Cooperation of cell biologists and physicists at IST Austria unravels physical basis of yolk-cytoplasm segregation in early fish embryo | Study published in Cell
The segregation of yolk from the surrounding cytoplasm in the very early fish embryo is a key process for the development of the fish larva. To identify its underlying mechanisms, biologists at the Institute of Science and Technology Austria (IST Austria) teamed up with their colleagues from theoretical physics. The discovery: Actin dynamics in the bulk of the cell drive phase segregation in zebrafish oocytes.
Illustration of the segregation process: The octopus represents the actin organizing center of the zebrafish oocyte. It pulls the cytoplasmic pockets up, while it pushes the bigger yolk granules down
IST Austria/Justine Renno
A single-cell fish egg evolves into a multi-cell embryo in less than two hours after fertilization. Within these two hours, the cytoplasm, which will later form the animal body, must separate completely from the yolk, which the larva is going to feed on.
Previously, cell biologists had proposed that local expansion of the cell surface at one pole of the egg mediates this segregation. However, direct evidence supporting this model was lacking.
Joined forces: lab experiments and physical theory
To understand the physical basis of this segregation process, Shayan Shamipour, PhD student in the research group of developmental biologist Carl-Philipp Heisenberg, teamed up with the research group of theoretical physicist Edouard Hannezo. Based on the combined expertise of these two groups, the authors, also including a third professor of IST Austria, Björn Hof, reveal that the forces exerted at the cell surface are dispensable for this process—as opposed to previous models.
Instead, they discovered that combined pulling and pushing forces within the embryo facilitate the segregation of cytoplasm from the yolk granules. Importantly, the theory developed to describe this process can be applied to any segregation due to the forces exerted from an active fluid and could thus also be used to examine potential similar processes in mammalian/human embryos.
But how are these concerted pulling and pushing movements generated? In the bulk of the cell, far away from the cell surface, filaments of actin and myosin—proteins also involved in muscle cell contraction—form a dense mesh. Polymerization and contraction of this mesh trigger actin flows towards the animal pole of the egg, the hemisphere that is going to differentiate into the later embryo. Via passive frictional forces, these actin flows drag along cytoplasm.
The bigger yolk granules, in contrast, are not dragged along by actin since their friction with actin is much lower. Instead, they are actively pushed, or rather squeezed, towards the opposite vegetal pole of the egg by comet-like actin structures—particular actin structures whose function had not been reported in developmental processes before. The combination of these pulling and pushing forces ensures a robust segregation of the cytoplasm and yolk granules within the developing embryo.
Bringing darkness into the light
By examining deeper parts of the cell more closely, the multi-disciplinary team has revealed that animal pole expansion at the cell surface, as previously proposed, is not essential for the yolk-cytoplasm segregation.
“The actin structures at the cell surface appear very bright and are therefore quite easy to study. Maybe that’s why scientists have so far simply missed to look more deeply into the much darker bulk area, which makes up most of the cell,” says Shayan Shamipour, lead author of the study.
Refined image processing allowed the IST Austria researchers to take a closer look at the developments in fish eggs during the moments right after fertilization. But, as Shamipour adds, another key to success was something else:
“To catch the very first moments of egg development, we had to be really fast: Whenever one of our fish had started to release its eggs into the water, I would press start on my stop watch and my colleagues would see me sprint from the fish facility to the microscopy room to observe and record the process.”
Curiosity-driven teamwork at its best
According to the cell biologist with a background in physics, Shamipour had been suspicious of the prevailing surface-based explanation for a while: “The embryo follows a big goal: It has to divide from one into thousands of cells in a very short amount of time. It was thus evident that the proposed surface mechanism alone could not accomplish this segregation and that the embryo would have to come up with some other mechanisms to accelerate the process.”
It is this curiosity-driven attitude of the young scientist paired with the interdisciplinary research culture of the Heisenberg and Hannezo groups—a mode of scientific work IST Austria particularly fosters—that enabled Shamipour to identify and analyze central cell processes that could be relevant in many other settings and organisms.
This project has received funding from the European Union (European Research Council Advanced Grant) and from the Austrian Science Fund (FWF).
The Institute of Science and Technology (IST Austria) is a PhD-granting research institution located in Klosterneuburg, 18 km from the center of Vienna, Austria. Inaugurated in 2009, the Institute is dedicated to basic research in the natural and mathematical sciences. IST Austria employs professors on a tenure-track system, postdoctoral fellows, and doctoral students. While dedicated to the principle of curiosity-driven research, the Institute owns the rights to all scientific discoveries and is committed to promote their use. The first president of IST Austria is Thomas A. Henzinger, a leading computer scientist and former professor at the University of California in Berkeley, USA, and the EPFL in Lausanne, Switzerland. The graduate school of IST Austria offers fully-funded PhD positions to highly qualified candidates with a bachelor's or master's degree in biology, neuroscience, mathematics, computer science, physics, and related areas. www.ist.ac.at
Understanding cell biological processes is only possible by studying real cells, in this case of zebrafish. No other methods can serve as alternatives. The animals were raised, kept and treated according to the strict regulations of Austrian law.
Carl-Philipp Heisenberg, firstname.lastname@example.org,
and Edouard Hannezo, email@example.com
Shayan Shamipour, Roland Karos, Shi-Lei Xue, Björn Hof, Edouard Hannezo & Carl-Philipp Heisenberg. 2019. Bulk actin dynamics drive phase segregation in zebrafish oocytes. Cell. DOI: 10.1016/j.cell.2019.04.030
https://ist.ac.at/en/research/life-sciences/heisenberg-group/ Heisenberg group
Dr. Elisabeth Guggenberger | idw - Informationsdienst Wissenschaft
Polarization of Br2 molecule in vanadium oxide cluster cavity and new alkane bromination
13.07.2020 | Kanazawa University
Researchers present concept for a new technique to study superheavy elements
13.07.2020 | Johannes Gutenberg-Universität Mainz
Biochemists at Martin Luther University Halle-Wittenberg (MLU) have used a standard electron cryo-microscope to achieve surprisingly good images that are on par with those taken by far more sophisticated equipment. They have succeeded in determining the structure of ferritin almost at the atomic level. Their results were published in the journal "PLOS ONE".
Electron cryo-microscopy has become increasingly important in recent years, especially in shedding light on protein structures. The developers of the new...
New insight into the spin behavior in an exotic state of matter puts us closer to next-generation spintronic devices
Aside from the deep understanding of the natural world that quantum physics theory offers, scientists worldwide are working tirelessly to bring forth a...
Kiel physics team observed extremely fast electronic changes in real time in a special material class
In physics, they are currently the subject of intensive research; in electronics, they could enable completely new functions. So-called topological materials...
Solar cells based on perovskite compounds could soon make electricity generation from sunlight even more efficient and cheaper. The laboratory efficiency of these perovskite solar cells already exceeds that of the well-known silicon solar cells. An international team led by Stefan Weber from the Max Planck Institute for Polymer Research (MPI-P) in Mainz has found microscopic structures in perovskite crystals that can guide the charge transport in the solar cell. Clever alignment of these "electron highways" could make perovskite solar cells even more powerful.
Solar cells convert sunlight into electricity. During this process, the electrons of the material inside the cell absorb the energy of the light....
Empa researchers have succeeded in applying aerogels to microelectronics: Aerogels based on cellulose nanofibers can effectively shield electromagnetic radiation over a wide frequency range – and they are unrivalled in terms of weight.
Electric motors and electronic devices generate electromagnetic fields that sometimes have to be shielded in order not to affect neighboring electronic...
07.07.2020 | Event News
02.07.2020 | Event News
19.05.2020 | Event News
13.07.2020 | Physics and Astronomy
13.07.2020 | Life Sciences
13.07.2020 | Life Sciences