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Researchers at Penn State University led by biologist Gong Chen have developed
technology to regenerate functional neurons after brain injury, and in
model systems used for research on Alzheimer's disease.
Red areas are stained nuclei of neuron cells.
Image Credit: Gong Chen lab, Penn State University
Brain repair possible after injury or Alzheimer's disease
Researchers at Penn State University have developed an innovative technology to regenerate functional neurons after brain injury, and also in model systems used for research on Alzheimer's disease. The scientists have used supporting cells of the central nervous system, glial cells, to regenerate healthy, functional neurons, which are critical for transmitting signals in the brain.
Gong Chen, a professor of biology, leader of the research team, and the Verne M. Willaman Chair in Life Sciences at Penn State, calls the method a breakthrough in the long journey toward brain repair.
Chen adds: "This technology may be developed into a new therapeutic treatment for traumatic brain and spinal cord injuries, stroke, Alzheimer's disease, Parkinson's disease, and other neurological disorders."
The research is posted Dec 19, 2013 online in the journal Cell Stem Cell.
When the brain is harmed by injury or disease, neurons often die or degenerate. However, glial cells become more branched and numerous. These "reactive glial cells" initially build a defense system to prevent bacteria and toxins from invading, but this process eventually forms glial scars that limit growth of healthy neurons.
"A brain-injury site is like a car-crash site," Chen explained. "Reactive glial cells are like police vehicles, ambulances, and fire trucks immediately rushing in to help — but these rescue vehicles can cause problems if too many of them get stuck at the scene. The problem with reactive glial cells is that they often stay at the injury site, forming a glial scar and preventing neurons from growing back into the injured areas."
So several years ago, Chen's lab tested new ways to transform glial scar tissue back to normal neural tissue. "There are more reactive glial cells and fewer functional neurons in the injury site," Chen said, "so we hypothesized that we might be able to convert glial cells in the scar into functional neurons at the site of injury in the brain. This research was inspired by the Nobel prize-winning technology of induced pluripotent stem cells (iPSCs) developed in Shinya Yamanaka's group, which showed how to reprogram skin cells into stem cells," Chen recalled.
Chen and his team began by studying how reactive glial cells respond to a specific protein, NeuroD1, which is known to be important in the formation of nerve cells in the hippocampus area of adult brains.
Chen's team hypothesized that expressing NeuroD1 protein into the reactive glial cells at the injury site might help to generate new neurons — just as it does in the hippocampus.
To test this hypothesis, his team infected reactive glial cells with a retrovirus specific for NeuroD1 protein.
Chen: "The retrovirus we used is replication-deficient and thus cannot kill infected cells like other viruses found in the wild. More importantly, a retrovirus can infect only dividing cells such as reactive glial cells, but it does not affect neurons, which makes it ideal for therapeutic use with minimal side effect on normal brain functions."
In a first test, Chen and his team investigated whether reactive glial cells can be converted into functional neurons after injecting NeuroD1 retrovirus into the cortex area of adult mice.
The scientists found that two types of reactive glial cells — star-shaped astroglial cells and NG2 glial cells — were reprogrammed into neurons within one week after being infected with the NeuroD1 retrovirus.
"Interestingly, the reactive astroglial cells were reprogrammed into excitatory neurons, whereas the NG2 cells were reprogrammed into both excitatory and inhibitory neurons, making it possible to achieve an excitation-inhibition balance in the brain after reprogramming,"
Gong Chen, professor of biology, leader of the research team, and the Verne M. Willaman Chair in Life Sciences at Penn State
His lab also performed electrophysiological tests, which demonstrated that the new neurons converted by the NeuroD1 retrovirus could receive neurotransmitter signals from other nerve cells, suggesting that the newly converted neurons had successfully integrated into local circuits.
In a second test, Chen and his team used a mouse model for Alzheimer's disease, and demonstrated that glial cells in the mouse's diseased brain can be converted back into functional neurons. Even in 14-month-old mice with Alzheimer's disease — an age roughly equivalent to 60 years old in humans — injection of the NeuroD1 retrovirus into a mouse cortex induced a large number of newborn neurons to be reprogrammed from reactive glial cells.
"Therefore, the conversion technology that we have demonstrated in the brains of mice potentially may be used to regenerate functional neurons in people with Alzheimer's disease," Chen said.
To ensure that the glial cell-to-neuron conversion method is not limited to rodent animals, Chen and his team further tested the method on cultured human glial cells.
"Within 3 weeks after expression of the NeuroD1 protein, under the microscope we saw human glial cells reinventing themselves: they changed their shape from flat sheet-like glial cells into normal-looking neurons with axon and dendritic branches," Chen observed.
They tested the function of these newly converted human neurons and found they were indeed, capable of both releasing and responding to neurotransmitters.
"Our dream is to develop this in vivo conversion method into a useful therapy to treat people suffering from neural injury or neurological disorders," Chen said. "Our passionate motivation for this research is the idea that an Alzheimer's patient, who for a long time was not able to remember things, could start to have new memories after regenerating new neurons as a result of our in vivo conversion method, and that a stroke victim who could not even move his legs might start to walk again."
• Conversion of reactive glial cells into functional neurons in vivo using NeuroD1
• NeuroD1 induces glia-neuron conversion in an Alzheimer’s disease mouse model
• NeuroD1 also reprograms NG2 cells into glutamatergic and GABAergic neurons
• Human astrocytes can also be converted into glutamatergic neurons with NeuroD1
Loss of neurons after brain injury and in neurodegenerative disease is often accompanied by reactive gliosis and scarring, which are difficult to reverse with existing treatment approaches. Here, we show that reactive glial cells in the cortex of stab-injured or Alzheimer’s disease (AD) model mice can be directly reprogrammed into functional neurons in vivo using retroviral expression of a single neural transcription factor, NeuroD1. Following expression of NeuroD1, astrocytes were reprogrammed into glutamatergic neurons, while NG2 cells were reprogrammed into glutamatergic and GABAergic neurons. Cortical slice recordings revealed both spontaneous and evoked synaptic responses in NeuroD1-converted neurons, suggesting that they integrated into local neural circuits. NeuroD1 expression was also able to reprogram cultured human cortical astrocytes into functional neurons. Our studies therefore suggest that direct reprogramming of reactive glial cells into functional neurons in vivo could provide an alternative approach for repair of injured or diseased brain.
In addition to Chen, other scientists who contributed to this research include Ziyuan Guo, Lei Zhang, Zheng Wu, Yuchen Chen, and Fan Wang, all from Penn State.
The research was funded by Penn State University and the National Institutes of Health.