Axonal transport of class II and III beta-tubulin: evidence that the slow component wave represents the movement of only a small fraction of the tubulin in mature motor axons

Adrenomedullin, a Novel Target for Neurodegenerative Diseases

Neurodegenerative diseases represent a heterogeneous group of disorders whose common characteristic is the progressive degeneration of neuronal structure and function. Although much knowledge has been accumulated on the pathophysiology of neurodegenerative diseases over the years, more efforts are needed to understand the processes that underlie these diseases and hence to propose new treatments. Adrenomedullin (AM) is a multifunctional peptide involved in vasodilation, hormone secretion, antimicrobial defense, cellular growth, and angiogenesis. In neurons, AM and related peptides are associated with some structural and functional cytoskeletal proteins that interfere with microtubule dynamics. Furthermore, AM may intervene in neuronal dysfunction through other mechanisms such as immune and inflammatory response, apoptosis, or calcium dyshomeostasis. Alterations in AM expression have been described in neurodegenerative processes such as Alzheimer's disease or vascular dementia. This review addresses the current state of knowledge on AM and its possible implication in neurodegenerative diseases.

Immunohistochemical Markers for Prospective Studies in Neurofibromatosis-1 Porcine Models

Neurofibromatosis type 1 (NF1) is a common, cancer-predisposing disease caused by mutations in the NF1 tumor gene. Patients with NF1 have an increased risk for benign and malignant tumors of the nervous system (e.g., neurofibromas, malignant peripheral nerve sheath tumors, gliomas) and other tissues (e.g., leukemias, rhabdomyosarcoma, etc.) as well as increased susceptibility to learning disabilities, chronic pain/migraines, hypertension, pigmentary changes, and developmental lesions (e.g., tibial pseudoarthrosis). Pigs are an attractive and upcoming animal model for future NF1 studies, but a potential limitation to porcine model research has been the lack of validated reagents for direct translational study to humans. To address that issue, we used formalin-fixed tissues (human and pigs) to evaluate select immunohistochemical markers (activated caspase-3, allograft inflammatory factor-1, beta-tubulin III, calbindin D, CD13, CD20, desmin, epithelial membrane antigen, glial fibrillary acidic protein, glucose transporter-1, laminin, myelin basic protein, myoglobin, proliferating cell nuclear antigen, S100, vimentin, and von Willebrand factor). The markers were validated by comparing known expression and localization in human and pig tissues. Validation of these markers on fixed tissues will facilitate prospective immunohistochemical studies of NF1 pigs, as well as other pig models, in a more efficient, reproducible, and translationally relevant manner.

Neurofilaments and Neurofilament Proteins in Health and Disease

SUMMARYNeurofilaments (NFs) are unique among tissue-specific classes of intermediate filaments (IFs) in being heteropolymers composed of four subunits (NF-L [neurofilament light]; NF-M [neurofilament middle]; NF-H [neurofilament heavy]; and α-internexin or peripherin), each having different domain structures and functions. Here, we review how NFs provide structural support for the highly asymmetric geometries of neurons and, especially, for the marked radial expansion of myelinated axons crucial for effective nerve conduction velocity. NFs in axons extensively cross-bridge and interconnect with other non-IF components of the cytoskeleton, including microtubules, actin filaments, and other fibrous cytoskeletal elements, to establish a regionally specialized network that undergoes exceptionally slow local turnover and serves as a docking platform to organize other organelles and proteins. We also discuss how a small pool of oligomeric and short filamentous precursors in the slow phase of axonal transport maintains this network. A complex pattern of phosphorylation and dephosphorylation events on each subunit modulates filament assembly, turnover, and organization within the axonal cytoskeleton. Multiple factors, and especially turnover rate, determine the size of the network, which can vary substantially along the axon. NF gene mutations cause several neuroaxonal disorders characterized by disrupted subunit assembly and NF aggregation. Additional NF alterations are associated with varied neuropsychiatric disorders. New evidence that subunits of NFs exist within postsynaptic terminal boutons and influence neurotransmission suggests how NF proteins might contribute to normal synaptic function and neuropsychiatric disease states.

Review : The Synthesis, Axonal Transport, and Phosphorylation of Neurofilaments Determine Axonal Caliber in Myelinated Nerve Fibers

Axonal diameter is the principal determinant of conduction velocity in myelinated nerve fibers, but, until recently, the factors that determine axonal diameter have not been understood. Recent studies indicate that neurofilaments (NFs), the principal intermediate (10-nm) filaments in neurons, are major intrinsic determinants of axonal caliber and have begun to elucidate the molecular mechanisms involved in NF deployment. The radial growth of myelinated axons during postnatal development reflects an increase in axonal NF content. The NF proteins are synthesized in the neuron cell body (soma) and transported somatofugally along axons in the slow component of axonal transport. The level of NF expression de termines the amount of NF protein transported in the slow component. In neurons with large axons, NF expression rises dramatically during neonatal development and is maintained at high steady-state levels as the NF content of axons increases during postnatal development. This increase in NF content, after NF expression has reached steady-state levels, appears to result from a progressive decline in the ve locity of NF transport, with increasing distance along nerve fibers (i.e., NFs enter a region of the axon faster than they leave). Phosphorylation of the NF proteins may regulate the spacing of axonal NFs and the velocity of NF transport. The Neuroscientist 1:76-83, 1995

Substantial Differentiation of Human Neural Stem Cells Into Motor Neurons on a Biomimetic Polyurea

To find the first restorative treatment for spinal cord injury (SCI), researchers have focused on stem cell therapies. However, one obstacle is the lack of an implantable cell scaffold that can support efficient motor neuron (MN) differentiation and proliferation. We aimed to overcome this through the use of an RGD functionalized novel biomimetic polyurea, optimized to encourage efficient differentiation of MNs. Images taken after 14-days showed increased differentiation (∼40%) of hNSCs into MNs as well as increased cell count on the biomimetic polymer compared to PDL-Laminin coating, indicating that the RGD-polyurea provides a favorable microenvironment for hNSC survival, having promising implications for future SCI therapies.

Bilateral nerve alterations in a unilateral experimental neurotrophic keratopathy model: a lateral conjunctival approach for trigeminal axotomy

To study bilateral nerve changes in a newly developed novel mouse model for neurotrophic keratopathy by approaching the trigeminal nerve from the lateral fornix. Surgical axotomy of the ciliary nerve of the trigeminal nerve was performed in adult BALB/c mice at the posterior sclera. Axotomized, contralateral, and sham-treated corneas were excised on post-operative days 1, 3, 5, 7 and 14 and immunofluorescence histochemistry was performed with anti-β-tubulin antibody to evaluate corneal nerve density. Blink reflex was evaluated using a nylon thread. The survival rate was 100% with minimal bleeding during axotomy and a surgical time of 8±0.5 minutes. The blink reflex was diminished at day 1 after axotomy, but remained intact in the contralateral eyes in all mice. The central and peripheral subbasal nerves were not detectable in the axotomized cornea at day 1 (p<0.001), compared to normal eyes (101.3±14.8 and 69.7±12.0 mm/mm² centrally and peripherally). Interestingly, the subbasal nerve density in the contralateral non-surgical eyes also decreased significantly to 62.4±2.8 mm/mm² in the center from day 1 (p<0.001), but did not change in the periphery (77.3±11.7 mm/mm², P = 0.819). Our novel trigeminal axotomy mouse model is highly effective, less invasive, rapid, and has a high survival rate, demonstrating immediate loss of subbasal nerves in axotomized eyes and decreased subbasal nerves in contralateral eyes after unilateral axotomy. This model will allow investigating the effects of corneal nerve damage and serves as a new model for neurotrophic keratopathy.

A conditioning lesion induces changes in gene expression and axonal transport that enhance regeneration by increasing the intrinsic growth state of axons

Injury of axons in the peripheral nervous system (PNS) induces transcription-dependent changes in gene expression and axonal transport that promote effective regeneration by increasing the intrinsic growth state of axons. Regeneration is enhanced in axons re-injured 1-2 weeks after the intrinsic growth state has been increased by such a prior conditioning lesion (CL). The intrinsic growth state does not increase after axons are injured in the mammalian central nervous system (CNS), where they lack the capacity for effective regeneration. Sensory neurons in the dorsal root ganglion (DRG) have two axonal branches that respond differently to injury. Peripheral branches, which are located entirely in the PNS, are capable of effective regeneration. Central branches regenerate in the PNS (i.e., in the dorsal root, which extends from the DRG to the spinal cord), but not in the CNS (i.e., the spinal cord). A CL of peripheral branches increases the intrinsic growth state of central branches in the dorsal columns of the spinal cord, enabling these axons to undergo lengthy regeneration in a segment of peripheral nerve transplanted into the spinal cord (i.e., a peripheral nerve graft). This regeneration does not occur in the absence of a CL. We will examine how changes in gene expression and axonal transport induced by a CL may promote this regeneration.

AS601245, a c-Jun NH2-terminal kinase (JNK) inhibitor, reduces axon/dendrite damage and cognitive deficits after global cerebral ischaemia in gerbils

BACKGROUND AND PURPOSE

Based on their proven ability, in animal models of stroke, to reduce damage to brain grey matter, many drugs have been tested in clinical trials but without success. Failure to save axons from injury and to protect functional outcome has been proposed as the major reason for this lack of success. We have previously demonstrated in two rodent models of cerebral ischaemia, that AS601245 (1,3-benzothiazol-2-yl (2-([2-(3-pyridinyl) ethyl] amino)-4 pyrimidinyl) acetonitrile), an inhibitor of the c-Jun NH(2)-terminal kinase (JNK), has neuroprotective properties. The aim of the present study was to further investigate if AS601245 in addition to its ability to protect neurons also could protect neurites and preserve memory after cerebral ischaemia, in gerbils.

EXPERIMENTAL APPROACH

Using immunohistochemical techniques and a behavioural test, we studied the effect of the compound AS601245 on neurodegeneration and cognitive deficits after global cerebral ischaemia in gerbils.

KEY RESULTS

At a dose of 80 mg kg(-1), i.p., AS601245 reduced damage to neurites by 67% (P<0.001 versus controls) and activation of astrocytes by 84% (P<0.001 versus controls). In addition, AS601245 (80 mg kg(-1), i.p.) prevented ischaemia-induced impairment of memory in the inhibitory avoidance task model.

CONCLUSIONS AND IMPLICATIONS

The present results suggest that AS601245 reduced damage to neurites and decreased astrogliosis following global ischaemia and also improved long-term memory, supporting JNK inhibition as a promising therapeutic strategy for ischaemic insults to the CNS.

Progressive motor neuronopathy: a critical role of the tubulin chaperone TBCE in axonal tubulin routing from the Golgi apparatus

Axonal degeneration represents one of the earliest pathological features in motor neuron diseases. We here studied the underlying molecular mechanisms in progressive motor neuronopathy (pmn) mice mutated in the tubulin-specific chaperone TBCE. We demonstrate that TBCE is a peripheral membrane-associated protein that accumulates at the Golgi apparatus. In pmn mice, TBCE is destabilized and disappears from the Golgi apparatus of motor neurons, and microtubules are lost in distal axons. The axonal microtubule loss proceeds retrogradely in parallel with the axonal dying back process. These degenerative changes are inhibited in a dose-dependent manner by transgenic TBCE complementation that restores TBCE expression at the Golgi apparatus. In cultured motor neurons, the pmn mutation, interference RNA-mediated TBCE depletion, and brefeldin A-mediated Golgi disruption all compromise axonal tubulin routing. We conclude that motor axons critically depend on axonal tubulin routing from the Golgi apparatus, a process that involves TBCE and possibly other tubulin chaperones.

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.

Axonal transport of mutant superoxide dismutase 1 and focal axonal abnormalities in the proximal axons of transgenic mice

Superoxide dismutase 1 (SOD1), a ubiquitously expressed enzyme, detoxifies superoxide radicals and participates in copper homeostasis. Mutations in this enzyme have been linked to a subset of autosomal dominant cases of familial amyotrophic lateral sclerosis (FALS), a disorder characterized by selective degeneration of motor neurons. Transgenic mice expressing FALS mutant human (Hu) SOD1 at high levels develop a motor neuron disease, indicating that mutant Hu SOD1 gains properties that are particularly toxic to motor neurons. In this report, we demonstrate that transgenic mice expressing Hu SOD1 with the G37R FALS mutation, but not mice expressing wild-type enzyme, develop focal increases in immunoreactivity in the proximal axons of spinal motor neurons. This SOD1 immunoreactivity and immunoreactivity to hypophosphorylated neurofilament H epitopes are found adjacent to small vacuoles in axons. Using metabolic radiolabeling methods, we show that mutant G37R HuSOD1 as well as endogenous mouse SOD1 are transported anterograde in slow component b in motor and sensory axons of the sciatic nerve. Together, these findings suggest that anterogradely transported mutant SOD1 may act locally to damage motor axons.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Changes in the isotype composition of beta-tubulin delivered to regenerating sensory axons by slow axonal transport

beta-Tubulin is encoded by a family of genes that produces at least five distinct polypeptide isotypes in neurons. Two of these isotypes (i.e., classes II and III) preferentially accumulate in axons, and the expression of one of them (i.e., class II) correlates closely with axonal outgrowth during development and regeneration. In dorsal root ganglion (DRG) neurons, expression of the class II isotype declines to relatively low levels during early postnatal development, and increases dramatically in mature neurons during axon regeneration (i.e., to a level comparable to that in developing neurons). In contrast, expression of the class III isotype, which rises slightly during postnatal development, increases much less than the class II isotype during regeneration. We now document that these changes in gene expression are associated with an increase in the relative amount of class II as compared to class III beta-tubulin delivered to regenerating sensory axons of rat sciatic nerve by slow axonal transport. In this study, the tubulin transported in sensory axons was labeled by injecting [35S]methionine into the L5 DRG either 7 or 14 days after crushing the sciatic nerve; pulse-labeled class II and class III beta-tubulin were identified using immunoprecipitation. This change in the isotype composition of beta-tubulin transported in regenerating axons may influence outgrowth by altering the assembly and dynamic properties of axonal microtubules.

Active transport of photoactivated tubulin molecules in growing axons revealed by a new electron microscopic analysis

To determine whether tubulin molecules transported in axons are polymers or oligomers, we carried out electron microscopic analysis of the movement of the tubulin molecules after photoactivation. Although previous optical microscopic analyses after photobleaching or photoactivation had suggested that most of the axonal microtubules were stationary, they were not sufficiently sensitive to allow detection of actively transported tubulin molecules which were expected to be only a small fraction of total tubulin molecules in axons. In addition, some recent studies using indirect approaches suggested active polymer transport as a mechanism for tubulin transport (Baas, P.W., F.J. Ahmad. 1993. J. Cell Biol. 120:1427-1437; Yu, W., V.E. Centonze, F.J. Ahmad, and P.W. Bass, 1993, J. Cell Biol. 122:349-359; Ahmad, F.J., and P.W. Bass. 1995. J. Cell Sci. 108:2761-2769). So, whether transported tubulin molecules are polymers or not remain to be determined. To clear up this issue, we made fluorescent marks on the tubulin molecules in the axons using a photoactivation technique and performed electron microscopic immunocytochemistry using anti-fluorescein antibody. Using this new method we achieved high resolution and high sensitivity for detecting the transported tubulin molecules. In cells fixed after permeabilization, we found no translocated microtubules. In those fixed without permeabilization, in which oligomers and heterodimers in addition to polymers were preserved, we found much more label in the regions distal to the photoactivated regions than in the proximal regions. These data indicated that tubulin molecules are transported not as polymers but as heterodimers or oligomers by an active mechanism rather than by diffusion.

Assembly of microfilaments and microtubules from axonally transported actin and tubulin after axotomy

The slow component (SC) of axonal transport conveys structural proteins, regulatory proteins, and glycolytic enzymes toward the axon tip at 1-6 mm/day. Following axon interruption (axotomy), the rate of outgrowth corresponds to the rate of SCb-the fastest subcomponent of SC. Both axonal outgrowth and SCb accelerate 20-25% after axotomy. Tubulin and actin are the major proteins being carried by SCb. To further characterize the acceleration of SCb, we measured the equilibrium between subunits and polymers for both actin and tubulin. We radiolabeled newly synthesized proteins in rat motor neurons by microinjecting [35S]methionine into the spinal cord 7 days after crushing the sciatic nerve (85 mm from the spinal cord). Nerves were removed 7 days later for homogenization in polymer-stabilizing buffer (PSB) and centrifugation, followed by SDS-PAGE of supernatants (S) and pellets (P). We removed beta-tubulin, actin, and the medium-weight neurofilament protein (NF-M) from each gel by using the fluorogram as a template. After solubilizing gel segments for liquid scintillation spectrometry, we expressed counts as a polymerization ratio: P/[S+P]. In the nerve segments that contained radiolabeled Scb proteins, located 24-36 mm from the spinal cord, axotomy increased the polymerization ratio of SCb actin from 0.23 to 0.36 (P < 0.05) but had no effect on SCb beta-tubulin. In a separate experiment, we added 12 microM taxol to PSB to stabilize newly assembled microtubules. Adding taxol did not alter the polymerization ratio for SCb beta-tubulin in sham-axotomized nerves but aid increase the ratio in axotomized nerves, from 0.44 to 0.63 (P < 0.05); polymerization ratios for SCb actin were unaffected. We conclude that the assembly of microfilaments and microtubules increases to provide cytoskeletal elements for axon sprouts. The resulting loss of actin and tubulin subunits may play a role in the acceleration of SCb.

The axonal transport of beta III-tubulin is altered in both branches of sensory axons after injury of the rat sciatic nerve

We have analyzed the axonal transport of beta III-tubulin in the central (dorsal root) and peripheral (sciatic nerve) branches of sensory axons after injury of the sciatic nerve. Our finding that the relative amount of beta III-tubulin transported in slow component b (SCb) is increased in both axonal branches does not support the generally accepted hypothesis that the transport of cytoskeletal proteins is altered in the peripheral, but not the central branch after injury of the sciatic nerve.

Axonal transport of type III intermediate filament protein peripherin in intact and regenerating motor axons of the rat sciatic nerve

Slow axonal transport of peripherin has been studied in the motor axons of both intact and regenerating rat sciatic nerves 7 days post-crush. The studies were done by two-dimensional gel electrophoresis after intraspinal injection of 35S-methionine. In the first experiment, the sciatic nerves were removed 3 weeks after the radiolabeling pulse and cut into 6 mm segments. Each nerve segment was submitted to two-dimensional gel electrophoresis and analyzed by an original procedure which allowed us to study the distribution along the nerve of the radioactivity associated with several proteins of the cytoskeleton, especially the intermediate filament proteins, peripherin, and the low molecular mass neurofilament protein, NF-L. Peripherin was transported at two main rates: 66% of the total radiolabeled peripherin moved at 1.42 mm/day and the remainder moved at 2.28 mm/day. The radioactivity associated with NF-L exhibited a similar pattern. In the second experiment, similar intraspinal injections were made 7 days after a unilateral crush of the sciatic nerve. Regenerating nerves exhibited a clear SCa wave. However, in contrast to the intact nerves, the SCb wave could not be precisely defined in the regenerating nerves. Thus, the changes in the amount of transported proteins were analyzed in the SCa wave only. Autoradiograms of 2D-PAGE revealed that in the regenerating axons, the quantity of transported peripherin in SCa was increased by 3.5-fold. In contrast, the quantity of transported NF-L was decreased by 1.6-fold. The regenerating motor axons conveyed significantly greater (approximately twofold) amounts of labeled tubulins and actin than did intact motor axons. Our results suggest that peripherin, although mainly conveyed by SCa, plays a role during the elongation process in addition to actin and tubulin.

Neuritic growth rate described by modeling microtubule dynamics

A model is developed to describe neuronal elongation as a result of the polymerization of microtubules and elastic stretching of the neurites by force produced by the growth cone. The model for a single segment with a single growth cone revealed a constant elongation rate, while the concentration of tubulin in the soma rises, and the concentration of tubulin becomes constant in the growth cone. Extending the model to a neurite with a single branch point and two growth cones revealed the same results. When the assembly or the disassembly rate of microtubules is unequal in both growth cones, transient retraction of one of the terminal segments occurs, which results in complete retraction of the segment when the difference in (dis)assembly rate between the two growth cones is large enough. When the model is applied to large trees, a maximal sustainable number of terminal segments as a function of the production rate of tubulin appears. Mechanisms to stop outgrowth are discussed in relation to the establishment of synaptical contacts between cells.

Microtubule behavior in PC12 neurites: variable results obtained with photobleach technology

We have examined the effects of various means of photobleaching on the recovery of fluorescence, movement, and morphology of the microtubules in the neurites of rhodamine-tubulin-injected PC12 cells. We find that, depending on power of and time of exposure to the bleaching beam, we can generate at least three different patterns of fluorescence recovery in regenerating PC12 neurites. If bleaching is performed with a relatively low-power beam for an extended period, fluorescence in polymer recovers very little after 1 hour. Under these conditions, however, tubulin immunostaining is seen extending through the bleach zone, and microtubules are present through the bleached zone in thin section electron micrographs. If bleaching is performed with a high-power laser, for 0.5-5 seconds, fluorescence recovery also is quite slow, but electron microscopic observations reveal that no microtubules extend through the bleached region of the neurite, and the uranyl acetate-stained cytoplasm appears more electron lucent than in the unbleached neurite. Finally, if bleaching is performed by very brief exposure to a high-intensity laser beam, resulting in an incomplete reduction of fluorescence intensity through the bleach zone, fluorescence recovery occurs within 20-30 minutes, and immunostained microtubules appear intact through the bleach zone; electron microscopy confirms that microtubules extend through the bleached zone of such neurites. In all three cases, movement of the bleach zone is observed in approximately half of the experimental neurites. These results indicate that highly variable microtubule behaviors can be obtained with photobleach technology, presumably due to different levels and pathways of photodamage generated by different bleach protocols. Nevertheless, it is clear that both turnover and movement of microtubules occur in PC12 neurites, and both are likely to be involved in neurite maintenance and growth.