Expression of poly(ADP-ribose) glycohydrolase in wild-type and PARG-110 knock-out retina

Fluorescent detection of PARP activity in unfixed tissue

Poly-ADP-ribose-polymerase (PARP) relates to a family of enzymes that can detect DNA breaks and initiate DNA repair. While this activity is generally seen as promoting cell survival, PARP enzymes are also known to be involved in cell death in numerous pathologies, including in inherited retinal degeneration. This ambiguous role of PARP makes it attractive to have a simple and fast enzyme activity assay, that allows resolving its enzymatic activity in situ, in individual cells, within complex tissues. A previously published two-step PARP activity assay uses biotinylated NAD+ and streptavidin labelling for this purpose. Here, we used the fluorescent NAD+ analogues ε-NAD+ and 6-Fluo-10-NAD+ to assess PARP activity directly on unfixed tissue sections obtained from wild-type and retinal degeneration-1 (rd1) mutant retina. In standard UV microscopy ε-NAD+ incubation did not reveal PARP specific signal. In contrast, 6-Fluo-10-NAD+ resulted in reliable detection of in situ PARP activity in rd1 retina, especially in the degenerating photoreceptor cells. When the 6-Fluo-10-NAD+ based PARP activity assay was performed in the presence of the PARP specific inhibitor olaparib, the activity signal was completely abolished, attesting to the specificity of the assay. The incubation of live organotypic retinal explant cultures with 6-Fluo-10-NAD+, did not produce PARP specific signal, indicating that the fluorescent marker may not be sufficiently membrane-permeable to label living cells. In summary, we present a new, rapid, and simple to use fluorescence assay for the cellular resolution of PARP activity on unfixed tissue, for instance in complex neuronal tissues such as the retina.

Identification of a common non-apoptotic cell death mechanism in hereditary retinal degeneration

Cell death in neurodegenerative diseases is often thought to be governed by apoptosis; however, an increasing body of evidence suggests the involvement of alternative cell death mechanisms in neuronal degeneration. We studied retinal neurodegeneration using 10 different animal models, covering all major groups of hereditary human blindness (rd1, rd2, rd10, Cngb1 KO, Rho KO, S334ter, P23H, Cnga3 KO, cpfl1, Rpe65 KO), by investigating metabolic processes relevant for different forms of cell death. We show that apoptosis plays only a minor role in the inherited forms of retinal neurodegeneration studied, where instead, a non-apoptotic degenerative mechanism common to all mutants is of major importance. Hallmark features of this pathway are activation of histone deacetylase, poly-ADP-ribose-polymerase, and calpain, as well as accumulation of cyclic guanosine monophosphate and poly-ADP-ribose. Our work thus demonstrates the prevalence of alternative cell death mechanisms in inherited retinal degeneration and provides a rational basis for the design of mutation-independent treatments.