Dividing cells regulate their lipid composition and localization

Yeast cells actively tune their membranes to phase separate at temperatures that scale with growth temperatures

Phase separation in membranes creates domains enriched in specific components. To date, the best example of micrometer-scale phase separation in the membrane of an unperturbed, living cell occurs in a yeast (Saccharomyces cerevisiae) organelle called the vacuole. Recent studies indicate that the phases are functionally important, enabling yeast survival during periods of stress. We discovered that yeast regulate this phase transition; the temperature at which membrane components mix into a single phase is ∼15 °C above the growth temperature. To maintain this offset, yeast may tune the level of ergosterol (a molecule that is structurally similar to cholesterol) in their membranes. Surprisingly, depleting sterols in vacuole membranes causes them to phase separate, in contrast to previous assumptions.

Membranes of vacuoles, the lysosomal organelles of Saccharomyces cerevisiae (budding yeast), undergo extraordinary changes during the cell’s normal growth cycle. The cycle begins with a stage of rapid cell growth. Then, as glucose becomes scarce, growth slows, and vacuole membranes phase separate into micrometer-scale domains of two liquid phases. Recent studies suggest that these domains promote yeast survival by organizing membrane proteins that play key roles in a central signaling pathway conserved among eukaryotes (TORC1). An outstanding question in the field has been whether cells regulate phase transitions in response to new physical conditions and how this occurs. Here, we measure transition temperatures and find that after an increase of roughly 15 °C, vacuole membranes appear uniform, independent of growth temperature. Moreover, populations of cells grown at a single temperature regulate this transition to occur over a surprisingly narrow temperature range. Remarkably, the transition temperature scales linearly with the growth temperature, demonstrating that the cells physiologically adapt to maintain proximity to the transition. Next, we ask how yeast adjust their membranes to achieve phase separation. We isolate vacuoles from yeast during the rapid stage of growth, when their membranes do not natively exhibit domains. Ergosterol is the major sterol in yeast. We find that domains appear when ergosterol is depleted, contradicting the prevalent assumption that increases in sterol concentration generally cause membrane phase separation in vivo, but in agreement with previous studies using artificial and cell-derived membranes.

Lipid droplet dynamics regulate adult muscle stem cell fate

The lipid droplet (LD) is a central hub for fatty acid metabolism in cells. Here we define the dynamics and explore the role of LDs in skeletal muscle satellite cells (SCs), a stem cell population responsible for muscle regeneration. In newly divided SCs, LDs are unequally distributed in sister cells exhibiting asymmetric cell fates, as the LDLow cell self-renews while the LDHigh cell commits to differentiation. When transplanted into regenerating muscles, LDLow cells outperform LDHigh cells in self-renewal and regeneration in vivo. Pharmacological inhibition of LD biogenesis or genetic inhibition of LD catabolism through knockout of Pnpla2 (encoding ATGL, the rate-limiting enzyme for lipolysis) disrupts cell fate homeostasis and impairs the regenerative capacity of SCs. Dysfunction of Pnpla2-null SCs is associated with energy insufficiency and oxidative stress that can be partially rescued by antioxidant (N-acetylcysteine) treatment. These results establish a direct link between LD dynamics and stem cell fate determination.

Mammalian Epidermis: A Compendium of Lipid Functionality

Mammalian epidermis is a striking example of the role of lipids in tissue biology. In this stratified epithelium, highly specialized structures are formed that leverage the hydrophobic properties of lipids to form an impermeable barrier and protect the humid internal environment of the body from the dry outside. This is achieved through tightly regulated lipid synthesis that generates the molecular species unique to the tissue. Beyond their fundamental structural role, lipids are involved in the active protection of the body from external insults. Lipid species present on the surface of the body possess antimicrobial activity and directly contribute to shaping the commensal microbiota. Lipids belonging to a variety of classes are also involved in the signaling events that modulate the immune responses to environmental stress as well as differentiation of the epidermal keratinocytes themselves. Recently, high-resolution methods are beginning to provide evidence for the involvement of newly identified specific lipid molecules in the regulation of epidermal homeostasis. In this review we give an overview of the wide range of biological functions of mammalian epidermal lipids.

Translational control of lipogenesis links protein synthesis and phosphoinositide signaling with nuclear division in Saccharomyces cerevisiae

Continuously dividing cells coordinate their growth and division. How fast cells grow in mass determines how fast they will multiply. Yet, there are few, if any, examples of a metabolic pathway that actively drives a cell cycle event instead of just being required for it. Here, we show that translational upregulation of lipogenic enzymes in Saccharomyces cerevisiae increased the abundance of lipids and promoted nuclear elongation and division. Derepressing translation of acetyl-CoA carboxylase and fatty acid synthase also suppressed cell cycle-related phenotypes, including delayed nuclear division, associated with Sec14p phosphatidylinositol transfer protein deficiencies, and the irregular nuclear morphologies of mutants defective in phosphatidylinositol 4-OH kinase activities. Our results show that increased lipogenesis drives a critical cell cycle landmark and report a phosphoinositide signaling axis in control of nuclear division. The broad conservation of these lipid metabolic and signaling pathways raises the possibility these activities similarly govern nuclear division in other eukaryotes.

Oncosuppressive and oncogenic activity of the sphingolipid-metabolizing enzyme β-galactosylceramidase

β-galactosylceramidase (GALC) is a lysosomal enzyme that removes β-galactose from β-galactosylceramide, leading to the formation of the oncosuppressor metabolite ceramide. Recent observations have shown that GALC may exert opposite effects on tumor growth by acting as an oncosuppressive or oncogenic enzyme depending on the different experimental approaches, in vitro versus in vivo observations, preclinical versus clinical findings, and tumor type investigated. This review will recapitulate and discuss the contrasting experimental evidence related to the impact of GALC on the biological behavior of cancer and stromal cells and its contribution to tumor progression.

Phospholipid Profiles for Phenotypic Characterization of Adipose-Derived Multipotent Mesenchymal Stromal Cells

Multipotent mesenchymal stromal cells (MSC) have emerged as therapeutic tools for a wide range of pathological conditions. Yet, the still existing deficits regarding MSC phenotype characterization and the resulting heterogeneity of MSC used in different preclinical and clinical studies hamper the translational success. In search for novel MSC characterization approaches to complement the traditional trilineage differentiation and immunophenotyping assays reliably across species and culture conditions, this study explored the applicability of lipid phenotyping for MSC characterization and discrimination. Human peripheral blood mononuclear cells (PBMC), human fibroblasts, and human and equine adipose-derived MSC were used to compare different mesodermal cell types and MSC from different species. For MSC, cells cultured in different conditions, including medium supplementation with either fetal bovine serum or platelet lysate as well as culture on collagen-coated dishes, were additionally investigated. After cell harvest, lipids were extracted by chloroform/methanol according to Bligh and Dyer. The lipid profiles were analysed by an untargeted approach using liquid chromatography coupled to mass spectrometry (LC-MS) with a reversed phase column and an ion trap mass spectrometer. In all samples, phospholipids and sphingomyelins were found, while other lipids were not detected with the current approach. The phospholipids included different species of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidylserine (PS) in all cell types, whereas phosphatidylglycerol (PG) species were only present in MSC. MSC from both species showed a higher phospholipid species diversity than PBMC and fibroblasts. Few differences were found between MSC from different culture conditions, except that human MSC cultured with platelet lysate exhibited a unique phenotype in that they exclusively featured PE O-40:4, PG 38:6 and PG 40:6. In search for specific and inclusive candidate MSC lipid markers, we identified PE O-36:3 and PG 40:7 as potentially suitable markers across culture conditions, at which PE O-36:3 might even be used across species. On that basis, phospholipid phenotyping is a highly promising approach for MSC characterization, which might condone some heterogeneity within the MSC while still achieving a clear discrimination even from fibroblasts. Particularly the presence or absence of PG might emerge as a decisive criterion for future MSC characterization.

From primordial germ cells to spermatids in Caenorhabditis elegans

Development of a syncytial germline for gamete formation requires complex regulation of cytokinesis and cytoplasmic remodeling. Recently, several uncovered cellular events have been investigated in the Caenorhabditis elegans (C. elegans) germline. In these cellular processes, the factors involved in contractility are highly conserved with those of mitosis and meiosis. However, the underlying regulatory mechanisms are far more complicated than previously thought, likely due to the single syncytial germline structure. In this review, we highlight how the proteins involved in contractility ensure faithful cell division in different cellular contexts and how they contribute to maintaining intercellular bridge stability. In addition, we discuss the current understanding of the cellular events of cytokinesis and cytoplasmic remodeling during the development of the C. elegans germline, including progenitor germ cells, germ cells, and spermatocytes. Comparisons are made with relevant systems in Drosophila melanogaster (D. melanogaster) and other animal models.

The ESCRT machinery counteracts Nesprin-2G-mediated mechanical forces during nuclear envelope repair

Transient nuclear envelope ruptures during interphase (NERDI) occur due to cytoskeletal compressive forces at sites of weakened lamina, and delayed NERDI repair results in genomic instability. Nuclear envelope (NE) sealing is completed by endosomal sorting complex required for transport (ESCRT) machinery. A key unanswered question is how local compressive forces are counteracted to allow efficient membrane resealing. Here, we identify the ESCRT-associated protein BROX as a crucial factor required to accelerate repair of the NE. Critically, BROX binds Nesprin-2G, a component of the linker of nucleoskeleton and cytoskeleton complex (LINC). This interaction promotes Nesprin-2G ubiquitination and facilitates the relaxation of mechanical stress imposed by compressive actin fibers at the rupture site. Thus, BROX rebalances excessive cytoskeletal forces in cells experiencing NE instability to promote effective NERDI repair. Our results demonstrate that BROX coordinates mechanoregulation with membrane remodeling to ensure the maintenance of nuclear-cytoplasmic compartmentalization and genomic stability.

•

Cytoskeletal forces exerted on the nucleus can rupture its membrane

•

BROX is recruited to sites of rupture by the ESCRT membrane remodeling machinery

•

BROX ubiquitinates the LINC complex protein Nesprin-2G, targeting it for degradation

•

BROX coordinates local relaxation of mechanical stress with membrane remodeling

Lipids in Pathophysiology and Development of the Membrane Lipid Therapy: New Bioactive Lipids

Membranes are mainly composed of a lipid bilayer and proteins, constituting a checkpoint for the entry and passage of signals and other molecules. Their composition can be modulated by diet, pathophysiological processes, and nutritional/pharmaceutical interventions. In addition to their use as an energy source, lipids have important structural and functional roles, e.g., fatty acyl moieties in phospholipids have distinct impacts on human health depending on their saturation, carbon length, and isometry. These and other membrane lipids have quite specific effects on the lipid bilayer structure, which regulates the interaction with signaling proteins. Alterations to lipids have been associated with important diseases, and, consequently, normalization of these alterations or regulatory interventions that control membrane lipid composition have therapeutic potential. This approach, termed membrane lipid therapy or membrane lipid replacement, has emerged as a novel technology platform for nutraceutical interventions and drug discovery. Several clinical trials and therapeutic products have validated this technology based on the understanding of membrane structure and function. The present review analyzes the molecular basis of this innovative approach, describing how membrane lipid composition and structure affects protein-lipid interactions, cell signaling, disease, and therapy (e.g., fatigue and cardiovascular, neurodegenerative, tumor, infectious diseases).

EAP45 association with budding HIV-1: Kinetics and domain requirements

A number of viruses including HIV use the ESCRT system to bud from the infected cell. We have previously confirmed biochemically that ESCRT-II is involved in this process in HIV-1 and have defined the molecular domains that are important for this. Here, using SNAP-tag fluorescent labelling and both fixed and live cell imaging we show that the ESCRT-II component EAP45 colocalises with the HIV protein Gag at the plasma membrane in a temporal and quantitative manner, similar to that previously shown for ALIX and Gag. We show evidence that a proportion of EAP45 may be packaged within virions, and we confirm the importance of the N terminus of EAP45 and specifically the H0 domain in this process. By contrast, the Glue domain of EAP45 is more critical for recruitment during cytokinesis, emphasising that viruses have ways of recruiting cellular components that may be distinct from those used by some cellular processes. This raises the prospect of selective interference with the pathway to inhibit viral function while leaving cellular functions relatively unperturbed.

β-Galactosylceramidase in cancer: friend or foe?

Lysosomal β-galactosylceramidase (GALC) removes β-galactose from β-galactosylceramide, thus generating the oncosuppressor metabolite ceramide. Recent observations have shown that GALC may exert opposite effects on tumor growth and differentiation, questioning its contribution to the sphingolipid metabolism in cancer cells and its role in tumor progression.

Cytokinetic Abscission Regulation in Neural Stem Cells and Tissue Development

Purpose of Review

How stem cells balance proliferation with differentiation, giving rise to specific daughter cells during development to build an embryo or tissue, remains an open question. Here, we discuss recent evidence that cytokinetic abscission regulation in stem cells, particularly neural stem cells (NSCs), is part of the answer. Abscission is a multi-step process mediated by the midbody, a microtubule-based structure formed in the intercellular bridge between daughter cells after mitosis.

Recent Findings

Human mutations and mouse knockouts in abscission genes reveal that subtle disruptions of NSC abscission can cause brain malformations. Experiments in several epithelial systems have shown that midbodies serve as scaffolds for apical junction proteins and are positioned near apical membrane fate determinants. Abscission timing is tightly controlled and developmentally regulated in stem cells, with delayed abscission in early embryos and faster abscission later. Midbody remnants (MBRs) contain over 400 proteins and may influence polarity, fate, and ciliogenesis.

Summary

As NSCs and other stem cells build tissues, they tightly regulate three aspects of abscission: midbody positioning, duration, and MBR handling. Midbody positioning and remnants establish or maintain cell polarity. MBRs are deposited on the apical membranes of epithelia, can be released or internalized by surrounding cells, and may sequester fate determinants or transfer information between cells. Work in cell lines and simpler systems has shown multiple roles for abscission regulation influencing stem cell polarity, potency, and daughter fates during development. Elucidating how the abscission process influences cell fate and tissue growth is important for our continued understanding of brain development and stem cell biology.

The biosynthesis of phospholipids is linked to the cell cycle in a model eukaryote

The structural challenges faced by eukaryotic cells through the cell cycle are key for understanding cell viability and proliferation. We tested the hypothesis that the biosynthesis of structural lipids is linked to the cell cycle. If true, this would suggest that the cell's structure is important for progress through and perhaps even control of the cell cycle. Lipidomics (31P NMR and MS), proteomics (Western immunoblotting) and transcriptomics (RT-qPCR) techniques were used to profile the lipid fraction and characterise aspects of its metabolism at seven stages of the cell cycle of the model eukaryote, Desmodesmus quadricauda. We found considerable, transient increases in the abundance of phosphatidylethanolamine during the G1 phase (+35%, ethanolamine phosphate cytidylyltransferase increased 2·5×) and phosphatidylglycerol (+100%, phosphatidylglycerol synthase increased 22×) over the G1/pre-replication phase boundary. The relative abundance of phosphatidylcholine fell by ~35% during the G1. N-Methyl transferases for the conversion of phosphatidylethanolamine into phosphatidylcholine were not found in the de novo transcriptome profile, though a choline phosphate transferase was found, suggesting that the Kennedy pathway is the principal route for the synthesis of PC. The fatty acid profiles of the four most abundant lipids suggested that these lipids were not generally converted between one another. This study shows for the first time that there are considerable changes in the biosynthesis of the three most abundant phospholipid classes in the normal cell cycle of D. quadricauda, by margins large enough to elicit changes to the physical properties of membranes.

Multi-dimensional and spatiotemporal correlative imaging at the plasma membrane of live cells to determine the continuum nano-to-micro scale lipid adaptation and collective motion

The primary cilium is a specialized plasma membrane protrusion with important receptors for signalling pathways. In polarized epithelial cells, the primary cilium assembles after the midbody remnant (MBR) encounters the centrosome at the apical surface. The membrane surrounding the MBR, namely remnant-associated membrane patch (RAMP), once situated next to the centrosome, releases some of its lipid components to form a centrosome-associated membrane patch (CAMP) from which the ciliary membrane stems. The RAMP undergoes a spatiotemporal membrane refinement during the formation of the CAMP, which becomes highly enriched in condensed membranes with low lateral mobility. To better understand this process, we have developed a correlative imaging approach that yields quantitative information about the lipid lateral packing, its mobility and collective assembly at the plasma membrane at different spatial scales over time. Our work paves the way towards a quantitative understanding of the spatiotemporal lipid collective assembly at the plasma membrane as a functional determinant in cell biology and its direct correlation with the membrane physicochemical state. These findings allowed us to gain a deeper insight into the mechanisms behind the biogenesis of the ciliary membrane of polarized epithelial cells.

Cell cycle dependence on the mevalonate pathway: Role of cholesterol and non-sterol isoprenoids

The mevalonate pathway is responsible for the synthesis of isoprenoids, including sterols and other metabolites that are essential for diverse biological functions. Cholesterol, the main sterol in mammals, and non-sterol isoprenoids are in high demand by rapidly dividing cells. As evidence of its importance, many cell signaling pathways converge on the mevalonate pathway and these include those involved in proliferation, tumor-promotion, and tumor-suppression. As well as being a fundamental building block of cell membranes, cholesterol plays a key role in maintaining their lipid organization and biophysical properties, and it is crucial for the function of proteins located in the plasma membrane. Importantly, cholesterol and other mevalonate derivatives are essential for cell cycle progression, and their deficiency blocks different steps in the cycle. Furthermore, the accumulation of non-isoprenoid mevalonate derivatives can cause DNA replication stress. Identification of the mechanisms underlying the effects of cholesterol and other mevalonate derivatives on cell cycle progression may be useful in the search for new inhibitors, or the repurposing of preexisting cholesterol biosynthesis inhibitors to target cancer cell division. In this review, we discuss the dependence of cell division on an active mevalonate pathway and the role of different mevalonate derivatives in cell cycle progression.

The viral restriction factor tetherin/BST2 tethers cytokinetic midbody remnants to the cell surface

The midbody at the center of the intercellular bridge connecting dividing cells recruits the machinery essential for the final steps of cytokinesis.^1-5 Successive abscission on both sides of the midbody generates a free midbody remnant (MBR) that can be inherited and accumulated in many cancer, immortalized, and stem cells, both in culture and in vivo.^6-12 Strikingly, this organelle was recently shown to contain information that induces cancer cell proliferation, influences cell polarity, and promotes dorso-ventral axis specification upon interaction with recipient cells.^13-16 Yet the mechanisms by which the MBR is captured by either a daughter cell or a distant cell are poorly described.^10,14 Here, we report that BST2/tetherin, a well-established restriction factor that blocks the release of numerous enveloped viruses from the surface of infected cells,^17-20 plays an analogous role in retaining midbody remnants. We found that BST2 is enriched at the midbody during cytokinesis and localizes at the surface of MBRs in a variety of cells. Knocking out BST2 induces the detachment of MBRs from the cell surface, their accumulation in the extracellular medium, and their transfer to distant cells. Mechanistically, the localization of BST2 at the MBR membrane is both necessary and sufficient for the interaction between MBRs and the cell surface. We thus propose that BST2 tethers post-cytokinetic midbody remnants to the cell surface. This finding reveals new parallels between cytokinesis and viral biology^21-26 that unexpectedly extend beyond the ESCRT-dependent abscission step.

Fatty Acid Unsaturation Degree of Plasma Exosomes in Colorectal Cancer Patients: A Promising Biomarker

Even though colorectal cancer (CRC) is one of the most preventable cancers, it is currently one of the deadliest. Worryingly, incidence in people <50 years has increased unexpectedly, and for unknown causes, despite the successful implementation of screening programs in the population aged >50 years. Thus, there is a need to improve early diagnosis detection strategies by identifying more precise biomarkers. In this scenario, the analysis of exosomes is given considerable attention. Previously, we demonstrated the exosome lipidome was able to classify CRC cell lines according to their malignancy. Herein, we investigated the use of the lipidome of plasma extracellular vesicles as a potential source of non-invasive biomarkers for CRC. A plasma exosome-enriched fraction was analyzed from patients undergoing colonoscopic procedure. Patients were divided into a healthy group and four pathological groups (patients with hyperplastic polyps; adenomatous polyps; invasive neoplasia (CRC patients); or hereditary non-polyposis CRC. The results showed a shift from 34:1- to 38:4-containing species in the pathological groups. We demonstrate that the ratio Σ34:1-containing species/Σ38:4-containing species has the potential to discriminate between healthy and pathological patients. Altogether, the results reinforce the utility of plasma exosome lipid fingerprint to provide new non-invasive biomarkers in a clinical context.

Reprogramming lipid metabolism prevents effector T cell senescence and enhances tumor immunotherapy

The functional state of T cells is a key determinant for effective antitumor immunity and immunotherapy. Cellular metabolism, including lipid metabolism, controls T cell differentiation, survival, and effector functions. Here, we report that development of T cell senescence driven by both malignant tumor cells and regulatory T cells is a general feature in cancers. Senescent T cells have active glucose metabolism but exhibit unbalanced lipid metabolism. This unbalanced lipid metabolism results in changes of expression of lipid metabolic enzymes, which, in turn, alters lipid species and accumulation of lipid droplets in T cells. Tumor cells and T_reg cells drove elevated expression of group IVA phospholipase A₂, which, in turn, was responsible for the altered lipid metabolism and senescence induction observed in T cells. Mitogen-activated protein kinase signaling and signal transducer and activator of transcription signaling coordinately control lipid metabolism and group IVA phospholipase A₂ activity in responder T cells during T cell senescence. Inhibition of group IVA phospholipase A₂ reprogrammed effector T cell lipid metabolism, prevented T cell senescence in vitro, and enhanced antitumor immunity and immunotherapy efficacy in mouse models of melanoma and breast cancer in vivo. Together, these findings identify mechanistic links between T cell senescence and regulation of lipid metabolism in the tumor microenvironment and provide a new target for tumor immunotherapy.

Phospholipid ebb and flow makes mitochondria go

Mitochondria, so much more than just being energy factories, also have the capacity to synthesize macromolecules including phospholipids, particularly cardiolipin (CL) and phosphatidylethanolamine (PE). Phospholipids are vital constituents of mitochondrial membranes, impacting the plethora of functions performed by this organelle. Hence, the orchestrated movement of phospholipids to and from the mitochondrion is essential for cellular integrity. In this review, we capture recent advances in the field of mitochondrial phospholipid biosynthesis and trafficking, highlighting the significance of interorganellar communication, intramitochondrial contact sites, and lipid transfer proteins in maintaining membrane homeostasis. We then discuss the physiological functions of CL and PE, specifically how they associate with protein complexes in mitochondrial membranes to support bioenergetics and maintain mitochondrial architecture.

A Model for Primary Cilium Biogenesis by Polarized Epithelial Cells: Role of the Midbody Remnant and Associated Specialized Membranes

Primary cilia are solitary, microtubule-based protrusions surrounded by a ciliary membrane equipped with selected receptors that orchestrate important signaling pathways that control cell growth, differentiation, development and homeostasis. Depending on the cell type, primary cilium assembly takes place intracellularly or at the cell surface. The intracellular route has been the focus of research on primary cilium biogenesis, whereas the route that occurs at the cell surface, which we call the "alternative" route, has been much less thoroughly characterized. In this review, based on recent experimental evidence, we present a model of primary ciliogenesis by the alternative route in which the remnant of the midbody generated upon cytokinesis acquires compact membranes, that are involved in compartmentalization of biological membranes. The midbody remnant delivers part of those membranes to the centrosome in order to assemble the ciliary membrane, thereby licensing primary cilium formation. The midbody remnant's involvement in primary cilium formation, the regulation of its inheritance by the ESCRT machinery, and the assembly of the ciliary membrane from the membranes originally associated with the remnant are discussed in the context of the literature concerning the ciliary membrane, the emerging roles of the midbody remnant, the regulation of cytokinesis, and the role of membrane compartmentalization. We also present a model of cilium emergence during evolution, and summarize the directions for future research.

Effects of lipid heterogeneity on model human brain lipid membranes

Cell membranes naturally contain a heterogeneous lipid distribution. However, homogeneous bilayers are commonly preferred and utilised in computer simulations due to their relative simplicity, and the availability of lipid force field parameters. Recently, experimental lipidomics data for the human brain cell membranes under healthy and Alzheimer's disease (AD) conditions were investigated, since disruption to the lipid composition has been implicated in neurodegenerative disorders, including AD [R. B. Chan et al., J. Biol. Chem., 2012, 287, 2678-2688]. In order to observe the effects of lipid complexity on the various bilayer properties, molecular dynamics simulations were used to study four membranes with increasing heterogeneity: a pure POPC membrane, a POPC and cholesterol membrane in a 1 : 1 ratio (POPC-CHOL), and to our knowledge, the first realistic models of a healthy brain membrane and an Alzheimer's diseased brain membrane. Numerous structural, interfacial, and dynamical properties, including the area per lipid, interdigitation, dipole potential, and lateral diffusion of the two simple models, POPC and POPC-CHOL, were analysed and compared to those of the complex brain models consisting of 27 lipid components. As the membranes gain heterogeneity, a number of alterations were found in the structural and dynamical properties, and more significant differences were observed in the lateral diffusion. Additionally, we observed snorkeling behaviour of the lipid tails that may play a role in the permeation of small molecules across biological membranes. In this work, atomistic description of realistic brain membrane models is provided, which can add insight towards the permeability and transport pathways of small molecules across these membrane barriers.

High-dimensional super-resolution imaging reveals heterogeneity and dynamics of subcellular lipid membranes

Lipid membranes are found in most intracellular organelles, and their heterogeneities play an essential role in regulating the organelles' biochemical functionalities. Here we report a Spectrum and Polarization Optical Tomography (SPOT) technique to study the subcellular lipidomics in live cells. Simply using one dye that universally stains the lipid membranes, SPOT can simultaneously resolve the membrane morphology, polarity, and phase from the three optical-dimensions of intensity, spectrum, and polarization, respectively. These high-throughput optical properties reveal lipid heterogeneities of ten subcellular compartments, at different developmental stages, and even within the same organelle. Furthermore, we obtain real-time monitoring of the multi-organelle interactive activities of cell division and successfully reveal their sophisticated lipid dynamics during the plasma membrane separation, tunneling nanotubules formation, and mitochondrial cristae dissociation. This work suggests research frontiers in correlating single-cell super-resolution lipidomics with multiplexed imaging of organelle interactome.

Kinase activity-independent role of EphA2 in the regulation of M-phase progression

Cell division is a tightly regulated, essential process for cell proliferation. Very recently, we reported that EphA2 is phosphorylated at Ser897, via the Cdk1/MEK/ERK/RSK pathway, during M phase and contributes to proper M-phase progression by maintaining cortical rigidity via the EphA2pSer897/ephexin4/RhoG pathway. Here, we show that EphA2 kinase activity is dispensable for M-phase progression. Although EphA2 knockdown delayed this progression, the delay was rescued by an EphA2 mutant expression with an Asp739 to Asn substitution, as well as by wild-type EphA2. Western blotting analysis confirmed that the Asp739Asn mutant lost its EphA2 kinase activity. Like wild-type EphA2, the Asp739Asn mutant was localized to the plasma membrane irrespective of cell cycle. While RhoG localization to the plasma membrane was decreased in EphA2 knockdown cells, it was rescued by re-expression of wild-type EphA2 but not via the mutant containing the Ser897 to Ala substitution. This confirmed our recent report that phosphorylation at Ser897 is responsible for RhoG localization to the plasma membrane. In agreement with the M-phase progression's rescue effect, the Asp739Asn mutant rescued RhoG localization in EphA2 knockdown cells. These results suggest that EphA2 regulates M-phase progression in a manner independent of its kinase activity.

Animal Cell Cytokinesis: The Rho-Dependent Actomyosin-Anilloseptin Contractile Ring as a Membrane Microdomain Gathering, Compressing, and Sorting Machine

Cytokinesis is the last step of cell division that partitions the cellular organelles and cytoplasm of one cell into two. In animal cells, cytokinesis requires Rho-GTPase-dependent assembly of F-actin and myosin II (actomyosin) to form an equatorial contractile ring (CR) that bisects the cell. Despite 50 years of research, the precise mechanisms of CR assembly, tension generation and closure remain elusive. This hypothesis article considers a holistic view of the CR that, in addition to actomyosin, includes another Rho-dependent cytoskeletal sub-network containing the scaffold protein, Anillin, and septin filaments (collectively termed anillo-septin). We synthesize evidence from our prior work in Drosophila S2 cells that actomyosin and anillo-septin form separable networks that are independently anchored to the plasma membrane. This latter realization leads to a simple conceptual model in which CR assembly and closure depend upon the micro-management of the membrane microdomains to which actomyosin and anillo-septin sub-networks are attached. During CR assembly, actomyosin contractility gathers and compresses its underlying membrane microdomain attachment sites. These microdomains resist this compression, which builds tension. During CR closure, membrane microdomains are transferred from the actomyosin sub-network to the anillo-septin sub-network, with which they flow out of the CR as it advances. This relative outflow of membrane microdomains regulates tension, reduces the circumference of the CR and promotes actomyosin disassembly all at the same time. According to this hypothesis, the metazoan CR can be viewed as a membrane microdomain gathering, compressing and sorting machine that intrinsically buffers its own tension through coordination of actomyosin contractility and anillo-septin-membrane relative outflow, all controlled by Rho. Central to this model is the abandonment of the dogmatic view that the plasma membrane is always readily deformable by the underlying cytoskeleton. Rather, the membrane resists compression to build tension. The notion that the CR might generate tension through resistance to compression of its own membrane microdomain attachment sites, can account for numerous otherwise puzzling observations and warrants further investigation using multiple systems and methods.

Human epidermal stem cell differentiation is modulated by specific lipid subspecies

While the lipids of the outer layers of mammalian epidermis and their contribution to barrier formation have been extensively described, the role of individual lipid species in the onset of keratinocyte differentiation remains unknown. A lipidomic analysis of primary human keratinocytes revealed accumulation of numerous lipid species during suspension-induced differentiation. A small interfering RNA screen of 258 lipid-modifying enzymes identified two genes that on knockdown induced epidermal differentiation: ELOVL1, encoding elongation of very long-chain fatty acids protein 1, and SLC27A1, encoding fatty acid transport protein 1. By intersecting lipidomic datasets from suspension-induced differentiation and knockdown keratinocytes, we pinpointed candidate bioactive lipid subspecies as differentiation regulators. Several of these-ceramides and glucosylceramides-induced differentiation when added to primary keratinocytes in culture. Our results reveal the potential of lipid subspecies to regulate exit from the epidermal stem cell compartment.

Applications of atomic force microscopy in immunology

Cellular mechanics, a major regulating factor of cellular architecture and biological functions, responds to intrinsic stresses and extrinsic forces exerted by other cells and the extracellular matrix in the microenvironment. Cellular mechanics also acts as a fundamental mediator in complicated immune responses, such as cell migration, immune cell activation, and pathogen clearance. The principle of atomic force microscopy (AFM) and its three running modes are introduced for the mechanical characterization of living cells. The peak force tapping mode provides the most delicate and desirable virtues to collect high-resolution images of morphology and force curves. For a concrete description of AFM capabilities, three AFM applications are discussed. These applications include the dynamic progress of a neutrophil-extracellular-trap release by neutrophils, the immunological functions of macrophages, and the membrane pore formation mediated by perforin, streptolysin O, gasdermin D, or membrane attack complex.

Understanding the Link between Lipid Diversity and the Biophysical Properties of the Neuronal Plasma Membrane

Cell membranes contain incredible diversity in the chemical structures of their individual lipid species and the ratios in which these lipids are combined to make membranes. Nevertheless, our current understanding of how each of these components affects the properties of the cell membrane remains elusive, in part due to the difficulties in studying the dynamics of membranes at high spatiotemporal resolution. In this work, we use coarse-grained molecular dynamics simulations to investigate how individual lipid species contribute to the biophysical properties of the neuronal plasma membrane. We progress through eight membranes of increasing chemical complexity, ranging from a simple POPC/CHOL membrane to a previously published neuronal plasma membrane [Ingólfsson, H. I., et al. (2017) Biophys. J. 113 (10), 2271-2280] containing 49 distinct lipid species. Our results show how subtle chemical changes can affect the properties of the membrane and highlight the lipid species that give the neuronal plasma membrane its unique biophysical properties. This work has potential far-reaching implications for furthering our understanding of cell membranes.

Mid-Gestation lethality of Atxn2l-Ablated Mice

Depletion of yeast/fly Ataxin-2 rescues TDP-43 overexpression toxicity. In mouse models of Amyotrophic Lateral Sclerosis via TDP-43 overexpression, depletion of its ortholog ATXN2 mitigated motor neuron degeneration and extended lifespan from 25 days to >300 days. There is another ortholog in mammals, named ATXN2L (Ataxin-2-like), which is almost uncharacterized but also functions in RNA surveillance at stress granules. We generated mice with Crispr/Cas9-mediated deletion of Atxn2l exons 5-8, studying homozygotes prenatally and heterozygotes during aging. Our novel findings indicate that ATXN2L absence triggers mid-gestational embryonic lethality, affecting female animals more strongly. Weight and development stages of homozygous mutants were reduced. Placenta phenotypes were not apparent, but brain histology showed lamination defects and apoptosis. Aged heterozygotes showed no locomotor deficits or weight loss over 12 months. Null mutants in vivo displayed compensatory efforts to maximize Atxn2l expression, which were prevented upon nutrient abundance in vitro. Mouse embryonal fibroblast cells revealed more multinucleated giant cells upon ATXN2L deficiency. In addition, in human neural cells, transcript levels of ATXN2L were induced upon starvation and glucose and amino acids exposure, but this induction was partially prevented by serum or low cholesterol administration. Neither ATXN2L depletion triggered dysregulation of ATXN2, nor a converse effect was observed. Overall, this essential role of ATXN2L for embryogenesis raises questions about its role in neurodegenerative diseases and neuroprotective therapies.

Cell cycle regulation in NAFLD: when imbalanced metabolism limits cell division

Cell division is essential for organismal growth and tissue homeostasis. It is exceptionally significant in tissues chronically exposed to intrinsic and external damage, like the liver. After decades of studying the regulation of cell cycle by extracellular signals, there are still gaps in our knowledge on how these two interact with metabolic pathways in vivo. Studying the cross-talk of these pathways has direct clinical implications as defects in cell division, signaling pathways, and metabolic homeostasis are frequently observed in liver diseases. In this review, we will focus on recent reports which describe various functions of cell cycle regulators in hepatic homeostasis. We will describe the interplay between the cell cycle and metabolism during liver regeneration after acute and chronic damage. We will focus our attention on non-alcoholic fatty liver disease, especially non-alcoholic steatohepatitis. The global incidence of non-alcoholic fatty liver disease is increasing exponentially. Therefore, understanding the interplay between cell cycle regulators and metabolism may lead to the discovery of novel therapeutic targets amenable to intervention.

Frustrated clathrin-mediated endocytosis – causes and possible functions

Clathrin-mediated endocytosis is the main entry route for most cell surface receptors and their ligands. It is regulated by clathrin-coated structures that are endowed with the ability to cluster receptors and to locally bend the plasma membrane, resulting in the formation of receptor-containing vesicles that bud into the cytoplasm. This canonical role of clathrin-coated structures has been shown to play a fundamental part in many different aspects of cell physiology. However, it has recently become clear that the ability of clathrin-coated structures to deform membranes can be perturbed. In addition to chemical or genetic alterations, numerous environmental conditions can physically prevent or slow down membrane bending and/or budding at clathrin-coated structures. The resulting ‘frustrated endocytosis’ is emerging as not merely a passive consequence, but one that actually fulfils some very specific and important cellular functions. In this Review, we provide an historical and defining perspective on frustrated endocytosis in the clathrin pathway of mammalian cells, before discussing its causes and highlighting the possible functional consequences in physiology and diseases.

Common and Differential Traits of the Membrane Lipidome of Colon Cancer Cell Lines and their Secreted Vesicles: Impact on Studies Using Cell Lines

Colorectal cancer (CRC) is the fourth leading cause of cancer death in the world. Despite the screening programs, its incidence in the population below the 50s is increasing. Therefore, new stratification protocols based on multiparametric approaches are highly needed. In this scenario, the lipidome is emerging as a powerful tool to classify tumors, including CRC, wherein it has proven to be highly sensitive to cell malignization. Hence, the possibility to describe the lipidome at the level of lipid species has renewed the interest to investigate the role of specific lipid species in pathologic mechanisms, being commercial cell lines, a model still heavily used for this purpose. Herein, we characterize the membrane lipidome of five commercial colon cell lines and their extracellular vesicles (EVs). The results demonstrate that both cell and EVs lipidome was able to segregate cells according to their malignancy. Furthermore, all CRC lines shared a specific and strikingly homogenous impact on ether lipid species. Finally, this study also cautions about the need of being aware of the singularities of each cell line at the level of lipid species. Altogether, this study firmly lays the groundwork of using the lipidome as a solid source of tumor biomarkers.

An acute exposure to perfluorooctanoic acid causes non-reversible plasma membrane injury in HeLa cells

Health and environmental risks regarding perfluorooctanoic acid, a well-known perfluorinated compound, are still a subject of great concern. Ubiquitous exposure and disparity of results make it difficult to determine the underlying mechanism of action, especially at the cellular level. This study proposes an experimental design to assess the reversibility of adverse effects after a one-time exposure to the compound, in comparison with other more conventional timings. Complementary endpoints including total protein content, neutral red uptake and MTT reduction tests along with division rates and microscopic observations were evaluated in HeLa cells. In addition, PFOA quantification inside the cells was performed. The cellular effects exerted after 24 h exposure to perfluorooctanoic acid are non-reversible after a 48 h recovery period. In addition, we describe for the first time the induction of plasma membrane blebbing and the activation of membrane repair mechanisms after recovery from non-cytotoxic treatments with the compound. This experimental design has provided relevant information regarding the toxicity of this perfluorinated compound, relating all the adverse effects detected to its interaction with the plasma membrane.

Cep55 promotes cytokinesis of neural progenitors but is dispensable for most mammalian cell divisions

In mammalian cell lines, the endosomal sorting complex required for transport (ESCRT)-III mediates abscission, the process that physically separates daughter cells and completes cell division. Cep55 protein is regarded as the master regulator of abscission, because it recruits ESCRT-III to the midbody (MB), the site of abscission. However, the importance of this mechanism in a mammalian organism has never been tested. Here we show that Cep55 is dispensable for mouse embryonic development and adult tissue homeostasis. Cep55-knockout offspring show microcephaly and primary neural progenitors require Cep55 and ESCRT for survival and abscission. However, Cep55 is dispensable for cell division in embryonic or adult tissues. In vitro, division of primary fibroblasts occurs without Cep55 and ESCRT-III at the midbody and is not affected by ESCRT depletion. Our work defines Cep55 as an abscission regulator only in specific tissue contexts and necessitates the re-evaluation of an alternative ESCRT-independent cell division mechanism.

Live-cell lipid biochemistry reveals a role of diacylglycerol side-chain composition for cellular lipid dynamics and protein affinities

Every cell produces thousands of lipid species, but studying the function of individual lipids in living cells is almost impossible with existing methodologies. Addressing this experimental bottleneck, we developed a strategy to quantify dissociation constants for lipid–protein interactions and transmembrane flip-flop rates of native lipids in live-cell experiments. Using a combination of plasma membrane-specific photochemical probes and mathematical modeling, we demonstrate that, for diacylglycerols as a model lipid class, the inherent lipid structural diversity caused by variations in acyl chain composition determines lipid protein affinities and transbilayer kinetics. In fact, subtle chemical differences change these values by orders of magnitude. Our approach represents a generally applicable method for elucidating the biological function of single lipid species on subcellular scales.

Every cell produces thousands of distinct lipid species, but insight into how lipid chemical diversity contributes to biological signaling is lacking, particularly because of a scarcity of methods for quantitatively studying lipid function in living cells. Using the example of diacylglycerols, prominent second messengers, we here investigate whether lipid chemical diversity can provide a basis for cellular signal specification. We generated photo-caged lipid probes, which allow acute manipulation of distinct diacylglycerol species in the plasma membrane. Combining uncaging experiments with mathematical modeling, we were able to determine binding constants for diacylglycerol–protein interactions, and kinetic parameters for diacylglycerol transbilayer movement and turnover in quantitative live-cell experiments. Strikingly, we find that affinities and kinetics vary by orders of magnitude due to diacylglycerol side-chain composition. These differences are sufficient to explain differential recruitment of diacylglycerol binding proteins and, thus, differing downstream phosphorylation patterns. Our approach represents a generally applicable method for elucidating the biological function of single lipid species on subcellular scales in quantitative live-cell experiments.

Abundances of transcripts, proteins, and metabolites in the cell cycle of budding yeast reveal coordinate control of lipid metabolism

Establishing the pattern of abundance of molecules of interest during cell division has been a long-standing goal of cell cycle studies. Here, for the first time in any system, we present experiment-matched datasets of the levels of RNAs, proteins, metabolites, and lipids from unarrested, growing, and synchronously dividing yeast cells. Overall, transcript and protein levels were correlated, but specific processes that appeared to change at the RNA level (e.g., ribosome biogenesis) did not do so at the protein level, and vice versa. We also found no significant changes in codon usage or the ribosome content during the cell cycle. We describe an unexpected mitotic peak in the abundance of ergosterol and thiamine biosynthesis enzymes. Although the levels of several metabolites changed in the cell cycle, by far the most significant changes were in the lipid repertoire, with phospholipids and triglycerides peaking strongly late in the cell cycle. Our findings provide an integrated view of the abundance of biomolecules in the eukaryotic cell cycle and point to a coordinate mitotic control of lipid metabolism.

Lipid droplet motility and organelle contacts

Lipid droplets (LDs) are fat storage organelles integral to energy homeostasis and a wide range of cellular processes. LDs physically and functionally interact with many partner organelles, including the ER, mitochondria, lysosomes, and peroxisomes. Recent findings suggest that the dynamics of LD inter-organelle contacts is in part controlled by LD intracellular motility. LDs can be transported directly by motor proteins along either actin filaments or microtubules, via Kinesin-1, Cytoplasmic Dynein, and type V Myosins. LDs can also be propelled indirectly, by hitchhiking on other organelles, cytoplasmic flows, and potentially actin polymerization. Although the anchors that attach motors to LDs remain elusive, other regulators of LD motility have been identified, ranging from modification of the tracks to motor co-factors to members of the perilipin family of LD proteins. Manipulating these regulatory pathways provides a tool to probe whether altered motility affects organelle contacts and has revealed that LD motility can promote interactions with numerous partners, with profound consequences for metabolism. LD motility can cause dramatic redistribution of LDs between a clustered and a dispersed state, resulting in altered organelle contacts and LD turnover. We propose that LD motility can thus promote switches in the metabolic state of a cell. Finally, LD motility is also important for LD allocation during cell division. In a number of animal embryos, uneven allocation results in a large difference in LD content in distinct daughter cells, suggesting cell-type specific LD needs.

Emerging roles for the nucleus during neutrophil signal relay and NETosis

The nucleus houses and protects genomic DNA, which is surrounded by the nuclear envelope. Owing to its size and stiffness, the nucleus is often a barrier to migration through confined spaces. Neutrophils are terminally differentiated, short-lived cells that migrate through tissues in response to injury and infections. The neutrophil nucleus is soft, multilobular, and exhibits altered levels of key nuclear envelope proteins. These alterations result in a multifunctional organelle that serves as a signaling hub during migration and NETosis, a process by which neutrophils release decondensed chromatin decorated with granular enzymes that entrap pathogens. In this review, we present emerging evidence suggesting that a unique, ambiguous cell-cycle state is critical for NETosis and migration. Finally, we discuss how the mechanisms underlying migration and NETosis are evolutionarily conserved.

A Coumarin Triflate Reagent Enables One-Step Synthesis of Photo-Caged Lipid Metabolites for Studying Cell Signaling

Photorelease of caged compounds is among the most powerful experimental approaches for studying cellular functions on fast timescales. However, its full potential has yet to be exploited, as the number of caged small molecules available for cell biological studies has been limited by synthetic challenges. Addressing this problem, a straightforward, one-step procedure for efficiently synthesizing caged compounds was developed. An in situ generated benzylic coumarin triflate reagent was used to specifically functionalize carboxylate and phosphate moieties in the presence of free hydroxy groups, generating various caged lipid metabolites, including a number of GPCR ligands. By combining the photo-caged ligands with the respective receptors, an easily implementable experimental platform for the optical control and analysis of GPCR-mediated signal transduction in living cells was developed. Ultimately, the described synthetic strategy allows rapid generation of photo-caged small molecules and thus greatly facilitates the analysis of their biological roles in live cell microscopy assays.

Who Needs a Contractile Actomyosin Ring? The Plethora of Alternative Ways to Divide a Protozoan Parasite

Cytokinesis, or the division of the cytoplasm, following the end of mitosis or meiosis, is accomplished in animal cells, fungi, and amoebae, by the constriction of an actomyosin contractile ring, comprising filamentous actin, myosin II, and associated proteins. However, despite this being the best-studied mode of cytokinesis, it is restricted to the Opisthokonta and Amoebozoa, since members of other evolutionary supergroups lack myosin II and must, therefore, employ different mechanisms. In particular, parasitic protozoa, many of which cause significant morbidity and mortality in humans and animals as well as considerable economic losses, employ a wide diversity of mechanisms to divide, few, if any, of which involve myosin II. In some cases, cell division is not only myosin II-independent, but actin-independent too. Mechanisms employed range from primitive mechanical cell rupture (cytofission), to motility- and/or microtubule remodeling-dependent mechanisms, to budding involving the constriction of divergent contractile rings, to hijacking host cell division machinery, with some species able to utilize multiple mechanisms. Here, I review current knowledge of cytokinesis mechanisms and their molecular control in mammalian-infective parasitic protozoa from the Excavata, Alveolata, and Amoebozoa supergroups, highlighting their often-underappreciated diversity and complexity. Billions of people and animals across the world are at risk from these pathogens, for which vaccines and/or optimal treatments are often not available. Exploiting the divergent cell division machinery in these parasites may provide new avenues for the treatment of protozoal disease.

Sphingolipid Modulation Activates Proteostasis Programs to Govern Human Hematopoietic Stem Cell Self-Renewal

Cellular stress responses serve as crucial decision points balancing persistence or culling of hematopoietic stem cells (HSCs) for lifelong blood production. Although strong stressors cull HSCs, the linkage between stress programs and self-renewal properties that underlie human HSC maintenance remains unknown, particularly at quiescence exit when HSCs must also dynamically shift metabolic state. Here, we demonstrate distinct wiring of the sphingolipidome across the human hematopoietic hierarchy and find that genetic or pharmacologic modulation of the sphingolipid enzyme DEGS1 regulates lineage differentiation. Inhibition of DEGS1 in hematopoietic stem and progenitor cells during the transition from quiescence to cellular activation with N-(4-hydroxyphenyl) retinamide activates coordinated stress pathways that coalesce on endoplasmic reticulum stress and autophagy programs to maintain immunophenotypic and functional HSCs. Thus, our work identifies a linkage between sphingolipid metabolism, proteostatic quality control systems, and HSC self-renewal and provides therapeutic targets for improving HSC-based cellular therapeutics.

Extrachromosomal Histone H2B Contributes to the Formation of the Abscission Site for Cell Division

Histones are constitutive components of nucleosomes and key regulators of chromatin structure. We previously observed that an extrachromosomal histone H2B (ecH2B) localizes at the intercellular bridge (ICB) connecting the two daughter cells during cytokinesis independently of DNA and RNA. Here, we show that ecH2B binds and colocalizes with CHMP4B, a key component of the ESCRT-III machinery responsible for abscission, the final step of cell division. Abscission requires the formation of an abscission site at the ICB where the ESCRT-III complex organizes into narrowing cortical helices that drive the physical separation of sibling cells. ecH2B depletion does not prevent membrane cleavage rather results in abscission delay and accumulation of abnormally long and thin ICBs. In the absence of ecH2B, CHMP4B and other components of the fission machinery, such as IST1 and Spastin, are recruited to the ICB and localize at the midbody. However, in the late stage of abscission, these fission factors fail to re-localize at the periphery of the midbody and the abscission site fails to form. These results show that extrachromosomal activity of histone H2B is required in the formation of the abscission site and the proper localization of the fission machinery.

The postmitotic midbody: Regulating polarity, stemness, and proliferation

Peterman and Prekeris review abscission and discuss the diverse roles for the postmitotic midbody in regulating polarity, tumorigenesis, and stemness.

Abscission, the final stage of cell division, requires well-orchestrated changes in endocytic trafficking, microtubule severing, actin clearance, and the physical sealing of the daughter cell membranes. These processes are highly regulated, and any missteps in localized membrane and cytoskeleton dynamics often lead to a delay or a failure in cell division. The midbody, a microtubule-rich structure that forms during cytokinesis, is a key regulator of abscission and appears to function as a signaling platform coordinating cytoskeleton and endosomal dynamics during the terminal stages of cell division. It was long thought that immediately following abscission and the conclusion of cell division, the midbody is either released or rapidly degraded by one of the daughter cells. Recently, the midbody has gained prominence for exerting postmitotic functions. In this review, we detail the role of the midbody in orchestrating abscission, as well as discuss the relatively new field of postabscission midbody biology, particularly focusing on how it may act to regulate cell polarity and its potential to regulate cell tumorigenicity or stemness.

Loss of SMPD4 Causes a Developmental Disorder Characterized by Microcephaly and Congenital Arthrogryposis

Sphingomyelinases generate ceramide from sphingomyelin as a second messenger in intracellular signaling pathways involved in cell proliferation, differentiation, or apoptosis. Children from 12 unrelated families presented with microcephaly, simplified gyral pattern of the cortex, hypomyelination, cerebellar hypoplasia, congenital arthrogryposis, and early fetal/postnatal demise. Genomic analysis revealed bi-allelic loss-of-function variants in SMPD4, coding for the neutral sphingomyelinase-3 (nSMase-3/SMPD4). Overexpression of human Myc-tagged SMPD4 showed localization both to the outer nuclear envelope and the ER and additionally revealed interactions with several nuclear pore complex proteins by proteomics analysis. Fibroblasts from affected individuals showed ER cisternae abnormalities, suspected for increased autophagy, and were more susceptible to apoptosis under stress conditions, while treatment with siSMPD4 caused delayed cell cycle progression. Our data show that SMPD4 links homeostasis of membrane sphingolipids to cell fate by regulating the cross-talk between the ER and the outer nuclear envelope, while its loss reveals a pathogenic mechanism in microcephaly.

Membrane Dynamics in Health and Disease: Impact on Cellular Signalling

Biological membranes display a staggering complexity of lipids and proteins orchestrating cellular functions. Superior analytical tools coupled with numerous functional cellular screens have enabled us to query their role in cellular signalling, trafficking, guiding protein structure and function-all of which rely on the dynamic membrane lipid properties indispensable for proper cellular functions. Alteration of these has led to emergence of various pathological conditions, thus opening an area of lipid-centric therapeutic approaches. This perspective is a short summary of the dynamic properties of membranes essential for proper cellular functions, dictating both protein and lipid functions, and mis-regulated in diseases. Towards the end, we focus on some challenges lying ahead and potential means to tackle the same, mainly underscored by multi-disciplinary approaches.

Ins and Outs of Interpreting Lipidomic Results

Membrane lipids are essential for life; however, research on how cells regulate cell lipid composition has been falling behind for quite some time. One reason was the difficulty in establishing analytical methods able to cope with the cell lipid repertoire. Development of a diversity of mass spectrometry-based technologies, including imaging mass spectrometry, has helped to demonstrate beyond doubt that the cell lipidome is not only greatly cell type dependent but also highly sensitive to any pathophysiological alteration such as differentiation or tumorigenesis. Interestingly, the current popularization of metabolomic studies among numerous disciplines has led many researchers to rediscover lipids. Hence, it is important to underscore the peculiarities of these metabolites and their metabolism, which are both radically different from protein and nucleic acid metabolism. Once differences in lipid composition have been established, researchers face a rather complex scenario, to investigate the signaling pathways and molecular mechanisms accounting for their results. Thus, a detail often overlooked, but of crucial relevance, is the complex networks of enzymes involved in controlling the level of each one of the lipid species present in the cell. In most cases, these enzymes are redundant and promiscuous, complicating any study on lipid metabolism, since the modification of one particular lipid enzyme impacts simultaneously on many species. Altogether, this review aims to describe the difficulties in delving into the regulatory mechanisms tailoring the lipidome at the activity, genetic, and epigenetic level, while conveying the numerous, stimulating, and sometimes unexpected research opportunities afforded by this type of studies.

Self-Assembly and Biogenesis of the Cellular Membrane are Dictated by Membrane Stretch and Composition

The cell plasma membrane is a highly dynamic organelle governing a wide range of cellular activities including ion transport, secretion, cell division, growth, and development. The fundamental process involved in the addition of new membranes to pre-existing plasma membranes, however, is unclear. Here, we report, using biophysical, morphological, biochemical, and molecular dynamic simulations, the selective incorporation of proteins and lipids from the cytosol into the cell plasma membrane dictated by membrane stretch and composition. Stretching of the cell membrane as a consequence of volume increase following incubation in a hypotonic solution and results in the incorporation of cytosolic proteins and lipids into the existing plasma membrane. Molecular dynamic simulations further confirm that increased membrane stretch results in the rapid insertion of lipids into the existing plasma membrane. Similarly, depletion of cholesterol from the cell plasma membrane selectively alters the incorporation of lipids into the membrane.

The lipid-dependent structure and function of LacY can be recapitulated and analyzed in phospholipid-containing detergent micelles

Membrane proteins play key roles in cellular functions, their activity mainly depending on their topological arrangement in membranes. Structural studies of membrane proteins have long adopted a protein-centric view regarding the determinants of membrane protein topology and function. Several studies have shown that the orientation of transmembrane domains of polytopic membrane proteins with respect to the plane of the lipid bilayer can be largely determined by membrane lipid composition. However, the mechanism by which membrane proteins exhibit structural and functional duality in the same membrane or different membranes is still unknown. Here we show that lipid-dependent structural and functional assessment of a membrane protein can be conducted in detergent micelles, opening the possibility for the determination of lipid-dependent high-resolution crystal structures. We found that the lactose permease purified from Escherichia coli cells exhibiting varied phospholipid compositions exhibits the same topology and similar function as in its membrane of origin. Furthermore, we found several conditions, including protein mutations and micelle lipid composition, that lead to increased protein stability, correlating with a higher yield of two-dimensional crystal formation. Altogether, our results demonstrate how the membrane lipid environment influences membrane protein topology and arrangement, both in native membranes and in mixed detergent micelles.