The Inner Nuclear Membrane Is a Metabolically Active Territory that Generates Nuclear Lipid Droplets

A quantitative ultrastructural timeline of nuclear autophagy reveals a role for dynamin-like protein 1 at the nuclear envelope

Autophagic mechanisms that maintain nuclear envelope homoeostasis are bulwarks to ageing and disease. Here we define a quantitative and ultrastructural timeline of nuclear macroautophagy (nucleophagy) in yeast by leveraging four-dimensional lattice light sheet microscopy and correlative light and electron tomography. Nucleophagy begins with a rapid accumulation of the selective autophagy receptor Atg39 at the nuclear envelope and finishes in ~300 s with Atg39-cargo delivery to the vacuole. Although there are several routes to the vacuole, at least one pathway incorporates two consecutive membrane fission steps: inner nuclear membrane (INM) fission to generate an INM-derived vesicle in the perinuclear space and outer nuclear membrane fission to liberate a double-membraned vesicle to the cytosol. Outer nuclear membrane fission occurs independently of phagophore engagement and instead relies surprisingly on dynamin-like protein 1 (Dnm1), which is recruited to sites of Atg39 accumulation by Atg11. Loss of Dnm1 compromises nucleophagic flux by stalling nucleophagy after INM fission. Our findings reveal how nuclear and INM cargo are removed from an intact nucleus without compromising its integrity, achieved in part by a non-canonical role for Dnm1 in nuclear envelope remodelling.

Phospholipid Biosynthesis: An Unforeseen Modulator of Nuclear Metabolism

Glycerophospholipid biosynthesis is crucial not only for providing structural components required for membrane biogenesis during cell proliferation but also for facilitating membrane remodeling under stress conditions. The biosynthetic pathways for glycerophospholipid tails, glycerol backbones, and diverse head group classes intersect with various other metabolic processes, sharing intermediary metabolites. Recent studies have revealed intricate connections between glycerophospholipid synthesis and nuclear metabolism, including metabolite-mediated crosstalk with the epigenome, signaling pathways that govern genome integrity, and CTP-involved regulation of nucleotide and antioxidant biosynthesis. This review highlights recent advances in understanding the functional roles of glycerophospholipid biosynthesis beyond their structural functions in budding yeast and mammalian cells. We propose that glycerophospholipid biosynthesis plays an integrative role in metabolic regulation, providing a new perspective on lipid biology.

Near-Native Imaging of Metal Ion-Initiated Cell State Transition

Metal ions are indispensable to life, as they can serve as essential enzyme cofactors to drive fundamental biochemical reactions, yet paradoxically, excess is highly toxic. Higher-order cells have evolved functionally distinct organelles that separate and coordinate sophisticated biochemical processes to maintain cellular homeostasis upon metal ion stimuli. Here, we uncover the remodeling of subcellular architecture and organellar interactome in yeast initiated by several metal ion stimulations, relying on near-native three-dimensional imaging, cryo-soft X-ray tomography. The three-dimensional architecture of intact yeast directly shows that iron or manganese triggers a hormesis-like effect that promotes cell proliferation. This process leads to the reorganization of organelles in the preparation for division, characterized by the polar distribution of mitochondria, an increased number of lipid droplets (LDs), volume shrinkage, and the formation of a hollow structure. Additionally, vesicle-like structures that detach from the vacuole are observed. Oppositely, cadmium or mercury causes stress-associated phenotypes, including mitochondrial fragmentation, LD swelling, and autophagosome formation. Notably, the organellar interactome, encompassing the interactions between mitochondria and LDs and those between the nuclear envelope and LDs, is quantified and exhibits alteration with multifaceted features in response to different metal ions. More importantly, the dynamics of organellar architecture render them more sensitive biomarkers than traditional approaches for assessing the cell state. Strikingly, yeast has a powerful depuration capacity to isolate and transform the overaccumulated cadmium in the vacuole, mitochondria, and cytoplasm as a high-value product, quantum dots. This work presents the possibility of discovering fundamental links between organellar morphological characteristics and the cell state.

Lipids and chromatin: a tale of intriguing connections shaping genomic landscapes

Recent studies in yeast reveal an intricate interplay between nuclear envelope (NE) architecture and lipid metabolism, and between lipid signaling and both epigenome and genome integrity. In this review, we highlight the reciprocal connection between lipids and histone modifications, which enable metabolic reprogramming in response to nutrients. The endoplasmic reticulum (ER)-NE regulates the compartmentalization and temporal availability of epigenetic metabolites and its lipid composition also impacts nuclear processes, such as transcriptional silencing and the DNA damage response (DDR). We also discuss recent work providing mechanistic insight into lipid droplet (LD) formation and sterols in the nucleus, and the collective data showing Opi1 as a central factor in both membrane sensing and transcriptional regulation of lipid-chromatin interrelated processes.

Ki-67 and CDK1 control the dynamic association of nuclear lipids with mitotic chromosomes

Nuclear lipids play roles in regulatory processes, such as signaling, transcriptional regulation, and DNA repair. In this report, we demonstrate that nuclear lipids may contribute to Ki-67-regulated chromosome integrity during mitosis. In COS-7 cells, nuclear lipids are enriched at the perichromosomal layer and excluded from intrachromosomal regions during early mitosis but are then detected in intrachromosomal regions during late mitosis, as revealed by TT-ExM (expansion microscopy with trypsin digestion and tyramide signal amplification), an improved expansion microscopy technique that enables high-sensitivity and super-resolution imaging of proteins, lipids, and nuclear DNA. The nuclear nonhistone protein Ki-67 acts as a surfactant to form a repulsive molecular brush around fully condensed sister chromatids in early mitosis, preventing the diffusion or penetration of nuclear lipids into intrachromosomal regions. Ki-67 is phosphorylated during mitosis by cyclin-dependent kinase 1 (CDK1), the best-known master regulator of the cell cycle. Both Ki-67 knockdown and reduced Ki-67 phosphorylation by CDK1 inhibitors allow nuclear lipids to penetrate chromosomal regions. Thus, both Ki-67 protein level and phosphorylation status during mitosis appear to influence the perichromosomal distribution of nuclear lipids. Ki-67 knockdown and CDK1 inhibition also lead to uneven chromosome disjunction between daughter cells, highlighting the critical role of this regulatory mechanism in ensuring accurate chromosome segregation. Given that Ki-67 has been proposed to promote chromosome individualization and establish chromosome-cytoplasmic compartmentalization during open mitosis in vertebrates, our results reveal that nuclear lipid enrichment at the perichromosomal layer enhances the ability of Ki-67 to form a protective perichromosomal barrier (chromosome envelope), which is critical for correct chromosome segregation and maintenance of genome integrity during mitosis.

Amphipathic helices sense the inner nuclear membrane environment through lipid packing defects

Amphipathic helices (AHs) are ubiquitous protein motifs that modulate targeting to organellar membranes by sensing differences in bulk membrane properties. However, the adaptation between membrane-targeting AHs and the nuclear membrane environment that surrounds the genome is poorly understood. Here, we computationally screened for candidate AHs in a curated list of characterized and putative human inner nuclear membrane (INM) proteins. Cell biological and in vitro experimental assays combined with computational calculations demonstrated that AHs detect lipid packing defects over electrostatics to bind to the INM, indicating that the INM is loosely packed under basal conditions. Membrane tension resulting from hypotonic shock further promoted AH binding to the INM, whereas cell-substrate stretch did not enhance recruitment of membrane tension-sensitive AHs. Together, our work demonstrates the rules driving lipid-protein interactions at the INM, and its implications in the response of the nucleus to different stimuli.

Nuclear lipid droplets: a novel regulator of nuclear homeostasis and ageing

Aging is a fundamental driver of numerous life-threatening diseases, significantly compromising cellular structures and functions, including the integrity of the nucleus. A consistent feature of aging across diverse species is the progressive accumulation of lipid droplets (nLDs) within the nuclear compartment, which disrupts nuclear architecture and functionality. Notably, aging is accompanied by a marked increase in nLD accumulation at the nuclear envelope. Interventions known to extend lifespan, such as caloric restriction and reduced insulin signaling, significantly reduce both the rate of accumulation and the size of nLDs. The triglyceride lipase ATGL-1, which localizes to the nuclear envelope, plays a critical role in limiting nLD buildup and maintaining nuclear lipid balance, especially in long-lived mutant worms. These findings establish excessive nuclear lipid deposition as a key hallmark of aging, with profound implications for nuclear processes such as chromatin organization, DNA repair, and gene regulation. In addition, ATGL-1 emerges as a promising therapeutic target for preserving nuclear health, extending organismal healthspan, and combating age-related disorders driven by lipid dysregulation.

Seipin governs phosphatidic acid homeostasis at the inner nuclear membrane

The nuclear envelope is a specialized subdomain of the endoplasmic reticulum and comprises the inner and outer nuclear membranes. Despite the crucial role of the inner nuclear membrane in genome regulation, its lipid metabolism remains poorly understood. Phosphatidic acid (PA) is essential for membrane growth as well as lipid storage. Using a genome-wide lipid biosensor screen in S. cerevisiae, we identify regulators of inner nuclear membrane PA homeostasis, including yeast Seipin, a known mediator of nuclear lipid droplet biogenesis. Here, we show that Seipin preserves nuclear envelope integrity by preventing its deformation and ectopic membrane formation. Mutations of specific regions of Seipin, some linked to human lipodystrophy, disrupt PA distribution at the inner nuclear membrane and nuclear lipid droplet formation. Investigating the Seipin co-factor Ldb16 reveals that a triacylglycerol binding site is crucial for lipid droplet formation, whereas PA regulation can be functionally separated. Our study highlights the potential of lipid biosensor screens for examining inner nuclear membrane lipid metabolism.

Behind the stoNE wall: A fervent activity for nuclear lipids

The four main types of biomolecules are nucleic acids, proteins, carbohydrates and lipids. The knowledge about their respective interactions is as important as the individual understanding of each of them. However, while, for example, the interaction of proteins with the other three groups is extensively studied, that of nucleic acids and lipids is, in comparison, very poorly explored. An iconic paradigm of physical (and likely functional) proximity between DNA and lipids is the case of the genomic DNA in eukaryotes: enclosed within the nucleus by two concentric lipid bilayers, the wealth of implications of this interaction, for example in genome stability, remains underassessed. Nuclear lipid-related phenotypes have been observed for 50 years, yet in most cases kept as mere anecdotical descriptions. In this review, we will bring together the evidence connecting lipids with both the nuclear envelope and the nucleoplasm, and will make critical analyses of these descriptions. Our exploration establishes a scenario in which lipids irrefutably play a role in nuclear homeostasis.

Sculpting nuclear envelope identity from the endoplasmic reticulum during the cell cycle

The nuclear envelope (NE) regulates nuclear functions, including transcription, nucleocytoplasmic transport, and protein quality control. While the outer membrane of the NE is directly continuous with the endoplasmic reticulum (ER), the NE has an overall distinct protein composition from the ER, which is crucial for its functions. During open mitosis in higher eukaryotes, the NE disassembles during mitotic entry and then reforms as a functional territory at the end of mitosis to reestablish nucleocytoplasmic compartmentalization. In this review, we examine the known mechanisms by which the functional NE reconstitutes from the mitotic ER in the continuous ER-NE endomembrane system during open mitosis. Furthermore, based on recent findings indicating that the NE possesses unique lipid metabolism and quality control mechanisms distinct from those of the ER, we explore the maintenance of NE identity and homeostasis during interphase. We also highlight the potential significance of membrane junctions between the ER and NE.

Mechanisms of nuclear envelope expansion

In actively dividing eukaryotic cells, the nuclear envelope membrane (NEM) expands during the cell cycle to accommodate increases in nuclear volume and formation of two nuclei as a cell passes through mitosis to form daughter cells. NEM expansion is driven by glycerophospholipid (GPL) synthesis that is regulated by the lipin family of phosphatidic acid phosphatases (PAPs). How, and when during the cell cycle, PAPs regulate membrane expansion differs between organisms undergoing a closed or open mitosis. Here, we discuss recent studies that shed light on the mechanisms of NE expansion. Moreover, we examine evidence that NEM expansion not only employs GPLs synthesized in the ER but also lipids whose synthesis is regulated by events at the inner nuclear membrane.

Nuclear membrane: A key potential therapeutic target for lipid metabolism

Lipid homeostasis plays a pivotal role in cellular growth, necessitating the engagement of numerous lipid metabolism genes and the cohesive functioning of organelles. While the nucleus is traditionally recognized for its genetic roles, emerging evidence highlights its significant contribution to lipid homeostasis maintenance. Certain nuclear membrane proteins or associated proteins have the capacity to directly catalyze lipid synthesis or modification processes. Mutations in the genes encoding these proteins can lead to disrupted lipid metabolism, contributing to a spectrum of metabolic disorders. This article provides a comprehensive reviews of the investigations exploring the interplay between nuclear membrane proteins and lipid metabolism. Additionally, it delves into the heterogeneity of the nuclear membrane, positioning it as a novel therapeutic target for managing metabolic disorders and mitigating adverse drug reactions.

A quantitative ultrastructural timeline of nuclear autophagy reveals a role for dynamin-like protein 1 at the nuclear envelope

Autophagic mechanisms that maintain nuclear envelope homeostasis are bulwarks to aging and disease. By leveraging 4D lattice light sheet microscopy and correlative light and electron tomography, we define a quantitative and ultrastructural timeline of nuclear macroautophagy (nucleophagy) in yeast. Nucleophagy begins with a rapid accumulation of the selective autophagy receptor Atg39 at the nuclear envelope and finishes in ~300 seconds with Atg39-cargo delivery to the vacuole. Although there are several routes to the vacuole, at least one pathway incorporates two consecutive membrane fission steps: inner nuclear membrane (INM) fission to generate an INM-derived vesicle in the perinuclear space and outer nuclear membrane (ONM) fission to liberate a double membraned vesicle to the cytosol. ONM fission occurs independently of phagophore engagement and instead relies surprisingly on dynamin like 1 (Dnm1), which is recruited to sites of Atg39 accumulation by Atg11. Loss of Dnm1 compromises nucleophagic flux by stalling nucleophagy after INM fission. Our findings reveal how nuclear and INM cargo are removed from an intact nucleus without compromising its integrity, achieved in part by a non-canonical role for Dnm1 in nuclear envelope remodeling.

Mechanisms coupling lipid droplets to MASLD pathophysiology

Hepatic steatosis, the buildup of neutral lipids in lipid droplets (LDs), is commonly referred to as metabolic dysfunction-associated steatotic liver disease when alcohol or viral infections are not involved. Metabolic dysfunction-associated steatotic liver disease encompasses simple steatosis and the more severe metabolic dysfunction-associated steatohepatitis, characterized by inflammation, hepatocyte injury, and fibrosis. Previously viewed as inert markers of disease, LDs are now understood to play active roles in disease etiology and have significant nonpathological and pathological functions in cell signaling and function. These dynamic properties of LDs are tightly regulated by hundreds of proteins that coat the LD surface, controlling lipid metabolism, trafficking, and signaling. The following review highlights various facets of LD biology with the primary goal of discussing key mechanisms through which LDs promote the development of advanced liver diseases, including metabolic dysfunction-associated steatohepatitis.

Mechanisms for assembly of the nucleoplasmic reticulum

The nuclear envelope consists of an outer membrane connected to the endoplasmic reticulum, an inner membrane facing the nucleoplasm and a perinuclear space separating the two bilayers. The inner and outer nuclear membranes are physically connected at nuclear pore complexes that mediate selective communication and transfer of materials between the cytoplasm and nucleus. The spherical shape of the nuclear envelope is maintained by counterbalancing internal and external forces applied by cyto- and nucleo-skeletal networks, and the nuclear lamina and chromatin that underly the inner nuclear membrane. Despite its apparent rigidity, the nuclear envelope can invaginate to form an intranuclear membrane network termed the nucleoplasmic reticulum (NR) consisting of Type-I NR contiguous with the inner nuclear membrane and Type-II NR containing both the inner and outer nuclear membranes. The NR extends deep into the nuclear interior potentially facilitating communication and exchanges between the nuclear interior and the cytoplasm. This review details the evidence that NR intrusions that regulate cytoplasmic communication and genome maintenance are the result of a dynamic interplay between membrane biogenesis and remodelling, and physical forces exerted on the nuclear lamina derived from the cyto- and nucleo-skeletal networks.

Determination of Initial Rates of Lipopolysaccharide Transport

Nonvesicular lipid trafficking pathways are an important process in every domain of life. The mechanisms of these processes are poorly understood in part due to the difficulty in kinetic characterization. One important class of glycolipids, lipopolysaccharides (LPS), are the primary lipidic component of the outer membrane of Gram-negative bacteria. LPS are synthesized in the inner membrane and then trafficked to the cell surface by the lipopolysaccharide transport proteins, LptB2FGCADE. By characterizing the interaction of a fluorescent probe and LPS, we establish a quantitative assay to monitor the flux of LPS between proteoliposomes on the time scale of seconds. We then incorporate photocaged ATP into this system, which allows for light-based control of the initiation of LPS transport. This control allows us to measure the initial rate of LPS transport (3.0 min-1 per LptDE). We also find that the rate of LPS transport by the Lpt complex is independent of the structure of LPS. In contrast, we find the rate of LPS transport is dependent on the proper function of the LptDE complex. Mutants of the outer membrane Lpt components, LptDE, that cause defective LPS assembly in live cells display attenuated transport rates and slower ATP hydrolysis compared to wild type proteins. Analysis of these mutants reveals that the rates of ATP hydrolysis and LPS transport are correlated such that 1.2 ± 0.2 ATP are hydrolyzed for each LPS transported. This correlation suggests a model where the outer membrane components ensure the coupling of ATP hydrolysis and LPS transport by stabilizing a transport-active state of the Lpt bridge.

Interplay between lipid metabolism, lipid droplets and RNA virus replication

Lipids play essential roles in the cell as components of cellular membranes, signaling molecules, and energy storage sources. Lipid droplets are cellular organelles composed of neutral lipids, such as triglycerides and cholesterol esters, and are also considered as cellular energy reserves, yet new functions have been recently associated with these structures, such as regulators of oxidative stress and cellular lipotoxicity, as well as modulators of pathogen infection through immune regulation. Lipid metabolism and lipid droplets participate in the infection process of many RNA viruses and control their replication and assembly, among others. Here, we review and discuss the contribution of lipid metabolism and lipid droplets over the replication cycle of RNA viruses, altogether pointing out potentially new pharmacological antiviral targets associated with lipid metabolism.

TorsinA is essential for neuronal nuclear pore complex localization and maturation

As lifelong interphase cells, neurons face an array of unique challenges. A key challenge is regulating nuclear pore complex (NPC) biogenesis and localization, the mechanisms of which are largely unknown. Here we identify neuronal maturation as a period of strongly upregulated NPC biogenesis. We demonstrate that the AAA+ protein torsinA, whose dysfunction causes the neurodevelopmental movement disorder DYT-TOR1A dystonia and co-ordinates NPC spatial organization without impacting total NPC density. We generated an endogenous Nup107-HaloTag mouse line to directly visualize NPC organization in developing neurons and find that torsinA is essential for proper NPC localization. In the absence of torsinA, the inner nuclear membrane buds excessively at sites of mislocalized nascent NPCs, and the formation of complete NPCs is delayed. Our work demonstrates that NPC spatial organization and number are independently determined and identifies NPC biogenesis as a process vulnerable to neurodevelopmental disease insults.

Nucleolar stress induces nucleolar stress body formation via the NOSR-1/NUMR-1 axis in Caenorhabditis elegans

Environmental stimuli not only alter gene expression profiles but also induce structural changes in cells. How distinct nuclear bodies respond to cellular stress is poorly understood. Here, we identify a subnuclear organelle named the nucleolar stress body (NoSB), the formation of which is induced by the inhibition of rRNA transcription or inactivation of rRNA processing and maturation in C. elegans. NoSB does not colocalize with other previously described subnuclear organelles. We conduct forward genetic screening and identify a bZIP transcription factor, named nucleolar stress response-1 (NOSR-1), that is required for NoSB formation. The inhibition of rRNA transcription or inactivation of rRNA processing and maturation increases nosr-1 expression. By using transcriptome analysis of wild-type animals subjected to different nucleolar stress conditions and nosr-1 mutants, we identify that the SR-like protein NUMR-1 (nuclear localized metal responsive) is the target of NOSR-1. Interestingly, NUMR-1 is a component of NoSB and itself per se is required for the formation of NoSB. We conclude that the NOSR-1/NUMR-1 axis likely responds to nucleolar stress and mediates downstream stress-responsive transcription programs and subnuclear morphology alterations in C. elegans.

3D Imaging of Lipid Droplet-Nuclear Membrane Contact Sites and Cirrhotic Lipid Droplet Overexpression

To coordinate cellular physiology, cells rely on the rapid exchange of molecules at specialized organelle-organelle contact sites. Lipid droplets (LDs) and nuclear membrane (NM) contact sites are particularly vital communication hubs, playing key roles in the exchange of signaling molecules, lipids, and metabolites. However, there is still a lack of understanding of the specific morphology of the contact sites. Here, we combine advanced three-dimensional (3D) imaging with a high-brightness fluorescent probe specifically targeting LDs to map the structural landscape of LD-NM contact sites. The probe exhibits exceptional photophysical properties, making it highly suitable for visualizing the changes occurring in LDs during the apoptosis process. In addition, we utilize the advantages of the probe to accurately monitor the overexpression of abnormal LDs in cirrhosis by 3D imaging for the first time. The outcomes of this investigation highlight that the probe has potential as a robust imaging tool to investigate intricate biological functions of LDs and their implications in related diseases.

Phosphatidylserine enrichment in the nuclear membrane regulates key enzymes of phosphatidylcholine synthesis

Phosphatidylserine (PS) is an important anionic phospholipid that is synthesized within the endoplasmic reticulum (ER). While PS shows the highest enrichment and serves important functional roles in the plasma membrane (PM) but its role in the nucleus is poorly explored. Using three orthogonal approaches, we found that PS is also uniquely enriched in the inner nuclear membrane (INM) and the nuclear reticulum (NR). Nuclear PS is critical for supporting the translocation of CCTα and Lipin1α, two key enzymes important for phosphatidylcholine (PC) biosynthesis, from the nuclear matrix to the INM and NR in response to oleic acid treatment. We identified the PS-interacting regions within the M-domain of CCTα and M-Lip domain of Lipin1α, and show that lipid droplet formation is altered by manipulations of nuclear PS availability. Our studies reveal an unrecognized regulatory role of nuclear PS levels in the regulation of key PC synthesizing enzymes within the nucleus.

Roles of lipid droplets and related proteins in metabolic diseases

Lipid droplets (LDs), which are active organelles, derive from the monolayer membrane of the endoplasmic reticulum and encapsulate neutral lipids internally. LD-associated proteins like RAB, those in the PLIN family, and those in the CIDE family participate in LD formation and development, and they are active players in various diseases, organelles, and metabolic processes (i.e., obesity, non-alcoholic fatty liver disease, and autophagy). Our synthesis on existing research includes insights from the formation of LDs to their mechanisms of action, to provide an overview needed for advancing research into metabolic diseases and lipid metabolism.

Uncovering the Role of the Yeast Lysine Acetyltransferase NuA4 in the Regulation of Nuclear Shape and Lipid Metabolism

Here, we report a novel role for the yeast lysine acetyltransferase NuA4 in regulating phospholipid availability for organelle morphology. Disruption of the NuA4 complex results in 70% of cells displaying nuclear deformations and nearly 50% of cells exhibiting vacuolar fragmentation. Cells deficient in NuA4 also show severe defects in the formation of nuclear-vacuole junctions (NJV), as well as a decrease in piecemeal microautophagy of the nucleus (PMN). To determine the cause of these defects we focused on Pah1, an enzyme that converts phosphatidic acid into diacylglycerol, favoring accumulation of lipid droplets over phospholipids that are used for membrane expansion. NuA4 subunit Eaf1 was required for Pah1 localization to the inner nuclear membrane and artificially tethering of Pah1 to the nuclear membrane rescued nuclear deformation and vacuole fragmentation defects, but not defects related to the formation of NVJs. Mutation of a NuA4-dependent acetylation site on Pah1 also resulted in aberrant Pah1 localization and defects in nuclear morphology and NVJ. Our work suggests a critical role for NuA4 in organelle morphology that is partially mediated through the regulation of Pah1 subcellular localization.

Disordered region of nuclear membrane protein Bqt4 recruits phosphatidic acid to the nuclear envelope to maintain its structural integrity

The nuclear envelope (NE) is a permeable barrier that maintains nuclear-cytoplasmic compartmentalization and ensures nuclear function; however, it ruptures in various situations such as mechanical stress and mitosis. Although the protein components for sealing a ruptured NE have been identified, the mechanism by which lipid components are involved in this process remains to be elucidated. Here, we found that an inner nuclear membrane (INM) protein Bqt4 directly interacts with phosphatidic acid (PA) and serves as a platform for NE maintenance in the fission yeast Schizosaccharomyces pombe. The intrinsically disordered region (IDR) of Bqt4, proximal to the transmembrane domain, binds to PA and forms a solid aggregate in vitro. Excessive accumulation of Bqt4 IDR in INM results in membrane overproliferation and lipid droplet formation in the nucleus, leading to centromere dissociation from the NE and chromosome missegregation. Our findings suggest that Bqt4 IDR controls nuclear membrane homeostasis by recruiting PA to the INM, thereby maintaining the structural integrity of the NE.

Cell-Impermeable Buffering Fluorogenic Probes for Live-Cell Super-Resolution Imaging of Plasma Membrane Morphology Dynamics

Super-resolution fluorescence imaging has emerged as a potent tool for investigating the nanoscale structure and function of the plasma membrane (PM). Nevertheless, the challenge persists in achieving super-resolution imaging of PM dynamics due to limitations in probe photostability and issues with cell internalization staining. Herein, we report assembly-mediated buffering fluorogenic probes BMP-14 and BMP-16 exhibiting fast PM labeling and extended retention time (over 2 h) on PM. The incorporation of alkyl chains proves effective in promoting the aggregation of BMP-14 and BMP-16 into nonfluorescent nanoparticles to realize fluorogenicity and regulate the buffering capacity to rapidly replace photobleached probes ensuring stable long-term super-resolution imaging of PM. Utilizing these PM-buffering probes, we observed dynamic movements of PM filopodia and continuous shrinkage, leading to the formation of extracellular vesicles (EVs) using structured illumination microscopy (SIM). Furthermore, we discovered two distinct modes of EV fusion: one involving fusion through adjacent lipids and the other through filamentous lipid traction. The entire process of EV fusion outside the PM was dynamically tracked. Additionally, BMP-16 exhibited a unique capability of inducing single-molecule fluorescence blinking when used for cell membrane staining. This property makes BMP-16 suitable for the PAINT imaging of cell membranes.

Nuclear lipid droplet: Guardian of nuclear membrane lipid homeostasis?

Lipid droplets (LDs) are cytoplasmic organelles, but they are also found within the nucleus in small numbers. Nuclear LDs that form at the inner nuclear membrane (INM) often increase in response to perturbation in phosphatidic acid (PA) and/or diacylglycerol (DAG), both implicated in various INM functions. Nuclear LDs also increase upon downregulation of seipin, a protein that can trap PA and DAG in the endoplasmic reticulum. Notably, both PA and DAG appear to be more densely distributed on the surface of nuclear LDs than in the INM. I propose that nuclear LDs play a role in regulating the PA and DAG level in the INM, thereby contributing to the lipid homeostasis in this compartment.

Nuclear lipid droplets in Caco2 cells originate from nascent precursors and in situ at the nuclear envelope

Intestinal epithelial cells convert excess fatty acids into triglyceride (TAG) for storage in cytoplasmic lipid droplets and secretion in chylomicrons. Nuclear lipid droplets (nLDs) are present in intestinal cells but their origin and relationship to cytoplasmic TAG synthesis and secretion is unknown. nLDs and related lipid-associated promyelocytic leukemia structures (LAPS) were abundant in oleate-treated Caco2 but less frequent in other human colorectal cancer cell lines and mouse intestinal organoids. nLDs and LAPS in undifferentiated oleate-treated Caco2 cells harbored the phosphatidate phosphatase Lipin1, its product diacylglycerol, and CTP:phosphocholine cytidylyltransferase (CCT)α. CCTα knockout Caco2 cells had fewer but larger nLDs, indicating a reliance on de novo PC synthesis for assembly. Differentiation of Caco2 cells caused large nLDs and LAPS to form regardless of oleate treatment or CCTα expression. nLDs and LAPS in Caco2 cells did not associate with apoCIII and apoAI and formed dependently of microsomal triglyceride transfer protein expression and activity, indicating they are not derived from endoplasmic reticulum luminal LDs precursors. Instead, undifferentiated Caco2 cells harbored a constitutive pool of nLDs and LAPS in proximity to the nuclear envelope that expanded in size and number with oleate treatment. Inhibition of TAG synthesis did affect the number of nascent nLDs and LAPS but prevented their association with promyelocytic leukemia protein, Lipin1α, and diacylglycerol, which instead accumulated on the nuclear membranes. Thus, nLD and LAPS biogenesis in Caco2 cells is not linked to lipoprotein secretion but involves biogenesis and/or expansion of nascent nLDs by de novo lipid synthesis.

The lipid droplet lipidome

Lipid droplets (LDs) are intracellular organelles with a hydrophobic core formed by neutral lipids surrounded by a phospholipid monolayer harboring a variety of regulatory and enzymatically active proteins. Over the last few decades, our understanding of LD biology has evolved significantly. Nowadays, LDs are appreciated not just as passive energy storage units, but rather as active players in the regulation of lipid metabolism and quality control machineries. To fulfill their functions in controlling cellular metabolic states, LDs need to be highly dynamic and responsive organelles. A large body of evidence supports a dynamic nature of the LD proteome and its contact sites with other organelles. However, much less is known about the lipidome of LDs. Numerous examples clearly indicate the intrinsic link between LD lipids and proteins, calling for a deeper characterization of the LD lipidome in various physiological and pathological settings. Here, we reviewed the current state of knowledge in the field of the LD lipidome, providing a brief overview of the lipid classes and their molecular species present within the neutral core and phospholipid monolayer.

A unifying mechanism for seipin-mediated lipid droplet formation

Lipid droplets (LDs) are dynamic organelles essential for cellular lipid homeostasis. Assembly of LDs occurs in the endoplasmic reticulum (ER), and the conserved ER membrane protein seipin emerged as a key player in this process. Here, we review recent advances provided by structural, biochemical, and in silico analysis that revealed mechanistic insights into the molecular role of the seipin complexes and led to an updated model for LD biogenesis. We further discuss how other ER components cooperate with seipin during LD biogenesis. Understanding the molecular mechanisms underlying seipin-mediated LD assembly is important to uncover the fundamental aspects of lipid homeostasis and organelle biogenesis and to provide hints on the pathogenesis of lipid storage disorders.

Zooming into lipid droplet biology through the lens of electron microscopy

Electron microscopy (EM), in its various flavors, has significantly contributed to our understanding of lipid droplets (LD) as central organelles in cellular metabolism. For example, EM has illuminated that LDs, in contrast to all other cellular organelles, are uniquely enclosed by a single phospholipid monolayer, revealed the architecture of LD contact sites with different organelles, and provided near-atomic resolution maps of key enzymes that regulate neutral lipid biosynthesis and LD biogenesis. In this review, we first provide a brief history of pivotal findings in LD biology unveiled through the lens of an electron microscope. We describe the main EM techniques used in the context of LD research and discuss their current capabilities and limitations, thereby providing a foundation for utilizing suitable EM methodology to address LD-related questions with sufficient level of structural preservation, detail, and resolution. Finally, we highlight examples where EM has recently been and is expected to be instrumental in expanding the frontiers of LD biology.

Lipotoxicity as a therapeutic target in obesity and diabetic cardiomyopathy

Unhealthy sources of fats, ultra-processed foods with added sugars, and a sedentary lifestyle make humans more susceptible to developing overweight and obesity. While lipids constitute an integral component of the organism, excessive and abnormal lipid accumulation that exceeds the storage capacity of lipid droplets disrupts the intracellular composition of fatty acids and results in the release of deleterious lipid species, thereby giving rise to a pathological state termed lipotoxicity. This condition induces endoplasmic reticulum stress, mitochondrial dysfunction, inflammatory responses, and cell death. Recent advances in omics technologies and analytical methodologies and clinical research have provided novel insights into the mechanisms of lipotoxicity, including gut dysbiosis, epigenetic and epitranscriptomic modifications, dysfunction of lipid droplets, post-translational modifications, and altered membrane lipid composition. In this review, we discuss the recent knowledge on the mechanisms underlying the development of lipotoxicity and lipotoxic cardiometabolic disease in obesity, with a particular focus on lipotoxic and diabetic cardiomyopathy.

Lipopolysaccharide binding protein resists hepatic oxidative stress by regulating lipid droplet homeostasis

Oxidative stress-induced lipid accumulation is mediated by lipid droplets (LDs) homeostasis, which sequester vulnerable unsaturated triglycerides into LDs to prevent further peroxidation. Here we identify the upregulation of lipopolysaccharide-binding protein (LBP) and its trafficking through LDs as a mechanism for modulating LD homeostasis in response to oxidative stress. Our results suggest that LBP induces lipid accumulation by controlling lipid-redox homeostasis through its lipid-capture activity, sorting unsaturated triglycerides into LDs. N-acetyl-L-cysteine treatment reduces LBP-mediated triglycerides accumulation by phospholipid/triglycerides competition and Peroxiredoxin 4, a redox state sensor of LBP that regulates the shuttle of LBP from LDs. Furthermore, chronic stress upregulates LBP expression, leading to insulin resistance and obesity. Our findings contribute to the understanding of the role of LBP in regulating LD homeostasis and against cellular peroxidative injury. These insights could inform the development of redox-based therapies for alleviating oxidative stress-induced metabolic dysfunction.

Chronic exposure to 3,6-dichlorocarbazole exacerbates non-alcoholic fatty liver disease in zebrafish by disrupting lipid metabolism and inducing special lipid biomarker accumulation

Most previous studies have focused primarily on the adverse effects of environmental chemicals on organisms of good healthy. Although global prevalence of non-alcoholic fatty liver disease (NAFLD) has reached approximately 25%, the impact of environmentally persistent organic chemicals on organisms with NAFLD is substantially unknown. Polyhalogenated carbazoles (PHCZs) as emerging contaminants have been frequently detected in the environment and organisms. In this study, we investigated the impact of the most frequently detected PHCZs, 3,6-dichlorocarbazole (36-CCZ), on zebrafish with high-fat diet (HFD)-induced NAFLD. After 4 weeks exposure to environmentally relevant concentrations of 36-CCZ (0.16-0.45 μg/L), the accumulation of lipid in zebrafish liver dramatically increased, and the transcription of genes involved in lipid synthesis, transport and oxidation was significantly upregulated, demonstrating that 36-CCZ had exacerbated the NAFLD in zebrafish. Lipidomic analysis indicated that 36-CCZ had significantly affected liver lipid metabolic pathways, mainly including glycerolipids and glycerophospholipids. Additionally, fifteen lipids were identified as potential lipid biomarkers for 36-CCZ exacerbation of NAFLD, including diacylglycerols (DGs), triglycerides (TGs), phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), phosphatidic acid (PA), and phosphatidylinositol (PI). These findings demonstrate that long-term exposure to 36-CCZ can promote the progression of NAFLD, which will contribute to raising awareness of the health risks of PHCZs.

Lipid- and phospho-regulation of CTP:Phosphocholine Cytidylyltransferase α association with nuclear lipid droplets

Fatty acids stored in triacylglycerol-rich lipid droplets are assembled with a surface monolayer composed primarily of phosphatidylcholine (PC). Fatty acids stimulate PC synthesis by translocating CTP:phosphocholine cytidylyltransferase (CCT) α to the inner nuclear membrane, nuclear lipid droplets (nLD) and lipid associated promyelocytic leukemia (PML) structures (LAPS). Huh7 cells were used to identify how CCTα translocation onto these nuclear structures are regulated by fatty acids and phosphorylation of its serine-rich P-domain. Oleate treatment of Huh7 cells increased nLDs and LAPS that became progressively enriched in CCTα. In cells expressing the phosphatidic acid phosphatase Lipin1α or 1β, the expanded pool of nLDs and LAPS had a proportional increase in associated CCTα. In contrast, palmitate induced few nLDs and LAPS and inhibited the oleate-dependent translocation of CCTα without affecting total nLDs. Phospho-memetic or phospho-null mutations in the P-domain revealed that a 70% phosphorylation threshold, rather than site-specific phosphorylation, regulated CCTα association with nLDs and LAPS. In vitro candidate kinase and inhibitor studies in Huh7 cells identified cyclin-dependent kinase (CDK) 1 and 2 as putative P-domain kinases. In conclusion, CCTα translocation onto nLDs and LAPS is dependent on available surface area and fatty acid composition, as well as threshold phosphorylation of the P-domain potentially involving CDKs.

Lipid droplets in pathogen infection and host immunity

As the hub of cellular lipid metabolism, lipid droplets (LDs) have been linked to a variety of biological processes. During pathogen infection, the biogenesis, composition, and functions of LDs are tightly regulated. The accumulation of LDs has been described as a hallmark of pathogen infection and is thought to be driven by pathogens for their own benefit. Recent studies have revealed that LDs and their subsequent lipid mediators contribute to effective immunological responses to pathogen infection by promoting host stress tolerance and reducing toxicity. In this comprehensive review, we delve into the intricate roles of LDs in governing the replication and assembly of a wide spectrum of pathogens within host cells. We also discuss the regulatory function of LDs in host immunity and highlight the potential for targeting LDs for the diagnosis and treatment of infectious diseases.

Diacylglycerol metabolism and homeostasis in fungal physiology

Diacylglycerol (DAG) is a relatively simple and primitive form of lipid, which does not possess a phospholipid headgroup. Being a central metabolite of the lipid metabolism network, DAGs are omnipresent in all life forms. While the role of DAG has been established in membrane and storage lipid biogenesis, it can impart crucial physiological functions including membrane shapeshifting, regulation of membrane protein activity, and transduction of cellular signalling as a lipid-based secondary messenger. Besides, the chemical diversity of DAGs, due to fatty acyl chain composition, has been proposed to be the basis of its functional diversity. Therefore, cells must regulate DAG level at a spatio-temporal scale for homeostasis and adaptation. The vast network of eukaryotic lipid metabolism has been unravelled majorly by studying yeast models. Here, we review the current understanding and the emerging concepts in metabolic and functional aspects of DAG regulation in yeast. The implications can be extended to understand pathogenic fungi and mammalian counterparts as well as disease aetiology.

Hard to handle: how lipid saturation affects the nuclear envelope

The nuclear envelope is a unique subdomain of the endoplasmic reticulum (ER) that encapsulates the genome and mediates communication between the nucleus and the rest of the cell via nuclear pore complexes. A recent study by Romanauska and Köhler shows that balanced lipid unsaturation is critical for nuclear envelope and nuclear pore complex architecture and function.

The Plasma Membrane H+ ATPase CsPMA2 Regulates Lipid Droplet Formation, Appressorial Development and Virulence in Colletotrichum siamense

Plasma membrane H+-ATPases (PMAs) play an important role in the pathogenicity of pathogenic fungi. Lipid droplets are important storage sites for neutral lipids in fungal conidia and hyphae and can be used by plant pathogenic fungi for infection. However, the relationship between plasma membrane H+-ATPase, lipid droplets and virulence remains unclear. Here, we characterized a plasma membrane H+-ATPase, CsPMA2, that plays a key role in lipid droplet formation, appresorial development and virulence in C. siamense. Deletion of CsPMA2 impaired C. siamense conidial size, conidial germination, appressorial development and virulence but did not affect hyphal growth. ΔCsPMA2 increased the sensitivity of C. siamense to phytic acid and oxalic acid. CsPMA2 was localized to lipids on the plasma membrane and intracellular membrane. Deletion of CsPMA2 significantly inhibited the accumulation of lipid droplets and significantly affected the contents of some species of lipids, including 12 species with decreased lipid contents and 3 species with increased lipid contents. Furthermore, low pH can inhibit CsPMA2 expression and lipid droplet accumulation. Overall, our data revealed that the plasma membrane H+-ATPase CsPMA2 is involved in the regulation of lipid droplet formation and affects appressorial development and virulence in C. siamense.

The interplay of nuclear pores and lipids

Nuclear pore complexes (NPCs) mediate the bidirectional transport of cargo across the nuclear envelope (NE). NPCs are also membrane remodeling machines with a capacity to curve and fuse the membranes of the NE. However, little is known about the interplay of NPCs and lipids at a mechanistic level. A full understanding of NPC structure and function needs to encompass how the NPC shapes membranes and is itself shaped by lipids. Here we attempt to connect recent findings in NPC research with the broader field of membrane biochemistry to illustrate how an interplay between NPCs and lipids may facilitate the conformational plasticity of NPCs and the generation of a unique pore membrane topology. We highlight the need to better understand the NPC's lipid environment and outline experimental avenues towards that goal.

Nuclear envelope budding and its cellular functions

The nuclear pore complex (NPC) has long been assumed to be the sole route across the nuclear envelope, and under normal homeostatic conditions it is indeed the main mechanism of nucleo-cytoplasmic transport. However, it has also been known that e.g. herpesviruses cross the nuclear envelope utilizing a pathway entitled nuclear egress or envelopment/de-envelopment. Despite this, a thread of observations suggests that mechanisms similar to viral egress may be transiently used also in healthy cells. It has since been proposed that mechanisms like nuclear envelope budding (NEB) can facilitate the transport of RNA granules, aggregated proteins, inner nuclear membrane proteins, and mis-assembled NPCs. Herein, we will summarize the known roles of NEB as a physiological and intrinsic cellular feature and highlight the many unanswered questions surrounding these intriguing nuclear events.

The leucine zipper domain of the transcriptional repressor Opi1 underlies a signal transduction mechanism regulating lipid synthesis

In Saccharomyces cerevisiae, the transcriptional repressor Opi1 regulates the expression of genes involved in phospholipid synthesis responding to the abundance of the phospholipid precursor phosphatidic acid at the endoplasmic reticulum. We report here the identification of the conserved leucine zipper (LZ) domain of Opi1 as a hot spot for gain of function mutations and the characterization of the strongest variant identified, Opi1N150D. LZ modeling posits asparagine 150 embedded on the hydrophobic surface of the zipper and specifying dynamic parallel homodimerization by allowing electrostatic bonding across the hydrophobic dimerization interface. Opi1 variants carrying any of the other three ionic residues at amino acid 150 were also repressing. Genetic analyses showed that Opi1N150D variant is dominant, and its phenotype is attenuated when loss of function mutations identified in the other two conserved domains are present in cis. We build on the notion that membrane binding facilitates LZ dimerization to antagonize an intramolecular interaction of the zipper necessary for repression. Dissecting Opi1 protein in three polypeptides containing each conserved region, we performed in vitro analyses to explore interdomain interactions. An Opi11-190 probe interacted with Opi1291-404, the C terminus that bears the activator interacting domain (AID). LZ or AID loss of function mutations attenuated the interaction of the probes but was unaffected by the N150D mutation. We propose a model for Opi1 signal transduction whereby synergy between membrane-binding events and LZ dimerization antagonizes intramolecular LZ-AID interaction and transcriptional repression.

Ultrabright organic fluorescent probe for quantifying the dynamics of cytosolic/nuclear lipid droplets

Lipid droplets (LDs) are extremely active organelles that play a crucial role in energy metabolism, membrane formation, and the production of lipid-derived signaling molecules by regulating lipid storage and release. Nevertheless, directly limited by the lack of superior fluorescent probes, studies of LDs dynamic motion velocity have been rarely reported, especially for nuclear LDs. Herein, a novel organic fluorescent probe Lipi-Bright has been rationally developed based on bridged cyclization of distyrylbenzene. The fully ring-fused molecule structure endows the probe with high photostability. Moreover, this new fluorescent probe displays the features of excellent LDs staining specificity as well as ultrahigh fluorescence brightness. Lipi-Bright labeled LDs was dozens of times brighter than representative probes BODIPY 493/503 or Nile Red. Consequently, by in-situ time-lapse fluorescence imaging, the dynamics of LDs have been quantitatively studied. For instance, the velocities of cytosolic LDs (37 ± 15 nm/s) are found to be obviously faster than those of nuclear LDs (24 ± 4 nm/s), and both the cytosolic LDs and the nuclear LDs would be moved faster or slower depend on the various stimulations. Overall, this work providing plentiful information on LDs dynamics will greatly facilitate the in-depth investigation of lipid metabolism.

Centrins control chicken cone cell lipid droplet dynamics through lipid-droplet-localized SPDL1

As evolutionarily conserved organelles, lipid droplets (LDs) carry out numerous functions and have various subcellular localizations in different cell types and species. In avian cone cells, there is a single apically localized LD. We demonstrated that CIDEA (cell death inducing DFFA like effector a) and microtubules promote the formation of the single LD in chicken cone cells. Centrins, which are well-known centriole proteins, target to the cone cell LD via their C-terminal calcium-binding domains. Centrins localize on cone cell LDs with the help of SPDL1-L (spindle apparatus coiled-coil protein 1-L), a previously uncharacterized isoform of the kinetochore-associated dynein adaptor SPDL1. The loss of CETN3 or overexpression of a truncated CETN1 abrogates the apical localization of the cone cell LD. Simulation analysis showed that multiple LDs or a single mispositioned LD reduces the light sensitivity. Collectively, our findings identify a role of centrins in the regulation of cone cell LD localization, which is important for the light sensitivity of cone cells.

Identification of novel proteins regulating lipid droplet biogenesis in filamentous fungi

Lipid droplets (LDs) are storage organelles for neutral lipids which are critical for lipid homeostasis. Current knowledge of fungal LD biogenesis is largely limited to budding yeast, while LD regulation in multinucleated filamentous fungi which exhibit considerable metabolic activity remains unexplored. In this study, 19 LD-associated proteins were identified in the multinucleated species Aspergillus oryzae using a colocalization screening of a previously established enhanced green fluorescent protein (EGFP) fusion library. Functional screening identified 12 lipid droplet-regulating (LDR) proteins whose loss of function resulted in irregular LD biogenesis, particularly in terms of LD number and size. Bioinformatics analysis, targeted mutagenesis, and microscopy revealed four LDR proteins that localize to LD via the putative amphipathic helices (AHs). Further analysis revealed that LdrA with an Opi1 domain is essential for cytoplasmic and nuclear LD biogenesis involving a novel AH. Phylogenetic analysis demonstrated that the patterns of gene evolution were predominantly based on gene duplication. Our study identified a set of novel proteins involved in the regulation of LD biogenesis, providing unique molecular and evolutionary insights into fungal lipid storage.

Scaffold, mechanics and functions of nuclear lamins

Nuclear lamins are type-V intermediate filaments that are involved in many nuclear processes. In mammals, A- and B-type lamins assemble into separate physical meshwork underneath the inner nuclear membrane, the nuclear lamina, with some residual fraction localized within the nucleoplasm. Lamins are the major part of the nucleoskeleton, providing mechanical strength and flexibility to protect the genome and allow nuclear deformability, while also contributing to gene regulation via interactions with chromatin. While lamins are the evolutionary ancestors of all intermediate filament family proteins, their ultimate filamentous assembly is markedly different from their cytoplasmic counterparts. Interestingly, hundreds of genetic mutations in the lamina proteins have been causally linked with a broad range of human pathologies, termed laminopathies. These include muscular, neurological and metabolic disorders, as well as premature aging diseases. Recent technological advances have contributed to resolving the filamentous structure of lamins and the corresponding lamina organization. In this review, we revisit the multiscale lamin organization and discuss its implications on nuclear mechanics and chromatin organization within lamina-associated domains.

Parallel CRISPR-Cas9 screens identify mechanisms of PLIN2 and lipid droplet regulation

Despite the key roles of perilipin-2 (PLIN2) in governing lipid droplet (LD) metabolism, the mechanisms that regulate PLIN2 levels remain incompletely understood. Here, we leverage a set of genome-edited human PLIN2 reporter cell lines in a series of CRISPR-Cas9 loss-of-function screens, identifying genetic modifiers that influence PLIN2 expression and post-translational stability under different metabolic conditions and in different cell types. These regulators include canonical genes that control lipid metabolism as well as genes involved in ubiquitination, transcription, and mitochondrial function. We further demonstrate a role for the E3 ligase MARCH6 in regulating triacylglycerol biosynthesis, thereby influencing LD abundance and PLIN2 stability. Finally, our CRISPR screens and several published screens provide the foundation for CRISPRlipid (http://crisprlipid.org), an online data commons for lipid-related functional genomics data. Our study identifies mechanisms of PLIN2 and LD regulation and provides an extensive resource for the exploration of LD biology and lipid metabolism.

A membrane-sensing mechanism links lipid metabolism to protein degradation at the nuclear envelope

Lipid composition determines organelle identity; however, whether the lipid composition of the inner nuclear membrane (INM) domain of the ER contributes to its identity is not known. Here, we show that the INM lipid environment of animal cells is under local control by CTDNEP1, the master regulator of the phosphatidic acid phosphatase lipin 1. Loss of CTDNEP1 reduces association of an INM-specific diacylglycerol (DAG) biosensor and results in a decreased percentage of polyunsaturated containing DAG species. Alterations in DAG metabolism impact the levels of the resident INM protein Sun2, which is under local proteasomal regulation. We identify a lipid-binding amphipathic helix (AH) in the nucleoplasmic domain of Sun2 that prefers membrane packing defects. INM dissociation of the Sun2 AH is linked to its proteasomal degradation. We suggest that direct lipid-protein interactions contribute to sculpting the INM proteome and that INM identity is adaptable to lipid metabolism, which has broad implications on disease mechanisms associated with the nuclear envelope.

Lipid saturation controls nuclear envelope function

The nuclear envelope (NE) is a spherical double membrane with elastic properties. How NE shape and elasticity are regulated by lipid chemistry is unknown. Here we discover lipid acyl chain unsaturation as essential for NE and nuclear pore complex (NPC) architecture and function. Increased lipid saturation rigidifies the NE and the endoplasmic reticulum into planar, polygonal membranes, which are fracture prone. These membranes exhibit a micron-scale segregation of lipids into ordered and disordered phases, excluding NPCs from the ordered phase. Balanced lipid saturation is required for NPC integrity, pore membrane curvature and nucleocytoplasmic transport. Oxygen deprivation amplifies the impact of saturated lipids, causing NE rigidification and rupture. Conversely, lipid droplets buffer saturated lipids to preserve NE architecture. Our study uncovers a fundamental link between lipid acyl chain structure and the integrity of the cell nucleus with implications for nuclear membrane malfunction in ischaemic tissues.

Comparative membrane proteomics reveals diverse cell regulators concentrated at the nuclear envelope

The nuclear envelope (NE) is a subdomain of the ER with prominent roles in nuclear organization, which are largely mediated by its distinctive protein composition. We developed methods to reveal low-abundance transmembrane (TM) proteins concentrated at the NE relative to the peripheral ER. Using label-free proteomics that compared isolated NEs with cytoplasmic membranes, we first identified proteins with apparent NE enrichment. In subsequent authentication, ectopically expressed candidates were analyzed by immunofluorescence microscopy to quantify their targeting to the NE in cultured cells. Ten proteins from a validation set were found to associate preferentially with the NE, including oxidoreductases, enzymes for lipid biosynthesis, and regulators of cell growth and survival. We determined that one of the validated candidates, the palmitoyltransferase Zdhhc6, modifies the NE oxidoreductase Tmx4 and thereby modulates its NE levels. This provides a functional rationale for the NE concentration of Zdhhc6. Overall, our methodology has revealed a group of previously unrecognized proteins concentrated at the NE and additional candidates. Future analysis of these can potentially unveil new mechanistic pathways associated with the NE.

SUMOylation at the inner nuclear membrane facilitates nuclear envelope biogenesis during mitosis

As eukaryotic cells progress through cell division, the nuclear envelope (NE) membrane must expand to accommodate the formation of progeny nuclei. In Saccharomyces cerevisiae, closed mitosis allows visualization of NE biogenesis during mitosis. During this period, the SUMO E3 ligase Siz2 binds the inner nuclear membrane (INM) and initiates a wave of INM protein SUMOylation. Here, we show these events increase INM levels of phosphatidic acid (PA), an intermediate of phospholipid biogenesis, and are necessary for normal mitotic NE membrane expansion. The increase in INM PA is driven by the Siz2-mediated inhibition of the PA phosphatase Pah1. During mitosis, this results from the binding of Siz2 to the INM and dissociation of Spo7 and Nem1, a complex required for the activation of Pah1. As cells enter interphase, the process is then reversed by the deSUMOylase Ulp1. This work further establishes a central role for temporally controlled INM SUMOylation in coordinating processes, including membrane expansion, that regulate NE biogenesis during mitosis.