NPY Neuron-Specific Y2 Receptors Regulate Adipose Tissue and Trabecular Bone but Not Cortical Bone Homeostasis in Mice

BACKGROUND: Y2 receptor signalling is known to be important in neuropeptide Y (NPY)-mediated effects on energy homeostasis and bone physiology. Y2 receptors are located post-synaptically as well as acting as auto receptors on NPY-expressing neurons, and the different roles of these two populations of Y2 receptors in the regulation of energy homeostasis and body composition are unclear. METHODOLOGY/PRINCIPAL FINDINGS: We thus generated two conditional knockout mouse models, Y2(lox/lox) and NPYCre/+;Y2(lox/lox), in which Y2 receptors can be selectively ablated either in the hypothalamus or specifically in hypothalamic NPY-producing neurons of adult mice. Specific deletion of hypothalamic Y2 receptors increases food intake and body weight compared to controls. Importantly, specific ablation of hypothalamic Y2 receptors on NPY-containing neurons results in a significantly greater adiposity in female but not male mice, accompanied by increased hepatic triglyceride levels, decreased expression of liver carnitine palmitoyltransferase (CPT1) and increased expression of muscle phosphorylated acetyl-CoA carboxylase (ACC). While food intake, body weight, femur length, bone mineral content, density and cortical bone volume and thickness are not significantly altered, trabecular bone volume and number were significantly increased by hypothalamic Y2 deletion on NPY-expressing neurons. Interestingly, in situ hybridisation reveals increased NPY and decreased proopiomelanocortin (POMC) mRNA expression in the arcuate nucleus of mice with hypothalamus-specific deletion of Y2 receptors in NPY neurons, consistent with a negative feedback mechanism between NPY expression and Y2 receptors on NPY-ergic neurons. CONCLUSIONS/SIGNIFICANCE: Taken together these data demonstrate the anti-obesogenic role of Y2 receptors in the brain, notably on NPY-ergic neurons, possibly via inhibition of NPY neurons and concomitant stimulation of POMC-expressing neurons in the arcuate nucleus of the hypothalamus, reducing lipogenic pathways in liver and/or skeletal muscle in females. These data also reveal as an anti-osteogenic effect of Y2 receptors on hypothalamic NPY-expressing neurons on trabecular but not on cortical bone

Neuropeptide Y knockout mice reveal a central role of NPY in the coordination of bone mass to body weight

Changes in whole body energy levels are closely linked to alterations in body weight and bone mass. Here, we show that hypothalamic signals contribute to the regulation of bone mass in a manner consistent with the central perception of energy status. Mice lacking neuropeptide Y (NPY), a well-known orexigenic factor whose hypothalamic expression is increased in fasting, have significantly increased bone mass in association with enhanced osteoblast activity and elevated expression of bone osteogenic transcription factors, Runx2 and Osterix. In contrast, wild type and NPY knockout (NPY (-/-)) mice in which NPY is specifically over expressed in the hypothalamus (AAV-NPY+) show a significant reduction in bone mass despite developing an obese phenotype. The AAV-NPY+ induced loss of bone mass is consistent with models known to mimic the central effects of fasting, which also show increased hypothalamic NPY levels. Thus these data indicate that, in addition to well characterized responses to body mass, skeletal tissue also responds to the perception of nutritional status by the hypothalamus independently of body weight. In addition, the reduction in bone mass by AAV NPY+ administration does not completely correct the high bone mass phenotype of NPY (-/-) mice, indicating the possibility that peripheral NPY may also be an important regulator of bone mass. Indeed, we demonstrate the expression of NPY specifically in osteoblasts. In conclusion, these data identifies NPY as a critical integrator of bone homeostatic signals; increasing bone mass during times of obesity when hypothalamic NPY expression levels are low and reducing bone formation to conserve energy under 'starving' conditions, when hypothalamic NPY expression levels are high

Effect of deleting Y2 receptors from hypothalamic NPY-expressing neurons on body weight change, feeding parameters, rectal temperature, oxygen consumption (VO2), respiratory exchange ratio (RER) and physical activity as determined during indirect calorimetry.

<p>Values are means ± SEM of 5–12 wild type or NPYCre/+;Y2^lox/lox mice injected into the hypothalamus with either saline or doxycyline (Dox). The number of mice in each group is indicated in parentheses.</p>#<p>p<0.05 versus sex-matched saline-injected WT mice.</p><p>*p<0.05 versus sex-matched saline-injected NPYCre/+;Y2^lox/lox mice. Body weight change is expressed as a percentage of initial body weight after 2 days in the indirect calorimetry chambers.</p

NPY neuron-specific Y2 deletion in the hypothalamus increases NPY and decreases POMC expression in the arcuate nucleus with no significant effects on body weight, spontaneous or fasting-induced food intake.

<p>(A, C) Bright-field photomicrographs of coronal brain sections, including the Arc, obtained from saline-injected control and doxycycline (Dox)-injected male NPYCre/+;Y2^−/− mice after <i>in situ</i> hybridization for NPY and proopiomelanocortin (POMC) mRNA. Scale bar, 40 µm. (B, D) Quantification of mean labeling intensity of neurons from <i>in situ</i> hybridization, given as percentage coverage of neuronal surface by silvergrains ± SEM of 5 male mice per group. *: p<0.05 versus saline-injected control mice. Arc: Arcuate nucleus of the hypothalamus; 3V: third cerebral ventricle. Spontaneous food intake (E, F), cumulative fasting-induced food intake (G, H) and body weight (I, J) are not altered by NPY neuron-specific Y2 receptor deletion in the hypothalamus. Data are mean ± SEM of 5 or more mice per group.</p