Nrf2 modulates cytosolic and mitochondrial calcium signal

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.