Toll-like receptors in mammalian sperm

Abstract

Background: Toll-like receptors (TLRs) are critical components of the innate immune system and are expressed in various cells, including the reproductive system. Although their roles in female reproductive tissues such as the ovaries and uterus, including their involvement in fertilization and implantation, have been extensively reviewed, their expression and function in male germ cells, particularly in sperm, remain underexplored.

Methods: This review provides a comprehensive summary of research on TLRs expressed in sperm, including findings from experimental models in mice, humans, and industrial livestock.

Results: The activation of TLR2 and TLR4, which detect Gram-positive and Gram-negative bacteria, has been shown to reduce sperm motility and viability, thereby impairing fertilization. Conversely, low levels of TLR2 activation have been reported to promote the fertilization of bull sperm, suggesting that TLR2/4 may act as regulators of fertilization. TLR7 and TLR8, which are exclusively expressed in X chromosome-bearing sperm (X-sperm), have attracted increasing research interest. These receptors modulate sperm metabolism, selectively reduce the motility of X sperm, and enable the separation of X and Y sperm.

Conclusion: TLRs in the sperm serve as immune receptors that detect bacterial and viral infections, thereby reducing sperm functionality, preventing miscarriage, protecting maternal health, and sex selection.

Keywords: fertilization; sperm; toll‐like receptor.

FIGURE 1

Toll‐like receptor and the agonists. In Toll‐like receptors (TLRs), TLR1, TLR2, TLR4, TLR5, and TLR6 are located on the plasma membrane, while TLR3, TLR7, TLR8, and TLR9 are found on endosomes. TLR1 binds to TLR2 and recognizes triacyl lipopeptides, while TLR6 also interacts with TLR2 to recognize diacyl lipopeptides. TLR2 can form homodimers and recognizes peptidoglycan and Zymosan. TLR4 on the plasma membrane detects lipopolysaccharides (LPS) from Gram‐negative bacteria, and TLR5 is responsible for recognizing flagellin. TLR3, located on endosomes, recognizes double‐stranded RNA; TLR7 and TLR8 recognize single‐stranded RNA, and TLR9 binds to CpG motifs in DNA.

FIGURE 2

Localization of TLRs in mammalian sperm. In mice, TLR1, TLR2, TLR3, TLR5, TLR9, TLR11, and TLR12 have been reported to localize to the acrosome of the sperm head, TLR8 and TLR9 are localized to the midpiece, and TLR7 is primarily found in the tail, especially in the principal piece. In humans, TLR2 is localized to the acrosome, while both TLR2 and TLR4 are present in the midpiece. Additionally, TLR2 is localized to the principal piece of the tail, and TLR4 is distributed along the entire tail. In bovine sperm, TLR2 is localized to the posterior segment of the head, TLR8 is found in the midpiece, and TLR7 is localized to the tail.

FIGURE 3

Function of TLR2/4 in mammalian sperm. TLR2/4 expressed in mammalian sperm are localized in the acrosome, midpiece, and tail. When activated by low concentrations of agonists, such as those from Gram‐positive bacteria, TLR2 enhances calcium influx, induces the acrosome reaction, and promotes hyperactivation, all of which are crucial for fertilization. In this context, TLR2 facilitates fertilization. However, when TLR2 is activated at high ligand concentrations, or when lipopolysaccharide (LPS) from Gram‐negative bacteria activates the receptor, mitochondrial oxidative stress is induced, leading to a decrease in ATP production via the activation of Myd88, IRAK1/4, PI3K, and GSK3α. Consequently, sperm death and reduced motility are observed, impairing fertilization.

FIGURE 4

Function of TLR7/8 in mammalian sperm. TLR7/8 expressed in mammalian sperm are activated by single‐strand RNA or the TLR7/8 agonist, R848. TLR8, which is localized in the midpiece, impairs mitochondrial function, while TLR7, expressed in the tail, suppresses the glycolytic pathway. In mice, cattle, and goats, TLR7/8 are exclusively expressed in spermatozoa bearing the X chromosome (X‐sperm). Activation of TLR7/8 specifically reduces the motility of X‐sperm, thereby creating a distinction between X‐ and Y‐sperm. This differential effect enables sex selection.