Biology subjects

Merta, H.

Publications and source records attributed to Merta, H..

5 recordsLinked to original sources

bioRxiv2024

Paraoxonase-like APMAP maintains endoplasmic reticulum-associated lipid and lipoprotein homeostasis

Oxidative stress perturbs lipid homeostasis and contributes to metabolic diseases. Though ignored compared to mitochondrial oxidation, the endoplasmic reticulum (ER) generates reactive oxygen species requiring antioxidant quality control. Using multi-organismal profiling featuring Drosophila, zebrafish, and mammalian cells, here we characterize the paraoxonase-like APMAP as an ER-localized protein that promotes redox and lipid homeostasis and lipoprotein maturation. APMAP-depleted mammalian cells exhibit defective ER morphology, elevated ER and oxidative stress, lipid droplet accumulation, and perturbed ApoB-lipoprotein homeostasis. Critically, APMAP loss is rescued with chemical antioxidant NAC. Organismal APMAP depletion in Drosophila perturbs fat and lipoprotein homeostasis, and zebrafish display increased vascular ApoB-containing lipoproteins, particles that are atherogenic in mammals. Lipidomics reveals altered polyunsaturated phospholipids and increased ceramides upon APMAP loss, which perturbs ApoB-lipoprotein maturation. These ApoB-associated defects are rescued by inhibiting ceramide synthesis. Collectively, we propose APMAP is an ER-localized antioxidant that promotes lipid and lipoprotein homeostasis. Key findings summaryO_LIAPMAP localizes primarily to the ER network in human cells and Drosophila fat body tissue, and is a type II integral membrane protein C_LIO_LILoss of APMAP or Drosophila APMAP (dAPMAP) causes ER membrane expansion, elevates CHOP-associated ER stress, promotes LD accumulation, and alters ApoB-lipoprotein secretion C_LIO_LIAPMAP-depleted cells and dAPMAP-depleted Drosophila fat tissue exhibit defective redox homeostasis; phenotypes associated with APMAP loss are rescued by antioxidant NAC C_LIO_LIZebrafish-based LipoGlo reporter reveals that loss of apmap in zebrafish causes increased vascular ApoB-containing lipoproteins C_LIO_LILipidomic profiling indicates that APMAP loss reduces PUFA-phospholipids and elevates intracellular ceramides, which perturbs ApoB maturation C_LI

cell biology↗

bioRxiv2024

Spatial proteomics of ER tubules reveals CLMN, an ER-actin tether at focal adhesions that promotes cell migration

The endoplasmic reticulum (ER) is structurally and functionally diverse, yet how its functions are organized within morphological subdomains is incompletely understood. Utilizing TurboID-based proximity labeling and CRISPR knock-in technologies, here we map the proteomic landscape of the human ER and nuclear envelope. Spatial proteomics reveals enrichments of proteins into ER tubules, sheets, and nuclear envelope. We uncover an ER-enriched actin-binding protein, Calmin (CLMN), and define it as an ER-actin tether that localizes to focal adhesions adjacent to ER tubules. CLMN depletion perturbs focal adhesion disassembly, actin dynamics, and cell movement. Mechanistically, CLMN-depleted cells also exhibit defects in calcium signaling near ER-actin interfaces, suggesting CLMN promotes calcium signaling near adhesions to facilitate their disassembly. Collectively, we map the sub-organelle proteome landscape of the ER, identify CLMN as an ER-actin tether, and describe a non-canonical mechanism by which ER tubules engage actin to regulate cell migration.

cell biology↗

bioRxiv2023

Clustering of cortical dynein regulates the mechanics of spindle orientation in human mitotic cells

The forces which orient the spindle in human cells remain poorly understood due to a lack of direct mechanical measurements in mammalian systems. We use magnetic tweezers to measure the force on human mitotic spindles. Combining the spindles measured resistance to rotation, the speed it rotates after laser ablating astral microtubules, and estimates of the number of ablated microtubules reveals that each microtubule contacting the cell cortex is subject to [~]1 pN of pulling force, suggesting that each is pulled on by an individual dynein motor. We find that the concentration of dynein at the cell cortex and extent of dynein clustering are key determinants of the spindles resistance to rotation, with little contribution from cytoplasmic viscosity, which we explain using a biophysically based mathematical model. This work reveals how pulling forces on astral microtubules determine the mechanics of spindle orientation and demonstrates the central role of cortical dynein clustering. HighlightsO_LICytoplasmic viscosity does not determine the spindles resistance to rotation C_LIO_LIEach astral microtubule that contacts the cell cortex is pulled on by a single dynein motor C_LIO_LIPulling forces on astral microtubules determine the mechanics of spindle orientation C_LIO_LIThe mechanics of spindle orientation is regulated by clustering of dynein motors at the cell cortex C_LI Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=178 HEIGHT=200 SRC="FIGDIR/small/557210v1_ufig1.gif" ALT="Figure 1"> View larger version (53K): org.highwire.dtl.DTLVardef@56e8daorg.highwire.dtl.DTLVardef@1d16449org.highwire.dtl.DTLVardef@f3006org.highwire.dtl.DTLVardef@119f6aa_HPS_FORMAT_FIGEXP M_FIG C_FIG

cell biology↗

bioRxiv2022

A membrane sensing mechanism couples local lipid metabolism to protein degradation at the inner nuclear membrane

Lipid composition is a determinant of organelle identity; however, whether the inner nuclear membrane (INM) domain of the endoplasmic reticulum (ER) harbors a unique lipid chemistry that contributes to its identity is not known. Here, we demonstrate that a unique INM lipid environment enriched in diacylglycerol protects the nucleo-cytoskeletal linker Sun2 from local degradation by the ubiquitin-proteasome system. A membrane binding amphipathic helix in the nucleoplasmic domain of Sun2 senses INM lipids and is essential to its protein stability. We show that the protein phosphatase CTDNEP1 localizes to the INM to maintain a distinct INM lipid environment necessary for Sun2 accumulation through regulation of the phosphatidic acid phosphatase lipin 1. Thus, the INM lipid environment sculpts the INM proteome via direct lipid-protein interactions that regulate protein stability, which has broad implications for mechanisms of diseases associated with the nuclear envelope.

cell biology↗

bioRxiv2021

A CTDNEP1-lipin 1-mTOR regulatory network restricts ER membrane biogenesis to enable chromosome motions necessary for mitotic fidelity

The endoplasmic reticulum (ER) dramatically restructures in open mitosis to become excluded from the mitotic spindle; however, the significance of ER reorganization to mitotic progression is not known. Here, we demonstrate that limiting ER membrane biogenesis enables mitotic chromosome movements necessary for chromosome biorientation and prevention of micronuclei formation. Aberrantly expanded ER membranes increase the effective viscosity of the mitotic cytoplasm to physically restrict chromosome dynamics - slowed chromosome motions impede correction of mitotic errors induced by transient spindle disassembly, leading to severe micronucleation. We define the mechanistic link between regulation of ER membrane biogenesis and mitotic fidelity by demonstrating that a CTDNEP1-lipin 1-mTOR regulatory network limits ER lipid synthesis to prevent chromosome missegregation. Together, this work shows that ER membranes reorganize in mitosis to enable chromosome movements necessary for mitotic error correction and reveal dysregulated lipid metabolism as a potential source of aneuploidy in cancer cells.

cell biology↗