Biology subjects

Melikov, K.

Publications and source records attributed to Melikov, K..

6 recordsLinked to original sources

bioRxiv2026

MULTIVALENT ACE2 ENGAGEMENT ACTIVATES PIEZO1-SPHINGOMYELINASE SIGNALING IN SARS-COV-2 SPIKE-MEDIATED CELL-CELL FUSION

SARS CoV 2 Spike not only fuses the viral envelope with host membranes to deliver viral RNA but also drives syncytium formation, a process that may contribute to viral spread and pulmonary pathology. Although the conformational transitions in Spike triggered by ACE2 engagement are well characterized, the host cell signaling events initiated by Spike-ACE2 interactions remain less understood. Here, we examine how Spike activates acid sphingomyelinase (ASM) in ACE2 expressing A549 cells and how this activity promotes cell-cell fusion. Using ASM-deficient and complemented cells, exogenous ceramide, pharmacological perturbations, and a split-NeonGreen fusion assay, we support a fusion-promoting pathway in which Spike-induced ACE2 clustering activates the mechanosensitive ion channel PIEZO1, triggering lysosomal exocytosis. Secreted ASM, together with intracellular neutral sphingomyelinase, generates ceramide that enhances syncytium formation, likely through activation of protein phosphatases PP1 and PP2A. Our results suggest that these phosphatases promote fusion by facilitating actin cortex detachment and reducing actomyosin tension through dephosphorylation of ERM proteins. Our findings highlight dynamic changes in membrane lipid composition and membrane-cytoskeleton interactions as regulators of fusion and signaling in SARS-CoV-2 infection and, potentially, other viral and cellular systems.

cell biology↗

bioRxiv2026

Interorganelle competition for linoleic acid underlies steatotic liver pathology

Lipid droplets (LDs) are traditionally viewed as protective organelles that sequester potentially cytotoxic lipids. However, whether and how LD biogenesis in pathological contexts actively rewires interorganelle lipid homeostasis to drive organelle dysfunction and disease progression remains unexplored. Here, we identify the adipocyte-enriched protein calsyntenin 3{beta} (CLSTN3B) as a critical promoter of metabolic dysfunction-associated steatotic liver disease (MASLD). CLSTN3B, an ER-LD contact protein previously shown to support LD maturation in adipocytes, is robustly induced in mouse hepatocytes by peroxisome proliferator-activated receptor {gamma} (PPAR{gamma}) in response to dietary caloric overload. CLSTN3B drives LD biogenesis and neutral lipid storage by stabilizing hemifusion-like ER-LD membrane bridges via its arginine-rich segment. These bridges preferentially recruit cone-shaped linoleoylated phosphatidic acid (PA), diverting linoleic acid (LA) into triacylglycerides (TAGs) rather than mitochondrial cardiolipin (CL), leading to disrupted cristae architecture, deficient ETC supercomplex assembly, elevated electron leak, and oxidative stress. Hepatocyte-specific CLSTN3B deletion impairs LD formation, reduces TAG accumulation, enhances fatty acid oxidation, restores CL maturation, and mitigates oxidative stress, collectively attenuating MASLD progression. Consistently, hepatic CLSTN3B expression correlates with fibrosis severity and progression in human MASLD. These findings position LDs as active regulators of interorganelle lipid partitioning and establish CLSTN3B as a key determinant of mitochondrial vulnerability, providing a general framework for how dysregulated organelle interfaces shape disease.

cell biology↗

bioRxiv2025

PHOSPHATIDYLSERINE EXPOSURE AND EXTRACELLULAR ANNEXIN A5 WEAKEN THE ACTIN CORTEX IN OSTEOCLAST FUSION

Diverse cell-cell fusions involve intracellular Ca2+ signaling, non-apoptotic exposure of phosphatidylserine (PS) at the surface of fusion-committed cells and binding of extracellular Annexin A5 (Anx A5). Here we focus on the cell fusion stage of formation of bone-resorbing multinucleated osteoclasts and report that each of the listed hallmarks of cell fusion represents a step in a novel bidirectional signaling pathway. A rise in intracellular Ca{superscript 2} activates a lipid scramblase that translocates PS from the inner to the outer leaflet of the plasma membrane. This redistribution is enhanced by extracellular Anx A5 binding to cell surface PS. Depletion of PS in the inner leaflet weakens actin cortex-plasma membrane attachment mediated by ezrin/radixin/moesin (ERM) proteins, as evidenced by the preferential localization of cortex detachment areas within PS-enriched regions at the surface of the cells. Weakening of the cortex-membrane connection by Anx A5 or by adding an inhibitor of the ERM proteins promotes osteoclast fusion. We propose that this pathway facilitates osteoclast fusion and other cell-cell fusions by promoting membrane deformations required for formation of prefusion membrane contacts. Additionally, the elevated tension in the cortex detachment region of the membrane, suggested by our theoretical analysis, promotes fusion pore expansion.

cell biology↗

bioRxiv2024

Formation of multinucleated osteoclasts depends on an oxidized species of cell surface associated La protein

The bone-resorbing activity of osteoclasts plays a critical role in the life-long remodeling of our bones that is perturbed in many bone loss diseases. Multinucleated osteoclasts are formed by the fusion of precursor cells, and larger cells - generated by an increased number of cell fusion events - have higher resorptive activity. We find that osteoclast fusion and bone-resorption are promoted by reactive oxygen species (ROS) signaling and by an unconventional low molecular weight species of La protein, located at the osteoclast surface. Here, we develop the hypothesis that Las unique regulatory role in osteoclast multinucleation and function is controlled by a ROS switch in La trafficking. Using antibodies that recognize reduced or oxidized species of La, we find that differentiating osteoclasts enrich an oxidized species of La at the cell surface, which is distinct from the reduced La species conventionally localized within cell nuclei. ROS signaling triggers the shift from reduced to oxidized La species, its dephosphorylation and delivery to the surface of osteoclasts, where La promotes multinucleation and resorptive activity. Moreover, intracellular ROS signaling in differentiating osteoclasts oxidizes critical cysteine residues in the C-terminal half of La, producing this unconventional La species that promotes osteoclast fusion. Our findings suggest that redox signaling induces changes in the location and function of La and may represent a promising target for novel skeletal therapies.

cell biology↗

bioRxiv2024

CLSTN3B enhances adipocyte lipid droplet structure and function via endoplasmic reticulum contact

Interorganelle contacts facilitate material exchanges and sustain the structural and functional integrity of organelles. Lipid droplets (LDs) of adipocytes are responsible for energy storage and mobilization responding to body needs. LD biogenesis defects compromise the lipid-storing capacity of adipocytes, resulting in ectopic lipid deposition and metabolic disorders, yet how the uniquely large LDs in adipocytes attain structural and functional maturation is incompletely understood. Here we show that the mammalian adipocyte-specific protein CLSTN3B is crucial for adipocyte LD maturation. CLSTN3B employs an arginine-rich segment to promote extensive contact and hemifusion-like structure formation between the endoplasmic reticulum (ER) and LD, allowing ER-to-LD phospholipid diffusion during LD expansion. CLSTN3B ablation results in reduced LD surface phospholipid density, increased turnover of LD-surface proteins, and impaired LD functions. Our results establish the central role of CLSTN3B in the adipocyte-specific LD maturation pathway that enhances lipid storage and maintenance of metabolic health under caloric overload.

cell biology↗

bioRxiv2022

Cell Surface-Bound La Protein Regulates The Cell Fusion Stage Of Osteoclastogenesis

Multinucleated osteoclasts, essential for skeletal remodeling in health and diseases, are formed by fusion of osteoclast precursors with each fusion event raising their bone-resorbing activity. Here we report that nuclear RNA chaperone, La protein moonlights as an osteoclast fusion regulator. Monocyte-to-osteoclast differentiation starts with a drastic decrease in La levels. Then La reappears as a proteasecleaved species at the cell surface where it promotes fusion by mechanisms independent of La-RNA interactions. Appearance-and-disappearance of cell-surface La act as an on-and-off switch of the fusion activity with fusion slowing down when surface La is replaced in mature osteoclasts by full-length nuclear La. Inhibiting surface La in a novel explant model of fibrous dysplasia inhibits excessive osteoclast formation characteristic of this disease, highlighting Las potential as a therapeutic target. One-Sentence SummaryA nuclear RNA chaperon moves to the surface of osteoclasts to control their formation and function in bone metabolism.

cell biology↗