Nrf2 modulates cytosolic and mitochondrial calcium signal

Abstract

Nrf2 is a transcription factor which regulates ∼1% of the mammalian genome and is responsible for orchestrating the cellular defense against oxidative, inflammatory and metabolic stress. Calcium (Ca²⁺) is a ubiquitous intracellular messenger which controls most cellular processes, from fertilization to cell death. Nrf2 and Ca²⁺ are involved in a large number of similar physiological processes, but it is not clear if they can regulate each other. Here, using primary co-cultures of neurons and astrocytes we asked if Nrf2 activation or deficiency alters physiological Ca²⁺ signaling and mitochondrial Ca²⁺ handling in brain cells. We found that activation of Nrf2 leads to an increase in the amplitude of Ca²⁺ peak and a faster Ca²⁺efflux in response to glutamate and ATP in neurons and astrocytes. Interestingly, Nrf2-deficient neurons and astrocytes also had higher Ca²⁺ peaks in response to glutamate and ATP, but the recovery in neurons was significantly delayed. Genetic (Keap1-knockdown) or pharmacological (ovameloxolone, RTA-408) activation of Nrf2 increases mitochondrial Ca²⁺ uptake and mitochondrial Ca²⁺ capacity, and this correlates with increased activity of the Na⁺/Ca²⁺/Li⁺ exchanger (NCLX) and inhibition of the mitochondrial permeability transition pore (mPTP). Conversely, mitochondria in neurons and astrocytes from Nrf2-knockout mice had a lower Ca²⁺ uptake, lower mitochondrial Ca²⁺ capacity and lower mitochondrial Ca²⁺efflux, making these cell vulnerable to Ca²⁺-induced cell death. Thus, Nrf2 modulates cytosolic calcium signaling and activates the mitochondrial NCLX, increasing the mitochondrial Ca²⁺ capacity, which adds another critical aspect to the multifaceted nature of Nrf2-mediated cytoprotection.

Keywords: Astrocyte; Calcium signal; Keap1; Mitochondria; Neuron; Nrf2.