The intravesicular cocktail and its role in the regulation of exocytosis

María Teresa Miras Portugal: a pioneer for vesicular nucleotide storage

Chromaffin granules are secretory granules present in adrenal medulla chromaffin cells. They contain high contents of catecholamines and nucleotides and have been regarded as a model system for the study of vesicular transmitter uptake and release. In 1988, Dr. María Teresa Miras Portugal, when studying catecholamine biosynthesis, detected a novel group of nucleotides, the diadenosine polyphosphates, in the adrenal chromaffin granules. Based on this finding, she unraveled the existence of diadenosine polyphosphate-mediated chemical transmission, leading to a paradigm shift in the field of purinergic signaling. She is also a pioneer in the studies on vesicular nucleotide storage. First, María Teresa and her group characterized nucleotide transport in chromaffin granules and synaptic vesicles using fluorescent nucleotide derivatives such as 1, N6-ethenoadenosine triphosphates. Then, they revealed the presence of a hypothetical vesicular nucleotide transporter with unique properties in terms of substrate specificity. In this article, we will describe her contributions to vesicular nucleotide storage and the foundations she laid for future studies.

Untapped Potential: Real-Time Measurements of Opioid Exocytosis at Single Cells

The endogenous opioid system is commonly targeted in pain treatment, but the fundamental nature of neuropeptide release remains poorly understood due to a lack of methods for direct detection of specific opioid neuropeptides in situ. These peptides are concentrated in, and released from, large dense-core vesicles in chromaffin cells. Although catecholamine release from these neuroendocrine cells is well characterized, the direct quantification of opioid peptide exocytosis events has not previously been achieved. In this work, a planar carbon-fiber microelectrode served as a "postsynaptic" sensor for probing catecholamine and neuropeptide release dynamics via amperometric monitoring. A constant potential of 500 mV was employed for quantification of catecholamine release, and a higher potential of 1000 mV was used to drive oxidation of tyrosine, the N-terminal amino acid in the opioid neuropeptides released from chromaffin cells. By discriminating the results collected at the two potentials, the data reveal unique kinetics for these two neurochemical classes at the single-vesicle level. The amplitude of the peptidergic signals decreased with repeat stimulation, as the halfwidth of these signals simultaneously increased. By contrast, the amplitude of catecholamine release events increased with repeat stimulation, but the halfwidth of each event did not vary. The chromogranin dense core was identified as an important mechanistic handle by which separate classes of transmitter can be kinetically modulated when released from the same population of vesicles. Overall, the data provide unprecedented insight into key differences between catecholamine and opioid neuropeptide release from isolated chromaffin cells.

Exploiting Microelectrode Geometry for Comprehensive Detection of Individual Exocytosis Events at Single Cells

Carbon fiber microelectrodes are commonly used for real-time monitoring of individual exocytosis events at single cells. Since the nature of an electrochemical signal is fundamentally governed by mass transport to the electrode surface, microelectrode geometry can be exploited to achieve precise and accurate measurements. Researchers traditionally pair amperometric measurements of exocytosis with a ∼10-μm diameter, disk microelectrode in an "artificial synapse" configuration to directly monitor individual release events from single cells. Exocytosis is triggered, and released molecules diffuse to the "post-synaptic" electrode for oxidation. This results in a series of distinct current spikes corresponding to individual exocytosis events. However, it remains unclear how much of the material escapes detection. In this work, the performance of 10- and 34-μm diameter carbon fiber disk microelectrodes was directly compared in monitoring exocytosis at single chromaffin cells. The 34-μm diameter electrode was more sensitive to catecholamines and enkephalins than its traditional, 10-μm diameter counterpart, and it more effectively covered the entire cell. As such, the larger sensor detected more exocytosis events overall, as well as a larger quantal size, suggesting that the traditional tools underestimate the above measurements. Both sensors reliably measured l-DOPA-evoked changes in quantal size, and both exhibited diffusional loss upon adjustment of cell-electrode spacing. Finite element simulations using COMSOL support the improved collection efficiency observed using the larger sensor. Overall, this work demonstrates how electrode geometry can be exploited for improved detection of exocytosis events by addressing diffusional loss─an often-overlooked source of inaccuracy in single-cell measurements.

Tuning the Size of Large Dense-Core Vesicles and Quantal Neurotransmitter Release via Secretogranin II Liquid-Liquid Phase Separation

Large dense-core vesicles (LDCVs) are larger in volume than synaptic vesicles, and are filled with multiple neuropeptides, hormones, and neurotransmitters that participate in various physiological processes. However, little is known about the mechanism determining the size of LDCVs. Here, it is reported that secretogranin II (SgII), a vesicle matrix protein, contributes to LDCV size regulation through its liquid-liquid phase separation in neuroendocrine cells. First, SgII undergoes pH-dependent polymerization and the polymerized SgII forms phase droplets with Ca²⁺ in vitro and in vivo. Further, the Ca²⁺ -induced SgII droplets recruit reconstituted bio-lipids, mimicking the LDCVs biogenesis. In addition, SgII knockdown leads to significant decrease of the quantal neurotransmitter release by affecting LDCV size, which is differently rescued by SgII truncations with different degrees of phase separation. In conclusion, it is shown that SgII is a unique intravesicular matrix protein undergoing liquid-liquid phase separation, and present novel insights into how SgII determines LDCV size and the quantal neurotransmitter release.

The Adrenal Medulla Modulates Mechanical Allodynia in a Rat Model of Neuropathic Pain

We have investigated whether the stress response mediated by the adrenal medulla in rats subjected to chronic constriction injury of the sciatic nerve (CCI) modulates their nocifensive behavior. Treatment with SK29661 (300 mg/kg; intraperitoneal (I.P.)), a selective inhibitor of phenylethanolamine N-methyltransferase (PNMT) that converts noradrenaline (NA) into adrenaline (A), fully reverted mechanical allodynia in the injured hind paw without affecting mechanical sensitivity in the contralateral paw. The effect was fast and reversible and was associated with a decrease in the A to NA ratio (A/NA) in the adrenal gland and circulating blood, an A/NA that was elevated by CCI. 1,2,3,4-tetrahydroisoquinoline-7-sulfonamide (SKF29661) did not affect exocytosis evoked by Ca²⁺ entry as well as major ionic conductances (voltage-gated Na⁺, Ca²⁺, and K⁺ channels, nicotinic acetylcholine receptors) involved in stimulus-secretion coupling in chromaffin cells, suggesting that it acted by changing the relative content of the two adrenal catecholamines. Denervation of the adrenal medulla by surgical splanchnectomy attenuated mechanical allodynia in neuropathic animals, hence confirming the involvement of the adrenal medulla in the pathophysiology of the CCI model. Inhibition of PNMT appears to be an effective and probably safe way to modulate adrenal medulla activity and, in turn, to alleviate pain secondary to the injury of a peripheral nerve.

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.

Intricacies of the Molecular Machinery of Catecholamine Biosynthesis and Secretion by Chromaffin Cells of the Normal Adrenal Medulla and in Pheochromocytoma and Paraganglioma

The adrenal medulla is composed predominantly of chromaffin cells producing and secreting the catecholamines dopamine, norepinephrine, and epinephrine. Catecholamine biosynthesis and secretion is a complex and tightly controlled physiologic process. The pathways involved have been extensively studied, and various elements of the underlying molecular machinery have been identified. In this review, we provide a detailed description of the route from stimulus to secretion of catecholamines by the normal adrenal chromaffin cell compared to chromaffin tumor cells in pheochromocytomas. Pheochromocytomas are adrenomedullary tumors that are characterized by uncontrolled synthesis and secretion of catecholamines. This uncontrolled secretion can be partly explained by perturbations of the molecular catecholamine secretory machinery in pheochromocytoma cells. Chromaffin cell tumors also include sympathetic paragangliomas originating in sympathetic ganglia. Pheochromocytomas and paragangliomas are usually locally confined tumors, but about 15% do metastasize to distant locations. Histopathological examination currently poorly predicts future biologic behavior, thus long term postoperative follow-up is required. Therefore, there is an unmet need for prognostic biomarkers. Clearer understanding of the cellular mechanisms involved in the secretory characteristics of pheochromocytomas and sympathetic paragangliomas may offer one approach for the discovery of novel prognostic biomarkers for improved therapeutic targeting and monitoring of treatment or disease progression.

Differential effects of the retinopathy of prematurity exam on the physiology of premature infants

OBJECTIVE

To compare the differential effects of the retinopathy of prematurity (ROP) examination on the physiology of premature infants with and without oxygen support.

STUDY DESIGN

We collected data from 42 premature infants (room air = 19, oxygen support = 23) and compared physiological metrics including heart rate (HR), systemic peripheral saturation (SpO₂), mesenteric tissue oxygen saturation (StO₂) and clinical events (oxygen desaturation episodes, bradycardia events, and gastric residuals).

RESULTS

We found significant differences between groups in HR during and briefly after the exam, and in mesenteric StO_2, during eye drop administration, eye exam, and up to 8 min after the exam. SpO₂ was significantly different between the groups at all time points. Gastric residuals were higher after the exam in infants on oxygen support, compared to baseline.

CONCLUSION

Premature infants on oxygen support may be at a higher risk of adverse physiologic effects in response to the ROP exam.

Phases of the exocytotic fusion pore

Membrane fusion and fission are fundamental processes in living organisms. Membrane fusion occurs through the formation of a fusion pore, which is the structure that connects two lipid membranes during their fusion. Fusion pores can form spontaneously, but cells endow themselves with a set of proteins that make the process of fusion faster and regulatable. The fusion pore starts with a narrow diameter and dilates relatively slowly; it may fluctuate in size or can even close completely, producing a transient vesicle fusion (kiss-and-run), or can finally expand abruptly to release all vesicle contents. A set of proteins control the formation, dilation, and eventual closure of the fusion pore and, therefore, the velocity at which the contents of secretory vesicles are released to the extracellular medium. Thus, the regulation of fusion pore expansion or closure is key to regulate the release of neurotransmitters and hormones. Here, we review the phases of the fusion pore and discuss the implications in the modes of exocytosis.

Electrochemical Investigation of the Interaction between Catecholamines and ATP

The study of the colligative properties of adenosine 5'-triphosphate (ATP) and catecholamines has received the attention of scientists for decades, as they could explain the capabilities of secretory vesicles (SVs) to accumulate neurotransmitters. In this Article, we have applied electrochemical methods to detect such interactions in vitro, at the acidic pH of SVs (pH 5.5) and examined the effect of compounds having structural similarities that correlate with functional groups of ATP (adenosine, phosphoric acid and sodium phosphate salts) and catecholamines (catechol). Chronoamperometry and fast scan cyclic voltammetry (FSCV) provide evidence compatible with an interaction of the catechol and adenine rings. This interaction is also reinforced by an electrostatic interaction between the phosphate group of ATP and the protonated ammonium group of catecholamines. Furthermore, chronoamperometry data suggest that the presence of ATP subtlety reduces the apparent diffusion coefficient of epinephrine in aqueous media that adds an additional factor leading to a slower rate of catecholamine exocytosis. This adds another plausible mechanism to regulate individual exocytosis events to alter communication.

Granin-derived peptides

The granin family comprises altogether 7 different proteins originating from the diffuse neuroendocrine system and elements of the central and peripheral nervous systems. The family is dominated by three uniquely acidic members, namely chromogranin A (CgA), chromogranin B (CgB) and secretogranin II (SgII). Since the late 1980s it has become evident that these proteins are proteolytically processed, intragranularly and/or extracellularly into a range of biologically active peptides; a number of them with regulatory properties of physiological and/or pathophysiological significance. The aim of this comprehensive overview is to provide an up-to-date insight into the distribution and properties of the well established granin-derived peptides and their putative roles in homeostatic regulations. Hence, focus is directed to peptides derived from the three main granins, e.g. to the chromogranin A derived vasostatins, betagranins, pancreastatin and catestatins, the chromogranin B-derived secretolytin and the secretogranin II-derived secretoneurin (SN). In addition, the distribution and properties of the chromogranin A-derived peptides prochromacin, chromofungin, WE14, parastatin, GE-25 and serpinins, the CgB-peptide PE-11 and the SgII-peptides EM66 and manserin will also be commented on. Finally, the opposing effects of the CgA-derived vasostatin-I and catestatin and the SgII-derived peptide SN on the integrity of the vasculature, myocardial contractility, angiogenesis in wound healing, inflammatory conditions and tumors will be discussed.