Control Of Ca2 Research Articles

Phosphatidylinositol 4,5-bisphosphate and calcium at ER-PM junctions — Complex interplay of simple messengers

Endoplasmic reticulum-plasma membrane contact sites (ER-PM MCS) are a specialised domain involved in the control of Ca2+ dynamics and various Ca2+-dependent cellular processes. Intracellular Ca2+ signals are broadly supported by Ca2+ release from intracellular Ca2+ channels such as inositol 1,4,5-trisphosphate receptors (IP3Rs) and subsequent store-operated Ca2+ entry (SOCE) across the PM to replenish store content. IP3Rs sit in close proximity to the PM where they can easily access newly synthesised IP3, interact with binding partners such as actin, and localise adjacent to ER-PM MCS populated by the SOCE machinery, STIM1–2 and Orai1–3, to possibly form a locally regulated unit of Ca2+ influx. PtdIns(4,5)P2 is a multiplex regulator of Ca2+ signalling at the ER-PM MCS interacting with multiple proteins at these junctions such as actin and STIM1, whilst also being consumed as a substrate for phospholipase C to produce IP3 in response to extracellular stimuli. In this review, we consider the mechanisms regulating the synthesis and turnover of PtdIns(4,5)P2 via the phosphoinositide cycle and its significance for sustained signalling at the ER-PM MCS. Furthermore, we highlight recent insights into the role of PtdIns(4,5)P2 in the spatiotemporal organization of signalling at ER-PM junctions and raise outstanding questions on how this multi-faceted regulation occurs.

Regulation of K+-Dependent Na+/Ca2+-Exchangers (NCKX).

Potassium-dependent sodium-calcium exchangers (NCKX) have emerged as key determinants of calcium (Ca2+) signaling and homeostasis, especially in environments where ion concentrations undergo large changes, such as excitatory cells and transport epithelia. The regulation of NCKX transporters enables them to respond to the changing cellular environment thereby helping to shape the extent and kinetics of Ca2+ signals. This review examines the current knowledge of the different ways in which NCKX activity can be modulated. These include (i) cellular and dynamic subcellular location (ii); changes in protein expression mediated at the gene, transcript, or protein level (iii); genetic changes resulting in altered protein structure or expression (iv); regulation via changes in substrate concentration (v); and post-translational modification, partner protein interactions, and allosteric regulation. Detailed mechanistic understanding of NCKX regulation is an emerging area of research with the potential to provide important new insights into transporter function, the control of Ca2+ signals, and possible interventions for dysregulated Ca2+ homeostasis.

The ataxia-linked E1081Q mutation affects the sub-plasma membrane Ca2+-microdomains by tuning PMCA3 activity

Calcium concentration must be finely tuned in all eukaryotic cells to ensure the correct performance of its signalling function. Neuronal activity is exquisitely dependent on the control of Ca2+ homeostasis: its alterations ultimately play a pivotal role in the origin and progression of many neurodegenerative processes. A complex toolkit of Ca2+ pumps and exchangers maintains the fluctuation of cytosolic Ca2+ concentration within the appropriate threshold. Two ubiquitous (isoforms 1 and 4) and two neuronally enriched (isoforms 2 and 3) of the plasma membrane Ca2+ATPase (PMCA pump) selectively regulate cytosolic Ca2+ transients by shaping the sub-plasma membrane (PM) microdomains. In humans, genetic mutations in ATP2B1, ATP2B2 and ATP2B3 gene have been linked with hearing loss, cerebellar ataxia and global neurodevelopmental delay: all of them were found to impair pump activity. Here we report three additional mutations in ATP2B3 gene corresponding to E1081Q, R1133Q and R696H amino acids substitution, respectively. Among them, the novel missense mutation (E1081Q) immediately upstream the C-terminal calmodulin-binding domain (CaM-BD) of the PMCA3 protein was present in two patients originating from two distinct families. Our biochemical and molecular studies on PMCA3 E1081Q mutant have revealed a splicing variant-dependent effect of the mutation in shaping the sub-PM [Ca2+]. The E1081Q substitution in the full-length b variant abolished the capacity of the pump to reduce [Ca2+] in the sub-PM microdomain (in line with the previously described ataxia-related PMCA mutations negatively affecting Ca2+ pumping activity), while, surprisingly, its introduction in the truncated a variant selectively increased Ca2+ extrusion activity in the sub-PM Ca2+ microdomains. These results highlight the importance to set a precise threshold of [Ca2+] by fine-tuning the sub-PM microdomains and the different contribution of the PMCA splice variants in this regulation.

P38 MAPK Activity Is Required to Prevent Hyperactivation of NLRP3 Inflammasome.

Inflammation contributes to the pathogenesis and morbidity of wide spectrum of human diseases. The inflammatory response must be actively controlled to prevent bystander damage to tissues. Yet, the mechanisms controlling excessive inflammatory responses are poorly understood. NLRP3 inflammasome plays an important role in innate immune response to cellular infection or stress. Its activation must be tightly regulated because uncontrolled inflammasome activation is associated with a number of human diseases. p38 MAPK signaling plays an essential role in the regulation of inflammation. The role of p38 MAPK in inflammatory response associated with the expression of proinflammatory molecules is known. However, the anti-inflammatory functions of p38 MAPK are largely unknown. In this study, we show that pharmacologic inhibition or genetic deficiency of p38 MAPK leads to hyperactivation of NLRP3 inflammasome, resulting in enhanced Caspase 1 activation and IL-1β and IL-18 production. The deficiency of p38 MAPK activity induced an increase of cytosolic Ca2+ and excessive mitochondrial Ca2+ uptake, leading to exacerbation of mitochondrial damage, which was associated with hyperactivation of NLRP3 inflammasome. In addition, mice with deficiency of p38 MAPK in granulocytes had evidence of in vivo hyperactivation of NLRP3 inflammasome and were more susceptible to LPS-induced sepsis compared with wild-type mice. Our results suggest that p38 MAPK negatively regulates NLRP3 inflammasome through control of Ca2+ mobilization. Hyperactivity of inflammasome in p38-deficient mice causes lung inflammation and increased susceptibility to septic shock.

Role of Sigma-1 Receptor in Calcium Modulation: Possible Involvement in Cancer.

Ca2+ signaling plays a pivotal role in the control of cellular homeostasis and aberrant regulation of Ca2+ fluxes have a strong impact on cellular functioning. As a consequence of this ubiquitous role, Ca2+ signaling dysregulation is involved in the pathophysiology of multiple diseases including cancer. Indeed, multiple studies have highlighted the role of Ca2+ fluxes in all the steps of cancer progression. In particular, the transfer of Ca2+ at the ER-mitochondrial contact sites, also known as mitochondrial associated membranes (MAMs), has been shown to be crucial for cancer cell survival. One of the proteins enriched at this site is the sigma-1 receptor (S1R), a protein that has been described as a Ca2+-sensitive chaperone that exerts a protective function in cells in various ways, including the modulation of Ca2+ signaling. Interestingly, S1R is overexpressed in many types of cancer even though the exact mechanisms by which it promotes cell survival are not fully elucidated. This review summarizes the findings describing the roles of S1R in the control of Ca2+ signaling and its involvement in cancer progression.

Impaired endoplasmic reticulum-mitochondrial signaling in ataxia-telangiectasia.

SummaryThere is evidence that ATM mutated in ataxia-telangiectasia (A-T) plays a key role in protecting against mitochondrial dysfunction, the mechanism for which remains unresolved. We demonstrate here that ATM-deficient cells are exquisitely sensitive to nutrient deprivation, which can be explained by defective cross talk between the endoplasmic reticulum (ER) and the mitochondrion. Tethering between these two organelles in response to stress was reduced in cells lacking ATM, and consistent with this, Ca2+ release and transfer between ER and mitochondria was reduced dramatically when compared with control cells. The impact of this on mitochondrial function was evident from an increase in oxygen consumption rates and a defect in mitophagy in ATM-deficient cells. Our findings reveal that ER-mitochondrial connectivity through IP3R1-GRP75-VDAC1, to maintain Ca2+ homeostasis, as well as an abnormality in mitochondrial fusion defective in response to nutrient stress, can account for at least part of the mitochondrial dysfunction observed in A-T cells.

14-3-3 binding creates a memory of kinase action by stabilizing the modified state of phospholamban.

The cardiac membrane protein phospholamban (PLN) is targeted by protein kinase A (PKA) at Ser16 and by Ca2+/calmodulin-dependent protein kinase II (CaMKII) at Thr17 β-Adrenergic stimulation and PKA-dependent phosphorylation of Ser16 acutely stimulate the sarcoplasmic reticulum calcium pump (SERCA) by relieving its inhibition by PLN. CaMKII-dependent phosphorylation may lead to longer-lasting SERCA stimulation and may sustain maladaptive Ca2+ handling. Here, we demonstrated that phosphorylation at either Ser16 or Thr17 converted PLN into a target for the phosphoadaptor protein 14-3-3 with different affinities. 14-3-3 proteins were localized within nanometers of PLN and endogenous 14-3-3 coimmunoprecipitated with pentameric PLN from cardiac membranes. Molecular dynamics simulations predicted different molecular contacts for peptides phosphorylated at Ser16 or Thr17 with the binding groove of 14-3-3, resulting in varied binding affinities. 14-3-3 binding protected either PLN phosphosite from dephosphorylation. β-Adrenergic stimulation of isolated adult cardiomyocytes resulted in the membrane recruitment of endogenous 14-3-3. The exogenous addition of 14-3-3 to β-adrenergic-stimulated cardiomyocytes led to prolonged SERCA activation, presumably because 14-3-3 protected PLN pentamers from dephosphorylation. Phosphorylation of Ser16 was disrupted by the cardiomyopathy-associated ∆Arg14 mutation, implying that phosphorylation of Thr17 by CaMKII may become crucial for 14-3-3 recruitment to ∆Arg14 PLN. Consistent with PLN acting as a dynamic hub in the control of Ca2+ handling, our results identify 14-3-3 binding to PLN as a contractility-augmenting mechanism.

The ER chaperone calnexin controls mitochondrial positioning and respiration.

Chaperones in the endoplasmic reticulum (ER) control the flux of Ca2+ ions into mitochondria, thereby increasing or decreasing the energetic output of the oxidative phosphorylation pathway. An example is the abundant ER lectin calnexin, which interacts with sarco/endoplasmic reticulum Ca2+ ATPase (SERCA). We found that calnexin stimulated the ATPase activity of SERCA by maintaining its redox state. This function enabled calnexin to control how much ER Ca2+ was available for mitochondria, a key determinant for mitochondrial bioenergetics. Calnexin-deficient cells compensated for the loss of this function by partially shifting energy generation to the glycolytic pathway. These cells also showed closer apposition between the ER and mitochondria. Calnexin therefore controls the cellular energy balance between oxidative phosphorylation and glycolysis.

Enhanced Biosorption of Sb(III) onto Living Rhodotorula mucilaginosa Strain DJHN070401: Optimization and Mechanism.

How to effectively remove excess Sb(III) in the water environment by biosorption is receiving close attention in the international scientific community. To obtain the maximum biosorption efficiency, response surface methodology (RSM) was employed to optimize a total of 13 factors for biosorption of Sb(III) onto living Rhodotorula mucilaginosa DJHN070401. The mechanism of biosorption and bioaccumulation was also studied. The results showed that biosorption reached 56.83% under the optimum conditions. Besides, pH, Fe2+, and temperature are significant influencing factors, and control of Ca2+ and Fe2+ has a beneficial impact on Sb(III) biosorption. The characterization explained that physical adsorption occurred readily on the loose and porous surface of DJHN070401 where carboxyl, amidogen, phosphate group, and polysaccharide C-O functional groups facilitated absorption by complexation with Sb(III), accompanied by ion exchange of Na+, Ca2+ ions with Sb(III). It was also noted that the living cell not only improved the removal efficiency in the presence of metabolic inhibitors but also prevented intracellular Sb(III) being re-released into the environment. The results of this study underpin improved and efficient methodology for biosorption of Sb(III) from wastewater.

The Ca2+ permeation mechanism of the ryanodine receptor revealed by a multi-site ion model

Ryanodine receptors (RyR) are ion channels responsible for the release of Ca2+ from the sarco/endoplasmic reticulum and play a crucial role in the precise control of Ca2+ concentration in the cytosol. The detailed permeation mechanism of Ca2+ through RyR is still elusive. By using molecular dynamics simulations with a specially designed Ca2+ model, we show that multiple Ca2+ ions accumulate in the upper selectivity filter of RyR1, but only one Ca2+ can occupy and translocate in the narrow pore at a time, assisted by electrostatic repulsion from the Ca2+ within the upper selectivity filter. The Ca2+ is nearly fully hydrated with the first solvation shell intact during the whole permeation process. These results suggest a remote knock-on permeation mechanism and one-at-a-time occupation pattern for the hydrated Ca2+ within the narrow pore, uncovering the basis underlying the high permeability and low selectivity of the RyR channels.

Sodium channel TRPM4 and sodium/calcium exchangers (NCX) cooperate in the control of Ca2+-induced mucin secretion from goblet cells

Regulated mucin secretion is essential for the formation of the mucus layer that protects the underlying epithelial cells from foreign particles. Alterations in the quantity or quality of secreted mucins are therefore detrimental to airway and colon physiology. Based on various biochemical assays in several human cell lines, we report here that Na+/Ca2+ exchanger 2 (NCX2) works in conjunction with transient receptor potential cation channel subfamily M member 4 (TRPM4), and perhaps TRPM5, Na+ channels to control Ca2+-mediated secretion of both mucin 2 (MUC2) and MUC5AC from HT29-18N2 colonic cancer cells. Differentiated normal bronchial epithelial (NHBE) cells and tracheal cells from patients with cystic fibrosis (CFT1-LC3) expressed only TRPM4 and all three isoforms of NCXs. Blocking the activity of TRPM4 or NCX proteins abrogated MUC5AC secretion from NHBE and CFT1-LC3 cells. Altogether, our findings reveal that NCX and TRPM4/TRPM5 are both required for mucin secretion. We therefore propose that these two proteins could be potential pharmacological targets to control mucus-related pathologies such as cystic fibrosis.

Endoplasmic reticulum-localized CCX2 is required for osmotolerance by regulating ER and cytosolic Ca2+ dynamics in Arabidopsis

Ca2+ signals in plant cells are important for adaptive responses to environmental stresses. Here, we report that the Arabidopsis CATION/Ca2+ EXCHANGER2 (CCX2), encoding a putative cation/Ca2+ exchanger that localizes to the endoplasmic reticulum (ER), is strongly induced by salt and osmotic stresses. Compared with the WT, AtCCX2 loss-of-function mutant was less tolerant to osmotic stress and displayed the most noteworthy phenotypes (less root/shoot growth) during salt stress. Conversely, AtCCX2 gain-of-function mutants were more tolerant to osmotic stress. In addition, AtCCX2 partially suppresses the Ca2+ sensitivity of K667 yeast triple mutant, characterized by Ca2+ uptake deficiency. Remarkably, Cameleon Ca2+ sensors revealed that the absence of AtCCX2 activity results in decreased cytosolic and increased ER Ca2+ concentrations in comparison with both WT and the gain-of-function mutants. This was observed in both salt and nonsalt osmotic stress conditions. It appears that AtCCX2 is directly involved in the control of Ca2+ fluxes between the ER and the cytosol, which plays a key role in the ability of plants to cope with osmotic stresses. To our knowledge, Atccx2 is unique as a plant mutant to show a measured alteration in ER Ca2+ concentrations. In this study, we identified the ER-localized AtCCX2 as a pivotal player in the regulation of ER Ca2+ dynamics that heavily influence plant growth upon salt and osmotic stress.

STIM1 Regulates Somatic Ca2+ Signals and Intrinsic Firing Properties of Cerebellar Purkinje Neurons.

Control of Ca2+ flux between the cytosol and intracellular Ca2+ stores is essential for maintaining normal cellular function. It has been well established in both neuronal and non-neuronal cells that stromal interaction molecule 1 (STIM1) initiates and regulates refilling Ca2+ into the ER. Here, we describe a novel, additional role for STIM1, the regulation of free cytosolic Ca2+, and the consequent control of spike firing in neurons. Among central neurons, cerebellar Purkinje neurons express the highest level of STIM1, and they fire continuously in the absence of stimulation, making somatic Ca2+ homeostasis of particular importance. By using Purkinje neuron-specific STIM1 knock-out (STIM1PKO) male mice, we found that the deletion of STIM1 delayed clearance of cytosolic Ca2+ in the soma during ongoing neuronal firing. Deletion of STIM1 also reduced the Purkinje neuronal excitability and impaired intrinsic plasticity without affecting long-term synaptic plasticity. In vestibulo-ocular reflex learning, STIM1PKO male mice showed severe deficits in memory consolidation, whereas they were normal in memory acquisition. Our results suggest that STIM1 is critically involved in the regulation of the neuronal excitability and the intrinsic plasticity of the Purkinje neurons as well as cerebellar memory consolidation.SIGNIFICANCE STATEMENT Stromal interaction molecule 1 (STIM1), which regulates the refilling of ER Ca2+, has been investigated in several systems including the CNS. In addition to a previous study showing that STIM1 regulates dendritic ER Ca2+ refilling and mGluR1-mediated synaptic transmission, we provide compelling evidence describing a novel role of STIM1 in spike firing Purkinje neurons. We found that STIM1 regulates cytosolic Ca2+ clearance of the soma during spike firing, and the interruption of this cytosolic Ca2+ clearing disrupts neuronal excitability and cerebellar memory consolidation. Our results provide new insights into neuronal functions of STIM1 from single neuronal Ca2+ dynamics to behavior level.

IP3 Receptor-Mediated Calcium Signaling and Its Role in Autophagy in Cancer.

Calcium ions (Ca2+) play a complex role in orchestrating diverse cellular processes, including cell death and survival. To trigger signaling cascades, intracellular Ca2+ is shuffled between the cytoplasm and the major Ca2+ stores, the endoplasmic reticulum (ER), the mitochondria, and the lysosomes. A key role in the control of Ca2+ signals is attributed to the inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs), the main Ca2+-release channels in the ER. IP3Rs can transfer Ca2+ to the mitochondria, thereby not only stimulating core metabolic pathways but also increasing apoptosis sensitivity and inhibiting basal autophagy. On the other hand, IP3-induced Ca2+ release enhances autophagy flux by providing cytosolic Ca2+ required to execute autophagy upon various cellular stresses, including nutrient starvation, chemical mechanistic target of rapamycin inhibition, or drug treatment. Similarly, IP3Rs are able to amplify Ca2+ signals from the lysosomes and, therefore, impact autophagic flux in response to lysosomal channels activation. Furthermore, indirect modulation of Ca2+ release through IP3Rs may also be achieved by controlling the sarco/endoplasmic reticulum Ca2+ ATPases Ca2+ pumps of the ER. Considering the complex role of autophagy in cancer development and progression as well as in response to anticancer therapies, it becomes clear that it is important to fully understand the role of the IP3R and its cellular context in this disease. In cancer cells addicted to ER–mitochondrial Ca2+ fueling, IP3R inhibition leads to cancer cell death via mechanisms involving enhanced autophagy or mitotic catastrophe. Moreover, IP3Rs are the targets of several oncogenes and tumor suppressors and the functional loss of these genes, as occurring in many cancer types, can result in modified Ca2+ transport to the mitochondria and in modulation of the level of autophagic flux. Similarly, IP3R-mediated upregulation of autophagy can protect some cancer cells against natural killer cells-induced killing. The involvement of IP3Rs in the regulation of both autophagy and apoptosis, therefore, directly impact cancer cell biology and contribute to the molecular basis of tumor pathology.

Control of Ca2+ Influx and Calmodulin Activation by SK-Channels in Dendritic Spines.

The key trigger for Hebbian synaptic plasticity is influx of Ca2+ into postsynaptic dendritic spines. The magnitude of [Ca2+] increase caused by NMDA-receptor (NMDAR) and voltage-gated Ca2+ -channel (VGCC) activation is thought to determine both the amplitude and direction of synaptic plasticity by differential activation of Ca2+ -sensitive enzymes such as calmodulin. Ca2+ influx is negatively regulated by Ca2+ -activated K+ channels (SK-channels) which are in turn inhibited by neuromodulators such as acetylcholine. However, the precise mechanisms by which SK-channels control the induction of synaptic plasticity remain unclear. Using a 3-dimensional model of Ca2+ and calmodulin dynamics within an idealised, but biophysically-plausible, dendritic spine, we show that SK-channels regulate calmodulin activation specifically during neuron-firing patterns associated with induction of spike timing-dependent plasticity. SK-channel activation and the subsequent reduction in Ca2+ influx through NMDARs and L-type VGCCs results in an order of magnitude decrease in calmodulin (CaM) activation, providing a mechanism for the effective gating of synaptic plasticity induction. This provides a common mechanism for the regulation of synaptic plasticity by neuromodulators.

Ca2+/H+ exchange by acidic organelles regulates cell migration in vivo.

Increasing evidence implicates Ca(2+) in the control of cell migration. However, the underlying mechanisms are incompletely understood. Acidic Ca(2+) stores are fast emerging as signaling centers. But how Ca(2+) is taken up by these organelles in metazoans and the physiological relevance for migration is unclear. Here, we identify a vertebrate Ca(2+)/H(+)exchanger (CAX) as part of a widespread family of homologues in animals. CAX is expressed in neural crest cells and required for their migration in vivo. It localizes to acidic organelles, tempers evoked Ca(2+) signals, and regulates cell-matrix adhesion during migration. Our data provide new molecular insight into how Ca(2+) is handled by acidic organelles and link this to migration, thereby underscoring the role of noncanonical Ca(2+) stores in the control of Ca(2+)-dependent function.

Thin Filament Structure and the Steric Blocking Model.

By interacting with the troponin-tropomyosin complex on myofibrillar thin filaments, Ca2+ and myosin govern the regulatory switching processes influencing contractile activity of mammalian cardiac and skeletal muscles. A possible explanation of the roles played by Ca2+ and myosin emerged in the early 1970s when a compelling "steric model" began to gain traction as a likely mechanism accounting for muscle regulation. In its most simple form, the model holds that, under the control of Ca2+ binding to troponin and myosin binding to actin, tropomyosin strands running along thin filaments either block myosin-binding sites on actin when muscles are relaxed or move away from them when muscles are activated. Evidence for the steric model was initially based on interpretation of subtle changes observed in X-ray fiber diffraction patterns of intact skeletal muscle preparations. Over the past 25 years, electron microscopy coupled with three-dimensional reconstruction directly resolved thin filament organization under many experimental conditions and at increasingly higher resolution. At low-Ca2+, tropomyosin was shown to occupy a "blocked-state" position on the filament, and switched-on in a two-step process, involving first a movement of tropomyosin away from the majority of the myosin-binding site as Ca2+ binds to troponin and then a further movement to fully expose the site when small numbers of myosin heads bind to actin. In this contribution, basic information on Ca2+-regulation of muscle contraction is provided. A description is then given relating the voyage of discovery taken to arrive at the present understanding of the steric regulatory model.

The two pore channel TPC2 is dispensable in pancreatic β-cells for normal Ca2+ dynamics and insulin secretion

Ca2+ signals are central to the stimulation of insulin secretion from pancreatic β-cells by glucose and other agents, including glucagon-like peptide-1 (GLP-1). Whilst Ca2+ influx through voltage-gated Ca2+ channels on the plasma membrane is a key trigger for glucose-stimulated secretion, mobilisation of Ca2+ from acidic stores has been implicated in the control of more localised Ca2+ changes and membrane potential. Nicotinic acid adenine dinucleotide phosphate (NAADP), generated in β-cells in response to high glucose, is a potent mobiliser of these stores, and has been proposed to act through two pore channels (TPC1 and TPC2, murine gene names Tpcn1 and Tpcn2). Whilst the role of TPC1 in the control of Ca2+ mobilisation and insulin secretion was recently confirmed, conflicting data exist for TPC2. Here, we used the selective and efficient deleter strain, Ins1Cre to achieve β-cell selective deletion of the Tpcn2 gene in mice. βTpcn2 KO mice displayed normal intraperitoneal and oral glucose tolerance, and glucose-stimulated Ca2+ dynamics and insulin secretion from islets were similarly normal. GLP-1-induced Ca2+ increases involved an increase in oscillation frequency from 4.35 to 4.84 per minute (p=0.04) at 8mM glucose, and this increase was unaffected by the absence of Tpcn2. The current data thus indicate that TPC2 is not absolutely required for normal glucose- or incretin-stimulated insulin secretion from the β-cell. Our findings suggest that TPC1, whose expression tended to increase in Tpcn2 null islets, might be sufficient to support normal Ca2+ dynamics in response to stimulation by nutrients or incretins.

Protein and Signaling Networks in Vertebrate Photoreceptor Cells.

Vertebrate photoreceptor cells are exquisite light detectors operating under very dim and bright illumination. The photoexcitation and adaptation machinery in photoreceptor cells consists of protein complexes that can form highly ordered supramolecular structures and control the homeostasis and mutual dependence of the secondary messengers cyclic guanosine monophosphate (cGMP) and Ca2+. The visual pigment in rod photoreceptors, the G protein-coupled receptor rhodopsin is organized in tracks of dimers thereby providing a signaling platform for the dynamic scaffolding of the G protein transducin. Illuminated rhodopsin is turned off by phosphorylation catalyzed by rhodopsin kinase (GRK1) under control of Ca2+-recoverin. The GRK1 protein complex partly assembles in lipid raft structures, where shutting off rhodopsin seems to be more effective. Re-synthesis of cGMP is another crucial step in the recovery of the photoresponse after illumination. It is catalyzed by membrane bound sensory guanylate cyclases (GCs) and is regulated by specific neuronal Ca2+-sensor proteins called guanylate cyclase-activating proteins (GCAPs). At least one GC (ROS-GC1) was shown to be part of a multiprotein complex having strong interactions with the cytoskeleton and being controlled in a multimodal Ca2+-dependent fashion. The final target of the cGMP signaling cascade is a cyclic nucleotide-gated (CNG) channel that is a hetero-oligomeric protein located in the plasma membrane and interacting with accessory proteins in highly organized microdomains. We summarize results and interpretations of findings related to the inhomogeneous organization of signaling units in photoreceptor outer segments.

Bcl-2 proteins, cell migration and embryonic development: lessons from zebrafish.

B-cell lymphoma-2 (Bcl-2) was cloned 30 years ago and associated with B-cell follicular lymphoma. A number of Bcl-2 homologs were identified later on. Importantly, the Bcl-2 family was found to control the mitochondrial outer membrane permeabilization: a key step of the mitochondrial pathway of apoptosis.1 Bcl-2 homologs are evolutionarily conserved throughout metazoans and considered as the hallmarks of multicellularity. Genetic manipulation in nematodes and mice demonstrated that the bcl-2 family has a pivotal role in tissue homeostasis by controlling cell death; however, an increased number of in vitro studies have identified additional non-apoptotic functions, suggesting that Bcl-2 proteins are in fact multitask factors (Figure 1a).2 Figure 1 Non-apoptotic functions of the Bcl-2 family of proteins. (a) Simplified representation of Bcl-2 proteins functions. Bcl-2 proteins are multitask factors and linked to non-apoptotic functions. The top panel represents the different Ca2+-independent ... Besides their mitochondrial localization, Bcl-2 proteins are also found in the endoplasmic reticulum (ER). In fact, a number of them contribute to apoptosis regulation though the control of Ca2+ exchanges at the level of the ER/mitochondria interface. Indeed physical proximity between these organelles creates intracellular microdomains, considered as Ca2+ hotspots.3 Mitochondria constantly uptake Ca2+ to ensure their physiological functions; they are also able to rapidly uptake Ca2+ when massively released from the ER, acting as a genuine Ca2+ buffer. This fast accumulation may lead to mitochondrial Ca2+ overload and, depending on Ca2+ levels, the cells will undergo apoptosis or necrosis.4 Bcl-2 proteins control Ca2+ exchanges through direct interactions with ER Ca2+ channels and pumps including the Inositol 1,4,5-Trisphosphate receptor (IP3R), the Ca2+-ATPase (SERCA) pump, the ryanodine receptor, the Bax inhibitor-1 channel, as well as, the voltage-dependent anion channel (VDAC) at the mitochondria (reviewed in Bonneau et al.2). It was reported that overexpression of Bcl-2 may lead to a decrease of the ER Ca2+ load,5 and the ability of Bcl-2 proteins to regulate intracellular Ca2+ homeostasis was linked to non-apoptotic functions (Figure 1a).