Synapses are specialized junctions through which neurons signal to each other and to other target cells such as muscles and are crucial to the functioning of the nervous system. However, the mechanism of how the synapses form during development remains poorly understood. In this study, we applied live imaging of fluorescent fusion proteins expressed in the target cells to visualize the process of synapse formation in developing embryos, and showed that a cell adhesion molecule, called Fasciclin2, mediates the cellular interaction between a neuron and its target, that is required for the accumulation of synaptic components. Our results elucidate a molecular mechanism of synapse formation that occurs in an intact organism.
How the innervating axons induce the assembly of synaptic components at the site of synaptic contact with their target cells is a fundamental issue in neuroscience. Synaptic induction is thought to be mediated by membrane-bound and/or diffusible molecules. Cell adhesion molecules (CAMs), in particular, have long been proposed to have instrumental roles in triggering synaptic assembly, for the following two reasons. First, CAMs may provide an adhesive link between pre- and postsynaptic cells by holding the synaptic contacts together. Second, by binding to other synaptic components such as synaptic scaffolding proteins, CAMs may induce the intracellular signalling necessary for synaptic differentiation. CAMs may therefore provide the earliest trans-synaptic scaffold upon which other synaptic components are later recruited (see Figure 1). Consistent with this idea, several cell adhesion molecules have previously been implicated in synapse formation in cultured neurons. However, their roles in developing animals remain largely unknown. Little is also known about the dynamics of CAMs or other synaptic molecules when the synapses are initially formed in the embryo. This is likely because it is usually difficult to identify newly forming synapses in the embryo and furthermore to image the process of molecular assembly. Here, we characterized the function and dynamics of a CAM, Fasciclin2, during synapse formation de novo, by applying high resolution live-imaging analyses to the highly accessible neuromuscular synapses in Drosophila (fruitfly) embryos.
We studied the function and dynamics of Fasciclin2 (Fas2), a CAM expressed both pre- and postsynaptically during neuromuscular synapse formation in Drosophila. We followed the dynamics of fluorescent Fas2 fusion proteins expressed in muscles and find that Fas2 accumulates at the synaptic contact site soon after the arrival of the nerve (Figure 1). Genetic, deletion, and photobleaching analyses suggest that Fas2-mediated trans-synaptic adhesion is important for the postsynaptic accumulation of Fas2 itself and other synaptic components such as a scaffolding protein called Discs large and neurotransmitter receptors. These results provide in vivo evidence for the role of trans-synaptic adhesion in triggering molecular assembly at nascent postsynaptic sites (Figure 2).
This work has been published online in the December 17th issue of The Journal of Cell Biology.
Hiroshi Kohsaka, Etsuko Takasu, and Akinao Nose. In vivo induction of postsynaptic molecular assembly by the cell adhesion molecule Fasciclin2. J. Cell Biol. 179 1289-1300 (2007).
Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, University of Tokyo
Akinao Nose (nosek.u-tokyo.ac.jp)
Hiroshi Kohsaka (kohsakaneuro.k.u-tokyo.ac.jp)
Figure 1. In vivo visualization of the cell adhesion molecule Fasciclin2 in a whole Drosophila embryo (left, green color). The motor axons are also visualized (red). The right panels are time-lapse images which show that Fasciclin2 (green) accumulates at the synaptic sites soon after the arrival of the nerve (red). The accumulation is seen in white because of the overlap of green and red color.
Figure 2. A model of synaptic induction by cell adhesion molecules.
A schematic diagram showing how cell adhesion molecules (CAMs) induce the clustering of other synaptic molecules (yellow) at the site of synaptic contact.