Role of STPK in Maintaining the Quiescent Nature of Human CD34⁺ Stem Cells

Haematopoietic stem cell normally exists in the hypoxic niche of bone marrow and in this high anaerobic condition phosphorylation is vital in understanding the stemness of these stem cells in bone marrow. Analysis of human aldehyde dehydrogenase (ALDH) and isocitrate dehydrogenase (IDH) we have observed the presence of serine threonine protein kinase (STPK) sites in the protein sequence of these enzymes conferring that functioning of ALDH and IDH is regulated largely by STPK through phosphorylation. Human CD34⁺ stem cells and mononuclear cells as a control isolated from peripheral blood and were propagated in DMEM media at 5% CO₂, 95% humidity and at 37°C. Thus obtained cells showed high enzyme activity for STPK, ALDH and low enzyme activity for IDH in CD34⁺ cells compare to control cells. These results were concurred with qRT-PCR studies with high gene expression levels of hypoxia inducing factor-1-alpha (HIF1α), STPK, ALDH and low IDH expression in CD34⁺ cells while normalized with β-actin. In addition the phosphorylating sites on ALDH and IDH proteins were identified and their importance in maintaining the anaerobic conditions in HSCs was demonstrated. In view of the importance of STPK signalling in the present study mechanism in cell division was addressed with phosphorylation of key regulating enzymes in the metabolic pathway of cell cycle was explored.