Genetic dissection of brown adipose tissue function in rat recombinant inbred strains

Michal Pravenec

Institute of Physiology of the Czech Academy of Sciences, Prague, Czech Republic

Recently, brown adipose tissue (BAT) was suggested to play an important role in lipid and glucose metabolism in rodents and possibly also in humans. In the current study, we used quantitative trait locus (QTL), genomic sequence and RNA expression analyses in the BXH/HXB recombinant inbred (RI) strains, derived from Brown Norway (BN) and spontaneously hypertensive rats (SHR), to identify some of the genetic determinants of BAT function and its role in the pathogenesis of metabolic disturbances. Linkage analyses revealed significant QTL associated with interscapular BAT mass on chromosome 4 and two closely linked QTL associated with glucose oxidation and glucose incorporation into BAT lipids on chromosome 2. Using quantitative transcriptional data and weighted gene co-expression network analysis (WGCNA), we identified 1,147 gene co-expression modules in the RNA extracted from BAT of the BXH/HXB panel. The “Coral14.1” co-expression module includes the hub gene Cd36 which plays an important role in fatty acid transport and triglyceride utilization in BAT. The eigengene QTL for the “Coral14.1” module overlaps with the QTL for interscapular fat mass on chromosome 4, and the module eigengene values are significantly correlated with the fat mass across the RI strains. The “Darkseagreen” module eigengene QTL overlaps the QTL associated with glucose oxidation and BAT lipid synthesis on chromosome 2, and this module’s eigengene values are significantly correlated with these biochemical parameters across the RI strains. The “Darkseagreen” module has the flavin-containing monooxygenase 5 (Fmo5) and sortilin 1 (Sort1) as the most connected transcripts. The functions of these genes can be clearly related to the associated phenotypes. We also searched the areas delineated by the phenotypic QTLs for functional polymorphisms. Cd36 harbors a deletion variant in the SHR. SHR-Cd36 transgenic rats with wild type Cd36, when compared to SHR, had reduced BAT mass. We also identified Wars2 (tryptophanyl  tRNA synthetase 2 (mitochondrial)) as a positional candidate which is dysfunctional in the SHR. SHR-Wars2 transgenic rats, with wild type Wars2 gene, showed increased glucose oxidation and incorporation into BAT lipids when compared to SHR. BAT mass, glucose oxidation and incorporation into BAT lipids significantly correlated with metabolic and hemodynamic parameters in RI strains. In summary, our results demonstrate an important role of both differences in transcriptional levels and functional genetic polymorphisms in regulating BAT mass and function and consequently lipid and glucose metabolism.