The 24,25(OH)₂D₃-TLCD3B signaling complex and reproductive function in mice and men

Vitamin D influences male reproductive function, and the expression of the vitamin D-inactivating enzyme CYP24A1 in spermatozoa has consistently been shown to be positively correlated with semen quality. The predominant CYP24A1-generated metabolite, 24,25(OH)₂D₃, previously considered to be inactive, has recently been shown to bind and activate TRAM-LAG1-CLN8 domain 3B (TLCD3B). Here, we demonstrate that TLCD3B is expressed in mouse and human testis, and in the midpiece of human spermatozoa. Short-term treatment of male mice with 24,25(OH)₂D₃ increased serum levels of LH and inhibin B. However, global and germ cell-specific TLCD3B-deficient mice showed no major changes in reproductive function. Conversely, in female global TLCD3B-deficient mice, serum levels of 1,25(OH)₂D₃ were elevated, accompanied by reduced ovarian Cyp24a1 expression, but no major changes in reproductive function were observed. In human spermatozoa, high concentrations of 24,25(OH)₂D₃-treatment elicited a rapid Ca²⁺ increase, but this was likely mediated via the vitamin D receptor. Our findings demonstrate that 24,25(OH)₂D₃-TLCD3B signaling is dispensable for reproductive function in mice and exerts limited effects in human spermatozoa. Our study highlights the complexity of vitamin D signaling and shows that 24,25(OH)₂D₃-TLCD3B has limited importance for male reproductive function in reproductively competent young adult mice.