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Redistribution of actin during assembly and reassembly of the contractile ring in grasshopper spermatocytes.

Alsop GB, Chen W, Foss M, Tseng KF, Zhang D - PLoS ONE (2009)

Bottom Line: The ring is highly dynamic; it assembles and disassembles during each cell cleavage, resulting in the recurrent redistribution of actin.Notably, actin filaments undergo a dramatic trajectory change as they enter the ring, implying the existence of a pulling force.Two other mechanisms of actin redistribution, cortical flow and de novo assembly, are also present in grasshopper, suggesting that actin converges at the nascent contractile ring from diffuse sources within the cytoplasm and cortex, mediated by spindle microtubules.

View Article: PubMed Central - PubMed

Affiliation: Department of Zoology, Oregon State University, Corvallis, OR, USA.

ABSTRACT
Cytokinesis in animal cells requires the assembly of an actomyosin contractile ring to cleave the cell. The ring is highly dynamic; it assembles and disassembles during each cell cleavage, resulting in the recurrent redistribution of actin. To investigate this process in grasshopper spermatocytes, we mechanically manipulated the spindle to induce actin redistribution into ectopic contractile rings, around reassembled lateral spindles. To enhance visualization of actin, we folded the spindle at its equator to convert the remnants of the partially assembled ring into a concentrated source of actin. Filaments from the disintegrating ring aligned along reorganizing spindle microtubules, suggesting that their incorporation into the new ring was mediated by microtubules. We tracked incorporation by speckling actin filaments with Qdots and/or labeling them with Alexa 488-phalloidin. The pattern of movement implied that actin was transported along spindle microtubules, before entering the ring. By double-labeling dividing cells, we imaged actin filaments moving along microtubules near the contractile ring. Together, our findings indicate that in one mechanism of actin redistribution, actin filaments are transported along spindle microtubule tracks in a plus-end-directed fashion. After reaching the spindle midzone, the filaments could be transported laterally to the ring. Notably, actin filaments undergo a dramatic trajectory change as they enter the ring, implying the existence of a pulling force. Two other mechanisms of actin redistribution, cortical flow and de novo assembly, are also present in grasshopper, suggesting that actin converges at the nascent contractile ring from diffuse sources within the cytoplasm and cortex, mediated by spindle microtubules.

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Related in: MedlinePlus

Tracking individual actin filaments labeled by Alexa 488-Phalloidin and speckled with Qdot 655-Phalloidin.As shown in the kymographs (Videos S8, S9, S10), actin filaments could be seen moving into the cleavage furrow (A, B) and moving away from the furrow (C). Arrows depict Qdot-marked reference points on the actin filaments, and a white vertical line marks the location of each cleavage furrow. Time in seconds. Bar, 10 µm.
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pone-0004892-g006: Tracking individual actin filaments labeled by Alexa 488-Phalloidin and speckled with Qdot 655-Phalloidin.As shown in the kymographs (Videos S8, S9, S10), actin filaments could be seen moving into the cleavage furrow (A, B) and moving away from the furrow (C). Arrows depict Qdot-marked reference points on the actin filaments, and a white vertical line marks the location of each cleavage furrow. Time in seconds. Bar, 10 µm.

Mentions: The contractile ring assembles during cytokinesis and remains highly dynamic as it constricts the cell (e.g., [12], [20], [23]). Although preexisting and newly polymerized actin filaments or bundles, as well as actin subunits, may all contribute to the ring dynamics, it is unclear whether preformed actin filaments are static or dynamic as they enter into the nascent contractile ring. Furthermore, it is unclear by what mechanism the filaments enter the ring. Are they released from the microtubules in the vicinity of the ring, and do they then passively drift toward the ring? Or are they actively pulled in by a component of the contractile ring? Finally, we wanted to determine whether the behavior of actin in live cells was consistent with our hypothesis that actin could travel along spindle microtubules toward the contractile ring. To address these questions, we labeled the actin filaments in dividing cells in a variety of ways (Figs. 3, 4, 5, 6).


Redistribution of actin during assembly and reassembly of the contractile ring in grasshopper spermatocytes.

Alsop GB, Chen W, Foss M, Tseng KF, Zhang D - PLoS ONE (2009)

Tracking individual actin filaments labeled by Alexa 488-Phalloidin and speckled with Qdot 655-Phalloidin.As shown in the kymographs (Videos S8, S9, S10), actin filaments could be seen moving into the cleavage furrow (A, B) and moving away from the furrow (C). Arrows depict Qdot-marked reference points on the actin filaments, and a white vertical line marks the location of each cleavage furrow. Time in seconds. Bar, 10 µm.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2654139&req=5

pone-0004892-g006: Tracking individual actin filaments labeled by Alexa 488-Phalloidin and speckled with Qdot 655-Phalloidin.As shown in the kymographs (Videos S8, S9, S10), actin filaments could be seen moving into the cleavage furrow (A, B) and moving away from the furrow (C). Arrows depict Qdot-marked reference points on the actin filaments, and a white vertical line marks the location of each cleavage furrow. Time in seconds. Bar, 10 µm.
Mentions: The contractile ring assembles during cytokinesis and remains highly dynamic as it constricts the cell (e.g., [12], [20], [23]). Although preexisting and newly polymerized actin filaments or bundles, as well as actin subunits, may all contribute to the ring dynamics, it is unclear whether preformed actin filaments are static or dynamic as they enter into the nascent contractile ring. Furthermore, it is unclear by what mechanism the filaments enter the ring. Are they released from the microtubules in the vicinity of the ring, and do they then passively drift toward the ring? Or are they actively pulled in by a component of the contractile ring? Finally, we wanted to determine whether the behavior of actin in live cells was consistent with our hypothesis that actin could travel along spindle microtubules toward the contractile ring. To address these questions, we labeled the actin filaments in dividing cells in a variety of ways (Figs. 3, 4, 5, 6).

Bottom Line: The ring is highly dynamic; it assembles and disassembles during each cell cleavage, resulting in the recurrent redistribution of actin.Notably, actin filaments undergo a dramatic trajectory change as they enter the ring, implying the existence of a pulling force.Two other mechanisms of actin redistribution, cortical flow and de novo assembly, are also present in grasshopper, suggesting that actin converges at the nascent contractile ring from diffuse sources within the cytoplasm and cortex, mediated by spindle microtubules.

View Article: PubMed Central - PubMed

Affiliation: Department of Zoology, Oregon State University, Corvallis, OR, USA.

ABSTRACT
Cytokinesis in animal cells requires the assembly of an actomyosin contractile ring to cleave the cell. The ring is highly dynamic; it assembles and disassembles during each cell cleavage, resulting in the recurrent redistribution of actin. To investigate this process in grasshopper spermatocytes, we mechanically manipulated the spindle to induce actin redistribution into ectopic contractile rings, around reassembled lateral spindles. To enhance visualization of actin, we folded the spindle at its equator to convert the remnants of the partially assembled ring into a concentrated source of actin. Filaments from the disintegrating ring aligned along reorganizing spindle microtubules, suggesting that their incorporation into the new ring was mediated by microtubules. We tracked incorporation by speckling actin filaments with Qdots and/or labeling them with Alexa 488-phalloidin. The pattern of movement implied that actin was transported along spindle microtubules, before entering the ring. By double-labeling dividing cells, we imaged actin filaments moving along microtubules near the contractile ring. Together, our findings indicate that in one mechanism of actin redistribution, actin filaments are transported along spindle microtubule tracks in a plus-end-directed fashion. After reaching the spindle midzone, the filaments could be transported laterally to the ring. Notably, actin filaments undergo a dramatic trajectory change as they enter the ring, implying the existence of a pulling force. Two other mechanisms of actin redistribution, cortical flow and de novo assembly, are also present in grasshopper, suggesting that actin converges at the nascent contractile ring from diffuse sources within the cytoplasm and cortex, mediated by spindle microtubules.

Show MeSH
Related in: MedlinePlus