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Developmental biology - Sperm Tail Machinery

Found: Molecule that Helps Sperm Find the Egg

A key molecule driving chemo-attraction between sperm and egg cells has been identified...


Scientists affiliated with the Marine Biological Laboratory (MBL) have identified a key molecule that drives the chemical attraction (chemo-attraction) between sperm and egg cells in marine invertebrates such as sea urchins. The study is recently published in Nature Communications.

More than 100 years ago, F.R. Lillie, University of Chicago MBL Director, discovered that eggs of marine invertebrates release a chemical to attract sperm, a process called chemotaxis. Sperm swim up this chemical trail which guarantees they reach the egg. Of course they are assisted by the pulsating rise in calcium ions (Ca2+) concentrated in and coordinating the beat of their whiplike tails.

In past years, many cell components have been identified that translate chemoattractants into a Ca2+ response. But, a crucial ingredient was missing. Before Ca2+ ions floating free in the environment can enter a sperm tail, the sperm cell's pH must be more alkaline to attrack the Ca2+. The molecule that brings about this pH change hadn't been known before. Now in this new report, and after 18 summers at MBL Whitman Center of Advanced European Studies in Bonn, Germany, U. Benjamin Kaupp has identified that molecule.

The molecule 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 the electrical activity in our brain.
This sodium/proton exchange in the sperm cell, just as in pacemaker channels, is activated by a stretch of positively charged amino acids called the voltage sensor. When sperm capture chemo-attractant molecules, their voltage becomes more negative, as potassium channels open up and potassium ions exit the cell.

The voltage-sensor registers this voltage change and the exchanger begins exporting protons out of the cell. This creates a more alkaline cell interior. When this mechanism is disabled, a Ca2+ pulse in the sperm tail is suppressed - and a sperm loses propulsion to make the voyage to the egg.

Abstract
Sperm are propelled by bending waves traveling along their flagellum. For steering in gradients of sensory cues, sperm adjust the flagellar waveform. Symmetric and asymmetric waveforms result in straight and curved swimming paths, respectively. Two mechanisms causing spatially asymmetric waveforms have been proposed: an average flagellar curvature and buckling. We image flagella of human sperm tethered with the head to a surface. The waveform is characterized by a fundamental beat frequency and its second harmonic. The superposition of harmonics breaks the beat symmetry temporally rather than spatially. As a result, sperm rotate around the tethering point. The rotation velocity is determined by the second-harmonic amplitude and phase. Stimulation with the female sex hormone progesterone enhances the second-harmonic contribution and, thereby, modulates sperm rotation. Higher beat frequency components exist in other flagellated cells; therefore, this steering mechanism might be widespread and could inspire the design of synthetic microswimmers.

Authors: Guglielmo Saggiorato, Luis Alvarez, Jan F. Jikeli, U. Benjamin Kaupp, Gerhard Gompper and Jens Elgeti.


Acknowledgements: Financial support by the Deutsche Forschungsgemeinschaft via the priority program SPP 1726 “Microswimmers” is gratefully acknowledged.

Authors’ contributions: The authors declare no competing financial interests.


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Aug 9, 2018   Fetal Timeline   Maternal Timeline   News   News Archive




Diagram of the kinesin motors that propel sperm flagella. Image Credit: Science


Phospholid by Wikipedia