Characterization of the A-Type potassium current in murine gastric fundus smooth muscles

Transient outward, or "A-type" currents are rapidly inactivating voltage gated potassium currents that operate at negative membrane potentials. A-type currents have not been reported in the gastric fundus, a tonic smooth muscle. We used whole-cell voltage-clamp to identify and characterize A-type currents in smooth muscle cells (SMCs) isolated from murine fundus. A-type currents were robust in these cells with peak amplitudes averaging 1.5nA at 0 mV. Inactivation was rapid with a time constant of 71ms at 0 mV; recovery from inactivation at -80 mV was similarly rapid with a time constant of 75ms. A-type currents in fundus were blocked by 4-aminopyridine (4-AP), flecainide and phrixotoxon-1 (PaTX1). Remaining currents after 4-AP and PaTX1 displayed half-activation potentials that were shifted to more positive potentials and showed incomplete inactivation. Currents after TEA displayed half inactivation at -48.1±1.0 mV. Conventional microelectrode and contractile experiments on intact fundus muscles showed that 4-AP depolarized membrane potential and increased tone under conditions in which enteric neurotransmission was blocked. These data suggest that A-type K+ channels in fundus SMCs are likely active at physiological membrane potentials, and sustained activation of A-type channels contributes to the negative membrane potentials of this tonic smooth muscle. Quantitative analysis of Kv4 expression showed that Kcnd3 was dominantly expressed in fundus SMCs. These data were confirmed by immunohistochemistry which revealed Kv4.3-like immunoreactivity within the tunica muscularis. These observations indicate that Kv4 channels likely form the A-type current in murine fundus SMCs.

Mutant huntingtin enhances activation of dendritic Kv4 K+ channels in striatal spiny projection neurons

Huntington’s disease (HD) is initially characterized by an inability to suppress unwanted movements, a deficit attributable to impaired synaptic activation of striatal indirect pathway spiny projection neurons (iSPNs). To better understand the mechanisms underlying this deficit, striatal neurons in ex vivo brain slices from mouse genetic models of HD were studied using electrophysiological, optical and biochemical approaches. Distal dendrites of iSPNs from symptomatic HD mice were hypoexcitable, a change that was attributable to increased association of dendritic Kv4 potassium channels with auxiliary KChIP subunits. This association was negatively modulated by TrkB receptor signaling. Dendritic excitability of HD iSPNs was rescued by knocking-down expression of Kv4 channels, by disrupting KChIP binding, by restoring TrkB receptor signaling or by lowering mutant-Htt (mHtt) levels with a zinc finger protein. Collectively, these studies demonstrate that mHtt induces reversible alterations in the dendritic excitability of iSPNs that could contribute to the motor symptoms of HD.

A quantitative hypermorphic CNGC allele confers ectopic calcium flux and impairs cellular development

The coordinated control of Ca2+ signaling is essential for development in eukaryotes. Cyclic nucleotide-gated channel (CNGC) family members mediate Ca2+ influx from cellular stores in plants (Charpentier et al., 2016; Gao et al., 2016; Frietsch et al., 2007; Urquhart et al., 2007). Here, we report the unusual genetic behavior of a quantitative gain-of-function CNGC mutation (brush) in Lotus japonicus resulting in a leaky tetrameric channel. brush resides in a cluster of redundant CNGCs encoding subunits which resemble metazoan voltage-gated potassium (Kv1-Kv4) channels in assembly and gating properties. The recessive mongenic brush mutation impaired root development and infection by nitrogen-fixing rhizobia. The brush allele exhibited quantitative behavior since overexpression of the cluster subunits was required to suppress the brush phenotype. The results reveal a mechanism by which quantitative competition between channel subunits for tetramer assembly can impact the phenotype of the mutation carrier.