Kathrin Leppek’s research while affiliated with University of Bonn and other places

Synopsis Internal ribosomal entry sites (IRES) contribute to translation regulation by the ribosome but are difficult to characterize. This study compiles a versatile toolbox of different technologies that allow robust characterization of mRNA IRESes in cells and embryonic tissues. Circular RNA (circRNA) split-EGFP reporters confirm IRES activity of multiple previously reported cellular IRESes including mouse Hoxa IRES-like elements. smFISH confirms and quantifies the tissue-specific expression of the IRES-containing Hoxa9 mRNA isoform during mouse early embryonic development. Promoterless constructs reveal artificial cryptic promoter activity of tested sequences independent of their IRES activity for various IRESes. PacBio long-read sequencing does not unambiguously detect mRNA isoforms with similar intron patterns and complex 5’ UTR variants, exemplified by not finding evidence for the existence of a Hoxa9/a10 fusion transcript in embryonic tissues. Polysome-qPCR can quantify the polysome distribution and translation rate of IRES-containing mRNA isoforms in an isoform-sensitive manner from embryonic tissues.

Therapeutic mRNAs and vaccines are being developed for a broad range of human diseases, including COVID-19. However, their optimization is hindered by mRNA instability and inefficient protein expression. Here, we describe design principles that overcome these barriers. We develop an RNA sequencing-based platform called PERSIST-seq to systematically delineate in-cell mRNA stability, ribosome load, as well as in-solution stability of a library of diverse mRNAs. We find that, surprisingly, in-cell stability is a greater driver of protein output than high ribosome load. We further introduce a method called In-line-seq, applied to thousands of diverse RNAs, that reveals sequence and structure-based rules for mitigating hydrolytic degradation. Our findings show that highly structured “superfolder” mRNAs can be designed to improve both stability and expression with further enhancement through pseudouridine nucleoside modification. Together, our study demonstrates simultaneous improvement of mRNA stability and protein expression and provides a computational-experimental platform for the enhancement of mRNA medicines. The authors develop an RNA sequencing-based platform, PERSIST-seq, to simultaneously delineate in-cell mRNA stability, ribosome load, and in-solution stability of a diverse mRNA library to derive design principles for improved mRNA therapeutics.

Therapeutic mRNAs and vaccines are being developed for a broad range of human diseases, including COVID-19. However, their optimization is hindered by mRNA instability and inefficient protein expression. Here, we describe design principles that overcome these barriers. We develop a new RNA sequencing-based platform called PERSIST-seq to systematically delineate in-cell mRNA stability, ribosome load, as well as in-solution stability of a library of diverse mRNAs. We find that, surprisingly, in-cell stability is a greater driver of protein output than high ribosome load. We further introduce a method called In-line-seq, applied to thousands of diverse RNAs, that reveals sequence and structure-based rules for mitigating hydrolytic degradation. Our findings show that superfolder mRNAs can be designed to improve both stability and expression that are further enhanced through pseudouridine nucleoside modification. Together, our study demonstrates simultaneous improvement of mRNA stability and protein expression and provides a computational-experimental platform for the enhancement of mRNA medicines.

Roles for ribosomal RNA (rRNA) in gene regulation remain largely unexplored. With hundreds of rDNA units positioned across multiple loci, it is not possible to genetically modify rRNA in mammalian cells, hindering understanding of ribosome function. It remains elusive whether expansion segments (ESs), tentacle-like rRNA extensions that vary in sequence and size across eukaryotic evolution, may have functional roles in translation control. Here, we develop variable expansion segment-ligand chimeric ribosome immunoprecipitation RNA sequencing (VELCRO-IP RNA-seq), a versatile methodology to generate species-adapted ESs and to map specific mRNA regions across the transcriptome that preferentially associate with ESs. Application of VELCRO-IP RNA-seq to a mammalian ES, ES9S, identified a large array of transcripts that are selectively recruited to ribosomes via an ES. We further characterize a set of 5′ UTRs that facilitate cap-independent translation through ES9S-mediated ribosome binding. Thus, we present a technology for studying the enigmatic ESs of the ribosome, revealing their function in gene-specific translation.

Ribosomes have been suggested to directly control gene regulation, but regulatory roles for ribosomal RNA (rRNA) remain largely unexplored. Expansion segments (ESs) consist of multitudes of tentacle-like rRNA structures extending from the core ribosome in eukaryotes. ESs are remarkably variable in sequence and size across eukaryotic evolution with largely unknown functions. In characterizing ribosome binding to a regulatory element within a Homeobox (Hox) 5′ UTR, we identify a modular stem-loop within this element that binds to a single ES, ES9S. Engineering chimeric, “humanized” yeast ribosomes for ES9S reveals that an evolutionary change in the sequence of ES9S endows species-specific binding of Hoxa9 mRNA to the ribosome. Genome editing to site-specifically disrupt the Hoxa9-ES9S interaction demonstrates the functional importance for such selective mRNA-rRNA binding in translation control. Together, these studies unravel unexpected gene regulation directly mediated by rRNA and how ribosome evolution drives translation of critical developmental regulators.

Roles for ribosomal RNA (rRNA) in gene regulation remain largely unexplored. With hundreds of rDNA units scattered across multiple chromosomal loci, it is not possible to genetically modify rRNA in mammalian cells, hindering understanding of ribosome function. Emerging evidence suggests that expansion segments (ESs), tentacle-like rRNA extensions that vary in sequence and size across eukaryotic evolution, may provide platforms for the binding of proteins and mRNAs. Here, we develop VELCRO-IP RNA-seq: a versatile methodology to generate species-adapted ESs and map specific mRNA regions across the transcriptome that preferentially associate with ESs. By applying VELCRO-IP RNA-seq to a mammalian ES, ES9S, we identified a large array of mRNAs that are selectively recruited to ribosomes via an ES. We further characterize a set of specific 5′ UTRs that facilitate cap-independent translation through ES9S-mediated ribosome recruitment. These data provide a novel technology for studying the enigmatic ESs of the ribosome in gene-specific translation.

Bacteria have previously been assumed to cope with environmental stress by tuning their total number of active ribosomes. Instead, a study in this issue of Nature Microbiology shows that from a heterogeneous pool of ribosomes, Vibrio vulnificus uses ribosomes with a particular ribosomal RNA variant to translate upregulated stress response mRNAs.

The legend of Figure 1 has been modified to remove misleading referencing to evolution. The title of the legend has been modified from 'Evolutionary expansion of eukaryotic 5' UTR lengths' to 'Interspecies variation in 5' UTR lengths'; the first sentence of the legend has been modified from 'The length of 5' untranslated regions (UTRs) has increased in eukaryotes during evolution…' to 'The length of 5' untranslated regions (UTRs) varies in eukaryotes…'. The changes have been made in the HTML and PDF versions of the manuscript.

RNA molecules can fold into intricate shapes that can provide an additional layer of control of gene expression beyond that of their sequence. In this Review, we discuss the current mechanistic understanding of structures in 5′ untranslated regions (UTRs) of eukaryotic mRNAs and the emerging methodologies used to explore them. These structures may regulate cap-dependent translation initiation through helicase-mediated remodelling of RNA structures and higher-order RNA interactions, as well as cap-independent translation initiation through internal ribosome entry sites (IRESs), mRNA modifications and other specialized translation pathways. We discuss known 5′ UTR RNA structures and how new structure probing technologies coupled with prospective validation, particularly compensatory mutagenesis, are likely to identify classes of structured RNA elements that shape post-transcriptional control of gene expression and the development of multicellular organisms.