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Christian Puller, Michael B. Manookin, Maureen Neitz, Jay Neitz; Syntaxin-4 Is Highly Enriched Beneath S-cone Pedicles In The Primate Retina. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6323.
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© ARVO (1962-2015); The Authors (2016-present)
Syntaxins are well known for their role as SNARE proteins in vesicular trafficking and exocytosis. In the mammalian retina, Syntaxin-4 has been shown to be expressed by horizontal cells and amacrine cells (Sherry et al., BMC Neurosci. 2006 13;7:54; Hirano et al., Vis Neurosci. 2007 24:489-502). Here, we investigated the localization of syntaxin-4 in the primate retina with a focus on its distribution in the outer plexiform layer.
We applied immunohistochemistry in combination with confocal microscopy on cryosections and whole-mount preparations of adult macaque monkeys. Antibodies against syntaxin-4 were used in double labeling experiments with various cell markers or different synaptic proteins to identify the relative location of syntaxin-4.
The staining pattern of syntaxin-4 in primates was similar to what has been observed in inner and outer plexiform layers of other species. In the outer plexiform layer, syntaxin-4 was expressed on dendrites and axon terminals of horizontal cells within the invaginations at rod spherules and cone pedicles. At the latter, syntaxin-4 appeared densely clustered primarily in two bands, above and below the cone pedicle base. The punctate bands were located at the horizontal cell dendritic tips and at the level of desmosome-like junctions. Interestingly, syntaxin-4 was highly enriched in the lower band beneath short-wavelength sensitive (S) cones. Other proteins, such as SNAP-25, a putative binding partner of syntaxin-4, or different transmitter receptors did not show an enhanced clustering at these sites. In experiments with mouse retina, this particular enrichment of syntaxin-4 at S-cones was not observed.
While syntaxin-4 appears to be expressed by both H1 and H2 horizontal cell types in primate, the intense clustering at S-cones points to an enhanced expression in H2 cells. It has been hypothesized that syntaxin-4 may play a role in vesicular GABA release by horizontal cells. Taking this idea into account, our results suggest increased levels of GABA beneath S-cones in comparison to middle- or long-wavelength sensitive cones in primate retina. This site of enhanced GABA release is in the right position to explain features of the color opponent response properties of small bistratified ganglion cells which are clearly distinct from color opponent midget cells.
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