Spermatic RXFP2 expression levels and seminal INSL3 concentrations among beef bull ejaculates with different levels of sperm morphological normality

The objective of this study was to examine the relationship between the insulin-like peptide 3 (INSL3) receptor (RXFP2) expression levels on spermatozoa and INSL3 concentrations in the seminal plasma of fresh semen from beef bulls with different levels of sperm morphological normality. Ejaculates (n = 44) were collected from 21 yearling Japanese Black beef bulls and categorized into three groups based on the levels of sperm morphological normality: High (normal morphology ≥ 80%; n = 23), Mid (< 80% & ≥ 65%; n = 10) and Low (< 65%; n = 11). Immunofluorescence was used to determine the localization and expression levels of RXFP2 in spermatozoa. Sperm RXFP2 was detected in the principal and equatorial segments of the acrosomal region, postacrosomal region, and neck in all groups. The levels of RXFP2 in the acrosomal principal segment, postacrosomal region, and neck in the Low group were significantly lower than those in the High and Mid groups, and those in the equatorial segment tended to be lower than those in the High group. The total level of RXFP2 in the Low group was also significantly reduced compared with that in the other two groups. Seminal plasma INSL3 concentrations were significantly higher in the Low group than in the other two groups, whereas testosterone levels did not differ significantly between the groups. In conclusion, RXFP2 levels were reduced in the sperm head and neck in bovine semen, with a lower level of sperm morphological normality, suggesting possible associations between increased sperm deformity and INSL3 receptor reduction. Higher seminal INSL3 concentrations in abnormal semen are probably related to fewer INSL3 receptors in spermatozoa.