Darstellung und Struktur kationischer η2-Thiocarbenkomplexe von Molybdän und Wolfram / Synthesis and Structure of Cationic η2-Thiocarbene Complexes of Molybdenum and Tungsten

The reaction of dimethyl(methylthio)sulfonium tetrafluoroborate [(CH3)2S–SCH3][BF4] with η/5-cyclopentadienyl(dicarbonyl)carbyne complexes of molybdenum and tungsten Cp(CO)2M ≡ C–R (M = Mo, W; R = CH3, C6H5, C6H4CH3) 1c, 2a–d results in an electrophilic attack of the SCH3 moiety at the metal carbon triple bond, providing orange to red, diamagnetic, cationic η2-thiocarbene complexes 3c, 4a–d in high yields. The new compounds were identified by X-ray structure analysis, NMR, IR and mass spectra.

Expression of voltage-activated chloride currents in acute slices of human gliomas

Using whole-cell patch-clamp recordings, we identified a novel voltage-activated chloride current that was selectively expressed in glioma cells from 23 patient biopsies. Chloride currents were identified in 64% of glioma cells studied in acute slices of nine patient biopsies. These derived from gliomas of various pathological grades. In addition, 98% of cells acutely isolated or in short-term culture from 23 patients diagnosed with gliomas showed chloride current expression. These currents, which we termed glioma chloride currents activated at potentials >45 mV, showed pronounced outward rectification, and were sensitive to bath application of the presumed Cl- channel specific peptide chlorotoxin (approximately 600 nM) derived from Leiurus scorpion venom. Interestingly, low grade tumours (e.g., pilocytic astrocytomas), containing more differentiated, astrocyte-like cells showed expression of glioma chloride currents in concert with voltage-activated sodium and potassium currents also seen in normal astrocytes. By contrast, high grade tumours (e.g., glioblastoma multiforme) expressed almost exclusively chloride currents, suggesting a gradual loss of Na+ currents and gain of Cl- currents with increasing pathological tumour grade. To expand on the observation that these chloride currents are glioma-specific, we introduced experimental tumours in scid mice by intracranial injection of D54MG glioma cells and subsequently recorded from tumour cells and adjacent normal glial cells in acute slices. We consistently observed expression of chlorotoxin-sensitive chloride channels in implanted glioma cells, but without evidence for expression of chloride channels in surrounding "normal" host glial cells, suggesting that these chloride channels are probably a glioma-specific feature. Finding of this novel glioma specific Cl- channel in gliomas in situ and it's selective binding of chlorotoxin may provide a way to identify or target glioma cells in the future.

Cell cycle-dependent expression of a glioma-specific chloride current: proposed link to cytoskeletal changes

We recently demonstrated expression of a novel, glioma-specific Cl- current in glial-derived tumor cells (gliomas), including stable cell lines such as STTG1, derived from a human anaplastic astrocytoma. We used STTG1 cells to study whether glioma Cl- channel (GCC) activity is regulated during cell cycle progression. Cells were arrested in defined stages of cell cycle (G0, G1, G1/S, S, and M phases) using serum starvation, mevastatin, hydroxyurea, demecolcine, and cytosine beta-D-arabinofuranoside. Cell cycle arrest was confirmed by measuring [3H]thymidine incorporation and by DNA flow cytometry. Using whole cell patch-clamp recordings, we demonstrate differential changes in GCC activity after cell proliferation and cell cycle progression was selectively altered; specifically, channel expression was low in serum-starved, G0-arrested cells, increased significantly in early G1, decreased during S phase, and increased after arrest in M phase. Although the link between the cell cycle and GCC activity is not yet clear, we speculate that GCCs are linked to the cytoskeleton and that cytoskeletal rearrangements associated with cell division lead to the observed changes in channel activity. Consistent with this hypothesis, we demonstrate the activation of GCC by disruption of F-actin using cytochalasin D or osmotic cell swelling.

Biophysical and pharmacological characterization of chloride currents in human astrocytoma cells

Expression of voltage-activated ion channels was studied in primary cultures from seven freshly resected human primary brain tumors and in an established human astrocytoma cell line, STTG1. Astrocytoma cells consistently expressed voltage-dependent outwardly rectifying currents. Currents activated at potentials > 45 mV and showed outward transients on termination of voltage steps. Currents reversed at the Cl equilibrium potential, suggesting that they were largely carried by Cl-. Altering extracellular K- or Na+ concentration did not alter currents; neither did replacement of intracellular K+ by Cs+ or intracellular Na+ by N-methyl-D-glucosamine. Anion-substitution experiments suggest the following permeability sequence, determined from shifts in tail current reversal potential: I- > NO3- > Br- > Cl- > acetate > isethionate > F- > glutamate. Currents were sensitive to the Cl- channel blockers chlorotoxin, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and 4,4'-dinitrostilbene-2,2' disulfonic acid (DNDS), with chlorotoxin being most effective, yielding > 80% block at 590 nM. DIDS (100 microM) and DNDS (100 microM) reduced currents by 33.5 and 38.2%, respectively. Currents were also sensitive to Zn2+ (100 microM, 47% block) and Cd2- (25 microM, 42% block). Reducing extracellular Ca2+ concentration decreased outward currents by 58% and almost completely eliminated transients, suggesting that Cl- currents are Ca2+ dependent. Cl channel block resulted in altered cell proliferation as determined by [3H]thymidine incorporation, suggesting that these channels may be involved in astrocytoma growth control.

Human astrocytoma cells express a unique chloride current

Human astrocytoma cells were studied using whole-cell patch-clamp recording. Voltage-dependent outwardly-rectifying anion currents were identified in primary cultures of six freshly resected human brain tumors and in seven established anaplastic astrocytoma/glioblastoma cell lines (U251MG, CH235MG, U373MG, U105MG, D54MG, SK-MG-1, and STTG1). Anion currents were not observed in normal, non-neoplastic glial cells, nor in human tumor-derived cells of non-glial origin (melanoma, breast cancer, neuroblastoma, rhabdomyosarcoma). Currents activated at potentials > 50 mV and showed large transients upon termination of voltage steps. Currents reversed at the predicted equilibrium potential for chloride ions and could also be recorded when Cl- was replaced by F-, Br- or I-. Currents were inhibited by the Cl- channel blockers chlorotoxin, DIDS, and DNDS. These Cl- currents may play a role in the growth control of astrocytoma cells.

Human astrocytoma cells express a unique chloride current

Human astrocytoma cells were studied using whole-cell patch-clamp recording. Voltage-dependent outwardly-rectifying anion currents were identified in primary cultures of six freshly resected human brain tumors and in seven established anaplastic astrocytoma/glioblastoma cell lines (U251MG, CH235MG, U373MG, U105MG, D54MG, SK-MG-1, and STTG1). Anion currents were not observed in normal, non-neoplastic glial cells, nor in human tumor-derived cells of non-glial origin (melanoma, breast cancer, neuroblastoma, rhabdomyosarcoma). Currents activated at potentials > 50 mV and showed large transients upon termination of voltage steps. Currents reversed at the predicted equilibrium potential for chloride ions and could also be recorded when Cl- was replaced by F-, Br- or I-. Currents were inhibited by the Cl- channel blockers chlorotoxin, DIDS, and DNDS. These Cl- currents may play a role in the growth control of astrocytoma cells.

Reduction of glial proliferation by K+ channel blockers is mediated by changes in pHi

Astrocyte proliferation was studied in primary cultures of rat spinal cord by [3H]thymidine uptake and was significantly reduced by culturing cells for 24 h in the presence of agents known to block astrocytic K+ channels: Cs+, Ba2+, 4-AP and tetraethylammonium (TEA). To determine whether effects were mediated by changes in Vm or pHi, these parameters were studied electrophysiologically or ratiometrically, using BCECF. Of the four K+ channel blockers, only Ba2+ depolarized astrocytes significantly. However, all four K+ channel blockers resulted in an alkaline shift in pHi. Under culture conditions that altered pHi in a defined way, proliferation strongly depended on pHi, with highest rates at pH approximately 6.7 and growth inhibition at more acidic or alkaline conditions. These observations suggest that astrocyte proliferation is sensitive to changes in pHi and that K+ channel blockers may exhibit their antiproliferative effects through changes in pHi.

Astrocyte Na+ channels are required for maintenance of Na+/K(+)-ATPase activity

Astrocytes in vitro and in situ have been shown to express voltage-activated ion channels previously thought to be restricted to excitable cells, including voltage-activated Na+, Ca2+, and K+ channels. However, unlike neurons, astrocytes do not generate action potentials, and the functional role of voltage-activated channels in astrocytes has been an enigma. In order to study the function of Na+ channels in glial cells, we carried out ion flux measurements, patch-clamp recordings, and ratiometric imaging of [Na+]i during blockade of Na+ channels on rat spinal cord astrocytes cultured for 7-10 d. Acute blockade of astrocyte Na+ channels by TTX had multiple effects: (1) TTX reduced, in a dose-dependent manner, Na+/K(+)-ATPase activity measured as unidirectional influx of 86Rb+; (2) TTX depolarized astrocyte membrane potential at a rate of approximately 1 mV/min; (3) TTX (100 microM) reduced [Na+]i; and (4) prolonged exposure to micromolar TTX induced astrocyte death. All these effects of TTX could be mimicked by ouabain or strophanthidin, specific blockers of the Na+/K(+)-ATPase. The effects of TTX and ouabain (or strophanthidin) were not additive. These results suggest that TTX-blockable Na+ channels in glial cells serve functions that do not require their participation in action potential electrogenesis; in particular, we propose that glial Na+ channels constitute a "return" pathway for Na+/K(+)-ATPase function, which permits Na+ ions to enter the cells to maintain [Na+]i at concentrations necessary for activity of the Na+/K(+)-ATPase. Since astrocyte Na+/K(+)-ATPase is believed to participate in [K+]o homeostasis in the CNS, the coupling of Na+ flux through voltage-activated Na+ channels to ATPase activity may provide a feedback loop that participates in the regulation of K+ ion levels in the extracellular space.