Abstract Inherited retinal degeneration refers to untreatable blinding diseases characterized by progressive photoreceptor loss. Photoreceptor degeneration is often associated with an excessive activation of poly(ADP-ribose) polymerase and Ca 2+ -dependent calpain-type proteases. To explore the interplay between poly(ADP-ribose) polymerase and calpain activity, we employed organotypic retinal explant cultures derived from wild-type mice and from the rd1 mouse model for inherited retinal degeneration. Retinae were treated with the poly(ADP-ribose) polymerase inhibitors INO1001 or Olaparib, the poly(ADP-ribose) glycohydrolase inhibitor JA2131, or the transient receptor potential channel M2 blocker 8-Br-ADPR. Readouts included the terminal deoxynucleotidyl transferase dUTP nick end labeling assay to detect cell death, in situ activity assays for histone-deacetylases, poly(ADP-ribose) polymerase, and calpain, as well as immunostaining for activated calpain-2, and poly(ADP-ribose). Poly(ADP-ribose) polymerase, poly(ADP-ribose) glycohydrolase, and transient receptor potential channel M2 inhibition reduced calpain activity and calpain-2 activation. Poly(ADP-ribose) polymerase activity was decreased by poly(ADP-ribose) polymerase and transient receptor potential channel M2 inhibitors but not by poly(ADP-ribose) glycohydrolase inhibition. Remarkably, the poly(ADP-ribose) polymerase inhibitor INO1001 increased histone-deacetylase activity unlike any of the other compounds. When combined with the poly(ADP-ribose) glycohydrolase inhibitor JA2131, INO1001 reduced photoreceptor cell death in a synergistic fashion, although such synergy was not observed for calpain or poly(ADP-ribose) polymerase activity. Moreover, synergistic photoreceptor preservation was not observed when JA2131 was combined with the poly(ADP-ribose) polymerase inhibitor Olaparib. Overall, these results indicate that in rd1 photoreceptors, poly(ADPribose) polymerase controls calpain activity via poly(ADP-ribose) glycohydrolase and transient receptor potential channel M2-induced Ca 2+ influx. We also characterized INO1001 as potentially more beneficial for inherited retinal degeneration treatment than Olaparib. Our study details the complexity of poly(ADP-ribose) polymerase-signaling in photoreceptors and identifies poly(ADP-ribose) glycohydrolase and transient receptor potential channel M2 as new targets for inherited retinal degeneration therapy development.
Yan et al. (Thu,) studied this question.