The influence of fibrillin-1 and physical activity upon tendon tissue morphology and mechanical properties in mice

Fibrillin-1 mutations cause pathological changes in connective tissue that constitute the complex phenotype of Marfan syndrome. In this study, we used fibrillin-1 hypomorphic and haploinsufficient mice (Fbn1^mgr/mgR and Fbn1^+/- mice, respectively) to investigate the impact of fibrillin-1 deficiency alone or in combination with regular physical activity on tendon tissue morphology and mechanical properties. Morphological and biomechanical analyses revealed that Fbn1^mgr/mgR but not Fbn1^+/- mice displayed smaller tendons with physical properties that were unremarkable when normalized to tendon size. Fbn1^mgR/mgR mice (n = 43) Fbn1^+/- mice (n = 27) and wild-type mice (WT, n = 25) were randomly assigned to either control cage conditions (n = 54) or to a running on a running wheel for 4 weeks (n = 41). Both fibrillin-1-deficient mice ran voluntarily on the running wheel in a manner similar to WT mice (3-4 km/24 h). Regular exercise did not mitigate aneurysm progression in Fbn1^mgR/mgR mice (P < 0.05) as evidenced by unmodified median survival. In spite of the smaller size, tendons of fibrillin-1-deficient mice subjected to regular exercise showed no evidence of overt histopathological changes or tissue overload. We therefore concluded that lack of optimal fibrillin-1 synthesis leads to a down regulation of integrated tendon formation, rather than to a loss of tendon quality, which also implies that fibrillin-1 deficiency in combination with exercise is not a suitable animal model for studying the development of tendon overuse (tendinopathy).