A missense mutation in Grm6 reduces but does not eliminate mGluR6 expression or rod depolarizing bipolar cell function

Abstract

encodes the metabotropic glutamate receptor 6 (mGluR6) used by retinal depolarizing bipolar cells (DBCs). Mutations in lead to DBC dysfunction and underlie the human condition autosomal recessive complete congenital stationary night blindness. Mouse mutants for are important models for this condition. Here we report a new mutant, identified in an electroretinogram (ERG) screen of mice maintained at The Jackson Laboratory. The mouse has a reduced-amplitude b-wave component of the ERG, which reflects light-evoked DBC activity. Sequencing identified a missense mutation that converts a highly conserved methionine within the ligand binding domain to leucine (p.Met66Leu). Consistent with prior studies of mutant mice, the laminar size and structure in the retina were comparable to control. The phenotype is distinguished from other mutants that carry a null allele by a reduced but not absent ERG b-wave, decreased but present expression of mGluR6 at DBC dendritic tips, and mislocalization of mGluR6 to DBC somas. Consistent with a reduced but not absent b-wave, there were a subset of retinal ganglion cells whose responses to light onset have times to peak within the range of those in control retinas. These data indicate that the p.Met66Leu mutant mGluR6 is trafficked less than control. However, the mGluR6 that is localized to the DBC dendritic tips is able to initiate DBC signal transduction. The mouse extends the allelic series and will be useful for elucidating the role of mGluR6 in DBC signal transduction and in human disease. This article describes a mouse model of the human disease complete congenital stationary night blindness in which the mutation reduces but does not eliminate GRM6 expression and bipolar cell function, a distinct phenotype from that seen in other mouse models.

LRIT3 is required for nyctalopin expression and normal ON and OFF pathway signaling in the retina

“…and similar to Grm6 − / − RGCs (median = 550 ms, n=35; p=?) we reported previously (Peachey et al, 2017b).…”

“…We used the same assays of RGC function that defined abnormal response properties of RGCs in other (no b-wave) mouse models of cCSNB, Nyx nob , and Grm6 − / − (Demas et al, 2006) (Peachey et al, 2017a). These abnormalities include the absence of normal latency ON responses (light onset) and rhythmic bursting in almost all RGCs.…”

“…This disruption in PNA expression, led us to examine dysfunction in OFF BC and RGC function in Lrit3 − mice. RGC response properties have been examined in several CSNB1 mouse lines (Nyx nob (Demas et al, 2006), Grm6 − / − (Renteria et al, 2006; Pinto et al, 2007; Peachey et al, 2017a), Trpm1 − / − (Takeuchi et al, 2018) and Lrit3 − / − (Fig. 4) and Lrit3 nob6 (Neuille et al, 2017).…”

“…Methods of the recording procedures for the multi-electrode array (MEA) have been published (Peachey et al, 2017a). Mice were dark-adapted overnight, deeply anesthetized with ketamine/xylazine, and killed by cervical dislocation under dim red light.…”

LRIT3 is Required for Nyctalopin Expression and Normal ON and OFF Pathway Signaling in the Retina

“…Procedures for recording extracellular spiking activity of RGCs, using the MEA have been published (Peachey et al, 2017). Mice were dark-adapted overnight, deeply anesthetized with IP injection of ketamine/xylazine and killed by cervical dislocation under dim red light.…”