Abstract
Many studies on retinal injury and repair following elevated intraocular pressure suggest that the survival ratio of retinal neurons has been improved by various measures. However, the visual function recovery is far lower than expected. The homeostasis of retinal synapses in the visual signal pathway is the key structural basis for the delivery of visual signals. Our previous studies found that complicated changes in the synaptic structure between retinal neurons occurred much earlier than obvious degeneration of retinal ganglion cells in rat retinae. The lack of consideration of these earlier retinal synaptic changes in the rescue strategy may be partly responsible for the limited visual function recovery with the types of protective methods for retinal neurons used following elevated intraocular pressure. Thus, research on the modulatory mechanisms of the synaptic changes after elevated intraocular pressure injury may give new light to visual function rescue. In this study, we found that thrombospondin 2, an important regulator of synaptogenesis in central nervous system development, was distributed in retinal macroglia cells, and its receptor α2δ-1 was in retinal neurons. Cell cultures including mixed retinal macroglia cells/neuron cultures and retinal neuron cultures were exposed to elevated hydrostatic pressure for 2 h. The expression levels of glial fibrillary acidic protein (the marker of activated macroglia cells), thrombospondin 2, α2δ-1 and presynaptic proteins were increased following elevated hydrostatic pressure in mixed cultures, but the expression levels of postsynaptic proteins were not changed. SiRNA targeting thrombospondin 2 could decrease the upregulation of presynaptic proteins induced by the elevated hydrostatic pressure. However, in retinal neuron cultures, elevated hydrostatic pressure did not affect the expression of presynaptic or postsynaptic proteins. Rather, the retinal neuron cultures with added recombinant thrombospondin 2 protein upregulated the level of presynaptic proteins. Finally, gabapentin decreased the expression of presynaptic proteins in mixed cultures by blocking the interaction of thrombospondin 2 and α2δ-1. Taken together, these results indicate that activated macroglia cells may participate in alterations of presynaptic proteins of retinal neurons following elevated hydrostatic pressure, and macroglia-derived thrombospondin 2 may modulate these changes via binding to its neuronal receptor α2δ-1.
Highlights
Elevated intraocular pressure (IOP) is an important risk factor for the degeneration of retinal neurons and causes visual deficits in some diseases such as glaucoma, diabetic retinopathy and age-related macular degeneration[1,2,3,4]
TSP1 is expressed in neurons and not significantly changed with IOP. These results suggest that TSP2 is the most likely factor to be secreted by activated macroglia cells and might be involved in synaptic alterations in retinal neurons after elevated IOP. α2δ-1 is a subunit of voltage-gated calcium channel and the neuronal TSP2 receptor identified by Eroglu and colleagues, which played an important role in central nervous system (CNS) synaptogenesis [29]
Double immunofluorescence staining was performed, and TSP2 staining was co-localized with glial fibrillary acidic protein (GFAP) immunoreactivity (Fig 2A). α2δ-1 was strongly expressed in the retinal neuron bodies, and reduced staining was observed in dendrites visualized by co-localization with Map2, which is consistent with Farrell’s report[36] (Fig 2B)
Summary
Elevated intraocular pressure (IOP) is an important risk factor for the degeneration of retinal neurons and causes visual deficits in some diseases such as glaucoma, diabetic retinopathy and age-related macular degeneration[1,2,3,4]. The retina is composed of five major neuron types: photoreceptor cells, interneurons (bipolar, horizontal, and amacrine cells) and retinal ganglion cells (RGCs), which form the synapses in the retinal outer and inner plexiform layers[8, 9] Through these synapses, the visual signal is preliminarily integrated and transmitted to the brain[10]. Park’s and our previous study found that the presynaptic functional protein synaptophysin (SYN) exhibited spatiotemporal alterations immediately after acute elevated IOP, which were changed before RGC death[13, 14] These changes were limited to the presynaptic components without being accompanied by alterations in postsynaptic elements, which means that no new and functional synapses were formed during this process[13]. The regulatory mechanisms of these synaptic changes after elevated IOP remain unclear and need to be explored
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