Influence of nonenzymatic glycation on biomechanical properties of cortical bone.

In this study, the influence of nonenzymatic glycation (NEG) on the mechanical properties of bone and bone collagen were investigated. Bovine cortical bone specimens were incubated in ribose to cause collagen cross-links in vitro, and nondestructive mechanical testing was used to determine tensile and compressive elastic modulus as a function of incubation time. Mechanical properties associated with yield, postyield, and final fracture of bone were determined at the end of the incubation period. The stiffness of the collagen network was measured using stress relaxation tests of demineralized bone cylinders extracted periodically throughout the incubation period. It was found that accumulation of nonenzymatic glycation end-products in cortical bone caused stiffening of the type I collagen network in bone (r2 = 0.92; p < 0.001) but did not significantly affect the overall stiffness of the mineralized bone (p = 0.98). The ribosylated group had significantly more NEG products and higher yield stress and strain than the control group (p < 0.05). Postyield properties including postyield strain and strain energy were lower in the ribosylated group but were not significantly different from the control group (p = 0.24). Compared with the control group, the ribosylated group was characterized by significantly higher secant modulus and lower damage fraction (p < 0.05). Taken together, the results of this study suggest that collagen in bone is susceptible to the same NEG-mediated changes as collagen in other connective tissues and that an increased stiffness of the collagen network in bone due to NEG may explain some of the age-related increase in skeletal fragility and fracture risk.

Calpain mediates eukaryotic initiation factor 4G degradation during global brain ischemia.

Global brain ischemia and reperfusion result in the degradation of the eukaryotic initiation factor (eIF) 4G, which plays a critical role in the attachment of the mRNA to the ribosome. Because eIF-4G is a substrate of calpain, these studies were undertaken to examine whether calpain I activation during global brain ischemia contributes to the degradation of eIF-4G in vivo. Immunoblots with antibodies against calpain I and eIF-4G were prepared from rat brain postmitochondrial supernatant incubated at 37 degrees C with and without the addition of calcium and the calpain inhibitors calpastatin or MDL-28,170. Addition of calcium alone resulted in calpain I activation (as measured by autolysis of the 80-kDa subunit) and degradation of eIF-4G; this effect was blocked by either 1 micromol/L calpastatin or 10 micromol/L MDL-28,170. In rabbits subjected to 20 minutes of cardiac arrest, immunoblots of brain postmitochondrial supernatants showed that the percentage of autolyzed calpain I increased from 1.9% +/- 1.1% to 15.8% +/- 5.0% and that this was accompanied by a 68% loss of eIF-4G. MDL-28,170 pretreatment (30 mg/kg) decreased ischemia-induced calpain I autolysis 40% and almost completely blocked eIF-4G degradation. We conclude that calpain I degrades eIF-4G during global brain ischemia.