1 research outputs found
Origins and heterogeneity of adipose tissue: investigating the role of the Wilmsâ tumour 1 (Wt1) gene
Largely as a consequence of the ongoing obesity epidemic, research into adipose tissue
biology has increased substantially in recent years. Worldwide, the number of people
classed as overweight or obese is growing, and this represents a major public health
concern. Adipose tissue is broadly divided into two types; white and brown. Whilst
white adipose tissue (WAT) functions to store and mobilise triglycerides, brown
adipose tissue burns chemical energy to generate heat. WAT is further divided into
visceral âbadâ fat and subcutaneous âgoodâ fat depots, and it is an increase in the
former that is linked to obesity-associated diseases. As well as adipocytes, several
other cell types including haematopoietic and endothelial are found within adipose
tissue, and comprise the stromal vascular fraction (SVF). Adipocyte precursor cells
(APCs) also reside within the SVF and are essential for the maintenance and expansion
of adipose tissue.
The protein encoded by the Wilmsâ tumour 1 (Wt1) gene is predominantly known to
function as a transcription factor, but also has a role in post-transcriptional processing.
Deletion of Wt1 in adult mice results in a considerable loss of fat tissue. Moreover,
recent work has revealed that a proportion of the APCs from all visceral WAT depots
express Wt1, therefore revealing heterogeneity within the APC population.
Additionally, visceral WAT depots are encapsulated by a WT1 expressing mesothelial
layer, which has its origins in the lateral plate mesoderm (LPM), and can give rise to
mature adipocytes. Lineage tracing has demonstrated that a significant proportion of
the mature adipocytes in all adult visceral WAT depots (but not subcutaneous) are
derived from cells that express Wt1 in late gestation. These findings uncovered key
ontogenetic differences between visceral and subcutaneous WAT and led us to ask
whether Wt1 functions in visceral adipose tissue biology.
Preliminary work has shown that adipocytes derived from Wt1 expressing (Wt1+)
precursor cells have fewer, larger lipid droplets than those derived from non-Wt1
expressing (Wt1-) precursors. In this thesis, this heterogeneity is explored further using
a Wt1GFP/+ knock-in mouse. When Wt1+ and Wt1- APCs are cultured separately, the
Wt1+ population differentiate into adipocytes more readily. Moreover, the Wt1+ APCs
are more proliferative than the Wt1-. Preliminary results also suggest that the Wt1+
APCs may secrete a factor(s) that causes the Wt1- APCs to exhibit improved
adipogenic differentiation, a result that is supported by data from comparative
transcriptomic analysis. Finally, the percentage of APCs decreases when mice are fed
a high fat diet. Interestingly, this decrease is more pronounced for the Wt1+ population.
Therefore, it appears that as well as exhibiting differing behaviours in vitro, the Wt1+
and Wt1- populations respond differently to physiologically relevant conditions in
vivo.
Whilst the LPM is a major source of visceral WAT, the origin of subcutaneous WAT
is currently unknown. Here, the Prx1-Cre and Prx1-CreERT2 mouse lines are used to
investigate this. It is shown that the majority of subcutaneous WAT adipocytes and
APCs are labelled by Prx1-Cre, however this is not the case for most of the visceral
WAT depots. The exception to this is the pericardial (heart fat) depot, in which
approximately 70% of the adipocytes and 40% of the APCs are labelled. Moreover, a
proportion of the Prx1-Cre labelled pericardial APCs also express Wt1, therefore
suggesting additional heterogeneity. Preliminary results show that this heterogeneity
may have functional consequences, at least in vitro. Additionally, lineage tracing
studies suggest that the somatic LPM may be one source of subcutaneous WAT and
pericardial visceral WAT
Finally, it is shown that the conditional deletion of Wt1 in the Prx1-Cre lineage results
in abnormal diaphragm development. Congenital diaphragmatic hernia (CDH) is
severe birth defect, the etiology of which is not well understood. Here, a new model
of CDH has been developed, and the cellular and molecular mechanisms responsible
for the defect in this model are investigated