Reduced Excitability and Increased Neurite Complexity of Cortical Interneurons in a Familial Mouse Model of Amyotrophic Lateral Sclerosis

Abstract

Cortical interneurons play a crucial role in regulating inhibitory-excitatory balance in brain circuits, filtering synaptic information and dictating the activity of pyramidal cells through the release of GABA. In the fatal motor neuron (MN) disease, amyotrophic lateral sclerosis (ALS), an imbalance between excitation and inhibition is an early event in the motor cortex, preceding the development of overt clinical symptoms. Patients with both sporadic and familial forms of the disease exhibit reduced cortical inhibition, including patients with mutations in the copper/zinc superoxide-dismutase-1 (SOD1) gene. In this study, we investigated the influence of the familial disease-causing hSOD1-G93A ALS mutation on cortical interneurons in neuronal networks. We performed whole-cell patch-clamp recordings and neurobiotin tracing from GFP positive interneurons in primary cortical cultures derived from Gad67-GFP::hSOD1G93A mouse embryos. Targeted recordings revealed no overt differences in the passive properties of Gad67-GFP::hSOD1G93A interneurons, however the peak outward current was significantly diminished and cells were less excitable compared to Gad67-GFP::WT controls. Post hoc neurite reconstruction identified a significantly increased morphological complexity of the Gad67-GFP::hSOD1G93A interneuron neurite arbor compared to Gad67-GFP::WT controls. Our results from the SOD1 model suggest that cortical interneurons have electrophysiological and morphological alterations that could contribute to attenuated inhibitory function in the disease. Determining if these phenomena are driven by the network or represent intrinsic alteration of the interneuron may help explain the emergence of inhibitory susceptibility and ultimately disrupted excitability, in ALS.

The Electrophysiological Determinants of Corticospinal Motor Neuron Vulnerability in ALS

“…The electrical properties of CSMN at this early stage of P30 demonstrated conflicting excitatory and inhibitory factors. Similar to our findings, there are reports to support hyperexcitability (van Zundert et al, 2008;Vucic et al, 2009;Wainger et al, 2014;Fogarty, 2018), but on the other hand also on reduced excitability (Mills, 2003;Delestrée et al, 2014;Leroy et al, 2014;King et al, 2016;Clark et al, 2018). We believe that hyper and/or hypo excitability is a dynamic phenomenon, and it is a function of disease state.…”

“…They are present throughout the motor column and numerous studies tried to reveal potential changes in their numbers concerning disease progression. Even though some results appear to contradict, overall analyses suggest that the distribution and the function of interneurons are an important component of UMN circuitry (Ziemann et al, 1997;Clark et al, 2018). The inhibitory input can be monosynaptic (direct) and disynaptic (via another neuron).…”

Cytoplasmic accumulation of FUS triggers early behavioral alterations linked to cortical neuronal hyperactivity and defects in inhibitory synapses

“…Others, however, found PV interneurons to be unaltered presymptomatically and to turn hyperexcitable during the symptomatic phase in the same SOD1 G93A mouse model 87 . In either case, those changes in PV excitability were always accompanied by hyperexcitability of layer V pyramidal neurons [84][85][86][87] . These findings in mouse models nicely recapitulate human ALS pathology, in which cortical hyperexcitability is a frequent and, most importantly, early finding in familial and sporadic cases, including FUS mutation carriers 16,88 .…”

“…Illustrating this, loss of Gabra1 77 , or of Gabra3 78 are sufficient to lead to locomotor hyperactivity and the FUS target Nrxn1, encoding a critical actor in the formation of GABAergic and glutamatergic synapses 79 , is critical in locomotor activity and social behavior in mice 80,81 . Indeed, deletion of all three neurexins from PV neurons is causing a decrease in the number of synapses in this neuronal type 82 [84][85][86] . Others, however, found PV interneurons to be unaltered presymptomatically and to turn hyperexcitable during the symptomatic phase in the same SOD1 G93A mouse model 87 .…”

Exciting Complexity: The Role of Motor Circuit Elements in ALS Pathophysiology

“…Mouse models of the disease have further substantiated the notion of compromised inhibition in ALS pathophysiology. Whole-cell patch-clamp recordings of cultured interneurons from embryonic Gad67-GFP:SOD1 G93A mice revealed that interneurons (subtype not specified) were morphologically less complex and less excitable (Clark et al, 2018). Subtype-specific investigations have furthermore unraveled a selective impairment of different interneuron populations.…”