Identifying the factors that result in the timely, massive cell suicide is important to understanding the developmental puzzle, the researchers say of the work featured on the cover of the Aug. 4 issue of the Journal of Cell Biology.
They say it also gives clues about cell death - and the brain's possible recovery - in devastating diseases such as Alzheimer's, Parkinson's and stroke.
MCG's Erhard Bieberich and UGA's Brian G. Condie have found that the lipid ceramide and the protein PAR-4 - each already implicated for playing a role in cell death - become deadly partners inside a dividing stem cell in the developing mouse brain.
"If PAR-4 is there and ceramide is high, the cell is lost, doomed to die," says Dr. Bieberich, biochemist at the Medical College of Georgia. "You can eliminate one of them, you can knock down the expression of PAR-4 or ceramide and the other stays up but the cell doesn't die. But if both signals are together up-regulated, then the cell is destined to die."
At a certain point in cell division, just before neurons begin forming, there is massive production of proteins and up-regulation of lipids. During that phase, decisions are made about which daughter cells get what composition of lipids and proteins, decisions that affect the cells' future function.
Typically at this point in division, the two daughter cells birthed from a single stem cell will have the same makeup and the same ultimate purpose.
Yet in a subpopulation of the stem cells involved in brain development, the scientists have documented increasing levels of ceramide in both resulting daughter cells while its death partner, PAR-4, gets handed off to only half the cells.
Cells destined to survive, and likely further divide and differentiate, are handed instead a protein called nestin. "Nestin is a marker for a particular stage of neuronal development," says Dr. Bieberich. "Nestin-bearing cells will develop into neural cells such as our neurons or astrocytes or other cells. So it makes sense that the cells that inherit nestin, but not PAR-4, will survive and develop into normal neuronal cells whereas the other ones will die."
It also makes sense that the lethal coupling that signals cell suicide, or apoptosis, comes at a point where the doomed cells seem to have lost their potential usefulness and where their continued survival would result in a malformed brain.
"During normal development in the central nervous system there is a great deal of cell death that occurs that seems to be required to create the final shape and structure of the brain," says Dr. Condie, developmental neurobiologist at the University of Georgia and MCG. "In cases where that process has been interfered with, you end up with this excess of cells that leads to a malformation of the developing brain.
"One of the ideas behind why there is an excess of cells generated during development is that it may be a mechanism for compensating for environmental stresses or other types of stresses that an embryo may encounter during development," says Dr. Condie. "So you actually generate an excess of the cells you need and then prune those cells back to an appropriate number and location for the brain to develop in a normal fashion." It's a typical characteristic of embryonic development for certain cells to survive and others to die, he says.
"During embryonic development, we would like to know how stem cell death is regulated because we know it needs to be regulated," says Dr. Bieberich. "You don't want the whole brain dying or overgrowing. You have to find a balance. How is that balanced maintained? What are the secrets for that?
"We have designed experiments showing that these two signals are necessary to make stem cells die, but you are talking about a whole signaling cascade that starts out with ceramide and PAR-4 and then there are a lot of unknown steps until we end up with the actual death of a cell," says Dr. Bieberich.
The MCG researcher recently received a grant from the National Institutes of Health so he and Dr. Condie can explore these unknowns such as how the expression of PAR-4 and ceramide is regulated, what accounts for the asymmetrical distribution of PAR-4 and just how the deadly duo interact.
But the two are excited about what they have found already. "If we don't know the signals, we don't know where to begin," says Dr. Bieberich.
They also are intrigued by where the work may lead, including helping minimize cell death that occurs when stem cells in the adult brain begin to once again divide in response to a stroke, as an example.
"We all know that even in adulthood, we have stem cells in the brain and they may be able to repair damaged areas," says Dr. Bieberich. "But if the same cell death mechanisms are still active, there will not be an increase in the number of stem cells because always one cell will die and one will survive. Maybe we can control this and increase the number of endogenous stem cells.
"Also during the neurodegeneration that occurs in diseases such as Alzheimer's and Parkinson's, we have a lot of cell death going on and we would like to know what signals are involved that make those brain cells die. They may be very similar or even exactly the same as the ones we investigate with our embryonic mouse stem cells."
Study co-authors include Scott Noggle, an MCG graduate student working with Dr. Condie at UGA; Sarah MacKinnon, a former participant in MCG's summer research programs for undergraduates who is now a graduate student at the University of Virginia; and Dr. Jeane Silva, Dr. Bieberich's research coordinator.