Skeletal muscle regeneration allows the repair of damage resulting from exercise, injury or degenerative diseases. This process depends on satellite cells, adult stem cells that remain quiescent until their function is required for repair. The activation of satellite cells involves their exit from quiescence followed by proliferation and differentiation into myofibers. Many questions remain unanswered, especially with regard to transcription and epigenetic regulation of this process. We have previously reported that Poly(ADP-ribose) Polymerase-1 (PARP-1), a regulator of chromatin function, is rapidly activated upon mitogen stimulation of quiescent fibroblasts and lymphocytes and that the enzyme activity is required for cell cycle re-entry. Preliminary results we obtained in an in vitro cell system that mimics the function of muscle satellite cells, strongly suggest that PARP-1 activity is required for the exit from quiescence also in the muscle system. In light of the enzyme ability to affect chromatin at a global level we hypothesize that PARP-1 may play an important role in switching the transcriptional program from quiescent to activated cells. Considering the possible involvement of PARP-1 in such a critical step of muscle regeneration, we plan to investigate this issue in more depth.
We propose to pursue the following specific objectives:
-provide conclusive evidence that PARP-1 and its activity are required for the function of muscle reserve cells, a faithful in vitro model of satellite cells, and in particular for the G0-G1 transition of the cell cycle and for their myogenic commitment.
-identify the wide range transcriptional targets of PARP-1 during reserve cell activation and the effects of the enzyme activity on chromatin organization.
This basic research will lay the foundations for identifying novel therapeutic targets and for developing effective therapeutic strategies aimed at improving the regenerative potential of satellite cells.
A large number of studies have elucidated the transcriptional networks and the epigenetic factors regulating muscle-specific transcription during development and differentiation. In contrast, the epigenetic mechanisms underlying satellite cell activation and muscle regeneration are just beginning to be investigated. In addition, unlike for other epigenetic regulators, the roles of PARP-1 and PARylation have never been addressed in the muscle system. PARP-1 is best recognized as an important actor in genome maintenance, and as a critical transcriptional regulator in cell death, inflammatory and cancer pathways. The proposed research, by disclosing its regulatory function in satellite cell activation, not only would reveal a novel transcriptional role of PARP-1 in a physiologically relevant process, but would also add a new drugable player that could be effectively exploited as a therapeutic target for enhanced muscle regeneration.
A more comprehensive understanding of the basic mechanisms underlying satellite cell activation will help in clarifying the possible role of their deregulation in pathologies associated with defective muscle regeneration. Moreover, the identification of a novel molecular pathway governing the transition from quiescence to proliferation will be helpful for devising therapeutic strategies aimed at promoting the expansion of satellite pools both in ex vivo and in vivo approaches and, at the same time, at preventing the exhaustion of the stem cell compartment due to excessive proliferation. The ability to manipulate the balance between quiescence and activation of satellite cells will allow to improve the efficacy of regeneration after muscle damage or in neuromuscular diseases.
It is worth mentioning that inhibition of PARP activity provide therapeutic benefits in cancer and inflammation-related pathologies and that some pharmacological PARP inhibitors are being used in clinical trials for the therapy of specific cancers (1). Remarkably, PARP inhibitors have been also shown to reduce muscle damage after ischemia/reperfusion (2), implying their possible use as therapeutic agents after muscle injury. Therefore the attainment of an integrated picture of the complex and multiple roles of PARP-activity in muscle, as well as in other tissues, is extremely important for increasing our awareness of the pleiotropic effects of modulating PARP function.