So I was browsing the internet for info on G-protein coupled receptors and ended up finding some interesting facts about sperm. It turns out sperm don’t just swim blindly, hoping to randomly bump into eggs. Instead, like bacteria, sperm can sense their chemical environment and adjust their swimming accordingly. Sperm have a sense of smell.
The (g-protein coupled) olfactory receptors in our noses that activate our sense of smell were discovered in 1991, an amazing discovery that earned the 2004 Nobel prize for physiology or medicine. The receptors sit on the surface of the cells up high in our nose, and smelly chemicals in the air we breath bounce around on them. When a molecule bounces onto the specialized receptor that recognizes it, the receptor turns on and activates the G-protein coupled to it, in this case G-olf. G-olf then goes on to activate a cascade of other proteins which end up opening protein channels in the cell’s membrane, allowing ions to flow through. The ion flow changes the electrical potential of the cell and starts the electrical signal that will make it all the way to the brain–that is if the receptor is in your nose.
After the initial discovery in the nose, olfactory receptors kept showing up in all sorts of tissues–in the heart, in the spleen, in the prostate, and even in sperm. On the sperm cell, what the receptors are doing is a lot harder to figure out than in the nose, especially in a way that is meaningful to how sperm swim in their natural environment. In vitro studies of sperm in a test tube identified chemicals that can activate the sperm olfactory receptors and others that will block them, but the model of how the receptors are connected to the waves of calcium ions that control the swimming motion of sperm is full of question marks. Even the chemical that sperm swim towards in vitro has no apparent physiological significance, bourgeonal being a chemical common in perfumery for its lily of the valley scent. Understanding the biochemistry of the sperm olfactory receptors in their physiological context may some day have impacts in fertility treatments, in vitro fertilization technology, and perhaps even hormone-free contraceptives, although the authors of the review that introduced me to world of sperm olfaction cautiously note that:
such speculation may still be a long way from future drug development and subsequent clinical trials. It will be challenging to demonstrate efficiency and inoffensiveness of potential pharmaca as well as to discover suitable ways of drug application.
Olfactory receptors turning up in unexpected places can expand our understanding of the way that cells interact with their environment, whether it’s how we navigate our smellscape or how sperm navigate to the egg. Findings like these also highlight the generalness of many of the proteins involved in cellular signaling and the flexibility of the olfactory receptors themselves–their general usefulness in sensing chemicals for all kinds of cells and the evolutionary conservation that allows even yeast to be engineered to have a sense of smell. So three cheers for olfaction and three cheers for all the G-protein coupled receptors!