Fine‐tuning of Hepatocyte Calcium Signaling and Liver Regeneration by the Mitochondrial Calcium Uniporter

Calcium (Ca2+) signaling regulates a variety of cellular processes such as metabolism, cell proliferation, division and differentiation, gene transcription, stimulus‐secretion coupling and cell death. Ca2+ is able to regulate these diverse cellular processes due to the versatility of the Ca2+ signaling mechanism in terms of speed, amplitude and spatiotemporal patterning. Mitochondria form an integral part of this machinery, both as regulators and targets of Ca2+ signaling. In recent years, leading‐edge research in the field of mCa2+ (mitochondrial Ca2+) handling and its implications for cellular Ca2+ homeostasis opened up the possibility to investigate the functional implications of mCa2+ uptake in shaping the spatiotemporal dynamics of Ca2+ signaling, and regulating cellular metabolism (through activation of Ca2+ sensitive TCA cycle enzymes) and mitochondria‐mediated cell death (through mitochondrial permeability transition). mCa2+ is mediated by the mitochondrial uniporter complex, consisting of the pore forming MCU (Mitochondrial Calcium Uniporter) and regulators such as MICU1 (mitochondrial calcium uptake 1), MICU2 and EMRE (essential MCU regulator). MCU deletion abolishes mCa2+ uptake in mouse hepatocytes driven by vasopressin‐evoked cCa2+ (cytosolic Ca2+) increases. This lack of mCa2+ uptake resulted in alterations in cCa2+ dynamics, including an elevated resting [cCa2+] and a reduced cCa2+ peak and the total Ca2+ mobilization in response to vasopressin stimulation. These changes in cCa2+ dynamics could lead to alterations in downstream Ca2+ responsive signaling events. Indeed, we see suppression of AMPK activation through CAMKK2, in response to hormone‐evoked Ca2+ signals in MCU KD hepatocytes. Activation of Creb, CAMKK2, AMPKα and ERK1/2 are also more transient in the MCU KD hepatocytes compared to controls. Our previous studies have highlighted the importance of regulating MCU (through MICU1) under stress conditions involving enhanced mCa2+ entry, especially during liver regeneration following partial hepatectomy (PHx). Enhanced mCa2+ entry during liver regeneration maybe vital to cellular signaling events and increased mitochondrial ATP production. The lack of mCa2+ uptake in hepatocyte‐specific MCU KD mice resulted in decelerated kinetics of cell cycle progression after PHx reflected by the inhibition in BrdU incorporation and Ki67 upregulation at 30h post‐PHx, which recovers by 36h. Interestingly, apart from an inhibition in Cyclin D1 expression, many cell cycle regulatory proteins remained unchanged in the KD compared to WT controls. Cell signaling events such as AMPK, CAMKK2 and mTOR activation post‐PHx are altered. Gene expression analysis at 30h post‐PHx revealed massive changes in gene expression profiles between KD and control livers, especially in genes involving metabolic processes and protein synthesis in response to PHx. Overall, the lack of mCa2+ uptake affects multiple signaling and metabolic pathways in hepatocytes and impacts the kinetics of the liver regeneration response.Support or Funding InformationAA018873 and U01EB023224 (JBH)This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.