Protein phosphorylation and the regulation of synaptic membrane traffic

doi:10.1016/S0166-2236(99)01436-8

Trends in Neurosciences

Volume 22, Issue 10, October 1999, Pages 459-464

https://doi.org/10.1016/S0166-2236(99)01436-8 Get rights and content

It is well established that protein phosphorylation has an important role in synaptic plasticity. This is achieved, in part, via the presynaptic modulation of neurotransmitter release by protein kinases and protein phosphatases. In recent years, the increase in information available about proteins that are involved in synaptic exocytosis and endocytosis has been exploited in order to study the effects of protein phosphorylation on synaptic-vesicle cycling at the molecular level. The best-characterized protein in this respect is synapsin, whose function in the release of synaptic vesicles from the reserve pool is regulated by phosphorylation. More recently, it has emerged that proteins that function at other stages of the synaptic-vesicle cycle, which include priming of vesicles for docking–fusion and endocytic recycling, are also controlled by phosphorylation. Furthermore, recent work suggests that this regulation of membrane traffic by phosphorylation also occurs postsynaptically, where it contributes to synaptic plasticity.

Section snippets

Release of synaptic vesicles from the reserve pool

In a given synapse only a small proportion of synaptic vesicles, the readily releasable pool, is available for fast (submillisecond) Ca²⁺-induced exocytosis. The majority of synaptic vesicles are sequestered in a filamentous network that contains synapsin. Protein kinases have an important role in mediating the release of synaptic vesicles from this reserve pool in order to refill the readily releasable pool and so prevent its depletion by exocytosis. Classical work of Greengard and co-workers⁶

Preparation for membrane fusion

Following recruitment to the active zone, synaptic vesicles must attach to exocytotic sites at the plasma membrane and be primed so that they are ready for fusion, in order to enter the functional pool of readily releasable vesicles. The increase in the size and rate of refilling of this pool by PKC activation suggests that phosphorylation of the proteins that mediate these pre-fusion events is an important regulatory mechanism in the synaptic-vesicle cycle. Many proteins have been implicated

Ca²⁺-activated membrane fusion

The lag period between an action potential and neurotransmitter release can be as short as 60 μs. This rules out an acute action of protein phosphorylation in the fastest presynaptic membrane-fusion events. Nevertheless, as discussed above, activity-dependent modulation of synaptic protein kinases is likely to have profound effects on subsequent neurotransmission by phosphorylation of proteins that have key roles in the synaptic-vesicle cycle, which include those proteins involved in Ca²⁺

Recycling of synaptic vesicles

Following exocytosis, synaptic vesicles must be reformed and refilled with neurotransmitter in order to allow neurotransmission to occur during repetitive stimulation. It now appears that multiple mechanisms of vesicle recycling exist, and that phosphorylation and dephosphorylation are important in determining which mechanism is employed⁵⁵. Three basic mechanisms have been proposed (Fig. 1). The simplest is the ‘kiss-andrun’ model whereby, after the transient opening of a fusion pore and

Postsynaptic membrane traffic

Long-term potentiation has both presynaptic and postsynaptic components, both of which are modulated by protein phosphorylation⁶⁸. Although presynaptic facilitation is widely accepted to involve increased synaptic-vesicle fusion, a role for membrane traffic in postsynaptic enhancement has emerged only recently. It has been found that hippocampal LTP in the CA1 region, which is expressed postsynaptically, is blocked by botulinum-B neurotoxin (which inactivates VAMP) and is enhanced by α-SNAP

Future directions

The importance of protein phosphorylation in the regulation of synaptic transmission is well established. More recently, candidates for the substrate proteins through which kinases and phosphatases mediate these effects have been identified. What is required now is a rigorous analysis of the functional implications of these putative phosphorylation events for synaptic physiology. There are important general questions to be addressed: are the substrates that have been identified in vitro

Acknowledgments

The authors would like to thank Reg Kelly for sharing unpublished data. Work in the authors’ laboratories is supported by grants from the Medical Research Council (A.M.) and the Wellcome Trust (A.M., R.D.B.), and by a Wellcome Prize PhD studentship (K.M.T.).

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Synaptic transmission involves the release of neurotransmitters from presynaptic terminals by exocytosis. Protein kinases both activate and inhibit neurotransmitter release in various contexts [25,26] and are important regulators of synaptic plasticity underlying higher brain functions, such as learning and memory. Synaptosomal-associated protein of 25 kDa (SNAP-25), a t-SNARE protein, plays an essential role in neurotransmitter release [18].
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Trends in Neurosciences

Protein phosphorylation and the regulation of synaptic membrane traffic

Section snippets

Release of synaptic vesicles from the reserve pool

Preparation for membrane fusion

Ca2+-activated membrane fusion

Recycling of synaptic vesicles

Postsynaptic membrane traffic

Future directions

Acknowledgments

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Ca²⁺-activated membrane fusion