Intracellular transport: ER and mitochondria meet and greet along designated tracks

MFN2 coordinates mitochondria motility with α-tubulin acetylation and this regulation is disrupted in CMT2A

Mitofusin-2 (MFN2), a large GTPase residing in the mitochondrial outer membrane and mutated in Charcot-Marie-Tooth type 2 disease (CMT2A), is a regulator of mitochondrial fusion and tethering with the ER. The role of MFN2 in mitochondrial transport has however remained elusive. Like MFN2, acetylated microtubules play key roles in mitochondria dynamics. Nevertheless, it is unknown if the α-tubulin acetylation cycle functionally interacts with MFN2. Here, we show that mitochondrial contacts with microtubules are sites of α-tubulin acetylation, which occurs through MFN2-mediated recruitment of α-tubulin acetyltransferase 1 (ATAT1). This activity is critical for MFN2-dependent regulation of mitochondria transport, and axonal degeneration caused by CMT2A MFN2 associated R94W and T105M mutations may depend on the inability to release ATAT1 at sites of mitochondrial contacts with microtubules. Our findings reveal a function for mitochondria in α-tubulin acetylation and suggest that disruption of this activity plays a role in the onset of MFN2-dependent CMT2A.

The α-tubulin acetyltransferase ATAT1: structure, cellular functions, and its emerging role in human diseases

The acetylation of α-tubulin on lysine 40 is a well-studied post-translational modification which has been associated with the presence of long-lived stable microtubules that are more resistant to mechanical breakdown. The discovery of α-tubulin acetyltransferase 1 (ATAT1), the enzyme responsible for lysine 40 acetylation on α-tubulin in a wide range of species, including protists, nematodes, and mammals, dates to about a decade ago. However, the role of ATAT1 in different cellular activities and molecular pathways has been only recently disclosed. This review comprehensively summarizes the most recent knowledge on ATAT1 structure and substrate binding and analyses the involvement of ATAT1 in a variety of cellular processes such as cell motility, mitosis, cytoskeletal organization, and intracellular trafficking. Finally, the review highlights ATAT1 emerging roles in human diseases and discusses ATAT1 potential enzymatic and non-enzymatic roles and the current efforts in developing ATAT1 inhibitors.

Depletion of a Toxoplasma porin leads to defects in mitochondrial morphology and contacts with the endoplasmic reticulum

The voltage-dependent anion channel (VDAC) is a ubiquitous channel in the outer membrane of the mitochondrion with multiple roles in protein, metabolite and small molecule transport. In mammalian cells, VDAC protein, as part of a larger complex including the inositol triphosphate receptor, has been shown to have a role in mediating contacts between the mitochondria and endoplasmic reticulum (ER). We identify VDAC of the pathogenic apicomplexan Toxoplasma gondii and demonstrate its importance for parasite growth. We show that VDAC is involved in protein import and metabolite transfer to mitochondria. Further, depletion of VDAC resulted in significant morphological changes in the mitochondrion and ER, suggesting a role in mediating contacts between these organelles in T. gondii.

This article has an associated First Person interview with the first author of the paper.

Summary: Depletion of the Toxoplasma voltage-dependent anion channel highlights the importance of endoplasmic reticulum–mitochondria membrane contact sites in maintaining organelle morphology.

Nicotine Modulates Mitochondrial Dynamics in Hippocampal Neurons

Mitochondria are widely recognized as fundamental organelles for cellular physiology and constitute the main energy source for different cellular processes. The location, morphology, and interactions of mitochondria with other organelles, such as the endoplasmic reticulum (ER), have emerged as critical events capable of determining cellular fate. Mitochondria-related functions have proven particularly relevant in neurons; mitochondria are necessary for proper neuronal morphogenesis and the highly energy-demanding synaptic transmission process. Mitochondrial health depends on balanced fusion-fission events, termed mitochondrial dynamics, to repair damaged organelles and/or improve the quality of mitochondrial function, ATP production, calcium homeostasis, and apoptosis, which represent some mitochondrial functions closely related to mitochondrial dynamics. Several neurodegenerative disorders, such as Alzheimer's, Parkinson's, and Huntington's diseases, have been correlated with severe mitochondrial dysfunction. In this regard, nicotine, which has been associated with relevant neuroprotective effects mainly through activation of the nicotinic acetylcholine receptor (nAChR), exerts its effects at least in part by acting directly on mitochondrial physiology and morphology. Additionally, a recent description of mitochondrial nAChR localization suggests a nicotine-dependent mitochondrial function. In the present work, we evaluated in cultured hipocampal neurons the effects of nicotine on mitochondrial dynamics by assessing mitochondrial morphology, membrane potential, as well as interactions between mitochondria, cytoskeleton and IP₃R, levels of the cofactor PGC-1α, and fission-fusion-related proteins. Our results suggest that nicotine modulates mitochondrial dynamics and influences mitochondrial association from microtubules, increasing IP₃ receptor clustering showing modulation between mitochondria-ER communications, together with the increase of mitochondrial biogenesis.

Microtubule-Based Transport and the Distribution, Tethering, and Organization of Organelles

SUMMARYMicrotubules provide long tracks along which a broad range of organelles and vesicles are transported by kinesin and dynein motors. Motor protein complexes also tether cargoes to cytoskeletal filaments, helping facilitate their interaction and communication. The generation of biochemically distinct microtubule subpopulations allows subsets of motors to recognize a given microtubule identity, allowing further organization within the cytoplasm. Both transport and tethering are spatiotemporally regulated through multiple modes, including acute modification of both motor-cargo and motor-track associations by various physiological signals. Strict regulation of intracellular transport is particularly important in specialized cell types such as neurons. Here, we review general mechanisms by which cargo transport is controlled and also highlight examples of transport regulated by multiple mechanisms.

AMPK activation promotes lipid droplet dispersion on detyrosinated microtubules to increase mitochondrial fatty acid oxidation

Lipid droplets (LDs) are intracellular organelles that provide fatty acids (FAs) to cellular processes including synthesis of membranes and production of metabolic energy. While known to move bidirectionally along microtubules (MTs), the role of LD motion and whether it facilitates interaction with other organelles are unclear. Here we show that during nutrient starvation, LDs and mitochondria relocate on detyrosinated MT from the cell centre to adopt a dispersed distribution. In the cell periphery, LD–mitochondria interactions increase and LDs efficiently supply FAs for mitochondrial beta-oxidation. This cellular adaptation requires the activation of the energy sensor AMPK, which in response to starvation simultaneously increases LD motion, reorganizes the network of detyrosinated MTs and activates mitochondria. In conclusion, we describe the existence of a specialized cellular network connecting the cellular energetic status and MT dynamics to coordinate the functioning of LDs and mitochondria during nutrient scarcity.

Lipid droplets (LDs) supply fatty acids to cellular processes and move bidirectionally on microtubules. Here the authors show that nutrient starvation causes dispersal of mitochondria and LD to the periphery of the cell along detyrosinated microtubules and increases LD–mitochondria interactions in an AMPK-dependent manner.

Fluorescence imaging of mitochondria in cultured skin fibroblasts: a useful method for the detection of oxidative phosphorylation defects

BACKGROUND

Protons are pumped from the mitochondrial matrix via oxidative phosphorylation (OXPHOS) into the intermembrane space, creating an electric membrane potential (ΔΨ) that is used for adenosine triphosphate (ATP) production. Defects in one or more of the OXPHOS complexes are associated with a variety of clinical symptoms, often making it difficult to pinpoint the causal mutation.

METHODS

In this article, a microscopic method for the quantitative evaluation of ΔΨ in cultured skin fibroblasts is described. The method using 5,5',6,6'-tetraethylbenzimidazolyl-carbocyanine iodide (JC-1) fluorescence staining was tested in a selection of OXPHOS-deficient cell lines.

RESULTS

A significant reduction of ΔΨ was found in the cell lines of patients with either an isolated defect in complex I, II, or IV or a combined defect (complex I + complex IV). ΔΨ was not reduced in the fibroblasts of two patients with severe complex V deficiency. Addition of the complex I inhibitor rotenone induced a significant reduction of ΔΨ and perinuclear relocalization of the mitochondria. In cells with a heteroplasmic mitochondrial DNA (mtDNA) defect, a more heterogeneous reduction of ΔΨ was detected.

CONCLUSION

Our data show that imaging of ΔΨ in cultured skin fibroblasts is a useful method for the evaluation of OXPHOS functioning in cultured cell lines.

Regulation of mouse oocyte microtubule and organelle dynamics by PADI6 and the cytoplasmic lattices

Organelle positioning and movement in oocytes is largely mediated by microtubules (MTs) and their associated motor proteins. While yet to be studied in germ cells, cargo trafficking in somatic cells is also facilitated by specific recognition of acetylated MTs by motor proteins. We have previously shown that oocyte-restricted PADI6 is essential for formation of a novel oocyte-restricted fibrous structure, the cytoplasmic lattices (CPLs). Here, we show that α-tubulin appears to be associated with the PADI6/CPL complex. Next, we demonstrate that organelle positioning and redistribution is defective in PADI6-null oocytes and that alteration of MT polymerization or MT motor activity does not induce organelle redistribution in these oocytes. Finally, we report that levels of acetylated microtubules are dramatically suppressed in the cytoplasm of PADI6-null oocytes, suggesting that the observed organelle redistribution failure is due to defects in stable cytoplasmic MTs. These results demonstrate that the PADI6/CPL superstructure plays a key role in regulating MT-mediated organelle positioning and movement.