Membrane Transport Group

Dr David Meredith


Understanding the structure-functional relationship of membrane transport proteins is of great importance, especially those involved in the uptake and delivery of nutrients across cell membranes such as those of intestinal enterocytes and renal proximal tubule absorptive epithelial cells, as these are potential routes for drug delivery and absorption.

The Membrane Transport Group’s research is focused on four main types of transporter: 

1. The proton-coupled di- and tri-peptide transporters (PepTs)

PepT1 is now well accepted as the major route by which protein digestion products enter the body via the intestine, and are retained by the kidney (along with PepT2). PepT1 is also considered to be an excellent route for drug delivery, as it accepts a wide range of structurally diverse natural substrates, in addition to a number of classes of pharmaceutical compounds such as the β-lactam antibiotics, angiotensin-converting enzyme (ACE) inhibitors, antivirals (eg valacyclovir) and anticancer agents such as bestatin. Using expression of mammalian PepT1 in Xenopus laevis oocytes as a model system, we have been able to identify essential residues in the functioning of the protein, that the protein is a multimer, and to characterise the binding site in terms of a substrate template model. 

2. The proton-coupled amino acid transporters (PATs)

Human PAT1 and PAT2 are well characterized as proton-coupled amino acid transporters, with glycine, alanine and proline as their main substrates. We have recently characterized human PAT4 as a non-proton coupled amino acid transporter with an unusually high affinity. I am also interested in PATs as nutrient receptors (“transceptors”), and in collaboration with Drs Boyd, Wilson and Goberdhan (Oxford University) we identified a Drosophila PAT homologue protein, PATH, which is also an extremely high affinity, low capacity amino acid transporter, and that is involved in cell growth and development via the target of rapamycin (TOR) intracellular signalling pathway. Thus PATs may play an important role in diseases such as diabetes and cancer. Further studies are ongoing into whether the human PAT4 behaves similarly, and into the function of the ‘orphan’ human transporter PAT3.

3. The organic anion transporting polypeptides (OATPs) - funded by the Medical Research Council and AstraZeneca PLC

The OATPs are a large family of mammalian transporters which transport a range of endogenous compounds, including bile salts and hormones, and many exogenous compounds (‘xenobiotics’) including pharmaceutically important classes of drugs, for example the ‘cholesteral busting’ statins. Due to their role in drug detoxification – they are largely expressed in the liver – they are of great interest to the pharmaceutical industry, not least as the site of drug-drug interactions due to their wide and overlapping substrate ranges. We are studying a number of the OATPs with the aim of better characterizing their function to better understand drug detoxification and to be able to predict drug-drug interactions.

4. The monocarboxylate transporter (MCT) family

There are 14 members of the human MCT family, of which the first characterized were MCTs 1 to 4, which are proton-coupled lactate / pyruvate transporters. Other members to be later characterized were MCT8 (thyroid hormone transporter) and TAT1 (‘MCT10’, aromatic amino acid transporter). We have been investigating the role of ‘orphan’ MCTs, one of which we believe is involved in the mechanism by which cancer cells avoid eliciting an immune response, and another (MCT9) which we have demonstrated as an efflux system for carnitine following its linking through a metabolomic / genome screening approach.

In addition, I have a more broad interest in membrane transport proteins in general: for example, in ongoing collaborations with Dr Wilkins (University of Oxford) investigating the transporters present in normal and diseased cartilage cells (chondrocytes), which are affected in osteoarthritis. 

Projects

The main areas of research are into the structure and function of proton-coupled nutrient transporters, PepT1 and the PATs, and the OATPs.

1. As for the vast majority of integral membrane proteins, there is no crystal structure for PepT1, yet its ability to accept such a diverse range of substrates means its binding site is of great interest. We are continuing to characterise this by two routes: firstly, by site-directed mutagenesis to identify important residues in the PepT1 sequence that contribute to its function (as assessed by radiolabelled dipeptide uptake and electrophysiology on protein expressed in Xenopus oocytes); and secondly, by testing novel peptidomimetic compounds synthesised by our collaborator colleagues at Keele University.

In addition to further illuminating the nature of the substrate binding site, the latter approach has particular impact on drug design, and there are two patents on approaches for drug carrier compounds that have been discovered through this process. We have also used homology modelling to try to computer predict the 3D arrangement of the PepT1 protein, based on the known structures of bacterial membrane transporters, most recently the soil bacterial peptide transporter PepTSo, in collaboration with colleagues in the Department of Biochemistry, University of Oxford.

2. It is now being appreciated that proteins that were thought to encode membrane transporters for nutrients may (also) act as sensors for the presence of that compound in the extracellular medium. After recently identifying a member of the Drosophila proton-coupled amino acid transport as one of these so-called “transceptors”, we are now investigating whether members of the homologous gene family in humans play the same role.

We have recently identified the function of one of the hitherto orphan proteins whose function was not known (hPAT4), and we are investigating whether these are transceptors. We are also performing studies to try to understand the structure-function relationship of the PAT proteins.

3. The OATPs are important in absorption, distribution, metabolism and excretion (ADME) of many drug compounds, being expressed particularly in intestine and liver, and their overlapping substrate ranges being important for understanding drug-drug interaction. This project aims to advance our understanding of the structure-function and substrate specificity of the OATPs, especially the OATP1B1.

People

Funding

  • Medical Research Council / AstraZeneca plc
  • Oxford Brookes University

Collaborations

  • Professor Pat Bailey, Faculty of Natural Sciences, Keele University
  • Dr David Foley, Nottingham University

  • Professor Mark Samson, Drs Simon Newstead & Philip Fowler, Department of Biochemistry, University of Oxford

  • Dr Sarah Kelly, AstraZeneca plc

  • Drs Richard Boyd, Helen Christian, Deborah Goberdhan, Clive Wilson & Robert Wilkins, Department of Physiology, Anatomy & Genetics, University of Oxford 

  • Professor Andrew Halestrap, Department of Biochemistry, University of Bristol

  • Professor Enzo Cerundolo & Dr Jon Silk, Weatherall Institute of Molecular Medicine, University of Oxford

  • Professor Karsten Suhre, Weill Cornell Medical College in Qatar, Doha, State of Qatar

  • Professor David Smith, Department of Pharmaceutical Sciences, University of Michigan, USA

Publications

Dr David Meredith

CONTACT US

Dr David Meredith
dmeredith@brookes.ac.uk
+44 (0) 1865 483613
Publications

Peptide transport Homology model of PepT1