The interaction of RNA with specific RNA-binding proteins (RBP) leads to the establishment of complex regulatory networks through which gene expression is controlled. Careful consideration should be given to the exact environment where a given RNA/RBP interplay occurs, as the functional responses might depend on the type of organism as well as the specific cellular or subcellular contexts. This requisite becomes particularly crucial for the study of long non-coding RNAs (lncRNA), as a consequence of their peculiar tissue-specificity and timely regulated expression. The functional characterization of lncRNAs has traditionally relied on the use of established cell lines that, although useful, are unable to fully recapitulate the complexity of a tissue or organ. Here, we detail an optimized protocol, with comments and tips, to identify the RNA interactome of given RBPs by performing cross-linking immunoprecipitation (CLIP) from mouse embryonal hearts. We tested the efficiency of this protocol on the murine pCharme, a muscle-specific lncRNA interacting with Matrin3 (MATR3) and forming RNA-enriched condensates of biological significance in the nucleus.

Key features

• The protocol refines previous methods of cardiac extracts preparation to use for CLIP assays.

• The protocol allows the quantitative RNA-seq analysis of transcripts interacting with selected proteins.

• Depending on the embryonal stage, a high number of hearts can be required as starting material.

• The steps are adaptable to other tissues and biochemical assays.

Graphical overview

Identification of RNA/protein interactions from developing hearts

The function of a protein within a cell critically depends on its interaction with other proteins as well as its subcellular localization. The expression of mutants of a particular protein that have selective perturbation of specific protein interaction motifs is a very useful strategy for resolving a protein’s mechanism of action in a cellular process. In addition, expression of fluorescent protein fusions is a key strategy for determining the subcellular localization of a protein. These strategies require tight regulation to avoid potential alterations in protein interactions or localizations that can result from protein overexpression. Previous work led to the development of a Sleeping Beauty transposon system that allows doxycycline-inducible expression of protein mutants or fusions; titration of doxycycline allows expression of protein fusions or mutants at near endogenous levels. When used in combination with siRNA gene silencing, this strategy allows for knockdown-rescue experiments to assess the function of specific protein mutants. In this protocol, we describe the use of this Sleeping Beauty strategy for expression of eGFP fusion or mutant proteins in ARPE-19 and MDA-MB-231 cells. This includes design of expression plasmids, transfection, and selection to obtain stable engineered cells, as well as doxycycline treatment for controlled induction of protein expression, either alone or in combination with siRNA silencing for knockdown-rescue experiments. This strategy is advantageous as it allows rapid generation of stable cells for controlled protein expression, suitable for functional studies that require knockdown-rescue as well as various forms of live cell fluorescence imaging.

Key features

• Highly versatile doxycycline-inducible expression system that can be used in various mammalian cell lines.

• Stable integration of transgene allows for sustained and stable expression.

• Titration of doxycycline levels allows expression of transgene at near endogenous levels.

In eukaryotic cells, RNA biogenesis generally requires processing of the nascent transcript as it is being synthesized by RNA polymerase. These processing events include endonucleolytic cleavage, exonucleolytic trimming, and splicing of the growing nascent transcript. Endonucleolytic cleavage events that generate an exposed 5′-monophosphorylated (5′-PO₄) end on the growing nascent transcript occur in the maturation of rRNAs, tRNAs, and mRNAs. These 5′-PO₄ ends can be a target of further processing or be subjected to 5′-3′ exonucleolytic digestion that may result in termination of transcription. Here, we describe how to identify 5′-PO₄ ends of intermediates in nascent RNA metabolism. We capture these species via metabolic labeling with bromouridine followed by immunoprecipitation and specific ligation of 5′-PO₄ RNA ends with the 3′-hydroxyl group of a 5′ adaptor (5′-PO₄ Bru-Seq) using RNA ligase I. These ligation events are localized at single nucleotide resolution via highthroughput sequencing, which identifies the position of 5′-PO₄ groups precisely. This protocol successfully detects the 5′monophosphorylated ends of RNA processing intermediates during production of mature ribosomal, transfer, and micro RNAs. When combined with inhibition of the nuclear 5′-3′ exonuclease Xrn2, 5′-PO₄ Bru-Seq maps the 5′ splice sites of debranched introns and mRNA and tRNA 3′ end processing sites cleaved by CPSF73 and RNaseZ, respectively.

Key features

• Metabolic labeling for brief periods with bromouridine focuses the analysis of 5′-PO₄ RNA ends on the population of nascent transcripts that are actively transcribed.

• Detects 5′-PO₄ RNA ends on nascent transcripts produced by all RNA polymerases.

• Detects 5′-PO₄ RNA ends at single nucleotide resolution.

Matriglycan is a linear polysaccharide of alternating xylose and glucuronic acid units [-Xyl-α1,3-GlcA-β1,3]_n that is uniquely synthesized on α-dystroglycan (α-DG) and is essential for neuromuscular function and brain development. It binds several extracellular matrix proteins that contain laminin-globular domains and is a receptor for Old World arenaviruses such as Lassa Fever virus. Monoclonal antibodies such as IIH6 are commonly used to detect matriglycan on α-DG. However, endogenous expression levels are not sufficient to detect and analyze matriglycan by mass spectrometry approaches. Thus, there is a growing need to independently confirm the presence of matriglycan on α-DG and possibly other proteins. We used an enzymatic approach to detect matriglycan, which involved digesting it with two thermophilic exoglycosidases: β-Glucuronidase from Thermotoga maritima and α-xylosidase from Sulfolobus solfataricus. This allowed us to identify and categorize matriglycan on α-DG by studying post-digestion changes in the molecular weight of α-DG using SDS-PAGE followed by western blotting with anti-matriglycan antibodies, anti-core α-DG antibodies, and/or laminin binding assay. In some tissues, matriglycan is capped by a sulfate group, which renders it resistant to digestion by these dual exoglycosidases. Thus, this method can be used to determine the capping status of matriglycan. To date, matriglycan has only been identified on vertebrate α-DG. We anticipate that this method will facilitate the discovery of matriglycan on α-DG in other species and possibly on other proteins.

Key features

• Analysis of endogenous matriglycan on dystroglycan from any animal tissue.

• Matriglycan is digested using thermophilic enzymes, which require optimum thermophilic conditions.

• Western blotting is used to assay the success and extent of digestion.

• Freshly purified enzymes work best to digest matriglycan.

Graphical overview

α-Dystroglycan (α-DG) from muscle is shown here modified by a phosphorylated core M3 glycan, which extends further and terminates in a repeating disaccharide of xylose (Xyl) and glucuronic acid (GlcA) called matriglycan. β-glucuronidase (Bgus) and α-xylosidase (Xyls) hydrolyze the β-1,3-linked GlcA and α-1,3 linked-Xyl, starting from the terminal residues.

Engineered aptamers for new compounds are typically produced by using in vitro selection methods. However, aptamers that are developed in vitro might not function as expected when introduced into complex cellular environments. One approach that addresses this concern is the design of initial RNA pools for selection that contain structural scaffolds from naturally occurring riboswitch aptamers. Here, we provide guidance on design and experimental principles for developing riboswitch-inspired aptamers for new ligands. The in vitro selection protocol (based on Capture-SELEX) is generalizable to diverse RNA scaffold types and amenable to multiplexing of ligand candidates. We discuss strategies to avoid propagation of selfish sequences that can easily dominate the selection. We also detail the identification of aptamer candidates using next-generation sequencing and bioinformatics, and subsequent biochemical validation of aptamer candidates. Finally, we describe functional testing of aptamer candidates in bacterial cell culture.

Key features

• Develop riboswitch-inspired aptamers for new ligands using in vitro selection.

• Ligand candidates can be multiplexed to conserve time and resources.

• Test aptamer candidates in bacterial cells by grafting the aptamer back onto its expression platform.

Graphical overview

This protocol describes the generation of chimeric mice in which the Y chromosome is deleted from a proportion of blood cells. This model recapitulates the phenomenon of hematopoietic mosaic loss of Y chromosome (mLOY), which is frequently observed in the blood of aged men. To construct mice with hematopoietic Y chromosome loss, lineage-negative cells are isolated from the bone marrow of ROSA26-Cas9 knock-in mice. These cells are transduced with a lentivirus vector encoding a guide RNA (gRNA) that targets multiple repeats of the Y chromosome centromere, effectively removing the Y chromosome. These cells are then transplanted into lethally irradiated wildtype C57BL6 mice. Control gRNAs are designed to target either no specific region or the fourth intron of Actin gene. Transduced cells are tracked by measuring the fraction of blood cells expressing the virally encoded reporter gene tRFP. This model represents a clinically relevant model of hematopoietic mosaic loss of Y chromosome, which can be used to study the impact of mLOY on various age-related diseases.

Graphical overview

Integral membrane proteins are an important class of cellular proteins. These take part in key cellular processes such as signaling transducing receptors to transporters, many operating within the plasma membrane. More than half of the FDA-approved protein-targeting drugs operate via interaction with proteins that contain at least one membrane-spanning region, yet the characterization and study of their native interactions with therapeutic agents remains a significant challenge. This challenge is due in part to such proteins often being present in small quantities within a cell. Effective solubilization of membrane proteins is also problematic, with the detergents typically employed in solubilizing membranes leading to a loss of functional activity and key interacting partners. In recent years, alternative methods to extract membrane proteins within their native lipid environment have been investigated, with the aim of producing functional nanodiscs, maintaining protein–protein and protein–lipid interactions. A promising approach involves extracting membrane proteins in the form of styrene maleic acid lipid particles (SMALPs) that allow the retention of their native conformation. This extraction method offers many advantages for further protein analysis and allows the study of the protein interactions with other molecules, such as drugs. Here, we describe a protocol for efficient SMALP extraction of functionally active membrane protein complexes within nanodiscs. We showcase the method on the isolation of a low copy number plasma membrane receptor complex, the nicotinic acetylcholine receptor (nAChR), from adult Drosophila melanogaster heads. We demonstrate that these nanodiscs can be used to study native receptor–ligand interactions. This protocol can be applied across many biological scenarios to extract the native conformations of low copy number integral membrane proteins.

Non-coding RNAs (ncRNAs) are defined as RNAs that do not encode proteins, but many ncRNAs do have the ability to regulate gene expression. These ncRNAs play a critical role in the epigenetic regulation of various physiological and pathological processes through diverse biochemical mechanisms. However, the existing screening methods to identify regulatory ncRNAs only focus on whole-cell expression levels and do not capture every ncRNA that targets certain genes. We describe a new method, chromatin-RNA in situ reverse transcription sequencing (CRIST-seq), that can identify all the ncRNAs that are associated with the regulation of any given gene. In this article, we targeted the ncRNAs that are associated with pluripotent gene Sox2, allowing us to catalog the ncRNA regulation network of pluripotency maintenance. This methodology is universally applicable for the study of epigenetic regulation of any genes by making simple changes on the CRISPR-dCas9 gRNAs.

Key features

• This method provides a new technique for screening ncRNAs and establishing chromatin interaction networks.

• The target gene for this method can be any gene of interest and any site in the entire genome.

• This method can be further extended to detect RNAs, DNAs, and proteins that interact with target genes.

Graphical overview

Epigenetic modifications of DNA, and especially cytosine, play a crucial role in regulating basic cellular processes and thereby the overall cellular metabolism. Their levels change during organismic and cellular development, but especially also in pathogenic aberrations such as cancer. Levels of respective modifications are often addressed in bulk by specialized mass spectrometry techniques or by employing dedicated ChIP-seq protocols, with the latter giving information about the sequence context of the modification. However, to address modification levels on a single cell basis, high- or low-content microscopy techniques remain the preferred methodology. The protocol presented here describes a straightforward method to detect and quantify different DNA modifications in human cell lines, which can also be adapted to other cultured mammalian cell types. To this end, cells are immunostained against two different cytosine modifications in combination with DNA counterstaining. Image acquisition takes place on a confocal microscopy system. A semi-automated analysis pipeline helps to gather data in a fast and reliable fashion. The protocol is comparatively simple, fast, and cost effective. By employing methodologies that are often well established in most molecular biology laboratories, many researchers are able to apply the described protocol straight away in-house.

In vitro translation systems are a useful biochemical tool to research translational regulation. Although the preparation of translation-competent cell extracts from mammals has often been a challenge, the commercially available rabbit reticulocyte lysate (RRL) is an exception. However, its valid use, investigating the mechanism of translation machinery such as ribosomes in RRL, presents an analytic hurdle. To overcome this issue, the hybrid translation system, which is based on the supplementation of purified human ribosomes into ribosome-depleted RRL, has been developed. Here, we describe the step-by-step protocol of this system to study translation driven by ribosomes lacking post-translational modifications of the ribosomal protein. Moreover, we combined this approach with a previously developed reporter mRNA to assess the processivity of translation elongation. This protocol could be used to study the potency of heterologous ribosomes.