Skeletal muscle contains a stem cell population called muscle derived stem cells or MDSCs that might have tremendous therapeutic potential. MDSCs have been isolated from skeletal muscle by means of their ability to adhere to culture flasks coated with collagen. Samples of muscle taken from a biopsy are mechanically mashed and then treated with enzymes the separate the cells. These cells are plated onto collagen-coated dishes and the cells either adhere quickly (fibroblasts and myoblasts), or slowly (MDSC-enriched fraction).
Skeletal muscle contains another cell population known as satellite cells. Satellite cells can divide and form muscle progenitor cells known as myoblasts that fuse to form myotubes. MDSCs, however, as distinct from satellite cells. They express different sets of genes: satellite cells typically express Pax7, whereas MDSCs are more heterogeneous but express Sca-1 consistently and often express CD34.
Studies in culture and in living animals have established that MDSCs can self-renew and differentiate into multiple lineages. They also have the potential to regenerate various adult tissues. See Usas A, et al Medicina (Kaunas) 2011;47:469–479; Cao B, et al Nat Cell Biol. 2003;5:640–646; Deasy BM, et al Blood Cells Mol Dis. 2001;27:924–933.
MDSCs also display a superior regenerative capacity relative to satellite cells following transplantation into mice with a form of rodent muscular dystrophy (mdx mice). MDSCs are at least partially invisible to the immune system. When transplanted into mdx mice and left for at least 3 months, no sign of immune rejection was detected.
The laboratory of Johnny Huard at the University of Pittsburgh has been genetically engineering MDSCs from mouse for use as cartilage making cells to treat rodents with osteoarthritis. In 2009, Huard’s group published an intriguing paper in the journal Arthritis and Rheumatism in which they genetically engineered MDSCs with two genes: Bone Morphogenetic Protein 4 (BMP-4) and Soluble Flt-1. If you are wondering what the heck these two genes encode, then you are not alone. BMP-4 is a secreted signaling protein that is very important for bone healing, but it also plays a central role in helping cartilage-making cells (chondrocytes) survive and divide. Flt-1 is one of the receptor proteins that binds the growth factor VEGF (vascular endothelial growth factor). Normally, VEGF forms blood vessels and remodels existing blood vessels. However, when it comes to cartilage, VEGF tends to cause cartilage to die back. Therefore, Huard’s group used a soluble version of Flt-1, which scavenged the available VEGF in the environment and bound it up.
In their 2009 paper, Huard and others showed that BMP-4/soluble Flt-1-expressing MDSCs did a remarkable job of making new cartilage and repairing damage joint cartilage in rodents. See Tomoyuki Matsumoto, et al ARTHRITIS & RHEUMATISM Vol. 60, No. 5, May 2009, pp 1390–1405.
In another paper that came out in January of this year, Huard has used platelet-rich plasma with his engineered MDSCs to determine with platelet-rich plasma (PRP) can increase the cartilage-making activity of engineered MDSCs.
Since PRP has been reported to promote the synthesis of collagen and cell proliferation, and increase cartilage repair, it is possible that, when paired with the right stem cells, PRP can enhance cartilage repair. To test this suspicion, MDSCs expressing BMP-4 and sFlt1 were mixed with PRP and injected into the knees of rats whose immune system did not work properly that had osteoarthritis. Osteoarthritis can be chemically induced in rats rather easily, and the rats were treated with MDSCs expressing BMP-4 and sFlt1 or MDSCs expressing BMP-4 and sFlt1 plus PRP. Tissue assessments of the arthritic joints were performed 4 and 12 weeks after cell transplantation. Other tests conducted in culture determined the cell proliferation, adhesion, migration and cartilage-making capacities of cells in culture.
The results showed that addition of PRP to MDSCs expressing BMP-4 and sFlt1 significantly improved joint cartilage repair at week 4 compared to MDSCs expressing BMP-4 and sFlt1 alone. The joints showed higher numbers of cells producing type II collagen and lower levels of chondrocyte cell death were observed by MDSCs expressing BMP-4 and sFlt1 and mixed with PRP. In culture, the addition of PRP promoted proliferation, adhesion and migration of the MDSCs. When pellets of cells were induced to make cartilage in culture, PRP tended to increase the number of type II collagen producing cells.
From this, Huard and his colleagues concluded that PRP can promote the cartilage-repairing capacities of MDSCs that express BMP-4 and sFlt1. This enhancement involves the promotion of collagen synthesis, the suppression of chondrocyte cell death, and by enhancing the integration of the transplanted cells in the repair process.
See Mifune Y, et al Osteoarthritis Cartilage. 2013 Jan;21(1):175-85. doi: 10.1016/j.joca.2012.09.018.