Tubulin stimulates adenylyl cyclase activity in rat striatal membranes via transfer of guanine nucleotide to Gs protein

The therapeutic efficacy of conjugated linoleic acid - paclitaxel on glioma in the rat

Considering the effects of conjugated linoleic acid (CLA) on anti-tumor and anti-angiogenic in brain tumor, synergistic anti-tumor activity with taxane as well as potential activity for transporting chemotherapeutic agents across the blood-brain barrier (BBB), the purpose of this study was to synthesize CLA-paclitaxel (CLA-PTX) conjugate which could reach to the brain tissue and target brain tumor. The CLA was covalently linked to PTX. The conjugate was stable in PBS and rat plasma in vitro and had no microtubule assembly activity in solution and slight effect of arresting cell cycle progression at the G(2)-M phase. The in vitro cytotoxicity of conjugate was lower than that of PTX (p < 0.05). The conjugate showed higher cellular uptake efficiency on C6 glioma cells. The entire pharmacokinetic index revealed the significant enhancement of the conjugate pharmacokinetics compared with that in PTX (p < 0.01). The conjugate, unlike PTX, could distribute in brain tissue and retained higher concentrations throughout 360 h. The anti-tumor efficacy in brain tumor-bearing rats after administering conjugate was significantly higher than that after giving Taxol (p < 0.01). In conclusion, this CLA-PTX conjugate showed great potential to become a new prodrug of PTX and the methodology can be applied to other anticancer drugs.

Tubulin polymerization modifies cardiac sodium channel expression and gating

AIMS

Treatment with the anticancer drug taxol (TXL), which polymerizes the cytoskeleton protein tubulin, may evoke cardiac arrhythmias based on reduced human cardiac sodium channel (Na(v)1.5) function. Therefore, we investigated whether enhanced tubulin polymerization by TXL affects Na(v)1.5 function and expression and whether these effects are beta1-subunit-mediated.

METHODS AND RESULTS

Human embryonic kidney (HEK293) cells, transfected with SCN5A cDNA alone (Na(v)1.5) or together with SCN1B cDNA (Na(v)1.5 + beta1), and neonatal rat cardiomyocytes (NRCs) were incubated in the presence and in the absence of 100 microM TXL. Sodium current (I(Na)) characteristics were studied using patch-clamp techniques. Na(v)1.5 membrane expression was determined by immunocytochemistry and confocal microscopy. Pre-treatment with TXL reduced peak I(Na) amplitude nearly two-fold in both Na(v)1.5 and Na(v)1.5 + beta1, as well as in NRCs, compared with untreated cells. Accordingly, HEK293 cells and NRCs stained with anti-Na(v)1.5 antibody revealed a reduced membrane-labelling intensity in the TXL-treated groups. In addition, TXL accelerated I(Na) decay of Na(v)1.5 + beta1, whereas I(Na) decay of Na(v)1.5 remained unaltered. Finally, TXL reduced the fraction of channels that slow inactivated from 31% to 18%, and increased the time constant of slow inactivation by two-fold in Na(v)1.5. Conversely, slow inactivation properties of Na(v)1.5 + beta1 were unchanged by TXL.

CONCLUSION

Enhanced tubulin polymerization reduces sarcolemmal Na(v)1.5 expression and I(Na) amplitude in a beta1-subunit-independent fashion and causes I(Na) fast and slow inactivation impairment in a beta1-subunit-dependent way. These changes may underlie conduction-slowing-dependent cardiac arrhythmias under conditions of enhanced tubulin polymerization, e.g. TXL treatment and heart failure.

IKs response to protein kinase A-dependent KCNQ1 phosphorylation requires direct interaction with microtubules

AIMS

KCNQ1 (alias KvLQT1 or Kv7.1) and KCNE1 (alias IsK or minK) co-assemble to form the voltage-activated K(+) channel responsible for I(Ks)-a major repolarizing current in the human heart-and their dysfunction promotes cardiac arrhythmias. The channel is a component of larger macromolecular complexes containing known and undefined regulatory proteins. Thus, identification of proteins that modulate its biosynthesis, localization, activity, and/or degradation is of great interest from both a physiological and pathological point of view.

METHODS AND RESULTS

Using a yeast two-hybrid screening, we detected a direct interaction between beta-tubulin and the KCNQ1 N-terminus. The interaction was confirmed by co-immunoprecipitation of beta-tubulin and KCNQ1 in transfected COS-7 cells and in guinea pig cardiomyocytes. Using immunocytochemistry, we also found that they co-localized in cardiomyocytes. We tested the effects of microtubule-disrupting and -stabilizing agents (colchicine and taxol, respectively) on the KCNQ1-KCNE1 channel activity in COS-7 cells by means of the permeabilized-patch configuration of the patch-clamp technique. None of these agents altered I(Ks). In addition, colchicine did not modify the current response to osmotic challenge. On the other hand, the I(Ks) response to protein kinase A (PKA)-mediated stimulation depended on microtubule polymerization in COS-7 cells and in cardiomyocytes. Strikingly, KCNQ1 channel and Yotiao phosphorylation by PKA-detected by phospho-specific antibodies-was maintained, as was the association of the two partners.

CONCLUSION

We propose that the KCNQ1-KCNE1 channel directly interacts with microtubules and that this interaction plays a major role in coupling PKA-dependent phosphorylation of KCNQ1 with I(Ks) activation.

Alteration in calcium channel properties is responsible for the neurotoxic action of a familial frontotemporal dementia tau mutation

Tau, a microtubule binding protein, is not only a major component of neurofibrillary tangles in Alzheimer's disease, but also a causative gene for hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). We show here that an FTDP-17 tau mutation (V337M) in SH-SY5Y cells reduces microtubule polymerization, increases voltage-dependent calcium current (ICa) density, and decreases ICa rundown. The reduced rundown of ICa by V337M was significantly inhibited by nifedipine (L-type Ca channel blocker), whereas omega-conotoxin GVIA (N-type Ca channel blocker) showed smaller effects, indicating that tau mutations affect L-type calcium channel activity. The depolarization-induced increase in intracellular calcium was also significantly augmented by the V337M tau mutation. Treatment with a microtubule polymerizing agent (taxol), an adenylyl cyclase inhibitor, or a protein kinase A (PKA) inhibitor, counteracted the effects of mutant tau on ICa. Taxol also attenuated the Ca2+ response to depolarization in cells expressing mutant tau. Apoptosis in SH-SY5Y cells induced by serum deprivation was exacerbated by the V337M mutation, and nifedipine, taxol, and a PKA inhibitor significantly protected cells against apoptosis. Our results indicate that a tau mutation which decreases its microtubule-binding ability augments calcium influx by depolymerizing microtubules and activating adenylyl cyclase and PKA.

G protein signaling and the molecular basis of antidepressant action

Over the past four decades, a variety of interventions have been used for the treatment of clinical depression and other affective disorders. Several distinct pharmacological compounds show therapeutic efficacy. There are three major classes of antidepressant drugs: monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), and tricyclic compounds. There are also a variety of atypical antidepressant drugs, which defy ready classification. Finally, there is electroconvulsive therapy, ECT. All require chronic (2-3 weeks) treatment to achieve a clinical response. To date, no truly inclusive hypothesis concerning a mechanism of action for these diverse therapies has been formed. This review is intended to give an overview of research concerning G protein signaling and the molecular basis of antidepressant action. In it, the authors attempt to discuss progress that has been made in this arena as well as the possibility that some point (or points) along a G protein signaling cascade represent a molecular target for antidepressant therapy that might lead toward a unifying hypothesis for depression. This review is not designed to address the clinical studies. Furthermore, as it is a relatively short paper, citations to the literature are necessarily selective. The authors apologize in advance to authors whose work we have failed to cite.

Colchicine attenuates left ventricular hypertrophy but preserves cardiac function of aortic-constricted rats

To investigate the effects of colchicine on left ventricular (LV) function and hypertrophy (LVH) of rats subjected to constriction of transverse aorta (TAoC), we evaluated SO (sham operated, vehicle; n = 25), SO-T (sham operated, colchicine 0.4 mg/kg body wt ip daily; n = 38), TAoC (vehicle; n = 37), and TAoC-T (TAoC, colchicine; n = 34) on the 2nd, 6th, and 15th day after surgery. Colchicine attenuated LVH of TAoC-T compared with TAoC rats, as evaluated by ratio between LV mass (LV(M)) and right ventricular mass, LV wall thickness, and average diameter of cardiac myocytes. Systolic gradient across TAoC ( approximately 45 mmHg), LV systolic pressure, LV end-diastolic pressure, and rate of LV pressure increase (+dP/dt) were comparable in TAoC-T and TAoC rats. However, the baseline and increases of LV systolic pressure-to-LV(M) and +dP/dt-to-LV(M) ratios induced by phenylephrine infusion were greater in TAoC-T and SO-T compared with SO rats. Baseline and increases of +dP/dt-to-LV(M) ratio were reduced in TAoC compared with SO rats. TAoC rats increased polymerized fraction of tubulin compared with SO, SO-T, and TAoC-T rats. Our results indicate that colchicine treatment reduced LVH to pressure overload but preserved LV function.

Microtubule disruption modulates the Rho-kinase pathway in vascular smooth muscle

Microtubules constitute one of the main cytoskeletal components in eukaryotic cells. Recent studies have shown that microtubule disruption induced significant vasoconstriction or enhanced agonist-induced contraction in vascular smooth muscle. However, the underlying mechanisms are not clear. We hypothesize that microtubule disruption may affect contractile signaling in vascular smooth muscle and lead to the enhanced contraction. The present study demonstrates that both colchicine and nocodazole induced a small but sustained contraction (4-6% P0) in rat aortic rings. This microtubule disruption-induced contraction was abolished by co-treatment with either HA 1077 or Y-27632, both of which are relatively specific Rho-kinase inhibitors. However, co-treatment with ML-9, an inhibitor of myosin light chain kinase, (MLCK) did not have a significant effect on the colchicine-induced contraction. The enhanced KCl-induced contraction due to treatment with colchicine was also blocked by inhibition of Rho-kinase, but not by inhibition of MLCK. These results indicate that microtubule disruption modulates contractile signaling in vascular smooth muscle, mainly through the Rho-kinase pathway, but not MLCK. Interestingly, the colchicine-enhanced, phenylephrine-induced contraction was not completely blocked by inhibition of Rho-kinase suggesting that other signaling pathways might also be involved.

Regulation and role of adenylyl cyclase isoforms

At least nine closely related isoforms of adenylyl cyclases (ACs), the enzymes responsible for the synthesis of cyclic AMP (cAMP) from ATP, have been cloned and characterized in mammals. Depending on the properties and the relative levels of the isoforms expressed in a tissue or a cell type at a specific time, extracellular signals received through the G-protein-coupled receptors can be differentially integrated. The present review deals with various aspects of such regulations, emphasizing the role of calcium/calmodulin in activating AC1 and AC8 in the central nervous system, the potential inhibitory effect of calcium on AC5 and AC6, and the changes in the expression pattern of the isoforms during development. A particular emphasis is given to the role of cAMP during drug and ethanol dependency and to some experimental limitations (pitfalls in the interpretation of cellular transfection, scarcity of the invalidation models, existence of complex macromolecular structures, etc).

Microtubule disruption by colchicine reversibly enhances calcium signaling in intact rat cardiac myocytes

Using the whole-cell patch-clamp configuration in rat ventricular myocytes, we recently reported that microtubule disruption increases calcium current (I(Ca)) and [Ca(2+)](i) transient and accelerates their kinetics by adenylyl cyclase activation. In the present report, we further analyzed the effects of microtubule disruption by 1 micromol/L colchicine on Ca(2+) signaling in cardiac myocytes with intact sarcolemma. In quiescent intact cells, it is possible to investigate ryanodine receptor (RyR) activity by analyzing the characteristics of spontaneous Ca(2+) sparks. Colchicine treatment decreased Ca(2+) spark amplitude (F/F(0): 1.78+/-0.01, n=983, versus 1.64+/-0.01, n=1660, recorded in control versus colchicine-treated cells; P<0.0001) without modifying the sarcoplasmic reticulum Ca(2+) load and enhanced their time to peak (in ms: 6.85+/-0.09, n=1185, versus 7.33+/-0.13, n=1647; P<0.0001). Microtubule disruption also induced the appearance of Ca(2+) sparks in doublets. These alterations may reflect RyR phosphorylation. To further investigate Ca(2+) signaling in cardiac myocytes with intact sarcolemma, we analyzed [Ca(2+)](i) transient evoked by field stimulation. Cells were loaded with the fluorescence Ca(2+) indicator, Fluo-3 cell permeant, and stimulated at 1 HZ: [Ca(2+)](i) transient amplitude was greater and its decay was accelerated in colchicine-treated, field-stimulated myocytes. This effect is reversible. When colchicine-treated myocytes were placed in a colchicine-free solution for 30 minutes, tubulin was repolymerized into microtubules, as shown by immunofluorescence, and the increase in [Ca(2+)](i) transient was reversed. In summary, we demonstrate that microtubule disruption by colchicine reversibly modulates Ca(2+) signaling in cardiac cells with intact sarcolemma.

Microtubule disruption modulates Ca(2+) signaling in rat cardiac myocytes

Microtubules have been shown to alter contraction in cardiac myocytes through changes in cellular stiffness. However, an effect on excitation-contraction coupling has not been examined. Here we analyze the effects of microtubule disruption by 1 micromol/L colchicine on calcium currents (I(Ca)) and [Ca(2+)](i) transients in rat ventricular myocytes. I(Ca) was studied using the whole-cell patch-clamp technique. Colchicine treatment increased I(Ca) density (peak values, -4.6+/-0.4 and -9.1+/-1.3 pA/pF in 11 control and 12 colchicine-treated myocytes, respectively; P<0.05). I(Ca) inactivation was well fitted by a biexponential function. The slow component of inactivation was unchanged, whereas the fast component was accelerated after colchicine treatment (at -10 mV, 11.8+/-1.0 versus 6.7+/-1.0 ms in control versus colchicine-treated cells; P<0.005). [Ca(2+)](i) transients were analyzed by fluo-3 epifluorescence simultaneously with I(Ca). Peak [Ca(2+)](i) transients were significantly increased in cardiac myocytes treated with colchicine. The values of F/F(0) at 0 mV were 1.1+/-0.02 in 9 control cells and 1.4+/-0.1 in 11 colchicine-treated cells (P<0.05). beta-Adrenergic stimulation with 1 micromol/L isoproterenol increased both I(Ca) and [Ca(2+)](i) transient in control cells. However, no significant change was induced by isoproterenol on colchicine-treated cells. Colchicine and isoproterenol effects were similar and not additive. Inhibition of adenylyl cyclase by 200 micromol/L 2'-deoxyadenosine 3'-monophosphate blunted the colchicine effect. We suggest that beta-adrenergic stimulation and microtubule disruption share a common pathway to enhance I(Ca) and [Ca(2+)](i) transient.

Effect of chronic ethanol treatment of Ca2+-inhibited adenylyl cyclase in mouse striatum

In the present study, to investigate the possibility that chronic ethanol treatment might alter Ca2+-inhibited type 5 adenylyl cyclase (AC) activity, we examined the effect of chronic ethanol treatment on striatal dopaminergic signal transduction, especially the AC system, in mice. We fed male C57BL/6 mice for 7 days with a 5% ethanol-containing or control liquid diet. Basal and forskolin-stimulated AC activities were reduced in striatal membranes of ethanol-treated mice. 5'-guanylylimidodiphosphate-stimulated AC activity was also decreased in ethanol-treated mice. But no significant differences were observed in the levels of the guanine nucleotide binding protein subunits Gs alpha and Gi1alpha&2alpha, determined by immunoblotting, between ethanol-treated and control mice. These results indicated that the function of the catalytic subunit of AC was decreased in the straitum of chronically ethanol-treated mice. We further examined the inhibitory regulation of AC activity in the context of a change of type 5 AC. Inhibition of forskolin-stimulated AC activity by 10 microM free Ca2+ was smaller in ethanol-treated mice than in control mice. However, the protein level of type 5 AC in the striatum, determined by immunoblotting, was not significantly different between ethanol-treated and control mice. These findings suggest that Ca2+-inhibited, presumably type 5, AC activity is reduced in mouse striatum by chronic ethanol treatment, and that this reduction is not due to a decrease in type 5 AC expression.

Dimeric tubulin-stimulated adenylyl cyclase activity is augmented after long-term amitriptyline treatment

We have investigated altered association of tubulin dimers interacting with G proteins and modulating adenylyl cyclase (AC) as a result of long-term amitriptyline (AMT) treatment. Gpp(NH)p-stimulated, but not basal or manganese-stimulated, AC activity was significantly augmented in cortex membranes prepared from rats chronically treated with AMT. The enhancement of AC activity by Gpp(NH)p-liganded tubulin (tubulin-Gpp(NH)p) was significantly higher in chronically AMT-treated rats than in control rats. Moreover, in cortex membranes from controls, tubulin-Gpp(NH)p prepared from chronically AMT-treated rats was more effective to activate AC activity than tubulin-Gpp(NH)p from controls. Immunoblotting and photoaffinity guanine nucleotide binding procedures showed no significant differences in the amount and the function of G proteins between controls and AMT-treated groups. It is suggested that long-term AMT treatment causes alteration in the functional interaction between tubulin and G protein, and this modification may participate in enhanced coupling of Gs to the catalytic subunit of AC induced by the chronic antidepressant treatment.