Culture Environment-Induced Pluripotency of SACK-Expanded Tissue Stem Cells
Figure 1
The SACK method for the expansion of tissue stem cells. (a) In vivo, tissue stem cells divide with asymmetric cell kinetics to produce progeny that eventually undergo a terminal cell cycle arrest and differentiate. Asymmetric cell kinetics maintains an equilibrium between renewal of differentiated tissue cells and preservation of constant tissue stem cell number. When tissue stem cells are cultured ex vivo, asymmetric cell kinetics results in their dilution and loss as a consequence of the accumulation of their differentiating progeny cells. The SACK method shifts tissue stem cells to symmetric cell kinetics that reversibly promotes their exponential expansion. Green circles, tissue stem cells; yellow circles, transient-amplifying lineage-committed cells; red circles, terminally arrested differentiated cells. (b) The guanine ribonucleotide biosynthesis pathway that controls tissue stem cells’ self-renewal pattern. The rate-limiting step for cellular guanine ribonucleotide biosynthesis is the formation of xanthosine-5′-monophosphate (XMP) from inosine-5′-monophosphate (IMP), the reaction catalyzed by inosine-5′-monophosphate dehydrogenase II (IMPDH II). The asymmetric cell kinetics of tissue stem cells requires downregulation of IMPDH II expression by the tumor suppressor protein p53. Supplementation with exogenous guanine ribonucleotide precursors, like xanthosine, which expands guanine ribonucleotide pools, converts tissue stem cells to symmetric cell kinetics despite normal p53 expression. Cells expressing a xanthine phosphoribosyl transferase (XPRT) transgene can use the highly membrane-permeable precursor xanthine to achieve symmetric cell kinetics. GMP, guanosine monophosphate; NsK, nucleoside kinase; PNP, purine nucleoside phosphorylase; XO, xanthine oxidase.
