Rapid Active Zone Remodeling during Synaptic Plasticity

Abstract

How can synapses change the amount of neurotransmitter released during synaptic plasticity? Although release in general is intensely investigated, its determinants during plasticity are still poorly understood. As a model for plastic strengthening of synaptic release, we here use the well-established presynaptic homeostatic compensation during interference with postsynaptic glutamate receptors at the Drosophila neuromuscular junction. Combining short-term plasticity analysis, cumulative EPSC analysis, fluctuation analysis, and quantal short-term plasticity modeling, we found an increase in the number of release-ready vesicles during presynaptic strengthening. High-resolution light microscopy revealed an increase in the amount of the active zone protein Bruchpilot and an enlargement of the presynaptic cytomatrix structure. Furthermore, these functional and structural alterations of the active zone were not only observed after lifelong but already after minutes of presynaptic strengthening. Our results demonstrate that presynaptic plasticity can induce active zone remodeling, which regulates the number of release-ready vesicles within minutes.

Glutamate Receptors in Synaptic Assembly and Plasticity: Case Studies on Fly NMJs

“…Moreover, this presynaptic adaptation to loss of GluRIIA was accompanied by a moderate but significant increase in the size of Brp clusters at AZs. Interestingly, an increase in synaptic Brp content was even traceable on a short-term scale after blocking GluRs acutely with PhTX (see above) (Weyhersm€ uller et al 2011). Increases in Brp at individual active zones triggered by loss of conductance through postsynaptic GluRIIA may therefore be part of the observed homeostatic response.…”

Homeostatic Control of Presynaptic Neurotransmitter Release

“…Several experiments at the Drosophila NMJ demonstrate that presynaptic homeostasis requires an increase in the number of release-ready vesicles, assayed either by quantification of pool size during a stimulus train or by EPSC amplitude fluctuation analysis during single-AP stimulation (Figure 2c) (51,52). Depending on the condition, the magnitude of the observed increase ranged between ∼160% and ∼200% of control.…”

“…Altered presynaptic release is often inferred from changes to the rate of spontaneous mEPSP release and/or from changes to pairedpulse stimulation. Although a change during a paired-pulse paradigm is indicative of altered release probability, the lack of an effect during a paired-pulse paradigm need not be diagnostic if altered release is achieved through changes in the size of the pool of releasable vesicles, a situation documented at the fly NMJ (51,52). The studies that have reported homeostatic alterations in presynaptic release in the central nervous system have done so by quantifying (a) changes in APdriven presynaptic calcium influx, (b) vesicle recycling with vital dyes or with GFP-based reporters of membrane recycling (42,45,47), and (c) correlated changes in synaptic ultrastructure (47).…”

“…Subsequent experiments showed that loss of rim prevents the homeostatic modulation of RRP size (52). Thus, presynaptic homeostasis seems to require two genetically separable processes: an increase in presynaptic calcium influx (57) and a parallel increase in the RRP (51,52). At present, how RIM could be modulated during homeostatic plasticity at the Drosophila NMJ is unclear.…”

Transmission, Development, and Plasticity of Synapses

“…Key presynaptic changes that increase quantal content during synaptic homeostasis include Ca 2+ influx through the voltage-gated Ca 2+ channel Cac (Frank et al 2006; and increased size of the readily releasable pool (RRP) (Weyhersmuller et al 2011). These changes appear to be separable; for example, mutants for rim (rab3-interacting molecule) block homeostatic changes in RRP size, but exhibit normal Ca 2+ influx .…”