Elsevier

Experimental Neurology

Volume 232, Issue 2, December 2011, Pages 149-153
Experimental Neurology

Markedly reduced axonal potassium channel expression in human sporadic amyotrophic lateral sclerosis: An immunohistochemical study

https://doi.org/10.1016/j.expneurol.2011.08.015Get rights and content

Abstract

Fasciculations are characteristic features of amyotrophic lateral sclerosis (ALS), suggesting abnormally increased excitability of motor axons. Previous nerve excitability studies have shown reduced axonal potassium currents in ALS patients that may contribute to the hyperexcitability and thereby generation of fasciculations. To clarify changes in axonal ion channel expression in motor axons of ALS, we performed immunohistochemistry of potassium and sodium channels in the C7 and L5 ventral/dorsal roots obtained from five autopsy cases of sporadic ALS. Compared to controls, the immunoreactivity of potassium channels (Kv1.2) was markedly reduced in the ventral roots, but normal in the dorsal roots of all the ALS patients. Nodal sodium channel expression was not significantly different in ALS patients and control subjects. Our results show prominently reduced expression of axonal potassium channels, and provide the neuropathological and biological basis for decreased accommodative potassium currents in motor axons of ALS patients. The axonal hyperexcitability would lead to generation of fasciculations, and possibly enhances motor neuron death in ALS.

Highlights

Fasciculations are prominent features of ALS, caused by axonal hyperexcitability. ► K+ channel expression was markedly reduced in the ventral roots of ALS autopsy cases. ► Reduced axonal K+ currents would lead to hyperexcitability and enhance neuronal death.

Introduction

Amyotrophic lateral sclerosis (ALS) is a progressive fatal neurodegenerative disorder characterized by upper and lower motor neuron loss with fasciculations (Cleveland and Rothstein, 2001, Ringel et al., 1993). Extensive fasciculations are prominent features of ALS, presumably mediated by increased excitability of some part of the motor neuronal membrane (de Carvalho, 2000, Layzer, 1994). Fasciculations may occur before any motor neuron death, as a precursor to ALS (de Carvalho and Swash, 1998). A better understanding of their membrane mechanism may therefore provide insights into the patholphysiology of ALS.

Although the mechanisms underlying the selective death of motor neurons remain poorly understood, there are several hypotheses, such as oxidative stress, protein aggregation, neuroinflammation, growth factor deficiency, impaired axonal transport, and excitotoxicity (Van Damme et al., 2005). Of these, excitotoxicity would lead to abnormal neuronal hyperexcitability of motor neurons through increased calcium ion influx into motor neuronal cell body resulting in frequent fasciculations (Brown, 1994, Henneberry et al., 1989, Novelli et al., 1988, Van Den Bosch et al., 2006). However, there are several lines of evidence that the predominant anatomical sites of origin of fasciculations are the distal motor axons (Layzer, 1994), although some fasciculations arise proximally early in the disease (de Carvalho and Swash, 1998). Therefore, most fasciculations arise from axons rather than neuronal cell bodies, suggesting altered membrane properties in motor axons of ALS.

Previous axonal excitability studies in ALS patients have shown two types of ion channel abnormalities, reduced fast potassium currents and increased persistent sodium currents, both of which lead to axonal hyperexcitability (Bostock et al., 1995, Kanai et al., 2006, Mogyoros et al., 1998, Vucic and Kiernan, 2006). However, excitability indices measured by the neurophysiological technique are indirect measures of axonal ion channel function, and so far direct morphological examinations of altered ion channel expression have never been performed. By using immunohistochemistry, we studied the changes in expression of the ion channels in motor axons obtained from five autopsy cases of sporadic ALS.

Section snippets

ALS patients and control subjects

We studied post-mortem materials obtained from five patients (3 men and 2 women) with pathologically-proven ALS. Clinical profiles are shown in Table 1. The median survival time was 33 months (range, 22 to 49 months). None of them had a family history of ALS. TDP-43 immunostaining in the cytoplasm of spinal motor neurons was positive in all five patients.

Two patients with Alzheimer disease or leukoencephalopathy with spheroids served as disease controls (Freeman et al., 2009). The research

Immunostaining of potassium channel clusters

In ALS patients, the immunoreactivity of potassium channel clusters (Kv1.2 channels) was prominently reduced in the ventral roots at both the C7 and L5 levels. Fig. 1 shows examples of immunostaining in the C7 roots of Patient 3 and a control subject with Alzheimer disease. In the ALS patient, the immunoreactivity of Kv1.2 channels was nearly lost in the C7 ventral root (Fig. 1C), but the potassium channel clusters were similarly stained in the ALS dorsal root and control ventral/dorsal roots.

Discussion

Our results show that in ALS patients, expression of Kv1.2 channels is markedly reduced in motor axons, and normal in sensory axons in both the C7 and L5 spinal roots levels, indicating selective loss of potassium channels in motor axons of ALS patients. These findings provide the morphological and biological basis for the previous neurophysiological data of reduced accommodative axonal potassium currents, and resulting increased axonal excitability in ALS motor axons (Bostock et al., 1995,

Acknowledgments

This work was partly supported by Grants-in-Aid from the Research Committee of CNS Degenerative Diseases, the Ministry of Health, Labour and Welfare of Japan (S.K.).

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