Neurons from a typical mouse (left) are longer and fuller than neurons from a mouse lacking SNX27 (right). (Credit: Image courtesy of Sanford-Burnham Medical Research Institute)
from Science Daily:
Researchers have discovered that the extra chromosome inherited in Down syndrome impairs learning and memory because it leads to low levels of SNX27 protein in the brain.
What is it about the extra chromosome inherited in Down syndrome -- chromosome 21 -- that alters brain and body development? Researchers at Sanford-Burnham Medical Research Institute (Sanford-Burnham) have new evidence that points to a protein called sorting nexin 27, or SNX27. SNX27 production is inhibited by a molecule encoded on chromosome 21. The study, published March 24 in Nature Medicine, shows that SNX27 is reduced in human Down syndrome brains. The extra copy of chromosome 21 means a person with Down syndrome produces less SNX27 protein, which in turn disrupts brain function. What's more, the researchers showed that restoring SNX27 in Down syndrome mice improves cognitive function and behavior.
"In the brain, SNX27 keeps certain receptors on the cell surface -- receptors that are necessary for neurons to fire properly," said Huaxi Xu, Ph.D., professor in Sanford-Burnham's Del E. Webb Neuroscience, Aging and Stem Cell Research Center and senior author of the study. "So, in Down syndrome, we believe lack of SNX27 is at least partly to blame for developmental and cognitive defects."
SNX27's role in brain function
Xu and colleagues started out working with mice that lack one copy of the snx27 gene. They noticed that the mice were mostly normal, but showed some significant defects in learning and memory. So the team dug deeper to determine why SNX27 would have that effect. They found that SNX27 helps keep glutamate receptors on the cell surface in neurons. Neurons need glutamate receptors in order to function correctly. With less SNX27, these mice had fewer active glutamate receptors and thus impaired learning and memory.
