Sperm 'see' it hot

In their arduous journey to the egg, sperm 'feel' the heat of the fallopian tube and 'taste' the chemical signals of the egg. Now, a new study shows sperm actually use sensors — mainly associated with the visual system — to detect ova.

Proteins better known as visual sensors play a role in the heat-seeking movement of sperm. If a sperm, about 46 microns (thousandths of a millimeter) long, were the size of a tall human male — about 6 feet — it would swim about 1.86411miles to reach its destination. Over such a long distance, sperm need guidance.

In earlier studies, Prof. Michael Eisenbach and his team at the Weizmann Institute of Science in Rehovot, Israel, had discovered two of these guidance mechanisms (1) heat-sensing and (2) chemical cues. The new report is published in Nature, Scientific Reports in November 2015.

Heat attraction works at the site of fertilization as it is warmer than the entrance to the fallopian tube. The increase in temperature directs sperm to the egg. As sperm move closer to the egg, chemical signals begin to be picked up.

"As in all important processes in nature, sperm rely on more than one mechanism in navigation, so that if one breaks down, other senses provide a back-up."

Michael Eisenbach PhD, Professor in the Department of Biological Chemistry, Weizmann Institute of Science, Rehovot, Israel

The heat sensitivity of sperm is extremely high. They can sense differences in temperature as miniscule as 0.0006 of a degree C, or less than one thousandth of a degree, from a distance equivalent to the length of a single sperm. This ability enables them to be guided to find the egg and fertilization site by very gradual temperature increases.

In the new study, Eisenbach's team set out to discover exactly how sperm sense heat. They examined a particular category of receptors discovered in previous work to convey signals between sperm. Within this category, they zeroed in on a family of proteins called opsins.

Opsins are best known for their role in an entirely different sphere: the visual system.

One major protein in this family called rhodopsin, is a photoreceptor in retinal cells.

But other research with fruit fly larvae found rhodopsin protein also responds to heat, hinting this could also be the case in sperm.

The Weizmann Institute scientists found that several proteins in the opsin family of receptors are present on the surface of mouse and human sperm.

Each opsin has its own distribution pattern on sperm, and each apparently makes a contribution through its own set of signals, to heat sensing. When researchers blocked the signals from these opsins, sperm failed to swim from a cooler to a warmer chamber.

These rather surprising findings may explain the enigmatic presence of opsins in organs that are not exposed to light, including the lungs and the liver. They suggest that mammalian opsins may also be performing heat-sensing functions in these parts of the body.

Abstract
A unique characteristic of mammalian sperm thermotaxis is extreme temperature sensitivity, manifested by the capacity of spermatozoa to respond to temperature changes of <0.0006 °C as they swim their body-length distance. The identity of the sensing system that confers this exceptional sensitivity on spermatozoa is not known. Here we show that the temperature-sensing system of mammalian spermatozoa involves opsins, known to be G-protein-coupled receptors that act as photosensors in vision. We demonstrate by molecular, immunological, and functional approaches that opsins are present in human and mouse spermatozoa at specific sites, which depend on the species and the opsin type, and that they are involved in sperm thermotaxis via two signalling pathways—the phospholipase C and the cyclic-nucleotide pathways. Our results suggest that, depending on the context and the tissue, mammalian opsins act not only as photosensors but also as thermosensors.

Team members
Dr. Serafín Pérez-Cerezales, Dr. Sergii Boryshpolets, Oshri Afanzar, Dr. Reinat Nevo and Vladimir Kiss of the Biological Chemistry Department and Dr. Alexander Brandis of Biological Services.

Prof. Michael Eisenbach's research is supported by the Benoziyo Endowment Fund for the Advancement of Science.

The Weizmann Institute of Science in Rehovot, Israel, is one of the world's top-ranking multidisciplinary research institutions. Noted for its wide-ranging exploration of the natural and exact sciences, the Institute is home to scientists, students, technicians and supporting staff. Institute research efforts include the search for new ways of fighting disease and hunger, examining leading questions in mathematics and computer science, probing the physics of matter and the universe, creating novel materials and developing new strategies for protecting the environment.

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Locations of different opsins on human sperm are
labeled with a fluorescent antibody (in bright yellow).

Image Credit: Weizmann Institute of Science