Mitochondrial cristae shape determines respiratory chain supercomplexes assembly and respiratory efficiency.

Respiratory chain complexes assemble into functional quaternary structures called supercomplexes (RCS) within the folds of the inner mitochondrial membrane, or cristae. Here, we investigate the relationship between respiratory function and mitochondrial ultrastructure and provide evidence that cristae shape determines the assembly and stability of RCS and hence mitochondrial respiratory efficiency. Genetic and apoptotic manipulations of cristae structure affect assembly and activity of RCS in vitro and in vivo, independently of changes to mitochondrial protein synthesis or apoptotic outer mitochondrial membrane permeabilization. We demonstrate that, accordingly, the efficiency of mitochondria-dependent cell growth depends on cristae shape. Thus, RCS assembly emerges as a link between membrane morphology and function.

Autophagy in antimicrobial immunity.

Autophagy plays a key role in cellular homeostasis, responding to various environmental stresses. In particular, pathogen invasion leads to rapid induction of autophagy, which is critical for both innate and adaptive immune responses. In this review, we focus on the emerging molecular mechanisms of pathogen elimination by autophagy (a process known as xenophagy) and on the strategies developed by pathogens to subvert autophagy. We also address other functions of autophagy proteins in restricting pathogen invasion, independent of the formation of a canonical double-membrane autophagosome.

Binding of OTULIN to the PUB domain of HOIP controls NF-κB signaling.

Linear ubiquitin chains are implicated in the regulation of the NF-κB pathway, immunity, and inflammation. They are synthesized by the LUBAC complex containing the catalytic subunit HOIL-1-interacting protein (HOIP) and are disassembled by the linear ubiquitin-specific deubiquitinase OTULIN. Little is known about the regulation of these opposing activities. Here we demonstrate that HOIP and OTULIN interact and act as a bimolecular editing pair for linear ubiquitin signals in vivo. The HOIP PUB domain binds to the PUB interacting motif (PIM) of OTULIN and the chaperone VCP/p97. Structural studies revealed the basis of high-affinity interaction with the OTULIN PIM. The conserved Tyr56 of OTULIN makes critical contacts with the HOIP PUB domain, and its phosphorylation negatively regulates this interaction. Functionally, HOIP binding to OTULIN is required for the recruitment of OTULIN to the TNF receptor complex and to counteract HOIP-dependent activation of the NF-κB pathway.

Mitochondrial morphology in mitophagy and macroautophagy.

Mitochondria are critical organelles in energy conversion, metabolism and amplification of signalling. They are however also major sources of reactive oxygen species and when dysfunctional they consume cytosolic ATP. Maintenance of a cohort of healthy mitochondria is therefore crucial for the overall cell fitness. Superfluous or damaged organelles are mainly degraded by mitophagy, a selective process of autophagy. In response to the triggers of mitophagy, mitochondria fragment: this morphological change accompanies the exposure of "eat-me" signals, resulting in the engulfment of the organelle by the autophagosomes. Conversely, during macroautophagy mitochondria fuse to be spared from degradation and to sustain ATP production in times of limited nutrient availability. Thus, mitochondrial shape defines different types of autophagy, highlighting the interplay between morphology of the organelle and complex cellular responses. This article is part of a Special Issue entitled: Mitochondrial dynamics and physiology.

High levels of Fis1, a pro-fission mitochondrial protein, trigger autophagy.

Damaged mitochondria can be eliminated in a process of organelle autophagy, termed mitophagy. In most cells, the organization of mitochondria in a network could interfere with the selective elimination of damaged ones. In principle, fission of this network should precede mitophagy; but it is unclear whether it is per se a trigger of autophagy. The pro-fission mitochondrial protein Fis1 induced mitochondrial fragmentation and enhanced the formation of autophagosomes which could enclose mitochondria. These changes correlated with mitochondrial dysfunction rather than with fragmentation, as substantiated by Fis1 mutants with different effects on organelle shape and function. In conclusion, fission associated with mitochondrial dysfunction stimulates an increase in autophagy.

Autophagy and modular restructuring of metabolism control germline tumor differentiation and proliferation in C. elegans.

Autophagy can act either as a tumor suppressor or as a survival mechanism for established tumors. To understand how autophagy plays this dual role in cancer, in vivo models are required. By using a highly heterogeneous C. elegans germline tumor, we show that autophagy-related proteins are expressed in a specific subset of tumor cells, neurons. Inhibition of autophagy impairs neuronal differentiation and increases tumor cell number, resulting in a shorter life span of animals with tumors, while induction of autophagy extends their life span by impairing tumor proliferation. Fasting of animals with fully developed tumors leads to a doubling of their life span, which depends on modular changes in transcription including switches in transcription factor networks and mitochondrial metabolism. Hence, our results suggest that metabolic restructuring, cell-type specific regulation of autophagy and neuronal differentiation constitute central pathways preventing growth of heterogeneous tumors.

Mitochondrial elongation during autophagy: a stereotypical response to survive in difficult times.

Mitochondrial morphological and structural changes play a role in several cellular processes, including apoptosis. We recently reported that mitochondrial elongation is also critical to sustain cell viability during macroautophagy. During macroautophagy unopposed mitochondrial fusion leads to organelle elongation both in vitro and in vivo. Longer mitochondria are protected from being degraded and possess more cristae where activity of the ATP synthase is increased, optimizing ATP production in times of nutrient restriction.

During autophagy mitochondria elongate, are spared from degradation and sustain cell viability.

A plethora of cellular processes, including apoptosis, depend on regulated changes in mitochondrial shape and ultrastructure. The role of mitochondria and of their morphology during autophagy, a bulk degradation and recycling process of eukaryotic cells' constituents, is not well understood. Here we show that mitochondrial morphology determines the cellular response to macroautophagy. When autophagy is triggered, mitochondria elongate in vitro and in vivo. During starvation, cellular cyclic AMP levels increase and protein kinase A (PKA) is activated. PKA in turn phosphorylates the pro-fission dynamin-related protein 1 (DRP1), which is therefore retained in the cytoplasm, leading to unopposed mitochondrial fusion. Elongated mitochondria are spared from autophagic degradation, possess more cristae, increased levels of dimerization and activity of ATP synthase, and maintain ATP production. Conversely, when elongation is genetically or pharmacologically blocked, mitochondria consume ATP, precipitating starvation-induced death. Thus, regulated changes in mitochondrial morphology determine the fate of the cell during autophagy.

Essential amino acids and glutamine regulate induction of mitochondrial elongation during autophagy.

Regulated changes in mitochondrial morphology and ultrastructure regulate several cellular processes, including apoptosis and, as we recently described, autophagy. Elongated mitochondria are spared from autophagic degradation and possess more cristae, where activity of the ATP synthase is increased, maintaining ATP levels in periods of nutrient depletion. Ultimately, mitochondrial elongation is crucial for cell survival during macroautophagy. Whether elongation is a widespread response to the lack of all nutrients, or if mitochondria respond differently to the presence of different ones is unclear. Here we show that mitochondrial shape responds differently to nutrients: elongation is inhibited when cells are starved in the presence of amino acids but not of glucose. Interestingly, starvation-induced mitochondrial elongation is a reversible process, but replenishment of amino acids is not sufficient to recover mitochondrial morphology after starvation. Intricate control pathways are likely to be in place to connect shape of the organelle with different energetic sources.