Neurofilament proteins of rat peripheral nerve and spinal cord

Neurofilaments and Neurofilament Proteins in Health and Disease

SUMMARYNeurofilaments (NFs) are unique among tissue-specific classes of intermediate filaments (IFs) in being heteropolymers composed of four subunits (NF-L [neurofilament light]; NF-M [neurofilament middle]; NF-H [neurofilament heavy]; and α-internexin or peripherin), each having different domain structures and functions. Here, we review how NFs provide structural support for the highly asymmetric geometries of neurons and, especially, for the marked radial expansion of myelinated axons crucial for effective nerve conduction velocity. NFs in axons extensively cross-bridge and interconnect with other non-IF components of the cytoskeleton, including microtubules, actin filaments, and other fibrous cytoskeletal elements, to establish a regionally specialized network that undergoes exceptionally slow local turnover and serves as a docking platform to organize other organelles and proteins. We also discuss how a small pool of oligomeric and short filamentous precursors in the slow phase of axonal transport maintains this network. A complex pattern of phosphorylation and dephosphorylation events on each subunit modulates filament assembly, turnover, and organization within the axonal cytoskeleton. Multiple factors, and especially turnover rate, determine the size of the network, which can vary substantially along the axon. NF gene mutations cause several neuroaxonal disorders characterized by disrupted subunit assembly and NF aggregation. Additional NF alterations are associated with varied neuropsychiatric disorders. New evidence that subunits of NFs exist within postsynaptic terminal boutons and influence neurotransmission suggests how NF proteins might contribute to normal synaptic function and neuropsychiatric disease states.

The dynamic properties of intermediate filaments during organelle transport

Intermediate filament (IF) dynamics during organelle transport and their role in organelle movement were studied using Xenopus laevis melanophores. In these cells, pigment granules (melanosomes) move along microtubules and microfilaments, toward and away from the cell periphery in response to alpha-melanocyte stimulating hormone (alpha-MSH) and melatonin, respectively. In this study we show that melanophores possess a complex network of vimentin IFs which interact with melanosomes. IFs form an intricate, honeycomb-like network that form cages surrounding individual and small clusters of melanosomes, both when they are aggregated and dispersed. Purified melanosome preparations contain a substantial amount of vimentin, suggesting that melanosomes bind to IFs. Analyses of individual melanosome movements in cells with disrupted IF networks show increased movement of granules in both anterograde and retrograde directions, further supporting the notion of a melanosome-IF interaction. Live imaging reveals that IFs, in turn, become highly flexible as melanosomes disperse in response to alpha-MSH. During the height of dispersion there is a marked increase in the rate of fluorescence recovery after photobleaching of GFP-vimentin IFs and an increase in vimentin solubility. These results reveal a dynamic interaction between membrane bound pigment granules and IFs and suggest a role for IFs as modulators of granule movement.

Quantitative evidence for neurofilament heavy subunit aggregation in motor neurons of spinal cords of patients with amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS), a neurodegenerative disease of unknown etiology, affects motor neurons leading to atrophy of skeletal muscles, paralysis and death. There is evidence for the accumulation of neurofilaments (NF) in motor neurons of the spinal cord in ALS cases. NF are major structural elements of the neuronal cytoskeleton. They play an important role in cell architecture and differentiation and in the determination and maintenance of fiber caliber. They are composed of three different polypeptides: light (NF-L), medium (NF-M) and heavy (NF-H) subunits. In the present study, we performed a morphological and quantitative immunohistochemical analysis to evaluate the accumulation of NF and the presence of each subunit in control and ALS cases. Spinal cords from patients without neurological disease and from ALS patients were obtained at autopsy. In all ALS cases there was a marked loss of motor neurons, besides atrophic neurons and preserved neurons with cytoplasmic inclusions, and extensive gliosis. In control cases, the immunoreaction in the cytoplasm of neurons was weak for phosphorylated NF-H, strong for NF-M and weak for NF-L. In ALS cases, anterior horn neurons showed intense immunoreactivity in focal regions of neuronal perikarya for all subunits, although the difference in the integrated optical density was statistically significant only for NF-H. Furthermore, we also observed dilated axons (spheroids), which were immunopositive for NF-H but negative for NF-M and NF-L. In conclusion, we present qualitative and quantitative evidence of NF-H subunit accumulation in neuronal perikarya and spheroids, which suggests a possible role of this subunit in the pathogenesis of ALS.

Identification of human brain from a tissue fragment by detection of neurofilament proteins

We developed a method for identifying human brain from a tissue-like fragment by detection of neurofilament protein (NF) using enzyme-linked immunosorbent assay (ELISA). NF was extracted from 0.1 g of organ/tissue homogenized with Tris-HCl buffer (pH 7.2) containing urea, phenylmethylsulfonyl fluoride (PMSF), EDTA and, EGTA. It was necessary to dilute the extract at more than 2(3)-fold to avoid immunosuppression by urea. Positive reaction was always obtained for NF-H in 2(3)-fold diluted extract of brain tissue, however, NF-L and NF-M were not always detected when a brain fragment contained gray matter. Human cerebral white matter could be easily distinguished from other organs/tissues by detecting any of the NF-subunits. Brains of human and some animals could be discriminated by detecting NF-L or NF-M, although the species specificity of NF-H was not good. Our findings suggested that detection of NF-H was more useful than NF-L and NF-M for identifying a brain from a tissue-like fragment. The present ELISA method for NF-H could identify human brain specimens under the following conditions: putrefied at 4 degrees C for up to 3 weeks, dried at 37 degrees C for at least 4 months, heated at 50 degrees C for at least 4 weeks. Our results showed that our method is useful for identification of brain tissue in forensic stain analysis. Two practical cases are described.

Relationship between functional deficiencies and the contribution of myelin nerve fibers derived from L-4, L-5, and L-6 spinolumbar branches in adult rat sciatic nerve

The distribution and relative intrafascicular contribution of myelin fibers derived from spinal segments L-4 to L-6 were analyzed in adult rat sciatic nerve and its main branches, using 200-kDa neurofilament subunit immunodetection in previously injured nerve sections in the L-4 or L-5 spinal branch or both. These branches' functional contribution was evaluated 16 days after the injury, using the method of J. Bain, S. Mackinnon, and D. Hunter (1988, Plast. Reconstr. Surg. 83: 129-136). A common topographic intrafascicular distribution was found in 69% of cases, with notable segregation of L-4 and L-5 fibers and a random distribution for L-6 fibers. At sciatic nerve main branch level, L-4 contributes almost entirely to the peroneal nerve, L-5 to the tibial nerve, and L-6 and other branches to the sural nerve. After injury to L-4, a significant reduction in peroneal nerve functional index (PFI) was observed, as was a reduction in print length (PL). Injury to L-5 caused a significant reduction in the sciatic (SFI) and tibial (TFI) functional nerve indices, an increase in PL, and a reduction in the spread between opposite toes (TS). Finally, transection of both L-4 and L-5 was followed by a significant reduction in all functional indices measured, an increase in PL, and a reduction in intermediate toe (ITS) and opposite toe spread (TS). The results indicate a direct relationship between the distribution and contribution of the spinal nerve fibers forming the sciatic nerve and the alteration in functional indices for sciatic, tibial, and peroneal nerves.

Somatostatin enhances neurite outgrowth in PC12 cells

The rat pheochromocytoma cell line PC12 forms neurites in response to nerve growth factor (NGF), and it was also reported to extend processes in the presence of somatostatin (somatotropin release-inhibiting factor, SRIF), a neuroactive peptide that seems to act as a morphogenetic factor in the developing nervous system. In the present study, we re-evaluated the effects of SRIF on PC12 cell differentiation. Our results indicate that SRIF alone is ineffective in promoting neurite outgrowth. Instead, SRIF or its analogue, octreotide (a SRIF agonist on the receptor subtypes 2, 3 and 5), potentiates neurite extension induced by NGF. These results suggest that SRIF enhances neurite formation in PC12 cells without directly promoting neurite outgrowth. SRIF potentiation of NGF-induced neurite outgrowth persists at least in part in the presence of pertussis toxin (PTX), suggesting the involvement of PTX-insensitive G-proteins. In addition, protein kinase-dependent pathways are likely to mediate SRIF effects on NGF-induced differentiation.

Postnatal expressions of non-phosphorylated and phosphorylated neurofilament proteins in the rat hippocampus and the Timm-stained mossy fiber pathway

Neurofilament proteins (NFPs), the cytoskeletal proteins that are essential for axogenesis and maintenance of neuron shape in the nervous system, were studied for their spatial distributions at nine postnatal days (PN 3, 5, 7, 10, 14, 17, 21, 28, and 120). Simultaneously non-phosphorylated (SMI-32; 150/200 kDa; Sternberger) and phosphorylated (SMI-31; 200 kDa) NFP immunoreactivity in the entire developing rat hippocampus was studied, quantified, and compared to that of mossy fiber (MF) axons and terminals using Neo-Timm's histochemistry, the most selective, sensitive, and reproducible technique. Differential developmental expressions were observed between the two NFP states. SMI-32 was initially expressed on PN 3 only in the perikarya of pyramidal neurons in CA3. As early as PN 5, SMI-31 appeared in the MF pathway, in parallel to the growth of MF axons. By contrast, SMI-32 did not appear at any age in the MF pathway, including the MF terminal zone of stratum lucidum. At PN 14, the distribution of both NFPs in the MF system (MFs and their target neurons, i.e., CA3/CA4 pyramidal neurons and hilar neurons) was nearly complete; however, the peak densities of SMI-32 and SMI-31 were later at PN 21 and statistically equal to the most adult level (PN 120). The temporal regulation and maximal levels of SMI-32 and SMI-31 expressions on MF target neurons (CA3: SMI-32) and in the MF terminal zone (stratum lucidum: SMI-31) were nearly parallel to the progressive and rapid PN growth of the MF axons and terminals occurring between PN 14 and PN 17, suggesting that the mechanisms for maturation of MF synaptogenesis occur after PN 17.

Heterogeneous expression of neurofilament proteins in forebrain and cerebellum during development: clinical implications for spinocerebellar ataxia

Using quantitative immunoblotting, we have measured the level of two mammalian neurofilament proteins, the 68-kDa NF-L and the 66-kDa NF-66 (alpha-internexin), in the rat CNS during development. NF-66 is localized in neurons and neuronal processes in both embryonic and postnatal brain. Importantly, NF-66 is more abundant than NF-L in both forebrain and cerebellum during development. The prevalence of NF-66 over NF-L is most pronounced in brain gray matter. The expression of both NF-66 and NF-L increases continuously during the first month after birth. In situ hybridization demonstrated that NF-66, but not NF-L is, expressed in the cerebellar granule cells. Our findings suggest that the neurofilaments are heterogeneous in developmental expression, among neuronal subtypes and in composition. Human NF-66 neurofilament has recently been mapped to chromosome 10q24. Careful analysis of the human genome map indicates NF-66 gene lies within the critical region of infantile-onset spinocerebellar ataxia (IOSCA). The characteristic developmental expression and spatial localization of the NF-66 gene suggests it as a candidate gene for the disease.

Age-dependent changes in the immunoreactivity for neurofilaments in rabbit hippocampus

The distribution of the three subunits of neurofilaments was examined in the hippocampus of young adult rabbits (three months of age), employing a panel of six monoclonal antibodies. Thereafter, age-dependent and subunit-selective changes in neurofilament immunoreactivity in the ageing rabbit hippocampus were studied, using animals of one, three, six, 12, 24, 30, 36, 48, and 60 months. Principal cells, interneurons, axons, and various fibre systems were immunoreactive for all three subunits, although the localization and staining intensity of neurofilament immunoreactivity depended on the antibody used. Small cells immunopositive for the low subunit of neurofilament (presumably glial cells) were found abundantly in the hippocampal formation at one month, and (occasionally) at 30-36 months. Young rabbits (one to three months of age) had high numbers of interneurons stained for the high subunit of neurofilament in the stratum oriens/pyramidale. The number declined and plateaued to approximately 78% at six to 30 months, and further declined and plateaued to approximately 56% at 36-60 months. The first decline may reflect a process of maturation, while the latter decline most likely relates to ageing. Ageing pyramidal cells in 48-60 months animals revealed a slight increase for the low subunit of neurofilament, but no changes for the other subunits. Transient changes in neurofilament immunoreactivity were a striking observation in dentate gyrus granule cells during ageing. The staining intensity for the low subunit of neurofilament decreased gradually from one to 24-30 months until it was no longer detectable in these cells. The immunoreactivity then reappeared, most notably in granule cells lining the hilus, at the age of 36-48 months. By 60 months all granule cells were nearly immunonegative for this subunit. Axonal aberrations, immunoreactive for all three subunits, were found throughout the hippocampal formation. These aberrations first appeared in 24-month-old animals and increased in number and maximal size in older rabbits. The alterations in neurofilament immunoreactivity in the ageing hippocampus correlated with age-associated learning disabilities in the acquisition of a hippocampally-dependent learning task. The potential relevance of changes in the cytoskeletal profile of hippocampal neurons to age-associated learning and memory disabilities is discussed.

Analysis of glial fibrillary acidic protein, neurofilament protein, actin and heat shock proteins in human fetal brain during the second trimester

Glial fibrillary acidic protein (GFAP), the 66 kDa neurofilament protein (NF-66), actin, the 27 kDa heat shock protein (HSP27) and the 70 kDa constitutive heat shock protein (HSC70) were analyzed in human fetal brain during the second trimester, from 10 to 24 gestational weeks (GW). By immunohistochemistry, cell-type specific localization of GFAP and NF-66 in astrocytes and neurons, respectively, was confirmed. HSP27 was expressed mostly in the nuclear region of neurons and non-neuronal cells, and HSC70 was widely distributed throughout the tissue. By quantitative immunoblotting, GFAP was not detectable in gray matter of prefrontal cortex prior to 16 GW. Between 16 and 21 GW, the content of GFAP rose slowly. Thereafter, GFAP accumulated rapidly. The content of GFAP in different brain regions (prefrontal, parietal, and occipital cortices) differed significantly at 22 GW. In contrast, NF-66 was already highly expressed at 10 GW, slowly rose to maximal values by 18 GW, and thereafter remained stationary. In contrast to GFAP, the content of NF-66 was similar in different brain regions at 22 GW. Although actin was abundant throughout the second trimester, a sharp drop in its content in the prefrontal cortex was detected at 17 GW. To explain such a decrease, two heat shock proteins were analyzed. HSP27, known to modulate actin polymerization, was found to increase sharply at 16-17 GW. In contrast, HSC70 remained constant during the second trimester and was highly expressed in the fetal brain, at a level comparable to that in the adult brain.

Relations between development and regeneration of tadpole spinal cord

1. The developing spinal cords of bullfrogs and transected cords of stage IV tadpoles were subjected to two-dimensional gel electrophoresis and histological analysis. During development, the level of actin, alpha-tubulin or beta-tubulin in the 7-10th spinal segments increased with time and reached a maximum around stage XIII followed by a decrease, as shown from quantitative assay on protein spots of 2-dimensional gels of cord homogenates. In contrast, the level of 68 kD neurofilament subunit (NF68) was low in tadpoles but high in frog. 2. Following a complete transection made at the level of the 8th spinal segment, the cord tissue of the lesion zone degenerated; regeneration from each cut end then occurred, which lengthened for approximate 0.35 mm by 28 days after transection. The content of actin, alpha-tubulin and beta-tubulin in the cord within 1-2 mm of the transection site was elevated to 124-192% of control values 7-28 days post-transection, whereas NF68 declined to near non-detectable extent. 3. The regeneration of each cord stump included outgrowth of neuroepithelial cells and nerve fibers, reconstituting a newly regenerated cord segment. Ultrastructural examination revealed that features of the regrowth of fibers and guidance of neuroepithelial cells to the axonal growth resembled that seen in the developing cord. Thus the biochemical and morphological data support that the regeneration of the nervous system recaptulates its developmental events, providing evidence for molecular mechanism underlying central axonal regeneration.

Overexpression of the human NFM subunit in transgenic mice modifies the level of endogenous NFL and the phosphorylation state of NFH subunits

Neurofilaments (NFs), the major intermediate filaments of central nervous system (CNS) and peripheral nervous system (PNS) neurons, are heteropolymers formed from the high (NFH), middle (NFM), and low (NFL) molecular weight NF subunits. To gain insights into how the expression of NF subunit proteins is regulated in vivo, two transgenes harboring coding sequences for human NFM (hNFM) with or without the hNFM multiphosphorylation repeat domain were introduced into mice. Expression of both hNFM constructs was driven by the hNFM promoter and resulted in increased levels of hNFM subunits concomitant with an elevation in the levels of mouse NFL (mNFL) proteins in the CNS of both lines of transgenic mice. The increased levels of mNFL appear specific to NFM because previous studies of transgenic mice overexpressing either NFL or NFH did not result in increased expression of either of the other two NF subunits. Further, levels of the most heavily phosphorylated isoforms of mouse NFH (mNFH) were reduced in the brains of these transgenic mice, and electron microscopic studies showed a higher packing density of NFs in large-diameter CNS axons of transgenic versus wild-type mice. Thus, reduced phosphorylation of the mNFH carboxy terminal domain may be a compensatory response of CNS neurons to the increase in NFs, and reduced negative charges on mNFH sidearms may allow axons to accommodate more NFs by increasing their packing density. Taken together, these studies imply that NFM may play a dominant role in the in vivo regulation of the levels of NFL protein, the stoichiometry of NF subunits, and the phosphorylation state of NFH. NFM and NFH proteins may assume similar functions in regulation of NF packing density in vivo.

Distribution of non-phosphorylated and phosphorylated neurofilament proteins in the spinal cord of an anuran amphibian during development and regeneration

The changes in the neurofilament medium and high molecular weight subunits (NF150 and NF200) in the developing and transected spinal cords of bullfrog tadpoles were studied. A monoclonal antibody recognizing the nonphosphorylated epitope of NF150, NF150D, stained the neuronal cell bodies and axons, whereas other antibodies against the phosphorylated NFs, NF150P or NF200P, labeled chiefly the axons. During development, the intensity of axonal staining by the anti-NF150D in the ventral fasciculi in younger tadpoles appeared stronger than older animals, but the reverse was seen for NF150P and NF200P. Complete signal transection of stage IV tadpoles resulted in degeneration and then regeneration of the cord tissue of both cut ends. Each stump lengthened by about 350 microns in the 4 weeks after the lesion. In the proximal stumps, the levels of NF150P or NF200P in the ventral axons at 550-350 microns proximal to the transection site increased notably by about 24-73% of the control value 7-28 days post-transection; however, the content of NF150D was decreased. The densities of NF150D and NF150P protein spots on the Coomassie blue-stained two-dimensional gels of the normal and injured cords also displayed alterations similar to the immunocytochemical data. Intense labeling by the anti-NF150P or NF200P was present in the cell bodies of axotomized motor neurons in the ventral horn. The results suggest that central axonal regeneration may be accompanied by upregulated phosphorylated neurofilament proteins.

Non-phosphorylated neurofilament protein immunoreactivity in adult and developing rat hippocampus: specificity and application in grafting studies

Neurofilament proteins are critical to the development and maintenance of neuronal shape in the nervous system. These proteins are developmentally regulated and several transition forms are expressed, prior to full neuronal stabilization. We have studied the spatial distribution and time course of expression of non-phosphorylated neurofilament protein (NPNFP) immunoreactivity in several preparations of rat hippocampus, using a mixture (SMI 311) of several monoclonal antibodies directed against NPNFP epitopes. Differential staining was observed in young and adult hippocampus. Large pyramidal neurons in CA3 and CA4 subfields were strongly immunoreactive in adult hippocampus whereas the smaller CA1 pyramidal neurons, most interneurons and dentate granule cells were immunonegative. SMI 311 staining initially appeared at postnatal day (P) 5 with positive staining in apical dendrites and soma in a few pyramidal neurons in CA3, but almost reached the adult pattern by P10. Compared to adult hippocampus, the number of immunoreactive interneurons in all subfields appeared increased at P10 and P15. In cultures of embryonic hippocampus, all neurons, regardless of their morphology, were SMI 311 positive, suggesting loss of differential expression in tissue culture conditions. However, SMI 311 expression in fetal hippocampal neurons grafted to adult hippocampus was similar to hippocampal neurons which had developed in situ. These results suggest that SMI 311 antibody identifies a distinct group of primarily CA3 and CA4 pyramidal cells in adult hippocampus. The application of SMI 311 immunostaining appears suitable for identification of large CA3 and CA4 pyramidal neurons within hippocampal transplants grafted to adult CNS but not in tissue culture.

Distribution of cytoskeletal proteins (neurofilaments, peripherin and MAP-tau) in the cochlea of the human fetus

We report here an immunohistochemical study of the distribution of intermediate filaments (neurofilament, peripherin) and a microtubule-associated protein, tau, in the human fetal cochlea at 27 weeks of gestation. Neurofilament immunoreactivity (160 and 200 KDa) was localized in afferent and efferent fibers of the cochlear innervation and restricted to a few small spiral ganglion neurons. Peripherin immunoreactivity was specifically distributed in some small ganglion neurons and in their central and peripheral extensions, particularly in fibers reaching the lower part of the outer hair cells. Double immuno-labelling studies with these neurofilaments and peripherin antibodies show that only small neuron cell bodies were stained. Morpholometrical data indicate that immunostained neurons could be related to the Type II neuron population in the spiral ganglion. Tau protein was localized in intraganglionic spiral bundle fibers and in fibers that reach the lower part of hair cells. These observations suggest that neurofilament and peripherin antibodies stain a particular population of human spiral ganglion neurons with Type II characteristics. Moreover, the specificity of peripherin labelling in Type II cells and their processes suggest that peripherin could be used as a probe for the developmental study of this system in the human cochlea. On the other hand, tau antibody appeared as a marker for efferent fibers during development and could give information on the ontogenesis of efferent innervation.

Co-localization of the beta-subtype of protein kinase C and phosphorylation-dependent immunoreactivity of neurofilaments in intact, decentralized and axotomized rat peripheral neurons

The localizations of protein kinase C-beta-immunoreactivity and phosphorylation-dependent immunoreactivity of neurofilaments were compared in rat dorsal root, hypogastric, and superior cervical ganglia. In all the ganglia studied, protein kinase C-beta and phosphorylation-dependent immunoreactivity of neurofilaments were co-localized in nerve fibres, and no fibres with only protein kinase C-beta-immunoreactivity or phosphorylation-dependent immunoreactivity of neurofilaments were observed. Most intense perikaryal protein kinase c-beta and phosphorylation-dependent neurofilament-staining were seen in large dorsal root ganglion neurons, whereas in the superior cervical ganglion only very faint protein-kinase C-beta and no phosphorylation-dependent staining was seen in the neuronal cell bodies. Both decentralization and axotomy of the superior cervical ganglion induced an accumulation of protein-kinase C-beta-immunoreactivity and phosphorylation-dependent immunoreactivity of neurofilaments in the majority of neuronal perikarya. The accumulation was first observed at 1-2 days postoperation and it persisted up to 6-10 days postoperation. In strongly labelled decentralized neuronal perikarya, precipitation of immunoreactivity was seen near the cell and nuclear membranes, whereas in axotomized neurons, immunoreactivity was often concentrated as a unipolar clump in the cytoplasm. The results show that protein kinase C-beta-immunoreactivity and phosphorylation-dependent immunoreactivity of neurofilaments are colocalized in intact rat peripheral ganglia and that both accumulate transiently in cell bodies of the superior cervical ganglion after decentralization and axotomy.

Glial and neuronal marker proteins in the silicone chamber model for nerve regeneration

In the present study, neuronal and Schwann cell marker proteins were used to biochemically characterize the spatiotemporal progress of degeneration/regeneration in the silicone chamber model for nerve regeneration. Rat sciatic nerves were transected and the proximal and distal stumps were inserted into a bridging silicone chamber with a 10-mm interstump gap. Using dot immunobinding assays, S-100 protein and neuronal intermediate filament polypeptides were measured in different parts of the nerve 0-30 days after transection. In the most proximal nerve segment, all the measured proteins were transiently increased. In the proximal and distal stumps adjacent to the transection, the studied proteins were decreased indicating degeneration of the nerve. Within the silicone chamber, the regenerating nerve expressed the Schwann cell S-100 protein already at 7 days, whereas the neurofilament polypeptides appeared later. These observations are corroborated by previous morphological studies. The biochemical method described provides a new and fast approach to the study of nerve regeneration.

Cryo-electron microscopy of nervous tissue. A review

The present work attempts to demonstrate that cryofixation is a valuable method for the study of the nervous tissue. The use of the newly developed methods of cryofixation and freeze-etching without fixatives or cryoprotectants allows new exciting perspectives for the electron microscopical observation of cellular components, emphasizing their three-dimensional morphological structures. Significant contributions have been made on the fine structure of the cytoskeleton, cell membranes and cell organelles. The components of the cytoskeleton are distributed in different composition through the perikarya, dendrites and axon. The ubiquitous presence of the cytoskeleton suggests a crucial role in the functional activities of the neurons, especially in relation to the intracellular communication and to developmental and regeneration processes. Vitrified cellular membranes of myelin sheaths and rod outer segments have been observed in hydrated state by using cryofixation and cryotransfer techniques. These procedures allow new insights into the supramolecular structure and an approximation of morphological data to the present biophysical membrane model including a critical comparison with the current descriptions gained by conventional electron microscopy.

Neurofilament reassembly in vitro: biochemical, morphological and immuno-electron microscopic studies employing monoclonal antibodies to defined epitopes

The reassembly process of purified native (phosphorylated) and enzymatically dephosphorylated bovine neurofilament (NF) subunits was studied to delineate how NF triplet proteins assemble together into intermediate-size filaments in vitro. We determined the time course for reassembly, the ultrastructural characteristics of reassembled NFs, and the topographical disposition of NF protein subdomains within reassembled NFs using quantitative biochemical techniques, negative staining and immunoelectron microscopy. Our data indicate that: (1) approximately 50% of the purified NF subunit proteins assembled within 30 min from the start of reassembly into 10- to 12-nm filaments, and by 90 min approximately 85-90% of the NF proteins reassembled, (2) low concentrations (0.15-0.5 mg/ml) of purified NF proteins were able to reassemble into long filaments, (3) the rate and ability of native phosphorylated and dephosphorylated NF proteins to assemble into NFs were comparable, (4) negative staining revealed a periodicity of approximately 18-22 nm and a protofilamentous substructure in reassembled NFs, (5) immunoelectron microscopy using domain specific anti-NF monoclonal antibodies (mAbs) to all 3 NF proteins demonstrated specific labeling patterns corresponding to the spatial relationships of subdomains within reassembled NFs, and (6) negative staining and immunolabeling revealed that reassembled NFs are very similar to isolated native NFs. We conclude that purified mammalian axonal NF triplet proteins, independent of their phosphorylation state, rapidly and efficiently reassemble in vitro to generate characteristic 10-nm filaments. Furthermore, immunological analysis reveals that the rod domains of NF-H, NF-M and NF-L are buried within the reassembled NF, whereas the head domain of NF-M and the tail domains of all 3 NF proteins remain exposed following reassembly.

Neurofilament protein phosphorylation in spinal cord of experimentally diabetic rats

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

Induction of NILE/L1 glycoprotein during neuronal differentiation of the embryonal carcinoma cell line EC1003

A new clone of the mouse embryonal carcinoma cell line 1003 (EC 1003.16) can be maintained in an undifferentiated state in serum-containing medium. Shifting these cells to serum-free, hormonally defined medium causes them to differentiate morphologically and acquire a number of molecular properties characteristic of neurons. Whereas undifferentiated cells lack the NILE/L1 glycoprotein, expression of this neuronal cell adhesion molecule is induced in the differentiating cells. Message for NILE/L1 becomes detectable after 5 days in serum-free medium, and cell-surface NILE/L1 can first be seen at this same time. Changes in two other cell adhesion molecules occur in parallel with the induction of NILE/L1. Fibronectin receptor is present on undifferentiated cells, but is down-regulated by the differentiating neurons. The neural cell adhesion molecule (N-CAM) undergoes a shift from the very adhesive adult form to the less adhesive, highly sialylated embryonic form. These changes would appear to emphasize the role of NILE/L1 in adhesive interactions involving differentiating neurons. Some changes in ganglioside expression also occur during EC 1003.16 differentiation. Undifferentiated cells express the D 1.1 ganglioside but lack gangliosides that are reactive with the monoclonal antibody A2B5. Differentiating cells lose D 1.1 and become A2B5-positive. Since D 1.1 is characteristic of undifferentiated neuroepithelial cells and A2B5 reactivity is a marker for several types of differentiated neurons, these changes in vitro appear to mimic events that occur in vivo.

Cochlear innervation in the developing rat: an immunocytochemical study of neurofilament and spectrin proteins

We have studied the innervation of the developing cochlea by immunocytochemical staining of the cytoskeletal proteins, neurofilament (NF), and spectrin (brain spectrin and erythrocyte spectrin). NF immunoreactivity was seen in spiral ganglion cell bodies and their processes and in fibers of the intraganglionic spiral bundle (IGSB) on gestational day 16. NF immunoreactivity with monoclonal antibodies to NF160 and NF68 was present beneath both inner hair cells (the IHC) and outer hair cells (OHCs) on gestational day 20. NF200 immunostaining was located only in the IGSB and in fibers reaching the IHC. The first NF200 immunoreactivity beneath the OHCs was seen in the basal turn at birth. NF labelling began to decrease on postnatal day 9 and its intensity became more like that of the adult. Brain spectrin immunostaining was first seen in the IGSB of the basal turn on gestational day 18. It reached the fibers between the spiral ganglion and the IHC on gestational day 20. Brain spectrin immunoreactivity was first seen beneath the OHCs in the basal turn at birth. It reached all the OHCs of the cochlea by postnatal day 4, and began to decrease 9 days after birth. Erythrocyte spectrin immunostaining was first observed during the second postnatal week, when it labelled spiral ganglion cells. The distribution of NF200 and brain spectrin immunoreactivity suggested that efferent innervation of OHCs is present at birth in the rat, and confirms previous studies showing the early efferent innervation of the OHCs of the mouse and the rat at birth, and the time lag between the appearance of the two spectrin isoforms during development.

Effect of phosphorylation on 68 KDa neurofilament subunit protein assembly by the cyclic AMP dependent protein kinase in vitro

The effect of phosphorylation by cyclic AMP dependent protein kinase on the assembly of the core-forming 68 KDa neurofilament subunit protein (NF-L) was studied in vitro by fluorescence energy transfer and electron microscopy. Phosphorylation of unassembled NF-L in a low ionic strength buffer by cyclic AMP dependent protein kinase led to the incorporation of 1-2 phosphate groups/mole protein. Assembly of this phosphorylated NF-L was inhibited significantly; compared to non-phosphorylated NF-L, the critical concentration of phosphorylated NF-L was raised by greater than 30-fold. Assembled NF-L filaments could also be phosphorylated by cyclic AMP dependent protein kinase indicating that the sites were accessible. Phosphorylation of NF-L in the filamentous state induced their disassembly. The results suggest that phosphorylation by cyclic AMP dependent protein kinase is a possible means to modulate the assembly state of NF-L.

Involvement of protein kinase C in the regulation of assembly-disassembly of neurofilaments in vitro

Protein kinase C phosphorylated the major mammalian neurofilament protein (NF-L) with approximately 3 mol phosphate per mol protein. The phosphorylated NF-L no longer formed the filaments. Sequential analysis of the tryptic phosphopeptides, together with the known primary sequence, revealed that Ser-12, Ser-27, Ser-33 and Ser-51 were phosphorylated by protein kinase C. These findings contribute toward elucidation of mechanisms regulating the functions of neurofilaments.

Expression of neurofilament proteins in granule cells of the cerebellum

We have used a panel of monoclonal antibodies directed against the low, middle and high molecular weight subunits of neurofilament triplet, to study their expression in mouse cerebellar granule cells. We demonstrate that in situ such cells only express the 2 lower molecular weight subunits either at various developmental stages or in the adult. The same results were obtained in vitro. This pattern of neurofilament protein expression in adult granule cells is therefore similar to that observed in developing neurons but differs from most neurons in the adult brain. The retention of such 'immature' pattern of neurofilament protein expression throughout adulthood could explain the lack of cytologically identifiable intermediate filaments in these neurons when examined with conventional electron microscopic techniques. It furthermore suggests that various neuronal populations might be characterized by the expression of specific subsets of neuronal intermediate filaments.

Ontogeny of the neuronal intermediate filament protein, peripherin, in the mouse embryo

The expression of peripherin, a type III neuron-specific intermediate filament protein, and the middle neurofilament subunit were studied in the mouse embryo using immunofluorescence staining. The earliest staining for both proteins is seen at embryonic day 9 in the myelencephalon, initially as fiber staining followed by cell body staining in the developing facial and acoustic nuclei. As the embryo develops, there is rostral as well as caudal extension of peripherin and staining is seen in the trigeminal ganglia, nerve fibers and in the enteric nervous system. As the spinal cord forms there is anti-peripherin staining in developing motoneurons of the anterior horns while little cell body staining is seen for the middle neurofilament subunit. Both antibodies stain the developing dorsal root and its entry zone, but peripherin is found in the secondary sensory and commissural fibers while the middle neurofilament subunit is not. While both proteins are found in the neurons of the dorsal root ganglia, their distribution varies. The larger peripheral cells of the ganglia contain both proteins while the smaller more central cells, constituting over 60% of the cells in the ganglia, contain only peripherin. A similar picture is found in the sympathetic ganglia where there are cells which contain peripherin. middle neurofilament subunit or both, but where the majority of the neurons have only peripherin in their cell bodies. Peripherin is not found in the developing retina or in the adrenal medulla. Peripherin is also completely absent from cell bodies in the cerebral and cerebellar cortices. These results indicate that peripherin is found in development only in regions in which it is found in the adult. It can either co-exist with neurofilaments in the same neuron or the two may be independently expressed.

Neurofilament protein-triplet immunoreactivity in distinct subpopulations of peptide-containing neurons in the guinea-pig coeliac ganglion

A battery of polyclonal and monoclonal antibodies raised against the triplet of identified neurofilament protein subunits was used to investigate neurofilament protein immunoreactivity in neurons of the guinea-pig coeliac ganglion. Using optimal conditions of fixation and tissue processing for each antibody we found that only 20% of the postganglionic sympathetic neurons in the guinea-pig coeliac ganglion contain neurofilament protein-triplet immunoreactivity. Double labelling with neurofilament protein-triplet antibodies raised in different species demonstrated that all of these antibodies labelled the same population of neurons. Double labelling using mouse monoclonal antibodies against neurofilament proteins in combination with rabbit polyclonals to neuronal markers showed that neurofilament protein-triplet immunoreactivity is restricted to specific chemically coded subpopulations of noradrenergic neurons. Approximately 52% of neurons in the ganglion contain neuropeptide Y and are presumed vasomotor neurons projecting to blood vessels in the submucosa of the small intestine. Virtually none of the neuropeptide Y-containing neurons were labelled with neurofilament protein-triplet antibodies. Neurons that contain somatostatin (21%) project to the submucous ganglia of the small intestine. Approximately two-thirds of neurons containing somatostatin are immunoreactive for the neurofilament protein-triplet. The other postganglionic neurons in the ganglion (27%) project to the myenteric plexus of the small intestine and do not contain either neuropeptide Y or somatostatin. Approximately a quarter of these neurons were labelled with neurofilament protein-triplet antibodies. These results suggest that the neurofilament protein-triplet may not be an intrinsic component of the cytoskeleton of all neurons. Furthermore the idea of a chemical coding of neurons should be extended to cytoskeletal proteins. The finding that these neurofilament proteins are confined to specific neuronal subpopulations has important implications for the search for a role of the neurofilament protein-triplet in neurons, for the interpretation of classical neurohistological silver impregnation techniques which appear to stain only neurofilament protein-triplet-containing neurons, as well as for neuropathological conditions that may involve these proteins in disease processes.

Polyclonal antisera to the individual neurofilament triplet proteins: a characterization using ELISA and immunoblotting

In this article, the preparation and characterization of polyclonal rabbit antisera against the individual polypeptides of bovine neurofilament (68, 150, and 200 kilodaltons) is described. Selected antisera against the 68- and 150-kilodalton neurofilament polypeptides were specific for the corresponding antigen in homogenates of bovine, rat, and human brain as judged by immunoblots. The antisera against the 200-kilodalton neurofilament polypeptide cross-reacted to some extent with the 150-kilodalton neurofilament polypeptide, especially with the human antigen. The most specific antisera were used to develop an enzyme-linked immunosorbent assay (ELISA), and the cross-reactivities between the antisera and the different bovine and rat neurofilament polypeptides were determined. Contrary to the results in the immunoblots, the antiserum against the 200-kilodalton neurofilament polypeptide was subunit-specific, as was the 150-kilodalton antiserum. The 68-kilodalton antiserum displayed a minute cross-reactivity against bovine 150- and 200-kilodalton neurofilaments, but it cross-reacted somewhat more with the rat 150- and 200-kilodalton antigens. Even so, the subunit specificity of the antisera is high enough to enable the development of a quantitative ELISA for determination of the individual bovine or rat neurofilament polypeptides in a mixture. This study is the necessary preparation for such an assay.

First appearance of type II neurons during ontogenesis in the spiral ganglion of the rat. An immunocytochemical study

Ontogenesis of spiral ganglion in the rat was studied using antibodies to three subunits of neurofilaments (NFs): NF 68 KDa, NF 160 KDa and NF 200 KDa. The expression of immunoreactivity was examined with 3 immunocytochemical methods: indirect immunofluorescence, peroxidase-antiperoxidase and avidin-biotin complex. Aim of the study was to detect the time of differentiation of the spiral ganglion type II neurons. At 16 and 18 days of gestation, most neuron cell bodies express immunoreactivity to only two NF subunits: NF 68 and NF 160, but at birth they react with the antibodies to all 3 subunits albeit weakly. Nevertheless, a small population (about 7%) of nerve cells that strongly reacts against all 3 NF subunits emerges in the basal turn, already at 20 days of gestation. Two to 3 days after birth, the strongly stained cells are dispersed throughout the entire ganglion. The intensity of their reaction to the NF antibodies is similar to that seen in the adult animal. The strong immunoreactivity of this selective neuronal population suggest, that they correspond to the type II spiral ganglion neurons. Our results imply that the differentiation between the type I and the type II of spiral neurons in the rat occurs perinatally.

Spatial and temporal expression of phosphorylated and non-phosphorylated forms of neurofilament proteins in the developing nervous system of Xenopus laevis

Immunocytochemical studies of developing Xenopus laevis embryos and tadpoles (stages 12 1/2 to 46) were performed using a panel of 11 monoclonal antibodies to phosphorylated and non-phosphorylated forms of the neurofilament proteins. These included nine antibodies to the middle molecular weight neurofilament protein (XNF-M, 175 kDa), and two additional antibodies to non-phosphorylated forms of the other two neurofilament proteins (XNF-L, 73 kDa; XNF-H, 205 kDa). The developmental expression of XNF-M, XNF-L and XNF-H, and the progressive phosphorylation of XNF-M in the rhombencephalon, spinal cord, and optic nerve were studied using these antibodies. In the spinal cord and rhombencephalon, non-phosphorylated forms of XNF-M were initially detected during neural tube stages (stages 22-26), one day before XNF-L and XNF-H at early tadpole stages (stage 35/36). In the eye, XNF-M was observed initially during tailbud stages (stage 29/30), but neither XNF-L nor XNF-H was seen even by stage 46 (swimming tadpole). The phosphorylation of XNF-M occurred over a protracted period of several days, both in the neural tube and visual system, and could be divided into four phases. (1) When initially expressed, XNF-M was hypophosphorylated. This was indicated by the early immunostaining of axons and cell bodies with antibodies to dephosphorylated epitopes on XNF-M and by the absence of staining with antibodies to phosphorylated epitopes. (2) After a short timelag (3-9 h) axons were stained by some, but not all antibodies to phosphorylated epitopes. (3) Approximately one day later, all antibodies to phosphorylated epitopes stained the relevant axons. However, XNF-M was not yet fully phosphorylated, as indicated by the continued staining of these axons with antibodies to dephosphorylated epitopes of XNF-M. (4) Two to 3 days after the initial expression of XNF-M, dephosphorylated epitopes disappeared from the axons, establishing the adult pattern. During development, the most heavily phosphorylated neurofilament proteins present at a given stage were found first in distal regions of the axons and progressed gradually toward the neuronal perikarya as development proceeded. This gradient of phosphorylation, established early within the axon, suggests that neurofilaments in the axons mature from their distal ends toward the cell body, a process which may be regulated by local factors within the axons themselves. The similarity of the basic features of NF-M phosphorylation in mammalian, avian, and amphibian axons underscores the importance of this phenomenon for the development of a mature axon.

Characterization of a novel 66 kd subunit of mammalian neurofilaments

A 66 kd protein, pl 5.4, was purified from the Triton-insoluble fraction of rat spinal cord. This protein formed 10 nm filaments in vitro. The 66 kd protein was unique, although it shared homology with the 70 kd neurofilament protein (NF-L) and vimentin. An antiserum (anti-66) specific to the 66 kd protein did not cross-react with any of the neurofilament triplet proteins. In the spinal cord, anti-66 intensely stained the axons of the anterior and lateral columns. However, afferents from dorsal root ganglia and the efferents from the motoneurons were negative. In the cerebellum, anti-66 intensely stained most axons. The 66 kd protein was readily detectable in homogenates of forebrain, cerebellum, brainstem, and spinal cord, but was found only in trace amounts in adult sciatic nerves and was not found in extraneural tissues. The 66 kd protein constituted 0.5% of total protein in the spinal cord, whereas NF-L constituted about 1.5%.

Chromatolysis of dorsal root ganglion cells studied by cryofixation

Cytoskeletal alterations in the cytoplasm of chromatolytic neurons of the dorsal root ganglia were studied in chickens after transection of the sciatic nerves. These studies were carried out using cryofixation with a nitrogen-cooled propane jet. By this method, the morphological complexity of the cytoskeleton in normal perikarya and cell processes can be visualized. The cytoskeleton of the dorsal root ganglion cells (DRG) is composed of an intricate network of microtubules, neurofilaments and microfilaments. The membrane-bounded cell organelles, as well as the cell nucleus and the plasmalemma, are linked to the microtubules and neurofilaments by microfilaments (or cross-linkers). As a result of the transection of the axon, chromatolysis takes place, characterized by dislocation of cell organelles, an eccentric position of the nucleus and dispersion of the parallel cisternae of the rough endoplasmic reticulum throughout the cytoplasm. This characteristic phenomenon coincides with a regression of the neurocytoskeletal network. The neurofilaments and microtubules become shorter, and the microfilaments are replaced by strands of globular or granular material. The temporary regression of the microfilaments leads to a dispersion of the cell organelles. During the remodelling of the cytoskeletal structures, proliferation of the neurofilaments in the regenerating neurons may occasionally be observed. These results show that the cytoskeletal structures are responsible not only for the preservation of cell shape, but also for the maintenance of the normal distributional pattern (location and mobility) of the intracellular components.

Early posttranslational modifications of the three neurofilament subunits in mouse retinal ganglion cells: neuronal sites and time course in relation to subunit polymerization and axonal transport

We have characterized stages in the posttranslational processing of the three neurofilament subunits, High (NF-H), Middle (NF-M), and Low (NF-L), in retinal ganglion cells in vivo during the interval between synthesis in cell bodies within the retina and appearance of these polypeptides in axons at the level of the optic nerve (optic axons). Neurofilament proteins pulse-labeled by injecting mice intravitreally with [35S]methionine or [32P]orthophosphate, were isolated from Triton-soluble and Triton-insoluble fractions of the retina or optic axons by immunoprecipitation or immunoaffinity chromatography. Within 2 h after [35S]methionine injection, the retina contained neurofilament-immunoreactive radiolabeled proteins with apparent molecular weights of 160, 139, and 70 kDa, which co-migrated with subunits of axonal neurofilaments that were dephosphorylated in vitro with alkaline phosphatase. The two larger polypeptides were not labeled with [32P]orthophosphate, indicating that they were relatively unmodified forms of NF-H and NF-M. About 75% of the subunits were Triton-insoluble by 2 h after isotope injection, and this percentage increased to 98% by 6 h. Labeled neurofilament polypeptides appeared in optic axons as early as 2 h after injection. These subunits exhibited apparent molecular weights of 160, 139, and 70 kDa and were Triton-insoluble. The time of appearance of fully modified polypeptide forms differed for each subunit (2 h for NF-L, 6-18 h for NF-M, 18-24 h for NF-H) and was preceded by the transient appearance of intermediate forms. The modified radiolabeled subunits in optic axons 3 days after synthesis were heavily labeled with [32P]orthophosphate and exhibited the same apparent molecular weights as subunits of axonal neurofilaments (70 kDa, 145 and 140 kDa, and 195-210 kDa, respectively). Whole mounts of retina immunostained with monoclonal antibodies against NF-H in different states of phosphorylation demonstrated a transition from non-phosphorylated neurofilaments to predominantly phosphorylated ones within a region of the axon between 200 and 1000 microns downstream from the cell body. These experiments demonstrate that the addition of most phosphate groups to NF-M and NF-H takes place within a proximal region of the axon. The rapid appearance of modified forms of NF-L after synthesis may imply that processing of this subunit occurs at least partly in the cell body. The presence of a substantial pool of Triton-insoluble, unmodified subunits early after synthesis indicates that the heaviest incorporation of phosphate occurs after neurofilament proteins are polymerized.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of neurofilament phosphorylation in cultured chromaffin cells

The distribution, structural organization and state of phosphorylation of neurofilaments have been examined in chromaffin cells from adult bovine adrenal medulla cultured under various conditions using a series of monoclonal antibodies directed against phosphorylated and nonphosphorylated epitopes of the 200,000 mol. wt subunit. Nonphosphorylated neurofilament epitopes were detected immunocytochemically to varying extents in chromaffin cells maintained under standard culture conditions for up to 3 weeks. Staining was usually limited to a perinuclear region from which fine filaments sometimes appeared to radiate around the nucleus. In marked contrast, none of the antibodies directed against phosphorylated neurofilament epitopes stained these structures. When cells were cultured under conditions favouring neurite outgrowth, in conditioned medium derived from intermediate lobe cultures, there was a more extensive expression of the nonphosphorylated neurofilament epitopes. In addition, phosphorylation of neurofilaments was induced. The phosphorylated neurofilament epitopes were restricted to the neurite, whereas the nonphosphorylated neurofilament epitopes were localized in both neurite extensions and perikarya. These results demonstrate that conditioned medium from intermediate lobe cells of the hypophysis not only provokes neurite outgrowth from chromaffin cells, but also supports neuronal maturation as demonstrated by the phosphorylation of neurofilaments in neurites.

Expression of Schwann cell characteristics in pigmented nevus. Immunohistochemical study using monoclonal antibody to Schwann cell associated antigen

A monoclonal antibody to the Schwann cell associated antigen (AHMY1) and a monoclonal antibody to neurofilament proteins (NFP) (AHNF1) each were produced. Using AHMY1, AHNF1, and anti-S-100 antibody, 90 pigmented nevi were examined immunohistochemically to clarify the relationship between nevus cells and Schwann cells. All nevus cells had S-100 protein, but did not have neurofilament proteins. The nevic corpuscle and Type C nevus cell were positively stained with AHMY1 indicating the presence of the Schwann cell associated antigen, while Type A and B nevus cells were entirely negative. This suggests that the nevic corpuscle and Type C nevus cell are closely related to the Schwann cell and differ from Type A and B nevus cells in their development.

Expression of neurofilament proteins by Purkinje cells: ultrastructural immunolocalization with monoclonal antibodies

Purkinje cell bodies in rodent cerebellum have been shown to express neurofilament protein epitopes but neurofilaments are rarely seen in these perikarya by classical morphological approaches. In an attempt to solve this enigma the ultrastructural distribution of two neurofilament epitopes was studied by immunoelectron microscopy with two monoclonal antibodies (Mabs) of divergent specificity: one, Mab 04-7 recognized a phosphorylated epitope, the other, Mab 02-135 a non-phosphorylated epitope. Longitudinal filamentous elements were heavily labeled in basket cell axons and afferent nerve fibers with both Mabs. While Mab 04-7 was unreactive with Purkinje cells, the immunoperoxidase reaction product with Mab 02-135 was distributed in the form of patches with no filamentous substructure throughout the cytoplasm of these cells. The data complement the results of other immunocytochemical studies showing the presence of all 3 neurofilament constituent proteins in Purkinje cell bodies, and lead to the conclusion that in these perikarya the majority of neurofilament proteins are not assembled in the form of neurofilaments.

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

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

Quantitative evaluation of axonal regeneration by immunochemical assay for neurofilament protein

In experiments on nerve regeneration requiring assessment of the rate and extent of axonal outgrowth, the availability of a simple and accurate method of quantification would be extremely useful. We approached this issue by modifying the conventional ELISA procedure so as to provide a sensitive, specific, and quantitative biochemical assay of the phosphorylated neurofilament content of homogenates or sections of nerve tissue. The technique involves four sequential steps: (i) adhesion of fixed or fresh homogenates or tissue sections to wells of microtiter plates, (ii) binding of a monoclonal antibody against phosphorylated neurofilament to the tissue, (iii) secondary binding to the anti-phosphorylated neurofilament of a phosphatase-labeled second antibody (antimouse IgG), and (iv) enzymatic assay of alkaline phosphatase activity using a fluorescent substrate (4-methylumbelliferyl phosphate). The technique is sufficiently sensitive to measure the phosphorylated neurofilament content of a 1:100,000 (w/v) homogenate of brain, spinal cord, or peripheral nerve and of single 10-microns paraffin sections of Bouin-fixed rat spinal cord. To validate the applicability of the procedure to the study of nerve regeneration, the sciatic nerve of adult rats was either crushed (to permit regeneration) or transected and ligated (to preclude regeneration). The animals were autopsied 1 to 16 weeks later, when four segments 3-mm in length taken from regions proximal and distal to the lesion site were assayed for phosphorylated filament content. The temporal course of its disappearance during degeneration and its reappearance during regeneration coincided with the known histologic changes in crushed and transected nerves. These findings demonstrate the validity of using the immunochemical assay for PNF in studies of nerve regeneration in the peripheral nervous system and the potential applicability of this procedure to studies on regeneration in the central nervous system.

A developmentally regulated isoform of 150,000 molecular weight neurofilament protein specifically expressed in autonomic and small sensory neurons

Neurofilament heterogeneity has been demonstrated using a monoclonal antibody (CH1) specific for the 150,000 molecular weight neurofilament subunit. In the peripheral nervous system of adult rats CH1 stained selectively sympathetic and parasympathetic neurons and a subpopulation of small neurons in the sensory dorsal root ganglia. Somatic motor neurons and large neurons in dorsal root ganglia were completely unreactive. In contrast, the anti-neurofilament antibody iC8, directed against the 150,000 molecular weight subunit, labelled all peripheral nervous system neurons. The immunostaining pattern with both antibodies was unchanged by phosphatase treatment. These data indicate that two antigenically distinct variants of the 150,000 molecular weight neurofilament subunit exist in somatic and autonomic neurons of adult animals. In addition, the phosphatase treatment suggests that the antigen recognized by CH1 is not masked by phosphorylation. In contrast, all neurons were labelled by this antibody in the peripheral nervous system of newborn rats. It is suggested that CH1 identifies a fetal 150,000 molecular weight neurofilament polypeptide isoform whose expression is prevented by the growth of somatic neurons and is selectively maintained in autonomic and small sensory neurons.

Early structural changes in the axoplasmic cytoskeleton after axotomy studied by cryofixation

Alterations in the cytoskeleton were studied in the axoplasm of neurites at the tips of proximal stumps of transected chicken sciatic nerves. The studies were carried out using cryofixation with a nitrogen-cooled propane jet. The most immediate effect is the almost complete disassembly of axoplasmic microtubules. This consequently causes the axonal transport of membrane-bounded organelles to cease and results in an accumulation of mitochondria and vesicles of the smooth endoplasmic reticulum. The neurofilament network is partially disorganized. Neurofilaments become shorter and fragmented, and are linked by a large number of anastomosed cross-linkers. The neurofilaments become newly aligned to the axis of the axoplasm and are of normal length 48-72 h after the transsection. At this stage the newly formed neurofilament bundles are in close proximity to the anastomosed cisternae and profiles of the smooth endoplasmic reticulum. The axonal sprouts always show a normally organized cytoskeletal network. These studies support the idea that the rapid remodelling of the neurofilament network is apparently a local event, not dependent on the slow transport of cytoskeletal materials to the tip of the proximal stump. The repair of the degraded cytoskeleton may be in accordance with the function of the endoplasmic reticulum as Ca2+-sequestering membrane system, which may be involved in restoring the physiological conditions of the axoplasm.

A rapid HPLC method to separate the triplet proteins of neurofilament

In this article a fast HPLC technique to separate the individual neurofilament proteins is described. Highly pure fractions of the three neurofilament proteins can be obtained. As much as 50 mg of each neurofilament polypeptide can be separated from a crude neurofilament protein preparation in one step in less than 2 h. The short separation time is of importance in minimizing degradation, especially of the 150-kilodalton neurofilament polypeptide.

The multimolecular cascade of spinal cord injury. Studies on prostanoids, calcium, and proteinases

Experimental spinal cord injury in animals induced by weight drop produces neurological deficit and paralysis. Correlation of the progressive morphological changes in the lesion by both light and electron microscopy with the biochemical alterations revealed ischemia, edema, hemorrhage, tissue necrosis, granular changes in axons, vesicular degeneration of myelin and axonal calcification. The biochemical pathology was that of degradation of axonal (neurofilaments) and myelin proteins (MBP and PLP) with increased activities of proteolytic enzymes and particularly the neutral proteinase. The level of total calcium increased progressively in the lesion to a peak at 8 hrs. and subsequently remained constant thereafter. The capacity of calcium for activating proteinases and lipases and fostering the degradation of axon and myelin proteins as well as the liberation of arachidonic acid required for the synthesis of prostanoids must be relevant. An increased production of prostanoids is indicated by elevation of thromboxane (TxB2), a stable metabolite of TXA2 at 1 hour after injury. The 6-keto-PG1(1)a was also increased but to a lesser extent. We suspect that the activation of arachidonic acid metabolism contributes to post-traumatic vascular injury and the progressive ischemia. These putative roles for calcium in proteolysis and lipolysis, inducing degradation of macromolecules and production of prostanoids which initiate edema, lysolecithin a myelinolytic factor and mitochondrial dysfunction in spinal cord injury are discussed.

Distribution of nerve fibers immunoreactive to neurofilament protein in rat molars and periodontium

The distribution of nerve fibers in molars, periodontal ligament and gingiva of the rat shows a complex pattern. Decalcified material including the alveolar bone was sectioned in three different planes and stained by means of immunohistochemistry for detection of the neurofilament protein (NFP); the immunoreactive neural elements were clearly visualized in three-dimensional analyses. NFP-positive nerve fibers formed a subodontoblastic plexus in the roof area of the dental pulp; some of them entered the predentin and dentin directly through the dentinal tubules. This penetration was found mainly in the pulp horn, and was limited to a distance of about 100 micrometers from the pulpo-dentinal junction. In the periodontal ligament, NFP-positive nerve fibers were found densely distributed in the lower half of the alveolar socket. Two types of nerve terminals were recognized in the periodontal ligament: free nerve endings with tree-like ramifications, and expanded nerve terminals showing button- or glove-like shapes. The former tapered among the periodontal fibers, some even reaching the cementoblastic layer. The latter were located, frequently in groups, within the ligament restricted to the lower third of the alveolar socket. A well-developed plexus of NFP-positive nerves was revealed in the lamina propria of the free gingiva, the innervation being denser toward the epithelium of the gingival crevice. The characteristic distribution of NFP-immunoreactive nerve fibers revealed in this study is discussed in relation to region-specific sensations in the teeth and surrounding tissues.

Posttranslational modification of neurofilament polypeptides in rabbit retina

Three polypeptides that compose neurofilaments, designated H, M, and L, are synthesized in the cell bodies of neurons and subsequently conveyed down their axons by the process of slow axonal transport. The axonal form of H, which is a component of the cross bridges between the neurofilaments, is antigenically different from the form in the cell bodies and dendrites. To understand how this special form of H is directed to the axon, and more generally how intracellular differentiation is established and maintained by the selective delivery of different molecular species to different compartments of a cell, we have studied the events that occur immediately after the synthesis of the three neurofilament polypeptides in the retinas of rabbits. We observed that H and M are synthesized in the retina as precursor polypeptides, EH and EM, that migrate markedly faster on SDS polyacrylamide gels than their mature axonal forms. The maturation of these precursors requires more than one day and appears to involve their phosphorylation. Only the electrophoretically mature forms appear in the axons of the retinal ganglion cells in the optic nerve. We consider the following interpretation of these observations. Shortly after they are translated in the cell body, the neurofilament polypeptides become phosphorylated at multiple sites. However, only after they have moved a distance of several hundred micrometers down the axon, H and M are phosphorylated at additional sites, causing their conformation or binding properties to change. This change, which is reflected in the reduction of their electrophoretic mobility and the appearance of new antigenic determinants, may function to alter the H-mediated crossbridges and produces the morphological and structural properties of the neurofilament lattice that is characteristic of axons.

Varying degrees of phosphorylation determine microheterogeneity of the heavy neurofilament polypeptide (Nf-H)

Two-dimensional immunoblots revealed a spectrum of 200 kDa neurofilament polypeptides (Nf-H) of apparent molecular weights ranging from 200 to 170 kDa. The entire spectrum was stained immunocytochemically by three monoclonal antibodies specific for nonphosphorylated neurofilaments, while more restricted staining was revealed by four monoclonal antibodies specific for phosphorylated neurofilament epitopes. Treatment with increasing amounts of phosphatase suggested the existence of various forms of partially phosphorylated neurofilaments that possess phosphoepitopes that differ in their ease of dephosphorylation. Immunoprecipitation in low detergent concentration confirmed the existence of microheterogeneous forms of Nf-H that differed in extent of phosphorylation or in distribution of phosphorylated sites.