Optic Nerve Injury Enhanced Mitochondrial Fission and Increased Mitochondrial Density without Altering the Uniform Mitochondrial Distribution in the Unmyelinated Axons of Retinal Ganglion Cells in a Mouse Model

Glaucomatous optic neuropathy (GON), a major cause of blindness, is characterized by the loss of retinal ganglion cells (RGCs) and the degeneration of their axons. Mitochondria are deeply involved in maintaining the health of RGCs and their axons. Therefore, lots of attempts have been made to develop diagnostic tools and therapies targeting mitochondria. Recently, we reported that mitochondria are uniformly distributed in the unmyelinated axons of RGCs, possibly owing to the ATP gradient. Thus, using transgenic mice expressing yellow fluorescent protein targeting mitochondria exclusively in RGCs within the retina, we assessed the alteration of mitochondrial distributions induced by optic nerve crush (ONC) via in vitro flat-mount retinal sections and in vivo fundus images captured with a confocal scanning ophthalmoscope. We observed that the mitochondrial distribution in the unmyelinated axons of survived RGCs after ONC remained uniform, although their density increased. Furthermore, via in vitro analysis, we discovered that the mitochondrial size is attenuated following ONC. These results suggest that ONC induces mitochondrial fission without disrupting the uniform mitochondrial distribution, possibly preventing axonal degeneration and apoptosis. The in vivo visualization system of axonal mitochondria in RGCs may be applicable in the detection of the progression of GON in animal studies and potentially in humans.