Modulation of mitochondrial calcium uptake by monoamine oxidases

Mitochondrial calcium (mtCa2+) uptake couples cellular Ca²⁺ signaling to mitochondrial metabolism and cell fate and is facilitated by Ca2+ transfer at endoplasmic reticulum (ER)–mitochondria contact sites. Monoamine oxidases (MAO-A and MAO-B) are outer mitochondrial membrane (OMM) enzymes that generate H2O2 and aldehydes during oxidative deamination, raising the possibility that MAO activity might modulate mtCa2+ handling. This study investigated whether MAO-A and MAO-B influence basal and histamine-induced mtCa2+ homeostasis in HeLa cells. Preliminary data demonstrated the presence of both isoforms in the mitochondria-associated membrane (MAM) fraction. HeLa cells were transiently transfected to overexpress MAO-A, MAO-B, or both isoforms, and expression was validated by Western blotting. Basal mtCa2+ levels were measured using mt-GCaMP6f, while histamine-induced mtCa2+ uptake was assessed using mitochondria-targeted aequorin. Endogenous MAO activity was stimulated using tyramine. Overexpression of MAO-A and/or MAO-B increased basal mtCa2+ levels, while tyramine also increased basal mtCa2+ in control cells. Following histamine stimulation, MAO-B and MAO-A/MAO-B co-overexpression increased mtCa2+ uptake, whereas MAO-A overexpression alone did not significantly alter the response. Tyramine increased histamine-induced mtCa2+ uptake in control cells but not in MAO-overexpressing cells. Overall, the findings indicate that MAO activity can influence mtCa2+ handling in HeLa cells, although further studies are required to establish the underlying mechanisms and isoform-specific contributions.