Easy to Draw Sea Anemone Easy to Draw Spider
Natural Products Structural Diversity-II Secondary Metabolites: Sources, Structures and Chemical Biology
Frank Marí , Jan Tytgat , in Comprehensive Natural Products II, 2010
2.15.7 Sea Anemones: Distribution, Diversity, Behavior, Feeding, and Defense
Sea anemones are ocean-dwelling members of the phylum Cnidaria. They are invertebrates belonging to the class of Anthozoa. The name Cnidaria (with a silent 'c') refers to the cnidae, or nematocysts, that is, the cellular entity of the venom apparatus, which all Cnidarians possess. The phylum Cnidaria includes anemones, corals, jellyfish (including box jellyfish), and hydras. Sea anemones, named after a terrestrial flower, have a basic radial symmetry with tentacles that surround a central mouth opening. The tentacles are used to catch food and transfer it to their mouth. Each stinging capsule in the tentacles, and other parts of the sea anemone, contains a coiled hollow filament, usually barbed, heavily loaded with venom. This is used to immobilize smaller organisms, for defense against predators, and to fight territorial disputes. When triggered by mechanical or chemical stimulation, the capsule 'explodes' and drives the filament into its prey, discharging its venom.
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Sea Anemone Venom Peptides
RAYMOND S. NORTON , in Handbook of Biologically Active Peptides, 2006
INTRODUCTION
Sea anemones, in common with other members of the phylum Cnidaria (Coelenterata), possess numerous tentacles containing specialized stinging cells or cnidocytes. These stinging cells are equipped with organelles known as nematocysts that contain small threads which are forcefully everted when stimulated mechanically or chemically. Anemones use this venom apparatus in the capture of prey (crustaceans, small fish), as well as for defense against predators and in intraspecific aggression. Accordingly, they contain a variety of potent and interesting biologically active compounds, including many peptide toxins.
Peptides and proteins characterized to date from sea anemones range in molecular mass from 3 to 300 kDa [3]. Even smaller peptides have been identified, such as the tetrapeptide Antho-RF amide and pentapeptides Antho-RW amide I and II from Anthopleura elegantissima, but they have potent biological activities as neurotransmitters or neuromodulators [16] and are unlikely to play a role in venom action. The two most thoroughly characterized classes of sea anemone polypeptides are the 5-kDa toxins that act by binding to the voltage-gated sodium channel [31] and the 16–20 kDa cytolysins, known as actinoporins [1]. More recently, new classes of potassium channel blockers have also been characterized. Here I summarize the salient features of these and other less abundant sea anemone peptides. In some cases it has been established that these molecules are localized to nematocysts and are genuine venom constituents, but in others this remains to be confirmed.
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Social Recognition
M.D. Breed , in Encyclopedia of Animal Behavior, 2010
Notable Examples
Clonal Recognition in Anemones
Sea anemones compete for space on rocks or large shells, engaging in territorial battles using their stinging cells. Because asexual reproduction by fission is common in anemones, adjacent anemones are likely to be clonemates. Aggression is reduced or absent among clonemates, and intense between unrelated anemones. This self/nonself recognition system seems to function analogously to the vertebrate MHC system: internal identification of tissues is extended to an external phenotype that shapes aggressive interactions.
Vertebrates
A fascinating thread that runs through the vertebrate kin recognition literature is in the intimate linkage between MHC variation and recognition phenotypes. This association between MHC and recognition draws an obvious inference that the recognition of self extends from the internal immune identification of tissue and organs to the external environment. Given this inference, it is unsurprising that MHC differences among animals correlate with urinary odorants.
Young salmonid fish swim in schools that are predominantly full sibs. Later in life, territorial interactions among adults can occur between highly related fish that have settled on adjacent territories. The cues used in maintaining sib associations are MHC related. The prevailing argument in studies of these fish is that MHC-related phenotype matching allows territorial fish to modulate their aggression to close relatives, resulting in inclusive fitness benefits to the fish.
One of the earliest areas of exploration of kin associations was schooling tadpoles. Larvae of many species of amphibia, including frogs, toads, and salamanders, preferentially school with close kin. The underlying ecological and evolutionary benefit of kin schooling in amphibia has been elusive. As in salmonid fish, kin associations may lead to modulation of competitive interactions among related animals. Alternatively, kin selection benefits from shared vigilance and predation risk may be important. In salamanders that develop cannibalistic morphs under food deprivation, cannibals avoid consuming close relatives. This is a clear example of kin recognition facilitating a kin-selected evolutionary response.
Rodent studies, using a wide variety of species, have provided key clues to the mechanisms and function of social recognition in mammals. Classic studies in mice link the cues used in phenotypic matching to the MHC complex. Mice that differ only at MHC loci can be discriminated using phenotypic matching mechanisms. Generally speaking, rodents show strong abilities to discriminate kin from nonkin using phenotype matching. This ability is important as many rodent populations may contain previously unmet close relatives; both nepotistic (aid-giving) and mating decisions can reflect information concerning kinship. Enough is known about rodent kin recognition to reveal that expression of phenotype matching as a source of kinship information in behavioral decisions is very much affected by life history. A 2003 review of rodent kin recognition by Mateo shows that while recognition by familiarity (presumably individual recognition) is nearly universal in rodents, phenotypic matching is more restricted. Hypothetically, phenotype matching is used by those species the life histories of which are likely to bring previously unmet relatives together ( Figure 2 ).
Figure 2. Mammals, such as this coati (Nasua narica), often have variable color patterns that might be used in making individual discriminations within social groups. Their highly developed olfactory senses could provide a platform for social recognition, as well. While individual recognition is often assumed with mammalian social groups, rigorous tests of this hypothesis are only available for a few species.
Human social biology is largely structured around individual recognition and classification. Familial similarity, at least in visual cues, provides at best weak evidence of relatedness, with the notable exception of monozygotic twins. The hypothesis that kin information, including possible phenotypic matching, operates at subconscious levels and shapes human social interactions is relatively unexplored.
Unlike all of the other animals discussed in this article, birds appear not to use olfactory information in social recognition. Studies of cooperatively breeding and communally nesting birds demonstrate that social recognition via distinctive calls, such as the contact call of the long-tailed tit, carry individually distinctive information that can be learned. Thus, kin are identified by association, but as yet there is no evidence for kin recognition by phenotypic matching in birds.
Eusocial Insects
Nestmate recognition in eusocial insects usually relies on phenotype matching. Colony-specific cues are learned by young workers and that information is used in social interactions, particularly in excluding nonnestmates from the colony. Transfers among colonies of larvae or newly emerged adults are usually fully tolerated, suggesting that the cues are acquired from the surrounding environment rather than distinct productions of each worker. A key early finding was that recognition cues in social wasps are usually transmitted among workers via the colony's nesting material. In honeybees, fatty acids that serve as strengtheners in the comb wax give all workers in the colony the same odor because of contact of the workers with the comb. Some ants use compound sequestration of hydrocarbons in the postpharyngeal gland as a method of establishing a colony-level recognition odor. These collective, or gestalt, labels simplify the recognition process in colonies that may have hundreds or thousands of members.
In eusocial insects, significant progress has been made in identifying the chemical compounds composing the recognition phenotype. Numerous studies show that young workers learn phenotypes, typically odors, of other workers in the colony. Most often the odors are metabolic products of the insects themselves; as discussed earlier, the products of colony members typically combine to form a colony-level recognition signature. The most commonly used compounds are waxy materials secreted as waterproofing on the surface of the insect. These compounds are typically alkanes, methyalkanes, and alkenes, with chain lengths from 21 to 37 carbons. In some species, though, odors acquired from the environment contribute to nestmate recognition cues. Chemical analyses of surface extracts of insects often yield 20 or more compounds, but the presence of a compound does not automatically translate in the use of that compound in recognition. Experimental studies suggest that alkanes are used less as signals, perhaps because alkanes lack an easily perceived functional group or conformational feature. Multivariate analyses of gas chromatographic results can easily separate colonies, but offer no proof of which compounds are used in discriminations by the insects. Bioassays of putative recognition cues have implicated alkenes and methylalkanes (in ants and wasps), fatty acids (in honeybees), and macrocyclic lactones (in halictid bees) as recognition cues.
Phenotypic variation among colonies in the relative proportion of these compounds (correlated with genotypic variation among colonies) provides the information needed to make discriminations. In most species, young workers learn the phenotype of their colony and use this to make nestmate/nonnestmate discriminations. Young workers are flexible enough to learn the phenotype of the colony in which they emerge, even if they are not genetic members of the colony. This extends to an ability to integrate into colonies of other species: a mechanism that facilitates slave-making in ants and other types of social parasitism.
A notable exception to the use of odors in nestmate recognition is the discovery that some eusocial wasps use interindividual variation in surface markings in individual recognition and as the basis for social classifications.
Explorations of differential nepotism within colonies have generally yielded negative results. The most intensely explored question is whether honeybee queen larvae, which may be either full- or half-sisters to the workers that tend them, are preferentially treated by full-sisters. While some experiments suggest the existence of such preferential treatment, others show no such effect. The opportunity for differential nepotism exists in many species of eusocial insect, either because the colony's queen mates several times, as in the honeybee, or because the colony has several queens. Worker policing mechanisms, in which competition among worker subgroups counterbalances possible nepotism, may prevent the emergence of measurable nepotistic biases.
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Social Behavior, Cooperation, and Kinship
Michael D. Breed , Janice Moore , in Animal Behavior (Second Edition), 2016
Social Recognition in Clonal Invertebrates
Some sea anemone species engage in "wars." The anemones live in clones, with new members produced by budding of old members. When one clone comes into contact with another, the anemones use their stinging capability to "fight," and ultimately one clone comes to dominate an area. This is an example of how animals that are phylogenetically far removed from, say, mammals or birds can nonetheless show well-developed mechanisms of kin discrimination and use these mechanisms to exclude unrelated individuals from habitat patches. Highly polymorphic (hypervariable) genetic systems give members of clones distinctive signatures that differ from those of other clones. 75,76
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Amino Acids, Peptides and Proteins
Nicolas Andreotti , ... Jean-Marc Sabatier , in Comprehensive Natural Products II, 2010
5.10.4 Peptides from Sea Anemone Venoms
The sea anemones (Phylum Cnidaria, Class Anthozoa, Order Actiniaria) are toxic species that are not lethal in humans ( Figure 4 ). They possess numerous tentacles containing cnidocytes, which are specialized stinging cells involved in defense against predators, intraspecific aggression, and capture of prey (fish and crustaceans). The cnidocytes are equipped with capsule-like organelles that are referred to as nematocysts. These are capable of everting upon either chemical or mechanical stimulations. The venom is basically a complex mixture of compounds with various functions and pharmacological activities, 37,38 such as protease inhibitors (including Kunitz and cysteine proteinase inhibitors), neurotransmitters or neuromodulators, phospholipases A2, epidermal growth factor (EGF)-like peptides (gigantoxin), actinoporins or cytolysins (16–20 kDa), and finally ion channel modulators. 39 The cytolysins/actinoporins are involved in pore formation in the membrane lipid bilayer leading to osmotic imbalance promoting cell lysis, 40 whereas ion channel-acting toxins target voltage-gated and Ca2+-activated K+ channels (K+ channel blockers of 5 kDa) as well as H+-dependent and voltage-gated Na+ channels (Na+ channel modulators). 41 The sea anemone toxins acting on K+ channels are classified into two categories depending on their amino acid sequence identities. In the first category, toxins (e.g., ShK, 42 HmK, 43 BgK, 44 kaliseptine 45 ) are folded according to combined helices of α and/or 310 type(s). In the second category, toxins (e.g., BDS-I and II blood depressing substances, 46 APETx1 47 ) are folded with an arrangement of β-sheets (β-defensin type). As for other venomous animal species, a number of characterized sea anemone toxins (e.g., ShK and BgK) are of therapeutic value as immunomodulators (immunosuppressants to treat autoimmune diseases). 48,49 These toxins block the voltage-gated Kv1.3 channel, which is critical for the activation of effector memory T cells, and a targeted channel in the prevention of graft rejection and treatment of (T-cell-mediated) autoimmune diseases such as multiple sclerosis. 18
Figure 4. Venomous sea anemones. (a) Sun anemone (Stichodactyla helianthus); (b) Bunodosoma granulifera; (c) snakelocks anemone (Anemonia viridis); (d) starburst anemone (Anthopleura sola); (e) discharged nematocysts; (f) a Nomarski micrograph of a ruthenium red-stained nematocyst from Aiptasia pallid. The red dye stains the polyanionic venom proteins found inside the partially discharged nematocyst. Photos from (a) to (f), by F. Charpin (http://www.Florent.us), Pline (Creative Commons Attribution ShareAlike License), M. Zinkova (Creative Commons Attribution ShareAlike License), J. Engman (The cnidarian lab), and D. Brand (Public domain), respectively.
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Chemical Ecology
Masaki Kita , ... Daisuke Uemura , in Comprehensive Natural Products II, 2010
4.06.5.1 Sea Anemone
The colonial sea anemone Anthopleura elegantissima responds with characteristic contraction to a pheromone released by wounded conspecifics. This alarm response is highly characteristic, includes rapid bending and shortening of the tentacles and depression of the oral disk. In 1975, by extensive ion exchange column chromatography, (3-carboxy-2,3-dihydroxy-N,N,N-trimethyl)-1-propanaminium chloride ( 50 ) was isolated as a pure crystalline substance. 116 It showed alarm pheromone activity with a median concentration of 0.35 nmol l−1 and was named anthopleurine. Comparison of spectral data between natural and synthetic compounds revealed that anthopleurine had a structure of 4-amino-4-deoxy-l-threonic acid betaine hydrochloride. 117
A. elegantissima is a preferred prey of the aeolid nudibranch Aeolidia papillosa. Interestingly, anthopleurine ( 50 ) remains in the tissue of nudibranch, and leakage of the pheromone causes the alarm response in other anemone individuals for several days. 118,119 Consequently, the predator may help in transmission of the alarm pheromone, which can reduce the severity of predation on Anthopleura.
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Phanerozoic evolution—Ediacaran biota
Nelson R. Cabej , in Epigenetic Mechanisms of the Cambrian Explosion, 2020
The genome
The starlet sea anemone Nematostella vectensis (class Anthozoa, phylum Cnidaria, a sister group of Bilateria) may be another "living fossil". Recent sequencing of its complex genome has shown that it has an estimated complement of 18,000 protein-coding genes. Its repertoire, structure, and organization is very conserved when compared with that of vertebrates but surprisingly different from that of fruit flies and nematodes, which have lost many genes and introns and have experienced genome rearrangements, indicating the genome of their common ancestor also was a complex genome (Putnam et al., 2007). The similarity of exons–intron structure of N. vectensis and vertebrates indicates that their common ancestor's eumetazoan genome has been intron-rich. Putative Hox genes in N. vectensis show a spatial pattern of expression that suggests a role of theirs in the embryonic development of their common ancestor. Based on their studies on the Nematostella genome and its conserved features in extant eumetazoans, Putnam et al. (2007) have described the common ancestor of cnidarians and vertebrates as an organism with "flagellated sperm, development through a process of gastrulation, multiple germ layers, true epithelia lying upon a basement membrane, a lined gut (enteron), a neuromuscular system, multiple sensory systems, and fixed body axes" (Putnam et al., 2007).
It is suggested that alternative splicing in cnidarians (Hydra and Nematostella) occurs, but its impact on the proteome in cnidarians is unclear (Steele et al., 2011). A clear correlation is observed, in the course of the metazoan evolution, between the proportion of the genes regulated by alternative splicing and the increase in size of the CNS.
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Comparative Reproduction
Shinya Shikina , Ching-Fong Chang , in Encyclopedia of Reproduction (Second Edition), 2018
Introduction
General Characteristics of Cnidarians
The Cnidaria phylum includes sea anemones, Hydra (sessile solitary species), corals, sea fans, sea whips (colonial species) and jellyfish (free-swimming species) that live in aquatic (mostly marine) environments (Ruppert et al., 2003). Cnidarians appear to have diverse morphologies, but all members of this phylum universally possess stinging cells called cnidoblasts. Cnidoblasts are unique to the animals of this phylum and are used to define the phylum. The bodies of individuals in the Cnidaria phylum exhibit radial or biradial symmetry, and almost all tissues have a basic structure composed of two cellular layers, the epidermis (outside) and the gastrodermis (inside). A gelatinous-like substance called the mesoglea is present between the two layers of the tissues and maintains the integrity of the tissues and the body. In addition, cnidarian bodies are relatively simple, and well-organized organs or systems that are found in vertebrates are not present in cnidarians, such as vascular and central nervous systems. Cnidarians only have a passive transport system that depends on diffusion; a simple nerve net; some functionally defined tissues such as tentacles for predation, attack and defense mechanisms; and a sac-like gastrovascular cavity used for digestion and absorption.
Body Forms
Cnidarians are present as either polyps or medusae as adults and planulae as larvae. Sea anemones are examples of the polyp forms, while jellyfish are examples of the medusa forms. In the current classification, the Cnidaria phylum is divided into two major groups, Anthozoa and Medusozoa, based on the structure of polyps and the existence of a medusa stage in the life cycles ( Fig. 1). Anthozoa is a class that consists of two subclasses, Hexacorallia (stony corals and sea anemones) and Octocorallia (soft corals and red corals). Medusozoa is a subphylum that consists of four classes, Hydrozoa (Hydra), Scyphozoa (true jellyfish), Cubozoa (box jellyfish), and Staurozoa (stalked jellyfish) (Technau and Steel, 2011).
Fig. 1. Phylogenic relationships of the classes in the phylum Cnidaria. The phylum Cnidaria is divided into two major groups, Anthozoa and Medusozoa. Anthozoa is a class that consists of two subclasses, Hexacorallia (stony corals and sea anemones) and Octocorallia (soft corals and red corals). Medusozoans is a subphylum that consists of four classes, Hydrozoa (freshwater Hydra), Scyphozoa (true jellyfish), Cubozoa (box jellyfish), and Staurozoa (stalked jellyfish).
Modified from Technau, U., Steele, R.E., 2011. Evolutionary crossroads in developmental biology: Cnidaria. Development 138, 1447–1458.Life Cycle
In anthozoans, the typical life cycle begins with the development of fertilized eggs into free-swimming larvae known as planula, which in turn transform into polyps that can undergo asexual reproduction to produce new polyps or form colonies. When the polyps (or colonies) reach certain ages or sizes, they begin to produce gametes for sexual reproduction under suitable environmental conditions (Fig. 2(A)). The medusa stage does not occur in the anthozoan life cycle. Many of the Medusozoa species alternate between the polyp and medusa stages during their life cycles. Similar to anthozoans, the typical life cycle of medusozoans begins with the development of fertilized eggs into planulae, which in turn transform into polyps. However, medusozoan polyps can produce medusae as well as polyps by asexual reproduction. Sexual reproduction generally occurs in the medusa stage (Fig. 2(B)). Considerable variations or modifications in the life cycle can be observed in many medusozoans.
Fig. 2. Schematic representation of cnidarian life cycle. (A) A typical life cycle of anthozoan. (B) A typical life cycle of medusozoan.
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Ion Channels: Channel Chemical Biology, Engineering, and Physiological Function
Ruiming Zhao , Steve A.N. Goldstein , in Methods in Enzymology, 2021
1 Introduction
Venom peptide toxins from sea anemone, scorpion, spider, snake and cone snail that act on ion channels either physically occlude the conduction pore or modify channel gating ( Kalia et al., 2015). Peptide toxins are powerful tools for studying the structure and function of ion channels (Bosmans, Martin-Eauclaire, & Swartz, 2008; Cordeiro et al., 2019; Goldstein, Pheasant, & Miller, 1994; Jiang et al., 2021; MacKinnon & Miller, 1989), and are used in medical diagnosis and therapy (Wulff, Christophersen, Colussi, Chandy, & Yarov-Yarovoy, 2019). The inspiration for tethered peptide toxins (T-toxin) was lynx1, an endogenous modulator of nicotinic acetylcholine receptors (nAChR) that is homologous to snake α-neurotoxins and naturally-tethered at the cell surface of mouse neurons via a glycosylphosphatidylinositol (GPI) anchor (Miwa et al., 1999). Subsequent to the description of lynx1, peptide toxins from a spider and a cone snail were purposefully tethered by encoding the toxin gene in-frame with the GPI signal and processing sequences of lynx1 and shown to express and modulate the function of voltage-gated sodium channels and calcium channels, respectively (Ibanez-Tallon et al., 2002, 2004). T-toxins expression constructs allow the encoded peptide toxins to pass through the protein secretory pathway, after which the N-terminal signal peptide sequence is cleaved by endogenous proteases, so the unadorned peptide is exposed on the surface, and the lynx1 GPI targeting sequence is removed so the toxin is linked covalently to GPI, anchoring the peptide to the extracellular leaflet of the plasma membrane in stable fashion. T-toxins appear to diffuse without limitation (on both poles of Xenopus oocytes) so they can interact with the external portions of surface ion channels, providing a new way to the regulate function in vitro and in vivo (Ibanez-Tallon & Nitabach, 2012). To-date, venom peptides from spiders, scorpions, snakes and sea anemone have been crafted as T-toxins and tested for potency on various membrane receptors (Gui et al., 2014; Ibanez-Tallon & Nitabach, 2012; Rupasinghe et al., 2020; Zhao, Dai, Mendelman, Chill, & Goldstein, 2020).
T-toxins have several useful attributes compared to their free peptide analogs. First, T-toxins can speed and reduce the costs of studies of toxin variants allowing their production by simple mutation of the expression construct; this is especially useful in scanning and, thereafter, in the iterative study of positions where examining successive hypothesis otherwise requires synthesis and purification of peptides, decreasing the time to study scores of variants from many months to just weeks. Second, T-toxins can be expressed at high levels close to their targets, concentrations that cannot be achieved for free peptides in solution, and thereby permit study of toxins with low affinities or to efficiently silent the target channels. Third, T-toxins can be selectively delivered to specific cell populations by viral vectors. Fourth, T-toxins tolerate engineering to incorporate additional useful modules, such as fluorescent proteins (Auer et al., 2010).
Thus, T-toxins have been successfully employed to silence neurotransmission (Auer et al., 2010), to dissect mammalian circuits in vivo (Choi et al., 2009), and to identify a natural peptide inhibitor of TRPA1 channels and study its residues by mutagenesis (Gui et al., 2014). Recently, we extended the use of T-toxins to scanning mutagenesis of two native sea anemone toxins (HmK and ShK) and one designer toxin (Hui1) that block the ion conduction pores in two K+ channels (KcsA and Kv1.3) to characterize the biophysical parameters of toxin blockade, including characterization of binding affinity and kinetic parameters of inhibition and to identify and vary residues important for binding, showing the approach can be used to help define modes and mechanisms of toxin action (Zhao et al., 2020).
For these reasons, we now employ T-toxins to scan peptide toxins and then confirm inferred mechanisms with a few soluble peptides. The most apparent limitations of the approach include the need to validate that the parent toxin acts similarly in soluble and tethered form, that surface expression of variants does not verify correct folding (although the same challenge exists for soluble congeners, HPLC purification and MS almost always provide forewarning), and an anticipated problem we have yet to observe that since the toxin C-terminus is tethered binding that requires its free exposure for interaction will be precluded.
In Sections 2–5, we describe methods for (1) determining T-toxin equilibrium affinity via quantification of current inhibition by two-electrode voltage clamp (TEVC) and estimation of local T-toxin concentration by enzyme-linked immunosorbent assay (ELISA) and single-molecule total internal reflection fluorescence (smTIRF) microscopy; (2) determining T-toxin association rate after rapid release from tetraethylammonium (TEA) blockade, and calculation of the T-toxin dissociation rate from the measured association rate and equilibrium affinity; (3) identification of T-toxin residues critical to binding via scanning mutagenesis; and (4) study of T-toxin blocking mechanism. We demonstrate the methods with HmK, a sea anemone type I (SAK1) toxin, and the pore of KcsA carried in a Shaker channel (KcsA-Shaker).
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Glial Cells: Invertebrate☆
J.A. Coles , in Reference Module in Biomedical Sciences, 2015
The Anatomical Distribution of Glial Cells in Invertebrates
Cnidaria: Nervous Systems without Glia
The phylum Cnidaria includes sea anemones, hydra, and jellyfish, all of which have nervous systems without glial cells. One or two ring nerves are found in cubozoans (box jellyfish) and hydrozoan jellyfish. In addition, the small (≈ 1 cm diameter) cubozoan jellyfish, notably Tripedalia cystophora, have elaborate visual systems with up to more than 24 eyes, each with advanced neural circuitry composed of many hundreds of neurons. Despite the absence of glia, these circuits allow rapid and complex hunting behavior.
Bilateria (Triploblastica)
Glial cells are reported in all Bilateria species that have been examined.
Nematoda
The hermaphrodite form of the nematode Caenorhabditis elegans ( Figure 2(a) ) has 56 glial cells. Each of the 24 sensilla has a sheath glial cell and a socket glial cell (or two socket cells for the phasmids). In addition, six glial cells, labeled GLR cells, extend sheetlike projections that contact muscle arms in the head. These associate with 302 neurons. The glia create a large part of the architecture of each sensillum. In the case of the amphid sensillum, the socket glial cell has a long process, which follows the dendrites of 12 neurons (only one of which is shown in ( Figure 1(b) ). At the cuticle, it takes a doughnut shape to create a pocket open to the exterior. At the inner end of the pocket, the sheath cell takes the form of a sieve with eight channels through which pass the sensory cilia of eight of the neurons. Laser ablation of single cells is contributing to understanding how the cells act together to form the sensillum.
Figure 2. Glia in the nematode Caenorhabditis elegans. (a) Schematic of an adult hermaphrodite, approximately 1 mm long, showing the outline of the pharynx. Some neuronal tracts are shown in green, and one amphid sheath glial cell is shown in red. (b) Enlarged view of the anterior region. The nerve ring (green) is a neuropil where most synaptic interactions between neurons occur. Elements of one amphid sensillum are shown: one of the 12 neuronal dendrites (green), the sheath glial cell (red), and the socket glial cell (blue).
Adapted from figures in www.wormatlas.org and from Procko, C, Lu Y, Shaham S, (2011) Glia delimit shape changes of sensory neuron receptor endings in C. elegans. Development 138: 1371–1381.Annelida
In leeches such as Hirudo medicinalis, the ventral nerve cord has 23 ganglia, each containing the cell bodies of monopolar neurons grouped in six packets. The neuronal somata and initial axon segments of each packet lie within a packet glial cell ( Figure 1 ). Two further giant neuropil glial cells invest the neuropil in the center of the ganglion. The axons connecting neighboring ganglia are enclosed in a glial cell sheath. There is no myelin (here or in any invertebrate); the larger axons are separated one from another by glial processes, but smaller axons are in bundles with no intervening glia.
The neuropil glial cell has a cell body more than 100 μm across and extensive processes; it is probably the largest of all glial cells. For this reason, it was chosen in the 1960s by Stephen Kuffler and colleagues for the first systematic study of glial cell physiology. They found that the ionic basis of the membrane potential depended more strongly on potassium ions than is the case for neurons. They showed that glucose (and other compounds) can diffuse freely from outside the ganglion to the surfaces of the neurons, and they concluded that these leech glial cells do not normally supply metabolic substrate to neurons. This is certainly not universally the case, and it may be exceptional. Over subsequent decades, it has been shown that these glial cells do, in fact, do a lot of interesting things (see Figure 3(a) ).
Figure 3. Invertebrate neuron–glia interactions are divers and usually have parallels in vertebrates. (a) Some of the receptors, channels, and transporters found on the giant neuropil glial cell of the leech. iGlu, ionotropic (AMPA/kainate) glutamate receptor; mGlu, metabotropic glutamate receptor; nACh, nicotinic acetylcholine receptor; 5-HTR, receptor for 5-hydroxytryptamine; MMR, receptor for myomodulin, a neurotransmitter in leech. (b) Signaling from giant axon to Schwann cell in the crayfish. Propagation of action potentials along the axon causes release of N-acetylaspartylglutamate (NAAG). NAAG is split into N-acetylaspartate (NAA) and glutamate (glu) by the extracellular enzyme glutamate carboxypeptidase II (GCPII). Glutamate acts on metabotropic glutamate receptors, which activate parallel signaling pathways, including signaling through extracellular acetylcholine. (c) On glia of Drosophila brain, glutamate transporters of the EAAT1 class reduce oxidative stress. In addition, the glia contain a cysteine protease, cathepsin, which is normally inhibited by Cer. Approximately 3 h after conditioned learning, the establishment of long-term memory involves a decrease in Cer concentration leading to increased cathepsin activity.
(a) Modified from a scheme kindly supplied by JW Deitmer. (b) Based on results in Urazaev AK, Grossfeld RM, and Lieberman EM (2005) Regulation of glutamate carboxypeptidase II hydrolysis of N-acetylaspartylglutamate (NAAG) in crayfish nervous tissue is mediated by glial glutamate and acetylcholine receptors. Journal of Neurochemistry 93: 605–610 and papers cited therein. (c) Based on results in Rival T, Soustelle L, Strambi C, Besson MT, Iche M, and Birman S (2004) Decreasing glutamate buffering capacity triggers oxidative stress and neuropil degeneration in the Drosophila brain. Current Biology 14: 599–605, and Comas D, Petit F, and Preat T (2004) Drosophila long-term memory involves regulation of cathepsin activity. Nature 430: 460–463.Insecta
In the central nervous system of insects (ventral nerve cord and brain) there are three main classes of glia: surface (or subperineurial) glia form a sheath around the entire central nervous system ( Figure 4 ), cortex glia provide a matrix in which are embedded neuronal cell bodies, and neuropil-associated glia send processes into the neuropil. Unlike the neuropil glia of the leech ( Figure 1 ), the cell bodies of insect neuropil-associated glia are numerous (25–30 in each neuromere segment of Drosophila) and are located outside the neuropil.
Figure 4. A glial blood–brain barrier. The ventral nerve chord (central nervous system) of the cockroach Periplaneta americana. (a) The connectives running between the ganglia are avascular and contain giant axons and small axons (all unmyelinated). (b) The neural lamella provides a connective tissue sheath. Some work suggests that the main barrier to extracellular diffusion of molecules to the axons is formed by tight junctions between the perineurial sheath cells, which are sometimes classed as glial cells. However, work on Drosophila suggests that these cells are of mesodermal origin and also that the barrier is constituted by the underlying glial cells.
Adapted from Treherne JE and Scholfield PK (1981) Mechanisms of homeostasis in the central nervous system of an insect. Journal of Experimental Biology 95: 61–73.In addition, several types of glia in the periphery can be defined, such as the antennal nerve glial cells of Manduca and the outer pigment cells of Apis retina ( Figure 6 ).
Urochordata
Ascidiacea (sessile tunicates)
The nervous systems of the pelagic larval forms of sea squirts (Urochordata and Ascidia) are laid out much like those of vertebrates. Nonneural cells, usually called ependymal cells, outnumber neurons. When the larvae become sessile, the brain is remodeled, becoming an order of magnitude larger. In this adult form, glial cells may be absent: the axons are unsheathed, but a few nonneural cells are present within the brain.
Thaliacea (pelagic tunicates and salps)
There are no glia in the peripheral nervous system. They are not in evidence in the brain: this is covered by a two-layered epithelium, but the cells of this differ markedly from most glia in that they conduct action potentials.
Cephalochordata
The amphioxus or lancelet, Branchiostoma lanceolatum, has glial cells. Thin glial lamellae containing gliofilaments enclose the rhabdom of the Joseph photoreceptor cells.
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