Changes in the amounts of cytoskeletal proteins within the perikarya and axons of regenerating frog motoneurons

Axotomy-induced cytoskeleton changes in unmyelinated mammalian central nervous system axons

Oligodendrocyte-derived myelin retards the ability of CNS axons to regenerate following transection. The intrinsic response of CNS axons to an axotomy insult may be vastly different in the absence of myelin. However, the paucity of adequate experimental models has limited detailed investigation of cellular behaviour following axon transection in an unmyelinated CNS environment. In this study we perform laser-induced axotomy of the porcine retinal ganglion cell axon, a physiologically unmyelinated, mature CNS axon that is structurally similar to humans to infer knowledge about axonal behaviour in the absence of myelin. Axotomy-induced changes to the neuronal cytoskeleton and supporting astrocytes during the early stages after transection are delineated by examining the sequence of neurofilament subunit, microtubule (TUB), microtubule associated protein (MAP), glial fibrillary acidic protein (GFAP) and terminal deoxynucleotidyl transferase biotin-dUTP nick end labelling (TUNEL) modification. Axonal transection induced an increase in the expression of neurofilament light at regions within, and immediately adjacent to, sites of axotomy. Other neurofilament subunits were not altered at sites of transection. Unlike myelinated axons where an increase in GFAP staining within hypertrophic glial scars have been shown to inhibit axonal repair we demonstrate a decrease in GFAP staining within regions of increased or preserved neurofilament expression. The behaviour of TUB and MAP proteins following transection of unmyelinated CNS axons are similar to what has previously been described in myelinated CNS axons. This study provides fundamental insights into astrocyte and axonal behaviour acutely after axotomy and demonstrates a series of degenerative events in unmyelinated CNS axons, which in comparison to prior reports are different to myelinated CNS axons. The findings of this report have relevance to understanding pathogenic mechanisms underlying neuro-degeneration in the CNS.

The role of the cytoskeleton in cell body enlargement, increased nuclear eccentricity and chromatolysis in axotomized spinal motor neurons

BACKGROUND

When spinal motor axons are injured, the nucleolus, nucleus and cell body of the injured cell transiently increase in size, the nucleus becomes more eccentrically placed, and the organization of polyribosomes into Nissl bodies is temporarily disrupted. The mechanisms for these classical morphological responses to axotomy have not been satisfactorily explained.

RESULTS

In this study we address the role of the cell body cytoskeleton in these structural changes. We show that the cytoskeleton of uninjured lumbar motor neuron cell bodies maintains nucleolar, nuclear and cell body size and nuclear position. When isolated, the relatively insoluble cell body cytoskeleton contains Nissl bodies and lipofuscin granules. After axotomy, protein labeling increases markedly and the cytoskeleton enlarges, increasing nucleolar, nuclear and cell body size, as well as nuclear eccentricity. Nearly all of the protein mass that accumulates in the cell body after axotomy appears to be added to the cytoskeleton.

CONCLUSION

We conclude that axotomy causes the conjugate enlargement of the nucleolus, nucleus and cell body and increases nuclear eccentricity in spinal motor neurons by adding protein to the cytoskeleton. The change in nuclear position, we propose, occurs when cytoskeletal elements of the axon cannot enter the shortened axon and "dam up" between the nucleus and axon hillock. As a consequence, we suggest that Nissl body-free axonal cytoskeleton accumulates between the nucleus and axon, displaces Nissl body-containing cytoskeleton, and produces central chromatolysis in that region of the cell.

Nitric oxide synthase in the frog cerebellum: response of Purkinje neurons to unilateral eighth nerve transection

When vestibular damage occurs, nitric oxide synthase (NOS) expression in rat cerebellar flocculus is affected. Since compensation for postural symptoms occurs and Purkinje cells play an important role in movement coordination and motor learning, we analyzed in situ the induction of NOS in the Purkinje cell population of the cerebellum (corpus cerebelli) of frog after unilateral transection of the eighth statoacoustic nerve to gain insight into the role of NO in neural plasticity after injury. Three days after neurectomy, the early effects induced NADPH diaphorase reactivity in most of the Purkinje cells on the ipsilateral side, while on the contralateral side the highest labeling was observed at 15 days. This finding can give information on the dynamics of vestibular compensation, in which NOS involvement was investigated. At 30 days, NADPH diaphorase reactivity was present in a large number of Purkinje cells of the whole cerebellum, while at 60 days a down-regulation for NADPH diaphorase reactivity was evident. A similar trend was observed for NOS-immunoreactivity, which was still present at 60 days in a high percentage of Purkinje cells, mainly on the ipsilateral side. On the basis of cell density evaluations, it was proposed that the early induction of NOS after neurectomy was linked to the degeneration of a part of the Purkinje neurons, while the permanence of NOS labeling might be due to a neuroprotective role of NO in the restoration phase of the vestibular compensation process.

Cytoskeleton abnormalities in axonopathies of unknown aetiology: correlations with morphometry

To determine if specific axonal cytoskeleton abnormalities could be demonstrated in axonopathies without aetiology, nerve biopsies from five controls and nine cases were analyzed by morphometry and immunocytochemistry with anti-neurofilament (NF, subunits L, M, H) and anti-beta tubulin (TUB) antibodies. Morphometry revealed either large fiber atrophy (decrease in large fiber density with increased density in small fibers), degeneration of large fibers (decrease in large fiber density and in total density of fibers) or of all diameter fibers. NF immunostaining density decreased (by 21-89%) only in cases with fiber loss, in parallel to myelinated fiber density as determined by morphometry. On the contrary, the density of fibers labelled for TUB increased significantly in all except two cases by 52-102% over controls. Nevertheless, in these two cases--with a severe loss of fibers--as well as in other cases, the ratio of the density of fibers labelled for TUB and NFL (TUB/NFL) increased by 48-404%. Thus, the total density of myelinated fibers was always inversely correlated with the TUB/NFL ratio. Similar abnormalities have been described only after axotomy; our cases could thus be compared to <<permanent axotomy>>.

Magnitude, laterality, and uniformity of swelling in axotomized spinal motoneurons: lack of evidence for influence by the distal stump

Injury to frog lumbar motor axons produces a coordinated, allometric enlargement of the nucleolus, nucleus, and cell body of the injured neuron. The mechanisms by which swelling is initiated and sustained are not known. In this study, we have sought evidence for a role of the severed distal stump in the magnitude, laterality, and uniformity of the swelling response in frog spinal motoneurons. We find that swelling of motoneuron nucleoli, nuclei, and perikarya after unilateral spinal nerve transection is exclusively ipsilateral and uniform among motoneurons of different sizes. Removal of the severed distal stump does not affect the magnitude, unilaterality, or uniformity of the swelling responses. Thus, the distal stump appears to play no role in initiating swelling following spinal nerve transection.

Immunocytochemical changes of cytoskeleton components and calmodulin in the frog cerebellum and optic tectum during hibernation

During hibernation, variation in the metabolism of nerve cells occurs. Since the cytoskeleton plays an important role in nerve cell function, we have analyzed the immunocytochemical expression of two cytoskeleton components, i.e. phosphorylated 200 kDa neurofilament protein, and microtubule-associated protein 2 in the cerebellum and optic tectum of hibernating frogs (Rana esculenta) in comparison with active animals. In addition, we have considered the immunocytochemical expression of calmodulin, which is known to be involved in neurofilament phosphorylation. In hibernating animals, there was a decrease in the immunoreactivity for phosphorylated 200 kDa neurofilament protein and microtubule-associated protein 2 of fibers in both the cerebellum and in the optic tectum. In contrast, in the large neurons of the cerebellum, i.e. Purkinje neurons, there was an increase in the immunoreactivity for microtubule-associated protein 2. The changes in the cytoskeleton components were accompanied by a decrease in calmodulin immunoreactivity in the cytoplasm of nerve cells of the cerebellum. All the changes observed are consistent with a low neuronal activity during hibernation, as also indicated by previous microdensitometric and microfluorometric data. This shows a higher degree of chromatin condensation in hibernating animals and suggests that hibernation represents a simple form of neuronal plasticity.

Radiolabeling motoneuron proteins in the isolated frog spinal cord preparation

An in vitro method for radiolabeling protein in adult frog spinal motoneurons is described, with per cell incorporations which are 2-3 orders of magnitude higher than previously reported for mammalian brain neurons. In the procedure, isolated lumbar spinal cord preparations from Rana pipiens are labeled with 3H-L-leucine, motoneuron cell bodies are recovered and TCA-precipitated protein is analyzed by scintillation counting. The higher levels of labeling (> 90 cpm/cell body) allow one to quantify newly synthesized protein within individual or small groups of identified nerve cell bodies. Motoneuronal labeling correlates directly with cell body size, and other sources of variation in labeling and their control are identified and discussed.

Neuronal protein NP185 is developmentally regulated, initially expressed during synaptogenesis, and localized in synaptic terminals

Evidence is presented here that demonstrates the presence of NP185 (AP3) in neuronal cells, specifically within syn-aptic terminals of the central nervous system and in the peripheral nervous system, particularly in the neuro-muscular junction of adult chicken muscle. Biochemical results obtained in our laboratories indicate that NP185 is associated with brain synaptic vesicles, with clathrin-coated vesicles, and with the synaptosomal plasma membrane. Also, NP185 binds to tubulin and clathrin light chains and the binding is regulated by phosphorylation (Su et al., 1991). Based on these properties and the data reported here, we advance the postulate that NP185 fulfills multiple functions in synaptic terminals. One function is that of a plasma membrane docking or channel protein, another of a signaling molecule for brain vesicles to reach the synaptic terminal region, and a third is that of a recycling molecule by binding to protein components on the lipid bilayer of the synaptic plasma membrane during the process of endocytosis. In support of these premises, a thorough study of NP185 using the developing chick brain, adult mouse brain, and chicken straited muscle was begun by temporally and spatially mapping the expression and localization of NP185 in evolving and mature nerve endings. To achieve these objectives, monoclonal antibodies to NP185 were used for immunocytochemistry in tissue sections of chicken and mouse cerebella. The distribution of NP185 was compared with those of other cytoskeletal and cytoplasmic proteins of axons and synapses, namely synaptophysin, vimentin, neurofilament NF68, and the intermediate filaments of glial cells (GFAP). The data indicate that expression of NP185 temporally coincides with synaptogenesis, and that the distribution of this protein is specific for synaptic terminal buttons of the CNS and the PNS.

Changes in cytoskeletal protein synthesis following axon injury and during axon regeneration

Injury to the axons of facial motoneurons stimulates increases in the synthesis of actin, tubulins, and GAP-43, and decreases in the synthesis of neurofilament proteins: mRNA levels change correspondingly. In contrast to this robust response of peripheral neurons to axotomy, injured central nervous system neurons show either an attenuated response that is subsequently aborted (rubrospinal neurons) or overall decreases in cytoskeletal protein mRNA expression (corticospinal and retinal ganglion neurons). There is evidence that these changes in synthesis are regulated by a variety of factors, including loss of endoneurially or target-derived trophic factors, positive signals arising from the site of injury, changes in the intraaxonal turnover of proteins, and substitution of target-derived trophic support by factors produced by glial cells. It is concluded that there is, as yet, no coherent explanation for the upregulation or downregulation of any of the cytoskeletal proteins following axotomy or during regeneration. In considering the relevance of these changes in cytoskeletal protein synthesis to regeneration, it is emphasized that they are unlikely to be involved in the initial outgrowth of the injured axons, both because transit times between cell body and injury site are too long, and because sprouting can occur in isolated axons. Injury-induced acceleration of the axonal transport of tubulin and actin in the proximal axon is likely to be more important in providing the cytoskeletal protein required for initial axonal outgrowth. Subsequently, the increased synthesis and transport velocity for actin and tubulin increase the delivery of these proteins to support the increased volume of the maturing regenerating axons. Reduction in neurofilament synthesis and changes in neurofilament phosphorylation may permit the increased transport velocity of the other cytoskeletal proteins. There is little direct evidence that alterations in cytoskeletal protein synthesis are necessary for successful regeneration, nor are they sufficient in the absence of a supportive environment. Nevertheless, the correlation that exists between a robust cell body response and successful regeneration suggests that an understanding of the regulation of cytoskeletal protein synthesis following axon injury must be a part of any successful strategy to improve the regenerative capacity of the central nervous system.

Selective decrease of small sensory neurons in lumbar dorsal root ganglia labeled with horseradish peroxidase after ND:YAG laser irradiation of the tibial nerve in the rat

Recent electrophysiological evidence indicates that Q-switched Nd:YAG laser irradiation might have selective effects on neural impulse transmission in small slow conducting sensory nerve fibers as compared to large diameter afferents. In an attempt to clarify the ultimate fate of sensory neurons after laser application to their peripheral axons, we have used horseradish peroxidase (HRP) as a cell marker to retrogradely label sensory neurons innervating the distal hindlimb in the rat. Pulsed Nd:YAG laser light was applied to the tibial nerve at pulse energies of 70 or 80 mJ/pulse for 5 min in experimental rats. Seven days later HRP was applied to the left (laser-treated) and to the contralateral (untreated) tibial nerve proximal to the site of laser irradiation. In control animals the numbers of HRP-labeled dorsal root ganglion cells were not significantly different between the right and the left side. In contrast, after previous laser irradiation labeling was always less on the laser-treated side (2183 +/- 513 cells, mean +/- SEM) as compared to the untreated side (3937 +/- 225). Analysis of the dimensions of labeled cells suggested that the reduction of labeled cells on the laser-treated side was mainly due to a deficit in small sensory neurons. Since the conduction velocity of nerve fibers is related to the size of their somata, our histological data imply that laser light selectively affects retrograde transport mechanisms for HRP in slow conducting sensory nerve fibers.

The expression of different cytochemical markers in normal and axotomised dorsal root ganglion cells projecting to the nucleus gracilis in the adult rat

Rat lumbar dorsal root ganglion neurones projecting to the nucleus gracilis in the brainstem were retrogradely labelled with Fluoro-Gold and analysed immunocytochemically for their expression of substance P-, calcitonin gene-related peptide-, galanin-, galanin message-associated peptide-, neuropeptide Y-, nitric oxide synthase- and carbonic anhydrase-like immunoreactivity as well as affinity to Griffonia (bandeiraea) simplicifolia lectin I--isolectin B4, RT97 and to choleragenoid. The analysis was made both in uninjured rats and in rats which had been subjected to unilateral sciatic nerve transection and partial resection 3 weeks earlier. The data showed that 6% of the L4 and L5 lumbar dorsal root ganglion cells that projected to the nucleus gracilis showed substance P-like immunoreactivity. Following nerve injury, none of the nucleus gracilis-projecting dorsal root ganglion cells showed substance P-like immunoreactivity. Nineteen per cent of the investigated cell population showed calcitonin gene-related peptide-like immunoreactivity in uninjured rats, but no nucleus gracilis-projecting calcitonin gene-related peptide-positive cells were found after nerve injury. Galanin- and galanin message-associated peptide-like immunoreactivity were found in 2% and 3%, respectively, of the Fluoro-Gold-labelled cell population normally and in 22% and 14%, respectively, after injury. No neuropeptide Y-positive cells were found in the Fluoro-Gold-labelled cell population normally, but after nerve injury, 96% of this population became neuropeptide Y-positive. Nitric oxide synthase-like immunoreactivity was found in 2% of the Fluoro-Gold-labelled cells normally and in 10% after injury. Two per cent of the Fluoro-Gold-labelled cells in the normal cases were stained by Griffonia (bandeiraea) simplicifolia lectin I--isolectin B4. After injury, however, no such double labelling was found. Thirty-four per cent of the Fluoro-Gold-labelled cell population was carbonic anhydrase positive normally, and 42% after injury. Seventy-five per cent of the Fluoro-Gold-labelled cells showed RT97 immunoreactivity normally and 12% after injury. Choleragenoid-like immunoreactivity was found in 99% of the Fluoro-Gold-labelled dorsal root ganglion cells normally and 81% after injury. Immunohistochemical visualisation of choleragenoid transganglionically transported from the injured sciatic nerve combined with neuropeptide Y immunocytochemistry showed that primary afferent fibres and terminals in the nucleus gracilis contain neuropeptide Y following peripheral nerve transection. Taken together, the results indicate that peripherally axotomised nucleus gracilis-projecting neurones undergo marked alterations in their cytochemical characteristics, which may be significant for the structural and functional plasticity of this system after injury.

Neurofilament immunoreactivity in vestibular ganglion neurons of the adult rat

Immunocytochemical methods were used to study the distribution of neurofilament (NF) proteins in vestibular ganglion neurons of the adult rat. Monoclonal antibodies against the three triplet proteins were used. By indirect immunofluorescence and the peroxidase-antiperoxidase method, two populations of neurons were distinguished. One population with large perikarya showed strong NF immunoreactivity. A second population of neurons presented only slight or no immunoreactivity. The strong NF immunoreactivity in the perikarya of certain neurons seems to be a general feature of many sensory ganglia.

Isolation of motoneuron cell bodies from spinal cord stored at -70 degrees C in ethylene glycol

Pretreatment of spinal cord with ethylene glycol permits long-term storage of the tissue at -70 degrees C prior to isolation and biochemical analysis of the cell bodies of spinal motoneurons. The method is useful for storing spinal tissue from laboratory animals, as well as from human post mortem specimens, where aliquots of tissue may then be used for motoneuron isolation over an indefinitely long period. In addition to inhibiting the loss of soluble proteins from the neurons during freezing and thawing, cryoprotection increases the yield and improves the appearance of the isolated cell bodies. The method should aid biochemical studies of many kinds of neuronal subpopulations isolated from small amounts of starting material.

Axonal sprouting after botulinum toxin does not elicit a histological axon reaction

In an attempt to determine which elements of the axon reaction are essential for early axonal outgrowth, axonal sprouting was induced with botulinum toxin (BoTx) and the nerve cell body changes compared with those accompanying axonal growth after nerve trauma. Anterior horn cells of mice were examined histologically at times ranging from 3 days to 3 weeks after either BoTx hindlimb injection or sciatic nerve crush. After sciatic nerve crush there was dispersion of Nissl substance, increase in cell body size, and an increase in neurofilament protein staining. None of these changes were found after BoTx-induced terminal axonal sprouting, suggesting that these morphological features of the axon reaction are not essential for early axonal outgrowth.

Intracellular injection of Ca2+ chelator does not affect spike repolarization of cat spinal motoneurons

Both intra- and extracellular injections of tetraethylammonium (TEA) prolonged the spike repolarization of motoneurons in the spinal cord of cats under pentabarbitone anaesthesia, but did not depress the afterhyperpolarization (AHP). Intracellular injections of EGTA and the fast-acting Ca2+ chelator, BAPTA, greatly depressed the AHP, but the spike shape remained unchanged. Extracellular applications of Cd2+ had similar effects. These observations suggest that a Ca2+-dependent K+ outward current is not a major mechanism of spike repolarization in motoneurons.

mRNA levels of all three neurofilament proteins decline following nerve transection

The control of neurofilament (NF) protein gene expression was studied by determining and comparing the levels of mRNA to the heavy (NF-H), mid-sized (NF-M) and light (NF-L) NF protein subunits in rat dorsal root ganglia (DRG) following sciatic nerve transection. mRNA to NF-H (4.5 kb), to NF-M (3.4 kb) and to NF-L (2.5 and 4.0 kb) were identified in Northern blots and quantitated in dot blot analyses, using specific cDNA probes for each NF protein. Following transection and continuing for at least 28 days. The early and co-terminal fall in mRNAs suggests that the 3 NF genes are regulated by common factor(s) and that the function of these factor(s) is influenced by the state of axonal continuity with the target organ.

Slow transport in a nerve with embryonic characteristics, the olfactory nerve

The kinetics for slowly transported polypeptides have been examined in intact garfish olfactory nerves. The shape of the slow peak is essentially determined by alpha-and beta-tubulin which are by far the major polypeptides of the entire wave. The proximal area of the peak is similar to the slow component a (Sca) subcomponent defined in other nerves and contains discretely moving neurofilament proteins. The distal peak area, however, is more reminiscent of Scb. The two subcomponents were found to overlap considerably. Traces of polypeptides comigrating with tubulin and actin move far ahead of the slow wave at rates similar to the rate of slow transport measured in growing fibers and to the maximal velocity of axonal elongation. One of the most striking properties of slow transport in this nerve is the difference in the spreading of the various transported polypeptides along the axon, following their release from the perikarya. Labeled tubulin and actin can cover more than 20 cm of nerve; while neurofilament proteins can be found only on a 6 cm segment. Comparisons between slow transport in garfish olfactory axons and other vertebrate nerves indicate that despite major differences, the basic characteristics of slow transport are conserved. The features specific to the olfactory nerve may reflect its specialized properties. The constant turnover of olfactory neurons implies that these cells have an excellent growth potential but a short life span and, therefore, never reach full maturity. It can, therefore, be expected that their molecular composition is reminiscent of that embryonic neurons with a high level of plasticity but a slow stability.

Changes in neurofilament transport coincide temporally with alterations in the caliber of axons in regenerating motor fibers

The delivery of neurofilaments via axonal transport has been proposed as an important mechanism for regulating axonal caliber. If this hypothesis is correct, alterations in axonal caliber should appear coincident with changes in the delivery of neurofilaments to the axon. The purpose of this study was to determine whether alterations in the caliber of axons in the proximal stumps of transected motor fibers precede, coincide with, or occur substantially later than changes in the delivery of neurofilaments via axonal transport. Between 3 d and 12 wk after crushing the sciatic nerves of 7-wk-old rats, lumbar motor neurons were labeled by the intraspinal injection of [35S]methionine. In neurons labeled between 3 d and 6 wk after axotomy, the relative amount of neurofilament protein in the slow component, as reflected by the ratio of the radioactivities of the 145-kD neurofilament protein to tubulin, was reduced to 30-40% of the control value. Moreover, as determined by immunoreactivity on blots, the amounts of neurofilament protein and tubulin in these nerve fibers were reduced fourfold and twofold, respectively. Thus, changes in the ratio of labeled neurofilament protein to tubulin correlated with comparable changes in the quantities of these proteins in nerve fibers. This decrease in the quantity of neurofilament proteins delivered to axons coincided temporally with reductions in axonal caliber. After regeneration occurred, the delivery of neurofilament proteins returned to pre-axotomy levels (i.e., 8 wk after axotomy), and caliber was restored with resumption of normal age-related radial growth of these axons. Thus, changes in axonal caliber coincided temporally with alterations in the delivery of neurofilament proteins. These results suggest that the majority of neurofilaments in these motor fibers continuously move in the anterograde direction as part of the slow component of axonal transport and that the transport of neurofilaments plays an important role in regulating the caliber of these axons.

Control of axonal caliber by neurofilament transport

The role of neurofilaments, the intermediate filaments of nerve cells, has been conjectural. Previous morphological studies have suggested a close relationship between neurofilament content and axonal caliber. In this study, the regenerating neuron was used as a model system for testing the hypotheses that neurofilaments are intrinsic determinants of axonal caliber, and that neurofilament content is controlled by the axonal transport of neurofilaments. This system was chosen because previous studies had shown that, after axotomy, axonal caliber was reduced within the proximal stump of the regenerating nerve and, because the relative amount of neurofilament protein undergoing axonal transport in regenerating axons was selectively reduced. The relationship between axonal caliber and neurofilament number was examined in a systematic fashion in both regenerating and control motor axons in rat L5 ventral root. Reconstruction of the spatial and temporal sequences of axonal atrophy in the proximal stump after axotomy showed that reductions in axonal caliber were first detected in the most proximal region of the root and subsequently progressed in a proximal-to-distal direction at a rate of 1.7 mm/day, which is identical to the rate of neurofilament transport in these neurons. Quantitative ultrastructural studies showed that these reductions in caliber correlated with a proportional decrease in the number of axonal neurofilaments but not microtubules. These results support the hypotheses that neurofilament content is a major intrinsic determinant of axonal caliber and that neurofilament content is controlled by the axonal transport of neurofilaments. On this basis, we suggest a role for neurofilaments in the control of axonal volume.