UNASSIGNED: Connexin 43 (Cx43) is the main gap junction (GJ) protein and hemichannel protein in bone tissue. It is involved in the formation of hemichannels and GJs and establishes channels that can communicate directly to exchange substances and signals, affecting the structure and function of osteocytes. CX43 is very important for the normal development of bone tissue and the establishment and balance of bone reconstruction. However, the molecular mechanisms by which CX43 regulates osteoblast function and homeostasis have been less well studied, and this article provides a review of research in this area.
UNASSIGNED: We searched the PubMed, EMBASE, Cochrane Library, and Web of Science databases for studies published up to June 2023 using the keywords Connexin 43/Cx43 and Osteocytes. Screening of literatures according to inclusion and exclusion guidelines and summarized the results.
UNASSIGNED: Osteocytes, osteoblasts, and osteoclasts all express Cx43 and form an overall network through the interaction between GJs. Cx43 is not only involved in the mechanical response of bone tissue but also in the regulation of signal transduction, which could provide new molecular markers and novel targets for the treatment of certain bone diseases.
UNASSIGNED: Cx43 is expressed in osteoblasts, osteoclasts, and osteoclasts and plays an important role in regulating the function, signal transduction, and mechanotransduction of osteocytes. This review offers a new contribution to the literature by summarizing the relationship between Cx43, a key protein of bone tissue, and osteoblasts.

While most of current models investigating bone remodelling are based on matrix deformation, intramedullary pressure also plays a role. Bone remodelling is orchestrated by the Lacuno-Canalicular Network (LCN) fluid-flow. The aim of this review was hence to assess the influence of intramedullary pressure on the fluid circulation within the LCN. Three databases (Science Direct, Web of Science, and PubMed) were used. The first phase of the search returned 731 articles, of which 9 respected the inclusion/exclusion criteria and were included. These studies confirm the association between intramedullary pressure and fluid dynamics in the LCN. Among the included studies, 7 experimental studies using animal models and 2 numerical models were found. The studies were then ranked according to the nature of the applied loading, either axial compression or direct cyclic intramedullary pressure. The current review revealed that there is an influence of intramedullary pressure on LCN fluid dynamics and that this influence depends on the magnitude and the frequency of the applied pressure. Two studies confirmed that the influence was effective even without bone matrix deformation. While intramedullary pressure is closely associated with LCN fluid, there is a severe lack of studies on this topic. STATEMENT OF SIGNIFICANCE: Since the 1990\'s, numerical models developed to investigate fluid flow in bone submicrometric porous network are based on the flow induced by matrix deformation. Bone fluid flow is known to be involved in cells stimulation and hence directly influences bone remodeling. Different studies have shown that intramedullary pressure is also associated with bone mechanosensitive adaptation. This pressure is developed in bone due to blood circulation and is increased during loading or muscle stimulation. The current article reviews the studies investigating the influence of this pressure on bone porous fluid flow. They show that fluid flow is involved by this pressure even without bone matrix deformation. The current review article highlights the severe lack of studies about this mechanism.

Osteoporosis is one of the skeletal diseases of major health concern worldwide. Homeostasis of bone occurs with the help of cells, namely, osteoblasts and osteoclasts. Physiological and pathological conditions involve the death of the cells by apoptosis, autophagy, and necrosis. Apoptosis is a key factor in the growth, development, and maintenance of the skeleton. Apoptosis is generated by two pathways: the intrinsic (mitochondria) and extrinsic (death receptor) pathways. Osteoblast apoptosis is governed by the factors like B cell lymphoma 2 (Bcl-2) family proteins, extracellular signal-regulated kinase (ERK), mitogen-activated protein kinases (MAPK), phosphoinositide- 3-kinase/ protein kinase B (PI3-K/Akt), Janus kinase 2 (JAK2), bone morphogenetic protein (BMP), and bone matrix protein. Cytokines interact with osteocytes and induce apoptosis. A pro-inflammatory signal stimulates osteocyte apoptosis and increases osteocyte cytokines production. Current therapies have adverse effects which limit their applications. Various plant metabolites have shown beneficial effects on bone. The present review converses about normal bone metabolism and the mechanism of apoptosis leading to bone deterioration. Furthermore, it discusses the role of plant metabolites on bone apoptosis with related indications of efficacy in various experimental models.

Periodontitis is the sixth most common chronic inflammatory disease, destroying the tissues supporting the teeth. There are three distinct stages in periodontitis: infection, inflammation, and tissue destruction, where each stage has its own characteristics and hence its line of treatment. Illuminating the underlying mechanisms of alveolar bone loss is vital in the treatment of periodontitis to allow for subsequent reconstruction of the periodontium. Bone cells, including osteoclasts, osteoblasts, and bone marrow stromal cells, classically were thought to control bone destruction in periodontitis. Lately, osteocytes were found to assist in inflammation-related bone remodeling besides being able to initiate physiological bone remodeling. Furthermore, mesenchymal stem cells (MSCs) either transplanted or homed exhibit highly immunosuppressive properties, such as preventing monocytes/hematopoietic precursor differentiation and downregulating excessive release of inflammatory cytokines. In the early stages of bone regeneration, an acute inflammatory response is critical for the recruitment of MSCs, controlling their migration, and their differentiation. Later during bone remodeling, the interaction and balance between proinflammatory and anti-inflammatory cytokines could regulate MSC properties, resulting in either bone formation or bone resorption. This narrative review elaborates on the important interactions between inflammatory stimuli during periodontal diseases, bone cells, MSCs, and subsequent bone regeneration or bone resorption. Understanding these concepts will open up new possibilities for promoting bone regeneration and hindering bone loss caused by periodontal diseases.

BACKGROUND: Osteoporosis is caused by the deterioration of bone density and microstructure, resulting in increased fracture risk. It transpires due to an imbalanced skeletal remodelling process favouring bone resorption. Various natural compounds can positively influence the skeletal remodelling process, of which naringenin is a candidate. Naringenin is an anti-inflammatory and antioxidant compound found in citrus fruits and grapefruit. This systematic review aims to present an overview of the available evidence on the skeletal protective effects of naringenin.
METHODS: A systematic literature search was conducted using the PubMed and Scopus databases in August 2022. Original research articles using cells, animals, or humans to investigate the bone protective effects of naringenin were included.
RESULTS: Sixteen eligible articles were included in this review. The existing evidence suggested that naringenin enhanced osteoblastogenesis and bone formation through BMP-2/p38MAPK/Runx2/Osx, SDF-1/CXCR4, and PI3K/Akt/c-Fos/c-Jun/AP-1 signalling pathways. Naringenin also inhibited osteoclastogenesis and bone resorption by inhibiting inflammation and the RANKL pathway.
CONCLUSIONS: Naringenin enhances bone formation while suppressing bone resorption, thus achieving its skeletal protective effects. It could be incorporated into the diet through fruit intake or supplements to prevent bone loss.

Bone is a highly dynamic tissue that is constantly adapting to micro-changes to facilitate movement. When the balance between bone building and resorption shifts more towards bone resorption, the result is reduced bone density and mineralization, as seen in osteoporosis or osteopenia. Current treatment strategies aimed to improve bone homeostasis and turnover are lacking in efficacy, resulting in the search for new preventative and nutraceutical treatment options. The myokine irisin, since its discovery in 2012, has been shown to play an important role in many tissues including muscle, adipose, and bone. Evidence indicate that irisin is associated with increased bone formation and decreased bone resorption, leading to reduced risk of osteoporosis in post-menopausal women. In addition, low serum irisin levels have been found in individuals with osteoporosis and osteopenia. Irisin targets key signaling proteins, promoting osteoblastogenesis and reducing osteoclastogenesis. The present review summarizes the existing evidence regarding the effects of irisin on bone homeostasis.

It is hypothesized that bone cells can sense mechanical force in the extracellular network via an electrical signal. This has led to the use of electrical stimulation (ES) to improve fracture repair and mitigate bone loss. Although overlap exists in bone maintenance and fracture healing mechanics, the processes involved in both are very different, resulting in dissimilar behaviors from the cells. Osteocytes are the most abundant cell type in bone tissue, and their basic structure and lineage are fairly well understood, but much debate is present regarding their behavior, with even less known about their behavior in electrical environments. A wide range of research exists on cell behavior under different types of ES, but it is difficult to draw conclusions due to the large variance in stimulation parameters, cell types, and origins (locations and species). By exploring behavior of multiple bone-cell types under different forms of ES, as well as mechanical stimulation through fluid flow, we can determine more about cell reactions to stimuli. In turn, a better understanding of cell response has the potential to improve and broaden therapeutic applications of ES for bone healing and bone loss mitigation, and enhance outcomes for osseointegration into implantable medical devices. These require greater understanding of the bone cellular environment from an electrical perspective as well as cellular responses to ES.

Bone regeneration is a critical problem in modern clinical practice. Osteocytes are the most abundant cell population of mature adult bone that play a major role in the regulation of bone formation. In humans, segmental bone defects cannot be repaired by endogenous regenerative mechanisms. Bone tissue engineering (BTE) is a promising option for the treatment of difficult segmental and skeletal defects. BTE requires suitable cell sources with rapid expansion and adequate function, inducible factors, and scaffolds to successfully regenerate or repair the bone injury. To overcome the disadvantages of using allogeneic and autologous tissue grafts, stem cell-based therapy has progressed in regenerative medicine. In the past few decades, numerous attempts have been made to generate osteocytes by using pluripotent stem cells (PSCs) to repair and regenerate bone defects. Human induced pluripotent stem cells (hiPSCs) are PSCs that can self-renew and differentiate into a variety of cell types. Reprogramming of human somatic cells into hiPSCs provides a new opportunity for regenerative medicine, cell-based drug discovery, disease modeling, and toxicity assessment. The ability to differentiate hiPSCs from mesenchymal stem cells (iPSC-MSCs) is essential for treating bone-related damages and injuries. Several in vitro studies revealed that the cell origin of iPSCs, a combination of transcription factors, the type of promoter in the vector, transduction methods, scaffolds, differentiating techniques, and culture medium, may affect the osteogenic differentiation potential of hiPSCs. This review will focus on several factors that influence the osteogenic differentiation of human iPSCs.

Between 5 and 10 percent of fractures do not heal, a condition known as nonunion. In clinical practice, stable fracture fixation associated with autologous iliac crest bone graft placement is the gold standard for treatment. However, some recalcitrant nonunions do not resolve satisfactorily with this technique. For these cases, biological alternatives are sought based on the molecular mechanisms of bone healing, whose most recent findings are reviewed in this article. The pro-osteogenic efficacy of morin (a pale yellow crystalline flavonoid pigment found in old fustic and osage orange trees) has recently been reported, and the combined use of bone morphogenetic protein-9 (BMP9) and leptin might improve fracture healing. Inhibition with methyl-piperidino-pyrazole of estrogen receptor alpha signaling delays bone regeneration. Smoking causes a chondrogenic disorder, aberrant activity of the skeleton\'s stem and progenitor cells, and an intense initial inflammatory response. Smoking cessation 4 weeks before surgery is therefore highly recommended. The delay in fracture consolidation in diabetic animals is related to BMP6 deficiency (35 kDa). The combination of bioceramics and expanded autologous human mesenchymal stem cells from bone marrow is a new and encouraging alternative for treating recalcitrant nonunions.

Bone plays critical roles in support, protection, movement, and metabolism. Although bone has an innate capacity for regeneration, this capacity is limited, and many bone injuries and diseases require intervention. Biomaterials are a critical component of many treatments to restore bone function and include non-resorbable implants to augment bone and resorbable materials to guide regeneration. Biomaterials can vary considerably in their biocompatibility and bioactivity, which are functions of specific material parameters. The success of biomaterials in bone augmentation and regeneration is based on their effects on the function of bone cells. Such functions include adhesion, migration, inflammation, proliferation, communication, differentiation, resorption, and vascularization. This review will focus on how different material parameters can enhance bone cell function both in vitro and in vivo.