Study On Browning In White Adipose Tissue And Limb Development Of Bats

Bats, the second largest order of mammals, possess more than 1,100 species. Bats are capable of storing fat before the hibernation, whose attribute might be the potential research subjects for obesity. Since bats are the only mammals of self-powered flight, the research on specific flight ability and limb development will play a vital importance on the diversification of mammalian limbs. In this article, we studied bats in the process of browning adipose tissue to indicate the molecular regulatory mechanism of adipocytes transdifferentiation and limb development in embryo to uncover the molecular pattern of regulating wing development.Inducing beige fat from white adipose tissue (WAT) is considered to be a shortcut to weight loss and increasingly becoming a key area in research into treatments for obesity and related diseases. However, currently, animal models of beige fat are restricted to rodents, where subcutaneous adipose tissue (sWAT, benign WAT) is more liable to develop into the beige fat under specific activators than the intra-abdominal adipose tissue (aWAT, malignant WAT) that is the major source of obesity related diseases in humans. Here we induced beige fat by cold exposure in the great roundleaf bat (Hipposideros armiger), and compared the molecular and morphological changes with those seen in the mouse. Expression of thermogenic genes (Ucpl and Pgcla) was measured by RT-qPCR and adipocyte morphology examined by HE staining at three adipose locations, sWAT, aWAT and iBAT (interscapular brown adipose tissue). Expression of Ucpl and Pgcla was significantly upregulated, by 729 and 23 fold, respectively, in aWAT of the great roundleaf bat after exposure to 10℃ for 7 days. Adipocyte diameters of WATs became significantly reduced and the white adipocytes became brown-like in morphology. In mice, similar changes were found in the sWAT, but much lower amounts of change in aWAT were seen. The great roundleaf bat is a potentially good animal model for human aWAT research. Combined with rodent models, this model should be helpful for finding therapies for reducing harmful aWAT in humans.Bats are the only mammals capable of self-powered flight by wings. Differing from mouse or human limbs, four elongated digits within a broad wing membrane support the bat wing, and the foot of the bat has evolved a long calcar that spread the interfemoral membrane. Our recent mRNA sequencing (mRNA-Seq) study found unique expression patterns for genes at the 5’end of the Hoxd gene cluster and for Tbx3 that are associated with digit elongation and wing membrane growth in bats. In this study, we focused on Mab21l2, identified from the mRNA-Seq data. Using whole-mount in situ hybridization (WISH) we validated the mRNA-Seq results for differences in the expression patterns of Mab21l2 between bat and mouse limbs, and further detected the timing and location of the gene expression. These analyses suggest that Mab21l2 may have a role in AP and DV axial patterning. In addition, we found that Tbx3 uniquely expressed in the calcar structure found in the bat hindlimb, suggesting its role for this gene in calcar growth and elongation. Together, our findings support the hypothesis that the modulation of the spatiotemporal expression patterns of multiple functional conserved genes control limb morphology and drive morphological change in the diversification of mammalian limbs.