Spatial regulation of Golgi phosphatidylinositol-4-phosphate is required for enzyme localization and glycosylation fidelity

Early signs of neurodegenerative diseases: Possible mechanisms and targets for Golgi stress

The Golgi apparatus plays a crucial role in mediating the modification, transport, and sorting of intracellular proteins and lipids. The morphological changes occurring in the Golgi apparatus are exceptionally important for maintaining its function. When exposed to external pressure or environmental stimulation, the Golgi apparatus undergoes adaptive changes in both structure and function, which are known as Golgi stress. Although certain signal pathway responses or post-translational modifications have been observed following Golgi stress, further research is needed to comprehensively summarize and understand the related mechanisms. Currently, there is evidence linking Golgi stress to neurodegenerative diseases; however, the role of Golgi stress in the progression of neurodegenerative diseases such as Alzheimer's disease remains largely unexplored. This review focuses on the structural and functional alterations of the Golgi apparatus during stress, elucidating potential mechanisms underlying the involvement of Golgi stress in regulating immunity, autophagy, and metabolic processes. Additionally, it highlights the pivotal role of Golgi stress as an early signaling event implicated in the pathogenesis and progression of neurodegenerative diseases. Furthermore, this study summarizes prospective targets that can be therapeutically exploited to mitigate neurodegenerative diseases by targeting Golgi stress. These findings provide a theoretical foundation for identifying novel breakthroughs in preventing and treating neurodegenerative diseases.

Orthogonal targeting of SAC1 to mitochondria implicates ORP2 as a major player in PM PI4P turnover

Oxysterol binding protein (OSBP)-related proteins (ORPs) 5 and 8 have been shown to deplete the lipid phosphatidylinositol 4-phosphate (PI4P) at sites of membrane contact between the endoplasmic reticulum (ER) and plasma membrane (PM). This is believed to be caused by transport of PI4P from the PM to the ER, where PI4P is degraded by an ER-localized SAC1 phosphatase. This is proposed to power the anti-port of phosphatidylserine (PS) lipids from ER to PM, up their concentration gradient. Alternatively, ORPs have been proposed to sequester PI4P, dependent on the concentration of their alternative lipid ligand. Here, we aimed to distinguish these possibilities in living cells by orthogonal targeting of PI4P transfer and degradation to PM-mitochondria contact sites. Surprisingly, we found that orthogonal targeting of SAC1 to mitochondria enhanced PM PI4P turnover independent of targeting to contact sites with the PM. This turnover could be slowed by knock-down of soluble ORP2, which also has a major impact on PM PI4P levels even without SAC1 over-expression. The data reveal a role for contact site-independent modulation of PM PI4P levels and lipid antiport.

Orthogonal Targeting of SAC1 to Mitochondria Implicates ORP2 as a Major Player in PM PI4P Turnover

Oxysterol-binding protein (OSBP)-related proteins (ORPs) 5 and 8 have been shown to deplete the lipid phosphatidylinositol 4-phosphate (PI4P) at sites of membrane contact between the endoplasmic reticulum (ER) and plasma membrane (PM). This is believed to be caused by transport of PI4P from the PM to the ER, where PI4P is degraded by an ER-localized SAC1 phosphatase. This is proposed to power the anti-port of phosphatidylserine (PS) lipids from ER to PM, up their concentration gradient. Alternatively, ORPs have been proposed to sequester PI4P, dependent on the concentration of their alternative lipid ligand. Here, we aimed to distinguish these possibilities in living cells by orthogonal targeting of PI4P transfer and degradation to PM-mitochondria contact sites. Surprisingly, we found that orthogonal targeting of SAC1 to mitochondria enhanced PM PI4P turnover independent of targeting to contact sites with the PM. This turnover could be slowed by knock-down of soluble ORP2, which also has a major impact on PM PI4P levels even without SAC1 over-expression. The data reveal a role for contact site-independent modulation of PM PI4P levels and lipid antiport.

Phosphoinositides in New Spaces

Phosphoinositides (PIs) are phospholipids derived from phosphatidylinositol. PIs are regulated via reversible phosphorylation, which is directed by the opposing actions of PI kinases and phosphatases. PIs constitute a minor fraction of the total cellular lipid pool but play pleiotropic roles in multiple aspects of cell biology. Genetic mutations of PI regulatory enzymes have been identified in rare congenital developmental syndromes, including ciliopathies, and in numerous human diseases, such as cancer and metabolic and neurological disorders. Accordingly, PI regulatory enzymes have been targeted in the design of potential therapeutic interventions for human diseases. Recent advances place PIs as central regulators of membrane dynamics within functionally distinct subcellular compartments. This brief review focuses on the emerging role PIs play in regulating cell signaling within the primary cilium and in directing transfer of molecules at interorganelle membrane contact sites and identifies new roles for PIs in subcellular spaces.

A Plethora of Functions Condensed into Tiny Phospholipids: The Story of PI4P and PI(4,5)P2

Phosphoinositides (PIs) are small, phosphorylated lipids that serve many functions in the cell. They regulate endo- and exocytosis, vesicular trafficking, actin reorganization, and cell mobility, and they act as signaling molecules. The most abundant PIs in the cell are phosphatidylinositol-4-monophosphate (PI4P) and phosphatidylinositol-4,5-bisphosphate [PI(4,5)P₂]. PI4P is mostly localized at the Golgi apparatus where it regulates the anterograde trafficking from the Golgi apparatus to the plasma membrane (PM), but it also localizes at the PM. On the other hand, the main localization site of PI(4,5)P₂ is the PM where it regulates the formation of endocytic vesicles. The levels of PIs are regulated by many kinases and phosphatases. Four main kinases phosphorylate the precursor molecule phosphatidylinositol into PI4P, divided into two classes (PI4KIIα, PI4KIIβ, PI4KIIIα, and PI4KIIIβ), and three main kinases phosphorylate PI4P to form PI(4,5)P₂ (PI4P5KIα, PI4P5KIβ, and PI4P5KIγ). In this review, we discuss the localization and function of the kinases that produce PI4P and PI(4,5)P₂, as well as the localization and function of their product molecules with an overview of tools for the detection of these PIs.

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.

Pathogen vacuole membrane contact sites - close encounters of the fifth kind

Vesicular trafficking and membrane fusion are well-characterized, versatile, and sophisticated means of 'long range' intracellular protein and lipid delivery. Membrane contact sites (MCS) have been studied in far less detail, but are crucial for 'short range' (10-30 nm) communication between organelles, as well as between pathogen vacuoles and organelles. MCS are specialized in the non-vesicular trafficking of small molecules such as calcium and lipids. Pivotal MCS components important for lipid transfer are the VAP receptor/tether protein, oxysterol binding proteins (OSBPs), the ceramide transport protein CERT, the phosphoinositide phosphatase Sac1, and the lipid phosphatidylinositol 4-phosphate (PtdIns(4)P). In this review, we discuss how these MCS components are subverted by bacterial pathogens and their secreted effector proteins to promote intracellular survival and replication.

Imaging of Intracellular and Plasma Membrane Pools of PI(4,5)P2 and PI4P in Human Platelets

Phosphoinositides (PIs) are phosphorylated membrane lipids that have a plethora of roles in the cell, including vesicle trafficking, signaling, and actin reorganization. The most abundant PIs in the cell are phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] and phosphatidylinositol-4-monophosphate (PI4P). The localization and roles of both PI(4,5)P2 and PI4P are well established, is the broadly accepted methodological approach for their immunocytochemical visualization in different cell compartments in several cell lines. However, not much is known about these PIs in platelets (PLTs), the smallest blood cells that detect vessel wall injury, activate, and stop the bleeding. Therefore, we sought to investigate the localization of PI(4,5)P2 and PI4P in resting and activated PLTs by antibody staining. Here, we show that the intracellular pools of PI(4,5)P2 and PI4P can be detected by the established staining protocol, and these pools can be modulated by inhibitors of OCRL phosphatase and PI4KIIIα kinase. However, although resting PLTs readily stain for the plasma membrane (PM) pools of PI(4,5)P2 and PI4P, just a few activated cells were stained with the established protocol. We show that optimized protocol allows for the visualization of PI(4,5)P2 and PI4P at PM in activated PLTs, which could also be modulated by OCRL and PI4KIIIα inhibitors. We conclude that PI(4,5)P2 and PI4P are more sensitive to lipid extraction by permeabilizing agents in activated than in resting human PLTs, which suggests their different roles during PLT activation.

GOLPH3 and GOLPH3L are broad-spectrum COPI adaptors for sorting into intra-Golgi transport vesicles

The fidelity of Golgi glycosylation is, in part, ensured by compartmentalization of enzymes within the stack. The COPI adaptor GOLPH3 has been shown to interact with the cytoplasmic tails of a subset of Golgi enzymes and direct their retention. However, other mechanisms of retention, and other roles for GOLPH3, have been proposed, and a comprehensive characterization of the clientele of GOLPH3 and its paralogue GOLPH3L is lacking. GOLPH3's role is of particular interest as it is frequently amplified in several solid tumor types. Here, we apply two orthogonal proteomic methods to identify GOLPH3+3L clients and find that they act in diverse glycosylation pathways or have other roles in the Golgi. Binding studies, bioinformatics, and a Golgi retention assay show that GOLPH3+3L bind the cytoplasmic tails of their clients through membrane-proximal positively charged residues. Furthermore, deletion of GOLPH3+3L causes multiple defects in glycosylation. Thus, GOLPH3+3L are major COPI adaptors that impinge on most, if not all, of the glycosylation pathways of the Golgi.

Should I stay or should I go? Golgi membrane spatial organization for protein sorting and retention

The Golgi complex is the membrane-bound organelle that lies at the center of the secretory pathway. Its main functions are to maintain cellular lipid homeostasis, to orchestrate protein processing and maturation, and to mediate protein sorting and export. These functions are not independent of one another, and they all require that the membranes of the Golgi complex have a well-defined biochemical composition. Importantly, a finely-regulated spatiotemporal organization of the Golgi membrane components is essential for the correct performance of the organelle. In here, we review our current mechanistic and molecular understanding of how Golgi membranes are spatially organized in the lateral and axial directions to fulfill their functions. In particular, we highlight the current evidence and proposed models of intra-Golgi transport, as well as the known mechanisms for the retention of Golgi residents and for the sorting and export of transmembrane cargo proteins. Despite the controversies, conflicting evidence, clashes between models, and technical limitations, the field has moved forward and we have gained extensive knowledge in this fascinating topic. However, there are still many important questions that remain to be completely answered. We hope that this review will help boost future investigations on these issues.

NCP2/RHD4/SAC7, SAC6 and SAC8 phosphoinositide phosphatases are required for PtdIns4P and PtdIns(4,5)P2 homeostasis and Arabidopsis development

Phosphoinositides play important roles in plant growth and development. Several SAC domain phosphoinositide phosphatases have been reported to be important for plant development. Here, we show functional analysis of SUPPRESSOR OF ACTIN 6 (SAC6) to SAC8 in Arabidopsis, a subfamily of phosphoinositide phosphatases containing SAC-domain and two transmembrane motifs. We isolated an Arabidopsis mutant ncp2 that lacked cotyledons in seedling and embryo in pid, a background defective in auxin signaling and transport. NCP2 encodes RHD4/SAC7 phosphoinositide phosphatase. SAC6, SAC7 and SAC8 exhibit overlapping and specific expression patterns in seedling and embryo. The sac6 sac7 embryos either fail to develop into seeds, or have three or four cotyledons. The embryo development of sac7 sac8 and sac6 sac7 sac8 mutants is significantly delayed or lethal, and the seedlings are arrested at early stages. Auxin maxima are decreased in double and triple sac mutants. The contents of PtdIns4P and PtdIns(4,5)P2 in sac6 sac7 and sac7 sac8 mutants are dramatically increased. Protein trafficking of the plasma membrane (PM)-localized protein PIN1 and PIN2 from trans-Golgi network/early endosome back to PM is delayed in sac7 sac8 mutants. These results indicate that SAC6-SAC8 are essential for maintaining homeostasis of PtdIns4P and PtdIns(4,5)P2, and auxin-mediated development in Arabidopsis.

The ER cholesterol sensor SCAP promotes CARTS biogenesis at ER-Golgi membrane contact sites

In response to cholesterol deprivation, SCAP escorts SREBP transcription factors from the endoplasmic reticulum to the Golgi complex for their proteolytic activation, leading to gene expression for cholesterol synthesis and uptake. Here, we show that in cholesterol-fed cells, ER-localized SCAP interacts through Sac1 phosphatidylinositol 4-phosphate (PI4P) phosphatase with a VAP-OSBP complex, which mediates counter-transport of ER cholesterol and Golgi PI4P at ER-Golgi membrane contact sites (MCSs). SCAP knockdown inhibited the turnover of PI4P, perhaps due to a cholesterol transport defect, and altered the subcellular distribution of the VAP-OSBP complex. As in the case of perturbation of lipid transfer complexes at ER-Golgi MCSs, SCAP knockdown inhibited the biogenesis of the trans-Golgi network-derived transport carriers CARTS, which was reversed by expression of wild-type SCAP or a Golgi transport-defective mutant, but not of cholesterol sensing-defective mutants. Altogether, our findings reveal a new role for SCAP under cholesterol-fed conditions in the facilitation of CARTS biogenesis via ER-Golgi MCSs, depending on the ER cholesterol.

Regulation of V-ATPase Activity and Organelle pH by Phosphatidylinositol Phosphate Lipids

Luminal pH and the distinctive distribution of phosphatidylinositol phosphate (PIP) lipids are central identifying features of organelles in all eukaryotic cells that are also critical for organelle function. V-ATPases are conserved proton pumps that populate and acidify multiple organelles of the secretory and the endocytic pathway. Complete loss of V-ATPase activity causes embryonic lethality in higher animals and conditional lethality in yeast, while partial loss of V-ATPase function is associated with multiple disease states. On the other hand, many cancer cells increase their virulence by upregulating V-ATPase expression and activity. The pH of individual organelles is tightly controlled and essential for function, but the mechanisms for compartment-specific pH regulation are not completely understood. There is substantial evidence indicating that the PIP content of membranes influences organelle pH. We present recent evidence that PIPs interact directly with subunit isoforms of the V-ATPase to dictate localization of V-ATPase subpopulations and participate in their regulation. In yeast cells, which have only one set of organelle-specific V-ATPase subunit isoforms, the Golgi-enriched lipid PI(4)P binds to the cytosolic domain of the Golgi-enriched a-subunit isoform Stv1, and loss of PI(4)P binding results in mislocalization of Stv1-containing V-ATPases from the Golgi to the vacuole/lysosome. In contrast, levels of the vacuole/lysosome-enriched signaling lipid PI(3,5)P₂ affect assembly and activity of V-ATPases containing the Vph1 a-subunit isoform. Mutations in the Vph1 isoform that disrupt the lipid interaction increase sensitivity to stress. These studies have decoded “zip codes” for PIP lipids in the cytosolic N-terminal domain of the a-subunit isoforms of the yeast V-ATPase, and similar interactions between PIP lipids and the V-ATPase subunit isoforms are emerging in higher eukaryotes. In addition to direct effects on the V-ATPase, PIP lipids are also likely to affect organelle pH indirectly, through interactions with other membrane transporters. We discuss direct and indirect effects of PIP lipids on organelle pH, and the functional consequences of the interplay between PIP lipid content and organelle pH.

ER-Golgi membrane contact sites

Membrane contact sites (MCSs) are sites where the membranes of two different organelles come into close apposition (10–30 nm). Different classes of proteins populate MCSs including factors that act as tethers between the two membranes, proteins that use the MCSs for their function (mainly lipid or ion exchange), and regulatory proteins and enzymes that can act in trans across the MCSs. The ER-Golgi MCSs were visualized by electron microscopists early in the sixties but have remained elusive for decades due to a lack of suitable methodological approaches. Here we report recent progress in the study of this class of MCSs that has led to the identification of their main morphological features and of some of their components and roles. Among these, lipid transfer proteins and lipid exchange have been the most studied and understood so far. However, many unknowns remain regarding their regulation and their role in controlling key TGN functions such as sorting and trafficking as well as their relevance in physiological and pathological conditions.

The Great Escape: how phosphatidylinositol 4-kinases and PI4P promote vesicle exit from the Golgi (and drive cancer)

Phosphatidylinositol 4-phosphate (PI4P) is a membrane glycerophospholipid and a major regulator of the characteristic appearance of the Golgi complex as well as its vesicular trafficking, signalling and metabolic functions. Phosphatidylinositol 4-kinases, and in particular the PI4KIIIβ isoform, act in concert with PI4P to recruit macromolecular complexes to initiate the biogenesis of trafficking vesicles for several Golgi exit routes. Dysregulation of Golgi PI4P metabolism and the PI4P protein interactome features in many cancers and is often associated with tumour progression and a poor prognosis. Increased expression of PI4P-binding proteins, such as GOLPH3 or PITPNC1, induces a malignant secretory phenotype and the release of proteins that can remodel the extracellular matrix, promote angiogenesis and enhance cell motility. Aberrant Golgi PI4P metabolism can also result in the impaired post-translational modification of proteins required for focal adhesion formation and cell–matrix interactions, thereby potentiating the development of aggressive metastatic and invasive tumours. Altered expression of the Golgi-targeted PI 4-kinases, PI4KIIIβ, PI4KIIα and PI4KIIβ, or the PI4P phosphate Sac1, can also modulate oncogenic signalling through effects on TGN-endosomal trafficking. A Golgi trafficking role for a PIP 5-kinase has been recently described, which indicates that PI4P is not the only functionally important phosphoinositide at this subcellular location. This review charts new developments in our understanding of phosphatidylinositol 4-kinase function at the Golgi and how PI4P-dependent trafficking can be deregulated in malignant disease.

Translation of genome to glycome: role of the Golgi apparatus

Glycans are one of the four biopolymers of the cell and they play important roles in cellular and organismal physiology. They consist of both linear and branched structures and are synthesized in a nontemplated manner in the secretory pathway of mammalian cells with the Golgi apparatus playing a key role in the process. In spite of the absence of a template, the glycans synthesized by a cell are not a random collection of possible glycan structures but a distribution of specific glycans in defined quantities that is unique to each cell type (Cell type here refers to distinct cell forms present in an organism that can be distinguished based on morphological, phenotypic and/or molecular criteria.) While information to produce cell type-specific glycans is encoded in the genome, how this information is translated into cell type-specific glycome (Glycome refers to the quantitative distribution of all glycan structures present in a given cell type.) is not completely understood. We summarize here the factors that are known to influence the fidelity of glycan biosynthesis and integrate them into known glycosylation pathways so as to rationalize the translation of genetic information to cell type-specific glycome.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

Imaging Lipid Metabolism at the Golgi Complex

The development of fluorescence-based molecular imaging has revolutionized cell biology allowing the visualization of specific biomolecules at the microscopic and, more recently, at the nanoscopic scale while in their relevant biological contexts. Nonetheless, despite the imaging toolkit for biologists interested in exploring the subcellular localization and dynamics of proteins and nucleic acids has expanded exponentially in the last decades, the means to visualize and track lipids in cells did not develop to the same extent until recently. Here we described some basic fluorescence-based techniques that can be used in standard cell biology laboratories to visualize subcellular pools of specific lipids and to evaluate their regional metabolism. Specifically, here we focus on the imaging-based analysis of phosphoinositide and sphingolipid metabolism at the Golgi complex.

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.

The activity of Sac1 across ER-TGN contact sites requires the four-phosphate-adaptor-protein-1

Venditti et al. identify FAPP1 as a new determinant of ER–trans-Golgi network contacts that interacts with the phosphoinositide phosphatase Sac1 and promotes its phosphatase activity. The results suggest that, by controlling PI4P levels, FAPP1 acts as a gatekeeper of cargo Golgi exit.

Phosphatidylinositol-4-phosphate (PI4P), a phosphoinositide with key roles in the Golgi complex, is made by Golgi-associated phosphatidylinositol-4 kinases and consumed by the 4-phosphatase Sac1 that, instead, is an ER membrane protein. Here, we show that the contact sites between the ER and the TGN (ERTGoCS) provide a spatial setting suitable for Sac1 to dephosphorylate PI4P at the TGN. The ERTGoCS, though necessary, are not sufficient for the phosphatase activity of Sac1 on TGN PI4P, since this needs the phosphatidyl-four-phosphate-adaptor-protein-1 (FAPP1). FAPP1 localizes at ERTGoCS, interacts with Sac1, and promotes its in-trans phosphatase activity in vitro. We envision that FAPP1, acting as a PI4P detector and adaptor, positions Sac1 close to TGN domains with elevated PI4P concentrations allowing PI4P consumption. Indeed, FAPP1 depletion induces an increase in TGN PI4P that leads to increased secretion of selected cargoes (e.g., ApoB100), indicating that FAPP1, by controlling PI4P levels, acts as a gatekeeper of Golgi exit.

Tied up: Does altering phosphoinositide-mediated membrane trafficking influence neurodegenerative disease phenotypes?

Phosphoinositides are a class of membrane lipids that are found on several intracellular compartments and play diverse roles inside cells, such as vesicle formation, protein trafficking, endocytosis etc. Intracellular distribution and levels of phosphoinositides are regulated by enzymes that generate and breakdown these lipids as well as other proteins that associate with phosphoinositides. These events lead to differing levels of specific phosphoinositides on different intracellular compartments. At these intracellular locations, phosphoinositides and their associated proteins, such as Rab GTPases, dynamin and BAR domain-containing proteins, regulate a variety of membrane trafficking pathways. Neurodegenerative phenotypes in disorders such as Parkinson's disease (PD) can arise as a consequence of altered or hampered intracellular trafficking. Altered trafficking can cause proteins such as α-synuclein to aggregate intracellularly. Several trafficking pathways are regulated bymaster regulators such as LRRK2,which is known to regulate the activity of phosphoinositide effector proteins. Perturbing either the levels of phosphoinositides or their interactions with different proteins disrupts intracellular trafficking pathways, contributing to phenotypes often observed in disorders such as Alzheimer's or PDs. Thus, studying phosphoinositide regulation and its role in trafficking can give us a deeper understanding of the contribution of disrupted trafficking to neurodegenerative phenotypes.

The phosphoinositide phosphatase Sac1 regulates cell shape and microtubule stability in the developing Drosophila eye

Epithelial patterning in the developing Drosophila melanogaster eye requires the Neph1 homolog Roughest (Rst), an immunoglobulin family cell surface adhesion molecule expressed in interommatidial cells (IOCs). Here, using a novel temperature-sensitive (ts) allele, we show that the phosphoinositide phosphatase Sac1 is also required for IOC patterning. Sac1ts mutants have rough eyes and retinal patterning defects that resemble rst mutants. Sac1ts retinas exhibit elevated levels of phosphatidylinositol 4-phosphate (PI4P), consistent with the role of Sac1 as a PI4P phosphatase. Indeed, genetic rescue and interaction experiments reveal that restriction of PI4P levels by Sac1 is crucial for normal eye development. Rst is delivered to the cell surface in Sac1ts mutants. However, Sac1ts mutant IOCs exhibit severe defects in microtubule organization, associated with accumulation of Rst and the exocyst subunit Sec8 in enlarged intracellular vesicles upon cold fixation ex vivo Together, our data reveal a novel requirement for Sac1 in promoting microtubule stability and suggest that Rst trafficking occurs in a microtubule- and exocyst-dependent manner.

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

The lipid phosphatase Sac1 dephosphorylates phosphatidylinositol 4-phosphate (PI4P), thereby holding levels of this crucial membrane signaling molecule in check. Sac1 regulates multiple cellular processes, including cytoskeletal organization, membrane trafficking and cell signaling. Here, we review the structure and regulation of Sac1, its roles in cell signaling and development and its links to health and disease. Remarkably, many of the diverse roles attributed to Sac1 can be explained by the recent discovery of its requirement at membrane contact sites, where its consumption of PI4P is proposed to drive interorganelle transfer of other cellular lipids, thereby promoting normal lipid homeostasis within cells.

SAC-1 ensures epithelial endocytic recycling by restricting ARF-6 activity

Arf6/ARF-6 is a crucial regulator of the endosomal phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) pool in endocytic recycling. To further characterize ARF-6 regulation, we performed an ARF-6 interactor screen in Caenorhabditis elegans and identified SAC-1, the homologue of the phosphoinositide phosphatase Sac1p in yeast, as a novel ARF-6 partner. In the absence of ARF-6, basolateral endosomes show a loss of SAC-1 staining in epithelial cells. Steady-state cargo distribution assays revealed that loss of SAC-1 specifically affected apical secretory delivery and basolateral recycling. PI(4,5)P2 levels and the endosomal labeling of the ARF-6 effector UNC-16 were significantly elevated in sac-1 mutants, suggesting that SAC-1 functions as a negative regulator of ARF-6. Further analyses revealed an interaction between SAC-1 and the ARF-6-GEF BRIS-1. This interaction outcompeted ARF-6(guanosine diphosphate [GDP]) for binding to BRIS-1 in a concentration-dependent manner. Consequently, loss of SAC-1 promotes the intracellular overlap between ARF-6 and BRIS-1. BRIS-1 knockdown resulted in a significant reduction in PI(4,5)P2 levels in SAC-1-depleted cells. Interestingly, the action of SAC-1 in sequestering BRIS-1 is independent of SAC-1's catalytic activity. Our results suggest that the interaction of SAC-1 with ARF-6 curbs ARF-6 activity by limiting the access of ARF-6(GDP) to its guanine nucleotide exchange factor, BRIS-1.

SAC1 degrades its lipid substrate PtdIns4P in the endoplasmic reticulum to maintain a steep chemical gradient with donor membranes

Gradients of PtdIns4P between organelle membranes and the endoplasmic reticulum (ER) are thought to drive counter-transport of other lipids via non-vesicular traffic. This novel pathway requires the SAC1 phosphatase to degrade PtdIns4P in a 'cis' configuration at the ER to maintain the gradient. However, SAC1 has also been proposed to act in 'trans' at membrane contact sites, which could oppose lipid traffic. It is therefore crucial to determine which mode SAC1 uses in living cells. We report that acute inhibition of SAC1 causes accumulation of PtdIns4P in the ER, that SAC1 does not enrich at membrane contact sites, and that SAC1 has little activity in 'trans', unless a linker is added between its ER-anchored and catalytic domains. The data reveal an obligate 'cis' activity of SAC1, supporting its role in non-vesicular lipid traffic and implicating lipid traffic more broadly in inositol lipid homeostasis and function.

The Function of the Golgi Ribbon Structure - An Enduring Mystery Unfolds!

The Golgi apparatus in vertebrate cells consists of individual Golgi stacks fused together in a continuous ribbon structure. The ribbon structure per se is not required to mediate the classical functions of this organelle and the relevance of the "ribbon" structure has been a mystery since first identified ultrastructurally in the 1950s. Recent advances recognize a role for the Golgi apparatus in a range of cellular processes, some mediated by signaling networks which are regulated at the Golgi. Here we review the cellular processes and signaling events regulated by the Golgi apparatus and, in particular, explore an emerging theme that the ribbon structure of the Golgi contributes directly to the regulation of these higher order functions.

Structural basis for the binding of tryptophan-based motifs by δ-COP

Coatomer consists of two subcomplexes: the membrane-targeting, ADP ribosylation factor 1 (Arf1):GTP-binding βγδζ-COP F-subcomplex, which is related to the adaptor protein (AP) clathrin adaptors, and the cargo-binding αβ'ε-COP B-subcomplex. We present the structure of the C-terminal μ-homology domain of the yeast δ-COP subunit in complex with the WxW motif from its binding partner, the endoplasmic reticulum-localized Dsl1 tether. The motif binds at a site distinct from that used by the homologous AP μ subunits to bind YxxΦ cargo motifs with its two tryptophan residues sitting in compatible pockets. We also show that the Saccharomyces cerevisiae Arf GTPase-activating protein (GAP) homolog Gcs1p uses a related WxxF motif at its extreme C terminus to bind to δ-COP at the same site in the same way. Mutations designed on the basis of the structure in conjunction with isothermal titration calorimetry confirm the mode of binding and show that mammalian δ-COP binds related tryptophan-based motifs such as that from ArfGAP1 in a similar manner. We conclude that δ-COP subunits bind Wxn(1-6)[WF] motifs within unstructured regions of proteins that influence the lifecycle of COPI-coated vesicles; this conclusion is supported by the observation that, in the context of a sensitizing domain deletion in Dsl1p, mutating the tryptophan-based motif-binding site in yeast causes defects in both growth and carboxypeptidase Y trafficking/processing.

PLCγ1 participates in protein transport and diacylglycerol production triggered by cargo arrival at the Golgi

Diacylglycerol (DAG) is required for membrane traffic and structural organization at the Golgi. DAG is a lipid metabolite of several enzymatic reactions present at this organelle, but the mechanisms by which they are regulated are still unknown. Here, we show that cargo arrival at the Golgi increases the recruitment of the DAG-sensing constructs C1-PKCθ-GFP and the PKD-wt-GFP. The recruitment of both constructs was reduced by PLCγ1 silencing. Post-Golgi trafficking of transmembrane and soluble proteins was impaired in PLCγ1-silenced cells. Under basal conditions, PLCγ1 contributed to the maintenance of the pool of DAG associated with the Golgi and to the structural organization of the organelle. Finally, we show that cytosolic phospholipase C (PLC) can hydrolyse phosphatidylinositol 4-phosphate in isolated Golgi membranes. Our results indicate that PLCγ1 is part of the molecular mechanism that couples cargo arrival at the Golgi with DAG production to co-ordinate the formation of transport carriers for post-Golgi traffic.

Sphingolipid metabolism and interorganellar transport: localization of sphingolipid enzymes and lipid transfer proteins

In recent decades, many sphingolipid enzymes, sphingolipid-metabolism regulators and sphingolipid transfer proteins have been isolated and characterized. This review will provide an overview of the intracellular localization and topology of sphingolipid enzymes in mammalian cells to highlight the locations where respective sphingolipid species are produced. Interestingly, three sphingolipids that reside or are synthesized in cytosolic leaflets of membranes (ceramide, glucosylceramide and ceramide-1-phosphate) all have cytosolic lipid transfer proteins (LTPs). These LTPs consist of ceramide transfer protein (CERT), four-phosphate adaptor protein 2 (FAPP2) and ceramide-1-phosphate transfer protein (CPTP), respectively. These LTPs execute functions that affect both the location and metabolism of the lipids they bind. Molecular details describing the mechanisms of regulation of LTPs continue to emerge and reveal a number of critical processes, including competing phosphorylation and dephosphorylation reactions and binding interactions with regulatory proteins and lipids that influence the transport, organelle distribution and metabolism of sphingolipids.

Golgi phosphoprotein 3 triggers signal-mediated incorporation of glycosyltransferases into coatomer-coated (COPI) vesicles

Newly synthesized membrane and secreted proteins undergo a series of posttranslational modifications in the Golgi apparatus, including attachment of carbohydrate moieties. The final structure of so-formed glycans is determined by the order of execution of the different glycosylation steps, which seems intimately related to the spatial distribution of glycosyltransferases and glycosyl hydrolases within the Golgi apparatus. How cells achieve an accurate localization of these enzymes is not completely understood but might involve dynamic processes such as coatomer-coated (COPI) vesicle-mediated trafficking. In yeast, this transport is likely to be regulated by vacuolar protein sorting 74 (Vps74p), a peripheral Golgi protein able to interact with COPI coat as well as with a binding motif present in the cytosolic tails of some mannosyltransferases. Recently, Golgi phosphoprotein 3 (GOLPH3), the mammalian homolog of Vps74, has been shown to control the Golgi localization of core 2 N-acetylglucosamine-transferase 1. Here, we highlight a role of GOLPH3 in the spatial localization of α-2,6-sialyltransferase 1. We show, for the first time, that GOLPH3 supports incorporation of both core 2 N-acetylglucosamine-transferase 1 and α-2,6-sialyltransferase 1 into COPI vesicles. Depletion of GOLPH3 altered the subcellular localization of these enzymes. In contrast, galactosyltransferase, an enzyme that does not interact with GOLPH3, was neither incorporated into COPI vesicles nor was dependent on GOLPH3 for proper localization.

Inositol lipid phosphatases in membrane trafficking and human disease

The specific interaction of phosphoinositides with proteins is critical for a plethora of cellular processes, including cytoskeleton remodelling, mitogenic signalling, ion channel regulation and membrane traffic. The spatiotemporal restriction of different phosphoinositide species helps to define compartments within the cell, and this is particularly important for membrane trafficking within both the secretory and endocytic pathways. Phosphoinositide homoeostasis is tightly regulated by a large number of inositol kinases and phosphatases, which respectively phosphorylate and dephosphorylate distinct phosphoinositide species. Many of these enzymes have been implicated in regulating membrane trafficking and, accordingly, their dysregulation has been linked to a number of human diseases. In the present review, we focus on the inositol phosphatases, concentrating on their roles in membrane trafficking and the human diseases with which they have been associated.

PtdIns(4)P signalling and recognition systems

The Golgi apparatus is a sorting platform that exchanges extensively with the endoplasmic reticulum (ER), endosomes (Es) and plasma membrane (PM) compartments. The last compartment of the Golgi, the trans-Golgi Network (TGN) is a large complex of highly deformed membranes from which vesicles depart to their targeted organelles but also are harbored from retrograde pathways. The phosphoinositide (PI) composition of the TGN is marked by an important contingent of phosphatidylinositol-4-phosphate (PtdIns(4)P). Although this PI is present throughout the Golgi, its proportion grows along the successive cisternae and peaks at the TGN. The levels of this phospholipid are controlled by a set of kinases and phosphatases that regulate its concentrations in the Golgi and maintain a dynamic gradient that determines the cellular localization of several interacting proteins. Though not exclusive to the Golgi, the synthesis of PtdIns(4)P in other membranes is relatively marginal and has unclear consequences. The significance of PtdIns(4)P within the TGN has been demonstrated for numerous cellular events such as vesicle formation, lipid metabolism, and membrane trafficking.

Cinderella story: PI4P goes from precursor to key signaling molecule

Phosphatidylinositol lipids are signaling molecules involved in nearly all aspects of cellular regulation. Production of phosphatidylinositol 4-phosphate (PI4P) has long been recognized as one of the first steps in generating poly-phosphatidylinositol phosphates involved in actin organization, cell migration, and signal transduction. In addition, progress over the last decade has brought to light independent roles for PI4P in membrane trafficking and lipid homeostasis. Here, we describe recent advances that reveal the breadth of processes regulated by PI4P, the spectrum of PI4P effectors, and the mechanisms of spatiotemporal control that coordinate crosstalk between PI4P and cellular signaling pathways.

The first transmembrane domain of lipid phosphatase SAC1 promotes Golgi localization

The lipid phosphatase Sac1 cycles between endoplasmic reticulum and cisternal Golgi compartments. In proliferating mammalian cells, a canonical dilysine motif at the C-terminus of Sac1 is required for coatomer complex-I (COP-I)-binding and continuous retrieval to the ER. Starvation triggers accumulation of Sac1 at the Golgi. The mechanism responsible for Golgi retention of Sac1 is unknown. Here we show that the first of the two transmembrane regions in human SAC1 (TM1) functions in Golgi localization. A minimal construct containing only TM1 and the adjacent flanking sequences is concentrated at the Golgi. Transplanting TM1 into transferrin receptor 2 (TfR2) induces Golgi accumulation of this normally plasma membrane and endosomal protein, indicating that TM1 is sufficient for Golgi localization. In addition, we determined that the N-terminal cytoplasmic domain of SAC1 also promotes Golgi localization, even when TM1 is mutated or absent. We conclude that the distribution of SAC1 within the Golgi is controlled via both passive membrane thickness-dependent partitioning of TM1 and a retention mechanism that requires the N-terminal cytoplasmic region.

RNAi screening reveals a large signaling network controlling the Golgi apparatus in human cells

RNAi screening and automated image analysis reveal 180 kinases and phosphatases regulating the organization of the Golgi apparatus. Most of these genes also control the expression of specific glycans, pointing to a web of interactions between signaling cascades and glycosylation at the Golgi.

Golgi organization was probed with three markers of different Golgi compartments and quantitative morphological analysis.

Knockdowns of ∼20% of all known kinases and phosphatases affected the Golgi globally or in a compartment-specific manner, and were comparable in degree to the depletion of known membrane traffic regulators such as SNAREs.

Several cell surface receptors, their cognate ligands and downstream effectors regulate Golgi organization, suggesting a large regulatory network.

Most signaling genes affected both Golgi morphology and the expression of specific glycans.

The Golgi apparatus has many important physiological functions, including sorting of secretory cargo and biosynthesis of complex glycans. These functions depend on the intricate and compartmentalized organization of the Golgi apparatus. To investigate the mechanisms that regulate Golgi architecture, we developed a quantitative morphological assay using three different Golgi compartment markers and quantitative image analysis, and performed a kinome- and phosphatome-wide RNAi screen in HeLa cells. Depletion of 159 signaling genes, nearly 20% of genes assayed, induced strong and varied perturbations in Golgi morphology. Using bioinformatics data, a large regulatory network could be constructed. Specific subnetworks are involved in phosphoinositides regulation, acto-myosin dynamics and mitogen activated protein kinase signaling. Most gene depletion also affected Golgi functions, in particular glycan biosynthesis, suggesting that signaling cascades can control glycosylation directly at the Golgi level. Our results provide a genetic overview of the signaling pathways that control the Golgi apparatus in human cells.

Inositol polyphosphate phosphatases in human disease

Phosphoinositide signalling molecules interact with a plethora of effector proteins to regulate cell proliferation and survival, vesicular trafficking, metabolism, actin dynamics and many other cellular functions. The generation of specific phosphoinositide species is achieved by the activity of phosphoinositide kinases and phosphatases, which phosphorylate and dephosphorylate, respectively, the inositol headgroup of phosphoinositide molecules. The phosphoinositide phosphatases can be classified as 3-, 4- and 5-phosphatases based on their specificity for dephosphorylating phosphates from specific positions on the inositol head group. The SAC phosphatases show less specificity for the position of the phosphate on the inositol ring. The phosphoinositide phosphatases regulate PI3K/Akt signalling, insulin signalling, endocytosis, vesicle trafficking, cell migration, proliferation and apoptosis. Mouse knockout models of several of the phosphoinositide phosphatases have revealed significant physiological roles for these enzymes, including the regulation of embryonic development, fertility, neurological function, the immune system and insulin sensitivity. Importantly, several phosphoinositide phosphatases have been directly associated with a range of human diseases. Genetic mutations in the 5-phosphatase INPP5E are causative of the ciliopathy syndromes Joubert and MORM, and mutations in the 5-phosphatase OCRL result in Lowe's syndrome and Dent 2 disease. Additionally, polymorphisms in the 5-phosphatase SHIP2 confer diabetes susceptibility in specific populations, whereas reduced protein expression of SHIP1 is reported in several human leukaemias. The 4-phosphatase, INPP4B, has recently been identified as a tumour suppressor in human breast and prostate cancer. Mutations in one SAC phosphatase, SAC3/FIG4, results in the degenerative neuropathy, Charcot-Marie-Tooth disease. Indeed, an understanding of the precise functions of phosphoinositide phosphatases is not only important in the context of normal human physiology, but to reveal the mechanisms by which these enzyme families are implicated in an increasing repertoire of human diseases.

The Sac domain-containing phosphoinositide phosphatases: structure, function, and disease

Phosphoinositides (PIs) have long been known to have an essential role in cell physiology. Their intracellular localization and concentration must be tightly regulated for their proper function. This spatial and temporal regulation is achieved by a large number of PI kinases and phosphatases that are present throughout eukaryotic species. One family of these enzymes contains a conserved PI phosphatase domain termed Sac. Although the Sac domain is homologous among different Sac domain-containing proteins, all appear to exhibit varied substrate specificity and subcellular localization. Dysfunctions in several members of this family are implicated in a range of human diseases such as cardiac hypertrophy, bipolar disorder, Down's syndrome, Charcot-Marie-Tooth disease (CMT) and Amyotrophic Lateral Sclerosis (ALS). In plant, several Sac domain-containing proteins have been implicated in the stress response, chloroplast function and polarized secretion. In this review, we focus on recent findings in the family of Sac domain-containing PI phosphatases in yeast, mammal and plant, including the structural analysis into the mechanism of enzymatic activity, cellular functions, and their roles in disease pathophysiology.

Polyphosphoinositide metabolism and Golgi complex morphology in hippocampal neurons in primary culture is altered by chronic ethanol exposure

AIMS

Ethanol affects not only the cytoskeletal organization and activity, but also intracellular trafficking in neurons in the primary culture. Polyphosphoinositide (PPIn) are essential regulators of many important cell functions, including those mentioned, cytoskeleton integrity and intracellular vesicle trafficking. Since information about the effect of chronic ethanol exposure on PPIn metabolism in neurons is scarce, this study analysed the effect of this treatment on three of these phospholipids.

METHODS

Phosphatidylinositol (PtdIns) levels as well as the activity and/or levels of enzymes involved in their metabolism were analysed in neurons chronically exposed to ethanol. The levels of phospholipases C and D, and phosphatidylethanol formation were also assessed. The consequence of the possible alterations in the levels of PtdIns on the Golgi complex (GC) was also analysed.

RESULTS

We show that phosphatidylinositol (4,5)-bisphosphate and phosphatidylinositol (3,4,5)-trisphosphate levels, both involved in the control of intracellular trafficking and cytoskeleton organization, decrease in ethanol-exposed hippocampal neurons. In contrast, several kinases that participate in the metabolism of these phospholipids, and the level and/or activity of phospholipases C and D, increase in cells after ethanol exposure. Ethanol also promotes phosphatidylethanol formation in neurons, which can result in the suppression of phosphatidic acid synthesis and, therefore, in PPIn biosynthesis. This treatment also lowers the phosphatidylinositol 4-phosphate levels, the main PPIn in the GC, with alterations in their morphology and in the levels of some of the proteins involved in structure maintenance.

CONCLUSIONS

The deregulation of the metabolism of PtdIns may underlie the ethanol-induced alterations on different neuronal processes, including intracellular trafficking and cytoskeletal integrity.

Golgi membrane dynamics and lipid metabolism

The striking morphology of the Golgi complex has fascinated cell biologists since its discovery over 100 years ago. Yet, despite intense efforts to understand how membrane flow relates to Golgi form and function, this organelle continues to baffle cell biologists and biochemists alike. Fundamental questions regarding Golgi function, while hotly debated, remain unresolved. Historically, Golgi function has been described from a protein-centric point of view, but we now appreciate that conceptual frameworks for how lipid metabolism is integrated with Golgi biogenesis and function are essential for a mechanistic understanding of this fascinating organelle. It is from a lipid-centric perspective that we discuss the larger question of Golgi dynamics and membrane trafficking. We review the growing body of evidence for how lipid metabolism is integrally written into the engineering of the Golgi system and highlight questions for future study.

Metabolic activation of the HOG MAP kinase pathway by Snf1/AMPK regulates lipid signaling at the Golgi

Phosphatidylinositol-4-phosphate (PI(4)P) is an important regulator of Golgi function. Metabolic regulation of Golgi PI(4)P requires the lipid phosphatase Sac1 that translocates between endoplasmic reticulum (ER) and Golgi membranes. Localization of Sac1 responds to changes in glucose levels, yet the upstream signaling pathways that regulate Sac1 traffic are unknown. Here, we report that mitogen-activated protein kinase (MAPK) Hog1 transmits glucose signals to the Golgi and regulates localization of Sac1. We find that Hog1 is rapidly activated by both glucose starvation and glucose stimulation, which is independent of the well-characterized response to osmotic stress but requires the upstream element Ssk1 and is controlled by Snf1, the yeast homolog of AMP-activated kinase (AMPK). Elimination of either Hog1 or Snf1 slows glucose-induced translocation of Sac1 lipid phosphatase from the Golgi to the ER and thus delays PI(4)P accumulation at the Golgi. We conclude that a novel cross-talk between the HOG pathway and Snf1/AMPK is required for the metabolic control of lipid signaling at the Golgi.

Local control of phosphatidylinositol 4-phosphate signaling in the Golgi apparatus by Vps74 and Sac1 phosphoinositide phosphatase

Signaling by phosphatidylinositol 4-kinases (PI4Ks) in the Golgi apparatus controls lipid homeostasis and protein-sorting pathways. Signaling is shown to be terminated on the medial cisterna by a complex of a PI4K effector, Vps74, and Sac1, the major PtdIns4P phosphatase in the cell.

In the Golgi apparatus, lipid homeostasis pathways are coordinated with the biogenesis of cargo transport vesicles by phosphatidylinositol 4-kinases (PI4Ks) that produce phosphatidylinositol 4-phosphate (PtdIns4P), a signaling molecule that is recognized by downstream effector proteins. Quantitative analysis of the intra-Golgi distribution of a PtdIns4P reporter protein confirms that PtdIns4P is enriched on the trans-Golgi cisterna, but surprisingly, Vps74 (the orthologue of human GOLPH3), a PI4K effector required to maintain residence of a subset of Golgi proteins, is distributed with the opposite polarity, being most abundant on cis and medial cisternae. Vps74 binds directly to the catalytic domain of Sac1 (K_D = 3.8 μM), the major PtdIns4P phosphatase in the cell, and PtdIns4P is elevated on medial Golgi cisternae in cells lacking Vps74 or Sac1, suggesting that Vps74 is a sensor of PtdIns4P level on medial Golgi cisternae that directs Sac1-mediated dephosphosphorylation of this pool of PtdIns4P. Consistent with the established role of Sac1 in the regulation of sphingolipid biosynthesis, complex sphingolipid homeostasis is perturbed in vps74Δ cells. Mutant cells lacking complex sphingolipid biosynthetic enzymes fail to properly maintain residence of a medial Golgi enzyme, and cells lacking Vps74 depend critically on complex sphingolipid biosynthesis for growth. The results establish additive roles of Vps74-mediated and sphingolipid-dependent sorting of Golgi residents.

Growth and metabolic control of lipid signalling at the Golgi

PtdIns4P is a key regulator of the secretory pathway and plays an essential role in trafficking from the Golgi. Our recent work demonstrated that spatial control of PtdIns4P at the ER (endoplasmic reticulum) and Golgi co-ordinates secretion with cell growth. The central elements of this regulation are specific phosphoinositide 4-kinases and the phosphoinositide phosphatase Sac1. Growth-dependent translocation of Sac1 between the ER and Golgi modulates the levels of PtdIns4P and anterograde traffic at the Golgi. In yeast, this mechanism is largely dependent on the availability of glucose, but our recent results in mammalian cells suggest that Sac1 phosphatases play evolutionarily conserved roles in the growth control of secretion. Sac1 lipid phosphatase plays also an essential role in the spatial control of PtdIns4P at the Golgi complex. A restricted pool of PtdIns4P at the TGN (trans-Golgi network) is required for Golgi integrity and for proper lipid and protein sorting. In mammalian cells, the stress-activated MAPK (mitogen-activated protein kinase) p38 appears to play a critical role in transmitting nutrient signals to the phosphoinositide signalling machinery at the ER and Golgi. These results suggest that temporal and spatial integration of metabolic and lipid signalling networks at the Golgi is required for controlling the secretory pathway.