The active zone protein RIM1α has multiple roles in late stages of neurotransmitter release.
Successful synaptic transmission involves intricate interactions of an array of proteins at the active zone. How these biochemical interactions are translated into physiological output is still largely unknown. Reporting in Neuron, Calakos and colleagues provide a detailed analysis of the molecular mechanism whereby an active zone protein, RIM1, regulates neurotransmitter release.
RIM1 is localized at the presynaptic active zone and has been implicated in short- and long-term plasticity. It contains several protein-binding domains, and is regarded as a scaffold that interacts with many proteins that are involved in the late stages of neurotransmitter release. Previous studies in mice and worms indicate that RIM1 functions at a stage after the docking of synaptic vesicles at the active zone. However, several regulated steps lead up to transmitter release, including vesicle priming, binding of calcium to presynaptic sensors and fusion of vesicles with the plasma membrane, so which steps require RIM1? Calakos et al. addressed this issue by carrying out a detailed analysis of presynaptic funtion in RIM1-deficient neurons.
The authors applied the whole-cell recording technique to hippocampal autaptic cultures prepared from RIM1-/- mice. They noted that RIM1-/- synapses had a 50% reduction in excitatory postsynaptic charge, which could result from a decrease in synapse numbers, postsynaptic receptor response or the synaptic probability of neurotransmitter release (Pr). RIM1-/- and wild-type mice had the same number of synapses, and there was no difference in the amplitude or frequency of miniature excitatory postsynaptic currents (mEPSCs), so the authors conjectured that compromised synaptic responses in RIM1-/- neurons were probably a result of decreased Pr.
RIM1 binds directly to Munc13-1, an active zone protein that is essential for synaptic vesicle priming, so Calakos et al. asked whether priming was affected in the absence of RIM1. The priming process influences Pr by determining the number of vesicles in the 'readily releasable pool' (RRP). The authors found that the RRP was reduced by 50% in RIM1-/- synapses, and that there was no evidence for a deficiency in subsequent vesicle exocytosis.
Having established RIM1 as a priming factor, Calokos et al. turned to examine the effect of RIM1 deficiency on short-term plasticity. They found that when RIM1-/- synapses were challenged with high-frequency stimulus trains, they could sustain responses throughout, whereas there was a 50% reduction in EPSC amplitude in wild-type cultures. Intriguingly, although there was a reduction in initial Pr in RIM1-/- synapses, Pr at steady state during high-frequency activity was not altered. The authors discovered that this was due not to a difference in activity-dependent refilling of RRP, but to an increase in the vesicle release probability.
Previous studies have indicated that calcium has an important role in regulating interactions of RIM1 with other synaptic proteins. Therefore, Calakos et al. suspected that calcium-dependent neurotransmitter release might be abnormal in RIM1-/- synapses. They found that although the overall calcium responsiveness was unchanged, the 'asynchronous' (slow) component of calcium-dependent release was markedly reduced in RIM1-deficient synapses.
This detailed analysis shows that RIM1 is a key regulator of vesicle maturation at the active zone, from priming to calcium-dependent triggering of synaptic vesicle fusion. These results support the idea that RIM1 acts as a scaffold to localize various active-zone components and to integrate their actions. The study also provides the first direct link between a synaptic protein and the poorly understood processes that control asynchronous neurotransmitter release.
Jane Qiu
References
Calakos, N. et al. Multiple roles for the active zone protein RIM1 in late stages of neurotransmitter release. Neuron42, 889–896 (2004) | Article | PubMed |
Rizo, J. & Sudhof, T. C. Snares and Munc18 in synaptic vesicle fusion. Nature Rev. Neurosci.3, 641–653 (2002) | Article | PubMed |
