Many studies have documented that
Many studies have documented that enolase acts as a plasminogen-binding protein and this interaction is involved in the tissue invasion and pathogenicity of the causative agents (Almeida et al., 2004; Avilan et al., 2000; Marcilla et al., 2007; Mundodi et al., 2008; Vanegas et al., 2007). Furthermore, plasminogen can be activated in vivo by tissue plasminogen activator (t-PA) to generate plasmin, which is responsible for the dissolution of fibrin blood clots and extracellular proteolysis in many physiological and pathological processes. In general, the two ways would contribute to the binding ability of plasminogen/enolase: one is by carboxy-terminal lysine residues in its amino hexokinase inhibitor sequence, and another way is using the special plasminogen-binding motif (FYDKERKVY)as a binding site (Bergmann et al., 2003; Ehinger et al., 2004). The present study demonstrated that His-Tseno could effectively bind to human plasminogen in vitro and enhance the activation of plasminogen into plasmin in the presence of t-PA, which was in agreement with the findings reported in T. multiceps and T. pisiformis (Li et al., 2015; Zhang et al., 2015), indicating enolases of tapeworm play an important role in facilitating worm invasion and migration in the host tissues. Additionally, the ability of His-Tseno to bind and activate plasminogen was dependent on lysine residues because 30 mM ε-ACA could inhibit the binding of His-Tseno to plasminogen, being in agreement with previous reports on other enolases of fungal, bacterial, protozoan and human systems (Almeida et al., 2004; Avilan et al., 2000; Boyle and Lottenberg, 1997; Crowe et al., 2003). Because of the absence of the carboxy-terminal lysine residues in Tseno, the binding ability of plasminogen-Tseno might be achieved by another way. Interestingly, Tseno does contain the deduced peptide 238-[GKVKIGMDVAASEFYQDGKYNLDF]-261, which exhibits similarity to an internal plasminogen-binding motif reported for F. hepatica and S. pneumonia enolases (Bergmann et al., 2003; Bernal et al., 2004) (Fig. 2), suggesting this region may be responsible for the plasminogen binding and activation. Based on this motif, the mutation experiments of three conserved lysines replaced by alanine (K239A, K241A, and K256A) were conducted. Unsatisfactorily, the plasminogen-binding activities of the mutated proteins did not significantly decrease in our experiment (data not shown). Further studies should analyze and verify the primary plasminogen-binding sites and the intervention of immunotherapeutic targets against taeniasis/cysticercosis. The tegument of cestodes is a highly efficient absorptive layer for the uptake of nutrients (carbohydrates, amino acids, and lipids). There are many important molecules on the tegumental surface, such as glucose and amino acid transporter molecules (Dalton et al., 2004). Numerous reports have indicated that enolase is a surface-located proteins (Pancholi, 2001). However, sequence analysis showed Tseno did not have the signal peptide or transmembrane region for its secretory expression, transport or anchor at the cell surface, which is similar to other enolases (Song et al., 2012). In S. bovis, the enolase is expressed on the tegument and facilitates the worm migration in blood vessel as the plasminogen receptor (Ramajo-Hernandez et al., 2007). The enolase from E. granulosus (EgEno) was detected in the protoscolex tegument, germinal layer cells and the hydatid fluid (Gan et al., 2010; Lorenzatto et al., 2012). Our previous study showed the enolase was also presented in the bladder wall and in the calcareous corpuscles of T. pisiformis larvae (Zhang et al., 2015). In the present study, Tseno was mainly localized on the tegument and eggs of adult T. solium. These findings well indicated that enolase of tapeworms was a surface-located protein. Furthermore, the real-time PCR also showed that Tseno was upregulated in the stage of adult worm, indicating that Tseno may be critical for the energy needs of the worm growth and egg production.