Exosomes in the Pathogenesis, Progression, and Treatment of Osteoarthritis

Abstract

Osteoarthritis (OA) is a prevalent and debilitating age-related joint disease characterized by articular cartilage degeneration, synovial membrane inflammation, osteophyte formation, as well as subchondral bone sclerosis. OA drugs at present are mainly palliative and do not halt or reverse disease progression. Currently, no disease-modifying OA drugs (DMOADs) are available and total joint arthroplasty remains a last resort. Therefore, there is an urgent need for the development of efficacious treatments for OA management. Among all novel pharmaco-therapeutical options, exosome-based therapeutic strategies are highly promising. Exosome cargoes, which include proteins, lipids, cytokines, and various RNA subtypes, are potentially capable of regulating intercellular communications and gene expression in target cells and tissues involved in OA development. With extensive research in recent years, exosomes in OA studies are no longer limited to classic, mesenchymal stem cell (MSC)-derived vesicles. New origins, structures, and functions of exosomes are constantly being discovered and investigated. This review systematically summarizes the non-classic origins, biosynthesis, and extraction of exosomes, describes modification and delivery techniques, explores their role in OA pathogenesis and progression, and discusses their therapeutic potential and hurdles to overcome in OA treatment.

Keywords: cartilage injury; chondrocyte; exosome; extracellular vesicle; osteoarthritis; regenerative medicine.

Figure 1

Tissue sources of exosomes in the knee joint. Exosomes are secreted by multiple types of cells of the joint, including adipocytes, adipose-derived stem cells (ADSCs), synovium-derived mesenchymal stem cells (MSCs), synovial fibroblasts and macrophages, chondrocytes, osteoblasts and osteocytes in the subchondral bone, vascular endothelial cells, immune cells such as T cells, B cells, and dendritic cells (DCs) meniscus cells, periodontal ligament cells, tenocytes, tendon stem cells, and bone marrow-derived MSCs. These exosomes are involved in the regulation of joint homeostasis, cell–cell communications, and the initiation and progression of OA.

Figure 2

Scheme of the biogenesis of exosomes. Endocytosis and plasma membrane invagination facilitate the entry of cell surface proteins and extracellular components such as lipids, proteins, metabolites, ions, and small molecules into cells, leading to the formation of early sorting exosomes (ESEs). The ESEs then fuse with the endoplasmic reticulum (ER) and/or trans-Golgi network (TGN) and result in late sorting exosome (LSE) formation. A second invagination in the LSEs leads to the generation of multivesicular bodies (MVBs). MVBs can then either fuse with lysosomes for degradation or be transported to the plasma membrane and undergo exocytosis—a process resulting in exosome release. Exosomes, filled with various cellular components such as proteins, mRNAs, miRNAs, lipids, enzymes, and carbohydrates, are released through exocytosis after MVBs fuse with the cell membrane. Released exosomes can be further taken up by adjacent or remote cells in various ways, including receptor-mediated endocytosis and fusion with the plasma membrane of cells.

Figure 3

Cargo loading, isolation, and delivery strategies for engineered exosomes. Bioactive molecules, such as nucleic acids, vectors, plasmids, drugs, ions, and other compounds were added in the cell culture medium. Exogenous cargo can be loaded into exosomes by several methods, such as electroporation, lipofection, sonication, and CaCl₂ treatment. Cells loaded with exogenous cargo secreted exosomes containing these bioactive molecules into cell culture medium. Cells expressing target peptides by plasmid transfection produce exosomes that can target specific cell populations. These engineered exosomes were isolated and purified from the culture medium via different methods. Through co-incubation or other strategies, exosomes loaded with endogenous and/or exogenous cargo can be taken up by recipient cells for the regulation of gene expression and cell function.

Figure 4

Schematic of fabricating AD/CS/RSF/EXO hydrogels for cartilage defect repair in a rat OA model. BMSCs were aseptically isolated from the bilateral femur marrow cavities of male Sprague-Dawley (SD) rats. When the cells reached 50–60% confluency in 2D culture flasks, they were rinsed and incubated for 48 h in serum-free medium. The collected conditioned medium was ultracentrifuged and ultrafiltered to obtain exosomes. The exosomes were mixed with AD/CS/RSF pre-gel solution, and then H₂O₂ and HRP were added to induce gelation. Subsequently, the cartilage defect was filled with the exosome-containing adhesive hydrogel. The exosomes released by the hydrogels recruited BMSCs that migrated and infiltrated the hydrogel and promoted BMSC proliferation and differentiation into chondrocytes. By inducing ECM production and neo-cartilage formation, the hydrogel facilitated the regeneration of cartilage defect in situ.