Neurofilament protein phosphorylation. Species generality and reaction characteristics

Regulation of neurofilament axonal transport by phosphorylation in optic axons in situ

Axonal transport of neurofilament (NFs) is considered to be regulated by phosphorylation. While existing evidence for this hypothesis is compelling, supportive studies have been largely restricted to correlative evidence and/or experimental systems involving mutants. We tested this hypothesis in retinal ganglion cells of normal mice in situ by comparing subunit transport with regional phosphorylation state coupled with inhibition of phosphatases. NF subunits were radiolabeled by intravitreal injection of 35S-methionine. NF axonal transport was monitored by following the location of the peak of radiolabeled subunits immunoprecipitated from 9x1.1 mm segments of optic axons. An abrupt decline transport rate was observed between days 1 and 6, which corresponded to translocation of the peak of radiolabeled subunits from axonal segment 2 into segment 3. Notably, this is far downstream from the only caliber increase of optic axons at 150 mu from the retina. Immunoblot analysis demonstrated a unique threefold increase between segments 2 and 3 in levels of a "late-appearing" C-terminal NF-H phospho-epitope (RT97). Intravitreal injection of the phosphatase inhibitor okadaic acid increased RT97 immunoreactivity within retinas and proximal axons, and markedly decreased NF transport rate out of retinas and proximal axons. These findings provide in situ experimental evidence for regulation of NF transport by site-specific phosphorylation.

C-terminal phosphorylation of the high molecular weight neurofilament subunit correlates with decreased neurofilament axonal transport velocity

We probed the relationship of NF axonal transport of neurofilaments (NFs) to their phosphorylation state by comparing these parameters in two closely-aged groups of young adult mice - 2 and 5 months of age. This particular time interval was selected since prior studies demonstrate that optic axons have already completed axonal caliber expansion and attained adult NF levels by 2 months but, as shown herein, continue to increase NF-H C-terminal phosphorylation. NF axonal transport was monitored by autoradiographic analysis of the distribution of radiolabeled subunits immunoprecipitated from optic axon segments at intervals following intravitreal injection of 35S-methionine. Both the peak and front of radiolabeled NFs translocated faster in 2- vs. 5-month-old mice. This developmental decline in NF transport rate was not due to reduced incorporation of NFs into the cytoskeleton, nor to an overall decline in slow axonal transport. By excluding or minimizing other factors, these findings support previous conclusions that C-terminal NF phosphorylation regulates NF axonal transport.

Protein phosphorylation in human peripheral nerve: altered phosphorylation of a 25-kDa glycoprotein in leprosy

Protein phosphorylation in a low speed supernatant of human peripheral nerve (tibial and sural) homogenate was investigated. The major phosphorylated proteins had molecular mass in the range of 70, 55, 45, and 25 kDa. Mg2+ or Mn2+ was essential for maximum phosphorylation although Zn2+, Co2+, and Ca2+ could partially support phosphorylation. External protein substrates casein and histone were also phosphorylated. The protein phosphatase inhibitor orthovanadate enhanced the phosphorylation of the 45 and 25 kDa proteins significantly. Concanavalin A-Sepharose chromatography of the phosphorylated peripheral nerve proteins showed that the 25 kDa protein was a glycoprotein. Protein phosphorylation of peripheral nerves from leprosy affected individuals was compared with normals. The phosphorylation of 25 kDa protein was decreased in most of the patients with leprosy.

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.

The regulation of neurofilament protein dynamics by phosphorylation: clues to neurofibrillary pathobiology

Neurofilament proteins are continuously modified during their lifetime by a succession of protein kinases and phosphatases. Site-specific phosphorylation or dephosphorylation within different polypeptide domains of each neurofilament subunit is now believed to regulate such behaviors of neurofilaments as subunit polymerization and exchange, axonal transport, interactions with other cytoskeletal proteins and degradation. Local regulation of phosphorylation events could account for variations in the size, morphology and dynamics of the neurofilament network in different regions of the neuron. The apparent greater plasticity of the neurofilament network in regions like the perikaryon, initial segment and nodes along the axon may provide some insight into the vulnerability of these regions in neurofibrillary disease.

Lithium chloride inhibits the phosphorylation of newly synthesized neurofilament protein, NF-M, in cultured chick sensory neurons

The middle and high molecular weight members of the neurofilament triplet, NF-M and NF-H, undergo extensive posttranslational polyphosphorylation, a process requiring 24 h or more for completion. We have investigated ways of perturbing this process in intact cells and have found that phosphorylation of newly synthesized NF-M in cultured chick sensory neurons is inhibited by Li+. [35S]Methionine pulse-chase experiments were carried out with pure neuronal cultures, and the phosphorylation of newly synthesized NF-M was monitored by following the accompanying change, with chase time, in apparent size and charge of the polypeptide. Addition of LiCl to the medium inhibited this mobility shift in a dose-dependent manner over concentrations between 2 and 25 mM. Incorporation of 32P into NF-M, as well as NF-H, was also inhibited, whereas incorporation into the low molecular weight neurofilament protein, beta-tubulin, and total protein was unaffected. Protein synthesis was not altered. Exposure to 25 mM LiCl for up to 72 h was not toxic, and the inhibition of NF-M phosphorylation was completely reversible. When 25 mM Li+ was added after NF-M had become partially phosphorylated, further progression was blocked, but there was no net dephosphorylation or degradation of NF-M. Additional experiments suggest that this action of Li+ is probably not due to effects on second messenger levels or to effects on tubulin metabolism and assembly state presented in our accompanying article, but rather to interference by Li+ itself, with the phosphorylation of NF-M and NF-H by specific neurofilament kinase(s).

Neurofilament protein phosphorylation in spinal cord of experimentally diabetic rats

This study was designed to determine if the known decrease in slow axonal transport of proteins in the sciatic nerve of experimentally diabetic rats is related to altered phosphorylation of neurofilament proteins (NFPs). Rats were rendered diabetic with 50 mg/kg of streptozotocin, i.p. At 3 and 6 weeks later, NFPs were prepared from spinal cord. The in vivo phosphorylation state of NFPs was examined by using phosphate-dependent (RT97) and -independent (RMd09) antibodies against high-molecular-mass NFPs on Western blots. Neurofilament-associated kinase activity was also measured in vitro by incubation of NFPs with [32P]ATP. Phosphorylation of all three NFPs (high, medium, and low molecular mass) occurred, as confirmed by gel electrophoresis and autoradiography. At 30 min of incubation, protein-bound radioactivity in NFPs from diabetic animals was reduced to 86.7 +/- 3.4 and 54.3 +/- 19.6% of that in nondiabetic animals at 3 and 6 weeks of diabetes, respectively (p less than 0.001 and p less than 0.05, respectively). NFPs were also incubated with acid phosphatase and rephosphorylated. Results showed that the increased in vivo phosphorylation contributed to the decreased in vitro phosphorylation. Extraction of protein kinases and addition back to the NFPs revealed, in addition, a reduced activity in the diabetic animals of the protein kinases measured in vitro.

Immunoelectron microscopical localization of the catalytic subunit of cAMP-dependent protein kinases in brain microtubules and neurofilaments

The catalytic subunit of cAMP-dependent protein kinases was localized in microtubules and neurofilaments by immunogold electron microscopy. In microtubules, the label was similarly distributed as an immunolabel for the microtubule associated protein MAP 2. The neurofilaments showed no reaction with the MAP 2-antiserum. Our results support the suggestion of an in vivo role of cAMP-dependent protein kinases in the regulation of microtubules. In addition, this is the first demonstration that cAMP-dependent protein kinase is associated with neurofilaments.

Characterization of neurofilament-associated protein kinase activities from bovine spinal cord

1. A neurofilament-enriched preparation from bovine spinal cord contains endogenous protein kinases that phosphorylate high, middle, and low molecular weight neurofilament subunits (NF-H, NF-M, and NF-L), as well as certain other endogenous and exogenous substrates. 2. Most of this associated kinase activity can be separated from the neurofilament subunits and the bulk of the protein by extraction of the neurofilament preparation with 0.8 M KCl. Assays using specific exogenous substrates, activators, and inhibitors for known kinases reveal significant levels of Ca2(+)-calmodulin-dependent, cyclic nucleotide-dependent, Ca2(+)-phosphatidylserine diglyceride-dependent, and regulator-independent kinase activities in the high-salt extract. 3. Fractionation of the salt extract on a gel filtration column resolves a regulator-independent kinase activity identified by its ability to phosphorylate purified NF-M. This preparation can phosphorylate all three neurofilament proteins either in purified form or in the assembled form, as well as alpha-casein. Only the regulator-independent kinase activity in this fraction is responsible for the phosphorylation of neurofilament proteins. 4. While this partially purified kinase activity does not show a strong substrate specificity between the three neurofilament subunits, the phosphorylation pattern it produces upon incubation with salt-extracted neurofilaments is similar to the regulator-independent phosphorylation pattern found in the original neurofilament preparation and, thus, represents a useful starting point for the further purification of this neurofilament-associated kinase activity.

Inhibition of phosphorylation of rat synaptosomal proteins by snake venom phospholipase A2 neurotoxins (beta-bungarotoxin, notexin) and enzymes (Naja naja atra, Naja nigricollis)

Some snake venom neurotoxins, such as beta-bungarotoxin (beta-BuTX) and notexin, which inhibit the release of neurotransmitter at both peripheral and central presynaptic terminals possess phospholipase A2 activity. In contrast, most snake venom phospholipase A2 enzymes such as those isolated from Naja naja atra and Naja nigricollis are structurally homologous to these neutrotoxins but do not have any specific or potent presynaptic action although they have higher enzymatic activities than the neurotoxins. In order to investigate the mechanisms of presynaptic action of the snake venom neurotoxins, we studied their effects on phosphorylation of rat brain synaptosomal proteins. It is known that phosphorylation of synapsin I, a neuron specific and synaptic vesicle associated phosphoprotein, increases neurotransmitter release. Incubation of cerebral cortical synaptosomes with 32P-orthophosphate at 37 degrees C for 30 min, caused significant phosphorylation of a wide mol. wt range of proteins including most markedly those proteins in the mol. wt range (81,000-86,000) of synapsin I. Both snake venom phospholipase A2 neurotoxins and enzymes (5, 15 and 50 nM) inhibited phosphorylation in a Ca2(+)-dependent manner with the following order of potencies: beta-BuTX greater than N.n. atra phospholipase A2 greater than or equal to notexin greater than N. nigricollis phospholipase A2. Five nanomoles of beta-BuTX, which has the lowest phospholipase A2 activity, inhibited phosphorylation of a wide range of mol. wt proteins (51,000-188,000) by 42-58%. At the same concentration, N.n. atra phospholipase A2 (which possesses the highest enzymatic activity), notexin and N. nigricollis phospholipase A2 caused less inhibition than beta-BuTX, ranging from 0-40% depending on the agent used. These results indicate that there is no correlation between their potencies in inhibiting phosphorylation and the levels of their phospholipase A2 activities. An inhibitory activity on phosphorylation may be at least partially responsible for a presynaptically-induced block of neurotransmitter release.

Visualization of differential neurofilament phosphorylation in the pre- and postsynaptic axoplasm of the squid giant synapse: an electron spectroscopic study

When inelastically scattered electrons with an energy loss specific for interaction with phosphorus atoms were used for visualization of sections of squid axons, bead-like domains of elongated proteins, presumably neurofilaments, exhibited distinct phosphorus signals. A marked asymmetry of these phosphorus signals was detectable between the pre- and the postsynaptic cytoskeleton of the giant synapse. Signals were very numerous and intense in the presynaptic terminal, while rare and weak in the postsynaptic axoplasm. The giant axon revealed a delayed appearance of phosphorus signals in its course from the cell bodies in the giant fibre lobe to its exit from the stellate ganglion. Numerous and intense phosphorylation signals were evident only in the peripheral giant axon. Asymmetry in the distribution of phosphorus signals between pre- and postsynaptic axoplasm paralleled differences in Ca(2+)-buffering mechanisms, as shown in a previous study. In the presynaptic terminal patterns of phosphorus signals correlated with precipitates which had formed after intra-axonal injection of calcium. Our observations suggest a role of phosphorylated neurofilaments in binding of calcium in the squid synapse.

Properties of several protein kinases that copurify with rat spinal cord neurofilaments

Several protein kinases that copurify with neurofilaments (NF) were identified and each kinase was assessed for its ability to phosphorylate NF proteins. NFs were isolated using an axonal flotation procedure and the kinases were extracted from NFs with 0.8 M KCl. NF kinases were incubated with peptide substrates for selected protein kinases, [32P]ATP and protein kinase cofactors and inhibitors to characterize the kinases. Using peptide substrates, three types of kinase were identified, and a fourth was identified using NF protein as substrate. The first three kinases were the catalytic subunit of cAMP-dependent protein kinase, calcium-calmodulin dependent protein kinase II and a cofactor-independent kinase that phosphorylated prepro VIP sequence 156-170 and was inhibited by heparin. Using NF proteins as substrate, a fourth kinase was identified which was cofactor-independent and was not inhibited by heparin. Neither cofactor-independent kinase was casein kinase II. NF proteins were phosphorylated in vitro on serine and threonine, primarily by the two cofactor-independent kinases. Using [alpha-32P]8-N3ATP for affinity labeling, one kinase of 43,800 Da was identified. Thus, in addition to cAMP-dependent protein kinase and calcium-calmodulin dependent protein kinase II, two kinases have been found which are primarily responsible for NF phosphorylation in vitro and are cofactor-independent.

Specificity of human anti-neurofilament autoantibodies

The specificities and isotypes of human antibodies that react with neurofilament (NF) proteins were examined by Western blot analysis. Two-thirds of the subjects tested had antibodies to the 200 kDa high molecular weight neurofilament protein (NF-H), and fewer had antibodies to the low and middle molecular weight neurofilament proteins (NF-L and NF-M respectively). Human autoantibodies bound to both native and dephosphorylated NF-H, but some antibodies bound to dephosphorylated NF-H only, indicating the presence of at least two target epitopes. They also recognized a fusion protein containing a segment of the NF-H protein produced by a cDNA clone in Escherichia coli, indicating that they bind to unmodified peptide epitopes. The anti-NF-H antibodies were mostly IgG, but were frequently complexed to IgA or IgM antibodies, possibly with rheumatoid factor or anti-idiotypic activity. These characteristics of anti-NF-H antibodies are most consistent with a secondary immune response that is antigen driven and T-cell dependent.

Identities, antigenic determinants, and topographic distributions of neurofilament proteins in the nervous systems of adult frogs and tadpoles of Xenopus laevis

Three proteins with nominal molecular weights of 73 kDa (XNF-L), 175 kDa (XNF-M), and 205 kDa (XNF-H) were identified as putative neurofilament proteins in the nervous system of the frog, Xenopus laevis. These conclusions were based on four criteria: (1) these proteins were enriched in cytoskeletal preparations; (2) they reacted with a monoclonal antibody (anti-IFA) that cross-reacts with an epitope found in all intermediate filament proteins; (3) they cross-reacted with monoclonal antibodies directed against specific mammalian neurofilaments; and (4) antibodies that reacted with these proteins on Western blots specifically stained neurons in immunohistochemical analyses. The neurofilament proteins in Xenopus were antigenically similar, but not identical to mammalian neurofilament proteins. The principal difference was that four antibodies that reacted on Western blots with rat NF-H reacted with XNF-M in Xenopus. However, similarly to mammals, antibodies against phosphorylated XNF-M specifically labeled axons, whereas an antibody that reacted only with dephosphorylated epitopes on XNF-M specifically labeled neuronal cell bodies in immunohistochemistry. Three other antibodies that reacted equally well with untreated or alkaline-phosphatase-treated XNF-M or XNF-H proteins also showed axonally restricted staining in the adult Xenopus nervous system. An XNF-L (XC5D10) antibody was produced which stained axons and cell bodies equivalently throughout the adult Xenopus nervous system. By 3 days of development (stage 42; Xenopus tadpoles), antibodies to all three molecular weight forms of the frog neurofilament proteins detected specific neurons in the brainstem and spinal cord; and antibodies to phosphorylated and dephosphorylated epitopes on XNF-M could discriminate between axons and cell bodies in the rhombencephalon. The immunoreactivities of four antibodies directed at XNF-L, -M, or -H, which were unaffected by alkaline phosphatase treatment, differed significantly in their immunohistochemical staining patterns in adult vs. premetamorphic frogs.

Acrylamide treatment of PtK1 cells causes dephosphorylation of keratin polypeptides

Treatment of PtKl cells with 5 mM acrylamide for 4 hr results in alterations in the distribution of keratin filaments within the cells. This effect is reversible within 18 hr. Labeling of PtKl cells with 32P demonstrates that there are four phosphorylated keratins, having Mr of 56 k, 53 k, 45 k, and 40 k. Phosphate associated with these polypeptides appears to turn over with a t1/2 of 12 hr. Incubation of labeled cells in 5 mM acrylamide results in approximately 50% dephosphorylation of keratins within 2 hr, which is 3 times faster than normal turnover. Recovery of cells from acrylamide is accompanied by rephosphorylation of keratins within 18 hr. Analysis by 31P NMR spectroscopy shows that acrylamide treatments are accompanied by a transient decrease in soluble inorganic phosphate. This is followed by a rapid increase in Pi which gradually returns to normal levels. These studies show a strong correlation between phosphorylation of PtKl cell keratins and morphological response of keratin filaments to acrylamide. These observations suggest that normal distribution of keratin filaments may be, in part, mediated by protein phosphorylation.

Distribution of specific intermediate-filament proteins in the goldfish retina

The intermediate-filament proteins expressed in the goldfish retina were investigated by immunohistochemistry and by immunoblotting. Polyclonal antibodies that previously had been raised against the goldfish optic nerve neurofilament (ON1 and ON2) and glial filament (ON3 and ON4) proteins were used in this study. Anti-ON1/ON2 antiserum reacted on a retinal immunoblot with two proteins having molecular weights and isoelectric points corresponding to those of ON1 and ON2. Histologically, the most pronounced anti-ON1/ON2 reactivity was observed in the ganglion cell layer of the goldfish retina. The anti-ON3/ON4 antiserum reacted with a single protein on a retinal immunoblot. This protein had a molecular weight and isoelectric point which corresponded to the goldfish optic nerve glial filament proteins. This anti-serum labeled horizontal cells in retina sections. Three previously unidentified goldfish visual-pathway intermediate-filament proteins sharing a molecular weight of 60K were observed on two-dimensional gels of retinal cytoskeletal proteins and on retinal immunoblots which were probed with a monoclonal antibody which recognizes an epitope common to all intermediate filament proteins. The possible existence of homologs of mammalian GFAP and vimentin in the goldfish retina was also explored. Antibodies directed against mammalian GFAP and vimentin labeled the Müller fibers and the cone horizontal cells, respectively. However, immunoblot analysis and a comparison of the two-dimensional gel electrophoresis patterns of goldfish retinal and rat spinal cord cytoskeletal proteins demonstrated a lack of goldfish proteins identical to the mammalian intermediate-filament proteins.

62K proteins constitute the major neurofilament proteins in the frog optic nerve

The frog optic nerve contains a major group of proteins at a molecular weight of 62K. These proteins are insoluble in nonionic detergents, reactive with a general antibody to intermediate filament proteins, and not labeled by ex vivo incubations of optic nerve. They were therefore considered neurofilament proteins. Axonal transport and enucleation studies were performed to characterize further the origin of these proteins. The results show that the 62K proteins are transported into the optic nerve at a very slow rate (0.1 mm/day). After enucleation, these proteins are substantially reduced in concentration to 20% of the control value at 13 weeks. The predominant neurofilament proteins of the frog optic nerve are 62K in molecular weight. These results are discussed in terms of the anatomy of the frog optic nerve and also contrasted to findings obtained for the goldfish optic nerve.

Appearance and phosphorylation of the 210 kDalton neurofilament protein in newborn rat brain, spinal cord, and sciatic nerve

The appearance and in vivo phosphorylation of the 210 kDalton (kD) neurofilament protein (NF210K) in newborn rat brain, spinal cord, and sciatic nerve were investigated. Electron microscopic examination of neurofilaments isolated from newborn rat brain and spinal cord demonstrated morphologically distinct filaments which contained cross-bridging side arms. Neurofilament proteins, phosphorylated in vivo, were separated by sodium dodecyl sulfate slab gel electrophoresis and were transferred from acrylamide gels to nitrocellulose sheets. The nitrocellulose sheets were treated with antiserum to the 70 kD, 145 kD and 210 kD neurofilament proteins by the immunoblot technique. The three neurofilament proteins were found to be present in newborn brain, spinal cord and sciatic nerve. The presence of NF210K in newborn rat brain was further confirmed by 2-dimensional gel electrophoresis followed by identification of this protein by the immunoblot technique. Exposure of the immunostained nitrocellulose sheets to x-ray film revealed that the NF210K, NF145K, and NF70K proteins were phosphorylated in filaments prepared from newborn rat central and peripheral nervous systems. These results suggest that the synthesis and posttranslational modification of the neurofilament proteins may be synchronized or developmentally regulated. It is feasible that phosphorylation of the NF210K subunit may be a prerequisite for the formation of neurofilament cross-bridging elements which are necessary for radial growth of axons.

Homology and diversity between intermediate filament proteins of neuronal and nonneuronal origin in goldfish optic nerve

The predominant intermediate filament proteins of the goldfish optic nerve have molecular weights of 58K. They can be separated into a series of four major isoelectric variants of neuronal (ON1 and ON2) and nonneuronal (ON3 and ON4) origin. The extent of homology between the goldfish 58K intermediate filament proteins themselves and to rat optic nerve vimentin and glial fibrillary acidic protein (GFAP) was investigated. Unlabeled and [32P]orthophosphate-labeled proteins were subjected to partial hydrolysis by V8 protease, chymotrypsin, and CNBr. The results show that the goldfish intermediate filament proteins share with vimentin and GFAP a 40K chymotrypsin-resistant core fragment. Phosphorylated moieties appear to be located outside the core region since they are preferentially cleaved off by chymotrypsin and not found associated with the 40K core. In addition, the goldfish ON proteins contain the antigenic site within the core that is common to most intermediate filaments. V8 or CNBr digestion indicates that many fragments that are common to ON1 and ON2 are clearly distinct from fragments that are common to ON3 and ON4. In addition, structural variability is observed between the goldfish intermediate filament proteins and vimentin and GFAP. The results are discussed in terms of intermediate filament structure and their possible role in nerve growth.

Altered phosphorylation of rat neuronal cytoskeletal proteins in acrylamide induced neuropathy

The activity of protein kinase has been assayed in neurofilament preparations from spinal cords of rats treated with acrylamide. Animals received 50 mg/kg, i.p., of acrylamide per day for a total of 5 or 10 days; these doses produce mild and marked symptoms of neurological damage, respectively. Incorporation of phosphate into proteins was determined using [gamma-32P]ATP followed by SDS-PAGE. Total phosphorylation of neurofilament preparations was significantly increased only in the animals treated with the 500 mg/kg cumulative dose of acrylamide. Phosphorylation of the 200 and 155 kdalton subunits of the neurofilaments was increased by 20-40% in the acrylamide treated groups. The phosphorylation of the 70 kdalton neurofilament subunit was unchanged in the 250 mg/kg group and was significantly decreased in the 500 mg/kg group. Phosphorylation of other protein bands was not altered. These results suggest a mechanism by which acrylamide might produce axonal neurofilamentous accumulations.

An immunocytochemical comparison of cytoskeletal proteins in aluminum-induced and Alzheimer-type neurofibrillary tangles

Exposure of the central nervous system (CNS) of rabbits to aluminum salts produces a progressive encephalopathy. Examination of CNS structures discloses widespread perikaryal neurofibrillary tangle (NFTs) formation. The aluminum-induced NFTs consist of collections of normal neurofilaments, and differ ultrastructurally and in their solubility characteristics from Alzheimer-type NFTs, the latter being composed of largely insoluble paired helical filaments. The present study compares NFTs found in the rabbit to those of Alzheimer's disease, using monoclonal antibodies (SMI 31, SMI 32) that recognize phosphorylated and non-phosphorylated determinants respectively in normal neurofilaments, and an antiserum raised against purified microtubules. Paraffin-embedded sections were stained by the avidin-biotin immunocytochemical method. Intense staining of aluminum-induced NFTs was found after processing with SMI 31 and SMI 32, while no staining of non-tangled perikarya of control rabbits or of Alzheimer-type NFTs was seen. Antimicrotubule anti-serum gave weak, nonfocal staining in the aluminum-treated and control rabbits, while Alzheimer-type NFTs were stained intensely. These results show that phosphorylated and non-phosphorylated neurofilaments accumulate in aluminum-induced NFTs, thus complementing the previously demonstrated specific slowing of the axonal transport of neurofilaments in aluminum intoxication. Further, they suggest that the presence of microtubular proteins may be necessary for altered neurofilaments to take on a paired helical configuration.

Presence of laminin receptors in Staphylococcus aureus

A characteristic feature of infection by Staphylococcus aureus is bloodstream invasion and widespread metastatic abscess formation. The ability to extravasate, which entails crossing the vascular basement membrane, appears to be critical for the organism's pathogenicity. Extravasation by normal and neoplastic mammalian cells has been correlated with the presence of specific cell surface receptors for the basement membrane glycoprotein laminin. Similar laminin receptors were found in Staphylococcus aureus but not in Staphylococcus epidermidis, a noninvasive pathogen. There were about 100 binding sites per cell, with an apparent binding affinity of 2.9 nanomolar. The molecular weight of the receptor was 50,000 and pI was 4.2. Eukaryotic laminin receptors were visualized by means of the binding of S. aureus in the presence of laminin. Prokaryotic and eukaryotic invasive cells might utilize similar, if not identical, mechanisms for invasion.

Survey of intermediate filament proteins in optic nerve and spinal cord: evidence for differential expression

The distribution of intermediate filament proteins in optic nerve and spinal cord from rat, hamster, goldfish, frog, and newt were analyzed by two-dimensional gel electrophoresis. General as well as specific monoclonal and polyclonal antibodies were reacted against putative intermediate filament proteins. In vitro incubations of excised optic nerve in the presence of [35S]methionine distinguished between neuronal and nonneuronal intermediate filament proteins. The proteins of the intermediate filament complex in the two tissues for rat and hamster were similar. The typical neurofilament triplet and glial fibrillary acidic protein (GFAP) were observed. Vimentin was more concentrated in the optic nerve than in the spinal cord. The goldfish, newt, and frog contained neurofilament proteins in the 145-150K range and in the 70-85K range. In addition, predominant neurofilament proteins in the 58-62K molecular-weight range were found in all three species. In contrast to mammalian species, the goldfish, newt, and frog displayed extensive heterogeneity between optic nerve and spinal cord in the expression of both neuronal and nonneuronal intermediate filament proteins. The distinctive presence of low-molecular-weight intermediate filament proteins and their high concentration in the optic nerve and spinal cord of these nonmammalian vertebrates is discussed in terms of neuronal development and regeneration.

Interaction in vitro of the neurofilament triplet proteins from porcine spinal cord with natural RNA and DNA

Neurofilaments were isolated from porcine spinal cord and separated into their subunit proteins (68 Kd NFP, 145 Kd NFP, 200 Kd NFP) by ion exchange chromatography on DEAE-cellulose in 6 M urea. The individual proteins were reacted with total rRNA from Ehrlich ascites tumor cells and the reaction products analysed by sucrose gradient centrifugation at low ionic strength and in the presence of EDTA. All three proteins interacted with rRNA with a preference for 18S rRNA. Competition experiments with native and heat-denatured calf thymus DNA showed that the affinities of the 68 Kd and 145 Kd NFPs were considerably higher for denatured DNA than for rRNA and that native DNA was only a weak competitor. The binding of the 200 Kd NFP to rRNA was unaffected by native and by denatured DNA. When denatured DNA was reacted with a mixture of the 68 Kd and 145 Kd NFPs, the two proteins interacted independently with the nucleic acid, giving rise to two different populations of deoxyribonucleoprotein particles. This segregation is the result of the cooperative interaction of the neurofilament proteins with single-stranded DNA. It could not be observed with rRNA or bacteriophage MS2 RNA. The results clearly show that the 68 Kd and 145 Kd NFPs are single-stranded RNA- and DNA-binding proteins, whereas the 200 Kd NFP seems to be only a single-stranded RNA-binding protein.

Posttranslational protein modification by amino acid addition in intact and regenerating axons of the rat sciatic nerve

Experiments were performed to determine whether posttranslational addition of amino acids to axonal proteins occurs in axons of the rat sciatic nerve. Two ligatures were placed 1 cm apart on sciatic nerves. Six days later, segments proximal to each ligature were removed, homogenized, centrifuged at 150,000 X g, and analyzed for the ability to incorporate 3H-amino acids into proteins. No incorporation of amino acids into proteins was found in the high-speed supernatant, but when the supernatant was passed through a Sephacryl S-200 chromatography column (removing molecules less than 20 kD), [3H]arginine, lysine, leucine and aspartic acid were incorporated into proteins in both proximal and distal nerve segments. Small but consistently greater amounts of radioactivity were incorporated into proteins in proximal segments compared with distal segments, indicating that the components necessary for the reaction are transported axonally. This reaction represents the posttranslational incorporation of a variety of amino acids into proteins of rat sciatic nerve axons. Other experiments showed that the incorporation of amino acids into proteins is by covalent bonding, that the amino acid donor is likely to be tRNA, and that the reaction is inhibited in vivo by a substance whose molecular mass is less than 20 kD. This inhibition is not affected by incubation with physiological concentrations of unlabeled amino acids, by boiling, or by treatment with Proteinase K. When the axonally transported component of the reaction was determined in regenerating nerves, the amount of incorporation of amino acids into protein was 15-150 times that in intact nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

58,000 dalton intermediate filament proteins of neuronal and nonneuronal origin in the goldfish visual pathway

A group of proteins in the goldfish optic nerve with a molecular weight of 58K daltons was analyzed by two-dimensional gel electrophoresis. Results show that the proteins are differentially phosphorylated and found exclusively in a cytoskeletal-enriched fraction. The proteins from this fraction can be reconstituted into typical intermediate filament structures, as shown by electron microscopy. Two components which are of neuronal origin are transported within the slow phase of transport. The 58K proteins are the most abundant proteins in the optic nerve, and they are distinct from actin and tubulin. It was concluded that they are intermediate filament proteins. Cytoskeletal preparations of rat spinal cord, rat optic nerve, and goldfish optic nerve were compared by one-dimensional gel electrophoresis. The rat spinal cord contains glial fibrillary acidic protein (GFAP), and the rat optic nerve contains vimentin and GFAP, in addition to the neurofilament triplet. A typical mammalian neurofilament triplet is not detected in the goldfish optic nerve, while the major cytoskeletal constituent is a 58K band which coelectrophoreses with vimentin in the rat optic nerve by one-dimensional gel electrophoresis.

Protein phosphorylation in the brain

Protein phosphorylation represents an approach, sometimes the only approach available, to study the molecular basis for a wide variety of neurophysiological phenomena. The injection of protein kinases or protein kinase inhibitors into neurones has provided direct evidence that activation of protein kinases has an obligatory role in the mechanisms by which numerous extracellular signals produce specific physiological responses in neurones. A diversity of substrate proteins for the kinases have already been found. In several instances, the identity and functional role of these substrate proteins have been established.

Limited proteolytic modification of a neurofilament protein involves a proteinase activated by endogenous levels of calcium

Posttranslational modification of a structural protein by limited proteolysis is demonstrated for the first time in the nervous system. The 145,000 dalton subunit of neurofilaments in mouse retinal ganglion cell (RGC) axons is selectively converted in vitro to the major 143,000 and 140,000 dalton neurofilament subunits by a neutral proteinase that is activated by endogenous levels of calcium and is distinguishable from other known brain proteinases. The close similarities between this in vitro process and the previously observed modification of the 145,000 dalton neurofilament protein during axoplasmic transport in vivo suggest that the same enzymatic mechanism is involved. These findings imply that limited proteolysis is an active process along central axons in vivo and that this enzyme may play a specific role in the function of the neuronal cytoskeleton.

Characteristics of the protein kinase activity associated with rat neurofilament preparations

Some properties of the protein kinase activity associated with neurofilaments isolated from the brain stem and spinal cord of rats have been investigated. The activity had an apparent Km for ATP of 20 microM, a pH optimum of 8.0 and phosphorylated both serine and threonine residues in neurofilament proteins. Cyclic AMP had no effect on the in vitro reaction and casein was a preferred exogenous substrate in comparison to histone. Phosphopeptide mapping of the 145 kDa subunit from neurofilaments phosphorylated in the presence and absence of microtubule proteins indicated that the neurofilament-associated activity was distinct from the microtubule-associated protein kinase. Limited proteolysis of neurofilaments with chymotrypsin indicated that the enzyme activity was not associated with a domain of the 200 kDa subunit which may form the side-arm projections on neurofilaments.