It’s thought that bitter taste receptors evolved to stop us from accidentally eating toxic plants. But the receptors aren’t just found in our mouths – far from it. They’re dotted right across the body, from the airways to the testes.
So what purpose could these taste receptors – or ‘TAS2R proteins’ - have in these random anatomical locations? There’s not a whole lot of tasting going on down there, that’s for sure.
Scientists have been pondering the answer to this question for many years. Along the way they’ve made some fascinating discoveries, some of which are laying the foundations for a new class of asthma medication.
Because TAS2Rs are thought to have evolved to help us avoid harmful foods, when the receptors were discovered in the airways, it was hypothesised that they were there to help us avoid harmful bitter gases.
However, not all bitter inhalants are toxic. Choloquine and quinine – both used in malaria medication, and quinine is also used to flavour tonic water – are both safe to inhale in aerosol form, and can be used to study TAS2R responses.
When scientists started experimenting with these bitter compounds, they saw that they did indeed trigger relaxation of the airway muscle tissue. The bitter ‘taste’ associated with noxious gases activates dilation of the airways, making it easier for us to expel the potentially harmful gas as we breathe.
This airway relaxation effect is an interesting finding on its own, but in the context of asthma – a disease which affects 1 in 9 Australians – the impact of these studies is huge.
“Conventional asthma therapies work in two main ways. Cortical steroids prevent inflammation and Beta2-agonists (i.e. such as those used in Ventolin) relax the airways,” explains Dr Pawan Sharma, Respiratory Research Group Leader at the University of Technology Sydney (UTS). “Ideally we should have a compound which can do both.”
Dr Sharma was born in India, and has worked on airway diseases across multiple institutions and industries throughout Asia, North America and Australia. He moved to Sydney in 2015 when he was offered the Chancellor's Fellowship at UTS.
In a scientific paper recently published by Sharma and his team, it's revealed that choloquine and quinine are indeed capable of modifying both inflammation and airway relaxation – thereby hitting the sweet spot of ideal asthma therapeutics.
So far these studies have been limited to animal and human cell culture models, so we’re still a long way away from seeing these aerosols in the pharmacy.
But the results are still extremely promising, not just because of the compounds’ double-whammy treatment capacity, but because there is also evidence that they could help treat a particularly vulnerable subset of asthma patients.
Although the majority of people with asthma can manage their condition using the two common forms of therapy, there is a group of asthma patients – those with ‘severe asthma’ - whose disease does not respond to these drugs.
“What happens with severe asthma is that over the course of the long term disease, there are structural changes in the lungs. The disease progresses to a different level and it's not easy to reverse the changes,” says Dr Sharma. “These are the patients who are often in and out of hospital.”
These structural changes are termed ‘airway remodelling’ and they include the build-up of scar tissue, the formation of too many blood vessels and of too much muscle tissue and disruption of the ordering of cellular layers.
“Severe asthma patients are often in and out of hospital.”
When the researchers treated their severely asthmatic mice with bitter aerosols, they saw many of these changes begin to be reversed. Airway muscle mass decreased, and there were fewer of the proteins which we typically associate with scarring being deposited.
The future of TAS2R research
While Dr Sharma is excited to relay his findings, he’s also enthusiastic about the future research of TAS2Rs and what other medical benefits might be found.
“The receptors are all over the body, even in the gut,” he reveals. “There are studies now which are aimed at understanding how they might play a role in the obesity epidemic, the microbiome, and how they might function in metabolic disease too.”
Unless you want to literally drown in your G&T, drinking bitter tonic water probably won’t help with your asthma.
While a few not-so-scientific reports are already boasting the benefits of tonic water for asthmatics, Dr Sharma assured me that, for therapeutic benefits to occur, the bitter compounds need to be delivered directly to the site of action (i.e. the lungs).
So unless you want to literally drown in your G&T, drinking tonic water probably won’t help with your asthma. Sure, it could trigger some molecular signalling pathways associated with asthma improvement. However, as death is one of the major consequences of having drowned, activation of those pathways would probably prove insignificant in the long term.