Immunolocalization of –synuclein in the Retina of Non–human and Human Primates

Abstract

Abstract: : Purpose: α–Synuclein is a neuron–specific protein highly enriched in presynaptic terminals throughout the brain and postulated to have a role in synapse maintenance and plasticity. A number of studies have linked mutations in its encoding gene and its accumulation in Lewy bodies to the development of juvenile Parkinson’s disease. Here we have made use of immunological methods to characterize α–synuclein expression in the distinct layers and neuronal types, in order to obtain clues on its physiological role in the primate retina. Methods: Retinal, iris and ciliary–body extracts from adult macaques were analyzed by immunoblotting using specific antibodies to human α–synuclein. Cryostat vertical sections of retinas from monkeys and humans were subjected to double and triple immunostaining with antibodies against α–synuclein and a set of molecular markers for the distinct retinal neuronal types. Images were obtained by means of laser–scanning fluorescent confocal microscopy. Results: Among the several ocular fractions analyzed by immunoblotting, α–synuclein expression was especially prominent in the primate neural retina. A strong α–synuclein immunoreactivity was found in the outer segments and axon terminals of photoreceptors (cone pedicles and rod spherules), and this protein was also present in the cone extracellular matrix. α–Synuclein immunolocated as well to the cell bodies and dendrites of bipolar and amacrine (GABAergic and glycinergic) cells, distributing along a complex plexus throughout the IPL. Among amacrine neurons, AII cells were highly enriched in α–synuclein. Interestingly, a partial colocalization was found between α–synuclein and synaptophysin at the synaptic terminals of bipolar and amacrine cells in the IPL. Conclusions: α–Synuclein distribution in photoreceptors suggests an involvement of this protein in outer segment disc formation. Additionally, a role on presynaptic vesicle formation and/or recycling leading to modulation of synaptic transmission at both the OPL and the IPL can be postulated.

Keywords: retinal connections, networks, circuitry • photoreceptors • gene/expression