Life without TTP: Apparent absence of an important anti-inflammatory protein in birds
AJP Regulatory Integrative and Comparative Physiology
Published online on July 31, 2013
Abstract
Both innate and adaptive immunity in birds are different from their mammalian counterparts. Understanding bird immunity is important because of the enormous potential impact of avian infectious diseases, both in their role as food animals and as potential carriers of zoonotic diseases in man. The anti-inflammatory protein tristetraprolin (TTP) is an important component of the mammalian innate immune response, in that it binds to and destabilizes key cytokine mRNAs. TTP KO mice exhibit a severe systemic inflammatory syndrome, and are abnormally sensitive to innate immune stimuli such as lipopolysaccharide (LPS). TTP orthologues have been found in most vertebrates studied, including frogs. Here we attempted to identify TTP orthologues in chicken and other birds, using database searches and deep mRNA sequencing. Although sequences encoding the two other widely expressed TTP family members, ZFP36L1 and ZFP36L2, were identified, we did not find sequences corresponding to TTP in any bird species. Sequences corresponding to TTP were identified in both lizards and alligators, close evolutionary relatives of birds. The induction kinetics of Zfp36l1 and Zfp36l2 mRNAs in LPS-stimulated chicken macrophages or serum-stimulated chick embryo fibroblasts did not resemble the normal mammalian TTP response to these stimuli, suggesting that the other two family members might not compensate for the TTP deficiency in regulating rapidly induced mRNA targets. Several mammalian TTP target transcripts have chicken counterparts that contain one or more potential TTP binding sites, raising the possibility that birds express other proteins that subsume TTP's function as a rapidly inducible regulator of AU-rich mRNA turnover.