Semaphorins in bone development, homeostasis, and disease

Inhibition of inflammatory osteoclasts accelerates callus remodeling in osteoporotic fractures by enhancing CGRP+TrkA+ signaling

Impaired callus remodeling significantly contributes to the delayed healing of osteoporotic fractures; however, the underlying mechanisms remain unclear. Sensory neuronal signaling plays a crucial role in bone repair. In this study, we aimed to investigate the pathological mechanisms hindering bone remodeling in osteoporotic fractures, particularly focusing on the role of sensory neuronal signaling. We demonstrate that in ovariectomized (OVX) mice, the loss of CGRP+TrkA+ sensory neuronal signaling during callus remodeling correlates with increased Cx3cr1+iOCs expression within the bone callus. Conditional knockout of Cx3cr1+iOCs restored CGRP+TrkA+ sensory neuronal, enabling normal callus remodeling progression. Mechanistically, we further demonstrate that Cx3cr1+iOCs secrete Sema3A in the osteoporotic fracture repair microenvironment, inhibiting CGRP+TrkA+ sensory neurons' axonal regeneration and suppressing nerve-bone signaling exchange, thus hindering bone remodeling. Lastly, in human samples, we observed an association between the loss of CGRP+TrkA+ sensory neuronal signaling and increased expression of Cx3cr1+iOCs. In conclusion, enhancing CGRP+TrkA+ sensory nerve signaling by inhibiting Cx3cr1+iOCs activity presents a potential strategy for treating delayed healing in osteoporotic fractures. Inhibition of inflammatory osteoclasts enhances CGRP+TrkA+ signaling and accelerates callus remodeling in osteoporotic fractures.

Sensory nerve regulation of bone homeostasis: Emerging therapeutic opportunities for bone-related diseases

Understanding the intricate interplay between sensory nerves and bone tissue cells is of paramount significance in the field of bone biology and clinical medicine. The regulatory role of sensory nerves in bone homeostasis offers a novel perspective for the development of targeted therapeutic interventions for a spectrum of bone-related diseases, including osteoarthritis, osteoporosis, and intervertebral disc degeneration. By elucidating the mechanisms through which sensory nerves and their neuropeptides influence the differentiation and function of bone tissue cells, this review aims to shed light on emerging therapeutic targets that harness the neuro-skeletal axis for the treatment and management of debilitating bone disorders. Moreover, a comprehensive understanding of sensory nerve-mediated bone regulation may pave the way for the development of innovative strategies to promote bone health and mitigate the burden of skeletal pathologies in clinical practice.

The intracellular domain of Sema6A is essential for development of the zebrafish retina

Semaphorin6A (Sema6A) is a repulsive guidance molecule that plays many roles in central nervous system, heart and bone development, as well as immune system responses and cell signaling in cancer. Loss of Sema6A or its receptor PlexinA2 in zebrafish leads to smaller eyes and improper retinal patterning. Here, we investigate a potential role for the Sema6A intracellular domain in zebrafish eye development and dissect which phenotypes rely on forward signaling and which rely on reverse signaling. We performed rescue experiments on zebrafish Sema6A morphants with either full-length Sema6A (Sema6A-FL) or Sema6A lacking its intracellular domain (Sema6A-ΔC). We identified that the intracellular domain is not required for eye size and retinal patterning, however it is required for retinal integrity, the number and end feet strength of Müller glia and protecting against retinal cell death. This novel function for the intracellular domain suggests a role for Sema6A reverse signaling in zebrafish eye development.

An analysis of semaphorin-mediated cellular interactions in the Caenorhabditis elegans epidermis using the IR-LEGO single-cell gene induction system

One of the major functions of the semaphorin signaling system is the regulation of cell shape. In the nematode Caenorhabditis elegans, membrane-bound semaphorins SMP-1/2 (SMPs) regulate the morphology of epidermal cells via their receptor plexin, PLX-1. In the larval male tail of the SMP-PLX-1 signaling mutants, the border between two epidermal cells, R1.p and R2.p, is displaced anteriorly, resulting in the anterior displacement of the anterior-most ray, ray 1, in the adult male. To elucidate how the intercellular signaling mediated by SMPs regulates the position of the intercellular border, we performed mosaic gene expression analyses by using infrared laser-evoked gene operator (IR-LEGO). We show that PLX-1 expressed in R1.p and SMP-1 expressed in R2.p are required for the proper positioning of ray 1. The result suggests that SMP signaling promotes extension, rather than retraction, of R1.p. This is in contrast to a previous finding that SMPs mediate inhibition of cell extension of vulval precursor cells, another group of epidermal cells of C. elegans, indicating the context dependence of cell shape control via the semaphorin signaling system.

Plexin B3 guides axons to cross the midline in vivo

During the development of neural circuits, axons are guided by a variety of molecular cues to navigate through the brain and establish precise connections with correct partners at the right time and place. Many axon guidance cues have been identified and they play pleiotropic roles in not only axon guidance but also axon fasciculation, axon pruning, and synaptogenesis as well as cell migration, angiogenesis, and bone formation. In search of receptors for Sema3E in axon guidance, we unexpectedly found that Plexin B3 is highly expressed in retinal ganglion cells of zebrafish embryos when retinal axons are crossing the midline to form the chiasm. Plexin B3 has been characterized to be related to neurodevelopmental disorders. However, the investigation of its pathological mechanisms is hampered by the lack of appropriate animal model. We provide evidence that Plexin B3 is critical for axon guidance in vivo. Plexin B3 might function as a receptor for Sema3E while Neuropilin1 could be a co-receptor. The intracellular domain of Plexin B3 is required for Semaphorin signaling transduction. Our data suggest that zebrafish could be an ideal animal model for investigating the role and mechanisms of Sema3E and Plexin B3 in vivo.

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.

Dementia, osteoporosis and fragility fractures: Intricate epidemiological relationships, plausible biological connections, and twisted clinical practices

Dementia, osteoporosis, and fragility fractures are chronic diseases, often co-existing in older adults. These conditions pose severe morbidity, long-term disability, and mortality, with relevant socioeconomic implications. While in the research arena, the discussion remains on whether dementia is the cause or the consequence of fragility fractures, healthcare professionals need a better understanding of the interplay between such conditions from epidemiological and physiological standpoints. With this review, we summarized the available literature surrounding the relationship between cognitive impairment, dementia, and both low bone mineral density (BMD) and fragility fractures. Given the strength of the bi-directional associations and their impact on the quality of life, we shed light on the biological connections between brain and bone systems, presenting the main mediators, including gut microbioma, and pathological pathways leading to the dysregulation of bone and brain metabolism. Ultimately, we synthesized the evidence about the impact of available pharmacological treatments for the prevention of fragility fractures on cognitive functions and individuals' outcomes when dementia coexists. Vice versa, the effects of symptomatic treatments for dementia on the risk of falls and fragility fractures are explored. Combining evidence alongside clinical practice, we discuss challenges and opportunities related to the management of older adults affected by cognitive impairment or dementia and at high risk for fragility fracture prevention, which leads to not only an improvement in patient health-related outcomes and survival but also a reduction in healthcare cost and socio-economic burden.

Fine-tuning osteoclastogenesis: An insight into the cellular and molecular regulation of osteoclastogenesis

Osteoclasts, the bone-resorbing cells, are essential for the bone remodeling process and are involved in the pathophysiology of several bone-related diseases. The extensive corpus of in vitro research and crucial mouse model studies in the 1990s demonstrated the key roles of monocyte/macrophage colony-stimulating factor, receptor activator of nuclear factor kappa B ligand (RANKL) and integrin αvβ3 in osteoclast biology. Our knowledge of the molecular mechanisms by which these variables control osteoclast differentiation and function has significantly advanced in the first decade of this century. Recent developments have revealed a number of novel insights into the fundamental mechanisms governing the differentiation and functional activity of osteoclasts; however, these mechanisms have not yet been adequately documented. Thus, in the present review, we discuss various regulatory factors including local and hormonal factors, innate as well as adaptive immune cells, noncoding RNAs (ncRNAs), etc., in the molecular regulation of the intricate and tightly regulated process of osteoclastogenesis. ncRNAs have a critical role as epigenetic controllers of osteoclast physiologic activities, including differentiation and bone resorption. The primary ncRNAs, which include micro-RNAs, circular RNAs, and long noncoding RNAs, form a complex network that affects gene transcription activities associated with osteoclast biological activity. Greater knowledge of the involvement of ncRNAs in osteoclast biological activities will contribute to the treatment and management of several skeletal diseases such as osteoporosis, osteoarthritis, rheumatoid arthritis, etc. Moreover, we further outline potential therapies targeting these regulatory pathways of osteoclastogenesis in distinct bone pathologies.

Group 2 innate lymphoid cells and their surrounding environment

Since the discovery of group 2 innate lymphoid cells (ILC2s) in 2010, subsequent studies have revealed their developmental pathways, mechanisms of activation and regulation, and immunological roles in tissue homeostasis and tissue-specific diseases in various organs. Although ILC2s are known to express tissue-specific features depending on where they reside, how the surrounding environment affects the functions of ILC2s remains to be fully elucidated. Recent histologic analyses revealed that ILC2s resides in specific perivascular regions in peripheral tissues with their function being controlled by the surrounding cells via cytokines, lipid mediators, neurotransmitters, and cell-cell interactions through surface molecules. This review summarizes the interactions between ILC2s and surrounding cells, including epithelial cells, neurons, immune cells, and mesenchymal cells, with the objective of promoting the development of novel diagnostic and therapeutic methods for ILC2-related diseases.

Pyroptosis in bone loss

Pyroptosis could be responsible for the bone loss from bone metabolic diseases, leading to the negative impact on people's health and life. It has been shown that osteoclasts, osteoblasts, macrophages, chondrocytes, periodontal and gingival cells may be involved in bone loss linked with pyroptosis. So far, the involved mechanisms have not been fully elucidated. In this review, we introduced the related cells involved in the pyroptosis associated with bone loss and summarized the role of these cells in the bone metabolism during the process of pyroptosis. We also discuss the clinical potential of targeting mechanisms in the osteoclasts, osteoblasts, macrophages, chondrocytes, periodontal and gingival cells touched upon pyroptosis to treat bone loss from bone metabolic diseases as well as the challenges of avoiding potential side effects and producing efficient treatment methods.

Downregulation of the Autism Spectrum Disorder Gene Shank2 Decreases Bone Mass in Male Mice

Mutations of the postsynaptic scaffold protein Shank2 lead to autism spectrum disorders (ASD). These patients frequently suffer from higher fracture risk. Here, we investigated whether Shank2 directly regulates bone mass. We show that Shank2 is expressed in bone and that Shank2 levels are increased during osteoblastogenesis. Knockdown of Shank2 by siRNA targeting the encoding regions for PDZ and SAM domain inhibits osteoblastogenesis of primary murine calvarial osteoblasts. Shank2 knockout mice (Shank2 ^-/-) have a decreased bone mass due to reduced osteoblastogenesis and bone formation, whereas bone resorption remains unaffected. Induced pluripotent stem cells (iPSCs)-derived osteoblasts from a loss-of-function Shank2 mutation in a patient showed a significantly reduced osteoblast differentiation potential. Moreover, silencing of known Shank2 interacting proteins revealed that a majority of them promote osteoblast differentiation. From this we conclude that Shank2 and interacting proteins known from the central nervous system are decisive regulators in osteoblast differentiation. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Semaphorin 6D-expressing mesenchymal cells regulate IL-10 production by ILC2s in the lung

Group 2 innate lymphoid cells (ILC2s) have features specific to the niches in which they reside, and we found that semaphorin 6D signaling in the lung niche controls IL-10 production by ILC2s.

Group 2 innate lymphoid cells (ILC2s) have been implicated in both physiologic tissue remodeling and allergic pathology, yet the niche signaling required for ILC2 properties is poorly understood. Here, we show that an axonal guidance cue semaphorin 6D (Sema6D) plays critical roles in the maintenance of IL-10–producing ILC2s. Sema6d^−/− mice exhibit a severe steady-state reduction in ILC2s in peripheral sites such as the lung, visceral adipose tissue, and mesentery. Interestingly, loss of Sema6D results in suppressed alarmin-driven type 2 cytokine production but increased IL-10 production by lung ILC2s both in vitro and in vivo. Consequently, Sema6d^−/− mice are resistant to the development of allergic lung inflammation. We further found that lung mesenchymal cells highly express Sema6D, and that niche-derived Sema6D is responsible for these phenotypes through plexin A1. Collectively, these findings suggest that niche-derived Sema6D is implicated in physiological and pathological characteristics of ILC2s.

The role of semaphorins in cardiovascular diseases: Potential therapeutic targets and novel biomarkers

Semaphorins (Semas), which belongs to the axonal guidance molecules, include 8 classes and could affect axon growth in the nervous system. Recently, semaphorins were found to regulate other pathophysiological processes, such as immune response, oncogenesis, tumor angiogenesis, and bone homeostasis, through binding with their plexin and neuropilin receptors. In this review, we summarized the detailed role of semaphorins and their receptors in the pathological progression of various cardiovascular diseases (CVDs), highlighting that semaphorins may be potential therapeutic targets and novel biomarkers for CVDs.

Spatial Organization of Osteoclastic Coupling Factors and Their Receptors at Human Bone Remodeling Sites

The strictly regulated bone remodeling process ensures that osteoblastic bone formation is coupled to osteoclastic bone resorption. This coupling is regulated by a panel of coupling factors, including clastokines promoting the recruitment, expansion, and differentiation of osteoprogenitor cells within the eroded cavity. The osteoprogenitor cells on eroded surfaces are called reversal cells. They are intermixed with osteoclasts and become bone-forming osteoblast when reaching a critical density and maturity. Several coupling factors have been proposed in the literature, but their effects and expression pattern vary between studies depending on species and experimental setup. In this study, we investigated the mRNA levels of proposed secreted and membrane-bound coupling factors and their receptors in cortical bone remodeling events within the femur of healthy adolescent human controls using high-sensitivity RNA in situ hybridization. Of the proposed coupling factors, human osteoclasts showed mRNA-presence of LIF, PDGFB, SEMA4D, but no presence of EFNB2, and OSM. On the other hand, the osteoblastic reversal cells proximate to osteoclasts presented with LIFR, PDGFRA and PLXNB1, but not PDGFRB, which are all known receptors of the proposed coupling factors. Although EFNB2 was not present in mature osteoclasts, the mRNA of the ligand-receptor pair EFNB2:EPHB4 were abundant near the central blood vessels within intracortical pores with active remodeling. EPHB4 and SEMA4D were also abundant in mature bone-forming osteoblasts. This study highlights that especially LIF:LIFR, PDGFB:PDGFRA, SEMA4D:PLXNB1 may play a critical role in the osteoclast-osteoblast coupling in human remodeling events, as they are expressed within the critical cells.

Semaphorin 3G exacerbates joint inflammation through the accumulation and proliferation of macrophages in the synovium

Objectives

Methotrexate (MTX) is an anchor drug for the treatment of rheumatoid arthritis (RA). However, the precise mechanisms by which MTX stalls RA progression and alleviates the ensuing disease effects remain unknown. The aim of the present study was to identify novel therapeutic target molecules, the expression patterns of which are affected by MTX in patients with RA.

Methods

CD4⁺ T cells from 28 treatment-naïve patients with RA before and 3 months after the initiation of MTX treatment were subjected to DNA microarray analyses. The expression levels of semaphorin 3G, a differentially expressed gene, and its receptor, neuropilin-2, were evaluated in the RA synovium and collagen-induced arthritis synovium. Collagen-induced arthritis and collagen antibody-induced arthritis were induced in semaphorin3G-deficient mice and control mice, and the clinical score, histological score, and serum cytokines were assessed. The migration and proliferation of semaphorin 3G-stimulated bone marrow-derived macrophages were analyzed in vitro. The effect of local semaphorin 3G administration on the clinical score and number of infiltrating macrophages during collagen antibody-induced arthritis was evaluated.

Results

Semaphorin 3G expression in CD4⁺ T cells was downregulated by MTX treatment in RA patients. It was determined that semaphorin 3G is expressed in RA but not in the osteoarthritis synovium; its receptor neuropilin-2 is primarily expressed on activated macrophages. Semaphorin3G deficiency ameliorated collagen-induced arthritis and collagen antibody-induced arthritis. Semaphorin 3G stimulation enhanced the migration and proliferation of bone marrow-derived macrophages. Local administration of semaphorin 3G deteriorated collagen antibody-induced arthritis and increased the number of infiltrating macrophages.

Conclusions

Upregulation of semaphorin 3G in the RA synovium is a novel mechanism that exacerbates joint inflammation, leading to further deterioration, through macrophage accumulation.

Axon guidance molecules in immunometabolic diseases

The global prevalence of metabolic diseases, such as obesity, diabetes, and atherosclerosis, is rapidly increasing and has now reached epidemic proportions. Chronic tissue inflammation is a characteristic of these metabolic diseases, indicating that immune responses are closely involved in the pathogenesis of metabolic disorders. However, the regulatory mechanisms underlying immunometabolic crosstalk in these diseases are not completely understood. Recent studies have revealed the multifaceted functions of semaphorins, originally identified as axon guidance molecules, in regulating tissue inflammation and metabolic disorders, thereby highlighting the functional coupling between semaphorin signaling and immunometabolism. In this review, we explore how semaphorin signaling transcends beyond merely guiding axons to controlling immune responses and metabolic diseases.

Differential Effects of Neurectomy and Botox-induced Muscle Paralysis on Bone Phenotype and Titanium Implant Osseointegration

Metabolic bone is highly innervated by both sensory and sympathetic nerves. In addition to skeletal development, neural regulation participates in local bone remodeling, which is important for successful osseointegration of titanium implants. Neurectomy is a model used to investigate the lack of neural function on bone homeostasis, but the relative impacts of direct denervation to bone or denervation-induced muscle paralysis are less well defined. To investigate this difference, we used two nerve intervention models, sciatic and femoral neurectomy (SFN) v. botox-induced muscle paralysis (BTX) and assessed the resulting femoral bone phenotype and Ti implant osseointegration. Male Sprague Dawley rats (19) were randomly divided into three groups: implant control (n = 5), SFN (n = 7), and BTX (n = 7). Ti implants (microrough/hydrophilic [modSLA], Institut Straumann AG) were placed in the distal metaphysis of each femur on day 24 post-SFN or BTX. Bone and muscle were examined on day 28 after implant insertion. Both nerve intervention models impaired osseointegration. MicroCT and histology indicated that both models had reduced trabecular bone formation. Only BTX reduced cortical bone formation and increased cortical bone porosity. BTX resulted in more bone loss characterized by the least trabecular and cortical bone, as well as osseointegration. Osteoblasts isolated from the tibia exhibited a model-specific phenotype when they were grown on Ti substrates in vitro. Neurectomy caused more severe muscle atrophy than botox injection. These results indicate that neural regulation directly modulates bone formation and osseointegration. Muscle paralysis modulated the effects of loss of neural inputs into bone, supporting the hypothesis that mechanical loading of bone is a factor in achieving successful osseointegration. The different effects of botox and neurectomy on bone phenotype indicated that the sensory and sympathetic nerves had a role in the osseointegration process.

Tenofovir Modulates Semaphorin 4D Signaling and Regulates Bone Homeostasis, Which Can Be Counteracted by Dipyridamole and Adenosine A2A Receptor

Semaphorin 4D (Sema4D) is a neurotrophin that is secreted by osteoclasts and binds to its receptor PlexinB1 on osteoblasts to inhibit their differentiation and function. Adenosine A2A activation inhibits osteoclast Sema4D-mediated secretion, diminishes inflammatory osteolysis and prevents bone loss following tenofovir (one of the most used antivirals in HIV). Therefore, tenofovir might activate Sema4D signaling to alter bone turnover. Female C57Bl/6/A2AKO mice were ovariectomized and treated with saline (control), tenofovir 75 mg/Kg/day, dipyridamole 25 mg/Kg/day or a combination for 5 weeks and long bones were prepared for histology. Primary murine-induced osteoclast/osteoblast were challenged with tenofovir/dipyridamole 1 μM each, and the expression of Sema4D/PlexinB1, RhoA/ROCK/IGF1R was studied by RT-PCR, Western blot and immunostaining. In vivo tenofovir showed an increased expression of Sema4D when compared to control mice, and dipyridamole reverted the expression in an A2A-dependent manner. In vitro, tenofovir increases Sema4D expression and secretion in osteoclast precursors, and pre-treatment with dipyridamole reverted this effect. pRhoA and ROCK1 activation were increased and IRS1/IGF1R expression was diminished by tenofovir in the Vav3/ARHGAP18 mechanism in osteoblast precursors and reverted by dipyridamole in an A2A-dependent manner. This suggests that tenofovir increases bone loss by activation of Sema4D/PlexinB1 signaling, which inhibits osteoblast differentiation. Agents that increase local adenosine concentrations, such as dipyridamole, might prevent bone loss following the inhibition of this pathway.

The Expression of Semaphorin 7A in Human Periapical Lesions

INTRODUCTION

Semaphorin 7A (SEMA7A) is a membrane-bound or secretory protein exerting multiple functions in the regulation of inflammation, neural degradation, and cancer progression. Human periapical lesions are chronic and infectious diseases mainly caused by bacteria. However, the involvement of SEMA7A in human periapical lesions is still unclear. This study aimed to explore the expression of SEMA7A in human periapical lesions accompanied by the potential association of SEMA7A with matrix metalloproteinase (MMP)-1 and MMP-3 during the progression of apical periodontitis.

METHODS

Samples of periapical lesions and healthy controls were collected. Total RNA and protein were extracted respectively for quantitative real-time polymerase chain reaction and Western blot analysis. Additionally, 6 healthy samples and 27 periapical lesion samples were fixed, dehydrated, and embedded for further histologic and immunochemical analysis. The expression of SEMA7A was quantified by average integrated optical density. Immunofluorescence analysis was conducted to explore the colocalization of SEMA7A/MMP-1 and SEMA7A/MMP-3.

RESULTS

Compared with healthy controls, the messenger RNA and protein expression of SEMA7A was markedly up-regulated in periapical lesions. A stronger expression of MMP-1, MMP-3, and inflammatory cytokines was exhibited in periapical lesions than in healthy groups. An increasing expression of SEMA7A can be observed in both the periapical granuloma group and the radicular cyst group compared with the normal group (P < .01). Immunofluorescence results showed the colocalization of SEMA7A with both MMP-1 and MMP-3 in vascular vessels and extracellular matrix.

CONCLUSIONS

SEMA7A was up-regulated in periapical periodontitis and might be involved in the tissue destruction and infiltration of immune cells in periapical lesions.

Semaphorins in Adult Nervous System Plasticity and Disease

Semaphorins, originally discovered as guidance cues for developing axons, are involved in many processes that shape the nervous system during development, from neuronal proliferation and migration to neuritogenesis and synapse formation. Interestingly, the expression of many Semaphorins persists after development. For instance, Semaphorin 3A is a component of perineuronal nets, the extracellular matrix structures enwrapping certain types of neurons in the adult CNS, which contribute to the closure of the critical period for plasticity. Semaphorin 3G and 4C play a crucial role in the control of adult hippocampal connectivity and memory processes, and Semaphorin 5A and 7A regulate adult neurogenesis. This evidence points to a role of Semaphorins in the regulation of adult neuronal plasticity. In this review, we address the distribution of Semaphorins in the adult nervous system and we discuss their function in physiological and pathological processes.

Naturally occurring coumestans from plants, their biological activities and therapeutic effects on human diseases

Naturally occurring coumestans are known as a collection of plant-derived polycyclic aromatic secondary metabolites which are characterized by the presence of an oxygen heterocyclic four-ring system comprising a coumarin moiety and a benzofuran moiety sharing a C˭C bond. Recently, there is an increasing attention in excavating the medicinal potential of coumestans, particularly coumestrol, wedelolactone, psoralidin and glycyrol, in a variety of diseases. This review is a comprehensive inventory of the chemical structures of coumestans isolated from various plant sources during the period of 1956-2020, together with their reported biological activities. 120 molecules were collected and further classified as coumestans containing core skeleton, dimethylpyranocoumestans, furanocoumestans, O-glycosylated coumestans and others, which showed a wide range of pharmacological activities including estrogenic, anti-cancer, anti-inflammatory, anti-osteoporotic, organ protective, neuroprotective, anti-diabetic and anti-obesity, antimicrobial, immunosuppressive, antioxidant and skin-protective activities. Furthermore, this review focuses on the counteraction of coumestans against bone diseases and organ damages, and the involved molecular mechanisms, which could provide important information to better understand the medicinal values of these compounds. This review is intended to be instructive for the rational design and development of less toxic and more effective drugs with a coumestan scaffold.

Skeleton-vasculature chain reaction: a novel insight into the mystery of homeostasis

Angiogenesis and osteogenesis are coupled. However, the cellular and molecular regulation of these processes remains to be further investigated. Both tissues have recently been recognized as endocrine organs, which has stimulated research interest in the screening and functional identification of novel paracrine factors from both tissues. This review aims to elaborate on the novelty and significance of endocrine regulatory loops between bone and the vasculature. In addition, research progress related to the bone vasculature, vessel-related skeletal diseases, pathological conditions, and angiogenesis-targeted therapeutic strategies are also summarized. With respect to future perspectives, new techniques such as single-cell sequencing, which can be used to show the cellular diversity and plasticity of both tissues, are facilitating progress in this field. Moreover, extracellular vesicle-mediated nuclear acid communication deserves further investigation. In conclusion, a deeper understanding of the cellular and molecular regulation of angiogenesis and osteogenesis coupling may offer an opportunity to identify new therapeutic targets.

Genomic association with pathogen carriage in bighorn sheep (Ovis canadensis)

Genetic composition can influence host susceptibility to, and transmission of, pathogens, with potential population-level consequences. In bighorn sheep (Ovis canadensis), pneumonia epidemics caused by Mycoplasma ovipneumoniae have been associated with severe population declines and limited recovery across North America. Adult survivors either clear the infection or act as carriers that continually shed M. ovipneumoniae and expose their susceptible offspring, resulting in high rates of lamb mortality for years following the outbreak event. Here, we investigated the influence of genomic composition on persistent carriage of M. ovipneumoniae in a well-studied bighorn sheep herd in the Wallowa Mountains of Oregon, USA. Using 10,605 SNPs generated using RADseq technology for 25 female bighorn sheep, we assessed genomic diversity metrics and employed family-based genome-wide association methodologies to understand variant association and genetic architecture underlying chronic carriage. We observed no differences among genome-wide diversity metrics (heterozygosity and allelic richness) between groups. However, we identified two variant loci of interest and seven associated candidate genes, which may influence carriage status. Further, we found that the SNP panel explained ~55% of the phenotypic variance (SNP-based heritability) for M. ovipneumoniae carriage, though there was considerable uncertainty in these estimates. While small sample sizes limit conclusions drawn here, our study represents one of the first to assess the genomic factors influencing chronic carriage of a pathogen in a wild population and lays a foundation for understanding genomic influence on pathogen persistence in bighorn sheep and other wildlife populations. Future research should incorporate additional individuals as well as distinct herds to further explore the genomic basis of chronic carriage.

Bone Vasculature and Bone Marrow Vascular Niches in Health and Disease

Bone vasculature and bone marrow vascular niches supply oxygen, nutrients, and secrete angiocrine factors required for the survival, maintenance, and self-renewal of stem and progenitor cells. In the skeletal system, vasculature creates nurturing niches for bone and blood-forming stem cells. Blood vessels regulate hematopoiesis and drive bone formation during development, repair, and regeneration. Dysfunctional vascular niches induce skeletal aging, bone diseases, and hematological disorders. Recent cellular and molecular characterization of the bone marrow microenvironment has provided unprecedented insights into the complexity, heterogeneity, and functions of the bone vasculature and vascular niches. The bone vasculature is composed of distinct vessel subtypes that differentially regulate osteogenesis, hematopoiesis, and disease conditions in bones. Further, bone marrow vascular niches supporting stem cells are often complex microenvironments involving multiple different cell populations and vessel subtypes. This review provides an overview of the emerging vascular cell heterogeneity in bone and the new roles of the bone vasculature and associated vascular niches in health and disease. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Semaphorins in health and disease

Cell-cell communication is pivotal to guide embryo development, as well as to maintain adult tissues homeostasis and control immune response. Among extracellular factors responsible for this function, are the Semaphorins, a broad family of around 20 different molecular cues conserved in evolution and widely expressed in all tissues. The signaling cascades initiated by semaphorins depend on a family of conserved receptors, called Plexins, and on several additional molecules found in the receptor complexes. Moreover, multiple intracellular pathways have been described to act downstream of semaphorins, highlighting significant diversity in the signaling cascades controlled by this family. Notably, semaphorin expression is altered in many human diseases, such as immunopathologies, neurodegenerative diseases and cancer. This underscores the importance of semaphorins as regulatory factors in the tissue microenvironment and has prompted growing interest for assessing their potential relevance in medicine. This review article surveys the main contexts in which semaphorins have been found to regulate developing and healthy adult tissues, and the signaling cascades implicated in these functions. Vis a vis, we will highlight the main pathological processes in which semaphorins are thought to have a role thereof.

Osteocytes control myeloid cell proliferation and differentiation through Gsα-dependent and -independent mechanisms

Osteocytes, the bone cells embedded in the mineralized matrix, control bone modeling, and remodeling through direct contact with adjacent cells and via paracrine and endocrine factors that affect cells in the bone marrow microenvironment or distant organs. Osteocytes express numerous G protein-coupled receptors (GPCRs) and thus mice lacking the stimulatory subunit of G-protein (Gsα) in osteocytes (Dmp1-Gsα^KO mice) have abnormal myelopoiesis, osteopenia, and reduced adipose tissue. We previously reported that the severe osteopenia and the changes in adipose tissue present in these mice were mediated by increased sclerostin, which suppress osteoblast functions and promote browning of white adipocytes. Inversely, the myeloproliferation was driven by granulocyte colony-stimulating factor (G-CSF) and administration of neutralizing antibodies against G-CSF only partially restored the myeloproliferation, suggesting that additional osteocyte-derived factors might be involved. We hypothesized that osteocytes secrete Gsα-dependent factor(s) which regulate the myeloid cells proliferation. To identify osteocyte-secreted proteins, we used the osteocytic cell line Ocy454 expressing or lacking Gsα expression (Ocy454-Gsα^cont and Ocy454-Gsα^KO ) to delineate the osteocyte "secretome" and its regulation by Gsα. Here we reported that factors secreted by osteocytes increased the number of myeloid colonies and promoted macrophage proliferation. The proliferation of myeloid cells was further promoted by osteocytes lacking Gsα expression. Myeloid cells can differentiate into bone-resorbing osteoclasts, therefore, we hypothesized that osteocyte-secreted factors might also regulate osteoclastogenesis in a Gsα-dependent manner. Conditioned medium (CM) from Ocy454 (both Gsα^cont and Gsα^KO ) significanlty increased the proliferation of bone marrow mononuclear cells (BMNC) and, at the same time, inhibited their differentiation into mature osteoclasts via a Gsα-dependent mechanism. Proteomics analysis of CM from Ocy454 Gsα^cont and Gsα^KO cells identified neuropilin-1 (Nrp-1) and granulin (Grn) as osteocytic-secreted proteins upregulated in Ocy454-Gsα^KO cells compared to Ocy454-Gsα^cont , whereas semaphorin3A was significantly suppressed. Treatment of Ocy454-Gsα^cont cells with recombinant proteins or knockdown of Nrp-1 and Grn in Ocy454-Gsα^KO cells partially rescued the inhibition of osteoclasts, demonstrating that osteocytes control osteoclasts differentiation through Nrp-1 and Grn which are regulated by Gsα signaling.

Regulation of mesenchymal stem cell differentiation on microstructured titanium surfaces by semaphorin 3A

Peri-implant bone formation depends on the ability of mesenchymal stem cells (MSCs) to colonize implant surfaces and differentiate into osteoblasts, but the precise mechanisms controlling this process remain unclear. In vitro, MSCs undergo osteoblastic differentiation on microstructured titanium (Ti) surfaces in the absence of exogenous media supplements and produce factors that promote osteogenesis while regulating osteoclast activity, including semaphorins. The goal of this study was to evaluate the role of semaphorin 3A (Sema3A) on surface-mediated osteoblastic differentiation and determine the hierarchy of this signaling cascade. Human MSCs were cultured on 15 mm grade 2 smooth (pretreatment, PT), hydrophobic-microrough (sand blasted/acid etched, SLA), hydrophilic-microrough Ti (mSLA) (Institut Straumann AG, Basel, Switzerland), or tissue culture polystyrene (TCPS). Expression of SEMA3A family proteins increased after 7 days of culture, and the increased expression in response to microstructured Ti was dependent on recognition of the surface by integrin α2β1. Exogenous Sema3A increased differentiation whereas differentiation was decreased in cells treated with a Sema3A antibody. Furthermore, Sema3A influenced the production of osteoprotegerin and osteopontin suggesting it as an important local regulator of bone remodeling. Inhibition of Wnt3A and Wnt5A revealed that activation of Sema3A occurs downstream of Wnt5A and may facilitate the translocation of β-catenin bypassing the canonical Wnt3A initiating signal associated with osteoblastic differentiation. Furthermore, chemical inhibition of calmodulin (CaM), Ca²⁺/calmodulin-dependent protein kinase (CaMKII), phospholipase A₂ (PLA2), protein kinase C (PKC), and BMP receptors suggest that Sema3A could serve as a feedback mechanism for both Wnt5A and BMP2. Here, we show novel roles for Sema3A family proteins in the surface-dependent modulation of MSCs as well as important interactions with pathways known to be associated with osteoblastic differentiation. Moreover, their effects on bone remodeling markers have significant implications for peri-implant bone remodeling and downstream modulation of osteoclastic activity. These results suggest that Sema3A aids in peri-implant bone formation through regulation on multiple stages of osseointegration, making it a potential target to promote osseointegration in patients with compromised bone remodeling.

The Effect of Boron-Containing Nano-Hydroxyapatite on Bone Cells

Metabolic diseases or injuries damage bone structure and self-renewal capacity. Trace elements and hydroxyapatite crystals are important in the development of biomaterials to support the renewal of bone extracellular matrix. In this study, it was assumed that the boron-loaded nanometer-sized hydroxyapatite composite supports the construction of extracellular matrix by controlled boron release in order to prevent its toxic effect. In this context, boron release from nanometer-sized hydroxyapatite was calculated by ICP-MS as in large proportion within 1 h and continuing release was provided at a constant low dose. The effect of the boron-containing nanometer-sized hydroxyapatite composite on the proliferation of SaOS-2 osteoblasts and human bone marrow-derived mesenchymal stem cells was evaluated by WST-1 and compared with the effects of nano-hydroxyapatite and boric acid. Boron increased proliferation of mesenchymal stem cells at high doses and exhibited different effects on osteoblastic cell proliferation. Boron-containing nano-hydroxyapatite composites increased osteogenic differentiation of mesenchymal stem cells by increasing alkaline phosphatase activity, when compared to nano-hydroxyapatite composite and boric acid. The molecular mechanism of effective dose of boron-containing hydroxyapatite has been assessed by transcriptomic analysis and shown to affect genes involved in Wnt, TGF-β, and response to stress signaling pathways when compared to nano-hydroxyapatite composite and boric acid. Finally, a safe osteoconductive dose range of boron-containing nano-hydroxyapatite composites for local repair of bone injuries and the molecular effect profile in the effective dose should be determined by further studies to validation of the regenerative therapeutic effect window.

Role of angiocrine signals in bone development, homeostasis and disease

Skeletal vasculature plays a central role in the maintenance of microenvironments for osteogenesis and haematopoiesis. In addition to supplying oxygen and nutrients, vasculature provides a number of inductive factors termed as angiocrine signals. Blood vessels drive recruitment of osteoblast precursors and bone formation during development. Angiogenesis is indispensable for bone repair and regeneration. Dysregulation of the angiocrine crosstalk is a hallmark of ageing and pathobiological conditions in the skeletal system. The skeletal vascular bed is complex, heterogeneous and characterized by distinct capillary subtypes (type H and type L), which exhibit differential expression of angiocrine factors. Furthermore, distinct blood vessel subtypes with differential angiocrine profiles differentially regulate osteogenesis and haematopoiesis, and drive disease states in the skeletal system. This review provides an overview of the role of angiocrine signals in bone during homeostasis and disease.

Functions of Plexins/Neuropilins and Their Ligands during Hippocampal Development and Neurodegeneration

There is emerging evidence that molecules, receptors, and signaling mechanisms involved in vascular development also play crucial roles during the development of the nervous system. Among others, specific semaphorins and their receptors (neuropilins and plexins) have, in recent years, attracted the attention of researchers due to their pleiotropy of functions. Their functions, mainly associated with control of the cellular cytoskeleton, include control of cell migration, cell morphology, and synapse remodeling. Here, we will focus on their roles in the hippocampal formation that plays a crucial role in memory and learning as it is a prime target during neurodegeneration.

Sema6A-plexin-A2 axis stimulates RANKL-induced osteoclastogenesis through PLCγ-mediated NFATc1 activation

Recently, several plexins and semaphorins have been associated with osteoclastogenesis, a vital process for bone remodeling. Plexin-A2 is implicated in bone homeostasis, however, whether it plays a role in osteoclastogenesis and the underlying mechanism remain unknown. We show that plexin-A2 expression is upregulated during RANKL-induced osteoclastogenesis. In addition, the soluble Sema6A fused with IgG1 Fc region (Fc-Sema6A) interacts with plexin-A2 from cell lysates of osteoclasts, suggesting that plexin-A2 acts as a receptor of Sema6A in osteoclasts. Moreover, Sema6A treatment stimulates RANKL-induced osteoclastogenesis, and this effect is abolished when plexin-A2 is neutralized, which illustrates an indispensable role of plexin-A2 in mediating Sema6A effect on osteoclastogenesis. Mechanistically, Sema6A-plexin-A2 axis enhances RANKL-induced activation of PLCγ as well as downstream target NFATc1, one master transcriptional factor of osteoclastogenesis. Lastly, inhibition of PLCγ by pharmacological inhibitor U73122 abrogates Sema6A-stimulated NFATc1 activation and RANKL-induced osteoclastogenesis, thus demonstrating that the PLCγ-mediated NFATc1 activation accounts for the promotive role of Sema6A-plexin-A2 axis in RANKL-induced osteoclastogenesis. Taken together, this study uncovers a novel role of Sema6A and plexin-A2 in osteoclastogenesis, and also offers them as possible therapeutic targets in the intervention of osteolytic diseases.

Role of Semaphorins in Immunopathologies and Rheumatic Diseases

Rheumatic diseases are disorders characterized by joint inflammation, in which other organs are also affected. There are more than two hundred rheumatic diseases, the most studied so far are rheumatoid arthritis, osteoarthritis, spondyloarthritis, systemic lupus erythematosus, and systemic sclerosis. The semaphorin family is a large group of proteins initially described as axon guidance molecules involved in nervous system development. Studies have demonstrated that semaphorins play a role in other processes such as the regulation of immunity, angiogenesis, bone remodeling, apoptosis, and cell migration and invasion. Moreover, semaphorins have been related to the pathogenesis of multiple sclerosis, asthma, Alzheimer, myocarditis, atherosclerosis, fibrotic diseases, osteopetrosis, and cancer. The aim of this review is to summarize current knowledge regarding the role of semaphorins in rheumatic diseases, and discuss their potential applications as therapeutic targets to treat these disorders.

Investigation of the effects of semaphorin 3A on new bone formation in a rat calvarial defect model

PURPOSE

This study investigates the effects of semaphorin 3A on new bone formation in an experimental rat model.

MATERIALS AND METHODS

Cortical bone defects, 5 mm, were created in the calvaria of 40 Wistar rats, which were then separated into three groups: empty defect (control) group, collagen group, collagen + semaphorin 3A group. The bone blocks were harvested after 4 and 8 weeks. New bone formation was assessed by micro-computed tomography (micro-CT), histology, histomorphometry, transmission electron microscope (TEM) and immunohistochemistry.

RESULTS

Increased bone formation was observed in collagen + semaphorin 3A groups both histologically and with micro-CT. In the histomorphometic analysis, the control group had significantly less bone formation compared to both the collagen and collagen + semaphorin 3A group at 4 weeks (p = 0.0001) and 8 weeks (p = 0.0001). The collagen group had significantly less bone formation compared to collagen + semaphorin 3A group both at 4 weeks (p = 0.002) and 8 weeks (p = 0.005). Immunohistochemical analysis revealed that semaphorin 3A inhibited receptor activator of nuclear factor-kB ligand (RANKL) expression and increased the expressions of osteoblastic bone markers at 4 weeks. In TEM analysis, the collagen + semaphorin 3A group had an increased proliferation and bone formation rate at 4 weeks, whereas bone quantity and maturation were enhanced at 8 weeks.

CONCLUSION

Locally applied semaphorin 3A increases callus formation at 4 weeks and bone formation at 8 weeks. Semaphorin 3A prevents bone resorption by inhibiting osteoclasts and increases bone formation by inducing osteoblasts.

Sema3a inhibits the differentiation of Raw264.7 cells to osteoclasts under 2Gy radiation by reducing inflammation

Astronauts and cancer patients receive different types of radiation, and radiation decreases bone strength and leads to radiation-induced osteoporosis. This effect is attributed to the activation of osteoclasts. Our aim was to study the effect of Sema3a on the differentiation of the murine macrophage cell line Raw264.7 into osteoclasts upon irradiation. Raw264.7 cells were divided into four groups: A, receiving no radiation; B, receiving no radiation + 50ngng/ml Sema3a; C, receiving 2Gy radiation; and D, receiving 2Gy radiation +50ngng/ml Sema3a. After treatment, cells were subjected to a proliferation assay, migration assay, live and apoptosis assay, and an ROS assay, along with analyses of bone resorption activity, TRAP staining and RT-PCR to assess the effect of Sema3a on Raw264.7 cells under 2Gy radiation. Sema3a inhibited the proliferation of Raw264.7 cells and showed statistical significance at a concentration of 100ngng/ml (P<0.05). Under 2Gy radiation, cell migration was reduced (P<0.05). In addition, 2Gy radiation resulted in more apoptotic cells, a higher level of ROS, larger bone resorption lacunae and more Trap-positive cells (p<0.05), and radiation increased CSTK, NFAT, TRAP-5b, Rankl/OPG, IL-1, IL-6, TNFa and P53 gene expression (P<0.05). Sema3a had an inhibitory effect on the differentiation of Raw264.7 cells and the migration and activity of osteoclasts upon irradiation but did not affect ROS. Sema3a also decreased the expression of CSTK, NFAT, TRAP-5b, Rankl/OPG, IL-1, IL-6 and TNFa on the 3rd and 7th days after irradiation (p<0.05), whereas P53 expression was increased (P<0.05). Sema3a reduced the inflammation induced by radiation and negatively regulated osteoclast differentiation. Sema3a promoted Raw264.7 cell apoptosis after irradiation, indicating that Sema3a could be a potential therapeutic target for radiation-induced osteoporosis.

Semaphorin 6D reverse signaling controls macrophage lipid metabolism and anti-inflammatory polarization

Polarization of macrophages into pro-inflammatory or anti-inflammatory states has distinct metabolic requirements, with mechanistic target of rapamycin (mTOR) kinase signaling playing a critical role. However, it remains unclear how mTOR regulates metabolic status to promote polarization of these cells. Here we show that an mTOR-Semaphorin 6D (Sema6D)-Peroxisome proliferator receptor γ (PPARγ) axis plays critical roles in macrophage polarization. Inhibition of mTOR or loss of Sema6D blocked anti-inflammatory macrophage polarization, concomitant with severe impairments in PPARγ expression, uptake of fatty acids, and lipid metabolic reprogramming. Macrophage expression of the receptor Plexin-A4 is responsible for Sema6D-mediated anti-inflammatory polarization. We found that a tyrosine kinase, c-Abl, which associates with the cytoplasmic region of Sema6D, is required for PPARγ expression. Furthermore, Sema6D is important for generation of intestinal resident CX3CR1^hi macrophages and prevents development of colitis. Collectively, these findings highlight crucial roles for Sema6D reverse signaling in macrophage polarization, coupling immunity, and metabolism via PPARγ.

Semaphorins and their Signaling Mechanisms

Semaphorins are extracellular signaling proteins that are essential for the development and maintenance of many organs and tissues. The more than 20-member semaphorin protein family includes secreted, transmembrane and cell surface-attached proteins with diverse structures, each characterized by a single cysteine-rich extracellular sema domain, the defining feature of the family. Early studies revealed that semaphorins function as axon guidance molecules, but it is now understood that semaphorins are key regulators of morphology and motility in many different cell types including those that make up the nervous, cardiovascular, immune, endocrine, hepatic, renal, reproductive, respiratory and musculoskeletal systems, as well as in cancer cells. Semaphorin signaling occurs predominantly through Plexin receptors and results in changes to the cytoskeletal and adhesive machinery that regulate cellular morphology. While much remains to be learned about the mechanisms underlying the effects of semaphorins, exciting work has begun to reveal how semaphorin signaling is fine-tuned through different receptor complexes and other mechanisms to achieve specific outcomes in various cellular contexts and physiological systems. These and future studies will lead to a more complete understanding of semaphorin-mediated development and to a greater understanding of how these proteins function in human disease.

Adenosine A2A receptor (A2AR) stimulation modulates expression of semaphorins 4D and 3A, regulators of bone homeostasis

The axonal guidance proteins semaphorin (Sema)4D and Sema3A play important roles in communication between osteoclasts and osteoblasts. As stimulation of adenosine A_2A receptors (A2AR) regulates both osteoclast and osteoblast function, we asked whether A2AR regulates both osteoclast and osteoblast expression of Semas. In vivo bone formation and Sema3A/PlexinA1/Neuropilin-1, Sema4D/PlexinB1 protein expression were studied in a murine model of wear particle-induced osteolysis. Osteoclast/osteoblast differentiation were studied in vitro as the number of tartrate-resistant acid phosphatase⁺/Alizarin Red⁺ cells after challenge with CGS21680 (A2AR agonist, 1 µM) or ZM241385 (A2AR antagonist, 1 µM), with or without Sema4D or Sema3A (10 ng/ml). Sema3A/PlexinA1/Neuropilin-1, Sema4D/PlexinB1, and receptor activator of NF-κB ligand/osteoprotegerin (RANKL/OPG) expression was studied by RT-PCR and Western blot. β-Catenin activation and cytoskeleton changes were studied by fluorescence microscopy and Western blot. In mice with wear particles implanted over the calvaria, CGS21680 treatment increased bone formation in vivo, reduced Sema4D, and increased Sema3A expression compared with mice with wear particle-induced osteolysis treated with vehicle alone. During osteoclast differentiation, CGS21680 abrogated RANKL-induced Sema4D mRNA expression (1.3 ± 0.3- vs. 2.5 ± 0.1-fold change, P < 0.001, n = 4). PlexinA1, but not Neuropilin-1, mRNA was enhanced by CGS21680 treatment. CGS21680 enhanced Sema3A mRNA expression during osteoblast differentiation (8.7 ± 0.2-fold increase, P < 0.001, n = 4); PlexinB1 mRNA was increased 2-fold during osteoblast differentiation and was not altered by CGS21680. Similar changes were observed at the protein level. CGS21680 decreased RANKL, increased OPG, and increased total/nuclear β-catenin expression in osteoblasts. Sema4D increased Ras homolog gene family, member A phosphorylation and focal adhesion kinase activation in osteoclast precursors, and CGS21680 abrogated these effects. In summary, A2AR activation diminishes secretion of Sema4D by osteoclasts, inhibits Sema4D-mediated osteoclast activation, and enhances secretion of Sema3A by osteoblasts, increasing osteoblast differentiation and diminishing inflammatory osteolysis.-Mediero, A., Wilder, T., Shah, L., Cronstein, B. N. Adenosine A_2A receptor (A2AR) stimulation modulates expression of semaphorins 4D and 3A, regulators of bone homeostasis.

Decreased Expression of Semaphorin3A/Neuropilin-1 Signaling Axis in Apical Periodontitis

Apical periodontitis (AP) is a chronic infection of endodontic origin accompanied with bone destruction around the apical region. Semaphorin3A (Sema3A) and neuropilin-1 (Nrp1) are regarded as a pair of immune regulators in bone metabolism. In this study, we firstly investigated the expression pattern of Sema3A/Nrp1 in apical periodontitis and its correlation with bone destruction. Using rat animal model, we analysed the level of mandibular bone destruction and the expression of Sema3A/Nrp1 on days 0, 7, 14, 21, 28, and 35 after pulp exposure. In addition, clinical samples from apical periodontitis patients were obtained to analyse the expression of Sema3A/Nrp1. These results indicated that the bone destruction level expanded from days 7 to 35. The number of positive cells and level of mRNA expression of Sema3A/Nrp1 were significantly decreased from days 7 to 35, with a negative correlation with bone destruction. Moreover, expression of Sema3A/Nrp1 in the AP group was reduced compared to the control group of clinical samples. In conclusion, decreased expression of Sema3A/Nrp1 was observed in periapical lesions and is potentially involved in the bone resorption of the periapical area, suggesting that Sema3A/Nrp1 may contribute to the pathological development of apical periodontitis.

Neuronal Control of Bone Remodeling

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

* Roughness and Hydrophilicity as Osteogenic Biomimetic Surface Properties

Successful dental and orthopedic implant outcomes are determined by the degree of osseointegration. Over the last 60 years, endosseous implants have evolved to stimulate osteogenesis without the need for exogenous biologics such as bone morphogenetic proteins. An understanding of the interaction between cells and the physical characteristics of their environments has led to development of bioactive implants. Implant surfaces that mimic the inherent chemistry, topography, and wettability of native bone have shown to provide cells in the osteoblast lineage with the structural cues to promote tissue regeneration and net new bone formation. Studies show that attachment, proliferation, differentiation, and local factor production are sensitive to these implant surface characteristics. This review focuses on how surface properties, including chemistry, topography, and hydrophilicity, modulate protein adsorption, cell behavior, biological reactions, and signaling pathways in peri-implant bone tissue, allowing the development of true biomimetics that promote osseointegration by providing an environment suitable for osteogenesis.

Class 3 semaphorins are transcriptionally regulated by 1,25(OH)2D3 in osteoblasts

The vitamin D endocrine system is essential for calcium metabolism and skeletal integrity. 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] regulates bone mineral homeostasis and acts directly on osteoblasts. In the present study we characterized the transcriptional regulation of the class 3 semaphorin (Sema3) gene family by 1,25(OH)₂D₃ in osteoblastic cells. Class 3 semaphorins are secreted proteins that regulate cell growth, morphology and migration, and were recently shown to be involved in bone homeostasis. In ST2, MC3T3-E1 and primary calvarial osteoblast cell cultures we found that all members of the Sema3 gene family were expressed, and that Sema3e and Sema3f were the most strongly induced 1,25(OH)₂D₃ target genes among the studied cell types. In addition, transcription of Sema3b and Sema3c was upregulated, whereas Sema3d and Sema3g was downregulated by 1,25(OH)₂D₃ in different osteoblastic cells. Chromatin immunoprecipitation analysis linked to DNA sequencing (ChIP-seq analysis) revealed the presence of the vitamin D receptor at multiple genomic loci in the proximity of Sema3 genes, demonstrating that the genes are primary 1,25(OH)₂D₃ targets. Furthermore, we showed that recombinant SEMA3E and SEMA3F protein were able to inhibit osteoblast proliferation. However, recombinant SEMA3s did not affect ST2 cell migration. The expression of class 3 semaphorins in osteoblasts together with their regulation by 1,25(OH)₂D₃ suggests that these genes, involved in the regulation of bone homeostasis, are additional mediators for 1,25(OH)₂D₃ signaling in osteoblasts.

Coordination of early cellular reactions during activation of bone resorption in the rat mandible periosteum: An immunohistochemical study

The activation step of bone remodeling remains poorly characterized. Activation comprises determination of the site to be remodeled, osteoclast precursor recruitment, their migration to the site of remodeling, and differentiation. These actions involve different compartments and cell types. The aim of this study was to investigate events and cell types involved during activation. We used a bone remodeling model in rats where extractions of the upper jaw molars initiate remodeling of the antagonist lower jaw (mandible) cortex along the periosteum. In this model osteoclastic resorption peaks 4 days after extractions. We previously reported that mast cell activation in the periosteum fibrous compartment is an early event of activation, associated with recruitment of circulating monocyte osteoclast precursors. By using immunohistochemistry, we observed 9 hours after induction a spatially oriented expression of InterCellular Adhesion Molecule-1 in the vessels that was inhibited by antagonists of histamine receptors 1 and 2. It was followed at 12 hours by the recruitment of ED1+ monocytes. In parallel, at 9 hours, Vascular Cellular Adhesion Molecule-1+ fibroblast-like cells scattered in the fibrous compartment of the periosteum between the vessels and the osteogenic compartment increased; these cells may be implicated in osteoclast precursor migration. Receptor Activator of NF KappaB Ligand+ cells increased at 12 hours in the osteogenic compartment and reached a peak at 18 hours. At 24 hours the numbers of osteogenic cells and subjacent osteocytes expressing semaphorin 3a, a repulsive for osteoclast precursors, decreased before returning to baseline at 48 hours. These data show that during activation the two periosteum compartments and several cell types are coordinated to recruit and guide osteoclast precursors towards the bone surface.

Semaphorin 7A as a potential immune regulator and promising therapeutic target in rheumatoid arthritis

Background

Semaphorin 7A (Sema7A) is expressed by several different classes of lymphoid and myeloid cells and is a potent immunomodulator. We examined the role of Sema7A in modulating cellular immune responses and to provide experimental data validating the therapeutic potential of Sema7A in rheumatoid arthritis (RA).

Methods

Soluble Sema7A (sSema7A) levels in the serum and synovial fluid from patients with RA or osteoarthritis, as well as cytokine secretions, were analyzed with an enzyme-linked immunosorbent assay. The cell surface levels and transcripts of Sema7A were evaluated in T cells and monocytes from patients with RA. The effect of Sema7A on the functions of primary T cells isolated from the peripheral blood of healthy donors was observed. Detection of the activation of the signal mediator focal adhesion kinase was performed by Western blotting. Shedding of sSema7A was evaluated in monocytes. The introduction of anti-Sema7A antibody to mice with collagen-induced arthritis (CIA) was observed in vivo.

Results

Upregulation of sSema7A levels in both the serum and synovial fluid of patients with RA was correlated with disease activity markers. sSema7A markedly increased Th1/Th17 cytokine secretion and induced evident upregulation of T-bet and retinoic acid receptor-related orphan nuclear receptor γt levels in T cells. Cell surface Sema7A was cleaved by a disintegrin and metalloprotease 17 (ADAM17) in monocytes. Interleukin-6 and tumor necrosis factor-α stimulated ADAM17 secretion in synovial macrophages. Blocking of β1-integrin abrogated the Sema7A-mediated cytokine secretion. Treatment with an anti-Sema7A antibody significantly attenuated CIA.

Conclusions

These findings indicate that Sema7A as a potent activator of T cells and monocytes in the immune response contributes to the inflammation and progression of RA, suggesting its therapeutic potential in the treatment of RA.

A reverse signaling pathway downstream of Sema4A controls cell migration via Scrib

Semaphorins comprise a large family of ligands that regulate key cellular functions through their receptors, plexins. In this study, we show that the transmembrane semaphorin 4A (Sema4A) can also function as a receptor, rather than a ligand, and transduce signals triggered by the binding of Plexin-B1 through reverse signaling. Functionally, reverse Sema4A signaling regulates the migration of various cancer cells as well as dendritic cells. By combining mass spectrometry analysis with small interfering RNA screening, we identify the polarity protein Scrib as a downstream effector of Sema4A. We further show that binding of Plexin-B1 to Sema4A promotes the interaction of Sema4A with Scrib, thereby removing Scrib from its complex with the Rac/Cdc42 exchange factor βPIX and decreasing the activity of the small guanosine triphosphatase Rac1 and Cdc42. Our data unravel a role for Plexin-B1 as a ligand and Sema4A as a receptor and characterize a reverse signaling pathway downstream of Sema4A, which controls cell migration.

BMP Signaling Mediated by BMPR1A in Osteoclasts Negatively Regulates Osteoblast Mineralization Through Suppression of Cx43

Osteoblasts and osteoclasts are well orchestrated through different mechanisms of communication during bone remodeling. Previously, we found that osteoclast-specific disruption of one of the BMP receptors, Bmpr1a, results in increased osteoblastic bone formation in mice. We hypothesized that BMPR1A signaling in osteoclasts regulates production of either membrane bound proteins or secreted molecules that regulated osteoblast differentiation. In our current study, we co-cultured wild-type osteoblasts with either control osteoclasts or osteoclasts lacking BMPR1A signaling activity. We found that loss of Bmpr1a in osteoclasts promoted osteoblast mineralization in vitro. Further, we found that the expression of Cx43/Gja1 in the mutant osteoclasts was increased, which encoded for one of the gap junction proteins connexin 43/gap junction alpha 1. Knockdown of Gja1 in the mutant osteoclasts for Bmpr1a reduced osteoblastic mineralization when co-cultured. Our findings suggest that GJA1 may be one of the downstream targets of BMPR1A signaling in osteoclasts that mediates osteoclast-osteoblast communication during bone remodeling. J. Cell. Biochem. 118: 605-614, 2017. © 2016 Wiley Periodicals, Inc.

Tissue-specific pathway association analysis using genome-wide association study summaries

MOTIVATION

Pathway association analysis has made great achievements in elucidating the genetic basis of human complex diseases. However, current pathway association analysis approaches fail to consider tissue-specificity.

RESULTS

We developed a tissue-specific pathway interaction enrichment analysis algorithm (TPIEA). TPIEA was applied to two large Caucasian and Chinese genome-wide association study summary datasets of bone mineral density (BMD). TPIEA identified several significant pathways for BMD [false discovery rate (FDR) < 0.05], such as KEGG FOCAL ADHESION and KEGG AXON GUIDANCE, which had been demonstrated to be involved in the development of osteoporosis. We also compared the performance of TPIEA and classical pathway enrichment analysis, and TPIEA presented improved performance in recognizing disease relevant pathways. TPIEA may help to fill the gap of classic pathway association analysis approaches by considering tissue specificity.

AVAILABILITY AND IMPLEMENTATION

The online web tool of TPIEA is available at https://sourceforge.net/projects/tpieav1/files CONTACT: fzhxjtu@mail.xjtu.edu.cnSupplementary information: Supplementary data are available at Bioinformatics online.

Class A Plexins Are Organized as Preformed Inactive Dimers on the Cell Surface

Plexins are single-pass transmembrane receptors that bind the axon guidance molecules semaphorins. Single-pass transmembrane proteins are an important class of receptors that display a wide variety of activation mechanisms, often involving ligand-dependent dimerization or conformational changes. Resolving the activation mechanism and dimerization state of these receptors is extremely challenging, especially in a live-cell environment. Here, we report on the dimerization state of PlexinA4 and its response to activation by semaphorin binding. Semaphorins are dimeric molecules that activate plexin by binding two copies of plexin simultaneously and inducing formation of a specific active dimer of plexin. An open question is whether there are preexisting plexin dimers that could act as autoinhibitory complexes. We address these questions with pulsed interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS). PIE-FCCS is a two-color fluorescence microscopy method that is directly sensitive to protein dimerization in a live-cell environment. With PIE-FCCS, we show that inactive PlexinA4 is dimerized in the live-cell plasma membrane. By comparing the cross correlation of full-length PlexinA4 to control proteins and plexin mutants, we show that dimerization of inactive PlexinA4 requires the Sema domain, but not the cytoplasmic domain. Ligand stimulation with Sema6A does not change the degree of cross correlation, indicating that plexin activation does not lead to higher-order oligomerization. Together, the results suggest that semaphorin activates plexin by disrupting an inhibitory plexin dimer and inducing the active dimer.