A small secreted protein NICOL regulates lumicrine-mediated sperm maturation and male fertility

Abstract

The mammalian spermatozoa produced in the testis require functional maturation in the epididymis for their full competence. Epididymal sperm maturation is regulated by lumicrine signalling pathways in which testis-derived secreted signals relocate to the epididymis lumen and promote functional differentiation. However, the detailed mechanisms of lumicrine regulation are unclear. Herein, we demonstrate that a small secreted protein, NELL2-interacting cofactor for lumicrine signalling (NICOL), plays a crucial role in lumicrine signalling in mice. NICOL is expressed in male reproductive organs, including the testis, and forms a complex with the testis-secreted protein NELL2, which is transported transluminally from the testis to the epididymis. Males lacking Nicol are sterile due to impaired NELL2-mediated lumicrine signalling, leading to defective epididymal differentiation and deficient sperm maturation but can be restored by NICOL expression in testicular germ cells. Our results demonstrate how lumicrine signalling regulates epididymal function for successful sperm maturation and male fertility.

Lumicrine signaling: Extracellular regulation of sperm maturation in the male reproductive tract lumen

“…These defects observed in Nicol KO males were all restored by the testis-specific expression of Nicol transgene (Kiyozumi et al, 2023). Interestingly, the in vitro experiment using purified recombinant proteins showed that NICOL forms a tight molecular complex with NELL2 (Kiyozumi et al, 2023). Collectively, the data suggest that NICOL-NELL2 complex is the functional unit of the testicular lumicrine factor that binds and stimulates ROS1 to trigger initial segment differentiation.…”

“…Thus, the defective initial segment differentiation, defective sperm maturation, defective sperm migration from uterus into oviduct, and male infertility observed in Nicol KO mice are phenocopies of defects observed in Nell2 KO or Ros1 KO mice. These defects observed in Nicol KO males were all restored by the testis-specific expression of Nicol transgene (Kiyozumi et al, 2023). Interestingly, the in vitro experiment using purified recombinant proteins showed that NICOL forms a tight molecular complex with NELL2 (Kiyozumi et al, 2023).…”

“…In addition to NELL2, a novel lumicrine factor was identified in the latest study. NELL2-interacting cofactor for lumicrine signaling (NICOL) is a small secreted protein (Figure 1c; Kiyozumi et al, 2023). Testicular Nicol expression is not restricted, but it starts during puberty like testicular Nell2 expression.…”

“…A disintegrin and metalloproteinase (ADAM) 3, a catalytically inactive pseudo-metalloproteinase, is abundantly expressed on the sperm surface. The processing or truncation of sperm cell surface ADAM3 appears critical to the acquisition of fertilizing ability by sperm during their maturation in the epididymis (Kiyozumi, 2022;Kiyozumi & Ikawa, 2022), but it is critically affected in the cauda spermatozoa of lumicrinedeficient Nell2 KO, Nicol KO, or Ros1 KO mice (Kiyozumi et al, 2020(Kiyozumi et al, , 2023. Ovochymase2 (OVCH2), a secreted serine protease, has been identified as a candidate ADAM3 processing enzyme (Kiyozumi et al, 2020).…”

“…Ovochymase2 (OVCH2), a secreted serine protease, has been identified as a candidate ADAM3 processing enzyme (Kiyozumi et al, 2020). OVCH2 is specifically expressed in the initial segment but absent in those of lumicrine-deficient or efferent duct-ligated mice (Kiyozumi et al, 2020(Kiyozumi et al, , 2023. The initial segment epididymis of Ovch2 KO mice does not show histological defects, but Ovch2 KO mice are male infertile.…”