Gene targeting in hemostasis. Hepsin.

Hepsin is a type II transmembrane serine protease abundantly expressed on the surface of hepatocytes. Biochemical studies have shown that hepsin is an enzyme of 51 kDa with the trypsin-like substrate specificity. Several in vitro studies have suggested that hepsin may play a role in blood coagulation, hepatocyte growth, and fertilization. To determine the functional importance of hepsin, hepsin-deficient mice were generated by homologous recombination. Homozygous hepsin-/- mice were viable and fertile, and grew normally. When analyzed in hemostasis assays, such as tail bleeding time and plasma clotting times, and in vivo modes, such as disseminated intravascular coagulation, septic shock, and acute liver regeneration, hepsin-/- mice had similar phenotypes as wild-type controls. Liver weight and serum concentrations of liver-derived proteins or enzymes were also similar in hepsin-/- and wild-type mice. No abnormalities were identified in major organs in hepsin-/- mice in histological examinations. These results indicate that hepsin is not an essential enzyme for normal hemostasis, embryogenesis, and maintenance of normal liver function. Unexpectedly, serum concentrations of bone-derived alkaline phosphatase were approximately two-fold higher in both male and female hepsin-/- mice than those in wild-type controls. The underlying mechanism for this phenotype and long-term effects of hepsin deficiency remain to be determined.