Sac1, a lipid phosphatase at the interface of vesicular and nonvesicular transport

Mitochondria-ER-PM contacts regulate mitochondrial division and PI(4)P distribution

The mitochondria-ER-cortex anchor (MECA) forms a tripartite membrane contact site between mitochondria, the endoplasmic reticulum (ER), and the plasma membrane (PM). The core component of MECA, Num1, interacts with the PM and mitochondria via two distinct lipid-binding domains; however, the molecular mechanism by which Num1 interacts with the ER is unclear. Here, we demonstrate that Num1 contains a FFAT motif in its C-terminus that interacts with the integral ER membrane protein Scs2. While dispensable for Num1's functions in mitochondrial tethering and dynein anchoring, the FFAT motif is required for Num1's role in promoting mitochondrial division. Unexpectedly, we also reveal a novel function of MECA in regulating the distribution of phosphatidylinositol-4-phosphate (PI(4)P). Breaking Num1 association with any of the three membranes it tethers results in an accumulation of PI(4)P on the PM, likely via disrupting Sac1-mediated PI(4)P turnover. This work establishes MECA as an important regulatory hub that spatially organizes mitochondria, ER, and PM to coordinate crucial cellular functions.

Metabolic channeling of lipids via the contact zones between different organelles

Various lipid transfer proteins (LTPs) mediate the inter-organelle transport of lipids. By working at membrane contact zones between donor and acceptor organelles, LTPs achieve rapid and accurate inter-organelle transfer of lipids. This article will describe the emerging paradigm that the action of LTPs at organelle contact zones generates metabolic channeling events in lipid metabolism, mainly referring to how ceramide synthesized in the endoplasmic reticulum is preferentially metabolized to sphingomyelin in the distal Golgi region, how cholesterol and phospholipids receive specific metabolic reactions in mitochondria, and how the hijacking of host LTPs by intracellular pathogens may generate new channeling-like events. In addition, the article will discuss how the function of LTPs is regulated, exemplified by a few representative LTP systems, and will briefly touch on experiments that will be necessary to establish the paradigm that LTP-mediated inter-organelle transport of lipids is one of the mechanisms of compartmentalization-based metabolic channeling events.

Sac1 links phosphoinositide turnover to cryptococcal virulence

Cryptococcus neoformans is an environmentally acquired fungal pathogen that causes over 140,000 deaths per year. Cryptococcal infection occurs when infectious particles are deposited into the lung, where they encounter host phagocytic cells. C. neoformans may be engulfed by these phagocytes, an important step of infection that leads to outcomes ranging from termination of infection to cryptococcal dissemination. To study this critical process, we screened approximately 4,700 cryptococcal gene deletion mutants for altered uptake, using primary mouse and human phagocytic cells. Among the hits of these two screens, we identified 93 mutants with perturbed uptake in both systems, as well as others with differences in uptake by only one cell type. We further screened the hits for changes in thickness of the capsule, a protective polysaccharide layer around the cell which is an important cryptococcal virulence factor. The combination of our three screens yielded 45 mutants, including one lacking the phosphatidylinositol-4-phosphate phosphatase Sac1. In this work, we implicate Sac1 in both host cell uptake and capsule production. We found that sac1 mutants exhibit lipid trafficking defects, reductions in secretory system function, and changes in capsule size and composition. Many of these changes occur specifically in tissue culture media, highlighting the role of Sac1 phosphatase activity in responding to the stress of host-like conditions. Overall, these findings show how genome-scale screening can identify cellular factors that contribute to our understanding of cryptococcal biology and demonstrate the role of Sac1 in determining fungal virulence.IMPORTANCECryptococcus neoformans is a fungal pathogen with significant impact on global health. Cryptococcal cells inhaled from the environment are deposited into the lungs, where they first contact the human immune system. The interaction between C. neoformans and host cells is critical because this step of infection can determine whether the fungal cells die or proliferate within the human host. Despite the importance of this stage of infection, we have limited knowledge of cryptococcal factors that influence its outcome. In this study, we identify cryptococcal genes that affect uptake by both human and mouse cells. We also identify mutants with altered capsule, a protective coating that surrounds the cells to shield them from the host immune system. Finally, we characterize the role of one gene, SAC1, in these processes. Overall, this study contributes to our understanding of how C. neoformans interacts with and protects itself from host cells.

Sac1 links phosphoinositide turnover to cryptococcal virulence

Cryptococcus neoformans is an environmentally-acquired fungal pathogen that causes over 140,000 deaths per year. Cryptococcal infection occurs when infectious particles are deposited into the lung, where they encounter host phagocytic cells. C. neoformans may be engulfed by these phagocytes, an important step of infection that leads to outcomes ranging from termination of infection to cryptococcal dissemination. To study this critical process, we screened approximately 4,700 cryptococcal gene deletion mutants for altered uptake, using primary mouse and human phagocytic cells. Among the hits of these two screens, we identified 93 mutants with perturbed uptake in both systems, as well as others with differences in uptake by only one cell type. We further screened the hits for changes in thickness of the capsule, a protective polysaccharide layer around the cell which is an important cryptococcal virulence factor. The combination of our three screens yielded 45 mutants, including one lacking the phosphatidylinositol-4-phosphate phosphatase Sac1. In this work, we implicate Sac1 in both host cell uptake and capsule production. We found that sac1 mutants exhibit lipid trafficking defects, reductions in secretory system function, and changes in capsule size and composition. Many of these changes occur specifically in tissue culture media, highlighting the role of Sac1 phosphatase activity in responding to the stress of host-like conditions. Overall, these findings show how genome-scale screening can identify cellular factors that contribute to our understanding of cryptococcal biology and demonstrate the role of Sac1 in determining fungal virulence.

IMPORTANCE

Cryptococcus neoformans is a fungal pathogen with significant impact on global health. Cryptococcal cells inhaled from the environment are deposited into the lungs, where they first contact the human immune system. The interaction between C. neoformans and host cells is critical because this step of infection can determine whether the fungal cells die or proliferate within the human host. Despite the importance of this stage of infection, we have limited knowledge of cryptococcal factors that influence its outcome. In this study, we identify cryptococcal genes that affect uptake by both human and mouse cells. We also identify mutants with altered capsule, a protective coating that surrounds the cells to shield them from the host immune system. Finally, we characterize the role of one gene, SAC1 , in these processes. Overall, this study contributes to our understanding of how C. neoformans interacts with and protects itself from host cells.

H3K4 methylation regulates development, DNA repair, and virulence in Mucorales

Mucorales are basal fungi that opportunistically cause a potentially fatal infection known as mucormycosis (black fungus disease), which poses a significant threat to human health due to its high mortality rate and its recent association with SARS-CoV-2 infections. On the other hand, histone methylation is a regulatory mechanism with pleiotropic effects, including the virulence of several pathogenic fungi. However, the role of epigenetic changes at the histone level never has been studied in Mucorales. Here, we dissected the functional role of Set1, a histone methyltransferase that catalyzes the methylation of H3K4, which is associated with the activation of gene transcription and virulence. A comparative analysis of the Mucor lusitanicus genome (previously known as Mucor circinelloides f. lusitanicus) identified only one homolog of Set1 from Candida albicans and Saccharomyces cerevisiae that contains the typical SET domain. Knockout strains in the gene set1 lacked H3K4 monomethylation, dimethylation, and trimethylation enzymatic activities. These strains also showed a significant reduction in vegetative growth and sporulation. Additionally, set1 null strains were more sensitive to SDS, EMS, and UV light, indicating severe impairment in the repair process of the cell wall and DNA lesions and a correlation between Set1 and these processes. During pathogen-host interactions, strains lacking the set1 gene exhibited shortened polar growth within the phagosome and attenuated virulence both in vitro and in vivo. Our findings suggest that the histone methyltransferase Set1 coordinates several cell processes related to the pathogenesis of M. lusitanicus and may be an important target for future therapeutic strategies against mucormycosis.

The Human Mutation K237_V238del in a Putative Lipid Binding Motif within the V-ATPase a2 Isoform Suggests a Molecular Mechanism Underlying Cutis Laxa

Vacuolar ATPases (V-ATPases), proton pumps composed of 16 subunits, are necessary for a variety of cellular functions. Subunit "a" has four isoforms, a1-a4, each with a distinct cellular location. We identified a phosphoinositide (PIP) interaction motif, KXnK(R)IK(R), conserved in all four isoforms, and hypothesize that a/PIP interactions regulate V-ATPase recruitment/retention to different organelles. Among the four isoforms, a2 is enriched on Golgi with a2 mutations in the PIP motif resulting in cutis laxa. We hypothesize that the hydrophilic N-terminal (NT) domain of a2 contains a lipid-binding domain, and mutations in this domain prevent interaction with Golgi-enriched PIPs, resulting in cutis laxa. We recreated the cutis laxa-causing mutation K237_V238del, and a double mutation in the PIP-binding motif, K237A/V238A. Circular dichroism confirmed that there were no protein structure alterations. Pull-down assays with PIP-enriched liposomes revealed that wildtype a2NT preferentially binds phosphatidylinositol 4-phosphate (PI(4)P), while mutants decreased binding to PI(4)P. In HEK293 cells, wildtype a2NT was localized to Golgi and co-purified with microsomal membranes. Mutants reduced Golgi localization and membrane association. Rapamycin depletion of PI(4)P diminished a2NT-Golgi localization. We conclude that a2NT is sufficient for Golgi retention, suggesting the lipid-binding motif is involved in V-ATPase targeting and/or retention. Mutational analyses suggest a molecular mechanism underlying how a2 mutations result in cutis laxa.

Peroxisome population control by phosphoinositide signaling at the endoplasmic reticulum-plasma membrane interface

Phosphoinositides are lipid signaling molecules acting at the interface of membranes and the cytosol to regulate membrane trafficking, lipid transport and responses to extracellular stimuli. Peroxisomes are multicopy organelles that are highly responsive to changes in metabolic and environmental conditions. In yeast, peroxisomes are tethered to the cell cortex at defined focal structures containing the peroxisome inheritance protein, Inp1p. We investigated the potential impact of changes in cortical phosphoinositide levels on the peroxisome compartment of the yeast cell. Here we show that the phosphoinositide, phosphatidylinositol-4-phosphate (PI4P), found at the junction of the cortical endoplasmic reticulum and plasma membrane (cER-PM) acts to regulate the cell's peroxisome population. In cells lacking a cER-PM tether or the enzymatic activity of the lipid phosphatase Sac1p, cortical PI4P is elevated, peroxisome numbers and motility are increased, and peroxisomes are no longer firmly tethered to Inp1p-containing foci. Reattachment of the cER to the PM through an artificial ER-PM "staple" in cells lacking the cER-PM tether does not restore peroxisome populations to the wild-type condition, demonstrating that integrity of PI4P signaling at the cell cortex is required for peroxisome homeostasis.

DGARM/C10orf76/ARMH3 for Ceramide Transfer Zone at the Endoplasmic Reticulum-Distal Golgi Contacts

Phosphatidylinositol 4-monophosphate (PtdIns(4)P) is one of the key membrane components which mark the membrane contact sites. In the mammalian Golgi complex, PtdIns(4)P is produced at various subregions via specific mechanisms for each site. Particularly, PtdIns(4)P pools generated at the distal Golgi regions are pivotal for the determination of membrane contacts between the endoplasmic reticulum (ER) and Golgi, at which inter-organelle lipid transport takes place. In this short review, we will focus on C10orf76 (or ARMH3), which we propose to rename as DGARM after a distal Golgi armadillo repeat protein, for its function in generating a PtdIns(4)P pool crucial for ER-to-distal Golgi ceramide transport. We further discuss from the viewpoint of the evolutionary conservation of DGARM.

Nutrient signaling: Starvation flips a phosphoinositide switch on lysosomal catabolism

Lysosomes are highly dynamic organelles that rapidly respond to changes in cellular nutrient status. A new study identifies a phosphoinositide switch that dictates lysosome function during nutrient starvation.

Nutrient-regulated control of lysosome function by signaling lipid conversion

Lysosomes serve dual antagonistic functions in cells by mediating anabolic growth signaling and the catabolic turnover of macromolecules. How these janus-faced activities are regulated in response to cellular nutrient status is poorly understood. We show here that lysosome morphology and function are reversibly controlled by a nutrient-regulated signaling lipid switch that triggers the conversion between peripheral motile mTOR complex 1 (mTORC1) signaling-active and static mTORC1-inactive degradative lysosomes clustered at the cell center. Starvation-triggered relocalization of phosphatidylinositol 4-phosphate (PI(4)P)-metabolizing enzymes reshapes the lysosomal surface proteome to facilitate lysosomal proteolysis and to repress mTORC1 signaling. Concomitantly, lysosomal phosphatidylinositol 3-phosphate (PI(3)P), which marks motile signaling-active lysosomes in the cell periphery, is erased. Interference with this PI(3)P/PI(4)P lipid switch module impairs the adaptive response of cells to altering nutrient supply. Our data unravel a key function for lysosomal phosphoinositide metabolism in rewiring organellar membrane dynamics in response to cellular nutrient status.

Novel findings about the mode of action of the antifungal protein PeAfpA against Saccharomyces cerevisiae

Antifungal proteins (AFPs) from filamentous fungi offer the potential to control fungal infections that threaten human health and food safety. AFPs exhibit broad antifungal spectra against harmful fungi, but limited knowledge of their killing mechanism hinders their potential applicability. PeAfpA from Penicillium expansum shows strong antifungal potency against plant and human fungal pathogens and stands above other AFPs for being active against the yeast Saccharomyces cerevisiae. We took advantage of this and used a model laboratory strain of S. cerevisiae to gain insight into the mode of action of PeAfpA by combining (i) transcriptional profiling, (ii) PeAfpA sensitivity analyses of deletion mutants available in the S. cerevisiae genomic deletion collection and (iii) cell biology studies using confocal microscopy. Results highlighted and confirmed the role of the yeast cell wall (CW) in the interaction with PeAfpA, which can be internalized through both energy-dependent and independent mechanisms. The combined results also suggest an active role of the CW integrity (CWI) pathway and the cAMP-PKA signalling in the PeAfpA killing mechanism. Besides, our studies revealed the involvement of phosphatidylinositol metabolism and the participation of ROX3, which codes for the subunit 19 of the RNA polymerase II mediator complex, in the yeast defence strategy. In conclusion, our study provides clues about both the killing mechanism of PeAfpA and the fungus defence strategies against the protein, suggesting also targets for the development of new antifungals. KEY POINTS: • PeAfpA is a cell-penetrating protein with inhibitory activity against S. cerevisiae. • The CW integrity (CWI) pathway is a key player in the PeAfpA killing mechanism. • Phosphatidylinositol metabolism and ROX3 are involved in the yeast defence strategy.

The Genetic Background of Abnormalities in Metabolic Pathways of Phosphoinositides and Their Linkage with the Myotubular Myopathies, Neurodegenerative Disorders, and Carcinogenesis

Myo-inositol belongs to one of the sugar alcohol groups known as cyclitols. Phosphatidylinositols are one of the derivatives of Myo-inositol, and constitute important mediators in many intracellular processes such as cell growth, cell differentiation, receptor recycling, cytoskeletal organization, and membrane fusion. They also have even more functions that are essential for cell survival. Mutations in genes encoding phosphatidylinositols and their derivatives can lead to many disorders. This review aims to perform an in-depth analysis of these connections. Many authors emphasize the significant influence of phosphatidylinositols and phosphatidylinositols' phosphates in the pathogenesis of myotubular myopathies, neurodegenerative disorders, carcinogenesis, and other less frequently observed diseases. In our review, we have focused on three of the most often mentioned groups of disorders. Inositols are the topic of many studies, and yet, there are no clear results of successful clinical trials. Analysis of the available literature gives promising results and shows that further research is still needed.

Regulation of phosphoinositide metabolism in Apicomplexan parasites

Phosphoinositides are a biologically essential class of phospholipids that contribute to organelle membrane identity, modulate membrane trafficking pathways, and are central components of major signal transduction pathways that operate on the cytosolic face of intracellular membranes in eukaryotes. Apicomplexans (such as Toxoplasma gondii and Plasmodium spp.) are obligate intracellular parasites that are important causative agents of disease in animals and humans. Recent advances in molecular and cell biology of Apicomplexan parasites reveal important roles for phosphoinositide signaling in key aspects of parasitosis. These include invasion of host cells, intracellular survival and replication, egress from host cells, and extracellular motility. As Apicomplexans have adapted to the organization of essential signaling pathways to accommodate their complex parasitic lifestyle, these organisms offer experimentally tractable systems for studying the evolution, conservation, and repurposing of phosphoinositide signaling. In this review, we describe the regulatory mechanisms that control the spatial and temporal regulation of phosphoinositides in the Apicomplexan parasites Plasmodium and T. gondii. We further discuss the similarities and differences presented by Apicomplexan phosphoinositide signaling relative to how these pathways are regulated in other eukaryotic organisms.

Increased Phospholipid Flux Bypasses Overlapping Essential Requirements for the Yeast Sac1p Phosphoinositide Phosphatase and ER-PM Membrane Contact Sites

In budding yeast cells, much of the inner surface of the plasma membrane (PM) is covered with the endoplasmic reticulum (ER). This association is mediated by seven ER membrane proteins that confer cortical ER-PM association at membrane contact sites (MCSs). Several of these membrane "tether" proteins are known to physically interact with the phosphoinositide phosphatase Sac1p. However, it is unclear how or if these interactions are necessary for their interdependent functions. We find that SAC1 inactivation in cells lacking the homologous synaptojanin-like genes INP52 and INP53 results in a significant increase in cortical ER-PM MCSs. We show in sac1Δ, sac1tsinp52Δ inp53Δ, or Δ-super-tether (Δ-s-tether) cells lacking all seven ER-PM tethering genes that phospholipid biosynthesis is disrupted and phosphoinositide distribution is altered. Furthermore, SAC1 deletion in Δ-s-tether cells results in lethality, indicating a functional overlap between SAC1 and ER-PM tethering genes. Transcriptomic profiling indicates that SAC1 inactivation in either Δ-s-tether or inp52Δ inp53Δ cells induces an ER membrane stress response and elicits phosphoinositide-dependent changes in expression of autophagy genes. In addition, by isolating high-copy suppressors that rescue sac1Δ Δ-s-tether lethality, we find that key phospholipid biosynthesis genes bypass the overlapping function of SAC1 and ER-PM tethers and that overexpression of the phosphatidylserine/phosphatidylinositol-4-phosphate transfer protein Osh6 also provides limited suppression. Combined with lipidomic analysis and determinations of intracellular phospholipid distributions, these results suggest that Sac1p and ER phospholipid flux controls lipid distribution to drive Osh6p-dependent phosphatidylserine/phosphatidylinositol-4-phosphate counter-exchange at ER-PM MCSs.

Targeting carnitine palmitoyltransferase 1 isoforms in the hypothalamus: A promising strategy to regulate energy balance

Tackling the growing incidence and prevalence of obesity urgently requires uncovering new molecular pathways with therapeutic potential. The brain, and in particular the hypothalamus, is a major integrator of metabolic signals from peripheral tissues that regulate functions such as feeding behavior and energy expenditure. In obesity, hypothalamic capacity to sense nutritional status and regulate these functions is altered. An emerging line of research is that hypothalamic lipid metabolism plays a critical role in regulating energy balance. Here, we focus on the carnitine palmitoyltransferase 1 (CPT1) enzyme family responsible for long-chain fatty acid metabolism. The evidence suggests that two of its isoforms expressed in the brain, CPT1A and CPT1C, play a crucial role in hypothalamic lipid metabolism, and their promise as targets in food intake and bodyweight management is currently being intensively investigated. In this review we describe and discuss the metabolic actions and potential up- and downstream effectors of hypothalamic CPT1 isoforms, and posit the need to develop innovative nanomedicine platforms for selective targeting of CPT1 and related nutrient sensors in specific brain areas as potential next-generation therapy to treat obesity.

The intracellular and plasma membrane pools of phosphatidylinositol-4-monophosphate control megakaryocyte maturation and proplatelet formation

Background

Megakaryocytes (MKs) develop from hematopoietic stem cells after stimulation by the cytokine thrombopoietin. During megakaryopoiesis, MKs enlarge, undergo the process of endomitosis, and develop intracellular membranes (demarcation membrane system, DMS). During DMS formation, there is active transport of proteins, lipids, and membranes from the Golgi apparatus to the DMS. The most important phosphoinositide that controls anterograde transport from the Golgi apparatus to the plasma membrane (PM) is phosphatidylinositol-4-monophosphate (PI4P), whose levels are controlled by suppressor of actin mutations 1-like protein (Sac1) phosphatase at the Golgi and endoplasmic reticulum.

Objectives

Here we investigated the role of Sac1 and PI4P in megakaryopoiesis.

Methods

We analyzed Sac1 and PI4P localization in primary MKs derived from fetal liver or bone marrow and in the DAMI cell line by immunofluorescence. The intracellular and PM pools of PI4P in primary MKs were modulated by expression of Sac1 constructs from retroviral vector and inhibition of PI4 kinase IIIα, respectively.

Results

We showed that in primary mouse MKs, PI4P is mostly found in the Golgi apparatus and the PM in immature MKs, while in mature MKs, it is found in the cell periphery and at the PM. The exogenous expression of wild-type but not C389S mutant (catalytically dead) Sac1 results in the perinuclear retention of the Golgi apparatus resembling immature MKs, with decreased ability to form proplatelets. The pharmacologic inhibition of PI4P production specifically at the PM also resulted in a significant decrease in MKs that form proplatelets.

Conclusion

These results indicate that both intracellular and PM pools of PI4P mediate MK maturation and proplatelet formation.

ORP9 and ORP10 form a heterocomplex to transfer phosphatidylinositol 4-phosphate at ER-TGN contact sites

Oxysterol-binding protein (OSBP) and its related proteins (ORPs) are a family of lipid transfer proteins (LTPs) that mediate non-vesicular lipid transport. ORP9 and ORP10, members of the OSBP/ORPs family, are located at the endoplasmic reticulum (ER)-trans-Golgi network (TGN) membrane contact sites (MCSs). It remained unclear how they mediate lipid transport. In this work, we discovered that ORP9 and ORP10 form a binary complex through intermolecular coiled-coil (CC) domain-CC domain interaction. The PH domains of ORP9 and ORP10 specially interact with phosphatidylinositol 4-phosphate (PI4P), mediating the TGN targeting. The ORP9-ORP10 complex plays a critical role in regulating PI4P levels at the TGN. Using in vitro reconstitution assays, we observed that while full-length ORP9 efficiently transferred PI4P between two apposed membranes, the lipid transfer kinetics was further accelerated by ORP10. Interestingly, our data showed that the PH domains of ORP9 and ORP10 participate in membrane tethering simultaneously, whereas ORDs of both ORP9 and ORP10 are required for lipid transport. Furthermore, our data showed that the depletion of ORP9 and ORP10 led to increased vesicle transport to the plasma membrane (PM). These findings demonstrate that ORP9 and ORP10 form a binary complex through the CC domains, maintaining PI4P homeostasis at ER-TGN MCSs and regulating vesicle trafficking.

To be or not to be a fat burner, that is the question for cpt1c in cancer cells

There is an urgent need to identify reliable genetic biomarkers for accurate diagnosis, prognosis, and treatment of different tumor types. Described as a prognostic marker for many tumors is the neuronal protein carnitine palmitoyltransferase 1 C (CPT1C). Several studies report that CPT1C is involved in cancer cell adaptation to nutrient depletion and hypoxia. However, the molecular role played by CPT1C in cancer cells is controversial. Most published studies assume that, like canonical CPT1 isoforms, CPT1C is a mediator of fatty acid transport to mitochondria for beta-oxidation, despite the fact that CPT1C has inefficient catalytic activity and is located in the endoplasmic reticulum. In this review, we collate existing evidence on CPT1C in neurons, showing that CPT1C is a sensor of nutrients that interacts with and regulates other proteins involved in lipid metabolism and transport, lysosome motility, and the secretory pathway. We argue, therefore, that CPT1C expression in cancer cells is not a direct regulator of fat burn, but rather is a regulator of lipid metabolic reprograming and cell adaptation to environmental stressors. We also review the clinical relevance of CPT1C as a prognostic indicator and its contribution to tumor growth, cancer invasiveness, and cell senescence. This new and integrated vision of CPT1C function can help better understand the metabolic plasticity of cancer cells and improve the design of therapeutic strategies.

Ca2+ and Annexins - Emerging Players for Sensing and Transferring Cholesterol and Phosphoinositides via Membrane Contact Sites

Maintaining lipid composition diversity in membranes from different organelles is critical for numerous cellular processes. However, many lipids are synthesized in the endoplasmic reticulum (ER) and require delivery to other organelles. In this scenario, formation of membrane contact sites (MCS) between neighbouring organelles has emerged as a novel non-vesicular lipid transport mechanism. Dissecting the molecular composition of MCS identified phosphoinositides (PIs), cholesterol, scaffolding/tethering proteins as well as Ca²⁺ and Ca²⁺-binding proteins contributing to MCS functioning. Compelling evidence now exists for the shuttling of PIs and cholesterol across MCS, affecting their concentrations in distinct membrane domains and diverse roles in membrane trafficking. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) not only controls endo-/exocytic membrane dynamics but is also critical in autophagy. Cholesterol is highly concentrated at the PM and enriched in recycling endosomes and Golgi membranes. MCS-mediated cholesterol transfer is intensely researched, identifying MCS dysfunction or altered MCS partnerships to correlate with de-regulated cellular cholesterol homeostasis and pathologies. Annexins, a conserved family of Ca²⁺-dependent phospholipid binding proteins, contribute to tethering and untethering events at MCS. In this chapter, we will discuss how Ca²⁺ homeostasis and annexins in the endocytic compartment affect the sensing and transfer of cholesterol and PIs across MCS.

Impact of interorganelle coordination between the conventional early secretory pathway and autophagy in cellular homeostasis and stress response

The conventional early secretory pathway and autophagy are two essential interconnected cellular processes that are crucial for maintaining cellular homeostasis. The conventional secretory pathway is an anabolic cellular process synthesizing and delivering proteins to distinct locations, including different organelles, the plasma membrane, and the extracellular media. On the other hand, autophagy is a catabolic cellular process that engulfs damaged organelles and aberrant cytosolic constituents into the double autophagosome membrane. After fusion with the lysosome and autolysosome formation, this process triggers digestion and recycling. A growing list of evidence indicates that these anabolic and catabolic processes are mutually regulated. While knowledge about the molecular actors involved in the coordination and functional cooperation between these two processes has increased over time, the mechanisms are still poorly understood. This review article summarized and discussed the most relevant evidence about the key molecular players implicated in the interorganelle crosstalk between the early secretory pathway and autophagy under normal and stressful conditions.

Sac1 phosphatidylinositol 4-phosphate phosphatase is a novel host cell factor regulating hepatitis B virus particles assembly and release

The life-cycle of the Hepatitis B Virus (HBV), an enveloped DNA virus affecting the lives of more than 296 million chronicallyinfected people, is tightly dependent on the lipid metabolism of the host cell. Fatty acids and cholesterol are among the lipid factors with documented roles in regulating HBV replication and infection, respectively, but little is known about the phosphoinositide metabolism in these processes. In this study, we investigated the role of Sac1, a highly conserved phosphatidylinositol-4-phosphate (PI4P) phosphatase, with essential functions in phospholipid metabolism, in HBV assembly, and release. PI4P is one of the most abundant cellular phosphoinositide with complex functions at the level of the secretory pathway. Owing to the highly specific phosphatase activity toward PI4P, Sac1 controls the levels and restricts the localization of this lipid particularly at the trans-Golgi network, where it regulates sphingolipid synthesis, proteins sorting, and vesicles budding, by recruiting specific adaptor proteins. As a complete loss of Sac1 function compromises cell viability, in this work, we first developed and characterized several HBV replication-permissive cellular models with a moderate, transient, or stable downregulation of Sac1 expression. Our results show that Sac1 depletion in hepatic cells results in increased levels and redistribution of intracellular PI4P pools and impaired trafficking of the HBV envelope proteins to the endosomal vesicular network. Importantly, virus envelopment and release from these cells are significantly inhibited, revealing novel roles for Sac1, as a key host cell factor regulating morphogenesis of a DNA virus.

Surface-enhanced Raman spectroscopy for characterization of filtrate portions of blood serum samples of typhoid patients

BACKGROUND

Surface-enhanced Raman spectroscopy (SERS) is explored to design a rapid screening method for the characterization and diagnosis of typhoid fever by employing filtrate fractions of blood serum samples obtained by centrifugal filtration with 50 KDa filters.

OBJECTIVES

The purpose of this study, to separate the filtrate portions of blood serum samples in this way contain proteins smaller than 50 kDa and removal of bigger size protein which allows to acquire the SERS spectral features of smaller proteins more effectively which are probably associated with typhoid disease. Disease caused by Salmonella typhi diagnose more effectively by using surface-enhanced Raman spectroscopy (SERS) and multivariate data analysis tools.

METHODS

SERS was used as a diagnostic tool for typhoid fever by comparison between healthy and diseased samples. For this purpose, all the samples were analyzed by comparing their SERS spectral features. Over the spectral range of 400-1800cm^-1, multivariate data analysis techniques such as Principal Component Analysis (PCA) and Partial Least Squares-Discriminant Analysis (PLS-DA) are applied to diagnose and differentiate different filtrate fractions of blood serum samples of patients of typhoid fever and healthy ones.

RESULTS

By comparing SERS spectra of healthy filtrate with that of filtrate of typhoid sample, the SERS spectral features associated with disease development are identified including PCA is found to be efficient for the qualitative differentiation of all of the samples analyzed. Moreover, PLS-DA successfully identified and classified healthy and typhoid positive blood serum samples with 97 % accuracy, 99 % specificity, 91 % sensitivity and 0.78 area under the receiver operating characteristic (AUROC) curve.

CONCLUSIONS

Surface enhanced Raman spectroscopy using silver nanoparticles SERS substrate, is found to be useful technique for the quick identification and evaluation of filtrate fractions of the blood serum samples of healthy and typhoid samples for disease diagnosis.

Hippo pathway regulation by phosphatidylinositol transfer protein and phosphoinositides

The Hippo pathway plays a key role in development, organ size control and tissue homeostasis, and its dysregulation contributes to cancer. The LATS tumor suppressor kinases phosphorylate and inhibit the YAP/TAZ transcriptional co-activators to suppress gene expression and cell growth. Through a screen of marine natural products, we identified microcolin B (MCB) as a Hippo activator that preferentially kills YAP-dependent cancer cells. Structure-activity optimization yielded more potent MCB analogs, which led to the identification of phosphatidylinositol transfer proteins α and β (PITPα/β) as the direct molecular targets. We established a critical role of PITPα/β in regulating LATS and YAP. Moreover, we showed that PITPα/β influence the Hippo pathway via plasma membrane phosphatidylinositol-4-phosphate. This study uncovers a previously unrecognized role of PITPα/β in Hippo pathway regulation and as potential cancer therapeutic targets.

Intracellular Cholesterol Synthesis and Transport

Cholesterol homeostasis is related to multiple diseases in humans, including cardiovascular disease, cancer, and neurodegenerative and hepatic diseases. The cholesterol levels in cells are balanced dynamically by uptake, biosynthesis, transport, distribution, esterification, and export. In this review, we focus on de novo cholesterol synthesis, cholesterol synthesis regulation, and intracellular cholesterol trafficking. In addition, the progression of lipid transfer proteins (LTPs) at multiple contact sites between organelles is considered.

PI4P/PS countertransport by ORP10 at ER-endosome membrane contact sites regulates endosome fission

Membrane contact sites (MCSs) serve as a zone for nonvesicular lipid transport by oxysterol-binding protein (OSBP)-related proteins (ORPs). ORPs mediate lipid countertransport, in which two distinct lipids are transported counterdirectionally. How such lipid countertransport controls specific biological functions, however, remains elusive. We report that lipid countertransport by ORP10 at ER–endosome MCSs regulates retrograde membrane trafficking. ORP10, together with ORP9 and VAP, formed ER–endosome MCSs in a phosphatidylinositol 4-phosphate (PI4P)-dependent manner. ORP10 exhibited a lipid exchange activity toward its ligands, PI4P and phosphatidylserine (PS), between liposomes in vitro, and between the ER and endosomes in situ. Cell biological analysis demonstrated that ORP10 supplies a pool of PS from the ER, in exchange for PI4P, to endosomes where the PS-binding protein EHD1 is recruited to facilitate endosome fission. Our study highlights a novel lipid exchange at ER–endosome MCSs as a nonenzymatic PI4P-to-PS conversion mechanism that organizes membrane remodeling during retrograde membrane trafficking.

Functions and Mechanisms of SAC Phosphoinositide Phosphatases in Plants

Phosphatidylinositol (PtdIns) is one type of phospholipid comprising an inositol head group and two fatty acid chains covalently linked to the diacylglycerol group. In addition to their roles as compositions of cell membranes, phosphorylated PtdIns derivatives, termed phosphoinositides, execute a wide range of regulatory functions. PtdIns can be phosphorylated by various lipid kinases at 3-, 4- and/or 5- hydroxyls of the inositol ring, and the phosphorylated forms, including PtdIns3P, PtdIns4P, PtdIns5P, PtdIns(3,5)P2, PtdIns(4,5)P2, can be reversibly dephosphorylated by distinct lipid phosphatases. Amongst many other types, the SUPPRESSOR OF ACTIN (SAC) family of phosphoinositide phosphatases recently emerged as important regulators in multiple growth and developmental processes in plants. Here, we review recent advances on the biological functions, cellular activities, and molecular mechanisms of SAC domain-containing phosphoinositide phosphatases in plants. With a focus on those studies in the model plant Arabidopsis thaliana together with progresses in other plants, we highlight the important roles of subcellular localizations and substrate preferences of various SAC isoforms in their functions.

Regulated inositol synthesis is critical for balanced metabolism and development in Drosophila melanogaster

Myo-inositol is a precursor of the membrane phospholipid, phosphatidylinositol (PI). It is involved in many essential cellular processes including signal transduction, energy metabolism, endoplasmic reticulum stress, and osmoregulation. Inositol is synthesized from glucose-6-phosphate by myo-inositol-3-phosphate synthase (MIPSp). The Drosophila melanogaster Inos gene encodes MIPSp. Abnormalities in myo-inositol metabolism have been implicated in type 2 diabetes, cancer, and neurodegenerative disorders. Obesity and high blood (hemolymph) glucose are two hallmarks of diabetes, which can be induced in Drosophila melanogaster third-instar larvae by high-sucrose diets. This study shows that dietary inositol reduces the obese-like and high-hemolymph glucose phenotypes of third-instar larvae fed high-sucrose diets. Furthermore, this study demonstrates Inos mRNA regulation by dietary inositol; when more inositol is provided there is less Inos mRNA. Third-instar larvae with dysregulated high levels of Inos mRNA and MIPSp show dramatic reductions of the obese-like and high-hemolymph glucose phenotypes. These strains, however, also display developmental defects and pupal lethality. The few individuals that eclose die within two days with striking defects: structural alterations of the wings and legs, and heads lacking proboscises. This study is an exciting extension of the use of Drosophila melanogaster as a model organism for exploring the junction of development and metabolism.

Summary: Inositol reduces obesity and high blood (hemolymph) glucose, but can cause dramatic developmental defects. This study uses the model organism Drosophila melanogaster to explore the junction of development and metabolism.

SAC1 regulates autophagosomal phosphatidylinositol-4-phosphate for xenophagy-directed bacterial clearance

Phosphoinositides are important molecules in lipid signaling, membrane identity, and trafficking that are spatiotemporally controlled by factors from both mammalian cells and intracellular pathogens. Here, using small interfering RNA (siRNA) directed against phosphoinositide kinases and phosphatases, we screen for regulators of the host innate defense response to intracellular bacterial replication. We identify SAC1, a transmembrane phosphoinositide phosphatase, as an essential regulator of xenophagy. Depletion or inactivation of SAC1 compromises fusion between Salmonella-containing autophagosomes and lysosomes, leading to increased bacterial replication. Mechanistically, the loss of SAC1 results in aberrant accumulation of phosphatidylinositol-4-phosphate [PI(4)P] on Salmonella-containing autophagosomes, thus facilitating recruitment of SteA, a PI(4)P-binding Salmonella effector protein, which impedes lysosomal fusion. Replication of Salmonella lacking SteA is suppressed by SAC-1-deficient cells, however, demonstrating bacterial adaptation to xenophagy. Our findings uncover a paradigm in which a host protein regulates the level of its substrate and impairs the function of a bacterial effector during xenophagy.

Biology of AMPA receptor interacting proteins - From biogenesis to synaptic plasticity

AMPA-type glutamate receptors mediate the majority of excitatory synaptic transmission in the central nervous system. Their signaling properties and abundance at synapses are both crucial determinants of synapse efficacy and plasticity, and are therefore under sophisticated control. Unique to this ionotropic glutamate receptor (iGluR) is the abundance of interacting proteins that contribute to its complex regulation. These include transient interactions with the receptor cytoplasmic tail as well as the N-terminal domain locating to the synaptic cleft, both of which are involved in AMPAR trafficking and receptor stabilization at the synapse. Moreover, an array of transmembrane proteins operate as auxiliary subunits that in addition to receptor trafficking and stabilization also substantially impact AMPAR gating and pharmacology. Here, we provide an overview of the catalogue of AMPAR interacting proteins, and how they contribute to the complex biology of this central glutamate receptor.

Functions of Oxysterol-Binding Proteins at Membrane Contact Sites and Their Control by Phosphoinositide Metabolism

Lipids must be correctly transported within the cell to the right place at the right time in order to be fully functional. Non-vesicular lipid transport is mediated by so-called lipid transfer proteins (LTPs), which contain a hydrophobic cavity that sequesters lipid molecules. Oxysterol-binding protein (OSBP)-related proteins (ORPs) are a family of LTPs known to harbor lipid ligands, such as cholesterol and phospholipids. ORPs act as a sensor or transporter of those lipid ligands at membrane contact sites (MCSs) where two different cellular membranes are closely apposed. In particular, a characteristic functional property of ORPs is their role as a lipid exchanger. ORPs mediate counter-directional transport of two different lipid ligands at MCSs. Several, but not all, ORPs transport their lipid ligand from the endoplasmic reticulum (ER) in exchange for phosphatidylinositol 4-phosphate (PI4P), the other ligand, on apposed membranes. This ORP-mediated lipid “countertransport” is driven by the concentration gradient of PI4P between membranes, which is generated by its kinases and phosphatases. In this review, we will discuss how ORP function is tightly coupled to metabolism of phosphoinositides such as PI4P. Recent progress on the role of ORP-mediated lipid transport/countertransport at multiple MCSs in cellular functions will be also discussed.

Phosphoregulation of the autophagy machinery by kinases and phosphatases

Eukaryotic cells use post-translational modifications to diversify and dynamically coordinate the function and properties of protein networks within various cellular processes. For example, the process of autophagy strongly depends on the balanced action of kinases and phosphatases. Highly conserved from the budding yeast Saccharomyces cerevisiae to humans, autophagy is a tightly regulated self-degradation process that is crucial for survival, stress adaptation, maintenance of cellular and organismal homeostasis, and cell differentiation and development. Many studies have emphasized the importance of kinases and phosphatases in the regulation of autophagy and identified many of the core autophagy proteins as their direct targets. In this review, we summarize the current knowledge on kinases and phosphatases acting on the core autophagy machinery and discuss the relevance of phosphoregulation for the overall process of autophagy.

Endoplasmic Reticulum-Plasma Membrane Contact Sites as an Organizing Principle for Compartmentalized Calcium and cAMP Signaling

In eukaryotic cells, ultimate specificity in activation and action-for example, by means of second messengers-of the myriad of signaling cascades is primordial. In fact, versatile and ubiquitous second messengers, such as calcium (Ca²⁺) and cyclic adenosine monophosphate (cAMP), regulate multiple-sometimes opposite-cellular functions in a specific spatiotemporal manner. Cells achieve this through segregation of the initiators and modulators to specific plasma membrane (PM) subdomains, such as lipid rafts and caveolae, as well as by dynamic close contacts between the endoplasmic reticulum (ER) membrane and other intracellular organelles, including the PM. Especially, these membrane contact sites (MCSs) are currently receiving a lot of attention as their large influence on cell signaling regulation and cell physiology is increasingly appreciated. Depletion of ER Ca²⁺ stores activates ER membrane STIM proteins, which activate PM-residing Orai and TRPC Ca²⁺ channels at ER-PM contact sites. Within the MCS, Ca²⁺ fluxes relay to cAMP signaling through highly interconnected networks. However, the precise mechanisms of MCS formation and the influence of their dynamic lipid environment on their functional maintenance are not completely understood. The current review aims to provide an overview of our current understanding and to identify open questions of the field.

Sensing of nutrients by CPT1C controls SAC1 activity to regulate AMPA receptor trafficking

Carnitine palmitoyltransferase 1C (CPT1C) is a sensor of malonyl-CoA and is located in the ER of neurons. AMPA receptors (AMPARs) mediate fast excitatory neurotransmission in the brain and play a key role in synaptic plasticity. In the present study, we demonstrate across different metabolic stress conditions that modulate malonyl-CoA levels in cortical neurons that CPT1C regulates the trafficking of the major AMPAR subunit, GluA1, through the phosphatidyl-inositol-4-phosphate (PI(4)P) phosphatase SAC1. In normal conditions, CPT1C down-regulates SAC1 catalytic activity, allowing efficient GluA1 trafficking to the plasma membrane. However, under low malonyl-CoA levels, such as during glucose depletion, CPT1C-dependent inhibition of SAC1 is released, facilitating SAC1’s translocation to ER-TGN contact sites to decrease TGN PI(4)P pools and trigger GluA1 retention at the TGN. Results reveal that GluA1 trafficking is regulated by CPT1C sensing of malonyl-CoA and provide the first report of a SAC1 inhibitor. Moreover, they shed light on how nutrients can affect synaptic function and cognition.

Endoplasmic Reticulum-Plasma Membrane Contact Sites: Regulators, Mechanisms, and Physiological Functions

The endoplasmic reticulum (ER) forms direct membrane contact sites with the plasma membrane (PM) in eukaryotic cells. These ER-PM contact sites play essential roles in lipid homeostasis, ion dynamics, and cell signaling, which are carried out by protein-protein or protein-lipid interactions. Distinct tethering factors dynamically control the architecture of ER-PM junctions in response to intracellular signals or external stimuli. The physiological roles of ER-PM contact sites are dependent on a variety of regulators that individually or cooperatively perform functions in diverse cellular processes. This review focuses on proteins functioning at ER-PM contact sites and highlights the recent progress in their mechanisms and physiological roles.

The function of apolipoproteins L (APOLs): relevance for kidney disease, neurotransmission disorders, cancer and viral infection

The discovery that apolipoprotein L1 (APOL1) is the trypanolytic factor of human serum raised interest about the function of APOLs, especially following the unexpected finding that in addition to their protective action against sleeping sickness, APOL1 C-terminal variants also cause kidney disease. Based on the analysis of the structure and trypanolytic activity of APOL1, it was proposed that APOLs could function as ion channels of intracellular membranes and be involved in mechanisms triggering programmed cell death. In this review, the recent finding that APOL1 and APOL3 inversely control the synthesis of phosphatidylinositol-4-phosphate (PI(4)P) by the Golgi PI(4)-kinase IIIB (PI4KB) is commented. APOL3 promotes Ca2+ -dependent activation of PI4KB, but due to their increased interaction with APOL3, APOL1 C-terminal variants can inactivate APOL3, leading to reduction of Golgi PI(4)P synthesis. The impact of APOLs on several pathological processes that depend on Golgi PI(4)P levels is discussed. I propose that through their effect on PI4KB activity, APOLs control not only actomyosin activities related to vesicular trafficking, but also the generation and elongation of autophagosomes induced by inflammation.

Machinery, regulation and pathophysiological implications of autophagosome maturation

Autophagy is a versatile degradation system for maintaining cellular homeostasis whereby cytosolic materials are sequestered in a double-membrane autophagosome and subsequently delivered to lysosomes, where they are broken down. In multicellular organisms, newly formed autophagosomes undergo a process called 'maturation', in which they fuse with vesicles originating from endolysosomal compartments, including early/late endosomes and lysosomes, to form amphisomes, which eventually become degradative autolysosomes. This fusion process requires the concerted actions of multiple regulators of membrane dynamics, including SNAREs, tethering proteins and RAB GTPases, and also transport of autophagosomes and late endosomes/lysosomes towards each other. Multiple mechanisms modulate autophagosome maturation, including post-translational modification of key components, spatial distribution of phosphoinositide lipid species on membranes, RAB protein dynamics, and biogenesis and function of lysosomes. Nutrient status and various stresses integrate into the autophagosome maturation machinery to coordinate the progression of autophagic flux. Impaired autophagosome maturation is linked to the pathogenesis of various human diseases, including neurodegenerative disorders, cancer and myopathies. Furthermore, invading pathogens exploit various strategies to block autophagosome maturation, thus evading destruction and even subverting autophagic vacuoles (autophagosomes, amphisomes and autolysosomes) for survival, growth and/or release. Here, we discuss the recent progress in our understanding of the machinery and regulation of autophagosome maturation, the relevance of these mechanisms to human pathophysiology and how they are harnessed by pathogens for their benefit. We also provide perspectives on targeting autophagosome maturation therapeutically.

A model of the PI cycle reveals the regulating roles of lipid-binding proteins and pitfalls of using mosaic biological data

The phosphatidylinositol (PI) cycle is central to eukaryotic cell signaling. Its complexity, due to the number of reactions and lipid and inositol phosphate intermediates involved makes it difficult to analyze experimentally. Computational modelling approaches are seen as a way forward to elucidate complex biological regulatory mechanisms when this cannot be achieved solely through experimental approaches. Whilst mathematical modelling is well established in informing biological systems, many models are often informed by data sourced from multiple unrelated cell types (mosaic data) or from purified enzyme data. In this work, we develop a model of the PI cycle informed by experimental and omics data taken from a single cell type, namely platelets. We were able to make a number of predictions regarding the regulation of PI cycle enzymes, the importance of the number of receptors required for successful GPCR signaling and the importance of lipid- and protein-binding proteins in regulating second messenger outputs. We then consider how pathway behavior differs, when fully informed by data for HeLa cells and show that model predictions remain consistent. However, when informed by mosaic experimental data model predictions greatly vary illustrating the risks of using mosaic datasets from unrelated cell types.

PtdIns4P restriction by hydrolase SAC1 decides specific fusion of autophagosomes with lysosomes

Biogenesis of autophagosomes is the early step of macroautophagy/autophagy and requires membrane acquisition mainly from ER-Golgi-sourced precursor vesicles. Matured autophagosomes fuse with lysosomes for final degradation. However, how this selective fusion is determined remains elusive. Here, we identified Sac1 by a high throughput screen in Saccharomyces cerevisiae to show it was critical for autophagosome-lysosome fusion through its PtdIns4P phosphatase activity. Sac1 deficiency caused a dramatic increase of PtdIns4P at early Golgi apparatus and abnormal incorporation of PtdIns4P into Atg9 vesicles and autophagosomes, which caused failure to recruit SNARE proteins for autophagosome fusion with vacuoles. Sac1 function in autophagy was highly conserved from yeast to mammalian cells. Our work thus suggested that correct upstream lipid incorporation was important for downstream fusion step of autophagy and that Sac1 played a critical and ancient role in this surveillance of lipid integration.Abbreviations: Ape1: aminopeptidase Ι; ATG: autophagy related; EBSS: Earle's balanced salt solution; ER: endoplasmic reticulum; ERGIC: Golgi apparatus and ER-Golgi intermediate compartment; HOPS: homotypic fusion and protein sorting complex; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; PtdIns3P: phosphatidylinositol-3-phosphate; PtdIns4K: phosphoinositide-4-kinase; PtdIns4P: phosphatidylinositol-4-phosphate; SD-N: nitrogen starvation medium; SNARE: soluble N-ethylamide-sensitive factor attachment protein receptor.

Co-opted Cellular Sac1 Lipid Phosphatase and PI(4)P Phosphoinositide Are Key Host Factors during the Biogenesis of the Tombusvirus Replication Compartment

Positive-strand RNA [(+)RNA] viruses assemble numerous membrane-bound viral replicase complexes (VRCs) with the help of viral replication proteins and co-opted host proteins within large viral replication compartments in the cytosol of infected cells. In this study, we found that deletion or depletion of Sac1 phosphatidylinositol 4-phosphate [PI(4)P] phosphatase reduced tomato bushy stunt virus (TBSV) replication in yeast (Saccharomyces cerevisiae) and plants. We demonstrate a critical role for Sac1 in TBSV replicase assembly in a cell-free replicase reconstitution assay. The effect of Sac1 seems to be direct, based on its interaction with the TBSV p33 replication protein, its copurification with the tombusvirus replicase, and its presence in the virus-induced membrane contact sites and within the TBSV replication compartment. The proviral functions of Sac1 include manipulation of lipid composition, sterol enrichment within the VRCs, and recruitment of additional host factors into VRCs. Depletion of Sac1 inhibited the recruitment of Rab5 GTPase-positive endosomes and enrichment of phosphatidylethanolamine in the viral replication compartment. We propose that Sac1 might be a component of the assembly hub for VRCs, likely in collaboration with the co-opted the syntaxin18-like Ufe1 SNARE protein within the TBSV replication compartments. This work also led to demonstration of the enrichment of PI(4)P phosphoinositide within the replication compartment. Reduction in the PI(4)P level due to chemical inhibition in plant protoplasts; depletion of two PI(4)P kinases, Stt4p and Pik1p; or sequestration of free PI(4)P via expression of a PI(4)P-binding protein in yeast strongly inhibited TBSV replication. Altogether, Sac1 and PI(4)P play important proviral roles during TBSV replication.IMPORTANCE Replication of positive-strand RNA viruses depends on recruitment of host components into viral replication compartments or organelles. Using TBSV, we uncovered the critical roles of Sac1 PI(4)P phosphatase and its substrate, PI(4)P phosphoinositide, in promoting viral replication. Both Sac1 and PI(4)P are recruited to the site of viral replication to facilitate the assembly of the viral replicase complexes, which perform viral RNA replication. We found that Sac1 affects the recruitment of other host factors and enrichment of phosphatidylethanolamine and sterol lipids within the subverted host membranes to promote optimal viral replication. In summary, this work demonstrates the novel functions of Sac1 and PI(4)P in TBSV replication in the model host yeast and in plants.

Cellular homeostasis in the Drosophila retina requires the lipid phosphatase Sac1

The complex functions of cellular membranes, and thus overall cell physiology, depend on the distribution of crucial lipid species. Sac1 is an essential, conserved, ER-localized phosphatase whose substrate, phosphatidylinositol 4-phosphate (PI4P), coordinates secretory trafficking and plasma membrane function. PI4P from multiple pools is delivered to Sac1 by oxysterol-binding protein and related proteins in exchange for other lipids and sterols, which places Sac1 at the intersection of multiple lipid distribution pathways. However, much remains unknown about the roles of Sac1 in subcellular homeostasis and organismal development. Using a temperature-sensitive allele (Sac1^ts), we show that Sac1 is required for structural integrity of the Drosophila retinal floor. The β_ps-integrin Myospheroid, which is necessary for basal cell adhesion, is mislocalized in Sac1^ts retinas. In addition, the adhesion proteins Roughest and Kirre, which coordinate apical retinal cell patterning at an earlier stage, accumulate within Sac1^ts retinal cells due to impaired endo-lysosomal degradation. Moreover, Sac1 is required for ER homeostasis in Drosophila retinal cells. Together, our data illustrate the importance of Sac1 in regulating multiple aspects of cellular homeostasis during tissue development.

APOL1 C-Terminal Variants May Trigger Kidney Disease through Interference with APOL3 Control of Actomyosin

The C-terminal variants G1 and G2 of apolipoprotein L1 (APOL1) confer human resistance to the sleeping sickness parasite Trypanosoma rhodesiense, but they also increase the risk of kidney disease. APOL1 and APOL3 are death-promoting proteins that are partially associated with the endoplasmic reticulum and Golgi membranes. We report that in podocytes, either APOL1 C-terminal helix truncation (APOL1Δ) or APOL3 deletion (APOL3KO) induces similar actomyosin reorganization linked to the inhibition of phosphatidylinositol-4-phosphate [PI(4)P] synthesis by the Golgi PI(4)-kinase IIIB (PI4KB). Both APOL1 and APOL3 can form K+ channels, but only APOL3 exhibits Ca2+-dependent binding of high affinity to neuronal calcium sensor-1 (NCS-1), promoting NCS-1-PI4KB interaction and stimulating PI4KB activity. Alteration of the APOL1 C-terminal helix triggers APOL1 unfolding and increased binding to APOL3, affecting APOL3-NCS-1 interaction. Since the podocytes of G1 and G2 patients exhibit an APOL1Δ or APOL3KO-like phenotype, APOL1 C-terminal variants may induce kidney disease by preventing APOL3 from activating PI4KB, with consecutive actomyosin reorganization of podocytes.

ORP3 phosphorylation regulates phosphatidylinositol 4-phosphate and Ca2+ dynamics at plasma membrane-ER contact sites

Oxysterol-binding protein (OSBP)-related proteins (ORPs) mediate non-vesicular lipid transfer between intracellular membranes. Phosphoinositide (PI) gradients play important roles in the ability of OSBP and some ORPs to transfer cholesterol and phosphatidylserine between the endoplasmic reticulum (ER) and other organelle membranes. Here, we show that plasma membrane (PM) association of ORP3 (also known as OSBPL3), a poorly characterized ORP family member, is triggered by protein kinase C (PKC) activation, especially when combined with Ca²⁺ increases, and is determined by both PI(4,5)P ₂ and PI4P After activation, ORP3 efficiently extracts PI4P and to a lesser extent phosphatidic acid from the PM, and slightly increases PM cholesterol levels. Full activation of ORP3 resulted in decreased PM PI4P levels and inhibited Ca²⁺ entry via the store-operated Ca²⁺ entry pathway. The C-terminal region of ORP3 that follows the strictly defined lipid transfer domain was found to be critical for the proper localization and function of the protein.

Integrated regulation of the phosphatidylinositol cycle and phosphoinositide-driven lipid transport at ER-PM contact sites

Among the structural phospholipids that form the bulk of eukaryotic cell membranes, phosphatidylinositol (PtdIns) is unique in that it also serves as the common precursor for low-abundance regulatory lipids, collectively referred to as polyphosphoinositides (PPIn). The metabolic turnover of PPIn species has received immense attention because of the essential functions of these lipids as universal regulators of membrane biology and their dysregulation in numerous human pathologies. The diverse functions of PPIn lipids occur, in part, by orchestrating the spatial organization and conformational dynamics of peripheral or integral membrane proteins within defined subcellular compartments. The emerging role of stable contact sites between adjacent membranes as specialized platforms for the coordinate control of ion exchange, cytoskeletal dynamics, and lipid transport has also revealed important new roles for PPIn species. In this review, we highlight the importance of membrane contact sites formed between the endoplasmic reticulum (ER) and plasma membrane (PM) for the integrated regulation of PPIn metabolism within the PM. Special emphasis will be placed on non-vesicular lipid transport during control of the PtdIns biosynthetic cycle as well as toward balancing the turnover of the signaling PPIn species that define PM identity.

Trafficking of Stretch-Regulated TRPV2 and TRPV4 Channels Inferred Through Interactomics

Transient receptor potential cation channels are emerging as important physiological and therapeutic targets. Within the vanilloid subfamily, transient receptor potential vanilloid 2 (TRPV2) and 4 (TRPV4) are osmo- and mechanosensors becoming critical determinants in cell structure and activity. However, knowledge is scarce regarding how TRPV2 and TRPV4 are trafficked to the plasma membrane or specific organelles to undergo quality controls through processes such as biosynthesis, anterograde/retrograde trafficking, and recycling. This review lists and reviews a subset of protein–protein interactions from the TRPV2 and TRPV4 interactomes, which is related to trafficking processes such as lipid metabolism, phosphoinositide signaling, vesicle-mediated transport, and synaptic-related exocytosis. Identifying the protein and lipid players involved in trafficking will improve the knowledge on how these stretch-related channels reach specific cellular compartments.

Phosphoinositide phosphatases in cancer cell dynamics-Beyond PI3K and PTEN

Phosphoinositides are a group of lipids that regulate intracellular signaling and subcellular biological events. The signaling by phosphatidylinositol-3,4,5-trisphosphate and Akt mediates the action of growth factors that are essential for cell proliferation, gene transcription, cell migration, and polarity. The hyperactivation of this signaling has been identified in different cancer cells; and, it has been implicated in oncogenic transformation and cancer cell malignancy. Recent studies have argued the role of phosphoinositides in cancer cell dynamics, including actin cytoskeletal rearrangement at the plasma membrane and the organization of intracellular compartments. The focus of this review is to summarize the impact of the activities of phosphoinositide phosphatases on intracellular signaling related to cancer cell dynamics and to discuss how the abnormalities in the activities of the enzymes alter the levels of phosphoinositides in cancer cells.

Inactivation of the PtdIns(4)P phosphatase Sac1 at the Golgi by H2O2 produced via Ca2+-dependent Duox in EGF-stimulated cells

Binding of epidermal growth factor (EGF) to its cell surface receptor induces production of H₂O₂, which serves as an intracellular messenger. We have shown that exogenous H₂O₂ reversibly inactivates the phosphatidylinositol 4-phosphate [PtdIns(4)P] phosphatase Sac1 (suppressor of actin 1) at the Golgi complex of mammalian cells by oxidizing its catalytic cysteine residue and thereby increases both the amount of Golgi PtdIns(4)P and the rate of protein secretion. Here we investigated the effects of EGF on Sac1 oxidation and PtdIns(4)P abundance at the Golgi in A431 cells. EGF induced a transient increase in Golgi PtdIns(4)P as well as a transient oxidation of Sac1 in a manner dependent on elevation of the intracellular Ca²⁺ concentration and on H₂O₂. Oxidation of Sac1 occurred at the Golgi, as revealed with the use of the Golgi-confined Sac1-K2A mutant. Knockdown of Duox enzymes implicated these Ca²⁺-dependent members of the NADPH oxidase family as the major source of H₂O₂ for Sac1 oxidation. Expression of a Golgi-targeted H₂O₂ probe revealed transient EGF-induced H₂O₂ production at this organelle. Our findings have thus uncovered a previously unrecognized EGF signaling pathway that links intracellular Ca²⁺ mobilization to events at the Golgi including Duox activation, H₂O₂ production, Sac1 oxidation, and PtdIns(4)P accumulation.

Accumulation of PtdIns(4)P at the Golgi mediated by reversible oxidation of the PtdIns(4)P phosphatase Sac1 by H2O2

Phosphatidylinositol 4-phosphate [PtdIns(4)P] plays a key role in the biogenesis of transport vesicles at the Golgi complex by recruiting coat proteins and their accessory factors. The PtdIns(4)P content of the Golgi is determined by the concerted action of PtdIns 4-kinase (PI4K) and PtdIns(4)P phosphatase enzymes. Sac1 (suppressor of actin 1) is the major PtdIns(4)P phosphatase and is localized to the Golgi and endoplasmic reticulum. The targeting of both PI4Ks and Sac1 to the Golgi membrane is extensively regulated, as is the catalytic activity of PI4Ks at the Golgi. However, regulation of the catalytic activity of Sac1 has been largely unexplored. Here we show that Sac1undergoes reversible inactivation in mammalian cells when its catalytic Cys³⁸⁹ residue is oxidized by exogenous H₂O₂ to form an intramolecular disulfide with Cys³⁹². The oxidative inactivation of Sac1 results in the accumulation of PtdIns(4)P at the Golgi, with this effect also being supported by the H₂O₂-induced activation of p38 mitogen-activated protein kinase (MAPK), which was previously shown to promote the translocation of Sac1 from the Golgi to the endoplasmic reticulum. The increase in Golgi PtdIns(4)P due to Sac1 inactivation, however, is faster than that due to Sac1 translocation. Exposure of cells to H₂O₂ also increased membrane protein trafficking from the Golgi to the plasma membrane as well as protein secretion.