Mechanisms and roles of muscarinic activation in guinea-pig adrenal medullary cells

Expression of Mitochondrial Uncoupling Proteins and GABA Signaling Molecules in Unstimulated and Nerve Growth Factor-Stimulated PC12 Cells: Models for Chromaffin Cells and Sympathetic Neurons

PC12 cells are a cell line originating from rat adrenal medullary chromaffin (AMC) cells. They extend a neurite-like structure in response to nerve growth factor (NGF). Thus, unstimulated and NGF-stimulated PC12 cells are used as models for AMC cells and sympathetic ganglion cells, respectively. However, how closely unstimulated and stimulated PC12 cells resemble AMC cells and sympathetic neurons, respectively, has not been elucidated sufficiently. We explored these issues by using biochemical and immunocytochemical methods. AMC cells and PC12 cells selectively expressed uncoupling protein 3 (UCP3) and uncoupling protein 4 (UCP4), respectively, and glucocorticoid activity inhibited UCP4 expression in PC12 cells. PC12 cells expressed extremely low levels of chromaffin granule-associated proteins, whereas the amount of synaptophysin, a synaptic vesicle-associated protein, was much higher than that in the adrenal medulla. Similar to AMC cells, the muscarinic receptor type 1 was located at the cell periphery in unstimulated PC12 cells, and its expression was markedly enhanced by NGF. Furthermore, NGF stimulation abolished the expression of GABA signaling molecules in PC12 cells. The results suggest that the properties of unstimulated PC12 cells are between those of AMC cells and sympathetic ganglion cells and GABA signaling is intrinsic to AMC cells.

Phospholipase C-ε defines a PACAP-stimulated pathway for secretion in the chromaffin cell

Adrenomedullary chromaffin cells respond to splanchnic (sympathetic) nerve stimulation by releasing stress hormones into the circulation. The signal for hormone secretion is encoded in the neurotransmitters - especially acetylcholine (ACh) and pituitary adenylate cyclase activating polypeptide (PACAP) - that are released into the splanchnic-chromaffin cell synapse. However, functional differences in the effects of ACh and PACAP on the chromaffin cell secretory response are not well defined. Here, selective agonists of PACAP receptors or nicotinic and muscarinic acetylcholine receptors were applied to chromaffin cells. The major differences in the effects of these agents were not on exocytosis, per se, but rather on the steps upstream of exocytosis. In almost every respect, the properties of individual fusion events triggered by PACAP and cholinergic agonists were similar. On the other hand, the properties of the Ca2+ transients evoked by PACAP differed in several ways from those evoked by muscarinic and nicotinic receptor stimulation. A defining feature of the PACAP-stimulated secretory pathway was its dependence on signaling through exchange protein directly activated by cAMP (Epac) and PLCε. However, the absence of PLCε did not disrupt Ca2+ transients evoked by cholinergic agonists. Accordingly, inhibition of Epac activity did not disrupt secretion triggered by acetylcholine or specific agonists of muscarinic and nicotinic receptors. Thus, PACAP and acetylcholine stimulate chromaffin cell secretion via separate and independent pathways. This feature of stimulus-secretion coupling may be important for sustaining hormone release from the adrenal medulla under conditions associated with the sympathetic stress response.

Enhancement of muscarinic receptor-mediated excitation in spontaneously hypertensive rat adrenal medullary chromaffin cells

One of the mechanisms for hypertension is an increase in blood catecholamines due to increased secretion from sympathetic nerve terminals and adrenal medullary chromaffin (AMC) cells. Spontaneously hypertensive rats (SHRs) are used as an animal model of hypertension. Catecholamine secretion in AMC cells occurs in response to humoral factors and neuronal inputs from the sympathetic nerve fibres. Acetylcholine (ACh) released from the nerve terminals activates nicotinic as well as muscarinic ACh receptors. The present experiment aimed to elucidate whether muscarinic receptor-mediated excitation is altered in SHR AMC cells and, if it is, how. Compared with normotensive rat AMC cells, muscarinic stimulation induced greater catecholamine secretion and larger depolarising inward currents in SHR AMC cells. In contrast to normotensive rat AMC cells, the muscarine-induced current consisted of quinine-sensitive and quinine-insensitive components. The former and the latter are possibly ascribed to nonselective cation channel activation and TWIK-related acid-sensitive K+ (TASK) channel inhibition, as noted in guinea pig AMC cells. In fact, immunoreactive material for TASK1 and several isoforms of transient receptor potential canonical (TRPC) channels was detected in SHR AMC cells. Stromal interaction molecule 1 (STIM1), which plays an essential role for heteromeric TRPC1-TRPC4 channel formation and is not expressed in normotensive rat AMC cells, was detected in the cytoplasm and co-localised with TRPC1. The expression of muscarinic M1 receptors was enhanced in SHR AMC cells compared with normotensive rats. The results indicate that muscarinic excitation is enhanced in SHR AMC cells, probably through facilitation of TRPC channel signalling.

Sodium background currents in endocrine/neuroendocrine cells: Towards unraveling channel identity and contribution in hormone secretion

In endocrine/neuroendocrine tissues, excitability of secretory cells is patterned by the repertoire of ion channels and there is clear evidence that extracellular sodium (Na⁺) ions contribute to hormone secretion. While voltage-gated channels involved in action potential generation are well-described, the background 'leak' channels operating near the resting membrane potential are much less known, and in particular the channels supporting a background entry of Na⁺ ions. These background Na⁺ currents (called here 'I_Nab') have the ability to modulate the resting membrane potential and subsequently affect action potential firing. Here we compile and analyze the data collected from three endocrine/neuroendocrine tissues: the anterior pituitary gland, the adrenal medulla and the endocrine pancreas. We also model how I_Nab can be functionally involved in cellular excitability. Finally, towards deciphering the physiological role of I_Nab in endocrine/neuroendocrine cells, its implication in hormone release is also discussed.

Mechanisms for establishment of GABA signaling in adrenal medullary chromaffin cells

γ-Aminobutyric acid (GABA) is thought to play a paracrine role in adrenal medullary chromaffin (AMC) cells. Comparative physiological and immunocytochemical approaches were used to address the issue of how the paracrine function of GABA in AMC cells is established. GABA_A receptor Cl^- channel activities in AMC cells of rats and mice, where corticosterone is the major glucocorticoid, were much smaller than those in AMC cells of guinea-pigs and cattle, where cortisol is the major. The extent of enhancement of GABA_A receptor α3 subunit expression in rat pheochromocytoma (PC12) cells by cortisol was larger than that by corticosterone in parallel with their glucocorticoid activities. Thus, the species difference in GABA_A receptor expression may be ascribed to a difference in glucocorticoid activity between corticosterone and cortisol. GABA_A receptor Cl^- channel activity in mouse AMC cells was enhanced by allopregnanolone, as noted with that in guinea-pig AMC cells, and the enzymes involved in allopregnanolone production were immunohistochemically detected in the zona fasciculata in both mice and guinea pigs. The expression of glutamic acid decarboxylase 67 (GAD67), one of the GABA synthesizing enzymes, increased after birth, whereas GABA_A receptors already developed at birth. Stimulation of pituitary adenylate cyclase-activating polypeptide (PACAP) receptors, but not nicotinic or muscarinic receptors, in PC12 cells, resulted in an increase in GAD67 expression in a protein-kinase A-dependent manner. The results indicate that glucocorticoid and PACAP are mainly responsible for the expressions of GABA_A receptors and GAD67 involved in GABA signaling in AMC cells, respectively.

Expression of p11 and Heteromeric TASK Channels in Rat Carotid Body Glomus Cells and Nerve Growth Factor-differentiated PC12 Cells

TWIK-related acid-sensitive K⁺ (TASK) homomeric channels, TASK1 and TASK3, are present in PC12 cells. The channels do not heteromerize due plausibly to a lack of p11 protein. Single-channel recording reveals that most of the rat carotid body (CB) glomus cells express heteromeric TASK1-TASK3 channels, but the presence of p11 in glomus cells has not yet been verified. TASK1, but not TASK3, binds to p11, which has a retention signal for the endoplasmic reticulum. We hypothesized that p11 could facilitate heteromeric TASK1-TASK3 formation in glomus cells. We investigated this hypothesis in isolated immunocytochemically identified rat CB glomus cells. The findings were that glomus cells expressed p11-like immunoreactive (IR) material, and TASK1- and TASK3-like IR material mainly coincided in the cytoplasm. The proximity ligation assay showed that TASK1 and TASK3 heteromerized. In separate experiments, supporting evidence for the major role of p11 for channel heteromerization was provided in PC12 cells stimulated by nerve growth factor. p11 production took place there via multiple signaling pathways comprising mitogen-activated protein kinase and phospholipase C, and heteromeric TASK1-TASK3 channels were formed. We conclude that p11 is expressed and TASK1 and TASK3 heteromerize in rat CB glomus cells.

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.

STIM1-dependent membrane insertion of heteromeric TRPC1-TRPC4 channels in response to muscarinic receptor stimulation

Muscarinic receptor stimulation results in activation of nonselective cation (NSC) channels in guinea pig adrenal medullary (AM) cells. The biophysical and pharmacological properties of the NSC channel suggest the involvement of heteromeric channels of TRPC1 with TRPC4 or TRPC5. This possibility was explored in PC12 cells and guinea pig AM cells. Proximity ligation assay (PLA) revealed that when exogenously expressed in PC12 cells, TRPC1 forms a heteromeric channel with TRPC4, but not with TRPC5, in a STIM1-dependent manner. The heteromeric TRPC1-TRPC4 channel was also observed in AM cells and trafficked to the cell periphery in response to muscarine stimulation. To explore whether heteromeric channels are inserted into the cell membrane, tags were attached to the extracellular domains of TRPC1 and TRPC4. PLA products developed between the tags in cells stimulated by muscarine, but not in resting cells, indicating that muscarinic stimulation results in the membrane insertion of channels. This membrane insertion required expression of full-length STIM1. We conclude that muscarinic receptor stimulation results in the insertion of heteromeric TRPC1-TRPC4 channels into the cell membrane in PC12 cells and guinea pig AM cells.

Differences among muscarinic agonists in M1 receptor-mediated nonselective cation channel activation and TASK1 channel inhibition in adrenal medullary cells

Muscarinic receptor stimulation induces depolarizing inward currents and catecholamine secretion in adrenal medullary (AM) cells from various mammals. In guinea-pig AM cells muscarine and oxotremorine at concentrations ≤ 1 μM produce activation of nonselective cation channels with a similar potency and efficacy, whereas muscarine at higher concentrations produces not only nonselective cation channel activation, but also TASK1 channel inhibition. In rat AM cells, the muscarinic M₁ receptor is involved in TASK1 channel inhibition in response to muscarinic agonists, and the efficacy of oxotremorine is half that of muscarine. These pharmacological findings might indicate that different muscarinic receptor subtypes are responsible for the regulation of nonselective cation and TASK1 channel activities. The present study aimed to determine the muscarinic receptor subtypes involved in nonselective cation channel activation in guinea-pig and mouse AM cells. The inward current evoked by 1 μM muscarine was completely suppressed by 100 μM quinine, whereas 30 μM muscarine-induced inward currents were comprised of quinine-sensitive and -insensitive components. The electrophysiological and pharmacological properties of the muscarine-induced currents indicated that the quinine-sensitive and insensitive components are due to nonselective cation channel activation and TASK1 channel inhibition, respectively. Muscarine at 30 μM failed to induce any current in AM cells treated with muscarinic toxin 7 or genetically deleted of the M₁ receptor. The K_D value of VU0255035 against the muscarinic receptor mediating nonselective cation channel activation was 17.5 nM. These results indicate that the M₁ receptor mediates nonselective cation channel activation as well as TASK1 channel inhibition.

Molecular mechanism for muscarinic M1 receptor-mediated endocytosis of TWIK-related acid-sensitive K+ 1 channels in rat adrenal medullary cells

KEY POINTS

The muscarinic acetylcholine receptor (mAChR)-mediated increase in excitability in rat adrenal medullary cells is at least in part due to inhibition of TWIK (tandem of P domains in a weak inwardly rectifying K⁺ channel)-related acid-sensitive K⁺ (TASK)1 channels. In this study we focused on the molecular mechanism of mAChR-mediated inhibition of TASK1 channels. Exposure to muscarine resulted in a clathrin-dependent endocytosis of TASK1 channels following activation of the muscarinic M₁ receptor (M₁ R). This muscarinic signal for the endocytosis was mediated in sequence by phospholipase C (PLC), protein kinase C (PKC), and then the non-receptor tyrosine kinase Src with the consequent tyrosine phosphorylation of TASK1. The present results establish that TASK1 channels are tyrosine phosphorylated and internalized in a clathrin-dependent manner in response to M₁ R stimulation and this translocation is at least in part responsible for muscarinic inhibition of TASK1 channels in rat AM cells.

ABSTRACT

Activation of muscarinic receptor (mAChR) in rat adrenal medullary (AM) cells induces depolarization through the inhibition of TWIK-related acid-sensitive K⁺ (TASK)1 channels. Here, pharmacological and immunological approaches were used to elucidate the molecular mechanism for this mAChR-mediated inhibition. TASK1-like immunoreactive (IR) material was mainly located at the cell periphery in dissociated rat AM cells, and its majority was internalized in response to muscarine. The muscarine-induced inward current and translocation of TASK1 were suppressed by dynasore, a dynamin inhibitor. The muscarinic translocation was suppressed by MT7, a specific M₁ antagonist, and the dose-response curves for muscarinic agonist-induced translocation were similar to those for the muscarinic inhibition of TASK1 currents. The muscarine-induced inward current and/or translocation of TASK1 were suppressed by inhibitors for phospholipase C (PLC), protein kinase C (PKC), and/or Src. TASK1 channels in AM cells and PC12 cells were transiently associated with Src and were tyrosine phosphorylated in response to muscarinic stimulation. After internalization, TASK1 channels were quickly dephosphorylated even while they remained in the cytoplasm. The cytoplasmic TASK1-like IR material quickly recycled back to the cell periphery after muscarine stimulation for 0.5 min, but not 10 min. We conclude that M₁ R stimulation results in internalization of TASK1 channels through the PLC-PKC-Src pathway with the consequent phosphorylation of tyrosine and that this M₁ R-mediated internalization is at least in part responsible for muscarinic inhibition of TASK1 channels in rat AM cells.

Src mediates endocytosis of TWIK-related acid-sensitive K+ 1 channels in PC12 cells in response to nerve growth factor

TWIK-related acid-sensitive K+ (TASK) channels produce background K+ currents. We elucidated that TASK1 channels in rat adrenal medullary cells and PC12 cells are internalized in a clathrin-dependent manner in response to nerve growth factor (NGF). Here, the molecular mechanism for this internalization in PC12 cells was explored. The combination of enzyme inhibitors with tropomyosin receptor kinase A mutants revealed that the internalization was mediated by both phospholipase C and phosphatidylinositol 3-kinase pathways that converge on protein kinase C with the consequent activation of Src, a nonreceptor tyrosine kinase. The NGF-induced endocytosis of TASK1 channels did not occur in the presence of the Src inhibitor or with the expression of a kinase-dead Src mutant. Additionally, NGF induced a transient colocalization of Src with the TASK1 channel, but not the TASK1 mutant, in which tyrosine at 370 was replaced with phenylalanine. This TASK1 mutant showed no increase in tyrosine phosphorylation and markedly diminished internalization in response to NGF. We concluded that NGF induces endocytosis of TASK1 channels via tyrosine phosphorylation in its carboxyl terminus.

Catecholamine secretion by chemical hypoxia in guinea-pig, but not rat, adrenal medullary cells: differences in mitochondria

The effects of mitochondrial inhibitors (CN(-), a complex IV inhibitor and CCCP, protonophore) on catecholamine (CA) secretion and mitochondrial function were explored functionally and biochemically in rat and guinea-pig adrenal chromaffin cells. Guinea-pig chromaffin cells conspicuously secreted CA in response to CN(-) or CCCP, but rat cells showed a little, if any, secretory response to either of them. The resting metabolic rates in rat adrenal medullae did not differ from those in guinea-pig adrenal medullae. On the other hand, the time course of depolarization of the mitochondrial membrane potential (ΔΨm) in guinea-pig chromaffin cells in response to CN(-) was slower than that in rat chromaffin cells, and this difference was abolished by oligomycin, an F1F0-ATPase inhibitor. The extent of CCCP-induced decrease in cellular ATP in guinea-pig chromaffin cells, which was indirectly measured using a Mg(2+) indicator, was smaller than that in rat chromaffin cells. Relative expression levels of F1F0-ATPase inhibitor factor in guinea-pig adrenal medullae were smaller than in rat adrenal medullae, and the opposite was true for F1F0-ATPase α subunit. The present results indicate that guinea-pig chromaffin cells secrete more CA in response to a mitochondrial inhibitor than rat chromaffin cells and this higher susceptibility in the former is accounted for by a larger extent of reversed operation of F1F0-ATPase with the consequent decrease in ATP under conditions where ΔΨm is depolarized.

Two-pore domain potassium channels in the adrenal cortex

The physiological control of steroid hormone secretion from the adrenal cortex depends on the function of potassium channels. The "two-pore domain K(+) channels" (K2P) TWIK-related acid sensitive K(+) channel 1 (TASK1), TASK3, and TWIK-related K(+) channel 1 (TREK1) are strongly expressed in adrenocortical cells. They confer a background K(+) conductance to these cells which is important for the K(+) sensitivity as well as for angiotensin II and adrenocorticotropic hormone-dependent stimulation of aldosterone and cortisol synthesis. Mice with single deletions of the Task1 or Task3 gene as well as Task1/Task3 double knockout mice display partially autonomous aldosterone synthesis. It appears that TASK1 and TASK3 serve different functions: TASK1 affects cell differentiation and prevents expression of aldosterone synthase in the zona fasciculata, while TASK3 controls aldosterone secretion in glomerulosa cells. TREK1 is involved in the regulation of cortisol secretion in fasciculata cells. These data suggest that a disturbed function of K2P channels could contribute to adrenocortical pathologies in humans.

Identification of muscarinic receptor subtypes involved in catecholamine secretion in adrenal medullary chromaffin cells by genetic deletion

BACKGROUND AND PURPOSE

Activation of muscarinic receptors results in catecholamine secretion in adrenal chromaffin cells in many mammals, and muscarinic receptors partly mediate synaptic transmission from the splanchnic nerve, at least in guinea pigs. To elucidate the physiological functions of muscarinic receptors in chromaffin cells, it is necessary to identify the muscarinic receptor subtypes involved in excitation.

EXPERIMENTAL APPROACH

To identify muscarinic receptors, pharmacological tools and strains of mice where one or several muscarinic receptor subtypes were genetically deleted were used. Cellular responses to muscarinic stimulation in isolated chromaffin cells were studied with the patch clamp technique and amperometry.

KEY RESULTS

Muscarinic M₁, M₄ and M₅ receptors were immunologically detected in mouse chromaffin cells, and these receptors disappeared after the appropriate gene deletion. Mouse cells secreted catecholamines in response to muscarinic agonists, angiotensin II and a decrease in external pH. Genetic deletion of M₁, but not M₃, M₄ or M₅, receptors in mice abolished secretion in response to muscarine, but not to other stimuli. The muscarine-induced secretion was suppressed by MT7, a snake peptide toxin specific for M₁ receptors. Similarly, muscarine failed to induce an inward current in the presence of MT7 in mouse and rat chromaffin cells. The binding affinity of VU0255035 for the inhibition of muscarine-induced currents agreed with that for the M₁ receptor.

CONCLUSIONS AND IMPLICATIONS

Based upon the effects of genetic deletion of muscarinic receptors and MT7, it is concluded that the M₁ receptor alone is responsible for muscarine-induced catecholamine secretion.