Characterisation of the role of mitochondrial translocator protein 18kDa (TSPO) in the regulation of energy homeostasis

Abstract

Obesity is a chronic condition where the body’s ability to regulate energy balance is compromised. Prolonged consumption of saturated fatty acids triggers inflammation throughout the body, including the brain. Within the brain, astrocytes and microglia respond to nutrients and inflammatory signals with reactive gliosis. The mitochondrial translocator protein of 18 kDa (TSPO) is used to mark reactive glia. However, the function of TSPO – or its relevance to gliosis - is not well-understood. In vitro studies suggest involvement in mitochondrial metabolism, including altering substrate use. In vivo work suggests a role for TSPO modulating systemic glucose homeostasis. The hypothesis underlying my project was: TSPO is involved in metabolic flexibility and is regulated in states of energy imbalance, and manipulation of TSPO expression will alter energy homeostasis. I validated a common TSPO antibody, which proved it to be unreliable for immunohistochemical characterisation of TSPO in the mouse brain. Comparison between brain tissue taken from TSPO knock-out (-/-) and wild-type mice indicated a low level of immunoreactivity throughout the mouse brain that was specific. This included immunoreactivity around the ventricles of the brain that was attributed to tanycytes. However, the majority of the immunoreactivity was not specific to TSPO and confounds results that use this antibody. I characterised the metabolic phenotype of the germline global TSPO -/- mice. These mice did not exhibit differences in body weight or food intake in response to an overnight fast compared to littermate controls. Male TSPO -/- mice consumed less high-fat diet than controls in the first week of exposure, but there were no long-term differences. Basal blood glucose levels or glucose clearance in the glucose tolerance test were also unaffected by genotype, though female TSPO -/- mice may have enhanced protection against diet-induced loss of glucose tolerance compared to wild-type. These data were consistent with experiments using PK11195, a TSPO ligand. These findings are important for comparisons across the literature that involve different knock-out mouse models. In conclusion, global modulation of TSPO does not impact energy homeostasis on the organismal level and its functions are likely cell-type specific. Therefore, systemic modulation of TSPO signalling it is unlikely to offer translational potential in treating obesity.