Washington DC: In a recent discovery, scientists have identified a key molecule that drives chemo attraction between sperm and egg cells in marine invertebrates, animals that don’t have a vertebral column.
More than 100 years ago, MBL Director F.R. Lillie of the University of Chicago discovered that eggs from marine invertebrates release a chemical factor that attracts sperm by a process called chemotaxis.
Sperm swims up a chemical gradient to reach the egg, assisted by a pulsatile rise in calcium ion concentration in the sperm tail that controls its beating.
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In past years, many of the cellular components that translate chemo-attractant stimulation into a calcium response have been revealed, but a crucial ingredient has been missing. A prerequisite for calcium 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 hard to pin down.
In this new report, U. Benjamin Kaupp, a MBL Whitman Center Scientist from the Center of Advanced European Studies (Caesar) in Bonn, Germany, identifies this molecule.
The molecule that Kaupp and colleagues 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. 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 is activated by a stretch of positively charged amino acids called the voltage sensor. When sperm capture chemo-attractant 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. When this mechanism is disabled, the calcium pulses in the sperm tail are suppressed, and sperm are lost on their voyage to the egg.