It happens quite a lot in neuroscience that something can be described without really knowing why it’s doing something. Bear with me a bit, as what I’m about to describe is probably unfamiliar to most people and also very much concerns the nitty-gritty of how the cells (neurons) of the brain and nervous system work.
Neuroscientists have known for some time that nerve endings near the critical junction points between nerve cells, the synapses, are filled with tiny sacs (like microscopic bubbles) containing chemicals called neurotransmitters. Neurons work by sending a pulse of electro-chemical energy along the length of the cell (through nerve fibers of the axons) until it reaches a synapse – a gap between the end of one neuron and the beginning of another. For the pulse to cross this gap it must trigger the release of neurotransmitters stored in vesicles (those tiny sacs) that cross the gap. Depending on how much and what kinds of neurotransmitters are released, they may fire triggers in the next neuron and so the nerve impulse passes on, from toe to brain in some cases. This is not, as you can tell, the convenient metaphor of an electrical charge travelling down a wire. We are not ‘wired,’ actually; it’s more like tenuously and conditionally connected, something like your computer connected to Wi-Fi when you are studying for your degrees online.
This is not the most efficient or high speed way of transmitting an electrical signal (parenthetically, some types of neuron actually do act like an efficient ‘wire’); so why are most neurons set up this way? That’s one of the big questions in neuroscience. I mention this because it’s also been clear for some time that the function of the synaptic vesicles and their neurotransmitters is to ‘filter’ or ‘weigh’ the meaning or importance of a pulse coming across the synapse.
An important sub-question for neuroscientists concerns the nature of the vesicles. There are two kinds, identified many years ago because some of them sit around the very end of neuron and release the neurotransmitters when an impulse arrives, and others (the majority) are pooled-up nearby. As far as scientists could tell, the two kinds of vesicles were identical, except that one was in a ‘resting’ pool and the others were in the ‘recycling’ pool. How do these two pools of vesicles interact? What roles are played? The chemistry involved, especially in the composition of the neurotransmitters and their function in transferring electrical information from one neuron to another, is very complicated. It all added up to a lot of unknowns, a mystery, if you like.
A research team at the University of California San Francisco (USA) under Robert Edwards and publishing in the journal Neuron [11 August 2011, paywalled, v-SNARE Composition Distinguishes Synaptic Vesicle Pools] has developed the first evidence that the two vesicles are not the same, that they are defined by having different proteins on their surface.
By using very high powered microscopes and ‘labeling’ specific proteins with glowing markers derived from jellyfish, they were able to determine that the vesicles in the resting pool have a high concentration of the protein VAMP7 on their surface. From this they were able to learn that the particular protein is involved in regulating the release of neurotransmitters, and that there are connections between the behavior of ‘resting’ and ‘recycling’ vesicles.
At this point, it would seem the appropriate reaction might be, “That’s it?” Yes, for the moment. Like a lot of important advances in science, this is one of those seemingly little keystone pieces. It’s a starting point, a piece of knowledge, upon which a much greater edifice will eventually be built. Now that scientists know that the two kinds of vesicles are different in their protein composition and that the protein(s) involved are significant in the release of neurotransmitters, it opens up a vast new area of research. It also points in the direction of that big question in neuroscience, how (and eventually why) do synapses use vesicles to transmit information? It appears that proteins may be at the basis of answering that question. This is consistent with the discoveries across the board in biochemistry that the role of proteins is far more crucial and complex than previously thought. Thus the burgeoning of the new field of proteomics, the study of proteins.
I was debating whether this story has real ‘impact’ or is it just another one of those many steps in the process of scientific discovery. Well, it is that, but I think that in this case science is dealing with one of the most fundamental biological mechanisms, finally dealing with it at the molecular level. In there, we can suspect, might be some answers to how the brain (all the trillions of connections and billions of neurons) turns into ‘mind’ and ‘consciousness.’ This discovery is a long way and maybe decades from that, but just learning that vesicles are different in their use of proteins points neuroscience in a new and potentially important direction.