Significance: The privileged interrelationship between mitochondria and the endoplasmic reticulum (ER) plays a key role in a variety of physiological functions, from lipid metabolism to Ca ²⁺ signalling, and its modulation influences apoptotic susceptibility, mitophagy, and cellular bioenergetics. Among the several proteins known to influence ER–mitochondria interactions, mitofusin 2 (Mfn2) has been proposed to form a physical tether. In this study, we demonstrate that Mfn2 instead works as an ER–mitochondria tethering antagonist preventing an excessive, potentially toxic, proximity between the two organelles. Cells in which Mfn2 is ablated or reduced have an increased number of ER–mitochondria close contacts, potentiated Ca ²⁺ transfer between the two organelles, and greater sensitivity to cell-death stimuli that implies mitochondria Ca ²⁺ overload toxicity.

Communication between organelles plays key roles in cell biology. In particular, physical and functional coupling of the endoplasmic reticulum (ER) and mitochondria is crucial for regulation of various physiological and pathophysiological processes. Here, we demonstrate that Presenilin 2 (PS2), mutations in which underlie familial Alzheimer's disease (FAD), promotes ER-mitochondria coupling only in the presence of mitofusin 2 (Mfn2). PS2 is not necessary for the antagonistic effect of Mfn2 on organelle coupling, although its abundance can tune it. The two proteins physically interact, whereas their homologues Mfn1 and PS1 are dispensable for this interplay. Moreover, PS2 mutants associated with FAD are more effective than the wild-type form in modulating ER-mitochondria tethering because their binding to Mfn2 in mitochondria-associated membranes is favored. We propose a revised model for ER-mitochondria interaction to account for these findings and discuss possible implications for FAD pathogenesis.

The molecular components of the mitochondrial Ca(2+) uptake machinery have been only recently identified. In the last months, in addition to the pore forming subunit and of one regulatory protein (named MCU and MICU1, respectively) other four components of this complex have been described. In addition, a MCU KO mouse model has been generated and a genetic human disease due to missense mutation of MICU1 has been discovered. In this contribution, we will first summarize the recent findings, discussing the roles of the different subunits of the mitochondrial Ca(2+) uptake complex, pointing to the current contradictions in the published data, as well as possible explanations. Finally we will speculate on the recent, totally unexpected, results obtained in the MCU knock-out (KO) mice.

Significance: Mitochondrial functions depend on the steep H ⁺ electrochemical gradient (ΔμH ⁺ ) across their inner membrane. The available tools for controlling this gradient are essentially limited to inhibitors of the respiratory chain or of the H ⁺ ATPase or to uncouplers, poisons plagued by important side effects and that lack both cell and spatial specificity. We show here that, by transfecting cells with the cDNA encoding channelrhodopsins specifically targeted to the inner mitochondrial membrane, we can obtain an accurate and spatially confined, light-dependent control of mitochondrial membrane potential and, as a consequence, of a series of mitochondrial activities ranging from electron transport to ATP synthesis and Ca ²⁺ signaling.

The recent paper by Naon et al. (1) claims that their new data “definitively” prove the role of Mitofusin 2 (Mfn2) as an endoplasmic reticulum (ER)–mitochondria tether, supporting their original proposal (2) and arguing against evidence presented by ourselves and others (3⇓⇓–6) suggesting that Mfn2 is a negative regulator of tethering. A careful reading of the paper highlights that Naon et al.’s (1) claims are not supported by the data presented. Below are some pivotal examples. First, the key parameter (number of ER–mitochondria contacts) that both we and others (3, 4) found doubled in Mfn2 −/− cells is simply not addressed. Second, the ER–mitochondria distance was determined by electron microscopy to be 2 nm larger in Mfn2 −/− cells, compared with wild-type; using the … [↵][1]1To whom correspondence may be addressed. Email: tullio.pozzan{at}unipd.it or paola.pizzo{at}unipd.it. [1]: #xref-corresp-1-1