Does reduced IGF-1R signaling in Igf1r+/- mice alter aging?

Abstract

Mutations in insulin/IGF-1 signaling pathway have been shown to lead to increased longevity in various invertebrate models. Therefore, the effect of the haplo-insufficiency of the IGF-1 receptor (Igf1r(+/-)) on longevity/aging was evaluated in C57Bl/6 mice using rigorous criteria where lifespan and end-of-life pathology were measured under optimal husbandry conditions using large sample sizes. Igf1r(+/-) mice exhibited reductions in IGF-1 receptor levels and the activation of Akt by IGF-1, with no compensatory increases in serum IGF-1 or tissue IGF-1 mRNA levels, indicating that the Igf1r(+/-) mice show reduced IGF-1 signaling. Aged male, but not female Igf1r(+/-) mice were glucose intolerant, and both genders developed insulin resistance as they aged. Female, but not male Igf1r(+/-) mice survived longer than wild type mice after lethal paraquat and diquat exposure, and female Igf1r(+/-) mice also exhibited less diquat-induced liver damage. However, no significant difference between the lifespans of the male Igf1r(+/-) and wild type mice was observed; and the mean lifespan of the Igf1r(+/-) females was increased only slightly (less than 5%) compared to wild type mice. A comprehensive pathological analysis showed no significant difference in end-of-life pathological lesions between the Igf1r(+/-) and wild type mice. These data show that the Igf1r(+/-) mouse is not a model of increased longevity and delayed aging as predicted by invertebrate models with mutations in the insulin/IGF-1 signaling pathway.

Figure 1. Igf1rβ expression.

The mRNA and protein levels of the β subunit of Igf1r were measured in the kidney, lung, and muscle (quadriceps) from male and female mice that were 6 (Graphs A, E, C, and G) and 25 (Graphs B, F, D, and H) months old. The graphs on the left represent data from qRT-PCR and those on the right represent data from Western blots. Three to 6 animals were used per group. Black bars represent WT mice and red bars represent Igf1r^+/− mice; the mean and SEM are shown, and asterisks indicate tissues showing a difference between WT and Igf1r^+/− where p<0.05. The Student's t-test was used for the comparisons.

Figure 2. Induction of AKT phosphorylation by IGF-1 in WT and Igf-1r^+/− mice.

Levels of phosphorylated AKT were measured in the muscle (quadriceps) of 6- (graphs A and C) and 25- (graphs B and D) month-old male and female mice following injection of saline or rhIGF-1 (1 mg/kg body wt.) using Western blots as described in Materials and Methods. Three to 4 animals were used per group.

Figure 3. Induction of GSK3β phosphorylation and levels of Igfbp5 mRNA transcript in WT and Igf1r^+/− mice.

Levels of phosphorylated GSK3β were measured in the muscle (quadriceps) of 25-month-old male (graph A) and female (graph B) mice following injection of saline or rhIGF-1 (1 mg/kg body wt.) using Western blots as described in Materials and Methods. Three animals were used per group. The expression of Igfbp5 was measured in the same samples using qRT-PCR (graphs C and D). The vertical axis represents expression levels relative to B2M and the error bars represent SEM. P-values of 0.01 and 0.005 are represented by * and **, respectively.

Figure 4. Glucose and Insulin Tolerance Tests.

GTTs (2 g/kg, i.p.) were performed in 6-month-old male (A) and female (B) as well as 25-month-old male (C) and female (D) mice after a 12 hr fast, and blood glucose was recorded at times indicated. ITTs (0.5 U/kg, i.p.) were performed in 6-month-old male (E) and female (F) as well as 25-month-old male (G) and female (H) mice after a 5 hr fast. The data were obtained from 6 to 8 animals per group and the SEM is shown. Black lines show the blood glucose levels of WT (black lines) and Igf1r^+/− mice (red lines). The Student's t-test was used for the comparisons of the areas under the curve (AUC) and the p-values are shown on each graph. Individual points were also compared in the same manner and corrected for multiple comparisons, and corrected p-values less than 0.05 are denoted by asterisks.

Figure 5. Peripheral Insulin Sensitivity.

Peripheral (muscle) insulin sensitivity was measured with a 90 min hyperinsulinemic euglycemic clamp performed in 4 WT and 5 Igf1r^+/− females, all 25 months old. Bars represent the average ± SE glucose infusion rate during the last 20 min of the clamp. A student's t-test was used to compare the infusion rates between the two groups, averaged over the 20 min period for each animal.

Figure 6. Sensitivity of WT and Igf1r^+/− mice to oxidative stress.

Paraquat (50 mg/kg) was administered to 37 male WT mice and 26 male Igf1r^+/− mice (graph A) as well as to 39 female WT mice and 24 female Igf1r^+/−mice (graph B). Diquat (50 mg/kg) was administered to 21 WT and 26 Igf1r^+/− male mice (graph C) as well as to 22 WT and 17 Igf1r^+/− female mice (graph D). The mice in graphs A–D were 5 to 9.5 months of age. Censored data points (due to uncertainty about the exact minute of an animal's death) are indicated by vertical tick-marks. Graphs E and F: Female WT and Igf1r^+/− mice (10 to 11 months of age) were treated with diquat (50 mg/kg). Six hours after treatment the mice were killed and the ALT activities in the plasma (graph E) and number of apoptotic cells per unit area in a liver cross section (graph F) of 7 WT and 8 Igf1r^+/−mice were determined. The mean and SEM are shown in the bar graphs. Black represents WT data and red represents Igf1r^+/−data. Survival data were analyzed using the log-rank test while the ALT and apoptosis data were analyzed using Student's t-test and the p-values are shown.

Figure 7. Longevity of WT and Igf1r^+/− mice.

The survival curves of 55 WT and 52 Igf1r^+/− male mice (A) and 68 WT and 47 Igf1r^+/− female (B) mice are shown in black for WT mice and red for Igf1r^+/−mice. The survival curves were compared using the log-rank test, and the p-values are shown.