Tendon-dependent Control of Longitudinal Bone Growth

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 DESCRIPTION (provided by applicant): A disproportionate increase of longitudinal bone growth that causes serious malformations of the limbs, anterior chest and spine is the clinical hallmark of patients afflicted with Marfan syndrome (MFS), a connective tissue disease caused by mutations in the extracellular matrix (ECM) protein and TGFβ regulator fibrillin-1. Our preliminary studies of mice with tissue-specific ablated Fbn1 gene activity have revealed an unsuspected causal relationship between tendon/ligament (T/L) dysfunction and longitudinal bone overgrowth (LBO). Specifically, we found that (1) Fbn1 inactivation in T/L cells was necessary and sufficient to promote linear bone overgrowth associated with dysregulated growth plate (GP) gene expression; (2) fibrillin-1-deficient tendons displayed abnormal tissue architecture and impaired mechanical properties, particularly at bone- insertion sites; (3) the relative amount of fibrillin-1 correlated with discrete changes in tendon mechanics; (4) tendon-derived stem/progenitor cell (TSPC) cultures deficient for fibrillin-1 differentiated improperly as result of increased latent TGFβ activation; and (5) ectopic tendon calcification of fibrillin-1-deficient tendons was commonly observed. We therefore hypothesize that fibrillin-1 assemblies normally restrict GP-driven linear growth of neighboring bones by specifying the mechanical properties of tendons through the control of ECM organization and TGF-regulated TSPC differentiation. Accordingly, the scope of our proposal is two-fold; first, to characterize how fibrillin-1 deficiency translates into tendon dysfunction and tendon-associated LBO, and second, to establish how local TGF hyperactivity in tendons promote tissue degeneration thereby leading to excessive linear growth of the adjacent, structurally normal bones. To this end, we will characterize the expression of molecular and cellular determinants of tendon development and maturation in mice deficient for fibrillin-1 in T/L matrices, in addition to employing computational approaches to identify probable disease-causing molecular abnormalities in the GP of these tendon-defective animals (Aim 1); apply data-driven statistical models to determine how graded fibrillin-1 deficiencies correlate with tendon mechanics and associated LBO (Aim 2); and assess whether systemic TGFβ neutralization modifies tendon pathology and LBO severity in fibrillin-1-deficient mice (Aim 3). The results of these investigations are expected to substantially advance our limited understanding of tendon function in health and disease and implicitly, of the cellular, molecular and tissue factors that coordinate the postnatal growth of musculoskeletal tissues.