Abstract 14: Central Nervous System Deletion of the Bbs1 Gene Causes Obesity and Hypertension in Mice

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Bardet-Biedl syndrome (BBS) is a pleiotropic autosomal recessive disorder with several features including obesity and hypertension. Systemic knock-out mouse models lacking expression of Bbs2, Bbs4 and Bbs6 genes, and Bbs1M390R knock-in recapitulated many of the BBS phenotypes including obesity. However, the role and contribution of different tissues to the various phenotypes associated with BBS including obesity and hypertension remains unclear. To address this, we generated a new conditional knockout mouse where exon 3 of the Bbs1 gene is floxed. Cre-mediated recombination causes a frame shift resulting in a premature stop. We assessed whether deletion of the Bbs1 gene in the central nervous system (CNS) affects body weight and arterial pressure. Breeding Bbs1flox with nestinCre mice created mice deficient in Bbs1 gene only in the CNS as indicated by the loss of Bbs1 gene expression (by RT-PCR) in the hypothalamus, hippocampus, cortex and brainstem, but not in peripheral tissues such as adipose tissue, liver, kidney and skeletal muscle. Importantly, Bbs1flox/nestinCre mice display an obesity phenotype as indicated by the increased (P<0.05) body weight (40±1 g vs. 31±1 g in controls) and fat mass measured by MRI (23±2 g vs. 9±1 g in controls) in 25 weeks old mice. We found that the obesity phenotype in Bbs1flox/nestinCre mice is due to both an increase (P<0.05) in food intake (4.0±0.2 g vs. 3.1±0.3 g in controls) and reduction in energy expenditure as indicated by the decreased (P<0.05) O2 consumption (2.8±0.3 mL/100g/min vs. 3.2±0.2 mL/100g/min in controls) and heat production (8.3±0.8 kcal/kg/h vs. 9.4±0.7 kcal/kg/h in controls). These results indicate that hyperphagia and low metabolic rate explain the development of obesity in Bbs1flox/nestinCre mice. Finally, we assessed by radiotelemetry the consequence on arterial pressure of ablating the Bbs1 gene throughout the CNS. Interestingly, CNS deletion of the Bbs1 gene recapitulates the hypertension phenotype of BBS as indicated by the elevated mean arterial pressure in Bbs1flox/nestinCre (123±3 mmHg) relative to littermate controls (112±4 mmHg, P=0.02). These findings demonstrate that Bbs genes in the CNS are critical for energy homeostasis and arterial pressure regulation.

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