Developing a Sense of Smell

    The world we live in bombards us with a plethora of signals, most of which humans are oblivious too. However, with the conventional five senses that we do have, the sense of smell adds an interesting dynamic to our interaction with the world. Moreover, the sense of smell adds another layer to our sense of taste; without smelling the food we eat, tasting it falls far short than when both senses are utilized. Such is the case with patients who have anosmia, either complete or partial, wherein the sufferer cannot smell at all or lose the sense of smell to certain stimuli. Also, the importance of smell can be seen in diagnostic practice because loss of smell is an early predictor of cognitive decline, hence, it can be used to diagnose early neurodegenerative diseases like Alzheimer's. But how does such an important sense of ours develop? How does something so complex arise in us? Well, researchers at Duke may have an answer. 

    Duke researchers, Pelin Volkan and colleagues, utilized the common fruit fly as a model to study how its sense of smell develops, from its small larval stage to the adult stage. Their research was published in Genetics and links a genetic network that works together to switch certain genes on and off at appropriate times to generate functional olfactory sensory cells. Also, the olfactory sensory cells are specific to the stimulus that they are detecting, and so a complex interaction between genes is at work during the process. Volkan and colleagues show that the same gene interaction mechanism occurring between a core of 5 genes to start precursor cells on the path to olfactory sensory cells, also occurs in the production of taste for the flies. The authors also saw that because the "toolkit" (ScienceDaily) of genes is the same between taste and smell, when manipulated, they were able to form other neuronal cell types in different tissues, indicating the immense diversity possible with just a basic paradigm of genes. The authors show that because we have networks in place as well, work such as this can elucidate how our own system develops from a basic genetic toolkit. 

     The sense of smell can get even more convoluted than complex genetic switches through the developmental process. Professor Thomas Bozza at Northwestern University presents an in-depth look at how certain specific receptors, TAAR, can work to influence our behavior by promoting aversion to amine containing compounds, smelled in potent sources like rotting fish. Bozza showed a very unique way of isolating specific TAAR4 from a family of TAAR that the mouse expresses and found that mice lacking the specific TAAR4 were not aversive to a potent stimulant PEA, seen in their natural predators (Pacifico et al). This work showed how specific the sense of smell is because the ligand is selective for that distinct receptor, but differing concentrations of the ligand or smell can activate other receptors, which the brain has to process effectively to perceive what the smell is. Consequently, this scenario creates a complex map for the olfactory system, which Bozza showed how the TAAR family contributes to a specific area in the olfactory map, where the receptors send their signals to, the olfactory bulb. 

      The work of Volkan shows how development consists of an interplay of genetic switches and Bozza shows how the perception of smell for a specific, horrendous class of smells, the volatile amines, occurs. Both research groups show just how powerful and complex our sense of smell is and just how much work still needs to be done to accurately gauge what is going on. Taking the research of both researchers as a story, the Duke researchers gave us the beginning of the story, with how development occurs, and Bozza told us the middle, with how specific receptors contribute to our perception of smell of certain amines. But the end still needs to be written and, hopefully, it will be soon. 


Sources: 

Pacifico et al., An Olfactory Subsystem that Mediates High-Sensitivity Detection of Volatile Amines, Cell Reports. (2012). http://dx.doi.org/10.1016/j.celrep.2012.06.006

Duke University. "Same switches program taste, smell in fruit flies: Findings help explain how complex nervous systems arise from few genes." ScienceDaily. ScienceDaily, 3 February 2016. <www.sciencedaily.com/releases/2016/02/160203161102.htm>.

Image: http://i.telegraph.co.uk/multimedia/archive/01628/smell_1628983c.jpg