Chemical senses

The outside world of humans basically consists of a collection of physical (mechanical, sound, vision and temperature) and chemical (taste and olfaction) facets, detected using our sensory systems.

Understanding how we sample chemicals is particularly important. Not only it helps to understand the way we form a picture of the world, but it is also important for food research. After all, chemical sensing is responsible for the pleasure and enjoyment of a meal. To control precisely perception of small molecules (odorant and tastants) we need to understand the underlying signaling processes. Because each flavoring ingredient is as an agonist, or an allosteric modulator targeting smell or taste receptors, we may say this ‘control of perception’ is what humans have always done in an imprecise way by adjusting the flavors of foods. To fine tune taste and smell sensations at will, we have to understand chemical sensing at the molecular level.

Odorant and taste sensing receptors are G-Protein Coupled Receptors (GPCRs), and, in the case of taste, also ion channels. We can say therefore that GPCRs are the key players of most chemical sensing. GPCRs are also very important as a family. They constitute the largest membrane-bound receptor family expressed by mammalians (encompassing more than 1% of the genome). GPCRs share a similar seven transmembrane (TM) helices domain connected by alternating extracellular and intracellular loops, an extracellular N-terminus, and an intracellular C-terminus. They may be assembled as dimers or form oligomeric complexes within the membrane. Despite sharing a common structure, they show considerable diverse primary structures. Ligand (or photon) binding to GPCRs activates a variety of downstream cascade of events.

Based upon evolutionary studies, GPCRs can be divided into six main classes, the so-called A–F classification system. Members of the largest class (Class A) are involved in chemical senses.

More than half GPCRs (about 900) are olfactory receptors (ORs). This underlies the crucial role of the sense of smell during evolution. ORs possess high affinity for thousands of volatile molecules associated with odor. With such a large number of different ORs, the olfactory system is capable to discriminate between ~10,000 different odors: one odorant can activate numerous types of ORs, while a single OR can be activated by several different odorants.

The initial part of the ORs cascade is the same across the GPCR family. In the cilia of olfactory sensory neurons, odorant molecules binding to ORs cause conformation changes, with the consequence of activating its cognate G-protein. The activation of the G-protein is associated with a decrease of affinity for GDP and an increase of affinity for GTP. This, in turn, stimulates the adenylate cyclase (AC) type III enzyme which converts ATP into cyclicAMP (cAMP). cAMP binds to and opens the cytoplasmic domains of the olfactory cyclic nucleotide gated (CNG) ion channels. This allows Na⁺ and Ca²⁺ cations to flow along their electrochemical gradients from the extracellular to the intracellular side of the membrane. The increased Ca²⁺ concentration in the cilia causes the opening of Ca²⁺-activated Cl⁻ channels and the subsequent Cl⁻ efflux, which further depolarizes the cell.

Thus, the chemical interactions of ORs with volatile molecules lead ultimately to the production of action potentials that will carry information about the external world to the brain. The axons of the olfactory sensory neurons from the nasal cavity send information to second-order neurons in the olfactory bulb, which in turn project to the olfactory cortex and then to other brain areas. The increase of Ca²⁺ concentration has an inhibitory effect, which eventually terminates the signal, as evidently required for the function of this apparatus. This is achieved by Ca²⁺ binding to calmodulin (CaM), which also stimulates the activity of a phosphodiesterase (PDE). Ca²⁺ is then extruded by a Na⁺/Ca²⁺ exchanger.

The lack of experimental structural information for most components of the pathway calls upon modeling at different levels, possibly with inclusion of experiments performed at the same time. We have predicted the structure of several of the components of the cascade [Lupieri et al. HFSP J. 3, 228­239 (2009)].

Taste receptors cause a handful of sensations from sweet to bitter, from sour to salty and savoury (or umami). Bitter is arguably the most complex taste. Bitter tastants have evolutionary been prevented from the ingestion of a large variety of poisonous and toxic substances by their aversion taste. They bind to and are discriminated by a family of roughly 30 bitter taste receptors (TAS2Rs). These are (GPCRs). Bitter molecules binding to their cognate target TAS2R initiates a downstream cascade of events inside the cell, which involves the dissociation of G protein gustducin. This may cause two different mechanisms. In the best characterized so far, the dissociated Gβγ-subunits activate the phospholipase Cβ2 enzyme and the inositol phosphate type III receptor. This produces inositol triphosphate, which in turn stimulates an increase of Ca²⁺ concentration from the endoplasmic reticulum. Calcium release activates the non-selective cation channel transient receptor potential–melastatin 5 , which transports mostly Na⁺ ions.

Recently, we have identified hTAS2R38 residues involved in binding to its agonist phenylthiocarbamide (PTC) as well as in receptor activation. We have first used state-of-the-art bioinformatics approaches based on multiple sequence alignment across the whole family of GPCRs. However, this procedure is likely not to be sufficient to identify residues in the binding site as ligand pockets vary largely in position and orientation across this family. Addressing this issue is hence aided by predicting the three-dimensional structure of the receptor, based on the former alignment and recent structural information on GPCRs along with massive virtual docking calculations [Biarnés et al., PLoS One. 5(8), e12394 (2010)].