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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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Cell division did not require the presence of spindle-associated mitochondria.A time-lapse sequence of DIC images. During anaphase (0 min), both groups of spindle-associated mitochondria (m) that flanked the central spindle (sp) were mechanically repositioned distal to opposing sets of segregating chromosomes (c; 12 min). Although the equatorial region was thus cleared of mitochondria, cleavage at the spindle equator was not inhibited (12 min onward). Arrows show the locations of the cleavage furrows: at the equator, and in two ectopic locations induced by the relocated, mitochondria-associated bundles of microtubules. The furrows produced two daughter cells (d), each associated with an anucleate membrane pocket (p). Time in minutes. Bar, 10 µm.
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pone-0004892-g009: Cell division did not require the presence of spindle-associated mitochondria.A time-lapse sequence of DIC images. During anaphase (0 min), both groups of spindle-associated mitochondria (m) that flanked the central spindle (sp) were mechanically repositioned distal to opposing sets of segregating chromosomes (c; 12 min). Although the equatorial region was thus cleared of mitochondria, cleavage at the spindle equator was not inhibited (12 min onward). Arrows show the locations of the cleavage furrows: at the equator, and in two ectopic locations induced by the relocated, mitochondria-associated bundles of microtubules. The furrows produced two daughter cells (d), each associated with an anucleate membrane pocket (p). Time in minutes. Bar, 10 µm.

Mentions: Recent research on mitochondrial function and morphology has shown that mitochondria are more than just the cell's ‘powerhouse’ (reviewed in [33]). For example, the mitochondrial membrane provides a surface on which Arp2/3 can nucleate formation of actin cables in yeast. Thus, mitochondria can promote their own anterograde transport along actin cables, apparently by recruiting their own tracks. Furthermore, the mitochondria (“m” in Fig. 9) in grasshopper spermatocytes are highly elongated and flank the central spindle. This spermatocyte-specific mitochondrial distribution, combined with the potential for interaction between actin and mitochondria [34] raised concerns that our observations and interpretations of actin redistribution could be confounded. Specifically, in late anaphase, these spindle-associated mitochondria splay towards the equatorial cortex, thus potentially co-transporting any associated actin filaments to the incipient contractile ring. Therefore, we hoped to determine whether the spindle-associated mitochondria played a specialized role in cytokinesis, in addition to their more general role in energy production. For example, spindle-associated mitochondria could conceivably play a role in actin assembly and/or delivery to the contractile ring. Alternatively, as mitochondria are known to function in multiple signal transduction cascades, perhaps spindle-associated mitochondria helped to signal the location or timing of furrowing.


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)

Cell division did not require the presence of spindle-associated mitochondria.A time-lapse sequence of DIC images. During anaphase (0 min), both groups of spindle-associated mitochondria (m) that flanked the central spindle (sp) were mechanically repositioned distal to opposing sets of segregating chromosomes (c; 12 min). Although the equatorial region was thus cleared of mitochondria, cleavage at the spindle equator was not inhibited (12 min onward). Arrows show the locations of the cleavage furrows: at the equator, and in two ectopic locations induced by the relocated, mitochondria-associated bundles of microtubules. The furrows produced two daughter cells (d), each associated with an anucleate membrane pocket (p). Time in minutes. Bar, 10 µm.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2654139&req=5

pone-0004892-g009: Cell division did not require the presence of spindle-associated mitochondria.A time-lapse sequence of DIC images. During anaphase (0 min), both groups of spindle-associated mitochondria (m) that flanked the central spindle (sp) were mechanically repositioned distal to opposing sets of segregating chromosomes (c; 12 min). Although the equatorial region was thus cleared of mitochondria, cleavage at the spindle equator was not inhibited (12 min onward). Arrows show the locations of the cleavage furrows: at the equator, and in two ectopic locations induced by the relocated, mitochondria-associated bundles of microtubules. The furrows produced two daughter cells (d), each associated with an anucleate membrane pocket (p). Time in minutes. Bar, 10 µm.
Mentions: Recent research on mitochondrial function and morphology has shown that mitochondria are more than just the cell's ‘powerhouse’ (reviewed in [33]). For example, the mitochondrial membrane provides a surface on which Arp2/3 can nucleate formation of actin cables in yeast. Thus, mitochondria can promote their own anterograde transport along actin cables, apparently by recruiting their own tracks. Furthermore, the mitochondria (“m” in Fig. 9) in grasshopper spermatocytes are highly elongated and flank the central spindle. This spermatocyte-specific mitochondrial distribution, combined with the potential for interaction between actin and mitochondria [34] raised concerns that our observations and interpretations of actin redistribution could be confounded. Specifically, in late anaphase, these spindle-associated mitochondria splay towards the equatorial cortex, thus potentially co-transporting any associated actin filaments to the incipient contractile ring. Therefore, we hoped to determine whether the spindle-associated mitochondria played a specialized role in cytokinesis, in addition to their more general role in energy production. For example, spindle-associated mitochondria could conceivably play a role in actin assembly and/or delivery to the contractile ring. Alternatively, as mitochondria are known to function in multiple signal transduction cascades, perhaps spindle-associated mitochondria helped to signal the location or timing of furrowing.

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