Mitochondrial calcium sequestration and protein kinase C cooperate in the regulation of cortical F-actin disassembly and secretion in bovine chromaffin cells

Mitochondrial modulation-induced activation of vagal sensory neuronal subsets by antimycin A, but not CCCP or rotenone, correlates with mitochondrial superoxide production

Inflammation causes nociceptive sensory neuron activation, evoking debilitating symptoms and reflexes. Inflammatory signaling pathways are capable of modulating mitochondrial function, resulting in reactive oxygen species (ROS) production, mitochondrial depolarization and calcium release. Previously we showed that mitochondrial modulation with antimycin A, a complex III inhibitor, selectively stimulated nociceptive bronchopulmonary C-fibers via the activation of transient receptor potential (TRP) ankyrin 1 (A1) and vanilloid 1 (V1) cation channels. TRPA1 is ROS-sensitive, but there is little evidence that TRPV1 is activated by ROS. Here, we used dual imaging of dissociated vagal neurons to investigate the correlation of mitochondrial superoxide production (mitoSOX) or mitochondrial depolarization (JC-1) with cytosolic calcium (Fura-2AM), following mitochondrial modulation by antimycin A, rotenone (complex I inhibitor) and carbonyl cyanide m-chlorophenyl hydrazone (CCCP, mitochondrial uncoupling agent). Mitochondrial modulation by all agents selectively increased cytosolic calcium in a subset of TRPA1/TRPV1-expressing (A1/V1+) neurons. There was a significant correlation between antimycin A-induced calcium responses and mitochondrial superoxide in wild-type 'responding' A1/V1+ neurons, which was eliminated in TRPA1-/- neurons, but not TRPV1-/- neurons. Nevertheless, antimycin A-induced superoxide production did not always increase calcium in A1/V1+ neurons, suggesting a critical role of an unknown factor. CCCP caused both superoxide production and mitochondrial depolarization but neither correlated with calcium fluxes in A1/V1+ neurons. Rotenone-induced calcium responses in 'responding' A1/V1+ neurons correlated with mitochondrial depolarization but not superoxide production. Our data are consistent with the hypothesis that mitochondrial dysfunction causes calcium fluxes in a subset of A1/V1+ neurons via ROS-dependent and ROS-independent mechanisms.

Membrane shaping by actin and myosin during regulated exocytosis

The cortical actin network in neurosecretory cells is a dense mesh of actin filaments underlying the plasma membrane. Interaction of actomyosin with vesicular membranes or the plasma membrane is vital for tethering, retention, transport as well as fusion and fission of exo- and endocytic membrane structures. During regulated exocytosis the cortical actin network undergoes dramatic changes in morphology to accommodate vesicle docking, fusion and replenishment. Most of these processes involve plasma membrane Phosphoinositides (PIP) and investigating the interactions between the actin cortex and secretory structures has become a hotbed for research in recent years. Actin remodelling leads to filopodia outgrowth and the creation of new fusion sites in neurosecretory cells and actin, myosin and dynamin actively shape and maintain the fusion pore of secretory vesicles. Changes in viscoelastic properties of the actin cortex can facilitate vesicular transport and lead to docking and priming of vesicle at the plasma membrane. Small GTPase actin mediators control the state of the cortical actin network and influence vesicular access to their docking and fusion sites. These changes potentially affect membrane properties such as tension and fluidity as well as the mobility of embedded proteins and could influence the processes leading to both exo- and endocytosis. Here we discuss the multitudes of actin and membrane interactions that control successive steps underpinning regulated exocytosis.

Tight mitochondrial control of calcium and exocytotic signals in chromaffin cells at embryonic life

Calcium buffering by mitochondria plays a relevant physiological function in the regulation of Ca(2+) and exocytotic signals in mature chromaffin cells (CCs) from various adult mammals. Whether a similar or different role of mitochondrial Ca(2+) buffering is present in immature CCs at early life has not been explored. Here we present a comparative study in rat embryonic CCs and rat mother CCs, of various physiological parameters that are known to be affected by mitochondrial Ca(2+) buffering during cell activation. We found that the clearance of cytosolic Ca(2+) transients ([Ca(2+)]c) elicited by high K(+) was 7-fold faster in embryo CCs compared to mother CCs. This strongly suggests that at embryonic life, the mitochondria play a more significant role in the clearance of [Ca(2+)]c loads compared to adult life. Consistent with this view are the following results concerning the transient suppression of mitochondrial Ca(2+) buffering by protonophore FCCP, in embryonic CCs compared to mother CCs: (i) faster and greater inactivation of inward calcium currents, (ii) higher K(+)-elicited [Ca(2+)]c transients with 25-fold faster clearance, (iii) higher increase of basal catecholamine release and (iv) higher potentiation of K(+)-evoked secretion. These pronounced differences could be explained by two additional features (embryo versus mother CCs): (a) slower recovery of mitochondrial resting membrane potential after the application of a transient FCCP pulse and (b) greater relative density of the mitochondria in the cytosol. This tighter control by the mitochondria of Ca(2+) and exocytotic signals may be relevant to secure a healthy catecholamine secretory response at early life.

The actin binding protein adseverin regulates osteoclastogenesis

Adseverin (Ads), a member of the Gelsolin superfamily of actin binding proteins, regulates the actin cytoskeleton architecture by severing and capping existing filamentous actin (F-actin) strands and nucleating the assembly of new F-actin filaments. Ads has been implicated in cellular secretion, exocytosis and has also been shown to regulate chondrogenesis and megakaryoblastic leukemia cell differentiation. Here we report for the first time that Ads is involved in regulating osteoclastogenesis (OCG). Ads is induced during OCG downstream of RANK-ligand (RANKL) stimulation and is highly expressed in mature osteoclasts. The D5 isoform of Ads is not involved in regulating OCG, as its expression is not induced in response to RANKL. Three clonal Ads knockdown RAW264.7 (RAW) macrophage cell lines with varying degrees of Ads expression and OCG deficiency were generated. The most drastic OCG defect was noted in the clonal cell line with the greatest degree of Ads knockdown as indicated by a lack of TRAcP staining and multinucleation. RNAi mediated knockdown of Ads in osteoclast precursors resulted in distinct morphological changes characterized by altered F-actin distribution and increased filopodia formation. Ads knockdown precursor cells experienced enhanced migration while fusion of knockdown precursors cells was limited. Transient reintroduction of de novo Ads back into the knockdown system was capable of rescuing TRAcP expression but not osteoclast multinucleation most likely due to the transient nature of Ads expression. This preliminary study allows us to conclude that Ads is a RANKL induced early regulator of OCG with a potential role in pre-osteoclast differentiation and fusion.

Hypoxia-elicited catecholamine release is controlled by L-type as well as N/PQ types of calcium channels in rat embryo chromaffin cells

At early life, the adrenal chromaffin cells respond with a catecholamine surge under hypoxic conditions. This response depends on Ca(2+) entry through voltage-activated calcium channels (VACCs). We have investigated here three unresolved questions that concern this response in rat embryo chromaffin cells (ECCs): 1) the relative contribution of L (α1D, Cav1.3), N (α1B, Cav2.2), and PQ (α1A, Cav2.1) to the whole cell Ca(2+) current (ICa); 2) the relative contribution of L and N/PQ channels to the cytosolic Ca(2+) elevations triggered by hypoxia (Δ[Ca(2+)]c); and 3) the role of L and non-L high-VACCs in the regulation of the catecholamine surge occurring during prolonged (1 min) hypoxia exposure of ECCs. Nimodipine halved peak ICa and blocked 60% the total Ca(2+) entry during a 50-ms depolarizing pulse to 0 mV (QCa). Combined ω-agatoxin IVA plus ω-conotoxin GVIA (Aga/GVIA) blocked 30% of both ICa peak and QCa. This relative proportion of L- and non-L VACCs was corroborated by Western blot that indicated 55, 23, and 25% relative expression of L, N, and PQ VACCs. Exposure of ECCs to hypoxia elicited a mild but sustained Δ[Ca(2+)]c; the area of Δ[Ca(2+)]c was blocked 50% by nifedipine and 10% by Aga/GVIA. Exposure of ECCs to 1-min hypoxia elicited an initial transient burst of amperometric secretory spikes followed by scattered spikes along the time of cell exposure to hypoxia. This bulk response was blocked 85% by nimodipine and 35% by Aga/GVIA. Histograms on secretory spike frequency vs. time indicated a faster initial inactivation when Ca(2+) entry took place through N/PQ channels; more sustained secretion but at a lower rate was associated to Ca(2+) entry through L channels. The results suggest that the HIS response may initially be controlled by L and P/Q channels, but later on, N/PQ channels inactivate and the delayed HIS response is maintained at lower rate by slow-inactivating L channels.

Novel features on the regulation by mitochondria of calcium and secretion transients in chromaffin cells challenged with acetylcholine at 37°C

From experiments performed at room temperature, we know that the buffering of Ca²⁺ by mitochondria contributes to the shaping of the bulk cytosolic calcium transient ([Ca²⁺]_c) and secretion transients of chromaffin cells stimulated with depolarizing pulses. We also know that the mitochondrial Ca²⁺ transporters and the release of catecholamine are faster at 37°C with respect to room temperature. Therefore, we planned this investigation to gain further insight into the contribution of mitochondrial Ca²⁺ buffering to the shaping of [Ca²⁺]_c and catecholamine release transients, using some novel experimental conditions that have not been yet explored namely: (1) perifusion of bovine chromaffin cells (BCCs) with saline at 37°C and their repeated challenging with the physiological neurotransmitter acetylcholine (ACh); (2) separate blockade of mitochondrial Ca²⁺ uniporter (mCUP) with Ru360 or the mitochondrial Na⁺/Ca²⁺ exchanger (mNCX) with CGP37157; (3) full blockade of the mitochondrial Ca²⁺ cycling (mCC) by the simultaneous inhibition of the mCUP and the mNCX. Ru360 caused a pronounced delay of [Ca²⁺]_c clearance and augmented secretion. In contrast, CGP37157 only caused a tiny delay of [Ca²⁺]_c clearance and a mild decrease in secretion. The mCC resulting in continued Ca²⁺ uptake and its release back into the cytosol was interrupted by combined Ru360 + CGP37157 (Ru/CGP), the protonophore carbonyl cyanide‐p‐trifluoromethoxyphenylhydrazone, or combined oligomycin + rotenone (O/R); these three treatments caused a mild but sustained elevation of basal [Ca²⁺]_c that, however, was not accompanied by a parallel increase in basal secretion. Nevertheless, all treatments caused a pronounced augmentation of ACh‐induced secretion, with minor changes of the ACh‐induced [Ca²⁺]_c transients. Combined Ru/CGP did not alter the resting membrane potential in current‐clamped cells. Additionally, Ru/CGP did not increase basal [Ca²⁺]_c near subplasmalemmal sites and caused a mild decrease in the size of the readily releasable vesicle pool. Our results provide new functional features in support of the view that in BCCs there are two subpopulations of mitochondria, M1 underneath the plasmalemma nearby exocytotic sites and M2 at the core cell nearby vesicle transport sites. While M1 serves to shape the ACh‐elicited exocytotic response through its efficient Ca²⁺ removal by the mCUP, M2 shapes the lower [Ca²⁺]_c elevations required for new vesicle supply to the exocytotic machinery, from the large reserve vesicle pool at the cell core. The mCUP of the M1 pool seems to play a more prominent role in controlling the ACh responses, in comparison with the mNCX.

Regulation by mitochondria of exocytosis at 37°C.

The cortical acto-Myosin network: from diffusion barrier to functional gateway in the transport of neurosecretory vesicles to the plasma membrane

Dysregulation of regulated exocytosis is linked to an array of pathological conditions, including neurodegenerative disorders, asthma, and diabetes. Understanding the molecular mechanisms underpinning neuroexocytosis including the processes that allow neurosecretory vesicles to access and fuse with the plasma membrane and to recycle post-fusion, is therefore critical to the design of future therapeutic drugs that will efficiently tackle these diseases. Despite considerable efforts to determine the principles of vesicular fusion, the mechanisms controlling the approach of vesicles to the plasma membrane in order to undergo tethering, docking, priming, and fusion remain poorly understood. All these steps involve the cortical actin network, a dense mesh of actin filaments localized beneath the plasma membrane. Recent work overturned the long-held belief that the cortical actin network only plays a passive constraining role in neuroexocytosis functioning as a physical barrier that partly breaks down upon entry of Ca(2+) to allow secretory vesicles to reach the plasma membrane. A multitude of new roles for the cortical actin network in regulated exocytosis have now emerged and point to highly dynamic novel functions of key myosin molecular motors. Myosins are not only believed to help bring about dynamic changes in the actin cytoskeleton, tethering and guiding vesicles to their fusion sites, but they also regulate the size and duration of the fusion pore, thereby directly contributing to the release of neurotransmitters and hormones. Here we discuss the functions of the cortical actin network, myosins, and their effectors in controlling the processes that lead to tethering, directed transport, docking, and fusion of exocytotic vesicles in regulated exocytosis.

Protein kinase C-delta deficiency perturbs bone homeostasis by selective uncoupling of cathepsin K secretion and ruffled border formation in osteoclasts

Bone homeostasis requires stringent regulation of osteoclasts, which secrete proteolytic enzymes to degrade the bone matrix. Despite recent progress in understanding how bone resorption occurs, the mechanisms regulating osteoclast secretion, and in particular the trafficking route of cathepsin K vesicles, remain elusive. Using a genetic approach, we describe the requirement for protein kinase C-delta (PKCδ) in regulating bone resorption by affecting cathepsin K exocytosis. Importantly, PKCδ deficiency does not perturb formation of the ruffled border or trafficking of lysosomal vesicles containing the vacuolar-ATPase (v-ATPase). Mechanistically, we find that cathepsin K exocytosis is controlled by PKCδ through modulation of the actin bundling protein myristoylated alanine-rich C-kinase substrate (MARCKS). The relevance of our finding is emphasized in vivo because PKCδ-/- mice exhibit increased bone mass and are protected from pathological bone loss in a model of experimental postmenopausal osteoporosis. Collectively, our data provide novel mechanistic insights into the pathways that selectively promote secretion of cathepsin K lysosomes independently of ruffled border formation, providing evidence of the presence of multiple mechanisms that regulate lysosomal exocytosis in osteoclasts.

Chromogranin A induces the biogenesis of granules with calcium- and actin-dependent dynamics and exocytosis in constitutively secreting cells

Chromogranins are a family of acidic glycoproteins that play an active role in hormone and neuropeptide secretion through their crucial role in secretory granule biogenesis in neuroendocrine cells. However, the molecular mechanisms underlying their granulogenic activity are still not fully understood. Because we previously demonstrated that the expression of the major component of secretory granules, chromogranin A (CgA), is able to induce the formation of secretory granules in nonendocrine COS-7 cells, we decided to use this model to dissect the mechanisms triggered by CgA leading to the biogenesis and trafficking of such granules. Using quantitative live cell imaging, we first show that CgA-induced organelles exhibit a Ca(2+)-dependent trafficking, in contrast to native vesicle stomatitis virus G protein-containing constitutive vesicles. To identify the proteins that confer such properties to the newly formed granules, we developed CgA-stably-expressing COS-7 cells, purified their CgA-containing granules by subcellular fractionation, and analyzed the granule proteome by liquid chromatography-tandem mass spectrometry. This analysis revealed the association of several cytosolic proteins to the granule membrane, including GTPases, cytoskeleton-based molecular motors, and other proteins with actin- and/or Ca(2+)-binding properties. Furthermore, disruption of cytoskeleton affects not only the distribution and the transport but also the Ca(2+)-evoked exocytosis of the CgA-containing granules, indicating that these granules interact with microtubules and cortical actin for the regulated release of their content. These data demonstrate for the first time that the neuroendocrine factor CgA induces the recruitment of cytoskeleton-, GTP-, and Ca(2+)-binding proteins in constitutively secreting COS-7 cells to generate vesicles endowed with typical dynamics and exocytotic properties of neuroendocrine secretory granules.

CD151 restricts the α6 integrin diffusion mode

Laminin-binding integrins (α3β1, α6β1, α6β4, α7β1) are almost always expressed together with tetraspanin CD151. In every coexpressing cell analyzed to date, CD151 makes a fundamental contribution to integrin-dependent motility, invasion, morphology, adhesion and/or signaling. However, there has been minimal mechanistic insight into how CD151 affects integrin functions. In MDA-MB-231 mammary cells, tetraspanin CD151 knockdown impairs α6 integrin clustering and functions without decreasing α6 integrin expression or activation. Furthermore, CD151 knockdown minimally affects the magnitude of α6 integrin diffusion, as measured using single particle tracking. Instead, CD151 knockdown has a novel and unexpected dysregulating effect on the mode of α6 integrin diffusion. In control cells α6 integrin shows mostly random-confined diffusion (RCD) and some directed motion (DMO). In sharp contrast, in CD151-knockdown cells α6 integrin shows mostly DMO. In control cells α6 diffusion mode is sensitive to actin disruption, talin knockdown and phorbol ester stimulation. By contrast, CD151 knockdown cell α6 integrin is sensitive to actin disruption but desensitized to talin knockdown or phorbol ester stimulation, indicating dysregulation. Both phorbol ester and EGF stimulate cell spreading and promote α6 RCD in control cells. By contrast, CD151-ablated cells retain EGF effects but lose phorbol-ester-stimulated spreading and α6 RCD. For α6 integrins, physical association with CD151 promotes α6 RCD, in support of α6-mediated cable formation and adhesion. By comparison, for integrins not associated with CD151 (e.g. αv integrins), CD151 affects neither diffusion mode nor αv function. Hence, CD151 support of α6 RCD is specific and functionally relevant, and probably underlies diverse CD151 functions in skin, kidney and cancer cells.

Calcium entry through slow-inactivating L-type calcium channels preferentially triggers endocytosis rather than exocytosis in bovine chromaffin cells

Calcium (Ca(2+))-dependent endocytosis has been linked to preferential Ca(2+) entry through the L-type (α(1D), Ca(V)1.3) of voltage-dependent Ca(2+) channels (VDCCs). Considering that the Ca(2+)-dependent exocytotic release of neurotransmitters is mostly triggered by Ca(2+) entry through N-(α(1B), Ca(V)2.2) or PQ-VDCCs (α(1A), Ca(V)2.1) and that exocytosis and endocytosis are coupled, the supposition that the different channel subtypes are specialized to control different cell functions is attractive. Here we have explored this hypothesis in primary cultures of bovine adrenal chromaffin cells where PQ channels account for 50% of Ca(2+) current (I(Ca)), 30% for N channels, and 20% for L channels. We used patch-clamp and fluorescence techniques to measure the exo-endocytotic responses triggered by long depolarizing stimuli, in 1, 2, or 10 mM concentrations of extracellular Ca(2+) ([Ca(2+)](e)). Exo-endocytotic responses were little affected by ω-conotoxin GVIA (N channel blocker), whereas ω-agatoxin IVA (PQ channel blocker) caused 80% blockade of exocytosis as well as endocytosis. In contrast, nifedipine (L channel blocker) only caused 20% inhibition of exocytosis but as much as 90% inhibition of endocytosis. Conversely, FPL67146 (an activator of L VDCCs) notably augmented endocytosis. Photoreleased caged Ca(2+) caused substantially smaller endocytotic responses compared with those produced by K(+) depolarization. Using fluorescence antibodies, no colocalization between L, N, or PQ channels with clathrin was found; a 20-30% colocalization was found between dynamin and all three channel antibodies. This is incompatible with the view that L channels are coupled to the endocytotic machine. Data rather support a mechanism implying the different inactivation rates of L (slow-inactivating) and N/PQ channels (fast-inactivating). Thus a slow but more sustained Ca(2+) entry through L channels could be a requirement to trigger endocytosis efficiently, at least in bovine chromaffin cells.

Permissive role of sphingosine on calcium-dependent endocytosis in chromaffin cells

Sphingosine has been shown to modulate neurotransmitter release. Because membrane fusion and fission involve lipid metabolism, we asked here whether sphingosine had a role in regulating endocytosis. To explore this hypothesis, we monitored changes of membrane capacitance (Cm) to study the effects of intracellular sphingosine on membrane retrieval after chromaffin cell stimulation with depolarising pulses (DPs). We found that: (1) sphingosine dialysis through the patch-clamp pipette (SpD) using the whole-cell configuration of the patch-clamp technique (WCC) favours the appearance of a pronounced endocytotic response; (2) SpD-elicited endocytosis was Ca(2+)-dependent but Ba(2+) did not substitute Ca(2+); (3) under WCC, such endocytotic response disappeared with repetitive DPs; (4) in cells preincubated with sphingomyelinase to augment endogenous sphingosine synthesis, and then voltage-clamped under the perforated-patch configuration of the patch-clamp technique (PPC), endocytosis decayed little with repeated stimulation; (5) sphingosine-1-phosphate (S1P), a metabolite of sphingosine, had a meagre effect on endocytosis; and (6) neither dynamin inhibitor dynasore nor calmodulin blocker calmidazolium affected the sphingosine elicited endocytosis. We believe this is the first report showing that sphingosine plays a permissive role in activating Ca(2+)-dependent endocytosis during cell depolarisation. This effect requires high subplasmalemmal cytosolic Ca(2+) concentrations and a cytosolic factor(s) that is dialysed with the pipette solution. Independence of dynamin and calmodulin suggests that sphingosine-dependent endocytosis could be a novel, more direct pathway for vesicle recycling under mild depolarisation stimuli.

Cortical F-actin, the exocytic mode, and neuropeptide release in mouse chromaffin cells is regulated by myristoylated alanine-rich C-kinase substrate and myosin II

Adrenal medullary chromaffin cells are innervated by the sympathetic splanchnic nerve and translate graded sympathetic firing into a differential hormonal exocytosis. Basal sympathetic firing elicits a transient kiss-and-run mode of exocytosis and modest catecholamine release, whereas elevated firing under the sympathetic stress response results in full granule collapse to release catecholamine and peptide transmitters into the circulation. Previous studies have shown that rearrangement of the cell actin cortex regulates the mode of exocytosis. An intact cortex favors kiss-and-run exocytosis, whereas disrupting the cortex favors the full granule collapse mode. Here, we investigate the specific roles of two actin-associated proteins, myosin II and myristoylated alanine-rich C-kinase substrate (MARCKS) in this process. Our data demonstrate that MARCKS phosphorylation under elevated cell firing is required for cortical actin disruption but is not sufficient to elicit peptide transmitter exocytosis. Our data also demonstrate that myosin II is phospho-activated under high stimulation conditions. Inhibiting myosin II activity prevented disruption of the actin cortex, full granule collapse, and peptide transmitter release. These results suggest that phosphorylation of both MARCKS and myosin II lead to disruption of the actin cortex. However, myosin II, but not MARCKS, is required for the activity-dependent exocytosis of the peptide transmitters.

F-actin and myosin II accelerate catecholamine release from chromaffin granules

The roles of nonmuscle myosin II and cortical actin filaments in chromaffin granule exocytosis were studied by confocal fluorescence microscopy, amperometry, and cell-attached capacitance measurements. Fluorescence imaging indicated decreased mobility of granules near the plasma membrane following inhibition of myosin II function with blebbistatin. Slower fusion pore expansion rates and longer fusion pore lifetimes were observed after inhibition of actin polymerization using cytochalasin D. Amperometric recordings revealed increased amperometric spike half-widths without change in quantal size after either myosin II inhibition or actin disruption. These results suggest that actin and myosin II facilitate release from individual chromaffin granules by accelerating dissociation of catecholamines from the intragranular matrix possibly through generation of mechanical forces.

Myosin II activation and actin reorganization regulate the mode of quantal exocytosis in mouse adrenal chromaffin cells

Chromaffin cells of the adrenal medulla are innervated by the sympathetic nervous system. Stimulation causes chromaffin cells to fire action potentials, leading to the exocytosis of various classes of transmitters into the circulation. Low-frequency electrical stimulation (action potentials delivered at 0.5 Hz) causes adrenal chromaffin cells to selectively release catecholamines through a kiss-and-run fusion event. Elevated electrical stimulation (action potentials at 15 Hz) evokes fusion pore dilation, full granule collapse, and additional release of the neuropeptide-containing proteinaceous granule core. Here we apply single-cell electrophysiological, electrochemical, and fluorescence measurements to investigate the cellular mechanism for this shift in exocytic behavior. We show that at low-frequency stimulation, a filamentous-actin cell cortex plays a key role in stabilizing the kiss-and-run fusion event. Increased stimulation disrupts the actin cortex, driving full granule collapse. We show that pharmacological perturbation of the actin cortex supersedes stimulus frequency in controlling exocytic mode. Finally, we show that nonmuscle myosin II activation contributes to the cytoskeleton-dependent control of the fusion event. Inhibition of myosin II or myosin light chain kinase under elevated stimulation frequencies inhibits fusion pore dilation and maintains the granule in a kiss-and-run mode of exocytosis. These results demonstrate an essential role for activity-evoked cytoskeletal rearrangement and the action of myosin II in the regulation of catecholamine and neuropeptide exocytosis and represent an essential element of the sympathetic stress response.

Pulses of extracellular K+ produce two cytosolic Ca2+ transients that display different temperature dependence and carbonyl cyanide m-chlorophenyl sensitivity in SH-SY5Y cells

In SH-SY5Y cells we have shown that stimulation with high extracellular K+ ([K+]e) evokes a transient increase in cytoplasmic Ca2+ ([Ca2+]cyt) (K+on) that is triggered by the opening of voltage-dependent Ca2+ channels and followed by Ca2+ -induced Ca2+ release from the endoplasmic reticulum (Xu, F., Zhang, J., Recio-Pinto, E. and Blanck, T.J., Halothane and isoflurane augment depolarization-induced cytosolic CA2+ transients and attenuate carbachol-stimulated CA2+ transients, Anesthesiology, 92 (2000) 1746-56). The removal of high-[K+]e results in a second transient increase in [Ca2+]cyt (K+off) that is independent of extracellular Ca2+ (Corrales, A., Montoya, G.J., Sutachan, J.J., Cornillez-Ty, G., Garavito-Aguilar, Z., Xu, F., Blanck, T.J. and Recio-Pinto, E., Transient increases in extracellular K+ produce two pharmacological distinct cytosolic Ca2+ transients, Brain Res., 1031 (2005) 174-184). In this study we further characterize the properties of K+off. We found that K+off was detectable at near physiological temperatures (34-36 degrees C) but, depending on the level of [K+]e, it was undetectable or highly diminished at room temperature. In contrast, K+on was increased by lowering the temperature. Extracellular Na+ -replacement with K+ did not affect K+off, indicating that K+off was not generated by osmolarity changes. Replacement of extracellular Na+ with choline+ did not affect K+off, indicating that K+off did not result from activity changes of the plasma membrane Na+/Ca2+ exchanger. Caffeine decreased K+on but not K+off. CCCP (carbonyl cyanide m-chlorophenyl), a protonophore uncoupler that decreases mitochondrial Ca2+ uptake, decreased K+on but not K+off. CGP37157, an inhibitor of the mitochondria Na+/Ca2+ exchanger, decreased K+off when added alone but not when added simultaneously with CCCP. Clonazepam had similar effects as CGP37157. These findings indicate that the generation of K+off is strongly temperature-dependent and its pharmacology is distinct from the [Ca2+]cyt changes observed previously at room temperature.

Cytoskeletal control of vesicle transport and exocytosis in chromaffin cells

Chromaffin cell exocytosis is a fascinating interplay between secretory vesicles and cellular components. One of these components is the cytoskeleton and its associated regulatory proteins. Transport of chromaffin secretory granules from their site of biosynthesis towards the active site of exocytosis requires both F-actin fine remodelling as well as microtubule trails. At least two molecular motors, myosins II and V, seem to play a crucial role in the control of F-actin dynamics and vectorial vesicle displacement respectively. Vesicle movement experiences spatial restrictions as they approach the cell cortical region, where the F-actin meshwork constitutes a barrier-limiting vesicle access to the plasmalemma. During secretion, cortical F-actin is locally disrupted providing access of vesicles to release sites on the plasmalemma. Removal of the stimulus restores cortical F-actin. Two pathways (Ca2+-scinderin and PKC-MARCKS) control F-actin changes during the secretory cycle . Furthermore, GTPases such as RhoA, that controls F-actin network integrity, and Cdc42 signalling which induces the formation of local actin filaments at active sites, provide additional evidence on the importance of F-actin as a key element in vesicle transport and in the exocytotic machinery of chromaffin cells.

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.

Barrier role of actin filaments in regulated mucin secretion from airway goblet cells

Airway goblet cells secrete mucin onto mucosal surfaces under the regulation of an apical, phospholipase C/G(q)-coupled P2Y(2) receptor. We tested whether cortical actin filaments negatively regulate exocytosis in goblet cells by forming a barrier between secretory granules and plasma membrane docking sites as postulated for other secretory cells. Immunostaining of human lung tissues and SPOC1 cells (an epithelial, mucin-secreting cell line) revealed an apical distribution of beta- and gamma-actin in ciliated and goblet cells. In goblet cells, actin appeared as a prominent subplasmalemmal sheet lying between granules and the apical membrane, and it disappeared from SPOC1 cells activated by purinergic agonist. Disruption of actin filaments with latrunculin A stimulated SPOC1 cell mucin secretion under basal and agonist-activated conditions, whereas stabilization with jasplakinolide or overexpression of beta- or gamma-actin conjugated to yellow fluorescent protein (YFP) inhibited secretion. Myristoylated alanine-rich C kinase substrate, a PKC-activated actin-plasma membrane tethering protein, was phosphorylated after agonist stimulation, suggesting a translocation to the cytosol. Scinderin (or adseverin), a Ca(2+)-activated actin filament severing and capping protein was cloned from human airway and SPOC1 cells, and synthetic peptides corresponding to its actin-binding domains inhibited mucin secretion. We conclude that actin filaments negatively regulate mucin secretion basally in airway goblet cells and are dynamically remodeled in agonist-stimulated cells to promote exocytosis.