Neurosecretion: what can we learn from chromaffin cells

Cab45G trafficking through the insulin secretory pathway is altered in human type 2 diabetes

In type 2 diabetes (T2D), the rate of insulin secretory granule biogenesis can limit insulin secretion from pancreatic β-cells. Using rat insulinoma INS1 β-cells, we show that the soluble Ca2+-binding/trafficking protein, Cab45G, serves as a non-essential chaperone for insulin granule biogenesis. In β-cells, Cab45G is stored within a cis-Golgi reservoir. Cab45G deletion dysregulates Ca2+ homeostasis and leads to secretory abnormality, but insulin granule biogenesis remains intact. Increasing Cab45G biosynthesis leads to anterograde trafficking into insulin granules, stimulating their production. Using human donor islets, we identify increased anterograde Cab45G trafficking in obese humans with and without T2D, consistent with the heightened demand for granule biogenesis. However, humans with T2D demonstrate decreased Golgi Cab45G localization and increased granule Cab45G localization compared to those without T2D. Our study provides the first insight into Cab45G function in specialized secretory cells and opens avenues of investigation into mechanisms associated with β-cell compensation and failure.

AMPK inhibits voltage-gated calcium channel-current in rat chromaffin cells

Metabolic changes are critical in the regulation of Ca2+ influx in central and peripheral neuroendocrine cells. To study the regulation of L-type Ca2+ channels by AMPK we used biochemical reagents and ATP/glucose-concentration manipulations in rat chromaffin cells. AICAR and Compound-C, at low concentration, significantly induce changes in L-type Ca2+ channel-current amplitude and voltage dependence. Remarkably, an overlasting decrease in the channel-current density can be induced by lowering the intracellular level of ATP. Accordingly, Ca2+ channel-current density gradually diminishes by decreasing the extracellular glucose concentration. By using immunofluorescence, a decrease in the expression of CaV1.2 is observed while decreasing extracellular glucose, suggesting that AMPK reduces the number of functional Ca2+ channels into the plasma membrane. Together, these results support for the first time the dependence of metabolic changes in the maintenance of Ca2+ channel-current by AMPK. They reveal a key step in Ca2+ influx in secretory cells.

The Role of Nicotinic Receptors on Ca2+ Signaling in Bovine Chromaffin Cells

Chromaffin cells have been used as a physiological model to understand neurosecretion in mammals for many years. Nicotinic receptors located in the cells' membrane are stimulated by acetylcholine, and they participate in the exocytosis of chromaffin granules, releasing catecholamines in response to stress. In this work, we discuss how the participation of nicotinic receptors and the localization of active zones in the borders of the cytoskeleton can generate local calcium signals leading to secretion. We use a computational model of a cytoskeleton cage to simulate Ca2+ levels in response to voltage and acetylcholine pulses. We find that nicotinic receptors are able to enhance the differences between local and average calcium values, as well as the heterogeneous distributions around the active zones, producing a non-linear, highly localized Ca2+ entry that, although consisting of a few ions, is able to improve secretion responses in chromaffin cells. Our findings emphasize the intricate interplay among nicotinic receptors, the cytoskeleton, and active zones within chromaffin cells as an example of Ca2+-dependent neurosecretion in mammals.

Forty years of the adrenal chromaffin cell through ISCCB meetings around the world

This historical review focuses on the evolution of the knowledge accumulated during the last two centuries on the biology of the adrenal medulla gland and its chromaffin cells (CCs). The review emerged in the context of a series of meetings that started on the Spanish island of Ibiza in 1982 with the name of the International Symposium on Chromaffin Cell Biology (ISCCB). Hence, the review is divided into two periods namely, before 1982 and from this year to 2022, when the 21st ISCCB meeting was just held in Hamburg, Germany. The first historical period extends back to 1852 when Albert Kölliker first described the fine structure and function of the adrenal medulla. Subsequently, the adrenal staining with chromate salts identified the CCs; this was followed by the establishment of the embryological origin of the adrenal medulla, and the identification of adrenaline-storing vesicles. By the end of the nineteenth century, the basic morphology, histochemistry, and embryology of the adrenal gland were known. The twentieth century began with breakthrough findings namely, the experiment of Elliott suggesting that adrenaline was the sympathetic neurotransmitter, the isolation of pure adrenaline, and the deciphering of its molecular structure and chemical synthesis in the laboratory. In the 1950s, Blaschko isolated the catecholamine-storing vesicles from adrenal medullary extracts. This switched the interest in CCs as models of sympathetic neurons with an explosion of studies concerning their functions, i.e., uptake of catecholamines by chromaffin vesicles through a specific coupled transport system; the identification of several vesicle components in addition to catecholamines including chromogranins, ATP, opioids, and other neuropeptides; the calcium-dependence of the release of catecholamines; the underlying mechanism of exocytosis of this release, as indicated by the co-release of proteins; the cross-talk between the adrenal cortex and the medulla; and the emission of neurite-like processes by CCs in culture, among other numerous findings. The 1980s began with the introduction of new high-resolution techniques such as patch-clamp, calcium probes, marine toxins-targeting ion channels and receptors, confocal microscopy, or amperometry. In this frame of technological advances at the Ibiza ISCCB meeting in 1982, 11 senior researchers in the field predicted a notable increase in our knowledge in the field of CCs and the adrenal medulla; this cumulative knowledge that occurred in the last 40 years of history of the CC is succinctly described in the second part of this historical review. It deals with cell excitability, ion channel currents, the exocytotic fusion pore, the handling of calcium ions by CCs, the kinetics of exocytosis and endocytosis, the exocytotic machinery, and the life cycle of secretory vesicles. These concepts together with studies on the dynamics of membrane fusion with super-resolution imaging techniques at the single-protein level were extensively reviewed by top scientists in the field at the 21st ISCCB meeting in Hamburg in the summer of 2022; this frontier topic is also briefly reviewed here. Many of the concepts arising from those studies contributed to our present understanding of synaptic transmission. This has been studied in physiological or pathophysiological conditions, in CCs from animal disease models. In conclusion, the lessons we have learned from CC biology as a peripheral model for brain and brain disease pertain more than ever to cutting-edge research in neurobiology. In the 22nd ISCCB meeting in Israel in 2024 that Uri Asheri is organizing, we will have the opportunity of seeing the progress of the questions posed in Ibiza, and on other questions that undoubtedly will arise.

Lytic granule exocytosis at immune synapses: lessons from neuronal synapses

Regulated exocytosis is a central mechanism of cellular communication. It is not only the basis for neurotransmission and hormone release, but also plays an important role in the immune system for the release of cytokines and cytotoxic molecules. In cytotoxic T lymphocytes (CTLs), the formation of the immunological synapse is required for the delivery of the cytotoxic substances such as granzymes and perforin, which are stored in lytic granules and released via exocytosis. The molecular mechanisms of their fusion with the plasma membrane are only partially understood. In this review, we discuss the molecular players involved in the regulated exocytosis of CTL, highlighting the parallels and differences to neuronal synaptic transmission. Additionally, we examine the strengths and weaknesses of both systems to study exocytosis.

The neurosciences at the Max Planck Institute for Biophysical Chemistry in Göttingen

The Max Planck Institute (MPI) for Biophysical Chemistry (Karl-Friedrich Bonhoeffer Institute) was founded in 1971 in Göttingen. Two of the 11 departments at the institute had a neuroscientific focus. Otto D. Creutzfeldt (1927-1992) and Victor P. Whittaker (1919-2016) were directors of the Neurobiological and Neurochemical Departments, respectively. Creutzfeldt's department researched the structure and function of the cerebral cortex, and Whittaker's department concentrated on the biochemical analysis of synapses and synaptic vesicles. Creutzfeldt and Whittaker were already internationally respected scientists when they were appointed to Göttingen. The next generation of departmental directors, Erwin Neher and Bert Sakmann, were "home-grown" researchers from the institute and, during their time as junior group leaders, they developed the so-called patch clamp technique, with which they were able to measure single ion channels in nerve cells. This technique revolutionized neurophysiology, and Neher and Sakmann were awarded the 1991 Nobel Prize in Physiology or Medicine for their work in this area. Neher was appointed director of the Membrane Biophysics Department in 1983 and, since then, his department has mainly examined the role of Ca²⁺ in the release of neurotransmitters at synapses and in the secretion of catecholamines from chromaffin cells. From 1985, Sakmann was director of the Cell Physiology Department, and his laboratory concentrated on the molecular and physiological characterization of transmitter receptors in postsynaptic membranes. In 1989, he was appointed to the MPI for Medical Research in Heidelberg. Reinhard Jahn became director of the Neurobiology Department in 1997, researching the molecular mechanisms of the release of neurotransmitters from the presynaptic terminals, and he discovered several proteins associated with the synaptic vesicles. With their work, Neher, Sakmann, and Jahn have made the MPI for Biophysical Chemistry one of the world's leading research centers for the transmission of signals at synapses.

Methodologies for Detecting Quantal Exocytosis in Adrenal Chromaffin Cells Through Diamond-Based MEAs

Diamond-based multiarray sensors are suitable to detect in real-time exocytosis and action potentials from cultured, spontaneously firing chromaffin cells, primary hippocampal neurons, and midbrain dopaminergic neurons. Here, we focus on how amperometric measurements of catecholamine release are performed on micrographitic diamond multiarrays (μG-D-MEAs) with high temporal and spatial resolution by 16 electrodes simultaneously.

Munc13 supports fusogenicity of non-docked vesicles at synapses with disrupted active zones

Active zones consist of protein scaffolds that are tightly attached to the presynaptic plasma membrane. They dock and prime synaptic vesicles, couple them to voltage-gated Ca²⁺ channels, and direct neurotransmitter release toward postsynaptic receptor domains. Simultaneous RIM + ELKS ablation disrupts these scaffolds, abolishes vesicle docking, and removes active zone-targeted Munc13, but some vesicles remain releasable. To assess whether this enduring vesicular fusogenicity is mediated by non-active zone-anchored Munc13 or is Munc13-independent, we ablated Munc13-1 and Munc13-2 in addition to RIM + ELKS in mouse hippocampal neurons. The hextuple knockout synapses lacked docked vesicles, but other ultrastructural features were near-normal despite the strong genetic manipulation. Removing Munc13 in addition to RIM + ELKS impaired action potential-evoked vesicle fusion more strongly than RIM + ELKS knockout by further decreasing the releasable vesicle pool. Hence, Munc13 can support some fusogenicity without RIM and ELKS, and presynaptic recruitment of Munc13, even without active zone anchoring, suffices to generate some fusion-competent vesicles.

Mechanism and function of phasic firing in vasopressin-releasing magnocellular neurosecretory cells

Magnocellular neurosecretory cells that release vasopressin (MNC^VP ) from axon terminals in the neurohypophysis display a unique pattern of action potential firing termed phasic firing. Under basal conditions, only a small proportion of MNC^VP display spontaneous phasic firing. However, acute and chronic conditions that stimulate vasopressin release, such as hemorrhage and dehydration, greatly enhance the number of MNC^VP that fire phasically. Phasic firing optimizes VP neurosecretion at axon terminals by allowing action potential broadening to promote calcium-dependent frequency-facilitation, at the same time as preventing the secretory fatigue caused by spike inactivation that occurs during prolonged continuous stimulation. This review provides an update on our mechanistic understanding of these processes and highlights important gaps in our knowledge that must be addressed in future experiments.

Glucagon-like peptide-1 receptor controls exocytosis in chromaffin cells by increasing full-fusion events

Agonists for glucagon-like-peptide-1 receptor (GLP-1R) are currently used for the treatment of type 2 diabetes and obesity. Their benefits have been centered on pancreas and hypothalamus, but their roles in other organ systems are not well understood. We studied the action of GLP-1R on secretions of adrenal medulla. Exendin-4, a synthetic analog of GLP-1, increases the synthesis and the release of catecholamines (CAs) by increasing cyclic AMP (cAMP) production, without apparent participation of cAMP-regulated guanine nucleotide exchange factor (Epac). Exendin-4, when incubated for 24 h, increases CA synthesis by promoting the activation of tyrosine hydroxylase. Short incubation (20 min) increases the quantum size of exocytotic events by switching exocytosis from partial to full fusion. Our results give a strong support to the role of GLP-1 in the fine control of exocytosis.

Spatial and Temporal Aspects of Exocytosis Studied on the Isolated Plasma Membranes

Exocytosis of large-dense core vesicles in neuroendocrine cells is a highly regulated, calcium-dependent process, mediated by networks of interrelated proteins and lipids. Here, I describe experimental procedures for studies of selective spatial and temporal aspects of exocytosis at the plasma membrane, or in its proximity, using adrenal chromaffin cells. The assay utilizes primary cells subjected to a brief ultrasonic pulse, resulting in the formation of thin, flat inside-out plasma membranes with attached secretory vesicles and elements of cell cytoskeleton. In this model, secretion of plasma membrane-attached secretory vesicles was found to be dependent on calcium and sensitive to clostridial neurotoxins. Depending on the probe selected for secretory vesicle cargo, protein, and/or lipid detection, this simple assay is versatile, fast and inexpensive, and offers excellent spatial resolution.

Synaptotagmin-7 places dense-core vesicles at the cell membrane to promote Munc13-2- and Ca2+-dependent priming

Synaptotagmins confer calcium-dependence to the exocytosis of secretory vesicles, but how coexpressed synaptotagmins interact remains unclear. We find that synaptotagmin-1 and synaptotagmin-7 when present alone act as standalone fast and slow Ca²⁺-sensors for vesicle fusion in mouse chromaffin cells. When present together, synaptotagmin-1 and synaptotagmin-7 are found in largely non-overlapping clusters on dense-core vesicles. Synaptotagmin-7 stimulates Ca²⁺-dependent vesicle priming and inhibits depriming, and it promotes ubMunc13-2- and phorbolester-dependent priming, especially at low resting calcium concentrations. The priming effect of synaptotagmin-7 increases the number of vesicles fusing via synaptotagmin-1, while negatively affecting their fusion speed, indicating both synergistic and competitive interactions between synaptotagmins. Synaptotagmin-7 places vesicles in close membrane apposition (<6 nm); without it, vesicles accumulate out of reach of the fusion complex (20-40 nm). We suggest that a synaptotagmin-7-dependent movement toward the membrane is involved in Munc13-2/phorbolester/Ca²⁺-dependent priming as a prelude to fast and slow exocytosis triggering.

Measurements of Exocytosis by Capacitance Recordings and Calcium Uncaging in Mouse Adrenal Chromaffin Cells

Fusion of vesicles with the plasma membrane and liberation of their contents is a multistep process involving several proteins. Correctly assigning the role of specific proteins and reactions in this cascade requires a measurement method with high temporal resolution. Patch-clamp recordings of cell membrane capacitance in combination with calcium measurements, calcium uncaging, and carbon-fiber amperometry allow for the accurate determination of vesicle pool sizes, their fusion kinetics, and their secreted oxidizable content. Here, we will describe this method in a model system for neurosecretion, the adrenal chromaffin cells, which secrete adrenaline.

The Decade of Super-Resolution Microscopy of the Presynapse

The presynaptic compartment of the chemical synapse is a small, yet extremely complex structure. Considering its size, most methods of optical microscopy are not able to resolve its nanoarchitecture and dynamics. Thus, its ultrastructure could only be studied by electron microscopy. In the last decade, new methods of optical superresolution microscopy have emerged allowing the study of cellular structures and processes at the nanometer scale. While this is a welcome addition to the experimental arsenal, it has necessitated careful analysis and interpretation to ensure the data obtained remains artifact-free. In this article we review the application of nanoscopic techniques to the study of the synapse and the progress made over the last decade with a particular focus on the presynapse. We find to our surprise that progress has been limited, calling for imaging techniques and probes that allow dense labeling, multiplexing, longer imaging times, higher temporal resolution, while at least maintaining the spatial resolution achieved thus far.

Ca2+ influx and clearance at hyperpolarized membrane potentials modulate spontaneous and stimulated exocytosis in neuroendocrine cells

Neuroendocrine adrenal chromaffin cells release neurohormones catecholamines in response to Ca²⁺ entry via voltage-gated Ca²⁺ channels (VGCCs). Adrenal chromaffin cells also express non-voltage-gated channels, which may conduct Ca²⁺ at negative membrane potentials, whose role in regulation of exocytosis is poorly understood. We explored how modulation of Ca²⁺ influx at negative membrane potentials affects basal cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and exocytosis in metabolically intact voltage-clamped bovine adrenal chromaffin cells. We found that in these cells, Ca²⁺ entry at negative membrane potentials is balanced by Ca²⁺ extrusion by the Na⁺/Ca²⁺ exchanger and that this balance can be altered by membrane hyperpolarization or stimulation with an inflammatory hormone bradykinin. Membrane hyperpolarization or application of bradykinin augmented Ca²⁺-carrying current at negative membrane potentials, elevated basal [Ca²⁺]_i, and facilitated synchronous exocytosis evoked by the small amounts of Ca²⁺ injected into the cell via VGCCs (up to 20 pC). Exocytotic responses evoked by the injections of the larger amounts of Ca²⁺ via VGCCs (> 20 pC) were suppressed by preceding hyperpolarization. In the absence of Ca²⁺ entry via VGCCs and Ca²⁺ extrusion via the Na⁺/Ca²⁺ exchanger, membrane hyperpolarization induced a significant elevation in [Ca²⁺]_i and asynchronous exocytosis. Our results indicate that physiological interferences, such as membrane hyperpolarization and/or activation of non-voltage-gated Ca²⁺ channels, modulate basal [Ca²⁺]_i and, consequently, segregation of exocytotic vesicles and their readiness to be released spontaneously and in response to Ca²⁺ entry via VGCCs. These mechanisms may play role in homeostatic plasticity of neuronal and endocrine cells.

Endophilin-A coordinates priming and fusion of neurosecretory vesicles via intersectin

Endophilins-A are conserved endocytic adaptors with membrane curvature-sensing and -inducing properties. We show here that, independently of their role in endocytosis, endophilin-A1 and endophilin-A2 regulate exocytosis of neurosecretory vesicles. The number and distribution of neurosecretory vesicles were not changed in chromaffin cells lacking endophilin-A, yet fast capacitance and amperometry measurements revealed reduced exocytosis, smaller vesicle pools and altered fusion kinetics. The levels and distributions of the main exocytic and endocytic factors were unchanged, and slow compensatory endocytosis was not robustly affected. Endophilin-A’s role in exocytosis is mediated through its SH3-domain, specifically via a direct interaction with intersectin-1, a coordinator of exocytic and endocytic traffic. Endophilin-A not able to bind intersectin-1, and intersectin-1 not able to bind endophilin-A, resulted in similar exocytic defects in chromaffin cells. Altogether, we report that two endocytic proteins, endophilin-A and intersectin-1, are enriched on neurosecretory vesicles and regulate exocytosis by coordinating neurosecretory vesicle priming and fusion.

Endophilins-A are conserved membrane-associated proteins required for endocytosis. Here, the authors report that endophilins-A also promote exocytosis of neurosecretory vesicles by coordinating priming and fusion through intersectin-1, independently of their roles in different types of endocytosis.

Phospholipases in neuronal function: A role in learning and memory?

Despite the human brain being made of nearly 60% fat, the vast majority of studies on the mechanisms of neuronal communication which underpin cognition, memory and learning, primarily focus on proteins and/or (epi)genetic mechanisms. Phospholipids are the main component of all cellular membranes and function as substrates for numerous phospholipid-modifying enzymes, including phospholipases, which release free fatty acids (FFAs) and other lipid metabolites that can alter the intrinsic properties of the membranes, recruit and activate critical proteins, and act as lipid signalling molecules. Here, we will review brain specific phospholipases, their roles in membrane remodelling, neuronal function, learning and memory, as well as their disease implications. In particular, we will highlight key roles of unsaturated FFAs, particularly arachidonic acid, in neurotransmitter release, neuroinflammation and memory. In light of recent findings, we will also discuss the emerging role of phospholipase A₁ and the creation of saturated FFAs in the brain.

SNAP-25 phosphorylation at Ser187 is not involved in Ca2+ or phorbolester-dependent potentiation of synaptic release

SNAP-25, one of the three SNARE-proteins responsible for synaptic release, can be phosphorylated by Protein Kinase C on Ser-187, close to the fusion pore. In neuroendocrine cells, this phosphorylation event potentiates vesicle recruitment into releasable pools, whereas the consequences of phosphorylation for synaptic release remain unclear. We mutated Ser-187 and expressed two mutants (S187C and S187E) in the context of the SNAP-25B-isoform in SNAP-25 knockout glutamatergic autaptic neurons. Whole-cell patch clamp recordings were performed to assess the effect of Ser-187 phosphorylation on synaptic transmission. Blocking phosphorylation by expressing the S187C mutant did not affect synapse density, basic evoked or spontaneous neurotransmission, the readily-releasable pool size or its Ca²⁺-independent or Ca²⁺-dependent replenishment. Furthermore, it did not affect the response to phorbol esters, which activate PKC. Expressing S187C in the context of the SNAP-25A isoform also did not affect synaptic transmission. Strikingly, the - potentially phosphomimetic - mutant S187E reduced spontaneous release and release probability, with the largest effect seen in the SNAP-25B isoform, showing that a negative charge in this position is detrimental for neurotransmission, in agreement with electrostatic fusion triggering. During the course of our experiments, we found that higher SNAP-25B expression levels led to decreased paired pulse potentiation, probably due to higher release probabilities. Under these conditions, the potentiation of evoked EPSCs by phorbol esters was followed by a persistent down-regulation, probably due to a ceiling effect. In conclusion, our results indicate that phosphorylation of Ser-187 in SNAP-25 is not involved in modulation of synaptic release by Ca²⁺ or phorbol esters.

MUNC18-1 regulates the submembrane F-actin network, independently of syntaxin1 targeting, via hydrophobicity in β-sheet 10

MUNC18-1 (also known as STXBP1) is an essential protein for docking and fusion of secretory vesicles. Mouse chromaffin cells (MCCs) lacking MUNC18-1 show impaired secretory vesicle docking, but also mistargeting of SNARE protein syntaxin1 and an abnormally dense submembrane F-actin network. Here, we tested the contribution of both these phenomena to docking and secretion defects in MUNC18-1-deficient MCCs. We show that an abnormal F-actin network and syntaxin1 targeting defects are not observed in Snap25- or Syt1-knockout (KO) MCCs, which are also secretion deficient. We identified a MUNC18-1 mutant (V263T in β-sheet 10) that fully restores syntaxin1 targeting but not F-actin abnormalities in Munc18-1-KO cells. MUNC18-2 and -3 (also known as STXBP2 and STXBP3, respectively), which lack the hydrophobic residue at position 263, also did not restore a normal F-actin network in Munc18-1-KO cells. However, these proteins did restore the normal F-actin network when a hydrophobic residue was introduced at the corresponding position. Munc18-1-KO MCCs expressing MUNC18-1(V263T) showed normal vesicle docking and exocytosis. These results demonstrate that MUNC18-1 regulates the F-actin network independently of syntaxin1 targeting via hydrophobicity in β-sheet 10. The abnormally dense F-actin network in Munc18-1-deficient cells is not a rate-limiting barrier in secretory vesicle docking or fusion.This article has an associated First Person interview with the first author of the paper.

Phosphatidylserine is critical for vesicle fission during clathrin-mediated endocytosis

Phosphatidylserine (PS), a negatively charged phospholipid present predominantly at the inner leaflet of the plasma membrane, has been widely implicated in many cellular processes including membrane trafficking. Along this line, PS has been demonstrated to be important for endocytosis, however, the involved mechanisms remain uncertain. By monitoring clathrin-mediated endocytosis (CME) of single vesicles in mouse chromaffin cells using cell-attached capacitance measurements that offer millisecond time resolution, we demonstrate in the present study that the fission-pore duration is reduced by PS addition, indicating a stimulatory role of PS in regulating the dynamics of vesicle fission during CME. Furthermore, our results show that the PS-mediated effect on the fission-pore duration is Ca²⁺ -dependent and abolished in the absence of synaptotagmin 1 (Syt1), implying that Syt1 is necessary for the stimulatory role of PS in vesicle fission during CME. Consistently, a Syt1 mutant with a defective PS-Syt1 interaction increases the fission-pore duration. Taken together, our study suggests that PS-Syt1 interaction may be critical in regulating fission dynamics during CME.

Chromaffin Cells of the Adrenal Medulla: Physiology, Pharmacology, and Disease

Chromaffin cells (CCs) of the adrenal gland and the sympathetic nervous system produce the catecholamines (epinephrine and norepinephrine; EPI and NE) needed to coordinate the bodily "fight-or-flight" response to fear, stress, exercise, or conflict. EPI and NE release from CCs is regulated both neurogenically by splanchnic nerve fibers and nonneurogenically by hormones (histamine, corticosteroids, angiotensin, and others) and paracrine messengers [EPI, NE, adenosine triphosphate, opioids, γ-aminobutyric acid (GABA), etc.]. The "stimulus-secretion" coupling of CCs is a Ca2+ -dependent process regulated by Ca2+ entry through voltage-gated Ca2+ channels, Ca2+ pumps, and exchangers and intracellular organelles (RE and mitochondria) and diffusible buffers that provide both Ca2+ -homeostasis and Ca2+ -signaling that ultimately trigger exocytosis. CCs also express Na+ and K+ channels and ionotropic (nAChR and GABAA ) and metabotropic receptors (mACh, PACAP, β-AR, 5-HT, histamine, angiotensin, and others) that make CCs excitable and responsive to autocrine and paracrine stimuli. To maintain high rates of E/NE secretion during stressful conditions, CCs possess a large number of secretory chromaffin granules (CGs) and members of the soluble NSF-attachment receptor complex protein family that allow docking, fusion, and exocytosis of CGs at the cell membrane, and their recycling. This article attempts to provide an updated account of well-established features of the molecular processes regulating CC function, and a survey of the as-yet-unsolved but important questions relating to CC function and dysfunction that have been the subject of intense research over the past 15 years. Examples of CCs as a model system to understand the molecular mechanisms associated with neurodegenerative diseases are also provided. Published 2019. Compr Physiol 9:1443-1502, 2019.

Ca2+-independent but voltage-dependent quantal catecholamine secretion (CiVDS) in the mammalian sympathetic nervous system

Action potential-induced vesicular exocytosis is considered exclusively Ca²⁺ dependent in Katz's Ca²⁺ hypothesis on synaptic transmission. This long-standing concept gets an exception following the discovery of Ca²⁺-independent but voltage-dependent secretion (CiVDS) and its molecular mechanisms in dorsal root ganglion sensory neurons. However, whether CiVDS presents only in sensory cells remains elusive. Here, by combining multiple independent recordings, we report that [1] CiVDS robustly presents in the sympathetic nervous system, including sympathetic superior cervical ganglion neurons and slice adrenal chromaffin cells, [2] uses voltage sensors of Ca²⁺ channels (N-type and novel L-type), and [3] contributes to catecholamine release in both homeostatic and fight-or-flight like states; [4] CiVDS-mediated catecholamine release is faster than that of Ca²⁺-dependent secretion at the quantal level and [5] increases Ca²⁺ currents and contractility of cardiac myocytes. Together, CiVDS presents in the sympathetic nervous system with potential physiological functions, including cardiac muscle contractility.

ELKS active zone proteins as multitasking scaffolds for secretion

Synaptic vesicle exocytosis relies on the tethering of release ready vesicles close to voltage-gated Ca²⁺ channels and specific lipids at the future site of fusion. This enables rapid and efficient neurotransmitter secretion during presynaptic depolarization by an action potential. Extensive research has revealed that this tethering is mediated by an active zone, a protein dense structure that is attached to the presynaptic plasma membrane and opposed to postsynaptic receptors. Although roles of individual active zone proteins in exocytosis are in part understood, the molecular mechanisms that hold the protein scaffold at the active zone together and link it to the presynaptic plasma membrane have remained unknown. This is largely due to redundancy within and across scaffolding protein families at the active zone. Recent studies, however, have uncovered that ELKS proteins, also called ERC, Rab6IP2 or CAST, act as active zone scaffolds redundant with RIMs. This redundancy has led to diverse synaptic phenotypes in studies of ELKS knockout mice, perhaps because different synapses rely to a variable extent on scaffolding redundancy. In this review, we first evaluate the need for presynaptic scaffolding, and we then discuss how the diverse synaptic and non-synaptic functional roles of ELKS support the hypothesis that ELKS provides molecular scaffolding for organizing vesicle traffic at the presynaptic active zone and in other cellular compartments.

Impaired chromaffin cell excitability and exocytosis in autistic Timothy syndrome TS2-neo mouse rescued by L-type calcium channel blockers

KEY POINTS

Tymothy syndrome (TS) is a multisystem disorder featuring cardiac arrhythmias, autism and adrenal gland dysfunction that originates from a de novo point mutation in the gene encoding the Cav1.2 (CACNA1C) L-type channel. To study the role of Cav1.2 channel signals in autism, the autistic TS2-neo mouse has been generated bearing the G406R point-mutation associated with TS type-2. Using heterozygous TS2-neo mice, we report that the G406R mutation reduces the rate of inactivation and shifts leftward the activation and inactivation of L-type channels, causing marked increase of resting Ca²⁺ influx ('window' Ca²⁺ current). The increased 'window current' causes marked reduction of Na_V channel density, switches normal tonic firing to abnormal burst firing, reduces mitochondrial metabolism, induces cell swelling and decreases catecholamine release. Overnight incubations with nifedipine rescue Na_V channel density, normal firing and the quantity of catecholamine released. We provide evidence that chromaffin cell malfunction derives from altered Cav1.2 channel gating.

ABSTRACT

L-type voltage-gated calcium (Cav1) channels have a key role in long-term synaptic plasticity, sensory transduction, muscle contraction and hormone release. A point mutation in the gene encoding Cav1.2 (CACNA1C) causes Tymothy syndrome (TS), a multisystem disorder featuring cardiac arrhythmias, autism spectrum disorder (ASD) and adrenal gland dysfunction. In the more severe type-2 form (TS2), the missense mutation G406R is on exon 8 coding for the IS6-helix of the Cav1.2 channel. The mutation causes reduced inactivation and induces autism. How this occurs and how Cav1.2 gating-changes alter cell excitability, neuronal firing and hormone release on a molecular basis is still largely unknown. Here, using the TS2-neo mouse model of TS we show that the G406R mutation altered excitability and reduced secretory activity in adrenal chromaffin cells (CCs). Specifically, the TS2 mutation reduced the rate of voltage-dependent inactivation and shifted leftward the activation and steady-state inactivation of L-type channels. This markedly increased the resting 'window' Ca²⁺ current that caused an increased percentage of CCs undergoing abnormal action potential (AP) burst firing, cell swelling, reduced mitochondrial metabolism and decreased catecholamine release. The increased 'window' Ca²⁺ current caused also decreased Na_V channel density and increased steady-state inactivation, which contributed to the increased abnormal burst firing. Overnight incubation with the L-type channel blocker nifedipine rescued the normal AP firing of CCs, the density of functioning Na_V channels and their steady-state inactivation. We provide evidence that CC malfunction derives from the altered Cav1.2 channel gating and that dihydropyridines are potential therapeutics for ASD.

Toward a unified picture of the exocytotic fusion pore

Neurotransmitter and hormone release involve calcium-triggered fusion of a cargo-loaded vesicle with the plasma membrane. The initial connection between the fusing membranes, called the fusion pore, can evolve in various ways, including rapid dilation to allow full cargo release, slow expansion, repeated opening-closing and resealing. Pore dynamics determine the kinetics of cargo release and the mode of vesicle recycling, but how these processes are controlled is poorly understood. Previous reconstitutions could not monitor single pores, limiting mechanistic insight they could provide. Recently developed nanodisc-based fusion assays allow reconstitution and monitoring of single pores with unprecedented detail and hold great promise for future discoveries. They recapitulate various aspects of exocytotic fusion pores, but comparison is difficult because different approaches suggested very different exocytotic fusion pore properties, even for the same cell type. In this Review, I discuss how most of the data can be reconciled, by recognizing how different methods probe different aspects of the same fusion process. The resulting picture is that fusion pores have broadly distributed properties arising from stochastic processes which can be modulated by physical constraints imposed by proteins, lipids and membranes.

Sensing Exocytosis and Triggering Endocytosis at Synapses: Synaptic Vesicle Exocytosis-Endocytosis Coupling

The intact synaptic structure is critical for information processing in neural circuits. During synaptic transmission, rapid vesicle exocytosis increases the size of never terminals and endocytosis counteracts the increase. Accumulating evidence suggests that SV exocytosis and endocytosis are tightly connected in time and space during SV recycling, and this process is essential for synaptic function and structural stability. Research in the past has illustrated the molecular details of synaptic vesicle (SV) exocytosis and endocytosis; however, the mechanisms that timely connect these two fundamental events are poorly understood at central synapses. Here we discuss recent progress in SV recycling and summarize several emerging mechanisms by which synapses can “sense” the occurrence of exocytosis and timely initiate compensatory endocytosis. They include Ca²⁺ sensing, SV proteins sensing, and local membrane stress sensing. In addition, the spatial organization of endocytic zones adjacent to active zones provides a structural basis for efficient coupling between SV exocytosis and endocytosis. Through linking different endocytosis pathways with SV fusion, these mechanisms ensure necessary plasticity and robustness of nerve terminals to meet diverse physiological needs.