Is there a classical nonsense-mediated decay pathway in trypanosomes?

Area of Science:

• Molecular Biology
• Eukaryotic Gene Regulation
• Parasitology

Background:

• Nonsense-mediated decay (NMD) is a conserved eukaryotic surveillance pathway that degrades mRNAs with premature termination codons (PTCs).
• NMD typically requires the RNA helicase Upf1 and its interacting partners, and is often influenced by the distance of PTCs from splice sites or polyadenylation signals.
• In the protist Trypanosoma brucei, mRNA degradation, particularly involving 3'-untranslated regions, is a primary mechanism for controlling mRNA abundance.

Purpose of the Study:

• To investigate the presence and function of the NMD pathway, specifically Upf1 homologs, in Trypanosoma brucei.
• To determine the role of premature termination codons and 3'-untranslated regions in mRNA decay in trypanosomes.
• To explore alternative mechanisms regulating mRNA stability in T. brucei, potentially involving translation and RNA-binding proteins.

Main Methods:

• Identified and characterized homologs of Upf1 (TbUPF1) and Upf2 (TbUPF2) in T. brucei.
• Investigated the interaction of TbUPF1 with TbUPF2 and poly(A) binding protein 1 (PABP1).
• Introduced premature termination codons into endogenous and reporter genes and manipulated open reading frame (ORF), 5'-untranslated region (5'-UTR), and 3'-untranslated region (3'-UTR) lengths to assess effects on mRNA abundance. Depleted TbUPF1 levels via RNA interference.

Main Results:

• TbUPF1 interacts with TbUPF2 and PABP1 in an RNA-dependent manner.
• Introduction of PTCs did not consistently decrease mRNA abundance, and significant depletion of TbUPF1 (>95%) had no discernible effect on parasite growth or the transcriptome.
• Increased ORF length generally increased mRNA abundance, while translation inhibition (via 5'-UTR structures or lengthening) decreased it. 3'-UTR length showed no consistent effect on mRNA levels.

Conclusions:

• The canonical nonsense-mediated decay pathway appears to be absent or non-functional in Trypanosoma brucei.
• mRNA stability in trypanosomes may be primarily regulated by translation efficiency, where active translation potentially inhibits mRNA decay.
• Interactions with multiple RNA-binding proteins might prevent mRNA degradation solely based on 3'-UTR length, suggesting a distinct regulatory mechanism in this parasite.