Slow transport of the cytoskeleton after axonal injury

The Mobility of Neurofilaments in Mature Myelinated Axons of Adult Mice

Studies in cultured neurons have shown that neurofilaments are cargoes of axonal transport that move rapidly but intermittently along microtubule tracks. However, the extent to which axonal neurofilaments move in vivo has been controversial. Some researchers have proposed that most axonally transported neurofilaments are deposited into a persistently stationary network and that only a small proportion of axonal neurofilaments are transported in mature axons. Here we use the fluorescence photoactivation pulse-escape technique to test this hypothesis in intact peripheral nerves of adult male hThy1-paGFP-NFM mice, which express low levels of mouse neurofilament protein M tagged with photoactivatable GFP. Neurofilaments were photoactivated in short segments of large, myelinated axons, and the mobility of these fluorescently tagged polymers was determined by analyzing the kinetics of their departure. Our results show that >80% of the fluorescence departed the window within 3 h after activation, indicating a highly mobile neurofilament population. The movement was blocked by glycolytic inhibitors, confirming that it was an active transport process. Thus, we find no evidence for a substantial stationary neurofilament population. By extrapolation of the decay kinetics, we predict that 99% of the neurofilaments would have exited the activation window after 10 h. These data support a dynamic view of the neuronal cytoskeleton in which neurofilaments cycle repeatedly between moving and pausing states throughout their journey along the axon, even in mature myelinated axons. The filaments spend a large proportion of their time pausing, but on a timescale of hours, most of them move.

Neuronal intrinsic mechanisms of axon regeneration

Failure of axon regeneration after central nervous system (CNS) injuries results in permanent functional deficits. Numerous studies in the past suggested that blocking extracellular inhibitory influences alone is insufficient to allow the majority of injured axons to regenerate, pointing to the importance of revisiting the hypothesis that diminished intrinsic regenerative ability critically underlies regeneration failure. Recent studies in different species and using different injury models have started to reveal important cellular and molecular mechanisms within neurons that govern axon regeneration. This review summarizes these observations and discusses possible strategies for stimulating axon regeneration and perhaps functional recovery after CNS injury.

Aldose reductase deficiency improves Wallerian degeneration and nerve regeneration in diabetic thy1-YFP mice

This study examined the role of aldose reductase (AR) in diabetes-associated impaired nerve regeneration using thy1-YFP (YFP) mice. Sciatic nerves of nondiabetic and streptozotocin-induced diabetic AR(+/+)YFP and AR(-/-)YFP mice were transected after 4 weeks of diabetes. Wallerian degeneration and nerve regeneration were evaluated at 1 and 2 weeks postaxotomy by fluorescence microscopy. Motor nerve conduction velocity recovery and regenerating nerve morphometric parameters were determined at 10 and 20 weeks, respectively. There was no difference in the extent of Wallerian degeneration, size of regenerating stump, motor nerve conduction velocity recovery, or caliber of regenerating fibers between nondiabetic AR(+/+)YFP and AR(-/-)YFP mice. In diabetic AR(+/+)YFP mice, Wallerian degeneration was delayed, associated with slower macrophage invasion and abnormal vascularization. Those mice had smaller regenerating stumps, slower motor nerve conduction velocity, and smaller regenerating fibers compared with nondiabetic mice. These features of impaired nerve regeneration were largely attenuated in diabetic AR(-/-)YFP mice. Retarded macrophage invasion and vascularization associated with Wallerian degeneration were normalized in diabetic AR(-/-)YFP mice. These results indicate that AR plays an important role in diabetes-associated impaired nerve regeneration, in part by affecting vascularization and macrophage invasion during Wallerian degeneration. The thy1-YFP mice are valuable tools for further investigation of the mechanism of diabetes-associated nerve regeneration.

A New Calcium Channel Antagonist, Lomerizine, Alleviates Secondary Retinal Ganglion Cell Death After Optic Nerve Injury in the Rat

PURPOSE

We investigated whether lomerizine, a new diphenylmethylpiperazine calcium channel blocker, exerted a neuroprotective effect on axonal or retinal damage induced by optic nerve injury in the rat.

METHODS

A partial crush lesion was inflicted unilaterally on the optic nerve, 2 mm behind the globe, in adult Wistar albino rats. Animals were treated with the vehicle, 10 or 30 mg/kg lomerizine. Each solution was given orally twice daily for 4 weeks. One week before euthanization, Fluoro-Gold (FG) was injected into both superior colliculi to retrogradely label surviving retinal ganglion cells (RGCs). Approximately 1 month after the optic nerve injury, the retinal damage was assessed morphologically, and the optic nerve axons surrounding the initial lesion were examined histologically.

RESULTS

The mean RGC density in the control group decreased to 65.9 +/- 1.32% of the contralateral eye, whereas the systemic application of 10 or 30 mg/kg of lomerizine significantly enhanced the RGC survival to 88.1 +/- 0.38% and 89.8 +/- 0.28%, respectively. Histological examination of damaged axons revealed no significant enhancement of the density or total number of axons of the retinal ganglion cells in the lomerizine-treated group. The crush force we employed caused no significant morphological differences in the retinal layers between the sham-operated animals and the animals from the experimental groups.

CONCLUSIONS

Our findings suggest that lomerizine alleviates secondary degeneration of RGCs induced by an optic nerve crush injury in the rat, presumably by improving the impaired axoplasmic flow.

L-type calcium channel antagonist nifedipine reduces neurofilament restitution following traumatic optic nerve injury

BACKGROUND

The aim of the present study was to observe if the use of the L-type calcium channel antagonist nifedipine would offer advantages for the retinal ganglion cells and the restitution of the axonal cytoskeleton after optic nerve crush.

METHODS

Retinal ganglion cells were retrogradely labeled with a fluorescent calcium marker. With the in vivo confocal neuroimaging (ICON) method we observed the fluorescent cell metabolism marker Oregon Green BAPTA in the same retinal ganglion cells over 3 weeks after optic nerve crush. 2 micromol nifedipine were injected intraocularly 30 minutes following optic nerve crush.

FINDINGS

Investigation of the optic nerve immunostained for NF-H presented decreased restitution of the neurofilaments in the axonal cytoskeleton after 3 weeks in the optic nerve crush group treated with nifedipine as compared to the optic nerve crush only group.

INTERPRETATION

These results show that a single injection of the calcium L-type antagonist nifedipine shortly after optic nerve injury has long-lasting negative effects on the recovery of the retinal ganglion cells.

Anatomical correlations of intrinsic axon repair after partial optic nerve crush in rats

About 15% of retinal ganglion cells survive diffuse axonal injury of the optic nerve in adult rats. Following initial blindness, discrimination of visual stimuli in behavioral tests recovers within three weeks. To investigate the mechanisms promoting this functional recovery the axonal transport and the neurofilaments were studied. Intraocularly applied MiniRuby is transported until the place of crush and accumulated in enlarged axon terminals. Three weeks after lesion the anterograde transport of MiniRuby recovers distal to the place of crush. At the same point in time the retrograde transport of surviving retinal ganglion cells is restored which was visualized by horseradish peroxidase injected into the superior colliculus. The heavy neurofilament was stained immunohistochemically and analyzed statistically up to three weeks after optic nerve crush. The stained filaments in the axon fibers of retinal ganglion cells appear wavelike and/or fragmented up to day 8, but first signs of heavy neurofilament restitution in the fibers of the optic nerve are seen at day 12 after axonal injury. Because these results cannot be explained by longlasting axon regeneration, the present results provide convincing evidence for intrinsic axon repair soon after diffuse axonal injury that correlates in time with recovery of vision.

Quantification of histological changes after calibrated crush of the intraorbital optic nerve in rats

BACKGROUND

Traumatic optic nerve lesions (TONL) are probable but unpredictable consequence after severe midface or skull base trauma. Based on a previously described rat model, the authors developed a new model in order to simulate optic nerve crush during trauma on the optic canal.

METHODS

To achieve a calibrated TONL, a microinjuring device was designed that made it possible to assess the correlation between a defined trauma and the neuronal degeneration in the rat retinal ganglion cell (RGC) layer. This device is based on a small dynamometer mounted onto a conventional micromanipulator. The supraorbital approach was chosen to expose the extracranial optic nerve.

RESULTS

In this rat model (n=100, Wistar strain) the parameters of "force" and "time" could be precisely monitored during the experiment. The decrease in the mean number of retinal neurons (N) according to the pressure exerted (2-30 cN x mm(-2)) on the optic nerve was linear for 1, 6, and 15 minutes of injuring time; the decrease in N for varying injuring forces also appears to be nearly linear.

CONCLUSION

The results show that this model provides a reliable method for studying quantitatively the anatomical effects of TONL on the RGC layer and the optic nerve itself, and may allow the design of treatment strategies following TONL.

Unilateral injury to the adult rat optic nerve causes multiple cellular responses in the contralateral site

This study was undertaken to examine whether unilateral injury to one optic nerve (ON) elicits a response in the microglia, neuroglia and ganglion cells of the retina and ON of the contralateral site as well. Bilateral activation of the transcription factor c-jun could be immunohistochemically detected in the ganglion cell layer 2 days after crush and later. Microglial cells were detected with the activation-specific antibodies MUC 102 and OX-42. They showed an immediate and clear pattern of activation within the contralateral ON and retina, although this response was less pronounced than in the directly lesioned site. Astrocytes and Müller cells showed a typical up-regulation of glial fibrillary acidic protein in the lesioned retina and only focal but virtually no generalized up-regulation in the contralateral eye. Ganglion cells whose axons had been crushed responded with vigorous axonal growth after 2 days in culture, in addition to exhibiting in situ reactions. However, ganglion cells of the contralateral retina responded with a moderate regeneration, too. Growth was less pronounced than in the crushed retina but significantly better than in retinas on untreated animals. The results suggest that unilateral lesion of the optic nerve elicits a defined response in the major cell types of the contralateral retinofugal system. The findings suggest that it is advisable to maintain caution in the use of the contralateral optic nerve and retina as a control in experiments dealing with cellular processes of de- and regeneration.

Intrinsic neuronal determinants that promote axonal sprouting and elongation

Nerve processes elongate, branch and form synaptic contacts in a highly regulated and specific manner. Long-distance axon elongation is restricted to the main phase of axon formation during development, but can be reinduced upon lesions in the adult (regeneration). It correlates with the expression of defined genes, including proteins involved in signalling (e.g. src, NCAM, integrins), transcription factors (e.g. c-jun) and structural proteins (e.g. actin and tubulin isoforms). Activation of an exon elongation program may require bcl-2. The formation and growth of local branches (sprouting) is controlled by mechanisms in the target region. In addition, the expression of growth-associated proteins such as GAP-43 and CAP-23 in neurons lowers the threshold for nerve sprouting and potentiates its vigour. Recent studies suggest that nerve sprouting and long-distance elongation depend on the expression of different intrinsic components in neurons. One implication of these findings is that the differential expression of genes facilitating local branching may affect structural plasticity in the intact adult nervous system.

Impaired myelinated fiber regeneration following freeze-injury in rats with streptozotocin-induced diabetes: involvement of the polyol pathway

This study examined the effects of streptozotocin-induced diabetes mellitus and aldose reductase inhibitor (ZD5522) treatment on myelinated fiber regeneration in rats. After 2 months of diabetes, with or without ZD5522 treatment (10 mg.kg-1.day-1) from induction, sciatic nerve degeneration was initiated by a punctate lesion using a liquid N2 cooled probe. Regeneration was studied over a subsequent 14-day period using in vitro electrophysiological techniques. There was a 21.4% (P < 0.001) deficit in the maximum fiber regeneration distance in diabetic rats, 14 days postlesion. This was partially (64.9%, P < 0.01) prevented by aldose reductase inhibitor treatment, the resultant regeneration distance being not significantly different from that of age-matched nondiabetic control rats. The regeneration rate, assessed from data collected 4, 9 and 14 days postlesion, was 23.7% (P < 0.001) reduced by diabetes and ZD5522 treatment provided 73.1% protection (P < 0.01). We conclude that polyol pathway activity is involved in impaired regeneration in experimental diabetes, potential pathophysiological mechanisms involving a reduction in neurotrophic support and impaired endoneurial blood supply.

Changes in retinal ganglion cell axons after optic nerve crush: neurofilament expression is not the sole determinant of calibre

After injury in the central nervous system of adult mammals, many of the axons that remain attached to their intact cell bodies degenerate and decrease in calibre. To understand this process better, we have investigated the relationship between axonal loss, cell loss, and the time course of changes in axonal calibre. Optic nerves (ONs) were crushed and the numbers and sizes of axons remaining close to the cell bodies (2 mm from the eye) and near the site of the lesion (6 mm from the eye) were determined for nerves examined between 1 week and 3 months after injury. Comparison with the retinal ganglion cell (RGC) counts from the same animals revealed that axonal loss was concomitant with cell body loss for at least the first 2 weeks after injury. However, there was no significant change in the calibre of the surviving neurons until 1 month after injury. Thereafter, the axonal calibre was decreased equally along the ON. No progressive somatofugal atrophy was observed. These decreases in axonal calibre occur much later than the immediate drop in neurofilament (NF) expression that also follows injury. The late effect of injury on axonal calibre suggests that NF expression is not the sole determinant of axon size of the RGC fibers in the ON. Other factors are likely additional contributing factors, such as the decreased rate of axonal transport that would help maintain the axonal neurofilament content.

Tubulin expression and axonal transport in injured and regenerating neurons in the adult mammalian central nervous system

Microtubules are essential components of the cytoskeleton required for axonal growth. To investigate how changes in tubulin transport and expression may affect axon regeneration, injury in the adult mammalian central nervous system was studied. Axotomized retinal ganglion cells (RGCs) that do not regenerate were compared with RGCs that regenerate their axons when the optic nerve is replaced with a peripheral nerve graft. When RGC axons regenerated through peripheral nerve grafts, the rate of slow transport increased but decreased when no regrowth occurred. To investigate the molecular mechanisms that mediate these responses, alterations in tubulin mRNA levels after injury were examined. Total tubulin mRNA levels fell after injury in the optic nerve but increased in those RGCs that regenerated their axons into a peripheral nerve graft. Further, the expression of four separate beta-tubulin isotypes in injured rat RGCs was characterized. mRNA levels for all four isotypes decreased in RGCs after injury in the optic nerve. How the autoregulation of tubulin expression may contribute to the changes in beta-tubulin isotype expression after injury is discussed.

Regulation of immediate-early gene expression in rat retinal ganglion cells after axotomy and during regeneration through a peripheral nerve graft

To determine mechanisms of structural plasticity in adult CNS neurons, we investigated the expression of immediate early genes (IEGs) in the rat retina. Gene products of different IEG families (JUN and FOS proteins) and cAMP-responsive element binding protein (CREBP) were examined by immunohistochemistry under three different paradigms. Normal rats which were not axotomized were compared with axotomized animals, were retinal ganglion cells (RGCs) were axotomized by intraorbital optic nerve cut and retrogradely labeled with fluorogold (FG). Under these circumstances, RGCs show only transient sprouting, followed by continuous retrograde RGC degeneration. In the third group, after the optic nerve lesion, adult rats additionally received a sciatic nerve graft to the transected optic nerve stump. This allows some RGCs to regenerate an axon into the grafted nerve. In both groups, the time course of RGC survival and JUN, CREB, and FOS protein expression was monitored. In normal animals, JUN-Immunoreactivity (JUN-Ir) was not detectable in the retinal ganglion cell layer. JUN-Ir was induced in about 70% of all FG-positive RGCs 5 days after axotomy. The expression of JUN-Ir stated to decline 8 days after axotomy. Only a few JUN-Ir-positive RGCs were found after 2 weeks. In transplanted animals, however, the numbers of JUN-Ir-positive RGCs were significantly higher 2 and 3 weeks after transplantation compared to animals that exclusively received axotomy. Furthermore, in grafted rats, about 70% of the regenerating RGCs expressed JUN-Ir 2 weeks after grafting as compared to only 38% JUN-positive RGCs among the surviving but not regenerating RGCs. In normal animals CREBP-Ir was constitutively expressed in nearly all cells of the retinal ganglion cell layer. The decline in number of CREBP-Ir-positive cells paralleled the axotomy-induced RGC death. FOS-Ir-positive cells were not found in the ganglion cell layer at any time. These results demonstrate a selective and transient JUN expression of RGCs after axotomy which is sustained during axonal regeneration. This suggests that sciatic nerve grafts are able to regulate the expression of JUN proteins in axotomized RGCs of adult rats.

Expression of JUN, KROX, and CREB transcription factors in goldfish and rat retinal ganglion cells following optic nerve lesion is related to axonal sprouting

Goldfish and rat optic nerves were cut and crushed, respectively, and the expression of the transcription factor proteins c-JUN, JUN B, JUN D, c-FOS, FOS B, KROX-24, and CREB was investigated in retinal ganglion cells (RGCs) by immunocytochemistry. Immunoreactivities (IRs) were followed up to 350 days in the goldfish and up to 22 days in the rat. In RGCs of untreated goldfish and rats, all JUN, FOS, and KROX proteins were absent whereas CREB was constitutively expressed. After optic nerve cut in goldfish, a JUN-like immunoreactivity (JUN-IR) appeared in a small number of RGCs of central retina after 24 h, reached a maximum within 5 days, declined after 30 days, and was on a half-maximal level after 50 days. Between 100 and 200 days, JUN-IR was only visible in a few RGCs and was completely absent after 350 days. Specific antibodies against c-JUN, JUN B, and JUN D gave no distinct immunoreactive signal. Thus, we could not determine which member of the JUN family contributed to the JUN-IR. The expression of CREB declined after 5 days. The number of CREB-labeled RGCs was reduced (not significant) and the intensity of labeling faded out. After 50 days, CREB-IR had returned to basal level. c-FOS, FOS B, and KROX-24 could not be detected in goldfish RGCs following optic nerve cut. After optic nerve crush in the rat, c-JUN, JUN D, and KROX-24 appeared in a substantial number of RGCs after 24 h, had a maximal expression after 5 days, and strongly declined after 8 days. c-JUN and KROX-24 were completely absent after 22 days whereas JUN D was still present in a few rat RGCs. The number of CREB-labeled RGCs decreased after 5 days and had declined by 50% after 22 days. Expression of JUN B, c-FOS, FOS B could not be detected in rat RGCs after optic nerve crush. Our data demonstrate that the decrease of CREB and the increase of JUN and KROX-24 transcription factors precedes and parallels both the alteration of de novo protein synthesis and the axonal sprouting, which are long lasting in goldfish and transient in rat.