Very small embryonic-like stem-cell optimization of isolation protocols: an update of molecular signatures and a review of current in vivo applications

Abstract

As the theory of stem cell plasticity was first proposed, we have explored an alternative hypothesis for this phenomenon: namely that adult bone marrow (BM) and umbilical cord blood (UCB) contain more developmentally primitive cells than hematopoietic stem cells (HSCs). In support of this notion, using multiparameter sorting we were able to isolate small Sca1(+)Lin(-)CD45(-) cells and CD133(+)Lin(-)CD45(-) cells from murine BM and human UCB, respectively, which were further enriched for the detection of various early developmental markers such as the SSEA antigen on the surface and the Oct4 and Nanog transcription factors in the nucleus. Similar populations of cells have been found in various organs by our team and others, including the heart, brain and gonads. Owing to their primitive cellular features, such as the high nuclear/cytoplasm ratio and the presence of euchromatin, they are called very small embryonic-like stem cells (VSELs). In the appropriate in vivo models, VSELs differentiate into long-term repopulating HSCs, mesenchymal stem cells (MSCs), lung epithelial cells, cardiomyocytes and gametes. In this review, we discuss the most recent data from our laboratory and other groups regarding the optimal isolation procedures and describe the updated molecular characteristics of VSELs.

Figure 1

IIS signaling and imprinted genes. In mammals, there are three insulin factors (insulin, Igf1 and Igf2) that bind to two tyrosine kinase receptors (insulin receptor (InsR) and Igf1 receptor (Igf1R)). Igf2R is a non-signaling mannose-type sink receptor for Igf2. Activation of InsR and Igf1R leads to metabolic or proliferative responses depending on the cell type. RasGrf1 is a small GEF that is involved in signaling from InsR and Igf1R. VSELs demonstrate a decrease in Igf2 and RasGrf1 expression and the overexpression of Igf2R (shown in red) due to changes in the epigenetic state of the imprinted genes. These epigenetic changes in genes regulate IIS and maintain quiescence in VSELs in adult tissues that are somehow resistant to in vitro expansion. We hypothesize that chronic exposure to IIS accelerates the premature depletion of VSELs from adult tissues.