RNA transcription modulates phase transition-driven nuclear body assembly

Area of Science:

• Cell Biology
• Biophysics
• Molecular Biology

Background:

• Nuclear bodies are membraneless organelles crucial for gene regulation.
• The nucleolus, the largest nuclear body, is vital for ribosome biogenesis, cell growth, and size homeostasis.
• Nuclear bodies exhibit liquid-like properties, suggesting formation via phase separation, but the role of active biological processes remains unclear.

Purpose of the Study:

• To investigate the assembly and disassembly dynamics of nucleoli and extranucleolar droplets (ENDs) in early Caenorhabditis elegans embryos.
• To determine how nonequilibrium biological activity, such as rRNA transcription, impacts phase separation in nuclear bodies.
• To characterize the physical mechanisms governing the formation and growth of nucleoli and ENDs.

Main Methods:

• Combined in vivo and in vitro experiments in C. elegans embryos.
• Theoretical modeling and computer simulations.
• Manipulation of C. elegans cell size to alter nucleolar component concentration.

Main Results:

• Observed transcription-independent extranucleolar droplets (ENDs) in addition to classical nucleoli.
• Demonstrated that nucleoli and ENDs grow via a first-order phase transition, with coarsening dynamics driven by Brownian coalescence and Ostwald ripening.
• Confirmed model predictions by altering cell size and nucleolar component concentration.

Conclusions:

• Nucleolar and END assembly are governed by passive phase separation mechanisms.
• While active biological processes like rRNA transcription can modulate thermodynamic parameters, they do not fundamentally alter the phase separation mechanism.
• These findings provide insights into the physical principles underlying the formation of nuclear bodies.