Changes in the Fracture Resistance of Bone with the Progression of Type 2 Diabetes in the ZDSD Rat

Abstract

Individuals with type 2 diabetes (T2D) have a higher fracture risk compared to non-diabetics, even though their areal bone mineral density is normal to high. Identifying the mechanisms whereby diabetes lower fracture resistance requires well-characterized rodent models of diabetic bone disease. Toward that end, we hypothesized that the bone toughness, more so than bone strength, decreases with the duration of diabetes in ZDSD rats. Bones were harvested from male CD(SD) control rats and male ZDSD rats at 16-wks (before the onset of hyperglycemia), at 22-wks (5–6 wks of hyperglycemia), and at 29-wks (12–13 wks of hyperglycemia). There were at least 12 rats per strain per age group. At 16-wks, there was no difference in either body weight or glucose levels between the 2 rat groups. Within 2 weeks of switching all rats to a diet with 48% of kcal from fat, only the ZDSD rats developed hyperglycemia (>250 mg/dl). They also began to lose body weight at 21-wks. CD(SD) rats remained normoglycemic (<110 mg/dl) on the high fat diet and became obese (>600 g). From micro-computed tomography (µCT) analysis of a lumbar vertebra and distal femur, trabecular bone volume did not vary with age among the non-diabetic rats but was lower at 29-wks than at 16-wks or at 22-wks for the diabetic rats. Consistent with that finding, µCT-derived intra-cortical porosity (femur diaphysis) was higher for ZDSD following ~12 wks of hyperglycemia than for age-matched CD(SD) rats. Despite an age-related increase in mineralization in both rat strains (µCT and Raman spectroscopy), material strength of cortical bone (from three-point bending testing) increased with age only in the non-diabetic CD(SD) rats. Moreover, two other material properties, toughness (radius) and fracture toughness (femur), significantly decreased with the duration of T2D in ZDSD rats. This was accompanied by the increase in the levels of the pentosidine (femur). However, pentosidine was not significantly higher in diabetic than in non-diabetic bone at any time point. The ZDSD rat, which has normal leptin signaling and becomes diabetic after skeletal maturity, provides a pre-clinical model of diabetic bone disease, but a decrease in body weight during prolonged diabetes and certain strain-related differences before the onset of hyperglycemia should be taken into consideration when interpreting diabetes-related differences.

Diabetes pharmacotherapy and effects on the musculoskeletal system

“…In diabetes, where glucose levels are elevated, AGEs are deposited in many tissues, including bone . AGEs are associated with increased bone fragility in diabetes; thus, by lowering glucose levels, insulin may help to decrease accumulation of AGEs in bone . Nevertheless, in vivo evidence also suggests that insulin has a direct effect on bone formation and osteogenesis, irrespective of its effect on glucose metabolism.…”

“…74 AGEs are associated with increased bone fragility in diabetes; thus, by lowering glucose levels, insulin may help to decrease accumulation of AGEs in bone. 75,76 Nevertheless, in vivo evidence also suggests that insulin has a direct effect on bone formation and osteogenesis, irrespective of its effect on glucose metabolism. For instance, several studies in mice have shown that impaired insulin signaling, via knock-down of the insulin receptor specifically in osteoblast progenitors and mature osteoblasts, results in impaired bone formation, abnormal trabecular architecture, smaller cortices, and increased fragility, dependent on the developmental stage in which the insulin receptor was eliminated and in a sex-specific manner.…”

Contributions of Material Properties and Structure to Increased Bone Fragility for a Given Bone Mass in the UCD-T2DM Rat Model of Type 2 Diabetes

“…AGEs can also inhibit the phenotypic expression of osteoblasts, interfere with osteoclast differentiation, and stimulate secretion of catabolic and proinflammatory factors . Accordingly, increased AGE accumulation with diabetes may underlie the impairments in cortical geometry and trabecular microarchitecture that have been reported in rodent models . In humans, cortical porosity is significantly higher at the distal radius and distal tibia in some cohorts with T2D, including cohorts with a fracture .…”

Altered Tissue Composition, Microarchitecture, and Mechanical Performance in Cancellous Bone From Men With Type 2 Diabetes Mellitus

“…There are reports of greater concentrations of tissue AGEs (35) and greater mean calcium content in bone from people with T2DM compared with non-DM controls (36) ; however, rodent models of T2DM provide the bulk of evidence that T2DM alters tissue material properties. Multiple rodent models of T2DM report increased mineral content (assessed by mineral:matrix ratio) in Zucker diabetic Sprague-Dawley (ZDSD) rats (37,38) and KK-Ay mice compared with controls. (39) The same studies report no differences in concentrations of mature enzymatic crosslinks (pyridinoline and deoxypyridinoline) or in the AGE pentosidine (37)(38)(39) ; however, the collagen maturity (ratio of mature to immature enzymatic crosslinks) of the KK-Ay mice was increased compared with controls.…”

“…Multiple rodent models of T2DM report increased mineral content (assessed by mineral:matrix ratio) in Zucker diabetic Sprague-Dawley (ZDSD) rats (37,38) and KK-Ay mice compared with controls. (39) The same studies report no differences in concentrations of mature enzymatic crosslinks (pyridinoline and deoxypyridinoline) or in the AGE pentosidine (37)(38)(39) ; however, the collagen maturity (ratio of mature to immature enzymatic crosslinks) of the KK-Ay mice was increased compared with controls. (39) In contrast, a study of WBN/Kob rats found a decrease in concentration of enzymatic crosslinks with a simultaneous increase in pentosidine concentration, (19) which suggests phenotype-specific changes in tissue material properties with T2DM.…”