Elsevier

Neuroscience Research

Volume 54, Issue 1, January 2006, Pages 11-14
Neuroscience Research

Underediting of GluR2 mRNA, a neuronal death inducing molecular change in sporadic ALS, does not occur in motor neurons in ALS1 or SBMA

https://doi.org/10.1016/j.neures.2005.09.006Get rights and content

Abstract

Deficient RNA editing of the AMPA receptor subunit GluR2 at the Q/R site is a primary cause of neuronal death and recently has been reported to be a tightly linked etiological cause of motor neuron death in sporadic amyotrophic lateral sclerosis (ALS). We quantified the RNA editing efficiency of the GluR2 Q/R site in single motor neurons of rats transgenic for mutant human Cu/Zn-superoxide dismutase (SOD1) as well as patients with spinal and bulbar muscular atrophy (SBMA), and found that GluR2 mRNA was completely edited in all the motor neurons examined. It seems likely that the death cascade is different among the dying motor neurons in sporadic ALS, familial ALS with mutant SOD1 and SBMA.

Introduction

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease with selective loss of both upper and lower motor neurons, and familial cases are rare. The etiology of sporadic ALS remains elusive but recently deficient RNA editing of AMPA receptor subunit GluR2 at the Q/R site is reported in motor neurons in ALS that occurs in a disease-specific and motor neuron-selective manner (Kawahara et al., 2004, Kwak and Kawahara, 2005). Moreover, underediting of the GluR2 Q/R site greatly increases the Ca2+ permeability of AMPA receptors (Hume et al., 1991, Verdoorn et al., 1991, Burnashev et al., 1992), which may cause neuronal death due to increased Ca2+ influx through the receptor channel, hence mice with RNA editing deficiencies at the GluR2 Q/R site die young (Brusa et al., 1995) and mice transgenic for an artificial Ca2+-permeable GluR2 develop motor neuron disease 12 months after birth (Kuner et al., 2005). Such evidence lends strong support to the close relevance of deficient RNA editing of the GluR2 at the Q/R site to death of motor neurons in sporadic ALS. However, although we and other researchers have demonstrated that dying neurons in several neurodegenerative diseases exhibit edited GluR2 (Kwak and Kawahara, 2005), it has not yet been demonstrated whether the underediting of GluR2 occurs in dying motor neurons in motor neuron diseases other than ALS. Such investigation is of particular importance since it will help clarify whether the molecular mechanism of motor neurons death is common among various subtypes of motor neurons.

ALS associated with the SOD1 mutation (ALS1) is the most frequent familial ALS (Rosen et al., 1993), and mutated human SOD1 transgenic animals have been studied extensively as a disease model of ALS1, yet the etiology of neuronal death in the animals has not been elucidated. Another example of non-ALS motor neuron disease is spinal and bulbar muscular atrophy (SBMA), which predominantly affects lower motor neurons with a relatively slow clinical course. Since the CAG repeat expansion in the androgen receptor gene has been demonstrated in SBMA (La Spada et al., 1991), and pharmacological castration is therapeutically effective in animal models (Katsuno et al., 2002, Katsuno et al., 2003), the death cascade responsible for SBMA is likely different from sporadic ALS. In this paper, an investigation is carried out into whether or not the dying mechanism underlying sporadic ALS is the same as ALS1 and SBMA by determining the editing status of the GluR2 Q/R site in single motor neurons.

Section snippets

Materials and methods

The animals used in this study were SOD1G93A and SOD1H46R transgenic male rats (Nagai et al., 2001) (n = 3 each) that had exhibited progressive neuromuscular weakness with their littermates as the control (n = 3 each) (Table 2). The first sign of disease in these rats was weakness of their hindlimbs, mostly exhibited by the dragging of one limb. Onset of motor neuron disease was scored as the first observation of abnormal gait or evidence of limb weakness. The mean age of onset of clinical weakness

Results

The number of motor neurons was severely decreased in the spinal cord of SBMA patients, and we analyzed 44 neurons dissected from three cases (12 from case 1, 16 from cases 2 and 3). Restriction digestion of the PCR products yielded only 116 and 66 bp fragments but no 81 or 35 bp fragments as seen in ALS motor neurons in all the SBMA motor neurons examined. Likewise, restriction digestion of the PCR products from motor neurons of mutated human SOD1 transgenic rats yielded only 219 and 59 bp

Discussion

Compared to the significant underediting reported for the GluR2 Q/R site in motor neurons of sporadic ALS (Kawahara et al., 2004), GluR2 mRNA in all the examined motor neurons of the mutated human SOD1 transgenic rats with two different mutation sites and SBMA patients was completely edited at the Q/R site. We have confirmed that postmortem delay hardly influenced the editing efficiency at the GluR2 Q/R site (Kawahara et al., 2003b), hence the significant difference in the postmortem delay

Acknowledgements

This investigation was supported in part by grants-in-aid for Scientific Research on Priority Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan and grants from the Ministry of Health, Labor and Welfare of Japan (to SK), and a grant from Japan ALS Association (to YK).

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Present address: The Wistar Institute, Philadelphia, PA, USA.

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