Novel Expression Patterns of Metabotropic Glutamate Receptor 6 in the Zebrafish Nervous System

Abstract

The metabotropic glutamate receptor 6 (mGluR6 or GRM6) belongs to the class III of the metabotropic glutamate receptor family. It is the only known mGluR that mediates direct synaptic transmission in the nervous system and is thought to mediate the ON-response in the ON-pathway of the vertebrate retina. Phylogenetic and gene structure analysis indicated that the zebrafish genome harbours two mglur6 paralogs, mglur6a and mglur6b. Besides expression in the inner nuclear layer and distinct regions in the brain, both mglur6 paralogs are expressed in ganglion cells of the retina, an expression pattern which can also be observed in the downstream effector molecules gnaoa and gnaob. This unexpected expression pattern is consistent with immunohistological labeling using a peptide antibody specific for the mGluR6b paralog. These expression patterns contradict the existing view that mGluR6 is solely located on ON-bipolar cells where it functions in signal transmission. Consistent with expression in ON-bipolar cells, we report a decreased b-wave amplitude in the electroretinogram after morpholino-based downregulation of mGluR6b, showing a function in the ON response. Our data suggest more widespread functions of mGluR6 mediated signaling in the central nervous system, possibly including sign reversing synapses in the inner retina.

Phylogeny and expression divergence of metabotropic glutamate receptor genes in the brain of zebrafish (Danio rerio)

“…Since rods mature later than cones (Branchek and Bremiller 1984), one could envision a scenario where the cone ON-pathway mainly relies on mglur6b and the rod ON-pathway on mglur6a, which is not expressed until rods are functionally mature. mGluR6 was thought of being exclusively involved in the postsynaptic reception of the ON-pathway signal, however, our finding of mglur6 expression in the inner retina proposes an additional modulatory role (Huang et al, 2012). The fact that mglur6 was recently detected in human retinal ganglion cell bodies (Klooster et al, 2011) indicates that this role might be more widespread in vertebrates than originally anticipated.…”

“…This suggests that the known function in the ON-bipolar cell pathway of the mammalian mGluR6 is fulfilled exclusively by the zebrafish mGluR6b in the larval retina. The expression of both mglur6 paralogs in the retinal bipolar cell layer of adult zebrafish (Huang et al, 2012) suggests -at least for mglur6a -a developmental regulation of gene expression in this retinal cell layer. mglur6 paralogs may have separate roles in the ON-bipolar cell pathway.…”

mglur6b:EGFP Transgenic zebrafish suggest novel functions of metabotropic glutamate signaling in retina and other brain regions

“…Because of its crucial role in retinal signaling, mutations in human MGLUR6 cause congenital stationary night blindness, accompanied by typical loss of the ON response in the electroretinogram (Dryja et al, 2005). Consistently, absence of mGluR6 results in impaired ON signaling in animal models (Huang et al, 2012;Masu et al, 1995;Zeitz et al, 2005). Besides mGluR6 playing a direct role in synaptic transmission, it is also necessary for postsynaptic localization of proteins involved in the ON signaling cascade (Cao et al, 2009;Morgans et al, 2007) and the Ca 2+ channel subunit Cacna1s (Tummala et al, 2014).…”

“…Fluorescent RNA in situ hybridization was performed as previously described (Huang et al, 2012). For signal development, Alexa Tyramide 488 (TSA Kit#12, Molecular Probes, Life…”

“…Work in zebrafish has clearly shown that the two mglur6 paralogs present in the zebrafish genome, mglur6a and mglur6b, are not only expressed in bipolar cells, but also in the proximal inner nuclear layer, retinal ganglion cells and various additional brain regions (Haug et al, 2013;Huang et al, 2012).…”