Expression of BK-type calcium-activated potassium channel splice variants during chick cochlear development.
The appearance of large-conductance, calcium-activated potassium (BK) current is a hallmark of functional maturation in auditory hair cells. Acquisition of this fast-activating current enables high-frequency, graded receptor potentials in all vertebrates and an electrical tuning mechanism in nonmammals. The gene encoding BK alpha subunits is highly alternatively spliced, and the resulting variations in channel isoforms may contribute to functional diversity at the onset of hearing. We examined the tissue specificity of nine BK alpha alternative exons and investigated changes in expression during chick cochlear development using quantitative polymerase chain reaction (qPCR). Each alternative was widely expressed in several tissues except for an insert near the C-terminus Ca(2+) sensing domain, which appeared brain-specific. The only alternative form in the membrane-bound core of the channel was expressed in brain and muscle but was undetected in cochlea. Of the remaining variants, three increased in expression prior to the onset of hearing and acquisition of BK currents. These three variants cause decreased Ca(2+) sensitivity or increased intracellular retention, traits that would not easily explain the advent of calcium-sensitive currents at embryonic day (E)18-19. Expression levels of other variants were mature and stable by E15, days before currents were acquired. Surface expression of C-terminal isoforms was examined using patch-clamp electrophysiology and immunocytochemistry. C-terminal variants that exhibit robust surface expression appeared in the membrane at E18, even though transcripts were unchanged during development starting from E12. These results indicate that delays in protein synthesis and trafficking/scaffolding of channel subunits underlie the late acquisition of BK currents in cochlear hair cells.
Kim
Jung-Min
JM
Kresge Hearing Research Institute, University of Michigan, Ann Arbor, Michigan 48109, USA.
Beyer
Ryan
R
Morales
Marti
M
Chen
Stephanie
S
Liu
Li Qian
LQ
Duncan
R Keith
RK
D000595:Amino Acid Sequence; D000818:Animals; D030161:Avian Proteins / Q000235:genetics / Q000378:metabolism*; D001921:Brain / Q000196:embryology / Q000254:growth & development / Q000378:metabolism; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002642:Chick Embryo; D002645:Chickens; D003051:Cochlea / Q000196:embryology / Q000254:growth & development* / Q000378:metabolism*; D005091:Exons; D006198:Hair Cells, Auditory / Q000378:metabolism; D006801:Humans; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000235:genetics / Q000378:metabolism*; D024510:Muscle Development / Q000502:physiology; D009132:Muscles / Q000196:embryology / Q000378:metabolism; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism
D030161:Avian Proteins; D051036:Large-Conductance Calcium-Activated Potassium Channels; D020033:Protein Isoforms