Abstract
Purpose :
The cause of corneal nerve fiber loss as an early sign of diabetic neuropathy remains unknown. We tested the hypothesis that preferentially centrally localized corneal nerves are damaged using 3D-multiphoton microscopic visualization of the total mouse corneal nerve plexus in vivo. We performed experiments in healthy and diabetic thy1-YFP and RAGE knockout mice to learn about the impact of the advanced glycation end products (AGEs) - receptor of AGEs (RAGE) axis in corneal nerve fiber loss.
Methods :
We investigated eyes of thy1-YFP and RAGE knockout mice (n = 5 per group) in vivo and ex vivo using an upright FVMPE-RS multiphoton microscope (Olympus) equipped with an InSight DS Dual-OL fs-laser system (Spectra-Physics). Diabetes was induced by streptozotocin injections and confirmed by blood glucose, HbA1c and AGEs measurements. RAGE expression was quantified using RT-PCR.
Results :
The total corneal nerve plexus was shown with high spatial resolution in thy1-YFP mice. The displayable quantity of subbasal nerve fibers including superficial terminals was comparable with ex vivo corneal whole mount analyses. The corneal nerve fiber length (CNFL) was significantly lower in the periphery than in the centre of the cornea (32.3 ± 1.4 vs. 58.6 ± 1.4 mm/mm2, p < 0,001). We used the second-harmonic generation signal of collagen fibers to distinguish between cornea and limbus. We observed nerve branching in the limbus supplying the subbasal plexus in the periphery. Diabetes (HbA1c 46.5 mmol/mol) significantly reduced (37.9 ± 1.3 mm/mm2, p < 0,001) and insulin treatment (HbA1c 27.9 mmol/mol) restored (49.4 ± 1.7 mm/mm2, p < 0,001) the CNFL in the centre of the cornea, whereas the periphery remained unaffected. Significant higher RAGE expression was found in the cornea than in brain and liver. RAGE knockout mice showed only a minor 9.8 % reduction of the CNFL after diabetes manifestation (HbA1c 48.8 mmol/mol).
Conclusions :
Our results confirm that diabetes affects corneal nerve fibers exclusively centrally but not peripherally. Significant RAGE expression in the cornea could explain the high susceptibility of corneal nerves to the increase in AGEs during diabetes manifestation. Further examination will be needed to test that peripheral corneal nerves are better protected against damage because of closed vicinity to the limbus.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.