Neurofilaments are obligate heteropolymers in vivo

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

The roles of the head and tail domains of alpha-internexin, a type IV neuronal intermediate filament protein, in its self-assembly and coassemblies with neurofilament triplet proteins, were examined by transient transfections with deletion mutants in a non-neuronal cell line lacking an endogenous cytoplasmic intermediate filament network. The results from the self-assembly studies showed that the head domain was essential for alpha-internexin's ability to self-assemble into a filament network and the tail domain was important for establishing a proper filament network. The data from the coassembly studies demonstrated that alpha-internexin interacted differentially with the neurofilament triplet protein subunits. Wild-type NF-L or NF-M, but not NF-H, was able to complement and form a normal filament network with the tailless alpha-internexin mutant, the alpha-internexin head-deletion mutant, or the alpha-internexin mutant missing the entire tail and some amino-terminal portion of the head domain. In contrast, neither the tailless NF-L mutant nor the NF-L head-deletion mutant was able to form a normal filament network with any of these alpha-internexin deletion mutants. However, coassembly of the tailless NF-M mutant with the alpha-internexin head-deletion mutant and coassembly of the NF-M head-deletion mutant with the tailless alpha-internexin mutant resulted in the formation of a normal filament network. Thus, the coassembly between alpha-internexin and NF-M exhibits some unique characteristics previously not observed with other intermediate filament proteins: only one intact tail and one intact head are required for the formation of a normal filament network, and they can be present within the same partner or separately in two partners.

Microtubule dynamics in living cells

Most people think of a skeleton as a solid and static framework upon which complex structures are elaborated. From what we have learned in the past decade about the cytoskeleton, it seems certain that the 'skeleton' part of the term is a bit misleading. It is clear now that the polymers that constitute the cytoskeleton, actin filaments, microtubules, and intermediate filaments, are all in fact ever-changing dynamic infrastructures of cells. Recently, advances have been made in the study of the cellular dynamics of one of the prominent components of the cytoskeleton, the microtubules. Observations in the past year have revealed some fundamental in vivo behaviors of these polymers, during interphase, during mitosis, and during the elaboration of postmitotic axonal microtubule arrays. These observations are important for the understanding of cytoplasmic organization in many types of cells.

Splice variant-specific interaction of the NMDA receptor subunit NR1 with neuronal intermediate filaments

NMDA receptors are excitatory neurotransmitter receptors critical for synaptic plasticity and neuronal development in the mammalian brain. These receptors are found highly concentrated in the postsynaptic membrane of glutamatergic synapses. To investigate the molecular mechanisms underlying NMDA receptor localization, we used the yeast two-hybrid system to identify proteins expressed in the brain that interact with the NMDA receptor subunit NR1. Here we report that the 68 kDa neurofilament subunit NF-L directly interacts with the NR1 subunit. This interaction occurs between the cytoplasmic C-terminal domain of NR1 and the rod domain of NF-L. However, NR1 splice variants lacking the first C-terminal exon cassette (C1) failed to associate with NF-L. Immunogold electron microscopy revealed a preferential localization of NR1 at the ends of in vitro-assembled neurofilaments. Overexpression of C1 cassette-containing NR1 constructs in fibroblast cells disrupted the assembly of recombinant neurofilaments. In addition, NR1 and NF-L cofractionated in detergent-treated rat brain synaptic plasma membranes. Furthermore, NR1 and NF-L colocalize in the dendrites and growth cones of cultured hippocampal neurons. These results demonstrate the splice variant-specific association of NR1 with neurofilaments and suggest a possible mechanism for anchoring or localizing NMDA receptors in the neuronal plasma membrane.

Localization of the giant axonal neuropathy gene to chromosome 16q24

Giant axonal neuropathy (GAN) is a degenerative disorder of the peripheral nerves that is inherited as an autosomal recessive trait, presenting in early childhood and progressing to death, usually by late adolescence. Diagnosis is made by peripheral nerve biopsy, in which a striking pathological finding is seen--fibers distorted by giant axonal swellings filled with densely packed bundles of neurofilaments (the primary intermediate filament in neurons), with segregation of other axoplasmic organelles. In addition to disorganized neurofilaments in nerve, disorganization of other members of the intermediate filament family of proteins is seen in other tissues; this implies that the underlying defect is one of generalized intermediate filament organization, with neurofilaments predominantly affected. We have pursued a genomewide search for regions of homozygosity of descent in 5 consanguineous families. A 5.3-cM region of homozygosity, shared in all 5 families, was found on chromosome 16q24, and linkage was established to this locus with a LOD score of 4.18 at theta = 0.00 at the most tightly linked marker, D16S3098. Determination of this locus is the first step toward characterizing the gene responsible for a fundamental property of intermediate filament organization and may shed light on other disorders (such as amyotrophic lateral sclerosis) in which neurofilament pathology is prominent.

Molecular characteristics of the novel intermediate filament protein paranemin. Sequence reveals EAP-300 and IFAPa-400 are highly homologous to paranemin

Paranemin was initially found to copurify with the intermediate filament (IF) proteins vimentin and desmin from embryonic chick skeletal muscle and was described as an IF-associated protein (IFAP). We have purified paranemin from embryonic chick skeletal muscle, prepared antibodies, and demonstrated that they label at the Z-lines of both adult avian and porcine cardiac and skeletal muscle myofibrils. We determined the cDNA sequence of paranemin by immunoscreening a lambdagt22A cDNA library from embryonic chick skeletal muscle. Northern blot analysis revealed a single transcript of 5.3 kilobases, which is much smaller than predicted from the size of paranemin (280 kDa) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The derived amino acid sequence of paranemin (1,606 residues; 178,161 kDa) contains the conserved IF rod domain (308 amino acids), which has highest homology to the rod domains of nestin and tanabin. Thus, paranemin is an IF protein rather than an IFAP. Sequence analysis also revealed that the partial cDNA sequences of two proteins, namely EAP-300 and IFAPa-400, are almost identical to regions of the cDNA sequence of paranemin. The complete paranemin cDNA was expressed in a cell line (SW13) with, and without, detectable cytoplasmic IFs. Antibody labeling of these cells suggests that paranemin does not form IFs by itself, but rather is incorporated into heteropolymeric IFs with vimentin.

Heterodimeric associations between neuronal intermediate filament proteins

Formation of protein dimers involving alpha-internexin, peripherin, and the neurofilament (NF) proteins NFH, NFM, and NFL was investigated by partial renaturation of various combinations of individually purified subunits in buffered 2 M urea. Oligomers that were formed were resolved by "blue" native electrophoresis (Schägger, H., Cramer, W. A., and von Jagow, G. (1994) Anal. Biochem. 217, 220-230) modified to include urea in the polyacrylamide gels. Combining this method with Western blot analysis, disulfide cross-linking, and SDS-polyacrylamide gel electrophoresis in the second dimension showed that NFL readily forms significant amounts of heterodimer with NFH, NFM, alpha-internexin, or peripherin in the presence of 2 M urea. alpha-Internexin and peripherin also formed heterodimers with NFH or NFM under these conditions. The modified version of blue native gel electrophoresis described here may be useful in monitoring the impact of post-translational modifications and mutations on the dimerization of intermediate filament proteins.

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.

Increased apoptosis arising from increased expression of the Alzheimer's disease-associated presenilin-2 mutation (N141I)

Mutations in the genes for presenilin 1 and 2 (PS-1 and PS-2) have been linked to development of early-onset Alzheimer's disease (AD). As neither the normal function of either presenilin is known nor why mutations cause disease, we examined the properties of wild-type, truncated, and mutant PS-2 upon expression in HeLa cells. Although HeLa cells are strongly predisposed to continued mitosis, expression of PS-2 induced programmed cell death (apoptosis). Direct evidence for apoptosis was obtained by double staining for terminal deoxynucleotide transferase nick end labeling (TUNEL) and PS-2 expression and by following green fluorescent protein-tagged PS-2 over time. Deletion analysis indicates that as little as 166 NH2-terminal residues of PS-2 are sufficient for endoplasmic reticulum (ER) localization and apoptosis. Moreover, the AD- associated PS-2 missense mutation (N141I) more efficiently induced cell death compared to wild-type PS-2 despite lower mutant protein accumulation. Expression of the presenilins in several other cell lines and transgenic mice has been accompanied by rapid protein cleavage without the induction of cell death. In contrast, PS-2 expressed in HeLa cells was not cleaved, and cell death occurred. We hypothesize that full-length but not cleaved PS-2 may be important in the regulation or induction of apoptosis.

Transient expression of the neurofilament proteins NF-L and NF-M by Schwann cells is regulated by axonal contact

Expression of the genes that encode neurofilament proteins is considered to be confined normally to neurons. However, in demyelinating peripheral nerves Schwann cells upregulate the mRNA for the medium-sized neurofilament protein (NF-M), and cultured Schwann cells of the myelin-forming phenotype can also synthesize and incorporate NF-M protein into their intermediate filament (IF) cytoskeleton. The purpose of this study was to establish how axonal contact might influence glial neurofilament gene expression and regulate the synthesis of neurofilament proteins. We show that the gene encoding NF-M is expressed at early stages of differentiation in myelin-forming Schwann cells in vivo; nevertheless, little NF-M protein can be detected in these cells. The transient induction of NF-M mRNA is also apparent in dedifferentiating Schwann cells during Wallerian degeneration. In these Schwann cells the mRNAs for NF-M and NF-L (the smallest polypeptide), but not NF-H (the largest neurofilament subunit), are coordinately expressed. In contrast to differentiating myelin-forming Schwann cells, the cells of degenerating nerves express both NF-M and NF-L polypeptides. Restoration of axonal contact in the growing nerve stimulates the recapitulation of Schwann cell differentiation including the elevation of NF-M and NF-L mRNA expression. These results demonstrate that the transient induction of neurofilament mRNAs in Schwann cells is a feature of both differentiation and dedifferentiation. However translation of these mRNAs is confined to Schwann cells deprived of axonal contact either by nerve injury or by culture in the absence of axons. These findings suggest that the expression of the NF-M and NF-L polypeptides is an important characteristic of those Schwann cells that will contribute to the repair of damaged peripheral nerves.

Slow axonal transport: the subunit transport model

A central problem concerning slow transport of cytoskeletal proteins along nerve axons is where they are assembled and the form in which they are transported. The polymer and subunit transport models are the two major hypotheses. Recent developments using molecular and cellular biophysics, molecular cell biology and gene technology have enabled visualization of moving forms of cytoskeletal proteins during their transport. Here, we argue that these studies support the subunit transport theory.

Coordinate induction of the three neurofilament genes by the Brn-3a transcription factor

The POU domain transcription factor Brn-3a is able to stimulate neurite outgrowth when overexpressed in the neuronal ND7 cell line, whereas the closely related Brn-3b factor does not have this effect. We show that Brn-3a overexpression also enhances the expression of the three neurofilament genes at both the mRNA and protein levels, whereas Brn-3b overexpression has no effect. In addition Brn-3a activates the three neurofilament gene promoters in co-transfection assays in both neuronal and non-neuronal cells. As observed for enhanced neurite outgrowth, the stimulation of neurofilament gene expression and activation of the neurofilament gene promoters is observed with the isolated POU domain of Brn-3a. A single amino acid change in the POU homeodomain of Brn-3a to the equivalent amino acid in Brn-3b abolishes its ability to activate the neurofilament promoters, whereas the reciprocal change converts Brn-3b to an activator of these promoters.

Developmentally regulated stabilization of neuronal intermediate filaments in rat cerebral cortex

The expression and Triton X-100 (Triton) solubility of neuronal intermediate filament proteins were determined in the developing rat cerebral cortex. The level of expression of alpha-internexin was unchanged from embryonic day 15 (E15) to postnatal day 15 (P15), whereas expression of the mid-sized neurofilament subunit increased continually during this interval concomitant with a reduction in Triton solubility of the two proteins. The low molecular weight neurofilament subunit, first barely detected at P2, was largely insoluble in Triton from the initial time point that its solubility could be assayed, at P5, to P15. Similar expression patterns and Triton solubility profiles were obtained for neuronal intermediate filament proteins in cultured neurons from E15 cerebral cortex. These results suggest that alpha-internexin is expressed earlier than neurofilament proteins to provide a more plastic network in the early developing brain. The incorporation of neurofilament proteins apparently results in the formation of the more stable intermediate filament network found in mature neurons.

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.

Identification of phosphorylation sites on neurofilament proteins by nanoelectrospray mass spectrometry

Neurofilament (NF) proteins are intermediate filaments found in the neuronal cytoskeleton. Phosphorylation of these proteins is considered an important factor in the assembly of filaments and determination of filament caliber and stability. Mammalian neurofilaments are composed of three polypeptide subunits, NF-L, NF-M, and NF-H, all of which are phosphorylated. Here we used techniques for the mass spectrometric sequencing of proteins from polyacrylamide gels to analyze in vivo phosphorylation sites on NF-M and NF-L. Neurofilaments were isolated from rat brain and enzymatically digested in gel. The resulting peptides were analyzed and sequence data obtained by nanoelectrospray mass spectrometry. Four phosphorylation sites have been found in the C-terminal domain of NF-M: serines 603, 608, 666, and 766. Two of these are found in lysine-serine-proline (KSP) motifs and two in the variant motifs, glutamic acid-serine-proline (ESP) and valine-serine-proline (VSP). Serine 55 in NF-L was not found to be phosphorylated, which confirms the possible role of phosphorylation and dephosphorylation of this site in early neurofilament assembly. The techniques used enable sequence data and characterization of posttranslational modifications to be obtained for each individual subunit directly from polyacrylamide gels.

Selective degeneration fo Purkinje cells with Lewy body-like inclusions in aged NFHLACZ transgenic mice

Transgenic (NFHLacZ) mice expressing a fusion protein composed of a truncated high-molecular-weight mouse neurofilament (NF) protein (NFH) fused to beta-galactosidase (LacZ) develop inclusions in neurons throughout the CNS. These inclusions persist from birth to advanced age and contain massive filamentous aggregates including all three endogenous NF proteins and the NFHLacZ fusion protein. Further, the levels of endogenous NF proteins are selectively reduced in NFHLacZ mice. Because these inclusions resemble NF-rich Lewy bodies (LBs) in Parkinson's disease and LB dementia, we asked whether these lesions compromised the viability of affected neurons during aging. We studied hippocampal CA1 neurons, nearly all of which harbored inclusions (type I) devoid of cellular organelles, and cerebellar Purkinje cells, nearly all of which accumulated inclusions (type II) containing numerous entrapped organelles. Purkinje cells with type II inclusions began to degenerate in the NFHLacZ mice at approximately 1 year of age, and most were eliminated by 18 months of age. In contrast, there was no significant loss of type I inclusion-bearing CA1 neurons with age. These data suggest that the sequestration of cellular organelles in type II inclusions may isolate and impair the function of these organelles, thereby rendering Purkinje cells selectively vulnerable to degeneration with age as in neurodegenerative diseases of the elderly characterized by accumulation of LBs.

Selective degeneration fo Purkinje cells with Lewy body-like inclusions in aged NFHLACZ transgenic mice

Transgenic (NFHLacZ) mice expressing a fusion protein composed of a truncated high-molecular-weight mouse neurofilament (NF) protein (NFH) fused to beta-galactosidase (LacZ) develop inclusions in neurons throughout the CNS. These inclusions persist from birth to advanced age and contain massive filamentous aggregates including all three endogenous NF proteins and the NFHLacZ fusion protein. Further, the levels of endogenous NF proteins are selectively reduced in NFHLacZ mice. Because these inclusions resemble NF-rich Lewy bodies (LBs) in Parkinson's disease and LB dementia, we asked whether these lesions compromised the viability of affected neurons during aging. We studied hippocampal CA1 neurons, nearly all of which harbored inclusions (type I) devoid of cellular organelles, and cerebellar Purkinje cells, nearly all of which accumulated inclusions (type II) containing numerous entrapped organelles. Purkinje cells with type II inclusions began to degenerate in the NFHLacZ mice at approximately 1 year of age, and most were eliminated by 18 months of age. In contrast, there was no significant loss of type I inclusion-bearing CA1 neurons with age. These data suggest that the sequestration of cellular organelles in type II inclusions may isolate and impair the function of these organelles, thereby rendering Purkinje cells selectively vulnerable to degeneration with age as in neurodegenerative diseases of the elderly characterized by accumulation of LBs.

Intermediate filaments in the nervous system: implications in cancer

In this review, we describe the different intermediate filament (IF) proteins, their assembly into IFs, the functions of IFs and their relation to disease with a particular emphasis on the intermediate filaments expressed in the nervous system. In the mammalian nervous system, seven intermediate filament proteins are known to be expressed in neurons or neuroblasts. These include the three neurofilament triplet proteins, which are present in both central and peripheral neurons; alpha-internexin, which is the first neuronal intermediate filament protein expressed in the developing mammalian nervous system and present primarily in CNS neurons in the adult nervous system; peripherin, which is most abundant in the PNS; vimentin, which is expressed in neuronal progenitor cells along with nestin, as well as in a few adult neurons. In contrast to these neuron-specific IF proteins, the glial fibrillary acidic protein (GFAP) is glial specific and expressed in mature astrocytes. Vimentin and nestin are also expressed in glial progenitor cells and vimentin is expressed along with GFAP in some mature astrocytes. As a whole, the expression of IF proteins is tissue specific and developmentally regulated. As a result, IF proteins are good markers for determining the cell origin and differentiation status of tumor cells. For example, peripherin is expressed in neuroblastomas, GFAP in astrocytomas and neurofilaments in tumors of neuronal origin. However, tumor cells may express IF patterns which are irrelevant to their cell origin. Therefore, one has to be very careful in using IF patterns as sole indicators of cell origin and differentiation status of tumors.

Aberrant stress-induced phosphorylation of perikaryal neurofilaments

The aberrant phosphorylation of the neurofilament high molecular weight subunit (NFH) in the neuronal perikaryon is a common feature of several neurological diseases. We demonstrated a strong correlation between hyperphosphorylation of the NFH carboxyl-terminal domain and activation of stress-activated protein kinase (SAPK) -gamma in PC12 cells. Agents that activated SAPKgamma in PC12 cells also caused the hyperphosphorylation of perikaryal NFH in cultured dorsal root ganglion neurons. The NFH carboxyl-terminal domain was phosphorylated by SAPKgamma in vitro, and the use of peptide substrates indicated that this event occurred preferentially at KSPXE motifs. We propose that SAPKgamma, perhaps in concert with other SAPKs, is involved in the abnormal phosphorylation of perikaryal NFH. This finding could lead to new insights into the etiology of several neurological diseases.

Neurofilament subunit NF-H modulates axonal diameter by selectively slowing neurofilament transport

To examine the mechanism through which neurofilaments regulate the caliber of myelinated axons and to test how aberrant accumulations of neurofilaments cause motor neuron disease, mice have been constructed that express wild-type mouse NF-H up to 4.5 times the normal level. Small increases in NF-H expression lead to increased total neurofilament content and larger myelinated axons, whereas larger increases in NF-H decrease total neurofilament content and strongly inhibit radial growth. Increasing NF-H expression selectively slow neurofilament transport into and along axons, resulting in severe perikaryal accumulation of neurofilaments and proximal axonal swellings in motor neurons. Unlike the situation in transgenic mice expressing modest levels of human NF-H (Cote, F., J.F. Collard, and J.P. Julien. 1993. Cell. 73:35-46), even 4.5 times the normal level of wild-type mouse NF-H does not result in any overt phenotype or enhanced motor neuron degeneration or loss. Rather, motor neurons are extraordinarily tolerant of wild-type murine NF-H, whereas wild-type human NF-H, which differs from the mouse homolog at > 160 residue positions, mediates motor neuron disease in mice by acting as an aberrant, mutant subunit.

Neurofilament triplet protein interactions: evidence for the preferred formation of NF-L-containing dimers and a putative function for the end domains

In this report we examine the molecular interactions that lead to formation of neurofilaments, the intermediate filaments in neurons. Using the yeast two-hybrid system, we found that the rod domains of all three NF triplet proteins interacted strongly with one another and with rod domains of the Type III IF proteins, vimentin and desmin. A slight preference toward NF-L-containing dimers was observed over ones not containing NF-L. Interactions among the full length NF triplet proteins exhibited more specificity. Full length NF-L had only a relatively weak interaction with another full length NF-L molecule, but reacted more robustly with full length NF-M or NF-H lacking only part of the head domain. No homologous or heterologous dimerization of NF-M and NF-H was detectable. These results support the hypothesis that neurofilaments are obligate heteropolymers and that heterodimeric subunits are the preferred building blocks. They further suggest that the mechanism that specifies heterodimeric interaction among the NF triplet proteins resides in the end domains.

Sequence variants in human neurofilament proteins: absence of linkage to familial amyotrophic lateral sclerosis

Neurofilaments, assembled from NF-L (68 kd), NF-M (95 kd), and NF-H (115 kd), are the most abundant structural components in large myelinated axons, particularly those of motor neurons. Aberrant neurofilament accumulation in cell bodies and axons of motor neurons is a prominent pathological feature of several motor neuron diseases, including sporadic and familial amyotrophic lateral sclerosis (ALS). Transgenic methods have proved in mice that mutation in or increased expression of neurofilament subunits can be primary causes of motor neuron disease that mimics the neurofilamentous pathology often reported in human disease. To examine whether mutation in neurofilament subunits causes or predisposes to ALS, we used single-strand conformation polymorphism coupled with DNA sequencing to search for mutations in the entirety of the human NF-L, NF-M, and NF-H genes from 100 familial ALS patients known not to carry mutations in superoxide dismutase 1 (SOD1), as well as from 75 sporadic ALS patients. Six polypeptide sequence variants were identified in rod and tail domains of NF-L, NF-M, or NF-H. However, all were found at comparable frequency in DNAs from normal individuals and no variant cosegregated with familial disease. Two deletions found previously in NF-H genes of sporadic ALS patients were not seen in this group of familial or sporadic ALS patients.

Mechanisms of selective motor neuron death in ALS: insights from transgenic mouse models of motor neuron disease

Concerning the mechanism(s) of disease underlying amyotrophic lateral sclerosis (ALS), transgenic mouse models have provided (i) a detailed look at the pathogenic progression of disease, (ii) a tool for testing hypotheses concerning the mechanism of neuronal death, and (iii) a host appropriate for testing therapeutic strategies. Thus far, these efforts have proved that mutation in a neurofilament subunit can cause progressive disease displaying both selective motor neuron death and aberrant neurofilament accumulation similar to that reported in human disease. Additional mice expressing point mutations in the cytoplasmic enzyme superoxide dismutase (SOD1), the only known cause of ALS, have proved that disease arises from a toxic property of the mutant enzyme rather than loss of enzymatic activity.

Alterations in neural intermediate filament organization: functional implications and the induction of pathological changes related to motor neuron disease

The properties regulating the supramolecular organization of neural intermediate filament (NIF) networks have been investigated in cultured dorsal root ganglion (DRG) neurons. The studies described take advantage of the ability of endogenous NIF to incorporate purified biotinylated neurofilament triplet (NFT) proteins, NF-L, NF-M and NF-H. When injected at concentrations of 0.8-1.0 mg/ml injection buffer, each of these proteins is incorporated without perturbing the endogenous NIF network. However, at progressively higher concentrations, NF-H induces the aggregation and accumulation of NIF in the cell body. Subsequent to the induction of these aggregates, numerous alterations in the cytoarchitecture of neurons can be detected. The latter occur in a temporal sequence which appears to begin with the fragmentation of the Golgi complex. At later times, accumulation of mitochondria within the proximal region of neurites, peripheralization of the nucleus, and a significant decrease in neurite caliber become obvious. After longer time periods, the NIF aggregates are seen to react with an antibody which reveals abnormally phosphorylated NF-H. These observations demonstrate that an imbalance in the normal stoichiometric relationships among the NFT proteins rapidly alters the supramolecular organization of the NIF network. These changes most likely reflect the normal functions of neurofilaments in cell shape and the organization and cytoplasmic distribution of membranous organelles. Interestingly, virtually all of these changes closely resemble those which have been reported in motor neuron diseases such as amyotrophic lateral sclerosis (ALS). These findings suggest that cultured neurons can be used as models for more precisely defining the relationships between the formation of NIF aggregates and the sequence of cytopathological events which typify neurodegenerative diseases.

An essential cytoskeletal linker protein connecting actin microfilaments to intermediate filaments

Typified by rapid degeneration of sensory neurons, dystonia musculorum mice have a defective BPAG1 gene, known to be expressed in epidermis. We report a neuronal splice form, BPAG1n, which localizes to sensory axons. Both isoforms have a coiled-coil rod, followed by a carboxy domain that associates with intermediate filaments. However, the amino terminus of BPAG1n differs from BPAG1e in that it contains a functional actin-binding domain. In transfected cells, BPAG1n coaligns neurofilaments and microfilaments, establishing this as a cytoskeletal protein interconnecting actin and intermediate filament cytoskeletons. In BPAG1 null mice, axonal architecture is markedly perturbed, consistent with a failure to tether neurofilaments to the actin cytoskeleton and underscoring the physiological relevance of this protein.

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.

Visualization of slow axonal transport in vivo

In axons, cytoskeletal constituents move by slow transport. However, it remains controversial whether axonal neurofilaments are dynamic structures in which only subunits are transported or whether filaments assemble in the proximal axon and are transported intact as polymers to the axon terminus. To investigate the form neurofilament proteins take during transport, neurons of transgenic mice lacking axonal neurofilaments were infected with a recombinant adenoviral vector encoding epitope-tagged neurofilament M. Confocal and electron microscopy revealed that the virally encoded neurofilament M was transported in unpolymerized form along axonal microtubules. Thus, neurofilament proteins are probably transported as subunits or small oligomers along microtubules, which are major routes for slow axonal transport.

Endoproteolysis of presenilin 1 and accumulation of processed derivatives in vivo

The majority of early-onset cases of familial Alzheimer's disease (FAD) are linked to mutations in two related genes, PS1 and PS2, located on chromosome 14 and 1, respectively. Using two highly specific antibodies against nonoverlapping epitopes of the PS1-encoded polypeptide, termed presenilin 1 (PS1), we document that the preponderant PS1-related species that accumulate in cultured mammalian cells, and in the brains of rodents, primates, and humans are approximately 27-28 kDa N-terminal and approximately 16-17 kDa C-terminal derivatives. Notably, a FAD-linked PS1 variant that lacks exon 9 is not subject to endoproteolytic cleavage. In brains of transgenic mice expressing human PS1, approximately 17 kDa and approximately 27 kDa PS1 derivatives accumulate to saturable levels, and at approximately 1:1 stoichiometry, independent of transgene-derived mRNA. We conclude that PS1 is subject to endoproteolytic processing in vivo.

Subunit composition of neurofilaments specifies axonal diameter

Neurofilaments (NFs), which are composed of NF-L, NF-M, and NF-H, are required for the development of normal axonal caliber, a property that in turn is a critical determinant of axonal conduction velocity. To investigate how each subunit contributes to the radial growth of axons, we used transgenic mice to alter the subunit composition of NFs. Increasing each NF subunit individually inhibits radial axonal growth, while increasing both NF-M and NF-H reduces growth even more severely. An increase in NF-L results in an increased filament number but reduced interfilament distance. Conversely, increasing NF-M, NF-H, or both reduces filament number, but does not alter nearest neighbor interfilament distance. Only a combined increase of NF-L with either NF-M or NF-H promotes radial axonal growth. These results demonstrate that both NF-M and NF-H play complementary roles with NF-L in determining normal axonal calibers.

Early upregulation of medium neurofilament gene expression in developing spinal cord of the wobbler mouse mutant

Homozygous wobbler mouse mutants develop a progressive paralysis due to spinal motoneuron degeneration. To understand the molecular aspect underlying the genetic defect we have studied the embryonic (from E13) and postnatal expression of the three neurofilament and choline acetyltransferase genes in each member from several wild-type (wt) and wobbler (wr) progenies. There are no variations among wt littermates at all ages studied. In contrast, analyses of neurofilament mRNA reveals a 3-4-fold increase of medium neurofilament (NFM) mRNA in wobbler mice (wr/wr). The pattern of increased NFM mRNA during development, prior to the appearance of the wobbler phenotype, among littermates (from heterozygous carriers) conforms to a mendelian inheritance of a single gene defect 1:2:1 (wr/wr:wr/+:+/+). Light and heavy neurofilament mRNA levels are also increased later in development exclusively in those individuals with high NFM mRNA values indicating that increase of the latter is associated with increase of the light and heavy subunit expression. Also NF proteins are increased. Expression of choline acetyltransferase gene is instead always comparable to normal control. Our study provides novel insights into the nature of the wobbler defect, strengthening the hypothesis that neurofilament accumulation plays a pivotal role in the etiopathogenesis of motoneuron degeneration.

Metabolism of the "Swedish" amyloid precursor protein variant in neuro2a (N2a) cells. Evidence that cleavage at the "beta-secretase" site occurs in the golgi apparatus

The 4-kDa beta-amyloid peptide (Abeta), a principal component of parenchymal amyloid deposits in Alzheimer's disease, is derived from amyloid precursor proteins (APP). To identify potential intracellular compartments involved in Abeta production, we expressed human APP-695 (APPwt) and APP-695 harboring the Swedish double mutation (APPswe) associated with familial early-onset Alzheimer's disease, in mouse N2a cells. We demonstrate that cells expressing APPswe secrete high levels of Abeta peptides and beta-secretase-generated soluble APP derivatives (APP s beta) relative to cells expressing APPwt. In addition, we observed a concomitant diminution in the levels of alpha-secretase-generated soluble APP derivatives (APP s alpha). Our interpretation of these findings is that beta-secretase cleavage occurs in an intracellular compartment and disables those substrates which would normally be cleaved by alpha-secretase. As anticipated, the levels of APPswe are diminished relative to the steady-state levels of surface-bound APPwt; moreover, surface-bound APPswe and APPwt molecules are released from the plasma membrane after cleavage by alpha-secretase, but not by beta-secretase. Finally, by examining the rate of appearance of specific APP metabolites generated by beta-secretase, we now unequivocally demonstrate that beta-secretase cleavage of APPswe occurs within the Golgi apparatus, as early as the medial compartment.

Compartmentation of alpha-internexin and neurofilament triplet proteins in cultured hippocampal neurons

Intermediate filaments comprise an integral part of the neuronal cytoskeleton. However, little is known about their function, and there remains some uncertainty about their precise subcellular localization. We examined the timing of expression and distribution of alpha-internexin, neurofilament triplet proteins and peripherin using immunocytochemistry in cultured hippocampal neurons. alpha-Internexin immunostaining was present in all neurons at all developmental stages. Immunostaining appeared as long filaments in axons and short fragments in dendrites which extended into dendritic spines. The presence of alpha-internexin in dendritic spines was confirmed in situ by electron microscopy of rat hippocampal tissue sections and suggests that this intermediate filament may serve as a link between cytoskeletal elements in dendritic shafts and spines. In culture, immunostaining using antibodies against individual triplet protein subunits indicated that light (NF-L) and middle (NF-M) subunits were first expressed in cells shortly after the initiation of axonal outgrowth. Expression of the heavy (NF-H) subunit occurred a few days later. Although timing and localization of expression did not correlate with the initiation of axonal or dendritic processes, it was coincident with periods of rapid outgrowth. Triplet proteins were more abundant in axons and appeared to be incorporated into lengthier filaments than in dendrites. Highly phosphorylated NFH/M immunoreactivity was polarized to axons after 6 days in culture. The distribution of one NF-H epitope was restricted to GABAergic neurons in mature cultures, suggesting a cell-type specific modification. Peripherin was not detectable at any time in hippocampal cultures. Our results show that intermediate filaments are integral components of the neuronal cytoskeleton of cultured hippocampal neurons throughout development. Furthermore, the localization of alpha-internexin suggests that it may be involved in the formation or maintenance of dendritic spines.

Filensin and phakinin form a novel type of beaded intermediate filaments and coassemble de novo in cultured cells

The fiber cells of the eye lens possess a unique cytoskeletal system known as the "beaded-chain filaments" (BFs). BFs consist of filensin and phakinin, two recently characterized intermediate filament (IF) proteins. To examine the organization and the assembly of these heteropolymeric IFs, we have performed a series of in vitro polymerization studies and transfection experiments. Filaments assembled from purified filensin and phakinin exhibit the characteristic 19-21-nm periodicity seen in many types of IFs upon low angle rotary shadowing. However, quantitative mass-per-length (MPL) measurements indicate that filensin/phakinin filaments comprise two distinct and dissociable components: a core filament and a peripheral filament moiety. Consistent with a nonuniform organization, visualization of unfixed and unstained specimens by scanning transmission electron microscopy (STEM) reveals the the existence of a central filament which is decorated by regularly spaced 12-15-nm-diam beads. Our data suggest that the filamentous core is composed of phakinin, which exhibits a tendency to self-assemble into filament bundles, whereas the beads contain filensin/phakinin hetero-oligomers. Filensin and phakinin copolymerize and form filamentous structures when expressed transiently in cultured cells. Experiments in IF-free SW13 cells reveal that coassembly of the lens-specific proteins in vivo does not require a preexisting IF system. In epithelial MCF-7 cells de novo forming filaments appear to grow from distinct foci and organize as thick, fibrous laminae which line the plasma membrane and the nuclear envelope. However, filament assembly in CHO and SV40-transformed lens-epithelial cells (both of which are fibroblast-like) yields radial networks which codistribute with the endogenous vimentin IFs. These observations document that the filaments formed by lens-specific IF proteins are structurally distinct from ordinary cytoplasmic IFs. Furthermore, the results suggest that the spatial arrangement of filensin/phakinin filaments in vivo is subject to regulation by host-specific factors. These factors may involve cytoskeletal networks (e.g., vimentin IFs) and/or specific sites associated with the cellular membranes.

Peripherin assembles into homopolymers in SW13 cells

The properties of full-length and mutant peripherins were studied in intermediate filament-less SW13 cells to define regions of peripherin that are essential for initiation of filament assembly. A full-length rat peripherin gene transfected into SW13 cells resulted in filament formation, consistent with the close structural relationship of peripherin to other type III intermediate filament proteins that readily form homopolymers. Translation of full-length rat peripherin is initiated predominantly at the second of two inframe AUGs. Deletions within the amino terminus of wild-type peripherin abolished its ability to form filaments in SW13 cells. In contrast, deletion of the entire carboxyl-terminal tail of peripherin did not affect its ability to form filamentous arrays in transfected SW13 cells. These results indicate that, of the intermediate filament proteins that are expressed in mature neurons, only peripherin and alpha-internexin are capable of making homopolymer intermediate filaments. In addition, mutations of the carboxyl tail of peripherin generally do not interfere with filament network formation.

Increasing neurofilament subunit NF-M expression reduces axonal NF-H, inhibits radial growth, and results in neurofilamentous accumulation in motor neurons

The carboxy-terminal tail domains of neurofilament subunits neurofilament NF-M and NF-H have been postulated to be responsible for the modulation of axonal caliber. To test how subunit composition affects caliber, transgenic mice were generated to increase axonal NF-M. Total neurofilament subunit content in motor and sensory axons remained essentially unchanged, but increases in NF-M were offset by proportionate decreases in both NF-H and axonal cross-sectional area. Increase in NF-M did not affect the level of phosphorylation of NF-H. This indicates that (a) in vivo NF-H and NF-M compete either for coassembly with a limiting amount of NF-L or as substrates for axonal transport, and (b) NF-H abundance is a primary determinant of axonal caliber. Despite inhibition of radial growth, increase in NF-M and reduction in axonal NF-H did not affect nearest neighbor spacing between neurofilaments, indicating that cross-bridging between nearest neighbors does not play a crucial role in radial growth. Increase in NF-M did not result in an overt phenotype or neuronal loss, although filamentous swellings in perikarya and proximal axons of motor neurons were frequently found.

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.

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.

Novel regulation of chondroitin sulfate glycosaminoglycan modification of amyloid precursor protein and its homologue, APLP2

Alzheimer's disease is characterized by the presence of parenchymal and cerebrovascular deposits of beta-amyloid (A beta). A beta is derived from larger amyloid precursor proteins (APP), a member of a family of related polypeptides that includes amyloid precursor-like proteins, APLP1 and APLP2. APP and APLP2 isoforms are encoded by several alternatively spliced APP and APLP2 transcripts, respectively. We previously reported that the APLP2-751 isoform is modified by the addition of chondroitin sulfate glycosaminoglycan (CS GAG) at Ser-614. In this report, we demonstrate that the APLP2-763 isoform, which contains an insertion of 12 amino acids immediately N-terminal to Ser-614, is not modified by CS GAG. Finally, we demonstrate that like APLP2-751, APP isoforms that lack sequences encoded by exon 15 (L-APP) are also modified by CS GAG, whereas APP forms containing exon 15 are not. We suggest that CS GAG modification of a subset of APP and APLP2 isoforms represents a means of generating functional diversity for these polypeptides.

Overexpression of the human NFM subunit in transgenic mice modifies the level of endogenous NFL and the phosphorylation state of NFH subunits

Neurofilaments (NFs), the major intermediate filaments of central nervous system (CNS) and peripheral nervous system (PNS) neurons, are heteropolymers formed from the high (NFH), middle (NFM), and low (NFL) molecular weight NF subunits. To gain insights into how the expression of NF subunit proteins is regulated in vivo, two transgenes harboring coding sequences for human NFM (hNFM) with or without the hNFM multiphosphorylation repeat domain were introduced into mice. Expression of both hNFM constructs was driven by the hNFM promoter and resulted in increased levels of hNFM subunits concomitant with an elevation in the levels of mouse NFL (mNFL) proteins in the CNS of both lines of transgenic mice. The increased levels of mNFL appear specific to NFM because previous studies of transgenic mice overexpressing either NFL or NFH did not result in increased expression of either of the other two NF subunits. Further, levels of the most heavily phosphorylated isoforms of mouse NFH (mNFH) were reduced in the brains of these transgenic mice, and electron microscopic studies showed a higher packing density of NFs in large-diameter CNS axons of transgenic versus wild-type mice. Thus, reduced phosphorylation of the mNFH carboxy terminal domain may be a compensatory response of CNS neurons to the increase in NFs, and reduced negative charges on mNFH sidearms may allow axons to accommodate more NFs by increasing their packing density. Taken together, these studies imply that NFM may play a dominant role in the in vivo regulation of the levels of NFL protein, the stoichiometry of NF subunits, and the phosphorylation state of NFH. NFM and NFH proteins may assume similar functions in regulation of NF packing density in vivo.

Metabolism of the amyloid precursor-like protein 2 in MDCK cells. Polarized trafficking occurs independent of the chondroitin sulfate glycosaminoglycan chain

Deposition of beta-amyloid peptide in senile plaques is a principal neuropathological hallmark of Alzheimer's disease. beta-Amyloid peptide is derived from larger amyloid precursor proteins. Amyloid precursor protein is a member of a family of integral membrane glycoproteins that includes amyloid precursor-like protein (APLP) 1 and 2. Alternatively spliced pre-mRNAs encode several APLP2 isoforms; the APLP2-751 isoform is a substrate for modifications by a chondroitin sulfate glycosaminoglycan (CS GAG) chain, whereas the APLP2-763 isoform does not undergo CS GAG modification. In this report, we have examined the sorting and metabolism of APLP2-751 and APLP2-763 in polarized epithelial Madin-Darby canine kidney (MDCK) cells. We demonstrate that, despite differences in post-translational modifications, both the APLP2-751 proteoglycan and APLP2-763 isoform were targeted and secreted to the basolateral compartment of MDCK cells. We document that the kinetics of intracellular maturation of full-length forms and secretion of soluble derivatives generated from each isoform were indistinguishable. Our results are consistent with the view that, in MDCK cells, the CS GAG chain of APLP2 has little influence on intracellular trafficking and that the principal basolateral targeting determinants are likely to reside in the APLP2 core protein.

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.

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.

Expression of neurofilament proteins by horizontal cells in the rabbit retina varies with retinal location

Classical neurofibrillar staining methods and immunocytochemistry with antibodies to the light, medium and heavy chain subunits of the neurofilament triplet have been used for in situ and in vitro investigation of the organization of neurofilaments in A- and B-type horizontal cells of the adult rabbit retina. Surprisingly, their expression and organization within a cell is dependent on its location along the dorso-ventral axis of the retina. A-type horizontal cells in superior retina consistently stained with a wide variety of neurofibrillar methods to reveal neurofibrillar bundles, which immunocytochemistry showed to contain all three neurofilament subunits. A-type horizontal cells in inferior retina were uniformly refractory to neurofibrillar staining, although they expressed all three subunits. However, there was less of the light and medium subunits; the organization of the filaments into bundles (neurofibrils) is minimal. B-type horizontal cells could not be stained with any neurofibrillar method and were not recognizable by in situ immunocytochemistry. However, B-type cells could be seen to express all three subunits in vitro, but the expression of the light and medium subunits was weak. There was only a slight difference between B-type cells taken from superior and inferior retina. Combined with the results of recent transfection studies, these findings suggest that the amount of the light neurofilament subunit present in a horizontal cell determines its content of neurofibrillar bundles, and that rabbit horizontal cells may contain more neurofilament protein, particularly of the heavy subunit, than is used for neurofilament formation.(ABSTRACT TRUNCATED AT 250 WORDS)

The single lamprey neurofilament subunit (NF-180) lacks multiphosphorylation repeats and is expressed selectively in projection neurons

The lamprey is considered the most primitive living vertebrate and its neurofilaments (NFs) are unique in being homopolymers of a single 180 kDa subunit (NF-180). Previous immunologic studies have suggested that the sidearm of NF-180 is highly phosphorylated selectively in the largest diameter axons. We report in this study the isolation and characterization of cDNA clones encoding the NF-180 lamprey protein. In situ hybridization with digoxigenin-labeled cRNA revealed NF-180 message exclusively in neurons with long axons, such as reticulospinal neurons and cranial motor neurons. The core of NF-180 was similar in structure to those of mammalian neurofilaments, but surprisingly, the carboxy sidearm lacked the multiphosphorylation repeats characteristic of higher vertebrate and invertebrate neurofilaments. Overall there was a paucity of potential phosphorylation sites in the NF-180 carboxy-terminus compared to NF-M and NF-H of mammals, fish and squid. This, along with the highly acidic nature of the NF-180 sidearm, makes it unlikely that phosphorylation of sidearm residues regulates interfilament spacing and axon diameter through global electrostatic repulsion of the carboxy-terminus away from the filament backbone. Furthermore, the expression of a single neurofilament subunit in the lamprey that is most similar to the NF-M of higher vertebrates suggests that all three mammalian neurofilament subunits evolved from a single NF-M-like precursor.

Intermediate filaments: new proteins, some answers, more questions

The past year has seen significant progress in the characterization of intermediate filament proteins. New proteins have been identified and physiologically significant differences between known proteins have been revealed. Changes in intermediate filament organization have been linked to changes in cell behavior, and mutational analyses are beginning to reveal the connection between intermediate filament expression, network formation, cellular behavior and disease.

The organization of neurofilaments accumulated in perikaryon following aluminum administration: relationship between structure and phosphorylation of neurofilaments

Neurofilaments accumulated in perikarya and dendrites of anterior horn cells and Purkinje cells of rabbit treated by aluminum chloride were analysed with a variety of techniques. Four different monoclonal antibodies against phosphorylated and nonphosphorylated epitopes on neurofilament H subunit were used to compare phosphorylation state of these accumulated neurofilaments with that of axonal neurofilaments. Although immunoblotting revealed no significant difference in phosphorylation between control and aluminum-treated brains, accumulated neurofilaments were immunocytochemically more phosphorylated than control perikaryal or dendritic neurofilaments. With detailed analysis of cryothin-section immunogold labeling, accumulated neurofilaments were, however, significantly less phosphorylated than axonal neurofilaments. With quick-freeze deep etching, core filaments of accumulated neurofilaments are as dense as axonal neurofilaments but much less regularly aligned. Cross-bridges of accumulated neurofilaments were less frequent and more branched than those of axonal neurofilaments, and when examined with combined immunocytochemistry and deep etching, were less phosphorylated. These results suggest that there is a relationship between the phosphorylation and the structural organization of neurofilaments. The phosphorylation of neurofilament H subunit may be necessary for formation of frequent and straight cross-bridges and resulting regular alignment of core filaments.

Determinants for intracellular sorting of cytoplasmic and nuclear intermediate filaments

The mechanism by which nuclear and cytoplasmic filaments are sorted in vivo was studied by examining which lamin sequences are required to target an otherwise cytoplasmic IF protein, the small neurofilament subunit (NF-L), to the nuclear lamina. By swapping corresponding domains between NF-L and lamin A, nuclear envelope targeting of NF-L was shown to require the presence of the "head" domain, a 42-amino acid sequence unique to lamin rod domains, a nuclear localization signal and the CAAX motif. Replacement of the entire COOH-terminal tail of lamin A with that of NF-L had no discernible effect on nuclear localization of lamin A, provided the substituted NF-L tail contained a NLS and a CAAX motif. This chimeric protein exhibited characteristics more typical of lamin B than that of the parental lamin A. With regard to cytoplasmic assembly properties, substitution of the head domain of lamin A for that of NF-L did not substantially affect the ability of NF-L to coassemble with vimentin in the cytoplasm. In contrast, insertion of a 42-amino acid sequence unique to lamin rod domains into NF-L profoundly affected NF-L coassembly with vimentin indicating that the 42-amino acid insertion in lamins may be important for sorting lamins from cytoplasmic IF proteins.