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micro-RNAs (miRNAs) are short (18–25 nucleotides), single-stranded, noncoding RNAs that function as negative regulators of gene expression at the post-transcriptional level by inhibiting messenger RNA (mRNA) translation and/or promoting mRNA degradation1. miRNAs are transcribed from introns or exons of coding or noncoding genes and are cleaved in the nucleus by DROSHA, to precursor miRNAs (pre-miRNAs), which are short stem-loop structures of 70 nucleotides2. Following cytoplasmic export, pre-miRNAs are further processed by DICER into mature miRNAs that span 18–25 nucleotides3,4. Subsequently, the RNA-induced silencing complex (RISC) incorporates these miRNAs as single-stranded RNAs, which allows for their binding to the 3' untranslated region (3'-UTR) of their target mRNAs to suppress their expression3,5.
Within the last three decades, since they were first identified, miRNAs have emerged to master regulators of gene expression, whose own expression levels are tightly controlled6. Roles for miRNAs have been described in organ development7,8,9,10,11,12, maintenance of homeostasis13,14, as well as disease contexts that include neurological15,16,17,18,19, cardiovascular20, autoimmune conditions21,22, cancers23,24, and others25. The increasing appreciation for the relevance of miRNA expression patterns has brought forward the need for reliable detection methods of miRNA transcripts. Such methods include Real Time PCR, microarrays, Northern Blotting, in situ hybridization and others, which vary in the sensitivity, specificity, and quantitative power, predominantly due to the fact that miRNA transcripts are comprised of short and highly homologous sequences6.
We recently reported an important role for miRNA-182 in the development of the myocardial hypertrophy26, a condition describing the structural adaptation of the heart in response to elevated hemodynamic demands27,28. Cardiac hypertrophy is characterized by the increase in the myocardial mass, which, if associated with maladaptive remodeling29, can lead to increased risk for heart failure, a condition accounting for 8.5% of all deaths attributable to cardiovascular disease30.
Here, we describe our protocol that combines in situ hybridization with a digoxigenin-labeled (DIG) Locked Nucleic Acid (LNA) probe and immunostaining for the concurrent detection of miRNA and protein molecules on mouse heart tissue sections, in our model of cardiac hypertrophy.