Ras proteins activate calcium channels in neuronal cells

Regulation of voltage-dependent calcium channels in rat sensory neurones involves a Ras-mitogen-activated protein kinase pathway

The small G-protein Ras, a critical component in the signalling pathways regulating cell growth, is involved in the tonic upregulation of voltage-dependent calcium channels (VDCCs) in rat sensory neurones. To investigate which downstream effector(s) of Ras is involved in this process, a series of Ras mutant cDNAs were co-expressed with green fluorescent protein (GFP) in primary cultured rat dorsal root ganglion neurones (DRGs). Constitutively active V12Ras (glycine 12 to valine) markedly increased basal calcium current density by 41 % compared with control cells (GFP alone). In contrast, a farnesylation-defective mutant, V12S186Ras (cysteine 186 to serine; activates no downstream effectors), significantly reduced calcium current density by 47 %. Ras effector region mutants V12C40 (tyrosine 40 to cysteine; activates the p110 alpha-subunit of phosphatidylinositol 3-kinase) and V12G37 (glutamic acid 37 to glycine; activates Ral guanine nucleotide dissociation stimulator) had no significant effect on VDCC current. However, V12S35Ras (threonine 35 to serine; activates Raf-1 and the mitogen-activated protein kinase (MAPK) pathway) markedly increased basal calcium current density by 67 %, suggesting that Raf-1 activation is sufficient for Ras enhancement of calcium current in these cells. Raf-1 activates MEK (MAPK kinase) in the MAPK pathway, and the MEK inhibitor U0126 reduced calcium current by 45 % after 10-15 min, whereas the inactive analogue U0124 had no effect. This rapid time course for MEK inhibition suggests direct modulation of VDCCs via the Ras-MAPK pathway rather than gene expression-mediated effects. The relative proportions of omega-conotoxin GVIA- and nicardipine-sensitive N- ( approximately 40 %) and L- ( approximately 40 %) type currents were unaffected by either V12S35Ras expression or U0126 pre-treatment, suggesting that all components of calcium current in DRGs, are enhanced via this pathway.

Aldosterone induces ras methylation in A6 epithelia

Aldosterone increases Na(+) reabsorption by renal epithelial cells: the acute actions (<4 h) appear to be promoted by protein methylation. This paper describes the relationship between protein methylation and aldosterone's action and describes aldosterone-mediated targets for methylation in cultured renal cells (A6). Aldosterone increases protein methylation from 7.90 +/- 0.60 to 20.1 +/- 0.80 methyl ester cpm/microg protein. Aldosterone stimulates protein methylation by increasing methyltransferase activity from 14.0 +/- 0.64 in aldosterone-depleted cells to 31.8 +/- 2.60 methyl ester cpm/microg protein per hour in aldosterone-treated cells. Three known methyltransferase inhibitors reduce the aldosterone-induced increase in methyltransferase activity. One of these inhibitors, the isoprenyl-cysteine methyltransferase-specific inhibitor, S-trans, trans-farnesylthiosalicylic acid, completely blocks aldosterone-induced protein methylation and also aldosterone-induced short-circuit current. Aldosterone induces protein methylation in two molecular weight ranges: near 90 kDa and around 20 kDa. The lower molecular weight range is the weight of small G proteins, and aldosterone does increase both Ras protein 1.6-fold and Ras methylation almost 12-fold. Also, Ras antisense oligonucleotides reduce the activity of Na(+) channels by about fivefold. We conclude that 1) protein methylation is essential for aldosterone-induced increases in Na(+) transport; 2) one target for methylation is p21(ras); and 3) inhibition of Ras expression or Ras methylation inhibits Na(+) channel activity.

Calcium influx triggers the sequential proteolysis of extracellular and cytoplasmic domains of E-cadherin, leading to loss of beta-catenin from cell-cell contacts

Cadherins are major cell-cell adhesion molecules in both tumor and normal tissues. Although serum levels of soluble E-cadherin have been shown to be higher in the cancer patients than in healthy volunteers, the detail mechanism regulating release of soluble E-cadherin remains to be elucidated. Here we show that the ectodomain of E-cadherin is proteolytically cleaved from some cancer cells by a membrane-bound metalloprotease to yield soluble form, and the residual membrane-tethered cleavage product is subsequently degraded by intracellular proteolytic pathway. Futhermore, we show that extracellular calcium influx, that is induced by mechanical scraping of cells or ionomycin treatment, enhances the metalloprotease-mediated E-cadherin cleavage and the subsequent degradation of the cytoplasmic domain. Immunocytochemical analysis demonstrates that the sequential proteolysis of E-cadherin triggered by the calcium influx results in translocation of beta-catenin from the cell-cell contacts to cytoplasm. Our data suggest that calcium influx-induced proteolysis of E-cadherin not only disrupts the cell-cell adhesion but also activates beta-catenin-mediated intracellular signaling pathway, potentially leading to alterations in motility and proliferation activity of cells.

The monomeric G-proteins Rac1 and/or Cdc42 are required for the inhibition of voltage-dependent calcium current by bradykinin

Although regulation of voltage-dependent calcium current (ICa,V) by neurotransmitters is a ubiquitous mechanism among nerve cells, the signaling pathways involved are not well understood. We have determined previously that in a neuroblastoma-glioma hybrid cell line (NG108-15), the heterotrimeric G-protein G13 mediates the inhibition of ICa,V produced by bradykinin (BK) via an unknown mechanism. Various reports indicate that G13 can couple to RhoA, Rac1, and Cdc42, which are closely related members of the Rho family of monomeric G-proteins. We have investigated their role as signaling intermediates in the pathway used by BK to inhibit ICa,V. Using immunoblot analysis and the PCR, we found evidence that RhoA, Rac1, and Cdc42 all are expressed in NG108-15 cells. Intracellularly perfused recombinant Rho-GDI (an inhibitor of guanine nucleotide exchange specific for the Rho family) attenuated the inhibition of ICa,V by BK. These findings indicate that activation of RhoA, Rac1, or Cdc42 may be required for the response to BK. To determine whether any of these monomeric G-proteins mediate the response to BK, we have intracellularly applied blocking antibodies specific for each of the candidate proteins. Only the anti-Rac1 antibody blocked the response to BK. In parallel experiments, peptides corresponding to the C-terminal regions of Rac1 and Cdc42 blocked the same response. These data indicate a novel functional contribution of Rac1 and possibly also of Cdc42 to the inhibition of ICa,V by neurotransmitters.

Regulation of rat neuronal voltage-dependent calcium channels by endogenous p21-ras

Influx of calcium through voltage-dependent calcium channels (VDCCs) has been implicated in the processes of cell growth and differentiation. Various signalling proteins, including nerve growth factor (NGF), p21-ras and src tyrosine kinases, have been suggested to have a role in the regulation of neuronal VDCCs. Using the whole-cell patch-clamp technique we have investigated the role of endogenous p21-ras in the regulation of VDCCs in primary cultured dorsal root ganglion (DRG) neurons obtained from neonatal rats. Neutralization of endogenous p21-ras by microinjection of p21-ras antibody (Y13-259) reduced the maximum peak barium current, I(max), whereas microinjection of oncogenic p21-K-ras increased the current. Thus, endogenous p21-ras is involved in the tonic regulation of calcium currents in these cells. Intracellular application of a phosphopeptide, Trk 490, which prevents the binding of the adaptor protein shc to the activated NGF receptor, so blocking p21-ras activation, reduced I(max). Similarly, deprivation of NGF by overnight incubation in NGF-free medium also reduced I(max). Together, these results suggest that NGF receptor tyrosine kinase activation of p21-ras is likely to be involved in the tonic regulation of VDCCs in DRG neurons. Deprivation of NGF combined with microinjection of p21-ras antibody (Y13-259), however, caused an even greater reduction of I(max). Thus, NGF activation can only partially explain the regulation of these currents by endogenous p21-ras. Src tyrosine kinases have been suggested to activate p21-ras. In DRG neurons, microinjection of purified src tyrosine kinase, pp60c-src, increased I(max) in these cells. However, co-microinjection of pp60c-src with Y13-259 antibody prevented the increase in I(max), implying that pp60c-src can also regulate calcium currents via the activation of endogenous p21-ras. Further support for the involvement of tyrosine kinases in VDCC regulation was provided by the application of the general tyrosine kinase inhibitor, genistein, which also reduced I(max). Thus, VDCCs in rat DRG neurons appear to be tonically up-regulated by endogenous p21-ras. This effect appears largely to involve NGF receptor tyrosine kinase activation of p21-ras. In addition, src tyrosine kinase may also regulate VDCCs, possibly via p21-ras.

Inhibition of the T-type Ca2+ current by the dopamine D1 receptor in rat adrenal glomerulosa cells: requirement of the combined action of the G betagamma protein subunit and cyclic adenosine 3',5'-monophosphate

Modulation of ionic Ca2+ currents by dopamine (DA) could play a pivotal role in the control of steroid secretion by the rat adrenal glomerulosa cells. In the present study, we report that DA decreases the T-type Ca2+ current amplitude in these cells. The use of pharmacological agonists and antagonists reveals that this effect is mediated by activation of the D1-like receptors. Modulation by cAMP is complex inasmuch as preincubation of the cells with 8-Br-cAMP or the specific adenylyl cyclase inhibitor, 2',3'-dideoxyadenosine, have no effect per se, but prevent the DA-induced inhibition. The inhibitory effect of DA was abolished by addition of GDPbetaS to the pipette medium but not by pertussis toxin. If a cell is dialyzed with medium containing G alpha(s)-GDP, the inhibitory effect is reduced and cannot be recovered by the addition of GTPgammaS, indicating that the alpha(s) is not involved, but rather the betagamma-subunit. Indeed, DA-induced inhibition was mimicked by G betagamma in the pipette and 8-Br-cAMP in the bath. Similarly, G betagamma release from the activation of the AT1 receptor of angiotensin II did affect the current amplitude only in the presence of 8-Br-cAMP in the bath. The mitogen-activated protein kinase cascade, which can be activated by receptors coupled to Gs, was not involved as shown by the lack of activation of p42mapk by DA and the absence of effect of the mitogen-activated protein kinase inhibitor, PD 098059, on the DA-induced inhibition. Because the binding of G betagamma-subunits to various effectors involves the motif QXXER, we therefore tested the effect of the QEHA peptide on the inhibition of the T-type Ca2+ current induced by DA. The peptide, added to the medium pipette (200 microM), abolished the effect of DA. We conclude that the presence of the G betagamma and an increase in cAMP concentration are both required to inhibit the T-type Ca2+ current in rat adrenal glomerulosa cells.

Receptor-activated increases in intracellular calcium and protein tyrosine phosphorylation in vascular smooth muscle cells

We studied the effects of protein tyrosine kinase inhibitors (genistein and tyrphostin) on receptor-activated increases in cellular Ca2+ ([Ca2+]i), and protein tyrosine phosphorylation in cultured canine femoral arterial smooth muscle cells. Fura-2 imaging analysis showed that each agonist evoked a transient increase in ([Ca2+]i) followed by a sustained plateau phase. Experiments in Ca(2+)-free medium showed that 70-80% of the transient increase in [Ca2+]i evoked by either agonist is due to influx of extracellular Ca2+ whereas the plateau phase is only due to Ca2+ entry. Pre-incubation with genistein or tyrphosin markedly inhibited the transient rise in [Ca2+]i evoked by serotonin or phenylephrine. Immunoblot analysis of cell extracts with antiphosphotyrosine antibodies revealed that serotonin and phenylephrine also evoked an increase in tyrosine phosphorylation of several substrates. These increases were abolished by tyrosine kinase inhibitors. One of the major substrates was recognized by an an antibody for rasGAP. These data suggest that receptor-activated increases in [Ca2+]i in vascular smooth muscle cells may be coupled to receptor-activated increases in protein tyrosine phosphorylation.

Insulin-like growth factor I modulates voltage-dependent Ca2+ channels in neuronal cells

Insulin and insulin-like growth factors are neuroactive peptides. We investigated the effect of insulin-like growth factor I (IGF-I) on Ca2+ channel currents in 108CC15 neuroblastoma x glioma (N x G) cells and a possible role of protein kinase C (PKC). Whereas the native IGF-I enhanced the Ca2+ channel current density in N x G cells, the boiled IGF-I had no effect. The effect of IGF-I occurred after 1-2 h incubation and reversed within 24 h. Ca2+ channel currents recorded in control cells were mainly of a low-threshold fast inactivating type and showed a mean density of 5.9 +/- 0.3 pA/pF. Current density in cells incubated with IGF-I (0.2 micrograms/ml) for 2 h increased to 9.2 +/- 0.8 pA/pF. Ca2+ channel currents in cells treated with IGF-I showed an enhanced amount of a high-threshold slowly inactivating Ca2+ current type sensitive to the dihydropyridine isradipine and the snail toxin omega-conotoxin. The effect of IGF-I was suppressed by coincubation with the PKC inhibitors 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine (H-7) and staurosporin which were both without effect on current density in control cells. Whereas the inactive phorbol ester phorbol 12-myristate 13-acetate (PMA) failed to modulate Ca2+ channels in N x G cells, stimulation of PKC by the active phorbol ester PMA mimicked the effect of IGF-I. The effects of IGF-I and phorbol ester were not additive. Our data suggest an intracellular mechanism dependent on PKC and we propose a physiological relevance of the observed Ca2+ channel modulation by IGF-I in the neuroactivity of the peptide.