GDF‐5 deficiency in mice alters the ultrastructure, mechanical properties and composition of the Achilles tendon

Abstract

Acromesomelic dysplasia of the Hunter-Thompson and Grebe types are rare human disorders based on growth/differentiation factor (GDF)-S/CDMP-l genetic mutations. Numerous skeletal abnormalities are present in these individuals, including shortened limb bones and severe dislocations of the knee. In the GDF-5 deficient brachypodisrn mouse, similar, although less severe, phenotypes are observed. It is unknown whether the joint dislocations observed in these disorders are due to a defect in the original formation of joints such as the knee, or to abnormalities in the tendons and ligaments themselves. We hypothesized that tendons from GDF-5 deficient mice would exhibit altered composition, mechanical properties, and ultrastructure when compared with heterozygous control littermates. GDF-5 deficient Achilles tendons were structurally weaker than controls, and structural strength differences appeared to be caused by compromised material properties: after normalizing by collagen per unit length, mutant tendons were still 50% weaker (P < 0.0001) and 50% more compliant ( P < 0.001) than controls. Despite comparable levels of skeletal maturity in the two cohorts, the majority of mutant tendon failures occurred in the mid-substance of the tendon (64% of all failures), whereas the majority of control failures occurred via avulsion (92% of all failures). Mutant Achilles tendons contained 40% less collagen per microgram of DNA when compared to controls ( P = 0.004). No significant difference in glycosaminoglycan (GAG)/DNA was detected. Ultrastructural analyses indicated a slight trend toward increased frequency of small diameter (30-100 nm) collagen fibrils in the mutant Achilles. Our findings suggest that increased tendon and ligament laxity may be the cause of the joint dislocations seen in patients with Hunter-Thompson and Grebe type dysplasia, rather than developmental abnormalities in the joints themselves.

Mechanical Load and BMP Signaling During Tendon Repair: A Role for Follistatin?

“…Exogenous GDFs stimulate tendon healing [1,6,7,15], and GDF-5-deficient mice have weaker tendons and slower tendon healing [2,3,11]. Tendon changes in GDF-7-deficient mice are more subtle, but show compensatory upregulation of both GDF-5 and -6 [10].…”

“…The bone morphogenetic protein (BMP) signaling system, with its ligands, antagonists, and receptors is important for bone repair and regeneration, but its role in the healing of tendon is largely unknown. Intramuscular injections of growth differentiation factor 5 (GDF-5), -6, and -7 have been reported to induce tendon-or ligament-like tissue in rats [17], and absence of GDF-5 has been reported to affect ultrastructure, composition, and biomechanical integrity of Achilles tendons in mice [11]. GDF-5 might therefore have a role in establishment and maintenance of tendon properties [11].…”

“…Intramuscular injections of growth differentiation factor 5 (GDF-5), -6, and -7 have been reported to induce tendon-or ligament-like tissue in rats [17], and absence of GDF-5 has been reported to affect ultrastructure, composition, and biomechanical integrity of Achilles tendons in mice [11]. GDF-5 might therefore have a role in establishment and maintenance of tendon properties [11]. Knockout mice lacking GDF-5 exhibit an increase of irregularly shaped Type I collagen fibrils and a time-dependent alternation in mechanical behavior [3].…”

Adhesions in a murine flexor tendon graft model: Autograft versus allograft reconstruction

“…The proximal end of the tendon was then secured between two square pieces of tape using a thin layer of cyanoacrylate as previously described. 13 The lower hind limb was fixed in a custom apparatus where the tibia was rigidly gripped to prevent rotation ( Fig. 2A).…”

“…Once the tendon was freed from the tunnel, the calcaneus was removed, freeing the proximal end of the tendon for direct gripping in the mechanical test as described by Mikic et al (2001). 13 The distal tendon and graft interfaces were not disrupted or dissected since the mechanical testing involved direct gripping of the distal bones of the foot without disrupting the graft or the branching tendon insertion into the phalanges. The specimens were placed in sterile gauze soaked with saline to maintain adequate tissue hydration.…”