Appearance and localization of phosphorylated variants of the high molecular weight neurofilament protein in NB2a/d1 cytoskeletons during differentiation

Growth and refinement of excitatory synapses in the human auditory cortex

We had earlier demonstrated a neurofilament-rich plexus of axons in the presumptive human auditory cortex during fetal development which became adult-like during infancy. To elucidate the origin of these axons, we studied the expression of the vesicular glutamate transporters (VGLUT) 1 and 2 in the human auditory cortex at different stages of development. While VGLUT-1 expression predominates in intrinsic and cortico-cortical synapses, VGLUT-2 expression predominates in thalamocortical synapses. Levels of VGLUT-2 mRNA were higher in the auditory cortex before birth compared to postnatal development. In contrast, levels of VGLUT-1 mRNA were low before birth and increased during postnatal development to peak during childhood and then began to decrease in adolescence. Both VGLUT-1 and VGLUT-2 proteins were present in the human auditory cortex as early as 15GW. Further, immunohistochemistry revealed that the supra- and infragranular layers were more immunoreactive for VGLUT-1 compared to that in Layer IV at 34GW and this pattern was maintained until adulthood. As for VGLUT-1 mRNA, VGLUT-1 synapses increased in density between prenatal development and childhood in the human auditory cortex after which they appeared to undergo attrition or pruning. The adult pattern of VGLUT-2 immunoreactivity (a dense band of VGLUT-2-positive terminals in Layer IV) also began to appear in the presumptive Heschl's gyrus at 34GW. The density of VGLUT-2-positive puncta in Layer IV increased between prenatal development and adolescence, followed by a decrease in adulthood, suggesting that thalamic axons which innervate the human auditory cortex undergo pruning comparatively late in development.

Interference with kinesin-based anterograde neurofilament axonal transport increases neurofilament-neurofilament bundling

Neurofilaments (NFs) associate with each other and with other cytoskeletal elements to form a lattice that supports the mature axon. Phosphorylation contributes to formation of this stationary population of NFs by fostering cation-dependent interactions among NF sidearms. Association of NFs with the stationary phase indirectly competes with NF axonal transport by withdrawing NFs from kinesin-dependent motility along microtubules. We therefore hypothesized that inhibition of anterograde NF transport may increase incorporation into the stationary phase. To test this hypothesis, we treated differentiated NB2a/d1 cells expressing GFP-tagged NF subunits with monastrol, a specific inhibitor of kinesin-5. Monastrol significantly inhibited anterograde axonal transport of NF-H but not NF-M, and increased the incorporation of newly-transported NF subunits into axonal NF bundles. These findings support the notion that NF transport and bundling exert opposing forces on axonal NF dynamics, and that inhibition of anterograde transport of NFs can increase their incorporation into the stationary phase.

Aluminum induces neurofilament aggregation by stabilizing cross-bridging of phosphorylated c-terminal sidearms

Exposure to neurotoxin aluminum neurotoxicity is accompanied by the perikaryal accumulation of tangles of phosphorylated neurofilaments (NFs). We examined their formation and reversibility under cell-free conditions. AlCl3 induced dose-dependent formation of NF aggregates, ultimately incorporating 100% of detectable NFs. The same concentration of CaCl2 induced approximately 25% of NFs to form longitudinal dimers and did not induce aggregation. AlCl3 induced similar percentages of aggregates in the presence or absence of CaCl2, and CaCl2 could not reduce pre-formed aggregates. CaCl(2)-induced dimers and AlCl(3)-induced aggregates were prevented by prior NF dephosphorylation. While CaCl(2)-induced dimers were dissociated by phosphatase treatment, AlCl(3)-induced aggregates were only reduced by approximately 50%, suggesting that aggregates may sequester phosphorylation sites. Since phosphatases regulate NF phosphorylation within perikarya, inhibition of NF dephosphorylation by aluminum would promote perikaryal NF phosphorylation and foster precocious phospho-dependent NF-NF associations. These findings are consistent with the notion that prolonged interactions induced among phospho-NFs by the trivalent aluminum impairs axonal transport and promotes perikaryal aggregation.

Pin1-dependent prolyl isomerization modulates the stress-induced phosphorylation of high molecular weight neurofilament protein

Aberrant phosphorylation of neuronal cytoskeletal proteins is a key pathological event in neurodegenerative disorders such as Alzheimer disease (AD) and amyotrophic lateral sclerosis, but the underlying mechanisms are still unclear. Previous studies have shown that Pin1, a peptidylprolyl cis/trans-isomerase, may be actively involved in the regulation of Tau hyperphosphorylation in AD. Here, we show that Pin1 modulates oxidative stress-induced NF-H phosphorylation. In an in vitro kinase assay, the addition of Pin1 substantially increased phosphorylation of NF-H KSP repeats by proline-directed kinases, Erk1/2, Cdk5/p35, and JNK3 in a concentration-dependent manner. In vivo, dominant-negative (DN) Pin1 and Pin1 small interfering RNA inhibited epidermal growth factor-induced NF-H phosphorylation. Because oxidative stress plays an important role in the pathogenesis of neurodegenerative diseases, we studied the role of Pin1 in stressed cortical neurons and HEK293 cells. Both hydrogen peroxide (H(2)O(2)) and heat stresses induce phosphorylation of NF-H in transfected HEK293 cells and primary cortical cultures. Knockdown of Pin1 by transfected Pin1 short interference RNA and DN-Pin1 rescues the effect of stress-induced NF-H phosphorylation. The H(2)O(2) and heat shock induced perikaryal phospho-NF-H accumulations, and neuronal apoptosis was rescued by inhibition of Pin1 in cortical neurons. JNK3, a brain-specific JNK isoform, is activated under oxidative and heat stresses, and inhibition of Pin1 by Pin1 short interference RNA and DN-Pin1 inhibits this pathway. These results implicate Pin1 as a possible modulator of stress-induced NF-H phosphorylation as seen in neurodegenerative disorders like AD and amyotrophic lateral sclerosis. Thus, Pin1 may be a potential therapeutic target for these diseases.

Discrete nuclear structures in actively growing neuroblastoma cells are revealed by antibodies raised against phosphorylated neurofilament proteins

BACKGROUND

Nuclear objects that have in common the property of being recognized by monoclonal antibodies specific for phosphoprotein epitopes and cytoplasmic intermediate filaments (in particular, SMI-31 and RT-97) have been reported in glial and neuronal cells, in situ and in vitro. Since neurofilament and glial filaments are generally considered to be restricted to the cytoplasm, we were interested in exploring the identity of the structures labeled in the nucleus as well as the conditions under which they could be found there.

RESULTS

Using confocal microscopy and western analysis techniques, we determined 1) the immunolabeled structures are truly within the nucleus; 2) the phosphoepitope labeled by SMI-31 and RT-97 is not specific to neurofilaments (NFs) and it can be identified on other intermediate filament proteins (IFs) in other cell types; and 3) there is a close relationship between DNA synthesis and the amount of nuclear staining by these antibodies thought to be specific for cytoplasmic proteins. Searches of protein data bases for putative phosphorylation motifs revealed that lamins, NF-H, and GFAP each contain a single tyrosine phosphorylation motif with nearly identical amino acid sequence.

CONCLUSION

We therefore suggest that this sequence may be the epitope recognized by SMI-31 and RT-97 mABs, and that the nuclear structures previously reported and shown here are likely phosphorylated lamin intermediate filaments, while the cytoplasmic labeling revealed by the same mABs indicates phosphorylated NFs in neurons or GFAP in glia.

Delayed expression of the NFH subunit in differentiating P19 cells

Pluripotent embryonal carcinoma (P19) cells differentiate into a neural phenotype in response to retinoic acid (RA). Expression of the low and medium molecular weight neurofilament subunits, but not the high molecular weight subunit (NFH), has been reported following RA treatment. In this study NFH expression was detected by Western blotting and immunofluorescence microscopy, but lagged behind the expression of the other subunits in a manner similar to that reported during in vivo neuronal development.

Selective accumulation of the high molecular weight neurofilament subunit within the distal region of growing axonal neurites

Axonal maturation in situ is accompanied by the transition of neurofilaments (NFs) comprised of only NF-M and NF-L to those also containing NF-H. Since NF-H participates in interactions of NFs with each other and with other cytoskeletal constituents, its appearance represents a critical event in the stabilization of axons that accompanies their maturation. Whether this transition is effected by replacement of "doublet" NFs with "triplet" NFs, or by incorporation of NF-H into existing doublet NFs is unclear. To address this issue, we examined the distribution of NF subunit immunoreactivity within axonal cytoskeletons of differentiated NB2a/d1 cell and DRG neurons between days 3-7 of outgrowth. Endogenous immunoreactivity either declined in a proximal-distal gradient or was relatively uniform along axons. This distribution was paralleled by microinjected biotinylated NF-L. By contrast, biotinylated NF-H displayed a bipolar distribution, with immunoreactivity concentrated within the proximal- and distal-most axonal regions. Proximal biotinylated NF-H accumulation paralleled that of endogenous NF immunoreactivity; however, distal-most biotinylated NF-H accumulation dramatically exceeded that of endogenous NFs and microinjected NF-L. This phenomenon was not due to co-polymerization of biotin-H with vimentin or alpha-internexin. This phenomenon declined with continued time in culture. These data suggest that NF-H can incorporate into existing cytoskeletal structures, and therefore suggest that this mechanism accounts for at least a portion of the accumulation of triplet NFs during axonal maturation. Selective NF-H accumulation into existing cytoskeletal structures within the distal-most region may provide de novo cytoskeletal stability for continued axon extension and/or stabilization.

Neurofilaments consist of distinct populations that can be distinguished by C-terminal phosphorylation, bundling, and axonal transport rate in growing axonal neurites

We examined the steady-state distribution and axonal transport of neurofilament (NF) subunits within growing axonal neurites of NB2a/d1 cells. Ultrastructural analyses demonstrated a longitudinally oriented "bundle" of closely apposed NFs that was surrounded by more widely spaced individual NFs. NF bundles were recovered during fractionation and could be isolated from individual NFs by sedimentation through sucrose. Immunoreactivity toward the restrictive C-terminal phospho-dependent antibody RT97 was significantly more prominent on bundled than on individual NFs. Microinjected biotinylated NF subunits, GFP-tagged NF subunits expressed after transfection, and radiolabeled endogenous subunits all associated with individual NFs before they associated with bundled NFs. Biotinylated and GFP-tagged NF subunits did not accumulate uniformly along bundled NFs; they initially appeared within the proximal portion of the NF bundle and only subsequently were observed along the entire length of bundled NFs. These findings demonstrate that axonal NFs are not homogeneous but, rather, consist of distinct populations. One of these is characterized by less extensive C-terminal phosphorylation and a relative lack of NF-NF interactions. The other is characterized by more extensive C-terminal NF phosphorylation and increased NF-NF interactions and either undergoes markedly slower axonal transport or does not transport and undergoes turnover via subunit and/or filament exchange with individual NFs. Inhibition of phosphatase activities increased NF-NF interactions within living cells. These findings collectively suggest that C-terminal phosphorylation and NF-NF interactions are responsible for slowing NF axonal transport.

The predominant form in which neurofilament subunits undergo axonal transport varies during axonal initiation, elongation, and maturation

The forms in which neurofilament (NF) subunits undergo axonal transport is controversial. Recent studies from have provided real-time visualization of the slow axonal transport of NF subunits by transfecting neuronal cultures with constructs encoding green fluorescent protein (GFP)-conjugated NF-M subunits. In our studies in differentiated NB2a/d1 cells, the majority NF subunits underwent transport in the form of punctate NF precursors, while studies in cultured neurons have demonstrated transport of NF subunits in predominantly filamentous form. Although different constructs were used in these studies, transfection of the same cultured neurons with our construct yielded the filamentous pattern observed by others, while transfection of our cultures with their construct generated punctate structures, confirming that the observed differences did not reflect variances in assembly-competence among the constructs. Manipulation of intracellular kinase, phosphatase, and protease activities shifted the predominant form of GFP-conjugated subunits between punctate and filamentous, confirming, as shown previously for vimentin, that punctate structures represent precursors for intermediate filament formation. Since these prior studies were conducted at markedly differing neuronal differentiation states, we tested the alternate hypothesis that these differing results reflected developmental alterations in NF dynamics that accompany various stages of neuritogenesis. We conducted time-course analyses of transfected NB2a/d1 cells, including monitoring of transfected cells over several days, as well as transfecting cells at varying intervals prior to and following induction of differentiation and axonal neurite outgrowth. GFP-conjugated subunits were predominantly filamentous during the period of most robust axonal outgrowth and NF accumulation, and presented a mixed profile of punctate and filamentous forms prior to neuritogenesis and following the developmental slowing of neurite outgrowth. These analyses demonstrate that NF subunits are capable of undergoing axonal transport in multiple forms, and that the predominant form in which NF subunits undergo axonal transport varies in accord with the rate of axonal elongation and accumulation of NFs within developing axons.

Neuronal intermediate filament protein alpha-internexin facilitates axonal neurite elongation in neuroblastoma cells

We examined the localization and role of alpha-IN vs. other neuronal intermediate filaments before and during differentiation. Vimentin but not alpha-IN localized within filopodia-like neurites of undifferentiated cells. During differentiation, alpha-IN immunoreactivity accumulated within axonal neurites following vimentin but, as previously describe in neurons in situ, before the appearance of NF-L. We therefore manipulated alpha-IN synthesis, accumulation, and function in attempts to determine whether or not this intermediate filament species played a role in axonal development. Intracellular delivery of anti-alpha-IN antisense oligonucleotides and antibodies was permissive for neuritogenesis, yet compromised neurite elongation; this effect was further reflected in diminished levels of stabilized axonal microtubules. These data suggest that alpha-IN plays a role in the development of neuronal polarity. Relatively more alpha-IN than NF-L accumulated within the plastic axonal neurites induced following serum-deprivation, while stable, dbcAMP-induced neurites treatment contained equivalent levels of each. Protease inhibition increased NF-L and NF-H but not alpha-IN immunoreactivity within serum-deprived neurites, suggesting that proteolysis restricts NF-L accumulation pending neurite stabilization. To test the possibility that NF-H accumulation is dependent upon NF-L and cannot be mediated by alpha-IN, we examined levels of NF-H co-precipitated from cells with alpha-IN and NF-L. Virtually all newly synthesized NF-H co-precipitated with NF-L, while only a small percentage co-precipitated with alpha-IN. Finally, NF-H or NF-M were absent from the axon hillock or perikaryal area at the base of neurites, where alpha-IN immunoreactivity is prominent. These data extend earlier cell-free demonstrations that NF-H preferentially associates with NF-L rather than alpha-IN.

Role of vimentin in early stages of neuritogenesis in cultured hippocampal neurons

Vimentin is expressed initially by nearly all neuronal precursors in vivo, and is replaced by neurofilaments shortly after the immature neurons become post-mitotic. Moreover, both vimentin and neurofilaments can be detected transiently within the same neurite, leaving open the possibility that vimentin may play a role in the early stages of neuritogenesis. In the present study, cultured hippocampal neurons, which transiently express vimentin in culture, were treated with sense- and antisense-oriented deoxyoligonucleotides encoding regions of the vimentin sequence that overlap the translation initiation codon. Antisense oligonucleotide treatment reduced vimentin-immunoreactivity to background levels. Moreover, while 90-100% of cultured hippocampal neurons elaborated neurites within the first 24 hr following plating, only 24-30% did so in the presence of vimentin antisense oligonucleotides. Inhibition of neurite outgrowth was reversible following removal of antisense oligonucleotide. These findings substantiate earlier studies in neuroblastoma cells, indicating a possible role for vimentin in the initiation of neurite outgrowth.

Differential synthesis and cytoskeletal deposition of neurofilament subunits before and during axonal outgrowth in NB2a/d1 cells: evidence that segregation of phosphorylated subunits within the axonal cytoskeleton involves selective deposition

NB2a/d1 cells constitutively express and extensively phosphorylate neurofilament (NF) triplet proteins. However, only hypophosphorylated NFs are observed within the Triton-insoluble perikaryal cytoskeletons of undifferentiated and differentiated cells, while phosphorylated NF isoforms accumulate exclusively within the axonal neurites elaborated following treatment with dbcAMP. We examined NF synthesis and distribution of newly synthesized subunits by immunoprecipitation from 35S-methionine-radiolabeled undifferentiated and dbcAMP-treated differentiated cells. Following a 15 min pulse radiolabeling, NF-H isoforms migrating from approximately 160-200 kDa, NF-M isoforms migrating from approximately 97 k-145 Da, and a single 70 kDa NF-L isoform were readily detectable within Triton-soluble fractions from both undifferentiated and differentiated cells. During chase analyses in the absence of radiolabel, the entire spectrum of isoforms was present in Triton-soluble and -insoluble fractions from both undifferentiated and differentiated cells. However, differentiated cells displayed a significant increase in radiolabel associated with each subunit and isoform. Normalization of their NF synthesis levels to those of undifferentiated cells revealed that differentiated cells deposited 10-fold more radiolabeled subunits into the Triton-insoluble cytoskeleton as compared to undifferentiated cells. Similar levels of radiolabeled subunits were observed throughout the 2 hr period in dbcAMP-treated cells. By contrast, radiolabeled subunits and isoforms increased in undifferentiated cytoskeletons during the chase period, although final levels remained substantially lower than those observed in cytoskeletons of dbcAMP-treated cells. These data were considered with respect to potential mechanisms by which the phosphorylated NFs are normally excluded from perikaryal cytoskeletons. The presence of extensively phosphorylated subunits within perikarya indicates the presence of necessary NF kinases.(ABSTRACT TRUNCATED AT 250 WORDS)

Respective roles of neurofilaments, microtubules, MAP1B, and tau in neurite outgrowth and stabilization

The respective roles of neurofilaments (NFs), microtubules (MTs), and the microtubule-associated proteins (MAPs) MAP 1B and tau on neurite outgrowth and stabilization were probed by the intracellular delivery of specific antisera into transiently permeabilized NB2a/d1 cells during treatment with dbcAMP. Intracellular delivery of antisera specific for the low (NF-L), middle (NF-M), or extensively phosphorylated high (NF-H) molecular weight subunits did not prevent initial neurite elaboration, nor did it induce retraction of existing neurites elaborated by cells that had been previously treated for 1 d with dbcAMP. By contrast, intracellular delivery of antisera directed against tubulin reduced the percentage of cells with neurites at both these time points. Intracellular delivery of anti-NF-L and anti-NF-M antisera did not induce retraction in cells treated with dbcAMP for 3 d. However, intracellular delivery of antisera directed against extensively phosphorylated NF-H, MAP1B, tau, or tubulin induced similar levels of neurite retraction at this time. Intracellular delivery of monoclonal antibodies (RT97 or SMI-31) directed against phosphorylated NF-H induced neurite retraction in cell treated with dbcAMP for 3 d; a monoclonal antibody (SMI-32) directed against nonphosphorylated NF-H did not induce neurite retraction at this time. By contrast, none of the above antisera induced retraction of neurites in cells treated with dbcAMP for 7 d. Neurites develop resistance to retraction by colchicine, first detectable in some neurites after 3 d and in the majority of neurites after 7 d of dbcAMP treatment. We therefore examined whether or not colchicine resistance was compromised by intracellular delivery of the above antisera. Colchicine treatment resulted in rapid neurite retraction after intracellular delivery of antisera directed against extensively phosphorylated NF-H, MAP1B, or tau into cells that had previously been treated with dbcAMP for 7 d. By contrast, colchicine resistance was not compromised by the intracellular delivery of antisera directed against NF-L, NF-M, or tubulin. These findings support previous studies indicating that MT polymerization mediates certain aspects of axonal neurite outgrowth and suggest that NFs do not directly participate in these events. These findings further suggest that NFs function in stabilization of the axonal cytoskeleton, apparently by interactions among NFs and MTs that are mediated by NF-H and MAPs.

Transient requirement for vimentin in neuritogenesis: intracellular delivery of anti-vimentin antibodies and antisense oligonucleotides inhibit neurite initiation but not elongation of existing neurites in neuroblastoma

Vimentin is initially expressed by nearly all neuronal precursors in vivo, and is gradually replaced by neurofilaments shortly after the immature neurons become postmitotic (Cochard and Paulin, 1984, J Neurosci 4:2080; Tapscott et al., 1981, Dev Biol 86:40). A transient increase in neuritic vimentin filaments occurs within the first day of dbcAMP-mediated neurite induction in NB2a/d1 neuroblastoma, after which vimentin levels rapidly decline and neurofilaments increase (Shea, 1990, Brain Res 521:343). In the present study, we tested the possibility that vimentin filaments may function in neurite elaboration by inducing neuritogenesis under conditions where vimentin expression and assembly was inhibited. Intracellular delivery of anti-vimentin antiserum into transiently permeabilized NB2a/d1 cells prevented the initial elaboration of neurites, but did not retract existing neurites. By contrast, intracellular delivery of antiserum directed against the low molecular weight neurofilament subunit or normal rabbit antiserum did not affect neurite outgrowth. Treatment with vimentin antisense oligonucleotides reversibly depleted vimentin synthesis and steady-state levels, and prevented neurite initiation, but did not induce retraction of existing neurites. These findings point toward an hitherto undetected role for vimentin in the initiation of neurite outgrowth.

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.

Evidence that the monoclonal antibodies SMI-31 and SMI-34 recognize different phosphorylation-dependent epitopes of the murine high molecular mass neurofilament subunit

The monoclonal antibodies SMI-31 and SMI-34 react with phosphate-dependent epitopes of the high molecular mass (200 kDa) neurofilament protein (Hphos). Determination of whether or not these monoclonals react with different epitopes would assist in interpretation of post mortem immunocytochemical analyses in neurodegenerative disorders and in normal aging. We therefore examined the relative immunoreactivity of these antibodies against Triton-insoluble (cytoskeleton-associated) and Triton-soluble Hphos variants in NB2a/d1 neuroblastoma and post-natal mouse brain in immunoblot analysis. Densitometric analysis yielded a 'reactivity ratio' (soluble Hphos/insoluble Hphos) for each antibody. This ratio was approximately 44% and 87% less for SMI-34 than for SMI-31 in neuroblastoma and brain, respectively. These findings confirm that the SMI-34 epitope is distinct from that recognized by SMI-31, and, in these systems, is preferentially associated with the cytoskeleton.

Methylation and expression of neurofilament genes in tissues and in cell lines of the mouse

The light (NF-L), mid-sized (NF-M) and heavy (NF-H) neurofilament (NF) genes were probed with methylation-sensitive restriction enzymes and patterns of methylation and expression of the NF genes were compared in tissues and cell lines of the mouse. The 5' regions of all three NF genes are identified as CpG islands that remain unmethylated in expressing and non-expressing tissues, although partial methylation occurs at -795 in NF-H and at -525 in NF-M. Methylation of the NF CpG islands is associated with the inactivation of NF genes in L cells and with the selective inactivation of NF-L and NF-M in Neuro 2a cells. We also show that methylation diminishes the ability of the NF promoters to drive transcription of a CAT reporter gene. Hence, the presence of CpG islands may be important in determining patterns of NF transcription in vitro. Moreover, the preservation of CpG islands may be an evolutionary link that bears upon the nature of the NF genes and the mechanisms that have evolved to limit NF expression.

Effect of the substrate on neurofilament phosphorylation in mixed cultures of rat embryo spinal cord and dorsal root ganglia

The effect of the substrate on neurofilament phosphorylation was studied in primary cultures of spinal cord and dorsal root ganglia dissociated from 15-day-old rat embryos. On polylysine and Primaria substrates, spinal cord neurons formed aggregates connected by bundles of neurites. (Primaria dishes have a modified plastic surface with a net positive charge). On both polylysine and Primaria substrates, spinal cord neurons were stained with neurofilament monoclonal antibodies reacting with phosphorylated epitopes appearing early in rat embryo development, i.e. soon after neurofilament expression. Conversely, immunoreactivity with antibodies recognizing late phosphorylation events was only observed on Primaria substrates. As reported by many investigators, fibronectin and laminin were excellent substrates for dorsal root ganglia neurons in culture. However, on both laminin and fibronectin substrates immunoreactivity with antibodies recognizing late phosphorylation events, was only observed on Primaria substrates. As reported by many investigators, fibronectin and laminin were excellent substrates for dorsal root ganglia neurons in culture. However, on both laminin and fibronectin substrates immunoreactivity with antibodies recognizing late phosphorylation events, only occurred after several days in culture, at a time when non-neuronal cells (mainly astrocytes) had formed a confluent monolayer.

Differential appearance of extensively phosphorylated forms of the high molecular weight neurofilament protein in regions of mouse brain during postnatal development

The appearance and accumulation of extensively phosphorylated forms of the high molecular weight neurofilament protein (H-phos) was studied in six regions of mouse brain during postnatal development by quantitative immunoblot analyses. H-phos (migrating at 200 kDa) was detected in brainstem, cerebellum, cortex and hippocampus as early as postnatal day 1. While NF-H levels increased dramatically during subsequent postnatal development in these regions, and reached levels similar to those observed in adult brain by postnatal day 14, quantitative differences were observed in both the rate and the extent of increase among individual regions. The most rapid accumulation of H-phos was observed in brainstem and cortex, where H-phos increased within the first postnatal week to levels comparable to those of adult brain. However, H-phos exhibited a slower developmental change in cerebellum, where the levels increased uniformly over the first two postnatal weeks. In hippocampus, the major increase in H-phos levels was delayed until the second postnatal week. In contrast to its early detection in the above regions, H-phos was not detected in immunoblot analyses of olfactory bulb or hypothalamus cytoskeletons at postnatal day 1, indicating that in these regions the accumulated levels of posttranslationally modified forms of this protein appeared relatively late. Furthermore, H-phos levels in hippocampus did not level off at postnatal day 14 and continued to increase until at least postnatal day 21. Immunoblot analyses of whole embryonic brain revealed the presence of H-phos as early as embryonic day 17, demonstrating that some mouse brain regions carry out extensive phosphorylation of NF-H during embryonic development.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamics of phosphorylation and assembly of the high molecular weight neurofilament subunit in NB2a/d1 neuroblastoma

In neuronal systems thus far studied, newly synthesized neurofilament subunits rapidly associate with the Triton-insoluble cytoskeleton and subsequently undergo extensive phosphorylation. However, in the present study we demonstrate by biochemical and immunological criteria that NB2a/d1 neuroblastoma cells also contain Triton-soluble, extensively phosphorylated 200-kDa high molecular weight neurofilament subunits (NF-H). High-speed centrifugation (100,000 g) of the Triton-soluble fraction for 1 h sedimented some, but not all, soluble NF-H subunits; immunoelectron microscopic analyses of the resulting pellet indicated that a portion of the NF-H subunits in this fraction are assembled into (Triton-soluble) neurofilaments. When cells were pulse labeled for 15 min with [35S]methionine, radiolabel was first associated with the Triton-soluble 200-kDa NF-H variants. Because only extensively phosphorylated NF-H subunits migrate at 200 kDa, whereas hypophosphorylated subunits migrate instead at 160 kDa, these findings suggest that some newly synthesized subunits were phosphorylated before they polymerized. In pulse-chase analyses, radiolabeled 200-kDa NF-H migrated into the 100,000 g particulate fraction of Triton-soluble extracts before its arrival in the Triton-insoluble cytoskeleton. Undifferentiated cells, which do not possess axonal neurites and lack a significant amount of Triton-insoluble, extensively phosphorylated NF-H, contain a sizeable pool of Triton-soluble extensively phosphorylated NF-H subunits and polymers. We interpret these data to indicate that the integration of newly synthesized NF-H into the cytoskeleton occurs in a progression of distinct stages, and that assembly of NF-H into neurofilaments and integration into the Triton-insoluble cytoskeleton are not prerequisites for the incorporation of certain phosphate groups on these polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient increase in vimentin in axonal cytoskeletons during differentiation in NB2a/d1 cells

The localization of vimentin (Vm) within the Triton-insoluble cytoskeleton was characterized during differentiation of mouse NB2a/dl neuroblastoma cells. Vm staining increased within neurites during the first day of differentiation, and then rapidly declined in both perikarya and neurites. By contrast, immunoreactivity against extensively phosphorylated forms of the high molecular weight neurofilament subunit (NF-H) was absent until the third day after differentiation. Immunoblot analyses confirmed that these alterations reflected specific changes in Vm and NF-H steady-state levels. Metabolic labeling demonstrated a decrease in the rate of Vm synthesis by the third day of differentiation. We conclude that changes in incorporation of intermediate filament species into the axonal cytoskeleton reflect distinct stages in neurite outgrowth and maturation; i.e., the Vm filament system may participate in initial stages of neuritogenesis during which outgrowth is most rapid, while NFPs may subsequently function in the establishment of a stabilized axonal cytoskeleton.