bims-mecosi 2022-12-25 papers

Issue of 2022–12–25
four papers selected by
Verena Kohler, University of Graz

Cell Calcium. 2022 Dec 11. pii: S0143-4160(22)00161-0. [Epub ahead of print]109 102688

Heterogeneity of the endoplasmic reticulum Ca2+ store determines colocalization with mitochondria.

Víctor Hugo Sánchez-Vázquez, Ericka Martínez-Martínez, Martín-Leonardo Gallegos-Gómez, Juan M Arias, Giorgia Pallafacchina, Rosario Rizzuto, Agustín Guerrero-Hernández.

Contact sites between the endoplasmic reticulum (ER) and mitochondria play a pivotal role in cell signaling, and the interaction between these organelles is dynamic and finely regulated. We have studied the role of ER Ca2+ concentration ([Ca2+]ER) in modulating this association in HeLa and HEK293 cells and human fibroblasts. According to Manders' coefficient, ER-mitochondria colocalization varied depending on the ER marker; it was the highest with ER-Tracker and the lowest with ER Ca2+ indicators (Mag-Fluo-4, erGAP3, and G-CEPIA1er) in both HeLa cells and human fibroblasts. Only GEM-CEPIA1er displayed a high colocalization with elongated mitochondria in HeLa cells, this ER Ca2+ indicator reveals low Ca2+ regions because this ion quenches its fluorescence. On the contrary, the typical rounded and fragmented mitochondria of HEK293 cells colocalized with Mag-Fluo-4 and, to a lesser extent, with GEM-CEPIA1er. The ablation of the three IP3R isoforms in HEK293 cells increased mitochondria-GEM-CEPIA1er colocalization. This pattern of colocalization was inversely correlated with the rate of ER Ca2+ leak evoked by thapsigargin (Tg). Moreover, Tg and Histamine in the absence of external Ca2+ increased mitochondria-ER colocalization. On the contrary, in the presence of external Ca2+, both Bafilomycin A1 and Tg reduced the mitochondria-ER interaction. Notably, knocking down MCU decreased mitochondria-ER colocalization. Overall, our data suggest that the [Ca2+] is not homogenous within the ER lumen and that mitochondria-ER interaction is modulated by the ER Ca2+ leak and the [Ca2+]i.

Keywords: Colocalization; ER Ca(2+) indicators; ER Ca(2+) leak, IP(3)R; ER-mitochondria association; Heterogeneous ER [Ca(2+)]

DOI: https://doi.org/10.1016/j.ceca.2022.102688

Cell Rep. 2022 Dec 20. pii: S2211-1247(22)01764-8. [Epub ahead of print]41(12): 111868

The Golgi-resident protein ACBD3 concentrates STING at ER-Golgi contact sites to drive export from the ER.

Kou Motani, Noriko Saito-Tarashima, Kohei Nishino, Shunya Yamauchi, Noriaki Minakawa, Hidetaka Kosako.

STING, an endoplasmic reticulum (ER)-resident receptor for cyclic di-nucleotides (CDNs), is essential for innate immune responses. Upon CDN binding, STING moves from the ER to the Golgi, where it activates downstream type-I interferon (IFN) signaling. General cargo proteins exit from the ER via concentration at ER exit sites. However, the mechanism of STING concentration is poorly understood. Here, we visualize the ER exit sites of STING by blocking its transport at low temperature or by live-cell imaging with the cell-permeable ligand bis-pivSATE-2'F-c-di-dAMP, which we have developed. After ligand binding, STING forms punctate foci at non-canonical ER exit sites. Unbiased proteomic screens and super-resolution microscopy show that the Golgi-resident protein ACBD3/GCP60 recognizes and concentrates ligand-bound STING at specialized ER-Golgi contact sites. Depletion of ACBD3 impairs STING ER-to-Golgi trafficking and type-I IFN responses. Our results identify the ACBD3-mediated non-canonical cargo concentration system that drives the ER exit of STING.

Keywords: ACBD3; APEX2; CP: Cell biology; CP: Immunology; ER exit site; ER-Golgi contact; ER-to-Golgi trafficking; STING; cyclic di-nucleotide; innate immune signaling; proximity proteomics; type-I interferon

DOI: https://doi.org/10.1016/j.celrep.2022.111868

Int J Mol Sci. 2022 Dec 09. pii: 15598. [Epub ahead of print]23(24):

Hepatic PLIN5 Deficiency Impairs Lipogenesis through Mitochondrial Dysfunction.

Enxiang Zhang.

Regulation of lipid droplets (LDs) metabolism is the core of controlling intracellular fatty acids (FAs) fluxes, and perilipin 5 (PLIN5) plays a key role in this process. Our previous studies have found that hepatic PLIN5 deficiency reduces LDs accumulation, but the trafficking of FAs produced from this pathway and the interaction between mitochondria and LDs in this process are largely unknown. Here, we found that the deficiency of PLIN5 decreases LDs accumulation by increasing FAs efflux. In addition, the decreased lipogenesis of PLIN5-deficient hepatocytes is accompanied by mitochondrial dysfunction, suggesting that PLIN5 plays an important role in mediating the interaction between LDs and mitochondria. Importantly, PLIN5 ablation negates oxidative capacity differences of peri-droplet and cytosolic mitochondria. In summary, these data indicate that PLIN5 plays a vital role in maintaining mitochondrial-mediated lipogenesis, which provides an important new perspective on the regulation of liver lipid storage and the relationship between PLIN5 and mitochondria.

Keywords: LDs-mitochondria interaction; PLIN5; fatty acids; lipid droplets; lipogenesis

DOI: https://doi.org/10.3390/ijms232415598

Cell Mol Life Sci. 2022 Dec 23. 80(1): 16

A duplex structure of SARM1 octamers stabilized by a new inhibitor.

Tami Khazma, Yarden Golan-Vaishenker, Julia Guez-Haddad, Atira Grossman, Radhika Sain, Michal Weitman, Alexander Plotnikov, Ran Zalk, Michael Hons, Yarden Opatowsky.

In recent years, there has been growing interest in SARM1 as a potential breakthrough drug target for treating various pathologies of axon degeneration. SARM1-mediated axon degeneration relies on its TIR domain NADase activity, but recent structural data suggest that the non-catalytic ARM domain could also serve as a pharmacological site as it has an allosteric inhibitory function. Here, we screened for synthetic small molecules that inhibit SARM1, and tested a selected set of these compounds in a DRG axon degeneration assay. Using cryo-EM, we found that one of the newly discovered inhibitors, a calmidazolium designated TK106, not only stabilizes the previously reported inhibited conformation of the octamer, but also a meta-stable structure: a duplex of octamers (16 protomers), which we have now determined to 4.0 Å resolution. In the duplex, each ARM domain protomer is engaged in lateral interactions with neighboring protomers, and is further stabilized by contralateral contacts with the opposing octamer ring. Mutagenesis of the duplex contact sites leads to a moderate increase in SARM1 activation in cultured cells. Based on our data we propose that the duplex assembly constitutes an additional auto-inhibition mechanism that tightly prevents pre-mature activation and axon degeneration.

Keywords: Cryo-EM; Drug discovery; NAD + metabolism; Neurodegeneration; SARM1; Structural biology

DOI: https://doi.org/10.1007/s00018-022-04641-3