ADRA2A is involved in neuro-endocrine regulation of bone resorption

Exploring the multicomponent synergy mechanism of Zuogui Wan on postmenopausal osteoporosis by a systems pharmacology strategy

OBJECTIVE

To explore the multi-component synergistic mechanism of Zuogui Wan (, ZGW) in treating postmenopausal osteoporosis (PMOP).

METHODS

The main components and target genes of ZGW were screened via the Traditional Chinese Medicine Systems Pharmacology (TCMSP). In addition, the target gene sets of PMOP were derived from the GeneCards and Online Mendelian Inheritance in Man databases. The search tool for recurring instances of neighbouring genes (STRING) 11.0 software was used to analyze the interaction among intersecting genes. Cytoscape 3.6.1 software and the Matthews correlation coefficient (MCC) algorithm were used to screen the core genes. Fifty Sprague-Dawley female rats were randomly divided into the sham-operated (Sham) group and the four ovariectomized (OVX) subgroups. Rats subjected to Sham or OVX were administered with the vehicle (OVX, 1 mL water/100 g weight), 17β-estradiol (E2, 50 μg·kg-1·d-1), and lyophilized powder of ZGW at a low dose of 2.3 (ZGW-L) and high dose of 4.6 (ZGW-H) g·kg-1·d-1 for three months. The bone density and bone strength were assessed using dual-energy X-ray and three-point bending tests, respectively. Furthermore, enzyme-linked immun-osorbent assay, Hematoxylin-eosin staining, and western blot analysis were used to determine the potential pharmacological mechanisms of action of ZGW in PMOP.

RESULTS

A total of 117 active compounds of ZGW were screened from the TCMSP. Furthermore, 108 intersecting genes of drugs and diseases were identified. Using STRING software and the MCC algorithm, ten core genes, including C-X-C chemokine living 8 (CXCL8), C-C chemokine receptor type 2 (CCR2), alpha-2a active receptor (ADRA2A), melatonin receptor type 1B (MTNR1B), and amyloid-beta A4 protein (APP), were identified. The anti-osteoporosis regulation network of ZGW was constructed using the Cytoscape software. The animal experiments demonstrated that ZGW groups significantly reduced the serum levels of β-C-terminal telopeptide of type I collagen (β-CTX) and increased serum levels of bone-specific alkaline phosphatase (BALP) (P < 0.05, P < 0.01). The OVX group exhibited a significant decrease in bone mineral density and bone strength compared with the Sham group (P < 0.01). Moreover, treatment with ZGW resulted in increased trabecular thickness, improved arrangement of trabecular structure, and reduced empty bone lacunae. Furthermore, treatment with ZGW significantly increased the protein expression of CXCL8, ADRA2A, and CCR2 (P < 0.05, P < 0.01), and significantly decreased the protein expression of Runx2 (P < 0.01). Furthermore, the ZGW and E2 groups demonstrated significantly increased BMD (P < 0.05, P < 0.01), improved bone strength (P < 0.05, P < 0.01), reduced expression of CXCL8, ADRA2A, and CCR2, and increased runt-related transcription factor 2 levels in bone tissue (P < 0.05, P < 0.01) compared with the OVX group. However, there were no significant differences in MTNR1B and APP expression among the groups.

CONCLUSION

ZGW shows synergistic mechanisms in PMOP through multiple components, targets, and pathways.

Autonomic neural regulation in mediating the brain-bone axis: mechanisms and implications for regeneration under psychological stress

Efficient regeneration of bone defects caused by disease or significant trauma is a major challenge in current medicine, which is particularly difficult yet significant under the emerging psychological stress in the modern society. Notably, the brain-bone axis has been proposed as a prominent new concept in recent years, among which autonomic nerves act as an essential and emerging skeletal pathophysiological factor related to psychological stress. Studies have established that sympathetic cues lead to impairment of bone homeostasis mainly through acting on mesenchymal stem cells (MSCs) and their derivatives with also affecting the hematopoietic stem cell (HSC)-lineage osteoclasts, and the autonomic neural regulation of stem cell lineages in bone is increasingly recognized to contribute to the bone degenerative disease, osteoporosis. This review summarizes the distribution characteristics of autonomic nerves in bone, introduces the regulatory effects and mechanisms of autonomic nerves on MSC and HSC lineages, and expounds the crucial role of autonomic neural regulation on bone physiology and pathology, which acts as a bridge between the brain and the bone. With the translational perspective, we further highlight the autonomic neural basis of psychological stress-induced bone loss and a series of pharmaceutical therapeutic strategies and implications toward bone regeneration. The summary of research progress in this field will add knowledge to the current landscape of inter-organ crosstalk and provide a medicinal basis for the achievement of clinical bone regeneration in the future.

The Sympathetic Nervous System in Dental Implantology

The sympathetic nervous system plays a vital role in various regulatory mechanisms. These include the well-known fight-or-flight response but also, for example, the processing of external stressors. In addition to many other tissues, the sympathetic nervous system influences bone metabolism. This effect could be highly relevant concerning osseointegration, which is responsible for the long-term success of dental implants. Accordingly, this review aims to summarize the current literature on this topic and to reveal future research perspectives. One in vitro study showed differences in mRNA expression of adrenoceptors cultured on implant surfaces. In vivo, sympathectomy impaired osseointegration in mice, while electrical stimulation of the sympathetic nerves promoted it. As expected, the beta-blocker propranolol improves histological implant parameters and micro-CT measurements. Overall, the present data are considered heterogeneous. However, the available publications reveal the potential for future research and development in dental implantology, which helps to introduce new therapeutic strategies and identify risk factors for dental implant failure.

Influence of Receptor Polymorphisms on the Response to α-Adrenergic Receptor Blockers in Pheochromocytoma Patients

Background: Presurgical treatment with an α-adrenergic receptor blocker is recommended to antagonize the catecholamine-induced α-adrenergic receptor mediated vasoconstriction in patients with pheochromocytoma or sympathetic paraganglioma (PPGL). There is, however, a considerable interindividual variation in the dose-response relationship regarding the magnitude of blood pressure reduction or the occurrence of side effects. We hypothesized that genetically determined differences in α-adrenergic receptor activity contribute to this variability in dose-response relationship. Methods: Thirty-one single-nucleotide polymorphisms (SNPs) of the α1A, α1B, α1D adrenoreceptor (ADRA1A, ADRA1B, ADRA1D) and α2A, α2B adrenoreceptor (ADRA2A, ADRA2B) genes were genotyped in a group of 116 participants of the PRESCRIPT study. Haplotypes were constructed after determining linkage disequilibrium blocks. Results: The ADRA1B SNP rs10515807 and the ADRA2A SNPs rs553668/rs521674 were associated with higher dosages of α-adrenergic receptor blocker (p < 0.05) and with a higher occurrence of side effects (rs10515807) (p = 0.005). Similar associations were found for haplotype block 6, which is predominantly defined by rs10515807. Conclusions: This study suggests that genetic variability of α-adrenergic receptor genes might be associated with the clinically observed variation in beneficial and adverse therapeutic drug responses to α-adrenergic receptor blockers. Further studies in larger cohorts are needed to confirm our observations.

Driving β2- While Suppressing α-Adrenergic Receptor Activity Suppresses Joint Pathology in Inflammatory Arthritis

Objective

Hypersympathetic activity is prominent in rheumatoid arthritis, and major life stressors precede onset in ~80% of patients. These findings and others support a link between stress, the sympathetic nervous system and disease onset and progression. Here, we extend previous research by evaluating how selective peripherally acting α/β2-adrenergic drugs affect joint destruction in adjuvant-induced arthritis.

Methods

Complete Freund's adjuvant induced inflammatory arthritis in male Lewis rats. Controls received no treatment. Arthritic rats then received vehicle or twice-daily treatment with the α-adrenergic antagonist, phentolamine (0.5 mg/day) and the β2-adrenergic agonist, terbutaline (1200 µg/day, collectively named SH1293) from day (D) of disease onset (D12) through acute (D21) and severe disease (D28). Disease progression was assessed in the hind limbs using dorsoplantar widths, X-ray analysis, micro-computed tomography, and routine histology on D14, D21, and D28 post-immunization.

Results

On D21, SH1293 significantly attenuated arthritis in the hind limbs, based on reduced lymphocytic infiltration, preservation of cartilage, and bone volume. Pannus formation and sympathetic nerve loss were not affected by SH1293. Bone area and osteoclast number revealed high- and low-treatment-responding groups. In high-responding rats, treatment with SH1293 significantly preserved bone area and decreased osteoclast number, data that correlated with drug-mediated joint preservation. SH1293 suppressed abnormal bone formation based on reduced production of osteophytes. On D28, the arthritic sparing effects of SH1293 on lymphocytic infiltration, cartilage and bone sparing were maintained at the expense of bone marrow adipocity. However, sympathetic nerves were retracted from the talocrural joint.

Conclusion and Significance

Our findings support a significant delay in early arthritis progression by treatment with SH1293. Targeting sympathetic neurotransmission may provide a strategy to slow disease progression.

Adrenergic signalling in osteoarthritis

Adrenoceptors (ARs) mediate the effects of the sympathetic neurotransmitters norepinephrine (NE) and epinephrine (E) in the human body and play a central role in physiologic and pathologic processes. Therefore, ARs have long been recognized as targets for therapeutic agents, especially in the field of cardiovascular medicine. During the past decades, the contribution of the sympathetic nervous system (SNS) and particularly of its major peripheral catecholamine NE to the pathogenesis of osteoarthritis (OA) attracted growing interest. OA is the most common degenerative joint disorder worldwide and a disease of the whole joint. It is characterized by progressive degradation of articular cartilage, synovial inflammation, osteophyte formation, and subchondral bone sclerosis mostly resulting in chronic pain. The subchondral bone marrow, the periosteum, the synovium, the vascular meniscus and numerous tendons and ligaments are innervated by tyrosine hydroxylase-positive (TH+) sympathetic nerve fibers that release NE into the synovial fluid and cells of all abovementioned joint tissues express at least one out of nine AR subtypes. During the past decades, several in vitro studies explored the AR-mediated effects of NE on different cell types in the joint. So far, only a few studies used animal OA models to investigate the contribution of distinct AR subtypes to OA pathogenesis in vivo. This narrative review shortly summarizes the current background knowledge about ARs and their signalling pathways at first. In the second part, we focus on recent findings in the field of NE-induced AR-mediated signalling in different joint tissues during OA pathogenesis and at the end, we will delineate the potential of targeting the adrenergic signalling for OA prevention or treatment. We used the PubMed bibliographic database to search for keywords such as 'joint' or 'cartilage' or 'synovium' or 'bone' and 'osteoarthritis' and/or 'trauma' and 'sympathetic nerve fibers' and/or 'norepinephrine' and 'adrenergic receptors / adrenoceptors' as well as 'adrenergic therapy'.

α 2-Adrenoceptors: Challenges and Opportunities-Enlightenment from the Kidney

It was indeed a Don Quixote-like pursuit of the mechanism of essential hypertension when we serendipitously discovered α₂-adrenoceptors (α₂-ARs) in skin-lightening experiments in the frog. Now α₂-ARs lurk on the horizon involving hypertension causality, renal denervation for hypertension, injury from falling in the elderly and prazosin's mechanism of action in anxiety states such as posttraumatic stress disorder (PTSD). Our goal here is to focus on this horizon and bring into clear view the role of α₂-AR-mediated mechanisms in these seemingly unrelated conditions. Our narrative begins with an explanation of how experiments in isolated perfused kidneys led to the discovery of a sodium-retaining process, a fundamental mechanism of hypertension, mediated by α₂-ARs. In this model system and in the setting of furosemide-induced sodium excretion, α₂-AR activation inhibited adenylate cyclase, suppressed cAMP formation, and caused sodium retention. Further investigations led to the realization that renal α₂-AR expression in hypertensive animals is elevated, thus supporting a key role for kidney α₂-ARs in the pathophysiology of essential hypertension. Subsequent studies clarified the molecular pathways by which α₂-ARs activate prohypertensive biochemical systems. While investigating the role of α₁-adrenoceptors (α₁-ARs) versus α₂-ARs in renal sympathetic neurotransmission, we noted an astonishing result: in the kidney α₁-ARs suppress the postjunctional expression of α₂-ARs. Here, we describe how this finding relates to a broader understanding of the role of α₂-ARs in diverse disease states. Because of the capacity for qualitative and quantitative monitoring of α₂-AR-induced regulatory mechanisms in the kidney, we looked to the kidney and found enlightenment.

Multi-omics Data Integration for Identifying Osteoporosis Biomarkers and Their Biological Interaction and Causal Mechanisms

Osteoporosis is characterized by low bone mineral density (BMD). The advancement of high-throughput technologies and integrative approaches provided an opportunity for deciphering the mechanisms underlying osteoporosis. Here, we generated genomic, transcriptomic, methylomic, and metabolomic datasets from 119 subjects with high (n = 61) and low (n = 58) BMDs. By adopting sparse multiple discriminative canonical correlation analysis, we identified an optimal multi-omics biomarker panel with 74 differentially expressed genes (DEGs), 75 differentially methylated CpG sites (DMCs), and 23 differential metabolic products (DMPs). By linking genetic data, we identified 199 targeted BMD-associated expression/methylation/metabolite quantitative trait loci (eQTLs/meQTLs/metaQTLs). The reconstructed networks/pathways showed extensive biomarker interactions, and a substantial proportion of these biomarkers were enriched in RANK/RANKL, MAPK/TGF-β, and WNT/β-catenin pathways and G-protein-coupled receptor, GTP-binding/GTPase, telomere/mitochondrial activities that are essential for bone metabolism. Five biomarkers (FADS2, ADRA2A, FMN1, RABL2A, SPRY1) revealed causal effects on BMD variation. Our study provided an innovative framework and insights into the pathogenesis of osteoporosis.

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.

Association of Adrenergic Receptor α2A (α2A-AR) Gene rs1800544 Polymorphism with Bone Mineral Density and Bone Turnover Markers in an Elderly Chinese Population

BACKGROUND Adrenergic receptor α2A (α2A-AR) is up-regulated in osteoporotic bone osteoblasts. Previous research demonstrated an association between polymorphism of a2A-AR gene and bone mineral density (BMD) as well as bone turnover markers (BTMs) in the Slovenian population. The present study aimed to investigate the association of rs1800544 polymorphism of α2A-AR gene with BMD and BTMs in the Chinese elderly population with osteoporosis (OP) or with osteoporotic fractures. MATERIAL AND METHODS A total of 346 unrelated elderly individuals were recruited in the study. Rs1800544 polymorphism was determined by Snapshot technology. BTMs were determined by electrochemiluminescence. BMDs at lumbar spine (LS) and proximal femur were measured with dual-energy X-ray absorptiometry (DEXA). Hardy-Weinberg equilibrium and distribution of genotype frequencies were verified using the chi-squared test. Analysis of co-variance (ANCOVA) adjusted for confounding factors was performed to explore the relationship of rs1800544 polymorphism with BMD and BTMs in all participants and in subgroups. RESULTS The genotype distributions in all subjects and in subgroups conformed to Hardy-Weinberg equilibrium (P>0.1). Distribution of genotype frequencies of subgroups showed no significant differences (P>0.05). Patients with GG genotype in the fracture group had significantly higher serum BTMs level compared with those carrying other genotypes (P<0.05). No significant association between rs1800544 and BTMs was detected in the elderly population with OP. Comparison of BMD at each site in all participants did not show any significant difference in subgroups with CC, CG, and GG genotypes (P>0.05). CONCLUSIONS Rs1800544 polymorphism is associated with BTMs level in Chinese elderly individuals with osteoporotic fractures, indicating the involvement of genetic variation of a2A-AR gene in bone metabolism.

Global Disruption of α2A Adrenoceptor Barely Affects Bone Tissue but Minimizes the Detrimental Effects of Thyrotoxicosis on Cortical Bone

Evidence shows that sympathetic nervous system (SNS) activation inhibits bone formation and activates bone resorption leading to bone loss. Because thyroid hormone (TH) interacts with the SNS to control several physiological processes, we raised the hypothesis that this interaction also controls bone remodeling. We have previously shown that mice with double-gene inactivation of α2A- and -adrenoceptors (α2A/2C-AR^-/-) present high bone mass (HBM) phenotype and resistance to thyrotoxicosis-induced osteopenia, which supports a TH-SNS interaction to control bone mass and suggests that it involves α2-AR signaling. Accordingly, we detected expression of α2A-AR, α2B-AR and α2C-AR in the skeleton, and that triiodothyronine (T3) modulates α2C-AR mRNA expression in the bone. Later, we found that mice with single-gene inactivation of α2C-AR (α2C-AR^-/-) present low bone mass in the femur and HBM in the vertebra, but that both skeletal sites are resistant to TH-induce osteopenia, showing that the SNS actions occur in a skeletal site-dependent manner, and that thyrotoxicosis depends on α2C-AR signaling to promote bone loss. To further dissect the specific roles of α2-AR subtypes, in this study, we evaluated the skeletal phenotype of mice with single-gene inactivation of α2A-AR (α2A-AR^-/-), and the effect of daily treatment with a supraphysiological dose of T3, for 4 or 12 weeks, on bone microarchitecture and bone resistance to fracture. Micro-computed tomographic (μCT) analysis revealed normal trabecular and cortical bone structure in the femur and vertebra of euthyroid α_2A-AR^-/- mice. Thyrotoxicosis was more detrimental to femoral trabecular bone in α2A-AR^-/- than in WT mice, whereas this bone compartment had been previously shown to present resistance to thyrotoxicosis in α2C-AR^-/- mice. Altogether these findings reveal that TH excess depends on α2C-AR signaling to negatively affect femoral trabecular bone. In contrast, thyrotoxicosis was more deleterious to femoral and vertebral cortical bone in WT than in α2A-AR^-/- mice, suggesting that α2A-AR signaling contributes to TH actions on cortical bone. These findings further support a TH-SNS interaction to control bone physiology, and suggest that α2A-AR and α2C-AR signaling pathways have key roles in the mechanisms through which thyrotoxicosis promotes its detrimental effects on bone remodeling, structure and resistance to fracture.

The continuing problem of poor transparency of reporting and use of inappropriate methods for RT-qPCR

Attendance at this year’s European Calcified Tissue Society’s (ECTS) Congress reveals that the methods used to obtain qPCR results continue to be significantly flawed and that and their reporting remain inadequate.

Evolution of the Marrow Adipose Tissue Microenvironment

Adipocytes of the marrow adipose tissue (MAT) are distributed throughout the skeleton, are embedded in extracellular matrix, and are surrounded by cells of the hematopoietic and osteogenic lineages. MAT is a persistent component of the skeletal microenvironment and has the potential to impact local processes including bone accrual and hematopoietic function. In this review, we discuss the initial evolution of MAT in vertebrate lineages while emphasizing comparisons to the development of peripheral adipose, hematopoietic, and skeletal tissues. We then apply these evolutionary clues to define putative functions of MAT. Lastly, we explore the regulation of MAT by two major components of its microenvironment, the extracellular matrix and the nerves embedded within. The extracellular matrix and nerves contribute to both rapid and continuous modification of the MAT niche and may help to explain evolutionary conserved mechanisms underlying the coordinated regulation of blood, bone, and MAT within the skeleton.

Peripheral Nerve Fibers and Their Neurotransmitters in Osteoarthritis Pathology

The importance of the nociceptive nervous system for maintaining tissue homeostasis has been known for some time, and it has also been suggested that organogenesis and tissue repair are under neuronal control. Changes in peripheral joint innervation are supposed to be partly responsible for degenerative alterations in joint tissues which contribute to development of osteoarthritis. Various resident cell types of the musculoskeletal system express receptors for sensory and sympathetic neurotransmitters, allowing response to peripheral neuronal stimuli. Among them are mesenchymal stem cells, synovial fibroblasts, bone cells and chondrocytes of different origin, which express distinct subtypes of adrenoceptors (AR), receptors for vasoactive intestinal peptide (VIP), substance P (SP) and calcitonin gene-related peptide (CGRP). Some of these cell types synthesize and secrete neuropeptides such as SP, and they are positive for tyrosine-hydroxylase (TH), the rate limiting enzyme for biosynthesis of catecholamines. Sensory and sympathetic neurotransmitters are involved in the pathology of inflammatory diseases such as rheumatoid arthritis (RA) which manifests mainly in the joints. In addition, they seem to play a role in pathogenesis of priori degenerative joint disorders such as osteoarthritis (OA). Altogether it is evident that sensory and sympathetic neurotransmitters have crucial trophic effects which are critical for joint tissue and bone homeostasis. They modulate articular cartilage, subchondral bone and synovial tissue properties in physiological and pathophysiological conditions, in addition to their classical neurological features.

β2-adrenergic receptor control of endosomal PTH receptor signaling via Gβγ

Cells express several G-protein-coupled receptors (GPCRs) at their surfaces, transmitting simultaneous extracellular hormonal and chemical signals into cells. A comprehensive understanding of mechanisms underlying the integrated signaling response induced by distinct GPCRs is thus required. Here we found that the β₂-adrenergic receptor, which induces a short cAMP response, prolongs nuclear cAMP and protein kinase A (PKA) activation by promoting endosomal cAMP production in parathyroid hormone (PTH) receptor signaling through the stimulatory action of G protein Gβγ subunits on adenylate cyclase type 2.

Clonidine increases bone resorption in humans

SUMMARY

Inhibition of sympathetic signaling to bone reduces bone resorption in rodents. In contrast, we show that pharmacological reduction of the sympathetic tone increases bone resorption in humans in vivo. This effect does not appear to be mediated via a direct pharmacological effect on the osteoclast.

INTRODUCTION

Inhibition of sympathetic signaling to bone reduces bone resorption in rodents. It is uncertain whether a similar role for the sympathetic nervous system exists in humans. The sympathetic tone can be reduced by clonidine, which acts via alpha-2-adrenergic receptors in the brainstem. Our objective was to determine the effect of clonidine on bone turnover in humans.

METHODS

The acute effect of a single oral dose of 0.3 mg clonidine on serum bone turnover markers (C-terminal cross-linking telopeptides of collagen type I (CTx), a marker for bone resorption, and procollagen type 1 N propeptide (P1NP), a marker for bone formation) was determined in a randomized crossover design in 12 healthy volunteers, aged 18-70 years. In addition, we assessed the effect of clonidine on the number of tartrate-resistant acid phosphatase-positive multinucleated cells (TRAcP(+) MNCs) and bone resorption.

RESULTS

CTx concentrations increased after clonidine treatment compared to the control condition (p = 0.035). P1NP concentrations were not affected by clonidine (p = 0.520). In vitro, clonidine had no effect on the number of TRAcP(+) MNCs (p = 0.513) or on bone resorption (p = 0.996).

CONCLUSIONS

We demonstrated that clonidine increases bone resorption in humans in vivo. This effect does not appear to be mediated via a direct effect on the osteoclast.

Lack of α2C-Adrenoceptor Results in Contrasting Phenotypes of Long Bones and Vertebra and Prevents the Thyrotoxicosis-Induced Osteopenia

A series of studies have demonstrated that activation of the sympathetic nervous system (SNS) causes osteopenia via β₂-adrenoceptor (β₂-AR) signaling. However, in a recent study, we found an unexpected and generalized phenotype of high bone mass in female mice with chronic sympathetic hyperactivity, due to double gene inactivation of adrenoceptors that negatively regulate norepinephrine release, α_2A-and α_2C-AR (α_2A/2C-AR^-/-). These findings suggest that β₂-AR is not the single adrenoceptor involved in bone turnover regulation and show that α₂-AR signaling may also mediate the SNS actions in the skeleton. In addition, we found that α_2A/2C-AR^-/- animals are resistant to the thyrotoxicosis-induced osteopenia, suggesting that thyroid hormone (TH), when in supraphysiological levels, interacts with the SNS to control bone mass and structure, and that this interaction may also involve α₂-AR signaling. In the present study, to further investigate these hypotheses and to discriminate the roles of α₂-AR subtypes, we have evaluated the bone phenotype of mice with the single gene inactivation of α_2C-AR subtype, which mRNA expression was previously shown to be down regulated by triiodothyronine (T3). A cohort of 30 day-old female α_2CAR^-/- mice and their wild-type (WT) controls were treated with a supraphysiological dose of T3 for 30 or 90 days, which induced a thyrotoxic state in both mouse lineages. The micro-computed tomographic (μCT) analysis showed that α_2C-AR^-/- mice present lower trabecular bone volume (BV/TV) and number (Tb.N), and increased trabecular separation (Tb.Sp) in the femur compared with WT mice; which was accompanied by decreased bone strength (determined by the three-point bending test) in the femur and tibia. The opposite was observed in the vertebra, where α_2C-AR^-/- mice show increased BV/TV, Tb.N and trabecular thickness (Tb.Th), and decreased Tb.Sp, compared with WT animals. In spite of the contrasting bone phenotypes of the femur and L5, thyrotoxicosis negatively regulated most of the micro architectural features of the trabecular bone in both skeletal sites of WT, but not of α_2C-AR^-/- mice. T3 treatment also decreased biomechanical properties (maximum load and ultimate load) in the femur and tibia of WT, but not of knockout mice. The mRNA expression of osteocalcin, a marker of mature osteoblasts, and tartrate-resistant acid phosphatase, which is expressed by osteoclasts and is involved in collagen degradation, was increased by T3 treatment only in WT, and not in α_2C-AR^-/- mice. Altogether, these findings suggest that α_2C-AR subtype mediates the effects of the SNS in the bone in a skeletal site-dependent manner, and that thyrotoxicosis depends on α_2C-AR signaling to promote bone loss, which sustains the hypothesis of a TH-SNS interaction to modulate bone remodeling and structure.