I often write on this blog that neurons in our brain are linked to each other by multiple connections. We call those connections synapses, but I never took the time to explain in detail how they work. It is a fascinating subject which constitutes an enormous field of research by itself. Here is an illustration of such a synapse and how one neuron sends signals to the next neuron.
Although what is described in this picture is a rather complex mechanism, it happens really fast – the whole story depicted in the figure from the electrical impulse to the release of the neurotransmitters to the next neuron can happen in less than 0.001 second for a synapse. On top of operating very quickly, there are so many such synapses in our brain it’s even difficult to imagine: at least 1 000 000 000 000 000, which is 100 trillion synapses. At any moment in time a part of these synapses are releasing neurotransmitters to their target neuron to pass on some signals. There is a good primer about neurons and synapses here.
There are hundreds of molecular mechanisms at play to allow neurons to release neurotransmitters and then recycle the vesicle so it can be used again later. There are actually many different ways in which those vesicles can be re-used, but for the rest of the article I will simply use the term “recycling” to describe what scientists have been calling endocytosis, the process by which the vesicles are re-absorbed in the neuron after releasing the neurotransmitters. For those interested in the details of the process, the Wikipedia page is very interesting and detailed. Understanding the role of any of the molecules involved at the synapse can take many years to a single scientist. The recent study which made the cover of the Journal of Neuroscience has been performed by Jennifer R. Morgan and colleagues1. The team of scientists has focused their attention on Nucleotide exchange factors. These proteins are abundant in biological cells and their job is to change molecules of low energetic potential that are attached to other proteins for molecules of high energetic potential. In particular, these proteins remove ADP or GDP, which you can see as “used” energy sources, from other proteins. The proteins can then bind to ATP or GTP, which I like to think of as fresh new batteries for the proteins. ATP and GTP are major sources of energy for many biological processes that are going on in all cells of our body.
The idea behind the study was to determine if the nucleotide exchange factor is implicated in the process of vesicle recycling. The process is important as neurons would quickly run out of vesicles if they would not have the appropriate mechanisms to recycle them.
To address this question, the researchers have used lampreys, a fish that resembles eels and that feeds by sucking the blood of other fishes. Lampreys are a very good animal model for those who study the brain because their neurons are among the biggest neurons in any vertebrate. Many currently active scientists including Sten Grillner and my thesis director Réjean Dubuc are using this animal to understand how the brain controls behaviors. The well-known psychologist Sigmund Freud has also debuted his research career on this animal.
Most neurons in most animals require high-end microscopes to be visualized, but some of the lamprey neurons are so big that you can actually see them to the naked eye simply looking at their brain – a big advantage because every microscopic manipulation on neurons becomes easier and more likely to succeed, including recording their activity or modifying their content.
The team of scientists has achieved a spectacular accomplishment by designing a molecule that would block the nucleotide exchange factor in lampreys. They did so by carefully studying the shape of the nucleotide exchange factor and how it works and then designed a molecule that had the perfect shape and properties to block it. This is called biomolecular engineering and whenever it works I applaud because it can involve many failed attempts and sometimes just never works.
The strategy was then to use that inhibitor to block the nucleotide exchange factor and see if it affected the recycling of vesicles. Now whenever such substances are used over the whole brain of the animal there are scientific problems that arise from the results – how do you know you are inhibiting specifically a molecule located at the synapse you are studying? The team therefore used the advantage they had in having an animal with such big neurons. They could insert a small glass pipette very close to the synapse, in the axon of the neuron that contains the vesicles, and with very small pressure pulses they injected their newly-developed nucleotide exchange factor inhibitor. Thus for the first time we can see how the synapses of just one neuron in the brain operate without this protein.
The question here is whether the blockade of the nucleotide exchange factor induces any change in the behavior of the synaptic vesicles, the purple spheres illustrated in the first figure. The authors found that it does have an impact indeed. Looking at the injected neuron with an electron microscope, they found that the membrane of the neuron at the synapse tended to grow much larger than usual. This means that the vesicles were indeed attaching to the membrane but were staying attached. The inhibitor has partly blocked the ability of neurons to recycle the vesicles.
This is a very interesting result and the inhibitor that has been developed in this study will likely become a useful tool in the future to study the role of nucleotide exchange factors in this very important cellular function by which neurons can recycle their synaptic vesicles.
1. Jennifer R. Morgan, Jianwen Jiang, Paul A. Oliphint, Suping Jin, Luis E. Gimenez, David J. Busch, Andrea E. Foldes, Yue Zhuo, Rui Sousa, Eileen M. Lafer (2013) A Role for an Hsp70 Nucleotide Exchange Factor in the Regulation of Synaptic Vesicle Endocytosis. Journal of Neuroscience 33: 8009-8021, doi: 10.1523/JNEUROSCI.4505-12.2013.