Olfactory rod cells: a putative mechanosensory and ionoregulatory rare cell type in the zebrafish olfactory epithelium that expresses the adhesion G-protein-coupled receptor adgrg6

Sensory systems contain a variety of specialised cell types, including receptors and non-sensory cells. Characterisation of every cell type is crucial for a full understanding of the neurophysiology behind these sensory systems. This thesis reports the presence of olfactory rod cells, a rare cell type in the zebrafish olfactory epithelium which differs from known classes of olfactory cells. Using a combination of staining, genetic, and imaging techniques, I have characterised the morphology, development, gene expression, and potential functions of these cells. Olfactory rod cells each bear a distinct actin-rich apical projection extending about 10 µm above the epithelial surface, which oscillates in live larvae, possibly as a result of ciliary beating. They have a rounded cell body positioned apically in the epithelium, and posterolaterally in the larval olfactory pit, near to multi-ciliated cells. Olfactory rod cells arise during embryogenesis and are present through to adulthood. Adgrg6/gpr126, a human disease-implicated gene which codes for an adhesion G-protein-coupled receptor with a proposed mechanosensory role, is specifically expressed in olfactory rod cells within the larval zebrafish olfactory epithelium. Transcription of cfos in olfactory rod cells following mechanical stimulation suggests that they are mechanosensory. Furthermore, a modulation in olfactory rod cell numbers following exposure to a low salinity environment, along with expression of a sodium-potassium ion transport protein in a subset of the cells, insinuate a potential function in ionoregulation. This work identifies a novel multimodal cell that may facilitate olfactory function by influencing ion balance and cilia beat dynamics. Given the implication of ionocytes and ciliary clearance in respiratory diseases, further work on olfactory rod cells may have clinical relevance.