Cytoskeleton Structure and Cell Motility (2A)
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MCAT Biological and Biochemical Foundations of Living Systems › Cytoskeleton Structure and Cell Motility (2A)
Migrating neurons were imaged as they advanced a leading process on a laminin substrate. Investigators quantified delivery of adhesion receptors to the front of the cell. Under control conditions, receptor-containing vesicles moved along linear tracks toward the leading edge and accumulated in the front membrane. After treatment with a microtubule motor inhibitor, vesicle movement became largely diffusive, front membrane receptor density decreased, and cells exhibited repeated protrusion attempts without sustained forward translocation.
Which conclusion about cytoskeletal dynamics is most consistent with these findings?
Actin filaments are static during migration; therefore, disrupting microtubule motors should not affect protrusion persistence.
Inhibiting microtubule motors should enhance receptor delivery by preventing vesicles from being pulled away from the front.
Intermediate filaments are the primary tracks for long-range vesicle delivery to the leading edge during migration.
Microtubule-based transport supports migration by delivering components needed for stable adhesions and coordinated forward movement.
Explanation
This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, migrating neurons rely on microtubule-based transport for delivering adhesion receptors to the leading edge. Choice D is correct because it highlights how microtubules support migration by enabling vesicle delivery for stable adhesions and forward movement. Choice B is incorrect because intermediate filaments do not serve as primary tracks for vesicle transport. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Assess how motor inhibition disrupts polarized delivery and impairs motility.
To test the mechanical role of intermediate filaments during migration, researchers compared wild-type cells to cells lacking a major intermediate filament protein. Both cell types were placed in a 3D matrix with narrow pores. Wild-type cells elongated and maintained integrity while squeezing through pores. Intermediate-filament–deficient cells initiated entry into pores but frequently developed localized membrane blebs and transient ruptures, followed by abrupt retraction; their average displacement over time decreased despite normal-looking actin-rich protrusions.
Which statement best describes the role of intermediate filaments in cell motility in this context?
Intermediate filaments replace microtubules as the main system for polarized vesicle trafficking during migration.
Intermediate filaments inhibit migration by increasing cytoplasmic viscosity; their loss should increase displacement.
Intermediate filaments primarily drive protrusion by polymerizing at the leading edge faster than actin filaments.
Intermediate filaments provide mechanical resilience that helps cells withstand deformation during confined migration.
Explanation
This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, cells in a 3D matrix with narrow pores require intermediate filaments for mechanical integrity during confined migration. Choice B is correct because it explains how intermediate filaments provide resilience to withstand deformation and prevent ruptures. Choice A is incorrect because intermediate filaments do not drive protrusion via rapid polymerization. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Note how filament deficiencies reveal roles in mechanical stress resistance.
A team examined how microtubules contribute to directional migration in epithelial cells moving into a scratch wound. Live imaging showed microtubule plus-ends repeatedly growing toward the leading edge, while vesicles carrying membrane components accumulated near the front. When cells were treated with a microtubule depolymerizing drug, overall speed decreased and cells frequently lost a stable front–rear axis; actin-rich protrusions still formed but were short-lived and appeared at multiple edges. A separate condition used a drug that stabilizes microtubules; these cells maintained a single front but turned slowly and showed delayed repositioning of internal organelles during direction changes.
Based on the vignette, which statement best describes the role of microtubules in cell motility?
Microtubules are dispensable for polarity because actin polymerization alone determines where the leading edge forms over time.
Microtubule depolymerization should increase migration speed by removing internal constraints and freeing actin networks to expand.
Microtubules primarily generate the protrusive force for lamellipodia extension by directly pushing the plasma membrane forward.
Microtubules help maintain polarity and support directed trafficking during migration, enabling persistent front–rear organization.
Explanation
This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, epithelial cells in a scratch wound depend on microtubules for polarity and trafficking during directional migration. Choice B is correct because it describes microtubules' role in maintaining polarity and supporting directed trafficking for persistent organization. Choice A is incorrect because microtubules do not directly generate protrusive force for lamellipodia. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Examine how microtubule perturbations affect overall directionality versus local protrusions.
In a scratch-wound assay, cells normally reorient their centrosome and microtubule network toward the wound edge before migrating. A drug that prevents microtubule polymerization was applied after the scratch. Cells still formed actin-rich protrusions but showed poor directional persistence and frequently changed direction. Based on the vignette, which statement best describes microtubule function in directional migration?
Microtubules contribute to front–rear organization and directional persistence, likely by coordinating polarized trafficking during migration.
Microtubules primarily generate contractile force at the rear; inhibiting polymerization should increase persistence by reducing rear contraction.
Microtubules directly polymerize to push the plasma membrane forward; actin-rich protrusions are incidental and not linked to movement.
Microtubules are static structural rods; preventing their polymerization should not affect directional persistence once protrusions form.
Explanation
This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, preventing microtubule polymerization after scratching leads to poor directional persistence despite protrusions. Choice A is correct because it accurately describes microtubules' role in front-rear organization and persistence. Choice B is incorrect because it wrongly attributes direct membrane pushing to microtubules. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Examine polarity markers like centrosome orientation in wound assays.
In migrating neurons, mitochondria accumulate near the leading process where ATP demand is high. A lab disrupted microtubules and observed that mitochondria became dispersed and leading-edge advance slowed, even though actin polymerization events were still detectable at the cortex. Which outcome would be expected if microtubule-based transport is inhibited during migration?
Reduced delivery of organelles and cargo to the front would impair sustained migration, even if actin polymerization can still initiate protrusions.
Migration speed would increase because disrupting microtubules frees actin monomers to polymerize more efficiently at the leading edge.
Migration would be unaffected because mitochondria move primarily by diffusion in the cytosol during motility.
Cells would lose all protrusive activity because microtubules, not actin, are required for lamellipodial formation at the membrane.
Explanation
This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, disrupting microtubules disperses mitochondria and slows leading-edge advance despite actin events. Choice C is correct because it accurately describes how impaired transport reduces sustained migration. Choice B is incorrect because it wrongly suggests speed increases by freeing actin monomers. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Monitor organelle positioning to assess transport's impact on motility.
In a study of collective migration, epithelial sheets were subjected to cyclic stretching while moving across a substrate. Cells at the leading edge maintained forward movement under moderate stretch, but when an intermediate filament–disrupting compound was added, the sheet began to fragment at cell–cell junctions during stretch cycles. Individual cells still formed actin-based protrusions, yet coordination across the sheet deteriorated and net advancement slowed.
What outcome would be expected if intermediate filament function is inhibited during mechanically stressed collective migration?
Improved coordination because actin networks become more rigid and can transmit forces between cells more effectively.
No change in sheet cohesion because microtubules alone determine the strength of cell–cell junctions under stretch.
Reduced tissue-level integrity and slower net advance because cells are less able to tolerate and distribute mechanical strain.
Faster net advance because intermediate filaments normally block protrusion formation at the leading edge.
Explanation
This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, epithelial sheets under cyclic stretching depend on intermediate filaments for maintaining integrity during collective migration. Choice B is correct because it predicts reduced integrity and slower advance due to impaired strain distribution without intermediate filaments. Choice A is incorrect because rigid actin would not improve coordination under stress. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Evaluate how mechanical perturbations highlight roles in tissue-level cohesion.
In a study of amoeboid motility, researchers tracked single-cell migration on a soft collagen-coated surface while imaging F-actin at the leading edge. Cells were briefly exposed to a low dose of an actin polymerization inhibitor that preferentially reduces new filament growth at barbed ends. During exposure, cells showed fewer membrane protrusions and a marked decrease in forward displacement, but remained viable and continued slow shape fluctuations. After washout, protrusions and net migration recovered within minutes. Based on these observations, which conclusion about actin dynamics is most consistent with the role of actin in amoeboid movement?
Simplified schematic:
Front (leading edge): G-actin → F-actin (polymerization) → protrusion
Rear: F-actin disassembly → monomers recycled
Inhibiting actin polymerization should increase migration speed by reducing cytoplasmic viscosity and allowing faster forward flow.
Microtubule polymerization at the leading edge is the primary driver of protrusion, so inhibiting actin should minimally affect net migration.
Intermediate filaments generate the protrusive force at the front, while actin is mainly static scaffolding that does not require turnover.
Actin polymerization at the leading edge provides a pushing force for protrusion, and reversible turnover enables rapid recovery after inhibitor washout.
Explanation
This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, a low-dose actin polymerization inhibitor reduces new filament growth, leading to fewer protrusions and decreased migration, with recovery after washout. Choice D is correct because it accurately describes actin polymerization providing protrusive force and reversible turnover enabling rapid recovery. Choice B is incorrect because it incorrectly attributes protrusion primarily to microtubules rather than actin. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Always verify how perturbations like inhibitors affect specific filament dynamics in motility assays.
Migrating epithelial cells were imaged while a fluorescently labeled vesicle marker tracked delivery of membrane components to the leading edge. When microtubules were selectively destabilized, vesicles accumulated near the cell center and leading-edge expansion slowed, even though cortical actin still exhibited transient polymerization. Which conclusion about microtubules in cell migration is most consistent with the findings?
Intermediate filaments are responsible for vesicle transport during migration; microtubule destabilization should not affect vesicle localization.
Microtubules inhibit migration by sequestering vesicles; destabilizing them should increase vesicle delivery and speed up expansion.
Microtubules are the primary source of protrusive force at the membrane, so vesicle accumulation indicates reduced microtubule pushing against the cortex.
Microtubules support migration by facilitating intracellular transport to the leading edge, enabling sustained protrusion and membrane remodeling.
Explanation
This question assesses understanding of the cytoskeleton's role in cell motility. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, destabilizing microtubules causes vesicle accumulation centrally and slows leading-edge expansion despite actin polymerization. Choice D is correct because it accurately describes microtubules' role in transport for sustained protrusion. Choice B is incorrect because it wrongly claims microtubules provide primary protrusive force. When evaluating cytoskeletal function, consider both structural support and dynamic changes essential for cellular processes. Track vesicle delivery to understand microtubule contributions to migration.
A research group compared two cell lines migrating on the same extracellular matrix. Cell line 1 displayed rapid, rounded amoeboid movement with transient protrusions; cell line 2 displayed slower, elongated movement with more persistent front–rear polarity. When both lines were treated with a mild actin polymerization inhibitor, line 1 showed a large drop in speed, whereas line 2 showed a modest drop but a pronounced loss of protrusion stability. The investigators proposed that differences in reliance on actin-driven protrusion dynamics contributed to the distinct motility modes. What outcome would be expected if actin polymerization is inhibited, consistent with the vignette?
Elongated movement is enhanced because reduced actin polymerization shifts force generation to microtubules, increasing polarity and speed.
Amoeboid-like movement is disproportionately impaired because rapid protrusion formation depends strongly on actin polymerization dynamics.
Amoeboid-like movement increases because inhibiting actin polymerization reduces membrane tension, allowing faster protrusions.
Both motility modes are unaffected because actin polymerization is redundant with intermediate filament assembly at the leading edge.
Explanation
This question assesses understanding of how different cell migration modes depend on actin polymerization dynamics. Amoeboid movement relies heavily on rapid, transient actin-based protrusions, while elongated mesenchymal movement involves more stable structures. In the vignette, the amoeboid cell line showed a larger speed reduction than the elongated line when actin polymerization was inhibited. Choice D is correct because it accurately describes how amoeboid-like movement is disproportionately impaired due to its strong dependence on rapid actin polymerization dynamics. Choice B is incorrect because inhibiting actin polymerization does not enhance movement or shift force generation to microtubules. When comparing migration modes, recognize that amoeboid movement's reliance on dynamic actin cycling makes it particularly sensitive to polymerization inhibitors.
In a study of amoeboid motility, researchers tracked single cells moving through a 3D collagen matrix. Live imaging showed repeated cycles of leading-edge protrusion followed by rear retraction. When cells were treated with a low dose of an actin polymerization inhibitor, protrusions became shorter-lived and net displacement over 10 minutes decreased, despite continued membrane ruffling. In a separate condition, mild stabilization of existing actin filaments reduced the frequency of protrusion–retraction cycles and also reduced net displacement. The authors concluded that efficient movement required both assembly and turnover of actin at the front. Based on these findings, which conclusion about cytoskeletal dynamics is most consistent with the observed changes in cell motility?
Actin filaments function as static struts; therefore, stabilizing them should preserve protrusions and increase migration speed in a collagen matrix.
Microtubule polymerization at the leading edge is the primary driver of protrusive force, so inhibiting actin polymerization should have minimal effect on net displacement.
Intermediate filaments generate the pushing force for membrane protrusion, so stabilizing actin should increase net displacement by reducing cytoskeletal noise.
Dynamic actin remodeling, including polymerization and depolymerization, is required to sustain productive protrusions that translate into forward movement.
Explanation
This question assesses understanding of the cytoskeleton's role in cell motility, specifically actin dynamics in amoeboid movement. The cytoskeleton, composed of actin filaments, microtubules, and intermediate filaments, facilitates cell movement through structural and dynamic functions. In the vignette, both actin polymerization inhibition and stabilization reduced net displacement, indicating that dynamic actin remodeling is essential for productive cell movement. Choice B is correct because it accurately describes how both polymerization and depolymerization of actin are required to sustain productive protrusions that translate into forward movement. Choice A is incorrect because microtubules do not drive protrusive force at the leading edge - this is primarily an actin-based process. When evaluating cytoskeletal function in motility, consider that efficient movement requires not just assembly but also turnover of cytoskeletal components to enable continuous remodeling and adaptation.