Depolarizing Bipolar Cell Dysfunction due to a Trpm 1 Point Mutation 1

Mutations in TRPM1 are found in humans with an autosomal recessive form of 25 complete congenital stationary night blindness (cCSNB). The Trpm1 mouse has been an 26 important animal model for this condition. Here we report a new mouse mutant, tvrm27, 27 identified in a chemical mutagenesis screen. Genetic mapping of the no b-wave 28 electroretinogram (ERG) phenotype of tvrm27 localized the mutation to a chromosomal region 29 that included Trpm1. Complementation testing with Trpm1 mice confirmed a mutation in 30 Trpm1. Sequencing identified a nucleotide change in exon 23, converting a highly conserved 31 alanine within the pore domain to threonine (p.A1068T). Consistent with prior studies of Trpm1 32 mice, no anatomical changes were noted in the Trpm1 retina. The Trpm1 33 phenotype is distinguished from that of Trpm1 by the retention of TRPM1 expression on the 34 dendritic tips of depolarizing bipolar cells (DBCs). While ERG b-wave amplitudes of Trpm1 35 heterozygotes are comparable to WT, those of Trpm1 mice are reduced by 32%. A similar 36 reduction in the response of Trpm1 DBCs to LY341495 or capsaicin is evident in whole 37 cell recordings. These data indicate that the p.A1068T mutant TRPM1 acts as a dominant 38 negative with respect to TRPM1 channel function. Further, these data indicate that the number 39 of functional TRPM1 channels at the DBC dendritic tips is a key factor in defining DBC response 40 amplitude. The Trpm1 mutant will be useful for elucidating the role of TRPM1 in DBC 41 signal transduction, for determining how Trpm1 mutations impact central visual processing, and 42 for evaluating experimental therapies for cCSNB. 43 . 44