• Extracellular vesicles and microRNA comprise the blastocyst secretome • Vesicles are elevated for blastocysts with diminished developmental competence • Vesicles may be rapidly detected by nanoparticle tracking analysis and flow cytometry • Ailing blastocysts release greater variety and amounts of microRNA • Studies have yet to show clear correlation between microRNA and embryo competence RBMO VOLUME 00 ISSUE 0 2020

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Extracellular microRNA (miRNA) sequences derived from the pre-implantation embryo have attracted interest for their possible contributions to the ongoing embryonic–uterine milieu, as well as their potential for use as accessible biomarkers indicative of embryonic health. Spent culture media microdroplets used to culture late-stage E4.0 murine blastocysts were screened for 641 mature miRNA sequences using a reverse transcription–quantitative polymerase chain reaction–based array. We report here 39 miRNAs exclusively detected in the conditioned media, including the implantation-relevant miR-126a-3p, miR-101a, miR-143, and miR-320, in addition to members of the highly expressed embryonic miR-125 and miR-290 families. Based on these results, an miRNA panel was assembled comprising five members of the miR-290 family (miR-291-295) and five conserved sequences with significance to the embryonic secretome (miR-20a, miR-30c, miR-142-3p, miR-191, and miR-320). Panel profiling of developing embryo cohort lysates and accompanying conditioned media microdroplets revealed extensive similarities in relative quantities of miRNAs and, as a biomarker proof of concept, enabled distinction between media conditioned with differently staged embryos (zygote, 4-cell, and blastocyst). When used to assess media conditioned with embryos of varying degrees of degeneration, the panel revealed increases in all extracellular panel sequences, suggesting cell death is an influential and identifiable factor detectable by this assessment. In situ hybridization of three panel sequences (miR-30c, miR-294, and miR-295) in late-stage blastocysts revealed primarily inner cell mass expression with a significant presence of miR-294 throughout the blastocyst cavity. Furthermore, extracellular miR-290 sequences responded significantly to high centrifugal force, suggesting a substantial fraction of these sequences may exist within a vesicle such as an exosome, microvesicle, or apoptotic bleb. Together, these results support the use of extracellular miRNA to assess embryonic health and enable development of a non-invasive viability diagnostic tool for clinical use.

Normalizing RT-qPCR miRNA datasets that encompass numerous preimplantation embryo stages requires the identification of miRNAs that may be used as stable reference genes. A need has also arisen for the normalization of the accompanying conditioned culture media as extracellular miRNAs may serve as biomarkers of embryo developmental competence. Here, we evaluate the stability of six commonly used miRNA normalization candidates, as well as small nuclear U6, using five different means of evaluation (BestKeeper, NormFinder, geNorm, the comparative Delta Ct method and RefFinder comprehensive analysis) to assess their stability throughout murine preimplantation embryo development from the oocyte to the late blastocyst stages, both in whole embryos and the associated conditioned culture media. In descending order of effectiveness, miR-16, miR-191 and miR-106 were identified as the most stable individual reference miRNAs for developing whole CD1 murine preimplantation embryos, while miR-16, miR-106 and miR-103 were ideal for the conditioned culture media. Notably, the widely used U6 reference was among the least appropriate for normalizing both whole embryo and conditioned media miRNA datasets. Incorporating multiple reference miRNAs into the normalization basis via a geometric mean was deemed beneficial, and combinations of each set of stable miRNAs are further recommended, pending validation on a per experiment basis.

No abstract