One of the ligaments most often damaged during sports-the anterior cruciate ligament (ACL)-has poor healing capacity. On damage, reconstructive surgery is performed to restore the mechanical stability of the knee and to reduce the inflammatory milieu otherwise present in the joint. A return to normal activities, however, takes between 9 and 12 months. Thus, strategies capable of improving ACL graft healing are needed. Embryonic development of tendon and ligament (T/L) is regulated by a crosstalk between different cell types. We hypothesized that terminally differentiated skeletal-derived cells such as osteoblasts, chondrocytes, and myoblasts modulate T/L healing. Using an indirect coculture system, we discovered that myoblast-secreted signals-but not osteoblasts, chondrocytes, or stromal-secreted signals-are capable of upregulating classical T/L markers such as scleraxis and tenomodulin on human hamstring tendon-derived cells (hTC), which contribute to ACL graft healing. Transcriptome analysis showed that coculturing hTC with myoblasts led to an upregulation of extracellular matrix (ECM) genes and resulted in enhanced ECM deposition. In vivo, using a rat model of ACL reconstruction showed that conditioned media derived from human muscle tissue accelerated femoral tunnel closure, a key step for autograft integration. Collectively, these results indicate that muscle-secreted signals can be used to improve ACL graft healing in a clinical setting where muscle remnants are often discarded.
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