Microcapsules Developed to Direct Neural Stem Cells
By Hanadie Yousef, The Tartan, November 12, 2007
In order to treat a variety of genetic disorders of the central nervous system, including Hunter syndrome, biomedical engineering professor Stefan Zappe and his research team are developing a neural stem cell therapy.
Stem cells can differentiate into a diverse range of cell types. These primal cells can multiply through cell division, and retain the ability to renew themselves.
Zappe said, “It’s like a wild card,” speaking of neural stem cells.
Since they can be matured into specialized cells when existing cells in the body become injured, adult stem cells act as a bodily repair mechanism and are found in adult body tissue.
A Carnegie Mellon press release stated, "since they can be harvested from a patient’s brain, adult neural stem cells were chosen by Zappe and his collaborator Raymond Sekula, who is a neurosurgeon at Allegheny General Hospital."
Any cell type in the brain can then be developed by manipulating and differentiating the neural stem cells.
An essential protein needed to help cells break down toxic waste is an enzyme called iduronate-2-sulfatase (IDS), patients with Hunter's syndrome lack this enzyme. One in 130,000 boys are born with Hunter syndrome according to a Carnegie Mellon press release.
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.”
The blood-brain barrier has presented a major obstacle in the past in regards to treating brain disorders resulting from lack of expression of a particular protein.
Regulating the pathway of drugs in the brain, the division between the blood vessels and the brain tissue is called the blood-brain barrier.
Containing genetically modified neural stem cells, cell-instructive microcapsules were developed by Zappe and Bakhru to overcome this barrier. The differentiation and proliferation of neural stem cells is controlled by these microcapsules.
“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.
Neural stem cells tend to “float around” as “neurospheres” (clusters of neural cells) as opposed to attaching themselves to 3-D surfaces said Zappe. 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.
Making up the inside of each microcapsule, are the neural stem cells which are suspended in microdroplets of a modified collagen solution.
Also located within microcapsules within the collagen matrix are other proteins and growth factors. The growth factors prevent the stem cells from differentiating 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].”
Harvesting a small number of stem cells from a brain tissue sample is the first step towards neural stem cell therapy.
Zappe said, “You start with a single stem cell...and you create many many stem cells.”
Finally, for expansion in the cell culture, the microencapsulation process seals the adult neural stem cells.
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.
Normally many of the implanted stem cells would differentiate into scar tissue, or die, but the microcapsules protect the stem cells within the brain from exposure to inflamed tissue after surgery.
Patients will be given a chemical inducer in pill form after inflammation has passed. An enzyme called alginase is produced and secreted by the stem cells due to chemical stimulation.
The cells are freed when the alginase selectively breaks down the alginate in the microcapsule wall. The cells may then migrate into the surrounding tissue.
The cells secrete the needed enzyme IDS to be used by neighboring neural cells upon administration of a second inducer. This drug system is effective in cell culture as researchers have proven.
Animal models will be the next step to show the success of this therapy.
“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.
Stem cells can be genetically modified to produce other proteins needed to treat a variety of genetic disorders of the central nervous system even though the current microcapsule delivery system is being developed to treat Hunter Syndrome.