Magnesium flux compendium: Discover ligands, channels, and metabolic signals

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ABSTRACT/SUMMARY Free ionized intracellular Mg2+ (iMg2+) is estimated to be in the range of 0.5–1.2 mM. In general, it is accepted that under resting conditions, the concentration of ionized cytosolic Mg2+ is `muffled' by phosphometabolites, nucleic acids and proteins. For example, ATP binds with a Kd value of 50 M-70 μM and therefore Mg2+ in the cytosol and the mitochondrial matrix is primarily complexed with ATP (Mg-ATP2-). Because of its abundance (~5 mM), ATP is considered to be the largest iMg2+ `store'. Fluctuations in free cytosolic (cMg2+) following hormonal stimuli have been touted as passive adjustments of Mg2+ dissociating from the exuberant Mg-ATP contingent and other `buffered' pools of Mg2+. Apart from iMg2+ `buffering' mechanism, Mg2+ ion channels and transporters controlling Mg2+ entry as well as efflux across the plasma membrane are thought to maintain the equilibrium of free cMg2+. Currently, several candidates are correlated to Mg2+ entry machinery (TRPM6, TRPM7, MagT1), but are still awaiting convincing biophysical and physiological evidence for such roles. The Mg2+/Na+ exchanger SLC41A1 was proposed to contribute Mg2+ efflux from the cell, whereas Mrs2 was proposed as a mitochondrial Mg2+ transporter. Very little is known about the molecular details of Mg2+ transport into/from cellular organelles like the ER, mitochondria, endosomes and lysosomes. A few studies have speculated that free [Mg2+] in the ER and mitochondria are likely to be similar to [cMg2+]. However, the temporal and spatial dynamics, let alone the biological relevance of iMg2+ mobilization, remain a mystery in cell biology. Nevertheless, Mg2+ is an essential cation controlling many biochemical reactions. Our recent work has shown that L-lactate acts as an activator that triggers a dynamic transfer of Mg2+ between the ER and mitochondria to shape bioenergetics and cellular metabolism (Cell 2020). Mechanistically, L-lactate stimulates Mg2+ release from the ER followed by Mg2+ uptake by mitochondria. The mitochondrial localized Mrs2 transporter was found to be responsible for the accumulation of Mg2+ in mitochondria. However, the L-lactate-induced ER release molecular machinery remains unidentified. I propose to identify ER Mg2+ release component, plasma membrane entry machinery and the resultant molecular signaling pathways. I will take advantage of unbiased RNAi screen and targeted CRISPR/Cas9 editing approaches to answer these mysteries in the Mg2+ signaling field. Identification of these molecular machineries would aid in our understanding of iMg2+ dynamics and the cause-effect relationships that exist between iMg2+ flux and cellular processes. Additionally, I will test and define the Mg2+-dependent signaling events based on the cellular and mouse model phenotypes. It is thrilling to define the molecular link between cellular Mg2+ homeostasis and physiological function. Our identification and characterization of the Mg2+ flux components will further investigate how, and if, these signaling routes impinge on the pathophysiology of a growing number of Mg2+ deficiency diseases in humankind. Overall, the R35/MIRA funding will support the testing of this unconventional hypothesis and my laboratory will address these major mysteries in the near future.