Human Sperm-Induced Cell-Cell Fusion Requiring JUNO (hSPICER): A paradigm shift to test sperm fertilizing potential

Current evaluation of male fertility is largely based on indirect sperm parameters such as viability, concentration, morphology, and motility; however, each of these parameters, alone or combined, has been shown to have limited predictive value for successful fertilization. To address this problem, we introduce hSPICER (human SPerm-Induced CEll-cell fusion Requiring JUNO), an assay that evaluates sperm function based on their ability to induce fusion of somatic cells expressing human JUNO (hJUNO), the egg-specific sperm receptor. Similarly to our previous discovery in mice, we found that human sperm can fuse with somatic cells expressing hJUNO on their surface (pseudo-eggs) and promote content mixing between cells in culture, as measured using a split GFP system. The assay is sensitive, specific, and species-dependent, requiring hJUNO for optimal signal. We generated a stable cell line expressing hJUNO, enhancing reproducibility and sensitivity. We also show that hSPICER is compatible with cryopreserved sperm and consistent over different days. Importantly, hSPICER values correlate with fertilization outcomes of patients during fertility treatments, indicating its potential as a functional diagnostic tool. Beyond diagnostic uses, hSPICER establishes a platform to explore sperm fusion mechanisms and to screen for therapeutic compounds and interventions to treat low fertility, enhance fertilization, and develop non-hormonal contraceptives for males and females, as well as quality assessment of semen samples in fertility clinics and sperm banks.