Dopamine D1 receptor-mediated activation of the ERK signaling pathway is involved in the osteogenic differentiation of bone mesenchymal stem cells

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.

Osteo-immunomodulatory effects of macrophage-derived extracellular vesicles treated with biphasic calcium phosphate ceramics on bone regeneration

Literature on osteoimmunology has demonstrated that macrophages have a great influence on biomaterial-induced bone formation. However, there are almost no reports clarifying the osteo-immunomodulatory capacity of macrophage-derived extracellular vesicles (EVs). This study comprehensively investigated the effects of EVs derived from macrophages treated with biphasic calcium phosphate (BCP) ceramics (BEVs) on vital events associated with BCP-induced bone formation such as immune response, angiogenesis, and osteogenesis. It was found that compared with EVs derived from macrophages alone (control, CEVs), BEVs preferentially promoted macrophage polarization towards a wound-healing M2 phenotype, enhanced migration, angiogenic differentiation, and tube formation of human umbilical vein endothelial cells, and induced osteogenic differentiation of mesenchymal stem cells. Analysis of 15 differentially expressed microRNAs (DEMs) related to immune, angiogenesis, and osteogenesis suggested that BEVs exhibited good immunomodulatory, pro-angiogenic, and pro-osteogenic abilities, which might be attributed to their specific miRNA cargos. These findings not only deepen our understanding of biomaterial-mediated osteoinduction, but also suggest that EVs derived from biomaterial-treated macrophages hold great promise as therapeutic agents with desired immunomodulatory capacity for bone regeneration.

Enhancing Osteogenic Potential: Controlled Release of Dopamine D1 Receptor Agonist SKF38393 Compared to Free Administration

Osteoporosis is the most common metabolic bone disorder and is characterized by decreased bone density, which has a relationship with the quality of life among the aging population. Previous research has found that activation of the dopamine D1 receptor can improve bone mass formation. SKF38393 is an agonist of dopamine D1 receptors. However, as a small-molecule drug, SKF38393 is unstable and releases quickly. The aim of this study was to prototype polylactic-co-glycolic acid (PLGA)/SKF38393 microspheres and assess their potential osteogenic effects compared to those under the free administration of SKF38393. The cytocompatibility of PLGA/SKF38393 was determined via CCK-8 and live/dead cell staining; the osteogenic effects in vitro were determined with ALP and alizarin red staining, qRT-PCR, and Western blotting; and the in vivo effects were assessed using 25 Balb/c mice. We also used a PCR array to explore the possible signaling pathway changes after employing PLGA/SKF38393. Our experiments demonstrated that the osteogenic effect of D1Rs activated by the PLGA/SKF38393 microsphere was better than that under free administration, both in vitro and in vivo. According to the PCR array, this result might be associated with six signaling pathways (graphical abstract). Ultimately, in this study, we prototyped PLGA/SKF38393, demonstrated its effectiveness, and preliminarily analyzed its mechanism of action.

Nerve growth factor promote osteogenic differentiation of dental pulp stem cells through MEK/ERK signalling pathways

Nerve growth factor (NGF) and its receptor, tropomyosin receptor kinase A (TrkA), are known to play important roles in the immune and nervous system. However, the effects of NGF on the osteogenic differentiation of dental pulp stem cells (DPSCs) remain unclear. This study aimed to investigate the role of NGF on the osteogenic differentiation of DPSCs in vitro and the underlying mechanisms. DPSCs were cultured in osteogenic differentiation medium containing NGF (50 ng/mL) for 7 days. Then osteogenic-related genes and protein markers were analysed using qRT-PCR and Western blot, respectively. Furthermore, addition of NGF inhibitor and small interfering RNA (siRNA) transfection experiments were used to elucidate the molecular signalling pathway responsible for the process. NGF increased osteogenic differentiation of DPSCs significantly compared with DPSCs cultured in an osteogenic-inducing medium. The NGF inhibitor Ro 08-2750 (10 μM) and siRNA-mediated gene silencing of NGF receptor, TrkA and ERK signalling pathways inhibitor U0126 (10 μM) suppressed osteogenic-related genes and protein markers on DPSCs. Furthermore, our data revealed that NGF-upregulated osteogenic differentiation of DPSCs may be associated with the activation of MEK/ERK signalling pathways via TrkA. Collectively, NGF was capable of promoting osteogenic differentiation of DPSCs through MEK/ERK signalling pathways, which may enhance the DPSCs-mediated bone tissue regeneration.

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.

Mitoribosomal synthetic lethality overcomes multidrug resistance in MYC-driven neuroblastoma

Mitochondria are central for cancer responses to therapy-induced stress signals. Refractory tumors often show attenuated sensitivity to apoptotic signaling, yet clinically relevant molecular actors to target mitochondria-mediated resistance remain elusive. Here, we show that MYC-driven neuroblastoma cells rely on intact mitochondrial ribosome (mitoribosome) processivity and undergo cell death following pharmacological inhibition of mitochondrial translation, regardless of their multidrug/mitochondrial resistance and stem-like phenotypes. Mechanistically, inhibiting mitoribosomes induced the mitochondrial stress-activated integrated stress response (ISR), leading to downregulation of c-MYC/N-MYC proteins prior to neuroblastoma cell death, which could be both rescued by the ISR inhibitor ISRIB. The ISR blocks global protein synthesis and shifted the c-MYC/N-MYC turnover toward proteasomal degradation. Comparing models of various neuroectodermal tumors and normal fibroblasts revealed overexpression of MYC proteins phosphorylated at the degradation-promoting site T58 as a factor that predetermines vulnerability of MYC-driven neuroblastoma to mitoribosome inhibition. Reducing N-MYC levels in a neuroblastoma model with tunable MYCN expression mitigated cell death induction upon inhibition of mitochondrial translation and functionally validated the propensity of neuroblastoma cells for MYC-dependent cell death in response to the mitochondrial ISR. Notably, neuroblastoma cells failed to develop significant resistance to the mitoribosomal inhibitor doxycycline over a long-term repeated (pulsed) selection. Collectively, we identify mitochondrial translation machinery as a novel synthetic lethality target for multidrug-resistant MYC-driven tumors.

Applications of Polydopamine in Implant Surface Modification

There is great clinical demand for orthopedic and dental implant surface modification methods to prevent osseointegration failure and improve implant biological functions. Notably, dopamine (DA) can be polymerized to form polydopamine (PDA), which is similar to the adhesive proteins secreted by mussels, to form a stable bond between the bone surface and implants. Therefore, PDA has the potential to be used as an implant surface modification material with good hydrophilicity, roughness, morphology, mechanical strength, biocompatibility, antibacterial activity, cellular adhesion, and osteogenesis. In addition, PDA degradation releases DA into the surrounding microenvironment, which is found to play an important role in regulating DA receptors on both osteoblasts and osteoclasts during the bone remodeling process. Furthermore, the adhesion properties of PDA suggest its use as an intermediate layer in assisting other functional bone remodeling materials, such as nanoparticles, growth factors, peptides, and hydrogels, to form "dual modifications." The purpose of this review is to summarize the recent progress in research on PDA and its derivatives as orthopedic and dental implant surface modification materials and to analyze the multiple functions of PDA.

Pramipexole has a neuroprotective effect in spinal cord injury and upregulates D2 receptor expression in the injured spinal cord tissue in rats

Spinal cord injury (SCI) has emerged as a prevalent condition with limited effective treatment options. The neuroprotective role of pramipexole (PPX) in inhibiting nerve cell apoptosis in central nervous system injuries is well established. Therefore, we investigated the effects of PPX in SCI. Adult Sprague-Dawley rats were divided into four groups (sham, SCI, PPX-0.25, and PPX-2.0 groups) according to the PPX therapy (n = 24). Then, SCI was induced using the modified Allen method, and PPX was intravenously administered into the tail at dosages of 0.25 or 2.0 mg/kg following the injury. Motor function was evaluated using the Rivlin-modified inclined plate apparatus and the Basso Beattie Bresnahan (BBB) workout scale. Western blotting assay was used to measure protein expression levels of DRD2, NeuN, Bax/Bcl-2, and caspase-3. Furthermore, immunohistochemistry assessed the effect of PPX on the quantity of NeuN-positive cells in the spinal cord tissue after SCI. Our findings revealed that the BBB and slanting board test scores of the PPX-treated model groups were considerably higher for the SCI group and significantly lower for the sham operation group (P < 0.001). Moreover, the PPX-2.0 group exhibited significantly higher NeuN expression levels than the SCI group (P < 0.01). Our findings indicate that PPX exerts a neuroprotective effect in secondary neuronal injury following SCI, facilitating the recovery of hind limb function by downregulating Bax/Bcl-2, caspase-3, and IL-1β.

Effects of Dopamine on stem cells and its potential roles in the treatment of inflammatory disorders: a narrative review

Inflammation is the host's protective response against harmful external stimulation that helps tissue repair and remodeling. However, excessive inflammation seriously threatens the patient's life. Due to anti-inflammatory effects, corticosteroids, immunosuppressants, and monoclonal antibodies are used to treat various inflammatory diseases, but drug resistance, non-responsiveness, and severe side effect limit their development and application. Therefore, developing other alternative therapies has become essential in anti-inflammatory therapy. In recent years, the in-depth study of stem cells has made them a promising alternative drug for the treatment of inflammatory diseases, and the function of stem cells is regulated by a variety of signals, of which dopamine signaling is one of the main influencing factors. In this review, we review the effects of dopamine on various adult stem cells (neural stem cells, mesenchymal stromal cells, hematopoietic stem cells, and cancer stem cells) and their signaling pathways, as well as the application of some critical dopamine receptor agonists/antagonists. Besides, we also review the role of various adult stem cells in inflammatory diseases and discuss the potential anti-inflammation function of dopamine receptors, which provides a new therapeutic target for regenerative medicine in inflammatory diseases.

Antipsychotic-induced bone loss: the role of dopamine, serotonin and adrenergic receptor signalling

Antipsychotics are commonly used in treating psychiatric disorders. These medications primarily target dopamine the serotonin receptors, they have some affinity to adrenergic, histamine, glutamate and muscarinic receptors. There is clinical evidence that antipsychotic use decreases BMD and increases fracture risk, with dopamine, serotonin and adrenergic receptor-signalling becoming an increasing area of focus where the presence of these receptors in osteoclasts and osteoblasts have been demonstrated. Osteoclasts and osteoblasts are the most important cells in the bone remodelling and the bone regeneration process where the activity of these cells determine the bone resorption and formation process in order to maintain healthy bone. However, an imbalance in osteoclast and osteoblast activity can lead to decreased BMD and increased fracture risk, which is also believed to be exacerbated by antipsychotics use. Therefore, the aim of this review is to provide an overview of the mechanisms of action of first, second and third generation antipsychotics and the expression profiles of dopamine, serotonin and adrenergic receptors during osteoclastogenesis and osteoblastogenesis.

Dopamine is involved in reparative dentin formation through odontoblastic differentiation of dental pulp stem cells

Conventional direct pulp-capping materials induce pulp cells to secrete various biomolecules in pulp tissues that promote reparative dentin formation through induction of odontoblastic differentiation of dental pulp stem cells (DPSCs). However, these biomolecules sometimes induce bone-like dentin with poor sealing properties. Therefore, exploration of biomolecules that allow tight sealing by tubular reparative dentin is required. We recently reported that dopamine (DA) is involved in dentinogenesis. Hence, we investigated the effect of DA on odontoblastic differentiation of DPSCs and reparative dentin formation. Both tyrosine hydroxylase (TH), a DA synthetase, and DA were expressed in odontoblast-like cells in vivo. In vitro, their expression was increased during odontoblastic differentiation of DPSCs. Furthermore, TH-overexpressing DPSCs had promoted odontoblastic differentiation and DA production. Moreover, DA stimulation promoted their differentiation and induced tubular reparative dentin. These results suggest that DA produced by TH is involved in odontoblastic differentiation of DPSCs and has an inductive capacity for reparative dentin formation similar to primary dentin. This study may lead to the development of therapy to preserve vital pulp tissues.

ANKRD1 activates the Wnt signaling pathway by modulating CAV3 expression and thus promotes BMSC osteogenic differentiation and bone formation in ovariectomized mice

Bone marrow-derived mesenchymal stem cells (BMSCs) are considered promising materials for treating bone diseases such as osteoporosis (OP). This research explored the functions and molecular mechanism of ankyrin repeat domain 1 (ANKRD1) in BMSC osteogenesis. An OP model in mice was established by bilateral ovariectomy. Manipulation of ANKRD1 expression in BMSCs or femurs was achieved by lentivirus infection. Increased ANKRD1 expression was observed in BMSCs during osteogenic induction. Silencing of ANKRD1 impaired the osteogenesis of BMSCs, as shown by the decreased alkaline phosphatase (ALP) activity, osteogenic gene (Runx2, Col1a1, Bglap, and Spp1) expression, and mineralized formation. ANKRD1-mediated promotion of osteogenesis was also reproduced in mouse MC3T3-E1 preosteoblastic cells. Activation of Wnt/β-catenin signaling, a well-known osteogenic stimulus, was also impaired in ANKRD1-silenced BMSCs. Overexpression of ANKRD1 resulted in the opposite effects on osteogenesis and Wnt/β-catenin signaling. Mechanistic studies revealed that ANKRD1 modulated caveolin-3 (CAV3) expression by reducing CAV3 ubiquitination, and the knockdown of CAV3 impaired the functions of ANKRD1. Additionally, a very low level of ANKRD1 was observed in the BMSCs from OP mice. Rescue of ANKRD1 significantly restored osteogenic differentiation and Wnt signaling activation in BMSCs from ovariectomized mice. The results of micro-CT, H&E staining, and IHC staining showed that ANKRD1 also promoted bone formation and Wnt activation and ameliorated pathological alterations in the femurs of OP mice. Collectively, this study demonstrated that ANKRD1 plays an important role in regulating the osteogenic differentiation of BMSCs and is a promising target for the treatment of OP and other bone diseases.

The Dopamine D1 Receptor Attenuates Titanium Particle-Induced Inhibition of Osteogenesis by Activating the Wnt Signaling Pathway

Periprosthetic osteolysis (PPO), caused by wear particles, has become a major cause of joint replacement failure. Secondary surgery after joint replacement poses a serious threat to public health worldwide. Therefore, determining how to effectively inhibit wear particle-induced PPO has become an urgent issue. Recently, the interaction between osteogenic inhibition and wear particles at the biological interface of the implant has been found to be an important factor in the pathological process. Previous studies have found that the central nervous system plays an important role in the regulation of bone formation and bone remodeling. Dopamine (DA), an important catecholamine neurotransmitter, plays an integral role in the physiological and pathological processes of various tissues through its corresponding receptors. Our current study found that upregulation of dopamine first receptors could be achieved by activating the Wnt/β-catenin pathway, improving osteogenesis in vivo and in vitro, and significantly reducing the inhibition of titanium particle-induced osteogenesis. Overall, these findings suggest that dopamine first receptor (D1R) may be a plausible target to promote osteoblast function and resist wear particle-induced PPO.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Surface modification of titanium with antibacterial porous N-halamine coating to prevent peri-implant infection

Peri-implant infection remains one of the greatest threats to orthopedics. The construction of bone implants with good antibacterial and osteogenic properties is beneficial for reducing the risk of implant-related infections and healing bone defects. In this study, N-halamine coating (namely N-Cl) was grafted onto alkali-heat treated titanium (Ti) using polydopamine to endow Ti-based orthopedic implants with strong bactericidal activity. Surface characterization revealed that the N-Cl coating has porous structure loaded with active chlorine (Cl⁺). The N-Cl coating also provided micro/nano-structured Ti surfaces with excellent antibacterial ability via transformation between N-H and N-Cl, and approximately 100% disinfection was achieved. Furthermore, the as-prepared N-Cl coating exhibited good biocompatibility and osteogenesis abilityin vitro. These results indicate that applying N-Cl coatings on Ti could prevent and treat peri-implant infections.

Dopamine Suppresses Osteogenic Differentiation of Rat Bone Marrow-Derived Mesenchymal Stem Cells via AKT/GSK-3β/β-Catenin Signaling Pathway

Nervous system is critically involved in bone homeostasis and osteogenesis. Dopamine, a pivotal neurotransmitter, plays a crucial role in sympathetic regulation, hormone secretion, immune activation, and blood pressure regulation. However, the role of dopamine on osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells (rBMSCs) remains poorly understood. In this study, we firstly investigated the effect of dopamine on the apoptosis, proliferation, and osteogenic differentiation of rBMSCs. Dopamine did not, however, interfere with the apoptosis and proliferation of rBMSCs. Interestingly, dopamine suppressed the osteogenic differentiation of rBMSCs, as characterized by reduced ALP staining, ALP activity, mineralized nodule formation, and the mRNA and protein levels of osteogenesis-related genes (Col1a1, Alp, Runx2, Opn, and Ocn). Furthermore, dopamine inactivated AKT/GSK-3β/β-catenin signaling pathway. Treatment of LiCl (GSK-3β inhibitor) rescued the inhibitory effects of dopamine on osteogenic differentiation of rBMSCs. LY294002 (AKT inhibitor) administration exacerbated the inhibitory effects of dopamine on osteogenic differentiation of rBMSCs. Taken together, these findings indicate that dopamine suppresses osteogenic differentiation of rBMSCs via AKT/GSK-3β/β-catenin signaling pathway. Our study provides new insights into the role of neurotransmitters in bone homeostasis.

Modulation of Dopamine Receptors on Osteoblasts as a Possible Therapeutic Strategy for Inducing Bone Formation in Arthritis

Rheumatoid arthritis (RA) is associated with systemic osteoporosis, which leads to severe disability and low quality of life. Current therapies target osteoclasts to reduce bone degradation, but more treatment options would be required to promote bone protection by acting directly on osteoblasts (OB). Recently, the local production of dopamine in inflamed joints of RA has been observed. Thus, in this project, we aimed to determine the implication of the neurotransmitter dopamine in the bone formation process in RA. Dopamine receptors (DR) in the human bone tissue of RA or osteoarthritis (OA) patients were examined by immunohistochemistry. DR in isolated human osteoblasts (OB) was analyzed by flow cytometry, and dopamine content was evaluated by ELISA. Osteoclasts (OC) were differentiated from the PBMCs of healthy controls (HC) and RA patients. Isolated cells were treated with specific dopamine agonists. The effect of dopamine on mineralization was evaluated by Alizarin red staining. Cytokine release in supernatants was measured by ELISA. Osteoclastogenesis was evaluated with TRAP staining. OC markers were analyzed via real-time PCR and bone resorption via staining of resorption pits with toluidine blue. All DR were observed in bone tissue, especially in the bone remodeling area. Isolated OB maintained DR expression, which allowed their study in vitro. Isolated OB expressed tyrosine hydroxylase, the rate-limiting enzyme for dopamine production, and contained dopamine. The activation of D2-like DR significantly increased bone mineralization in RA osteoblasts and increased osteoclastogenesis but did not alter the expression of OC markers nor bone resorption. DR were found in the bone remodeling area of human bone tissue and dopamine can be produced by osteoblasts themselves, thus suggesting a local autocrine/paracrine pathway of dopamine in the bone. D2-like DRs are responsible for bone mineralization in osteoblasts from RA patients without an increase in bone resorption, thus suggesting the D2-like DR pathway as a possible future therapeutic target to counteract bone resorption in arthritis.

Activation of dopamine receptor D1 promotes osteogenic differentiation and reduces glucocorticoid-induced bone loss by upregulating the ERK1/2 signaling pathway

Background

The inhibition of osteogenic differentiation is a major factor in glucocorticoid-induced bone loss, but there is currently no effective treatment. Dopamine, a major neurotransmitter, transmits signals via five different seven-transmembrane G protein-coupled receptors termed D1 to D5. Although the relevance of the neuroendocrine system in bone metabolism has emerged, the precise effects of dopamine receptor signaling on osteoblastogenesis remain unknown.

Methods

In vitro, western blotting and immunofluorescence staining were used to observe the expression of dopamine receptors in MC3T3-E1 and BMSCs cells treated with dexamethasone (Dex). In addition, Alizarin red S (ARS) and alkaline phosphatase (ALP) staining and western blotting were used to evaluate the effect of D1R activation on osteogenic differentiation in Dex-induced MC3T3-E1 cells via the ERK1/2 signaling pathway. In vivo, micro-CT and hematoxylin and eosin (H&E), toluidine blue and immunohistochemical staining were used to determine the effect of D1R activation on Dex-induced bone loss.

Results

We demonstrated that the trend in D1R but not D2-5R was consistent with that of osteogenic markers in the presence of Dex. We also demonstrated that the activation of D1R promoted Dex-induced osteogenic differentiation by activating the ERK1/2 pathway in vitro. We further demonstrated that a D1R agonist could reduce Dex-induced bone loss, while pretreatment with a D1R inhibitor blocked the effect of a D1R agonist in vivo.

Conclusions

Activation of D1R promotes osteogenic differentiation and reduces Dex-induced bone loss by activating the ERK1/2 pathway. Hence, D1R could serve as a potential therapeutic target for glucocorticoid-induced osteoporosis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-022-00453-0.

The Neuropeptide Spexin Promotes the Osteoblast Differentiation of MC3T3-E1 Cells via the MEK/ERK Pathway and Bone Regeneration in a Mouse Calvarial Defect Model

BACKGROUND

The neural regulation of bone regeneration has emerged recently. Spexin (SPX) is a novel neuropeptide and regulates multiple biological functions. However, the effects of SPX on osteogenic differentiation need to be further investigated. Therefore, the aim of this study is to investigate the effects of SPX on osteogenic differentiation, possible underlying mechanisms, and bone regeneration.

METHODS

In this study, MC3T3-E1 cells were treated with various concentrations of SPX. Cell proliferation, osteogenic differentiation marker expressions, alkaline phosphatase (ALP) activity, and mineralization were evaluated using the CCK-8 assay, reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR), ALP staining, and alizarin red S staining, respectively. To determine the underlying molecular mechanism of SPX, the phosphorylation levels of signaling molecules were examined via western blot analysis. Moreover, in vivo bone regeneration by SPX (0.5 and 1 µg/µl) was evaluated in a calvarial defect model. New bone formation was analyzed using micro-computed tomography (micro-CT) and histology.

RESULTS

The results indicated that cell proliferation was not affected by SPX. However, SPX significantly increased ALP activity, mineralization, and the expression of genes for osteogenic differentiation markers, including runt-related transcription factor 2 (Runx2), Alp, collagen alpha-1(I) chain (Col1a1), osteocalcin (Oc), and bone sialoprotein (Bsp). In contrast, SPX downregulated the expression of ectonucleotide pyrophosphatase/phosphodiesterase 1 (Enpp1). Moreover, SPX upregulated phosphorylated mitogen-activated protein kinase kinase (MEK1/2) and extracellular signal-regulated kinase (ERK1/2). In vivo studies, micro-CT and histologic analysis revealed that SPX markedly increased a new bone formation.

CONCLUSION

Overall, these results demonstrated that SPX stimulated osteogenic differentiation in vitro and increased in vivo bone regeneration via the MEK/ERK pathway.

Intrinsic osteoinductivity of PCL-DA/PLLA semi-IPN shape memory polymer scaffolds

Engineering osteoinductive, self-fitting scaffolds offers a potential treatment modality to repair irregularly shaped craniomaxillofacial bone defects. Recently, we innovated on osteoinductive poly(ε-caprolactone)-diacrylate (PCL-DA) shape memory polymers (SMPs) to incorporate poly-L-lactic acid (PLLA) into the PCL-DA network, forming a semi-interpenetrating network (semi-IPN). Scaffolds formed from these PCL-DA/PLLA semi-IPNs display stiffnesses within the range of trabecular bone and accelerated degradation relative to scaffolds formed from slowly degrading PCL-DA SMPs. Herein, we demonstrate for the first time that PCL-DA/PLLA semi-IPN SMP scaffolds show increased intrinsic osteoinductivity relative to PCL-DA. We also confirm that application of a bioinspired polydopamine (PD) coating further improves the osteoinductive capacity of these PCL-DA/PLLA semi-IPN SMPs. In the absence of osteogenic supplements, protein level assessment of human mesenchymal stem cells (h-MSCs) cultured in PCL-DA/PLLA scaffolds revealed an increase in expression of osteogenic markers osterix, bone morphogenetic protein-4 (BMP-4), and collagen 1 alpha 1 (COL1A1), relative to PCL-DA scaffolds and osteogenic medium controls. Likewise, the expression of runt-related transcription factor 2 (RUNX2) and BMP-4 was elevated in the presence of PD-coating. In contrast, the chondrogenic and adipogenic responses associated with the scaffolds matched or were reduced relative to osteogenic medium controls, indicating that the scaffolds display intrinsic osteoinductivity.

Controlled release of dopamine coatings on titanium bidirectionally regulate osteoclastic and osteogenic response behaviors

Bone diseases, for example, osteoporosis, cause excessive differentiation of osteoclasts and decreased bone formation, resulting in imbalance of bone remodeling and poor osseointegration, which can be considered a relative contraindication for titanium implants. Dopamine (DA) might provide a solution to this problem by inhibiting osteoclasts and promoting osteoblasts at different concentrations. However, current commercial implants cannot load bone-active molecules, such as DA. Therefore, this study aimed to develop a surface modification method for implants to achieve a controlled release of DA and enhance the resistance of titanium implants to bone resorption and bone regeneration. DA-loaded alginate-arginine-glycine-aspartic acid (RGD) (AlgR) coatings on a vaterite-modified titanium surface were successfully assembled, which continuously and steadily released DA. In vitro studies have shown that materials showing good biocompatibility can not only inhibit receptor activator of nuclear factor-kappa B (NFκB) ligand (RANKL)-induced osteoclastogenesis but also enhance the adhesion and osteogenic differentiation of human bone marrow mesenchymal stem cells (hBMSCs). The optimal DA-loaded concentration of this bidirectional regulation is 100 μM. Interestingly, DA more effectively attenuated osteoclastogenesis when released in a sustained manner from titanium coatings than it did via traditional, free administration, and the alginate-RGD coating and DA clearly exhibited great synergy. This study provides a design of titanium implant surface modification to improve bone remodeling around implants.

Apomorphine Reduces A53T α-Synuclein-Induced Microglial Reactivity Through Activation of NRF2 Signalling Pathway

The chiral molecule, apomorphine, is currently used for the treatment of Parkinson’s disease (PD). As a potent dopamine receptor agonist, this lipophilic compound is especially effective for treating motor fluctuations in advanced PD patients. In addition to its receptor-mediated actions, apomorphine has also antioxidant and free radical scavenger activities. Neuroinflammation, oxidative stress, and microglia reactivity have emerged as central players in PD. Thus, modulating microglia activation in PD may be a valid therapeutic strategy. We previously reported that murine microglia are strongly activated upon exposure to A53T mutant α-synuclein. The present study was designed to investigate whether apomorphine enantiomers could modulate this A53T-induced microglial activation. Taken together, the results provided evidence that apomorphine enantiomers decrease A53T-induced microgliosis, through the activation of the NRF2 signalling pathway, leading to a lower pro-inflammatory state and restoring the phagocytic activity. Suppressing NRF2 recruitment (trigonelline exposure) or silencing specifically Nfe2l2 gene (siRNA treatment) abolished or strongly decreased the anti-inflammatory activity of apomorphine. In conclusion, apomorphine, which is already used in PD patients to mimic dopamine activity, may also be suitable to decrease α-synuclein-induced microglial reactivity.

Amino acid metabolism and autophagy in skeletal development and homeostasis

Bone is an active organ that is continuously remodeled throughout life via formation and resorption; therefore, a fine-tuned bone (re)modeling is crucial for bone homeostasis and is closely connected with energy metabolism. Amino acids are essential for various cellular functions as well as an energy source, and their synthesis and catabolism (e.g., metabolism of carbohydrates and fatty acids) are regulated through numerous enzymatic cascades. In addition, the intracellular levels of amino acids are maintained by autophagy, a cellular recycling system for proteins and organelles; under nutrient deprivation conditions, autophagy is strongly induced to compensate for cellular demands and to restore the amino acid pool. Metabolites derived from amino acids are known to be precursors of bioactive molecules such as second messengers and neurotransmitters, which control various cellular processes, including cell proliferation, differentiation, and homeostasis. Thus, amino acid metabolism and autophagy are tightly and reciprocally regulated in our bodies. This review discusses the current knowledge and potential links between bone diseases and deficiencies in amino acid metabolism and autophagy.

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?

HSPB7 regulates osteogenic differentiation of human adipose derived stem cells via ERK signaling pathway

Background

Heat shock protein B7 (HSPB7), which belongs to small heat shock protein family, has been reported to be involved in diverse biological processes and diseases. However, whether HSPB7 regulates osteogenic differentiation of human adipose derived stem cells (hASCs) remains unexplored.

Methods

The expression level of HSPB7 during the osteogenesis of hASCs was examined by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and Western blot analysis. Lentivirus transfection was used to knock down or overexpress HSPB7, which enabled us to investigate the effect of HSPB7 on osteogenic differentiation of hASCs. U0126 and extracellular signal-regulated protein kinase 1/2 (ERK1/2) siRNA were used to identify the mechanism of the HSPB7/ERK1/2 axis in regulating osteogenic differentiation of hASCs. Moreover, ectopic bone formation in nude mice and osteoporosis mice model was used to investigate the effect of HSPB7 on osteogenesis in vivo.

Results

In this study, we found the expression of HSPB7 was significantly downregulated during the osteogenic differentiation of hASCs. HSPB7 knockdown remarkably promoted osteogenic differentiation of hASCs, while HSPB7 overexpression suppressed osteogenic differentiation of hASCs both in vitro and in vivo. Moreover, we discovered that the enhancing effect of HSPB7 knockdown on osteogenic differentiation was related to the activation of extracellular signal-regulated protein kinase (ERK) signaling pathway. Inhibition of ERK signaling pathway with U0126 or silencing ERK1/2 effectively blocked the stimulation of osteogenic differentiation induced by HSPB7 knockdown. Additionally, we found that HSPB7 expression was markedly increased in mouse bone marrow mesenchymal stem cells (mBMSCs) from the osteoporotic mice which suggested that HSPB7 might be utilized as a potential target in the development of effective therapeutic strategies to treat osteoporosis and other bone diseases.

Conclusion

Taken together, these findings uncover a previously unrecognized function of HSPB7 in regulating osteogenic differentiation of hASCs, partly via the ERK signaling pathway.

Crosstalk of Brain and Bone-Clinical Observations and Their Molecular Bases

As brain and bone disorders represent major health issues worldwide, substantial clinical investigations demonstrated a bidirectional crosstalk on several levels, mechanistically linking both apparently unrelated organs. While multiple stress, mood and neurodegenerative brain disorders are associated with osteoporosis, rare genetic skeletal diseases display impaired brain development and function. Along with brain and bone pathologies, particularly trauma events highlight the strong interaction of both organs. This review summarizes clinical and experimental observations reported for the crosstalk of brain and bone, followed by a detailed overview of their molecular bases. While brain-derived molecules affecting bone include central regulators, transmitters of the sympathetic, parasympathetic and sensory nervous system, bone-derived mediators altering brain function are released from bone cells and the bone marrow. Although the main pathways of the brain-bone crosstalk remain ‘efferent’, signaling from brain to bone, this review emphasizes the emergence of bone as a crucial ‘afferent’ regulator of cerebral development, function and pathophysiology. Therefore, unraveling the physiological and pathological bases of brain-bone interactions revealed promising pharmacologic targets and novel treatment strategies promoting concurrent brain and bone recovery.