TBX15 regulates a network of immune response genes in adipose tissue and alters fat mass and depot weight in heterozygous knockout mice

Adipose tissue distribution in the body is an indicator of metabolic disease risk, independent of body mass index (BMI), and is indirectly measured by waist-hip-ratio (WHR). T-Box transcription factor-15 (TBX15) has been implicated in regulation of adipose distribution in multiple human and mouse studies, and the TBX15-WARS2 genome-wide association study locus has been associated with BMI-adjusted-WHR signals in multiple investigations. As a potential mediator of this signal, we investigated the role of Tbx15 using heterozygous and homozygous mouse knockout models to determine if loss of this gene alters adipose physiology, and to identify the transcriptional network regulated by Tbx15 in adipose tissue and preadipocyte cells. In a metabolic phenotyping experiment we provided either low fat diet (LFD) or high fat diet (HFD) to male and female heterozygous Tbx15+/- and wildtype Tbx15+/+mice from weaning and maintained for 24 weeks. Only Tbx15+/-mice maintained on LFD weighed less than wildtype LFD controls, and female LFD Tbx15+/- mice had lower fat mass overall. We found that in LFD Tbx15+/- mice, multiple visceral fat depots weighed less than wildtype controls, and this was maintained when corrected for body mass for both gonadal and mesenteric visceral adipose depots. When comparing adipocyte size in multiple adipose depots, some reduction in number of larger adipocytes was detected in the perirenal adipose tissue of female HFD Tbx15+/- vs Tbx15+/+ mice, mesenteric adipose tissue from female LFD Tbx15+/- vs Tbx15+/+ mice and male HFD Tbx15+/- vs Tbx15+/+mice. RNA-sequencing of subcutaneous (inguinal) adipose tissues from 12-week old male and female knockout Tbx15-/-, Tbx15+/- and Tbx15+/+ mice raised on a standard chow diet identified 897 upregulated genes and 2328 downregulated genes in female Tbx15-/- mice compared to Tbx15+/+mice. We then combined this dataset with TBX15 ChIP-sequencing data from mouse preadipocyte 3T3-L1 cells overexpressing TBX15 to identify a credible set of genes directly regulated by TBX15. These 52 genes were enriched for B- and T-cell receptor signalling, JAK-STAT signalling and haematopoietic cell lineage pathways; suggesting a direct regulatory role for TBX15 in these pathways in adipose tissue. Together, these data highlight a role for TBX15 in regulation of differential adipose tissue expansion, particularly under low caloric conditions. Further, we identify a potentially important role for TBX15 in the well described adipocyte-immune cell crosstalk associated with obesity and type 2 diabetes mellitus.