Targeting cortical hyperexcitability and neurodegeneration in amyotrophic lateral sclerosis (ALS)

Abstract

This thesis focused on the contributions of both astrocytes and neurons to disease progression in ALS. As extracellular K+ concentration can affect neuronal excitability and astrocytes are mainly responsible for maintaining K+ homeostasis, we hypothesise that dysfunction of astrocytic K+ clearance is a mechanism that contributes towards MN hyperexcitability. The first aim of this thesis was – using the SOD1G93A mouse model of ALS – to investigate astrocytic K+ clearance in the primary motor cortex of young control and SOD1 mice (p60-90) and aged control and SOD1 mice (p120+). The first, second and third experimental chapters (recently published in Glia) used electrophysiological recordings from acute brain slices, to show that astrocytic K+ clearance from the extracellular space is significantly reduced in the primary motor cortex in a mouse model of ALS (SOD1G93A). This decrease was accompanied by significant changes in astrocytic morphology, impaired conductivity via Kir4.1 channels and low coupling ratio in astrocytic networks in the motor cortex, which affected their ability to form the K+ gradient needed to disperse K+ through the astrocytic syncytium. Observed alterations in astrocytic K+ clearance in symptomatic ALS mice, could indicate that the supportive function astrocytes normally provide to nearby MNs is diminished. As such, alteration in astrocytic Kir4.1 channel function may explain why specific populations of MNs are vulnerable to degeneration in ALS; illuminating complicated neuron-astrocyte interactions that are vital for the advancement of treatments for neurodegenerative diseases. This alteration in astrocytic K+ clearance appears to be region specific, as it was not observed in the somatosensory cortex as demonstrated in a recent publication by our lab using the same mice (Stevenson et al., 2023). The data presented in this thesis provides new insights into the contributions of astrocytes towards disease progression in ALS. Previous work in our lab (Buskila, Kékesi, et al., 2019) revealed that aged SOD1 mice demonstrate reduced expression of HCN1 channels, (in addition to HCN2 and HCN3 channels) compared to presymptomatic SOD1 mice. This important finding led to the hypothesis that low expression of HCN1 channels is a contributing factor to MN vulnerability in ALS. Therefore, the second aim of this thesis was to investigate ionic mechanisms which contribute towards motor neuron vulnerability in ALS using injection of adeno-associated viruses (AAVs) to overexpress HCN1 channels in layer V cortical neurons in the primary motor cortex in aged SOD1G93A mice and littermate controls. A specific population of MNs are vulnerable in ALS (corticostriatal motor neurons). Research in our lab indicates that these are the first to degenerate early in disease progression. Therefore, it is hypothesised that due to their low expression of HCN channels – which are responsible for H-current (Ih) – these neurons are more vulnerable to degeneration in ALS.

Date of Award	2023
Original language	English
Awarding Institution	Western Sydney University
Supervisor	Yossi Buskila (Supervisor), Lezanne Ooi (Supervisor) & John Morley (Supervisor)