bims-mecosi 2022-06-12 papers

Issue of 2022–06–12
two papers selected by
Verena Kohler, Stockholm University

Proc Natl Acad Sci U S A. 2022 Jun 14. 119(24): e2200513119

Metabolic and immune-sensitive contacts between lipid droplets and endoplasmic reticulum reconstituted in vitro.

Sukrut Kamerkar, Jagjeet Singh, Subham Tripathy, Hemangi Bhonsle, Mukesh Kumar, Roop Mallik.

Coordinated cell function requires a variety of subcellular organelles to exchange proteins and lipids across physical contacts that are also referred to as membrane contact sites. Such organelle-to-organelle contacts also evoke interest because they can appear in response to metabolic changes, immune activation, and possibly other stimuli. The microscopic size and complex, crowded geometry of these contacts, however, makes them difficult to visualize, manipulate, and understand inside cells. To address this shortcoming, we deposited endoplasmic reticulum (ER)-enriched microsomes purified from rat liver or from cultured cells on a coverslip in the form of a proteinaceous planar membrane. We visualized real-time lipid and protein exchange across contacts that form between this ER-mimicking membrane and lipid droplets (LDs) purified from the liver of rat. The high-throughput imaging possible in this geometry reveals that in vitro LD-ER contacts increase dramatically when the metabolic state is changed by feeding the animal and also when the immune system is activated. Contact formation in both cases requires Rab18 GTPase and phosphatidic acid, thus revealing common molecular targets operative in two very different biological pathways. An optical trap is used to demonstrate physical tethering of individual LDs to the ER-mimicking membrane and to estimate the strength of this tether. These methodologies can potentially be adapted to understand and target abnormal contact formation between different cellular organelles in the context of neurological and metabolic disorders or pathogen infection.

Keywords: Rab18; lipid droplets; membrane contact sites; optical trap; supported lipid bilayer

DOI: https://doi.org/10.1073/pnas.2200513119

Metabolism. 2022 Jun 06. pii: S0026-0495(22)00117-2. [Epub ahead of print] 155239

TBC1D15-Drp1 interaction-mediated mitochondrial homeostasis confers cardioprotection against myocardial ischemia/reperfusion injury.

Shiqun Sun, Wenjun Yu, Haixia Xu, Congye Li, Rongjun Zou, Ne N Wu, Li Wang, Junbo Ge, Jun Ren, Yingmei Zhang.

OBJECTIVE: Mitochondria are essential for myocardial ischemia/reperfusion (I/R) injury. TBC domain family member 15 (TBC1D15) participates in the regulation of mitochondrial homeostasis although its role remains elusive in I/R injury.
METHODS AND MATERIALS: This study examined the role of TBC1D15 in mitochondrial homeostasis under myocardial I/R injury using inducible cardiac-specific TBC1D15 knockin (TBC1D15CKI) and knockout (TBC1D15CKO) mice.
RESULTS: TBC1D15 mRNA/protein levels were downregulated in human ischemic cardiomyopathy samples, mouse I/R hearts and neonatal mouse cardiomyocytes with H/R injury, consistent with scRNA sequencing finding from patients with coronary heart disease. Cardiac-specific knockin of TBC1D15 attenuated whereas cardiac-specific knockout of TBC1D15 overtly aggravated I/R-induced cardiomyocyte apoptosis and cardiac dysfunction. TBC1D15CKI mice exhibited reduced mitochondrial damage and mitochondrial fragmentation following myocardial I/R injury, while TBC1D15CKO mice displayed opposite results. TBC1D15 preserved mitochondrial function evidenced by safeguarding MMP and oxygen consumption capacity, antagonizing ROS accumulation and cytochrome C release, which were nullified by TBC1D15 knockdown. Time-lapse confocal microscopy revealed that TBC1D15 activated asymmetrical mitochondrial fission through promoting mitochondria-lysosome contacts untethering in NMCMs under H/R injury, whereas overexpression of TBC1D15 mutants (R400K and ∆231-240) failed to regulate asymmetrical fission and knockdown of TBC1D15 slowed down asymmetrical fission. Moreover, TBC1D15-offered benefits were mitigated by knockdown of Fis1 and Drp1. Mechanistically, TBC1D15 recruited Drp1 to mitochondria-lysosome contact sites via direct interaction with Drp1 through its C terminus (574-624) domain. Interfering with interaction between TBC1D15 and Drp1 abrogated asymmetrical mitochondrial fission and mitochondrial function. Cardiac phenotypes of TBC1D15CKO mice upon I/R injury were rescued by adenovirus-mediated overexpression of wild-type but not mutants (R400K, ∆231-240 and ∆574-624) TBC1D15.
CONCLUSIONS: TBC1D15 ameliorated I/R injury through a novel modality to preserve mitochondrial homeostasis where mitochondria-lysosome contacts (through the TBC1D15/Fis1/RAB7 cascade) regulate asymmetrical mitochondrial fission (TBC1D15/Drp1 interaction), suggesting promises of targeting TBC1D15 in the management of myocardial I/R injury.

Keywords: Drp1; I/R injury; Mitochondrial fission; Mitochondria–lysosome contacts; TBC1D15

DOI: https://doi.org/10.1016/j.metabol.2022.155239