Immunolocalization of Water Channel Aquaporins in the Vomeronasal Organ of the Rat: Expression of AQP4 in Neuronal Sensory Cells

Chem. Senses 33: 481–488, 2008 doi:10.1093/chemse/bjn015 Advance Access publication April 11, 2008 Immunolocalization of Water Channel Aquaporins in the Vomeronasal Organ of the Rat: Expression of AQP4 in Neuronal Sensory Cells Abduxukur Ablimit1, Takeo Aoki1, Toshiyuki Matsuzaki1, Takeshi Suzuki1, Haruo Hagiwara1, Shigeru Takami2 and Kuniaki Takata1 1 Department of Anatomy and Cell Biology, Gunma University Graduate School of Medicine, 3-39-22 Showa-machi, Maebashi, Gunma 371-8511, Japan and 2Laboratory of Anatomy and Cellular Biology, Faculty and Graduate School of Health Sciences, Kyorin University, Hachioji, Tokyo 192-8508, Japan Correspondence to be sent to: Kuniaki Takata, Department of Anatomy and Cell Biology, Gunma University Graduate School of Medicine, 3-39-22 Showa-machi, Maebashi, Gunma 371-8511, Japan. e-mail: Abstract The vomeronasal organ comprises a pair of narrow tubes in the mammalian nasal septum, serving as a chemosensory system for pheromones. We examined the expression and localization of water channel aquaporins (AQPs) in the rat vomeronasal organ. AQP1 was localized in blood vessels, being particularly abundant in cavernous tissues of the nonsensory mucosa. AQP5 was found in the apical membrane of the gland acinar cells in the vomeronasal organ. AQP3 was detected in the basal cells of the nonsensory epithelium, whereas it was absent in the sensory epithelium. AQP4 was found in both the sensory and the nonsensory epithelia. Interestingly, AQP4 was highly concentrated in the sensory cells of the sensory epithelium. Immunoelectron microscopic examination clearly showed that AQP4 was localized at the plasma membrane in the cell body and lateral membrane of the dendrite, except for the microvillous apical membrane. Nerve fiber bundles emanating from neuronal sensory cells were positive for AQP4, whereby the plasma membrane of each axon was positive for AQP4. These observations clearly show that neuronal sensory cells in the vomeronasal organ are unique in that they express abundant AQP4 at their plasma membrane. This is in marked contrast to the olfactory and central nervous systems, where AQPs are not detectable in neurons, and instead, AQP4 is abundant in the supporting cells and astrocytes surrounding them. The present findings suggest a unique water-handling feature in neuronal sensory cells in the vomeronasal organ. Key words: immunohistochemistry, nerve, pheromone, sensory cell, supporting cell, ultrastructure Introduction Aquaporins (AQPs) are integral membrane proteins that serve as channels in the transfer of water and small solutes such as glycerol and urea (Agre et al. 2002; Takata et al. 2004; Verkman 2005). At present, 13 isoforms of AQPs (AQP0–AQP12) have been identified and cloned in mammalian cells (Morishita et al. 2004). They are divided into 3 groups based on their sequence homology and functions: aquaporin (AQP), aquaglyceroporin, and superaquaporin subfamilies (Agre et al. 2002; Morishita et al. 2004). The AQP subfamily is composed of AQP0, AQP1, AQP2, AQP4, AQP5, AQP6, and AQP8 and is specifically involved in the transfer of water. The aquaglyceroporin subfamily is made up of AQP3, AQP7, AQP9, and AQP10 and is involved in the transfer of water as well as glycerol and urea. The superaquaporin subfamily comprises AQP11 and AQP12 and is more distantly related to other mammalian AQPs (Morishita et al. 2004; Ishibashi 2006). Water transport via AQPs is important in maintaining homeostasis. They are highly and differentially expressed in water-handling organs including the kidney, exocrine glands, and blood vessels (Nielsen et al. 2002; Takata et al. 2004; Verkman 2005). Sensory organs such as the eye and inner ear are watery organs, where the transfer of water is critical in maintaining the sensing of external stimuli. In the eye, at least 5 isoforms of AQPs are expressed in a cell type–specific manner and play important roles in regulating the lens transparency, intraocular pressure, microenvironment of the retina, and tear secretion (Verkman 2003). In the ear, AQP1, AQP3, AQP4, AQP5, AQP7, and AQP8 are expressed, and they must work in concert ª The Author 2008. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: 482 A. Ablimit et al. in order to maintain fluid regulation in the inner ear (Huang et al. 2002). In the olfactory system, we previously showed the distinct expression pattern of AQPs in the rat nasal mucosa (Ablimit et al. 2006). AQP3 and AQP4 are differentially expressed in the olfactory and respiratory epithelia. AQP3 is strongly expressed in the supporting cells of the olfactory epithelium and may serve in maintaining the specific microenvironment around sensory cells for olfaction (Ablimit et al. 2006). The vomeronasal organ of animals was discovered by Ludvig Jacobson at the beginning of 19th century (Døving and Trotier 1998; Takami 2002). In many terrestrial tetrapods, a pair of vomeronasal organs are situated at the base of the nasal septum in the anterior nasal cavity (Døving and Trotier 1998; Takami 2002). The vomeronasal organ is part of the nasal chemosensory system, is distinct anatomically and physiologically from the olfactory system (Wysocki 1979; Halpern et al. 1998), and is regarded as a chemosensory organ for pheromones. Many animals use their vomeronasal organs to detect chemical cues released by congeners and in biological fluids (Døving and Trotier 1998). In mammals, vomeronasal organs seem to play a role in regulated social behavior and sexual preference, necessary for their existence and propagation. Anatomically, the organ comprises a pair of tubes encapsulated in the vomer (Døving and Trotier 1998; Takami 2002). Posteriorly, the central lumen of the tube is blind ending, whereas the anterior opens into the nasopalatine or incisive ducts. The lumen is filled with fluid, and fluid handling seems to be closely related to the function of the vomeronasal organ (Takami 2002). The lumen is lined with 2 types of opposing epithelia: sensory epithelium and nonsensory epithelium. The sensory epithelium is located on the medial wall and the nonsensory epithelium on the opposite side. The sensory epithelium is composed of a sensory cell layer and supporting cell layer. The supporting cell layer is located in the upper portion of the epithelium and is made up of supporting cells. The sensory cell layer is located in the middle to the base of the epithelium, where the cell bodies of neuronal sensory cells are packed. They extend dendrites upward, which penetrate the supporting cell layer and reach the surface of the epithelium for chemical reception. Supporting cells and dendrites of neuronal sensory cells form junctions between them. The sensory cells extend axons downward, which leave the epithelium, form bundles, and run along the nasal septa as the vomeronasal nerves. The vomeronasal nerves finally enter the cranial cavity and project to the accessory olfact (...truncated)