Background: INDY (I’m Not Dead Yet) is a plasma membrane citrate transporter and is highly expressed in liver and brain. In mammals, whole body deletion of the coding gene (mIndy, Slc13a5) increased energy expenditure and protected mice from diet-induced obesity and insulin resistance. Generation of neuronal mIndy-KO (NINKO) mice revealed improved insulin sensitivity (IS) in these mice, mediated through better hepatic IS. Gene expression studies in wildtype C57Bl/6 mice revealed high expression of mIndy in the hypothalamus (HTM), which is the main brain area regulating hepatic glucose metabolism. One mechanisms is the regulation through AMPK. Reduced hypothalamic AMPK phosphorylation is known to reduce hepatic glucose output, probably mediated via vagal nerve signaling.
Hypothesis: Hepatic glucose production is regulated via mIndy expression in the HTM.
Methods: Hyperinsulinemic-euglycemic clamp studies were performed to assess IS after hepatic vagotomy of NINKO mice. NestinCre+ controls were sham-denervated. Ex vivo brain slices of C57Bl/6 mice and the neuronal hypothalamic cell line CLU183 were acutely stimulated with 1 mM sodium citrate to investigate the effect of citrate on the brain and neurons.
Results: Acute stimulation of brain slices and CLU183 cells with citrate decreased cellular respiration (-19.7%, p≤0.01), ATP production (-37.7%, p≤0.01) and increased AMPK phosphorylation (+24.7%, p≤0.001). In line with that, AMPK phosphorylation in the HTM of NINKO mice was significantly reduced (-68.1%, p≤0.05). With hepatic branch vagotomy, no differences in hepatic IS could be observed anymore (suppression of basal hepatic glucose output (%); NestinCre+: 100.0±27.2, NINKO: 99.4±8.8).
Conclusion: These data suggest that neuronal mIndy is a critical regulator of glucose homeostasis in mammals, probably regulated via AMPK phosphorylation in the HTM and vagal nerve signaling. Further studies will address the exact mechanisms involved in the effect.
A. Kurzbach: None. M. Schell: None. D.M. Willmes: None. N. El-Agroudy: None. A. Kleinridders: None. A.L. Birkenfeld: None.