Metabolism and trafficking of N-type voltage-operated calcium channels in neurosecretory cells

De novo transcriptome assembly for the lobster Homarus americanus and characterization of differential gene expression across nervous system tissues

BACKGROUND

The American lobster, Homarus americanus, is an important species as an economically valuable fishery, a key member in marine ecosystems, and a well-studied model for central pattern generation, the neural networks that control rhythmic motor patterns. Despite multi-faceted scientific interest in this species, currently our genetic resources for the lobster are limited. In this study, we de novo assemble a transcriptome for Homarus americanus using central nervous system (CNS), muscle, and hybrid neurosecretory tissues and compare gene expression across these tissue types. In particular, we focus our analysis on genes relevant to central pattern generation and the identity of the neurons in a neural network, which is defined by combinations of genes distinguishing the neuronal behavior and phenotype, including ion channels, neurotransmitters, neuromodulators, receptors, transcription factors, and other gene products.

RESULTS

Using samples from the central nervous system (brain, abdominal ganglia), abdominal muscle, and heart (cardiac ganglia, pericardial organs, muscle), we used RNA-Seq to characterize gene expression patterns across tissues types. We also compared control tissues with those challenged with the neuropeptide proctolin in vivo. Our transcriptome generated 34,813 transcripts with known protein annotations. Of these, 5,000-10,000 of annotated transcripts were significantly differentially expressed (DE) across tissue types. We found 421 transcripts for ion channels and identified receptors and/or proteins for over 20 different neurotransmitters and neuromodulators. Results indicated tissue-specific expression of select neuromodulator (allostatin, myomodulin, octopamine, nitric oxide) and neurotransmitter (glutamate, acetylcholine) pathways. We also identify differential expression of ion channel families, including kainite family glutamate receptors, inward-rectifying K(+) (IRK) channels, and transient receptor potential (TRP) A family channels, across central pattern generating tissues.

CONCLUSIONS

Our transcriptome-wide profiles of the rhythmic pattern generating abdominal and cardiac nervous systems in Homarus americanus reveal candidates for neuronal features that drive the production of motor output in these systems.

The involvement of actin, calcium channels and exocytosis proteins in somato-dendritic oxytocin and vasopressin release

Hypothalamic magnocellular neurons release vasopressin and oxytocin not only from their axon terminals into the blood, but also from their somata and dendrites into the extracellular space of the brain, and this can be regulated independently. Differential release of neurotransmitters from different compartments of a single neuron requires subtle regulatory mechanisms. Somato-dendritic, but not axon terminal release can be modulated by changes in intracellular calcium concentration [(Ca²⁺)] by release of calcium from intracellular stores, resulting in priming of dendritic pools for activity-dependent release. This review focuses on our current understanding of the mechanisms of priming and the roles of actin remodeling, voltage-operated calcium channels (VOCCs) and SNARE proteins in the regulation somato-dendritic and axon terminal peptide release.

N- and P/Q-type Ca2+ channels in adrenal chromaffin cells

Ca2+ is the most ubiquitous second messenger found in all cells. Alterations in [Ca2+]i contribute to a wide variety of cellular responses including neurotransmitter release, muscle contraction, synaptogenesis and gene expression. Voltage-dependent Ca2+ channels, found in all excitable cells (Hille 1992), mediate the entry of Ca2+ into cells following depolarization. Ca2+ channels are composed of a large pore-forming subunit, called the alpha1 subunit, and several accessory subunits. Ten different alpha1 subunit genes have been identified and classified into three families, Ca(v1-3) (Dunlap et al. 1995, Catterall 2000). Each alpha1 gene produces a unique Ca2+ channel. Although chromaffin cells express several different types of Ca2+ channels, this review will focus on the Cav(2.1) and Cav(2.2) channels, also known as P/Q- and N-type respectively (Nowycky et al. 1985, Llinas et al. 1989b, Wheeler et al. 1994). These channels exhibit physiological and pharmacological properties similar to their neuronal counterparts. N-, P/Q and to a lesser extent R-type Ca2+ channels are known to regulate neurotransmitter release (Hirning et al. 1988, Horne & Kemp 1991, Uchitel et al. 1992, Luebke et al. 1993, Takahashi & Momiyama 1993, Turner et al. 1993, Regehr & Mintz 1994, Wheeler et al. 1994, Wu & Saggau 1994, Waterman 1996, Wright & Angus 1996, Reid et al. 1997). N- and P/Q-type Ca2+ channels are abundant in nerve terminals where they colocalize with synaptic vesicles. Similarly, these channels play a role in neurotransmitter release in chromaffin cells (Garcia et al. 2006). N- and P/Q-type channels are subject to many forms of regulation (Ikeda & Dunlap 1999). This review pays particular attention to the regulation of N- and P/Q-type channels by heterotrimeric G-proteins, interaction with SNARE proteins, and channel inactivation in the context of stimulus-secretion coupling in adrenal chromaffin cells.

ω-Conotoxin MVIIA inhibits amygdaloid kindled seizures in Sprague–Dawley rats

omega-Conotoxin MVIIA (omega-CTX MVIIA) is a reversible and potent antagonist of N-type voltage-dependent calcium channels (VDCCs) in neurons. In this study, we evaluated the effect of a fusion form of omega-CTX MVIIA with glutathione S-transferase (GST) on amygdaloid kindled seizures. Intracerebraventricular (i.c.v.) injection of the fusion protein of GST-omega-CTX MVIIA significantly decreased seizure stage and shortened afterdischarge duration and generalized seizure duration in a dose-dependent and time-related manner. In addition, GST-omega-CTX MVIIA significantly increased the GABA levels in the cortex and glycine levels in the brainstem. In contrast, GST alone did not have any effect on seizure behavior or neurochemical levels. These findings firstly demonstrate that the N-type VDCC is a potential therapeutic target for temporal lobe epilepsy. The mechanism of the anticonvulsant function of omega-CTX MVIIA is related to the blockade of N-type VDCC-mediated neurotransmitter release in the brain.

The role of voltage-gated calcium channels in pancreatic beta-cell physiology and pathophysiology

Voltage-gated calcium (CaV) channels are ubiquitously expressed in various cell types throughout the body. In principle, the molecular identity, biophysical profile, and pharmacological property of CaV channels are independent of the cell type where they reside, whereas these channels execute unique functions in different cell types, such as muscle contraction, neurotransmitter release, and hormone secretion. At least six CaValpha1 subunits, including CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1, have been identified in pancreatic beta-cells. These pore-forming subunits complex with certain auxiliary subunits to conduct L-, P/Q-, N-, R-, and T-type CaV currents, respectively. beta-Cell CaV channels take center stage in insulin secretion and play an important role in beta-cell physiology and pathophysiology. CaV3 channels become expressed in diabetes-prone mouse beta-cells. Point mutation in the human CaV1.2 gene results in excessive insulin secretion. Trinucleotide expansion in the human CaV1.3 and CaV2.1 gene is revealed in a subgroup of patients with type 2 diabetes. beta-Cell CaV channels are regulated by a wide range of mechanisms, either shared by other cell types or specific to beta-cells, to always guarantee a satisfactory concentration of Ca2+. Inappropriate regulation of beta-cell CaV channels causes beta-cell dysfunction and even death manifested in both type 1 and type 2 diabetes. This review summarizes current knowledge of CaV channels in beta-cell physiology and pathophysiology.

Chronic PKC-beta activation in HT-29 Cl.19a colonocytes prevents cAMP-mediated ion secretion by inhibiting apical membrane current generation

We investigated the effects of PKC-stimulating 12-deoxyphorbol 13-phenylacetate 20-acetate (DOPPA) and phorbol 12-myristate 13-acetate (PMA) phorbol esters on cAMP-dependent, forskolin (FSK)-stimulated, short-circuit Cl- current (ISC-cAMP) generation by colonocyte monolayers. These agonists elicited different actions depending on their dose and incubation time; PMA effects at the onset (<5 min) were independent of cAMP agonist and were characterized by transient anion-dependent transcellular and apical membrane ISC generation. DOPPA failed to elicit similar responses. Whereas chronic (24 h) exposure to both agents inhibited FSK-stimulated transcellular and apical membrane ISC-cAMP, the effects of DOPPA were more complex: this conventional PKC-beta-specific agonist also stimulated Ba2+-sensitive basolateral membrane-dependent facilitation of transcellular ISC-cAMP. PMA did not elicit a similar phenomenon. Prolonged exposure to high-dose PMA but not DOPPA led to apical membrane ISC-cAMP recovery. Changes in PKC alpha-, beta1-, gamma-, and epsilon-isoform membrane partitioning and expression correlated with these findings. PMA-induced transcellular ISC correlated with PKC-alpha membrane association, whereas low doses of both agents inhibited transcellular and apical membrane ISC-cAMP, increased PKC-beta1, decreased PKC-beta2 membrane association, and caused reciprocal changes in isoform mass. During the apical membrane ISC-cAMP recovery after prolonged high-dose PMA exposure, an almost-complete depletion of cellular PKC-beta1 and a significant reduction in PKC-epsilon mass occurred. Thus activated PKC-beta1 and/or PKC-epsilon prevented, whereas activated PKC-alpha facilitated, apical membrane ISC-cAMP. PKC-beta-dependent augmentation of transcellular ISC-cAMP at the level of the basolateral membrane demonstrated that transport events with geographically distinct subcellular membranes can be independently regulated by the PKC beta-isoform.

Role of calcium channels in carboxyl-terminal parathyroid hormone receptor signaling

Parathyroid hormone (PTH), an 84-amino acid polypeptide, is a major systemic regulator of calcium homeostasis that activates PTH/PTHrP receptors (PTH1Rs) on target cells. Carboxyl fragments of PTH (CPTH), secreted by the parathyroids or generated by PTH proteolysis in the liver, circulate in blood at concentrations much higher than intact PTH-(1–84) but cannot activate PTH1Rs. Receptors specific for CPTH fragments (CPTHRs), distinct from PTH1Rs, are expressed by bone cells, especially osteocytes. Activation of CPTHRs was previously reported to modify intracellular calcium within chondrocytes. To further investigate the mechanism of action of CPTHRs in osteocytes, cytosolic free calcium concentration ([Ca2+]i) was measured in the PTH1R-null osteocytic cell line OC59, which expresses abundant CPTHRs but no PTH1Rs. [Ca2+]iwas assessed by single-cell ratiometric microfluorimetry in fura-2-loaded OC59 cells. A rapid and transient increase in [Ca2+]iwas observed in OC59 cells in response to the CPTH fragment hPTH-(53–84) (250 nM). No [Ca2+]isignal was observed in COS-7 cells, in which CPTHR binding also cannot be detected. Neither hPTH-(1–34) nor a mutant CPTH analog, [Ala55–57]hPTH-(53–84), that does not to bind to CPTHRs, increased [Ca2+]iin OC59 cells. The [Ca2+]iresponse to hPTH-(53–84) required the presence of extracellular calcium and was blocked by inhibitors of voltage-dependent calcium channels (VDCCs), including nifedipine (100 nM), ω-agatoxin IVA (10 nM), and ω-conotoxin GVIA (100 nM). We conclude that activation of CPTHRs in OC59 osteocytic cells leads to a rapid increase in influx of extracellular calcium, most likely through the opening of VDCCs.

Blockade of presynaptic voltage-gated calcium channels in the medial prefrontal cortex of neonatal rats leads to post-pubertal alterations in locomotor behavior

Although the etiology of neurodevelopmental mental disorders remains obscure, converging lines of evidence using animal modeling suggest a critical role for activity-dependent neurodevelopmental processes during neonatal life. Here, we report the behavioral effects of a novel technique designed to induce targeted, transient disruption of activity-dependent processes in early development via reduction of calcium-mediated neurotransmitter release. We examined the post-pubertal behavioral effects of neonatal (postnatal day 7) medial prefrontal cortex infusion of either vehicle or N-type and P/Q-type presynaptic voltage-dependent calcium channel blockers (omega-conotoxins MVIIA and MVIIC respectively; 6.8 and 45 pmol infused respectively) in rat pups. In a test of amphetamine-induced behavioral sensitization, neonatal omega-conotoxin MVIIA treatment significantly increased locomotion following repeated amphetamine injections (1.5 mg/kg i.p.) and significantly decreased locomotion following repeated saline injections relative to animals treated neonatally with vehicle. However, there was no effect of conotoxin treatment on the long-term expression of amphetamine sensitization. Neonatal treatment with omega-conotoxins had no effect on the other behaviors assayed, namely, acoustic startle response, prepulse inhibition of startle, novelty- and amphetamine-induced (1.5 mg/kg i.p.) locomotion, and anxiety-like behavior in the elevated plus-maze. These data confirm that transient, region-specific disruption of synaptic transmission during early development can have long-term effects on behaviors relevant to neurodevelopmental mental disorders.

Role of high-voltage-activated calcium channels in glucose-regulated beta-cell calcium homeostasis and insulin release

High-voltage-activated (HVA) calcium channels are known to be the primary source of calcium for glucose-stimulated insulin secretion. However, few studies have investigated how these channels can be regulated by chronically elevated levels of glucose. In the present study, we determined the level of expression of the four major HVA calcium channels (N-type, P/Q-type, L(C)-type, and L(D)-type) in rat pancreatic beta-cells. Using quantitative real-time PCR (QRT-PCR), we found the expression of all four HVA genes in rat insulinoma cells (INS-1) and in primary isolated rat islet cells. We then determined the role of each channel in insulin secretion by using channel-selective antagonists. Insulin secretion analysis revealed that N- and L-type channels are both involved in immediate glucose-induced insulin secretion. However, L-type was preferentially coupled to secretion at later time points. P/Q-type channels were not found to play a role in insulin secretion at any stage. It was also found that long-term exposure to elevated glucose increases basal calcium in these cells. Interestingly, chronically elevated glucose decreased the mRNA expression of the channels involved with insulin secretion and diminished the level of stimulated calcium influx in these cells. Using whole cell patch clamp, we found that N- and L-type channel currents increase gradually subsequent to lower intracellular calcium perfusion, suggesting that these channels may be regulated by glucose-induced changes in calcium.

Targeting mechanisms of high voltage-activated Ca2+ channels

Functional voltage-dependent Ca2+ channel complexes are assembled by three to four subunits: alpha1, beta, alpha2delta subunits (C. Leveque et al., 1994, J. Biol Chem. 269, 6306-6312; M. W. McEnery et al., 1991, Proc. Natl. Acad. Sci. U.S.A. 88, 11095-11099) and at least in muscle cells also y subunits (B. M. Curtis and W. A. Catterall, 1984, Biochemistry 23, 2113-2118). Ca2+ channels mediate the voltage-dependent Ca2+ influx in subcellular compartments, triggering such diverse processes as neurotransmitter release, dendritic action potentials, excitation-contraction, and excitation-transcription coupling. The targeting of biophysically defined Ca2+ channel complexes to the correct subcellular structures is, thus, critical to proper cell and physiological functioning. Despite their importance, surprisingly little is known about the targeting mechanisms by which Ca2+ channel complexes are transported to their site of function. Here we summarize what we know about the targeting of Ca2+ channel complexes through the cell to the plasma membrane and subcellular structures.

Release of beta-nicotinamide adenine dinucleotide upon stimulation of postganglionic nerve terminals in blood vessels and urinary bladder

Chemical signaling in autonomic neuromuscular transmission involves agents that function as neurotransmitters and/or neuromodulators. Using high performance liquid chromatography techniques with fluorescence and electrochemical detection we observed that, in addition to ATP and norepinephrine (NE), electrical field stimulation (EFS, 4-16 Hz, 0.1-0.3 ms, 15 V, 60-120 s) of isolated vascular and non-vascular preparations co-releases a previously unidentified compound with apparent nucleotide or nucleoside structure. Extensive screening of more than 25 nucleotides and nucleosides followed by detailed peak identification revealed that beta-nicotinamide adenine dinucleotide (beta-NAD) is released in tissue superfusates upon EFS of canine mesenteric artery (CMA), canine urinary bladder, and murine urinary bladder in the amounts of 7.1 +/- 0.7, 26.5 +/- 4.5, and 15.1 +/- 3.2 fmol/mg of tissue, respectively. Smaller amounts of the beta-NAD metabolites cyclic adenosine 5'-diphosphoribose (cADPR) and ADPR were also present in the superfusates collected during EFS of CMA (2.5 +/- 0.9 and 5.8 +/- 0.8 fmol/mg of tissue, respectively), canine urinary bladder (1.8 +/- 0.5 and 9.0 +/- 6.0 fmol/mg of tissue, respectively), and murine urinary bladder (1.4 +/- 0.1 and 6.2 +/- 2.4 fmol/mg of tissue, respectively). The three nucleotides were also detected in the samples collected before EFS (0.2-1.6 fmol/mg of tissue). Exogenous beta-NAD, cADPR, and ADPR (all 100 nm) reduced the release of NE in CMA at 16 Hz from 27.8 +/- 6.0 fmol/mg of tissue to 15.5 +/- 5.0, 12 +/- 3.0, and 10.0 +/- 4.0 fmol/mg of tissue, respectively. In conclusion, we detected constitutive and nerve-evoked overflow of beta-NAD, cADPR, and ADPR in vascular and non-vascular smooth muscles, beta-NAD being the prevailing compound. These substances modulate the release of NE, implicating novel nucleotide mechanisms of autonomic nervous system control of smooth muscle.

Modulation of the N-type calcium channel gene expression by the alpha subunit of Go

Go, a heterotrimeric G-protein, is enriched in brain and neuronal growth cones. Although several reports suggest that Go may be involved in modulation of neuronal differentiation, the precise role of Go is not clear. To investigate the function of Go in neuronal differentiation, we determined the effect of Goalpha, the alpha subunit of Go, on the expression of Ca(v)2.2, the pore-forming unit of N-type calcium channels, at the transcription level. Treatment with cyclic AMP (cAMP), which triggers neurite outgrowth in neuroblastoma F11 cells, increased the mRNA level and the promoter activity of the Ca(v)2.2 gene. Overexpression of Goalpha inhibited neurite extension in F11 cells and simultaneously repressed the stimulatory effect of cAMP on the Ca(v)2.2 gene expression to the basal level. Targeted mutation of the Goalpha gene also increased the level of Ca(v)2.2 in the brain. These results suggest that Go may regulate neuronal differentiation through modulation of gene expression of target genes such as N-type calcium channels.

Effects of prolonged exposure to nanomolar concentrations of methylmercury on voltage-sensitive sodium and calcium currents in PC12 cells

The neurotoxicant methylmercury (CH(3)Hg(+)) inhibits voltage-sensitive Na(+) and Ca(2+) currents in neuronal preparations following acute, in vitro, exposure. In the present study, effects on voltage-sensitive Na(+) (I(Na)) and Ca(2+) (I(Ca)) currents in pheochromocytoma (PC12) cells were examined following prolonged exposure to CH(3)Hg(+). When PC12 cells cultured in the presence of nerve growth factor (NGF) for 7 days ('primed') were replated in the presence of NGF and 30 nM CH(3)Hg(+), I(Ca), but not I(Na), amplitude was reduced (29%) significantly approximately 24 h later. Quantitative assessment of morphology indicated that this approximately 24 h exposure to CH(3)Hg(+) significantly reduced neurite length. The N-type voltage-sensitive Ca(2+) channel (VSCC) antagonist omega-conotoxin GVIA (500 pM) was without significant effect on current amplitude or morphology in this exposure protocol. When undifferentiated cells were cultured in the presence of NGF and 10 nM CH(3)Hg(+) for 6 days, I(Ca) and I(Na) amplitude were reduced by 36 and 52%, respectively. I(Ca) at the end of a 150 ms test pulse was also reduced by 40% in CH(3)Hg(+)-treated cells. Thus, both inactivating and non-inactivating I(Ca) were reduced equally. There was no change in [(3)H]saxitoxin or omega-[(125)I]conotoxin GVIA binding, nor were there any morphological alterations in cells treated with CH(3)Hg(+) for 6 days. Omega-conotoxin GVIA (500 pM, 6 days), reduced significantly I(Ca), but not I(Na), but was without effect on morphology. These results demonstrate that prolonged exposure to low concentrations of CH(3)Hg(+) reduces cationic currents in differentiating PC12 cells, but that current reduction is not always associated with morphological alteration.

Developmental expression of retinal cone cGMP-gated channels: evidence for rapid turnover and trophic regulation

The cyclic GMP-gated cationic channels of vertebrate photoreceptors are essential for visual phototransduction. We have examined the developmental regulation of cGMP-gated channels in morphologically identified cones in the chick retina. Expression of cone-type cGMP-gated channel mRNA can be detected at embryonic day 6 (E6), but expression of functional channels, as accessed by patch-clamp recordings, cannot be detected until E8. Plasma membrane channels in embryonic cones have a high turnover rate because inhibition of protein synthesis or disruption of the Golgi apparatus causes an almost complete loss of functional cGMP-gated channels within 12 hr. Different subpopulations of cones begin to express functional channels at different developmental stages, but all cones express channels by E10. Expression of cGMP-gated channels in at least one cone subpopulation appears to require one or more soluble differentiation factors, which are presumably present in the normal microenvironment of the developing retina. Application of chick embryo extract (CEE), a rich source of trophic factors, causes marked stimulation of cGMP-gated channel expression in chick cones at E8, but not at E6. Inhibition of MAP kinase (Erk) signaling using PD98059, or inhibition of PI3 kinase signaling by LY294002, blocked the stimulatory effects of CEE on E8 cones. Several recombinant trophic factors were also tested, but none could mimic the stimulatory effects of CEE on channel expression. In summary, the developmental expression of cGMP-gated cationic channels in embryonic cones appears to be regulated by epigenetic factors. The ability of cones to respond to these epigenetic factors is also developmentally regulated.

Overview of voltage-dependent calcium channels

Voltage-dependent calcium channels couple electrical signals to cellular responses in excitable cells. Calcium channels are crucial for excitation-secretion coupling in neurons and endocrine cells, and excitation-contraction coupling in muscle. Several pharmacologically and kinetically distinct calcium channel types have been identified at the electrophysiological and molecular levels. This review summarizes the basic properties of voltage-dependent calcium channels, including mechanisms of ion permeation and block, gating kinetics, and modulation by G proteins and second messengers.