Size- and position-dependent cytoplasm viscoelasticity through hydrodynamic interactions with the cell surface.

Area of Science:

• Cell biology
• Biophysics
• Rheology

Background:

• Cytoplasm rheology studies often focus on submicrometer components.
• Large organelles (nuclei, spindles) significantly impact cell mechanics and function.
• Understanding the movement of large organelles is crucial for cell division and polarization.

Purpose of the Study:

• Investigate the rheological properties of cytoplasm for large, micron-sized objects.
• Determine how object size influences cytoplasmic viscoelasticity.
• Explore the role of hydrodynamic interactions in organelle movement.

Main Methods:

• Translated large passive components (up to 50% cell diameter) in live sea urchin eggs using calibrated magnetic forces.
• Analyzed creep and relaxation responses of the cytoplasm.
• Performed flow analysis and simulations to model hydrodynamic interactions.

Main Results:

• Cytoplasm behaves as a Jeffreys material for objects >1 micron, showing viscoelasticity at short timescales and fluidization at longer times.
• Cytoplasmic viscoelastic resistance non-monotonically increased as object size approached cell size.
• Hydrodynamic interactions between the moving object and the static cell surface caused size-dependent and position-dependent viscoelasticity.

Conclusions:

• Cytoplasm hydrodynamically couples large organelles to the cell surface, restraining their motion.
• This coupling influences cell shape sensing and cellular organization.
• Findings reveal a novel mechanism for controlling large organelle dynamics within the cell.