Length control of the metaphase spindle

Area of Science:

• Cell Biology
• Molecular Biology
• Biophysics

Background:

• Metaphase spindle length is generally constant within a cell type, including vertebrate oocytes.
• Microtubules within the spindle are highly dynamic.
• Factors determining metaphase spindle length are not fully understood.

Purpose of the Study:

• To investigate how proteins affecting microtubule dynamics, sliding forces, and sister-chromatid cohesion influence metaphase spindle length.
• To develop a quantitative model explaining spindle length regulation.

Main Methods:

• Utilized the Drosophila S2 cell line for experiments.
• Employed RNA interference (RNAi) to deplete or overexpress key proteins.
• Conducted high-throughput automated microscopy and image analysis of over 4000 spindles.
• Developed a quantitative model to interpret findings.

Main Results:

• Spindle size decreased with RNAi of microtubule-polymerizing factors or overexpression of Kinesin-8.
• Spindle length increased upon knockdown of Rad21, Kinesin-8, or Kinesin-13.
• Bipolar spindle length showed insensitivity to increased motor-generated sliding forces.
• Observed an ultrasensitive transition from monopolar to bipolar spindles at a critical Kinesin-5 concentration.

Conclusions:

• Metaphase spindle length is sensitive to changes in microtubule dynamics and sister-chromatid cohesion.
• Spindle length is robust against alterations in microtubule sliding forces.
• A model coupling microtubule depolymerization rates and sliding forces explains observed phenomena.