Growth and Mechanobiology of the Achilles Enthesis in Mice

Entheses are connective tissues that connect tendon to bone, two vastly different hierarchical materials with different structural and mechanical properties. As a result of this material mismatch, entheses are prone to local peaks in mechanical stress (stress concentrations) that increase their susceptibility to overuse injuries, especially during rapid growth (postnatal maturation) and in young athletes. The enthesis matures postnatally in a mechanoadaptive process, similar to the growing bone, and forms a graded transition to dampen the stress concentrations at the attachment site. Despite decades of research, the key biological and mechanoadaptive processes that govern the adaptation of the enthesis under repeated loading and onset of injury during its postnatal maturation remain unknown. The objective of this research was to investigate the role of mechanical and biological cues on the mechanoadaptation of growing and adult entheses. I did that through four main aims: (1) developing and confirming the feasibility and

repeatability of a novel non-invasive in vivo model for repeated loading of the tendon and enthesis, using optogenetics; (2) investigating the age-dependent mechanically-induced structural and functional adaptation of the enthesis during growth and adulthood; (3) exploring the structural and functional relationships and possible mechanisms of damage (i.e., disrupted interdigitation vs. collagen denaturation) in disruption of the toughening mechanism of the maturing enthesis; and (4) identifying FGFs signaling, a known mediator of bone growth, as a critical regulator of the

structural gradation, and therefore, mechanical properties of maturing entheses. This study is innovative in taking an interdisciplinary approach to put forth a novel model for skeletal adaptation to loading during growth, elucidating the structural adaptation of maturing and adult enthesis under repeated loading, and proposing new biological pathways involved in the mechanoadaptation of the maturing enthesis. The results and tools developed in this work can be used to investigate the adaptation of the enthesis by investigating the mechanobiology of enthesis formation, with the long term

goal of improving the diagnosis and treatment of overuse injuries in maturing attachment.