PARP1 Gene Knock-Out Increases Resistance to Retinal Degeneration without Affecting Retinal Function

Abstract

Retinitis pigmentosa (RP) is a group of inherited neurodegenerative diseases affecting photoreceptors and causing blindness in humans. Previously, excessive activation of enzymes belonging to the poly-ADP-ribose polymerase (PARP) group was shown to be involved in photoreceptor degeneration in the human homologous rd1 mouse model for RP. Since there are at least 16 different PARP isoforms, we investigated the exact relevance of the predominant isoform - PARP1 - for photoreceptor cell death using PARP1 knock-out (KO) mice. In vivo and ex vivo morphological analysis using optic coherence tomography (OCT) and conventional histology revealed no major alterations of retinal phenotype when compared to wild-type (wt). Likewise, retinal function as assessed by electroretinography (ERG) was normal in PARP1 KO animals. We then used retinal explant cultures derived from wt, rd1, and PARP1 KO animals to test their susceptibility to chemically induced photoreceptor degeneration. Since photoreceptor degeneration in the rd1 retina is triggered by a loss-of-function in phosphodiesterase-6 (PDE6), we used selective PDE6 inhibition to emulate the rd1 situation on non-rd1 genotypes. While wt retina subjected to PDE6 inhibition showed massive photoreceptor degeneration comparable to rd1 retina, in the PARP1 KO situation, cell death was robustly reduced. Together, these findings demonstrate that PARP1 activity is in principle dispensable for normal retinal function, but is of major importance for photoreceptor degeneration under pathological conditions. Moreover, our results suggest that PARP dependent cell death or PARthanatos may play a major role in retinal degeneration and highlight the possibility to use specific PARP inhibitors for the treatment of RP.

Inhibition of Mitochondrial Pyruvate Transport by Zaprinast Causes Massive Accumulation of Aspartate at the Expense of Glutamate in the Retina

“…Interestingly, in long term organotypic retinal explant cultures, at concentrations of up to 200 M, Zaprinast induces selective photoreceptor death without affecting other retinal neurons (37). Why does Zaprinast not cause generalized cell death and instead appears to affect only PDE6 under culture conditions (37)(38)(39)(40)? Tissue culture medium, such as R16 (41), used for organotypic retinal culture contains both fatty acids and glutamine.…”

Light-Driven Calcium Signals in Mouse Cone Photoreceptors

“…Another way of influencing the open probability of CNG channels is to inhibit PDE6, which slows down cGMP hydrolysis in the OS (Sahaboglu et al, 2010) and, thus, promotes Ca 2ϩ influx through CNG channels. To this end, we used the PDE6-selective inhibitor zaprinast (Fig.…”

Poly (ADP-ribose) polymerase 1 expression in fibroblasts of Down syndrome subjects

“…The C-terminal catalytic domain of PARP-1 protein is activated via an unknown mechanism, causing formation and addition of the polyadenosine-ribose (PAR) complex to acceptor proteins including PARP-1 itself [13]. PARP-1 activation has been connected to Retinal Degeneration, a neurodegenerative disease affecting photoreceptors and causing blindness in humans [14]. In fact, uncontrolled poly-ADP-ribosylation reactions can result in tissue damage and massive necrotic cell death, which in turn often leads to severe inflammatory or neurodegenerative disorders [9,15].…”

“…In addition, recent studies using DNA damaging agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), hydrogen peroxide (H 2 O 2 ) or peroxynitrite, which are well known to induce necrosis at high concentrations, showed that pharmacological inhibition of PARP-1 activity or knockout of the PARP-1 gene blocks programmed-necrotic cell death induced by these agents [14,17,18]. All together these evidences lead to the hypothesis of using inhibitors of PARP-1 gene as a protective therapy also to prevent neurodegeneration and premature aging in individuals with DS.…”