Time-resolved profiling of RNA binding proteins throughout the mRNA life cycle

Large-scale purifications reveal yeast and human stress granule cores are heterogeneous particles with complex transcriptomes and proteomes

Stress granules are a conserved response of eukaryotic cells to environmental insults. These cytoplasmic ribonucleoprotein condensates have hitherto been primarily studied by microscopy, which showed previously that they comprise dense ∼200 nm cores embedded in a diffuse shell. We have developed large-scale purifications of budding yeast and mammalian (HEK293T cell) stress granule cores that do not rely on immunoprecipitation of candidate protein constituents. These unbiased preparations reveal that stress granule cores are discrete particles with variable size (average, 135 and 225 nm for yeast and human, respectively) and shape. Proteomics and transcriptomics demonstrate complex composition. The results of hybridization chain reaction fluorescence in situ hybridization (FISH) analyses in HEK293T cells are consistent with stress granule cores having heterogeneous composition, i.e., each stress granule core particle contains only a limited number of mRNA species. Biochemical purification now opens the way to mechanistic analysis of the heterogeneity and complexity of stress granules.

Dissecting RNA selectivity mediated by tandem RNA-binding domains

RNA-protein interactions are pivotal to proper gene regulation. Many RNA-binding proteins possess multiple RNA-binding domains; however, how these domains interplay to select and regulate RNA targets remains poorly understood. Here, we investigate three multidomain proteins, Musashi-1, Musashi-2, and unkempt, which share a high degree of RNA specificity, a common feature across RNA-binding proteins. We used massively parallel in vitro assays with unprecedented depth with random or naturally derived RNA sequences and find that individual domains within a protein can have differing affinities, specificities, and motif spacing preferences. We conducted large scale competition assays between these proteins and determined how individual protein specificities and affinities influence competitive binding. Integration of binding and regulation in cells with in vitro specificities showed that target selection involves a combination of the protein intrinsic specificities described here, but cellular context is critical to drive these proteins to motifs in specific transcript regions. Finally, evolutionarily conserved RNA regions displayed evidence of binding multiple RBPs in cultured cells, and these RNA regions represent the highest affinity targets. This work emphasizes the importance of in vitro and in cultured cells studies to fully profile RNA-binding proteins and highlights the complex modes of RNA-protein interactions and the contributing factors in target selection.

ZNFX1: a multifunctional modulator of the innate immune response

Recent research has identified ZNFX1 as a critical modulator of the innate immune response. Individuals with loss of function mutations in ZNFX1 have chronic inflammation and increased susceptibility to various pathogens. Several potential functions of ZNFX1 have been proposed, including binding double-stranded RNA to activate antiviral innate immunity, inhibiting the NLRP3 inflammasome, and regulating the stability of host mRNAs. Notably, homologs of ZNFX1 are implicated in innate immunity across a wide range of species, including contributing to transgenerational epigenetic inheritance of small RNA-based defense in C. elegans. In this review, we discuss the significance of ZNFX1 and explore the potential underlying mechanisms that govern its diverse functions.

Cellular dsRNA interactome captured by K1 antibody reveals the regulatory map of exogenous RNA sensing

RNA-binding proteins (RBPs) provide a critical post-transcriptional regulatory layer in determining RNA fate. Currently, UV crosslinking followed by oligo-dT pull-down is the gold standard in identifying the RBP repertoire of poly-adenylated RNAs, but such method is ineffective in capturing RBPs that recognize double-stranded RNAs (dsRNAs). Here, we utilize anti-dsRNA K1 antibody immunoprecipitation followed by quantitative mass spectrometry to comprehensively identify RBPs bound to cellular dsRNAs without external stimulus. Notably, our dsRNA interactome contains proteins involved in sensing N6-methyladenosine RNAs and stress granule components. We further perform targeted CRISPR-Cas9 knockout functional screening and discover proteins that can regulate the interferon (IFN) response during exogenous RNA sensing. Interestingly, most dsRBPs promote IFN-β secretion in response to dsRNA stimulation and act as antiviral factors during HCoV-OC43 infection. Our dsRNA interactome capture provides an unbiased and comprehensive characterization of putative dsRBPs and will facilitate our understanding of dsRNA sensing in physiological and pathological contexts.

Profiling local translatomes and RNA binding proteins of somatosensory neurons reveals specializations of individual axons

Individual neurons have one or more axons that often extend long distances and traverse multiple microenvironments. However, it is not known how the composition of individual axons is established or locally modulated to enable neuronal function and plasticity. Here, we use spatial translatomics to identify local axonal translatomes in anatomically and functionally specialized neurons in the dorsal root ganglia (DRG). DRG neurons extend long central and peripheral axons in opposite directions and distinct microenvironments to enable somatosensation. Using Translating Ribosome Affinity Purification and RNA sequencing, we generated a comprehensive resource of mRNAs preferentially translated within each axon. Locally translated proteins include pain receptors, ion channels, and translational machinery, which establish distinct electrophysiologic properties and regenerative capacities for each axon. We identify RNA-binding proteins associated with sorting and transporting functionally related mRNAs. These findings provide resources for addressing how axonal translation shapes the spatial organization of neurons and enables subcellular neuroplasticity.

HIGHLIGHTS

Distinct mRNAs are localized to and translated in individual axons.Axonal translatomes govern regenerative capacity, translational machinery, and electrophysiology.The RBP, SFPQ, coordinates mRNA sorting towards peripheral somatosensory axons.Axonal translatome data can be explored at painseq.shinyapps.io/CompartmentTRAP/.

RNA binding protein Pumilio2 promotes chemoresistance of pancreatic cancer via focal adhesion pathway and interacting with transcription factor EGR1

Pancreatic cancer (PCa) has insidious onset, high malignancy and poor prognosis. Gemcitabine (GEM) is one of the first-line chemotherapy drugs for PCa. However, GEM resistance has always been a bottleneck problem leading to recurrence and death of PCa patients. RNA-binding proteins (RBPs) are important proteins that regulate transportation, splicing, stability and translation of RNA. Abnormal expression of RBPs often lead to a series of abnormal accumulation or degradation of downstream RNA resulting in various diseases. In our study, we utilized RIP seq, RIP-qPCR, in vitro and in vivo experiments and found that pumilio2 (PUM2) was high expression in PCa, and promoted GEM resistance of PCa by regulating mRNA stability of integrin Alpha 3 (ITGA3) and other genes in focal adhesion pathway, and there was positive feedback regulation between PUM2 and transcription factor early growth response gene 1 (EGR1), that is PUM2 binding to 3'UTR region of EGR1 mRNA, and EGR1 binding to promoter region of PUM2 gene. The discovery of EGR1/PUM2/ITGA3 axis provided a solid experimental basis for the selection of chemotherapy regiments for PCa patients and exploration of combined regimens to reverse GEM resistance in the future.

Distinct interactomes of ADAR1 nuclear and cytoplasmic protein isoforms and their responses to interferon induction

The RNA editing enzyme adenosine deaminase acting on RNA 1 (ADAR1) is essential for correct functioning of innate immune responses. The ADAR1p110 isoform is mainly nuclear and ADAR1p150, which is interferon (IFN) inducible, is predominately cytoplasmic. Using three different methods - co-immunoprecipitation (co-IP) of endogenous ADAR1, Strep-tag co-IP and BioID with individual ADAR1 isoforms - a comprehensive interactome was generated during both homeostasis and the IFN response. Both known and novel interactors as well as editing regulators were identified. Nuclear proteins were detected as stable interactors with both ADAR1 isoforms. In contrast, BioID identified distinct protein networks for each ADAR1 isoform, with nuclear components observed with ADAR1p110 and components of cytoplasmic cellular condensates with ADAR1p150. RNase A digestion distinguished between distal and proximal interactors, as did a double-stranded RNA (dsRNA)-binding mutant of ADAR1 which demonstrated the importance of dsRNA binding for ADAR1 interactions. IFN treatment did not affect the core ADAR1 interactomes but resulted in novel interactions, the majority of which are proximal interactions retained after RNase A treatment. Short treatment with high molecular weight poly(I:C) during the IFN response resulted in dsRNA-binding-dependent changes in the proximal protein network of ADAR1p110 and association of the ADAR1p150 proximal protein network with some components of antiviral stress granules.

Subcellular mRNA kinetic modeling reveals nuclear retention as rate-limiting

Eukaryotic mRNAs are transcribed, processed, translated, and degraded in different subcellular compartments. Here, we measured mRNA flow rates between subcellular compartments in mouse embryonic stem cells. By combining metabolic RNA labeling, biochemical fractionation, mRNA sequencing, and mathematical modeling, we determined the half-lives of nuclear pre-, nuclear mature, cytosolic, and membrane-associated mRNAs from over 9000 genes. In addition, we estimated transcript elongation rates. Many matured mRNAs have long nuclear half-lives, indicating nuclear retention as the rate-limiting step in the flow of mRNAs. In contrast, mRNA transcripts coding for transcription factors show fast kinetic rates, and in particular short nuclear half-lives. Differentially localized mRNAs have distinct rate constant combinations, implying modular regulation. Membrane stability is high for membrane-localized mRNA and cytosolic stability is high for cytosol-localized mRNA. mRNAs encoding target signals for membranes have low cytosolic and high membrane half-lives with minor differences between signals. Transcripts of nuclear-encoded mitochondrial proteins have long nuclear retention and cytoplasmic kinetics that do not reflect co-translational targeting. Our data and analyses provide a useful resource to study spatiotemporal gene expression regulation.

Challenges in Therapeutically Targeting the RNA-Recognition Motif

The RNA recognition motif (RRM) is the most common RNA binding domain found in the human proteome. RRM domains provide RNA-binding proteins with sequence specific RNA recognition allowing them to participate in RNA-centric processes such as mRNA maturation, translation initiation, splicing, and RNA degradation. They are drivers of various diseases through overexpression or mutation, making them attractive therapeutic targets and addressing these proteins through their RRM domains with chemical compounds is gaining ever more attention. However, it is still very challenging to find selective and potent RNA-competitors due to the small size of the domain and high structural conservation of its RNA binding interface. Despite these challenges, a selection of compounds has been reported for several RRM containing proteins, but often with limited biophysical evidence and low selectivity. A solution to selectively targeting RRM domains might be through avoiding the RNA-binding surface altogether, but rather look for composite pockets formed with other proteins or for protein-protein interaction sites that regulate the target's activity but are less conserved. Alternative modalities, such as oligonucleotides, peptides, and molecular glues, are exciting new approaches to address these challenging targets and achieve the goal of therapeutic intervention at the RNA regulatory level.

Understanding nuclear mRNA export: Survival under stress

Nuclear messenger RNA (mRNA) export is vital for cell survival under both physiological and stress conditions. To cope with stress, cells block bulk mRNA export while selectively exporting stress-specific mRNAs. Under physiological conditions, nuclear adaptor proteins recruit the mRNA exporter to the mRNA for export. By contrast, during stress conditions, the mRNA exporter is likely directly recruited to stress-specific mRNAs at their transcription sites to facilitate selective mRNA export. In this review, we summarize our current understanding of nuclear mRNA export. Importantly, we explore insights into the mechanisms that block bulk mRNA export and facilitate transcript-specific mRNA export under stress, highlighting the gaps that still need to be filled.

Altered mRNA transport and local translation in iNeurons with RNA binding protein knockdown

Neurons rely on mRNA transport and local translation to facilitate rapid protein synthesis in processes far from the cell body. These processes allow precise spatial and temporal control of translation and are mediated by RNA binding proteins (RBPs), including those known to be associated with neurodegenerative diseases. Here, we use proteomics, transcriptomics, and microscopy to investigate the impact of RBP knockdown on mRNA transport and local translation in iPSC-derived neurons. We find thousands of transcripts enriched in neurites and that many of these transcripts are locally translated, possibly due to the shorter length of transcripts in neurites. Loss of frontotemporal dementia/amyotrophic lateral sclerosis (FTD/ALS)-associated RBPs TDP-43 and hnRNPA1 lead to distinct alterations in the neuritic proteome and transcriptome. TDP-43 knockdown (KD) leads to increased neuritic mRNA and translation. In contrast, hnRNPA1 leads to increased neuritic mRNA, but not translation, and more moderate effects on local mRNA profiles, possibly due to compensation by hnRNPA3. These results highlight the crucial role of FTD/ALS-associated RBPs in mRNA transport and local translation in neurons and the importance of these processes in neuron health and disease.

Nuclear mRNA export

In eukaryotic cells, gene expression begins with transcription in the nucleus, followed by the maturation of messenger RNAs (mRNAs). These mRNA molecules are then exported to the cytoplasm through the nuclear pore complex (NPC), a process that serves as a critical regulatory phase of gene expression. The export of mRNA is intricately linked to precursor mRNA (pre-mRNA) processing, ensuring that only properly processed mRNA reaches the cytoplasm. This coordination is essential, as recent studies have revealed that mRNA export factors not only assist in transport but also influence upstream processing steps, adding a layer of complexity to gene regulation. Furthermore, the export process competes with RNA processing and degradation pathways, maintaining a delicate balance vital for accurate gene expression. While these mechanisms are generally conserved across eukaryotes, significant differences exist between yeast and higher eukaryotic cells, particularly due to the more genome complexity of the latter. This review delves into the current research on mRNA export in higher eukaryotic cells, focusing on its role in the broader context of gene expression regulation and highlighting how it interacts with other gene expression processes to ensure precise and efficient gene functionality in complex organisms.

Genome-wide quantification of RNA flow across subcellular compartments reveals determinants of the mammalian transcript life cycle

Dissecting the regulatory mechanisms controlling mammalian transcripts from production to degradation requires quantitative measurements of mRNA flow across the cell. We developed subcellular TimeLapse-seq to measure the rates at which RNAs are released from chromatin, exported from the nucleus, loaded onto polysomes, and degraded within the nucleus and cytoplasm in human and mouse cells. These rates varied substantially, yet transcripts from genes with related functions or targeted by the same transcription factors and RNA-binding proteins flowed across subcellular compartments with similar kinetics. Verifying these associations uncovered a link between DDX3X and nuclear export. For hundreds of RNA metabolism genes, most transcripts with retained introns were degraded by the nuclear exosome, while the remaining molecules were exported with stable cytoplasmic lifespans. Transcripts residing on chromatin for longer had extended poly(A) tails, whereas the reverse was observed for cytoplasmic mRNAs. Finally, machine learning identified molecular features that predicted the diverse life cycles of mRNAs.