Poly(ADP-ribose) polymerase regulates transient receptor potential channel M2-dependent calpain activation in rd1 mouse retinal degeneration

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 Ca2+-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 Ca2+ 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.