The active zones are specific sites of presynaptic terminals where the docking and the release of synaptic vesicles occur. They are complex, organized protein structures that anchor synaptic vesicles to the plasma membrane. Once bound to the active zone and docked to the plasma membrane, synaptic vesicles can properly respond to the changes of action potential mediated by the entry of Ca++, and they can be released in the post-synaptic zone with the help of other proteins like SNARE complex.
A functional active zone helps not only to recruit synaptic vesicles but also determines their chances of release by controlling two parameters: the readily releasable pool (RRP) of vesicles and the vesicular release probability (P). The RRP is the subset of plasma membrane fusion competent vesicles that can be released after the arrive of an action potential, and P is the probability of release of the RRP vesicles. The product of RRP x P gives the so called synaptic strength, that is, the number of vesicles released from a synapse.
Given these premises, researchers from Harvard Medical School, Boston, wanted to assess the importance of the active zone in the vesicle release. They created a knockout mice strain that lacked two key proteins of the active zone complex, called RIM and ELKS, and performed electronic microscopy analysis of the presynaptic sites of hippocampal neurons. The aim was to disrupt the active zone to test its role in vesicle fusion with plasma membrane and its influence on the synaptic strength.
As expected, RIM and ELKS lack led to the elimination or to a strong reduction of the other proteins of the active zone, like Munc13-1 or Bassoon, while other protein complexes like SNARE remained unaltered. But, what it was unexpected, there was not a complete loss of neurotransmitter release. The researchers found that the probability P of the release of vesicles was strongly reduced, but the RRP vesicle amount was higher than expected: more than 40% of vesicles remained in the presynaptic terminal, thus the neurotransmitter release was impaired partially, but not completely.
Thus, the lack of the docking sites provided by the active zone did not reduce the availability of the RRP vesicles to zero. It has been hypothesized that the absence of docking sites could be overcome by the storage of fusion competent vesicles in other sites of the neurons, without the need of being stably docked on the presynaptic membrane. So, the vesicles seem to have a “compensatory” behavior that let the neurons to maintain a low level of neurotransmitter release even when an important part of the protein machinery is disrupted.
It is important to notice that this experiment has been the first one so far in which the presynaptic active zone has been completely disrupted. So, more studies will have to be performed for a better understanding of the molecular mechanisms that regulate the release of the neurotransmitter vesicles.
For the first time, neurons lacking the proteins of the active zone at the presynaptic sites have been studied. Surprisingly, the neurotransmitter release has not been completely stopped, thus underlining that neurotransmitter vesicles have mechanisms to release their content other than the docking to the active zone. Further molecular studies will be needed to give a better explanation of these findings.
Wang et al. Fusion competent synaptic vesicles persist upon active zone disruption and loss of vesicle docking. Neuron 91, 777–791, August 17, 2016
The elaboration of this post has been financed by the project PI15/01082, as a part of the National Plan of I+D+I and co-financed by the ISCIII – General Deputy Direction for Evaluation and Development of Health Research – and the European Regional Development Fund (ERDF).