The longevity of certain types of messenger RNA increases considerably in response to monocyte adhesion during an innate immune response, said University of Medicine and Dentistry, New Jersey prof. Gary Brewer, who proposed a model for this finding on Thursday in Grand Rounds at the Norris Cotton Center.
Under normal circumstances, there are two primary means of RNA degradation: the deadenylation-dependent pathway and the deadenylation-independent pathway. In the former, degradation begins when the poly(A) tail at the 3’ end of the mRNA is degraded by deadenylase, triggering the decapping of the 5’ end.
Deadenylation-independent degradation occurs when a nonsense mutation in the mRNA is detected. The 5’ end of the mRNA is decapped, followed by exonucleolytic decay. This pathway can also be initiated by endonucleases that cleave the mRNA before exonucleolytic degradation occurs.
One determinant of the stability of mRNA – how quickly the mRNA is degraded – is the interaction between AU-rich elements (ARE) and a protein complex that includes proteins AUF-1 and Hsp-27, said Brewer.
In Brewer’s general model of ARE-mRNA degradation, an AUF-1 dimer first binds the mRNA, recruiting other proteins that form a complex and, in turn, recruits ribonuclease and proteasomes to degrade the mRNA. The proteasomes, Brewer explained, are necessary for a less well-understood step in ARE-mRNA decay: the degradation of AUF-1.
When an immune response elicits the migration and adhesion of monocytes to injured tissue, however, the half-life of cytokine mRNA is substantially increased, said Brewer. This is advantageous to the purposes of the immune response, as more robust cytokine mRNA allows for the recruitment of more monocytes to the injury site. Brewer and his team sought a molecular explanation for this phenomenon.
Brewer found a couple of key ideas underlying the increased longevity of mRNA in activated monocytes. First, he found that p40 AUF-1 (an isoform of AUF-1), which is normally found phosphorylated, is not phosphorylated during monocyte adhesion. Moreover, when this non-phosphorylated form of p40 AUF-1 binds to mRNA at the start of the ARE-mRNA degradation process, it induces the compacting of the bound mRNA.
Second, he found that when Hsp-27 is phosphorylated at any three of its serine sites, as it is during monocyte adhesion, it actively recruits proteasomes, which destabilize AUF-1 proteins. This in turn facilitates an important step in ARE-mRNA degradation. “The Hsp-27 phosphorylation state dictates the half-life of AUF-1 proteins,” he said.
To piece together his results, Brewer presented a tentative model – “It’s a hypothesis,” he emphasized. In a non-activated monocyte, phosphorylated p40 AUF-1 and Hsp-27 bind the ARE. Hsp-27 facilitates the degradation of the AUF-1, opening the poly(A) tail to degradation.
In an activated monocyte, however, the p40 AUF-1 is not phosphorylated – a characteristic that prevents it from being degraded, said Brewer. Thus, it remains bound to the mRNA and does not open the mRNA up for degradation.
Brewer and his team are actively continuing their research in this area in order to answer remaining questions about the true molecular model of RNA degradation.