# Inhibition of cGMP‐Signalling Rescues Retinal Ganglion Cells From Axotomy‐Induced Degeneration

## TL;DR

Blocking cGMP signaling can protect retinal ganglion cells from degeneration after optic nerve damage, offering new treatment possibilities for blinding diseases like glaucoma.

## Contribution

The study identifies the NO/cGMP/PKG pathway as a key driver of RGC degeneration and shows that PKG and Kv1-channel inhibitors can rescue RGCs.

## Key findings

- Inhibiting PKG with CN238 or blocking Kv1.3/1.6 channels with Margatoxin significantly reduced RGC death after axotomy.
- NO/cGMP/PKG signaling is implicated in delayed RGC degeneration following optic nerve damage.
- Organotypic retinal explant cultures are a viable model for studying optic nerve damage and testing potential treatments.

## Abstract

The axons of retinal ganglion cells (RGCs) form the optic nerve, which relays visual information to the brain. RGC degeneration is the root cause of a variety of blinding diseases linked to optic nerve damage, including glaucoma, the second leading cause of blindness worldwide. The underlying cellular mechanisms of RGC degeneration are largely unclear; yet, they have been connected to excessive production of the signalling molecule nitric oxide (NO) by nitric oxide synthase (NOS). NO activates soluble guanylate cyclase (sGC), which subsequently produces the second messenger cyclic guanosine monophosphate (cGMP). This, in turn, activates protein kinase G (PKG), which can phosphorylate downstream protein targets. To study the role of NO/cGMP/PKG signalling in RGC degeneration, we used organotypic retinal explant cultures in which the optic nerve had been severed. We assessed the activity of NOS, RGC death and survival at different times after optic nerve transection. While NOS activity was high right after optic nerve transection, significant RGC loss occurred with a 24–48‐h delay. We then treated retinal explants with inhibitors selectively targeting either NOS, sGC, PKG, or Kv1.3 and Kv1.6 voltage‐gated potassium channels. While all four treatments reduced RGC death, the PKG inhibitor CN238 and the Kv‐channel blocker Margatoxin (MrgX) showed the most pronounced rescue effects. Our results confirm an involvement of NO/cGMP/PKG signalling in RGC degeneration, highlight the potential of PKG and Kv1‐channel targeting drugs for treatment development, and further suggest organotypic retinal explant cultures as a useful model for investigations into optic nerve damage.

Retinal ganglion cell (RGC) axons are myelinated only beyond the lamina cribrosa, that is, when leaving the eye. Axotomy‐induced RGC degeneration is likely to be related to excessive nitric oxide (NO) production, leading to an over‐production of cGMP, over‐activation of protein kinase G (PKG) and excessive phosphorylation of target proteins, such as voltage‐gated Kv1‐type potassium channels. The ensuing excessive outflow of K+ ions drains ATP supplies, eventually triggering cell death.