Membrane phospholipid asymmetry counters the adverse effects of sterol overloading in the Golgi membrane of Drosophila

Axonal Endoplasmic Reticulum Dynamics and Its Roles in Neurodegeneration

The physical continuity of axons over long cellular distances poses challenges for their maintenance. One organelle that faces this challenge is endoplasmic reticulum (ER); unlike other intracellular organelles, this forms a physically continuous network throughout the cell, with a single membrane and a single lumen. In axons, ER is mainly smooth, forming a tubular network with occasional sheets or cisternae and low amounts of rough ER. It has many potential roles: lipid biosynthesis, glucose homeostasis, a Ca2+ store, protein export, and contacting and regulating other organelles. This tubular network structure is determined by ER-shaping proteins, mutations in some of which are causative for neurodegenerative disorders such as hereditary spastic paraplegia (HSP). While axonal ER shares many features with the tubular ER network in other contexts, these features must be adapted to the long and narrow dimensions of axons. ER appears to be physically continuous throughout axons, over distances that are enormous on a subcellular scale. It is therefore a potential channel for long-distance or regional communication within neurons, independent of action potentials or physical transport of cargos, but involving its physiological roles such as Ca2+ or organelle homeostasis. Despite its apparent stability, axonal ER is highly dynamic, showing features like anterograde and retrograde transport, potentially reflecting continuous fusion and breakage of the network. Here we discuss the transport processes that must contribute to this dynamic behavior of ER. We also discuss the model that these processes underpin a homeostatic process that ensures both enough ER to maintain continuity of the network and repair breaks in it, but not too much ER that might disrupt local cellular physiology. Finally, we discuss how failure of ER organization in axons could lead to axon degenerative diseases, and how a requirement for ER continuity could make distal axons most susceptible to degeneration in conditions that disrupt ER continuity.

Multiscale Simulations of Biological Membranes: The Challenge To Understand Biological Phenomena in a Living Substance

Biological membranes are tricky to investigate. They are complex in terms of molecular composition and structure, functional over a wide range of time scales, and characterized by nonequilibrium conditions. Because of all of these features, simulations are a great technique to study biomembrane behavior. A significant part of the functional processes in biological membranes takes place at the molecular level; thus computer simulations are the method of choice to explore how their properties emerge from specific molecular features and how the interplay among the numerous molecules gives rise to function over spatial and time scales larger than the molecular ones. In this review, we focus on this broad theme. We discuss the current state-of-the-art of biomembrane simulations that, until now, have largely focused on a rather narrow picture of the complexity of the membranes. Given this, we also discuss the challenges that we should unravel in the foreseeable future. Numerous features such as the actin-cytoskeleton network, the glycocalyx network, and nonequilibrium transport under ATP-driven conditions have so far received very little attention; however, the potential of simulations to solve them would be exceptionally high. A major milestone for this research would be that one day we could say that computer simulations genuinely research biological membranes, not just lipid bilayers.

A Neuronal Relay Mediates a Nutrient Responsive Gut/Fat Body Axis Regulating Energy Homeostasis in Adult Drosophila

The control of systemic metabolic homeostasis involves complex inter-tissue programs that coordinate energy production, storage, and consumption, to maintain organismal fitness upon environmental challenges. The mechanisms driving such programs are largely unknown. Here, we show that enteroendocrine cells in the adult Drosophila intestine respond to nutrients by secreting the hormone Bursicon α, which signals via its neuronal receptor DLgr2. Bursicon α/DLgr2 regulate energy metabolism through a neuronal relay leading to the restriction of glucagon-like, adipokinetic hormone (AKH) production by the corpora cardiaca and subsequent modulation of AKH receptor signaling within the adipose tissue. Impaired Bursicon α/DLgr2 signaling leads to exacerbated glucose oxidation and depletion of energy stores with consequent reduced organismal resistance to nutrient restrictive conditions. Altogether, our work reveals an intestinal/neuronal/adipose tissue inter-organ communication network that is essential to restrict the use of energy and that may provide insights into the physiopathology of endocrine-regulated metabolic homeostasis.

Bridging the molecular and biological functions of the oxysterol-binding protein family

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute a large eukaryotic gene family that transports and regulates the metabolism of sterols and phospholipids. The original classification of the family based on oxysterol-binding activity belies the complex dual lipid-binding specificity of the conserved OSBP homology domain (OHD). Additional protein- and membrane-interacting modules mediate the targeting of select OSBP/ORPs to membrane contact sites between organelles, thus positioning the OHD between opposing membranes for lipid transfer and metabolic regulation. This unique subcellular location, coupled with diverse ligand preferences and tissue distribution, has identified OSBP/ORPs as key arbiters of membrane composition and function. Here, we will review how molecular models of OSBP/ORP-mediated intracellular lipid transport and regulation at membrane contact sites relate to their emerging roles in cellular and organismal functions.

A P4-ATPase subunit of the Cdc50 family plays a role in iron acquisition and virulence in Cryptococcus neoformans

The pathogenic fungus Cryptococcus neoformans delivers virulence factors such as capsule polysaccharide to the cell surface to cause disease in vertebrate hosts. In this study, we screened for mutants sensitive to the secretion inhibitor brefeldin A to identify secretory pathway components that contribute to virulence. We identified an ortholog of the cell division control protein 50 (Cdc50) family of the noncatalytic subunit of type IV P-type ATPases (flippases) that establish phospholipid asymmetry in membranes and function in vesicle-mediated trafficking. We found that a cdc50 mutant in C. neoformans was defective for survival in macrophages, attenuated for virulence in mice and impaired in iron acquisition. The mutant also showed increased sensitivity to drugs associated with phospholipid metabolism (cinnamycin and miltefosine), the antifungal drug fluconazole and curcumin, an iron chelator that accumulates in the endoplasmic reticulum. Cdc50 is expected to function with catalytic subunits of flippases, and we previously documented the involvement of the flippase aminophospholipid translocases (Apt1) in virulence factor delivery. A comparison of phenotypes with mutants defective in genes encoding candidate flippases (designated APT1, APT2, APT3, and APT4) revealed similarities primarily between cdc50 and apt1 suggesting a potential functional interaction. Overall, these results highlight the importance of membrane composition and homeostasis for the ability of C. neoformans to cause disease.

Studying the effects of asymmetry on the bending rigidity of lipid membranes formed by microfluidics

In this article we detail a robust high-throughput microfluidic platform capable of fabricating either symmetric or asymmetric giant unilamellar vesicles (GUVs) and characterise the mechanical properties of their membranes.

OSBP-Related Protein Family: Mediators of Lipid Transport and Signaling at Membrane Contact Sites

Oxysterol-binding protein (OSBP) and its related protein homologs, ORPs, constitute a conserved family of lipid-binding/transfer proteins (LTPs) expressed ubiquitously in eukaryotes. The ligand-binding domain of ORPs accommodates cholesterol and oxysterols, but also glycerophospholipids, particularly phosphatidylinositol-4-phosphate (PI4P). ORPs have been implicated as intracellular lipid sensors or transporters. Most ORPs carry targeting determinants for the endoplasmic reticulum (ER) and non-ER organelle membrane. ORPs are located and function at membrane contact sites (MCSs), at which ER is closely apposed with other organelle limiting membranes. Such sites have roles in lipid transport and metabolism, control of Ca(2+) fluxes, and signaling events. ORPs are postulated either to transport lipids over MCSs to maintain the distinct lipid compositions of organelle membranes, or to control the activity of enzymes/protein complexes with functions in signaling and lipid metabolism. ORPs may transfer PI4P and another lipid class bidirectionally. Transport of PI4P followed by its hydrolysis would in this model provide the energy for transfer of the other lipid against its concentration gradient. Control of organelle lipid compositions by OSBP/ORPs is important for the life cycles of several pathogenic viruses. Targeting ORPs with small-molecular antagonists is proposed as a new strategy to combat viral infections. Several ORPs are reported to modulate vesicle transport along the secretory or endocytic pathways. Moreover, antagonists of certain ORPs inhibit cancer cell proliferation. Thus, ORPs are LTPs, which mediate interorganelle lipid transport and coordinate lipid signals with a variety of cellular regimes.

Inner workings and biological impact of phospholipid flippases

The plasma membrane, trans-Golgi network and endosomal system of eukaryotic cells are populated with flippases that hydrolyze ATP to help establish asymmetric phospholipid distributions across the bilayer. Upholding phospholipid asymmetry is vital to a host of cellular processes, including membrane homeostasis, vesicle biogenesis, cell signaling, morphogenesis and migration. Consequently, defining the identity of flippases and their biological impact has been the subject of intense investigations. Recent work has revealed a remarkable degree of kinship between flippases and cation pumps. In this Commentary, we review emerging insights into how flippases work, how their activity is controlled according to cellular demands, and how disrupting flippase activity causes system failure of membrane function, culminating in membrane trafficking defects, aberrant signaling and disease.

The Drosophila melanogaster phospholipid flippase dATP8B is required for odorant receptor function

The olfactory systems of insects are fundamental to all aspects of their behaviour, and insect olfactory receptor neurons (ORNs) exhibit exquisite specificity and sensitivity to a wide range of environmental cues. In Drosophila melanogaster, ORN responses are determined by three different receptor families, the odorant (Or), ionotropic-like (IR) and gustatory (Gr) receptors. However, the precise mechanisms of signalling by these different receptor families are not fully understood. Here we report the unexpected finding that the type 4 P-type ATPase phospholipid transporter dATP8B, the homologue of a protein associated with intrahepatic cholestasis and hearing loss in humans, is crucial for Drosophila olfactory responses. Mutations in dATP8B severely attenuate sensitivity of odorant detection specifically in Or-expressing ORNs, but do not affect responses mediated by IR or Gr receptors. Accordingly, we find dATP8B to be expressed in ORNs and localised to the dendritic membrane of the olfactory neurons where signal transduction occurs. Localisation of Or proteins to the dendrites is unaffected in dATP8B mutants, as is dendrite morphology, suggesting instead that dATP8B is critical for Or signalling. As dATP8B is a member of the phospholipid flippase family of ATPases, which function to determine asymmetry in phospholipid composition between the outer and inner leaflets of plasma membranes, our findings suggest a requirement for phospholipid asymmetry in the signalling of a specific family of chemoreceptor proteins.

The olfactory systems of insects are fundamental to critical behaviours such as finding mates, food and host plants. Insects can detect a wide range of environmental cues using three different families of olfactory receptor proteins. Why insects have three different families of receptor genes, and how they function together, is not fully understood. Here we identified a new gene, dATP8B, which is critically and specifically required for the function of only one of these receptor families in Drosophila. dATP8B is a member of the P4-type ATPases, or phospholipid flippases; these enzymes function in establishing a difference or asymmetry in lipid composition between the outer and inner leaflets of plasma membranes. This is thought to be important for many cellular membrane processes; however, specific functions of individual flippase proteins are not well described. We find that dATP8B is required for the function of the odorant receptor family, but not the ionotropic-like and gustatory receptor families. This further highlights the functional differences between these receptor families and suggests a role for phospholipids in the signalling of a specific family of receptor proteins.

Using natural variation in Drosophila to discover previously unknown endoplasmic reticulum stress genes

Natural genetic variation is a rich resource for identifying novel elements of cellular pathways such as endoplasmic reticulum (ER) stress. ER stress occurs when misfolded proteins accumulate in the ER and cells respond with the conserved unfolded protein response (UPR), which includes large-scale gene expression changes. Although ER stress can be a cause or a modifying factor of human disease, little is known of the amount of variation in the response to ER stress and the genes contributing to such variation. To study natural variation in ER stress response in a model system, we measured the survival time in response to tunicamycin-induced ER stress in flies from 114 lines from the sequenced Drosophila Genetic Reference Panel of wild-derived inbred strains. These lines showed high heterogeneity in survival time under ER stress conditions. To identify the genes that may be driving this phenotypic variation, we profiled ER stress-induced gene expression and performed an association study. Microarray analysis identified variation in transcript levels of numerous known and previously unknown ER stress-responsive genes. Survival time was significantly associated with polymorphisms in candidate genes with known (i.e., Xbp1) and unknown roles in ER stress. Functional testing found that 17 of 25 tested candidate genes from the association study have putative roles in ER stress. In both approaches, one-third of ER stress genes had human orthologs that contribute to human disease. This study establishes Drosophila as a useful model for studying variation in ER stress and identifying ER stress genes that may contribute to human disease.

Oxysterol-binding proteins: functions in cell regulation beyond lipid metabolism

Oxysterol-binding (OSBP)-related proteins (ORPs) constitute a family of sterol and phosphoinositide binding/transfer proteins in eukaryotes from yeast to man. While their functions have mainly been addressed in cellular lipid metabolism or sterol transport, increasing evidence points to more versatile regulatory roles in a spectrum of cellular regimes. In fact ORPs do not appear to be robust controllers of lipid homeostasis. Several ORPs localize at membrane contacts sites (MCS), where endoplasmic reticulum (ER) is apposed with other organelle limiting membranes. Apparently, ORPs have the capacity to control the formation of MCS or activity of enzymatic machineries at these sites. Thereby, ORPs most likely affect organelle membrane lipid compositions, with impacts on signaling and vesicle transport, but also cellular lipid metabolism. Moreover, an increasing number of protein interaction partners of ORPs have been identified, connecting these proteins with various aspects of cell regulation. Small molecular anti-proliferative compounds, ORPphilins, were recently found to target two members of the ORP family, OSBP and ORP4, revealing an essential function of ORPs in cancer cell proliferation and survival. Further functions assigned for ORPs include regulation of extracellular signal regulated kinase (ERK) activity (OSBP), control of ER-late endosome MCS and late endosome motility (ORP1L), regulation of β1-integrin activity (ORP3), modulation of hepatocyte insulin signaling and macrophage migration (ORP8), as well as post-Golgi vesicle transport, phosphatidylinositol-4-phosphate and target of rapamycin complex 1 signaling and nitrogen sensing (Saccharomyces cerevisiae Osh4p). These and other recent observations shed light on the ORPs as integrators of lipid signals with an unforeseen variety of vital cellular processes.