Calcium dynamics and buffering in oculomotor neurones from mouse that are particularly resistant during amyotrophic lateral sclerosis (ALS)‐related motoneurone disease

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Abstract
1 Motoneurones are particularly vulnerable both in human forms of amyotrophic lateral sclerosis (ALS) and corresponding animal models of the disease. While most motoneurone populations are selectively impaired, oculomotor neurones are essentially resistant to ALS-related damage. 2 Motoneurone vulnerability has been closely linked to disruptions of calcium signalling. To investigate underlying events, we performed a quantitative analysis of calcium homeostasis in oculomotor neurones from mice by simultaneous patch-clamp recordings in sliced tissue and microfluorometric-calcium measurements. 3 Somatic calcium dynamics were investigated by using a computer-controlled microfluorometric system. In oculomotor neurones, basal calcium concentrations were around 80 nm and depolarisation-induced calcium responses were observed for membrane voltages positive to −40 mV. 4 Endogenous calcium homeostasis was quantified by using the ‘added buffer’ approach. The recovery phase of depolarisation-induced calcium transients was well approximated by a mono-exponential function with a decay time constant that showed a linear dependence on dye concentration. The extrapolated time constant in the absence of indicator dye was 1.7 ± 0.2 s (n= 11 cells, 21°C). 5 Endogenous calcium binding ratios (κs) were found to be 264 ± 25 (n= 11 cells), indicating that 99.6 % of cytosolic calcium ions were taken up by endogenous buffers. Recovery of calcium transients was characterised by an ‘effective’ extrusion rate γ= 156 ± 20 s−1 (n= 11 cells, 21 °C). 6 Endogenous calcium binding ratios in oculomotor neurones were 5- to 6-fold larger compared with those of more vulnerable motoneurones in the nucleus hypoglossus and spinal cord. In a first order approximation, they reduced the volume of local calcium elevations around open calcium channels, lowered peak amplitudes of global calcium transients for a given influx and prolonged calcium recovery times for a given set of uptake and extrusion mechanisms. 7 With respect to motoneurone degeneration, our measurements suggest that the exceptional stability of oculomotor neurones partially results from a specialised calcium homeostasis based on high buffering capacities. Furthermore, they indicate that cellular adaptations that account for rapid calcium signalling in hypoglossal and spinal motoneurones enhance their vulnerability during ALS-related motoneurone disease.