Bone marrow cells contain several populations that are useful for regenerating injured/aged tissue. These cells include hematopoietic stem cells, mesenchymal stem cells, endothelial progenitor cells, and some argue, progenitor cells left over from embryonic periods that are still capable of differentiating into numerous injured tissue. It has been known for some time that bone marrow cells are capable of treating liver failure both in vitro and in early clinical trials, as can be seen on this video: Stem Cell Therapy for Liver Failure. Other types of stem cells useful for treatment of liver failure, such as cord blood stem cells, may be seen on this video: Cord Blood and Bone Marrow Stem Cells for Liver Failure.
One of the major questions with adult stem cell therapy is how do the stem cells go to where they are needed? Some people have made the argument that stem cells administered intravenously do not cause systemic effect because the majority get stuck in the lung and liver. Although cell sequestration is an issue, numerous studies have demonstrated therapeutic effects after intravenous administration of stem cells. Perhaps the most well-known stem cell homing molecule is stromal derived factor (SDF-1), which is made by injured and/or hypoxic tissue and causes stem cell mobilization and migration through activation of the CXCR4 receptor. The SDF-1/CXCR4 axis has been found in numerous conditions of tissue injury such as: stroke, heart attack, acoustic injured ear, liver failure, and post-transplant reconstitution of bone marrow. To understand how this “chemokine” works, the following video will describe it as relevant to stem cell repopulation post-irradiation: Homing of Stem Cells to Target Tissue
In a study published today scientists examined another signal made by injured tissue in order to assess whether it may act like SDF-1 and “call in” stem cells. The signal chosen was the amino acid adenosine, which is released from injured/necrotic cells. They found that adenosine did not by itself induce chemotaxis of mesenchymal stem cells (MSC) but dramatically inhibited MSC chemotaxis in response to the chemoattractant hepatocyte growth factor (HGF). Inhibition of HGF-induced chemotaxis by adenosine requires the A2a receptor and is mediated via up-regulation of the cyclic adenosine monophosphate (AMP)/protein kinase A pathway. Additionally, the investigators found that adenosine induces the expression of some key endodermal and hepatocyte-specific genes in mouse and human MSCs in vitro.
The ability of adenosine to modulate migration/differentiation processes implies that numerous paracrine/autocrine interactions are occurring during tissue injury. It will be critical to identify how to manipulate such factors to obtain maximal therapeutic responses.