Identification of membrane and secreted proteins in anterior cruciate ligament derived cells using “signal-sequence-trap”, a retrovirus-mediated expression screening method

Ligament injuries are one of most common injuries to the knee joint; however, little is known about how ligaments heal and what kinds of precursor cells participate in ligament repair. Ligament cells were thought to be members of one of the lineages from mesenchymal stem cells (MSCs). However, there have been no reports on the differentiation mechanism of MSC to ligament cells because they do not have known phenotypic markers. To identify the anterior cruciate ligament (ACL)-specific phenotypic marker, we screened a cDNA library derived from rabbit ACL cells, using the signal sequence trap by the retrovirus-mediated expression screening (SST-REX) method. We detected known clones representing 135 independent proteins. Among them, 120 were secreted proteins and 15 were membrane proteins. Some of the clones encoded proteins that have never been reported to be expressed in ACLs. Therefore, the data could provide a better understanding of ACL cells and could help in finding their phenotypic markers.