*7.5.3 PARs (receptors coupled to proteases)*

It is a family of receptors coupled to the G-protein, which is formed by seven transmembrane domains with an amino-terminal extracellular domain and a C-terminal domain. PARs are activated by proteolytic cleavage in the N-terminal domain by serine proteases. N-terminal serves as a ligand to carry out intracellular signaling. They are expressed in epithelial, endothelial and immune cells such as leukocytes, mast cells, eosinophils, neutrophils and mastoid cells. Four types of receptors have been described (PAR-1–4). PAR-1, 3 and 4 are activated mainly by thrombin and are involved in the aggregation of platelets. PAR-2 is activated by trypsin, tryptase from mastoid cells, protease 3 from neutrophils and tissue factor, factor VIIa and Xa [81]. Recently, PARs have been implicated in the regulation of the expression of antimicrobial peptides found in epithelial cells such as defensins [65, 82]. The researchers identified the expression of the HBD2 in human

gingiva by *P. gingivalis* infection. Further, proteases of the bacteria induced the expression of the defensin through PAR2. The authors suggest that this signaling pathway can lead to the development of preventive therapies in mucosal infections [65].
