Extracellular matrices of bone marrow stroma regulate cell phenotype and contribute to distinct stromal niches in vivo

Abstract Background Bone marrow stromal cells (BMSCs) are highly heterogeneous, which may reflect their diverse biological functions, including tissue maintenance, haematopoietic support and immune control. The current understanding of the mechanisms that drive the onset and resolution of heterogeneity, and how BMSCs influence other cells in their environment is limited. Here, we determined how the secretome of clonally-derived BMSC subtypes was able to direct cellular phenotype. Methods We used two immortalised clonal BMSC lines isolated from the same heterogeneous culture as model stromal subtypes with distinct phenotypic traits; a multipotent stem cell-like stromal line (Y201) and a nullipotent non-stem cell stromal line (Y202), isolated from the same donor BMSC pool. Label-free ptychographic imaging was used to track cell morphology and migration of the BMSC lines over 96 hours in colony-forming assays. We quantified the secreted factors of each cell line by mass spectrometry and confirmed presence of proteins in human bone marrow by immunofluorescence. Results Transfer of secreted signals from a stem cell to a non-stem cell resulted in a change in morphology and enhanced migration to more closely match stem cell-like features. Mass spectrometry analysis revealed a significant enrichment of extracellular matrix (ECM) proteins in the Y201 stem cell secretome compared to Y202 stromal cells. We confirmed that the stem cell line produced a more robust ECM in culture and that this ECM was capable of changing migration and morphology of non-stem cells. The most highly enriched proteins, aggrecan and periostin, were identified at rare sites on the endosteal surfaces of mouse and human bone, underlying CD271-positive stromal cells, indicating that they may represent key non-cellular niche-components important for cell maintenance and phenotype in vivo. Conclusions We identified plasticity in BMSC morphology and migratory characteristics that can be modified through secreted proteins, particularly from multipotent stem cells. Overall, we demonstrate the importance of specific ECM proteins in co-ordination of cellular phenotype and highlight how non-cellular components of the BMSC niche may provide insights into cell population heterogeneity and the role of BMSCs in health and disease.