Molecular architecture of the neurofilament. II. Reassembly process of neurofilament L protein in vitro

In vitro assembly properties of human type I and II hair keratins

Multiple type I and II hair keratins are expressed in hair-forming cells but the role of each protein in hair fiber formation remains obscure. In this study, recombinant proteins of human type I hair keratins (K35, K36 and K38) and type II hair keratins (K81 and K85) were prepared using bacterial expression systems. The heterotypic subunit interactions between the type I and II hair keratins were characterized using two-dimensional gel electrophoresis and surface plasmon resonance (SPR). Gel electrophoresis showed that the heterotypic complex-forming urea concentrations differ depending on the combination of keratins. K35-K85 and K36-K81 formed relatively stable heterotypic complexes. SPR revealed that soluble K35 bound to immobilized K85 with a higher affinity than to immobilized K81. The in vitro intermediate filament (IF) assembly of the hair keratins was explored by negative-staining electron microscopy. While K35-K81, K36-K81 and K35-K36-K81 formed IFs, K35-K85 afforded tight bundles of short IFs and large paracrystalline assemblies, and K36-K85 formed IF tangles. K85 promotes lateral association rather than elongation of short IFs. The in vitro assembly properties of hair keratins depended on the combination of type I and II hair keratins. Our data suggest the functional significance of K35-K85 and K36-K81 with distinct assembly properties in the formation of macrofibrils.

SELF-CONSISTENT FIELD MODELING OF THE NEUROFILAMENT NETWORK

We have investigated, on a self-consistent field level, the equilibrium structure of the neurofilament network formed by the NF -H, NF -M and NF -L proteins, using the one-gradient version of the numerical model of Scheutjens and Fleer. We demonstrate a reticulation of NFs in parallel bundles that occurs due to hydrophobic attractions between apolar aminoacid residues in the terminal parts of the M- and H-tails. We elaborate on the feasibility that the stability of the NF network can be enhanced by specific interactions between the projection domains, possibly induced by accessary proteins. We demonstrate that the phosphorylation of KSP repeats in the M- and H-tails promotes the cross-bridging between the NFs and therefore helps form the NF network.

Phosphatidylinositol phosphates directly bind to neurofilament light chain (NF-L) for the regulation of NF-L self assembly

Phosphatidylinositol phosphates (PtdInsPs) are ubiquitous membrane phospholipids that play diverse roles in cell growth and differentiation. To clarify the regulation mechanism acting on neurofilament light chain (NF-L) self assembly, we examined the effects of various PtdInsPs on this process. We found that PtdInsPs, including PI(4,5)P((2)), directly bind to the positively charged Arg(54) of murine NF-L, and this binding promotes NF-L self assembly in vitro. Mutant NF-L (R53A/R54A) proteins lacking binding affinity to PtdInsPs did not have the same effect, but the mutant NF-L proteins showed greater self assembly than the wild-type in the absence of any PtdInsP. These results collectively suggest that Arg(54) plays a pivotal role in NF-L self assembly by binding with PtdInsPs.

How the projection domains of NF-L and alpha-internexin determine the conformations of NF-M and NF-H in neurofilaments

Making use of a numerical self-consistent field method and polymer brush concepts, we model the solvated corona of neurofilaments (NF) composed of projection domains (unstructured tails) of constituent proteins. Projections are modeled with amino acid resolution. We focus on the importance of the two shortest ones (alpha-internexin and NF-L) in regulating the conformations of the two longer ones (NF-M and NF-H) in an isolated NF. We take the wild-type NF with no alpha-internexin as the reference, for which the phosphorylation-induced translocation of M- and H-tails has been examined previously. We demonstrate that a subbrush of L-tails creates an electrostatic potential profile with an approximately parabolic shape. An experimentally relevant (2:1) ratio of L- to alpha-projections reduces the charge density of the L subbrush and shifts the translocation transition of the H-tails to slightly higher degrees of phosphorylation. Replacing all L-tails by alpha-projections destroys the substructure of the NF corona and this alters the NF response to the phosphorylation of long tails.

Structural properties of neurofilament sidearms: sequence-based modeling of neurofilament architecture

Neurofilaments (NFs) are essential cytoskeletal filaments that impart mechanical integrity to nerve cells. They are assembled from three distinct molecular mass proteins that bind to each other to form a 10-nm-diameter filamentous rod with sidearm extensions. The sidearms are considered to play a critical role in modulating interfilament spacing and axonal caliber. However, the precise mechanism by which NF protrusions regulate axonal diameter remains to be well understood. In particular, the role played by individual NF protrusions in specifying interfilament distances is yet to be established. To gain insight into the role of individual proteins, we investigated the structural organization of NF architecture under different phosphorylation conditions. To this end, a physically motivated sequence-based coarse-grain model of NF brush has been developed based on the three-dimensional architecture of NFs. The model incorporates the charge distribution of sidearms, including charges from the phosphorylation sites corresponding to Lys-Ser-Pro repeat motifs. The model also incorporates the proper grafting of the real NF sidearms based on the stoichiometry of the three subunits. The equilibrium structure of the NF brush is then investigated under different phosphorylation conditions. The phosphorylation of NF modifies the structural organization of sidearms. Upon phosphorylation, a dramatic change involving a transformation from a compact conformation to an extended conformation is found in the heavy NF (NF-H) protein. However, in spite of extensive phosphorylation sites present in the NF-H subunit, the tails of the medium NF subunit are found to be more extended than the NF-H sidearms. This supports the notion that medium NF protrusions are critical in regulating NF spacings and, hence, axonal caliber.

Role of the aromatic residues in the near-amino terminal motif of vimentin in intermediate filament assembly in vitro

Type III and IV intermediate filament (IF) proteins share a conserved sequence motif of -Tyr-Arg-Arg-X-Phe- at the near-amino termini. To characterize significance of the aromatic residues in the motif, we prepared vimentin mutants in which Tyr-10 and Phe-14 are substituted with Asn and Ser (Vim[Y10N], Vim[F14S] and Vim[Y10N, F14S]), and examined assembly properties in vitro by electron microscopy and viscosity measurements. At 2 s after initiation of assembly reaction at pH 7.2 and 150 mM NaCl, all the vimentin mutants formed so-called unit-length filaments (ULFs) that were slightly larger than ULFs of wild-type vimentin. In following filament elongation, Vim[Y10N, F14S] and Vim[Y10N] performed longitudinal annealing of ULFs very rapidly and formed IFs within only 2.5 and 5 min, respectively, while Vim[F14S] and wild-type vimentin gave IFs by 40-60 min. The IFs of Vim[Y10N, F14S] and Vim[Y10N], however, tended to intertwine each other and formed bundles in parts of the specimens. The intertwinements decreased as the salt concentration decreased, and optimal salt concentration for the two mutants to form normal IFs was 50 mM. These results suggest that the aromatic residues, especially Tyr-10, in the motif have a role in controlling intermolecular interactions involved in IF assembly in vitro and suppress undesirable filament intertwinements at physiological ionic strength.

Direct imaging of aligned neurofilament networks assembled using in situ dialysis in microchannels

We report a technique to produce aligned neurofilament networks for direct imaging and diffraction studies using in situ dialysis in a microfluidic device. The alignment is achieved by assembling neurofilaments from protein subunits confined within microchannels. Resulting network structure was probed by polarized optical microscopy and atomic force microscopy, which confirmed a high degree of protein alignment inside the microchannels. This technique can be expanded to facilitate structural studies of a wide range of filamentous proteins and their hierarchical assemblies under varying assembly conditions.

Interplay between liquid crystalline and isotropic gels in self-assembled neurofilament networks

Neurofilaments (NFs) are a major constituent of nerve cell axons that assemble from three subunit proteins of low (NF-L), medium (NF-M), and high (NF-H) molecular weight into a 10 nm diameter rod with radiating sidearms to form a bottle-brush-like structure. Here, we reassemble NFs in vitro from varying weight ratios of the subunit proteins, purified from bovine spinal cord, to form homopolymers of NF-L or filaments composed of NF-L and NF-M (NF-LM), NF-L and NF-H (NF-LH), or all three subunits (NF-LMH). At high protein concentrations, NFs align to form a nematic liquid crystalline gel with a well-defined spacing determined with synchrotron small angle x-ray scattering. Near physiological conditions (86 mM monovalent salt and pH 6.8), NF-LM networks with a high NF-M grafting density favor nematic ordering whereas filaments composed of NF-LH transition to an isotropic gel at low protein concentrations as a function of increasing mole fraction of NF-H subunits. The interfilament distance decreases with NF-M grafting density, opposite the trend seen with NF-LH networks. This suggests a competition between the more attractive NF-M sidearms, forming a compact aligned nematic gel, and the repulsive NF-H sidearms, favoring a more expansive isotropic gel, at 86 mM monovalent salt. These interactions are highly salt dependent and the nematic gel phase is stabilized with increasing monovalent salt.

Effect of the ionic strength and pH on the equilibrium structure of a neurofilament brush

Using the numerical model of Scheutjens and Fleer, we investigated, on a self-consistent field level, the equilibrium structure of the neurofilament brush formed by projection domains of the constituent NF-H, NF-M, and NF-L proteins. The phosphorylation of such a brush is a major regulatory process that triggers the relocation of the H tails from the NF core to the brush periphery. We explore how the pH and the ionic strength affect the rearrangements in the NF brush structure upon phosphorylation. We demonstrate that the translocation of H tails in an individual NF occurs as a sharp cooperative transition below and up to the physiological salt concentration. Regularities of this process are reminiscent of the collapse-to-stretching transition in a cylindrical polyelectrolyte brush in a poor solvent. The effect of pH at physiological ionic strength is noticeable only in the acidic range and is more pronounced for a dephosphorylated NF.

A self-consistent field analysis of the neurofilament brush with amino-acid resolution

Using the numerical model of Scheutjens and Fleer we investigated, on a self-consistent field level, the equilibrium structure of the neurofilament brush formed by the projection domains of NF-H, NF-M, and NF-L proteins. Although the actual amino-acid sequences in the projection domains are coarse-grained, the different (realistic) solubilities of amino-acid residues and the specific distribution of its intrinsic charges inside the arms of the NF proteins are taken explicitly into account. We collect strong evidence that the electrostatic interactions are a dominant force that controls the NF brush structure. There exists a remarkable spatial separation of the H, M, and L tails. In a dephosphorylated NF we found confined and flowerlike conformations for the H and M projection domains, respectively. We demonstrate that the ionization of KSP repeats in NF proteins triggers a conformational transition in the H tail that leads to the expulsion of its terminal (KEP) domain to the periphery of the NF brush. We argue that the phosphorylation of the NF proteins in axons can both increase the interfilament distance and stabilize cross bridges between neurofilaments.

Squid (Loligo pealei) giant fiber system: a model for studying neurodegeneration and dementia?

In many neurodegenerative disorders that lead to memory loss and dementia, the brain pathology responsible for neuronal loss is marked by accumulations of proteins in the form of extracellular plaques and intracellular filamentous tangles, containing hyperphosphorylated cytoskeletal proteins. These are assumed to arise as a consequence of deregulation of a normal pattern of topographic phosphorylation-that is, an abnormal shift of cytoskeletal protein phosphorylation from the normal axonal compartment to cell bodies. Although decades of studies have been directed to this problem, biochemical approaches in mammalian systems are limited: neurons are too small to permit separation of cell body and axon compartments. Since the pioneering studies of Hodgkin and Huxley on the giant fiber system of the squid, however, the stellate ganglion and its giant axons have been the focus of a large literature on the physiology and biochemistry of neuron function. This review concentrates on a host of studies in our laboratory and others on the factors regulating compartment-specific patterns of cytoskeletal protein phosphorylation (primarily neurofilaments) in an effort to establish a normal baseline of information for further studies on neurodegeneration. On the basis of these data, a model of topographic regulation is proposed that offers several possibilities for further studies on potential sites of deregulation that may lead to pathologies resembling those seen in mammalian and human brains showing neurodegeneration, dementia, and neuronal cell death.

Neurofilament phosphoforms: surrogate markers for axonal injury, degeneration and loss

This review on the role of neurofilaments as surrogate markers for axonal degeneration in neurological diseases provides a brief background to protein synthesis, assembly, function and degeneration. Methodological techniques for quantification are described and a protein nomenclature is proposed. The relevance for recognising anti-neurofilament autoantibodies is noted. Pathological implications are discussed in view of immunocytochemical, cell-culture and genetic findings. With reference to the present symposium on multiple sclerosis, the current literature on body fluid levels of neurofilaments in demyelinating disease is summarised.

Allelic variants of the canine heavy neurofilament (NFH) subunit and extensive phosphorylation in dogs with motor neuron disease

Aberrant accumulation of extensively phosphorylated heavy (high molecular weight) neurofilament (NFH) and neurodegeneration are features of hereditary canine spinal muscular atrophy (HCSMA), an animal model of human motor neuron disease. In this study, the canine NFH gene was mapped, cloned, and sequenced, and electrospray/mass spectrometry was used to evaluate the phosphorylation state of NFH protein from normal dogs and dogs with HCSMA. The canine NFH gene was localized to a region on canine chromosome 26 that corresponds to human NFH on chromosome 22q. The predicted length of the canine NFH protein is 1135 amino acids, and it shares an 80.3% identity with human NFH and >74.6% with murine NFH proteins. Direct sequencing of NFH cDNA from HCSMA dogs revealed no mutations, although cDNA sequence and restriction fragment length polymorphism (RFLP) analysis indicates that there are at least three canine NFH alleles, differing in the position and number (61 or 62) of Lys-Ser-Proline (KSP) motifs. The two longest alleles (L1 and L2), each with 62 KSP repeats, contain an additional 24-base insert and were observed in both normal and HCSMA dogs. However, the shorter allele (the C allele), with 61 KSP sites and lacking the 24-base insertion, was absent in dogs with HCSMA. Mass spectrometry data indicated that almost all of the NFH KSP phosphorylation sites were occupied. No new or extra sites were identified in native NFH purified from the HCSMA dogs. The predominance of the two longest NFH alleles and the additional KSP phosphorylation sites they confer probably account for the presence of extensively phosphorylated NFs detected immunohistochemically in dogs with HCSMA.

Morphological analysis of glutaraldehyde-fixed vimentin intermediate filaments and assembly-intermediates by atomic force microscopy

Atomic force microscopy (AFM) was used to study the morphology of vimentin intermediate filaments (IFs) and their assembly intermediates. At each time after initiation of IF assembly in vitro of recombinant mouse vimentin, the sample was fixed with 0.1% glutaraldehyde and then applied to AFM analysis. When mature vimentin IFs were imaged in air on mica, they appeared to have a width of approximately 28 nm, a height of approximately 4 nm and a length of several micrometers. Taking into account the probe tip's distortion effect, the exact width was evaluated to be approximately 25 nm, suggesting that the filaments flatten on the substrate rather than be cylindrical with a diameter of approximately 10 nm. Vimentin IFs in air clearly demonstrated approximately 21-nm repeating patterns along the filament axis. The three-dimensional profiles of vimentin IFs indicated that the characteristic patterns were presented by repeating segments with a convex surface. The repeating patterns close to 21 nm were also observed by AFM analysis in a physiological solution condition, suggesting that the segments along the filaments are an intrinsic substructure of vimentin IFs. In the course of IF assembly, assembly intermediates were analyzed in air. Many short filaments with a full-width and an apparent length of approximately 78 nm (evaluated length approximately 69 nm) were observed immediately after initiation of the assembly reaction. Interestingly, the short full-width filaments appeared to be composed of the four segments. Further incubation enabled the short full-width filaments to anneal longitudinally into longer filaments with a distinct elongation step of approximately 40 nm, which corresponds to the length of the two segments. To explain these observations, we propose a vimentin IF formation model in which vimentin dimers are supercoiling around the filament axis.

In vitro assembly properties of mutant and chimeric intermediate filament proteins: insight into the function of sequences in the rod and end domains of IF

The factors and mechanisms regulating assembly of intermediate filament (IF) proteins to produce filaments with their characteristic 10 nm diameter are not fully understood. All IF proteins contain a central rod domain flanked by variable head and tail domains. To elucidate the role that different domains of IF proteins play in filament assembly, we used negative staining and electron microscopy (EM) to study the in vitro assembly properties of purified bacterially expressed IF proteins, in which specific domains of the proteins were either mutated or swapped between a cytoplasmic (mouse neurofilament-light (NF-L) subunit) and nuclear intermediate filament protein (human lamin A). Our results indicate that filament formation is profoundly influenced by the composition of the assembly buffer. Wild type (wt) mouse NF-L formed 10 nm filaments in assembly buffer containing 175 mM NaCl, whereas a mutant deleted of 18 NH2-terminal amino acids failed to assemble under similar conditions. Instead, the mutant assembled efficiently in buffers containing CaCl2 > or = 6 mM forming filaments that were 10 times longer than those formed by wt NF-L, although their diameter was significantly smaller (6-7 nm). These results suggest that the 18 NH2-terminal sequence of NF-L might serve two functions, to inhibit filament elongation and to promote lateral association of NF-L subunits. We also demonstrate that lengthening of the NF-L rod domain, by inserting a 42 aa sequence unique to nuclear IF proteins, does not compromise filament assembly in any noticeable way. Our results suggests that the known inability of nuclear lamin proteins to assemble into 10 nm filaments in vitro cannot derive solely from their longer rod domain. Finally, we demonstrate that the head domain of lamin A can substitute for that of NF-L in filament assembly, whereas substitution of both the head and tail domains of lamins for those of NF-L compromises assembly. Therefore, the effect of lamin A "tail" domain alone, or the synergistic effect of lamin "head" and the "tail" domains together, interferes with assembly into 10-nm filaments.

Neurofilament formation and synaptic activity are delayed in the myenteric neurons of the rat fetus with gastroschisis

Gastroschisis is a malformation characterized by prenatal evisceration of the midgut into the amniotic cavity. Because of the harmful effects of the amniotic fluid, the intestinal loops appear matted, thickened, and covered by a peel. At birth, the newborn presents altered intestinal motility. In a previous publication, we reported a delay in the myenteric ganglia organization and neuronal maturity in a rat model of gastroschisis. In the present study, the neurofilament formation and synaptic activity were immunohistochemically investigated in the myenteric neurons of this animal model. The expression of low, medium and high molecular weight neurofilament proteins and of a protein of the synaptic vesicles, the synaptophysin, were similar to those found at earlier embryonic ages. These findings demonstrate delayed cytoskeletal organization and reduced synaptic activity in the myenteric neurons in the rat model of gastroschisis.

The single neurofilament subunit of lamprey may need another element for filament assembly

Regenerating axon tips in transected lamprey spinal cord contain dense accumulations of neurofilaments (NFs), suggesting that NFs may play a role in the mechanism of axonal regeneration. Compared with heteropolymeric assemblies of NF triplet proteins in mammals, NF in lampreys has been thought to contain only a single subunit (NF180). This would imply that NF180 self-assembles, which would be important for manipulating its expression in studies of axonal regeneration. In order to study the possible role of NF in process outgrowth and to determine whether NF180 can self-assemble, its gene was transfected into mammalian and fish cell lines that either contain or lack vimentin. In transfected NIH3T3 cells, NF180 was poorly phosphorylated and its expression did not alter the length or number of cell processes. Nor did it appear to form typical intermediate filaments, suggesting that it may not self-assemble. NF180 also did not form typical filaments in SW13cl cells that either possessed or lacked vimentin, nor in transfected fish cells that were cultured at 18 degrees C. In vitro, NF180 could not self-assemble but interacted with NF-L to interrupt its self-assembly. When cotransfected with rat NF-L into SW13c1.2vim(-) cells, NF180 did form thick, rod-like filamentous structures on immunofluorescence. More typical NFs were observed when NF180 was cotransfected with both NF-L and NF-M. Thus, NF180 cannot self-assemble but appears to require one or more additional elements for incorporation into NFs.

Spontaneous central nervous system remyelination is not altered in NFH-lacZ transgenic mice after chemical demyelination

Harmonious functioning of the nervous system depends on neuron-glia interactions, particularly between the axons and their myelinating cells, i.e., oligodendrocytes (OL) in the central nervous system (CNS). In human demyelinating diseases such as multiple sclerosis (MS), demyelination may be associated with axonal damage, but alterations of the axonal cytoskeleton, which is composed mainly of neurofilaments (NF) and microtubules, are largely unknown, as are the consequences on remyelination. In a model of demyelination induced by lysophosphatidylcholine (LPC), we have shown that demyelination was correlated with a decrease in NF immunolabelling, and that these axonal abnormalities were reduced by platelet-derived growth factor (PDGF)-enhanced remyelination in adult rats. We have analysed the spontaneous remyelination after LPC stereotaxic injection in the CNS of transgenic NFH-lacZ mice, which present axonal atrophy caused by abnormal distribution of NF, associated with hypermyelination in the PNS, and normal myelin thickness in the CNS. Axonal atrophy in the CNS of NFH-lacZ mice was confirmed, but it was not worsened by demyelination. On the contrary, demyelination induced axonal atrophy in wild-type mice, demonstrating that NF are essential for axonal calibre determination. Moreover, an efficient spontaneous remyelination occurred in NFH-lacZ as well as in wild-type mice, indicating that the NF are not necessary for CNS remyelination. These findings point out that NF modifications observed in MS may not be responsible for the lack of remyelination in this disease.

Phosphorylation of the head domain of neurofilament protein (NF-M): a factor regulating topographic phosphorylation of NF-M tail domain KSP sites in neurons

In neurons the phosphorylation of neurofilament (NF) proteins NF-M and NF-H is topographically regulated. Although kinases and NF subunits are synthesized in cell bodies, extensive phosphorylation of the KSP repeats in tail domains of NF-M and NF-H occurs primarily in axons. The nature of this regulation, however, is not understood. As obligate heteropolymers, NF assembly requires interactions between the core NF-L with NF-M or NF-H subunits, a process inhibited by NF head domain phosphorylation. Phosphorylation of head domains at protein kinase A (PKA)-specific sites seems to occur transiently in cell bodies after NF subunit synthesis. We have proposed that transient phosphorylation of head domains prevents NF assembly in the soma and inhibits tail domain phosphorylation; i.e. assembly and KSP phosphorylation in axons depends on prior dephosphorylation of head domain sites. Deregulation of this process leads to pathological accumulations of phosphorylated NFs in the soma as seen in some neurodegenerative disorders. To test this hypothesis, we studied the effect of PKA phosphorylation of the NF-M head domain on phosphorylation of tail domain KSP sites. In rat cortical neurons we showed that head domain phosphorylation of endogenous NF-M by forskolin-activated PKA inhibits NF-M tail domain phosphorylation. To demonstrate the site specificity of PKA phosphorylation and its effect on tail domain phosphorylation, we transfected NIH3T3 cells with NF-M mutated at PKA-specific head domain serine residues. Epidermal growth factor stimulation of cells with mutant NF-M in the presence of forskolin exhibited no inhibition of NF-tail domain phosphorylation compared with the wild type NF-M-transfected cells. This is consistent with our hypothesis that transient phosphorylation of NF-M head domains inhibits tail domain phosphorylation and suggests this as one of several mechanisms underlying topographic regulation.

Characterization of distinct early assembly units of different intermediate filament proteins

We have determined the mass-per-length (MPL) composition of distinct early assembly products of recombinant intermediate filament (IF) proteins from the four cytoplasmic sequence homology classes, and compared these values with those of the corresponding mature filaments. After two seconds under standard assembly conditions (i.e. 25 mM Tris-HCl (pH 7.5), 50 mM NaCl, 37 degrees C), vimentin, desmin and the neurofilament triplet protein NF-L aggregated into similar types of "unit-length filaments" (ULFs), whereas cytokeratins (CKs) 8/18 already yielded long IFs at this time point, so the ionic strength had to be reduced. The number of molecules per filament cross-section, as deduced from the MPL values, was lowest for CK8/18, i.e. 16 and 25 at two seconds compared to 16 and 21 at one hour. NF-L exhibited corresponding values of 26 and 30. Vimentin ULFs yielded a pronounced heterogeneity, with major peak values of 32 and 45 at two seconds and 30, 37 and 44 after one hour. Desmin formed filaments of distinctly higher mass with 47 molecules per cross-section, at two seconds and after one hour of assembly. This indicates that individual types of IF proteins generate filaments with distinctly different numbers of molecules per cross-section. Also, the observed significant reduction of apparent filament diameter of ULFs compared to the corresponding mature IFs is the result of a "conservative" radial compaction-type reorganization within the filament, as concluded from the fact that both the immature and mature filaments contain very similar numbers of subunits per cross-section. Moreover, the MPL composition of filaments is strikingly dependent on the assembly conditions employed. For example, vimentin fibers formed in 0.7 mM phosphate (pH 7.5), 2.5 mM MgCl2, yield a significantly increased number of molecules per cross-section (56 and 84) compared to assembly under standard conditions. Temperature also strongly influences assembly: above a certain threshold temperature "pathological" ULFs form that are arrested in this state, indicating that the system is forced into strong but unproductive interactions between subunits. Similar "dead-end" structures were obtained with vimentins mutated to introduce principal alterations in subdomains presumed to be of general structural importance, indicating that these sequence changes led to new modes of intermolecular interactions.

The pathway of assembly of intermediate filaments from recombinant alpha-internexin

The pathway of filament assembly from the neuronal intermediate filament alpha-intermexin was investigated. Optimal assembly occurred in solutions of pH 6.5 to 7 and moderate ionic strength at 37 degrees C. Short filaments formed upon dialysis at 24 degrees C, which elongated further when incubated at 37 degrees C. Soluble forms of alpha-internexin were characterized by analytical ultracentrifugation and electron microscopy. In 10 mM Tris, pH 8, conditions that favor formation of tetramers and other small oligomers for other intermediate filament proteins, alpha-internexin formed 10.5 S particles, apparently unit-length half-filaments in the form of rods 10.6 nm in diameter and 68 nm long. Dialysis vs the same buffer with added 10 mM NaCl yielded 16 S rods, probably unit-length filaments, of the same length but 13.0 nm in diameter. At 50 mM NaCl, rods about 13 nm in diameter and heterogeneous in length were observed in electron micrographs, apparently formed from longitudinal annealing of unit-length rods. The results favor a model of assembly in which coiled coil dimers aggregate laterally to form first "unit-length half-filaments" (Herrmann, H., and Aebi, U. (1998) Curr. Opin. Struct. Biol. 8, 177-185) and then "unit-length filaments," which subsequently elongate by annealing.

Transgenic mice in the study of ALS: the role of neurofilaments

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurological disorder of multiple etiologies that affects primarily motor neurons in the brain and spinal cord. Abnormal accumulations of neurofilaments (NFs) in motor neurons and a down-regulation of mRNA for the NF light subunit (NF-L) are associated with ALS, but it remains unclear to what extent these NF perturbations contribute to human disease. Transgenic mouse studies demonstrated that overexpression of normal and mutant NF proteins can sometimes provoke a motor neuronopathy characterized by the presence of abnormal NF accumulations resembling those found in ALS. Remarkably, the motor neuronopathy in transgenic mice overexpressing human NF heavy (NF-H) subunits was rescued by the co-expression of a human NF-L transgene at levels that restored a correct stoichiometry of NF-L to NF-H subunits. Transgenic approaches have also been used to investigate the role of NFs in disease caused by Cu/Zn superoxide dismutase (SOD1) mutations, which is responsible for approximately 2% cases of ALS. Studies with transgenic mice expressing low levels of a fusion NF-H/lacZ protein, in which NFs are withheld from the axonal compartment, suggested that axonal NFs are not toxic intermediates required for SOD1-mediated disease. On the contrary, overexpression of human NF-H proteins was found to confer an effective protection against mutant SOD1 toxicity in transgenic mice, a phenomenon that may be due to the ability of NF proteins to chelate calcium. In conclusion, transgenic studies showed that disorganized NFs can sometimes have noxious effects resulting in neuronopathy. However, in the context of motor neuron disease caused by mutant SOD1, there is emerging evidence that NF proteins rather play a protective role.

Neurofilaments in health and disease

This article reviews current knowledge of neurofilament structure, phosphorylation, and function and neurofilament involvement in disease. Neurofilaments are obligate heteropolymers requiring the NF-L subunit together with either the NF-M or the NF-H subunit for polymer formation. Neurofilaments are very dynamic structures; they contain phosphorylation sites for a large number of protein kinases, including protein kinase A (PKA), protein kinase C (PKC), cyclin-dependent kinase 5 (Cdk5), extracellular signal regulated kinase (ERK), glycogen synthase kinase-3 (GSK-3), and stress-activated protein kinase gamma (SAPK gamma). Most of the neurofilament phosphorylation sites, located in tail regions of NF-M and NF-H, consist of the repeat sequence motif, Lys-Ser-Pro (KSP). In addition to the well-established role of neurofilaments in the control of axon caliber, there is growing evidence based on transgenic mouse studies that neurofilaments can affect the dynamics and perhaps the function of other cytoskeletal elements, such as microtubules and actin filaments. Perturbations in phosphorylation or in metabolism of neurofilaments are frequently observed in neurodegenerative diseases. A down-regulation of mRNA encoding neurofilament proteins and the presence of neurofilament deposits are common features of human neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Parkinson's disease, and Alzheimer's disease. Although the extent to which neurofilament abnormalities contribute to pathogenesis in these human diseases remains unknown, emerging evidence, based primarily on transgenic mouse studies and on the discovery of deletion mutations in the NF-H gene of some ALS eases, suggests that disorganized neurofilaments can provoke selective degeneration and death of neurons. An interference of axonal transport by disorganized neurofilaments has been proposed as one possible mechanism of neurofilament-induced pathology. Other factors that can potentially lead to the accumulation of neurofilaments will be discussed as well as the emerging evidence for neurofilaments as being possible targets of oxidative damage by mutations in the superoxide dismutase enzyme (SOD1); such mutations are responsible for approximately 20% of familial ALS cases.

Delayed maturation of regenerating myelinated axons in mice lacking neurofilaments

Using the technique of homologous recombination in embryonic stem cells, we generated mice bearing a targeted disruption of the gene encoding the neurofilament light (NF-L) protein. The absence of NF-L protein in mice resulted in dramatic declines of approximately 20-fold in the levels of neurofilament medium and heavy proteins in the brain and sciatic nerve while increases were detected for other cytoskeletal proteins such as tubulin and GAP-43. Despite a lack of neurofilaments and hypotrophy of axons, the NF-L knockout mice develop normally and do not exhibit overt phenotypes. However, in both NF-L -/- and NF-L +/- mice, the regeneration of myelinated axons following crush injury of peripheral nerves was found to be abnormal. In the second week after axotomy, the number of newly regenerated myelinated axons in the sciatic nerve and facial nerve of NF-L -/- mice corresponded to only approximately 25 and approximately 5% of the number of myelinated axons found in normal mice, respectively. At this early postaxotomy stage, electron microscopy of nerve segments distal to the crush site in NF-L -/- mice revealed abundant clusters of axonal sprouts that were indicative of retarded maturation of regenerating fibers. The analysis of the distal sciatic nerve at 2 months after crush indicated that neurofilament-deficient axons have the capacity to regrow for a long distance and to remyelinate, albeit at a slower rate. These results provide the first direct evidence for a role of neurofilaments in the maturation of regenerating myelinated axons.

Neurofilaments and motor neuron disease

Amyotrophic lateral sclerosis (ALS) is an adult-onset and heterogeneous neurological disorder that affects primarily motor neurons in the brain and spinal cord. Although multiple genetic and environmental factors might be implicated in ALS, the striking similarities in the clinical and pathological features of sporadic ALS and familial ALS suggest that similar mechanisms of disease may occur. A common and perhaps universal pathological finding in ALS is the presence of abnormal accumulations of neurofilaments (often called spheroids or Lewy body-like deposits) in the cell body and proximal axon of surviving motor neurons. Such neurofilament deposits have been widely viewed as a consequence of neuronal dysfunction, perhaps reflecting axonal transport defects. This review discusses the emerging evidence, based primarily on transgenic mouse studies and on the discovery of deletion mutations in a neurofilament gene associated with ALS, that neurofilament proteins can play a causative role in motor neuron disease.

PKN associates and phosphorylates the head-rod domain of neurofilament protein

PKN is a fatty acid-activated serine/threonine kinase that has a catalytic domain highly homologous to that of protein kinase C in the carboxyl terminus and a unique regulatory region in the amino terminus. Recently, we reported that the small GTP-binding protein Rho binds to the amino-terminal region of PKN and activates PKN in a GTP-dependent manner, and we suggested that PKN is located on the downstream of Rho in the signal transduction pathway (Amano, M., Mukai, H., Ono, Y., Chihara, K., Matsui, T., Hamajima, Y., Okawa, K., Iwamatsu, A., and Kaibuchi, K. (1996) Science 271, 648-650; Watanabe, G., Saito, Y., Madaule, P., Ishizaki, T., Fujisawa, K., Morii, N., Mukai, H., Ono, Y. Kakizuka, A., and Narumiya, S. (1996) Science 271, 645-648). To identify other components of the PKN pathway such as substrates and regulatory proteins of PKN, the yeast two-hybrid strategy was employed. By this screening, a clone encoding the neurofilament L protein, a subunit of neuron-specific intermediate filament, was isolated. The amino-terminal regulatory region of PKN was shown to associate with the head-rod domains of other subunits of neurofilament (neurofilament proteins M and H) as well as neurofilament L protein in yeast cells. The direct binding between PKN and each subunit of neurofilament was confirmed by using the in vitro translated amino-terminal region of PKN and glutathione S-transferase fusion protein containing the head-rod domain of each subunit of neurofilament. PKN purified from rat testis phosphorylated each subunit of the native neurofilament purified from bovine spinal cord and the bacterially synthesized head-rod domain of each subunit of neurofilament. Polymerization of neurofilament L protein in vitro was inhibited by phosphorylation of neurofilament L protein by PKN. The identification and characterization of the novel interaction with PKN may contribute toward the elucidation of mechanisms regulating the function of neurofilament.

A nontetrameric species is the major soluble form of keratin in Xenopus oocytes and rabbit reticulocyte lysates

Inside the interphase cell, approximately 5% of the total intermediate filament protein exists in a soluble form. Past studies using velocity gradient sedimentation (VGS) indicate that soluble intermediate filament protein exists as an approximately 7 S tetrameric species. While studying intermediate filament assembly dynamics in the Xenopus oocyte, we used both VGS and size-exclusion chromatography (SEC) to analyze the soluble form of keratin. Previous studies (Coulombe, P. A., and E. Fuchs. 1990. J. Cell Biol. 111:153) report that tetrameric keratins migrate on SEC with an apparent molecular weight of approximately 150,000; the major soluble form of keratin in the oocyte, in contrast, migrates with an apparent molecular weight of approximately 750,000. During oocyte maturation, the keratin system disassembles into a soluble form (Klymkowsky, M. W., L. A. Maynell, and C. Nislow. 1991. J. Cell Biol. 114:787) and the amount of the 750-kD keratin complex increases dramatically. Immunoprecipitation analysis of soluble keratin from matured oocytes revealed the presence of type I and type II keratins, but no other stoichiometrically associated polypeptides, suggesting that the 750-kD keratin complex is composed solely of keratin. To further study the formation of the 750-kD keratin complex, we used rabbit reticulocyte lysates (RRL). The 750-kD keratin complex was formed in RRLs contranslating type I and type II Xenopus keratins, but not when lysates translated type I or type II keratin RNAs alone. The 750-kD keratin complex could be formed posttranslationally in an ATP-independent manner when type I and type II keratin translation reactions were mixed. Under conditions of prolonged incubation, such as occur during VGS analysis, the 750-kD keratin complex disassembled into a 7 S (by VGS), 150-kD (by SEC) form. In urea denaturation studies, the 7 S/150-kD form could be further disassembled into an 80-kD species that consists of cofractionating dimeric and monomeric keratin. Based on these results, the 750-kD species appears to be a supratetrameric complex of keratins and is the major, soluble form of keratin in both prophase and M-phase oocytes, and RRL reactions.

A role for neurofilaments in the pathogenesis of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a late-onset degenerative disease of motor neurons, characterized by abnormal accumulation of neurofilaments (NFs) in perikarya and proximal axons. Two lines of evidence suggest that neurofilament accumulation can play a crucial role in ALS pathogenesis. First, transgenic mouse models overexpressing NF proteins were found to develop motor neuron degeneration and, second, variant alleles of the NF heavy-subunit (NF-H) gene have been found in some human ALS patients. Our axonal transport studies with transgenic mice overexpressing the human NF-H gene, a model of ALS, revealed defects of intracellular transport not only for neurofilament proteins but also for other cytoskeletal proteins and organelles such as mitochondria. Therefore, we propose that neurofilament accumulation in mice causes neurodegeneration by disrupting axonal transport, a mechanism that may account for the pathogenesis of ALS.

Alterations of intermediate filaments in various histopathological conditions

Intermediate filament proteins belong to a multigene family and constitute an important cytoskeletal component of most vertebrate cells. Their pattern of expression is tissue specific and is highly controlled during embryonic development. Numerous pathologies are known to be associated with modifications of intermediate filament organisation, although their precise role has not yet been elucidated. The present review focuses on the most recent data concerning the possible causes of intermediate filaments disorganization in specific pathologic conditions affecting the epidermis, the liver, and the nervous system. We discuss the formation of abnormal intermediate filament networks that arise as a consequence of mutations that directly affect intermediate filament structure or are induced by multifactorial causes such as modifications of post-translational processes and changes in the levels of expression.

Mice overexpressing the human neurofilament heavy gene as a model of ALS

We discuss the evidence, based on the analysis of transgenic mice overexpressing the human neurofilament (NF) heavy gene, that abnormal NF accumulations can provoke neurodegeneration of motor neurons. Transgenic mice overexpressing by two-fold the normal levels of human NF-H proteins develop a progressive motor neuron disease with several pathologic features reminiscent of those found in amyotrophic lateral sclerosis (ALS). A plausible mechanism for the selective motor neuron degeneration is that exceeding levels of NF-H cross-linkages impede transport of newly synthesized NF structures. The abnormal NF accumulations in perikarya and proximal axons is accompanied by a disruption in axonal transport of not only NF proteins but also of other components required for maintenance of axons. The relevance of the NF-H transgenics as a model of ALS is discussed in light of our current knowledge of motor neuron disease.

Neurofilament protein heterotetramers as assembly intermediates

Evidence is presented for the existence of a soluble heterotetramer containing the low and middle molecular weight neurofilament (NF) proteins, NF-L and NF-M, and one containing the low and high molecular weight proteins, NF-L and NF-H, and for their role in filament assembly. When a mixture of either pair of proteins was renatured in 2 M urea, 20 mM Tris, pH 7.2, a new band representing a complex was observed in native gel electrophoresis. No new band was observed with a mixture of NF-M and NF-H. Two-dimensional gel electrophoresis showed that treatment of the complexes with SDS caused them to dissociate into their constituent polypeptide chains. Native neurofilaments dissociated in 2 M urea into a mixture of LM and LH complexes. Titration of NF-L with NF-M indicated that complex formation was complete at an approximately equimolar ratio of the two proteins. The LM complex had a sedimentation coefficient, s20,w, of 4.4 S, consistent with a tetrameric structure. Dialysis of a solution of the LM complex against 50 mM 4-morpholineethanesulfonic acid, 0.17 M NaCl, pH 6.25, led to the formation of 10-nm filaments in good yield. These results suggest that NF protein heterooligomers are intermediates in NF assembly and disassembly.

Phosphorylation and transport of neurofilament proteins in the rat spinal ganglion

Neurofilament proteins (NFs) in rat spinal ganglia were labeled with [32P]orthophosphate injected into ganglia and analyzed by two-dimensional autoradiography and immunoblotting. Three polypeptides of NF were labeled irrespective of the extent of phosphorylation. Most of the labeled NFs were transported from cell bodies to proximal axons within 24 h. A major fraction of low phosphorylated NF-H changed to high phosphorylated form in intraganglionic nerve fibers and peripheral nerves adjacent to spinal ganglia. A small fraction of low phosphorylated NF-H appeared earlier than the high phosphorylated form in adjacent peripheral nerves, suggesting that newly synthesized NF-H in cell bodies migrate a long distance before they are extensively phosphorylated and assembled into the cytoskeleton in proximal axons.

Two distinct functions of the carboxyl-terminal tail domain of NF-M upon neurofilament assembly: cross-bridge formation and longitudinal elongation of filaments

Neurofilaments are the major cytoskeletal elements in the axon that take highly ordered structures composed of parallel arrays of 10-nm filaments linked to each other with frequent cross-bridges, and they are believed to maintain a highly polarized neuronal cell shape. Here we report the function of rat NF-M in this characteristic neurofilament assembly. Transfection experiments were done in an insect Sf9 cell line lacking endogenous intermediate filaments. NF-L and NF-M coassemble to form bundles of 10-nm filaments packed in a parallel manner with frequent cross-bridges resembling the neurofilament domains in the axon when expressed together in Sf9 cells. Considering the fact that the expression of either NF-L or NF-M alone in these cells results in neither formation of any ordered network of 10-nm filaments nor cross-bridge structures, NF-M plays a crucial role in this parallel filament assembly. In the case of NF-H the carboxyl-tail domain has been shown to constitute the cross-bridge structures. The similarity in molecular architecture between NF-M and NF-H suggests that the carboxyl-terminal tail domain of NF-M also constitutes cross-bridges. To examine this and to further investigate the function of the carboxyl-terminal tail domain of NF-M, we made various deletion mutants that lacked part of their tail domains, and we expressed these with NF-L. From this deletion mutant analysis, we conclude that the carboxyl-terminal tail domain of NF-M has two distinct functions. First, it is the structural component of cross-bridges, and these cross-bridges serve to control the spacing between core filaments. Second, the portion of the carboxyl-terminal tail domain of NF-M that is directly involved in cross-bridge formation affects the core filament assembly by helping them to elongate longitudinally so that they become straight.

Characterization of multiple oligomeric vimentin intermediate filament units by transient electric birefringence measurements

In this work we have studied the structure of soluble vimentin units from which intermediate filaments (IFs) are built. Several oligomeric forms have been presented in the literature as IF "building blocks", but there is still no agreement on this matter. By comparing our data with various models as proposed in the literature we can favour certain models and reject others. Transient electric birefringence (TEB) measurements were performed from which information is obtained concerning electric and hydrodynamic properties of the particles under investigation. TEB decay analysis at pH 6.8 after 70 microseconds pulses (at 20 degrees C in aqueous solution) yielded three decay times: 1.1(+/- 0.3) microseconds, 4.0(+/- 1.0) microseconds and 20.0(+/- 5.0) microseconds, with amplitudes of 45% to 60%, 30% to 45%, and less than 10%, respectively. At pH 8.5 after 70 microseconds pulses, more than 90% of the TEB signal with the second decay time is obtained, while the remainder had a decay time of 15.0(+/- 4.0) microseconds. Only when the pulse duration was decreased, the fast decay time around 1 microsecond was observed, suggesting that only a minor fraction of the particles at this pH value causes such a short decay time. At both pH values, the TEB measurements indicated that, at least in part, the molecules are oriented by a permanent dipole moment. It will be shown that the shortest decay time originates from bent or flexible dimers, and the second decay time from particles with a length of 54 to 65 nm containing, at least in part, a relatively large overall dipole moment. The longest decay time is probably due to larger aggregates. These results are consistent with a model in which single dimers, antiparallel staggered tetramers and hexamers coexist. Alternatively, but less likely on the basis of literature data, a model of parallel in-register tetramers with a considerable length contribution of the head groups would fit our research.

Phosphorylation of native and reassembled neurofilaments composed of NF-L, NF-M, and NF-H by the catalytic subunit of cAMP-dependent protein kinase

Phosphorylation of neurofilament-L protein (NF-L) by the catalytic subunit of cAMP-dependent protein kinase (A-kinase) inhibits the reassembly of NF-L and disassembles filamentous NF-L. The effects of phosphorylation by A-kinase on native neurofilaments (NF) composed of three distinct subunits: NF-L, NF-M, and NF-H, however, have not yet been described. In this paper, we examined the effects of phosphorylation of NF proteins by A-kinase on both native and reassembled filaments containing all three NF subunits. In the native NF, A-kinase phosphorylated each NF subunit with stoichiometries of 4 mol/mol for NF-L, 6 mol/mol for NF-M, and 4 mol/mol for NF-H. The extent of NF-L phosphorylation in the native NF was nearly the same as that of purified NF-L. However, phosphorylation did not cause the native NFs to disassemble into oligomers, as was the case for purified NF-L. Instead, partial fragmentation was detected in sedimentation experiments and by electron microscopic observations. This is probably not due to the presence of the three NF subunits in NF or to differences in phosphorylation sites because reassembled NF containing all three NF subunits were disassembled into oligomeric forms by phosphorylation with A-kinase and the phosphorylation by A-kinase occurred at the head domain of NF-L whether NF were native or reassembled. Disassembling intermediates of reassembled NF containing all three NF subunits were somewhat different from disassembling intermediates of NF-L. Thinning and loosening of filaments was frequently observed preceding complete disassembly. From the fact that the thinning was also observed in the native filaments phosphorylated by A-kinase, it is reasonable to propose the native NF is fragmented through a process of thinning that is stimulated by phosphorylation in the head domain of the NF subunits.

The rod domain of NF-L determines neurofilament architecture, whereas the end domains specify filament assembly and network formation

Neurofilaments, assembled from NF-L, NF-M, and NF-H subunits, are the most abundant structural elements in myelinated axons. Although all three subunits contain a central, alpha-helical rod domain thought to mediate filament assembly, only NF-L self-assembles into 10-nm filaments in vitro. To explore the roles of the central rod, the NH2-terminal head and the COOH-terminal tail domain in filament assembly, full-length, headless, tailless, and rod only fragments of mouse NF-L were expressed in bacteria, purified, and their structure and assembly properties examined by conventional and scanning transmission electron microscopy (TEM and STEM). These experiments revealed that in vitro assembly of NF-L into bona fide 10-nm filaments requires both end domains: whereas the NH2-terminal head domain promotes lateral association of protofilaments into protofibrils and ultimately 10-nm filaments, the COOH-terminal tail domain controls lateral assembly of protofilaments so that it terminates at the 10-nm filament level. Hence, the two end domains of NF-L have antagonistic effects on the lateral association of protofilaments into higher-order structures, with the effect of the COOH-terminal tail domain being dominant over that of the NH2-terminal head domain. Consideration of the 21-nm axial beading commonly observed with 10-nm filaments, the approximate 21-nm axial periodicity measured on paracrystals, and recent cross-linking data combine to support a molecular model for intermediate filament architecture in which the 44-46-nm long dimer rods overlap by 1-3-nm head-to-tail, whereas laterally they align antiparallel both unstaggered and approximately half-staggered.

Assembly of type IV neuronal intermediate filaments in nonneuronal cells in the absence of preexisting cytoplasmic intermediate filaments

We report here on the in vivo assembly of alpha-internexin, a type IV neuronal intermediate filament protein, in transfected cultured cells, comparing its assembly properties with those of the neurofilament triplet proteins (NF-L, NF-M, and NF-H). Like the neurofilament triplet proteins, alpha-internexin coassembles with vimentin into filaments. To study the assembly characteristics of these proteins in the absence of a preexisting filament network, transient transfection experiments were performed with a non-neuronal cell line lacking cytoplasmic intermediate filaments. The results showed that only alpha-internexin was able to self-assemble into extensive filamentous networks. In contrast, the neurofilament triplet proteins were incapable of homopolymeric assembly into filamentous arrays in vivo. NF-L coassembled with either NF-M or NF-H into filamentous structures in the transfected cells, but NF-M could not form filaments with NF-H. alpha-internexin could coassemble with each of the neurofilament triplet proteins in the transfected cells to form filaments. When all but 2 and 10 amino acid residues were removed from the tail domains of NF-L and NF-M, respectively, the resulting NF-L and NF-M deletion mutants retained the ability to coassemble with alpha-internexin into filamentous networks. These mutants were also capable of forming filaments with other wild-type neurofilament triplet protein subunits. These results suggest that the tail domains of NF-L and NF-M are dispensable for normal coassembly of each of these proteins with other type IV intermediate filament proteins to form filaments.

Progressive neuronopathy in transgenic mice expressing the human neurofilament heavy gene: a mouse model of amyotrophic lateral sclerosis

We generated four transgenic mice with a 34 kb genomic fragment including the complete human neurofilament heavy (NF-H) gene. This human NF-H fragment contained all regulatory elements for tissue-specific expression, and in two transgenic lines, human NF-H proteins were produced at levels up to 2-fold the levels of endogenous mouse NF-H protein. By 3-4 months of age, these NF-H transgenics progressively develop neurological defects and abnormal neurofilamentous swellings that are highly reminiscent of those found in amyotrophic lateral sclerosis (ALS). We propose that a modest up-regulation of NF-H cross-linkers can result in an impairment of neurofilament transport, causing neuronal swellings with ensuing axonopathy and muscle atrophy, a mechanism of pathogenesis pertinent to the possible etiology of ALS.

Dynamics of the neuronal intermediate filaments

We have analyzed the dynamics of neuronal intermediate filaments in living neurons by using the method of photobleaching of fluorescently-labeled neurofilament L protein and immunoelectron microscopy of incorporation sites of biotinylated neurofilament L protein. Low-light-level imaging and photobleaching of growing axons of mouse sensory neurons did not affect the rate of either axonal growth or the addition of intermediate filament structures at the axon terminal, suggesting that any perturbations caused by these optical methods would be minimal. After laser photobleaching, recovery of fluorescence did occur slowly with a recovery half-time of 40 min. Furthermore, we observed a more rapid fluorescence recovery in growing axons than in quiescent ones, indicating a growth-dependent regulation of the turnover rate. Incorporation sites of biotin-labeled neurofilament L protein were localized as numerous discrete sites along the axon, and they slowly elongated to become continuous arrays 24 h after injection. Collectively, these results indicate that neuronal intermediate filaments in growing axons turn over within the small area of the axoplasm possibly by the mechanism of lateral and segmental incorporation of new subunits.

Acceleration of bovine neurofilament L assembly by deprivation of acidic tail domain

Functions of the tail region of neurofilament L have, to date, not been clearly elucidated. Bovine neurofilament L was cleaved into tail-less neurofilament L (50 kDa) and a tail fragment (19 kDa), by thrombin. Tail-less neurofilament L was deficit of the highly acidic domain of the tail region (approximately 77% of the entire region). Assembly of tail-less neurofilament L. was observed to be accelerated by both fluorometric and centrifugal measurements, compared with intact neurofilament L. The critical concentration of tail-less neurofilament L, which constitutes the constant unassembled pool, was approximately 0.25-times lower than that of neurofilament L. Under physiological conditions, tail-less neurofilament L formed a ribbon-like structure, whereas tail-less neurofilament L could form 10-nm filaments in an extremely low ionic-strength buffer in the presence of 1 mM MgCl2. An affinity-purified antibody directed against the tail fragment also accelerated neurofilament L assembly. The tail fragment neither coassembled with neurofilament L nor affect neurofilament L assembly. The acidic domain of the tail region may regulate neurofilament assembly and may be involved in 10-nm filament formation under physiological conditions.

Chemical cross-linking indicates a staggered and antiparallel protofilament of desmin intermediate filaments and characterizes one higher-level complex between protofilaments

Tetrameric rods, protofilaments and assembled filaments of desmin, the intermediate filament protein of muscle, have been chemically cross-linked with the lysine specific cross-linkers EGS [ethylene glycol bis(succinimidylsuccinate), 1.61 nm span] and bis(sulfosuccinimidyl) suberate (1.14 nm span). One bis(sulfosuccinimidyl)suberate and two EGS cross-links were isolated from the rod and characterized. They show that the two coiled coils in the rod tetramer are staggered by approximately 15-20 nm and strongly indicate an antiparallel arrangement in which the inner overlapping part of the rod is formed by the amino-terminal helices 1A, 1B and 2A. Both EGS cross-links identified in the rod were also isolated from cross-linked filaments. The isolated rod, therefore, represents a complex also present in identical, or very similar form in protofilaments and in assembled filaments. Cross-linked filaments yielded a third EGS cross-link that must have been formed between neighboring protofilaments. It connects the highly conserved carboxy-terminus of helix 2B of the first protofilament to the overlap region formed by helices 1A and 2A of the second protofilament. The restrictions posed by these cross-links on current filament models are discussed.

Intermediate filaments formed de novo from tail-less cytokeratins in the cytoplasm and in the nucleus

The roles of the different molecular domains of intermediate filament (IF) proteins in the assembly and higher order organization of IF structures have recently been studied by various groups but with partially controversial results. To examine the requirement of the aminoterminal (head) and the carboxyterminal (tail) domain of cytokeratins (CKs) for de novo IF formation in the living cell, we have constructed cDNAs coding for intact as well as head- and/or tail-less human CKs 8 and 18 and the naturally tail-less human CK 19, all under the control of the human beta-actin promoter. After transient and stable transfections of mouse 3T3-L1 cells, which are devoid of any CKs, we have studied, with such constructs, the resulting gene products by gel electrophoresis and immunolocalization techniques. By light and electron microscopy we show that extended cytoplasmic IF meshworks are formed from pairs of the type II CK 8 with the type I CKs 18 or 19 as well as from pairs of tail-less CK 8 with tail-less CKs 18 or 19 in the transfected cells, proving that the absence of the tail domain in both types of CKs does not prevent the de novo formation of regular IFs. Most surprisingly, however, we have observed spectacular alterations in the nucleocytoplasmic distribution of the IFs formed from tail-less CKs. In many of the transfected cells, a large part, or all, of the detectable CKs was found to occur in extensive IF bundles in the nucleoplasm. Intranuclear accumulations of CK deposits, however mostly nonfibrillar, were also observed when the cells had been transfected with cDNAs encoding tail-less CKs also lacking their head domains, whereas CKs deleted only in the head domain were found exclusively in the cytoplasm. The specific domain requirements for the assembly of cytoplasmic IF bundles are discussed and possible mechanisms of intranuclear accumulation of IFs are proposed.

Organization of coiled-coil molecules in native mouse keratin 1/keratin 10 intermediate filaments: evidence for alternating rows of antiparallel in-register and antiparallel staggered molecules

There is considerable diversity of opinion in the literature concerning the organization of two-chain coiled-coil molecules in intermediate filaments. I have reexplored this issue using the limited proteolysis paradigm with native mouse epidermal keratin intermediate filaments (KIF), consisting of keratins 1 and 10. KIF were harvested as cytoskeletal pellets, dissociated into subfilamentous forms at pH 9.8, 9.0, or 2.6, and were subjected to limited proteolytic digestion to recover alpha-helix-enriched particles that derived from the rod domains of the constituent chains, using conditions that do not promote reorganization of the constituent protein chains or coiled-coil molecules. The multichain particles were subjected to physicochemical analyses, amino acid sequencing, and electron microscopy in order to determine their composition, structure, and organization within the intact KIF. The results predict two principal modes of alignment: neighboring molecules may be aligned in register and antiparallel or staggered and antiparallel. From known structural constraints, this permits construction of a two-dimensional surface lattice for KIF which consists of alternating antiparallel rows of in-register and staggered molecules. These data establish the level of hierarchy at which the well-known antiparallelity and staggered features of KIF are introduced. This model supports the proposals of KIF structure based on theoretical considerations of ionic interactions scores (Crewther et al., 1983). When the KIF are dissociated at extremes of pH, this structural model allows for disruption along alternate axes; the in-register antiparallel alignment is seen only when KIF are dissociated at high pH values; below pH 9, only the staggered antiparallel alignment is seen. The process of molecule realignment especially in concentrated urea solutions indicates that the staggered antiparallel alignment is the more thermodynamically stable form in solution.