ABSTRACT: Currently, micro RNAs (miRNAs) constitute a promising models for cell-to-cell communication since they are transferred between cells to execute essential roles in many processes. Their structure and size, in addition to various transport mechanisms, allow them to remain stable within various biological fluids, surviving in extremely adverse conditions, including low pH, boiling, and freezing. The presence of miRNAs in reproductive fluids, such as follicular, uterine and seminal fluid, indicate potential roles in the reproductive system. These extracellular miRNAs can be transported by lipoproteins (both HDL and LDL) or other proteins, including Argonaute2 (AGO2) and nucleophosmin1 (NPM1). Another transport system is mediated by extracellular vesicles (EVs), such as apoptotic bodies, microvesicles (MVs) and/or exosome-like vesicles. EVs protect miRNAs from degradation and contribute to their stability within biological fluids. Furthermore, EVs can transport a wide range of components packaged in a selective way. For instance, Squadrito et al. (2014), used macrophages and endothelial cells to demonstrate that the sorting of miRNAs into EVs for heterotrophic cell communication is altered by both, the presence of target transcripts and the self-presence of the respectively miRNA. In addition, the signature of miRNAs found in the exosomes significantly differed for those detected in the parent cells. To date, mechanisms controlling the specific loading of miRNAs into exosomes remain unclear. Indeed, several mechanisms may govern exosome sorting of specific subsets of miRNAs. MiRNA sorting appears to be influenced by different pathways and molecules in different cell types and tissues, and miRNAs contain well defined motifs (i.e., EXOmotifs), that direct the miRNA allocation into exosomes before delivery into recipient cells. A recent study showed that this RNA sequence can be recognized by the sumoylated form of the heterogeneous ribonucleoprotein A2B1 (hnRNPA2B1). Moreover, a terminal 30 nucleotide addition in miRNAs affects their selective sorting in B cells. Another hypothesis suggests that RNAs are selectively sorted depending on the differential affinity of RNA motifs towards the raft-like region of the cytoplasmic surface of microvesicular body (MVB) limiting membranes. Current data indicate that cells can communicate with each other through the transfer of miRNA-loaded exosomes. For example, monocyte-derived exosomes deliver miR-150 to endothelial cells and enhance endothelial cell migration by reducing c-myb expression. The miRNA content of exosomes plays a critical role in such cell-to-cell communication and determines the fate of recipient cells. Thus, exosomes derived from the bone marrow mesenchymal stromal cells of myeloma patients promote tumor growth depending on the content of miR-15a in exosomes. Our group has described a novel cell-to-cell communication mechanism involving the delivery of endometrial miRNAs from the maternal endometrium to the trophectoderm cells of preimplantation embryos. Specifically, in B6C3 derived mouse embryos, we found EV-associated and free miR-30d to cause overexpression of genes involved in embryonic adhesion processes, including Itb3, Itga7 and Cdh5. Furthermore, supplementing murine embryos with miR-30d significantly improved embryo adhesion, suggesting that external miRNAs may have a functional role as transcriptomic modifiers of preimplantation embryos. Based on profiling of miRNAs in endometrial fluid, maternally-derived miRNAs are present within EVs in the uterine microenvironment. The internalization of maternally-derived exosomes has been visualized, but the mechanism by which miR-30d becomes incorporated into exosomes remains unknown. The present study aimed to elucidate the underlying mechanism of hsa-miR-30d transfer from human endometrial epithelial cells (hEECs) to the interior of exosomes and eventually to early-stage blastocysts, using a mir-30d knockout murine mode.
Luz Valero 
