Cytoskeletal abnormalities in amyotrophic lateral sclerosis: beneficial or detrimental effects?

Abstract

Cytoskeletal abnormalities have been reported in cases of amyotrophic lateral sclerosis (ALS) including abnormal inclusions containing neurofilaments (NFs) and/or peripherin, reduced mRNA levels for the NF light (NF-L) protein and mutations in the NF heavy (NF-H) gene. Recently, transgenic mouse approaches have been used to address whether cytoskeletal changes may contribute to motor neuron disease. Mice lacking one of the three NF subunits are viable and do not develop motor neuron disease. Nonetheless, mice with null mutations for NF-L or for both NF-M and NF-H genes developed severe atrophy of ventral and dorsal root axons. The atrophic process is associated with hind limb paralysis during aging in mice deficient for both NF-M and NF-H proteins. The overexpression in mice of transgenes coding for wild-type or mutant NF proteins can provoke abnormal NF accumulations, axonal atrophy and sometimes motor dysfunction. However, the perikaryal NF accumulations are generally well tolerated by motor neurons and, except for expression of a mutant NF-L transgene, they did not provoke massive motor neuron death. Increasing the levels of perikaryal NF proteins may even confer protection in motor neuron disease caused by ALS-linked mutations in the superoxide dismutase (SOD1). In contrast, the overexpression of wild-type peripherin, a type of IF gene upregulated by inflammatory cytokines, provoked the formation of toxic IF inclusions with the high-molecular-weight NF proteins resulting in the death of motor neurons during aging. These results together with the detection of peripherin inclusions at early stage of disease in mice expressing mutant SOD1 suggest that IF inclusions containing peripherin may play a contributory role in ALS pathogenesis.

Introduction

Amyotrophic lateral sclerosis (ALS) is an adult-onset and heterogeneous neurological disorder that affects primarily large motor neurons in the brain and spinal cord. The degeneration of motor neurons leads to denervation atrophy of skeletal muscles and, ultimately, to paralysis and death. Current evidence suggests that multiple genetic and environmental factors may be implicated in ALS pathogenesis. For the familial ALS cases, which correspond to ∼10% ALS cases, the disease is inherited in an autosomal dominant manner with varying degree of penetrance. Mutations in the gene coding for the Cu/Zn superoxide dismutase (SOD1) located on chromosome 21 have been found in ∼20% of familial cases [1], [2] but for the vast majority of ALS cases the etiology remains unknown.

A common and perhaps universal pathological finding in ALS is the presence of abnormal accumulations of intermediate filaments (IF) proteins in the perikaryon and axon of motor neurons [3], [4], [5]. Whether IF accumulations contribute to pathogenesis of human ALS remains unknown. The accumulations of IFs in ALS could simply be the consequence of neuronal dysfunction. However, there is emerging evidence suggesting that IF aggregates can sometimes have detrimental as well as protective effects in motor neuron disease. This review focuses on the recent studies aiming to clarify the role of IF proteins in motor neuron disease with particular emphasis on transgenic mouse data.

The neuronal cytoskeleton is composed of three interconnected filaments: the actin microfilaments, microtubules and IFs. Neurofilaments (NFs), which are the major type of IFs in adult motor neurons, are made by the copolymerization of three proteins, the NF light (NF-L, 61 kDa), medium (NF-M, 90 kDa) and heavy (NF-H, 115 kDa) proteins. There is evidence of NF abnormalities in human ALS. NF accumulations are frequently detected in motor neurons of both sporadic and familial ALS cases [3], [4], [5]. However, it remains unknown how these NF accumulations are formed and whether they contribute to disease. It seems paradoxical that in situ hybridization studies revealed declines of NF-L mRNA levels by ∼60% in degenerating motor neurons of ALS patients [6]. The reduction of NF-L mRNA levels (∼87%) is even more pronounced in neurons with NF accumulations.

The discovery of mutations in the NF-H gene of a small number of ALS patients provided compelling evidence for NF involvement in ALS. In 1994, our group reported the discovery of codon deletions in the tail NF-H domain in five sporadic ALS cases [7] but subsequent studies failed to find NF-H mutations in familial ALS cases [8], [9]. Recently, two other groups discovered four novel codon deletions and one insertion of 84 bp in the same tail domain of the NF-H gene from ALS patients [10], [11]. The combined results suggest that variations in the NF-H tail domain can be a primary cause for a small percentage (∼1%) of ALS cases. While genetic mutations in NF genes may account for only a small number of ALS cases, there is a possibility that NF abnormalities in ALS may occur as a result of post-translational protein modifications. Chou et al. [12] detected the presence advance glycation products in NF inclusions associated with ALS. In addition, the NF-L protein is very susceptible to peroxynitrite-mediated nitration and this phenomenon has been suggested as one possible mechanism of ALS pathogenesis [13]. However, Strong et al. [14] examined the extent of nitration of tyrosine residues in the NF-L protein from the spinal cord of sporadic ALS cases and the results show that the extent of NF-L nitration did not differ from age-matched controls.

NF proteins are not the only types of IF proteins present in ALS inclusion bodies. Peripherin is another IF protein detected in the majority of IF inclusions (89%) in motor neurons of ALS patients [15], [16], [17]. Peripherin is a type III IF protein of 57 kDa that is normally expressed in subsets of peripheral sensory neurons and at low levels in motor neurons [18], [19], [20], [21], [22], [23]. However, peripherin gene expression can be upregulated by nerve injury [23], [24] and by inflammatory cytokines such as IL-6 (Interleukin 6) and LIF (Leukemia Inhibitory Factor) [25], [26].