The mouse remains the most appropriate species to create transgenic animals and to analyse the consequences of overexpressed genes and modifications introduced into the genome.10 Rats are also used for transgenic studies; however, practical and technical disadvantages are the reasons for favouring mice for transgenic studies. Although rabbits are more relevant for the study of some human diseases,14 this animal has not been used frequently for transgenic studies because the success rate for generating transgenic rabbits is very low (1%) and the costs of maintaining a rabbit colony are high.10
The main focus of transgenic animal models in the study of adipose tissue biology has been the expression or knockout of selected genes specifically in adipose tissue. Adipose-tissue-specific expression/knockout of genes became possible with the identification and characterization of promoter regions conferring adipose-tissue-specific expression. The promoters for the adipocyte lipid binding protein aP215 and for phosphoenolpyruvate carboxykinase (PEPCK)16 are commonly used to target both white and brown adipose tissue (WAT/BAT), whereas the UCP-1 promoter only targets BAT.17 Using the recombinase activity of the cyclization recombination (Cre) gene from the P1 bacteriophage, conditional transgenesis and knockout of specific genes became a commonly used method to create transgenic animal models. The Cre recombinase, a 38-kDa site-specific DNA recombinase, catalyses the recombination of two 34-bp-long loxP (locus of X-over of P1) sites, making it possible to excise loxP-flanked DNA segments.10 and 18 The major application of the Cre/LoxP system is the generation of spatially or temporally regulated gene knockouts.19 Cre/LoxP-mediated recombination requires two steps. First, the endogenous gene to be targeted is modified by homologous recombination in murine embryonic stem (ES) cells so that LoxP sites flank a portion of the gene, and mice are generated from these recombinant ES cells to obtain flox (flanked by lox) mice. Second, the heterozygous flox mice are bred with mice expressing a Cre transgene to generate double heterozygous mice. These double heterozygous mice are bred with single heterozygous flox mice to generate conditional knockout mice