Regional variation in the abundance of axonal cytoskeletal proteins

Different Clinical Contexts of Use of Blood Neurofilament Light Chain Protein in the Spectrum of Neurodegenerative Diseases

One of the most pressing challenges in the clinical research of neurodegenerative diseases (NDDs) is the validation and standardization of pathophysiological biomarkers for different contexts of use (CoUs), such as early detection, diagnosis, prognosis, and prediction of treatment response. Neurofilament light chain (NFL) concentration is a particularly promising candidate, an indicator of axonal degeneration, which can be analyzed in peripheral blood with advanced ultrasensitive methods. Serum/plasma NFL concentration is closely correlated with cerebrospinal fluid NFL and directly reflects neurodegeneration within the central nervous system. Here, we provide an update on the feasible CoU of blood NFL in NDDs and translate recent findings to potentially valuable clinical practice applications. As NFL is not a disease-specific biomarker, however, blood NFL is an easily accessible biomarker with promising different clinical applications for several NDDs: (1) early detection and diagnosis (i.e., amyotrophic lateral sclerosis, Creutzfeldt-Jakob disease, atypical parkinsonisms, sporadic late-onset ataxias), (2) prognosis (Huntington's disease and Parkinson's disease), and (3) prediction of time to symptom onset (presymptomatic mutation carriers in genetic Alzheimer's disease and spinocerebellar ataxia type 3).

The third wave: Intermediate filaments in the maturing nervous system

Intermediate filaments are critical for the extreme structural specialisations of neurons, providing integrity in dynamic environments and efficient communication along axons a metre or more in length. As neurons mature, an initial expression of nestin and vimentin gives way to the neurofilament triplet proteins and α-internexin, substituted by peripherin in axons outside the CNS, which physically consolidate axons as they elongate and find their targets. Once connection is established, these proteins are transported, assembled, stabilised and modified, structurally transforming axons and dendrites as they acquire their full function. The interaction between these neurons and myelinating glial cells optimises the structure of axons for peak functional efficiency, a property retained across their lifespan. This finely calibrated structural regulation allows the nervous system to maintain timing precision and efficient control across large distances throughout somatic growth and, in maturity, as a plasticity mechanism allowing functional adaptation.

CSF neurofilament light chain reflects corticospinal tract degeneration in ALS

Objective

Diffusion tensor imaging (DTI) is sensitive to white matter tract pathology. A core signature involving the corticospinal tracts (CSTs) has been identified in amyotrophic lateral sclerosis (ALS). Raised neurofilament light chain protein (NfL) in cerebrospinal fluid (CSF) is thought to reflect axonal damage in a range of neurological disorders. The relationship between these two measures was explored.

Methods

CSF and serum NfL concentrations and DTI acquired at 3 Tesla on the same day were obtained from ALS patients (n = 25 CSF, 40 serum) and healthy, age-similar controls (n = 17 CSF, 25 serum). Within-group correlations between NfL and DTI measures of microstructural integrity in major white matter tracts (CSTs, superior longitudinal fasciculi [SLF], and corpus callosum) were performed using tract-based spatial statistics.

Results

NfL levels were higher in patients compared to controls. CSF levels correlated with clinical upper motor neuron burden and rate of disease progression. Higher NfL levels were significantly associated with lower DTI fractional anisotropy and increased radial diffusivity in the CSTs of ALS patients, but not in controls.

Interpretation

Elevated CSF and serum NfL is, in part, a result of CST degeneration in ALS. This highlights the wider potential for combining neurochemical and neuroimaging-based biomarkers in neurological disease.

Quantitative analysis of the relationship between intra- axonal neurofilament compaction and impaired axonal transport following diffuse traumatic brain injury

Traumatic axonal injury (TAI) following traumatic brain injury (TBI) contributes to morbidity and mortality. TAI involves intra-axonal changes assumed to progress to impaired axonal transport (IAT), disconnection, and axonal bulb formation. Immunocytochemical studies employing antibodies to amyloid precursor protein (APP), a marker of IAT and RMO14, a marker of neurofilament compaction (NFC), have shown that TAI involves both NFC and IAT, with the suggestion that NFC leads to IAT. Recently, new data has suggested that NFC may occur independently of IAT. The objective of this study was to determine quantitatively the precise relationship between NFC and IAT. Following TBI, rats were studied at 30 min, 3 h, and 24 h. Using single-label immunocytochemistry employing the antibodies RM014, APP, or a combined labeling strategy targeting APP/RMO14 in aggregate, the immunoreactive (IR) profiles were counted in the corticospinal tract (CSpT) and medial lemniscus (ML). In the CSpT, the number of axons demonstrating RMO14-IR approximated the number of axons showing APP-IR, with the APP-IR population showing a significant increase over 24 h (p < 0.05). The sum of both single-label counts equaled the aggregate APP/RMO14 numbers, demonstrating little relationship between NFC and IAT. In the ML, 75% of fibers demonstrated a separation of APP-IR and NFC-IR; however, 25% of the ML fibers showed co-localization of APP-IR and RMO14. The results of these studies indicate that, in the majority of damaged axons, NFC is not associated with IAT. Our findings argue for the use of multiple markers when evaluating the extent of TAI or the efficacy of therapies targeting the treatment of TAI.

Administration of the immunophilin ligand FK506 differentially attenuates neurofilament compaction and impaired axonal transport in injured axons following diffuse traumatic brain injury

Traumatic axonal injury (TAI) following traumatic brain injury (TBI) remains a clinical problem for which no effective treatment exists. TAI was thought to involve intraaxonal changes that universally led to impaired axonal transport (IAT), disconnection and axonal bulb formation. However, recent, immunocytochemical studies employing antibodies to amyloid precursor protein (APP), a marker of IAT and antibodies to neurofilament compaction (NFC), RM014, demonstrated that NFC typically occurs independent of IAT, indicating the existence of different populations of damaged axons. FK506 administration has been shown to attenuate IAT. However, in light of the above, the ability of FK506 to attenuate axonal damage demonstrating NFC requires evaluation. The current study explored the potential of FK506 to attenuate both populations of damaged axons. Rats were administered FK506 (3 mg/kg) or vehicle 30 min preinjury. Three hours post-TBI, tissue was prepared for the visualization of TAI using antibodies targeting IAT (APP) or NFC (RMO14) or a combined labeling strategy. Confirming previous reports, FK506 treatment reduced the number of axons demonstrating IAT in the CSpT, from 411 +/- 54.70 to 91.00 +/- 33.87 (P <or= 0.05) and in the ML from 78.62 +/- 16.87 to 41.00 +/- 5.80 (P <or= 0.05). FK506 treatment failed to reduce the number of axons demonstrating NFC in either the CSpT or ML. FK506's failure to attenuate NFC suggests that additional therapeutic agents may be necessary to blunt the full burden of TAI. Because FK506 targets IAT, calcineurin appears to be a major target for neuroprotection in damaged axons demonstrating IAT.

Differential brain growth in the infant born preterm: current knowledge and future developments from brain imaging

Preterm birth is associated with a high prevalence of neuropsychiatric impairment in childhood and adolescence, but the neural correlates underlying these disorders are not fully understood. Quantitative magnetic resonance imaging techniques have been used to investigate subtle differences in cerebral growth and development among children and adolescents born preterm or with very low birth weight. Diffusion tensor imaging and computer-assisted morphometric techniques (including voxel-based morphometry and deformation-based morphometry) have identified abnormalities in tissue microstructure and cerebral morphology among survivors of preterm birth at different ages, and some of these alterations have specific functional correlates. This chapter reviews the literature reporting differential brain development following preterm birth, with emphasis on the morphological changes that correlate with neuropsychiatric impairment.

The internal capsule in neonatal imaging

The internal capsule is highly visible on conventional magnetic resonance imaging (MRI). It is myelinating rapidly at term, and the time course of its maturation is well known. It carries the major motor and sensory pathways to and from the cortex and the spinal cord. Additionally, fibres from the thalamus pass through it connecting to most regions of the cortex. It is therefore of vital importance, and damage to it has severe consequences. Its abnormal appearance on conventional MRI is a good predictor of an abnormal motor outcome in different clinical situations encountered in perinatal medicine. Its normal appearance on conventional MR images at term age is usually associated with a relatively normal motor outcome. More recently, diffusion-weighted and diffusion tensor imaging have allowed a much more sophisticated assessment of its maturation and connectivity; this has already led to a better understanding of how its development is affected by preterm birth and by hypoxic-ischaemic brain injury. Future studies will assess the relevance of these findings not only for motor outcome but also for cognitive, visual and sensory abilities.

Cerebrospinal fluid biomarkers in experimental spinal nerve root injury

STUDY DESIGN

Cerebrospinal fluid biomarkers were evaluated in a setup using established pig models to mimic clinical disc herniation.

OBJECTIVES

To investigate biomarkers for nerve tissue injury, inflammation, and pain in cerebrospinal fluid after mechanical compression and/or nucleus pulposus application to spinal nerve roots.

SUMMARY OF BACKGROUND DATA

The association between mechanical compression, biochemical effects of nucleus pulposus, and nerve root injury in degenerative disc disorders is incompletely investigated.

METHODS

The unilateral S1 nerve root was exposed in 20 pigs. The animals were divided into four groups (n = 5 each): 1) slow-onset mechanical compression with an ameroid constrictor; 2) autologous nucleus pulposus application; 3) mechanical compression plus nucleus pulposus; and 4) sham operation. After 1 week, 6 mL of cerebrospinal fluid was collected, and four structural nerve proteins, neurofilaments, S-100, glial fibrillary acidic protein, neuron-specific enolase, the proinflammatory cytokine interleukin-8, the neurotransmitter nociceptin, and substance P endopeptidase activity were analyzed using immunoassays.

RESULTS

The concentration of neurofilament was increased in the mechanical compression group (17.0 microg/L +/- 5.0) and in the mechanical compression plus nucleus pulposus group (19.8 +/- 12.1 microg/L) compared with the sham group (0.9 +/- 0.9 microg/L) and the nucleus pulposus group (0.4 +/- 0.1 microg/L) (P < 0.01 for both). The concentration of nociceptin was increased significantly in the mechanical compression group (24.0 +/- 8.6 fm/mL) and in the mechanical compression plus nucleus pulposus group (31.2 +/- 6.6 fm/mL) compared with the sham group (7.0 +/- 1.3 fm/mL) (P < 0.05 and P < 0.01, respectively). A correlation was found between concentrations of neurofilament and nociceptin (r = 0.50, P < 0.05). There were no intergroup differences regarding glial fibrillary acidic protein, neuron-specific enolase, S-100, interleukin-8, or substance P endopeptidase activity.

CONCLUSIONS

The present study demonstrates increased concentrations of neurofilament and nociceptin in cerebrospinal fluid after nerve root compression. A simultaneous application of nucleus pulposus did not increase the response.

Selective changes in the neurofilament and microtubule cytoskeleton of NF-H/LacZ mice

This study focused mainly on changes in the microtubule cytoskeleton in a transgenic mouse where beta-galactosidase fused to a truncated neurofilament subunit led to a decrease in neurofilament triplet protein expression and a loss in neurofilament assembly and abolished transport into neuronal processes in spinal cord and brain. Although all neurofilament subunits accumulated in neuronal cell bodies, our data suggest an increased solubility of all three subunits, rather than increased precipitation, and point to a perturbed filament assembly. In addition, reduced neurofilament phosphorylation may favor an increased filament degradation. The function of microtubules seemed largely unaffected, in that tubulin and microtubule-associated proteins (MAP) expression and their distribution were largely unchanged in transgenic animals. MAP1A was the only MAP with a reduced signal in spinal cord tissue, and differences in immunostaining in various brain regions corroborate a relationship between MAP1A and neurofilaments.

Transglutaminase bonds in neurofibrillary tangles and paired helical filament tau early in Alzheimer's disease

Transglutaminase-catalyzed epsilon(gamma-glutamyl)lysine cross-links exist in Alzheimer's disease (AD) paired helical filament (PHF) tau protein but not normal soluble tau. To test the hypothesis that these cross-links could play a role in the formation of neurofibrillary tangles (NFT), we used single- and double-label immunofluorescence confocal microscopy and immunoaffinity purification and immunoblotting to examine epsilon(gamma-glutamyl)lysine cross-links in AD and control brains. The number of neurons that are immunoreactive with an antibody directed at the epsilon-(gamma-glutamyl)lysine bond was significantly higher in AD cortex compared with age-matched controls and schizophrenics. PHF tau-directed antibodies AT8, MC-1 and PHF-1 co-localized with epsilon(gamma-glutamyl)lysine immunolabeling in AD NFT. Immunoaffinity purification and immunoblotting experiments demonstrated that PHF tau contains epsilon(gamma-glutamyl)lysine bonds in parietal and frontal cortex in AD. In control cases with NFT present in the entorhinal cortex and hippocampus, indicative of Braak and Braak stage II, epsilon(gamma-glutamyl)lysine bonds were present in PHF tau in parietal and frontal cortex, despite the lack of microscopically detectable NFT or senile plaques in these cortical regions. The presence of PHF tau with epsilon(gamma-glutamyl)lysine bonds in brain regions devoid of NFT in stage II (but regions, which would be expected to contain NFT in stage III) suggests that these bonds occur early in the formation of NFT.

Markers of nerve tissue injury in the cerebrospinal fluid in patients with lumbar disc herniation and sciatica

STUDY DESIGN

The light subunit of neurofilament protein, S-100 protein, neuron-specific enolase, and glial fibrillary acidic protein were determined in the cerebrospinal fluid in patients with lumbar disc herniation and in control patients.

OBJECTIVES

To determine whether nerve root injury caused by disc herniation increases the levels of nerve and glial cell injury markers in the cerebrospinal fluid.

SUMMARY OF BACKGROUND DATA

Markers of nerve tissue injury can be analyzed in the cerebrospinal fluid, allowing characterization of the cell types involved and the degree of disease in patients with neurologic disorders.

METHODS

Cerebrospinal fluid samples were obtained by preoperative lumbar puncture in patients who underwent surgery for lumbar disc herniation and in patients who underwent lower extremity surgery (control group), neurofilament protein (light subunit) and glial fibrillary acidic protein were analyzed by enzyme-linked immunosorbent assay and S-100 protein and neuron-specific enolase by radioimmunoassay and luminescence immunoassay, respectively. In the disc herniation group the concentrations of the four markers were evaluated regarding possible correlation to patient history, computed tomographic findings, and clinical findings.

RESULTS

Cerebrospinal fluid concentration of neurofilament protein (light subunit) and S-100 were increased in the disc herniation group compared with that in control subjects (1158 +/- 383 ng/L vs. 152 +/- 14 ng/L, P < 0.01; 1963 +/- 231 ng/L vs. 1003 +/- 152 ng/L, P < 0.05, respectively). No statistical differences in neuron-specific enolase and glial fibrillary acidic protein concentrations were observed between the groups. Disc herniation patients with fewer than 3 months' duration of subjective symptoms had higher neurofilament protein levels than did patients with longer duration. None of the markers was related to preoperative clinical or computed tomographic findings. Patients with persistent neurologic findings at follow-up 2-3 months after surgery had higher levels of neurofilament protein before surgery compared with-those without sequelae.

CONCLUSIONS

Patients with disc herniation and sciatica have increased concentrations of neurofilament protein and S-100 in the cerebrospinal fluid, which indicates damage of axons and Schwann cells in the affected nerve root.

A mechanistic analysis of nondisruptive axonal injury: a review

Axons are particularly at risk in human diffuse head injury. Use of immunocytochemical labeling techniques has recently demonstrated that axonal injury (AI) and the ensuing reactive axonal change is, probably, more widespread and occurs over a longer posttraumatic time in the injured brain than had previously been appreciated. But the characterization of morphologic or reactive changes occurring after nondisruptive AI has largely been defined from animal models. The comparability of AI in animal models to human diffuse AI (DAI) is discussed and the conclusion drawn that, although animal models allow the analysis of morphologic changes, the spatial distribution within the brain and the time course of reactive axonal change differs to some extent both between species and with the mode of brain injury. Thus, the majority of animal models do not reproduce exactly the extent and time course of AI that occurs in human DAI. Nonetheless, these studies provide good insight into reactive axonal change. In addition, there is developing in the literature considerable variance in the terminology applied to injured axons or nerve fibers. We explain our current understanding of a number of terms now present in the literature and suggest the adoption of a common terminology. Recent work has provided a consensus that reactive axonal change is linked to pertubation of the axolemma resulting in disruption of ionic homeostatic mechanisms within injured nerve fibers. But quantitative data for changes for different ion species is lacking and is required before a better definition of this homeostatic disruption may be provided. Recent studies of responses by the axonal cytoskeleton after nondisruptive AI have demonstrated loss of axonal microtubules over a period up to 24 h after injury. The biochemical mechanisms resulting in loss of microtubules are, hypothetically, mediated both by posttraumatic influx of calcium and activation of calmodulin. This loss results in focal accumulation of membranous organelles in parts of the length of damaged axons where the axonal diameter is greater than normal to form axonal swellings. We distinguish, on morphologic grounds, between axonal swellings and axonal bulbs. There is also a growing consensus regarding responses by neurofilaments after nondisruptive AI. Initially, and rapidly after injury, there is reduced spacing or compaction of neurofilaments. This compaction is stable over at least 6 h and results from the loss or collapse of neurofilament sidearms but retention of the filamentous form of the neurofilaments. We posit that sidearm loss may be mediated either through proteolysis of sidearms via activation of microM calpain or sidearm dephosphorylation via posttraumatic, altered interaction between protein phosphatases and kinase(s), or a combination of these two, after calcium influx, which occurs, at least in part, as a result of changes in the structure and functional state of the axolemma. Evidence for proteolysis of neurofilaments has been obtained recently in the optic nerve stretch injury model and is correlated with disruption of the axolemma. But the earliest posttraumatic interval at which this was obtained was 4 h. Clearly, therefore, no evidence has been obtained to support the hypothesis that there is rapid, posttraumatic proteolysis of the whole axonal cytoskeleton mediated by calpains. Rather, we hypothesize that such proteolysis occurs only when intra-axonal calcium levels allow activation of mM calpain and suggest that such proteolysis, resulting in the loss of the filamentous structure of neurofilaments occurs either when the amount of deformation of the axolemma is so great at the time of injury to result in primary axotomy or, more commonly, is a terminal degenerative change that results in secondary axotomy or disconnection some hours after injury.

Node-paranode regions as local degradative centres in alpha-motor axons

Lysosomes play an important role for the maintenance of a normal internal milieu in the cell. In neurons lysosomes are abundant in the perikaryon and dendrites, but have been observed to a much lesser degree in the axon. A general opinion has therefore formed among biologists interested in the nervous system that axonal material destined for degradation has to be transported to the neuronal perikaryon. The lysosomal occurrence and distribution at the level of the axon have, however, not been investigated systematically. This review summarizes recent morphological data based on light, fluorescence, and electron microscopic observations in peripheral nerve fibres of cats and rats providing evidence that node-paranode regions mainly along the peripheral parts of alpha motor axons, where the axon cross-section area decreases to 10-25% of internodal values, can control the passage and participate in a lysosome-mediated degradation of axonally transported materials directed towards the neuronal perikaryon. An important role is played by the paranodal axon-Schwann cell networks, which are lysosome-rich entities whereby the Schwann cells can sequester material from the axoplasm of large myelinated peripheral nerve fibres. The networks also seem to serve as depots for axonal waste products. The degradative ability of node-paranode regions in alpha-motor axons could be of some significance for the protection of the motor neuron perikarya from being flooded with and perhaps injured by indigestible materials as well as potentially deleterious, exogenous substances imbibed by the axon terminals in the muscle. A similar degradative capacity may not be needed in nerve fibres with synaptic terminals in the CNS where the local environment is regulated by the blood-brain barrier.

The organization of neurofilaments accumulated in perikaryon following aluminum administration: relationship between structure and phosphorylation of neurofilaments

Neurofilaments accumulated in perikarya and dendrites of anterior horn cells and Purkinje cells of rabbit treated by aluminum chloride were analysed with a variety of techniques. Four different monoclonal antibodies against phosphorylated and nonphosphorylated epitopes on neurofilament H subunit were used to compare phosphorylation state of these accumulated neurofilaments with that of axonal neurofilaments. Although immunoblotting revealed no significant difference in phosphorylation between control and aluminum-treated brains, accumulated neurofilaments were immunocytochemically more phosphorylated than control perikaryal or dendritic neurofilaments. With detailed analysis of cryothin-section immunogold labeling, accumulated neurofilaments were, however, significantly less phosphorylated than axonal neurofilaments. With quick-freeze deep etching, core filaments of accumulated neurofilaments are as dense as axonal neurofilaments but much less regularly aligned. Cross-bridges of accumulated neurofilaments were less frequent and more branched than those of axonal neurofilaments, and when examined with combined immunocytochemistry and deep etching, were less phosphorylated. These results suggest that there is a relationship between the phosphorylation and the structural organization of neurofilaments. The phosphorylation of neurofilament H subunit may be necessary for formation of frequent and straight cross-bridges and resulting regular alignment of core filaments.

Dephosphorylation of the largest neurofilament subunit protein influences the structure of crossbridges in reassembled neurofilaments

Phosphorylation-dependent change in electrophoretic mobility is the most unique characteristic of NF-H, the largest molecular mass subunit of the neurofilament. We dephosphorylated NF-H using Escherichia coli alkaline phosphatase, then reassembled it into neurofilaments with NF-M and NF-L, and into NF-H filaments with NF-H alone. We compared these dephosphorylated filaments with control: projections by low-angle rotary-shadow, crossbridges by quick-freeze deep-etch, and core filament packing density by thin-section electron microscopy. Projections in the dephosphorylated filaments were basically similar in structure to those in control, although there was a tendency for them to be wider and less dense, especially in NF-H filaments. Dephosphorylated filaments were still able to form crossbridges between core filaments, but their crossbridges were significantly wider, less dense, more branched and more irregular than crossbridges in control, and core filaments were more densely packed. These structural differences may be brought about by the removal of phosphate groups from NF-H tail and consequent reduction of electrostatic repulsion between adjacent crossbridges extending from the same core filament. The results indicate that phosphorylation of NF-H is necessary for forming well developed crossbridges, straight and at constant intervals, like those of in vivo axonal neurofilaments.

Phosphorylation of neurofilament H subunit as related to arrangement of neurofilaments

To find out what causes differences in phosphorylation states in neurofilaments (NF), we selected two types of dendrite, one provided with very few NFs (Purkinje cell) and the other with relatively many (anterior horn cell). We examined these with four monoclonal antibodies selected by the Western blot analysis, two (NE14 and SMI31) recognizing only phosphorylated, SMI32 recognizing only nonphosphorylated, and N52 recognizing phosphorylation-independent epitopes of NF-H. The immunoperoxidase labeling of dendrites, and also of perikarya, in both neurons was detectable with all four antibodies. After the tissue was treated with Triton X-100, the labeling was still detectable with SMI32 or N52, but undetectable with NE14 and SMI31. The brain homogenate Triton-extracted supernatant after centrifugation at 100,000g for 1 hr showed the staining of NE14, SMI31, and N52 but not that of SMI32. In Purkinje cell dendrite and perikaryon, NFs always appeared singly. In the immunogold labeling, they were labeled only with SMI32 or N52. Labeling by NE14 or SMI31 was distributed throughout the cytoplasm and hardly associated with NFs. In the anterior horn cell dendrite and perikaryon, NFs appeared both singly and in bundles. They were predominantly labeled with SMI32 or N52 when they were single, and with NE14, SMI31, or N52 when they were bundled. Even in one NF, portions that appeared single were labeled mostly with SMI32 or N52, while the remainder, to which other NFs approached closely, were labeled mostly with NE14, SMI31, or N52. Thus, when NFs appear singly, NF-H in their projections or cross-bridges with other organelles is not phosphorylated, while when NFs are bundled, NF-H is phosphorylated in crossbridges between NF core filaments. These data may explain why the NF-H is heavily phosphorylated in axons, where NFs are abundant, and not in dendrites and perikarya, where NFs are sparse.

Transport of cytoskeletal proteins in axons of hippocampal pyramidal cells

Axonal transport of cytoskeletal proteins has not yet been extensively studied in the brain proper, in contrast to the peripheral nerves and optic nerves. The authors have developed a means for the study of transport of cytoskeletal proteins in axons of hippocampal pyramidal cells. Proteins of intrinsic neurons of the dorsal hippocampus were labeled by microinjection of 35S methionine, and the subsequent transport of labeled proteins was characterized in the axons projecting into the fimbria-fornix. A peak of labeled proteins was present in the fimbria-fornix at 4-12 days after labeling, corresponding to transport rates 0.2-0.7 mm/day. The most abundant proteins at each time studied exhibited one-dimensional electrophoretic mobilities of actin and tubulin; neurofilaments were less intensely labeled. The observed specializations of cytoskeletal transport, especially the paucity of tubulin transport at rates of 2-4 mm/day, may predispose hippocampal pyramidal cells to accumulate tubulin and microtubule-associated proteins in their cell bodies in various disease states.

Regional distribution and biochemical characteristics of high molecular weight tau in the nervous system

The present study examined the distribution of the high molecular weight (HMW) tau protein isoform in the nervous system by immunoblotting and immunohistochemistry. Some of the biochemical properties of this 110 kDa tau protein were explored, including its heat stability, phosphorylation and partitioning with cold/Ca2+ stable vs. soluble microtubules. Qualitative western blot analysis revealed that HMW tau is preferentially expressed in neurons with peripherally projecting axons. For example, this isotype was present in sciatic nerve, ventral and dorsal roots, trigeminal nerve, vagus nerve, dorsal root ganglia (DRG) and spinal cord, but was present in only trace amounts in CNS regions. Another tau isoform of slightly smaller size (90-100 kDa), termed mid-molecular weight (MMW) tau, was present in abundant quantity in optic nerve samples and detectable in several other CNS regions, including hippocampus and cerebellum. The 110 kDa HMW tau as well as MMW tau and the other tau isoforms were found to be heat stable proteins. The HMW and MMW tau isoforms preferentially partitioned with the cold and Ca+2 insoluble tubulin fraction, but the association of HMW tau with stable microtubules was very susceptible to proteolysis. Dephosphorylation of fresh tissue with alkaline phosphatase produced no apparent shift in the mobility of HMW tau on SDS-PAGE but did alter the mobility of other brain tau isoforms, including MMW tau. Immunocytochemical staining with tau-1 antibody in the DRG, which contains HMW tau but no other tau isotypes, showed localization to mainly small neurons and was not altered by dephosphorylation of the histological sections.(ABSTRACT TRUNCATED AT 250 WORDS)