Ultrastructural and transcriptional changes in aging and glaucomatous murine optic nerve heads

Abstract

Purpose : Age is a major and non-modifiable risk factor for glaucoma. Aging causes multiple detrimental effects on the eye and the optic nerve. We focused on the glial lamina in the unmyelinated portion of the optic nerve head (ONH) because previous findings indicate that the first insult to ganglion cell axons occurs in this region. The first purpose of this study was to identify differences and similarities between aging and glaucoma in the ONH. The second aim was to find genetic pathways that may explain the morphological changes.

Methods : Naïve C57bl/6 mice at ages 3 to 30 months were used in this study. In addition, intraocular pressure (IOP) elevation was induced in a group of young C57bl/6 mice by injection of microbeads into the anterior chamber. The glial laminas of the ONHs were imaged by transmission electron microscopy. Axon densities, axon diameters, mitochondrial densities and diameters were measured. We also performed RNA sequencing on ONHs of young and old naïve mice, microbead-injected mice, and a saline-injected control group.

Results : As was described before, mice lose ganglion cell axons with age. However, axon diameter and especially the variance of axon diameters were most closely correlated with age (R² = 0.6017 and 0.8998, respectively). In addition, there were signs of pathology in axonal mitochondria that exacerbated with age. The mitochondrial diameters increased from 0.366 ± 0.149 μm in young mice to 0.469 ± 0.219 μm in 30-month old mice. Older nerves also contained many profiles of degenerating mitochondria without cristae. Astrocytic mitochondria appeared morphologically normal even in old animals. In the young glaucomatous group axonal diameters, variances, and mitochondrial diameters were significantly increased and comparable to those in naïve mice of 18-30 months. In RNA-seq, 968 genes were differentially regulated in 10-months old versus 3-months old optic nerve heads and 361 genes were differentially regulated in the microbead-injected group versus the saline group. Of these, 46.6% were also differentially regulated in the aged mice.

Conclusions : One month of moderately elevated IOP leads to morphological changes in the ONH that are as profound as the effects of at least one year of normal aging. The overlap in the differentially expressed genes suggests that similar pathways are involved in both processes.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.