Cloning and expression of three K+ channel cDNAs from Xenopus muscle

Dorsal-ventral gradient for neuronal plasticity in the embryonic spinal cord

Within the developing Xenopus spinal cord, voltage-gated potassium (Kv) channel genes display different expression patterns, many of which occur in opposing dorsal-ventral gradients. Regional differences in Kv gene expression would predict different patterns of potassium current (I(Kv)) regulation. However, during the first 24 h of postmitotic differentiation, all primary spinal neurons undergo a temporally coordinated upregulation of I(Kv) density that shortens the duration of the action potential. Here, we tested whether spinal neurons demonstrate regional differences in I(Kv) regulation subsequent to action potential maturation. We show that two types of neurons, I and II, can be identified in culture on the basis of biophysical and pharmacological properties of I(Kv) and different firing patterns. Chronic increases in extracellular potassium, a signature of high neuronal activity, do not alter excitability properties of either neuron type. However, elevating extracellular potassium acutely after the period of action potential maturation leads to different changes in membrane properties of the two types of neurons. I(Kv) of type I neurons gains sensitivity to the blocker XE991, whereas type II neurons increase I(Kv) density and fire fewer action potentials. Moreover, by recording from neurons in vivo, we found that primary spinal neurons can be identified as either type I or type II. Type I neurons predominate in dorsal regions, whereas type II neurons localize to ventral regions. The findings reveal a dorsal-ventral gradient for I(Kv) regulation and a novel form of neuronal plasticity in spinal cord neurons.

Properties of Xenopus Kv1.10 channels expressed in HEK293 cells

Voltage-gated K+ channels play important roles in shaping the characteristics of action potentials and electrical activity. In a previous study, we isolated cDNAs encoding several distinct K+ channel isoforms, including a novel isoform (XKv1.10) expressed in Xenopus laevis spinal cord neurons and myocytes. Here, we report the biophysical characterization of XKv1.10 expressed in transiently transfected HEK293 cells. Whole cell patch clamp recordings revealed a voltage-gated, rapidly activating and inactivating K+ current. Interestingly, the rate of inactivation of XKv1.10 channels showed apparent voltage dependence, with time constants between 77.7-213.3 ms. The predicted protein sequence of XKv1.10 does not appear to encode an N-terminal inactivating "ball and chain" domain, and instead these channels may inactivate via a C/P-type mechanism. Consistent with this, either increasing the external concentration of K+ or external application of tetraethylammonium caused a decrease in the rate of inactivation. Pharmacologically, XKv1.10 K+ channels were sensitive to 4-aminopyridine and tetraethylammonium with apparent IC50 values of 68.5 microM and 17.1 mM, respectively. When simulated action potentials were used as a voltage command, XKv1.10 was similar to XKv1.4 in that it carried more repolarizing current during the action potential than XKv1.2. However, while XKv1.4 was active during the interspike interval, XKv1.10 and XKv1.2 were not. Overall, the data suggest that XKv1.10 channels make a unique contribution to the developmental maturation of electrical signaling in Xenopus laevis.

Expression of voltage-dependent potassium channels in the developing visual system of Xenopus laevis

Accumulating evidence suggests that voltage-dependent potassium (Kv) channels have important and varied roles in the development of neuronal and non-neuronal cell types. They have been implicated in processes such as proliferation, cell adhesion, migration, neurite outgrowth, and axon guidance. In this study, we used antibodies against several electrically active Kv channel alpha-subunits (Kv1-4) to describe the spatial and temporal expression patterns of Kv channel subunits in Xenopus laevis retinal ganglion cell (RGC) somata, axons, and growth cones. We found that RGCs express Kv1.3-, Kv1.5-, Kv3.4-, and Kv4.2-like subunits. Each subunit displayed unique cellular and subcellular distributions. Moreover, the expression patterns changed considerably over the major period of Xenopus retinal cell genesis and differentiation. Weak or no immunoreactivity was observed with antibodies against Kv1.1, Kv1.2, Kv1.4, Kv1.6, and Kv3.2 subunits in RGCs or other retinal cell types. In support of our previous pharmacologic evidence implicating Kv channels in RGC axon outgrowth, we found that Kv1.5-, Kv3.4-, and Kv4.2-like proteins, but not Kv1.3-like subunits, are abundantly expressed in RGC growth cones.

In vivo analysis of Kvbeta2 function in Xenopus embryonic myocytes

Kv1 potassium channels consist of pore-forming alpha subunits as well as auxiliary beta subunits. In heterologous systems, Kv1alpha subunits suffice for induction of voltage-dependent potassium current (I(Kv)). Although Kv1 channels can be expressed without auxiliary subunits in heterologous systems, coexpression with Kvbeta subunits has dramatic effects on surface expression and kinetic properties. Much less is known about the functional roles of Kvbeta subunits in vivo, despite their presence in the majority of native Kv1 channel complexes. We used an antisense approach to probe the contribution of Kvbeta2 subunits to native Kv1 channel function in embryonic myocytes. We compared the effects of antisense Kvbeta2 treatment on the whole cell I(Kv) to those produced by overexpression of a dominant-negative Kv1alpha subunit. The reductions in the maximal potassium conductance produced by antisense Kvbeta2 treatment and elimination of Kv1alpha subunit function were not significantly different from each other. In addition, simultaneous elimination of Kv1alpha and Kvbeta2 subunit function resulted in no further reduction of the maximal conductance. The Kv channel complexes targeted by Kvbeta2 and/or Kv1alpha subunit elimination contributed to action potential repolarization because elimination of either or both subunits led to increases in the duration of the action potential. As for potassium conductance, the effects of elimination of both alpha and beta subunits on the duration of the action potential were not additive. Taken together, the results suggest that Kv1 potassium channel complexes in vivo have a strong requirement for both alpha and beta subunits.

A Shaker homologue encodes an A-type current in Xenopus laevis

In Xenopus laevis, several distinct K(+)-channels (xKv1.1, xKv1.2, xKv2,1, xKv2.2, xKv3.1) have been cloned, sequenced, and electrophysiologically characterized. K(+)-channels significantly shape neuronal excitability by setting the membrane potential, and latency and duration of action potentials. We identified a further Shaker homologue, xKv1.4, in X. laevis. The open reading frame encodes a K(+)-channel that shares 72% of its 698 amino acids with the human Shaker homologue, hKv1.4. Northern blot analysis revealed xKv1.4 in the brain, muscle, and spleen but not in the ovary, intestine, heart, liver, kidney, lung, and skin. Whole-cell patch clamp recording from rat basophilic leukaemia (RBL) cells transfected with xKv1.4 revealed a voltage-gated, outward rectifying, transient A-type, K(+) selective current. xKv1.4 was strongly dependent on extracellular K(+). Exposure of cells to K(+) free bath solution almost completely abolished the current, whereas in the presence of high K(+), inactivation in response to a maintained depolarizing step and the frequency-dependent cumulative inactivation decreased. Ion channels encoded by xKv1.4 are sensitive to 4-aminopyridine and quinidine but insensitive to tetraethylammonium and the peptide toxins, charybdotoxin, margatoxin, and dendrotoxin. In conclusion, our results indicate that the biophysical and pharmacological signature of xKv1.4 closely resemble those of the A-current described in Xenopus embryonic neurons and is similar to the human Shaker homologue, hKv1.4.