7.8
Spindle Assembly
Spindle assembly occurs through three, often coexisting, pathways – the centrosome-mediated pathway, the chromatin-mediated pathway, and the microtubu…
During eukaryotic cell division, microtubules assemble around the duplicated chromosomes in a bipolar array, forming the spindle apparatus. Spindle assembly is aided by multiple collaborating mechanisms.
Spindle assembly begins after two centrosomes are positioned at opposite ends of the cell. In animal cells, spindle poles are located at the centrosomes.
The centrosomes mature and nucleate bipolar microtubules, where the minus-end is anchored to the spindle pole, and the plus-end radiates outwards.
Simultaneously, multifunctional enzyme complexes, called M-Cdks, phosphorylate several nuclear envelope components, triggering the nuclear envelope to breakdown and expose the condensed chromosomes to the cytoplasm.
Microtubule nucleation at the centrosomes generates three types of microtubules. Interpolar microtubules come from opposite poles whose plus ends overlap, creating an antiparallel array at the spindle midzone. Kinetochore microtubules have their plus-ends connect with the kinetochores of exposed chromosomes. Astral microtubules have their plus-ends contact and interact with the cell cortex, positioning the spindle pole.
In the absence of centrosomes, mitotic chromosomes aid in acentrosomal spindle assembly. Mitotic chromosomes activate Ran-GTP, a nuclear protein. Activated Ran-GTP induces the release of microtubule-stabilizing proteins from protein complexes in the cytosol. The local activation of these factors promotes localized microtubule nucleation and stabilization.
Several microtubule-dependent motor proteins also contribute to spindle assembly and stabilization.
Dynein links astral microtubule plus-ends with cell-cortex components and pulls the spindle poles toward the cell-cortex. In the spindle midzone, kinesin-5 associates with interpolar microtubule plus-ends, to slide them past each other and generate a force that pushes the poles apart.
Kinesin-14 cross-links the interpolar microtubules at the spindle midzone and generates tension that pulls the poles together. Kinesin-4 and kinesin-10, the chromokinesins, connect microtubules with chromosomal arms to push chromosomes away from the poles.
The balance of these opposing forces generated by the motor proteins determines the final length and position of the assembled spindle.
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Q1: What are the three types of microtubules that form during spindle assembly?
Spindle assembly generates three distinct microtubule types. Kinetochore microtubules connect chromosome kinetochores to spindle poles. Astral microtubules extend toward the cell cortex to position the spindle poles. Interpolar microtubules originate from opposite poles, overlap at the spindle midzone in an antiparallel arrangement, and help separate the poles during cell division.
Q2: How do centrosomes contribute to spindle assembly in animal cells?
In animal cells, centrosomes serve as primary microtubule nucleation centers. During the G2-prophase transition, centrosomes mature and nucleate bipolar microtubules with minus-ends anchored at the spindle pole and plus-ends radiating outward. Two centrosomes position at opposite cell ends, establishing the bipolar spindle architecture essential for chromosome segregation.
Q3: What role does Ran-GTP play in acentrosomal spindle assembly?
Ran-GTP, a nuclear protein activated by mitotic chromosomes, drives acentrosomal spindle assembly. When activated, Ran-GTP binds importin-beta and releases spindle assembly factors from cytoplasmic protein complexes. These factors promote localized microtubule nucleation and stabilization near chromosomes, enabling spindle formation without centrosomes.
Q4: How do motor proteins balance forces during spindle assembly?
Motor proteins generate opposing forces that determine spindle length and position. Kinesin-5 slides interpolar microtubules apart, pushing poles away from each other. Kinesin-14 cross-links interpolar microtubules and pulls poles together. Dynein pulls astral microtubules toward the cell cortex. Chromokinesins push chromosomes away from poles. The balance of these forces stabilizes the final spindle structure.
Q5: What triggers nuclear envelope breakdown during spindle assembly?
M-Cdks, multifunctional enzyme complexes, phosphorylate nuclear envelope components during spindle assembly initiation. This phosphorylation triggers nuclear envelope breakdown, exposing condensed chromosomes to the cytoplasm. This exposure allows microtubules to access and attach to kinetochores, enabling chromosome capture and proper spindle organization.
Q6: How does the microtubule-mediated pathway contribute to spindle robustness?
The microtubule-mediated pathway uses existing microtubules to nucleate additional microtubules. The augmin protein complex associates with existing microtubules and recruits gamma-tubulin ring complex to initiate new microtubule formation. This pathway increases microtubule density within the spindle, enhancing its structural stability and ensuring robust chromosome segregation.
Q7: What happens to spindle assembly when centrosomes are absent?
When centrosomes are absent, mitotic chromosomes activate alternative spindle assembly pathways. Chromosomes activate Ran-GTP, which releases spindle assembly factors that promote localized microtubule nucleation around chromosomes. Additionally, the microtubule-mediated pathway recruits gamma-tubulin complexes to existing microtubules, collectively generating a functional spindle apparatus without centrosomal nucleation.