Single-cell transcriptomics reveal cellular diversity in the human first-trimester placenta and decidua.

Human LIN28A and LIN28B are RNA-binding proteins (RBPs) conserved in animals with important roles during development and stem cell reprogramming. We used Photoactivatable-Ribonucleoside-Enhanced Crosslinking and Immunoprecipitation (PAR-CLIP) in HEK293 cells and identified a largely overlapping set of ∼3000 mRNAs at ∼9500 sites located in the 3 ′ UTR and CDS. In vitro and in vivo, LIN28 preferentially bound single-stranded RNA containing a uridine-rich element and one or more flanking guanosines and appeared to be able to disrupt base-pairing to access these elements when embedded in predicted secondary structure. In HEK293 cells, LIN28 protein binding mildly stabilized target mRNAs and increased protein abundance. The top targets were its own mRNAs and those of other RBPs and cell cycle regulators. Alteration of LIN28 protein levels also negatively regulated the abundance of some but not all let-7 miRNA family members, indicating sequence-specific binding of let-7 precursors to LIN28 proteins and competition with cytoplasmic miRNA biogenesis factors.

SignificanceNucleic acids mediate storage and expression of genetic information. Extracellular DNA (exDNA) and exRNA are traces of nucleic acids released from cells into the extracellular environment. Their use as disease biomarkers has been limited by technical challenges in their isolation caused by abundant RNA- and DNA-degrading enzymes in biofluids. Using isolation protocols developed especially for biofluids, we generated plasma and serum exRNA reference profiles from 13 healthy volunteers over time and determined the effect of critical clinical parameters such as gender and fasting. Surprisingly, we encountered one participant with dramatically increased endocrine-origin exRNA contributions stable over 1 year and detectable in all of his samples, thereby demonstrating the robustness of this approach and the clinical potential of circulating RNAs as biomarkers.