Mechanically induced Ca2+ oscillations in osteocytes release extracellular vesicles and enhance bone formation

High-Yield Bioproduction of Extracellular Vesicles from Stem Cell Spheroids via Millifluidic Vortex Transport

Extracellular vesicles (EVs) are emerging as novel therapeutics, particularly in cancer and degenerative diseases. Nevertheless, from both market and clinical viewpoints, high-yield production methods using minimal cell materials are still needed. Herein, a millifluidic cross-slot chip is proposed to induce high-yield release of biologically active EVs from less than three million cells. Depending on the flow rate, a single vortex forms in the outlet channels, exposing transported cellular material to high viscous stresses. Importantly, the chip accommodates producer cells within their physiological environment, such as human mesenchymal stem cells (hMSCs) spheroids, while facilitating their visualization and individual tracking within the vortex. This precise control of viscous stresses at the spheroid level allows for the release of up to 30000 EVs per cell at a Reynolds number of ≈400, without compromising cellular integrity. Additionally, it reveals a threshold initiating EV production, providing evidence for a stress-dependent mechanism governing vesicle secretion. EVs mass-produced at high Reynolds displayed pro-angiogenic and wound healing capabilities, as confirmed by proteomic and cytometric analysis of their cargo. These distinct molecular signatures of these EVs, compared to those derived from monolayers, underscore the critical roles of the production method and the 3D cellular environment in EV generation.

3D printed porous PEEK scaffolds with stable and durable gelatin composite hydrogel coating loaded Yoda1 for in vivo osseointegration

Polyetheretherketone (PEEK) has emerged as a promising material for bone substitution; however, its limited osseointegration hinders its clinical applications. This research employs a porous structural design alongside surface modification techniques to improve the osseointegration properties of PEEK. Porous PEEK scaffolds were fabricated via 3D printing technology, followed by the application of a stable coating of Yoda1-loaded methacrylated gelatin (GelMA) on their surfaces. After modification, the hydrophilicity of the PEEK scaffolds was considerably improved, and the GelMA coating facilitated the sustained release of Yoda1. In vitro experiments demonstrated that the modified surfaces promoted cell proliferation and adhesion, facilitated angiogenesis, and enhanced osteoblast differentiation and mineral deposition. Furthermore, porous PEEK scaffolds were implanted into the femoral condyles of SD rats for 6 weeks to evaluate in vivo osseointegration The results showed that the tailored three-dimensional porous structure, along with the Yoda1-loaded GelMA coating, promoted bone ingrowth and enhanced osseointegration within the scaffold. This study offers a viable strategy for enhancing PEEK osseointegration through a combination of structural design and surface modification, and introduces new avenues for the application of Yoda1 in bone tissue engineering.

Mineralised bone properties in a child with recessive osteogenesis imperfecta type XIV and in a conditional Tmem38b knockout murine model (Runx2-Cre; Tmem38bfl/fl)

INTRODUCTION

OI type XIV is caused by variants in the TMEM38B gene, encoding for the ubiquitously expressed endoplasmic reticulum trimeric intracellular cation channel type B (TRIC-B), causing disruptions in calcium homeostasis and collagen synthesis. Patients with OI type XIV present with a highly variable clinical phenotype, ranging from asymptomatic to severe. We present here data from a 6 year clinical follow-up of two affected siblings and bone tissue characterisation obtained during corrective surgery from one of the patients, as well as tibiae from a novel Tmem38b conditional knockout murine model (Runx2-Cre; Tmem38bfl/fl).

METHODS

Clinical examinations of the patients include bone mineral density (BMD) measurements using dual-energy x-ray absorptiometry (DXA) scanning and gait analyses. Quantitative backscattered electron imaging (qBEI) was used to investigate bone mineralisation density distribution (BMDD) and osteocyte lacunae properties, and confocal laser scanning microscopy was used to quantify the osteocyte lacuno-canalicular network (OLCN) in both human and murine specimens.

RESULTS

Both patients (P1, P2) presented with muscular hypotension, fatigue, progression of lower limb deformities, and fractures. BMDD of the osteonal bone region of the tibia and fibula specimens obtained from P1 revealed no significant shift towards higher mineral content as seen in "classical" OI. Osteocyte lacunae porosity was elevated and analyses of the OLCN revealed a reduction in canalicular density and lacunar degree. Runx2-Cre; Tmem38bfl/fl mice exhibited a very severe skeletal phenotype, with 10/12 of the tibiae showing evidence of fractures, bone deformations, or calluses. In contrast to the patient samples, both the cortex and metaphysis of mutant mice demonstrated a significant increase in the average mineral content (CaMean) and the peak of the distribution (CaPeak), as well as in osteocyte lacunae porosity (P < 0.0001), whereas canalicular density (P < 0.0001), and lacunar degree (P = 0.0004) were decreased.

CONCLUSION

While Runx2-Cre; Tmem38bfl/fl mice exhibit hypermineralisation of the bone matrix, this is not apparent in bone specimens obtained from the OI type XIV patient. However, both human and murine bone tissue with absence of TRIC-B demonstrate the same abnormalities of the osteocyte lacunae porosity and osteocyte lacuno-canalicular network, indicating disruption to the OLCN which is likely a general hallmark of OI bone.

Natural collagen scaffold with intrinsic piezoelectricity for enhanced bone regeneration

Materials-mediated piezoelectric signals have been widely applied in bone regeneration. Collagen is the most abundant protein in the human body, and native collagen with complete tertiary structure shows efficient piezoelectricity. However, the traditional collagen scaffolds are lack of piezoelectricity due to the destruction of the complete tertiary structure. Here, natural collagen scaffolds with the complete tertiary structure were prepared. Alkali treatment made the collagen scaffold lose piezoelectricity. The collagen with/without piezoelectricity (PiezoCol/NCol) scaffolds both possessed good cytocompatibility and promoted cell adhesion. After being implanted subcutaneously, the NCol scaffold almost did not affect bone regeneration with/without ultrasound treatment. However, under ultrasound treatment, the PiezoCol scaffold promoted the new bone formation with enhanced osteogenic differentiation, angiogenesis, and neural differentiation, meaning that piezoelectricity endows collagen with satisfactory promotion for bone regeneration. Meanwhile, the PiezoCol scaffold can also accelerate bone formation without ultrasound treatment, which should be attributed to the daily exercise-caused weak piezoelectric stimulation. Further, the proteomic analysis revealed the mechanism by which the PiezoCol scaffold promoted bone tissue formation via mainly upregulating the PI3K-Akt signaling pathway. This study provides a new strategy to enhance the osteoinduction of collagen scaffold for bone regeneration by maintaining intrinsic piezoelectricity.

Soy Protein-Cultured Mesenchymal Stem Cell-Secreted Extracellular Vesicles Target the Neurovascular Unit: Insights from a Zebrafish Brain Injury Model

Cerebral vascular disorders often accompany hypoxia-induced brain injury. In this study, we develop a zebrafish model of hypoxia-induced cerebral vascular injury to replicate the associated phenotypic changes, including cerebrovascular damage, neuronal apoptosis, and neurological dysfunction. We then explored the therapeutic potential of extracellular vesicles derived from Wharton's jelly-derived mesenchymal stem cells (WJ-MSCs) cultured on soy protein-coated surfaces. These vesicles demonstrated superior recovery efficacy, especially in restoring the blood-brain barrier integrity and improving neurological function. Our findings suggest that these potent therapeutic extracellular vesicles, easily produced from WJ-MSCs cultured in the presence of soy proteins, may mitigate hypoxia-induced brain injury by decreasing the severity of vascular disorder caused by oxidative stress. Protein-protein interactome analysis further suggests that multiple signaling pathways are likely involved in restoring normal neurovascular unit function.

Shear stress-induced influx of extracellular calcium ions: a pivotal trigger amplifying the production of mesenchymal stem cell-derived extracellular vesicles

Extracellular vesicles (EVs) have drawn attention as promising therapeutic agents whose characteristics resemble their parent cells. However, their practical utility is limited by low EV yields. Cell culture under fluidic flow to enhance EV secretion has been proposed to address this challenge. However, the precise mechanism of increased EV production in response to flow conditions has not been studied thoroughly. We investigated the mechanism of higher release of EVs from mesenchymal stem cells under flow conditions, focusing on the correlation between intracellular calcium ions and EV production. Shear stress was applied to cells through shaking cultures, and stimulated cells showed increased EV production. Results suggested that the stimulation of EV secretion was promoted by an increasing intracellular concentration of calcium ions, primarily due to their transport through calcium ion channels in the plasma membrane, which was induced by shear stress. Furthermore, we confirmed that the essential characteristics of the EVs released under shear stress remained intact by analyzing individual EVs and assessing their regeneration efficacy in a model of kidney injury in vitro. Unveiling the reason for the high production of EVs under shear stress is expected to contribute to the development of EV-application research by increasing the reliability of EV utilization.

Isolation of Extracellular Vesicle Subtypes from Primary Osteoblast Cultures

This chapter describes the isolation and characterization of extracellular vesicles (EVs) isolated from primary rodent osteoblasts and extracellular matrix. The key advantages of this assay are that it allows the isolation of EVs from the conditioned media of primary osteoblasts at varying stages of differentiation and subsequent characterization prior to downstream applications.

Ank-mediated pyrophosphate regulates shear stress-induced small extracellular vesicle production in 3D-cultured osteocytes

Osteocytes are located in the lacunae of fluid-filled bone and communicate with neighboring or distant cells by secreting small extracellular vesicles (sEVs) and growth factors as well as via dendrite-dendrite direct connections. However, the mechanism regulating sEV production in osteocytes is yet to be elucidated. In this study, we investigated sEV production and its underlying mechanism in osteocytes cultured on a three dimensional (3D) scaffold. We employed a perfusion system to apply shear stress stimulation to MLO-Y4 cells cultured on a 3D biphasic calcium phosphate (BCP) scaffold and analyzed sEV production and gene expression using RNA sequencing. We found that the expression of genes associated with sEV biogenesis and the secretory pathway were enhanced by fluid shear stress in MLO-Y4 cells cultured on a 3D BCP scaffold. In particular, fluid shear stress induced the expression of Ank, a pyrophosphate transporter, in 3D-cultured MLO-Y4 cells. The role of Ank in sEV production was further examined. Probenecid, an Ank inhibitor, significantly suppressed shear stress-induced sEV production, whereas Ank cDNA overexpression stimulated it. The inhibition of shear stress-induced sEV production by probenecid was recovered by the exogenous addition of pyrophosphate to MLO-Y4 cells. These findings suggest that shear stress-mediated sEV production in 3D-cultured osteocytes is regulated by extracellular pyrophosphate transported by Ank.

Piezo1 expression in mature osteocytes is dispensable for the skeletal response to mechanical loading

Mechanical loading is essential for bone growth and homeostasis, with osteocytes considered the primary mechanosensors. Deleting the mechanosensitive ion channel Piezo1 from Dmp1-Cre-targeted cells, which include both osteoblasts and osteocytes, resulted in reduced bone mass and impaired skeletal responses to mechanical stimuli. To specifically isolate Piezo1's role in osteocytes, we used Sost-Cre mice to generate mice with Piezo1 deletion exclusively in mature osteocytes. These mice exhibited lower bone mineral density, decreased cancellous bone mass, and reduced cortical thickness with decrease periosteal expansion. However, unlike mice lacking Piezo1 in both osteoblasts and osteocytes, those with Piezo1 deletion in mature osteocytes responded normally to mechanical loading. These findings suggest that Piezo1 expression in mature osteocytes contributes to bone maintenance under normal physiological condition, but is dispensable for the skeletal response to mechanical loading.

Extracellular vesicle-functionalized bioactive scaffolds for bone regeneration

The clinical need for effective bone regeneration in compromised conditions continues to drive demand for innovative solutions. Among emerging strategies, extracellular vesicles (EVs) have shown promise as an acellular approach for bone regeneration. However, their efficacy is hindered by rapid sequestration and clearance when administered via bolus injection. To address this challenge, EV-functionalized scaffolds have recently been proposed as an alternative delivery strategy to enhance EV retention and subsequent healing efficacy. This review aims to consolidate recent advancements in the development of EV-functionalized scaffolds for augmenting bone regeneration. It explores various sources of EVs and different strategies for integrating them into biomaterials. Furthermore, the mechanisms underlying their therapeutic effects in bone regeneration are elucidated. Current limitations in clinical translation and perspectives on the design of more efficient EVs for improved therapeutic efficacy are also presented. Overall, this review can provide inspiration for the development of novel EV-assisted grafts with superior bone regeneration potential.

Bone regeneration by a bone substitute biomaterial containing hydroxyapatite, chitosan, xanthan and graphene oxide supplemented with conditioned medium from mesenchymal stem cells

This study analyzed a recently developed bone substitute biomaterial made of chitosan-xanthanhydroxyapatite-graphene oxide (CXHAG). The CXHAG particles underwent in vitro structural and morphological characterization, and in vivo testing with or without osteogenic conditioned medium from mesenchymal stem cells. Aim: The aim of this study was to determine whether the CXHAG novel biomaterial, supplemented with conditioned medium from mesenchymal stem cells, could be useful for bone regeneration. Materials and Method: For the in vitro study, cells were incubated with 20mg of CXHAG granules for 24 hours and a MTT assay was performed to tests for cytotoxicity. For the in vivo study, critical size calvarial bone defects were created in twenty-five rats. One animal had the defect unfilled (Control Group-CG) and was euthanized after 42 days. Twelve rats received the CXHAG particles (Group 1-G1) and the other twelve received the CXHAG particles supplemented with the conditioned medium (Group 2-G2). All G1/G2 grafts were covered with a CXHAG membrane. G1/G2 animals were euthanized after 14 days (T1) or 42 days (T2). The specimens were processed and histologically evaluated. Results: SEM analysis of the CXHAG particles showed granules of 300-400μm, with a rough irregular surface. They were not cytotoxic to dental pulp stem cells in vitro. The CG specimen showed loose immature connective tissue and no bone formation at the center of the defect. G1 and G2 presented remnant biomaterial particles at both time points, but only G2 had bone formation at the enter of the defect. Conclusions: The conditioned medium had a positive effect on bone regeneration in rat calvarial critical size defects when associated with the novel bone substitute biomaterial.

Mechanical loading on osteocytes regulates thermogenesis homeostasis of brown adipose tissue by influencing osteocyte-derived exosomes

Background

Osteocytes are the main stress-sensing cells in bone. The substances secreted by osteocytes under mechanical loading play a crucial role in maintaining body homeostasis. Osteocytes have recently been found to release exosomes into the circulation, but whether they are affected by mechanical loading or participate in the regulation of systemic homeostasis remains unclear.

Methods

We used a tail-suspension model to achieve mechanical unloading on osteocytes. Osteocyte-specific CD63 reporter mice were used for osteocyte exosome tracing. Exosome detection and inhibitor treatment were performed to confirm the effect of mechanical loading on exosome secretion by osteocytes. Co-culture, GW4869 and exosome treatment were used to investigate the biological functions of osteocyte-derived exosomes on brown adipose tissue (BAT) and primary brown adipocytes. Osteocyte-specific Dicer KO mice were used to screen for loading-sensitive miRNAs. Dual luciferase assay was performed to validate the selected target gene.

Results

Firstly, we found the thermogenic activity was increased in BAT of mice subjected to tail suspension, which is due to the effect of unloaded bone on circulating exosomes. Further, we showed that the secretion of exosomes from osteocytes is regulated by mechanical loading, and osteocyte-derived exosomes can reach BAT and affect thermogenic activity. More importantly, we confirmed the effect of osteocyte exosomes on BAT both in vivo and in vitro. Finally, we discovered that let-7e-5p contained in exosomes is under regulation of mechanical loading and regulates thermogenic activity of BAT by targeting Ppargc1a.

Conclusion

Exosomes derived from osteocytes are loading-sensitive, and play a vital role in regulation on BAT, suggesting that regulation of exosomes secretion can restore homeostasis.

The translational potential of this article

This study provides a biological rationale for using osteocyte exosomes as potential agents to modulate BAT and even whole-body homeostasis. It also provides a new pathological basis and a new treatment approach for mechanical unloading conditions such as spaceflight.

The roles of extracellular vesicles released by mechanically stimulated osteocytes in regulating osteoblast and osteoclast functions

Bone adapts to mechanical loading by changing its shape and mass. Osteocytes, as major mechanosensors, are critical for bone modeling/remodeling in response to mechanical stimuli. Intracellular calcium oscillation is one of the early responses in osteocytes, and this further facilitates bone cell communication through released biochemical signals. Our previous study has found that mechanically induced calcium oscillations in osteocytes enhance the release of extracellular vesicles (EVs), and those released EVs can elevate bone formation activity. However, the mechanism of mechanically stimulated EVs' regulation of bone formation and resorption is still unclear. Here, using in vitro studies, we exposed OCY454 cells, with relatively high sclerostin expression, to steady fluid flow (SFF) and characterized the functions of rapidly released EVs in osteoblast and osteoclast regulation. Our study demonstrates that SFF stimulates intracellular calcium response in OCY454 cells and further induces sclerostin, osteoprotegerin (OPG), receptor activator of NF-κB ligand (RANKL) inside or outside EVs to regulate osteoblast and osteoclast activities. This load-induced protein and EVs release is load-duration dependent. Moreover, stimulated osteocytes rapidly regulate osteoclast maturation through EVs capsulated RANKL. In contrast, other regulating proteins, OPG, and sclerostin, are mainly released directly into the medium without EV capsulation.

Rescuing SERCA2 pump deficiency: A novel approach to improve bone mechano-responsiveness in type 2 diabetes

A recent study published in Nature Communications demonstrated that restoring SERCA2 pump deficiency can enhance bone mechano-responsiveness in type 2 diabetes (T2D) by modulating osteocyte calcium dynamics. The findings revealed that in T2D mice, the ability of the bone to respond to mechanical stress is compromised, primarily due to attenuated calcium oscillatory dynamics within osteocytes rather than in osteoblasts or osteoclasts. The underlying mechanism of this reduction in bone mechano-responsiveness in T2D was identified as a specific decrease in osteocytic SERCA2 expression mediated by PPARα. Additionally, mice overexpressing SERCA2 in osteocytes exhibited reduced deterioration of bone mechano-responsiveness induced by T2D. Collectively, this study provides mechanistic insights into T2D-induced deterioration in bone mechano-responsiveness and identifies a promising therapeutic approach to counteract T2D-associated fragility fractures.

Mechanically strained osteocyte-derived exosomes contained miR-3110-5p and miR-3058-3p and promoted osteoblastic differentiation

BACKGROUND

Osteocytes are critical mechanosensory cells in bone, and mechanically stimulated osteocytes produce exosomes that can induce osteogenesis. MicroRNAs (miRNAs) are important constituents of exosomes, and some miRNAs in osteocytes regulate osteogenic differentiation; previous studies have indicated that some differentially expressed miRNAs in mechanically strained osteocytes likely influence osteoblastic differentiation. Therefore, screening and selection of miRNAs that regulate osteogenic differentiation in exosomes of mechanically stimulated osteocytes are important.

RESULTS

A mechanical tensile strain of 2500 με at 0.5 Hz 1 h per day for 3 days, elevated prostaglandin E2 (PGE2) and insulin-like growth factor-1 (IGF-1) levels and nitric oxide synthase (NOS) activity of MLO-Y4 osteocytes, and promoted osteogenic differentiation of MC3T3-E1 osteoblasts. Fourteen miRNAs differentially expressed only in MLO-Y4 osteocytes which were stimulated with mechanical tensile strain, were screened, and the miRNAs related to osteogenesis were identified. Four differentially expressed miRNAs (miR-1930-3p, miR-3110-5p, miR-3090-3p, and miR-3058-3p) were found only in mechanically strained osteocytes, and the four miRNAs, eight targeted mRNAs which were differentially expressed only in mechanically strained osteoblasts, were also identified. In addition, the mechanically strained osteocyte-derived exosomes promoted the osteoblastic differentiation of MC3T3-E1 cells in vitro, the exosomes were internalized by osteoblasts, and the up-regulated miR-3110-5p and miR-3058-3p in mechanically strained osteocytes, were both increased in the exosomes, which was verified via reverse transcription quantitative polymerase chain reaction (RT-qPCR).

CONCLUSIONS

In osteocytes, a mechanical tensile strain of 2500 με at 0.5 Hz induced the fourteen differentially expressed miRNAs which probably were in exosomes of osteocytes and involved in osteogenesis. The mechanically strained osteocyte-derived exosomes which contained increased miR-3110-5p and miR-3058-3p (two of the 14 miRNAs), promoted osteoblastic differentiation.

Young osteocyte-derived extracellular vesicles facilitate osteogenesis by transferring tropomyosin-1

BACKGROUND

Bone marrow mesenchymal stem cells (BMSCs) can undergo inadequate osteogenesis or excessive adipogenesis as they age due to changes in the bone microenvironment, ultimately resulting in decreased bone density and elevated risk of fractures in senile osteoporosis. This study aims to investigate the effects of osteocyte senescence on the bone microenvironment and its influence on BMSCs during aging.

RESULTS

Primary osteocytes were isolated from 2-month-old and 16-month-old mice to obtain young osteocyte-derived extracellular vesicles (YO-EVs) and senescent osteocyte-derived EVs (SO-EVs), respectively. YO-EVs were found to significantly increase alkaline phosphatase activity, mineralization deposition, and the expression of osteogenesis-related genes in BMSCs, while SO-EVs promoted BMSC adipogenesis. Neither YO-EVs nor SO-EVs exerted an effect on the osteoclastogenesis of primary macrophages/monocytes. Our constructed transgenic mice, designed to trace osteocyte-derived EV distribution, revealed abundant osteocyte-derived EVs embedded in the bone matrix. Moreover, mature osteoclasts were found to release osteocyte-derived EVs from bone slices, playing a pivotal role in regulating the functions of the surrounding culture medium. Following intravenous injection into young and elderly mouse models, YO-EVs demonstrated a significant enhancement of bone mass and biomechanical strength compared to SO-EVs. Immunostaining of bone sections revealed that YO-EV treatment augmented the number of osteoblasts on the bone surface, while SO-EV treatment promoted adipocyte formation in the bone marrow. Proteomics analysis of YO-EVs and SO-EVs showed that tropomyosin-1 (TPM1) was enriched in YO-EVs, which increased the matrix stiffness of BMSCs, consequently promoting osteogenesis. Specifically, the siRNA-mediated depletion of Tpm1 eliminated pro-osteogenic activity of YO-EVs both in vitro and in vivo.

CONCLUSIONS

Our findings suggested that YO-EVs played a crucial role in maintaining the balance between bone resorption and formation, and their pro-osteogenic activity declining with aging. Therefore, YO-EVs and the delivered TPM1 hold potential as therapeutic targets for senile osteoporosis.

Peripheral extracellular vesicles in neurodegeneration: pathogenic influencers and therapeutic vehicles

Neurodegenerative diseases (NDDs) such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis epitomize a class of insidious and relentless neurological conditions that are difficult to cure. Conventional therapeutic regimens often fail due to the late onset of symptoms, which occurs well after irreversible neurodegeneration has begun. The integrity of the blood-brain barrier (BBB) further impedes efficacious drug delivery to the central nervous system, presenting a formidable challenge in the pharmacological treatment of NDDs. Recent scientific inquiries have shifted focus toward the peripheral biological systems, investigating their influence on central neuropathology through the lens of extracellular vesicles (EVs). These vesicles, distinguished by their ability to breach the BBB, are emerging as dual operatives in the context of NDDs, both as conveyors of pathogenic entities and as prospective vectors for therapeutic agents. This review critically summarizes the burgeoning evidence on the role of extracerebral EVs, particularly those originating from bone, adipose tissue, and gut microbiota, in modulating brain pathophysiology. It underscores the duplicity potential of peripheral EVs as modulators of disease progression and suggests their potential as novel vehicles for targeted therapeutic delivery, positing a transformative impact on the future landscape of NDD treatment strategies. Search strategy A comprehensive literature search was conducted using PubMed, Web of Science, and Scopus from January 2000 to December 2023. The search combined the following terms using Boolean operators: "neurodegenerative disease" OR "Alzheimer's disease" OR "Parkinson's disease" OR "Amyotrophic lateral sclerosis" AND "extracellular vesicles" OR "exosomes" OR "outer membrane vesicles" AND "drug delivery systems" AND "blood-brain barrier". MeSH terms were employed when searching PubMed to refine the results. Studies were included if they were published in English, involved human subjects, and focused on the peripheral origins of EVs, specifically from bone, adipose tissue, and gut microbiota, and their association with related diseases such as osteoporosis, metabolic syndrome, and gut dysbiosis. Articles were excluded if they did not address the role of EVs in the context of NDDs or did not discuss therapeutic applications. The titles and abstracts of retrieved articles were screened using a dual-review process to ensure relevance and accuracy. The reference lists of selected articles were also examined to identify additional relevant studies.

Extracellular Vesicles and Exosomes in the Control of the Musculoskeletal Health

PURPOSE OF REVIEW

The present review will highlight recent reports supporting the relevance of extracellular vesicles to the musculoskeletal system in health and disease.

RECENT FINDINGS

Preserving the health of the musculoskeletal system is important to maintain a good quality of life, and the bone-muscle crosstalk is crucial in this regard. This latter is largely mediated by extracellular vesicles released by the different cell populations residing in muscle and bone, which deliver cargoes, microRNAs, and proteins being the most relevant ones, to target cells. Extracellular vesicles could be exploited as therapeutic tools, in view of their resistance to destruction in the biological fluid and of the possibility to be functionalized according to the need. Extracellular vesicles are recognized as crucial players in the bone-muscle cross-talk. Additional studies however are required to refine their use as biomarkers of early alterations of the musculoskeletal system, and as potential therapeutic tools.

Dynamic cultivation of human mesenchymal stem/stromal cells for the production of extracellular vesicles in a 3D bioreactor system

PURPOSE

3D cell culture and hypoxia have been demonstrated to increase the therapeutic effects of mesenchymal stem/stromal cells (MSCs)-derived extracellular vesicles (EVs). In this study, a process for the production of MSC-EVs in a novel 3D bioreactor system under normoxic and hypoxic conditions was established and the resulting EVs were characterized.

METHODS

Human adipose-derived MSCs were seeded and cultured on a 3D membrane in the VITVO® bioreactor system for 7 days. Afterwards, MSC-EVs were isolated and characterized via fluorescence nanoparticle tracking analysis, flow cytometry with staining against annexin V (Anx5) as a marker for EVs exposing phosphatidylserine, as well as CD73 and CD90 as MSC surface markers.

RESULTS

Cultivation of MSC in the VITVO® bioreactor system demonstrated a higher concentration of MSC-EVs from the 3D bioreactor (9.1 × 109 ± 1.5 × 109 and 9.7 × 109 ± 3.1 × 109 particles/mL) compared to static 2D culture (4.2 × 109 ± 7.5 × 108 and 3.9 × 109 ± 3.0 × 108 particles/mL) under normoxic and hypoxic conditions, respectively. Also, the particle-to-protein ratio as a measure for the purity of EVs increased from 3.3 × 107 ± 1.1 × 107 particles/µg protein in 2D to 1.6 × 108 ± 8.3 × 106 particles/µg protein in 3D. Total MSC-EVs as well as CD73-CD90+ MSC-EVs were elevated in 2D normoxic conditions. The EV concentration and size did not differ significantly between normoxic and hypoxic conditions.

CONCLUSION

The production of MSC-EVs in a 3D bioreactor system under hypoxic conditions resulted in increased EV concentration and purity. This system could be especially useful in screening culture conditions for the production of 3D-derived MSC-EVs.

Temporal dynamics of immune-stromal cell interactions in fracture healing

Bone fracture repair is a complex, multi-step process that involves communication between immune and stromal cells to coordinate the repair and regeneration of damaged tissue. In the US, 10% of all bone fractures do not heal properly without intervention, resulting in non-union. Complications from non-union fractures are physically and financially debilitating. We now appreciate the important role that immune cells play in tissue repair, and the necessity of the inflammatory response in initiating healing after skeletal trauma. The temporal dynamics of immune and stromal cell populations have been well characterized across the stages of fracture healing. Recent studies have begun to untangle the intricate mechanisms driving the immune response during normal or atypical, delayed healing. Various in vivo models of fracture healing, including genetic knockouts, as well as in vitro models of the fracture callus, have been implemented to enable experimental manipulation of the heterogeneous cellular environment. The goals of this review are to (1): summarize our current understanding of immune cell involvement in fracture healing (2); describe state-of-the art approaches to study inflammatory cells in fracture healing, including computational and in vitro models; and (3) identify gaps in our knowledge concerning immune-stromal crosstalk during bone healing.

Enhancing osteoporosis treatment with engineered mesenchymal stem cell-derived extracellular vesicles: mechanisms and advances

As societal aging intensifies, the incidence of osteoporosis (OP) continually rises. OP is a skeletal disorder characterized by reduced bone mass, deteriorated bone tissue microstructure, and consequently increased bone fragility and fracture susceptibility, typically evaluated using bone mineral density (BMD) and T-score. Not only does OP diminish patients' quality of life, but it also imposes a substantial economic burden on society. Conventional pharmacological treatments yield limited efficacy and severe adverse reactions. In contemporary academic discourse, mesenchymal stem cells (MSCs) derived extracellular vesicles (EVs) have surfaced as auspicious novel therapeutic modalities for OP. EVs can convey information through the cargo they carry and have been demonstrated to be a crucial medium for intercellular communication, playing a significant role in maintaining the homeostasis of the bone microenvironment. Furthermore, various research findings provide evidence that engineered strategies can enhance the therapeutic effects of EVs in OP treatment. While numerous reviews have explored the progress and potential of EVs in treating degenerative bone diseases, research on using EVs to address OP remains in the early stages of basic experimentation. This paper reviews advancements in utilizing MSCs and their derived EVs for OP treatment. It systematically examines the most extensively researched MSC-derived EVs for treating OP, delving not only into the molecular mechanisms of EV-based OP therapy but also conducting a comparative analysis of the strengths and limitations of EVs sourced from various cell origins. Additionally, the paper emphasizes the technical and engineering strategies necessary for leveraging EVs in OP treatment, offering insights and recommendations for future research endeavors.

Understanding exosomes: Part 2-Emerging leaders in regenerative medicine

Exosomes are the smallest subset of extracellular signaling vesicles secreted by most cells with the ability to communicate with other tissues and cell types over long distances. Their use in regenerative medicine has gained tremendous momentum recently due to their ability to be utilized as therapeutic options for a wide array of diseases/conditions. Over 5000 publications are currently being published yearly on this topic, and this number is only expected to dramatically increase as novel therapeutic strategies continue to be developed. Today exosomes have been applied in numerous contexts including neurodegenerative disorders (Alzheimer's disease, central nervous system, depression, multiple sclerosis, Parkinson's disease, post-traumatic stress disorders, traumatic brain injury, peripheral nerve injury), damaged organs (heart, kidney, liver, stroke, myocardial infarctions, myocardial infarctions, ovaries), degenerative processes (atherosclerosis, diabetes, hematology disorders, musculoskeletal degeneration, osteoradionecrosis, respiratory disease), infectious diseases (COVID-19, hepatitis), regenerative procedures (antiaging, bone regeneration, cartilage/joint regeneration, osteoarthritis, cutaneous wounds, dental regeneration, dermatology/skin regeneration, erectile dysfunction, hair regrowth, intervertebral disc repair, spinal cord injury, vascular regeneration), and cancer therapy (breast, colorectal, gastric cancer and osteosarcomas), immune function (allergy, autoimmune disorders, immune regulation, inflammatory diseases, lupus, rheumatoid arthritis). This scoping review is a first of its kind aimed at summarizing the extensive regenerative potential of exosomes over a broad range of diseases and disorders.

Rescuing SERCA2 pump deficiency improves bone mechano-responsiveness in type 2 diabetes by shaping osteocyte calcium dynamics

Type 2 diabetes (T2D)-related fragility fractures represent an increasingly tough medical challenge, and the current treatment options are limited. Mechanical loading is essential for maintaining bone integrity, although bone mechano-responsiveness in T2D remains poorly characterized. Herein, we report that exogenous cyclic loading-induced improvements in bone architecture and strength are compromised in both genetically spontaneous and experimentally-induced T2D mice. T2D-induced reduction in bone mechano-responsiveness is directly associated with the weakened Ca2+ oscillatory dynamics of osteocytes, although not those of osteoblasts, which is dependent on PPARα-mediated specific reduction in osteocytic SERCA2 pump expression. Treatment with the SERCA2 agonist istaroxime was demonstrated to improve T2D bone mechano-responsiveness by rescuing osteocyte Ca2+ dynamics and the associated regulation of osteoblasts and osteoclasts. Moreover, T2D-induced deterioration of bone mechano-responsiveness is blunted in mice with osteocytic SERCA2 overexpression. Collectively, our study provides mechanistic insights into T2D-mediated deterioration of bone mechano-responsiveness and identifies a promising countermeasure against T2D-associated fragility fractures.

The role and applications of extracellular vesicles in osteoporosis

Osteoporosis is a widely observed condition characterized by the systemic deterioration of bone mass and microarchitecture, which increases patient susceptibility to fragile fractures. The intricate mechanisms governing bone homeostasis are substantially impacted by extracellular vesicles (EVs), which play crucial roles in both pathological and physiological contexts. EVs derived from various sources exert distinct effects on osteoporosis. Specifically, EVs released by osteoblasts, endothelial cells, myocytes, and mesenchymal stem cells contribute to bone formation due to their unique cargo of proteins, miRNAs, and cytokines. Conversely, EVs secreted by osteoclasts and immune cells promote bone resorption and inhibit bone formation. Furthermore, the use of EVs as therapeutic modalities or biomaterials for diagnosing and managing osteoporosis is promising. Here, we review the current understanding of the impact of EVs on bone homeostasis, including the classification and biogenesis of EVs and the intricate regulatory mechanisms of EVs in osteoporosis. Furthermore, we present an overview of the latest research progress on diagnosing and treating osteoporosis by using EVs. Finally, we discuss the challenges and prospects of translational research on the use of EVs in osteoporosis.

Mechanical loading intensities affect the release of extracellular vesicles from mouse bone marrow-derived hematopoietic progenitor cells and change their osteoclast-modulating effect

Low-intensity loading maintains or increases bone mass, whereas lack of mechanical loading and high-intensity loading decreases bone mass, possibly via the release of extracellular vesicles by mechanosensitive bone cells. How different loading intensities alter the biological effect of these vesicles is not fully understood. Dynamic fluid shear stress at low intensity (0.7 ± 0.3 Pa, 5 Hz) or high intensity (2.9 ± 0.2 Pa, 1 Hz) was used on mouse hematopoietic progenitor cells for 2 min in the presence or absence of chemical compounds that inhibit release or biogenesis of extracellular vesicles. We used a Receptor activator of nuclear factor kappa-Β ligand-induced osteoclastogenesis assay to evaluate the biological effect of different fractions of extracellular vesicles obtained through centrifugation of medium from hematopoietic stem cells. Osteoclast formation was reduced by microvesicles (10 000× g) obtained after low-intensity loading and induced by exosomes (100 000× g) obtained after high-intensity loading. These osteoclast-modulating effects could be diminished or eliminated by depletion of extracellular vesicles from the conditioned medium, inhibition of general extracellular vesicle release, inhibition of microvesicle biogenesis (low intensity), inhibition of ESCRT-independent exosome biogenesis (high intensity), as well as by inhibition of dynamin-dependent vesicle uptake in osteoclast progenitor cells. Taken together, the intensity of mechanical loading affects the release of extracellular vesicles and change their osteoclast-modulating effect.

Connexin 43 in the function and homeostasis of osteocytes: a narrative review

Background and Objective

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.

Methods

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.

Key Content and Findings

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.

Conclusions

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.

Thermostable Basic Fibroblast Growth Factor Enhances the Production and Activity of Human Wharton's Jelly Mesenchymal Stem Cell-Derived Extracellular Vesicles

Wharton's jelly-derived mesenchymal stem cell (WJ-MSC)-derived exosomes contain a diverse cargo and exhibit remarkable biological activity, rendering them suitable for regenerative and immune-modulating functions. However, the quantity of secretion is insufficient. A large body of prior work has investigated the use of various growth factors to enhance MSC-derived exosome production. In this study, we evaluated the utilization of thermostable basic fibroblast growth factor (TS-bFGF) with MSC culture and exosome production. MSCs cultured with TS-bFGF displayed superior proliferation, as evidenced by cell cycle analysis, compared with wild-type bFGF (WT-bFGF). Stemness was assessed through mRNA expression level and colony-forming unit (CFU) assays. Furthermore, nanoparticle tracking analysis (NTA) measurements revealed that MSCs cultured with TS-bFGF produced a greater quantity of exosomes, particularly under three-dimensional culture conditions. These produced exosomes demonstrated substantial anti-inflammatory and wound-healing effects, as confirmed by nitric oxide (NO) assays and scratch assays. Taken together, we demonstrate that utilization of TS-bFGF for WJ-MSC-derived exosome production not only increases exosome yield but also enhances the potential for various applications in inflammation regulation and wound healing.

Convergence of Calcium Channel Regulation and Mechanotransduction in Skeletal Regenerative Biomaterial Design

Cells are known to perceive their microenvironment through extracellular and intracellular mechanical signals. Upon sensing mechanical stimuli, cells can initiate various downstream signaling pathways that are vital to regulating proliferation, growth, and homeostasis. One such physiologic activity modulated by mechanical stimuli is osteogenic differentiation. The process of osteogenic mechanotransduction is regulated by numerous calcium ion channels-including channels coupled to cilia, mechanosensitive and voltage-sensitive channels, and channels associated with the endoplasmic reticulum. Evidence suggests these channels are implicated in osteogenic pathways such as the YAP/TAZ and canonical Wnt pathways. This review aims to describe the involvement of calcium channels in regulating osteogenic differentiation in response to mechanical loading and characterize the fashion in which those channels directly or indirectly mediate this process. The mechanotransduction pathway is a promising target for the development of regenerative materials for clinical applications due to its independence from exogenous growth factor supplementation. As such, also described are examples of osteogenic biomaterial strategies that involve the discussed calcium ion channels, calcium-dependent cellular structures, or calcium ion-regulating cellular features. Understanding the distinct ways calcium channels and signaling regulate these processes may uncover potential targets for advancing biomaterials with regenerative osteogenic capabilities.

Cell culture-derived extracellular vesicles: Considerations for reporting cell culturing parameters

Cell culture-conditioned medium (CCM) is a valuable source of extracellular vesicles (EVs) for basic scientific, therapeutic and diagnostic applications. Cell culturing parameters affect the biochemical composition, release and possibly the function of CCM-derived EVs (CCM-EV). The CCM-EV task force of the Rigor and Standardization Subcommittee of the International Society for Extracellular Vesicles aims to identify relevant cell culturing parameters, describe their effects based on current knowledge, recommend reporting parameters and identify outstanding questions. While some recommendations are valid for all cell types, cell-specific recommendations may need to be established for non-mammalian sources, such as bacteria, yeast and plant cells. Current progress towards these goals is summarized in this perspective paper, along with a checklist to facilitate transparent reporting of cell culturing parameters to improve the reproducibility of CCM-EV research.

The RANK-RANKL-OPG System: A Multifaceted Regulator of Homeostasis, Immunity, and Cancer

The RANK-RANKL-OPG system is a complex signaling pathway that plays a critical role in bone metabolism, mammary epithelial cell development, immune function, and cancer. RANKL is a ligand that binds to RANK, a receptor expressed on osteoclasts, dendritic cells, T cells, and other cells. RANKL signaling promotes osteoclast differentiation and activation, which leads to bone resorption. OPG is a decoy receptor that binds to RANKL and inhibits its signaling. In cancer cells, RANKL expression is often increased, which can lead to increased bone resorption and the development of bone metastases. RANKL-neutralizing antibodies, such as denosumab, have been shown to be effective in the treatment of skeletal-related events, including osteoporosis or bone metastases, and cancer. This review will provide a comprehensive overview of the functions of the RANK-RANKL-OPG system in bone metabolism, mammary epithelial cells, immune function, and cancer, together with the potential therapeutic implications of the RANK-RANKL pathway for cancer management.

Mesenchymal Stem Cell Culture within Perfusion Bioreactors Incorporating 3D-Printed Scaffolds Enables Improved Extracellular Vesicle Yield with Preserved Bioactivity

Extracellular vesicles (EVs) are implicated as promising therapeutics and drug delivery vehicles in various diseases. However, successful clinical translation will depend on the development of scalable biomanufacturing approaches, especially due to the documented low levels of intrinsic EV-associated cargo that may necessitate repeated doses to achieve clinical benefit in certain applications. Thus, here the effects of a 3D-printed scaffold-perfusion bioreactor system are assessed on the production and bioactivity of EVs secreted from bone marrow-derived mesenchymal stem cells (MSCs), a cell type widely implicated in generating EVs with therapeutic potential. The results indicate that perfusion bioreactor culture induces an ≈40-80-fold increase (depending on measurement method) in MSC EV production compared to conventional cell culture. Additionally, MSC EVs generated using the perfusion bioreactor system significantly improve wound healing in a diabetic mouse model, with increased CD31+ staining in wound bed tissue compared to animals treated with flask cell culture-generated MSC EVs. Overall, this study establishes a promising solution to a major EV translational bottleneck, with the capacity for tunability for specific applications and general improvement alongside advancements in 3D-printing technologies.

Osteoclast-Derived Exosomal miR-5134-5p Interferes with Alveolar Bone Homeostasis by Targeting the JAK2/STAT3 Axis

Background

In chronic periodontitis, exosomes transport various informative substances between osteoclasts and osteoblasts in alveolar bone. Herein, we aimed to investigate the effect of exosomal micro-ribonucleic acid (miRNA/miR)-5134-5p derived from osteoclasts on osteoblastic proliferation and differentiation and the development of periodontitis in vivo and in vitro.

Methods

The effects of OC-Exos on the proliferation and differentiation of osteoblasts were identified by Real-time quantitative reverse polymerase chain reaction (qRT-PCR), Western blot(WB), alkaline phosphatase(ALP) staining, etc. Exosomal miRNA expression was analyzed by sequencing. The sites of miRNA action were predicted through TargetScan and tested by double luciferase assay. After transfecting miR-5134-5p mimic/inhibitor into osteoblasts, we measured the proliferation and differentiation of osteoblasts by ALP staining and WB, etc. Furthermore, OC-Exos were injected into the gingival sulcus at the ligation site. Inflammation was observed by Hematoxylin-eosin (H&E) staining, the expression of inflammatory factors were detected by qRT-PCR, the resorption of alveolar bone was observed by Micro CT.

Results

Osteoblastic proliferation and differentiation were negatively regulated by OC-Exos in vitro. miRNA sequencing analysis revealed that miR-5134-5p expression was significantly elevated in OC-Exos, which also increased in osteoblasts following OC-Exo intervention. The dual-luciferase assay revealed that miR-5134-5p and Janus kinase 2 (JAK2) had binding sites. miR-5134-5p-mimics could upregulate miR-5134-5p expression in osteoblasts while downregulating Runt-related transcription factor 2(Runx2), phosphorylated-JAK2 (p-JAK2), and phosphorylated-signal transducer and activator of transcription 3 (p-STAT3) expression and inhibited osteogenic differentiation. However, miR-5134-5p-inhibitor had the opposite effect. In vivo, the OC-Exo group demonstrated morphological disruption of periodontal tissue, massive inflammatory cell infiltration, upregulation of inflammatory factors mRNA expression, a significant decrease in BV/TV, and an increase in the cementoenamel junction and alveolar bone crest distance.

Conclusion

Osteoclast-derived exosomal miR-5134-5p inhibits osteoblastic proliferation and differentiation via the JAK2/STAT3 pathway. OC-Exos exacerbate periodontal tissue inflammation and accelerate alveolar bone resorption in mice with experimental periodontitis.

Examining Mechanisms for Voltage-Sensitive Calcium Channel-Mediated Secretion Events in Bone Cells

In addition to their well-described functions in cell excitability, voltage-sensitive calcium channels (VSCCs) serve a critical role in calcium (Ca2+)-mediated secretion of pleiotropic paracrine and endocrine factors, including those produced in bone. Influx of Ca2+ through VSCCs activates intracellular signaling pathways to modulate a variety of cellular processes that include cell proliferation, differentiation, and bone adaptation in response to mechanical stimuli. Less well understood is the role of VSCCs in the control of bone and calcium homeostasis mediated through secreted factors. In this review, we discuss the various functions of VSCCs in skeletal cells as regulators of Ca2+ dynamics and detail how these channels might control the release of bioactive factors from bone cells. Because VSCCs are druggable, a better understanding of the multiple functions of these channels in the skeleton offers the opportunity for developing new therapies to enhance and maintain bone and to improve systemic health.

Sympathetic Innervation Regulates Osteocyte-Mediated Cortical Bone Resorption during Lactation

Bone undergoes constant remodeling by osteoclast bone resorption coupled with osteoblast bone formation at the bone surface. A third major cell type in the bone is osteocytes, which are embedded in the matrix, are well-connected to the lacunar network, and are believed to act as mechanical sensors. Here, it is reported that sympathetic innervation directly regulates lacunar osteocyte-mediated bone resorption inside cortical bone. It is found that sympathetic activity is elevated in different mouse models of bone loss, including lactation, ovariectomy, and glucocorticoid treatment. Further, during lactation elevated sympathetic outflow induces netrin-1 expression by osteocytes to further promote sympathetic nerve sprouting in the cortical bone endosteum in a feed-forward loop. Depletion of tyrosine hydroxylase-positive (TH+ ) sympathetic nerves ameliorated osteocyte-mediated perilacunar bone resorption in lactating mice. Moreover, norepinephrine activates β-adrenergic receptor 2 (Adrb2) signaling to promote secretion of extracellular vesicles (EVs) containing bone-degrading enzymes for perilacunar bone resorption and inhibit osteoblast differentiation. Importantly, osteocyte-specific deletion of Adrb2 or treatment with a β-blocker results in lower bone resorption in lactating mice. Together, these findings show that the sympathetic nervous system promotes osteocyte-driven bone loss during lactation, likely as an adaptive response to the increased energy and mineral demands of the nursing mother.

Targeting strategies for bone diseases: signaling pathways and clinical studies

Since the proposal of Paul Ehrlich's magic bullet concept over 100 years ago, tremendous advances have occurred in targeted therapy. From the initial selective antibody, antitoxin to targeted drug delivery that emerged in the past decades, more precise therapeutic efficacy is realized in specific pathological sites of clinical diseases. As a highly pyknotic mineralized tissue with lessened blood flow, bone is characterized by a complex remodeling and homeostatic regulation mechanism, which makes drug therapy for skeletal diseases more challenging than other tissues. Bone-targeted therapy has been considered a promising therapeutic approach for handling such drawbacks. With the deepening understanding of bone biology, improvements in some established bone-targeted drugs and novel therapeutic targets for drugs and deliveries have emerged on the horizon. In this review, we provide a panoramic summary of recent advances in therapeutic strategies based on bone targeting. We highlight targeting strategies based on bone structure and remodeling biology. For bone-targeted therapeutic agents, in addition to improvements of the classic denosumab, romosozumab, and PTH1R ligands, potential regulation of the remodeling process targeting other key membrane expressions, cellular crosstalk, and gene expression, of all bone cells has been exploited. For bone-targeted drug delivery, different delivery strategies targeting bone matrix, bone marrow, and specific bone cells are summarized with a comparison between different targeting ligands. Ultimately, this review will summarize recent advances in the clinical translation of bone-targeted therapies and provide a perspective on the challenges for the application of bone-targeted therapy in the clinic and future trends in this area.

Extracellular vesicles: From bone development to regenerative orthopedics

Regenerative medicine aims to promote the replacement of tissues lost to damage or disease. While positive outcomes have been observed experimentally, challenges remain in their clinical translation. This has led to growing interest in applying extracellular vesicles (EVs) to augment or even replace existing approaches. Through the engineering of culture environments or direct/indirect manipulation of EVs themselves, multiple avenues have emerged to modulate EV production, targeting, and therapeutic potency. Drives to modulate release using material systems or functionalize implants for improved osseointegration have also led to outcomes that could have real-world impact. The purpose of this review is to highlight advantages in applying EVs for the treatment of skeletal defects, outlining the current state of the art in the field and emphasizing avenues for further investigation. Notably, the review identifies inconsistencies in EV nomenclature and outstanding challenges in defining a reproducible therapeutic dose. Challenges also remain in the scalable manufacture of a therapeutically potent and pure EV product, with a need to address scalable cell sources and optimal culture environments. Addressing these issues will be critical if we are to develop regenerative EV therapies that meet the demands of regulators and can be translated from bench to bedside.

Tanycyte, the neuron whisperer

Reciprocal communication between neurons and glia is essential for normal brain functioning and adequate physiological functions, including energy balance. In vertebrates, the homeostatic process that adjusts food intake and energy expenditure in line with physiological requirements is tightly controlled by numerous neural cell types located within the hypothalamus and the brainstem and organized in complex networks. Within these neural networks, peculiar ependymoglial cells called tanycytes are nowadays recognized as multifunctional players in the physiological mechanisms of appetite control, partly by modulating orexigenic and anorexigenic neurons. Here, we review recent advances in tanycytes' impact on hypothalamic neuronal activity, emphasizing on arcuate neurons.

Oscillating Fluid Flow Activated Osteocyte Lysate-Based Hydrogel for Regulating Osteoblast/Osteoclast Homeostasis to Enhance Bone Repair

As major regulators on bone formation/resorption in response to mechanical stimuli, osteocytes have shown great promise for restoring bone injury. However, due to the unmanageable and unabiding cell functions in unloading or diseased environments, the efficacy of osteogenic induction by osteocytes has been enormously limited. Herein, a facile method of oscillating fluid flow (OFF) loading for cell culture is reported, which enables osteocytes to initiate only osteogenesis and not the osteolysis process. After OFF loading, multiple and sufficient soluble mediators are produced in osteocytes, and the collected osteocyte lysates invariably induce robust osteoblastic differentiation and proliferation while restraining osteoclast generation and activity under unloading or pathological conditions. Mechanistic studies confirm that elevated glycolysis and activation of the ERK1/2 and Wnt/β-catenin pathways are the major contributors to the initiation of osteoinduction functions induced by osteocytes. Moreover, an osteocyte lysate-based hydrogel is designed to establish a stockpile of "active osteocytes" to sustainably deliver bioactive proteins, resulting in accelerated healing through regulation of endogenous osteoblast/osteoclast homeostasis.

Bioengineering extracellular vesicles: smart nanomaterials for bone regeneration

In the past decade, extracellular vesicles (EVs) have emerged as key regulators of bone development, homeostasis and repair. EV-based therapies have the potential to circumnavigate key issues hindering the translation of cell-based therapies including functional tissue engraftment, uncontrolled differentiation and immunogenicity issues. Due to EVs' innate biocompatibility, low immunogenicity, and high physiochemical stability, these naturally-derived nanoparticles have garnered growing interest as potential acellular nanoscale therapeutics for a variety of diseases. Our increasing knowledge of the roles these cell-derived nanoparticles play, has made them an exciting focus in the development of novel pro-regenerative therapies for bone repair. Although these nano-sized vesicles have shown promise, their clinical translation is hindered due to several challenges in the EV supply chain, ultimately impacting therapeutic efficacy and yield. From the biochemical and biophysical stimulation of parental cells to the transition to scalable manufacture or maximising vesicles therapeutic response in vivo, a multitude of techniques have been employed to improve the clinical efficacy of EVs. This review explores state of the art bioengineering strategies to promote the therapeutic utility of vesicles beyond their native capacity, thus maximising the clinical potential of these pro-regenerative nanoscale therapeutics for bone repair.

The role of biomechanical stress in extracellular vesicle formation, composition and activity

Extracellular vesicles (EVs) are cornerstones of intercellular communication with exciting fundamental, clinical, and more broadly biotechnological applications. However, variability in EV composition, which results from the culture conditions used to generate the EVs, poses significant fundamental and applied challenges and a hurdle for scalable bioprocessing. Thus, an understanding of the relationship between EV production (and for clinical applications, manufacturing) and EV composition is increasingly recognized as important and necessary. While chemical stimulation and culture conditions such as cell density are known to influence EV biology, the impact of biomechanical forces on the generation, properties, and biological activity of EVs remains poorly understood. Given the omnipresence of these forces in EV preparation and in biomanufacturing, expanding the understanding of their impact on EV composition-and thus, activity-is vital. Although several publications have examined EV preparation and bioprocessing and briefly discussed biomechanical stresses as variables of interest, this review represents the first comprehensive evaluation of the impact of such stresses on EV production, composition and biological activity. We review how EV biogenesis, cargo, efficacy, and uptake are uniquely affected by various types, magnitudes, and durations of biomechanical forces, identifying trends that emerge both generically and for individual cell types. We also describe implications for scalable bioprocessing, evaluating processes inherent in common EV production and isolation methods, and propose a path forward for rigorous EV quality control.

Healthy and Osteoarthritis-Affected Joints Facing the Cellular Crosstalk

Osteoarthritis (OA) is a chronic, progressive, severely debilitating, and multifactorial joint disease that is recognized as the most common type of arthritis. During the last decade, it shows an incremental global rise in prevalence and incidence. The interaction between etiologic factors that mediate joint degradation has been explored in numerous studies. However, the underlying processes that induce OA remain obscure, largely due to the variety and complexity of these mechanisms. During synovial joint dysfunction, the osteochondral unit undergoes cellular phenotypic and functional alterations. At the cellular level, the synovial membrane is influenced by cartilage and subchondral bone cleavage fragments and extracellular matrix (ECM) degradation products from apoptotic and necrotic cells. These "foreign bodies" serve as danger-associated molecular patterns (DAMPs) that trigger innate immunity, eliciting and sustaining low-grade inflammation in the synovium. In this review, we explore the cellular and molecular communication networks established between the major joint compartments-the synovial membrane, cartilage, and subchondral bone of normal and OA-affected joints.

Osteocyte Remodeling of the Lacunar-Canalicular System: What's in a Name?

PURPOSE OF REVIEW

Osteocytes directly modify the bone surrounding the expansive lacunar-canalicular system (LCS) through both resorption and deposition. The existence of this phenomenon is now widely accepted, but is referred to as "osteocyte osteolysis," "LCS remodeling," and "perilacunar remodeling," among other names. The uncertainty in naming this physiological process reflects the many persistent questions about why and how osteocytes interact with local bone matrix. The goal of this review is to examine the purpose and nature of LCS remodeling and its impacts on multiscale bone quality.

RECENT FINDINGS

While LCS remodeling is clearly important for systemic calcium mobilization, this process may have additional potential drivers and may impact the ability of bone to resist fracture. There is abundant evidence that the osteocyte can resorb and replace bone mineral and does so outside of extreme challenges to mineral homeostasis. The impacts of the osteocyte on organic matrix are less certain, especially regarding whether osteocytes produce osteoid. Though multiple lines of evidence point towards osteocyte production of organic matrix, definitive work is needed. Recent high-resolution imaging studies demonstrate that LCS remodeling influences local material properties. The role of LCS remodeling in the maintenance and deterioration of bone matrix quality in aging and disease are active areas of research. In this review, we highlight current progress in understanding why and how the osteocyte removes and replaces bone tissue and the consequences of these activities to bone quality. We posit that answering these questions is essential for evaluating whether, how, when, and why LCS remodeling may be manipulated for therapeutic benefit in managing bone fragility.

Characterization of sclerostin's response within white adipose tissue to an obesogenic diet at rest and in response to acute exercise in male mice

Introduction: It is well established that sclerostin antagonizes the anabolic Wnt signalling pathway in bone, however, its physiological role in other tissues remains less clear. This study examined the effect of a high-fat diet (HFD) on sclerostin content and downstream markers of the Wnt signaling pathway (GSK3β and β-catenin) within subcutaneous inguinal white adipose tissue (iWAT), and visceral epididymal WAT (eWAT) depots at rest and in response to acute aerobic exercise. Methods: Male C57BL/6 mice (n = 40, 18 weeks of age) underwent 10 weeks of either a low-fat diet (LFD) or HFD. Within each diet group, mice were assigned to either remain sedentary (SED) or perform 2 h of endurance treadmill exercise at 15 m min^-1 with 5° incline (EX), creating four groups: LFD + SED (N = 10), LFD + EX (N = 10), HFD + SED (N = 10), and HFD + EX (N = 10). Serum and WAT depots were collected 2 h post-exercise. Results: Serum sclerostin showed a diet-by-exercise interaction, reflecting HFD + EX mice having higher concentration than HFD + SED (+31%, p = 0.03), and LFD mice being unresponsive to exercise. iWAT sclerostin content decreased post-exercise in both 28 kDa (-31%, p = 0.04) and 30 kDa bands (-36%, main effect for exercise, p = 0.02). iWAT β-catenin (+44%, p = 0.03) and GSK3β content were higher in HFD mice compared to LFD (+128%, main effect for diet, p = 0.005). Monomeric sclerostin content was abolished in eWAT of HFD mice (-96%, main effect for diet, p < 0.0001), was only detectable as a 30 kDa band in LFD mice and was unresponsive to exercise. β-catenin and GSK3β were both unresponsive to diet and exercise within eWAT. Conclusion: These results characterized sclerostin's content to WAT depots in response to acute exercise, which appears to be specific to a reduction in iWAT and identified a differential regulation of sclerostin's form/post-translational modifications depending on diet and WAT depot.

Circulating Extracellular Vesicles Express Receptor Activator of Nuclear Factor κB Ligand and Other Molecules Informative of the Bone Metabolic Status of Mouse Models of Experimentally Induced Osteoporosis

Extracellular vesicles (EVs) are potent means of cell-to-cell communication. They are released in biological fluids, including blood, urine, and saliva, and can be exploited to identify new biomarkers of diseases. We hypothesized that EVs contain molecular cargos involved in bone metabolism, possibly mirroring biological differences between postmenopausal and disuse osteoporosis. We tested this hypothesis in primary murine osteoblasts subjected to steroid depletion or to unloading, and in the serum of animal models of osteoporosis induced by ovariectomy or hindlimb tail suspension. EVs were isolated by ultracentrifugation and analysed by transmission electron microscopy, cytofluorimetry, immunoblotting and RT-PCR. Large-scale analyses were performed by Real-Time arrays and Proteome Profiler™ Antibody arrays. Finally, precise titration of analytes was carried out by ELISA assay. In vitro, we confirmed an increased release of EVs enriched in surface RANKL by primary mouse osteoblasts subjected to steroid depletion or simulated microgravity compared to controls. In vivo, circulating EVs isolated from the sera of control female mice expressed RANKL along with other genes associated with bone metabolism. Serum EVs from ovariectomized or hindlimb tail-suspended mice showed distinct molecular profiles. They expressed RANKL with different kinetics, while transcriptomic and proteomic profiles uncovered unique molecular signatures that discriminated the two conditions, unveiling exclusive molecules expressed in time- and osteoporosis type-dependent manner. These results suggest that circulating EVs could represent a new tool for monitoring the onset and the progression of diverse types of the disease in mice, paving the way for their exploitation to diagnose human osteoporosis in liquid biopsies.

Lowry M, Almasri R, Gorgun C, Eichholz K, Stavenschi E, Hokamp K, Roche F, O’Driscoll L, Hoey D. Mechanically activated mesenchymal-derived bone cells drive vessel formation via an extracellular vesicle mediated mechanism

Blood vessel formation is an important initial step for bone formation during development as well as during remodelling and repair in the adult skeleton. This results in a heavily vascularized tissue where endothelial cells and skeletal cells are constantly in crosstalk to facilitate homeostasis, a process that is mediated by numerous environmental signals, including mechanical loading. Breakdown in this communication can lead to disease and/or poor fracture repair. Therefore, this study aimed to determine the role of mature bone cells in regulating angiogenesis, how this is influenced by a dynamic mechanical environment, and understand the mechanism by which this could occur. Herein, we demonstrate that both osteoblasts and osteocytes coordinate endothelial cell proliferation, migration, and blood vessel formation via a mechanically dependent paracrine mechanism. Moreover, we identified that this process is mediated via the secretion of extracellular vesicles (EVs), as isolated EVs from mechanically stimulated bone cells elicited the same response as seen with the full secretome, while the EV-depleted secretome did not elicit any effect. Despite mechanically activated bone cell-derived EVs (MA-EVs) driving a similar response to VEGF treatment, MA-EVs contain minimal quantities of this angiogenic factor. Lastly, a miRNA screen identified mechanoresponsive miRNAs packaged within MA-EVs which are linked with angiogenesis. Taken together, this study has highlighted an important mechanism in osteogenic-angiogenic coupling in bone and has identified the mechanically activated bone cell-derived EVs as a therapeutic to promote angiogenesis and potentially bone repair.

Mechanosensitive piezo1 calcium channel activates connexin 43 hemichannels through PI3K signaling pathway in bone

BACKGROUND

Mechanical loading promotes bone formation and osteocytes are a major mechanosensory cell in the bone. Both Piezo1 channels and connexin 43 hemichannels (Cx43 HCs) in osteocytes are important players in mechanotransduction and anabolic function by mechanical loading. However, the mechanism underlying mechanotransduction involving Piezo1 channels and Cx43 HCs in osteocytes and bone remains unknown.

RESULTS

We showed that, like mechanical loading, Piezo1 specific agonist Yoda1 was able to increase intracellular Ca2+ signaling and activate Cx43 HCs, while Yoda1 antagonist Dooku1 inhibited Ca2+ and Cx43 HC activation induced by both mechanical loading and Yoda1. Moreover, the intracellular Ca2+ signal activated by Yoda1 was reduced by the inhibition of Cx43 HCs and pannexin1 (Panx1) channels, as well as ATP-P2X receptor signaling. Piezo1 and Cx43 HCs were co-localized on the osteocyte cell surface, and Yoda1-activated PI3K-Akt signaling regulated the opening of Cx43 HCs. Furthermore, Cx43 HCs opening by mechanical loading on tibias was ablated by inhibition of Piezo1 activation in vivo.

CONCLUSION

We demonstrated that upon mechanical stress, increased intracellular Ca2+ activated by Piezo1 regulates the opening of HCs through PI3K-Akt and opened Cx43 HCs, along with Panx1 channels, and ATP-P2X signaling sustain the intracellular Ca2+ signal, leading to bone anabolic function.

In vitromodel to study confined osteocyte networks exposed to flow-induced mechanical stimuli

Osteocytes are considered the primary mechanical sensor in bone tissue and orchestrate the coupled bone remodeling activity of adjacent osteoblast and osteoclast cells.In vivoinvestigation of mechanically induced signal propagation through networks of interconnected osteocytes is confounded by their confinement within the mineralized bone matrix, which cannot be modeled in conventional culture systems. In this study, we developed a new model that mimics thisin vivoconfinement using gelatin methacrylate (GelMA) hydrogel or GelMA mineralized using osteoblast-like model cells. This model also enables real-time optical examination of osteocyte calcium (Ca2+) signaling dynamics in response to fluid shear stimuli cultured under confined conditions. Using this system, we discovered several distinct and previously undescribed patterns of Ca2+responses that vary across networks of interconnected osteocytes as a function of space, time and connectivity. Heterogeneity in Ca2+signaling may provide new insights into bone remodeling in response to mechanical loading. Overall, such a model can be extended to study signaling dynamics within cell networks exposed to flow-induced mechanical stimuli under confined conditions.