Learning and memory depend on the sprouting and pruning of synapses, which in turn depend on the emergence and disappearance of neuronal receptors, which in turn depend on a sort of recycling service within brain cells. The recycling service is carried out with shuttles called endosomes, vesicles that may swallow up surface receptors, take them into the cell’s interior, and then return them.
Ordinarily, the endosomal shuttles keep a brisk pace, but their schedules may slip if a certain protein is faulty, reports a team of scientists based at Johns Hopkins University School of Medicine. This protein is called GRASP1, short for GRIP-associated protein 1. This protein was previously shown to help recycle certain protein complexes that act as chemical signal receptors in the brain. These findings come, for the most part, from studies of laboratory-grown cells. To build on these findings, the Johns Hopkins researchers, led by Richard Huganir, Ph.D., studied GRASP1 in a mouse model and measured changes not only in neuronal properties, but also in the animals’ behavior. The researchers also considered how mutations affecting GRASP1 could be related to cognitive disorders in humans.
Removing GRASP1 reduced mice's ability to learn and recall information. “We see deficits in learning tasks,” said Dr. Huganir.
Additional findings appeared March 9 in the journal Neuron, in an article entitled, “GRASP1 Regulates Synaptic Plasticity and Learning through Endosomal Recycling of AMPA Receptors.” In this article, the team describes how it found mutations in the gene that produces GRASP1 in a few patients with intellectual disability, and how those genetic errors affected neural connections when introduced into mouse brain cells.
“We provide direct evidence for physiological roles of the recycling endosome protein GRASP1 in glutamatergic synapse function and animal behavior,” wrote the article’s authors. “Mice lacking GRASP1 showed abnormal excitatory synapse number, synaptic plasticity, and hippocampal-dependent learning and memory due to a failure in learning-induced synaptic AMPAR [α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor] incorporation.”
Dr. Huganir’s team genetically engineered so-called knockout mice that lacked GRASP1 and recorded electrical currents from the animals' synapses. In mice without GRASP1, neurons appeared to fire spontaneously an average of 28% less frequently than in normal mice, suggesting that they had fewer synaptic connections.
Next, Dr. Huganir's team counted protrusions on the mice's brain cells called spines, which have synapses at their tips. The average density of spines in knockout mice was 15% lower than in normal mice, perhaps because defects in receptor recycling had caused spines to be “pruned” or retracted. Neurons from mice without GRASP1 also showed weaker long-term potentiation, a measure of synapse strengthening, in response to electrical stimulation.
The team then tested the mice's learning and memory. First, the animals were placed in a tub of milky water and trained to locate a hidden platform. The normal mice needed five training sessions to quickly find the platform in the opaque water, while the knockout mice required seven; the next day, the normal mice spent more time swimming in that location than in other parts of the tub, but the knockout mice seemed to swim around randomly.
Second, the mice were put in a box with light and dark chambers and given a slight shock when they entered the dark area. The next day, the normal mice hesitated for an average of about 4 minutes before crossing into the dark chamber, while the knockout mice paused for less than 2 minutes. “Their memory was not quite as robust,” Dr. Huganir said.
To assess the importance of GRASP1 in humans, the team identified two mutations in the gene that produce the protein in three young male patients with intellectual disabilities, who had an IQ of less than 70 and were diagnosed at an early age. When the researchers replaced the rodent version of the normal GRASP1 gene with the two mutated mouse versions in mouse brain cells, the spine density decreased by 11% to 16% and the long-term potentiation response disappeared.
Dr. Huganir speculates that defects in GRASP1 might cause learning and memory problems because the cells aren't efficiently recycling receptors back to the surface. Normally, GRASP1 attaches to traveling cellular compartments called vesicles, which carry the receptors, and somehow helps receptors get transferred from ingoing to outgoing vesicles.
“We identified two GRASP1 point mutations from intellectual disability (ID) patients that showed convergent disruptive effects on AMPAR recycling and glutamate uncaging-induced structural and functional plasticity,” the article’s authors continued. “Wild-type GRASP1, but not ID mutants, rescued spine loss in hippocampal CA1 neurons in Grasp1 knockout mice.”
Essentially, when the GRASP1 mutations were in mouse cells, receptors accumulated inside recycling compartments instead of being shuttled to the surface.
Dr. Huganir cautions that the results don't prove that the GRASP1 mutations caused the patients' intellectual disability. But the study may encourage geneticists to start testing other patients for mutations in this gene, he says. If more cases are found, researchers may be able to design drugs that target the pathway. Dr. Huganir's team is now studying GRASP1's role in the receptor recycling process in more detail.