Cod106, a novel synaptic protein expressed in sensory hair cells of the inner ear and in CNS neurons Ellen Reisinger, a Ulrike Zimmermann, b Marlies Knipper, b Jost Ludwig, c Nikolaj Klfcker, a Bernd Fakler, a and Dominik Oliver a, * a Department of Physiology II, University of Freiburg, 79104 Freiburg, Germany b Tu ¨bingen Hearing Research Centre, University of Tu ¨bingen, Molecular Neurobiology, 72076 Tu ¨ bingen, Germany c Department of Physiological Chemistry, University of Tu ¨bingen, 72076 Tu ¨ bingen, Germany Received 6 May 2004; revised 23 July 2004; accepted 30 August 2004 The exquisite performance of the highly specialized mammalian inner ear requires a multitude of specific proteins. Yet, only a subset of these proteins has been identified and studied in detail. Here, we describe a novel gene expressed in the organ of Corti that encodes a membrane- associated protein of 106 kDa. The new protein, termed Cod106, lacks sequence homology to characterized gene products. As shown by in situ hybridization, it is expressed in auditory and vestibular hair cells, as well as in distinct sets of CNS neurons with particularly high abundance in hippocampus and cerebellum. Immunohistochemistry detected Cod106 at the basal, synaptic pole of cochlear outer hair cells and vestibular hair cells. In cultured hippocampal neurons, Cod106 immunofluorescence co-localized with the postsynaptic density protein 95 (PSD95), indicating a postsynaptic localization. Cell-type specificity and subcellular localization may be consistent with an involvement of Cod106 in synaptic processes. D 2004 Elsevier Inc. All rights reserved. Introduction The mammalian hearing organ is exquisitely sensitive in terms of dynamic range and frequency selectivity (Dallos, 1992). Its impressive performance is realized by the coordinated function of a variety of specialized tissues that are organized into a highly ordered microarchitecture within a comparatively small space (Dallos, 1992; Hudspeth, 1997). These tissues serve different functions, including ion transport and homeostasis, microme- chanics, sensory transduction, and electrical signal propagation. On the cellular level, this specialization is reflected in the ordered array of a multitude of specialized cell types (Slepecky, 1996). Thus, the sensory epithelium, termed organ of Corti, includes the sensory hair cells that transduce soundborne vibrations into receptor potentials, different types of epithelial supporting cells that are involved in potassium homeostasis and provide a micromechanical scaffold, as well as afferent and efferent neuronal elements. In turn, the precise functioning of these cells (and thus of the hearing process) requires a multitude of specific proteins. This notion is reflected in the large number of gene loci that have been shown to underlie inherited forms of deafness, both in humans and mouse models (Petit et al., 2001; Steel and Kros, 2001). Positional cloning strategies that identified genes and proteins underlying hereditary forms of deafness have proved to be a very powerful tool for the identification of proteins that are essential for normal hearing (Steel and Kros, 2001). Some of these proteins are specific to the inner ear, with mutations resulting in non-syndromic hearing loss, while others serve functions in a variety of organs, with mutational defects giving rise to either syndromic or non-syndromic deafness. To date, many genes underlying inherited deafness still await identification and examination. We have used a different approach to identify proteins that may be involved in cochlear function. In rodents, pups are born deaf and the inner ear undergoes extensive remodeling during postnatal development (Pujol et al., 1997) culminating in the onset of hearing, which occurs around postnatal day 12 (P12) in rats (Ru ¨bsamen and Lippe, 1997). Consequently, many proteins involved in hearing are likely to experience an up-regulation on the transcriptional level during this period with a high abundance of the respective mRNA before the onset of hearing. Therefore, we screened cDNA libraries from the cochlear sensory epithelium of rats at P0 and P9 for genes that only appear at P9 and code for proteins of unknown function. Here, we describe the identification of a novel protein whose expression in the inner ear is restricted to sensory hair cells of both 1044-7431/$ - see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.mcn.2004.08.018 * Corresponding author. Physiologisches Institut II, Universit7t Frei- burg, Hermann-Herder-Str. 7, 79104 Freiburg, Germany. Fax: +49 761 2035191. E-mail address: dominik.oliver@physiologie.uni-freiburg.de (D. Oliver). Available online on ScienceDirect (www.sciencedirect.com). www.elsevier.com/locate/ymcne Mol. Cell. Neurosci. 28 (2005) 106– 117