Transcriptomic models of brown versus white adipocytes: targeting cell fate

Fat tissue in mammals exists in two forms, white adipose tissue which stores energy reserves in the form of triglycerides, and brown adipose tissue which combusts calories so as to generate heat to help defend body temperature in a cold environment and to help defend body weight in a situation of excess caloric intake. While brown adipose tissue has earlier been considered active only in neonatal mammals and in small adult mammals, it has recently become evident that active brown adipose tissue exists in most adult humans. Expanding brown adipose tissue mass can thus be of potential therapeutic interest as a means of manipulating body mass in obesity by increasing caloric expenditure. It is therefore of considerable importance that the regulation of differentiation of the two adipose tissues is understood. We have recently found that the precursor cells of these two fat storing cell types can be distinguishable, prior to them becoming a white or a brown fat cell. It is through the use of this information that we can understand the control of the differentiation process and potentially how we can regulate these processes to increase the amount of brown adipose tissue. We generated microarray data to make these discoveries. This is a tool to capture the expression level of all genes in one experiment, producing 20,000 data points for each study. It is of utmost importance for cell function that gene expression levels can be accurately regulated. For this, the cell has numerous means of controlling expression levels, with a recent addition for the regulation of gene expression levels being the ability to inhibit the translation of the gene message into protein by the synthesis by microRNA molecules. microRNAs are RNA-only genes that can block the translation of multiple genes (so preventing protein expression). In preliminary experiments, we have found that some of the microRNAs present in the brown and white adipocytes are turned on and off during the differentiation process when the cells develop from immature to mature cells. We will continue these studies of miRNA regulation during differentiation with the aim of obtaining tools to influence and possibly redirect these developmental processes. Following the completion of the gene expression studies in brown and white fat cells, we aim to study the effects of manipulating the levels of genes specific to brown or white adipocytes. We will determine the ability of the fat cell to maintain it's phenotype in the absence of one or more of its marker genes. Studies will be performed both in cell lines that are as representative as possible for these normal cells and also in precursor cells derived directly from tissue. These robust studies will allow us to work out ways through a substantial list of interesting genes, and provide the best guidance as to which genes and mechanisms to focus on for future research. The original study that provided evidence for the distinction between brown and white adipocytes has allowed for the creation of a data base containing information on the expression levels of numerous genes of interest specifying a brown or a white phnotype. Further studies will investigate the effects of stimulation by the natural nervous system transmitter substance noradrenaline. Noradrenaline is the agent which activates heat production in brown adipocytes and also helps to regulate the number of cells found under different circumstances. These comparisons will pin-point genes that are activated in brown adipocytes and enhance caloric expenditure, and these genes could provide as potential targets for reversing obesity in humans. Our data base of results still holds large quantities of information that require further analysis, which will also be pursued within the project. This can give information about means by which the brown adipose tissue can be activated as a possibility to decrease body weight.

We have recently demonstrated that brown and white adipocytes derive from distinct cell lineages and that a myogenic gene signature is associated with the brown progenitors but only at an early stage of their differentiation. This suggests that the white progenitors branch off from the common precursor prior to the distinction between brown progenitors and muscle progenitors. The current proposal is designed to extend these findings by studies in three sub-programmes: 1) the genes which were identified as being cell type-specific will be over-expressed/silenced in the progenitors. The studies will be performed initially in verified cell lines and subsequently in primary cultures of the cells derived from mouse tissues. 2) the extensive data base which has been generated from the microarray study will be further enhanced through completion of miRNA profiling and then analysed to identify potential gene-networks of interest (identifying mRNA, protein and non-coding RNA points of regulation) involved in both cell proliferation and responsiveness to the sympathetic transmitter norepinephrine, the physiological agent to stimulate brown adipocyte thermogenesis and white adipocyte lipolysis; 3) microRNA profiles have been obtained from both types of progenitor cells during their development. We will study the influence of manipulation of novel non-coding RNAs on mRNA and translation of predicted gene targets to provide a robust validation from dry-lab to wet-lab. These programmes of work deliver a novel and robust data-base for the biosciences community, they identify novel regulators of progenitor cell phenotype and differentiation and valuable wet-lab data to aid the development of bioinformatic tools for the non-coding RNA fields.