A protein called ACTL7B, found only in the testis, plays a crucial role in spermatogenesis, according to recent research. This protein shares up to 60% amino acid identity with conventional actins and is highly conserved in rodents and primates. ACTL7B is expressed exclusively in the testis in both mice and humans, indicating its likely involvement in sperm production.
Previous animal studies have suggested a link between ACTL7B and fertility, while research in humans has found single nucleotide polymorphisms (SNPs) in the gene’s coding sequence among men with infertility. However, no study has directly connected the ACTL7B gene to infertility, and the exact molecular function of the protein has not been fully understood.
About the Study
In this study, researchers used CRISPR/Cas9 technology to create Actl7b-deficient mice, enabling a closer look at the gene’s role in spermatogenesis. Two types of Actl7b-knockout (KO) mice were created: heterozygous (Actl7b+/−) and homozygous (Actl7b−/−). These mice were then analyzed for structural changes in their sperm and testicular tissue. The researchers used immunohistochemical staining and transmission electron microscopy (TEM) to observe abnormalities in sperm formation, including issues with acrosome biogenesis, DNA condensation, manchette formation, and sperm tail development.
Additionally, the research team studied the testicular proteome of these mice to identify how ACTL7B interacts with other proteins. Through mass spectrometry (MS), they found that ACTL7B interacts with dynein light chains LC8-Type 1 and Type 2 (DYNLL1 and DYNLL2), which are critical in the later stages of spermatogenesis. This interaction likely occurs via the microtubule network or the dynein 1 motor complex, rather than the actin cytoskeleton.
Study Findings
The study revealed that Actl7b-deficient mice experienced spermatid arrest during development, leading to various abnormalities in sperm, including malformed flagella. As a result, many spermatids were degraded and removed by Sertoli cells. Increased levels of autophagy marker proteins in the testes confirmed this degradation process.
The MS analysis revealed that ACTL7B interacts with specific proteins involved in the later stages of spermatogenesis. Interestingly, Actl7b+/− male mice, which had reduced ACTL7B levels, maintained fertility similar to wild-type mice. However, Actl7b−/− mice, which lacked ACTL7B, showed severe infertility, with sperm counts drastically reduced.
The study also noted that previously reported cases of infertility in Actlt7b-KO mice involved severe sperm malformations and a dramatic decrease in sperm count. In this study, sperm counts in the KO mice were around 32,000, a 1,000-fold reduction compared to the normal count of 32 million in wild-type mice. This difference may be due to variations in the parental mouse strains used.
Both heterozygous and homozygous knockout models showed reduced germ cell survival, a decrease in epididymal sperm number, and a higher rate of immature germ cell loss.
Conclusions
Given the similarities between human and mouse ACTL7B genes, the study suggests that genetic variants in ACTL7B could contribute to failed spermatogenesis and male infertility. Additionally, the presence of ACTL7B could serve as a useful biomarker to distinguish between obstructive and non-obstructive azoospermia, potentially helping to diagnose male infertility in the future.
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