Neuropeptide Y is expressed by osteocytes and can inhibit osteoblastic activity

Emerging roles of nerve-bone axis in modulating skeletal system

Over the past decades, emerging evidence in the literature has demonstrated that the innervation of bone is a crucial modulator for skeletal physiology and pathophysiology. The nerve-bone axis sparked extensive preclinical and clinical investigations aimed at elucidating the contribution of nerve-bone crosstalks to skeleton metabolism, homeostasis, and injury repair through the perspective of skeletal neurobiology. To date, peripheral nerves have been widely reported to mediate bone growth and development and fracture healing via the secretion of neurotransmitters, neuropeptides, axon guidance factors, and neurotrophins. Relevant studies have further identified several critical neural pathways that stimulate profound alterations in bone cell biology, revealing a complex interplay between the skeleton and nerve systems. In addition, inspired by nerve-bone crosstalk, novel drug delivery systems and bioactive materials have been developed to emulate and facilitate the process of natural bone repair through neuromodulation, eventually boosting osteogenesis for ideal skeletal tissue regeneration. Overall, this work aims to review the novel research findings that contribute to deepening the current understanding of the nerve-bone axis, bringing forth some schemas that can be translated into the clinical scenario to highlight the critical roles of neuromodulation in the skeletal system.

Vasoactive intestinal peptide exerts an osteoinductive effect in human mesenchymal stem cells

Several neuropeptides present in bone tissues, produced by nerve fibers and bone cells, have been reported to play a role in regulating the fine-tuning of osteoblast and osteoclast functions to maintain bone homeostasis. This study aims to characterize the influence of the neuropeptide vasoactive intestinal peptide (VIP) on the differentiation process of human mesenchymal stem cells (MSCs) into osteoblasts and on their anabolic function. We describe the mRNA and protein expression profile of VIP and its receptors in MSCs as they differentiate into osteoblasts, suggesting the presence of an autocrine signaling pathway in these cells. Our findings reveal that VIP enhances the expression of early osteoblast markers in MSCs under osteogenic differentiation and favors both bone matrix formation and proper cytoskeletal reorganization. Finally, our data suggest that VIP could be exerting a direct modulatory role on the osteoblast to osteoclast signaling by downregulating the receptor activator of nuclear factor-κB ligand/osteoprotegerin ratio. These results highlight the potential of VIP as an osteoinductive differentiation factor, emerging as a key molecule in the maintenance of human bone homeostasis.

Bone-nerve crosstalk: a new state for neuralizing bone tissue engineering-A mini review

Neuro bone tissue engineering is a multidisciplinary field that combines both principles of neurobiology and bone tissue engineering to develop innovative strategies for repairing and regenerating injured bone tissues. Despite the fact that regeneration and development are considered two distinct biological processes, yet regeneration can be considered the reactivation of development in later life stages to restore missing tissues. It is noteworthy that the regeneration capabilities are distinct and vary from one organism to another (teleost fishes, hydra, humans), or even in the same organism can vary dependent on the injured tissue itself (Human central nervous system vs. peripheral nervous system). The skeletal tissue is highly innervated, peripheral nervous system plays a role in conveying the signals and connecting the central nervous system with the peripheral organs, moreover it has been shown that they play an important role in tissue regeneration. Their regeneration role is conveyed by the different cells' resident in it and in its endoneurium (fibroblasts, microphages, vasculature associated cells, and Schwann cells) these cells secrete various growth factors (NGF, BDNF, GDNF, NT-3, and bFGF) that contribute to the regenerative phenotype. The peripheral nervous system and central nervous system synchronize together in regulating bone homeostasis and regeneration through neurogenic factors and neural circuits. Receptors of important central nervous system peptides such as Serotonin, Leptin, Semaphorins, and BDNF are expressed in bone tissue playing a role in bone homeostasis, metabolism and regeneration. This review will highlight the crosstalk between peripheral nerves and bone in the developmental stages as well as in regeneration and different neuro-bone tissue engineering strategies for repairing severe bone injuries.

Cracking the Code: The Role of Peripheral Nervous System Signaling in Fracture Repair

PURPOSE OF REVIEW

The traditionally understated role of neural regulation in fracture healing is gaining prominence, as recent findings underscore the peripheral nervous system's critical contribution to bone repair. Indeed, it is becoming more evident that the nervous system modulates every stage of fracture healing, from the onset of inflammation to repair and eventual remodeling.

RECENT FINDINGS

Essential to this process are neurotrophins and neuropeptides, such as substance P, calcitonin gene-related peptide, and neuropeptide Y. These molecules fulfill key roles in promoting osteogenesis, influencing inflammation, and mediating pain. The sympathetic nervous system also plays an important role in the healing process: while local sympathectomies may improve fracture healing, systemic sympathetic denervation impairs fracture healing. Furthermore, chronic activation of the sympathetic nervous system, often triggered by stress, is a potential impediment to effective fracture healing, marking an important area for further investigation. The potential to manipulate aspects of the nervous system offers promising therapeutic possibilities for improving outcomes in fracture healing. This review article is part of a series of multiple manuscripts designed to determine the utility of using artificial intelligence for writing scientific reviews.

Crosstalk Between the Neuroendocrine System and Bone Homeostasis

The homeostasis of bone microenvironment is the foundation of bone health and comprises 2 concerted events: bone formation by osteoblasts and bone resorption by osteoclasts. In the early 21st century, leptin, an adipocytes-derived hormone, was found to affect bone homeostasis through hypothalamic relay and the sympathetic nervous system, involving neurotransmitters like serotonin and norepinephrine. This discovery has provided a new perspective regarding the synergistic effects of endocrine and nervous systems on skeletal homeostasis. Since then, more studies have been conducted, gradually uncovering the complex neuroendocrine regulation underlying bone homeostasis. Intriguingly, bone is also considered as an endocrine organ that can produce regulatory factors that in turn exert effects on neuroendocrine activities. After decades of exploration into bone regulation mechanisms, separate bioactive factors have been extensively investigated, whereas few studies have systematically shown a global view of bone homeostasis regulation. Therefore, we summarized the previously studied regulatory patterns from the nervous system and endocrine system to bone. This review will provide readers with a panoramic view of the intimate relationship between the neuroendocrine system and bone, compensating for the current understanding of the regulation patterns of bone homeostasis, and probably developing new therapeutic strategies for its related disorders.

Cornus officinalis: a potential herb for treatment of osteoporosis

Osteoporosis (OP) is a systemic metabolic skeletal disorder characterized by a decline in bone mass, bone mineral density, and deterioration of bone microstructure. It is prevalent among the elderly, particularly postmenopausal women, and poses a substantial burden to patients and society due to the high incidence of fragility fractures. Kidney-tonifying Traditional Chinese medicine (TCM) has long been utilized for OP prevention and treatment. In contrast to conventional approaches such as hormone replacement therapy, TCM offers distinct advantages such as minimal side effects, low toxicity, excellent tolerability, and suitability for long-term administration. Extensive experimental evidence supports the efficacy of kidney-tonifying TCM, exemplified by formulations based on the renowned herb Cornus officinalis and its bioactive constituents, including morroniside, sweroside, flavonol kaempferol, Cornuside I, in OP treatment. In this review, we provide a comprehensive elucidation of the underlying pathological principles governing OP, with particular emphasis on bone marrow mesenchymal stem cells, the homeostasis of osteogenic and osteoclastic, and the regulation of vascular and immune systems, all of which critically influence bone homeostasis. Furthermore, the therapeutic mechanisms of Cornus officinalis-based TCM formulations and Cornus officinalis-derived active constituents are discussed. In conclusion, this review aims to enhance understanding of the pharmacological mechanisms responsible for the anti-OP effects of kidney-tonifying TCM, specifically focusing on Cornus officinalis, and seeks to explore more efficacious and safer treatment strategies for OP.

The role of sensory and sympathetic nerves in craniofacial bone regeneration

Multiple factors regulate the regeneration of craniofacial bone defects. The nervous system is recognized as one of the critical regulators of bone mass, thereby suggesting a role for neuronal pathways in bone regeneration. However, in the context of craniofacial bone regeneration, little is known about the interplay between the nervous system and craniofacial bone. Sensory and sympathetic nerves interact with the bone through their neuropeptides, neurotransmitters, proteins, peptides, and amino acid derivates. The neuron-derived factors, such as semaphorin 3A (SEMA3A), substance P (SP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), and vasoactive intestinal peptide (VIP), possess a remarkable role in craniofacial regeneration. This review summarizes the roles of these factors and recently published factors such as secretoneurin (SN) and spexin (SPX) in the osteoblast and osteoclast differentiation, bone metabolism, growth, remodeling and discusses the novel application of nerve-based craniofacial bone regeneration. Moreover, the review will facilitate understanding the mechanism of action and provide potential treatment direction for the craniofacial bone defect.

FGF23 Actions in CKD-MBD and other Organs During CKD

Fibroblast growth factor 23 (FGF23) is a new endocrine product discovered in the past decade. In addition to being related to bone diseases, it has also been found to be related to kidney metabolism and parathyroid metabolism, especially as a biomarker and a key factor to be used in kidney diseases. FGF23 is upregulated as early as the second and third stages of chronic kidney disease (CKD) in response to relative phosphorus overload. The early rise of FGF23 has a protective effect on the body and is essential for maintaining phosphate balance. However, with the decline in renal function, eGFR (estimated glomerular filtration rate) declines, and the phosphorus excretion effect caused by FGF23 is weakened. It eventually leads to a variety of complications, such as bone disease (Chronic Kidney Disease-Mineral and Bone Metabolism Disorder), vascular calcification (VC), and more. Monoclonal antibodies against FGF23 are currently used to treat genetic diseases with increased FGF23. CKD is also a state of increased FGF23. This article reviews the current role of FGF23 in CKD and discusses the crosstalk between various organs under CKD conditions and FGF23. Studying the effect of hyperphosphatemia on different organs of CKD is important. The prospect of FGF23 for therapy is also discussed.

The PYY/Y2R-deficient male mouse is not protected from bone loss due to Roux-en-Y gastric bypass

BACKGROUND

Peptide YY (PYY) is an anorexigenic gut hormone that also has anti-osteogenic effects, inhibiting osteoblastic activity and inducing catabolic effects. It has been postulated that increases in PYY after Roux-en-Y gastric bypass (RYGB) contribute to declines in bone mineral density (BMD) and increases in bone turnover. The aim of this study is to determine the role of the PYY Y2-receptor in mediating bone loss post-RYGB in mice.

METHODS

We compared adult male wildtype (WT) and PYY Y2 receptor-deficient (KO) C57BL/6 mice that received RYGB (WT: n = 8; KO: n = 9), with sham-operated mice (Sham; WT: n = 9; KO: n = 10) and mice that were food-restricted to match the weights of the RYGB-treated group (Weight-Matched, WM; WT: n = 7; KO: n = 5). RYGB or sham surgery was performed at 15-16 weeks of age, and mice sacrificed 21 weeks later. We characterized bone microarchitecture with micro-computed tomography (μCT) at the distal femur (trabecular) and femoral midshaft (cortical). Differences in body weight, bone microarchitecture and biochemical bone markers (parathyroid hormone, PTH; C-telopeptide, CTX; and type 1 procollagen, P1NP) were compared using 2-factor ANOVA with Tukey's adjustments for multiple comparisons.

RESULTS

Body weights were similar in the WT-RYGB, WT-WM, KO-RYGB, and KO-WM: 41-44 g; these groups weighed significantly less than the Sham surgery groups: 55-57 g. Trabecular BMD was 31-43 % lower in RYGB mice than either Sham or WM in WT and KO groups. This deficiency in trabecular bone was accompanied by a lower trabecular number (19 %-23 %), thickness (22 %-30 %) and increased trabecular spacing (25 %-34 %) in WT and KO groups (p < 0.001 for all comparisons vs. RYGB). RYGB led to lower cortical thickness, cortical tissue mineral density, and cortical bone area fraction as compared to Sham and WM in WT and KO groups (p ≤ 0.004 for all). There were no interactions between genotype and bone microarchitecture, with patterns of response to RYGB similar in both WT and KO groups. CTX and P1NP were significantly higher in RYGB mice than WM in WT and KO groups. PTH did not differ among groups.

CONCLUSIONS

RYGB induced greater trabecular and cortical deficits and high bone turnover than observed in weight-matched mice, with a similar pattern in the WT and Y2RKO mice. Thus, skeletal effects of RYGB are independent of weight loss, and furthermore, PYY signaling through Y2R is not a key mediator of bone loss post-RYGB.

Hallmarks of peripheral nerve function in bone regeneration

Skeletal tissue is highly innervated. Although different types of nerves have been recently identified in the bone, the crosstalk between bone and nerves remains unclear. In this review, we outline the role of the peripheral nervous system (PNS) in bone regeneration following injury. We first introduce the conserved role of nerves in tissue regeneration in species ranging from amphibians to mammals. We then present the distribution of the PNS in the skeletal system under physiological conditions, fractures, or regeneration. Furthermore, we summarize the ways in which the PNS communicates with bone-lineage cells, the vasculature, and immune cells in the bone microenvironment. Based on this comprehensive and timely review, we conclude that the PNS regulates bone regeneration through neuropeptides or neurotransmitters and cells in the peripheral nerves. An in-depth understanding of the roles of peripheral nerves in bone regeneration will inform the development of new strategies based on bone-nerve crosstalk in promoting bone repair and regeneration.

Peripheral nerves in the tibial subchondral bone : the role of pain and homeostasis in osteoarthritis

Osteoarthritis (OA) is a highly prevalent degenerative joint disorder characterized by joint pain and physical disability. Aberrant subchondral bone induces pathological changes and is a major source of pain in OA. In the subchondral bone, which is highly innervated, nerves have dual roles in pain sensation and bone homeostasis regulation. The interaction between peripheral nerves and target cells in the subchondral bone, and the interplay between the sensory and sympathetic nervous systems, allow peripheral nerves to regulate subchondral bone homeostasis. Alterations in peripheral innervation and local transmitters are closely related to changes in nociception and subchondral bone homeostasis, and affect the progression of OA. Recent literature has substantially expanded our understanding of the physiological and pathological distribution and function of specific subtypes of neurones in bone. This review summarizes the types and distribution of nerves detected in the tibial subchondral bone, their cellular and molecular interactions with bone cells that regulate subchondral bone homeostasis, and their role in OA pain. A comprehensive understanding and further investigation of the functions of peripheral innervation in the subchondral bone will help to develop novel therapeutic approaches to effectively prevent OA, and alleviate OA pain.

Cite this article: Bone Joint Res 2022;11(7):439–452.

The Multiple Biological Functions of Dipeptidyl Peptidase-4 in Bone Metabolism

Dipeptidyl peptidase-4 (DPP4) is a ubiquitously occurring protease involved in various physiological and pathological processes ranging from glucose homeostasis, immunoregulation, inflammation to tumorigenesis. Recently, the benefits of DPP4 inhibitors as novel hypoglycemic agents on bone metabolism have attracted extensive attraction in many studies, indicating that DPP4 inhibitors may regulate bone homeostasis. The effects of DPP4 on bone metabolism are still unclear. This paper thoroughly reviews the potential mechanisms of DPP4 for interaction with adipokines, bone cells, bone immune cells, and cytokines in skeleton system. This literature review shows that the increased DPP4 activity may indirectly promote bone resorption and inhibit bone formation, increasing the risk of osteoporosis. Thus, bone metabolic balance can be improved by decreasing DPP4 activities. The substantial evidence collected and analyzed in this review supports this implication.

The osteocyte as a signaling cell

Osteocytes, former osteoblasts encapsulated by mineralized bone matrix, are far from being passive and metabolically inactive bone cells. Instead, osteocytes are multifunctional and dynamic cells capable of integrating hormonal and mechanical signals and transmitting them to effector cells in bone and in distant tissues. Osteocytes are a major source of molecules that regulate bone homeostasis by integrating both mechanical cues and hormonal signals that coordinate the differentiation and function of osteoclasts and osteoblasts. Osteocyte function is altered in both rare and common bone diseases, suggesting that osteocyte dysfunction is directly involved in the pathophysiology of several disorders affecting the skeleton. Advances in osteocyte biology initiated the development of novel therapeutics interfering with osteocyte-secreted molecules. Moreover, osteocytes are targets and key distributors of biological signals mediating the beneficial effects of several bone therapeutics used in the clinic. Here we review the most recent discoveries in osteocyte biology demonstrating that osteocytes regulate bone homeostasis and bone marrow fat via paracrine signaling, influence body composition and energy metabolism via endocrine signaling, and contribute to the damaging effects of diabetes mellitus and hematologic and metastatic cancers in the skeleton.

The Role of NPY in the Regulation of Bone Metabolism

Bone diseases are the leading causes of disability and severely compromised quality of life. Neuropeptide Y (NPY) is a multifunctional neuropeptide that participates in various physiological and pathological processes and exists in both the nerve system and bone tissue. In bone tissue, it actively participates in bone metabolism and disease progression through its receptors. Previous studies have focused on the opposite effects of NPY on bone formation and resorption through paracrine modes. In this review, we present a brief overview of the progress made in this research field in recent times in order to provide reference for further understanding the regulatory mechanism of bone physiology and pathological metabolism.

The Osteocyte Transcriptome: Discovering Messages Buried Within Bone

PURPOSE OF THE REVIEW

Osteocytes are cells embedded within the bone matrix, but their function and specific patterns of gene expression remain only partially defined; this is beginning to change with recent studies using transcriptomics. This unbiased approach can generate large amounts of data and is now being used to identify novel genes and signalling pathways within osteocytes both at baseline conditions and in response to stimuli. This review outlines the methods used to isolate cell populations containing osteocytes, and key recent transcriptomic studies that used osteocyte-containing preparations from bone tissue.

RECENT FINDINGS

Three common methods are used to prepare samples to examine osteocyte gene expression: digestion followed by sorting, laser capture microscopy, and the isolation of cortical bone shafts. All these methods present challenges in interpreting the data generated. Genes previously not known to be expressed by osteocytes have been identified and variations in osteocyte gene expression have been reported with age, sex, anatomical location, mechanical loading, and defects in bone strength. A substantial proportion of newly identified transcripts in osteocytes remain functionally undefined but several have been cross-referenced with functional data. Future work and improved methods (e.g. scRNAseq) likely provide useful resources for the study of osteocytes and important new information on the identity and functions of this unique cell type within the skeleton.

Neuronal Induction of Bone-Fat Imbalance through Osteocyte Neuropeptide Y

A differentiation switch of bone marrow mesenchymal stem/stromal cells (BMSCs) from osteoblasts to adipocytes contributes to age- and menopause-associated bone loss and marrow adiposity. Here it is found that osteocytes, the most abundant bone cells, promote adipogenesis and inhibit osteogenesis of BMSCs by secreting neuropeptide Y (NPY), whose expression increases with aging and osteoporosis. Deletion of NPY in osteocytes generates a high bone mass phenotype, and attenuates aging- and ovariectomy (OVX)-induced bone-fat imbalance in mice. Osteocyte NPY production is under the control of autonomic nervous system (ANS) and osteocyte NPY deletion blocks the ANS-induced regulation of BMSC fate and bone-fat balance. γ-Oryzanol, a clinically used ANS regulator, significantly increases bone formation and reverses aging- and OVX-induced osteocyte NPY overproduction and marrow adiposity in control mice, but not in mice lacking osteocyte NPY. The study suggests a new mode of neuronal control of bone metabolism through the ANS-induced regulation of osteocyte NPY.

Early treadmill running delays rotator cuff healing via Neuropeptide Y mediated inactivation of the Wnt/β-catenin signaling

Background

Defining the optimal rehabilitation programs for rotator cuff healing remains a challenge. Early treadmill running may have negative effects on tendon-bone interface (TBI) healing with increased expression of Neuropeptide Y (NPY). However, the underlying mechanism is still unknown.

Methods

The mice were randomly assigned to four groups: control group, treadmill group, treadmill ​+ ​BIBO3304 group and BIBO3304 group alone. Specifically, the control group was allowed free cage activity without any treatment after surgery. The treadmill group received early treadmill running initiated from postoperative day 2. The treadmill ​+ ​BIBO3304 group received treadmill running combined with intra-articular injection of BIBO3304 postoperatively. The BIBO3304 group only received type 1 NPY receptor (Y1 receptor, Y1R) antagonist BIBO3304 postoperatively. Healing outcomes of the rotator cuff were evaluated by histological analysis, synchrotron radiation micro-computed tomography (SR-μCT) scanning, and biomechanical testing at 4 and 8 weeks after surgery. The expression of NPY and its Y1 receptor during the treadmill running were tested by immunofluorescence. In addition, the related signaling pathway of Neuropeptide Y among all groups was detected by immunohistochemistry and western-blot.

Results

Immunofluorescence results show that early treadmill training could lead to a significant increase in the expression of NPY at the healing site, and Y1R was widely expressed in both normal or injured rotator cuff without statistical difference. At the same time, early treadmill running delayed the healing of rotator cuff, as indicated with unsatisfactory outcomes, including a significantly lower histological score, decreased bone formation and inferior biomechanical properties at postoperative week 4 and 8. Moreover, the use of BIBO3304 could partly alleviate the negative effects of early treadmill running on the healing of rotator cuff and promote the natural healing process of rotator cuff, as evidenced by significant differences observed between the treadmill and treadmill ​+ ​BIBO3304 groups, as well as observed between the control and BIBO3304 groups. On the other hand, the expressions of Wnt3a and β-catenin in the treadmill group were significantly lower compared with the other groups, while the expression in the BIBO3304 group was the highest, as evaluated by immunohistochemistry and western-blot.

Conclusions

Early treadmill running increased the expression of NPY at the RC healing site, which might burden the expression of Wnt3a/β-catenin and delay the healing process, inhibition of Y1 receptor with BIBO3304 could promote bone-tendon healing through the Wnt/β-catenin signaling.

The translational potential of this article: This is the first study to evaluate the specific role of the NPY-Y1R axis and its underlying mechanism by which early treadmill running delays bone-tendon healing. Further, our study may provide references of precise and individualized exercise-based rehabilitation strategies for TBI healing in clinic.

The translational potential of this article

This is the first study to evaluate the specific role of the NPY-Y1R axis and its underlying mechanism by which early treadmill running delays bone-tendon healing. Further, our study may provide references of precise and individualized exercise-based rehabilitation strategies for TBI healing in clinic.

The Effects of NPY1 Receptor Antagonism on Intervertebral Disc and Bone Changes in Ovariectomized Rats

STUDY DESIGN

Basic science.

OBJECTIVE

To compare the effects of a neuropeptide Y1 receptor antagonist (NPY-1RA) to estrogen on maintaining vertebral bone microarchitecture and disc height in a rat model of menopause.

METHODS

This study was an institutional animal care approved randomized control study with 104 ovariectomized rats and 32 intact control animals. Comparison of disc height, trabecular bone, body weights, circulating levels of NPY and estrogen, and distribution of Y1 receptors in the intervertebral disc in an established rodent osteoporotic model were made at baseline and after 2, 4, and 8 weeks after receiving either an implant containing estrogen or an antagonist to the neuropeptide Y1 receptor. Data was compared statistically using One-way analysis of variance.

RESULTS

Circulating levels of estrogen increased and NPY decreased following estrogen replacement, with values comparable to ovary-intact animals. NPY-1RA-treated animals had low estrogen and high NPY circulating levels and were similar to ovariectomized control rats. Both NPY-1RA and estrogen administration were able reduce, menopause associated weight gain. NPY-1RA appeared to restore bone formation and maintain disc height, while estrogen replacement prevented further bone loss.

CONCLUSION

NPY-1RA in osteoporotic rats activates osteoblast production of bone and decreased marrow and body fat more effectively than estrogen replacement when delivered in similar concentrations. Annulus cells had NPY receptors, which may play a role in disc nutrition, extracellular matrix production, and pain signaling cascades.

Endocrine role of bone in the regulation of energy metabolism

Bone mainly functions as a supportive framework for the whole body and is the major regulator of calcium homeostasis and hematopoietic function. Recently, an increasing number of studies have characterized the significance of bone as an endocrine organ, suggesting that bone-derived factors regulate local bone metabolism and metabolic functions. In addition, these factors can regulate global energy homeostasis by altering insulin sensitivity, feeding behavior, and adipocyte commitment. These findings may provide a new pathological mechanism for related metabolic diseases or be used in the diagnosis, treatment, and prevention of metabolic diseases such as osteoporosis, obesity, and diabetes mellitus. In this review, we summarize the regulatory effect of bone and bone-derived factors on energy metabolism and discuss directions for future research.

Bone-to-Brain: A Round Trip in the Adaptation to Mechanical Stimuli

Besides the classical ones (support/protection, hematopoiesis, storage for calcium, and phosphate) multiple roles emerged for bone tissue, definitively making it an organ. Particularly, the endocrine function, and in more general terms, the capability to sense and integrate different stimuli and to send signals to other tissues, has highlighted the importance of bone in homeostasis. Bone is highly innervated and hosts all nervous system branches; bone cells are sensitive to most of neurotransmitters, neuropeptides, and neurohormones that directly affect their metabolic activity and sensitivity to mechanical stimuli. Indeed, bone is the principal mechanosensitive organ. Thanks to the mechanosensing resident cells, and particularly osteocytes, mechanical stimulation induces metabolic responses in bone forming (osteoblasts) and bone resorbing (osteoclasts) cells that allow the adaptation of the affected bony segment to the changing environment. Once stimulated, bone cells express and secrete, or liberate from the entrapping matrix, several mediators (osteokines) that induce responses on distant targets. Brain is a target of some of these mediator [e.g., osteocalcin, lipocalin2, sclerostin, Dickkopf-related protein 1 (Dkk1), and fibroblast growth factor 23], as most of them can cross the blood-brain barrier. For others, a role in brain has been hypothesized, but not yet demonstrated. As exercise effectively modifies the release and the circulating levels of these osteokines, it has been hypothesized that some of the beneficial effects of exercise on brain functions may be associated to such a bone-to-brain communication. This hypothesis hides an interesting clinical clue: may well-addressed physical activities support the treatment of neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases?

Treadmill exercise facilitated rotator cuff healing is coupled with regulating periphery neuropeptides expression in a murine model

Postoperative exercise has been found able to accelerate bone-tendon (B-T) healing. In this study, we systematically compared tendon-to-bone healing in mice subjected to postoperative treadmill exercise and free cage recovery in a murine rotator cuff repair model. Specifically, C57BL/6 mice underwent unilateral supraspinatus tendon (SST) detachment and repair were randomly allocated into treadmill group and control group. Treadmill group received daily treadmill running initiated from postoperative day 7 while the control group was allowed free cage activity. Mice were euthanized at postoperative 4 and 8 weeks for synchrotron radiation micro-computed tomography (SR-μCT), histology and biomechanical tests to investigate the effect of treadmill running on B-T healing. The results indicated that treadmill running initiated at day 7 postoperatively was able to accelerate B-T healing, as evidenced by better tendon-to-bone maturation and increased mechanical property. Recent studies show that peripheral neuropeptides are closely associated with musculoskeletal tissue repair. We furtherly conducted quantitative reverse transcription-polymerase chain reaction and immunofluorescence staining to investigate the temporal-spatial expression of calcitonin gene-related peptide (CGRP), substance P (SP), and peripheral neuropeptide Y (NPY) to verify whether they are related to rotator cuff healing. Our results show increased expression of CGRP, SP, and NPY at the healing site under the effect of mechanical stimulation. In conclusion, delayed postoperative exercise with moderate strength appears to accelerate the early phase of B-T healing, a process that may prove to be linked to increased expression of periphery neuropeptides known to play a role in tissue healing.

Mechanical Loading-Driven Tumor Suppression Is Mediated by Lrp5-Dependent and Independent Mechanisms

Simple Summary

Advanced breast cancer and prostate cancer metastasize to varying organs including the bone. We show here that mechanical loading to the knee suppresses tumor growth in the loaded bone and the non-loaded mammary pad. Although lipoprotein receptor-related protein 5 (Lrp5) in osteocytes is necessary to induce loading-driven bone formation, loading-driven tumor suppression is regulated by Lrp5-dependent and independent mechanisms. Lrp5 overexpression in osteocytes enhances tumor suppression, but without Lrp5 in osteocytes, mechanical loading elevates dopamine, chemerin, p53, and TNF-related apoptosis-inducing ligand (TRAIL) and reduces cholesterol and nexin. Their systemic changes contribute to inhibiting tumors without Lrp5. Osteoclast development is also inhibited by the load-driven regulation of chemerin and nexin.

Abstract

Bone is mechanosensitive and lipoprotein receptor-related protein 5 (Lrp5)-mediated Wnt signaling promotes loading-driven bone formation. While mechanical loading can suppress tumor growth, the question is whether Lrp5 mediates loading-driven tumor suppression. Herein, we examined the effect of Lrp5 using osteocyte-specific Lrp5 conditional knockout mice. All mice presented noticeable loading-driven tumor suppression in the loaded tibia and non-loaded mammary pad. The degree of suppression was more significant in wild-type than knockout mice. In all male and female mice, knee loading reduced cholesterol and elevated dopamine. It reduced tumor-promoting nexin, which was elevated by cholesterol and reduced by dopamine. By contrast, it elevated p53, TNF-related apoptosis-inducing ligand (TRAIL), and chemerin, and they were regulated reversely by dopamine and cholesterol. Notably, Lrp5 overexpression in osteocytes enhanced tumor suppression, and osteoclast development was inhibited by chemerin. Collectively, this study identified Lrp5-dependent and independent mechanisms for tumor suppression. Lrp5 in osteocytes contributed to the loaded bone, while the Lrp5-independent regulation of dopamine- and cholesterol-induced systemic suppression.

The Osteocyte: New Insights

Osteocytes are an ancient cell, appearing in fossilized skeletal remains of early fish and dinosaurs. Despite its relative high abundance, even in the context of nonskeletal cells, the osteocyte is perhaps among the least studied cells in all of vertebrate biology. Osteocytes are cells embedded in bone, able to modify their surrounding extracellular matrix via specialized molecular remodeling mechanisms that are independent of the bone forming osteoblasts and bone-resorbing osteoclasts. Osteocytes communicate with osteoclasts and osteoblasts via distinct signaling molecules that include the RankL/OPG axis and the Sost/Dkk1/Wnt axis, among others. Osteocytes also extend their influence beyond the local bone environment by functioning as an endocrine cell that controls phosphate reabsorption in the kidney, insulin secretion in the pancreas, and skeletal muscle function. These cells are also finely tuned sensors of mechanical stimulation to coordinate with effector cells to adjust bone mass, size, and shape to conform to mechanical demands.

Is There a Governing Role of Osteocytes in Bone Tissue Regeneration?

PURPOSE OF REVIEW

Bone regeneration plays an important role in contemporary clinical treatment. Bone tissue engineering should result in successful bone regeneration to restore congenital or acquired bone defects in the human skeleton. Osteocytes are thought to have a governing role in bone remodeling by regulating osteoclast and osteoblast activity, and thus bone loss and formation. In this review, we address the so far largely unknown role osteocytes may play in bone tissue regeneration.

RECENT FINDINGS

Osteocytes release biochemical signaling molecules involved in bone remodeling such as prostaglandins, nitric oxide, Wnts, and insulin-like growth factor-1 (IGF-1). Treatment of mesenchymal stem cells in bone tissue engineering with prostaglandins (e.g., PGE₂, PGI₂, PGF_2α), nitric oxide, IGF-1, or Wnts (e.g., Wnt3a) improves osteogenesis. This review provides an overview of the functions of osteocytes in bone tissue, their interaction with other bone cells, and their role in bone remodeling. We postulate that osteocytes may have a pivotal role in bone regeneration as well, and consequently that the bone regeneration process may be improved effectively and rapidly if osteocytes are optimally used and stimulated.

Mechanisms of action of neuropeptide Y on stem cells and its potential applications in orthopaedic disorders

Musculoskeletal disorders are the leading causes of disability and result in reduced quality of life. The neuro-osteogenic network is one of the most promising fields in orthopaedic research. Neuropeptide Y (NPY) system has been reported to be involved in the regulations of bone metabolism and homeostasis, which also provide feedback to the central NPY system via NPY receptors. Currently, potential roles of peripheral NPY in bone metabolism remain unclear. Growing evidence suggests that NPY can regulate biological actions of bone marrow mesenchymal stem cells, hematopoietic stem cells, endothelial cells, and chondrocytes via a local autocrine or paracrine manner by different NPY receptors. The regulative activities of NPY may be achieved through the plasticity of NPY receptors, and interactions among the targeted cells as well. In general, NPY can influence proliferation, apoptosis, differentiation, migration, mobilization, and cytokine secretion of different types of cells, and play crucial roles in the development of bone delayed/non-union, osteoporosis, and osteoarthritis. Further basic research should clarify detailed mechanisms of action of NPY on stem cells, and clinical investigations are also necessary to comprehensively evaluate potential applications of NPY and its receptor-targeted drugs in management of musculoskeletal disorders.

Neuropeptide Y upregulates Runx2 and osterix and enhances osteogenesis in mouse MC3T3‑E1 cells via an autocrine mechanism

The neuropeptide Y (NPY) system is considered one of the primary neural signaling pathways. NPY, produced by osteoblasts and other peripheral tissues, is known to inhibit biological functions of osteoblasts. However, until recently, little was known of the autocrine mechanism by which NPY is regulated. To investigate this mechanism, overexpression plasmids and small interfering RNA (siRNA) targeting NPY were transfected into the MC3T3-E1 cell line to observe its effects on osteogenesis. NPY overexpression was found to markedly enhance the osteogenic ability of MC3T3-E1 cells by an autocrine mechanism, coincident with the upregulation of osterix and runt-related transcription factor 2 (Runx2). Furthermore, NPY increased the activities of alkaline phosphatase (ALP) and osteocalcin (OCN) by upregulating their osteoblastic expression in vitro (as well as that of osterix and Runx2). Following transfection with NPY-siRNA, the osteoblastic ability of MC3T3-E1 cells was markedly decreased, and NPY deficiency inhibited the protein expression of osterix, Runx2, OCN and ALP in primary osteoblasts. Collectively, these results indicated that NPY played an important role in osteoblast differentiation by regulating the osterix and Runx2 pathways.

Effects of Early Life Stress on Bone Homeostasis in Mice and Humans

Bone pathology is frequent in stressed individuals. A comprehensive examination of mechanisms linking life stress, depression and disturbed bone homeostasis is missing. In this translational study, mice exposed to early life stress (MSUS) were examined for bone microarchitecture (μCT), metabolism (qPCR/ELISA), and neuronal stress mediator expression (qPCR) and compared with a sample of depressive patients with or without early life stress by analyzing bone mineral density (BMD) (DXA) and metabolic changes in serum (osteocalcin, PINP, CTX-I). MSUS mice showed a significant decrease in NGF, NPYR1, VIPR1 and TACR1 expression, higher innervation density in bone, and increased serum levels of CTX-I, suggesting a milieu in favor of catabolic bone turnover. MSUS mice had a significantly lower body weight compared to control mice, and this caused minor effects on bone microarchitecture. Depressive patients with experiences of childhood neglect also showed a catabolic pattern. A significant reduction in BMD was observed in depressive patients with childhood abuse and stressful life events during childhood. Therefore, future studies on prevention and treatment strategies for both mental and bone disease should consider early life stress as a risk factor for bone pathologies.

Methionine Enkephalin Suppresses Osteocyte Apoptosis Induced by Compressive Force through Regulation of Nuclear Translocation of NFATc1

Mechanical stress stimulates bone remodeling, which occurs through bone formation and resorption, resulting in bone adaptation in response to the mechanical stress. Osteocytes perceive mechanical stress loaded to bones and promote bone remodeling through various cellular processes. Osteocyte apoptosis is considered a cellular process to induce bone resorption during mechanical stress-induced bone remodeling, but the underlying molecular mechanisms are not fully understood. Recent studies have demonstrated that neuropeptides play crucial roles in bone metabolism. The neuropeptide, methionine enkephalin (MENK) regulates apoptosis positively and negatively depending on cell type, but the role of MENK in osteocyte apoptosis, followed by bone resorption, in response to mechanical stress is still unknown. Here, we examined the roles and mechanisms of MENK in osteocyte apoptosis induced by compressive force. We loaded compressive force to mouse parietal bones, resulting in a reduction of MENK expression in osteocytes. A neutralizing connective tissue growth factor (CTGF) antibody inhibited the compressive force-induced reduction of MENK. An increase in osteocyte apoptosis in the compressive force-loaded parietal bones was inhibited by MENK administration. Nuclear translocation of NFATc1 in osteocytes in the parietal bones was enhanced by compressive force. INCA-6, which inhibits NFAT translocation into nuclei, suppressed the increase in osteocyte apoptosis in the compressive force-loaded parietal bones. NFATc1-overexpressing MLO-Y4 cells showed increased expression of apoptosis-related genes. MENK administration reduced the nuclear translocation of NFATc1 in osteocytes in the compressive force-loaded parietal bones. Moreover, MENK suppressed Ca²⁺ influx and calcineurin and calmodulin expression, which are known to induce the nuclear translocation of NFAT in MLO-Y4 cells. In summary, this study shows that osteocytes expressed MENK, whereas the MENK expression was suppressed by compressive force via CTGF signaling. MENK downregulated nuclear translocation of NFATc1 probably by suppressing Ca²⁺ signaling in osteocytes and consequently inhibiting compressive force-induced osteocyte apoptosis, followed by bone resorption. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Neuropeptide Y1 receptor antagonist promotes osteoporosis and microdamage repair and enhances osteogenic differentiation of bone marrow stem cells via cAMP/PKA/CREB pathway

Osteoporosis is a common metabolic bone disorder in the elderly population. The accumulation of bone microdamage is a critical factor of osteoporotic fracture. Neuropeptide Y (NPY) has been reported to regulated bone metabolism through Y1 receptor (Y1R). In this study the effects and mechanisms of Y1R antagonist on prevention for osteoporosis were characterized. In the clinical experiment, compared with osteoarthritis (OA), osteoporosis (OP) showed significant osteoporotic bone microstructure and accumulation of bone microdamage. NPY and Y1R immunoreactivity in bone were stronger in OP group, and were both correlated with bone volume fraction (BV/TV). In vivo experiment, Y1R antagonist significantly improved osteoporotic microstructure in the ovariectomized (OVX) rats. And Y1R antagonist promoted RUNX2, OPG and inhibit RANKL, MMP9 in bone marrow. In vitro cell culture experiment, NPY inhibited osteogenesis, elevated RANKL/OPG ratio and downregulated the expression of cAMP, p-PKAs and p-CREB in BMSCs, treated with Y1R antagonist or 8-Bromo-cAMP could inhibit the effects of NPY. Together, Y1R antagonist improved the bone microstructure and reduced bone microdamage in OVX rats. NPY-Y1R could inhibit osteoblast differentiation of BMSCs via cAMP/PKA/CREB pathway. Our findings highlight the regulation of NPY-Y1R in bone metabolism as a potential therapy strategy for the prevention of osteoporosis and osteoporotic fracture.

Neuropeptide Y enhances proliferation and chondrogenic differentiation of ATDC5 cells

In recent years, emerging evidence has illustrated the indispensable role of sympathetic neurotransmitters and their receptors in cartilage mediation. The presence of neuropeptide Y (NPY)-positive sympathetic nerve fibres in cartilage and NPY-secretion function in chondrocytes raises the possibility of NPY directly regulating the function of chondrocytes. Therefore, this study intended to evaluate the effect of NPY and its receptors on the proliferation and chondrogenic differentiation of ATDC5 cells. Results showed NPY, especially at a concentration of 10^-10 M, to significantly enhance proliferation of ATDC5 cells. Moreover, NPY effectively facilitated early chondrogenesis and late hypertrophy/mineralisation of ATDC5 cells via Y1 receptor signalling, rather than via Y2 receptor signalling. Taken together, the results help us to understand how NPY and its receptors affect the function of chondrocytes.

Neuropeptide Y1 Receptor Antagonist Alters Gut Microbiota and Alleviates the Ovariectomy-Induced Osteoporosis in Rats

A plethora of evidence has suggested that gut microbiota is involved in the occurrence and development of postmenopausal osteoporosis (PMO). It has been suggested that neuropeptide Y (NPY) modulates the bone metabolism through Y1 receptor (Y1R), and might be associated with gut microbiota. The present study aims to evaluate the anti-osteoporotic effects of Y1R antagonist and to investigate the potential mechanism by which Y1R antagonist regulates gut microbiota. In this study, eighteen female rats were randomly divided into three groups: the sham surgery (SHAM) group, the ovariectomized (OVX) group, and OVX+BIBO3304 group. After 6 weeks following surgery, Y1R antagonist BIBO3304 was administered to the rats in OVX+BIBO3304 group for 7 days. The bone microstructure and serum biochemical parameters were measured at 12 weeks after operation. The differences in the gut microbiota were analyzed by 16S rDNA gene sequencing. Heat-map and Spearman's correlation analyses were constructed to investigate the correlations between microbiota and bone metabolism-related parameters. The results indicated that OVX+BIBO3304 group showed significantly higher BMD, BV/TV, Tb.Th, Tb.N, Conn.D, and serum Ca²⁺ level than those in OVX group. Additionally, Y1R antagonist changed the gut microbiota composition with lower Firmicutes/Bacteroidetes ratio and higher proportions of some probiotics, including Lactobacillus. The correlation analysis showed that the changes of gut microbiota were closely associated with bone microstructure and serum Ca²⁺ levels. Our results suggested that Y1R antagonist played an anti-osteoporotic effect and regulated gut microbiota in OVX rats, indicating the potential to utilize Y1R antagonist as a novel treatment for PMO.

The lack of neuropeptide Y-Y1 receptor signaling modulates the chemical and mechanical properties of bone matrix

Genetic and pharmacological functional studies have provided evidence that the lack of Neuropeptide Y-Y₁  receptor (Y₁ R) signaling pathway induces a high bone mass phenotype in mice. However, clinical observations have shown that drug or genetic mediated improvement of bone mass might be associated to alterations to bone extracellular matrix (ECM) properties, leading to bone fragility. Hence, in this study we propose to characterize the physical, chemical and biomechanical properties of mature bone ECM of germline NPY-Y₁ R knockout (Y₁ R^-/- ) mice, and compare to their wild-type (WT) littermates. Our results demonstrated that the high bone mass phenotype observed in Y₁ R^-/- mice involves alterations in Y₁ R^-/-  bone ECM ultrastructure, as a result of accelerated deposition of organic and mineral fractions. In addition, Y₁ R^-/- bone ECM displays enhanced matrix maturation characterized by greater number of mature/highly packed collagen fibers without pathological accumulation of immature/mature collagen crosslinks nor compromise of mineral crystallinity. These unique features of Y₁ R^-/-  bone ECM improved the biochemical properties of Y₁ R^-/-  bones, reflected by mechanically robust bones with diminished propensity to fracture, contributing to greater bone strength. These findings support the future usage of drugs targeting Y₁ R signaling as a promising therapeutic strategy to treat bone loss-related pathologies.

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.

The Role of Nerves in Skeletal Development, Adaptation, and Aging

The skeleton is well-innervated, but only recently have the functions of this complex network in bone started to become known. Although our knowledge of skeletal sensory and sympathetic innervation is incomplete, including the specific locations and subtypes of nerves in bone, we are now able to reconcile early studies utilizing denervation models with recent work dissecting the molecular signaling between bone and nerve. In total, sensory innervation functions in bone much as it does elsewhere in the body-to sense and respond to stimuli, including mechanical loading. Similarly, sympathetic nerves regulate autonomic functions related to bone, including homeostatic remodeling and vascular tone. However, more study is required to translate our current knowledge of bone-nerve crosstalk to novel therapeutic strategies that can be effectively utilized to combat skeletal diseases, disorders of low bone mass, and age-related decreases in bone quality.

Changes in the osteocyte lacunocanalicular network with aging

Osteoporosis is an aging-related disease of reduced bone mass that is particularly prevalent in post-menopausal women, but also affects the aged male population and is associated with increased fracture risk. Osteoporosis is the result of an imbalance whereby bone formation by osteoblasts no longer keeps pace with resorption of bone by osteoclasts. Osteocytes are the most abundant cells in bone and, although previously thought to be quiescent, they are now known to be active, multifunctional cells that play a key role in the maintenance of bone mass by regulating both osteoblast and osteoclast activity. They are also thought to regulate bone mass through their role as mechanoresponsive cells in bone that coordinate adaptive responses to mechanical loading. Osteocytes form an extensive interconnected network throughout the mineralized bone matrix and receive their nutrients as well as hormones and signaling factors through the lacunocanalicular system. Several studies have shown that the extent and connectivity of the lacunocanalicular system and osteocyte networks degenerates in aged humans as well as in animal models of aging. It is also known that the bone anabolic response to loading is decreased with aging. This review summarizes recent research on the degenerative changes that occur in osteocytes and their lacunocanalicular system as a result of aging and discusses the implications for skeletal health and homeostasis as well as potential mechanisms that may underlie these degenerative changes. Since osteocytes are such key regulators of skeletal homeostasis, maintaining the health of the osteocyte network would seem critical for maintenance of bone health. Therefore, a more complete understanding of the structure and function of the osteocyte network, its lacunocanalicular system, and the degenerative changes that occur with aging should lead to advances in our understanding of age related bone loss and potentially lead to improved therapies.

Skeletal phenotype of the neuropeptide Y knockout mouse

Neuropeptide Y (NPY) is involved in multiple processes such as behavior, energy and bone metabolism. Previous studies have relied on global NPY depletion to examine its effects on bone. However, this approach is unable to distinguish the central or local source of NPY influencing bone. Our aim was to identify which cells within the skeleton express Npy and establish a model that will enable us to differentiate effects of NPY derived from different cell types. We have generated the NPY floxed (NPY^flox) mice using CRISPR technology. By crossing the NPY^flox mice with Hypoxanthine Phosphoribosyltransferase 1 (Hprt)-cre to generate a global knockout, we were able to validate and confirm loss of Npy transcript and protein in our global NPYKO. Global deletion of NPY results in a smaller femoral cortical cross-sectional area (-12%) and reduced bone strength (-18%) in male mice. In vitro, NPY-deficient bone marrow stromal cells (BMSCs) showed increase in osteogenic differentiation detected by increases in alkaline phosphatase staining and bone sialoprotein and osteocalcin expression. Despite both sexes presenting with increased adiposity, female mice had no alterations in bone mass, suggesting that NPY may have sex-specific effects on bone. In this study we identified Npy expression in the skeleton and examined the effect of global NPY depletion to bone mass. The differential impact of NPY deletion in cortical and cancellous compartments along with differences in phenotypes between in vitro and in vivo, highlights the complex nature of NPY signaling, indicative of distinct sources that can be dissected in the future using this NPY^flox model.

Impact of the Autonomic Nervous System on the Skeleton

It is from the discovery of leptin and the central nervous system as a regulator of bone remodeling that the presence of autonomic nerves within the skeleton transitioned from a mere histological observation to the mechanism whereby neurons of the central nervous system communicate with cells of the bone microenvironment and regulate bone homeostasis. This shift in paradigm sparked new preclinical and clinical investigations aimed at defining the contribution of sympathetic, parasympathetic, and sensory nerves to the process of bone development, bone mass accrual, bone remodeling, and cancer metastasis. The aim of this article is to review the data that led to the current understanding of the interactions between the autonomic and skeletal systems and to present a critical appraisal of the literature, bringing forth a schema that can put into physiological and clinical context the main genetic and pharmacological observations pointing to the existence of an autonomic control of skeletal homeostasis. The different types of nerves found in the skeleton, their functional interactions with bone cells, their impact on bone development, bone mass accrual and remodeling, and the possible clinical or pathophysiological relevance of these findings are discussed.

Diet-induced obesity suppresses cortical bone accrual by a neuropeptide Y-dependent mechanism

OBJECTIVE

To determine whether age and neuropeptide Y (NPY) were involved in the skeletal response to extended periods of diet-induced obesity.

METHODS

Male wild-type (WT) and NPY null (NPYKO) mice were fed a mild (23% fat) high-fat diet for 10 weeks from 6 or 16 weeks of age. Metabolism and bone density were assessed during feeding. Skeletal changes were assessed by microCT and histomorphometry.

RESULTS

High-fat feeding in 6-week-old WT mice led to significantly increased body weight, adiposity and serum leptin levels, accompanied with markedly suppressed cortical bone accrual. NPYKO mice were less susceptible to fat accrual but, importantly, displayed a complete lack of suppression of bone accrual or cortical bone loss. In contrast, when skeletally mature (16 week old) mice underwent 10 weeks of fat feeding, the metabolic response to HFD was similar to younger mice, however bone mass was not affected in either WT or NPYKO. Thus, growing mice are particularly susceptible to the detrimental effects of HFD on bone mass, through suppression of bone accrual involving NPY signalling.

CONCLUSION

This study provides new insights into the relationship between the opposing processes of a positive weight/bone relationship and the negative 'metabolic' effect of obesity on bone mass. This negative effect is particularly active in growing skeletons, which have heightened sensitivity to changes in obesity. In addition, NPY is identified as a fundamental driver of this negative 'metabolic' pathway to bone.

Do Neuroendocrine Peptides and Their Receptors Qualify as Novel Therapeutic Targets in Osteoarthritis?

Joint tissues like synovium, articular cartilage, meniscus and subchondral bone, are targets for neuropeptides. Resident cells of these tissues express receptors for various neuroendocrine-derived peptides including proopiomelanocortin (POMC)-derived peptides, i.e., α-melanocyte-stimulating hormone (α-MSH), adrenocorticotropin (ACTH) and β-endorphin (β-ED), and sympathetic neuropeptides like vasoactive intestinal peptide (VIP) and neuropeptide y (NPY). Melanocortins attained particular attention due to their immunomodulatory and anti-inflammatory effects in several tissues and organs. In particular, α-MSH, ACTH and specific melanocortin-receptor (MCR) agonists appear to have promising anti-inflammatory actions demonstrated in animal models of experimentally induced arthritis and osteoarthritis (OA). Sympathetic neuropeptides have obtained increasing attention as they have crucial trophic effects that are critical for joint tissue and bone homeostasis. VIP and NPY are implicated in direct and indirect activation of several anabolic signaling pathways in bone and synovial cells. Additionally, pituitary adenylate cyclase-activating polypeptide (PACAP) proved to be chondroprotective and, thus, might be a novel target in OA. Taken together, it appears more and more likely that the anabolic effects of these neuroendocrine peptides or their respective receptor agonists/antagonists may be exploited for the treatment of patients with inflammatory and degenerative joint diseases in the future.

Neuronal Control of Bone Remodeling

Although the brain is well established as a master regulator of homeostasis in peripheral tissues, central regulation of bone mass represents a novel and rapidly expanding field of study. This review examines the current understanding of central regulation of the skeleton, exploring several of the key pathways connecting brain to bone and their implications both in mice and the clinical setting. Our understanding of central bone regulation has largely progressed through examination of skeletal responses downstream of nutrient regulatory pathways in the hypothalamus. Mutations and modulation of these pathways, in cases such as leptin deficiency, induce marked bone phenotypes, which have provided vital insights into central bone regulation. These studies have identified several central neuropeptide pathways that stimulate well-defined changes in bone cell activity in response to changes in energy homeostasis. In addition, this work has highlighted the endocrine nature of the skeleton, revealing a complex cross talk that directly regulates other organ systems. Our laboratory has studied bone-active neuropeptide pathways and defined osteoblast-based actions that recapitulate central pathways linking bone, fat, and glucose homeostasis. Studies of neural control of bone have produced paradigm-shifting changes in our understanding of the skeleton and its relationship with the wider array of organ systems.

Inhibition of neuropeptide Y Y1 receptor induces osteoblast differentiation in MC3T3‑E1 cells

Neuropeptide Y (NPY) is a major neural signaling molecule. NPY is produced by peripheral tissues, such as osteoblasts, and binds to the corresponding Y1 receptor that belongs to the G‑protein‑coupled receptor family. Osteoblast‑specific Y1 receptor knockout mice exhibit high bone mass, indicating a role of the NPY‑Y1 receptor axis in the regulation of bone homeostasis. In the bone microenvironment, peripheral nerve fibers and osteoblasts produce NPY. However, the effects of the Y1 receptor on osteoblasts remain unexplored. In the present study, an RNA interference approach was employed to target the Y1 receptor, in order to determine whether it may function to regulate the growth, differentiation and viability of osteoblasts. Knockdown of the Y1 receptor by small interfering RNA (siRNA) lead to induction of alkaline phosphatase (ALP) activity and mineralization in mouse MC3T3‑E1 osteoblast cells. In addition, the mRNA expression levels of ALP, osteocalcin, collagen (I) α1, and bone sialoprotein were significantly increased following transfection of a Y1 receptor siRNA. Furthermore, the mRNA expression levels of Runx2 and osterix were significantly increased; however, no significant alterations in cell proliferation and caspase‑3/7 activity were observed in Y1 receptor siRNA‑transfected cells when compared with non‑targeting controls. The results demonstrate that Y1 receptor inhibition may increase osteoblastic differentiation, which indicates a role of the Y1 receptor in the regulation of osteoblastic differentiation.

Bone Cell Bioenergetics and Skeletal Energy Homeostasis

The rising incidence of metabolic diseases worldwide has prompted renewed interest in the study of intermediary metabolism and cellular bioenergetics. The application of modern biochemical methods for quantitating fuel substrate metabolism with advanced mouse genetic approaches has greatly increased understanding of the mechanisms that integrate energy metabolism in the whole organism. Examination of the intermediary metabolism of skeletal cells has been sparked by a series of unanticipated observations in genetically modified mice that suggest the existence of novel endocrine pathways through which bone cells communicate their energy status to other centers of metabolic control. The recognition of this expanded role of the skeleton has in turn led to new lines of inquiry directed at defining the fuel requirements and bioenergetic properties of bone cells. This article provides a comprehensive review of historical and contemporary studies on the metabolic properties of bone cells and the mechanisms that control energy substrate utilization and bioenergetics. Special attention is devoted to identifying gaps in our current understanding of this new area of skeletal biology that will require additional research to better define the physiological significance of skeletal cell bioenergetics in human health and disease.

Neuropeptide Y1 Receptor Regulates Glucocorticoid-Induced Inhibition of Osteoblast Differentiation in Murine MC3T3-E1 Cells via ERK Signaling

High dose glucocorticoid (GC) administration impairs the viability and function of osteoblasts, thus causing osteoporosis and osteonecrosis. Neuropeptide Y1 receptor (Y1 receptor) is expressed in bone tissues and cells, and regulates bone remodeling. However, the role of Y1 receptor in glucocorticoid-induced inhibition of osteoblast differentiation remains unknown. In the present study, osteoblastic cell line MC3T3-E1 cultured in osteogenic differentiation medium was treated with or without of 10⁻⁷ M dexamethasone (Dex), Y1 receptor shRNA interference, Y1 receptor agonist [Leu³¹, Pro³⁴]-NPY, and antagonist BIBP3226. Cell proliferation and apoptosis were assessed by cell counting kit-8 (CCK-8) assay and cleaved caspase expression, respectively. Osteoblast differentiation was evaluated by Alizarin Red S staining and osteogenic marker gene expressions. Protein expression was detected by Western blot analysis. Dex upregulated the expression of Y1 receptor in MC3T3-E1 cells associated with reduced osteogenic gene expressions and mineralization. Blockade of Y1 receptor by shRNA transfection and BIBP3226 significantly attenuated the inhibitory effects of Dex on osteoblastic activity. Y1 receptor signaling modulated the activation of extracellular signal-regulated kinases (ERK) as well as the expressions of osteogenic genes. Y1 receptor agonist inhibited ERK phosphorylation and osteoblast differentiation, while Y1 receptor blockade exhibited the opposite effects. Activation of ERK signaling by constitutive active mutant of MEK1 (caMEK) abolished Y1 receptor-mediated Dex inhibition of osteoblast differentiation in MC3T3-E1 cells. Taken together, Y1 receptor regulates Dex-induced inhibition of osteoblast differentiation in murine MC3T3-E1 cells via ERK signaling. This study provides a novel role of Y1 receptor in the process of GC-induced suppression in osteoblast survival and differentiation.

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.

Bone Injury and Repair Trigger Central and Peripheral NPY Neuronal Pathways

Bone repair is a specialized type of wound repair controlled by complex multi-factorial events. The nervous system is recognized as one of the key regulators of bone mass, thereby suggesting a role for neuronal pathways in bone homeostasis. However, in the context of bone injury and repair, little is known on the interplay between the nervous system and bone. Here, we addressed the neuropeptide Y (NPY) neuronal arm during the initial stages of bone repair encompassing the inflammatory response and ossification phases in femoral-defect mouse model. Spatial and temporal analysis of transcriptional and protein levels of NPY and its receptors, Y1R and Y2R, reported to be involved in bone homeostasis, was performed in bone, dorsal root ganglia (DRG) and hypothalamus after femoral injury. The results showed that NPY system activity is increased in a time- and space-dependent manner during bone repair. Y1R expression was trigged in both bone and DRG throughout the inflammatory phase, while a Y2R response was restricted to the hypothalamus and at a later stage, during the ossification step. Our results provide new insights into the involvement of NPY neuronal pathways in bone repair.

A Single Chance to Contact Multiple Targets: Distinct Osteocyte Morphotypes Shed Light on the Cellular Mechanism Ensuring the Robust Formation of Osteocytic Networks

The formation of the complex osteocytic network relies on the emission of long cellular processes involved in communication, mechanical strain sensing, and bone turnover control. Newly deposited osteocytic processes rapidly become trapped within the calcifying matrix, and, therefore, they must adopt their definitive conformation and contact their targets in a single morphogenetic event. However, the cellular mechanisms ensuring the robustness of this unique mode of morphogenesis remain unknown. To address this issue, we examined the developing calvaria of the amphibian Xenopus tropicalis by confocal, two-photon, and super-resolution imaging, and described flattened osteocytes lying within a woven bone structured in lamellae of randomly oriented collagen fibers. While most cells emit peripheral and perpendicular processes, we report two osteocytes morphotypes, located at different depth within the bone matrix and exhibiting distinct number and orientation of perpendicular cell processes. We show that this pattern is conserved with the chick Gallus gallus and suggest that the cellular microenvironment, and more particularly cell-cell contact, plays a fundamental role in the induction and stabilization of osteocytic processes. We propose that this intrinsic property might have been evolutionarily selected for its ability to robustly generate self-organizing osteocytic networks harbored by the wide variety of bone shapes and architectures found in extant and extinct vertebrates.

Correlation between neuropeptide distribution, cancellous bone microstructure and joint pain in postmenopausal women with osteoarthritis and osteoporosis

OBJECTIVES

To explore the relationship between the distribution of neuropeptides, cancellous bone microstructure and joint pain in postmenopausal women with osteoarthritis (OA) and osteoporosis (OP).

METHODS

Cancellous bone of the femoral head was obtained at the time of hip arthroplasty from 20 postmenopausal women, 10 with OA and 10 with OP. Pain intensity was evaluated using the visual analog scale (VAS) before the operation. The microstructural parameters were measured with micro-CT and the neuropeptides of the cancellous bone were stained by an immunohistochemical method.

RESULTS

We observed that BV/TV, Tb.Th and Th.N values in the OP were significantly decreased compared to those in the OA. Immunohistochemical analysis revealed that the mean optical density (MOD) values for SP, CGRP, and VIP in the OA group were significantly higher than those in the OP, and the MOD value for NPY in the OA was significantly lower than that in the OP. We also observed that the MOD values for SP were positively correlated with AD, BV/TV, Tb.Th, Tb.N and Conn.D and negatively with MD, Tb.Sp and SMI in all patients. The MOD values for CGRP were positively correlated with AD, BV/TV and Tb.Th. MOD values for VIP were positively correlated with BV/TV and Tb.Th and negatively with SMI. The VAS score was correlated positively with the MOD values for SP, CGRP, VIP and negatively with NPY in all patients.

CONCLUSIONS

Neuropeptides play an important role in the pathogenesis of OA and OP, which may cause pain and influence the bone microstructure.

Neuropeptide Y stimulates osteoblastic differentiation and VEGF expression of bone marrow mesenchymal stem cells related to canonical Wnt signaling activating in vitro

Neuropeptide Y (NPY) is a neuropeptide secreted by sensory nerve fibers distributed in the marrow and vascular canals of bone tissue. However, the effect of NPY on the osteogenic ability of bone marrow mesenchymal stem cells (BMSCs) remains controversial and has not been thoroughly investigated. To explore the osteogenic activity and the migration and VEGF expression capabilities of BMSCs affected by NPY, as well as the underlying mechanisms, we investigated the potential relationships among NPY, osteoblastic differentiation, angiogenesis and canonical Wnt signaling in BMSCs. NPY was observed to regulate osteoblastic differentiation at concentrations ranging from 10(-8) to 10(-12)mol/L, and the effects of NPY on the levels of Wnt signaling proteins were detected using Western blotting. To unravel the underlying mechanism, BMSCs were treated with NPY after pretreatment with the NPY-1R antagonist PD160170 or the Wnt pathway antagonist DKK1, and gene expression levels of Wnt signaling molecules and osteoblastic markers were determined by qPCR. Our results indicated that NPY significantly promoted osteoblastic differentiation of BMSCs in a concentration-dependent manner and up-regulated the expression levels of proteins including β-catenin and p-GSK-3β and the mRNA level of β-catenin. Moreover, NPY promoted the translocation of β-catenin into nucleus. The effects of NPY were inhibited by PD160170 or DKK1. Additionally, NPY enhanced the ability of BMSCs to migrate and promoted the expression of vascular endothelial growth factor (VEGF) as measured by immunocytochemical staining, qPCR and Western blot. These results suggested that NPY may stimulate osteoblastic differentiation via activating canonical Wnt signaling and enhance the angiogenic capacity of BMSCs.

The y6 receptor suppresses bone resorption and stimulates bone formation in mice via a suprachiasmatic nucleus relay

The neuropeptide Y system is known to play an important role in the regulation of bone homeostasis and while the functions of its major receptors, Y1R and Y2R, in this process have become clearer, the contributions of other Y-receptors, like the y6 receptor (y6R), are unknown. Y6R expression is restricted to the suprachiasmatic nucleus (SCN) of the hypothalamus, an area known to regulate circadian rhythms, and the testis. Here we show that lack of y6R signalling, results in significant reduction in bone mass, but no changes in bone length. Male and female y6R knockout (KO) mice display reduced cortical and cancellous bone volume in axial and appendicular bones. Mechanistically, the reduction in cancellous bone is the result of an uncoupling of bone remodelling, leading to an increase in osteoclast surface and number, and a reduction in osteoblast number, osteoid surface, mineralizing surface and bone formation rate. y6R KO mice displayed increased numbers of osteoclast precursors and produced greater numbers of osteoclasts in RANKL-treated cultures. They also produced fewer CFU-ALP osteoblast precursors in the marrow and showed reduced mineralization in primary osteoblastic cultures, as well as reduced expression for the osteoblast lineage marker, alkaline phosphatase, in bone isolates. The almost exclusive location of y6Rs in the hypothalamus suggests a critical role of central neuronal pathways controlling this uncoupling of bone remodelling which is in line with known actions or other Y-receptors in the brain. In conclusion, y6R signalling is required for maintenance of bone mass, with loss of y6R uncoupling bone remodelling and resulting in a negative bone balance. This study expands the scope of hypothalamic regulation of bone, highlighting the importance for neural/endocrine coordination and their marked effect upon skeletal homeostasis.