The application of stem cells to regenerative medicine is one of the most crucial field of research, the encouraging developments of which have been already employed in the clinics, although some lacunas in the understanding of stem cell functioning remain still unexplained.

In particular, the basic requirement for the use of stem cells in regenerative medicine is the steady maintenance of their properties during proliferation in prolonged cultures. Specific markers of the undifferentiated state are fundamental for checking cell phenotypes stability in the course of time and comparing cell lines among different laboratories. In fact, human stem cell lines, cultured for extended periods of time, show changes in karyotype stability, expression of cell surface markers, transcription factors and telomerase activity [1] [2] [3]. These modifications are an obstacle to scientists' capability to study and employ adult stem cells ex vivo. In the last years, several parameters were characterized to be directly correlated to the functions of stem cells and mitochondrial activity has been considered as particularly important for sustaining cell viability. Mitochondria, in fact, not only supply cells with the bulk of their ATP, but also refill cellular GTP, as well as control amino acid turnover and accomplish fatty acids beta-oxidation [4]. Moreover, these subcellular organelles, together with the endoplasmic reticulum, serve also as a reservoir of cell calcium.

Some recent papers have analyzed mitochondrial activity and oxygen consumption rate alteration [5] [6][7][8][9][10]. In the very last years, new interesting results were obtained and, in particular, some studies documented that each cell phase depend on a specific metabolic state [4] [11]. In fact, the metabolism of "pluripotent embryonic stem cells" (pESCs) is based on a steady high level of glycolysis while, during cell differentiation, glucose disposal through the glycolytic pathway decreases and mitochondrial oxidative phosphorylation increases [5] [12] [13]. Moreover, during stem cell differentiation, mitochondria modify their number, morphology and localization [10] [12] [13] [14] .

Interestingly, when differentiated cells are transformed to "induced Pluripotent Stem Cells" (iPSCs), they switch their metabolism from oxidative phosphorylation back to glycolysis [15], whose inhibition blocks the de-differentiation process [15].

Some adult stem cells, such as Mesenchymal stem cells (MSCs) and long-term hematopoietic stem cells (LT-HSCs), require low level of oxygen to keep undifferentiated conditions. These cells live in niches, under hypoxic condition and they mainly use glycolysis to obtain ATP, thus limiting reactive oxygen species (ROS) production and subsequent damages to DNA and RNA [12] [13]. The relation between aerobic and anaerobic metabolism is much more varied and complex but, if a bit of simplification is allowed, all these results seem to indicate that cell pluripotency (stemness) is related to a really modest oxidative phosphorylation activity linked to the hypoxic conditions in which stem cells are kept living. Nevertheless, many metabolic aspects of stem cells (both adult and embryonic cells) are still not well understood and, for these reasons, is not always possible to obtain in vitro terminally differentiated cells from progenitor cells. The determination of these features might help us to establish whether the stem cells are in quiescent state and when they are fully capable of differentiating. This knowledge could permit the discovery of the linkages of metabolic profiles with different surface markers of stem cells (i.e., CD29, CD44, CD73, CD90, CD105 and CD166) and cell signaling pathways and useful for checking eventual deviations from the normal behavior of stem cells in cultures and sorting out defective cells for the prosecution of work.

Keywords: Mitochondrial function, Stem cells, Pluripotency

We are very grateful to Dr. Marinella Magini, Dr. Federica Vincenzoni of the Università Cattolica del Sacro Cuore and Dr. Alessandro Lupi of the Istituto di Chimica del Riconoscimento Molecolare, CNR, for valuable discussions, encouragement and great interest.

1. Niwa H. Molecular mechanism to maintain stem cell renewal of ES cells. Cell Struct Funct 2001 Jun;26(3):137–48.   [CrossRef]   [Pubmed]    
2. Brimble SN, Zeng X, Weiler DA, et al. Karyotypic stability, genotyping, differentiation, feeder-free maintenance, and gene expression sampling in three human embryonic stem cell lines derived prior to August 9, 2001. Stem Cells Dev 2004 Dec;13(6):585–97.   [CrossRef]   [Pubmed]    
3. Rosler ES, Fisk GJ, Ares X, et al. Long-term culture of human embryonic stem cells in feeder-free conditions. Dev Dyn 2004 Feb;229(2):259–74.   [CrossRef]   [Pubmed]    
4. Ito K, Suda T. Metabolic requirements for the maintenance of self-renewing stem cells. Nat Rev Mol Cell Biol 2014 Apr;15(4):243–56.   [CrossRef]   [Pubmed]    
5. Cho YM, Kwon S, Pak YK, et al. Dynamic changes in mitochondrial biogenesis and antioxidant enzymes during the spontaneous differentiation of human embryonic stem cells. Biochem Biophys Res Commun 2006 Oct 6;348(4):1472–8.   [CrossRef]   [Pubmed]    
6. Lonergan T, Brenner C, Bavister B. Differentiation-related changes in mitochondrial properties as indicators of stem cell competence. J Cell Physiol 2006 Jul;208(1):149–53.   [CrossRef]   [Pubmed]    
7. von Heimburg D, Hemmrich K, Zachariah S, Staiger H, Pallua N. Oxygen consumption in undifferentiated versus differentiated adipogenic mesenchymal precursor cells. Respir Physiol Neurobiol 2005 Apr 15;146(2-3):107–16.   [CrossRef]   [Pubmed]    
8. Piccoli C, Ria R, Scrima R, et al. Characterization of mitochondrial and extra-mitochondrial oxygen consuming reactions in human hematopoietic stem cells. Novel evidence of the occurrence of NAD(P)H oxidase activity. J Biol Chem 2005 Jul 15;280(28):26467–76.   [CrossRef]   [Pubmed]    
9. Plotnikov EY, Marei MV, Podgornyi OV, Aleksandrova MA, Zorov DB, Sukhikh GT. Functional activity of mitochondria in cultured neural precursor cells. Bull Exp Biol Med 2006 Jan;141(1):142–6.   [CrossRef]   [Pubmed]    
10. Lonergan T, Bavister B, Brenner C. Mitochondria in stem cells. Mitochondrion 2007 Sep;7(5):289–96.   [CrossRef]   [Pubmed]    
11. Folmes CD, Dzeja PP, Nelson TJ, Terzic A. Metabolic plasticity in stem cell homeostasis and differentiation. Cell Stem Cell 2012 Nov 2;11(5):596–606.   [CrossRef]   [Pubmed]    
12. Suda T, Takubo K, Semenza GL. Metabolic regulation of hematopoietic stem cells in the hypoxic niche. Cell Stem Cell 2011 Oct 4;9(4):298–310.   [CrossRef]   [Pubmed]    
13. Shyh-Chang N, Daley GQ, Cantley LC. Stem cell metabolism in tissue development and aging. Development 2013 Jun;140(12):2535–47.   [CrossRef]   [Pubmed]    
14. Xu X, Duan S, Yi F, Ocampo A, Liu GH, Izpisua Belmonte JC. Mitochondrial regulation in pluripotent stem cells. Cell Metab 2013 Sep 3;18(3):325–32.   [CrossRef]   [Pubmed]    
15. Folmes CD, Nelson TJ, Martinez-Fernandez A, et al. Somatic oxidative bioenergetics transitions into pluripotency-dependent glycolysis to facilitate nuclear reprogramming. Cell Metab 2011 Aug 3;14(2):264–71.   [CrossRef]   [Pubmed]