Establishment of Transgenic Fluorescent Mice for Screening Synaptogenic Adhesion Molecules

Synapse is the fundamental structure for neurons to transmit information between cells. The proper synapse formation is crucial for developing neural circuits and cognitive functions of the brain. The aberrant synapse formation has been proved to cause many neurological disorders, including autism spectrum disorders and intellectual disability. Synaptic cell adhesion molecules (CAMs) are thought to play a major role in achieving mechanistic cell-cell recognition and initiating synapse formation via trans-synaptic interactions. Although several synaptic CAMs, such as neurexins, neuroligins, SynCAMs, and LRRTMs, have been identified as synaptogenic molecules, these molecules so far as we know cannot fully explain the mechanism of synapse formation. There should be other synaptogenic adhesion molecules that remain undiscovered. Artificial synapse formation (ASF) assays, the commonly used method for screening synaptogenesis molecules, is time-consuming and labor-intensive due to the long-lasting immunostaining step. To skip this step, we generated synaptotagmin 1-tdTomato (Syt1-TDT) transgenic mice by inserting the tdTomato-fused synaptotagmin 1 coding sequence into the genome of C57BL/6J mice. In the brain of Syt1-TDT transgenic mice, the Syt1-TDT signals were widely observed in different areas. In the cultured hippocampal neurons, the Syt1-TDT signals merged with several synaptic markers, suggesting the well synaptic localization of Syt1-TDT. Compared to the wild-type (WT) mouse neurons, cultured hippocampal neurons from Syt1-TDT transgenic mice presented normal synaptic neurotransmission. In ASF assays, neurons from Syt1-TDT transgenic mice could form synaptic connections with HEK293T cells expressing neuroligin2, LRRTM2, and Slitrk2 without immunostaining. Therefore, our work suggested that the Syt1-TDT transgenic mice with the ability to label synapses by tdTomato will be a convenient tool for screening synaptogenic molecules.