Restoring cellular copper homeostasis in Alzheimer disease: a novel peptide shuttle is internalized by an ATP-dependent endocytosis pathway involving Rab5- and Rab14-endosomes

CPPs, or Cell-Penetrating Peptides, offer invaluable utility in disease treatment due to their ability to transport various therapeutic molecules across cellular membranes. Their unique characteristics, such as biocompatibility and low immunogenicity, make them ideal candidates for delivering drugs, genes, or imaging agents directly into cells. This targeted delivery enhances treatment efficacy while minimizing systemic side effects. CPPs exhibit versatility, crossing biological barriers and reaching intracellular targets that conventional drugs struggle to access. This capability holds promise in treating a wide array of diseases, including cancer, neurodegenerative disorders, and infectious diseases, offering a potent avenue for innovative and targeted therapies, yet their precise mechanism of cell entry is far from being fully understood. In order to correct Cu dysregulation found in various pathologies such as Alzheimer disease, we have recently conceived a peptide Cu(II) shuttle, based on the αR5W4 CPP, which, when bound to Cu(II), is able to readily enter a neurosecretory cell model, and release bioavailable Cu in cells. Furthermore, this shuttle has the capacity to protect cells in culture against oxidative stress-induced damage which occurs when Cu binds to the Aβ peptide. The aim of this study was therefore to characterize the cell entry route used by this shuttle and determine in which compartment Cu is released. Pharmacological treatments, siRNA silencing and colocalization experiments with GFP-Rab fusion proteins, indicate that the shuttle is internalized by an ATP-dependent endocytosis pathway involving both Rab5 and Rab14 endosomes route and suggest an early release of Cu from the shuttle.

V-ATPase modulates exocytosis in neuroendocrine cells through the activation of the ARNO-Arf6-PLD pathway and the synthesis of phosphatidic acid

Although there is mounting evidence indicating that lipids serve crucial functions in cells and are implicated in a growing number of human diseases, their precise roles remain largely unknown. This is particularly true in the case of neurosecretion, where fusion with the plasma membrane of specific membrane organelles is essential. Yet, little attention has been given to the role of lipids. Recent groundbreaking research has emphasized the critical role of lipid localization at exocytotic sites and validated the essentiality of fusogenic lipids, such as phospholipase D (PLD)-generated phosphatidic acid (PA), during membrane fusion. Nevertheless, the regulatory mechanisms synchronizing the synthesis of these key lipids and neurosecretion remain poorly understood. The vacuolar ATPase (V-ATPase) has been involved both in vesicle neurotransmitter loading and in vesicle fusion. Thus, it represents an ideal candidate to regulate the fusogenic status of secretory vesicles according to their replenishment state. Indeed, the cytosolic V1 and vesicular membrane-associated V0 subdomains of V-ATPase were shown to dissociate during the stimulation of neurosecretory cells. This allows the subunits of the vesicular V0 to interact with different proteins of the secretory machinery. Here, we show that V0a1 interacts with the Arf nucleotide-binding site opener (ARNO) and promotes the activation of the Arf6 GTPase during the exocytosis in neuroendocrine cells. When the interaction between V0a1 and ARNO was disrupted, it resulted in the inhibition of PLD activation, synthesis of phosphatidic acid during exocytosis, and changes in the timing of fusion events. These findings indicate that the separation of V1 from V0 could function as a signal to initiate the ARNO-Arf6-PLD1 pathway and facilitate the production of phosphatidic acid, which is essential for effective exocytosis in neuroendocrine cells.

Dysfunction of calcium-regulated exocytosis at a single-cell level causes catecholamine hypersecretion in patients with pheochromocytoma

Neuroendocrine tumors constitute a heterogeneous group of tumors arising from hormone-secreting cells and are generally associated with a dysfunction of secretion. Pheochromocytoma (Pheo) is a neuroendocrine tumor that develops from chromaffin cells of the adrenal medulla, and is responsible for an excess of catecholamine secretion leading to severe clinical symptoms such as hypertension, elevated stroke risk and various cardiovascular complications. Surprisingly, while the hypersecretory activity of Pheo is well known to pathologists and clinicians, it has never been carefully explored at the cellular and molecular levels. In the present study, we have combined catecholamine secretion measurement by carbon fiber amperometry on human tumor cells directly cultured from freshly resected Pheos, with the analysis by mass spectrometry of the exocytotic proteins differentially expressed between the tumor and the matched adjacent non-tumor tissue. In most patients, catecholamine secretion recordings from single Pheo cells revealed a higher number of exocytic events per cell associated with faster kinetic parameters. Accordingly, we unravel significant tumor-associated modifications in the expression of key proteins involved in different steps of the calcium-regulated exocytic pathway. Altogether, our findings indicate that dysfunction of the calcium-regulated exocytosis at the level of individual Pheo cell is a cause of the tumor-associated hypersecretion of catecholamines.

Development of Cu(ii)-specific peptide shuttles capable of preventing Cu–amyloid beta toxicity and importing bioavailable Cu into cells

Copper (Cu) in its ionic forms is an essential element for mammals and its homeostasis is tightly controlled. Accordingly, Cu-dyshomeostasis can be lethal as is the case in the well-established genetic Wilson's and Menkes diseases. In Alzheimer's disease (AD), Cu-accumulation occurs in amyloid plaques, where it is bound to the amyloid-beta peptide (Aβ). In vitro, Cu–Aβ is competent to catalyze the production of reactive oxygen species (ROS) in the presence of ascorbate under aerobic conditions, and hence Cu–Aβ is believed to contribute to the oxidative stress in AD. Several molecules that can recover extracellular Cu from Aβ and transport it back into cells with beneficial effects in cell culture and transgenic AD models were identified. However, all the Cu-shuttles currently available are not satisfactory due to various potential limitations including ion selectivity and toxicity. Hence, we designed a novel peptide-based Cu shuttle with the following properties: (i) it contains a Cu(ii)-binding motif that is very selective to Cu(ii) over all other essential metal ions; (ii) it is tagged with a fluorophore sensitive to Cu(ii)-binding and release; (iii) it is made of a peptide platform, which is very versatile to add new functions. The work presented here reports on the characterization of AKH-αR5W4^NBD, which is able to transport Cu ions selectively into PC12 cells and the imported Cu appeared bioavailable, likely via reductive release induced by glutathione. Moreover, AKH-αR5W4^NBD was able to withdraw Cu from the Aβ_1–16 peptide and consequently inhibited the Cu-Aβ based reactive oxygen species production and related cell toxicity. Hence, AKH-αR5W4^NBD could be a valuable new tool for Cu-transport into cells and suitable for mechanistic studies in cell culture, with potential applications in restoring Cu-homeostasis in Cu-related diseases such as AD.

The synthetic peptide AKH-αR5W4^NBD was designed as a shuttle to counteract copper imbalance in Alzheimer’s disease. In vitro, this shuttle is able to abstract Cu(ii) selectively from amyloid-β and transport it into cells in a bioavailable form.

Gold labelling of a green fluorescent protein (GFP)-tag inside cells using recombinant nanobodies conjugated to 2.4 nm thiolate-coated gold nanoparticles

Advances in microscopy technology have prompted efforts to improve the reagents required to recognize specific molecules within the intracellular environment. For high-resolution electron microscopy, conjugation of selective binders originating from the immune response arsenal to gold nanoparticles (AuNPs) as contrasting agents is the method of choice to obtain labeling tools. However, conjugation of the minimal sized 15 kDa nanobody (Nb) to AuNPs remains challenging in comparison to the conjugation of 150 kDa IgG to AuNPs. Herein, effective Nb-AuNP assemblies are built using the selective and almost irreversible non-covalent associations between two peptide sequences deriving from a p53 heterotetramer domain variant. The 15 kDa GFP-binding Nb is fused to one dimerizing motif to obtain a recombinant Nb dimer with improved avidity for GFP while the other complementing dimerizing motif is equipped with thiols and grafted to a 2.4 nm substituted thiobenzoate-coordinated AuNP via thiolate exchange. After pegylation, the modified AuNPs are able to non-covalently anchor Nb dimers and the subsequent complexes demonstrate the ability to form immunogold label GFP-protein fusions within various subcellular locations. These tools open an avenue for precise localization of targets at high resolution by electron microscopy.

Phospholipase D1-generated phosphatidic acid modulates secretory granule trafficking from biogenesis to compensatory endocytosis in neuroendocrine cells

Calcium-regulated exocytosis is a multi-step process that allows specialized secretory cells to release informative molecules such as neurotransmitters, neuropeptides, and hormones for intercellular communication. The biogenesis of secretory vesicles from the Golgi cisternae is followed by their transport towards the cell periphery and their docking and fusion to the exocytic sites of the plasma membrane allowing release of vesicular content. Subsequent compensatory endocytosis of the protein and lipidic constituents of the vesicles maintains cell homeostasis. Despite the fact that lipids represent the majority of membrane constituents, little is known about their contribution to these processes. Using a combination of electrochemical measurement of single chromaffin cell catecholamine secretion and electron microscopy of roof-top membrane sheets associated with genetic, silencing and pharmacological approaches, we recently reported that diverse phosphatidic acid (PA) species regulates catecholamine release efficiency by controlling granule docking and fusion kinetics. The enzyme phospholipase D1 (PLD1), producing PA from phosphatidylcholine, seems to be the major responsible of these effects in this model. Here, we extended this work using spinning disk confocal microscopy showing that inhibition of PLD activity also reduced the velocity of granules undergoing a directed motion. Furthermore, a dopamine β-hydroxylase (DβH) internalization assay revealed that PA produced by PLD is required for an optimal recovery of vesicular membrane content by compensatory endocytosis. Thus, among numerous roles that have been attributed to PA our work gives core to the key regulatory role in secretion that has been proposed in different cell models. Few leads to explain these multiple functions of PA along the secretory pathway are discussed.

Leptin brain entry via a tanycytic LepR-EGFR shuttle controls lipid metabolism and pancreas function

Metabolic health depends on the brain's ability to control food intake and nutrient use versus storage, processes that require peripheral signals such as the adipocyte-derived hormone, leptin, to cross brain barriers and mobilize regulatory circuits. We have previously shown that hypothalamic tanycytes shuttle leptin into the brain to reach target neurons. Here, using multiple complementary models, we show that tanycytes express functional leptin receptor (LepR), respond to leptin by triggering Ca²⁺ waves and target protein phosphorylation, and that their transcytotic transport of leptin requires the activation of a LepR-EGFR complex by leptin and EGF sequentially. Selective deletion of LepR in tanycytes blocks leptin entry into the brain, inducing not only increased food intake and lipogenesis but also glucose intolerance through attenuated insulin secretion by pancreatic β-cells, possibly via altered sympathetic nervous tone. Tanycytic LepRb-EGFR-mediated transport of leptin could thus be crucial to the pathophysiology of diabetes in addition to obesity, with therapeutic implications.

Heteromerization of Endogenous Mu and Delta Opioid Receptors Induces Ligand-Selective Co-Targeting to Lysosomes

Increasing evidence indicates that native mu and delta opioid receptors can associate to form heteromers in discrete brain neuronal circuits. However, little is known about their signaling and trafficking. Using double-fluorescent knock-in mice, we investigated the impact of neuronal co-expression on the internalization profile of mu and delta opioid receptors in primary hippocampal cultures. We established ligand selective mu–delta co-internalization upon activation by 1-[[4-(acetylamino)phenyl]methyl]-4-(2-phenylethyl)-4-piperidinecarboxylic acid, ethyl ester (CYM51010), [d-Ala2, NMe-Phe4, Gly-ol5]enkephalin (DAMGO), and deltorphin II, but not (+)-4-[(αR)-α-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80), morphine, or methadone. Co-internalization was driven by the delta opioid receptor, required an active conformation of both receptors, and led to sorting to the lysosomal compartment. Altogether, our data indicate that mu–delta co-expression, likely through heteromerization, alters the intracellular fate of the mu opioid receptor, which provides a way to fine-tune mu opioid receptor signaling. It also represents an interesting emerging concept for the development of novel therapeutic drugs and strategies.

The atypical Rho GTPase RhoU interacts with intersectin-2 to regulate endosomal recycling pathways

Rho GTPases play a key role in various membrane trafficking processes. RhoU is an atypical small Rho GTPase related to Rac/Cdc42, which possesses unique N- and C-terminal domains that regulate its function and its subcellular localization. RhoU localizes at the plasma membrane, on endosomes and in cell adhesion structures where it governs cell signaling, differentiation and migration. However, despite its endomembrane localization, RhoU function in vesicular trafficking has been unexplored. Here, we identified intersectins (ITSNs) as new binding partners for RhoU and showed that the second PxxP motif at the N terminus of RhoU mediated interactions with the SH3 domains of ITSNs. To evaluate the function of RhoU and ITSNs in vesicular trafficking, we used fluorescent transferrin as a cargo for uptake experiments. We showed that silencing of either RhoU or ITSN2, but not ITSN1, increased transferrin accumulation in early endosomes, resulting from a defect in fast vesicle recycling. Concomitantly, RhoU and ITSN2 colocalized to a subset of Rab4-positive vesicles, suggesting that a RhoU-ITSN2 interaction may occur on fast recycling endosomes to regulate the fate of vesicular cargos.

αII-spectrin controls calcium-regulated exocytosis in neuroendocrine chromaffin cells through neuronal Wiskott-Aldrich Syndrome protein interaction

Besides a fundamental structural role at the plasma membrane, spectrin- and actin-based skeletons have been proposed to participate in various processes including vesicular trafficking. Neuroendocrine cells release hormones and neuropeptides through calcium-regulated exocytosis, a process that is coordinated by a fine remodeling of the actin cytoskeleton. We describe here that calcium-regulated exocytosis is impaired in chromaffin and PC12 cells with reduced αII-spectrin expression levels. Using yeast two-hybrid screening, we show that neuronal Wiskott-Aldrich Syndrome protein (N-WASP) is a partner of the αII-spectrin SH3 domain and demonstrate that secretagogue-evoked N-WASP recruitment at cell periphery is blocked in the absence of αII-spectrin. Additionally, experiments performed with ectopically expressed αII-spectrin mutant unable to bind N-WASP indicated that the interaction between SH3 domain and N-WASP is pivotal for neuroendocrine secretion. Our results extend the list of spectrin interactors and strengthen the idea that αII-spectrin is an important scaffold protein that gathers crucial actin-related players of the exocytic machinery.

Impact du nombre d’interventions infirmières sur la durée d’administration des catécholamines chez les patients de réanimation : étude INTERVAL

Objectifs : Bien que quotidiennement géré par les infirmier(ière)s diplômé(e)s d’État (IDE) de réanimation, le sevrage des catécholamines a rarement été étudié. L’hypothèse de notre étude était que l’augmentation du nombre d’interventions menées sur la vitesse de perfusion des catécholamines permettait de raccourcir la durée d’administration de celles-ci.

Patients et méthodes : Il s’agissait d’une étude prospective observationnelle dans 21 réanimations. Tous les patients traités par vasopresseurs pendant la période d’étude ont été inclus. Le nombre d’interventions effectuées par l’IDE en charge du patient sur la vitesse d’administration des catécholamines était noté toutes les quatre heures. La posologie de catécholamines en cours ainsi que la pression artérielle moyenne (PAM) du patient étaient également relevées. Les facteurs influençant la rapidité du sevrage des catécholamines étaient également analysés.

Résultats : Nous avons inclus 226 patients dont l’âge moyen était de 65 ± 14 ans. La durée moyenne du traitement par catécholamines était de 71,6 (±81) heures. Le nombre d’interventions IDE par quatre heures était de 1,9 (±2,8) intervention. La durée du traitement par catécholamines était significativement moindre lorsque le nombre d’interventions augmentait (p = 0,04). L’existence d’un protocole spécifique à la gestion des catécholamines raccourcissait leur durée de sevrage. Un IGS2 élevé, une PAM élevée ainsi que la période diurne étaient associés à une augmentation du nombre des interventions IDE.

Conclusion : Le nombre d’interventions par les IDEs menées sur la perfusion de catécholamines était inversement lié à la durée du traitement par catécholamines. Des études complémentaires permettront d’évaluer l’impact de ces interventions sur le pronostic des patients.

RCAN1 Knockdown Reverts Defects in the Number of Calcium-Induced Exocytotic Events in a Cellular Model of Down Syndrome

In humans, Down Syndrome (DS) is a condition caused by partial or full trisomy of chromosome 21. Genes present in the DS critical region can result in excess gene dosage, which at least partially can account for DS phenotype. Although regulator of calcineurin 1 (RCAN1) belongs to this region and its ectopic overexpression in neurons impairs transmitter release, synaptic plasticity, learning and memory, the relative contribution of RCAN1 in a context of DS has yet to be clarified. In the present work, we utilized an in vitro model of DS, the CTb neuronal cell line derived from the brain cortex of a trisomy 16 (Ts16) fetal mouse, which reportedly exhibits acetylcholine release impairments compared to CNh cells (a neuronal cell line established from a normal littermate). We analyzed single exocytotic events by using total internal reflection fluorescence microscopy (TIRFM) and the vesicular acetylcholine transporter fused to the pH-sensitive green fluorescent protein (VAChT-pHluorin) as a reporter. Our analyses showed that, compared with control CNh cells, the trisomic CTb cells overexpress RCAN1, and they display a reduced number of Ca²⁺-induced exocytotic events. Remarkably, RCAN1 knockdown increases the extent of exocytosis at levels comparable to those of CNh cells. These results support a critical contribution of RCAN1 to the exocytosis process in the trisomic condition.

18F-FDOPA PET/CT Uptake Parameters Correlate with Catecholamine Secretion in Human Pheochromocytomas

BACKGROUND

18F-FDOPA positron emission tomography/computed tomography (PET/CT) is a sensitive nuclear imaging technology for the diagnosis of pheochromocytomas (PHEO). However, its utility in determining predictive factors for the secretion of catecholamines remains poorly studied.

METHODS

Thirty-nine histologically confirmed PHEO were included in this retrospective single-center study. Patients underwent 18F-FDOPA PET/CT before surgery, with an evaluation of several uptake parameters (standardized uptake values [SUVmax and SUVmean] and the metabolic burden [MB] calculated as follows: MB = SUVmean × tumor volume) and measurement of plasma and/or urinary metanephrine (MN), normetanephrine (NM), and chromogranin A. Thirty-five patients were screened for germline mutations in the RET, SDHx, and VHL genes. Once resected, primary cultures of 5 PHEO were used for real-time measurement of catecholamine release by carbon fiber amperometry.

RESULTS

The MB of the PHEO positively correlated with 24-h urinary excretion of NM (r = 0.64, p < 0.0001), MN (r = 0.49, p = 0.002), combined MN and NM (r = 0.75, p < 0.0001), and eventually plasma free levels of NM (r = 0.55, p = 0.006). In the mutated patients (3 SDHD, 2 SDHB, 3 NF1, 1 VHL, and 3 RET), a similar correlation was observed between MB and 24-h urinary combined MN and NM (r = 0.86, p = 0.0012). For the first time, we demonstrate a positive correlation between the PHEO-to-liver SUVmax ratio and the mean number of secretory granule fusion events of the corresponding PHEO cells revealed by amperometric spikes (p = 0.01).

CONCLUSION

While the 18F-FDOPA PET/CT MB of PHEO strongly correlates with the concentration of MN, amperometric recordings suggest that 18F-FDOPA uptake could be enhanced by overactivity of catecholamine exocytosis.

Distribution of Water in Synthetic Calcium Silicate Hydrates

Understanding calcium silicate hydrates (CSHs) is of paramount importance for understanding the behavior of cement materials because they control most of the properties of these man-made materials. The atomic scale water content and structure have a major influence on their properties, as is analogous with clay minerals, and we should assess these. Here, we used a multiple analytical approach to quantify water distribution in CSH samples and to determine the relative proportions of water sorbed on external and internal (interlayer) surfaces. Water vapor isotherms were used to explain the water distribution in the CSH microstructure. As with many layered compounds, CSHs have external and internal (interlayer) surfaces displaying multilayer adsorption of water molecules on external surfaces owing to the hydrophilic surfaces. Interlayer water was also quantified from water vapor isotherm, X-ray diffraction (XRD), and thermal gravimetric analyses (TGA) data, displaying nonreversible swelling/shrinkage behavior in response to drying/rewetting cycles. From this quantification and balance of water distribution, we were able to explain most of the widely dispersed data already published according to the various relative humidity (RH) conditions and measurement techniques. Stoichiometric formulas were proposed for the different CSH samples analyzed (0.6 < Ca/Si < 1.6), considering the interlayer water contribution.

Inhibition of Cdc42 and Rac1 activities in pheochromocytoma, the adrenal medulla tumor

Altered Rho GTPase signaling has been linked to many types of cancer. As many small G proteins, Rho GTPases cycle between an active and inactive state thanks to specific regulators that catalyze exchange of GDP into GTP (Rho-GEF) or hydrolysis of GTP into GDP (Rho-GAP). Recent studies have shown that alteration takes place either at the level of Rho proteins themselves (expression levels, point mutations) or at the level of their regulators, mostly RhoGEFs and RhoGAPs. Most reports describe Rho GTPases gain of function that may participate to the tumorigenesis processes. In contrast, we have recently reported that decreased activities of Cdc42 and Rac1 as well as decreased expression of 2 Rho-GEFs, FARP1 and ARHGEF1, correlate with pheochromocytomas, a tumor developing in the medulla of the adrenal gland (Croisé et al., Endocrine Related Cancer, 2016). Here we highlight the major evidence and further study the correlation between Rho GTPases activities and expression levels of ARHGEF1 and FARP1. Finally we also discuss how the decrease of Cdc42 and Rac1 activities may help human pheochromocytomas to develop and comment the possible relationship between FARP1, ARHGEF1 and the 2 Rho GTPases Cdc42 and Rac1 in tumorigenesis.

Cdc42 and Rac1 activity is reduced in human pheochromocytoma and correlates with FARP1 and ARHGEF1 expression

Among small GTPases from the Rho family, Cdc42, RAC, and Rho are well known to mediate a large variety of cellular processes linked with cancer biology through their ability to cycle between an inactive (GDP-bound) and an active (GTP-bound) state. Guanine nucleotide exchange factors (GEFs) stimulate the exchange of GDP for GTP to generate the activated form, whereas the GTPase-activating proteins (GAPs) catalyze GTP hydrolysis, leading to the inactivated form. Modulation of Rho GTPase activity following altered expression of RHO-GEFs and/or RHO-GAPs has already been reported in various human tumors. However, nothing is known about the Rho GTPase activity or the expression of their regulators in human pheochromocytomas, a neuroendocrine tumor (NET) arising from chromaffin cells of the adrenal medulla. In this study, we demonstrate, through an ELISA-based activity assay, that Rac1 and Cdc42 activities decrease in human pheochromocytomas (PCCs) compared with the matched adjacent non-tumor tissue. Furthermore, through quantitative mass spectrometry (MS) approaches, we show that the expression of two RHO-GEF proteins, namely ARHGEF1 and FARP1, is significantly reduced in tumors compared with matched non-tumor tissue, whereas ARHGAP36 expression is increased. Moreover, siRNA-based knockdown of ARHGEF1 and FARP1 in PC12 cells leads to a significant inhibition of Rac1 and Cdc42 activities, respectively. Finally, a principal component analysis (PCA) of our dataset was able to discriminate PCC from non-tumor tissue and indicates a close correlation between Cdc42/Rac1 activity and FARP1/ARHGEF1 expression. Altogether, our findings reveal for the first time the importance of modulation of Rho GTPase activities and expression of their regulators in human PCCs.

Rho GTPases, phosphoinositides, and actin: a tripartite framework for efficient vesicular trafficking

Rho GTPases are well known regulators of the actin cytoskeleton that act by binding and activating actin nucleators. They are therefore involved in many actin-based processes, including cell migration, cell polarity, and membrane trafficking. With the identification of phosphoinositide kinases and phosphatases as potential binding partners or effectors, Rho GTPases also appear to participate in the regulation of phosphoinositide metabolism. Since both actin dynamics and phosphoinositide turnover affect the efficiency and the fidelity of vesicle transport between cell compartments, Rho GTPases have emerged as critical players in membrane trafficking. Rho GTPase activity, actin remodeling, and phosphoinositide metabolism need to be coordinated in both space and time to ensure the progression of vesicles along membrane trafficking pathways. Although most molecular pathways are still unclear, in this review, we will highlight recent advances made in our understanding of how Rho-dependent signaling pathways organize actin dynamics and phosphoinositides and how phosphoinositides potentially provide negative feedback to Rho GTPases during endocytosis, exocytosis and membrane exchange between intracellular compartments.

Exocytosis and endocytosis in neuroendocrine cells: inseparable membranes!

Although much has been learned concerning the mechanisms of secretory vesicle formation and fusion at donor and acceptor membrane compartments, relatively little attention has been paid toward understanding how cells maintain a homeostatic membrane balance through vesicular trafficking. In neurons and neuroendocrine cells, release of neurotransmitters, neuropeptides, and hormones occurs through calcium-regulated exocytosis at the plasma membrane. To allow recycling of secretory vesicle components and to preserve organelles integrity, cells must initiate and regulate compensatory membrane uptake. This review relates the fate of secretory granule membranes after full fusion exocytosis in neuroendocrine cells. In particular, we focus on the potential role of lipids in preserving and sorting secretory granule membranes after exocytosis and we discuss the potential mechanisms of membrane retrieval.

Dynamin-2 regulates fusion pore expansion and quantal release through a mechanism that involves actin dynamics in neuroendocrine chromaffin cells

Over the past years, dynamin has been implicated in tuning the amount and nature of transmitter released during exocytosis. However, the mechanism involved remains poorly understood. Here, using bovine adrenal chromaffin cells, we investigated whether this mechanism rely on dynamin’s ability to remodel actin cytoskeleton. According to this idea, inhibition of dynamin GTPase activity suppressed the calcium-dependent de novo cortical actin and altered the cortical actin network. Similarly, expression of a small interfering RNA directed against dynamin-2, an isoform highly expressed in chromaffin cells, changed the cortical actin network pattern. Disruption of dynamin-2 function, as well as the pharmacological inhibition of actin polymerization with cytochalasine-D, slowed down fusion pore expansion and increased the quantal size of individual exocytotic events. The effects of cytochalasine-D and dynamin-2 disruption were not additive indicating that dynamin-2 and F-actin regulate the late steps of exocytosis by a common mechanism. Together our data support a model in which dynamin-2 directs actin polymerization at the exocytosis site where both, in concert, adjust the hormone quantal release to efficiently respond to physiological demands.

Intersectin: The Crossroad between Vesicle Exocytosis and Endocytosis

Intersectins (ITSNs) are a family of highly conserved proteins with orthologs from nematodes to mammals. In vertebrates, ITSNs are encoded by two genes (itsn1 and itsn2), which act as scaffolds that were initially discovered as proteins involved in endocytosis. Further investigation demonstrated that ITSN1 is also implicated in several other processes including regulated exocytosis, thereby suggesting a role for ITSN1 in the coupling between exocytosis and endocytosis in excitatory cells. Despite a high degree of conservation amongst orthologs, ITSN function is not so well preserved as they have acquired new properties during evolution. In this review, we will discuss the role of ITSN1 and its orthologs in exo- and endocytosis, in particular in neurons and neuroendocrine cells.

Chromogranin A induces the biogenesis of granules with calcium- and actin-dependent dynamics and exocytosis in constitutively secreting cells

Chromogranins are a family of acidic glycoproteins that play an active role in hormone and neuropeptide secretion through their crucial role in secretory granule biogenesis in neuroendocrine cells. However, the molecular mechanisms underlying their granulogenic activity are still not fully understood. Because we previously demonstrated that the expression of the major component of secretory granules, chromogranin A (CgA), is able to induce the formation of secretory granules in nonendocrine COS-7 cells, we decided to use this model to dissect the mechanisms triggered by CgA leading to the biogenesis and trafficking of such granules. Using quantitative live cell imaging, we first show that CgA-induced organelles exhibit a Ca(2+)-dependent trafficking, in contrast to native vesicle stomatitis virus G protein-containing constitutive vesicles. To identify the proteins that confer such properties to the newly formed granules, we developed CgA-stably-expressing COS-7 cells, purified their CgA-containing granules by subcellular fractionation, and analyzed the granule proteome by liquid chromatography-tandem mass spectrometry. This analysis revealed the association of several cytosolic proteins to the granule membrane, including GTPases, cytoskeleton-based molecular motors, and other proteins with actin- and/or Ca(2+)-binding properties. Furthermore, disruption of cytoskeleton affects not only the distribution and the transport but also the Ca(2+)-evoked exocytosis of the CgA-containing granules, indicating that these granules interact with microtubules and cortical actin for the regulated release of their content. These data demonstrate for the first time that the neuroendocrine factor CgA induces the recruitment of cytoskeleton-, GTP-, and Ca(2+)-binding proteins in constitutively secreting COS-7 cells to generate vesicles endowed with typical dynamics and exocytotic properties of neuroendocrine secretory granules.

Identification of a bipartite focal adhesion localization signal in RhoU/Wrch-1, a Rho family GTPase that regulates cell adhesion and migration

BACKGROUND INFORMATION

Rho GTPases are important regulators of cytoskeleton dynamics and cell adhesion. RhoU/Wrch-1 is a Rho GTPase which shares sequence similarities with Rac1 and Cdc42 (cell division cycle 42), but has also extended N- and C-terminal domains. The N-terminal extension promotes binding to SH3 (Src homology 3)-domain-containing adaptors, whereas the C-terminal extension mediates membrane targeting through palmitoylation of its non-conventional CAAX box. RhoU/Wrch-1 possesses transforming activity, which is negatively regulated by its N-terminal extension and depends on palmitoylation.

RESULTS

In the present study, we have shown that RhoU is localized to podosomes in osteoclasts and c-Src-expressing cells, and to focal adhesions of HeLa cells and fibroblasts. The N-terminal extension and the palmitoylation site were dispensable, whereas the C-terminal extension and effector binding loop were critical for RhoU targeting to focal adhesions. Moreover, the number of focal adhesions was reduced and their distribution changed upon expression of activated RhoU. Conversely, RhoU silencing increased the number of focal adhesions. As RhoU was only transiently associated with adhesion structures, this suggests that RhoU may modify adhesion turnover and cell migration rate. Indeed, we found that migration distances were increased in cells expressing activated RhoU and decreased when RhoU was knocked-down.

CONCLUSIONS

Our data indicate that RhoU localizes to adhesion structures, regulates their number and distribution and increases cell motility. It also suggests that the RhoU effector binding and C-terminal domains are critical for these functions.

How important are Rho GTPases in neurosecretion?

Rho GTPases are small GTP binding proteins belonging to the Ras superfamily which act as molecular switches that regulate many cellular function including cell morphology, cell to cell interaction, cell migration and adhesion. In neuronal cells, Rho GTPases have been proposed to regulate neuronal development and synaptic plasticity. However, the role of Rho GTPases in neurosecretion is poorly documented. In this review, we discuss data that highlight the importance of Rho GTPases and their regulators into the control of neurotransmitter and hormone release in neurons and neuroendocrine cells, respectively.

[Use of mitoxantrone in early multiple sclerosis with malignant disease course. Observational study in 30 patients with clinical and MRI outcomes after one year]

INTRODUCTION

In an observational multicenter study, we analyzed retrospectively 30 patients with malignant form of multiple sclerosis (MS) treated with mitoxantrone the year following the first neurological event.

METHODS

The 30 patients were selected according to Weinshenker criteria of malignant MS (either a "catastrophic" relapse or a quickly aggressive form). We compared clinical and MRI findings the year before with the year following mitoxantrone onset treatment: annualized relapse rates (ARR), EDSS score and percentage of patients with gadolinium enhancing lesions on MRI.

RESULTS

A total of 87 relapses were observed in the 5.7 months before and 10 during the year following onset of mitoxantrone treatment. The ARR decreased by 95% (6.0+/-2 before and 0.3+/-0.7 after). Twenty-four patients (80%) were relapse-free one year after onset of mitoxantrone treatment. The EDSS score improved in 87% of MS patients and the mean EDSS decreased by 1.9. Ninety-seven percent had at least gadolinium enhancing lesions before the start of mitoxantrone treatment as compared to 17% after.

CONCLUSION

In our experience, mitoxantrone had a rapid and strong impact on the malignant forms of MS with a short disease duration. In this MS subgroup, mitoxantrone should be considered as an early treatment option.

Rho GTPases in osteoclasts: orchestrators of podosome arrangement

Cells from the myeloid lineage, namely macrophages, dendritic cells and osteoclasts, develop podosomes instead of stress fibers and focal adhesions to adhere and migrate. Podosomes share many components with focal adhesions but differ in their molecular organization, with a dense core of polymerized actin surrounded by scaffolding proteins, kinases and integrins. Podosomes are found either isolated both in macrophages and dendritic cells or arranged into superstructures in osteoclasts. When osteoclasts resorb bone, they form an F-actin rich sealing zone, which is a dense array of connected podosomes that firmly anchors osteoclasts to bone. It delineates a compartment in which protons and proteases are secreted to dissolve and degrade the mineralized matrix. Since Rho GTPases have been shown to control F-actin stress fibers and focal adhesions in mesenchymal cells, the question of whether they could also control podosome formation and arrangement in cells from the myeloid lineage, and particularly in osteoclasts, rapidly emerged. This article considers recent advances made in our understanding of podosome arrangements in osteoclasts and how Rho GTPases may control it.

Expression profile of RhoGTPases and RhoGEFs during RANKL-stimulated osteoclastogenesis: identification of essential genes in osteoclasts

RhoGTPases regulate actin cytoskeleton dynamics, a key element in osteoclast biology. We identified three novel genes induced during RANKL-stimulated osteoclastogenesis among RhoGTPases and their exchange factors that are essential in osteoclast biology.

INTRODUCTION

During the process of differentiation, adhesion to the bone matrix or osteolysis, the actin cytoskeleton of osteoclasts undergoes profound reorganization. RhoGTPases are key regulators of actin dynamics. They control cell adhesion, migration, and morphology through their action on actin cytoskeleton. In mice, there are 18 low molecular weight RhoGTPases. They are activated by guanine nucleotide exchange factors: the RhoGEFs. There are 76 RhoGEFs in mice: 65 belong to the Dbl family and 11 to the CZH family. To identify novel genes among RhoGTPases and RhoGEFs important in osteoclasts, we established the expression profiles of the complete families of RhoGTPases and RhoGEFs during RANKL-stimulated osteoclastogenesis.

MATERIALS AND METHODS

The RAW264.7 cell line, mouse bone marrow macrophages, and hematopoietic stem cells were used as precursors for RANKL-induced osteoclastogenesis. Gene arrays and real-time quantitative PCR analyses were performed to establish the transcription profiles of RhoGTPase and RhoGEF genes during differentiation. Small hairpin RNA was used to knock down genes of interest.

RESULTS

Of the 18 RhoGTPases and 76 RhoGEFs, the expression of three genes was upregulated by RANKL: the RhoGTPase RhoU/Wrch1, the Dbl family exchange factor Arhgef8/Net1, and the CZH family exchange factor Dock5. The inductions were observed in gene array and real-time quantitative PCR experiments performed in RAW264.7 cells. They were further confirmed in bone marrow macrophages and hematopoietic stem cells. Silencing of Wrch1 and Arhgef8 expression severely inhibited differentiation and affected osteoclast morphology. Dock5 suppression was lethal in osteoclast precursors while having no effect in fibroblasts.

CONCLUSIONS

We identified three genes among RhoGTPase signaling pathways that are upregulated during RANKL-induced osteoclastogenesis. These genes are novel essential actors in osteoclasts, most likely through the control of actin cytoskeleton dynamics.

A novel Rho-mDia2-HDAC6 pathway controls podosome patterning through microtubule acetylation in osteoclasts

Osteoclast maturation is accompanied by changes in podosome patterning, resulting in the formation of a peripheral belt, which requires an intact microtubule network. Here, we report that by inhibiting Rho, the podosome belt is maintained at the cell periphery despite depolymerisation of microtubules by nocodazole. Rho inhibition was correlated to the increase in microtubule stabilisation and microtubule acetylation. By microinjecting activated Rho or its activated effector mDia2 in osteoclasts, we found that the podosome belt was disrupted and the level of microtubule acetylation dramatically decreased. We further characterised the molecular mechanism responsible for microtubule deacetylation by co-immunoprecipitation experiments. We found that not only was mDia2 coprecipitating with the recently identified microtubule deacetylase HDAC6 but that it also activated the microtubule deacetylase activity of HDAC6 in an in vitro deacetylase assay. Finally, we found that during osteoclastogenesis, there is a correlation between the increase in microtubule acetylation and the podosome belt stabilisation and that if Rho is inhibited in the early stages of osteoclast differentiation, it accelerates both microtubule acetylation and podosome belt stabilisation. Altogether, our data reveal a pathway in which Rho interferes with the osteoclast maturation process by controlling the level of microtubule acetylation and actin organisation through mDIA2 and HDAC6.

Signal Transduction: IMPlications for Ras-Dependent ERK Signaling

Ras interacts with numerous downstream effectors to transmit a diverse array of cellular signals. A new study shows that a protein known as Impedes Mitogenic signal Propagation, IMP, is an E3 ubiquitin ligase that binds Ras and modulates MAP kinase signaling by regulating the scaffolding activity of KSR.

Protein phosphatase 2A positively regulates Ras signaling by dephosphorylating KSR1 and Raf-1 on critical 14-3-3 binding sites

BACKGROUND

Kinase Suppressor of Ras (KSR) is a conserved component of the Ras pathway that acts as a molecular scaffold to facilitate signal transmission through the MAPK cascade. Although recruitment of KSR1 from the cytosol to the plasma membrane is required for its scaffolding function, the precise mechanism(s) regulating the translocation of KSR1 have not been fully elucidated.

RESULTS

Using mass spectrometry to analyze the KSR1-scaffolding complex, we identify the serine/threonine protein phosphatase PP2A as a KSR1-associated protein and show that PP2A is a critical regulator of KSR1 activity. We find that the enzymatic core subunits of PP2A (PR65A and catalytic C) constitutively associate with the N-terminal domain of KSR1, whereas binding of the regulatory PR55B subunit is induced by growth factor treatment. Specific inhibition of PP2A activity prevents the growth factor-induced dephosphorylation event involved in the membrane recruitment of KSR1 and blocks the activation of KSR1-associated MEK and ERK. Moreover, we find that PP2A activity is required for activation of the Raf-1 kinase and that both Raf and KSR1 must be dephosphorylated by PP2A on critical regulatory 14-3-3 binding sites for KSR1 to promote MAPK pathway activation.

CONCLUSIONS

These findings identify KSR1 as novel substrate of PP2A and demonstrate the inducible dephosphorylation of KSR1 in response to Ras pathway activation. Further, these results elucidate a common regulatory mechanism for KSR1 and Raf-1 whereby their localization and activity are modulated by the PP2A-mediated dephosphorylation of critical 14-3-3 binding sites.

p60(v-src) and serum control cell shape and apoptosis via distinct pathways in quail neuroretina cells

We made use of QNR cells transformed by a thermosensitive (tsNY68) strain of the Rous sarcoma virus (RSV) to compare the effect of p60(v-src) and serum in cultured nerve cells. In this system, both p60(v-src) heat inactivation and serum removal resulted in growth arrest in G1. In both cases, growth arrest was reversible since cell proliferation was rapidly re-induced following respectively p60v-src renaturation or serum re-addition. However, cells did not fully recover their ability to grow in soft agar, suggesting that, in contrast to the cell cycle machinery, the transforming capacities of these cells have been irreversibly altered. We found that p60(v-src) kinase activity prevented detachment from the substratum and cell death following serum removal. Thermal inactivation of p60(v-src) at restrictive temperature (41.5 degrees C), but not serum removal, resulted in dramatic morphological changes, which occurred 4 h after temperature shift up to 41.5 degrees C. Later on, typical features of apoptotic cells could be observed. Cell death was greatly reduced by the caspase-3 inhibitor ZVAD.FMK, but not by the caspase-1 inhibitor Ac-YVAD.CHO. Together, these results suggested that p60(v-src) and serum factors act on distinct pathways, at least in part. In an attempt to identify the signalling pathways involved in the cell response to p60(v-src) down regulation, we found that Erk and Rac were rapidly inactivated following temperature shift up to 41.5 degrees C. Thus, the combined effects of p60(v-src) and serum factors on the cytoskeleton dynamics and the apoptosis machinery are essential for full neoplastic transformation of neuroretina cells.

CD46/CD3 costimulation induces morphological changes of human T cells and activation of Vav, Rac, and extracellular signal-regulated kinase mitogen-activated protein kinase

Efficient T cell activation requires at least two signals, one mediated by the engagement of the TCR-CD3 complex and another one mediated by a costimulatory molecule. We recently showed that CD46, a complement regulatory receptor for C3b as well as a receptor for several pathogens, could act as a potent costimulatory molecule for human T cells, highly promoting T cell proliferation. Indeed, we show in this study that CD46/CD3 costimulation induces a synergistic activation of extracellular signal-related kinase mitogen-activated protein kinase. Furthermore, whereas T lymphocytes primarily circulate within the bloodstream, activation may induce their migration toward secondary lymphoid organs or other tissues to encounter APCs or target cells. In this study, we show that CD46/CD3 costimulation also induces drastic morphological changes of primary human T cells, as well as actin relocalization. Moreover, we show that the GTP/GDP exchange factor Vav is phosphorylated upon CD46 stimulation alone, and that CD46/CD3 costimulation induces a synergistic increase of Vav phosphorylation. These results prompted us to investigate whether CD46/CD3 costimulation induced the activation of GTPases from the Rho family. Indeed, we report that the small GTPase Rac is also activated upon CD46/CD3 costimulation, whereas no change of Rho and Cdc42 activity could be detected. Therefore, CD46 costimulation profoundly affects T cell behavior, and these results provide important data concerning the biology of primary human T cells.

C-TAK1 regulates Ras signaling by phosphorylating the MAPK scaffold, KSR1

Kinase suppressor of Ras (KSR) is a conserved component of the Ras pathway that interacts directly with MEK and MAPK. Here we show that KSR1 translocates from the cytoplasm to the cell surface in response to growth factor treatment and that this process is regulated by Cdc25C-associated kinase 1 (C-TAK1). C-TAK1 constitutively associates with mammalian KSR1 and phosphorylates serine 392 to confer 14-3-3 binding and cytoplasmic sequestration of KSR1 in unstimulated cells. In response to signal activation, the phosphorylation state of S392 is reduced, allowing the KSR1 complex to colocalize with activated Ras and Raf-1 at the plasma membrane, thereby facilitating the phosphorylation reactions required for the activation of MEK and MAPK.

Rho and Rac exert antagonistic functions on spreading of macrophage-derived multinucleated cells and are not required for actin fiber formation

Multinucleated giant cells (MNGC) derived from avian blood monocytes present, like osteoclasts, an unusual cytoskeletal organization characterized by (1) cortical rings of actin filaments, (2) unique adhesion structures called podosomes and (3) vinculin containing focal complexes which are not visibly connected to F-actin structures. The Rho family of small GTPases plays an essential role in the regulation and organization of cellular cytoskeletal structures including F-actin and vinculin associated structures. Using bacterial toxins such as modified exoenzyme C3 (C3B) and toxin B or overexpression of constitutively active Rac and Rho proteins fused to the green fluorescent protein (GFP), we show that Rac and Rho play antagonistic roles in regulating the morphology of osteoclast-like cells. Inhibition of Rho by C3B triggered MNGC spreading whereas activated Rho promoted cell retraction. However, inhibition or activation of Rho led to complete disorganization of fibrillar actin structures, including podosomes. Toxin B inhibition of Rho, Rac and Cdc42 induced a time dependent F-actin and vinculin reorganization. Initially, actin fibers with associated adhesion plaques formed and disappeared subsequently. Finally, only small focal complexes remained at the MNGC periphery before retracting. At the time when actin fibers formed, we observed that Rac was already inhibited by toxin B. By combining C3B treatment and overexpression of a dominant negative form of Rac (N17Rac), we show that the formation of these focal adhesion and actin fiber structures required neither Rho nor Rac activity. Moreover, our results show that podosomes are extremely unstable structures since any modifications of Rho or Rac activity resulted in their dissociation.

Co-culture with assisted hatching of human embryos using Buffalo rat liver cells

Commercially obtained Buffalo rat liver (BRL) cells were grown in monolayer culture. The effect of BRL cell co-culture with assisted hatching on embryo development, implantation and pregnancy was investigated in a population of 200 'first-time' in-vitro fertilization (IVF) patients, subdivided into three groups according to the methods of fertilization [IVF; intracytoplasmic sperm injection (ICSI); ICSI/IVF]. Assisted hatching was performed on all embryos chosen for transfer. Following co-culture, the overall embryo quality, implantation rate and pregnancy rates were not significantly different from the controls. However, when grouped according to fertilization method, co-culture was found to have an impact on pregnancy and implantation rates in the group undergoing conventional IVF. Using co-culture with assisted hatching, we were able to achieve a 58% (38/65) clinical pregnancy rate with a 49% (32/65) live birth rate and a 26% (60/235) implantation rate. No changes in the pregnancy and implantation rates were apparent in ICSI or ICSI/IVF subgroups. This is the first prospective, randomly controlled study which reports the use of BRL cell co-culture for human IVF for a large number of patients undergoing IVF for the first time.