Stem Cells, Regenerative Medicine, and Muscle Tissue
In many adult tissues, adult stem cells are responsible for maintaining the tissue and replacing any lost tissue. To do so, adult stem cells are generally multipotent, having the ability to differentiate into the multiple differentiated cell types of a given tissue.
Perhaps the most important goal of regenerative medicine is to replace lost tissue, due to trauma or certain degenerative diseases or pathologies. For example, replacing beta cells (the insulin-producing cells of the pancreas) for patients with type 1 diabetes or regenerating rods and cones (the light-sensing cells of the eye) for patients with macular degeneration could help millions of people with debilitating conditions.
One of the major challenges in regenerating tissue within the body is doing it safely. Current approaches include reprogramming terminally differentiated adult cells (often fibroblasts from the skin) into multipotent stem cells that have the ability to differentiate into multiple cell types. However, these approaches are associated with a certain amount of risk for tumor development and improving these methods is the focus of research groups around the world, including one discussed below.
In particular, skeletal muscle satellite cells are multipotent quiescent cells present in skeletal muscle that are activated upon muscle injury and in response, begin to divide and differentiate to generate new skeletal muscle tissue. Skeletal muscle (found in muscles attached to the skeleton) is one of the three types of muscle present in the body, in addition to the cardiac muscle (found in the heart) and smooth muscle (found in hollow organs throughout the body. Unlike smooth muscle, which is controlled involuntarily, skeletal muscle is controlled voluntarily.
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Lifeline Skeletal Muscle Satellite Cells in Regenerative Medicine Research
Alternatives to current methods of cell reprogramming are needed, particularly to reduce the risk of tumor development following implantation of reprogrammed cells. In a previous study, Zheng and colleagues (opens in new window) described the development of a new human dermal fibroblast reprogramming method to generate multipotent cells using uses continuous recombinant human fibromodulin (FMOD) stimulation. Importantly, in contrast to induced pluripotent stem cells (iPSCs), FMOD-reprogrammed (FReP) cells did not induce tumor formation in immunodeficient mice. This research group previously demonstrated that FReP cells could form bone tissue, and in the current study, set out to determine whether these reprogrammed cells could also form skeletal muscle.
First, using differentiated Lifeline skeletal muscle satellite cells as a positive control for successful muscle differentiation, the authors confirmed that following a 2-stage skeletal myogenic differentiation protocol, FReP cells expressed myogenic protein markers, as well as 84 myogenesis-related genes, confirmed their myogenic potential. Next, the authors tested the in vivo myogenic differentiation capacity of FReP cells by implanting non-myogenic-stimulated FReP cells into the muscle of immunodeficient SCID mice. Compared with iPSCs, FReP cells generated higher muscle mass six weeks after implantation. Importantly, no tumors were observed in mice implanted with FReP cells, while 25% of mice implanted with iPSCs developed tumors. Similarly, FReP cells had a lower proliferation rate and did not form colonies in a soft agar colony formation assay, suggesting they have decreased tumorigenic potential compared with iPSCs. To further confirm the lower tumorigenic potential of FReP cells, the authors implanted iPSCs and FReP cells intratesticularly in immunodeficient mice and found that 100% of mice implanted with iPSCs developed teratomas, while no mice implanted with FReP cells developed tumors after four months.
To further parse out why iPSCs formed tumors when implanted into mice and FReP cells did not, the authors compared gene expression profiles of the two reprogrammed cell types. Interestingly, they found that FReP cells expressed lower levels of proto-oncogenes (tumor-promoting genes) and higher levels of tumor suppressor genes than did iPSCs. In particular, the authors investigated the specific role of the tumor suppressor CDKN2B in maintaining the low tumorigenic potential of FReP cells. When CDKN2B was knocked down in FReP cells, multipotent potential was not affected; however, CDKN2B-knockdown FReP cells exhibited increased proliferation, formed colonies on soft agar, and induced teratoma formation in 100% of the mice implanted.
Together, the results of this study demonstrate that FReP cells have myogenic differentiation capacity, with very low risk of tumorigenic potential, likely due to expression of CDKN2B.
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