Otology Seminar Hair cell tuning 2001-12-19 R3 周昱甫 Extrinsic tuning vs intrinsic tuning Electrical vs mechanical tuning Electrical tuning Hair cell tuning in turtle basilar papilla 1. No mechanical tuning 2. Low-frequency hearing, 30-600 Hz---basilar papilla 3. Resonant frequency vary along the length od basilar papilla. 4. Voltage-dependent activation of membrane K+ conductance. BK calcium-activated potassium channel tune hair cells 1. Interaction between voltage-gated Ca 2+ current and large conductance Ca 2+ -activated K+ channel (BK channel)----oscillatory response. a. Depolarization---open voltage-gated Ca 2+ channel--- Ca 2+ ↑--- activate BK channel b. Large outward K+ ---hyperpolarize---closing Ca channel ----1st cycle of ossilation c. BK partially closed---continuous extrinsic current---another cycle 2. BK channel---negative feedback (K+: -80 mV, resting potential: -50 mV) 3. Resonant frequency: influence by size and speed of the feedback. 4. Intrinsic kinetic differences in BK channel Open & relaxation time constant: 30 fold Accont for 40-600 HZ 5. Below 40 Hz: other voltaged-gated K+ channels Voltage-gated Ca 2+ channel 1. rapid gating kinetics 2.more negative activation range 3.α1D channel in chick 4. BK channel relatively high calcium to open (K1/2 =12μM at –50 mV), located in proximity. 6. Number: Ca 2+ channel: BK channel=2:1 BK channel---alternatively spliced slo gene products 1.slo gene: originally found in Drosophila----mammals 2.8 splice site, 15 alternative exons 3.In situ hybridization X3=4 in middle third of chick’s basilar papilla, X2=31 excluded from low frequency quarter………. 4.Addition of β subunit: increased calcium affinity, slowed channel deactivation rates---lying in apical portion(low frequency)---gradient of distribution 5.Chimeric hair cell---contain αβ &α, or multipleαsplice variant ? Tuning with other K+ channels 1.Turtle, frog, alligator, chick: Hair cell < 100 Hz: slowly activated Ca- dependent retifier K+ current ; inward retifier K+ current---membrane conductance----tuning to low frequency 2.Voltage-gated K+ channel Determinants of ion channel expression 1. Synaptic organization---cluster at transmiter release sites 2. Highest frequency: larger CA current, greatest number of release site 3. Gradient of ion channel along and across sensory epithelium. 4. Efferent innervation---rapidly inacvtivating A- type K channel; afferent: delayed rectifier and inward rectifier 5. Potential regulatory mechanism: later expression of BK channel Mechanical tuning Passive resonance of hair cell bundle 1. Vibration of basilar membrane—to and fro motion of hair bundle---tip link---mechanotransducer channel 2. Resonant frequency: function of stiffness & loading mass 3. Stiffness: number of stereocilia, links, maximal height 4. FB=kKB0.5L-1.5 5. Contribute little to frequency selectivity? (11 fold change only0---lizard 6. Reduce bundle height & increse stereocilia number---enhance sensitivity—10 fold Transducer current and adaptation 1.adaptation time constant 0.3 ~>30 ms 2.Turtle, from low-frequency end to high frequency---5 fold increase transducer current & 10 fold decrease in adaptation time constant 3.Contribute to hair cell frequency selectivity---high pass filter. 4.Mechanism: incompletely understood a. Changes in stereocilia Ca concentration b. Reset the range of displacements detected by the channel c. Myosin motor: Ca inhibit actomyosin interaction---channel slip down the side wall of stereocilia d. More direct action of Ca on transducer channel---rapid e. Negative feedback control of transducer channel---sharply tuned transducer current. Active hair bundle movements 1.Unclear mechanism 2.Ca-dependent process a. Slower component: myosin-actin-cytoskeloton---ferrying channel & adjust tension of tip-links b. Faster: modulation of transducer channel 3.Role: undefined, higherfrequency ? 4.Avian: (short) outer hair cell---supply tuned energy into vibration of the cochlear partition Electromachanical behavior of mammalian outer hair cells 1. Extended auditory frequency range. a. Modification of middle ear structure b. Increase length of basilar membrane---traveling wave, sharp mechanical tuning c. Specialization of cell types 2. Active mechanical feedback of outer hair cell---generat force to cancel viscous damping of the cochlear partition. 3. Contracting longitudinally like aminiature muscle fiber a.Fast, voltage-controlled b. Motor protein in lateral wall c.Deform organ of Corti d. Out frequency: functions of kinetics of transducer channel & speed of the motor e.Limited by membrane time constant τ=CR 1) High threshold channel activated at > -35 mV---apical 2) Low threshold channel activated at –90~-50 mV---basal 3) Ca 2+ -activated K+ channel 4. Cochlear gradient in outer cell membrane capacitance & K channel 5. From low—high freq: 5 fold reduction in capacitance, 7 fold decrease in resistance----faster time constant in high frequency outer hair cells. ---Minimize high frequency attenuation of receptor potentials---drive motor cycle by cycle 6. Contractile behavior---cochlear amplifier 7. Active bundle feedback---? Contribute to mechanical feedback esp. in high frequency Evolution 1. Turtle: electrical tuning 2. Recent amphibians and reptiles: increased high frequency range---unique organ, passive mechanicaltuning 3. Bird: electrical tuning, elevated temperature, inner and outer hair cells---mechanical tuning 4.Mammal: somatic contractions of outer hair cell, electrical tuning over any frequency? Referrences 1. Fettiplace R, Fuchs PA: Mechanisms of hair cell tuning. Annu. Rev. Physiol. 1999;61:809-834 2. Ramanathan K, Michael TH, Fuchs PA: βsubunit modulate alternatively spliced, large conductance, calcium –activated potassium channels of avian hair cells. J Neurosc 2000, 20:1675-1684 3. The calcium-activated potassium channels of turtle hair cells. J Gen Physiol 1995; 105:49-72 4. Ramanathan K et al: A molecular mechanism for electrical tuning of cochlear hair cells. Science 1999;283:215-217 5. Engel JE, Wu CF: Genetic dissection of functional contributions od specific potassium channel subunits in habituation of an escape circuit in Drosophila. J Neurosc 1998; 18:2254-2267. 6. Ruggero MA, Rich NC: Furosemide alters organ of Corti mechanics: evidence for feedback of outer hair cells upon the basilar membrane. J Neurosc 1991;11:1057-1067 7. Art JJ, Fettiplace R: Variation of membrane properties in hair cells isolated from the turtle cochlea J Physiol 1987;385:207-242.