Functional roles of MICU1 and MICU2 in mitochondrial Ca2 + uptake

• MICU1 and MICU2, the two main regulators of the mitochondrial Ca2 + uniporter (MCU) have separate and defined functions.
• MICU1 and MICU2 both inhibit MCU at low [Ca2 +]c, but these effects are additive and kinetically different.
• MICU1 activates MCU at high [Ca2 +]c (> 2.5 μM), while MICU2 has no effect at high [Ca2 +]c.

MICU1 and MICU2 are the main regulators of the mitochondrial Ca2 +-uniporter (MCU), but their precise functional role is still under debate. We show here that MICU2 behaves as a pure inhibitor of MCU at low cytosolic [Ca2 +] ([Ca2 +]c), though its effects decrease as [Ca2 +]c is increased and disappear above 7 μM. Regarding MICU1, studying its effects is more difficult because knockdown of MICU1 leads also to loss of MICU2. However, while knockdown of MICU2 induces only a persistent increase in mitochondrial Ca2 + uptake, knockdown of MICU1 also induces a peculiar use-dependent transient activation of MCU that cannot be attributed to the parallel loss of MICU2. Therefore, MICU1 is endowed with a specific inhibitory effect on MCU at low [Ca2 +]c, separate and kinetically different from that of MICU2. On the other hand, we and others have shown previously that MICU1 activates MCU at [Ca2 +]c above 2.5 μM. Thus, MICU1 has a double role in MCU regulation, inhibitory at low [Ca2 +]c and activatory at high [Ca2 +]c.

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