We have performed a series of studies to investigate the potential of stem cell-based therapies for the treatment of tendon injuries. We first found that adipose-derived stem cells (ASCs), either alone or in combination with such tenogenic growth factors as BMP12 and Connective Tissue Growth Factor, could potentially improve tendon healing by both attenuating inflammation and promoting tendon regeneration after injury. We later discovered that extracellular vesicles produced by inflammation-primed ASCs (iEVs) have similar functions to ASCs for the treatment of tendon injury. Extracellular vesicles are nanosized vesicles released by cells. They mediate the functions of parent cells by delivering diverse active molecules from parent cells to selected target cells. Compared to ASCs, iEVs can be better delivered to the fibrous tendon tissue and exhibit higher bioavailability in the hostile injury milieu due to their small size, stable membrane structure, and the ability to selectively target tendon cells and macrophages. Moreover, the cell-free feature of iEVs eliminates concerns about the tumorigenicity of stem cells. Using a preclinical mouse tendon injury and repair model, we have shown that iEVs can inhibit macrophage inflammatory response and increase tendon cell accumulation and collagen deposition at the injury site, leading to accelerated tendon structural and functional recovery. In collaboration with Drs. Farshid Guilak, Audrey McAlinden, and Ratna B. Ray (Saint Louis University), we are investigating the active components and the mechanism of iEV action to develop component-defined and disease-selective EV drugs to treat tendon injuries effectively and safely.
Intrasynovial flexor tendons fail to mount an effective healing response after injury, which contrasts with extrasynovial flexor tendons showing much higher healing capacity. In collaboration with Drs. Richard Gelberman, Stavros Thomopoulos (Columbia University), and Shelly Sakiyama-Elbert (University of Washington), we recently discovered that flexor tendons from the intrasynovial and extrasynovial regions exhibit distinct metabolic profiles. Hypovascular intrasynovial flexor tendons enriched in glycolytic enzymes preferentially generate ATP via glycolysis, while well-vascularized extrasynovial flexor tendons possess higher levels of enzymes involved in oxidative phosphorylation (OXPHOS) and mostly use the pathway for energy production. Intrasynovial flexor tendons also express higher levels of pyruvate dehydrogenase kinase 1 (PDK1), which inhibits OXPHOS. Systemic inhibitions of PDK1 with dichloroacetate (DCA) shifted the metabolic preference of intrasynovial flexor tendons and led to increased tenogenic gene expression and neovascularization in the early phase of tendon healing. We are following up these findings and testing the hypothesis that shifting tendon metabolism from glycolysis to oxidative phosphorylation will improve healing outcomes after intrasynovial tendon injury and repair.
The tendon is a dense fibrous tissue with a small cross-sectional area and limited peritendinous space. It is difficult to deliver sufficient bioactive drugs to the cells surrounded by tendon fibers in an injury environment. Moreover, tendon healing is a slow process consisting of three overlapping phases: inflammation (one week), proliferation (several weeks), and remodeling (several months). As different treatments are needed during different phases of tendon healing, it is even more challenging to deliver selected drugs to targeted cells during a specific treatment window. Small extracellular vesicles (sEVs), besides their therapeutic potential, are attractive natural drug carriers due to their excellent biocompatibility, low immunogenicity, and the ability of intracellular delivery of diverse molecules (e.g., microRNA, mRNA, proteins, and other small molecule drugs). We previously showed that sEVs from adipose-derived stem cells (ASCs) have great tissue penetration and homing capabilities and can be taken up by residing tendon cells and infiltrating inflammatory cells at the injury site when applied to the surface of injured tendons during surgical repair. However, not all tendon injuries require surgical repair. Because many commonly injured tendons are close to the skin surface, and intradermal injection can be performed in any phase of healing, we recently administrated ASC-sEVs intradermally with hollow microneedles that create micron size pathways directly to the pain-free upper dermis region. Our results demonstrated that intradermally delivered ASC-sEVs can efficiently reach the tendon and be taken up by tendon cells. In collaboration with Dr. Srikanth Singamaneni, we are developing self-administrable ASC-sEV microneedle patches that enable non-invasive drug delivery for the treatment of tendon injury.
