Mechanisms of dual modulatory effects of spermine on the mitochondrial calcium uniporter complex

Abstract

The mitochondrial ${\text{Ca}}^{2+}$ uniporter mediates the crucial cellular process of mitochondrial ${\text{Ca}}^{2+}$ uptake, which regulates cell bioenergetics, intracellular ${\text{Ca}}^{2+}$ signaling, and cell death initiation. The uniporter contains the pore-forming MCU subunit, an EMRE protein that binds to MCU, and the regulatory MICU1 subunit, which can dimerize with MICU1 or MICU2 and under resting cellular [${\text{Ca}}^{2+}$] occludes the MCU pore. It has been known for decades that spermine, which is ubiquitously present in animal cells, can enhance mitochondrial ${\text{Ca}}^{2+}$ uptake, but the underlying mechanisms remain unclear. Here, we show that spermine exerts dual modulatory effects on the uniporter. In physiological concentrations of spermine, it enhances uniporter activity by breaking the physical interactions between MCU and the MICU1-containing dimers to allow the uniporter to constitutively take up ${\text{Ca}}^{2+}$ even in low [${\text{Ca}}^{2+}$] conditions. This potentiation effect does not require MICU2 or the EF-hand motifs in MICU1. When [spermine] rises to millimolar levels, it inhibits the uniporter by targeting the pore region in a MICU-independent manner. The MICU1-dependent spermine potentiation mechanism proposed here, along with our previous finding that cardiac mitochondria have very low MICU1, can explain the puzzling observation in the literature that mitochondria in the heart show no response to spermine.