Coupling exo- and endocytosis: an essential role for PIP₂ at the synapse

Endogenous Protein-Protein Interaction Network of the NPC Cholesterol Transporter 1 in the Cerebral Cortex

NPC intracellular cholesterol transporter 1 (NPC1) is a multipass, transmembrane glycoprotein mostly recognized for its key role in facilitating cholesterol efflux. Mutations in the NPC1 gene result in Niemann-Pick disease, type C (NPC), a fatal, lysosomal storage disease. Due to the progressively expanding implications of NPC1-related disorders, we investigated endogenous NPC1 protein-protein interactions in the mouse cortex and human-derived iPSCs neuronal models of the disease through coimmunoprecipitation-coupled with LC-MS based proteomics. The current study investigated protein-protein interactions specific to the wild-type and the most prevalent NPC1 mutation (NPC1I1061T) while filtering out any protein interactor identified in the Npc1-/- mouse model. Additionally, the results were matched across the two species to map the parallel interactome of wild-type and mutant NPC1I1061T. Most of the identified wild-type NPC1 interactors were related to cytoskeleton organization, synaptic vesicle activity, and translation. We found many putative NPC1 interactors not previously reported, including two SCAR/WAVE complex proteins that regulate ARP 2/3 complex actin nucleation and multiple membrane proteins important for neuronal activity at synapse. Moreover, we identified proteins important in trafficking specific to wild-type and mutant NPC1I1061T. Together, the findings are essential for a comprehensive understanding of NPC1 biological functions in addition to its classical role in sterol efflux.

MARCKS and PI(4,5)P2 reciprocally regulate actin-based dendritic spine morphology

Myristoylated, alanine-rich C-kinase substrate (MARCKS) is an F-actin and phospholipid binding protein implicated in numerous cellular activities, including the regulation of morphology in neuronal dendrites and dendritic spines. MARCKS contains a lysine-rich effector domain that mediates its binding to plasma membrane phosphatidylinositol-4,5-biphosphate (PI(4,5)P2) in a manner controlled by PKC and calcium/calmodulin. In neurons, manipulations of MARCKS concentration and membrane targeting strongly affect the numbers, shapes, and F-actin properties of dendritic spines, but the mechanisms remain unclear. Here, we tested the hypothesis that the effects of MARCKS on dendritic spine morphology are due to its capacity to regulate the availability of plasma membrane PI(4,5)P2. We observed that the concentration of free PI(4,5)P2 on the dendritic plasma membrane was inversely proportional to the concentration of MARCKS. Endogenous PI(4,5)P2 levels were increased or decreased, respectively, by acutely overexpressing either phosphatidylinositol-4-phosphate 5-kinase (PIP5K) or inositol polyphosphate 5-phosphatase (5ptase). PIP5K, like MARCKS depletion, induced severe spine shrinkage; 5ptase, like constitutively membrane-bound MARCKS, induced aberrant spine elongation. These phenotypes involved changes in actin properties driven by the F-actin severing protein cofilin. Collectively, these findings support a model in which neuronal activity regulates actin-dependent spine morphology through antagonistic interactions of MARCKS and PI(4,5)P2.

Synaptotagmin 1-triggered lipid signaling facilitates coupling of exo- and endocytosis

Exocytosis and endocytosis are essential physiological processes and are of prime importance for brain function. Neurotransmission depends on the Ca2+-triggered exocytosis of synaptic vesicles (SVs). In neurons, exocytosis is spatiotemporally coupled to the retrieval of an equal amount of membrane and SV proteins by compensatory endocytosis. How exocytosis and endocytosis are balanced to maintain presynaptic membrane homeostasis and, thereby, sustain brain function is essentially unknown. We combine mouse genetics with optical imaging to show that the SV calcium sensor Synaptotagmin 1 couples exocytic SV fusion to the endocytic retrieval of SV membranes by promoting the local activity-dependent formation of the signaling lipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at presynaptic sites. Interference with these mechanisms impairs PI(4,5)P2-triggered SV membrane retrieval but not exocytic SV fusion. Our findings demonstrate that the coupling of SV exocytosis and endocytosis involves local Synaptotagmin 1-induced lipid signaling to maintain presynaptic membrane homeostasis in central nervous system neurons.

Activation of TRPV1 Channels Inhibits the Release of Acetylcholine and Improves Muscle Contractility in Mice

TRPV1 represents a non-selective transient receptor potential cation channel found not only in sensory neurons, but also in motor nerve endings and in skeletal muscle fibers. However, the role of TRPV1 in the functioning of the neuromuscular junction has not yet been fully established. In this study, the Levator Auris Longus muscle preparations were used to assess the effect of pharmacological activation of TRPV1 channels on neuromuscular transmission. The presence of TRPV1 channels in the nerve terminal and in the muscle fiber was confirmed by immunohistochemistry. It was verified by electrophysiology that the TRPV1 channel agonist capsaicin inhibits the acetylcholine release, and this effect was completely absent after preliminary application of the TRPV1 channel blocker SB 366791. Nerve stimulation revealed an increase of amplitude of isometric tetanic contractions upon application of capsaicin which was also eliminated after preliminary application of SB 366791. Similar data were obtained during direct muscle stimulation. Thus, pharmacological activation of TRPV1 channels affects the functioning of both the pre- and postsynaptic compartment of the neuromuscular junction. A moderate decrease in the amount of acetylcholine released from the motor nerve allows to maintain a reserve pool of the mediator to ensure a longer signal transmission process, and an increase in the force of muscle contraction, in its turn, also implies more effective physiological muscle activity in response to prolonged stimulation. This assumption is supported by the fact that when muscle was indirect stimulated with a fatigue protocol, muscle fatigue was attenuated in the presence of capsaicin.

PLCγ1 in dopamine neurons critically regulates striatal dopamine release via VMAT2 and synapsin III

Dopamine neurons are essential for voluntary movement, reward learning, and motivation, and their dysfunction is closely linked to various psychological and neurodegenerative diseases. Hence, understanding the detailed signaling mechanisms that functionally modulate dopamine neurons is crucial for the development of better therapeutic strategies against dopamine-related disorders. Phospholipase Cγ1 (PLCγ1) is a key enzyme in intracellular signaling that regulates diverse neuronal functions in the brain. It was proposed that PLCγ1 is implicated in the development of dopaminergic neurons, while the physiological function of PLCγ1 remains to be determined. In this study, we investigated the physiological role of PLCγ1, one of the key effector enzymes in intracellular signaling, in regulating dopaminergic function in vivo. We found that cell type-specific deletion of PLCγ1 does not adversely affect the development and cellular morphology of midbrain dopamine neurons but does facilitate dopamine release from dopaminergic axon terminals in the striatum. The enhancement of dopamine release was accompanied by increased colocalization of vesicular monoamine transporter 2 (VMAT2) at dopaminergic axon terminals. Notably, dopamine neuron-specific knockout of PLCγ1 also led to heightened expression and colocalization of synapsin III, which controls the trafficking of synaptic vesicles. Furthermore, the knockdown of VMAT2 and synapsin III in dopamine neurons resulted in a significant attenuation of dopamine release, while this attenuation was less severe in PLCγ1 cKO mice. Our findings suggest that PLCγ1 in dopamine neurons could critically modulate dopamine release at axon terminals by directly or indirectly interacting with synaptic machinery, including VMAT2 and synapsin III.

Long-term depression in neurons involves temporal and ultra-structural dynamics of phosphatidylinositol-4,5-bisphosphate relying on PIP5K, PTEN and PLC

Synaptic plasticity involves proper establishment and rearrangement of structural and functional microdomains. Yet, visualization of the underlying lipid cues proved challenging. Applying a combination of rapid cryofixation, membrane freeze-fracturing, immunogold labeling and electron microscopy, we visualize and quantitatively determine the changes and the distribution of phosphatidylinositol-4,5-bisphosphate (PIP₂) in the plasma membrane of dendritic spines and subareas thereof at ultra-high resolution. These efforts unravel distinct phases of PIP₂ signals during induction of long-term depression (LTD). During the first minutes PIP₂ rapidly increases in a PIP5K-dependent manner forming nanoclusters. PTEN contributes to a second phase of PIP₂ accumulation. The transiently increased PIP₂ signals are restricted to upper and middle spine heads. Finally, PLC-dependent PIP₂ degradation provides timely termination of PIP₂ cues during LTD induction. Together, this work unravels the spatial and temporal cues set by PIP₂ during different phases after LTD induction and dissects the molecular mechanisms underlying the observed PIP₂ dynamics.

PI(4,5)P2 and Cholesterol: Synthesis, Regulation, and Functions

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is the most abundant membrane phosphoinositide and cholesterol is an essential component of the plasma membrane (PM). Both lipids play key roles in a variety of cellular functions including as signaling molecules and major regulators of protein function. This chapter provides an overview of these two important lipids. Starting from a brief description of their structure, synthesis, and regulation, the chapter continues to describe the primary functions and signaling processes in which PI(4,5)P₂ and cholesterol are involved. While PI(4,5)P₂ and cholesterol can act independently, they often act in concert or affect each other's impact. The chapters in this volume on "Cholesterol and PI(4,5)P₂ in Vital Biological Functions: From Coexistence to Crosstalk" focus on the emerging relationship between cholesterol and PI(4,5)P₂ in a variety of biological systems and processes. In this chapter, the next section provides examples from the ion channel field demonstrating that PI(4,5)P₂ and cholesterol can act via common mechanisms. The chapter ends with a discussion of future directions.

Hippocampal neurons maintain a large PtdIns(4)P pool that results in faster PtdIns(4,5)P2 synthesis

PtdIns(4,5)P₂ is a signaling lipid central to the regulation of multiple cellular functions. It remains unknown how PtdIns(4,5)P₂ fulfills various functions in different cell types, such as regulating neuronal excitability, synaptic release, and astrocytic function. Here, we compared the dynamics of PtdIns(4,5)P₂ synthesis in hippocampal neurons and astrocytes with the kidney-derived tsA201 cell line. The experimental approach was to (1) measure the abundance and rate of PtdIns(4,5)P₂ synthesis and precursors using specific biosensors, (2) measure the levels of PtdIns(4,5)P₂ and its precursors using mass spectrometry, and (3) use a mathematical model to compare the metabolism of PtdIns(4,5)P₂ in cell types with different proportions of phosphoinositides. The rate of PtdIns(4,5)P₂ resynthesis in hippocampal neurons after depletion by cholinergic or glutamatergic stimulation was three times faster than for tsA201 cells. In tsA201 cells, resynthesis of PtdIns(4,5)P₂ was dependent on the enzyme PI4K. In contrast, in hippocampal neurons, the resynthesis rate of PtdIns(4,5)P₂ was insensitive to the inhibition of PI4K, indicating that it does not require de novo synthesis of the precursor PtdIns(4)P. Measurement of phosphoinositide abundance indicated a larger pool of PtdIns(4)P, suggesting that hippocampal neurons maintain sufficient precursor to restore PtdIns(4,5)P₂ levels. Quantitative modeling indicates that the measured differences in PtdIns(4)P pool size and higher activity of PI4K can account for the experimental findings and indicates that high PI4K activity prevents depletion of PtdIns(4)P. We further show that the resynthesis of PtdIns(4,5)P₂ is faster in neurons than astrocytes, providing context to the relevance of cell type–specific mechanisms to sustain PtdIns(4,5)P₂ levels.

AT1 Receptors: Their Actions from Hypertension to Cognitive Impairment

Hypertension is one of the most prevalent cardiovascular disorders worldwide, affecting 1.13 billion people, or 14% of the global population. Hypertension is the single biggest risk factor for cerebrovascular dysfunction. According to the American Heart Association, high blood pressure (BP), especially in middle-aged individuals (~ 40 to 60 years old), is associated with an increased risk of dementia, later in life. Alzheimer’s disease and cerebrovascular disease are the two leading causes of dementia, accounting for around 80% of the total cases and usually combining mixed pathologies from both. Little is known regarding how hypertension affects cognitive function, so the impact of its treatment on cognitive impairment has been difficult to assess. The brain renin-angiotensin system (RAS) is essential for BP regulation and overactivity of this system has been established to precede the development and maintenance of hypertension. Angiotensin II (Ang-II), the main peptide within this system, induces vasoconstriction and impairs neuro-vascular coupling by acting on brain Ang-II type 1 receptors (AT₁R). In this review, we systemically analyzed the association between RAS and biological mechanisms of cognitive impairment, from the perspective of AT₁R located in the central nervous system. Additionally, the possible contribution of brain AT₁R to global cognition decline in COVID-19 cases will be discussed as well.

Long-term exposure to high glucose induces changes in the expression of AMPA receptor subunits and glutamate transmission in primary cultured cortical neurons

Hyperglycemia, which occurs under the diabetic conditions, induces serious diabetic complications. Diabetic encephalopathy has been defined as one of the major complications of diabetes, and is characterized by neurochemical and neurodegenerative changes. However, little is known about the effect of long-term exposure to high glucose on neuronal cells. In the present study, we showed that exposure to glutamate (100 mM) for 7 days induced toxicity in primary cortical neurons using the MTT assay. Additionally, high glucose increased the sensitivity of AMPA- or NMDA-induced neurotoxicity, and decreased extracellular glutamate levels in primary cortical neurons. In Western blot analyses, the protein levels of the GluA1 and GluA2 subunits of the AMPA receptor as well as synaptophysin in neurons treated with high glucose were significantly increased compared with the control (25 mM glucose). Therefore, long-term exposure to high glucose induced neuronal death through the disruption of glutamate homeostasis.

Acyl-chain saturation regulates the order of phosphatidylinositol 4,5-bisphosphate nanodomains

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) plays a critical role in the regulation of various plasma membrane processes and signaling pathways in eukaryotes. A significant amount of cellular resources are spent on maintaining the dominant 1-stearoyl-2-arachidonyl PI(4,5)P₂ acyl-chain composition, while less abundant and more saturated species become more prevalent in response to specific stimuli, stress or aging. Here, we report the impact of acyl-chain structure on the biophysical properties of cation-induced PI(4,5)P₂ nanodomains. PI(4,5)P₂ species with increasing levels of acyl-chain saturation cluster in progressively more ordered nanodomains, culminating in the formation of gel-like nanodomains for fully saturated species. The formation of these gel-like domains was largely abrogated in the presence of 1-stearoyl-2-arachidonyl PI(4,5)P_2. This is, to the best of our knowledge, the first report of the impact of PI(4,5)P₂ acyl-chain composition on cation-dependent nanodomain ordering, and provides important clues to the motives behind the enrichment of PI(4,5)P₂ with polyunsaturated acyl-chains. We also show how Ca²⁺-induced PI(4,5)P₂ nanodomains are able to generate local negative curvature, a phenomenon likely to play a role in membrane remodeling events.

On the Need to Tell Apart Fraternal Twins eEF1A1 and eEF1A2, and Their Respective Outfits

eEF1A1 and eEF1A2 are paralogous proteins whose presence in most normal eukaryotic cells is mutually exclusive and developmentally regulated. Often described in the scientific literature under the collective name eEF1A, which stands for eukaryotic elongation factor 1A, their best known activity (in a monomeric, GTP-bound conformation) is to bind aminoacyl-tRNAs and deliver them to the A-site of the 80S ribosome. However, both eEF1A1 and eEF1A2 are endowed with multitasking abilities (sometimes performed by homo- and heterodimers) and can be located in different subcellular compartments, from the plasma membrane to the nucleus. Given the high sequence identity of these two sister proteins and the large number of post-translational modifications they can undergo, we are often confronted with the dilemma of discerning which is the particular proteoform that is actually responsible for the ascribed biochemical or cellular effects. We argue in this review that acquiring this knowledge is essential to help clarify, in molecular and structural terms, the mechanistic involvement of these two ancestral and abundant G proteins in a variety of fundamental cellular processes other than translation elongation. Of particular importance for this special issue is the fact that several de novo heterozygous missense mutations in the human EEF1A2 gene are associated with a subset of rare but severe neurological syndromes and cardiomyopathies.

Two StAR-related lipid transfer proteins play specific roles in endocytosis, exocytosis, and motility in the parasitic protist Entamoeba histolytica

Lipid transfer proteins (LTPs) are the key contributor of organelle-specific lipid distribution and cellular lipid homeostasis. Here, we report a novel implication of LTPs in phagocytosis, trogocytosis, pinocytosis, biosynthetic secretion, recycling of pinosomes, and motility of the parasitic protist E. histolytica, the etiological agent of human amoebiasis. We show that two StAR-related lipid transfer (START) domain-containing LTPs (named as EhLTP1 and 3) are involved in these biological pathways in an LTP-specific manner. Our findings provide novel implications of LTPs, which are relevant to the elucidation of pathophysiology of the diseases caused by parasitic protists.

We showed that EhLTP1, but not EhLTP3, is involved in secretion of cysteine protease, the well-established degrading factor of host cells and the extracellular matrix, and in pseudopod formation and migration. In contrast, EhLTP3, but not EhLTP1, is exclusively involved in pinocytosis of the fluid-phase marker. Both EhLTP1 and EhLTP3 are also involved in trogocytosis (ingestion by nibbling) of live mammalian cells and phagocytosis of dead cells. In trogocytosis and phagcytosis, these two LTPs displayed distinct patterns of recruitment: e.g., EhLTP1 was associated at the ligand attachment site at the initiation of trogocytosis, followed by the recruitment of EhLTP3 onto the “trogocytic tunnel” at the intermediate stage of trogocytosis before the closure of the trogosome. Such tempo-spatially coordinated involvement of LTPs in the course of trogo- and phagocytosis has never been demonstrated in unicellular eukaryotes. Neither has LTP been shown to be involved in both endocytosis and exocytosis.

Disposition of Proteins and Lipids in Synaptic Membrane Compartments Is Altered in Q175/Q7 Huntington's Disease Mouse Striatum

Dysfunction at synapses is thought to be an early change contributing to cognitive, psychiatric and motor disturbances in Huntington's disease (HD). In neurons, mutant Huntingtin collects in aggregates and distributes to the same sites as wild-type Huntingtin including on membranes and in synapses. In this study, we investigated the biochemical integrity of synapses in HD mouse striatum. We performed subcellular fractionation of striatal tissue from 2 and 6-month old knock-in Q175/Q7 HD and Q7/Q7 mice. Compared to striata of Q7/Q7 mice, proteins including GLUT3, Na+/K+ ATPase, NMDAR 2b, PSD95, and VGLUT1 had altered distribution in Q175/Q7 HD striata of 6-month old mice but not 2-month old mice. These proteins are found on plasma membranes and pre- and postsynaptic membranes supporting hypotheses that functional changes at synapses contribute to cognitive and behavioral symptoms of HD. Lipidomic analysis of mouse fractions indicated that compared to those of wild-type, fractions 1 and 2 of 6 months Q175/Q7 HD had altered levels of two species of PIP2, a phospholipid involved in synaptic signaling, increased levels of cholesterol ester and decreased cardiolipin species. At 2 months, increased levels of species of acylcarnitine, phosphatidic acid and sphingomyelin were measured. EM analysis showed that the contents of fractions 1 and 2 of Q7/Q7 and Q175/Q7 HD striata had a mix of isolated synaptic vesicles, vesicle filled axon terminals singly or in clusters, and ER and endosome-like membranes. However, those of Q175/Q7 striata contained significantly fewer and larger clumps of particles compared to those of Q7/Q7. Human HD postmortem putamen showed differences from control putamen in subcellular distribution of two proteins (Calnexin and GLUT3). Our biochemical, lipidomic and EM analysis show that the presence of the HD mutation conferred age dependent disruption of localization of synaptic proteins and lipids important for synaptic function. Our data demonstrate concrete biochemical changes suggesting altered integrity of synaptic compartments in HD mice that may mirror changes in HD patients and presage cognitive and psychiatric changes that occur in premanifest HD.

Synaptotagmin-1 interacts with PI(4,5)P2 to initiate synaptic vesicle docking in hippocampal neurons

Synaptic vesicle (SV) docking is a dynamic multi-stage process that is required for efficient neurotransmitter release in response to nerve impulses. Although the steady-state SV docking likely involves the cooperation of Synaptotagmin-1 (Syt1) and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), where and how the docking process initiates remains unknown. Phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) can interact with Syt1 and SNAREs to contribute to vesicle exocytosis. In the present study, using the CRISPRi-mediated multiplex gene knockdown and 3D electron tomography approaches, we show that in mouse hippocampal synapses, SV docking initiates at ∼12 nm to the active zone (AZ) by Syt1. Furthermore, we demonstrate that PI(4,5)P2 is the membrane partner of Syt1 to initiate SV docking, and disrupting their interaction could abolish the docking initiation. In contrast, the SNARE complex contributes only to the tight SV docking within 0-2 nm. Therefore, Syt1 interacts with PI(4,5)P2 to loosely dock SVs within 2-12 nm to the AZ in hippocampal neurons.

PI(4,5)P2 Clustering and Its Impact on Biological Functions

Located at the inner leaflet of the plasma membrane (PM), phosphatidyl-inositol 4,5-bisphosphate [PI(4,5)P₂] composes only 1-2 mol% of total PM lipids. With its synthesis and turnover both spatially and temporally regulated, PI(4,5)P₂ recruits and interacts with hundreds of cellular proteins to support a broad spectrum of cellular functions. Several factors contribute to the versatile and dynamic distribution of PI(4,5)P₂ in membranes. Physiological multivalent cations such as Ca²⁺ and Mg²⁺ can bridge between PI(4,5)P₂ headgroups, forming nanoscopic PI(4,5)P₂-cation clusters. The distinct lipid environment surrounding PI(4,5)P₂ affects the degree of PI(4,5)P₂ clustering. In addition, diverse cellular proteins interacting with PI(4,5)P₂ can further regulate PI(4,5)P₂ lateral distribution and accessibility. This review summarizes the current understanding of PI(4,5)P₂ behavior in both cells and model membranes, with emphasis on both multivalent cation- and protein-induced PI(4,5)P₂ clustering. Understanding the nature of spatially separated pools of PI(4,5)P₂ is fundamental to cell biology.

Structural Cues for Understanding eEF1A2 Moonlighting

Spontaneous mutations in the EEF1A2 gene cause epilepsy and severe neurological disabilities in children. The crystal structure of eEF1A2 protein purified from rabbit skeletal muscle reveals a post-translationally modified dimer that provides information about the sites of interaction with numerous binding partners, including itself, and maps these mutations onto the dimer and tetramer interfaces. The spatial locations of the side chain carboxylates of Glu301 and Glu374, to which phosphatidylethanolamine is uniquely attached via an amide bond, define the anchoring points of eEF1A2 to cellular membranes and interorganellar membrane contact sites. Additional bioinformatic and molecular modeling results provide novel structural insight into the demonstrated binding of eEF1A2 to SH3 domains, the common MAPK docking groove, filamentous actin, and phosphatidylinositol-4 kinase IIIβ. In this new light, the role of eEF1A2 as an ancient, multifaceted, and articulated G protein at the crossroads of autophagy, oncogenesis and viral replication appears very distant from the "canonical" one of delivering aminoacyl-tRNAs to the ribosome that has dominated the scene and much of the thinking for many decades.

Structure and Lateral Organization of Phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is a minor but ubiquitous component of the inner leaflet of the plasma membrane of eukaryotic cells. However, due to its particular complex biophysical properties, it stands out from its neighboring lipids as one of the most important regulators of membrane-associated signaling events. Despite its very low steady-state concentration, PI(4,5)P₂ is able to engage in a multitude of simultaneous cellular functions that are temporally and spatially regulated through the presence of localized transient pools of PI(4,5)P₂ in the membrane. These pools are crucial for the recruitment, activation, and organization of signaling proteins and consequent regulation of downstream signaling. The present review showcases some of the most important PI(4,5)P2 molecular and biophysical properties as well as their impact on its membrane dynamics, lateral organization, and interactions with other biochemical partners.

Activity and Cytosolic Na+ Regulate Synaptic Vesicle Endocytosis

Retrieval of synaptic vesicles via endocytosis is essential for maintaining sustained synaptic transmission, especially for neurons that fire action potentials at high frequencies. However, how neuronal activity regulates synaptic vesicle recycling is largely unknown. Here we report that Na⁺ substantially accumulated in the mouse calyx of Held terminals of either sex during repetitive high-frequency spiking. Elevated presynaptic Na⁺ accelerated both slow and rapid forms of endocytosis and facilitated endocytosis overshoot, but did not affect the readily releasable pool size, Ca²⁺ influx, or exocytosis. To examine whether this facilitation of endocytosis is related to the Na⁺-dependent vesicular content change, we dialyzed glutamate into the presynaptic cytosol or blocked the vesicular glutamate uptake with bafilomycin and found that the rate of endocytosis was not affected by regulating the vesicular glutamate content. Endocytosis is critically dependent on intracellular Ca²⁺, and the activity of Na⁺/Ca²⁺ exchanger (NCX) may be altered when the Na⁺ gradient is changed. However, neither NCX inhibitor nor change of extracellular Na⁺ concentration affected the endocytosis rate. Moreover, two-photon Ca²⁺ imaging showed that presynaptic Na⁺ did not affect the action potential-evoked intracellular Ca²⁺ transient and decay. Therefore, we revealed a novel mechanism of cytosolic Na⁺ in accelerating vesicle endocytosis. During high-frequency synaptic transmission, when large numbers of synaptic vesicles were fused, the rapid buildup of presynaptic cytosolic Na⁺ promoted vesicle recycling and sustained synaptic transmission.SIGNIFICANCE STATEMENT High-frequency firing neurons are widely distributed in the CNS. A large number of synaptic vesicles are released during high-frequency synaptic transmission; accordingly, synaptic vesicles need to be recycled rapidly to replenish the vesicle pool. Synaptic vesicle exocytosis and endocytosis are tightly coupled, and their coupling is essential for synaptic function and structural stability. We showed here that intracellular Na⁺ concentration at the calyx of Held terminal increased rapidly during spike activity and the increased Na⁺ accelerated endocytosis. Thus, when large numbers of synaptic vesicles are released during high-frequency synaptic transmission, Na⁺ accumulated in terminals and facilitated vesicle recycling. These findings represent a novel cellular mechanism that supports reliable synaptic transmission at high frequency in the CNS.

Restoration of MARCKS enhances chemosensitivity in cancer

Purpose

Increased ATP-binding-cassette (ABC) transporter activity is a major cause of chemotherapy resistance in cancer. The ABC transporter family member ABCB1 is often overexpressed in colorectal cancer (CRC). Phosphatidylinositol-4,5-bisphosphat (PI(4,5)P₂)-dependent pathways are involved in the regulation of ABCB1 function. The protein Myristoylated Alanine-Rich C-Kinase Substrate (MARCKS) is a pivotal regulator of PI(4,5)P₂ and inactivated in many CRC cancers via genetic deletion or hyperphosphorylation. Therefore, MARCKS may critically impact ABCB1.

Methods

CRC samples as well as CRC cell lines were tested for a connection between MARCKS and ABCB1 via immunofluorescence and Western-blot analysis. ABCB1 function was studied via calcein influx assay under treatment with known ABCB1 inhibitors (verapamil, tariquidar) as well as the kinase inhibitor bosutinib. ABCB1 internalization and MARCKS translocation was analyzed via confocal microscopy exploiting the endocytosis inhibitors chlorpromazine and dynasore. Abundance of PI(4,5)P₂ was monitored by intramolecular fluorescence resonance energy transfer (FRET). Reproductive cell survival was studied via colorimetric WST-1 and clonogenic assays in combination with exposure to the chemotherapeutics doxorubicin and 5-fuorouracil (5-FU).

Results

We found increased ABCB1 expression in MARCKS negative CRC patient tumor samples and established CRC cell lines. Mechanistically, the reconstitution of MARCKS function via recombinant expression or the pharmacological inhibition of MARCKS phosphorylation led to a substantial decrease in ABCB1 activity. In CRC cells, bosutinib treatment resulted in a MARCKS translocation from the cytosol to the plasma membrane, while simultaneously, ABCB1 was relocated to intracellular compartments. Inhibition of MARCKS phosphorylation via bosutinib rendered cells more sensitive to the chemotherapeutics doxorubicin and 5-FU.

Conclusions

Cells devoid of MARCKS function showed incomplete ABCB1 internalization, leading to higher ABCB1 activity enhancing chemoresistance. Vice versa our data suggest the prevention of MARCKS inhibition by reversing hyperphosphorylation or genomic restoration after deletion as two promising approaches to overcome tumor cell resistance towards chemotherapeutic ABCB1 substrates.

The CHD Protein, Kismet, is Important for the Recycling of Synaptic Vesicles during Endocytosis

Chromatin remodeling proteins of the chromodomain DNA-binding protein family, CHD7 and CHD8, mediate early neurodevelopmental events including neural migration and differentiation. As such, mutations in either protein can lead to neurodevelopmental disorders. How chromatin remodeling proteins influence the activity of mature synapses, however, is relatively unexplored. A critical feature of mature neurons is well-regulated endocytosis, which is vital for synaptic function to recycle membrane and synaptic proteins enabling the continued release of synaptic vesicles. Here we show that Kismet, the Drosophila homolog of CHD7 and CHD8, regulates endocytosis. Kismet positively influenced transcript levels and bound to dap160 and endophilin B transcription start sites and promoters in whole nervous systems and influenced the synaptic localization of Dynamin/Shibire. In addition, kismet mutants exhibit reduced VGLUT, a synaptic vesicle marker, at stimulated but not resting synapses and reduced levels of synaptic Rab11. Endocytosis is restored at kismet mutant synapses by pharmacologically inhibiting the function of histone deacetyltransferases (HDACs). These data suggest that HDAC activity may oppose Kismet to promote synaptic vesicle endocytosis. A deeper understanding of how CHD proteins regulate the function of mature neurons will help better understand neurodevelopmental disorders.

At the intersection of exocytosis and endocytosis in plants

Vesicle exocytosis and endocytosis control the activities and turnover of plasma membrane proteins required for signaling triggering or attenuating at the cell surface. In recent years, the diverse exocytic and endocytic trafficking pathways have been uncovered in plants. The balance between conventional and unconventional protein secretion provides an efficient strategy to respond to stress conditions. Similarly, clathrin-dependent and -independent endocytosis cooperatively regulate the dynamics of membrane proteins in response to environmental cues. In fact, many aspects of plant growth and development, such as tip growth, immune response, and protein polarity establishment, involve the tight deployment of exo-endocytic trafficking. However, our understanding of their intersection is limited. Here, we discuss recent advances in the molecular factors coupling plant exo-endocytic trafficking and the biological significance of balance between exocytosis and endocytosis in plants.

Regulation of Membrane Turnover by Phosphatidic Acid: Cellular Functions and Disease Implications

Phosphatidic acid (PA) is a simple glycerophospholipid with a well-established role as an intermediate in phospholipid biosynthesis. In addition to its role in lipid biosynthesis, PA has been proposed to act as a signaling molecule that modulates several aspects of cell biology including membrane transport. PA can be generated in eukaryotic cells by several enzymes whose activity is regulated in the context of signal transduction and enzymes that can metabolize PA thus terminating its signaling activity have also been described. Further, several studies have identified PA binding proteins and changes in their activity are proposed to be mediators of the signaling activity of this lipid. Together these enzymes and proteins constitute a PA signaling toolkit that mediates the signaling functions of PA in cells. Recently, a number of novel genetic models for the analysis of PA function in vivo and analytical methods to quantify PA levels in cells have been developed and promise to enhance our understanding of PA functions. Studies of several elements of the PA signaling toolkit in a single cell type have been performed and are presented to provide a perspective on our understanding of the biochemical and functional organization of pools of PA in a eukaryotic cell. Finally, we also provide a perspective on the potential role of PA in human disease, synthesizing studies from model organisms, human disease genetics and analysis using recently developed PLD inhibitors.

RIM C2B Domains Target Presynaptic Active Zone Functions to PIP2-Containing Membranes

Rapid and efficient synaptic vesicle fusion requires a pool of primed vesicles, the nearby tethering of Ca²⁺ channels, and the presence of the phospholipid PIP₂ in the target membrane. Although the presynaptic active zone mediates the first two requirements, it is unclear how fusion is targeted to membranes with high PIP₂ content. Here we find that the C₂B domain of the active zone scaffold RIM is critical for action potential-triggered fusion. Remarkably, the known RIM functions in vesicle priming and Ca²⁺ influx do not require RIM C₂B domains. Instead, biophysical experiments reveal that RIM C₂ domains, which lack Ca²⁺ binding, specifically bind to PIP₂. Mutational analyses establish that PIP₂ binding to RIM C₂B and its tethering to the other RIM domains are crucial for efficient exocytosis. We propose that RIM C₂B domains are constitutive PIP₂-binding modules that couple mechanisms for vesicle priming and Ca²⁺ channel tethering to PIP₂-containing target membranes.

Safeguards of Neurotransmission: Endocytic Adaptors as Regulators of Synaptic Vesicle Composition and Function

Communication between neurons relies on neurotransmitters which are released from synaptic vesicles (SVs) upon Ca²⁺ stimuli. To efficiently load neurotransmitters, sense the rise in intracellular Ca²⁺ and fuse with the presynaptic membrane, SVs need to be equipped with a stringently controlled set of transmembrane proteins. In fact, changes in SV protein composition quickly compromise neurotransmission and most prominently give rise to epileptic seizures. During exocytosis SVs fully collapse into the presynaptic membrane and consequently have to be replenished to sustain neurotransmission. Therefore, surface-stranded SV proteins have to be efficiently retrieved post-fusion to be used for the generation of a new set of fully functional SVs, a process in which dedicated endocytic sorting adaptors play a crucial role. The question of how the precise reformation of SVs is achieved is intimately linked to how SV membranes are retrieved. For a long time both processes were believed to be two sides of the same coin since Clathrin-mediated endocytosis (CME), the proposed predominant SV recycling mode, will jointly retrieve SV membranes and proteins. However, with the recent proposal of Clathrin-independent SV recycling pathways SV membrane retrieval and SV reformation turn into separable events. This review highlights the progress made in unraveling the molecular mechanisms mediating the high-fidelity retrieval of SV proteins and discusses how the gathered knowledge about SV protein recycling fits in with the new notions of SV membrane endocytosis.

MoEnd3 regulates appressorium formation and virulence through mediating endocytosis in rice blast fungus Magnaporthe oryzae

Eukaryotic cells respond to environmental stimuli when cell surface receptors are bound by environmental ligands. The binding initiates a signal transduction cascade that results in the appropriate intracellular responses. Studies have shown that endocytosis is critical for receptor internalization and signaling activation. In the rice blast fungus Magnaporthe oryzae, a non-canonical G-protein coupled receptor, Pth11, and membrane sensors MoMsb2 and MoSho1 are thought to function upstream of G-protein/cAMP signaling and the Pmk1 MAPK pathway to regulate appressorium formation and pathogenesis. However, little is known about how these receptors or sensors are internalized and transported into intracellular compartments. We found that the MoEnd3 protein is important for endocytic transport and that the ΔMoend3 mutant exhibited defects in efficient internalization of Pth11 and MoSho1. The ΔMoend3 mutant was also defective in Pmk1 phosphorylation, autophagy, appressorium formation and function. Intriguingly, restoring Pmk1 phosphorylation levels in ΔMoend3 suppressed most of these defects. Moreover, we demonstrated that MoEnd3 is subject to regulation by MoArk1 through protein phosphorylation. We also found that MoEnd3 has additional functions in facilitating the secretion of effectors, including Avr-Pia and AvrPiz-t that suppress rice immunity. Taken together, our findings suggest that MoEnd3 plays a critical role in mediating receptor endocytosis that is critical for the signal transduction-regulated development and virulence of M. oryzae.

Exocytosis, Endocytosis, and Their Coupling in Excitable Cells

Evoked exocytosis in excitable cells is fast and spatially confined and must be followed by coupled endocytosis to enable sustained exocytosis while maintaining the balance of the vesicle pool and the plasma membrane. Various types of exocytosis and endocytosis exist in these excitable cells, as those has been found from different types of experiments conducted in different cell types. Correlating these diversified types of exocytosis and endocytosis is problematic. By providing an outline of different exocytosis and endocytosis processes and possible coupling mechanisms here, we emphasize that the endocytic pathway may be pre-determined at the time the vesicle chooses to fuse with the plasma membrane in one specific mode. Therefore, understanding the early intermediate stages of vesicle exocytosis may be instrumental in exploring the mechanism of tailing endocytosis.

Phosphatidic acid and neurotransmission

Lipids play a vital role in the health and functioning of neurons and interest in the physiological role of neuronal lipids is certainly increasing. One neuronal function in which neuronal lipids appears to play key roles in neurotransmission. Our understanding of the role of lipids in the synaptic vesicle cycle and neurotransmitter release is becoming increasingly more important. Much of the initial research in this area has highlighted the major roles played by the phosphoinositides (PtdIns), diacylglycerol (DAG), and phosphatidic acid (PtdOH). Of these, PtdOH has not received as much attention as the other lipids although its role and metabolism appears to be extremely important. This lipid has been shown to play a role in modulating both exocytosis and endocytosis although its precise role in either process is not well defined. The currently evidence suggest this lipid likely participates in key processes by altering membrane architecture necessary for membrane fusion, mediating the penetration of membrane proteins, serving as a precursor for other important SV cycling lipids, or activating essential enzymes. In this review, we address the sources of PtdOH, the enzymes involved in its production, the regulation of these enzymes, and its potential roles in neurotransmission in the central nervous system.

CAPS1 stabilizes the state of readily releasable synaptic vesicles to fusion competence at CA3-CA1 synapses in adult hippocampus

Calcium-dependent activator protein for secretion 1 (CAPS1) regulates exocytosis of dense-core vesicles in neuroendocrine cells and of synaptic vesicles in neurons. However, the synaptic function of CAPS1 in the mature brain is unclear because Caps1 knockout (KO) results in neonatal death. Here, using forebrain-specific Caps1 conditional KO (cKO) mice, we demonstrate, for the first time, a critical role of CAPS1 in adult synapses. The amplitude of synaptic transmission at CA3–CA1 synapses was strongly reduced, and paired-pulse facilitation was significantly increased, in acute hippocampal slices from cKO mice compared with control mice, suggesting a perturbation in presynaptic function. Morphological analysis revealed an accumulation of synaptic vesicles in the presynapse without any overall morphological change. Interestingly, however, the percentage of docked vesicles was markedly decreased in the Caps1 cKO. Taken together, our findings suggest that CAPS1 stabilizes the state of readily releasable synaptic vesicles, thereby enhancing neurotransmitter release at hippocampal synapses.

Phosphoinositide Binding Inhibits Actin Crosslinking and Polymerization by Palladin

Actin cytoskeleton remodeling requires the coordinated action of a large number of actin binding proteins that reorganize the actin cytoskeleton by promoting polymerization, stabilizing filaments, causing branching, or crosslinking filaments. Palladin is a key cytoskeletal actin binding protein whose normal function is to enable cell motility during development of tissues and organs of the embryo and in wound healing, but palladin is also responsible for regulating the ability of cancer cells to become invasive and metastatic. The membrane phosphoinositide phosphatidylinositol (PI) 4,5-bisphosphate [PI(4,5)P₂] is a well-known precursor for intracellular signaling and a bona fide regulator of actin cytoskeleton reorganization. Our results show that two palladin domains [immunoglobulin (Ig) 3 and 34] interact with the head group of PI(4,5)P₂ with moderate affinity (apparent K_d=17μM). Interactions with PI(4,5)P₂ decrease the actin polymerizing activity of Ig domain 3 of palladin (Palld-Ig3). Furthermore, NMR titration and docking studies show that residues K38 and K51, which are present on the β-sheet C and D, form salt bridges with the head group of PI(4,5)P₂. Moreover, charge neutralization at lysine 38 in the Palld-Ig3 domain severely limits the actin polymerizing and bundling activity of Palld-Ig3. Our results provide biochemical proof that PI(4,5)P₂ functions as a moderator of palladin activity and have also identified residues directly involved in the crosslinking activity of palladin.

Cholesterol-Dependent Phase-Demixing in Lipid Bilayers as a Switch for the Activity of the Phosphoinositide-Binding Cytoskeletal Protein Gelsolin

The lateral distribution of phosphatidylinositol 4,5-bisphosphate (PIP2) in lipid bilayers is affected both by divalent cation-mediated attractions and cholesterol-dependent phase demixing. The effects of lateral redistribution of PIP2 within a membrane on PIP2-protein interactions are explored with an N-terminal fragment of gelsolin (NtGSN) that severs actin in a Ca(2+)-insensitive manner. The extent of NtGSN inhibition by PIP2-containing large unilamellar vesicles (LUVs) depends on the lateral organization of the membrane as quantified by an actin-severing assay. At a fixed PIP2 mole fraction, the inhibition is largely enhanced by the segregation of liquid ordered/liquid disordered (Lo/Ld) phases that is induced by altering either cholesterol content or temperature, whereas the presence of Ca(2+) only slightly improves the inhibition. Inhibition of gelsolin induced by demixed LUVs is more effective with decreasing temperature, coincident with increasing membrane order as determined by Laurdan generalized polarization and is reversible as the temperature increases. This result suggests that PIP2-mediated inhibition of gelsolin function depends not only on changes in global concentration but also on lateral distribution of PIP2. These observations imply that gelsolin, and perhaps other PIP2-regulated proteins, can be activated or inactivated by the formation of nanodomains or clusters without changing PIP2 bulk concentration in the cell membrane.

Vesicular glutamate transporter 1 orchestrates recruitment of other synaptic vesicle cargo proteins during synaptic vesicle recycling

A long standing question in synaptic physiology is how neurotransmitter-filled vesicles are rebuilt after exocytosis. Among the first steps in this process is the endocytic retrieval of the transmembrane proteins that are enriched in synaptic vesicles (SVs). At least six types of transmembrane proteins must be recovered, but the rules for how this multiple cargo selection is accomplished are poorly understood. Among these SV cargos is the vesicular glutamate transporter (vGlut). We show here that vGlut1 has a strong influence on the kinetics of retrieval of half of the known SV cargos and that specifically impairing the endocytosis of vGlut1 in turn slows down other SV cargos, demonstrating that cargo retrieval is a collective cargo-driven process. Finally, we demonstrate that different cargos can be retrieved in the same synapse with different kinetics, suggesting that additional post-endocytic sorting steps likely occur in the nerve terminal.

Botulinum Neurotoxins Can Enter Cultured Neurons Independent of Synaptic Vesicle Recycling

Botulinum neurotoxins (BoNTs) are the causative agent of the severe and long-lasting disease botulism. At least seven different serotypes of BoNTs (denoted A-G) have been described. All BoNTs enter human or animal neuronal cells via receptor mediated endocytosis and cleave cytosolic SNARE proteins, resulting in a block of synaptic vesicle exocytosis, leading to the flaccid paralysis characteristic of botulism. Previous data have indicated that once a neuronal cell has been intoxicated by a BoNT, further entry of the same or other BoNTs is prevented due to disruption of synaptic vesicle recycling. However, it has also been shown that cultured neurons exposed to BoNT/A are still capable of taking up BoNT/E. In this report we show that in general BoNTs can enter cultured human or mouse neuronal cells that have previously been intoxicated with another BoNT serotype. Quantitative analysis of cell entry by assessing SNARE cleavage revealed none or only a minor difference in the efficiency of uptake of BoNTs into previously intoxicated neurons. Examination of the endocytic entry pathway by specific endocytosis inhibitors indicated that BoNTs are taken up by clathrin coated pits in both non pre-exposed and pre-exposed neurons. LDH release assays indicated that hiPSC derived neurons exposed consecutively to two different BoNT serotypes remained viable and healthy except in the case of BoNT/E or combinations of BoNT/E with BoNT/B, /D, or /F. Overall, our data indicate that previous intoxication of neuronal cells with BoNT does not inhibit further uptake of BoNTs.

A role for novel lipid interactions in the dynamic recruitment of SNX27 to the T-cell immune synapse

SNX27 is a member of the sorting nexin family that plays an important role in the recycling of receptors from endosomes to the cell surface. In addition to a PX (Phox homology) domain that regulates its endosomal localization, SNX27 has a unique PDZ (Psd-95/Dlg/ZO1) domain and an atypical FERM (4.1, ezrin, radixin, moesin) domain that both function to bind short peptide sequence motifs in the cytoplasmic domains of the cargo receptors. Using the T cell immune synapse (IS) as a model for polarized protein recycling, we recently identified an additional mechanism that enhances SNX27 localization to the endosomal recycling compartment (ERC). Our study defined a phosphoinositide (PI) lipid-binding site within the SNX27 FERM domain, with a clear preference for bi- and triphosphorylated PIs, which may promote SNX27 localization to phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) and/or PtdIns(3,4,5)P3-enriched membrane domains. Using fluorescently tagged lipid-binding probes, we studied the kinetics of distinct PIs in living T cells during IS formation. Our results suggest that PtdIns(3,4,5)P3 accumulates at the contact site simultaneously with early SNX27 recruitment to the plasma membrane (PM), and this is partly controlled by by lipid binding through the FERM domain. These studies define 2 independent binding sites for PtdIns-derived lipids in SNX27, that contribute to the dynamic recruitment of SNX27 to distinct membranes during T cell activation.

Voltage-sensing phosphatase modulation by a C2 domain

The voltage-sensing phosphatase (VSP) is the first example of an enzyme controlled by changes in membrane potential. VSP has four distinct regions: the transmembrane voltage-sensing domain (VSD), the inter-domain linker, the cytosolic catalytic domain, and the C2 domain. The VSD transmits the changes in membrane potential through the inter-domain linker activating the catalytic domain which then dephosphorylates phosphatidylinositol phosphate (PIP) lipids. The role of the C2, however, has not been established. In this study, we explore two possible roles for the C2: catalysis and membrane-binding. The Ci-VSP crystal structures show that the C2 residue Y522 lines the active site suggesting a contribution to catalysis. When we mutated Y522 to phenylalanine, we found a shift in the voltage dependence of activity. This suggests hydrogen bonding as a mechanism of action. Going one step further, when we deleted the entire C2 domain, we found voltage-dependent enzyme activity was no longer detectable. This result clearly indicates the entire C2 is necessary for catalysis as well as for modulating activity. As C2s are known membrane-binding domains, we tested whether the VSP C2 interacts with the membrane. We probed a cluster of four positively charged residues lining the top of the C2 and suggested by previous studies to interact with phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] (Kalli et al., 2014). Neutralizing those positive charges significantly shifted the voltage dependence of activity to higher voltages. We tested membrane binding by depleting PI(4,5)P₂ from the membrane using the 5HT2C receptor and found that the VSD motions as measured by voltage clamp fluorometry (VCF) were not changed. These results suggest that if the C2 domain interacts with the membrane to influence VSP function it may not occur exclusively through PI(4,5)P₂. Together, this data advances our understanding of the VSP C2 by demonstrating a necessary and critical role for the C2 domain in VSP function.

Flies with Parkinson's disease

Parkinson's disease is an incurable neurodegenerative disease. Most cases of the disease are of sporadic origin, but about 10% of the cases are familial. The genes thus far identified in Parkinson's disease are well conserved. Drosophila is ideally suited to study the molecular neuronal cell biology of these genes and the pathogenic mutations in Parkinson's disease. Flies reproduce quickly, and their elaborate genetic tools in combination with their small size allow researchers to analyze identified cells and neurons in large numbers of animals. Furthermore, fruit flies recapitulate many of the cellular and molecular defects also seen in patients, and these defects often result in clear locomotor and behavioral phenotypes, facilitating genetic modifier screens. Hence, Drosophila has played a prominent role in Parkinson's disease research and has provided invaluable insight into the molecular mechanisms of this disease.

Capsaicin modulates acetylcholine release at the myoneural junction

Transient receptor potential (TRP) proteins are non-selective cation channel proteins that are expressed throughout the body. Previous studies demonstrated the expression of TRP Vanilloid 1 (TRPV1), capsaicin (CAP) receptor, in sensory neurons. Recently, we reported TRPV1 expression in mouse motor nerve terminals [MNTs; (Thyagarajan et al., 2009)], where we observed that CAP protected MNTs from botulinum neurotoxin A (BoNT/A). Phrenic nerve diaphragm nerve muscle preparations (NMP) isolated from isoflurane anesthetized adult mice were analyzed for twitch tension, spontaneous (mEPCs) and nerve stimulus evoked (EPCs) acetylcholine release. When acutely applied to isolated NMP, CAP produced a concentration-dependent decline of twitch tension and produced a significant decline in the amplitude of EPCs and quantal content without any effect on the mEPCs. The suppression of nerve stimulus evoked acetylcholine release by CAP was antagonized by capsazepine (CPZ), a TRPV1 antagonist. CAP did not suppress phrenic nerve stimulus evoked acetylcholine release in TRPV1 knockout mice. Also, CAP treatment, in vitro, interfered with the localization of adapter protein 2 in cholinergic Neuro 2a cells. Wortmannin, (WMN; non-selective phosphoinositol kinase inhibitor), mimicked the effects of CAP by inhibiting the acetylcholine exocytosis. Our data suggest that TRPV1 proteins expressed at the MNT are coupled to the exo-endocytic mechanisms to regulate neuromuscular functions.

PI(4,5)P₂-binding effector proteins for vesicle exocytosis

PI(4,5)P₂participates directly in priming and possibly in fusion steps of Ca²⁺-triggered vesicle exocytosis. High concentration nanodomains of PI(4,5)P₂reside on the plasma membrane of neuroendocrine cells. A subset of vesicles that co-localize with PI(4,5)P₂ domains appear to undergo preferential exocytosis in stimulated cells. PI(4,5)P₂directly regulates vesicle exocytosis by recruiting and activating PI(4,5)P₂-binding proteins that regulate SNARE protein function including CAPS, Munc13-1/2, synaptotagmin-1, and other C2 domain-containing proteins. These PI(4,5)P₂effector proteins are coincidence detectors that engage in multiple interactions at vesicle exocytic sites. The SNARE protein syntaxin-1 also binds to PI(4,5)P₂, which promotes clustering, but an activating role for PI(4,5)P₂in syntaxin-1 function remains to be fully characterized. Similar principles underlie polarized constitutive vesicle fusion mediated in part by the PI(4,5)P₂-binding subunits of the exocyst complex (Sec3, Exo70). Overall, focal vesicle exocytosis occurs at sites landmarked by PI(4,5)P2, which serves to recruit and/or activate multifunctional PI(4,5)P₂-binding proteins. This article is part of a Special Issue entitled Phosphoinositides.

Phosphoinositide control of membrane protein function: a frontier led by studies on ion channels

Anionic phospholipids are critical constituents of the inner leaflet of the plasma membrane, ensuring appropriate membrane topology of transmembrane proteins. Additionally, in eukaryotes, the negatively charged phosphoinositides serve as key signals not only through their hydrolysis products but also through direct control of transmembrane protein function. Direct phosphoinositide control of the activity of ion channels and transporters has been the most convincing case of the critical importance of phospholipid-protein interactions in the functional control of membrane proteins. Furthermore, second messengers, such as [Ca(2+)]i, or posttranslational modifications, such as phosphorylation, can directly or allosterically fine-tune phospholipid-protein interactions and modulate activity. Recent advances in structure determination of membrane proteins have allowed investigators to obtain complexes of ion channels with phosphoinositides and to use computational and experimental approaches to probe the dynamic mechanisms by which lipid-protein interactions control active and inactive protein states.

Preferential localization of type I phosphatidylinositol 4-phosphate 5-kinase γ at the periactive zone of mouse photoreceptor ribbon synapses

Type I phosphatidylinositol 4-phosphate 5 kinase γ (PIP5KIγ) constitutes a major pathway for the generation of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) that regulates a variety of neuronal functions at both presynaptic and postsynaptic compartments. In this study, we examined the expression and localization of PIP5KIγ in the adult mouse retina. RT-PCR analysis revealed that PIP5KIγ_v2 was predominantly expressed in the retina while PIP5KIγ_v3 was also expressed faintly. Immunostaining of the adult mouse retina revealed intense PIP5KIγ-immunoreactivity in the inner and outer plexiform layers in a punctate manner. In the photoreceptor ribbon synapse, PIP5KIγ was highly concentrated at the periactive zone. These findings suggest that PIP5KIγ, especially PIP5KIγ_i2, is localized at the periactive zone, a functionally suitable compartment for the endocytosis of synaptic vesicles in photoreceptor ribbon synapses.

CAPS and Munc13 utilize distinct PIP2-linked mechanisms to promote vesicle exocytosis

Phosphoinositides provide compartment-specific signals for membrane trafficking. Plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP2) is required for Ca(2+)-triggered vesicle exocytosis, but whether vesicles fuse into PIP2-rich membrane domains in live cells and whether PIP2 is metabolized during Ca(2+)-triggered fusion were unknown. Ca(2+)-dependent activator protein in secretion 1 (CAPS-1; CADPS/UNC31) and ubMunc13-2 (UNC13B) are PIP2-binding proteins required for Ca(2+)-triggered vesicle exocytosis in neuroendocrine PC12 cells. These proteins are likely effectors for PIP2, but their localization during exocytosis had not been determined. Using total internal reflection fluorescence microscopy in live cells, we identify PIP2-rich membrane domains at sites of vesicle fusion. CAPS is found to reside on vesicles but depends on plasma membrane PIP2 for its activity. Munc13 is cytoplasmic, but Ca(2+)-dependent translocation to PIP2-rich plasma membrane domains is required for its activity. The results reveal that vesicle fusion into PIP2-rich membrane domains is facilitated by sequential PIP2-dependent activation of CAPS and PIP2-dependent recruitment of Munc13. PIP2 hydrolysis only occurs under strong Ca(2+) influx conditions sufficient to activate phospholipase Cη2 (PLCη2). Such conditions reduce CAPS activity and enhance Munc13 activity, establishing PLCη2 as a Ca(2+)-dependent modulator of exocytosis. These studies provide a direct view of the spatial distribution of PIP2 linked to vesicle exocytosis via regulation of lipid-dependent protein effectors CAPS and Munc13.

Apart from its known function, the plasma membrane Ca²⁺ATPase can regulate Ca²⁺ signaling by controlling phosphatidylinositol 4,5-bisphosphate levels

Plasma membrane Ca(2+) ATPases (PMCAs, also known as ATP2B1-ATP2B4) are known targets of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂], but if and how they control the PtdIns(4,5)P₂ pool has not been considered. We demonstrate here that PMCAs protect PtdIns(4,5)P₂ in the plasma membrane from hydrolysis by phospholipase C (PLC). Comparison of active and inactive PMCAs indicates that the protection operates by two mechanisms; one requiring active PMCAs, the other not. It appears that the mechanism requiring activity is the removal of the Ca(2+) required for sustained PLC activity, whereas the mechanism not requiring activity is PtdIns(4,5)P₂ binding. We show that in PMCA overexpressing cells, PtdIns(4,5)P₂ binding can lead to less inositol 1,4,5-triphosphate (InsP₃) and diminished Ca(2+) release from intracellular Ca(2+) pools. Inspection of a homology model of PMCA suggests that PMCAs have a conserved cluster of basic residues forming a 'blue collar' at the interface between the membrane core and the cytoplasmic domains. By molecular dynamics simulation, we found that the blue collar forms four binding pockets for the phosphorylated inositol head group of PtdIns(4,5)P₂; these pockets bind PtdIns(4,5)P₂ strongly and frequently. Our studies suggest that by having the ability to bind PtdIns(4,5)P₂, PMCAs can control the accessibility of PtdIns(4,5)P₂ for PLC and other PtdIns(4,5)P₂-mediated processes.

Biogenesis and transport of membrane domains-potential implications in brain pathologies

Lipids in biological membranes show astonishing chemical diversity, but they also show some key conserved structures in different organisms. In addition, some of their biophysical properties have been related to specific functions. In this review, we aim to discuss the role of sphingolipids- and cholesterol-rich micro- and nano-membrane domains (MD) and highlight their pivotal role in lipid-protein clustering processes, vesicle biogenesis and membrane fusion. We further review potential connections between human pathologies and defects in MD biosynthesis, recycling and homeostasis. Brain, which is second only to the adipose tissues in term of lipid abundance, is particularly affected by MD defects which are linked to neurodegenerative disorders. Finally we propose a potential connection between MD and several nutrient-related processes and envision how diet and autophagy could bring insights towards understanding the impact of global lipid homeostasis on human health and disease.

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.

Lipids in Regulated Exocytosis: What are They Doing?

The regulated secretory pathway in neuroendocrine cells ends with the release of hormones and neurotransmitters following a rise in cytosolic calcium. This process known as regulated exocytosis involves the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, the synaptic vesicle VAMP (synaptobrevin), and the plasma membrane proteins syntaxin and SNAP-25. Although there is much evidence suggesting that SNARE proteins play a key role in the fusion machinery, other cellular elements regulating the kinetics, the extent of fusion, and the preparation of vesicle for release have received less attention. Among those factors, lipids have also been proposed to play important functions both at the level of secretory vesicle recruitment and late membrane fusion steps. Here, we will review the latest evidence supporting the concept of the fusogenic activity of lipids, and also discuss how this may be achieved. These possibilities include the recruitment and sequestration of the components of the exocytotic machinery, regulation of protein function, and direct effects on membrane topology.

Tumor protein D52 controls trafficking of an apical endolysosomal secretory pathway in pancreatic acinar cells

Zymogen granule (ZG) formation in acinar cells involves zymogen cargo sorting from trans-Golgi into immature secretory granules (ISGs). ISG maturation progresses by removal of lysosomal membrane and select content proteins, which enter endosomal intermediates prior to their apical exocytosis. Constitutive and stimulated secretion through this mechanism is termed the constitutive-like and minor-regulated pathways, respectively. However, the molecular components that control membrane trafficking within these endosomal compartments are largely unknown. We show that tumor protein D52 is highly expressed in endosomal compartments following pancreatic acinar cell stimulation and regulates apical exocytosis of an apically directed endolysosomal compartment. Secretion from the endolysosomal compartment was detected by cell-surface antigen labeling of lysosome-associated membrane protein LAMP1, which is absent from ZGs, and had incomplete overlap with surface labeling of synaptotagmin 1, a marker of ZG exocytosis. Although culturing (16-18 h) of isolated acinar cells is accompanied by a loss of secretory responsiveness, the levels of SNARE proteins necessary for ZG exocytosis were preserved. However, levels of endolysosomal proteins D52, EEA1, Rab5, and LAMP1 markedly decreased with culture. When D52 levels were restored by adenoviral delivery, the levels of these regulatory proteins and secretion of both LAMP1 (endolysosomal) and amylase was strongly enhanced. These secretory effects were absent in alanine and aspartate substitutions of serine 136, the major D52 phosphorylation site, and were inhibited by brefeldin A, which does not directly affect the ZG compartment. Our results indicate that D52 directly regulates apical endolysosomal secretion and are consistent with previous studies, suggesting that this pathway indirectly regulates ZG secretion of digestive enzymes.