Intracellular Ca2+ microdomain-triggered exocytosis in neuroendocrine cells

Neuropeptides and small-molecule amine transmitters: cooperative signaling in the nervous system

Neuropeptides are expressed in cell-specific patterns throughout mammalian brain. Neuropeptide gene expression has been useful for clustering neurons by phenotype, based on single-cell transcriptomics, and for defining specific functional circuits throughout the brain. How neuropeptides function as first messengers in inter-neuronal communication, in cooperation with classical small-molecule amine transmitters (SMATs) is a current topic of systems neurobiology. Questions include how neuropeptides and SMATs cooperate in neurotransmission at the molecular, cellular and circuit levels; whether neuropeptides and SMATs always co-exist in neurons; where neuropeptides and SMATs are stored in the neuron, released from the neuron and acting, and at which receptors, after release; and how neuropeptides affect 'classical' transmitter function, both directly upon co-release, and indirectly, via long-term regulation of gene transcription and neuronal plasticity. Here, we review an extensive body of data about the distribution of neuropeptides and their receptors, their actions after neuronal release, and their function based on pharmacological and genetic loss- and gain-of-function experiments, that addresses these questions, fundamental to understanding brain function, and development of neuropeptide-based, and potentially combinatorial peptide/SMAT-based, neurotherapeutics.

Sustained Exocytosis after Action Potential-Like Stimulation at Low Frequencies in Mouse Chromaffin Cells Depends on a Dynamin-Dependent Fast Endocytotic Process

Under basal conditions the action potential firing rate of adrenal chromaffin cells is lower than 0.5 Hz. The maintenance of the secretory response at such frequencies requires a continuous replenishment of releasable vesicles. However, the mechanism that allows such vesicle replenishment remains unclear. Here, using membrane capacitance measurements on mouse chromaffin cells, we studied the mechanism of replenishment of a group of vesicles released by a single action potential-like stimulus (AP_ls). The exocytosis triggered by AP_ls (ETAP) represents a fraction (40%) of the immediately releasable pool, a group of vesicles highly coupled to voltage dependent calcium channels. ETAP was replenished with a time constant of 0.73 ± 0.11 s, fast enough to maintain synchronous exocytosis at 0.2–0.5 Hz stimulation. Regarding the mechanism involved in rapid ETAP replenishment, we found that it depends on the ready releasable pool; indeed depletion of this vesicle pool significantly delays ETAP replenishment. On the other hand, ETAP replenishment also correlates with a dynamin-dependent fast endocytosis process (τ = 0.53 ± 0.01 s). In this regard, disruption of dynamin function markedly inhibits the fast endocytosis and delays ETAP replenishment, but also significantly decreases the synchronous exocytosis during repetitive AP_ls stimulation at low frequencies (0.2 and 0.5 Hz). Considering these findings, we propose a model in where both the transfer of vesicles from ready releasable pool and fast endocytosis allow rapid ETAP replenishment during low stimulation frequencies.

Calcium in health and disease

Evolution has exploited the chemical properties of Ca(2+), which facilitate its reversible binding to the sites of irregular geometry offered by biological macromolecules, to select it as a carrier of cellular signals. A number of proteins bind Ca(2+) to specific sites: those intrinsic to membranes play the most important role in the spatial and temporal regulation of the concentration and movements of Ca(2+) inside cells. Those which are soluble, or organized in non-membranous structures, also decode the Ca(2+) message to be then transmitted to the targets of its regulation. Since Ca(2+) controls the most important processes in the life of cells, it must be very carefully controlled within the cytoplasm, where most of the targets of its signaling function reside. Membrane channels (in the plasma membrane and in the organelles) mediate the entrance of Ca(2+) into the cytoplasm, ATPases, exchangers, and the mitochondrial Ca(2+) uptake system remove Ca(2+) from it. The concentration of Ca(2+) in the external spaces, which is controlled essentially by its dynamic exchanges in the bone system, is much higher than inside cells, and can, under conditions of pathology, generate a situation of dangerous internal Ca(2+) overload. When massive and persistent, the Ca(2+) overload culminates in the death of the cell. Subtle conditions of cellular Ca(2+) dyshomeostasis that affect individual systems that control Ca(2+), generate cell disease phenotypes that are particularly severe in tissues in which the signaling function of Ca(2+) has special importance, e.g., the nervous system.

Optical control of calcium-regulated exocytosis

BACKGROUND

Neurons signal to each other and to non-neuronal cells as those in muscle or glands, by means of the secretion of neurotransmitters at chemical synapses. In order to dissect the molecular mechanisms of neurotransmission, new methods for directly and reversibly triggering neurosecretion at the presynaptic terminal are necessary. Here we exploit the calcium permeability of the light-gated channel LiGluR in order to reversibly manipulate cytosolic calcium concentration, thus controlling calcium-regulated exocytosis.

METHODS

Bovine chromaffin cells expressing LiGluR were stimulated with light. Exocytic events were detected by amperometry or by whole-cell patch-clamp to quantify membrane capacitance and calcium influx.

RESULTS

Amperometry reveals that optical stimulation consistently triggers exocytosis in chromaffin cells. Secretion of catecholamines can be adjusted between zero and several Hz by changing the wavelength of illumination. Differences in secretion efficacy are found between the activation of LiGluR and native voltage-gated calcium channels (VGCCs). Our results show that the distance between sites of calcium influx and vesicles ready to be released is longer when calcium influx is triggered by LiGluR instead of native VGCCs.

CONCLUSION

LiGluR activation directly and reversibly increases the intracellular calcium concentration. Light-gated calcium influx allows for the first time to control calcium-regulated exocytosis without the need of applying depolarizing solutions or voltage clamping in chromaffin cells.

GENERAL SIGNIFICANCE

LiGluR is a useful tool to study the secretory mechanisms and their spatiotemporal patterns in neurotransmission, and opens a window to study other calcium-dependent processes such as muscular contraction or cell migration.

Role of annexin A6 isoforms in catecholamine secretion by PC12 cells: distinct influence on calcium response

Noradrenaline and adrenaline are secreted by adrenal medulla chromaffin cells via exocytosis. Exocytosis of catecholamines occurs after cell stimulation with various endogenous activators such as nicotine or after depolarization of the plasma membrane and is regulated by calcium ions. Cytosolic [Ca(2+)] increases in response to cell excitation and triggers a signal-initiated secretion. Annexins are known to participate in the regulation of membrane dynamics and are also considered to be involved in vesicular trafficking. Some experimental evidence suggests that annexins may participate in Ca(2+)-regulated catecholamine secretion. In this report the effect of annexin A6 (AnxA6) isoforms 1 and 2 on catecholamine secretion has been described. Overexpression of AnxA6 isoforms and AnxA6 knock-down in PC12 cells were accompanied by almost complete inhibition or a 20% enhancement of dopamine secretion, respectively. AnxA6-1 and AnxA6-2 overexpression reduced Delta[Ca(2+)](c) upon depolarization by 32% and 58%, respectively, while AnxA6 knock-down increased Delta[Ca(2+)](c) by 44%. The mechanism of AnxA6 action on Ca(2+) signalling is not well understood. Experimental evidence suggests that two AnxA6 isoforms interact with different targets engaged in regulation of calcium homeostasis in PC12 cells.

Loose coupling between calcium channels and sites of exocytosis in chromaffin cells

Calcium microdomains generated by tight clusters of calcium channels regulate fusion of small vesicles at the synaptic terminal and have also been suggested to trigger exocytosis of large dense-core vesicles from neuroendocrine cells. To test this idea, we have compared sites of exocytosis and the spatial distribution of calcium channels in chromaffin cells. Fusion of individual vesicles was visualized using interference reflection microscopy and the submembranous calcium signal was assessed using total internal reflection fluorescence microscopy. Depolarization triggered a burst of exocytosis from up to seven sites in a membrane area of 11 microm(2), but these sites did not colocalize with calcium microdomains. Instead, calcium influx occurred in large patches (averaging 34 microm(2)) containing a mixture of P/Q- and N-type channels. About 20% of fusion events occurred outside calcium channel patches. Further, the delay between the onset of stimulation and a burst of exocytosis was prolonged for several seconds by increasing the concentration of the slow calcium chelator EGTA from 1.5 to 5 mM. These results demonstrate that while calcium channels and release sites tend to congregate in specialized regions of the surface membrane, these have dimensions of several micrometres. The dominant calcium signal regulating release in chromaffin cells is generated by the cooperative action of many channels operating over distances of many micrometres rather than discrete clusters of calcium channels generating localized microdomains.

Modulation of calcium signalling by intracellular organelles seen with targeted aequorins

The cytosolic Ca(2+) signals that trigger cell responses occur either as localized domains of high Ca(2+) concentration or as propagating Ca(2+) waves. Cytoplasmic organelles, taking up or releasing Ca(2+) to the cytosol, shape the cytosolic signals. On the other hand, Ca(2+) concentration inside organelles is also important in physiology and pathophysiology. Comprehensive study of these matters requires to measure [Ca(2+)] inside organelles and at the relevant cytosolic domains. Aequorins, the best-known chemiluminescent Ca(2+) probes, are excellent for this end as they do not require stressing illumination, have a large dynamic range and a sharp Ca(2+)-dependence, can be targeted to the appropriate location and engineered to have the proper Ca(2+) affinity. Using this methodology, we have evidenced the existence in chromaffin cells of functional units composed by three closely interrelated elements: (1) plasma membrane Ca(2+) channels, (2) subplasmalemmal endoplasmic reticulum and (3) mitochondria. These Ca(2+)-signalling triads optimize Ca(2+) microdomains for secretion and prevent propagation of the Ca(2+) wave towards the cell core. Oscillatory cytosolic Ca(2+) signals originate also oscillations of mitochondrial Ca(2+) in several cell types. The nuclear envelope slows down the propagation of the Ca(2+) wave to the nucleus and filters high frequencies. On the other hand, inositol-trisphosphate may produce direct release of Ca(2+) to the nucleoplasm in GH(3) pituitary cells, thus providing mechanisms for selective nuclear signalling. Aequorins emitting at different wavelengths, prepared by fusion either with green or red fluorescent protein, permit simultaneous and independent monitorization of the Ca(2+) signals in different subcellular domains within the same cell.

Cytoplasmic organelles determine complexity and specificity of calcium signalling in adrenal chromaffin cells

Complex and coordinated fluctuations of intracellular free Ca2+ concentration ([Ca2+]c) regulate secretion of adrenaline from chromaffin cells. The physiologically relevant intracellular Ca2+ signals occur either as localized microdomains of high Ca2+ concentrations or as propagating Ca2+ waves, which give rise to global Ca2+ elevations. Intracellular organelles, the endoplasmic reticulum (ER), mitochondria and nuclear envelope, are endowed with powerful Ca2+ transport systems. Calcium uptake and Ca2+ release from these organelles determine the spatial and temporal parameters of Ca2+ signalling events. Furthermore, the ER and mitochondria form close relations with the sites of plasmalemmal Ca2+ entry, creating 'Ca2+ signalling triads' which act as elementary operational units, which regulate exocytosis. Ca2+ ions accumulating in the ER and mitochondria integrate exocytotic activity with energy production and protein synthesis.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

Identification of a novel regulatory mechanism for norepinephrine transporter activity by the IP3 receptor

The norepinephrine transporter (NET) plays a crucial role in noradrenergic neurotransmission and is a target of many antidepressants and psychostimulants. Intracellular Ca2+ is reportedly involved in regulating NET activity, but the detailed mechanism is not clear. We employed a norepinephrine uptake assay using SH-SY5Y cells and found that the IP3 receptor inhibitors, 2-aminoethoxydiphenyl borate and xestospongin C, reduced the NET Vmax. These reductions were accompanied by the decreased cell surface expression of NET. Our findings suggest that intracellular Ca2+ mobilized by IP3 receptor is required for the maintenance of NET activity. This adds another pathway involving Ca2+ for the regulation of NET to other known mechanisms providing intracellular Ca2+.