ß-Actin mRNA interactome mapping by proximity biotinylation.

The molecular function and fate of mRNAs are controlled by RNA-binding proteins (RBPs). Identification of the interacting proteome of a specific mRNA in vivo remains very challenging, however. Based on the widely used technique of RNA tagging with MS2 aptamers for RNA visualization, we developed a RNA proximity biotinylation (RNA-BioID) technique by tethering biotin ligase (BirA*) via MS2 coat protein at the 3' UTR of endogenous MS2-tagged ß-actin mRNA in mouse embryonic fibroblasts. We demonstrate the dynamics of the ß-actin mRNA interactome by characterizing its changes on serum-induced localization of the mRNA. Apart from the previously known interactors, we identified more than 60 additional ß-actin-associated RBPs by RNA-BioID. Among these, the KH domain-containing protein FUBP3/MARTA2 has been shown to be required for ß-actin mRNA localization. We found that FUBP3 binds to the 3' UTR of ß-actin mRNA and is essential for ß-actin mRNA localization, but does not interact with the characterized ß-actin zipcode element. RNA-BioID provides a tool for identifying new mRNA interactors and studying the dynamic view of the interacting proteome of endogenous mRNAs in space and time.

Crystallin gene expression: Insights from studies of transcriptional bursting.

Cellular differentiation is marked by temporally and spatially regulated gene expression. The ocular lens is one of the most powerful mammalian model system since it is composed from only two cell subtypes, called lens epithelial and fiber cells. Lens epithelial cells differentiate into fiber cells through a series of spatially and temporally orchestrated processes, including massive production of crystallins, cellular elongation and the coordinated degradation of nuclei and other organelles. Studies of transcriptional and posttranscriptional gene regulatory mechanisms in lens provide a wide range of opportunities to understand global molecular mechanisms of gene expression as steady-state levels of crystallin mRNAs reach very high levels comparable to globin genes in erythrocytes. Importantly, dysregulation of crystallin gene expression results in lens structural abnormalities and cataracts. The mRNA life cycle is comprised of multiple stages, including transcription, splicing, nuclear export into cytoplasm, stabilization, localization, translation and ultimate decay. In recent years, development of modern mRNA detection methods with single molecule and single cell resolution enabled transformative studies to visualize the mRNA life cycle to generate novel insights into the sequential regulatory mechanisms of gene expression during embryogenesis. This review is focused on recent major advancements in studies of transcriptional bursting in differentiating lens fiber cells, analysis of nascent mRNA expression from bi-directional promoters, transient nuclear accumulation of specific mRNAs, condensation of chromatin prior lens fiber cell denucleation, and outlines future studies to probe the interactions of individual mRNAs with specific RNA-binding proteins (RBPs) in the cytoplasm and regulation of translation and mRNA decay.

Imaging mRNA and protein interactions within neurons.

RNA-protein interactions are essential for proper gene expression regulation, particularly in neurons with unique spatial constraints. Currently, these interactions are defined biochemically, but a method is needed to evaluate them quantitatively within morphological context. Colocalization of two-color labels using wide-field microscopy is a method to infer these interactions. However, because of chromatic aberrations in the objective lens, this approach lacks the resolution to determine whether two molecules are physically in contact or simply nearby by chance. Here, we developed a robust super registration methodology that corrected the chromatic aberration across the entire image field to within 10 nm, which is capable of determining whether two molecules are physically interacting or simply in proximity by random chance. We applied this approach to image single-molecule FISH in combination with immunofluorescence (smFISH-IF) and determined whether the association between an mRNA and binding protein(s) within a neuron was significant or accidental. We evaluated several mRNA-binding proteins identified from RNA pulldown assays to determine which of these exhibit bona fide interactions. Surprisingly, many known mRNA-binding proteins did not bind the mRNA in situ, indicating that adventitious interactions are significant using existing technology. This method provides an ability to evaluate two-color registration compatible with the scale of molecular interactions.

CPEB1 coordinates alternative 3'-UTR formation with translational regulation.

More than half of mammalian genes generate multiple messenger RNA isoforms that differ in their 3' untranslated regions (3' UTRs) and therefore in regulatory sequences, often associated with cell proliferation and cancer; however, the mechanisms coordinating alternative 3'-UTR processing for specific mRNA populations remain poorly defined. Here we report that the cytoplasmic polyadenylation element binding protein 1 (CPEB1), an RNA-binding protein that regulates mRNA translation, also controls alternative 3'-UTR processing. CPEB1 shuttles to the nucleus, where it co-localizes with splicing factors and mediates shortening of hundreds of mRNA 3' UTRs, thereby modulating their translation efficiency in the cytoplasm. CPEB1-mediated 3'-UTR shortening correlates with cell proliferation and tumorigenesis. CPEB1 binding to pre-mRNAs not only directs the use of alternative polyadenylation sites, but also changes alternative splicing by preventing U2AF65 recruitment. Our results reveal a novel function of CPEB1 in mediating alternative 3'-UTR processing, which is coordinated with regulation of mRNA translation, through its dual nuclear and cytoplasmic functions.

mRNA on the move: the road to its biological destiny.

Cells have evolved to regulate the asymmetric distribution of specific mRNA targets to institute spatial and temporal control over gene expression. Over the last few decades, evidence has mounted as to the importance of localization elements in the mRNA sequence and their respective RNA-binding proteins. Live imaging methodologies have shown mechanistic details of this phenomenon. In this minireview, we focus on the advanced biochemical and cell imaging techniques used to tweeze out the finer aspects of mechanisms of mRNA movement.

Continuous nucleolar ribosomal RNA synthesis in differentiating lens fiber cells until abrupt nuclear degradation required for ocular lens transparency.

Cellular differentiation requires highly coordinate action of all three transcriptional systems to produce rRNAs, mRNAs, and various "short" and "long" non-coding RNAs by RNA Polymerase I, II, and III systems, respectively. The RNA Polymerase I catalyzes transcription of about 400 copies of rDNA genes generating 18S, 5.8S, and 28S rRNA molecules from the individual primary transcript. Lens fiber cell differentiation is a unique process to study transcriptional mechanisms of individual crystallin genes as their very high transcriptional outputs are directly comparable only to globin genes in erythrocytes. Importantly, both terminally differentiated lens fiber cells and mammalian erythrocytes degrade their nuclei though by different mechanisms. In lens, generation of organelle-free zone (OFZ) includes degradation of mitochondria, endoplasmic reticulum, Golgi apparatus, and nuclei; nevertheless, very little is known about their nucleoli and rRNA transcription. Here, using RNA fluorescence in situ hybridization (FISH) we evaluated nascent rRNA transcription during the entire process of lens fiber cell differentiation. The lens fiber cell nuclei undergo morphological changes prior their denucleation, including chromatin condensation; remarkably, the nascent rRNA transcription persists in all nuclei next to the OFZ. The changes in both nuclei and nucleoli shape and microarchitecture were evaluated by immunofluorescence to detect fibrillarin, nucleolin, UBF, and other nuclear proteins. These studies demonstrate for the first time that highly condensed lens fiber cell nuclei have the capacity to support rRNA transcription. Thus, "late" production of rRNA molecules and consequently the ribosomes contribute to the terminal translational mechanisms to produce maximal quantities of the crystallin proteins.

Single-Cell Single-Molecule RNA-FISH Combined with Immunofluorescence and High-Speed and High-Resolution Scanning Analysis to Visualize the Reactivation of Latent HIV-1.

Latent HIV-1 reservoirs are a major obstacle to the eradication of HIV-1. Several cure strategies have been proposed to eliminate latent reservoirs. One of the key strategies involves the reactivation of latent HIV-1 from cells using latency-reversing agents. However, currently it is unclear whether any of the latency-reversing agents are able to completely reactivate HIV-1 provirus transcription in all latent cells. An understanding of the reactivation of HIV-1 provirus at single-cell single-molecule level is necessary to fully comprehend the reactivation of HIV-1 in the reservoirs. Furthermore, since reactivable viruses in the pool of latent reservoirs are rare, combining single-cell imaging techniques with the ability to visualize a large number of reactivated single cells that express both viral RNA and proteins in a pool of uninfected and non-reactivated cells will provide unprecedented information about cell-to-cell variability in reactivation. Here, we describe the single-cell single-molecule RNA-FISH (smRNA-FISH) method to visualize HIV-1 gag RNA combined with the immunofluorescence (IF) method to detect Gag protein to characterize the reactivated cells. This method allows the visualization of subcellular localization of RNA and proteins before and after reactivation and facilitates absolute quantitation of the number of transcripts per cell using FISH-quant. In addition, we describe a high-speed and high-resolution scanning (HSHRS) fluorescence microscopy imaging method to visualize rare and reactivated cells in a pool of non-reactivated cells with high efficiency.

Single-molecule RNA-FISH analysis reveals stochasticity in reactivation of latent HIV-1 regulated by Nuclear Orphan Receptors NR4A and cMYC.

HIV-1 eradication strategies require complete reactivation of HIV-1 latent cells by Latency Reversing Agents (LRA). Current methods lack effectiveness due to incomplete proviral reactivation. We employed a single-molecule RNA-FISH (smRNA-FISH) and FISH-Quant analysis and found that proviral reactivation is highly variable from cell-to-cell, stochastic, and occurs in bursts and waves, with different kinetics in response to diverse LRAs. Approximately 1-5% of latent cells exhibited stochastic reactivation without LRAs. Through single-cell RNA-seq analysis, we identified NR4A3 and cMYC as extrinsic factors associated with stochastic HIV-1 reactivation. Concomitant with HIV-1 reactivation cMYC was downregulated and NR4A3 was upregulated in both latent cell lines and primary CD4+ T-cells from aviremic patients. By inhibiting cMYC using SN-38, an active metabolite of irinotecan, we induced NR4A3 and HIV-1 expression. Our results suggest that inherent stochasticity in proviral reactivation contributes to cell-to-cell variability, which could potentially be modulated by drugs targeting cMYC and NR4A3.

Visualization of Early RNA Replication Kinetics of SARS-CoV-2 by Using Single Molecule RNA-FISH Combined with Immunofluorescence.

SARS-CoV-2 infection remains a global burden. Despite intensive research, the mechanism and dynamics of early viral replication are not completely understood, such as the kinetics of the formation of genomic RNA (gRNA), sub-genomic RNA (sgRNA), and replication centers/organelles (ROs). We employed single-molecule RNA-fluorescence in situ hybridization (smRNA-FISH) to simultaneously detect viral gRNA and sgRNA and immunofluorescence to detect nsp3 protein, a marker for the formation of RO, and carried out a time-course analysis. We found that single molecules of gRNA are visible within the cytoplasm at 30 min post infection (p.i.). Starting from 2 h p.i., most of the viral RNA existed in clusters/speckles, some of which were surrounded by single molecules of sgRNA. These speckles associated with nsp3 protein starting at 3 h p.i., indicating that these were precursors to ROs. Furthermore, RNA replication was asynchronous, as cells with RNA at all stages of replication were found at any given time point. Our probes detected the SARS-CoV-2 variants of concern, and also suggested that the BA.1 strain exhibited a slower rate of replication kinetics than the WA1 strain. Our results provide insights into the kinetics of SARS-CoV-2 early post-entry events, which will facilitate identification of new therapeutic targets for early-stage replication to combat COVID-19.

Spatial hepatocyte plasticity of gluconeogenesis during the metabolic transitions between fed, fasted and starvation states.

The liver acts as a master regulator of metabolic homeostasis in part by performing gluconeogenesis. This process is dysregulated in type 2 diabetes, leading to elevated hepatic glucose output. The parenchymal cells of the liver (hepatocytes) are heterogeneous, existing on an axis between the portal triad and the central vein, and perform distinct functions depending on location in the lobule. Here, using single cell analysis of hepatocytes across the liver lobule, we demonstrate that gluconeogenic gene expression ( Pck1 and G6pc ) is relatively low in the fed state and gradually increases first in the periportal hepatocytes during the initial fasting period. As the time of fasting progresses, pericentral hepatocyte gluconeogenic gene expression increases, and following entry into the starvation state, the pericentral hepatocytes show similar gluconeogenic gene expression to the periportal hepatocytes. Similarly, pyruvate-dependent gluconeogenic activity is approximately 10-fold higher in the periportal hepatocytes during the initial fasting state but only 1.5-fold higher in the starvation state. In parallel, starvation suppresses canonical beta-catenin signaling and modulates expression of pericentral and periportal glutamine synthetase and glutaminase, resulting in an enhanced pericentral glutamine-dependent gluconeogenesis. These findings demonstrate that hepatocyte gluconeogenic gene expression and gluconeogenic activity are highly spatially and temporally plastic across the liver lobule, underscoring the critical importance of using well-defined feeding and fasting conditions to define the basis of hepatic insulin resistance and glucose production.

Visualization of early RNA replication kinetics of SARS-CoV-2 by using single molecule RNA-FISH.

SARS-CoV-2 infection has caused a major global burden. Despite intensive research, the mechanism and dynamics of early viral replication are not completely understood including the kinetics of formation of plus stranded genomic and subgenomic RNAs (gRNA and sgRNA) starting from the RNA from the first virus that enters the cell. We employed single-molecule RNA-fluorescence in situ hybridization (smRNA-FISH) to simultaneously detect viral gRNA and sgRNA in infected cells and carried out a time course analysis to determine the kinetics of their replication. We visualized the single molecules of gRNA within the cytoplasm of infected cells 30 minutes post-infection and detected the co-expression of gRNA and sgRNA within two hours post-infection. Furthermore, we observed the formation of a replication organelle (RO) from a single RNA, which led to the formation of multiple ROs within the same cells. Single molecule analysis indicated that while gRNA resided in the center of these ROs, the sgRNAs were found to radiate and migrate out of these structures. Our results also indicated that after the initial delay, there was a rapid but asynchronous replication, and the gRNA and sgRNAs dispersed throughout the cell within 4-5 hours post-infection forming multiple ROs that filled the entire cytoplasm. These results provide insight into the kinetics of early post-entry events of SARS-CoV-2 and the formation of RO, which will help to understand the molecular events associated with viral infection and facilitate the identification of new therapeutic targets that can curb the virus at a very early stage of replication to combat COVID-19. Author Summary: SARS-CoV-2 infection continues to be a global burden. Soon after the entry, SARS-CoV-2 replicates by an elaborate process, producing genomic and subgenomic RNAs (gRNA and sgRNAs) within specialized structures called replication organelles (RO). Many questions including the timing of multiplication of gRNA and sgRNA, the generation, subcellular localization, and function of the ROs, and the mechanism of vRNA synthesis within ROs is not completely understood. Here, we have developed probes and methods to simultaneously detect the viral gRNA and a sgRNA at single cell single molecule resolution and have employed a method to scan thousands of cells to visualize the early kinetics of gRNA and sgRNA synthesis soon after the viral entry into the cell. Our results reveal that the replication is asynchronous and ROs are rapidly formed from a single RNA that enters the cell within 2 hours, which multiply to fill the entire cell cytoplasm within ~4 hours after infection. Furthermore, our studies provide a first glimpse of the gRNA and sgRNA synthesis within ROs at single molecule resolution. Our studies may facilitate the development of drugs that inhibit the virus at the earliest possible stages of replication to minimize the pathogenic impact of viral infection.

Zipcode Binding Protein 1 (ZBP1; IGF2BP1): A Model for Sequence-Specific RNA Regulation.

The fate of an RNA, from its localization, translation, and ultimate decay, is dictated by interactions with RNA binding proteins (RBPs). ß-actin mRNA has functioned as the classic example of RNA localization in eukaryotic cells. Studies of ß-actin mRNA over the past three decades have allowed understanding of how RBPs, such as ZBP1 (IGF2BP1), can control both RNA localization and translational status. Here, we summarize studies of ß-actin mRNA and focus on how ZBP1 serves as a model for understanding interactions between RNA and their binding protein(s). Central to the study of RNA and RBPs were technological developments that occurred along the way. We conclude with a future outlook highlighting new technologies that may be used to address still unanswered questions about RBP-mediated regulation of mRNA during its life cycle, within the cell.

The structural basis for RNA selectivity by the IMP family of RNA-binding proteins.

The IGF2 mRNA-binding proteins (ZBP1/IMP1, IMP2, IMP3) are highly conserved post-transcriptional regulators of RNA stability, localization and translation. They play important roles in cell migration, neural development, metabolism and cancer cell survival. The knockout phenotypes of individual IMP proteins suggest that each family member regulates a unique pool of RNAs, yet evidence and an underlying mechanism for this is lacking. Here, we combine systematic evolution of ligands by exponential enrichment (SELEX) and NMR spectroscopy to demonstrate that the major RNA-binding domains of the two most distantly related IMPs (ZBP1 and IMP2) bind to different consensus sequences and regulate targets consistent with their knockout phenotypes and roles in disease. We find that the targeting specificity of each IMP is determined by few amino acids in their variable loops. As variable loops often differ amongst KH domain paralogs, we hypothesize that this is a general mechanism for evolving specificity and regulation of the transcriptome.

RNP transport in cell biology: the long and winding road.

Regulation of gene expression is key determinant to cell structure and function. RNA localization, where specific mRNAs are transported to subcellular regions and then translated, is highly conserved in eukaryotes ranging from yeast to extremely specialized and polarized cells such as neurons. Messenger RNA and associated proteins (mRNP) move from the site of transcription in the nucleus to their final destination in the cytoplasm both passively through diffusion and actively via directed transport. Dysfunction of RNA localization, transport and translation machinery can lead to pathology. Single-molecule live-cell imaging techniques have revealed unique features of this journey with unprecedented resolution. In this review, we highlight key recent findings that have been made using these approaches and possible implications for spatial control of gene function.

Translation dynamics of single mRNAs in live cells and neurons.

Translation is the fundamental biological process converting mRNA information into proteins. Single-molecule imaging in live cells has illuminated the dynamics of RNA transcription; however, it is not yet applicable to translation. Here, we report single-molecule imaging of nascent peptides (SINAPS) to assess translation in live cells. The approach provides direct readout of initiation, elongation, and location of translation. We show that mRNAs coding for endoplasmic reticulum (ER) proteins are translated when they encounter the ER membrane. Single-molecule fluorescence recovery after photobleaching provides direct measurement of elongation speed (5 amino acids per second). In primary neurons, mRNAs are translated in proximal dendrites but repressed in distal dendrites and display "bursting" translation. This technology provides a tool with which to address the spatiotemporal translation mechanism of single mRNAs in living cells.

Key contribution of CPEB4-mediated translational control to cancer progression.

Malignant transformation, invasion and angiogenesis rely on the coordinated reprogramming of gene expression in the cells from which the tumor originated. Although deregulated gene expression has been extensively studied at genomic and epigenetic scales, the contribution of the regulation of mRNA-specific translation to this reprogramming is not well understood. Here we show that cytoplasmic polyadenylation element binding protein 4 (CPEB4), an RNA binding protein that mediates meiotic mRNA cytoplasmic polyadenylation and translation, is overexpressed in pancreatic ductal adenocarcinomas and glioblastomas, where it supports tumor growth, vascularization and invasion. We also show that, in pancreatic tumors, the pro-oncogenic functions of CPEB4 originate in the translational activation of mRNAs that are silenced in normal tissue, including the mRNA of tissue plasminogen activator, a key contributor to pancreatic ductal adenocarcinoma malignancy. Taken together, our results document a key role for post-transcriptional gene regulation in tumor development and describe a detailed mechanism for gene expression reprogramming underlying malignant tumor progression.

Spindle-localized CPE-mediated translation controls meiotic chromosome segregation.

Meiotic progression requires the translational activation of stored maternal mRNAs, such as those encoding cyclin B1 or mos. The translation of these mRNAs is regulated by the cytoplasmic polyadenylation element (CPE) present in their 3'UTRs, which recruits the CPE-binding protein CPEB. This RNA-binding protein not only dictates the timing and extent of translational activation by cytoplasmic polyadenylation but also participates, together with the translational repressor Maskin, in the transport and localization, in a quiescent state, of its targets to subcellular locations where their translation will take place. During the early development of Xenopus laevis, CPEB localizes at the animal pole of oocytes and later on at embryonic spindles and centrosomes. Disruption of embryonic CPEB-mediated translational regulation results in abnormalities in the mitotic apparatus and inhibits embryonic mitosis. Here we show that spindle-localized translational activation of CPE-regulated mRNAs, encoding for proteins with a known function in spindle assembly and chromosome segregation, is essential for completion of the first meiotic division and for chromosome segregation in Xenopus oocytes.