Metabolic Disturbances Caused by Haploinsufficiency of RanBP2
The growth rates of inbred RanBP2+/− mice on high-fat (~10% fat) diet were significantly slower than RanBP2+/+ mice (Figure 6A). Beginning at around 4 mo of age, RanBP2+/− mice exhibit a significant slower gain in body mass than wild-type mice (Figure 6A). In addition, RanBP2+/− inbred mice presented deficits in body mass that were erased by changing the genetic background to a mixed C57BL/6J/129Ola (Figure 6B). Food consumption did not account for the body weight differences observed (Figure 6C).
Figure 6  RanBP2+/− Mice on High-Fat Diet Exhibit Deficits in Growth
(A) In comparison to wild-type mice, RanBP2+/− mice show slower growth rates beginning at 4 mo of age (arrow), and the difference in body weight between these is maintained afterward. Note that RanBP2+/− mice lack the growth spur observed in wild-type mice between 3 and 4 mo of age.
(B) In comparison to inbred RanBP2+/−mice (129Ola genetic background), the difference in body weight between RanBP2+/+ and RanBP2+/− mice is masked upon placing these on a mixed 129Ola/C57Bl6 genetic background.
(C) RanBP2+/+ and RanBP2+/−inbred mice exhibit similar rates of food consumption. Mice in (A), (B), and (C) were placed on a high-fat diet since birth (n = 5). HKI in the CNS (brain and retina) accounts virtually for all expression of HK isozymes and glucose utilization in the CNS [33,34]. Moreover, glucose is the sole reliance source of energy in the CNS under normal conditions, the CNS lacks glucose storage sources, and despite the disproportionate mass of the CNS to the rest of the body, the CNS consumes daily about 60% of the body's glucose and 25% of the total oxygen [35,36]. To determine the impact of RanBP2 haploinsufficiency on the utilization, formation, and uptake of glucose, we carried out several physiological assays. In contrast to mice placed on a normal chow diet (~5% fat; unpublished data), RanBP2+/−mice on a higher fat diet (~10% fat) performed significantly worse in the glucose tolerance test beginning at 6 mo of age (Figure 7A and 7B), thus supporting that the RanBP2+/− mice exhibited a deficit in glucose clearance. This deficit was rescued in RanBP2+/− mice of mixed C57BL/6J/129Ola background (Figure S3). Glucose clearance was not affected due to a disturbance in insulin-mediated glucose uptake (Figure 7C). Then, we probed whether RanBP2 induces impairment of gluconeogenesis, which could contribute to the pathophysiological production and clearance of glucose. To this end, the administration of the gluconeogenic substrate precursor, pyruvate (pyruvate tolerance test), showed that there was no difference in glucose production in RanBP2+/− mice (Figure 7D). Hence, partial loss-of-function of RanBP2 had no impact on the gluconeogenesis pathway. However, upon glucose production (15 min), the clearance rates of glucose were again significantly slower in RanBP2+/− than in RanBP2+/+ mice (Figure 7D), confirming an impairment in glucose breakdown.
Figure 7  Metabolic Phenotypes of RanBP2+/− Inbred Mice on High-Fat Diet
(A) 3-mo-old inbred RanBP2+/− mice (n = 5) have normal glucose clearance rates upon glucose challenge and overnight fasting.
(B) In contrast, 6-mo-old inbred RanBP2+/− mice (n = 5) have significantly decreased glucose clearance rates upon glucose challenge and overnight fasting.
(C) Fasted 6- to 8-mo-old RanBP2+/+ and RanBP2+/− mice have no difference in insulin-mediated glucose uptake as assayed by insulin tolerance test (n = 5).
(D) Pyruvate tolerance test shows normal rise in glucose but decreased glucose clearance between inbred RanBP2+/+ and RanBP2+/− mice (n = 5).