NPY Signalling Pathway in Bone Homeostasis: Y1 Receptor as a Potential Drug Target

Exploring the effect of the "quaternary regulation" theory of "peripheral nerve-angiogenesis-osteoclast-osteogenesis" on osteoporosis based on neuropeptides

Osteoporosis is a common bone metabolic disease among the middle-aged and elderly, with its high incidence rate and a major cause of disability and mortality. Early studies found that bone metabolic homeostasis is achieved through osteogenesis-osteoclast coupling. Although current anti-osteoporosis drugs can attenuate bone loss caused by aging, they present specific side effects. With the discovery of CD31^hi Emcn^hi blood vessels in 2014, the effect of H-type blood vessels on bone metabolism has been valued by researchers, and the ternary regulation theory of bone metabolism of "Angiogenesis-Osteoclast-Osteogenesis" has also been recognized. Nowadays, more studies have confirmed that peripheral nerves substantially impact bone metabolism. However, due to the complex function of peripheral nerves, the crosstalk mechanism of "Peripheral nerve-Angiogenesis-Osteoclast-Osteogenesis" has not yet been fully revealed. Neuropeptide serves as signaling molecules secreted by peripheral nerves that regulate blood vessels, osteoblasts, and osteoclasts' functions. It is likely to be the breakthrough point of the quaternary regulation theory of "Peripheral nerve-Angiogenesis-Osteoclast-Osteogenesis". Here, we discuss the effect of peripheral nerves on osteoporosis based on neuropeptides.

Differential regulation of NPY and SP receptor expression in STRO-1+ve PDLSCs by inflammatory cytokines

OBJECTIVES

The aims of this study were to investigate neuropeptide receptor expression regulation on STRO-1 +ve periodontal ligament stem cells (PDLSCs) in response to inflammatory cytokines and to investigate a potential osteogenic effect of neuropeptides.

BACKGROUND

Nerve fibres innervating the periodontal tissues in humans contain several neuropeptides including neuropeptide Y and substance P. The role of neuropeptide receptors on PDLSCs, including their response to the local inflammatory environment of periodontitis, is currently unknown.

METHODS

A homogenous population of STRO-1 +ve PDLSCs was prepared by immunomagnetic separation of cells obtained by the tissue out-growth method from healthy premolar teeth from a single donor. Regulation of gene expression of the neuropeptide Y Y1 receptor and substance P receptor tachykinin receptor 1 was investigated. A potential osteogenic effect of neuropeptide Y and substance P was also investigated by measuring alkaline phosphatase (ALP) activity, Alizarin red staining and quantifying osteogenic gene expression.

RESULTS

Treatment of STRO-1 +ve PDLSCs with tumour necrosis factor-alpha or interleukin 1-beta up-regulated the expression of the neuropeptide Y's Y1 receptor, but down-regulated substance P's receptor. Significantly increased ALP activity was observed in STRO-1 +ve PDLSCs treated with neuropeptide Y but not substance P. Further studies showed that neuropeptide Y had a modest osteogenic effect on cells at both a functional level and a gene level.

CONCLUSIONS

Expression of the neuropeptide Y Y1 receptor gene on STRO-1 +ve PDLSCs was sensitive to local inflammatory cytokines. Treatment of cells with neuropeptide Y was found to produce a modest enhanced osteogenic effect.

Neuropeptide Y1 receptor antagonist but not neuropeptide Y itself increased bone mineral density when locally injected with hyaluronic acid in male Wistar rats

Background/aim

The nervous system controls bone mass via both the central (CNS) and the peripheral (PNS) nervous systems. Intriguingly, neuropeptide Y (NPY) signaling occurs in both. Less is known on how the PNS stimulated NPY signaling controls bone metabolism. The objective of this study was to evaluate whether NPY or NPY1 receptor antagonist changes local bone mineral density (BMD) when injected into a Wistar rat tibia.

Materials and methods

Tibial intramedullary area of 24 wild type male Wistar rats (average weight = 350 ± 50 g, average age = 4 ± 0.5 months) were injected with NPY (1 × 10-5 M and 1 × 10-6 M) and NPY1 receptor antagonist (1 × 10-4 M) dissolved in hyaluronic acid (HA) separately. Tibiae were collected after one and two weeks. BMD was measured with dual-energy X-ray absorptiometry (DXA) and micro quantitative computer tomography (QCT). Histological changes were analyzed with light microscopy, Goldner's Masson trichrome (MT), and hematoxylin-eosin staining.

Results

According to DXA, the mean BMD of NPY dose 1 (1 × 10-5 M) was significantly lower than that of the control (HA applied) group and not significantly but still lower than that of the NPY dose 2 and NPY1 antagonist applied groups. QCT results indicated the same pattern statistically insignificantly in the trabecular area but not in the cortex of the bones. Histologically, only NPY1 antagonist applied tibiae revealed young spongiosis bone trabeculae formed in the borderline of the cortical bones. HA was remarkably biocompatible and late degrading in the tissues.

Conclusion

Local administration of NPY and NPY1 antagonists may hold regulating potential of BMD and bone formation. NPY1 antagonist caused new bone formation in trabecular bone when applied locally. NPY dissolved in HA however can be used to suppress bone formation.

Neurological Regulation of the Bone Marrow Niche

The bone marrow (BM) hematopoietic niche is the microenvironment where in the adult hematopoietic stem and progenitor cells (HSPCs) are maintained and regulated. This regulation is tightly controlled through direct cell-cell interactions with mesenchymal stromal stem (MSCs) and reticular cells, adipocytes, osteoblasts and endothelial cells, through binding to extracellular matrix molecules and through signaling by cytokines and hematopoietic growth factors. These interactions provide a healthy environment and secure the maintenance of the HSPC pool, their proliferation, differentiation and migration. Recent studies have shown that innervation of the BM and interactions with the peripheral sympathetic neural system are important for maintenance of the hematopoietic niche, through direct interactions with HSCPs or via interactions with other cells of the HSPC microenvironment. Signaling through adrenergic receptors (ARs), opioid receptors (ORs), endocannabinoid receptors (CRs) on HSPCs and MSCs has been shown to play an important role in HSPC homeostasis and mobilization. In addition, a wide range of neuropeptides and neurotransmitters, such as Neuropeptide Y (NPY), Substance P (SP) and Tachykinins, as well as neurotrophins and neuropoietic growth factors have been shown to be involved in regulation of the hematopoietic niche. Here, a comprehensive overview is given of their role and interactions with important cells in the hematopoietic niche, including HSPCs and MSCs, and their effect on HSPC maintenance, regulation and mobilization.

Inhibition of Y1 Receptor Promotes Osteogenesis in Bone Marrow Stromal Cells via cAMP/PKA/CREB Pathway

Inhibition of neuropeptide Y1 receptor stimulates osteogenesis in vitro and in vivo. However, the underlying mechanisms involved in these effects remain poorly understood. Here we identify the effects of Y1 receptor deficiency on osteogenic differentiation in human bone marrow stromal cells (BMSCs) by using genetic and pharmacological regulation, and to explore the pathways mediating these effects. In BMSCs, inhibition of Y1 receptor stimulates osteogenesis and upregulates the expression levels of the master transcriptional factor RUNX2. Mechanistically, Y1 receptor deficiency increases the levels of intracellular cAMP, which via protein kinase A (PKA) mediated pathways results in activation of phospho-CREB (p-CREB). We find RUNX2 activation induced by Y1 receptor deficiency is reversed by H-89, a PKA inhibitor. These results indicate Y1 receptor deficiency activates PKA-mediated phosphorylation of CREB, leading to activation of RUNX2 and enhances osteogenic differentiation in BMSCs. In conclusion, these data indicate that Y1 receptor deficiency promotes osteogenic differentiation by RUNX2 stimulation through cAMP/PKA/CREB pathway.

[Neuropeptide Y Y1 receptor antagonist PD160170 promotes osteogenic differentiation of rat bone marrow mesenchymal stem cells in vitro and femoral defect repair in rats]

OBJECTIVE

To investigate the effects of neuropeptide Y (NPY) Y1 receptor antagonist PD160170 in promoting osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) and accelerating healing of femoral defect in rats.

METHODS

The third generation of rat BMSCs were treated with PBS (control) or 10^-6, 10^-7, or 10^-8 mol/L NPY Y1 receptor antagonist PD160170. After 7 and 14 days of treatment, the cells were examined for osteogenic differentiation with alkaline phosphatase (ALP) and alizarin red staining. At 7 and 21 days of treatment, the mRNA and protein expressions of collagen type I (COLI), osteocalcin (OCN) and Runt-related transcription factor 2 (Runx2) in the cells were detected using q-PCR and Westem Blotting. In a male SD rat model (body weight 300∓20 g) of bilateral femoral condyle defects (2.5 mm in diameter), the effect of daily local injection of 0.2 mL PD160170 (10^-6 and 10^-8 mol/L, for 28 consecutive days) in promoting bone defect repair was evaluated with micro-CT scans.

RESULTS

ALP and alizarin red staining showed that the BMSCs treated with PD160170, at the optimal concentration of 10^-8 mol/L, contained more intracellular cytoplasmic brown particles and mineralized nodules in extracellular matrix than PBS-treated cells. PD160170 (10^-8 mol/L) significantly up-regulated the mRNA and protein expressions of COLI at day 7 and those of OCN and Runx2 at day 21 (P<0.05). In the rat models of femoral bone defect, the volume/tissue volume ratio, bone mineral density and the number of bone trabeculae were significantly greater in 10^-6 mol/L PD160170 group than in the control group (P<0.05), but the bone trabecular thickness (P=0.07) and bone volume (P=0.35) were similar between the two groups.

CONCLUSION

NPY Y1 receptor antagonist PD160170 can promote osteogenic differentiation of BMSCs and healing of femoral defects in rats, suggesting the potential of therapeutic strategies targeting NPY Y1 receptor signaling in the prevention and treatment of bone fracture and osteoporosis.

Structural basis of ligand binding modes at the neuropeptide Y Y1 receptor

Neuropeptide Y (NPY) receptors belong to the G-protein-coupled receptor superfamily and have important roles in food intake, anxiety and cancer biology 1,2 . The NPY-Y receptor system has emerged as one of the most complex networks with three peptide ligands (NPY, peptide YY and pancreatic polypeptide) binding to four receptors in most mammals, namely the Y1, Y2, Y4 and Y5 receptors, with different affinity and selectivity 3 . NPY is the most powerful stimulant of food intake and this effect is primarily mediated by the Y1 receptor (Y1R) 4 . A number of peptides and small-molecule compounds have been characterized as Y1R antagonists and have shown clinical potential in the treatment of obesity 4 , tumour 1 and bone loss 5 . However, their clinical usage has been hampered by low potency and selectivity, poor brain penetration ability or lack of oral bioavailability 6 . Here we report crystal structures of the human Y1R bound to the two selective antagonists UR-MK299 and BMS-193885 at 2.7 and 3.0 Å resolution, respectively. The structures combined with mutagenesis studies reveal the binding modes of Y1R to several structurally diverse antagonists and the determinants of ligand selectivity. The Y1R structure and molecular docking of the endogenous agonist NPY, together with nuclear magnetic resonance, photo-crosslinking and functional studies, provide insights into the binding behaviour of the agonist and for the first time, to our knowledge, determine the interaction of its N terminus with the receptor. These insights into Y1R can enable structure-based drug discovery that targets NPY receptors.

Current views on neuropeptide Y and diabetes-related atherosclerosis

Diabetes-induced atherosclerotic cardiovascular disease is the leading cause of death of diabetic patients. Neuronal regulation plays a critical role in glucose metabolism and cardiovascular function under physiological and pathological conditions, among which, neurotransmitter neuropeptide Y has been shown to be closely involved in these two processes. Elevated central neuropeptide Y level promotes food intake and reduces energy expenditure, thereby increasing adiposity. Neuropeptide Y is co-localized with noradrenaline in central and sympathetic nervous systems. As a major peripheral vascular contractive neurotransmitter, through interactions with its receptors, neuropeptide Y has been implicated in the pathology and progression of diabetes, by promoting the proliferation of endothelial cells and vascular fibrosis, which may contribute to diabetes-induced cardiovascular disease. Neuropeptide Y also participates in the pathogenesis of atherosclerosis, the major form of cardiovascular disease, via aggravating endothelial dysfunction, growth of vascular smooth muscle cells, formation of foam cells and platelets aggregation. This review highlights the causal role of neuropeptide Y and its receptor system in the development of diabetes mellitus and one of its complications: atherosclerotic cardiovascular disease. The information from this review provides both critical insights onto the mechanisms underlying the pathogenesis of atherosclerosis and evidence for the development of therapeutic strategies.

Boning up on DPP4, DPP4 substrates, and DPP4-adipokine interactions: Logical reasoning and known facts about bone related effects of DPP4 inhibitors

Dipeptidyl peptidase 4 (DPP4) is a conserved exopeptidase with an important function in protein regulation. The activity of DPP4, an enzyme which can either be anchored to the plasma membrane or circulate free in the extracellular compartment, affects the glucose metabolism, cellular signaling, migration and differentiation, oxidative stress and the immune system. DPP4 is also expressed on the surface of osteoblasts, osteoclasts and osteocytes, and was found to play a role in collagen metabolism. Many substrates of DPP4 have an established role in bone metabolism, among which are incretins, gastrointestinal peptides and neuropeptides. In general, their effects favor bone formation, but some effects are complex and have not been completely elucidated. DPP4 and some of its substrates are known to interact with adipokines, playing an essential role in the energy metabolism. The prolongation of the half-life of incretins through DPP4 inhibition led to the development of these inhibitors to improve glucose tolerance in diabetes. Current literature indicates that the inhibition of DPP4 activity might also result in a beneficial effect on the bone metabolism, but the long-term effect of DPP4 inhibition on fracture outcome has not been entirely established. Diabetic as well as postmenopausal osteoporosis is associated with an increased activity of DPP4, as well as a shift in the expression levels of DPP4 substrates, their receptors, and adipokines. The interactions between these factors and their relationship in bone metabolism are therefore an interesting field of study.

Ablation of Y1 receptor impairs osteoclast bone-resorbing activity

Y₁ receptor (Y₁R)-signalling pathway plays a pivotal role in the regulation of bone metabolism. The lack of Y₁R-signalling stimulates bone mass accretion that has been mainly attributed to Y₁R disruption from bone-forming cells. Still, the involvement of Y₁R-signalling in the control of bone-resorbing cells remained to be explored. Therefore, in this study we assessed the role of Y₁R deficiency in osteoclast formation and resorption activity. Here we demonstrate that Y₁R germline deletion (Y₁R^−/−) led to increased formation of highly multinucleated (n > 8) osteoclasts and enhanced surface area, possibly due to monocyte chemoattractant protein-1 (MCP-1) overexpression regulated by RANKL-signalling. Interestingly, functional studies revealed that these giant Y₁R^−/− multinucleated cells produce poorly demineralized eroded pits, which were associated to reduce expression of osteoclast matrix degradation markers, such as tartrate-resistant acid phosphatase-5b (TRAcP5b), matrix metalloproteinase-9 (MMP-9) and cathepsin-K (CTSK). Tridimensional (3D) morphologic analyses of resorption pits, using an in-house developed quantitative computational tool (BonePit), showed that Y₁R^−/− resorption pits displayed a marked reduction in surface area, volume and depth. Together, these data demonstrates that the lack of Y₁Rs stimulates the formation of larger multinucleated osteoclasts in vitro with reduced bone-resorbing activity, unveiling a novel therapeutic option for osteoclastic bone diseases based on Y₁R-signalling ablation.

Identification of candidate genes for myeloma-induced osteocyte death based on microarray data

Background

The study was aimed to investigate the molecular mechanisms of osteocyte death in multiple myeloma (MM) patients.

Methods

GSE27372 was downloaded from Gene Expression Omnibus, including three HOB-01 (osteocyte cell line) control samples and three HOB-01 samples co-cultured with JJN3 (human MM cell line). After the differentially expressed genes (DEGs) were identified by Student’s t test method, enrichment analyses were performed for them using DAVID software. Using TRANSFAC, TSGene, and tumor-associated gene (TAG) databases, functional annotation was conducted for the DEGs. Additionally, protein-protein interaction (PPI) network and sub-network analyses were performed using STRING database and Cytoscape software.

Results

Total 393 DEGs were identified, including 22 transcription factors (e.g., KLF4 and IRF8) and 37 TAGs. Enrichment analysis suggested that EGF, S1PR1, and NPY1R were enriched in the function of circulatory system development. EGF (degree = 31) and EGR1 (degree = 19) had high degrees and interactions in the PPI network. In the sub-network, S1PR1, C3AR1, and NPY1R could interact with each other.

Conclusions

These DEGs might participate in the osteocyte apoptosis induced by myeloma cells. These findings might provide a theoretical basis for a better understanding of the osteolysis in MM patients.

Functionalization of biomaterials with small osteoinductive moieties

Human mesenchymal stem cells (MSCs) are currently recognized as a powerful cell source for regenerative medicine, notably for their capacity to differentiate into multiple cell types. The combination of MSCs with biomaterials functionalized with instructive cues can be used as a strategy to direct specific lineage commitment, and can thus improve the therapeutic efficacy of these cells. In terms of biomaterial design, one common approach is the functionalization of materials with ligands capable of directly binding to cell receptors and trigger specific differentiation signaling pathways. Other strategies focus on the use of moieties that have an indirect effect, acting, for example, as sequesters of bioactive ligands present in the extracellular milieu that, in turn, will interact with cells. Compared with complex biomolecules, the use of simple compounds, such as chemical moieties and peptides, and other small molecules can be advantageous by leading to less expensive and easily tunable biomaterial formulations. This review describes different strategies that have been used to promote substrate-mediated guidance of osteogenic differentiation of immature osteoblasts, osteoprogenitors and MSCs, through chemically conjugated small moieties, both in two- and three-dimensional set-ups. In each case, the selected moiety, the coupling strategy and the main findings of the study were highlighted. The latest advances and future perspectives in the field are also discussed.

Neuropeptide Y modulates fracture healing through Y1 receptor signaling

Neuropeptide Y acting via it's Y1 receptor represents a powerful pathway in the control of bone mass. The global or osteoblast-specific Y1 receptor deletion induces pronounced bone anabolic effects in mice. However, the contribution of Y1 receptor deletion in bone repair/healing remained to be clarified. Therefore, in this study we characterized the role of Y1 receptor deletion in fracture healing. Closed tibial fractures were generated in germline (Y1 (-/-) ) and osteoblastic-specific Y1 receptor knockout mice. The progression of tibial repair monitored from 1- until 6-weeks post-fracture demonstrated that in Y1 (-/-) mice there is a delay in fracture repair, as seen by a decrease in bone callus volume and callus strength. Moreover, the histological features included elevated avascular and cartilage area and consequently delayed cartilage removal, and hence impaired union. Interestingly, this delay in bone repair was not related directly to Y1 receptors expressed by mature osteoblasts. These findings suggest that the global absence of the Y1 receptor delays fracture healing, through impairing the early phases of fracture repair to achieve bony union. The data acquired on the role of Y1 receptor signaling disruption in bone regeneration is critical for the design of future therapeutic strategies.

Development of neuropeptide Y-containing neurons in sympathetic ganglia of rats

Expression of neuropeptide Y (NPY) in the sympathetic ganglia was investigated by immunohistochemistry and tract tracing. The distribution of NPY immunoreactivity (IR) was studied in the superior cervical ganglion (SCG), stellate ganglion (SG) and celiac ganglion (CG) from rats of different ages (newborn, 10-day-old, 20-day-old, 30-day-old, 2-month-old, 6-month-old, 24-month-old). We observed that the percentage of NPY-IR neuronal profiles increased during early postnatal development. In the SCG and SG, the percentage of NPY-IR profiles enlarged in the first month of life from 43±3.6% (SCG) and 46±3.8% (SG) until 64±4.1% (SCG) and 58±3.5% (SG). The percentage of NPY-IR profiles in the CG increased during the period between 20days (65±3.8%) and 30days (82±5.1%) of animals' life and did not change in further development. In newborn and 10-day-old rats, a large portion of NPY-IR neurons was also calbindin D28K (CB)-IR in all sympathetic ganglia. The proportion of CB-IR substantially decreased during next 10days in the SCG, SG and CG. NPY-IR was approximately present in a half of the postganglionic neurons innervating muscle vessels of the neck and forearm, and the percentage of labeled NPY-IR profiles did not change during the development. Only single Ki67-IR neurons were also NPY-IR in the SCG, SG and CG in newborns and not in older animals. No NPY+/caspase 3+IR neurons were observed. Finally, the process of morphological changes in the size and percentages of NPY-IR profiles is complete in rats by the first month of life.

Stress hormone epinephrine enhances adipogenesis in murine embryonic stem cells by up-regulating the neuropeptide Y system

Prenatal stress, psychologically and metabolically, increases the risk of obesity and diabetes in the progeny. However, the mechanisms of the pathogenesis remain unknown. In adult mice, stress activates NPY and its Y2R in a glucocorticoid-dependent manner in the abdominal fat. This increased adipogenesis and angiogenesis, leading to abdominal obesity and metabolic syndrome which were inhibited by intra-fat Y2R inactivation. To determine whether stress elevates NPY system and accelerates adipogenic potential of embryo, here we "stressed" murine embryonic stem cells (mESCs) in vitro with epinephrine (EPI) during their adipogenic differentiation. EPI was added during the commitment stage together with insulin, and followed by dexamethasone in the standard adipogenic differentiation medium. Undifferentiated embryonic bodies (EBs) showed no detectable expression of NPY. EPI markedly up-regulated the expression NPY and the Y1R at the commitment stage, followed by increased Y2R mRNA at the late of the commitment stage and the differentiation stage. EPI significantly increased EB cells proliferation and expression of the preadipocyte marker Pref-1 at the commitment stage. EPI also accelerated and amplified adipogenic differentiation detected by increasing the adipocyte markers FABP4 and PPARγ mRNAs and Oil-red O-staining at the end of the differentiation stage. EPI-induced adipogenesis was completely prevented by antagonists of the NPY receptors (Y1R+Y2R+Y5R), indicating that it was mediated by the NPY system in mESC's. Taken together, these data suggest that stress may play an important role in programming ESCs for accelerated adipogenesis by altering the stress induced hormonal regulation of the NPY system.

Preproenkephalin (Penk) is expressed in differentiated osteoblasts, and its deletion in Hyp mice partially rescues their bone mineralization defect

Although our understanding of the molecular mechanisms controlling osteoblast differentiation and function is steadily increasing, there are still many open questions, especially regarding the regulation of bone matrix mineralization. For instance, while there is hallmark evidence for the importance of the endopeptidase Phex, whose inactivation in Hyp mice or human patients causes X-linked hypophosphatemic rickets, it is still largely unknown how Phex controls bone mineralization since a physiological substrate for its endopeptidase activity has not been identified yet. Using a genome-wide expression analysis comparing primary calvarial osteoblasts, we have identified preproenkephalin (Penk) as a gene that is selectively expressed in mineralized cultures. Since a role of enkephalin in the regulation of bone remodeling has been suggested previously and since Leu-enkephalin is known to be cleaved by Phex, we analyzed whether Penk expression in osteoblasts is physiologically relevant. Through skeletal analysis of a Penk-deficient mouse model, we found that Penk expression is dispensable for bone development and remodeling since we could not detect any defect following nondecalcified bone histology and histomorphometry compared to wild-type littermates. When Penk was deleted in Phex-deficient Hyp mice, however, we observed a significant reduction of the osteoid enrichment at 24 weeks of age, whereas their disturbance of mineral homeostasis was not affected by the additional absence of the Penk gene. Taken together, our data provide the first in vivo analysis concerning the role of Penk in osteoblasts.

Molecular variation in neuropeptide Y and bone mineral density among men of African ancestry

Neuropeptide Y (NPY) is a physiological candidate gene for the regulation of body weight and has more recently been implicated in regulating bone mass. The current study sought to test if inherited variation in NPY might influence BMD in a population of African-ancestry men who have high bone mineral density (BMD). We genotyped 17 tagging single-nucleotide polymorphisms (SNPs) across the NPY gene region in 1,113 randomly selected men of African ancestry aged >or=40 years and tested for association with anthropometric characteristics and proximal femur BMD. The homozygous rare genotype of four SNPs was associated with a 0.92-1.59% decrease in stature (corrected P < 0.05). No SNP was associated with body mass index or body weight. Two SNPs in a 5-kb linkage disequilibrium block encompassing exons 3 and 4 were associated with proximal femur BMD, adjusted for age, body weight, and height (corrected P < 0.05). These results suggest that genetic variation at the NPY locus may contribute to bone density, independently of body weight.