Rasmus Klinck:
The role of Hes1 in pancreas development. Expression, Interdependancy and Notch signalling

Date: 31-05-2011    Supervisor: Olaf Nielsen

Diabetes is becoming a worldwide epidemic due to increased living standards throughout the world. Although the disease is treatable with a wide variety of insulins, affordable by most, complications will inevitably arise, and on average the disease takes six years off the average life expectancy in a western European country. Therefore, the medical industry is looking for a cure or a treatment that will regulate the blood glucose levels of the diabetic patient much tighter than what is currently possible. The most prevailing strategies seen from a basic research point is currently the use of; directed differentiation of stem cells, and induced β-cell regeneration. To eventually be able to either differentiate stem cells or discover a drugable target that will lead to an increase in β-cell numbers, it is necessary to know how the pancreas is developed and what network of factors that are at play during both development and disease.

Based on several transgenic mouse lines and immonuhistochemical techniques, this work applies basic research to uncover a small part of the transcription factor network necessary for correct pancreatic development. The work focuses on the transcriptional repressor, Hairy Enhancer of Split 1 (Hes1), in relation to the formation of the pancreas and the pancreatic cell types.

In manuscript I we use a novel BAC transgenic mouse reporter line, Tg(Hes1-EGFP)1Hri, to analyze Hes1 expression from embryonic day 7.0 (E7.0) to postnatal day 8 (P8). At E7.0 Hes1 is expressed exclusively in the definitive endoderm. At E8.5 in the posterior endoderm Hes1 is expressed with a distinct boundary at the 4th somite pair. Anteriorly, Hes1 is expressed in the anterior intestinal portal (AIP). After gut tube closure, Hes1 is expressed in the dorsal part of the gut tube, including the pancreatic bud epithelium. Later at E10.5 we describe Hes1 expression in both the dorsal and ventral pancreatic epithelium. At later stages, Hes1 is expressed in cell types of the developing intestine, kidney, lung, and pancreas, matching known expression patterns.

Manuscript II focuses on the Hes1-null mutant phenotype in relation to pancreatic organogenesis. In the Hes1-null mutant the pancreas is hypoplastic and demonstrates excessive endocrine differentiation of primarily glucagon expressing cells. The endocrine cells are also ectopic and can be found along the dorsal stomach extending anterior from the pancreas. The pro-endocrine factor Neurog3 is necessary to confer the aberrant morphogenesis which is caused by excessive Neurog3 mediated migration rather than a patterning defect.

Manuscript III discusses Hes1 in relation to the well described Notch signalling pathway. It has been documented that Notch signalling and its down-stream target Hes1, regulate pancreatic endocrine development, possibly through lateral inhibition, where Neurog3 expressing endocrine progenitors via Dll1, instruct neighboring cells not to acquire an endocrine fate via Hes1 inhibition of Neurog3. Although Notch indeed is active during the early specification of the pancreas, we elaborate on the interdependency of Hes1, Dll1 and Ptf1a. We show that Dll1 expression is dependent on Ptf1a in MPCs. Hes1 is required for Dll1 and Ptf1 expression during a brief time window, and Hes1 expression is independent of Dll1 mediated Notch signalling up until E10.5.

Together, these results provide new insight in to the expression and function of Hes1 during early pancreatic development and provide the scientific field with a new tool for further investigating the function of Hes1.