Calpain and Poly-ADP-Ribose-Polymerase (PARP) Activity in Rd1 Mouse Retinal Degeneration

Abstract

Purpose: : Retinitis Pigmentosa (RP) is an inherited blinding disease that is characterized by a progressive loss of photoreceptors. The underlying degeneration mechanisms are poorly understood and so far there is no treatment available. Excessive activation of calpain type proteases and poly-ADP-ribose-polymerase (PARP) has been shown to be involved in a number of neurodegenerative disorders. Previously we have identified an increased activity of these enzymes during photoreceptor cell death in the rd1 mouse model for RP.

Methods: : To investigate the possibility of a causal connection between excessive activity and cell death, we studied activities of calpain and PARP and the effects of specific inhibitors on short-term and long-term photoreceptor viability in vitro, on rd1 retinal explants, and in vivo using intravitreal injections. Treatment effects were analyzed using activity assays, immunofluorescence, TUNEL staining, cell counts, and western blotting.

Results: : Activation of calpain correlates and colocalizes with PARP activation in rd1 photoreceptors. The inhibition of either calpains or PARP was beneficial for photoreceptor viability, although calpain inhibition was also detrimental in certain treatment paradigms. PARP inhibition did not affect activation of calpains but, interestingly, calpain inhibition led to reduced PARP activation, suggesting that calpains could in part mediate PARP activation. PARP activity in turn co-localizes with markers for oxidative stress and nuclear translocation of apoptosis inducing factor (AIF).

Conclusions: : Taken together, the data suggest that calpain may mediate cell death via the activation of PARP, a subsequent energetic collapse and AIF nuclear translocation. Calpain activity may however also prevent cell death. These ambiguous results could be due to calpain isoform specific effects and should be an important consideration as the use of calpain inhibitors to prevent or delay retinal degeneration is further studied.

Keywords: cell survival • retinal degenerations: cell biology • neuroprotection