Unicellular Ca2+ signaling 'toolkit' at the origin of metazoa

Extracellular DNA traps in a ctenophore demonstrate immune cell behaviors in a non-bilaterian

The formation of extracellular DNA traps (ETosis) is a first response mechanism by specific immune cells following exposure to microbes. Initially characterized in vertebrate neutrophils, cells capable of ETosis have been discovered recently in diverse non-vertebrate taxa. To assess the conservation of ETosis between evolutionarily distant non-vertebrate phyla, we observed and quantified ETosis using the model ctenophore Mnemiopsis leidyi and the oyster Crassostrea gigas. Here we report that ctenophores - thought to have diverged very early from the metazoan stem lineage - possess immune-like cells capable of phagocytosis and ETosis. We demonstrate that both Mnemiopsis and Crassostrea immune cells undergo ETosis after exposure to diverse microbes and chemical agents that stimulate ion flux. We thus propose that ETosis is an evolutionarily conserved metazoan defense against pathogens.

What is it like to be a choanoflagellate? Sensation, processing and behavior in the closest unicellular relatives of animals

All animals evolved from a single lineage of unicellular precursors more than 600 million years ago. Thus, the biological and genetic foundations for animal sensation, cognition and behavior must necessarily have arisen by modifications of pre-existing features in their unicellular ancestors. Given that the single-celled ancestors of the animal kingdom are extinct, the only way to reconstruct how these features evolved is by comparing the biology and genomic content of extant animals to their closest living relatives. Here, we reconstruct the Umwelt (the subjective, perceptive world) inhabited by choanoflagellates, a group of unicellular (or facultatively multicellular) aquatic microeukaryotes that are the closest living relatives of animals. Although behavioral research on choanoflagellates remains patchy, existing evidence shows that they are capable of chemosensation, photosensation and mechanosensation. These processes often involve specialized sensorimotor cellular appendages (cilia, microvilli, and/or filopodia) that resemble those that underlie perception in most animal sensory cells. Furthermore, comparative genomics predicts an extensive "sensory molecular toolkit" in choanoflagellates, which both provides a potential basis for known behaviors and suggests the existence of a largely undescribed behavioral complexity that presents exciting avenues for future research. Finally, we discuss how facultative multicellularity in choanoflagellates might help us understand how evolution displaced the locus of decision-making from a single cell to a collective, and how a new space of behavioral complexity might have become accessible in the process.

The premetazoan ancestry of the synaptic toolkit and appearance of first neurons

Neurons, especially when coupled with muscles, allow animals to interact with and navigate through their environment in ways unique to life on earth. Found in all major animal lineages except sponges and placozoans, nervous systems range widely in organization and complexity, with neurons possibly representing the most diverse cell-type. This diversity has led to much debate over the evolutionary origin of neurons as well as synapses, which allow for the directed transmission of information. The broad phylogenetic distribution of neurons and presence of many of the defining components outside of animals suggests an early origin of this cell type, potentially in the time between the first animal and the last common ancestor of extant animals. Here, we highlight the occurrence and function of key aspects of neurons outside of animals as well as recent findings from non-bilaterian animals in order to make predictions about when and how the first neuron(s) arose during animal evolution and their relationship to those found in extant lineages. With advancing technologies in single cell transcriptomics and proteomics as well as expanding functional techniques in non-bilaterian animals and the close relatives of animals, it is an exciting time to begin unraveling the complex evolutionary history of this fascinating animal cell type.

Comments on the evolution of TRPV6

It is well known that not all biological findings derived from animals can be directly applied to humans. The TRPV6 protein may serve as an example which highlights these inter-species differences as an example of parallel evolutionary pathways. TRPV6 (and TRPV5) belong to a family of ion channels from the transient receptor potential group but are selectively permeable for Ca²⁺, in contrast to other members of the family. Sequences with recognizable similarity to TRPV6 can already be found in archaebacteria. These ancient sequences show clear similarity to the ion-conducting pore of TRPV6. Over the course of evolution, the duplication of the TRPV6 gene gave rise to TRPV5. Duplications of the complete genome as well as subsequent loss of genetic material have led to a variety of different TRPV5/6 combinations. In addition, there is an N-terminal extension of the protein in placental animals. This extension causes translation of TRPV6 to be initiated from an ACG codon. Inactivation of one TRPV6 allele can be correlated with alcohol-independent pancreatitis in humans while inactivation of both alleles leads to skeletal dysplasia of newborn babies. The latter effect is not observed in mice, implying that the effects due to perturbations in TRPV6 levels are much more pronounced in humans.

Transient receptor potential channels: current perspectives on evolution, structure, function and nomenclature

The transient receptor potential superfamily of ion channels (TRP channels) is widely recognized for the roles its members play in sensory nervous systems. However, the incredible diversity within the TRP superfamily, and the wide range of sensory capacities found therein, has also allowed TRP channels to function beyond sensing an organism's external environment, and TRP channels have thus become broadly critical to (at least) animal life. TRP channels were originally discovered in Drosophila and have since been broadly studied in animals; however, thanks to a boom in genomic and transcriptomic data, we now know that TRP channels are present in the genomes of a variety of creatures, including green algae, fungi, choanoflagellates and a number of other eukaryotes. As a result, the organization of the TRP superfamily has changed radically from its original description. Moreover, modern comprehensive phylogenetic analyses have brought to light the vertebrate-centricity of much of the TRP literature; much of the nomenclature has been grounded in vertebrate TRP subfamilies, resulting in a glossing over of TRP channels in other taxa. Here, we provide a comprehensive review of the function, structure and evolutionary history of TRP channels, and put forth a more complete set of non-vertebrate-centric TRP family, subfamily and other subgroup nomenclature.

Evolutionary Aspects of TRPMLs and TPCs

Transient receptor potential (TRP) or transient receptor potential channels are a highly diverse family of mostly non-selective cation channels. In the mammalian genome, 28 members can be identified, most of them being expressed predominantly in the plasma membrane with the exception of the mucolipins or TRPMLs which are expressed in the endo-lysosomal system. In mammalian organisms, TRPMLs have been associated with a number of critical endo-lysosomal functions such as autophagy, endo-lysosomal fusion/fission and trafficking, lysosomal exocytosis, pH regulation, or lysosomal motility and positioning. The related non-selective two-pore cation channels (TPCs), likewise expressed in endosomes and lysosomes, have also been found to be associated with endo-lysosomal trafficking, autophagy, pH regulation, or lysosomal exocytosis, raising the question why these two channel families have evolved independently. We followed TRP/TRPML channels and TPCs through evolution and describe here in which species TRP/TRPMLs and/or TPCs are found, which functions they have in different species, and how this compares to the functions of mammalian orthologs.

Reconstruction of protein domain evolution using single-cell amplified genomes of uncultured choanoflagellates sheds light on the origin of animals

Understanding the origins of animal multicellularity is a fundamental biological question. Recent genome data have unravelled the role that co-option of pre-existing genes played in the origin of animals. However, there were also some important genetic novelties at the onset of Metazoa. To have a clear understanding of the specific genetic innovations and how they appeared, we need the broadest taxon sampling possible, especially among early-branching animals and their unicellular relatives. Here, we take advantage of single-cell genomics to expand our understanding of the genomic diversity of choanoflagellates, the sister-group to animals. With these genomes, we have performed an updated and taxon-rich reconstruction of protein evolution from the Last Eukaryotic Common Ancestor (LECA) to animals. Our novel data re-defines the origin of some genes previously thought to be metazoan-specific, like the POU transcription factor, which we show appeared earlier in evolution. Moreover, our data indicate that the acquisition of new genes at the stem of Metazoa was mainly driven by duplications and protein domain rearrangement processes at the stem of Metazoa. Furthermore, our analysis allowed us to reveal protein domains that are essential to the maintenance of animal multicellularity. Our analyses also demonstrate the utility of single-cell genomics from uncultured taxa to address evolutionary questions. This article is part of a discussion meeting issue 'Single cell ecology'.

The acidocalcisome inositol-1,4,5-trisphosphate receptor of Trypanosoma brucei is stimulated by luminal polyphosphate hydrolysis products

Acidocalcisomes are acidic calcium stores rich in polyphosphate (polyP) and are present in trypanosomes and also in a diverse range of other organisms. Ca²⁺ is released from these organelles through a channel, inositol 1,4,5-trisphosphate receptor (TbIP₃R), which is essential for growth and infectivity of the parasite Trypanosoma brucei However, the mechanism by which TbIP₃R controls Ca²⁺ release is unclear. In this work, we expressed TbIP₃R in a chicken B lymphocyte cell line in which the genes for all three vertebrate IP₃Rs were stably ablated (DT40-3KO). We show that IP₃-mediated Ca²⁺ release depends on Ca²⁺ but not on ATP concentration and is inhibited by heparin, caffeine, and 2-aminomethoxydiphenyl borate (2-APB). Excised patch clamp recordings from nuclear membranes of DT40 cells expressing only TbIP₃R disclosed that luminal inorganic orthophosphate (P_i) or pyrophosphate (PP_i), and neutral or alkaline pH can stimulate IP₃-generated currents. In contrast, polyP or acidic pH did not induce these currents, and nuclear membranes obtained from cells expressing rat IP₃R were unresponsive to polyP or its hydrolysis products. Our results are consistent with the notion that polyP hydrolysis products within acidocalcisomes or alkalinization of their luminal pH activate TbIP₃R and Ca²⁺ release. We conclude that TbIP₃R is well-adapted to its role as the major Ca²⁺ release channel of acidocalcisomes in T. brucei.

Mode of Action of Farnesol, the "Noble Unknown" in Particular in Ca2+ Homeostasis, and Its Juvenile Hormone-Esters in Evolutionary Retrospect

Farnesol, the sesquiterpenoid precursor of insect juvenile hormones (JH) that itself has JH activity, existed already long before animals and their hormones came into being. Although it is omnipresent in all eukaryotes, this molecule remains a "noble unknown" in cell physiology. It is neither documented as a hormone nor as another type of signaling molecule. To date, its function as an intermediate in the synthesis of squalene-cholesterol-steroids in chordates/vertebrates, and of the insect/arthropod JHs, esters of farnesol, in the mevalonate biosynthetic pathway is assumed to be the only one. This assumption neglects that already two decades ago, farnesol has been shown to be a potent endogenous inhibitor of N-type voltage-gated Ca²⁺ channels in some mammalian cell types. The tandem mevalonate pathway and Ca²⁺ channels originated early in eukaryotic evolution, and has since been well conserved, "promoting" it as a ubiquitous player in Ca²⁺ homeostasis in all eukaryotes. This paper accentuates how this drastic change in thinking gained momentum after the discovery by Paroulek and Sláma that the huge amounts of JH I in male accessory glands of the Cecropia moth, are actually synthesized in these glands themselves and not in the corpora allata, the hitherto assumed unique synthesis site of such compounds. In addition, MAG-JHs have no hormonal- but an exocrine function. Here we hypothesize that MAG-JHs may function in protecting the spermatozoa against toxic Ca²⁺ concentrations, and in enabling their flagellum to undulate. They may do so by acting through membrane receptors. Our novel paradigm assigns to farnesol/JHs a function of flexible hydrophobic molecular valves for restricting untimely Ca²⁺-passage through some types of canonical Ca²⁺channels, using covalently bound farnesyl- or geranyl-geranyl group attachment as well as GPCRs-G proteins all containing a prenyl group. The high rotatable bond count, and their horseshoe-shape are instrumental to their valve function. In our paradigm, Met/Tai and Gce, to date generally thought to be the (only) functional (nuclear) receptors for JHs, are classified as probable Ca²⁺-sensitive transcription factors. Some theoretical and practical considerations for possible applications in a medical context will be discussed.

The Evolution of Calcium-Based Signalling in Plants

The calcium-based intracellular signalling system is used ubiquitously to couple extracellular stimuli to their characteristic intracellular responses. It is becoming clear from genomic and physiological investigations that while the basic elements in the toolkit are common between plants and animals, evolution has acted in such a way that, in plants, some components have diversified with respect to their animal counterparts, while others have either been lost or have never evolved in the plant lineages. In comparison with animals, in plants there appears to have been a loss of diversity in calcium-influx mechanisms at the plasma membrane. However, the evolution of the calcium-storing vacuole may provide plants with additional possibilities for regulating calcium influx into the cytosol. Among the proteins that are involved in sensing and responding to increases in calcium, plants possess specific decoder proteins that are absent from the animal lineage. In seeking to understand the selection pressures that shaped the plant calcium-signalling toolkit, we consider the evolution of fast electrical signalling. We also note that, in contrast to animals, plants apparently do not make extensive use of cyclic-nucleotide-based signalling. It is possible that reliance on a single intracellular second-messenger-based system, coupled with the requirement to adapt to changing environmental conditions, has helped to define the diversity of components found in the extant plant calcium-signalling toolkit.

Identification of a novel fused gene family implicates convergent evolution in eukaryotic calcium signaling

Background

Both calcium signals and protein phosphorylation responses are universal signals in eukaryotic cell signaling. Currently three pathways have been characterized in different eukaryotes converting the Ca²⁺ signals to the protein phosphorylation responses. All these pathways have based mostly on studies in plants and animals.

Results

Based on the exploration of genomes and transcriptomes from all the six eukaryotic supergroups, we report here in Metakinetoplastina protists a novel gene family. This family, with a proposed name SCAMK, comprises SnRK3 fused calmodulin-like III kinase genes and was likely evolved through the insertion of a calmodulin-like3 gene into an SnRK3 gene by unequal crossover of homologous chromosomes in meiosis cell. Its origin dated back to the time intersection at least 450 million-year-ago when Excavata parasites, Vertebrata hosts, and Insecta vectors evolved. We also analyzed SCAMK’s unique expression pattern and structure, and proposed it as one of the leading calcium signal conversion pathways in Excavata parasite. These characters made SCAMK gene as a potential drug target for treating human African trypanosomiasis.

Conclusions

This report identified a novel gene fusion and dated its precise fusion time in Metakinetoplastina protists. This potential fourth eukaryotic calcium signal conversion pathway complements our current knowledge that convergent evolution occurs in eukaryotic calcium signaling.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4685-y) contains supplementary material, which is available to authorized users.

Neuronal Calcium Signaling in Metabolic Regulation and Adaptation to Nutrient Stress

All organisms can respond physiologically and behaviorally to environmental fluxes in nutrient levels. Different nutrient sensing pathways exist for specific metabolites, and their inputs ultimately define appropriate nutrient uptake and metabolic homeostasis. Nutrient sensing mechanisms at the cellular level require pathways such as insulin and target of rapamycin (TOR) signaling that integrates information from different organ systems like the fat body and the gut. Such integration is essential for coordinating growth with development. Here we review the role of a newly identified set of integrative interneurons and the role of intracellular calcium signaling within these neurons, in regulating nutrient sensing under conditions of nutrient stress. A comparison of the identified Drosophila circuit and cellular mechanisms employed in this circuit, with vertebrate systems, suggests that the identified cell signaling mechanisms may be conserved for neural circuit function related to nutrient sensing by central neurons. The ideas proposed are potentially relevant for understanding the molecular basis of metabolic disorders, because these are frequently linked to nutritional stress.

TRPV4: a Sensor for Homeostasis and Pathological Events in the CNS

Transient receptor potential vanilloid type 4 (TRPV4) was originally described as a calcium-permeable nonselective cation channel. TRPV4 is now recognized as a polymodal ionotropic receptor: it is a broadly expressed, nonselective cation channel (permeable to calcium, potassium, magnesium, and sodium) that plays an important role in a multitude of physiological processes. TRPV4 is involved in maintaining homeostasis, serves as an osmosensor and thermosensor, can be activated directly by endogenous or exogenous chemical stimuli, and can be activated or sensitized indirectly via intracellular signaling pathways. Additionally, TRPV4 is upregulated in a variety of pathological conditions. In this review, we focus on the role of TRPV4 in mediating homeostasis and pathological events in the central nervous system (CNS). This review is composed of three parts. Section 1 describes the role of TRPV4 in maintaining homeostatic processes, including the volume of body water, ionic concentrations, volume, and the temperature. Section 2 describes the effects of activation and inhibition of TRPV4 in the CNS. Section 3 focuses on the role of TRPV4 during pathological events in CNS.

Inseparable tandem: evolution chooses ATP and Ca2+ to control life, death and cellular signalling

From the very dawn of biological evolution, ATP was selected as a multipurpose energy-storing molecule. Metabolism of ATP required intracellular free Ca(2+) to be set at exceedingly low concentrations, which in turn provided the background for the role of Ca(2+) as a universal signalling molecule. The early-eukaryote life forms also evolved functional compartmentalization and vesicle trafficking, which used Ca(2+) as a universal signalling ion; similarly, Ca(2+) is needed for regulation of ciliary and flagellar beat, amoeboid movement, intracellular transport, as well as of numerous metabolic processes. Thus, during evolution, exploitation of atmospheric oxygen and increasingly efficient ATP production via oxidative phosphorylation by bacterial endosymbionts were a first step for the emergence of complex eukaryotic cells. Simultaneously, Ca(2+) started to be exploited for short-range signalling, despite restrictions by the preset phosphate-based energy metabolism, when both phosphates and Ca(2+) interfere with each other because of the low solubility of calcium phosphates. The need to keep cytosolic Ca(2+) low forced cells to restrict Ca(2+) signals in space and time and to develop energetically favourable Ca(2+) signalling and Ca(2+) microdomains. These steps in tandem dominated further evolution. The ATP molecule (often released by Ca(2+)-regulated exocytosis) rapidly grew to be the universal chemical messenger for intercellular communication; ATP effects are mediated by an extended family of purinoceptors often linked to Ca(2+) signalling. Similar to atmospheric oxygen, Ca(2+) must have been reverted from a deleterious agent to a most useful (intra- and extracellular) signalling molecule. Invention of intracellular trafficking further increased the role for Ca(2+) homeostasis that became critical for regulation of cell survival and cell death. Several mutually interdependent effects of Ca(2+) and ATP have been exploited in evolution, thus turning an originally unholy alliance into a fascinating success story.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Role of TRP Channels in Dinoflagellate Mechanotransduction

Transient receptor potential (TRP) ion channels are common components of mechanosensing pathways, mainly described in mammals and other multicellular organisms. To gain insight into the evolutionary origins of eukaryotic mechanosensory proteins, we investigated the involvement of TRP channels in mechanosensing in a unicellular eukaryotic protist, the dinoflagellate Lingulodinium polyedra. BLASTP analysis of the protein sequences predicted from the L. polyedra transcriptome revealed six sequences with high similarity to human TRPM2, TRPM8, TRPML2, TRPP1, and TRPP2; and characteristic TRP domains were identified in all sequences. In a phylogenetic tree including all mammalian TRP subfamilies and TRP channel sequences from unicellular and multicellular organisms, the L. polyedra sequences grouped with the TRPM, TPPML, and TRPP clades. In pharmacological experiments, we used the intrinsic bioluminescence of L. polyedra as a reporter of mechanoresponsivity. Capsaicin and RN1734, agonists of mammalian TRPV, and arachidonic acid, an agonist of mammalian TRPV, TRPA, TRPM, and Drosophila TRP, all stimulated bioluminescence in L. polyedra. Mechanical stimulation of bioluminescence, but not capsaicin-stimulated bioluminescence, was inhibited by gadolinium (Gd³⁺), a general inhibitor of mechanosensitive ion channels, and the phospholipase C (PLC) inhibitor U73122. These pharmacological results are consistent with the involvement of TRP-like channels in mechanosensing by L. polyedra. The TRP channels do not appear to be mechanoreceptors but rather are components of the mechanotransduction signaling pathway and may be activated via a PLC-dependent mechanism. The presence and function of TRP channels in a dinoflagellate emphasize the evolutionary conservation of both the channel structures and their functions.

Evolutionary origin of synapses and neurons - Bridging the gap

The evolutionary origin of synapses and neurons is an enigmatic subject that inspires much debate. Non-bilaterian metazoans, both with and without neurons and their closest relatives already contain many components of the molecular toolkits for synapse functions. The origin of these components and their assembly into ancient synaptic signaling machineries are particularly important in light of recent findings on the phylogeny of non-bilaterian metazoans. The evolution of synapses and neurons are often discussed only from a metazoan perspective leaving a considerable gap in our understanding. By taking an integrative approach we highlight the need to consider different, but extremely relevant phyla and to include the closest unicellular relatives of metazoans, the ichthyosporeans, filastereans and choanoflagellates, to fully understand the evolutionary origin of synapses and neurons. This approach allows for a detailed understanding of when and how the first pre- and postsynaptic signaling machineries evolved.

Evolutionary Cell Biology of Proteins from Protists to Humans and Plants

During evolution, the cell as a fine-tuned machine had to undergo permanent adjustments to match changes in its environment, while "closed for repair work" was not possible. Evolution from protists (protozoa and unicellular algae) to multicellular organisms may have occurred in basically two lineages, Unikonta and Bikonta, culminating in mammals and angiosperms (flowering plants), respectively. Unicellular models for unikont evolution are myxamoebae (Dictyostelium) and increasingly also choanoflagellates, whereas for bikonts, ciliates are preferred models. Information accumulating from combined molecular database search and experimental verification allows new insights into evolutionary diversification and maintenance of genes/proteins from protozoa on, eventually with orthologs in bacteria. However, proteins have rarely been followed up systematically for maintenance or change of function or intracellular localization, acquirement of new domains, partial deletion (e.g. of subunits), and refunctionalization, etc. These aspects are discussed in this review, envisaging "evolutionary cell biology." Protozoan heritage is found for most important cellular structures and functions up to humans and flowering plants. Examples discussed include refunctionalization of voltage-dependent Ca²⁺ channels in cilia and replacement by other types during evolution. Altogether components serving Ca²⁺ signaling are very flexible throughout evolution, calmodulin being a most conservative example, in contrast to calcineurin whose catalytic subunit is lost in plants, whereas both subunits are maintained up to mammals for complex functions (immune defense and learning). Domain structure of R-type SNAREs differs in mono- and bikonta, as do Ca²⁺ -dependent protein kinases. Unprecedented selective expansion of the subunit a which connects multimeric base piece and head parts (V0, V1) of H⁺ -ATPase/pump may well reflect the intriguing vesicle trafficking system in ciliates, specifically in Paramecium. One of the most flexible proteins is centrin when its intracellular localization and function throughout evolution is traced. There are many more examples documenting evolutionary flexibility of translation products depending on requirements and potential for implantation within the actual cellular context at different levels of evolution. From estimates of gene and protein numbers per organism, it appears that much of the basic inventory of protozoan precursors could be transmitted to highest eukaryotic levels, with some losses and also with important additional "inventions."

Role of STIM2 in cell function and physiopathology

An endoplasmic reticulum (ER)-resident protein that regulates cytosolic and ER free-Ca²⁺ concentration by induction of store-operated calcium entry: that is the original definition of STIM2 and its function. While its activity strongly depends on the amount of calcium stored in the ER, its function goes further, to intracellular signalling and gene expression. Initially under-studied owing to the prominent function of STIM1, STIM2 came to be regarded as vital in mice, gradually emerging as an important player in the nervous system, and cooperating with STIM1 in the immune system. STIM2 has also been proposed as a relevant player in pathological conditions related to ageing, Alzheimer's and Huntington's diseases, autoimmune disorders and cancer. The discovery of additional functions, together with new splicing forms with opposite roles, has clarified existing controversies about STIM2 function in SOCE. With STIM2 being essential for life, but apparently not for development, newly available data demonstrate a complex and still intriguing behaviour that this review summarizes, updating current knowledge of STIM2 function.

A calcium sensor - protein kinase signaling module diversified in plants and is retained in all lineages of Bikonta species

Calcium (Ca²⁺) signaling is a universal mechanism of signal transduction and involves Ca²⁺ signal formation and decoding of information by Ca²⁺ binding proteins. Calcineurin B-like proteins (CBLs), which upon Ca²⁺ binding activate CBL-interacting protein kinases (CIPKs) regulate a multitude of physiological processes in plants. Here, we combine phylogenomics and functional analyses to investigate the occurrence and structural conservation of CBL and CIPK proteins in 26 species representing all major clades of eukaryotes. We demonstrate the presence of at least singular CBL-CIPK pairs in representatives of Archaeplastida, Chromalveolates and Excavates and their general absence in Opisthokonta and Amoebozoa. This denotes CBL-CIPK complexes as evolutionary ancient Ca²⁺ signaling modules that likely evolved in the ancestor of all Bikonta. Furthermore, we functionally characterize the CBLs and CIPK from the parabasalid human pathogen Trichomonas vaginalis. Our results reveal strict evolutionary conservation of functionally important structural features, preservation of biochemical properties and a remarkable cross-kingdom protein-protein interaction potential between CBLs and CIPKs from Arabidopsis thaliana and T. vaginalis. Together our findings suggest an ancient evolutionary origin of a functional CBL-CIPK signaling module close to the root of eukaryotic evolution and provide insights into the initial evolution of signaling networks and Ca²⁺ signaling specificity.

Drosophila larval to pupal switch under nutrient stress requires IP3R/Ca(2+) signalling in glutamatergic interneurons

Neuronal circuits are known to integrate nutritional information, but the identity of the circuit components is not completely understood. Amino acids are a class of nutrients that are vital for the growth and function of an organism. Here, we report a neuronal circuit that allows Drosophila larvae to overcome amino acid deprivation and pupariate. We find that nutrient stress is sensed by the class IV multidendritic cholinergic neurons. Through live calcium imaging experiments, we show that these cholinergic stimuli are conveyed to glutamatergic neurons in the ventral ganglion through mAChR. We further show that IP3R-dependent calcium transients in the glutamatergic neurons convey this signal to downstream medial neurosecretory cells (mNSCs). The circuit ultimately converges at the ring gland and regulates expression of ecdysteroid biosynthetic genes. Activity in this circuit is thus likely to be an adaptation that provides a layer of regulation to help surpass nutritional stress during development.

Choanoflagellate models - Monosiga brevicollis and Salpingoeca rosetta

Choanoflagellates are the closest single-celled relatives of animals and provide fascinating insights into developmental processes in animals. Two species, the choanoflagellates Monosiga brevicollis and Salpingoeca rosetta are emerging as promising model organisms to reveal the evolutionary origin of key animal innovations. In this review, we highlight how choanoflagellates are used to study the origin of multicellularity in animals. The newly available genomic resources and functional techniques provide important insights into the function of choanoflagellate pre- and postsynaptic proteins, cell-cell adhesion and signaling molecules and the evolution of animal filopodia and thus underscore the relevance of choanoflagellate models for evolutionary biology, neurobiology and cell biology research.

From damage response to action potentials: early evolution of neural and contractile modules in stem eukaryotes

Eukaryotic cells convert external stimuli into membrane depolarization, which in turn triggers effector responses such as secretion and contraction. Here, we put forward an evolutionary hypothesis for the origin of the depolarization-contraction-secretion (DCS) coupling, the functional core of animal neuromuscular circuits. We propose that DCS coupling evolved in unicellular stem eukaryotes as part of an 'emergency response' to calcium influx upon membrane rupture. We detail how this initial response was subsequently modified into an ancient mechanosensory-effector arc, present in the last eukaryotic common ancestor, which enabled contractile amoeboid movement that is widespread in extant eukaryotes. Elaborating on calcium-triggered membrane depolarization, we reason that the first action potentials evolved alongside the membrane of sensory-motile cilia, with the first voltage-sensitive sodium/calcium channels (Nav/Cav) enabling a fast and coordinated response of the entire cilium to mechanosensory stimuli. From the cilium, action potentials then spread across the entire cell, enabling global cellular responses such as concerted contraction in several independent eukaryote lineages. In animals, this process led to the invention of mechanosensory contractile cells. These gave rise to mechanosensory receptor cells, neurons and muscle cells by division of labour and can be regarded as the founder cell type of the nervous system.

The origin and evolution of synaptic proteins - choanoflagellates lead the way

The origin of neurons was a key event in evolution, allowing metazoans to evolve rapid behavioral responses to environmental cues. Reconstructing the origin of synaptic proteins promises to reveal their ancestral functions and might shed light on the evolution of the first neuron-like cells in metazoans. By analyzing the genomes of diverse metazoans and their closest relatives, the evolutionary history of diverse presynaptic and postsynaptic proteins has been reconstructed. These analyses revealed that choanoflagellates, the closest relatives of metazoans, possess diverse synaptic protein homologs. Recent studies have now begun to investigate their ancestral functions. A primordial neurosecretory apparatus in choanoflagellates was identified and it was found that the mechanism, by which presynaptic proteins required for secretion of neurotransmitters interact, is conserved in choanoflagellates and metazoans. Moreover, studies on the postsynaptic protein homolog Homer revealed unexpected localization patterns in choanoflagellates and new binding partners, both which are conserved in metazoans. These findings demonstrate that the study of choanoflagellates can uncover ancient and previously undescribed functions of synaptic proteins.

Signalling in ciliates: long- and short-range signals and molecular determinants for cellular dynamics

In ciliates, unicellular representatives of the bikont branch of evolution, inter- and intracellular signalling pathways have been analysed mainly in Paramecium tetraurelia, Paramecium multimicronucleatum and Tetrahymena thermophila and in part also in Euplotes raikovi. Electrophysiology of ciliary activity in Paramecium spp. is a most successful example. Established signalling mechanisms include plasmalemmal ion channels, recently established intracellular Ca²⁺ -release channels, as well as signalling by cyclic nucleotides and Ca²⁺ . Ca²⁺ -binding proteins (calmodulin, centrin) and Ca²⁺ -activated enzymes (kinases, phosphatases) are involved. Many organelles are endowed with specific molecules cooperating in signalling for intracellular transport and targeted delivery. Among them are recently specified soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), monomeric GTPases, H⁺ -ATPase/pump, actin, etc. Little specification is available for some key signal transducers including mechanosensitive Ca²⁺ -channels, exocyst complexes and Ca²⁺ -sensor proteins for vesicle-vesicle/membrane interactions. The existence of heterotrimeric G-proteins and of G-protein-coupled receptors is still under considerable debate. Serine/threonine kinases dominate by far over tyrosine kinases (some predicted by phosphoproteomic analyses). Besides short-range signalling, long-range signalling also exists, e.g. as firmly installed microtubular transport rails within epigenetically determined patterns, thus facilitating targeted vesicle delivery. By envisaging widely different phenomena of signalling and subcellular dynamics, it will be shown (i) that important pathways of signalling and cellular dynamics are established already in ciliates, (ii) that some mechanisms diverge from higher eukaryotes and (iii) that considerable uncertainties still exist about some essential aspects of signalling.

Comparative sequence analysis suggests a conserved gating mechanism for TRP channels

The transient receptor potential (TRP) channel superfamily plays a central role in transducing diverse sensory stimuli in eukaryotes. Although dissimilar in sequence and domain organization, all known TRP channels act as polymodal cellular sensors and form tetrameric assemblies similar to those of their distant relatives, the voltage-gated potassium (Kv) channels. Here, we investigated the related questions of whether the allosteric mechanism underlying polymodal gating is common to all TRP channels, and how this mechanism differs from that underpinning Kv channel voltage sensitivity. To provide insight into these questions, we performed comparative sequence analysis on large, comprehensive ensembles of TRP and Kv channel sequences, contextualizing the patterns of conservation and correlation observed in the TRP channel sequences in light of the well-studied Kv channels. We report sequence features that are specific to TRP channels and, based on insight from recent TRPV1 structures, we suggest a model of TRP channel gating that differs substantially from the one mediating voltage sensitivity in Kv channels. The common mechanism underlying polymodal gating involves the displacement of a defect in the H-bond network of S6 that changes the orientation of the pore-lining residues at the hydrophobic gate.

Tracing the Evolutionary History of Inositol, 1, 4, 5-Trisphosphate Receptor: Insights from Analyses of Capsaspora owczarzaki Ca2+ Release Channel Orthologs

Cellular Ca(2+) homeostasis is tightly regulated and is pivotal to life. Inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) are the major ion channels that regulate Ca(2+) release from intracellular stores. Although these channels have been extensively investigated in multicellular organisms, an appreciation of their evolution and the biology of orthologs in unicellular organisms is largely lacking. Extensive phylogenetic analyses reveal that the IP3R gene superfamily is ancient and diverged into two subfamilies, IP3R-A and IP3R-B/RyR, at the dawn of Opisthokonta. IP3R-B/RyR further diversified into IP3R-B and RyR at the stem of Filozoa. Subsequent evolution and speciation of Holozoa is associated with duplication of IP3R-A and RyR genes, and loss of IP3R-B in the vertebrate lineages. To gain insight into the properties of IP3R important for the challenges of multicellularity, the IP3R-A and IP3R-B family orthologs were cloned from Capsaspora owczarzaki, a close unicellular relative to Metazoa (designated as CO.IP3R-A and CO.IP3R-B). Both proteins were targeted to the endoplasmic reticulum. However, CO.IP3R-A, but strikingly not CO.IP3R-B, bound IP3, exhibited robust Ca(2+) release activity and associated with mammalian IP3Rs. These data indicate strongly that CO.IP3R-A as an exemplar of ancestral IP3R-A orthologs forms bona fide IP3-gated channels. Notably, however, CO.IP3R-A appears not to be regulated by Ca(2+), ATP or Protein kinase A-phosphorylation. Collectively, our findings explore the origin, conservation, and diversification of IP3R gene families and provide insight into the functionality of ancestral IP3Rs and the added specialization of these proteins in Metazoa.

Copper-induced activation of TRP channels promotes extracellular calcium entry, activation of CaMs and CDPKs, copper entry and membrane depolarization in Ulva compressa

In order to identify channels involved in membrane depolarization, Ulva compressa was incubated with agonists of TRP channels C5, A1 and V1, and the level of intracellular calcium was detected. Agonists of TRPC5, A1 and V1 induced increases in intracellular calcium at 4, 9, and 11 min of exposure, respectively, and antagonists of TRPC5, A1, and V1 corresponding to SKF-96365 (SKF), HC-030031 (HC), and capsazepin (CPZ), respectively, inhibited calcium increases indicating that functional TRPs exist in U. compressa. In addition, copper excess induced increases in intracellular calcium at 4, 9, and 12 min which were inhibited by SKF, HC, and CPZ, respectively, indicating that copper activate TRPC5, A1, and V1 channels. Moreover, copper-induced calcium increases were inhibited by EGTA, a non-permeable calcium chelating agent, but not by thapsigargin, an inhibitor of endoplasmic reticulum (ER) calcium ATPase, indicating that activation of TRPs leads to extracellular calcium entry. Furthermore, copper-induced calcium increases were not inhibited by W-7, an inhibitor of CaMs, and staurosporine, an inhibitor of CDPKs, indicating that extracellular calcium entry did not require activation of CaMs and CDPKs. In addition, copper induced membrane depolarization events at 4, 8, and 11 min and these events were inhibited by SKF, HC, CPZ, and bathocuproine, a specific copper chelating agent, indicating that copper entry through TRP channels leads to membrane depolarization. Moreover, membrane depolarization events were inhibited by W-7 and staurosporine, indicating that activation of CaMs and CDPKs is required to allow copper entry through TRPs. Interestingly, copper-induced calcium increases and depolarization events were light-dependent and were inhibited by DCMU, an inhibitor of photosystem II, and ATP-γ-S, a non-hydrolizable analog of ATP, suggesting that ATP derived from photosynthesis is required to activate TRPs. Thus, light-dependent copper-induced activation TRPC5, A1 and V1 promotes extracellular calcium entry leading to activation of CaMs and CDPKs which, in turn, promotes copper entry through TRP channels and membrane depolarization.

Insights into the early evolution of animal calcium signaling machinery: a unicellular point of view

The basic principles of Ca(2+) regulation emerged early in prokaryotes. Ca(2+) signaling acquired more extensive and varied functions when life evolved into multicellular eukaryotes with intracellular organelles. Animals, fungi and plants display differences in the mechanisms that control cytosolic Ca(2+) concentrations. The aim of this review is to examine recent findings from comparative genomics of Ca(2+) signaling molecules in close unicellular relatives of animals and in common unicellular ancestors of animals and fungi. Also discussed are the evolution and origins of the sperm-specific CatSper channel complex, cation/Ca(2+) exchangers and four-domain voltage-gated Ca(2+) channels. Newly identified evolutionary evidence suggests that the distinct Ca(2+) signaling machineries in animals, plants and fungi likely originated from an ancient Ca(2+) signaling machinery prior to early eukaryotic radiation.

Structural insights into endoplasmic reticulum stored calcium regulation by inositol 1,4,5-trisphosphate and ryanodine receptors

The two major calcium (Ca²⁺) release channels on the sarco/endoplasmic reticulum (SR/ER) are inositol 1,4,5-trisphosphate and ryanodine receptors (IP3Rs and RyRs). They play versatile roles in essential cell signaling processes, and abnormalities of these channels are associated with a variety of diseases. Structural information on IP3Rs and RyRs determined using multiple techniques including X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy and cryo-electron microscopy (EM), has significantly advanced our understanding of the mechanisms by which these Ca²⁺ release channels function under normal and pathophysiological circumstances. In this review, structural advances on the understanding of the mechanisms of IP3R and RyR function and dysfunction are summarized. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Calcium signalling in the ciliated protozoan model, Paramecium: strict signal localisation by epigenetically controlled positioning of different Ca²⁺-channels

The Paramecium tetraurelia cell is highly organised, with regularly spaced elements pertinent to Ca(2+) signalling under epigenetic control. Vesicles serving as stationary Ca(2+) stores or undergoing trafficking contain Ca(2+)-release channels (PtCRCs) which, according to sequence and domain comparison, are related either to inositol 1,4,5-trisphosphate (InsP3) receptors (IP3R) or to ryanodine receptor-like proteins (RyR-LP) or to both, with intermediate characteristics or deviation from conventional domain structure. Six groups of such PtCRCs have been found. The ryanodine-InsP3-receptor homology (RIH) domain is not always recognisable, in contrast to the channel domain with six trans-membrane domains and the pore between transmembrane domain 5 and 6. Two CRC subtypes tested more closely, PtCRC-II and PtCRC-IV, with and without an InsP3-binding domain, reacted to InsP3 and to caffeine, respectively, and hence represent IP3Rs and RyR-LPs. IP3Rs occur in the contractile vacuole complex where they allow for stochastic constitutive Ca(2+) reflux into the cytosol. RyR-LPs are localised to cortical Ca(2+) stores; they are engaged in dense core-secretory vesicle exocytosis by Ca(2+) release, superimposed by Ca(2+)-influx via non-ciliary Ca(2+)-channels. One or two different types of PtCRCs also occur in other vesicles undergoing trafficking. Since the PtCRCs described combine different features they are considered derivatives of primitive precursors. The highly regular, epigenetically controlled design of a Paramecium cell allows it to make Ca(2+) available very locally, in a most efficient way, along predetermined trafficking pathways, including regulation of exocytosis, endocytosis, phagocytosis and recycling phenomena. The activity of cilia is also regulated by Ca(2+), yet independently from any CRCs, by de- and hyperpolarisation of the cell membrane potential.

Early evolution of the eukaryotic Ca2+ signaling machinery: conservation of the CatSper channel complex

Calcium signaling is one of the most extensively employed signal transduction mechanisms in life. As life evolved into increasingly complex organisms, Ca(2+) acquired more extensive and varied functions. Here, we compare genes encoding proteins that govern Ca(2+) entry and exit across cells or organelles within organisms of early eukaryotic evolution into fungi, plants, and animals. Recent phylogenomics analyses reveal a complex Ca(2+) signaling machinery in the apusozoan protist Thecamonas trahens, a putative unicellular progenitor of Opisthokonta. We compare T. trahens Ca(2+) signaling to that in a marine bikont protist, Aurantiochytrium limacinum, and demonstrate the conservation of key Ca(2+) signaling molecules in the basally diverging alga Cyanophora paradoxa. Particularly, our findings reveal the conservation of the CatSper channel complex in Au. limacinum and C. paradoxa, suggesting that the CatSper complex likely originated from an ancestral Ca(2+) signaling machinery at the root of early eukaryotic evolution prior to the unikont/bikont split.

Evolution of TRP channels inferred by their classification in diverse animal species

The functions of TRP channels have primarily been characterized in model organisms within a limited evolutionary context. We thus characterize the TRP channels in choanoflagellate, sponge, Cnidaria, Lophotrochozoa, and arthropods to understand how they emerged during early evolution of animals and have changed during diversification of various species. As previously reported, five metazoan TRP subfamily members (TRPA, TRPC, TRPM, TRPML, and TRPV) were identified in choanoflagellates, demonstrating that they evolved before the emergence of multicellular animals. TRPN was identified in Hydra magnipapillata, and therefore emerged in the last common ancestor of Cnidaria-Bilateria. A novel subfamily member (TRPVL) was identified in Cnidaria and Capitella teleta, indicating that it was present in the last common ancestor of Cnidaria-Bilateria but has since been lost in most bilaterians. The characterization of arthropod TRP channels revealed that Daphnia pulex and insects have specifically expanded the TRPA subfamily, which diverged from the ancient TRPA1 channel gene. The diversity of TRPA channels except TRPA1 was detectable even within a single insect family, namely the ant lineage. The present study demonstrates the evolutionary history of TRP channel genes, which may have diverged in conjunction with the specific habitats and life histories of individual species.

Evolutionary insights into premetazoan functions of the neuronal protein homer

Reconstructing the evolution and ancestral functions of synaptic proteins promises to shed light on how neurons first evolved. The postsynaptic density (PSD) protein Homer scaffolds membrane receptors and regulates Ca²⁺ signaling in diverse metazoan cell types (including neurons and muscle cells), yet its ancestry and core functions are poorly understood. We find that the protein domain organization and essential biochemical properties of metazoan Homer proteins, including their ability to tetramerize, are conserved in the choanoflagellate Salpingoeca rosetta, one of the closest living relatives of metazoans. Unlike in neurons, Homer localizes to the nucleoplasm in S. rosetta and interacts directly with Flotillin, a protein more commonly associated with cell membranes. Surprisingly, we found that the Homer/Flotillin interaction and its localization to the nucleus are conserved in metazoan astrocytes. These findings suggest that Homer originally interacted with Flotillin in the nucleus of the last common ancestor of metazoans and choanoflagellates and was later co-opted to function as a membrane receptor scaffold in the PSD.

Calcium entry in Toxoplasma gondii and its enhancing effect of invasion-linked traits

During invasion and egress from their host cells, Apicomplexan parasites face sharp changes in the surrounding calcium ion (Ca(2+)) concentration. Our work with Toxoplasma gondii provides evidence for Ca(2+) influx from the extracellular milieu leading to cytosolic Ca(2+) increase and enhancement of virulence traits, such as gliding motility, conoid extrusion, microneme secretion, and host cell invasion. Assays of Mn(2+) and Ba(2+) uptake do not support a canonical store-regulated Ca(2+) entry mechanism. Ca(2+) entry was blocked by the L-type Ca(2+) channel inhibitor nifedipine and stimulated by the increase in cytosolic Ca(2+) and by the specific L-type Ca(2+) channel agonist Bay K-8644. Our results demonstrate that Ca(2+) entry is critical for parasite virulence. We propose a regulated Ca(2+) entry mechanism activated by cytosolic Ca(2+) that has an enhancing effect on invasion-linked traits.

The genomic and cellular foundations of animal origins

The first animals arose more than six hundred million years ago, yet they left little impression in the fossil record. Nonetheless, the cell biology and genome composition of the first animal, the Urmetazoan, can be reconstructed through the study of phylogenetically relevant living organisms. Comparisons among animals and their unicellular and colonial relatives reveal that the Urmetazoan likely possessed a layer of epithelium-like collar cells, preyed on bacteria, reproduced by sperm and egg, and developed through cell division, cell differentiation, and invagination. Although many genes involved in development, body patterning, immunity, and cell-type specification evolved in the animal stem lineage or after animal origins, several gene families critical for cell adhesion, signaling, and gene regulation predate the origin of animals. The ancestral functions of these and other genes may eventually be revealed through studies of gene and genome function in early-branching animals and their closest non-animal relatives.

Phylogenetic analysis of CDK and cyclin proteins in premetazoan lineages

Background

The molecular history of animal evolution from single-celled ancestors remains a major question in biology, and little is known regarding the evolution of cell cycle regulation during animal emergence. In this study, we conducted a comprehensive evolutionary analysis of CDK and cyclin proteins in metazoans and their unicellular relatives.

Results

Our analysis divided the CDK family into eight subfamilies. Seven subfamilies (CDK1/2/3, CDK5, CDK7, CDK 20, CDK8/19, CDK9, and CDK10/11) are conserved in metazoans and fungi, with the remaining subfamily, CDK4/6, found only in eumetazoans. With respect to cyclins, cyclin C, H, L, Y subfamilies, and cyclin K and T as a whole subfamily, are generally conserved in animal, fungi, and amoeba Dictyostelium discoideum. In contrast, cyclin subfamilies B, A, E, and D, which are cell cycle-related, have distinct evolutionary histories. The cyclin B subfamily is generally conserved in D. discoideum, fungi, and animals, whereas cyclin A and E subfamilies are both present in animals and their unicellular relatives such as choanoflagellate Monosiga brevicollis and filasterean Capsaspora owczarzaki, but are absent in fungi and D. discoideum. Although absent in fungi and D. discoideum, cyclin D subfamily orthologs can be found in the early-emerging, non-opisthokont apusozoan Thecamonas trahens. Within opisthokonta, the cyclin D subfamily is conserved only in eumetazoans, and is absent in fungi, choanoflagellates, and the basal metazoan Amphimedon queenslandica.

Conclusions

Our data indicate that the CDK4/6 subfamily and eumetazoans emerged simultaneously, with the evolutionary conservation of the cyclin D subfamily also tightly linked with eumetazoan appearance. Establishment of the CDK4/6-cyclin D complex may have been the key step in the evolution of cell cycle control during eumetazoan emergence.

What do we really know and what do we need to know: some controversies, perspectives, and surprises

TRP channels comprise one of the most rapid growing research topics in ion channel research, in fields related to ion channels including channelopathies and translational medicine. We provide here a critical survey on our current knowledge of TRP channels and highlight some of the still open or controversial questions. This comprises questions related to evolution of TRP channels; biophysics, i.e., permeation; pore properties and gating; modulation; the still-elusive 3D structure; and channel subunits but also their role as general sensory channels and in human diseases. We will conclude that our knowledge on TRP channels is still at the very beginning of an exciting research journey.

Loss of IP3 receptor function in neuropeptide secreting neurons leads to obesity in adult Drosophila

BACKGROUND

Intracellular calcium signaling regulates a variety of cellular and physiological processes. The inositol 1,4,5 trisphosphate receptor (IP3R) is a ligand gated calcium channel present on the membranes of endoplasmic reticular stores. In previous work we have shown that Drosophila mutants for the IP3R (itprku) become unnaturally obese as adults with excessive storage of lipids on a normal diet. While the phenotype manifests in cells of the fat body, genetic studies suggest dysregulation of a neurohormonal axis.

RESULTS

We show that knockdown of the IP3R, either in all neurons or in peptidergic neurons alone, mimics known itpr mutant phenotypes. The peptidergic neuron domain includes, but is not restricted to, the medial neurosecretory cells as well as the stomatogastric nervous system. Conversely, expression of an itpr+ cDNA in the same set of peptidergic neurons rescues metabolic defects of itprku mutants. Transcript levels of a gene encoding a gastric lipase CG5932 (magro), which is known to regulate triacylglyceride storage, can be regulated by itpr knockdown and over-expression in peptidergic neurons. Thus, the focus of observed itpr mutant phenotypes of starvation resistance, increased body weight, elevated lipid storage and hyperphagia derive primarily from peptidergic neurons.

CONCLUSIONS

The present study shows that itpr function in peptidergic neurons is not only necessary but also sufficient for maintaining normal lipid metabolism in Drosophila. Our results suggest that intracellular calcium signaling in peptidergic neurons affects lipid metabolism by both cell autonomous and non-autonomous mechanisms.

Calcium signalling and calcium channels: evolution and general principles

Calcium as a divalent cation was selected early in evolution as a signaling molecule to be used by both prokaryotes and eukaryotes. Its low cytosolic concentration likely reflects the initial concentration of this ion in the primordial soup/ocean as unicellular organisms were formed. As the concentration of calcium in the ocean subsequently increased, so did the diversity of homeostatic molecules handling calcium. This includes the plasma membrane channels that allowed the calcium entry, as well as extrusion mechanisms, i.e., exchangers and pumps. Further diversification occurred with the evolution of intracellular organelles, in particular the endoplasmic reticulum and mitochondria, which also contain channels, exchanger(s) and pumps to handle the homeostasis of calcium ions. Calcium signalling system, based around coordinated interactions of the above molecular entities, can be activated by the opening of voltage-gated channels, neurotransmitters, second messengers and/or mechanical stimulation, and as such is all-pervading pathway in physiology and pathophysiology of organisms.

Intracellular calcium channels in protozoa

Ca(2+)-signaling pathways and intracellular Ca(2+) channels are present in protozoa. Ancient origin of inositol 1,4,5-trisphosphate receptors (IP3Rs) and other intracellular channels predates the divergence of animals and fungi as evidenced by their presence in the choanoflagellate Monosiga brevicollis, the closest known relative to metazoans. The first protozoan IP3R cloned, from the ciliate Paramecium, displays strong sequence similarity to the rat type 3 IP3R. This ciliate has a large number of IP3- and ryanodine(Ry)-like receptors in six subfamilies suggesting the evolutionary adaptation to local requirements for an expanding diversification of vesicle trafficking. IP3Rs have also been functionally characterized in trypanosomatids, where they are essential for growth, differentiation, and establishment of infection. The presence of the mitochondrial calcium uniporter (MCU) in a number of protozoa indicates that mitochondrial regulation of Ca(2+) signaling is also an early appearance in evolution, and contributed to the discovery of the molecular nature of this channel in mammalian cells. There is only sequence evidence for the occurrence of two-pore channels (TPCs), transient receptor potential Ca(2+) channels (TRPCs) and intracellular mechanosensitive Ca(2+)-channels in Paramecium and in parasitic protozoa.

Themes and variations in ER/SR calcium release channels: structure and function

Calcium (Ca(2+)) release from reticular stores is a vital regulatory signal in eukaryotes. Recent structural data on large NH(2)-terminal regions of IP(3)Rs and RyRs and their tetrameric arrangement in the full-length context reveal striking mechanistic similarities in Ca(2+) release channel function. A common ancestor found in unicellular genomes underscores the fundamentality of these elements to Ca(2+) release channels.

Calcium regulation in the protozoan model, Paramecium tetraurelia

Early in eukaryotic evolution, the cell has evolved a considerable inventory of proteins engaged in the regulation of intracellular Ca(2+) concentrations, not only to avoid toxic effects but beyond that to exploit the signaling capacity of Ca(2+) by small changes in local concentration. Among protozoa, the ciliate Paramecium may now be one of the best analyzed models. Ciliary activity and exo-/endocytosis are governed by Ca(2+) , the latter by Ca(2+) mobilization from alveolar sacs and a superimposed store-operated Ca(2+) -influx. Paramecium cells possess plasma membrane- and endoplasmic reticulum-resident Ca(2+) -ATPases/pumps (PMCA, SERCA), a variety of Ca(2+) influx channels, including mechanosensitive and voltage-dependent channels in the plasma membrane, furthermore a plethora of Ca(2+) -release channels (CRC) of the inositol 1,4,5-trisphosphate and ryanodine receptor type in different compartments, notably the contractile vacuole complex and the alveolar sacs, as well as in vesicles participating in vesicular trafficking. Additional types of CRC probably also occur but they have not been identified at a molecular level as yet, as is the equivalent of synaptotagmin as a Ca(2+) sensor for exocytosis. Among established targets and sensors of Ca(2+) in Paramecium are calmodulin, calcineurin, as well as Ca(2+) /calmodulin-dependent protein kinases, all with multiple functions. Thus, basic elements of Ca(2+) signaling are available for Paramecium.

Ca2+ signalling early in evolution--all but primitive

Early in evolution, Ca(2+) emerged as the most important second messenger for regulating widely different cellular functions. In eukaryotic cells Ca(2+) signals originate from several sources, i.e. influx from the outside medium, release from internal stores or from both. In mammalian cells, Ca(2+)-release channels represented by inositol 1,4,5-trisphosphate receptors and ryanodine receptors (InsP3R and RyR, respectively) are the most important. In unicellular organisms and plants, these channels are characterised with much less precision. In the ciliated protozoan, Paramecium tetraurelia, 34 molecularly distinct Ca(2+)-release channels that can be grouped in six subfamilies, based on criteria such as domain structure, pore, selectivity filter and activation mechanism have been identified. Some of these channels are genuine InsP3Rs and some are related to RyRs. Others show some--but not all--features that are characteristic for one or the other type of release channel. Localisation and gene silencing experiments revealed widely different--yet distinct--localisation, activation and functional engagement of the different Ca(2+)-release channels. Here, we shall discuss early evolutionary routes of Ca(2+)-release machinery in protozoa and demonstrate that detailed domain analyses and scrutinised functional analyses are instrumental for in-depth evolutionary mapping of Ca(2+)-release channels in unicellular organisms.

Ryanodine receptor calcium release channels: lessons from structure-function studies

Ryanodine receptors (RyRs) are the largest known ion channels. They are Ca(2+) release channels found primarily on the sarcoplasmic reticulum of myocytes. Several hundred mutations in RyRs are associated with skeletal or cardiomyocyte disease in humans. Many of these mutations can now be mapped onto the high resolution structures of individual RyR domains and on full-length tetrameric cryo-electron microscopy structures. A closely related Ca(2+) release channel, the inositol 1,4,5-trisphospate receptor (IP3 R), shows a conserved structural architecture at the N-terminus, suggesting that both channels evolved from an ancestral unicellular RyR/IP3 R. The functional insights provided by recent structural studies for both channels will aid in the development of rationale treatments for a myriad of Ca(2+)-signaled malignancies.

NALCN ion channels have alternative selectivity filters resembling calcium channels or sodium channels

NALCN is a member of the family of ion channels with four homologous, repeat domains that include voltage-gated calcium and sodium channels. NALCN is a highly conserved gene from simple, extant multicellular organisms without nervous systems such as sponges and placozoans and mostly remains a single gene compared to the calcium and sodium channels which diversified into twenty genes in humans. The single NALCN gene has alternatively-spliced exons at exons 15 or exon 31 that splices in novel selectivity filter residues that resemble calcium channels (EEEE) or sodium channels (EKEE or EEKE). NALCN channels with alternative calcium, (EEEE) and sodium, (EKEE or EEKE) -selective pores are conserved in simple bilaterally symmetrical animals like flatworms to non-chordate deuterostomes. The single NALCN gene is limited as a sodium channel with a lysine (K)-containing pore in vertebrates, but originally NALCN was a calcium-like channel, and evolved to operate as both a calcium channel and sodium channel for different roles in many invertebrates. Expression patterns of NALCN-EKEE in pond snail, Lymnaea stagnalis suggest roles for NALCN in secretion, with an abundant expression in brain, and an up-regulation in secretory organs of sexually-mature adults such as albumen gland and prostate. NALCN-EEEE is equally abundant as NALCN-EKEE in snails, but is greater expressed in heart and other muscle tissue, and 50% less expressed in the brain than NALCN-EKEE. Transfected snail NALCN-EEEE and NALCN-EKEE channel isoforms express in HEK-293T cells. We were not able to distinguish potential NALCN currents from background, non-selective leak conductances in HEK293T cells. Native leak currents without expressing NALCN genes in HEK-293T cells are NMDG(+) impermeant and blockable with 10 µM Gd(3+) ions and are indistinguishable from the hallmark currents ascribed to mammalian NALCN currents expressed in vitro by Lu et al. in Cell. 2007 Apr 20;129(2):371-83.

Acidocalcisomes of Trypanosoma brucei have an inositol 1,4,5-trisphosphate receptor that is required for growth and infectivity

Acidocalcisomes are acidic calcium stores rich in polyphosphate and found in a diverse range of organisms. The mechanism of Ca(2+) release from these organelles was unknown. Here we present evidence that Trypanosoma brucei acidocalcisomes possess an inositol 1,4,5-trisphosphate receptor (TbIP(3)R) for Ca(2+) release. Localization studies in cell lines expressing TbIP(3)R in its endogenous locus fused to an epitope tag revealed its partial colocalization with the vacuolar proton pyrophosphatase, a marker of acidocalcisomes. IP(3) was able to stimulate Ca(2+) release from a chicken B-lymphocyte cell line in which the genes for all three vertebrate IP(3)Rs have been stably ablated (DT40-3KO) and that were stably expressing TbIP(3)R, providing evidence of its function. IP(3) was also able to release Ca(2+) from permeabilized trypanosomes or isolated acidocalcisomes and photolytic release of IP(3) in intact trypanosomes loaded with Fluo-4 elicited a transient Ca(2+) increase in their cytosol. Ablation of TbIP(3)R by RNA interference caused a significant reduction of IP(3)-mediated Ca(2+) release in trypanosomes and resulted in defects in growth in culture and infectivity in mice. Taken together, the data provide evidence of the presence of a functional IP(3)R as a Ca(2+) release channel in acidocalcisomes of trypanosomes and suggest that a Ca(2+) signaling pathway that involves acidocalcisomes is required for growth and establishment of infection.