mRNA psi profiling using nanopore DRS reveals cell type-specific pseudouridylation

Pseudouridine (psi) is one of the most abundant human mRNA modifications generated from the isomerization of uridine via psi synthases, including TRUB1 and PUS7 . Nanopore direct RNA sequencing combined with our recent tool, Mod- p ID, enables psi mapping, transcriptome-wide, without chemical derivatization of the input RNA and/or conversion to cDNA. This method is highly efficient for detecting changes in positional psi occupancies across cell types, which can inform our understanding of the impact on gene expression. We sequenced, mapped, and compared the positional psi occupancy across six immortalized human cell lines derived from diverse tissue types. We found that lung-derived cells have the highest proportion of psi, while liver-derived cells have the lowest. Further, among a list of highly conserved sites across cell types, most are TRUB1 substrates and fall within the coding sequence. Interestingly, we identify cell type-specific sites of psi modification in ubiquitously expressed genes. We validate these sites by ruling out single-nucleotide variants, analyzing current traces, and performing enzymatic knockdowns of psi synthases. Finally, we characterize sites with multiple psi modifications on the same transcript (hypermodification type II) and found that these can be conserved or cell type-specific. Among these, we discovered examples of multiple psi modifications within the same k-mer for the first time and analyzed the effect on current distribution. Our data support the hypothesis that motif sequence and the presence of psi synthase are insufficient to drive modifications and that cell type-specific trans-acting factors play a major role in driving pseudouridylation.

Probing enzyme-dependent pseudouridylation using direct RNA sequencing to assess neuronal epitranscriptome plasticity

Chemical modifications in mRNAs such as pseudouridine (psi) can regulate gene expression, although our understanding of the functional impact of individual psi modifications, especially in neuronal cells, is limited. We apply nanopore direct RNA sequencing to investigate psi dynamics under cellular perturbations in SH-SY5Y cells. We assign sites to psi synthases using siRNA-based knockdown. A steady-state enzyme-substrate model reveals a strong correlation between psi synthase and mRNA substrate levels and psi modification frequencies. Next, we performed either differentiation or lead-exposure to SH-SY5Y cells and found that, upon lead exposure, not differentiation, the modification frequency is less dependent on enzyme levels suggesting translational control. Finally, we compared the plasticity of psi sites across cellular states and found that plastic sites can be condition-dependent or condition-independent; several of these sites fall within transcripts encoding proteins involved in neuronal processes. Our psi analysis and validation enable investigations into the dynamics and plasticity of RNA modifications.

Paired Capture and FISH Detection of Individual Virions Enable Cell-Free Determination of Infectious Titers

Early detection of viruses can prevent the uncontrolled spread of viral infections. Determination of viral infectivity is also critical for determining the dosage of gene therapies, including vector-based vaccines, CAR T-cell therapies, and CRISPR therapeutics. In both cases, for viral pathogens and viral vector delivery vehicles, fast and accurate measurement of infectious titers is desirable. The most common methods for virus detection are antigen-based (rapid but not sensitive) and polymerase chain reaction (PCR)-based (sensitive but not rapid). Current viral titration methods heavily rely on cultured cells, which introduces variability within labs and between labs. Thus, it is highly desirable to directly determine the infectious titer without using cells. Here, we report the development of a direct, fast, and sensitive assay for virus detection (dubbed rapid capture fluorescence in situ hybridization (FISH) or rapture FISH) and cell-free determination of infectious titers. Importantly, we demonstrate that the virions captured are "infectious," thus serving as a more consistent proxy of infectious titers. This assay is unique because it first captures viruses bearing an intact coat protein using an aptamer and then detects genomes directly in individual virions using fluorescence in situ hybridization (FISH); thus, it is selective for infectious particles (i.e., positive for coat proteins and positive for genomes).

Enzymatic synthesis of RNA standards for mapping and quantifying RNA modifications in sequencing analysis

Synthesis of RNA standards that contain an internal site-specific modification is important for mapping and quantification of the modified nucleotide in sequencing analysis. While RNA containing a site-specific modification can be readily synthesized by solid-state coupling for less than 100-mer nucleotides, longer RNA must be synthesized by enzymatic ligation in the presence of a DNA splint. However, long RNAs have structural heterogeneity, and those generated by in vitro transcription have 3'-end sequence heterogeneity, which together substantially reduce the yield of ligation. Here we describe a method of 3-part splint ligation that joins an in vitro transcribed left-arm RNA, an in vitro transcribed right-arm RNA, and a chemically synthesized modification-containing middle RNA, with an efficiency higher than previously reported. We report that the improved efficiency is largely attributed to the inclusion of a pair of DNA disruptors proximal to the ligation sites, and to a lesser extent to the homogeneous processing of the 3'-end of the left-arm RNA. The yields of the ligated long RNA are sufficiently high to afford purification to homogeneity for practical RNA research. We also verify the sequence accuracy at each ligation junction by nanopore sequencing.

Multicellular, IVT-derived, unmodified human transcriptome for nanopore direct RNA analysis

Nanopore direct RNA sequencing (DRS) enables measurements of native RNA modifications. Modification-free transcripts are an important control for DRS. Additionally, it is advantageous to have canonical transcripts from multiple cell lines to better account for human transcriptome variation. Here we generated and analyzed Nanopore DRS datasets for five human cell lines using in vitro transcribed (IVT) RNA. We compared performance statistics amongst biological replicates. We also documented nucleotide and ionic current level variation across cell lines. These data will serve as a resource to the community for RNA modification analysis.

Semi-quantitative detection of pseudouridine modifications and type I/II hypermodifications in human mRNAs using direct long-read sequencing

Here, we develop and apply a semi-quantitative method for the high-confidence identification of pseudouridylated sites on mammalian mRNAs via direct long-read nanopore sequencing. A comparative analysis of a modification-free transcriptome reveals that the depth of coverage and specific k-mer sequences are critical parameters for accurate basecalling. By adjusting these parameters for high-confidence U-to-C basecalling errors, we identify many known sites of pseudouridylation and uncover previously unreported uridine-modified sites, many of which fall in k-mers that are known targets of pseudouridine synthases. Identified sites are validated using 1000-mer synthetic RNA controls bearing a single pseudouridine in the center position, demonstrating systematic under-calling using our approach. We identify mRNAs with up to 7 unique modification sites. Our workflow allows direct detection of low-, medium-, and high-occupancy pseudouridine modifications on native RNA molecules from nanopore sequencing data and multiple modifications on the same strand.

Click chemistry-based amplification and detection of endogenous RNA and DNA molecules in situ using clampFISH probes

Direct labeling and measurement of gene expression in single cells show the tremendous variability otherwise hidden in bulk measurements. Single-molecule RNA fluorescence in situ hybridization (FISH) has become a mainstay in laboratories worldwide for measuring gene expression with precision. However, this method remains relatively low throughput because the total fluorescent signal produced is weak and requires long exposure times and high magnification microscopy, which limits the total number of cells sampled in each image. As such, it is experimentally difficult and time-consuming to sample a large enough population of cells to visualize and quantify specific gene expression of rare cells directly. Several FISH-based tools were recently developed that retain single-molecule sensitivity and specificity while greatly amplifying the fluorescent signal, thus making FISH-based analysis possible using standard microscopes with low magnification objectives. These tools have also enabled the detection of smaller and more specific targets like splice junctions or single nucleotide polymorphisms. Here we will describe one such tool, clampFISH, an oligonucleotide-based fluorescence amplification strategy for visualizing genomic loci and individual RNA transcripts in fixed cells. ClampFISH maintains specificity while amplifying fluorescent signals, making it amenable to high throughput assays such as low magnification microscopy, spatial transcriptomics, and flow sorting. The clampFISH technique involves probing the target RNA or DNA using a series of C-shaped oligonucleotide probes, each with a 3' azide and a 5' alkyne. Hybridization of the probe with the target nucleic acid brings the azide and the alkyne in close proximity, allowing for ligation via bioorthogonal click chemistry (CuAAC). As a result, the probe forms a closed loop around the target sequence, thus enabling stringent washes to remove nonspecific binding in further rounds of amplification and retention of signal throughout liquid handling steps. Iterative rounds of hybridization with C-shaped, fluorescently labeled probes exponentially amplify the fluorescent signal. ClampFISH is simple to implement and expands the utility of in situ hybridization for multiple high throughput techniques such as low magnification microscopy, flow cytometry, and sorting based on RNA expression levels.

ClampFISH detects individual nucleic acid molecules using click chemistry-based amplification

Methods for detecting single nucleic acids in cell and tissues, such as fluorescence in situ hybridization (FISH), are limited by relatively low signal intensity and nonspecific probe binding. Here we present click-amplifying FISH (clampFISH), a method for fluorescence detection of nucleic acids that achieves high specificity and high-gain (>400-fold) signal amplification. ClampFISH probes form a 'C' configuration upon hybridization to the sequence of interest in a double helical manner. The ends of the probes are ligated together using bio-orthogonal click chemistry, effectively locking the probes around the target. Iterative rounds of hybridization and click amplify the fluorescence intensity. We show that clampFISH enables the detection of RNA species with low-magnification microscopy and in RNA-based flow cytometry. Additionally, we show that the modular design of clampFISH probes allows multiplexing of RNA and DNA detection, that the locking mechanism prevents probe detachment in expansion microscopy, and that clampFISH can be applied in tissue samples.

Engineered Glycocalyx Regulates Stem Cell Proliferation in Murine Crypt Organoids

At the base of the intestinal crypt, long-lived Lgr5⁺ stem cells are intercalated by Paneth cells that provide essential niche signals for stem cell maintenance. This unique epithelial anatomy makes the intestinal crypt one of the most accessible models for the study of adult stem cell biology. The glycosylation patterns of this compartment are poorly characterized, and the impact of glycans on stem cell differentiation remains largely unexplored. We find that Paneth cells, but not Lgr5⁺ stem cells, express abundant terminal N-acetyllactosamine (LacNAc). Employing an enzymatic method to edit glycans in cultured crypt organoids, we assess the functional role of LacNAc in the intestinal crypt. We discover that blocking access to LacNAc on Paneth cells leads to hyperproliferation of the neighboring Lgr5⁺ stem cells, which is accompanied by the downregulation of genes that are known as negative regulators of proliferation.

Neutrophils and Ly6Chi monocytes collaborate in generating an optimal cytokine response that protects against pulmonary Legionella pneumophila infection

Early responses mounted by both tissue-resident and recruited innate immune cells are essential for host defense against bacterial pathogens. In particular, both neutrophils and Ly6Chi monocytes are rapidly recruited to sites of infection. While neutrophils and monocytes produce bactericidal molecules, such as reactive nitrogen and oxygen species, both cell types are also capable of synthesizing overlapping sets of cytokines important for host defense. Whether neutrophils and monocytes perform redundant or non-redundant functions in the generation of anti-microbial cytokine responses remains elusive. Here, we sought to define the contributions of neutrophils and Ly6Chi monocytes to cytokine production and host defense during pulmonary infection with Legionella pneumophila, responsible for the severe pneumonia Legionnaires' disease. We found that both neutrophils and monocytes are critical for host defense against L. pneumophila. Both monocytes and neutrophils contribute to maximal IL-12 and IFNγ responses, and monocytes are also required for TNF production. Moreover, natural killer (NK) cells, NKT cells, and γδ T cells are sources of IFNγ, and monocytes direct IFNγ production by these cell types. Thus, neutrophils and monocytes cooperate in eliciting an optimal cytokine response that promotes effective control of bacterial infection.

CHoMP: a chemoenzymatic histology method using clickable probes

The characterization of aberrant glycosylation patterns in biopsied patient samples represents a remarkable challenge for scientists and medical doctors due to the lack of specific methods for detection. Here, we report the development of a histological method, dubbed CHoMP-chemoenzymatic histology of membrane polysaccharides-for analyzing glycosylation patterns in mammalian tissues. This method exploits a recombinant glycosyltransferase to transfer a monosaccharide analogue equipped with a chemical handle to a specific cell-surface glycan target, which can then be derivatized with imaging probes by using bioorthogonal click chemistry for visualization. We applied CHoMP to survey changes in expression of N-acetyllactosamine (LacNAc) in human samples from patients afflicted with lung adenocarcinoma and observed a sharp decrease in expression levels between normal and early grade tumors, thus suggesting a potential application of this technique in early cancer diagnosis.

Chemical probing of glycans in cells and organisms

Among the four major building blocks of life, glycans play essential roles in numerous physiological and pathological processes. Due to their non-templated biosynthesis, advances towards elucidating the molecular details of glycan functions are relatively slow compared with the pace of protein and nucleic acid research. Over the past 30 years, chemical tools have emerged as powerful allies to genetics and molecular biology in the study of glycans in their native environment. This tutorial review will provide an overview of the recent technological developments in the field, as well as the progress in the application of these techniques to probe glycans in cells and organisms.

Detection and isolation of dendritic cells using Lewis X-functionalized magnetic nanoparticles

dendritic cell (DC)-specific intracellular adhesion molecule-3 grabbing nonintegrin (DC-SIGN) is a receptor found on DCs that recognizes antigens bearing mannose-rich or fucosylated glycans, including Lewis X (Le(X)). Here, we report the fabrication of magnetic nanoparticles coated with multivalent Le(X) glycans using Cu (I)-catalyzed azide-alkyne cycloaddition. The resulting nanoparticles are selective and biocompatible, serving as a highly efficient tool for DC detection and enrichment.

Click Triazoles for Bioconjugation

Click Chemistry is a set of rapid, selective and robust reactions that give near-quantitative yield of the desired product in aqueous solutions. The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) that forms 1,4-disubstituted triazoles is a prototypical example of click chemistry that features exquisite selectivity and bioorthogonality-that is, non-interacting with biological components while proceeding under physiological conditions. Over the past ten years, CuAAC has found extensive applications in the field of chemical biology. In this chapter, we describe the discovery of Cu(I) catalysts for this transformation and the recent development of the strain-promoted azide-alkyne cycloaddition that eliminate the use of copper. We also highlight several recent applications toward conjugating biomolecules, including proteins, nucleic acids, lipids and glycans, with biophysical probes for both in vitro and in vivo studies.

The PTEN-regulating microRNA miR-26a is amplified in high-grade glioma and facilitates gliomagenesis in vivo

Activated oncogenic signaling is central to the development of nearly all forms of cancer, including the most common class of primary brain tumor, glioma. Research over the last two decades has revealed the particular importance of the Akt pathway, and its molecular antagonist PTEN (phosphatase and tensin homolog), in the process of gliomagenesis. Recent studies have also demonstrated that microRNAs (miRNAs) may be responsible for the modulation of cancer-implicated genes in tumors. Here we report the identification miR-26a as a direct regulator of PTEN expression. We also show that miR-26a is frequently amplified at the DNA level in human glioma, most often in association with monoallelic PTEN loss. Finally, we demonstrate that miR-26a-mediated PTEN repression in a murine glioma model both enhances de novo tumor formation and precludes loss of heterozygosity and the PTEN locus. Our results document a new epigenetic mechanism for PTEN regulation in glioma and further highlight dysregulation of Akt signaling as crucial to the development of these tumors.

miRNA in situ hybridization in formaldehyde and EDC-fixed tissues

MicroRNAs (miRNAs) are small regulatory RNAs with many biological functions and disease associations. We showed that in situ hybridization (ISH) using conventional formaldehyde fixation results in significant miRNA loss from mouse tissue sections, which can be prevented by fixation with 1–ethyl–3–(3–dimethylaminopropyl) carbodiimide (EDC) that irreversibly immobilizes the miRNA at its 5' phosphate. We determined optimal hybridization parameters for 130 locked nucleic acid (LNA) probes by recording nucleic acid melting temperature during ISH.

MicroRNA-21 contributes to myocardial disease by stimulating MAP kinase signalling in fibroblasts

MicroRNAs comprise a broad class of small non-coding RNAs that control expression of complementary target messenger RNAs. Dysregulation of microRNAs by several mechanisms has been described in various disease states including cardiac disease. Whereas previous studies of cardiac disease have focused on microRNAs that are primarily expressed in cardiomyocytes, the role of microRNAs expressed in other cell types of the heart is unclear. Here we show that microRNA-21 (miR-21, also known as Mirn21) regulates the ERK-MAP kinase signalling pathway in cardiac fibroblasts, which has impacts on global cardiac structure and function. miR-21 levels are increased selectively in fibroblasts of the failing heart, augmenting ERK-MAP kinase activity through inhibition of sprouty homologue 1 (Spry1). This mechanism regulates fibroblast survival and growth factor secretion, apparently controlling the extent of interstitial fibrosis and cardiac hypertrophy. In vivo silencing of miR-21 by a specific antagomir in a mouse pressure-overload-induced disease model reduces cardiac ERK-MAP kinase activity, inhibits interstitial fibrosis and attenuates cardiac dysfunction. These findings reveal that microRNAs can contribute to myocardial disease by an effect in cardiac fibroblasts. Our results validate miR-21 as a disease target in heart failure and establish the therapeutic efficacy of microRNA therapeutic intervention in a cardiovascular disease setting.