Tankyrase Regulates Neurite Outgrowth through Poly(ADP-ribosyl)ation-Dependent Activation of ß-Catenin Signaling.

Poly(ADP-ribosyl)ation is a post-translational modification of proteins by transferring poly(ADP-ribose) (PAR) to acceptor proteins by the action of poly(ADP-ribose) polymerase (PARP). Two tankyrase (TNKS) isoforms, TNK1 and TNK2 (TNKS1/2), are ubiquitously expressed in mammalian cells and participate in diverse cellular functions, including wnt/ß-catenin signaling, telomere maintenance, glucose metabolism and mitosis regulation. For wnt/ß-catenin signaling, TNKS1/2 catalyze poly(ADP-ribosyl)ation of Axin, a key component of the ß-catenin degradation complex, which allows Axin's ubiquitination and subsequent degradation, thereby activating ß-catenin signaling. In the present study, we focused on the functions of TNKS1/2 in neuronal development. In primary hippocampal neurons, TNKS1/2 were detected in the soma and neurites, where they co-localized with PAR signals. Treatment with XAV939, a selective TNKS1/2 inhibitor, suppressed neurite outgrowth and synapse formation. In addition, XAV939 also suppressed norepinephrine uptake in PC12 cells, a rat pheochromocytoma cell line. These effects likely resulted from the inhibition of ß-catenin signaling through the stabilization of Axin, which suggests TNKS1/2 enhance Axin degradation by modifying its poly(ADP-ribosyl)ation, thereby stabilizing wnt/ß-catenin signaling and, in turn, promoting neurite outgrowth and synapse formation.

PARP1 is activated by membrane damage and is involved in membrane repair through poly(ADP-ribosyl)ation.

Mono(ADP-ribosyl)ation and poly(ADP-ribosyl)ation are posttranslational modifications evolutionarily conserved in prokaryotes and eukaryotes. They entail transfer of one or more ADP-ribose moieties from NAD+ to acceptor proteins with the simultaneous release of nicotinamide. The resultant ADP-ribosylated acceptor proteins regulate diverse cellular functions. For instance, ADP-ribosyltransferase 1 (ART1) catalyzes mono(ADP-ribosyl)ation of arginine residues in Trim72, a protein specifically expressed in muscle cells and involved in cell membrane repair, which is enhanced upon its ADP-ribosylation. By contrast, the contribution made by ADP-ribosylation to membrane repair in epithelial cells remains unclear. In this study, we investigated the involvement of ADP-ribosylation in cell membrane repair in HEK293T and HeLa cells. We found that upon induction of membrane damage using streptolysin-O, poly(ADP-ribose) polymerase 1 (PARP1) catalyzed poly(ADP-ribosyl)ation. In scratch assays, inhibition of PARP1 activity using the nonspecific PARP inhibitor PJ34 or shRNA targeting PARP1 delayed wound healing, suggesting that PARP1-catalyzed poly(ADP-ribosyl)ation plays a key role in membrane repair in epithelial cells.