What's in a Connection? A Look at Protein Patterns Within Synapses

What's in a Connection? A Look at Protein Patterns Within Synapses
Monday, May 05, 2003

Overview A new study has begun to unravel the mysteries of protein interactions that govern the strength of nerve cell connections, or synapses, in the brain. The findings give researchers a better understanding of how synapses function during learning and memory, and they may lead to new insights about such neurological disorders as Parkinson's and Alzheimer's diseases.

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A new study has begun to unravel the mysteries of protein interactions that govern the strength of nerve cell connections, or synapses, in the brain. The findings give researchers a better understanding of how synapses function during learning and memory, and they may lead to new insights about such neurological disorders as Parkinson's and Alzheimer's diseases.

The complex processes of learning and memory formation rely heavily on the constant chemical fluctuations within synapses. A synapse is a tiny space between nerve cells, across which information is passed from one neuron to another. A structure called the post-synaptic density (PSD) is a thickening of the synaptic membrane where one nerve cell receives information from other neurons in the form of chemical signals, or neurotransmitters. Neurotransmitters trigger the receiving nerve cell to initiate a nerve impulse.

Previous research has shown that the PSD changes its shape in response to increases and decreases in signaling activity, which occurs during learning and other activities. Many researchers believe malfunctions of this process may play a role in many neurodegenerative disorders.

"Past work showed that the PSD is an important specialized ‘machine' for receiving chemical signals from the pre-synaptic nerve cell," said study author Michael Ehlers, M.D., Ph.D., of Duke University Medical Center. "In this study, I wanted to take a closer look at the PSD, especially the patterns of protein accumulation and loss within it." The study appears in the March 2003 issue of Nature Neuroscience¹ and was funded in part by the National Institute of Neurological Disorders and Stroke (NINDS).

To produce a molecular "fingerprint" of the PSD, Dr. Ehlers developed a technique that creates profiles of proteins that are produced at specific points in time. He used this technique to examine how changes in protein patterns in the PSDs of embryonic rat nerve cells correlate with increases and decreases in nerve signaling activity.

Dr. Ehlers manipulated nerve signals in a way that simulates the fluctuations that take place during learning or memory formation. To do this, he used a common calcium channel blocker to decrease signaling activity, and to increase activity he blocked the normal inhibition of nerve signals. He then measured the levels of 30 specific proteins contained in the PSD.

The tests revealed that particular sets of proteins acted in an ensemble-type fashion. As nerve-signaling activity increased, the amount of one set of proteins increased, while another decreased, and a third set stayed the same. Dr. Ehlers saw the same effect in the opposite direction when nerve activity decreased. The changes occurred with the same magnitude over the same period of time, suggesting some kind of co-dependent relationship among the sets of proteins, he says.

Protein degradation and removal correlated closely with signaling activity, suggesting a complex system of regulation within the synaptic machinery. In a sample of 10 PSD proteins, increasing neural activity enhanced protein turnover, while a decrease in activity slowed the turnover rate.

"The advantage of looking at patterns of several proteins, rather than at single molecules, is that we can start to glean more complex information about the PSD," says Dr. Ehlers. "I hope that this technique will eventually allow us to study how synapses function in various disease states."

Previous studies have shown that synapses continually remodel themselves by degrading, removing, and replacing proteins. To measure the rates of protein breakdown and removal within the synapses, Dr. Ehlers radioactively labeled some of the proteins. The turnover rate was much higher than he had expected. "At 1 week, there were no radioactively labeled proteins left," said Dr. Ehlers. "Eventually I determined that the entire molecular composition of the synapse was replaced in only about 10 hours. I was shocked."

Dr. Ehlers says the high turnover of proteins within the PSD brings about many new questions and possible research angles. He notes that while most nervous system research focuses on how neural circuits change, he's now puzzled as to how they manage to stay the same.

"I think the dynamic, constant remodeling of the PSD is remarkable," says Dr. Ehlers. "How does it stay stable? Why the enormous daily expenditure of cellular energy? These results show just how little we fully understand about synaptic machinery."

Another avenue to explore is whether protein turnover slows with age. Since this study looked at embryonic nerve cells, Dr. Ehlers says future studies should measure protein removal and degradation in mature cells for comparison.

Several neurological disorders are marked by mutations that disrupt the normal processes of protein removal and degradation, which are regulated by a substance called ubiquitin. A rare form of familial Parkinson's disease, for instance, arises from genetic mutations that disrupt how ubiquitin functions.

When ubiquitin attaches to a protein, that protein gets degraded and removed from the cell. Dr. Ehlers found that the amount of nerve-signaling activity appears to regulate ubiquitin's attachment to PSD proteins. Increased activity encouraged ubiquitin attachment, while inhibiting ubiquitin completely prevented protein turnover and remodeling.

Dr. Ehlers says he and many other researchers in the field believe subtle synaptic defects, such as disruptions in the ubiquitin pathway, could be associated with the early stages of some neurological disorders, such as Alzheimer's, in which patients experience only mild mental decline at first.

"This study lays the groundwork for new pathways to understanding those subtle defects," says Dr. Ehlers. "And we hope that these new insights will help us find ways to treat some neurological disorders early."

The NINDS is a component of the National Institutes of Health, within the U.S. Department of Health and Human Services, and is the nation's primary supporter of biomedical research on the brain and nervous system.

Reference:
¹Ehlers, MD. "Activity level controls postsynaptic composition and signaling via the ubiquitin-proteasome system." Nature Neuroscience, Vol. 6, No. 3, March 2003, pp. 231-242.

- By Tania Zeigler

Reviewed May 5, 2003