Highly Specific Alternative Splicing of Transcripts Encoding BK Channels in the Chicken's Cochlea Is a Minor Determinant of the Tonotopic Gradient

Abstract

The frequency sensitivity of auditory hair cells in the inner ear varies with their longitudinal position in the sensory epithelium. Among the factors that determine the differential cellular response to sound is the resonance of a hair cell's transmembrane electrical potential, whose frequency correlates with the kinetic properties of the high-conductance Ca 2؉ -activated K ؉ (BK) channels encoded by a Slo (kcnma1) gene. It has been proposed that the inclusion of specific alternative axons in the Slo transcripts along the cochlea underlies the gradient of BK-channel kinetics. By analyzing the complete sequences of chicken Slo gene (cSlo) cDNAs from the chicken's cochlea, we show that most transcripts lack alternative exons. Transcripts with more than one alternative exon constitute only 10% of the total. Although the fraction of transcripts containing alternative exons increases from the cochlear base to the apex, the combination of alternative exons is not regulated. There is also a clear increase in the expression of BK transcripts with long carboxyl termini toward the apex. When long and short BK transcripts are expressed in HEK-293 cells, the kinetics of single-channel currents differ only slightly, but they are substantially slowed when the channels are coexpressed with the auxiliary ␤ subunit that occurs more widely at the apex. These results argue that the tonotopic gradient is not established by the selective inclusion of highly specific cSlo exons. Instead, a gradient in the expression of ␤ subunits slows BK channels toward the low-frequency apex of the cochlea.The auditory system maps continuous sensory variables, such as the frequency of a sound or the spatial location of its source, onto cellular detector arrays whose individual elements are narrowly tuned. How sensory maps of any kind are established and maintained is an open question. In only a single case, the tonotopic frequency map in the inner ear, is the physiological mechanism responsible for the individual cell's differential tuning understood at the molecular level.In the process of electrical resonance, a combination of voltage-and ion-dependent conductances operates in conjunction with the passive electrical properties of a hair cell's membrane to accentuate cellular responsiveness to a particular range of frequencies while attenuating that to frequencies outside this range. A dynamic interaction between two types of ion channels mediates this band-pass behavior. Voltage-sensitive Ca 2ϩ channels activated by the current through mechanotransduction channels depolarize the hair cell's membrane and increase the intracellular concentration of Ca 2ϩ . With some delay, these ions activate the Ca 2ϩ -sensitive high-conductance Ca 2ϩ -activated K ϩ (BK) channels that repolarize the membrane, thereby closing the Ca 2ϩ channels, decreasing the intracellular concentration of Ca 2ϩ , and deactivating the K ϩ channels. How rapidly this dynamic system progresses through the cycle depends on several factors, most notably on the kinetic properties of th...

β4-Subunit increases Slo responsiveness to physiological Ca2+ concentrations and together with β1 reduces surface expression of Slo in hair cells

“…RNA encoding the very COOH terminus of the protein is also alternatively spliced, and the sequence we chose for these experiments encoded the amino acids KQKYVQEDRL at its COOH terminus. This variant is the most abundantly expressed form in the chick cochlea (34,36).…”

Development of tonotopy in the auditory periphery

“…(Duncan and Fuchs, 2003;Miranda-Rottmann et al, 2010;Ramanathan et al, 2000) Faster deactivation kinetics in basal hair cells. (Fuchs and Sokoloski, 1990b;Ramanathan et al, 2000) Turtle Differential a-subunit splicing but precise pattern unclear.…”

“…In addition to differences in a-subunit splicing, co-expression of an auxiliary b-subunit provides further modulation of channel kinetics (Jones et al, 1999;MirandaRottmann et al, 2010;Ramanathan et al, 2000). The presence of the b-subunit results in a higher Ca 2þ affinity and slowed deactivation rates, a characteristic well suited to the "slower" apical cells (Miranda-Rottmann et al, 2010;Ramanathan et al, 2000).…”

Hair cell BK channels interact with RACK1, and PKC increases its expression on the cell surface by indirect phosphorylation

“…Currents carried by BK channels and the number of BK channel clusters increase in high-frequency hair cells (7,40). The increase in channel clusters is at variance with the amounts of Slo transcripts that show an opposite decrease in high-frequency hair cells (21,32). The seeming paradox between mRNA and Slo surface expression gradients along the tonotopic axis is likely to be mediated by complex mechanisms.…”