Imaging dynamics of endogenous mitochondrial RNA in single living cells

Approaches and challenges in identifying, quantifying, and manipulating dynamic mitochondrial genome variations

Mitochondria, the cellular powerhouses, possess their own unique genetic system, including replication, transcription, and translation. Studying these processes is crucial for comprehending mitochondrial disorders, energy production, and their related diseases. Over the past decades, various approaches have been applied in detecting and quantifying mitochondrial genome variations with also the purpose of manipulation of mitochondria or mitochondrial genome for therapeutics. Understanding the scope and limitations of above strategies is not only fundamental to the understanding of basic biology but also critical for exploring disease-related novel target(s), as well to develop innovative therapies. Here, this review provides an overview of different tools and techniques for accurate mitochondrial genome variations identification, quantification, and discuss novel strategies for the manipulation of mitochondria to develop innovative therapeutic interventions, through combining the insights gained from the study of mitochondrial genetics with ongoing single cell omics combined with advanced single molecular tools.

Opticool: Cutting-edge transgenic optical tools

Only a few short decades have passed since the sequencing of GFP, yet the modern repertoire of transgenically encoded optical tools implies an exponential proliferation of ever improving constructions to interrogate the subcellular environment. A myriad of tags for labeling proteins, RNA, or DNA have arisen in the last few decades, facilitating unprecedented visualization of subcellular components and processes. Development of a broad array of modern genetically encoded sensors allows real-time, in vivo detection of molecule levels, pH, forces, enzyme activity, and other subcellular and extracellular phenomena in ever expanding contexts. Optogenetic, genetically encoded optically controlled manipulation systems have gained traction in the biological research community and facilitate single-cell, real-time modulation of protein function in vivo in ever broadening, novel applications. While this field continues to explosively expand, references are needed to assist scientists seeking to use and improve these transgenic devices in new and exciting ways to interrogate development and disease. In this review, we endeavor to highlight the state and trajectory of the field of in vivo transgenic optical tools.

De Novo Single-Stranded RNA-Binding Peptides Discovered by Codon-Restricted mRNA Display

RNA-binding proteins participate in diverse cellular processes, including DNA repair, post-transcriptional modification, and cancer progression through their interactions with RNAs, making them attractive for biotechnological applications. While nature provides an array of naturally occurring RNA-binding proteins, developing de novo RNA-binding peptides remains challenging. In particular, tailoring peptides to target single-stranded RNA with low complexity is difficult due to the inherent structural flexibility of RNA molecules. Here, we developed a codon-restricted mRNA display and identified multiple de novo peptides from a peptide library that bind to poly(C) and poly(A) RNA with KDs ranging from micromolar to submicromolar concentrations. One of the newly identified peptides is capable of binding to the cytosine-rich sequences of the oncogenic Cdk6 3'UTR RNA and MYU lncRNA, with affinity comparable to that of the endogenous binding protein. Hence, we present a novel platform for discovering de novo single-stranded RNA-binding peptides that offer promising avenues for regulating RNA functions.

Split Luciferase-Fragment Reconstitution for Unveiling RNA Localization and Dynamics in Live Cells

The intracellular distribution and dynamics of RNAs play pivotal roles in various physiological phenomena. The ability to monitor the amount and localization of endogenous RNAs in living cells allows for elucidating the mechanisms of various intracellular events. Protein-based fluorescent RNA probes are now widely used to visualize and analyze RNAs in living cells. However, continuously monitoring the temporal changes in RNA localization and dynamics in living cells is challenging. In this study, we developed a bioluminescent probe for spatiotemporal monitoring of RNAs in living cells by using a split-luciferase reconstitution technique. The probe consists of split fragments of a bioluminescent protein, NanoLuc, connected with RNA-binding protein domains generated from a custom-made mutation of a PUM-HD. The probe showed rapid luminescence intensity changes in response to an increase or decrease in the amount of a target RNA in vitro. In live-cell imaging, temporal alteration of the intracellular distribution of endogenous β-actin mRNA was visualized in response to extracellular stimulation. Furthermore, the application of the probe to the visualization of the specific localization of β-actin mRNA in primary hippocampal neurons was conducted. These results demonstrate the capability of the bioluminescent RNA probe to monitor the changes in localization, dynamics, and the amount of target RNA in living cells.

Live-Cell Imaging of Endogenous RNA with a Genetically Encoded Fluorogenic Allosteric Aptamer

Imaging and tracking tools for natural cellular RNA with improved biocompatibility, specificity, and sensitivity are critical to understanding RNA function and providing insights into disease therapeutics. We developed a new genetically encoded sensor using fluorogenic allosteric aptamer (FaApt) for the sensitive imaging of the localization and dynamics of RNA targets in live cells. Target RNAs can be specifically recognized with our sensor by forming perfectly complementary duplexes, which in turn can induce allosteric structural changes of the sensor to refold the native conformation of fluorogenic RNA aptamers. We demonstrated the ability of the sensor to monitor the effect of tumor necrosis factor and small-molecule inhibitor on the expression abundance of CXCL1 and survivin mRNA in human cancer cells, respectively. The asymmetrical distribution of endogenous Squint mRNA was confirmed in developing zebrafish embryos through microinjection of FaApt probes. This study provides an effective molecular tool for sensitive imaging and tracking endogenous RNA in living cells. Due to the high specificity and small size of our sensor system, it is expected to be applied to early diagnosis of RNA marker-related diseases and real-time evaluation of the treatment process.

Quantitative Analysis of Mitochondrial RNA in Living Cells with a Dual-Color Imaging System

Accurate quantification and dynamic expression profiling of mitochondrial RNA (mtRNA for short) are critical for illustrating their cellular functions. However, there lack methods for precise detection of mtRNA in situ due to the delivery restrictions and complicated cellular interferences. Herein, a dual-color imaging system featured with signal amplification and normalization capability for quantitative analysis of specific mtRNA is established. As a proof-of-concept example, an enzyme-free hairpin DNA cascade amplifier fine-tailored to specifically recognize mtRNA encoding NADH dehydrogenase subunit 6 (ND6) is employed as the signal output module and integrated into the biodegradable mitochondria-targeting black phosphorus nanosheet (BP-PEI-TPP) to monitor spatial-temporal dynamics of ND6 mtRNA. An internal reference module targeting β-actin mRNA is sent to the cytoplasm via BP-PEI for signal normalization, facilitating mtRNA quantification inside living cells with a degree of specificity and sensitivity as high as reverse transcription-quantitative polymerase chain reaction (RT-qPCR). With negligible cytotoxicity, this noninvasive "RT-qPCR mimic" can accurately indicate target mtRNA levels across different cells, providing a new strategy for precise analysis of subcellular RNAs in living systems.

Genetically encodable tagging and sensing systems for fluorescent RNA imaging

Live cell imaging of RNAs is crucial to interrogate their fundamental roles in various biological processes. The highly spatiotemporal dynamic nature of RNA abundance and localization has presented great challenges for RNA imaging. Genetically encodable tagging and sensing (GETS) systems that can be continuously produced in living systems have afforded promising tools for imaging and sensing RNA dynamics in live cells. Here we review the recent advances of GETS systems that have been developed for RNA tagging and sensing in live cells. We first describe the various GETS systems using MS2-bacteriophage-MS2 coat protein, pumilio homology domain and clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9/13 for RNA labeling and tracking. The progresses of GETS systems for fluorogenic labeling and/or sensing RNAs by engineering light-up RNA aptamers, CRISPR-Cas9 systems and RNA aptamer stabilized fluorogenic proteins are then elaborated. The challenges and future perspectives in this field are finally discussed. With the continuing development, GETS systems will afford powerful tools to elucidate RNA biology in living systems.

Construction of a Versatile, Programmable RNA-Binding Protein Using Designer PPR Proteins and Its Application for Splicing Control in Mammalian Cells

RNAs play many essential roles in gene expression and are involved in various human diseases. Although genome editing technologies have been established, the engineering of sequence-specific RNA-binding proteins that manipulate particular cellular RNA molecules is immature, in contrast to nucleotide-based RNA manipulation technology, such as siRNA- and RNA-targeting CRISPR/Cas. Here, we demonstrate a versatile RNA manipulation technology using pentatricopeptide-repeat (PPR)-motif-containing proteins. First, we developed a rapid construction and evaluation method for PPR-based designer sequence-specific RNA-binding proteins. This system has enabled the steady construction of dozens of functional designer PPR proteins targeting long 18 nt RNA, which targets a single specific RNA in the mammalian transcriptome. Furthermore, the cellular functionality of the designer PPR proteins was first demonstrated by the control of alternative splicing of either a reporter gene or an endogenous CHK1 mRNA. Our results present a versatile protein-based RNA manipulation technology using PPR proteins that facilitates the understanding of unknown RNA functions and the creation of gene circuits and has potential for use in future therapeutics.

Programmable RNA base editing with a single gRNA-free enzyme

Programmable RNA editing enables rewriting gene expression without changing genome sequences. Current tools for specific RNA editing dependent on the assembly of guide RNA into an RNA/protein complex, causing delivery barrier and low editing efficiency. We report a new gRNA-free system, RNA editing with individual RNA-binding enzyme (REWIRE), to perform precise base editing with a single engineered protein. This artificial enzyme contains a human-originated programmable PUF domain to specifically recognize RNAs and different deaminase domains to achieve efficient A-to-I or C-to-U editing, which achieved 60-80% editing rate in human cells, with a few non-specific editing sites in the targeted region and a low level off-target effect globally. The RNA-binding domain in REWIREs was further optimized to improve editing efficiency and minimize off-target effects. We applied the REWIREs to correct disease-associated mutations and achieve both types of base editing in mice. As a single-component system originated from human proteins, REWIRE presents a precise and efficient RNA editing platform with broad applicability.

Computational study on the binding of Mango-II RNA aptamer and fluorogen using the polarizable force field AMOEBA

Fluorescent light-up aptamers (FLAPs) are well-performed biosensors for cellular imaging and the detection of different targets of interest, including RNA, non-nucleic acid molecules, metal ions, and so on. They could be easily designed and emit a strong fluorescence signal once bound to specified fluorogens. Recently, one unique aptamer called Mango-II has been discovered to possess a strong affinity and excellent fluorescent properties with fluorogens TO1-Biotin and TO3-Biotin. To explore the binding mechanisms, computational simulations have been performed to obtain structural and thermodynamic information about FLAPs at atomic resolution. AMOEBA polarizable force field, with the capability of handling the highly charged and flexible RNA system, was utilized for the simulation of Mango-II with TO1-Biotin and TO3-Biotin in this work. The calculated binding free energy using published crystal structures is in excellent agreement with the experimental values. Given the challenges in modeling complex RNA dynamics, our work demonstrates that MD simulation with a polarizable force field is valuable for understanding aptamer-fluorogen binding and potentially designing new aptamers or fluorogens with better performance.

Mechanisms and Strategies for Determining m6 A RNA Modification Sites by Natural and Engineered m6 A Effector Proteins

N⁶ -Methyladenosine (m⁶ A) is the most common internal RNA modification in the consensus sequence of 5'-RRACH-3'. The methyl mark is added by writer proteins (METTL3/METTL14 metyltransferase complex) and removed by eraser proteins (m⁶ A demethylases; FTO and ALKBH5). Recognition of this methyl mark by m⁶ A reader proteins leads to changes in RNA metabolism. How the writer and eraser proteins determine their targets is not well-understood, despite the importance of this information in understanding the regulatory mechanisms and physiological roles of m⁶ A. However, approaches for targeted manipulation of the methylation state at specific sites are being developed. In this review, I summarize the recent findings on the mechanisms of target identification of m⁶ A regulatory proteins, as well as recent approaches for targeted m⁶ A modifications.

Live imaging of RNA and RNA splicing in mammalian cells via the dcas13a-SunTag-BiFC system

Dynamic tracking of the localization of RNA molecules (nucleus and/or cytoplasm) and RNA splicing in living cells plays an important role in understanding their functions. However, a lack of dynamic imaging and high background fluorescence have been reported in the fluorescence in situ hybridization (FISH). Here, we developed a new tool, the dcas13a-SunTag-BiFC system, which fused the dLwacas13a and SunTag systems. dLwacas13a is used as a tracker to target specific RNAs, while SunTag recruits split Venus fluorescent proteins to label targeted RNAs. Our results showed that 4 × NLS-dCas13a-24 × SunTag-BiFC and 2 × NLS- dCas13a-24 × SunTag-BiFC systems can be used for imaging of endogenous RNA foci in the nucleus (Xist) and cytoplasm (Ppib and stress granules) in living cells, respectively. Compared to 12x MS2-MCP system, the dcas13a-SunTag-BiFC system showed a better performance of mRNA foci tracking in live cells. Furthermore, we confirmed the premature termination codon (PTC)-induced exon skipping of Oxt RNA using the dcas13a-SunTag-BiFC and MS2-MCP systems in the nucleus. Thus, the dcas13a-SunTag-BiFC system will facilitate the study of RNA localization in living cells and provide new insights into RNA translocation and splicing.

Live-Cell Imaging of RNA Transport in Axons of Cultured Primary Neurons

The use of fluorescent proteins has revolutionized the study of protein localization and transport. However, the visualization of other molecules and specifically RNA during live-cell imaging remains challenging. In this chapter, we provide guidance to the available methods, their advantages and drawbacks as well as provide a detailed protocol for the detection of RNA transport using the MS2/PP7-split-Venus system for background-free RNA imaging.

Expanding the binding specificity for RNA recognition by a PUF domain

The ability to design a protein to bind specifically to a target RNA enables numerous applications, with the modular architecture of the PUF domain lending itself to new RNA-binding specificities. For each repeat of the Pumilio-1 PUF domain, we generate a library that contains the 8,000 possible combinations of amino acid substitutions at residues critical for RNA contact. We carry out yeast three-hybrid selections with each library against the RNA recognition sequence for Pumilio-1, with any possible base present at the position recognized by the randomized repeat. We use sequencing to score the binding of each variant, identifying many variants with highly repeat-specific interactions. From these data, we generate an RNA binding code specific to each repeat and base. We use this code to design PUF domains against 16 RNAs, and find that some of these domains recognize RNAs with two, three or four changes from the wild type sequence.

RNA imaging in bacteria

The spatiotemporal regulation of gene expression plays an essential role in many biological processes. Recently, several imaging-based RNA labeling and detection methods, both in fixed and live cells, were developed and now enable the study of transcript abundance, localization and dynamics. Here, we review the main single-cell techniques for RNA visualization with fluorescence microscopy and describe their applications in bacteria.

Poly(A)+ Sensing of Hybridization-Sensitive Fluorescent Oligonucleotide Probe Characterized by Fluorescence Correlation Methods

Ribonucleic acid (RNA) plays an important role in many cellular processes. Thus, visualizing and quantifying the molecular dynamics of RNA directly in living cells is essential to uncovering their role in RNA metabolism. Among the wide variety of fluorescent probes available for RNA visualization, exciton-controlled hybridization-sensitive fluorescent oligonucleotide (ECHO) probes are useful because of their low fluorescence background. In this study, we apply fluorescence correlation methods to ECHO probes targeting the poly(A) tail of mRNA. In this way, we demonstrate not only the visualization but also the quantification of the interaction between the probe and the target, as well as of the change in the fluorescence brightness and the diffusion coefficient caused by the binding. In particular, the uptake of ECHO probes to detect mRNA is demonstrated in HeLa cells. These results are expected to provide new insights that help us better understand the metabolism of intracellular mRNA.

Imaging of native transcription and transcriptional dynamics in vivo using a tagged Argonaute protein

A flexible method to image unmodified transcripts and transcription in vivo would be a valuable tool to understand the regulation and dynamics of transcription. Here, we present a novel approach to follow native transcription, with fluorescence microscopy, in live C. elegans. By using the fluorescently tagged Argonaute protein NRDE-3, programmed by exposure to defined dsRNA to bind to nascent transcripts of the gene of interest, we demonstrate transcript labelling of multiple genes, at the transcription site and in the cytoplasm. This flexible approach does not require genetic manipulation, and can be easily scaled up by relying on whole-genome dsRNA libraries. We apply this method to image the transcriptional dynamics of the heat-shock inducible gene hsp-4 (a member of the hsp70 family), as well as two transcription factors: ttx-3 (a LHX2/9 orthologue) in embryos, and hlh-1 (a MyoD orthologue) in larvae, respectively involved in neuronal and muscle development.

Recent Development of Fluorescent Light-Up RNA Aptamers

Technologies for RNA imaging in live cells play an important role in understanding the function and regulatory process of RNAs. One approach for genetically encoded fluorescent RNA imaging involves fluorescent light-up aptamers (FLAPs), which are short RNA sequences that can bind cognate fluorogens and activate their fluorescence greatly. Over the past few years, FLAPs have emerged as genetically encoded RNA-based fluorescent biosensors for the cellular imaging and detection of various targets of interest. In this review, we first give a brief overview of the development of the current FLAPs based on various fluorogens. Then we further discuss on the photocycles of the reversibly photoswitching properties in FLAPs and their photostability. Finally, we focus on the applications of FLAPs as genetically encoded RNA-based fluorescent biosensors in biosensing and bioimaging, including RNA, non-nucleic acid molecules, metal ions imaging and quantitative imaging. Their design strategies and recent cellular applications are emphasized and summarized in detail.

Mini-III RNase-based dual-color system for in vivo mRNA tracking

Mini-III RNase (mR3), a member of RNase III endonuclease family, can bind to and cleave double-stranded RNAs (dsRNAs). Inactive mR3 protein without the α5β-α6 loop loses the dsRNA cleavage activity, but retains dsRNA binding activity. Here, we establish an inactive mR3-based non-engineered mR3/dsRNA system for RNA tracking in zebrafish embryos. In vitro binding experiments show that inactive Staphylococcus epidermidis mR3 (dSmR3) protein possesses the highest binding affinity with dsRNAs among mR3s from other related species, and its binding property is retained in zebrafish embryos. Combined with a fluorescein-labeled antisense RNA probe recognizing the target mRNAs, dSmR3 tagged with a nuclear localization sequence and a fluorescent protein could allow visualization of the dynamics of endogenous target mRNAs. The dSmR3/antisense probe dual-color system provides a new approach for tracking non-engineered RNAs in real-time, which will help understand how endogenous RNAs dynamically move during embryonic development.

Summary: A fluorescent antisense probe and the inactive form of Staphylococcus epidermidis Mini-III RNase with a fluorescent tag may be used together to visualize endogenous mRNAs in zebrafish embryos.

Effective RNA Regulation by Combination of Multiple Programmable RNA-Binding Proteins

RNAs play important roles in gene expression through translation and RNA splicing. Regulation of specific RNAs is useful to understand and manipulate specific transcripts. Pumilio and fem-3 mRNA-binding factor (PUF) proteins, programmable RNA-binding proteins, are promising tools for regulating specific RNAs by fusing them with various functional domains. The key question is: How can PUF-based molecular tools efficiently regulate RNA functions? Here, we show that the combination of multiple PUF proteins, compared to using a single PUF protein, targeting independent RNA sequences at the 3′ untranslated region (UTR) of a target transcript caused cooperative effects to regulate the function of the target RNA by luciferase reporter assays. It is worth noting that a higher efficacy was achieved with smaller amounts of each PUF expression vector introduced into the cells compared to using a single PUF protein. This strategy not only efficiently regulates target RNA functions but would also be effective in reducing off-target effects due to the low doses of each expression vector.

Illuminating RNA Biology: Tools for Imaging RNA in Live Mammalian Cells

The central dogma teaches us that DNA makes RNA, which in turn makes proteins, the main building blocks of the cell. But this over simplified linear transmission of information overlooks the vast majority of the genome produces RNAs that do not encode proteins and the myriad ways that RNA regulates cellular functions. Historically, one of the challenges in illuminating RNA biology has been the lack of tools for visualizing RNA in live cells. But clever approaches for exploiting RNA binding proteins, in vitro RNA evolution, and chemical biology have resulted in significant advances in RNA visualization tools in recent years. This review provides an overview of current tools for tagging RNA with fluorescent probes and tracking their dynamics, localization andfunction in live mammalian cells.

Plant PUF RNA-binding proteins: A wealth of diversity for post-transcriptional gene regulation

PUF proteins are a conserved group of sequence-specific RNA-binding proteins that typically function to negatively regulate mRNA stability and translation. PUFs are well characterized at the molecular, structural and functional levels in Drosophila, Caenorhabditis elegans, budding yeast and human systems. Although usually encoded by small gene families, PUFs are over-represented in the plant genome, with up to 36 genes identified in a single species. PUF gene expansion in plants has resulted in extensive variability in gene expression patterns, diversity in predicted RNA-binding domain structure, and novel combinations of key amino acids involved in modular nucleotide binding. Reports on the characterization of plant PUF structure and function continue to expand, and include RNA target identification, subcellular distribution, crystal structure, and molecular mechanisms. Arabidopsis PUF mutant analysis has provided insight into biological function, and has identified roles related to development and environmental stress tolerance. The diversity of plant PUFs implies an extensive role for this family of proteins in post-transcriptional gene regulation. This diversity also holds the potential for providing novel RNA-binding domains that could be engineered to produce designer PUFs to alter the metabolism of target RNAs in the cell.

A Multi-responsive Fluorescent Probe Reveals Mitochondrial Nucleoprotein Dynamics with Reactive Oxygen Species Regulation through Super-resolution Imaging

Understanding the biomolecular interactions in a specific organelle has been a long-standing challenge because it requires super-resolution imaging to resolve the spatial locations and dynamic interactions of multiple biomacromolecules. Two key difficulties are the scarcity of suitable probes for super-resolution nanoscopy and the complications that arise from the use of multiple probes. Herein, we report a quinolinium derivative probe that is selectively enriched in mitochondria and switches on in three different fluorescence modes in response to hydrogen peroxide (H₂ O₂ ), proteins, and nucleic acids, enabling the visualization of mitochondrial nucleoprotein dynamics. STED nanoscopy reveals that the proteins localize at mitochondrial cristae and largely fuse with nucleic acids to form nucleoproteins, whereas increasing H₂ O₂ level leads to disassociation of nucleic acid-protein complexes.

Quantification of native mRNA dynamics in living neurons using fluorescence correlation spectroscopy and reduction-triggered fluorescent probes

RNA localization in subcellular compartments is essential for spatial and temporal regulation of protein expression in neurons. Several techniques have been developed to visualize mRNAs inside cells, but the study of the behavior of endogenous and nonengineered mRNAs in living neurons has just started. In this study, we combined reduction-triggered fluorescent (RETF) probes and fluorescence correlation spectroscopy (FCS) to investigate the diffusion properties of activity-regulated cytoskeleton-associated protein (Arc) and inositol 1,4,5-trisphosphate receptor type 1 (Ip3r1) mRNAs. This approach enabled us to discriminate between RNA-bound and unbound fluorescent probes and to quantify mRNA diffusion parameters and concentrations in living rat primary hippocampal neurons. Specifically, we detected the induction of Arc mRNA production after neuronal activation in real time. Results from computer simulations with mRNA diffusion coefficients obtained in these analyses supported the idea that free diffusion is incapable of transporting mRNA of sizes close to those of Arc or Ip3r1 to distal dendrites. In conclusion, the combined RETF-FCS approach reported here enables analyses of the dynamics of endogenous, unmodified mRNAs in living neurons, affording a glimpse into the intracellular dynamics of RNA in live cells.

Imaging of DNA and RNA in Living Eukaryotic Cells to Reveal Spatiotemporal Dynamics of Gene Expression

This review focuses on imaging DNA and single RNA molecules in living cells to define eukaryotic functional organization and dynamic processes. The latest advances in technologies to visualize individual DNA loci and RNAs in real time are discussed. Single-molecule fluorescence microscopy provides the spatial and temporal resolution to reveal mechanisms regulating fundamental cell functions. Novel insights into the regulation of nuclear architecture, transcription, posttranscriptional RNA processing, and RNA localization provided by multicolor fluorescence microscopy are reviewed. A perspective on the future use of live imaging technologies and overcoming their current limitations is provided.

Propargylamine-selective dual fluorescence turn-on method for post-synthetic labeling of DNA

We have developed a propargylamine-selective dual fluorescence turn-on system, using ylidenemalononitrile enamines, for post-synthetic DNA labeling, allowing the direct monitoring of DNA using dual emission in living cells.

Techniques for investigating mitochondrial gene expression

The mitochondrial genome encodes 13 proteins that are components of the oxidative phosphorylation system (OXPHOS), suggesting that precise regulation of these genes is crucial for maintaining OXPHOS functions, including ATP production, calcium buffering, cell signaling, ROS production, and apoptosis. Furthermore, heteroplasmy or mis-regulation of gene expression in mitochondria frequently is associated with human mitochondrial diseases. Thus, various approaches have been developed to investigate the roles of genes encoded by the mitochondrial genome. In this review, we will discuss a wide range of techniques available for investigating the mitochondrial genome, mitochondrial transcription, and mitochondrial translation, which provide a useful guide to understanding mitochondrial gene expression.

Live-cell imaging of single mRNA dynamics using split superfolder green fluorescent proteins with minimal background

The MS2 system, with an MS2 binding site (MBS) and an MS2 coat protein fused to a fluorescent protein (MCP-FP), has been widely used to fluorescently label mRNA in live cells. However, one of its limitations is the constant background fluorescence signal generated from free MCP-FPs. To overcome this obstacle, we used a superfolder GFP (sfGFP) split into two or three nonfluorescent fragments that reassemble and emit fluorescence only when bound to the target mRNA. Using the high-affinity interactions of bacteriophage coat proteins with their corresponding RNA binding motifs, we showed that the nonfluorescent sfGFP fragments were successfully brought close to each other to reconstitute a complete sfGFP. Furthermore, real-time mRNA dynamics inside the nucleus as well as the cytoplasm were observed by using the split sfGFPs with the MS2-PP7 hybrid system. Our results demonstrate that the split sfGFP systems are useful tools for background-free imaging of mRNA with high spatiotemporal resolution.

Programmable RNA methylation and demethylation using PUF RNA binding proteins

A new method manipulating local RNA methylation was developed by fusing the programmable RNA binding protein and the m⁶A demethylase or methyltransferase.

Beyond classic editing: innovative CRISPR approaches for functional studies of long non-coding RNA

Long non-coding RNAs (lncRNAs) makeup a considerable part of the non-coding human genome and had been well-established as crucial players in an array of biological processes. In spite of their abundance and versatile roles, their functional characteristics remain largely undiscovered mainly due to the lack of suitable genetic manipulation tools. The emerging CRISPR/Cas9 technology has been widely adapted in several studies that aim to screen and identify novel lncRNAs as well as interrogate the functional properties of specific lncRNAs. However, the complexity of lncRNAs genes and the regulatory mechanisms that govern their transcription, as well as their unique functionality pose several limitations the utilization of classic CRISPR methods in lncRNAs functional studies. Here, we overview the unique characteristics of lncRNAs transcription and function and the suitability of the CRISPR toolbox for applications in functional characterization of lncRNAs. We discuss some of the novel variations to the classic CRISPR/Cas9 system that have been tailored and applied previously to study several aspects of lncRNAs functionality. Finally, we share perspectives on the potential applications of various CRISPR systems, including RNA-targeting, in the direct editing and manipulation of lncRNAs.

Manipulating and elucidating mitochondrial gene expression with engineered proteins

Many conventional, modern genome engineering tools cannot be used to study mitochondrial genetics due to the unusual structure and physiology of the mitochondrial genome. Here, we review a number of newly developed, synthetic biology-based approaches for altering levels of mutant mammalian mitochondrial DNA and mitochondrial RNAs, including transcription activator-like effector nucleases, zinc finger nucleases and engineered RNA-binding proteins. These approaches allow researchers to manipulate and visualize mitochondrial processes and may provide future therapeutics. This article is part of the theme issue 'Linking the mitochondrial genotype to phenotype: a complex endeavour'.

The potential of engineered eukaryotic RNA-binding proteins as molecular tools and therapeutics

Eukaroytic RNA-binding proteins (RBPs) recognize and process RNAs through recognition of their sequence motifs via RNA-binding domains (RBDs). RBPs usually consist of one or more RBDs and can include additional functional domains that modify or cleave RNA. Engineered RBPs have been used to answer basic biology questions, control gene expression, locate viral RNA in vivo, as well as many other tasks. Given the growing number of diseases associated with RNA and RBPs, engineered RBPs also have the potential to serve as therapeutics. This review provides an in depth description of recent advances in engineered RBPs and discusses opportunities and challenges in the field. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Methods > RNA Nanotechnology RNA in Disease and Development > RNA in Disease.

Deep-Red Light-up Signaling of Benzo[c,d]indole-Quinoline Monomethine Cyanine for Imaging of Nucleolar RNA in Living Cells and for Sequence-Selective RNA Analysis

RNA-binding small probes with deep-red emission are promising for RNA analysis in biological media without suffering from background fluorescence. Here benzo[c,d]indole-quinoline (BIQ), an asymmetric monomethine cyanine analogue, was newly developed as a novel RNA-selective probe with light-up signaling ability in the deep-red spectral range. BIQ features a significant light-up response (105-fold) with an emission maximum at 657 nm as well as improved photostability over the commercially available RNA-selective probe, SYTO RNA select. BIQ was successfully applied to the fluorescence imaging of nucleolar RNAs in living cells with negligible cytotoxicity. Furthermore, we found the useful ability of BIQ as a base surrogate integrated in peptide nucleic acid (PNA) oligonucleotides for RNA sequence analysis. BIQ base surrogate functioned as a deep-red light-up base surrogate in forced intercalation (FIT) and triplex-forming FIT (tFIT) systems for the sequence-selective detection of single-stranded and double-stranded RNAs, respectively.

Multiplex live single-cell transcriptional analysis demarcates cellular functional heterogeneity

A fundamental goal in the biological sciences is to determine how individual cells with varied gene expression profiles and diverse functional characteristics contribute to development, physiology, and disease. Here, we report a novel strategy to assess gene expression and cell physiology in single living cells. Our approach utilizes fluorescently labeled mRNA-specific anti-sense RNA probes and dsRNA-binding protein to identify the expression of specific genes in real-time at single-cell resolution via FRET. We use this technology to identify distinct myocardial subpopulations expressing the structural proteins myosin heavy chain α and myosin light chain 2a in real-time during early differentiation of human pluripotent stem cells. We combine this live-cell gene expression analysis with detailed physiologic phenotyping to capture the functional evolution of these early myocardial subpopulations during lineage specification and diversification. This live-cell mRNA imaging approach will have wide ranging application wherever heterogeneity plays an important biological role.

Modulation of miRNA function by natural and synthetic RNA-binding proteins in cancer

RNA-binding proteins (RBPs) and microRNAs (miRNAs) are the most important regulators of mRNA stability and translation in eukaryotic cells; however, the complex interplay between these systems is only now coming to light. RBPs and miRNAs regulate a unique set of targets in either a positive or negative manner and their regulation is mainly opposed to each other on overlapping targets. In some cases, the levels of RBPs or miRNAs regulate the cellular levels of one another and decreased levels of either results in changes in translation of their targets. There is growing evidence that these regulatory circuits are crucial in the development and progression of cancer; however, the rules underlying synergism and antagonism between miRNAs and RNA-binding proteins remain unclear. Synthetic biology seeks to develop artificial systems to better understand their natural counterparts and to develop new, useful technologies for manipulation of gene expression at the RNA level. The recent development of artificial RNA-binding proteins promises to enable a much greater understanding of the importance of the functional interactions between RNA-binding proteins and miRNAs, as well as enabling their manipulation for therapeutic purposes.

Recent progress in single-molecule studies of mRNA localization in vivo

From biogenesis to degradation, mRNA goes through diverse types of regulation and interaction with other biomolecules. Uneven distribution of mRNA transcripts and the diverse isoforms and modifications of mRNA make us wonder how cells manage the complexity and keep the functional integrity for the normal development of cells and organisms. Single-molecule microscopy tools have expanded the scope of RNA research with unprecedented spatiotemporal resolution. In this review, we highlight the recent progress in the methods for labeling mRNA targets and analyzing the quantitative information from fluorescence images of single mRNA molecules. In particular, the MS2 system and its various applications are the main focus of this article. We also review how recent studies have addressed biological questions related to the significance of mRNA localization in vivo. Efforts to visualize the dynamics of single mRNA molecules in live cells will push forward our knowledge on the nature of heterogeneity in RNA sequence, structure, and distribution as well as their molecular function and coordinated interaction with RNA binding proteins.

Searching for a Match: Structure, Function and Application of Sequence-Specific RNA-Binding Proteins

Plants encode over 1800 RNA-binding proteins (RBPs) that modulate a myriad of steps in gene regulation from chromatin organization to translation, yet only a small number of these proteins and their target transcripts have been functionally characterized. Two classes of eukaryotic RBPs, pentatricopeptide repeat (PPR) and pumilio/fem-3 binding factors (PUF), recognize and bind to specific sequential RNA sequences through protein-RNA interactions. These modular proteins possess helical structural units containing key residues with high affinity for specific nucleotides, whose sequential order determines binding to a specific target RNA sequence. PPR proteins are nucleus-encoded, but largely regulate post-transcriptional gene regulation within plastids and mitochondria, including splicing, translation and RNA editing. Plant PUFs are involved in gene regulatory processes within the cell nucleus and cytoplasm. The modular structures of PPRs and PUFs that determine sequence specificity has facilitated identification of their RNA targets and biological functions. The protein-based RNA-targeting of PPRs and PUFs contrasts to the prokaryotic cluster regularly interspaced short palindromic repeats (CRISPR)-associated proteins (Cas) that target RNAs in prokaryotes. Together the PPR, PUF and CRISPR-Cas systems provide varied opportunities for RNA-targeted engineering applications.

SRB-2: a promiscuous rainbow aptamer for live-cell RNA imaging

The SRB-2 aptamer originally selected against sulforhodamine B is shown here to promiscuously bind to various dyes with different colors. Binding of SRB-2 to these dyes results in either fluorescence increase or decrease, making them attractive for fluorescence microscopy and biological assays. By systematically varying fluorophore structural elements and measuring dissociation constants, the principles of fluorophore recognition by SRB-2 were analyzed. The obtained structure-activity relationships allowed us to rationally design a novel, bright, orange fluorescent turn-on probe (TMR-DN) with low background fluorescence, enabling no-wash live-cell RNA imaging. This new probe improved the signal-to-background ratio of fluorescence images by one order of magnitude over best previously known probe for this aptamer. The utility of TMR-DN is demonstrated by imaging ribosomal and messenger RNAs, allowing the observation of distinct localization patterns in bacteria and mammalian cells. The SRB-2 / TMR-DN system is found to be orthogonal to the Spinach/DFHBI and MG/Malachite green aptamer/dye systems.

Ribosomal RNA-Selective Light-Up Fluorescent Probe for Rapidly Imaging the Nucleolus in Live Cells

RNA-based fluorescent probes are currently limited by their low selectivity toward RNA versus DNA, and low specificity to different RNA structures. Poor membrane permeability is another defect of existing fluorogenic RNA probes for intracellular imaging. In this work, a naphthalimide derivative, probe 1, was developed for the rapid and selective detection of intracellular rRNA (rRNA). Probe 1 exhibited a 32-fold fluorescent enhancement in response to rRNA binding and showed desirable selectivity for rRNA versus DNA and other nucleic acids in phosphate buffer at pH 7.2. Importantly, probe 1 displayed excellent permeability of the nucleolus, could be taken up in 1 min by four different cell lines, and may be the fastest nucleolus dye. The excellent selectivity of probe 1 toward rRNA is attributed to the specific interaction between the complicated 3D structures of rRNA, which was confirmed by quantum calculations using molecular docking simulations. An appropriate lipophilic balance in 1 with the hydrophilic amine group and hydrophobic naphthalimide, as well as its high water solubility, guarantees the high permeability of 1 in cell membranes and nucleolus pores, compared to other analogues (e.g., probes 2-8 in this work). Furthermore, enlarged confocal laser micro images of nucleoli and RNase digestion tests revealed that 1 remained highly selective toward rRNA, even for intracellular imaging. As a live cell probe, 1 also exhibited better photostability than the commercial RNA dye, SYTO RNA select.

Live-cell imaging of small nucleolar RNA tagged with the broccoli aptamer in yeast

The development of the RNA 'vegetable' aptamers, Spinach and Broccoli, has simplified RNA imaging, especially in live cells. These RNA aptamers interact with a fluorophore (DFHBI or DFHBI-1T) to produce a green fluorescence signal. Although used in mammalian and Escherichia coli cells, the use of these aptamers in yeast has been limited. Here we describe how the Saccharomyces cerevisiae snoRNA, snR30, was tagged with the Spinach or the Broccoli aptamers and observed in live cells. The ability to observe aptamer fluorescence in polyacrylamide gels stained with a fluorophore or with a microplate reader can ease preliminary screening of the aptamers in different RNA scaffolds. In snR30 a tandem repeat of the Broccoli aptamer produced the best signal in vitro. Multiple factors in cell preparation were vital for obtaining a good fluorescence signal. These factors included the clearance of the native unmodified snR30, the amount and length of dye incubation and the rinsing of cells. In cells, the aptamers did not interfere with the structure or essential function of snR30, as the tagged RNA localized to the nucleolus and directed processing of ribosomal RNA in yeast. High-resolution images of the tagged snoRNA were obtained with live cells immobilized by a microcompressor.

Advances in CRISPR-Cas systems for RNA targeting, tracking and editing

Clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein (Cas) systems, especially type II (Cas9) systems, have been widely used in gene/genome targeting. Modifications of Cas9 enable these systems to become platforms for precise DNA manipulations. However, the utilization of CRISPR-Cas systems in RNA targeting remains preliminary. The discovery of type VI CRISPR-Cas systems (Cas13) shed light on RNA-guided RNA targeting. Cas13d, the smallest Cas13 protein, with a length of only ~930 amino acids, is a promising platform for RNA targeting compatible with viral delivery systems. Much effort has also been made to develop Cas9, Cas13a and Cas13b applications for RNA-guided RNA targeting. The discovery of new RNA-targeting CRISPR-Cas systems as well as the development of RNA-targeting platforms with Cas9 and Cas13 will promote RNA-targeting technology substantially. Here, we review new advances in RNA-targeting CRISPR-Cas systems as well as advances in applications of these systems in RNA targeting, tracking and editing. We also compare these Cas protein-based technologies with traditional technologies for RNA targeting, tracking and editing. Finally, we discuss remaining questions and prospects for the future.

Import of Non-Coding RNAs into Human Mitochondria: A Critical Review and Emerging Approaches

Mitochondria harbor their own genetic system, yet critically depend on the import of a number of nuclear-encoded macromolecules to ensure their expression. In all eukaryotes, selected non-coding RNAs produced from the nuclear genome are partially redirected into the mitochondria, where they participate in gene expression. Therefore, the mitochondrial RNome represents an intricate mixture of the intrinsic transcriptome and the extrinsic RNA importome. In this review, we summarize and critically analyze data on the nuclear-encoded transcripts detected in human mitochondria and outline the proposed molecular mechanisms of their mitochondrial import. Special attention is given to the various experimental approaches used to study the mitochondrial RNome, including some recently developed genome-wide and in situ techniques.

RNA Localization in Bacteria

Diverse mechanisms and functions of posttranscriptional regulation by small regulatory RNAs and RNA-binding proteins have been described in bacteria. In contrast, little is known about the spatial organization of RNAs in bacterial cells. In eukaryotes, subcellular localization and transport of RNAs play important roles in diverse physiological processes, such as embryonic patterning, asymmetric cell division, epithelial polarity, and neuronal plasticity. It is now clear that bacterial RNAs also can accumulate at distinct sites in the cell. However, due to the small size of bacterial cells, RNA localization and localization-associated functions are more challenging to study in bacterial cells, and the underlying molecular mechanisms of transcript localization are less understood. Here, we review the emerging examples of RNAs localized to specific subcellular locations in bacteria, with indications that subcellular localization of transcripts might be important for gene expression and regulatory processes. Diverse mechanisms for bacterial RNA localization have been suggested, including close association to their genomic site of transcription, or to the localizations of their protein products in translation-dependent or -independent processes. We also provide an overview of the state of the art of technologies to visualize and track bacterial RNAs, ranging from hybridization-based approaches in fixed cells to in vivo imaging approaches using fluorescent protein reporters and/or RNA aptamers in single living bacterial cells. We conclude with a discussion of open questions in the field and ongoing technological developments regarding RNA imaging in eukaryotic systems that might likewise provide novel insights into RNA localization in bacteria.

Exploring the suitability of RanBP2-type Zinc Fingers for RNA-binding protein design

Transcriptomes consist of several classes of RNA that have wide-ranging but often poorly described functions and the deregulation of which leads to numerous diseases. Engineering of functionalized RNA-binding proteins (RBPs) could therefore have many applications. Our previous studies suggested that the RanBP2-type Zinc Finger (ZF) domain is a suitable scaffold to investigate the design of single-stranded RBPs. In the present work, we have analyzed the natural sequence specificity of various members of the RanBP2-type ZF family and characterized the interaction with their target RNA. Surprisingly, our data showed that natural RanBP2-type ZFs with different RNA-binding residues exhibit a similar sequence specificity and therefore no simple recognition code can be established. Despite this finding, different discriminative abilities were observed within the family. In addition, in order to target a long RNA sequence and therefore gain in specificity, we generated a 6-ZF array by combining ZFs from the RanBP2-type family but also from different families, in an effort to achieve a wider target sequence repertoire. We showed that this chimeric protein recognizes its target sequence (20 nucleotides), both in vitro and in living cells. Altogether, our results indicate that the use of ZFs in RBP design remains attractive even though engineering of specificity changes is challenging.

Split Green Fluorescent Proteins: Scope, Limitations, and Outlook

Many proteins can be split into fragments that spontaneously reassemble, without covalent linkage, into a functional protein. For split green fluorescent proteins (GFPs), fragment reassembly leads to a fluorescent readout, which has been widely used to investigate protein-protein interactions. We review the scope and limitations of this approach as well as other diverse applications of split GFPs as versatile sensors, molecular glues, optogenetic tools, and platforms for photophysical studies.

Mitochondrial Heterogeneity

Cell-to-cell heterogeneity drives a range of (patho)physiologically important phenomena, such as cell fate and chemotherapeutic resistance. The role of metabolism, and particularly of mitochondria, is increasingly being recognized as an important explanatory factor in cell-to-cell heterogeneity. Most eukaryotic cells possess a population of mitochondria, in the sense that mitochondrial DNA (mtDNA) is held in multiple copies per cell, where the sequence of each molecule can vary. Hence, intra-cellular mitochondrial heterogeneity is possible, which can induce inter-cellular mitochondrial heterogeneity, and may drive aspects of cellular noise. In this review, we discuss sources of mitochondrial heterogeneity (variations between mitochondria in the same cell, and mitochondrial variations between supposedly identical cells) from both genetic and non-genetic perspectives, and mitochondrial genotype-phenotype links. We discuss the apparent homeostasis of mtDNA copy number, the observation of pervasive intra-cellular mtDNA mutation (which is termed "microheteroplasmy"), and developments in the understanding of inter-cellular mtDNA mutation ("macroheteroplasmy"). We point to the relationship between mitochondrial supercomplexes, cristal structure, pH, and cardiolipin as a potential amplifier of the mitochondrial genotype-phenotype link. We also discuss mitochondrial membrane potential and networks as sources of mitochondrial heterogeneity, and their influence upon the mitochondrial genome. Finally, we revisit the idea of mitochondrial complementation as a means of dampening mitochondrial genotype-phenotype links in light of recent experimental developments. The diverse sources of mitochondrial heterogeneity, as well as their increasingly recognized role in contributing to cellular heterogeneity, highlights the need for future single-cell mitochondrial measurements in the context of cellular noise studies.

Engineering a conserved RNA regulatory protein repurposes its biological function in vivo

PUF (PUmilio/FBF) RNA-binding proteins recognize distinct elements. In C. elegans, PUF-8 binds to an 8-nt motif and restricts proliferation in the germline. Conversely, FBF-2 recognizes a 9-nt element and promotes mitosis. To understand how motif divergence relates to biological function, we first determined a crystal structure of PUF-8. Comparison of this structure to that of FBF-2 revealed a major difference in a central repeat. We devised a modified yeast 3-hybrid screen to identify mutations that confer recognition of an 8-nt element to FBF-2. We identified several such mutants and validated structurally and biochemically their binding to 8-nt RNA elements. Using genome engineering, we generated a mutant animal with a substitution in FBF-2 that confers preferential binding to the PUF-8 element. The mutant largely rescued overproliferation in animals that spontaneously generate tumors in the absence of puf-8. This work highlights the critical role of motif length in the specification of biological function.