Nature: A new mechanism for the regulation of synaptic plasticity: demystifying the role of synaptic vesicles in rapidly replenishing the key molecule SYT3

The release of neurotransmitters is limited by the size of the easily released pool of vesicles (RRPs), which are rapidly depleted
by synaptic activity.
Vesicle depletion reduces the release of neurotransmitters, which reduces the fidelity of neuronal signals
.
Sustained neuronal activity requires rapid replenishment of synaptic vesicles to maintain synaptic transmission
.
This vesicle replenishment is accelerated by submolar presynaptic Ca2+ signaling and mediated by as-yet-unidentified high-affinity Ca2+ sensors
.

Recently, the Skyler L.
Jackman research team of the Vollum Institute of Oregon Health and Science University published a study in Nature to determine that synaptic binding protein-3 (SYT3) is a presynaptic high-affinity Ca2+ sensor, which drives vesicle replenishment and short-term synaptic plasticity
.

SYT3 is present at the presynaptic terminal

Immunolabeling was performed in the brainstem, and SYT3 was found to be evident in the calyx synapse of the trapezoidal medial nucleus (MNTB) and co-standard with synaptic vesicles (VGLUT1) and presynaptic active region (Bassoon) molecular markers [Fig.
1a].

The fluorescence level of SYT3 from the presynaptic terminal to the postsynaptic cell was quantified, and the SYT3 fluorescence overlapped with VGLUT1 but closer to the synaptic gap [Fig.
1b, c], indicating that SYT3 was concentrated near
the presynaptic active region.

Further labeling synaptosomes with SYT3 found that SYT3 was also present in a larger proportion of VGLUT1 (presynaptic) labeled synaptosomes than PSD-95 (postsynaptic ), indicating more presynaptic enrichment [Fig.
1f].

Figure 1 SYT3 is localized to the anterior terminal of the synapse

SYT3 is required for calcium-dependent recovery

Next, the researchers performed patch-clamp recordings of MNTB neurons, measuring neurotransmitter release
at synapses.
Firstly, knocking out SYT3 does not affect the basic characteristics
such as postsynaptic receptor density and initial release probability.
However, the effect on short-term plasticity, Syt3 KO synapses are much more strongly inhibited than WT, in WT synapses, EPSC charge transfer increases almost linearly from 10-200Hz, but there is no [Fig.
2a-c]
in Syt3 KO synapses.
It was shown that SYT3 is required
to maintain transmitter release during high-frequency stimulation.

Calcium-dependent recovery (CDR) plays a key role in maintaining neurotransmitter release during high-frequency stimulation, while Syt3 KO synapses recover approximately twice as slowly as WT synapses after high-frequency stimulation [Fig.
2e-g].

Illustrate that SYT3
is required for CDR and sustained neurotransmitter release in calyx synapses.

Figure 2 SYT3 accelerates vesicle refilling in calyx synapses

Ca2+ binds to presynaptic SYT3

Accelerates synaptic inhibition recovery

The researchers rescued presynaptic Syt3 expression
in KO mice with the adeno-associated virus (AAV) vector Syt3.
It was found that SYT3-positive synapses exhibited reduced synaptic inhibition and faster recovery compared to WT synapses [Fig.
3a-e].

However, the Ca2+ binding defective mutant SYT3 (Syt3D/N) failed to improve synaptic inhibition or restore
synaptic inhibition.
Indicates that CDR requires Ca2+ to bind to
presynaptic SYT3.

Vesicles in the active zone have two states, vesicles at < 2 nm from the plasma membrane are considered to be tightly docked and ready for release, while vesicles farther away from the plasma membrane (5–10 nm) are said to be loosely docked
.
To explore how SYT3 affects synaptic transmission, the researchers constructed a vesicle transport and fusion model by which vesicles are recruited from an infinite reserve pool into a loosely docked state, which can then be reversibly transitioned to a tight docking state
.
The model that found that SYT3 KO affects the release probability of tightly docked vesicles failed to reproduce the behavior of WT synapses [Fig.
3f,g], indicating that SYT3 binds to residual Ca2+ to accelerate vesicle docking, thereby having a great effect
on vesicle replenishment during high-frequency activity.

Figure 3 Binding of Ca2+ to presynaptic SYT3 accelerates the recovery of synaptic inhibition

Wide range of effects of vesicle supplementation

CDR is also evident in climbing fiber synapses on cerebellar Purkinje cells, and SYT3 is abundantly expressed in the cerebellar molecular layer and colocalized with VGLUT2, a marker of climbing fiber bundles [Fig.
4a].

The researchers also performed patch-clamp recordings, which were found to be consistent
with those in calyx synapses.

In addition, cerebellar lichen fibers to the end of granule cells were also tested, and the results were still similar [Fig.
4].

Illustrating that SYT3 is required
for rapid vesicle replenishment in synapses with CDR and widely varying release properties.
Figure 4 SYT3 promotes the recovery of cerebellar climbing fibers from synaptic inhibition

Summary

This study reveals the critical role
of presynaptic SYT3 in maintaining reliable high-frequency synaptic transmission.
Many forms of short-term plasticity may focus on
reversible, Ca2+-dependent vesicle docking mechanisms.
Cell type-specific removal of Syt3 could reveal how activity-dependent vesicle supplementation promotes neural circuit function and neuropsychiatric disorders
.