A microtubule-based, dynein-dependent force induces local cell protrusions: Implications for neurite initiation

Intrabundle microtubule dynamics in the Arabidopsis cortical array

We tested the general hypothesis that bundling stabilizes the dynamic properties of the constituent microtubules (MTs) in vivo. We quantified the assembly dynamics of bundled and unbundled MTs in the interphase cortical array of Arabidopsis hypocotyl cells using high dynamic range spinning disk confocal microscopy. We find no evidence that bundled MTs are stabilized against depolymerization through changes to their dynamic properties. Our observations of MT plus and minus ends indicate that both bundled and unbundled polymers undergo persistent treadmilling in this system. We conclude that the temporal persistence of MT subassemblies in the Arabidopsis cortical array is largely dependent upon recruitment or nucleation of new treadmilling MTs and not on polymer stabilization. Monte Carlo simulations suggest that small differences discovered in the dynamic properties between bundled and unbundled polymers would produce relatively small macroscopic effects on the larger MT array.

Paclitaxel induces axonal microtubules polar reconfiguration and impaired organelle transport: implications for the pathogenesis of paclitaxel-induced polyneuropathy

In differentiated axons almost all microtubules (MTs) uniformly point their plus ends towards the axonal tip. The uniform polar pattern provides the structural substrate for efficient organelle transport along axons. It is generally believed that the mass and pattern of MTs polar orientation remain unchanged in differentiated neurons. Here we examined long-term effects of the MTs stabilizing reagent paclitaxel (taxol) over MTs polar orientation and organelle transport in cultured Aplysia neurons. Unexpectedly, we found that rather than stabilizing the MTs, paclitaxel leads to their massive polar reconfiguration, accompanied by impaired organelle transport. Washout of paclitaxel does not lead to recovery of the polar orientation indicating that the new pattern is self-maintained. Taken together the data suggest that MTs in differentiated neurons maintain the potential to be reconfigured. Such reconfiguration may serve physiological functions or lead to degeneration. In addition, our observations offer a novel mechanism that could account for the development of peripheral neuropathy in patients receiving paclitaxel as an antitumor drug.

Dendrites differ from axons in patterns of microtubule stability and polymerization during development

Background

Dendrites differ from axons in patterns of growth and development, as well as in morphology. Given that microtubules are key structural elements in cells, we assessed patterns of microtubule stability and polymerization during hippocampal neuron development in vitro to determine if these aspects of microtubule organization could distinguish axons from dendrites.

Results

Quantitative ratiometric immunocytochemistry identified significant differences in microtubule stability between axons and dendrites. Most notably, regardless of developmental stage, there were high levels of dynamic microtubules throughout the dendritic arbor, whereas dynamic microtubules were predominantly concentrated in the distal end of axons. Analysis of microtubule polymerization using green fluorescent protein-tagged EB1 showed both developmental and regional differences in microtubule polymerization between axons and dendrites. Early in development (for example, 1 to 2 days in vitro), polymerization events were distributed equally in both the anterograde and retrograde directions throughout the length of both axons and dendrites. As development progressed, however, polymerization became biased, with a greater number of polymerization events in distal than in proximal and middle regions. While polymerization occurred almost exclusively in the anterograde direction for axons, both anterograde and retrograde polymerization was observed in dendrites. This is in agreement with predicted differences in microtubule polarity within these compartments, although fewer retrograde events were observed in dendrites than expected.

Conclusion

Both immunocytochemical and live imaging analyses showed that newly formed microtubules predominated at the distal end of axons and dendrites, suggesting a common mechanism that incorporates increased microtubule polymerization at growing process tips. Dendrites had more immature, dynamic microtubules throughout the entire arbor than did axons, however. Identifying these differences in microtubule stability and polymerization is a necessary first step toward understanding how they are developmentally regulated, and may reveal novel mechanisms underlying neuron maturation and dendritic plasticity that extend beyond the initial specification of polarity.

Motor-induced sliding of microtubule and actin bundles

Interactions of multiple molecular motors with bundles of actin and microtubule filaments form the basis for many cytoskeletal processes including axonal growth, muscle contraction, cell division and platelet formation. Continuum models based on generalized diffusion equations have been suggested to quantify the dynamics of such active bundles. In highly cross-linked and densely packed filament bundles, however, a major complication arises due to the multiple interactions that each filament forms with its neighbors. To explore the effects of these interactions, we used detailed computer simulations and studied the bundles with different types of motors at different densities and boundary conditions. We found that highly cross-linked bundles exhibit effects of long-ranged interactions that are sensitive to the boundary conditions. In open bundles, these give rise to 'telescopic' patterns resulting in significant acceleration of the filaments at the edges. In contrast, in ringed bundles, the long-ranged interactions 'lock' filaments and slow down their movements. The filaments in loosely connected bundles, on the other hand, undergo local diffusion-drift dynamics consistent with previous continuum models. Our simulations also demonstrate the sorting phenomena in the mixed-polarity bundles and reveal characteristic scales and conditions for spontaneous pattern formation in the bundle. We discuss the relevance of our results for cytoskeleton systems such as microtubules in axons, platelet formation, kinetochore fibers and actin bundles in motile cells.

Lysine-specific demethylase 1 is strongly expressed in poorly differentiated neuroblastoma: implications for therapy

Aberrant epigenetic changes in DNA methylation and histone acetylation are hallmarks of most cancers, whereas histone methylation was previously considered to be irreversible and less versatile. Recently, several histone demethylases were identified catalyzing the removal of methyl groups from histone H3 lysine residues and thereby influencing gene expression. Neuroblastomas continue to remain a clinical challenge despite advances in multimodal therapy. Here, we address the functional significance of the chromatin-modifying enzyme lysine-specific demethylase 1 (LSD1) in neuroblastoma. LSD1 expression correlated with adverse outcome and was inversely correlated with differentiation in neuroblastic tumors. Differentiation of neuroblastoma cells resulted in down-regulation of LSD1. Small interfering RNA-mediated knockdown of LSD1 decreased cellular growth, induced expression of differentiation-associated genes, and increased target gene-specific H3K4 methylation. Moreover, LSD1 inhibition using monoamine oxidase inhibitors resulted in an increase of global H3K4 methylation and growth inhibition of neuroblastoma cells in vitro. Finally, targeting LSD1 reduced neuroblastoma xenograft growth in vivo. Here, we provide the first evidence that a histone demethylase, LSD1, is involved in maintaining the undifferentiated, malignant phenotype of neuroblastoma cells. We show that inhibition of LSD1 reprograms the transcriptome of neuroblastoma cells and inhibits neuroblastoma xenograft growth. Our results suggest that targeting histone demethylases may provide a novel option for cancer therapy.

Neuronal polarization: old cells can learn new tricks

Regeneration was once thought to be exclusive to young neurons. Now, a new study shows that functional and interconnected hippocampal neurons have the potential to quickly recover from losing an axon. They do so by signaling a dendrite to change its specification and replace the missing axon by rearranging the microtubule cytoskeleton.

A novel mouse model with impaired dynein/dynactin function develops amyotrophic lateral sclerosis (ALS)-like features in motor neurons and improves lifespan in SOD1-ALS mice

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative condition characterized by progressive motor neuron degeneration and muscle paralysis. Genetic evidence from man and mouse has indicated that mutations in the dynein/dynactin motor complex are correlated with motor neuron degeneration. In this study, we have generated transgenic mice with neuron-specific expression of Bicaudal D2 N-terminus (BICD2-N) to chronically impair dynein/dynactin function. Motor neurons expressing BICD2-N showed accumulation of dynein and dynactin in the cell body, Golgi fragmentation and several signs of impaired retrograde trafficking: the appearance of giant neurofilament swellings in the proximal axon, reduced retrograde labelling by tracer injected in the muscle and delayed expression of the injury transcription factor ATF3 after axon transection. Despite these abnormalities, BICD2-N mice did not develop signs of motor neuron degeneration and motor abnormalities. Interestingly, the BICD2-N transgene increased lifespan in 'low copy' SOD1-G93A ALS transgenic mice. Our findings indicate that impaired dynein/dynactin function can explain several pathological features observed in ALS patients, but may be beneficial in some forms of ALS.

Filopodia are required for cortical neurite initiation

Extension of neurites from a cell body is essential to form a functional nervous system; however, the mechanisms underlying neuritogenesis are poorly understood. Ena/VASP proteins regulate actin dynamics and modulate elaboration of cellular protrusions. We recently reported that cortical axon-tract formation is lost in Ena/VASP-null mice and Ena/VASP-null cortical neurons lack filopodia and fail to elaborate neurites. Here, we report that neuritogenesis in Ena/VASP-null neurons can be rescued by restoring filopodia formation through ectopic expression of the actin nucleating protein mDia2. Conversely, wild-type neurons in which filopodia formation is blocked fail to elaborate neurites. We also report that laminin, which promotes the formation of filopodia-like actin-rich protrusions, rescues neuritogenesis in Ena/VASP-deficient neurons. Therefore, filopodia formation is a key prerequisite for neuritogenesis in cortical neurons. Neurite initiation also requires microtubule extension into filopodia, suggesting that interactions between actin-filament bundles and dynamic microtubules within filopodia are crucial for neuritogenesis.

Neurite Outgrowth: A Flick of the Wrist

A new study has shown that, near the tip of a growing axon, dephosphorylation of the microtubule-associated protein Doublecortin is controlled by protein phosphatase 1 and its regulator spinophilin. This results in spatially regulated microtubule bundling within the axon and more efficient axon outgrowth.