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Lauren Lee Daniele, Farida Emran, Glenn Lobo, Robert Gaivin, Brian D Perkins; Wrb functions in a TRC40-dependent pathway required for zebrafish photoreceptor ribbon synapse function. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):4372.
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© ARVO (1962-2015); The Authors (2016-present)
Ribbon synapses of sensory neurons are specialized for graded and sustained glutamate release, with release rates dynamically adjusting to changing stimulus intensity. While the molecular specializations required for transmitter release at ribbon synapses have been studied extensively, the mechanisms for their biosynthesis, trafficking and assembly remain poorly understood. The goal of this study was to investigate the role of the Wrb protein in ribbon synapse function. Wrb is required for the post-translational insertion of tail - anchored membrane proteins via the transmembrane domain recognition complex (TRC) pathway. Zebrafish wrb mutants show defects in visual and mechanosensory function, and ribbon synapse architecture, suggesting a role for the TRC pathway in ribbon synapse function.
Electroretinography (ERG), optokinetic response measurements (OKR), immunohistochemistry and electron microscopy analyses were employed to assess ribbon synapse function, protein expression, and ultrastructure in 5 day old zebrafish larvae. Expression of wrb was investigated with real-time qRT-PCR and in-situ hybridization.
In wrb mutants, OKR saccades were infrequent and contrast response functions were flat. ERG b-wave amplitudes were reduced in mutants, while a-waves were unaffected, indicating defective synaptic transmission between photoreceptors and bipolar cels. Bipolar cells of mutants made fewer projections into cone pedicles and synaptic proteins SV2 and ribeye were partially mislocalized in mutant photoreceptors. Wrb is expressed throughout the CNS and Wrb localized to photoreceptor ER and synapses. Morpholino knockdown of TRC40, a key TRC pathway member, resulted in selective impairments similar to wrb mutants, including reduced OKR contrast sensitivity. Overexpression of wrb fully restored contrast sensitivity in mutants, while overexpression of mutant wrbR73A, which cannot bind TRC40, did not.
Wrb and TRC40 are required for synaptic transmission between photoreceptors and bipolar cells, indicating that TA protein insertion by the TRC pathway is a critical step in ribbon synapse assembly and function
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