Male infertility accounts for nearly 50% of fertility issues globally, affecting around 8-12% of couples. A common cause is errors during spermatogenesis, the process through which sperm cells are developed, hindering their ability to swim toward the egg for fertilization.
A key player in sperm motility is the centriole, an organelle responsible for forming spindle fibers during cell division and flagella for sperm movement. Despite its critical role, the molecular mechanisms and structural changes of centrioles during spermatogenesis remain poorly understood.
In a groundbreaking study led by Hiroki Shibuya at the RIKEN Center for Biosystems Dynamics Research, researchers used advanced microscopy techniques to observe unexpected structural changes in centrioles in mouse germ cells. Their findings, published in Science Advances, reveal a critical molecular mechanism that could be key to reproductive success.
Shibuya’s team employed a technique known as ultrastructure expansion microscopy, which uses a swellable polymer hydrogel to expand cells, allowing for the visualization of nanoscale structures at high resolution. The method was optimized for mouse sperm cells, enabling detailed observation of the barrel-like microtubule structures within centrioles.
By tracing centrioles through various stages of male germ cell development, from spermatocytes to spermatids, the researchers identified gradual structural changes. They fluorescently labeled centrin and POC5, two proteins essential for centriole assembly and maintenance. The team found that as sperm cells matured, these proteins formed complexes within the centriole’s inner scaffold, especially at the distal tip of the sperm.
To explore the function of these centrin-POC5 complexes, the researchers created Poc5 knockout mice using CRISPR-Cas9 gene editing. These mice were completely infertile, with no sperm cells found in their epididymis. Microscopic analysis revealed that without POC5, centrin could not localize properly within the centriole, disrupting flagellar development. In contrast, flagella were present and functional in normal sperm cells, highlighting the importance of the centrin-POC5 complex in flagellar assembly.
Immunofluorescence microscopy showed that in wild-type spermatids, flagella emerged from the distal tip of centrioles and gradually elongated as the cells matured. However, in Poc5 knockout spermatids, flagella-like structures were disorganized and eventually degenerated, explaining the infertility observed in these mice.
Shibuya noted that their enhanced expansion microscopy technique could be applied to human sperm cells, offering a new approach to studying structural abnormalities that contribute to male infertility. In the future, this research could lead to innovative diagnostic and therapeutic advancements in reproductive medicine.
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