Expression of calcium-activated chloride channels TMEM16A and TMEM16B in adult mouse vomeronasal epithelium and during embryonic development of the olfactory epithelium

Olfaction enables animals to be familiar with the surrounding environmental changes. Exchange of odor molecules between animals is a way to communicate with each other and is necessary for various physiological processes, like reproduction, food preferences, prey detection, etc. The olfactory epithelium is always in contact with the inhaled air that is accompanied by odor molecules. Olfactory sensory neurons are the primary neurons of the olfactory epithelium. These neurons follow the “one receptor one neuron” rule, i.e. each individual olfactory sensory neuron expresses only one type of olfactory receptor out of ~1300 types in mouse. These neurons are specialized to convert chemical interaction, between odor molecules and olfactory receptor, into electrical signals by specific transduction mechanisms, which occur in the cilia of these neurons. The ciliary membrane contains cyclic nucleotide-gated channels and calcium-activated chloride channels. It is well documented that calcium-activated chloride channels are used to enhance signal to noise ratio in olfactory sensory neurons but we do not know about their involvement in the development of olfactory epithelium. TMEM16A and TMEM16B, the members of transmembrane proteins 16 (TMEM16) family, are responsible for the calcium-activated chloride current in various cells. In present work, I studied expression of TMEM16A and TMEM16B proteins during embryonic development of mouse and tried to find their role in olfactory epithelium development. I found expression of TMEM16A and TMEM16B in the developing olfactory epithelium at different embryonic ages. At embryonic day 12.5 (E12.5), TMEM16A immunoreactivity was present at the apical surface of the entire olfactory epithelium, but from E16.5 became restricted to a region near the transition zone with the respiratory epithelium. Olfactory sensory neurons are devoid of TMEM16A but this channel is expressed in the apical organelle free region and microvilli of supporting cells. Nasal septal glands and lateral nasal glands also express TMEM16A at the luminal surface of glands. In contrast, TMEM16B immunoreactivity was observed at E14.5 at the apical surface of the olfactory epithelium. Its expression was observed only in mature olfactory sensory neurons. With the maturation of olfactory sensory neurons and elongation of cilia TMEM16B expression is increased along with ACIII, CNGA2 and acetylated-tubulin. Interestingly, olfactory sensory neurons express only TMEM16B, but I found expression of TMEM16A as well as of TMEM16B in microvilli of vomeronasal sensory neurons. These findings indicate different physiological roles for TMEM16A and TMEM16B in the developing as well as in the postnatal olfactory and vomeronasal epithelia. Taking into account the previous evidences, I hypothesized that the presence of TMEM16A at the apical part and in microvilli of the supporting cells as well as in nasal glands is involved (1) in the regulation of the chloride ionic composition of the mucus covering the apical surface of the olfactory epithelium and/or (2) in proliferation and development during embryonic development. By comparing immunohistochemistry experiments on TMEM16A-/- and TMEM16A+/+ littermate mice I excluded the hypothesis that TMEM16A is involved in proliferation or development of the olfactory epithelium. So, either TMEM16A does not play a central role in the development of the olfactory epithelium or its genetic ablation does not affect olfactory development. Supporting cells, Bowman’s and nasal glands morphology remained unchanged in TMEM16A-/- mice, but at present we do not know whether the mucus composition is same as in TMEM16A+/+ littermate mice. Localization of TMEM16B to the cilia of mature olfactory sensory neurons and in microvilli of vomeronasal sensory neurons is consistent with a role in sensory signal transduction mechanism. In conclusion, the present work explored the dynamic expression pattern of TMEM16A and TMEM16B. It might be possible that different physiological roles of these proteins depend on the intracellular and extracellular factors expressed in the corresponding cells.