To stay or to leave: Stem cells and progenitor cells navigating the S1P gradient

Figure 3 Stem cell trafficking to injured tissues. A: Retention of hematopoietic stem progenitor cells (HSPCs) in local tissue. HSPCs can down-regulate the expression of sphingosine-1-phosphate (S1P)₁ in response to lipopolysaccharide and thus stayed in local tissue and could differentiate to multilineage cells to help clear the infection; B: Trafficking of mesenchymal stem cell (MSC) to injured liver. S1P is elevated in response to liver damage, which attract bone marrow MSC to the injured liver for liver remodeling in a S1P₃ dependent manner.Sph, sphingosine; SPHK, sphingosine kinase; C: Targeting of neural progenitor cell (NPC) to the site of brain injury. NPCs are found to migrate to the site of injury for local regeneration. This is regulated by the down-regulation of S1P₂ receptors; D: Migration of endothelial precursor cell (EPC) to sites of vascular injury. EPCs migrate to vascular injury due to high S1P generated from local vessel leakage and platelet clotting. This is probably mediated by the S1P₃ receptor subtype. BV: Blood vessel.