Kinesin accumulation in chick spinal axonal swellings with beta,beta'-iminodipropionitrile (IDPN) intoxication

Axonal Transport Defect in Gigaxonin Deficiency Rescued by Tubastatin A

Giant axonal neuropathy (GAN) is a disease caused by a deficiency of gigaxonin, a mediator of the degradation of intermediate filament (IF) proteins. A lack of gigaxonin alters the turnover of IF proteins, provoking accumulation and disorganization of neurofilaments (NFs) in neurons, a hallmark of the disease. However, the effects of IF disorganization on neuronal function remain unknown. Here, we report that cultured embryonic dorsal root ganglia (DRG) neurons derived from Gan-/- mice exhibit accumulations of IF proteins and defects in fast axonal transport of organelles. Kymographs generated by time-lapse microscopy revealed substantial reduction of anterograde movements of mitochondria and lysosomes in axons of Gan-/- DRG neurons. Treatment of Gan-/- DRG neurons with Tubastatin A (TubA) increased the levels of acetylated tubulin and it restored the normal axonal transport of these organelles. Furthermore, we tested the effects of TubA in a new mouse model of GAN consisting of Gan-/- mice with overexpression of peripherin (Prph) transgene. Treatment of 12-month-old Gan-/-;TgPer mice with TubA led to a slight amelioration of motor function, especially a significant improvement of gait performance as measured by footprint analyses. Moreover, TubA treatment reduced the abnormal accumulations of Prph and NF proteins in spinal neurons and it boosted the levels of Prph transported into peripheral nerve axons. These results suggest that drug inhibitors of histone deacetylase aiming to enhance axonal transport should be considered as a potential treatment for GAN disease.

Signaling mechanisms and disrupted cytoskeleton in the diphenyl ditelluride neurotoxicity

Evidence from our group supports that diphenyl ditelluride (PhTe)₂ neurotoxicity depends on modulation of signaling pathways initiated at the plasma membrane. The (PhTe)₂-evoked signal is transduced downstream of voltage-dependent Ca²⁺ channels (VDCC), N-methyl-D-aspartate receptors (NMDA), or metabotropic glutamate receptors activation via different kinase pathways (protein kinase A, phospholipase C/protein kinase C, mitogen-activated protein kinases (MAPKs), and Akt signaling pathway). Among the most relevant cues of misregulated signaling mechanisms evoked by (PhTe)₂ is the cytoskeleton of neural cells. The in vivo and in vitro exposure to (PhTe)₂ induce hyperphosphorylation/hypophosphorylation of neuronal and glial intermediate filament (IF) proteins (neurofilaments and glial fibrillary acidic protein, resp.) in different brain structures of young rats. Phosphorylation of IFs at specific sites modulates their association/disassociation and interferes with important physiological roles, such as axonal transport. Disrupted cytoskeleton is a crucial marker of neurodegeneration and is associated with reactive astrogliosis and apoptotic cell death. This review focuses the current knowledge and important results on the mechanisms of (PhTe)₂ neurotoxicity with special emphasis on the cytoskeletal proteins and their differential regulation by kinases/phosphatases and Ca²⁺-mediated mechanisms in developmental rat brain. We propose that the disrupted cytoskeletal homeostasis could support brain damage provoked by this neurotoxicant.

Effect of the microtubule-associated protein tau on dynamics of single-headed motor proteins KIF1A

Intracellular transport based on molecular motors and its regulation are crucial to the functioning of cells. Filamentary tracks of the cells are abundantly decorated with nonmotile microtubule-associated proteins, such as tau. Motivated by experiments on kinesin-tau interactions [Dixit et al., Science 319, 1086 (2008)] we developed a stochastic model of interacting single-headed motor proteins KIF1A that also takes into account the interactions between motor proteins and tau molecules. Our model reproduces experimental observations and predicts significant effects of tau on bound time and run length which suggest an important role of tau in regulation of kinesin-based transport.

Charcot-Marie-Tooth causing HSPB1 mutations increase Cdk5-mediated phosphorylation of neurofilaments

Mutations in the small heat shock protein HSPB1 (HSP27) are a cause of axonal Charcot-Marie-Tooth neuropathy (CMT2F) and distal hereditary motor neuropathy. To better understand the effect of mutations in HSPB1 on the neuronal cytoskeleton, we stably transduced neuronal cells with wild-type and mutant HSPB1 and investigated axonal transport of neurofilaments (NFs). We observed that mutant HSPB1 affected the binding of NFs to the anterograde motor protein kinesin, reducing anterograde transport of NFs. These deficits were associated with an increased phosphorylation of NFs and cyclin-dependent kinase Cdk5. As Cdk5 mediates NF phosphorylation, inhibition of Cdk5/p35 restored NF phosphorylation level, as well as NF binding to kinesin in mutant HSPB1 neuronal cells. Altogether, we demonstrate that HSPB1 mutations induce hyperphosphorylation of NFs through Cdk5 and reduce anterograde transport of NFs.

Topographic regulation of neuronal intermediate filaments by phosphorylation, role of peptidyl-prolyl isomerase 1: significance in neurodegeneration

The neuronal cytoskeleton is tightly regulated by phosphorylation and dephosphorylation reactions mediated by numerous associated kinases, phosphatases and their regulators. Defects in the relative kinase and phosphatase activities and/or deregulation of compartment-specific phosphorylation result in neurodegenerative disorders. The largest family of cytoskeletal proteins in mammalian cells is the superfamily of intermediate filaments (IFs). The neurofilament (NF) proteins are the major IFs. Aggregated forms of hyperphosphorylated tau and phosphorylated NFs are found in pathological cell body accumulations in the central nervous system of patients suffering from Alzheimer's disease, Parkinson's disease, and Amyotrophic Lateral Sclerosis. The precise mechanisms for this compartment-specific phosphorylation of cytoskeletal proteins are not completely understood. In this review, we focus on the mechanisms of neurofilament phosphorylation in normal physiology and neurodegenerative diseases. We also address the recent breakthroughs in our understanding the role of different kinases and phosphatases involved in regulating the phosphorylation status of the NFs. In addition, special emphasis has been given to describe the role of phosphatases and Pin1 in phosphorylation of NFs.

Shock detection in dynamics of single-headed motor proteins KIF1A via Jensen-Shannon divergence

Information theoretic quantities are useful tools to characterize symbolic sequences. In this paper, we use the Jensen-Shannon divergence to study symbolic binary sequences that represent the stationary state of a lattice-gas model describing the traffic of monomeric kinesin KIF1A. More specifically, the constructed binary sequences represent the state of a microtubule protofilament at different adenosine triphosphate (ATP) and KIF1A motor concentrations in the cytosol. The model presents some stationary regimes with phase coexistence. By using the Jensen-Shannon divergence, we develop a method of analysis that allows us to identify cases in which phase coexistence occurs and, for these cases, to locate the position of the interphase that separates the regions with different phase.

Neurofilament dynamics: a tug of war by microtubule motors

Structural support for axons, which can consist of volumes thousands of times larger than the neuronal perikaryon, is provided in part by neurofilaments (NFs), the major fibrous constituent of the axonal cytoskeleton. Most NFs undergo anterograde transport (towards the synapse or growth cone), while a few undergo retrograde transport (back towards the perikaryon). Some NFs exhibit an extended residence time along axons, which allows NFs to provide structural support to the axon yet minimizes NF turnover, which would otherwise impart a prohibitive metabolic burden upon the neuron. Herein, we explore known and hypothesized roles for microtubule motors in transport and distribution of NFs along axons. We present evidence that those NFs that display extended residence along axons are critically dependent upon surrounding microtubules, and that simultaneous interaction with multiple microtubule motors provides the architectural force regulating their distribution.

Organizational dynamics, functions, and pathobiological dysfunctions of neurofilaments

Neurofilament phosphorylation has long been considered to regulate their axonal transport rate, and in doing so it provides stability to mature axons. We evaluate the collective evidence to date regarding how neurofilament C-terminal phosphorylation may regulate axonal transport. We present a few suggestions for further experimentation in this area, and expand upon previous models for axonal NF dynamics. We present evidence that the NFs that display extended residence along axons are critically dependent upon the surrounding microtubules, and that simultaneous interaction with multiple microtubule motors provides the architectural force that regulates their distribution. Finally, we address how C-terminal phosphorylation is regionally and temporally regulated by a balance of kinase and phosphatase activities, and how misregulation of this balance might contribute to motor neuron disease.

Regulation of neurofilament dynamics by phosphorylation

Neurofilament (NF) phosphorylation has long been considered to regulate axonal transport rate and in doing so to provide stability to mature axons. Studies utilizing mice in which the C-terminal region of NF subunits (which contains the vast majority of phosphorylation sites) has been deleted has prompted an ongoing challenge to this hypothesis. We evaluate the collective evidence to date for and against a role for NF C-terminal phosphorylation in regulation of axonal transport and in providing structural support for axons, including some novel studies from our laboratory. We present a few suggestions for further experimentation in this area, and expand upon previous models for axonal NF dynamics. Finally, we address how C-terminal phosphorylation is regionally and temporally regulated by a balance of kinase and phosphatase activities, and how misregulation of this balance can contribute to motor neuron disease.

Inhibition of dynein but not kinesin induces aberrant focal accumulation of neurofilaments within axonal neurites

Studies from several laboratories indicate that the microtubule motors kinesin and dynein respectively participate in anterograde and retrograde axonal transport of neurofilaments. Inhibition of dynein function by transfection with a construct expressing dynamitin or intracellular delivery of anti-dynein antibodies accelerates anterograde transport, which has been interpreted to indicate that the opposing action of both motors mediates the normal distribution of neurofilaments along axons. Herein, we demonstrate that, while expression of relatively low levels of exogenous dynamitin indeed accelerated anterograde neurofilament transport along axonal neurites in culture, expression of progressively increasing levels of dynamitin induced focal accumulation of neurofilaments within axonal neurites and eventually caused neurite retraction. Inhibition of kinesin inhibited anterograde transport, but did not induce similar focal accumulations. These findings are consistent with studies indicating that perturbations in dynein activity can contribute to the aberrant accumulations of neurofilaments that accompany ALS/motor neuron disease.

Traffic jams: dynamic models for neurofilament accumulation in motor neuron disease

The dynamics of axonal transport are often colloquially described using highway traffic as a model system. Examination of the physics of traffic patterns, with emphasis on traffic jams and accidents, provides unique and perhaps counterintuitive insight into the aberrant accumulation of neurofilaments that accompanies amyotrophic lateral sclerosis/motor neuron disease.

Dynein mediates retrograde neurofilament transport within axons and anterograde delivery of NFs from perikarya into axons: Regulation by multiple phosphorylation events

We examined the respective roles of dynein and kinesin in axonal transport of neurofilaments (NFs). Differentiated NB2a/d1 cells were transfected with green fluorescent protein-NF-M (GFP-M) and dynein function was inhibited by co-transfection with a construct expressing myc-tagged dynamitin, or by intracellular delivery of purified dynamitin and two antibodies against dynein's cargo domain. Monitoring of the bulk distribution of GFP signal within axonal neurites, recovery of GFP signal within photobleached regions, and real-time monitoring of individual NFs/punctate structures each revealed that pertubation of dynein function inhibited retrograde transport and accelerated anterograde, confirming that dynein mediated retrograde axonal transport, while intracellular delivery of two anti-kinesin antibodies selectively inhibited NF anterograde transport. In addition, dynamitin overexpression inhibited the initial translocation of newly-expressed NFs out of perikarya and into neurites, indicating that dynein participated in the initial anterograde delivery of NFs into neurites. Delivery of NFs to the axon hillock inner plasma membrane surface, and their subsequent translocation into neurites, was also prevented by vinblastine-mediated inhibition of microtubule assembly. These data collectively suggest that some NFs enter axons as cargo of microtubues that are themselves undergoing transport into axons via dynein-mediated interactions with the actin cortex and/or larger microtubules. C-terminal NF phosphorylation regulates motor association, since anti-dynein selectively coprecipitated extensively phosphorylated NFs, while anti-kinesin selectively coprecipitated less phosphorylated NFs. In addition, however, the MAP kinase inhibitor PD98059 also inhibited transport of a constitutively-phosphorylated NF construct, indicating that one or more additional, non-NF phosphorylation events also regulated NF association with dynein or kinesin.

The interaction of neurofilaments with the microtubule motor cytoplasmic dynein

Neurofilaments are synthesized in the cell body of neurons and transported outward along the axon via slow axonal transport. Direct observation of neurofilaments trafficking in live cells suggests that the slow outward rate of transport is due to the net effects of anterograde and retrograde microtubule motors pulling in opposition. Previous studies have suggested that cytoplasmic dynein is required for efficient neurofilament transport. In this study, we examine the interaction of neurofilaments with cytoplasmic dynein. We used fluid tapping mode atomic force microscopy to visualize single neurofilaments, microtubules, dynein/dynactin, and physical interactions between these neuronal components. AFM images suggest that neurofilaments act as cargo for dynein, associating with the base of the motor complex. Yeast two-hybrid and affinity chromatography assays confirm this hypothesis, indicating that neurofilament subunit M binds directly to dynein IC. This interaction is blocked by monoclonal antibodies directed either to NF-M or to dynein. Together these data suggest that a specific interaction between neurofilament subunit M and cytoplasmic dynein is involved in the saltatory bidirectional motility of neurofilaments undergoing axonal transport in the neuron.

Differential screening of mutated SOD1 transgenic mice reveals early up-regulation of a fast axonal transport component in spinal cord motor neurons

In the present study we analyze the molecular mechanisms underlying motor neuron degeneration in familial amyotrophic lateral sclerosis (FALS). For this, we used a transgenic mouse model expressing the Cu/Zn superoxide dismutase (SOD1) gene with a Gly(86) to Arg (G86R) mutation equivalent to that found in a subset of human FALS. Using an optimized suppression subtractive hybridization method, a cDNA specifically up-regulated during the asymptomatic phase in the lumbar spinal cord of G86R mice was identified by sequence analysis as the KIF3-associated protein (KAP3), a regulator of fast axonal transport. RT-PCR analysis revealed that KAP3 induction was an early event arising long before axonal degeneration. Immunohistochemical studies further revealed that KAP3 protein predominantly accumulates in large motor neurons of the ventral spinal cord. We further demonstrated that KAP3 up-regulation occurs independent of any change in the other components of the kinesin II complex. However, since the ubiquitous KIF1A motor is up-regulated, our results show an early and complex rearrangement of the fast axonal transport machinery in the course of FALS pathology.

Kinesin-mediated transport of neurofilament protein oligomers in growing axons

We examined cytoskeleton-associated forms of NF proteins during axonal neuritogenesis in cultured dorsal root ganglion (DRG) neurons and NB2a/d1 neuroblastoma. In addition to filamentous immunoreactivity, we observed punctate NF immunoreactivity throughout perikarya and neurites. Immuno-electron microscopy revealed this punctate immunoreactivity to consist of non-membrane-bound 75 nm round/ovoid structures consisting of amorphous, fibrous material. Endogenous and microinjected NF subunits incorporated into dots prior to their accumulation within filaments. A transfected GFP-conjugated NF-M incorporated into dots and translocated at a rate consistent with slow axonal transport in real-time video analyses. Some dots converted into a filamentous form or exuded filamentous material during transport. Dots contained conventional kinesin immunoreactivity, associated with microtubules, and their transport into axons was blocked by anti-kinesin antibodies and nocodazole. These oligomeric structures apparently represent one form in which NF subunits are transported in growing axons and may utilize kinesin as a transport motor.