The Presynaptic Particle Web

148

181

Signal integration in the brain is determined by the size and kinetics of rapid synaptic responses. The latter, in turn, depends on the concentration profile of neurotransmitter in the synaptic cleft. According to a traditional view, narrower clefts should correspond to higher intracleft concentrations of neurotransmitter, and therefore to the enhanced activation of synaptic receptors. Here, we argue that narrowing the cleft also increases electrical resistance of the intracleft medium and therefore reduces local receptor currents. We employ detailed theoretical analyses and Monte Carlo simulations to propose that these two contrasting phenomena result in a relatively narrow range of cleft heights at which the synaptic receptor current reaches its maximum. Over a physiological range of synaptic parameters, the ''optimum'' height falls between Ϸ12 and 20 nm. This range is consistent with the structure of central synapses reported by electron microscopy. Therefore, our results suggest that a simple fundamental principle may underlie the synaptic cleft architecture: to maximize synaptic strength.T he waveform of rapid synaptic responses shapes the temporal domain of signal integration in the brain (1, 2). In turn, the kinetics of synaptic receptor currents is constrained by diffusion of neurotransmitter in the synaptic cleft (3, 4). This relationship, however, remains poorly understood, mainly because events inside the cleft are beyond the powers of direct experimental observation. There is little doubt, however, that synaptic cleft geometry is an important factor in shaping synaptic currents (3, 5-7). The straightforward logic of physics predicts that decreasing the cleft height should increase the intracleft concentration of neurotransmitter and therefore enhance activation of synaptic receptors (8). In turn, increasing the lateral cleft size could generally slow down neurotransmitter escape from the cleft and thus prolong synaptic responses (9). Indeed, faster AMPA receptor-mediated EPSCs have recently been associated with smaller synaptic apposition zones in the developing cerebellar synapses (10). However, lateral dimensions of synapses fluctuate considerably (even within homogeneous synaptic populations), whereas the cleft height remains remarkably stable. For instance, at the common excitatory synapses in hippocampal area CA1, the lateral cleft area varies several-fold (11) whereas the cleft height shows the coefficient of variation (an upper estimate) of only Ϸ26% (12). Interestingly, osmotic challenge or other physiological manipulations that affect dramatically the overall extracellular space dimensions (13, 14) do not appear to modify the synaptic cleft height (15). This parameter also remains virtually unchanged during development (16-18). Remarkably, in electron micrographs of synaptosomes (a preparation obtained using strong mechanical forces of centrifugal separation), the distance between the apposing synaptic membranes is indistinguishable from that in the intact neuropil. Across many synaptic type...

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

The optimal height of the synaptic cleft

Savtchenko

Rusakov

2007

148

181

“…Subcellular fractionation and solubility analysis of chicken brain proteins were performed as described previously (25,34).…”

Section: Methodsmentioning

confidence: 99%

Association of β-catenin with the α-subunit of neuronal large-conductance Ca ²⁺ -activated K+ channels

Lesage

Hibino

Hudspeth

2003

, and neurons contributes to rapid repolarization of the membrane after excitation. Ca 2؉ channels have been shown to bind to a large set of synaptic proteins, but the proteins interacting with Ca 2؉ -activated K ؉ channels remain unknown. Here, we report that the large-conductance Ca 2؉ -activated K ؉ channel of the chicken's cochlear hair cell interacts with ␤-catenin. Yeast two-hybrid assays identified the S10 region of the K ؉ channel's ␣-subunit and the ninth armadillo repeat and carboxyl terminus of ␤-catenin as necessary for the interaction. An antiserum directed against the ␣-subunit specifically coprecipitated ␤-catenin from brain synaptic proteins. ␤-Catenin is known to associate with the synaptic protein Lin7͞Velis͞MALS, whose interaction partner Lin2͞CASK also binds voltage-gated Ca 2؉ channels. ␤-Catenin may therefore provide a physical link between the two types of channels at the presynaptic active zone.T he hair cell, the sensory receptor of the internal ear, releases neurotransmitter at presynaptic active zones studding its basolateral membrane surface (for a review, see ref. 1). Examination of hair cells by loose-seal patch recording demonstrated that each active zone bears a cluster of voltage-gated Ca 2ϩ (Ca V ) channels and Ca 2ϩ -activated K ϩ (K Ca ) channels (2-4). When Ca V channels are activated by a hair cell's depolarization in response to acoustical stimulation, the resultant Ca 2ϩ influx not only triggers synaptic-vesicle release but also opens K Ca channels. The efflux of K ϩ through these channels causes a rapid repolarization that plays a key role in the electrical oscillation that tunes some hair cells (5) and in fast inhibitory synaptic transmission (6). Nerve terminals of the central and peripheral nervous system, including those at the neuromuscular junction, use a similar mechanism to effect repolarization in anticipation of the next action potential (7-11). The presynaptic colocalization of K Ca channels with Ca V channels is thus indispensable for normal electrical signaling by excitatory cells.Electrophysiological and molecular-biological analyses have shown that hair cells express L-type Ca V channels and largeconductance (BK or Maxi) K Ca channels (12-15) that contain, respectively, ␣ 1D -subunits (16-18) and slowpoke (slo) ␣-and slo ␤-subunits (19-22). In hippocampal neurons, both large-and small-conductance K ϩ channels occur in presynaptic regions and are associated, respectively, with L-and N-type Ca V channels (7, 23). The Ca V channels at the presynaptic active zone form a protein complex with several anchoring proteins (24-26), such as Mint-1, CASK, and Rab3-interacting molecule (RIM) binding proteins (RBP). In Drosophila, two cytosolic regulatory proteins, Slob and dSLIP, and two protein kinases, Src and PKAc, are able to assemble with the slo ␣-subunit (27-29). In vertebrates, however, only kinases, such as Src and Syk, have been reported to interact with this subunit (30-32). To seek other proteins that associate with K Ca channels and mediate their presynapti...

“…Presynaptic elements adjacent to SVs and plasma membranes also appear with different aspects. In most cases, they are fuzzy but there are also observations of a regular organization (12)(13)(14).…”

mentioning

confidence: 99%

The mammalian central nervous synaptic cleft contains a high density of periodically organized complexes

Zuber

Nikonenko

Klauser

et al. 2005

213

176

Cryo-electron microscopy of vitreous section makes it possible to observe cells and tissues at high resolution in a close-to-native state. The specimen remains hydrated; chemical fixation and staining are fully avoided. There is minimal molecular aggregation and the density observed in the image corresponds to the density in the object. Accordingly, organotypic hippocampal rat slices were vitrified under high pressure and controlled cryoprotection conditions, cryosectioned at a final thickness of Ϸ70 nm and observed below ؊170°C in a transmission electron microscope. The general aspect of the tissue compares with previous electron microscopy observations. The detailed analysis of the synapse reveals that the density of material in the synaptic cleft is high, even higher than in the cytoplasm, and that it is organized in 8.2-nm periodic transcleft complexes. Previously undescribed structures of presynaptic and postsynaptic elements are also described.Cryo-electron microscopy of vitreous section ͉ high-pressure freezing ͉ hippocampus S ynapses of the central nervous system (CNS) play a key role in neuronal information processing. Their physiology and structural organization have been extensively characterized (1, 2). The presynaptic compartment contains synaptic vesicles (SVs) filled with neurotransmitters. There is an intensive tethering and fusion activity between SVs and the presynaptic membrane. The postsynaptic membrane is covered with neurotransmitter receptors, which detect variations in neurotransmitter concentration. Postsynaptic submembrane cytoplasm is occupied by a complex network of proteins, the postsynaptic density, which modulates the strength of synaptic transmission. The space between both cells is the synaptic cleft. It contains neurotransmitter receptors and various adhesion molecules, such as N-cadherin and neural cell adhesion molecule. Their function in synapse formation and regulation is only beginning to be understood, and their precise spatial organization remains unclear (3).Resolving the molecular structure of the synapse is difficult because, on one hand, high resolution x-ray diffraction data are obtained outside their cellular context (4, 5). On the other hand, conventional electron microscopy generally fails to resolve individual proteins in their natural environment because the preparation relies on chemical fixation, dehydration, resinembedding, and heavy-metal staining and, thus, produces aggregation artifacts and artificial contrast, resulting in a limited resolution and a difficulty of interpreting the nature of the observed structures (6, 7). Freeze substitution and freeze etching, by which the specimen is immobilized by freezing and dehydrated at low temperature before being embedded in resin or replicated, were developed to reduce preparation artifacts but, even under these conditions, aggregation cannot be completely avoided. Depending on the protocols used for specimen preparation, the synaptic cleft presents different aspects. A dense central plaque was observed in some s...