bims-mecosi 2022-11-20 papers

Issue of 2022–11–20
five papers selected by
Verena Kohler, University of Graz

Emerg Top Life Sci. 2022 Nov 14. pii: ETLS20220028. [Epub ahead of print]

Creating and sensing asymmetric lipid distributions throughout the cell.

A key feature of eukaryotic cells is the asymmetric distribution of lipids along their secretory pathway. Because of the biological significance of these asymmetries, it is crucial to define the mechanisms which create them. Extensive studies have led to the identification of lipid transfer proteins (LTPs) that work with lipid-synthesizing enzymes to carry lipids between two distinct membranes in a directional manner, and are thus able to create asymmetries in lipid distribution throughout the cell. These networks are often in contact sites where two organelle membranes are in close proximity for reasons we have only recently started to understand. A question is whether these networks transfer lipids en masse within the cells or adjust the lipid composition of organelle membranes. Finally, recent data have confirmed that some networks organized around LTPs do not generate lipid asymmetries between membranes but sense them and rectify the lipid content of the cell.

Keywords: asymmetry; cellular membrane; lipid synthesis; lipid transfer; membrane contact sites

DOI: https://doi.org/10.1042/ETLS20220028

J Biol Chem. 2022 Nov 14. pii: S0021-9258(22)01145-0. [Epub ahead of print] 102702

ATP-binding cassette A1 and cholesterol transfer protein Aster-A promote an asymmetric cholesterol distribution in the plasma membrane.

Fumihiko Ogasawara, Kazumitsu Ueda.

Cholesterol is a major and essential component of the mammalian cell plasma membrane (PM), and the loss of cholesterol homeostasis leads to various pathologies. Cellular cholesterol uptake and synthesis are regulated by a cholesterol sensor in the endoplasmic reticulum (ER). However, it remains unclear how changes in the cholesterol level of the PM are recognized. Here we show that the sensing of cholesterol in the PM depends on ATP-binding cassette A1 (ABCA1) and the cholesterol transfer protein Aster-A, which cooperatively maintain the asymmetric transbilayer cholesterol distribution in the PM. We demonstrate that ABCA1 translocates (flops) cholesterol from the inner leaflet of the PM to the outer leaflet to maintain a low inner leaflet cholesterol level. We also found when inner cholesterol levels were increased, Aster-A was recruited to the PM-ER contact site to transfer cholesterol to the ER. These results suggest that ABCA1 could promote an asymmetric cholesterol distribution to suppress Aster-A recruitment to the PM-ER contact site to maintain intracellular cholesterol homeostasis.

Keywords: ATP-binding cassette A1 (ABCA1); Aster-A (GramD1a); PM-ER contact site; asymmetric transbilayer cholesterol distribution; cholesterol flop; intracellular cholesterol homeostasis

DOI: https://doi.org/10.1016/j.jbc.2022.102702

J Cell Sci. 2023 Mar 01. pii: jcs259689. [Epub ahead of print]136(5):

Choreographing the motor-driven endosomal dance.

Marlieke L M Jongsma, Nina Bakker, Jacques Neefjes.

The endosomal system orchestrates the transport of lipids, proteins and nutrients across the entire cell. Along their journey, endosomes mature, change shape via fusion and fission, and communicate with other organelles. This intriguing endosomal choreography, which includes bidirectional and stop-and-go motions, is coordinated by the microtubule-based motor proteins dynein and kinesin. These motors bridge various endosomal subtypes to the microtubule tracks thanks to their cargo-binding domain interacting with endosome-associated proteins, and their motor domain interacting with microtubules and associated proteins. Together, these interactions determine the mobility of different endosomal structures. In this Review, we provide a comprehensive overview of the factors regulating the different interactions to tune the fascinating dance of endosomes along microtubules.

Keywords: Dynactin; Dynein; Endoplasmic reticulum; Endosomes; Kinesin; Membrane contact sites; Microtubules

DOI: https://doi.org/10.1242/jcs.259689

EMBO J. 2022 Nov 18. e112006

LUBAC assembles a ubiquitin signaling platform at mitochondria for signal amplification and transport of NF-κB to the nucleus.

Mitochondria are increasingly recognized as cellular hubs to orchestrate signaling pathways that regulate metabolism, redox homeostasis, and cell fate decisions. Recent research revealed a role of mitochondria also in innate immune signaling; however, the mechanisms of how mitochondria affect signal transduction are poorly understood. Here, we show that the NF-κB pathway activated by TNF employs mitochondria as a platform for signal amplification and shuttling of activated NF-κB to the nucleus. TNF treatment induces the recruitment of HOIP, the catalytic component of the linear ubiquitin chain assembly complex (LUBAC), and its substrate NEMO to the outer mitochondrial membrane, where M1- and K63-linked ubiquitin chains are generated. NF-κB is locally activated and transported to the nucleus by mitochondria, leading to an increase in mitochondria-nucleus contact sites in a HOIP-dependent manner. Notably, TNF-induced stabilization of the mitochondrial kinase PINK1 furthermore contributes to signal amplification by antagonizing the M1-ubiquitin-specific deubiquitinase OTULIN. Overall, our study reveals a role for mitochondria in amplifying TNF-mediated NF-κB activation, both serving as a signaling platform, as well as a transport mode for activated NF-κB to the nuclear.

Keywords: HOIP; NEMO; OTULIN; PINK1; ubiquitin

DOI: https://doi.org/10.15252/embj.2022112006

Front Cell Dev Biol. 2022 ;10 1044672

Analysis of mitochondrial dynamics and function in the retinal pigment epithelium by high-speed high-resolution live imaging.

Li Xuan Tan, Jianlong Li, Colin J Germer, Aparna Lakkaraju.

Mitochondrial dysfunction is strongly implicated in neurodegenerative diseases including age-related macular degeneration (AMD), which causes irreversible blindness in over 50 million older adults worldwide. A key site of insult in AMD is the retinal pigment epithelium (RPE), a monolayer of postmitotic polarized cells that performs essential functions for photoreceptor health and vision. Recent studies from our group and others have identified several features of mitochondrial dysfunction in AMD including mitochondrial fragmentation and bioenergetic defects. While these studies provide valuable insight at fixed points in time, high-resolution, high-speed live imaging is essential for following mitochondrial injury in real time and identifying disease mechanisms. Here, we demonstrate the advantages of live imaging to investigate RPE mitochondrial dynamics in cell-based and mouse models. We show that mitochondria in the RPE form extensive networks that are destroyed by fixation and discuss important live imaging considerations that can interfere with accurate evaluation of mitochondrial integrity such as RPE differentiation status and acquisition parameters. Our data demonstrate that RPE mitochondria show localized heterogeneities in membrane potential and ATP production that could reflect focal changes in metabolism and oxidative stress. Contacts between the mitochondria and organelles such as the ER and lysosomes mediate calcium flux and mitochondrial fission. Live imaging of mouse RPE flatmounts revealed a striking loss of mitochondrial integrity in albino mouse RPE compared to pigmented mice that could have significant functional consequences for cellular metabolism. Our studies lay a framework to guide experimental design and selection of model systems for evaluating mitochondrial health and function in the RPE.

Keywords: RPE; live imaging; mitochondria; pigmented and albino mice; retina

DOI: https://doi.org/10.3389/fcell.2022.1044672