Using CXCR4 to Make Stem Cells Stay Put: Regenerating Intervertebral Discs

The migration of several different types of stem cells is regulated by a receptor called “CXCR4” and the molecule that binds to this receptor, SDF-1. SDF-1 is a powerful summoner of white blood cells. During early development, SDF-1 mediates the migration of hematopoietic cells from fetal liver to bone marrow and plays a role in the formation of large blood vessels. During adult hood, SDF-1 plays an important role in making new blood vessels by recruiting endothelial progenitor cells (EPCs) from the bone marrow. Consequently, SDF-1 has a role in tumor metastasis where cancer cells that express the receptor CXCR4 are attracted to metastasis target tissues that release SDF-1. SDF-1 also attracts mesenchymal stem cells and helps them suppress the breakdown of bone.

Hopefully, I have convinced you that SDF-1 and its receptor CXC4 are important molecules. Can overexpression the CXCR4 receptor improve the retention of stem cells within an injured tissue?

Xiao-Tao Wu and Feng Wang from Zhongda Hospital in Nanjing, China and their colleagues have used this CXCR4 receptor/SDF-1 system to test this question in the damaged spinal cord.  This work was published in the journal DNA and Cell Biology (doi:10.1089/dna.2015.3118).

Isolated MSCs were treated with genetically engineered viruses to so that would overexpress the CXCR4 receptor. In order to track these cells under medical imaging scans, the MSCs were also labeled with superparamagnetic iron oxide (SPIO). Next, rabbits that had suffered injuries to their intervertebral discs that lie between the vertebrae were given infusions of these labeled, genetically engineered MSCs. Images of the spine were taken at 0, 8, and 16 weeks after the surgery. The degeneration of the damaged intervertebral discs were also evaluated by disc height (damaged, degenerating intervertebral discs tend to shrink and lose height).

The SPIO-labeled CXCR4-MSC could be detected within the intervertebral discs by MRI 16 weeks post-transplantation. The MSCs that had been engineered to overexpress CXCR4 showed better retention within the discs, relative to implanted MSCs that had not been engineered to overexpress CXCR4.

Did the implanted MSCs affect the integrity of the intervertebral discs? Indeed they did. Compared to the control group, loss of disc height was slowed in the animals that received the CXCR4-overexpressing MSCs. Also, the genetically engineered MSCs seemed to make more cartilage-specific materials, like the giant molecule aggrecan and type II collagen. There is a caveat here, since there is no indication that measured protein directly; only mRNAs. Until the quantities of these molecules can be directly shown to increase in the disc, the increases in these cartilage-building molecules can be said to be presumptive, but not proven.

From these experiments, it seems reasonable to conclude that CXCR4 overexpression promoted MSC retention within the damaged intervertebral discs and the increased stem cell retention enhanced stem cell-based disc regeneration. Therefore this SDF-1/CXCR4 signaling pathway might be a way to drive stem cell migration and infiltration within degenerated intervertebral discs.

Intravenous Administration of Lipitor-treated Stem Cells on the Heart

Hao Zhang and colleagues from the Chinese Academy of Medical Sciences and Peking Union Medical College have published a rather unusual experiment in the American Journal of Translational Research. This experiment, however, could have implications for stem cell therapy in heart attack patients.

When heart attack patients are treated with stem cells, they are either injected directly into the heart muscle or released into the heart through the coronary arteries by means of angioplasty. Injecting stem cells directly into the heart requires special equipment and training. Releasing cells into the coronary arteries causes most cells to end up in the lungs or other organs, and the retention of the stem cells is poor. Introducing cells by means intravenous administration would be supremely simple, but in animal experiments, intravenously administered stem cells almost never get to their target organ.

When the heart undergoes a heart attack, the damaged heart cells release a molecule called SDF1 or stromal cell-derived factor 1. SDF1 summons stem cells to the damaged areas by binding to the surfaces of stem cells and drawing them to the higher concentrations of SDF1. SDF1 binds to a receptor on the surfaces of stem cells called CXCR4. Unfortunately, when stem cells are administered intravenously to animals that have just experienced a heart attack, the stem cells do not have enough CXCR4 on their surfaces to properly respond to the SDF1 being secreted by the damaged heart.

Zhang and his colleagues capitalized on an observation made several years ago. When stem cells are exposed to statin drugs that are normally used to lower serum cholesterol levels, the stem cells increase the number of CXCR4 molecules on their surfaces. Statins have also been shown to increase stem cell survival once the cells get to the heart, but Zhang and his team wanted to know if pre-treating stem cells with statins could increase their migration to the damaged heart.

The Zhang group isolated mesenchymal stem cells from rat bone marrow and treated these cells with increasing concentrations of the drug Lipitor (atorvastatin). Indeed, increasing amounts of Lipitor increased the number of CXCR4 molecules on the surfaces of the mesenchymal stem cells (MSCs), This increase in CXCR4 molecules peaked at 24 hours, after which the number of receptors declined. These Lipitor-treated MSCs also migrated much more robustly in culture when treated with SDF1.

Next, Zhang’s group pre-treated MSCs with Lipitor and labeled them with an innocuous tracking molecule. 24 hours after giving some laboratory rats heart attacks, these MSCs were administered to the rats in their tail veins. Two other groups of similarly treated rats were given either MSCs that had not been pre-treated with Lipitor, or just buffer.

The Lipitor-treated MSCs were found in significantly higher quantities in the hearts of laboratory animals, relative to the other animals. Secondly, these Lipitor pre-treated MSCs cut the size of the heart scar in half, and there was also substantially less inflammation in hearts from animals treated with Lipitor pre-treated MSCs than the other groups. Heart function was also increased in the pre-treated group.

Live MSCs were observed in the hearts of the animals given Lipitor pre-treated MSCs. This is a remarkable finding, because most experiments have shown that MSCs administered to the heart after a heart attack ad usually dead within 21 day after administration. However the Lipitor pre-treated MSCs survive and flourish in the damaged heart, which suggests that SDF1 not only attracts stem cells but also increases their rates of survival.

This is a somewhat off-beat experiment at first glance, but if MSCs could be pre-treated with a drug like Lipitor and then administered to heart patients intravenously, they would survive in the heart, convey greater benefits, and their administration would be safer, and not require special equipment or training. With a little luck, this idea will reach human clinical trials in a few years; provided that further animal and cell culture studies confirm these results, elucidate the mechanism of SDF1-mediated survival, and show that such augmentation of function is also observed in human MSCs.