Mechanistic investigations into pro-survival and pro-death neuronal Ca2+ signalling pathways
Márkus, Nóra Mercedes
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Ca2+ is an important second messenger which modulates a variety of signalling pathways in both excitable and non-excitable cells. In the CNS, Ca2+ plays an important role in neurons both physiologically and pathologically. Ca2+ influx following synaptic activity, is important in development, plasticity, redox balance, as well as in neuroprotection, largely through activation of pro-survival pathways downstream of synaptic NMDAR activation, including upregulation of antioxidant defences. However, excessive Ca2+ influx in neurons leads to neuronal damage and excitotoxicity, in which mitochondrial Ca2+ uptake through the mitochondrial Ca2+ uniporter (Mcu) resulting in mitochondrial dysfunction is a key player. Excitotoxicity occurs due to glutamate efflux from astrocytes following stroke, traumatic brain injury and in chronic neurodegenerative diseases, leading to excessive neuronal NMDAR activation and triggering of its downstream pro-death pathways. This thesis focuses on understanding the pro-survival and pro-death effects of signalling pathways activated by Ca2+ in neurons, as well as the potential effect of neuronal synaptic activity on influencing neuroprotective gene transcription in astrocytes. I investigated the role of AMPK, a master regulator of metabolism, in NMDA excitotoxicity in cortical neurons as a potential downstream effector of Mcu-dependent excitotoxic death; and found the deletion of AMPKα1/2 to be neuroprotective against NMDA-mediated excitotoxicity, however I found AMPK activation to be independent of Mcu. I also investigated the expression pattern of Mcu and other mitochondrial calcium regulatory genes (MCRGs), and found MCRGs to be differentially expressed in different neural cells (primary neurons vs astrocytes), and neuronal subtypes (CA1 vs CA3 region of the hippocampus), suggesting differing dependence on the various MCRGs in mitochondrial Ca2+ handling in these cell types. A better functional understanding of these genes will allow for investigation of their importance in mitochondrial Ca2+ handling, including their potential role in excitotoxicity. I next investigated the neuroprotective effects of synaptic activity induced Ca2+ influx, focusing on antioxidant target genes of Nrf2, a transcription factor and major regulator of antioxidant genes. I found that unlike astrocytes, neurons express very low levels of Nrf2. However, synaptic activity increased the expression of several Nrf2 target genes in neurons, independently of astrocytes and Nrf2. Additionally, I found no effect of synaptic activity on increasing Nrf2 protein levels, despite previous reports in literature of Nrf2 pathway activation following synaptic activity. Finally, using RNA-seq I identified a list of genes strongly upregulated by a known Nrf2 activator in astrocytes, and found no evidence that neuronal activity triggers expression of these genes independently of neurons, providing further evidence that neuronal activity does not activate the Nrf2 pathway in astrocytes. This suggests that synaptic activity via pathways activated by Ca2+ signalling provides neurons with cell-autonomous antioxidant defences, independently of Nrf2; thus providing a distinct pathway for antioxidant defences in neurons from the Nrf2 pathway, which is activated in astrocytes providing neurons with non-cell autonomous antioxidant support. These results give us further insight into the mechanisms that underlie synaptic and non-synaptic Ca2+ signalling pathways mediating neuronal survival and death, which could help in identifying therapeutic targets to combat excitotoxicity and oxidative stress in various neurological diseases.