Scientists estimate that our brain consists of about ten to one hundred billions of nerve cells. In order to fulfill their respective tasks as long as possible, these cells have to constantly control their internal proteins with regard to quality and functionality. Otherwise the proteins might clump together and thereby paralyze or even kill the cells. Once the cell recognizes a defect protein, this is marked for degradation and a kind of a molecular shredder, the so-called proteasome, chops it into pieces that are eventually recycled.
For the first time now, researchers have succeeded in visualizing this process in intact nerve cells, which previously could only be investigated in the test tube. Electron cryo-tomography was essential for obtaining the described images. Hereby, cells are cooled down to minus 170°C in a fraction of a second. In a consecutive step, pictures of the interior of the cells are taken from many different angles, which then are merged computationally into a three-dimensional image.
“First time in intact cells”
In the current study, the use of specific technical innovations allowed the researchers to achieve a unprecedented imaging quality, enabling them to distinguish single proteasomes within the cell. “For the first time it is possible to qualitatively and quantitatively describe this important enzyme complex in intact cells”, Asano classifies the results. In the following experiments, the scientists focused on the activity of the proteasomes. For the interpretation of the single particles it is important to know that there are cap-like structures, the so-called regulatory particles, attached to the ends of proteasomes (see picture). They bind proteins that are designated to be degraded and thereby change their shape. The scientists were able to distinguish these states and consequently could deduce how many of the proteasomes were actively degrading proteins.
The flip of a single molecular switch helps create the mature neuronal connections that allow the brain to bridge the gap between
Image credit: www.telegraph.co.uk
adolescent impressionability and adult stability. Now Yale School of Medicine researchers have reversed the process, recreating a youthful brain that facilitated both learning and healing in the adult mouse.
Scientists have long known that the young and old brains are very different. Adolescent brains are more malleable or plastic, which allows them to learn languages more quickly than adults and speeds recovery from brain injuries. The comparative rigidity of the adult brain results in part from the function of a single gene that slows the rapid change in synaptic connections between neurons.
By monitoring the synapses in living mice over weeks and months, Yale researchers have identified the key genetic switch for brain maturation a study released March 6 in the journal Neuron. The Nogo Receptor 1 gene is required to suppress high levels of plasticity in the adolescent brain and create the relatively quiescent levels of plasticity in adulthood. In mice without this gene, juvenile levels of brain plasticity persist throughout adulthood. When researchers blocked the function of this gene in old mice, they reset the old brain to adolescent levels of plasticity.
“These are the molecules the brain needs for the transition from adolescence to adulthood,” said Stephen Strittmatter. Vincent Coates Professor of Neurology, Professor of Neurobiology and senior author of the paper. “It suggests we can turn back the clock in the adult brain and recover from trauma the way kids recover.”
Rehabilitation after brain injuries like strokes requires that patients re-learn tasks such as moving a hand. Researchers found that adult mice lacking Nogo Receptor recovered from injury as quickly as adolescent mice and mastered new, complex motor tasks more quickly than adults with the receptor.
“This raises the potential that manipulating Nogo Receptor in humans might accelerate and magnify rehabilitation after brain injuries like strokes,” said Feras Akbik, Yale doctoral student who is first author of the study.
Researchers also showed that Nogo Receptor slows loss of memories. Mice without Nogo receptor lost stressful memories more quickly, suggesting that manipulating the receptor could help treat post-traumatic stress disorder.
“We know a lot about the early development of the brain,” Strittmatter said, “But we know amazingly little about what happens in the brain during late adolescence.”
A new finding by Harvard stem cell biologists turns one of the basics of neurobiology on its head, demonstrating that it is possible to turn one type of already differentiated neuron into another within the brain.
The discovery by Paola Arlotta and Caroline Rouaux “tells you that maybe the brain is not as immutable as we always thought, because at least during an early window of time one can reprogram the identity of one neuronal class into another,” said Arlotta, an Associate Professor in Harvard’sDepartment of Stem Cell and Regenerative Biology(SCRB).
The principle of direct lineage reprogramming of differentiated cells within the body was first proven by SCRB co-chair and Harvard Stem Cell Institute(HSCI) co-director Doug Melton and colleagues five years ago, when they reprogrammed exocrine pancreatic cells directly into insulin producing beta cells.
This is an image of a cerebral cortex slice. The image is credited to the US Government.
Arlotta and Rouaux now have proven that neurons too can change their mind. The work is being published on-line by the journal Nature Cell Biology.
In their experiments, Arlotta targeted callosal projection neurons, which connect the two hemispheres of the brain, and turned them into neurons similar to corticospinal motor neurons, one of two populations of neurons destroyed in Amyotrophic Lateral Sclerosis (ALS), also known as Lou Gehrig’s disease. To achieve such reprogramming of neuronal identity, the researchers used a transcription factor called Fezf2, which long has been known for playing a central role in the development of corticospinal neurons in the embryo.
What makes the finding even more significant is that the work was done in the brains of living mice, rather than in collections of cells in laboratory dishes. The mice were young, so researchers still do not know if neuronal reprogramming will be possible in older laboratory animals and humans. If it is possible, this has enormous implications for the treatment of neurodegenerative diseases.
“Neurodegenerative diseases typically effect a specific population of neurons, leaving many others untouched. For example, in ALS it is corticospinal motor neurons in the brain and motor neurons in the spinal cord, among the many neurons of the nervous system, that selectively die,” Arlotta said. “What if one could take neurons that are spared in a given disease and turn them directly into the neurons that die off? In ALS, if you could generate even a small percentage of corticospinal motor neurons, it would likely be sufficient to recover basic functioning,” she said.
The experiments that led to the new finding began five years ago, when “we wondered: in nature you never seen a neuron change identity; are we just not seeing it, or is this the reality? Can we take one type of neuron and turn it into another?” Arlotta and Rouaux asked themselves.
Over the course of the five years, the researchers analyzed “thousands and thousands of neurons, looking for many molecular markers as well as new connectivity that would indicate that reprogramming was occurring,” Arlotta said. “We could have had this two years ago, but while this was a conceptually very simple set of experiments, it was technically difficult. The work was meant to test important dogmas on the irreversible nature of neurons in vivo. We had to prove, without a shadow of a doubt, that this was happening.”
The work in Arlotta’s lab is focused on the cerebral cortex, but “it opens the door to reprogramming in other areas of the central nervous system,” she said.
Arlotta, an HSCI principal faculty member, is now working with colleague Takao Hensch, of Harvard’sDepartment of Molecular and Cellular Biology, to explicate the physiology of the reprogrammed neurons, and learn how they communicate within pre-existing neuronal networks.
“My hope is that this will facilitate work in a new field of neurobiology that explores the boundaries and power of neuronal reprogramming to re-engineer circuits relevant to disease,” said Paola Arlotta.
Notes about this neurobiology article
The work was financed by a seed grant from HSCI, and by support from the National Institutes of Health and the Spastic Paraplegia Foundation.
Written by: BD Colen – Harvard University Contact: Paola Arlotta – Harvard University Source:Harvard University press release Image Source: The image of a cerebral cortex slice is credited to the US Government. The image is available via Wikimedia Commons and is licensed as Public Domain. Please feel free to share or use. Original Research:Abstract for “Direct lineage reprogramming of post-mitotic callosal neurons into corticofugal neurons in vivo” by Caroline Rouaux and Paola Arlotta in Nature Cell Biology. Published online January 20 2013 doi:10.1038/ncb2660