岸 敏郎 先生 Toshiro Kishi, MD, PhD

島根医大精神医学講座

Department of Neurology and Program in Neuroscience

Beth Israel Deaconess Medical Center , Harvard Medical School


演題: 

Anatomy of central melanocortin and neuropeptide Y systems controlling food

intake and body weight.


要旨:

The melanocortin-4 receptor (MC4-R), expressed exclusively in the central nervous system (CNS), is now established as a fundamental regulator of food intake and body weight. For example, MC4-R deletion or mutation produces obesity, hyperphagia, and insulin resistance in rodents and humans. Accumulating evidence suggests that leptin, an adipocyte-derived hormone, activates sympathetic outflow via the central melanocortin system, resulting in several coordinated responses including increased energy expenditure. Therefore, anatomic analyses of melanocortin-autonomic pathways are significant to better understand neural bases underlying energy homeostasis and leptin action.

The arcuate hypothalamic nucleus (Arc) is a primary action site of leptin, and contains two distinct populations of leptin-responsive neurons producing distinct melanocortins; i.e., -melanocyte-stimulating hormone (-MSH, a MC3/4-R agonist) and agouti-related protein (AgRP, a MC3/4-R antagonist). Using in situ hybridization histochemistry (ISHH), we observed MC4-R mRNA in a number of rat CNS sites receiving -MSH- and/or AgRP axons, such as the paraventricular hypothalamic nucleus (PVH), perifornical area, dorsal motor nucleus of the vagus (DMV), and the intermediolateral nucleus of the spinal cord (IML). These observations suggest a post-synaptic action of melanocortins on MC4-Rs in key autonomic control sites. Moreover, our dual-label ISHH study demonstrated MC4-R mRNA expression in cholinergic autonomic preganglionic neurons in the DMV and IML. Notably, our laboratory previously demonstrated leptin-activated Arc -MSH neurons directly projecting to the IML. The expression of MC4-R mRNA in cholinergic IML cells also suggests the existence of an Arc-IML pathway through which leptin may activate sympathetic outflow.

To chemically define MC4-R cells, we have also used a transgenic mouse model in which green fluorescent protein (GFP) is produced under the control of the MC4-R promoter. This model revealed GFP expression marking MC4-Rs in oxytocin-producing PVH cells and cholinergic DMV cells. Moreover, we observed cells co-expressing GFP and GAD67 in the perifornical area that provides GABAergic inputs to the PVH. This observation supports a previous electrophysiological study indicating that -MSH acts pre-synaptically to increase GABA release in the PVH.

Neuropeptide Y (NPY) is another established regulator of energy balance, which exerts a potent stimulatory effect on feeding behavior. Using the MC4-R-GFP transgenic mice, we found PVH cells co-expressing GFP and NPY Y-1 receptor (Y1-R) mRNA. It is well known that Arc NPY neurons co-express AgRP and that NPY- and AgRP fibers densely terminate in the PVH. Taken together, our data supports a proposed model that subsets of PVH neurons integrate NPY- and melanocortin inputs from the Arc to coordinate autonomic, endocrine, and behavioral responses to changes in energy availability.

Finally, we have generated a lox-modified MC4-R -/- mouse model to determine specific MC4-R-expressing CNS sites critical for energy metabolism. The null allele contains lox-P sites flanking a transcription termination cassette so that MC4-R expression can be re-activated by expression of Cre-recombinase (Cre). To direct Cre expression in a site-specific manner, we have used an adeno-associated viral vector containing the Cre gene (AAV-Cre). Indeed, we have observed that stereotaxic injections of AAV-Cre re-activate MC4-R mRNA in this mouse model. We are currently assessing the effects of AAV-Cre injections into the candidate MC4-R-expressing sites on the obese MC4-R-null phenotype.

We believe that the use of multidisciplinary molecular anatomic techniques illustrated above will elucidate CNS pathways regulating food intake and body weight, and thus will facilitate pharmaceutical intervention for treating obesity and associated comorbidities such as diabetes mellitus.

 
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