In order to determine how CREB acts to regulate HDAC, Berdeaux and colleagues looked for phosphorylation sites in the HDAC protein. They discovered that one of many proteins turned on and off by the CREB switch, a little-known enzyme called salt-inducible kinase-1, or SIK1, specifically recognizes and phosphorylates HDAC.
Then, to explore the possibility that SIK1 is the enzyme controlling the HDAC brake in muscle cells, the researchers reduced SIK1 activity in muscle cells grown in the lab using a technique that silences gene expression. Less SIK1 activity resulted in less phosphorylated HDAC. With the unphosphorylated HDAC brake now firmly in place, expression of muscle-specific target genes was also reduced. Conversely, when the researchers boosted SIK1 levels or inhibited HDAC activity with a drug, muscle cell health was restored in mice genetically engineered to lack CREB and suffering from muscle weakness as a result.
"Weve discovered that SIK1 provides a completely unexpected link between two important mechanisms of gene regulation, CREB and HDAC, and have shown that this pathway plays a major role in maintaining normal muscle function," said Berdeaux. "Now it remains to be seen how this pathway works in muscle under different conditions, such as during exercise, as well as what part it plays in maintaining cell survival in other tissue types and in other species."
Indeed, all mammals have SIK1, not just mice, and even worms and fruit flies use variations of the enzyme. And as Montminy adds, "Application of this same pathway over and over across species further emphasizes its importance."