Heterologous Expression In HEK293 Cells Research Articles

Sustained intracellular calcium oscillations induced by activation of the extracellular calcium sensing receptor do not require receptor phosphorylation by protein kinase C

Activation of the calcium sensing receptor (CaR) by small increments of extracellular calcium (Ca2+e), phenylalkylamines (calcimimetics) or aromatic amino acids induces intracellular calcium (Ca2+i) oscillations. Phosphorylation / dephosphorylation of G protein-coupled receptors by protein kinase C (PKC) can represent a feedback mechanism necessary for the initiation and maintenance of Ca2+i oscillations. However, the importance of PKC-mediated phosphorylation / dephosphorylation of CaR is debated. Whereas addition of PKC inhibitors did not affect the initiation of CaR-mediated Ca2+i oscillations (Breitwieser & Gama 2001, Am J Physiol 280: C1412–21), mutation of the major PKC phosphorylation site appeared to eliminate the Ca2+i oscillatory behaviour (Young et al. 2002, J Biol Chem 277: 46871–46876). We utilized heterologous expression in HEK-293 cells to clarify whether alanine mutations in all five predicted PKC phosphorylation sites of CaR would affect the initiation and maintenance of Ca2+i oscillations. We find that small increments of Ca2+e induce Ca2+i oscillations in cells transfected with the mutant receptors, however at lower [Ca2+]e compared to wildtype CaR and over a narrow range. Abolishing all PKC sites did not affect the maintenance of Ca2+i oscillations. Desensitization of CaR appeared to be independent of receptor phosphorylation by PKC. Depletion of thapsigargin-sensitive Ca2+i stores in the presence of [Ca2+]e above the EC50 of mutant and wildtype CaR respectively, or the PKC inhibitors negatively affected the maintenance of Ca2+i oscillations and potentiated receptor desensitization. PKC activation by phorbol esters eliminated the highly cooperative Ca2+e dependent activation of CaR as well as Ca2+i oscillations. We conclude that initiation and maintenance of Ca2+i oscillations are not dependent on PKC-mediated phosphorylation / dephosphorylation of CaR. Receptor cooperativity as well as the content of thapsigargin-sensitive Ca2+i stores are critical determinants for initiation and maintenance of Ca2+i oscillations.

Two Arginines in the Cytoplasmic C-terminal Domain Are Essential for Voltage-dependent Regulation of A-type K+ Current in the Kv4 Channel Subfamily

Contributions of the C-terminal domain of Kv4.3 to the voltage-dependent gating of A-type K+ current (IA) were examined by (i) making mutations in this region, (ii) heterologous expression in HEK293 cells, and (iii) detailed voltage clamp analyses. Progressive deletions of the C terminus of rat Kv4.3M (to amino acid 429 from the N terminus) did not markedly change the inactivation time course of IA but shifted the voltage dependence of steady state inactivation in the negative direction to a maximum of -17 mV. Further deletions (to amino acid 420) shifted this parameter in the positive direction, suggesting a critical role for the domain 429-420 in the voltage-dependent regulation of IA. There are four positively charged amino acids in this domain: Lys423, Lys424, Arg426, and Arg429. The replacement of the two arginines with alanines (R2A) resulted in -23 and -13 mV shifts of inactivation and activation, respectively. Additional replacement of the two lysines with alanines did not result in further shifts. Single replacements of R426A or R429A induced -15 and -10 mV shifts of inactivation, respectively. R2A did not significantly change the inactivation rate but did markedly change the voltage dependence of recovery from inactivation. These two arginines are conserved in Kv4 subfamily, and alanine replacement of Arg429 and Arg432 in Kv4.2 gave essentially the same results. These effects of R2A were not modulated by co-expression of the K+ channel beta subunit, KChIPs. In conclusion, the two arginines in the cytosolic C-terminal domain of alpha-subunits of Kv4 subfamily strongly regulate the voltage dependence of channel activation, inactivation, and recovery.

Dissection of the Functional Differences between Sarco(endo)plasmic Reticulum Ca2+-ATPase (SERCA) 1 and 2 Isoforms and Characterization of Darier Disease (SERCA2) Mutants by Steady-state and Transient Kinetic Analyses

Steady-state and rapid kinetic studies were conducted to functionally characterize the overall and partial reactions of the Ca2+ transport cycle mediated by the human sarco(endo)plasmic reticulum Ca2+-ATPase 2 (SERCA2) isoforms, SERCA2a and SERCA2b, and 10 Darier disease (DD) mutants upon heterologous expression in HEK-293 cells. SERCA2b displayed a 10-fold decrease in the rate of Ca2+ dissociation from E1Ca2 relative to SERCA2a (i.e. SERCA2b enzyme manifests true high affinity at cytosolic Ca2+ sites) and a lower rate of dephosphorylation. These fundamental kinetic differences explain the increased apparent affinity for activation by cytosolic Ca2+ and the reduced catalytic turnover rate in SERCA2b. Relative to SERCA1a, both SERCA2 isoforms displayed a 2-fold decrease of the rate of E2 to E1Ca2 transition. Furthermore, seven DD mutants were expressed at similar levels as wild type. The expression level was 2-fold reduced for Gly23 --> Glu and Ser920 --> Tyr and 10-fold reduced for Gly749 --> Arg. Uncoupling between Ca2+ translocation and ATP hydrolysis and/or changes in the rates of partial reactions account for lack of function for 7 of 10 mutants: Gly23 --> Glu (uncoupling), Ser186 --> Phe, Pro602 --> Leu, and Asp702 --> Asn (block of E1 approximately P(Ca2) to E2-P transition), Cys318 --> Arg (uncoupling and 3-fold reduction of E2-P to E2 transition rate), and Thr357 --> Lys and Gly769 --> Arg (lack of phosphorylation). A 2-fold decrease in the E1 approximately P(Ca2) to E2-P transition rate is responsible for the 2-fold decrease in activity for Pro895 --> Leu. Ser920 --> Tyr is a unique DD mutant showing an enhanced molecular Ca2+ transport activity relative to wild-type SERCA2b. In this case, the disease may be a consequence of the low expression level and/or reduction of Ca2+ affinity and sensitivity to inhibition by lumenal Ca2+.

Modulation of the two-pore domain acid-sensitive K+ channel TASK-2 (KCNK5) by changes in cell volume.

The molecular identity of K(+) channels involved in Ehrlich cell volume regulation is unknown. A background K(+) conductance is activated by cell swelling and is also modulated by extracellular pH. These characteristics are most similar to those of newly emerging TASK (TWIK-related acid-sensitive K(+) channels)-type of two pore-domain K(+) channels. mTASK-2, but not TASK-1 or -3, is present in Ehrlich cells and mouse kidney tissue from where the full coding sequences were obtained. Heterologous expression of mTASK-2 cDNA in HEK-293 cells generated K(+) currents in the absence intracellular Ca(2+). Exposure to hypotonicity enhanced mTASK-2 currents and osmotic cell shrinkage led to inhibition. This occurred without altering voltage dependence and with only slight decrease in pK(a) in hypotonicity but no change in hypertonicity. Replacement with other cations yields a permselectivity sequence for mTASK-2 of K(+) > Rb(+) Cs(+) > NH(4)(+) > Na(+) congruent with Li(+), similar to that for the native conductance (I(K, vol)). Clofilium, a quaternary ammonium blocker of I(K, vol), blocked the mTASK-2-mediated K(+) current with an IC(50) of 25 microm. The presence of mTASK-2 in Ehrlich cells, its functional similarities with I(K, vol), and its modulation by changes in cell volume suggest that this two-pore domain K(+) channel participates in the regulatory volume decrease phenomenon.

Calcium-sensing receptor activation induces intracellular calcium oscillations.

Parathyroid hormone secretion is exquisitely sensitive to small changes in serum Ca2+ concentration, and these responses are transduced via the Ca2+-sensing receptor (CaR). We utilized heterologous expression in HEK-293 cells to determine the effects of small, physiologically relevant perturbations in extracellular Ca2+ on CaR signaling via phosphatidylinositol-phospholipase C, using changes in fura 2 fluorescence to quantify intracellular Ca2+. Chronic exposure of CaR-transfected cells to Ca2+ in the range from 0.5 to 3 mM modulated the resting intracellular Ca2+ concentration and the subsequent cellular responses to acute extracellular Ca2+ perturbations but had no effect on thapsigargin-sensitive Ca2+ stores. Modest, physiologically relevant increases in extracellular Ca2+ concentration (0.5 mM increments) caused sustained (30-40 min) low-frequency oscillations of intracellular Ca2+ (approximately 45 s peak to peak interval). Oscillations were eliminated by 1 microM thapsigargin but were insensitive to protein kinase inhibitors (staurosporine, KN-93, or bisindolylmaleimide I). Staurosporine did increase the fraction of cells oscillating at a given extracellular Ca2+ concentration. Serum Ca2+ concentrations thus chronically regulate cells expressing CaR, and small perturbations in extracellular Ca2+ alter both resting intracellular Ca2+ as well as Ca2+ dynamics.

Regional expression of the splice variants of Kv4.3 in rat brain and effects of C-terminus deletion on expressed K + currents

In situ hybridization and RT-PCR analyses have revealed that, among three Kv4.3 splice variants (a, b, and c) with distinct C-terminal cytoplasmic domains, the mRNA for Kv4.3a is abundant in cerebral cortex, cerebellum, olfactory bulb, and medulla oblongata, whereas the mRNA for Kv4.3c is localized mainly to hippocampus. Three new distinct splice variants of Kv4.3 (Kv4.3d, e and f), which consist of 601, 635, and 628 amino acids, respectively, and have distinct C-terminal cytoplasmic domains, were isolated from rat brain by RT-PCR. Kv4.3b, d, e and f were expressed at much lower levels in brain. Mutagenesis which removed 149 amino acids in C-terminal domain of Kv4.3a significantly slowed its rate of recovery from inactivation as measured in heterologous expression in HEK293 cells. Surprisingly, however, neither the rate of inactivation nor voltage dependence of the activation and inactivation were changed.

Effects of temperature and mexiletine on the F1473S Na+ channel mutation causing paramyotonia congenita.

The F1473S mutation of the adult human skeletal muscle Na+ channel causes paramyotonia congenita, a disease characterized by muscle stiffness sometimes followed by weakness in a cold environment. The symptoms are relieved by the local anaesthetic mexiletine. This mutation, which resides in the cytoplasmic S4-S5 loop in domain IV of the alpha-subunit, was studied by heterologous expression in HEK293 cells using standard patch-clamp techniques. Compared to wild-type (WT) channels, those with the F1473S mutation exhibit a twofold slowing of fast inactivation, an increased persistent Na+ current, a +18-mV shift in steady-state inactivation and a fivefold acceleration of recovery from fast inactivation; slow inactivation was similar for both clones. Single-channel recordings for the F1473S mutation revealed a prolonged mean open time and an increased number of channel reopenings that increased further upon cooling. The pharmacological effects of mexiletine on cells expressing either WT, F1473S or G1306E channels were studied. G1306E is a myotonia-causing mutation located within the inactivation gate that displays similar but stronger inactivation defects than F1473S. The hyperpolarizing shift in steady-state inactivation induced by mexiletine was almost identical for all three clones. In contrast, this agent had a reduced effectiveness on the phasic (use-dependent) block of Na+ currents recorded from the mutants: the relative order of block was WT>F1473S>G1306E. We suggest that the relative effectiveness of mexiletine is associated with the degree of abnormal channel inactivation and that the relative binding affinity of mexiletine is not substantially different between the mutations or the WT.

The Putative Rat Choline Transporter CHOT1 Transports Creatine and Is Highly Expressed in Neural and Muscle-Rich Tissues

A rabbit homolog of the putative rat choline transporter CHOT1 has recently been shown to mediate creatine transport (Guimbal and Kilimann, J. Biol. Chem., 1993). Here, the functional properties of the CHOT1 cDNA were studied by heterologous expression in HEK-293 cells. After transfection, the cells displayed a 7-8 fold stimulation of [14C]creatine uptake with a Km of 46.2 μM. This transport was inhibited by the creatine uptake inhibitor β-guanidinopropionate (Ki of 23 μM). Northern blot analysis revealed a major transcript of 4.8 kb in brain, heart, skeletal muscle and kidney. In situ hybridization showed high transcript levels in the cerebellum, hippocampus and other brain regions. High expression was also seen in the rat embryo along the entire neuraxis and in some non-neuronal tissues. These data establish CHOT1 as a widely expressed rat creatine transporter.