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S. D. Crish, I. D. Raymond, N. C. Brecha, D. J. Calkins; Structural Persistence of the Retinal Ganglion Cell Projection in the DBA/2J Mouse Model of Pigmentary Glaucoma. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2777.
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© ARVO (1962-2015); The Authors (2016-present)
We used the DBA/2J mouse model of pigmentary glaucoma to compare the progression of deficits in anterograde axonal transport with synapse loss and axon degeneration in the superior colliculus (SC), the primary target for retinal ganglion cells (RGCs) in the rodent.
We injected the neuronal tracer cholera toxin β-subunit (CTB) intravitreally in DBA/2J mice 3-22 months of age to examine active anterograde transport to the brain. Forty-eight hours after injection, the animals were perfused and the brains harvested and sectioned to quantify transport to the SC. We used antibodies against the pre-synaptic protein VGluT2 (vesicular glutamate transporter 2) to label RGC synapses in the SC and the myelin stain Black Gold II to investigate the integrity of the axon itself. We also sectioned brains from thy1-CFP DBA/2J transgenic mice to directly visualize RGC axons in the SC.
C57/BL6 and DBA/2J mice with full retinotopic representation of CTB transport in the SC also demonstrated a complete and co-localizingVGluT2 distribution, supporting the finding that VGluT2 is selective for RGC pre-synaptic terminals. VGluT2 persisted in the SC even in the complete absence of intact active transport, which occurred at 10-12 months of age. Loss of VGluT2 in older SC (up to 22 months) was retinotopically discrete, and only a few SC exhibited widespread reductions of VGluT2. Conversely, some of the oldest SC (22 months) retained normal VGluT2 distributions. Strong myelin staining was evident in both the optic tract and in the RGC axon-rich layer III of the SC, even the oldest animals in which CTB was completely absent. Similarly, in the Thy1 reporter mouse RGC axons persisted in the optic tract from the chiasm to layer III of the SC even in aged (15 month) animals.
Even with loss of distal transport, components of the RGC projection in the optic tract and SC persist. RGC synapses (VGluT2), myelin (Black Gold II), and axonal surface proteins (Thy1.1) are evident even in the oldest animals demonstrating complete loss of active axonal transport. These data suggest that transport deficits are pre-degenerative for the RGC in glaucoma, since structural elements of the axons and synapses persist long after functional axonopathy has occurred.
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