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Abstract
The membranes of olfactory receptor neurons (ORNs) need to aid the detection of odourants and the generation of appropriate signals that are transmitted to the central nervous system. Odourant receptors and their signalling machinery are located in thin membrane protrusions called cilia. Ciliary membranes must withstand the harsh environment of the nasal cavity while maintaining a fluid lipid bilayer that enables regulation of the olfactory signalling cascade and generation of the olfactory response. The organisation of ORN ciliary membranes and the functional consequences of this organisation are poorly understood.
To quantitatively assess the organisation of plasma membranes of functionally and anatomically different membrane domains of ORN, environmentally sensitive membrane dyes, Laurdan and di-4-ANEPPDHQ in conjunction with two-photon and confocal laser-scanning microscopy were employed. The emission spectra of these fluorophores correlate with the degree of membrane hydration that can be quantified as a Generalised Polarisation (GP).
In this study it was found, that the membrane order (MO) in different membrane domains on the surface of the ORNs had similar GP values, in isolated amphibian ORNs as well as in mammalian ORNs, which were still embedded within olfactory epithelium (OE). However, when membrane lipid content was altered or when odourants were present, MO exhibited domain-specific changes. Also it was demonstrated, that odourants from chemically different groups had different and domain-specific effect in OE.
To better understand the complex relationship between olfactory signalling proteins, odourants and MO, a heterologous system was employed. The A2 subunit of CNG channel and isoform B of CaCC, ANO2, was transiently expressed in HEK293T/17 cells. The lipid composition of these cells was manipulated and cells were exposed to different concentrations of the odourant 1-heptanol.
The findings confirmed that odourant can directly affect MO. Effects of the odourant were not only dose-dependent, but also dependent on membrane lipid composition. Both of the investigated olfactory signalling proteins demonstrated the ability to regulate MO when membrane lipid content was altered and/or odourant was present. Hence, the role of the signalling proteins in olfaction may not be limited to signalling processes but may also be extended to the maintenance of membrane organisation.