Immunohistochemical studies on neurofilamentous hypertrophy in degenerating retinal terminals of the olivary pretectal nucleus in the rat

Immunoelectron microscopy reveals the presence of neurofilament proteins in retinal terminals undergoing dark degeneration

After nerve crushing or section, the distal stump undergoes morphological changes described as Wallerian degeneration (WD). Immediately after nerve injury, early ultrastructural alterations occur in the terminal boutons, a process known as terminal degeneration (TD), which occurs before degeneration of the axon and leads to electrophysiological impairment. In this study we investigated the presence of neurofilament (NF) proteins in TD and compared the results with degeneration in the optic nerve. Young adult Wistar rats were submitted to bilateral enucleation and perfused after 24 h, 48 h and 1 week. Optic nerves (ON) and superior colliculus (SC) segments were processed for electron microscopy (EM) and immunoelectron microscopy (IEM) for NF subunits. Analysis of ultrathin sections of SC, at 24 h, revealed terminals undergoing TD. At 48 h and 1 week after enucleation, there was a clear increase in the number of degenerating terminals. The cytoarchitecture of the optic nerve did not change considerably at 24 h, but it was progressively altered at 48 h and 1 week after enucleation, when we observed intense astrogliosis, and most fibers exhibited dark degeneration (DD). The IEM for the NF subunits of normal ON showed gold particles located along the filaments, but we did not observe labeling for neurofilament proteins in normal retinal terminals. However, 48 h after lesion, we observed immunogold particles for the NF proteins in fibers undergoing DD and on terminals undergoing TD. Therefore, we can conclude that NF proteins participate in the process of TD, and this event occurs before complete axonal degeneration, suggesting different mechanisms for TD and DD.

Identification and characterization of SMI32-immunoreactive amacrine cells in the mouse retina

Mammalian neurons express the neural intermediate filament protein neurofilament (NF). In the retina, NFs have been detected primarily in the axons and processes of retinal ganglion and horizontal cells. We found an amacrine cell type that was immunolabeled with an antibody against SMI32, a non-phosphorylated epitope on neurofilament proteins of high molecular weight, in the mouse retina. This type of amacrine cell was non-randomly distributed, and these cells exhibited a central-peripheral density gradient. Most of these cells co-expressed GABA and ChAT, but not glycine or any other amacrine cell marker. These results suggest that some SMI32-immunoreactive amacrine cells belong to a GABAergic population, and that SMI32 can therefore be used as a marker for a subset of amacrine cells in addition to ganglion cells and horizontal cells in the mouse retina.

Activity-Driven Synaptic and Axonal Degeneration in Canine Motor Neuron Disease

The role of neuronal activity in the pathogenesis of neurodegenerative disease is largely unknown. In this study, we examined the effects of increasing motor neuron activity on the pathogenesis of a canine version of inherited motor neuron disease (hereditary canine spinal muscular atrophy). Activity of motor neurons innervating the ankle extensor muscle medial gastrocnemius (MG) was increased by denervating close synergist muscles. In affected animals, 4 wk of synergist denervation accelerated loss of motor-unit function relative to control muscles and decreased motor axon conduction velocities. Slowing of axon conduction was greatest in the most distal portions of motor axons. Morphological analysis of neuromuscular junctions (NMJs) showed that these functional changes were associated with increased loss of intact innervation and with the appearance of significant motor axon and motor terminal sprouting. These effects were not observed in the MG muscles of age-matched, normal animals with synergist denervation for 5 wk. The results indicate that motor neuron action potential activity is a major contributing factor to the loss of motor-unit function and degeneration in inherited canine motor neuron disease.

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.

Neuronal response to physical injury and its relationship to the pathology of Alzheimer's disease

1. Central nerve cells undergo a stereotyped regenerative response following physical injury. 2. This reaction involves adaptive changes within the axon and cell body of origin, directed at sprouting and synaptogenesis. 3. Intimately associated with the regenerative response are specific alterations to cytoskeletal proteins, including the neurofilament (NF) triplet. 4. The morphological and neurochemical alterations to NF within axons following injury are reminiscent of plaque-associated dystrophic neurites (DN) in early Alzheimer's disease (AD). 5. Associated changes in perikaryal NF resemble Alzheimer neurofibrillary tangle pathology, while growth-associated sprouting markers are localized to the abnormal neurites of AD. 6. The present review postulates that beta-amyloid plaques in AD cause physical damage to local nerve cell processes and it is the chronic stimulation of the stereotyped response to injury that results in the end-stage pathology and neurodegeneration associated with AD.

An immunohistochemical marker for Wallerian degeneration of fibers in the central and peripheral nervous system

This work was prompted by the accidental observation that a newly developed, affinity purified polyclonal antibody against the C-terminus of the neuropeptide tyrosine (NPY) Y1-receptor protein decorates degenerating fibers in the central nervous system (CNS). This staining did not appear in control animals in which the antibody marked perikarya and dendrites at previously described locations [X. Zhang, L. Bao, Z.-Q. Xu, J. Kopp, U. Arvidsson, R. Elde, T. Hökfelt, Localization of neuropeptide Y Y1-receptors in the rat nervous system with special reference to somatic receptors on small dorsal root ganglion neurons, Proc. Natl. Acad. Sci. USA 91 (1994) 11738-11742]. Three models of experimental lesions were studied: sciatic nerve transection, spinal cord transection and parietal cortex thermocoagulation. In each model, animals were divided in groups (n=2) and processed for indirect immunofluorescence at different time intervals up to 28 days post-lesion (PL) (see below). All three experimental lesions produced a very intense immunolabeling of fibers in the projection pathways of the lesioned structures, strongly reminding of Wallerian degeneration (WD). In the sciatic nerve, the staining first appeared on day 1 PL, was strongly increased on day 3 PL, then declined after 7 days and had almost completely disappeared after 14 days. In the CNS, the staining appeared later and was first observed on day 3 PL and remained for a longer period, thus showing different time courses in the brain and spinal cord as compared to the sciatic nerve. The labeling was completely abolished, both in the CNS and in the sciatic nerve, by pre-incubation of the Y1-R antibody with the immunogenic peptide at a dilution of 10-6 M. The appearance of the staining and its time course strongly suggest that the process was related to degenerating axons. Although the protein actually detected remains to be determined, it is suggested that the staining ability of this antibody could be used as a positive marker of axonal degeneration following experimental or naturally occurring lesions of the nervous system.

Phosphorylated and non-phosphorylated neurofilament proteins: distribution in the rat hippocampus and early changes after kainic acid induced seizures

The regional distribution of neurofilament proteins in the rat hippocampus and their early changes after kainic acid induced seizures were investigated immunocytochemically with antibodies against light weight neurofilament, phosphorylated and non-phosphorylated heavy weight neurofilament. The light weight and non-phosphorylated heavy weight neurofilaments were distributed more unevenly than the phosphorylated neurofilament. The perikarya and processes of pyramidal cells in the CA3 field contained the highest light weight and non-phosphorylated heavy weight neurofilaments, while the perikarya of granule cells contained only few light weight neurofilament and the perikarya of CA1 pyramidal cells were even devoid of immunoreactivity of both light and heavy weight neurofilaments. The fiber staining of the light weight and non-phosphorylated heavy weight neurofilaments, especially the former, was less in the CA1 field and molecular layer of dentate gyrus. The phosphorylated neurofilament immunoreactivity was identified only in axons. Mossy fibers, the axons of granule cells, contained the light weight and phosphorylated heavy weight neurofilaments, but not the non-phosphorylated neurofilament. Seven days after the kainic acid induced seizures, the phosphorylated neurofilament staining was greatly reduced in the CA1 and inner molecular layer of the dentate gyrus, probably resulting from the axonal degeneration of the Schaffer collaterals and the commissural/associational fibers. Furthermore, the nonphosphorylated neurofilament appeared in the mossy fibers of the CA3 stratum lucidum, which normally do not express such immunoreactivity. The results indicate that the neurofilaments are altered following the neuronal degeneration and postlesional plasticity caused by the kainic acid administration. Therefore, the examination of various phosphorylated neurofilaments may offer a comprehensive understanding of major hippocampal pathways, axonal plasticity and the possible roles of neurofilaments in the hippocampus following excitotoxic insults.

Transient immunohistochemical labelling of rat retinal axons during Wallerian degeneration by a monoclonal antibody to neurofilaments

Immunohistochemical labelling with the monoclonal antibody SMI32 to non-phosphorylated epitopes on neurofilament proteins of high molecular weight class was low in rat central optic fibers of controls. After unilateral transection of optic nerve, a strong, transient increase of labelling with SMI32 occurred in degenerating fibers of optic tract at 2 and 4 days, which then declined at 8 and remained low at 21 days. Consequently, immunostaining with SMI32 may serve as a positive marker for degenerating fibers in rat optic system.