The New Life of tRNAs - From Passive Translators to Active Regulators

tRNAs are usually portrayed as abundant, ubiquitous, passive mRNA decoders and protein translators. tRNAs carrying the same amino acid are called isoacceptors, while tRNAs sharing an anticodon but differing in body sequence are isodecoders. Recent work has overturned the passive view: tRNA isoacceptors and isodecoders carry specific, non-translational regulatory functions that can profoundly influence biology and disease. tRNAs are also highly abundant in biofluids, making them attractive biomarkers. Studying these hidden functions opens a new chapter in tRNA biology.

• Ordinary, passive RNAs in protein translation.
• Abundant, ubiquitous.
• Cell-, tissue-, disease-, and temporal-specific expression.
• Wobble decoding of isoacceptors affects translation.
• Isodecoders impact biological processes and diseases.
• tRNAs and derived fragments are the most abundant biomarkers in biofluids.

Cell, Tissue, and Disease Specific Expression

tRNA isoacceptor and isodecoder profiles are tied to cell differentiation and proliferation (Fig. 1A), with strong tissue, cell type, spatial, and temporal specificity during embryonic development (Fig. 1B), and disease specificity in brain, thymus, blood, spleen, liver, testis, ovary, and breast cancer (Fig. 1C). These expression specificities are linked to biological function and pathophysiology, and they also improve tRNA biomarker performance.

Fig. 1. (A) Proliferating and differentiating cells have distinct tRNA repertoire signatures [1]. (B) tRNAs show specific expression in tissues, organs, and developmental stages [2]. (C) Differential tRNA expression in human tissues and diseases; cancer cells generally express higher nuclear-encoded tRNA than every tissue examined [3].

Wobble Decoding of Isoacceptors Affects Translation

The tRNA pool, built from isoacceptor families for each amino acid, informs studies of codon usage, translation efficiency and accuracy, biological processes, and human diseases. A codon can be read by a perfectly matched cognate anticodon or by a near-cognate tRNA with a single wobble-position mismatch. The cognate-to-near-cognate ratios in the pool are dynamically regulated, influencing translation efficiency, fidelity, and transcript stability (Fig. 2) [4]. Under different conditions, tRNA pools shift to maintain homeostasis or favor a particular gene expression program [5]. Perturbed isoacceptor levels contribute to diseases such as cancer (Fig. 3A) [6] and neurodegeneration (Fig. 3B) [7].

Fig. 2. (A) An optimal codon has cognate tRNA supply exceeding translation demand; a non-optimal codon faces short supply or near-cognate usage. (B) Optimal codons favor faster translation and higher decoding accuracy; non-optimal codons are slower but favor accurate protein folding, often in inter-domain linker regions. (C) Optimal codons at fast elongation protect mRNA from decay, while slow non-optimal codons expose mRNA to degradation and shorten half-lives.

Fig. 3. (A) Overexpression of tRNA-Glu-UUC and tRNA-Arg-CCG drives oncogenic metastasis, with their optimal codons upregulating oncoprotein EXOSC2 and GRIPAP1 translation [6]. (B) Normal tRNA-Gln-CUG levels correctly translate the poly-Q repeat in huntingtin; reduced levels cause -1 frameshift translation producing erroneous poly-alanines. The Q/A ratio determines aggregation state and Huntington's disease severity [7].

Isodecoders Impact Biological Processes and Diseases

Highly conserved isodecoder sequences can differ in tissue-specific expression and function. One of five mouse tRNA-Arg-UCU isodecoders is central-nervous-system-specific (Fig. 4A). Its loss causes ribosome stalling on mRNAs, an ATF4-driven stress response, and widespread neurodegeneration (Fig. 4B-D). This CNS-specific isodecoder thus maintains neuronal homeostasis and prevents neurodegeneration [8].

Fig. 4. (A) CNS-specific and housekeeping isodecoder genes of tRNA-Arg-UCU. (B) The CNS-specific isodecoder is vital for translation in wild-type neurons; without it, cells undergo neurodegeneration. (C) Normal wild-type brain. (D) Neurodegenerative brain lacking CNS-specific tRNA-Arg-UCU isodecoder [8].

Non-Canonical, Non-Translational Regulatory tRNA Functions

tRNAs can regulate cellular functions without direct protein translation. For example, tRNAs bind and inhibit cytochrome C from forming the apoptosome in apoptosis [9, 10], acting as an anti-apoptosis factor.

tRNAs are also first responders to cellular stress such as heat shock, hypoxic shock, or nutrient deprivation, shutting down global translation through multiple regulatory mechanisms (Fig. 5) [11].

Fig. 5. tRNAs respond to cellular stress by (A) re-importing tRNAs from cytoplasm to nucleus, depleting the cytosolic pool; (B) deactivating the tRNA 3-prime CCA arm; (C) hypoxia-induced tRNA cleavage generating tRF and tiRNA fragments that inhibit translation initiation; (D) reprogramming tRNA modifications during starvation to favor translation of amino-acid-biosynthesis enzymes [11].

tRNA Biomarkers in Biofluids

Small RNAs such as microRNAs are widely explored as biomarkers. tRNAs are now emerging as a new biomarker class thanks to several favorable properties. They are abundant small RNAs and particularly enriched in biofluids, even more so than miRNAs (Fig. 6A) [12, 13]. tRNAs are dramatically enriched in exosomes, becoming the most abundant RNA class after rRNAs, far exceeding miRNAs (Fig. 6B) [14].

High abundance, stability conferred by heavy modifications, and biofluid enrichment translate into better biomarker sensitivity. Combined with the tight association between perturbed tRNA repertoires and disease (Fig. 1), tRNAs represent a biomarker class with great potential.

Fig. 6. (A) Relative proportions of miRNA vs tRNA in biofluids; many biofluids contain much more tRNA than miRNA [12, 13]. (B) tRNAs are highly enriched in exosomal RNA compared with intracellular RNA, much more so than miRNAs [14].

nrStar™ tRNA PCR Arrays

References

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