By David N. Leff
When a woman's egg mates with a man's sperm, their first progeny is the primordial embryonic stem cell. This speck of DNA and protoplasm is master of its universe, that is, totipotent. Its ineffable complexity packages all the instructions and actions that will give rise, after gestation, to a complete human, mouse, chick - or whatever form of life called that first shot.
Step One in this process is for that initial embryonic stem cell to divide into second-generation stem cells, which will in turn order up the cell, tissue and organ types of the newborn - and future adult.
"As a general concept," observed neuroscientist Jonas Frisin, at the Karolinska Institute in Stockholm, Sweden, "it was generally thought that stem cells in adult tissues generate only the type of cells that are expressed in that particular tissue. For example, blood stem cells generate only blood, skin stem cells only skin, and brain cells only brain cells. But in the last few years we have seen indications that this may not be entirely true.
"What we have done now," he related, "is to examine the usual types of cells found in the brain - such as neurons, glial cells, astrocytes and oligodendrocytes. First we asked ourselves, Do they generate only brain cells - perhaps because they're in a nervous-system environment? All their neighbors are neurons or glial cells. Maybe they get instructive cues from these cells.
"Then we asked, What if we placed them in another context, another environment? Could they maybe respond to these unfamiliar signals, and generate completely different types of cells? That speculation and a few other indications suggested that maybe some adult stem cells have other, wider potential than previously assumed. So we decided to test their limits."
Frisin is senior author of the research paper reporting that limit-testing in the current issue of Science, dated June 2, 2000. Its title: "Generalized potential of adult neural stem cells."
"What we did," he told BioWorld Today, "was expose adult brain stem cells to various inductive environments. We supposed that the most rich system for a lot of such cellular neighborhoods must be the very early embryo. Because the starting cell there gives rise to all different cell types, there must be inductive signals to make that happen."
For starters, Frisin and his co-authors chose two neighborhoods - one murine (mice), the other avian (chicks). "First, we exposed adult brain stem cells from mice to embryonic stem cells in vitro," he recounted. "Our rationale was that these cells may secrete some factors that could instruct the adult brain stem cells to generate something else than brain cells. That's exactly what happened. So then we went to an in vivo situation: We injected stem cells from the adult mouse brain into very, very early mouse embryos.
"They distributed throughout the embryos," Frisin continued, "and contributed to the formation of various organs. We could see liver cells, heart muscle cells and, for example, kidney cells developing from these adult mouse brain stem cells. That indicated that these cells have a very broad potential to generate a variety of tissues if just exposed to the correct neighborhood signals."
Chick, Mouse Stem Cells Mix It Up
"We then chose chick embryos," Frisin recounted, "because they are so easily accessible. You just take the fertilized egg, knock a small hole in the shell - and there the embryo is. When we injected murine neural brain stem cells into the amniotic cavity of chicks, they mixed in to chick cells. We then let them develop a few more days, so they were still embryos - we never let any of these chicks hatch - but we saw that they had taken on typical features of different differentiated cell types.
"For example" he continued, "we saw that neural stem cell-derived liver cells produced albumin, which is a liver-specific protein. In kidney we looked for another marker, and so on. The hearts - fully formed mosaic mouse/chick organs - were beating. They were morphologically indistinguishable from their neighbors, and as far as we could tell, looked just like normal embryonic heart cells."
As their all-purpose progenitor neural stem cell, the Karolinska team fixed on ependymal - ventricle-lining - cells in the brain. "We found," Frisin pointed out, "that this cell population has the broad potential we described." (See BioWorld Today, Jan. 12, 1999, p. 1.)
That potential has not escaped the notice of Anders Haegerstrand, president and CEO of Stockholm-based NeuroNova AB, which Frisin co-founded in 1998 to commercialize the stem-cell discoveries.
"If one reflects upon Karolinska's recent findings reported in the Science paper," Haegerstrand told BioWorld Today, "I think it tells you that the ependymal cell is proprietary to NeuroNova - licensed from the Karolinska. It shows that that brain-derived cell is really versatile. If it has the same potential in humans - close to totipotent or whatever - I think it would show that the company's potential to exploit this particular cell type with a human corresponding cell is great. We are sitting on a pot of gold in that sense, in that the cell type is really useful for understanding stem cell differentiation and function."
In Pursuit Of Therapeutic Potential
Looking to the future, Frisin observed, "What we want to do in the line of this research is to test other cell populations. If one thinks of a therapeutic situation way down the road, brain cells are really not acceptable. You don't want to drill another hole in the skull, unless you really need to. Another cell source would be so much better."
Haegerstrand's take: "We've talked to neurosurgeons who would say, 'It's a piece of cake.' But a normal person, say a patient, would say it's a major surgical procedure."
Frisin said, "Although shifting our focus onto non-neural cells, most of our work really focuses on the nervous system. The main project we're working on right now with NeuroNova is to specifically generate the type of dopamine-secreting neurons that are lost in Parkinson's disease."
Harvard neurobiologist Evan Snyder commented on Frisin's paper in Science: "It's pleasing that the neural stem cell conceivably may be trained to differentiate into stem-like cells that give rise to other organs. I suspect that in about two years we will start knowing what is the best way of harnessing stem cell biology for treating neurologic and other diseases."