Ribosome slowdown triggers codon‐mediated mRNA decay independently of ribosome quality control

  1. Yuichiro Mishima
  2. Peixun Han
  3. Kota Ishibashi
  4. Seisuke Kimura
  5. Shintaro Iwasaki

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  1. Version published to 10.15252/embj.2021109256
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    Reply to the reviewers

    Reviewer #1 (Evidence, reproducibility and clarity (Required)):

    In this manuscript, Mishima et al., designed a reporter system (dubbed PACE, for Parallel Analysis of Codon Effects) to assess the effect of codon usage in regulating mRNA stability in a controlled sequence context. This reporter corresponds to a stretch of 20 repetitions of a given codon (to be tested for its effect on mRNA stability), each repetition being separated by one codon corresponding to each of the 20 canonical amino acids. This stretch is inserted at the 3' end of the coding sequence of a superfolder GFP flanked with fixed 5' and 3' untranslated regions. In vitro transcribed capped and polyadenylated RNAs are then produced from these reporters (each with a specific stretch of repetitions of a given codon), pooled together and injected into zebrafish zygotes to monitor their relative abundance at different time points upon injection.

    Using the PACE reporter, the authors were able to obtain a quantitative estimation of the impact of 58 out of the 61 sense codons on modulating mRNA stability. Their results are in agreement with a previous report that estimated the effect of codon usage on mRNA stability using endogenous mRNAs and an ORFeome library (Bazzini et al., 2016). However, contrary to relying on endogenous mRNAs and ORFeome reporters, the advantage of the PACE strategy is that the effect of the codon to be studied can be probed in a defined context, thus avoiding the presence of other motifs or transcript features that could also regulate mRNA stability. Similarly to results from Bazzini et al., 2016, the authors show that blocking translation completely abrogates the effect of codon usage, indicating that translation is required to drive codon-dependent mRNA degradation from their reporters. Also, the extent of codon-dependent mRNA decay is correlated with tRNA abundance and occurs through a process involving mRNA deadenylation as previously described in the zebrafish (Mishima et al., 2016 and Bazzini et al., 2016).

    Having validated their PACE protocol, the authors performed ribosome profiling to test whether ribosome occupancy on tested codons is correlated with their capacity to drive mRNA degradation. Their results indicate that, at least for polar amino acids, there is indeed an inverse correlation between ribosome occupancy at tested codons and mRNA stability thus suggesting that slow decoding of codons due to low levels of available cognate tRNA can induce mRNA degradation. The authors further validate this finding by reducing the levels of aminoacylated tRNAAsn (corresponding a polar amino acid) and showing that stability of the reporter RNA carrying a stretch of AAC codons (decoded by tRNAAsnGUU) is reduced. To test whether codon-dependent mRNA degradation in the context of slow ribosome decoding lead to ribosome stalling and collisions, the authors generated a mutant zebrafish strain with impaired expression of ZNF598 (an essential factor of the No-Go decay (NGD) pathway in yeast). They also integrated a known ribosome stalling sequence from hCMV (and a mutant version that does not trigger ribosome stalling) in their sfGFP reporter construct as a positive control for NGD in their assays. Their results indicate that although ZNF598 depletion impairs degradation of the hCMV reporter (as expected), it does not affect codon-dependent mRNA degradation, which appears to occur for most codons through a NGD-independent manner. Finally, through the use of a tandem ORF reporter assay separated by codon tags to be tested, the authors show that destabilizing codons do not stall ribosomes but only lead to their transient slowdown which induces mRNA deadenylation and degradation in a ZNF598-independent manner.

    Overall, the manuscript is very well written and pleasant to read. The introduction is well documented and relevant to the study as it allows readers to place the study in the current context of the field while highlighting open questions that have not been addressed yet. The results are clearly presented, the technical approaches are elegant and the conclusions convincing.

    Below you will find some major and minor points that, in my opinion, should be addressed by the authors.

    **Major point:**

    • One interesting aspect of the PACE reporter assay is the possibility to monitor ribosome occupancy in parallel for all codon-tags tested, which the authors did in Figure 3. However, instead of using RNA-seq data to normalize ribosome footprints and obtain ribosome occupancy, the authors used an alternative normalization approach consisting, for each codon-tag, to calculate the number of ribosome footprints with test codons in the A site divided by the number of ribosome footprints with spacer codons in the A site. This approach is elegant and appears to work with codons corresponding to polar amino acids. However, it might have its limitations for other codons.

    Indeed, ribosome dwell times (in yeast and mammals) have been shown to respond both to tRNA availability but also to other features such as the nature of the pair of adjacent codons, and the nature of the amino acid within the exit channel (Gobet C et al., 2020 PNAS; Gamble CE et al., 2016 Cell; Pavlov MY et al., 2009 PNAS). However, based on the work of "Buschauer R et al., 2020 Science", only ribosomes lacking an accommodated tRNA at the A site are able to recruit Ccr4-Not to mediate mRNA deadenylation and degradation. Other events that increase ribosome dwell time (and thus occupancy), such as slow peptidyl-transfer, do not lead to Ccr4-Not recruitment and are resolved by eIF5A. It is therefore possible that depending on the nature of the codon that is being tested, ribosome occupancy at test and spacer codons can be biased by the nature of codon-pairs and "dilute" the effects of tRNA availability.

    If the authors performed RNA-seq together with the ribosome profiling experiment, it might be interesting to use the RNA-seq data to calculate ribosome occupancy on "tested" and "spacer" codons to check whether using this normalization, they do find a negative correlation between ribosome occupancy and PACE stability. A different approach would be to perform ribosome run-off experiments using harringtonine and estimate the elongation speed across the codon tag. However, I am aware that this experiment could be tedious an expensive.

    • Figure 6: Insertion of the Lys x8 AAA stretch in the tandem ORF reporter leads to a decrease in HA-DsRedEx expression compared to that of Myc-EGFP. However, results from "Juszkiewicz and Hedge, 2017" using a similar reporter in mammalian cells indicate that stretches of Lys AAA below 20 repetitions only elicit poor RQC (less than 10% of true ribosome stalling for 12 repetitions of the AAA codon). Instead, most of the loss in RFP signal results from a change in the reading frame of ribosomes due to the "slippery" translation of the poly(A) stretch. I therefore think that it could be important to perform the experiment in ZNF598 KO embryos to validate that the observed reduction in HA-dsRedEx does indeed result from stalling and RQC and not from a change in the reading frame of ribosomes.

    On a similar note, how do the authors explain the decrease in signal of the Flag-EGFP and HA-DsRedEx observed when using the Flag-EGFP with non-optimal codons? I understand that RQC occurring through NGD leads to ribosome disassembly at the stalling site and possibly mRNA cleavage (thus explaining the decrease in HA-DsRedEx signal compared to Myc-EGFP). However, I would assume that codon-mediated mRNA decay (even for ORF longer than 200 of non-optimal codons) should trigger mRNA deadenylation, followed by decapping and co-translational 5'to3' mRNA degradation, following the last translating ribosome. I would therefore expect not to see any change in the HA-DsRedEx/Myc-EGFP ratio even for the non-optimal Flag-EGFP reporter. Could the 200 non-optimal codons trigger some background RQC through NGD? Or could there be some ribosome drop-off? It might be interesting to test the optimal and non-optimal Flag-EGFP reporters in the ZNF598 KO background to check whether the observed decrease in the relative amount of HA-DsRedEx results from stalling-dependent RQC.

    **Minor comments:**

    • The color-coded CSC results from "Bazzini et al., 2016" presented at the bottom of panel B in figure 2 are misleading because many codons (such as PheUUU, AsnAAU, TyrUAC...) are lacking information. I have the impression that the authors used the combined data from the rCSCI (obtained from the reporter RNAs) and CSC (obtained from endogenous transcripts) corresponding to Figure 1F from Bazzini et al., 2016. This data set excluded all codons that were not concordant between the endogenous and reporter CSCs (which are those that are lacking a color code in Figure 2B from this study). However, in the scatter-plot of PACE Vs CSC (from Supplemental Figure 1D of this study), the authors used the complete set of CSC values from Bazzini et al .,2016. Could the authors please use the complete set of CSC values from Bazzini et al., 2016 to color code codons in their Figure 2B?
    • Figure 4B. The charged tRNA measurements seem to have been done in a single biological replicate (there aren't any error bars in the chart). I understand that the procedure is tedious and requires a large amount of total RNA to begin with, but it would be preferable to perform it in three biological replicates.
    • Supplementary Figure 2B. I do not understand what the figure represents. The legend is quite cryptic and states that the panel corresponds to the information content of each reading frame. More information should be given so that readers can understand how to interpret de figure and extract periodicity information.

    Reviewer #1 (Significance (Required)):

    Since the seminal work from Jeff Coller's laboratory in 2015 (Presnyak et al., 2015 Cell) showing a global and major role for codon optimality in determining mRNA half-lives in yeast, the role of codon usage in modulating translation and stability of mRNAs has been widely studied in different organisms (including zebrafish and mammals). As stated by the authors in the introduction, most studies have relied on correlation analyses between codon usage and mRNA half-lives from endogenous transcripts or from ORF libraries with fixed 5'UTR and 3'UTRs. This approaches could suffer from the presence of transcript features that can participate in other mRNA degradation pathways, which could limit their use when performing further mechanistic studies.

    The work by Mishima and collaborators presents an original reporter assay that allows to evaluate the role of codon usage on regulating mRNA stability in a defined context, thus avoiding the impact of confounding factors that could bias the measurement of mRNA stability. Results obtained using this reporter are in good agreement with previous reports from Zebrafish (Bazzini et al 2016., and Mishima et al., 2016). From this validated reporter approach, the authors further show that codon-dependent mRNA degradation is directly related to tRNA availability and (at least partially) to ribosome occupancy (two factors already suggested as being important for codon-mediated decay in zebrafish, although they were based on correlation analyses). Furthermore, the authors show that codon-mediated mRNA decay occurs during productive mRNA translation and that it is functionally distinct from RQC induced by ribosome stalling. As a consequence, codon-mediated mRNA degradation is independent from the RQC factor ZNF598 (which they also validate for the first time as an important RQC factor in zebrafish). This information is new within metazoans since only in yeast it has been clearly shown that codon-mediated mRNA decay is distinct from RQC induced by ribosome stalling and collisions.

    Taken together, the reported findings will be of interest to the community working on mRNA metabolism and translation. It could also interest, more broadly, scientists working on translational selection and genome evolution.

    Reviewer #2 (Evidence, reproducibility and clarity (Required)):

    In this manuscript, Mishima et al aim to determine if the RNA-mediated decay determined by codon optimality is part of the ribosome quality control pathway, triggered by slowed codon decoding and ribosome stalling or it is an independent pathway.

    To this end, the authors capitalize on their previous work to design a very elegant high-throughput reporter system that can analyze individually codon usage, ribosome occupancy and tRNA abundance. This reporter system, called PACE, is rigorously validated throughout the manuscript, because blocking translation with a morpholino blocking the AUG codon demonstrated that the effects no RNA stability are translation dependent.

    When most of the available codons are tested using the PACE system, the authors recapitulate codon optimality profiles similar to the ones previously uncovered using transcriptome-wide approaches.

    Thanks to the design of the reporter, which alternates repeats of a test codon with random codons, the authors can calculate how quickly a ribosome decodes the test codon on average. With this approach, the authors uncover a negative correlation between RNA stability and ribosome density on codons for polar amino acids and suggest that codon optimality is related to a slower decoding of the codons.

    With the PACE reporter validated, the authors can interrogate the system to gain mechanistic insights of codon optimality. First, they test if RNA decay and deadenylation mediated by codon optimality is determined, in part, by the levels of aminoacylated tRNAs available. The authors use a very elegant approach, as they overexpress a bacterial enzyme (AnsB) in zebrafish that degrades asparagine, effectively reducing the levels of tRNA-Asn. The authors demonstrate that AnsB turns a previously optimal Asn codon, AAC, into a non-optimal one. This effect is translated into RNA destabilization and deadenylation, but this effect in not extended to other codons encoding amino acids not affected by Asn. These results provide a direct experimental validation of the previously published observation of tRNA levels and codon optimality.

    Finally, the authors interrogate the relationship between the codon optimality pathways and the ribosome quality control pathways, that takes care of stalled ribosomes. The authors generate a zebrafish mutant of Znf598, a vertebrate homolog of the yeast protein in charge of resolving stalled ribosomes. Using a maternal-and-zygotic mutant, the authors demonstrate that in these mutant's codon optimality proceeds as usual but ribosome stalling is not resolved, providing evidence for first time that Znf598 is involved in ribosome quality control in vertebrates.

    Altogether, this manuscript presents work that builds on the previous findings of the authors and other labs but it is a qualitative leap forward rather than a marginal increment, because the body of work in the current manuscript i) establishes a reporter to dissect the mechanisms of codon optimality, ii) demonstrates that ribosome slow-down but not stalling is part of the trigger of RNA decay mediated by codon optimality, iii) demonstrates that this pathway is independent of ribosome quality control pathway and finally iv) demonstrates that vertebrate Znf598 is involved in the RNA decay mediated by ribosome stalling.

    Due to these novel findings, and the rigor of the experimental design, this manuscript should be accepted for publication. The authors should first address the following comments:

    **Major comment:**

    1. The authors very elegantly demonstrate the impact of AnsB on the stability of the RNA reporter, and it is precisely the simplicity of the reporter that allows the authors to draw clear conclusions. Nevertheless, it would be interesting to determine if the reporter results in embryos injected with AnsB also translate to endogenous genes rich in AAC codons. The authors could perform a polyA-selected RNA-Seq in embryos treated with AnsB to determine if the transcripts rich in AAC codons are destabilized compared to wild-type, thus validating the reporter results in endogenous genes. **Minor comments:**

    In figure 5J the authors plot the normalized codon tag levels of the PACE reporter run in the MZznf598 mutant. The authors color code the labels in the x-axis following the PACE results in wild-type (figure 2B). The authors should also plot the wild-type values to have a direct visual comparison of the results trend in both genotypes. The authors focus in the title on the role of Znf598 or the lack thereof in RNA decay induced by codon optimality. However, for the non-aficionados in codon-optimality, ZnF598 is an unknown protein and adds little information to the title. The authors should provide a more informative title, directly pinpointing that codon-optimality is independent of the ribosome quality control pathway.

    Reviewer #2 (Significance (Required)):

    This manuscript presents work that builds on the previous findings made by the authors and other laboratories but it is a qualitative leap forward rather than a marginal increment, because the body of work in the current manuscript i) establishes a reporter to dissect the mechanisms of codon optimality, ii) demonstrates that ribosome slow-down but not stalling is part of the trigger of RNA decay mediated by codon optimality, iii) demonstrates that this pathway is independent of ribosome quality control pathway and finally iv) demonstrates that vertebrate Znf598 is involved in the RNA decay mediated by ribosome stalling.

    In addition to the conceptual findings, the authors establish a new high-throughput reporter system to evaluate the influence of codon optimality in RNA decay.

    The work its done in zebrafish embryos, an in vivo model system where codon optimality has been extensively tested by the authors and others, following the stability of reporter and endogenous genes.

    Reviewer #3 (Evidence, reproducibility and clarity (Required)):

    Mishima et al. address a very timely topic of how the codon composition of the ORF and the associated translation elongation speed affect mRNA stability. Several studies have already shown a strong correlation between codon optimality and mRNA stability - meaning the more "optimal" the codons, the faster supposedly the elongation speed and the more stable the mRNA. Most of these studies were done by analyzing global expression data, with limited follow up, therefore being also impacted by other co-translational mRNA decay pathways and in addition these studies could also not test directly the effect of each single codon on mRNA stability. The authors took a systematic reporter-based assay approach, called PACE, which allowed them to test systematically the effect of codon composition on mRNA decay. By integrating also ribosome profiling data, the authors could nicely show that the speed of translation (measured by ribosome density) correlates with their determined mRNA stability effect of each codon and also the corresponding tRNA levels. However, interestingly this seems to be the case only for codons encoding polar amino acids, but not the ones that encode charged or non-polar amino acids. It will be very interesting to find out why that is? Finally, the authors address if some of the effects they see might be due to ribosome collisions and associated no go decay (NGD). For this they generated a Znf598 mutant by CRISPR-Cas9. Znf598 is the proposed homolog of Hel2, the protein in yeast that is essential for NGD. The authors go on to show that NGD is defective in this mutant, but that codon mediated decay, which is elongation dependent, is not to a large part not dependent on Znf598.

    **All minor comments:**

    1. It is intriguing why only polar AAs show a tRNA amount specific effect in the ribosome footprint data. Some hypothesis/discussion about this could be expanded further in the discussion or results.
    2. On the same token some additional analysis might be helpful. For example, in Figure 2E, the authors group codons in weak, neutral and strong based on PACE measurements and then look at the tRNA expression range for each of the three groups. Could the authors do this also separately for the codons of polar, non-polar and charged amino acids? What do you see - still the same pattern as for all the codons or do again only polar amino acids show the trend?
    3. Can the authors elaborate on the development of their PACE system? Why is it designed the way it is? What parameters did they test? For example, why the 20 amino acids tail, did you you test shorter sequences of the amino acid, spacer repeats, etc?
    4. The next few questions are a bit more of a technical nature regarding the reporter construct used for PACE.
    5. Did all AA pairs (Codon of interest + spacer codon) behave the same in the footprint assay? Does the data have enough information and resolution for this?
    6. Was the order of the spacer codons always the same in all the constructs? Could the specific order, if it is consistent, have any unseen consequences (ie. interaction with the exit tunnel)? Did the authors test other orders?
    7. Are the spacer codons optimized?
    8. Are the codons affected in the NGD mutant the ones that are most different in the Bazzini data?
    9. The authors inject directly mRNA into the embryos, therefore avoiding that the reporter mRNA is ever in the nucleus. However, there could be nuclear events (e.g. loading of particular proteins) that might affect the fate of an mRNA in the cytosol, among these the translation efficiency and also stability. Maybe some comment in the discussion as to the effect of missing nuclear factors would be welcome. This is not a criticism; it would just be nice to hear the authors' thoughts on that.
    10. Page 6; final paragraph: "Finally, we compared the speed of the ribosome translating mRNA destabilizing codons to that of an aberrantly stalled ribosome." Not sure the authors did that actually. They tested the effect of ribosome slowing down on protein production and mRNA levels and compared that to stalling ribosomes, but did not compare the "speed" directly and I am not even sure what they mean by that in this context. Probably good to rephrase.

    Page 7, upper half: ".....by taking the positional effect of codon-mediated decay into account (Mishima and Tomari, 2016)."

    This is my limited knowledge of the literature, but I think you should mention what this positional effect is and not just cite a paper.

    Very minor, but on page 8 when PACE is introduced, the authors show the different destabilizing effects of the three Ile codons. While that is ok, in the section before, when the authors tested their construct by qRT-PCR, they focused on the two Leu codons. I would also mention them here and do a direct comparison of the qRT-PCR results with the pooled PACE result for these two codons. Based on the figure the two codons seem to behave qualitatively like expected, but I am not sure how good the quantitative behavior matches. The AnsB experiment - the authors only mention data about one of the two Asn codons (AAC), but what about the second Asn codon (AAU) - do you also see an effect on that codon upon overexpression of AnsB as well? AAU is already a quite destabilizing codon and you might not see a further increase in destabilization, but it would be great to know if there was or not. Page 13, second paragraph: More out of interest, but it is quite intriguing that GCG turned into a destabilizing codon (opposite of what one would expect if NGD would play a bit a role). Any speculation why? Page 14, end of page and related to Figure 6C: AAU seems much more destabilizing than AAC. Therefore, I would have expected that the inserted sequence with the AAU codons would lead actually to downregulation of the mRNA and therefore the EGFP and DsRFP total protein signal relative to the construct with the AAC inserted in between, even if the ratio of EGFP/DsRed seems unchanged. However, based on the western blot in 6C the total protein levels seem very similar. Isn't that surprising? Although, AAU obviously allows translation to proceed it should still induce a stronger mRNA decay than AAC and therefore result in less total mRNA (and protein level as a consequence). Did the authors quantify the exact levels of the reporter proteins and mRNA and compared them between the two constructs? Page 15, last sentence: Somehow for me the word "transient" is a bit hard to grasp in this context. What do you mean by that - do you really mean "impermanent" or "lasting only for a short amount of time"? Don't you simply mean "weaker", "less strong"? Page 17, second sentence: I think the authors want to reference here Figure 2E and not Figure 2D.

    Reviewer #3 (Significance (Required)):

    All in all, I have to say that it was a real pleasure to read this manuscript. The authors were extremely thorough with their experiments and did nearly never overstate any of their conclusions. It is a very insightful story, which in my opinion will contribute greatly to the field of gene expression and posttranscriptional gene expression regulation in particular. The PACE assay, although a bit artificial, gave very clean results, which agree with the previous literature and could be very useful for future studies. Generating the Znf598 mutant and showing that the codon-dependent decay is independent from NGD is a great addition to this paper. Although it is a bit of a pity that we do not see more of a characterization of the Znf598 mutant in this paper, I do agree with the authors that this might take away a bit of the focus of this manuscript and that the mutant deserves actually its own story. I only have very minor comments/questions for the authors that they should be able to address easily. Finally, I can only repeat myself by saying: congrats on this great paper and I fully support publication.

  3. Review Commons

    Note: This preprint has been reviewed by subject experts for Review Commons. Content has not been altered except for formatting.

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    Referee #3

    Evidence, reproducibility and clarity

    Mishima et al. address a very timely topic of how the codon composition of the ORF and the associated translation elongation speed affect mRNA stability. Several studies have already shown a strong correlation between codon optimality and mRNA stability - meaning the more "optimal" the codons, the faster supposedly the elongation speed and the more stable the mRNA. Most of these studies were done by analyzing global expression data, with limited follow up, therefore being also impacted by other co-translational mRNA decay pathways and in addition these studies could also not test directly the effect of each single codon on mRNA stability. The authors took a systematic reporter-based assay approach, called PACE, which allowed them to test systematically the effect of codon composition on mRNA decay. By integrating also ribosome profiling data, the authors could nicely show that the speed of translation (measured by ribosome density) correlates with their determined mRNA stability effect of each codon and also the corresponding tRNA levels. However, interestingly this seems to be the case only for codons encoding polar amino acids, but not the ones that encode charged or non-polar amino acids. It will be very interesting to find out why that is? Finally, the authors address if some of the effects they see might be due to ribosome collisions and associated no go decay (NGD). For this they generated a Znf598 mutant by CRISPR-Cas9. Znf598 is the proposed homolog of Hel2, the protein in yeast that is essential for NGD. The authors go on to show that NGD is defective in this mutant, but that codon mediated decay, which is elongation dependent, is not to a large part not dependent on Znf598.

    All minor comments:

    1. It is intriguing why only polar AAs show a tRNA amount specific effect in the ribosome footprint data. Some hypothesis/discussion about this could be expanded further in the discussion or results.
    2. On the same token some additional analysis might be helpful. For example, in Figure 2E, the authors group codons in weak, neutral and strong based on PACE measurements and then look at the tRNA expression range for each of the three groups. Could the authors do this also separately for the codons of polar, non-polar and charged amino acids? What do you see - still the same pattern as for all the codons or do again only polar amino acids show the trend?
    3. Can the authors elaborate on the development of their PACE system? Why is it designed the way it is? What parameters did they test? For example, why the 20 amino acids tail, did you you test shorter sequences of the amino acid, spacer repeats, etc?
    4. The next few questions are a bit more of a technical nature regarding the reporter construct used for PACE. a. Did all AA pairs (Codon of interest + spacer codon) behave the same in the footprint assay? Does the data have enough information and resolution for this? b. Was the order of the spacer codons always the same in all the constructs? Could the specific order, if it is consistent, have any unseen consequences (ie. interaction with the exit tunnel)? Did the authors test other orders? c. Are the spacer codons optimized?
    5. Are the codons affected in the NGD mutant the ones that are most different in the Bazzini data?
    6. The authors inject directly mRNA into the embryos, therefore avoiding that the reporter mRNA is ever in the nucleus. However, there could be nuclear events (e.g. loading of particular proteins) that might affect the fate of an mRNA in the cytosol, among these the translation efficiency and also stability. Maybe some comment in the discussion as to the effect of missing nuclear factors would be welcome. This is not a criticism; it would just be nice to hear the authors' thoughts on that.
    7. Page 6; final paragraph: "Finally, we compared the speed of the ribosome translating mRNA destabilizing codons to that of an aberrantly stalled ribosome." Not sure the authors did that actually. They tested the effect of ribosome slowing down on protein production and mRNA levels and compared that to stalling ribosomes, but did not compare the "speed" directly and I am not even sure what they mean by that in this context. Probably good to rephrase.
    8. Page 7, upper half: ".....by taking the positional effect of codon-mediated decay into account (Mishima and Tomari, 2016)." This is my limited knowledge of the literature, but I think you should mention what this positional effect is and not just cite a paper.
    9. Very minor, but on page 8 when PACE is introduced, the authors show the different destabilizing effects of the three Ile codons. While that is ok, in the section before, when the authors tested their construct by qRT-PCR, they focused on the two Leu codons. I would also mention them here and do a direct comparison of the qRT-PCR results with the pooled PACE result for these two codons. Based on the figure the two codons seem to behave qualitatively like expected, but I am not sure how good the quantitative behavior matches.
    10. The AnsB experiment - the authors only mention data about one of the two Asn codons (AAC), but what about the second Asn codon (AAU) - do you also see an effect on that codon upon overexpression of AnsB as well? AAU is already a quite destabilizing codon and you might not see a further increase in destabilization, but it would be great to know if there was or not.
    11. Page 13, second paragraph: More out of interest, but it is quite intriguing that GCG turned into a destabilizing codon (opposite of what one would expect if NGD would play a bit a role). Any speculation why?
    12. Page 14, end of page and related to Figure 6C: AAU seems much more destabilizing than AAC. Therefore, I would have expected that the inserted sequence with the AAU codons would lead actually to downregulation of the mRNA and therefore the EGFP and DsRFP total protein signal relative to the construct with the AAC inserted in between, even if the ratio of EGFP/DsRed seems unchanged. However, based on the western blot in 6C the total protein levels seem very similar. Isn't that surprising? Although, AAU obviously allows translation to proceed it should still induce a stronger mRNA decay than AAC and therefore result in less total mRNA (and protein level as a consequence). Did the authors quantify the exact levels of the reporter proteins and mRNA and compared them between the two constructs?
    13. Page 15, last sentence: Somehow for me the word "transient" is a bit hard to grasp in this context. What do you mean by that - do you really mean "impermanent" or "lasting only for a short amount of time"? Don't you simply mean "weaker", "less strong"?
    14. Page 17, second sentence: I think the authors want to reference here Figure 2E and not Figure 2D.

    Significance

    All in all, I have to say that it was a real pleasure to read this manuscript. The authors were extremely thorough with their experiments and did nearly never overstate any of their conclusions. It is a very insightful story, which in my opinion will contribute greatly to the field of gene expression and posttranscriptional gene expression regulation in particular. The PACE assay, although a bit artificial, gave very clean results, which agree with the previous literature and could be very useful for future studies. Generating the Znf598 mutant and showing that the codon-dependent decay is independent from NGD is a great addition to this paper. Although it is a bit of a pity that we do not see more of a characterization of the Znf598 mutant in this paper, I do agree with the authors that this might take away a bit of the focus of this manuscript and that the mutant deserves actually its own story. I only have very minor comments/questions for the authors that they should be able to address easily. Finally, I can only repeat myself by saying: congrats on this great paper and I fully support publication.

  4. Review Commons

    Note: This preprint has been reviewed by subject experts for Review Commons. Content has not been altered except for formatting.

    Learn more at Review Commons

    Referee #2

    Evidence, reproducibility and clarity

    In this manuscript, Mishima et al aim to determine if the RNA-mediated decay determined by codon optimality is part of the ribosome quality control pathway, triggered by slowed codon decoding and ribosome stalling or it is an independent pathway.

    To this end, the authors capitalize on their previous work to design a very elegant high-throughput reporter system that can analyze individually codon usage, ribosome occupancy and tRNA abundance. This reporter system, called PACE, is rigorously validated throughout the manuscript, because blocking translation with a morpholino blocking the AUG codon demonstrated that the effects no RNA stability are translation dependent.

    When most of the available codons are tested using the PACE system, the authors recapitulate codon optimality profiles similar to the ones previously uncovered using transcriptome-wide approaches.

    Thanks to the design of the reporter, which alternates repeats of a test codon with random codons, the authors can calculate how quickly a ribosome decodes the test codon on average. With this approach, the authors uncover a negative correlation between RNA stability and ribosome density on codons for polar amino acids and suggest that codon optimality is related to a slower decoding of the codons.

    With the PACE reporter validated, the authors can interrogate the system to gain mechanistic insights of codon optimality. First, they test if RNA decay and deadenylation mediated by codon optimality is determined, in part, by the levels of aminoacylated tRNAs available. The authors use a very elegant approach, as they overexpress a bacterial enzyme (AnsB) in zebrafish that degrades asparagine, effectively reducing the levels of tRNA-Asn. The authors demonstrate that AnsB turns a previously optimal Asn codon, AAC, into a non-optimal one. This effect is translated into RNA destabilization and deadenylation, but this effect in not extended to other codons encoding amino acids not affected by Asn. These results provide a direct experimental validation of the previously published observation of tRNA levels and codon optimality.

    Finally, the authors interrogate the relationship between the codon optimality pathways and the ribosome quality control pathways, that takes care of stalled ribosomes. The authors generate a zebrafish mutant of Znf598, a vertebrate homolog of the yeast protein in charge of resolving stalled ribosomes. Using a maternal-and-zygotic mutant, the authors demonstrate that in these mutant's codon optimality proceeds as usual but ribosome stalling is not resolved, providing evidence for first time that Znf598 is involved in ribosome quality control in vertebrates.

    Altogether, this manuscript presents work that builds on the previous findings of the authors and other labs but it is a qualitative leap forward rather than a marginal increment, because the body of work in the current manuscript i) establishes a reporter to dissect the mechanisms of codon optimality, ii) demonstrates that ribosome slow-down but not stalling is part of the trigger of RNA decay mediated by codon optimality, iii) demonstrates that this pathway is independent of ribosome quality control pathway and finally iv) demonstrates that vertebrate Znf598 is involved in the RNA decay mediated by ribosome stalling.

    Due to these novel findings, and the rigor of the experimental design, this manuscript should be accepted for publication. The authors should first address the following comments:

    Major comment:

    1. The authors very elegantly demonstrate the impact of AnsB on the stability of the RNA reporter, and it is precisely the simplicity of the reporter that allows the authors to draw clear conclusions. Nevertheless, it would be interesting to determine if the reporter results in embryos injected with AnsB also translate to endogenous genes rich in AAC codons. The authors could perform a polyA-selected RNA-Seq in embryos treated with AnsB to determine if the transcripts rich in AAC codons are destabilized compared to wild-type, thus validating the reporter results in endogenous genes.

    Minor comments:

    1. In figure 5J the authors plot the normalized codon tag levels of the PACE reporter run in the MZznf598 mutant. The authors color code the labels in the x-axis following the PACE results in wild-type (figure 2B). The authors should also plot the wild-type values to have a direct visual comparison of the results trend in both genotypes.
    2. The authors focus in the title on the role of Znf598 or the lack thereof in RNA decay induced by codon optimality. However, for the non-aficionados in codon-optimality, ZnF598 is an unknown protein and adds little information to the title. The authors should provide a more informative title, directly pinpointing that codon-optimality is independent of the ribosome quality control pathway.

    Significance

    This manuscript presents work that builds on the previous findings made by the authors and other laboratories but it is a qualitative leap forward rather than a marginal increment, because the body of work in the current manuscript i) establishes a reporter to dissect the mechanisms of codon optimality, ii) demonstrates that ribosome slow-down but not stalling is part of the trigger of RNA decay mediated by codon optimality, iii) demonstrates that this pathway is independent of ribosome quality control pathway and finally iv) demonstrates that vertebrate Znf598 is involved in the RNA decay mediated by ribosome stalling.

    In addition to the conceptual findings, the authors establish a new high-throughput reporter system to evaluate the influence of codon optimality in RNA decay.

    The work its done in zebrafish embryos, an in vivo model system where codon optimality has been extensively tested by the authors and others, following the stability of reporter and endogenous genes.

  5. Review Commons

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    Referee #1

    Evidence, reproducibility and clarity

    In this manuscript, Mishima et al., designed a reporter system (dubbed PACE, for Parallel Analysis of Codon Effects) to assess the effect of codon usage in regulating mRNA stability in a controlled sequence context. This reporter corresponds to a stretch of 20 repetitions of a given codon (to be tested for its effect on mRNA stability), each repetition being separated by one codon corresponding to each of the 20 canonical amino acids. This stretch is inserted at the 3' end of the coding sequence of a superfolder GFP flanked with fixed 5' and 3' untranslated regions. In vitro transcribed capped and polyadenylated RNAs are then produced from these reporters (each with a specific stretch of repetitions of a given codon), pooled together and injected into zebrafish zygotes to monitor their relative abundance at different time points upon injection.

    Using the PACE reporter, the authors were able to obtain a quantitative estimation of the impact of 58 out of the 61 sense codons on modulating mRNA stability. Their results are in agreement with a previous report that estimated the effect of codon usage on mRNA stability using endogenous mRNAs and an ORFeome library (Bazzini et al., 2016). However, contrary to relying on endogenous mRNAs and ORFeome reporters, the advantage of the PACE strategy is that the effect of the codon to be studied can be probed in a defined context, thus avoiding the presence of other motifs or transcript features that could also regulate mRNA stability. Similarly to results from Bazzini et al., 2016, the authors show that blocking translation completely abrogates the effect of codon usage, indicating that translation is required to drive codon-dependent mRNA degradation from their reporters. Also, the extent of codon-dependent mRNA decay is correlated with tRNA abundance and occurs through a process involving mRNA deadenylation as previously described in the zebrafish (Mishima et al., 2016 and Bazzini et al., 2016). Having validated their PACE protocol, the authors performed ribosome profiling to test whether ribosome occupancy on tested codons is correlated with their capacity to drive mRNA degradation. Their results indicate that, at least for polar amino acids, there is indeed an inverse correlation between ribosome occupancy at tested codons and mRNA stability thus suggesting that slow decoding of codons due to low levels of available cognate tRNA can induce mRNA degradation. The authors further validate this finding by reducing the levels of aminoacylated tRNAAsn (corresponding a polar amino acid) and showing that stability of the reporter RNA carrying a stretch of AAC codons (decoded by tRNAAsnGUU) is reduced. To test whether codon-dependent mRNA degradation in the context of slow ribosome decoding lead to ribosome stalling and collisions, the authors generated a mutant zebrafish strain with impaired expression of ZNF598 (an essential factor of the No-Go decay (NGD) pathway in yeast). They also integrated a known ribosome stalling sequence from hCMV (and a mutant version that does not trigger ribosome stalling) in their sfGFP reporter construct as a positive control for NGD in their assays. Their results indicate that although ZNF598 depletion impairs degradation of the hCMV reporter (as expected), it does not affect codon-dependent mRNA degradation, which appears to occur for most codons through a NGD-independent manner. Finally, through the use of a tandem ORF reporter assay separated by codon tags to be tested, the authors show that destabilizing codons do not stall ribosomes but only lead to their transient slowdown which induces mRNA deadenylation and degradation in a ZNF598-independent manner.

    Overall, the manuscript is very well written and pleasant to read. The introduction is well documented and relevant to the study as it allows readers to place the study in the current context of the field while highlighting open questions that have not been addressed yet. The results are clearly presented, the technical approaches are elegant and the conclusions convincing.

    Below you will find some major and minor points that, in my opinion, should be addressed by the authors.

    Major point:

    • One interesting aspect of the PACE reporter assay is the possibility to monitor ribosome occupancy in parallel for all codon-tags tested, which the authors did in Figure 3. However, instead of using RNA-seq data to normalize ribosome footprints and obtain ribosome occupancy, the authors used an alternative normalization approach consisting, for each codon-tag, to calculate the number of ribosome footprints with test codons in the A site divided by the number of ribosome footprints with spacer codons in the A site. This approach is elegant and appears to work with codons corresponding to polar amino acids. However, it might have its limitations for other codons.

    Indeed, ribosome dwell times (in yeast and mammals) have been shown to respond both to tRNA availability but also to other features such as the nature of the pair of adjacent codons, and the nature of the amino acid within the exit channel (Gobet C et al., 2020 PNAS; Gamble CE et al., 2016 Cell; Pavlov MY et al., 2009 PNAS). However, based on the work of "Buschauer R et al., 2020 Science", only ribosomes lacking an accommodated tRNA at the A site are able to recruit Ccr4-Not to mediate mRNA deadenylation and degradation. Other events that increase ribosome dwell time (and thus occupancy), such as slow peptidyl-transfer, do not lead to Ccr4-Not recruitment and are resolved by eIF5A. It is therefore possible that depending on the nature of the codon that is being tested, ribosome occupancy at test and spacer codons can be biased by the nature of codon-pairs and "dilute" the effects of tRNA availability.

    If the authors performed RNA-seq together with the ribosome profiling experiment, it might be interesting to use the RNA-seq data to calculate ribosome occupancy on "tested" and "spacer" codons to check whether using this normalization, they do find a negative correlation between ribosome occupancy and PACE stability. A different approach would be to perform ribosome run-off experiments using harringtonine and estimate the elongation speed across the codon tag. However, I am aware that this experiment could be tedious an expensive.

    • Figure 6: Insertion of the Lys x8 AAA stretch in the tandem ORF reporter leads to a decrease in HA-DsRedEx expression compared to that of Myc-EGFP. However, results from "Juszkiewicz and Hedge, 2017" using a similar reporter in mammalian cells indicate that stretches of Lys AAA below 20 repetitions only elicit poor RQC (less than 10% of true ribosome stalling for 12 repetitions of the AAA codon). Instead, most of the loss in RFP signal results from a change in the reading frame of ribosomes due to the "slippery" translation of the poly(A) stretch. I therefore think that it could be important to perform the experiment in ZNF598 KO embryos to validate that the observed reduction in HA-dsRedEx does indeed result from stalling and RQC and not from a change in the reading frame of ribosomes. On a similar note, how do the authors explain the decrease in signal of the Flag-EGFP and HA-DsRedEx observed when using the Flag-EGFP with non-optimal codons? I understand that RQC occurring through NGD leads to ribosome disassembly at the stalling site and possibly mRNA cleavage (thus explaining the decrease in HA-DsRedEx signal compared to Myc-EGFP). However, I would assume that codon-mediated mRNA decay (even for ORF longer than 200 of non-optimal codons) should trigger mRNA deadenylation, followed by decapping and co-translational 5'to3' mRNA degradation, following the last translating ribosome. I would therefore expect not to see any change in the HA-DsRedEx/Myc-EGFP ratio even for the non-optimal Flag-EGFP reporter. Could the 200 non-optimal codons trigger some background RQC through NGD? Or could there be some ribosome drop-off? It might be interesting to test the optimal and non-optimal Flag-EGFP reporters in the ZNF598 KO background to check whether the observed decrease in the relative amount of HA-DsRedEx results from stalling-dependent RQC.

    Minor comments:

    • The color-coded CSC results from "Bazzini et al., 2016" presented at the bottom of panel B in figure 2 are misleading because many codons (such as PheUUU, AsnAAU, TyrUAC...) are lacking information. I have the impression that the authors used the combined data from the rCSCI (obtained from the reporter RNAs) and CSC (obtained from endogenous transcripts) corresponding to Figure 1F from Bazzini et al., 2016. This data set excluded all codons that were not concordant between the endogenous and reporter CSCs (which are those that are lacking a color code in Figure 2B from this study). However, in the scatter-plot of PACE Vs CSC (from Supplemental Figure 1D of this study), the authors used the complete set of CSC values from Bazzini et al .,2016. Could the authors please use the complete set of CSC values from Bazzini et al., 2016 to color code codons in their Figure 2B?
    • Figure 4B. The charged tRNA measurements seem to have been done in a single biological replicate (there aren't any error bars in the chart). I understand that the procedure is tedious and requires a large amount of total RNA to begin with, but it would be preferable to perform it in three biological replicates.
    • Supplementary Figure 2B. I do not understand what the figure represents. The legend is quite cryptic and states that the panel corresponds to the information content of each reading frame. More information should be given so that readers can understand how to interpret de figure and extract periodicity information.

    Significance

    Since the seminal work from Jeff Coller's laboratory in 2015 (Presnyak et al., 2015 Cell) showing a global and major role for codon optimality in determining mRNA half-lives in yeast, the role of codon usage in modulating translation and stability of mRNAs has been widely studied in different organisms (including zebrafish and mammals). As stated by the authors in the introduction, most studies have relied on correlation analyses between codon usage and mRNA half-lives from endogenous transcripts or from ORF libraries with fixed 5'UTR and 3'UTRs. This approaches could suffer from the presence of transcript features that can participate in other mRNA degradation pathways, which could limit their use when performing further mechanistic studies.

    The work by Mishima and collaborators presents an original reporter assay that allows to evaluate the role of codon usage on regulating mRNA stability in a defined context, thus avoiding the impact of confounding factors that could bias the measurement of mRNA stability. Results obtained using this reporter are in good agreement with previous reports from Zebrafish (Bazzini et al 2016., and Mishima et al., 2016). From this validated reporter approach, the authors further show that codon-dependent mRNA degradation is directly related to tRNA availability and (at least partially) to ribosome occupancy (two factors already suggested as being important for codon-mediated decay in zebrafish, although they were based on correlation analyses). Furthermore, the authors show that codon-mediated mRNA decay occurs during productive mRNA translation and that it is functionally distinct from RQC induced by ribosome stalling. As a consequence, codon-mediated mRNA degradation is independent from the RQC factor ZNF598 (which they also validate for the first time as an important RQC factor in zebrafish). This information is new within metazoans since only in yeast it has been clearly shown that codon-mediated mRNA decay is distinct from RQC induced by ribosome stalling and collisions.

    Taken together, the reported findings will be of interest to the community working on mRNA metabolism and translation. It could also interest, more broadly, scientists working on translational selection and genome evolution.

  6. Version published to 10.1101/2021.05.12.443743v1 on bioRxiv