Biomechanical characterization of tibial articular cartilage in Hyp mice, a murine model of osteoarthritis in X-linked hypophosphatemia

The clinical picture of adult XLH is dominated by a pervasive, early-onset osteoarthritis (OA). Using a murine XLH model, we have shown that the femoral/tibial joint of Hyp mice lacks the mineralized zone of articular cartilage, a transitional tissue proposed to reduce mechanical stress at the interface between cartilage and subchondral bone. Although there is a significant compensatory increase of sulfated proteoglycans (PG), the cartilages ultimately undergo an age-dependent thinning. To better understand the etiology of OA in XLH, we compared the structural stiffness (N/mm) of articular cartilage of wildtype (WT) and Hyp mice by measuring stress relaxation under time-dependent loading conditions of the tibial plateau. Because articular cartilage has variable viscoelastic material properties, indentation mapping was performed over the entire surface, including the area contacted by the meniscus medial and lateral components. Multiaxial automated indentation was performed using a Mach-1 mechanical tester (model v500css) to ensure perpendicular force displacement (indentation amplitude: 0.03 mm, indentation velocity: 0.03 mm/s, relaxation time: 20 s). The average structural stiffness of WT was 3.11±0.69 N/mm (n=3) and was significantly higher than Hyp mice, 0.60±0.11 N/mm (n=3), p < 0.02. The WT tibial plateau regions are stiffest in the regions contacted by the medial and lateral component of the meniscus. However, the topographical pattern of stiffness in Hyp mice was inverted with the central cartilage regions being stiffest. Because joint immobilization reduces PG expression, we conducted experiments using botulinum toxin (2U/100g BW) to evaluate the impact on structural stiffness. Preliminary data show that structural stiffness decreases further in both Hyp and WT, relative to contralateral vehicle-treated joints. In summary, the Hyp mouse represents a unique opportunity to better understand the role of the mineralized zone and PGs to withstand compressive loading forces. Despite an increase in PG expression, the mechanical properties of cartilage in Hyp mice are significantly diminished compared to controls. We conclude that the mineralized zone of cartilage significantly defines the biomechanical properties of the joint, which we have previously demonstrated restoration of using standard therapy. These data underscore the critical importance of therapies that maintain phosphate homeostasis to prohibit the progression of OA in adult XLH.