Slow axonal transport: fast motors in the slow lane

Magnetically-actuated microposts stimulate axon growth

New strategies to promote neuronal regeneration should aim to increase the speed of axonal elongation. Biochemical signaling is a key factor in axon growth, but recent discoveries have shown that mechanical force, through a process referred to as stretch growth, can significantly influence the elongation rate. Here, we develop a method to apply forces to primary hippocampal neurons from mice using magnetic microposts that actuate in response to an external magnetic field. Neurons are cultured onto these microposts and subjected to an average displacement of 0.2 μm at a frequency of 5 Hz. We find that the mechanical stimulation promotes an increase in the length of the axons compared to control conditions. In addition, there is an increase in the density of microtubules and in the amount of cisternae of the endoplasmic reticulum, providing evidence that stretch growth is accompanied by a mass addition to the neurite. Together, these results indicate that magnetically-actuated microposts can accelerate the rate of axon growth, paving the way for future applications in neuronal regeneration. VIDEO ABSTRACT.

Estrogen effects on oxytocinergic pathways that regulate food intake

Multiple stimulatory and inhibitory neural circuits control eating, and these circuits are influenced by an array of hormonal, neuropeptide, and neurotransmitter signals. For example, estrogen and oxytocin (OT) both are known to decrease food intake, but the mechanisms by which these signal molecules influence eating are not fully understood. These studies investigated the interaction between estrogen and OT in the control of food intake. RT-qPCR studies revealed that 17β-estradiol benzoate (EB)-treated rats showed a two-fold increase in OT mRNA in the paraventricular nucleus of the hypothalamus (PVN) compared to Oil-treated controls. Increased OT mRNA expression may increase OT protein levels, and immunohistochemistry studies showed that EB-treated rats had more intense OT labeling in the nucleus of the solitary tract (NTS), a region known to integrate signals for food intake. Food intake measurements showed that EB treatment reduced food intake, as expected. EB-treated rats lost weight over the course of the experiment, as expected, and EB-treated rats that received the highest dose of OT lost more weight than EB-treated rats that did not receive OT. Finally, OT antagonist administered to EB-treated rats reversed the effect of EB on food intake, suggesting that estrogen effects to decrease food intake may involve the oxytocinergic pathway.

Dynein binds and stimulates axonal motility of the endosome adaptor and NEEP21 family member, calcyon

The neuron-enriched, endosomal protein Calcyon (Caly) regulates endocytosis and vesicle sorting, and is important for synaptic plasticity and brain development. In the current investigation of Caly interacting proteins in brain, the microtubule retrograde motor subunit, cytoplasmic dynein 1 heavy chain (DYNC1H), and microtubule structural proteins, α and β tubulin, were identified as Caly associated proteins by MALDI-ToF/ToF. Direct interaction of the Caly-C terminus with dynein and tubulin was further confirmed in in vitro studies. In Cos-7 cells, mCherry-Caly moved along the microtubule network in organelles largely labeled by the late endosome marker Rab7. Expression of the dynein inhibitor CC1, produced striking alterations in Caly distribution, consistent with retrograde motors playing a prominent role in Caly localization and movement. In axons of cultured adult rat sensory neurons, Caly-positive organelles co-localized with dynein intermediate chain (DYNC1I1-isoform IC-1B) and the dynein regulator, lissencephaly 1 (LIS1), both of which co-precipitated from brain with the Caly C-terminus. Manipulation of dynein function in axons altered the motile properties of Caly indicating that Caly vesicles utilize the retrograde motor. Altogether, the current evidence for association with dynein motors raises the possibility that the endocytic and cargo sorting functions of Caly in neurons could be regulated by interaction with the microtubule transport system.

Utilization of the bootstrap method for determining confidence intervals of parameters for a model of MAP1B protein transport in axons

This paper develops a model of axonal transport of MAP1B protein. The problem of determining parameter values for the proposed model utilizing limited available experimental data is addressed. We used a global minimum search algorithm to find parameter values that minimize the discrepancy between model predictions and published experimental results. By analyzing the best fit parameter values it was established that some processes can be dropped from the model without losing accuracy, thus a simplified version of the model was formulated. In particular, our analysis suggests that reversals in MAP1B transport are infrequent and can be neglected. The detachment of anterogradely-biased MAP1B from microtubules (MTs) and the attachment of retrogradely-biased MAP1B to MTs can also be neglected. An analytical solution for the simplified model was obtained. Confidence intervals for the determined parameters were found by bootstrapping model residuals. The resultant analysis heavily constrained the values of some parameters while showing that some could vary without significantly impacting model error. For example, our analysis suggests that, above a certain threshold value, the kinetic constant determining the rate of MAP1B transition from the retrograde pausing state to the off-track state has little impact on model error. On the contrary, the kinetic constant describing MAP1B transition from a pausing to a running state has great impact on model error.

Deciphering the axonal transport kinetics of neurofilaments using the fluorescence photoactivation pulse-escape method

Neurofilaments are transported along axons stochastically in a stop-and-go manner, cycling between brief bouts of rapid movement and pauses that can vary from seconds to hours in length. Presently the only way to analyze neurofilament pausing experimentally on both long and short time scales is the pulse-escape method. In this method, fluorescence photoactivation is used to mark a population of axonal neurofilaments and then the loss of fluorescence from the activated region due to neurofilament movement is monitored by time-lapse imaging. Here we develop a mathematical description of the pulse-escape kinetics in terms of the rate constants of a tested mathematical model and we show how this model can be used to characterize neurofilament transport kinetics from fluorescence photoactivation pulse-escape experiments. This combined experimental and computational approach is a powerful tool for the analysis of the moving and pausing behavior of neurofilaments in axons.

White matter as a transport system

There are two ways to picture white matter: as a grid of electrical wires or a network of roads. The first metaphor captures the classical function of an axon as conductor of action potentials (and information) from one brain region to another. The second one points to the important role of axons in a bi-directional transport of biological molecules and organelles between the cell body and synapse. Given the wide variety of such cargoes, a well-functioning axonal transport is critical for a number of processes, including neurotransmission, metabolism and viability of neurons. This selective review will emphasize the need for considering axonal transport when interpreting functional consequences of inter-individual variations in the structural properties of white matter. We start by describing the space occupied by white matter and techniques used in vivo for its characterization. We then provide examples of key features of maturation and aging of white matter, as well as some of the common abnormalities observed in neurodevelopmental and neurodegenerative disorders. Next, we review work that motivated our focus on axonal diameter, and explain the relationships between transport and cytoskeleton within the axon. We will conclude by describing molecular machinery of axonal transport and genes that may contribute to inter-individual variations in axonal diameter and axonal transport.

α-Synuclein: the long distance runner

Parkinson's disease is characterized by α-synuclein pathology in the form of Lewy bodies and Lewy neurites. Braak et al described the spatial and temporal spread of α-synuclein pathology in Parkinson's disease. Recent experimental studies have demonstrated that α-synuclein can transfer from cell to cell. In this review, we highlight the involvement of α-synuclein in Parkinson's disease and in Braak's staging of Parkinson's disease pathology. We discuss whether a prion-like mechanism of α-synuclein spread might contribute to Parkinson's disease pathology. We describe recent studies investigating cell-to-cell transfer of α-synuclein and focus our review on the long-distance axonal transport of α-synuclein along neurons.

Endoplasmic reticulum sorting and kinesin-1 command the targeting of axonal GABAB receptors

In neuronal cells the intracellular trafficking machinery controls the availability of neurotransmitter receptors at the plasma membrane, which is a critical determinant of synaptic strength. Metabotropic γ amino-butyric acid (GABA) type B receptors (GABA_BRs) are neurotransmitter receptors that modulate synaptic transmission by mediating the slow and prolonged responses to GABA. GABA_BRs are obligatory heteromers constituted by two subunits, GABA_BR1 and GABA_BR2. GABA_BR1a and GABA_BR1b are the most abundant subunit variants. GABA_BR1b is located in the somatodendritic domain whereas GABA_BR1a is additionally targeted to the axon. Sushi domains located at the N-terminus of GABA_BR1a constitute the only difference between both variants and are necessary and sufficient for axonal targeting. The precise targeting machinery and the organelles involved in sorting and transport have not been described. Here we demonstrate that GABA_BRs require the Golgi apparatus for plasma membrane delivery but that axonal sorting and targeting of GABA_BR1a operate in a pre-Golgi compartment. In the axon GABA_BR1a subunits are enriched in the endoplasmic reticulum (ER), and their dynamic behavior and colocalization with other secretory organelles like the ER-to-Golgi intermediate compartment (ERGIC) suggest that they employ a local secretory route. The transport of axonal GABA_BR1a is microtubule-dependent and kinesin-1, a molecular motor of the kinesin family, determines axonal localization. Considering that progression of GABA_BRs through the secretory pathway is regulated by an ER retention motif our data contribute to understand the role of the axonal ER in non-canonical sorting and targeting of neurotransmitter receptors.

Neurogenetics of slow axonal transport: from cells to animals

Slow axonal transport is a multivariate phenomenon implicated in several neurodegenerative disorders. Recent reports have unraveled the molecular basis of the transport of certain slow component proteins, such as the neurofilament subunits, tubulin, and certain soluble enzymes such as Ca(2+)/calmodulin-dependent protein kinase IIa (CaM kinase IIa), etc., in tissue cultured neurons. In addition, genetic analyses also implicate microtubule-dependent motors and other housekeeping proteins in this process. However, the biological relevance of this phenomenon is not so well understood. Here, the authors have discussed the possibility of adopting neurogenetic analyses in multiple model organisms to correlate molecular level measurements of the slow transport phenomenon to animal behavior, thus facilitating the investigation of its biological efficacy.

Role of kinesin-1 in the pathogenesis of SPG10, a rare form of hereditary spastic paraplegia

Molecular protein motors play key roles in processes such as intracellular cargo transport and brain wiring, and failure of function can give rise to serious diseases. Kinesin-1, a member of the kinesin superfamily (also known as KIFs) is a two-headed motor protein that uses energy derived from ATP hydrolysis to transport diverse types of intracellular cargo toward the plus-ends of microtubules within axons. Recent studies at the level of a single molecule have provided extensive knowledge on how kinesin-1 moves along microtubules. Further elucidation of kinesin-1 movement may shed light on its influence on axon generation, thereby leading to therapies for diseases such as spastic paraplegia type 10 (SPG10), the subject of this review. SPG10 is an autosomal dominant form of hereditary spastic paraplegia caused by mutations in KIF5A, which encodes one of the isoforms of kinesin-1 (KIF5A, KIF5B, and KIF5C). Although little is known about the cargo of KIF5A, a recent study revealed an axonal transport defect of mitochondria in a KIF5A (-/-) mouse model. This review discusses the consensus moving model of kinesin-1 and the pathogenicity of SPG10 caused by defective KIF5A function.

Intraflagellar transport delivers tubulin isotypes to sensory cilium middle and distal segments

Sensory cilia are assembled and maintained by kinesin-2-dependent intraflagellar transport (IFT). We investigated whether two Caenorhabditis elegans α- and β-tubulin isotypes, identified through mutants that lack their cilium distal segments, are delivered to their assembly sites by IFT. Mutations in conserved residues in both tubulins destabilize distal singlet microtubules. One isotype, TBB-4, assembles into microtubules at the tips of the axoneme core and distal segments, where the microtubule tip tracker EB1 is found, and localizes all along the cilium, whereas the other, TBA-5, concentrates in distal singlets. IFT assays, fluorescence recovery after photobleaching analysis and modelling indicate that the continual transport of sub-stoichiometric numbers of these tubulin subunits by the IFT machinery can maintain sensory cilia at their steady-state length.

Expression of kinesin superfamily genes in cultured hippocampal neurons

The nature of the different kinesin family members that function in a single, specific neuron type has not been systematically investigated. Here, we used quantitative real-time PCR to analyze the developmental expression patterns of kinesin family genes in cultured mouse hippocampal neurons, a highly homogeneous population of nerve cells. For purposes of comparison, we also determined the set of kinesins expressed in embryonic and adult hippocampal tissue. Twenty kinesins are expressed at moderate-to-high levels in mature hippocampal cultures. These include 9 plus-end directed kinesins from the Kinesin-1, -2, and -3 families that are known to mediate organelle transport and 6 other members of the Kinesin-3 and -4 families that are candidate organelle motors. Hippocampal cultures express high levels of a Kinesin-13, which regulates microtubule depolymerization, and moderate-to-high levels of Kinesin-9 and -14 family members, whose functions are not understood. Twelve additional kinesins, including 10 known mitotic kinesins, are expressed at moderate levels in embryonic hippocampus but at very low levels in mature cultures and the adult hippocampus. Collectively, our findings suggest that kinesins subserve diverse functions within a single type of neuron.

Kinesin's light chains inhibit the head- and microtubule-binding activity of its tail

Kinesin-1 is a microtubule-based motor comprising two heavy chains (KHCs) and two light chains (KLCs). Motor activity is precisely regulated to avoid futile ATP consumption and to ensure proper intracellular localization of kinesin-1 and its cargoes. The KHC tail inhibits ATPase activity by interacting with the enzymatic KHC heads, and the tail also binds microtubules. Here, we present a role for the KLCs in regulating both the head- and microtubule-binding activities of the kinesin-1 tail. We show that KLCs reduce the affinity of the head-tail interaction over tenfold and concomitantly repress the tail's regulatory activity. We also show that KLCs inhibit tail-microtubule binding by a separate mechanism. Inhibition of head-tail binding requires steric and electrostatic factors. Inhibition of tail-microtubule binding is largely electrostatic, pH dependent, and mediated partly by a highly negatively charged linker region between the KHC-interacting and cargo-binding domains of the KLCs. Our data support a model wherein KLCs promote activation of kinesin-1 for cargo transport by simultaneously suppressing tail-head and tail-microtubule interactions. KLC-mediated inhibition of tail-microtubule binding may also influence diffusional movement of kinesin-1 on microtubules, and kinesin-1's role in microtubule transport/sliding.

Growth of white matter in the adolescent brain: myelin or axon?

White matter occupies almost half of the human brain. It contains axons connecting spatially segregated modules and, as such, it is essential for the smooth flow of information in functional networks. Structural maturation of white matter continues during adolescence, as reflected in age-related changes in its volume, as well as in its microstructure. Here I review recent observations obtained with magnetic resonance imaging in typically developing adolescents and point out some of the known variations in structural properties of white matter vis-à-vis brain function in health and disease. I conclude by re-focusing the interpretations of MR-based studies of white matter from myelin to axon.

Stretch growth of integrated axon tracts: extremes and exploitations

Although virtually ignored in the literature until recently, the process of 'stretch growth of integrated axon tracts' is perhaps the most remarkable axon growth mechanism of all. This process can extend axons at seemingly impossible rates without the aid of chemical cues or even growth cones. As animals grow, the organization and extremely rapid expansion of the nervous system appears to be directed purely by mechanical forces on axon tracts. This review provides the first glimpse of the astonishing features of axon tracts undergoing stretch growth and how this natural process can be exploited to facilitate repair of the damaged nervous system.

Upregulation of CRABP1 in human neuroblastoma cells overproducing the Alzheimer-typical Abeta42 reduces their differentiation potential

BACKGROUND

Alzheimer's disease (AD) is characterized by neurodegeneration and changes in cellular processes, including neurogenesis. Proteolytic processing of the amyloid precursor protein (APP) plays a central role in AD. Owing to varying APP processing, several beta-amyloid peptides (Abeta) are generated. In contrast to the form with 40 amino acids (Abeta40), the variant with 42 amino acids (Abeta42) is thought to be the pathogenic form triggering the pathological cascade in AD. While total-Abeta effects have been studied extensively, little is known about specific genome-wide effects triggered by Abeta42 or Abeta40 derived from their direct precursor C99.

METHODS

A combined transcriptomics/proteomics analysis was performed to measure the effects of intracellularly generated Abeta peptides in human neuroblastoma cells. Data was validated by real-time polymerase chain reaction (real-time PCR) and a functional validation was carried out using RNA interference.

RESULTS

Here we studied the transcriptomic and proteomic responses to increased or decreased Abeta42 and Abeta40 levels generated in human neuroblastoma cells. Genome-wide expression profiles (Affymetrix) and proteomic approaches were combined to analyze the cellular response to the changed Abeta42- and Abeta40-levels. The cells responded to this challenge with significant changes in their expression pattern. We identified several dysregulated genes and proteins, but only the cellular retinoic acid binding protein 1 (CRABP1) was up-regulated exclusively in cells expressing an increased Abeta42/Abeta40 ratio. This consequently reduced all-trans retinoic acid (RA)-induced differentiation, validated by CRABP1 knock down, which led to recovery of the cellular response to RA treatment and cellular sprouting under physiological RA concentrations. Importantly, this effect was specific to the AD typical increase in the Abeta42/Abeta40 ratio, whereas a decreased ratio did not result in up-regulation of CRABP1.

CONCLUSION

We conclude that increasing the Abeta42/Abeta40 ratio up-regulates CRABP1, which in turn reduces the differentiation potential of the human neuroblastoma cell line SH-SY5Y, but increases cell proliferation. This work might contribute to the better understanding of AD neurogenesis, currently a controversial topic.

Creatine kinase B deficient neurons exhibit an increased fraction of motile mitochondria

BACKGROUND

Neurons require an elaborate system of intracellular transport to distribute cargo throughout axonal and dendritic projections. Active anterograde and retrograde transport of mitochondria serves in local energy distribution, but at the same time also requires input of ATP. Here we studied whether brain-type creatine kinase (CK-B), a key enzyme for high-energy phosphoryl transfer between ATP and CrP in brain, has an intermediary role in the reciprocal coordination between mitochondrial motility and energy distribution. Therefore, we analysed the impact of brain-type creatine kinase (CK-B) deficiency on transport activity and velocity of mitochondria in primary murine neurons and made a comparison to the fate of amyloid precursor protein (APP) cargo in these cells, using live cell imaging.

RESULTS

Comparison of average and maximum transport velocities and global transport activity showed that CK-B deficiency had no effect on speed of movement of mitochondria or APP cargo, but that the fraction of motile mitochondria was significantly increased by 36% in neurons derived from CK-B knockout mice. The percentage of motile APP vesicles was not altered.

CONCLUSION

CK-B activity does not directly couple to motor protein activity but cells without the enzyme increase the number of motile mitochondria, possibly as an adaptational strategy aimed to enhance mitochondrial distribution versatility in order to compensate for loss of efficiency in the cellular network for ATP distribution.

Intracellular Transport and Kinesin Superfamily Proteins, KIFs: Structure, Function, and Dynamics

Various molecular cell biology and molecular genetic approaches have indicated significant roles for kinesin superfamily proteins (KIFs) in intracellular transport and have shown that they are critical for cellular morphogenesis, functioning, and survival. KIFs not only transport various membrane organelles, protein complexes, and mRNAs for the maintenance of basic cellular activity, but also play significant roles for various mechanisms fundamental for life, such as brain wiring, higher brain functions such as memory and learning and activity-dependent neuronal survival during brain development, and for the determination of important developmental processes such as left-right asymmetry formation and suppression of tumorigenesis. Accumulating data have revealed a molecular mechanism of cargo recognition involving scaffolding or adaptor protein complexes. Intramolecular folding and phosphorylation also regulate the binding activity of motor proteins. New techniques using molecular biophysics, cryoelectron microscopy, and X-ray crystallography have detected structural changes in motor proteins, synchronized with ATP hydrolysis cycles, leading to the development of independent models of monomer and dimer motors for processive movement along microtubules.

Effect of spastic paraplegia mutations in KIF5A kinesin on transport activity

Hereditary spastic paraplegia (HSP) is a neurodegenerative disease caused by motoneuron degeneration. It is linked to at least 30 loci, among them SPG10, which causes dominant forms and originates in point mutations in the neuronal Kinesin-1 gene (KIF5A). Here, we investigate the motility of KIF5A and four HSP mutants. All mutations are single amino-acid exchanges and located in kinesin's motor or neck domain. The mutation in the neck (A361V) did not change the gliding properties in vitro, the others either reduced microtubule affinity or gliding velocity or both. In laser-trapping assays, none of the mutants moved more than a few steps along microtubules. Motility assays with mixtures of homodimeric wild-type, homodimeric mutant and heterodimeric wild-type/mutant motors revealed that only one mutant (N256S) reduces the gliding velocity at ratios present in heterozygous patients, whereas the others (K253N, R280C) do not. Attached to quantum dots as artificial cargo, mixtures involving N256S mutants produced slower cargo populations lagging behind in transport, whereas mixtures with the other mutants led to populations of quantum dots that rarely bound to microtubules. These differences indicate that the dominant inheritance of SPG10 is caused by two different mechanisms that both reduce the gross cargo flux, leading to deficient supply of the synapse.

Process outgrowth in oligodendrocytes is mediated by CNP, a novel microtubule assembly myelin protein

Oligodendrocytes (OLs) extend arborized processes that are supported by microtubules (MTs) and microfilaments. Little is known about proteins that modulate and interact with the cytoskeleton during myelination. Several lines of evidence suggest a role for 2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNP) in mediating process formation in OLs. In this study, we report that tubulin is a major CNP-interacting protein. In vitro, CNP binds preferentially to tubulin heterodimers compared with MTs and induces MT assembly by copolymerizing with tubulin. CNP overexpression induces dramatic morphology changes in both glial and nonglial cells, resulting in MT and F-actin reorganization and formation of branched processes. These morphological effects are attributed to CNP MT assembly activity; branched process formation is either substantially reduced or abolished with the expression of loss-of-function mutants. Accordingly, cultured OLs from CNP-deficient mice extend smaller outgrowths with less arborized processes. We propose that CNP is an important component of the cytoskeletal machinery that directs process outgrowth in OLs.

Role of neuronal activity and kinesin on tract tracing by manganese-enhanced MRI (MEMRI)

MEMRI offers the exciting possibility of tracing neuronal circuits in living animals by MRI. Here we use the power of mouse genetics and the simplicity of the visual system to test rigorously the parameters affecting Mn2+ uptake, transport and trans-synaptic tracing. By measuring electrical response to light before and after injection of Mn2+ into the eye, we determine the dose of Mn2+ with the least toxicity that can still be imaged by MR at 11.7 T. Using mice with genetic retinal blindness, we discover that electrical activity is not necessary for uptake and transport of Mn2+ in the optic nerve but is required for trans-synaptic transmission of this tracer to distal neurons in this pathway. Finally, using a kinesin light chain 1 knockout mouse, we find that conventional kinesin is a participant but not essential to neuronal transport of Mn2+ in the optic tract. This work provides a molecular and physiological framework for interpreting data acquired by MEMRI of circuitry in the brain.

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.

The genetics of axonal transport and axonal transport disorders

Neurons are specialized cells with a complex architecture that includes elaborate dendritic branches and a long, narrow axon that extends from the cell body to the synaptic terminal. The organized transport of essential biological materials throughout the neuron is required to support its growth, function, and viability. In this review, we focus on insights that have emerged from the genetic analysis of long-distance axonal transport between the cell body and the synaptic terminal. We also discuss recent genetic evidence that supports the hypothesis that disruptions in axonal transport may cause or dramatically contribute to neurodegenerative diseases.

A peptide zipcode sufficient for anterograde transport within amyloid precursor protein

Fast anterograde transport of membrane-bound organelles delivers molecules synthesized in the neuronal cell body outward to distant synapses. Identification of the molecular "zipcodes" on organelles that mediate attachment and activation of microtubule-based motors for this directed transport is a major area of inquiry. Here we identify a short peptide sequence (15 aa) from the cytoplasmic C terminus of amyloid precursor protein (APP-C) sufficient to mediate the anterograde transport of peptide-conjugated beads in the squid giant axon. APP-C beads travel at fast axonal transport rates (0.53 mum/s average velocity, 0.9 mum/s maximal velocity) whereas beads coupled to other peptides coinjected into the same axon remain stationary at the injection site. This transport appears physiologic, because it mimics behavior of endogenous squid organelles and of beads conjugated to C99, a polypeptide containing the full-length cytoplasmic domain of amyloid precursor protein (APP). Beads conjugated to APP lacking the APP-C domain are not transported. Coinjection of APP-C peptide reduces C99 bead motility by 75% and abolishes APP-C bead motility, suggesting that the soluble peptide competes with protein-conjugated beads for axoplasmic motor(s). The APP-C domain is conserved (13/15 aa) from squid to human, and peptides from either squid or human APP behave similarly. Thus, we have identified a conserved peptide zipcode sufficient to direct anterograde transport of exogenous cargo and suggest that one of APP's roles may be to recruit and activate axonal machinery for endogenous cargo transport.

Dynamics of outgrowth in a continuum model of neurite elongation

Neurite outgrowth (dendrites and axons) should be a stable, but easily regulated process to enable a neuron to make its appropriate network connections during development. We explore the dynamics of outgrowth in a mathematical continuum model of neurite elongation. The model describes the construction of the internal microtubule cytoskeleton, which results from the production and transport of tubulin dimers and their assembly into microtubules at the growing neurite tip. Tubulin is assumed to be largely synthesised in the cell body from where it is transported by active mechanisms and by diffusion along the neurite. It is argued that this construction process is a fundamental limiting factor in neurite elongation. In the model, elongation is highly stable when tubulin transport is dominated by either active transport or diffusion, but oscillations in length may occur when both active transport and diffusion contribute. Autoregulation of tubulin production can eliminate these oscillations. In all cases a stable steady-state length is reached, provided there is intrinsic decay of tubulin. Small changes in growth parameters, such as the tubulin production rate, can lead to large changes in length. Thus cytoskeleton construction can be both stable and easily regulated, as seems necessary for neurite outgrowth during nervous system development.

Neurofilament polymer transport in axons

Neurofilament proteins are known to be transported along axons by slow axonal transport, but the form in which they move is controversial. In previous studies on cultured rat sympathetic neurons, we found that green fluorescent protein-tagged neurofilament proteins move predominantly in the form of filamentous structures, and we proposed that these structures are single-neurofilament polymers. In the present study, we have tested this hypothesis by using a rapid perfusion technique to capture these structures as they move through naturally occurring gaps in the axonal neurofilament array. Because the gaps lack neurofilaments, they permit unambiguous identification of the captured structure. Using quantitative immunofluorescence microscopy and correlative light and electron microscopy, we show that the captured structures are single continuous neurofilament polymers. Thus, neurofilament polymers are one of the cargo structures of slow axonal transport.

Neurofilaments can undergo axonal transport and cytoskeletal incorporation in a discontinuous manner

Neurofilaments (NFs) are thought to provide structural support for axons. Some NFs exhibit an extended residence time along axons, the nature of which remains unclear. In prior studies in NB2a/d1 cells, hypophosphorylated NFs were demonstrated to be dispersed throughout the axon and to undergo relatively rapid axonal transport, while extensively phosphorylated NFs organized into a "bundle" localized along the center of the axon. It was not conclusively determined whether bundled NFs underwent transport or instead underwent turnover via exchange with transporting individual NFs. Herein, using transfection with multiple constructs and regional photobleaching, we demonstrate that bundled NFs undergo relatively slow transport as well as exchange with surrounding individual NFs. We also demonstrate that newly synthesized NFs disperse nonhomogenously throughout axonal neurites and perikarya. These findings provide a mechanism by which some NFs exhibit extended residence time within axons, which lessens the metabolic burden of cytoskeletal turnover.

Linking molecular motors to Alzheimer's disease

It is currently thought that Alzheimer's disease develops due to aberrant generation of amyloid-beta peptides. However, the mechanisms underlying the aberrant generation of amyloid-beta peptides remain unknown. An emerging concept suggests that impaired axonal transport may play a pivotal role in the aberrant generation of amyloid-beta peptides. Here we review and discuss advances in understanding AD with the primary focus on the possible role of molecular motors and axonal transport in its pathogenesis.

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.

Cooperative cargo transport by several molecular motors

The transport of cargo particles that are pulled by several molecular motors in a cooperative manner is studied theoretically in this article. The transport properties depend primarily on the maximal number N of motor molecules that may pull simultaneously on the cargo particle. Because each motor must unbind from the filament after a finite number of steps but can also rebind to it again, the actual number of pulling motors is not constant but varies with time between zero and N. An increase in the maximal number N leads to a strong increase of the average walking distance (or run length) of the cargo particle. If the cargo is pulled by up to N kinesin motors, for example, the walking distance is estimated to be 5(N-1)/N micrometers, which implies that seven or eight kinesin molecules are sufficient to attain an average walking distance in the centimeter range. If the cargo particle is pulled against an external load force, this force is shared between the motors, which provides a nontrivial motor-motor coupling and a generic mechanism for nonlinear force-velocity relationships. With increasing load force, the probability distribution of the instantaneous velocity is shifted toward smaller values, becomes broader, and develops several peaks. Our theory is consistent with available experimental data and makes quantitative predictions that are accessible to systematic in vitro experiments.

Neurofilament transport is dependent on actin and myosin

Real-time analyses have revealed that some newly synthesized neurofilament (NF) subunits translocate into and along axonal neurites by moving along the inner plasma membrane surface, suggesting that they may translocate against the submembrane actin cortex. We therefore examined whether or not NF axonal transport was dependent on actin and myosin. Perturbation of filamentous actin in NB2a/d1 cells with cytochalasin B inhibited translocation of subunits into axonal neurites and inhibited bidirectional translocation of NF subunits within neurites. Intravitreal injection of cytochalasin B inhibited NF axonal transport in optic axons in a dose-response manner. NF subunits were coprecipitated from NB2a/d1 cells by an anti-myosin antibody, and myosin colocalized with NFs in immunofluorescent analyses. The myosin light chain kinase inhibitor ML-7 and the myosin ATPase inhibitor 2,3-butanedione-2-monoxime perturbed NF translocation within NB2a/d1 axonal neurites. These findings suggest that some NF subunits may undergo axonal transport via myosin-mediated interactions with the actin cortex.

Tubulin and CRMP-2 complex is transported via Kinesin-1

The transport of tubulin and microtubules in a growing axon is essential for axonal growth and maintenance. However, the molecular mechanism underlying the linkage of tubulin and microtubules to motor proteins is not yet clear. Collapsin response mediator protein-2 (CRMP-2) is enriched at the distal part of growing axons in primary hippocampal neurons and plays a critical role in axon differentiation through its interaction with tubulin dimer and Numb. In this study, we show that CRMP-2 regulates tubulin transport by linking tubulin and Kinesin-1. The C-terminal region of CRMP-2 directly binds to the tetratricopeptide repeat domain of kinesin light chain 1 (KLC1). Soluble tubulin binds to the middle of CRMP-2 and forms a trimeric complex with CRMP-2/KLC1. Furthermore, the movement of GFP-tubulin in the photobleached area is weakened by knockdown of KLCs or CRMP-2. These results indicate that the CRMP-2/Kinesin-1 complex regulates soluble tubulin transport to the distal part of the growing axon.

Extreme stretch growth of integrated axons

Large animals can undergo enormous growth during development, suggesting that axons in nerves and white matter tracts rapidly expand as well. Because integrated axons have no growth cones to extend from, it has been postulated that mechanical forces may stimulate axon elongation matching the growth of the animal. However, this distinct form of rapid and sustained growth of integrated axons has never been demonstrated. Here, we used a microstepper motor system to evaluate the effects of escalating rates of stretch on integrated axon tracts over days to weeks in culture. We found that axon tracts could be stretch grown at rates of 8 mm/d and reach lengths of 10 cm without disconnection. Despite dynamic and long-term elongation, stretched axons increased in caliber by 35%, while the morphology and density of cytoskeletal constituents and organelles were maintained. These data provide the first evidence that mechanical stimuli can induce extreme "stretch growth" of integrated axon tracts, far exceeding any previously observed limits of axon growth.

The progressive nature of Wallerian degeneration in wild-type and slow Wallerian degeneration (WldS) nerves

BACKGROUND

The progressive nature of Wallerian degeneration has long been controversial. Conflicting reports that distal stumps of injured axons degenerate anterogradely, retrogradely, or simultaneously are based on statistical observations at discontinuous locations within the nerve, without observing any single axon at two distant points. As axon degeneration is asynchronous, there are clear advantages to longitudinal studies of individual degenerating axons. We recently validated the study of Wallerian degeneration using yellow fluorescent protein (YFP) in a small, representative population of axons, which greatly improves longitudinal imaging. Here, we apply this method to study the progressive nature of Wallerian degeneration in both wild-type and slow Wallerian degeneration (WldS) mutant mice.

RESULTS

In wild-type nerves, we directly observed partially fragmented axons (average 5.3%) among a majority of fully intact or degenerated axons 37-42 h after transection and 40-44 h after crush injury. Axons exist in this state only transiently, probably for less than one hour. Surprisingly, axons degenerated anterogradely after transection but retrogradely after a crush, but in both cases a sharp boundary separated intact and fragmented regions of individual axons, indicating that Wallerian degeneration progresses as a wave sequentially affecting adjacent regions of the axon. In contrast, most or all WldS axons were partially fragmented 15-25 days after nerve lesion, WldS axons degenerated anterogradely independent of lesion type, and signs of degeneration increased gradually along the nerve instead of abruptly. Furthermore, the first signs of degeneration were short constrictions, not complete breaks.

CONCLUSIONS

We conclude that Wallerian degeneration progresses rapidly along individual wild-type axons after a heterogeneous latent phase. The speed of progression and its ability to travel in either direction challenges earlier models in which clearance of trophic or regulatory factors by axonal transport triggers degeneration. WldS axons, once they finally degenerate, do so by a fundamentally different mechanism, indicated by differences in the rate, direction and abruptness of progression, and by different early morphological signs of degeneration. These observations suggest that WldS axons undergo a slow anterograde decay as axonal components are gradually depleted, and do not simply follow the degeneration pathway of wild-type axons at a slower rate.

Two-photon fluorescent microlithography for live-cell imaging

Fluorescent dyes added to UV-cure resins allow the rapid fabrication of fluorescent micropatterns on standard glass coverslips by two-photon optical lithography. We use this lithographic method to tailor fiduciary markers, focal references, and calibration tools, for fluorescence and laser scanning microscopy. Fluorescent microlithography provides spatial landmarks to quantify molecular transport, cell growth and migration, and to compensate for focal drift during time-lapse imaging. We show that the fluorescent patterned microstructures are biocompatible with cultures of mammalian cell lines and hippocampal neurons. Furthermore, the high-relief topology of the lithographed substrates is utilized as a mold for poly(dimethylsiloxane) stamps to create protein patterns by microcontact printing, representing an alternative to the current etching techniques. We present two different applications of such protein patterns for localizing cell adhesion and guidance of neurite outgrowth.

Transport limited effects in a model of dendritic branching

A variety of stochastic models of dendritic growth in developing neurons have been formulated previously. Such models indicate that the probability of a new branch forming in a growing tree may be modulated by factors such as the number of terminals in the tree and their centrifugal order. However, these models cannot identify any underlying biophysical mechanisms that may cause such dependencies. Here, we explore a new model in which branching depends on the concentration of a branch-determining substance in each terminal segment. The substance is produced in the cell body and is transported by active transport and diffusion to the terminals. The model reveals that transport-limited effects may give rise to the same modulation of branching as indicated by the stochastic models. Different limitations arise if transport is dominated by active transport or by diffusion.

Neural system-enriched gene expression: relationship to biological pathways and neurological diseases

To understand the commitment of the genome to nervous system differentiation and function, we sought to compare nervous system gene expression to that of a wide variety of other tissues by gene expression database construction and mining. Gene expression profiles of 10 different adult nervous tissues were compared with that of 72 other tissues. Using ANOVA, we identified 1,361 genes whose expression was higher in the nervous system than other organs and, separately, 600 genes whose expression was at least threefold higher in one or more regions of the nervous system compared with their median expression across all organs. Of the 600 genes, 381 overlapped with the 1,361-gene list. Limited in situ gene expression analysis confirmed that identified genes did represent nervous system-enriched gene expression, and we therefore sought to evaluate the validity and significance of these top-ranked nervous system genes using known gene literature and gene ontology categorization criteria. Diverse functional categories were present in the 381 genes, including genes involved in intracellular signaling, cytoskeleton structure and function, enzymes, RNA metabolism and transcription, membrane proteins, as well as cell differentiation, death, proliferation, and division. We searched existing public sites and identified 110 known genes related to mental retardation, neurological disease, and neurodegeneration. Twenty-one of the 381 genes were within the 110-gene list, compared with a random expectation of 5. This suggests that the 381 genes provide a candidate set for further analyses in neurological and psychiatric disease studies and that as a field, we are as yet, far from a large-scale understanding of the genes that are critical for nervous system structure and function. Together, our data indicate the power of profiling an individual biologic system in a multisystem context to gain insight into the genomic basis of its structure and function.

Quantitative Analysis of Microtubule Transport in Growing Nerve Processes

In neurons, tubulin is synthesized primarily in the cell body, whereas the molecular machinery for neurite extension and elaboration of microtubule (MT) array is localized to the growth cone region. This unique functional and biochemical compartmentalization of neuronal cells requires transport mechanisms for the delivery of newly synthesized tubulin and other cytoplasmic components from the cell body to the growing axon. According to the polymer transport model, tubulin is transported along the axon as a polymer. Because the majority of axonal MTs are stationary at any given moment, it has been assumed that only a small fraction of MTs translocates along the axon by saltatory movement reminiscent of the fast axonal transport. Such intermittent "stop and go" MT transport has been difficult to detect or to exclude by using direct video microscopy methods. In this study, we measured the translocation of MT plus ends in the axonal shaft by expressing GFP-EB1 in Xenopus embryo neurons in culture. Formal quantitative analysis of MT assembly/disassembly indicated that none of the MTs in the axonal shaft were rapidly transported. Our results suggest that transport of axonal MTs is not required for delivery of newly synthesized tubulin to the growing nerve processes.

Myosin-dependent transport in neurons

Axonal transport in neurons has been shown to be microtubule dependent, driven by the molecular motor proteins kinesin and dynein. However, organelles undergoing fast transport can often pause or rapidly change directions without apparent dissociation from their transport tracks. Cytoskeletal polymers such as neurofilaments and microtubules have also been shown to make infrequent but rapid movements in axons indicating that their transport is likely to involve molecular motors. In addition, neurons have multiple compartments that are devoid of microtubules where transport of organelles is still seen to occur. These areas are rich in other cytoskeletal polymers such as actin filaments. Transported organelles have been shown to associate with multiple motor proteins including myosins. This suggests that nonmicrotubule-based transport may be myosin driven. In this review we will focus our attention on myosin motors known to be present in neurons and evaluate the evidence that they contribute to transport or other functions in the different compartments of the neuron.

Axonal transport of human α-synuclein slows with aging but is not affected by familial Parkinson's disease-linked mutations

Biochemical and genetic abnormalities of alpha-synuclein (alpha-Syn) are implicated in the pathogenesis of Parkinson's disease (PD) and other alpha-synucleinopathies. The abnormal intraneuronal accumulations of alpha-Syn in Lewy bodies (LBs) and Lewy neurites (LNs) have implicated defects in axonal transport of alpha-Syn in the alpha-synucleinopathies. Using human (Hu) alpha-Syn transgenic (Tg) mice, we have examined whether familial PD (FPD)-linked mutations (A30P and A53T) alter axonal transport of Hualpha-Syn. Our studies using peripheral nerves show that Hualpha-Syn and Moalpha-Syn are almost exclusively transported in the slow component (SC) of axonal transport and that the FPD-linked alpha-Syn mutations do not have obvious effects on the axonal transport of alpha-Syn. Moreover, older pre-symptomatic A53T Hualpha-Syn Tg mice do not show gross alterations in the axonal transport of alpha-Syn and other proteins in the SC, indicating that the early stages of alpha-synucleinopathy in A53T alpha-Syn Tg mice are not associated with gross alterations in the slow axonal transport. However, the axonal transport of alpha-Syn slows significantly with aging. Because the rate of axonal transport affects the stability and accumulation of proteins in axons, age-dependent-slowing alpha-Syn is a likely contributor to axonal aggregation of alpha-Syn in alpha-synucleinopathy.

The neurofilament middle molecular mass subunit carboxyl-terminal tail domains is essential for the radial growth and cytoskeletal architecture of axons but not for regulating neurofilament transport rate

The phosphorylated carboxyl-terminal "tail" domains of the neurofilament (NF) subunits, NF heavy (NF-H) and NF medium (NF-M) subunits, have been proposed to regulate axon radial growth, neurofilament spacing, and neurofilament transport rate, but direct in vivo evidence is lacking. Because deletion of the tail domain of NF-H did not alter these axonal properties (Rao, M.V., M.L. Garcia, Y. Miyazaki, T. Gotow, A. Yuan, S. Mattina, C.M. Ward, N.S. Calcutt, Y. Uchiyama, R.A. Nixon, and D.W. Cleveland. 2002. J. Cell Biol. 158:681-693), we investigated possible functions of the NF-M tail domain by constructing NF-M tail-deleted (NF-MtailDelta) mutant mice using an embryonic stem cell-mediated "gene knockin" approach that preserves normal ratios of the three neurofilament subunits. Mutant NF-MtailDelta mice exhibited severely inhibited radial growth of both motor and sensory axons. Caliber reduction was accompanied by reduced spacing between neurofilaments and loss of long cross-bridges with no change in neurofilament protein content. These observations define distinctive functions of the NF-M tail in regulating axon caliber by modulating the organization of the neurofilament network within axons. Surprisingly, the average rate of axonal transport of neurofilaments was unaltered despite these substantial effects on axon morphology. These results demonstrate that NF-M tail-mediated interactions of neurofilaments, independent of NF transport rate, are critical determinants of the size and cytoskeletal architecture of axons, and are mediated, in part, by the highly phosphorylated tail domain of NF-M.

Neurofilament transport in vivo minimally requires hetero-oligomer formation

Neurofilament assembly requires at minimum the polymerization of neurofilament light chain (NF-L) with either neurofilament medium chain (NF-M) or neurofilament heavy chain (NF-H) subunits, but requirements for their axonal transport have long been controversial. Using a gene deletion approach, we generated mice containing only NF-L or NF-M. In vivo pulse radiolabeling analyses in retinal ganglion cell neurons revealed that NF-L alone is incapable of efficient transport, whereas nearly one-half of the normal level of NF-M is transported along optic axons in the absence of the other triplet subunits. Under these conditions, however, NF-M transport is completely abolished by deleting alpha-internexin. Our results strongly suggest that efficient neurofilament protein transport in vivo minimally requires hetero-oligomer formation. They also show that NF-M can partner with intermediate filament proteins other than the NF-H and NF-L subunits in neurons to support slow transport and possibly other functions of neuronal intermediate filaments.

Defective neurofilament transport in mouse models of amyotrophic lateral sclerosis: a review

Neurofilament proteins synthesized in the cell body of neurons are assembled and transported into axons, where they influence axon radial growth, axonal transport, and nerve conduction velocities. In diseased states, neurofilaments accumulate in cell bodies and proximal axons of affected neurons, and these lesions are characteristic of many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), spinal muscular atrophy (SMA), Charcot-Marie-Tooth disease type 2 (CMT2), and hereditary sensory motor neuropathy. Although the molecular mechanisms that contribute to these accumulations are not yet identified, transgenic mouse models are beginning to provide insight into the role of neurofilament transport in disease-related dysfunction of neurons. This review addresses axonal transport in mouse models of ALS and the special significance of neurofilament transport in this disease.

Mutations in dynein link motor neuron degeneration to defects in retrograde transport

Degenerative disorders of motor neurons include a range of progressive fatal diseases such as amyotrophic lateral sclerosis (ALS), spinal-bulbar muscular atrophy (SBMA), and spinal muscular atrophy (SMA). Although the causative genetic alterations are known for some cases, the molecular basis of many SMA and SBMA-like syndromes and most ALS cases is unknown. Here we show that missense point mutations in the cytoplasmic dynein heavy chain result in progressive motor neuron degeneration in heterozygous mice, and in homozygotes this is accompanied by the formation of Lewy-like inclusion bodies, thus resembling key features of human pathology. These mutations exclusively perturb neuron-specific functions of dynein.

Sensory neuron proteins interact with the intracellular domains of sodium channel NaV1.8

Voltage-gated sodium channels initiate and propagate action potentials in excitable cells. The tetrodotoxin-resistant Na(+) channel (Na(V)1.8/SNS) is expressed in damage-sensing neurons (nociceptors) and plays an important role in pain pathways. Expression of high levels of functional Na(V)1.8 in heterologous cells has proved problematic, even in the presence of known sodium channel accessory beta-subunits. This suggests that other regulatory proteins are required for normal levels of Na(V)1.8 expression. Here we report the use of a yeast two-hybrid system and a rat dorsal root ganglion cDNA library to identify 28 different clones encoding proteins which interact with intracellular domains of Na(V)1.8. Many clones are expressed at high levels in small diameter DRG neurons as judged by in situ hybridization. Interacting proteins include cytoplasmic elements and linker proteins (e.g. beta-actin and moesin), enzymes (e.g. inositol polyphosphate 5-phosphatase and TAO2 thousand and one protein kinase), channels and membrane-associated proteins (voltage-dependent anion channel VDAC3V and tetraspanin), as well as motor proteins (dynein intermediate and light chain) and transcripts encoding previously undescribed proteins. Immunoprecipitation (pull-down) assays confirm that some of the proteins interact with, and may hence regulate, Na(V)1.8 in vivo.