Selective solubilization of high-molecular-mass neurofilament subunit during nerve regeneration

Neuroplastic change of cytoskeleton in inferior colliculus after auditory deafferentation

Neural plasticity is a characteristic of the brain that helps it adapt to changes in sensory input. We hypothesize that auditory deafferentation may induce plastic changes in the cytoskeleton of the neurons in the inferior colliculus (IC). In this study, we evaluated the dynamic status of neurofilament (NF) phosphorylation in the IC after hearing loss. We induced auditory deafferentation via unilateral or bilateral cochlear ablation in rats, aged 4 weeks. To evaluate cytoskeletal changes in neurons, we evaluated mRNA fold changes in NF heavy chain expression, non-phosphorylated NF protein fold changes using SMI-32 antibody, and the ratio of SMI-32 immunoreactive (SMI-32-ir) neurons to the total neuronal population in the IC at 4 and 12 weeks after deafness. In the bilateral deafness (BD) group, the ratios of SMI-32-ir neurons significantly increased at 4 weeks after ablation in the right and left IC (6.1 ± 4.4%, 5.0 ± 3.4%, respectively), compared with age-matched controls (P < 0.01, P < 0.01). At 12 weeks after ablation, the ratio of SMI-32 positive neurons was higher (right, 3.4 ± 2.0%; left, 3.2 ± 2.3%) than that in the age-matched control group, albeit not significant in the right and left side (P = 0.38, P = 0.24, respectively). Consistent with the results of the ratio of SMI-32-ir neurons, SMI-32-ir protein expression was increased at 4 weeks after BD, and the changes at 12 weeks after bilateral ablation were not significant in the right or left IC. The age-matched control fold changes of NF mRNA expression after bilateral deafness were not significant at 4 and 12 weeks after deafness in right and left IC. Unilateral deafness did not induce significant change of NF mRNA expression, SMI-32-ir protein expression, and the ratio of SMI-32-ir neurons in the IC at 4 and 12 weeks after hearing loss. Bilateral auditory deafferentation induces structural changes in the neuronal cytoskeleton within the IC, which is prominent at 4 weeks after BD. The structural remodeling of neurons stabilized at 12 weeks after BD. Unlike BD, unilateral auditory deafferentation did not affect the dynamic status of NFs in the IC.

Neurofilament heavy chain expression and neuroplasticity in rat auditory cortex after unilateral and bilateral deafness

Deafness induces many plastic changes in the auditory neural system. For instance, dendritic changes cause synaptic changes in neural cells. SMI-32, a monoclonal antibody reveals auditory areas and recognizes non-phosphorylated epitopes on medium- and high-molecular-weight subunits of neurofilament proteins in cortical pyramidal neuron dendrites. We investigated SMI-32-immunoreactive (-ir) protein levels in the auditory cortices of rats with induced unilateral and bilateral deafness. Adult male Sprague-Dawley rats were divided into unilateral deafness (UD), bilateral deafness (BD), and control groups. Deafness was induced by cochlear ablation. All rats were sacrificed, and the auditory cortices were harvested for real-time quantitative polymerase chain reaction (RT-qPCR) and western blot analyses at 2, 4, 6, and 12 weeks after deafness was induced. Immunohistochemical staining was performed to evaluate the location of SMI-32-ir neurons. Neurofilament heavy chain (NEFH) mRNA expression and SMI-32-ir protein levels were increased in the BD group. In particular, SMI-32-ir protein levels increased significantly 6 and 12 weeks after deafness was induced. In contrast, no significant changes in protein level were detected in the right or left auditory cortices at any time point in the UD group. NEFH mRNA level decreased at 4 weeks after deafness was induced in the UD group, but recovered thereafter. Taken together, BD induced plastic changes in the auditory cortex, whereas UD did not affect the auditory neural system sufficiently to show plastic changes, as measured by neurofilament protein level.

Axonal outgrowth and dendritic plasticity in the cortical peri-infarct area after experimental stroke

BACKGROUND AND PURPOSE

Axonal remodeling is critical to brain repair after stroke. The present study investigated axonal outgrowth after stroke and the signaling pathways mediating axonal outgrowth in cortical neurons.

METHODS

Using a rodent model of middle cerebral artery occlusion, we examined high-molecular weight neurofilament (NFH) immunoreactive axons and myelin basic protein-positive oligodendrocytes in the peri-infarct area. In vitro, using cultured cortical neurons in a microfluidic chamber challenged by oxygen-glucose deprivation (OGD), we investigated mechanisms selectively regulating axonal outgrowth after OGD.

RESULTS

NFH(+) axons and MBP(+) oligodendrocytes substantially increased in the peri-infarct area during stroke recovery, concomitantly with an increase in dendrites and spines identified by Golgi-Cox staining. In vitro, cortical neurons subjected to OGD exhibited significant increases in axonal outgrowth and in phosphorylated NFH protein levels, concurrently with downregulation of phosphatase tensin homolog deleted on chromosome 10, activation of Akt, and inactivation of glycogen synthase kinase-3β in regenerated axons. Blockage of phosphoinositide 3-kinase with pharmacological inhibitors suppressed Akt activation and attenuated phosphorylation of glycogen synthase kinase-3β, which resulted in suppression of phosphorylated NFH and axonal outgrowth after OGD; whereas GSK-3 inhibitors augmented axonal regeneration and elevated phosphorylated NFH levels after OGD.

CONCLUSIONS

Stroke induces axonal outgrowth and myelination in rodent ischemic brain during stroke recovery, and the phosphoinositide 3-kinase/Akt/glycogen synthase kinase-3β signaling pathway mediates axonal regeneration of cortical neurons after OGD.

The polymer brush model of neurofilament projections: effect of protein composition

Applying self-consistent field theory, we consider a coarse-grained model for the polymerlike projections of neurofilament (NF) proteins that form a brush structure around neurofilaments. We focus on effects of molecular composition, which is the relative occurrence of NF-H, NF-M, and NF-L proteins, on the organization of NF projection domains. We consider NF brushes with selectively truncated projections, and with a varied ratio L:H:M of constituent tails. Our conclusion is that the NF brush structure is remarkably tolerant with respect to the variation in M and H chains. Results compare favorably with experimental data on model animals, provided that due attention is paid on the level of phosphorylation of the KSP repeats.

Interplay between liquid crystalline and isotropic gels in self-assembled neurofilament networks

Neurofilaments (NFs) are a major constituent of nerve cell axons that assemble from three subunit proteins of low (NF-L), medium (NF-M), and high (NF-H) molecular weight into a 10 nm diameter rod with radiating sidearms to form a bottle-brush-like structure. Here, we reassemble NFs in vitro from varying weight ratios of the subunit proteins, purified from bovine spinal cord, to form homopolymers of NF-L or filaments composed of NF-L and NF-M (NF-LM), NF-L and NF-H (NF-LH), or all three subunits (NF-LMH). At high protein concentrations, NFs align to form a nematic liquid crystalline gel with a well-defined spacing determined with synchrotron small angle x-ray scattering. Near physiological conditions (86 mM monovalent salt and pH 6.8), NF-LM networks with a high NF-M grafting density favor nematic ordering whereas filaments composed of NF-LH transition to an isotropic gel at low protein concentrations as a function of increasing mole fraction of NF-H subunits. The interfilament distance decreases with NF-M grafting density, opposite the trend seen with NF-LH networks. This suggests a competition between the more attractive NF-M sidearms, forming a compact aligned nematic gel, and the repulsive NF-H sidearms, favoring a more expansive isotropic gel, at 86 mM monovalent salt. These interactions are highly salt dependent and the nematic gel phase is stabilized with increasing monovalent salt.

Distinct patterns of motor nerve terminal sprouting induced by ciliary neurotrophic factor vs. botulinum toxin

Both diffusible and surface-bound molecules are thought to induce sprouting of motor nerve terminals in response to paralysis. Here we report that the sprouting induced by ciliary neurotrophic factor (CNTF) is qualitatively different from the sprouting induced by botulinum toxin (BoTX). We show first that subcutaneous application of CNTF to levator auris longus muscles of adult mice evokes sprouting from nearly all nerve terminals. Surprisingly, however, most terminal sprouts remain within the boundaries of the endplate region and rarely grow extrasynaptically even if CNTF is administered chronically. In contrast, terminal sprouts induced by BoTX extend vigorously along the extrasynaptic muscle surface. The different patterns of sprout elongation are attributable in part to different patterns of initiation: whereas CNTF-induced sprouts emerge randomly from the surface of terminal branches, BoTX-induced sprouts emerge exclusively along the perimeter of terminal branches in direct apposition to muscle fiber membranes. Combined treatment with CNTF and BoTX produces exceptionally robust extraterminal sprouting with little if any intrasynaptic growth of terminal sprouts. We interpret these results as showing that paralysis induces sprouting primarily by muscle-associated, surface-bound molecules rather than by diffusible factors. Our findings may be useful in defining the physiological role of the numerous candidate sprouting-inducers and in promoting compensatory sprouting after nerve injury for therapeutic benefit.

Developmental changes in the neuronal protein composition: A study by high resolution 2D-gel electrophoresis

Cerebellar granular neurons were grown in culture up to 21 days and the protein compositions of undifferentiated (day 1), partially differentiated (day 7) and fully differentiated (day 21) neurons were analyzed by high-resolution 2D-gel electrophoresis. During neuronal differentiation there were not only increase in the amount of several known proteins, viz. actin, tubulin (both alpha and beta subunits), myosin (heavy and light chains), but very interesting changes were also observed in the expressions of different subunits and isoforms of those proteins. Furthermore, both in the acidic (pI 4.0-4.5) and alkaline (pI 7.0-8.5) regions interesting up and down regulations of several unidentified proteins were observed during the neuronal differentiation. These results indicated that there were several unidentified proteins that might be very valuable targets for studying regulation of neuronal differentiation. Research is going on for further characterization of those proteins using recently developed proteomics technology.

Morphological and biochemical changes of neurofilaments in aged rat sciatic nerve axons

We have made a detailed comparison of neurofilaments (NFs) in the axons of the sciatic nerves between young and aged rats. In young rats, NF density was similar between proximal and distal sciatic nerve, but it became higher in the proximal region of sciatic nerve of aged rats. In accordance with this morphological change, NF protein content decreased dramatically in the middle region of the sciatic nerves of aged rats. The ratio of NF-M to NF-H in aged rats was lower than that in young rats at the proximal region of sciatic nerves and further decreased in the distal region of sciatic nerve. We analyzed transcription and axonal transport of NF proteins in motor neurons in spinal cord which are the major constituents of sciatic nerve axons. Of the transcripts of the NF subunits, NF-M mRNA was particularly reduced in aged rats. Examination of slow axonal transport revealed that the transport rate for NF-M was slightly faster than that for NF-H in young rats, but slightly slower in aged rats. A decrease in both the synthesis and transport rate of NF-M with aging may contribute to the relative reduction in NF-M in the aged rat sciatic nerve. Although the relationship between NF packing and reduced NF-M is not clear at present, these changes in NFs may be associated with age-dependent axonal degeneration diseases.

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.

Nogo-A expressed in Schwann cells impairs axonal regeneration after peripheral nerve injury

Injured axons in mammalian peripheral nerves often regenerate successfully over long distances, in contrast to axons in the brain and spinal cord (CNS). Neurite growth-inhibitory proteins, including the recently cloned membrane protein Nogo-A, are enriched in the CNS, in particular in myelin. Nogo-A is not detectable in peripheral nerve myelin. Using regulated transgenic expression of Nogo-A in peripheral nerve Schwann cells, we show that axonal regeneration and functional recovery are impaired after a sciatic nerve crush. Nogo-A thus overrides the growth-permissive and -promoting effects of the lesioned peripheral nerve, demonstrating its in vivo potency as an inhibitor of axonal regeneration.

Effect of the branched-chain alpha-ketoacids accumulating in maple syrup urine disease on the high molecular weight neurofilament subunit (NF-H) in rat cerebral cortex

In this study we investigated the effects of the branched chain alpha-ketoacids accumulating in maple syrup urine disease (MSUD) on the concentrations of the high molecular weight neurofilament subunit (NF-H) associated with the cytoskeletal fraction of the cerebral cortex of 12-day-old rats. Cortical slices were incubated with alpha-ketoisocaproic acid (KIC), alpha-keto beta-methylvaleric acid (KMV) and alpha-ketoisovaleric acid (KIV) at concentrations ranging from 0.5 to 1.0 mM. The cytoskeletal fraction was extracted and the immunoreactivity for phosphorylated and total NF-H was analyzed by immunoblotting. The in vitro 32P incorporation into NF-H was also determined. Results showed that treatment of tissue slices induced with KMV increased Triton-insoluble phosphorylated NF-H immunoreactivity, with no alteration in total NF-H immunoreactivity. Furthermore, KIC treatment drastically increased the total amount of NF-H, whereas KIV did not change either phosphorylated or total NF-H immunoreactivity. KMV also increased the in vitro 32P incorporation into NF-H, confirming the highly phosphorylated NF-H levels detected in the immunoblot. These findings demonstrate that KIC and KMV alter the dynamic regulation of NF-H assembly in the cytoskeletal fraction. Therefore we may suggest that cytoskeletal disorganization may be one of the factors associated with the neurodegeneration characteristic of MSUD disease.

p35 and p39 are essential for cyclin-dependent kinase 5 function during neurodevelopment

Cyclin-dependent kinase 5 (Cdk5) plays a pivotal role in brain development and neuronal migration. Cdk5 is abundant in postmitotic, terminally differentiated neurons. The ability of Cdk5 to phosphorylate substrates is dependent on activation by its neuronal-specific activators p35 and p39. There exist striking differences in the phenotypic severity of Cdk5-deficient mice and p35-deficient mice. Cdk5-null mutants show a more severe disruption of lamination in the cerebral cortex, hippocampus, and cerebellum. In addition, Cdk5-null mice display perinatal lethality, whereas p35-null mice are viable. These discrepancies have been attributed to the function of other Cdk5 activators, such as p39. To understand the roles of p39 and p35, we created p39-null mice and p35/p39 compound-mutant mice. Interestingly, p39-null mice show no obvious detectable abnormalities, whereas p35(-/-)p39(-/-) double-null mutants are perinatal lethal. We show here that the p35(-/-)p39(-/-) mutants exhibit phenotypes identical to those of the Cdk5-null mutant mice. Other compound-mutant mice with intermediate phenotypes allow us to determine the distinct and redundant functions between p35 and p39. Our data strongly suggest that p35 and p39 are essential for Cdk5 activity during the development of the nervous system. Thus, p35 and p39 are likely to be the principal, if not the only, activators of Cdk5.

Massive accumulation of M and H subunits of neurofilament proteins in spinal motor neurons of neurofilament deficient Japanese quail, Quv

Quiver (Quv) is a non-sense mutation of neurofilament protein L subunit (NF-L) that causes neurofilament deficiency with preserved microtubules in Japanese quail. Anti-NF-M and anti-NF-H mAbs stained cell bodies of motor neurons in Quv embryo spinal cords much more intense than those in control spinal cords. Volume of motor neurons in Quv spinal cords increased to 2.3 times of control motor neurons. Immunoblot of Quv spinal cords revealed a relative increase in non- and hypo-phosphorylated NF-M and NF-H, and a decrease in the total amount of NFs. Quv sciatic nerves showed faintly reacted phosphorylated NF-M and NF-H. These results suggest that deficiency of assembled neurofilament results in decreased axonal transport of NFs and accumulation of NFs in cell bodies of spinal motor neurons.