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 PPAR? and MED1 binding compared to white-specific TSSs. Finally, we validate beige specific isoform expression at the protein level for two thermogenic regulators, PPAR? 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. Overall design: To study the differentiation of beige adipocytes in vitro, six human primary adipose stem cell (hASC) lines from female subjects were selected for similar white adipose differentiation capacity based on marker genes and proteins. hASC cell lines from six subjects were differentiated in triplicate into both beige and white adipocytes in the presence or absence of rosiglitazone, respectively, and harvested for RNA-seq. We investigated the difference between the white and beige adipocytes using differential gene and exon junction expression. Further, differential exon junctions were annotated with functionality scores from TRIFID.