Cloning and expression of Shaker alpha- and beta-subunits during inner ear development

The activity of spontaneous action potentials in developing hair cells is regulated by Ca(2+)-dependence of a transient K+ current

Spontaneous action potentials have been described in developing sensory systems. These rhythmic activities may have instructional roles for the functional development of synaptic connections. The importance of spontaneous action potentials in the developing auditory system is underpinned by the stark correlation between the time of auditory system functional maturity, and the cessation of spontaneous action potentials. A prominent K(+) current that regulates patterning of action potentials is I(A). This current undergoes marked changes in expression during chicken hair cell development. Although the properties of I(A) are not normally classified as Ca(2+)-dependent, we demonstrate that throughout the development of chicken hair cells, I(A) is greatly reduced by acute alterations of intracellular Ca(2+). As determinants of spike timing and firing frequency, intracellular Ca(2+) buffers shift the activation and inactivation properties of the current to more positive potentials. Our findings provide evidence to demonstrate that the kinetics and functional expression of I(A) are tightly regulated by intracellular Ca(2+). Such feedback mechanism between the functional expression of I(A) and intracellular Ca(2+) may shape the activity of spontaneous action potentials, thus potentially sculpting synaptic connections in an activity-dependent manner in the developing cochlea.

Kvbeta1.1 associates with Kvalpha1.4 in Chinese hamster ovary cells and pigeon type II vestibular hair cells and enhances the amplitude, inactivation and negatively shifts the steady-state inactivation range

Although A-type potassium currents are found in type II hair cells in the inner ear of most species, the molecular mechanisms for activation and inactivation of the A-type potassium current (I(A)) remain unknown. In frog semicircular canal hair cells, for example, there appear to be two classes of currents having either fast or slow inactivation [Norris CH, Ricci AJ, Housley GD, Guth PS (1992) The inactivating potassium currents of hair cells isolated from the crista ampullaris of the frog. J Neurophysiol 68:1642-1653; Russo G, Calzi D, Martini M, Rossi ML, Fesce R, Prigioni I (2007) Potassium currents in the hair cells of vestibular epithelium: position-dependent expression of two types of A channels. Eur J Neurosci 25:695-704]. It has been suggested that somehow the "ball and chain" mechanism (NH(3) (N) terminus motif) is modified by alternative splicing to account for the two classes of inactivation. To examine other possibilities, we cloned alpha and beta subunits that comprise the A-type potassium channel complex in adult and embryonic pigeon brain, cochlea and labyrinth. By sequence homology, we concluded that the subunits present were Kvalpha1.4 and Kvbeta1.1. The sequence of the open reading frame for Kvalpha1.4 contained the N-terminus, pore and COOH (C) terminus motifs for N-and C-type inactivation. The sequence for Kvbeta1.1 displayed amino acids consistent with assembly and association with Kvalpha1.4 alpha subunits. Kvalpha1.4 and Kvbeta1.1 were transfected either singly or in combination into Chinese hamster ovary (CHO) cells. These cells and native hair cells from the pigeon utricle were patch clamped and the inactivation properties of the A-type current were studied. In the native hair cells, the A-type current was identified by its pharmacological (4-aminopyridine (4-AP); IC(50)=11 microM) and voltage dependent inactivation properties. A comparison of the mean time constants from best-fitted single exponential and sum of two exponential equations to the ionic current inactivation revealed the following. In CHO cells when Kvalpha1.4 was expressed alone, the mean time constant (tau(1)=107 ms+/-19, N=32) was significantly (P<0.001) longer and the mean peak amplitude (2.28 nA+/-0.39, N=32) was smaller than when Kvalpha1.4 and Kvbeta1.1 were expressed in CHO cells. Moreover, the co-transfection of Kvalpha1.4 and Kvbeta1.1 into CHO cells caused a shift in the steady state inactivation curve parameter Vo 30 mV in the hyperpolarized direction relative to CHO cells expressing only Kvalpha1.4. Similarly, Kvalpha1.4-transfected CHO cells produced longer time constants and smaller amplitudes than those found for native utricular hair cells. These data lead us to conclude that while the amino acid motifs are present in Kvalpha1.4 and Kvbeta1.1 to suggest N-and C-type inactivation, co-assembly and association of Kvalpha1.4 and Kvbeta1.1 may also produce changes in the time dependent inactivation properties of vestibular hair cells.

Protein-protein interactions of a Kv? subunit in the cochlea

Accessory subunits associated with voltage-gated potassium (Kv) channels can influence the biophysical properties and promote the surface expression of channel-forming alpha-subunits. Previously, we cloned several alpha-subunits and a beta-subunit from a cDNA library of the chicken cochlea. In the present study, we raised an antibody against the N-terminus of chicken Kvbeta1.1 (cKvbeta1.1) and characterized the Kvbeta-related polypeptide in cochlear tissues and heterologous cells. The anti-cKvbeta1.1 antibody recognizes a 45-kDa polypeptide in chick cochlear extracts as well as in Chinese hamster ovary (CHO) cells transfected with cKvbeta1.1. The accessory subunit was localized to the ganglion cells of the chick cochlea using immunohistochemistry and in situ hybridization. Coimmunoprecipitation studies show that Kvbeta1.1 interacts with Shaker channel members Kvalpha1.2 and 1.3, both of which colocalize with beta to the cochlear ganglion cells. Additionally, coimmunoprecipitation studies show that Kvalpha1.2 and 1.3 interact with each other, suggesting that these ion channels are formed by heteromultimers. In comparison, Kvbeta did not coprecipitate with a member of the Shal subfamily. The presence of Kvbeta in the cochlea suggests that this subunit contributes to the modulation of auditory signals in the ganglion cells, presumably by regulating properties of inactivation as well as surface expression of Kvalpha channels.

Cloning and developmental expression of Shaker potassium channels in the cochlea of the chicken

Signal coding by the receptor and neuronal cells of the auditory system involves various ion channels that modulate a sound stimulus. The genes that encode a number of these ion channels and their accessory subunits are presently unknown for channels found in the sensory epithelium and cochlear nerve. Among these genes are those that encode delayed rectifier and transient type potassium channels found in both the sensory cells and the ganglion. Here, we report the cloning and developmental expression of Shaker family members that include cKv1.2, cKv1.3, cKv1.5, and the Shaker-related cGMP-gated potassium channel cKCNA10. Clones were obtained by screening a chicken embryonic cochlea cDNA library using, as a probe, a mixture of two DNA fragments of cKv1.2 and cKv1.3 obtained by the reverse transcription polymerase chain reaction (RT-PCR). Sequence analysis revealed chicken homologues of Kv1.2, Kv1.3, Kv1.5 and cGMP-gated potassium channels with a deduced amino acid homology of 96-98%, 82-84%, 67-71% and 67-79% to correspondent mammalian homologues. During development of chicken inner ear, RT-PCR studies show expression of cKv1.2, cKv1.3 and cKv1.5 as early as Embryonic Day (ED) 3, while cKCNA10 was detected at low levels beginning on ED6 and was highly expressed by ED9. Additionally, analysis of expression in different parts of the cochlea showed that these genes were co-expressed in different regions of the cochlea, including the cochlear ganglion, sensory epithelium, lagena, and tegmentum. This expression pattern suggests the potential for the formation of heteromeric channels from the corresponding alpha-subunits in these various tissues.

Xenopus embryonic spinal neurons express potassium channel Kvbeta subunits

Developmental regulation of voltage-dependent delayed rectifier potassium current (I(Kv)) of Xenopus primary spinal neurons regulates the waveform of the action potential. I(Kv) undergoes a tripling in density and acceleration of it activation kinetics during the initial day of its appearance. Another voltage-dependent potassium current, the A current, is acquired during the subsequent day and contributes to further shortening of the impulse duration. To decipher the molecular mechanisms underlying this functional differentiation, we are identifying potassium channel genes expressed in the embryonic amphibian nervous system. Potassium channels consist of pore-forming (alpha) as well as auxiliary (beta) subunits. Here, we report the primary sequence, developmental localization, and functional properties of two Xenopus Kvbeta genes. On the basis of primary sequence, one of these (xKvbeta2) is highly conserved with Kvbeta2 genes identified in other species, whereas the other (xKvbeta4) appears to identify a new member of the Kvbeta family. Both are expressed in developing spinal neurons during the period of impulse maturation but in different neuronal populations. In a heterologous system, coexpression of xKvbeta subunits modulates properties of potassium current that are developmentally regulated in the endogenous I(Kv). Consistent with xKvbeta4's unique primary sequence, the repertoire of functional effects it has on coexpressed Kv1alpha subunits is novel. Taken together, the results implicate auxiliary subunits in regulation of potassium current function and action potential waveforms in subpopulations of embryonic primary spinal neurons.