By David N. Leff
About one in 10 human infants is born into this world with one or more anatomical anomalies ¿ minor or major. These imperfections range from a relatively harmless sixth finger or toe to a cleft lip to cardiac malformations to spina bifida. That failure of vertebrae to close over the growing neural tube during the first two months of gestation results in an open, exposed neural tube ¿ with horrendous complications.
Neural tube defect (NTD) is a multigenic, partially inherited disorder, largely caused by an environmental component ¿ inadequate nutrition early in pregnancy. That¿s why expectant mothers at genetic risk are prescribed folic acid, vitamin B6. That risk, in North American whites, plays out in one to two NTDs per thousand live births. Folic acid from one month before conception to three months after cuts their risk by 50 percent to 70 percent.
Unaffected parents of a first affected offspring carry many harmful genes that raise the odds of a second NTD baby to 3 percent. Prenatal diagnosis by amniocentesis commonly detects NTD in the unborn child.
¿The neural tube is actually the rudimental stage of the brain and spinal cord,¿ observed neurobiologist Su-Chun Zhang, an anatomy professor at the University of Wisconsin in Madison. ¿Our latest reported research,¿ he added, ¿found a neural tube-like structure for the first time in human embryonic stem cell culture.¿
Zhang is lead author of a paper in the December 2001 issue of Nature Biotechnology titled: ¿In vitro differentiation of transplantable neural precursors from human embryonic stem cells.¿ Its senior author is developmental biologist James Thomson, at Wisconsin, who first isolated human embryonic stem cells (hESCs) in 1998. (See BioWorld Today, Nov. 6, 1998, and Sept. 5, 2001, both, p. 1.)
A closely parallel research article follows Thomson¿s paper, back-to-back, titled: ¿Neural progenitors from human embryonic stem cells.¿ Its authors are at Hadassah University Hospital in Jerusalem.
¿Our main finding in this paper,¿ Zhang told BioWorld Today, ¿is that human embryonic stem cells can be used as a source to derive neural precursor cells ¿ brain cells. These further differentiate into mature neurons and glia, both in culture and transplanted to animals¿ brains.
¿We have designed a culture system,¿ he continued, ¿to study human brain development, particularly neural cell specification. In other words, how these neurons in the brain are first specified from the totipotent stem cells. This paper also confirms that hESCs are indeed multipotential: They can generate many different neural cell types.¿
Steps From Stem Cells To Brain Cells
¿It builds on our previous paper in showing that we can generate large numbers of these neural precursor cells,¿ Zhang went on. ¿We get over 70 percent of the stem cells to become neural precursors by a very simple technique. We demonstrate that these cells ¿ seeded into the intact brains of baby mice ¿ do indeed differentiate into neurons and glia after transplantation.¿
Zhang recounted the step-wise procedure by which he and his co-authors accomplished these advances:
¿Knowing the time course of human brain development, we detached the stem cell from the mouse fetal layer cells, and aggregated them to make the so-called embryoid body, which induces the first differentiation process. But then we stopped in the middle. We didn¿t want them to randomly differentiate toward everything. We stopped them and cultured this embryoid body through a special growth-factor cocktail, to stimulate neural cell development. We cultured them in this system for seven to 10 days, and most of the cells developed into neural precursor cells. Most strikingly, in that culture system we found a structure like a neural tube.
¿In the next step, because not 100 percent of the cells are neural cells, we separated them out, leaving the non-neural cells behind. It then remained to confirm that the cells were pure, truly neural cells. When we cultured them further in neural differentiation medium, they were developing into brain neurons and glia.
¿Then, to see whether they do the same work in the brain, we transplanted these neural precursor cells into new mouse brains, and verified that the transplanted human cells become in fact neurons and glia.
¿We also paid attention,¿ Zhang observed, ¿to whether they potentially generated tumors, because previous work had shown that transplanted ES cells directly can become tumorous. Because our cells were relatively pure, it was not surprising that we did not find any such teratoma formation.¿
Turning to the clinical potential of their work, Zhang said: ¿The paper we presented in Nature Biotechnology is the first step. We have not yet done anything related to the clinic. That¿s our current ongoing project ¿ to test whether these stem-cell-derived neurons and glia are functional. So we are preparing to transplant these neural cells into animals with neurological injury or diseases. At this point, we are mainly using rodents. However, we are making a few monkey models of Parkinson¿s disease [PD].
¿Currently, we are planning to test both monkey and human embryonic stem cell-generated neural cells in rat models of PD. Then if that works we will put them into monkey models of PD. Moreover, we are trying to get the neural precursor cells to make dopamine-producing neurons, and transplant those cells into the PD models, which lack dopamine.¿
Biotech Firms Sense A Good Thing
Zhang¿s laboratory in the university¿s Waisman Center focuses on neurodegenerative disorders. ¿We use stem cells as a potential source for cell replacement, and as a model system for the mechanism of neural development.
¿I get a lot of calls from biotechnology companies,¿ he observed. ¿They are particularly interested in this model system we developed, and ask what is its status. A few companies are working on it. And they know that will lead to a lot of new discoveries. Our system can look at early human development, which would otherwise not be possible, because we cannot get early human embryos to study that aspect. Also, we try to combine this stem cell approach with genomics ¿ the molecular and cellular aspects of the processes in the early phase of human brain development.
¿I think a lot of biotech companies are interested simply to generate cells for tissue transplantation purposes. That aspect intrigues a lot of people in the biotech community,¿ Zhang concluded. ¿They are pretty keen about it.¿