S1P-S1PR1 Signaling: the "Sphinx" in Osteoimmunology

Differences in collateral vessel formation after experimental retinal vein occlusion in spontaneously hypertensive rats and wild-type rats

Objective

Retinal vein occlusion (RVO) can lead to visual impairment, but the development of collateral vessels can sometimes mitigate significant damage. This study aimed to investigate the relationship between collateral vessels and hypertension, the most common underlying condition associated with RVO, by comparing spontaneously hypertensive rats (SHRs) and wild-type Wister rats (WWRs). We also examined the differences between WWRs and SHRs in terms of sphingosine 1-phosphate receptor 1 (S1PR1) expression and its product nitric oxide synthase 3 (NOS3) expression, which are involved in the formation of collateral vessels after vascular occlusion.

Methods

Laser photocoagulation (PC) was used to occlude one randomly selected retinal vein in WWRs and SHRs, and the area surrounding the occluded vessel was examined using optical coherence tomography angiography. If reperfusion of the occluded vessel occurred within 2 weeks, the vessel was re-occluded repeatedly by PC. The number of eyes with successfully occluded vessels accompanied by collateral vessels was recorded. Then, WWRs and SHRs were divided into the following four groups: 1) control (no treatment), 2) vehicle (20% DMSO), 3) S1PR1 agonist (2 mg/mL SEW2871), and 4) S1PR1 antagonist (0.25 mg/mL VPC 23019) groups. The drugs were administered intravitreally in all groups except the control. The number of laser shots required for successful RVO was recorded. Histological evaluation and quantitative real-time PCR of S1PR1 and NOS3 were performed to elucidate the mechanisms underlying collateral vessel formation.

Results

The proportion of eyes achieving successful vein occlusion was lower in SHRs (4/12 eyes, 33.3%) than in WWRs (8/10 eyes, 80%, p = 0.043). NOS3 expression at 6 h after PC was significantly higher in WWRs than in SHRs (p = 0.021). In WWRs treated with SEW2871, vein occlusion failed in 7 of 10 eyes (70%). The expression of NOS3 was significantly higher in the SEW2871 treatment group than in the untreated group (p < 0.001). Furthermore, NOS3 expression was significantly higher after SEW2871 treatment in WWRs than in SHRs (p = 0.011).

Conclusion

In hypertensive environments, collateral vessels are less likely to develop, and S1PR1 may be involved in this phenomenon.

S1P/S1PR signaling pathway advancements in autoimmune diseases

Sphingosine-1-phosphate (S1P) is a versatile sphingolipid that is generated through the phosphorylation of sphingosine by sphingosine kinase (SPHK). S1P exerts its functional effects by binding to the G protein-coupled S1P receptor (S1PR). This lipid mediator plays a pivotal role in various cellular activities. The S1P/S1PR signaling pathway is implicated in the pathogenesis of immune-mediated diseases, significantly contributing to the functioning of the immune system. It plays a crucial role in diverse physiological and pathophysiological processes, including cell survival, proliferation, migration, immune cell recruitment, synthesis of inflammatory mediators, and the formation of lymphatic and blood vessels. However, the full extent of the involvement of this signaling pathway in the development of autoimmune diseases remains to be fully elucidated. Therefore, this study aims to comprehensively review recent research on the S1P/S1PR axis in diseases related to autoimmunity.

The role of sphingosine-1-phosphate in autophagy and related disorders

S1P, also referred to as sphingosine-1-phosphate, is a lipid molecule with bioactive properties involved in numerous cellular processes such as cell growth, movement, programmed cell death, self-degradation, cell specialization, aging, and immune system reactions. Autophagy is a meticulously controlled mechanism in which cells repurpose their elements to maintain cellular balance. There are five stages in autophagy: initiation, nucleation, elongation and maturation, fusion, and degradation. New research has provided insight into the complex connection between S1P and autophagy, uncovering their interaction in both normal and abnormal circumstances. Gaining knowledge about the regulatory mechanism of S1P signaling on autophagy can offer a valuable understanding of its function in well-being and illness, potentially leading to innovative therapeutic concepts for diverse ailments. Hence, this review analyzes the essential stages in mammalian autophagy, with a specific emphasis on recent research exploring the control of each stage by S1P. Additionally, it sheds light on the roles of S1P-induced autophagy in various disorders.

Eggerthella lenta augments preclinical autoantibody production and metabolic shift mimicking senescence in arthritis

Although the etiology of rheumatoid arthritis (RA) is unknown, a strong genetic predisposition and the presence of preclinical antibodies before the onset of symptoms is documented. An expansion of Eggerthella lenta is associated with severe disease in RA. Here, using a humanized mouse model of collagen-induced arthritis, we determined the impact of E. lenta abundance on RA severity. Naïve mice gavaged with E. lenta produce preclinical rheumatoid factor and, when induced for arthritis, develop severe disease. The augmented antibody response was much higher in female mice, and among patients with RA, women had higher average load of E. lenta. Expansion of E. lenta increased CXCL5 and CD4 T cells, and both interleukin-17- and interferon-γ-producing B cells. Further, E. lenta gavage caused gut dysbiosis and decline in amino acids and nicotinamide adenine dinucleotide with an increase in microbe-dependent bile acids and succinyl carnitine causing systemic senescent-like inflammation.

Potential therapeutic targets of macrophages in inhibiting immune damage and fibrotic processes in musculoskeletal diseases

Macrophages are a heterogeneous cell type with high plasticity, exhibiting unique activation characteristics that modulate the progression and resolution of diseases, serving as a key mediator in maintaining tissue homeostasis. Macrophages display a variety of activation states in response to stimuli in the local environment, with their subpopulations and biological functions being dependent on the local microenvironment. Resident tissue macrophages exhibit distinct transcriptional profiles and functions, all of which are essential for maintaining internal homeostasis. Dysfunctional macrophage subpopulations, or an imbalance in the M1/M2 subpopulation ratio, contribute to the pathogenesis of diseases. In skeletal muscle disorders, immune and inflammatory damage, as well as fibrosis induced by macrophages, are prominent pathological features. Therefore, targeting macrophages is of great significance for maintaining tissue homeostasis and treating skeletal muscle disorders. In this review, we discuss the receptor-ligand interactions regulating macrophages and identify potential targets for inhibiting collateral damage and fibrosis in skeletal muscle disorders. Furthermore, we explore strategies for modulating macrophages to maintain tissue homeostasis.

Dysregulation of sphingosine-1-phosphate (S1P) and S1P receptor 1 signaling in the 5xFAD mouse model of Alzheimer's disease

Sphingolipid-1-phosphate (S1P) signaling through the activation S1P receptors (S1PRs) plays critical roles in cellular events in the brain. Aberrant S1P metabolism has been identified in the brains of Alzheimer's disease (AD) patients. Our recent studies have shown that treatment with fingolimod, an analog of sphingosine, provides neuroprotective effects in five familiar Alzheimer disease (5xFAD) transgenic mice, resulting in the reduction of amyloid-β (Aβ) neurotoxicity, inhibition of activation of microglia and astrocytes, increased hippocampal neurogenesis, and improved learning and memory. However, the pathways by which dysfunctional S1P and S1PR signaling may associate with the development of AD-like pathology remain unknown. In this study, we investigated the alteration of signaling of S1P/S1P receptor 1 (S1PR1), the most abundant S1PR subtype in the brain, in the cortex of 5xFAD transgenic mice at 3, 8, and 14 months of age. Compared to non-transgenic wildtype (WT) littermates, we found significant decreased levels of sphingosine kinases (SphKs), increased S1P lyase (S1PL), and increased S1PR1 in 8- and 14-month-old, but not in 3-month-old 5xFAD mice. Furthermore, we detected increased activation of the S1PR1 downstream pathway of Akt/mTor/Tau signaling in aging 5xFAD mice. Treatment with fingolimod from 1 to 8 months of age reversed the levels of SphKs, S1PL, and furthermore, those of S1PR1 and its downstream pathway of Akt/mTor/Tau signaling. Together the data reveal that dysregulation of S1P and S1PR signaling may associate with the development of AD-like pathology through Akt/mTor/Tau signaling.

Controlled mechanical loading improves bone regeneration by regulating type H vessels in a S1Pr1-dependent manner

Despite the best treatment, approximately 10% of fractures still face undesirable repair and result in delayed unions or non-unions. Dynamic mechanical stimulation promotes bone formation, when applied at the correct time frame, with optimal loading magnitude, frequency, and repetition. Controlled mechanical loading significantly increases osteogenic cells during the matrix deposition phase of bone repair. In the bone defect, the blood vessel network guides the initial bone formation activities. A unique blood vessel subtype (Type H) exists in bone, which expresses high levels of CD31 and endomucin, and functions to couple angiogenesis and osteogenesis. However, how this form of controlled mechanical loading regulates the Type H vessels and promotes bone formation is still not clear. Sphingosine 1-phosphate (S1P) participates in the bone anabolic process and is a key regulator of the blood vessel. Its receptor, sphingosine 1-phosphate receptor 1 (S1Pr1), is a mechanosensitive protein that regulates vascular integrity. Therefore, we hypothesis that controlled anabolic mechanical loading promotes bone repair by acting on Type H vessels. To study the effect of S1Pr1 on loading induced-bone repair, we utilized a stabilized tibial defect model, which allows for the application of anabolic mechanical loading. Mechanical loading upregulated S1Pr1 within the entire defect, with up to 80% expressed in blood vessels, as observed by deep tissue imaging. Additionally, S1Pr1 antagonism by W146 inhibited the anabolic effects of mechanical loading. We showed that mechanical loading or activating S1Pr1 could induce YAP nuclear translocation, a key regulator in the cell's mechanical response, in endothelial cells (ECs) in vitro. Inhibition of S1Pr1 in endothelial cells by siRNA reduced loading-induced YAP nuclear translocation and expressions of angiogenic genes. In vivo, YAP nuclear translocation in Type H vessels was up-regulated after mechanical loading but was inhibited by antagonizing S1Pr1. S1Pr1 agonist, FTY720, increased bone volume and Type H vessel volume, similar to that of mechanical stimulation. In conclusion, controlled anabolic mechanical loading enhanced bone formation mainly through Type H vessels in a S1Pr1-dependent manner.

HIF signaling: A new propellant in bone regeneration

Bone tissue destruction leads to severe pain, physical flaws, and loss of motility. Bone repair using biocompatible and osteo-inductive scaffolds is regarded as a viable and potential therapeutic approach. However, for large-scale bone regeneration, oxygen and nutrient supply have become limiting factors. Further, a considerable need exists for recruited cell activities and blood vessel growth. Hypoxia-inducible factor (HIF) signaling pathways induced by hypoxia are involved in angiogenesis and osteogenesis. As an important transcription factor, HIF-1 functions by modulating vital genes, such as VEGF, PDK1, and EPO, and is a crucial regulator that influences the final fate of bone regeneration. Collectively, to achieve better osteogenesis results, the in-depth molecular mechanisms that underpin the links between materials, cells, and HIF signaling pathways must be determined. This review aimed to provide an in-depth insight into recent progress in HIF-regulated bone regeneration. Hypoxia and cellular oxygen-sensing mechanisms and their correlations with osteogenesis were determined, and recent studies on hypoxia-inducing and hypoxia-mimicking strategies were briefly described. Finally, the potential applications of HIF signaling in bone regeneration were highlighted. This review provides theoretical support for establishing a novel and viable bone repair strategy in the clinic by harnessing HIF signaling.

A Foxo1-Klf2-S1pr1-Gnai1-Rac1 signaling axis is a critical mediator of Ostm1 regulatory network in T lymphopoiesis

Ostm1 mutations cause the severe form of osteopetrosis with bone marrow deficiency in humans and mice, yet a role in T cell ontogeny remains to be determined. Herein, we show that thymi of the Ostm1-null mice (gl/gl) from P8-to-P15 become markedly hypocellular with disturbed architecture. Analysis of gl/gl early T cell program determined a major decrease of 3-fold in bone marrow common lymphoid precursors (CLP), 35-fold in early thymic precursors (ETPs) and 100-fold in T cell double positive subpopulations. Ostm1 ablation in T cell double negative (DN) also appears to induce fast-paced differentiation kinetics with a transitory intermediate CD44⁺CD25^int subpopulation. Transgenic targeting Ostm1 expression from the gl/gl DN1 population partially rescued T cell subpopulations from ETP onwards and normalized the accelerated DN differentiation, indicating a cell-autonomous role for Ostm1. Transcriptome of early DN1 population identified an Ostm1 crosstalk with a Foxo1-Klf2-S1pr1-Gnai1-Rac1 signaling axis. Our findings establish that Ostm1 is an essential regulator of T cell ontogeny.

•

Loss of Ostm1 causes severe thymus hypocellularity

•

Ostm1 is a modulator of the T cell differentiation program from the CLPs onwards

•

Targeted CD2-Ostm1 in Ostm1 null mice leads to partial rescue of DN differentiation

•

Ostm1 null DN1 transcriptome identifies a Foxo1-Klf2-S1pr1-Gnai1-Rac1 signaling axis

Review article: the sphingosine 1 phosphate/sphingosine 1 phosphate receptor axis - a unique therapeutic target in inflammatory bowel disease

BACKGROUND

Ozanimod, a high selective sphingosine 1 phosphate (S1P) receptor (S1PR) 1/5 modulator was approved by the Food and Drug Administration for the treatment of adult patients with moderately to severely active ulcerative colitis. Additional S1PR modulators are being tested in clinical development programmes for both ulcerative colitis and Crohn's disease.

AIM

To provide an overview of advances in understanding S1PRs biology and summarise preclinical and clinical investigations of S1P receptor modulators in chronic inflammatory disease with special emphasis on inflammatory bowel diseases (IBD).

METHODS

We performed a narrative review using PubMed and ClinicalTrials.gov.

RESULTS

Through S1PRs, S1P regulates multiple cellular processes, including proliferation, migration, survival, and vascular barrier integrity. The S1PRs function of regulating lymphocyte trafficking is well known, but new functions of S1PRs expand our knowledge of S1PRs biology. Several S1PR modulators are in clinical development for both ulcerative colitis and Crohn's disease and have shown promise in phase II and III studies with ozanimod now being approved for ulcerative colitis.

CONCLUSIONS

S1P receptor modulators constitute a novel, promising, safe, and convenient strategy for the treatment of IBD.

Molecular Mechanism of Sphingosine-1-Phosphate Receptor 1 Regulating CD4+ Tissue Memory in situ T Cells in Primary Sjogren's Syndrome

Objective

Although extensive research has been carried out on CD4⁺T cells infiltrating the labial glands in patients with primary Sjögren’s Syndrome (pSS), it is still unclear how CD4⁺T cells remain in the labial gland tissue and develop into tissue resident cells. The aim of this study was to investigate the molecular mechanism by which CD4⁺T reside in labial glandular tissue of pSS patients.

Methods

Lymphocyte infiltration in labial salivary glands (LSG) of pSS patients was detected by H&E staining. Expression of sphingosine-1-phosphate receptor 1 (S1PR1) in LSG was examined by Immunohistochemistry. Immunofluorescence analyses were utilized to detect the co-expression of CD4, CD69 and S1PR1 in T cells of LSG of pSS patients. Expression of gene S1pr1 in peripheral blood CD4⁺T cells of healthy controls and pSS patients was detected by quantitative real-time PCR (QPCR). QPCR was used to examine the expression of gene S1pr1, Klf2, and Cd69 in the CD4⁺T cells that were co-cultured in vitro with cytokines TNF-α, TGF-β, and IL-33.

Results

S1PR1 was expressed in the infiltrating monocytes in LSG of pSS patients, and S1PR1 was weakly or even not expressed in cytoplasm of CD4⁺CD69⁺T_RM cells of LSG in patients with pSS. Expression of gene S1pr1 in peripheral blood CD4⁺T cells of pSS patients was about three-fifths of that of healthy controls (P < 0.05). Expression of genes S1pr1 (P < 0.001) and Klf-2 (P < 0.001) was significantly decreased, and the expression of gene Cd69 (P < 0.05) was significantly increased in peripheral blood CD4⁺T cells of pSS patients co-cultured in vitro with cytokines TNF-α, TGF-β, and IL-33.

Conclusion

Our study suggests that the decrease of S1pr1 gene expression may provide a molecular basis for promoting the tissue retention and development of CD4⁺CD69⁺T_RM cells.

S1P Increases VEGF Production in Osteoblasts and Facilitates Endothelial Progenitor Cell Angiogenesis by Inhibiting miR-16-5p Expression via the c-Src/FAK Signaling Pathway in Rheumatoid Arthritis

Angiogenesis is a critical process in the formation of new capillaries and a key participant in rheumatoid arthritis (RA) pathogenesis. Vascular endothelial growth factor (VEGF) stimulation of endothelial progenitor cells (EPCs) facilitates angiogenesis and the progression of RA. Phosphorylation of sphingosine kinase 1 (SphK1) produces sphingosine-1-phosphate (S1P), which increases inflammatory cytokine production, although the role of S1P in RA angiogenesis is unclear. In this study, we evaluated the impact of S1P treatment on VEGF-dependent angiogenesis in osteoblast-like cells (MG-63 cells) and the significance of SphK1 short hairpin RNA (shRNA) on S1P production in an in vivo model. We found significantly higher levels of S1P and VEGF expression in synovial fluid from RA patients compared with those with osteoarthritis by ELISA analysis. Treating MG-63 cells with S1P increased VEGF production, while focal adhesion kinase (FAK) and Src siRNAs and inhibitors decreased VEGF production in S1P-treated MG-63 cells. Conditioned medium from S1P-treated osteoblasts significantly increased EPC tube formation and migration by inhibiting miR-16-5p synthesis via proto-oncogene tyrosine-protein kinase src (c-Src) and FAK signaling in chick chorioallantoic membrane (CAM) and Matrigel plug assays. Infection with SphK1 shRNA reduced angiogenesis, articular swelling and cartilage erosion in the ankle joints of mice with collagen-induced arthritis (CIA). S1P appears to have therapeutic potential in RA treatment.

Current Understanding of Osteoimmunology in Certain Osteoimmune Diseases

The skeletal system and immune system seem to be two independent systems. However, there in fact are extensive and multiple crosstalk between them. The concept of osteoimmunology was created to describe those interdisciplinary events, but it has been constantly updated over time. In this review, we summarize the interactions between the skeletal and immune systems in the co-development of the two systems and the progress of certain typical bone abnormalities and bone regeneration on the cellular and molecular levels according to the mainstream novel study. At the end of the review, we also highlighted the possibility of extending the research scope of osteoimmunology to other systemic diseases. In conclusion, we propose that osteoimmunology is a promising perspective to uncover the mechanism of related diseases; meanwhile, a study from the point of view of osteoimmunology may also provide innovative ideas and resolutions to achieve the balance of internal homeostasis.

Biased and allosteric modulation of bone cell-expressing G protein-coupled receptors as a novel approach to osteoporosis therapy

On the cellular level, osteoporosis (OP) is a result of imbalanced bone remodeling, in which osteoclastic bone resorption outcompetes osteoblastic bone formation. Currently available OP medications include both antiresorptive and bone-forming drugs. However, their long-term use in OP patients, mainly in postmenopausal women, is accompanied by severe side effects. Notably, the fundamental coupling between bone resorption and formation processes underlies the existence of an undesirable secondary outcome that bone anabolic or anti-resorptive drugs also reduce bone formation. This drawback requires the development of anti-OP drugs capable of selectively stimulating osteoblastogenesis and concomitantly reducing osteoclastogenesis. We propose that the application of small synthetic biased and allosteric modulators of bone cell receptors, which belong to the G-protein coupled receptors (GPCR) family, could be the key to resolving the undesired anti-OP drug selectivity. This approach is based on the capacity of these GPCR modulators, unlike the natural ligands, to trigger signaling pathways that promote beneficial effects on bone remodeling while blocking potentially deleterious effects. Under the settings of OP, an optimal anti-OP drug should provide fine-tuned regulation of downstream effects, for example, intermittent cyclic AMP (cAMP) elevation, preservation of Ca²⁺ balance, stimulation of osteoprotegerin (OPG) and estrogen production, suppression of sclerostin secretion, and/or preserved/enhanced canonical β-catenin/Wnt signaling pathway. As such, selective modulation of GPCRs involved in bone remodeling presents a promising approach in OP treatment. This review focuses on the evidence for the validity of our hypothesis.

Increased Sphingosine-1-Phosphate Serum Concentrations in Subjects with Periodontitis: A Matter of Inflammation

PURPOSE

Periodontitis is an inflammatory disease of the oral cavity with an alarmingly high prevalence within the adult population. The signaling lipid sphingosine-1-phosphate (S1P) plays a crucial role in inflammatory and immunomodulatory responses. In addition to cardiovascular disease, sepsis and tumor entities, S1P has been recently identified as both mediator and biomarker in osteoporosis. We hypothesized that S1P may play a role in periodontitis as an inflammation-prone bone destructive disorder. The goal of our study was to evaluate associations between periodontitis and S1P serum concentrations in the Study of Health in Pomerania (SHIP)-Trend cohort. In addition, we investigated the expression of S1P metabolizing enzymes in inflamed gingival tissue.

PATIENTS AND METHODS

We analyzed data from 3371 participants (51.6% women) of the SHIP-Trend cohort. Periodontal parameters and baseline characteristics were assessed. Serum S1P was measured by liquid chromatography tandem mass spectrometry. The expression of S1P metabolizing enzymes was determined by immunofluorescence staining of human gingival tissue.

RESULTS

S1P serum concentrations were significantly increased in subjects with both moderate and severe periodontitis, assessed as probing depth and clinical attachment loss. In contrast, no significant association of S1P was seen with caries variables (number and percentage of decayed or filled surfaces). S1P concentrations significantly increased with increasing high-sensitivity C-reactive protein (hs-CRP) levels. Interestingly, inflamed compared to normal human gingival tissue exhibited elevated expression levels of the S1P-generating enzyme sphingosine kinase 1 (SphK1).

CONCLUSION

We report an intriguingly significant association of various periodontal parameters with serum levels of the inflammatory lipid mediator S1P. Our data point towards a key role of S1P during periodontitis pathology. Modulation of local S1P levels or its signaling properties may represent a potential future therapeutic strategy to prevent or to retard periodontitis progression and possibly reduce periodontitis-related tooth loss.

The Role of Sphingolipids in Cancer Immunotherapy

Immunotherapy is now considered an innovative and strong strategy to beat metastatic, drug-resistant, or relapsing tumours. It is based on the manipulation of several mechanisms involved in the complex interplay between cancer cells and immune system that culminates in a form of immune-tolerance of tumour cells, favouring their expansion. Current immunotherapies are devoted enforcing the immune response against cancer cells and are represented by approaches employing vaccines, monoclonal antibodies, interleukins, checkpoint inhibitors, and chimeric antigen receptor (CAR)-T cells. Despite the undoubted potency of these treatments in some malignancies, many issues are being investigated to amplify the potential of application and to avoid side effects. In this review, we discuss how sphingolipids are involved in interactions between cancer cells and the immune system and how knowledge in this topic could be employed to enhance the efficacy of different immunotherapy approaches. In particular, we explore the following aspects: how sphingolipids are pivotal components of plasma membranes and could modulate the functionality of surface receptors expressed also by immune cells and thus their functionality; how sphingolipids are related to the release of bioactive mediators, sphingosine 1-phosphate, and ceramide that could significantly affect lymphocyte egress and migration toward the tumour milieu, in addition regulating key pathways needed to activate immune cells; given the renowned capability of altering sphingolipid expression and metabolism shown by cancer cells, how it is possible to employ sphingolipids as antigen targets.

Potential role of bioactive lipids in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease, which involves a pathological inflammatory response against articular cartilage in multiple joints throughout the body. It is a complex disorder associated with comorbidities such as depression, lymphoma, osteoporosis and cardiovascular disease (CVD), which significantly deteriorate patients' quality of life and prognosis. This has ignited a large initiative to elucidate the physiopathology of RA, aiming to identify new therapeutic targets and approaches in its multidisciplinary management. Recently, various lipid bioactive products have been proposed to have an essential role in this process; including eicosanoids, specialized pro-resolving mediators, phospholipids/sphingolipids, and endocannabinoids. Dietary interventions using omega-3 polyunsaturated fatty acids or treatment with synthetic endocannabinoids agonists have been shown to significantly ameliorate RA symptoms. Indeed, the modulation of lipid metabolism may be crucial in the pathophysiology and treatment of autoimmune diseases.

Macrophages at Low-Inflammatory Status Improved Osteogenesis via Autophagy Regulation

Accumulating evidence indicates that the interaction between immune and skeletal systems is vital in bone homeostasis. However, the detailed mechanisms between macrophage polarization and osteogenic differentiation of mesenchymal stromal cells (bone marrow-derived stromal cells [BMSCs]) remain largely unknown. We observed enhanced macrophage infiltration along with bone formation in vivo, which showed a transition from early-stage M1 phenotype to later stage M2 phenotype, cells at the transitional stage expressed both M1 and M2 markers that actively participated in osteogenesis, which was mimicked by stimulating macrophages with lower inflammatory stimulus (compared with typical M1). Using conditioned medium (CM) from M0, typical M1, low-inflammatory M1 (M1semi), and M2 macrophages, it was found that BMSCs treated with M1semi CM showed significantly induced migration, osteogenic differentiation, and mineralization, compared with others. Along with the induced osteogenesis, the autophagy level was the highest in M1semi CM-treated BMSCs, which was responsible for BMSC migration and osteogenic differentiation, as autophagy interruption significantly abolished this effect. This study indicated that low-inflammatory macrophages could activate autophagy in BMSCs to improve osteogenesis.

Aberrant Bone Homeostasis in AML Is Associated with Activated Oncogenic FLT3-Dependent Cytokine Networks

Acute myeloid leukaemia (AML) is a haematopoietic malignancy caused by a combination of genetic and epigenetic lesions. Activation of the oncoprotein FLT3 ITD (Fms-like tyrosine kinase with internal tandem duplications) represents a key driver mutation in 25–30% of AML patients. FLT3 is a class III receptor tyrosine kinase, which plays a role in cell survival, proliferation, and differentiation of haematopoietic progenitors of lymphoid and myeloid lineages. Mutant FLT3 ITD results in an altered signalling quality, which causes cell transformation. Recent evidence indicates an effect of FLT3 ITD on bone homeostasis in addition to haematological aberrations. Using gene expression data repositories of FLT3 ITD-positive AML patients, we identified activated cytokine networks that affect the formation of the haematopoietic niche by controlling osteoclastogenesis and osteoblast functions. In addition, aberrant oncogenic FLT3 signalling of osteogenesis-specific cytokines affects survival of AML patients and may be used for prognosis. Thus, these data highlight the intimate crosstalk between leukaemic and osteogenic cells within the osteohaematopoietic niche.

S1P facilitates IL-1β production in osteoblasts via the JAK and STAT3 signaling pathways

Rheumatoid arthritis (RA) is a systemic autoimmune inflammatory disease, in which the immune system attacks synovial joint tissues. Interleukin (IL)-1β is a critical proinflammatory cytokine in RA progression. Sphingosine-1-phosphate (S1P), a platelet-derived lysophospholipid mediator, reportedly regulates osteoimmunology. Here, we investigated how S1P mediates IL-1β expression in osteoblasts. Our analysis of records from the Gene Expression Omnibus (GEO) database demonstrate higher levels of IL-1β in patients with RA compared with those with osteoarthritis. Stimulation of osteoblasts with S1P concentration dependently increased mRNA and protein expression of IL-1β. Elevations in IL-1β mRNA expression induced by S1P were reduced by the small interfering RNA (siRNA) against the S1P₁ receptor. S1P also augmented JAK and STAT3 molecular cascades. We also found that JAK and STAT3 inhibitors and their siRNAs antagonized S1P-promoted IL-1β expression. Our results indicate that S1P promotes the expression of IL-1β in osteoblasts via the S1P₁ receptor and the JAK and STAT3 signaling pathways.

Overcoming barriers confronting application of protein therapeutics in bone fracture healing

Bone fracture is a major contributor to debilitation and death among patients with bone diseases. Thus, osteogenic protein therapeutics and their delivery to bone have been extensively researched as strategies to accelerate fracture healing. To prevent morbidity and mortality of fractures, which occur frequently in the aging population, there is a critical need for development of first-line therapeutics. Bone morphogenic protein-2 (BMP-2) has been at the forefront of bone regeneration research for its potent osteoinduction, despite safety concerns and biophysiological obstacles of delivery to bone. However, continued pursuit of osteoinductive proteins as a therapeutic option is largely aided by drug delivery systems, playing an imperative role in enhancing safety and efficacy. In this work, we highlighted several types of drug delivery platforms and their biomaterials, to evaluate the suitability in overcoming challenges of therapeutic protein delivery for bone regeneration. To showcase the clinical considerations for each type of platform, we have assessed the most common route of administration strategies for bone regeneration, classifying the platforms as implantable or injectable. Additionally, we have analyzed the commonly utilized models and methodology for safety and efficacy evaluation of these osteogenic protein-loaded systems, to present clinical opinions for future directions of research in this field. It is hoped that this review will promote research and development of clinically translatable osteogenic protein therapeutics, while targeting first-line treatment status for achieving desired outcomes of fracture healing. Graphical abstract.

Bioactive lipids, inflammation and chronic diseases

Endogenous bioactive lipids are part of a complex network that modulates a plethora of cellular and molecular processes involved in health and disease, of which inflammation represents one of the most prominent examples. Inflammation serves as a well-conserved defence mechanism, triggered in the event of chemical, mechanical or microbial damage, that is meant to eradicate the source of damage and restore tissue function. However, excessive inflammatory signals, or impairment of pro-resolving/anti-inflammatory pathways leads to chronic inflammation, which is a hallmark of chronic pathologies. All main classes of endogenous bioactive lipids - namely eicosanoids, specialized pro-resolving lipid mediators, lysoglycerophopsholipids and endocannabinoids - have been consistently involved in the chronic inflammation that characterises pathologies such as cancer, diabetes, atherosclerosis, asthma, as well as autoimmune and neurodegenerative disorders and inflammatory bowel diseases. This review gathers the current knowledge concerning the involvement of endogenous bioactive lipids in the pathogenic processes of chronic inflammatory pathologies.

NF-κB/TWIST1 Mediates Migration and Phagocytosis of Macrophages in the Mice Model of Implant-Associated Staphylococcus aureus Osteomyelitis

Staphylococcus aureus (S. aureus) infection-induced osteomyelitis is a great challenge in clinic treatment. Identification of the essential genes and biological processes that are specifically changed in mononuclear cells at an early stage of S. aureus osteomyelitis is of great clinical significance. Based on transcriptional dataset GSE16129 available publicly, a bioinformatic analysis was performed to identify the differentially expressed genes of osteomyelitis caused by S. aureus infection. ERBB2, TWIST1, and NANOG were screened out as the most valuable osteomyelitis-related genes (OMRGs). A mice model of implant-associated S. aureus osteomyelitis was used to verify the above genes. We found significantly up-regulated expression of TWIST1 in macrophages and accumulation of macrophages around the infected implant. Meanwhile, S. aureus infection increased the expression of TWIST1, MMP9, and MMP13, and stimulated the migration and phagocytosis function of Raw 264.7 cells. Additionally, knock-down of the expression of TWIST1 by siRNA could significantly down-regulate MMP9 and MMP13 and suppress the migration and phagocytosis ability of macrophages in response to S. aureus infection. Furthermore, we found that NF-κB signaling was activated in Raw 264.7 cells by S. aureus and that inhibition of NF-κB signaling by Bay11-7082 blocked the expression of TWIST1, MMP9, and MMP13 as well as cell migration and phagocytosis evoked by S. aureus. Our findings demonstrate that NF-κB/TWIST1 is necessary for migration and phagocytosis of macrophages in response to S. aureus infection. Our study highlights the essential role of NF-κB/TWIST1 in early innate immune response to S. aureus infection in bone.

Functional Lipids in Autoimmune Inflammatory Diseases

Lipids are apolar small molecules known not only as components of cell membranes but also, in recent literature, as modulators of different biological functions. Herein, we focused on the bioactive lipids that can influence the immune responses and inflammatory processes regulating vascular hyperreactivity, pain, leukocyte trafficking, and clearance. In the case of excessive pro-inflammatory lipid activity, these lipids also contribute to the transition from acute to chronic inflammation. Based on their biochemical function, these lipids can be divided into different families, including eicosanoids, specialized pro-resolving mediators, lysoglycerophospholipids, sphingolipids, and endocannabinoids. These bioactive lipids are involved in all phases of the inflammatory process and the pathophysiology of different chronic autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, type-1 diabetes, and systemic lupus erythematosus.

Sphingosine-1-phosphate (S1P) receptors: Promising drug targets for treating bone-related diseases

Sphingosine-1-phosphate (S1P) is a natural bioactive lipid molecule and a common first or second messenger in the cardiovascular and immune systems. By binding with its receptors, S1P can serve as mediator of signalling during cell migration, differentiation, proliferation and apoptosis. Although the predominant role of S1P in bone regeneration has been noted in many studies, this role is not as well-known as its roles in the cardiovascular and immune systems. In this review, we summarize previous research on the role of S1P receptors (S1PRs) in osteoblasts and osteoclasts. In addition, S1P is regarded as a bridge between bone resorption and formation, which brings hope to patients with bone-related diseases. Finally, we discuss S1P and its receptors as therapeutic targets for treating osteoporosis, inflammatory osteolysis and bone metastasis based on the biological effects of S1P in osteoclastic/osteoblastic cells, immune cells and tumour cells.

IL-33/IL-31 Axis in Osteoporosis

The study of the immunoskeletal interface has led to the discovery of numerous cytokines involved in the regulation of bone remodeling, providing valuable information on the pathogenesis of osteoporosis. The role of inflammatory cytokines of the Th1 and Th17 profile in osteoporosis is well known. Here we focus on two newly discovered Th2 cytokines, IL-31 and IL-33, whose implications in osteoporosis are recently emerging. Clinical and experimental observations suggest an important role of the IL-33/IL-31 axis in osteoporosis. IL-33 induces IL-31 secretion by Th2 cells and inhibits RANKL-dependent osteoclastogenesis, thus counteracting bone loss. IL-31 influences Th1/Th17 osteoclastogenetic inflammation and limits Th2 osteoprotective processes, thus favoring osteoporosis. Better knowledge of the role of IL-31 and IL-33 and their receptor complexes in osteoporosis could provide an interesting perspective for the development of new and more effective therapies, possibly with less side effects.

S1P promotes IL-6 expression in osteoblasts through the PI3K, MEK/ERK and NF-κB signaling pathways

Rheumatoid arthritis (RA) is a systemic autoimmune inflammatory disease, in which the immune system attacks joint tissue. Interleukin (IL)-6 is a key proinflammatory cytokine in RA progression. Sphingosine-1-phosphate (S1P), a platelet-derived lysophospholipid mediator, reportedly regulates osteoimmunology. Here, we examined the effects of S1P on IL-6 expression in osteoblasts. Our results and records from the Gene Expression Omnibus (GEO) database demonstrate higher levels of IL-6 in patients with RA compared with those with osteoarthritis. Stimulation of osteoblasts with S1P increased mRNA and protein expression of IL-6. PI3K, MEK, ERK and NF-κB inhibitors and their small interfering RNAs (siRNAs) reduced S1P-promoted IL-6 expression. S1P also facilitated PI3K, MEK/ERK and NF-κB signaling cascades. Our results indicate that S1P promotes the expression of IL-6 in osteoblasts via the PI3K, MEK/ERK and NF-κB signaling pathways.