Significance: Organelles engage in heterotypic interactions crucial for metabolic and signaling cascades. The best-studied case of this heterotypic interaction is that between the mitochondria and endoplasmic reticulum (ER), crucial for transfer of lipids and especially Ca ²⁺ between the two organelles. The original discovery that the mitochondria-shaping protein Mitofusin 2 (Mfn2) physically tethers the ER to mitochondria was recently challenged. Here, electron microscopy and fluorescent probes of organelle proximity provide definitive evidence that constitutive or acute Mfn2 ablation increases the distance between the ER and mitochondria. Functionally, this process reduces mitochondrial Ca ²⁺ uptake without altering the mitochondrial Ca ²⁺ uniporter complex in multiple tissues. Thus, the discoveries of the role of ER–mitochondria juxtaposition in cell biology based on Mfn2 as a tool remain unchallenged.

Contact sites are discrete areas of organelle proximity that coordinate essential physiological processes across membranes, including Ca²⁺ signaling, lipid biosynthesis, apoptosis, and autophagy. However, tools to easily image inter-organelle proximity over a range of distances in living cells and in vivo are lacking. Here we report a split-GFP-based contact site sensor (SPLICS) engineered to fluoresce when organelles are in proximity. Two SPLICS versions efficiently measured narrow (8–10 nm) and wide (40–50 nm) juxtapositions between endoplasmic reticulum and mitochondria, documenting the existence of at least two types of contact sites in human cells. Narrow and wide ER–mitochondria contact sites responded differently to starvation, ER stress, mitochondrial shape modifications, and changes in the levels of modulators of ER–mitochondria juxtaposition. SPLICS detected contact sites in soma and axons of D. rerio Rohon Beard (RB) sensory neurons in vivo, extending its use to analyses of organelle juxtaposition in the whole animal.

Huntington9s disease (HD), a genetic neurodegenerative disease caused by a polyglutamine expansion in the Huntingtin (Htt) protein, is accompanied by multiple mitochondrial alterations. Here, we show that mitochondrial fragmentation and cristae alterations characterize cellular models of HD and participate in their increased susceptibility to apoptosis. In HD cells, the increased basal activity of the phosphatase calcineurin dephosphorylates the pro‐fission dynamin related protein 1 (Drp1), increasing its mitochondrial translocation and activation, and ultimately leading to fragmentation of the organelle. The fragmented HD mitochondria are characterized by cristae alterations that are aggravated by apoptotic stimulation. A genetic analysis indicates that correction of mitochondrial elongation is not sufficient to rescue the increased cytochrome c release and cell death observed in HD cells. Conversely, the increased apoptosis can be corrected by manoeuvres that prevent fission and cristae remodelling. In conclusion, the cristae remodelling of the fragmented HD mitochondria contributes to their hypersensitivity to apoptosis. See accompanying Closeup by Oliveira and Lightowlers DOI 10.1002/emmm.201000104.