mGluR6 is required for cone terminal targeting and synaptic layer assembly during human retinal development

Metabotropic glutamate receptor 6 (mGluR6), encoded by GRM6, is a core component of the ON-bipolar signaling cascade in the retina, but its role in human retinal development remains unclear. Here, we used temporally controlled CRISPR-based genetic ablation in human-induced pluripotent stem cell-derived retinal organoids to define the developmental functions of mGluR6. Unexpectedly, we found that mGluR6 is expressed not only in ON-bipolar cells but also transiently in cone photoreceptors during human retinal development, a pattern not observed in the mouse retina. Early loss of GRM6 before synaptogenesis disrupted cone pedicle architecture, leading to mislocalization of synaptic proteins including Bassoon, ELFN2, and TRPM1, and ultimately resulting in widening or duplication of the outer plexiform layer. In contrast, deletion after synapse formation did not alter retinal architecture. These findings uncover a previously unrecognized, temporally restricted role for mGluR6 in cone terminal targeting and synaptic layer assembly during human retinal development.