Biology subjects

Zouiouich, M.

Publications and source records attributed to Zouiouich, M..

4 recordsLinked to original sources

bioRxiv2026

Triglyceride/Cholesterol Ester Ratio Encodes Lipid Droplet Size and Diversity

Lipid droplet (LD) heterogeneity is a hallmark of their pathophysiological relevance. This is especially evident when cells sequester toxic cholesterol by converting it into cholesterol esters (CEs) within LDs. Since CEs can form liquid crystals (LCs), it remains unclear how such ordered structures are accommodated within the inherently dynamic environment of LDs. Here, we show that the fluidizing properties of triglycerides (TGs) help CE incorporation and influence LD growth and heterogeneity. Seipin, the key regulator of TG-LD formation and size, does not significantly impact CE-rich LD size. Instead, the CE/TG ratio, the sequence of neutral lipid deposition, and the activity of diacylglycerol acyltransferases, especially DGAT2, determine whether LDs enlarge, remain fluid, or lock into LC phases. We found that the LC phase resists LD ripening and acts as a kinetic barrier to lipid entry. Lastly, we observe that perilipins, the most abundant LD surface proteins, differentially target CE-rich LDs: Plin3 and 5 are excluded from them, whereas Plin2 and 4 are favored. These findings highlight the CE/TG ratio as a key organizing principle of lipid storage and LD function, with immediate relevance for diseases linked to sterol accumulation.

cell biology↗

bioRxiv2025

ORP5 controls the partitioning of phosphatidic acid between triacylglycerol and cardiolipin synthesis at mitochondria-ER-lipid droplet contact sites

Phosphatidic acid (PA) is a central metabolic intermediate that can fuel triacylglycerol (TAG) synthesis in lipid droplets (LDs) or cardiolipin production in mitochondria, but how cells partition PA between these competing fates has remained a fundamental unresolved question in lipid cell biology. We identify the lipid transfer protein ORP5 as a key regulator of PA partitioning at Mitochondria-Associated endoplasmic reticulum Membranes (MAM) that contact lipid droplets (LD), referred to as MAM-LD junctions. Cell imaging analysis shows that ORP5 stabilizes PA levels at MAM to promote TAG synthesis. On the other hand, loss of ORP5 causes PA accumulation on mitochondrial membranes, leading to excess cardiolipin synthesis and mitochondrial hyperfusion, while impairing triacylglycerol (TAG) synthesis and LD formation. Finally, reconstitution assays using liposomes or giant organelles further demonstrate that ORP5 can transfer PA from mitochondria to the ER via its ORD domain. Together, these findings reveal that ORP5 functions as a PA lipid transfer protein at tripartite MAM-LD contacts, where it balances LD formation with mitochondrial lipid metabolism, protecting mitochondria from cardiolipin overload.

cell biology↗

bioRxiv2025

STARD3 coordinates Endoplasmic Reticulum-late endosome/lysosome contacts and organelle positioning through a GSK3-regulated phosphorylation switch

Membrane contact sites (MCS) are dynamic regions where the membranes of two organelles come into close apposition. MCSs play many roles in cellular homeostasis by facilitating inter-organelle lipid and ion exchange as well as organelle positioning. The late endosome/lysosome (LE/Lys) cholesterol transfer protein STARD3 (StAR-related lipid transfer (START) domain containing protein 3) forms reversible contacts between the LE/Lys and the endoplasmic reticulum (ER). This tether protein contains a Phospho-FFAT motif (two phenylalanines (FF) in an acidic tract (AT)) whose interaction with ER-resident VAP proteins is phosphorylation-dependent. In this study, we identify Glycogen Synthase Kinase 3 (GSK3 and GSK3{beta}) as the kinases responsible for phosphorylating serine 209 within the Phospho-FFAT motif of STARD3. This phosphorylation event is both necessary and sufficient to activate STARD3s tethering activity, thereby promoting ER-LE/Lys contacts. Furthermore, we show that the ER-LE/Lys tethering activity made by STARD3 regulates endosome positioning, revealing an additional function for STARD3 on endosome biology. Our findings establish a direct and critical role for GSK3 in regulating MCS dynamics via STARD3 phosphorylation. This novel insight into GSK3-mediated phosphorylation expands our understanding of the molecular mechanisms governing inter-organelle communication.

cell biology↗

bioRxiv2024

Seipin Regulates Caveolin-1 Trafficking and Organelle Crosstalk

Caveolin-1 (CAV1), the main structural component of caveolae, is essential in various biological processes, including mechanotransduction, lipid metabolism, and endocytosis1-4. Deregulation of CAV1 dynamics is linked to various pathologies, including cellular senescence, cancer, insulin resistance, and lipodystrophy5-9. However, mechanisms regulating CAV1 trafficking and function remain poorly understood. Here, we show that seipin, a crucial lipid droplet (LD) biogenesis factor10, modulates CAV1 trafficking. Deletion of seipin resulted in the accumulation of saturated lipids, leading to ceramide and sphingomyelin overproduction, which disrupted the membrane order of the trans-Golgi network (TGN). In seipin deficiency, CAV1 location to the plasma membrane (PM) was impaired, reducing caveolae. Instead, CAV1 accumulated in TGN and late endosome compartments, which fused with LDs and delivered the protein. In wild-type (WT) cells, this process was minimal but significantly enhanced by treatment with palmitate, ceramide, or Stearoyl-CoA desaturase-1 (SCD1) inhibition. Conversely, in seipin-deficient cells, inhibiting Fatty Acid Synthase (FASN) or overexpressing SCD1 restored CAV1 localization to the PM and reduced its accumulation in LDs. Our findings reveal that seipin controls the funneling of palmitate toward glycerolipids synthesis and storage in LDs versus conversion to ceramides in the ER. This balance is crucial to cellular protein trafficking by controlling the TGN membrane order. Therefore, our study identifies seipin as a critical regulator of cellular lipid metabolism, protein trafficking, and organelle homeostasis. These findings shed light on the processes regulating CAV1 trafficking and show that convergent pathophysiological mechanisms associated with defects in CAV1 and seipin contribute to metabolic disorders, including insulin resistance and lipodystrophies11-14.

cell biology↗