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Sarcoglycanopathies, limb-girdle muscular dystrophies (LGMD) caused by genetic loss-of-function of the membrane proteins sarcoglycans (SGs), are characterized by progressive degeneration of skeletal muscle. In these disorders, muscle necrosis is associated with immune-mediated damage, whose triggering and perpetuating molecular mechanisms are not fully elucidated yet. Extracellular adenosine triphosphate (eATP) seems to represent a crucial factor, with eATP activating purinergic receptors. Indeed, in vivo blockade of the eATP/P2X7 purinergic pathway ameliorated muscle disease progression. P2X7 inhibition improved the dystrophic process by restraining the activity of P2X7 receptors on immune cells. Whether P2X7 blockade can display a direct action on muscle cells is not known yet. In this study, we investigated eATP effects in primary cultures of myoblasts isolated from patients with LGMDR3 (α-sarcoglycanopathy) and in immortalized cells isolated from a patient with LGMDR5 (γ-sarcoglycanopathy). Our results demonstrated that, owing to a reduced ecto-ATPase activity and/or an enhanced release of ATP, patient cells are exposed to increased juxtamembrane concentrations of eATP and display a higher susceptivity to eATP signals. The purinoceptor P2Y2, which proved to be overexpressed in patient cells, was identified as a pivotal receptor responsible for the enhanced ATP-induced or UTP-induced Ca 2+ increase in affected myoblasts. Moreover, P2Y2 stimulation in LDMDR3 muscle cells induced chemotaxis of immune cells and release of interleukin-8. In conclusion, a higher eATP concentration and sensitivity in primary human muscle cells carrying different α-SG or γ-SG loss-of-function mutations indicate that eATP/P2Y2 is an enhanced signaling axis in cells from patients with α-/γ-sarcoglycanopathy. Understanding the basis of the innate immune-mediated damage associated with the dystrophic process may be critical in overcoming the immunologic hurdles associated with emerging gene therapies for these disorders., (Copyright © 2022 United States & Canadian Academy of Pathology. Published by Elsevier Inc. All rights reserved.)

In muscle ATP is primarily known for its function as an energy source and as a mediator of the "excitation-transcription" process, which guarantees muscle plasticity in response to environmental stimuli. When quickly released in massive concentrations in the extracellular space as in presence of muscle membrane damage, ATP acts as a damage-associated molecular pattern molecule (DAMP). In experimental murine models of muscular dystrophies characterized by membrane instability, blockade of eATP/P2X7 receptor (R) purinergic signaling delayed the progression of the dystrophic phenotype dampening the local inflammatory response and inducing Foxp3 + T Regulatory lymphocytes. These discoveries highlighted the relevance of ATP as a harbinger of immune-tissue damage in muscular genetic diseases. Given the interactions between the immune system and muscle regeneration, the comprehension of ATP/purinerigic pathway articulated organization in muscle cells has become of extreme interest. This review explores ATP release, metabolism, feedback control and cross-talk with members of muscle inflammasome in the context of muscular dystrophies.

In muscular dystrophies, muscle membrane fragility results in a tissue-specific increase of danger-associated molecular pattern molecules (DAMPs) and infiltration of inflammatory cells. The DAMP extracellular ATP (eATP) released by dying myofibers steadily activates muscle and immune purinergic receptors exerting dual negative effects: a direct damage linked to altered intracellular calcium homeostasis in muscle cells and an indirect toxicity through the triggering of the immune response and inhibition of regulatory T cells. Accordingly, pharmacologic and genetic inhibition of eATP signaling improves the phenotype in models of chronic inflammatory diseases. In α-sarcoglycanopathy, eATP effects may be further amplified because α-sarcoglycan extracellular domain binds eATP and displays an ecto-ATPase activity, thus controlling eATP concentration at the cell surface and attenuating the magnitude and/or the duration of eATP-induced signals. Herein, we show that in vivo blockade of the eATP/P2X purinergic pathway by a broad-spectrum P2X receptor-antagonist delayed the progression of the dystrophic phenotype in α-sarcoglycan-null mice. eATP blockade dampened the muscular inflammatory response and enhanced the recruitment of forkhead box protein P3-positive immunosuppressive regulatory CD4 + T cells. The improvement of the inflammatory features was associated with increased strength, reduced necrosis, and limited expression of profibrotic factors, suggesting that pharmacologic purinergic antagonism, altering the innate and adaptive immune component in muscle infiltrates, might provide a therapeutic approach to slow disease progression in α-sarcoglycanopathy., (Copyright © 2019 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)