Diminished transcriptional activity and splicing changes drive gene length-biased rewiring in the aging transcriptome

Transcription by RNA polymerase II (RNAPII), which is essential for protein-coding gene expression and cellular function, is increasingly understood to become dysregulated with aging. Here, we use a multimodal approach to comprehensively characterize age-dependent changes in RNAPII-mediated transcription in both mouse and human tissues. Short-read total RNA sequencing (RNA-seq) to profile nascent transcription reveals a global reduction in overall transcriptional activity/frequency in aged tissues, without apparent change in elongation rates. Transcriptomic analysis reveals a shift toward preferential expression of short genes in aged tissues, with notable upregulation of short stress-response genes and downregulation of long neurodevelopmental genes in the aged mouse brain. These results are recapitulated by analysis of total RNA-seq data from human tissues. Leveraging long-read RNA-seq, we determine that the representation of aberrant mono-exonic and intron-retention splice isoforms is increased in the aged mouse brain. Finally, we characterize the composition of RNAPII transcriptional machinery, finding that interactions between RNAPII and the Mediator complex are decreased in the chromatin of aged mouse liver and brain. Collectively, these analyses provide insight for future aging studies and reveal potential transcriptional control targets for anti-aging drug development.