Spartin-mediated lipid transfer facilitates lipid droplet turnover

Specificity of lipid transfer proteins: An in vitro story

Lipids, which are highly diverse, are finely distributed between organelle membranes and the plasma membrane (PM) of eukaryotic cells. As a result, each compartment has its own lipid composition and molecular identity, which is essential for the functional fate of many proteins. This distribution of lipids depends on two main processes: lipid synthesis, which takes place in different subcellular regions, and the transfer of these lipids between and across membranes. This review will discuss the proteins that carry lipids throughout the cytosol, called LTPs (Lipid Transfer Proteins). More than the modes of action or biological roles of these proteins, we will focus on the in vitro strategies employed during the last 60 years to address a critical question: What are the lipid ligands of these LTPs? We will describe the extent to which these strategies, combined with structural data and investigations in cells, have made it possible to discover proteins, namely ORPs, Sec14, PITPs, STARDs, Ups/PRELIs, START-like, SMP-domain containing proteins, and bridge-like LTPs, which compose some of the main eukaryotic LTP families, and their lipid ligands. We will see how these approaches have played a central role in cell biology, showing that LTPs can connect distant metabolic branches, modulate the composition of cell membranes, and even create new subcellular compartments.

The membrane curvature-inducing REEP1-4 proteins generate an ER-derived vesicular compartment

The endoplasmic reticulum (ER) is shaped by abundant membrane curvature-generating proteins that include the REEP family member REEP5. The REEP1 subfamily, consisting of four proteins in mammals (REEP1-4), is less abundant and lack a N-terminal region. Mutations in REEP1 and REEP2 cause Hereditary Spastic Paraplegia, but the function of these four REEP proteins remains enigmatic. Here we show that REEP1-4 reside in a unique vesicular compartment and identify features that determine their localization. Mutations in REEP1-4 that compromise curvature generation, including those causing disease, relocalize the proteins to the bulk ER. These mutants interact with wild-type proteins to retain them in the ER, consistent with their autosomal-dominant disease inheritance. REEP1 vesicles contain the membrane fusogen atlastin-1, but not general ER proteins. We propose that REEP1-4 generate these vesicles themselves by budding from the ER, and that they cycle back to the ER by atlastin-mediated fusion. The vesicles may serve to regulate ER tubule dynamics.

Mechanisms of lipid droplet degradation

Lipid droplets (LDs) are subcellular organelles that play an integral role in lipid metabolism by regulating the storage and release of fatty acids, which are essential for energy production and various cellular processes. Lipolysis and lipophagy are the two major LD degradation pathways that mediate the utilization of lipids stored in these organelles. Recent studies have further uncovered alternative pathways, including direct lysosomal LD degradation and LD exocytosis. Here, we highlight recent findings that dissect the molecular basis of these diverse LD degradation pathways. Then, we discuss speculations on the crosstalk among these pathways and the potential unconventional roles of LD degradation.

Emerging functions of the mitochondria-ER-lipid droplet three-way junction in coordinating lipid transfer, metabolism, and storage in cells

Over the past two decades, we have witnessed a growing appreciation for the importance of membrane contact sites (CS) in facilitating direct communication between organelles. CS are tiny regions where the membranes of two organelles meet but do not fuse and allow the transfer of metabolites between organelles, playing crucial roles in the coordination of cellular metabolic activities. The significant advancements in imaging techniques and molecular and cell biology research have revealed that CS are more complex than what originally thought, and as they are extremely dynamic, they can remodel their shape, composition, and functions in accordance with metabolic and environmental changes and can occur between more than two organelles. Here, we describe how recent studies led to the identification of a three-way mitochondria-ER-lipid droplet CS and discuss the emerging functions of these contacts in maintaining lipid storage, homeostasis, and balance. We also summarize the properties and functions of key protein components localized at the mitochondria-ER-lipid droplet interface, with a special focus on lipid transfer proteins. Understanding tripartite CS is essential for unraveling the complexities of inter-organelle communication and cooperation within cells.

Lysosomal damage sensing and lysophagy initiation by SPG20-ITCH

Cells respond to lysosomal membrane permeabilization by membrane repair or selective macroautophagy of damaged lysosomes, termed lysophagy, but it is not fully understood how this decision is made. Here, we uncover a pathway in human cells that detects lipid bilayer perturbations in the limiting membrane of compromised lysosomes, which fail to be repaired, and then initiates ubiquitin-triggered lysophagy. We find that SPG20 binds the repair factor IST1 on damaged lysosomes and, importantly, integrates that with the detection of damage-associated lipid-packing defects of the lysosomal membrane. Detection occurs via sensory amphipathic helices in SPG20 before rupture of the membrane. If lipid-packing defects are extensive, such as during lipid peroxidation, SPG20 recruits and activates ITCH, which marks the damaged lysosome with lysine-63-linked ubiquitin chains to initiate lysophagy and thus triages the lysosome for destruction. With SPG20 being linked to neurodegeneration, these findings highlight the relevance of a coordinated lysosomal damage response for cellular homeostasis.

Spartin is a Lipid Transfer Protein That Facilitates Lipid Droplet Turnover

One means by which cells reutilize neutral lipids stored in lipid droplets is to degrade them by autophagy. This process involves spartin, mutations of which cause the rare inherited disorder Troyer syndrome (or spastic paraplegia-20, SPG20). A recently published paper from the team led by Karin Reinsich (Yale) suggests that the molecular function of spartin and its unique highly conserved "senescence" domain is as a lipid transfer protein. Spartin binds to and transfers all lipid species found in lipid droplets, from phospholipids to triglycerides and sterol esters. This lipid transfer activity correlates with spartin's ability to sustain lipid droplet turnover. The senescence domain poses an intriguing question around the wide range of its cargoes, but intriguingly it has yet to yield up its secrets because attempts at crystallization failed and AlphaFold's prediction is unconvincing.