Research in stem cell structure, manipulation, and therapy has given rise to many potential disease treatments. Stem cell research is commonly associated with the use of controversial embryonic stem cells (ESCs), although scientists are beginning to use non-embryonic stem cells more frequently. Known as induced pluripotent stem cells (iPSCs), these cells are derived from a patient’s somatic cell line (all cells except those involved in reproduction). Like ESCs, iPSCs are capable of differentiating into any cell type and have no predetermined end structure.
In addition to overcoming this ethical controversy surrounding stem cell use, recent work has shown that using iPSCs may be more effective than MSCs, as the latter is associated with aging drawbacks.
Another type of stem cell frequently emphasized in research is the tissue-specific (also referred to as somatic/adult) stem cell. These stem cells are more specific than the ESCs or iPSCs, and typically differentiate into cell types that comprise a specific tissue. The MSC, for example, is an adult stem cell that gives rise to bone, cartilage, muscle, marrow fat cells. For more on stem cell types, click here.
Applications for Stem Cell Therapy
Given the variety of stem cell types and their potential utility in medicine, much research has been devoted to their use. Described below are several areas in which stem cell therapies and research have been applied.
In a previous paper published in Nature, researchers applied genetically modified stem cells to regenerate skin in a patient with a disease known as epidermolysis bullosa. This heritable condition arises from mutations in genes related to the collagen and laminin proteins, and can cause severe damage to the skin. The researchers obtained tissue-specific stem cells from a biopsy of the patient’s healthy skin, genetically modified these cells to express functioning laminin, and re-implanted these cells to the affected area. This technique was able to reproduce 80 percent of the skin lost in this patient. Though use of MSCs and iPSC-derived skin cells has occured in similar transplants, this tissue-specific stem cell technique is more commonly used in skin regeneration.
Cardiac muscle is a tissue that is not easily regenerated in the body. Claims regarding tissue-specific stem cells in the heart have been disputed, and the documented turnover in heart muscle cells is extremely slow. When one has a heart attack (myocardial infarction), blood flow to the heart is blocked and these tissues begin to die as a result. With no natural means of regenerating this tissue, heart transplants are the only option for people with severe cardiac damage. Researchers have attempted to use MSCs in treatment, but long-term results have not been achieved. ESCs from cardiac cells, however, have been found to rebuild cardiac muscle after infarction in animal models. Rodent studies yielded optimistic results, but studies in primates showed complications in some. The heart’s structure is complex and comprised of many different cell types, and researchers continue working to address issues regarding ESC and iPSC use in regenerating such tissues.
Stem cell therapy also holds utility in treating those with visual loss, with therapies aimed at restoring integrity of the retinal pigment epithelium (RPE). Delivering stem cells to the retina (in the back of the eye) is a difficult task, but evidence from clinical trials supports the use of ESC- and iPSC-derived RPE cells in treating age-related macular degeneration. This disease is characterized by loss of photoreceptors and RPE cells in the retina, and can lead to full vision loss. Treating the disease early would require replenishing these RPE cells, but later stages need to target photoreceptor regeneration as well. Transplanting healthy RPE cells from a donor brings potential immune response risks, and use of stem cells could potentially overcome this barrier.
Recent research has aimed to use muscle stem cells (MuSCs) in regenerating skeletal muscle for those with conditions such as muscular dystrophy. These patients’ lack proteins essential to muscular integrity, and experience gradual loss in muscle mass. It has been noted that these MuSCs are challenging to grow in a cell culture, causing drawbacks for their potential applications. Although it has not been used in clinical trials, another approach to this issue is stimulating MuSCs in situ, which has been modeled in rodents. iPSCs have emerged as a candidate, in that they would allow gene replacement before implantation. Alternatively, gene therapy has been cited as an area of strong interest in regenerating muscle mass.
Similar to cardiac muscle, neural tissue does not actively regenerate. In fact, neurogenesis is a process that occurs almost fully in utero, and once the brain tissue becomes damaged there is no natural healing process. Unlike the cardiac tissue, no transplantation option for neural tissue currently exists, making it an item of particular interest for stem cell research. Studies in rodents have indicated that pluripotent stem cell (PSC)-derived neurons could be effective in treating Parkinson’s disease, and similar trials involving iPSCs and ESCs are soon to come.
The pancreas does not regenerate insulin-producing cells from its own stem cells. As such, PSCs are being investigated by researchers seeking to treat diabetes. ESCs have been reported to yield viable insulin-producing cells both in vitro and after transplantation to animal models, and human trials involving ESCs are currently being conducted.
PSCs offer potential for creating red blood cells in vitro, offering an alternative to use of donor blood. These in vitro red blood cells created with PSCs remained immature (nucleated) until injected into rodent models, where they matured into functional cells. To increase supply of platelets in a similar manner, researchers are using PSCs to create megakaryocyte populations in vitro as well. Although these solutions could be effective, PSC-derived blood cell production remains costly and inefficient. Creation of PSC-derived T-cells for use in cancer therapies is also of interest, with cancer patients having extremely low levels of these white blood cells.
We’ve been hearing about stem cells for treating disease for so many years. Finally, a 1st rate review to set the record straight, and future directions (perhaps a bit overly optimistic) https://t.co/VrxPZd83uR @NEJM by Helen Blau & @G_Q_Daley pic.twitter.com/2XLY8tCywB
— Eric Topol (@EricTopol) May 1, 2019