No requirement of alpha-internexin for nervous system development and for radial growth of axons

Predicting Key Genes and Therapeutic Molecular Modelling to Explain the Association between Porphyromonas gingivalis (P. gingivalis) and Alzheimer’s Disease (AD)

The association between Porphyromonas gingivalis (P. gingivalis) and Alzheimer’s disease (AD) remains unclear. The major aim of this study was to elucidate the role of genes and molecular targets in P. gingivalis-associated AD. Two Gene Expression Omnibus (GEO) datasets, GSE5281 for AD (n = 84 Alzheimer’s, n = 74 control) and GSE9723 (n = 4 P. gingivalis, n = 4 control), were downloaded from the GEO database. Differentially expressed genes (DEGs) were obtained, and genes common to both diseases were drawn. Additionally, Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) analysis was performed from the top 100 genes (50 upregulated and 50 downregulated genes). We then proceeded with CMap analysis to screen for possible small drug molecules targeting these genes. Subsequently, we performed molecular dynamics simulations. A total of 10 common genes (CALD1, HES1, ID3, PLK2, PPP2R2D, RASGRF1, SUN1, VPS33B, WTH3DI/RAB6A, and ZFP36L1) were identified with a p-value < 0.05. The PPI network of the top 100 genes showed UCHL1, SST, CHGB, CALY, and INA to be common in the MCC, DMNC, and MNC domains. Out of the 10 common genes identified, only 1 was mapped in CMap. We found three candidate small drug molecules to be a fit for PLK2, namely PubChem ID: 24971422, 11364421, and 49792852. We then performed molecular docking of PLK2 with PubChem ID: 24971422, 11364421, and 49792852. The best target, 11364421, was used to conduct the molecular dynamics simulations. The results of this study unravel novel genes to P. gingivalis-associated AD that warrant further validation.

Tumor Spheroids of an Aggressive Form of Central Neurocytoma Have Transit-Amplifying Progenitor Characteristics with Enhanced EGFR and Tumor Stem Cell Signaling

Central neurocytoma (CN) has been known as a benign neuronal tumor. In rare cases, CN undergoes malignant transformation to glioblastomas (GBM). Here we examined its cellular origin by characterizing differentiation potential and gene expression of CN-spheroids. First, we demonstrate that both CN tissue and cultured primary cells recapitulate the hierarchal cellular composition of subventricular zone (SVZ), which is comprised of neural stem cells (NSCs), transit amplifying progenitors (TAPs), and neuroblasts. We then derived spheroids from CN which displayed EGFR+/MASH+ TAP and BLBP+ radial glial cell (RGC) characteristic, and mitotic neurogenesis and gliogenesis by single spheroids were observed with cycling multipotential cells. CN-spheroids expressed increased levels of pluripotency and tumor stem cell genes such as KLF4 and TPD5L1, when compared to their differentiated cells and human NSCs. Importantly, Gene Set Enrichment Analysis showed that gene sets of GBM-Spheroids, EGFR Signaling, and Packaging of Telomere Ends are enriched in CN-spheroids in comparison with their differentiated cells. We speculate that CN tumor stem cells have TAP and RGC characteristics, and upregulation of EGFR signaling as well as downregulation of eph-ephrin signaling have critical roles in tumorigenesis of CN. And their ephemeral nature of TAPs destined to neuroblasts, might reflect benign nature of CN.

Cytoskeletal organization of axons in vertebrates and invertebrates

The maintenance of axons for the lifetime of an organism requires an axonal cytoskeleton that is robust but also flexible to adapt to mechanical challenges and to support plastic changes of axon morphology. Furthermore, cytoskeletal organization has to adapt to axons of dramatically different dimensions, and to their compartment-specific requirements in the axon initial segment, in the axon shaft, at synapses or in growth cones. To understand how the cytoskeleton caters to these different demands, this review summarizes five decades of electron microscopic studies. It focuses on the organization of microtubules and neurofilaments in axon shafts in both vertebrate and invertebrate neurons, as well as the axon initial segments of vertebrate motor- and interneurons. Findings from these ultrastructural studies are being interpreted here on the basis of our contemporary molecular understanding. They strongly suggest that axon architecture in animals as diverse as arthropods and vertebrates is dependent on loosely cross-linked bundles of microtubules running all along axons, with only minor roles played by neurofilaments.

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.

Filaments and phenotypes: cellular roles and orphan effects associated with mutations in cytoplasmic intermediate filament proteins

Cytoplasmic intermediate filaments (IFs) surround the nucleus and are often anchored at membrane sites to form effectively transcellular networks. Mutations in IF proteins (IFps) have revealed mechanical roles in epidermis, muscle, liver, and neurons. At the same time, there have been phenotypic surprises, illustrated by the ability to generate viable and fertile mice null for a number of IFp-encoding genes, including vimentin. Yet in humans, the vimentin ( VIM) gene displays a high probability of intolerance to loss-of-function mutations, indicating an essential role. A number of subtle and not so subtle IF-associated phenotypes have been identified, often linked to mechanical or metabolic stresses, some of which have been found to be ameliorated by the over-expression of molecular chaperones, suggesting that such phenotypes arise from what might be termed "orphan" effects as opposed to the absence of the IF network per se, an idea originally suggested by Toivola et al. and Pekny and Lane.

Myelinated axons fail to develop properly in a genetically authentic mouse model of Charcot-Marie-Tooth disease type 2E

We have analyzed a mouse model of Charcot-Marie-Tooth disease 2E (CMT2E) harboring a heterozygous p.Asn98Ser (p.N98S) Nefl mutation, whose human counterpart results in a severe, early-onset neuropathy. Behavioral, electrophysiological, and pathological analyses were done on separate cohorts of Nefl^N98S/+ mutant mice and their wild type Nefl^+/+ littermates between 8 and 48 weeks of age. The motor performance of Nefl^N98S/+ mice, as evidenced by altered balance and gait measures, was impaired at every age examined (from 6 to 25 weeks of age). At all times examined, myelinated axons were smaller and contained markedly fewer neurofilaments in Nefl^N98S/+ mice, in all examined aspects of the PNS, from the nerve roots to the distal ends of the sciatic and caudal nerves. Similarly, the myelinated axons in the various tracts of the spinal cord and in the optic nerves were smaller and contained fewer neurofilaments in mutant mice. The myelinated axons in both the PNS and the CNS of mutant mice had relatively thicker myelin sheaths. The amplitude and the nerve conduction velocity of the caudal nerves were reduced in proportion with the diminished sizes of myelinated axons. Conspicuous aggregations of neurofilaments were only seen in primary sensory and motor neurons, and were largely confined to the cell bodies and proximal axons. There was evidence of axonal degeneration and regeneration of myelinated axons, mostly in distal nerves. In summary, the p.N98S mutation causes a profound reduction of neurofilaments in the myelinated axons of the PNS and CNS, resulting in substantially reduced axonal diameters, particularly of large myelinated axons, and distal axon loss in the PNS.

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.

Composite bottlebrush mechanics: α-internexin fine-tunes neurofilament network properties

Neuronal cytoplasmic intermediate filaments are principal structural and mechanical elements of the axon. Their expression during embryonic development follows a differential pattern, while their unregulated expression is correlated to neurodegenerative diseases. The largest neurofilament proteins of medium (NF-M) and high molecular weight (NF-H) were shown to modulate the axonal architecture and inter-filament spacing. However, the individual roles of the remaining α-internexin (α-Inx) and neurofilament of low molecular weight (NF-L) proteins in composite filaments remained elusive. In contrast to previous predictions, we show that when co-assembled with NF-M, the shortest and the least charged α-Inx protein increases inter-filament spacing. These findings suggest a novel structural explanation for the expression pattern of neurofilament proteins during embryonic development. We explain our results by an analysis of ionic cross-links between the disordered polyampholytic C-terminal tails and suggest that a collapsed conformation of the α-Inx tail domain interferes with tail cross-linking near the filament backbone.

Neurofilament dynamics and involvement in neurological disorders

Neurons are extremely polarised cells in which the cytoskeleton, composed of microtubules, microfilaments and neurofilaments, plays a crucial role in maintaining structure and function. Neurofilaments, the 10-nm intermediate filaments of neurons, provide structure and mechanoresistance but also provide a scaffolding for the organization of the nucleus and organelles such as mitochondria and ER. Disruption of neurofilament organization and expression or metabolism of neurofilament proteins is characteristic of certain neurological syndromes including Amyotrophic Lateral Sclerosis, Charcot-Marie-Tooth sensorimotor neuropathies and Giant Axonal Neuropathy. Microfluorometric live imaging techniques have been instrumental in revealing the dynamics of neurofilament assembly and transport and their functions in organizing intracellular organelle networks. The insolubility of neurofilament proteins has limited identifying interactors by conventional biochemical techniques but yeast two-hybrid experiments have revealed new roles for oligomeric, nonfilamentous structures including vesicular trafficking. Although having long half-lives, new evidence points to degradation of subunits by the ubiquitin-proteasome system as a mechanism of normal turnover. Although certain E3-ligases ubiquitinating neurofilament proteins have been identified, the overall process of neurofilament degradation is not well understood. We review these mechanisms of neurofilament homeostasis and abnormalities in motor neuron and peripheral nerve disorders. Much remains to discover about the disruption of processes that leads to their pathological aggregation and accumulation and the relevance to pathogenesis. Understanding these mechanisms is crucial for identifying novel therapeutic strategies.

Insights into intermediate filament regulation from development to ageing

Intermediate filament (IF) proteins comprise a large family with more than 70 members. Initially, IFs were assumed to provide only structural reinforcement for the cell. However, IFs are now known to be dynamic structures that are involved in a wide range of cellular processes during all stages of life, from development to ageing, and during homeostasis and stress. This Commentary discusses some lesser-known functional and regulatory aspects of IFs. We specifically address the emerging roles of nestin in myogenesis and cancer cell migration, and examine exciting evidence on the regulation of nestin and lamin A by the notch signalling pathway, which could have repercussions for our understanding of the roles of IF proteins in development and ageing. In addition, we discuss the modulation of the post-translational modifications of neuronally expressed IFs and their protein-protein interactions, as well as IF glycosylation, which not only has a role in stress and ageing, but might also regulate IFs during development. Although many of these recent findings are still preliminary, they nevertheless open new doors to explore the functionality of the IF family of proteins.

Nestin is required for the proper self-renewal of neural stem cells

The intermediate filament protein, nestin, is a widely employed marker of multipotent neural stem cells (NSCs). Recent in vitro studies have implicated nestin in a number of cellular processes, but there is no data yet on its in vivo function. Here, we report the construction and functional characterization of Nestin knockout mice. We found that these mice show embryonic lethality, with neuroepithelium of the developing neural tube exhibiting significantly fewer NSCs and much higher levels of apoptosis. Consistent with this in vivo observation, NSC cultures derived from knockout embryos show dramatically reduced self-renewal ability that is associated with elevated apoptosis but no overt defects in cell proliferation or differentiation. Unexpectedly, nestin deficiency has no detectable effect on the integrity of the cytoskeleton. Furthermore, the knockout of Vimentin, which abolishes nestin's ability to polymerize into intermediate filaments in NSCs, does not lead to any apoptotic phenotype. These data demonstrate that nestin is important for the proper survival and self-renewal of NSCs, and that this function is surprisingly uncoupled from nestin's structural involvement in the cytoskeleton.

A novel recessive Nefl mutation causes a severe, early-onset axonal neuropathy

OBJECTIVE

To report the first cases of a homozygous recessive mutation in NEFL, the gene that encodes the light subunit of neurofilaments.

METHODS

Clinical and electrophysiologic data were evaluated, and a sural nerve biopsy from one affected child was examined by immunohistochemistry and electron microscopy. The ability of the mutant protein to form filaments was characterized in an established cell culture system.

RESULTS

Four of five siblings developed of a severe, progressive neuropathy beginning in early childhood. Serial nerve conduction studies showed progressively reduced amplitudes with age and pronounced slowing at all ages. Visual-evoked responses were slowed in three children, indicating that central nervous system axons were subclinically involved. All four affected children were homozygous for a nonsense mutation at glutamate 210 (E210X) in the NEFL gene; both parents were heterozygous carriers. A sural nerve biopsy from an affected patient showed markedly reduced numbers of myelinated axons; the remaining myelinated axons were small and lacked intermediate filaments. The E210X mutant protein did not form an intermediate filament network and did not interfere with the filament formation by wild-type human light subunit of neurofilaments in SW-13 vim(-) cells.

INTERPRETATION

This is the first demonstration of a recessive NEFL mutation, which appears to cause a simple loss of function, resulting in a severe, early-onset axonal neuropathy with unique features. These results confirm that neurofilaments are the main determinant of axonal caliber and conduction velocity, and demonstrate for the first time that neurofilaments are required for the maintenance of myelinated peripheral nervous system axons.

Dysfunctions of neuronal and glial intermediate filaments in disease

Intermediate filaments (IFs) are abundant structures found in most eukaryotic cells, including those in the nervous system. In the CNS, the primary components of neuronal IFs are alpha-internexin and the neurofilament triplet proteins. In the peripheral nervous system, a fifth neuronal IF protein known as peripherin is also present. IFs in astrocytes are primarily composed of glial fibrillary acidic protein (GFAP), although vimentin is also expressed in immature astrocytes and some mature astrocytes. In this Review, we focus on the IFs of glial cells (primarily GFAP) and neurons as well as their relationship to different neurodegenerative diseases.

Amyotrophic lateral sclerosis – The tools of the trait

The aim of this review is to analyze how our knowledge on the etiology, pathology, and treatment of amyotrophic lateral sclerosis (ALS) has profited from the application of biotechnology tools for the identification of disease markers, the development of animal disease models, and the design of innovative therapeutics. In humans, ALS-specific clinical, genetic or protein biomarkers, or panels of biomarkers stemming from genomics and proteomics analyses can be critical for early diagnosis, monitoring of disease progression, drug validation in clinical trials, and identification of therapeutic targets for subsequent drug development. At the same time, animal models representing a number of human superoxide dismutase 1 mutations, intermediate-filament disorganization or axonal-transport defects have been invaluable in unraveling aspects of the pathophysiology of the disease; in each case, these only represent a small proportion of all ALS patients. Preclinical and clinical trials, although at present heavily concentrating on pharmacological approaches, are embracing the emerging alternative strategies of stem-cell and gene therapy. In combination with a further subcategorization of patients and the development of corresponding model systems for functional analyses, they will significantly influence the already changing face of ALS therapy.

Intermediate filaments: a historical perspective

Intracellular protein filaments intermediate in size between actin microfilaments and microtubules are composed of a surprising variety of tissue specific proteins commonly interconnected with other filamentous systems for mechanical stability and decorated by a variety of proteins that provide specialized functions. The sequence conservation of the coiled-coil, alpha-helical structure responsible for polymerization into individual 10 nm filaments defines the classification of intermediate filament proteins into a large gene family. Individual filaments further assemble into bundles and branched cytoskeletons visible in the light microscope. However, it is the diversity of the variable terminal domains that likely contributes most to different functions. The search for the functions of intermediate filament proteins has led to discoveries of roles in diseases of the skin, heart, muscle, liver, brain, adipose tissues and even premature aging. The diversity of uses of intermediate filaments as structural elements and scaffolds for organizing the distribution of decorating molecules contrasts with other cytoskeletal elements. This review is an attempt to provide some recollection of how such a diverse field emerged and changed over about 30 years.

New movements in neurofilament transport, turnover and disease

Revealing the mechanisms by which neurofilament transport and turnover are regulated has proven difficult over the years but recent studies have given new insight into these processes. Mature neurofilament fibers may incorporate a fourth functional subunit, alpha-internexin, as new evidence suggests. Recent findings have made the role of phosphorylation in regulating neurofilament transport velocity controversial. Kinesin and dynein may transport neurofilaments in slow axonal transport as they have been found to associate with neurofilaments. Neurofilament transport and turnover rates may be reduced depending on the existing stationary neurofilament network. Finally, mutations in neurofilament light that have been linked to Charcot-Marie-Tooth disease as well as other neurofilament abnormalities in human disease are discussed.

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.

Transgenic mouse models of amyotrophic lateral sclerosis

The discovery of missense mutations in the gene coding for the Cu/Zn superoxide dismutase 1 (SOD1) in subsets of familial cases was rapidly followed by the generation of transgenic mice expressing various forms of SOD1 mutants. The mice overexpressing high levels of mutant SOD1 mRNAs do develop motor neuron disease but unraveling the mechanisms of pathogenesis has been very challenging. Studies with mouse lines suggest that the toxicity of mutant SOD1 is unrelated to copper-mediated catalysis but rather to propensity of a subfraction of mutant SOD1 proteins to form misfolded protein species and aggregates. However, the mechanism of toxicity of SOD1 mutants remains to be elucidated. Involvement of cytoskeletal components in ALS pathogenesis is supported by several mouse models of motor neuron disease with neurofilament abnormalities and with genetic defects in microtubule-based transport. Here, we describe how transgenic mouse models have been used for understanding pathogenic pathways of motor neuron disease and for pre-clinical drug testing.

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.

Overexpression of neuronal intermediate filament protein alpha-internexin in PC12 cells

The neuronal intermediate filaments include not only the neurofilament triplet proteins but also peripherin and alpha-internexin. To determine whether neurite outgrowth is enhanced by alpha-internexin, the cDNA of rat alpha-internexin tagged with enhanced green fluorescent protein (EGFP) was transfected into a rat adrenal pheochromocytoma cell line PC12 that responds to nerve growth factor (NGF) by induction of the neuronal phenotype. Selected stable clones were induced by NGF and examined for expression patterns of neuronal intermediate filaments by Western blot and immunocytochemistry. Differentiating neurons were also collected after NGF induction for RT-PCR analysis. Overexpressed alpha-internexin-EGFPs were found mainly in cell bodies and the proximal part of neurites. It was also found that overexpression of alpha-internexin-EGFPs enhanced the neurite outgrowth of PC12 cells at the early stages of NGF induction. Meantime, NF-L and NF-M were upregulated by the overexpression of alpha-internexin-EGFPs. Interestingly, alpha-internexin-EGFP-transfected cells obviously detached from culture plates at the later stages of NGF induction. Massive IF accumulations, swelling mitochondria, and degenerating neurites with numerous electron-dense granules were observed ultrastructurally in the alpha-internexin-EGFP-transfected cells. In addition, neuronal death was also characterized positively by the TUNEL assay. These observations may imply that cell death was occurring in alpha-internexin-EGFP-transfected cells. From this study, it could be suggested that alpha-internexin plays an important role in neurite outgrowth and regulates the expression of other neurofilaments during neuronal development. Apoptosis-like cell death could also be induced by the overexpression of alpha-internexin-EGFP in PC12 cells after NGF induction.

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.

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.

Discrete gene sets depend on POU domain transcription factor Brn3b/Brn-3.2/POU4f2 for their expression in the mouse embryonic retina

Brn3b/Brn-3.2/POU4f2 is a POU domain transcription factor that is essential for retinal ganglion cell (RGC) differentiation, axonal outgrowth and survival. Our goal was to establish a link between Brn3b and the downstream events leading to RGC differentiation. We sought to determine both the number and types of genes that depend on Brn3b for their expression. RNA probes from wild-type and Brn3b(-/-) E14.5, E16.5 and E18.5 mouse retinas were hybridized to a microarray containing 18,816 retina-expressed cDNAs. At E14.5, we identified 87 genes whose expression was significantly altered in the absence of Brn3b and verified the results by real-time PCR and in situ hybridization. These genes fell into discrete sets that encoded transcription factors, proteins associated with neuron integrity and function, and secreted signaling molecules. We found that Brn3b influenced gene expression in non RGCs of the retina by controlling the expression of secreted signaling molecules such as sonic hedgehog and myostatin/Gdf8. At later developmental stages, additional alterations in gene expression were secondary consequences of aberrant RGC differentiation caused by the absence of Brn3b. Our results demonstrate that a small but crucial fraction of the RGC transcriptome is dependent on Brn3b. The Brn3b-dependent gene sets therefore provide a unique molecular signature for the developing retina.

Neurobehavioral characteristics of mice with modified intermediate filament genes

Intermediate proteins comprise cytoskeletal elements that preserve the shape and structure of neurons. These proteins have been proposed to be involved in the onset and progression of amyotrophic lateral sclerosis (ALS), mainly characterized by motoneuron atrophy and paresis. In support of this hypothesis are the findings that genetically modified mice for intermediate filaments successfully mimic certain neuropathological aspects of ALS, such as reduced axonal caliber and retarded conduction speed in peripheral nerves, although often without leading to paresis. Nevertheless, even in those models with no overt phenotype, the involvement of intermediate proteins in motor function is underlined by the deficits in tests of balance and equilibrium revealed in mice containing transgenes for neurofilament of heavy molecular weight (NFH), alpha-internexin, peripherin, and vimentin. In addition, spatial learning was impaired in transgenic mice expressing transgenes for NFH and NFM, similar to the memory deficits reported in patients with ALS.

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.

Increased expression of multiple neurofilament mRNAs during regeneration of vertebrate central nervous system axons

Characteristic changes in the expression of neuronal intermediate filaments (nIFs), an abundant cytoskeletal component of vertebrate axons, accompany successful axon regeneration. In mammalian regenerating PNS, expression of nIFs that are characteristic of mature neurons becomes suppressed throughout regeneration, whereas that of peripherin, which is abundant in developing axons, increases. Comparable changes are absent from mammalian injured CNS; but in goldfish and lamprey CNS, expression of several nIFs increases during axon regrowth. To obtain a broader view of the nIF response of successfully regenerating vertebrate CNS, in situ hybridization and video densitometry were used to track multiple nIF mRNAs during optic axon regeneration in Xenopus laevis. As in other successfully regenerating systems, peripherin expression increased rapidly after injury and expression of those nIFs characteristic of mature retinal ganglion cells decreased. Unlike the decrease in nIF mRNAs of regenerating PNS, that of Xenopus retinal ganglion cells was transient, with most nIF mRNAs increasing above normal during axon regrowth. At the peak of regeneration, increases in each nIF mRNA resulted in a doubling of the total amount of nIF mRNA, as well as a shift in the relative proportions contributed by each nIF. The relative proportions of peripherin and NF-M increased above normal, whereas proportions of xefiltin and NF-L decreased and that of XNIF remained the same. The increases in peripherin and NF-M mRNAs were accompanied by increases in protein. These results are consistent with the hypothesis that successful axon regeneration involves changes in nIF subunit composition conducive to growth and argue that a successful injury response differs between CNS and PNS.

Cytoskeletal and morphological alterations underlying axonal sprouting after localized transection of cortical neuron axons in vitro

We examined the cytoskeletal dynamics that characterize neurite sprouting after axonal injury to cortical neurons maintained in culture for several weeks and compared these with initial neurite development. Cultured neocortical neurons, derived from embryonic day 18 rats, were examined at 3 d in vitro (DIV) and at various time points after axotomy at 21 DIV. The postinjury neuritic response was highly dynamic, progressing through an initial phase of retraction, followed by substantial axonal sprouting within 4-6 hr. Postinjury sprouts were motile and slender with expanded growth cone-like end structures. Microtubule markers were localized to sprout shafts and the proximal regions of putative growth cones and filamentous actin was distributed throughout growth cones, whereas neurofilament proteins were restricted to sprout shafts. A similar distribution of cytoskeletal proteins was present in developing neurites at 3 DIV. Exposure of developing and mature, injured cultures to the microtubule stabilizing agent taxol (10 microg/ml) caused growth inhibition, process distension, the transformation of growth cones into bulbous structures, and abnormal neurite directionality. Microtubule and neurofilament segregation occurred after taxol exposure in developing neurites and postinjury sprouts. Exposure to the microtubule destabilizing agent nocodazole (100 microg/ml) resulted in substantial morphological alteration of developing neurons and inhibited neurite growth and postinjury axonal sprouting. Our results indicate that the axons of cortical neurons have an intrinsic ability to sprout after transection, and similar cytoskeletal dynamics underlie neurite development and postinjury axonal sprouting.

Pathways to motor neuron degeneration in transgenic mouse models

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurological disorder characterized by the selective loss of motor neurons. A pathological hallmark of both sporadic and familial ALS is the presence of abnormal accumulations of neurofilament and peripherin proteins in motor neurons. In the past decade, transgenic mouse approaches have been used to address the role of such cytoskeletal abnormalities in motor neuron disease and also to unravel the pathogenesis caused by mutations in the gene coding for superoxide dismutase 1 (SOD1) that account for ~20% of familial ALS cases. In mouse models, disparate effects could result from different types of intermediate filament (IF) aggregates. Perikaryal IF accumulations induced by the overexpression of any of the three wild-type neurofilament proteins were quite well tolerated by motor neurons. Indeed, perikaryal swellings provoked by NF-H overexpression can even confer protection against toxicity of mutant SOD1. Other types of IF aggregates seem neurotoxic, such as those found in transgenic mice overexpressing either peripherin or an assembly-disrupting NF-L mutant. Moreover, understanding the toxicity of SOD1 mutations has been surprisingly difficult. The analysis of transgenic mice expressing mutant SOD1 has yielded complex results, suggesting that multiple pathways may contribute to disease that include the involvement of non-neuronal cells.

Axonally transported peripheral signals regulate alpha-internexin expression in regenerating motoneurons

The class IV neuronal intermediate filament (IF) family proteins includes the neurofilament (NF) triplet proteins NF-L, NF-M, and NF-H and also the more recently characterized alpha-internexin-NF66. It is well established that NF-L, -M, and -H protein and mRNA are downregulated after peripheral nerve injury. We examined alpha-internexin protein expression after three facial nerve lesion paradigms: crush, transection, and resection. Alpha-internexin immunoreactivity was absent in the perikarya of uninjured facial motoneurons but increased dramatically in all three injury paradigms, with maximum immunoreactivity observed at 7 d after injury. Twenty-eight days after nerve crush or transection, there was a dramatic decrease in the number of alpha-internexin-positive cells. In contrast, alpha-internexin remained elevated 28 d after nerve resection, an injury that hinders regeneration and target reinnervation. In situ hybridization studies showed an increase in alpha-internexin mRNA expression in the facial nucleus at 7 and 14 d after injury. Retrograde transport of fluorogold from the whisker pads to the facial nucleus was seen only in motoneurons that lacked alpha-internexin immunoreactivity, supporting the idea that target reinnervation and inhibitory signals from the periphery regulate the expression of alpha-internexin. Blockage of axonal transport through local colchicine application induced strong immunoreactivity in motoneurons. Alpha-internexin expression was also examined after central axotomy of rubrospinal neurons, which constitutively show alpha-internexin immunoreactivity. After rubrospinal tractotomy, alpha-internexin immunoreactivity transiently increased by 7 d after injury but returned to control levels by 14 d. We conclude that alpha-internexin upregulation in injured motoneurons suggests a role for this IF protein in neuronal regeneration.

Reduced number of unmyelinated sensory axons in peripherin null mice

Peripherin is a type III intermediate filament (IF) abundantly expressed in developing neurons, but in the adult, it is primarily found in neurons extending to the peripheral nervous system. It has been suggested that peripherin may play a role in axonal elongation and/or cytoskeletal stabilization during development and regeneration. To further clarify the function of peripherin, we generated and characterized mice with a targeted disruption of the peripherin gene. The peripherin null mice were viable, reproduced normally and did not exhibit overt phenotypes. Microscopic analysis revealed no gross morphological defects in the ventral and dorsal roots, spinal cord, retina and gut, but protein analyses showed increased levels of the type IV IF alpha-internexin in ventral roots of peripherin null mice. Whereas the number and caliber of myelinated motor and sensory axons in the L5 roots remained unchanged in peripherin knockout mice, there was a substantial reduction ( approximately 34%) in the number of L5 unmyelinated sensory fibers that correlated with a decreased binding of the lectin IB4. These results demonstrate a requirement of peripherin for the proper development of a subset of sensory neurons.

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.

Localization of the gene for the intermediate form of Charcot-Marie-Tooth to chromosome 10q24.1-q25.1

Intermediate Charcot-Marie-Tooth neuropathy (CMT) is an inherited sensory motor neuropathy characterized by motor median nerve conduction velocities of 25-45 m/s. We performed a genomewide search in an Italian family with autosomal dominant intermediate CMT and mapped the locus on chromosome 10q. Analysis of key recombinants maps the gene for autosomal dominant intermediate CMT to a 10.7-Mb interval on chromosome 10q24.1-q25.1, between simple tandem repeat markers D10S1709 and D10S1795.

Cytoskeletal abnormalities in amyotrophic lateral sclerosis: beneficial or detrimental effects?

Cytoskeletal abnormalities have been reported in cases of amyotrophic lateral sclerosis (ALS) including abnormal inclusions containing neurofilaments (NFs) and/or peripherin, reduced mRNA levels for the NF light (NF-L) protein and mutations in the NF heavy (NF-H) gene. Recently, transgenic mouse approaches have been used to address whether cytoskeletal changes may contribute to motor neuron disease. Mice lacking one of the three NF subunits are viable and do not develop motor neuron disease. Nonetheless, mice with null mutations for NF-L or for both NF-M and NF-H genes developed severe atrophy of ventral and dorsal root axons. The atrophic process is associated with hind limb paralysis during aging in mice deficient for both NF-M and NF-H proteins. The overexpression in mice of transgenes coding for wild-type or mutant NF proteins can provoke abnormal NF accumulations, axonal atrophy and sometimes motor dysfunction. However, the perikaryal NF accumulations are generally well tolerated by motor neurons and, except for expression of a mutant NF-L transgene, they did not provoke massive motor neuron death. Increasing the levels of perikaryal NF proteins may even confer protection in motor neuron disease caused by ALS-linked mutations in the superoxide dismutase (SOD1). In contrast, the overexpression of wild-type peripherin, a type of IF gene upregulated by inflammatory cytokines, provoked the formation of toxic IF inclusions with the high-molecular-weight NF proteins resulting in the death of motor neurons during aging. These results together with the detection of peripherin inclusions at early stage of disease in mice expressing mutant SOD1 suggest that IF inclusions containing peripherin may play a contributory role in ALS pathogenesis.

Formation of intermediate filament protein aggregates with disparate effects in two transgenic mouse models lacking the neurofilament light subunit

Protein aggregates containing intermediate filaments (IFs) are a hallmark of degenerating spinal motor neurons in amyotrophic lateral sclerosis (ALS). Recently, we reported that a deficiency in neurofilament light subunit (NF-L), a phenomenon associated with ALS, promoted the formation of IF inclusions with ensuing motor neuron death in transgenic mice overproducing peripherin, a type III IF protein detected in axonal inclusions of ALS patients. To further assess the role of NF-L in the formation of abnormal IF inclusions, we generated transgenic mice overexpressing human neurofilament heavy subunits (hNF-H) in a context of targeted disruption of the NF-L gene (hH;L-/- mice). The hH;L-/- mice exhibited motor dysfunction, and they developed nonfilamentous protein aggregates containing NF-H and peripherin proteins in the perikarya of spinal motor neurons. However, the perikaryal protein aggregates in the hH;L-/- mice did not provoke motor neuron death, unlike toxic IF inclusions induced by peripherin overexpression in NF-L null mice (Per;L-/- mice). Our results indicate that different types of IF protein aggregates with distinct properties may occur in a context of NF-L deficiency and that an axonal localization of such aggregates may be an important factor of toxicity.

Intermediate filaments and their associates: multi-talented structural elements specifying cytoarchitecture and cytodynamics

The assembly of intermediate filament (IF) arrays involves the recruitment of a complex set of cell-type-specific IF-associated proteins. Some of them are integral membrane proteins, others act as crosslinking proteins with vectorial binding activities, and yet others comprise motor proteins. In vivo IFs appear to be predominantly heteropolymers, although in vitro several IF proteins (e.g. vimentin, desmin, neurofilament (NF)-L and the nuclear lamins) do self-assemble into IF-like polymers. In contrast, NF-M, NF-H, nestin, synemin and paranemin, all bona fide IF proteins, are unable to self-assemble into IFs either in vitro or in vivo. The individual IF proteins of this large multigene family are chemically heterogeneous, exhibiting different assembly kinetics and yielding discrete types of filaments. The unique physical properties and interaction capabilities of these distinct IF molecular building blocks, in combination with accessory proteins, mediate the generation of a highly dynamic and interconnected, cell-type-specific cytoarchitecture.

Neurofilament functions in health and disease

Transgenic approaches have recently been used to investigate the functions of neuronal intermediate filaments. Gene knockout studies have demonstrated that neurofilaments are not required for axogenesis and that individual neurofilament proteins play distinct roles in filament assembly and in the radial growth of axons. The involvement of neurofilaments in disease is supported by the discovery of novel mutations in the neurofilament heavy gene from cases of amyotrophic lateral sclerosis and by reports of neuronal death in mouse models expressing neurofilament and alpha-internexin transgenes. However, mouse studies have shown that axonal neurofilaments are not required for pathogenesis caused by mutations in superoxide dismutase and that increasing perikaryal levels of neurofilament proteins may even confer protection in this disease.