Distinct effects of cardiac mitochondrial calcium uniporter inactivation via EMRE deletion in the short and long term

Abstract

Transport of Ca²⁺ into mitochondria is thought to stimulate the production of ATP, a critical process in the heart's fight or flight response, but excess Ca²⁺ can trigger cell death. The mitochondrial Ca²⁺ uniporter complex is the primary route of Ca²⁺ transport into mitochondria, in which the channel-forming protein MCU and the regulatory protein EMRE are essential for activity. In previous studies, chronic Mcu or Emre deletion differed from acute cardiac Mcu deletion in response to adrenergic stimulation and ischemia/reperfusion (I/R) injury, despite equivalent inactivation of rapid mitochondrial Ca²⁺ uptake. To explore this discrepancy between chronic and acute loss of uniporter activity, we compared short-term and long-term Emre deletion using a novel conditional cardiac-specific, tamoxifen-inducible mouse model. After short-term Emre deletion (3 weeks post-tamoxifen) in adult mice, cardiac mitochondria were unable to take up Ca²⁺, had lower basal mitochondrial Ca²⁺ levels, and displayed attenuated Ca²⁺-induced ATP production and mPTP opening. Moreover, short-term EMRE loss blunted cardiac response to adrenergic stimulation and improved maintenance of cardiac function in an ex vivo I/R model. We then tested whether the long-term absence of EMRE (3 months post-tamoxifen) in adulthood would lead to distinct outcomes. After long-term Emre deletion, mitochondrial Ca²⁺ handling and function, as well as cardiac response to adrenergic stimulation, were similarly impaired as in short-term deletion. Interestingly, however, protection from I/R injury was lost in the long-term. These data suggest that several months without uniporter function are insufficient to restore bioenergetic response but are sufficient to restore susceptibility to I/R.

Keywords: ATP; Calcium; Ischemia/reperfusion; Mitochondria; Mitochondrial calcium uniporter; Permeability transition pore.