Expression and subunit interaction of voltage-dependent Ca2+ channels in PC12 cells

Cavβ1 regulates T cell expansion and apoptosis independently of voltage-gated Ca2+ channel function

TCR stimulation triggers Ca2+ signals that are critical for T cell function and immunity. Several pore-forming α and auxiliary β subunits of voltage-gated Ca2+ channels (VGCC) were reported in T cells, but their mechanism of activation remains elusive and their contribution to Ca2+ signaling in T cells is controversial. We here identify CaVβ1, encoded by Cacnb1, as a regulator of T cell function. Cacnb1 deletion enhances apoptosis and impairs the clonal expansion of T cells after lymphocytic choriomeningitis virus (LCMV) infection. By contrast, Cacnb1 is dispensable for T cell proliferation, cytokine production and Ca2+ signaling. Using patch clamp electrophysiology and Ca2+ recordings, we are unable to detect voltage-gated Ca2+ currents or Ca2+ influx in human and mouse T cells upon depolarization with or without prior TCR stimulation. mRNAs of several VGCC α1 subunits are detectable in human (CaV3.3, CaV3.2) and mouse (CaV2.1) T cells, but they lack transcription of many 5' exons, likely resulting in N-terminally truncated and non-functional proteins. Our findings demonstrate that although CaVβ1 regulates T cell function, these effects are independent of VGCC channel activity.

A multimodal electrochemical approach to measure the effect of zinc on vesicular content and exocytosis in a single cell model of ischemia

Zinc ion is essential for normal brain function that modulates synaptic activity and neuronal plasticity and it is associated with memory formation. Zinc is considered to be a contributing factor to the pathogenesis of ischemia, but the association between zinc and ischemia on vesicular exocytosis is unclear. In this study, we used a combination of chemical analysis methods and a cell model of ischemia/reperfusion to investigate exocytotic release and vesicular content, as well as the effect of zinc alteration on vesicular exocytosis. Oxygen–glucose deprivation and reperfusion (OGDR) was used as an in vitro model of ischemia in a model cell line. Exocytotic release and vesicular storage of catecholamine content were increased following OGDR, resulting in a higher fraction of release during exocytosis. However, zinc eliminated these increases following OGDR and the fraction of release remained unchanged. Understanding the consequences of zinc accumulation on vesicular exocytosis at the early stage of OGDR should aid in the development of therapeutic strategies to reduce ischemic brain injury. As the fraction released has been suggested to be related to presynaptic plasticity, insights are gained towards deciphering ischemia related memory impairment.

The Puzzling Role of Neuron-Specific PMCA Isoforms in the Aging Process

The aging process is a physiological phenomenon associated with progressive changes in metabolism, genes expression, and cellular resistance to stress. In neurons, one of the hallmarks of senescence is a disturbance of calcium homeostasis that may have far-reaching detrimental consequences on neuronal physiology and function. Among several proteins involved in calcium handling, plasma membrane Ca²⁺-ATPase (PMCA) is the most sensitive calcium detector controlling calcium homeostasis. PMCA exists in four main isoforms and PMCA2 and PMCA3 are highly expressed in the brain. The overall effects of impaired calcium extrusion due to age-dependent decline of PMCA function seem to accumulate with age, increasing the susceptibility to neurotoxic insults. To analyze the PMCA role in neuronal cells, we have developed stable transfected differentiated PC12 lines with down-regulated PMCA2 or PMCA3 isoforms to mimic age-related changes. The resting Ca²⁺ increased in both PMCA-deficient lines affecting the expression of several Ca²⁺-associated proteins, i.e., sarco/endoplasmic Ca²⁺-ATPase (SERCA), calmodulin, calcineurin, GAP43, CCR5, IP₃Rs, and certain types of voltage-gated Ca²⁺ channels (VGCCs). Functional studies also demonstrated profound changes in intracellular pH regulation and mitochondrial metabolism. Moreover, modification of PMCAs membrane composition triggered some adaptive processes to counterbalance calcium overload, but the reduction of PMCA2 appeared to be more detrimental to the cells than PMCA3.

Peripheral nerve injury increases contribution of L-type calcium channels to synaptic transmission in spinal lamina II: Role of α2δ-1 subunits

Background

Following peripheral nerve chronic constriction injury, the accumulation of the α2δ–1 auxiliary subunit of voltage-gated Ca²⁺ channels in primary afferent terminals contributes to the onset of neuropathic pain. Overexpression of α2δ–1 in Xenopus oocytes increases the opening properties of Ca_v1.2 L-type channels and allows Ca²⁺ influx at physiological membrane potentials. We therefore posited that L-type channels play a role in neurotransmitter release in the superficial dorsal horn in the chronic constriction injury model of neuropathic pain.

Results

Whole-cell recording from lamina II neurons from rats, subject to sciatic chronic constriction injury, showed that the L-type Ca²⁺ channel blocker, nitrendipine (2 µM) reduced the frequency of spontaneous excitatory postsynaptic currents. Nitrendipine had little or no effect on spontaneous excitatory postsynaptic current frequency in neurons from sham-operated animals. To determine whether α2δ–1 is involved in upregulating function of Ca_v1.2 L-type channels, we tested the effect of the α2δ–1 ligand, gabapentin (100 µM) on currents recorded from HEK293F cells expressing Ca_v1.2/β4/α2δ–1 channels and found a significant decrease in peak amplitude with no effect on control Ca_v1.2/β4/α2δ–3 expressing cells. In PC-12 cells, gabapentin also significantly reduced the endogenous dihydropyridine-sensitive calcium current. In lamina II, gabapentin reduced spontaneous excitatory postsynaptic current frequency in neurons from animals subject to chronic constriction injury but not in those from sham-operated animals. Intraperitoneal injection of 5 mg/kg nitrendipine increased paw withdrawal threshold in animals subject to chronic constriction injury.

Conclusion

We suggest that L-type channels show an increased contribution to synaptic transmission in lamina II dorsal horn following peripheral nerve injury. The effect of gabapentin on Cav1.2 via α2δ–1 may contribute to its anti-allodynic action.

LRRK2 Regulates Voltage-Gated Calcium Channel Function

Voltage-gated Ca²⁺ (Ca_V) channels enable Ca²⁺ influx in response to membrane depolarization. Ca_V2.1 channels are localized to the presynaptic membrane of many types of neurons where they are involved in triggering neurotransmitter release. Several signaling proteins have been identified as important Ca_V2.1 regulators including protein kinases, G-proteins and Ca²⁺ binding proteins. Recently, we discovered that leucine rich repeat kinase 2 (LRRK2), a protein associated with inherited Parkinson’s disease, interacts with specific synaptic proteins and influences synaptic transmission. Since synaptic proteins functionally interact with Ca_V2.1 channels and synaptic transmission is triggered by Ca²⁺ entry via Ca_V2.1, we investigated whether LRRK2 could impact Ca_V2.1 channel function. Ca_V2.1 channel properties were measured using whole cell patch clamp electrophysiology in HEK293 cells transfected with Ca_V2.1 subunits and various LRRK2 constructs. Our results demonstrate that both wild type (wt) LRRK2 and the G2019S LRRK2 mutant caused a significant increase in whole cell Ca²⁺ current density compared to cells expressing only the Ca_V2.1 channel complex. In addition, LRRK2 expression caused a significant hyperpolarizing shift in voltage-dependent activation while having no significant effect on inactivation properties. These functional changes in Ca_V2.1 activity are likely due to a direct action of LRRK2 as we detected a physical interaction between LRRK2 and the β3 Ca_V channel subunit via coimmunoprecipitation. Furthermore, effects on Ca_V2.1 channel function are dependent on LRRK2 kinase activity as these could be reversed via treatment with a LRRK2 inhibitor. Interestingly, LRRK2 also augmented endogenous voltage-gated Ca²⁺ channel function in PC12 cells suggesting other Ca_V channels could also be regulated by LRRK2. Overall, our findings support a novel physiological role for LRRK2 in regulating Ca_V2.1 function that could have implications for how mutations in LRRK2 contribute to Parkinson’s disease pathophysiology.

Regulation of GAP43/calmodulin complex formation via calcineurin-dependent mechanism in differentiated PC12 cells with altered PMCA isoforms composition

Several lines of evidence suggest the contribution of age-related decline in plasma membrane calcium pump (PMCA) to the onset of neurodegenerative diseases. From four PMCA isoforms, PMCA2, and PMCA3 respond to a rapid removal of Ca²⁺ and are expressed predominantly in excitable cells. We have previously shown that suppression of neuron-specific PMCAs in differentiated PC12 cells accelerated cell differentiation, but increased apoptosis in PMCA2-deficient line. We also demonstrated that altered expression of voltage-dependent calcium channels correlated with their higher contribution to Ca²⁺ influx, which varied between PMCA-reduced lines. Here, we propose a mechanism unique for differentiated PC12 cells by which PMCA2 and PMCA3 regulate pGAP43/GAP43 ratio and the interaction between GAP43 and calmodulin (CaM). Although down-regulation of PMCA2 or PMCA3 altered the content of GAP43/pGAP43, of paramount importance for the regulatory mechanism is a disruption of isoform-specific inhibitory PMCA/calcineurin interaction. In result, higher endogenous calcineurin (CaN) activity leads to hypophosphorylation of GAP43 in PMCA2- or PMCA3-deficient lines and intensification of GAP43/CaM complex formation, thus potentially limiting the availability of free CaM. In overall, our results indicate that both “fast” PMCA isoforms could actively regulate the local CaN function and CaN-downstream processes. In connection with our previous observations, we also suggest a negative feedback of cooperative action of CaM, GAP43, and CaN on P/Q and L-type channels activity. PMCAs- and CaN-dependent mechanism presented here, may signify a protective action against calcium overload in neuronal cells during aging, as well a potential way for decreasing neuronal cells vulnerability to neurodegenerative insults.

In vitro models for neurotoxicology research

The nervous system has a highly complex organization, including many cell types with multiple functions, with an intricate anatomy and unique structural and functional characteristics; the study of its (dys)functionality following exposure to xenobiotics, neurotoxicology, constitutes an important issue in neurosciences.

The effect of Cyclin-dependent kinase 5 on voltage-dependent calcium channels in PC12 cells varies according to channel type and cell differentiation state

Cyclin-dependent kinase 5 (Cdk5) is a Ser/Thr kinase that plays an important role in the release of neurotransmitter from pre-synaptic terminals triggered by Ca(2+) influx into the pre-synaptic cytoplasm through voltage-dependent Ca(2+) channels (VDCCs). It is reported that Cdk5 regulates L-, P/Q-, or N-type VDCC, but there is conflicting data as to the effect of Cdk5 on VDCC activity. To clarify the mechanisms involved, we examined the role of Cdk5 in regulating the Ca(2+) -channel property of VDCCs, using PC12 cells expressing endogenous, functional L-, P/Q-, and N-type VDCCs. The Ca(2+) influx, induced by membrane depolarization with high K(+) , was monitored with a fluorescent Ca(2+) indicator protein in both undifferentiated and nerve growth factor (NGF)-differentiated PC12 cells. Overall, Ca(2+) influx was increased by expression of Cdk5-p35 in undifferentiated PC12 cells but suppressed in differentiated PC12 cells. Moreover, we found that different VDCCs are distinctly regulated by Cdk5-p35 depending on the differentiation states of PC12 cells. These results indicate that Cdk5-p35 regulates L-, P/Q-, or N-type VDCCs in a cellular context-dependent manner. Calcium (Ca(2+) ) influx through voltage-dependent Ca(2+) channels (VDCCs) triggers neurotransmitter release from pre-synaptic terminal of neurons. The channel activity of VDCCs is regulated by Cdk5-p35, a neuronal Ser/Thr kinase. However, there have been debates about the regulation of VDCCs by Cdk5. Using PC12 cells, we show that Cdk5-p35 regulates VDCCs in a type (L, P/Q, and N) and differentiation-dependent manner. NGF = nerve growth factor.

The Ca2+ channel β2 subunit is selectively targeted to the axon terminals of supraoptic neurons

The assembly of high voltage-activated Ca(2+) channels with different β subunits influences channel properties and possibly subcellular targeting. We studied β subunit expression in the somata and axon terminals of the magnocellular neurosecretory cells, which are located in the supraoptic nucleus (SON) and neurohypophysis, respectively. Antibodies directed against the 4 Ca(V)β subunits (Ca(V)β(1)-Ca(V)β(4)) were used for immunoblots and for immunostaining of slices of these two tissues. We found that all 4 β subunits are expressed in both locations, but that Ca(V)β(2) had the highest relative expression in the neurohypophysis. These data suggest that the Ca(V)β(2) subunit is selectively targeted to axon terminals and may play a role in targeting and/or regulating the properties of Ca(2+) channels.

MKP3 eliminates depolarization-dependent neurotransmitter release through downregulation of L-type calcium channel Cav1.2 expression

Release of neurotransmitters is a fundamental and regulated process that is essential for normal brain functioning. Regulation of this process is potentially important for any neuronal process, and disruption of the release process may contribute to the pathophysiology associated with psychiatric diseases. In this work it is shown that expression of the negative regulator of mitogen-activated protein kinase (MAPK) signaling the MAPK phosphatase MKP3/DUSP6 eliminates depolarization-dependent release of dopamine in rat PC12 cells. Pharmacologic interventions with latrotroxin (LTX) or A23187, which make the cells permeable to calcium, reestablish the dopamine release. Calcium imaging also reveals that calcium influx is impaired in MKP3-expressing cells. Because acute pharmacologic inhibition of MAPKs has no effect on dopamine release in naïve PC12 cells, the MKP3-mediated elimination of neurotransmitter release must be caused by a long-term process, such as changes in gene expression. In support of this the expression of the L-type calcium channel cav1.2 alpha subunit (Cacna1c) is decreased in MKP3-expressing PC12 cells. With the reintroduction of cav1.2 expression, neurotransmitter release is restored in the MKP3-expressing PC12 cells. Thus, MKP3 expression reduces neurotransmitter release by decreasing the expression of cav1.2. Because MKP3 is increased when neuronal activity is elevated, this process could play a role in regulating neurotransmitter homeostasis.

Downregulation of PMCA2 or PMCA3 reorganizes Ca(2+) handling systems in differentiating PC12 cells

Changes in PMCA2 and PMCA3 expression during neuronal development are tightly linked to structural and functional modifications in Ca(2+) handling machinery. Using antisense strategy we obtained stably transfected PC12 lines with reduced level of PMCA2 or PMCA3, which were then subjected to dibutyryl-cAMP differentiation. Reduced level of neuron-specific PMCAs led to acceleration of differentiation and formation of longer neurites than in control PC12 line. Treatment with dibutyryl-cAMP was associated with retraction of growth cones and intensified formation of varicosities. In PMCA2-reduced cells development of apoptosis and DNA laddering were detected. Higher amounts of constitutive isoforms PMCA1 and PMCA4, their putative extended location to gaps left after partial removal of PMCA2 or PMCA3, together with increased SERCA may indicate the induction of compensatory mechanism in modified cells. Functional studies showed altered expression of certain types of VDCCs in PMCA-reduced cells, which correlated with their higher contribution to Ca(2+) influx. The cell response to PMCAs suppression suggests the interplay between transcription level of two opposite calcium-transporting systems i.e. voltage- and store depletion-activated channels facilitating Ca(2+) influx and calcium pumps responsible for Ca(2+) clearance, as well highlights the role of both neuron-specific PMCA isoforms in the control of PC12 cells differentiation.

Identification of a disulfide bridge essential for structure and function of the voltage-gated Ca(2+) channel α(2)δ-1 auxiliary subunit

Voltage-gated calcium (Ca(V)) channels are transmembrane proteins that form Ca(2+)-selective pores gated by depolarization and are essential regulators of the intracellular Ca(2+) concentration. By providing a pathway for rapid Ca(2+) influx, Ca(V) channels couple membrane depolarization to a wide array of cellular responses including neurotransmission, muscle contraction and gene expression. Ca(V) channels fall into two major classes, low voltage-activated (LVA) and high voltage-activated (HVA). The ion-conducting pathway of HVA channels is the α(1) subunit, which typically contains associated β and α(2)δ ancillary subunits that regulate the properties of the channel. Although it is widely acknowledged that α(2)δ-1 is post-translationally cleaved into an extracellular α(2) polypeptide and a membrane-anchored δ protein that remain covalently linked by disulfide bonds, to date the contribution of different cysteine (Cys) residues to the formation of disulfide bridges between these proteins has not been investigated. In the present report, by predicting disulfide connectivity with bioinformatics, molecular modeling and protein biochemistry experiments we have identified two Cys residues involved in the formation of an intermolecular disulfide bond of critical importance for the structure and function of the α(2)δ-1 subunit. Site directed-mutagenesis of Cys404 (located in the von Willebrand factor-A region of α(2)) and Cys1047 (in the extracellular domain of δ) prevented the association of the α(2) and δ peptides upon proteolysis, suggesting that the mature protein is linked by a single intermolecular disulfide bridge. Furthermore, co-expression of mutant forms of α(2)δ-1 Cys404Ser and Cys1047Ser with recombinant neuronal N-type (Ca(V)2.2α(1)/β(3)) channels, showed decreased whole-cell patch-clamp currents indicating that the disulfide bond between these residues is required for α(2)δ-1 function.

Functional coupling of Rab3-interacting molecule 1 (RIM1) and L-type Ca2+ channels in insulin release

Insulin release by pancreatic β-cells is regulated by diverse intracellular signals, including changes in Ca(2+) concentration resulting from Ca(2+) entry through voltage-gated (Ca(V)) channels. It has been reported that the Rab3 effector RIM1 acts as a functional link between neuronal Ca(V) channels and the machinery for exocytosis. Here, we investigated whether RIM1 regulates recombinant and native L-type Ca(V) channels (that play a key role in hormone secretion) and whether this regulation affects insulin release. Whole-cell patch clamp currents were recorded from HEK-293 and insulinoma RIN-m5F cells. RIM1 and Ca(V) channel expression was identified by RT-PCR and Western blot. RIM1-Ca(V) channel interaction was determined by co-immunoprecipitation. Knockdown of RIM1 and Ca(V) channel subunit expression were performed using small interference RNAs. Insulin release was assessed by ELISA. Co-expression of Ca(V)1.2 and Ca(V)1.3 L-type channels with RIM1 in HEK-293 cells revealed that RIM1 may not determine the availability of L-type Ca(V) channels but decreases the rate of inactivation of the whole cell currents. Co-immunoprecipitation experiments showed association of the Ca(V)β auxiliary subunit with RIM1. The lack of Ca(V)β expression suppressed channel regulation by RIM1. Similar to the heterologous system, an increase of current inactivation was observed upon knockdown of endogenous RIM1. Co-immunoprecipitation showed association of Ca(V)β and RIM1 in insulin-secreting RIN-m5F cells. Knockdown of RIM1 notably impaired high K(+)-stimulated insulin secretion in the RIN-m5F cells. These data unveil a novel functional coupling between RIM1 and the L-type Ca(V) channels via the Ca(V)β auxiliary subunit that contribute to determine insulin secretion.

Bafilomycin A1 activates respiration of neuronal cells via uncoupling associated with flickering depolarization of mitochondria

Bafilomycin A1 (Baf) induces an elevation of cytosolic Ca(2+) and acidification in neuronal cells via inhibition of the V-ATPase. Also, Baf uncouples mitochondria in differentiated PC12 ((d)PC12), (d)SH-SY5Y cells and cerebellar granule neurons, and markedly elevates their respiration. This respiratory response in (d)PC12 is accompanied by morphological changes in the mitochondria and decreases the mitochondrial pH, Ca(2+) and ΔΨm. The response to Baf is regulated by cytosolic Ca(2+) fluxes from the endoplasmic reticulum. Inhibition of permeability transition pore opening increases the depolarizing effect of Baf on the ΔΨm. Baf induces stochastic flickering of the ΔΨm with a period of 20 ± 10 s. Under conditions of suppressed ATP production by glycolysis, oxidative phosphorylation impaired by Baf does not provide cells with sufficient ATP levels. Cells treated with Baf become more susceptible to excitation with KCl. Such mitochondrial uncoupling may play a role in a number of (patho)physiological conditions induced by Baf.

Allethrin differentially modulates voltage-gated calcium channel subtypes in rat PC12 cells

Pyrethroid insecticides are one of the most widely used classes of insecticides. Previous studies revealed that pyrethroids potently affect the insect voltage-gated sodium (Na(+)) channel (VGSC), resulting in prolonged channel open time. However, recent findings have suggested that pyrethroids may affect targets other than the VGSC. In particular, several studies have shown that pyrethroids can modulate the activity of voltage-gated calcium (Ca(2+)) channels (VGCCs). However, these studies often reported conflicting results; some studies observed stimulatory effects, whereas others observed inhibitory effects of pyrethroids on VGCCs. This study investigated whether allethrin (AL), a well-characterized type I pyrethroid, altered VGCC characteristics measured by whole-cell recording in rat pheochromocytoma cells (PC12) differentiated with nerve growth factor (NGF). AL (5 microM) increased peak, end, and tail composite VGCC current independent of its effects on VGSCs. After blocking VGCC subtype-specific current with omega-conotoxin GVIA (GVIA, an N-type VGCC antagonist) or nimodipine (NIM, an L-type VGCC antagonist), our data further suggest that AL differentially affects VGCC subtypes. Thus, AL apparently stimulated GVIA-insensitive current while inhibiting NIM-insensitive current. AL also significantly altered the voltage dependency of activation and inactivation of L-type VGCCs. The differential modulation of VGCC subtypes by AL may explain some of the conflicting observations of other studies.

Rab3-interacting molecule gamma isoforms lacking the Rab3-binding domain induce long lasting currents but block neurotransmitter vesicle anchoring in voltage-dependent P/Q-type Ca2+ channels

Assembly of voltage-dependent Ca(2+) channels (VDCCs) with their associated proteins regulates the coupling of VDCCs with upstream and downstream cellular events. Among the four isoforms of the Rab3-interacting molecule (RIM1 to -4), we have previously reported that VDCC beta-subunits physically interact with the long alpha isoform of the presynaptic active zone scaffolding protein RIM1 (RIM1alpha) via its C terminus containing the C(2)B domain. This interaction cooperates with RIM1alpha-Rab3 interaction to support neurotransmitter exocytosis by anchoring vesicles in the vicinity of VDCCs and by maintaining depolarization-triggered Ca(2+) influx as a result of marked inhibition of voltage-dependent inactivation of VDCCs. However, physiological functions have not yet been elucidated for RIM3 and RIM4, which exist only as short gamma isoforms (gamma-RIMs), carrying the C-terminal C(2)B domain common to RIMs but not the Rab3-binding region and other structural motifs present in the alpha-RIMs, including RIM1alpha. Here, we demonstrate that gamma-RIMs also exert prominent suppression of VDCC inactivation via direct binding to beta-subunits. In the pheochromocytoma PC12 cells, this common functional feature allows native RIMs to enhance acetylcholine secretion, whereas gamma-RIMs are uniquely different from alpha-RIMs in blocking localization of neurotransmitter-containing vesicles near the plasma membrane. Gamma-RIMs as well as alpha-RIMs show wide distribution in central neurons, but knockdown of gamma-RIMs attenuated glutamate release to a lesser extent than that of alpha-RIMs in cultured cerebellar neurons. The results suggest that sustained Ca(2+) influx through suppression of VDCC inactivation by RIMs is a ubiquitous property of neurons, whereas the extent of vesicle anchoring to VDCCs at the plasma membrane may depend on the competition of alpha-RIMs with gamma-RIMs for VDCC beta-subunits.

Calcium-related processes involved in the inhibition of depolarization-evoked calcium increase by hydroxylated PBDEs in PC12 cells

In vitro studies indicated that hydroxylated polybrominated diphenyl ethers (OH-PBDEs) have an increased toxic potential compared to their parent congeners. An example is the OH-PBDE-induced increase of basal intracellular Ca(2+) concentration ([Ca(2+)](i)) by release of Ca(2+) from endoplasmic reticulum (ER) and mitochondria and/or influx of extracellular Ca(2+). ER and mitochondria regulate Ca(2+) homeostasis in close association with voltage-gated Ca(2+) channels (VGCCs). Therefore, effects of (OH-)PBDEs on the depolarization-evoked (100 mM K(+)) net increase in [Ca(2+)](i) (depolarization-evoked [Ca(2+)](i)) were measured in neuroendocrine pheochromocytoma cells using the Ca(2+)-responsive dye Fura-2. OH-PBDEs dose dependently inhibited depolarization-evoked [Ca(2+)](i). This inhibition was potentiated by a preceding increase in basal [Ca(2+)](i). Especially at higher concentrations of OH-PBDEs (5-20 microM), large increases in basal [Ca(2+)](i) strongly inhibited depolarization-evoked [Ca(2+)](i). The inhibition appeared more sensitive to increases in basal [Ca(2+)](i) by Ca(2+) release from intracellular stores (by 3-OH-BDE-47 or 6'-OH-BDE-49) compared to those by influx of extracellular Ca(2+) (by 6-OH-BDE-47 or 5-OH-BDE-47). The expected [Ca(2+)](i) difference close to the membrane suggests involvement of Ca(2+)-dependent regulatory processes close to VGCCs. When coapplied with depolarization, some OH-PBDEs induced also moderate direct inhibition of depolarization-evoked [Ca(2+)](i). Polybrominated diphenyl ethers and methoxylated BDE-47 affected neither basal nor depolarization-evoked [Ca(2+)](i), except for BDE-47, which moderately increased fluctuations in basal [Ca(2+)](i) and depolarization-evoked [Ca(2+)](i). These findings demonstrate that OH-PBDEs inhibit depolarization-evoked [Ca(2+)](i) depending on preceding basal [Ca(2+)](i). Related environmental pollutants that affect Ca(2+) homeostasis (e.g., polychlorinated biphenyls) may thus also inhibit depolarization-evoked [Ca(2+)](i), justifying further investigation of possible mixture effects of environmental pollutants on Ca(2+) homeostasis.

Dissociation between rat hippocampal CA1 and dentate gyrus cells in their response to corticosterone: effects on calcium channel protein and current

Stress and corticosterone affect, via glucocorticoid receptors, cellular physiology in the rodent brain. A well-documented example concerns corticosteroid effects on high-voltage activated (L type) calcium currents in the hippocampal CA1 area. We tested whether corticosterone also affects calcium currents in another hippocampal area that highly expresses glucocorticoid receptors, i.e. the dentate gyrus (DG). Remarkably, corticosterone (100 nm, given for 20 min, 1-4.5 hr before recording) did not change high-voltage activated calcium currents in the DG, whereas currents in the CA1 area of the same rats were increased. Follow-up studies revealed that no apparent dissociation between the two areas was observed with respect to transcriptional regulation of calcium channel subunits; thus, in both areas corticosterone increased mRNA levels of the calcium channel-beta4 but not the (alpha) Ca(v)1.2 subunit. At the protein level, however, beta4 and Ca(v)1.2 levels were significantly up-regulated by corticosterone in the CA1 but not the DG area. These data suggest that stress-induced elevations in the level of corticosterone result in a regionally differentiated physiological response that is not simply determined by the glucocorticoid receptor distribution and that the observed regional differentiation may be caused by a gene involved in the translational machinery or in mechanisms regulating mRNA or protein stability.

Novel splice variants of rat CaV2.1 that lack much of the synaptic protein interaction site are expressed in neuroendocrine cells

Voltage-gated Ca(2+) channels are responsible for the activation of the Ca(2+) influx that triggers exocytotic secretion. The synaptic protein interaction (synprint) site found in the II-III loop of Ca(V)2.1 and Ca(V)2.2 mediates a physical association with synaptic proteins that may be crucial for fast neurotransmission and axonal targeting. We report here the use of nested PCR to identify two novel splice variants of rat Ca(V)2.1 that lack much of the synprint site. Furthermore, we compare immunofluorescence data derived from antibodies directed against sequences in the Ca(V)2.1 synprint site and carboxyl terminus to show that channel variants lacking a portion of the synprint site are expressed in two types of neuroendocrine cells. Immunofluorescence data also suggest that such variants are properly targeted to neuroendocrine terminals. When expressed in a mammalian cell line, both splice variants yielded Ca(2+) currents, but the variant containing the larger of the two deletions displayed a reduced current density and a marked shift in the voltage dependence of inactivation. These results have important implications for Ca(V)2.1 function and for the mechanisms of Ca(V)2.1 targeting in neurons and neuroendocrine cells.

Dynamics of intracellular oxygen in PC12 Cells upon stimulation of neurotransmission

Neurotransmission, synaptic plasticity, and maintenance of membrane excitability require high mitochondrial activity in neurosecretory cells. Using a fluorescence-based intracellular O2 sensing technique, we investigated the respiration of differentiated PC12 cells upon depolarization with 100 mm K+. Single cell confocal analysis identified a significant depolarization of the plasma membrane potential and a relatively minor depolarization of the mitochondrial membrane potential following K+ exposure. We observed a two-phase respiratory response: a first intense spike lasting approximately 10 min, during which average intracellular O2 was reduced from 85-90% of air saturation to 55-65%, followed by a second wave of smaller amplitude and longer duration. The fast rise in O2 consumption coincided with a transient increase in cellular ATP by approximately 60%, which was provided largely by oxidative phosphorylation and by glycolysis. The increase of respiration was orchestrated mainly by Ca2+ release from the endoplasmic reticulum, whereas the influx of extracellular Ca2+ contributed approximately 20%. Depletion of Ca2+ stores by ryanodine, thapsigargin, and 4-chloro-m-cresol reduced the amplitude of respiratory spike by 45, 63, and 71%, respectively, whereas chelation of intracellular Ca2+ abolished the response. Uncoupling of the mitochondria with the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone amplified the responses to K+; elevated respiration induced a profound deoxygenation without increasing the cellular ATP levels reduced by carbonyl cyanide p-trifluoromethoxyphenylhydrazone. Cleavage of synaptobrevin 2 by tetanus toxin, known to reduce neurotransmission, did not affect the respiratory response to K+, whereas the general excitability of d PC12 cells increased.

Muscarinic acetylcholine receptors activate TRPC6 channels in PC12D cells via Ca2+ store-independent mechanisms

In this paper we report that stimulation of mAChRs in PC12D cells activates Ca2+ channels that are regulated independently of intracellular Ca2+ stores. In nominally Ca2+-free medium, exposure of PC12D cells to carbachol stimulates a robust influx of Ba2+, a Ca2+ substitute. This influx is blocked by atropine, but not by inhibitors of the nicotinic acetylcholine receptor or L-, N-, or T-type voltage-regulated Ca2+ channels. By contrast, depletion of intracellular Ca2+ stores with thapsigargin only weakly stimulates Ba2+ influx. Unlike store-operated Ca2+ channels (SOCCs), which close only after intracellular Ca2+ stores refill, channels mediating carbachol-stimulated Ba2+ influx rapidly close following the inactivation of mAChRs with atropine. Ba2+ influx is inhibited by extracellular Ca2+, by the Ca2+ channel blocker SKF-96365, and by activation of protein kinase C (PKC). Exogenous expression of antisense RNA encoding the rat canonical-transient receptor potential Ca2+ channel subtype 6 (TRPC6) or the N-terminal domain of TRPC6 blocks carbachol-stimulated Ba2+ influx in PC12D cells. Expression of TRPC6 antisense RNA or the TRPC6 N-terminal domain also blocks Ba2+ influx stimulated by 1-oleoyl-2-acetyl-sn-glycerol (OAG), a diacylglycerol analog previously shown to activate exogenously expressed TRPC6 channels. These data show that mAChRs in PC12D cells activate endogenous Ca2+ channels that are regulated independently of Ca2+ stores and require the expression of TRPC6.

Myotonic dystrophy CTG repeat expansion alters Ca2+channel functional expression in PC12 cells

We previously reported that expression of myotonic dystrophy (DM1) expanded CUG repeats impedes NGF-induced differentiation in a PC12 clone (CTG90 cells). Here, we present evidence for changes in the fractional contribution of distinct voltage-gated Ca(2+) channels, key elements in neurotrophin-promoted differentiation, to the total Ca(2+) current in the CTG90 cells. Patch-clamp recordings showed that the relative proportion of pharmacologically isolated Ca(2+) channel types differed between control and CTG90 cells. Particularly, the functional expression of N-type channels was significantly reduced. Though quantitative real-time RT-PCR revealed that transcripts for the pore-forming subunit encoding the N-type channels remained unchanged, the protein level analyzed by semi-quantitative Western blotting was down-regulated in the CTG90 cells. These data suggest modifications in the processing of N-type Ca(2+) channels in PC12 cells expressing the DM1 mutation.

IL1-receptor accessory protein-like 1 (IL1RAPL1), a protein involved in cognitive functions, regulates N-type Ca2+-channel and neurite elongation

Null mutations in the IL1-receptor accessory protein-like 1 gene (IL1RAPL1) are responsible for an inherited X-linked form of cognitive impairment. IL1RAPL1 protein physically interacts with neuronal calcium sensor-1 (NCS-1), but the functional impact of the IL1RAPL1/NCS-1 interaction remains unknown. Here, we demonstrate that stable expression of IL1RAPL1 in PC12 cells induces a specific silencing of N-type voltage-gated calcium channels (N-VGCC) activity that explains a secretion deficit observed in these IL1RAPL1 cells. Importantly, this modulation of VGCC activity is mediated by NCS-1. Indeed, a specific loss-of-function of N-VGCC was observed in PC12 cells overexpressing NCS-1, and a total recovery of N-VGCC activity was obtained by a down-regulation of NCS-1 in IL1RAPL1 cells. The functional relevance of the interaction between IL1RAPL1 and NCS-1 was also suggested by the reduction of neurite elongation observed in nerve growth factor (NGF)-treated IL1RAPL1 cells, a phenotype rescued by NCS-1 inactivation. Because both proteins are highly expressed in neurons, these results suggest that IL1RAPL1-related mental retardation could result from a disruption of N-VGCC and/or NCS-1-dependent synaptic and neuronal activities.

RIM1 confers sustained activity and neurotransmitter vesicle anchoring to presynaptic Ca2+ channels

The molecular organization of presynaptic active zones is important for the neurotransmitter release that is triggered by depolarization-induced Ca2+ influx. Here, we demonstrate a previously unknown interaction between two components of the presynaptic active zone, RIM1 and voltage-dependent Ca2+ channels (VDCCs), that controls neurotransmitter release in mammalian neurons. RIM1 associated with VDCC beta-subunits via its C terminus to markedly suppress voltage-dependent inactivation among different neuronal VDCCs. Consistently, in pheochromocytoma neuroendocrine PC12 cells, acetylcholine release was significantly potentiated by the full-length and C-terminal RIM1 constructs, but membrane docking of vesicles was enhanced only by the full-length RIM1. The beta construct beta-AID dominant negative, which disrupts the RIM1-beta association, accelerated the inactivation of native VDCC currents, suppressed vesicle docking and acetylcholine release in PC12 cells, and inhibited glutamate release in cultured cerebellar neurons. Thus, RIM1 association with beta in the presynaptic active zone supports release via two distinct mechanisms: sustaining Ca2+ influx through inhibition of channel inactivation, and anchoring neurotransmitter-containing vesicles in the vicinity of VDCCs.

Differential coupling of α7 and non-α7 nicotinic acetylcholine receptors to calcium-induced calcium release and voltage-operated calcium channels in PC12 cells

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation channels that can modulate various neuronal processes by altering intracellular Ca(2+) levels. Following nAChR stimulation Ca(2+) can enter cells either directly, through the intrinsic ion channel, or indirectly following voltage-operated Ca(2+) channel (VOCC) activation; Ca(2+) levels can subsequently be amplified via Ca(2+)-induced Ca(2+) release from intracellular stores. We have used subtype-selective nAChR agonists to investigate the Ca(2+) sources contributing to alpha7 and non-alpha7 nAChR-mediated increases in intracellular Ca(2+) in PC12 cells. Application of the alpha7 nAChR positive allosteric modulator PNU 120596 (10 mum), in conjunction with the alpha7 nAChR agonist, compound A [(R)-N-(1-azabicyclo[2.2.2]oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide), 10 nm], produces a rapid increase in fluo-3 fluorescence that is prevented by the selective alpha7 nAChR antagonist alpha-bungarotoxin. The non-alpha7 nAChR agonist 5-Iodo-A-85380 produces alpha-bungarotoxin-insensitive increases in intracellular Ca(2+) (EC(50) = 11.2 mum). Using these selective agonists or KCl in conjunction with general and selective VOCC inhibitors, we demonstrate that the primary route of Ca(2+) entry following either non-alpha7 nAChR activation or KCl stimulation is via L-type VOCCs. In contrast, the alpha7 nAChR-mediated response is unaffected by VOCC blockers but is inhibited by modulators of intracellular Ca(2+) stores. These results indicate that alpha7 and non-alpha7 nAChRs are differentially coupled to Ca(2+)-induced Ca(2+) release and VOCCs, respectively.

The marine secondary metabolites 2,4-dibromophenol and 2,4,6-tribromophenol differentially modulate voltage dependent ion currents in neuroendocrine (PC12) cells

2,4-Dibromophenol (2,4-DBP) and 2,4,6-tribromophenol (2,4,6-TBP) are marine secondary metabolites, with 2,4,6-tribromophenol playing an important role as industrially produced flame retardant and pesticide. Both substances disturb cellular calcium signals in neuroendocrine cells as previously shown by Hassenklöver et al. (2006) [Hassenklöver, T., Predehl, S., Pilli, J., Ledwolorz, J., Assmann, M., Bickmeyer, U., 2006. Bromophenols, both present in marine organisms and in industrial flame retardants, disturb cellular Ca(2+) signaling in neuroendocrine cells (PC12). Aquat. Toxicol. 76, 37-45]. We investigated calcium channel currents in detail and outward membrane currents as potential cellular targets of both bromophenols. In this electrophysiological approach, 2,4-DBP reduced voltage dependent calcium channel currents with a half-maximal concentration of 45+/-32 microM (S.D.) and a Hill coefficient of 0.87+/-0.49 (S.D.). 2,4,6-TBP reduced calcium channel currents with a half-maximal concentration of 28+/-19 microM (S.D.) and a Hill coefficient of 0.79+/-0.31 (S.D.). The major contribution to calcium channel currents was mediated by L-type (67%) and N-type channels (30%) in PC12 cells; both bromophenols modulated both current types. Whole cell outward currents, mainly carried by potassium ions, were reduced by 2,4-DBP with a half-maximal concentration of 41+/-9 microM (S.D.) showing a Hill coefficient of 1.71+/-0.31 (S.D.). 2,4,6-TBP showed a weak reduction of outward currents at high concentrations of 300 microM. 2,4,6-TBP selectively decreased calcium entry via calcium channels as revealed in whole cell patch clamp experiments, whereas 2,4-DBP reduced both in- and outward currents.

Secretion of brain-derived neurotrophic factor from PC12 cells in response to oxidative stress requires autocrine dopamine signaling

Expression of brain-derived neurotrophic factor (BDNF) is sensitive to changes in oxygen availability, suggesting that BDNF may be involved in adaptive responses to oxidative stress. However, it is unknown whether or not oxidative stress actually increases availability of BDNF by stimulating BDNF secretion. To approach this issue we examined BDNF release from PC12 cells, a well-established model of neurosecretion, in response to hypoxic stimuli. BDNF secretion from neuronally differentiated PC12 cells was strongly stimulated by exposure to intermittent hypoxia (IH). This response was inhibited by N-acetyl-l-cysteine, a potent scavenger of reactive oxygen species (ROS) and mimicked by exogenous ROS. IH-induced BDNF release requires activation of tetrodotoxin sensitive Na+ channels and Ca2+ influx through N- and L-type channels, as well as mobilization of internal Ca2+ stores. These results demonstrate that oxidative stress can stimulate BDNF release and that underlying mechanisms are similar to those previously described for activity-dependent BDNF secretion from neurons. Surprisingly, we also found that IH-induced secretion of BDNF was blocked by dopamine D2 receptor antagonists or by inhibition of dopamine synthesis with alpha-methyl-p-tyrosine. These data indicate that oxidative stress can stimulate BDNF release through an autocrine or paracrine loop that requires dopamine receptor activation.

C31 enhances voltage-gated calcium channel currents in undifferentiated PC12 cells

C31, consisting of 31 amino acid residues, is generated from the carboxyl terminal fragments (CTFs) of amyloid precursor protein (APP). It has been shown that C31 causes apoptosis in neurons and is present in brains of Alzheimer disease (AD) patients. Using whole-cell patch clamp techniques, we investigated effects of C31 on voltage-gated calcium channel (VGCC) currents and the protective effects of beta-estradiol on PC12 cells. The results demonstrated that C31 induced a significant increase of the VGCC currents in PC12 cells, which was blocked by beta-estradiol. These results suggest that modulation of intracellular calcium levels by VGCC may in part be involved in C31 induced neuronal death associated with AD.

Modal gating of human CaV2.1 (P/Q-type) calcium channels: I. The slow and the fast gating modes and their modulation by beta subunits

The single channel gating properties of human Ca_V2.1 (P/Q-type) calcium channels and their modulation by the auxiliary β_1b, β_2e, β_3a, and β_4a subunits were investigated with cell-attached patch-clamp recordings on HEK293 cells stably expressing human Ca_V2.1 channels. These calcium channels showed a complex modal gating, which is described in this and the following paper (Fellin, T., S. Luvisetto, M. Spagnolo, and D. Pietrobon. 2004. J. Gen. Physiol. 124:463–474). Here, we report the characterization of two modes of gating of human Ca_V2.1 channels, the slow mode and the fast mode. A channel in the two gating modes differs in mean closed times and latency to first opening (both longer in the slow mode), in voltage dependence of the open probability (larger depolarizations are necessary to open the channel in the slow mode), in kinetics of inactivation (slower in the slow mode), and voltage dependence of steady-state inactivation (occurring at less negative voltages in the slow mode). Ca_V2.1 channels containing any of the four β subtypes can gate in either the slow or the fast mode, with only minor differences in the rate constants of the transitions between closed and open states within each mode. In both modes, Ca_V2.1 channels display different rates of inactivation and different steady-state inactivation depending on the β subtype. The type of β subunit also modulates the relative occurrence of the slow and the fast gating mode of Ca_V2.1 channels; β_3a promotes the fast mode, whereas β_4a promotes the slow mode. The prevailing mode of gating of Ca_V2.1 channels lacking a β subunit is a gating mode in which the channel shows shorter mean open times, longer mean closed times, longer first latency, a much larger fraction of nulls, and activates at more positive voltages than in either the fast or slow mode.

Polyglutamine repeats of spinocerebellar ataxia 6 impair the cell-death-preventing effect of CaV2.1 Ca2+ channel—loss-of-function cellular model of SCA6

Spinocerebellar ataxia (SCA) 6 is caused by small expansion of a polyglutamine sequence, encoded by CAG trinucleotide repeats, at the C-terminal end of the human CaV2.1 (P/Q-type) Ca2+ channel alpha12.1 subunit and it manifests itself as slowly progressive cerebellar ataxia. To elucidate the pathogenic mechanisms underlying SCA6, we introduced CAG repeats of various lengths into the Ca2+ channel alpha12.1 subunit cDNA and expressed them in baby hamster kidney cells stably expressing the auxiliary subunits (alpha2delta and beta4). The occurrence of cell death differed between cells transfected with the normal and mutant Ca2+ channels under the condition of serum starvation plus potassium-induced depolarization, and Cdk inhibition elucidated the differences more clearly. The CaV2.1 (P/Q-type) Ca2+ channel-specific blocker omega-agatoxin IVA abolished the cell-death-preventing effect of the normal Ca2+ channel. Together with our previous finding that the polyglutamine expansion in SCA6 interferes with the Ca2+ channel to reduce Ca2+ influx, these results indicate that impaired function of the mutant Ca2+ channels rendered them unable to prevent cell death.

Ion channel regulation by chronic hypoxia in models of acute oxygen sensing

Several potentially life-threatening cardiovascular and respiratory disorders result in prolonged deprivation of oxygen, which in turn results in significant cellular adaptation, or remodelling. An important component of this functional adaptation arises as a direct consequence of altered ion channel expression by chronic hypoxia. In this review, we discuss current understanding of this hypoxic remodelling process, with particular reference to regulation of L-type Ca2+ channels and high-conductance, Ca2+-sensitive K+ (BK) channels. In systems where this remodelling occurs, changes in functional expression of these particular channels evokes marked alteration in, or responses to, Ca2+-dependent events. Evidence to date indicates that channel expression can be modulated at the transcriptional level but, additionally, that crucial post-transcriptional events are also regulated by chronic hypoxia. Importantly, such remodelling is, in some cases, strongly associated with production of amyloid peptides of Alzheimer's disease, implicating chronic hypoxia as a causative factor in the progression of specific pathology. Moreover, subtle changes in functional expression of BK channels implicates chronic hypoxia as an important regulator of cell excitability.

Quantal release of ATP from clusters of PC12 cells

Although ATP is important for intercellular communication, little is known about the mechanism of endogenous ATP release due to a dearth of suitable models. Using PC12 cells known to express the P2X2 subtype of ATP receptors and to store ATP with catecholamines inside dense-core vesicles, we found that clusters of PC12 cells cultured for 3-7 days generated small transient inward currents (STICs) after an inward current elicited by exogenous ATP. The amplitude of STICs in individual cells correlated with the peak amplitude of ATP-induced currents. STICs appeared as asynchronous responses (approximately 20 pA average amplitude) for 1-20 s and were investigated with a combination of patch clamping, Ca2+ imaging, biochemistry and electron microscopy. Comparable STICs were produced by focal KCl pulses and were dependent on extracellular Ca2+. STICs were abolished by the P2X antagonist PPADS and potentiated by Zn2+, suggesting they were mediated by P2X2 receptor activation. The highest probability of observing STICs was after the peak of intracellular Ca2+ increase caused by KCl. Biochemical measurements indicated that KCl application induced a significant release of ATP from PC12 cells. Electron microscopy studies showed narrow clefts without 'synaptic-like' densities between clustered cells. Our data suggest that STICs were caused by quantal release of endogenous ATP by depolarized PC12 cells in close juxtaposition to the recorded cell. Thus, STICs may be a new experimental model to characterize the physiology of vesicular release of ATP and to study the kinetics and pharmacology of P2X2 receptor-mediated quantal currents.

Distinct Intracellular Calcium Profiles Following Influx Through N- Versus L-Type Calcium Channels: Role of Ca2+-Induced Ca2+Release

Selective activation of neuronal functions by Ca2+is determined by the kinetic profile of the intracellular calcium ([Ca2+]i) signal in addition to its amplitude. Concurrent electrophysiology and ratiometric calcium imaging were used to measure transmembrane Ca2+current and the resulting rise and decay of [Ca2+]iin differentiated pheochromocytoma (PC12) cells. We show that equal amounts of Ca2+entering through N-type and L-type voltage-gated Ca2+channels result in significantly different [Ca2+]itemporal profiles. When the contribution of N-type channels was reduced by ω-conotoxin MVIIA treatment, a faster [Ca2+]idecay was observed. Conversely, when the contribution of L-type channels was reduced by nifedipine treatment, [Ca2+]idecay was slower. Potentiating L-type current with BayK8644, or inactivating N-type channels by shifting the holding potential to −40 mV, both resulted in a more rapid decay of [Ca2+]i. Channel-specific differences in [Ca2+]idecay rates were abolished by depleting intracellular Ca2+stores with thapsigargin or by blocking ryanodine receptors with ryanodine, suggesting the involvement of Ca2+-induced Ca2+release (CICR). Further support for involvement of CICR is provided by the demonstration that caffeine slowed [Ca2+]idecay while ryanodine at high concentrations increased the rate of [Ca2+]idecay. We conclude that Ca2+entering through N-type channels is amplified by ryanodine receptor mediated CICR. Channel-specific activation of CICR provides a mechanism whereby the kinetics of intracellular Ca2+leaves a fingerprint of the route of entry, potentially encoding the selective activation of a subset of Ca2+-sensitive processes within the neuron.

Non-genomic modulation of dopamine release by bisphenol-A in PC12 cells

An endocrine disruptor chemical, bisphenol-A (BPA), is reported to have several short-term actions in various tissues and/or cells; however, the mechanisms of these actions have not been fully elucidated. We investigated short-term actions evoked by BPA in pheochromocytoma PC12 cells. BPA elicited dopamine release in PC12 cells in a dose-dependent manner. A selective N-type calcium channel antagonist (omega-conotoxin GVIA) and a ryanodine receptor blocker (ryanodine) inhibited the BPA-induced dopamine release. The expression of ryanodine receptor mRNA was detected by RT-PCR in PC12 cells. Subsequently, in order to prove whether membrane receptors participate in BPA-evoked dopamine release, a guanine nucleotide-binding protein inhibitor [guanosine 5'-(beta-thio) diphosphate], cyclic AMP antagonist (Rp-cAMPS) or protein kinase A inhibitor (H7 or H89) was added to PC12 cells prior to BPA-treatment. All of these agents suppressed BPA-evoked dopamine release, indicating that multiple signaling pathways may be involved in BPA-evoked dopamine release in PC12 cells. In conclusion, we demonstrated that BPA induced dopamine release in a non-genomic manner through guanine nucleotide-binding protein and N-type calcium channels. These findings illustrate a novel function of BPA and suggest that exposure to BPA influences the function of dopaminergic neurons.

Expression of recombinant calcium channels support secretion in a mouse pheochromocytoma cell line

We have characterized a recently established mouse pheochromocytoma cell line (MPC 9/3L) as a useful model for studying neurotransmitter release and neuroendocrine secretion. MPC 9/3L cells express many of the proteins involved in Ca2+-dependent neurotransmitter release but do not express functional endogenous Ca2+-influx pathways. When transfected with recombinant N-type Ca2+ channel subunits alpha1B,beta2a,alpha2delta (Cav2.2), the cells expressed robust Ca2+ currents that were blocked by omega-conotoxin GVIA. Activation of N-type Ca2+ currents caused rapid increases in membrane capacitance of the MPC 9/3L cells, indicating that the Ca2+ influx was linked to exocytosis of vesicles similar to that reported in chromaffin or PC12 cells. Synaptic protein interaction (synprint) sites, like those found on N-type Ca2+ channels, are thought to link voltage-dependent Ca2+ channels to SNARE proteins involved in synaptic transmission. Interestingly, MPC 9/3L cells transfected with either LC-type (alpha1C, beta2a, alpha2delta, Cav1.2) or T-type (alpha1G, beta2a, alpha2delta, Cav3.1) Ca2+ channel subunits, which do not express synprint sites, expressed appropriate Ca2+ currents that supported rapid exocytosis. Thus MPC 9/3L cells provide a unique model for the study of exocytosis in cells expressing specific Ca2+ channels of defined subunit composition without complicating contributions from endogenous channels. This model may help to distinguish the roles that different Ca2+ channels play in Ca2+-dependent secretion.

Calcium channel upregulation in response to activation of neurotrophin and surrogate neurotrophin receptor tyrosine kinases

Modulation of calcium channel expression and function in the context of neurotrophin induced neuronal differentiation remains incompletely understood at a mechanistic level. We addressed this issue in the PC12 model neuronal system using patch clamp electrophysiology combined with ectopic expression of the human beta platelet-derived growth factor (betaPDGF) receptor as a surrogate neurotrophin receptor system. PC12 cells ectopically expressing the human betaPDGF receptor were treated with PDGF or nerve growth factor (NGF) for up to 7 days, and Ca2+ channel subtype expression was analyzed using selective pharmacological agents in both whole-cell and cell-attached single channel patch clamp configurations. PDGF-induced upregulation of N- and P/Q-type Ca2+ channel currents completely mimicked upregulation of these currents caused by NGF stimulation of the endogenous TrkA receptor tyrosine kinase (RTK). Neither PDGF nor NGF significantly altered L- or R-type currents. Single channel recordings together with immunocytochemistry implied that growth factor-induced increases in whole-cell Ca2+ currents were a result of synthesis of new channels, and that whereas increased N channel density was apparent in the soma, additional P/Q channels distributed preferentially to extrasomal locations, most likely the proximal neurites. Finally, specific signaling-deficient mutant forms of the betaPDGF receptor were used to show that activation of Src, PI3-kinase, RasGAP, PLCgamma or SHP-2 (some of which are implicated in certain other aspects of PC12 cell differentiation) by RTKs is not required for growth factor-induced Ca2+ channel upregulation. In contrast, activation of the Ras-related G-protein Rap1 was found critical to this process.

The neuronal beta 4 subunit increases the unitary conductance of L-type voltage-gated calcium channels in PC12 cells

beta subunits of voltage-gated calcium channels influence channel behavior in numerous ways, including enhancing the targeting of alpha1 subunits to the plasma membrane and shifting the voltage dependence of activation and inactivation. Of the four beta subunits that have been identified, beta 4 is of particular interest because mutation of its alpha1 subunit interaction domain produces severe neurological defects. Its differential distribution in the hippocampus prompted us to examine whether this subunit was responsible for the heterogeneity of hippocampal L-type calcium channels. To study the functional effects of the beta 4 subunit on native L-type calcium channels, we transfected beta 4 cDNA subcloned out of embryonic hippocampal neurons into PC12 cells, a cell line that contains the beta 1, beta 2, and beta 3 subunits but not the beta 4 subunit. Cell-attached single-channel recordings of L-type channel activity from untransfected and transfected PC12 cells compared with recordings obtained from hippocampal neurons revealed an effect of the beta 4 subunit on single-channel conductance. L-type channels in untransfected PC12 cells had a significantly smaller conductance (19.8 picosiemens (pS)) than L-type channels in hippocampal neurons (22 pS). After transfection of beta 4, however, L-type single-channel conductance was indistinguishable between the two cell types. Our data suggest that calcium channel beta 4 subunits affect the conductance of L-type calcium channels and that native hippocampal L-type channels contain the beta 4 subunit.

The calcium channel ligand FPL 64176 enhances L-type but inhibits N-type neuronal calcium currents

One strategy for isolating neuronal L-type calcium (Ca(2+)) currents, which typically comprise a minority of the whole cell current in neurons, has been to use pharmacological agents that increase channel activity. This study examines the effects of the benzoyl pyrrole FPL 64176 (FPL) on L-type Ca(2+) currents and compares them to those of the dihydropyridine (+)-202-791. At micromolar concentrations, both agonists increased whole cell current amplitude in PC12 cells. However, FPL also significantly slowed the rate of activation and elicited a longer-lasting slow component of the tail current compared to (+)-202-791. In single channel cell-attached patch recordings, FPL increased open probability, first latency, mean closed time and mean open time more than (+)-202-791, with no difference in unitary conductance. These gating differences suggest that, compared to (+)-202-791, FPL decreases transition rates between open and closed conformations. Where examined, the actions of FPL and (+)-202-791 on whole cell L-type currents in sympathetic neurons appeared similar to those in PC12 cells. In contrast to its effects on L-type current, 10 microM FPL inhibited the majority of the whole cell current in HEK cells expressing a recombinant N-type Ca(2+) channel, raising caution concerning the use of FPL as a selective L-type Ca(2+) channel agonist in neurons.

Induction of T-type calcium channel gene expression by chronic hypoxia

Cellular responses to hypoxia can be acute or chronic. Acute responses mainly depend on oxygen-sensitive ion channels, whereas chronic responses rely on the hypoxia-inducible transcription factors (HIFs), which up-regulate the expression of enzymes, transporters, and growth factors. It is unknown whether the expression of genes coding for ion channels is also influenced by hypoxia. We report here that the alpha1H gene of T-type voltage-gated calcium channels is highly induced by lowering oxygen tension in PC12 cells. Accumulation of alpha1H mRNA in response to hypoxia is time- and dose-dependent and paralleled by an increase in the density of T-type calcium channel current recorded in patch clamped cells. HIF appears to be involved in the response to hypoxia, since cobalt chloride, desferrioxamine, and dimethyloxalylglycine, compounds that mimic HIF-regulated gene expression, replicate the hypoxic effect. Moreover, functional inhibition of HIF-2alpha protein accumulation using antisense HIF-2alpha oligonucleotides reverses the effect of hypoxia on T-type Ca2+ channel expression. Importantly, regulation by oxygen tension is specific for T-type calcium channels, since it is not observed with the L-, N-, and P/Q-channel types. These findings show for the first time that hypoxia induces an ion channel gene via a HIF-dependent mechanism and define a new role for the T-type calcium channels as regulators of cellular excitability and calcium influx under chronic hypoxia.

Toosendanin induces outgrowth of neuronal processes and apoptosis in PC12 cells

In the present study, the effects of toosendanin on cell differentiation and apoptosis were investigated in PC12 cells. The results showed that after 24-48 h of culture in a medium containing toosendanin (approximately 1-10x10(-7) M), cell differentiation and outgrowth of neuronal processes were promoted. Combined treatment with toosendanin and a calcium channel blocker, nifedipine or omega-conotoxin GVIA, resulted in a significant inhibition of the toosendanin-induced effects. Pretreatment of PC12 cells with BAPTA-AM also inhibited the toosendanin-induced effects; however, these effects were not inhibited by pertussis toxin and H-7 in the medium. Toosendanin also induced cell apoptosis. Based on the DNA content determined by flow cytometric analysis, the number of apoptotic cells significantly increased when the incubation time in the toosendanin-containing medium was lasted up to 72 h. Toosendanin at a higher concentration (> or =1 x 10(-6) M) caused cell death while it had no effect on cell division at concentrations lower than 1 x 10(-7) M.

Attenuation of the P2Y receptor-mediated control of neuronal Ca2+ channels in PC12 cells by antithrombotic drugs

1. In PC12 cells, adenine nucleotides inhibit voltage-activated Ca(2+) currents and adenylyl cyclase activity, and the latter effect was reported to involve P2Y(12) receptors. To investigate whether these two effects are mediated by one P2Y receptor subtype, we used the antithrombotic agents 2-methylthio-AMP (2-MeSAMP) and N(6)-(2-methyl-thioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP (AR-C69931MX). 2. ADP reduced A(2A) receptor-dependent cyclic AMP synthesis with half maximal effects at 0.1-0.17 micro M. In the presence of 30 micro M 2-MeSAMP or 100 nM AR-C69931MX, concentration response curves were shifted to the right by factors of 39 and 30, indicative of pA(2) values of 6.1 and 8.5, respectively. 3. The inhibition of Ca(2+) currents by ADP was attenuated by 10-1000 nM AR-C69931MX and by 3-300 micro M 2-MeSAMP. ADP reinhibited Ca(2+) currents after removal of 2-MeSAMP within less than 15 s, but required 2 min to do so after removal of AR-C69931MX. 4. ADP inhibited Ca(2+) currents with half maximal effects at 5-20 micro M. AR-C69931MX (10-100 nM) displaced concentration response curves to the right, and the resulting Schild plot showed a slope of 1.09 and an estimated pK(B) value of 8.7. Similarly, 10-100 micro M 2-MeSAMP also caused rightward shifts resulting in a Schild plot with a slope of 0.95 and an estimated pK(B) of 5.4. 5. The inhibition of Ca(2+) currents by 2-methylthio-ADP and ADPbetaS was also antagonized by AR-C69931MX, which (at 30 nM) caused a rightward shift of the concentration response curve for ADPbetaS by a factor of 3.8, indicative of a pA(2) value of 8.1. 6. These results show that antithrombotic drugs antagonize the inhibition of neuronal Ca(2+) channels by adenine nucleotides, which suggests that this effect is mediated by P2Y(12) receptors.

Signaling pathways involved in Ca2+- and Pb2+-induced vesicular catecholamine release from rat PC12 cells

Since Pb(2+) substitutes for Ca(2+) in essential steps leading to exocytosis, we have investigated whether Ca(2+) and Pb(2+) induce exocytosis through similar pathways. Vesicular catecholamine release was measured from dexamethasone-differentiated PC12 cells using carbon fiber microelectrode amperometry. Effects of drugs known to modulate PKC (PMA, staurosporine), calcineurin (cyclosporin A), calmodulin (W7), and CaM kinase II (KN-62) activity were investigated in intact and in ionomycin-permeabilized PC12 cells. Activation of PKC and inhibition of calmodulin decrease the frequency of exocytotic events evoked by high K(+) stimulation in intact cells. In addition, inhibition of calmodulin enhances the frequency of basal exocytosis from intact cells. Activation of PKC and inhibition of calcineurin enhance the frequency of basal exocytosis in intact as well as in ionomycin-permeabilized cells. Inhibition of PKC and of CaM kinase II cause no significant effects. None of the treatments has a significant effect on vesicle contents. The combined results indicate that PKC and calcineurin enhance and inhibit exocytosis through direct effects on the exocytotic machinery, whereas calmodulin and CaM kinase II exert indirect effects only. Conversely, Pb(2+)-evoked exocytosis in permeabilized cells is strongly reduced by inhibition of CaM kinase II, but is not sensitive to modulation of PKC and calcineurin activity. Inhibition of calmodulin only reduces the delay to onset of Pb(2+)-evoked exocytosis. Synaptotagmin I- and II-deficient PC12-F7 cells exhibit vesicular catecholamine release following depolarization or superfusion with Pb(2+). However, the frequency of exocytosis and the contents of vesicles released are strongly reduced as compared to PC12 cells. It is concluded that Ca(2+)-evoked exocytosis is modulated mainly by PKC and calcineurin, whereas Pb(2+)-evoked exocytosis is mainly modulated by CaM kinase II.

P2Y-receptors mediating an inhibition of the evoked entry of calcium through N-type calcium channels at neuronal processes

In the search for P2-receptors modulating the stimulation-evoked entry of calcium at processes of PC12 cells differentiated in the presence of nerve growth factor and neurotrophin-3, electrically evoked increases in free calcium were assessed by fura-2 microfluorimetry. Omission of calcium and addition of cadmium (100 microM) or the N-type calcium channel blocker omega-conotoxin GVIA (0.5 microM) abolished or markedly reduced the evoked responses. The P2Y-receptor agonists 2-methylthio adenosine 5'-diphosphate (2-methylthio-ADP), ADP, and adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS) inhibited the electrically evoked entry of calcium without any changes in basal calcium concentrations. 2-Methylthio-ADP was the most potent agonist. Adenosine, P(1),P(4)-di(adenosine-5')-tetraphosphate (Ap4A), UDP, and UTP (30 microM each) had no effect. The effect of ADPbetaS (30 microM) was abolished by the P2-antagonists reactive blue 2 (3 microM), suramin (100 microM), 2-methylthio-AMP (10 microM), p-chloromercuriphenyl sulfonic acid (1 microM), and AR-C 69931MX [N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene adenosine 5'-triphosphate] (300 nM). In contrast, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (10 microM), the selective P2Y1-receptor antagonist MRS 2179 (N(6)-methyl-2'-deoxyadenosine 3',5'-bisphosphate; 10 microM), as well as the adenosine A(1)-receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine; 100 nM), caused no change. Pretreatment with pertussis toxin abolished the effect of ADPbetaS. Reverse transcriptase-polymerase chain reaction revealed the presence of mRNA for P2Y12-receptors in nondifferentiated and differentiated PC12 cells. The results indicate that processes of differentiated PC12 cells possess P2Y12-receptors coupling to pertussis toxin-sensitive G-proteins and mediating an inhibition of the stimulation-evoked entry of calcium through omega-conotoxin GVIA-sensitive calcium channels. This suggests a role of P2Y12-receptors in neuromodulation in addition to their involvement in platelet aggregation.

Hypoxia potentiates exocytosis and Ca2+ channels in PC12 cells via increased amyloid beta peptide formation and reactive oxygen species generation

Exposure of PC12 cells to chronic hypoxia (CH; 10 % O(2), 24 h) augments catecholamine secretion via formation of a Cd2+-resistant Ca2+ influx pathway, and up-regulates native L-type Ca2+ channels. These effects are mimicked by exposure of cells to Alzheimer's disease-associated amyloid beta peptides (AbetaPs). Since pathological effects of AbetaPs have been associated with increased levels of reactive oxygen species (ROS), the involvement of ROS in hypoxia-mediated up-regulation of exocytosis and Ca2+ channel activity was examined. Both melatonin and ascorbic acid (two structurally unrelated antioxidants) fully blocked the enhancement of catecholamine secretion caused by CH (as determined amperometrically). Enhanced immunofluorescence, observed in chronically hypoxic cells using a primary monoclonal antibody raised against the N-terminus of AbetaP, was also suppressed by melatonin. Ascorbic acid, melatonin and ebselen (an additional antioxidant) also fully prevented augmentation of whole-cell Ca2+ currents caused by CH (as monitored using whole-cell patch-clamp recordings). Exposure of normoxic cells to H(2)O(2) (40 microM, 24 h), like hypoxia, caused Ca2+ channel up-regulation. Importantly, AbetaP formation appeared to be an absolute requirement for the effects of hypoxia, since the ability of CH to augment exocytosis and Ca2+ channel activity was blocked by two novel inhibitors of gamma secretase, an enzyme complex required for AbetaP formation. Our results indicate that the effects of hypoxia require ROS generation from AbetaPs, and suggest that elevated levels of ROS mediate hypoxic and AbetaP-mediated pathological remodelling of Ca2+ homeostasis.