An essential step in intricate visual processing is the segregation of visual signals into ON and OFF pathways by retinal bipolar cells (BCs). Glutamate released from photoreceptors modulates the photoresponse of ON BCs via metabotropic glutamate receptor 6 (mGluR6) and G protein (Go) that regulates a cation channel. However, the cation channel has not yet been unequivocally identified. Here, we report a mouse TRPM1 long form (TRPM1-L) as the cation channel. We found that TRPM1-L localization is developmentally restricted to the dendritic tips of ON BCs in colocalization with mGluR6. TRPM1 null mutant mice completely lose the photoresponse of ON BCs but not that of OFF BCs. In the TRPM1-L-expressing cells, TRPM1-L functions as a constitutively active nonselective cation channel and its activity is negatively regulated by Go in the mGluR6 cascade. These results demonstrate that TRPM1-L is a component of the ON BC transduction channel downstream of mGluR6 in ON BCs.egregation of visual signals into ON and OFF pathways originates in BCs, the second-order neurons in the retina (1, 2). ON and OFF BCs express metabotropic glutamate receptors, mGluR6, and ionotropic glutamate receptors (iGluRs), respectively, on their dendrites (3-5). Reduction of glutamate released from photoreceptors by light stimulation depolarizes ON BCs and hyperpolarizes OFF BCs (6-8) mediated through respective glutamate receptors. The mGluR6 couples to a heterotrimeric G protein complex, Go (9, 10). Signals require Goα, which ultimately closes a downstream nonselective cation channel in ON BCs (6, 9, 11-13). However, this transduction cation channel in ON BCs has not been identified, despite intensive investigation.In our screen to identify functionally important molecules in the retina, we found that TRPM1 is predominantly expressed in retinal BCs. Most members of the TRP superfamily, which are found in a variety of sense organs, are non-voltage-gated cation channels (14-16). The founding member of the TRP family was discovered as a key component of the light response in Drosophila photoreceptors (17). TRPM1, also known as melastatin, was the first member of the melanoma-related transient receptor potential (TRPM) subfamily to be discovered (18,19). TRPM1 is alternatively spliced, resulting in the production of a long form (TRPM1-L) and a short N-terminal form devoid of transmembrane segments (TRPM1-S) (18,20). Although mouse TRPM1-S was previously identified as melastatin, mouse TRPM1-L has not been identified (18). The distinct physiological and biological functions of TRPM1 still remain elusive, although some recent evidences including us suggested that TRPM1 might contribute to retinal BC function (21-23). Here, we show that TRPM1-L is the transduction cation channel of retinal ON BCs in the downstream of mGluR6 cascade.

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.

No abstract

No abstract

Self-assembly of thiol-terminated oligonucleotides on gold substrates provides a convenient and versatile route to DNA-functionalised surfaces. Here we show that the square-wave voltammetric peak position of methylene blue complexed to thiol-terminated single-stranded oligonucleotides immobilised on gold electrodes differs from that of methylene blue complexed to thiol-terminated double-stranded oligonucleotides immobilised on gold electrodes. The peak potential of methylene blue at the single-stranded oligonucleotide array was consistently found to occur at potentials ca. 10–15 mV more positive than that at double-stranded oligonucleotide arrays, the precise difference being dependent on the direction of the voltammetry. This voltammetric behaviour mirrors that found for methylene blue bound to freely diffusing single- and double-stranded calf thymus DNA and suggests that the immobilised oligonucleotides retain the methylene blue binding properties of their freely diffusing counterparts. Thus methylene blue provides a simple electrochemical indicator for the status of oligonucleotide-functionalised gold surfaces