Contribution of the porcine aminopeptidase N (CD13) receptor density to porcine epidemic diarrhea virus infection. 
Porcine epidemic diarrhea virus (PEDV) and transmissible gastroenteritis virus (TGEV), which belong to group 1 coronaviruses, are important viral pathogens in pigs causing lethal diarrhea.  As with the other members in the group 1, theses viruses are also known to use the host aminopeptidase N (APN) as the major receptor for cell entry.  Remarkably, it was found that they utilize distinct cultured cell lines for in vitro virus propagation, since PEDV could not be replicated in swine testis (ST) cells expressing native porcine APN (pAPN), which are highly susceptible to TGEV.  To explain the mechanism causing this discrimination, we postulated that there may be a correlation between the pAPN expression level and PEDV infection.  As a first step toward understanding the role of cellular receptor density in PEDV replication, therefore, sub-lines of ST cells stably overexpressing recombinant pAPN were generated.  We initially confirmed that the control ST cells do express relatively low levels of endogenous pAPN.  In contrast, in the engineered stable cell lines, a high level of recombinant pAPN expression was demonstrated.  The introduction of a pAPN gene into nonpermissive ST cells was further found to be fully sufficient to support productive infection, revealing that constitutive overexpression of pAPN can directly rescue PEDV multiplication.  We further assessed whether the pAPN enzymatic function is relevant to PEDV infection.  The enzymatic active motif-null mutant pAPN still retained the ability to exert its receptor activity and consequently, to cause infectious virus production.  Moreover, the only APN inhibitor blocking the protease activity site had no obvious negative effect on viral infection, indicating that the enzymatic role of APN is dispensable for the process of virus replication.  Taken together, our results suggest that pAPN receptor density appears to be an important factor in contributing to efficient PEDV infection.