Copyright @ 2009 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited. Nogo in the Mammalian Cochlea *Antje Caelers, †Arianne Monge, †Jochen Michael, ‡Martin E. Schwab, and *Daniel Bodmer *Inner Ear Research, Department of Biomedicine University Hospital Basel and the Klinik fu ¨ r Ohren-, Nasen-, Halskrankheiten University Hospital Basel, Basel; ÞInner Ear Research, Clinic for Otolaryngology, Head & Neck Surgery, University Hospital Zurich; and þBrain Research Institute, University and ETH Zurich, Zurich, Switzerland Hypothesis: Different members of the Nogo system are ex- pressed in the mammalian cochlea. Background: The protein Nogo has gained a lot of attention during the last couple of years because it inhibits neurite out- growth in the adult central nervous system. In contrast to the central nervous system, very little is known regarding the expression and possible function of the Nogo system within the inner ear. Methods: Using reverse-transcriptaseYpolymerase chain reac- tion and immunohistochemistry, we analyzed for the expression of members of the Nogo system within the cochlea. In addition, we determined hearing levels of Nogo A knockout and wild- type mice with auditory brainstem response audiometry. Results: In this study, we demonstrate the expression of Nogo A, B, C, and of Nogo receptor mRNA in the organ of Corti, spiral ganglion, and stria vascularis. Immunohistochemistry revealed that Nogo A and Nogo receptor localize to the spiral ganglion neurons. Interestingly, Nogo A expression was also observed in the outer and inner hair cells of the organ of Corti. As revealed by light microscopy, deletion of Nogo A does not alter cochlear microanatomy. We have assessed hearing levels in 10-month old wild-type and Nogo A knockout mice, and thereby, we could not detect any differences between these 2 groups. Conclusion: Different members of the Nogo family are ex- pressed in the mammalian cochlea. Deletion of Nogo A does not alter cochlea microanatomy or hearing levels compared with wild-type mice. Key Words: ApoptosisVHair cellsVInner earVNogo A knockout miceVNogoR. Otol Neurotol 30:668Y675, 2009. Development of the nervous system is characterized by extensive but highly controlled axon outgrowth. With maturation of the central nervous system, the growth capacity of axons decreases so that, in contrast to the peripheral nervous system, central nervous system axons do not regenerate after injury in the adult animal (1). Myelin especially impedes axon growth in the adult central nervous system (2,3). The discovery and charac- terization of specific regeneration and growth of inhibi- tory proteins of the adult central nervous system, in particular, Nogo A (3), which is one of the most potent, was a breakthrough. The 3 Nogo isoforms A, B, and C (4Y6) share a common carboxy-terminal region that indi- cates their membership to the reticulon family (RTN4) of proteins (7). The receptor for Nogo A is the glycoprotein Nogo receptor (NgR) (3). Nogo A is a neurite growth inhibitory protein of 1,163 amino acids (rat sequence; apparent molecular weight, 200 kDa). In contrast to Nogo B and C, which show no or only weak neurite growth inhibitory activity and occur in many tissues (8), Nogo A is predominantly present in the central ner- vous system. Although Nogo A mRNA is present in oligodendrocytes (9), the protein is found in the most inner and outer myelin membranes (8,10). Interestingly, Nogo A appears very late in evolution, at the Bfrog level[ (11). This could explain the well-known high-regeneration potential of the spinal cord after lesion in fish and salamanders, which lack Nogo A (12). In fish, amphibians, and birds, regeneration of hair cells is also possible (13,14). However, the regeneration of hair cells in the avian auditory epithelium occurs only in response to externally triggered hair cell trauma (15,16). Impor- tantly, hair cell regeneration in birds occurs after hair cell and hearing function have matured (17). The neurite growth inhibitory activity of Nogo A has been studied both in vitro and in vivo (3). For example, after spinal cord lesion, Nogo A knockout mice and rats or monkeys treated with Nogo A function blocking anti- bodies showed increased regenerative sprouting and elongation of corticospinal tract fibers, in parallel with enhanced functional recovery. Compensatory growth of Address correspondence and reprint requests to Daniel Bodmer, MD, Clinic for Otolaryngology, Head & Neck Surgery, University Hospital Basel, Petersgraben 4, CH-4031 Basel, Switzerland; E-mail: dbodmer@ uhbs.ch This work was supported by Grant 1140 from the Hartmann Mu ¨ller- Stiftung for Medical Research. Otology & Neurotology 30:668Y675 Ó 2009, Otology & Neurotology, Inc. 668