Norepinephrine inhibition of mesenchymal stem cell and chondrogenic progenitor cell chondrogenesis and acceleration of chondrogenic hypertrophy

Sympathetic innervation induces exosomal miR-125 transfer from osteoarthritic chondrocytes, disrupting subchondral bone homeostasis and aggravating cartilage damage in aging mice

INTRODUCTION

Osteoarthritis (OA) is a progressive disease that poses a significant threat to human health, particularly in aging individuals: Although sympathetic activation has been implicated in bone metabolism, its role in the development of OA related to aging remains poorly understood. Therefore, this study aimed to investigate how sympathetic regulation impacts aging-related OA through experiments conducted both in vivo and in vitro.

METHODS

To analyze the effect of sympathetic regulation on aging-related OA, we conducted experiments using various mouse models. These models included a natural aging model, a medial meniscus instability model, and a load-induced model, which were used to examine the involvement of sympathetic nerves. In order to evaluate the expression levels of β1-adrenergic receptor (Adrβ1) and sirtuin-6 (Sirt6) in chondrocytes of naturally aging OA mouse models, we performed assessments. Additionally, we investigated the influence of β1-adrenergic receptor knockout or treatment with a β1-adrenergic receptor blocker on the progression of OA in aging mice and detected exosome release and detected downstream signaling expression by inhibiting exosome release. Furthermore, we explored the impact of sympathetic depletion through tyrosine hydroxylase (TH) on OA progression in aging mice. Moreover, we studied the effects of norepinephrine(NE)-induced activation of the β1-adrenergic receptor signaling pathway on the release of exosomes and miR-125 from chondrocytes, subsequently affecting osteoblast differentiation in subchondral bone.

RESULTS

Our findings demonstrated a significant increase in sympathetic activity, such as NE levels, in various mouse models of OA including natural aging, medial meniscus instability, and load-induced models. Notably, we observed alterations in the expression levels of β1-adrenergic receptor and Sirt6 in chondrocytes in OA mouse models associated with natural aging, leading to an improvement in the progression of OA. Critically, we found that the knockout of β1-adrenergic receptor or treatment with a β1-adrenergic receptor blocker attenuated OA progression in aging mice and the degraded cartilage explants produced more exosome than the nondegraded ones, Moreover, sympathetic depletion through TH was shown to ameliorate OA progression in aging mice. Additionally, we discovered that NE-induced activation of the β1-adrenergic receptor signaling pathway facilitated the release of exosomes and miR-125 from chondrocytes, promoting osteoblast differentiation in subchondral bone.

CONCLUSION

In conclusion, our study highlights the role of sympathetic innervation in facilitating the transfer of exosomal miR-125 from osteoarthritic chondrocytes, ultimately disrupting subchondral bone homeostasis and exacerbating cartilage damage in aging mice. These findings provide valuable insights into the potential contribution of sympathetic regulation to the pathogenesis of aging-related OA.

Viral infections of the central nervous system increase the risk of knee osteoarthritis: a two-sample mendelian randomization study

OBJECTIVE

Osteoarthritis (OA) represents a condition under the influence of central nervous system (CNS) regulatory mechanisms. This investigation aims to examine the causal association between viral infections of the central nervous system (VICNS) and inflammatory diseases of the central nervous system (IDCNS) and knee osteoarthritis (KOA) at the genetic level.

METHODS

In this investigation, VICNS and IDCNS were considered as primary exposure variables, while KOA served as the primary outcome. Employing a two-sample mendelian randomization (MR) approach, we conducted an analysis utilizing summary data derived from genome-wide association studies (GWAS). The GWAS summary data pertaining to VICNS and IDCNS were procured from the Finnish consortium, whereas the IEU OpenGWAS database furnished the requisite data for KOA. To ensure the robustness of our genetic causal assessment, a comprehensive array of sensitivity analyses was undertaken, encompassing evaluations of heterogeneity, horizontal pleiotropy, outlier identification, leave-one-out analyses, and assessment of the normal distribution.

RESULTS

The results of the MR analyses revealed a suggestive positive genetic causal relationship between VICNS and KOA (P = 0.012, odds ratio [OR] with a 95% confidence interval [CI] of 1.033 [1.007-1.059]). Conversely, the MR analyses did not indicate any evidence of genetic causation between IDCNS and KOA (P = 0.575, OR 95% CI = 0.986 [0.940-1.035]). Importantly, the genetic causal assessment of the exposure and outcome variables did not demonstrate any indications of heterogeneity, horizontal pleiotropy, or outliers. Furthermore, this assessment remained robust against the influence of individual single nucleotide polymorphisms (SNPs) and exhibited adherence to a normal distribution.

CONCLUSION

The result of this study has elucidated a suggestive positive genetic causal link between the VICNS and KOA. However, no such genetic causal relationship was observed between the IDCNS and KOA. These findings substantiate the genetic underpinnings supporting the association between the CNS and OA.

β2-adrenoceptors kick osteoarthritis - Time to rethink prevention and therapy

Although, during the past decades, substantial advances emerged in identifying major local and systemic factors contributing to initiation and progression of osteoarthritis (OA), some neuroendocrine mechanisms are still not understood or even neglected when thinking about novel therapeutic options. One of which is the sympathetic nervous system that exhibits various OA-promoting effects in different tissues of the joint. Interestingly, the β2-adrenoceptor (AR) mediates the majority of these effects as demonstrated by several in vitro, in vivo as well as in clinical studies. This review article does not only summarize studies of the past two decades demonstrating that the β2-AR plays an OA-promoting role in different tissues of the joint but also aims to encourage the reader to think about next-level research to discover novel and innovative preventive and/or therapeutic strategies targeting the β2-AR in OA.

A History of Psycho-Neuro-Endocrine Immune Interactions in Rheumatic Diseases

BACKGROUND

All active scientists stand on the shoulders of giants and many other more anonymous scientists, and this is not different in our field of psycho-neuro-endocrine immunology in rheumatic diseases. Too often, the modern world of publishing forgets about the collective enterprise of scientists. Some journals advise the authors to present only literature from the last decade, and it has become a natural attitude of many scientists to present only the latest publications. In order to work against this general unempirical behavior, neuroimmunomodulation devotes the 30th anniversary issue to the history of medical science in psycho-neuro-endocrine immunology.

SUMMARY

Keywords were derived from the psycho-neuro-endocrine immunology research field very well known to the authors (R.H.S. has collected a list of keywords since 1994). We screened PubMed, the Cochran Library of Medicine, Embase, Scopus database, and the ORCID database to find relevant historical literature. The Snowballing procedure helped find related work. According to the historical appearance of discoveries in the field, the order of presentation follows the subsequent scheme: (1) the sensory nervous system, (2) the sympathetic nervous system, (3) the vagus nerve, (4) steroid hormones (glucocorticoids, androgens, progesterone, estrogens, and the vitamin D hormone), (5) afferent pathways involved in fatigue, anxiety, insomnia, and depression (includes pathophysiology), and (6) evolutionary medicine and energy regulation - an umbrella theory.

KEY MESSAGES

A brief history on psycho-neuro-endocrine immunology cannot address all relevant aspects of the field. The authors are aware of this shortcoming. The reader must see this review as a viewpoint through the biased eyes of the authors. Nevertheless, the text gives an overview of the history in psycho-neuro-endocrine immunology of rheumatic diseases.

Neuronal regulation of bone and tendon injury repair: a focused review

Beyond the sensation of pain, peripheral nerves have been shown to play crucial roles in tissue regeneration and repair. As a highly innervated organ, bone can recover from injury without scar formation, making it an interesting model in which to study the role of nerves in tissue regeneration. As a comparison, tendon is a musculoskeletal tissue that is hypo-innervated, with repair often resulting in scar formation. Here, we reviewed the significance of innervation in 3 stages of injury repair (inflammatory, reparative, and remodeling) in 2 commonly injured musculoskeletal tissues: bone and tendon. Based on this focused review, we conclude that peripheral innervation is essential for phases of proper bone and tendon repair, and that nerves may dynamically regulate the repair process through interactions with the injury microenvironment via a variety of neuropeptides or neurotransmitters. A deeper understanding of neuronal regulation of musculoskeletal repair, and the crosstalk between nerves and the musculoskeletal system, will enable the development of future therapies for tissue healing.

Coordination between circadian neural circuit and intracellular molecular clock ensures rhythmic activation of adult neural stem cells

The circadian clock throughout the day organizes the activity of neural stem cells (NSCs) in the dentate gyrus (DG) of adult hippocampus temporally. However, it is still unclear whether and how circadian signals from the niches contribute to daily rhythmic variation of NSC activation. Here, we show that norepinephrinergic (NEergic) projections from the locus coeruleus (LC), a brain arousal system, innervate into adult DG, where daily rhythmic release of norepinephrine (NE) from the LC NEergic neurons controlled circadian variation of NSC activation through β3-adrenoceptors. Disrupted circadian rhythmicity by acute sleep deprivation leads to transient NSC overactivation and NSC pool exhaustion over time, which is effectively ameliorated by the inhibition of the LC NEergic neuronal activity or β3-adrenoceptors-mediated signaling. Finally, we demonstrate that NE/β3-adrenoceptors-mediated signaling regulates NSC activation through molecular clock BMAL1. Therefore, our study unravels that adult NSCs precisely coordinate circadian neural circuit and intrinsic molecular circadian clock to adapt their cellular behavior across the day.

Fracture healing research: Recent insights

Bone has the rare capability of scarless regeneration that enables the complete restoration of the injured bone area. In recent decades, promising new technologies have emerged from basic, translational and clinical research for fracture treatment; however, 5-10 % of all bone fractures still fail to heal successfully or heal in a delayed manner. Several comorbidities and risk factors have been identified which impair bone healing and might lead to delayed bone union or non-union. Therefore, a considerable amount of research has been conducted to elucidate molecular mechanisms of successful and delayed fracture healing to gain further insights into this complex process. One focus of recent research is to investigate the complex interactions of different cell types and the action of progenitor cells during the healing process. Of particular interest is also the identification of patient-specific comorbidities and how these affect fracture healing. In this review, we discuss the recent knowledge about progenitor cells for long bone repair and the influence of comorbidities such as diabetes, postmenopausal osteoporosis, and chronic stress on the healing process. The topic selection for this review was made based on the presented studies at the 2022 annual meeting of the European Calcified Tissue Society (ECTS) in Helsinki.

The effect of local sympatholysis on bone-tendon interface healing in a murine rotator cuff repair model

Background

Although neuroregulation plays an important role in tissue healing, the key neuroregulatory pathways and related neurotransmitters involved in bone-tendon interface (BTI) healing are still unknown. It is reported that sympathetic nerves can regulate cartilage and bone metabolism, which are the basic aspects of BTI repair after injury, through the release of norepinephrine (NE). Thus, the purpose of this study was to explore the effect of local sympatholysis (LS) on BTI healing in a murine rotator cuff repair model.

Methods

Specifically, C57BL/6 mice underwent unilateral supraspinatus tendon (SST) detachment and repair was established on a total of 174 mature C57BL/6 mice (12 weeks old): 54 mice were used to examine the sympathetic fibers and its neurotransmitter NE for the representation of sympathetic innervation of BTI, while the rest of them were randomly allocated into (LS) group and control group to verify the effect of sympathetic denervation during BTI healing. The LS group were intervened with fibrin sealant containing 10 ​ng/ml guanethidine, while the control group received fibrin sealant only. Mice were euthanized at postoperative 2, 4 and 8 weeks for immunofluorescent, qRT-PCR, ELISA, Micro-computed tomography (CT), histology and biomechanical evaluations.

Results

Immunofluorescence, qRT-PCR and ELISA evaluations indicated that there were the expression of tyrosine hydroxylase (TH), NE and β2-adrenergic receptor (β2-AR) at the BTI site. All the above showed a trend of increasing at the early postoperative stage and they started to decrease with the healing time after a significant peak. Meanwhile, local sympathetic denervation of BTI was achieved after the use of guanethidine as shown in the NE ELISA outcomes in two groups. QRT-PCR analysis revealed that the healing interface in the LS group expressed more transcription factors, such as Runx2, Bmp2, Sox9, and Aggrecan, than the control group. Further, radiographic data showed that the LS group significantly possessed higher bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), and lower trabecular spacing (Tb.Sp) than the control group. Also, histological test results showed that there was more fibrocartilage regenerated at the healing interface in the LS group compared with the control group. Mechanical testing results demonstrated that the failure load, ultimate strength and stiffness in the LS group were significantly higher at postoperative week 4 (P ​< ​0.05), but not at postoperative week 8 (P ​> ​0.05), compared to the control group.

Conclusion

The regulation of sympathetic innervation was involved in the healing process of injured BTI, and local sympathetic denervation by using guanethidine was beneficial for BTI healing outcomes.The translational potential of this article: This is the first study to evaluate the expression and specific role of sympathetic innervation during BTI healing. The findings of this study also imply that the antagonists of β2-AR could serve as a potential therapeutic strategy for BTI healing. Also, we firstly successfully constructed a local sympathetic denervation mouse model by using guanethidine loaded fibrin sealant, which provided a new effective methodology for future neuroskeletal biology study.

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.

The role of nerve fibers and their neurotransmitters in regulating intervertebral disc degeneration

Intervertebral disc degeneration (IVDD) has been the major contributor to chronic lower back pain (LBP). Abnormal apoptosis, senescence, and pyroptosis of IVD cells, extracellular matrix (ECM) degradation, and infiltration of immune cells are the major molecular alternations during IVDD. Changes at tissue level frequently occur at advanced IVD tissue. Ectopic ingrowth of nerves within inner annulus fibrosus (AF) and nucleus pulposus (NP) tissue has been considered as the primary cause for LBP. Innervation at IVD tissue mainly included sensory and sympathetic nerves, and many markers for these two types of nerves have been detected since 1940. In fact, in osteoarthritis (OA), beyond pain transmission, the direct regulation of neuropeptides on functions of chondrocytes have attracted researchers' great attention recently. Many physical and pathological similarities between joint and IVD have shed us the light on the neurogenic mechanism involved in IVDD. Here, an overview of the advances in the nervous system within IVD tissue will be performed, with a discussion on in the role of nerve fibers and their neurotransmitters in regulating IVDD. We hope this review can attract more research interest to address neuromodulation and IVDD itself, which will enhance our understanding of the contribution of neuromodulation to the structural changes within IVD tissue and inflammatory responses and will help identify novel therapeutic targets and enable the effective treatment of IVDD disease.

β2-Adrenoceptor Deficiency Results in Increased Calcified Cartilage Thickness and Subchondral Bone Remodeling in Murine Experimental Osteoarthritis

Purpose

Recent studies demonstrated a contribution of adrenoceptors (ARs) to osteoarthritis (OA) pathogenesis. Several AR subtypes are expressed in joint tissues and the β2-AR subtype seems to play a major role during OA progression. However, the importance of β2-AR has not yet been investigated in knee OA. Therefore, we examined the development of knee OA in β2-AR-deficient (Adrb2^-/-) mice after surgical OA induction.

Methods

OA was induced by destabilization of the medial meniscus (DMM) in male wildtype (WT) and Adrb2^-/- mice. Cartilage degeneration and synovial inflammation were evaluated by histological scoring. Subchondral bone remodeling was analyzed using micro-CT. Osteoblast (alkaline phosphatase - ALP) and osteoclast (cathepsin K - CatK) activity were analyzed by immunostainings. To evaluate β2-AR deficiency-associated effects, body weight, sympathetic tone (splenic norepinephrine (NE) via HPLC) and serum leptin levels (ELISA) were determined. Expression of the second major AR, the α2-AR, was analyzed in joint tissues by immunostaining.

Results

WT and Adrb2^-/- DMM mice developed comparable changes in cartilage degeneration and synovial inflammation. Adrb2^-/- DMM mice displayed elevated calcified cartilage and subchondral bone plate thickness as well as increased epiphyseal BV/TV compared to WTs, while there were no significant differences in Sham animals. In the subchondral bone of Adrb2^-/- mice, osteoblasts activity increased and osteoclast activity deceased. Adrb2^-/- mice had significantly higher body weight and fat mass compared to WT mice. Serum leptin levels increased in Adrb2^-/- DMM compared to WT DMM without any difference between the respective Shams. There was no difference in the development of meniscal ossicles and osteophytes or in the subarticular trabecular microstructure between Adrb2^-/- and WT DMM as well as Adrb2^-/- and WT Sham mice. Number of α2-AR-positive cells was lower in Adrb2^-/- than in WT mice in all analyzed tissues and decreased in both Adrb2^-/- and WT over time.

Conclusion

We propose that the increased bone mass in Adrb2^-/- DMM mice was not only due to β2-AR deficiency but to a synergistic effect of OA and elevated leptin concentrations. Taken together, β2-AR plays a major role in OA-related subchondral bone remodeling and is thus an attractive target for the exploration of novel therapeutic avenues.

Mechanical loading alleviated the inhibition of β2-adrenergic receptor agonist terbutaline on bone regeneration

The long-term use of adrenergic medication in treating various conditions, such as asthma, increases the chances of bone fracture. Dynamic mechanical loading at a specific time is a method for improving bone quality and promoting healing. Therefore, we hypothesized that precisely controlling the mechanical environment can contribute to the alleviation of the negative effects of chronic treatment with the common asthma drug terbutaline, which is a β2-adrenergic receptor agonist that facilitates bone homeostasis and defect repair through its anabolic effect on osteogenic cells. Our in vitro results showed that terbutaline can directly inhibit osteogenesis by impairing osteogenic differentiation and mineralization. Chronic treatment in vivo was simulated by administering terbutaline to C57BL/6J mice for 4 weeks before bone defect surgery and mechanical loading. We utilized a stabilized tibial defect model, which allowed the application of anabolic mechanical loading. During homeostasis, chronic terbutaline treatment reduced the bone formation rate, the fracture toughness of long bones, and the concentrations of bone formation markers in the sera. During defect repair, terbutaline decreased the bone volume, type H vessel, and total blood vessel volume. Terbutaline treatment reduced the number of osteogenic cells. Periostin, which was secreted mainly by Prrx1+ osteoprogenitors and F4/80+ macrophages, was inhibited by treating the bone defect with terbutaline. Interestingly, controlled mechanical loading facilitated the recovery of bone volume and periostin expression and the number of osteogenic cells within the defect. In conclusion, mechanical loading can rescue negative effects on new bone accrual and repair induced by chronic terbutaline treatment.

Norepinephrine modulates IL-1β-induced catabolic response of human chondrocytes

Background

The influence of the sympathetic nervous system (SNS) on metabolism of bone and cartilage expressing β-adrenergic receptors (AR) was suggested. Here, we investigated whether the SNS functions as a modulator of cartilage metabolism induced by interleukin-1beta (IL-1β).

Methods

Human articular chondrocytes and articular cartilage were collected from patients with osteoarthritis (OA). Chondrocyte monolayer and cartilage explant culture were stimulated with IL-1β. The activity of β-ARs was modulated by an agonist, norepinephrine (NE), and antagonists, including propranolol, atenolol, nebivolol, and nadolol.

Results

The levels of β₁-, β₂-, and β₃-AR in OA cartilage and IL-1β-treated chondrocytes were lower than normal cartilage and untreated cells. Treatment of chondrocytes with IL-1β and β-blockers, including propranolol, atenolol, nebivolol, and nadolol, for 6 h significantly upregulated IL-1β-induced expression of MMP-1, -3, and − 13, compared to chondrocytes treated with IL-1β alone, indicating that antagonism of β-AR confers catabolic signals. On the other hand, NE antagonized IL-1β-induced catabolic response. In addition, NE significantly inhibited IL-1β-induced release of glycosaminoglycan (GAG) from cartilage explant culture. In addition, β-AR activity significantly affected IL-1β-stimulated phosphorylation of JNK and ERK. These results indicate that β-AR signal is associated with cartilage metabolism.

Conclusions

Our findings showed that β-ARs is a regulator of cartilage catabolism induced with IL-1β.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04598-7.

Effect and mechanism of propranolol on promoting osteogenic differentiation and early implant osseointegration

The present study aimed to investigate the effect of β‑receptor blocker propranolol on early osseointegration of pure titanium implants and the underlying molecular regulatory mechanisms. An implant osseointegration model using the tibial metaphysis of New Zealand rabbits was established. The rabbits were divided into control and low‑, medium‑ and high‑dose propranolol groups. The formation of implant osseointegration was detected by X‑ray scanning. Mesenchymal stem cells (MSCs) and osteoblasts (OBs) were isolated and cultured in vitro, isoproterenol was supplemented to simulate sympathetic action and propranolol was subsequently administrated. The effect of propranolol on cell proliferation and osteogenic differentiation were assessed by EdU, flow cytometry, alizarin red staining and alkaline phosphatase (ALP) detection. The expression levels of bone morphogenetic protein (BMP)2, RUNX family transcription factor (RunX)2, collagen (COL)‑1, osteocalcin (OCN) and β2‑adrenergic receptor (AR) were detected by immunofluorescence, reverse transcription‑quantitative PCR and western blot assay. Propranolol effectively promoted implant osseointegration in vivo, facilitated proliferation of OBs, inhibited proliferation of MSCs and enhanced osteogenic differentiation of OBs and MSCs. The calcium content and ALP activity of cells treated with propranolol were markedly higher than in the control group. Propranolol also elevated mRNA and protein expression levels of BMP2, RunX2, COL‑1 and OCN in tissue and cells, and decreased the expression of β2‑AR. The present study demonstrated that the β‑receptor blocker propranolol promoted osteogenic differentiation of OBs and MSCs and enhanced implant osseointegration. The present study provided a novel insight into the application and regulatory mechanisms of propranolol.

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.

Enhancing the chondrogenic potential of chondrogenic progenitor cells by deleting RAB5C

Osteoarthritis (OA) is the most prevalent chronic joint disease that affects a large proportion of the elderly population. Chondrogenic progenitor cells (CPCs) reside in late-stage OA cartilage tissue, producing a fibrocartilaginous extracellular matrix; these cells can be manipulated in vitro to deposit proteins of healthy articular cartilage. CPCs are under the control of SOX9 and RUNX2. In our earlier studies, we showed that a knockdown of RUNX2 enhanced the chondrogenic potential of CPCs. Here we demonstrate that CPCs carrying a knockout of RAB5C, a protein involved in endosomal trafficking, exhibited elevated expression of multiple chondrogenic markers, including the SOX trio, and increased COL2 deposition, whereas no changes in COL1 deposition were observed. We report RAB5C as an attractive target for future therapeutic approaches designed to increase the COL2 content in the diseased joint.

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'.

Norepinephrine Inhibits the Proliferation of Human Bone Marrow-Derived Mesenchymal Stem Cells via β2-Adrenoceptor-Mediated ERK1/2 and PKA Phosphorylation

Bone marrow-derived mesenchymal stem cells (BMSCs) represent an alternative to chondrocytes to support cartilage regeneration in osteoarthritis (OA). The sympathetic neurotransmitter norepinephrine (NE) has been shown to inhibit their chondrogenic potential; however, their proliferation capacity under NE influence has not been studied yet. Therefore, we used BMSCs obtained from trauma and OA donors and compared the expression of adrenergic receptors (AR). Then, BMSCs from both donor groups were treated with NE, as well as with combinations of NE and α1-, α2- or β1/2-AR antagonists (doxazosin, yohimbine or propranolol). Activation of AR-coupled signaling was investigated by analyzing ERK1/2 and protein kinase A (PKA) phosphorylation. A similar but not identical subset of ARs was expressed in trauma (α2B-, α2C- and β2-AR) and OA BMSCs (α2A-, α2B-, and β2-AR). NE in high concentrations inhibited the proliferation of both trauma and OA BMCSs significantly. NE in low concentrations did not influence proliferation. ERK1/2 as well as PKA were activated after NE treatment in both BMSC types. These effects were abolished only by propranolol. Our results demonstrate that NE inhibits the proliferation and accordingly lowers the regenerative capacity of human BMSCs likely via β2-AR-mediated ERK1/2 and PKA phosphorylation. Therefore, targeting β2-AR-signaling might provide novel OA therapeutic options.

Adrenoceptor Expression during Intervertebral Disc Degeneration

Healthy and degenerating intervertebral discs (IVDs) are innervated by sympathetic nerves, however, adrenoceptor (AR) expression and functionality have never been investigated systematically. Therefore, AR gene expression was analyzed in both tissue and isolated cells from degenerated human IVDs. Furthermore, human IVD samples and spine sections of wildtype mice (WT) and of a mouse line that develops spontaneous IVD degeneration (IVDD, in SM/J mice) were stained for ARs and extracellular matrix (ECM) components. In IVD homogenates and cells α1a-, α1b-, α2a-, α2b-, α2c-, β1-, and β2-AR genes were expressed. In human sections, β2-AR was detectable, and its localization parallels with ECM alterations. Similarly, in IVDs of WT mice, only β2-AR was expressed, and in IVDs of SM/J mice, β2AR expression was stronger accompanied by increased collagen II, collagen XII, decorin as well as decreased cartilage oligomeric matrix protein expression. In addition, norepinephrine stimulation of isolated human IVD cells induced intracellular signaling via ERK1/2 and PKA. For the first time, the existence and functionality of ARs were demonstrated in IVD tissue samples, suggesting that the sympathicus might play a role in IVDD. Further studies will address relevant cellular mechanisms and thereby help to develop novel therapeutic options for IVDD.

Norepinephrine Inhibits Synovial Adipose Stem Cell Chondrogenesis via α2a-Adrenoceptor-Mediated ERK1/2 Activation

In recent years, first evidences emerged that sympathetic neurotransmitters influence osteoarthritis (OA) manifestation. Joint-resident stem cells might contribute to cartilage repair, however, their chondrogenic function is reduced. The neurotransmitter norepinephrine (NE) was detected in the synovial fluid of trauma and OA patients. Therefore, the aim of this study was to analyse how NE influences the chondrogenesis of synovial adipose tissue-derived stem cells (sASCs). sASCs were isolated from knee-OA patients synovia. After adrenoceptor (AR) expression analysis, proliferation and chondrogenic differentiation in presence of NE and/or α- and β-AR antagonist were investigated. Cell count, viability, chondrogenic and hypertophic gene expression, sulfated glycosaminoglycan (sGAG) and type II collagen content were determined. Key AR-dependent signaling (ERK1/2, PKA) was analyzed via western blot. sASC expressed α1A-, α1B-, α2A-, α2B-, α2C-, and β2-AR in monolayer and pellet culture. NE did not affect proliferation and viability, but 10⁻⁷ and 10⁻⁶ M NE significantly reduced sGAG and type II collagen content as well as ERK1/2 phosphorylation. These effects were fully reversed by yohimbine (α2-AR antagonist). Our study confirms the important role of NE in sASC chondrogenic function and provides new insights in OA pathophysiology. Future studies might help to develop novel therapeutic options targeting neuroendocrine pathways for OA treatment.

Role of Norepinephrine in IL-1β-Induced Chondrocyte Dedifferentiation under Physioxia

As part of the pathogenesis of osteoarthritis (OA), chondrocytes lose their phenotype and become hypertrophic, or dedifferentiate, mainly driven by interleukin-1β (IL-1β). The contribution of other factors to the dedifferentiation process is not completely understood. Recent studies suggested a dose-dependent role for the sympathetic neurotransmitter norepinephrine (NE) in OA chondrocyte metabolism. Therefore, the aim of this study was to analyze the contribution of NE (10^-8 M, 10^-6 M) to human articular OA chondrocyte dedifferentiation in the absence or presence of IL-1β (0.5 ng/mL). Here, we demonstrate that OA chondrocytes express α2A-, α2C- and β2-adrenoceptors (AR) and show the characteristic shift towards a fibroblast-like shape at day 7 in physioxic monolayer culture. NE alone did not affect morphology but, in combination with IL-1β, markedly accelerated this shift. Moderate glycosaminoglycan (GAG) staining was observed in untreated and NE-treated cells, while IL-1β strongly decreased GAG deposition. IL-1β alone or in combination with NE decreased SOX9, type II collagen, COMP, and aggrecan, and induced MMP13 and ADAMTS4 gene expression, indicating an accelerated dedifferentiation. NE alone did not influence gene expression and did not modulate IL-1β-mediated effects. In conclusion, these results indicate that low-grade inflammation exerts a dominant effect on chondrocyte dedifferentiation and should be targeted early in OA therapy.

The Effect of Dexamethasone, Adrenergic and Cholinergic Receptor Agonists on Phospholipid Metabolism in Human Osteoarthritic Synoviocytes

Phospholipids (PLs) possess the unique ability to contribute to synovial joint lubrication. The aim of our study was to determine for the first time the effect of dexamethasone and some adrenergic and cholinergic agonists on the biosynthesis and release of PLs from human fibroblast-like synoviocytes (FLS). Osteoarthritic human knee FLS were treated with dexamethasone, terbutaline, epinephrine, carbachol, and pilocarpine, or the glucocorticoid receptor antagonist RU 486. Simultaneously PL biosynthesis was determined through the incorporation of stable isotope-labeled precursors into PLs. Radioactive isotope-labeled precursors were used to radiolabel PLs for the subsequent quantification of their release into nutrient media. Lipids were extracted and quantified using electrospray ionization tandem mass spectrometry or liquid scintillation counting. Dexamethasone significantly decreased the biosynthesis of phosphatidylcholine, phosphatidylethanolamine (PE), PE-based plasmalogen, and sphingomyelin. The addition of RU 486 abolished these effects. A release of PLs from FLS into nutrient media was not recognized by any of the tested agents. None of the adrenergic or cholinergic receptor agonists modulated the PL biosynthesis. We demonstrate for the first time an inhibitory effect of dexamethasone on the PL biosynthesis of FLS from human knees. Moreover, our study indicates that the PL metabolism of synovial joints and lungs are differently regulated.

Tissue regeneration: Impact of sleep on stem cell regenerative capacity

The circadian rhythm orchestrates many cellular functions, such as cell division, cell migration, metabolism and numerous intracellular biological processes. The physiological changes during sleep are believed to promote a suitable microenvironment for stem cells to proliferate, migrate and differentiate. These effects are mediated either directly by circadian clock genes or indirectly via hormones and cytokines. Hormones, such as melatonin and cortisol, are secreted in response to neural optic signals and act in harmony to regulate many biological functions during sleep. Herein, we correlate the effects of the main circadian genes on the expression of certain stem cell genes responsible for the regeneration of different tissues, including bone, cartilage, skin, and intestine. We also review the effects of different hormones and cytokines on stem cell activation or suppression and their relationship to the day/night cycle. The correlation of circadian rhythm with tissue regeneration could have implications in understanding the biology of sleep and tissue regeneration and in enhancing the efficacy and timing of surgical procedures.

β-Blockade use for Traumatic Injuries and Immunomodulation: A Review of Proposed Mechanisms and Clinical Evidence

Sympathetic nervous system activation and catecholamine release are important events following injury and infection. The nature and timing of different pathophysiologic insults have significant effects on adrenergic pathways, inflammatory mediators, and the host response. Beta adrenergic receptor blockers (β-blockers) are commonly used for treatment of cardiovascular disease, and recent data suggests that the metabolic and immunomodulatory effects of β-blockers can expand their use. β-blocker therapy can reduce sympathetic activation and hypermetabolism as well as modify glucose homeostasis and cytokine expression. It is the purpose of this review to examine either the biologic basis for proposed mechanisms or to describe current available clinical evidence for the use of β-blockers in traumatic brain injury, spinal cord injury, hemorrhagic shock, acute traumatic coagulopathy, erythropoietic dysfunction, metabolic dysfunction, pulmonary dysfunction, burns, immunomodulation, and sepsis.

Chronic psychosocial stress disturbs long-bone growth in adolescent mice

Although a strong association between psychiatric and somatic disorders is generally accepted, little is known regarding the interrelationship between mental and skeletal health. Although depressive disorders have been shown to be strongly associated with osteoporosis and increased fracture risk, evidence from post-traumatic stress disorder (PTSD) patients is less consistent. Therefore, the present study investigated the influence of chronic psychosocial stress on bone using a well-established murine model for PTSD. C57BL/6N mice (7 weeks old) were subjected to chronic subordinate colony housing (CSC) for 19 days, whereas control mice were singly housed. Anxiety-related behavior was assessed in the open-field/novel-object test, after which the mice were euthanized to assess endocrine and bone parameters. CSC mice exhibited increased anxiety-related behavior in the open-field/novel-object test, increased adrenal and decreased thymus weights, and unaffected plasma morning corticosterone. Microcomputed tomography and histomorphometrical analyses revealed significantly reduced tibia and femur lengths, increased growth-plate thickness and reduced mineral deposition at the growth plate, suggesting disturbed endochondral ossification during long-bone growth. This was associated with reduced Runx2 expression in hypertrophic chondrocytes in the growth plate. Trabecular thicknesses and bone mineral density were significantly increased in CSC compared to singly housed mice. Tyrosine hydroxylase expression was increased in bone marrow cells located at the growth plates of CSC mice, implying that local adrenergic signaling might be involved in the effects of CSC on the skeletal phenotype. In conclusion, chronic psychosocial stress negatively impacts endochondral ossification in the growth plate, affecting both longitudinal and appositional bone growth in adolescent mice.

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.

Activation of α2A-adrenergic signal transduction in chondrocytes promotes degenerative remodelling of temporomandibular joint

This study tested whether activation of adrenoreceptors in chondrocytes has roles in degenerative remodelling of temporomandibular joint (TMJ) and to determine associated mechanisms. Unilateral anterior crossbite (UAC) was established to induce TMJ degeneration in rats. Saline vehicle, α2- and β-adrenoreceptor antagonists or agonists were injected locally into the TMJ area of UAC rats. Cartilage degeneration, subchondral bone microarchitecture and the expression of adrenoreceptors, aggrecans, matrix metalloproteinases (MMPs) and RANKL by chondrocytes were evaluated. Chondrocytes were stimulated by norepinephrine to investigate signal transduction of adrenoreceptors. Increased α2A-adrenoreceptor expression was observed in condylar cartilage of UAC rats, together with cartilage degeneration and subchondral bone loss. Norepinephrine depresses aggrecans expression but stimulates MMP-3, MMP-13 and RANKL production by chondrocytes through ERK1/2 and PKA pathway; these effects were abolished by an α2A-adrenoreceptor antagonist. Furthermore, inhibition of α2A-adrenoreceptor attenuated degenerative remodelling in the condylar cartilage and subchondral bone, as revealed by increased cartilage thickness, proteoglycans and aggrecan expression, and decreased MMP-3, MMP-13 and RANKL expressions in cartilage, increased BMD, BV/TV, and decreased Tb.Sp in subchondral bone. Conversely, activation of α2A-adrenoreceptor intensified aforementioned degenerative changes in UAC rats. It is concluded that activation of α2A-adrenergic signal in chondrocytes promotes TMJ degenerative remodelling by chondrocyte-mediated pro-catabolic activities.

Mesenchymal Stem Cells after Polytrauma: Actor and Target

Mesenchymal stem cells (MSCs) are multipotent cells that are considered indispensable in regeneration processes after tissue trauma. MSCs are recruited to damaged areas via several chemoattractant pathways where they function as “actors” in the healing process by the secretion of manifold pro- and anti-inflammatory, antimicrobial, pro- and anticoagulatory, and trophic/angiogenic factors, but also by proliferation and differentiation into the required cells. On the other hand, MSCs represent “targets” during the pathophysiological conditions after severe trauma, when excessively generated inflammatory mediators, complement activation factors, and damage- and pathogen-associated molecular patterns challenge MSCs and alter their functionality. This in turn leads to complement opsonization, lysis, clearance by macrophages, and reduced migratory and regenerative abilities which culminate in impaired tissue repair. We summarize relevant cellular and signaling mechanisms and provide an up-to-date overview about promising future therapeutic MSC strategies in the context of severe tissue trauma.

Norepinephrine modulates osteoarthritic chondrocyte metabolism and inflammatory responses

OBJECTIVE

Norepinephrine (NE) was measured in synovial fluid of trauma patients and sympathetic nerve fibers were detected in healthy and osteoarthritic (OA) joint tissues indicating that cartilage pathophysiology might be influenced by sympathetic neurotransmitters. The aim of this study was to elucidate the mostly unknown role of NE in OA chondrocyte metabolism and inflammatory responses.

METHODS

Articular cartilage was received after total knee replacement surgery from OA patients. Expression of adrenergic receptors (AR) and tyrosine hydroxylase (TH) was tested with end point polymerase chain reaction (PCR) and immunohistochemistry. 3-dimensional (3D) cell cultures were employed to analyze effects of NE on chondrocyte cell metabolism and the expression of interleukins (ILs), matrix metalloproteases (MMPs), tissue inhibitor of metalloproteases (TIMPs), glycosaminoglycan (GAG) and collagen II under non- and inflammatory conditions. Chondrocyte monolayer cultures were used to specify AR subtypes, to analyze cell cycle distribution and to determine catecholamines in cell culture supernatants.

RESULTS

AR subtypes and TH were detected in chondrocytes, whereas NE was not released in measurable amounts. 10(-6) M NE reversed IL-1β induced changes in IL-8, MMP-13, GAG and collagen II expression/production indicating for β-AR signaling. Additionally, NE caused cell cycle slow down and decreased proliferation via β-AR signaling. 10(-8) M NE increased the number of proliferating cells and induced apoptosis via α1-AR signaling.

CONCLUSIONS

NE affects chondrocytes from OA cartilage regarding inflammatory response and its cell metabolism in a dose dependent manner. The sympathetic nervous system (SNS) may have a dual function in OA pathology with preserving a stable chondrocyte phenotype via β-AR signaling and OA pathogenesis accelerating effects via α-AR signaling.

The role of peripheral nerve fibers and their neurotransmitters in cartilage and bone physiology and pathophysiology

The peripheral nervous system is critically involved in bone metabolism, osteogenesis, and bone remodeling. Nerve fibers of sympathetic and sensory origin innervate synovial tissue and subchondral bone of diathrodial joints. They modulate vascularization and matrix differentiation during endochondral ossification in embryonic limb development, indicating a distinct role in skeletal growth and limb regeneration processes. In pathophysiological situations, the innervation pattern of sympathetic and sensory nerve fibers is altered in adult joint tissues and bone. Various resident cell types of the musculoskeletal system express receptors for sensory and sympathetic neurotransmitters. Osteoblasts, osteoclasts, mesenchymal stem cells, synovial fibroblasts, and different types of chondrocytes produce distinct subtypes of adrenoceptors, receptors for vasointestinal peptide, for substance P and calcitonin gene-related peptide. Many of these cells even synthesize neuropeptides such as substance P and calcitonin gene-related peptide and are positive for tyrosine-hydroxylase, the rate-limiting enzyme for biosynthesis of catecholamines. Sensory and sympathetic neurotransmitters modulate osteo-chondrogenic differentiation of mesenchymal progenitor cells during endochondral ossification in limb development. In adults, sensory and sympathetic neurotransmitters are critical for bone regeneration after fracture and are involved in the pathology of inflammatory diseases as rheumatoid arthritis which manifests mainly in joints. Possibly, they might also play a role in pathogenesis of degenerative joint disorders, such as osteoarthritis. All together, accumulating data imply that sensory and sympathetic neurotransmitters have crucial trophic effects which are critical for proper limb formation during embryonic skeletal growth. In adults, they modulate bone regeneration, bone remodeling, and articular cartilage homeostasis in addition to their classic neurological actions.

Dynamic hydrostatic pressure enhances differentially the chondrogenesis of meniscal cells from the inner and outer zone

This study analyses the influence of dynamic hydrostatic pressure on chondrogenesis of human meniscus-derived fibrochondrocytes and explores the differences in chondrogenic differentiation under loading conditions between cells derived from the avascular inner zone and vascularized outer region of the meniscus. Aggregates of human fibrochondrocytes with cell origin from the inner region or with cell origin from the outer region were generated. From the two groups of either cell origin, aggregates were treated with dynamic hydrostatic pressure (1Hz for 4h; 0.55-5.03MPa, cyclic sinusoidal) from day 1 to day 7. The other aggregates served as unloaded controls. At day 0, 7, 14 and 21 aggregates were harvested for evaluation including histology, immunostaining and ELISA analysis for glycosaminoglycan (GAG) and collagen II. Loaded aggregates were found to be macroscopically larger and revealed immunohistochemically enhanced chondrogenesis compared to the corresponding controls. Loaded or non-loaded meniscal cells from the outer zone showed a higher potential and earlier onset of chondrogenesis compared to the cells from the inner part of the meniscus. This study suggests that intrinsic factors like cell properties in the different areas of the meniscus and their reaction on mechanical load might play important roles in designing Tissue Engineering strategies for meniscal repair in vivo.

Absence of substance P and the sympathetic nervous system impact on bone structure and chondrocyte differentiation in an adult model of endochondral ossification

OBJECTIVE

Sensory and sympathetic nerve fibers (SNF) innervate bone and epiphyseal growth plate. The role of neuronal signals for proper endochondral ossification during skeletal growth is mostly unknown. Here, we investigated the impact of the absence of sensory neurotransmitter substance P (SP) and the removal of SNF on callus differentiation, a model for endochondral ossification in adult animals, and on bone formation.

METHODS

In order to generate callus, tibia fractures were set in the left hind leg of wild type (WT), tachykinin 1-deficient (Tac1-/-) mice (no SP) and animals without SNF. Locomotion was tested in healthy animals and touch sensibility was determined early after fracture. Callus tissue was prepared for immunofluorescence staining for SP, neurokinin1-receptor (NK1R), tyrosine-hydroxylase (TH) and adrenergic receptors α1, α2 and β2. At the fracture site, osteoclasts were stained for TRAP, osteoblasts were stained for RUNX2, and histomorphometric analysis of callus tissue composition was performed. Primary murine bone marrow derived macrophages (BMM), osteoclasts, and osteoblasts were tested for differentiation, activity, proliferation and apoptosis in vitro. Femoral fractures were set in the left hind leg of all the three groups for mechanical testing and μCT-analysis.

RESULTS

Callus cells stained positive for SP, NK1R, α1d- and α2b adrenoceptors and remained β2-adrenoceptor and TH-negative. Absence of SP and SNF did not change the general locomotion but reduces touch sensitivity after fracture. In mice without SNF, we detected more mesenchymal callus tissue and less cartilaginous tissue 5 days after fracture. At day 13 past fracture, we observed a decrease of the area covered by hypertrophic chondrocytes in Tac1-/- mice and mice without SNF, a lower number of osteoblasts in Tac1-/- mice and an increase of osteoclasts in mineralized callus tissue in mice without SNF. Apoptosis rate and activity of osteoclasts and osteoblasts isolated from Tac1-/- and sympathectomized mice were partly altered in vitro. Mechanical testing of fractured- and contralateral legs 21 days after fracture, revealed an overall reduced mechanical bone quality in Tac1-/- mice and mice without SNF. μCT-analysis revealed clear structural alteration in contralateral and fractured legs proximal of the fracture site with respect to trabecular parameters, bone mass and connectivity density. Notably, structural parameters are altered in fractured legs when related to unfractured legs in WT but not in mice without SP and SNF.

CONCLUSION

The absence of SP and SNF reduces pain sensitivity and mechanical stability of the bone in general. The micro-architecture of the bone is profoundly impaired in the absence of intact SNF with a less drastic effect in SP-deficient mice. Both sympathetic and sensory neurotransmitters are indispensable for proper callus differentiation. Importantly, the absence of SP reduces bone formation rate whereas the absence of SNF induces bone resorption rate. Notably, fracture chondrocytes produce SP and its receptor NK1 and are positive for α-adrenoceptors indicating an endogenous callus signaling loop. We propose that sensory and sympathetic neurotransmitters have crucial trophic effects which are essential for proper bone formation in addition to their classical neurological actions.