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Jessica Agostinone, Adriana Di Polo; Activation of mammalian target of rapamycin complex 1 (mTORC1) and 2 (mTORC2) is required for retinal ganglion cell dendrite regeneration after axonal injury.. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):4830.
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© ARVO (1962-2015); The Authors (2016-present)
Emerging data indicate that early retraction of retinal ganglion cell (RGC) dendrites plays a prominent role in neurodegeneration. We recently demonstrated that the mammalian target of rapamycin complex 1 (mTORC1) is a key mediator of dendrite stability in adult RGCs. The purpose of this study was: 1) to determine whether RGC dendrites, once retracted, can regenerate to their initial (pre-injury) length, surface area and complexity; 2) to establish whether mTORC1 activation is sufficient to stimulate RGC dendrite regeneration.
Unilateral optic nerve axotomy was performed in transgenic mice expressing yellow fluorescent protein (YFP) in RGCs under control of the Thy1 promoter. Insulin, a potent activator of both mTORC1 and mTORC2, was administered daily (i.p. 30 u/kg) starting at three days after axotomy, a time when RGC dendrites have already undergone substantial retraction. Retinal whole-mounts were prepared at seven days after axotomy and YFP-positive RGC dendritic trees were reconstructed using Imaris software (Bitplane). Rapamycin, a specific inhibitor of mTORC1, and KU0063794, an inhibitor of both mTORC1 and mTORC2, were used to assess the mechanism regulating dendritic changes.
Our data show that insulin-mediated mTORC1/2 activation promoted dendrite regeneration and restored dendritic length and field area to values similar to those in non-injured RGCs (N=8-10/group, # RGCs: 40/group). Sholl analysis confirmed that the complexity of RGC dendritic trees was also restored with insulin treatment. Importantly, combined administration of insulin with KU0063794, which inhibits both mTORC1 and mTORC2, completely abrogated dendrite regeneration. Interestingly, combined administration of insulin with rapamycin, which blocks mTORC1, resulted only in loss of dendritic tree complexity while dendrite length and field area were preserved. RGC dendrites in retinas treated with vehicle did not regenerate.
Our data support two major conclusions: 1) RGC dendrites are capable of regenerative growth after substantial injury-induced retraction, and 2) mTORC1 is required for dendritic tree complexity, while mTORC2 promotes dendrite extension determining process length and arbor area. The ability to regenerate dendrites in injured RGCs may have implications to prevent synaptic loss and visual deficits in glaucoma.
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