Phosphatidylinositol 4-kinase type IIalpha is responsible for the phosphatidylinositol 4-kinase activity associated with synaptic vesicles

Synaptic vesicle fusion promotes phosphatidylinositol 4-phosphate synthesis for efficient synaptic transmission

Efficient synaptic vesicle (SV) recycling is essential for sustaining synaptic transmission. While the multiple roles of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) in SV recycling are well documented, presynaptic regulation of phosphatidylinositol 4-phosphate (PI(4)P) synthesis and its potential role in SV recycling remain poorly understood. Here, we identify phosphatidylinositol 4-kinase IIIα (PI4KIIIα) as the key enzyme responsible for both the maintenance and activity-dependent production of presynaptic PI(4)P. Notably, we find that SVs are nearly devoid of PI(4)P and PI(4,5)P2 but are rich in phosphatidylinositol (PI) and that PI(4)P synthesis is triggered upon SV fusion as vesicular PI is delivered to the plasma membrane. Furthermore, when PI(4)P levels are selectively reduced without affecting basal PI(4,5)P2 levels, SV exo-endocytosis is significantly impaired, primarily due to reduced conductivity of voltage-gated Ca2+ channels. This reveals a PI(4,5)P2-independent homeostatic mechanism in which continuous PI(4)P production, driven by SV fusion, sustains efficient synaptic transmission.

Differential functions of phosphatidylinositol 4-kinases in neurotransmission and synaptic development

Phosphoinositides, such as PI(4,5)P2, are known to function as structural components of membranes, signalling molecules, markers of membrane identity, mediators of protein recruitment and regulators of neurotransmission and synaptic development. Phosphatidylinositol 4-kinases (PI4Ks) synthesize PI4P, which are precursors for PI(4,5)P2, but may also have independent functions. The roles of PI4Ks in neurotransmission and synaptic development have not been studied in detail. Previous studies on PI4KII and PI4KIIIβ at the Drosophila larval neuromuscular junction have suggested that PI4KII and PI4KIIIβ enzymes may serve redundant roles, where single PI4K mutants yielded mild or no synaptic phenotypes. However, the precise synaptic functions (neurotransmission and synaptic growth) of these PI4Ks have not been thoroughly studied. Here, we used PI4KII and PI4KIIIβ null mutants and presynaptic-specific knockdowns of these PI4Ks to investigate their roles in neurotransmission and synaptic growth. We found that PI4KII and PI4KIIIβ appear to have non-overlapping functions. Specifically, glial PI4KII functions to restrain synaptic growth, whereas presynaptic PI4KIIIβ promotes synaptic growth. Furthermore, loss of PI4KIIIβ or presynaptic PI4KII impairs neurotransmission. The data presented in this study uncover new roles for PI4K enzymes in neurotransmission and synaptic growth.

Research progress on phosphatidylinositol 4-kinase inhibitors

Phosphatidylinositol 4-kinases (PI4Ks) could phosphorylate phosphatidylinositol (PI) to produce phosphatidylinositol 4-phosphate (PI4P) and maintain its metabolic balance and location. PI4P, the most abundant monophosphate inositol in eukaryotic cells, is a precursor of higher phosphoinositols and an essential substrate for the PLC/PKC and PI3K/Akt signaling pathways. PI4Ks regulate vesicle transport, signal transduction, cytokinesis, and cell unity, and are involved in various physiological and pathological processes, including infection and growth of parasites such as Plasmodium and Cryptosporidium, replication and survival of RNA viruses, and the development of tumors and nervous system diseases. The development of novel drugs targeting PI4Ks and PI4P has been the focus of the research and clinical application of drugs, especially in recent years. In particular, PI4K inhibitors have made great progress in the treatment of malaria and cryptosporidiosis. We describe the biological characteristics of PI4Ks; summarize the physiological functions and effector proteins of PI4P; and analyze the structural basis of selective PI4K inhibitors for the treatment of human diseases in this review. Herein, this review mainly summarizes the developments in the structure and enzyme activity of PI4K inhibitors.

IP3R1 underlies diastolic ANO1 activation and pressure-dependent chronotropy in lymphatic collecting vessels

Pressure-dependent chronotropy of murine lymphatic collecting vessels relies on the activation of the Ca2+-activated chloride channel encoded by Anoctamin 1 (Ano1) in lymphatic muscle cells. Genetic ablation or pharmacological inhibition of ANO1 results in a significant reduction in basal contraction frequency and essentially complete loss of pressure-dependent frequency modulation by decreasing the rate of the diastolic depolarization phase of the ionic pacemaker in lymphatic muscle cells (LMCs). Oscillating Ca2+ release from sarcoendoplasmic reticulum Ca2+ channels has been hypothesized to drive ANO1 activity during diastole, but the source of Ca2+ for ANO1 activation in smooth muscle remains unclear. Here, we investigated the role of the inositol triphosphate receptor 1 (Itpr1; Ip3r1) in this process using pressure myography, Ca2+ imaging, and membrane potential recordings in LMCs of ex vivo pressurized inguinal-axillary lymphatic vessels from control or Myh11CreERT2;Ip3r1fl/fl (Ip3r1ismKO) mice. Ip3r1ismKO vessels had significant reductions in contraction frequency and tone but an increased contraction amplitude. Membrane potential recordings from LMCs of Ip3r1ismKO vessels revealed a depressed diastolic depolarization rate and an elongation of the plateau phase of the action potential (AP). Ca2+ imaging of LMCs using the genetically encoded Ca2+ sensor GCaMP6f demonstrated an elongation of the Ca2+ flash associated with an AP-driven contraction. Critically, diastolic subcellular Ca2+ transients were absent in LMCs of Ip3r1ismKO mice, demonstrating the necessity of IP3R1 activity in controlling ANO1-mediated diastolic depolarization. These findings indicate a critical role for IP3R1 in lymphatic vessel pressure-dependent chronotropy and contractile regulation.

Phosphatidylinositol 4-kinase IIα is a glycogen synthase kinase 3-regulated interaction hub for activity-dependent bulk endocytosis

Phosphatidylinositol 4-kinase IIα (PI4KIIα) generates essential phospholipids and is a cargo for endosomal adaptor proteins. Activity-dependent bulk endocytosis (ADBE) is the dominant synaptic vesicle endocytosis mode during high neuronal activity and is sustained by glycogen synthase kinase 3β (GSK3β) activity. We reveal the GSK3β substrate PI4KIIα is essential for ADBE via its depletion in primary neuronal cultures. Kinase-dead PI4KIIα rescues ADBE in these neurons but not a phosphomimetic form mutated at the GSK3β site, Ser-47. Ser-47 phosphomimetic peptides inhibit ADBE in a dominant-negative manner, confirming that Ser-47 phosphorylation is essential for ADBE. Phosphomimetic PI4KIIα interacts with a specific cohort of presynaptic molecules, two of which, AGAP2 and CAMKV, are also essential for ADBE when depleted in neurons. Thus, PI4KIIα is a GSK3β-dependent interaction hub that silos essential ADBE molecules for liberation during neuronal activity.

zDHHC9 Regulates Cardiomyocyte Rab3a Activity and Atrial Natriuretic Peptide Secretion Through Palmitoylation of Rab3gap1

Production and release of natriuretic peptides by the stressed heart reduce cardiac workload by promoting vasodilation, natriuresis, and diuresis, which has been leveraged in the recent development of novel heart-failure pharmacotherapies, yet the mechanisms regulating cardiomyocyte exocytosis and natriuretic peptide release remain ill defined. We found that the Golgi S-acyltransferase zDHHC9 palmitoylates Rab3gap1 resulting in its spatial segregation from Rab3a, elevation of Rab3a-GTP levels, formation of Rab3a-positive peripheral vesicles, and impairment of exocytosis that limits atrial natriuretic peptide release. This novel pathway potentially can be exploited for targeting natriuretic peptide signaling in the treatment of heart failure.

EMT-activated secretory and endocytic vesicular trafficking programs underlie a vulnerability to PI4K2A antagonism in lung cancer

Hypersecretory malignant cells underlie therapeutic resistance, metastasis, and poor clinical outcomes. However, the molecular basis for malignant hypersecretion remains obscure. Here, we showed that epithelial-mesenchymal transition (EMT) initiates exocytic and endocytic vesicular trafficking programs in lung cancer. The EMT-activating transcription factor zinc finger E-box-binding homeobox 1 (ZEB1) executed a PI4KIIIβ-to-PI4KIIα (PI4K2A) dependency switch that drove PI4P synthesis in the Golgi and endosomes. EMT enhanced the vulnerability of lung cancer cells to PI4K2A small-molecule antagonists. PI4K2A formed a MYOIIA-containing protein complex that facilitated secretory vesicle biogenesis in the Golgi, thereby establishing a hypersecretory state involving osteopontin (SPP1) and other prometastatic ligands. In the endosomal compartment, PI4K2A accelerated recycling of SPP1 receptors to complete an SPP1-dependent autocrine loop and interacted with HSP90 to prevent lysosomal degradation of AXL receptor tyrosine kinase, a driver of cell migration. These results show that EMT coordinates exocytic and endocytic vesicular trafficking to establish a therapeutically actionable hypersecretory state that drives lung cancer progression.

Effect of the Rho-Kinase/ROCK Signaling Pathway on Cytoskeleton Components

The mechanical properties of cells are important in tissue homeostasis and enable cell growth, division, migration and the epithelial-mesenchymal transition. Mechanical properties are determined to a large extent by the cytoskeleton. The cytoskeleton is a complex and dynamic network composed of microfilaments, intermediate filaments and microtubules. These cellular structures confer both cell shape and mechanical properties. The architecture of the networks formed by the cytoskeleton is regulated by several pathways, a key one being the Rho-kinase/ROCK signaling pathway. This review describes the role of ROCK (Rho-associated coiled-coil forming kinase) and how it mediates effects on the key components of the cytoskeleton that are critical for cell behaviour.

PI(4,5)P2-dependent and -independent roles of PI4P in the control of hormone secretion by pituitary cells

Plasma membrane and organelle membranes are home to seven phosphoinositides, an important class of low-abundance anionic signaling lipids that contribute to cellular functions by recruiting cytoplasmic proteins or interacting with the cytoplasmic domains of membrane proteins. Here, we briefly review the functions of three phosphoinositides, PI4P, PI(4,5)P2, and PI(3,4,5)P3, in cellular signaling and exocytosis, focusing on hormone-producing pituitary cells. PI(4,5)P2, acting as a substrate for phospholipase C, plays a key role in the control of pituitary cell functions, including hormone synthesis and secretion. PI(4,5)P2 also acts as a substrate for class I PI3-kinases, leading to the generation of two intracellular messengers, PI(3,4,5)P3 and PI(3,4)P2, which act through their intracellular effectors, including Akt. PI(4,5)P2 can also influence the release of pituitary hormones acting as an intact lipid to regulate ion channel gating and concomitant calcium signaling, as well as the exocytic pathway. Recent findings also show that PI4P is not only a precursor of PI(4,5)P2, but also a key signaling molecule in many cell types, including pituitary cells, where it controls hormone secretion in a PI(4,5)P2-independent manner.

Type II phosphatidylinositol 4-kinases function sequentially in cargo delivery from early endosomes to melanosomes

Melanosomes are pigment cell-specific lysosome-related organelles in which melanin pigments are synthesized and stored. Melanosome maturation requires delivery of melanogenic cargoes via tubular transport carriers that emanate from early endosomes and that require BLOC-1 for their formation. Here we show that phosphatidylinositol-4-phosphate (PtdIns4P) and the type II PtdIns-4-kinases (PI4KIIα and PI4KIIβ) support BLOC-1-dependent tubule formation to regulate melanosome biogenesis. Depletion of either PI4KIIα or PI4KIIβ with shRNAs in melanocytes reduced melanin content and misrouted BLOC-1-dependent cargoes to late endosomes/lysosomes. Genetic epistasis, cell fractionation, and quantitative live-cell imaging analyses show that PI4KIIα and PI4KIIβ function sequentially and non-redundantly downstream of BLOC-1 during tubule elongation toward melanosomes by generating local pools of PtdIns4P. The data show that both type II PtdIns-4-kinases are necessary for efficient BLOC-1-dependent tubule elongation and subsequent melanosome contact and content delivery during melanosome biogenesis. The independent functions of PtdIns-4-kinases in tubule extension are downstream of likely redundant functions in BLOC-1-dependent tubule initiation.

GSK3β mediates the spatiotemporal dynamics of NLRP3 inflammasome activation

Subcellular machinery of NLRP3 is essential for inflammasome assembly and activation. However, the stepwise process and mechanistic basis of NLRP3 engagement with organelles remain unclear. Herein, we demonstrated glycogen synthase kinase 3β (GSK3β) as a molecular determinant for the spatiotemporal dynamics of NLRP3 inflammasome activation. Using live cell multispectral time-lapse tracking acquisition, we observed that upon stimuli NLRP3 was transiently associated with mitochondria and subsequently recruited to the Golgi network (TGN) where it was retained for inflammasome assembly. This occurred in relation to the temporal contact of mitochondria to Golgi apparatus. NLRP3 stimuli initiate GSK3β activation with subsequent binding to NLRP3, facilitating NLRP3 recruitment to mitochondria and transition to TGN. GSK3β activation also phosphorylates phosphatidylinositol 4-kinase 2 Α (PI4k2A) in TGN to promote sustained NLRP3 oligomerization. Our study has identified the interplay between GSK3β signaling and the organelles dynamics of NLRP3 required for inflammasome activation and opens new avenues for therapeutic intervention.

Striking a balance: PIP2 and PIP3 signaling in neuronal health and disease

Phosphoinositides are membrane phospholipids involved in a variety of cellular processes like growth, development, metabolism, and transport. This review focuses on the maintenance of cellular homeostasis of phosphatidylinositol 4,5-bisphosphate (PIP₂), and phosphatidylinositol 3,4,5-trisphosphate (PIP₃). The critical balance of these PIPs is crucial for regulation of neuronal form and function. The activity of PIP₂ and PIP₃ can be regulated through kinases, phosphatases, phospholipases and cholesterol microdomains. PIP₂ and PIP₃ carry out their functions either indirectly through their effectors activating integral signaling pathways, or through direct regulation of membrane channels, transporters, and cytoskeletal proteins. Any perturbations to the balance between PIP₂ and PIP₃ signaling result in neurodevelopmental and neurodegenerative disorders. This review will discuss the upstream modulators and downstream effectors of the PIP₂ and PIP₃ signaling, in the context of neuronal health and disease.

Phosphoinositides as membrane organizers

Phosphoinositides are signalling lipids derived from phosphatidylinositol, a ubiquitous phospholipid in the cytoplasmic leaflet of eukaryotic membranes. Initially discovered for their roles in cell signalling, phosphoinositides are now widely recognized as key integrators of membrane dynamics that broadly impact on all aspects of cell physiology and on disease. The past decade has witnessed a vast expansion of our knowledge of phosphoinositide biology. On the endocytic and exocytic routes, phosphoinositides direct the inward and outward flow of membrane as vesicular traffic is coupled to the conversion of phosphoinositides. Moreover, recent findings on the roles of phosphoinositides in autophagy and the endolysosomal system challenge our view of lysosome biology. The non-vesicular exchange of lipids, ions and metabolites at membrane contact sites in between organelles has also been found to depend on phosphoinositides. Here we review our current understanding of how phosphoinositides shape and direct membrane dynamics to impact on cell physiology, and provide an overview of emerging concepts in phosphoinositide regulation.

NPC1 regulates the distribution of phosphatidylinositol 4-kinases at Golgi and lysosomal membranes

Cholesterol and phosphoinositides (PI) are two critically important lipids that are found in cellular membranes and dysregulated in many disorders. Therefore, uncovering molecular pathways connecting these essential lipids may offer new therapeutic insights. We report that loss of function of lysosomal Niemann-Pick Type C1 (NPC1) cholesterol transporter, which leads to neurodegenerative NPC disease, initiates a signaling cascade that alters the cholesterol/phosphatidylinositol 4-phosphate (PtdIns4P) countertransport cycle between Golgi-endoplasmic reticulum (ER), as well as lysosome-ER membrane contact sites (MCS). Central to these disruptions is increased recruitment of phosphatidylinositol 4-kinases-PI4KIIα and PI4KIIIβ-which boosts PtdIns4P metabolism at Golgi and lysosomal membranes. Aberrantly increased PtdIns4P levels elevate constitutive anterograde secretion from the Golgi complex, and mTORC1 recruitment to lysosomes. NPC1 disease mutations phenocopy the transporter loss of function and can be rescued by inhibition or knockdown of either key phosphoinositide enzymes or their recruiting partners. In summary, we show that the lysosomal NPC1 cholesterol transporter tunes the molecular content of Golgi and lysosome MCS to regulate intracellular trafficking and growth signaling in health and disease.

An overview of the synaptic vesicle lipid composition

Chemical neurotransmission is the major mechanism of neuronal communication. Neurotransmitters are released from secretory organelles, the synaptic vesicles (SVs) via exocytosis into the synaptic cleft. Fusion of SVs with the presynaptic plasma membrane is balanced by endocytosis, thus maintaining the presynaptic membrane at steady-state levels. The protein machineries responsible for exo- and endocytosis have been extensively investigated. In contrast, less is known about the role of lipids in synaptic transmission and how the lipid composition of SVs is affected by dynamic exo-endocytotic cycling. Here we summarize the current knowledge about the composition, organization, and function of SV membrane lipids. We also cover lipid biogenesis and maintenance during the synaptic vesicle cycle.

Proteomic Analysis of Tear Film Obtained from Diabetic Dogs

Simple Summary

Canine diabetes is a serious disease, which can lead to severe complications, eventually even death. Currently, all the diagnostic procedures are the invasive ones, with blood collection remaining as a golden standard for both initial diagnosis, and later follow-up. Tears can be obtained in a non-invasive manner, which makes them a perfect candidate for a screening tool in canine diabetes. In this study we aimed to analyze the protein composition of the tears collected from the healthy animals and compared it to the diabetic group. There are significant differences between these two groups, and we believe that the identified proteins hold promise as a potential diagnostic tool, which can be later on used both in clinical practice, and for better understanding of the disease.

Abstract

Canine diabetes mellitus is a significant health burden, followed with numerous systemic complications, including diabetic cataracts and retinopathy, leading to blindness. Diabetes should be considered as a disease damaging all the body organs, including gastrointestinal tract, through a complex combination of vascular and metabolic pathologies, leading to impaired gut function. Tear film can be obtained in a non-invasive way, which makes it a feasible biomarker source. In this study we compared proteomic changes ongoing in tear film of diabetic dogs. The study group consisted of 15 diabetic dogs, and 13 dogs served as a control group. After obtaining tear film with Schirmer strips, we performed 2-dimensional electrophoresis, followed by Delta2D software analysis, which allowed to select statistically significant differentially expressed proteins. After their identification with MALDI-TOF (matrix assisted laser desorption and ionisation time of flight) spectrometry we found one up-regulated protein in tear film of diabetic dogs—SRC kinase signaling inhibitor 1 (SRCIN1). Eight proteins were down-regulated: phosphatidylinositol-4 kinase type 2 alpha (PI4KIIα), Pro-melanin concentrating hormone (Pro-MCH), Flotillin-1, Protein mono-ADP ribosyltransferase, GRIP and coiled coil domain containing protein 2, tetratricopeptide repeat protein 36, serpin, and Prelamin A/C. Identified proteins were analyzed by Panther Gene Ontology software, and their possible connections with diabetic etiopathology were discussed. We believe that this is the first study to target tear film proteome in canine diabetes. We believe that combined with traditional examination, the tear film proteomic analysis can be a new source of biomarkers both for clinical practice, and experimental research.

Lipid transfer proteins and instructive regulation of lipid kinase activities: Implications for inositol lipid signaling and disease

Cellular membranes are critical platforms for intracellular signaling that involve complex interfaces between lipids and proteins, and a web of interactions between a multitude of lipid metabolic pathways. Membrane lipids impart structural and functional information in this regulatory circuit that encompass biophysical parameters such as membrane thickness and fluidity, as well as chaperoning the interactions of protein binding partners. Phosphatidylinositol and its phosphorylated derivatives, the phosphoinositides, play key roles in intracellular membrane signaling, and these involvements are translated into an impressively diverse set of biological outcomes. The phosphatidylinositol transfer proteins (PITPs) are key regulators of phosphoinositide signaling. Found in a diverse array of organisms from plants, yeast and apicomplexan parasites to mammals, PITPs were initially proposed to be simple transporters of lipids between intracellular membranes. It now appears increasingly unlikely that the soluble versions of these proteins perform such functions within the cell. Rather, these serve to facilitate the activity of intrinsically biologically insufficient inositol lipid kinases and, in so doing, promote diversification of the biological outcomes of phosphoinositide signaling. The central engine for execution of such functions is the lipid exchange cycle that is a fundamental property of PITPs. How PITPs execute lipid exchange remains very poorly understood. Molecular dynamics simulation approaches are now providing the first atomistic insights into how PITPs, and potentially other lipid-exchange/transfer proteins, operate.

An early endosome-derived retrograde trafficking pathway promotes secretory granule maturation

Regulated secretion is a fundamental cellular process in which biologically active molecules stored in long-lasting secretory granules (SGs) are secreted in response to external stimuli. Many studies have described mechanisms responsible for biogenesis and secretion of SGs, but how SGs mature remains poorly understood. In a genetic screen, we discovered a large number of endolysosomal trafficking genes required for proper SG maturation, indicating that maturation of SGs might occur in a manner similar to lysosome-related organelles (LROs). CD63, a tetraspanin known to decorate LROs, also decorates SG membranes and facilitates SG maturation. Moreover, CD63-mediated SG maturation requires type II phosphatidylinositol 4 kinase (PI4KII)-dependent early endosomal sorting and accumulation of phosphatidylinositol 4-phosphate (PI4P) on SG membranes. In addition, the PI4P effector Past1 is needed for formation of stable PI4KII-containing endosomal tubules associated with this process. Our results reveal that maturation of post-Golgi-derived SGs requires trafficking via the endosomal system, similar to mechanisms employed by LROs.

PIP4Ks Suppress Insulin Signaling through a Catalytic-Independent Mechanism

Insulin stimulates the conversion of phosphatidylino-sitol-4,5-bisphosphate (PI(4,5)P₂) to phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P₃), which mediates downstream cellular responses. PI(4,5)P₂ is produced by phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks) and by phosphatidylinositol-5-phos-phate 4-kinases (PIP4Ks). Here, we show that the loss of PIP4Ks (PIP4K2A, PIP4K2B, and PIP4K2C) in vitro results in a paradoxical increase in PI(4,5)P₂ and a concomitant increase in insulin-stimulated production of PI(3,4,5)P₃. The reintroduction of either wild-type or kinase-dead mutants of the PIP4Ks restored cellular PI(4,5)P₂ levels and insulin stimulation of the PI3K pathway, suggesting a catalytic-independent role of PIP4Ks in regulating PI(4,5)P₂ levels. These effects are explained by an increase in PIP5K activity upon the deletion of PIP4Ks, which normally suppresses PIP5K activity through a direct binding interaction mediated by the N-terminal motif VMLϕFPDD of PIP4K. Our work uncovers an allosteric function of PIP4Ks in suppressing PIP5K-mediated PI(4,5)P₂ synthesis and insulin-dependent conversion to PI(3,4,5)P₃ and suggests that the pharmacological depletion of PIP4K enzymes could represent a strategy for enhancing insulin signaling.

PI(4,5)P₂ is produced by both phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks) and by phosphatidylinositol-5-phosphate 4-kinases (PIP4Ks). Wang et al. report an allosteric function of a conserved N-terminal motif of PIP4Ks in suppressing PIP5K-mediated PI(4,5)P₂ synthesis and insulin-dependent conversion to PI(3,4,5) P₃. This non-catalytic role has implications for the development of PIP4K targeted therapies.

Pi4KIIα Regulates Unconditioned Stimulus-Retrieval-Induced Fear Memory Reconsolidation through Endosomal Trafficking of AMPA Receptors

Targeting memory reconsolidation is an effective intervention for treating posttraumatic stress disorder (PTSD). Disrupting unconditioned stimulus (US)-retrieval-induced fear memory reconsolidation has become an effective therapeutic approach to attenuate fear memory, but the underlying molecular mechanisms remain unknown. Here, we report that US-retrieval-dependent increase in phosphatidylinositol 4-kinase IIα (Pi4KIIα) promotes early endosomal trafficking of AMPA receptors, leading to the enhancement of synaptic efficacy in basolateral amygdala (BLA) neurons. The inhibition of Pi4KIIα by an inhibitor or short hairpin RNA impaired contextual fear memory reconsolidation. This disruptive effect persisted for at least 2 weeks, which was restored by Pi4KIIα overexpression with TAT-Pi4KIIα. Furthermore, the blockade of early endosomal trafficking following US retrieval reduced synaptosomal membrane GluA1 levels and decreased subsequent fear expression. These data demonstrate that Pi4KIIα in the BLA is crucial for US-retrieval-induced fear memory reconsolidation, the inhibition of which might be an effective therapeutic strategy for treating PTSD.

•

Unconditioned stimulus (US) retrieval induces a transient increase in Pi4KIIα expression

•

Pi4KIIα regulates early endosomal trafficking of AMPARs during memory reconsolidation

•

Pi4KIIα contributes to US-retrieval-induced synaptic enhancement in rat BLA

•

Pi4KIIα inhibition after US retrieval impairs fear expression and shows long-term effects

Novel Cellular Functions of Very Long Chain-Fatty Acids: Insight From ELOVL4 Mutations

Elongation of Very Long chain fatty acids-4 (ELOVL4) protein is a member of the ELOVL family of fatty acid elongases that is collectively responsible for catalyzing formation of long chain fatty acids. ELOVL4 is the only family member that catalyzes production of Very Long Chain Saturated Fatty Acids (VLC-SFA) and Very Long Chain Polyunsaturated Fatty Acids (VLC-PUFA) with chain lengths ≥28 carbons. ELOVL4 and its VLC-SFA and VLC-PUFA products are emerging as important regulators of synaptic signaling and neuronal survival in the central nervous system (CNS). Distinct sets of mutations in ELOVL4 cause three different neurological diseases in humans. Heterozygous inheritance of one set of autosomal dominant ELOVL4 mutations that leads to truncation of the ELOVL4 protein causes Stargardt-like macular dystrophy (STGD3), an aggressive juvenile-onset retinal degeneration. Heterozygous inheritance of a different set of autosomal dominant ELOVL4 mutations that leads to a full-length protein with single amino acid substitutions causes spinocerebellar ataxia 34 (SCA34), a late-onset neurodegenerative disease characterized by gait ataxia and cerebellar atrophy. Homozygous inheritance of a different set of ELOVL4 mutations causes a more severe disease with infantile onset characterized by seizures, spasticity, intellectual disability, ichthyosis, and premature death. ELOVL4 is expressed widely in the CNS and is found primarily in neurons. ELOVL4 is expressed in cell-specific patterns within different regions of the CNS that are likely to be related to disease symptoms. In the retina, ELOVL4 is expressed exclusively in photoreceptors and produces VLC-PUFA that are incorporated into phosphatidylcholine and enriched in the light sensitive membrane disks of the photoreceptor outer segments. VLC-PUFA are enzymatically converted into "elovanoid" compounds that appear to provide paracrine signals that promote photoreceptor and neuronal survival. In the brain, the main ELOVL4 products are VLC-SFA that are incorporated into sphingolipids and enriched in synaptic vesicles, where they regulate kinetics of presynaptic neurotransmitter release. Understanding the function of ELOVL4 and its VLC-SFA and VLC-PUFA products will advance our understanding of basic mechanisms in neural signaling and has potential for developing novel therapies for seizure and neurodegenerative diseases.

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.

Phosphoinositides: Regulators of Nervous System Function in Health and Disease

Phosphoinositides, the seven phosphorylated derivatives of phosphatidylinositol have emerged as regulators of key sub-cellular processes such as membrane transport, cytoskeletal function and plasma membrane signaling in eukaryotic cells. All of these processes are also present in the cells that constitute the nervous system of animals and in this setting too, these are likely to tune key aspects of cell biology in relation to the unique structure and function of neurons. Phosphoinositides metabolism and function are mediated by enzymes and proteins that are conserved in evolution, and analysis of knockouts of these in animal models implicate this signaling system in neural function. Most recently, with the advent of human genome analysis, mutations in genes encoding components of the phosphoinositide signaling pathway have been implicated in human diseases although the cell biological basis of disease phenotypes in many cases remains unclear. In this review we evaluate existing evidence for the involvement of phosphoinositide signaling in human nervous system diseases and discuss ways of enhancing our understanding of the role of this pathway in the human nervous system's function in health and disease.

Adaptor protein-3 complex is required for Vangl2 trafficking and planar cell polarity of the inner ear

Planar cell polarity (PCP) regulates coordinated cellular polarity among neighboring cells to establish a polarity axis parallel to the plane of the tissue. Disruption in PCP results in a range of developmental anomalies and diseases. A key feature of PCP is the polarized and asymmetric localization of several membrane PCP proteins, which is essential to establish the polarity axis to orient cells coordinately. However, the machinery that regulates the asymmetric partition of PCP proteins remains largely unknown. In the present study, we show Van gogh-like 2 (Vangl2) in early and recycling endosomes as made evident by colocalization with diverse endosomal Rab proteins. Vangl2 biochemically interacts with adaptor protein-3 complex (AP-3). Using short hairpin RNA knockdown, we found that Vangl2 subcellular localization was modified in AP-3–depleted cells. Moreover, Vangl2 membrane localization within the cochlea is greatly reduced in AP-3–deficient mocha mice, which exhibit profound hearing loss. In inner ears from AP-3–deficient mocha mice, we observed PCP-dependent phenotypes, such as misorientation and deformation of hair cell stereociliary bundles and disorganization of hair cells characteristic of defects in convergent extension that is driven by PCP. These findings demonstrate a novel role of AP-3–mediated sorting mechanisms in regulating PCP proteins.

Recent advances in understanding phosphoinositide signaling in the nervous system

Polyphosphoinositides (PPIn) are essential signaling phospholipids that make remarkable contributions to the identity of all cellular membranes and signaling cascades in mammalian cells. They exert regulatory control over membrane homeostasis via selective interactions with cellular proteins at the membrane–cytoplasm interface. This review article briefly summarizes our current understanding of the key roles that PPIn play in orchestrating and regulating crucial electrical and chemical signaling events in mammalian neurons and the significant neuro-pathophysiological conditions that arise following alterations in their metabolism.

PI4KIIα regulates insulin secretion and glucose homeostasis via a PKD-dependent pathway

Insulin release by pancreatic β cells plays a key role in regulating blood glucose levels in humans, and to understand the mechanism for insulin secretion may reveal therapeutic strategies for diabetes. We found that PI4KIIα transgenic (TG) mice have abnormal glucose tolerance and higher serum glucose levels than wild-type mice. Glucose-stimulated insulin secretion was significantly reduced in both PI4KIIα TG mice and PI4KIIα-overexpressing pancreatic β cell lines. A proximity-based biotin labeling technique, BioID, was used to identify proteins that interact with PI4KIIα, and the results revealed that PI4KIIα interacts with PKD and negatively regulates its activity. The effect of PI4KIIα on insulin secretion was completely rescued by altering PKD activity. PI4KIIα overexpression also worsened glucose tolerance in streptozotocin/high-fat diet-induced diabetic mice by impairing insulin secretion. Our study has shed new light on PI4KIIα function and mechanism in diabetes and identified PI4KIIα as an important regulator of insulin secretion.

The structural basis for calcium inhibition of lipid kinase PI4K IIalpha and comparison with the apo state

PI4K IIalpha is a critical enzyme for the maintenance of Golgi and is also known to function in the synaptic vesicles. The product of its catalytical function, phosphatidylinositol 4-phosphate (PI4P), is an important lipid molecule because it is a hallmark of the Golgi and TGN, is directly recognized by many proteins and also serves as a precursor molecule for synthesis of higher phosphoinositides. Here, we report crystal structures of PI4K IIalpha enzyme in the apo-state and inhibited by calcium. The apo-structure reveals a surprising rigidity of the active site residues important for catalytic activity. The structure of calcium inhibited kinase reveals how calcium locks ATP in the active site.

PtdIns4KIIα generates endosomal PtdIns(4)P and is required for receptor sorting at early endosomes

PtdIns4KIIα has been implicated in the regulation of endosomal traffic, but the role of its enzymatic activity and the site of its action have not been elucidated. Depletion of PtdIns4KIIα significantly reduced the amount of vesicular PtdIns(4)P on early endosomes, leaving cells with an impaired ability to sort molecules from early endosomes.

Phosphatidylinositol 4-kinase IIα (PtdIns4KIIα) localizes to the trans-Golgi network and endosomal compartments and has been implicated in the regulation of endosomal traffic, but the roles of both its enzymatic activity and the site of its action have not been elucidated. This study shows that PtdIns4KIIα is required for production of endosomal phosphatidylinositol 4-phosphate (PtdIns(4)P) on early endosomes and for the sorting of transferrin and epidermal growth factor receptor into recycling and degradative pathways. Depletion of PtdIns4KIIα with small interfering RNA significantly reduced the amount of vesicular PtdIns(4)P on early endosomes but not on Golgi membranes. Cells depleted of PtdIns4KIIα had an impaired ability to sort molecules destined for recycling from early endosomes. We further identify the Eps15 homology domain–containing protein 3 (EHD3) as a possible endosomal effector of PtdIns4KIIα. Tubular endosomes containing EHD3 were shortened and became more vesicular in PtdIns4KIIα-depleted cells. Endosomal PtdIns(4,5)P₂ was also significantly reduced in PtdIns4KIIα-depleted cells. These results show that PtdIns4KIIα regulates receptor sorting at early endosomes through a PtdIns(4)P-dependent pathway and contributes substrate for the synthesis of endosomal PtdIns(4,5)P₂.

Lipid kinases as therapeutic targets for chronic pain

Existing analgesics are not efficacious in treating all patients with chronic pain and have harmful side effects when used long term. A deeper understanding of pain signaling and sensitization could lead to the development of more efficacious analgesics. Nociceptor sensitization occurs under conditions of inflammation and nerve injury where diverse chemicals are released and signal through receptors to reduce the activation threshold of ion channels, leading to an overall increase in neuronal excitability. Drugs that inhibit specific receptors have so far been unsuccessful in alleviating pain, possibly because they do not simultaneously target the diverse receptors that contribute to nociceptor sensitization. Hence, the focus has shifted toward targeting downstream convergence points of nociceptive signaling. Lipid mediators, including phosphatidylinositol 4,5-bisphosphate (PIP2), are attractive targets, as these molecules are required for signaling downstream of G-protein-coupled receptors and receptor tyrosine kinases. Furthermore, PIP2 regulates the activity of various ion channels. Thus, PIP2 sits at a critical convergence point for multiple receptors, ion channels, and signaling pathways that promote and maintain chronic pain. Decreasing the amount of PIP2 in neurons was recently shown to attenuate pronociceptive signaling and could provide a novel approach for treating pain. Here, we review the lipid kinases that are known to regulate pain signaling and sensitization and speculate on which additional lipid kinases might regulate signaling in nociceptive neurons.

Rab27a controls HIV-1 assembly by regulating plasma membrane levels of phosphatidylinositol 4,5-bisphosphate

During the late stages of the HIV-1 replication cycle, the viral polyprotein Pr55(Gag) is recruited to the plasma membrane (PM), where it binds phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and directs HIV-1 assembly. We show that Rab27a controls the trafficking of late endosomes carrying phosphatidylinositol 4-kinase type 2 α (PI4KIIα) toward the PM of CD4(+) T cells. Hence, Rab27a promotes high levels of PM phosphatidylinositol 4-phosphate and the localized production of PI(4,5)P2, therefore controlling Pr55(Gag) membrane association. Rab27a also controls PI(4,5)P2 levels at the virus-containing compartments of macrophages. By screening Rab27a effectors, we identified that Slp2a, Slp3, and Slac2b are required for the association of Pr55(Gag) with the PM and that Slp2a cooperates with Rab27a in the recruitment of PI4KIIα to the PM. We conclude that by directing the trafficking of PI4KIIα-positive endosomes toward the PM, Rab27a controls PI(4,5)P2 production and, consequently, HIV-1 replication.

PIPs in neurological diseases

Phosphoinositide (PIP) lipids regulate many aspects of cell function in the nervous system including receptor signalling, secretion, endocytosis, migration and survival. Levels of PIPs such as PI4P, PI(4,5)P2 and PI(3,4,5)P3 are normally tightly regulated by phosphoinositide kinases and phosphatases. Deregulation of these biochemical pathways leads to lipid imbalances, usually on intracellular endosomal membranes, and these changes have been linked to a number of major neurological diseases including Alzheimer's, Parkinson's, epilepsy, stroke, cancer and a range of rarer inherited disorders including brain overgrowth syndromes, Charcot-Marie-Tooth neuropathies and neurodevelopmental conditions such as Lowe's syndrome. This article analyses recent progress in this area and explains how PIP lipids are involved, to varying degrees, in almost every class of neurological disease. This article is part of a Special Issue entitled Brain Lipids.

PI4KIIα phosphorylation by GSK3 directs vesicular trafficking to lysosomes

Glycogen synthase kinase 3 (GSK3) is essential for normal development and function of the central nervous system. It is especially important for regulating neurotransmission, although the downstream substrates mediating this function are not yet clear. In the present paper, we report the lipid kinase phosphatidylinositol 4-kinase II α (PI4KIIα) is a novel substrate of GSK3 that regulates trafficking and cell-surface expression of neurotransmitter receptors in neurons. GSK3 phosphorylates two distinct sites in the N-terminus of PI4KIIα (Ser5 and Ser47), promoting binding to the adaptor protein 3 (AP-3) complex for trafficking to the lysosome to be degraded. Blocking phosphorylation reduces trafficking to the lysosome, stabilizing PI4KIIα and its cargo proteins for redistribution throughout the cell. Importantly, a reduction in PI4KIIα expression or phosphorylation increases α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor expression at the surface of hippocampal neurons. These studies implicate signalling between GSK3 and PI4KIIα as a novel regulator of vesicular trafficking and neurotransmission in the brain.

Endosomal sorting of VAMP3 is regulated by PI4K2A

Specificity of membrane fusion in vesicular trafficking is dependent on proper subcellular distribution of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). Although SNARE complexes are fairly promiscuous in vitro, substantial specificity is achieved in cells owing to the spatial segregation and shielding of SNARE motifs prior to association with cognate Q-SNAREs. In this study, we identified phosphatidylinositol 4-kinase IIα (PI4K2A) as a binding partner of vesicle-associated membrane protein 3 (VAMP3), a small R-SNARE involved in recycling and retrograde transport, and found that the two proteins co-reside on tubulo-vesicular endosomes. PI4K2A knockdown inhibited VAMP3 trafficking to perinuclear membranes and impaired the rate of VAMP3-mediated recycling of the transferrin receptor. Moreover, depletion of PI4K2A significantly decreased association of VAMP3 with its cognate Q-SNARE Vti1a. Although binding of VAMP3 to PI4K2A did not require kinase activity, acute depletion of phosphatidylinositol 4-phosphate (PtdIns4P) on endosomes significantly delayed VAMP3 trafficking. Modulation of SNARE function by phospholipids had previously been proposed based on in vitro studies, and our study provides mechanistic evidence in support of these claims by identifying PI4K2A and PtdIns4P as regulators of an R-SNARE in intact cells.

Dual inhibition of EGFR at protein and activity level via combinatorial blocking of PI4KIIα as anti-tumor strategy

Our previous studies indicate that phosphatidylinositol 4-kinase IIα can promote the growth of multi-malignant tumors via HER-2/PI3K and MAPK pathways. However, the molecular mechanisms of this pathway and its potential for clinical application remain unknown. In this study, we found that PI4KIIα could be an ideal combinatorial target for EGFR treatment via regulating EGFR degradation. Results showed that PI4KIIα knockdown reduced EGFR protein level, and the expression of PI4KIIα shows a strong correlation with EGFR in human breast cancer tissues (r = 0.77, P < 0.01). PI4KIIα knockdown greatly prolonged the effects and decreased the effective dosage of AG-1478, a specific inhibitor of EGFR. In addition, it significantly enhanced AG1478-induced inhibition of tumor cell survival and strengthened the effect of the EGFR-targeting anti-cancer drug Iressa in xenograft tumor models. Mechanistically, we found that PI4KIIα suppression increased EGFR ligand-independent degradation. Quantitative proteomic analysis by stable isotope labeling with amino acids in cell culture (SILAC) and LC-MS/MS suggested that HSP90 mediated the effect of PI4KIIα on EGFR. Furthermore, we found that combined inhibition of PI4KIIα and EGFR suppressed both PI3K/AKT and MAPK/ERK pathways, and resulted in downregulation of multiple oncogenes like PRDX2, FASN, MTA2, ultimately leading to suppression of tumor growth. Therefore, we conclude that combined inhibition of PI4KIIα and EGFR exerts a multiple anti-tumor effect. Dual inhibition of EGFR at protein and activity level via combinatorial blocking of PI4KIIα presents a novel strategy to combat EGFR-dependent tumors.

Molecular insights into the membrane-associated phosphatidylinositol 4-kinase IIα

Phosphatidylinositol 4-kinase IIα (PI4KIIα), a membrane-associated PI kinase, plays a central role in cell signalling and trafficking. Its kinase activity critically depends on palmitoylation of its cysteine-rich motif (-CCPCC-) and is modulated by the membrane environment. Lack of atomic structure impairs our understanding of the mechanism regulating kinase activity. Here we present the crystal structure of human PI4KIIα in ADP-bound form. The structure identifies the nucleotide-binding pocket that differs notably from that found in PI3Ks. Two structural insertions, a palmitoylation insertion and an RK-rich insertion, endow PI4KIIα with the ‘integral’ membrane-binding feature. Molecular dynamics simulations, biochemical and mutagenesis studies reveal that the palmitoylation insertion, containing an amphipathic helix, contributes to the PI-binding pocket and anchors PI4KIIα to the membrane, suggesting that fluctuation of the palmitoylation insertion affects PI4KIIα’s activity. We conclude from our results that PI4KIIα’s activity is regulated indirectly through changes in the membrane environment.

Type II PI4-kinase dysfunction is associated with diseases including cancer and Alzheimer's disease; however, the development of specific modulators has been hampered by a lack of structural information. Zhou et al. present the crystal structure of PI4KIIα in its ADP-bound form, providing insight into its regulation.

Cholesterol and F-actin are required for clustering of recycling synaptic vesicle proteins in the presynaptic plasma membrane

Synaptic vesicles (SVs) and their proteins must be recycled for sustained synaptic transmission. We tested the hypothesis that SV cholesterol is required for proper sorting of SV proteins during recycling in live presynaptic terminals. We used the reversible block of endocytosis in the Drosophila temperature-sensitive dynamin mutant shibire-ts1 to trap exocytosed SV proteins, and then examined the effect of experimental treatments on the distribution of these proteins within the presynaptic plasma membrane by confocal microscopy. SV proteins synaptotagmin, vglut and csp were clustered following SV trapping in control experiments but dispersed in samples treated with the cholesterol chelator methyl-β-cyclodextrin to extract SV cholesterol. There was accumulation of phosphatidylinositol (4,5)-bisphosphate (PIP2) in presynaptic terminals following SV trapping and this was reduced following SV cholesterol extraction. Reduced PIP2 accumulation was associated with disrupted accumulation of actin in presynaptic terminals. Similar to vesicular cholesterol extraction, disruption of actin by latrunculin A after SV proteins had been trapped on the plasma membrane resulted in the dispersal of SV proteins and prevented recovery of synaptic transmission due to impaired endocytosis following relief of the endocytic block. Our results demonstrate that vesicular cholesterol is required for aggregation of exocytosed SV proteins in the presynaptic plasma membrane and are consistent with a mechanism involving regulation of PIP2 accumulation and local actin polymerization by cholesterol. Thus, alteration of membrane or SV lipids may affect the ability of synapses to undergo sustained synaptic transmission by compromising the recycling of SV proteins.

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.

Presynaptic membrane retrieval and endosome biology: defining molecularly heterogeneous synaptic vesicles

The release and uptake of neurotransmitters by synaptic vesicles is a tightly controlled process that occurs in response to diverse stimuli at morphologically disparate synapses. To meet these architectural and functional synaptic demands, it follows that there should be diversity in the mechanisms that control their secretion and retrieval and possibly in the composition of synaptic vesicles within the same terminal. Here we pay particular attention to areas where such diversity is generated, such as the variance in exocytosis/endocytosis coupling, SNAREs defining functionally diverse synaptic vesicle populations and the adaptor-dependent sorting machineries capable of generating vesicle diversity. We argue that there are various synaptic vesicle recycling pathways at any given synapse and discuss several lines of evidence that support the role of the endosome in synaptic vesicle recycling.

Modulation of lipid kinase PI4KIIα activity and lipid raft association of presenilin 1 underlies γ-secretase inhibition by ginsenoside (20S)-Rg3

Amyloid β-peptide (Aβ) pathology is an invariant feature of Alzheimer disease, preceding any detectable clinical symptoms by more than a decade. To this end, we seek to identify agents that can reduce Aβ levels in the brain via novel mechanisms. We found that (20S)-Rg3, a triterpene natural compound known as ginsenoside, reduced Aβ levels in cultured primary neurons and in the brains of a mouse model of Alzheimer disease. The (20S)-Rg3 treatment induced a decrease in the association of presenilin 1 (PS1) fragments with lipid rafts where catalytic components of the γ-secretase complex are enriched. The Aβ-lowering activity of (20S)-Rg3 directly correlated with increased activity of phosphatidylinositol 4-kinase IIα (PI4KIIα), a lipid kinase that mediates the rate-limiting step in phosphatidylinositol 4,5-bisphosphate synthesis. PI4KIIα overexpression recapitulated the effects of (20S)-Rg3, whereas reduced expression of PI4KIIα abolished the Aβ-reducing activity of (20S)-Rg3 in neurons. Our results substantiate an important role for PI4KIIα and phosphoinositide modulation in γ-secretase activity and Aβ biogenesis.

MeCP2 regulates the synaptic expression of a Dysbindin-BLOC-1 network component in mouse brain and human induced pluripotent stem cell-derived neurons

Clinical, epidemiological, and genetic evidence suggest overlapping pathogenic mechanisms between autism spectrum disorder (ASD) and schizophrenia. We tested this hypothesis by asking if mutations in the ASD gene MECP2 which cause Rett syndrome affect the expression of genes encoding the schizophrenia risk factor dysbindin, a subunit of the biogenesis of lysosome-related organelles complex-1 (BLOC-1), and associated interacting proteins. We measured mRNA and protein levels of key components of a dysbindin interaction network by, quantitative real time PCR and quantitative immunohistochemistry in hippocampal samples of wild-type and Mecp2 mutant mice. In addition, we confirmed results by performing immunohistochemistry of normal human hippocampus and quantitative qRT-PCR of human inducible pluripotent stem cells (iPSCs)-derived human neurons from Rett syndrome patients. We defined the distribution of the BLOC-1 subunit pallidin in human and mouse hippocampus and contrasted this distribution with that of symptomatic Mecp2 mutant mice. Neurons from mutant mice and Rett syndrome patients displayed selectively reduced levels of pallidin transcript. Pallidin immunoreactivity decreased in the hippocampus of symptomatic Mecp2 mutant mice, a feature most prominent at asymmetric synapses as determined by immunoelectron microcopy. Pallidin immunoreactivity decreased concomitantly with reduced BDNF content in the hippocampus of Mecp2 mice. Similarly, BDNF content was reduced in the hippocampus of BLOC-1 deficient mice suggesting that genetic defects in BLOC-1 are upstream of the BDNF phenotype in Mecp2 deficient mice. Our results demonstrate that the ASD-related gene Mecp2 regulates the expression of components belonging to the dysbindin interactome and these molecular differences may contribute to synaptic phenotypes that characterize Mecp2 deficiencies and ASD.

The WASH complex, an endosomal Arp2/3 activator, interacts with the Hermansky-Pudlak syndrome complex BLOC-1 and its cargo phosphatidylinositol-4-kinase type IIα

The WASH complex, an endosomal activator of the Arp2/3 complex involved in branched actin polymerization, is identified as a new factor in vesicle traffic mediated by the Hermansky–Pudlak syndrome complex BLOC-1.

Vesicle biogenesis machinery components such as coat proteins can interact with the actin cytoskeleton for cargo sorting into multiple pathways. It is unknown, however, whether these interactions are a general requirement for the diverse endosome traffic routes. In this study, we identify actin cytoskeleton regulators as previously unrecognized interactors of complexes associated with the Hermansky–Pudlak syndrome. Two complexes mutated in the Hermansky–Pudlak syndrome, adaptor protein complex-3 and biogenesis of lysosome-related organelles complex-1 (BLOC-1), interact with and are regulated by the lipid kinase phosphatidylinositol-4-kinase type IIα (PI4KIIα). We therefore hypothesized that PI4KIIα interacts with novel regulators of these complexes. To test this hypothesis, we immunoaffinity purified PI4KIIα from isotope-labeled cell lysates to quantitatively identify interactors. Strikingly, PI4KIIα isolation preferentially coenriched proteins that regulate the actin cytoskeleton, including guanine exchange factors for Rho family GTPases such as RhoGEF1 and several subunits of the WASH complex. We biochemically confirmed several of these PI4KIIα interactions. Of importance, BLOC-1 complex, WASH complex, RhoGEF1, or PI4KIIα depletions altered the content and/or subcellular distribution of the BLOC-1–sensitive cargoes PI4KIIα, ATP7A, and VAMP7. We conclude that the Hermansky–Pudlak syndrome complex BLOC-1 and its cargo PI4KIIα interact with regulators of the actin cytoskeleton.

Mammalian phosphatidylinositol 4-kinases as modulators of membrane trafficking and lipid signaling networks

The four mammalian phosphatidylinositol 4-kinases modulate inter-organelle lipid trafficking, phosphoinositide signalling and intracellular vesicle trafficking. In addition to catalytic domains required for the synthesis of PI4P, the phosphatidylinositol 4-kinases also contain isoform-specific structural motifs that mediate interactions with proteins such as AP-3 and the E3 ubiquitin ligase Itch, and such structural differences determine isoform-specific roles in membrane trafficking. Moreover, different permutations of phosphatidylinositol 4-kinase isozymes may be required for a single cellular function such as occurs during distinct stages of GPCR signalling and in Golgi to lysosome trafficking. Phosphatidylinositol 4-kinases have recently been implicated in human disease. Emerging paradigms include increased phosphatidylinositol 4-kinase expression in some cancers, impaired functioning associated with neurological pathologies, the subversion of PI4P trafficking functions in bacterial infection and the activation of lipid kinase activity in viral disease. We discuss how the diverse and sometimes overlapping functions of the phosphatidylinositol 4-kinases present challenges for the design of isoform-specific inhibitors in a therapeutic context.