Oxidative modification of neurofilament-L and neuronal cell death induced by the catechol neurotoxin, tetrahydropapaveroline

Tetrahydropapaveroline (THP), which is an endogenous neurotoxin, has been suspected to be associated with dopaminergic neurotoxicity of l-DOPA. In this study, we examined oxidative modification of neurofilament-L (NF-L) and neuronal cell death induced by THP. When disassembled NF-L was incubated with THP, protein aggregation was increased in a time- and THP dose-dependent manner. The formation of carbonyl compounds and dityrosine were observed in the THP-mediated NF-L aggregates. Radical scavengers reduced THP-mediated NF-L modification. These results suggest that the modification of NF-L by THP may be due to oxidative damage resulting from the generation of reactive oxygen species (ROS). When THP exposed NF-L was subjected to amino acid analysis, glutamate, proline and lysine residues were found to be particularly sensitive. We also investigated the effects of copper ions on THP-mediated NF-L modification. At a low concentration of THP, copper ions enhanced the modification of NF-L. Treatment of C6 astrocyte cells with THP led to a concentration-dependent reduction in cell viability. When these cells were treated with 100μM THP, the levels of ROS increased 3.5-fold compared with control cells. Furthermore, treatment of cells with THP increased NF-L aggregate formation, suggesting the involvement of NF-L modification in THP-induced cell damage.

Identification of nitrated proteins in the normal rat brain using a proteomics approach

BACKGROUND

The nitration of tyrosine has been suggested to play a role in the pathogenesis of neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and Alzheimer's disease (AD).

METHODS

In the present study, we identified four targets of protein nitration, T-complex polypeptide 1 alpha subunit (TCP-1), neurofilament L (NFL), glial fibrillary acidic protein (GFAP) and clathrin heavy chain (CHC), in the normal rat cortex using a proteomics approach.

CONCLUSIONS

There have been no reports on these proteins being identified by proteomics as nitrated forms in the brain. For further study, we have to investigate alterations in these nitrated proteins during aging and in neurodegenerative disorders.

Lamprey neurofilaments contain a previously unreported 50-kDa protein

We have previously hypothesized that regeneration of axons after spinal cord injury in the lamprey may involve assembly and transport of neurofilaments (NFs) into the growing tip. A single NF, NF-180, has been cloned in this laboratory and until now was thought to be the only NF subunit in lamprey nervous system. However, homopolymerization of NF-180 has not been observed either in experiments on transfected cells or in self-assembly tests in vitro. Forty-three monoclonal antibodies designated as LCM series were generated previously against cytoskeletal proteins of the lamprey nervous system. Seven LCMs were NF specific, and five were keratin specific, as demonstrated by immunohistochemistry. In the present study, one antibody, LCM40, selectively labeled axons in immunohistochemical sections and recognized a single 50-kDa protein in Western blots. Other neuron-specific LCMs and anti-NF antibodies, e.g., LCM39, recognized a known NF subunit, NF-180. Two-dimensional (2-D) gel electrophoresis was employed to separate otherwise indistinguishable individual cytoskeletal proteins. Western blot analysis with an antibody (IFA) that selectively labels all known intermediate filaments indicated that this 50-kDa protein is an intermediate filament (IF). The new protein was incorporated into IF polymers in vitro. Immunoelectron microscopy confirmed that neuronal IFs contain this novel protein. These results suggest that the 50-kDa protein is a previously unrecognized neuronal IF subunit in the lamprey.

Identification of a neurofilament-like protein in the protocerebral tract of the crab Ucides cordatus

Neurofilaments (NFs) have not been observed in crustaceans using conventional electron microscopy, and intermediate filaments have never been described in crustaceans and other arthropods by immunocytochemistry. Since polypeptides, labeled by the NN18-clone antibody, were revealed on microtubule side-arms of crayfish, we have tested, in this study, whether proteins similar to mammalian NFs are present in the protocerebral tract (PCT) of the crab Ucides cordatus. We used immunohistochemistry for light microscopy with monoclonal antibodies against three different NF subunits, high (NF-H), medium (NF-M), and light (NF-L). Labeling was observed with the NN18-clone, which recognizes NF-M. In order to confirm the results obtained with the immunohistochemical reactions, Western blotting, using the three primary antibodies, was performed and the presence of NF-M was confirmed. The NN18-clone monoclonal antibody recognized a protein of approximately 160 kDa, similar to the mammalian NF-M protein, but NF-L and NF-H were not recognized. Conventional transmission electron microscopy was used to observe the ultrastructural components of the axons and immunoelectron microscopy was used to show the distribution of the NF-M-like polypeptides along cytoskeletal elements of the PCT. Our results agree with previous studies on crustacean NF proteins that have reported negative immunoreactions against NF-H and NF-L subunits and positive immunoreactions against the mammalian NF-M subunit. However, the protein previously referred to as P600 and recognized by the NN18-clone, has a very high molecular weight, thus, being different from mammalian NF-M subunit and from the protein revealed now in our study.

Identification of endogenous phosphorylation sites of bovine medium and low molecular weight neurofilament proteins by tandem mass spectrometry

Neurofilament proteins (NFP) are intermediate filaments found in the neuronal cytoskeleton. They are highly phosphorylated, a condition that is believed to be responsible for the assembly and stability of the filaments. Matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) shows molecular masses for bovine NFP subunits of 63, 105, and 125 kDa for NFL, NFM, and NFH. Mass spectrometric de novo sequencing was used to determine the N-terminal sequence of bovine NFM (115 amino acids), which was previously unknown. Molecular mass information shows that there is one-half equivalent phosphate group on NFL and 24 on NFM. For the first time, it is shown that bovine NFL has three phosphorylation sites (Ser(55), Ser(66), and Ser(472)) and NFM has 22 (Ser(512), Ser(546), Ser(554), Ser(560), Thr(627), Ser(629), Ser(634), Ser(639), Thr(646), Ser(649), Ser(654), Ser(664), Ser(669), Thr(676), Ser(679), Ser(684), Ser(694), Ser(726), Ser(750), Ser(756), Ser(770), and Ser(846)) and two tentative sites (Ser(659)/Thr(661) and Thr(840)). Ser(66) was previously not known to be phosphorylated in NFL of other species, while two sites (Ser(55) and Ser(472)) are consistent with the phosphorylations observed in other mammalian NFLs. The three sites, Ser(55), Ser(66), Ser(472), are heterogeneously phosphorylated. Phosphorylation in bovine NFM occurs mainly in the Lys-Ser-Pro (KSP) region, but the Val-Ser-Pro and Ser-Glu-Lys motifs are also phosphorylated. Most of the phosphorylation sites are in accordance with those previously identified in other mammalian NFMs. In bovine NFM, 16 out of the 22 sites are always phosphorylated (Ser(512), Thr(627), Ser(629), Ser(634), Ser(639), Thr(646), Ser(649), Ser(654), Ser(664), Ser(669), Thr(676), Ser(679), Ser(684), Ser(694), Ser(726), and Ser(750)), all of which are contained in the KSP region, and six are sometimes phosphorylated (Ser(546), Ser(554), Ser(560), Ser(756), Ser(770), and Ser(846)). The NFPs have other modifications, including deamidation, oxidation, and N-terminal acetylation. Pyroglutamic acid formation also occurs.

Cdk5 regulates axonal transport and phosphorylation of neurofilaments in cultured neurons

Phosphorylation has long been considered to regulate neurofilament (NF) interaction and axonal transport, and, in turn, to influence axonal stability and their maturation to large-caliber axons. Cdk5, a serine/threonine kinase homologous to the mitotic cyclin-dependent kinases, phosphorylates NF subunits in intact cells. In this study, we used two different haptenized NF subunits and manipulated cdk5 activity by microinjection, transfection and pharmacological inhibition to monitor the effect of Cdk5-p35 on NF dynamics and transport. We demonstrate that overexpression of cdk5 increases NF phosphorylation and inhibits NF axonal transport, whereas inhibition both reduces NF phosphorylation and enhances NF axonal transport in cultured chicken dorsal-root-ganglion neurons. Large phosphorylated-NF `bundles' were prominent in perikarya following cdk5 overexpression. These findings suggest that Cdk5-p35 activity regulates normal NF distribution and that overexpression of Cdk5-p35 induces perikaryal accumulation of phosphorylated-NFs similar to those observed under pathological conditions.

In vitro phosphorylation of cytoskeletal proteins from cerebral cortex of rats

Procedures for the preparation of high- and low-salt Triton insoluble cytoskeletal fractions from rat brain suitable for studying in vitro phosphorylation by endogenous kinases and phosphatases are described. The high-salt Triton insoluble cytoskeletal fraction is enriched in neurofilament subunits (NF-H, NF-M and NF-L), vimentin and glial fibrillary acidic protein (GFAP), while the low-salt Triton insoluble cytoskeletal fraction contains detergent insoluble cytoskeletal elements such as intermediate filament subunits and tubulins. One of our approaches is to incubate cerebral cortex slices with [32P]orthophosphate before the cytoskeletal fraction extraction, which allows the in vitro phosphorylation of cytoskeletal constituents in an intact intracellular environment. On the other hand, we also incubate low- or high-salt cytoskeletal fractions previously prepared with [gamma(32)P]ATP. By doing so, we are able to study the direct effects of substances on the kinase and phosphatase activities associated with the cytoskeletal fraction. Moreover by using specific activators or inhibitors of protein kinases and phosphatases we can obtain more detailed information on the alterations provoked by these substances. These approaches are useful for the investigation of the neurotoxic effects of various drugs and metabolites affecting the cytoskeletal-associated phosphorylation system in the brain.

Identification and characterization of Scp15, a protein from Streptomyces coelicolor A3(2) inducing neurites in PC12 cells

We previously showed that a fungal protein, p15, induces neurite outgrowth and differentiation of rat pheochromocytoma PC12 cells. We report here the identification and characterization of a protein similar to p15, found in Streptomyces coelicolor A3(2). This hypothetical protein, tentatively named Scp15, has significant similarity with p15, including conserved positions of four cysteine residues involved in the formation of essential disulfide bonds in p15. Hexahistidine-tagged recombinant Scp15 proteins were produced in Escherichia coli, purified, and analyzed for their neurite-inducing activity. Although they were less active than p15, they dose-dependently induced neurites and the expression of neurofilament M. Neurite outgrowth by Scp15 was inhibited by nicardipine, suggesting that Scp15 induces neurites via activation of a calcium signaling pathway.

Phosphorylation state of the native high-molecular-weight neurofilament subunit protein from cervical spinal cord in sporadic amyotrophic lateral sclerosis

The intraneuronal aggregation of phosphorylated high-molecular-weight neurofilament protein (NFH) in spinal cord motor neurons is considered to be a key pathological marker of amyotrophic lateral sclerosis (ALS). In order to determine whether this observation is due to the aberrant or hyper-phosphorylation of NFH, we have purified and characterized NFH from the cervical spinal cords of ALS patients and controls. We observed no differences between ALS and normal controls in the physicochemical properties of NFH in Triton X-100 insoluble protein fractions, with respect to migration patterns on 2D-iso electrofocusing (IEF) gels, the rate of Escherichia coli alkaline phosphatase mediated dephosphorylation, or the rate of calpain-mediated proteolysis. The rate of calpain-mediated proteolysis was unaffected by either exhaustive NFH dephosphorylation or by the addition of calmodulin to the reaction. Phosphopeptides and the phosphorylated motifs characterized by liquid chromatography tandem mass spectroscopy (LC/MS/MS) analysis demonstrated that all the phosphorylated residues found in ALS NFH were also found to be phosphorylated in normal human NFH samples. Hence, we have observed no difference in the physicochemical properties of normal and ALS NFH extracted from cervical spinal cords, suggesting that the perikaryal aggregation of highly phosphorylated NF in ALS neurons reflects the aberrant somatotopic localization of normally phosphorylated NFH.

Characterization of the phosphorylation sites of the squid (Loligo pealei) high-molecular-weight neurofilament protein from giant axon axoplasm

Axonal caliber in vertebrates is attributed, in part, to the extensive phosphorylation of NFM and NFH C-terminal tail domain KSP repeats by proline-directed kinases. The squid giant axon, primarily involved in rapid impulse conduction during jet propulsion motility, is enriched in squid-specific neurofilaments, particularly the highly phosphorylated NF-220. Of the 228 serine-threonine candidate phosphate acceptor sites in the NF-220 tail domain (residues 401-1220), 82 are found in numerous repeats of three different motifs SAR/K, SEK/R, K/RSP, with 62 of these tightly clustered in the C-terminal repeat segment (residues 840-1160). Characterization of the in vivo NF-220 phosphorylated sites should provide clues as to the relevant kinases. To characterize these sites, proteolytic digests of NF-220 were analyzed by a combination of HPLC, electrospray tandem mass spectrometry and database searching. A total of 53 phosphorylation sites were characterized, with 47 clustered in the C-terminal repeat segment (residues 840-1160), representing 76% (47/62) of the total acceptor sites in the region. As in mammalian NFH, approximately 64% of the K/RSP sites (14/22) in this region were found to be phosphorylated implicating proline-directed kinases. Significantly, 78% of serines (31/40) in the KAES*EK and EKS*ARSP motifs were also phosphorylated suggesting that non proline-directed kinases such as CKI may also be involved. This is consistent with previous studies showing that CKI is the principal kinase associated with axoplasmic NF preparations. It also suggests that phosphorylation of large macromolecules with multiple phospho-sites requires sequential phosphorylation by several kinases.

gamma-diketone peripheral neuropathy. II. Neurofilament subunit content

Quantitative morphometric analyses have demonstrated that axon atrophy is the primary neuropathic alteration in peripheral nerve of 2,5-hexanedione (HD)-intoxicated rats (Lehning et al., Toxicol. Appl. Pharmacol. 165, 127-140, 2000). Research suggests that axon caliber is regulated by neurofilament (NF) content and density. Therefore, as a possible mechanism of atrophy, NF subunit (NF-L, -M, and -H) proteins were quantitated in moderately affected rats intoxicated with HD at three daily dosing rates (175, 250, and 400 mg/kg/day). Analyses of subunit protein contents in proximal sciatic nerves indicated uniformly small decreases, which corresponded to minimal changes in axon area occurring in this region. In distal tibial nerve, subunit proteins were decreased substantially (40-70%) when rats were exposed to the 175 and 250 mg/kg/day doses. These reductions in NFs corresponded to significant decreases (approximately 50%) in tibial axon area induced by lower dosing rates. In contrast, 400 mg/kg/day produced similar changes in caliber but smaller reductions (18-25%) in NF-L, -M, and -H levels. This suggests that a decrement in axonal NF content is unlikely to be solely responsible for gamma-diketone-induced axon atrophy and that the corresponding mechanism probably involves additional changes in factors regulating NF density. Analysis of NF content in peripheral nerve also identified the presence of anomolous higher molecular weight NF-H proteins. However, the neurotoxicological significance of these abnormal subunits is uncertain based on their limited occurrence and inconsistent spatiotemporal expression.

Neurofilament-L is a protein phosphatase-1-binding protein associated with neuronal plasma membrane and post-synaptic density

Far Westerns with digoxigenin-conjugated protein phosphatase-1 (PP1) catalytic subunit identified PP1-binding proteins in extracts from bovine, rat, and human brain. A major 70-kDa PP1-binding protein was purified from bovine brain cortex plasma membranes, using affinity chromatography on the immobilized phosphatase inhibitor, microcystin-LR. Mixed peptide sequencing following cyanogen bromide digestion identified the 70-kDa membrane-bound PP1-binding protein as bovine neurofilament-L (NF-L). NF-L was the major PP1-binding protein in purified preparations of bovine spinal cord neurofilaments and the cytoskeletal compartment known as post-synaptic density, purified from rat brain cortex. Bovine neurofilaments, at nanomolar concentrations, inhibited the phosphorylase phosphatase activity of rabbit skeletal muscle PP1 catalytic subunit but not the activity of PP2A, another major serine/threonine phosphatase. PP1 binding to bovine NF-L was mapped to the head region. This was confirmed by both binding and inhibition of PP1 by recombinant human NF-L fragments. Together, these studies indicate that NF-L fulfills many of the biochemical criteria established for a PP1-targeting subunit and suggest that NF-L may target the functions of PP1 in membranes and cytoskeleton of mammalian neurons.

Topographic regulation of cytoskeletal protein phosphorylation by multimeric complexes in the squid giant fiber system

In mammalian and squid nervous systems, the phosphorylation of neurofilament proteins (NFs) seems to be topographically regulated. Although NFs and relevant kinases are synthesized in cell bodies, phosphorylation of NFs, particularly in the lys-ser-pro (KSP) repeats in NF-M and NF-H tail domains, seem to be restricted to axons. To explore the factors regulating the cellular compartmentalization of NF phosphorylation, we separated cell bodies (GFL) from axons in the squid stellate ganglion and compared the kinase activity in the respective lysates. Although total kinase activity was similar in each lysate, the profile of endogenous phosphorylated substrates was strikingly different. Neurofilament protein 220 (NF220), high-molecular-weight NF protein (HMW), and tubulin were the principal phosphorylated substrates in axoplasm, while tubulin was the principal GFL phosphorylated substrate, in addition to highly phosphorylated low-molecular-weight proteins. Western blot analysis showed that whereas both lysates contained similar kinases and cytoskeletal proteins, phosphorylated NF220 and HMW were completely absent from the GFL lysate. These differences were highlighted by P13(suc1) affinity chromatography, which revealed in axoplasm an active multimeric phosphorylation complex(es), enriched in cytoskeletal proteins and kinases; the equivalent P13 GFL complex exhibited six to 20 times less endogenous and exogenous phosphorylation activity, respectively, contained fewer cytoskeletal proteins and kinases, and expressed a qualitatively different cdc2-like kinase epitope, 34 kDa rather than 49 kDa. Cell bodies and axons share a similar repertoire of molecular consitutents; however, the data suggest that the cytoskeletal/kinase phosphorylation complexes extracted from each cellular compartment by P13 are fundamentally different.

[The assembly of low molecular weight neurofilament protein (NF-L) in vitro]

Neurofilaments were isolated from bovine spinal cords by ultra-speed centrifugation and examined by negative staining. The neurofilament triplet proteins: NF-L, NF-M and NF-H were purified by DE-52 anion exchange chromatography in the presence of 6 mol/L urea. The reassembly of NF-L under controlled conditions was studied. NF-L can reassemble into 10 nm width filaments within 60 minutes at physiological condition of around 0.15 mol/L NaCl, 2 mmol/L MgCl2, neutral pH(pH 6.8) and 37 degrees C. In 6 mol/L urea, NF-L was examined as 12 nm-diameter particle by low angle rotary shadowing. When dialyzed against reassembly buffer for 20 minutes, some irregular filaments were formed. Further dialyzed for another 40 minutes, the long smooth filaments appeared. Some filaments were unraveled at the end regions, where existed 2-4 subfilaments. Four subfilaments were more often observed. That is to say, the 10 nm-width filament was composed of 4 subfilaments. While dialyzed against the alkaline buffer containing 0.15 mol/L NaCl, NF-L reconstituted into 45-180 nm-long, 10 nm-width filaments, which were not able to elongate into long filaments.

Identification of novel in vitro PKA phosphorylation sites on the low and middle molecular mass neurofilament subunits by mass spectrometry

Phosphorylation of the head domains of intermediate filament proteins by second messenger-dependent kinases is important in regulating filament assembly. In the case of neurofilaments, head domain phosphorylation is known to be important in assembly, but few sites have been identified. Using matrix-assisted laser desorption-ionization (MALDI) and nano-electrospray mass spectrometry, we report the identification of several novel in vitro cAMP-dependent protein kinase (PKA) phosphorylation sites in the low (NF-L) and middle (NF-M) molecular mass neurofilament subunits. Neurofilament polypeptides were purified from adult rat brain, and fractions containing a mixture of NF-L and NF-M were nonradioisotopically phosphorylated with PKA prior to proteolytic digestion of the polypeptides in situ in polyacrylamide excised from SDS gels. Sites of phosphorylation were determined by mass spectrometric analysis of mixtures enriched in tryptic phosphopeptides. In NF-L, four novel sites were identified: serines 12, 41, and 49 in the head domain and serine 435 in the carboxyl-terminal tail domain, and data consistent with phosphorylation of serine 2 were obtained. Recombinant rat NF-L protein was also phosphorylated with PKA, and the same serines were identified as phosphorylation sites, with two additional sites, serine 43 and probable phosphorylation of serine 55. In NF-M, one novel site, serine 1 in the amino-terminal head domain, was found to be phosphorylated, and serine 46, also in the amino-terminal head domain, was confirmed as a PKA phosphorylation site.

Protein serine/threonine phosphatase 1 and 2A associate with and dephosphorylate neurofilaments

The phosphorylation state of neurofilaments plays an important role in the control of cytoskeletal integrity, axonal transport, and axon diameter. Immunocytochemical analyses of spinal cord revealed axonal localization of all protein phosphatase subunits. To determine whether protein phosphatases associate with axonal neurofilaments, neurofilament proteins were isolated from bovine spinal cord white matter by gel filtration. approximately 15% of the total phosphorylase a phosphatase activity was present in the neurofilament fraction. The catalytic subunits of PP1 and PP2A, as well as the A and B alpha regulatory subunits of PP2A, were detected in the neurofilament fraction by immunoblotting, whereas PP2B and PP2C were found exclusively in the low molecular weight soluble fractions. PP1 and PP2A subunits could be partially dissociated from neurofilaments by high salt but not by phosphatase inhibitors, indicating that the interaction does not involve the catalytic site. In both neurofilament and soluble fractions, 75% of the phosphatase activity towards exogenous phosphorylase a could be attributed to PP2A, and the remainder to PP1 as shown with specific inhibitors. Neurofilament proteins were phosphorylated in vitro by associated protein kinases which appeared to include protein kinase A, calcium/calmodulin-dependent protein kinase, and heparin-sensitive and -insensitive cofactor-independent kinases. Dephosphorylation of phosphorylated neurofilament subunits was mainly (60%) catalyzed by associated PP2A, with PP1 contributing minor activity (10-20%). These studies suggest that neurofilament-associated PP1 and PP2A play an important role in the regulation of neurofilament phosphorylation.

Enhanced ex vivo cosedimentation of high molecular weight neurofilament protein (NFH) with microtubules following in vivo aluminum chloride exposure: inhibition of dephosphorylation-dependent dissociation

We have investigated the effect of acute in vivo aluminum exposure on the subsequent ex vivo cross-linking of the high molecular weight neurofilament protein (NFH) with polymerized microtubules. Young adult female New Zealand white rabbits were inoculated intracisternally with 1000 micrograms of AlCl3 in 0.9% NaCl or with 0.9% NaCl alone, and killed 48 hours later. Following isolation of a cytoskeletal-enriched protein fraction from the cervical spinal cord, NFH was purified by either electroelution or column chromatography. Tubulin was isolated from New Zealand white rabbit brains by repeated temperature-dependent polymerization and depolymerization, purified over phosphocellulose, and cosedimented with either phosphorylated or dephosphorylated NFH. Following incubation for 30 minutes at 32 degrees C with tubulin in the presence of 20 microM Taxol, 1.0 mM MgCl2 and 1.0 mM GTP, the insoluble pellet containing NFH/microtubules was isolated. Both the pellet and supernatent were fractionated by SDS.PAGE and the amount of NFH present quantified by transmission densitometry following silver-staining. Results were identical regardless of the technique utilized for the purification of NFH. Control NFH preferentially cosedimented with microtubules when in the fully phosphorylated isoform, but remained in the soluble fraction following dephosphorylation. Phosphorylated NFH derived from AlCl3-inoculated rabbits demonstrated similar binding characteristics to control NFH, but following exhaustive dephosphorylation, exhibited a 4.5 fold induction of NFH/microtubule binding (p = 0.0314). Incubating dephosphorylated control NFH with microtubules in the presence of increasing concentrations of AlCl3 failed to induce similar cosedimentation. These experiments suggest that phosphorylation promotes NFH cross-linking to microtubules. In addition, the phosphorylation/dephosphorylation dependent regulation of NFH cross-linking to microtubules is disrupted following in vivo AlCl3 exposure by a mechanism that s independent of NFH/Al3+ binding.

Triton-soluble phosphovariants of the heavy neurofilament subunit in developing and mature mouse central nervous system

The low abundance of soluble neurofilament (NF) subunits in mature axons has suggested that newly synthesized NF proteins rapidly assemble into highly stable polymers and associate with the Triton X-100-insoluble cytoskeleton. The dynamic nature of these subunit associations in vivo remains unresolved, and the applicability of this assembly model to NFs in other neuronal compartments or to developing neurons is unknown. Here, we report that a unique pool of Triton X-100-soluble, extensively phosphorylated, high molecular weight NF subunits (NF-H, or H-200) are abundantly expressed in the mouse CNS during early postnatal development and persist in the perikaryal compartment of some mature neurons. Triton-soluble H-200 subunits appeared at postnatal day 14 (P14) and remained high through P60, beyond which the percentage declined to marginal levels by P120. Medium and low molecular weight NF (NF-M and NF-L, respectively) were at all times only detectable within the cytoskeleton. Comparison of soluble and cytoskeleton-associated H-200 immunoreactivity indicated that certain phosphorylation-dependent epitopes were confined to the cytoskeleton. Pulse-chase radiolabeling analyses in optic pathway demonstrated that some Triton-soluble NF-H subunits are extensively phosphorylated within retinal perikarya before they are incorporated into Triton-insoluble structures. These findings indicate that the assembly behaviors of NF-H differ substantially from those of NF-M and NF-L, and that the interaction of NF-H with NFs may be more dynamic than is generally recognized, especially during brain development and within specific compartments of mature neurons.

Changes in brain protease activity in aging

We measured changes in protease activity with aging, conducting assays of cathepsin D and calpain II activities and the rate of degradation of cytoskeletal proteins, preparing the enzymes and substrates from young and aged brains. Calpain preparations added to the young and to the aged substrates were standardized with casein as substrate so that age-related changes in calpain specificity and substrate susceptibility were measured. Several age-related differences were observed in substrate susceptibility and in enzyme activity. With respect to substrate, the neurofilament protein from young animals was somewhat more susceptible to calpain action than that from older animals. With respect to enzyme activity, calpain from aged brain cleaved neurofilament protein at a faster rate than did calpain from young. With neurofilaments, the most rapid breakdown usually occurred when enzyme from aged tissue was incubated with substrate from young. Kidney enzyme of aged rats incubated with neurofilament substrate of aged rats resulted in a more rapid breakdown than enzyme of young kidney incubated with substrate of young. The age dependence of tubulin breakdown was somewhat different from that of neurofilament breakdown. The most rapid breakdown usually occurred when using enzyme from young with tubulin from young. Incubation of neurofilament protein or tubulin with cathepsin D did not reveal any differences with aging. These studies suggest that an increase in enzyme activity observed previously during aging may also include changes in the properties of the enzyme (substrate specificity) and/or in the properties of their endogenous substrates (susceptibility to breakdown).

Comparison of biochemical extraction techniques for the detection of scrapie-associated fibrils in the central nervous system of sheep naturally affected with scrapie

Standardized samples of brain material from four sheep naturally affected with scrapie and from four healthy control sheep were subjected to six different extraction techniques used for the detection of scrapie-associated fibrils by negative-contrast transmission electron microscopy. The six methods were compared in respect of fibril yield and clarity of ultrastructure. The simplest method consisting of a single N-lauroylsarcosine detergent extraction and differential centrifugation, followed by proteinase K enzyme digestion, gave the best overall results. The use of proteinase and nuclease inhibitors made no apparent difference to the yield or ultrastructural clarity of fibrils. Density gradient centrifugation appeared to reduce tungstate stain penetration and often obscured the ultrastructural clarity. The results suggested that the preferred technique could be improved by the use of a double homogenization stage at the beginning of the procedure and by adding an ultrasonic disintegration step to resuspend the final pellet prior to tungstate staining.

Ultrastructural instability of paired helical filaments from corticobasal degeneration as examined by scanning transmission electron microscopy

Paired helical filaments (PHFs) accumulate in the brains of subjects affected with Alzheimer's disease (AD) and certain other neurodegenerative disorders, including corticobasal degeneration (CBD). Electron microscope studies have shown that PHFs from CBD differ from those of AD by being wider and having a longer periodicity of the helical twist. Moreover, PHFs from CBD have been shown to be primarily composed of two rather than three highly phosphorylated polypeptides of tau (PHF-tau), with these polypeptides expressing no exons 3 and 10. To further explore the relationship between the heterogeneity of PHF-tau and the appearance of abnormal filaments, the ultrastructure and physical parameters such as mass per unit length and dimensions were compared in filaments from CBD and AD using high resolution scanning transmission electron microscopy (STEM). Filament-enriched fractions were isolated as Sarcosyl-insoluble pellets and for STEM studies, samples were freeze-dried without prior fixation or staining. Ultrastructurally, PHFs from CBD were shown to be a heterogeneous population as double- and single-stranded filaments could be identified based on their width and physical mass per unit length expressed in kilodaltons (kd) per nanometer (nm). Less abundant, double-stranded filaments had a maximal width of 29 nm and a mass per unit length of 133 kd/nm, whereas three times more abundant single-stranded filaments were 15 nm wide and bad a mass per unit length of 62 kd/nm. Double-stranded filaments also displayed a distinct axial region of less dense mass, which appeared to divide the PHFs into two protofilament-like strands. Furthermore, these filaments were frequently observed to physically separate along the long axis into two single strands or to break longitudinally. In contrast, PHFs from AD were ultrastructurally stable and uniform both in their width (22 nm) and physical mass per unit length (104 kd/nm). The ultrastructural features indicate that filaments of CBD and AD differ both in stability and packing of tau and that CBD filaments, composed of two distinct protofilaments, are more labile under STEM conditions. As fixed and stained filaments from CBD have been shown to be stable and uniform in size by conventional transmission electron microscopy, STEM studies may be particularly suitable for detecting instability of unstained and unfixed filaments. The results also suggest that molecular heterogeneity and/or post-translational modifications of tau may strongly influence the morphology and stability of abnormal filaments.

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

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

Neurofilament-associated protein phosphatase 2A: its possible role in preserving neurofilaments in filamentous states

Neurofilament phosphatase (NF-phosphatase) activity, which dephosphorylates NF proteins phosphorylated by cyclic AMP-dependent protein kinase (A-kinase), was detected in NF fractions prepared from bovine spinal cords. This phosphatase was suggested to be associated with NFs by gel filtration and sedimentation analysis and was further demonstrated by dephosphorylation-dependent binding assay of NFs to microtubules. The NF-associated NF-phosphatase was identified as a type of protein phosphatase 2A (PP2A) by (i) its complete inhibition with 100 nM okadaic acid, at which concentration the purified type 1 protein phosphatase (PP1) was inhibited only 25%; (ii) the absence of effect of inhibitor-2, a specific inhibitor of PP1, on the NF-phosphatase activity; and (iii) the detection of 38-kDa catalytic and 65-kDa regulatory subunits of PP2A by immunoblotting. The NF-associated PP2A was partially solubilized from NFs by a high concentration of MgSO4, and the solubilized PP2A was suggested by gel filtration to be a dimeric holoenzyme consisting of a 38-kDa catalytic and a 65-kDa regulatory subunit. Phosphorylated NF-L, which is assembly incompetent, was induced to assemble into filaments by dephosphorylation with PP2A. These results suggest a role of NF-associated PP2A in preserving filamentous forms of NF in neurons.

Metal-catalyzed oxidation of bovine neurofilaments in vitro

Neurofilaments (NF) are important determinants of the shape and size of nerve cells. The oxidation of NF, relevant to aging, neurodegenerative disorders, and axonal (Wallerian) degeneration, has not been studied. In this investigation, we have combined biochemical and ultrastructural methods to study the metal-catalyzed oxidation (MCO) of bovine NF using an ascorbate/Fe+3/O2 system. The oxidation of NF proteins was documented by increases in carbonyl content, which were time- and concentration-dependent. Polyacrylamide gel electrophoresis (PAGE) and immunoblot analyses revealed the fragmentation of oxidized NF proteins, predominantly NF-H and NF-M. Electron microscopy (EM) showed that oxidized NF formed dense aggregates and bundles of laterally aggregated filaments. Finally, we also demonstrated that oxidized NF proteins were more susceptible to calpain proteolysis. In view of the growing evidence supporting increased oxidative stress on the nervous system in aging and the report of Cu/Zn superoxide dismutase mutation in familial motor neuron disease, oxidative injury of NF may be relevant to cell atrophy and degeneration of nerve cells and to the formation of abnormal cytoskeletal structures.

Porcine brain neurofilament-H tail domain kinase: its identification as cdk5/p26 complex and comparison with cdc2/cyclin B kinase

Using dephosphorylated neurofilament (NF) proteins as substrates, the kinase with a higher activity for the dephosphorylated NF-H than the phosphorylated form of NF-H was searched for in the porcine brain extract. Most NF-H kinase activity in the brain extract pelleted with microtubules. The NF-H kinase purified from a high salt extract of the microtubule pellets was composed of cdk5 and a 26 kDa protein, a fragment of the 35 kDa regulatory subunit of cdk5. In contrast to the association of the active kinase with microtubules, each of uncomplexed cdk5 and the 35 kDa regulatory subunit was differently distributed in the supernatant fraction and the pellet, respectively, by ultracentrifugation of the brain extract. Dephosphorylated forms of NF-H and NF-M became reactive to antibodies recognizing in vivo phosphorylation sites (SMI31, 34, and 36, JJ31 and 51) by phosphorylation with cdk5/p26. cdk5/p26 showed similar enzymatic properties to p34cdc2/cyclin B kinase; the substrate specificity and inhibition by a p34cdc2 kinase specific inhibitor, butyrolactone I. However, p34cdc2/cyclin B kinase was distinguished from cdk5/p26 by its binding to p13suc1 protein and by its reactivity to anti-p34cdc2 antibodies. In spite of similar enzymatic properties of cdk5/p26 and p34cdc2/cyclin B kinase, cdk5/p26 did not display M-phase promoting activity when assayed with a cell-free system of Xenopus egg extract.

[32P]orthophosphate and [35S]methionine label separate pools of neurofilaments with markedly different axonal transport kinetics in mouse retinal ganglion cells in vivo

Newly synthesized neurofilament proteins become highly phosphorylated within axons. Within 2 days after intravitreously injecting normal adult mice with [32P]orthophosphate, we observed that neurofilaments along the entire length of optic axons were radiolabeled by a soluble 32P-carrier that was axonally transported faster than neurofilaments. 32P-incorporation into neurofilament proteins synthesized at the time of injection was comparatively low and minimally influenced the labeling pattern along axons. 32P-incorporation into axonal neurofilaments was considerably higher in the middle region of the optic axons. This characteristic non-uniform distribution of radiolabel remained nearly unchanged for at least 22 days. During this interval, less than 10% of the total 32P-labeled neurofilaments redistributed from the optic nerve to the optic tract. By contrast, newly synthesized neurofilaments were selectively pulse-labeled in ganglion cell bodies by intravitreous injection of [35S]methionine and about 60% of this pool translocated by slow axoplasmic transport to the optic tract during the same time interval. These findings indicate that the steady-state or resident pool of neurofilaments in axons is not identical to the newly synthesized neurofilament pool, the major portion of which moves at the slowest rate of axoplasmic transport. Taken together with earlier studies, these results support the idea that, depending in part on their phosphorylation state, transported neurofilaments can interact for short or very long periods with a stationary but dynamic neurofilament lattice in axons.

Phosphorylation on carboxyl terminus domains of neurofilament proteins in retinal ganglion cell neurons in vivo: influences on regional neurofilament accumulation, interneurofilament spacing, and axon caliber

The high molecular weight subunits of neurofilaments, NF-H and NF-M, have distinctively long carboxyl-terminal domains that become highly phosphorylated after newly formed neurofilaments enter the axon. We have investigated the functions of this process in normal, unperturbed retinal ganglion cell neurons of mature mice. Using in vivo pulse labeling with [35S]methionine or [32P]orthophosphate and immunocytochemistry with monoclonal antibodies to phosphorylation-dependent neurofilament epitopes, we showed that NF-H and NF-M subunits of transported neurofilaments begin to attain a mature state of phosphorylation within a discrete, very proximal region along optic axons starting 150 microns from the eye. Ultrastructural morphometry of 1,700-2,500 optic axons at each of seven levels proximal or distal to this transition zone demonstrated a threefold expansion of axon caliber at the 150-microns level, which then remained constant distally. The numbers of neurofilaments nearly doubled between the 100- and 150-microns level and further increased a total of threefold by the 1,200-microns level. Microtubule numbers rose only 30-35%. The minimum spacing between neurofilaments also nearly doubled and the average spacing increased from 30 nm to 55 nm. These results show that carboxyl-terminal phosphorylation expands axon caliber by initiating the local accumulation of neurofilaments within axons as well as by increasing the obligatory lateral spacing between neurofilaments. Myelination, which also began at the 150-microns level, may be an important influence on these events because no local neurofilament accumulation or caliber expansion occurred along unmyelinated optic axons. These findings provide evidence that carboxyl-terminal phosphorylation triggers the radial extension of neurofilament sidearms and is a key regulatory influence on neurofilament transport and on the local formation of a stationary but dynamic axonal cytoskeletal network.

Increased phosphorylation of the amino-terminal domain of the low molecular weight neurofilament subunit in okadaic acid-treated neurons

Treatment of rat dorsal root ganglion cultures with 1 microM okadaic acid leads to a fragmentation of neurofilaments and a reduction in the electrophoretic mobilities of the three subunits on SDS-polyacrylamide gels (Sacher, M. G., Athlan, E. S., and Mushynski, W. E. (1992) Biochem. Biophys. Res. Commun. 186, 524-530). Based on the observed response to varying concentrations of okadaic acid, fragmentation was inferred to be due to inhibition of protein phosphatase-2A activity and reduction in electrophoretic mobility to inhibition of protein phosphatase-1. Okadaic acid treatment led to an increase in amino-terminal, relative to carboxyl-terminal, domain phosphorylation in the low molecular weight (NF-L) subunit in the Triton X-100-soluble and -insoluble fractions. The purified catalytic subunit of protein phosphatase-2A dephosphorylated 32P-labeled NF-L and the middle molecular weight subunit from okadaic acid-treated cultures, whereas the catalytic subunit of protein phosphatase-1 had no effect. In the case of NF-L, phosphate moieties were preferentially removed from the amino-terminal domain. These results show that the amino-terminal domain of NF-L can be phosphorylated in situ and implicate protein phosphatase-2A in the turnover of phosphate moieties in this domain.

Dephosphorylation of the largest neurofilament subunit protein influences the structure of crossbridges in reassembled neurofilaments

Phosphorylation-dependent change in electrophoretic mobility is the most unique characteristic of NF-H, the largest molecular mass subunit of the neurofilament. We dephosphorylated NF-H using Escherichia coli alkaline phosphatase, then reassembled it into neurofilaments with NF-M and NF-L, and into NF-H filaments with NF-H alone. We compared these dephosphorylated filaments with control: projections by low-angle rotary-shadow, crossbridges by quick-freeze deep-etch, and core filament packing density by thin-section electron microscopy. Projections in the dephosphorylated filaments were basically similar in structure to those in control, although there was a tendency for them to be wider and less dense, especially in NF-H filaments. Dephosphorylated filaments were still able to form crossbridges between core filaments, but their crossbridges were significantly wider, less dense, more branched and more irregular than crossbridges in control, and core filaments were more densely packed. These structural differences may be brought about by the removal of phosphate groups from NF-H tail and consequent reduction of electrostatic repulsion between adjacent crossbridges extending from the same core filament. The results indicate that phosphorylation of NF-H is necessary for forming well developed crossbridges, straight and at constant intervals, like those of in vivo axonal neurofilaments.

Neuromodulin (GAP-43) can regulate a calmodulin-dependent target in vitro

The calmodulin-binding polypeptide neuromodulin (GAP-43) was tested in vitro for its ability to modulate a typical calmodulin target, the enzyme nitric oxide synthase. The titration of enzyme with increasing neuromodulin concentrations demonstrated a concentration-dependent decrease in enzyme activity. Subsequent analysis of the ability of increased calcium concentrations to activate the enzyme was tested in the presence or absence of neuromodulin. The effect of neuromodulin on the calcium-dependent activation of the enzyme was to depress enzyme activity in the range of 0.2 to approximately 6 microM calcium. Treatment of the neuromodulin polypeptide with protein kinase C eliminated its ability to inhibit nitric oxide synthase activation. Subsequent treatment of the phosphorylated neuromodulin with calcineurin (phosphatase 2b) caused it to regain its inhibitory action on the enzyme. The results from these in vitro studies have indicated that neuromodulin has the ability to affect the activation of a calmodulin-dependent enzyme at levels of the polypeptide that exist in neurons. They also demonstrated that the regulation occurred within a physiological range of calcium concentrations. Since the inhibition of enzyme activity appeared to be occurring through the interaction of neuromodulin with calmodulin, it seems likely that neuromodulin has a general ability to impede activation of calmodulin-dependent targets.

Naftidrofuryl, a putative activator of neuron survival, stimulates the expression of neurofilament heavy subunit in cultivated spinal cord neurons from chicken

The effect of naftidrofuryl, a drug used in ischemia for its vasodilator properties and its protective effect on neuronal survival, was investigated on the maturation of cultured chicken spinal cord neurons, focusing on the presence of proteins specific for the developing neuronal cytoskeleton. Although no influence of naftidrofuryl on the rate of growth of neurites was observed, the drug enhanced the relative amount of the high molecular weight neurofilament subunit without affecting the concentration of a microtubule-associated protein, MAP2. These findings suggest that the effect of naftidrofuryl on cultured spinal cord neurons might involve molecular events directly associated with the induction of a mature cytoskeleton architecture, instead of stimulating undifferentiated neurite growth.

The protein phosphatase inhibitor okadaic acid increases axonal neurofilaments and neurite caliber, and decreases axonal microtubules in NB2a/d1 cells

When cells were treated with dbcAMP for 3 days to induce the outgrowth of axonal neurites, the addition of the phosphatase inhibitor okadaic acid (OA; 5 nM) for the last 24 hr markedly increased neurofilament subunit immunoreactivity including phosphate-dependent NF-H epitopes in axonal neurites, increased axonal neurite caliber by approximately 30%, but did not increase neurite contour length. Ultrastructural analysis demonstrated a > 2-fold increase in neurofilaments and indicated that neurofilaments were phosphorylated to a similar extent in the presence and absence of OA. Vimentin immunoreactivity, which undergoes down-regulation during dbcAMP-mediated differentiation, was not increased by OA. OA did not induce the precocious appearance of delayed phosphate-dependent neurofilament epitopes suggesting that it did not induce the activation of additional neurofilament kinases. NF-H subunits from cytoskeletons of OA-treated cells were less susceptible to degradation by an endogenous calcium-dependent protease, providing a possible mechanism for neurofilament accumulation during OA treatment. By contrast, OA decreased axonal neurite microtubules, and eliminated stabilized (acetylated) axonal microtubules. OA treatment at earlier times prevented and reversed neurite outgrowth. Despite increased deposition of phosphorylated neurofilaments, OA did not hasten the development of colchicine resistance to neurites, suggesting that stabilization of the axonal cytoskeletal lattice requires neurofilament-microtubule interaction.

The maximum rate of neurofilament transport in axons: a view of molecular transport mechanisms continuously engaged

Neurofilaments (NFs) were radiolabeled in the optic systems of mice. The leading edge of the radiolabeled NF waveform was distinguished near the injection site (the eye) both by liquid scintillation spectroscopy and visually from fluorographs. The fastest NFs were found to be translocated at rates of between 72 and 144 mm/day. It appears that the continuous (maximal) operation of the slow axonal transport machinery can move polymers intra-axonally at rates one hundred times greater than those previously reported.

Identification of chaperonin particles in mammalian brain cytosol and of T-complex polypeptide 1 as one of their components

An approximately 950-kDa heteromeric particle was purified from guinea-pig and rat brain by sucrose gradient fractionation of post-mitochondrial supernatants. Further purification, by affinity chromatography on ATP-Sepharose and anion exchange FPLC on MonoQ, yielded a particle with typical chaperonin ultrastructure. One of the component polypeptides was recognized by a monoclonal antibody to murine T-complex polypeptide 1. Brain cytosolic chaperonin particles formed a binary complex with unfolded tubulin subunits. The polypeptide compositions of the cytosolic chaperonin particles appeared very similar between brain and testicular tissues of the same animal, but differed subtly between the guinea-pig and rat.

A critical assessment of the RNAse protection assay as a means of determining exon sizes

The RNAse protection assay is a highly sensitive assay which is commonly used to detect specific hybridization between complementary RNAs and to determine exon sizes in gene characterization studies. Unfortunately, each of the numerous steps involved in the assay could give artifacts depending on the probe used. In this study, common causes of artifacts have been identified using riboprobes which identify exons of known sizes. The RNAse concentration and duration of digestion used were found to be critical factors affecting exon size estimations. Five different riboprobes were tested to obtain a consensus optimum RNAse condition--10 micrograms/ml RNAse A, 0.5 microgram/ml RNAse T1--enabling the correct determination of exon sizes. This condition was further analyzed for its specificity when RNAse protection assays were performed between highly homologous RNA fragments from two different species. Results show that this concentration of RNAse would efficiently cleave a minimum of two nucleotide mismatches. Single nucleotide mismatches were frequently not cleaved by the same RNAse concentration making it possible to detect the correct exon size regardless of such sequence polymorphisms in gene sequences.

Brain proline-directed protein kinase is a neurofilament kinase which displays high sequence homology to p34cdc2

The carboxyl-terminal regions of neurofilament high (NF-H) and middle (NF-M) molecular weight proteins have been suggested to be phosphorylated in vivo by a p34cdc2-like protein kinase, on the basis of the in vivo phosphorylation site motif and in vitro phosphorylation of the proteins by p34cdc2 kinase (Hisanaga, S.I., Kusubata, M., Okumura, E. and Kishimoto, T. (1991) J. Biol. Chem. 266, 21798-21803). A novel proline-directed protein kinase previously identified and purified from bovine brain has been found in this study to phosphorylate NF-H and NF-M at sites identical to those phosphorylated by HeLa cell p34cdc2 kinase. The proline-directed kinase is composed of a 33-kDa and a 25-kDa subunit. The 33-kDa kinase subunit was partially sequenced, and degenerate oligonucleotide primers corresponding to the amino acid sequence information were used to clone the subunit by polymerase chain reaction (PCR). Two overlapping PCR products comprised a complete open reading frame of 292 amino acids. The sequence contains all features of a protein kinase, suggesting that the 33-kDa peptide represents the catalytic subunit of the kinase. The 33-kDa subunit shows high and approximately equal homology to human p34cdc2 and human cdk2, with about 58 and 59% amino acid identity, respectively. These results suggest that the brain kinase represents a new category of the cdc2 family, and that some members of the cdc2 kinase family may have major functions unrelated to cell cycle control.

Axo-glial interactions at the dorsal root transitional zone regulate neurofilament protein synthesis in axotomized sensory neurons

After dorsal root crush, dramatic ultrastructural differences are observed between regenerated dorsal root axonal endings that are physically blocked at a ligation neuroma and those that are allowed to form axo-glial endings among the astrocytes at the dorsal root transitional zone (DRTZ). Physically blocked axonal endings swell immensely with membranous organelles and neurofilaments (NFs) while axo-glial endings do not, suggesting that DRTZ astrocytes stop axonal growth by activating a physiological stop pathway within those endings. Since protease-dependent NF degradation at axonal endings is a part of this pathway, this study addresses the question of whether NF subunit synthesis in the dorsal root ganglion (DRG) is regulated by the pathway. Lumbar dorsal roots were crushed and, at various postinjury times, the attached DRGs were removed and pulse-labeled in vitro with 35S-methionine for subsequent analysis of protein synthesis by electrophoresis and fluorography. Within 24 hr of axotomy, there was a down-regulation of the 68 kDa (NF-L) and 145 kDa (NF-M) NF subunits. At 14 d postcrush, a time when most of the regenerating axons have reached and been stopped by DRTZ astrocytes, NF protein synthesis returned to control levels. By contrast, when the axons were prevented from reaching the DRTZ by ligating or removing segments of the roots, NF synthesis failed to return to normal levels. These data suggest that activation of the physiological stop pathway by DRTZ astrocytes regulates NF protein synthesis in the DRG.

Solubilization and fractionation of paired helical filaments

Paired helical filaments isolated from brains of two different patients with Alzheimer's disease were extensively treated with the ionic detergent, sodium dodecyl sulphate. Filaments were solubilized at different extents, depending on the brain examined, thus suggesting the existence of two types of paired helical filaments: sodium dodecyl sulphate-soluble and insoluble filaments. In the first case, the number of structures resembling paired helical filaments greatly decreased after the detergent treatment, as observed by electron microscopy. Simultaneously, a decrease in the amount of sedimentable protein was also observed upon centrifugation of the sodium dodecyl sulfate-treated paired helical filaments. A sodium dodecyl sulphate-soluble fraction was isolated as a supernatant after low-speed centrifugation of the sodium dodecyl sulphate-treated paired helical filaments. The addition of the non-ionic detergent Nonidet-P40 to this fraction resulted in the formation of paired helical filament-like structures. When the sodium dodecyl sulphate-soluble fraction was further fractionated by high-speed centrifugation, three subfractions were observed: a supernatant, a pellet and a thin layer between these two subfractions. No paired helical filaments were observed in any of these subfractions, even after addition of Nonidet P-40. However, when they were mixed back together, the treatment with Nonidet P-40 resulted in the visualization of paired helical filament-like structures. These results suggest that at least two different components are needed for the reconstitution of paired helical filaments as determined by electron microscopy. The method described here may allow the study of the components involved in the formation of paired helical filaments and the identification of possible factors capable of blocking this process.

Evidence for unequal crossing over in the evolution of the neurofilament polypeptide H

Two allelic forms of the rabbit neurofilament protein H, designated H1 and H2, differ by approximately 6% in their electrophoretic mobilities. We have used techniques of peptide and nucleic acid analysis to determine that this difference is located in the central portion of the COOH-terminal tail, a region of the H protein that can project from the filament, and may form cross-bridges. This region comprises tandem amino acid motifs containing the sequence KSP, in which the serine residues are sites of phosphorylation. The sequence of this repetitive region of H2 is 132 nucleotides (44 amino acids) shorter than H1 and differs significantly in the arrangement of the repeated motifs. The difference suggests that both forms have evolved from a common ancestor over the past several million years by the process of unequal crossing over and that this process figured importantly in generating the repetitive region. In addition, the results indicate that this region is functionally redundant and that the length and structure of the tail of rabbit H resemble H from other mammalian species more closely than was suggested by a previously reported cDNA, on account of a single nucleotide difference.

Limited and selective adduction of carboxyl-terminal lysines in the high molecular weight neurofilament proteins by 2,5-hexanedione in vitro

2,5-Hexanedione (2,5-HD) induces a toxic neuropathy characterized by massive, focal axonal neurofilament (NF) accumulation. Covalent interaction of 2,5-HD with NF protein amines, resulting in pyrrole adduct formation, has been proposed as a critical step in its mechanism. The present study was undertaken to evaluate the hypothesis of selective 2,5-HD/lysine modification, by quantitating in vitro adduction in the NF proteins and in specific polypeptide domains of each protein. Native rat spinal cord NFs were exposed to 0-212.5 mM [14C]2,5-HD for 2-16 h (37 degrees C under argon), followed by removal of non-covalently bound radioactivity. Incorporation of radioactivity and pyrrole formation in NFs increased linearly with 2,5-HD concentration and biphasically with time. SDS-PAGE and fluorography demonstrated prominent labeling of the three NF subunit proteins (H, M, and L), in addition to high-MW, crosslinked material derived from NF-H and -M. Mild chymotryptic cleavage was employed to isolate the carboxyl-terminal 'tail' domains of NF-H and -M, and the pooled amino-terminal NF 'rod' regions, all of which were radiolabeled. Specific activity (mol adduct/mol protein) of adducted NF proteins and polypeptide domains was determined by scintillation counting of electroeluted proteins. Stable binding in the NF-H and -M proteins was 4- to 6-fold higher than in the NF-L protein at all 2,5-HD concentrations, with specific activities of approximately 6.9, 4.7, and 1.3 mol/mol protein, respectively, at 212.5 mM. Approximately 70-80% of NF-H and -M binding was localized to the tail domains. In contrast, NF-L and pooled rod domain adduction did not substantially exceed 1 mol/mol protein. These findings provide the first direct evidence for limited and selective pyrrole adduction in the NF proteins following 2,5-HD exposure.

Okadaic acid induces the rapid and reversible disruption of the neurofilament network in rat dorsal root ganglion neurons

Treatment of 15-17 day old dissociated cultures of rat dorsal root ganglia with 1 microM okadaic acid caused a reduction in the mobilities of neurofilament subunits on SDS-polyacrylamide gels, signifying an increase in their phosphorylation levels. When cultures were exposed to okadaic acid for 0.5 hrs and harvested in buffer containing Triton X-100, NF-H was nearly completely redistributed to the detergent- soluble fraction while NF-M and NF-L required a longer exposure to the drug before undergoing a similar shift. This redistribution of subunits corresponded with striking changes in the immunofluorescence staining pattern for neurofilaments. Upon removal of okadaic acid from the culture medium following a 0.5 hr treatment, NF-L and NF-M returned to the Triton X-100 insoluble fraction within 2 hrs while NF-H required 10 hrs for recovery.

Experimental diabetic neuropathy. Effect of ganglioside treatment on axonal transport of cytoskeletal proteins

Abnormalities in axonal transport of proteins are thought to play an important role in the pathogenesis of diabetic neuropathy. Gangliosides exert a positive action on numerous alterations in biochemistry and physiology of diabetic nerves. This study was undertaken to assess the effects of exogenous gangliosides on the axonal transport of structural proteins such as actin and tubulin in the sensory fibers of short-term (9-wk) and long-term (6-mo) diabetic rats. Adult Sprague-Dawley rats were made diabetic with a single injection of 70 mg/kg streptozocin i.p. Subgroups were injected daily with either highly purified ganglioside mixture (10 mg/kg i.p.) or saline for 1 mo, beginning either 2 or 17 wk after streptozocin injection. Age-matched rats were used as controls. Axonal transport was studied by the pulse-labeling technique. Three weeks after labeling, sciatic nerves were dissected out and processed for sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. In diabetic rats of both experimental designs, the transport rate of tubulin and actin was decreased by approximately 30% compared with control rats. Ganglioside treatment counteracted such alterations in both 9-wk and 6-mo diabetic rats. These data suggest a pharmacological effect that could be correlated with molecular interactions between integral membrane glycolipids and cytoskeletal elements.

Isolation and characterization of hamster brain polyribosome-cytomatrix complexes

We have developed a method for the isolation of a brain subcellular fraction enriched in both highly aggregated polyribosomes and cytoskeletal proteins. This method is based on gentle dispersion of brain tissue and low speed centrifugation. This fraction is enriched in typical cytoskeletal proteins as glial fibrillary protein, neurofilament proteins and actin. Messenger RNA did not seem to be involved in the polyribosome association to the cytomatrix as shown by the effect of exposure to micrococcal nuclease. On the other hand, in vivo disruption of protein synthesis by acute experimental phenylketonuria, hypothermia or heat-shock did not cause the release of ribosomes from the cytomatrix.

Reversible intracellular displacement of the cytoskeletal protein and organelles by ultracentrifugation of the sympathetic ganglion

Superior cervical ganglia of rats were centrifuged at 40,000 rpm (160,500 g) for 1 h at 4 degrees C. Most neuronal somata exhibit a minor centripetal domain free of organelles and a major centrifugal domain rich in organelles. The former is occupied by numerous fine granules having low electron density unlike ribosomes in epoxy sections stained with uranium and lead, and is occupied by a meshwork of microtrabecular or filamentous elements similar to that of the centrifugal domain as well as that of the normal cells in PEG (polyethylene glycol)-processed embedment-free sections without staining. The latter centrifugal domain contains regular cell organelles except for neurofilaments without stratification. All the organelles are suspended in the meshwork of microtrabecular or filamentous elements. In immunolight microscopy, NFPs (neurofilament proteins) are confined to the centripetal domain. In immunoelectron microscopy using ultrathin cryosections and the protein A-gold labeling, numerous gold-particles for NFPs were deposited randomly in the centripetal cytoplasmic domain without long linear alignment. In the PEG sections the gold-labelings for NFPs are randomly deposited on portions of the microtrabecular strands in the centripetal domain. After incubation of the centrifuged ganglia in the anterior eye chamber overnight, NFPs-immunoreactivity appears again diffusely throughout the entire cytoplasm of all neuronal somata in immunolight microscopy. The organelle-free domain of the cytoplasm is no longer visible in electron microscopy. The present findings are discussed in relation to the state of the cytoplasmic soluble proteins and the reality of the microtrabecular or filamentous elements in the cytoplasm.

Phosphatase activity against neurofilament proteins from bovine spinal cord: effect of aluminium and neuropsychoactive drugs

Protein phosphatase activity associated with neurofilament (NF) rich (Triton X-100 insoluble) fraction was extracted and partially characterised by using known inhibitors of protein phosphatases such as vanadate and fluoride. Protein phosphatase activity was demonstrated with reference to the dephosphorylation of endogenous substrate, NF protein and exogenous protein substrates, casein and phosvitin. Phosphoamino acids and beta-glycerophosphate were found to be poor substrates. Further, new observations have been made regarding the in vitro inhibitory effect of aluminium and the differential effects of some of the neuropsychoactive drugs. The findings could possibly lead to studies explaining the biochemical basis of aluminium induced neurotoxicity as well as the side effects associated with the long term medication of neuropsychoactive drugs.

A new series of trpE vectors that enable high expression of nonfusion proteins in bacteria

Expression of recombinant proteins in bacteria has facilitated the characterization of many gene products. However, the biochemical characterization of recombinant proteins is limited since the bacterially expressed proteins are often synthesized as fusion polypeptides. The presence of bacterial sequences in fusion proteins further limits the use of these proteins for generating antibodies since the bacterial sequences are also antigenic. We describe two new bacterial expression vectors based on the pATH series of plasmids. These vectors were made by precisely deleting all of the trpE coding sequences found in pATH. The new vectors have enabled us to express eukaryotic genes as nonfusion polypeptides. These altered plasmids can be used to insert any DNA sequence of interest through a multiple cloning site located just 3' of an ATG start codon. Protein expression is still under the control of the trp operon and is carried out at great efficiency when the bacteria are tryptophan deprived. Studies presented here test the expression system with neurofilament subunits, NF-L and NF-H. Large amounts of recombinant nonfusion proteins were produced. Also, a time course of induction shows that the production of the nonfusion proteins was under the control of the trp operon which is readily inducible after tryptophan starvation and addition of indoleacrylic acid. These vectors may be useful for the overexpression of many proteins in a form closely approximating their native state.