Synaptic vesicles — the tiniest circles — clump together at the membrane, ready to release neurotransmitter

We all have similar storage rooms in our body, except these ones we definitely don't want to empty out. They are known as synaptic vesicles, tiny sacs in our brains that store the chemical information that allow messages to pass from neuron to neuron.

Dr. James Cheetham studies how certain proteins interact with these tiny synaptic vesicles to encourage neurotransmission and stabilize these vesicles. His work offers a key to understanding how some diseases such as Parkinson's can be overcome with proteins and how drug addiction works.

Cheetham uses his laptop's PowerPoint presentation with over 50 slides to talk about the work he is doing. He starts by explaining neurotransmission and how synaptic vesicles are involved in this process.

The invisible world

Picture neurotransmission as a set of dominoes that are placed on their edge in a random pattern, but close enough to one another so if one fell, the others follow. The synaptic vesicles are the white dots on the domino. 

When the first electrical signal, (your finger), stimulates the first neuron, (the domino), the synaptic vesicles fuse to the membrane of that neuron and release their neurotransmitter. Then that neuron becomes excited and sends another electrical signal to another neuron. In essence, it's like each domino falling over, forcing another one to do the same until the desired action is carried out.

The electrical signal that stimulates the first neuron is called the 'action potential' and it moves down the axon between two neurons. It is like a signal travelling down a telegraph line. "The action potential either happens or does not happen, like flushing a toilet or ringing a doorbell," Cheetham says. "When the action potential gets to the nerve terminal, it can cause the release of variable amounts of neurotransmitter."

There are four main neurotransmitters. Glutamate gives you the excited feeling, serotonin deals with reward pathways - the "high" feeling - GABBA is inhibitory and dopamine deals with movement.

After the desired neurotransmitter is released, you have to get rid of the empty vesicles and this is done through a re-uptake process. This is when the recycling factory kicks in and the pre-synaptic cell, or the cell from which the synaptic vesicles were released, sucks the used ones back up like a vacuum cleaner so they can refuel them with neurotransmitter.

Building armies

If the body is working hard and quickly to send messages across the brain, there is the danger this re-uptake process can't keep pace. What that means is the vesicles that have been emptied can't get refuelled fast enough before they are needed again, Cheetham says.

This is where Cheetham's work with proteins comes into play. Synaptic vesicles form clusters around the membrane. To carry out a normal action, the front-line vesicles would be released, then sucked back up, refuelled and sent back out. But if a long-term action is required and the re-uptake process can't keep up with the demand, the cluster acts as a back up army.

Instead of firing, then refuelling and firing the same vesicles again, this back up army supplies the front lines with neurotransmitter-full synaptic vesicles. But without proteins, this back up army wouldn't exist.

Because synaptic vesicles are so tiny, their membranes are stretched to the limit. Cheetham says when membranes curl that tightly, they have a tendency to want to join together to make a bigger membrane. Proteins help stop this from happening.

Cheetham has also found, through his work with a certain family of proteins called synapsins, that the proteins help keep the vesicles clustered together so the backup army is there. Without the backup army, the brain doesn't get enough neurotransmitter to carry out a longer-term desired action, he says.

Mapping proteins

"If you know how a protein works on synaptic vesicles, it gives you a better idea of how synapses work. This gives you a better understanding of the molecular basis for diseases. If you know that, you can develop ways to counteract it," Cheetham says.

Ryan Doherty is a fourth year undergraduate student at Carleton. He works in Cheetham's lab developing three-dimensional models of proteins from the synapsin family by mapping out the protein's sequence.

This gives them both insight into how that protein may be of use and how it interacts with other proteins and neurotransmitters. For example, you can map out the region of the protein that could bind to the lipid bilayer of a cell, Doherty says.

The lipid bilayer is a membrane with two levels of lipid cells (commonly referred to as fat cells) which combine with proteins and carbohydrates to make up the structure of the cell. It's like drawing a circle in a circle - the double outline is your lipid bilayer.

With this information, Cheetham has been able to discover the way in which synapsins target and act on synaptic vesicles and the interaction these proteins have with the lipids in the cell. Because he knows how the protein family of synapsins interacts with the lipid membrane of a cell, he can tell how the protein will stabilize the vesicles inside the cell.

Doherty says "once you understand the interactions, you can make a model that will help in looking at, for example, how drugs interact with proteins to cure diseases." Then you can model drugs to target the specific parts of the protein that will have the desired effect.

Targeting diseases

This kind of work is being used in researching and understanding Parkinson's disease. "The neurons that produce and use dopamine as a neurotransmitter (dopaminergic neurons) in the basal ganglia (a part of the brain involved in controlling movement) selectively die off," Cheetham says.

In some people, this happens faster than in others which means they can't produce enough dopamine to regulate their movements.

Cheetham's work could one day provide the needed information to tell us how proteins can be targeted to help the cell produce more dopamine, but he says right now, he is not very close to having these techniques used for medical purposes.

Currently, to treat Parkinson's disease, doctors are injecting stem cells, cells which have not yet designated themselves to a given role in the body, into the basal ganglia so they will become dopamine-producing cells. More commonly, they just inject dopamine into the body.

Dopamine is just one of the four main neurotransmitters previously mentioned. Another is serotonin, the neurotransmitter that stimulates the reward pathways in the brain. Synaptic vesicles are therefore instrumental in drug users' "highs".

Highs and lows

The drugs cocaine and ecstasy inhibit the re-uptake of serotonin, Cheetham says. So when it is released, it stays in the system and the user gets a prolonged 'high' feeling. This can easily lead to drug addiction.

The more you use a certain pathway, whether it be by playing the same piece of music on a piano, or taking the same drug over and over again, the pathway used to carry out the action strengthens. This is a process known as long-term potentiation.

This principle applies to drug users in that by taking a drug, the re-uptake process fails. The pathways that use serotonin are therefore strengthened by the excess chemical, so you have to take more of the drug to get the same high. In other words, the user builds up a tolerance.

Eventually, what happens is the user over-stimulates the serotonin pathways and the neurons shut down, Cheetham says. If you know how this works, you can intervene with medications, he adds.

Synaptic vesicles are therefore involved in everything we do be it turning a page in a book or running a marathon. Without them, the brain can't continue to send messages through the body to continue a desired action.

These vesicles are also involved in how certain diseases such as Parkinson's effects the body and how drug addiction works.

Exciting work

Cheetham is not alone in studying synaptic vesicles. He as been at Carleton for six years, but studies in how proteins effect synaptic vesicles have been going on for around thirty years, he says. In the November 17, 2001 issue of ScienceDaily Magazine, scientists discussed how a protein can be used to mature synapses, the gaps between neurons over which the electrical signal must pass to transmit information.

The protein, dubbed PSD-95 "helps build the physical scaffolding of the synapse that cells use to transmit the chemical messenger . . . glutamate." Glutamate is also an excitatory neurotransmitter. In this case, by maturing the synapse, the protein allows for an increased number of synaptic vesicles to adhere to the membrane and thus increases the number of them that release their neurotransmitter, so more neurotransmitter is released.

According to the article, "glutamate enables the brain to develop, language to be learned, a new math equation to be grasped, tennis to be mastered and walking to be relearned following a stroke. But without the synapses that allow the chemical signal's transmission from one nerve cell to the next, glutamate has no more luck in communicating its messages than a train has luck in reaching its destination without tracks to follow."

This research was important in learning about how to strengthen the pathways and their ability to transmit a stronger signal in order to help treat mental retardation which is caused when the post-synaptic cell can't receive the glutamate messages effectively.

Research continues in the area of synaptic vesicles and how proteins interact with the vesicles to create different outcomes. Not only have scientists found ways to mature the synapses to allow for stronger neurotransmission, but Cheetham has found how the protein family synapsins stabilize synaptic vesicles and prevent them from binding together when they shouldn't.

With the PowerPoint screen finally blank, Cheetham leans back in his chair and contemplates the question of "where to next?"

"You just keep on going," he says.

Scientists working in this area have a vast array of proteins to chose from. Cheetham has just been working with one family.

"There is so much, you never run out of things to work on," he says.