Genome-wide analysis of murine bone marrow-derived very small embryonic-like stem cells reveals that mitogenic growth factor signaling pathways play a crucial role in the quiescence and ageing of these cells

It has been postulated that the most primitive population of stem cells, Oct4(+)Sca-1(+)Lin(-)CD45(-) very small embryonic-like stem cells (VSELs), differentiate into tissue-committed stem cells in adult mice. However, Oct4(+) VSELs remain quiescent in adult tissues and do not form teratomas. In thi study, we report the characteristics of the VSEL transcriptome by gene set enrichment analysis employing a microarray database established from 20 murine bone marrow-derived, FACS-sorted VSELs in comparison with hematopoietic stem cells and embryonic stem cells. In the Oct4(+) VSELs, we observed the upregulation of tissue-specific gene sets and a gene set encoding the complement-coagulation cascade. By contrast, in the VSELs, we observed the downregulation of genes involved in the UV radiation response, mRNA processing and mitogenic growth factor signaling [e.g., insulin-like growth factor‑1 (IGF-1) and neurotrophic tyrosine kinase receptor A (TRKA), as well as the ERK and PI3K pathways]. Employing leading-edge subset analysis and real-time PCR assays, we observed that several genes, such as growth factor receptor-bound protein 2 (Grb2), son of sevenless homolog 1 (Sos1), SHC (Src homology 2 domain containing) transforming protein 1 (Shc1), mitogen-activated protein kinase kinase 1 (Map2k1), v-akt murine thymoma viral oncogene homolog 3 (Akt3), Elk1, ribosomal protein S6 kinase, 90 kDa, polypeptide 3 (Rps6kA3), glycogen synthase kinase 3β (Gsk3β) and casein kinase 2, alpha 1 polypeptide (Csnk2A1), which are involved in mitogenic growth factor signaling pathways, were commonly downregulated in the VSELs. Notably, this repression was reversed in the VSELs co-cultured over a C2C12 supportive cell-line, whereby they are induced to form VSEL-derived spheres (VSEL-DSs); thus, they are enriched, forming more differentiated stem cells. Therefore, we suggest that the repression of mitogenic growth factor signaling (e.g., through the IGF-1 receptor) may prevent uncontrolled Oct4(+) VSEL proliferation and teratoma formation. Thus, restoring the responsiveness to mitogenic growth factors may be a crucial step in employing these cells in regenerative medicine.