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The incretin effect, which in normal health is responsible for ~80% of insulin secretion at mealtimes, is impaired in type 2 diabetes (T2D)1. GLP-1 is thought to be the most important incretin hormone which regulates blood glucose levels, making it a successful therapy for diabetes2. However, active GLP-1 is prone to rapid degradation by a group of proteolytic enzymes called dipeptidyl peptidase-IV (DPP-IV)3. Thus, prolongation of GLP-1 activity through inhibition of DPP-IV is an attractive therapeutic opportunity for enhancement of insulin secretion in diabetes. Interestingly, the gut hormone peptide tyrosine tyrosine (PYY) is also degraded by DPP-IV action and PYY has been shown to play a vital role in restoration of diabetes following Roux-En-Y gastric bypass (RYGB)4,5. Recently, in a double-blind randomised control trial, the DPP-IV inhibitor sitagliptin was shown to have no effect on plasma GLP-1 levels but led to an increase in PYY6.

 We have preliminary data demonstrating that PYY is secreted locally by islet cells and it can affect glucose-stimulated insulin secretion (GSIS) in both rat and human islets. In addition, we have observed potentiation of GSIS by sitagliptin in islets isolated from GLP-1 receptor knockout mice, indicating that the action of DPP-IV in islets is independent of GLP-15. We have also shown for the first time that PYY levels in islets are markedly reduced in diabetes and that the levels can be restored upon RYGB, indicating a critical role of PYY in islet physiology and diabetes5. Whilst pancreatic islets express DPP-IV, its role and function on local GLP-1 or PYY system remain obscure7. Further work establishing PYY as a mediator of glycaemic control with DPP-IV inhibition is required, as well as gaining further understanding of DPP-IV action within islets on local GLP-1 and PYY.   PYY levels are known to vary with adiposity and non-specific metabolomic analysis of serum pre-and post-surgery in man has shown changes in metabolites suggesting potential lipid biomarkers of diabetes remission following RYGB8.9.  More detailed metabolomic investigation is required on key metabolites involved in glucose and metabolic regulation, and as to whether these could potentially augment reversal of hyperglycaemia post-surgery. PYY is currently not used clinically to treat diabetes or obesity - this work may contribute to its use as a potential therapy in diabetes management.

The main goals of this project are: 1) investigate the effect of DPP-IV inhibition therapy on plasma PYY and GLP-1 levels in T2D; 2) explore the role of DPP-IV inhibitors on insulin and glucagon secretion and local GLP-1 and PYY levels in mouse and human islets; and 3) metabolomic analysis of human serum pre-and post-bariatric surgery to determine which metabolic changes could potentially mediate reversal of hyperglycaemia.

The study will involve use of serum and plasma samples from patients with T2D undergoing DPP-IV inhibition treatment and RYGB with subsequent application of basic science techniques to explore the role of PYY in DPP-IV action and islet function, providing an excellent opportunity for engaging in translational work in a clinically-relevant area.

Training Opportunities

Serum samples from patients on DPP-IV inhibition therapy and relevant controls are available via an existing collaboration with Dr Angus Jones (University of Exeter Medical School).   Samples will be analysed for incretins, insulin and glucagon.  Metabolomic analysis will be done on serum samples from patients pre- and post-surgery with samples available via an existing collaboration with Dr John Ryan at the Translational Gastroenterology Unit in Oxford. In vitro work will be conducted using isolated mouse and donor human islets using a combination of optical, molecular and genetic approaches to study the mechanisms underlying DPP-IV inhibition on pancreatic islet function. Secretion studies will be performed to investigate the effects of DPP-IV blockers on insulin and glucagon secretion and hormone release will be measured by MesoScale Multiplex platform. Selective inhibitors will be used to study the intracellular signal transduction mechanisms involved in DPP-IV inhibition and GLP-1 and PYY action. The project will also provide training and experience in range of other broad techniques, including light microscopy, islet isolation and hand-picking, cell culture, immunohistochemistry, gene expression and Western blotting and biochemical assays.

As well as the specific training detailed above, students will have access to a wide-range of seminars and training opportunities through the many research institutes and centres based in Oxford. Students are also able to attend the Methods and Techniques course run by the MRC Weatherall Institute of Molecular Medicine. This course runs through the year, ensuring that students have the opportunity to build a broad-based understanding of differing research techniques.

Generic skills training is offered through the Medical Sciences Division's Skills Training Programme. This programme offers a comprehensive range of courses covering many important areas of researcher development: knowledge and intellectual abilities, personal effectiveness, research governance and organisation, and engagement, influence and impact. Students are actively encouraged to take advantage of the training opportunities available to them.

The department has a successful mentoring scheme, open to graduate students, which provides an additional possible channel for personal and professional development outside the regular supervisory framework. We hold an Athena SWAN Silver Award in recognition of our efforts to support the careers of female students and staff.


1 MARI A, BAGGER JI, FERRANNINI E, HOLST JJ, KNOP FK, VILSBØLL T. 2013. Mechanisms of the incretin effect in subjects with normal glucose tolerance and patients with type 2 diabetes. PLoS ONE, 8 (9), pp. e73154
2 HARRIS KB, MCCARTY DJ   Efficacy and tolerability of glucagon-like peptide-1 receptor agonists in patients with type 2 diabetes mellitus. Ther Adv Endocrinol Metab. 2015 Feb;6(1):3-18
3 HEARD KR, WU W, LI Y, ZHAO P, WOZNICA I, LAI JH, BEINBORN M, SANFORD DG, DIMARE MT, CHILUWAL AK, PETERS DE, WHICHER D, SUDMEIER JL, BACHOVCHIN WW. 2013. A general method for making peptide therapeutics resistant to serine protease degradation: application to dipeptidyl peptidase IV substrates. J. Med. Chem., 56 (21), pp. 8339-51
4 RAMRACHEYA RD, MCCULLOCH LJ, CLARK A, WIGGINS D, JOHANNESSEN H, OLSEN MK, CAI X, ZHAO CM, CHEN D, RORSMAN P. 2016. PYY-Dependent Restoration of Impaired Insulin and Glucagon Secretion in Type 2 Diabetes following Roux-En-Y Gastric Bypass Surgery.Cell Rep, 15 (5), pp. 944-50
5 Guida C, McCulloch LJ, Godazgar M, Stephen SD, Baker C, Basco D, Dong J, Chen D, Clark A, Ramracheya RD. Sitagliptin and Roux-en-Y gastric bypass modulate insulin secretion via regulation of intra-islet PYY. Diabetes Obes Metab. Sept 2017; in press
6 AABOE, K. et al. Twelve weeks treatment with the DPP-4 inhibitor, sitagliptin, prevents degradation of peptide YY and improves glucose and non-glucose induced insulin secretion in patients with type 2 diabetes mellitus. Diabetes Obes Metab 12, 323-333, doi:10.1111/j.1463-1326.2009.01167.x (2010)
7 OMAR BA, LIEHUA L, YAMADA Y, SEINO Y, MARCHETTI P, AHRÉN B. 2014. Dipeptidyl peptidase 4 (DPP-4) is expressed in mouse and human islets and its activity is decreased in human islets from individuals with type 2 diabetes. Diabetologia, 57 (9), pp. 1876-83
8 Jamie A. Cooper. Factors affecting circulating levels of peptide YY in humans: a comprehensive review Nutrition Reviews Volume 27, Issue 1 June 2014, pp. 186-197
9 Luo P, Yu H, Zhao X, Bao Y, Hong CS, Zhang P, Tu Y, Yin P, Gao P, Wei L, Zhuang Z, Jia W, Xu G. Metabolomics Study of Roux-en-Y Gastric Bypass Surgery (RYGB) to Treat Type 2 Diabetes Patients Based on Ultraperformance Liquid Chromatography-Mass Spectrometry. J Proteome Res. 2016 Apr 1;15(4):1288-99

Research Themes, Tools and Technologies


Additional Supervisors

Dr John Ryan, Senior Researcher in Gastroenterology, Translational Gastroenterology Unit, Nuffield Department of Medicine, University of Oxford

Dr Angus Jones, NIHR Clinician Scientist / Honorary Consultant Physician, University of Exeter 

Key Dates for October 2018 Entry

Application deadline:  12 noon GMT on Monday 8 January 2018

Interviews for shortlisted candidates: Week commencing Monday 22 January 

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How to apply

To apply for a place on the DPhil in Medical Sciences you will need to submit an application using the online application form.

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