IDG-SW3 Osteocyte Differentiation and Bone Extracellular Matrix Deposition Are Enhanced in a 3D Matrix Metalloproteinase-Sensitive Hydrogel

A 3D bioreactor model to study osteocyte differentiation and mechanobiology under perfusion and compressive mechanical loading

Osteocytes perceive and process mechanical stimuli in the lacuno-canalicular network in bone. As a result, they secrete signaling molecules that mediate bone formation and resorption. To date, few three-dimensional (3D) models exist to study the response of mature osteocytes to biophysical stimuli that mimic fluid shear stress and substrate strain in a mineralized, biomimetic bone-like environment. Here we established a biomimetic 3D bone model by utilizing a state-of-art perfusion bioreactor platform where immortomouse/Dmp1-GFP-derived osteoblastic IDG-SW3 cells were differentiated into mature osteocytes. We evaluated proliferation and differentiation properties of the cells on 3D microporous scaffolds of decellularized bone (dBone), poly(L-lactide-co-trimethylene carbonate) lactide (LTMC), and beta-tricalcium phosphate (β-TCP) under physiological fluid flow conditions over 21 days. Osteocyte viability and proliferation were similar on the scaffolds with equal distribution of IDG-SW3 cells on dBone and LTMC scaffolds. After seven days, the differentiation marker alkaline phosphatase (Alpl), dentin matrix acidic phosphoprotein 1 (Dmp1), and sclerostin (Sost) were significantly upregulated in IDG-SW3 cells (p = 0.05) on LTMC scaffolds under fluid flow conditions at 1.7 ml/min, indicating rapid and efficient maturation into osteocytes. Osteocytes responded by inducing the mechanoresponsive genes FBJ osteosarcoma oncogene (Fos) and prostaglandin-endoperoxide synthase 2 (Ptgs2) under perfusion and dynamic compressive loading at 1 Hz with 5 % strain. Together, we successfully created a 3D biomimetic platform as a robust tool to evaluate osteocyte differentiation and mechanobiology in vitro while recapitulating in vivo mechanical cues such as fluid flow within the lacuno-canalicular network. STATEMENT OF SIGNIFICANCE: This study highlights the importance of creating a three-dimensional (3D) in vitro model to study osteocyte differentiation and mechanobiology, as cellular functions are limited in two-dimensional (2D) models lacking in vivo tissue organization. By using a perfusion bioreactor platform, physiological conditions of fluid flow and compressive loading were mimicked to which osteocytes are exposed in vivo. Microporous poly(L-lactide-co-trimethylene carbonate) lactide (LTMC) scaffolds in 3D are identified as a valuable tool to create a favorable environment for osteocyte differentiation and to enable mechanical stimulation of osteocytes by perfusion and compressive loading. The LTMC platform imitates the mechanical bone environment of osteocytes, allowing the analysis of the interaction with other cell types in bone under in vivo biophysical stimuli.

Synthetic biodegradable microporous hydrogels for in vitro 3D culture of functional human bone cell networks

Generating 3D bone cell networks in vitro that mimic the dynamic process during early bone formation remains challenging. Here, we report a synthetic biodegradable microporous hydrogel for efficient formation of 3D networks from human primary cells, analysis of cell-secreted extracellular matrix (ECM) and microfluidic integration. Using polymerization-induced phase separation, we demonstrate dynamic in situ formation of microporosity (5-20 µm) within matrix metalloproteinase-degradable polyethylene glycol hydrogels in the presence of living cells. Pore formation is triggered by thiol-Michael-addition crosslinking of a viscous precursor solution supplemented with hyaluronic acid and dextran. The resulting microporous architecture can be fine-tuned by adjusting the concentration and molecular weight of dextran. After encapsulation in microporous hydrogels, human mesenchymal stromal cells and osteoblasts spread rapidly and form 3D networks within 24 hours. We demonstrate that matrix degradability controls cell-matrix remodeling, osteogenic differentiation, and deposition of ECM proteins such as collagen. Finally, we report microfluidic integration and proof-of-concept osteogenic differentiation of 3D cell networks under perfusion on chip. Altogether, this work introduces a synthetic microporous hydrogel to efficiently differentiate 3D human bone cell networks, facilitating future in vitro studies on early bone development.

Osteocytes support bone metastasis of melanoma cells by CXCL5

Bone metastasis is a common complication of certain cancers such as melanoma. The spreading of cancer cells into the bone is supported by changes in the bone marrow environment. The specific role of osteocytes in this process is yet to be defined. By RNA-seq and chemokines screening we show that osteocytes release the chemokine CXCL5 when they are exposed to melanoma cells. Osteocytes-mediated CXCL5 secretion enhanced the migratory and invasive behaviour of melanoma cells. When the expression of the CXCL5 receptor, CXCR2, was down-regulated in melanoma cells in vitro, we observed a significant decrease in melanoma cell migration in response to osteocytes. Furthermore, melanoma cells with down-regulated CXCR2 expression showed less bone metastasis and less bone loss in the bone metastasis model in vivo. Furthermore, when simultaneously down-regulating CXCL5 in osteocytes and CXCR2 in melanoma cells, melanoma progression was abrogated in vivo. In summary, these data suggest a significant role of osteocytes in bone metastasis of melanoma, which is mediated through the CXCL5-CXCR2 pathway.

Interpenetrating network hydrogels for studying the role of matrix viscoelasticity in 3D osteocyte morphogenesis

During bone formation, osteoblasts are embedded in a collagen-rich osteoid tissue and differentiate into an extensive 3D osteocyte network throughout the mineralizing matrix. However, how these cells dynamically remodel the matrix and undergo 3D morphogenesis remains poorly understood. Although previous reports investigated the impact of matrix stiffness in osteocyte morphogenesis, the role of matrix viscoelasticity is often overlooked. Here, we report a viscoelastic alginate-collagen interpenetrating network (IPN) hydrogel for 3D culture of murine osteocyte-like IDG-SW3 cells. The IPN hydrogels consist of an ionically crosslinked alginate network to tune stress relaxation as well as a permissive collagen network to promote cell adhesion and matrix remodeling. Two IPN hydrogels were developed with comparable stiffnesses (4.4-4.7 kPa) but varying stress relaxation times (t1/2, 1.5 s and 14.4 s). IDG-SW3 cells were pre-differentiated in 2D under osteogenic conditions for 14 days to drive osteoblast-to-osteocyte transition. Cellular mechanosensitivity to fluid shear stress (2 Pa) was confirmed by live-cell calcium imaging. After embedding in the IPN hydrogels, cells remained highly viable following 7 days of 3D culture. After 24 h, osteocytes in the fast-relaxing hydrogels showed the largest cell area and long dendritic processes. However, a significantly larger increase of some osteogenic markers (ALP, Dmp1, hydroxyapatite) as well as intercellular connections via gap junctions were observed in slow-relaxing hydrogels on day 14. Our results imply that fast-relaxing IPN hydrogels promote early cell spreading, whereas slow relaxation favors osteogenic differentiation. These findings may advance the development of 3D in vivo-like osteocyte models to better understand bone mechanobiology.

Bone-homing metastatic breast cancer cells impair osteocytes' mechanoresponse in a 3D loading model

Breast cancer predominantly metastasizes to the skeleton. Mechanical loading is reliably anabolic in bone, and also inhibits bone metastatic tumor formation and bone loss in vivo. To study the underlying mechanisms, we developed a 3D culture model for osteocytes, the primary bone mechanosensor. We verified that MLO-Y4s responded to perfusion by reducing their rankl and rankl:opg gene expression. We next cultured MLO-Y4s with tumor-conditioned media (TCM) collected from human breast cancer cells (MDA-MB-231s) and a corresponding bone-homing subclone to test the impacts on osteocytes' mechanosensation. We found that TCM from the bone-homing subclone was more detrimental to MLO-Y4 growth and viability, and it abrogated loading-induced changes to rankl:opg. Our studies demonstrate that MLO-Y4s, including their mechanoresponse to perfusion, were more negatively impacted by soluble factors from bone-homing breast cancer cells compared to those from parental cells.

Bone on-a-chip: a 3D dendritic network in a screening platform for osteocyte-targeted drugs

Age-related musculoskeletal disorders, including osteoporosis, are frequent and associated with long lasting morbidity, in turn significantly impacting on healthcare system sustainability. There is therefore a compelling need to develop reliable preclinical models of disease and drug screening to validate novel drugs possibly on a personalized basis, without the need ofin vivoassay. In the context of bone tissue, although the osteocyte (Oc) network is a well-recognized therapeutic target, currentin vitropreclinical models are unable to mimic its physiologically relevant and highly complex structure. To this purpose, several features are needed, including an osteomimetic extracellular matrix, dynamic perfusion, and mechanical cues (e.g. shear stress) combined with a three-dimensional (3D) culture of Oc. Here we describe, for the first time, a high throughput microfluidic platform based on 96-miniaturized chips for large-scale preclinical evaluation to predict drug efficacy. We bioengineered a commercial microfluidic device that allows real-time visualization and equipped with multi-chips by the development and injection of a highly stiff bone-like 3D matrix, made of a blend of collagen-enriched natural hydrogels loaded with hydroxyapatite nanocrystals. The microchannel, filled with the ostemimetic matrix and Oc, is subjected to passive perfusion and shear stress. We used scanning electron microscopy for preliminary material characterization. Confocal microscopy and fluorescent microbeads were used after material injection into the microchannels to detect volume changes and the distribution of cell-sized objects within the hydrogel. The formation of a 3D dendritic network of Oc was monitored by measuring cell viability, evaluating phenotyping markers (connexin43, integrin alpha V/CD51, sclerostin), quantification of dendrites, and responsiveness to an anabolic drug. The platform is expected to accelerate the development of new drug aimed at modulating the survival and function of osteocytes.

Stimuli-responsive dynamic hydrogels: design, properties and tissue engineering applications

The field of tissue engineering and regenerative medicine has been evolving at a rapid pace with numerous novel and interesting biomaterials being reported. Hydrogels have come a long way in this regard and have been proven to be an excellent choice for tissue regeneration. This could be due to their innate properties such as water retention, and ability to carry and deliver a multitude of therapeutic and regenerative elements to aid in better outcomes. Over the past few decades, hydrogels have been developed into an active and attractive system that can respond to various stimuli, thereby presenting a wider control over the delivery of the therapeutic agents to the intended site in a spatiotemporal manner. Researchers have developed hydrogels that respond dynamically to a multitude of external as well as internal stimuli such as mechanics, thermal energy, light, electric field, ultrasonics, tissue pH, and enzyme levels, to name a few. This review gives a brief overview of the recent developments in such hydrogel systems which respond dynamically to various stimuli, some of the interesting fabrication strategies, and their application in cardiac, bone, and neural tissue engineering.

Applications of Stimuli-Responsive Hydrogels in Bone and Cartilage Regeneration

Bone and cartilage regeneration is an area of tremendous interest and need in health care. Tissue engineering is a potential strategy for repairing and regenerating bone and cartilage defects. Hydrogels are among the most attractive biomaterials in bone and cartilage tissue engineering, mainly due to their moderate biocompatibility, hydrophilicity, and 3D network structure. Stimuli-responsive hydrogels have been a hot topic in recent decades. They can respond to external or internal stimulation and are used in the controlled delivery of drugs and tissue engineering. This review summarizes current progress in the use of stimuli-responsive hydrogels in bone and cartilage regeneration. The challenges, disadvantages, and future applications of stimuli-responsive hydrogels are briefly described.

The effects of prostaglandin E2 on gene expression of IDG-SW3-derived osteocytes in 2D and 3D culture

Prostaglandin E2 (PGE2) is a key signaling molecule produced by osteocytes in response to mechanical loading, but its effect on osteocytes is less understood. This work examined the effect of PGE2 on IDG-SW3-derived osteocytes in standard 2D culture (collagen-coated tissue culture polystyrene) and in a 3D degradable poly(ethylene glycol) hydrogel. IDG-SW3 cells were differentiated for 35 days into osteocytes in 2D and 3D cultures. 3D culture led to a more mature osteocyte phenotype with 100-fold higher Sost expression. IDG-SW3-derived osteocytes were treated with PGE2 and assessed for expression of genes involved in PGE2, anabolic, and catabolic signaling. In 2D, PGE2 had a rapid (1 h) and sustained (24 h) effect on many PGE2 signaling genes, a rapid stimulatory effect on Il6, and a sustained inhibitory effect on Tnfrsf11b and Bglap. Comparing culture environment without PGE2, osteocytes had higher expression of all four EP receptors and Sost but lower expression of Tnfrsf11b, Bglap, and Gja1 in 3D. Osteocytes were more responsive to PGE2 in 3D. With increasing PGE2, 3D led to increased Gja1 and decreased Sost expressions and a higher Tnfrsf11b/Tnfsf11 ratio, indicating an anabolic response. Further analysis in 3D revealed that EP4, the receptor implicated in PGE2 signaling in bone, was not responsible for the PGE2-induced gene expression changes in osteocytes. In summary, osteocytes are highly responsive to PGE2 when cultured in an in vitro 3D hydrogel model suggesting that autocrine and paracrine PGE2 signaling in osteocytes may play a role in bone homeostasis.

The Mechanosensory Role of Osteocytes and Implications for Bone Health and Disease States

Bone homeostasis is a dynamic equilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts. This process is primarily controlled by the most abundant and mechanosensitive bone cells, osteocytes, that reside individually, within chambers of porous hydroxyapatite bone matrix. Recent studies have unveiled additional functional roles for osteocytes in directly contributing to local matrix regulation as well as systemic roles through endocrine functions by communicating with distant organs such as the kidney. Osteocyte function is governed largely by both biochemical signaling and the mechanical stimuli exerted on bone. Mechanical stimulation is required to maintain bone health whilst aging and reduced level of loading are known to result in bone loss. To date, both in vivo and in vitro approaches have been established to answer important questions such as the effect of mechanical stimuli, the mechanosensors involved, and the mechanosensitive signaling pathways in osteocytes. However, our understanding of osteocyte mechanotransduction has been limited due to the technical challenges of working with these cells since they are individually embedded within the hard hydroxyapatite bone matrix. This review highlights the current knowledge of the osteocyte functional role in maintaining bone health and the key regulatory pathways of these mechanosensitive cells. Finally, we elaborate on the current therapeutic opportunities offered by existing treatments and the potential for targeting osteocyte-directed signaling.

Impact of FasL Stimulation on Sclerostin Expression and Osteogenic Profile in IDG-SW3 Osteocytes

Simple Summary

FasL used to be considered as a classical ligand triggering cell death (apoptosis) via its receptor, Fas and thefollowing caspase cascade. As such, it is known to be involved in regulation within the bone. Recently, however, the knowledge has expanded about the non-apoptotic and caspase-independent engagement of the Fas/FasL pathway. The present investigation identified that stimulation of osteocytic IDG-SW3 cells by FasL leads to a dramatic decrease in expression of the major osteocytic marker, sclerostin. Additionally, other key components of the osteogenic pathways were impacted, notably in a caspase-independent manner. Such findings are of importance for basic biology as well as biomedical applications since osteocytes are the major population within adult bones and Fas signalling is one of therapeutical targets, e.g., in the anti-osteoporotic treatment.

Abstract

The Fas ligand (FasL) is known from programmed cell death, the immune system, and recently also from bone homeostasis. As such, Fas signalling is a potential target of anti-osteoporotic treatment based on the induction of osteoclastic cell death. Less attention has been paid to osteocytes, although they represent the majority of cells within the mature bone and are the key regulators. To determine the impact of FasL stimulation on osteocytes, differentiated IDG-SW3 cells were challenged by FasL, and their osteogenic expression profiles were evaluated by a pre-designed PCR array. Notably, the most downregulated gene was the one for sclerostin, which is the major marker of osteocytes and a negative regulator of bone formation. FasL stimulation also led to significant changes (over 10-fold) in the expression of other osteogenic markers: Gdf10, Gli1, Ihh, Mmp10, and Phex. To determine whether these alterations involved caspase-dependent or caspase-independent mechanisms, the IDG-SW3 cells were stimulated by FasL with and without a caspase inhibitor: Q-VD-OPh. The alterations were also detected in the samples treated by FasL along with Q-VD-OPh, pointing to the caspase-independent impact of FasL stimulation. These results contribute to an understanding of the recently emerging pleiotropic effects of Fas/FasL signalling and specify its functions in bone cells.

Structurally Dynamic Hydrogels for Biomedical Applications: Pursuing a Fine Balance between Macroscopic Stability and Microscopic Dynamics

Owing to their unique chemical and physical properties, hydrogels are attracting increasing attention in both basic and translational biomedical studies. Although the classical hydrogels with static networks have been widely reported for decades, a growing number of recent studies have shown that structurally dynamic hydrogels can better mimic the dynamics and functions of natural extracellular matrix (ECM) in soft tissues. These synthetic materials with defined compositions can recapitulate key chemical and biophysical properties of living tissues, providing an important means to understanding the mechanisms by which cells sense and remodel their surrounding microenvironments. This review begins with the overall expectation and design principles of dynamic hydrogels. We then highlight recent progress in the fabrication strategies of dynamic hydrogels including both degradation-dependent and degradation-independent approaches, followed by their unique properties and use in biomedical applications such as regenerative medicine, drug delivery, and 3D culture. Finally, challenges and emerging trends in the development and application of dynamic hydrogels are discussed.

Antimicrobial and enzyme-responsive multi-peptide surfaces for bone-anchored devices

Functionalization of dental and orthopedic implants with multiple bioactivities is desirable to obtain surfaces with improved biological performance and reduced infection rates. While many approaches have been explored to date, nearly all functionalized surfaces are static, i.e., non-responsive to biological cues. However, tissue remodeling necessary for implant integration features an ever-changing milieu of cells that demands a responsive biomaterial surface for temporal synchronization of interactions between biomaterial and tissue. Here, we successfully synthesized a multi-functional, dynamic coating on titanium by co-immobilizing GL13K antimicrobial peptide and an MMP-9 - a matrix metalloproteinase secreted by bone-remodeling osteoclasts - responsive peptide. Our co-immobilized peptide surface showed potent anti-biofilm activity, enabled effective osteoblast and fibroblast proliferation, and demonstrated stability against a mechanical challenge. Finally, we showed peptide release was triggered for up to seven days when the multi-peptide coatings were cultured with MMP-9-secreting osteoclasts. Our MMP-9 cleavable peptide can be conjugated with osteogenic or immunomodulatory motifs for enhanced bone formation in future work. Overall, we envisage our multifunctional, dynamic surface to reduce infection rates of percutaneous bone-anchored devices via strong anti-microbial activity and enhanced tissue regeneration via temporal synchronization between biomaterial cues and tissue responses.

Bone Matrix Non-Collagenous Proteins in Tissue Engineering: Creating New Bone by Mimicking the Extracellular Matrix

Engineering biomaterials that mimic the extracellular matrix (ECM) of bone is of significant importance since most of the outstanding properties of the bone are due to matrix constitution. Bone ECM is composed of a mineral part comprising hydroxyapatite and of an organic part of primarily collagen with the rest consisting on non-collagenous proteins. Collagen has already been described as critical for bone tissue regeneration; however, little is known about the potential effect of non-collagenous proteins on osteogenic differentiation, even though these proteins were identified some decades ago. Aiming to engineer new bone tissue, peptide-incorporated biomimetic materials have been developed, presenting improved biomaterial performance. These promising results led to ongoing research focused on incorporating non-collagenous proteins from bone matrix to enhance the properties of the scaffolds namely in what concerns cell migration, proliferation, and differentiation, with the ultimate goal of designing novel strategies that mimic the native bone ECM for bone tissue engineering applications. Overall, this review will provide an overview of the several non-collagenous proteins present in bone ECM, their functionality and their recent applications in the bone tissue (including dental) engineering field.

Decellularized Disc Hydrogels for hBMSCs tissue-specific differentiation and tissue regeneration

Tissue specificity, a key factor in the decellularized tissue matrix (DTM), has shown bioactive functionalities in tuning cell fate-e.g., the differentiation of mesenchymal stem cells. Notably, cell fate is also determined by the living microenvironment, including material composition and spatial characteristics. Herein, two neighboring tissues within intervertebral discs, the nucleus pulposus (NP) and annulus fibrosus (AF), were carefully processed into DTM hydrogels (abbreviated DNP-G and DAF-G, respectively) to determine the tissue-specific effects on stem cell fate, such as specific components and different culturing methods, as well as in vivo regeneration. Distinct differences in their protein compositions were identified by proteomic analysis. Interestingly, the fate of human bone marrow mesenchymal stem cells (hBMSCs) also responds to both culturing methods and composition. Generally, hBMSCs cultured with DNP-G (3D) differentiated into NP-like cells, while hBMSCs cultured with DAF-G (2D) underwent AF-like differentiation, indicating a close correlation with the native microenvironments of NP and AF cells, respectively. Furthermore, we found that the integrin-mediated RhoA/LATS/YAP1 signaling pathway was activated in DAF-G (2D)-induced AF-specific differentiation. Additionally, the activation of YAP1 determined the tendency of NP- or AF-specific differentiation and played opposite regulatory effects. Finally, DNP-G and DAF-G specifically promoted tissue regeneration in NP degeneration and AF defect rat models, respectively. In conclusion, DNP-G and DAF-G can specifically determine the fate of stem cells through the integrin-mediated RhoA/LATS/YAP1 signaling pathway, and this tissue specificity is both compositional and spatial, supporting the utilization of tissue-specific DTM in advanced treatments of intervertebral disc degeneration.

Probing Osteocyte Functions in Gelatin Hydrogels with Tunable Viscoelasticity

Bone is an attractive site for metastatic cancer cells and has been considered as "soil" for promoting tumor growth. However, accumulating evidence suggests that some bone cells (e.g., osteocytes) can actually suppress cancer cell migration and invasion via direct cell-cell contact and/or through cytokine secretion. Toward designing a biomimetic niche for supporting 3D osteocyte culture, we present here a gelatin-based hydrogel system with independently tunable matrix stiffness and viscoelasticity. In particular, we synthesized a bifunctional macromer, gelatin-norbornene-boronic acid (i.e., GelNB-BA), for covalent cross-linking with multifunctional thiol linkers [e.g., four-arm poly(ethylene glycol)-thiol or PEG4SH] to form thiol-NB hydrogels. The immobilized BA moieties in the hydrogel readily formed reversible boronate ester bonds with 1,3-diols on physically entrapped poly(vinyl alcohol) (PVA). Adjusting the compositions of GelNB-BA, PEG4SH, and PVA afforded hydrogels with independently tunable elasticity and viscoelasticity. With this new dynamic hydrogel platform, we investigated matrix mechanics-induced growth and cytokine secretion of encapsulated MLO-A5 pre-osteocytes. We discovered that more compliant or viscoelastic gels promoted A5 cell growth. On the other hand, cells encapsulated in stiffer gels secreted higher amounts of pro-inflammatory cytokines and chemokines. Finally, conditioned media (CM) collected from the encapsulated MLO-A5 cells (i.e., A5-CM) strongly inhibited breast cancer cell proliferation, invasion, and expression of tumor-activating genes. This new biomimetic hydrogel platform not only serves as a versatile matrix for investigating mechano-sensing in osteocytes but also provides a means to produce powerful anti-tumor CM.

Rational design of hydrogels to enhance osteogenic potential

Bone tissue engineering (BTE) encompasses the field of biomaterials, cells, and bioactive molecules to successfully guide the growth and repair of bone tissue. Current BTE strategies rely on delivering osteogenic molecules or cells via scaffolding materials. However, growth factor- and stem cell-based treatments have several limitations, such as source restriction, low stability, difficulties in predicting long-term efficacy, and high costs, among others. These issues have promoted the development of material-based therapy with properties of accessibility, high stability, tunable efficacy, and low-cost production. Hydrogels are widely used in BTE applications because of their unique hydrophilic nature and tunable physicochemical properties to mimic the native bone environment. However, current hydrogel materials are not ideal candidates due to minimal osteogenic capability on their own. Therefore, recent studies of BTE hydrogels attempt to counterbalance these issues by modifying their biophysical properties. In this article, we review recent progress in the design of hydrogels to instruct osteogenic potential, and present strategies developed to precisely control its bone healing properties.

A 3D, Dynamically Loaded Hydrogel Model of the Osteochondral Unit to Study Osteocyte Mechanobiology

Osteocytes are mechanosensitive cells that orchestrate signaling in bone and cartilage across the osteochondral unit. The mechanisms by which osteocytes regulate osteochondral homeostasis and degeneration in response to mechanical cues remain unclear. This study introduces a novel 3D hydrogel bilayer composite designed to support osteocyte differentiation and bone matrix deposition in a bone-like layer and to recapitulate key aspects of the osteochondral unit's complex loading environment. The bilayer hydrogel is fabricated with a soft cartilage-like layer overlaying a stiff bone-like layer. The bone-like layer contains a stiff 3D-printed hydrogel structure infilled with a soft, degradable, cellular hydrogel. The IDG-SW3 cells embedded within the soft hydrogel mature into osteocytes and produce a mineralized collagen matrix. Under dynamic compressive strains, near-physiological levels of strain are achieved in the bone layer (≤ 0.08%), while the cartilage layer bears the majority of the strains (>99%). Under loading, the model induces an osteocyte response, measured by prostaglandin E2, that is frequency, but not strain, dependent: a finding attributed to altered fluid flow within the composite. Overall, this new hydrogel platform provides a novel approach to study osteocyte mechanobiology in vitro in an osteochondral tissue-mimetic environment.

Control of Bone Matrix Properties by Osteocytes

Osteocytes make up 90-95% of the cellular content of bone and form a rich dendritic network with a vastly greater surface area than either osteoblasts or osteoclasts. Osteocytes are well positioned to play a role in bone homeostasis by interacting directly with the matrix; however, the ability for these cells to modify bone matrix remains incompletely understood. With techniques for examining the nano- and microstructure of bone matrix components including hydroxyapatite and type I collagen becoming more widespread, there is great potential to uncover novel roles for the osteocyte in maintaining bone quality. In this review, we begin with an overview of osteocyte biology and the lacunar-canalicular system. Next, we describe recent findings from in vitro models of osteocytes, focusing on the transitions in cellular phenotype as they mature. Finally, we describe historical and current research on matrix alteration by osteocytes in vivo, focusing on the exciting potential for osteocytes to directly form, degrade, and modify the mineral and collagen in their surrounding matrix.