Geraldine Seydoux (Johns Hopkins / HHMI) 2: How to Polarize the Cytoplasm

Summary

In this engaging presentation, Professor Geraldine Seydoux explores the intricate process of how the zygote becomes polarized, focusing on the role of the MEX-5 protein in establishing anterior-posterior gradients. Through various experiments, she demonstrates that MEX-5 redistribution, governed by PAR-1 kinase, is crucial for this polarization. The findings reveal that MEX-5 exists in two species—slow and fast—whose proportions differ between the anterior and posterior sides, leading to a concentration gradient. The research highlights the importance of reversible phosphorylation in controlling diffusion rates, paving the way for a deeper understanding of embryonic development.

Takeaways

• 🎓 Geraldine Seydoux discusses the polarization of the zygote's cytoplasm and how it contributes to body axis formation.
• 🔬 The PAR proteins play a crucial role in establishing polarity within the one-cell embryo.
• 📈 MEX-5, an RNA-binding protein, forms an anterior-posterior gradient during the polarization process.
• ⚗️ Erik Griffin's experiments aim to determine how the MEX-5 gradient is established, considering various hypotheses.
• 💡 One hypothesis suggests that MEX-5 levels could either increase in the anterior or decrease in the posterior, while another proposes redistribution of existing protein.
• 🧪 Erik created a photoactivatable version of MEX-5 fused with a fluorescent protein, allowing for tracking of its distribution during polarization.
• 📊 Experiments showed that total MEX-5 levels remain constant, indicating that redistribution is key to the gradient formation.
• 🌊 The diffusion of MEX-5 was found to vary in speed across different regions of the cytoplasm during polarization.
• 🔄 PAR-1 kinase phosphorylates MEX-5, which increases its diffusion rate, while a phosphatase can reverse this effect.
• 💻 Computational modeling by David Odde successfully recreated the MEX-5 gradient based on experimental data, showing that local reversible phosphorylation can explain gradient formation without directed movement.

Q & A

What is the primary focus of Geraldine Seydoux's presentation?

The presentation focuses on how the cytoplasm of the zygote in Caenorhabditis elegans becomes polarized and how this process is controlled by PAR proteins and the distribution of MEX-5 protein.

How do the PAR proteins influence the one-cell embryo?

The PAR proteins control various aspects of the polarity of the one-cell embryo by directing the distribution of proteins and organelles in the cytoplasm along the anterior-posterior axis.

What role does MEX-5 play in the polarization process?

MEX-5 is an RNA-binding protein that forms an anterior-posterior gradient in the cytoplasm during the polarization process, which is crucial for determining the fate of the resulting cells.

What experimental approach did Erik Griffin use to study MEX-5 distribution?

Erik Griffin created a fusion protein of MEX-5 and a photoactivatable fluorescent protein, allowing him to label existing MEX-5 in red and newly synthesized MEX-5 in green to study the protein's redistribution.

What did Erik's experiments reveal about the necessity of new MEX-5 synthesis during polarization?

Erik's experiments indicated that new MEX-5 synthesis is not necessary for gradient formation; instead, existing MEX-5 protein can redistribute to create the gradient.

How did the diffusion rates of MEX-5 differ between the anterior and posterior cytoplasm?

Before polarization, MEX-5 diffused slowly in both anterior and posterior regions, but during polarization, it became faster in the posterior cytoplasm while remaining slow in the anterior.

What impact does PAR-1 have on MEX-5 diffusion?

PAR-1 phosphorylates MEX-5, which is essential for increasing the diffusion rate of MEX-5 in the posterior cytoplasm while keeping it sluggish in the anterior cytoplasm.

What conclusion did Erik draw from the fluorescence correlation spectroscopy experiments?

Erik concluded that MEX-5 exists in two species: a slow, sluggish form and a faster form, with the proportion of these species differing between the anterior and posterior cytoplasm.

What is the significance of reversible phosphorylation in the MEX-5 gradient formation?

Reversible phosphorylation allows for a dynamic change in MEX-5's diffusion rate, contributing to the establishment of the concentration gradient without requiring directed movement.

How did David Odde contribute to understanding the MEX-5 gradient?

David Odde utilized computational modeling to recreate the MEX-5 gradient using experimentally determined values, confirming the relationship between PAR-1 kinase activity and MEX-5 diffusion rates.