Novel methods to detect ROS in viable spermatozoa of native semen samples

A crucial issue in male infertility work-up is to have reliable methods to detect oxidative stress in native semen samples. Here, we explored flow cytometric detection of Reactive Oxygen Species (ROS) in viable spermatozoa using native semen samples. To this aim, we challenged three fluorescent probes: CM-H2DCFDA, CellROX Green and MitoSOX Red. After excluding all non-sperm cells, each probe was coupled to a suitable stain to eliminate also semen apoptotic bodies and non-viable spermatozoa: Merocyanine 540 (M540) for CM-H2DCFDA and CellROX Green, and LIVE/DEAD Fixable Green Dead Cell Stain (LD-G) for MitoSOX Red. We found that CM-H2DCFDA was confined in the sperm midpiece, whereas CellROX Green and MitoSOX Red were localized in the head of spermatozoa. Treatment with H2O2 highly increased MitoSOX Red fluorescence (36.20 ± 5.24 vs 18.02 ± 2.25, %, p < 0.01), but not, or only slightly, the labelling with CMH2DCFDA (2.57 ± 1.70 vs 2.77 ± 1.43, p > 0.05) and CellROX Green (5.34 ± 3.18 vs 3.76 ± 2.04, p < 0.05), respectively. Menadione treatment highly increased CellROX Green (10.13 ± 5.85 vs 3.82 ± 2.70, p < 0.01) and MitoSOX Red (69.20 ± 27.14 vs 21.18 ± 7.96, %, p < 0.05), but not CM-H2DCFDA fluorescence (8.30 ± 11.56 vs 7.30 ± 9.19, p > 0.05). Further, only MitoSOX Red was able to detect spontaneous ROS generation during in vitro sperm incubation. We also detected DNA fragmentation by Comet and SCD Assay after sorting MitoSOX Red positive and negative sperm viable fractions. Results indicated that MitoSOX labelling in viable spermatozoa was strictly associated to sperm DNA fragmentation. In conclusion, MitoSOX Red/LD-G appears to be a promising method to detect oxidative stress in human semen for male infertility work-up.