The Interaction Between Niche and Hematopoietic Stem Cells

Single-cell RNA profiling identifies diverse cellular responses to EWSR1/FLI1 downregulation in Ewing sarcoma cells

BACKGROUND

The EWSR1/FLI1 gene fusion is the most common rearrangement leading to cell transformation in Ewing sarcoma (ES). Previous studies have indicated that expression at the cellular level is heterogeneous, and that levels of expression may oscillate, conferring different cellular characteristics. In ES the role of EWSR1/FLI1 in regulating subpopulation dynamics is currently unknown.

METHODS

We used siRNA to transiently suppress EWSR1/FLI1 expression and followed population dynamics using both single cell expression profiling, CyTOF and functional assays to define characteristics of exponentially growing ES cells and of ES cells in which EWSR1/FLI1 had been downregulated. Novel transcriptional states with distinct features were assigned using random forest feature selection in combination with machine learning. Cells isolated from ES xenografts in immune-deficient mice were interrogated to determine whether characteristics of specific subpopulations of cells in vitro could be identified. Stem-like characteristics were assessed by primary and secondary spheroid formation in vitro, and invasion/motility was determined for each identified subpopulation. Autophagy was determined by expression profiling, cell sorting and immunohistochemical staining.

RESULTS

We defined a workflow to study EWSR1/FLI1 driven transcriptional states and phenotypes. We tracked EWSR1/FLI1 dependent proliferative activity over time to discover sources of intra-tumoral diversity. Single-cell RNA profiling was used to compare expression profiles in exponentially growing populations (si-Control) or in two dormant populations (D1, D2) in which EWSR1/FLI1 had been suppressed. Three distinct transcriptional states were uncovered contributing to ES intra-heterogeneity. Our predictive model identified ~1% cells in a dormant-like state and ~ 2-4% cells with stem-like and neural stem-like features in an exponentially proliferating ES cell line and in ES xenografts. Following EWSR1/FLI1 knockdown, cells re-entering the proliferative cycle exhibited greater stem-like properties, whereas for those cells remaining quiescent, FAM134B-dependent dormancy may provide a survival mechanism.

CONCLUSIONS

We show that time-dependent changes induced by suppression of oncogenic EWSR1/FLI1 expression induces dormancy, with different subpopulation dynamics. Cells re-entering the proliferative cycle show enhanced stem-like characteristics, whereas those remaining dormant for prolonged periods appear to survive through autophagy. Cells with these characteristics identified in exponentially growing cell populations and in tumor xenografts may confer drug resistance and could potentially contribute to metastasis.

Role of microvascular endothelial cells on proliferation, migration and adhesion of hematopoietic stem cells

Background: The present study investigated the effects of microvascular endothelial cells (MECs) on the chemotaxis, adhesion and proliferation of bone marrow hematopoietic stem cells (HSCs) ex vivo. Methods and Results: MECs were collected from the lung tissue of C57BL/6 mice, and HSCs were isolated with immunomagnetic beads from bone marrow of GFP mice. MECs and HSCs were co-cultured with or without having direct cell–cell contact in Transwell device for the measurement of chemotaxis and adhesion of MECs to HSCs. Experimental results indicate that the penetration rate of HSCs from the Transwell upper chamber to lower chamber in ‘co-culture’ group was significantly higher than that of ‘HSC single culture’ group. Also, the HSCs in co-culture group were all adherent at 24 h, and the co-culture group with direct cell–cell contact had highest proliferation rate. The HSC number was positively correlated with vascular endothelial growth factor (VEGF) and stromal cell-derived factor-1 (SDF-1) levels in supernatants of the culture. Conclusions: Our study reports that MECs enhance the chemotaxis, adhesion and proliferation of HSCs, which might be related to cytokines SDF-1 and VEGF secreted by MECs, and thus MECs enhance the HSC proliferation through cell–cell contact. The present study revealed the effect of MECs on HSCs, and provided a basis and direction for effective expansion of HSCs ex vivo.