Children and adolescents with autism have a surplus of synapses in the brain, and this excess is due to a slowdown in a normal brain “pruning” process during development, according to a study by neuroscientists at Columbia University Medical Center (CUMC).
Because synapses are the points where neurons connect and communicate with each other, the excessive synapses may have profound effects on how the brain functions. The study was published in the online issue of the journal Neuron.
A drug that restores normal synaptic pruning can improve autistic-like behaviors in mice, the researchers found, even when the drug is given after the behaviors have appeared.
“This is an important finding that could lead to a novel and much-needed therapeutic strategy for autism,” said Jeffrey Lieberman, MD, Lawrence C. Kolb Professor and Chair of Psychiatry at CUMC and director of the New York State Psychiatric Institute, who was not involved in the study.
Although the drug, rapamycin, has side effects that may preclude its use in people with autism, “the fact that we can see changes in behavior suggests that autism may still be treatable after a child is diagnosed, if we can find a better drug,” said the study’s senior investigator, David Sulzer, PhD, professor of neurobiology in the Departments of Psychiatry, Neurology, and Pharmacology at CUMC.
During normal brain development, a burst of synapse formation occurs in infancy, particularly in the cortex, a region involved in autistic behaviours; pruning eliminates about half of these cortical synapses by late adolescence. Synapses are known to be affected by many genes linked to autism, and some researchers have hypothesized that people with autism may have more synapses.
To test this hypothesis, coauthor Guomei Tang, PhD, assistant professor of neurology at CUMC, examined brains from children with autism who had died from other causes. Thirteen brains came from children ages 2 to 9, and thirteen brains came from children ages 13 to 20. Twenty-two brains from children without autism were also examined for comparison.
Dr. Tang measured synapse density in a small section of tissue in each brain by counting the number of tiny spines that branch from these cortical neurons; each spine connects with another neuron via a synapse.
By late childhood, she found, spine density had dropped by about half in the control brains, but by only 16 percent in the brains from autism patients.
“It’s the first time that anyone has looked for, and seen, a lack of pruning during development of children with autism,” Dr. Sulzer said, “although lower numbers of synapses in some brain areas have been detected in brains from older patients and in mice with autistic-like behaviors.”
Clues to what caused the pruning defect were also found in the patients’ brains; the autistic children’s brain cells were filled with old and damaged parts and were very deficient in a degradation pathway known as “autophagy.” Cells use autophagy (a term from the Greek for self-eating) to degrade their own components.
Autophagy is regulated by a protein called mTOR. Children with autism have overactive mTOR, which prevents autophagy from ‘cleaning house’ and getting rid of the damaged synapses.
“What’s remarkable about the findings,” said Dr. Sulzer, “is that hundreds of genes have been linked to autism, but almost all of our human subjects had overactive mTOR and decreased autophagy, and all appear to have a lack of normal synaptic pruning. This says that many, perhaps the majority, of genes may converge onto this mTOR/autophagy pathway, the same way that many tributaries all lead into the Mississippi River. Overactive mTOR and reduced autophagy, by blocking normal synaptic pruning that may underlie learning appropriate behavior, may be a unifying feature of autism.”