Platelet peptidylarginine deiminase 4 is not essential for platelet function in homeostasis or fibrosis during normal aging.

BACKGROUND AND OBJECTIVES: Fibrotic remodeling is a key feature of cardiac aging and a major contributor to heart failure. The bidirectional interplay between platelets and neutrophil extracellular traps (NETs), with peptidylarginine deiminase 4 (PAD4) facilitating NET formation, has emerged as a mediator of fibrotic remodeling. Systemic PAD4-deficiency protects mice against age-related cardiac fibrosis and platelet accumulation in response to pressure overload injury. While PAD4 has been extensively studied in neutrophils, its functional significance in platelets and megakaryocytes (MK), which also express PAD4, remains unexplored. METHODS: This study investigates the role of platelet PAD4 in platelet function and its contribution to age-related fibrotic remodeling. Platelet function was assessed using flow cytometry and light transmission aggregometry in murine and human samples, using platelet/MK-specific PAD4-knockout mice or pharmacological inhibition of PAD4 by GSK484. RESULTS: Cytoplasmic expression of PAD4 was confirmed in human and mouse platelets and megakaryocytes. At the gene level, Padi4 was the second highest expressed of the PAD family members in mouse platelets and megakaryocytes. Genetic deletion or pharmacological inhibition of PAD4 with GSK484 did not impair platelet activation or aggregation. Moreover, platelet/MK PAD4 deficiency did not prevent the development of age-related cardiac fibrosis in mice, indicating that acting on platelet-derived PAD4 is insufficient to phenocopy the cardioprotective phenotype seen in systemic PAD4-deficient mice. CONCLUSION: Collectively, these findings suggest that non-platelet sources of PAD4 are likely driving age-related cardiac fibrosis and provide supporting evidence that PAD4 inhibition to combat excessive NET formation in disease is unlikely to negatively impact platelet function.