Elusive molecule that helps sperm find egg identified

Scientists have identified a key molecule driving chemo-attraction between sperm and egg cells in marine invertebrates.

Over 100 years ago, researchers at the University of Chicago in the US discovered that eggs from marine invertebrates release a chemical factor that attracts sperm, a process called chemotaxis.

Sperm, for their part, swim up a chemical gradient to reach the egg, assisted by a pulsatile rise in calcium ion (Ca2+) concentration in the sperm tail that controls its beating, said researchers at Marine Biological Laboratory (MBL) in the US.

In the past years, many of the cellular components that translate chemoattractant stimulation into a Ca2+ response have been revealed, but a crucial ingredient has been missing, according to the study published in the journal Nature Communications.

A prerequisite for Ca2+ ions from the sperm's environment being able to enter the tail is that the sperm cell's pH becomes more alkaline.

The molecule that brings about this change in pH has been elusive.

In the latest study, Benjamin Kaupp, an MBL Whitman Center Scientist from the Center of Advanced European Studies in Germany identified this molecule.

The molecule that Kaupp identified allows sodium ions to flow into the sperm cell and, in exchange, transports protons out of the cell.

Such so-called sodium/proton exchangers have been known for a long time, but this one is special, researchers said.

It is a chimaera that shares structural features with ion channels, called pacemaker channels, which control our heartbeat and electrical activity in the brain.

This sodium/proton exchange in the sperm cell, like in the pacemaker channels, is activated by a stretch of positively charged amino acids called the voltage sensor.

When sperm capture chemoattractant molecules, the voltage becomes more negative, because potassium channels open and potassium ions leave the cell.

The voltage-sensor registers this voltage change and the exchanger begins exporting protons from the cell; the cell's interior becomes more alkaline, researchers said.

When this mechanism is disabled, the Ca2+ pulses in the sperm tail are suppressed, and sperm are lost on their voyage to the egg, they said.