Will Space Travel Affect Reproduction?

Will Space Travel Affect Reproduction?

Dateline: June 2002. Studies of sea urchin sperm indicate that gravity, or the lack thereof, could indeed have a significant effect on fertilization in space.



Above: The purple sea urchin, Strongelocentrotus purpuratus, is a widely used model for studying the biology of fertilization because the mechanisms of its sperm movement and fertilization are well-known. Sperm and eggs are collected during the spawning season. On the left, a spawning female sea urchin has deposited eggs, and on the right, a male sea urchin has deposited sperm. Photo credit - Joseph Tash

Colonizing other planets and living and working in space for entire lifetimes were once the stuff of science fiction, but these days spaceflight itself has become somewhat routine, and space stations (Skylab, Russia's Mir, and recently the International Space Station) have provided people with the opportunity to live and work in space for extended periods of time. People now speculate that the ability to explore and colonize other planets is simply a matter of time. But some practical issues that go with traveling to and inhabiting other planets must still be addressed. One of the most fundamental biological questions posed by space travel is that of the effects of microgravity on reproduction.
Sperm and Serendipity

In the course of a literature search pertaining to his research in the field of male reproductive issues and male contraceptives, NASA Principal Investigator Joseph Tash, of the University of Kansas Medical Center, came upon a paper by Ute Engelmann, of Medical Consulting in Munich, Germany, and her co-investigators. The paper described experimental results in which bull sperm motility was increased when subjected to freefall. Tash's discovery of the Engelmann article coincided with a NASA announcement seeking research proposals for studying the effects of microgravity on the ability of species to reproduce, and Tash believed that his own research would benefit from a microgravity environment, so he submitted a research proposal.

Tash was interested in signal transduction, the process by which sperm are "told" to travel toward and fertilize an egg. He says, "We proposed to examine whether the signal transduction associated with the activation of sperm, and also the signaling that occurs in the sperm in association with signaling from the egg, were altered under the effects of microgravity." The proposal was selected for further ground-based studies and subsequently for flight studies.
Sperm vs. Eggs

Tash and his co-investigators chose to study sperm not only because that was where Tash's initial research interest lay, but also because sperm are very easy to collect, store, and study without affecting their function. With eggs, it's difficult to assess possible changes in their function resulting from the effects of microgravity without first fertilizing them. Notes Tash, "With sperm, you don't have to do that in order to get a good idea of whether they're working or not."

Sperm cells are considered to be terminally differentiated cells. They have just two functions: moving, and fertilizing the egg. Fertilization is not possible without sperm movement, so studying the fundamental ability of sperm cells to move is a relatively simple way of assessing sperm functionality.

For his research, Tash chose to use sea urchin sperm because the sperm are more uniform than sperm obtained from humans or other mammals, but their function and mode of movement are very similar to those of sperm from higher species. Tash notes that sea urchins are a long-standing, widely used model for studying the biology of fertilization. Common genetic origins, or homologies, between the sea urchin system and mammalian systems make the sea urchin a good model for obtaining basic information that can point to important questions to be addressed by studying mammalian systems. Sea urchin sperm also provide the added benefit of survivability - they are able to tolerate delays that sometimes occur with flight research.
First Steps

To send the sperm into space, Tash and his co-investigators used the European Space Agency's (ESA's) Biorack facility, a multiuser biological research facility originally designed for shuttle missions. The investigators were supplied with the hardware a year ahead of time. They used this period to demonstrate that the hardware itself did not affect the outcome of their studies and that they could ask and answer the questions they wanted to before the experiment was manifested. "I think that's a real critical component of why we were so successful," says Tash.

A key aspect of the experiment was that the sperm were not in an active state - that is, they were not moving - when they were sent into orbit aboard the space shuttle. During fertilization in sea urchins, activation of the sperm occurs in less than a minute. Sperm are activated by a chemical process called phosphorylation, which sets off reactions within the sperm cells that start them swimming toward an egg. A separate chemical process stops sperm movement. During their preflight experiments, the researchers proved that the sperm could be collected and maintained in an inactive state for at least 20 hours before launch until the beginning of the experiment, which occurred a minimum of 20 hours after launch.

This preflight research involved developing new technology for sperm storage, which led to a patent for the team. The researchers have been able to adapt the technology for sperm from different species, and they hope that the technology will find application in the agriculture industry, specifically for the collection, storage, and transport of semen for use in breeding, such as when a farmer wishes to breed his cattle to a bull that is located in another part of the country.
A Moving Experience

The experiment involved looking at specific proteins associated with sperm motility. Sperm were held in chambers in the Biorack; each chamber held experiment hardware for six samples of sperm, and there were two chambers for each of the time points at which the sperm were examined (0, 30, and 60 seconds). Once the sperm were activated by the introduction of seawater, their movement was stopped at either 30 seconds or 60 seconds. The researchers were then able to use antibodies to compare how the proteins associated with motility changed at each of the time points.

"During our ground-based studies we found that two key sets of proteins, called FP 130 and FP 160, were likely associated with dynein, the main motor protein that is responsible for sperm tail movement," explains Tash, referring to a paper he published in Biochemical and Biophysical Research Communications in 1998 (see below for full reference).

"These proteins are phosphorylated [a phosphorous group is attached to them] during activation of sperm, which starts the whole chemical cascade within the sperm cell that leads to onset of motility. Under microgravity conditions, the phosphorylation of FP 130 and FP 160 occurred much more rapidly than it did under normal-gravity conditions," says Tash. This result is consistent with those obtained from the earlier sounding rocket experiments conducted by Engelmann. The researchers learned that the sperm will begin to move sooner and will move more rapidly in space than they will on Earth, but it is not clear whether faster sperm will actually lead to increased fertilization.

In a second flight experiment, Tash and his co-investigators were also able to examine the reaction of sperm to egg peptides that act as chemotactic factors to the sperm, guiding the sperm to the egg. The researchers discovered that the same proteins that were phosphorylated during the activation of motility were also modified in response to the presence of egg factors. In this second experiment, a delay occurred in the reduction of phosphorylation under microgravity conditions, meaning that the process that stops the movement of sperm occurred more slowly than under normal gravity. Tash explains, "This implies that microgravity could have an effect on fertilization itself, on the efficiency and timing thereof."

Tash comments that the effects on phosphorylation could be either positive or negative - only further studies will tell. He remarks, "For example, the signal trans-duction [phosphorylation] that occurs during activation of sperm motility occurs faster, but because it's occurring faster, does that mean that it will also deteriorate faster? Because the sperm begin to move more quickly, does that mean that they will get to the eggs more quickly and fertilize better? There are all sorts of questions that this research raises that obviously have not been looked at in detail yet but hopefully will be."

Having found that microgravity caused an increase in sperm motility, Tash and his co-investigators decided to study the move-ment of sea urchin sperm and fertilization of the egg under increased gravity. Using a centrifuging microscope in Germany, Tash was able to study individual sperm in environments up to 5 g. He found that sperm motility was affected under hyper-gravity conditions as low as 1.3 g. The phosphorylation processes that were stimulated under microgravity were inhibited under increased gravity, and Tash observed a concurrent 50 percent decrease in sperm/egg binding and fertilization of the egg. The fact that both binding and fertilization were reduced by 50 percent under hypergravity conditions suggests that hypergravity exerts a primary effect on the sperm rather than on the egg. Had the primary effect been on the egg, then Tash would have expected to see a much greater drop in the egg response as compared with the drop in sperm/egg binding. Tash's experiments left him with no doubt that the level of gravity does indeed have a significant effect on sperm motility.
The Mammalian Connection

On the basis of these sea urchin sperm motility studies, Tash received another NASA grant to study reproductive issues in a mammalian system. His research team has done some preliminary work with rats under simulated microgravity. Early results, published in the Journal of Applied Physiology in 2001, indicate that sperm production is severely blocked after six weeks of simulated microgravity. So before continuing to study sperm motility, the investigators will now address the more basic issue of the effect of microgravity on sperm formation.


Left: To be able to study the effects of microgravity on sperm motility, sperm had to be collected and stored in an inactive state prior to launch. Tash and his co-workers developed a new sperm storage method that did not require freezing of the sperm.
Graphic credit - Stanton Fernald, Imaging Core, University of Kansas Medical Ce