Impact of diet and aging on the mouse corneal innervation at the epithelial-stromal interface

Purpose: The cornea is the most highly innervated structure in the body, undergoing changes in sensitivity and distribution throughout life and in response to insult such as the introduction of an obesogenic diet. However, the mechanisms governing corneal nerve reorganization are currently unknown. While imaging corneal nerves using SBF-SEM, we discovered a novel event whereby axons fused with basal epithelial cells such that portions of the nerve bundle do not penetrate into the epithelium. Here we morphologically define neuronal-epithelial cell fusion, and determine its correlation with age and obesogenic diet-induced nerve loss. Methods: Corneas were collected from C57BL/6 mice, prepared for either SBF-SEM or light-microscopy, and evaluated for neuronal-epithelial cell fusion frequency. In aim 1, 9 week-old mice were evaluated for fusion frequency. In aim 2, 9, 16, and 24 week-old mice were assessed for correlation between aging and frequency of fusion. In aim 3, 5 week-old C57BL/6J mice were fed a normal diet, or an obesogenic diet, for 10 weeks before being assessed for a correlation between diet and fusion frequency. Results: In 9 week-old animals 47% of central cornea stromal nerve bundles contain axons that fuse with basal epithelial cells. The average surface-to-volume ratio of a penetrating nerve was 3.32, while the average fusing nerve was 1.39. Fusing axons were swollen, electron-translucent, contained autophagic vesicles, and lacked mitochondria near sites of fusion. Penetrating and fusing nerves passed through similar sized basal lamina discontinuities. Most corneal nerves labeled with DiI (membrane tracer) after trigeminal ganglion application, and fusion sites were identified by DiI transfer to basal epithelial cells. Fusion frequency increased with aging and the introduction of an obesogenic diet. 74% of central nerves showed fusion in 24 week-old animals, 82% showed fusion in 6 week-old mice fed a 10 week obesogenic diet, and 57% contained fusion in 16 week-old normal diet controls. Conclusion: This is, to our knowledge, the first report of neuronal-epithelial cell fusion in the mouse cornea. The fusion event is positively correlated with nerve loss reported in aging and obesogenic-diet fed mice and as such, it represents a potential mechanism for nerve organization and loss in the central cornea.