Research background: Stem cells as a potential treatment for diabetic skin wounds
Diabetes is on the rise in the United States and worldwide. Chronically high blood sugar in patients with diabetes damages blood vessels and nerves and leads to a range of complications.
Impaired wound healing is a common complication of diabetes. Skin injuries in patients with diabetes are more susceptible to infections and can lead to chronic wounds (ulcers) and amputations. Diabetic foot and leg ulcers are the primary cause for hospitalization among patients with diabetes. The annual cost of treating lower extremity diabetic ulcers exceeds $1.5 billion in the United States.
Despite the impact of chronic wounds on patients with diabetes, effective therapies are lacking.
Stem cells are a potential treatment for diabetic skin wounds. Stem cells migrate to a wound area and secrete a cocktail of growth factors that promote wound healing. The precise factors and processes that attract skin stem cells to wounded areas are not well characterized.
Stem cell factor (SCF) — a cell signaling protein — and its receptor are expressed in many types of skin cells, and the SCF signaling pathway has been shown to be important for wound healing.
Investigators Ken Liechty, MD, former Director of Pediatric Surgery Basic and Translational Research at Children's Hospital Colorado, and Carlos Zgheib, PhD, postdoctoral fellow and instructor of surgery at the University of Colorado School of Medicine, previously demonstrated that certain stem cells (mesenchymal stem cells) improved the healing of diabetic wounds. In cell culture, they showed that SCF can induce the migration of these stem cells. However, the role of SCF in skin stem cell migration in diabetic skin wounds was unknown. The investigators have now explored the action of SCF in recruiting stem cells and healing diabetic skin wounds.
Research methods: Mouse model studies and role of stem cell factor in wound healing
The investigators studied wound healing in a widely used mouse model of diabetes. The mice have a genetic mutation that results in morbid obesity, chronically high blood sugar levels and death of insulin-producing pancreatic beta cells, as well as delayed wound healing.
The diabetic mice also had small wounds (8 millimeters in diameter) on their backs. The wounds were injected with a virus expressing either SCF or a green fluorescent protein (GFP) as a control. The researchers evaluated wound closure zero, five and seven days following wounding. They also analyzed gene expression at the wound site for: SCF, hypoxia-inducible factor (HIF)-1alpha and vascular endothelial growth factor (VEGF).
To study skin stem cell migration, the investigators developed a unique mouse model. They injected a gene-transfer virus expressing a red fluorescent protein (RFP) into the amniotic fluid of pregnant mice at the eighth day of gestation. The RFP gene incorporated into the DNA of dividing fetal cells, including skin stem cells, which were visualized using fluorescence microscopy.
The investigators examined wound healing and the role of SCF in wound healing in the RFP-labeled mouse model.
Billion annual cost to treat lower extremity diabetic ulcers in U.S.
Have been used to treat diabetic wound impairment
Research results: Stem cell factor treatment improves healing of impaired diabetic wounds
In the diabetic mouse model, SCF:
- Reduced the size of wounds five and seven days following treatment compared with GFP (control)
- Decreased the size of the gap between epithelial cells in diabetic wounds at day seven
- Increased the expression of genes involved in wound repair — SCF, HIF-1alpha and VEGF
In the unique mouse model of RFP-labeled cells:
- RFP-labeled cells were present in a mosaic pattern in a skin sample.
- RFP-labeled cells were present at wound sites on the day of wound generation (day 0).
- SCF treatment significantly increased the migration of RFP-labeled cells to the wound at day seven compared with GFP treatment (control).
- In day-seven wounds, RFP-labeled cells were distinct from a population of characterized hair follicle stem cells that express the protein Keratin 15 (K15).
Research conclusions: Mouse model of diabetes showed wound healing
SCF significantly improved wound healing in a mouse model of diabetes. Using a unique mouse model that they generated, investigators showed that SCF enhanced the migration of a population of fluorescently labeled skin cells to wounded areas.
The labeled skin cells were distinct from stem cells known to reside in the hair follicle, suggesting that the researchers have identified a novel population of skin stem cells involved in wound healing.
Hypoxia and the expression of HIF-1alpha play an important role in tissue repair and wound healing. VEGF is key to building new blood vessels for wound healing. The researchers found that both HIF-1alpha and VEGF increased in response to SCF in diabetic wounds. The findings suggest that SCF not only may recruit skin stem cells directly, but create a suitable hypoxic environment by promoting new blood vessel formation.
This study highlights the potential therapeutic application of SCF to promote and enhance diabetic wound healing.