Protein phosphorylation is required for endocytosis in nerve terminals: potential role for the dephosphins dynamin I and synaptojanin, but not AP180 or amphiphysin

Mini-review: Synaptojanin 1 and its implications in membrane trafficking

This mini-review aims to summarize a growing body of literature on synaptojanin 1 (Synj1), a phosphoinositide phosphatase that was initially known to have a prominent role in synaptic vesicle recycling. Synj1 is coded by the SYNJ1 gene, whose mutations and variants are associated with an increasing number of neurological disorders. To better understand the mechanistic role of Synj1 in disease pathogenesis, we review details of phosphoinositide signaling pathways and the reported involvement of Synj1 in membrane trafficking with a specific focus on Parkinson’s disease (PD). Recent studies have tremendously advanced our understanding of Synj1 protein structure and function while broadening our view of how Synj1 regulates synaptic membrane trafficking and endosomal trafficking in various organisms and cell types. A growing body of evidence points to inefficient membrane trafficking as key pathogenic mechanisms in neurodegenerative diseases associated with abnormal Synj1 expression. Despite significant progress made in the field, the mechanism by which Synj1 connects to trafficking, signaling, and pathogenesis is lacking and remains to be addressed.

Synaptic AP2 CCV life cycle regulation by the Eps15, ITSN1, Sgip1/AP2, synaptojanin1 interactome

The AP1/σ1B knockout causes impaired synaptic vesicle recycling and enhanced protein sorting into endosomes, leading to severe intellectual disability. These disturbances in synaptic protein sorting induce as a secondary phenotype the upregulation of AP2 CCV mediated endocytosis. Synapses contain canonical AP2 CCV and AP2 CCV with a more stable coat and thus extended life time. In AP1/σ1B knockout synapses, pool sizes of both CCV classes are doubled. Additionally, stable CCV of the knockout are more stabilised than stable wt CCV. One mechanism responsible for enhanced CCV stabilisation is the reduction of synaptojanin1 CCV levels, the PI-4,5-P2 phosphatase essential for AP2 membrane dissociation. To identify mechanisms regulating synaptojanin1 recruitment, we compared synaptojanin1 CCV protein interactome levels and CCV protein interactions between both CCV classes from wt and knockout mice. We show that ITSN1 determines synaptojanin1 CCV levels. Sgip1/AP2 excess hinders synaptojanin1 binding to ITSN1, further lowering its levels. ITSN1 levels are determined by Eps15, not Eps15L1. In addition, the data reveal that reduced amounts of pacsin1 can be counter balanced by its enhanced activation. These data exemplify the complexity of CCV life cycle regulation and indicate how cargo proteins determine the life cycle of their CCV.

Protein Phosphorylation in Depolarized Synaptosomes: Dissecting Primary Effects of Calcium from Synaptic Vesicle Cycling

Synaptic transmission is mediated by the regulated exocytosis of synaptic vesicles. When the presynaptic membrane is depolarized by an incoming action potential, voltage-gated calcium channels open, resulting in the influx of calcium ions that triggers the fusion of synaptic vesicles (SVs) with the plasma membrane. SVs are recycled by endocytosis. Phosphorylation of synaptic proteins plays a major role in these processes, and several studies have shown that the synaptic phosphoproteome changes rapidly in response to depolarization. However, it is unclear which of these changes are directly linked to SV cycling and which might regulate other presynaptic functions that are also controlled by calcium-dependent kinases and phosphatases. To address this question, we analyzed changes in the phosphoproteome using rat synaptosomes in which exocytosis was blocked with botulinum neurotoxins (BoNTs) while depolarization-induced calcium influx remained unchanged. BoNT-treatment significantly alters the response of the synaptic phoshoproteome to depolarization and results in reduced phosphorylation levels when compared with stimulation of synaptosomes by depolarization with KCl alone. We dissect the primary Ca2+-dependent phosphorylation from SV-cycling-dependent phosphorylation and confirm an effect of such SV-cycling-dependent phosphorylation events on syntaxin-1a-T21/T23, synaptobrevin-S75, and cannabinoid receptor-1-S314/T322 on exo- and endocytosis in cultured hippocampal neurons.

Signalling Pathways Implicated in Alzheimer's Disease Neurodegeneration in Individuals with and without Down Syndrome

Down syndrome (DS), the most common cause of intellectual disability of genetic origin, is characterized by alterations in central nervous system morphology and function that appear from early prenatal stages. However, by the fourth decade of life, all individuals with DS develop neuropathology identical to that found in sporadic Alzheimer's disease (AD), including the development of amyloid plaques and neurofibrillary tangles due to hyperphosphorylation of tau protein, loss of neurons and synapses, reduced neurogenesis, enhanced oxidative stress, and mitochondrial dysfunction and neuroinflammation. It has been proposed that DS could be a useful model for studying the etiopathology of AD and to search for therapeutic targets. There is increasing evidence that the neuropathological events associated with AD are interrelated and that many of them not only are implicated in the onset of this pathology but are also a consequence of other alterations. Thus, a feedback mechanism exists between them. In this review, we summarize the signalling pathways implicated in each of the main neuropathological aspects of AD in individuals with and without DS as well as the interrelation of these pathways.

The interaction of assembly protein AP180 and clathrin is inhibited by multi-site phospho-mimetics

Clathrin-mediated endocytosis at the nerve terminal is dependent on assembly protein 180 (AP180) and adapter protein complex 2 (AP2). Both membrane adapter proteins bind to each other and to clathrin, to drive assembly of the clathrin coat over nascent synaptic vesicles. Using knowledge of in vivo phosphorylation sites, AP180 was mutated to determine the effect on binding. N-terminally truncated AP180 exhibited phospho-mimetic (Ser/Thr to Glu)-dependent interaction with AP2, but not clathrin. C-terminally truncated and full length phospho-mutant AP180 bound less AP2 than wild type. However, there was no difference in AP2 binding for the phospho-mimetic or phospho-deficient (Ser/Thr to Ala) AP180 mutants. Thus, the phospho-mutant approach did not provide clarity for the role of phosphorylation in AP180-AP2 binding. Clathrin exhibited a phospho-mimetic-dependent interaction with full-length AP180. Furthermore, phospho-mimetic AP180 was deficient at assembling clathrin cages. These latter discoveries support a model where AP180 phosphorylation inhibits clathrin binding and assembly.

Selective regulation of clathrin-mediated epidermal growth factor receptor signaling and endocytosis by phospholipase C and calcium

Clathrin-mediated endocytosis is a major regulator of cell-surface protein internalization. Clathrin and other proteins assemble into small invaginating structures at the plasma membrane termed clathrin-coated pits (CCPs) that mediate vesicle formation. In addition, epidermal growth factor receptor (EGFR) signaling is regulated by its accumulation within CCPs. Given the diversity of proteins regulated by clathrin-mediated endocytosis, how this process may distinctly regulate specific receptors is a key question. We examined the selective regulation of clathrin-dependent EGFR signaling and endocytosis. We find that perturbations of phospholipase Cγ1 (PLCγ1), Ca²⁺, or protein kinase C (PKC) impair clathrin-mediated endocytosis of EGFR, the formation of CCPs harboring EGFR, and EGFR signaling. Each of these manipulations was without effect on the clathrin-mediated endocytosis of transferrin receptor (TfR). EGFR and TfR were recruited to largely distinct clathrin structures. In addition to control of initiation and assembly of CCPs, EGF stimulation also elicited a Ca²⁺- and PKC-dependent reduction in synaptojanin1 recruitment to clathrin structures, indicating broad control of CCP assembly by Ca²⁺ signals. Hence EGFR elicits PLCγ1-calcium signals to facilitate formation of a subset of CCPs, thus modulating its own signaling and endocytosis. This provides evidence for the versatility of CCPs to control diverse cellular processes.

Putting a brake on synaptic vesicle endocytosis

In chemical synapses, action potentials evoke synaptic vesicle fusion with the presynaptic membrane at the active zone to release neurotransmitter. Synaptic vesicle endocytosis (SVE) then follows exocytosis to recapture vesicle proteins and lipid components for recycling and the maintenance of membrane homeostasis. Therefore, SVE plays an essential role during neurotransmission and is one of the most precisely regulated biological processes. Four modes of SVE have been characterized and both positive and negative regulators have been identified. However, our understanding of SVE regulation remains unclear, especially the identity of negative regulators and their mechanisms of action. Here, we review the current knowledge of proteins that function as inhibitors of SVE and their modes of action in different forms of endocytosis. We also propose possible physiological roles of such negative regulation. We believe that a better understanding of SVE regulation, especially the inhibitory mechanisms, will shed light on neurotransmission in health and disease.

Synaptic Vesicle-Recycling Machinery Components as Potential Therapeutic Targets

Presynaptic nerve terminals are highly specialized vesicle-trafficking machines. Neurotransmitter release from these terminals is sustained by constant local recycling of synaptic vesicles independent from the neuronal cell body. This independence places significant constraints on maintenance of synaptic protein complexes and scaffolds. Key events during the synaptic vesicle cycle-such as exocytosis and endocytosis-require formation and disassembly of protein complexes. This extremely dynamic environment poses unique challenges for proteostasis at synaptic terminals. Therefore, it is not surprising that subtle alterations in synaptic vesicle cycle-associated proteins directly or indirectly contribute to pathophysiology seen in several neurologic and psychiatric diseases. In contrast to the increasing number of examples in which presynaptic dysfunction causes neurologic symptoms or cognitive deficits associated with multiple brain disorders, synaptic vesicle-recycling machinery remains an underexplored drug target. In addition, irrespective of the involvement of presynaptic function in the disease process, presynaptic machinery may also prove to be a viable therapeutic target because subtle alterations in the neurotransmitter release may counter disease mechanisms, correct, or compensate for synaptic communication deficits without the need to interfere with postsynaptic receptor signaling. In this article, we will overview critical properties of presynaptic release machinery to help elucidate novel presynaptic avenues for the development of therapeutic strategies against neurologic and neuropsychiatric disorders.

Endocytosis and Signal Transduction: Basic Science Update

Endocytosis can be separated into the categories of phagocytosis and pinocytosis. Phagocytosis can be distinguished from pinocytosis primarily by the size of particle ingested and by its dependence on actin polymerization as a key step in particle ingestion. Several specific forms of pinocytosis have been identified that can be distinguished based on their dependence on clathrin or caveolin. Both clathrin and caveolin-dependent pinocytosis appear to require the participation of dynamin to internalize the plasma membrane. Other, less well-characterized forms of pinocytosis have also been described. Although endocytosis has long been known to affect receptor density, recent studies have demonstrated that endocytosis through clathrin- and caveolin-dependent processes plays a key role in receptor-mediated signal transduction. In some cases, blockade of these processes attenuates, or even prevents, signal transduction from taking place. This information, coupled with a better understanding of endocytosis mechanisms, will help advance the field of cell biology as well as present new targets for drug development and disease treatment.

The role of molecular chaperones in clathrin mediated vesicular trafficking

The discovery that the 70 kD “uncoating ATPase,” which removes clathrin coats from vesicles after endocytosis, is the constitutively expressed Hsc70 chaperone was a surprise. Subsequent work, however, revealed that uncoating is an archetypal Hsp70 reaction: the cochaperone auxilin, which contains a clathrin binding domain and an Hsc70 binding J domain, recruits Hsc70^*ATP to the coat and, concomitant with ATP hydrolysis, transfers it to a hydrophobic Hsc70-binding element found on a flexible tail at the C-terminus of the clathrin heavy chain. Release of clathrin in association with Hsc70^*ADP follows, and the subsequent, persistent association of clathrin with Hsc70 is important to prevent aberrant clathrin polymerization. Thus, the two canonical functions of Hsp70—dissociation of existing protein complexes or aggregates, and binding to a protein to inhibit its inappropriate aggregation—are recapitulated in uncoating. Association of clathrin with Hsc70 in vivo is regulated by Hsp110, an Hsp70 NEF that is itself a member of the Hsp70 family. How Hsp110 activity is itself regulated to make Hsc70-free clathrin available for endocytosis is unclear, though at synapses it's possible that the influx of calcium that accompanies depolarization activates the Ca⁺⁺/calmodulin dependent calcineurin phosphatase which then dephosphorylates and activates Hsp110 to stimulate ADP/ATP exchange and release clathrin from Hsc70^*ADP:clathrin complexes.

Loss of cyclin-dependent kinase 5 from parvalbumin interneurons leads to hyperinhibition, decreased anxiety, and memory impairment

Perturbations in fast-spiking parvalbumin (PV) interneurons are hypothesized to be a major component of various neuropsychiatric disorders; however, the mechanisms regulating PV interneurons remain mostly unknown. Recently, cyclin-dependent kinase 5 (Cdk5) has been shown to function as a major regulator of synaptic plasticity. Here, we demonstrate that genetic ablation of Cdk5 in PV interneurons in mouse brain leads to an increase in GABAergic neurotransmission and impaired synaptic plasticity. PVCre;fCdk5 mice display a range of behavioral abnormalities, including decreased anxiety and memory impairment. Our results reveal a central role of Cdk5 expressed in PV interneurons in gating inhibitory neurotransmission and underscore the importance of such regulation during behavioral tasks. Our findings suggest that Cdk5 can be considered a promising therapeutic target in a variety of conditions attributed to inhibitory interneuronal dysfunction, such as epilepsy, anxiety disorders, and schizophrenia.

Regulating dynamin dynamics during endocytosis

Dynamin is a large GTPase that mediates plasma membrane fission during clathrin-mediated endocytosis. Dynamin assembles into polymers on the necks of budding membranes in cells and has been shown to undergo GTP-dependent conformational changes that lead to membrane fission in vitro. Recent efforts have shed new light on the mechanisms of dynamin-mediated fission, yet exactly how dynamin performs this function in vivo is still not fully understood. Dynamin interacts with a number of proteins during the endocytic process. These interactions are mediated by the C-terminal proline-rich domain (PRD) of dynamin binding to SH3 domain-containing proteins. Three of these dynamin-binding partners (intersectin, amphiphysin and endophilin) have been shown to play important roles in the clathrin-mediated endocytosis process. They promote dynamin-mediated plasma membrane fission by regulating three important sequential steps in the process: recruitment of dynamin to sites of endocytosis; assembly of dynamin into a functional fission complex at the necks of clathrin-coated pits (CCPs); and regulation of dynamin-stimulated GTPase activity, a key requirement for fission.

The regulation of synaptic vesicle recycling by cGMP-dependent protein kinase type II in cerebellar granule cells under strong and sustained stimulation

From the early periods of neurogenesis and migration, up until synaptogenesis, both nitric oxide (NO) and its downstream messenger, cGMP, are thought to influence the development of neurons. The NO/cGMP/cGMP-dependent protein kinase (cGK) pathway regulates the clustering and recruitment of synaptic proteins and vesicles to the synapse, adjusting the exoendocytic cycle to the intensity of activity and accelerating endocytosis following large-scale exocytosis. Here, we show that blockage of the N-methyl-D-aspartate receptor impairs the cycling of synaptic vesicles in a subset of boutons on cerebellar granule cells, an effect that was reversed by increasing cGMP. Furthermore, we demonstrate that presynaptic cGK type II (cGKII) plays a major role in this process. Using the FM1-43 dye to track vesicle recycling, we found that knockdown of cGKII and/or the application of a cGK inhibitor reduced the efficiency of synaptic vesicle recycling to a similar extent. Likewise, in cerebellar granule cells transfected with vGlut1-pHluorin to follow the exoendocytotic cycle, application of a cGK inhibitor slowed vesicle endocytosis when exocytosis was accelerated through strong and sustained stimulation. Additionally, ultrastructural analysis showed that cGKII knockdown or inhibition favored the formation of endosomal-like structures after strong and sustained stimulation. We conclude that cGKII controls the homeostatic balance of vesicle exocytosis and endocytosis in synaptic boutons of rat cerebellar granule cells.

The Biochemical Properties and Functions of CALM and AP180 in Clathrin Mediated Endocytosis

Clathrin-mediated endocytosis (CME) is a fundamental process for the regulated internalization of transmembrane cargo and ligands via the formation of vesicles using a clathrin coat. A vesicle coat is initially created at the plasma membrane by clathrin assembly into a lattice, while a specific cargo sorting process selects and concentrates proteins for inclusion in the new vesicle. Vesicles formed via CME traffic to different parts of the cell and fuse with target membranes to deliver cargo. Both clathrin assembly and cargo sorting functions are features of the two gene family consisting of assembly protein 180 kDa (AP180) and clathrin assembly lymphoid myeloid leukemia protein (CALM). In this review, we compare the primary structure and domain organization of CALM and AP180 and relate these properties to known functions and roles in CME and disease.

Activity-dependent facilitation of Synaptojanin and synaptic vesicle recycling by the Minibrain kinase

Phosphorylation has emerged as a crucial regulatory mechanism in the nervous system to integrate the dynamic signalling required for proper synaptic development, function and plasticity, particularly during changes in neuronal activity. Here we present evidence that Minibrain (Mnb; also known as Dyrk1A), a serine/threonine kinase implicated in autism spectrum disorder and Down syndrome, is required presynaptically for normal synaptic growth and rapid synaptic vesicle endocytosis at the Drosophila neuromuscular junction (NMJ). We find that Mnb-dependent phosphorylation of Synaptojanin (Synj) is required, in vivo, for complex endocytic protein interactions and to enhance Synj activity. Neuronal stimulation drives Mnb mobilization to endocytic zones and triggers Mnb-dependent phosphorylation of Synj. Our data identify Mnb as a synaptic kinase that promotes efficient synaptic vesicle recycling by dynamically calibrating Synj function at the Drosophila NMJ, and in turn endocytic capacity, to adapt to conditions of high synaptic activity.

Synaptic vesicle generation from activity-dependent bulk endosomes requires calcium and calcineurin

Activity-dependent bulk endocytosis (ADBE) is the dominant mode of synaptic vesicle (SV) endocytosis during high-frequency stimulation in central nerve terminals. ADBE generates endosomes direct from the plasma membrane, meaning that high concentrations of calcium will be present in their interior due to fluid phase uptake from the extracellular space. Morphological and fluorescent assays were used to track the generation of SVs from bulk endosomes in primary neuronal culture. This process was functionally uncoupled from both SV exocytosis and plasma membrane retrieval events by intervening only after SV fusion and endocytosis were completed. Either intracellular (BAPTA-AM) or intra-endosomal (Rhod-dextran) calcium chelation inhibited SV generation from bulk endosomes, indicating that calcium efflux from this compartment is critical for this process. The V-type ATPase antagonist bafilomycin A1 also arrested SV generation from bulk endosomes, indicating endosomal acidification may be required for calcium efflux. Finally, pharmacological inhibition of the calcium-dependent protein phosphatase calcineurin blocked endosomal SV generation, identifying it as a key downstream effector in this process. These results reveal a novel and key role for the fluid phase uptake of extracellular calcium and its subsequent efflux in the SV lifecycle.

Role of Scd5, a protein phosphatase-1 targeting protein, in phosphoregulation of Sla1 during endocytosis

Phosphorylation regulates assembly and disassembly of proteins during endocytosis. In yeast, Prk1 and Ark1 phosphorylate factors after vesicle internalization leading to coat disassembly. Scd5, a protein phosphatase-1 (PP1)-targeting subunit, is proposed to regulate dephosphorylation of Prk1/Ark1 substrates to promote new rounds of endocytosis. In this study we analyzed scd5-PP1Δ2, a mutation causing impaired PP1 binding. scd5-PP1Δ2 caused hyperphosphorylation of several Prk1 endocytic targets. Live-cell imaging of 15 endocytic components in scd5-PP1Δ2 revealed that most factors arriving before the invagination/actin phase of endocytosis had delayed lifetimes. Severely affected were early factors and Sla2 (Hip1R homolog), whose lifetime was extended nearly fourfold. In contrast, the lifetime of Sla1, a Prk1 target, was extended less than twofold, but its cortical recruitment was significantly reduced. Delayed Sla2 dynamics caused by scd5-PP1Δ2 were suppressed by SLA1 overexpression. This was dependent on the LxxQxTG repeats (SR) of Sla1, which are phosphorylated by Prk1 and bind Pan1, another Prk1 target, in the dephosphorylated state. Without the SR, Sla1ΔSR was still recruited to the cell surface, but was less concentrated in cortical patches than Pan1. sla1ΔSR severely impaired endocytic progression, but this was partially suppressed by overexpression of LAS17, suggesting that without the SR region the SH3 region of Sla1 causes constitutive negative regulation of Las17 (WASp). These results demonstrate that Scd5/PP1 is important for recycling Prk1 targets to initiate new rounds of endocytosis and provide new mechanistic information on the role of the Sla1 SR domain in regulating progression to the invagination/actin phase of endocytosis.

Prediction of functional phosphorylation sites by incorporating evolutionary information

Protein phosphorylation is a ubiquitous protein post-translational modification, which plays an important role in cellular signaling systems underlying various physiological and pathological processes. Current in silico methods mainly focused on the prediction of phosphorylation sites, but rare methods considered whether a phosphorylation site is functional or not. Since functional phosphorylation sites are more valuable for further experimental research and a proportion of phosphorylation sites have no direct functional effects, the prediction of functional phosphorylation sites is quite necessary for this research area. Previous studies have shown that functional phosphorylation sites are more conserved than non-functional phosphorylation sites in evolution. Thus, in our method, we developed a web server by integrating existing phosphorylation site prediction methods, as well as both absolute and relative evolutionary conservation scores to predict the most likely functional phosphorylation sites. Using our method, we predicted the most likely functional sites of the human, rat and mouse proteomes and built a database for the predicted sites. By the analysis of overall prediction results, we demonstrated that protein phosphorylation plays an important role in all the enriched KEGG pathways. By the analysis of protein-specific prediction results, we demonstrated the usefulness of our method for individual protein studies. Our method would help to characterize the most likely functional phosphorylation sites for further studies in this research area.

Analysis of synaptic vesicle endocytosis in synaptosomes by high-content screening

Small molecules modulating synaptic vesicle endocytosis (SVE) may ultimately be useful for diseases where pathological neurotransmission is implicated. Only a small number of specific SVE modulators have been identified to date. Slow progress is due to the laborious nature of traditional approaches to study SVE, in which nerve terminals are identified and studied in cultured neurons, typically yielding data from 10-20 synapses per experiment. We provide a protocol for a quantitative, high-throughput method for studying SVE in thousands of nerve terminals. Rat forebrain synaptosomes are attached to 96-well microplates and depolarized; SVE is then quantified by uptake of the dye FM4-64, which is imaged by high-content screening. Synaptosomes that have been frozen and stored can be used in place of fresh synaptosomes, reducing the experimental time and animal numbers required. With a supply of frozen synaptosomes, the assay can be performed within a day, including data analysis.

The Rhodadyns, a New Class of Small Molecule Inhibitors of Dynamin GTPase Activity

Six focused rhodanine-based libraries, 60 compounds in total, were synthesized and evaluated as potential dynamin I GTPase inhibitors. Twenty-six were more potent than the lead compound with 13 returning IC50 values ≤10 μM, making the Rhodadyn series among the most active dynamin inhibitors reported. Two analogues were highly effective at blocking receptor-mediated endocytosis: C10 and D10 with IC50(RME) = 7.0 ± 2.2 and 5.9 ± 1.0 μM, respectively. These compounds are equipotent with the best reported in-cell dynamin inhibitors.

Phosphorylation by Dyrk1A of clathrin coated vesicle-associated proteins: identification of the substrate proteins and the effects of phosphorylation

Dyrk1A phosphorylated multiple proteins in the clathrin-coated vesicle (CCV) preparations obtained from rat brains. Mass spectrometric analysis identified MAP1A, MAP2, AP180, and α- and β-adaptins as the phosphorylated proteins in the CCVs. Each protein was subsequently confirmed by [(32)P]-labeling and immunological methods. The Dyrk1A-mediated phosphorylation released the majority of MAP1A and MAP2 and enhanced the release of AP180 and adaptin subunits from the CCVs. Furthermore, Dyrk1A displaced adaptor proteins physically from CCVs in a kinase-concentration dependent manner. The clathrin heavy chain release rate, in contrast, was not affected by Dyrk1A. Surprisingly, the Dyrk1A-mediated phosphorylation of α- and β-adaptins led to dissociation of the AP2 complex, and released only β-adaptin from the CCVs. AP180 was phosphorylated by Dyrk1A also in the membrane-free fractions, but α- and β-adaptins were not. Dyrk1A was detected in the isolated CCVs and was co-localized with clathrin in neurons from mouse brain sections and from primary cultured rat hippocampus. Previously, we proposed that Dyrk1A inhibits the onset of clathrin-mediated endocytosis in neurons by phosphorylating dynamin 1, amphiphysin 1, and synaptojanin 1. Current results suggest that besides the inhibition, Dyrk1A promotes the uncoating process of endocytosed CCVs.

Lessons from yeast for clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) is the major pathway for internalization of membrane proteins from the cell surface. Half a century of studies have uncovered tremendous insights into how a clathrin-coated vesicle is formed. More recently, the advent of live-cell imaging has provided a dynamic view of this process. As CME is highly conserved from yeast to humans, budding yeast provides an evolutionary template for this process and has been a valuable system for dissecting the underlying molecular mechanisms. In this review we trace the formation of a clathrin-coated vesicle from initiation to uncoating, focusing on key findings from the yeast system.

Role of Phosphorylation in the Control of Clathrin-Mediated Internalization of GPCR

The process by which G protein-coupled receptors (GPCRs) are internalized through the clathrin-coated vesicles involves interactions of multifunctional adaptor proteins. These interactions are tightly controlled by phosphorylation and dephosphorylation mechanisms resulting in the regulation of receptor endocytosis. However, the identities of the kinases involved in this process remained largely unknown until recently. This paper discusses advances in our knowledge of the important role played by protein phosphorylation in the regulation of the endocytic machinery and how phosphorylation controls the coated vesicle cycle.

Overexpression of Dyrk1A causes the defects in synaptic vesicle endocytosis

Trisomy 21-linked Dyrk1A (dual-specificity tyrosine phosphorylation-regulated kinase 1A) overexpression is implicated in pathogenic mechanisms underlying mental retardation in Down syndrome (DS). It is known to phosphorylate multiple substrates including endocytic proteins in vitro, but the functional consequence of Dyrk1A-mediated phosphorylation on endocytosis has never been investigated. Here, we show that overexpression of Dyrk1A causes defects in clathrin-mediated endocytosis and specifically, in the recruitment of endocytic proteins to clathrin-coated pits in fibroblasts. Synaptic vesicle endocytosis also significantly slowed down as a result of Dyrk1A overexpression in cultured hippocampal neurons. These effects are dependent on Dyrk1A kinase activity. The inhibitory effect of Dyrk1A on synaptic vesicle endocytosis was confirmed in neuronal cultures derived from transgenic mice overexpressing Dyrk1A at levels found in DS. Pharmacological blockade of Dyrk1A with epigallocatechin gallate rescued the endocytic phenotypes found in transgenic neurons. Together, our results suggest that aberrant Dyrk1A-mediated phosphorylation of the endocytic machinery perturbs synaptic vesicle endocytosis, which may contribute to synaptic dysfunctions and cognitive deficits associated with DS.

Taking a back seat: synaptic vesicle clustering in presynaptic terminals

Central inter-neuronal synapses employ various molecular mechanisms to sustain neurotransmitter release during phases of high-frequency synaptic activity. One of the features ensuring this property is the presence of a pool of synaptic vesicles (SVs) in the presynaptic terminal. At rest and low rates of stimulation, most of the vesicles composing this pool remain in a tight cluster. They are actively utilized when neurons fire action potentials at higher rates and the capability of the recycling machinery is limited. In addition, SV clusters are capable of migrating between release sites and reassemble into clusters at neighboring active zones (AZs). Within the cluster, thin "tethers" interconnect SVs. These dynamic filamentous structures are reorganized during stimulation thereby releasing SVs from the cluster. So far, one protein family, the synapsins, which bind actin filaments and vesicles in a phosphorylation-dependent manner, has been implicated in SV clustering in vertebrate synapses. As evident from recent studies, many endocytic proteins reside in the SV cluster in addition to synapsin. Here we discuss alternative possible mechanisms involved in the organization of this population of SVs. We propose a model in which synapsins together with other synaptic proteins, a large proportion of which is involved in SV recycling, form a dynamic proteinaceous "matrix" which limits the mobility of SVs. Actin filaments, however, do not seem to contribute to SV crosslinking within the SV cluster, but instead they are present peripherally to it, at sites of neurotransmitter release, and at sites of SV recycling.

SH3 domains from a subset of BAR proteins define a Ubl-binding domain and implicate parkin in synaptic ubiquitination

Mutations in the parkin gene are responsible for a common inherited form of Parkinson's disease (PD). Parkin is a RING-type E3 ubiquitin ligase with an N-terminal ubiquitin-like domain (Ubl). We report here that the parkin Ubl binds SH3 domains from endocytic BAR proteins such as endophilin-A with an affinity comparable to proline-rich domains (PRDs) from well-established SH3 partners. The NMR structure of the Ubl-SH3 complex identifies the PaRK extension, a unique C-terminal motif in the parkin Ubl required for SH3 binding and for parkin-mediated ubiquitination of endophilin-A in vitro. In nerve terminals, conditions that promote phosphorylation enhance the interaction between parkin and endophilin-A and increase the levels of ubiquitinated proteins within PRD-associated synaptic protein complexes in wild-type but not parkin knockout brain. The findings identify a pathway for the recruitment of synaptic substrates to parkin with the potential to explain the defects in synaptic transmission observed in recessive forms of PD.

Mental retardation and associated neurological dysfunctions in Down syndrome: a consequence of dysregulation in critical chromosome 21 genes and associated molecular pathways

Down syndrome (DS), affecting 1/700 live births, is the major genetic cause of mental retardation (MR), a cognitive disorder with hard impact on public health. DS brain is characterized by a reduced cerebellar volume and number of granular cells, defective cortical lamination and reduced cortical neurons, malformed dendritic trees and spines, and abnormal synapses. These neurological alterations, also found in trisomic mouse models, result from gene-dosage effects of Human Chromosome 21 (HC21) on the expression of critical developmental genes. HC21 sequencing, mouse ortholog gene identification and DS mouse model generation lead to determine HC21 gene functions and the effects of protein-dosage alterations in neurodevelopmental and metabolic pathways in DS individuals. Trisomic brain transcriptome of DS patients and trisomic mouse models identified some molecular changes determined by gene-overdosage and associated dysregulation of some disomic gene expression in DS brains. These transcriptional variations cause developmental alterations in neural patterning and signal transduction pathways that may lead to defective neuronal circuits responsible for the pathogenesis of MR in DS. Recently, the first altered molecular pathway responsible of some DS phenotypes, including neurological and cognitive disorders has been identified. In this pathway, two critical HC21 genes (DYRK1A and DSCR1) act synergistically to control the phosphorylation levels of NFATc and NFATc-regulated gene expression. Interestingly, the NFATc mice show neurological dysfunctions similar to those seen in DS patients and trisomic mouse models. Treatment of DS mouse model Ts65Dn with GABA(A) antagonists allowed post-drug rescue of cognitive defects, indicating a hopeful direction in clinical therapies for MR in children with DS.

The in vivo phosphorylation sites in multiple isoforms of amphiphysin I from rat brain nerve terminals

Amphiphysin I (amphI) is dephosphorylated by calcineurin during nerve terminal depolarization and synaptic vesicle endocytosis (SVE). Some amphI phosphorylation sites (phosphosites) have been identified with in vitro studies or phosphoproteomics screens. We used a multifaceted strategy including 32P tracking to identify all in vivo amphI phosphosites and determine their relative abundance and potential relevance to SVE. AmphI was extracted from 32P-labeled synaptosomes, phosphopeptides were isolated from proteolytic digests using TiO2 chromatography, and mass spectrometry revealed 13 sites: serines 250, 252, 262, 268, 272, 276, 285, 293, 496, 514, 539, and 626 and Thr-310. These were distributed into two clusters around the proline-rich domain and the C-terminal Src homology 3 domain. Hierarchical phosphorylation of Ser-262 preceded phosphorylation of Ser-268, -272, -276, and -285. Off-line HPLC separation and two-dimensional tryptic mapping of 32P-labeled amphI revealed that Thr-310, Ser-293, Ser-285, Ser-272, Ser-276, and Ser-268 contained the highest 32P incorporation and were the most stimulus-sensitive. Individually Thr-310 and Ser-293 were the most abundant phosphosites, incorporating 16 and 23% of the 32P. The multiple phosphopeptides containing Ser-268, Ser-276, Ser-272, and Ser-285 had 27% of the 32P. Evidence for a role for at least one proline-directed protein kinase and one non-proline-directed kinase was obtained. Four phosphosites predicted for non-proline-directed kinases, Ser-626, -250, -252, and -539, contained low amounts of 32P and were not depolarization-responsive. At least one alternatively spliced amphI isoform was identified in synaptosomes as being constitutively phosphorylated because it did not incorporate 32P during the 1-h labeling period. Multiple phosphosites from amphI-co-migrating synaptosomal proteins were also identified, including SGIP (Src homology 3 domain growth factor receptor-bound 2 (Grb2)-like (endophilin)-interacting protein 1), AAK1, eps15R, MAP6, alpha/beta-adducin, and HCN1. The results reveal two sets of amphI phosphosites that are either dynamically turning over or constitutively phosphorylated in nerve terminals and improve understanding of the role of individual amphI sites or phosphosite clusters in synaptic SVE.

The synapsin cycle: a view from the synaptic endocytic zone

Although the synapsin phosphoproteins were discovered more than 30 years ago and are known to play important roles in neurotransmitter release and synaptogenesis, a complete picture of their functions within the nerve terminal is lacking. It has been shown that these proteins play an important role in the clustering of synaptic vesicles (SVs) at active zones and function as modulators of synaptic strength by acting at both pre- and postdocking levels. Recent studies have demonstrated that synapsins migrate to the endocytic zone of central synapses during neurotransmitter release, which suggests that there are additional functions for these proteins in SV recycling.

The interplay between clathrin-coated vesicles and cell signalling

Internalization of cargo proteins and lipids at the cell surface occurs in both a constitutive and signal-regulated manner through clathrin-mediated and other endocytic pathways. Clathrin-coated vesicle formation is a principal uptake route in response to signalling events. Protein-lipid and protein-protein interactions control both the targeting of signalling molecules and their binding partners to membrane compartments and the assembly of clathrin coats. An emerging aspect of membrane trafficking research is now addressing how signalling cascades and vesicle coat assembly and subsequently disassembly are integrated.

Major Cdk5-dependent phosphorylation sites of amphiphysin 1 are implicated in the regulation of the membrane binding and endocytosis

Amphiphysin 1 (amph 1) is an endocytic protein enriched in the nerve terminals that functions in the clathrin-mediated endocytosis. It acts as membrane curvature sensor, a linker of clathrin coat proteins, and an enhancer of dynamin Guanosine Triphosphatase (GTPase) activity. Amph 1 undergoes phosphorylation by cyclin-dependent kinase 5 (Cdk5), at five phosphorylation sites, serine 262, 272, 276, 285, and threonine 310, as determined by mass spectrometry (MS). We show here that Cdk5-dependent phosphorylation of amph 1 is enhanced in the presence of lipid membranes. Analysis by tandem liquid chromatograph MS revealed that the phosphorylation occurs at two phosphorylation sites. The phosphorylation was markedly decreased by mutation either Ser276 or Ser285 of amph 1 to alanine (S276A and S285A). Furthermore, mutation of both sites (S276, 285A) completely eliminated the phosphorylation. Functional studies indicated that binding of amph 1 to lipid membrane was attenuated by Cdk5-dependent phosphorylation of wild type amph 1, but not of the S276, 285A form. Interestingly, endocytosis was increased in rat pheochromocytoma cells expressing amph 1 S276, 285A in comparison with wild type. These results suggest that Ser276 and Ser285 are regulatory Cdk5 phosphorylation sites of amph 1 in the lipid-bound state. Phosphorylation at these sites alters binding of amph 1 to lipid membranes, and may be an important regulatory aspect in the regulation of synaptic vesicle endocytosis.

Role of activation of PIP5Kgamma661 by AP-2 complex in synaptic vesicle endocytosis

Synaptic vesicles (SVs) are retrieved by clathrin-mediated endocytosis at the nerve terminals. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] drives this event by recruiting the components of the endocytic machinery. However, the molecular mechanisms that result in local generation of PI(4,5)P2 remain unclear. We demonstrate here that AP-2 complex directly interacts with phosphatidylinositol 4-phosphate 5-kinase gamma661 (PIP5Kgamma661), the major PI(4,5)P2-producing enzyme in the brain. The beta2 subunit of AP-2 was found to bind to the C-terminal tail of PIP5Kgamma661 and cause PIP5Kgamma661 activation. The interaction is regulated by PIP5Kgamma661 dephosphorylation, which is triggered by depolarization in mouse hippocampal neurons. Finally, overexpression of the PIP5Kgamma661 C-terminal region in hippocampal neurons suppresses depolarization-dependent SV endocytosis. These findings provide evidence for the molecular mechanism through which PIP5Kgamma661 locally generates PI(4,5)P2 in hippocampal neurons and suggest a model in which the interaction trigger SV endocytosis.

Activity-dependent control of slow synaptic vesicle endocytosis by cyclin-dependent kinase 5

The stimulated dephosphorylation of the dephosphin group of endocytic proteins by calcineurin and their subsequent rephosphorylation by cyclin-dependent kinase 5 (cdk5) is required for synaptic vesicle (SV) retrieval in central nerve terminals. However, the specific endocytic pathway(s) controlled by these enzymes is unknown. To address this issue, we combined functional and morphological assays of endocytosis in primary neuronal cultures with pharmacological and molecular ablation of calcineurin and cdk5 activity. During strong stimulation, inhibition of calcineurin or cdk5 blocked uptake of the activity-dependent membrane marker FM1-43, but not the more hydrophilic FM2-10. However, FM2-10 uptake-measured poststimulation was sensitive to cdk5 and calcineurin inhibition, indicating that a slow form of endocytosis persists after termination of stimulation. In parallel EM studies, inhibition of cdk5 during strong stimulation greatly reduced horseradish peroxidase labeling of plasma membrane-derived nerve terminal endosomes, but not SVs. Furthermore, during mild stimulation, FM1-43 uptake was unaffected by cdk5 inhibition and the SV membrane was exclusively retrieved via a single SV route, suggesting that recruitment of the endosomal route of membrane retrieval is activity dependent. Thus, we propose that the calcineurin/cdk5-dependent phosphorylation cycle of the dephosphins specifically controls a slow endocytic pathway that proceeds via endosomal intermediates and is activated by strong physiological stimulation in central nerve terminals.

Intersectin is a negative regulator of dynamin recruitment to the synaptic endocytic zone in the central synapse

Intersectin is a multidomain dynamin-binding protein implicated in numerous functions in the nervous system, including synapse formation and endocytosis. Here, we demonstrate that during neurotransmitter release in the central synapse, intersectin, like its binding partner dynamin, is redistributed from the synaptic vesicle pool to the periactive zone. Acute perturbation of the intersectin-dynamin interaction by microinjection of either intersectin antibodies or Src homology 3 (SH3) domains inhibited endocytosis at the fission step. Although the morphological effects induced by the different reagents were similar, antibody injections resulted in a dramatic increase in dynamin immunoreactivity around coated pits and at constricted necks, whereas synapses microinjected with the GST (glutathione S-transferase)-SH3C domain displayed reduced amounts of dynamin in the neck region. Our data suggest that intersectin controls the amount of dynamin released from the synaptic vesicle cluster to the periactive zone and that it may regulate fission of clathrin-coated intermediates.

Nerve growth factor-induced phosphorylation of amphiphysin-1 by casein kinase 2 regulates clathrin-amphiphysin interactions

Amphiphysins interact directly with clathrin and have a function in clathrin-mediated synaptic vesicle recycling and clathrin-mediated endocytosis. The neuronal isoform amphiphysin-1 is a serine/threonine phosphoprotein that is dephosphorylated upon stimulation of synaptic vesicle endocytosis. Rephosphorylation was stimulated by nerve growth factor. We analysed the regulation of amphiphysin-clathrin interactions by phosphorylation. The N-terminal domain of clathrin bound to unphosphorylated amphiphysin-1, but not to the phosphorylated protein. A search for possible phosphorylation sites revealed two casein kinase 2 consensus motifs in close proximity to the clathrin binding sites in amphiphysin-1 and -2. We mutagenized these residues (T350 and T387) to glutamate, mimicking a constitutive phosphorylation. The double mutant showed a strong reduction in clathrin binding. The assumption that casein kinase 2 phosphorylates amphiphysin-1 at T350 and T387 was corroborated by experiments showing that: (i) casein kinase 2 phosphorylated these residues directly in vitro, (ii) when expressed in HeLa cells, the glutamate mutant showed reduced phosphorylation, and (iii) casein kinase 2 inhibitors blocked nerve growth factor-induced phosphorylation of endogenous amphiphysin-1 in PC12 cells. These observations are consistent with the hypothesis that, upon activation by nerve growth factor, casein kinase 2 phosphorylates amphiphysin-1 and thereby regulates the endocytosis of clathrin-coated vesicles via the interaction between clathrin and amphiphysin.

Alzheimer's disease and endocytic dysfunction: clues from the Down syndrome-related proteins, DSCR1 and ITSN1

Down syndrome (DS) is a genetically-based disorder which results in multiple conditions for sufferers. Amongst these is a common early incidence of Alzheimer's disease (AD) which usually affects DS individuals by their mid 40s. This fact provides a clue that one or more of the genes located on chromosome 21 may be involved in the onset of AD. Current evidence suggests that endosomal disorders may underlie the earliest pathology of AD, preceding the classical pathological markers of beta-amyloid plaque deposition and neurofibrillary tangles. Therefore, any genes involved in endocytosis and vesicle trafficking which are over-expressed in DS are novel candidates in the pathogenesis of AD. Intersectin-1 (ITSN1) and Down syndrome candidate region 1 (DSCR1) are two such genes. Extensive in vitro data and data from Drosophila indicates that the over-expression of either of these genes or their products results in inhibition or ablation of endocytosis in neuronal as well as non-neuronal cells. This review discusses in detail the known and potential roles of ITSN1 and DSCR1 in DS, AD, endocytosis and vesicle trafficking.

14-3-3 proteins: regulation of endoplasmic reticulum localization and surface expression of membrane proteins

The density and composition of cell surface proteins are major determinants for cellular functions. Regulation of cell surface molecules occurs at several levels, including the efficiency of surface transport, and is therefore of great interest. As the major phosphoprotein-binding modules, 14-3-3 proteins are known for their crucial roles in a wide range of cellular activities, including the subcellular localization of target proteins. Accumulating evidence suggests a role for 14-3-3 in surface transport of membrane proteins, in which 14-3-3 binding reduces endoplasmic reticulum (ER) localization, thereby promoting surface expression of membrane proteins. Here, we focus on recent evidence of 14-3-3-mediated surface transport and discuss the possible molecular mechanisms.

Endocytic adaptors: recruiters, coordinators and regulators

Clathrin-dependent endocytosis allows cells to bring plasma membrane and extracellular molecules into the cell. Forming a clathrin-coated vesicle requires the sequential action of numerous factors, beginning with endocytic adaptors. Adaptors are thought to initiate the process in two ways: by selecting cargo for packaging into the vesicle and assembling the clathrin coat and other components necessary to shape the vesicle. Here, we review recent work focusing on the sequential and cooperative interactions of adaptors with their binding partners, and how adaptors contribute to initial stages of endocytic internalization. The regulation of adaptors might be a key step for controlling endocytosis, and thus aid in homeostasis and cell physiology.

Phosphorylation of amphiphysin I by minibrain kinase/dual-specificity tyrosine phosphorylation-regulated kinase, a kinase implicated in Down syndrome

Minibrain kinase/dual-specificity tyrosine phosphorylation-regulated kinase (Mnb/Dyrk1A) is a proline-directed serine/threonine kinase encoded in the Down syndrome critical region of human chromosome 21. This kinase has been shown to phosphorylate dynamin 1 and synaptojanin 1. Here we report that amphiphysin I (Amph I) is also a Mnb/Dyrk1A substrate. This kinase phosphorylated native Amph I in rodent brains and recombinant human Amph I expressed in Escherichia coli. Serine 293 (Ser-293) was identified as the major site, whereas serine 295 and threonine 310 were found as minor kinase sites. In cultured cells, recombinant Amph I was phosphorylated at Ser-293 by endogenous kinase(s). Because mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) has been suggested to phosphorylate Amph I at Ser-293, our efforts addressed whether Ser-293 is phosphorylated in vivo by MAPK/ERK or by Mnb/Dyrk1A. Overnight serum-withdrawal inactivated MAPK/ERK; nonetheless, Ser-293 was phosphorylated in Chinese hamster ovary and SY5Y cells. Epigallocatechin-3-gallate, a potent Mnb/Dyrk1A inhibitor in vitro, apparently reduced the phosphorylation at Ser-293, whereas PD98059, a potent MAPK/ERK inhibitor, did not. High frequency stimulation of mouse hippocampal slices reduced the phosphorylation at Ser-293, albeit in the midst of MAPK/ERK activation. The endophilin binding in vitro was inhibited by phosphorylating Amph I with Mnb/Dyrk1A. However, phosphorylation at Ser-293 did not appear to alter cellular distribution patterns of the protein. Our results suggest that Mnb/Dyrk1A, not MAPK/ERK, is responsible for in vivo phosphorylation of Amph I at Ser-293 and that phosphorylation changes the recruitment of endophilin at the endocytic sites.

The BAR domain proteins: molding membranes in fission, fusion, and phagy

The Bin1/amphiphysin/Rvs167 (BAR) domain proteins are a ubiquitous protein family. Genes encoding members of this family have not yet been found in the genomes of prokaryotes, but within eukaryotes, BAR domain proteins are found universally from unicellular eukaryotes such as yeast through to plants, insects, and vertebrates. BAR domain proteins share an N-terminal BAR domain with a high propensity to adopt alpha-helical structure and engage in coiled-coil interactions with other proteins. BAR domain proteins are implicated in processes as fundamental and diverse as fission of synaptic vesicles, cell polarity, endocytosis, regulation of the actin cytoskeleton, transcriptional repression, cell-cell fusion, signal transduction, apoptosis, secretory vesicle fusion, excitation-contraction coupling, learning and memory, tissue differentiation, ion flux across membranes, and tumor suppression. What has been lacking is a molecular understanding of the role of the BAR domain protein in each process. The three-dimensional structure of the BAR domain has now been determined and valuable insight has been gained in understanding the interactions of BAR domains with membranes. The cellular roles of BAR domain proteins, characterized over the past decade in cells as distinct as yeasts, neurons, and myocytes, can now be understood in terms of a fundamental molecular function of all BAR domain proteins: to sense membrane curvature, to bind GTPases, and to mold a diversity of cellular membranes.

Developmental change in the calcium sensor for synaptic vesicle endocytosis in central nerve terminals

Synaptic vesicle endocytosis is stimulated by calcium influx in mature central nerve terminals via activation of the calcium-dependent protein phosphatase, calcineurin. However, in different neuronal preparations calcineurin activity is either inhibitory, stimulatory or irrelevant to the process. We addressed this inconsistency by investigating the requirement for calcineurin activity in synaptic vesicle endocytosis during development, using vesicle recycling assays in isolated nerve terminals. We show that endocytosis occurs independently of calcineurin activity in immature nerve terminals, and that a calcineurin requirement develops 2-4 weeks after birth. Calcineurin-independent endocytosis is not due to the absence of calcineurin activity, since calcineurin is present in immature nerve terminals and its substrate, dynamin I, is dephosphorylated on depolarization. Calcineurin-independent endocytosis is calcium-dependent, since substitution of the divalent cation, barium, inhibits the process. Finally, we demonstrated that in primary neuronal cultures derived from neonatal rats, endocytosis that was initially calcineurin-independent developed a calcineurin requirement on maturation in culture. Our data account for the apparent inconsistencies regarding the role of calcineurin in synaptic vesicle endocytosis, and we propose that an unidentified calcium sensor exists to couple calcium influx to endocytosis in immature nerve terminals.

Inhibition of dynamin-dependent endocytosis increases shedding of the amyloid precursor protein ectodomain and reduces generation of amyloid beta protein

Background

The amyloid precursor protein (APP) is transported via the secretory pathway to the cell surface, where it may be cleaved within its ectodomain by α-secretase, or internalized within clathrin-coated vesicles. An alternative proteolytic pathway occurs within the endocytic compartment, where the sequential action of β- and γ-secretases generates the amyloid β protein (Aβ). In this study, we investigated the effects of modulators of endocytosis on APP processing.

Results

Human embryonic kidney cells were transfected with a dominant negative mutant of dynamin I, an important mediator of clathrin-dependent endocytosis, and APP proteolysis was analyzed. Overexpression of the mutant dynamin (dyn I K44A) resulted in increased shedding of the APP ectodomain (sAPPα), accumulation of the C-terminal α-secretase product C83, and a reduction in the release of Aβ. Levels of mature APP on the cell surface were increased in cells expressing dyn I K44A, and internalization of surface-immunolabeled APP, assessed by fluorescence microscopy, was inhibited. Dynamin is a substrate for protein kinase C (PKC), and it was hypothesized that activators of PKC, which are known to stimulate α-secretase-mediated cleavage of APP, might exert their effects by inhibiting dynamin-dependent endocytosis. However, the internalization of surface-biotinylated APP was unaffected by treatment of cells with phorbol 12-myristate 13-acetate in the presence of the α-secretase inhibitor TAPI-1.

Conclusion

The results indicate that APP is internalized by a dynamin-dependent process, and suggest that alterations in the activity of proteins that mediate endocytosis might lead to significant changes in Aβ production.

Dynamin I phosphorylation and the control of synaptic vesicle endocytosis

The GTPase dynamin I is essential for synaptic vesicle endocytosis in nerve terminals. It is a nerve terminal phosphoprotein that is dephosphorylated on nerve terminal stimulation by the calcium-dependent protein phosphatase calcineurin and then rephosphorylated by cyclin-dependent kinase 5 on termination of the stimulus. Because of its unusual phosphorylation profile, the phosphorylation status of dynamin I was assumed to be inexorably linked to synaptic vesicle endocytosis; however, direct proof of this link has been elusive until very recently. This review will describe current knowledge regarding dynamin I phosphorylation in nerve terminals and how this regulates its biological function with respect to synaptic vesicle endocytosis.