Alpha synuclein gene expression profile in the retina of vertebrates

Alpha-synuclein is a Parkinson's disease-linked protein of ubiquitous expression in the central nervous system. It has a proposed role in the modulation of neurotransmission and synaptic function. This study was aimed at analyzing expression of the alpha-synuclein gene in the normal retina, and characterizing its pattern of distribution in the different retinal cell types and layers in a variety of vertebrates, ranging from fish to humans.

Reverse transcriptase-polymerase chain reaction and immunoblotting were used to assess alpha-synuclein expression at both mRNA and protein levels. Its retinal distribution profile was characterized by immunohistochemical methods. With this purpose, retinal sections were analyzed under fluorescent confocal microscopy using specific antibodies against alpha-synuclein, alone and in double or triple combinations with a set of antibodies to molecular markers for the distinct retinal neuronal types. Also, synaptophysin was used as a marker for synaptic vesicles in the retina.

Alpha-synuclein mRNA and protein were expressed by both retinal pigment epithelium (RPE) and neural retinal cells. The pattern of alpha-synuclein distribution in the retina was quite consistent across all vertebrate species examined. A strong immunoreactivity was found in the outer segments (OS) of photoreceptors and in their axon terminals (cone pedicles and rod spherules) in the outer plexiform layer (OPL) of the retina. Alpha-synuclein was also present in rod and cone bipolar cells, as well as in GABAergic and glycinergic amacrines, distributing along a complex plexus throughout the inner plexiform layer (IPL). Additionally, colocalization was found between alpha-synuclein and synaptophysin at presynaptic terminals of the retina. Alpha-synuclein-positive phagosome-like structures were observed in the cytoplasm of RPE cells.

An involvement of alpha-synuclein can be postulated in neurotransmission at axon terminals of photoreceptors in the OPL, and at presynaptic endings of bipolar and amacrine cells in the IPL. As well, this protein could have a role in the function as well as the maintenance of photoreceptor OS. Alpha-synuclein contained in RPE cells should derive not only from protein expression by this cell type, but also from their phagocytosis of OS disc membranes.