Cochlea and Hearing Loss
❷ Biological Battery in the Cochlea
【Outline of the Research Project】
Figure 1 show a cross section of the cochlea, which is made up of three tubular structures. Upper and lower tubules contain ‘perilymph’, which resembles a regular extracellular solution or blood plasma. Center tubule is filled with an unusual extracellular solution, ‘endolymph’, which shows 150 mM [K+] and 2 – 5 mM [Na+]. This profile looks like the properties in an intracellular solution. In addition, the endolymph always exhibits a highly positive potential of +80 – +100 mV with reference to the perilymph. These unique electrochemical properties are essential for high sensitivity observed in mammalian hearing. In particular, the high potential is a key; its disruption results in deafness.
The milieu of the endolymph has been considered to be maintained by K+-circulation or K+-recycling between this fluid and the perilymph. This unidirectional ion transport is likely to be driven by ‘stria vascularis’, an epithelial-like tissue composed of inner and outer layers. However, it has remained uncertain how the K+-circulation actually contributes to the endolymph in vivo.
We have focused on the high potential of the endolymph and addressed the mechanism underlying this electrical element on the basis of the results described by our and other groups, as follows. Firstly, the K+-circulation depends on functional coupling of ion channels and transporters in the stria vascularis (Fig. 2). Secondly, the origin of the endolymphatic potential is strial two batteries, which are connected in series are controlled by the K+-circulation. Thirdly, these two issues are integrated and linked with hair-cell’s activity in vivo, resulting in maintenance of the endolymphatic potential [refs: 5-12]. In this context, acoustic stimuli, which excite hair cells, induce the K+-circulation and thereby affected the batteries [ref: 1]. Moreover, interference with a crucial factor involved in the K+-circulation by optogenetic approach caused repetitive acute hearing loss observed in clinical sites [ref: 4].
【Experimental Procedures】
Go to the page entitled ‘Analytical instruments’ to see the details.
(1) Molecular biological and histological methods [refs: 4, 5, 13]
These methods are used to identify the molecular basis and tissue and cellular localization of the ion channels and transporters involved in driving the K+-circulation.
(2) Comprehensive analysis of the proteins [refs: 2, 14]
With comprehensive approaches using HPLC and LC-MS/MS, we are working on clarification of the profile of the proteins that contribute to the K+-circulation. This project is carried out by collaborating with Profs. Yoshikatsu Kanai and Shuji Nagamori.
(3) Ion selective microelectrodes [refs: 5-11]
The K+-circulation maintains the high potential in the endolymph by regulating [K+] in the lateral wall. Therefore, it is crucial for the project to examine the dynamics of K+ profile. We fabricate ion selective microelectrode that monitor the potential and [K+] in the microenvironment ‘simultaneously and in real time’ and use this sensor for in vivo measurements.
(4) Computer simulation [refs: 1, 5, 10]
Some of vital phenomena cannot be measured by any experimental techniques. Indeed, in this project, we are unable to directly detect ionic flows through the channels or transporters in vivo. This issue is accessible by means of theoretical approaches. We have constructed a number of equations that represent the ion channels and transporters in the stria vascularis and integrated these ionic flows to those in sensory hair cells in silico. This ‘Nin-Hibino-Kurachi (NHK) model’ can reconstitute the K+-circulation as well as the electrochemical properties of the stria and endolymph in different conditions [ref: 10]. This model has recently renewed with the experimental observations (fi-NHK model) (for source code, see the linkpage) [5]. With this updated version, we showed the response of the stria vascularis when the hair cells are stimulated by sounds. We will further improve the model via feedback from the experimental results and use for a variety of projects in the future.
Publication List
(1) Zhang Q*, Ota T*, Yoshida T, Ino D, Sato MP, Doi K, Horii A, Nin F#, Hibino H# (2021). Electrochemical properties of the non-excitable tissue stria vascularis of the mammalian cochlea are sensitive to sounds. The Journal of Physiology 599(19):4497-4516. [*: equal contributors]
(2) Nonomura Y, Sawamura S, Hanzawa K, Nishikaze T, Sekiya S, Higuchi T, Nin F, Uetsuka S, Inohara H, Okuda S, Miyoshi E, Horii A, Takahashi S, Natsuka S, Hibino H. Characterisation of N-glycans in the epithelial-like tissue of the rat cochlea. Scientific Reports, (2019), 9, Article number:1551.
(3) Watabe T, Xu M, Watanabe M, Nabekura J, Higuchi T, Hori K, Sato MP, Nin F, Hibino H, Ogawa K, Masuda M, Tanaka KF. Time-controllable Nkcc1 knockdown replicates reversible hearing loss in postnatal mice. Scientific Reports, (2017), 7, Article number:13605.
(4) Sato MP, Higuchi T, Nin F, Ogata G, Sawamura S, Yoshida T, Ota T, Hori K, Komune S, Uetsuka S, Choi S, Masuda M, Watabe T, Kanzaki S, Ogawa K, Inohara H, Sakamoto S, Takebayashi H, Doi K, Tanaka KF, Hibino H. Hearing loss controlled by optogenetic stimulation of nonexcitable nonglial cells in the cochlea of the inner ear. Frontiers in Molecular Neuroscience, (2017), 10(300): 1-16.
(5) Nin F, Yoshida T, Murakami S, Ogata G, Uetsuka S, Choi S, Doi K, Sawamura S, Inohara H, Komune S, Kurachi Y, Hibino H. Computer modeling defines the system driving a constant current crucial for homeostasis in the mammalian cochlea by integrating unique ion transports. npj Systems Biology and Applications, (2017), 3, Article number:24.
(6) Yoshida T, Nin F, Murakami S, Ogata G, Uetsuka S, Choi S, Nakagawa T, Inohara H, Komune S, Kurachi Y, Hibino H. The unique ion permeability profile of cochlear fibrocytes and its contribution to establishing their positive resting membrane potential. Pflügers Archiv - European Journal of Physiology, (2016), 468(9): 1609-1619.
(7) Nin F, Yoshida T, Sawamura S, Ogata G, Ota T, Higuchi T, Murakami S, Doi K, Kurachi Y, Hibino H. The unique electrical properties in an extracellular fluid of the mammalian cochlea; their functional roles, homeostatic processes, and pathological significance. Pflügers Archiv - European Journal of Physiology, (2016), 468(10): 1637-1649.
(8) Yoshida T, Nin F, Ogata G, Uetsuka S, Kitahara T, Inohara H, Akazawa K, Komune S, Kurachi Y, Hibino H. NKCCs in the fibrocytes of the spiral ligament are silent on the unidirectional K+-transport that controls the electrochemical properties in the mammalian cochlea. Pflügers Archiv - European Journal of Physiology, (2015), 467(7): 1577-1589.
(9) Adachi N, Yoshida T, Nin F, Ogata G, Yamaguchi S, Suzuki T, Komune S, Hisa Y, Hibino H#, Kurachi Y#. The mechanism underlying maintenance of the endocochlear potential by the K+-transport system in the fibrocytes of the inner ear. Journal of Physiology (London), (2013), 591(18): 4459-4472. [#: equal corresponding authors]
(10) Nin F, Hibino H#, Murakami S, Suzuki T, Hisa Y, Kurachi Y#. A computational model of a circulation current that controls electrochemical properties in the mammalian cochlea. Proceedings of National Academy Sciences of the United States of America, (2012), 109(23): 9191-9196. [#: equal corresponding authors]
(11) Nin F*, Hibino H*, Doi K, Suzuki T, Hisa Y, Kurachi Y. The endocochlear potential depends on two K+ diffusion potentials and an electrical barrier in the stria vascularis of the inner ear. Proceedings of National Academy Sciences of the United States of America, (2008), 105(5):1751-1756. [*: equal contributors]
(12) Hibino H, Kurachi Y. Molecular and physiological bases of the K+ circulation in the mammalian inner ear. Physiology (Bethesda), (2006), 21:336-345.
(13) Hibino H, Horio Y, Inanobe A, Doi K, Ito M, Yamada M, Gotow T, Uchiyama Y, Kawamura M, Kubo T, Kurachi Y. An ATP-dependent inwardly rectifying potassium channel, KAB-2 (Kir4.1), in cochlear stria vascularis of inner ear: its specific subcellular localization and correlation with the formation of endocochlear potential. Journal of Neuroscience, (1997), 17(12):4711-4721.
(14) Uetsuka S*, Ogata G*, Nagamori S*, Isozumi N, Nin F, Yoshida T, Komune S, Kitahara T, Kikkawa Y, Inohara H, Kanai Y, Hibino H. Molecular architecture of the stria vascularis membrane transport system, which is essential for physiological function of the mammalian cochlea. European Journal of Neuroscience, (2015), 42: 1984-2002. [*: equal contributors]