitter storage in individual vesicles. In this review, we first introduce the recent advances of single-cell amperometry (SCA) and the nanoscale interface between two immiscible electrolyte solutions (nanoITIES), which can monitor the quantity and release the kinetics of electrochemically and non-electrochemically active neurotransmitters, respectively. Then, the development and application of the vesicle impact electrochemical cytometry (VIEC) and intracellular vesicle impact electrochemical cytometry (IVIEC) and their combination with other advanced techniques can further explain the mechanism of neurotransmitter storage in vesicles before exocytosis. It has been proved that these electrochemical techniques have great potential in the field of neuroscience.

Electrochemical quantification of transmitter concentration in single nanoscale vesicles isolated from PC12 cells

We use an electrochemical platform, nanoparticle tracking analysis, and differential centrifugation of single catecholamine vesicles to study the properties of nanometer transmitter vesicles, including the number of molecules, size, and catecholamine concentration inside. Vesicle impact electrochemical cytometry (VIEC) was used to quantify the catecholamine content of single vesicles in different batches isolated from pheochromocytoma (PC12) cells with different ultracentrifugation speeds. We show that, vesicles containing less catecholamine are obtained at subsequent centrifugation steps with higher speed (force). Important to quantification, the cumulative content after subsequent centrifugation steps is equivalent to that of one-step centrifugation at the highest speed, 70 000g. Moreover, as we count molecules in the vesicles, we compared molecular numbers from VIEC, flow VIEC, and intracellular VIEC to corresponding vesicle size measured by nanoparticle tracking analysis to evaluate catecholamine concentration in vesicles. The data suggest that vesicular catecholamine concentration is relatively constant and independent of the vesicular size, indicating vesicular transmitter content as a main factor regulating the vesicle size.

Histamine causes influx via T-type voltage-gated calcium channels in an enterochromaffin tumor cell line: potential therapeutic target in adverse food reactions

The P-STS human ileal neuroendocrine tumor cells, as a model for gut enterochromaffin cells, are strongly and synergistically activated by histamine plus acetylcholine (ACh), presumably via histamine 4 receptors, and weakly activated by histamine alone. Sensing these signals, enterochromaffin cells could participate in intestinal intolerance or allergic reactions to food constituents associated with elevated histamine levels. In this study we aimed to analyze the underlying molecular mechanisms. Inhibition by mepyramine and mibefradil indicated that histamine alone caused a rise in intracellular calcium concentration ([Ca²⁺]_i) via histamine 1 receptors involving T-type voltage-gated calcium channels (VGCCs). Sensitivity to histamine was enhanced by pretreatment with the inflammatory cytokine tumor necrosis factor-α (TNF-α). In accordance with the relief it offers some inflammatory bowel disease patients, otilonium bromide, a gut-impermeable inhibitor of T-type (and L-type) VGCCs and muscarinic ACh receptors, efficiently inhibited the [Ca²⁺]_i responses induced by histamine plus ACh or by histamine alone in P-STS cells. It will take clinical studies to show whether otilonium bromide has promise for the treatment of adverse food reactions. The cells did not react to the nutrient constituents glutamate, capsaicin, cinnamaldehyde, or amylase-trypsin inhibitors and the transient receptor potential channel vanilloid 4 agonist GSK-1016790A. The bacterial product butyrate evoked a rise in [Ca²⁺]_i only when added together with ACh. Lipopolysaccharide had no effect on [Ca²⁺]_i despite the presence of Toll-like receptor 4 protein. Our results indicate that inflammatory conditions with elevated levels of TNF-α might enhance histamine-induced serotonin release from intestinal neuroendocrine cells. NEW & NOTEWORTHY We show that histamine synergistically enhances the intracellular calcium response to the physiological agonist acetylcholine in human ileal enterochromaffin tumor cells. This synergistic activation and cell activation by histamine alone largely depend on T-type voltage-gated calcium channels and are inhibited by the antispasmodic otilonium bromide. The cells showed no response to wheat amylase-trypsin inhibitors, suggesting that enterochromaffin cells are not directly involved in nongluten wheat sensitivity.

Altered exocytosis in chromaffin cells from mouse models of neurodegenerative diseases

Chromaffin cells from the adrenal gland (CCs) have extensively been used to explore the molecular structure and function of the exocytotic machinery, neurotransmitter release and synaptic transmission. The CC is integrated in the sympathoadrenal axis that helps the body maintain homoeostasis during both routine life and in acute stress conditions. This function is exquisitely controlled by the cerebral cortex and the hypothalamus. We propose the hypothesis that damage undergone by the brain during neurodegenerative diseases is also affecting the neurosecretory function of adrenal medullary CCs. In this context, we review here the following themes: (i) How the discharge of catecholamines is centrally and peripherally regulated at the sympathoadrenal axis; (ii) which are the intricacies of the amperometric techniques used to study the quantal release of single-vesicle exocytotic events; (iii) which are the alterations of the exocytotic fusion pore so far reported, in CCs of mouse models of neurodegenerative diseases; (iv) how some proteins linked to neurodegenerative pathologies affect the kinetics of exocytotic events; (v) finally, we try to integrate available data into a hypothesis to explain how the centrally originated neurodegenerative diseases may alter the kinetics of single-vesicle exocytotic events in peripheral adrenal medullary CCs.

Nanopore Opening at Flat and Nanotip Conical Electrodes during Vesicle Impact Electrochemical Cytometry

The oxidation of catecholamine at a microelectrode, following its release from individual vesicles, allows interrogation of the content of single nanometer vesicles with vesicle impact electrochemical cytometry (VIEC). Previous to this development, there were no methods available to quantify the chemical load of single vesicles. However, accurate quantification of the content is hampered by uncertainty in the proportion of substituent molecules reaching the electrode surface (collection efficiency). In this work, we use quantitative modeling to calculate this collection efficiency. For all vesicles except those at the very edge of the electrode, modeling shows that ∼100% oxidation efficiency is achieved when employing a 33 μm diameter disk microelectrode for VIEC, independent of the location of the vesicle release pore. We use this to experimentally determine a precise distribution of catecholamine in individual vesicles extracted from PC12 cells. In contrast, we calculate that when a nanotip conical electrode (∼4 μm length, ∼1.5 μm diameter at the base) is employed, as in intracellular VIEC (IVIEC), the current-time response depends strongly on the position of the catecholamine-releasing pore in the vesicle membrane. When vesicle release occurs with the pore opening occurring far from the electrode, lower currents and partial oxidation (∼75%) of the catecholamine are predicted, as compared to higher currents and ∼100% oxidation, when the pore is close to/at the electrode surface. As close agreement is observed between the experimentally measured vesicular content in intracellular and extracted vesicles from the same cell line using nanotip and disk electrodes, respectively, we conclude that pores open at the electrode surface. Not only does this suggest that electroporation of the vesicle membrane is the primary driving force for catecholamine release from vesicles at polarized electrodes, but it also indicates that IVIEC with nanotip electrodes can directly assess vesicular content without correction.

Mechanistic Aspects of Vesicle Opening during Analysis with Vesicle Impact Electrochemical Cytometry

Vesicle impact electrochemical cytometry (VIEC) has been used to quantify the vesicular transmitter content in mammalian vesicles. In the present study, we studied the mechanism of VIEC by quantifying the catecholamine content in single vesicles isolated from pheochromocytoma (PC12) cells. These vesicles contain about one tenth of the catecholamine compared with adrenal chromaffin vesicles. The existence of a prespike foot for many events suggests the formation of an initial transiently stable pore at the beginning of vesicle rupture. Increasing the detection temperature from 6 to 30 °C increases the possibility of vesicle rupture on the electrode, implying that there is a temperature-dependent process that facilitates electroporation. Natively larger vesicles are shown to rupture earlier and more frequently than smaller ones in VIEC. Likewise, manipulating vesicle content and size with drugs leads to similar trends. These data support the hypothesis that electroporation is the primary force for pore opening in VIEC. We further hypothesize that a critical step for initiating vesicle opening by electroporation is diffusion of membrane proteins away from the membrane region of contact with the electrode to allow closer contact, increasing the lateral potential field and thus facilitating electroporation.

The quantal catecholamine release from mouse chromaffin cells challenged with repeated ACh pulses is regulated by the mitochondrial Na+ /Ca2+ exchanger

KEY POINTS

Upon repeated application of short ACh pulses to C57BL6J mouse chromaffin cells, the amperometrically monitored secretory responses promptly decayed to a steady-state level of around 25% of the initial response. A subsequent K⁺ pulse, however, overcame such decay. These data suggest that mouse chromaffin cells have a ready release-vesicle pool that is selectively recruited by the physiological neurotransmitter ACh. The ACh-sensitive vesicle pool is refilled and maintained by the rate of Ca²⁺ delivery from mitochondria to the cytosol, through the mitochondrial Na⁺ /Ca²⁺ exchanger (mNCX). ITH12662, a novel blocker of the mNCX, prevented the decay of secretion elicited by ACh pulses and delayed the rate of [Ca²⁺ ]_c clearance. This regulatory pathway may be physiologically relevant in situations of prolonged stressful conflicts where a sustained catecholamine release is regulated by mitochondrial Ca²⁺ circulation through the mNCX, which couples respiration and ATP synthesis to long-term stimulation of chromaffin cells by endogenously released ACh.

ABSTRACT

Using caged-Ca²⁺ photorelease or paired depolarising pulses in voltage-clamped chromaffin cells (CCs), various pools of secretory vesicles with different readiness to undergo exocytosis have been identified. Whether these pools are present in unclamped CCs challenged with ACh, the physiological neurotransmitter at the splanchnic nerve-CC synapse, is unknown. We have explored here whether an ACh-sensitive ready-release vesicle pool (ASP) is present in C57BL6J mouse chromaffin cells (MCCs). Single cells were fast perfused with a Tyrode solution at 37°C, and challenged with 12 sequential ACh pulses (100 μm, 2 s, every 30 s) plus a K⁺ pulse given at the end (75 mm K⁺ ). After the first 2-3 ACh pulses the amperometrically monitored secretory responses promptly decayed to a steady-state level of around 25% of the initial response. The last K⁺ pulse, however, overcame such decay. Repeated ACh pulses to voltage-clamped cells elicited non-desensitising nicotinic currents. Also, the [Ca²⁺ ]_c transients elicited by repeated ACh pulses that were superimposed on a stable baseline elevation did not undergo decay. The novel blocker of the mitochondrial Na⁺ /Ca²⁺ exchanger (mNCX) ITH12662 prevented the decay of secretion elicited by ACh pulses and delayed the rate of [Ca²⁺ ]_c clearance. The experiments are compatible with the idea that C57BL6J MCCs have an ASP vesicle pool that is selectively recruited by the physiological neurotransmitter ACh and is regulated by the rate of Ca²⁺ delivery from mitochondria to the cytosol, through the mNCX.