Roles of head and tail domains in alpha-internexin's self-assembly and coassembly with the neurofilament triplet proteins

Are the Head and Tail Domains of Intermediate Filaments Really Unstructured Regions?

Intermediate filaments (IFs) are integral components of the cytoskeleton which provide cells with tissue-specific mechanical properties and are involved in a plethora of cellular processes. Unfortunately, due to their intricate architecture, the 3D structure of the complete molecule of IFs has remained unresolved. Even though most of the rod domain structure has been revealed by means of crystallographic analyses, the flanked head and tail domains are still mostly unknown. Only recently have studies shed light on head or tail domains of IFs, revealing certainsecondary structures and conformational changes during IF assembly. Thus, a deeper understanding of their structure could provide insights into their function.

Distribution patterns of the zebrafish neuronal intermediate filaments inaa and inab

It has been reported that the neuronal intermediate filament (IF) α-internexin may plays a role in the formation of the neuronal cytoskeleton during mammalian development. From a phylogenetic viewpoint, zebrafish express inaa and inab as homologs of mammalian α-internexin. However, the distribution patterns of the inaa and inab proteins throughout zebrafish development have not been well-characterized. We generated antibodies specific for zebrafish inaa and inab and analyzed the distribution of these two proteins in developing zebrafish. Inaa was identified in the major subdivisions of embryonic and larval brains as early as 1 day postfertilization (dpf), including the telencephalon, optic tectum, and cerebellum, and inab was also detected in the same regions from 3 dpf to the adult stage. Moreover, we demonstrated for the first time that inaa was distinctively expressed in the photoreceptor-like cells of the pineal gland, where inab was sparsely detected. Besides, the expression of inaa in male adult fish was found to be stable under different photoperiod conditions. Thus, we suggest that inaa is one of useful markers for studies of zebrafish cone photoreceptors not only in the retina but also in the pineal gland. In conclusion, we report that the distribution patterns of inaa and inab are phylogenetically conserved in the telencephalon, optic tectum, and cerebellum. Moreover, inaa and inab had different expression patterns in the pineal gland and retina during zebrafish development. Both inaa and inab are neuronal IFs and their functional roles may be different in various aspects of zebrafish neuronal development.

Developmental pattern of the neuronal intermediate filament inaa in the zebrafish retina

α-Internexin is a member of the neuronal intermediate filament (nIF) protein family, which also includes peripherin and neurofilament (NF) triplet proteins. Previous studies found that expression of α-internexin precedes that of the NF triplet proteins in mammals and suggested that α-internexin plays a key role in the neuronal cytoskeleton network during development. In this study, we aimed to analyze the expression patterns and function of internexin neuronal intermediate filament protein-alpha a (inaa), the encoding gene of which is a homolog of the mammalian α-internexin, during retinal development in zebrafish. Via in vitro and in vivo studies, we demonstrated that zebrafish inaa is an α-internexin homolog that shares characteristics with nIFs. An immunohistochemical analysis of zebrafish revealed that inaa was distributed dynamically in the developing retina. It was widely localized in retinal neuroepithelial cells at 1 day postfertilization (dpf), and was mainly found in the ganglion cell layer (GCL) and inner part of the inner nuclear layer (INL) from 3-9 dpf; after 14 dpf, it was restricted to the outer nuclear layer (ONL). Moreover, we demonstrated for the first time that inaa acted distinctively from the cytoskeletal scaffold of zebrafish cone photoreceptors during development. In conclusion, we demonstrated the morphological features of a novel nIF, inaa, and illustrated its developmental expression pattern in the zebrafish retina. J. Comp. Neurol. 524:3810-3826, 2016. © 2016 Wiley Periodicals, Inc.

A plasmid-based expression system to study protein-protein interactions at the Golgi in vivo

There is still an unmet need for simple methods to verify, visualize, and confirm protein-protein interactions in vivo. Here we describe a plasmid-based system to study such interactions. The system is based on the transmembrane domain (TMD) of the EF-hand Ca(2+) sensor protein calneuron-2. We show that fusion of 28 amino acids that include the TMD of calneuron-2 to proteins of interest results in prominent localization on the cytoplasmic side of the Golgi. The recruitment of binding partners to the protein of interest fused to this sequence can then be easily visualized by fluorescent tags.

Expression pattern of neuronal intermediate filament α-internexin in anterior pituitary gland and related tumors

PURPOSE

α-Internexin (INA) is a class IV neuronal intermediate filament protein that maintains the morphogenesis of neurons. It is expressed in developing neuroblasts and represents the major component of the cytoskeleton in cerebellar granule cells of adult central nervous system tissue. Data concerning INA expression in the human frontal pituitary lobe and related adenomas (PA) is missing.

METHODS

Using immunohistochemistry we examined the distribution pattern of INA in a large cohort of 152 PA, 11 atypical PA, 4 pituitary carcinomas and 20 normal pituitaries (overall n = 187). Quantity of INA protein expression was semi-quantitatively evaluated and grouped into five categories (0 = 0%; 1 = >0-5%; 2 = >5-35%; 3 = >35-80%; 4 = >80% of cells).

RESULTS

Cellular staining intensity of INA appeared significantly higher in gonadotropinomas (Go, n = 62), null cell adenomas (NC, n = 7) and thyrotropinomas (TSHomas, n = 7) compared to the other tumor subtypes (p ≤ 0.001). Furthermore, Go and NC showed a peculiar pseudorosette-like staining pattern surrounding blood vessels in 85.5% (59/69) of cases. Interestingly, areas exhibiting homogenous INA staining were often associated with oncocytic cell changes and decreased immunohistochemically detectable hormone expression. Only 8.5% (8/94) of other PA showed a comparable INA distribution (p ≤ 0.001).

CONCLUSION

Go, NC as well as TSHomas exhibit high levels of intracellular INA protein indicating neuronal transdifferentiation. A possible impact on pathogenesis and endocrine activity needs further investigation.

Neurofilament light polypeptide gene N98S mutation in mice leads to neurofilament network abnormalities and a Charcot-Marie-Tooth Type 2E phenotype

Charcot-Marie-Tooth disease (CMT) is the most commonly inherited neurological disorder with a prevalence of 1 in 2500 people worldwide. Patients suffer from degeneration of the peripheral nerves that control sensory information of the foot/leg and hand/arm. Multiple mutations in the neurofilament light polypeptide gene, NEFL, cause CMT2E. Previous studies in transfected cells showed that expression of disease-associated neurofilament light chain variants results in abnormal intermediate filament networks associated with defects in axonal transport. We have now generated knock-in mice with two different point mutations in Nefl: P8R that has been reported in multiple families with variable age of onset and N98S that has been described as an early-onset, sporadic mutation in multiple individuals. Nefl(P8R/+) and Nefl(P8R/P8R) mice were indistinguishable from Nefl(+/+) in terms of behavioral phenotype. In contrast, Nefl(N98S/+) mice had a noticeable tremor, and most animals showed a hindlimb clasping phenotype. Immunohistochemical analysis revealed multiple inclusions in the cell bodies and proximal axons of spinal cord neurons, disorganized processes in the cerebellum and abnormal processes in the cerebral cortex and pons. Abnormal processes were observed as early as post-natal day 7. Electron microscopic analysis of sciatic nerves showed a reduction in the number of neurofilaments, an increase in the number of microtubules and a decrease in the axonal diameters. The Nefl(N98S/+) mice provide an excellent model to study the pathogenesis of CMT2E and should prove useful for testing potential therapies.

Molecular cloning and characterization of chicken neuronal intermediate filament protein α-internexin

α-Internexin is one of the neuronal intermediate filament (IF) proteins, which also include low-, middle-, and high-molecular-weight neurofilament (NF) triplet proteins, designated NFL, NFM, and NFH, respectively. The expression of α-internexin occurs in most neurons as they begin differentiation and precedes the expression of the NF triplet proteins in mammals. However, little is known about the gene sequence and physiological function of α-internexin in avians. In this study we describe the molecular cloning of the mRNA sequence encoding the chicken α-internexin (chkINA) protein from embryonic brains. The gene structure and predicted amino acid sequence of chkINA exhibited high similarity to those of its zebrafish, mouse, rat, bovine, and human homologs. Data from transient-transfection experiments show that the filamentous pattern of chkINA was found in transfected cells and colocalized with other endogenous IFs, as demonstrated via immunocytochemistry using a chicken-specific antibody. The expression of chkINA was detected at the early stage of development and increased during the developmental process of the chicken. chkINA was expressed widely in chicken brains and colocalized with NF triplet proteins in neuronal processes, as assessed using immunohistochemistry. We also found that chkINA was expressed abundantly in the developing cerebellum and was the major IF protein in the parallel processes of granule neurons. Thus, we suggest that chkINA is a neuron-specific IF protein that may be a useful marker for studies of chicken brain development.

A neuronal death model: overexpression of neuronal intermediate filament protein peripherin in PC12 cells

BACKGROUND

Abnormal accumulation of neuronal intermediate filament (IF) is a pathological indicator of some neurodegenerative disorders. However, the underlying neuropathological mechanisms of neuronal IF accumulation remain unclear. A stable clone established from PC12 cells overexpressing a GFP-Peripherin fusion protein (pEGFP-Peripherin) was constructed for determining the pathway involved in neurodegeneration by biochemical, cell biology, and electronic microscopy approaches. In addition, pharmacological approaches to preventing neuronal death were also examined.

RESULTS

Results of this study showed that TUNEL positive reaction could be detected in pEGFP-Peripherin cells. Swollen mitochondria and endoplasmic reticulum (ER) were seen by electron microscopy in pEGFP-Peripherin cells on day 8 of nerve growth factor (NGF) treatment. Peripherin overexpression not only led to the formation of neuronal IF aggregate but also causes aberrant neuronal IF phosphorylation and mislocation. Western blots showed that calpain, caspase-12, caspase-9, and caspase-3 activity was upregulated. Furthermore, treatment with calpain inhibitor significantly inhibited cell death.

CONCLUSIONS

These results suggested that the cytoplasmic neuronal IF aggregate caused by peripherin overexpression may induce aberrant neuronal IF phosphorylation and mislocation subsequently trapped and indirectly damaged mitochondria and ER. We suggested that the activation of calpain, caspase-12, caspase-9, and caspase-3 were correlated to the dysfunction of the ER and mitochondria in our pEGFP-Peripherin cell model. The present study suggested that pEGFP-Peripherin cell clones could be a neuronal death model for future studies in neuronal IFs aggregate associated neurodegeneration.

Spatial patterns of FUS-immunoreactive neuronal cytoplasmic inclusions (NCI) in neuronal intermediate filament inclusion disease (NIFID)

Neuronal intermediate filament inclusion disease (NIFID), a rare form of frontotemporal lobar degeneration (FTLD), is characterized neuropathologically by focal atrophy of the frontal and temporal lobes, neuronal loss, gliosis, and neuronal cytoplasmic inclusions (NCI) containing epitopes of ubiquitin and neuronal intermediate filament (IF) proteins. Recently, the 'fused in sarcoma' (FUS) protein (encoded by the FUS gene) has been shown to be a component of the inclusions of NIFID. To further characterize FUS proteinopathy in NIFID, we studied the spatial patterns of the FUS-immunoreactive NCI in frontal and temporal cortex of 10 cases. In the cerebral cortex, sectors CA1/2 of the hippocampus, and the dentate gyrus (DG), the FUS-immunoreactive NCI were frequently clustered and the clusters were regularly distributed parallel to the tissue boundary. In a proportion of cortical gyri, cluster size of the NCI approximated to those of the columns of cells was associated with the cortico-cortical projections. There were no significant differences in the frequency of different types of spatial patterns with disease duration or disease stage. Clusters of NCI in the upper and lower cortex were significantly larger using FUS compared with phosphorylated, neurofilament heavy polypeptide (NEFH) or α-internexin (INA) immunohistochemistry (IHC). We concluded: (1) FUS-immunoreactive NCI exhibit similar spatial patterns to analogous inclusions in the tauopathies and synucleinopathies, (2) clusters of FUS-immunoreactive NCI are larger than those revealed by NEFH or ΙΝΑ, and (3) the spatial patterns of the FUS-immunoreactive NCI suggest the degeneration of the cortico-cortical projections in NIFID.

Proteomic identification of protein targets for 15-deoxy-Δ(12,14)-prostaglandin J2 in neuronal plasma membrane

15-deoxy-Δ(12,14)-prostaglandin J(2) (15d-PGJ(2)) is one of factors contributed to the neurotoxicity of amyloid β (Aβ), a causative protein of Alzheimer's disease. Type 2 receptor for prostaglandin D(2) (DP2) and peroxysome-proliferator activated receptorγ (PPARγ) are identified as the membrane receptor and the nuclear receptor for 15d-PGJ(2), respectively. Previously, we reported that the cytotoxicity of 15d-PGJ(2) was independent of DP2 and PPARγ, and suggested that 15d-PGJ(2) induced apoptosis through the novel specific binding sites of 15d-PGJ(2) different from DP2 and PPARγ. To relate the cytotoxicity of 15d-PGJ(2) to amyloidoses, we performed binding assay [(3)H]15d-PGJ(2) and specified targets for 15d-PGJ(2) associated with cytotoxicity. In the various cell lines, there was a close correlation between the susceptibilities to 15d-PGJ(2) and fibrillar Aβ. Specific binding sites of [(3)H]15d-PGJ(2) were detected in rat cortical neurons and human bronchial smooth muscle cells. When the binding assay was performed in subcellular fractions of neurons, the specific binding sites of [(3)H]15d-PGJ(2) were detected in plasma membrane, nuclear and cytosol, but not in microsome. A proteomic approach was used to identify protein targets for 15d-PGJ(2) in the plasma membrane. By using biotinylated 15d-PGJ(2), eleven proteins were identified as biotin-positive spots and classified into three different functional proteins: glycolytic enzymes (Enolase2, pyruvate kinase M1 (PKM1) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH)), molecular chaperones (heat shock protein 8 and T-complex protein 1 subunit α), cytoskeletal proteins (Actin β, F-actin-capping protein, Tubulin β and Internexin α). GAPDH, PKM1 and Tubulin β are Aβ-interacting proteins. Thus, the present study suggested that 15d-PGJ(2) plays an important role in amyloidoses not only in the central nervous system but also in the peripheral tissues.

The spectrum and severity of FUS-immunoreactive inclusions in the frontal and temporal lobes of ten cases of neuronal intermediate filament inclusion disease

Neuronal intermediate filament inclusion disease (NIFID), a rare form of frontotemporal lobar degeneration (FTLD), is characterized neuropathologically by focal atrophy of the frontal and temporal lobes, neuronal loss, gliosis, and neuronal cytoplasmic inclusions (NCI) containing epitopes of ubiquitin and neuronal intermediate filament proteins. Recently, the 'fused in sarcoma' (FUS) protein (encoded by the FUS gene) has been shown to be a component of the inclusions of familial amyotrophic lateral sclerosis with FUS mutation, NIFID, basophilic inclusion body disease, and atypical FTLD with ubiquitin-immunoreactive inclusions (aFTLD-U). To further characterize FUS proteinopathy in NIFID, and to determine whether the pathology revealed by FUS immunohistochemistry (IHC) is more extensive than α-internexin, we have undertaken a quantitative assessment of ten clinically and neuropathologically well-characterized cases using FUS IHC. The densities of NCI were greatest in the dentate gyrus (DG) and in sectors CA1/2 of the hippocampus. Anti-FUS antibodies also labeled glial inclusions (GI), neuronal intranuclear inclusions (NII), and dystrophic neurites (DN). Vacuolation was extensive across upper and lower cortical layers. Significantly greater densities of abnormally enlarged neurons and glial cell nuclei were present in the lower compared with the upper cortical laminae. FUS IHC revealed significantly greater numbers of NCI in all brain regions especially the DG. Our data suggest: (1) significant densities of FUS-immunoreactive NCI in NIFID especially in the DG and CA1/2; (2) infrequent FUS-immunoreactive GI, NII, and DN; (3) widely distributed vacuolation across the cortex, and (4) significantly more NCI revealed by FUS than α-internexin IHC.

The molecular basis of frontotemporal dementia

Frontotemporal dementia (FTD) is a clinical syndrome with a heterogeneous molecular basis. Familial FTD has been linked to mutations in several genes, including those encoding the microtubule-associated protein tau (MAPT), progranulin (GRN), valosin-containing protein (VCP) and charged multivescicular body protein 2B (CHMP2B). The associated neuropathology is characterised by selective degeneration of the frontal and temporal lobes (frontotemporal lobar degeneration, FTLD), usually with the presence of abnormal intracellular protein accumulations. The current classification of FTLD neuropathology is based on the identity of the predominant protein abnormality, in the belief that this most closely reflects the underlying pathogenic process. Major subgroups include those characterised by the pathological tau, TDP-43, intermediate filaments and a group with cellular inclusions composed of an unidentified ubiquitinated protein. This review will focus on the current understanding of the molecular basis of each of the major FTLD subtypes. It is anticipated that this knowledge will provide the basis of future advances in the diagnosis and treatment of FTD.

RE1 silencing transcription factor is involved in regulating neuron-specific expression of alpha-internexin and neurofilament genes

Alpha-internexin and the neurofilament triplet proteins (NF-L, NF-M, and NF-H) co-assemble into intermediate filament networks in neurons. We have found that the RE1 silencing transcription factor (REST) plays a contributory role in the neuron-specific expression of the alpha-internexin, NF-H and NF-M genes. Chromatin immunoprecipitation and transient transfection experiments performed with catecholaminergic neuronal Cath a.-differentiated (CAD) cells and non-neuronal NIH3T3 cells demonstrated that REST repressed transcription of these genes in NIH3T3 cells by binding and recruiting mSin3A, CoREST, histone deacetylase (HDAC) 1 and MeCP2 to the RE1 sites in the intron-1 of alpha-internexin and the 5' flanking regions of NF-H and NF-M. No repression effect of the RE1 sites was observed in CAD cells, which express these neuronal genes but not REST. Treatment of NIH3T3 cells with trichostatin A, a HDAC inhibitor, relieved the REST-mediated repression and induced ectopic activation of alpha-internexin, NF-H and NF-M. The trichostatin A treatment did not affect the levels of REST occupancy but caused coordinated changes in acetylation and methylation of histones around the RE1 sites of these genes in NIH3T3 cells consistent with a transition from transcriptional repression to transcriptional activation. Thus, REST regulates expression of these neuronal genes, partly by a HDAC-dependent epigenetic mechanism.

Development of starburst cholinergic amacrine cells in the retina of Tupaia belangeri

"Starburst" cholinergic amacrines specify the response of direction-selective ganglion cells to image motion. Here, development of cholinergic amacrines was studied in the tree shrew Tupaia belangeri (Scandentia) by immunohistochemistry with antibodies against choline acetyltransferase (ChAT) and neurofilament proteins. Starburst amacrines expressed ChAT much earlier than previously thought. From embryonic day 34 (E34) onward, orthotopic and displaced subpopulations segregated from a single cluster of immunoreactive precursor cells. Orthotopic starburst amacrines rapidly took up positions in the inner nuclear layer. Displaced starburst amacrines were first arranged in a monocellular row in the inner plexiform layer, and, with a delay of 1 week, they descended to the ganglion cell layer. Conversely, dendritic stratification of displaced amacrines slightly preceded that of orthotopic ones. Starburst amacrines expressed the medium-molecular-weight neurofilament protein (NF-M) from E34 to postnatal day 11 (P11) and coexpressed alpha-internexin from E36.5 to P11. Consequently, neurofilaments composed of alpha-internexin and NF-M may stabilize developing dendrites of starburst amacrines. During the first 2 postnatal weeks, subpopulations of anti-NF-M-labeled ganglion cells costratified with the preexisting dendritic strata of starburst amacrines in the ON sublamina, OFF sublamina, or both. Hence, anti-NF-M-labeled ganglion cells may include direction-selective ones. Thereafter, NF-M and alpha-internexin proteins disappeared from starburst amacrines, and NF-M immunoreactivity was lost in the dendrites of ganglion cells. Our findings suggest that NF-M and alpha-internexin are important for starburst amacrines and ganglion cells to recognize each other and, thus, contribute to the formation of early developing retinal circuits in the inner plexiform layer.

Spatial patterns of the pathological changes in neuronal intermediate filament inclusion disease (NIFID): an α-internexin immunohistochemical study

Neuronal intermediate filament inclusion disease (NIFID) is characterized by alpha-internexin positive neuronal cytoplasmic inclusions (NCI), swollen achromatic neurons (SN), neuronal loss, and gliosis. This study tested: 1) whether the spatial patterns of the lesions was topographically organized in areas of the frontal and temporal lobe and 2) whether a spatial relationship exists between the NCI and SN. The NCI were distributed in regular clusters and in a quarter of these areas, the clusters were 400-800 microm in diameter approximating to the size of the cells of origin of the cortico-cortical pathways. Variations in the density of the NCI were positively correlated with the SN. Hence, cortical degeneration in NIFID appears to be topographically organized and may affect the cortico-cortical projections, the clusters of NCI and SN developing within the same vertical columns of cells.

Topography of alpha-internexin-positive neuronal aggregates in 10 patients with neuronal intermediate filament inclusion disease

Abnormal neuronal intermediate filament (IF) inclusions immunopositive for the type IV IF alpha-internexin have been identified as the pathological hallmark of neuronal intermediate filament inclusion disease (NIFID). We studied the topography of these inclusions in the frontal and temporal lobe in 68 areas from 10 cases of NIFID. In the cerebral cortex, CA sectors of the hippocampus, and dentate gyrus granule cell layer, the inclusions were distributed mainly in regularly distributed clusters, 50-800 microm in diameter. In seven cortical areas, there was a more complex pattern in which the clusters of inclusions were aggregated into larger super clusters. In 11 cortical areas, the size of the clusters approximated to those of the cells of origin of the cortico-cortical pathways but in the majority of the remaining areas, cluster size was smaller than 400 microm. The topography of the lesions suggests that there is degeneration of the cortico-cortical projections in NIFID with the formation of alpha-internexin-positive aggregates within vertical columns of cells. Initially, only a subset of cells within a vertical column develops inclusions but as the disease progresses, the whole of the column becomes affected. The corticostriate projection appears to have little effect on the cortical topography of the inclusions.

Neuropathological changes in ten cases of neuronal intermediate filament inclusion disease (NIFID): a study using α-internexin immunohistochemistry and principal components analysis (PCA)

Ten cases of neuronal intermediate filament inclusion disease (NIFID) were studied quantitatively. The alpha-internexin positive neurofilament inclusions (NI) were most abundant in the motor cortex and CA sectors of the hippocampus. The densities of the NI and the swollen achromatic neurons (SN) were similar in laminae II/III and V/VI but glial cell density was greater in V/VI. The density of the NI was positively correlated with the SN and the glial cells. Principal components analysis (PCA) suggested that PC1 was associated with variation in neuronal loss in the frontal/temporal lobes and PC2 with neuronal loss in the frontal lobe and NI density in the parahippocampal gyrus. The data suggest: 1) frontal and temporal lobe degeneration in NIFID is associated with the widespread formation of NI and SN, 2) NI and SN affect cortical laminae II/III and V/VI, 3) the NI and SN affect closely related neuronal populations, and 4) variations in neuronal loss and in the density of NI were the most important sources of pathological heterogeneity.

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.

The cytoskeleton in neurodegenerative diseases

Abundant abnormal aggregates of cytoskeletal proteins are neuropathological signatures of many neurodegenerative diseases that are broadly classified by filamentous aggregates of neuronal intermediate filament (IF) proteins, or by inclusions containing the microtubule-associated protein (MAP) tau. The discovery of mutations in neuronal IF and tau genes firmly establishes the importance of neuronal IF proteins and tau in the pathogenesis of neurodegenerative diseases. Multiple IF gene mutations are pathogenic for Charcot-Marie-Tooth (CMT) disease and amyotrophic lateral sclerosis (ALS)--in addition to those in the copper/zinc superoxide dismutase-1 (SOD1) gene. Tau gene mutations are pathogenic for frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), and tau polymorphisms are genetic risk factors for sporadic progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). Thus, IF and tau abnormalities are linked directly to the aetiology and pathogenesis of neurodegenerative diseases. In vitro and transgenic animal models are being used to demonstrate that different mutations impair protein function, promote tau fibrilization, or perturb tau gene splicing, leading to aberrant and distinct tau aggregates. For recognition of these disorders at neuropathological examination, immunohistochemistry is needed, and this may be combined with biochemistry and molecular genetics to properly determine the nosology of a particular case. As reviewed here, the identification of molecular genetic defects and biochemical alterations in cytoskeletal proteins of human neurodegenerative diseases has facilitated experimental studies and will promote the development of assays of molecules which inhibit abnormal neuronal IF and tau protein inclusions.

alpha-internexin is present in the pathological inclusions of neuronal intermediate filament inclusion disease

Neuronal intermediate filament (IF) inclusion disease (NIFID) is a novel neurological disease of early onset with a variable clinical phenotype including frontotemporal dementia, pyramidal, and extrapyramidal signs. Pathologically, in affected areas, there is neuronal loss, astrocytosis, and neuronal intracytoplasmic aggregates of abnormal neuronal IFs that contain neither tau nor alpha-synuclein. Thus, to characterize the neuronal IF protein profile of inclusions in NIFID, immunohistochemistry (IHC) was performed on 10 cases of NIFID, four normal aged controls (NL), and two cases of Alzheimer's disease (AD) using a panel of anti-neuronal IF proteins. Immunoelectron microscopy was performed on selected cases and frozen tissue from the frontal lobe of four cases was used for biochemical studies including sequential extractions and Western blotting. Based on these studies, we report here for the first time that alpha-internexin, a neuronal IF protein, is present within the inclusions of NIFID as are all three neurofilament subunits: heavy, medium, and light. Thus, all class IV neuronal IF proteins are present within the pathological inclusions of this disease. Biochemistry revealed that IF aggregates were soluble in sodium dodecyl sulfate (SDS) and no post-translational modification was detected when compared with Alzheimer's disease or aged control brains. Hence, we conclude that NIFID is characterized by the pathological cytoplasmic aggregation of all class IV neuronal IF proteins in brain. The discovery of alpha-internexin in the cytoplasmic inclusions implicates novel mechanisms of pathogenesis in NIFID and other neurological diseases with pathological accumulations of IFs.

A specific ELISA for measuring neurofilament heavy chain phosphoforms

Neurofilaments (Nf) are the major constitutents of the axoskeleton and body fluid Nf levels are an important tool for estimating axonal degeneration in vivo. This paper presents a new sandwich ELISA allowing quantification of the NfH(SMI35) phosphoform from CSF, brain tissue and cell culture homogenates. The sensitivity of the NfH(SMI35) ELISA is 0.2 ng/ml with a recovery of 119% and a mean within- and between-batch precision of 10.6% and 23%, respectively. CSF NfH(SMI35) was stable at 4 degrees C, is not influenced by freeze-thaw cycles, and proteolysis present at room temperature could be prevented by adding protease inhibitors. Aggregate formation was observed for HPLC-purified bovine NfH and could be resolved by sonication. The upper reference value for CSF NfH(SMI35) levels (0.73 ng/ml) was defined as the 95% cumulative frequency from 416 CSF samples. Based on this cutoff, a significantly higher proportion of patients with amyotrophic lateral sclerosis, space-occupying lesions, disc prolapse and subarachnoid haemorrhage had pathologically elevated NfH(SMI35) levels compared to patients with cluster headache or demyelinating disease.A new nomenclature is proposed to facilitate the comparison between ELISA, immunoblotting and immunocytochemistry.

Effects of Charcot-Marie-Tooth-linked mutations of the neurofilament light subunit on intermediate filament formation

Neurofilaments (NFs) are the major intermediate filaments (IFs) of mature neurons. They play important roles in the structure and function of axons. Recently, two mutations in the neurofilament light (NFL) subunit have been identified in families affected by Charcot-Marie-Tooth (CMT) neuropathy type 2. We have characterized the effects of these NFL mutations on the formation of IF networks using a transient transfection system. Both mutations disrupted the self-assembly of human NFL. The Q333P mutant in the rod domain of NFL also disrupted the formation of rat and human NFL/NFM heteropolymers. The phenotypes produced by the P8R mutation in the head domain of NFL were less severe. The P8R mutant NFL co-polymerized with NFM to form bundled filaments and, less often, aggregates. Our results suggest that alterations in the formation of a normal IF network in neurons elicited by these NFL mutations may contribute to the development of Charcot-Marie-Tooth neuropathy.

Analysis of the roles of the head domains of type IV rat neuronal intermediate filament proteins in filament assembly using domain-swapped chimeric proteins

Type IV neuronal intermediate filament proteins consist of alpha-internexin, which can self-assemble into filaments and the neurofilament triplet proteins, which are obligate heteropolymers, at least in rodents. These IF proteins therefore provide good systems for elucidating the mechanism of intermediate filament assembly. To analyze the roles of the head domains of these proteins in contributing to their differential assembly properties, we generated chimeric proteins by swapping the head domains between rat alpha-internexin and either rat NF-L or NF-M and examined their assembly properties in transfected cells that lack their own cytoplasmic intermediate filament network. Lalphaalpha and Malphaalpha, the chimeric proteins generated by replacing the head domain of alpha-internexin with those of NF-L and NF-M, respectively, were unable to self-assemble into filaments. In contrast, alphaLL, a chimeric NF-L protein generated by replacing the head domain of NF-L with that of alpha-internexin, was able to self-assemble into filaments, whereas MLL, a chimeric NF-L protein containing the NF-M head domain, was unable to do so. These results demonstrate that the alpha-internexin head domain is essential for alpha-internexin's ability to self-assemble. While coassembly of Lalphaalpha with NF-M and coassembly of Malphaalpha with NF-L resulted in formation of filaments, coassembly of Lalphaalpha with NF-L and coassembly of Malphaalpha with NF-M yielded punctate patterns. These coassembly results show that heteropolymeric filament formation requires that one partner has the NF-L head domain and the other partner has the NF-M head domain. Thus, the head domains of rat NF-L and NF-M play important roles in determining the obligate heteropolymeric nature of filament formation. The data obtained from these self-assembly and coassembly studies provide some new insights into the mechanism of intermediate filament assembly.

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

Alpha-Internexin is a type IV intermediate filament protein that is expressed abundantly in neurons during development of the peripheral and central nervous systems as well as in few neurons of the adult central nervous system. It has been suggested that alpha-internexin may act as a scaffold for the formation of neuronal intermediate filaments during early development. In addition, recent reports suggest that alpha-internexin could play a major role in two degenerative neurological disorders. We report here an analysis of mice with a targeted disruption of alpha-internexin gene. Unexpectedly, alpha-internexin -/- mice developed normally and did not exhibit overt phenotypes. Moreover, the absence of alpha-internexin did not interfere with neurite extension of cultured DRG neurons. The number and caliber of L4 ventral root axons remained unchanged in alpha-internexin -/- mice. In the retina, alpha-internexin begins to be expressed in retinal ganglion cells when their first axons reach the optic chiasma. Using HRP tracer, we show that the projection pattern of the RGC axons is not modified by the absence of alpha-internexin. Electron microscopy did not reveal significant differences in axonal calibers, in myelination of axons and in neurofilament structures between alpha-internexin -/- and control mice during development and at adult stage. These data indicate that alpha-internexin is not required for the polymerization of neurofilament in vivo. Mice deficient for both alpha-internexin and neurofilament light chain (NF-L) exhibited no over phenotypes as well. No intermediate filament structures were detectable in optic nerve of alpha-internexin -/-; NF-L -/- mice. Ours results do not support the hypothesis of a role for type IV intermediate filaments in axonal outgrowth during development of nervous system.

The intermediate filament protein peripherin is the specific interaction partner of mouse BPAG1-n (dystonin) in neurons

The dystonia musculorum (dt) mouse suffers from severe degeneration of primary sensory neurons. The mutated gene product is named dystonin and is identical to the neuronal isoform of bullous pemphigoid antigen 1 (BPAG1-n). BPAG1-n contains an actin-binding domain at its NH2 terminus and a putative intermediate filament-binding domain at its COOH terminus. Because the degenerating sensory neurons of dt mice display abnormal accumulations of intermediate filaments in the axons, BPAG1-n has been postulated to organize the neuronal cytoskeleton by interacting with both the neurofilament triplet proteins (NFTPs) and microfilaments. In this paper we show by a variety of methods that the COOH-terminal tail domain of mouse BPAG1 interacts specifically with peripherin, but in contrast to a previous study (Yang, Y., J. Dowling, Q.C. Yu, P. Kouklis, D.W. Cleveland, and E. Fuchs. 1996. Cell. 86:655-665), mouse BPAG1 fails to associate with full-length NFTPs. The tail domains interfered with the association of the NFTPs with BPAG1. In dt mice, peripherin is present in axonal swellings of degenerating sensory neurons in the dorsal root ganglia and is downregulated even in other neural regions, which have no obvious signs of pathology. Since peripherin and BPAG1-n also display similar expression patterns in the nervous system, we suggest that peripherin is the specific interaction partner of BPAG1-n in vivo.