Evidence for a Metabolic Stem Cell Niche.

The hematopoietic stem cell niche presents a localised environment supporting the balanced maintenance, self-renewal and occasional expansion of the stem cell pool. These options are widely assumed to be regulated exclusively by signalling from specific combinations of stroma-bound or soluble ligands. However, a consideration of the rare conditions under which absolute numbers of stem cells increase in vivo as well as the selective pressures acting on regenerative systems during evolution has led us to propose a metabolic component to the stem cell niche which serves to limit cumulative damage, to avoid the selection of potentially oncogenic mutations and to tie symmetric division to slow proliferation. This would mean that traditional cell culture media based on “systemic” substrates such as glucose and glutamine may actively prevent the symmetric amplification of high quality stem cells, offering a possible explanation for the limited success in this area to date. To investigate this possibility, we have examined the effects of range of carbon and energy sources on the proliferation and maintenance of stem and progenitor cells. Our strategy is to screen a wide variety of culture conditions using murine FDCPmix cells, which are non-tumorigenic but have an innate tendency to amplify symmetrically in the presence of IL-3, and then to test key observations in human UCB CD133⁺ cells provided with SCF, TPO and FLT-3L. In both cell systems, we do indeed find an unusually low requirement for the systemic substrates glucose and glutamine normally included as major energy and carbon sources in cell culture media. Reducing glucose reduces the yield of committed cells from CD133⁺ cultures without affecting the accumulation of CD133⁺CD34⁺cKit⁺ progenitors. When provided with alternative substrates more likely to reflect a “niche” type environment, FDCPmix cells can be maintained for long periods in media containing only the trace levels of glucose or glutamine derived from dialysed serum, and show improved self-renewal under these conditions. We have also found that raising osmolarity reduces glucose dependence and simultaneously favours the maintenance both of self-renewing CFU (FDCPmix culture) and of CAFCweek13 (CD133⁺ culture). In parallel, the use of NMR and mass spectrometry techniques to profile intracellular metabolites in self-renewing and differentiating FDCPmix cells reveals a shift in the metabolite balance indicating reduced glycolysis in the early cells. Taken together, these results suggest that hematopoietic stem cells do indeed have remarkable metabolic characteristics consistent with adaptation to a metabolically limiting niche environment. It may therefore be necessary to identify niche substrates and to combine these with the relevant signalling environment in vitro in order to effectively increase stem cell numbers for research, stem cell transplantation and tissue engineering applications.