Differential distribution of vimentin and neurofilament protein immunoreactivity in NB2a/d1 neuroblastoma cells following neurite retraction distinguishes two separate intermediate filament systems

Deleting the phosphorylated tail domain of the neurofilament heavy subunit does not alter neurofilament transport rate in vivo

Phosphorylation of the carboxyl tail domains of the neurofilament heavy (NF-H) and middle molecular weight (NF-M) subunits has been proposed to regulate the axonal transport of neurofilaments. To test this hypothesis, we recently constructed mice lacking the extensively phosphorylated NF-H tail domain (NF-HtailDelta) and showed that the transport rate of neurofilaments in optic axons is unaltered in the absence of this domain [M.V. Rao, M.L. Garcia, Y. Miyazaki, T. Gotow, A. Yuan, S. Mattina, C.M. Ward, N.A. Calcutt, Y. Uchiyama, R.A. Nixon, D.W. Cleveland, Gene replacement in mice reveals that the heavily phosphorylated tail of neurofilament heavy subunit does not affect axonal caliber or the transit of cargoes in slow axonal transport, J. Cell Biol. 158 (2002) 681-693]. However, Shea et al. proposed that deletion of NF-H carboxyl-terminal region accelerates the transport of a subpopulation of neurofilaments based on minor differences between tail-deleted and control mice in our axonal transport analysis. Here, we present additional evidence that neurofilament transport rate is unchanged after deleting the phosphorylated NF-H tail domain, establishing unequivocally that the NF-H tail domain alone does not regulate the rate of neurofilament transport in optic axons in vivo. Possible roles for tail domains as cross-bridges between a neurofilament and its neighbors or other cytoskeletal elements is discussed.

Regulation of the transition from vimentin to neurofilaments during neuronal differentiation

Vimentin (Vm) is initially expressed by nearly all neuronal precursors in vivo, and is replaced by neurofilaments (NFs) shortly after the immature neurons become post-mitotic. Both Vm and NFs can be transiently detected within the same neurite, and Vm is essential for neuritogenesis at least in culture. How neurons effect the orderly transition from expression of Vm as their predominant intermediate filament to NFs remains unclear. We examined this phenomenon within growing axonal neurites of NB2a/d1 cells. Transfection of cells with a construct expressing Vm conjugated to green fluorescent protein confirmed that axonal transport machinery for Vm persisted following the developmental decrease in Vm, but that the amount undergoing transport decreased in parallel to the observed developmental increase in NF transport. Immunoprecipitation from pulse-chase radiolabeled cells demonstrated transient co-precipitation of newly synthesized NF-H with Vm, followed by increasing co-precipitation with NF-L. Immunofluorescent and immuno-electron microscopic analyses demonstrated that some NF and Vm subunits were incorporated into the same filamentous profiles, but that Vm was excluded from the longitudinally-oriented "bundle" of closely-apposed NFs that accumulates within developing axons and is known to undergo slower turnover than individual NFs. These data collectively suggest that developing neurons are able to replace their Vm-rich cytoskeleton with one rich in NFs simply by down-regulation of Vm expression and upregulation of NFs, coupled with turnover of existing Vm filaments and Vm-NF heteropolymers.

Mutually exclusive expression of beta(III)-tubulin and vimentin in adrenal cortex carcinoma SW13 cells

During embryogenesis, the maturation of neuroblasts into neurones is accompanied by the down-regulation of vimentin and by the expression of neuronal microtubular proteins. Here, we show that human adrenal cortex SW13 cells express beta(III)-tubulin, MAP2b and tau. Analysis of vimentin-positive and -negative subclones of SW13 cells revealed that, under defined cultured conditions, beta(III)-tubulin and MAP2b were present only in vimentin-deficient cells and that beta(III)-tubulin repression occurred at the transcriptional level in vimentin-positive cells. These results suggest that vimentin repression and beta(III)-tubulin expression are co-ordinated by an upstream mechanism relevant to the control of cytoskeletal protein expression during neuronal development.

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.

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

NB2a/d1 cells incorporate neurofilaments (NFs) containing extensively phosphorylated high (NF-H) molecular weight subunits into the Triton-insoluble cytoskeleton of axonal neurites elaborated during differentiation with dibutyryl cAMP. However, immunocytochemical and biochemical analyses demonstrate the constitutive expression and extensive phosphorylation of a sizeable pool of (200 kDa) NF-H. We examined by cell-free analyses whether or not this Triton-soluble NF-H pool was assembly-competent in cell-free analyses. Triton-soluble fractions from 35S-radiolabeled NB2a/d1 cells were incubated with dissociated mouse CNS Triton-insoluble cytoskeletons that had been dissociated by treatment with 6 M urea. Following overnight dialysis to remove urea, low-speed centrifugation to sediment Triton-insoluble cytoskeletons resulted in the co-sedimentation of radiolabeled NF-H, indicating that Triton-soluble NF-H was capable of association with Triton-insoluble structures. Triton-soluble, extensively phosphorylated NF-H from NB2a/d1 cells was also capable of co-assembling with purified NF-L. Following high-speed centrifugation (100,000 x g for 1 h) to sediment any oligomeric assemblies, the Triton-soluble fraction from NB2a/d1 cells was mixed with purified NF-L that had been solubilized by 6 M urea. Following overnight dialysis to remove urea, high-speed centrifugation sedimented both NF-L and Triton-soluble NF-H from NB2a/d1 cells, demonstrating that Triton-soluble NF-H variants are assembly-competent. These data suggest that NF-H variants represent precursors for NF assembly, and indicate that their assembly within NB2a/d1 cells, demonstrating that Triton-soluble NF-H variants are assembly-competent. These data suggest that NF-H variants represent precursors for NF assembly, and indicate that their assembly within NB2a/d1 cells must be under temporal and spatial regulation.

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.

Expression of the neurofilament protein NF-H in L cells

We have inserted a Not1-Sal1 fragment of the mouse gene coding for the neurofilament protein NF-H behind the dexamethasone-inducible transcription promoter of MMTV in a vector derived from pMAMneo (Clonetech Labs). This construct, which includes all four exons of the NF-H gene, was amplified and incorporated into liposomes for transfection of L cells. Transfectants were selected in G418-containing medium and cloned. Clones were grown in serum-containing medium and screened for expression of the NF-H mRNA by extraction of total RNA, generation of cDNAs by reverse transcription, and amplification of a 900-base portion of the NF-H cDNA by PCR. Positive clones were detected by the presence of a band of the correct size on agarose gels. This was confirmed by Southern blotting of the gels probed with a 185-base segment of the amplified region. Immunofluorescent analysis of two positive clones, C33 and C34, showed that C33 cells grown in serum-containing medium or in serum-free medium in the presence of dexamethasone have a network of SMI32 (Sternberger/Meyer Inc.--monoclonal antibody against a nonphosphorylated epitope on NF-H)-positive filaments with the same distribution as filaments stained with antibodies to vimentin, while C34 cells do not react with antibodies against neurofilament proteins. Neither clone reacted with antibodies against highly phosphorylated NF-H (SMI31).

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.

Soluble, phosphorylated forms of the high molecular weight neurofilament protein in perikarya of cultured neuronal cells

The high molecular weight subunit of neurofilaments (NF-H) in mouse NB2a/d1 neuroblastoma cells is extensively phosphorylated and exhibits an apparent molecular weight of 200 kDa by SDS gel electrophoresis. In this study, we observed that extensively phosphorylated NF-H variants exist as both Triton-soluble and -insoluble forms, which display different cellular distributions. Perikarya and neurites of differentiated NB2a/d1 cells were immunostained by a polyclonal antiserum (anti-NF-H) that specifically recognizes the extensively phosphorylated NF-H forms and a monoclonal antibody (SMI-31) that recognizes phosphorylated epitopes of neurofilament proteins (NFPs). When cells were extracted with Triton X-100 to remove soluble proteins, however, only axonal neurites remained immunoreactive. Immunoblot analyses established the specificity of anti-NF-H and SMI-31 and demonstrated that both Triton-soluble and -insoluble NF-H subunits exhibit an apparent molecular weight of 200 kDa. Incorporation of radiolabeled phosphate into Triton-soluble NF-H following incubation of intact NB2a/d1 cells with 32P-orthophosphate confirmed that the Triton-soluble form of NF-H is a phosphoprotein. Most NF-H subunits in the Triton-soluble fraction sedimented after centrifugation at 100,000 g for 1 h, indicating that they may be present as oligomers. The implications of these data for the development of neurofibrillary pathology are discussed.