Challenges and solutions: G protein-coupled receptors as novel drug targets
G protein-coupled receptors (GPCRs) are a superfamily of integral membrane proteins that have been linked to a wide range of human diseases. They are unique to eukaryotic cells (containing a nucleus, unlike bacteria and other cells) with roughly 800 genes encoding them in humans. Upon binding of a hormone, such as adrenaline, the GPCR undergoes a conformational change which causes it to interact with a nearby G protein, which then in turn becomes activated and alters the cells’ biochemistry.
GPCRs perform a wide variety of functions and are vital in cell signalling processes. As such, they transmit signals from outside a cell across its membrane. Their modulatory properties include adaptation of neurotransmission, sensory perception, metabolism and homeostasis. Their involvement in inflammatory and metabolic diseases, cardiac dysfunction, diabetes, neurological disorders and cancer, make them important therapeutic targets. For example, research has found that malignant cells can influence GPCR function to proliferate and avoid immunological responses. A better understanding of GPCR structure, together with enhanced modelling techniques, is likely to reveal additional functions of receptors and membrane proteins, providing great potential for drug development.
GPCR stability influences structure determination
As of now, around 30–40% of prescription drugs target GPCRs. Due to this, they are among the most frequently investigated drug targets in the pharmaceutical industry. However, developing drugs to GPCRs continues to be a challenge, as advances in scientific knowledge of receptor structure and function have been relatively slow. Structure determination of GPCRs faces high rates of failure because the receptors are very unstable. Dr Chris Tate’s research has developed techniques to stabilise any GPCR through systematic scanning mutagenesis, thus increasing the probability that structures of any GPCR can be determined. Each mutation can increase thermostability by up to 14˚C, and when a number of mutations are combined, a highly stable receptor is generated that is ideal for structure determination because well-diffracting crystals can be grown in detergent solution. Using this technique, the structures of many different GPCRs have been determined bound to many different ligands. For example, the structure of the β1-adrenergic receptor (β1AR) has been determined bound to many different molecules, such as the beta blocker bucindolol and the anti-asthma drug salbutamol.
Discovering how GPCRs work
Understanding how GPCRs function is essential to develop appropriate drugs. GPCRs exist in two different states, an inactive state and an active state that binds G proteins. Some drugs need to stop the receptor from working and therefore have to be targeted to the inactive state, such as beta blockers binding to β1AR. However, some drugs need to activate receptors and therefore have to be targeted to the active state, such as salbutamol binding to β2AR used to treat asthma. Unfortunately, the active states of receptors are even less stable than the inactive states, so it is far more difficult to determine their structures.
GPCRs are vital in cell signalling processes and their structures can now be determined routinely after thermostabilisation
Recently, the lab of Dr Tate has engineered a minimal G protein, mini-Gs, that improves the chances of getting a high-resolution structure of a receptor in the fully active state. As proof of principle, the structure of the adenosine A2A receptor was determined coupled to mini-Gs. This gave valuable insights into the role of the G protein in binding to the receptor and how it affected the receptor’s structure around the ligand binding pocket. Novel minimal G proteins have now been derived from other types of G proteins, such as Gq, Gi and Go, which now permits the structure determination of many more receptors, thus accelerating drug discovery.
Thermostabilisation enhances drug discovery
Thermostabilisation offers some distinct advantages in drug discovery with receptors being more easily purified and crystallised. It is at the heart of Heptares Therapeutics’ structure-based drug discovery (SBDD) platform, which is built to overcome receptor stability issues and aid development of high-quality medicines. The company’s stabilised receptor technology (StaR) identifies point mutations, which enhance the thermostability of proteins – enabling GPCRs to become better targets for SBDD. StaR proteins are easier to crystallise which has enabled researchers to develop novel clinical candidates that may in the future be used for the treatment of such diseases as cancer, obesity and loss of brain function in dementia.
Heptares continues to develop new technologies to accelerate drug discovery, such as CHESS and SaBRE, which were developed by its Swiss operation HeptaresZurich, for the rapid optimisation of expression of GPCRs in E. coli and yeast and to thermostabilise them using evolutionary approaches.
Ten years strong
Celebrating its tenth anniversary in 2017, Heptares recently announced a multi-million-dollar drug discovery and licensing agreement with Daiichi Sankyo Company to focus on a single GPCR involved in pain reduction mechanisms. Not only that, but the company has also recently entered into a collaboration with AstraZeneca to take adenosine A2A receptor antagonists into the clinic for the treatment of cancer. The original compounds were developed in Heptares using SBDD, and the lead candidate AZD4635 has recently been found to reduce tumour growth in preclinical trials through the reversal of adenosine-mediated T-cell suppression. These deals followed the $3.3 billion partnership between Heptares and Allergan to take a series of compounds that activate the muscarinic M1 receptor with the aim of improving cognition during neural degeneration associated with dementia in diseases such as Alzheimer’s disease.
Compounds derived from the StaR technology of Heptares are now in clinic trials for cancer and Alzheimer’s disease
Heptares was founded in 2007 with the goal to commercialise research from the MRC Laboratory of Molecular Biology as well as the National Institute of Medical Research, London. The company has since generated a host of new clinical candidates targeting neurological, immune-oncology, metabolic and orphan diseases.
I have always been fascinated by integral membrane proteins such as transporters, ion channels and GPCRs, and how their molecular structures facilitate the movement of small molecules or structural changes across the membrane. My work with GPCRs started at the LMB due to the work of Gebhard Schertler and Reinhard Grisshammer, who were endeavouring to try and grow crystals of β1AR, A2AR and the neurotensin receptor. I thought that the way forward would be to thermostabilise them through systematic mutagenesis. Richard Henderson and I managed to get funding from Pfizer and MRCT for four postdocs, who started work in early 2015 and, by the summer of 2016, we were excited to see that the thermostabilisation strategy was an outstanding success.
What are you currently working on?
Our work is focusing on the structure determination of GPCR complexes. To fully understand how GPCRs function, structures need to be determined of each receptor bound to the same ligand in complex with binding partners such as G proteins and arrestins. The new mini-G proteins we have engineered will facilitate this work and we hope to emulate this with arrestins. This will be essential to understand how biased agonists work, that is to understand why some ligands signal through the G protein pathway compared to the arrestin pathway. Understanding biased signalling will enable the development of drugs with reduced side effects.
GPCRs are the cornerstone of signalling between organs
and cells throughout our whole body
Which clinical disease do you see the potentially most successful application of your research in?
All diseases! GPCRs are the cornerstone of signalling between organs and cells throughout our whole body, so potentially any disturbance in signalling can be rectified by using an appropriate drug. However, one of the major problems is identifying pathways in the body that can be used to target a particular disease or ailment, but once this is achieved and a single type of receptor identified as the key player, then drugs can be developed to it.
Does the thermostabilisation strategy work for other proteins apart from GPCRs?
Yes, it can work for potentially any protein. So far, it has been shown to work for transporters and ion channels as well as GPCRs, but I am convinced that it will work for any protein. This is because most proteins in humans have not evolved to be extremely stable, as the cell relies on protein flexibility to be able to control their function. I am also excited to see that a number of other research groups have adopted the thermostabilisation strategy and have published structures of membrane proteins that were unable to yield well-diffracting crystals before thermostabilisation.
What’s next for Heptares?
Heptares has built a broad pipeline of novel candidates that are advancing towards clinical trials in multiple indications. The company is building its clinical capability so that it can progress these compounds through to advanced clinical studies and ultimately to market.
Heptares is at an exciting point where it will be testing compounds in patients, which were developed by structure-based design using the StaR technology. This will be the ultimate proof of the value of the research and the thermostabilisation technology developed in our group at the LMB.
Dr Tate studies the structure and function of G protein-coupled receptors (GPCRs) and has developed a number of novel methodologies to allow structures of GPCRs to be determined. These GPCRs represent a superfamily of receptors linked to a wide range of human diseases.
- Medical Research Council (MRC)
- European Research Council (ERC)
- Heptares Therapeutics
- Wellcome Trust
- MRC Technology
- Richard Henderson (LMB)
- Andrew Leslie (LMB)
- Gebhard Schertler (Paul Scherrer Institute)
- Reinhard Grisshammer (NIH)
- Nagarajan Vaidehi (City of Hope)
- Fiona Marshall (Heptares Therapeutics)
Dr Tate is a Programme Leader at the MRC LMB, Cambridge, working on G protein-coupled receptors. He completed a PhD in biochemistry (University of Bristol, 1989) and a postdoc at the University of Cambridge (1989-1992), before moving to the LMB. He became a co-founder of Heptares Therapeutics back in 2007.
Dr Christopher G. Tate
MRC Laboratory of Molecular Biology
Cambridge Biomedical Campus
Francis Crick Avenue
T: +44 1223-267073