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Front Mol Neurosci. 2012 Feb 16;5:18. doi: 10.3389/fnmol.2012.00018. eCollection 2012.

Transgenic strategy for identifying synaptic connections in mice by fluorescence complementation (GRASP).

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Department of Molecular and Cellular Biology and Center for Brain Science, Harvard University, Cambridge MA, USA.


In the "GFP reconstitution across synaptic partners" (GRASP) method, non-fluorescent fragments of GFP are expressed in two different neurons; the fragments self-assemble at synapses between the two to form a fluorophore. GRASP has proven useful for light microscopic identification of synapses in two invertebrate species, Caenorhabditis elegans and Drosophila melanogaster, but has not yet been applied to vertebrates. Here, we describe GRASP constructs that function in mammalian cells and implement a transgenic strategy in which a Cre-dependent gene switch leads to expression of the two fragments in mutually exclusive neuronal subsets in mice. Using a transgenic line that expresses Cre selectively in rod photoreceptors, we demonstrate labeling of synapses in the outer plexiform layer of the retina. Labeling is specific, in that synapses made by rods remain labeled for at least 6 months whereas nearby synapses made by intercalated cone photoreceptors on many of the same interneurons remain unlabeled. We also generated antisera that label reconstituted GFP but neither fragment in order to amplify the GRASP signal and thereby increase the sensitivity of the method.


GFP; GRASP; neurexin; neuroligin; photoreceptor; retina; rod; synapse

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