Plasma Membrane Receptors Involved in the Binding and Response of Osteoclasts to Noncellular Components of the Bone

Bioactive Self-Regulated Liquified Microcompartments to Bioengineer Bone-Like Microtissues

Designing a microenvironment that drives autonomous stromal cell differentiation toward osteogenesis while recapitulating the complexity of bone tissue remains challenging. In the current study, bone-like microtissues are created using electrohydrodynamic atomization to form two distinct liquefied microcapsules (mCAPs): i) hydroxypyridinone (HOPO)-modified gelatin (GH mCAPs, 7.5% w/v), and ii) HOPO-modified gelatin and dopamine-modified gelatin (GH+GD mCAPs, 7.5%+1.5% w/v). The ability of HOPO to coordinate with iron ions at physiological pH allows the formation of a semipermeable micro-hydrogel shell. In turn, the dopamine affinity for calcium ions sets a bioactive milieu for bone-like microtissues. After 21 days post encapsulation, GH and GH+GD mCAPs potentiate autonomous osteogenic differentiation of mesenchymal stem cells accompanied by collagen type-I gene upregulation, increased alkaline phosphatase (ALP) expression, and formation of mineralized extracellular matrix. However, the GH+GD mCAPs show higher levels of osteogenic markers starting on day 14, translating into a more advanced and organized mineralized matrix. The GH+GD system also shows upregulation of the receptor activator of nuclear factor kappa-B ligand (RANK-L) gene, enabling the autonomous osteoclastic differentiation of monocytes. These catechol-based mCAPs offer a promising approach to designing multifunctional and autonomous bone-like microtissues to study in vitro bone-related processes at the cell-tissue interface, angiogenesis, and osteoclastogenesis.

Metabolomic signatures distinguish extracellular vesicles from osteoclasts and odontoclasts

AIMS

Pathological dental root resorption and alveolar bone loss are often detected only after irreversible damage. Biomarkers in the gingival crevicular fluid or saliva could provide a means for early detection; however, such biomarkers have proven elusive. We hypothesize that a multiomic approach might yield reliable diagnostic signatures for root resorption and alveolar bone loss. Previously, we showed that extracellular vesicles (EVs) from osteoclasts and odontoclasts differ in their protein composition. In this study, we investigated the metabolome of EVs from osteoclasts, odontoclasts and clasts (non-resorbing clastic cells).

MATERIALS AND METHODS

Mouse haematopoietic precursors were cultured on dentine, bone or plastic, in the presence of recombinant RANKL and CSF-1 to trigger differentiation along the clastic line. On Day 7, the cells were fixed and the differentiation state and resorptive status of the clastic cells were confirmed. EVs were isolated from the conditioned media on Day 7 and characterized by nanoparticle tracking and electron microscopy to ensure quality. Global metabolomic profiling was performed using a Thermo Q-Exactive Orbitrap mass spectrometer with a Dionex UHPLC and autosampler.

RESULTS

We identified 978 metabolites in clastic EVs. Of those, 79 are potential biomarkers with Variable Interdependent Parameters scores of 2 or greater. Known metabolites cytidine, isocytosine, thymine, succinate and citrulline were found at statistically higher levels in EVs from odontoclasts compared with osteoclasts.

CONCLUSION

We conclude that numerous metabolites found in odontoclast EVs differ from those in osteoclast EVs, and thus represent potential biomarkers for root resorption and periodontal tissue destruction.

Physiological functions of podosomes: From structure and function to therapy implications in osteoclast biology of bone resorption

With increasing age, bone tissue undergoes significant alterations in composition, architecture, and metabolic functions, probably causing senile osteoporosis. Osteoporosis possess the vast majority of bone disease and associates with a reduction in bone mass and increased fracture risk. Bone loss is on account of the disorder in osteoblast-induced bone formation and osteoclast-induced bone resorption. As a unique bone resorptive cell type, mature bone-resorbing osteoclasts exhibit dynamic actin-based cytoskeletal structures called podosomes that participate in cell-matrix adhesions specialized in the degradation of mineralized bone matrix. Podosomes share many of the same molecular constitutions as focal adhesions, but they have a unique structural organization, with a central core abundant in F-actin and encircled by scaffolding proteins, kinases and integrins. Here, we conclude recent advancements in our knowledge of the architecture and the functions of podosomes. We also discuss the regulatory pathways in osteoclast podosomes, providing a reference for future research on the podosomes of osteoclasts and considering podosomes as a therapeutic target for inhibiting bone resorption.

Multimodal regulation of the osteoclastogenesis process by secreted group IIA phospholipase A2

Increasing evidence points to the involvement of group IIA secreted phospholipase A₂ (sPLA₂-IIA) in pathologies characterized by abnormal osteoclast bone-resorption activity. Here, the role of this moonlighting protein has been deepened in the osteoclastogenesis process driven by the RANKL cytokine in RAW264.7 macrophages and bone-marrow derived precursor cells from BALB/cJ mice. Inhibitors with distinct selectivity toward sPLA₂-IIA activities and recombinant sPLA₂-IIA (wild-type or catalytically inactive forms, full-length or partial protein sequences) were instrumental to dissect out sPLA₂-IIA function, in conjunction with reduction of sPLA₂-IIA expression using small-interfering-RNAs and precursor cells from Pla2g2a knock-out mice. The reported data indicate sPLA₂-IIA participation in murine osteoclast maturation, control of syncytium formation and resorbing activity, by mechanisms that may be both catalytically dependent and independent. Of note, these studies provide a more complete understanding of the still enigmatic osteoclast multinucleation process, a crucial step for bone-resorbing activity, uncovering the role of sPLA₂-IIA interaction with a still unidentified receptor to regulate osteoclast fusion through p38 SAPK activation. This could pave the way for the design of specific inhibitors of sPLA₂-IIA binding to interacting partners implicated in osteoclast syncytium formation.