The challenge of bone pathologies and the limitations of current therapies

Bone pathologies, including osteoporosis, osteoarthritis and fractures, constitute a major cause of global disability. Despite the existing therapeutic arsenal, current pharmacological treatments (anti-resorptive or anabolic agents) are hindered by poor targeting efficacy and significant systemic adverse effects during prolonged use. In clinical practice, the challenge remains to restore physiological homeostasis without disrupting other biological systems. Faced with these limitations, the exploration of new bone regeneration and osseointegration strategies becomes a clinical priority.

Extracellular vesicles as precision vectors: study objectives

This scientific review synthesises recent advances in extracellular vesicles (EVs), endogenous lipid nanoparticles capable of protecting and delivering bioactive molecules (nucleic acids, proteins) to target cells. The main objective is to compare the biological characteristics and therapeutic potential of EVs derived from three distinct sources: mammalian cells, plants, and the gut microbiota. The authors explore the hypothesis that gut microbiota-derived EVs (GM-EVs) act as fundamental mediators of the "gut-bone" axis. Capable of crossing the intestinal barrier to influence distant skeletal tissues, these GM-EVs could constitute a promising alternative to traditional probiotics for regulating bone remodelling and reducing local inflammation.

Methodology of the synthesis

This literature review compiles and synthesises recent data on the use of extracellular vesicles (EVs) as endogenous nanocarriers in the treatment of bone pathologies. The analytical framework encompasses osteoporosis, osteoarthritis, rheumatoid arthritis, fractures as well as avascular necrosis of the femoral head.

The evaluation is based on the systematic comparison of three sources of VE:

• EVs derived from mammalian cells;
• Plant-derived EVs ;
• EVs derived from the gut microbiota (GM-EVs), key vectors of the "gut-bone" axis.

The authors detail the physical properties of these particles (diameter of 30 to 1,000 nm) and their lipid bilayer structure, analysing their ability to protect bioactive substances (nucleic acids, proteins, lipids) against enzymatic degradation. The review specifically examines the three modes of functional transfer to target cells: endocytosis, direct membrane fusion and receptor-ligand interactions.

The comparative methodology evaluates the biological advantages (biocompatibility, low immunogenicity), the technical limitations and the clinical translation potential of each source for the regulation of skeletal homeostasis and bone tissue repair.

Molecular Regulation of Bone Remodelling by MEVs

This literature review highlights the dual role of mammalian cell-derived extracellular vesicles (MEVs) in skeletal homeostasis. The compiled data indicate that MEVs act as signalling vectors carrying specific nucleic acids and proteins to modulate the balance between osteoblastogenesis and osteoclastogenesis.

Modulation of Inflammation and Oxidative Stress

The authors report that EVs, particularly those derived from human umbilical cord mesenchymal stem cells (hUCMSCs), exert strict immunological control. They deliver the microRNAs miR-122-5p and miR-148a-3p, as well as the A2M and ALB proteins, to activate the PI3K-Akt pathway. This activation leads to:

• A suppression of the nuclear translocation of NF-κB.
• A polarisation of macrophages towards the anti-inflammatory M2 phenotype.
• A reduction in pro-inflammatory cytokines (IL-6, IL-1β, TNF-α).
• An increase in anti-inflammatory cytokines (IL-10, TGF-β).

Furthermore, the use of MEVs derived from ADSCs pretreated with H2O2 activates the Nrf2/HO-1 pathway, reducing the levels of reactive oxygen species (ROS) and enhancing osteogenic differentiation in diabetic bone defects.

Emergence of the Gut-Bone Axis via GM-EVs

The review highlights the potential of gut microbiota-derived extracellular vesicles (GM-EVs). These particles, 30 to 1,000 nm in diameter, possess a robust lipid bilayer protecting their bioactive content from enzymatic degradation. The included studies demonstrate that GM-EVs are capable of crossing the intestinal barrier to enter the systemic circulation, directly influencing bone metabolism at a distance.

The rise of endogenous nanocarriers in bone surgery

This review highlights a paradigm shift: the use of extracellular vesicles (EVs) as an alternative to synthetic carriers. Their lipid structure, protecting biomolecules (proteins, nucleic acids) from enzymatic degradation, offers superior biocompatibility and extremely low immunogenicity. For the practitioner, this means a prospect of more targeted therapies, capable of crossing biological barriers to act locally, thereby minimising the systemic adverse effects of current pharmacological treatments for osteoporosis or osteoarthritis.

The major innovation lies in the importance of the "gut-bone" axis mediated by gut microbiota extracellular vesicles (GM-EVs). Unlike traditional approaches such as probiotics or faecal microbiota transplantation, which sometimes face colonisation failures or systemic infectious risks, GM-EVs can cross the intestinal barrier to directly influence distant bone tissues. This capacity for inter-organ communication via the systemic circulation paves the way for metabolic modulation of the bone without the constraints of bacterial survival.

However, the clinical transition presents limitations. While EVs derived from mesenchymal stem cells (BMSCs, ADSCs) show promising results in promoting osteoblastic differentiation and fracture repair, the standardisation of their production remains a challenge. The current challenge for research is to transform these natural vectors into reliable regenerative medical devices for the practice, ensuring a functional and safe delivery of bone growth factors.

Summary of results

This review highlights the potential of extracellular vesicles (EVs) measuring 30 to 1000 nm as precision vectors. Their lipid bilayer protects bioactive cargoes (proteins, nucleic acids) from enzymatic degradation, ensuring superior stability compared to synthetic nanocarriers. The compiled data emphasise that EVs derived from mammalian cells (CSM) and the gut microbiota (GM-EVs) regulate bone homeostasis, with GM-EVs crossing the digestive barrier to act remotely on bone via the gut-bone axis.

In practical terms, for the practitioner:

• Towards "cell-free" therapies: EVs make it possible to harness the regenerative potential of stem cells (osteoblastic differentiation) while eliminating the risks of immune rejection or infectious colonisation.
• Innovation in GBR: the integration of EVs into barrier membranes or bone substitutes could optimise osseointegration by actively modulating the inflammatory microenvironment and local oxidative stress.
• Leveraging the gut-bone axis: the stability of microbiota-derived EVs paves the way for targeted oral supplementation protocols to support bone regeneration and implant longevity in at-risk patients.

Technical glossary of the study

Extracellular Vesicles (EVs): Nanometric particles (30 to 1,000 nm) enclosed by a stable lipid bilayer, secreted by prokaryotic and eukaryotic cells to mediate intercellular communication via the transport of proteins, nucleic acids and metabolites.

Gut-bone axis: Complex regulatory system integrating immune, metabolic and endocrine signals, through which the gut microbiota influences the homeostasis and metabolism of bone tissue at a distance.

GM-EVs (Gut microbiota-derived EVs): Extracellular vesicles produced by the gut microbiota, capable of crossing the intestinal barrier and entering the systemic circulation to act on bone cells.

Mesenchymal Stem Cells (MSCs): Multipotent cells (derived from bone marrow, adipose tissue or the umbilical cord) whose secreted vesicles play a key role in osteoblastic differentiation and the regulation of inflammation.

Guided Bone Regeneration (GBR/ROG): Surgical technique using barrier membranes to guide the growth of new bone, mentioned here in the context of the development of advanced adhesive tissue membranes.

Osseointegration: Process of structural and functional connection between living bone and the surface of an implant, the optimisation of which is targeted by the application of nanotechnologies and functional biomaterials cited in the study.

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