Abstract
The transcription factor Prox1 is expressed in multiple cells in the retina during eye development. This study has focused on neuronal Prox1 expression in the inner nuclear layer (INL) of the adult mouse retina. Prox1 immunostaining was evaluated in vertical retinal sections and whole mount preparations using a specific antibody directed to the C-terminus of Prox1. Strong immunostaining was observed in numerous amacrine cell bodies and in all horizontal cell bodies in the proximal and distal INL, respectively. Some bipolar cells were also weakly immunostained. Prox1-immunoreactive amacrine cells expressed glycine, and they formed 35 ± 3% of all glycinergic amacrine cells. Intracellular Neurobiotin injections into AII amacrine cells showed that all gap junction-coupled AII amacrine cells express Prox1, and no other Prox1-immunostained amacrine cells were in the immediate area surrounding the injected AII amacrine cell. Prox1-immunoreactive amacrine cell bodies were distributed across the retina, with their highest density (3887 ± 160 cells/mm2) in the central retina, 0.5 mm from the optic nerve head, and their lowest density (3133 ± 350 cells/mm2) in the mid-peripheral retina, 2 mm from the optic nerve head. Prox1-immunoreactive amacrine cell bodies comprised ~9.8% of the total amacrine cell population, and they formed a non-random mosaic with a regularity index (RI) of 3.4, similar to AII amacrine cells in the retinas of other mammals. Together, these findings indicate that AII amacrine cells are the predominant and likely only amacrine cell type strongly expressing Prox1 in the adult mouse retina, and establish Prox1 as a marker of AII amacrine cells.
Highlights
Amacrine cells are the most diverse group of cells within the mammalian retina with more than 40 types, distinguished by their size, axonal and dendritic architecture and neurotransmitter content
The transcription factor Prox1 is strongly expressed in a single row of amacrine cell bodies in the inner nuclear layer (INL) at the inner plexiform layer (IPL) border, corresponding to AII amacrine cells
Our immunohistochemical experiments demonstrated that Prox1-immunoreactive amacrine cells label ∼35% of the narrow-field glycine-immunoreactive cells
Summary
Amacrine cells are the most diverse group of cells within the mammalian retina with more than 40 types, distinguished by their size, axonal and dendritic architecture and neurotransmitter content (for reviews see Wässle and Boycott, 1991; MacNeil and Masland, 1998; MacNeil et al, 1999; Masland, 2001, 2012). The glycineimmunoreactive amacrine cells have narrow-field processes that span multiple IPL laminae, and several types contain a second immunohistochemical marker, including parvalbumin, calretinin and Disabled 1 (Wässle et al, 1993; Haverkamp and Wässle, 2000, 2004; Rice and Curran, 2000; Lee et al, 2006, 2016). An exception to this general principle is that VGluT3immunoreactive amacrine cells, which have medium-field processes distributed to multiple IPL laminae, exhibit both glycine and glutamate immunoreactivity (Haverkamp and Wässle, 2004; Johnson et al, 2004; Grimes et al, 2011; Kim et al, 2015)
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