The Heterogeneity and Metastability of Multipotent Hematopoietic Cells

Increasing numbers of studies are reporting phenotypic and functional heterogeneity within a variety of stem cell types as well as the existence of inter-convertible subsets within hematopoietic stem cells (HSCs), suggesting that these cells are alternating between several metastable states. The extent of heterogeneity and metastability in HSCs remains to be explored in depth. The metastability of HSCs is most likely positively and negatively regulated by currently unknown cell intrinsic and extrinsic mechanisms emanating from HSC niches. But the regulation of HSC metastability by bone marrow (BM) niche cells is not currently understood. Numerous studies have shown that phenotypic, functional and molecular features of EML cells closely resemble or are identical to the features of adult HSCs and multipotent hematopoietic progenitors (MPPs). Thus, EML cells represent the closest known in vitro counterpart of HSC/MPPs. This investigation sought to analyze in depth the heterogeneity and metastability of the EML cell line, as well as investigate the effect of BM niche cells on their metastability. Our data illustrate that EML cells are very heterogeneous and consist of several cell subsets with distinct expression of HSC markers, cell cycle profile and capacity to differentiate. Remarkably, each cell subset has the capacity to self-renew, as well as to produce other subsets and restore the parental EML cell population. Our findings show that EML cells oscillate between several inter-convertible metastable states with distinct phenotypic and functional features. Equally important, our in depth studies have also found that the metastability of EML cell line is homeostatically maintained and tightly regulated by currently unidentified cell intrinsic mechanisms. We therefore sought to investigate whether bone marrow (BM) niche cells will affect the metastability of EML cells in a co-culture environment. Our results indicate that mesenchymal and osteoblast cells do affect the metastability of EML cells by shifting the metastability process towards increased production and maintenance of more quiescent cell subsets. Understanding how the metastability of HSCs is regulated is imperative to deciphering the characteristics of HSCs when in a particular “state”. This knowledge will ultimately help provide us the ability to alter the state of HSCs for specific purposes, for example, more effective bone marrow transplant, accelerated differentiation of HSCs post-transplant, reduced GVHD, lower susceptibility to chemotherapy, etc