Distinct ClC-6 and ClC-7 Cl- sensitivities provide insight into ClC-7's role in lysosomal Cl- homeostasis.

ClC-6 and ClC-7 are closely related, intracellular Cl- /H+ antiporters belonging to the CLC family of channels and transporters. They localize to acidic late endosomes and lysosomes and probably function in ionic homeostasis of these contiguous compartments. ClC-7 transport function requires association with the accessory protein Ostm1, whereas ClC-6 transport does not. To elucidate their roles in endo-lysosomes, we measured Cl- - and pH-dependences of over-expressed wild-type ClC-6 and ClC-7, as well as disease-associated mutants, using high-resolution recording protocols. Lowering extracellular Cl- (corresponding to luminal Cl- in endo-lysosomes) reduced ClC-6 currents, whereas it increased transport activity of ClC-7/Ostm1. Low extracellular Cl- activated ClC-7/Ostm 1 under acidic extracellular conditions, as well as under conditions of low intracellular chloride. Activation is conserved in ClC-7Y713C , a variant displaying disrupted PI(3,5)P2 inhibition. Detailed biophysical analysis of disease-associated ClC-6 and ClC-7 gain-of-function (GoF) variants, ClC-6Y553C and ClC-7Y713C , and the ClC-7Y577C and ClC-6Y781C correlates, identified additional functional nuances distinguishing ClC-6 and ClC-7. ClC-7Y577C recapitulated GoF produced by ClC-6Y553C . ClC-6Y781C displayed transport activation qualitatively similar to ClC-7Y713C , although current density did not differ from that of wild-type ClC-6. Finally, rClC-7R760Q , homologous to hClC-7R762Q , an osteopetrosis variant with fast gating kinetics, appeared indifferent to extracellular Cl- , identifying altered Cl- sensitivity as a plausible mechanism underlying disease. Collectively, the present studies underscore the distinct roles of ClC-6 and ClC-7 within the context of their respective localization to late endosomes and lysosomes. In particular, we suggest the atypical inhibition of ClC-7 by luminal Cl- serves to limit excessive intraluminal Cl- accumulation. KEY POINTS: ClC-6 and ClC-7 are late endosomal and lysosomal 2 Cl- /1 H+ exchangers, respectively. When targeted to the plasma membrane, both activate slowly at positive voltages. ClC-6 activity is decreased in low extracellular (i.e. luminal) chloride, whereas ClC-7 is activated by low luminal chloride, even at acidic pH. The functional gain-of-function phenotypes of the ClC-6 and ClC-7 disease mutations ClC-6Y553C and ClC-7Y715C are maintained when introduced in their respective homologues, ClC-7Y577C and ClC-6Y781C , with all mutations retaining chloride dependence of the respective wild type (WT). An osteopetrosis mutation of ClC-7 displaying fast gating kinetics (R762Q) was less sensitive to extracellular chloride compared to WT. The opposing substrate dependences of ClC-6 and ClC-7 Cl- / H+ exchangers point to non-overlapping physiological functions, leading us to propose that inhibition of ClC-7 by luminal chloride and protons serves to prevent osmotic stress imposed by hyper-accumulation of chloride.

Voltage-Gated Sodium Channel Dysfunctions in Neurological Disorders.

The pore-forming subunits (α subunits) of voltage-gated sodium channels (VGSC) are encoded in humans by a family of nine highly conserved genes. Among them, SCN1A, SCN2A, SCN3A, and SCN8A are primarily expressed in the central nervous system. The encoded proteins Nav1.1, Nav1.2, Nav1.3, and Nav1.6, respectively, are important players in the initiation and propagation of action potentials and in turn of the neural network activity. In the context of neurological diseases, mutations in the genes encoding Nav1.1, 1.2, 1.3 and 1.6 are responsible for many forms of genetic epilepsy and for Nav1.1 also of hemiplegic migraine. Several pharmacological therapeutic approaches targeting these channels are used or are under study. Mutations of genes encoding VGSCs are also involved in autism and in different types of even severe intellectual disability (ID). It is conceivable that in these conditions their dysfunction could indirectly cause a certain level of neurodegenerative processes; however, so far, these mechanisms have not been deeply investigated. Conversely, VGSCs seem to have a modulatory role in the most common neurodegenerative diseases such as Alzheimer's, where SCN8A expression has been shown to be negatively correlated with disease severity.

Hyperexcitable interneurons trigger cortical spreading depression in an Scn1a migraine model.

Cortical spreading depression (CSD), a wave of depolarization followed by depression of cortical activity, is a pathophysiological process implicated in migraine with aura and various other brain pathologies, such as ischemic stroke and traumatic brain injury. To gain insight into the pathophysiology of CSD, we generated a mouse model for a severe monogenic subtype of migraine with aura, familial hemiplegic migraine type 3 (FHM3). FHM3 is caused by mutations in SCN1A, encoding the voltage-gated Na+ channel NaV1.1 predominantly expressed in inhibitory interneurons. Homozygous Scn1aL1649Q knock-in mice died prematurely, whereas heterozygous mice had a normal lifespan. Heterozygous Scn1aL1649Q knock-in mice compared with WT mice displayed a significantly enhanced susceptibility to CSD. We found L1649Q to cause a gain-of-function effect with an impaired Na+-channel inactivation and increased ramp Na+ currents leading to hyperactivity of fast-spiking inhibitory interneurons. Brain slice recordings using K+-sensitive electrodes revealed an increase in extracellular K+ in the early phase of CSD in heterozygous mice, likely representing the mechanistic link between interneuron hyperactivity and CSD initiation. The neuronal phenotype and premature death of homozygous Scn1aL1649Q knock-in mice was partially rescued by GS967, a blocker of persistent Na+ currents. Collectively, our findings identify interneuron hyperactivity as a mechanism to trigger CSD.

Regulatory-auxiliary subunits of CLC chloride channel-transport proteins.

The CLC family of chloride channels and transporters is composed by nine members, but only three of them, ClC-Ka/b, ClC-7 and ClC-2, have been found so far associated with auxiliary subunits. These CLC regulatory subunits are small proteins that present few common characteristics among them, both structurally and functionally, and their effects on the corresponding CLC protein are different. Barttin, a protein with two transmembrane domains, is essential for the membrane localization of ClC-K proteins and their activity in the kidney and inner ear. Ostm1 is a protein with a single transmembrane domain and a highly glycosylated N-terminus. Unlike the other two CLC auxiliary subunits, Ostm1 shows a reciprocal relationship with ClC-7 for their stability. The subcellular localization of Ostm1 depends on ClC-7 and not the other way around. ClC-2 is active on its own, but GlialCAM, a transmembrane cell adhesion molecule with two extracellular immunoglobulin (Ig)-like domains, regulates its subcellular localization and activity in glial cells. The common theme for these three proteins is their requirement for a proper homeostasis, since their malfunction leads to distinct diseases. We will review here their properties and their role in normal chloride physiology and the pathological consequences of their improper function.

The role of individual caspases in cell death induction by taxanes in breast cancer cells.

BACKGROUND: In previous study we showed that caspase-2 plays the role of an apical caspase in cell death induction by taxanes in breast cancer cells. This study deals with the role of other caspases. We tested breast cancer cell lines SK-BR-3 (functional caspase-3) and MCF-7 (nonfunctional caspase-3). METHODS AND RESULTS: Using western blot analysis we demonstrated the activation of initiator caspase-8 and -9 as well as executioner caspase-6 and -7 in both tested cell lines after application of taxanes (paclitaxel, SB-T-1216) at death-inducing concentrations. Caspase-3 activation was also found in SK-BR-3 cells. Employing specific siRNAs after taxane application, suppression of caspase-3 expression significantly increased the number of surviving SK-BR-3 cells. Inhibition of caspase-7 expression also increased the number of surviving SK-BR-3 and MCF-7 cells. On the other hand, suppression of caspase-8 and caspase-9 expression had no significant effect on cell survival. However, caspase-9 seemed to be involved in the activation of caspase-3 and caspase-7. Caspase-3 and caspase-7 appeared to activate mutually. Furthermore, we observed a significant decrease in mitochondrial membrane potential (flow cytometric analysis) and cytochrome c release (confocal microscopy, western blot after cell fractionation) from mitochondria in SK-BR-3 cells. No such changes were observed in MCF-7 cells after taxane treatment. CONCLUSION: We conclude that the activation of apical caspase-2 results in the activation of caspase-3 and -7 without the involvement of mitochondria. Caspase-9 can be activated directly via caspase-2 or alternatively after cytochrome c release from mitochondria. Subsequently, caspase-9 activation can also lead to caspase-3 and -7 activations. Caspase-3 and caspase-7 activate mutually. It seems that there is also a parallel pathway involving mitochondria that can cooperate in taxane-induced cell death in breast cancer cells.

Taxanes with high potency inducing tubulin assembly overcome tumoural cell resistances.

We have found that four taxanes with chemical modifications at positions C10 and C13 were active against all types of taxane resistant cell lines, resistant by P-gp overexpression, by mutations in the ß-tubulin binding site or by overexpression of the highly dynamic ßIII-tubulin isotype. We have characterized the interaction of taxanes with high activity on chemotherapy resistant tumoural cells with microtubules, and also studied their cellular effects. The biochemical property enhanced in comparison with other taxanes is their potency at inducing tubulin assembly, despite the fact that their interactions with the microtubule binding sites (pore and luminal) are similar as studied by NMR and SAXS. A differential interaction with the S7-S9 loop (M-loop) is responsible for their enhanced assembly induction properties. The chemical changes in the structure also induce changes in the thermodynamic properties of the interaction, indicating a higher hydrophilicity and also explaining their properties on P-gp and ßIII overexpressing cells and on mutant cells. The effect of the compounds on the microtubular network is different from those observed with the classical (docetaxel and paclitaxel) taxanes, inducing different bundling in cells with microtubules being very short, indicating a very fast nucleation effect and reflecting their high assembly induction power.

Caspase-2 is involved in cell death induction by taxanes in breast cancer cells.

BACKGROUND: We studied the role of caspase-2 in apoptosis induction by taxanes (paclitaxel, novel taxane SB-T-1216) in breast cancer cells using SK-BR-3 (nonfunctional p53, functional caspase-3) and MCF-7 (functional p53, nonfunctional caspase-3) cell lines. RESULTS: Both taxanes induced apoptosis in SK-BR-3 as well as MCF-7 cells. Caspase-2 activity in SK-BR-3 cells increased approximately 15-fold within 48 h after the application of both taxanes at the death-inducing concentration (100 nM). In MCF-7 cells, caspase-2 activity increased approximately 11-fold within 60 h after the application of taxanes (300 nM). Caspase-2 activation was confirmed by decreasing levels of procaspase-2, increasing levels of cleaved caspase-2 and the cleavage of caspase-2 substrate golgin-160. The inhibition of caspase-2 expression using siRNA increased the number of surviving cells more than 2-fold in MCF-7 cells, and at least 4-fold in SK-BR-3 cells, 96 h after the application of death-inducing concentration of taxanes. The inhibition of caspase-2 expression also resulted in decreased cleavage of initiator caspases (caspase-8, caspase-9) as well as executioner caspases (caspase-3, caspase-7) in both cell lines after the application of taxanes. In control cells, caspase-2 seemed to be mainly localized in the nucleus. After the application of taxanes, it was released from the nucleus to the cytosol, due to the long-term disintegration of the nuclear envelope, in both cell lines. Taxane application led to some formation of PIDDosome complex in both cell lines within 24 h after the application. After taxane application, p21WAF1/CIP1 expression was only induced in MCF-7 cells with functional p53. However, taxane application did not result in a significant increase of PIDD expression in either SK-BR-3 or MCF-7 cells. The inhibition of RAIDD expression using siRNA did not affect the number of surviving SK-BR-3 and MCF-7 cells after taxane application at all. CONCLUSION: Caspase-2 is required, at least partially, for apoptosis induction by taxanes in tested breast cancer cells. We suggest that caspase-2 plays the role of an apical caspase in these cells. Caspase-2 seems to be activated via other mechanism than PIDDosome formation. It follows the release of caspase-2 from the nucleus to the cytosol.

An optical assay of the transport activity of ClC-7.

Osteoporosis, characterized by excessive osteoclast mediated bone resorption, affects millions of people worldwide representing a major public health problem. ClC-7 is a chloride-proton exchanger localized in lysosomes and in the resorption lacuna in osteoclasts where it is essential for bone resorption. Thus, drugs targeted at ClC-7 have been proposed for ameliorating osteoporosis. However, functional assays suited for high throughput screening (HTS) of ClC-7 function are lacking. Here we describe two complementary variants of purely optical assays of the transport activity of ClC-7, redirected to the plasma membrane employing a genetically encoded fluorescent Cl⁻/pH indicator fused to the ClC-7 protein. These simple and robust functional assays of ClC-7 transport are well-suited to be applied in HTS of small-molecule inhibitors and may help to develop drugs suited for the treatment of osteoporosis.

On the mechanism of gating charge movement of ClC-5, a human Cl(-)/H(+) antiporter.

ClC-5 is a Cl(-)/H(+) antiporter that functions in endosomes and is important for endocytosis in the proximal tubule. The mechanism of transport coupling and voltage dependence in ClC-5 is unclear. Recently, a transport-deficient ClC-5 mutant (E268A) was shown to exhibit transient capacitive currents. Here, we studied the external and internal Cl(-) and pH dependence of the currents of E268A. Transient currents were almost completely independent of the intracellular pH. Even though the transient currents are modulated by extracellular pH, we could exclude that they are generated by proton-binding/unbinding reactions. In contrast, the charge movement showed a nontrivial dependence on external chloride, strongly supporting a model in which the movement of an intrinsic gating charge is followed by the voltage-dependent low-affinity binding of extracellular chloride ions. Mutation of the external Glu-211 (a residue implicated in the coupling of Cl(-) and proton transport) to aspartate abolished steady-state transport, but revealed transient currents that were shifted by ~150 mV to negative voltages compared to E268A. This identifies Glu(ext) as a major component of the gating charge underlying the transient currents of the electrogenic ClC-5 transporter. The molecular events underlying the transient currents of ClC-5 emerging from these results can be explained by an inward movement of the side chain of Glu(ext), followed by the binding of extracellular Cl(-) ions.

The Arabidopsis central vacuole as an expression system for intracellular transporters: functional characterization of the Cl-/H+ exchanger CLC-7.

Functional characterization of intracellular transporters is hampered by the inaccessibility of animal endomembranes to standard electrophysiological techniques. Here, we used Arabidopsis mesophyll protoplasts as a novel heterologous expression system for the lysosomal chloride­proton exchanger CLC-7 from rat. Following transient expression of a rCLC-7:EGFP construct in isolated protoplasts, the fusion protein efficiently targeted to the membrane of the large central vacuole, the lytic compartment of plant cells. Membrane currents recorded from EGFP-positive vacuoles were almost voltage independent and showed time-dependent activation at elevated positive membrane potentials as a hallmark. The shift in the reversal potential of the current induced by a decrease of cytosolic pH was compatible with a 2Cl(-)/1H(+) exchange stoichiometry. Mutating the so-called gating glutamate into alanine (E245A) uncoupled chloride fluxes from the movement of protons, transforming the transporter into a chloride channel-like protein. Importantly, CLC-7 transport activity in the vacuolar expression system was recorded in the absence of the auxiliary subunit Ostm1, differently to recent data obtained in Xenopus oocytes using a CLC-7 mutant with partial plasma membrane expression. We also show that plasma membrane-targeted CLC-7(E245A) is non-functional in Xenopus oocytes when expressed without Ostm1. In summary, our data suggest the existence of an alternative CLC-7 operating mode, which is active when the protein is not in complex with Ostm1. The vacuolar expression system has the potential to become a valuable tool for functional studies on intracellular ion channels and transporters from animal cells.

Second-generation taxanes effectively suppress subcutaneous rat lymphoma: role of disposition, transport, metabolism, in vitro potency and expression of angiogenesis genes.

The study investigated possible mechanisms by which second-generation taxanes, established as significantly more effective than paclitaxel in vitro, suppress a rat lymphoma model in vivo. The studied mechanisms included taxane pharmacokinetics, expression of genes dominating their metabolism (Cyp3a1/2) and transport (Abcb1) and genes controlling tumour angiogenesis (growth factors and receptors). SB-T-1214, SB-T-12854 and IDN5109 suppressed rat lymphoma more effectively than paclitaxel, SB-T-12851, SB-T-12852, SB-T-12853 or IDN5390 as well as P388D1 leukaemia cells in vitro. The greater anti-lymphoma effects of SB-T-1214 in rats corresponded to a higher bioavailability than with SB-T-12854, and lower systemic toxicity of SB-T-1214 for rats reflected its lower cytotoxicity for P388D1 cells in vitro. Suppression of Abcb1 and CYP3a1 expression by SB-T-1214 and IDN5109 could partly explain their anti-lymphoma effects, but not that of SB-T-12854. Growth factors genes Egf, Fgf, Pdgf, and Vegf associated with tumour angiogenesis had significantly lower expression following treatment with anti-lymphoma effective IDN5109 and their receptors were unaffected, whereas inefficient IDN5390 increased expression of the most important Vegf. The effective SB-T-12854 inhibited Egf, Egfr, Fgfr and Pdgfr expression, while the ineffective SB-T-12851, SB-T-12852 and SB-T-12853 inhibited only Egf or Egfr expression. Vegfr expression was inhibited significantly by SB-T-12851 and SB-T-12854, but effect of SB-T-12851 was compromised by induced Vegf expression. The very effective SB-T-1214 decreased the expression of Vegf, Egf and all receptors most prominently indicating the possible supporting role of these genes in anti-lymphoma effects. In conclusion, SB-T-1214, SB-T-12854 and IDN5109 are good candidates for further study.

Novel trisubstituted benzimidazoles, targeting Mtb FtsZ, as a new class of antitubercular agents.

Libraries of novel trisubstituted benzimidazoles were created through rational drug design. A good number of these benzimidazoles exhibited promising MIC values in the range of 0.5-6 µg/mL (2-15 µM) for their antibacterial activity against Mtb H37Rv strain. Moreover, five of the lead compounds also exhibited excellent activity against clinical Mtb strains with different drug-resistance profiles. All lead compounds did not show appreciable cytotoxicity (IC(50) > 200 µM) against Vero cells, which inhibited Mtb FtsZ assembly in a dose dependent manner. The two lead compounds unexpectedly showed enhancement of the GTPase activity of Mtb FtsZ. The result strongly suggests that the increased GTPase activity destabilizes FtsZ assembly, leading to efficient inhibition of FtsZ polymerization and filament formation. The TEM and SEM analyses of Mtb FtsZ and Mtb cells, respectively, treated with a lead compound strongly suggest that lead benzimidazoles have a novel mechanism of action on the inhibition of Mtb FtsZ assembly and Z-ring formation.

Cell death induced by novel fluorinated taxanes in drug-sensitive and drug-resistant cancer cells.

The aim of this study is to compare the effects of new fluorinated taxanes SB-T-12851, SB-T-12852, SB-T-12853, and SB-T-12854 with those of the classical taxane paclitaxel and novel non-fluorinated taxane SB-T-1216 on cancer cells. Paclitaxel-sensitive MDA-MB-435 and paclitaxel-resistant NCI/ADR-RES human cancer cell lines were used. Cell growth and survival evaluation, colorimetric assessment of caspases activities, flow cytometric analyses of the cell cycle and the assessment of mitochondrial membrane potential, reactive oxygen species (ROS) and the release of cytochrome c from mitochondria were employed. Fluorinated taxanes have similar effects on cell growth and survival. For MDA-MB-435 cells, the C(50) of SB-T-12851, SB-T-12852, SB-T-12853 and SB-T-12854 was 3 nM, 4 nM, 3 nM and 5 nM, respectively. For NCI/ADR-RES cells, the C(50) of SB-T-12851, SB-T-12852, SB-T-12853, and SB-T-12854 was 20 nM, 20 nM, 10 nM and 10 nM, respectively. Selected fluorinated taxanes, SB-T-12853 and SB-T-12854, at the death-inducing concentrations (30 nM for MDA-MB-435 and 300 nM for NCI/ADR-RES) were shown to activate significantly caspase-3, caspase-9, caspase-2 and also slightly caspase-8. Cell death was associated with significant accumulation of cells in the G(2)/M phase. Cytochrome c was not released from mitochondria and other mitochondrial functions were not significantly impaired. The new fluorinated taxanes appear to use the same or similar mechanisms of cell death induction as compared with SB-T-1216 and paclitaxel. New fluorinated and non-fluorinated taxanes are more effective against drug-resistant cancer cells than paclitaxel. Therefore, new generation of taxanes, either non-fluorinated or fluorinated, are excellent candidates for further and detailed studies.

Comparison of cell death-inducing effect of novel taxane SB-T-1216 and paclitaxel in breast cancer cells.

BACKGROUND: In this study, the effect of novel taxane SB-T-1216 and paclitaxel on sensitive MDA-MB-435 and resistant NCI/ADR-RES human breast cancer cells was compared. MATERIALS AND METHODS: Cell growth and survival were evaluated after 96-hour incubation with tested concentrations of taxanes. The effect on the formation of microtubule bundles was assessed employing fluorescence microscopy and on the cell cycle employing flow cytometric analysis. The activity of caspases was assessed employing commercial colorimetric kits. RESULTS: The IC(50) (concentration resulting in 50% of living cells in comparison with the control) of SB-T-1216 in sensitive cells was 0.6 nM versus 1 nM for paclitaxel. However, the IC(50) of SB-T-1216 in resistant cells was 1.8 nM versus 300 nM for paclitaxel. Both SB-T-1216 and paclitaxel at death-inducing concentrations induced the formation of microtubule bundles in sensitive as well as resistant cells. Cell death induced in sensitive and resistant cells by paclitaxel was associated with the accumulation of cells in the G(2)/M phase. On the contrary, cell death induced by SB-T-1216 took place without the accumulation of cells in the G(2)/M phase but with a decreased number of G(1) cells and the accumulation of hypodiploid cells. Both SB-T-1216 and paclitaxel activated caspase-3, caspase-9, caspase-2 and caspase-8 in sensitive as well as resistant cells. CONCLUSION: Cell death induced by both paclitaxel and novel taxane SB-T-1216 in breast cancer cells is associated with caspase activation and with the formation of interphase microtubule bundles. Novel taxane SB-T-1216, but not paclitaxel, seems to be capable of inducing cell death without the accumulation of cells in the G(2)/M phase.

Novel C-seco-taxoids possessing high potency against paclitaxel-resistant cancer cell lines overexpressing class III beta-tubulin.

Novel C-seco-taxoids were synthesized from 10-deacetylbaccatin III and their potencies evaluated against drug-sensitive and drug-resistant cancer cell lines. The drug-resistant cell lines include ovarian cancer cell lines resistant to cisplatin, topotecan, adriamycin and paclitaxel overexpressing class III beta-tubulin, A2780TC1 and A2780TC3. The last two cell lines were selected through chronic exposure of A2780wt to paclitaxel and Pgp blocker cyclosporine. All novel C-seco-taxoids exhibited remarkable potency against A2780TC1 and A2780TC3 cell lines, and no cross resistance to cisplatin- and topotecan-resistant cell lines, A2780CIS and A2780TOP. Four of those C-seco-taxoids exhibit much higher activities than IDN5390 against paclitaxel-resistant cell lines, A2780ADR, A2780TC1 and A2780TC3. SB-CST-10202 possesses the best all-round high potencies across different drug-resistant cell lines. Molecular modeling studies, including molecular dynamics simulations, on the drug-protein complexes of class I and III beta-tubulins were performed to identify possible cause of the remarkable potency of these C-seco-taxoids against paclitaxel-resistant cell lines overexpressing class III beta-tubulin.

Scorpion toxins that block transient currents (I(A)) of rat cerebellum granular cells.

This communication is a revision of the state-of-the-art knowledge of the field of scorpion toxins specific for the K(+)-channels, responsible for the I(A) currents of granular cells of rat cerebellum, maintained in vitro culture. There are 6 members of the sub-family alpha-KTx15 known to affect the I(A) currents. They are: toxins Aa1 from Androctonus australis Garzoni, BmTx3 from Buthusmartensi Karch, AmmTx3 from Androctonus mauretanicus mauretanicus, AaTx1 and AaTx2 from A. australis Garzoni and Discrepin from Tityus discrepans. They share high sequence similarity, apart from Discrepin, which causes an irreversible effect on the I(A) currents and is the most thoroughly studied toxin of the sub-family alpha-KTx15. The three-dimensional structure of Discrepin was determined and a series of mutants were synthesized and assayed in the system with the aim of identifying possible amino acids or sequence segments responsible for the irreversible effect found. In this revision some unpublished original data are also included to foster future work on the field, as well as a short discussion on some relevant aspects still pending and possible limitations associated with the strategy proposed.

Molecular determinants of multiple effects of nickel on NMDA receptor channels.

Nickel (Ni2+) is a toxic metal that affects the function of several neuronal ionic channels. Ni2+ inhibits N-methyl-D: -aspartate receptor (NR) channel in a voltage-dependent manner, but also causes enhancement of NR2B-containing channel activity and voltage-independent inhibition of those containing NR2A. The present work was aimed to find the sites of Ni2+ interaction on the NR2A and NR2B subunits by expressing wild-type and mutated NRs in either HEK293 cells or Xenopus laevis oocytes. The point mutation N616G in the pore region of the NR2B subunit completely removed the voltage-dependent block. In NR2 subunits deleted for their entire amino terminal domain (ATD) and expressed with wild-type NR1 subunit, voltage-independent inhibition of NR2A-containing channels was not modified, but the potentiation effect was abolished in NR2B-containing channels. In the latter channels, potentiation of the current was also removed by H127A, D101A, D104A point mutations and by the double mutation H127AD101A, all located in lobe I of ATD, and reduced by the point mutation T233A in lobe II, suggesting that the interaction site that causes potentiation shares common determinants with the Zn2+ and ifenprodil binding sites. In contrast, in NR2A-containing channels, we postulate the existence of an additional divalent cation binding site in the M3-M4 extracellular loop. In these channels, the point mutation H801A in the NR2A subunit caused an important reduction of the voltage-independent block, with a 7-time increase in IC(50). The block was also partially, but not as prominently, reduced by the double mutation H705AH709A in the same region of NR1. This additional binding site can be responsible of specific heavy metal interaction with NR channels.

Design, synthesis, and biological evaluation of new-generation taxoids.

Novel second-generation taxoids with systematic modifications at the C2, C10, and C3'N positions were synthesized and their structure-activity relationships studied. A number of these taxoids exhibited exceptionally high potency against multidrug-resistant cell lines, and several taxoids exhibited virtually no difference in potency against the drug-sensitive and drug-resistant cell lines. These exceptionally potent taxoids were termed "third-generation taxoids". 19 (SB-T-1214), 14g (SB-T-121303), and 14i (SB-T-1213031) exhibited excellent activity against paclitaxel-resistant ovarian cancer cell lines with mutations in beta-tubulin as well, wherein the drug resistance is mediated by the beta-tubulin mutation. These taxoids were found to possess exceptional activity in promoting tubulin assembly, forming numerous very short microtubules similar to those formed by discodermolide. Taxoids 19 and 14g also showed excellent cytotoxicity against four pancreatic cancer cell lines, expressing three to four multidrug-resistant genes. Moreover, taxoid 19 exhibited excellent in vivo efficacy against highly drug-resistant CFPAC-1 pancreatic as well as DLD-1 human colon tumor xenografts in mice.

Molecular determinants of Pb2+ interaction with NMDA receptor channels.

Lead (Pb2+) is a potent neurotoxin that acts as a non-competitive, voltage-independent antagonist of the NMDA receptor (NR) channel. Pb2+ action partially overlaps with that of zinc (Zn2+), but precise coincidence with Zn2+ binding site is debated. We investigated the site of Pb2+ interaction in NR channels expressed in Xenopus laevis oocytes from the clones zeta1, epsilon1 or epsilon2 and mutated epsilon1 or epsilon2 forms. For each epsilon subunit we chose two mutations that have been identified as 'strong mutations' for Zn2+ binding and examined the effect of Pb2+ on channels that contained those mutations. In epsilon1-containing channels, mutations D102A and H128A caused a decrease of Pb2+ inhibition with a 10-fold (D102A) and four-fold (H128A) shift of IC50. In epsilon2-containing channels, the most effective mutation in removing Pb2+ inhibition was H127A, with a five-fold increase of IC50, while D101A was virtually ineffective. Other mutations, D104A, T103A, and T233A, were less effective. The double mutation D101AH127A, while reducing Zn2+ inhibition by nearly nine-fold, caused a minor (less than two-fold) shift in Pb2+ IC50. Competition experiments showed that increasing doses of Zn2+ reduced the apparent affinity for Pb2+ in epsilon1-containing receptors, but not in epsilon2-containing receptors. In addition the effect of Pb2+ on epsilon2-containing channels was additive with that of ifenprodil, with no competition for the site. Although none of the mutations that we have tested abolished the block by Pb2+, our results indicate that the action of this toxic metal on NR channels is more dependent on the receptor composition than previously thought, because Zn2+ is able to displace Pb2+ from its binding site in epsilon1-containing channels, but not in epsilon2-containing channels.