XenopusEmbryonic Spinal Neurons Express Potassium Channel Kvβ Subunits

Abstract

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.

Dorsal–Ventral Gradient for Neuronal Plasticity in the Embryonic Spinal Cord

“…When expressed heterologously, Kv2.2 mRNA alone generates currents typical of type I neurons (Burger and Ribera, 1996), a result that does not support Kv2.2 as a molecular determinant of Kv current in type II neurons. Similar results have been obtained for the other Kv genes expressed in the embryonic Xenopus spinal cord (Ribera and Nguyen, 1993;Lazaroff et al, 1999;Gurantz et al, 2000;Fry et al, 2004 (Fantozzi et al, 2006). Changes in neuronal activity have been shown to induce homeostatic changes allowing neurons to respond appropriately (Turrigiano et al, 1994;Golowasch et al, 1999;Turrigiano, 1999;Davis and Bezprozvanny, 2001).…”

“…Kv genes expressed in the embryonic Xenopus spinal cord include Kv1.1, Kv1.2, Kv1.4, Kv1.10, Kv2.2, Kv3.1, Kv␤4, and Kv␤2 (Ribera and Nguyen, 1993;Burger and Ribera, 1996;Lazaroff et al, 1999;Gurantz et al, 2000;Fry et al, 2001). Interestingly, the expression patterns of the majority of these genes show regional differences along the dorsal-ventral axis.…”

Kv1 Potassium Channel Complexes In Vivo Require Kvβ2 Subunits in Dorsal Spinal Neurons

“…The known molecular determinants of potassium current in the dorsal spinal cord of Xenopus embryos consist of Kv1 ␣-subunits and Kv␤2 subunits (Lazaroff et al 1999;Ribera and Nguyen 1993). To investigate the functional role of Kv␤2 subunits in vivo, we developed a semiintact preparation that allowed us to identify two different types of dorsal spinal cord neurons.…”

“…Voltage-gated potassium currents (I Kv ) have been studied in vitro as well as in vivo in spinal neurons of the Xenopus embryo (Desarmenien et al 1993;O'Dowd et al 1988;Pineda and Ribera 2008). The Xenopus embryo expresses Kv␤2 mRNA in spinal cord neurons during the same developmental period during which extensive regulation of voltage-gated potassium current occurs (Lazaroff et al 1999). Further, the developmental changes in I Kv consist of increases in current density and acceleration of activation kinetics (Barish et al 1986;Lockery and Spitzer 1992;O'Dowd et al 1988), potassium current properties that are modulated by Kv␤2 subunits when coexpressed heterologously with Kv1 ␣-subunits.…”

“…Further, the developmental changes in I Kv consist of increases in current density and acceleration of activation kinetics (Barish et al 1986;Lockery and Spitzer 1992;O'Dowd et al 1988), potassium current properties that are modulated by Kv␤2 subunits when coexpressed heterologously with Kv1 ␣-subunits. Moreover, dorsal spinal neurons express Kv1.1 ␣-subunit and Kv␤2 transcripts, providing an experimentally accessible and relevant neuronal population for the study of the in vivo roles of Kv␤2 subunits (Lazaroff et al 1999;Ribera and Nguyen 1993).…”