Rolling blackout is required for bulk endocytosis in non-neuronal cells and neuronal synapses

A Potential Role of EFR3A in Human Disease States

EFR3A is a conserved peripheral membrane protein required for the plasma membrane localization of the phosphatidylinositol-4 kinase (PI4KIIIα/PI4KA) complex and for regulating the responsiveness of G-protein-coupled receptors. Additionally, it was implicated in several other potentially unrelated physiological functions. In metazoan organisms, EFR3A is ubiquitously co-expressed with its paralog EFR3B which shares similar biological roles. This brief review summarizes the current knowledge regarding the potential roles of EFR3A in human disease states, including neurological and cardiovascular disorders, as well as various neoplasia-based diseases.

The CHD family chromatin remodeling enzyme, Kismet, promotes both clathrin-mediated and activity-dependent bulk endocytosis

Chromodomain helicase DNA binding domain (CHD) proteins, including CHD7 and CHD8, remodel chromatin to enable transcriptional programs. Both proteins are important for proper neural development as heterozygous mutations in Chd7 and Chd8 are causative for CHARGE syndrome and correlated with autism spectrum disorders, respectively. Their roles in mature neurons are poorly understood despite influencing the expression of genes required for cell adhesion, neurotransmission, and synaptic plasticity. The Drosophila homolog of CHD7 and CHD8, Kismet (Kis), promotes neurotransmission, endocytosis, and larval locomotion. Endocytosis is essential in neurons for replenishing synaptic vesicles, maintaining protein localization, and preserving the size and composition of the presynaptic membrane. Several forms of endocytosis have been identified including clathrin-mediated endocytosis, which is coupled with neural activity and is the most prevalent form of synaptic endocytosis, and activity-dependent bulk endocytosis, which occurs during periods of intense stimulation. Kis modulates the expression of gene products involved in endocytosis including promoting shaggy/GSK3β expression while restricting PI3K92E. kis mutants electrophysiologically phenocopy a liquid facets mutant in response to paradigms that induce clathrin-mediated endocytosis and activity-dependent bulk endocytosis. Further, kis mutants do not show further reductions in endocytosis when activity-dependent bulk endocytosis or clathrin-mediated endocytosis are pharmacologically inhibited. We find that Kis is important in postsynaptic muscle for proper endocytosis but the ATPase domain of Kis is dispensable for endocytosis. Collectively, our data indicate that Kis promotes both clathrin-mediated endocytosis and activity-dependent bulk endocytosis possibly by promoting transcription of several endocytic genes and maintaining the size of the synaptic vesicle pool.

Efr3b is essential for social recognition by modulating the excitability of CA2 pyramidal neurons

CA2 pyramidal neurons (PNs) are associated with social behaviors. The mechanisms, however, remain to be fully investigated. Here, we report that Efr3b, a protein essential for phospholipid metabolism at the plasma membrane, is widely expressed in the brain, especially in the hippocampal CA2/CA3 areas. To assess the functional significance of Efr3b in the brain, we generated Efr3bf/f mice and crossed them with Nestin-cre mice to delete Efr3b specifically in the brain. We find that Efr3b deficiency in the brain leads to deficits of social novelty recognition and hypoexcitability of CA2 PNs. We then knocked down the expression of Efr3b specifically in CA2 PNs of C57BL/6J mice, and our results showed that reducing Efr3b in CA2 PNs also resulted in deficits of social novelty recognition and hypoexcitability of CA2 PNs. More interestingly, restoring the expression of Efr3b in CA2 PNs enhances their excitability and improves social novelty recognition in Efr3b-deficient mice. Furthermore, direct activation of CA2 PNs with chemogenetics improves social behaviors in Efr3b-deficient mice. Together, our data suggest that Efr3b is essential for social novelty by modulating the excitability of CA2 PNs.

EFR3 and phosphatidylinositol 4-kinase IIIα regulate insulin-stimulated glucose transport and GLUT4 dispersal in 3T3-L1 adipocytes

Insulin stimulates glucose transport in muscle and adipocytes. This is achieved by regulated delivery of intracellular glucose transporter (GLUT4)-containing vesicles to the plasma membrane where they dock and fuse, resulting in increased cell surface GLUT4 levels. Recent work identified a potential further regulatory step, in which insulin increases the dispersal of GLUT4 in the plasma membrane away from the sites of vesicle fusion. EFR3 is a scaffold protein that facilitates localization of phosphatidylinositol 4-kinase type IIIα to the cell surface. Here we show that knockdown of EFR3 or phosphatidylinositol 4-kinase type IIIα impairs insulin-stimulated glucose transport in adipocytes. Using direct stochastic reconstruction microscopy, we also show that EFR3 knockdown impairs insulin stimulated GLUT4 dispersal in the plasma membrane. We propose that EFR3 plays a previously unidentified role in controlling insulin-stimulated glucose transport by facilitating dispersal of GLUT4 within the plasma membrane.

Hypoxia controls plasma membrane targeting of polarity proteins by dynamic turnover of PI4P and PI(4,5)P2

Phosphatidylinositol 4-phosphate (PI4P) and phosphatidylinositol 4,5-biphosphate (PIP2) are key phosphoinositides that determine the identity of the plasma membrane (PM) and regulate numerous key biological events there. To date, mechanisms regulating the homeostasis and dynamic turnover of PM PI4P and PIP2 in response to various physiological conditions and stresses remain to be fully elucidated. Here, we report that hypoxia in Drosophila induces acute and reversible depletion of PM PI4P and PIP2 that severely disrupts the electrostatic PM targeting of multiple polybasic polarity proteins. Genetically encoded ATP sensors confirmed that hypoxia induces acute and reversible reduction of cellular ATP levels which showed a strong real-time correlation with the levels of PM PI4P and PIP2 in cultured cells. By combining genetic manipulations with quantitative imaging assays we showed that PI4KIIIα, as well as Rbo/EFR3 and TTC7 that are essential for targeting PI4KIIIα to PM, are required for maintaining the homeostasis and dynamic turnover of PM PI4P and PIP2 under normoxia and hypoxia. Our results revealed that in cells challenged by energetic stresses triggered by hypoxia, ATP inhibition and possibly ischemia, dramatic turnover of PM PI4P and PIP2 could have profound impact on many cellular processes including electrostatic PM targeting of numerous polybasic proteins.

Secreted C-type lectin regulation of neuromuscular junction synaptic vesicle dynamics modulates coordinated movement

The synaptic cleft manifests enriched glycosylation, with structured glycans coordinating signaling between presynaptic and postsynaptic cells. Glycosylated signaling ligands orchestrating communication are tightly regulated by secreted glycan-binding lectins. Using the Drosophila neuromuscular junction (NMJ) as a model glutamatergic synapse, we identify a new Ca2+-binding (C-type) lectin, Lectin-galC1 (LGC1), which modulates presynaptic function and neurotransmission strength. We find that LGC1 is enriched in motoneuron presynaptic boutons and secreted into the NMJ extracellular synaptomatrix. We show that LGC1 limits locomotor peristalsis and coordinated movement speed, with a specific requirement for synaptic function, but not NMJ architecture. LGC1 controls neurotransmission strength by limiting presynaptic active zone (AZ) and postsynaptic glutamate receptor (GluR) aligned synapse number, reducing both spontaneous and stimulation-evoked synaptic vesicle (SV) release, and capping SV cycling rate. During high-frequency stimulation (HFS), mutants have faster synaptic depression and impaired recovery while replenishing depleted SV pools. Although LGC1 removal increases the number of glutamatergic synapses, we find that LGC1-null mutants exhibit decreased SV density within presynaptic boutons, particularly SV pools at presynaptic active zones. Thus, LGC1 regulates NMJ neurotransmission to modulate coordinated movement.

A positive feedback loop between Flower and PI(4,5)P2 at periactive zones controls bulk endocytosis in Drosophila

Synaptic vesicle (SV) endocytosis is coupled to exocytosis to maintain SV pool size and thus neurotransmitter release. Intense stimulation induces activity-dependent bulk endocytosis (ADBE) to recapture large quantities of SV constituents in large endosomes from which SVs reform. How these consecutive processes are spatiotemporally coordinated remains unknown. Here, we show that Flower Ca²⁺ channel-dependent phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) compartmentalization governs control of these processes in Drosophila. Strong stimuli trigger PI(4,5)P₂ microdomain formation at periactive zones. Upon exocytosis, Flower translocates from SVs to periactive zones, where it increases PI(4,5)P₂ levels via Ca²⁺ influxes. Remarkably, PI(4,5)P₂ directly enhances Flower channel activity, thereby establishing a positive feedback loop for PI(4,5)P₂ microdomain compartmentalization. PI(4,5)P₂ microdomains drive ADBE and SV reformation from bulk endosomes. PI(4,5)P₂ further retrieves Flower to bulk endosomes, terminating endocytosis. We propose that the interplay between Flower and PI(4,5)P₂ is the crucial spatiotemporal cue that couples exocytosis to ADBE and subsequent SV reformation.

FM Dye Cycling at the Synapse: Comparing High Potassium Depolarization, Electrical and Channelrhodopsin Stimulation

FM dyes are used to study the synaptic vesicle (SV) cycle. These amphipathic probes have a hydrophilic head and hydrophobic tail, making them water-soluble with the ability to reversibly enter and exit membrane lipid bilayers. These styryl dyes are relatively non-fluorescent in aqueous medium, but insertion into the outer leaflet of the plasma membrane causes a >40X increase in fluorescence. In neuronal synapses, FM dyes are internalized during SV endocytosis, trafficked both within and between SV pools, and released with SV exocytosis, providing a powerful tool to visualize presynaptic stages of neurotransmission. A primary genetic model of glutamatergic synapse development and function is the Drosophila neuromuscular junction (NMJ), where FM dye imaging has been used extensively to quantify SV dynamics in a wide range of mutant conditions. The NMJ synaptic terminal is easily accessible, with a beautiful array of large synaptic boutons ideal for imaging applications. Here, we compare and contrast the three ways to stimulate the Drosophila NMJ to drive activity-dependent FM1-43 dye uptake/release: 1) bath application of high [K+] to depolarize neuromuscular tissues, 2) suction electrode motor nerve stimulation to depolarize the presynaptic nerve terminal, and 3) targeted transgenic expression of channelrhodopsin variants for light-stimulated, spatial control of depolarization. Each of these methods has benefits and disadvantages for the study of genetic mutation effects on the SV cycle at the Drosophila NMJ. We will discuss these advantages and disadvantages to assist the selection of the stimulation approach, together with the methodologies specific to each strategy. In addition to fluorescent imaging, FM dyes can be photoconverted to electron-dense signals visualized using transmission electron microscopy (TEM) to study SV cycle mechanisms at an ultrastructural level. We provide the comparisons of confocal and electron microscopy imaging from the different methods of Drosophila NMJ stimulation, to help guide the selection of future experimental paradigms.

Notum coordinates synapse development via extracellular regulation of Wingless trans-synaptic signaling

Synaptogenesis requires orchestrated communication between pre- and postsynaptic cells via coordinated trans-synaptic signaling across the extracellular synaptomatrix. The first Wnt signaling ligand discovered, Drosophila Wingless (Wg; Wnt1 in mammals), plays crucial roles in synaptic development, regulating synapse architecture as well as functional differentiation. Here, we investigate synaptogenic functions of the secreted extracellular deacylase Notum, which restricts Wg signaling by cleaving an essential palmitoleate moiety. At the glutamatergic neuromuscular junction (NMJ) synapse, we find that Notum secreted from the postsynaptic muscle acts to strongly modulate synapse growth, structural architecture, ultrastructural development and functional differentiation. In Notum null flies, we find upregulated extracellular Wg ligand and nuclear trans-synaptic signal transduction, as well as downstream misregulation of both pre- and postsynaptic molecular assembly. Structural, functional and molecular synaptogenic defects are all phenocopied by Wg overexpression, suggesting that Notum acts solely by inhibiting Wg trans-synaptic signaling. Moreover, these synaptic development phenotypes are suppressed by genetically correcting Wg levels in Notum null mutants, indicating that Notum normally functions to coordinate synaptic structural and functional differentiation via negative regulation of Wg trans-synaptic signaling in the extracellular synaptomatrix.

Secreted tissue inhibitor of matrix metalloproteinase restricts trans-synaptic signaling to coordinate synaptogenesis

Synaptogenesis is coordinated by trans-synaptic signals that traverse the specialized synaptomatrix between presynaptic and postsynaptic cells. Matrix metalloproteinase (Mmp) activity sculpts this environment, balanced by secreted tissue inhibitors of Mmp (Timp). Here, we use the simplified Drosophila melanogaster matrix metalloproteome to test the consequences of eliminating all Timp regulatory control of Mmp activity at the neuromuscular junction (NMJ). Using in situ zymography, we find Timp limits Mmp activity at the NMJ terminal and shapes extracellular proteolytic dynamics surrounding individual synaptic boutons. In newly generated timp null mutants, NMJs exhibit architectural overelaboration with supernumerary synaptic boutons. With cell-targeted RNAi and rescue studies, we find that postsynaptic Timp limits presynaptic architecture. Functionally, timp null mutants exhibit compromised synaptic vesicle cycling, with activity that is lower in amplitude and fidelity. NMJ defects manifest in impaired locomotor function. Mechanistically, we find that Timp limits BMP trans-synaptic signaling and the downstream synapse-to-nucleus signal transduction. Pharmacologically restoring Mmp inhibition in timp null mutants corrects bone morphogenetic protein (BMP) signaling and synaptic properties. Genetically restoring BMP signaling in timp null mutants corrects NMJ structure and motor function. Thus, Timp inhibition of Mmp proteolytic activity restricts BMP trans-synaptic signaling to coordinate synaptogenesis.

Downregulation of RBO-PI4KIIIα Facilitates Aβ42 Secretion and Ameliorates Neural Deficits in Aβ42-Expressing Drosophila

Phosphoinositides and their metabolizing enzymes are involved in Aβ₄₂ metabolism and Alzheimer's disease pathogenesis. In yeast and mammals, Eighty-five requiring 3 (EFR3), whose Drosophila homolog is Rolling Blackout (RBO), forms a plasma membrane-localized protein complex with phosphatidylinositol-4-kinase Type IIIα (PI4KIIIα) and a scaffold protein to tightly control the level of plasmalemmal phosphatidylinositol-4-phosphate (PI₄P). Here, we report that RBO binds to Drosophila PI4KIIIα, and that in an Aβ₄₂-expressing Drosophila model, separate genetic reduction of PI4KIIIα and RBO, or pharmacological inhibition of PI4KIIIα ameliorated synaptic transmission deficit, climbing ability decline, premature death, and reduced neuronal accumulation of Aβ₄₂ Moreover, we found that RBO-PI4KIIIa downregulation increased neuronal Aβ₄₂ release and that PI4P facilitated the assembly or oligomerization of Aβ₄₂ in/on liposomes. These results indicate that RBO-PI4KIIIa downregulation facilitates neuronal Aβ₄₂ release and consequently reduces neuronal Aβ₄₂ accumulation likely via decreasing Aβ₄₂ assembly in/on plasma membrane. This study suggests the RBO-PI4KIIIα complex as a potential therapeutic target and PI4KIIIα inhibitors as drug candidates for Alzheimer's disease treatment.SIGNIFICANCE STATEMENT Phosphoinositides and their metabolizing enzymes are involved in Aβ₄₂ metabolism and Alzheimer's disease pathogenesis. Here, in an Aβ₄₂-expressing Drosophila model, we discovered and studied the beneficial role of downregulating RBO or its interacting protein PI4KIIIα-a protein that tightly controls the plasmalemmal level of PI₄P-against the defects caused by Aβ₄₂ expression. Mechanistically, RBO-PI4KIIIα downregulation reduced neuronal Aβ₄₂ accumulation, and interestingly increased neuronal Aβ₄₂ release. This study suggests the RBO-PI4KIIIα complex as a novel therapeutic target, and PI4KIIIα inhibitors as new drug candidates.

A Ca2+ channel differentially regulates Clathrin-mediated and activity-dependent bulk endocytosis

Clathrin-mediated endocytosis (CME) and activity-dependent bulk endocytosis (ADBE) are two predominant forms of synaptic vesicle (SV) endocytosis, elicited by moderate and strong stimuli, respectively. They are tightly coupled with exocytosis for sustained neurotransmission. However, the underlying mechanisms are ill defined. We previously reported that the Flower (Fwe) Ca2+ channel present in SVs is incorporated into the periactive zone upon SV fusion, where it triggers CME, thus coupling exocytosis to CME. Here, we show that Fwe also promotes ADBE. Intriguingly, the effects of Fwe on CME and ADBE depend on the strength of the stimulus. Upon mild stimulation, Fwe controls CME independently of Ca2+ channeling. However, upon strong stimulation, Fwe triggers a Ca2+ influx that initiates ADBE. Moreover, knockout of rodent fwe in cultured rat hippocampal neurons impairs but does not completely abolish CME, similar to the loss of Drosophila fwe at the neuromuscular junction, suggesting that Fwe plays a regulatory role in regulating CME across species. In addition, the function of Fwe in ADBE is conserved at mammalian central synapses. Hence, Fwe exerts different effects in response to different stimulus strengths to control two major modes of endocytosis.

Efr3a Insufficiency Attenuates the Degeneration of Spiral Ganglion Neurons after Hair Cell Loss

Sensorineural hearing loss (SNHL) is caused by an irreversible impairment of cochlear hair cells and subsequent progressive degeneration of spiral ganglion neurons (SGNs). Eighty-five requiring 3 (Efr3) is a plasma membrane protein conserved from yeast to human, and knockout of Efr3a was reported to facilitate the survival of hippocampal newborn neurons in adult mice. Previously, we found Efr3a expression in the auditory neural pathway is upregulated soon after the destruction of hair cells. Here we conducted a time-course analysis of drug-caused damage to hearing ability, hair cells and SGNs in Efr3a knocking down mice (Efr3a^−/+, Efr3a KD) and their wild type littermates. Functional examination showed that both groups of mice suffered from serious hearing loss with a higher level of severity in wild type (WT) mice. Morphologic observation following drugs administration showed that both WT and Efr3a KD mice went through progressive loss of hair cells and SGNs, in association with degenerative changes in the perikarya, intracellular organelles, cell body conformation in SGNs, and the changes of SGNs in WT mice were more severe than in Efr3a KD mice. These beneficial effects of Efr3a KD could be ascribed to an increase in the expression of some neurotrophic factors and their receptors in Efr3a KD mice. Our results indicate that Efr3a insufficiency suppresses drug-caused SNHL neurodegeneration in association with an increase in the expression of some neurotrophic factors and their receptors, which may be targeted in the treatment of neurodegeneration.

Using the Drosophila Nephrocyte to Model Podocyte Function and Disease

Glomerular disorders are a major cause of end-stage renal disease and effective therapies are often lacking. Nephrocytes are considered to be part of the Drosophila excretory system and form slit diaphragms across cellular membrane invaginations. Nehphrocytes have been shown to share functional, morphological, and molecular features with podocytes, which form the glomerular filter in vertebrates. Here, we report the progress and the evolving tool-set of this model system. Combining a functional, accessible slit diaphragm with the power of the genetic tool-kit in Drosophila, the nephrocyte has the potential to greatly advance our understanding of the glomerular filtration barrier in health and disease.

The role of Efr3a in age-related hearing loss

Efr3a has been found to be involved in the functional maintenance and structural degeneration of sensory and motor nervous tissues. Our previous data have suggested that Efr3a may be associated with the initiation of the degeneration of spiral ganglion neurons (SGNs). In this study, we used Efr3a knockdown (Efr3a KD) and Efr3a overexpression (Efr3a OE) mice to determine the role of Efr3a in age-related hearing loss. Measurements of hearing thresholds showed that Efr3a had little or no influence on the hearing threshold at all frequencies in adult mice, whereas in an early stage of senescence, Efr3a reduction resulted in better hearing function, especially at 10 and 12months of age. No significant differences were observed in hair cell loss among the three groups until 14months. The number of surviving hair cells in the OE mice was lower than that in the KD mice. As indicated by the density of SGNs in the upper basal turn, the Efr3a OE mice displayed earlier and more severe degeneration than the KD mice. In addition, the p-Akt levels in the cochlear spiral ganglions were higher in adult Efr3a KD mice than in WT and OE mice, although there was no difference in Akt expression among the three groups. Our study suggests that down-regulation of Efr3a might improve hearing function and alleviate the degeneration of SGNs in an early stage of senescence, probably via enhancing the basal expression of activated Akt.

Proximity biotinylation provides insight into the molecular composition of focal adhesions at the nanometer scale

Focal adhesions are protein complexes that link metazoan cells to the extracellular matrix through the integrin family of transmembrane proteins. Integrins recruit many proteins to these complexes, referred to as the "adhesome." We used proximity-dependent biotinylation (BioID) in U2OS osteosarcoma cells to label proteins within 15 to 25 nm of paxillin, a cytoplasmic focal adhesion protein, and kindlin-2, which directly binds β integrins. Using mass spectrometry analysis of the biotinylated proteins, we identified 27 known adhesome proteins and 8 previously unknown components close to paxillin. However, only seven of these proteins interacted directly with paxillin, one of which was the adaptor protein Kank2. The proteins in proximity to β integrin included 15 of the adhesion proteins identified in the paxillin BioID data set. BioID also correctly established kindlin-2 as a cell-cell junction protein. By focusing on this smaller data set, new partners for kindlin-2 were found, namely, the endocytosis-promoting proteins liprin β1 and EFR3A, but, contrary to previous reports, not the filamin-binding protein migfilin. A model adhesome based on both data sets suggests that focal adhesions contain fewer components than previously suspected and that paxillin lies away from the plasma membrane. These data not only illustrate the power of using BioID and stable isotope-labeled mass spectrometry to define macromolecular complexes but also enable the correct identification of therapeutic targets within the adhesome.

Aging Drosophila melanogaster display altered pre- and postsynaptic ultrastructure at adult neuromuscular junctions

Although age-related changes in synaptic plasticity are an important focus within neuroscience, little is known about ultrastructural changes of synaptic morphology during aging. Here we report how aging affects synaptic ultrastructure by using fluorescence and electron microscopy at the adult Drosophila neuromuscular junction (NMJ) of ventral abdominal muscles. Mainly four striking morphological changes of aging NMJs were revealed. 1) Bouton size increases with proportionally rising number of active zones (AZs). 2) Synaptic vesicle density at AZs is increased in old flies. 3) Late endosomes, cisternae, and multivesicular bodies accumulate in the presynaptic terminal, and vesicles accumulate between membranes of the terminal bouton and the subsynaptic reticulum. 4) The electron-dense pre- and postsynaptic apposition is expanded in aging NMJs, which is accompanied by an expansion of the postsynaptic glutamate receptor fields. These findings suggest that aging is possibly accompanied by impaired synaptic vesicle release and recycling and a potentially compensatory expansion of AZs and postsynaptic densities.

Expression of EFR3A in the mouse cochlea during degeneration of spiral ganglion following hair cell loss

Retrograde degeneration of spiral ganglion cells in the cochlea following hair cell loss is similar to dying back in pathology. The EFR3A gene has recently been discovered to be involved in the pathogenesis of dying back. The relationship of EFR3A and spiral ganglion degeneration, however, was rarely investigated. In this study, we destroyed the hair cells of the mouse cochlea by co-administration of kanamycin and furosemide and then investigated the EFR3A expression during the induced spiral ganglion cell degeneration. Our results revealed that co-administration of kanamycin and furosemide quickly induced hair cell loss in the C57BL/6J mice and then resulted in progressive degeneration of the spiral ganglion beginning at day 5 following drug administration. The number of the spiral ganglion cells began to decrease at day 15. The expression of EFR3A increased remarkably in the spiral ganglion at day 5 and then decreased to near normal level within the next 10 days. Our study suggested that the change of EFR3A expression in the spiral ganglion was coincident with the time of the spiral ganglion degeneration, which implied that high expression of EFR3A may be important to prompt initiation of spiral ganglion degeneration following hair cell loss.

Synaptic vesicle generation from central nerve terminal endosomes

Central nerve terminals contain a small number of synaptic vesicles (SVs) that must sustain the fidelity of neurotransmission across a wide range of stimulation intensities. For this to be achieved, nerve terminals integrate a number of complementary endocytosis modes whose activation spans the breadth of these neuronal stimulation patterns. Two such modes are ultrafast endocytosis and activity-dependent bulk endocytosis, which are triggered by stimuli at either end of the physiological range. Both endocytosis modes generate endosomes directly from the nerve terminal plasma membrane, before the subsequent production of SVs from these structures. This review will discuss the current knowledge relating to the molecular mechanisms involved in the generation of SVs from nerve terminal endosomes, how this relates to other mechanisms of SV production and the functional role of such SVs.

EFR3s are palmitoylated plasma membrane proteins that control responsiveness to G-protein-coupled receptors

The yeast Efr3p protein is a main regulator of the Stt4p phosphatidylinositol 4-kinase at contact sites between the endoplasmic reticulum and the plasma membrane. A mutation in its fly homologue Rbo, leads to diminished light responses in the eye attributed to progressively impaired PLC signaling. Here, we find that Efr3s plays a role in maintaining responsiveness to the type-I angiotensin II (AngII) receptors. siRNA-mediated depletion of EFR3A and EFR3B impaired the sustained phase of cytosolic Ca(2+) response to high concentration of AngII in HEK293 cells that express wild type but not truncated AGTR1 (AT1a receptor), missing the phosphorylation sites. Efr3 depletion had minimal effect on the recovery of plasma membrane phosphoinositides during stimulation, and AT1 receptors still underwent ligand-induced internalization. A higher level of basal receptor phosphorylation and a larger response was observed after stimulation. Moreover, Gq activation more rapidly desensitized after AngII stimulation in Efr3 downregulated cells. A similar but less pronounced effect of EFR3 depletion was observed on the desensitization of the cAMP response after stimulation with isoproterenol. These data suggest that mammalian Efr3s contribute to the control of the phosphorylation state and, hence, desensitization of AT1a receptors, and could affect responsiveness of G-protein-coupled receptors in higher eukaryotes.

The cell polarity scaffold Lethal Giant Larvae regulates synapse morphology and function

Lethal Giant Larvae (LGL) is a cytosolic cell polarity scaffold whose loss dominantly enhances neuromuscular junction (NMJ) synaptic overgrowth caused by loss of the Fragile X Mental Retardation Protein (FMRP). However, direct roles for LGL in NMJ morphological and functional development have not before been tested. Here, we use confocal imaging and two-electrode voltage-clamp electrophysiology at the Drosophila larval NMJ to define the synaptic requirements of LGL. We find that LGL is expressed both pre- and postsynaptically, where the scaffold localizes at the membrane on both sides of the synaptic interface. We show that LGL has a cell autonomous presynaptic role facilitating NMJ terminal branching and synaptic bouton formation. Moreover, loss of both pre- and postsynaptic LGL strongly decreases evoked neurotransmission strength, whereas the frequency and amplitude of spontaneous synaptic vesicle fusion events is increased. Cell-targeted RNAi and rescue reveals separable pre- and postsynaptic LGL roles mediating neurotransmission. We show that presynaptic LGL facilitates the assembly of active zone vesicle fusion sites, and that neuronally targeted rescue of LGL is sufficient to ameliorate increased synaptic vesicle cycling imaged with FM1-43 dye labeling. Postsynaptically, we show that loss of LGL results in a net increase in total glutamate receptor (GluR) expression, associated with the selective elevation of GluRIIB subunit-containing receptors. Taken together, these data indicate that the presynaptic LGL scaffold facilitates the assembly of active zone fusion sites to regulate synaptic vesicle cycling, and that the postsynaptic LGL scaffold modulates glutamate receptor composition and function.

The Drosophila nephrocyte

PURPOSE OF REVIEW

The functioning kidney requires proper organization in multiple cell types that mediate filtration and removal of wastes. Interest has increasingly focused on the podocyte as an important mediator of kidney function; defects in podocyte function likely mediate a broad palate of kidney dysfunctions. Here I explore recent work that establishes the Drosophila nephrocyte as a useful model for podocyte function and dysfunction.

RECENT FINDINGS

Although described many decades in the past, recent evidence has emphasized important similarities in the molecules that construct the 'nephrocyte diaphragm' and its vertebrate cousin the 'podocyte diaphragm'. For example, loss of Nephrin and its associated proteins lead to collapse of these structures and loss of proper filtration.

SUMMARY

These data emphasize both differences between the podocyte and nephrocyte and also useful similarities. These similarities provide the promise of bringing Drosophila genetics--strongly successful in other disciplines--to the complex problem of how podocyte dysfunction leads to disease. To further explore this point I discuss work on Nephrin in a better understood tissue, the Drosophila eye, in which the role of Nephrin and its connection to actin dynamics is under intense study.

Regulation of synaptic vesicle budding and dynamin function by an EHD ATPase

Eps15 homology domain-containing proteins (EHDs) are conserved ATPases implicated in membrane remodeling. Recently, EHD1 was found to be enriched at synaptic release sites, suggesting a possible involvement in the trafficking of synaptic vesicles. We have investigated the role of an EHD1/3 ortholog (l-EHD) in the lamprey giant reticulospinal synapse. l-EHD was detected by immunogold at endocytic structures adjacent to release sites. In antibody microinjection experiments, perturbation of l-EHD inhibited synaptic vesicle endocytosis and caused accumulation of clathrin-coated pits with atypical, elongated necks. The necks were covered with helix-like material containing dynamin. To test whether l-EHD directly interferes with dynamin function, we used fluid-supported bilayers as in vitro assay. We found that l-EHD strongly inhibited vesicle budding induced by dynamin in the constant presence of GTP. l-EHD also inhibited dynamin-induced membrane tubulation in the presence of GTPγS, a phenomenon linked with dynamin helix assembly. Our in vivo results demonstrate the involvement of l-EHD in clathrin/dynamin-dependent synaptic vesicle budding. Based on our in vitro observations, we suggest that l-EHD acts to limit the formation of long, unproductive dynamin helices, thereby promoting vesicle budding.

The Role of GSK3 in Presynaptic Function

The past ten years of research have identified a number of key roles for glycogen synthase kinase 3 (GSK3) at the synapse. In terms of presynaptic physiology, critical roles for GSK3 have been revealed in the growth and maturation of the nerve terminal and more recently a key role in the control of activity-dependent bulk endocytosis of synaptic vesicles. This paper will summarise the major roles assigned to GSK3 in both immature and mature nerve terminals, the substrates GSK3 phosphorylates to exert its action, and how GSK3 activity is regulated by different presynaptic signalling cascades. The number of essential roles for GSK3, coupled with the numerous signalling cascades all converging to regulate its activity, suggests that GSK3 is a key integrator of multiple inputs to modulate the strength of neurotransmission. Modulation of these pathways may point to potential mechanisms to overcome synaptic failure in neurodegenerative disorders such as Alzheimer's disease.

The fragile X mental retardation protein developmentally regulates the strength and fidelity of calcium signaling in Drosophila mushroom body neurons

Fragile X syndrome (FXS) is a broad-spectrum neurological disorder characterized by hypersensitivity to sensory stimuli, hyperactivity and severe cognitive impairment. FXS is caused by loss of the fragile X mental retardation 1 (FMR1) gene, whose FMRP product regulates mRNA translation downstream of synaptic activity to modulate changes in synaptic architecture, function and plasticity. Null Drosophila FMR1 (dfmr1) mutants exhibit reduced learning and loss of protein synthesis-dependent memory consolidation, which is dependent on the brain mushroom body (MB) learning and memory center. We targeted a transgenic GFP-based calcium reporter to the MB in order to analyze calcium dynamics downstream of neuronal activation. In the dfmr1 null MB, there was significant augmentation of the calcium transients induced by membrane depolarization, as well as elevated release of calcium from intracellular organelle stores. The severity of these calcium signaling defects increased with developmental age, although early stages were characterized by highly variable, low fidelity calcium regulation. At the single neuron level, both calcium transient and calcium store release defects were exhibited by dfmr1 null MB neurons in primary culture. Null dfmr1 mutants exhibit reduced brain mRNA expression of calcium-binding proteins, including calcium buffers calmodulin and calbindin, predicting that the inability to appropriately sequester cytosolic calcium may be the common mechanistic defect causing calcium accumulation following both influx and store release. Changes in the magnitude and fidelity of calcium signals in the absence of dFMRP likely contribute to defects in neuronal structure/function, leading to the hallmark learning and memory dysfunction of FXS.

Drosophila rolling blackout displays lipase domain-dependent and -independent endocytic functions downstream of dynamin

Drosophila temperature-sensitive rolling blackout (rbo(ts) ) mutants display a total block of endocytosis in non-neuronal cells and a weaker, partial defect at neuronal synapses. RBO is an integral plasma membrane protein and is predicted to be a serine esterase. To determine if lipase activity is required for RBO function, we mutated the catalytic serine 358 to alanine in the G-X-S-X-G active site, and assayed genomic rescue of rbo mutant non-neuronal and neuronal phenotypes. The rbo(S358A) mutant is unable to rescue rbo null 100% embryonic lethality, indicating that the lipase domain is critical for RBO essential function. Likewise, the rbo(S358A) mutant cannot provide any rescue of endocytic blockade in rbo(ts) Garland cells, showing that the lipase domain is indispensable for non-neuronal endocytosis. In contrast, rbo(ts) conditional paralysis, synaptic transmission block and synapse endocytic defects are all fully rescued by the rbo(S358A) mutant, showing that the RBO lipase domain is dispensable in neuronal contexts. We identified a synthetic lethal interaction between rbo(ts) and the well-characterized dynamin GTPase conditional shibire (shi(ts1)) mutant. In both non-neuronal cells and neuronal synapses, shi(ts1); rbo(ts) phenocopies shi(ts1) endocytic defects, indicating that dynamin and RBO act in the same pathway, with dynamin functioning upstream of RBO. We conclude that RBO possesses both lipase domain-dependent and scaffolding functions with differential requirements in non-neuronal versus neuronal endocytosis mechanisms downstream of dynamin GTPase activity.

The nonsense-mediated decay pathway maintains synapse architecture and synaptic vesicle cycle efficacy

A systematic Drosophila forward genetic screen for photoreceptor synaptic transmission mutants identified no-on-and-no-off transient C (nonC) based on loss of retinal synaptic responses to light stimulation. The cloned gene encodes phosphatidylinositol-3-kinase-like kinase (PIKK) Smg1, a regulatory kinase of the nonsense-mediated decay (NMD) pathway. The Smg proteins act in an mRNA quality control surveillance mechanism to selectively degrade transcripts containing premature stop codons, thereby preventing the translation of truncated proteins with dominant-negative or deleterious gain-of-function activities. At the neuromuscular junction (NMJ) synapse, an extended allelic series of Smg1 mutants show impaired structural architecture, with decreased terminal arbor size, branching and synaptic bouton number. Functionally, loss of Smg1 results in a ~50% reduction in basal neurotransmission strength, as well as progressive transmission fatigue and greatly impaired synaptic vesicle recycling during high-frequency stimulation. Mutation of other NMD pathways genes (Upf2 and Smg6) similarly impairs neurotransmission and synaptic vesicle cycling. These findings suggest that the NMD pathway acts to regulate proper mRNA translation to safeguard synapse morphology and maintain the efficacy of synaptic function.

Identifying cellular pathways modulated by Drosophila palmitoyl-protein thioesterase 1 function

Infantile-onset Neuronal Ceroid Lipofuscinosis (INCL) is a severe pediatric neurodegenerative disorder produced by mutations in the gene encoding palmitoyl-protein thioesterase 1 (Ppt1). This enzyme is responsible for the removal of a palmitate post-translational modification from an unknown set of substrate proteins. To better understand the function of Ppt1 in neurons, we performed an unbiased dominant loss-of-function genetic modifier screen in Drosophila using a previously characterized Ppt1 gain-of-function system. The enhancers and suppressors identified in our screen make novel connections between Ppt1 and genes involved in cellular trafficking and the modulation of synaptic growth. We further support the relevance of our screen by demonstrating that Garland cells from Ppt1 loss-of-function mutants have defects in endocytic trafficking. Endocytic tracer uptake and ultrastructural analysis of these non-neuronal cells points to Ppt1 playing a role in modulating the early stages of vesicle formation. This work lays the groundwork for further experimental exploration of these processes to better understand their contributions to the INCL disease process.

The molecular physiology of activity-dependent bulk endocytosis of synaptic vesicles

Central nerve terminals release neurotransmitter in response to a wide variety of stimuli. Because maintenance of neurotransmitter release is dependent on the continual supply of synaptic vesicles (SVs), nerve terminals possess an array of endocytosis modes to retrieve and recycle SV membrane and proteins. During mild stimulation conditions, single SV retrieval modes such as clathrin-mediated endocytosis predominate. However, during increased neuronal activity, additional SV retrieval capacity is required, which is provided by activity-dependent bulk endocytosis (ADBE). ADBE is the dominant SV retrieval mechanism during elevated neuronal activity. It is a high capacity SV retrieval mode that is immediately triggered during such stimulation conditions. This review will summarize the current knowledge regarding the molecular mechanism of ADBE, including molecules required for its triggering and subsequent steps, including SV budding from bulk endosomes. The molecular relationship between ADBE and the SV reserve pool will also be discussed. It is becoming clear that an understanding of the molecular physiology of ADBE will be of critical importance in attempts to modulate both normal and abnormal synaptic function during intense neuronal activity.