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Microtubule-based filopodium-like protrusions form after axotomy
1Department of Pharmacology, Columbia University, New York, New York 10032.
Summary
Newly discovered filopodium-like protrusions (FLPs) in transected Aplysia axons are primarily microtubule-based, not actin-based. These structures drive rapid neurite precursor extension, challenging previous assumptions about growth cone dynamics.
Area of Science:
- Neuroscience
- Cell Biology
- Axon Regeneration
Background:
- Neurite growth cones utilize F-actin-rich filopodia and lamellipodia for protrusion.
- Microtubules and other components subsequently fill these protrusions to extend neurites.
- Axon transection can initiate neurite regrowth.
Purpose of the Study:
- To investigate the early cellular events following axon transection in Aplysia.
- To determine the molecular basis of rapid filopodium-like protrusion (FLP) formation.
- To elucidate the roles of actin and microtubules in FLP formation and transport.
Main Methods:
- Video-enhanced contrast-differential interference contrast microscopy of transected Aplysia axons in culture.
- Pharmacological disruption of actin polymerization (dihydrocytochalasin B) and microtubule dynamics (nocodazole).
- Post-experiment plasma membrane removal and immunofluorescence staining for microtubules.
Main Results:
- Rapid formation of FLPs from axon stumps within minutes of transection.
- Some FLPs exhibited bidirectional transport of swellings, similar to fast axonal transport.
- Dihydrocytochalasin B reduced FLP formation but increased transport; nocodazole reduced FLP formation and abolished transport.
- Microtubules were identified within FLPs in untreated axons.
Conclusions:
- A significant proportion of early FLPs following axotomy are microtubule-dependent.
- Microtubules, not actin, are the primary drivers of rapid neuritic precursor extension in this context.
- Findings challenge the traditional view of actin's dominant role in initial growth cone protrusion.