Differential expression of the three neurofilament polypeptides

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.

Cytoplasmic mechanisms of axonal and dendritic growth in neurons

The structural mechanisms responsible for the gradual elaboration of the cytoplasmic elongation of neurons are reviewed. In addition to discussing recent work, important older work is included to inform newcomers to the field how the current perspective arose. The highly specialized axon and the less exaggerated dendrite both result from the advance of the motile growth cone. In the area of physiology, studies in the last decade have directly confirmed the classic model of the growth cone pulling forward and the axon elongating from this tension. Particularly in the case of the axon, cytoplasmic elongation is closely linked to the formation of an axial microtubule bundle from behind the advancing growth cone. Substantial progress has been made in understanding the expression of microtubule-associated proteins during neuronal differentiation to stiffen and stabilize axonal microtubules, providing specialized structural support. Studies of membrane organelle transport along the axonal microtubules produced an explosion of knowledge about ATPase molecules serving as motors driving material along microtubule rails. However, most aspects of the cytoplasmic mechanisms responsible for neurogenesis remain poorly understood. There is little agreement on mechanisms for the addition of new plasma membrane or the addition of new cytoskeletal filaments in the growing axon. Also poorly understood are the mechanisms that couple the promiscuous motility of the growth cone to the addition of cytoplasmic elements.

The Purkinje cell in olivopontocerebellar atrophy. A Golgi and immunocytochemical study

Purkinje cells were examined in three familial cases of olivopontocerebellar atrophy (OPCA) by means of the Golgi method, and neurofilament and calcium-binding protein immunocytochemistry. Reduced dendritic arborizations, as seen with different techniques, early formation of axonal spheroids, and abnormal accumulation of phosphorylated neurofilament epitopes in dendrites, somata and axonal spheroids, together with limited formation of proximal spine-like protrusions were the main changes in Purkinje cells. These lesions are unlikely to be the consequence of anterograde degeneration secondary to olivary atrophy, as postulated by some investigators, but probably represent primary damage to Purkinje cells in patients with OPCA. Reduced dendritic arborizations result in a decrease of receptor sites for parallel fibres and deprive granule cells of their main targets. Abnormal accumulation of neurofilaments in somata, dendrites and axonal spheroids may contribute to an abnormal transport and may impair protein turnover in the distal regions of Purkinje cells.

Neurofilament-deficient axons and perikaryal aggregates in viable transgenic mice expressing a neurofilament-beta-galactosidase fusion protein

Interactions between neurofilament side arms may modulate axon caliber. To investigate this hypothesis, we derived transgenic mice expressing a fusion protein in which the carboxyl terminus of the high molecular weight neurofilament protein (NFH) was replaced by beta-galactosidase. The transgene, regulated by NFH sequences, was expressed in projection neurons. However, the fusion protein remained in perikarya precipitating large filamentous aggregates. Axons were not invested with neurofilaments and developed only small calibers. Perikaryal aggregates, with similar structural features, are associated with neurodegenerative diseases, but these mice showed few ill effects and their neurons rarely degenerated. We conclude that an organized neurofilament cytoskeleton is required by axons to achieve large calibers but is not essential for neuronal function or extended survival.

Neurofilaments in rat and cat spinal cord; a comparative immunocytochemical study of phosphorylated and non-phosphorylated subunits

Neurofilament immunoreactivity was examined in spinal cords of rats and cats with antibodies to all three subunits (68 kD, 155 kD and 200 kD) and to different phosphorylation states of 200 kD. NFHP-, an antibody against non-phosphorylated 200 kD, labelled all rat neuronal perikarya but failed to label cat neurofilaments. In both species, the perikarya and processes of motoneurones were immunoreactive for all three subunits but most dorsal horn neuronal perikarya were not immunoreactive for 68 kD and 155 kD. Motoneuronal perikarya and proximal processes showed filamentous labelling for 68 kD but not for 155 kD in the rat, while in neither species did these show labelling with RT97, an antibody against a highly phosphorylated form of 200 kD; immunoreactivity for 200 kD was present in both filamentous (probably partially phosphorylated) and non-filamentous (non-phosphorylated) forms, but in dorsal horn neurones only the latter was present. Interpretations consistent with this data are: in rat and possibly also cat, motoneuronal neurofilaments consist of a 68 kD backbone with partially phosphorylated 200 kD sidearms, with both 155 kD and 200 kD (nonphosphorylated) subunits in a non-filamentous form; this neurofilament becomes more highly phosphorylated along the proximal processes. The dorsal horn neurones probably contain 200 kD in a non-filamentous form but may lack the other subunits.

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

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

Developmental changes in the heavy subunit of neurofilaments in the corpus callosum of the cat

In the corpus callosum of the cat, the heavy subunit of neurofilaments (NFH) can be demonstrated with the monoclonal antibody NE14, as early as P11, not at P3, and only in a few axons. At P18-19 and more markedly at P29, many more callosal axons have become positive to NE14 and this is similar to what is found in the adult. In contrast, callosal axons become positive to the neurofilament antibody SMI-32 only between P29 and P39 and remain positive in the adult. Treatment with alkaline phosphatase prevents axonal staining with NE14, but results in SMI-32 staining of a few callosal axons as early as P11, but not at P3. Between P11 and P19 the number of axons stained with SMI-32 after alkaline phosphatase treatment increases, in parallel with that of axons stained with NE14. Thus NE14 appears to recognize a phosphorylated form of NFH, while SMI-32 appears to recognize an epitope of NFH which is either masked by phosphate or inaccessible until between P29 and P39, unless the tissue is treated with alkaline phosphatase. These two forms of NFH appear towards the end of the period of massive developmental elimination of callosal axons. They are also synchronous with changes in the spacing of neurofilaments quantified in a separate ultrastructural study. These cytoskeletal changes may terminate the juvenile-labile state of callosal axons and allow further axial growth of the axon.

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

We used immunoblot and immunocytochemical methodologies to characterize the appearance and intracellular localization of the high molecular weight neurofilament subunit (NF-H) within the Triton-insoluble cytoskeleton during the first 5 days of differentiation of mouse NB2a/d1 neuroblastoma cells. Hypophosphorylated and partially phosphorylated forms of NF-H were detected in cells before and throughout differentiation. By contrast, some extensively phosphorylated forms of NF-H were first detected on the third day of differentiation and at least one additional 200 kDa isoform was visualized in cytoskeletons only after five days of differentiation. Extensively phosphorylated forms of NF-H were restricted to axonal neurites; by contrast, hypophosphorylated and partially phosphorylated forms of NF-H were present throughout undifferentiated and differentiated cells.

Developmentally regulated epitopes on a neurofilament protein visualized by monoclonal antibodies

Two monoclonal antibodies (mabs) which recognized the 68 kDa subunit of the rat neurofilament triplet were isolated. In immunoblots with SDS-solubilized and reduced proteins, these mabs recognized their epitopes equally well in embryonic and in adult tissue. However, these epitopes were developmentally regulated in paraformaldehyde-fixed rat brain sections. They were abundant in all compartments of differentiating neurons, whereas in mature neurons their presence was markedly attenuated, with a moderate abundance in perikarya and larger dendrites and low concentrations in axons. Thus, a differential developmental modification, possibly involving the masking of an epitope, is demonstrated for the small neurofilament polypeptide in rat and monkey brain tissue.

On the role of the 200-kDa neurofilament protein at the developing neuromuscular junction

To examine whether the 200-kDa neurofilament protein (200K NFP) is involved in mechanically stabilizing axons, we studied the developmental appearance of immunoreactivity to nonphosphorylated and phosphorylated 200K NFP at the neuromuscular junction. Polyinnervated rat muscle fibers become singly innervated during the first 3 weeks of postnatal life through the process of synapse elimination. If production or post-translational modification of the 200K NFP is actively involved in imparting mechanical stability on neuromuscular synapses, then the selective presence of this protein in only one of several axons at each developing end plate region might make that one axon selectively resistant to elimination. The remaining axons would then be eliminated. Immunoreactivity to the 200K NFP is present on Gestational Day 14 and can be seen in more than one preterminal axon in the end plate region of a muscle fiber during the period of synapse elimination. These results suggest that the 200K NFP is present and phosphorylated early in development and, although the 200K NFP may increase the mechanical stability of axons, this increased stability does not determine the final outcome of synapse elimination.

The development of the corpus callosum in cats: a light- and electron-microscopic study

Changes in the size and shape of the corpus callosum (CC)--and in number, size, and structure of callosal axons--between embryonic day 38 (E38) and postnatal day 150 (P150) were studied by light and electron microscope in 25 kittens. The development of the CC was divided into three phases: 1. Embryonic development (E38, 53, 58): At E38, only part of the body of the CC was formed. At E53 and E58, the CC was still very short, but its different parts (genu, body, and splenium) had formed. The cross-sectional callosal area (CCA) was 5.4 mm2 at E53 and 5.6 mm2 at E58. The CC contained 46.3 and 56.4 million axons at E53 and E58 respectively. Mean axon diameters were 0.26 micron at E53 and 0.27 micron at E58. 2. Early postnatal development (P4, 9, 15, 18, 21, 26): The CC at P4 was much longer than at E58 and still slightly elongated during this phase; CCA reached 8.55 mm2 at P4 and 8.88 mm2 at P26. There was a substantial axonal loss (66.8 million at P4 and 52.6 million at P26). From P15 onward, premyelinated and myelinated axons were seen. Mean axon diameter increased from 0.30 micron at P4 to 0.33 micron at P26. 3. Late postnatal development (P39, 57, 92, 107, 150). The CC grew dramatically in both length and thickness, the latter especially in the genu. CCA was 10.1 mm2 at P39 and 15.3 mm2 at P150. The number of axons still decreased (46.5 million at P39 and 31.9 million at P150). The growth of the CCA paralleled the increase of myelinated axons (0.5% at P26 and 29.6% at P150 and in the mean axon diameters (0.34 micron at P39 and 0.42 micron at P150). A number of axonal ultrastructural peculiarities (electron-dense bodies, large vacuoles, lamellated bodies, etc., including those mentioned below) were noticed; their frequency at different ages was estimated as the percent of total axons. Interestingly, accumulations of vesicles inside axons increased from 4.1% at E53 to 8.9% at P26, dropped to 0.2% at P39, and remained below 1% thereafter. Swollen mitochondria increased from 0.2% at E53 to 0.9% at P26 and dropped to 0.06% (on the average) from P39 onward. Accumulations of vesicles and swollen mitochondria increased during the phase of rapid axonal elimination; thus, they may indicate axonal retraction and/or degeneration. Microglia-gitter cells and astrocytes showing signs of phagocytosis were found during the embryonic and early postnatal development and may be involved in axon elimination.

Differential expression of neurofilament subunits in the developing corpus callosum

In the course of development, corticocortical axons seem to first appear in a labile state from which they either mature into a stable state or are eliminated. These state transitions may be related to cytoskeletal modifications. By immunohistochemistry and immunobiochemistry we found that, in the corpus callosum of the cat, the heavy (200 kDa) subunit of neurofilaments (NF) becomes progressively more visible during the first postnatal month. This aspect of cytoskeletal maturation parallels the developmental loss of callosal axons, i.e. probably the stabilization of the axons which are not eliminated. A similar maturation of the heavy subunit was observed in the visual cortical areas 17 and 18. The medium (150 kDa) and to a lesser extent the light (70 kDa) NF subunits are already present a few days after birth.

Monoclonal antibodies to human neuron-specific enolase reveal heterogeneity of the enzyme in neurons of the central nervous system

Three monoclonal antibodies to human neuron-specific enolase (NSE) were used to survey the human brain and spinal cord for immunoreactivity. Two of the antibodies (EB and CF) recognized the same population of cells and cell processes. Reactivity was restricted to myelinated axons, basket cell bodies and processes, and a small population of pyramidal cell bodies in the visual cortex. The third antibody (AD) reacted with some, but not all, of the neuronal cell bodies in cerebral cortex, hippocampus, midbrain, and spinal cord. Many neurons did not react with any of the antibodies. The epitope recognized by AD was trypsin-sensitive, while those recognized by EB and CF were not. These studies suggest that NSE may have multiple conformational or structural forms which are segregated between the cell body and axon.

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.

Distribution of N-CAM in synaptic and extrasynaptic portions of developing and adult skeletal muscle

Previous studies of denervated and cultured muscle have shown that the expression of the neural cell adhesion molecule (N-CAM) in muscle is regulated by the muscle's state of innervation and that N-CAM might mediate some developmentally important nerve-muscle interactions. As a first step in learning whether N-CAM might regulate or be regulated by nerve-muscle interactions during normal development, we have used light and electron microscopic immunohistochemical methods to study its distribution in embryonic, perinatal, and adult rat muscle. In embryonic muscle, N-CAM is uniformly present on the surface of myotubes and in intramuscular nerves; N-CAM is also present on myoblasts, both in vivo and in cultures of embryonic muscle. N-CAM is lost from the nerves as myelination proceeds, and from myotubes as they mature. The loss of N-CAM from extrasynaptic portions of the myotube is a complex process, comprising a rapid rearrangement as secondary myotubes form, a phase of decline late in embryogenesis, a transient reappearance perinatally, and a more gradual disappearance during the first two postnatal weeks. Throughout embryonic and perinatal life, N-CAM is present at similar levels in synaptic and extrasynaptic regions of the myotube surface. However, N-CAM becomes concentrated in synaptic regions postnatally: it is present in postsynaptic and perisynaptic areas of the muscle fiber, both on the surface and intracellularly (in T-tubules), but undetectable in portions of muscle fibers distant from synapses. In addition, N-CAM is present on the surfaces of motor nerve terminals and of Schwann cells that cap nerve terminals, but absent from myelinated portions of motor axons and from myelinating Schwann cells. Thus, in the adult, N-CAM is present in synaptic but not extrasynaptic portions of all three cell types that comprise the neuromuscular junction. The times and places at which N-CAM appears are consistent with its playing several distinct roles in myogenesis, synaptogenesis, and synaptic maintenance, including alignment of secondary along primary myotubes, early interactions of axons with myotubes, and adhesion of Schwann cells to nerve terminals.