Watery and dark axons in Wallerian degeneration of the opossum's optic nerve: different patterns of cytoskeletal breakdown?

Traumatic Axonal Injury in the Optic Nerve: The Selective Role of SARM1 in the Evolution of Distal Axonopathy

Traumatic axonal injury (TAI), thought to be caused by rotational acceleration of the head, is a prevalent neuropathology in traumatic brain injury (TBI). TAI in the optic nerve is a common finding in multiple blunt-force TBI models and hence a great model to study mechanisms and treatments for TAI, especially in view of the compartmentalized anatomy of the visual system. We have previously shown that the somata and the proximal, but not distal, axons of retinal ganglion cells (RGC) respond to DLK/LZK blockade after impact acceleration of the head (IA-TBI). Here, we explored the role of the sterile alpha and TIR-motif containing 1 (SARM1), the key driver of Wallerian degeneration (WD), in the progressive breakdown of distal and proximal segments of the optic nerve following IA-TBI with high-resolution morphological and classical neuropathological approaches. Wild type and Sarm1 knockout (KO) mice received IA-TBI or sham injury and were allowed to survive for 3, 7, 14, and 21 days. Ultrastructural and microscopic analyses revealed that TAI in the optic nerve is characterized by variable involvement of individual axons, ranging from apparent early disconnection of a subpopulation of axons to a range of ongoing axonal and myelin perturbations. Traumatic axonal injury resulted in the degeneration of a population of axons distal and proximal to the injury, along with retrograde death of a subpopulation of RGCs. Quantitative analyses on proximal and distal axons and RGC somata revealed that different neuronal domains exhibit differential vulnerability, with distal axon segments showing more severe degeneration compared with proximal segments and RGC somata. Importantly, we found that Sarm1 KO had a profound effect in the distal optic nerve by suppressing axonal degeneration by up to 50% in the first 2 weeks after IA-TBI, with a continued but lower effect at 3 weeks, while also suppressing microglial activation. Sarm1 KO had no evident effect on the initial traumatic disconnection and did not ameliorate the proximal optic axonopathy or the subsequent attrition of RGCs, indicating that the fate of different axonal segments in the course of TAI may depend on distinct molecular programs within axons.

Traumatic axonopathy in spinal tracts after impact acceleration head injury: Ultrastructural observations and evidence of SARM1-dependent axonal degeneration

Traumatic axonal injury (TAI) and the associated axonopathy are common consequences of traumatic brain injury (TBI) and contribute to significant neurological morbidity. It has been previously suggested that TAI activates a highly conserved program of axonal self-destruction known as Wallerian degeneration (WD). In the present study, we utilize our well-established impact acceleration model of TBI (IA-TBI) to characterize the pathology of injured myelinated axons in the white matter tracks traversing the ventral, lateral, and dorsal spinal columns in the mouse and assess the effect of Sterile Alpha and TIR Motif Containing 1 (Sarm1) gene knockout on acute and subacute axonal degeneration and myelin pathology. In silver-stained preparations, we found that IA-TBI results in white matter pathology as well as terminal field degeneration across the rostrocaudal axis of the spinal cord. At the ultrastructural level, we found that traumatic axonopathy is associated with diverse types of axonal and myelin pathology, ranging from focal axoskeletal perturbations and focal disruption of the myelin sheath to axonal fragmentation. Several morphological features such as neurofilament compaction, accumulation of organelles and inclusions, axoskeletal flocculation, myelin degeneration and formation of ovoids are similar to profiles encountered in classical examples of WD. Other profiles such as excess myelin figures and inner tongue evaginations are more typical of chronic neuropathies. Stereological analysis of pathological axonal and myelin profiles in the ventral, lateral, and dorsal columns of the lower cervical cord (C6) segments from wild type and Sarm1 KO mice at 3 and 7 days post IA-TBI (n = 32) revealed an up to 90% reduction in the density of pathological profiles in Sarm1 KO mice after IA-TBI. Protection was evident across all white matter tracts assessed, but showed some variability. Finally, Sarm1 deletion ameliorated the activation of microglia associated with TAI. Our findings demonstrate the presence of severe traumatic axonopathy in multiple ascending and descending long tracts after IA-TBI with features consistent with some chronic axonopathies and models of WD and the across-tract protective effect of Sarm1 deletion.

Demyelination of the Optic Nerve: An Underlying Factor in Glaucoma?

Neurodegenerative disorders are characterized by typical neuronal degeneration and axonal loss in the central nervous system (CNS). Demyelination occurs when myelin or oligodendrocytes experience damage. Pathological changes in demyelination contribute to neurodegenerative diseases and worsen clinical symptoms during disease progression. Glaucoma is a neurodegenerative disease characterized by progressive degeneration of retinal ganglion cells (RGCs) and the optic nerve. Since it is not yet well understood, we hypothesized that demyelination could play a significant role in glaucoma. Therefore, this study started with the morphological and functional manifestations of demyelination in the CNS. Then, we discussed the main mechanisms of demyelination in terms of oxidative stress, mitochondrial damage, and immuno-inflammatory responses. Finally, we summarized the existing research on the relationship between optic nerve demyelination and glaucoma, aiming to inspire effective treatment plans for glaucoma in the future.

Metformin treatment confers protection of the optic nerve following photoreceptor degeneration

Acquired or inherited or photoreceptor loss causes retinal ganglion cell loss and ultimately axonal transport alteration. Thus, therapies should be applied early during photoreceptors degeneration before the remodeling process reaches the inner retina. This study aimed to evaluate the protective effect of metformin on the rat optic nerve following photoreceptors loss induced by N-Ethyl-N-nitrosourea (ENU). Eighteen adults male Wistar rats were divided into two groups. Group I: normal vehicle control (n=6). Group II: ENU-induced photoreceptors degeneration (n=12) received a single intraperitoneal injection of ENU at a dose of 600 mg/kg. Rats in group II were equally divided into two subgroups: IIa: photoreceptor degeneration induced group and IIb: metformin treated group (200 mg/kg) for 7 days. Specimens from the optic nerve were processed for light and electron microscopy. In ENU treated group, the optic nerve revealed reduction in the diameter of the optic nerve fibers and thinning of myelin sheath with morphological changes in the glia (astrocytes, oligodendrocytes, and microglia). Caspase-3 (apoptotic marker), iNOS (oxidative stress marker) and CD68 (macrophage marker) expression increased. In metformin-treated group, the diameter of optic nerve fibers and myelin sheath thickness increased with improvement of the deterioration in the glia. Caspase-3, iNOS and CD68 expression decreased. Metformin ameliorates the histological changes of the rat optic nerve following photoreceptors loss induced by ENU.

Myelination of regenerating optic nerve axons occurs in conjunction with an increase of the oligodendrocyte precursor cell population in the adult mice

Recently, regeneration of CNS tracts has been partially accomplished by strategies of intrinsic neuronal growth stimulation. However, restoration of function is dependent on proper myelination of regenerating axons. Previous work from our group (Goulart et al., 2018) has shown an increase in oligodendrocyte staining in the regenerating optic nerve, 2 weeks after crush, in animals that were submitted to conditional deletion of pten gene in retinal ganglion cells and intravitreal injection of zymosan + cAMP. Thus, in the present study we aimed to investigate the maturation of the oligodendroglial lineage and myelination during the regeneration of the optic nerve under the same conditions of our previous work. We showed that the combined treatment promoted an increase of myelinated fibers within the optic nerve, 12 weeks after lesion, as well as an increase in Sox10 positive cells. Early-OPCs, positive to A2B5, were also increased at 12 weeks, whereas O4 positive, late-OPCs, were increased from 2 until 12 weeks after crush. At 12 weeks after crush, the optic nerve of Regenerating group presented more CC1 positive oligodendrocytes and increased MRF positive myelinating oligodendrocytes, culminating in CTB traced regenerating axons superimposed to MBP staining, suggestive of myelination. Thus, our work showed that conditional deletion of pten gene in retinal ganglion cells and intravitreal inflammatory stimuli + cAMP stimulate full maturation of the olidodendroglial lineage, from OPC proliferation and differentiation to myelination of regenerating CNS axons.

Myelination of axons emerging from neural progenitor grafts after spinal cord injury

Neural progenitor cells (NPCs) grafted to sites of spinal cord injury (SCI) extend numerous axons over long distances and form new synaptic connections with host neurons. In the present study we examined the myelination of axons emerging from NPC grafts. Rat embryonic day 14 (E14) multipotent NPCs constitutively expressing GFP were grafted into adult C5 spinal cord hemisection lesions; 3months later we examined graft-derived axonal diameter and myelination using transmission electron microscopy. 104 graft-derived axons were characterized. Axon diameter ranged from 0.15 to 1.70μm, and 24% of graft-derived axons were myelinated by host oligodendrocytes caudal to the lesion. The average diameter of myelinated axons (0.72±0.3μm) was significantly larger than that of non-myelinated axons (0.61±0.2μm, p<0.05). Notably, the G-ratio of myelinated graft-derived axons (0.77±0.01) was virtually identical to that of the normal, intact spinal cord described in published reports. These findings indicate that axons emerging from early stage neural grafts into the injured spinal cord recapitulate both the small/medium size range and myelin thickness of intact spinal cord axons.

Effect of chronic administration of sildenafil citrate (Viagra) on the histology of the retina and optic nerve of adult male rat

BACKGROUND

Abnormal vision has been reported by 3% of patients treated with sildenafil citrate (Viagra). Although many men use Viagra for an extended period for treatment of erectile dysfunction, the implications of the long term-daily use of it on the retina and optic nerve are unclear.

AIM OF THE WORK

To investigate the effect of chronic daily use of sildenafil citrate in a dose equivalent to men preferred therapeutic dose on the histology of the retina and optic nerve of adult male rat.

MATERIAL & METHODS

Eighteen adult male Wistar rats were equally divided into three groups. Group I: control. Group II: treated with sildenafil citrate orally (10mg/kg/day) for 8 weeks. Group III (withdrawal): treated as group II and then left for 4 weeks without treatment. Specimens from the retina and optic nerve were processed for light and electron microscopy.

RESULTS

In sildenafil citrate treated group, the retina and optic nerve revealed vacuolations and congested blood capillaries with apoptotic endothelial and pericytic cells, and thickened basal lamina. Caspase-3 (apoptotic marker) and CD31 (endothelial marker) expression increased. Glial cells revealed morphological changes: Müller cells lost their processes, activated microglia, astrocytic clasmatodendrosis, degenerated oligodendrocytes surrounded by disintegrated myelin sheathes of the optic nerve fibers. The retina and optic nerve of the withdrawal group revealed less vacuolations and congestion, and partial recovery of the glial cells.

CONCLUSION

Chronic treatment with sildenafil citrate (Viagra) caused toxic effect on the structure of the retina and optic nerve of the rat. Partial recovery was observed after drug withdrawal.

Rescuing axons from degeneration does not affect retinal ganglion cell death

After a traumatic injury to the central nervous system, the distal stumps of axons undergo Wallerian degeneration (WD), an event that comprises cytoskeleton and myelin breakdown, astrocytic gliosis, and overexpression of proteins that inhibit axonal regrowth. By contrast, injured neuronal cell bodies show features characteristic of attempts to initiate the regenerative process of elongating their axons. The main molecular event that leads to WD is an increase in the intracellular calcium concentration, which activates calpains, calcium-dependent proteases that degrade cytoskeleton proteins. The aim of our study was to investigate whether preventing axonal degeneration would impact the survival of retinal ganglion cells (RGCs) after crushing the optic nerve. We observed that male Wistar rats (weighing 200-400 g; n=18) treated with an exogenous calpain inhibitor (20 mM) administered via direct application of the inhibitor embedded within the copolymer resin Evlax immediately following optic nerve crush showed a delay in the onset of WD. This delayed onset was characterized by a decrease in the number of degenerated fibers (P<0.05) and an increase in the number of preserved fibers (P<0.05) 4 days after injury. Additionally, most preserved fibers showed a normal G-ratio. These results indicated that calpain inhibition prevented the degeneration of optic nerve fibers, rescuing axons from the process of axonal degeneration. However, analysis of retinal ganglion cell survival demonstrated no difference between the calpain inhibitor- and vehicle-treated groups, suggesting that although the calpain inhibitor prevented axonal degeneration, it had no effect on RGC survival after optic nerve damage.

Wallerian degeneration in the optic nerve stretch-injury model of traumatic brain injury: a stereological analysis

Patients with chronic traumatic encephalopathy (CTE) show loss of central white matter, central gray matter, and cortical gray matter with increasing post-traumatic survival. The majority of experimental studies using animals have, however, discussed only the ultrastructural pathophysiology of injured central white matter leading to secondary axotomy and the formation of axonal terminal bulbs. Using the stretch-injured optic nerve model in adult guinea pigs, the present study provides novel quantitative data concerning Wallerian degeneration of disconnected axonal fragments following secondary axotomy out to 12 weeks after injury to an optic nerve. The time course of Wallerian degeneration at the level of an individual nerve fiber is comparable to that reported in earlier studies over 48 h to two weeks after secondary axotomy. But only a relatively small proportion of nerve fibers within the optic tract degenerate via Wallerian degeneration during the first two weeks. Rather, examples of each of the three stages of Wallerian degeneration-acute axonal degeneration, latency of the distal axonal segment, and granular fragmentation-occur within the optic tract across the entire experimental survival of 12 weeks used in the present study. This data suggests that some nerve fibers initiate Wallerian degeneration days and weeks after the initial time of mechanical injury to an optic nerve. The number of intact nerve fibers continues to fall over at least three months after injury in the stretch-injury model of traumatic axonal injury. It is suggested that these novel findings relate to the mechanism(s) whereby central white matter volume decreases over months and years in CTE patients.

Role of calpains in the injury-induced dysfunction and degeneration of the mammalian axon

Axonal injury and degeneration, whether primary or secondary, contribute to the morbidity and mortality seen in many acquired and inherited central nervous system (CNS) and peripheral nervous system (PNS) disorders, such as traumatic brain injury, spinal cord injury, cerebral ischemia, neurodegenerative diseases, and peripheral neuropathies. The calpain family of proteases has been mechanistically linked to the dysfunction and degeneration of axons. While the direct mechanisms by which transection, mechanical strain, ischemia, or complement activation trigger intra-axonal calpain activity are likely different, the downstream effects of unregulated calpain activity may be similar in seemingly disparate diseases. In this review, a brief examination of axonal structure is followed by a focused overview of the calpain family. Finally, the mechanisms by which calpains may disrupt the axonal cytoskeleton, transport, and specialized domains (axon initial segment, nodes, and terminals) are discussed.

Damage to myelin and oligodendrocytes: a role in chronic outcomes following traumatic brain injury?

There is increasing evidence in the experimental and clinical traumatic brain injury (TBI) literature that loss of central myelinated nerve fibers continues over the chronic post-traumatic phase after injury. However, the biomechanism(s) of continued loss of axons is obscure. Stretch-injury to optic nerve fibers in adult guinea-pigs was used to test the hypothesis that damage to the myelin sheath and oligodendrocytes of the optic nerve fibers may contribute to, or facilitate, the continuance of axonal loss. Myelin dislocations occur within internodal myelin of larger axons within 1–2 h of TBI. The myelin dislocations contain elevated levels of free calcium. The volume of myelin dislocations increase with greater survival and are associated with disruption of the axonal cytoskeleton leading to secondary axotomy. Waves of Ca²⁺ depolarization or spreading depression extend from the initial locus injury for perhaps hundreds of microns after TBI. As astrocytes and oligodendrocytes are connected via gap junctions, it is hypothesized that spreading depression results in depolarization of central glia, disrupt axonal ionic homeostasis, injure axonal mitochondria and allow the onset of axonal degeneration throughout an increasing volume of brain tissue; and contribute toward post-traumatic continued loss of white matter.

Calpains mediate axonal cytoskeleton disintegration during Wallerian degeneration

In both the central nervous system (CNS) and peripheral nervous system (PNS), transected axons undergo Wallerian degeneration. Even though Augustus Waller first described this process after transection of axons in 1850, the molecular mechanisms may be shared, at least in part, by many human diseases. Early pathology includes failure of synaptic transmission, target denervation, and granular disintegration of the axonal cytoskeleton (GDC). The Ca(2+)-dependent protease calpains have been implicated in GDC but causality has not been established. To test the hypothesis that calpains play a causal role in axonal and synaptic degeneration in vivo, we studied transgenic mice that express human calpastatin (hCAST), the endogenous calpain inhibitor, in optic and sciatic nerve axons. Five days after optic nerve transection and 48 h after sciatic nerve transection, robust neurofilament proteolysis observed in wild-type controls was reduced in hCAST transgenic mice. Protection of the axonal cytoskeleton in sciatic nerves of hCAST mice was nearly complete 48 h post-transection. In addition, hCAST expression preserved the morphological integrity of neuromuscular junctions. However, compound muscle action potential amplitudes after nerve transection were similar in wild-type and hCAST mice. These results, in total, provide direct evidence that calpains are responsible for the morphological degeneration of the axon and synapse during Wallerian degeneration.

Ultrastructural characterization of tryptophan hydroxylase 2-specific cortical serotonergic fibers and dorsal raphe neuronal cell bodies after MDMA treatment in rat

RATIONALE

3,4-Methylenedioxymethamphetamine (MDMA, "ecstasy") is a widely used recreational drug known to cause selective long-term serotonergic damage.

OBJECTIVES

The aim of this study was to characterize the ultrastructure of serotonergic pericarya and proximal neurites in the dorsal raphe nucleus as well as the ultrastructure of serotonergic axons in the frontal cortex of adolescent Dark Agouti rats 3 days after treatment with 15 mg/kg i.p. MDMA.

METHODS

Light microscopic immunohistochemistry and pre-embedding immunoelectron microscopy with a novel tryptophan hydroxylase-2 (Tph2) specific antibody, as a marker of serotonergic structures.

RESULTS

Light microscopic analysis showed reduced serotonergic axon density and aberrant swollen varicosities in the frontal cortex of MDMA-treated animals. According to the electron microscopic analysis, Tph2 exhibited diffuse cytoplasmic immunolocalization in dorsal raphe neuronal cell bodies. The ultrastructural-morphometric analysis of these cell bodies did not indicate pathological changes or significant alteration in the cross-sectional areal density of any examined organelles. Proximal serotonergic neurites in the dorsal raphe exhibited no ultrastructural alteration. However, in the frontal cortex among intact fibers, numerous serotonergic axons with destructed microtubules were found. Most of their mitochondria were intact, albeit some injured axons also contained degenerating mitochondria; moreover, a few of them comprised confluent membrane whorls only.

CONCLUSIONS

Our treatment protocol does not lead to ultrastructural alteration in the serotonergic dorsal raphe cell bodies and in their proximal neurites but causes impairment in cortical serotonergic axons. In these, the main ultrastructural alteration is the destruction of microtubules although a smaller portion of these axons probably undergo an irreversible damage.

Local anesthetic sciatic nerve block and nerve fiber damage in diabetic rats

BACKGROUND AND OBJECTIVES

A concern for anesthesiologists is whether local anesthetics are more toxic to peripheral nerves in diabetic patients. A previous study in streptozotocin-induced diabetic rats showed that larger doses of lidocaine produce moderate nerve injury after nerve block in normal rats and worse injury in diabetic rats. However, it is not clear whether a smaller local anesthetic dose that produces negligible nerve fiber damage in normal rats will produce significant nerve damage in diabetic rats and if adding adjuvant drugs modulates this effect.

METHODS

Rats were intravenously injected with 50 mg/kg streptozotocin to induce diabetes (blood glucose levels 9250 mg/dL) and diabetic neuropathy. After waiting 35 days, an injection (0.1 mL) of 1% lidocaine alone, or with 5 kg/mL epinephrine or 7.5 kg/mL clonidine added, or 0.5% ropivacaine alone was performed at the left sciatic notch in both diabetic and nondiabetic rats. The duration of sensory (pin prick) and motor (toe spreading reflex) nerve block in the hind paws was determined.For histologic controls, all rats also received saline vehicle injection into the right sciatic notch. Another group of uninjected rats was used as naive controls. Left and right nerves were removed 2 days after injection and fixed in situ with a 4% glutaraldehyde solution. Myelinated axon profiles suggestive of neuropathy (myelin figures, pale and swollen,or dark-staining axoplasm) were counted and expressed as a percentage of the total number of fibers in each rat sciatic nerve.

RESULTS

All streptozotocin-injected rats became diabetic and had pronounced tactile allodynia. All rats had sensory and motor nerve blocks lasting for at least 50 mins after injection of local anesthetic. The duration of sensory and motor nerve block was longer in diabetic rats than in nondiabetic rats for all drug groups tested. None of the sciatic nerves examined showed greater than 3% nerve fiber degeneration. Although lidocaine in diabetic rats did not produce nerve fiber damage,diabetic rats receiving lidocaine/clonidine or ropivacaine had more abnormal myelinated axon profiles than did nondiabetic rats receiving the same drug.

CONCLUSIONS

The duration of sciatic nerve block with local anesthetics is longer in diabetic compared with nondiabetic rats. A small, but statistically significant, increase in nerve damage occurred in diabetic rats after nerve block with ropivacaine alone or when duration of lidocaine block was extended with clonidine. These findings may have implications for dosing of local anesthetics in diabetic patients undergoing regional analgesia with nerve blocks.

Wallerian-like axonal degeneration in the optic nerve after excitotoxic retinal insult: an ultrastructural study

Background

Excitotoxicity is involved in the pathogenesis of a number neurodegenerative diseases, and axonopathy is an early feature in several of these disorders. In models of excitotoxicity-associated neurological disease, an excitotoxin delivered to the central nervous system (CNS), could trigger neuronal death not only in the somatodendritic region, but also in the axonal region, via oligodendrocyte N-methyl-D-aspartate (NMDA) receptors. The retina and optic nerve, as approachable regions of the brain, provide a unique anatomical substrate to investigate the "downstream" effect of isolated excitotoxic perikaryal injury on central nervous system (CNS) axons, potentially providing information about the pathogenesis of the axonopathy in clinical neurological disorders.

Herein, we provide ultrastructural information about the retinal ganglion cell (RGC) somata and their axons, both unmyelinated and myelinated, after NMDA-induced retinal injury. Male Sprague-Dawley rats were killed at 0 h, 24 h, 72 h and 7 days after injecting 20 nM NMDA into the vitreous chamber of the left eye (n = 8 in each group). Saline-injected right eyes served as controls. After perfusion fixation, dissection, resin-embedding and staining, ultrathin sections of eyes and proximal (intraorbital) and distal (intracranial) optic nerve segments were evaluated by transmission electron tomography (TEM).

Results

TEM demonstrated features of necrosis in RGCs: mitochondrial and endoplasmic reticulum swelling, disintegration of polyribosomes, rupture of membranous organelle and formation of myelin bodies. Ultrastructural damage in the optic nerve mimicked the changes of Wallerian degeneration; early nodal/paranodal disturbances were followed by the appearance of three major morphological variants: dark degeneration, watery degeneration and demyelination.

Conclusion

NMDA-induced excitotoxic retinal injury causes mainly necrotic RGC somal death with Wallerian-like degeneration of the optic nerve. Since axonal degeneration associated with perikaryal excitotoxic injury is an active, regulated process, it may be amenable to therapeutic intervention.

The fate of axons subjected to traumatic ultrastructural (neurofilament) compaction: an electron-microscopic study

By means of a new head-injury apparatus, a 0.75-mm-deep depression was produced momentarily at a predetermined site of the rat calvaria. This immediately evoked ultrastructural (neurofilament) compaction in many myelinated axon segments in layers IV and V of the neocortex under the impact site. The affected axon segments run quasi-parallel to the brain surface in a diffuse distribution among normal axons. Other kinds of damage to the brain tissue were insignificant; the conditions were therefore favorable for investigation of the fate of the compacted axons. Quantitative analysis of the findings on groups of ten rats that were sacrificed either immediately after the head injury or following a 1 day or a 1 week survival period showed that around 50% of the compacted axons recovered in 1 day, and a further less than 10% did so in 1 week. Electron microscopy revealed that the non-recovering compacted axons underwent a sequence of degenerative morphological changes including homogenization, fragmentation and resorption of the fragments. However, the myelin sheaths around these degenerating axons remained apparently unchanged even in the long-surviving rats, and hardly any phagocytotic cells were encountered. On the other hand, many such myelin sheaths contained axolemma-bound, normal-looking axoplasm besides the above morphological signs of axon-degeneration. It is concluded that the non-recovering compacted axons undergo an uncommon (non-Wallerian) kind of degeneration, which is mostly reversible.

Age-related neural degeneration in nuclear-factor κB p50 knockout mice

Nuclear factor-kappaB is a transcription factor that regulates a variety of genes involved not only with immune and inflammatory responses, but also in cell survival. Nuclear-factor kappaB in the CNS is an area of current research interest; however, its role in age-related neural degeneration is obscure. The present study examines developmental degeneration changes in wild type and nuclear factor-kappaB p50 subunit knockout mice (p50-/-) using various morphological methodologies. P50-/- mice appeared normal at birth. At 6 and 10 months old, the body weight of p50-/- mice was significantly less than that of wild type mice and they started to die from aging. Consistently, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end-labeling positive cells in the cortex were significantly more in p50-/- mice than that in wild type mice, and neuronal cells in the cortex, hippocampus and caudate nucleus-putamen decreased in p50-/- mice. Fewer myelinated axons of the optic nerve were found in p50-/- mice than in wild type mice at 6 months. In p50-/- mice, morphological examinations showed: 1) aging and degenerative changes in the cortex and hippocampus including increased lipofuscin granules in neural cytoplasm, 2) abnormal capillaries, 3) dark and watery alterations and organelle accumulations, 4) apoptotic glia cells, and 5) terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end-labeling and caspase-3 positive neurons. These results suggest that nuclear-factor kappaB may play an important role in neurovascular development, cell survival, and the aging process in the CNS. This new evidence linking nuclear-factor kappaB to myelination and aging may be of considerable importance for several areas of basic and clinical neuroscience.

Ultrastructural Study of Normal and Degenerating Nerve Fibers in the Protocerebral Tract of the Crab Ucides cordatus

Wallerian degeneration is a very well described phenomenon in the vertebrate nervous system. In arthropods, and especially in crustaceans, nerve fiber degeneration has not been described extensively. In addition, literature shows that the events do not follow the same patterns as in vertebrates. In this study we report, by qualitative and quantitative ultrastructural analyses, the features and time course of the protocerebral tract degeneration following extirpation of the optic stalk. No remarkable changes were observed seven days after lesion. After 28 days the protocerebral tracts presented apparently preserved small and large diameter axons and some degenerating medium axons, with irregular contours and empty-looking aspect of the axoplasm. Forty days after the ablation of the optic stalks, both small (type I) and medium (type II and III) axons revealed signs of partial or total degeneration, but large nerve fibers (type IV) were still intact. After 45 days, the tract showed signs of advanced stage of degeneration and, apart from large axons, normal-looking fibers were almost absent. At these 3 last time points, degenerating axons displayed different electron densities and aspects, probably correlating to different onset times of the process. In addition, cells with granules in their cytoplasm, possibly hemocytes, were quite distinct, especially at 40 and 45 days after axotomy. These cells might share with glial cells the function of phagocytosis of cellular debris during the protocerebral tract degeneration. Quantitative analysis showed that the number of degenerating fibers increased significantly from 28 to 40 days after lesion, whereas the number of normal fibers decreased accordingly. Measurements of cross-sectional areas of normal and degenerating axons showed that types II and III (medium) start to degenerate before type I (small). Type IV (large) axons do not degenerate, even after 40 days. Therefore, we can conclude that degeneration in these afferent fibers starts late after axotomy, but proceeds at a faster rate afterwards until the complete degeneration of small and medium axons.

Calpain inhibitor 2 prevents axonal degeneration of opossum optic nerve fibers

The ultrastructural change that characterizes the onset of Wallerian degeneration is the disintegration of axoplasmic microtubules and neurofilaments, which are converted into an amorphous and granular material, followed by myelin breakdown. The mechanism underlying such processes is an increase in the amount of intracellular calcium, leading to activation of proteases called calpains. The aim of this study was to evaluate by quantitative ultrastructural analysis whether nerve fibers can be preserved by the use of an exogenous inhibitor of these proteases (calpain inhibitor-2, Mu-F-hF-FMK), after optic nerve crush. For that, the left optic nerves of opossums, Didelphis aurita, were crushed with the aid of a fine forceps, and half of them received a calpain inhibitor mixed with Elvax resin. Ninety-six hours after the lesion, the animals were reanesthetized and transcardially perfused, and the optic nerves were removed, the right ones being used as normal nerves. Afterward, the optic nerves were dissected and processed for routine transmission electron microscopy and quantitative and statistical analysis. The results of this analysis showed that the group that received the calpain inhibitor presented a reduction of astrogliosis, maintaining the optic nerve structure in an organized state; a significant decrease in the number of degenerating fibers; and a significant increase in the number of fibers with preserved cytoskeleton and preservation of axonal and myelin area and integrity, reducing the enlargement and edema of the axon. In conclusion, our findings suggest that calpain inhibitor is able to provide neuroprotection of the central nervous system fibers after a crush lesion.

Participation of neurofilament proteins in axonal dark degeneration of rat's optic nerves

Neurofilaments (NF) are neuronal intermediate filaments formed by three different subunits: high (NF-H), medium (NF-M) and light (NF-L). They are responsible for the determination and maintenance of axon caliber. Accumulation of NF or their immunoreactive products are components of several neurodegenerative disease lesions, such as neurofibrillary tangles, Lewy bodies and the spheroids of amyotrophic lateral sclerosis. Also, cytoskeletal breakdown is one of the first ultrastructural changes occurring after nerve crush or section. In the present study, Wistar rats were subjected to bilateral enucleation to induce Wallerian degeneration of optic nerve fibers and perfused 24 h, 48 h and 1 week later. Optic nerve segments were processed for electron microscopy (EM), light microscopy immunofluorescence (LM) and immunoelectronmicroscopy (IEM) for NF subunit detection. LM for NF of control nerves showed a slightly different pattern and intensity for each subunit, with more intense staining of NF-M and NF-H and less intense staining of NF-L. This reaction did not change considerably at 48 h, but was severely reduced 1 week after enucleation. Results of EM showed fibers in: (1) partial cytoskeleton degeneration or (2) watery degeneration or (3) dark degeneration. The number of dark degenerating axons was statistically higher at the latest time-interval studied. Neurofilament clumping areas and dark degenerating axons showed positive immunostaining for the three neurofilaments subunits when examined by IEM. These results suggest that dark degenerating axons develop from areas of neurofilament aggregation. We may also conclude that NF proteins participate in the process of axonal dark degeneration.