Regulation of mRNA Translation in Neurons-A Matter of Life and Death

Abstract

Dynamic regulation of mRNA translation initiation and elongation is essential for the survival and function of neural cells. Global reductions in translation initiation resulting from mutations in the translational machinery or inappropriate activation of the integrated stress response may contribute to pathogenesis in a subset of neurodegenerative disorders. Aberrant proteins generated by non-canonical translation initiation may be a factor in the neuron death observed in the nucleotide repeat expansion diseases. Dysfunction of central components of the elongation machinery, such as the tRNAs and their associated enzymes, can cause translational infidelity and ribosome stalling, resulting in neurodegeneration. Taken together, dysregulation of mRNA translation is emerging as a unifying mechanism underlying the pathogenesis of many neurodegenerative disorders.

Keywords: GTPBP2; aminoacyl tRNA synthetase; angiogenin; eIF2α phosphorylation; integrated stress response; mTOR; mistranslation; n-Tr20; n-Trtct5; repeat-associated non-ATG translation.

Figure 1. Eukaryotic Cap-Dependent Translation Initiation and Key Regulatory Pathways

Initiation begins with the assembly of the 43S preinitiation complex (PIC), consisting of the 40S small ribosomal subunit, eIF1, eIF1A, eIF3, eIF5, and the ternary complex eIF2-GTP-Met-tRNA_i. The PIC is recruited to the 5’ cap of the mRNA by the eIF4F complex (eIF4E, eIF4G, and eIF4A) and eIF4B. Circularization of the mRNA is promoted by interaction between eIF4G and PABP. The mRNA is scanned in a 5’ to 3’ direction until a start (AUG) codon is recognized, triggering the release of eIF1, inorganic phosphate (Pi), eIF5, and eIF2-GDP. The joining of the 60S ribosomal subunit to the PIC and release of several initiation factors, including eIF1A, is catalyzed by eIF5B, leading to the formation of the elongation-competent 80S ribosome. eIF2-GDP is recycled to eIF2-GTP by the exchange factor eIF2B. Under stress conditions, phosphorylation of the α-subunit of eIF2 (eIF2α) impairs recycling and ternary complex formation. Translation initiation is also regulated by mTOR, which can directly or indirectly phosphorylate eIF4G, eIF4B, 4E-BP, and PDCD4, promoting cap-dependent initiation.

Figure 2. Eukaryotic Translation Elongation, Termination, and Ribosome Recycling

In each elongation cycle, an eEF1A-GTP-aminoacyl (aa)-tRNA ternary complex binds the ribosome, with the anticodon loop of the tRNA in contact with the mRNA codon at the A site. Recognition of the codon triggers GTP hydrolysis and the dissociation of eEF1A-GDP. Peptide bond formation leads to large conformational changes in the ribosome, and the tRNAs transition into hybrid states, with their anticodon loops in the A and P sites, and their acceptor stems shifted to the P and E sites respectively. Binding of eEF2-GTP to the A site and subsequent GTP hydrolysis promotes the translocation of the ribosome, shifting the tRNAs into the canonical P and E sites and ratcheting the mRNA by exactly one codon. The ribosome is now poised for another cycle of elongation, with binding of an appropriate eEF1A-GTP-aminoacyl-tRNA complex to the A site, and release of the deacylated tRNA from the E site. The elongation process continues until a stop codon is reached that is recognized by eRF1-eRF3-GTP. Binding of this complex to the ribosome leads to GTP hydrolysis, and the release of eRF3-GDP and the polypeptide. Finally, ABCE1 facilitates subunit dissociation and recycling, regenerating the components for another round of translation.

Figure 3. tRNA Processing in Health and in Neurodegeneration

Precursor tRNAs undergo numerous processing steps in order to form mature tRNAs that can be charged by their cognate aminoacyl tRNA synthetases. A small subset of tRNAs contain introns that are removed by splicing, and the tRNA splicing endonuclease (TSEN) complex, along with the kinase CLP1, play an essential role in this process. Disease-linked mutations in TSEN subunits or in CLP1 disrupt the assembly and activity of the TSEN/CLP1 complex, impairing the processing of precursor tRNAs. This leads to the accumulation of intermediate fragments, and in some cases, a reduction in the level of mature tRNA available for charging. Mature tRNAs can be cleaved by a stress-activated ribonuclease, angiogenin (ANG). Cleavage within the anticodon loop releases tRNA halves, and specific 5’ tRNA halves can displace the eukaryotic initiation factors (eIF) 4G and eIF4A from the 7-methylguanosine (m⁷G) cap of the mRNA, repressing translation initiation. ANG can also remove the CCA tail (added by TRNT1) from the 3’ end of the tRNA, preventing charging by the synthetase, and thus inhibiting translation elongation.

Figure 4. Ribosome stalling-mediated neurodegeneration

A processing mutation in n-Tr20, an arginine tRNA gene, in C57BL/6J mice significantly reduces the pool of tRNA^ArgUCU available for translation. This reduction leads to increased ribosome pausing on cognate AGA codons. GTPBP2 and Pelota play a role in the resolution of these paused ribosomes, and ribosome recycling may be accompanied by the degradation of the nascent protein and of the mRNA. In the absence of GTPBP2, ribosome pausing at the AGA codons is not resolved, leading to neurodegeneration. Ribosome stalling activates the integrated stress response (ISR) via the eIF2a kinase GCN2, and neurodegeneration is exacerbated in the absence of ISR activation.