YAF2-Mediated YY1-Sirtuin6 Interactions Responsible for Mitochondrial Downregulation in Aging Tunicates

Abstract

In budding tunicates, aging accompanies a decrease in the gene expression of mitochondrial transcription factor A (<i>Tfam</i>), and the <i>in vivo</i> transfection of <i>Tfam</i> mRNA stimulates the mitochondrial respiratory activity of aged animals. The gene expression of both the transcriptional repressor Yin-Yang-1 (<i>YY1</i>) and corepressor Sirtuin6 (<i>Sirt6</i>) increased during aging, and the cotransfection of synthetic mRNA of <i>YY1</i> and <i>Sirt6</i> synergistically downregulated <i>Tfam</i> gene expression. Pulldown assays of proteins indicated that YY1-associated factor 2 (YAF2) was associated with both YY1 and SIRT6. Protein cross-linking confirmed that YAF2 bound YY1 and SIRT6 with a molar ratio of 1:1. YY1 was bound to CCAT- or ACAT-containing oligonucleotides in the 5' flanking region of the <i>Tfam</i> gene. Chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) showed that SIRT6 specifically induced the histone H3 lysine 9 (H3K9) deacetylation of the <i>Tfam</i> upstream region. YY1 and YAF2 accelerated SIRT6-induced H3K9 deacetylation. <i>YY1</i> and <i>Sirt6</i> mRNA transfection attenuated mitochondrial respiratory gene expression and blocked MitoTracker fluorescence. In contrast, the SIRT6 inhibitor and <i>Tfam</i> mRNA antagonized the inhibitory effects of <i>YY1</i> and <i>Sirt6</i>, indicating that <i>Tfam</i> acts on mitochondria downstream of <i>YY1</i> and <i>Sirt6</i>. We concluded that in the budding tunicate Polyandrocarpa misakiensis, YY1 recruits SIRT6 via YAF2 to the <i>TFAM</i> gene, resulting in aging-related mitochondrial downregulation.