Molecular Biology

Many single nucleotide polymorphisms (SNPs) identified by genome-wide association studies exert their effects on disease risk as expression quantitative trait loci (eQTL) via allele-specific expression (ASE). While databases for probing eQTLs in tissues from normal individuals exist, one may wish to ascertain eQTLs or ASE in specific tissues or disease-states not characterized in these databases. Here, we present a protocol to assess ASE of two possible target genes (GPNMB and KLHL7) of a known genome-wide association study (GWAS) Parkinson’s disease (PD) risk locus in postmortem human brain tissue from PD and neurologically normal individuals. This was done using a sequence of RNA isolation, cDNA library generation, enrichment for transcripts of interest using customizable cDNA capture probes, paired-end RNA sequencing, and subsequent analysis. This method provides increased sensitivity relative to traditional bulk RNAseq-based and a blueprint that can be extended to the study of other genes, tissues, and disease states.

Key features

• Analysis of GPNMB allele-specific expression (ASE) in brain lysates from cognitively normal controls (NC) and Parkinson’s disease (PD) individuals.

• Builds on the ASE protocol of Mayba et al. (2014) and extends application from cells to human tissue.

• Increased sensitivity by enrichment for desired transcript via RNA CaptureSeq (Mercer et al., 2014).

• Optimized for human brain lysates from cingulate gyrus, caudate nucleus, and cerebellum.

Graphical overview

Information on RNA localisation is essential for understanding physiological and pathological processes, such as gene expression, cell reprogramming, host–pathogen interactions, and signalling pathways involving RNA transactions at the level of membrane-less or membrane-bounded organelles and extracellular vesicles. In many cases, it is important to assess the topology of RNA localisation, i.e., to distinguish the transcripts encapsulated within an organelle of interest from those merely attached to its surface. This allows establishing which RNAs can, in principle, engage in local molecular interactions and which are prevented from interacting by membranes or other physical barriers. The most widely used techniques interrogating RNA localisation topology are based on the treatment of isolated organelles with RNases with subsequent identification of the surviving transcripts by northern blotting, qRT-PCR, or RNA-seq. However, this approach produces incoherent results and many false positives. Here, we describe Controlled Level of Contamination coupled to deep sequencing (CoLoC-seq), a more refined subcellular transcriptomics approach that overcomes these pitfalls. CoLoC-seq starts by the purification of organelles of interest. They are then either left intact or lysed and subjected to a gradient of RNase concentrations to produce unique RNA degradation dynamics profiles, which can be monitored by northern blotting or RNA-seq. Through straightforward mathematical modelling, CoLoC-seq distinguishes true membrane-enveloped transcripts from degradable and non-degradable contaminants of any abundance. The method has been implemented in the mitochondria of HEK293 cells, where it outperformed alternative subcellular transcriptomics approaches. It is applicable to other membrane-bounded organelles, e.g., plastids, single-membrane organelles of the vesicular system, extracellular vesicles, or viral particles.

Key features

• Tested on human mitochondria; potentially applicable to cell cultures, non-model organisms, extracellular vesicles, enveloped viruses, tissues; does not require genetic manipulations or highly pure organelles.

• In the case of human cells, the required amount of starting material is ~2,500 cm² of 80% confluent cells (or ~3 × 10⁸ HEK293 cells).

• CoLoC-seq implements a special RNA-seq strategy to selectively capture intact transcripts, which requires RNases generating 5′-hydroxyl and 2′/3′-phosphate termini (e.g., RNase A, RNase I).

• Relies on nonlinear regression software with customisable exponential functions.

Graphical overview

Inflammation of the gastrointestinal tract is a prevalent pathology in diseases such as inflammatory bowel disease (IBD). Currently, there are no therapies to prevent IBD, and available therapies to treat IBD are often sub-optimal. Thus, an unmet need exists to better understand the molecular mechanisms underlying intestinal tissue responses to damage and regeneration. The recent development of single-cell RNA (sc-RNA) sequencing-based techniques offers a unique opportunity to shed light on novel signaling pathways and cellular states that govern tissue adaptation or maladaptation across a broad spectrum of diseases. These approaches require the isolation of high-quality cells from tissues for downstream transcriptomic analyses. In the context of intestinal biology, there is a lack of protocols that ensure the isolation of epithelial and non-epithelial compartments simultaneously with high-quality yield. Here, we report two protocols for the isolation of epithelial and stromal cells from mouse and human colon tissues under inflammatory conditions. Specifically, we tested the feasibility of the protocols in a mouse model of dextran sodium sulfate (DSS)-induced colitis and in human biopsies from Crohn’s patients. We performed sc-RNA sequencing analysis and demonstrated that the protocol preserves most of the epithelial and stromal cell types found in the colon. Moreover, the protocol is suitable for immunofluorescence staining of surface markers for epithelial, stromal, and immune cell lineages for flow cytometry analyses. This optimized protocol will provide a new resource for scientists to study complex tissues such as the colon in the context of tissue damage and regeneration.

Key features

• This protocol allows the isolation of epithelial and stromal cells from colon tissues.

• The protocol has been optimized for tissues under inflammatory conditions with compromised cell viability.

• This protocol is suitable for experimental mouse models of colon inflammation and human biopsies.

Graphical overview

Graphical representation of the main steps for the processing of colon tissue from dextran sodium sulfate (DSS)-treated mice (upper panel) and frozen biopsies from Crohn’s patients (lower panel)

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.

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

Polysome profiling is widely used to isolate and analyze polysome fractions, which consist of actively translating mRNAs and ribosomes. Compared to ribosome profiling and translating ribosome affinity purification, polysome profiling is simpler and less time consuming in sample preparation and library constructions. Spermiogenesis, i.e., the post-meiotic phase of male germ cell development, is a highly coordinated developmental process in which transcription and translation are decoupled because of nuclear condensation, resulting in translation regulation as the major mode for the regulation of gene expression in post-meiotic spermatids. To understand the translation regulation during spermiogenesis, an overview of translational state of spermiogenic mRNAs is required. Here, we describe a protocol to identify translating mRNAs using polysome profiling. Briefly, mouse testes are gently homogenized to release polysomes containing translating mRNAs, following polysome-bound mRNAs isolated by sucrose density gradient purification and characterized by RNA-seq. This protocol allows to quickly isolate translating mRNAs from testes and analyze the discrepancy of translational efficiency in mouse testes from different mouse lines.

Key features

• Quickly obtain polysome RNAs from testes.

• Omit RNase digestion and RNA recovery from gel.

• High efficiency and robustness compared to ribo-seq.

Graphical overview

Schematic illustrating the experimental design for polysome profiling in mouse testes. Mouse testes are homogenized and lysed in Sample preparation, and polysome RNAs are enriched by sucrose gradient centrifugation and used to calculate translation efficiency in Sample analysis.

Single-nucleus RNA sequencing (snRNA-seq) provides a powerful tool for studying cell type composition in heterogenous tissues. The liver is a vital organ composed of a diverse set of cell types; thus, single-cell technologies could greatly facilitate the deconvolution of liver tissue composition and various downstream omics analyses at the cell-type level. Applying single-cell technologies to fresh liver biopsies can, however, be very challenging, and snRNA-seq of snap-frozen liver biopsies requires some optimization given the high nucleic acid content of the solid liver tissue. Therefore, an optimized protocol for snRNA-seq specifically targeted for the use of frozen liver samples is needed to improve our understanding of human liver gene expression at the cell-type resolution. We present a protocol for performing nuclei isolation from snap-frozen liver tissues, as well as guidance on the application of snRNA-seq. We also provide guidance on optimizing the protocol to different tissue and sample types.

Long noncoding RNAs (lncRNAs) play essential roles in normal physiology and in disease but their mechanisms of action can be challenging to identify. For mechanistic studies, it is often useful to know a lncRNA’s intracellular abundance, i.e., approximately how many molecules of the lncRNA are present in a typical cell of a cell-type of interest. At least two approaches have been used to approximate lncRNA intracellular abundance: single-molecule sensitivity RNA fluorescence in situ hybridization (smFISH) and single-gene, calibrated reverse-transcription followed by quantitative PCR (RT-qPCR). However, like all experimental approaches, these methods have their limitations. smFISH, when analyzed using diffraction-limited microscopy, can underestimate intracellular abundance, especially for lncRNAs that accumulate in focused subcellular regions. Calibrated RT-qPCR may return inaccurate estimates of abundance because individual PCR amplicons spaced across the length of a transcript can vary in their efficiency of reverse transcription. Here, we describe a sequencing-based approach that is straightforward, orthogonal to smFISH and RT-qPCR, and can be used to approximate the intracellular abundance for most expressed long RNAs (lncRNAs and mRNAs) in a cell type of interest. Firstly, the average weight of total RNA per cell for the cell type of interest is estimated by replicate rounds of RNA purification from a known number of cells. Secondly, an rRNA-depletion RNA-Seq protocol is performed after adding spike-in control RNAs to a known quantity of total cellular RNA. Lastly, by comparing read counts per transcript to a standard curve derived from the spiked-in RNAs, the intracellular abundance for each transcript is estimated. The sequencing-based approach provides a powerful complement to existing methods, particularly in situations where it is desirable to quantify the abundance of multiple lncRNAs and/or mRNAs simultaneously.

RNA-Seq is a powerful method for transcriptome analysis used in varied field of biology. Although several commercial products and hand-made protocols enable us to prepare RNA-Seq library from total RNA, their cost are still expensive. Here, we established a low-cost and multiplexable whole mRNA-Seq library preparation method for illumine sequencers. In order to reduce cost, we used cost-effective and robust commercial regents with small reaction volumes. This method is a whole mRNA-Seq, which can be applied even to non-model organisms lacking the transcriptome references. In addition, we designed large number of 3′ PCR primer including 8 nucleotides barcode sequences for multiplexing up to three hundreds samples. To summarize, it is possible with this protocol to prepare 96 directional RNA-Seq libraries from purified total RNA in three days and can be pooled for up to three hundred libraries. This is beneficial for large scale transcriptome analysis in many fields of animals and plant biology.

Cell heterogeneity is high in tissues like lung. Research conducted on pure population of cells usually offers more insights than bulk tissues, such as circadian clock work. In this protocol, we provide a detailed work flow on how to do circadian clock study by RNA seq in laser capture micro-dissected mouse lung club cells. The method uses frozen tissues and is highly reproduciable.