MICU1 controls cristae junction and spatially anchors mitochondrial Ca2+ uniporter complex

LncRNAs regulate metabolism in cancer

Metabolic reprogramming is a hallmark of cancer. Mammalian genome is characterized by pervasive transcription, generating abundant non-coding RNAs (ncRNAs). Long non-coding RNAs (lncRNAs) are freshly discovered functional ncRNAs exerting extensive regulatory impact through diverse mechanisms. Emerging studies have revealed widespread roles of lncRNAs in the regulation of various cellular activities, including metabolic pathways. In this review, we summarize the latest advances regarding the regulatory roles of lncRNAs in cancer metabolism, particularly their roles in mitochondrial function, glucose, glutamine, and lipid metabolism. Moreover, we discuss the clinical application and challenges of targeting lncRNAs in cancer metabolism. Understanding the complex and special behavior of lncRNAs will allow a better depiction of cancer metabolic networks and permit the development of lncRNA-based clinical therapies by targeting cancer metabolism.

The mRNA Decay Factor CAR-1/LSM14 Regulates Axon Regeneration via Mitochondrial Calcium Dynamics

mRNA decay factors regulate mRNA turnover by recruiting non-translating mRNAs and targeting them for translational repression and mRNA degradation. How mRNA decay pathways regulate cellular function in vivo with specificity is poorly understood. Here, we show that C. elegans mRNA decay factors, including the translational repressors CAR-1/LSM14 and CGH-1/DDX6, and the decapping enzymes DCAP-1/DCP1, function in neurons to differentially regulate axon development, maintenance, and regrowth following injury. In neuronal cell bodies, CAR-1 fully colocalizes with CGH-1 and partially colocalizes with DCAP-1, suggesting that mRNA decay components form at least two types of cytoplasmic granules. Following axon injury in adult neurons, loss of CAR-1 or CGH-1 results in increased axon regrowth and growth cone formation, whereas loss of DCAP-1 or DCAP-2 results in reduced regrowth. To determine how CAR-1 inhibits regrowth, we analyzed mRNAs bound to pan-neuronally expressed GFP::CAR-1 using a crosslinking and immunoprecipitation-based approach. Among the putative mRNA targets of CAR-1, we characterized the roles of micu-1, a regulator of the mitochondrial calcium uniporter MCU-1, in axon injury. We show that loss of car-1 results increased MICU-1 protein levels, and that enhanced axon regrowth in car-1 mutants is dependent on micu-1 and mcu-1. Moreover, axon injury induces transient calcium influx into axonal mitochondria, dependent on MCU-1. In car-1 loss-of-function mutants and in micu-1 overexpressing animals, the axonal mitochondrial calcium influx is more sustained, which likely underlies enhanced axon regrowth. Our data uncover a novel pathway that controls axon regrowth through axonal mitochondrial calcium uptake.

Role of Mitochondrial Calcium and the Permeability Transition Pore in Regulating Cell Death

Adult cardiomyocytes are postmitotic cells that undergo very limited cell division. Thus, cardiomyocyte death as occurs during myocardial infarction has very detrimental consequences for the heart. Mitochondria have emerged as an important regulator of cardiovascular health and disease. Mitochondria are well established as bioenergetic hubs for generating ATP but have also been shown to regulate cell death pathways. Indeed many of the same signals used to regulate metabolism and ATP production, such as calcium and reactive oxygen species, are also key regulators of mitochondrial cell death pathways. It is widely hypothesized that an increase in calcium and reactive oxygen species activate a large conductance channel in the inner mitochondrial membrane known as the PTP (permeability transition pore) and that opening of this pore leads to necroptosis, a regulated form of necrotic cell death. Strategies to reduce PTP opening either by inhibition of PTP or inhibiting the rise in mitochondrial calcium or reactive oxygen species that activate PTP have been proposed. A major limitation of inhibiting the PTP is the lack of knowledge about the identity of the protein(s) that form the PTP and how they are activated by calcium and reactive oxygen species. This review will critically evaluate the candidates for the pore-forming unit of the PTP and discuss recent data suggesting that assumption that the PTP is formed by a single molecular identity may need to be reconsidered.

Visualization of Sirtuin 4 Distribution between Mitochondria and the Nucleus, Based on Bimolecular Fluorescence Self-Complementation

Mitochondrial sirtuins (Sirts) control important cellular processes related to stress. Despite their regulatory importance, however, the dynamics and subcellular distributions of Sirts remain debatable. Here, we investigate the subcellular localization of sirtuin 4 (Sirt4), a sirtuin variant with a mitochondrial targeting sequence (MTS), by expressing Sirt4 fused to the superfolder green fluorescent protein (Sirt4-sfGFP) in HeLa and pancreatic β-cells. Super resolution fluorescence microscopy revealed the trapping of Sirt4-sfGFP to the outer mitochondrial membrane (OMM), possibly due to slow mitochondrial import kinetics. In many cells, Sirt4-sfGFP was also present within the cytosol and nucleus. Moreover, the expression of Sirt4-sfGFP induced mitochondrial swelling in HeLa cells. In order to bypass these effects, we applied the self-complementing split fluorescent protein (FP) technology and developed mito-STAR (mitochondrial sirtuin 4 tripartite abundance reporter), a tripartite probe for the visualization of Sirt4 distribution between mitochondria and the nucleus in single cells. The application of mito-STAR proved the importation of Sirt4 into the mitochondrial matrix and demonstrated its localization in the nucleus under mitochondrial stress conditions. Moreover, our findings highlight that the self-complementation of split FP is a powerful technique to study protein import efficiency in distinct cellular organelles.

Tracking intra- and inter-organelle signaling of mitochondria

Mitochondria are as highly specialized organelles and masters of the cellular energy metabolism in a constant and dynamic interplay with their cellular environment, providing adenosine triphosphate, buffering Ca2+ and fundamentally contributing to various signaling pathways. Hence, such broad field of action within eukaryotic cells requires a high level of structural and functional adaptation. Therefore, mitochondria are constantly moving and undergoing fusion and fission processes, changing their shape and their interaction with other organelles. Moreover, mitochondrial activity gets fine-tuned by intra- and interorganelle H+ , K+ , Na+ , and Ca2+ signaling. In this review, we provide an up-to-date overview on mitochondrial strategies to adapt and respond to, as well as affect, their cellular environment. We also present cutting-edge technologies used to track and investigate subcellular signaling, essential to the understanding of various physiological and pathophysiological processes.

Development and Application of Sub-Mitochondrial Targeted Ca2 + Biosensors

Mitochondrial Ca2+ uptake into the mitochondrial matrix is a well-established mechanism. However, the sub-organellar Ca2+ kinetics remain elusive. In the present work we identified novel site-specific targeting sequences for the intermembrane space (IMS) and the cristae lumen (CL). We used these novel targeting peptides to develop green- and red- Ca2+ biosensors targeted to the IMS and to the CL. Based on their distinctive spectral properties, and comparable sensitivities these novel constructs were suitable to visualize Ca2+-levels in various (sub) compartments in a multi-chromatic manner. Functional studies that applied these new biosensors revealed that knockdown of MCU and EMRE yielded elevated Ca2+ levels inside the CL but not the IMS in response to IP3-generating agonists. Knockdown of VDAC1, however, strongly impeded the transfer of Ca2+ through the OMM while the cytosolic Ca2+ signal remained unchanged. The novel sub-mitochondrially targeted Ca2+ biosensors proved to be suitable for Ca2+ imaging with high spatial and temporal resolution in a multi-chromatic manner allowing simultaneous measurements. These informative biosensors will facilitate efforts to dissect the complex sub-mitochondrial Ca2+ signaling under (patho)physiological conditions.