Functional consequences of Kir2.1/Kir2.2 subunit heteromerization

Kir2 subunits form channels that underlie classical strongly inwardly rectifying potassium currents. While homomeric Kir2 channels display a number of distinct and physiologically important properties, the functional properties of heteromeric Kir2 assemblies, as well as the stoichiometries and the arrangements of Kir2 subunits in native channels, remain largely unknown. Therefore, we have implemented a concatemeric approach, whereby all four cloned Kir2 subunits were linked in tandem, in order to study the effects of Kir2.1 and Kir2.2 heteromerization on properties of the resulting channels. Kir2.2 subunits contributed stronger to single-channel conductance than Kir2.1 subunits, and channels containing two or more Kir2.2 subunits displayed conductances indistinguishable from that of a Kir2.2 homomeric channel. In contrast, single-channel kinetics was a more discriminating property. The open times were significantly shorter in Kir2.2 channels compared with Kir2.1 channels and decreased nearly proportionally to the number of Kir2.2 subunits in the heteromeric channel. Similarly, the sensitivity to block by barium also depended on the proportions of Kir2.1 to Kir2.2 subunits. Overall, the results showed that Kir2.1 and Kir2.2 subunits exert neither a dominant nor an anomalous effect on any of the properties of heteromeric channels. The data highlight opportunities and challenges of using differential properties of Kir2 channels in deciphering the subunit composition of native inwardly rectifying potassium currents

Inwardly rectifying Kir2 potassium channels: Differential properties and contributions to channels underlying cardiac IK1.

The cardiac inwardly rectifying potassium current IK1 is responsible for stabilizing the resting membrane potential and shaping the late phase of repolarization of the action potential. IK1 channels are composed of subunits belonging to the Kir2 subfamily of inward rectifier potassium channels. Genetic and pharmacological manipulation of Kir2 channels in experimental animals and identification of mutations in the human Kir2.1 gene have highlighted the role of IK1 in the genesis of cardiac arrhythmias. Nevertheless, significant gaps remain in understanding of the essential properties of Kir2 channels and contributions of specific Kir2 subunits to IK1. The results showed that different homomeric Kir2 channels exhibit distinct sensitivities to block by the polyamine spermine (a major intracellular factor underlying inward rectification in Kir2 channels) with Kir2 2 and Kir2.3 channels being the most and least sensitive, respectively. The rates of spermine unblock (channel activation) were significantly slower in Kir2.3 channels compared with that in Kir2.1 and Kir2.2 channels. Experiments using a concatemeric approach showed that the activation of heteromeric Kir2.1/Kir2.3 channels was significantly slower than that of homomeric Kir2.1 channels. In mouse, the kinetics of activation of IK1 was fast in both ventricular and atrial myocytes, suggesting no significant contribution of the Kir2.3 subunit. The strength of rectification in atrial myocytes was weaker than that in ventricular myocytes, and it also varied greatly between individual myocytes isolated from both tissues. The data support the hypothesis that variation in concentration of free intracellular polyamines contributes to regional and cellular heterogeneity of IK1. The contribution of Kir2.1 and Kir2.2 subunits to the function of heteromeric channels was studied using a concatemeric approach. Measurements of single-channel conductances, mean open times and Ba2+ sensitivities showed that Kir2.1 or Kir2.2 subunits exert neither a 'dominant' nor an 'anomalous' effect on any of the properties of heteromeric channels and provided a set of unique parameters useful for deciphering the subunit composition of IK1. The results have provided insights into the function of Kir2 channels and the contributions of distinct Kir2 subunits to IK1, and will be important for the development of future treatments for arrhythmias.Ph.D.Biological SciencesBiophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127028/2/3305056.pd