Although the binding of synaphin (also called complexin) to the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex is critical for synaptic vesicle exocytosis, the exact role of synaphin remains unclear. Here, we show that synaphin directly binds to synaptotagmin 1, a major Ca2+ sensor for fast neurotransmitter release, in a 1:1 stoichiometry. Mapping of the synaphin site involved in synaptotagmin 1 binding revealed that the C-terminal region is essential for the interaction between these two proteins. Binding was sensitive to ionic strength, suggesting the involvement of charged residues in the C-terminus region. Mutation of the seven consecutive glutamic acid residues (residues 108–114) at the C-terminal region of synaphin to alanines or glutamines resulted in a dramatic reduction in synaptotagmin 1 binding activity. Furthermore, a peptide from the C-terminus of synaphin (residues 91–124) blocked the binding of synaptotagmin 1 to synaphin, an effect that was abolished by mutating the consecutive glutamic acid residues to alanine. Immunoprecipitation experiments with brain membrane extracts showed the presence of a complex consisting of synaphin, synaptotagmin 1, and SNAREs. We propose that synaphin recruits synaptotagmin 1 to the SNARE-based fusion complex and synergistically functions with synaptotagmin 1 in mediating fast synaptic vesicle exocytosis.
For quantitative measurements of Ca2+ concentration ([Ca2+]), ratiometric dyes are preferable, because the use of such dyes allows for correction of uneven loading or partitioning of dye within the cell as well as variations in cell thickness. Although dual-excitation ratiometric dyes for measuring [Ca2+], such as Fura-2, Fura-Red, and ratiometric-pericam, are widely used for a variety of applications, it has been difficult to use them for monitoring very fast Ca2+ dynamics or Ca2+ changes in highly motile cells. To overcome this problem, we have developed three new dual-excitation ratiometry systems. (1) A system in which two laser beams are alternated on every scanning line, allowing us to obtain confocal images using dual-excitation ratiometric dyes. This system increases the rate at which ratio measurements can be made to 200 Hz and provides confocal images at 1–10 Hz depending on the image size. (2) A truly simultaneous dual-excitation ratiometry system that used linearly polarized excitation light and polarization detection, allowing us to obtain ratiometric images without any time lag. This system, however, is based on statistical features of the fluorescence polarization and is limited to samples that contain a large number of fluorophores. In addition, this method requires complicated calculations. (3) An efficient, nearly simultaneous dual-excitation ratiometry system that allows us to rapidly switch between two synchronized excitation-detection components by employing two high-power light-emitting diodes (LEDs) and two high-speed liquid crystal shutters. The open/close operation of the two shutters is synchronized with the on/off switching of the two LEDs. This system increases the rate at which ratio measurements are made to 1 kHz, and provides ratio images at 10–100 Hz depending on the signal intensity.
Over the past few years, the light-gated cation channel Channelrhodopsin-2 (ChR2) has seen a remarkable diversity of applications in neuroscience. However, commonly used wide-field illumination provides poor spatial selectivity for cell stimulation. We explored the potential of focal laser illumination to map photocurrents of individual neurons in sparsely transfected hippocampal slice cultures. Interestingly, the best spatial resolution of photocurrent induction was obtained at the lowest laser power. By adjusting the light intensity to a neuron’s spike threshold, we were able to trigger action potentials with a spatial selectivity of less than 30 μm. Experiments with dissociated hippocampal cells suggested that the main factor limiting the spatial resolution was ChR2 current density rather than scattering of the excitation light. We conclude that subcellular resolution can be achieved only in cells with a high ChR2 expression level and that future improved variants of ChR2 are likely to extend the spatial resolution of photocurrent induction to the level of single dendrites.
Using pHluorin-tagged synaptic vesicle proteins we have examined the partitioning of these probes into recycling and nonrecycling pools at hippocampal nerve terminals in cell culture. Our studies show that for three of the major synaptic vesicle components, vGlut-1, VAMP-2, and Synaptotagmin I, ~50–60% of the tagged protein appears in a recycling pool that responds readily to sustained action potential stimulation by mobilizing and fusing with the plasma membrane, while the remainder is targeted to a nonrecycling, acidic compartment. The fraction of recycling and nonrecycling (or resting) pools varied significantly across boutons within an individual axon, from 100% resting (silent) to 100% recycling. Single-bouton bleaching studies show that recycling and resting pools are dynamic and exchange between synaptic boutons. The quantitative parameters that can be extracted with the approaches outlined here should help elucidate the potential functional role of the resting vesicle pool.
I regret to inform our readers that this is the final issue of Brain Cell Biology. It seems that the first issue of BCB appeared only yesterday; in fact, it has been less than 2 years since BCB first appeared. Over this time, BCB has made steady progress toward its goal of becoming a primary source for research into the cellular and molecular biology of the nervous system. We have published many cutting-edge articles that have advanced this field and the recent special issue on ‘‘Optogenetic probes of neuronal function and circuitry’’ promises to be a landmark in this area of research. The alltoo-brief lifespan of BCB recalls a line from the classic Beatles song Hello, Goodbye: ‘‘You say goodbye and I say hello’’. It seems that our journal is saying ‘‘goodbye’’ just at the moment it was really starting to say ‘‘hello’’ to its audience. In saying ‘‘goodbye’’, it is important to acknowledge the contributions of those who contributed to BCB. Certainly everyone on our Editorial Board is to be thanked for providing excellent scientific leadership. These editors and the reviewers that they selected are to be commended for their exceptionally fair treatment of articles that were submitted to BCB. None of our authors ever complained of unfair editorial treatment, a remarkable record that is entirely consistent with our goal of making BCB a journal where decisions were made solely on the basis of scientific merit. Finally, we are exceptionally grateful both to the authors who submitted their articles to BCB and to the readers who made production of BCB worthwhile. I will miss reading BCB and expect that many others will as well. Fortunately, BCB articles will continue to be accessible online. This means that BCB will continue to be read and cited, thereby providing an on-going legacy for our journal.
