Science & Technology

The science dispatches (05/08/18): Elusive molecule that draws sperm to egg found.

These are the Scisco Media science dispatches for the first week of August 2018, including; The rules of attraction, finding the chemical which leads sperm to the egg to enable conception. High-resolution imaging of nanoparticle surface structures achieved for the first time.
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These are the Scisco Media science dispatches for the first week of August 2018, including; The rules of attraction, finding the chemical which leads sperm to the egg to enable conception in marine organisms. High-resolution imaging of nanoparticle surface structures achieved for the first time.

The rules of attraction: Researchers find the molecule which leads sperm to the egg enabling conception.

Whilst it has been known years that marine invertebrate eggs release a chemical factor that attracts sperm, a process known as chemotaxis, a key ingredient has been missing.  Researchers from the Marine Biological Laboratory (MBL) have identified the key molecule driving the attraction between sperm and egg cells in marine invertebrates, a study published in Nature Communications (1) this week confirms.

Egg and sperm photo illustration. The sperm drawings are from an article by Antoine van Leeuwenhoek, who built the first microscope and discovered sperm, published in Philosophical Transactions 1678.

Egg and sperm photo illustration. The sperm drawings are from an article by Antoine van Leeuwenhoek, who built the first microscope and discovered sperm, published in Philosophical Transactions 1678. (René Pascal of Caesar, Bonn)

Chemicals released by the egg create a gradient that allows sperm to swim towards the egg. The process is assisted by a pulsatile (2) rise in calcium ion (Ca2+) concentration in the sperm tail that controls its beating. Whilst many of the chemical ingredients required by the process have been identified, the element responsible for allowing Ca2+ ions from the sperm’s environment to enter the tail and thus causing the sperm cell’s pH to become more alkaline has eluded researchers.

U. Benjamin Kaupp holds a sea urchin in the Marine Resources Center at the Marine Biological Laboratory, Woods Hole, Mass. (Megan Costello)

U. Benjamin Kaupp holds a sea urchin in the Marine Resources Center at the Marine Biological Laboratory, Woods Hole, Mass. (Megan Costello)

After 18 summer’s of searching, a team led by biologist U Benjamin Kaupp from the Center of Advanced European Studies (Caesar) in Bonn, Germany, has identified the molecule responsible for this pH change.

The molecule allows sodium ions to flow into the sperm cell whilst simultaneously transporting 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, 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. When this mechanism is disabled, the Ca2+ pulses in the sperm tail are suppressed, and sperm is lost on their voyage to the egg.

High-resolution imaging of nanoparticle surface structures is now possible

The use of scanning tunnelling microscopy (STM) allows extremely high-resolution imaging of the molecule-covered surface structures of silver nanoparticles, even down to the recognition of individual parts of the molecules protecting the surface, joint research (3) between teams of scientists in China and Finland has found. 

 

Left: High-resolution STM image of a silver nanoparticle of 374 silver atoms covered by 113 TBBT molecules. Right: a simulated STM image from one orientation of the particle. Center: the atomic structure of the particle.

Left: High-resolution STM image of a silver nanoparticle of 374 silver atoms covered by 113 TBBT molecules. Right: a simulated STM image from one orientation of the particle. Center: the atomic structure of the particle. (Hannu Häkkinen)

The Study of the surface structures of nanoparticles at atomic resolution is vital in understanding their chemical properties, molecular interactions and the functioning of particles in their environments. Thus far research of surface structures involved imaging techniques suitable for nanometer-level resolution, the most common of which are based on electron tunnelling, the abovementioned scanning tunnelling microscopy (STM), and atomic force microscopy (AFM) based on the measurement of small, atomic-scale forces.

Achieving a finer resolution has been extremely challenging thus due to the similarity in size and curvature of the nanoparticle and the scanning tip of the machine that conducts the examination. Molecular measurements are also extremely sensitive to environmental disturbances. Slight changes in temperature can cause the thermal excitement of the molecule being examined, for example.

structure of a typical silver nanoparticle

structure of a typical silver nanoparticle

To combat this, the scientists behind this new research published in Nature Communications imaged previously characterised silver nanoparticles with a known atomic structure at extremely low temperatures. Clear sequential modulations were observed in the tunnelling current formed by the image. Similar modulations were noted when individual surface molecules of the nanoparticle were imaged on a flat surface.

The research team led by Professor Hannu Häkkinen of the University of Jyväskylä showed that each of the three carbon groups of the TBBT molecules which coat the outside of the nanoparticle, provides its own current maximum in the STM image and that the distances between the maxima corresponded to the STM measurement results. This confirmed that measurement was successful at the sub-molecular level. The simulations also predicted that accurate STM measurement can no longer be successful at room temperature, as the thermal movement of the molecules is so high that the current maxima of individual carbon groups blend into the background.

The team performed the STM imaging from over 1,600 different orientations and developed a computer-based algorithm to determine which images best matched experimental results.

“This is the first time that STM imaging of nanoparticle surface structures has been able to ‘see’ the individual parts of molecules. Our computational work was important to verify the experimental results. However, we wanted to go one step further. As the atomic structure of particles is well known, we had grounds for asking whether the precise orientation of the imaged particle could be identified using simulations,” says Häkkinen, describing the research.

Häkkinen also described the possible future applications of this research “We believe that our work demonstrates a new useful strategy for the imaging of nanostructures. In the future, pattern recognition algorithms and artificial intelligence based on machine learning will become indispensable to the interpretation of images of nanostructures. Our work represents the first step in that direction.”

References and further reading

(1) Windler, F. et al (2018) The solute carrier SLC9C1 is a Na+/H+-exchanger gated by an S4-type voltage-sensor and cyclic-nucleotide binding. Nature Communications, DOI: 10.1038/s41467-018-05253-xDOI: 10.1038

(2) https://en.wikipedia.org/wiki/Pulsatile_secretion

(3) : Qin Zhou, Sami Kaappa, Sami Malola, Hui Lu, Dawei Guan, Yajuan Li, Haochen Wang, Zhaoxiong Xie, Zhibo Ma, Hannu Häkkinen, Nanfeng Zheng Xueming Yang & Lansun Zheng, “Real-space imaging with pattern recognition of a ligand-protected Ag374 nanocluster at sub-molecular resolution”, Nature Communications 9, 2948 (2018), DOI 10.1038/s41467-018-05372-5.

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