ABSTRACT: Hematopoietic stem cell transplantation (HSCT) is a common practice in the treatment of (malignant-) hematopoietic diseases. Umbilical Cord Blood (UCB) has become an important hematopoietic stem and progenitor cell (HSPC) source for allogeneic HSCT. However, the relatively low number of HSPCs that are present in a single UCB unit is associated with a delayed engraftment and a higher risk for graft failure. To overcome these limitations, the discovery of new therapeutics and the development of efficient ex vivo HSPC expansion conditions will help towards the successful treatment of malignant hematopoietic disease. Bone marrow derived mesenchymal stromal cells (BMSCs) are known to support the characteristic properties of HSPCs in the bone marrow microenvironment. Recently, we and others have illustrated that extracellular vesicles (EVs) have the potential to support HSPC expansion, however, the mechanism and processes responsible for the cell-cell communication of EVs are still unknown. In the current study, we investigate whether primary human BMSC EVs isolated from two different origins, fetal (fEV) and adult (aEV) tissue respectively, can increase the relative low number of HSPC found in UCB and which EV-derived components are responsible for ex vivo HSPC expansion. Interestingly, aEVs but not fEVs showed supportive ex vivo expansion capacity of UCB-HSPC. Taking advantage of the two BMSC sources with different supportive effects, we performed small RNA sequencing and proteomics analyses on the EV cargo and investigated how gene expression is modulated in HSPC after incubation with aEVs and fEVs. Proteomics analyses of the protein cargo composition of the supportive aEV versus the non-supportive fEV identified 90% of the Top100 exosome proteins present in the ExoCarta database. We identified well-defined EV markers such as ANXA1, ANXA2, ANXA5 and GAPDH as the most abundant proteins detected in both aEV and fEV. Gene Ontology (GO) analyses of the enriched proteins in aEVs and fEVs illustrated that of the proteins overrepresented in aEVs were annotated to oxidation-reduction process (eg. HADHA, HADHB), mitochondrial ATP synthesis coupled proton transport (eg. ATP5A1, ATP5B and ATP5O) or protein folding (eg. FKBP11, FKBP10, FKBP2, MESDC2). In contrast, the proteins overrepresented in fEVs were annotated to extracellular matrix organization (eg. FN1, COL12A1, COL1A1, COL6A1, CCDC80) or positive regulation of cell migration (eg. ITGA4, ITGA6, GDF15, SEMA3, APDSD6, MMP14, ICAM1, FGFR1, PDGFRA, ADAMTS1). Interestingly, we found various proteins that were overrepresented in the non-supportive fEV cargo, that are involved in transforming growth factor beta receptor (TGFBR) signaling pathway (TGFBR1, TGFBR2, TGFB1, TGFB2, LTBP1, LTBP2, BMPR1A, GDF5, GDF15, PARP1, RPS27A and COL3A1). Together this illustrates the large differences in the protein content of EV populations derived from two different origins. Small RNA sequencing identified different molecular signatures between the supportive aEVs and the non-supportive fEVs. miRNA, yRNA and piRNA were abundantly found in aEV, while fEVs contained a significant enrichment of snoRNAs. Interestingly, the microRNA cluster miR-99b/let-7e/miR-125a, previously identified to increase the number of HSPC by targeting multiple proapoptotic genes, was highly and significantly enriched in aEV in comparison to fEV. Although we identified a significant difference in EV cargo and supportive effects of aEVs and fEVs, RNAseq analyses of 24hrs treated HSPCs with aEVs or fEVs indicated that only a limited set of genes was differentially regulated when compared to cells that were only treated with cytokines. Interestingly, we identified 5 genes, (eg MTF1, PER1, HOMER1, HSPA6, ENSG00000260534) that were significantly upregulated upon aEV compared to fEV treatment of HSPC. Moreover, HSPC treated with fEVs, resulted in increased expression of genes (SKIL, SMAD7, ENG, LDLRAD4) that are involved in the negative regulation of the TGFbeta signalling pathway. Together with the TGFbeta pathway proteins that were specifically identified in the fEVs, our results suggest that HSPCs may activate a negative feedback loop upon fEV treatment that consolidate HSPC expansion. In conclusion, our study gives novel insights into the complex biological role of EVs in the bone marrow microenvironment. Systematic analyses of supportive and non-supportive human BMSC derived extracellular vesicles indicated known molecules, eg. microRNA cluster miR-99b/let-7e/miR-125a, and the presence of TGFbeta pathway components that are important regulators in the cell-cell communication via EVs and open new means for the application of EVs in the discovery of therapeutics for more efficient ex vivo HSPC expansion.
Floris van Alphen 
