Reduced metabolism in the hypothalamus of the anorectic anx/anx mouse.

The anorectic anx/anx mouse exhibits a mitochondrial complex I dysfunction that is related to aberrant expression of hypothalamic neuropeptides and transmitters regulating food intake. Hypothalamic activity, i.e. neuronal firing and transmitter release, is dependent on glucose utilization and energy metabolism. To better understand the role of hypothalamic activity in anorexia, we assessed carbohydrate and high-energy phosphate metabolism, in vivo and in vitro, in the anx/anx hypothalamus. In the fasted state, hypothalamic glucose uptake in the anx/anx mouse was reduced by ~50% of that seen in wild-type (wt) mice (P < 0.05). Under basal conditions, anx/anx hypothalamus ATP and glucose 6-P contents were similar to those in wt hypothalamus, whereas phosphocreatine was elevated (~2-fold; P < 0.001) and lactate was reduced (~35%; P < 0.001). The anx/anx hypothalamus had elevated total AMPK (~25%; P < 0.05) and GLUT4 (~60%; P < 0.01) protein contents, whereas GLUT1 and GLUT3 were similar to that of wt hypothalamus. Interestingly, the activation state of AMPK (ratio of phosphorylated AMPK/total AMPK) was significantly decreased in hypothalamus of the anx/anx mouse (~60% of that in wt; P < 0.05). Finally, during metabolic stress (ischemia), accumulation of lactate (measure of glycolysis) and IMP and AMP (breakdown products of ATP) were ~50% lower in anx/anx vs wt hypothalamus. These data demonstrate that carbohydrate and high-energy phosphate utilization in the anx/anx hypothalamus are diminished under basal and stress conditions. The decrease in hypothalamic metabolism may contribute to the anorectic behavior of the anx/anx mouse, i.e. its inability to regulate food intake in accordance with energy status.

Anorexia and hypothalamic degeneration.

Anorexia, meaning poor appetite, occurs in many human conditions, for example, anorexia nervosa, cachexia, and failure to thrive in infants. A key player in the regulation of appetite/food intake in general, as well as conditions of anorexia, is the hypothalamus, in particular, the AGRP/NPY and POMC/CART neurons in the arcuate nucleus. In this chapter, we review the hypothalamic aberrances seen in the anorectic anx/anx mouse. This mouse displays deviations in neuropeptidergic/-transmitter systems, including selective hypothalamic degeneration and inflammation that have been associated with mitochondrial dysfunction. In addition, we discuss data from other animal models, as well as clinical data relating hypothalamic inflammation/degeneration, neurogenesis, and mitochondrial dysfunction to conditions of disturbed regulation of food intake.

Hypothalamic mitochondrial dysfunction associated with anorexia in the anx/anx mouse.

The anorectic anx/anx mouse exhibits disturbed feeding behavior and aberrances, including neurodegeneration, in peptidergic neurons in the appetite regulating hypothalamic arcuate nucleus. Poor feeding in infants, as well as neurodegeneration, are common phenotypes in human disorders caused by dysfunction of the mitochondrial oxidative phosphorylation system (OXPHOS). We therefore hypothesized that the anorexia and degenerative phenotypes in the anx/anx mouse could be related to defects in the OXPHOS. In this study, we found reduced efficiency of hypothalamic OXPHOS complex I assembly and activity in the anx/anx mouse. We also recorded signs of increased oxidative stress in anx/anx hypothalamus, possibly as an effect of the decreased hypothalamic levels of fully assembled complex I, that were demonstrated by native Western blots. Furthermore, the Ndufaf1 gene, encoding a complex I assembly factor, was genetically mapped to the anx interval and found to be down-regulated in anx/anx mice. These results suggest that the anorexia and hypothalamic neurodegeneration of the anx/anx mouse are associated with dysfunction of mitochondrial complex I.

Evidence of hypothalamic degeneration in the anorectic anx/anx mouse.

Mice homozygous for the anorexia (anx) mutation are characterized by poor food intake and death by three to five weeks after birth. By P21 these mice display lower density of hypothalamic neuropeptides, including Agouti gene-related protein (AGRP). The AGRP/neuropeptide Y (NPY) system of the anx/anx mice develops normally until postnatal day (P) 12, then the normal increase in fiber density ceases, in some areas even distinctly decreases. This overlaps with activation of microglia, indicating an inflammatory and/or degenerative process. Here we studied, by in situ hybridization and immunohistochemistry (IHC), the expression of major histocompatibility complex (MHC) class I-related molecules and markers for cellular reactivity in hypothalamus of anx/anx mice. MHC class I transcript and -related proteins were found in arcuate nucleus (Arc), presumably both in neurons and glia, the latter also in areas innervated by AGRP (NPY) neurons. In the anx/anx hypothalamus, using TUNEL labeling, significantly higher number of apoptotic cells were found compared with +/+ mice, and active caspase 6 immunoreactivity was detected in degenerating NPY-fibers as well as signs of "microglia-associated cell death". In addition, Y1 receptor-labeled processes and soma of pro-opiomelanocortin (POMC) neurons, were markedly decreased at P21. These results support the hypothesis of degeneration of hypothalamic arcuate neuron populations in the anx/anx mice, whereby the AGRP system may be first affected, the changes in the POMC system being secondary in this process.