Unclouding the human brain
Biomedical engineering professor Stefan Zappe and his research team are developing a neural stem cell therapy to treat a variety of genetic disorders of the central nervous system, including Hunter syndrome.
Stem cells are primal cells that retain the ability to renew themselves and multiply through cell division. These cells can also differentiate into a diverse range of cell types.
Zappe said, “It’s like a wild card,” speaking of neural stem cells.
Adult stem cells are found in adult tissue and act as a bodily repair mechanism because they can be matured into specialized cells when these cells become injured.
According to a Carnegie Mellon press release, Zappe and his collaborator Raymond Sekula, a neurosurgeon at Allegheny General Hospital, are working with adult neural stem cells because they can be harvested from a patient’s brain.
Neural stem cells can then be manipulated to multiply and differentiate into any cell type of the brain.
Hunter syndrome is a disease in which patients lack the enzyme iduronate-2-sulfatase (IDS), an essential protein needed to help cells break down toxic waste. According to a Carnegie Mellon press release, one in 130,000 boys are born with Hunter syndrome.
Sasha Bakhru, a graduate student in Zappe’s lab, stated in an e-mail, “The present therapy for Hunter syndrome involves delivery of an active, recombinant version of the lacking enzyme, IDS, through the blood. This works to relieve the problem in almost all tissues of the body, but not the brain.”
In the past, a major barrier to treating brain disorders resulting from lack of expression of a particular protein has been the blood-brain barrier. The blood-brain barrier is the division between brain tissue and blood vessels that regulates the pathway of drugs in the brain.
To overcome this barrier, Zappe and Bakhru have developed cell-instructive microcapsules that contain genetically modified neural stem cells. These microcapsules control proliferation and differentiation of neural stem cells.
“Our microcapsules containing stem cells are implanted on the other side of the blood-brain barrier, providing the cells access to the affected tissue of the brain, into which they may migrate and integrate,” Bakhru stated.
Zappe said that neural stem cells tend not to attach to 3-D surfaces, but instead, they “float around” as “neurospheres” (clusters of neural cells). Cells at the center of these clusters do not receive nutrients, and they die or spontaneously differentiate.
“The idea would be to adapt the microcapsule culture to human neural stem cells,” he said.
The microcapsules are made out of modified collagen held together by positively charged collagen and a negatively charged polysaccharide, alginate.
The neural stem cells are suspended in microdroplets of modified collagen solution, which make up the inside of each microcapsule.
In addition, the microcapsules contain growth factors and other proteins, all of which are located within the collagen matrix. Growth factors prevent differentiation of the stem cells into other cell types.
Zappe said, “Our idea is to create new stem cells that are implanted into the brain. They are engineered so that they can produce the missing enzyme [in Hunter syndrome patients].”
The neural stem cell therapy begins by harvesting a small number of stem cells from a brain tissue sample.
Zappe said, “You start with a single stem cell...and you create many many stem cells.”
The adult neural stem cells are then microencapsulated for expansion in the cell culture.
Bakhru stated, “These cells are then harvested from microcapsules and genetically modified to produce [the enzyme] alginase (for delivery) and IDS (to treat the underlying problem) on demand, in response to administration of two chemical inducers.”
“The genetically engineered cells are then re-encapsulated for expansion and implantation into the brain,” Bakhru stated.
The microcapsules are then implanted into the brain.
Within the brain, the microcapsules protect the stem cells from exposure to inflamed tissue after surgery, which would otherwise cause many of the implanted stem cells to differentiate into scar tissue, or die.
After inflammation has passed, patients will be given a chemical inducer in pill form. This chemical will stimulate the stem cells and cause them to produce and secrete an enzyme called alginase.
The alginase selectively breaks down the alginate in the microcapsule wall, thereby freeing the cells. The cells may then migrate into the surrounding tissue.
Upon administration of a second inducer, the cells secrete the needed enzyme IDS to be used by neighboring neural cells.
So far, researchers have shown that this drug therapy system is effective in cell culture.
The next step is to show successful therapy in an animal model.
“We have made progress in vitro, but now will focus on demonstration of system efficacy in vivo in a mouse model of Hunter syndrome,” Bakhru stated.
While the current microcapsule delivery system is being developed to treat Hunter Syndrome, stem cells can be genetically modified to produce other proteins needed to treat a variety of genetic disorders of the central nervous system.