Differential junction usage in paired RNA-seq of white and beige differentiated adipocytes from six human subjects

Background: The beneficial effect of thermogenic adipocytes in maintaining body weight and protecting against metabolic disorders has raised interest in understanding the regulatory mechanisms defining white and beige adipocyte identity. Although alternative splicing has been shown to propagate adipose browning signals in mice, this has yet to be thoroughly investigated in human adipocytes. Methods: We performed parallel white and beige adipogenic differentiation using primary adipose stem cells from 6 unrelated healthy subjects, and assessed differential gene and isoform expression in mature adipocytes by RNA sequencing. Results: We find 693 exon junctions with robust differential usage between white and beige adipocytes in all 6 subjects, mapping to 507 genes. Importantly, only 8% of these differentially spliced genes are also differentially expressed, indicating that alternative splicing constitutes an additional layer of gene expression regulation during beige adipocyte adipogenic differentiation. Functional classification of alternative isoforms point to a gain of function for key thermogenic regulators such as PPARG, CITED1 and PEMT. We find that a large majority of the splice variants arise from differential usage of transcription start sites (TSSs), with beige-specific TSSs being enriched for PPARγ and MED1 binding compared to white-specific TSSs. Finally, we validate beige specific isoform expression at the protein level for two thermogenic regulators, PPARγ and PEMT. Discussion: These results indicate that differential isoform expression through alternative TSS usage is an important regulatory mechanism for human adipocyte thermogenic specification. Code for data processing and analysis are available at https://github.com/sarahhp/splicing_thermogenesis.