The ability to quantitate and visualize microRNAs (miRNAs) in situ in single cells would greatly facilitate the elucidation of miRNA-mediated regulatory circuits and their disease associations. A toehold-initiated strand-displacement process was used to initiate rolling circle amplification of specific miRNAs, an approach that achieves both stringent recognition and in situ amplification of the target miRNA. This assay, termed toehold-initiated rolling circle amplification (TIRCA), can be utilized to identify miRNAs at physiological temperature with high specificity and to visualize individual miRNAs in situ in single cells within 3 h. TIRCA is a competitive candidate technique for in situ miRNA imaging and may help us to understand the role of miRNAs in cellular processes and human diseases in more detail.

MicroRNAs (miRNAs) are a class of small noncoding RNAs that act as pivotal post-transcriptional regulators of gene expression, thus involving in many fundamental cellular processes such as cell proliferation, migration, and canceration. The detection of miRNAs has attracted significant interest, as abnormal miRNA expression is identified to contribute to serious human diseases such as cancers. Particularly, miRNAs in peripheral blood have recently been recognized as important biomarkers potential for liquid biopsy. Furthermore, as miRNAs are expressed heterogeneously in different cells, investigations into single-cell miRNA expression will be of great value for resolving miRNA-mediated regulatory circuits and the complexity and heterogeneity of miRNA-related diseases. Thus, the development of miRNA detection methods, especially for complex clinic samples and single cells is in great demand. In this Account, we will present recent progress in the design and application of isothermal amplification enabling miRNA detection transition from the test tube to the clinical sample and single cell, which will significantly advance our knowledge of miRNA functions and disease associations, as well as its translation in clinical diagnostics. miRNAs present a huge challenge in detection because of their extremely short length (∼22 nucleotides) and sequence homology (even with only single-nucleotide variation). The conventional golden method for nucleic acid detection, quantitative PCR (qPCR), is not amenable to directly detecting short RNAs and hardly enables distinguishing between miRNA family members with very similar sequences. Alternatively, isothermal amplification has emerged as a powerful method for quantification of nucleic acids and attracts broad interest for utilization in developing miRNA assays. Compared to PCR, isothermal amplification can be performed without precise control of temperature cycling and is well fit for detecting short RNA or DNA. We and other groups are seeking methods based on isothermal amplification for detecting miRNA with high specificity (single-nucleotide resolution) and sensitivity (detection limit reaching femtomolar or even attomolar level). These methods have recently been demonstrated to quantify miRNA in clinical samples (tissues, serum, and plasma). Remarkably, attributed to the mild reaction conditions, isothermal amplification can be performed inside cells, which has recently enabled miRNA detection in single cells. The localized in situ amplification even enables imaging of miRNA at the single-molecule level. The single-cell miRNA profiling data clearly shows that genetically identical cells exhibit significant cell-to-cell variation in miRNA expression. The leap of miRNA detection achievements will significantly contribute to its full clinical adoption and translation and give us new insights into miRNA cellular functions and disease associations.

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Purpose: Although many biological processes are involved in the modification of N 6 -methyladenosine (m 6 A), the exact role of m 6 A in the development of malignant tumors remains unclear. Methyltransferase 3 (METTL3) is a major RNA N 6 -methyladenosine methyltransferase. We aimed to explore the role of METTL3 in colorectal cancer (CRC) carcinogenesis and disease progression. Methods: In this study, immunohistochemistry was performed with a tissue microarray. qRT-PCR and Western blots were used to evaluate the expression of METTL3 in CRC cells. The effect of METTL3 on cell proliferation, migration and invasion of CRC cells was examined by IncuCyte Live Cell Analysis System and transwell assay, respectively. Results: The results suggested that positive expression of METTL3 was significantly associated with longer survival time ( P =0.011). We next demonstrated that overexpression of METTL3 could inhibit proliferation, migration and invasion in CRC cells, while downregulation of METTL3 shows the opposite result. Furthermore, downregulation of METTL3 resulted in activation of p-p38 and p-ERK. Moreover, the inhibitors of p38 or ERK kinase could significantly reverse the effect of migration and invasion, which was induced by knockdown of METTL3. Conclusion: We concluded that METTL3 played a tumor-suppressive role in CRC cell proliferation, migration and invasion through p38/ERK pathways, which indicated that METTL3 might be a novel marker for CRC carcinogenesis, progression and survival.

The expression and spatial profile of multiple RNA species at high precision in single cells is key information for understanding cellular behaviors and functions. Fluorescence microscopy is a powerful tool, but its multiplexing ability is limited by the number of spectrally distinct fluorophores. Here, a DNA-sequence-encoded fluorescence barcoding method based on thermodynamically tuning DNA hybridization was applied for fluorescence tagging RNAs, which allows for highly multiplexed imaging of RNA in single cells with single-molecule and singlenucleotide resolution.