Quantitative evaluation of axonal regeneration by immunochemical assay for neurofilament protein

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.

Bone marrow transplants provide tissue protection and directional guidance for axons after contusive spinal cord injury in rats

Contusive spinal cord injury (SCI) produces large fluid-, debris- and inflammatory cell-filled cystic cavities that lack structure to support significant axonal regeneration. The recent discovery of stem cells capable of generating central nervous system (CNS) tissues, coupled with success in neurotransplantation strategies, has renewed hope that repair and recovery from CNS trauma is possible. Based on results from several studies using bone marrow stromal cells (MSCs) to promote CNS repair, we transplanted MSCs into the rat SCI lesion cavity to further investigate their effects on functional recovery, lesion morphology, and axonal growth. We found that transplanted MSCs induced hindlimb airstepping--a spontaneous locomotor movement associated with activation of the stepping control circuitry--but did not alter the time course or extent of overground locomotor recovery. Using stereological techniques to describe spinal cord anatomy, we show that MSC transplants occupied the lesion cavity and were associated with preservation of host tissue and white matter (myelin), demonstrating that these cells exert neuroprotective effects. The tissue matrix formed by MSC grafts supported greater axonal growth than that found in specimens without grafts. Moreover, uniform random sampling of axon profiles revealed that the majority of neurites in MSC grafts were oriented with their long axis parallel to that of the spinal cord, suggesting longitudinally directed growth. Together, these studies support further investigation of marrow stromal cells as a potential SCI repair strategy.

Damage, reorganization, and abnormal neocortical hyperexcitability in the pilocarpine model of temporal lobe epilepsy

PURPOSE

Clinical, neuropathological, and electrophysiological data have shown that limbic structures are involved in the pathogenesis of temporal lobe epilepsy (TLE). In most cases, limbic-originated seizures frequently spread to extrahippocampal areas. It is unclear whether such distant circuitries, especially the neocortex, exhibit abnormal electrophysiology as consequences of a chronic epileptogenic process. The present research studied neuropathological abnormalities and in vitro electrophysiological properties of sensorimotor neocortex in pilocarpine-treated epileptic rats.

METHODS

Adult epileptic animals showing six to seven seizures/week and saline-injected rats were selected for neurohistology. Coronal sections were sampled throughout the anteroposterior extent of the diencephalon and stained with cresyl violet (Nissl). Immunocytochemistry (ICC) was performed using anti-neurofilament (SMI-311) antibody. Extracellular (layer II/III) and intracellular (layer V) recordings were performed in coronal sensorimotor neocortical slices. Several electrophysiological aspects were examined such as evoked responses, intrinsic properties, and firing patterns of layer V pyramidal cells.

RESULTS

Nissl staining showed a significant decrease of cortical thickness in epileptic rats when compared with controls, particularly in superficial layers (II-IV). Such abnormalities were also revealed by SMI-311 staining. SMI-311-labeled dendrite arborizations were more complex in layers I-II of epileptic rats. Epileptic rats manifested several abnormalities in extracellular field responses including hyperresponsiveness and presence of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-mediated polysynaptic activity. Although no significant changes were observed concerning passive intrinsic properties, it was possible to detect a higher proportion of bursting neurons distributed in layer V (60%) of epileptic rats compared with 22% in control slices.

CONCLUSIONS

Taken together, our findings indicate damage, reorganization, and chronic hyperexcitability of sensorimotor neocortex in experimental TLE.

Increased expression of the urokinase plasminogen-activator receptor in amyloid beta peptide-treated human brain microglia and in AD brains

The urokinase plasminogen-activator receptor (uPAR) is involved in many processes in inflammation including the migration of inflammatory-associated cells to sites of tissue damage. This receptor, also designated as CD87, is induced in response to a range of stimuli and is a marker of macrophage activation. Its role in inflammatory responses of microglia in Alzheimer's disease (AD) has not been previously investigated. In this study we demonstrate that uPAR mRNA and protein expression is induced following incubation of human post-mortem brain-derived microglia with fibrillar amyloid beta (Abeta) peptide. This response was stronger with Abeta peptide than with other tested pro-inflammatory agents. Induction of uPAR surface expression by microglia was inhibited by the antioxidant N-acetyl-cysteine, indicating that this gene may be induced as a result of oxidative stress-related mechanisms. The significance of these findings to AD was investigated. UPAR protein levels were significantly increased in human brain tissues from the hippocampus, superior frontal gyrus and inferior temporal gyrus of AD cases compared with similar tissues from non-demented cases. Increased uPAR expression was not demonstrated in AD cerebellum. Finally, increased uPAR immunoreactivity was demonstrated in activated microglia in AD brain samples using two different antibodies to uPAR. These results provide a connection between the induction of oxidative stress in AD and microglial activation, and establish a possible involvement of uPAR in AD pathogenesis.

Intracerebroventricular kainic acid administration in adult rat alters hippocampal calbindin and non-phosphorylated neurofilament expression

Calbindin and non-phosphorylated neurofilament proteins were assessed in hippocampus following a unilateral intracerebroventricular kainic acid injection at 4, 26, and 60 days post-lesion, using immunocytochemical expression. The density of calbindin-positive non-pyramidal neurons throughout the hippocampus showed no significant alteration at 4 days post-lesion, a significant decrease at 26 days post-lesion, and a partial recovery at 60 days post-lesion. In addition, calbindin immunoreactivity was dramatically reduced at 26 days post-lesion in the CA1 pyramidal and dentate granule cell layers and the mossy fibers, bilaterally. Although not significant statistically, most of these reductions showed signs of reversal at 60 days post-lesion except the CA1 pyramidal cell layer where the dramatic reductions persisted. Neurofilaments were also altered throughout the post-lesion period, particularly in abnormal expression of non-phosphorylated neurofilament proteins in mossy fibers. The apparent return of calbindin immunoreactivity in non-pyramidal neurons by 60 days post-lesion suggests that recovery from the lesion may involve remaining neuronal elements which either become reactivated with time or have the capability to express normal levels of calbindin with re-innervation. On the other hand, prolonged calbindin reductions in superficial CA1 pyramidal cells suggest sustained down-regulation of calbindin expression owing to persistent reductions in the activity of these neurons. The temporal correlation of the expression of non-phosphorylated neurofilaments in mossy fibers with their sprouting response following target loss suggests a potential role for non-phosphorylated neurofilaments in neuronal plasticity involving axonal sprouting. Alternatively, it may also suggest that injury-induced neurofilament modifications are either conducive or permissive for axonal sprouting.

Phosphorylated neurofilament antigen redistribution in intercostal nerve subsequent to retrograde axonal transport of diphtheria toxin

A novel enzyme-linked immunosorbent assay technique using specific monoclonal antibodies has been used to examine the proximal-distal distribution of phosphorylated neurofilament proteins (pNF) in normal feline intercostal nerve, and to compare it with that following retrograde axonal transport of the ADP-ribosylating protein diphtheria toxin (DTX) to thoracic motoneurones. The molecular target of DTX is elongation factor 2 which resides solely in the cell body. Normal intercostal nerves exhibited significantly higher amounts of the 200-kDa pNF-H, 160-kDA pNF-M, and 68-kDA pNF-L in proximal nerve than in the distal nerve. The overall content of all three triplet pNF proteins decreased 3 days after injection of DTX, but the normal proximal-distal gradient was retained. By 8 days post DTX injection, the proximal-distal gradient had reversed, with proximal nerve starved of pNF-H and pNF-M and distal nerve showing abnormally high pNF-L content. Correlative immunocytochemistry of spinal cords from normal animals verified that pNF-H and pNF-M are confined to efferent axons in the spinal grey matter, and that motoneurones are only reactive for pNF-L. At 8 days following toxin treatment, motoneurones in the ipsilateral ventral horn were strongly immunoreactive for all pNF. Contralateral motoneurones were non-reactive. Onset of abnormal perikaryal pNF immunoreactivity at 3 days precedes onset of ultrastructural cytopathology. Together these results indicate an early deficit in transference to the axon of NF proteins synthesised prior to full toxicity, probably because of a toxin-induced failure in regulation of phosphorylation-dependent NF assembly and turnover immediately prior to entry into the proximal axon. Results are discussed in relation to diphtheritic motoneuronopathy.

Early and late appearance of neurofilament phosphorylation events in nerve regeneration

Neurofilament phosphorylation in regenerating rat sciatic nerve was studied by indirect immunofluorescence with monoclonal antibodies reacting with phosphorylated epitopes of the 2 large polypeptides of the neurofilament protein triplet (NF 150K, NF 200K). One group of antibodies decorated axons early in the process. In fact, no differences were seen in double labeled sections between these antibodies and polyclonal neurofilament antibodies as to their reactivity with the distal stump of transected sciatic nerves. Another group stained axons after they had completed their elongation, i.e., after they had reached the distal part of the denervated sciatic nerve. In general, the epitopes recognized by antibodies in this group appeared more sensitive to phosphatase digestion as compared to the first group. Furthermore, there was a good correlation between the thickness of the regenerated axons and staining with these monoclonal antibodies. Thick axons (like those observed in normal nerves) were stained, while bundles of thin axons remained unstained. Monoclonal II32 stained regenerated axons in a remarkable segmental pattern. With this antibody, continuous decoration of the axons was still not observed 7 weeks after transection, the longest follow-up period in this study. We suggest that some neurofilament phosphorylation events may contribute to the stabilization of the axonal cytoskeleton and that abnormalities persist in regenerated axons as to the extent of neurofilament phosphorylation.

Neurofilament elongation into regenerating facial nerve axons

Immunocytochemistry was used to show that neurofilaments advance into regenerating facial nerve axons at 2.5 mm/day, which is less than the rate of axonal elongation (4.3 mm/day), measured from the transport of radiolabeled protein into the axons. Thus, the distal region of the newly-regenerated axons is deficient in neurofilaments, and this was confirmed by electron microscopy. These neurofilament-free regenerating axons could also be detected by immunocytochemistry using antibody to protein B50 (GAP43), a component of growth-cones. Immunoblots of nerve segments, incubated with monoclonal antibodies against the three neurofilament proteins, showed that all three proteins were present in the neurofilaments elongating into the regenerating axons, and confirmed the more distal extensions of B50 immunoreactivity. These results show that neurofilament immunocytochemistry underestimates the extent of axonal regeneration, and it is suggested that this technique should be employed with caution in regeneration studies. When the facial nerve received a conditioning lesion 7 days prior to a test lesion, axonal regeneration rate increased to 6.0 mm/day, and there was a proportional increase in neurofilament elongation rate to 4.4 mm/day. This occurred in spite of the reduction in cell body neurofilament protein synthesis induced by the lesions. It is concluded that the rate of neurofilament extension into regenerating axons is not governed by cell body synthesis but by local interactions with other cytoskeletal materials which support the increased regeneration rate of conditioned axons.