Regulation of dendritic protein synthesis by miniature synaptic events

Minis: whence and wherefore?

In this issue of Neuron, Sara et al. find that spontaneously released miniature synaptic potentials arise from a pool of vesicles distinct from those released by neural activity. This modification of a basic tenet of the quantal hypothesis has important implications for the analysis of changes in synaptic transmission.

Dopaminergic stimulation of local protein synthesis enhances surface expression of GluR1 and synaptic transmission in hippocampal neurons

The use-dependent modification of synapses is strongly influenced by dopamine, a transmitter that participates in both the physiology and pathophysiology of animal behavior. In the hippocampus, dopaminergic signaling is thought to play a key role in protein synthesis-dependent forms of synaptic plasticity. The molecular mechanisms by which dopamine influences synaptic function, however, are not well understood. Using a GFP-based reporter, as well as a small-molecule reporter of endogenous protein synthesis, we show that dopamine D1/D5 receptor activation stimulates local protein synthesis in the dendrites of hippocampal neurons. We also identify the GluR1 subunit of AMPA receptors as one protein upregulated by dopamine receptor activation, with increased incorporation of surface GluR1 at synaptic sites. The insertion of new GluRs is accompanied by an increase in the frequency of miniature synaptic events. Together, these data suggest a local protein synthesis-dependent activation of previously silent synapses as a result of dopamine receptor stimulation.

Partial behavioral compensation is revealed in balance tasked mutant mice lacking otoconia

We describe for the first time behavioral tests which show that mammals with congenital absence of otoconia can learn a motor task that normally relies on gravity perception. The mouse mutation tilted (tlt) occurs in the otopetrin 1 gene (Otop1(tlt/tlt)) and eliminates an essential component necessary for the formation of otoconia. Our data show that even in the absence of otoconia, tlt mutant mice, like normal mice, learn to cross a bar suspended between two boxes and, with practice, improve their speed of crossing. Despite this learned compensatory skills, tlt mutant mice show balance impairments, such as falling from the bar, not observed in wild type (WT) or heterozygous (het) Otop1(+/)(tlt) littermates. The tlt mutant mice also use their tail as additional support, a behavior that is rarely exhibited in the control littermates. Interestingly, the Otop1(+/)(tlt) heterozygous littermates show in many aspects an intermediate phenotype between wild type and tlt mutant mice, suggestive of a gene dosage effect. Overall, these data support the notion that mammals can use other otic and extraotic receptors such as semicircular canals and limb proprioreceptors, respectively, to compensate for the absence of otoconia-mediated gravity perception in a balance task.

An Isolated Pool of Vesicles Recycles at Rest and Drives Spontaneous Neurotransmission

Spontaneous synaptic vesicle fusion is a common property of all synapses. To trace the origin of spontaneously fused vesicles in hippocampal synapses, we tagged vesicles with fluorescent styryl dyes, antibodies against synaptotagmin-1, or horseradish peroxidase. We could show that synaptic vesicles recycle at rest, and after spontaneous exo-endocytosis, they populate a reluctantly releasable pool of limited size. Interestingly, vesicles in this spontaneously labeled pool were more likely to re-fuse spontaneously compared to vesicles labeled with activity. We found that blocking vesicle refilling at rest selectively depleted neurotransmitter from spontaneously fusing vesicles without significantly altering evoked transmission. Furthermore, in the absence of the vesicle SNARE protein synaptobrevin (VAMP), activity-dependent and spontaneously recycling vesicles could mix, suggesting a role for synaptobrevin in the separation of the two pools. Taken together these results suggest that spontaneously recycling vesicles and activity-dependent recycling vesicles originate from distinct pools with limited cross-talk with each other.

Biochemical mechanisms for translational regulation in synaptic plasticity

Changes in gene expression are required for long-lasting synaptic plasticity and long-term memory in both invertebrates and vertebrates. Regulation of local protein synthesis allows synapses to control synaptic strength independently of messenger RNA synthesis in the cell body. Recent reports indicate that several biochemical signalling cascades couple neurotransmitter and neurotrophin receptors to translational regulatory factors in protein synthesis-dependent forms of synaptic plasticity and memory. In this review, we highlight these translational regulatory mechanisms and the signalling pathways that govern the expression of synaptic plasticity in response to specific types of neuronal stimulation.

Translational regulatory mechanisms in persistent forms of synaptic plasticity

Memory and synaptic plasticity exhibit distinct temporal phases, with long-lasting forms distinguished by their dependence on macromolecular synthesis. Prevailing models for the molecular mechanisms underlying long-lasting synaptic plasticity have largely focused on transcriptional regulation. However, a growing body of evidence now supports a crucial role for neuronal activity-dependent mRNA translation, which may occur in dendrites for a subset of neuronal mRNAs. Recent work has begun to define the signaling mechanisms coupling synaptic activation to the protein synthesis machinery. The ERK and mTOR signaling pathways have been shown to regulate the activity of the general translational machinery, while the translation of particular classes of mRNAs is additionally controlled by gene-specific mechanisms. Rapid enhancement of the synthesis of a diverse array of neuronal proteins through such mechanisms provides the components necessary for persistent forms of LTP and LTD. These findings have important implications for the synapse specificity and associativity of protein synthesis-dependent changes in synaptic strength.