The Kv2.2 voltage-gated potassium channel: From the gene to its potential physiological role in arousal

Abstract

Voltage-gated potassium (Kv) channels are involved in various physiological processes such as repolarization of neuronal and cardiac action potentials, calcium signaling, cellular proliferation, migration and behavioral rhythms. These channels exhibit distinct biophysical and biochemical characteristics primarily through the formation of heteromeric tetramers into a functional channel. Mammalian Kv2 delayed rectifier channels are, however, the unique exception in this subfamily because previous studies have shown these isotypes are localized in distinct domains of neuronal membranes and lack the capability to form heteromeric channels. In this dissertation, we report a novel form of rat Kv2.2, which has not been previously recognized. Our data indicate that this novel form of Kv2.2 is indeed the predominant form expressed in the brain and is co-localized in the same neuronal membrane domains with Kv2.1. In addition, co-immunoprecipitation and electrophysiology experiments showed that Kv2.1 and Kv2.2 are capable of forming heteromeric channels. However, through specific immunostaining, we found that Kv2.2 is expressed in a specific set of neurons, which have negligible levels of Kv2.1, suggesting a unique physiological role of Kv2.2. These neurons are located in the magnocellular peroptic area (MCPO) and the horizontal limb of diagonal band of Broca (HDB) of the basal forebrain (BF). It has been shown that the MCPO/HDB are implicated in the regulation of cortical activity and the sleep-wake cycle. Using specific immunolabeling and knock-in mice in which GFP is expressed in GABAergic neurons, we found that Kv2.2 is abundantly expressed in a sub-population of GABAergic neurons in this region which establishes Kv2.2 as a molecular target to study the role of this specific sub-population of BF GABAergic neurons. In conclusion we hypothesize, that the Kv2.2-GABAergic neurons has a functional role in the regulation of the sleep-wake cycle. Using c-FOS immunolabeling and polysomnographic recordings of Kv2.2 knockout mice, we found that Kv2.2-GABAergic neurons are preferentially active during the wake state and that the knockout mice exhibit an increase wakefulness phenotype, respectively. Taken all together, these results challenges the present dogma of Kv2 channels as well as reveal a significant aspect of BF GABAergic neurons in the promotion of wakefulness.