Neuronal intermediate filaments: new progress on an old subject

Changes in neurofilament light chain protein (NEFL) in children and adolescents with Schizophrenia and Bipolar Disorder: Early period neurodegeneration

AIM

Neurofilament light chain protein (NEFL), is defined as a structural protein which exists particularly in axones of neurons and is released to the cerum in consequence of neuroaxonal damage. The aim of this study is to investigate the peripheral cerumNEFLlevels of children and adolescents with early onset schizophrenia and bipolar disorder.

METHOD

In this study, we evaluated serum levels of NEFL in children and adolescents (13-17 years) with schizophrenia, bipolar disorder and healthy control group. The study is conducted with 35 schizophrenia, 38 bipolar disorder manic episode patients and 40 healthy controls.

RESULTS

The median age of the patient and control groups was 16 (IQR- Interquartile Range: 2). There was no statistical difference in median age (p = 0.52) and gender distribution(p = 0.53) between groups. NEFL levels of the patients with schizophrenia were significantly higher than the controls. NEFL levels of the patients with bipolar disorder were significantly higher than the controls. Serum levels of NEFL of the schizophrenia were higher than the bipolar disorder; however, the difference was not statistically significant.

CONCLUSION

In conclusion, serum NEFL level, as a confidential marker of neural damage, is increased in the children and adolescents with bipolar disorder and schizophrenia. This result may indicatea degenerative period in neurons of children and adolescents with schizophrenia or bipolar disorder and may play a role in the pathophisiology of these disorders. This result shows that there is neuronal damage in both diseases, but neuronal damage may be more in schizophrenia.

Neuronal intermediate filament IgGs in CSF: Autoimmune Axonopathy Biomarkers

Objectives

To describe CSF‐defined neuronal intermediate filament (NIF) autoimmunity.

Methods

NIF‐IgG CSF‐positive patients (41, 0.03% of 118599 tested, 1996–2019) were included (serum was neither sensitive nor specific). Criteria‐based patient NIF‐IgG staining of brain and myenteric NIFs was detected by indirect immunofluorescence assay (IFA); NIF‐specificity was confirmed by cell‐based assays (CBAs, alpha internexin, neurofilament light [NF‐L]), heavy‐[NF‐H] chain).

Results

Sixty‐one percent of 41 patients were men, median age, 61 years (range, 21–88). Syndromes were encephalopathy predominant (23), cerebellar ataxia predominant (11), or myeloradiculoneuropathies (7). MRI abnormalities (T2 hyperintensities of brain, spinal cord white matter tracts. and peripheral nerve axons) and neurophysiologic testing (EEG, EMG, evoked potentials) co‐localized with clinical neurological phenotypes (multifocal in 29%). Thirty patients (73%) had ≥ 1 immunological perturbation: cancer (paraneoplastic), 22; systemic infection (parainfectious [including ehrlichosis, 3] or HIV), 7; checkpoint‐inhibitor cancer immunotherapy, 4; other, 5. Cancers were as follows: neuroendocrine‐lineage carcinomas, 12 (small cell, 6; Merkel cell, 5; pancreatic, 1 [11/12 had NF‐L‐IgG detected, versus 8/29 others, P = 0.0005]) and other, 11. Onset was predominantly subacute (92%) and accompanied by inflammatory CSF (75%), and immunotherapy response (77%). In contrast, CSF controls (15684 total) demonstrated NIF‐IgG negativity (100% of test validation controls), and low frequencies of autoimmune diagnoses (20% of consecutively referred clinical specimens) and neuroendocrine‐lineage carcinoma diagnosis (3.1% vs. 30% of NIF cases), P < 0.0001. Median NF‐L protein concentration was higher in 8 NF‐L‐IgG‐positive patients (median, 6718 ng/L) than 16 controls.

Interpretation

Neurological autoimmunity, defined by CSF‐detected NIF‐IgGs, represents a continuum of treatable axonopathies, sometimes paraneoplastic or parainfectious.

Signaling mechanisms and disrupted cytoskeleton in the diphenyl ditelluride neurotoxicity

Evidence from our group supports that diphenyl ditelluride (PhTe)₂ neurotoxicity depends on modulation of signaling pathways initiated at the plasma membrane. The (PhTe)₂-evoked signal is transduced downstream of voltage-dependent Ca²⁺ channels (VDCC), N-methyl-D-aspartate receptors (NMDA), or metabotropic glutamate receptors activation via different kinase pathways (protein kinase A, phospholipase C/protein kinase C, mitogen-activated protein kinases (MAPKs), and Akt signaling pathway). Among the most relevant cues of misregulated signaling mechanisms evoked by (PhTe)₂ is the cytoskeleton of neural cells. The in vivo and in vitro exposure to (PhTe)₂ induce hyperphosphorylation/hypophosphorylation of neuronal and glial intermediate filament (IF) proteins (neurofilaments and glial fibrillary acidic protein, resp.) in different brain structures of young rats. Phosphorylation of IFs at specific sites modulates their association/disassociation and interferes with important physiological roles, such as axonal transport. Disrupted cytoskeleton is a crucial marker of neurodegeneration and is associated with reactive astrogliosis and apoptotic cell death. This review focuses the current knowledge and important results on the mechanisms of (PhTe)₂ neurotoxicity with special emphasis on the cytoskeletal proteins and their differential regulation by kinases/phosphatases and Ca²⁺-mediated mechanisms in developmental rat brain. We propose that the disrupted cytoskeletal homeostasis could support brain damage provoked by this neurotoxicant.

Topographic regulation of neuronal intermediate filaments by phosphorylation, role of peptidyl-prolyl isomerase 1: significance in neurodegeneration

The neuronal cytoskeleton is tightly regulated by phosphorylation and dephosphorylation reactions mediated by numerous associated kinases, phosphatases and their regulators. Defects in the relative kinase and phosphatase activities and/or deregulation of compartment-specific phosphorylation result in neurodegenerative disorders. The largest family of cytoskeletal proteins in mammalian cells is the superfamily of intermediate filaments (IFs). The neurofilament (NF) proteins are the major IFs. Aggregated forms of hyperphosphorylated tau and phosphorylated NFs are found in pathological cell body accumulations in the central nervous system of patients suffering from Alzheimer's disease, Parkinson's disease, and Amyotrophic Lateral Sclerosis. The precise mechanisms for this compartment-specific phosphorylation of cytoskeletal proteins are not completely understood. In this review, we focus on the mechanisms of neurofilament phosphorylation in normal physiology and neurodegenerative diseases. We also address the recent breakthroughs in our understanding the role of different kinases and phosphatases involved in regulating the phosphorylation status of the NFs. In addition, special emphasis has been given to describe the role of phosphatases and Pin1 in phosphorylation of NFs.

Changes of cytoskeletal proteins in nerve tissues and serum of rats treated with 2,5-hexanedione

To investigate the mechanisms and biomarker of the neuropathy induced by 2,5-hexanedione (HD), male Wistar rats were administrated HD at dosage of 200 or 400mg/kg for 8 weeks (five-times per week). All rats were sacrificed after 8 weeks of treatment and the cerebrum cortex (CC), spinal cord (SC) and sciatic nerves (SN) were dissected, homogenized and used for the determination of cytoskeletal proteins by western blotting. The levels of neurofilaments (NFs) subunits (NF-L, NF-M and NF-H) in nerve tissues of 200 and 400mg/kg HD rats significantly decreased in both the supernatant and pellet fractions. Furthermore, significant negative correlations between NFs levels and gait abnormality were observed. As for microtubule (MT) and microfilament (MF) proteins, the levels of alpha-tubulin, beta-tubulin and beta-actin in the supernatant and pellet fraction of SN significantly decreased in 200 and 400mg/kg HD rats and correlated negatively with gait abnormality. However, the contents of MT and MF proteins in CC and SC were inconsistently affected and had no significant correlation with gait abnormality. The levels of NF-L and NF-H in serum significantly increased, while NF-M, alpha-tubulin, beta-tubulin and beta-actin contents remain unchanged. A significant positive correlation (R=0.9427, P<0.01) was observed between gait abnormality and NF-H level in serum as the intoxication went on. These findings suggested that HD intoxication resulted in a progressive decline of cytoskeletal protein contents, which might be relevant to the mechanisms of HD-induced neuropathy. NF-H was the most sensitive index, which may serve as a good indicator for neurotoxicity of n-hexane or HD.

Timing of the retinoid-signalling pathway determines the expression of neuronal markers in neural progenitor cells

By culturing neural progenitor cells in the presence of retinoid receptor agonists, we have defined the components of the retinoid-signalling pathway that are important for the birth and maintenance of neuronal cells. We provide evidence that depending on the order and combination of retinoid receptors activated, different neuronal cells are obtained. Astrocytes and oligodendrocytes are predominantly formed in the presence of activated retinoic acid receptor (RAR) alpha, whereas motoneurons are formed when RARbeta is activated. We have looked at the regulation of two transcription factors islet-1/2 which are involved in neuronal development. We find that activated RARbeta up-regulates islet-1 expression, whereas activation of RARalpha can either act in combination with RARbeta signalling to maintain islet-1 expression or induce islet-2 expression in the absence of activated RARbeta. RARgamma cannot directly regulate islet-1/2 but can down-regulate RARbeta expression, which results in loss of islet-1 expression. We finally show that activated RARalpha is one of the final steps required for a mature motoneuron phenotype.

2,5-Hexanedione Induced Decrease in Cytoskeletal Proteins of Rat Sciatic?tibial Nerve

Exposure chronically to n-hexane produces peripheral-central axonopathy mediated by 2,5-hexanedione (HD). Previous studies have demonstrated decreases in neurofilament (NF) contents of peripheral and central nervous regions from rats intoxicated with HD, and recent analysis has demonstrated that axonal atrophy, instead of NF-filled swellings, is a specific component of morphologic alterations. To deeply investigate the alterations of cytoskeletal proteins in HD peripheral neuropathy, the relative levels of NF-L, NF-M, NF-H, alpha-tubulin, beta-tubulin and beta-actin of rat sciatic-tibial nerves were determined by SDS-PAGE and immunoblotting. HD was administrated to Wistar rats by intraperitoneal injection at dosage of 200 or 400 mg/kg/day (five-times per week). Rats were sacrificed after 6 weeks of treatment, and sciatic-tibial nerves were dissected, homogenized, and used for the determination of cytoskeletal proteins. Except for supernatant NF-L that could not be assayed, the results showed HD intoxication was associated with significant decreases in NF subunits in both of the supernatant and the pellet fractions of sciatic-tibial nerve homogenates (P < 0.01), and obvious reductions in alpha-tubulin, beta-tubulin and beta-actin only in the supernatant (P < 0.05 or P < 0.01). Among these alterations, the falls in the levels of NF subunits tended to be greater compared to those of the other cytoskeletal proteins in all HD-exposed groups, and the trend for decrements in NF-M was greater than those in the other NF subunits. Thus, HD intoxication was associated with significant declines in cytoskeletal protein contents in rat sciatic-tibial nerves, and the decreases might be related to the involvement of the peripheral axonopathy induced by HD.

Chronological alterations of neurofilament 150 immunoreactivity in the gerbil hippocampus and dentate gyrus after transient forebrain ischemia

In this study, we observed the chronological alterations of neurofilament 150 (NF-150) immunoreactivity in the gerbil hippocampus and dentate gyrus after 5 min transient forebrain ischemia. NF-150 immunoreactivity in the sham-operated group was mainly detected in mossy fibers and in the hilar region of the dentate gyrus. NF-150 immunoreactivity and protein contents of NF-150 and RT 97 (polyphosphorylation epitopes of neurofilament) were significantly decreased at 15 min after ischemic insult. Between 30 min and 12 h after ischemic insult, NF-150 immunoreactivity and protein content were significantly increased as compared with the sham-operated group. Thereafter, NF-150 immunoreactivity and protein content started to decrease. At 12 h after ischemic insult, unlike dentate gyrus, NF-150 immunoreactivity increased in pyramidal cells of the CA1 region. Thereafter, NF-150 immunoreactivity in the CA1 region started to decrease, and 4 days after ischemic insult, NF-150 immunoreactivity nearly was similar to that of the sham-operated group. These biphasic patterns of NF-150 immunoreactivity in the hippocampus and dentate gyrus are reverse correlated with that of the intracellular calcium influx. For calcium detection in the CA1 region, we also conducted alizarin red staining. Alizarin red positive neurons were detected in some neurons at 15-30 min after ischemic insult. At 12 h after ischemia, alizarin red positive neurons were decreased. Thereafter, alizarin red positive neurons started to decrease, but alizarin positive neurons were significantly increased in dying neurons 4 days after ischemia. These results suggest that ischemia-related changes of NF-150 expression may be caused by the calcium following transient forebrain ischemia.

Neuronal cyclin-dependent kinase 5: role in nervous system function and its specific inhibition by the Cdk5 inhibitory peptide

Cyclin-dependent kinase 5 (Cdk5) is a member of the cyclin-dependent kinase family that is involved in the regulation of the cell cycle. As their name suggests, the Cdks require association with activator proteins called cyclins for their activity. Cdk5, however, is unique to this family of proline-directed serine/threonine kinases on two accounts. Firstly, Cdk5 has not been found to function in the cell cycle and, although expressed in a number of tissues, its activity is restricted to the nervous system. Secondly, unlike the other members of the Cdk family, Cdk5 is not activated by association with a cyclin, although it can bind them. Instead, Cdk5 is activated by the activator proteins p35 and p39 that are structurally distinct from cyclins and have, for the most part, a neuronal-specific expression pattern. In the past decade of research on Cdk5, it is now established that Cdk5 activity is critical for the proper formation and function of the brain. Moreover, its role as a central kinase, phosphorylating its substrates in its 'cross-talk' control of other kinase and signal transduction pathways, has also been determined. In addition to the normal physiological role of Cdk5, the kinase has been implicated in certain neurodegenerative disorders. For example, Cdk5 associates with the proteolytic, more active p25 fragment that is derived through the cleavage of p35. In turn, the p25/Cdk5 complex aberrantly phosphorylates its substrates tau and neurofilaments, which has been implicated in the pathogenesis of these disorders. Here, we attempt to review the past decade of research on Cdk5 from our laboratory and others, on the roles of Cdk5 in nervous system function. Additionally, our research has recently uncovered a possible therapeutic avenue of research, focusing on inhibition of aberrant Cdk5 hyperactivity which may well be used to treat the symptoms of a number of neurodegenerative diseases. The elucidation of a specific inhibitor of p25/Cdk5, termed CIP, also inhibits p25/Cdk5-mediated tau phosphorylation. This may well provide us with avenues of research focusing on the inhibition of pathologically damaging p25/Cdk5 species.

Molecular Abnormalities of the Glutamate Synapse in the Thalamus in Schizophrenia

Schizophrenia has been associated with dysfunction of glutamatergic neurotransmission. Synaptic glutamate activates pre- and postsynaptic ionotropic NMDA, AMPA, and kainate and metabotropic receptors, is removed from the synapse via five cell surface-expressed transporters, and is packaged for release by three vesicular transporters. In addition, there is a family of intracellular molecules enriched in the postsynaptic density (PSD) that target glutamate receptors to the synaptic membrane, modulate receptor activity, and coordinate glutamate receptor-related signal transduction. Each family of PSD proteins is selective for a given glutamate receptor subtype, the most well characterized being the NMDA receptor binding proteins PSD93, PSD95, NF-L, and SAP102. Besides binding glutamate receptors, many of these proteins also interact with cell surface proteins like cell adhesion molecules, ion channels, cytoskeletal elements, and signal transduction molecules. Given the complexity of the glutamate neurotransmitter system, there are many locations where disruption of normal signaling could occur and give rise to abnormal glutamatergic neurotransmission in schizophrenia. Using multiple cohorts of postmortem tissue, we have examined these synaptic molecules in schizophrenic thalamus. The expression of NR1 and NR2C subunit transcripts is decreased in the thalamus in schizophrenia. Interestingly, three intracellular PSD molecules that link the NMDA receptor to signal transduction pathways are also abnormally expressed. Additionally, several of the cell surface and vesicular transporters are abnormal in the schizophrenic thalamus. While occasional findings of abnormal receptor expression are made, the most dramatic and consistent alterations that we have found in the thalamus in schizophrenia involve the family of intracellular signaling/scaffolding molecules. We propose that schizophrenia has a glutamatergic component that involves alterations in the intracellular machinery that is coupled to glutamate receptors, in addition to abnormalities of the receptors themselves. Our data suggest that schizophrenia is associated with abnormal glutamate receptor-related intracellular signaling in the thalamus, and point to novel targets for innovative drug discovery.

Promotion of axonal regeneration in the injured CNS

Molecules that are found in the extracellular environment at a CNS lesion site, or that are associated with myelin, inhibit axon growth. In addition, neuronal changes--such as an age-dependent reduction in concentrations of cyclic AMP--render the neuron less able to respond to axotomy by a rapid, forward, actin-dependent movement. An alternative mechanism, based on the protrusive forces generated by microtubule elongation or the anterograde transport of cytoskeletal elements, may underlie a slower form of axon elongation that happens during regeneration in the mature CNS. Therapeutic approaches that restore the extracellular CNS environment or the neuron's characteristics back to a more embryonic state increase axon regeneration and improve functional recovery after injury. These advances in the understanding of regeneration in the CNS have major implications for neurorehabilitation and for the use of axonal regeneration as a therapeutic approach to disorders of the CNS such as spinal-cord injury.

NFH-LacZ transgenic mice: regional brain activity of cytochrome oxidase

Expression of the NFH-LacZ fusion protein in transgenic mice causes an early accumulation of neurofilament proteins in the cell bodies of neurons, as well as a reduction of motor neuron axonal caliber and Purkinje cell number in the cerebellum. Young (3 month old) and older (12-20 months) NFH-LacZ transgenic mice were compared to normal controls for regional brain metabolism, as assessed by cytochrome oxidase (CO) activity. Irrespective of age, CO activity was reduced in three cerebellar-related regions of NFH-LacZ transgenic mice: (1) the lateral reticular nucleus, (2) the parvicellular red nucleus, and (3) the superior colliculus, possibly as a secondary consequence of cerebellar Purkinje cell histopathology. Aged NFH-LacZ mice had lower CO activity relative to either age-matched controls or young transgenic mice in the following regions: the motor nucleus of the vagus nerve, the trapezoid nucleus, the subiculum, the motor cortex, the superior olive, and the lateral dorsal thalamus. These results indicate regional and age-selective deficits of brain metabolism in a transgenic model with neurofilament maldistribution.

Chronic nicotine treatment changes the axonal distribution of 68 kDa neurofilaments in the rat ventral tegmental area

Region-specific decreases of neurofilament proteins (NF) were described in the ventral tegmental area (VTA) of rats treated chronically with morphine, cocaine or alcohol. In a previous study, we demonstrated that NF levels were also changed in the VTA after chronic treatment with nicotine. The aim of this study was to clarify the submicroscopic basis of decreased immunoreactivity for NF-68, NF-160 and NF-200, as determined by using NR4, BF10 and RT97 antibodies, respectively. Microdensitometric analysis of brain sections showed that immunoreactivity for all NF was reduced in the VTA of animals exposed chronically to nicotine (0.4 mg/kg per day, 6 days of treatment), when compared to rats exposed to saline. Reduction in immunoreactivity was significant for NF-68 (P < 0.05), NF-160 (P < 0.01) and NF-200 (P < 0.05), showing a relative reduction of 34%, 42% and 38%, respectively, when compared to saline-treated rats. No difference was observed for any of the NF under study when immunoreactivity measurements in the substantia nigra were compared. Ultrastructural analysis was applied to evaluate changes in NF-68, NF-160 and NF-200 immunoreactivity in regions of the VTA that contain dopaminergic neurons following chronic nicotine treatment. At the electron microscopic level, no degenerative changes were found in neurons or glial cells of the VTA. With ultrastructural immunohistochemistry, evaluation of the homogeneity parameter of NF distribution showed a loss of homogeneity for NF-68 linked to the nicotine treatment. In areas in which NF organization appeared well preserved, analysis of the numerical density of NF revealed no significant difference for NF-68 (897/ micro m2 vs. 990/ micro m2), NF-160 (970/ micro m2 vs. 820/ micro m2) and NF-200 (1107/ micro m2 vs. 905/ micro m2) in nicotine-treated rats when compared to saline-treated rats. These results confirm that nicotine shares the same properties with cocaine and morphine in reducing NF in the VTA, a key brain structure of the rewards system, and that chronic nicotine treatment changes the axonal distribution of 68 kDa neurofilaments in the rat VTA.

Normal dendritic arborization in spinal motoneurons requires neurofilament subunit L

Neurofilaments, composed of three polypeptide subunits, NF-L, NF-M, and NF-H, are major cytoskeletal elements in large neurons with long axons. Neurofilaments play a critical role in the development of axonal diameter; however, their role in the development of dendrites is largely unknown. By overexpressing different neurofilament subunits, we previously demonstrated that alteration of neurofilament subunit composition resulted in dramatic changes in dendritic arborization. To further determine the role of neurofilaments in dendritic growth, we examined and compared the dendritic architecture of spinal cord neurons in young NF-L knockout (-/-), heterozygous (+/-), and wild-type (+/+) mice. We show that an absence or reduction in the expression of NF-L inhibited dendritic growth most dramatically in large motoneurons, mildly in medium neurons, but had no effect on small neurons. We also reveal that a decrease in NF-L leads to an increase in NF-M and NF-H subunits in cell bodies and their reduction in dendrites. These results demonstrate that NF-L is a critical intrinsic factor for dendritic growth in large motoneurons.

The exact expression of glial fibrillary acidic protein (GFAP) in trigeminal ganglion and dental pulp

The expression in various cell types of peripheral tissues of glial fibrillary acidic protein (GFAP), first discovered as an intermediate filament specific for astrocytes, remains controversial owing to numerous reports of a wide distribution for GFAP-immunoreactivity in various cells. The present study employed immunohistochemistry to investigate the precise expression of GFAP in the dental pulp and trigeminal ganglion of adult rats and wild-type mice as well as GFAP-knockout mice. The exhibition of GFAP-immunoreactivity in the trigeminal ganglion was further examined by a reverse transcription polymerase chain reaction (RT-PCR) technique, and in situ hybridization histochemistry using a specific cRNA probe prepared by us. The immunoreaction for GFAP was recognizable in the axons, Schwann cells, and the fibroblasts in the dental pulp of rats and wild-type littermate mice. However, mice with null mutations in the GFAP gene remained immunoreactive for GFAP in all these locations. Intense GFAP-immunoreactivity was found in a small number of satellite cells in the trigeminal ganglion in all animals examined in this study. RT-PCR analysis demonstrated bands for the GFAP gene corresponding to the length expected from the primer design in the samples of trigeminal ganglion and dental pulp. In situ hybridization histochemistry also showed intense signals for GFAP mRNA in some satellite cells of the trigeminal ganglion, but never in the neurons. These data suggest that the GFAP-immunoreactive molecules in the pulpal axons and fibroblasts react non-specifically with the polyclonal antibody and are probably a closely related type of intermediate filament.

Auditory nerve fiber differences in the normal and neurofilament deficient Japanese quail

A primary axonal disease affecting the central and peripheral nervous system was discovered in a mutant strain of the Japanese quail, named quiver (Quv). We have previously demonstrated altered auditory evoked potentials in the neurofilament (NF) deficient quail. In this current study we attempt to find relationships between the auditory evoked potential results and the histo-pathological abnormalities of the auditory neurons. No abnormalities in the external auditory meatus and tympanic cavity were observed in either Quv or control quails and the ganglion cell bodies and their nuclei appeared normal by light microscopy. The myelin staining pattern was found to be similar in both strains with hematoxylin and eosin and Klüver-Barrera staining. The frequency histograms of fiber and axonal diameters of myelinated fibers showed an unimodal pattern in both strains. In Quv quails myelinated fibers and their axoplasm were smaller in diameter than in controls resulting in smaller neural tissue mass. In electron microscopic observation the axons of the Quv quail were composed of mitochondria and microtubules and smooth endoplasmic reticuli. In Quv quail electron micrographs of cochlear nerve myelinated fibers NFs were not seen in the axons and the neuronal cell bodies. Our current findings indicate that the previously reported reduction of conduction velocity of auditory evoked potentials may be due to smaller fiber and/or axonal diameter. The g-ratio, myelin thickness and fiber circularity were found to be the same for both strains. In conclusion, loss of axonal cytoskeletal elements (NFs) correlates well with our electrophysiological findings. Reduced conduction velocity and severely distorted auditory evoked potentials in NF deficient quails seem to be primarily due to axonal hypotrophy.

Temporal and spatial variations in slow axonal transport velocity along peripheral motoneuron axons

Neuronal axons are cellular extensions that can reach more than a meter in length. To maintain such a structure, macromolecules synthesized in cell bodies must be transported to the distal axons. Proteins associated with membranous organelles are generally transported in several fast transported groups, while cytoplasmic proteins, mostly composed of cytoskeletal proteins, are transported in slowly transported groups. Neurofilaments are a main component in the slowly transported group. Composed of three polypeptide subunits (NF-H, NF-M and NF-L), they are the most abundant cytoskeletal element in large myelinated axons. In various neurological or neurotoxic disorders, selective accumulation of neurofilaments was observed in different compartments of a neuron (cell bodies, proximal or distal axons). The underlying mechanism for this regional selectivity has been unclear. Using the classical pulse labeling method, we examined the changes in neurofilament transport velocity in transgenic mice that overexpress different neurofilament subunits. We present evidence that at least three velocities of neurofilament transport exist along peripheral motor axons. Each of these velocities was altered differently depending on which neurofilament subunit was overexpressed. We suggest that neurofilament transport in motor axons consists of multiple successive stages and that each of these stages is carried out by different transport mechanisms. These differences provide a basis for the regional deficiencies in axonal transport associated with several neurological disorders.

Overexpression of neurofilament subunit M accelerates axonal transport of neurofilaments

Neurofilaments are composed of three polypeptide subunits (NF-H, NF-M and NF-L). They are the most abundant cytoskeletal element in large myelinated axons and play a central role in development of axonal caliber. To perform this role, neurofilaments are transported from their site of synthesis, the cell bodies, to the distal axons. Previous studies showed that overexpression of NF-M in transgenic mice led to accumulation of neurofilaments in neurons and a reduction in the number of neurofilaments in axons, suggesting that axonal transport of neurofilaments was slowed. To determine whether this was the case, we measured axonal transport velocities in the wild type and transgenic mice overexpressing NF-M by the classical pulse-labeling method using 35S-methionine. We found that neurofilament transport in peripheral motor axons can be described with a model consistent with two linear velocities. Contrary to expectations, both velocities were accelerated by overexpression of NF-M. These results suggest that subunit composition in neurofilaments play a regulatory role in neurofilament transport. In addition, these results show that there are regional differences in neurofilament transport along long axons and these differences may be the basis for selective regional accumulation of neurofilaments in various neurological disorders.

Age-dependent changes in the midsized neurofilament subunit in sensory-motor systems of the cat brainstem: an immunocytochemical study

This study documents age-related changes in the immunoreactivity of the medium-molecular weight subunit of neurofilaments in sensory and motor neurons in the brainstem of the cat. In old age, there was a clear decrease in immunoreactivity in the following brainstem sensory and motor nuclei: sensory trigeminal, gracile, cuneate, and facial motor. Only a few neuronal perikarya and dendrites were labeled in these nuclei in old cats; moreover, when present, the labeling was weak. In contrast, in adult cats, these nuclei contained intensely stained neuronal perikarya and dendrites. In other sensory and motor nuclei of the brainstem, there was an obvious age-related increase in the immunoreactivity of the medium-molecular weight subunit of neurofilaments in the perikarya. Despite different patterns of age-related alterations in immunoreactivity within perikarya and dendrites in distinct brainstem regions, most sensory and motor axons in old cats were smaller than those in adult cats. A decrease in the medium-molecular weight neurofilament subunit in the dendrites may be the basis for the dendritic atrophy that has been shown to occur in sensory nuclei in old animals. The decrease in axonal size is likely to be one of the causes of the decrease in axonal conduction velocity, in these neurons, that was reported in our previous studies.

Chronic nicotine treatment decreases neurofilament immunoreactivity in the rat ventral tegmental area

Region-specific decreases of neurofilament proteins have been described in the ventral tegmental area of rats chronically treated with either morphine or cocaine. The aim of the present study was to assess if the levels of neurofilament proteins are changed in the ventral tegmental area by chronic treatment with nicotine. Immunoreactivity for NF-68, NF-160 and NF-200 was determined using NR4, BF10 and RT97 antibodies, respectively. Measurements were performed using computer-assisted microdensitometry of brain sections from rats exposed to chronic nicotine treatment (0.4 mg/kg/dayx6 days) or to saline. Chronic nicotine treatment reduced NF-160 and NF-200 immunoreactivity by 44.5% (P<0.01) and 22.5% (P<0. 05), respectively, in the ventral tegmental area but not in the substantia nigra. A trend towards reduction was observed for NF-68 immunoreactivity in the ventral tegmental area. These preliminary results suggest that nicotine shares the same properties with cocaine and morphine to reduce neurofilament proteins in the ventral tegmental area, a key brain structure of the reward system.

Changes in brain neurofilament and beta-tubulin proteins after cerebral hypoxia-ischemia in rabbits

Neurofilaments (NF) and tubulin are highly phosphorylated proteins that are important in neuronal structure and function. Changes in phosphorylation alter their antigenicity, and previous studies examining the effect of ischemia on these proteins failed to consider this factor. Using phosphate-independent antibodies and a quantitative immunoassay, we examined whether the amount of NF 68-kD (NF 68), NF 160-kD (NF 160), NF 200-kD (NF 200) and class III beta-tubulin proteins in the brain are reduced after cerebral injury. Rabbits were subjected to 8 min of hypoxia and then to 8 min of ischemia. After 4 h of reperfusion, NF 68 decreased from an overall group mean (+/- SEM) of 17.5+/-2.3 ng NF 68 microg/total protein in the noninjured controls (n = 8) to 12.9+/-1.2 ng/microg total protein in the hypoxic-ischemic group (n = 9). Conversely, NF 200 increased from 31.6+/-3.3 ng/microg in controls to 47.7+/-3.2 ng/microg. The amount of NF 160 and beta-tubulin was unchanged. The response of the NF proteins to brain injury is more complicated than described previously. Additional studies examining the regulation and metabolism of the NF are warranted, especially regarding the role of phosphorylation.

Immunohistochemical study of rabbit choroidal innervation

Immunocytochemical methods with antibodies to the light (68 kDa), medium (160 kDa), and heavy (200 kDa) chain subunits of the neurofilament triplet have been used to visualize neuronal structures in rabbit choroids. Choroidal nerve fibers were present in the suprachoroid and vascular laminae and absent in the choriocapillary layer. These fibers may be classified as perivascular and intervascular. Perivascular fibers surround all arterial and venous blood vessels and form a network; these fibers were labeled with the three NF antibodies, although they were more easily visualized with anti NF-160 and anti NF-200 than anti NF-68. Intervascular fibers formed two groups. The first group consisted of fibers situated between the blood vessels and parallel to the blood vessel wall surface (paravascular fibers); these fibers were better observed using anti NF-160 and NF-200 than anti NF-68. The second group consisted of fibers which travel the entire length of the choroid until they reach the nerve plexus of the ciliary body (long tract fibers). The plexus was observed with anti NF-68, anti NF-160 and anti NF-200; however, the long tract fibers were more clearly visualized with anti NF-160 and anti NF-200 than with anti NF-68. Two types of choroidal cell were also labeled: ganglion cells and melanocytes. Ganglion cells are small, scarce neurons situated in the peripheral choroid; they were labeled with anti NF-160 and anti NF-200. The melanocytes were only labeled with anti NF-200 and they were the only non neuronal structure visualized using antibodies against neurofilaments.

Requirement of heavy neurofilament subunit in the development of axons with large calibers

Neurofilaments (NFs) are prominent components of large myelinated axons. Previous studies have suggested that NF number as well as the phosphorylation state of the COOH-terminal tail of the heavy neurofilament (NF-H) subunit are major determinants of axonal caliber. We created NF-H knockout mice to assess the contribution of NF-H to the development of axon size as well as its effect on the amounts of low and mid-sized NF subunits (NF-L and NF-M respectively). Surprisingly, we found that NF-L levels were reduced only slightly whereas NF-M and tubulin proteins were unchanged in NF-H-null mice. However, the calibers of both large and small diameter myelinated axons were diminished in NF-H-null mice despite the fact that these mice showed only a slight decrease in NF density and that filaments in the mutant were most frequently spaced at the same interfilament distance found in control. Significantly, large diameter axons failed to develop in both the central and peripheral nervous systems. These results demonstrate directly that unlike losing the NF-L or NF-M subunits, loss of NF-H has only a slight effect on NF number in axons. Yet NF-H plays a major role in the development of large diameter axons.

Mouse models of myelin diseases

Dys- and demyelination are the common endpoints of several inherited diseases of glial cells, which elaborate myelin and which maintain the myelin sheath very much like an "external" cellular organelle. Whereas some of the genes that are affected by mutations appear to be glial-specific, other genes are expressed in many cell types but their defect is restricted to oligodendrocytes or Schwann cells. Many of the disease genes and their encoded proteins have been studied with the help of mouse models, and a number of different molecular pathomechanisms have emerged which have been summarized in Figure 8. Some of the new concepts in the field, which have been addressed in this review, have only emerged because similar pathomechanisms were discovered for different myelin proteins. Mouse models have clearly helped to address both, the molecular pathology of myelin diseases and the normal function of myelin genes, but as discussed in this review, these questions turned out to be very different. Despite the progress in understanding the role of the abundant myelin proteins, there also remain a number of open questions that concern, among other things, the initial axon-glia recognition, the assembly process of the myelin sheath, and the long-term interaction of axons with their myelinating glia. Finally, animal models of human neurological diseases should not be restricted to the study of pathology, but they should also contribute to the development of experimental treatments. It is encouraging that a few attempts have been made.

Age-related intra-axonal accumulation of neurofilaments in the dorsal column nuclei of the cat brainstem: a light and electron microscopic immunohistochemical study

In the present study, we examined the age-related intra-axonal accumulation of neurofilaments in the dorsal column nuclei of the cat by using immunohistochemical techniques combined with light and electron microscopy. Light microscopic analysis revealed oval or circular immunostained structures in the dorsal column nuclei of old cats. These immunostained structures were not observed in the material obtained from adult controls. Under the electron microscope, it was discovered that the immunostained structures were greatly enlarged axons with disrupted myelin sheaths. These enlarged axons contained massive accumulations of neurofilaments, some mitochondria, vacuoles and dense granules. The abnormalities of the myelin sheaths included the breaking of myelin at several locations, a splitting and ballooning in the myelin lamellae of the sheath and a distended periaxonal space between the axon and myelin sheaths. These ultrastructural changes resembled the degenerative alterations that have been observed in the axons of human and animals suffering from a number of pathological conditions, including giant axonal neuropathy and toxic neuropathy. Therefore, severely altered axons with intra-axonal accumulation of neurofilaments appear to reflect chronic degenerative changes that are a component of the aging process.

Antagonistic roles of neurofilament subunits NF-H and NF-M against NF-L in shaping dendritic arborization in spinal motor neurons

Dendrites play important roles in neuronal function. However, the cellular mechanism for the growth and maintenance of dendritic arborization is unclear. Neurofilaments (NFs), a major component of the neuronal cytoskeleton, are composed of three polypeptide subunits, NF-H, NF-M, and NF-L, and are abundant in large dendritic trees. By overexpressing each of the three NF subunits in transgenic mice, we altered subunit composition and found that increasing NF-H and/or NF-M inhibited dendritic arborization, whereas increasing NF-L alleviated this inhibition. Examination of cytoskeletal organization revealed that increasing NF-H and/or NF-M caused NF aggregation and dissociation of the NF network from the microtubule (MT) network. Increasing NF-H or NF-H together with NF-M further reduced NFs from dendrites. However, these changes were reversed by elevating the level of NF-L with either NF-H or NF-M. Thus, NF-L antagonizes NF-H and NF-M in organizing the NF network and maintaining a lower ratio of NF-H and NF-M to NF-L is critical for the growth of complex dendritic trees in motor neurons.

Age-related alterations in immunoreactivity of the midsized neurofilament subunit in the brainstem reticular formation of the cat

In the present study, we compared the immunoreactivity of the midsized subunit of neurofilaments (NF-M) in the brainstem reticular formation of adult and old cats. There was a dramatic decrease in immunoreactivity in most reticular nuclei in the old cats. The most obvious reduction in these regions occurred in dendritic arborizations. In contrast, a small number of nuclei showed a slight increase in immunoreactivity in the aged animals. The age-related changes in immunoreactivity indicate that there is an alteration of NF-M content in reticular neurons and their processes in old age. Such changes in NF-M content may be the basis for the alterations in the morphology of reticular neurons in aged animals.

Alcohol reduces neurofilament protein levels in primary cultured hippocampal neurons

High concentrations of alcohol (> or = 1.8%) were shown previously to impair health and viability of cultured hippocampal neurons. Because neurofilament proteins are essential for neuronal process outgrowth and differentiation, the effects of alcohol on these proteins were determined in the neuronal processes of primary cultured gestational day 18 rat hippocampal neurons. At the relatively lower concentrations used in the present study, alcohol caused a concentration-dependent reduction (< or = 47%) in 68 and 200 kDa neurofilament proteins (p < 0.05). Alcohol caused a 32% downward trend in 160 kDa neurofilament protein levels. Alcohol up to 1% (72-h exposure) produced no obvious alterations in neurite extension or explant morphology, and there were no visual signs of cell death. The sensitive MTT dye reduction assay showed no biochemical evidence of decreased cell viability at < or = 0.5% alcohol. The 32-47% reductions in neurofilament protein levels in vitro may hold implications for later hippocampal neuronal differentiation events in animals prenatally exposed to alcohol.

Phosphorylation of human high molecular weight neurofilament protein (hNF-H) by neuronal cyclin-dependent kinase 5 (cdk5)

Neurofilaments (NFs), the neuron-specific intermediate (i.e. approximately 10-nm diameter) filaments are major cytoskeletal components of most neurons. In a mature mammalian neuron, NFs are co-assembled from three subunits, NF-L (low), NF-M (medium), and NF-H (high), with molecular masses of 68, 95, and 115 kDa, respectively. Neurofilament proteins (NF-Ps), particularly, NF-H, are most extensively phosphorylated in large myelinated axons under normal conditions. This phosphorylation occurs on the serine residues of the lysine (Lys)-serine (Ser)-proline (Pro) (KSP) multiple amino acid repeats of the carboxy-terminal tail domain. Phosphorylation of KSP motifs affects physical, biochemical, and immunological properties of NF-H. For example, phosphorylation is thought to play a pivotal role in the maintenance of the neuronal cytoskeletal structure which influences the conduction velocity of the nerve fiber. The key components responsible for phosphorylation are not known. In this study, an identified cyclin-dependent kinase 5 (cdk5), isolated from nervous tissue, has been shown to phosphorylate the human NF-H (hNF-H) and affects its electrophoretic mobility. On the basis of the following observations, it is suggested that neuronal cdk5 (cdk5) phosphorylates KSPXK motifs in the human high molecular weight neurofilament (hNF-H) and affects its electrophoretic mobility. (1) A 14-mer synthetic peptide (KSPEKAKSPVKEEA) derived from hNF-H; (2) a bacterially expressed protein containing 14 KSPXK multiple repeats of hNF-H in C-terminal tail domain; and (3) a dephosphorylated hNF-H in neurofilament preparation are phosphorylated by cdk5. The decrease in molecular mass of hNF-H caused by dephosphorylated was completely recovered upon cdk5 phosphorylation. It is proposed that neuronal cdk5 regulates phosphorylation of the KSPXK motif in hNF-H and other cytoskeletal proteins with similar motifs in the nervous system.

Cytoplasmic O-GlcNAc modification of the head domain and the KSP repeat motif of the neurofilament protein neurofilament-H

Neurofilaments, the major intermediate filaments in large myelinated neurons, are essential for specifying proper axonal caliber. Mammalian neurofilaments are obligate heteropolymers assembled from three polypeptides, neurofilament (NF)-H, NF-M, and NF-L, each of which undergoes phosphorylation at multiple sites. NF-M and NF-L are known to be modified by O-linked N-acetylglucosamine (O-GlcNAc) (Dong, D. L.-Y., Xu, Z.-S., Chevrier, M. R., Cotter, R. J., Cleveland, D. W., and Hart, G. W. (1993) J. Biol. Chem. 268, 16679-16687). Here we further report that NF-H is extensively modified by O-GlcNAc at Thr53, Ser54, and Ser56 in the head domain and, somewhat surprisingly, at multiple sites within the Lys-Ser-Pro repeat motif in the tail domain, a region in assembled neurofilaments known to be nearly stoichiometrically phosphorylated on each of the approximately 50 KSP repeats. Beyond the earlier identified sites on NF-M and NF-L, O-GlcNAc sites on Thr19 and Ser34 of NF-M and Ser34 and Ser48 of NF-L are also determined here, all of which are localized in head domain sequences critical for filament assembly. The proximity of O-GlcNAc and phosphorylation sites in both head and tail domains of each subunit indicates that these modifications may influence one another and play a role in filament assembly and network formation.

A cdc2-like kinase distinct from cdk5 is associated with neurofilaments

An immunoprecipitation assay was used to identify protein kinases which are physically associated with neurofilaments (NF) in mouse brain extracts. Using this approach, we show that a cdc2-related kinase is associated with NF. The cdc2-related kinase was found to be distinct from cdk5 and the authentic cdc2 by a number of criteria. Firstly, it has a molecular mass on SDS-PAGE gels of 34 kDa, similar to that of cdc2, but differing from cdk5 (31 kDa). Secondly, it is not recognized by an antibody specific for cdk5. Thirdly, it is recognized by an antibody raised against the C-terminal region of authentic cdc2, but not by an antibody specific for the PSTAIRE motif. Using immunoblotting, we further show that the cdc2-related kinase copurifies with NF isolated from rat tissues. In vitro kinase assays further demonstrated that immunoprecipitated cdc2-related kinase phosphorylates recombinant NF-H protein. Phosphorylation of NF-H by the cdc2-like activity was not affected by 3 microM olomoucine but was inhibited by 10 microM of this kinase inhibitor. Phosphoamino acid analysis of in vitro phosphorylated NF-H indicates that the immunoprecipitated cdc2-related kinase phosphorylates serine residues.

Distribution of immunoreactivity for cytoskeletal (microtubule, microtubule-associated, and neurofilament) proteins in adult human dorsal root ganglia

BACKGROUND

The cytoskeleton of mature neurons consists of three main types of filamentous structures: microtubules (or neurotubules) neurofilaments and microfilaments, and of the so-called associated proteins. Neurotubules are formed by alpha- and beta-tubulin; neurofilaments are comprised of three protein subunits (68, 160, and 200 kDa of molecular weight), referred to here as neurofilament proteins (NFPs). The microtubule-associated proteins (MAPs) and tau-proteins form cross bridges between microtubules and other cytoskeletal constituents, as well as cellular organelles. This study analyzes the distribution of several cytoskeletal proteins in adult human dorsal root ganglia (DRG).

METHODS

Sections of formaldehyde-fixed, paraffin-embedded adult human DRG were processed for PAP immunohistochemistry. Mouse monoclonal antibodies against specific epitopes of alpha- and beta-tubulin, MAP-1, MAP-2, MAP-5, tau-protein, and NFPs (68, 160, and 200 kDa) were used. Furthermore, a quantitative image analysis (optic microdensitometry) was performed to establish the relationship between neuronal size and intensity of immunostaining.

RESULTS

Most of DRG neuron cell bodies displayed immunoreactivity for all assessed antibodies, with the exception of MAP2, which was absent. Nevertheless, the neuronal perikarya showed an heterogeneous pattern of immunoreactivity, which was not related to neuronal profile size. Positive immunolabelling was also observed in satellite cells and Schwann cells for microtubule and MAP1 proteins, and for tau-protein in Schwann cells.

CONCLUSIONS

Adult human primary sensory neurons in DRG express immunoreactivity for neurotubule and neurofilament proteins, as well as for some microtubule-associated proteins. However, since large heterogeneity was observed in the expression of those proteins, we conclude that the expression of cytoskeletal proteins is not a criterion to establish DRG neuronal subtypes.

The first intron of the mouse neurofilament light gene (NF-L) increases gene expression

Neurofilament expression is developmentally and post-transcriptionally controlled. Using transient transfection assays in mouse L cells, we demonstrate that the expression of the mouse neurofilament light subunit (NF-L) is influenced by intron sequences. NF-L expression was decreased twenty fold upon deletion of the three intron sequences. Elements contained principally within a 350 bp region of intron 1 were responsible for enhanced NF-L expression. Enhancement of expression did not occur when intron I was placed 3' to a heterologous chloramphenicol acetyl transferase (CAT) gene whose expression was driven by NF-L 5' sequences. The intron enhancement of NF-L expression was not promoter-specific and also occurred with the mouse sarcoma virus (MSV) LTR promoter. These data suggest intron sequences may be important in regulating NF gene expression.

Neurofilament phosphorylation

Neurofilament proteins (NFPs) are highly phosphorylated molecules in the axonal compartment of the adult nervous system. The phosphorylation of NFP is considered an important determinant of filament caliber, plasticity, and stability. This process reflects the function of NFs during the lifetime of a neuron from differentiation in the embryo through long-term activity in the adult until aging and environmental insult leads to pathology and ultimately death. NF function is modulated by phosphorylation-dephosphorylation in each of these diverse neuronal states. In this review, we have summarized some of these properties of NFP in adult nervous tissue, mostly from work in our own laboratory. Identification of sites phosphorylated in vivo in high molecular weight NFP (NF-H) and properties of NF-associated and neural-specific kinases phosphorylating specific sites in NFP are described. A model to explain the role of NF phosphorylation in determining filament caliber, plasticity, and stability is proposed.