In vivo imaging of mitochondrial axonal transport in the diseased and aged mammalian CNS

Abstract

Purpose: Mitochondrial axonal transport is essential for functions and survival of neurons. The disturbance of the dynamics in neurodegenerative models has been shown using mainly neurons in vitro and in Drosophila, whereas in vivo imaging of mitochondrial axonal transport in the mammalian central nervous system (CNS) has not been achieved. This has limited our knowledge of the dynamics in the diseased and aged mammalian CNS. The purpose of the study is to reveal the disturbance of mitochondrial axonal transport of retinal ganglion cells (RGCs) in a mouse glaucoma model and aged mice.

Methods: Under general anesthesia, in vivo imaging of the mice in which fluorescent proteins are expressed specifically in neuronal mitochondria was conducted. As a glaucoma model, ocular hypertension was induced by laser treatment of limbal and episcleral veins in mice 4 months of age.

Results: In vivo imaging of mouse RGCs showed active mitochondrial axonal transport. In the glaucoma model, the density of RGCs decreased (P < 0.0001) at 14 days after treatment, whereas the density did not significantly decrease at 3 days after treatment. The length of mitochondria transported in RGC axons (measured parallel to the axonal long axis) in the mouse glaucoma model at 3 days after treatment (1.81 ± 0.06 μm; n = 275) was shortened (P < 0.0001), compared with that in control mice (2.23 ± 0.08 μm; n = 512). The number of mitochondria transported in RGC axons in the mouse glaucoma model at 3 days after treatment (65 ± 7 per mm; n = 67) decreased (P = 0.0001), compared with that in control mice (102 ± 6 per mm; n = 78). The length of mitochondria transported in RGC axons in aged mice 12 months of age (1.57 ± 0.04 μm; n = 579) was also shortened, compared with that in young mice 4 months of age (P < 0.0001). However, the number of mitochondria transported in RGC axons in aged mice (125 ± 8 per mm; n = 85) increased, compared with that in young mice (P = 0.048).

Conclusions: In vivo imaging of RGCs in mice reveals the disturbance of mitochondrial axonal transport in the mouse glaucoma model before RGC death and in aged mice. In both the mouse glaucoma model and aged mice, the length of mitochondria transported in RGC axons is shortened. In contrast with the reduction in the number of mitochondria transported in RGC axons in the mouse glaucoma model, that in aged mice does not decrease.

Keywords: 691 retina: proximal (bipolar, amacrine, and ganglion cells) • 629 optic nerve • 413 aging