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ZEINABSADAT Mohammadi, Jiang P. Zhang, Kevin Gregory-Evans, Joanne A. Matsubara, Orson L. Moritz, Cheryl Y. Gregory-Evans; Rod And Cone Photoreceptors Die By Different Cell-death Mechanisms In The Pde6c Mutant Zebrafish Model Of Achromatopsia. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2580.
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© ARVO (1962-2015); The Authors (2016-present)
Achromatopsia is an autosomal recessive trait characterized by loss of colour discrimination, photophobia and poor visual acuity. Mutations in CNGA3, CNGB3, GNAT2 and PDE6C account for 90% of cases. Due to this genetic heterogeneity, mutation-independent treatment strategies aimed at preventing cell death are very attractive for such photoreceptor disorders. The aim of this study was to determine the mechanism of photoreceptor cell death in the pde6c zebrafish model of achromatopsia.
Cell death in pde6c-/- mutant zebrafish model was analysed by histology, TUNEL staining and immunohistochemistry for the following cell death-associated markers: cGMP, cleaved caspase-3, cleaved poly-ADP-ribose polymerase (PARP), calpain, RIP1 and RIP3. Necrostatin-1 small molecule drug treatment was used in vivo to inhibit RIP1 activity.
Although cone photoreceptors are formed initially in the retina, rapid cell death occurs between 3-4 days post-fertilization (dpf). At this stage of development, TUNEL-positive cells were detected in the mutant retina yet the expression of activated caspase-3 was not elevated in the photoreceptors. At 4 dpf in homozygous pde6c mutant zebrafish immunohistochemistry demonstrated that rod photoreceptors cells were immunopositive for PARP and calpain, whereas cone photoreceptors expressed high levels of RIP1 and RIP3, demonstrating activation of the caspase-independent cell death pathways in this retinal degeneration model. Treatment with necrostatin-1 delayed cone degeneration by several days.
Collectively, these results show in the pde6c mutant retina that cone and rod photoreceptors die by different caspase-independent mechanisms. Elevated levels of RIP1and RIP3 suggest that the necroptosis cell death pathway is activated in cone photoreceptors. Surprisingly though, elevation of PARP and calpain in rod photoreceptors implies an alternative cell death pathway causing the bystander rod cell death. This suggests that a combinatorial approach of inhibitors for necroptosis and PARP might be needed in studies to treat this disease. Bystander cell death is an important feature of many retinal degenerations and so combinatorial approaches may evolve as an important principle in treatment.
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