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NEUROSCIENCE Teaching an Old Dogma New TricksHealing the Brain from The Inside Out? Life is all about making choices, and in cell life, the choices come early. As cells differentiate to take up their life work in the body, there are few second chances—for most brain nerve cells in mammals, there are supposed to be none. Now comes the revelation from an HMS and Children's Hospital research group in the June 22 Nature that our neurons may be induced into having second thoughts about healing themselves.
In the lab of Jeffrey Macklis (right), graduate student Sanjay Magavi switched on apoptosis in neocortical neurons and then watched endogenous precursor cells take up the work of their vanished predecessors. The paper from Jeffrey Macklis, HMS associate professor of neurology (neuroscience) at Children's Hospital, graduate student Sanjay Magavi, postdoctoral fellow Blair Leavitt, and colleagues adds to other recent findings that fly in the face of a century of neuroscience dogma that in mammals the brain and particularly the cerebral cortex is unhealing. The Macklis group was able to induce stem cells deep in the cerebral cortex of adult mice to replace damaged neurons. The new neurons grew from already present immature precursor cells into fully formed, connected, and mature replacements. These homegrown neurons demonstrate that the brain can heal itself from the inside out, without transplantation. This breakthrough in fundamental neural cell biology is a long way from clinical application, but Macklis says that if the mechanisms at work here can be understood and controlled, it may open a new avenue someday for treatment of degenerative brain diseases and central nervous system injuries. Brain DesignThe evolutionary choice for mammals was supposed to be between a brain that was fixable and a brain that was too complex to tinker with after it was formed, even from the inside. "Somewhere during evolution," Macklis says, "our brain, unlike the brains of other lower vertebrates, decided it would no longer do self-repair. The assumption has been that because we as mammals build a very complex brain, we don't want to mess around with it later."But what if neurons could go back in cellular time to when the nervous system was assembling in the developing embryo? Neural precursor cells were plastic then as they changed into differentiated neurons. Extra neurons could be removed and new ones inserted to take up new tasks. Then, after birth, this option for plasticity had to be closed. The genes that controlled this pathway were switched off, supposedly forever. Yet this view has been badly shaken of late. The dogma held that neural precursor cells should not exist in adults, and yet recent research has uncovered them in the forebrains of mice. Other work has shown that precursors can form new neurons in two limited areas of the brain. Prevailing theory said that diseased or damaged neurons in the cerebral cortex could not be replaced, and yet Macklis's lab has had success with injecting lab-grown neural precursors into the cortex of mice and watching them replace dying cells. Even while working with transplantation, the investigators proceded along another route, pursuing what Macklis calls "the futuristic idea that one might be able to activate neuronal repopulation and repair from the inside out."
The trick was to reopen the genetically controlled pathway that once allowed nerve cells to change. Macklis and his colleagues reasoned that, even suppressed, the instructions for the pathway must still be encoded in DNA. Could there be a set of signals that would reopen the forgotten path? Glimpsing Nerve GrowthMacklis and company found the signals by the equivalent of entering a dark basement and throwing every switch on the circuit board. The "lights on" signal was apoptosis, or cellular suicide, triggered by an elegant piece of biophysical targeting (see Focus, Oct. 17, 1997) and a very bright light. Magavi zeroed in on neurons connecting to the thalamus deep in layer VI of the cortex by injecting their axonal terminal fields with a light-sensitive dye. Two weeks later, the dye that had been taken up by the neurons into lysosomes was chemically activated by a near-infrared light, shining down through the tissue layers.The dye doesn't "kill" the neuron in the way poisoning or blocking its metabolic pathways would, Macklis says. The targeted neuron does not lyse, dumping its contents into the neocortex and touching off inflammation. Instead, the dye inflicts a "subcritical injury" causing the cell to turn on apoptosis. "The neuron is very neat and tidy. It's like a cat in a litter box," Macklis says. "When it decides to kill itself, it cuts itself into little apoptotic bodies, packets that are small enough to be eaten by other cells. Phagocytes come through and engulf them." In the meantime, the investigators had chemically labeled precursor cells already on the scene in the cortex to see if they would multiply, turn into the right kind of neuron, and take the place of the cellular suicides. Contrary to neurological common knowledge, the precursors did just that. The endogenous stem cells underwent mitosis, as evidenced by the appearance of labeled BrdU, a marker of DNA replication. Then the investigators spotted Doublecortin, a protein expressed only by migrating neurons; Hu, an early neuronal marker; and NeuN, a marker expressed only by mature neurons, indicating the precursors were progressively developing into mature neurons. Confirmation that the replacements were projecting their axons to make connections with other neurons came from anatomical labeling with dyes. But did the replacement neurons actually function? Macklis says that it's very difficult to know if a mouse's neurons are working correctly just from observing its behavior, so his lab is pursuing parallel studies of mice with regenerated neurons derived from transplanted precursors. The goal is to spot clear biochemical signals that the new neurons and their neighbors are, indeed, working together. Still, Macklis warns that his lab's demonstration of neurons healing from within is only a first step. "Not for a moment would any of us suggest that to repair the brain we want to go around inducing cell death. Rather, it's that we want to use this as an experimental tool to dissect out what the controls are. Our approach of targeted apoptosis has given us a crude external lever over a whole program of genes that we're investigating now. What we've done in this study is to turn on the whole program, all at once. What we'd really like to do is to define what the sequence and combination of the individual molecules is. "Now comes the hard work," says Macklis. —John Fleischman |
