Origins and heterogeneity of adipose tissue: investigating the role of the Wilms’ tumour 1 (Wt1) gene

Field: Biochemistry, Genetics and Molecular Biology (Life Sciences)

Largely as a consequence of the ongoing obesity epidemic, research into adipose tissue \nbiology has increased substantially in recent years. Worldwide, the number of people \nclassed as overweight or obese is growing, and this represents a major public health \nconcern. Adipose tissue is broadly divided into two types; white and brown. Whilst \nwhite adipose tissue (WAT) functions to store and mobilise triglycerides, brown \nadipose tissue burns chemical energy to generate heat. WAT is further divided into \nvisceral “bad” fat and subcutaneous “good” fat depots, and it is an increase in the \nformer that is linked to obesity-associated diseases. As well as adipocytes, several \nother cell types including haematopoietic and endothelial are found within adipose \ntissue, and comprise the stromal vascular fraction (SVF). Adipocyte precursor cells \n(APCs) also reside within the SVF and are essential for the maintenance and expansion \nof adipose tissue. \n \nThe protein encoded by the Wilms’ tumour 1 (Wt1) gene is predominantly known to \nfunction as a transcription factor, but also has a role in post-transcriptional processing. \nDeletion of Wt1 in adult mice results in a considerable loss of fat tissue. Moreover, \nrecent work has revealed that a proportion of the APCs from all visceral WAT depots \nexpress Wt1, therefore revealing heterogeneity within the APC population. \nAdditionally, visceral WAT depots are encapsulated by a WT1 expressing mesothelial \nlayer, which has its origins in the lateral plate mesoderm (LPM), and can give rise to \nmature adipocytes. Lineage tracing has demonstrated that a significant proportion of \nthe mature adipocytes in all adult visceral WAT depots (but not subcutaneous) are \nderived from cells that express Wt1 in late gestation. These findings uncovered key \nontogenetic differences between visceral and subcutaneous WAT and led us to ask \nwhether Wt1 functions in visceral adipose tissue biology. \n \nPreliminary work has shown that adipocytes derived from Wt1 expressing (Wt1+) \nprecursor cells have fewer, larger lipid droplets than those derived from non-Wt1 \nexpressing (Wt1-) precursors. In this thesis, this heterogeneity is explored further using \na Wt1GFP/+ knock-in mouse. When Wt1+ and Wt1- APCs are cultured separately, the \nWt1+ population differentiate into adipocytes more readily. Moreover, the Wt1+ APCs \nare more proliferative than the Wt1-. Preliminary results also suggest that the Wt1+ \nAPCs may secrete a factor(s) that causes the Wt1- APCs to exhibit improved \nadipogenic differentiation, a result that is supported by data from comparative \ntranscriptomic analysis. Finally, the percentage of APCs decreases when mice are fed \na high fat diet. Interestingly, this decrease is more pronounced for the Wt1+ population. \nTherefore, it appears that as well as exhibiting differing behaviours in vitro, the Wt1+ \nand Wt1- populations respond differently to physiologically relevant conditions in \nvivo. \n \nWhilst the LPM is a major source of visceral WAT, the origin of subcutaneous WAT \nis currently unknown. Here, the Prx1-Cre and Prx1-CreERT2 mouse lines are used to \ninvestigate this. It is shown that the majority of subcutaneous WAT adipocytes and \nAPCs are labelled by Prx1-Cre, however this is not the case for most of the visceral \nWAT depots. The exception to this is the pericardial (heart fat) depot, in which \napproximately 70% of the adipocytes and 40% of the APCs are labelled. Moreover, a \nproportion of the Prx1-Cre labelled pericardial APCs also express Wt1, therefore \nsuggesting additional heterogeneity. Preliminary results show that this heterogeneity \nmay have functional consequences, at least in vitro. Additionally, lineage tracing \nstudies suggest that the somatic LPM may be one source of subcutaneous WAT and \npericardial visceral WAT \n \nFinally, it is shown that the conditional deletion of Wt1 in the Prx1-Cre lineage results \nin abnormal diaphragm development. Congenital diaphragmatic hernia (CDH) is \nsevere birth defect, the etiology of which is not well understood. Here, a new model \nof CDH has been developed, and the cellular and molecular mechanisms responsible \nfor the defect in this model are investigated.