Calcium influx is required for endocytotic membrane retrieval

The Structural Basis of Long-Term Potentiation in Hippocampal Synapses, Revealed by Electron Microscopy Imaging of Lanthanum-Induced Synaptic Vesicle Recycling

Hippocampal neurons in dissociated cell cultures were exposed to the trivalent cation lanthanum for short periods (15–30 min) and prepared for electron microscopy (EM), to evaluate the stimulatory effects of this cation on synaptic ultrastructure. Not only were characteristic ultrastructural changes of exaggerated synaptic vesicle turnover seen within the presynapses of these cultures—including synaptic vesicle depletion and proliferation of vesicle-recycling structures—but the overall architecture of a large proportion of the synapses in the cultures was dramatically altered, due to large postsynaptic “bulges” or herniations into the presynapses. Moreover, in most cases, these postsynaptic herniations or protrusions produced by lanthanum were seen by EM to distort or break or “perforate” the so-called postsynaptic densities (PSDs) that harbor receptors and recognition molecules essential for synaptic function. These dramatic EM observations lead us to postulate that such PSD breakages or “perforations” could very possibly create essential substrates or “tags” for synaptic growth, simply by creating fragmented free edges around the PSDs, into which new receptors and recognition molecules could be recruited more easily, and thus, they could represent the physical substrate for the important synaptic growth process known as “long-term potentiation” (LTP). All of this was created simply in hippocampal dissociated cell cultures, and simply by pushing synaptic vesicle recycling way beyond its normal limits with the trivalent cation lanthanum, but we argued in this report that such fundamental changes in synaptic architecture—given that they can occur at all—could also occur at the extremes of normal neuronal activity, which are presumed to lead to learning and memory.

Single and combined toxicity of amino-functionalized polystyrene nanoparticles with potassium dichromate and copper sulfate on brine shrimp Artemia franciscana larvae

The increasing use and disposal of plastics has become a persistent problem in the marine environment, calling for studies that refer to realistic scenarios to understand their effects on biota. Particularly, the understanding about the effects of co-exposure with nanoplastic particles and metals on aquatic organisms is still limited. The present work aimed to investigate the acute toxicity of amino-functionalized polystyrene nanoparticles (PS-NH₂; 50 nm) as proxy for nanoplastics on brine shrimp Artemia franciscana larvae under different culture conditions and at different stages of development, as well as the combined effect with two reference toxicants - potassium dichromate (K₂Cr₂O₇) and copper sulfate (CuSO₄). Nauplii (instar II or III larval stages) were exposed to different concentrations of PS-NH₂ (0.005 to 5 μg mL^-1) for up to 48 h, with or without agitation in order to mimic a more realistic environmental scenario. Larval mobility and PS-NH₂ accumulation were monitored under microscopy. PS-NH₂ alone showed toxicity only at the highest concentration tested (5 μg mL^-1) regardless the incubation method used (61.2 + 3.1% and 65.0 + 4.5% with and without agitation, respectively). Moreover, instar III stage was the most sensitive to PS-NH₂ exposure (38.2% immobility in 24 h of exposure; 5 μg mL^-1). Evidence of PS-NH₂ retention in the gastrointestinal tract in a concentration- and time-dependent manner was also obtained. Mixtures of PS-NH₂ (0.005 and 5 μg mL^-1) with different concentrations of K₂Cr₂O₇ increased the immobilization rate of the larvae after 48 h of exposure, when compared to the K₂Cr₂O₇ alone. Similar results were observed for CuSO₄ in the co-exposure conditions at different concentrations. However, exposing nauplii to a mixture of PS-NH₂ (0.005 μg mL^-1) and CuSO₄ decreased immobilization rate, in comparison to the group exposed to CuSO₄ alone. The present work highlights the potential risk posed by nanoplastics to zooplanktonic species through their interaction with other toxicants.

Opening doors with ultrasound and microbubbles: Beating biological barriers to promote drug delivery

Apart from its clinical use in imaging, ultrasound has been thoroughly investigated as a tool to enhance drug delivery in a wide variety of applications. Therapeutic ultrasound, as such or combined with cavitating nuclei or microbubbles, has been explored to cross or permeabilize different biological barriers. This ability to access otherwise impermeable tissues in the body makes the combination of ultrasound and therapeutics very appealing to enhance drug delivery in situ. This review gives an overview of the most important biological barriers that can be tackled using ultrasound and aims to provide insight on how ultrasound has shown to improve accessibility as well as the biggest hurdles. In addition, we discuss the clinical applicability of therapeutic ultrasound with respect to the main challenges that must be addressed to enable the further progression of therapeutic ultrasound towards an effective, safe and easy-to-use treatment tailored for drug delivery in patients.

Copine-7 binds to the cell surface receptor, nucleolin, and regulates ciliogenesis and Dspp expression during odontoblast differentiation

Tooth development is a progressive process regulated by interactions between epithelial and mesenchymal tissues. Our previous studies showed that copine-7 (Cpne7), a dental epithelium-derived protein, is a signalling molecule that is secreted by preameloblasts and regulates the differentiation of preodontoblasts into odontoblasts. However, the mechanisms involved in the translocation of Cpne7 from preameloblasts to preodontoblasts and the functions of Cpne7 during odontogenesis are poorly understood. Here, we showed that the internalization of Cpne7 was mediated primarily by caveolae. This process was initiated by Cpne7 binding to the cell surface protein, nucleolin. Treatment with recombinant Cpne7 protein (rCpne7) in human dental pulp cells (hDPCs) caused an increase in the number of ciliated cells. The expression level of cilium components, Ift88 and Kif3a, and Dspp were increased by rCpne7. Treatment with Ift88 siRNA in hDPCs and MDPC-23 cells significantly down-regulated the expression of Dspp, an odontoblastic differentiation marker gene. Furthermore, the treatment with nucleolin siRNA in MDPC-23 cells decreased the expression of Dmp1, Dspp, and cilium components. Our findings suggested that the binding of Cpne7 with its receptor, nucleolin, has an important function involving Cpne7 internalization into preodontoblasts and regulation of Dspp expression through ciliogenesis during odontoblast differentiation.

Effects of waterborne cadmium on energy metabolism in the tropical sea cucumber, Stichopus horrens, and a comparison of tissue-specific cadmium accumulation with the temperate sea cucumber Australostichopus mollis

The sea cucumber Stichopus horrens is an important component of near-shore ecosystems, and in the Kingdom of Tonga it also comprises an important commercial and subsistence fishery. To assess the sensitivity of this species to the toxic trace metal cadmium (Cd), adult S. horrens were exposed for 96h to an environmental (15µgL^-1) or effect (765µgL^-1) concentration of waterborne Cd. The respiratory tree and intestine accumulated higher concentrations of Cd than the muscle and body wall, but there were no effects of Cd on tissue ions (sodium, potassium, magnesium, calcium). For comparison, Cd accumulation was also examined in the Australasian sea cucumber Australostichopus mollis. This species displayed a similar pattern of tissue-specific accumulation to S. horrens, but exhibited lower tissue Cd burdens, likely a consequence of lower experimental temperature. Effects on gonad ion content were also seen in this species. At the highest Cd exposure concentration, S. horrens showed impaired ammonia excretion rates and an increased molar oxygen:nitrogen ratio (O:N), indicative of a decreased reliance on protein metabolism. Overall, this study suggests that S. horrens is relatively tolerant of Cd exposure, but raises concerns regarding the subsistence fishery practice of consuming the viscera of this species.

Rab3A, a possible marker of cortical granules, participates in cortical granule exocytosis in mouse eggs

Fusion of cortical granules with the oocyte plasma membrane is the most significant event to prevent polyspermy. This particular exocytosis, also known as cortical reaction, is regulated by calcium and its molecular mechanism is still not known. Rab3A, a member of the small GTP-binding protein superfamily, has been implicated in calcium-dependent exocytosis and is not yet clear whether Rab3A participates in cortical granules exocytosis. Here, we examine the involvement of Rab3A in the physiology of cortical granules, particularly, in their distribution during oocyte maturation and activation, and their participation in membrane fusion during cortical granule exocytosis. Immunofluorescence and Western blot analysis showed that Rab3A and cortical granules have a similar migration pattern during oocyte maturation, and that Rab3A is no longer detected after cortical granule exocytosis. These results suggested that Rab3A might be a marker of cortical granules. Overexpression of EGFP-Rab3A colocalized with cortical granules with a Pearson correlation coefficient of +0.967, indicating that Rab3A and cortical granules have almost a perfect colocalization in the egg cortical region. Using a functional assay, we demonstrated that microinjection of recombinant, prenylated and active GST-Rab3A triggered cortical granule exocytosis, indicating that Rab3A has an active role in this secretory pathway. To confirm this active role, we inhibited the function of endogenous Rab3A by microinjecting a polyclonal antibody raised against Rab3A prior to parthenogenetic activation. Our results showed that Rab3A antibody microinjection abolished cortical granule exocytosis in parthenogenetically activated oocytes. Altogether, our findings confirm that Rab3A might function as a marker of cortical granules and participates in cortical granule exocytosis in mouse eggs.

Agonist-dependent modulation of cell surface expression of the cold receptor TRPM8

The spatial and temporal distribution of receptors constitutes an important mechanism for controlling the magnitude of cellular responses. Several members of the transient receptor potential (TRP) ion channel family can regulate their function by modulating their expression at the plasma membrane (PM) through rapid vesicular translocation and fusion. The mechanisms underlying this regulation are not completely understood, and the contribution of vesicular trafficking to physiological function is unknown. TRPM8 receptors are expressed in mammalian peripheral sensory neurons and are essential for the detection of cold temperatures. Previously, we showed that TRPM8-containing vesicles are segregated into three main pools, immobile at the PM, simple diffusive and corralled-hopping. Here, we show that channel expression at the PM is modulated by TRPM8 agonists in F11 and HEK293T cells. Our results support a model in which the activation of TRPM8 channels, located at the PM, induces a short-lived recruitment of a TRPM8-containing vesicular pool to the cell surface causing a transitory increase in the number of functional channels, affecting intrinsic properties of cold receptor responses. We further demonstrate the requirement of intact vesicular trafficking to support sustained cold responses in the skin of mice.

Selective modulation of cellular voltage-dependent calcium channels by hyperbaric pressure-a suggested HPNS partial mechanism

Professional deep sea divers experience motor and cognitive impairment, known as High Pressure Neurological Syndrome (HPNS), when exposed to pressures of 100 msw (1.1 MPa) and above, considered to be the result of synaptic transmission alteration. Previous studies have indicated modulation of presynaptic Ca²⁺ currents at high pressure. We directly measured for the first time pressure effects on the currents of voltage dependent Ca²⁺ channels (VDCCs) expressed in Xenopus oocytes. Pressure selectivity augmented the current in Ca_V1.2 and depressed it in Ca_V3.2 channels. Pressure application also affected the channels' kinetics, such as Ʈ_Rise, Ʈ_Decay. Pressure modulation of VDCCs seems to play an important role in generation of HPNS signs and symptoms.

Ca2+ influx and tyrosine kinases trigger Bordetella adenylate cyclase toxin (ACT) endocytosis. Cell physiology and expression of the CD11b/CD18 integrin major determinants of the entry route

Humans infected with Bordetella pertussis, the whooping cough bacterium, show evidences of impaired host defenses. This pathogenic bacterium produces a unique adenylate cyclase toxin (ACT) which enters human phagocytes and catalyzes the unregulated formation of cAMP, hampering important bactericidal functions of these immune cells that eventually cause cell death by apoptosis and/or necrosis. Additionally, ACT permeabilizes cells through pore formation in the target cell membrane. Recently, we demonstrated that ACT is internalised into macrophages together with other membrane components, such as the integrin CD11b/CD18 (CR3), its receptor in these immune cells, and GM1. The goal of this study was to determine whether ACT uptake is restricted to receptor-bearing macrophages or on the contrary may also take place into cells devoid of receptor and gain more insights on the signalling involved. Here, we show that ACT is rapidly eliminated from the cell membrane of either CR3-positive as negative cells, though through different entry routes, which depends in part, on the target cell physiology and characteristics. ACT-induced Ca(2+) influx and activation of non-receptor Tyr kinases into the target cell appear to be common master denominators in the different endocytic strategies activated by this toxin. Very importantly, we show that, upon incubation with ACT, target cells are capable of repairing the cell membrane, which suggests the mounting of an anti-toxin cell repair-response, very likely involving the toxin elimination from the cell surface.

Vesicular calcium channels as regulators of the exocytotic post-fusion phase

Regulated secretion is a fundamental cellular process in many different types of eukaryotic cells with Ca2(+-)triggered exocytosis taking centre stage. Elevations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) regulate multiple steps from vesicle fusion with the plasma membrane to fusion pore dilation and subsequent retrieval of spent vesicles. The general view is that the rise in [Ca(2+)](c) initiates during the pre-fusion stage and either results from Ca(2+)-influx via Ca(2+) channels in the plasma membrane or from release from intracellular Ca(2+)-stores. However, there is increasing evidence that exocytosis of secretory vesicles triggers additional, localised Ca(2+) signals via insertion of vesicle-associated Ca(2+) channels into the cell surface. These restricted Ca(2+) signals following fusion are ideally suited for regulating the post-fusion fate of individual secretory vesicles. In invertebrates they have been shown to trigger compensatory endocytosis.  Recently we have reported that exocytosis of lamellar bodies in alveolar type II epithelial cells results in a localized Ca(2+)-influx via vesicular P2X(4) receptors which regulates fusion pore expansion and vesicle content release. This finding expands the emerging picture that post-fusion Ca(2+) entry via vesicle-associated Ca(2+) channels plays a central role for regulated exocytosis.

Observations of calcium dynamics in cortical secretory vesicles

Calcium (Ca(2+)) dynamics were evaluated in fluorescently labeled sea urchin secretory vesicles using confocal microscopy. 71% of the vesicles examined exhibited one or more transient increases in the fluorescence signal that was damped in time. The detection of transient increases in signal was dependent upon the affinity of the fluorescence indicator; the free Ca(2+) concentration in the secretory vesicles was estimated to be in the range of ∼10 to 100 μM. Non-linear stochastic analysis revealed the presence of extra variance in the Ca(2+) dependent fluorescence signal. This noise process increased linearly with the amplitude of the Ca(2+) signal. Both the magnitude and spatial properties of this noise process were dependent upon the activity of vesicle p-type (Ca(v)2.1) Ca(2+) channels. Blocking the p-type Ca(2+) channels with ω-agatoxin decreased signal variance, and altered the spatial noise pattern within the vesicle. These fluorescence signal properties are consistent with vesicle Ca(2+) dynamics and not simply due to obvious physical properties such as gross movement artifacts or pH driven changes in Ca(2+) indicator fluorescence. The results suggest that the free Ca(2+) content of cortical secretory vesicles is dynamic; this property may modulate the exocytotic fusion process.

Massive calcium-activated endocytosis without involvement of classical endocytic proteins

We describe rapid massive endocytosis (MEND) of >50% of the plasmalemma in baby hamster kidney (BHK) and HEK293 cells in response to large Ca transients. Constitutively expressed Na/Ca exchangers (NCX1) are used to generate Ca transients, whereas capacitance recording and a membrane tracer dye, FM 4–64, are used to monitor endocytosis. With high cytoplasmic adenosine triphosphate (ATP; >5 mM), Ca influx causes exocytosis followed by MEND. Without ATP, Ca transients cause only exocytosis. MEND can then be initiated by pipette perfusion of ATP, and multiple results indicate that ATP acts via phosphatidylinositol-bis 4,5-phosphate (PIP₂) synthesis: PIP₂ substitutes for ATP to induce MEND. ATP-activated MEND is blocked by an inositol 5-phosphatase and by guanosine 5′-[γ-thio]triphosphate (GTPγS). Block by GTPγS is overcome by the phospholipase C inhibitor, U73122, and PIP₂ induces MEND in the presence of GTPγS. MEND can occur in the absence of ATP and PIP₂ when cytoplasmic free Ca is clamped to 10 µM or more by Ca-buffered solutions. ATP-independent MEND occurs within seconds during Ca transients when cytoplasmic solutions contain polyamines (e.g., spermidine) or the membrane is enriched in cholesterol. Although PIP₂ and cholesterol can induce MEND minutes after Ca transients have subsided, polyamines must be present during Ca transients. MEND can reverse over minutes in an ATP-dependent fashion. It is blocked by brief β-methylcyclodextrin treatments, and tests for involvement of clathrin, dynamins, calcineurin, and actin cytoskeleton were negative. Therefore, we turned to the roles of lipids. Bacterial sphingomyelinases (SMases) cause similar MEND responses within seconds, suggesting that ceramide may be important. However, Ca-activated MEND is not blocked by reagents that inhibit SMases. MEND is abolished by the alkylating phospholipase A₂ inhibitor, bromoenol lactone, whereas exocytosis remains robust, and Ca influx causes MEND in cardiac myocytes without preceding exocytosis. Thus, exocytosis is not prerequisite for MEND. From these results and two companion studies, we suggest that Ca promotes the formation of membrane domains that spontaneously vesiculate to the cytoplasmic side.

Delineating biological pathways unique to embryonic stem cell-derived insulin-producing cell lines from their noninsulin-producing progenitor cell lines

To identify unique biochemical pathways in embryonic stem cell-derived insulin-producing cells as potential therapeutic targets to prevent or delay beta-cell dysfunction or death in diabetic patients, comparative genome-wide gene expression studies of recently derived mouse insulin-producing cell lines and their progenitor cell lines were performed using microarray technology. Differentially expressed genes were functionally clustered to identify important biochemical pathways in these insulin-producing cell lines. Biochemical or cellular assays were then performed to assess the relevance of these pathways to the biology of these cells. A total of 185 genes were highly expressed in the insulin-producing cell lines, and computational analysis predicted the pentose phosphate pathway (PPP), clathrin-mediated endocytosis, and the peroxisome proliferator-activated receptor (PPAR) signaling pathway as important pathways in these cell lines. Insulin-producing ERoSHK cells were more resistant to hydrogen peroxide (H(2)O(2))-induced oxidative stress. Inhibition of PPP by dehydroepiandrosterone and 6-aminonicotinamide abrogated this H(2)O(2) resistance with a concomitant decrease in PPP activity as measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. Clathrin-mediated endocytosis, which is essential in maintaining membrane homeostasis in secreting cells, was up-regulated by glucose in ERoSHK but not in their progenitor ERoSH cells. Its inhibition by chlorpromazine at high glucose concentration was toxic to the cells. Troglitazone, a PPARG agonist, up-regulated expression of Ins1 and Ins2 but not Glut2. Gene expression analysis has identified the PPP, clathrin-mediated endocytosis, and the PPAR signaling pathway as the major delineating pathways in these insulin-producing cell lines, and their biological relevance was confirmed by biochemical and cellular assays.

Membrane wounding triggers ATP release and dysferlin-mediated intercellular calcium signaling

Dysferlin is a Ca(2+)-binding protein found in many different cell types. It is required for membrane wound repair in muscle, but it is not known whether it has the same function in other cells. Here we report the activation of an intercellular signaling pathway in sea urchin embryos by membrane wounding that evokes Ca(2+) spikes in neighboring cells. This pathway was mimicked by ATP application, and inhibited by apyrase, cadmium, and omega-agatoxin-IVA. Microinjection of dysferlin antisense phosphorodiamidate morpholino oligonucleotides blocked this pathway, whereas control morpholinos did not. Co-injection of mRNA encoding human dysferlin with the inhibitory morpholino rescued signaling activity. We conclude that in sea urchin embryos dysferlin mediates Ca(2+)-triggered intercellular signaling in response to membrane wounding.

Channeling calcium: a shared mechanism for exocytosis-endocytosis coupling

Cell surface area is maintained in most cells by coupling exocytotic activity to compensatory endocytosis, a process that specifically retrieves membrane inserted by exocytosis. Although such coupling mechanisms seem to be ubiquitous, the mechanisms through which these membrane trafficking events are linked have remained elusive. A mechanism for coupling exocytosis to endocytosis in fruit fly nerve terminals that depends on the exocytotic insertion of vesicular calcium channels into the plasma membrane has recently been identified. This coupling mechanism resembles one previously described in sea urchin eggs. Here, I compare the similarities and differences of the processes involved in linking exocytosis to endocytosis in these two invertebrate systems and speculate on whether the vertebrate coupling mechanism might also depend on vesicular channels.

Two independent forms of endocytosis maintain embryonic cell surface homeostasis during early development

Eukaryotic cells have multiple forms of endocytosis which maintain cell surface homeostasis. One explanation for this apparent redundancy is to allow independent retrieval of surface membranes derived from different types of vesicles. Consistent with this hypothesis we find that sea urchin eggs have at least two types of compensatory endocytosis. One is associated with retrieving cortical vesicle membranes, and formed large endosomes by a mechanism that was inhibited by agatoxin, cadmium, staurosporine and FK506. The second type is thought to compensate for constitutive exocytosis, and formed small endosomes using a mechanism that was insensitive to the above mentioned reagents, but was inhibited by phenylarsine oxide (PAO), and by microinjection of mRNA encoding Src kinase. Both mechanisms could act concurrently, and account for all of the endocytosis occurring during early development. Inhibition of either form did not trigger compensation by the other form, and phorbol ester treatment rescued the endocytotic activity blocked by agatoxin, but not the retrieval blocked by PAO.

Calcium increases endocytotic vesicle size and accelerates membrane fission in insulin-secreting INS-1 cells

In many cells, endocytotic membrane retrieval is accelerated by Ca2+. The effect of Ca2+ on single endocytotic vesicles and fission pore kinetics was examined by measuring capacitance and conductance changes in small membrane patches of insulin-secreting INS-1 cells. In intact cells, elevation of Ca2+ by glucose stimulation induced a 1.8-fold increase in membrane internalisation. This surprisingly resulted from an increased unitary capacitance of endocytotic vesicles whereas the frequency of endocytosis was unaltered. This effect of glucose was prevented by inhibition of L- or R-type Ca2+ channels. Extracellular (pipette) Ca2+ was found to regulate endocytotic vesicle capacitance in a bimodal manner. Vesicle capacitance was increased at intermediate Ca2+ (2.6 mM), but not at high Ca2+ (10 mM). Similar results were obtained upon direct application of 100 nM and 0.5 mM Ca2+ to the intracellular surface of inside-out excised membrane patches, and in these experiments the increase in vesicle capacitance was prevented by the calcineurin inhibitor deltamethrin. Endocytotic fission pore kinetics were accelerated by Ca2+ in both the intact cells and isolated membrane patches; however, the effect in this case was neither bimodal nor deltamethrin sensitive. Membrane retrieval can therefore be upregulated by a Ca2+-dependent increase in endocytotic vesicle size and acceleration of membrane fission in insulin-secreting INS-1 cells.

Apical endocytosis in outer hair cells of the mammalian cochlea

Outer hair cells (OHCs), the sensory-motor cells of the mammalian cochlea, contain an endocytic tubulovesicular compartment below their apical stereocilia. We have used two-photon imaging of FM1-43 in the intact epithelium to show that these cells take up membrane in a Ca(2+)-dependent manner from a distinct apical site. The uptake rate was 0.8 microm(2)/s and internalized membrane was trafficked rapidly to a compartment along the lateral wall and distinct intracellular compartments. Double labelling with FM1-43 and DiOC(6), an endoplasmic reticulum (ER) marker, showed that these compartments are part of the tubulovesicular endoplasmic reticulum of OHCs. Labelling with a lysosomal marker showed that OHC lysosomes are restricted to the apex. Using the protein marker wheat germ agglutinin (WGA-FITC) we demonstrate that apical protein internalization and trafficking is about eight times slower than membrane internalization. Using double labelling with FM1-43 and WGA-FITC, we show that membrane and protein internalization are apically colocalized but that patterns of protein and membrane traffic differ. Protein was targeted only to the most apical third of the lateral wall. In control conditions, OHCs displayed only weak WGA-FITC surface labelling at the site of endocytosis. Lowering the rate of apical endocytosis increased this surface signal. The results suggest that OHCs endocytose membrane and membrane proteins with a high turnover rate and that these cells may use apical endocytosis to sort proteins via an indirect pathway to the lateral membrane.

An increase in surface area is not required for cell division in early sea urchin development

Cell division requires an increase in surface area to volume ratio. During early development, surface area can increase, volume can decrease, or surface topography can be optimized to allow for division. While exocytosis is thought to be essential for division [Mol. Biol. Cell 10 (1999), 2735; Proc. Natl. Acad. Sci. USA 99 (2002), 3633], exocytosis doesn't always yield an increase in surface area [Proc. Natl. Acad. Sci. USA 79 (1982), 6712]. We used multiphoton laser scanning microscopy, fluorescence spectroscopy, and electron microscopy to monitor membrane trafficking, surface area, volume, and surface topography during early sea urchin development. Despite extensive membrane trafficking monitored by FM 1-43 fluorescence, we find that the net surface area of the embryo does not change prior to the eight-cell stage. During this period, embryo volume decreases by 15%, and microvilli disappear from interior facing membrane segments. Thus, the first three cell divisions utilize residual membrane liberated by decreasing cytoplasmic volume, and reducing microvilli density on interior facing membranes. Only after the eight-cell stage was a net increase in FM 1-43 fluorescence from the embryo surface detected. Our data suggest that compensatory endocytosis is downregulated after this developmental stage to yield an increase in surface area for cell division.

Low pH inhibits compensatory endocytosis at a step between depolarization and calcium influx

Cell function can be modulated by the insertion and removal of ion channels from the cell surface. The mechanism used to keep channels quiescent prior to delivery to the cell surface is not known. In eggs, cortical vesicle exocytosis inserts voltage-gated calcium channels into the cell surface. Calcium influx through these channels triggers compensatory endocytosis. Secretory vesicles contain high concentrations of calcium and hydrogen ions. We propose that lumenal hydrogen ions inhibit vesicular calcium channel gating prior to exocytosis, discharge of lumenal protons upon vesicle-plasma membrane fusion enables calcium channel gating. Consistent with this hypothesis we find that cortical vesicle lumens are acidic, and exocytosis releases lumenal hydrogen ions. Acidic extracellular pH reversibly blocks endocytosis, and the windows of opportunity for inhibition with a calcium-channel blocker or hydrogen ions are indistinguishable. Calcium ionophore treatment circumvents the low pH block, suggesting that calcium influx, or an upstream step, is obstructed. Inhibition of calcium influx by preventing membrane depolarization is unlikely, as elevation of the extracellular potassium concentration failed to overcome the pH block, and low extracellular pH was found to depolarize the membrane potential. We conclude that low pH inhibits endocytosis at a step between membrane depolarization and calcium influx.

Patterns of gene expression in the developing adult sea urchin central nervous system reveal multiple domains and deep-seated neural pentamery

The adult sea urchin central nervous system (CNS) is composed of five radial nerve cords connected to a circular nerve ring. Although much is known about the molecular mechanisms underlying the development and function of the nervous systems of many invertebrate and vertebrate species, virtually nothing is known about these processes in echinoderms. We have isolated a set of clones from a size-selected cDNA library prepared from the nervous system of the sea urchin Heliocidaris erythrogramma for use as probes. A total of 117 expressed sequence clones were used to search the GenBank database. Identified messages include genes that encode signaling proteins, cytoskeletal elements, cell surface proteins and receptors, cell proliferation and differentiation factors, transport and channel proteins, and a RNA DEAD box helicase. Expression was analyzed by RNA gel blot hybridization to document expression through development. Many of the genes have apparently neural limited expression and function, but some have been co-opted into new roles, notably associated with exocytotic events at fertilization. Localization of gene expression by whole-mount in situ hybridization shows that the morphologically simple sea urchin radial CNS exhibits complex organization into localized transcriptional domains. The transcription patterns reflect the morphological pentamery of the echinoderm CNS and provide no indication of an underlying functional bilateral symmetry in the CNS.

Pronounced infracuticular endocytosis in mammalian outer hair cells

Endocytosis in cochlear hair cells was investigated by staining with the vital fluorescent dye FM 1-43, that partitions reversibly into membranes and is trapped in vesicles during endocytosis. The temporal development and spatial distribution of FM 1-43 induced fluorescence was investigated using confocal laser-scanning microscopy. FM 1-43 rapidly and intensely stained cochlear hair cells, leaving the supporting cells unstained. For short application (0.2-30 s), only the infracuticular region of outer hair cells (OHCs) was labeled, whereas for long application (30-60 s), the OHCs were also labeled in the infranuclear zone and along a central strand extending from the infracuticular zone down to the nucleus, as well as along the entire cell membrane. Except for the cell membrane, the infracuticular zone, directly below the cuticular plate, showed the most rapid and intense staining, and in most cases staining was spherically shaped with a diameter of 3-7 microm. Localization and size of this infracuticular staining coincided with Hensen's body, a specialized variant of the endoplasmic reticulum. In contrast to the OHCs, apical fluorescence of inner hair cells presented a homogeneous distribution. When OHCs were incubated in FM 1-43 for longer than 1 min, many points of contact between the central strand, the infracuticular zone and the lateral cell membrane were observed. Since Hensen's bodies are a specialty of OHCs and the fluorescent staining pattern of these cells was unique, it is proposed that Hensen's body is involved in the turnover of OHC-specific proteins, such as those involved in the molecular machinery of the motor action of the plasma membrane.

NAADP+ initiates the Ca2+ response during fertilization of starfish oocytes

We have explored the role of the recently discovered second messenger nicotinic acid adenine nucleotide phosphate (NAADP+) in Ca2+ swings that accompany the fertilization process in starfish oocytes. The injection of NAADP+ deep into the cytoplasm of oocytes matured by the hormone 1-methyladenine (1-MA), mobilized Ca2+ exclusively in the cortical layer, showing that the NAADP+-sensitive Ca2+ pool is restricted to the subplasma membrane region of the cell. At variance with this, InsP3 initiated the liberation of Ca2+ next to the point of injection in the center of the cell. The initial cortical Ca2+ liberation induced by NAADP+ was followed by a spreading of the Ca2+ wave to the remainder of the cell and by a massive cortical granule exocytosis similar to that routinely observed on injection of InsP3. A striking difference in the responses to NAADP+ and InsP3 was revealed by the removal of the nucleus from immature oocytes, i.e., from oocytes not treated with 1-MA. Whereas the Ca2+ response and the cortical granule exocytosis induced by NAADP+ were unaffected by the removal of the nucleus, the Ca2+ response promoted by InsP3 was significantly slowed. In addition, the cortical granule exocytosis was completely abolished. When enucleated oocytes were fertilized, the spermatozoon still promoted the Ca2+ wave and normal cortical exocytosis, strongly suggesting that the Ca2+ response was mediated by NAADP+ and not by InsP3. InsP3-sensitive Ca2+ stores may mediate the propagation of the wave initiated by NAADP+ since its spreading was strongly affected by removal of the nucleus.

Plasma membrane resident 'fusion complexes' mediate reconstituted exocytosis

Calcium-triggered exocytosis is thought to be mediated by membrane-associated protein complexes. In sea urchin eggs, high concentrations of calcium activate multiple 'fusion complexes' per cortical vesicle-plasma membrane docking site. Some of these fusion complexes are known to reside in the vesicle membrane. It is not known if fusion complexes also reside in the plasma membrane, or if plasma membrane-resident fusion complexes require cognate partners in the vesicle membrane. Using reconstitution, we show that N-ethylmaleimide treatment of either vesicles or plasma membrane fragments prior to reconstitution does not completely inhibit exocytosis. Treatment of both components did result in complete inhibition. Upon reconstitution, cortical vesicles and the early endosomes formed by compensatory endocytosis both contributed, on average, two fusion complexes per reconstituted docking site. The plasma membrane contributed, on average, two fusion complexes per docking site when assembled with cortical vesicles, but only one complex when reconstituted with endosomes. We conclude that there are at least two types of plasma membrane-resident fusion complexes that participate in reconstituted cortical vesicle-plasma membrane fusion. The activity of one of these fusion complexes is target-specific for cortical vesicles, while the second type also supports fusion with endosomes.

Two endocytic recycling routes selectively fill two vesicle pools in frog motor nerve terminals

We have identified and characterized two vesicle recycling pathways in frog motor nerve terminals. We exploited the differential staining properties of FM dyes of varying hydrophobicity to label selectively two different vesicle pools, using optical imaging and electron microscopy of photoconverted dyes. During a 1 min tetanus, a rapidly recycling route places vesicles selectively into a small readily releasable pool comprising about 20% of vesicles. After the tetanus, a much slower pathway (from which FM2-10 but not FM1-43 can be rinsed) delivers vesicles via infoldings and cisternae selectively to a reserve pool with a halftime of about 8 min. Mixing between the two pools is slow. During stimulation at 30 Hz, 10-15 s is required to mobilize and release dye from the reserve pool.

The molecular biology of invertebrate voltage-gated Ca(2+) channels

The importance of voltage-gated Ca(2+) channels in cellular function is illustrated by the many distinct types of Ca(2+) currents found in vertebrate tissues, a variety that is generated in part by numerous genes encoding Ca(2+) channel subunits. The degree to which this genetic diversity is shared by invertebrates has only recently become apparent. Cloning of Ca(2+) channel subunits from various invertebrate species, combined with the wealth of information from the Caenorhabditis elegans genome, has clarified the organization and evolution of metazoan Ca(2+) channel genes. Functional studies have employed novel structural information gained from invertebrate Ca(2+) channels to complement ongoing research on mammalian Ca(2+) currents, while demonstrating that the strict correspondence between pharmacological and molecular classes of vertebrate Ca(2+) channels does not fully extend to invertebrate tissues. Molecular structures can now be combined with physiological data to develop a more cogent system of categorizing invertebrate channel subtypes. In this review, we examine recent progress in the characterization of invertebrate Ca(2+) channel genes and its relevance to the diversity of invertebrate Ca(2+) currents.

Exocytotic insertion of calcium channels constrains compensatory endocytosis to sites of exocytosis

Proteins inserted into the cell surface by exocytosis are thought to be retrieved by compensatory endocytosis, suggesting that retrieval requires granule proteins. In sea urchin eggs, calcium influx through P-type calcium channels is required for retrieval, and the large size of sea urchin secretory granules permits the direct observation of retrieval. Here we demonstrate that retrieval is limited to sites of prior exocytosis. We tested whether channel distribution can account for the localization of retrieval at exocytotic sites. We find that P-channels reside on secretory granules before fertilization, and are translocated to the egg surface by exocytosis. Our study provides strong evidence that the transitory insertion of P-type calcium channels in the surface membrane plays an obligatory role in the mechanism coupling exocytosis and compensatory endocytosis.

Ablation of P/Q-type Ca(2+) channel currents, altered synaptic transmission, and progressive ataxia in mice lacking the alpha(1A)-subunit

The Ca(2+) channel alpha(1A)-subunit is a voltage-gated, pore-forming membrane protein positioned at the intersection of two important lines of research: one exploring the diversity of Ca(2+) channels and their physiological roles, and the other pursuing mechanisms of ataxia, dystonia, epilepsy, and migraine. alpha(1A)-Subunits are thought to support both P- and Q-type Ca(2+) channel currents, but the most direct test, a null mutant, has not been described, nor is it known which changes in neurotransmission might arise from elimination of the predominant Ca(2+) delivery system at excitatory nerve terminals. We generated alpha(1A)-deficient mice (alpha(1A)(-/-)) and found that they developed a rapidly progressive neurological deficit with specific characteristics of ataxia and dystonia before dying approximately 3-4 weeks after birth. P-type currents in Purkinje neurons and P- and Q-type currents in cerebellar granule cells were eliminated completely whereas other Ca(2+) channel types, including those involved in triggering transmitter release, also underwent concomitant changes in density. Synaptic transmission in alpha(1A)(-/-) hippocampal slices persisted despite the lack of P/Q-type channels but showed enhanced reliance on N-type and R-type Ca(2+) entry. The alpha(1A)(-/-) mice provide a starting point for unraveling neuropathological mechanisms of human diseases generated by mutations in alpha(1A).