Retinal degeneration: Multilevel protection of photoreceptor and ganglion cell viability and function with the novel PKG inhibitor CN238

Abstract

Hereditary retinal degeneration (RD) is often associated with excessive cGMP-signaling in photoreceptors. Previous research has shown that inhibition of cGMP-dependent protein kinase G (PKG) can slow down the loss of photoreceptors in different RD animal models. In this study, we identified a novel PKG inhibitor, the cGMP analogue CN238, with strong protective effects on photoreceptors in retinal degeneration rd1 and rd10 mutant mice. In long-term organotypic retinal explants, CN238 preserved rd1 and rd10 photoreceptor viability and function. Surprisingly, in explanted retinae CN238 also protected retinal ganglion cells from axotomy induced retrograde degeneration and preserved their functionality. Together, these results confirm the strong neuroprotective capacity of PKG inhibitors for both photoreceptors and retinal ganglion cells, thereby significantly broadening their potential applications for the treatment of retinal diseases and possibly neurodegenerative diseases in general.

In vitro Model Systems for Studies Into Retinal Neuroprotection

“…One way to study the mechanism of cell death triggered by abnormal cGMP accumulation is the inhibition of its two prototypic targets: Cyclic nucleotide-gated (CNG) channels and protein kinase G (PKG), both of which have been connected to photoreceptor cell death ( Farber and Lolley, 1974 ; Paquet-Durand et al, 2009 ). Both photoreceptor-like cell cultures and organotypic retinal explant cultures have been employed to target CNG channels and PKG and have contributed valuable insights into the photoreceptor death pathway ( Vighi et al, 2018 ; Tolone et al, 2021 ; Das et al, 2022 ).…”

“…PKG-inhibiting cGMP analogs carry a phosphorothioate configured as Rp, which allows them to counteract PKG activation ( Zhao et al, 1997 ). Two novel cGMP analogs with strong protective abilities on degenerating photoreceptors have recently been identified using in vitro systems: Photoreceptor-like cell cultures allowed an initial screening of several compounds, while organotypic retinal explants cultures derived from different IRD mouse models confirmed the neuroprotective capacities of these cGMP analogs ( Vighi et al, 2018 ; Tolone et al, 2021 ). Furthermore, the use of retinal explant cultures treated with inhibitory cGMP analogs also led to the identification of novel PKG targets that may be involved in the cell death mechanism of IRD ( Roy et al, 2021 ; Roy et al, 2022 ).…”

Drug Discovery Strategies for Inherited Retinal Degenerations

“…A subsequent study using organotypic culture was useful to test the effect of analogs on the survival of cone and rod photoreceptors [ 56 ]. The resulting cGMP-analogs, CN03 and CN238, effectively prevented rod photoreceptor cell death in the rd10 mouse model [ 56 , 57 ]. These studies also exemplify the effective use of quantitative phenotypic assays for drug development which we further discuss in the section addressing protein destabilizing point mutations (also see Figure 1 ).…”

cGMP Signaling in the Neurovascular Unit—Implications for Retinal Ganglion Cell Survival in Glaucoma

“…Within the neuron itself, cGMP and its downstream effector PKG are key mediators for the neuroprotective effects of NO [ 110 ], and multiple studies have confirmed the neuroprotective effect of stimulating the NO-cGMP-PKG pathway in RGCs [ 28 , 111 , 112 , 113 ] and in other neurons of the CNS [ 110 , 114 ]. However, it is worth noting that activation of PKG has shown neurotoxicity, rather than neuroprotective effects, in RGCs [ 115 , 116 ], suggesting that alternative downstream effectors of cGMP, such as cGMP-gated channels and transcription factors, such as cAMP response element binding protein (CREB), may underpin the neuroprotective capacity of cGMP. cGMP-gated channels have been identified in RGCs and are responsible for modulation of neuron excitability [ 117 , 118 ].…”