Complexity and Computation at the Synapse: Multilayer Architecture and Role of Diffusion in Shaping Synaptic Activity and Computation - Bioinspired Approaches for Human-Centric Technologies - page 269

Axons of primary hippocampal neurons typically form en passant synaptic termi-
nals at relatively short pitches (2-10 m), indicating that (1) SV migration by

stochastic or regulated lateral diffusion could represent an important informational
cross talk among adjacent terminals and (2) SV traffic may not be limited to
immediately adjacent synapses, but rather SVs can be shared across multiple
nerve terminals. The existence of functional SV sharing among different synapses
provides novel perspectives for synaptic physiology: migrating SVs could supply a
reservoir for synaptic release to depressed synapses or contribute to the regulation
of synaptic strength through remodeling of SV pools at host synapses. Moreover,
since heterogeneity among SV populations exists, the SV sharing mechanism could
involve SV populations that are specific for distinct release modalities, thus propa-
gating SV heterogeneity among distinct synapses. It is tempting to speculate that
lateral diffusion of SVs carries activity signals and modifies host synapses based on
the previous history of their neighbors. At present, the regulatory mechanisms that
might control the release/capture of SVs at individual terminals are unknown. One
such candidate could be BDNF that was reported to induce a TrkB-linked SV
declustering (Bamji et al. 2006 ; Staras et al. 2010 ). In the opposite direction,
synapsin I and synapsin II, in addition to their involvement in the structural
organization of synapses, represent an inhibitory clamp to SV diffusion by rever-
sibly tethering SVs to the synaptic region in a phosphorylation-dependent fashion.
Accordingly, genetically altered mice in which either or both synapsin genes have
been deleted exhibit a decreased density of SVs at individual boutons and an
increased size of the superpool of SVs migrating along the axonal shafts
(Fig. 9.2 ; Fornasiero et al. 2012 ; Orenbuch et al. 2012 ).
Advanced imaging techniques allow to investigate in detail not only the quantal
parameters of release, but also the dynamic trafficking of SVs within the terminal.
The latter techniques include: (1) super-resolution optical imaging techniques, such
as STED confocal microscopy that allow to resolve, under optimal conditions and
with suitable fluorophores, the size of single SVs or of very small SV clusters
(Galiani et al. 2012 ); (2) dynamic electron microscopy in which nerve terminals are
“frozen” under specific stimulation paradigms and in the presence of extracellular
tracers (e.g., horseradish peroxidase) that are taken up by SVs during exo-endo-
cytotic cycling (Fig. 9.3 ; Lignani et al. 2013 ); and (3) genetically encoded reporters
of SV cycling represented by superecliptic pH-sensitive GFP (pHluorin) targeted to
the intravesicular space of SVs by fusion with the intravesicular domains of either
VAMP2, synaptophysin, or VGLUT1. The pHluorin fluorescence (p K a ¼

ʼ

7.1) is
quenched at the acidic pH of the SV interior and strongly increases when exposed to
the neutral extracellular medium during exocytosis (Miesenb¨ck et al. 1998 ;
Sankaranarayanan et al. 2000 ; Burrone et al. 2006 ). The latter tool allows an
extremely detailed kinetic analysis of exocytosis from the RRP, depletion of the
RP, endocytosis, and respective size of the three pools of SVs. Acute addition of the
proton pump inhibitor bafilomycin blocks reacidification of SVs after endocytosis
and thus allows the study of the net exocytotic traffic, while intracellular basifi-
cation with ammonia/ammonium chloride unquenches both recycling and