Mice that are homozygous for this knockout allele are viable and fertile, but exhibit sensitivity to cold temperatures. The Donating Investigator notes that homozygous offspring have increased survival rates to weaning age when maintained at 25°C to 28°C. No gene product (mRNA or protein) is detected by Northern or Western blot analysis of brown adipose tissue from homozygotes. When maintained at 20°C, homozygotes are resistant to diet induced obesity with reduced white fat depot weights, exhibit a 0.1-0.3°C higher body temperature, and a slightly lowered respiratory quotient compared to controls. Notably, when maintained at 27°C, homozygous mutant mice are no longer resistant to diet induced obesity and gain weight at a rate similar to wildtype controls. An increase in the number of brown adipocytes in the inguinal fat depots is observed in mutant mice maintained at 20°C, on either diet. Homozygotes have reduced levels of plasma fatty acids, circulating triglyceride levels, and T4 levels. Spleen cell numbers are reduced by approximately 3-fold, CD8 single positive cells are reduced to approximately half and CD4/CD8 double positive cells in both the spleen and thymus are increased in homozygous animals. Homozygotes have no heat production from brown adipose tissue. Mitochondria isolated from the muscles of cold-acclimated homozygotes display an increased total ATP production capacity, a metabolic shift for increased lipid oxidation and reduced carbohydrate catabolism capacity with a corresponding increased serum level of beta-hydroxybuterate, and decreased sensitivity to fatty acids as uncoupling agents. Mitochondria isolated from brown adipose tissue from homozygotes exhibit increased rate of reactive oxygen species (ROS) production when compared to wildtype controls. The Donating Investigator reports that mice carrying this allele are less cold sensitive on the C57BL/6 genetic background compared to the 129 genetic background.
