Selective mitochondrial Ca²⁺ uptake deficit in disease endstage vulnerable motoneurons of the SOD1^G93A mouse model of amyotrophic lateral sclerosis

Abstract

Key points• So far, increased excitability and calcium handling problems have been discussed as causes for motoneuron death in amyotrophic lateral sclerosis (ALS) mainly on the basis of studies in juvenile presymptomatic mice.• We developed a brainstem preparation to analyse excitability and calcium handling during disease progression up to disease endstage of motoneurons in an ALS mouse model. • Increased excitability of motoneurons is not seen at disease endstage, challenging this factor as a direct cause for motoneuron death in ALS.• We show that calcium handling is remodelled during disease progression from mitochondrial uptake to mitochondrial uptake failure and increased plasma membrane extrusion, providing a compensatory mechanism that fails at disease endstage and might lead to a toxic calcium overload of the cells. • Supporting this newly described compensatory endeavour of the motoneurons might be a promising therapeutic strategy.Abstract Amyotrophic lateral sclerosis is a progressive neurodegenerative disease that targets some somatic motoneuron populations, while others, e.g. those of the oculomotor system, are spared. The pathophysiological basis of this pattern of differential vulnerability, which is preserved in a transgenic mouse model of amyotrophic lateral sclerosis (SOD1 G93A ), and the mechanism of neurodegeneration in general are unknown. Hyperexcitability and calcium dysregulation have been proposed by others on the basis of data from juvenile mice that are, however, asymptomatic. No studies have been done with symptomatic mice following disease progression to the disease endstage. Here, we developed a new brainstem slice preparation for whole-cell patch-clamp recordings and single cell fura-2 calcium imaging to study motoneurons in adult wild-type and SOD1 G93A mice up to disease endstage. We analysed disease-stage-dependent electrophysiological properties and intracellular Ca 2+ handling of vulnerable hypoglossal motoneurons in comparison to resistant oculomotor neurons. Thereby, we identified a transient hyperexcitability in presymptomatic but not in endstage vulnerable motoneurons. Additionally, we revealed a remodelling of intracellular Ca 2+ clearance within vulnerable but not resistant motoneurons at disease endstage characterised by a reduction of uniporter-dependent mitochondrial Ca 2+ uptake and enhanced Ca 2+ extrusion across the plasma membrane. Our study challenged the notion that hyperexcitability is a direct cause

The role of SIGMAR1 gene mutation and mitochondrial dysfunction in amyotrophic lateral sclerosis

“…Knock-out or down-regulation of fission or fusion proteins reduces mitochondrial respiration and ATP generation (15). In addition, decreased mitochondrial calcium transport through the mitochondrial calcium uniporter (MCU) is observed in brainstem neurons of aging SOD1 G93A mice (16). Consistent with these findings, levels of stored mitochondrial calcium decline in these neurons (17).…”

Resveratrol Improves Motoneuron Function and Extends Survival in SOD1G93A ALS Mice

“…Although there is no consensus about the exact role of mitochondrial abnormalities [61], it is accepted that mitochondrial dysfunction is an important hallmark of ALS pathogenesis [5,62,63]. Several authors have shown deficits in mitochondrial function in the spinal cord and muscles of both human patients [64] and animal models of ALS [65][66][67]. Resveratrol improves mitochondrial function and induces biogenesis, although there is some controversy about whether this effect is mediated by AMPK activation [18] or by Sirt1 [16,68].…”

Therapeutic progress in amyotrophic lateral sclerosis-beginning to learning

“…It has also been observed that calcium buffering capacity of mitochondria is affected in the brain and spinal cord of SOD1 G93A mice [127]. Furthermore, mitochondrial calcium uptake is also decreased in SOD1 G93A mice [128]. These disturbances of calcium buffering activity in motor neurons make them vulnerable to glutamatemediated excitotoxicity.…”