The Ribosome Hypothesis: Decoding Mood Disorder Complexity.

Several types of mood disorders lie along a continuum, with nebulous boundaries between them. Understanding the mechanisms that contribute to mood disorder complexity is critical for effective treatment. However, present treatments are largely centered around neurotransmission and receptor-based hypotheses, which, given the high instance of treatment resistance, fail to adequately explain the complexities of mood disorders. In this opinion piece, based on our recent results, we propose a ribosome hypothesis of mood disorders. We suggest that any hypothesis seeking to explain the diverse nature of mood disorders must incorporate infrastructure diversity that results in a wide range of effects. Ribosomes, with their mobility across neurites and complex composition, have the potential to become specialized during stress; thus, ribosome diversity and dysregulation are well suited to explaining mood disorder complexity. Here, we first establish a framework connecting ribosomes to the current state of knowledge associated with mood disorders. Then, we describe the potential mechanisms through which ribosomes could homeostatically regulate systems to manifest diverse mood disorder phenotypes and discuss approaches for substantiating the ribosome hypothesis. Investigating these mechanisms as therapeutic targets holds promise for transdiagnostic avenues targeting mood disorders.

45S rDNA Diversity In Natura as One Step towards Ribosomal Heterogeneity in Arabidopsis thaliana.

The keystone of ribosome biogenesis is the transcription of 45S rDNA. The Arabidopsis thaliana genome contains hundreds of 45S rDNA units; however, they are not all transcribed. Notably, 45S rDNA units contain insertions/deletions revealing the existence of heterogeneous rRNA genes and, likely, heterogeneous ribosomes for rRNAs. In order to obtain an overall picture of 45S rDNA diversity sustaining the synthesis of rRNAs and, subsequently, of ribosomes in natura, we took advantage of 320 new occurrences of Arabidopsis thaliana as a metapopulation named At66, sampled from 0 to 1900 m of altitude in the eastern Pyrenees in France. We found that the 45S rDNA copy number is very dynamic in natura and identified new genotypes for both 5' and 3' External Transcribed Spacers (ETS). Interestingly, the highest 5'ETS genotype diversity is found in altitude while the highest 3'ETS genotype diversity is found at sea level. Structural analysis of 45S rDNA also shows conservation in natura of specific 5'ETS and 3'ETS sequences/features required to control rDNA expression and the processing of rRNAs. In conclusion, At66 is a worthwhile natural laboratory, and unraveled 45S rDNA diversity represents an interesting starting material to select subsets for rDNA transcription and alter the rRNA composition of ribosomes both intra- and inter-site.

Proteome diversification by mRNA translation in cancer.

mRNA translation is a highly conserved and tightly controlled mechanism for protein synthesis and is well known to be altered by oncogenes to promote cancer development. This distorted mRNA translation is accompanied by the vulnerability of cancer to inhibitors of key mRNA translation components. Novel studies also suggest that these alternations could be utilized for immunotherapy. Ribosome heterogeneity and alternative responses to nutrient shortages, which aid cancer growth and spread, are proposed to elicit aberrant protein production but may also result in previously unidentified therapeutic targets, such as the presentation of cancer-specific peptides at the surface of cancer cells (neoepitopes). This review will assess the driving forces in tRNA and ribosome function that underlie proteome diversification due to alterations in mRNA translation in cancer cells.

Promising Assays for Examining a Putative Role of Ribosomal Heterogeneity in COVID-19 Susceptibility and Severity.

The heterogeneity of ribosomes, characterized by structural variations, arises from differences in types, numbers, and/or post-translational modifications of participating ribosomal proteins (RPs), ribosomal RNAs (rRNAs) sequence variants plus post-transcriptional modifications, and additional molecules essential for forming a translational machinery. The ribosomal heterogeneity within an individual organism or a single cell leads to preferential translations of selected messenger RNA (mRNA) transcripts over others, especially in response to environmental cues. The role of ribosomal heterogeneity in SARS-CoV-2 coronavirus infection, propagation, related symptoms, or vaccine responses is not known, and a technique to examine these has not yet been developed. Tools to detect ribosomal heterogeneity or to profile translating mRNAs independently cannot identify unique or specialized ribosome(s) along with corresponding mRNA substrate(s). Concurrent characterizations of RPs and/or rRNAs with mRNA substrate from a single ribosome would be critical to decipher the putative role of ribosomal heterogeneity in the COVID-19 disease, caused by the SARS-CoV-2, which hijacks the host ribosome to preferentially translate its RNA genome. Such a protocol should be able to provide a high-throughput screening of clinical samples in a large population that would reach a statistical power for determining the impact of a specialized ribosome to specific characteristics of the disease. These characteristics may include host susceptibility, viral infectivity and transmissibility, severity of symptoms, antiviral treatment responses, and vaccine immunogenicity including its side effect and efficacy. In this study, several state-of-the-art techniques, in particular, chemical probing of ribosomal components or rRNA structures, proximity ligation to generate rRNA-mRNA chimeras for sequencing, nanopore gating of individual ribosomes, nanopore RNA sequencing and/or structural analyses, single-ribosome mass spectrometry, and microfluidic droplets for separating ribosomes or indexing rRNAs/mRNAs, are discussed. The key elements for further improvement and proper integration of the above techniques to potentially arrive at a high-throughput protocol for examining individual ribosomes and their mRNA substrates in a clinical setting are also presented.

Angiogenin (ANG)-Ribonuclease Inhibitor (RNH1) System in Protein Synthesis and Disease.

Protein synthesis is a highly complex process executed by well-organized translation machinery. Ribosomes, tRNAs and mRNAs are the principal components of this machinery whereas RNA binding proteins and ribosome interacting partners act as accessory factors. Angiogenin (ANG)-Ribonuclease inhibitor (RNH1) system is one such accessory part of the translation machinery that came into focus afresh due to its unconventional role in the translation. ANG is conventionally known for its ability to induce blood vessel formation and RNH1 as a "sentry" to protect RNAs from extracellular RNases. However, recent studies suggest them to be important in translation regulation. During cell homeostasis, ANG in the nucleus promotes rRNA transcription. While under stress, ANG translocates to the cytosol and cleaves tRNA into fragments which inhibit ribosome biogenesis and protein synthesis. RNH1, which intimately interacts with ANG to inhibit its ribonucleolytic activity, can also bind to the 40S ribosomes and control translation by yet to be known mechanisms. Here, we review recent advancement in the knowledge of translation regulation by the ANG-RNH1 system. We also gather information about this system in cell homeostasis as well as in pathological conditions such as cancer and ribosomopathies. Additionally, we discuss the future research directions and therapeutic potential of this system.

Loss of m1acp3Ψ Ribosomal RNA Modification Is a Major Feature of Cancer.

The ribosome is an RNA-protein complex that is essential for translation in all domains of life. The structural and catalytic core of the ribosome is its ribosomal RNA (rRNA). While mutations in ribosomal protein (RP) genes are known drivers of oncogenesis, oncogenic rRNA variants have remained elusive. We identify a cancer-specific single-nucleotide variation in 18S rRNA at nucleotide 1248.U in up to 45.9% of patients with colorectal carcinoma (CRC) and present across >22 cancer types. This is the site of a unique hyper-modified base, 1-methyl-3-α-amino-α-carboxyl-propyl pseudouridine (m1acp3Ψ), a >1-billion-years-conserved RNA modification at the peptidyl decoding site of the ribosome. A subset of CRC tumors we call hypo-m1acp3Ψ shows sub-stoichiometric m1acp3Ψ modification, unlike normal control tissues. An m1acp3Ψ knockout model and hypo-m1acp3Ψ patient tumors share a translational signature characterized by highly abundant ribosomal proteins. Thus, m1acp3Ψ-deficient rRNA forms an uncharacterized class of "onco-ribosome" which may serve as a chemotherapeutic target for treating cancer patients.

Is the cellular initiation of translation an exclusive property of the initiator tRNAs?

Translation of mRNAs is the primary function of the ribosomal machinery. Although cells allow for a certain level of translational errors/mistranslation (which may well be a strategic need), maintenance of the fidelity of translation is vital for the cellular function and fitness. The P-site bound initiator tRNA selects the start codon in an mRNA and specifies the reading frame. A direct P-site binding of the initiator tRNA is a function of its special structural features, ribosomal elements, and the initiation factors. A highly conserved feature of the 3 consecutive G:C base pairs (3 GC pairs) in the anticodon stem of the initiator tRNAs is vital in directing it to the P-site. Mutations in the 3 GC pairs diminish/abolish initiation under normal physiological conditions. Using molecular genetics approaches, we have identified conditions that allow initiation with the mutant tRNAs in Escherichia coli. During our studies, we have uncovered a novel phenomenon of in vivo initiation by elongator tRNAs. Here, we recapitulate how the cellular abundance of the initiator tRNA, and nucleoside modifications in rRNA are connected with the tRNA selection in the P-site. We then discuss our recent finding of how a conserved feature in the mRNA, the Shine-Dalgarno sequence, influences tRNA selection in the P-site.