Rhodopsin signaling mediates light-induced photoreceptor cell death in rd10 mice through a transducin-independent mechanism

Abstract

Retinitis pigmentosa (RP) is a debilitating blinding disease affecting over 1.5 million people worldwide, but the mechanisms underlying this disease are not well understood. One of the common models used to study RP is the retinal degeneration-10 (rd10) mouse, which has a mutation in Phosphodiesterase-6b (Pde6b) that causes a phenotype mimicking the human disease. In rd10 mice, photoreceptor cell death occurs with exposure to normal light conditions, but as demonstrated in this study, rearing these mice in dark preserves their retinal function. We found that inactivating rhodopsin signaling protected photoreceptors from degeneration suggesting that the pathway activated by this G-protein-coupled receptor is causing light-induced photoreceptor cell death in rd10 mice. However, inhibition of transducin signaling did not prevent the loss of photoreceptors in rd10 mice reared under normal light conditions implying that the degeneration caused by rhodopsin signaling is not mediated through its canonical G-protein transducin. Inexplicably, loss of transducin in rd10 mice also led to photoreceptor cell death in darkness. Furthermore, we found that the rd10 mutation in Pde6b led to a reduction in the assembled PDE6αβγ2 complex, which was corroborated by our data showing mislocalization of the γ subunit. Based on our findings and previous studies, we propose a model where light activates a non-canonical pathway mediated by rhodopsin but independent of transducin that sensitizes cyclic nucleotide gated channels to cGMP and causes photoreceptor cell death. These results generate exciting possibilities for treatment of RP patients without affecting their vision or the canonical phototransduction cascade.

Gradual Increase in Environmental Light Intensity Induces Oxidative Stress and Inflammation and Accelerates Retinal Neurodegeneration

“…The fact that rd10 mice housed at 5 lux had better preserved retinal function and structure agrees with previous studies showing that dark-adapted animals suffering from RP exhibited a slower rate of degeneration compared with light-exposed animals. 9,27,48 As the results of Sundar et al 9 show, the retina of rd10 mice kept in total darkness also degenerates, remaining approximately four photoreceptors rows in the ONL at P45, being probably slightly more preserved than our results found in rd10 mice housed at 5-lux illumination. Nevertheless, the absence of a dark-reared rd10 mice experimental group in our study may be a limitation to better determining the threshold of light-induced retinal degeneration in rd10 mice.…”

“…25,27,48 In many studies, retinal degeneration has been assessed by testing a single light intensity or complete darkness. 9,25,27,49 In this study, we evaluated whether slight changes in light-intensity conditions may affect the rate of the neurodegeneration process in RP, using the rd10 mouse model. One of the main novelties of this study compared to other published works is the analysis of three lighting conditions, which represented the three categories of vision: scotopic, very low levels of lighting (5 lux); mesopic, low levels of lighting (50 lux); and photopic, indoor illumination conditions (300 lux).…”

“…It is important to keep in mind that rd10 mice have a mutation in the Pde6b gene that causes an accumulation of cGMP and keeps cGMP-gated cation channels open, generating an excessive accumulation of cations within the cell and inducing oxidative stress and photoreceptors degeneration. [7][8][9]75 One commonly proposed hypothesis that explains retinal photodamage is based on the light-induced accumulation of all-trans-retinal, one of the products of the phototransduction cascade. The processing of all-trans-retinal and its transformation back into the 11-cis-retinal chromophore is essential for the survival of photoreceptors.…”

Suppression of cGMP-Dependent Photoreceptor Cytotoxicity With Mycophenolate Is Neuroprotective in Murine Models of Retinitis Pigmentosa

“…A recent study indicates that these rhodopsin-mediated transducin-independent pathways may very well exist in rd10 mice. 87 The mechanisms of photoreceptor pathophysiology upstream of cGMP cytotoxicity require further elucidation. Nevertheless, these studies demonstrate that manipulation of cGMP cytotoxicity via the use of cGMP analogues or IMPDH inhibitors is a viable strategy that may represent a new class of neuroprotective agents.…”

Divergent Effects of HSP70 Overexpression in Photoreceptors During Inherited Retinal Degeneration

“… 45 The rd10 allele produces a hypomorphic PDE6B that is partially defective in assembling into the PDE6αβγ2 holoenzyme with decreased transport to the OS, leading to higher free cGMP, increased channel opening and Ca 2+ influx, and rapid photoreceptor death. 58 – 60 We were intrigued by the accelerated photoreceptor degeneration in rd10 retina by HSP70 overexpression. The mutant PDE6B is expressed at low levels in rd10 retina, exhibits residual catalytic activity, and is partially mislocalized, consistent with a misfolding defect.…”