INWARDLY RECTIFYING POTASSIUM CHANNEL KIR4.1 IS LOCALIZED AT THE CALYX ENDINGS OF VESTIBULAR AFFERENTS T. UDAGAWA, a,b N. TATSUMI, b T. TACHIBANA, b Y. NEGISHI, b H. SAIJO, b,c T. KOBAYASHI, a Y. YAGUCHI, a H. KOJIMA, a H. MORIYAMA a AND M. OKABE b * a Department of Otorhinolaryngology, The Jikei University School of Medicine, Tokyo 105-8461, Japan b Department of Anatomy, The Jikei University School of Medicine, Tokyo 105-8461, Japan c Division of Gastroenterology and Hepatology, Department of Internal Medicine, The Jikei University School of Medicine, Tokyo 105-8461, Japan Abstract—Inwardly rectifying potassium (Kir) channel Kir4.1 (also called Kcnj10) is expressed in various cells such as satellite glial cells. It is suggested that these cells would absorb excess accumulated K + from intercellular space which is surrounded by these cell membranes expressing Kir4.1. In the vestibular system, loss of Kir4.1 results in selec- tive degeneration of type I hair cells despite normal develop- ment of type II hair cells. The mechanisms underlying this developmental disorder have been unclear, because it was thought that Kir4.1 is only expressed in glial cells throughout the entire nervous system. Here, we show that Kir4.1 is expressed not only in glial cells but also in neurons of the mouse vestibular system. In the vestibular ganglion, Kir4.1 mRNA is transcribed in both satellite cells and neuronal somata, whereas Kir4.1 protein is expressed only in satellite cells. On the other hand, in the vestibular sensory epithelia, Kir4.1 protein is localized at the calyx endings of vestibular afferents, which surround type I hair cells. Kir4.1 protein expression in the vestibular sensory epithelia is detected beginning after birth, and its localization gradually adopts a calyceal shape until type I hair cells are mature. Kir4.1 local- ized at the calyx endings may play a role in the K + -buffering action of vestibular afferents surrounding type I hair cells. Ó 2012 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: inwardly rectifying potassium channel, Kir4.1, synapse, calyx ending, sensation of balance, inner ear. INTRODUCTION Vestibular afferents transmit essential information for the balance into the brain. In the vestibular ganglion, these neuronal somata are surrounded by satellite glial cells. In the vestibular sensory epithelia, vestibular afferents form synapses with vestibular hair cells. These hair cells differ in terms of their innervation. Type I hair cells are innervated by vestibular calyx afferents, the endings of which have a unique morphology with a calyceal shape, whereas type II hair cells are innervated by vestibular bouton afferents (Eatock et al., 1998, 2008). It is known that type I hair cells gradually mature with changes in K + conductances until 3 weeks after birth (Hurley et al., 2006). In the vestibular sensory epithelia, the flow of K + ions plays a major role in the transmission of information related to the sensation of the balance. Head movement and tilt causes vibration of endolymph, which contains high concentrations of K + . This mechanical stimulation opens mechanoelectrical channels on the apical mem- branes of hair cells and K + flows from the endolymph into hair cells. K + is released from type I hair cells into the cal- yceal synaptic cleft (Lim et al., 2011). It is considered that the accumulation of K + in the calyceal synaptic cleft plays a role in calyceal synaptic transmission, except for that mediated by glutamic acid, and can lead to depolarization of the calyx ending (Goldberg, 1996; Soto et al., 2002; Holt et al., 2007; Lim et al., 2011). Although it has not yet been revealed where this accumulated K + goes, it is assumed that K + channels expressed in the calyx end- ing allow passage of K + into peripheral vestibular affer- ents (Kharkovets et al., 2000). Inwardly rectifying potassium (Kir) channel Kir4.1 (also called Kcnj10) is expressed in various glial cells such as brain astrocytes, retinal Mu¨ller cells, cochlear Deiters’ cells and satellite glial cells (Hibino et al., 1997, 1999; Ishii et al., 1997; Higashi et al., 2001). It is sug- gested that these cells would absorb excess accumulated K + from intercellular space which is surrounded by these cell membranes expressing Kir4.1 (Hibino and Kurachi, 2006; Hibino et al., 2010). Mutations in the KIR4.1 cause epilepsy, ataxia, sensorineural deafness, and tubulopathy (EAST) syndrome (Bockenhauer et al., 2009). Although it is considered that ataxia such as gait disturbance of EAST syndrome is caused by developmental disorder of the central nervous system (Bockenhauer et al., 2009), Kir4.1 knockout mice also show hypertrophic degenera- tion of the peripheral vestibular calyx afferents (Rozengurt et al., 2003). It has been thought that degeneration of these neurons occurs secondary to a developmental dis- order of satellite glial cells, because Kir4.1 is only expressed in satellite cells in the vestibular system (Rozengurt et al., 2003). Kir4.1 knockout mice show 0306-4522/12 $36.00 Ó 2012 IBRO. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.neuroscience.2012.04.037 * Corresponding author. Address: Department of Anatomy, The Jikei University School of Medicine, 3-25-8 Nishi-Shimbashi, Minato-ku, Tokyo 105-8461, Japan. Tel: +81-3-3433-1111x220; fax: +81-3- 3433-2065. E-mail address: maokabe@jikei.ac.jp (M. Okabe). Abbreviations: EAST, epilepsy, ataxia, sensorineural deafness, and tubulopathy; Kir, inwardly rectifying potassium; P, postnatal day; PBS, phosphate buffer saline; PCR, polymerase chain reaction; RT, room temperature. Neuroscience 215 (2012) 209–216 209