Mitochondria-Associated Endoplasmic Reticulum Membrane (MAM) Integrity Is Required for Insulin Signaling and Is Implicated in Hepatic Insulin Resistance

Abstract

Mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) Mitochondria and endoplasmic reticulum (ER) networks are interconnected, sharing structural and functional interactions essential for the maintenance of cellular homeostasis. The contacts between ER and mitochondria, known as mitochondria-associated ER membranes (MAMs), play a pivotal role in calcium (Ca 2+ ) signaling, lipid transport, energy metabolism, and cell survival (1). The physical interactions between both organelles depend on complementary membrane proteins, which tether the two organelles together at specific sites. For example, the voltage-dependent anion channel (VDAC) of the outer mitochondrial membrane interacts with the inositol 1,4,5-triphosphate receptor (IP3R) on the ER through the molecular chaperone glucoseregulated protein 75 (Grp75), allowing Ca 2+ transfer from the ER to mitochondria (2). Recently, mitofusin 2 (Mfn2)

Disruption of calcium transfer from ER to mitochondria links alterations of mitochondria-associated ER membrane integrity to hepatic insulin resistance

“…Pharmacological and genetic inhibition of CYPD inhibits Ca 2+ transfer from ER to mitochondria in HuH7 cells We recently demonstrated by in situ PLA that CYPD interacted with the VDAC1-GRP75-IP3R1 calcium-channelling complex [9,10]. Here, we challenged the interactions of CYPD with this complex using pharmacological and genetic loss of function studies, and investigated the repercussions on both ER-mitochondria interactions and Ca 2+ transfer.…”

“…Particularly, we recently demonstrated that cyclophilin D (CYPD), a mitochondrial protein known to modulate the opening of the PTP [8], also interacts with the VDAC-GRP75-IP3R complex at the MAM interface in both heart [9] and liver [10]. In cardiomyocytes, we found that the loss of CYPD reduced mitochondrial Ca 2+ overload by depressing ER-mitochondria interactions and protected cells against lethal reperfusion injury [9], suggesting that CYPD regulates Ca 2+ transfer from ER to mitochondria.…”

“…In cardiomyocytes, we found that the loss of CYPD reduced mitochondrial Ca 2+ overload by depressing ER-mitochondria interactions and protected cells against lethal reperfusion injury [9], suggesting that CYPD regulates Ca 2+ transfer from ER to mitochondria. In liver, the loss of CYPD reduced organelle interactions and induced hepatic insulin resistance, pointing to a new role of MAM integrity in the control of insulin's action [10]. Whereas other studies in mice also suggest a role for MAMs in the control of glucose homeostasis [11][12][13][14], the mechanisms by which disruption of MAMs alters insulin signalling are unknown.…”

“…ER-mitochondria interactions ER-mitochondria interactions were measured both by subcellular fractionation and by in situ proximity ligation assay (PLA), as previously described and thoroughly validated [10]. See ESM Methods for further details.…”

“…They were cultured as previously described [10] and stimulated either with CYPD (ciclosporin [cyclosporin A, CsA] or NIM811) or with IP3R (2-APB and Xestospongin C) pharmacological inhibitors. Inhibition of CYPD was also achieved by silencing of CYPD.…”