Calcium-dependent association of annexin VI, protein kinase C alpha, and neurocalcin alpha on the raft fraction derived from the synaptic plasma membrane of rat brain

Niemann-Pick type C disease: cellular pathology and pharmacotherapy

Niemann-Pick type C disease (NPCD) was first described in 1914 and affects approximately 1 in 150 000 live births. It is characterized clinically by diverse symptoms affecting liver, spleen, motor control, and brain; premature death invariably results. Its molecular origins were traced, as late as 1997, to a protein of late endosomes and lysosomes which was named NPC1. Mutation or absence of this protein leads to accumulation of cholesterol in these organelles. In this review, we focus on the intracellular events that drive the pathology of this disease. We first introduce endocytosis, a much-studied area of dysfunction in NPCD cells, and survey the various ways in which this process malfunctions. We briefly consider autophagy before attempting to map the more complex pathways by which lysosomal cholesterol storage leads to protein misregulation, mitochondrial dysfunction, and cell death. We then briefly introduce the metabolic pathways of sphingolipids (as these emerge as key species for treatment) and critically examine the various treatment approaches that have been attempted to date.

Enriched Expression of Neutral Sphingomyelinase 2 in the Striatum is Essential for Regulation of Lipid Raft Content and Motor Coordination

Sphingomyelinases are a family of enzymes that hydrolyze sphingomyelin to generate phosphocholine and ceramide. The brain distribution and function of neutral sphingomyelinase 2 (nSMase2) were elucidated in this study. nSMase2 mRNA expression was greatest in the striatum, followed by the prefrontal cortex, hippocampus, cerebellum, thalamus, brainstem, and olfactory bulb. The striatum had the highest level of nSMase2 protein expression, followed by the prefrontal cortex, thalamus, hippocampus, brainstem, and cerebellum. Dense immunolabeling was observed in the striatum, including the caudate-putamen, while moderately dense staining was found in the olfactory bulb and cerebral neocortex. Electron microscopy of the caudate-putamen showed nSMase2 immunoreaction product was present in small diameter dendrites or dendritic spines, that formed asymmetrical synapses with unlabeled axon terminals containing small round vesicles; and characteristics of glutamatergic axons. Lipidomic analysis of the striatum showed increase in long chain sphingomyelins, SM36:1 and SM38:1 after inhibition of nSMase activity. Quantitative proteomic analysis of striatal lipid raft fraction showed many proteins were downregulated by more than 2-fold after inhibition or antisense knockdown of nSMase; consistent with the notion that nSMase2 activity is important for aggregation or clustering of proteins in lipid rafts. Inhibition or antisense knockdown of nSMase2 in the caudate-putamen resulted in motor deficits in the rotarod and narrow beam tests; as well as decreased acoustic startle and improved prepulse inhibition of the startle reflex. Together, results indicate an important function of nSMase2 in the striatum.

Annexin A6 regulates interleukin-2-mediated T-cell proliferation

Annexin A6 (AnxA6) has been implicated in cell signalling by contributing to the organisation of the plasma membrane. Here we examined whether AnxA6 regulates signalling and proliferation in T cells. We used a contact hypersensitivity model to immune challenge wild-type (WT) and AnxA6(-/-) mice and found that the in vivo proliferation of CD4(+) T cells, but not CD8(+) T cells, was impaired in AnxA6(-/-) relative to WT mice. However, T-cell migration and signalling through the T-cell receptor ex vivo was similar between T cells isolated from AnxA6(-/-) and WT mice. In contrast, interleukin-2 (IL-2) signalling was reduced in AnxA6(-/-) compared with WT T cells. Further, AnxA6-deficient T cells had reduced membrane order and cholesterol levels. Taken together, our data suggest that AnxA6 regulates IL-2 homeostasis and sensitivity in T cells by sustaining a lipid raft-like membrane environment.

The proteome of the presynaptic active zone from mouse brain

Neurotransmitter release as well as the structural and functional dynamics of the presynaptic active zone is controlled by proteinaceous components. Here we describe for the first time an experimental approach for the isolation of the presynaptic active zone from individual mouse brains, a prerequisite for understanding the functional inventory of the presynaptic protein network and for the later analysis of changes occurring in mutant mice. Using a monoclonal antibody against the ubiquitous synaptic vesicle protein SV2 we immunopurified synaptic vesicles docked to the presynaptic plasma membrane. Enrichment studies by means of Western blot analysis and mass spectrometry identified 485 proteins belonging to an impressive variety of functional categories. Our data suggest that presynaptic active zones represent focal hot spots that are not only involved in the regulation of neurotransmitter release but also in multiple structural and functional alterations the adult nerve terminal undergoes during neural activity in adult CNS. They furthermore open new avenues for characterizing alterations in the active zone proteome of mutant mice and their corresponding controls, including the various mouse models of neurological diseases.

Annexin A6 is an organizer of membrane microdomains to regulate receptor localization and signalling

Annexin A6 (AnxA6) belongs to the conserved annexin protein family--a group of Ca(2+) -dependent membrane binding proteins. It is the largest of all annexin proteins and upon activation, binds to negatively charged phospholipids in the plasma membrane and endosomes. In addition, AnxA6 associates with cholesterol-rich membrane microdomains termed lipid rafts. Membrane cholesterol triggers Ca(2+) -independent translocation of AnxA6 to membranes and AnxA6 levels determine the number of caveolae, a form of specialized rafts at the cell surface. AnxA6 also has an F-actin binding domain and interacts with cytoskeleton components. Taken together, this suggests that AnxA6 has a scaffold function to link membrane microdomains with the organization of the cytoskeleton. Such a link facilitates AnxA6 to participate in plasma membrane repair and it would also impact on receptor signalling at the cell surface, growth factor, and lipoprotein receptor trafficking, Ca(2+) -channel activity and T cell activation. Hence, the regulation of cell surface receptors by AnxA6 may be facilitated by its unique structure that allows recruitment of interaction partners and simultaneously bridging specialized membrane domains with cortical actin surrounding activated receptors.

Annexin A6 is recruited into lipid rafts of Niemann-Pick type C disease fibroblasts in a Ca2+-dependent manner

Niemann-Pick type C (NPC) disease is characterized by excessive accumulation of cholesterol in the late endosome/lysosome compartment. Some members of the annexin family of proteins such as annexin A2 (AnxA2) and annexin A6 (AnxA6) follow the same route as cholesterol during the endocytic pathway and are found, as AnxA6, attached to the membranes of the cholesterol storage compartment in NPC disease fibroblasts. Therefore, the purpose of this work was to test the hypothesis that AnxA6 participates in the NPC-induced changes in the organization of membrane microdomains resistant to solubilization by a nonionic detergent, Triton X-100, i.e., detergent-resistant microdomains (DRMs). Using cellular fractionation, fluorescence microscopy and specific antibodies we observed that in the absence of calcium AnxA6 was found in the DRM-depleted membrane fractions isolated from NPC and control fibroblasts. In the presence of calcium, AnxA6 re-located to the fractions enriched in DRMs only in the NPC cells, suggestive of AnxA6 participation in organization of these microdomains.

Annexin A6-Linking Ca(2+) signaling with cholesterol transport

Annexin A6 (AnxA6) belongs to a conserved family of Ca(2+)-dependent membrane-binding proteins. Like other annexins, the function of AnxA6 is linked to its ability to bind phospholipids in cellular membranes in a dynamic and reversible fashion, in particular during the regulation of endocytic and exocytic pathways. High amounts of AnxA6 sequester cholesterol in late endosomes, thereby lowering the levels of cholesterol in the Golgi and the plasma membrane. These AnxA6-dependent redistributions of cellular cholesterol pools give rise to reduced cytoplasmic phospholipase A2 (cPLA(2)) activity, retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. In addition to regulating cholesterol and caveolin distribution, AnxA6 acts as a scaffold/targeting protein for several signaling proteins, the best characterized being the Ca(2+)-dependent membrane targeting of p120GAP to downregulate Ras activity. AnxA6 also stimulates the Ca(2+)-inducible involvement of PKC in the regulation of HRas and possibly EGFR signal transduction pathways. The ability of AnxA6 to recruit regulators of the EGFR/Ras pathway is likely potentiated by AnxA6-induced actin remodeling. Accordingly, AnxA6 may function as an organizer of membrane domains (i) to modulate intracellular cholesterol homeostasis, (ii) to create a scaffold for the formation of multifactorial signaling complexes, and (iii) to regulate transient membrane-actin interactions during endocytic and exocytic transport. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Proteomic comparison of spherical aggregates and adherent cells of cardiac stem cells

BACKGROUND

Previously published papers showed that cardiac stem cells (CSCs) form (cardio)sphere. However, recent studies questioned the significance of the sphere-formation as one of the characteristics of CSCs. We isolated c-kit-positive cardiac stem cells, cultured as bulk (CSC-BC) and characterized them previously. Among them, CSC-BC21 formed an extraordinary number of spheres. Using a clone derived from this bulk culture, we investigated the effect of sphere-formation on differentiation and performed proteomics analysis comparing two statuses, cardiosphere and dish substrate attachment.

METHODS

We performed sphere-forming assay to compare the sphere-forming ability among CSC-BCs. The cloned cells from CSC-BC21, which had distinct sphere-forming ability, were cultured in a differentiation medium (DM) to induce cardiac myocyte differentiation. We performed RT-PCR analysis to investigate if cardiosphere-formation affects cardiac myocyte gene expression level. Furthermore, proteome analysis was performed to compare floating cardiosphere (flCS) and dish-attached cardiosphere-derived cells (daCS).

RESULTS

One of the cloned cells, CSC-21E expressed higher troponin I message than CSC-BC21. Moreover, the message level of troponin I was enhanced when they had experienced cardiosphere prior to the treatment of myocyte differentiation medium. The change from flCS to daCS accompanied up-regulation of chaperones and down regulation of glycolytic and other metabolic enzymes. Calreticulin and Hsp 90 were among the up-regulated chaperons. Calreticulin is known to be an essential component of cardiogenesis.

CONCLUSION

These results suggest that the switch from aggregated sphere to the cell attachment, is important for advancing the cardiac cell differentiation.

The annexins: spatial and temporal coordination of signaling events during cellular stress

Annexins are a family of structurally related, Ca2+-sensitive proteins that bind to negatively charged phospholipids and establish specific interactions with other lipids and lipid microdomains. They are present in all eukaryotic cells and share a common folding motif, the "annexin core", which incorporates Ca2+- and membrane-binding sites. Annexins participate in a variety of intracellular processes, ranging from the regulation of membrane dynamics to cell migration, proliferation, and apoptosis. Here we focus on the role of annexins in cellular signaling during stress. A chronic stress response triggers the activation of different intracellular pathways, resulting in profound changes in Ca2+ and pH homeostasis and the production of lipid second messengers. We review the latest data on how these changes are sensed by the annexins, which have the ability to simultaneously interact with specific lipid and protein moieties at the plasma membrane, contributing to stress adaptation via regulation of various signaling pathways.

Annexins--modulators of EGF receptor signalling and trafficking

At the cell surface, activation of the epidermal growth factor (EGF) receptor triggers a complex network of signalling events that regulate a variety of cellular processes. For signal termination, the activated EGF receptor is internalised and targeted to lysosomes for degradation. Microdomain localization at the plasma membrane and endocytic transport of the EGFR is important for the formation of compartment-specific signalling complexes and is regulated by scaffolding and targeting proteins. This includes Ca2+-effector proteins, such as calmodulin and annexins (Anx), in particular AnxA1, AnxA2, AnxA6 and as shown recently,AnxA8. Given that these annexins show differences in their expression patterns, subcellular localization and mode of action, they are likely to differentially contribute and cooperate in the fine-tuning of EGFR activity. In support of this hypothesis, current literature suggests these annexins to be involved in different steps that control the endocytic transport and signalling of the EGF receptor. This review summarizes how the coordinated activity of AnxA1, AnxA2, AnxA6 and AnxA8 can contribute to regulate EGF receptor localization and activity.

The proteome of the presynaptic active zone: from docked synaptic vesicles to adhesion molecules and maxi-channels

The presynaptic proteome controls neurotransmitter release and the short and long term structural and functional dynamics of the nerve terminal. Using a monoclonal antibody against synaptic vesicle protein 2 we immunopurified a presynaptic compartment containing the active zone with synaptic vesicles docked to the presynaptic plasma membrane as well as elements of the presynaptic cytomatrix. Individual protein bands separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were subjected to nanoscale-liquid chromatography electrospray ionization-tandem mass spectrometry. Combining this method with 2-dimensional benzyldimethyl-n-hexadecylammonium chloride/sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization time of flight and immunodetection we identified 240 proteins comprising synaptic vesicle proteins, components of the presynaptic fusion and retrieval machinery, proteins involved in intracellular signal transduction, a large variety of adhesion molecules and proteins potentially involved in regulating the functional and structural dynamics of the pre-synapse. Four maxi-channels, three isoforms of voltage-dependent anion channels and the tweety homolog 1 were co-isolated with the docked synaptic vesicles. As revealed by in situ hybridization, tweety homolog 1 reveals a distinct expression pattern in the rodent brain. Our results add novel information to the proteome of the presynaptic active zone and suggest that in particular proteins potentially involved in the short and long term structural modulation of the mature presynaptic compartment deserve further detailed analysis.

Annexin A6 at the cardiac myocyte sarcolemma--evidence for self-association and binding to actin

The plasma membrane of the heart muscle cell and its underlying cytoskeleton are vitally important to the function of the heart. Annexin A6 is a major cellular calcium and phospholipid binding protein. Here we show that annexin A6 copurifies with sarcolemma isolated from pig heart. Two pools of annexin A6 are present in the sarcolemma fraction, one dependent on calcium and one that resists extraction by the calcium chelator EGTA. Potential annexin A6 binding proteins in the sarcolemma fraction were identified using Far Western blotting. Two major annexin A6 binding proteins were identified as actin and annexin A6 itself. Annexin A6 bound to itself both in the presence and in the absence of calcium ions. Sites for self association were mapped by performing Western blots on proteolytic fragments of recombinant annexin A6. Annexin A6 bound preferentially not only to the N terminal fragment (domains I-IV, residues 1-352) but also to C-terminal fragments corresponding to domains V+VI and domains VII+VIII. Actin binding to annexin A6 was calcium-dependent and exclusively to the N-terminal fragment of annexin A6. A calcium-dependent complex of annexin A6 and actin may stabilize the cardiomyocyte sarcolemma during cell stimulation.

Molecular interaction of neurocalcin alpha with alsin (ALS2)

Membrane microdomains (MDs), or lipid rafts, are recently identified dynamic membrane domains on which various signal-transductions are performed. Intracellular Ca(2+)-binding proteins participate in the Ca(2+) signaling through interaction with various proteins. Neurocalcin alpha (NCalpha) is a member of neuronal calcium sensor (NCS) protein family and shows Ca(2+)-dependent binding to the cell membrane through N-terminal myristoyl moiety. Since NCalpha was identified as a Ca(2+)-dependent binding protein to neuronal MDs, its binding proteins may participate in the signal-transduction on the MDs. In an immunoprecipitate using anti-NCalpha antibody, alsin (ALS2), a protein product of one of the responsive genes for amyotrophic lateral sclerosis, was detected through LC-MS/MS. Specific antibody to alsin was produced and immunoprecipitation using this antibody showed co-sedimentation of NCalpha. Some part of alsin bound to brain-derived MD fraction in the presence of Ca(2+) ions and eluted out by the chelation of Ca(2+) ions, as in the case of NCalpha. Immunostaining of cultured neurons showed broad distribution of alsin and NCalpha, and membrane association of these proteins were increased through Ca(2+) loading by maitotoxin. These results suggest that alsin binds cell membrane in a Ca(2+)-dependent manner through NCalpha and regulates membrane dynamics.

Annexin A6-induced alterations in cholesterol transport and caveolin export from the Golgi complex

Annexin A6 (AnxA6) belongs to a family of Ca(2+)-dependent membrane-binding proteins and is involved in the regulation of endocytic and exocytic pathways. We previously demonstrated that AnxA6 regulates receptor-mediated endocytosis and lysosomal targeting of low-density lipoproteins and translocates to cholesterol-enriched late endosomes (LE). As cholesterol modulates the membrane binding and the cellular location of AnxA6, but also affects the intracellular distribution of caveolin, we investigated the localization and trafficking of caveolin in AnxA6-expressing cells. Here, we show that cells expressing high levels of AnxA6 are characterized by an accumulation of caveolin-1 (cav-1) in the Golgi complex. This is associated with a sequestration of cholesterol in the LE and lower levels of cholesterol in the Golgi and the plasma membrane, both likely contributing to retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. Further strengthening these findings, knock down of AnxA6 and the ectopic expression of the Niemann-Pick C1 protein in AnxA6-overexpressing cells restore the cellular distribution of cav-1 and cholesterol, respectively. In summary, this study demonstrates that elevated expression levels of AnxA6 perturb the intracellular distribution of cholesterol, which indirectly inhibits the exit of caveolin from the Golgi complex.

Identification of a novel interaction between the Ca2+-binding protein S100A11 and the Ca2+- and phospholipid-binding protein annexin A6

S100A11 is a member of the S100 family of EF-hand Ca2+-binding proteins, which is expressed in smooth muscle and other tissues. Ca2+binding to S100A11 induces a conformational change that exposes a hydrophobic surface for interaction with target proteins. Affinity chromatography with immobilized S100A11 was used to isolate a 70-kDa protein from smooth muscle that bound to S100A11 in a Ca2+-dependent manner and was identified by mass spectrometry as annexin A6. Direct Ca2+-dependent interaction between S100A11 and annexin A6 was confirmed by affinity chromatography of the purified bacterially expressed proteins, by gel overlay of annexin A6 with purified S100A11, by chemical cross-linking, and by coprecipitation of S100A11 with annexin A6 bound to liposomes. The expression of S100A11 and annexin A6 in the same cell type was verified by RT-PCR and immunocytochemistry of isolated vascular smooth muscle cells. The site of binding of S100A11 on annexin A6 was investigated by partial tryptic digestion and deletion mutagenesis. The unique NH2terminal head region of annexin A6 was not required for S100A11 binding, but binding sites were identified in both NH2- and COOH-terminal halves of the molecule. We hypothesize that an agonist-induced increase in cytosolic free [Ca2+] leads to formation of a complex of S100A11 and annexin A6, which forms a physical connection between the plasma membrane and the cytoskeleton, or plays a role in the formation of signaling complexes at the level of the sarcolemma.

Identification of members of the annexin family in the detergent-insoluble fraction of rat Morris hepatoma plasma membranes

For proteomic analysis, plasma membranes of rat hepatocellular carcinoma Morris hepatoma 7777 were selectively solubilized according to the previously developed method [D. Josic, K. Zeilinger, Methods Enzymol. 271 (1996) 113-134]. If the Triton X100 insoluble pellet is subsequently extracted, several proteins can be solubilized. These proteins can be classified in two groups according to their molecular size. The proteins with apparent molecular weights in SDS-PAGE between 70 and 75 kDa belong to the first group. Smaller proteins, with apparent molecular weights between 30 and 45 kDa, are members of the second group. The main protein of higher molecular weight was also found in the Triton X100 insoluble extract from normal rat liver plasma membranes. This protein was identified as Annexin A6. The proteins from the second group are practically absent in the Triton X100 insoluble extract from rat liver. These proteins are present in relatively high concentrations in plasma membranes of Morris hepatoma 7777. Both groups of detergent-insoluble proteins from Morris hepatoma 7777 were further analyzed with SELDI-TOF and LC electrospray ionization mass spectrometry. From the first group, Annexin A6, together with two other integral plasma membrane proteins, was identified. In the second group of proteins with apparent molecular weights between 30 and 45kDa, further members of the annexin family, Annexins A1, A2, A4, A5 and A7 were identified. The possible role of these low molecular size annexins as potential cancer biomarkers is discussed.

Protein kinase C delta (deltaPKC)-annexin V interaction: a required step in deltaPKC translocation and function

Protein kinase C (PKC) plays a critical role in diseases such as cancer, stroke, and cardiac ischemia, and participates in a variety of signal transduction pathways such as apoptosis, cell proliferation, and tumor suppression. Though much is known about PKC downstream signaling events, the mechanisms of regulation of PKC activation and subsequent translocation have not been elucidated. Protein-protein interactions regulate and determine the specificity of many cellular signaling events. Such a specific protein-protein interaction is described here between deltaPKC and annexin V. We demonstrate, at physiologically relevant conditions, that a transient interaction between annexin V and deltaPKC occurs in cells after deltaPKC stimulation, but before deltaPKC translocates to the particulate fraction. Evidence of deltaPKC-annexin V binding is provided also by FRET and by in vitro binding studies. Dissociation of the deltaPKC-annexin V complex requires ATP and microtubule integrity. Furthermore, depletion of endogenous annexin V, but not annexin IV, with siRNA inhibits deltaPKC translocation following PKC stimulation. A rationally designed eight amino acid peptide, corresponding to the interaction site for deltaPKC on annexin V, inhibits deltaPKC translocation and deltaPKC-mediated function as evidenced by its protective effect in a model of myocardial infarction. Our data indicate that translocation of deltaPKC is not simply a diffusion-driven process, but is instead a multi-step event regulated by protein-protein interactions. We show that following cell activation, deltaPKC-annexin V binding is a transient and an essential step in the function of deltaPKC, thus identifying a new role for annexin V in PKC signaling and a new step in PKC activation.

Annexin V associates with the IFN-gamma receptor and regulates IFN-gamma signaling

Many of the biological activities of IFN-gamma are mediated through the IFN-gammaR3-linked Jak-Stat1alpha pathway. However, regulation of IFN-gamma signaling is not fully understood, and not all responses to IFN-gamma are Stat1alpha dependent. To identify novel elements involved in IFN-gamma cell regulation, the cytoplasmic domain of the R2 subunit of the human IFN-gammaR was used as bait in a yeast two-hybrid screen of a human monocyte cDNA library. This identified annexin A5 (AxV) as a putative IFN-gammaR binding protein. The interaction was confirmed in pull-down experiments in which a GST-R2 cytoplasmic domain fusion protein was incubated with macrophage lysates. Furthermore, immunoprecipitation using anti-IFN-gammaR2 Abs showed that AxV interacted with IFN-gammaR2 to form a stable complex following incubation of cells with IFN-gamma. In 293T cells with reduced expression of AxV, brought about by small interfering RNA targeting, activation of Jak2 and Stat1alpha in response to IFN-gamma was enhanced. Inhibition of cell proliferation, a hallmark of the IFN-gamma response, also was potentiated in HeLa cells treated with small interfering RNA directed at AxV. Taken together, these results suggest that through an inducible association with the R2 subunit of the IFN-gammaR, AxV modulates cellular responses to IFN-gamma by modulating signaling through the Jak-Stat1 pathway.

Molecular mechanisms involved in Ras inactivation: the annexin A6–p120GAP complex

In mammalian cells, a complex network of signaling pathways tightly regulates a variety of cellular processes, such as proliferation and differentiation. New insights from one of the most-important signaling cascades involved in oncogenesis, the Ras-Raf-MAPK pathway, suggest that the subcellular localisation and assembly of signaling modules of this pathway is crucial to control the biological response. This commonly requires membrane targeting events that are mediated by adaptor/scaffold proteins. Of particular interest is the translocation and complex formation of GTPase-activating proteins (GAPs), such as p120GAP, at the plasma membrane to inactivate Ras. Recent studies indicate that one member of the annexin family, annexin A6 acts as a targeting protein for p120GAP. This review discusses how annexin A6 modulates the involvement of negative regulators of the Ras-Raf-MAPK pathway contributing to Ras inactivation.

Biochemical and morphologic evidence of the interaction of oligodendrocyte membrane rafts with actin filaments

Cytoskeletal structures under the cell membrane carry out pivotal roles in the maintenance and remodeling of the cell structures. Reforming of the cytoskeletal networks after partial extraction of membrane components could be a good clue to identify molecular components pertaining the interaction of cytoskeleton with membrane. A detergent extract from 3-week-old rat brain membrane fraction was found to make an actin-based gel upon incubation at 25 degrees C. Some protein components of the gelation products were recovered in a Triton-insoluble low-density microdomain fraction (raft) after depolymerization of actin filaments. Some of these proteins were identified as 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase), proteolipid protein (PLP), and myelin oligodendrocyte glycoprotein (MOG) through electrospray time-of-flight (ESI-TOF) analysis and Western blotting. Because these proteins are well-known marker proteins of oligodendrocytes, localization of these proteins and cholesterol, a raft-localized lipid, with actin filaments was studied using cultured oligodendrocytes. Clear colocalization of these proteins and cholesterol with actin filaments was observed after Triton treatment at 4 degrees C before fixation. These results indicate that raft microdomains participate in the formation of cell shape through interaction with microfilaments in oligodendrocytes.

Neuronal Ca2+ sensor protein VILIP-1 affects cGMP signalling of guanylyl cyclase B by regulating clathrin-dependent receptor recycling in hippocampal neurons

The family of neuronal Ca2+ sensor (NCS) proteins is known to influence a variety of physiological and pathological processes by affecting signalling of different receptors and ion channels. Recently, it has been shown that the NCS protein VILIP-1 influences the activity of the receptor guanylyl cyclase GC-B. In transfected cell lines, VILIP-1 performs a Ca2+-dependent membrane association, the reversible Ca2+-myristoyl switch of VILIP-1, which leads to an increase in natriuretic peptide-stimulated cGMP levels. In this study, we have investigated the effect of VILIP-1 on cGMP signalling in C6 cells and in primary hippocampal neurons, where VILIP-1 and GC-B are co-expressed in many but not all neurons and partially co-localize in the soma and in dendrites. Our data indicate that VILIP-1 modulates GC-B activity by influencing clathrin-dependent receptor recycling. These data support a general physiological role for VILIP-1 in membrane trafficking in the intact hippocampus, where the NCS protein may affect processes, such as neuronal differentiation and synaptic plasticity e.g. by influencing cGMP-signalling.

Biologically active milli-calpain associated with caveolae is involved in a spatially compartmentalised signalling involving protein kinase C alpha and myristoylated alanine-rich C-kinase substrate (MARCKS)

We have previously shown that calpain promotes myoblast fusion by acting on protein kinase C-alpha and the cytosolic phosphorylated form of MARCKS. In other cell types, various isoforms of calpain, PKC alpha and MARCKS were found associated with caveolae. These vesicular invaginations of the plasma membrane are essential for myoblast fusion and differentiation. We have isolated caveolae from myoblasts and studied the presence of calpain isoforms and their possible effects on signalling mediated by caveolae-associated PKC. Our results show that milli-calpain co-localizes with myoblast caveolae. Futhermore we provide evidence, using a calcium ionophore and a specific inhibitor of calpains (calpastatin peptide), that milli-calpain reduces the PKC alpha and MARCKS content in these structures. Purified milli-calpain causes the appearance of the active catalytic fragment of PKC alpha (PKM), without having an effect on MARCKS. Addition of phorbol myristate acetate, an activator of PKC, induces tranlocation of PKC alpha towards caveolae and results in a significant reduction of MARCKS associated with caveolae. This phenomenon is not observed when a PKC alpha inhibitor is added at the same time. We conclude that the presence of biologically active milli-calpain within myoblast caveolae induces, in a PKC alpha-dependent manner, MARCKS translocation towards the cytosol. Such a localised signalling event may be essential for myoblast fusion and differentiation.

Annexin A6 stimulates the membrane recruitment of p120GAP to modulate Ras and Raf-1 activity

Annexin A6 is a calcium-dependent membrane-binding protein that interacts with signalling proteins, including the GTPase-activating protein p120GAP, one of the most important inactivators of Ras. Since we have demonstrated that annexin A6 inhibits EGF- and TPA-induced Ras signalling, we investigated whether modulation of Ras activity by annexin A6 was mediated via altered subcellular localization of p120GAP. First, we exploited our observation that high-density lipoproteins (HDL) can activate the Ras/MAP kinase pathway. Expression of annexin A6 caused a significant reduction in HDL-induced activation of Ras and Raf-1. Annexin A6 promoted membrane binding of p120GAP in vitro, and plasma membrane targeting of p120GAP in living cells, both in a Ca(2+)-dependent manner, which is consistent with annexin A6 promoting the Ca(2+)-dependent assembly of p120GAP-Ras at the plasma membrane. We then extended these studies to other cell types and stimuli. Expression of annexin A6 in A431 cells reduced, while RNAi-mediated suppression of annexin A6 in HeLa cells enhanced EGF-induced Ras and Erk activation. Importantly, the enhancement of Ras activation following RNAi-mediated reduction in p120GAP levels was more marked in annexin A6-expressing A431 cells than controls, indicating that the effect of annexin A6 on Ras was mediated via p120GAP. Finally, we demonstrated that annexin A6 promotes plasma membrane targeting of p120GAP in A431 cells in response to a variety of stimuli, resulting in colocalization with H-Ras. These findings demonstrate an important role for annexin A6 in regulating plasma membrane localization of p120GAP and hence Ras activity.

Inhibition of H-Ras and MAPK is compensated by PKC-dependent pathways in annexin A6 expressing cells

High-density lipoprotein (HDL)-induced activation of the Ras/MAPK pathway can be mediated by protein kinase C (PKC)-dependent and independent pathways. Although both pathways co-exist in cells, we showed that binding of HDL to scavenger receptor BI (SR-BI) in CHO cells activates Ras and MAPK in a PKC-independent manner. We have recently identified that HDL-induced activation of Ras and Raf-1 is reduced in annexin A6 expressing CHO cells (CHOanx6). In the present study we demonstrate that despite the loss of Ras and Raf-1 activity, HDL induces MAPK phosphorylation in CHOanx6 cells. Since annexin A6 is a PKCalpha-binding protein we therefore investigated the possible involvement of PKC in HDL-induced Ras and MAPK activation in CHOanx6 cells. Taken together our findings demonstrate that HDL-induced H-Ras and MAPK activation is PKC-dependent in cells expressing annexin A6 to compensate for the loss of PKC-independent activation of H-Ras and MAPK.

A novel protein kinase C α-dependent signal to ERK1/2 activated by αVβ3 integrin in osteoclasts and in Chinese hamster ovary (CHO) cells

We identified a novel protein kinase C (PKC)α-dependent signal to extracellular signal-regulated kinase (ERK)1/2 in mouse osteoclasts and Chinese hamster ovary (CHO) cells, specifically activated by the αVβ3 integrin. It involves translocation (i.e. activation) of PKCα from the cytosol to the membrane and/or the Triton X-100-insoluble subcellular fractions, with recruitment into a complex with αVβ3 integrin, growth factor receptor-bound protein (Grb2), focal adhesion kinase (FAK) in CHO cells and proline-rich tyrosine kinase (PYK2) in osteoclasts. Engagement of αvβ3 integrin triggered ERK1/2 phosphorylation, but the underlying molecular mechanism was surprisingly independent of the well known Shc/Ras/Raf-1 cascade, and of phosphorylated MAP/ERK kinase (MEK)1/2, so far the only recognized direct activator of ERK1/2. In contrast, PKCα was involved in ERK1/2 activation because inhibition of its activity prevented ERK1/2 phosphorylation. The tyrosine kinase c-Src also contributed to ERK1/2 activation, however, it did not interact with PKCα in the same molecular complex. The αVβ3/PKCα complex formation was fully dependent upon the intracellular calcium concentration ([Ca2+]i), and the use of the intracellular Ca2+ chelator 1,2-bis(o-amino-phenoxy)ethane-N,N,N′,N′-tetraaceticacidtetra (acetoxymethyl) ester (BAPTA-AM) also inhibited PKCα translocation and ERK1/2 phosphorylation. Functional studies showed that αVβ3 integrin-activated PKCα was involved in cell migration and osteoclast bone resorption, but had no effect on the ability of cells to attach to LM609, suggesting a role in events downstream of αVβ3 integrin engagement.

Motor, sensory and autonomic nerve terminals containing NAP-22 immunoreactivity in the rat muscle

Neuron-enriched acidic protein having a molecular mass of 22 kDa, NAP-22, is a Ca(2+)-dependent calmodulin-binding protein and is phosphorylated with protein kinase C (PKC). This protein is localized to the biological membrane via myristoylation and found in the membrane fraction of the brain and in the synaptic vesicle fraction. Recent studies showed that NAP-22 is localized in the membrane raft domain in a cholesterol-dependent manner and suggest a role for NAP-22 in maturation and/or maintenance of nerve terminals by controlling cholesterol-dependent membrane dynamics. The present study revealed the immunohistochemical distribution of NAP-22 in the peripheral nerves in rat muscles. In all examined muscles, nerve terminals in the motor endplates showed NAP-22 immunoreactivity associated with the membranes of synaptic vesicles and nerve terminals. In the muscle spindles, annulospiral endings, which made spirals around the intrafusal muscles, showed intense NAP-22 immunoreactivity. Autonomic nerve fibers around the intramuscular blood vessels also showed the immunoreactivity for NAP-22. NAP-22 immunoreactivity in these peripheral nerves was observed from birth to adulthood (100 days after birth). Though growth-associated protein-43 (GAP-43) immunoreactivity in these nerves was observed from birth, this immunoreactivity decreased from 20 days after birth. These findings suggest that NAP-22 is distributed and regulates functions in the motor, sensory and autonomic nerve terminals in the peripheral nervous system.

Accumulation of glycosphingolipids in Niemann-Pick C disease disrupts endosomal transport

Glycosphingolipids are endocytosed and targeted to the Golgi apparatus but are mistargeted to lysosomes in sphingolipid storage disorders. Substrate reduction therapy utilizes imino sugars to inhibit glucosylceramide synthase and potentially abrogate the effects of storage. Niemann-Pick type C (NPC) disease is a disorder of intracellular transport where glycosphingolipids (GSLs) and cholesterol accumulate in endosomal compartments. The mechanisms of altered intracellular trafficking are not known but may involve the mistargeting and disrupted function of proteins associated with GSL membrane microdomains. Membrane microdomains were isolated by Triton X-100 and sucrose density gradient ultracentrifugation. High pressure liquid chromatography and mass spectrometric analysis of NPC1(-/-) mouse brain revealed large increases in GSL. Sphingosine was also found to be a component of membrane microdomains, and in NPC liver and spleen, large increases in cholesterol and sphingosine were found. GSL and cholesterol levels were increased in mutant NPC1-null Chinese hamster ovary cells as well as U18666A and progesterone induced NPC cell culture models. However, inhibition of GSL synthesis in NPC cells with N-butyldeoxygalactonojirimycin led to marked decreases in GSL but only small decreases in cholesterol levels. Both annexin 2 and 6, membrane-associated proteins that are important in endocytic trafficking, show distorted distributions in NPC cells. Altered BODIPY lactosylceramide targeting, decreased endocytic uptake of a fluid phase marker, and mistargeting of annexin 2 (phenotypes associated with NPC) are reversed by inhibition of GSL synthesis. It is suggested that accumulating GSL is part of a mislocalized membrane microdomain and is responsible for the deficit in endocytic trafficking found in NPC disease.

Prenatal exposure of rats to Ginkgo biloba extract (EGb 761) increases neuronal survival/growth and alters gene expression in the developing fetal hippocampus

Hippocampal neuron survival/growth and gene expression have been examined after prenatal (in utero) exposure of rats to EGb 761, a leaf extract of Ginkgo biloba. Oral administration of EGb 761 (100 or 300 mg/kg/day) to pregnant dams for 5 days increased the number of hippocampal neurons (maintained in culture) of their fetuses, indicating a neurotrophic effect of the extract. Using large-scale oligonucleotide microarrays containing over 8000 combined rat genes and expressed sequence tag clusters, it was shown that treatment of pregnant dams with EGb 761 (25, 50 or 100 mg/kg/day for 5 days) altered the expression of 187 genes in the hippocampi of male fetuses and 160 genes in those of female fetuses. Using gene-cluster analysis, these genes were grouped into 18 distinct clusters for males and 17 distinct clusters for females. Among these clusters, 35 genes shared a common expression pattern in male and female hippocampal development. Of these genes, the changes observed in insulin growth factor II, insulin growth factor binding protein 2, testosterone repressed prostate message-2, glutathione-dependent dehydroascorbate reductase, lipoprotein lipase, guanylate cyclase and DNA binding protein Brn-2 were confirmed by real-time quantitative polymerase chain reaction. These findings, which have provided the first genetic profile of the effects of EGb 761 on the developing rat hippocampus, increase our understanding of the molecular and genetic programs that are activated by the extract. These effects of EGb 761 may underlie its neuroprotective properties.

Identification of upregulated SCG10 mRNA expression associated with late-phase long-term potentiation in the rat hippocampal Schaffer-CA1 pathway in vivo

The maintenance of long-term potentiation (LTP) depends on alteration of gene transcription. By screening a subtracted cDNA library that is enriched in upregulated transcripts in rat hippocampus 3 hr after Schaffer-CA1 LTP induction in vivo, we identified a neural growth-associated protein SCG10 (superior cervical ganglia clone 10) gene. The semiquantitative reverse transcription-PCR and Northern blot experiments confirmed that SCG10 mRNA levels were elevated in tetanized rat hippocampi compared with those of sham controls that received only low-frequency stimulation. Both 1 and 2 kb forms of SCG10 mRNAs contributed to the increased expression. Using a riboprobe with a sequence specific to the 3'-untranslated region of rat SCG10 mRNA, in situ hybridization further revealed a significant increase of the SCG10 mRNA 2 kb form in the ipsilateral CA3 and CA1 regions of LTP animals. In addition, we systemically injected the competitive NMDA receptor antagonist d,l-3[(+/-)-2-carboxypiperazine-4-yl]-propyl-1-phosphonic acid (CPP) to determine whether the alteration of SCG10 expression depends on NMDA receptor activation or tetanus alone. Administration of CPP 1 hr before tetanus completely blocked LTP induction and the increase of SCG10 mRNA levels. Thus, these results suggest that the transcription of SCG10 in vivo is regulated by long-lasting synaptic activity and may contribute to the maintenance of long-term synaptic plasticity via a presynaptic remodeling mechanism.

Molecular characterization of the detergent-insoluble cholesterol-rich membrane microdomain (raft) of the central nervous system

Many fundamental neurological issues such as neuronal polarity, the formation and remodeling of synapses, synaptic transmission, and the pathogenesis of the neuronal cell death are closely related to the membrane dynamics. The elucidation of functional roles of a detergent-insoluble cholesterol-rich domain (raft) could therefore provide good clues to the molecular understanding of these important phenomena, for the participation of the raft in the fundamental cell functions, such as signal transduction and selective transport of lipids and proteins, has been elucidated in nonneural cells. Interestingly, the brain is rich in raft and the brain-derived raft differs in its lipid and protein components from other tissue-derived rafts. Since many excellent reviews are written on the membrane lipid dynamics of this microdomain, signal transduction, and neuronal glycolipids, we review on the characterization of the raft proteins recovered in the detergent-insoluble low-density fraction from rat brain. Special focus is addressed on the biochemical characterization of a neuronal enriched protein, NAP-22, for the lipid organizing activity of this protein has become increasingly clear.

Localization of cyclic nucleotide phosphodiesterase 2 in the brain-derived Triton-insoluble low-density fraction (raft)

The cyclic nucleotides perform a variety of roles in the formation and remodeling of the neuronal interaction. The membrane microdomain called "raft" has been paid much attention, for this domain contains many signal-transducing molecules including trimeric G proteins and cytoskeletal proteins. The raft domain is recovered in a low-density fraction after the treatment of the membrane with a non-ionic detergent such as Triton X-100. The enrichment of cholesterol and sphingolipids is ascribed to be responsible for the detergent insolubility. In this study we focused on the cyclic nucleotide signaling process in rafts prepared from the cerebral cortex of 10-day-old rat and the synaptic plasma membrane fraction and found the presence of a high cAMP and cGMP phosphodiesterase (PDE) activity. The activity was effectively inhibited with erythro-9-(2-hydroxy-3-nonyl)adenine, a PDE2-specific inhibitor but not with other inhibitors such as vinpocetine, quazione, or zaprinast. Further western blotting analysis confirmed the localization of PDE2 in the raft fraction. The presence of adenylyl cyclase V/VI and PKA in the raft fraction was also shown with Western blotting. These results suggest the participation of the raft in the cyclic nucleotide signaling cascade in neurons.

Intercellular transfer of the cellular prion protein

The cellular prion protein (PrP(C)) is a glycosylphosphatidylinositol (GPI)-anchored protein. We investigated whether PrP(C) can move from one cell to another cell in a cell model. Little PrP(C) transfer was detected when a PrP(C) expressing human neuroblastoma cell line was cultured with the human erythroleukemia cells IA lacking PrP(C). Efficient transfer of PrP(C) was detected with the presence of phorbol 12-myristate 13-acetate, an activator of protein kinase C. Maximum PrP(C) transfer was observed when both donor and recipient cells were activated. Furthermore, PrP(C) transfer required the GPI anchor and direct cell to cell contact. However, intercellular protein transfer is not limited to PrP(C), another GPI-anchored protein, CD90, also transfers from the donor cells to acceptor cells after cellular activation. Therefore, this transfer process is GPI-anchor and cellular activation dependent. These findings suggest that the intercellular transfer of GPI-anchored proteins is a regulated process, and may have implications for the pathogenesis of prion disease.

Lipid binding activity of a neuron-specific protein NAP-22 studied in vivo and in vitro

There exists a microdomain called "raft" in the cell membrane. The enrichment of cholesterol and sphingolipids in its outer leaflet is well recognized. In contrast, little is known of the lipid composition of the inner leaflet of raft, where many acylated signal-transducing molecules, such as trimeric G proteins and protein tyrosine kinases, associate. NAP-22 is a neuronal protein localized on the inner leaflet of raft domain. This protein was found to bind cholesterol in the liposome. In this study, we further analyze the lipid binding activity of NAP-22 using eukaryotic and bacterial expression systems. In addition to cholesterol, NAP-22 showed a phosphatidylethanolamine (PE)- and polyphosphoinositide-dependent membrane binding in the liposome assay. The N-terminal myristoylation was essential for the liposome binding. The C-terminal deletion up to D61 showed little effect on the binding. The lipid binding region was hence judged to be in the N-terminal 60-amino-acid sequence. NAP-22 was then expressed in COS7 cells, and the intracellular localization was studied. Biochemical analysis showed the localization of NAP-22 in a Triton-insoluble low-density fraction. Cell staining analysis showed colocalization patterns of NAP-22 with PE and cholesterol in the membrane.

Reversible translocation and activity-dependent localization of the calcium-myristoyl switch protein VILIP-1 to different membrane compartments in living hippocampal neurons

Visinin-like protein-1 (VILIP-1) belongs to the family of neuronal calcium sensor (NCS) proteins, a neuronal subfamily of EF-hand [corrected] calcium-binding proteins that are myristoylated at their N termini. NCS proteins are discussed to play roles in calcium-dependent signal transduction of physiological and pathological processes in the CNS. The calcium-dependent membrane association, the so-called calcium-myristoyl switch, localizes NCS proteins to a distinct cellular signaling compartment and thus may be a critical mechanism for the coordinated regulation of signaling cascades. To study whether the biochemically defined calcium-myristoyl switch of NCS proteins can occur in living neuronal cells, the reversible and stimulus-dependent translocation of green fluorescent protein (GFP)-tagged VILIP-1 to subcellular targets was examined by fluorescence microscopy in transfected cell lines and hippocampal primary neurons. In transiently transfected NG108-15 and COS-7 cells, a translocation of diffusely distributed VILIP-1-GFP but not of myristoylation-deficient VILIP-1-GFP to the plasma membrane and to intracellular targets, such as Golgi membranes, occurred after raising the intracellular calcium concentration with a calcium ionophore. The observed calcium-dependent localization was completely reversed after depletion of intracellular calcium by EGTA. Interestingly, a fast and reversible translocation of VILIP-1-GFP and translocation of endogenous VILIP-1 to specialized membrane structures was also observed after a depolarizing stimulus or activation of glutamate receptors in hippocampal neurons. These results show for the first time the reversibility and stimulus-dependent occurrence of the calcium-myristoyl switch in living neurons, suggesting a physiological role as a signaling mechanism of NCS proteins, enabling them to activate specific targets localized in distinct membrane compartments.

Cholesterol modulates the membrane binding and intracellular distribution of annexin 6

Annexins are Ca(2+)- and phospholipid-binding proteins that are widely expressed in mammalian tissues and that bind to different cellular membranes. In recent years its role in membrane traffic has emerged as one of its predominant functions, but the regulation of its intracellular distribution still remains unclear. We demonstrated that annexin 6 translocates to the late endocytic compartment in low density lipoprotein-loaded CHO cells. This prompted us to investigate whether cholesterol, one of the major constituents of low density lipoprotein, could influence the membrane binding affinity and intracellular distribution of annexin 6. Treatment of crude membranes or early and late endosomal fractions with digitonin, a cholesterol-sequestering agent, displayed a strong reduction in the binding affinity of a novel EDTA-resistant and cholesterol-sensitive pool of annexin 6 proteins. In addition, U18666A-induced accumulation of cholesterol in the late endosomal compartment resulted in a significant increase of annexin 6 in these vesicles in vivo. This translocation/recruitment correlates with an increased membrane binding affinity of GST-annexin 6 to late endosomes of U18666A-treated cells in vitro. In conclusion, the present study shows that changes in the intracellular distribution and concentration of cholesterol in different subcellular compartments participate in the reorganization of intracellular pools of Ca(2+)-dependent and -independent annexin 6.

Annexins: from structure to function

Annexins are Ca2+ and phospholipid binding proteins forming an evolutionary conserved multigene family with members of the family being expressed throughout animal and plant kingdoms. Structurally, annexins are characterized by a highly alpha-helical and tightly packed protein core domain considered to represent a Ca2+-regulated membrane binding module. Many of the annexin cores have been crystallized, and their molecular structures reveal interesting features that include the architecture of the annexin-type Ca2+ binding sites and a central hydrophilic pore proposed to function as a Ca2+ channel. In addition to the conserved core, all annexins contain a second principal domain. This domain, which NH2-terminally precedes the core, is unique for a given member of the family and most likely specifies individual annexin properties in vivo. Cellular and animal knock-out models as well as dominant-negative mutants have recently been established for a number of annexins, and the effects of such manipulations are strikingly different for different members of the family. At least for some annexins, it appears that they participate in the regulation of membrane organization and membrane traffic and the regulation of ion (Ca2+) currents across membranes or Ca2+ concentrations within cells. Although annexins lack signal sequences for secretion, some members of the family have also been identified extracellularly where they can act as receptors for serum proteases on the endothelium as well as inhibitors of neutrophil migration and blood coagulation. Finally, deregulations in annexin expression and activity have been correlated with human diseases, e.g., in acute promyelocytic leukemia and the antiphospholipid antibody syndrome, and the term annexinopathies has been coined.