Comment

Genetics of obesity: advances from rodent studies

This chapter demonstrates that there are many expressed non-protein coding regions in the mammalian genome. The core power of positional genetics approaches is that one can identify the causative factor whether it is an expressed gene, microRNA, antisense, or other unknown mechanisms. New phenotyping resources may also facilitate gene discovery programs. The Mouse Phenome Database includes extensive phenotyping information on dozens of strains. One possibility is that this phenotyping can be combined with high-density mouse haplotyping to find genes for many complex traits. Using genetic approaches leads to unpredictable results; only a small fraction may identify genes that can be targeted by weight loss drugs. Although each obesity gene may have its own interesting story, not all genes have human orthologs that cause obesity; and not all human obesity genes provide safe and effective drug targets. It is not always clear that weight gain or loss in mice can be predicted from measurements of food intake and energy expenditure, possibly because the assays used are not accurate enough to capture the small imbalances of intake and expenditure that cause fat accumulation or because the assays are used for cross-sectional studies at one time point, but the obesity-causing energy imbalance occurs at another time. Settling these questions will allow for a more precise understanding of the mechanisms by which alleles cause fat accumulation.

This chapter demonstrates that there are many expressed non-protein coding regions in the mammalian genome. The core power of positional genetics approaches is that one can identify the causative factor whether it is an expressed gene, microRNA, antisense, or other unknown mechanisms. New phenotyping resources may also facilitate gene discovery programs. The Mouse Phenome Database includes extensive phenotyping information on dozens of strains. One possibility is that this phenotyping can be combined with high-density mouse haplotyping to find genes for many complex traits. Using genetic approaches leads to unpredictable results; only a small fraction may identify genes that can be targeted by weight loss drugs. Although each obesity gene may have its own interesting story, not all genes have human orthologs that cause obesity; and not all human obesity genes provide safe and effective drug targets. It is not always clear that weight gain or loss in mice can be predicted from measurements of food intake and energy expenditure, possibly because the assays used are not accurate enough to capture the small imbalances of intake and expenditure that cause fat accumulation or because the assays are used for cross-sectional studies at one time point, but the obesity-causing energy imbalance occurs at another time. Settling these questions will allow for a more precise understanding of the mechanisms by which alleles cause fat accumulation.

NADPH is an essential cofactor for many enzymatic reactions including glutathione metabolism and fat and cholesterol biosynthesis. We have reported recently an important role for mitochondrial NADP⁺-dependent isocitrate dehydrogenase in cellular defense against oxidative damage by providing NADPH needed for the regeneration of reduced glutathione. However, the role of cytosolic NADP⁺-dependent isocitrate dehydrogenase (IDPc) is still unclear. We report here for the first time that IDPc plays a critical role in fat and cholesterol biosynthesis. During differentiation of 3T3-L1 adipocytes, both IDPc enzyme activity and its protein content were increased in parallel in a time-dependent manner. Increased expression of IDPc by stable transfection of IDPc cDNA positively correlated with adipogenesis of 3T3-L1 cells, whereas decreased IDPc expression by an antisense IDPc vector retarded adipogenesis. Furthermore, transgenic mice with overexpressed IDPc exhibited fatty liver, hyperlipidemia, and obesity. In the epididymal fat pads of the transgenic mice, the expressions of adipocyte-specific genes including peroxisome proliferator-activated receptor γ were markedly elevated. The hepatic and epididymal fat pad contents of acetyl-CoA and malonyl-CoA in the transgenic mice were significantly lower, whereas the total triglyceride and cholesterol contents were markedly higher in the liver and serum of transgenic mice compared with those measured in wild type mice, suggesting that the consumption rate of those lipogenic precursors needed for fat biosynthesis must be increased by elevated IDPc activity. Taken together, our findings strongly indicate that IDPc would be a major NADPH producer required for fat and cholesterol synthesis.

The ubiquitin-dependent proteolytic pathway targets many key regulatory proteins for rapid intracellular degradation. Specificity in protein ubiquitination derives from E3 ubiquitin protein ligases, which recognize substrate proteins. Recently, analysis of the E3s that regulate cell division has revealed common themes in structure and function. One particularly versatile class of E3s, referred to as Skp1p–Cdc53p–F-box protein (SCF) complexes, utilizes substrate-specific adaptor subunits called F-box proteins to recruit various substrates to a core ubiquitination complex. A vast array of F-box proteins have been revealed by genome sequencing projects, and the early returns from genetic analysis in several organisms promise that F-box proteins will participate in the regulation of many processes, including cell division, transcription, signal transduction and development.

Drug repositioning is an emerging strategy for filling the innovation gap in the pharmaceutical industry. Successful drug repositioning examples are nearly always explained by observations made in clinical settings or in in vivo therapeutic models. We have established an in vivo phenotypic platform, termed theraTRACE^® that systematically evaluates and repositions drugs to new indications. This review article describes examples of drugs found using phenotypic models and the concepts behind the theraTRACE^® approach.