Stem cell plasticity revisited: the continuum marrow model and phenotypic changes mediated by microvesicles

Abstract

The phenotype of marrow hematopoietic stem cells is determined by cell-cycle state and microvesicle entry into the stem cells. The stem cell population is continually changing based on cell-cycle transit and can only be defined on a population basis. Purification of marrow stem cells only addresses the heterogeneity of these populations. When whole marrow is studied, the long-term repopulating stem cells are in active cell cycle. However, with some variability, when highly purified stem cells are studied, the cells appear to be dormant. Thus, the study of purified stem cells is intrinsically misleading. Tissue-derived microvesicles enhanced by injury effect the phenotype of different cell classes. We propose that previously described stem cell plasticity is due to microvesicle modulation. We further propose a stem cell population model in which the individual cell phenotypes continually change, but the population phenotype is relatively stable. This, in turn, is modulated by microvesicle and microenvironmental influences.

Figure 1. Population model of stem cell phenotype

Numbers and circles represent different phenotypic cell classes. The concept here is that the phenotypes change at different points in cell cycle and eventually return to the original phenotype. For example, cell #1 is a long-term repopulating cell in G2/M/G0 and becomes a different cell in G1, a CFU-Meg in S phase, then returns to the original phenotype. In this model, the individual cell phenotype continuously changes while the population remains stable.

Figure 2. Marrow conversion to epithelial lung cell

This shows conversion of a marrow stem cell phenotype to a pulmonary epithelial cell which is affected by host irradiation, treatment of host or exogenous marrow cells with G-CSF and stem cell phenotype.

Figure 3. Marrow-lung co-culture

Marrow cells were co-cultured across from lung fragments but separated from them by a cell impermeable (0.4 micron) membrane for two or seven days and expression of pulmonary epithelial genes in marrow cells determined by RT-PCR analysis.

Figure 4. Lung-derived microvesicles

A-D shows a marrow cell with incorporated PKH26 and CFSE-labeled lung-derived microvesicles. A, merged image; B, DAPI filter; C, Texas Red filter; D, FITC filter. Image E is an electron micrograph of FACS-sorted lung-derived microvesicles. Red bar = 10 microns, black bar = 100 nanometers.

Figure 5. Injury induction of microvesicles

Irradiation injures a non-hematopoietic cell which releases bioactive microvesicles containing protein, mRNA and microRNA. These microvesicles enter marrow cells and alter their phenotype to that of the cell of microvesicle origin.

Figure 6. Effect of microvesicles on the stem cell population model

This indicates that microvesicles impose a different order of phenotypic change on stem cells progressing through a cell cycle-related stem cell continuum.

Figure 7. Stem cell modulogram

Stem cells progressing though cycle continuously change individual cell phenotypes while maintaining the population phenotype. This is further modulated by microvesicle cell entry and the final cell fate determined by interactions with different microenvironments.

Figure 8. Concepts of stem cell plasticity

Panel 1 indicates that marrow-derived microvesicles may enter lung cells and induce marrow characteristics in the lung cells. Panel 2 indicates than lung-derived microvesicles may enter marrow cells and alter their phenotype towards that of a lung cell.