Uncovering the Invisible: Mono-ADP-ribosylation Moved into the Spotlight

Adenosine diphosphate (ADP)-ribosylation is a nicotinamide adenine dinucleotide (NAD+)-dependent post-translational modification that is found on proteins as well as on nucleic acids. While ARTD1/PARP1-mediated poly-ADP-ribosylation has extensively been studied in the past 60 years, comparably little is known about the physiological function of mono-ADP-ribosylation and the enzymes involved in its turnover. Promising technological advances have enabled the development of innovative tools to detect NAD+ and NAD+/NADH (H for hydrogen) ratios as well as ADP-ribosylation. These tools have significantly enhanced our current understanding of how intracellular NAD dynamics contribute to the regulation of ADP-ribosylation as well as to how mono-ADP-ribosylation integrates into various cellular processes. Here, we discuss the recent technological advances, as well as associated new biological findings and concepts.

MARTs and MARylation in the Cytosol: Biological Functions, Mechanisms of Action, and Therapeutic Potential

Mono(ADP-ribosyl)ation (MARylation) is a regulatory post-translational modification of proteins that controls their functions through a variety of mechanisms. MARylation is catalyzed by mono(ADP-ribosyl) transferase (MART) enzymes, a subclass of the poly(ADP-ribosyl) polymerase (PARP) family of enzymes. Although the role of PARPs and poly(ADP-ribosyl)ation (PARylation) in cellular pathways, such as DNA repair and transcription, is well studied, the role of MARylation and MARTs (i.e., the PARP 'monoenzymes') are not well understood. Moreover, compared to PARPs, the development of MART-targeted therapeutics is in its infancy. Recent studies are beginning to shed light on the structural features, catalytic targets, and biological functions of MARTs. The development of new technologies to study MARTs have uncovered essential roles for these enzymes in the regulation of cellular processes, such as RNA metabolism, cellular transport, focal adhesion, and stress responses. These insights have increased our understanding of the biological functions of MARTs in cancers, neuronal development, and immune responses. Furthermore, several novel inhibitors of MARTs have been developed and are nearing clinical utility. In this review, we summarize the biological functions and molecular mechanisms of MARTs and MARylation, as well as recent advances in technology that have enabled detection and inhibition of their activity. We emphasize PARP-7, which is at the forefront of the MART subfamily with respect to understanding its biological roles and the development of therapeutically useful inhibitors. Collectively, the available studies reveal a growing understanding of the biochemistry, chemical biology, physiology, and pathology of MARTs.