FROM CA²⁺ BINDING TO THE HOST-PARASITE INTERFACE: EXPLORING THE MOLECULAR ROLE OF SmE16 IN Schistosoma mansoni

Schistosomiasis is a neglected tropical disease that affects millions of people worldwide, causing high morbidity and mortality among infected individuals and significantly impacting the socioeconomic development of endemic countries. In the Americas, the etiological agent responsible for schistosomiasis is Schistosoma mansoni. Praziquantel remains the primary treatment for schistosomiasis, however, reports of reduced parasite susceptibility highlight the need for novel therapeutic strategies. SmE16 is an immunoreactive calcium-binding protein expressed at different stages of the parasite life cycle and potentially involved in processes relevant to parasite survival and host interactions. Proteomic studies of S. mansoni have identified SmE16 in parasite secretions. In addition, our studies demonstrated an enrichment of SmE16 transcripts in the parasite esophageal glands suggesting that this protein is present at the host-parasite interface. Our biophysical studies demonstrated that SmE16 undergoes significant conformational changes in the presence of calcium, accompanied by a marked increase in thermal stability. The aim of this study was to investigate the potential association of SmE16 with host proteins using proteomic and biophysical approaches to identify and characterize potential host binding partners. Pull-down assays were performed using recombinant SmE16 fused to either His or GST tags and immobilized on Ni-NTA or glutathione affinity resins, respectively. Immobilized SmE16 was incubated with mouse serum, human serum, mouse blood lysate, and extracts from RAW 264.7 macrophages in the presence or absence of calcium ions. Associated proteins were subsequently identified by GeLC-MS/MS analysis. Our results identified several candidate host  proteins potentially associated with SmE16, including proteins involved in innate immunity, the complement system, inflammatory regulation, and lipid metabolism, such as SERPINA1, MBL1, APOA1, APOA4, APOE, and vitronectin. Among these, SERPINA1 was identified under different experimental conditions and in samples from both species, highlighting it as a promising candidate for subsequent biophysical validation of its potential interaction with SmE16. MBL1 was identified specifically under calcium-containing conditions, suggesting that the calcium-dependent conformational state of SmE16 may influence its association with host components. These findings identify potential host-associated partners, providing insights into its role at the host-parasite interface and its potential contribution to host immune response modulation.