How sperm cells pack their energy makers

New insights into sperm mitochondria could provide clues about male infertility

Studying sperm cells in unprecedented detail, researchers from Utrecht University discovered how some of the cell’s vital energy producers are arranged. The research team uncovered the molecular pins the hold togethers these cell components, called mitochondria. The discovery provides new insights into the workings of sperm mitochondria and sperm movement, as well as unknown possible causes of male infertility.

Dr. Tzviya Zeev Ben Mordehai
Dr. Tzviya Zeev-Ben-Mordehai

Sperm cells across different mammalian species all share the same goals —to find and fuse with the egg— but the details of their inner structures appear to vary. Researchers from Utrecht University discovered that crucial energy producing cell structures called mitochondria have different sizes and different internal arrangements. They publish their results today in Proceedings of the National Academy of Sciences.

The research team, led by structural biologist Tzviya Zeev-Ben-Mordehai, found that mouse sperm mitochondria are the largest, while horse sperm contains the densest mitochondria. Despite these differences, the researchers observed that the way mitochondria are clustered within sperm cells is very similar across different species.

Small cells, big numbers of mitochondria

Due to their high energy demand during swimming, sperm cells have big numbers of mitochondria. However, sperm cells are one of the smallest cells in mammals. Most likely due to the limited space and to facilitate efficient energy coupling, in sperm cells mitochondria are clustered in a tight spiral. This spiral is strategically located in the sperm tail but close to the head, defining a region called the midpiece.

Cross-section of a horse sperm midpiece, showing mitochondria (mito) and other cell components.

The researchers discovered that neighbouring mitochondria are ‘glued’ by specialised linker proteins. Next, the mitochondria are anchored to a structure called the cytoskeleton, that helps cells maintain their shape and internal organization. This happens through an array of proteins on the outer mitochondrial membrane. With image processing techniques the researchers determined the structure of this array.

The team identified glycerol kinase-like proteins as the most likely components of the arrays. According to the researchers, these protein arrays may help maintain the mitochondrial spiral and provide stability when the sperm cell moves its tail to propel itself forward.

Observing a more natural condition

Zeev-Ben-Mordehai and her team were able to visualise details of sperm cells that have never been observed before, thanks to a relatively new electron microscopic technique called cryogenic electron microscopy (cryo-EM). A major benefit of cryo-EM over standard electron microscopy is that samples requires much less preparation. This way, samples are better preserved and retain their natural condition more closely.

The researchers were able to visualise details of sperm cells that have never been observed before

With standard electron microscopy, samples can only be observed after being dehydrated and coated with heavy atoms. But with cryo-EM, none of these steps are required. The only preparation required is rapid freezing of the sample. “This is the best sample preservation for a cell you can get”, says Zeev-Ben-Mordehai.

Unravelling the sperm cell’s innards

Besides studying sperm mitochondria, the team aims to reveal how various structures in sperm cells interact, to form a compact and streamlined cell. In a study that was published in The EMBO Journal earlier this year, the team revealed how the tail’s core structure, knows as the axoneme, is ornamented to support motility through diverse fluid environments.

In a follow up study published in Nature Communications the team provided an explanation to the atypical structure of a sperm cell component that coordinates the sperm cell’s tail-to-head movement. They showed that this component, called a centriole, facilitates a cascade of internal deformations that couple the tail beating with head movement, serving as a mechanical transducer. This study was performed together with their collaborator Tomer Avidor-Reiss from the University of Toledo.

Looking deeper into male infertility

The team’s findings could be relevant for diagnosing infertility in humans, since malformation of mitochondrial spirals and other cell structures cause infertility. “When analysing male fertility in the clinic, mainly sperm numbers, motility and progression are examined, as they can be easily assessed with a light microscope”, says Tzviya Zeev-Ben-Mordehai.

Malformations of mitochondria and other cell structures cause infertility

“But more than 60% of male infertility remains unexplained. Now that we established protocols to image animal sperm with cryo-EM, we are expanding to healthy and infertile humans. If fertility doctors are better informed of the cause of infertility, they can choose a more personalised fertilization treatment that will have potentially higher chances of success.”

Hormone-free male contraceptives 

On the other hand, the results of this research could open up possibilities for developing male contraceptives that don’t involve hormones, says Zeev-Ben-Mordehai. “We need to understand the molecular details of how movement is generated by sperm. Once we understand it well, we can find ways to block it, put a spoke in the wheel.”

Cooperation

Zeev-Ben-Mordehai’s studies are supported by a START-UP grant from the Dutch Research Council (NWO). Sample preparation and imaging were enabled by the Utrecht University Electron Microscopy Centre. This team uses electron microscopes at the Netherlands Centre for Electron Nanoscopy (NeCEN) which is sponsored by the Netherlands Electron Microscopy Infrastructure (NEMI).

Publication

In-cell structures of conserved supramolecular protein arrays at the mitochondria-cytoskeleton interface in mammalian sperm

Proceedings of the National Academy of Sciences, October 2021

Miguel Ricardo Leung, Riccardo Zenezini Chiozzi, Marc C. Roelofs, Johannes F. Hevler, Ravi Teja Ravi, Paula Maitan, Min Zhang, Heiko Henning, Elizabeth G. Bromfield, Stuart C. Howes, Bart M. Gadella, Albert J.R. Heck, Tzviya Zeev-Ben-Mordehai