RESUMEN
Long non-coding RNAs (lncRNAs) constitute the largest and most diverse class of non-coding RNAs. They localize to the nucleus, cytoplasm, or both compartments, and regulate gene expression through various mechanisms at multiple levels. LncRNAs tend to evolve faster and present higher tissue- and developmental stage-specific expression than protein-coding genes. Initially considered byproducts of erroneous transcription without biological function, lncRNAs are now recognized for their involvement in numerous biological processes, such as immune response, apoptosis, pluripotency, reprogramming, and differentiation. In this study, we focused on Heart Brake lncRNA 1 (HBL1), a lncRNA recently reported to modulate the process of pluripotent stem cell differentiation toward cardiomyocytes. We employed RT-qPCR and high-resolution RNA FISH to monitor the expression and localization of HBL1 during the differentiation progression. Our findings indicate a significant increase in HBL1 expression during mesodermal and cardiac mesodermal stages, preceding an anticipated decrease in differentiated cells. We detected the RNA in discrete foci in both the nucleus and in the cytoplasm. In the latter compartment, we observed colocalization of HBL1 with Y-box binding protein 1 (YB-1), which likely results from an interaction between the RNA and the protein, as the two were found to be coimmunoprecipitated in RNP-IP experiments. Finally, we provide evidence that HBL1, initially reported as an independent lncRNA gene, is part of the LINC00458 (also known as lncRNA-ES3 or ES3) gene, forming the last exon of some LINC00458 splice isoforms.