Polycystin 2 regulates mitochondrial Ca ²⁺ signaling, bioenergetics, and dynamics through mitofusin 2

Abstract

Mitochondria and the endoplasmic reticulum (ER) have an intimate functional relationship due to tethering proteins that bring their membranes in close (~30 nm) apposition. One function of this inter-organellar junction is to increase the efficiency of Ca2+ transfer into mitochondria, thus stimulating mitochondrial respiration. Here we show that the ER cation-permeant channel polycystin 2 (PC2) functions to reduce mitochondria-ER contacts. In cell culture models, PC2 knockdown led to a 50% increase in mitofusin-2 (MFN2) expression, an outer mitochondrial membrane GTPase. Live-cell super-resolution and electron microscopy analyses revealed enhanced MFN2-dependent tethering between the ER and mitochondria in PC2 knockdown cells. PC2 knockdown also led to increased ER-mediated mitochondrial Ca2+ signaling, bioenergetic activation, and mitochondrial density. Mutation or deletion of the gene encoding for PC2 results in autosomal dominant polycystic kidney disease (ADPKD), a condition characterized by numerous fluid-filled cysts. In cell culture models and mice with kidney-specific PC2 knockout, knockdown of MFN2 rescued defective mitochondrial Ca2+ transfer, and markedly diminished cell proliferation in kidney cysts. Consistent with these results, cyst-lining epithelial cells from human ADPKD kidneys had a 2-fold increase in mitochondria and MFN2 expression. Our data suggest that PC2 normally serves to limit key mitochondrial proteins at the ER-mitochondrial interface, and acts as a checkpoint for mitochondrial biogenesis and bioenergetics through a transcriptional mechanism. Loss of this regulation may contribute to the increased oxidative metabolism and aberrant cell proliferation typical of kidney cysts in ADPKD.

Increased mitochondrial fragmentation in polycystic kidney disease acts as a modifier of disease progression

“…While all the data reported to date in cellular and animal models carrying Pkd1 mutations provide a unified evidence of reduced mitochondrial calcium uptake, reduced mitochondria biogenesis, and reduced mitochondrial function, one recent study seems to suggest that the opposite occurs in Pkd2 mutants. Indeed, increased MAM contacts, increased calcium uptake, increased mitochondrial activity (oxygen consumption rate) were all reported in Pkd2 cellular and animal models, all effects mediated by an increase in MFN2 expression 13 . Here, we show that on the contrary, Pkd1 mutants do not have increased MFN2 levels, in line with a previous study showing decreased Ca 2+ uptake in the mitochondria 9 .…”

“…Thus, our data indicate a downregulation of pro‐fusion proteins OPA1 and MFN1 and a reduction in mitochondrial mass. Of interest, we did not observe any decrease in the expression levels of MFN2 (Figure S3), a molecule that is relevant not only for mitochondrial fusion, but also for tethering of the ER membrane to mitochondria, being it localized also on the ER side 13 . These data further highlight that the increase in DRP1 is not secondary to mitochondria alterations and it is very likely to contribute and sustain the increase in mitochondrial fragmentation.…”

“…These alterations correlate in human and mouse ADPKD cells with increased expression of miR‐17, which negatively modulates mitochondrial function, through repression of the master regulator of mitochondrial biogenesis, PPARalpha 12 . Moreover, PC1 and PC2 proteins localize also in the mitochondria‐associated membranes (MAMs), where they can mediate calcium signaling from the ER lumen to the mitochondria matrix thus modulating mitochondrial metabolism 7,9,13 . Of note, a cleaved carboxyl‐terminal fragment of PC1 has been proposed to accumulate in mitochondria, indicating that PC1 may directly modulate mitochondrial function 11 …”

Molecular Mechanism for Protection Against Liver Failure in Human Yellow Fever Infection

“…Transmission of calcium into mitochondria depends on which ITPR isoforms are expressed at the MAM, ( 15,16 ) which varies among cell types and can change in disease states. ( 13,14,17,18 ) Therefore, we examined whether expression of each of the three ITPR isoforms was altered in YF.…”

“…ITPR2 can preferentially transmit calcium into mitochondria in some cell types, ( 15,40 ) and ITPR2 expression was increased in YFV‐infected hepatocytes, but there is disagreement about whether this isoform localizes to the MAM and does not regulate steatosis in hepatocytes. ( 14,41 ) ITPR3 can preferentially transmit calcium into mitochondria in other cell types ( 15‐17,42 ) . Indeed, we found that loss of ITPR3 sensitized YFV‐infected cells to apoptosis; however, effects of ITPR3 on steatosis may instead reflect concomitant changes in expression of enzymes involved in fatty acid oxidation and lipogenesis.…”