The centuries-old question of how to stop aging became the topic of study for researchers from a variety of institutions across the globe, including Hopkins.
In previous studies, the acetylation and deacetylation of histones had been investigated as a means of lifespan regulation. However, this new study found that many nonhistone substrates play an important role in cellular lifespan. It was these nonhistones that were then investigated on yeast cells.
Researchers focused their attention on the complex Snf1 and more specifically on the Sip2 regulator, a subunit that regulates the Snf1 complex and is also a known life span regulator. Furthermore, they found that the Sip2 subunit is acetylated and deacetylated by the NuA4/Esa1 and Rpd3 enzymes, respectively, which in turn alters the way Sip2 interacts with the gene of study, the Snf1.
According to the study, the acetylation of Sip2 has the ability to extend lifespan by increasing Sip2-Snf1 interactions in the cell. With this understanding, being able to acetylate Sip2 seems to be the next line of study.
Within the Sip2 subunit, researchers identified four regions of acetylated lysine residues, then constructed unacetylable mutants to replace those four regions in varying combinations.
What were the results? By mixing and matching the regions with mutants, the data revealed that the first three were highly necessary for Sip2 acetylation. Furthermore, they confirmed that when nonmutant regions were present, the Rpd3 enzyme eliminated the acetylation signals of Sip2.
How exactly does this lead us to alter the lifespan of an organism? By altering acetylation points in the gene, scientists were able to see that when either one of the first three regions were mutated, or when Rpd3 was present, the yeast cells had a shortened life span. However, when the first three acetylation regions were in their native state and Rpd3 was eliminated, the shortened life span was reversed and that the NuA4/Esa-1 enzymes aided in acetylation of Sip2. So all together, NuA4/Esa-1 along with the presence of the three acetylated lysine residues, cell lifespan could be extended. However, without those three residues or with the Rpd3 enzyme present cell lifespan was shortened.
Now with this understanding, one could ask why yeast cells die at all if they can simply alter their Sip2 subunits to interact with Snf1? The answer lies in the fact that young cells can and do acetylate Sip2 and the studies were able to see a high amount of interaction there. However, as the yeast cells aged there was almost no acetylation of Sip2, resulting in death.
To go even further, researchers found that another enzyme called the Sch9 kinase regulates life span by phosporylating the ribosomal protein Rps6, a protein which was found to slow cell growth but also increase cell lifespan. This function operated by acting as a "critical downstream effector" of the Snf1 complex, drawing another line of evidence supporting the hypothesis that Snf1 regulated lifespan.
From this data, researchers were able to gain valuable insight about the mechanics of nonhistone regulators, a newer field of study which holds some promise for the future.
