Molecular motors in axonal transport. Cellular and molecular biology of kinesin

Multiscale Mechanobiology in Brain Physiology and Diseases

Increasing evidence suggests that mechanics play a critical role in regulating brain function at different scales. Downstream integration of mechanical inputs into biochemical signals and genomic pathways causes observable and measurable effects on brain cell fate and can also lead to important pathological consequences. Despite recent advances, the mechanical forces that influence neuronal processes remain largely unexplored, and how endogenous mechanical forces are detected and transduced by brain cells into biochemical and genetic programs have received less attention. In this review, we described the composition of brain tissues and their pronounced microstructural heterogeneity. We discuss the individual role of neuronal and glial cell mechanics in brain homeostasis and diseases. We highlight how changes in the composition and mechanical properties of the extracellular matrix can modulate brain cell functions and describe key mechanisms of the mechanosensing process. We then consider the contribution of mechanobiology in the emergence of brain diseases by providing a critical review on traumatic brain injury, neurodegenerative diseases, and neuroblastoma. We show that a better understanding of the mechanobiology of brain tissues will require to manipulate the physico-chemical parameters of the cell microenvironment, and to develop three-dimensional models that can recapitulate the complexity and spatial diversity of brain tissues in a reproducible and predictable manner. Collectively, these emerging insights shed new light on the importance of mechanobiology and its implication in brain and nerve diseases.

Molecular and cellular mechanisms of spastin in neural development and disease (Review)

Spastin is a microtubule (MT)‑severing enzyme identified from mutations of hereditary spastic paraplegia in 1999 and extensive studies indicate its vital role in various cellular activities. In the past two decades, efforts have been made to understand the underlying molecular mechanisms of how spastin is linked to neural development and disease. Recent studies on spastin have unraveled the mechanistic processes of its MT‑severing activity and revealed that spastin acts as an MT amplifier to mediate its remodeling, thus providing valuable insight into the molecular roles of spastin under physiological conditions. In addition, recent research has revealed multiple novel molecular mechanisms of spastin in cellular biological pathways, including endoplasmic reticulum shaping, calcium trafficking, fatty acid trafficking, as well as endosomal fission and trafficking. These processes are closely involved in axonal and dendritic development and maintenance. The current review presents recent biological advances regarding the molecular mechanisms of spastin at the cellular level and provides insight into how it affects neural development and disease.

Salivary gland proteome analysis of developing adult female Haemaphysalis longicornis ticks: molecular motor and TCA cycle-related proteins play an important role throughout development

Background

Ticks are notorious blood-feeding arthropods that can spread a variety of deadly diseases. The salivary gland is an important organ for ticks to feed on blood, and this organ begins to develop rapidly when ixodid ticks suck blood. When these ticks reach a critical weight, the salivary glands stop developing and begin to degenerate. The expression levels of a large number of proteins during the development and degeneration of salivary glands change, which regulate the biological functions of the salivary glands. Furthermore, to the best of our knowledge, there are only a few reports on the role of molecular motor and TCA cycle-related proteins in the salivary glands of ticks.

Results

We used iTRAQ quantitative proteomics to study the dynamic changes in salivary gland proteins in female Haemaphysalis longicornis at four feeding stages: unfed, partially fed, semi-engorged and engorged. Using bioinformatics methods to analyze the dynamic changes of a large number of proteins, we found that molecular motor and TCA cycle-related proteins play an important role in the physiological changes of the salivary glands. The results of RNAi experiments showed that when dynein, kinesin, isocitrate dehydrogenase and citrate synthase were knocked down independently, the weight of the engorged female ticks decreased by 63.5%, 54.9%, 42.6% and 48.6%, respectively, and oviposition amounts decreased by 83.1%, 76.0%, 50.8%, and 55.9%, respectively, and the size of type III acini of females salivary glands decreased by 35.6%, 33.3%, 28.9%, and 20.0%, respectively.

Conclusions

The results showed that the expression of different types of proteins change in different characteristics in salivary glands during the unfed to engorged process of female ticks. Corresponding expression changes of these proteins at different developmental stages of female ticks are very important to ensure the orderly development of the organ. By analyzing these changes, some proteins, such as molecular motor and TCA cycle-related proteins, were screened and RNAi carried out. When these mRNAs were knocked down, the female ticks cannot develop normally. The research results provide a new protein target for the control of ticks and tick-borne diseases.

Microtubules cut loose at the cell cortex

The ability of the microtubule cytoskeleton to rapidly and locally reorganize itself in response to intra- and extracellular signals is essential to its wide range of functions. A site of tightly regulated microtubule dynamics--and the major interface between the microtubule cytoskeleton and the extracellular environment--is the cell cortex, where the selective stabilization and destabilization of microtubule plus-ends is required for normal cell division, morphogenesis and migration. In a recent study, we found that the cortex of Drosophila S2 and D17 cells is coated with the microtubule severing enzyme and plus-end depolymerase, Kat-60, which actively suppresses microtubule growth and stability along the cell edge. We have proposed that cortical Kat-60 functions by uncapping plus-ends, thereby activating another microtubule depolymerase, KLP10A, preloaded onto the end. The localized destruction of microtubule plus-ends at a specific cortical could feed into larger regulatory pathways, such as those in control of the actin cytoskeleton, to influence cell polarization and motility.

Tri-partite complex for axonal transport drug delivery achieves pharmacological effect

BACKGROUND

Targeted delivery of pharmaceutical agents into selected populations of CNS (Central Nervous System) neurons is an extremely compelling goal. Currently, systemic methods are generally used for delivery of pain medications, anti-virals for treatment of dermatomal infections, anti-spasmodics, and neuroprotectants. Systemic side effects or undesirable effects on parts of the CNS that are not involved in the pathology limit efficacy and limit clinical utility for many classes of pharmaceuticals. Axonal transport from the periphery offers a possible selective route, but there has been little progress towards design of agents that can accomplish targeted delivery via this intraneural route. To achieve this goal, we developed a tripartite molecular construction concept involving an axonal transport facilitator molecule, a polymer linker, and a large number of drug molecules conjugated to the linker, then sought to evaluate its neurobiology and pharmacological behavior.

RESULTS

We developed chemical synthesis methodologies for assembling these tripartite complexes using a variety of axonal transport facilitators including nerve growth factor, wheat germ agglutinin, and synthetic facilitators derived from phage display work. Loading of up to 100 drug molecules per complex was achieved. Conjugation methods were used that allowed the drugs to be released in active form inside the cell body after transport. Intramuscular and intradermal injection proved effective for introducing pharmacologically effective doses into selected populations of CNS neurons. Pharmacological efficacy with gabapentin in a paw withdrawal latency model revealed a ten fold increase in half life and a 300 fold decrease in necessary dose relative to systemic administration for gabapentin when the drug was delivered by axonal transport using the tripartite vehicle.

CONCLUSION

Specific targeting of selected subpopulations of CNS neurons for drug delivery by axonal transport holds great promise. The data shown here provide a basic framework for the intraneural pharmacology of this tripartite complex. The pharmacologically efficacious drug delivery demonstrated here verify the fundamental feasibility of using axonal transport for targeted drug delivery.

Tau binding to microtubules does not directly affect microtubule-based vesicle motility

Tau protein is a major microtubule (MT)-associated brain protein enriched in axons. Multiple functional roles are proposed for tau protein, including MT stabilization, generation of cell processes, and targeting of phosphotransferases to MTs. Recently, experiments involving exogenous tau expression in cultured cells suggested a role for tau as a regulator of kinesin-1-based motility. Tau was proposed to inhibit attachment of kinesin-1 to MTs by competing for the kinesin-1 binding site. In this work, we evaluated effects of tau on fast axonal transport (FAT) by using vesicle motility assays in isolated squid axoplasm. Effects of recombinant tau constructs on both kinesin-1 and cytoplasmic dynein-dependent FAT rates were evaluated by video microscopy. Exogenous tau binding to endogenous squid MTs was evidenced by a dramatic change in individual MT morphologies. However, perfusion of tau at concentrations approximately 20-fold higher than physiological levels showed no effect on FAT. In contrast, perfusion of a cytoplasmic dynein-derived peptide that competes with kinesin-1 and cytoplasmic dynein binding to MTs in vitro rapidly inhibited FAT in both directions. Taken together, our results indicate that binding of tau to MTs does not directly affect kinesin-1- or cytoplasmic dynein-based motilities. In contrast, our results provide further evidence indicating that the functional binding sites for kinesin-1 and cytoplasmic dynein on MTs overlap.

A kinesin medley: biochemical and functional heterogeneity

Over the past decade, a remarkable number and diversity of molecular motors have been described in eukaryotic cells. In addition to the identification of novel forms of myosin and dynein, the kinesins have been defined as an entirely new family of molecular motors. There may be as many as 30 different genes in a single organism encoding members of the kinesin superfamily. Why is such diversity in molecular motors needed? The biochemical and functional diversity of the originally defined form of kinesin provides some insights into the roles of molecular motors in cellular dynamics.

Glucocorticoid hormone (cortisol) affects axonal transport in human cortex neurons but shows resistance in Alzheimer's disease

1 The changes of tissue sensitivity to glucocorticoids are associated with many pathological states including neurological diseases. In the present study, using a novel in vitro post-mortem tracing method on human brain slices, we demonstrated that cortisol, a major glucocorticoid hormone in humans, affected axonal transport both in the cortex neurons in four Alzheimer's disease (AD) patients and four nondemented controls. 2 Cortisol appeared to affect axonal transport of prefrontal cortex (PFC) and temporal cortex (TC) neurons in AD patients and controls in a dose-dependent way at concentrations of 30, 60, 120 and 240 microg dl(-1). 3 Higher doses of cortisol were needed for TC neurons to achieve a similar axonal transport effect as obtained in PFC neurons in AD patients. The maximum effect (Emax) on axonal transport was achieved in PFC slices at relatively low contraction (30-120 microg dl(-1)), while in TC slices, a maximum effect was only reached at relatively high concentrations (120-240 microg dl(-1)). 4 For PFC and TC slices from nondemented aging subjects, lower doses of cortisol (30-60 microg dl(-1)) on axonal transport were sufficient to achieve the maximum effect as compared to those used in AD brain slices, while levels of more than 60 microg dl(-1) of cortisol mostly depressed axonal transport. 5 These results suggest that glucocorticoid resistance, which is thought to contribute to the pathogenesis of a number of common human disorders, may exist in AD brains and play an important role in neuropathological mechanisms and dementia.

Myosin function in nervous and sensory systems

Development of the nervous system requires remarkable changes in cell structure that are dependent upon the cytoskeleton. The importance of specific components of the neuronal cytoskeleton, such as microtubules and neurofilaments, to neuronal function and development has been well established. Recently, increasing focus has been put on understanding the functional role of the actin cytoskeleton in neurons. Important modulators of the actin cytoskeleton are the large family of myosins, many of which (classes I, II, III, V, VI, VII, IX, and XV; Fig. 1) are expressed in developing neurons or sensory cells. Myosins are force-producing proteins that have been implicated in a wide variety of cellular functions in the developing nervous system, including neuronal migration, process outgrowth, and growth cone motility, as well as other aspects of morphogenesis, axonal transport, and synaptic and sensory functions. We review the roles that neuronal myosins play in these functions with particular focus on the first three events listed above, as well as sensory function.

Impaired axonal transport of cortical neurons in Alzheimer's disease is associated with neuropathological changes

Using a novel in vitro post mortem tracing method, we demonstrate a decrease of axonal transport in the temporal cortex neurons as compared to axonal transport in the prefrontal cortex neurons in AD patients, but not in non-demented controls. The decrease of axonal transport is related to the degree of neuropathological changes, as the temporal cortex undergoes more severe neuropathological changes in AD. The present study provides, for the first time, direct evidence of the presence of impaired axonal transport in AD brains.

High-resolution imaging demonstrates dynein-based vesicular transport of activated Trk receptors

Target-derived neurotrophins signal from nerve endings to the cell body to influence cellular and nuclear responses. The retrograde signal is conveyed by neurotrophin receptors (Trks) themselves. To accomplish this, activated Trks may physically relocalize from nerve endings to the cell bodies. However, alternative signaling mechanisms may also be used. To identify the vehicle wherein the activated Trks are located and transported, and to identify associated motor proteins that would facilitate transport, we use activation-state specific antibodies in concert with immunoelectron microscopy and deconvolution microscopy. We show that the'activated Trks within rat sciatic nerve axons are preferentially localized to coated and uncoated vesicles. These vesicles are moving in a retrograde direction and so accumulate distal to a ligation site. The P-Trk containing vesicles, in turn, colocalize with dynein components, and not with kinesins. Collectively, these results indicate activated Trk within axons travel in vesicles and dynein is the motor that drives these vesicles towards the cell bodies.

Cytoplasmic dynein is associated with slow axonal transport

Neuronal function is dependent on the transport of materials from the cell body to the synapse via anterograde axonal transport. Anterograde axonal transport consists of several components that differ in both rate and protein composition. In fast transport, membranous organelles are moved along microtubules by the motor protein kinesin. The cytoskeleton and the cytomatrix proteins move in the two components of slow transport. While the mechanisms underlying slow transport are unknown, it has been hypothesized that the movement of microtubules in slow transport is generated by sliding. To determine whether dynein, a motor protein that causes microtubule sliding in flagella, may play a role in slow axonal transport, we identified the transport rate components with which cytoplasmic dynein is associated in rat optic nerve. Nearly 80% of the anterogradely moving dynein was associated with slow transport, whereas only approximately 15% of the dynein was associated with the membranous organelles of anterograde fast axonal transport. A segmental analysis of the transport of dynein through contiguous regions of the optic nerve and tract showed that dynein is associated with the microfilaments and other proteins of slow component b. Dynein from this transport component has the capacity to bind microtubules in vitro. These results are consistent with the hypothesis that cytoplasmic dynein generates the movement of microtubules in slow axonal transport. A model is presented to illustrate how dynein attached to the slow component b complex of proteins is appropriately positioned to generate force of the correct polarity to slide microtubules down the axon.

Acrylamide arrests mitosis and prevents chromosome migration in the absence of changes in spindle microtubules

Cultured HT 1080 fibrosarcoma cells were exposed to acrylamide (ACR), an industrial neurotoxicant that disrupts neuronal intracellular transport, to determine if mitosis (another microtubule-based intracellular transport system) was adversely affected. The number of cells arrested in mitosis increased, in a concentration-dependent manner, from 1 to 10 mM acrylamide. A 4-h exposure to 10 mM acrylamide increased the mitotic index by 4.5-fold over control, comparable to the arrest caused by colchicine. In mitotic acrylamide-exposed cells, the chromosomes remained at the metaphase plate; no changes in spindle microtubules (MTs), as seen with tubulin immunofluorescence, were observed. The distance between spindle poles (interaster) was the same in control and experimental cells. The non-neurotoxic analogue methylene bisacrylamide had no effect in the same concentration range. The data suggest potential molecular mechanisms of action for general toxicity and neurotoxicity to be disruption in MT disassembly or MT-kinetochore interactions and/or cellular homeostasis.

The role of microtubule dynamics in growth cone motility and axonal growth

The growth cone contains dynamic and relatively stable microtubule populations, whose function in motility and axonal growth is uncharacterized. We have used vinblastine at low doses to inhibit microtubule dynamics without appreciable depolymerization to probe the role of these dynamics in growth cone behavior. At doses of vinblastine that interfere only with dynamics, the forward and persistent movement of the growth cone is inhibited and the growth cone wanders without appreciable forward translocation; it quickly resumes forward growth after the vinblastine is washed out. Direct visualization of fluorescently tagged microtubules in these neurons shows that in the absence of dynamic microtubules, the remaining mass of polymer does not invade the peripheral lamella and does not undergo the usual cycle of bundling and splaying and the growth cone stops forward movement. These experiments argue for a role for dynamic microtubules in allowing microtubule rearrangements in the growth cone. These rearrangements seem to be necessary for microtubule bundling, the subsequent coalescence of the cortex around the bundle to form new axon, and forward translocation of the growth cone.

The role of microtubules in growth cone turning at substrate boundaries

To understand the role of microtubules in growth cone turning, we observed fluorescently labeled microtubules in neurons as they encountered a substrate boundary. Neurons growing on a laminin-rich substrate avoided growing onto collagen type IV. Turning growth cones assumed heterogeneous morphologies and behaviors that depended primarily in their extent of adhesion to the substrate. We grouped these behaviors into three categories-sidestepping, motility, and growth-mediated reorientation. In sidestepping and motility-mediated reorientation, the growth cone and parts of the axon were not well attached to the substrate so the acquisition of an adherent lamella caused the entire growth cone to move away from the border and consequently reoriented the axon. In these cases, since the motility of the growth cone dominates its reorientation, the microtubules were passive, and reorientation occurred without significant axon growth. In growth-mediated reorientation, the growth cone and axon were attached to the substrate. In this case, microtubules reoriented within the growth cone to stabilize a lamella. Bundling of the reoriented microtubules was followed by growth cone collapse to form new axon, and further, polarized lamellipodial extension. These observations indicate that when the growth cone remains adherent to the substrate during turning, the reorientation and bundling of microtubules is an important, early step in growth cone turning.

Axonal transport and MR imaging: prospects for contrast agent development

Axonal transport plays a critical role in the physiology and pathology of neurons, yet there have been virtually no clinical tools for its evaluation in human subjects. A wide variety of molecules that can act as axonal transport facilitators have been discovered and, in many cases, used to deliver labels detectable with histologic methods. Recently a number of investigators have reported preliminary success in developing intraneural contrast agents based on various versions of dextran-coated magnetite that may render magnetic resonance imaging capable of depicting axonal transport. It is not yet clear whether any clinically useful agents will eventually be developed, but there has been considerable progress in identifying design factors for such a pharmaceutical agent.