Mouse metabotropic glutamate receptor 6 (GRM6) minimal enhancer/SV40 basal promoter (0.4 kb)
Compact regulatory element combining a 200 bp enhancer from the mouse Grm6 (mGluR6) gene with a basal SV40 minimal promoter, conferring selective expression in retinal ON bipolar cells. First used to target channelrhodopsin-2 to ON bipolar cells for optogenetic restoration of light responses in mouse models of retinal degeneration.
Origin: Mouse Grm6 gene, 200 bp enhancer fused to SV40 minimal promoter
Characteristics
The 200 bp Grm6 enhancer (200En) alone is insufficient to drive expression and requires fusion to a basal SV40 promoter for activity (Lagali et al. 2008). In vivo electroporation of this 200En-SV40 cassette into rd1 (Pde6b-mutant) mouse retinas restricted channelrhodopsin-2 expression to ON bipolar cells and restored light-evoked responses and visually guided behavior. When delivered by AAV rather than electroporation, this minimal cassette produces comparatively weak transduction and some off-target expression in non-bipolar inner nuclear layer neurons (Cronin et al. 2016).
Applications: Used to restrict optogenetic actuators such as channelrhodopsin-2 to retinal ON bipolar cells for vision restoration in mouse models of retinal degeneration (e.g., the rd1 line), delivered by in vivo electroporation or packaged into AAV vectors where its small size (~0.4 kb) leaves room for larger effector transgenes.
Limitations: AAV-mediated expression from this minimal 200En-SV40 cassette is relatively weak and shows some off-target expression in non-bipolar inner nuclear layer neurons; the longer grm6-long cassette achieves over 20-fold higher and more selective AAV-mediated expression in bipolar cells (Cronin et al. 2016).