Neuronal intermediate filament protein alpha-internexin facilitates axonal neurite elongation in neuroblastoma cells

Intermediate filament proteins are reliable immunohistological biomarkers to help diagnose multiple tissue-specific diseases

Cytoskeletal networks are proteins that effectively maintain cell integrity and provide mechanical support to cells by actively transmitting mechanical signals. Intermediate filaments, which are from the cytoskeleton family and are 10 nanometres in diameter, are unlike actin and microtubules, which are highly dynamic cytoskeletal elements. Intermediate filaments are flexible at low strain, harden at high strain and resist breaking. For this reason, these filaments fulfil structural functions by providing mechanical support to the cells through their different strain-hardening properties. Intermediate filaments are suitable in that cells both cope with mechanical forces and modulate signal transmission. These filaments are composed of fibrous proteins that exhibit a central α-helical rod domain with a conserved substructure. Intermediate filament proteins are divided into six groups. Type I and type II include acidic and basic keratins, type III, vimentin, desmin, peripheralin and glial fibrillary acidic protein (GFAP), respectively. Type IV intermediate filament group includes neurofilament proteins and a fourth neurofilament subunit, α-internexin proteins. Type V consists of lamins located in the nucleus, and the type VI group consists of lens-specific intermediate filaments, CP49/phakinin and filen. Intermediate filament proteins show specific immunoreactivity in differentiating cells and mature cells of various types. Various carcinomas such as colorectal, urothelial and ovarian, diseases such as chronic pancreatitis, cirrhosis, hepatitis and cataract have been associated with intermediate filaments. Accordingly, this section reviews available immunohistochemical antibodies to intermediate filament proteins. Identification of intermediate filament proteins by methodological methods may contribute to the understanding of complex diseases.

Proteomic characterization of spontaneously regrowing spinal cord following injury in the teleost fish Apteronotus leptorhynchus, a regeneration-competent vertebrate

In adult mammals, spontaneous repair after spinal cord injury (SCI) is severely limited. By contrast, teleost fish successfully regenerate injured axons and produce new neurons from adult neural stem cells after SCI. The molecular mechanisms underlying this high regenerative capacity are largely unknown. The present study addresses this gap by examining the temporal dynamics of proteome changes in response to SCI in the brown ghost knifefish (Apteronotus leptorhynchus). Two-dimensional difference gel electrophoresis (2D DIGE) was combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and tandem mass spectrometry (MS/MS) to collect data during early (1 day), mid (10 days), and late (30 days) phases of regeneration following caudal amputation SCI. Forty-two unique proteins with significant differences in abundance between injured and intact control samples were identified. Correlation analysis uncovered six clusters of spots with similar expression patterns over time and strong conditional dependences, typically within functional families or between isoforms. Significantly regulated proteins were associated with axon development and regeneration; proliferation and morphogenesis; neuronal differentiation and re-establishment of neural connections; promotion of neuroprotection, redox homeostasis, and membrane repair; and metabolism or energy supply. Notably, at all three time points examined, significant regulation of proteins involved in inflammatory responses was absent.

Intermediate filaments in developing neurons: Beyond structure

Neuronal development relies on a highly choreographed progression of dynamic cellular processes by which newborn neurons migrate, extend axons and dendrites, innervate their targets, and make functional synapses. Many of these dynamic processes require coordinated changes in morphology, powered by the cell's cytoskeleton. Intermediate filaments (IFs) are the third major cytoskeletal elements in vertebrate cells, but are rarely considered when it comes to understanding axon and dendrite growth, pathfinding and synapse formation. In this review, we first introduce the many new and exciting concepts of IF function, discovered mostly in non-neuronal cells. These roles include dynamic rearrangements, crosstalk with microtubules and actin filaments, mechano-sensing and -transduction, and regulation of signaling cascades. We then discuss the understudied roles of neuronally expressed IFs, with a particular focus on IFs expressed during development, such as nestin, vimentin and α-internexin. Lastly, we illustrate how signaling modulation by the unconventional IF nestin shapes neuronal morphogenesis in unexpected and novel ways. Even though the first IF knockout mice were made over 20 years ago, the study of the cell biological functions of IFs in the brain still has much room for exciting new discoveries.

Bottom up proteomics reveals novel differentiation proteins in neuroblastoma cells treated with 13-cis retinoic acid

Neuroblastoma, a cancer of the sympathetic nervous system, is the second most common pediatric cancer. A unique feature of neuroblastoma is remission in some patients due to spontaneous differentiation of metastatic tumors. 13-cis retinoic acid (13-cis RA) is currently used in the clinic to treat neuroblastoma due to its differentiation inducing effects. In this study, we used shotgun proteomics to identify proteins affected by 13-cis RA treatment in neuroblastoma SK-N-SH cells. Our results showed that 13-cis RA reduced proteins involved in extracellular matrix synthesis and organization and increased proteins involved in cell adhesion and neurofilament formation. These changes indicate that 13-cis RA induces tumor cell differentiation by decreasing extracellular matrix rigidity and increasing neurite overgrowth. Differentially-affected proteins identified in this study may be novel biomarkers of drug efficacy in the treatment of neuroblastoma. SIGNIFICANCE: As neuroblastoma can spontaneously differentiate, determining which proteins are involved in differentiation can guide development of novel treatments. 13-cis retinoic acid is currently used in the clinic as a differentiation inducer. Here we have established a proteome map of SK-N-SH cells treated with 13-cis retinoic acid. Bioinformatic analysis revealed the involvement of development, differentiation, extracellular matrix assembly, collagen biosynthesis, and neurofilament bundle association. This proteome map provides information as to which proteins are important for differentiation and identifies networks that can be targeted by drugs to treat neuroblastoma [1].

Neurofilaments form flexible bundles during neuritogenesis in culture and in mature axons in situ

Neurofilaments (NFs) undergo cation-dependent phospho-mediated associations with each other and other cytoskeletal elements that support axonal outgrowth. Progressive NF-NF associations generate a resident, bundled population that undergoes exchange with transporting NFs. We examined the properties of bundled NFs. Bundles did not always display a fully linear profile but curved and twisted at various points along the neurite length. Bundles retracted faster than neurites and retracted bundles did not expand following extraction with Triton, indicating that they coiled passively rather than due to pressure from the cell. Bundles consisted of helically wound NFs, which may provide flexibility necessary for turning of growing axons during pathfinding. Interactions between NFs and other cytoskeletal elements may be disrupted en masse during neurite retraction or regionally during remodeling. It is suggested that bundles within long axons that cannot be fully retracted into the soma could provide maintain proximal support yet still allow more distal flexibility for remodeling and changing direction during pathfinding.

Nestin in immature embryonic neurons affects axon growth cone morphology and Semaphorin3a sensitivity

Correct wiring in the neocortex requires that responses to an individual guidance cue vary among neurons in the same location, and within the same neuron over time. Nestin is an atypical intermediate filament expressed strongly in neural progenitors and is thus used widely as a progenitor marker. Here we show a subpopulation of embryonic cortical neurons that transiently express nestin in their axons. Nestin expression is thus not restricted to neural progenitors, but persists for 2-3 d at lower levels in newborn neurons. We found that nestin-expressing neurons have smaller growth cones, suggesting that nestin affects cytoskeletal dynamics. Nestin, unlike other intermediate filament subtypes, regulates cdk5 kinase by binding the cdk5 activator p35. Cdk5 activity is induced by the repulsive guidance cue Semaphorin3a (Sema3a), leading to axonal growth cone collapse in vitro. Therefore, we tested whether nestin-expressing neurons showed altered responses to Sema3a. We find that nestin-expressing newborn neurons are more sensitive to Sema3a in a roscovitine-sensitive manner, whereas nestin knockdown results in lowered sensitivity to Sema3a. We propose that nestin functions in immature neurons to modulate cdk5 downstream of the Sema3a response. Thus, the transient expression of nestin could allow temporal and/or spatial modulation of a neuron's response to Sema3a, particularly during early axon guidance.

The third wave: Intermediate filaments in the maturing nervous system

Intermediate filaments are critical for the extreme structural specialisations of neurons, providing integrity in dynamic environments and efficient communication along axons a metre or more in length. As neurons mature, an initial expression of nestin and vimentin gives way to the neurofilament triplet proteins and α-internexin, substituted by peripherin in axons outside the CNS, which physically consolidate axons as they elongate and find their targets. Once connection is established, these proteins are transported, assembled, stabilised and modified, structurally transforming axons and dendrites as they acquire their full function. The interaction between these neurons and myelinating glial cells optimises the structure of axons for peak functional efficiency, a property retained across their lifespan. This finely calibrated structural regulation allows the nervous system to maintain timing precision and efficient control across large distances throughout somatic growth and, in maturity, as a plasticity mechanism allowing functional adaptation.

The Proteomics of Intrathecal Analgesic agents for Chronic Pain

Chronic pain remains a challenging clinical problem with a growing socio-economic burden for the state. Its prevalence is high and many of the patients are of work age. Our knowledge regarding the pathophysiology of chronic pain is poor. The consensus view is that the central nervous system plays a key role in the persistence of pain after an initiating event has long ceased. However the specifics of this biological response to an initiating event remains unclear. There is a growing body of evidence to support the concept that a central neuroimmune response is initiated and a number of small peptides have been implicated in this process following cerebrospinal fluid analysis in patients with chronic pain. This central biosynthetic peptide response leads to a process called central sensitization. Therapy is aimed at modulating and even inhibiting this response. However current pharmacological therapeutic options are limited in efficacy with significant deleterious side effect profiles. Proteomic studies extend single molecule analysis by identifying the components of biological networks and pathways and defining their interactions. This tool offers the potential to provide a molecular overview of the biological processes involved in chronic pain. It will also facilitate examination of gene-drug interactions. This technique offers a mechanism of defining the central biological responses that result in chronic pain and this information may facilitate the development of better therapies.

α-Internexin and Peripherin: Expression, Assembly, Functions, and Roles in Disease

α-Internexin and peripherin are neuronal-specific intermediate filament (IF) proteins. α-Internexin is a type IV IF protein like the neurofilament triplet proteins (NFTPs, which include neurofilament light chain, neurofilament medium chain, and neurofilament high chain) that are generally considered to be the primary components of the neuronal IFs. However, α-internexin is often expressed together with the NFTPs and has been proposed as the fourth subunit of the neurofilaments in the central nervous system. α-Internexin is also expressed earlier in the development than the NFTPs and is a maker for neuronal IF inclusion disease. α-Internexin can self-polymerize in vitro and in transfected cells and it is present in the absence of the NFTP in development and in granule cells in the cerebellum. In contrast, peripherin is a type III IF protein. Like α-internexin, peripherin is specific to the nervous system, but it is expressed predominantly in the peripheral nervous system (PNS). Peripherin can also self-assemble both in vitro and in transfected cells. It is as abundant as the NFTPs in the sciatic nerve and can be considered a fourth subunit of the neurofilaments in the PNS. Peripherin has multiple isoforms that arise from intron retention, cryptic intron receptor site or alternative translation initiation. The functional significance of these isoforms is not clear. Peripherin is a major component found in inclusions of patients with amyotrophic lateral sclerosis (ALS) and peripherin expression is upregulated in ALS patients.

Hypergravity stimulation enhances PC12 neuron-like cell differentiation

Altered gravity is a strong physical cue able to elicit different cellular responses, representing a largely uninvestigated opportunity for tissue engineering/regenerative medicine applications. Our recent studies have shown that both proliferation and differentiation of C2C12 skeletal muscle cells can be enhanced by hypergravity treatment; given these results, PC12 neuron-like cells were chosen to test the hypothesis that hypergravity stimulation might also affect the behavior of neuronal cells, in particular promoting an enhanced differentiated phenotype. PC12 cells were thus cultured under differentiating conditions for either 12 h or 72 h before being stimulated with different values of hypergravity (50 g and 150 g). Effects of hypergravity were evaluated at transcriptional level 1 h and 48 h after the stimulation, and at protein level 48 h from hypergravity exposure, to assess its influence on neurite development over increasing differentiation times. PC12 differentiation resulted strongly affected by the hypergravity treatments; in particular, neurite length was significantly enhanced after exposure to high acceleration values. The achieved results suggest that hypergravity might induce a faster and higher neuronal differentiation and encourage further investigations on the potential of hypergravity in the preparation of cellular constructs for regenerative medicine and tissue engineering purposes.

The high molecular weight neurofilament subunit plays an essential role in axonal outgrowth and stabilization

Neurofilaments (NFs) are thought to provide structural support to mature axons via crosslinking of cytoskeletal elements mediated by the C-terminal region of the high molecular weight NF subunit (NF-H). Herein, we inhibited NF-H expression in differentiating mouse NB2a/d1 cells with shRNA directed against murine NF-H without affecting other NF subunits, microtubules or actin. shRNA-mediated NF-H knockdown not only in compromised of late-stage axonal neurite stabilization but also compromised early stages of axonal neurite elongation. Expression of exogenous rat NF-H was able to compensate for knockdown of endogenous NF-H and restored the development and stabilization of axonal neurites. This rescue was prevented by simultaneous treatment with shRNA that inhibited both rat and murine NF-H, or by expression of exogenous rat NF-H lacking the C-terminal sidearm during knockdown of endogenous NF-H. Demonstration of a role for NF-H in the early stages of axonal elaboration suggests that axonal stabilization is not delayed until synaptogenesis, but rather that the developing axon undergoes sequential NF-H-mediated stabilization along its length in a proximal–distal manner, which supports continued pathfinding in distal, unstabilized regions.

Cytoskeletal and signaling mechanisms of neurite formation

The formation of a neurite, the basis for axons and dendrites, begins with the concerted accumulation and organization of actin and microtubules. Whereas much is known about the proteins that play a role in these processes, because they perform similar functions in axon branching and filopodia formation, much remains to be discovered concerning the interaction of these individual cytoskeletal regulators during neurite formation. Here, we review the literature regarding various models of filopodial formation and the way in which proteins that control actin organization and polymerization induce neurite formation. Although several different regulators of actin polymerization are involved in neurite initiation, redundancy occurs between these regulators, as the effects of the loss of a single regulator can be mitigated by the addition of neurite-promoting substrates and proteins. Similar to actin dynamics, both microtubule stabilizing and destabilizing proteins play a role in neurite initiation. Furthermore, interactions between the actin and microtubule cytoskeleton are required for neurite formation. Several lines of evidence indicate that the interactions between these two components of the cytoskeleton are needed for force generation and for the localization of microtubules at sites of nascent neurites. The general theme that emerges is the existence of several central regulatory pathways on which extracellular cues converge to control and organize both actin and microtubules to induce the formation of neurites.

Phosphoproteomics and bioinformatics analyses of spinal cord proteins in rats with morphine tolerance

Introduction

Morphine is the most effective pain-relieving drug, but it can cause unwanted side effects. Direct neuraxial administration of morphine to spinal cord not only can provide effective, reliable pain relief but also can prevent the development of supraspinal side effects. However, repeated neuraxial administration of morphine may still lead to morphine tolerance.

Methods

To better understand the mechanism that causes morphine tolerance, we induced tolerance in rats at the spinal cord level by giving them twice-daily injections of morphine (20 µg/10 µL) for 4 days. We confirmed tolerance by measuring paw withdrawal latencies and maximal possible analgesic effect of morphine on day 5. We then carried out phosphoproteomic analysis to investigate the global phosphorylation of spinal proteins associated with morphine tolerance. Finally, pull-down assays were used to identify phosphorylated types and sites of 14-3-3 proteins, and bioinformatics was applied to predict biological networks impacted by the morphine-regulated proteins.

Results

Our proteomics data showed that repeated morphine treatment altered phosphorylation of 10 proteins in the spinal cord. Pull-down assays identified 2 serine/threonine phosphorylated sites in 14-3-3 proteins. Bioinformatics further revealed that morphine impacted on cytoskeletal reorganization, neuroplasticity, protein folding and modulation, signal transduction and biomolecular metabolism.

Conclusions

Repeated morphine administration may affect multiple biological networks by altering protein phosphorylation. These data may provide insight into the mechanism that underlies the development of morphine tolerance.

Anti-alpha-internexin autoantibody from neuropsychiatric lupus induce cognitive damage via inhibiting axonal elongation and promote neuron apoptosis

BACKGROUND

Neuropsychiatric systemic lupus erythematosus (NPSLE) is a major complication for lupus patients, which often leads to cognitive disturbances and memory loss and contributes to a significant patient morbidity and mortality. The presence of anti-neuronal autoantibodies (aAbs) has been identified; as examples, anti-NMDA receptors and anti-Ribsomal P aAbs have been linked to certain pathophysiological features of NPSLE.

METHODS AND FINDINGS

In the current study, we used a proteomic approach to identify an intermediate neurofilament alpha-internexin (INA) as a pathogenetically relevant autoantigen in NPSLE. The significance of this finding was then validated in an expanded of a cohort of NPSLE patients (n = 67) and controls (n = 270) by demonstrating that high titers of anti-INA aAb was found in both the serum and cerebrospinal fluid (CSF) of approximately 50% NPSLE. Subsequently, a murine model was developed by INA immunization that resulted in pronounced cognitive dysfunction that mimicked features of NPSLE. Histopathology in affected animals displayed cortical and hippocampal neuron apoptosis. In vitro studies further demonstrated that anti-INA Ab mediated neuronal damage via inhibiting axonal elongation and eventually driving the cells to apoptosis.

CONCLUSIONS

Taken together, this study identified a novel anti-neurofilament aAb in NPSLE, and established a hitherto undescribed mechanism of aAb-mediated neuron damage that could have relevance to the pathophysiology of NPSLE.

Identification and characterization of a population of motile neurons in long-term cortical culture

The specific phenotypes and progression to maturity of primary cortical neurons in long-term culture correlate well with neurons in vivo. Utilizing a model of neuronal injury in long-term cultures at 21 days in vitro (DIV), we have identified a distinct population of neurons that translocate into the injury site. 5-bromo-2'-deoxyUridine (BrdU) incorporation studies demonstrated that neurons with the capacity to translocate were 21 days old. However, this motile ability is not consistent with the traditional view of the maturation and structural stability of neurons in long-term culture. Therefore, we examined the neurons' cytoskeletal profile using immunocytochemistry, to establish relative stage of maturation and phenotype. Expression of marker proteins including beta-III-tubulin, alpha-internexin, NF-L and NF-M, tau and L1 indicated the neurons were differentiated, and in some cases polarized. The neurons did not immunolabel with NF-H or MAP2, which might suggest they had not reached the level of maturity of other neurons in culture. They did not express the microtubule-associated migration marker doublecortin (DCX). Cytoskeletal disrupting agents were used to further investigate the role of the microtubule cytoskeleton in translocation, and microtubule destabilization significantly enhanced aspects of their motility. Finally, molecular guidance cues affected their motility in a similar manner to that reported for both axon guidance and early neuron migration. Therefore, this study has identified and characterized a population of motile neurons in vitro that have the capacity to migrate into a site of injury. These studies provide new information on the structurally dynamic features of subsets of neurons.

Alpha-internexin is structurally and functionally associated with the neurofilament triplet proteins in the mature CNS

Alpha-internexin, a neuronal intermediate filament protein implicated in neurodegenerative disease, coexists with the neurofilament (NF) triplet proteins (NF-L, NF-M, and NF-H) but has an unknown function. The earlier peak expression of alpha-internexin than the triplet during brain development and its ability to form homopolymers, unlike the triplet, which are obligate heteropolymers, have supported a widely held view that alpha-internexin and neurofilament triplet form separate filament systems. Here, we demonstrate, however, that despite a postnatal decline in expression, alpha-internexin is as abundant as the triplet in the adult CNS and exists in a relatively fixed stoichiometry with these subunits. Alpha-internexin exhibits transport and turnover rates identical to those of triplet proteins in optic axons and colocalizes with NF-M on single neurofilaments by immunogold electron microscopy. Alpha-internexin also coassembles with all three neurofilament proteins into a single network of filaments in quadruple-transfected SW13vim(-) cells. Genetically deleting NF-M alone or together with NF-H in mice dramatically reduces alpha-internexin transport and content in axons throughout the CNS. Moreover, deleting alpha-internexin potentiates the effects of NF-M deletion on NF-H and NF-L transport. Finally, overexpressing a NF-H-LacZ fusion protein in mice induces alpha-internexin and neurofilament triplet to aggregate in neuronal perikarya and greatly reduces their transport and content selectively in axons. Our data show that alpha-internexin and the neurofilament proteins are functionally interdependent. The results strongly support the view that alpha-internexin is a fourth subunit of neurofilaments in the adult CNS, providing a basis for its close relationship with neurofilaments in CNS diseases associated with neurofilament accumulation.

Neurodegeneration with features of NIFID and ALS--extended clinical and neuropathological spectrum

Heterogeneous clinical and neuropathological features have been observed in the recently described neuronal intermediate filament inclusion disease (NIFID). The immunohistological findings common to all cases are alpha-internexin and neurofilament-positive neuronal cytoplasmic inclusions, which have not been found in comparable density in other neurodegenerative disorders. Notwithstanding these common features, the cases reported so far have shown differences concerning age at onset, constellation and dominance of symptoms as well as type and distribution of additional neuropathological findings. Here we present the first NIFID case that exhibits severe involvement of lower motor neurons. Also, this patient may have had a clinical onset of disease in early childhood, as she was diagnosed as having dysarthria, which could not be attributed to any other cause at the age of 3 years. This case is a further contribution to the spectrum of this novel neurodegenerative disease.

A Possible Cellular Mechanism of Neuronal Loss in the Dorsal Root Ganglia of Dystonia musculorum (dt) Mice

Dystonia musculorum (dt) is a mutant mouse with hereditary sensory neuropathy. A defective bullous pemphigoid antigen 1 (BPAG1) gene is responsible for this mutation. In the present study, we examined the distribution of neuronal intermediate filament proteins in the central and peripheral processes of the dorsal root ganglia (DRG) in adult dt mice using different approaches. We found that not only BPAG1, but also alpha-internexin was absent in the DRG neurons in adult dt mice. To study the relationship between the absence of alpha-internexin and the progressive neuronal loss in the DRG of dt mice, we further cultured DRG neurons from embryonic dt mutants. Immunocytochemical assay of cultured DRG neurons from dt embryos revealed that alpha-internexin was aggregated in the proximal region of axons and juxtanuclear region of the cytoplasma, yet the other intermediate filament proteins were widely distributed in all processes. The active caspase-3 activity was observed in the dt neuron with massive accumulation of alpha-internexin. From our observations, we suggest that the interaction between BPAG1 and alpha-internexin may be one of the key factors involved in neuronal degeneration, and abnormal accumulation of alpha-internexin may impair the axonal transport and subsequently turns on the cascade of neuronal apoptosis in dt mice.

Cytoskeletal organization of the developing mouse olfactory nerve layer

Olfactory sensory neuron (OSN) axonal extension and targeting occur within the olfactory nerve layer (ONL) of the olfactory bulb (OB). The ONL can be differentiated into sublaminae: the outer (ONLo), where axons broadly target regions of the OB in tight fascicles, and inner (ONLi), where axons perform final targeting in loosely organized fascicles. During perinatal development, cadherin-2 and its binding partner, gamma-catenin, are preferentially expressed by OSN axons in the ONLo vs. the ONLi. Given the expression of these cytoskeleton-associated molecules, we hypothesized that cytoskeletal elements of OSN axons may be differentially expressed across the ONL. We therefore examined cytoskeletal organization of OSN axons in the ONL, focusing on the day of birth (P0). We show that microfilaments, microtubules, and the intermediate filament (IF) vimentin are homogeneously expressed across the ONL at P0. In contrast, the IFs peripherin and alpha-internexin are preferentially localized to the ONLo at P0, with alpha-internexin expressed by a restricted subset of OSNs. We also show that OSN axons in the ONLo are significantly smaller than those in the ONLi. The data demonstrate that, as OSN axons begin to exit the ONLo and target a specific region of the OB, there is a down-regulation of cytoskeletal elements and bound extracellular adhesion molecules. The increase in axon diameter may reflect additional mechanisms involved in glomerular targeting or the formation of the large terminal boutons of OSN axons within glomeruli.

Phosphoproteome and transcriptome analysis of the neuronal response to a CDK5 inhibitor

In Alzheimer's disease and amyotrophic lateral sclerosis deregulation of cyclin-dependent kinase 5 (CDK5) causes hyperphosphorylation of tau and neurofilament proteins, respectively, leading to neuronal cell death. We have demonstrated recently that pharmacological inhibition of CDK5 protects neurons under various stressful conditions (Weishaupt J. H., et al., Molec. Cell. Neurosci. 2003, 24, 489-502). To get an overview on the cellular mechanisms of action we analyzed global changes in protein phosphorylation in cultured cerebellar granule neurons by [(32)P]orthophosphate labeling after administration of a CDK5 inhibitor. Since CDK5 has recently been shown to phosphorylate and inactivate transcription factor MEF2, we included gene expression profiling using cDNA microarrays. By two-dimensional gel electrophoresis and matrix assisted laser desorption/ionisation-time of flight (MALDI-TOF)-mass spectrometry we identified several phosphoproteins that were modulated by compound administration. Among them syndapin I which is involved in vesicle recycling, and dynein light intermediate chain 2 which represents a regulatory subunit of the dynein protein complex. These findings are consistent with the known physiological function of CDK5 in synaptic signaling and axonal transport. Moreover, we detected phosphoproteins acting in neuronal surival and/or neurite outgrowth, such as cofilin and collapsin response mediator protein. Subsequent testing in cell cultures revealed that the CDK5 inhibitor blocked mitochondrial translocation of pro-apoptotic cofilin in cerebellar granule neurons and enhanced neurite outgrowth in dorsal root ganglia. Numerous genes exhibiting MEF2 consensus binding sequences were modulated by CDK5 inhibitor treatment. Among them some that may contribute to neurite elongation or neuronal survival, but also several genes functioning in synaptic transmission. Taken together, phosphoproteome and transcriptome analysis indicate that the compound promotes both neuronal survival and neurite outgrowth, but also may affect synaptic function in cultured neurons.

α-Internexin immunoreactivity reflects variable neuronal vulnerability in Alzheimer's disease and supports the role of the β-amyloid plaques in inducing neuronal injury

This study investigated the role of alpha-internexin in the neuronal alterations associated with beta-amyloid plaque formation in Alzheimer's disease (AD). Cortical neurons could be defined by their variable content of neurofilament (NF) triplet and alpha-internexin proteins, with a distinct population of supragranular pyramidal cells containing alpha-internexin alone. Both NF triplet and alpha-internexin were localized to reactive axonal structures in physically damaged neurons in experimental trauma models. Similarly, NF triplet and alpha-internexin immunoreactive neurites were localized to plaques densely packed with beta-amyloid fibrils in preclinical AD cases, indicating that certain plaques may cause structural injury or impediment of local axonal transport. However, alpha-internexin, and not NF triplet, ring-like reactive neurites were present in end-stage AD cases, indicating the relatively late involvement of neurons that selectively contain alpha-internexin. These results implicate the expression of specific intermediate filament proteins in a distinct hierarchy of differential neuronal vulnerability to AD.

Reexpression of vimentin in differentiated neuroblastoma cells enhances elongation of axonal neurites

Vimentin (Vm) is initially expressed by early neuronal precursors in situ and in culture. Vm is essential for neuritogenesis at least in culture and is gradually replaced by neurofilaments (NFs) because of down-regulation of Vm expression. This period is accompanied by a slowing of axonal elongation. We examined whether continued expression of Vm could foster continued axonal elongation. NB2a/d1 cells differentiated with dibutyryl cAMP were transfected with constructs expressing Vm or the middle-molecular-weight NF subunit (NF-M) each conjugated to green fluorescent protein (GFP). Axonal neurites of cells expressing GFP-Vm were 30% longer than those of nonexpressing cells, or cells expressing GFP-M, and exhibited a decrease in neurite caliber. Expression of GFP-M did not enhance axonal neurite length but significantly increased caliber. These findings provide further evidence of a role for Vm in axonal outgrowth. Culturing of nontransfected cells on laminin increased neurite length, but cells expressing GFP-Vm demonstrated an equivalent increase whether cultured on laminin or culture plastic. Axonal neurites of cells expressing GFP-Vm turned to avoid a nonfavorable substrate (nitrocellulose), but culturing of these cells on nitrocellulose did not impair axonal outgrowth. These latter findings indicate that the more robust outgrowth following reexpression of Vm is independent of a favorable or nonfavorable substrate but that axonal neurites of these cells still interact with the substrate to the extent that the substrate can influence directionality.

Intermediate filament steady-state mRNA levels in amyotrophic lateral sclerosis

We have examined the steady-state levels of intermediate filament mRNA in amyotrophic lateral sclerosis using the RNAse protection assay (NFL, NFM, NFH; corrected against GAPDH) or by PCR (peripherin, alpha-internexin, nestin, and vimentin; corrected against beta-actin). Significant elevations of NFL and peripherin mRNA levels were observed within the ALS cervical and lumbar spinal cord, with all other IF mRNA levels being comparable between control and ALS cases. These findings suggest that disturbances in both NFL and peripherin expression, independently known to contribute to the generation of motor neuron dysfunction in transgenic mice, are evident in ALS.

Mechanisms of action of the antidepressants fluoxetine and the substance P antagonist L-000760735 are associated with altered neurofilaments and synaptic remodeling

Antidepressants are widely prescribed in the treatment of depression, although the mechanism of how they exert their therapeutic effects is poorly understood. To shed further light on their mode of action, we have attempted to identify a common proteomic signature in guinea pig brains after chronic treatment with two different antidepressants. Both fluoxetine and the substance P receptor (NK(1)R) antagonist (SPA) L-000760735 altered cortical expression of multiple heat shock protein 60 forms along with neurofilaments and related proteins that are critical determinants of synaptic structure and function. Analysis of NK(1)R-/- mice showed similar alterations of neurofilaments confirming the specificity of the effects observed with chronic NK(1)R antagonist treatment. To determine if these changes were associated with structural modification of synapses, we carried out electron microscopic analysis of cerebral cortices from fluoxetine-treated guinea pigs. This showed an increase in the percentage of synapses with split postsynaptic densities (PSDs), a phenomenon that is characteristic of activity-dependent synaptic rearrangement. These findings suggest that cortical alterations of the neurofilament pathway and increased synaptic remodeling are associated with the mechanism of these two antidepressant drug treatments and may contribute to their psychotherapeutic actions.

Functions of intermediate filaments in neuronal development and disease

Five major types of intermediate filament (IF) proteins are expressed in mature neurons: the three neurofilament proteins (NF-L, NF-M, and NF-H), alpha-internexin, and peripherin. While the differential expression of IF genes during embryonic development suggests potential functions of these proteins in axogenesis, none of the IF gene knockout experiments in mice caused gross developmental defects of the nervous system. Yet, deficiencies in neuronal IF proteins are not completely innocuous. Substantial developmental loss of motor axons was detected in mice lacking NF-L and in double knockout NF-M;NF-H mice, supporting the view of a role for IFs in axon stabilization. Moreover, the absence of peripherin resulted in approximately 30% loss of small sensory axons. Mice lacking NF-L had a scarcity of IF structures and exhibited a severe axonal hypotrophy, causing up to 50% reduction in conduction velocity, a feature that would be very detrimental for large animal species. Unexpectedly, the NF-M rather than NF-H protein turned out to be required for proper radial growth of large myelinated axons. Studies with transgenic mice suggest that some types of IF accumulations, reminiscent of those found in amyotrophic lateral sclerosis (ALS), can have deleterious effects and even cause neurodegeneration. Additional evidence for the involvement of IFs in pathogenesis came from the recent discovery of neurofilament gene mutations linked to ALS and Charcot-Marie-Tooth disease (CMT2E). Conversely, we discuss how certain types of perikaryal neurofilament aggregates might confer protection in motor neuron disease.

Time-lapse analysis of ethanol's effects on axon growth in vitro

The cortical abnormalities found in animal models of fetal alcohol syndrome (FAS) suggest a disruption of axon growth. After emerging from the cell body, axons exhibit saltatory growth, cycling between periods of extension and periods of retraction. The timing of neuronal process outgrowth an the balance between extension and retraction together determine the net rate of axon elongation, and may be independently regulated. In this study, we used time-lapse digital microscopy and custom-designed analytic software to assess the effects of ethanol on the growth of axons from embryonic rat hippocampal pyramidal neurons in culture during 24 h of development, beginning approximately 7 h after plating. We recorded the amount of time elapsed before axons emerged, the relative amount of time spent in periods of growth and nongrowth, and the rate and direction of change in axon length during both periods of growth and nongrowth. The initiation of axonal outgrowth was significantly delayed by ethanol in a dose-dependent fashion at concentrations in the medium at or above 100 mg/dl. However, once established, axons exhibited accelerated growth in the presence of ethanol. This increase in overall growth rate was primarily due to a significant decrease in axon retraction during nongrowth periods. Ethanol did not affect the duration or frequency of growth and nongrowth periods. We propose, therefore, that mechanisms underlying ethanol-mediated changes in axon growth are linked to signaling events that differentially regulate outgrowth and retraction.

Regulation of the transition from vimentin to neurofilaments during neuronal differentiation

Vimentin (Vm) is initially expressed by nearly all neuronal precursors in vivo, and is replaced by neurofilaments (NFs) shortly after the immature neurons become post-mitotic. Both Vm and NFs can be transiently detected within the same neurite, and Vm is essential for neuritogenesis at least in culture. How neurons effect the orderly transition from expression of Vm as their predominant intermediate filament to NFs remains unclear. We examined this phenomenon within growing axonal neurites of NB2a/d1 cells. Transfection of cells with a construct expressing Vm conjugated to green fluorescent protein confirmed that axonal transport machinery for Vm persisted following the developmental decrease in Vm, but that the amount undergoing transport decreased in parallel to the observed developmental increase in NF transport. Immunoprecipitation from pulse-chase radiolabeled cells demonstrated transient co-precipitation of newly synthesized NF-H with Vm, followed by increasing co-precipitation with NF-L. Immunofluorescent and immuno-electron microscopic analyses demonstrated that some NF and Vm subunits were incorporated into the same filamentous profiles, but that Vm was excluded from the longitudinally-oriented "bundle" of closely-apposed NFs that accumulates within developing axons and is known to undergo slower turnover than individual NFs. These data collectively suggest that developing neurons are able to replace their Vm-rich cytoskeleton with one rich in NFs simply by down-regulation of Vm expression and upregulation of NFs, coupled with turnover of existing Vm filaments and Vm-NF heteropolymers.

The basic aspects of therapeutics in amyotrophic lateral sclerosis

Once thought to be a single pathological disease state, amyotrophic lateral sclerosis (ALS) is now recognized to be the limited phenotypic expression of a complex, heterogeneous group of biological processes, resulting in an unrelenting loss of motor neurons. On average, individuals affected with the disease live <5 years. In this article, the complex nature of the pathogenesis of ALS, including features of age dependency, environmental associations, and genetics, is reviewed. Once held to be uncommon, it is now clear that ALS is associated with a frontotemporal dementia and that this process may reflect disturbances in the microtubule-associated tau protein metabolism. The motor neuron ultimately succumbs in a state where significant disruptions in neurofilament metabolism, mitochondrial function, and management of oxidative stress exist. The microenvironment of the neuron becomes a complex milieu in which high levels of glutamate provide a source of chronic excitatory neurotoxicity, and the contributions of activated microglial cells lead to further cascades of motor neuron death, perhaps serving to propagate the disease once established. The final process of motor neuron death encompasses many features of apoptosis, but it is clear that this alone cannot account for all features of motor neuron loss and that aspects of a necrosis-apoptosis continuum are at play. Designing pharmacological strategies to mitigate against this process thus becomes an increasingly complex issue, which is reviewed in this article.

Microarray comparison of normal and W/Wv mice in the gastric fundus indicates a supersensitive phenotype

Interstitial cells of Cajal (ICC) have been identified in specific areas throughout the smooth musculature of the gastrointestinal (GI) tract. Located within the circular and longitudinal muscle layers of the gastric fundus lies a specific type of ICC, termed "intramuscular" ICC or IC-IM. The principal function of this cell type is to act as "mediators of excitatory and inhibitory enteric neurotransmission." The functional role of these cells has been investigated using W/W(v) mutant mice that specifically lack IC-IM, resulting in disrupted enteric neurotransmission. The aim of the present study was to investigate differential gene expression in W/W(v) mutant mice, from the tunica muscularis of the gastric fundus using a mouse cDNA microarray containing 1,081 known genes. Verification of the microarray data was attained using real-time "quantitative" PCR (qPCR). Of the 1,081 arrayed genes, 36 demonstrated differential expression by >2-fold in the W/W(v) mice. An agreement rate of 50% (7 of 14 tested) was obtained using qPCR. Of the seven confirmed changes in expression, several were indicative of a supersensitive phenotype, observed in denervation models. Expression of several putative neurotransmitter receptors including P2Y, the receptor for the inhibitory neurotransmitter ATP, was upregulated. The functional role of the P2Y receptor was also investigated using electrophysiological recordings. These results offer a new insight into the molecular changes that occur in W/W(v) fundic smooth muscle and may also provide novel information with regard to the importance of IC-IM in enteric neurotransmission.

Spontaneous differentiation of porcine and bovine embryonic stem cells (epiblast) into astrocytes or neurons

The culture of porcine or bovine epiblasts, i.e., embryonic stem cells, on STO feeder cells resulted in their spontaneous differentiation into multiple cell types that were subsequently isolated as separate cell lines. Some of these cell lines were "neuron-like" in morphology. Immunofluorescent analysis of two porcine epiblast-derived cell lines demonstrated that the cells were positive for the expression of vimentin and the glial fibrillary acidic protein (GFAP). Because of their stellate morphology and lack of neurofilament expression, it is possible that the cells are type 2 astrocytes. Similar analysis of a bovine epiblast-derived cell line showed that the cells were positive for vimentin but that they did not express GFAP. However, a few cells within the population expressed neurofilaments and alpha-internexin. It is possible that the bovine cells are neural precursor cells. The results confirm and extend the demonstrated in vitro pluripotency of porcine and bovine epiblast cultures and provide evidence for an in vitro model of embryonic neuroectoderm development.

Loss of neurofilaments alters axonal growth dynamics

The highly regulated expression of neurofilament (NF) proteins during axon outgrowth suggests that NFs are important for axon development, but their contribution to axon growth is unclear. Previous experiments in Xenopus laevis embryos demonstrated that antibody-induced disruption of NFs stunts axonal growth but left unresolved how the loss of NFs affects the dynamics of axon growth. In the current study, dissociated cultures were made from the spinal cords of embryos injected at the two-cell stage with an antibody to the middle molecular mass NF protein (NF-M), and time-lapse videomicroscopy was used to study early neurite outgrowth in descendants of both the injected and uninjected blastomeres. The injected antibody altered the growth dynamics primarily in long neurites (>85 microm). These neurites were initiated just as early and terminated growth no sooner than did normal ones. Rather, they spent relatively smaller fractions of time actively extending than normal. When growth occurred, it did so at the same velocity. In very young neurites, which have NFs made exclusively of peripherin, NFs were unaffected, but in the shaft of older neurites, which have NFs that contain NF-M, NFs were disrupted. Thus growth was affected only after NFs were disrupted. In contrast, the distributions of alpha-tubulin and mitochondria were unaffected; thus organelles were still transported into neurites. However, mitochondrial staining was brighter in descendants of injected blastomeres, suggesting a greater demand for energy. Together, these results suggest a model in which intra-axonal NFs facilitate elongation of long axons by making it more efficient.

Selective accumulation of the high molecular weight neurofilament subunit within the distal region of growing axonal neurites

Axonal maturation in situ is accompanied by the transition of neurofilaments (NFs) comprised of only NF-M and NF-L to those also containing NF-H. Since NF-H participates in interactions of NFs with each other and with other cytoskeletal constituents, its appearance represents a critical event in the stabilization of axons that accompanies their maturation. Whether this transition is effected by replacement of "doublet" NFs with "triplet" NFs, or by incorporation of NF-H into existing doublet NFs is unclear. To address this issue, we examined the distribution of NF subunit immunoreactivity within axonal cytoskeletons of differentiated NB2a/d1 cell and DRG neurons between days 3-7 of outgrowth. Endogenous immunoreactivity either declined in a proximal-distal gradient or was relatively uniform along axons. This distribution was paralleled by microinjected biotinylated NF-L. By contrast, biotinylated NF-H displayed a bipolar distribution, with immunoreactivity concentrated within the proximal- and distal-most axonal regions. Proximal biotinylated NF-H accumulation paralleled that of endogenous NF immunoreactivity; however, distal-most biotinylated NF-H accumulation dramatically exceeded that of endogenous NFs and microinjected NF-L. This phenomenon was not due to co-polymerization of biotin-H with vimentin or alpha-internexin. This phenomenon declined with continued time in culture. These data suggest that NF-H can incorporate into existing cytoskeletal structures, and therefore suggest that this mechanism accounts for at least a portion of the accumulation of triplet NFs during axonal maturation. Selective NF-H accumulation into existing cytoskeletal structures within the distal-most region may provide de novo cytoskeletal stability for continued axon extension and/or stabilization.

Differential expression and localization of neuronal intermediate filament proteins within newly developing neurites in dissociated cultures of Xenopus laevis embryonic spinal cord

The molecular subunit composition of neurofilaments (NFs) progressively changes during axon development. In developing Xenopus laevis spinal cord, peripherin emerges at the earliest stages of neurite outgrowth. NF-M and XNIF (an alpha-internexin-like protein) appear later, as axons continue to elongate, and NF-L is expressed after axons contact muscle. Because NFs are the most abundant component of the vertebrate axonal cytoskeleton, we must understand why these changes occur before we can fully comprehend how the cytoskeleton regulates axon growth and morphology. Knowing where these proteins are localized within developing neurites and how their expression changes with cell contact is essential for this understanding. Thus, we examined by immunofluorescence the expression and localization of these NF subunits within dissociated cultures of newly differentiating spinal cord neurons. In young neurites, peripherin was most abundant in distal neuritic segments, especially near branch points and extending into the central domain of the growth cone. In contrast, XNIF and NF-M were usually either absent from very young neurites or exhibited a proximal to distal gradient of decreasing intensity. In older neurites, XNIF and NF-M expression increased, whereas that of peripherin declined. All three of these proteins became more evenly distributed along the neurites, with some branches staining more intensely than others. At 24 h, NF-L appeared, and in 48-h cultures, its expression, along with that of NF-M, was greater in neurites contacting muscle cells, arguing that the upregulation of these two subunits is dependent on contact with target cells. Moreover, this contact had no effect on XNIF or peripherin expression. Our findings are consistent with a model in which peripherin plays an important structural role in growth cones, XNIF and NF-M help consolidate the intermediate filament cytoskeleton beginning in the proximal neurite, and increased levels of NF-L and NF-M help further solidify the cytoskeleton of axons that successfully reach their targets.

Characterization of neuronal intermediate filament protein expression in cervical spinal motor neurons in sporadic amyotrophic lateral sclerosis (ALS)

Because transgenic mice expressing an altered stoichiometry of neurofilament proteins develop a motor neuron degeneration associated with neurofilamentous aggregate formation similar to that found in amyotrophic lateral sclerosis (ALS), we studied the expression of intermediate filament proteins in sporadic ALS. Archival cervical spinal cord paraffin-embedded sections from 11 disease and 11 control cases were studied by either in situ hybridization using 35S-labeled riboprobes or immunohistochemically using specific antibodies for the individual neurofilament subunit proteins, alpha-internexin, nestin, peripherin, vimentin, beta-actin, or Talpha1-tubulin. Median NFL, alpha-internexin, and peripherin steady-state mRNA levels were significantly reduced in the lateral motor neuron cell column (p < 0.05) of ALS cases, while neither NFM nor NFH mRNA levels were altered. ALS cases demonstrated an elevation of beta-actin mRNA levels (p < 0.01) with no increase in Talpha1-tubulin mRNA levels. No motor neuronal expression of nestin or vimentin was observed. Ubiquitin-immunoreactive perikaryal aggregates were immunoreactive for NFH or beta-actin, but not for peripherin, alpha-internexin, vimentin, or nestin. In contrast, neuroaxonal spheroids were strongly immunoreactive for NFH and peripherin, but not for beta-actin, alpha-internexin, vimentin, or nestin. These findings suggest that the stoichiometry of cytoskeletal protein expression in ALS spinal motor neurons is significantly altered in a pattern conducive to the formation of neurofilamentous aggregates.