The effects of age on mitochondria, axonal transport, and axonal degeneration after chronic IOP elevation using a murine ocular explant model

Abstract

The purpose of this study was to compare younger and older mice after chronic intraocular pressure (IOP) elevation lasting up to 4 days with respect to mitochondrial density, structure, and movement, as well as axonal integrity, in an ex vivo explant model. We studied 2 transgenic mouse strains, both on a C57BL/6J background, one expressing yellow fluorescent protein (YFP) in selected axons and one expressing cyan fluorescent protein (CFP) in all mitochondria. Mice of 4 months or 14 months of age were exposed to chronic IOP by anterior chamber microbead injection for 14 h, 1, 3, or 4 days. The optic nerve head of globe--optic nerve explants were examined by laser scanning microscopy. Mitochondrial density, structure, and movement were quantified in the CFP explants, and axonal integrity was quantified in YFP explants. In control mice, there was a trend towards decreased mitochondrial density (# per mm2) with age when comparing younger to older, control mice, but this was not significant (1947 ± 653 vs 1412 ± 356; p = 0.19). Mitochondrial density decreased after IOP elevation, significantly, by 31%, in younger mice (p = 0.04) but trending towards a decrease, by 22%, in older mice (p = 0.82) compared to age matched controls. Mitochondrial mean size was not altered after chronic IOP elevation for 14 h or more (p ≥ 0.16). When assessing mitochondrial movement, in younger mice, 5% were mobile at any given time; 4% in the anterograde direction and 1% retrograde. In younger untreated tissue, only 75% of explants had moving mitochondria (mean = 15.8 moving/explant), while after glaucoma induction only 24% of explants had moving mitochondria (mean = 4.2 moving/explant; difference from control, p = 0.03). The distance mitochondria traveled in younger mice was unchanged after glaucoma exposure, but in older glaucoma explants the distance traveled was less than half of older controls (p < 0.0003). In younger mice, mitochondrial speed increased after 14 h of elevated IOP (p = 0.006); however, in older glaucoma explants, movement was actually slower than controls (p = 0.02). In RGC-YFP explants, axonal integrity declined significantly after 4 days of IOP elevation to a similar degree in both younger and older mice. Older mice underwent greater loss of mitochondrial movement with chronic IOP elevation than younger mice, but suffered similar short-term axonal fragmentation in C57BL/6J mice. These transgenic strains, studied in explants, permit observations of alterations in intracellular structure and organelle activity in experimental glaucoma.

Pressure-Induced Changes in Astrocyte GFAP, Actin, and Nuclear Morphology in Mouse Optic Nerve

“…R1 is also the region where axonal transport is blocked in mice. 7 , 39 – 41 Similarly, in larger mammalian eyes and humans, transport block in axons occurs in the connective tissue zone of the LC and myelination begins just posterior to this. 3 , 42 – 44 The accumulation of amyloid precursor protein 33 during axonal obstruction can lead to axonal swelling, 39 , 45 resulting in the increase in axonal area measured in this work.…”

“…46 Likewise, the axonal transport obstruction was barely detectable at 24 hours, but was significant at 3 days. 7 More than 1 week of IOP elevation led to substantial axonal degeneration and represented a different phase of the remodeling process. There were also large changes in strain response after 3 days of OHT.…”

Subtype-specific response of retinal ganglion cells to optic nerve crush

“…In addition, neurotrophic factors and neurotrophins have been used for RGC and axon survival 52 – 55 . Mitochondrial dysfunction as well as oxidative stress has been shown to contribute to RGC loss and axonal degeneration during injury 56 – 60 . The immune system also contributes to pathophysiology of glaucoma both in the retina and the optic nerve.…”

Effectiveness and Safety of Trabeculectomy along with Amniotic Membrane Transplantation on Glaucoma: A Systematic Review

“…As the main target of glaucoma, elevated IOP is associated with glaucomatous optic nerve damage and visual field loss. Kimball et al [ 32 ] found that IOP elevation in elder mice caused clear abnormalities in the density and movement of mitochondria as well as axonal integrity. The stability of IOP seems to be essential.…”