Energetics of African trypanosome infection

This project seeks to understand the physiological basis of cachexia in African trypanosomiasis and its relationship to appetite control and body-weight regulation. African trypanosome infections cause cachexia in livestock and experimental animals. Although increased catabolism triggered by inflammatory cytokines is one factor in this condition, it is certainly not the only one and almost nothing is known about the energetic basis of cachexia in trypanosomiasis raising fundamental questions such as whether infected-hosts undergo suppression of appetite, increased resting metabolic rate, or reduced digestive efficiency. This project will use a mouse model of trypanosome infection and define longitudinal changes in energy budgets in relation to parasitaemia and body-weight. A preliminary study in our lab has already demonstrated that weight loss is directly attributable to anorexia that is most profound during a period lagging about 5 days beyond peak parasitaemia, and from which animals recover without any hyperphagic response. Therefore it appears that cachexia in trypansomiasis involves a loss of appetite. The fact that this inappetance lags beyond peak parasitemia suggests that it is not simply a consequence of malaise, but that the infection is interfering with specific cytokine and endocrine regulatory circuits controlling appetite. Therefore experimental trypanosome infections have the potential to offer new insights into the fundamental physiological mechanisms of appetite regulation. The project will involve three phases. 1. Using a standardised infection-host model, detailed measurements will be made of the components of energy balance including changes in food intake, energy excretion and hence assimilation efficiency, resting metabolic rate, body temperature and physical activity. These will be monitored using a combination of oxymax calorimetry (CLAMS) and vital view (Minimitter Inc) measures of telemetered body temperature and activity. The time course of changes in energy balance will be linked to the record of parasitamaemia. This will allow us to determine the primary disrupted aspects of energy balance that accompany the infection. 2. We will sacrifice mice at various stages in the infection. These will be used to generate tissues that will be used to further explore the mechanisms underpinning the disturbances in energy balance. In particular we will use in situ hybridisation of three key areas in the brain (the hypothalamus, the nucleus accumbens and the brain stem - including the VTA and nucleus of the solitary tract). In these areas we will explore the changes in gene expression of key neuropeptides know to mediate energy balance responses. These will include in the hypothalamus - neuropeptide Y, agouti-regulated protein, Pro-opiomelanocortin, Melanocortin 4 receptor, Suppressor of cytokine signalling 3, Leptin receptor, Orexin A and TNFalpha receptor 1. In the nucleus accumbens we will measure tyrosine hydroxylase and the dopamine receptors (DR1 and DR2) as well as the mu-opioid receptor. In the NTS we will measure leptin receptor and CCK receptors. This will cover aspects of both the hedonic and homeostatic control mechanisms known to control food intake and energy expenditure. 3. The parasite may exert its action through secretions that interact directly with the host regulation system or indirectly affect signalling molecules in the periphery that have the centrally mediated effect. We will use using fractionated conditioned medium in which trypanosomes have been raised inoculated at concentrations adjusted to parasitaemia equivalent load to explore the possibility that the parasite produces compounds that cause the changes characterised under phases 2 and 3. We will characterise these media and plasma from infected mice to identify mediators of anorexia using metabolomics and proteomics. This latter aspect provides the most prospect for novel target discovery.