mRNA Translation Gone Awry: Translation Fidelity and Neurological Disease

Abstract

Errors during mRNA translation can lead to a reduction in the levels of functional proteins and an increase in deleterious molecules. Advances in next-generation sequencing have led to the discovery of rare genetic disorders, many caused by mutations in genes encoding the mRNA translation machinery, as well as to a better understanding of translational dynamics through ribosome profiling. We discuss here multiple neurological disorders that are linked to errors in tRNA aminoacylation and ribosome decoding. We draw on studies from genetic models, including yeast and mice, to enhance our understanding of the translational defects observed in these diseases. Finally, we emphasize the importance of tRNA, their associated enzymes, and the inextricable link between accuracy and efficiency in the maintenance of translational fidelity.

Keywords: elongation factor; mistranslation; neurodegeneration; ribosome stalling; tRNA modifications.

Figure 1. tRNA Aminoacylation and Editing by Aminoacyl tRNA Synthetases

Aminoacyl tRNA synthetases (aaRS) activate an amino acid via ATP hydrolysis to form an aminoacyl adenylate. These enzymes then ligate the activated amino acid to the 3′ end of their cognate tRNA to generate an aminoacylated tRNA (aa-tRNA). Usually, aaRSs efficiently select the correct amino acid from the cellular pool, correctly discriminating between it and other related amino acids. However, if the non-cognate amino acid is activated, it can be hydrolyzed either directly or after ligation to the tRNA. Misaminoacylated tRNAs that escape these proofreading mechanisms may be edited after release from the synthetase (i.e., in trans) by the appropriate aaRS.

Figure 2. Ribosome Decoding and Translocation during Translation Elongation

There are multiple opportunities for rejection of incorrect tRNAs during the selection of the cognate aminoacyl-tRNA for the codon in the ribosomal A site. Non-cognate and most near-cognate tRNAs are rejected during initial screening of EF-Tu•aminoacyl-tRNA•GTP ternary complexes. Near-cognate complexes that make it past initial screening can be rejected after GTP hydrolysis, or even after the release of EF-Tu•GDP. Peptide bond formation leads to a spontaneous rotation of the ribosomal subunits to form the rotated state. Binding of EF-G stabilizes this rotated state, and GTP hydrolysis catalyzes the translocation of the ribosome, and its return to the non-rotated state.

Figure 3. Modifications in the tRNA Anticodon Stem Loop Modulate Translational Fidelity

(A) tRNAs decoding ANN codons (where N is any nucleotide) have the t⁶A modification at nucleotide 37. Loss of this modification leads to recognition of non-AUG codons by the initiator methionyl tRNA. (B) The modification of uridine to pseudouridine (Ψ) at nucleotides 38 and 39 broadens the decoding capacity of tRNAs. Modified tRNA^Leu_CAA can decode stop (UAG) codons, leading to stop codon readthrough. Loss of this modification at these nucleotides narrows the decoding ability of the tRNA, preventing basepairing with UAG stop codons. (C) Uridine at the wobble position (nucleotide 34) of most tRNAs is modified to ncm⁵U, mcm⁵U, or mcm⁵s²U. Loss of these modifications disrupts decoding of the cognate codon, and leads to codon specific ribosome pausing.

Figure I. The Integrated Stress Response, and its Activation by Translational Infidelity

Activation of the eIF2α kinases by different stressors leads to repression of global translation and selective upregulation of translation from transcripts such as ATF4 (in mammals; GCN4 in yeast) that are regulated by uORFs. Translational infidelity caused by changes in either tRNA modifications or levels can activate this stress response in both canonical and non-canonical ways.