Large excitatory synapses with multiple active zones ensure reliable and fast information transfer at specific points in neuronal circuits. However, the mechanisms that determine synapse size in CNS circuits are largely unknown. Here we use the calyx of Held synapse, a major relay in the auditory system, to identify and study signaling pathways that specify large nerve terminal size and fast synaptic transmission. Using genome-wide screening, we identified bone morphogenetic proteins (BMPs) as candidate signaling molecules in the area of calyx synapses. Conditional deletion of BMP receptors in the auditory system of mice led to aberrations of synapse morphology and function specifically at the calyx of Held, with impaired nerve terminal growth, loss of monoinnervation and less mature transmitter release properties. Thus, BMP signaling specifies large and fast-transmitting synapses in the auditory system in a process that shares homologies with, but also extends beyond, retrograde BMP signaling at Drosophila neuromuscular synapses.
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We thank A. Grapin-Botton, E. Robertson, P. Scheiffele, M. Müller, O. Kochubey and D. Perkel for discussions, R. Behringer (University of Texas) and K. Lyons (University of California Los Angeles) for sharing mice, D. Constam (Ecole Polytechnique Fédérale de Lausanne) for providing ISH probes, H. Murray for expert technical assistance and O. Burri for help with image analysis. The cDNA array analyses were performed in the DNA array facility Lausanne (DAFL); image acquisition was done at the Bio-optical imaging platform of EPFL. The work was supported by the Swiss National Foundation Science (SNF; Sinergia CRSI33_127440/1), the National Center of Competence in Research of the SNF 'Synaptic Bases of Mental Disease', the Deutsche Forschungsgemeinschaft (DFG; PP 1608) (all to R.S.) and a Marie-Curie Fellowship (IEF-235223-CALYX MMFF; to N.M.).
The authors declare no competing financial interests.
Supplementary Figures 1–5 and Supplementary Table 1 (PDF 1888 kb)
Video of a 3D-reconstructed MNTB neuron and its adjacent calyx synapse in a wild-type P8 mouse. Serial block face scanning EM (SBF-sEM) was used to reconstruct a single postsynaptic MNTB neuron and its adjacent synapses. The postsynaptic neuron is shown in gray; the adjacent excitatory nerve terminals are shown in color code according to their reconstructed volumes (green: 779 μm3; violet: < 10 μm3; see also color scale bar in Figure 5a. (MP4 1269 kb)
Video of a 3D-reconstructed MNTB neuron and its adjacent nerve terminals in a BMPR1a/1b cDKO mouse at P8. Serial block face scanning EM (SBF-sEM) was used to reconstruct several adjacent MNTB neurons, of which one is shown here. The postsynaptic neuron is shown in gray; the adjacent excitatory nerve terminals are shown in color code according to their reconstructed volumes (see color scale bar in Figure 5a). Note that in the BMPR1a/1b cDKO mice, several nerve terminals of intermediate size innervate a given MNTB neuron, suggesting that BMP signaling is involved in synapse elimination at the calyx of Held. (MP4 1624 kb)
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Xiao, L., Michalski, N., Kronander, E. et al. BMP signaling specifies the development of a large and fast CNS synapse. Nat Neurosci 16, 856–864 (2013). https://doi.org/10.1038/nn.3414