Microtubule End-Clustering Maintains a Steady-State Spindle Shape.

Area of Science:

• Cell Biology
• Cytoskeletal Dynamics
• Biophysics

Background:

• The metaphase spindle, a key structure in cell division, maintains a stable geometry despite dynamic microtubule turnover and internal forces.
• While factors like motor proteins and microtubule crosslinking influence spindle size and shape, the mechanisms ensuring steady-state geometry remain incompletely understood.

Purpose of the Study:

• To identify the molecular mechanisms responsible for maintaining the steady-state geometry of mammalian metaphase spindles.
• To investigate the role of microtubule end-binding proteins and motor proteins in spindle stability and dynamics.

Main Methods:

• Utilized deletion studies of dynein and NuMA in mammalian cells to observe effects on spindle structure.
• Employed live-cell imaging to analyze microtubule network dynamics and assess spindle geometry.
• Investigated the role of kinesin-5 Eg5 in driving microtubule network turbulence through inhibition studies.
• Analyzed microtubule nematic order and aster dynamics within the turbulent spindle network.

Main Results:

• Deletion of dynein or NuMA leads to a "turbulent" microtubule network characterized by unstable spindle shape.
• The homotetrameric kinesin-5 Eg5 was identified as the driver of this spindle turbulence.
• Acute inhibition of Eg5 in turbulent spindles restored normal spindle geometry and stability.
• Turbulent spindles exhibited decreased nematic order and facilitated cytoplasmic organelle flow and cell motility.

Conclusions:

• End-clustering mediated by dynein and NuMA is essential for mammalian spindles to achieve and maintain a steady-state geometry.
• In the absence of end-clustering, kinesin-5 Eg5 activity generates a turbulent microtubule network within mitotic cells.
• Spindle turbulence can lead to altered cellular mechanics, including organelle transport and cell movement.