Brown adipose tissue development and cardiovascular parameters in Gnasxl knock-out mice

Nicolas Nunn, Richard Barrett-Jolley, Antonius Plagge. University of Liverpool, Liverpool, United Kingdom.

Gnasxl encodes XLαs, an extra-large variant of the stimulatory G-protein α subunit (Gsα), which links G-protein coupled receptor activation to cAMP production. Its expression is restricted to a number of neuroendocrine tissues, including discrete areas of the brain (e.g. hypothalamus and medulla), as well as brown adipose tissue (BAT). Mice lacking XLαs have a complex phenotype, which includes failure to thrive postnatally, greatly reduced adiposity throughout life and, in adulthood, hypermetabolism, glucose tolerance and insulin sensitivity. Adult Gnasxl knock-outs show a BAT gene expression profile that indicates increased lipolysis, which, together with elevated levels of noradrenaline in urine, suggest sympathetic nervous system (SNS) hyperactivity. Our aim is to define the causes of the knock-out phenotype, in particular the role of the SNS, and to analyse potential differences in affected tissues between adult and neonatal stages. We found that in neonatal BAT of Gnasxl knock-out mice, UCP1 expression was significantly downregulated (to 48% of wild-type, p<0.001, n=14). This is in contrast to adult BAT, which has been shown to have increased UCP1 expression; since UCP1 is an SNS-inducible gene, this indicates that increased SNS activity might be limited to adults. Necdin was also shown to be downregulated (to 40% of wild-type, p<0.001, n=14); reduced expression of Necdin provides evidence for the paucity of BAT in Gnasxl-deficient neonates, as it has been shown to be a marker for proliferative brown preadipocytes and could potentially be an attenuator of apoptosis. In addition, preliminary data, showing increased numbers of activated Caspase-3 positive cells in neonatal BAT of Gnasxl knock-outs, hint at increased levels of apoptosis. Furthermore, to clarify whether any potential global changes to sympathetic nerve activity, apart from BAT innervation, occur as a consequence of XLαs deficiency, we are currently performing in vivo work to examine blood pressure and heart rate. Initial recordings of urethane-anaesthetised adult mice show an increased blood pressure in Gnasxl knock-outs (84.4 ± 6.3 mmHg in knock-outs; 63.9 ± 4.3 mmHg in wild-types, p<0.01 by ANOVA). The increase in blood pressure seen here, together with the increase in BAT lipolysis shown previously, is consistent with a global increase in SNS activity.