Objective Age is an important risk factor for osteoarthritis (OA). During aging, nonenzymatic glycation results in the accumulation of advanced glycation end products (AGEs) in cartilage collagen. We studied the effect of AGE crosslinking on the stiffness of the collagen network in human articular cartilage. Methods To increase AGE levels, human adult articular cartilage was incubated with threose. The stiffness of the collagen network was measured as the instantaneous deformation (ID) of the cartilage and as the change in tensile stress in the collagen network as a function of hydration (osmotic stress technique). AGE levels in the collagen network were determined as: N^ε‐(carboxy[m]ethyl)lysine, pentosidine, amino acid modification (loss of arginine and [hydroxy‐]lysine), AGE fluorescence (360/460 nm), and digestibility by bacterial collagenase. Results Incubation of cartilage with threose resulted in a dose‐dependent increase in AGEs and a concomitant decrease in ID (r = −0.81, P < 0.001; up to a 40% decrease at 200 mM threose), i.e., increased stiffness, which was confirmed by results from the osmotic stress technique. The decreased ID strongly correlated with AGE levels (e.g., AGE fluorescence r = −0.81, P < 0.0001). Coincubation with arginine or lysine (glycation inhibitors) attenuated the threose‐induced decrease in ID (P < 0.05). Conclusion Increasing cartilage AGE crosslinking by in vitro incubation with threose resulted in increased stiffness of the collagen network. Increased stiffness by AGE crosslinking may contribute to the age‐related failure of the collagen network in human articular cartilage to resist damage. Thus, the age‐related accumulation of AGE crosslinks presents a putative molecular mechanism whereby age is a predisposing factor for the development of OA.

Cells in the musculoskeletal system can respond to mechanical stimuli, supporting tissue homeostasis and remodeling. Recent studies have suggested that mechanical stimulation also influences the differentiation of MSCs, whereas the effect on embryonic cells is still largely unknown. In this study, we evaluated the influence of dynamic mechanical compression on chondrogenesis of bone marrow-derived MSCs and embryonic stem cell-derived (human embryoid body-derived [hEBd]) cells encapsulated in hydrogels and cultured with or without transforming growth factor β-1 (TGF-β1). Cells were cultured in hydrogels for up to 3 weeks and exposed daily to compression for 1, 2, 2.5, and 4 hours in a bioreactor. When MSCs were cultured, mechanical stimulation quantitatively increased gene expression of cartilage-related markers, Sox-9, type II collagen, and aggrecan independently from the presence of TGF-β1. Extracellular matrix secretion into the hydrogels was also enhanced. When hEBd cells were cultured without TGF-β1, mechanical compression inhibited their differentiation as determined by significant downregulation of cartilage-specific genes. However, after initiation of chondrogenic differentiation by administration of TGF-β1, the hEBd cells quantitatively increased expression of cartilage-specific genes when exposed to mechanical compression, similar to the bone marrow-derived MSCs. Therefore, when appropriately directed into the chondrogenic lineage, mechanical stimulation is beneficial for further differentiation of stem cell tissue engineered constructs. Disclosure of potential conflicts of interest is found at the end of this article.

Objective To provide evidence for the hypothesis that the loss of tensile strength of osteoarthritic (OA) cartilage (resulting in swelling—the hallmark of OA) is due to an impaired collagen network and not to loss or degradation of proteoglycans. Methods The amount of degraded collagen molecules, the fixed charge density (FCD) on a dry‐weight basis, the degree of swelling in saline, and the instantaneous deformation (ID; a test reflecting the tensile stiffness of the collagen network) were measured in full‐depth OA femoral condyle samples. In addition, levels of the crosslink hydroxylysylpyridinoline (HP), the amount of degraded collagen molecules, and the degree of swelling were determined in the 3 zones (surface, middle, and deep) of OA cartilage. We also compared the ID of normal and OA cartilage. Results In full‐depth OA cartilage, a close relationship was found between swelling and ID. Swelling and ID correlated strongly with the amount of degraded collagen molecules, and were not related to FCD. OA cartilage showed the same zonal pattern in HP levels as normal cartilage (i.e., an increase with depth). No relationship was found between collagen crosslinking and swelling of the surface, middle, and deep zones. In all 3 zones, swelling was proportional to the amount of degraded collagen molecules. Compared with that of normal cartilage, the change in ID of OA cartilage was most pronounced at the surface in a direction parallel to the direction of the collagen fibrils. Conclusion The decreased stiffness of the OA collagen network (as measured by swelling and ID) is strongly related to the amount of degraded collagen molecules. The anisotropy in ID parallel and perpendicular to the direction of the fibrils revealed that the impairment of strength resides mainly in, and not between, the fibrils. Proteoglycans play only a minor role in the degeneration of the tensile stiffness of OA cartilage.