Pikachurin, a dystroglycan ligand, is essential for photoreceptor ribbon synapse formation Shigeru Sato 1–3 , Yoshihiro Omori 1 , Kimiko Katoh 1 , Mineo Kondo 4 , Motoi Kanagawa 5 , Kentaro Miyata 4 , Kazuo Funabiki 6 , Toshiyuki Koyasu 4 , Naoko Kajimura 7 , Tomomitsu Miyoshi 8 , Hajime Sawai 8 , Kazuhiro Kobayashi 5 , Akiko Tani 1 , Tatsushi Toda 5 , Jiro Usukura 9 , Yasuo Tano 2 , Takashi Fujikado 2,3 & Takahisa Furukawa 1 Exquisitely precise synapse formation is crucial for the mammalian CNS to function correctly. Retinal photoreceptors transfer information to bipolar and horizontal cells at a specialized synapse, the ribbon synapse. We identified pikachurin, an extracellular matrix–like retinal protein, and observed that it localized to the synaptic cleft in the photoreceptor ribbon synapse. Pikachurin null-mutant mice showed improper apposition of the bipolar cell dendritic tips to the photoreceptor ribbon synapses, resulting in alterations in synaptic signal transmission and visual function. Pikachurin colocalized with both dystrophin and dystroglycan at the ribbon synapses. Furthermore, we observed direct biochemical interactions between pikachurin and dystroglycan. Together, our results identify pikachurin as a dystroglycan-interacting protein and demonstrate that it has an essential role in the precise interactions between the photoreceptor ribbon synapse and the bipolar dendrites. This may also advance our understanding of the molecular mechanisms underlying the retinal electrophysiological abnormalities observed in muscular dystrophy patients. The establishment of precise synaptic connections between neurons in the developing and mature CNS is crucial for normal nervous system functions, including perception, memory and cognition. Thus, eluci- dating the mechanisms by which synapses develop and are modified is a central aim in neurobiology. Over the past few decades, a large number of protein components have been identified that are required for synapse morphogenesis and neurotransmitter release 1,2 . However, the molecules and mechanisms underlying specific synapse connections in the vertebrate CNS are still poorly understood. The neural retina is developmentally a part of the CNS and is where the first stage of visual signal processing occurs. Visual information is transmitted from photoreceptor cells to the ganglion cells via bipolar interneurons. The photoreceptor axon terminal forms a specialized structure, the ribbon synapse, which specifically connects photorecep- tor synaptic terminals with bipolar and horizontal cell terminals in the outer plexiform layer (OPL) of the retina. Although various presynaptic factors that are required for synaptic ribbon structure, such as CtBp2/ RIBEYE, piccolo and bassoon, have been identified 3,4 , the mechanism of ribbon synapse apposition specific to bipolar and horizontal terminals remains totally unknown. Mutations in the dystrophin-glycoprotein complex (DGC) cause various forms of muscular dystrophy 5 . Dystroglycan, a central compo- nent of the DGC, functions as a cellular receptor that is expressed in a variety of tissues, including the CNS 6 . Dystroglycan precursor protein is cleaved into two subunits, a-dystroglycan and b-dystroglycan 7 . a-dystroglycan is a heavily glycosylated extracellular protein and has the potential to bind to several extracellular proteins containing the laminin-G domain, including laminin-a1, laminin-a2, agrin, perlecan and neurexins 8–11 . The DGC components are also expressed in the retina 12–15 . Altered electroretinograms (ERGs) are frequently found in individuals with Duchenne and Becker muscular dystrophy, indicating that the DGC is necessary for normal retinal physiology 16–18 . However, the functional role of DGC in the retina is elusive. We isolated and characterized mouse pikachurin, a dystroglycan ligand in the retina. To the best of our knowledge, pikachurin is the first dystroglycan ligand to interact with the presynaptic dystro- glycan. Our results demonstrate that pikachurin is critically involved in both the normal photoreceptor ribbon synapse formation and physiological functions of visual perception. This may also shed light on the molecular mechanisms underlying the retinal Received 1 May; accepted 12 June; published online 20 July 2008; doi:10.1038/nn.2160 1 Department of Developmental Biology, Osaka Bioscience Institute, 6-2-4 Furuedai, Suita, Osaka, 565-0874, Japan. 2 Department of Ophthalmology, Osaka University Graduate School of Medicine, 2-2 Yamadaoka, Suita, Osaka, 565-0871, Japan. 3 Department of Visual Science, Osaka University Graduate School of Medicine, 2-2 Yamadaoka, Suita, Osaka, 565-0871, Japan. 4 Department of Ophthalmology, Nagoya University Graduate School of Medicine, 65 Tsuruma-cho, Showa-ku, Nagoya, 466-8550, Japan. 5 Division of Clinical Genetics, Department of Medical Genetics, Osaka University Graduate School of Medicine, 2-2 Yamadaoka, Suita, Osaka, 565-0871, Japan. 6 Department of Systems Biology, Osaka Bioscience Institute, 6-2-4 Furuedai, Suita, Osaka, 565-0874, Japan. 7 Research Center for Ultrahigh-Voltage Electron Microscopy, Osaka University, 7-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan. 8 Department of Physiology, Osaka University Graduate School of Medicine, 2-2 Yamadaoka, Suita, Osaka, 565-0871, Japan. 9 Department of Materials Physics and Engineering, Nagoya University Graduate School of Engineering, 1-1 Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan. Correspondence should be addressed to T.F. (furukawa@obi.or.jp). NATURE NEUROSCIENCE VOLUME 11 [ NUMBER 8 [ AUGUST 2008 923 ARTICLES © 2008 Nature Publishing Group http://www.nature.com/natureneuroscience