TLR4 Promotes and DAP12 Limits Obesity-Induced Osteoarthritis in Aged Female Mice

Aging and female sex are the strongest risk factors for nontraumatic osteoarthritis (OA); whereas obesity is a modifiable risk factor accelerating OA. Prior studies indicate that the innate immune receptor toll-like receptor 4 (TLR4) mediates obesity-induced metabolic inflammation and cartilage catabolism via recognition of damage-associated molecular patterns and is increased with aging in OA joints. TLR4 responses are limited by innate immunoreceptor adapter protein DNAX-activating protein of 12kDA (DAP12). We undertook this study to test the hypothesis that TLR4 promotes, whereas DAP12 limits, obesity-accelerated OA in aged female mice. We fed 13- to 15-month-old female WT, TLR4 KO, and DAP12 KO mice a high-fat diet (HFD) or a control diet for 12 weeks, and changes in body composition, glucose tolerance, serum cytokines, and insulin levels were compared. Knee OA was evaluated by histopathology and μCT. Infrapatellar fat pads (IFPs) were analyzed by histomorphometry and F4/80+ crown-like structures were quantified. IFPs and synovium gene expression were analyzed using a targeted insulin resistance and inflammation array. All HFD-treated mice became obese, but only WT and TLR4 KO mice developed glucose intolerance. HFD induced cartilage catabolism in WT and DAP12 KO female mice, but not in TLR4 KO mice. Gene-expression analysis of IFPs and synovium showed significant differences in insulin signaling, adipokines, and inflammation between genotypes and diets. Unlike young mice, systemic inflammation was not induced by HFD in the older female mice independent of genotype. Our findings support the conclusion that TLR4 promotes and DAP12 limits HFD-induced cartilage catabolism in middle-aged female mice.

Differential effects of IGF-1 deficiency during the life span on structural and biomechanical properties in the tibia of aged mice

Advanced aging is associated with the loss of structural and biomechanical properties in bones, which increases the risk for bone fracture. Aging is also associated with reductions in circulating levels of the anabolic signaling hormone, insulin-like growth factor (IGF)-1. While the role of IGF-1 in bone development has been well characterized, the impact of the age-related loss of IGF-1 on bone aging remains controversial. Here, we describe the effects of reducing IGF-1 at multiple time points in the mouse life span--early in postnatal development, early adulthood, or late adulthood on tibia bone aging in both male and female igf (f/f) mice. Bone structure was analyzed at 27 months of age using microCT. We find that age-related reductions in cortical bone fraction, cortical thickness, and tissue mineral density were more pronounced when IGF-1 was reduced early in life and not in late adulthood. Three-point bone bending assays revealed that IGF-1 deficiency early in life resulted in reduced maximum force, maximum bending moment, and bone stiffness in aged males and females. The effects of IGF-1 on bone aging are microenvironment specific, as early-life loss of IGF-1 resulted in decreased cortical bone structure and strength along the diaphysis while significantly increasing trabecular bone fraction and trabecular number at the proximal metaphysis. The increases in trabecular bone were limited to males, as early-life loss of IGF-1 did not alter bone fraction or number in females. Together, our data suggest that the age-related loss of IGF-1 influences tibia bone aging in a sex-specific, microenvironment-specific, and time-dependent manner.

IGF-1 Regulates Vertebral Bone Aging Through Sex-Specific and Time-Dependent Mechanisms

Advanced aging is associated with increased risk of bone fracture, especially within the vertebrae, which exhibit significant reductions in trabecular bone structure. Aging is also associated with a reduction in circulating levels of insulin-like growth factor (IGF-1). Studies have suggested that the reduction in IGF-1 compromises healthspan, whereas others report that loss of IGF-1 is beneficial because it increases healthspan and lifespan. To date, the effect of decreases in circulating IGF-1 on vertebral bone aging has not been thoroughly investigated. Here, we delineate the consequences of a loss of circulating IGF-1 on vertebral bone aging in male and female Igf(f/f) mice. IGF-1 was reduced at multiple specific time points during the mouse lifespan: early in postnatal development (crossing albumin-cyclic recombinase [Cre] mice with Igf(f/f) mice); and in early adulthood and in late adulthood using hepatic-specific viral vectors (AAV8-TBG-Cre). Vertebrae bone structure was analyzed at 27 months of age using micro-computed tomography (μCT) and quantitative bone histomorphometry. Consistent with previous studies, both male and female mice exhibited age-related reductions in vertebral bone structure. In male mice, reduction of circulating IGF-1 induced at any age did not diminish vertebral bone loss. Interestingly, early-life loss of IGF-1 in females resulted in a 67% increase in vertebral bone volume fraction, as well as increased connectivity density and increased trabecular number. The maintenance of bone structure in the early-life IGF-1-deficient females was associated with increased osteoblast surface and an increased ratio of osteoprotegerin/receptor-activator of NF-κB-ligand (RANKL) levels in circulation. Within 3 months of a loss of IGF-1, there was a 2.2-fold increase in insulin receptor expression within the vertebral bones of our female mice, suggesting that local signaling may compensate for the loss of circulating IGF-1. Together, these data suggest the age-related loss of vertebral bone density in females can be reduced by modifying circulating IGF-1 levels early in life.

Detrimental effects of reloading recovery on force, shortening velocity, and power of soleus muscles from hindlimb-unloaded rats

To better understand how atrophied muscles recover from prolonged nonweight-bearing, we studied soleus muscles (in vitro at optimal length) from female rats subjected to normal weight bearing (WB), 15 days of hindlimb unloading (HU), or 15 days HU followed by 9 days of weight bearing reloading (HU-R). HU reduced peak tetanic force (P(o)), increased maximal shortening velocity (V(max)), and lowered peak power/muscle volume. Nine days of reloading failed to improve P(o), while depressing V(max) and intrinsic power below WB levels. These functional changes appeared intracellular in origin as HU-induced reductions in soleus mass, fiber cross-sectional area, and physiological cross-sectional area were partially or completely restored by reloading. We calculated that HU-induced reductions in soleus fiber length were of sufficient magnitude to overextend sarcomeres onto the descending limb of their length-tension relationship upon the resumption of WB activity. In conclusion, the force, shortening velocity, and power deficits observed after 9 days of reloading are consistent with contraction-induced damage to the soleus. HU-induced reductions in fiber length indicate that sarcomere hyperextension upon the resumption of weight-bearing activity may be an important mechanism underlying this response.

Blood lactate responses to leg presses performed against inertial resistance

INTRODUCTION

A flywheel ergometer has been devised which employs gravity-independent inertial resistance. Concentric and eccentric actions, which are integral to weight-bearing and ambulation activities, may be done on this ergometer. However, blood lactate responses to exercise on the ergometer by contractile mode and training volume are unknown.

METHODS

Workout order was balanced so that subjects performed each type of ergometer leg press workout twice in a nonsequential manner. Per workout, 10 repetitions were performed per set. Workouts were as follows: three sets with concentric and eccentric (CE3) actions, three sets with concentric-only (CO3) actions, and six sets with concentric-only (CO6) actions. Pre- and 5-min post-exercise lactate was measured from a fingertip blood drop. Lactate means were compared with a 2 x 3 (time X workout) ANOVA with repeated measures applied to both independent variables and Scheffe's post hoc test. With body mass and performance measures as predictor variables, multivariate regression attempted to explain post-exercise and delta (post-pre) lactate variance.

RESULTS

Post-exercise blood lactate values (mmol x L(-1), mean +/- SEM) were as follows: CE3, 8.08 +/- 0.44; CO3, 7.57 +/- 0.49; and CO6, 6.96 +/- 0.43. CE3 and CO6 workouts produced comparable volumes of work. Though several CE3 performance measures were significantly correlated to post-exercise and delta lactate values, power indices had the strongest relationship.

DISCUSSION

Factors related to lactate production and clearance caused CE3 values to be highest. Power indices were most correlated to lactate as they denote a higher work rate and reliance on glycolysis.

Metabolic responses to the seated calf press exercise performed against inertial resistance

INTRODUCTION

Future in-flight strength training devices may use inertial resistance to abate mass and strength losses to muscle groups such as the triceps surae, which incurs pronounced deficits from space travel. Yet little data exist regarding physiological outcomes to triceps surae exercise performed against inertial resistance. Two sets of subjects were employed to note either blood lactate (La-) or net caloric cost responses to seated calf presses done on an inertial resistance ergometer.

METHODS

Both sets of subjects performed 3 identical 3-set 10-repetition workouts. Blood La- measurements were made pre- and 5 min post-exercise. During workouts, breath-by-breath O2 uptake values were also recorded to help determine the net caloric cost of exercise.

RESULTS

Compared to pre-exercise (mean +/- SEM) blood La- (2.01 +/- 0.08 mmol x L(-1)) values, post-exercise (4.73 +/- 0.24 mmol x L(-1)) measurements showed a significant increase. Delta (post/pre differences) La- correlated significantly (r = 0.31-0.34) to several workout performance measures. Net caloric cost averaged 52.82 +/- 3.26 kcals for workouts; multivariate regression showed a subject's height, body mass, and body surface area described the variance associated with energy expenditure.

CONCLUSIONS

Workouts evoked minimal energy expenditure, though anaerobic glycolysis likely played a major role in ATP resynthesis. Metabolic and exercise performance measures were likely influenced by series elastic element involvement of the triceps surae-Achilles tendon complex. Ergometer calf presses provided a high-intensity workout stimulus with a minimal metabolic cost.