Mitochondrial-derived vesicles retain membrane potential and contain a functional ATP synthase

Privileged proteins with a second residence: dual targeting and conditional re-routing of mitochondrial proteins

Almost all mitochondrial proteins are encoded by nuclear genes and synthesized in the cytosol as precursor proteins. Signals in the amino acid sequence of these precursors ensure their targeting and translocation into mitochondria. However, in many cases, only a certain fraction of a specific protein is transported into mitochondria, while the rest either remains in the cytosol or undergoes reverse translocation to the cytosol, and can populate other cellular compartments. This phenomenon is called dual localization which can be instigated by different mechanisms. These include alternative start or stop codons, differential transcripts, and ambiguous or competing targeting sequences. In many cases, dual localization might serve as an economic strategy to reduce the number of required genes; for example, when the same groups of enzymes are required both in mitochondria and chloroplasts or both in mitochondria and the nucleus/cytoplasm. Such cases frequently employ ambiguous targeting sequences to distribute proteins between both organelles. However, alternative localizations can also be used for signaling, for example when non-imported precursors serve as mitophagy signals or when they represent transcription factors in the nucleus to induce the mitochondrial unfolded stress response. This review provides an overview regarding the mechanisms and the physiological consequences of dual targeting.

Mitochondrial derived vesicles- Quo Vadis?

Mitochondria are dynamic, intracellular organelles with a separate genome originating from prokaryotes. They perform numerous functions essential for cellular metabolism and energy production. Mitochondrial-derived vesicles (MDVs) are single or double membrane-enclosed vesicles, formed and released from the mitochondrial sub-compartments into the cytosol, in response to various triggers. MDVs interact with other organelles such as lysosomes and peroxisomes or may be incorporated and excreted via extracellular vesicles (EVs). MDVs selectively incorporate diverse protein and lipid cargoes and are involved in various functions such as mitochondrial quality control, immunomodulation, energy complementation, and compartmentalization and transport. This review aims to provide a summary of the current knowledge of MDVs biogenesis, release, cargoes, and roles.

mtDNA extramitochondrial replication mediates mitochondrial defect effects

A high ratio of severe mitochondrial defects causes multiple human mitochondrial diseases. However, until now, the in vivo rescue signal of such mitochondrial defect effects has not been clear. Here, we built fly mitochondrial defect models by knocking down the essential mitochondrial genes dMterf4 and dMrps23. Following genome-wide RNAi screens, we found that knockdown of Med8/Tfb4/mtSSB/PolG2/mtDNA-helicase rescued dMterf4/dMrps23 RNAi-mediated mitochondrial defect effects. Extremely surprisingly, they drove mtDNA replication outside mitochondria through the Med8/Tfb4-mtSSB/PolG2/mtDNA-helicase axis to amplify cytosolic mtDNA, leading to activation of the cGAS-Sting-like IMD pathway to partially mediate dMterf4/dMrps23 RNAi-triggered effects. Moreover, we found that the Med8/Tfb4-mtSSB/PolG2/mtDNA-helicase axis also mediated other fly mitochondrial gene defect-triggered dysfunctions and Drosophila aging. Overall, our study demarcates the Med8/Tfb4-mtSSB/PolG2/mtDNA-helicase axis as a candidate mechanism to mediate mitochondrial defect effects through driving mtDNA extramitochondrial replication; dysfunction of this axis might be used for potential treatments for many mitochondrial and age-related diseases.

The case for considering the term 'mitochondrial vesicle' as a misnomer in publications about assisted reproductive technologies (ART) for amphibians

The term 'mitochondrial vesicle' was first used in 2003 in a description of anuran sperm and persists to this day throughout the literature on assisted reproductive technologies (ART) for amphibians. In the present paper, we argue that the term is inappropriate because the widely accepted definition of a 'vesicle' refers to an integral structure with an enclosing lipid bilayer/membrane. Moreover, there are no electron micrographs that show a vesicular structure encapsulating mitochondria on amphibian sperm heads in the literature. In fact, in 1993, the mitochondria in the anuran sperm head had been described as positioned in 'mitochondrial collars' or 'mitochondrial sheaths' surrounded by the plasma membrane of the sperm head. On the other hand, mitochondrial-derived vesicles are defined as vesicles shed from mitochondria surfaces, potentially creating confusion. Therefore, our view is that the term 'mitochondrial vesicle' should be avoided in describing the positioning of mitochondria on sperm.

Tollip deficiency exaggerates airway type 2 inflammation in mice exposed to allergen and influenza A virus: role of the ATP/IL-33 signaling axis

Toll-interacting protein (Tollip) is a negative regulator of the pro-inflammatory response to viruses, including influenza A virus (IAV). Genetic variation of Tollip has been associated with reduced airway epithelial Tollip expression and poor lung function in patients with asthma. Whether Tollip deficiency exaggerates type 2 inflammation (e.g., eosinophils) and viral infection in asthma remains unclear. We sought to address this critical, but unanswered question by using a Tollip deficient mouse asthma model with IAV infection. Further, we determined the underlying mechanisms by focusing on the role of the ATP/IL-33 signaling axis. Wild-type and Tollip KO mice were intranasally exposed to house dust mite (HDM) and IAV with or without inhibitors for IL-33 (i.e., soluble ST2, an IL-33 decoy receptor) and ATP signaling (i.e., an antagonist of the ATP receptor P2Y13). Tollip deficiency amplified airway type 2 inflammation (eosinophils, IL-5, IL-13 and mucins), and the release of ATP and IL-33. Blocking ATP receptor P2Y13 decreased IL-33 release during IAV infection in HDM-challenged Tollip KO mice. Furthermore, soluble ST2 attenuated airway eosinophilic inflammation in Tollip KO mice treated with HDM and IAV. HDM challenges decreased lung viral load in wild-type mice, but Tollip deficiency reduced the protective effects of HDM challenges on viral load. Our data suggests that during IAV infection, Tollip deficiency amplified type 2 inflammation and delayed viral clearance, in part by promoting ATP signaling and subsequent IL-33 release. Our findings may provide several therapeutic targets, including ATP and IL-33 signaling inhibition for attenuating excessive airway type 2 inflammation in human subjects with Tollip deficiency and IAV infection.