Ovulation in mammals is a tricky business, regulated differently in different species. Many mammalian females, like cattle, mice, and humans, ovulate on a schedule, irrespective of sexual activity. Others, like camels, llamas, and mink, only ovulate after intercourse. Researchers had identified a substance they dubbed ovulation inducing factor (OIF) in camel semen that they could use to induce ovulation in other “induced” ovulators, such as llamas. Surprisingly, research showed that these induced animals will also ovulate when semen from on-schedule or “spontaneous” ovulators, such as cattle, is used. In research published today (August 20) scientists in Chile and Canada identify the substance in spontaneous ovulators: nerve growth factor beta (NGF-beta)—a neurotrophic protein involved in nerve development, which has the same chemical structure as OIF. The research may help shed light on undiscovered causes of infertility.
The finding is “very exciting,” said Sergio Ojeda, a neuroscientist at Oregon Health & Science University, who did not participate in the research. “It’s a step toward negating the dogma that neurotrophins are only important in nervous system development."
This is not, however, the first time researchers have demonstrated a critical function for neurotrophins in reproduction. Ojeda’s own work has identified a role for a different neurotrophic factor in the development and fertilization of oocytes.
Previous work by lead author Gregg Adams at the University of Saskatchewan and colleagues also demonstrated that OIF injection altered the follicle maturation dynamics in cows—which ovulate spontaneously—suggesting crossover in function between these two types of ovulators. This led Adams and collaborator Marcelo Ratto at Universidad Austral de Chile to hypothesize that OIF was a single protein factor, conserved across species.
In order to identify the chemical structure of OIF, Addams and Ratto collaborated with protein chemist Louis Delbaere at the University of Saskatchewan. They fractionated semen from llamas (induced ovulators), and subjected the OIF-containing fraction to mass spectrometry to identify its amino acid sequence, which was highly homologous to human, cow, pig, and mouse NGF-beta. X-ray crystallography of OIF and NGF-beta confirmed their structural and sequence homology.
To determine whether NGF and OIF had the same functions, Adams and Ratto treated neurons with OIF and demonstrated that it promoted neurite outgrowth in an NGF-fashion. They also injected NGF into llamas, which promoted ovulation at a similar rate as OIF.
The work suggests a new mechanism of action for NGF, said Ojeda. Previous work focused on its ability to stimulate cell growth near its neuron of origin—but NGF in the semen must be acting on distant tissues. “The concept that NGF may be acting as a hormone is novel,” notes Ojeda, “but the evidence is very strong.”
“I suspect this work will inspire investigations of NGF's role in reproduction” beyond ovulation, Daniel Bernard, who studies signal transduction at McGill University, wrote in an email. “For example,” said Bernard, who did not participate in the study, “does NGF play important roles in uterine and/or ovarian function that might contribute to reproductive success” in spontaneous ovulators, including humans?
Though Adams acknowledges NGF’s role in reproduction has yet to be studied in humans, he hypothesizes that it may play some purpose. During the menstrual cycle, a small subset of follicles in an ovary will begin to grow and mature, with just a few achieving the potential to ovulate, Adams explains. In 2003, Adams’s group demonstrated that this actually happens twice in women—raising the possibility that ovulation may occur twice in one cycle. Adams thinks that NGF in semen may act in humans to promote a second round of ovulation.
Additionally, OIF is important for development of the gland that produces progesterone, which is necessary for pregnancy. Likewise, NGF may act to facilitate this process in spontaneous ovulators, rather than prompting ovulation, said Adams. Inadequate development of this gland, called the corpeus leuteum, underlies many cases of infertility. It may be that women with leuteal insufficiency are insensitive to OIF or their partners produce inadequate amounts, Adams speculated. A female’s “own NGF is probably not playing a role in ovulation,” said Warren Foster, who studies the affect of environment on reproduction at McMaster University, but was not involved in the work. NGF from semen may produce an effect because it’s concentrated over background levels, he added.
If NGF proves important in human fertility, Ojeda suspects that scientists may be able to employ it and other neurotrophins to enhance success of in vitro fertility strategies. In the meantime, says Ojeda, the paper is a “boost” to research focusing on substances important in both nervous system development and reproduction.
M. H. Ratto et al., “The nerve of ovulation inducing factor in semen,” Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1206273109, 2012.