Distinct nanoscale calcium channel and synaptic vesicle topographies contribute to the diversity of synaptic function

Abstract

SUMMARYThe nanoscale topographical arrangement of voltage-gated calcium channels (VGCC) and synaptic vesicles (SVs) determines synaptic strength and plasticity, but whether distinct spatial distributions underpin diversity of synaptic function is unknown. We performed single bouton Ca2+ imaging, Ca2+ chelator competition, immunogold electron microscopic (EM) localization of VGCCs and the active zone (AZ) protein Munc13-1, at two cerebellar synapses. Unexpectedly, we found that weak synapses exhibited 3-fold more VGCCs than strong synapses, while the coupling distance was 5-fold longer. Reaction-diffusion modelling could explain both functional and structural data with two strikingly different nanotopographical motifs: strong synapses are composed of SVs that are tightly coupled (∼10 nm) to VGCC clusters, whereas at weak synapses VGCCs were excluded from the vicinity (∼50 nm) of docked vesicles. The distinct VGCC-SV topographical motifs also confer differential sensitivity to neuromodulation. Thus VGCC-SV arrangements are not canonical across CNS synapses and their diversity could underlie functional heterogeneity.

Neocortical High Probability Release Sites Are Formed by Distinct Ca2+ Channel-to-Release Sensor Topographies during Development

“…Nakamura et al, 2015), while Ca 2+ signals remained unaltered at parallel-fiber boutons (Baur et al, 2015). We found that neither amplitudes nor time courses or durations of influx of single-bouton Ca 2+ transients were different between age groups (Figure 3), indicating that neither Ca 2+ influx nor buffering were different between age groups (Sabatini and Regehr, 1998;Schmidt et al, 2003;Bornschein et al, 2013;Rebola et al, 2018). In addition, the CV of Ca 2+ amplitudes upon repeated activation of individual boutons was not different between age groups (Figure 3), indicating that similar numbers of VGCCs opened during an AP at young and mature terminals (Sabatini and Svoboda, 2000).…”

“…It is still conceivable that AP duration shortens developmentally as in the calyx of Held (Taschenberger and von Gersdorff, 2000;Nakamura et al, 2015), if it were counterbalanced by changes in VGCC gating. Since electrophysiological recordings from small cortical boutons in slices still pose a challenge, we analyzed the rise times of Ca 2+ signals and the temporal derivatives of the signals as a proxy for the duration of the presynaptic Ca 2+ current (Sabatini and Regehr, 1998;Rebola et al, 2018). The 10%-90% rise times (young: 0.75 ms, 0.69-0.86 ms, n = 7; mature: 0.90 ms, 0.80-0.76 ms, n = 5; p = 0.639, MWU) and the SDs of Gaussian functions fit to the derivatives of the Ca 2+ signals (young: 217 ms, 191-234 ms; mature: 235 ms, 189-242 ms; p = 0.343, MWU) were not different among age windows ( Figure 3G), supporting the notion of similarity in Ca 2+ influx.…”

“…In the mature brain, loose coupling was reported for highly plastic, low-p v synapses (Vyleta and Jonas 2014) and tight coupling at synapses with elevated p v specialized for reliable signaling (Eggermann et al, 2011). Synapses with tight coupling include all previously investigated central inhibitory synapses (Bucurenciu et al, 2008;Bornschein et al, 2013;Arai and Jonas, 2014) and excitatory synapses processing sensory information in the brainstem (Nakamura et al, 2015) and cerebellum (Schmidt et al, 2013, but see Rebola et al, 2018). At the excitatory synapses, coupling tightened during development, p v increased, and the number of VGCCs gating release decreased (Fedchyshyn and Wang, 2005;Baur et al, 2015;Nakamura et al, 2015;Kusch et al, 2018).…”

Primary and secondary motoneurons use different calcium channel types to control escape and swimming behaviors in zebrafish

“…At zebrafish NMJ and mouse mossy fiber synapses, the P/Q type has been shown to have a higher open probability than N type, which would be expected to result in greater calcium influx upon action potential (Li et al, 2007;Naranjo et al, 2015). In addition, calcium accumulation also depends on both the density and location of calcium channels (Dittrich et al, 2018;Rebola et al, 2019). Should the P/Q channels occur at higher density the domains formed by calcium influx would experience greater overlap, thereby increasing local calcium concentration (Stanley, 2016).…”

“…Should the P/Q channels occur at higher density the domains formed by calcium influx would experience greater overlap, thereby increasing local calcium concentration (Stanley, 2016). Furthermore, there might be channel type specific differences in coupling between presynaptic calcium influx and the calcium sensor for release, a critical determinant for synaptic efficacy (Eggermann et al, 2011;Rebola et al, 2019). Tighter coupling has been shown for the several central synapses that rely on P/Q type, while looser coupling is often associated with weaker synapses that use the N-type for release (Iwasaki and Takahashi, 1998;Wu et al, 1999;Forti et al, 2000;Stephens et al, 2001;Hefft and Jonas, 2005;Bucurenciu et al, 2010).…”