Duke University Medical Center researchers have discovered that neurons take in receptors and other molecules from their surface membranes through discrete "doorways" -- specialized domains on the surface of nerve cells that regulate such entry.
The discovery of such entry points drastically revises a long-held theory that surface molecules such as receptors are enveloped right where they rest in the fatty membrane, to be drawn into the cell’s interior.
This uptake process, called endocytosis, is part of the constant cycling of receptors to and from the membrane surface. The balance of this cycling is the principal means by which neurons regulate the number of surface receptors for such chemical triggers as neurotransmitters and drugs -- thereby controlling neuronal sensitivity to such external chemical triggers. The newly discovered "endocytic zones" are also entry points for nutrients and pathogens such as viruses.
The researchers said their discovery of the zones raises the possibility that drugs affecting receptor transport to and through the zones could prove useful in treating addiction, depression, stroke, epilepsy, and other neurological disorders involving abnormal transport of receptors into the neuron’s interior.
The researchers, led by neurobiologist Michael Ehleers , reported their discoveries in the October 24, 2002, issue of the journal Neuron. Co-authors of the paper are neurobiologists Thomas Blanpied and Derek Scott. The research was sponsored principally by the National Institutes of Health.
"We have found that the nerve cell is in a way like a room with only certain entry points or doorways into that room," said Ehlers. "Before, it had been thought that the cell membrane might be like one big curtain that substances could move through at any point."
In their studies, Ehlers and his colleagues concentrated on the "postsynaptic" regions of the neuron -- the parts of neurons that receive chemical signals, called neurotransmitters, from neighboring neurons that trigger impulses in the receiving neuron. These postsynaptic regions of neurons are also sensitive to drugs that plug into the same protein receptors.
The brain establishes memory pathways by adjusting the strength of the connections, called synapses, among certain neurons. Synapses are located on doorknob-shaped spines that extend from neuronal branches called dendrites.
"We first began to look for specific endocytic zones, because studies over the past five years had indicated that synaptic strength could be controlled by removing or inserting postsynaptic receptors from the membrane," said Ehlers. "And a major unresolved question was where exactly do these receptors get removed from that membrane."
While microscopic studies of neurons had revealed that receptors tend to cluster in "postsynaptic densities" in the membrane, there was no evidence for specialized doorways through which they would enter the neuronal cell. To detect such regions, Ehlers and his colleagues first attached fluorescent molecules to a molecule called clathrin that is part of the structural mold that shapes the infinitesimal bubbles in the membrane that carry receptors and other protein cargoes into the cell. Specifically, clathrin forms a coat on the membrane surface, pinching off the cargo-carrying bubble, or vesicle.
Infusing the tagged clathrin molecules into cultures of neurons, the researchers then used high-resolution imaging to see where the tagged clathrin molecules migrated in carrying out their vesicle-coating duties. These imaging studies revealed specific "hot spots" where clathrin coats tended to cluster. What’s more, the researchers found that these hot spots tended to change in character as neurons aged.
"Initially, we found a lot of dynamic behavior of the hot spots in young neurons as they were growing and forming their synapses," said Ehlers. "But still mysterious is that, as the neurons in culture mature and age, these hot spots seem to stabilize and specialize. They become much more well-defined in location and not to appear and disappear as often as they do in young cells," he said.
"We believe that this change with maturation provides important clues about how nerve cells differentiate and specialize during brain development," said Ehlers. "And, these changes give some new insight into the diminished plasticity of the brain with aging."
Since entry of nutrients and pathogens such as viruses is through such clathrin-coated vesicles, further understanding of the endocytic zones could lead to better understanding of nutrient uptake by nerve cells and the process by which they are invaded by pathogens, said Ehlers.
Importantly, he said, the finding could also lead to new strategies for treating drug addiction and maintaining sensitivity of patients to therapeutic drugs.
"It seems quite reasonable to imagine developing techniques to control drug tolerance or sensitivity to a therapeutic agent such as an antidepressant by preventing the relevant receptors from migrating to or entering the endocytic zones to undergo endocytosis," said Ehlers. More broadly, he said, the discovery of endocytic zones could represent the beginning of a new paradigm for understanding neuronal cell membranes.
"These zones could represent the first of a series of specialized membrane structures dedicated to anchoring, sorting and trafficking of proteins in the postsynaptic membrane or the dendritic spines," said Ehlers. "We could end up developing an entirely new ‘microanatomy’ of neurons."
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