Triton-soluble phosphovariants of the high molecular weight neurofilament subunit from NB2a/d1 cells are assembly-competent. Implications for normal and abnormal neurofilament assembly

AMP-activated protein kinase and p38 MAPK activate O-GlcNAcylation of neuronal proteins during glucose deprivation

We have demonstrated previously that a wide array of stress signals induces O-GlcNAc transferase (OGT) expression and increases O-GlcNAcylation of many intracellular proteins, a response that is critical for cell survival. Here, we describe a mechanism by which glucose deprivation induces OGT expression and activity in Neuro-2a neuroblastoma cells. Glucose deprivation increases OGT mRNA and protein expression in an AMP-activated protein kinase-dependent manner, whereas OGT enzymatic activity is regulated in a p38 MAPK-dependent manner. OGT is not phosphorylated by p38, but rather it interacts directly with p38 through its C terminus; this interaction increases with p38 activation during glucose deprivation. Surprisingly, the catalytic activity of OGT, as measured toward peptide substrates, is not altered by glucose deprivation. Instead, p38 regulates OGT activity within the cell by recruiting it to specific targets, including neurofilament H. Neurofilament H is O-GlcNAcylated during glucose deprivation in a p38-dependent manner. Interestingly, neurofilament H solubility is increased by glucose deprivation in an O-GlcNAc-dependent manner, suggesting that O-GlcNAcylation of neurofilament H regulates its disassembly from filaments. Not only do these data help to reveal how OGT is regulated by stress, but these findings also describe a possible mechanism by which defective brain glucose metabolism, as found in aging and ischemia, may directly affect axonal structure.

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.

Local control of neurofilament accumulation during radial growth of myelinating axons in vivo. Selective role of site-specific phosphorylation

The accumulation of neurofilaments required for postnatal radial growth of myelinated axons is controlled regionally along axons by oligodendroglia. Developmentally regulated processes previously suspected of modulating neurofilament number, including heavy neurofilament subunit (NFH) expression, attainment of mature neurofilament subunit stoichiometry, and expansion of interneurofilament spacing cannot be primary determinants of regional accumulation as we show each of these factors precede accumulation by days or weeks. Rather, we find that regional neurofilament accumulation is selectively associated with phosphorylation of a subset of Lys-Ser-Pro (KSP) motifs on heavy neurofilament subunits and medium-size neurofilament subunits (NFMs), rising >50-fold selectively in the expanding portions of optic axons. In mice deleted in NFH, substantial preservation of regional neurofilament accumulation was accompanied by increased levels of the same phosphorylated KSP epitope on NFM. Interruption of oligodendroglial signaling to axons in Shiverer mutant mice, which selectively inhibited this site-specific phosphorylation, reduced regional neurofilament accumulation without affecting other neurofilament properties or aspects of NFH phosphorylation. We conclude that phosphorylation of a specific KSP motif triggered by glia is a key aspect of the regulation of neurofilament number in axons during axonal radial growth.

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.

Nitration of the low molecular weight neurofilament is equivalent in sporadic amyotrophic lateral sclerosis and control cervical spinal cord

To determine the extent to which enhanced nitration of the low molecular weight neurofilament subunit protein (NFL) is of pathogenic significance in sporadic ALS, we isolated the neurofilament (NF) from the cervical spinal cord of 15 cases of sporadic ALS and 11 age-matched control cases. Of the three NF subunits, only NFL demonstrated consistent nitrotyrosine immunoreactivity on immunoblots against mouse monoclonal anti-nitrotyrosine antibodies. Regardless of whether the NFL was isolated from the Triton X-100 soluble or insoluble cytoskeletal fractions, the extent of NFL nitration did not differ between ALS and control tissue. Similarly, no differences were observed on either two dimensional isoelectric focusing or NFL peptide maps. These findings suggest that NFL is particularly susceptible to peroxynitrite-mediated nitration in vivo, but reveal no significant qualitative or quantitative modifications in the nitration of NFL isolated from sporadic ALS cervical spinal cord tissue as compared to non-ALS controls.

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)