Cloning and Characterization of Glioma BK, a Novel BK Channel Isoform Highly Expressed in Human Glioma Cells

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Abstract
Voltage-dependent large-conductance Ca2+-activated K+ channels (BK channels) are widely expressed in excitable and nonexcitable cells. BK channels exhibit diverse electrophysiological properties, which are attributable in part to alternative splicing of their α-subunits. BK currents have been implicated in the growth control of glial cells, and BK channels with novel biophysical properties have recently been characterized in human glioma cells. Here we report the isolation, cloning, and functional characterization of glioma BK (gBK), a novel splice isoform of hSlo, the gene that encodes the α-subunits of human BK channels. The primary sequence of gBK is 97% identical to its closest homolog hbr5, but it contains an additional 34-amino-acid exon at splice site 2 in the C-terminal tail of BK channels. hSlo transcripts containing this novel exon are expressed ubiquitously in various normal tissues as well as in neoplasmic samples, suggesting that the novel exon may modulate important physiological functions of BK channels. Expression of gBK inXenopus oocytes gives rise to iberiotoxin-sensitive (IbTX) currents, with an IC50 for IbTX of 5.7 nm and a Hill coefficient of 0.76. Single gBK channels have a unitary conductance of ∼250 pS, and the currents show significantly slower activation and higher Ca2+ sensitivity than hbr5. Ca2+ sensitivity was enhanced specifically at physiologically relevant [Ca2+]i(100–500 nm). Examination of biopsies from patients with malignant gliomas has revealed specific overexpression of BK channels in gliomas compared with nonmalignant human cortical tissues. Importantly, tumor malignancy grades have correlated positively with BK channel expression, suggesting an important role for the gBK channel in glioma biology.