Synergistic Effects of Y2 and Y4 Receptors on Adiposity and Bone Mass Revealed in Double Knockout Mice

Abstract

Neuropeptide Y regulates numerous physiological processes via at least five different Y receptors, but the specific roles of each receptor are still unclear. We previously demonstrated that Y2 receptor knockout results in a lean phenotype, increased cancellous bone volume, and an increase in plasma pancreatic polypeptide (PP), a ligand for Y4 receptors. PP-overexpressing mice are also known to have a lean phenotype. Deletion of the Y4 receptor also produced a lean phenotype and increased plasma PP levels. We therefore hypothesized that part of the Y2 phenotype results from increased PP action on Y4 receptors and tested this in PP transgenic Y4 and Y2؊/؊ Y4 ؊/؊ double knockout mice. Bone mass was not altered in Y4 knockout mice. Surprisingly, despite significant hyperphagia, Y2؊/؊ Y4 ؊/؊ mice retained a markedly lean phenotype, with reduced body weight, white adipose tissue mass, leptinemia, and insulinemia. Furthermore, bone volume was also increased threefold in Y2 ؊/؊ Y4 ؊/؊ mice, and this was associated with enhanced osteoblastic activity. These changes were more pronounced than those observed in Y2 ؊/؊ mice, suggesting synergy between Y2 and Y4 receptor pathways. The lack of bone changes in PP transgenic mice suggests that PP alone is not responsible for the bone mass increases but might play a major role in the lean phenotype. However, a synergistic interaction between Y2 and Y4 pathways seems to regulate bone volume and adiposity and could have important implications for possible interventions in obesity and for anabolic treatment of osteoporotic bone loss.Neuropeptide Y (NPY) is an important regulator of numerous physiological processes, such as food intake (9, 43), energy homeostasis (22, 42), and growth and reproduction (2, 10). These processes are mediated at least partially within the hypothalamus, via five different Y receptors (Y1, Y2, Y4, Y5, and y6). All of these receptors are expressed in the hypothalamus, the brain stem, and peripheral tissues (12). Despite having highly divergent primary structures, different Y receptors have very similar pharmacological profiles, with few agonists and antagonists that are selective for specific Y receptors and active in vivo being presently available. Consequently, the specific roles of individual receptors and the mechanisms that mediate the diverse physiological effects of NPY are not yet clearly understood.Recent studies with germ line and conditional Y receptor knockout mouse models have begun to shed some light on the functions of different Y receptors. We have shown that both hypothalamus-specific and germ line Y2 receptor knockout resulted in a lean phenotype in male mice (reduced body weight and white adipose tissue [WAT] mass) (35) and a twofold increase in cancellous bone volume associated with accelerated bone formation (3). Consistent with the general inhibitory functions of NPY, it appears from these data that Y2 receptors directly or indirectly mediate the tonic stimulation of adipocyte activity while also mediating tonic inhibition of osteoblast...

Adult-onset PYY overexpression in mice reduces food intake and increases lipogenic capacity

“…For instance, lack of Y1 signalling promotes lipid oxidation (Henry et al, 2005 andZhang et al, 2010b), whereas lack of Y2 signalling appears to have opposite effects on oxidative fuel selection (Shi et al, 2010, Shi et al, 2011and Zhang et al, 2010c. Furthermore, Y1 receptor deficiency leads to pronounced hyperinsulinemia which is not observed in Y2 receptor deficient mice (Kushi et al, 1998, Sainsbury et al, 2003and Zhang et al, 2010b, suggesting that Y1 but not Y2 signalling plays an important role in regulating insulin secretion. Based on the differential effects of Y1 and Y2 signalling on energy and glucose homeostasis, and considering that PYY1-36 and PYY3-36 co-exist in the circulation in varying ratios depending on nutritional status (Grandt et al, 1994), PYY1-36 and PYY3-36 may act in a co-ordinate or synergistic manner to regulate energy and glucose metabolism in fasted and postprandial states.…”

Peptide YY (PYY) levels and bone mineral density (BMD) in women with anorexia nervosa

“…Recent data suggest that the hypothalamus plays a direct role in the regulation of BMD [1][2][3][4][5]. The discovery of the molecular circuitry that links leptin signaling to alterations in BMD initiated an active area of investigation [1].…”

“…However, a decline in NPY does not appear to be the mechanism for leptin's negative effect on BMD, as intracerebroventricular infusion of NPY also has a deleterious impact on BMD [1]. These data have inspired studies, primarily via knockout mouse models, to examine the role of NPY neuron signaling in the regulation of BMD [3,4].…”

“…These peptides bind with differing affinities to receptors of the Y-receptor family (Y1, Y2, Y3, Y4, and Y5 in humans, plus y6 in rodents) [10]. Whereas Y1 and Y5 receptors are believed to primarily mediate feeding behavior [11,12], studies of Y2 and Y4 receptors suggest a role in controlling BMD [3,4]. Y2 receptors are expressed on NPY-secreting neurons in the arcuate nucleus and are believed to act as autoreceptors to decrease the release of NPY [13][14][15].…”

Compensatory changes in [125I]-PYY binding in Y receptor knockout mice suggest the potential existence of further Y receptor(s)

“…Germ-line deletion of different Y receptor genes was achieved as previously described (Howell et al, 2003;Sainsbury et al, 2003Sainsbury et al, , 2002a. Animals were group-housed (2-3 animals per cage) and maintained under standard laboratory conditions with a 12:12 h light:dark schedule.…”