Reduced expression of the nob8 gene does not normalize the distribution or function of mGluR6 in the mouse retina

PURPOSE: The Grm6(nob8) mouse carries a missense mutation in the Grm6 gene (p.Met66Leu), and exhibits a reduced b-wave of the electroretinogram (ERG), abnormal localization of metabotropic glutamate receptor 6 (mGluR6) to the depolarizing bipolar cell (DBC) soma, and a reduced level of mGluR6 at the DBC dendritic tips. Although the underlying mechanism remains unknown, one possible explanation is that DBCs cannot efficiently traffic the mutant mGluR6. In that scenario, reducing the total amount of mutant mGluR6 protein might normalize localization, and thus, improve the ERG phenotype as well. The second purpose of this study was to determine whether the abnormal cellular distribution of mutant mGluR6 in Grm6(nob8) retinas might induce late onset DBC degeneration. METHODS: We crossed Grm6(nob8) animals with Grm6(nob3) mice, which carry a null mutation in Grm6, to generate Grm6(nob3/nob8) compound heterozygotes. We used western blotting to measure the total mGluR6 content, and immunohistochemistry to document mGluR6 localization within DBCs. In addition, we examined outer retinal function with ERG and retinal architecture in vivo with spectral domain optical coherence tomography (SD-OCT). RESULTS: The retinal content of mGluR6 was reduced in the retinas of the Grm6(nob3/nob8) compound heterozygotes compared to the Grm6(nob8) homozygotes. The cellular distribution of mGluR6 in the Grm6(nob3/nob8) compound heterozygotes matched that of the Grm6(nob8) homozygotes, with extensive expression throughout the DBC cell body and limited expression at the DBC dendritic tips. The dark-adapted ERG b-waves of the Grm6(nob3/nob8) mice were reduced in comparison to those of the Grm6(nob8) homozygotes at postnatal day 21 and 28. The overall ERG waveforms obtained from 4- through 68-week old Grm6(nob8) mice were in general agreement for dark- and light-adapted conditions. The maximum response and sensitivity of the dark-adapted ERG b-wave did not change statistically significantly with age. SD-OCT revealed the maintained laminar structure of the retina, including a clear inner nuclear layer (INL) at each age examined (from 11 to 57 weeks old), although the INL in the mice older than 39 weeks of age was somewhat thinner than that seen at 11 weeks. CONCLUSIONS: Mislocalization of mutant mGluR6 is not normalized by reducing the total mGluR6. Mislocalized mutant mGluR6 does not trigger substantial loss of DBCs.