cnidaria - units.it

CNIDARIA • Radially symmetric

• Dimorphic: two body forms (except for Anthozoa)

– Polyp

• Sessile, cylindrical body, ring of tentacles on oral surface

– Medusa

• Flattened, mouth-down version of polyp

• Free-swimming

• ~10,000 species

• Marine animals

• Only few freswater (Hydra)

CNIDARIA • Basic body plan of ALL cnidarians

– Sac with a central digestive compartment (GVC)

– Single opening serving as both mouth and anus

– Ring of tentacles on oral surface

– Ectoderm and endoderm separated by acellular mesoglea

CNIDARIA

• All cnidarians are carnivores

– Tentacles capture and push food into mouth

– Tentacles are armed with stinging cells

• Cnidoblasts / cnidocytes

• Contain capsules (nematocysts)

– The tentacle is stimulated (on “trigger”)

– Nematocyst is discharged on the prey

• Some species produce toxins (Injection of the toxin)

CNIDARIA

CNIDARIA

• Different types of nematocystis:

• Generic nematocyst (all)

– Double-walled capsule with toxic mixture of phenols + proteins

– Spines or barbs for penetration, anchor in victim

• Spirocyst (Anthozoa)

– Spring-like mechanism

– Adhesive tubules wrap around and stick to victim

• Ptychocyst (tube anemones)

– Sticky nematocyst

CNIDARIA

• Prey is forced into the GVC

• Extracellular digestion begins (coelenteron)

– Enzymes secreted into GVC

• Intracellular digestion completes process

– Partially digested food engulfed by endoderm cells

Phylum CNIDARIA • Anthozoa

– Subclass Hexacorallia

– Subclass Octocorallia

• Cubozoa

• Hydrozoa

– Subclass Hydroidolina

– Subclass Trachylina

• Scyphozoa

• Staurozoa

CLASS ANTHOZOA • Anemones and corals

• Lack medusa form

• Gonads endodermal (found in the gastrodermis).

• Form colonies

• Exclusively marine

CLASS ANTHOZOA

• Subclass Hexacorallia

– Hexacorallia = 6-fold symmetry (6x tentacles)

– Both anemone and hard corals

• Hard corals precipitate Ca3(CO3)2 from sea water to produce skeletal structures (coral reefs): exoskeleton

• Contain symbiotic dinoflagellates

– Zooxanthella

CLASS ANTHOZOA • Zooxanthella:

– Unicellular microalgae able to live in symbiosis with marine invertebrates

– The majority belongs to the genus Symbiodinium and Amphidinium

– Photosynthetic organisms (chlorophyll a and c) and the dinoflagellate pigments peridinin and diadinoxanthin.

– Provide their host with the organic carbon products of photosynthesis (up to 90% of their host's energy needs for metabolism, growth and reproduction)

– In return, they receive nutrients

CLASS ANTHOZOA

• Subclass Octacorallia

– Octacorallia = 8-fold symmetry (8x tentacles)

– Soft corals

• Form complex tube-like skeletal structures

• Lack zooxanthella

Class Anthozoa

Subclass Hexacorallia

Order Antipatharia

Subclass Octacorallia

Order Zoanthidae

Order Actinaria

Order Scleractinia

Zoanthids Sea anemones Stony corals Precious black coral Wire coral

Sea fans Sea whips Sea pens

Organ pipe coral Precious gold coral

CLASS ANTHOZOA – TOXIC SPECIES • Hard corals:

– Hard corals can cause abrasion injuries if a swimmer simply brushes against them

– Certain coral colonies also possess stinging nematocysts which can leave a rash if touched (i.e. Goniopora)

CLASS ANTHOZOA – TOXIC SPECIES • Soft corals:

– Zoanthus and Palythoa

(Indo Pacific)

• Anemones:

– Actinia equina (eastern Atlantic)

– Condylactis gigantea (Red Sea, death)

CLASS CUBOZOA

• Name means «cube animals»

• Box-shape medusa stage

• Generally live in tropical oceans

• They are equipped with highly toxic cnidocytes

CLASS CUBOZOA

• Polyps and medusae stages, but medusae dominate with polyp stage reduced.

– polyp stage develops directly into medusa.

• Tetramerous radial symmetry; bell cube-shaped with tentacles arising from each corner.

• Gonads endodermal

• Includes some 15 marine species.

• Includes box jellies and sea wasps.

CLASS CUBOZOA – TOXIC SPECIES

• The most dangerous cnidarians

– Most stings result in only a short-lived burning sensation but can be dangerous if the swimmer has a severe allergic reaction

– Some species of jellyfish can be fatal

• Several species of box jellyfish have been implicated in human deaths:

– Chiropsalmus quadrigatus

(20–50 deaths/year in the Philippines)

CLASS CUBOZOA – TOXIC SPECIES

• Chironex fleckeri (tropical waters of Australia):

– The most venomous of all marine creatures: respiratory failure may occur within few minutes

– Size of human head with tentacles up to almost 3 meters long

– 5-7 meters of tentacles can deliver enough poison to kill in <5 min

CLASS HYDROZOA

• Most varied and derived of the cnidarian groups

– Includes freshwater species (Hydra spp)

• Polyps and medusa stage (polyp is dominant)

– Examples of polyp-only forms (hydra)

– Examples of medusa-only forms

• Tetramerous radial symmetry

• Gonads are ectodermal (found in the epidermis)

• Solitary or colonials

CLASS HYDROZOA

• Colony of specialized hydranths:

– Gonozooids: reproduction

– Gastrozooids: feeding

– Dactylozooids: catching prey

CLASS HYDROZOA – TOXIC SPECIES

• Most of the 2700 species of hydrozoa are harmless, but some can inflict painful injuries on humans

– Millepora alcicornis (fire corals) have nematocysts that can cause a painful skin rash (Indo-Pacific, Red Sea and the Caribbean)

– Aglaophenia cupresina (fire-weed) causes a nettle-like rash lasting several days (Indo-Pacific)

CLASS HYDROZOA – TOXIC SPECIES

• Physalia spp. (Portuguese man-of-war) is a free-swimming colony of openwater hydrozoans that lives at the sea–air interface.

– Different species of Physalia are widespread throughout all oceanic regions, except the Arctic and Antarctic

– May be blown onto beaches in swarms after strong onshore winds

– The nematocysts remain active even when beached.

– The tentacles may reach a length of up to 10 metres.

– Physalia physalis is the most dangerous and has been responsible for some severe stings and few deaths

CLASS SCYPHOZOA • Typically jellyfish

• Most have atypical dimorphic life cycle

– Polyp stage is atypical

– Majority of life cycle spent in medusa form

• Tetramerous radial symmetry.

• Gut divided into a complex system of radial canals.

• Some with a simple single mouth, but many with thousands of microscopic “mouths” at the ends of oral arms.

• Gonads endodermal (found in the gastrodermis).

CLASS SCYPHOZOA – TOXIC SPECIES

• All jellyfish are capable of stinging, but only a few species are considered a significant hazard to human health:

– Stomolophus nomurai

– Sanderia malayensis

• Species of some genera, such as Cyanea, Catostylus and Pelagia, may occur in large groups or swarms

CNIDARIA– TOXIN

• Analytical and clinical observations have established the toxicological diversity of cnidarian venoms

– high molecular weight proteins

– enzymes

– hemolysins

– non-proteinaceous compounds (e.g. purines, biogenic amines)

• Some toxins identified previously in other venomous animals comprise the venom arsenal of cnidarians

CNIDARIA– TOXIN

Palytoxins Anthozoa Na+/K+ ATPase opening 2-3

PORE-FORMING TOXINS (PFTs) • Present in all cnidarian venoms

• The mechanism of action is their penetration through the target cell membrane

– Leakage of small molecules and solutes

– Osmotic imbalance and cell lysis

• PFTs exhibit dual structure

– A stable water-soluble structure: monomeric and binds to the receptors on the target cell

– A membrane-bound structure: oligomeric molecules forming integral membrane pores

• PFTs are classified in two groups based on their secondary structure: α-PFTs and β-PFTs

PORE-FORMING TOXINS (PFTs)

• α-PFTs and β-PFTs displays different mechanisms of pore formation:

– α-PFTs: toroidal mode

• Requires a protein binding to stabilize

– β-PFTs: toroidal pore or octameric pore

• No role of lipids in the pore wall is necessary

• High stability of β-barrel is assured by interstrand hydrogen bonds

ACTINOPORINS

– Cardiovascular and respiratory arrest in rats

– Lysis of chicken, goat, human and sheep erythrocytes

• The best characterized are:

– Equinatoxin II from the sea anemone Actinia equina

• α-PFTs present in Anthozoa and Hydrozoa

• Mediate various types of toxicity and bioactivity all caused by a pore-forming mechanism:

ACTINOPORINS – Sticholysin I and II from the anemone Stichodactyla helianthus

– Fragaceatoxin C from the anemone Actinia fragacea.

ACTINOPORINS • The mechanism of membrane penetration requires several steps:

– Initial binding to the target membrane

• Specific recognition of sphingomyelin using aromatic rich region

– Insertion of the N-terminal amphiphilic α-helix segment to the lipid membrane

– Oligomerization on the surface of the membrane

• α-helices of 3 or 4 monomers insert into the membrane and form the ion conductive pathway

– This last step is most likely arranged in a toroidal pore mode

JELLYFISH TOXINS (JFTs) • Cubozoan-related porins are the most potent and rapid-acting toxins

secreted by jellyfish species

• α-PFTs

• Exhibit varying target specify towards various vertebrate tissue, dependent on the toxin member:

– Cardiovascular collapse

– Hemolytic

• Toxin originally identified in Cubozoa species:

– CaTX-A/B from Carybdea alata

JELLYFISH TOXINS (JFTs) – CrTX-A/B from Carybdea rastoni

– CqTX-A from Chiropsalmus quadrigatus

– CfTX-1/2 and CfTX-A/B/Bt from Chironex fleckeri

JELLYFISH TOXINS (JFTs) • Homologues of cubozoan porins were reported also in:

Scyphozoa (Aurelia aurita)

Hydrozoa (Hydra magnipapillata)

Anthozoa (Aiptasia pallida)

JELLYFISH TOXINS (JFTs)

• The hypothetical mechanism involves oligomerization of several amphiphilic and hydrophobic α-helices in the N-terminal region of the toxin

– distortion of the plasma membrane

– cell death

• The same mechanism stays at the basis of the hemolytic effect

• However, no in depth mechanicistic and structural studies have been performed so far

MEMBRANE ATTACK COMPLEX-PERFORIN • β-PFTs

• Detected in Japan in the venoms extracted from the sea anemones:

– Phyllodiscus semoni

– Actinaria villosa

MEMBRANE ATTACK COMPLEX-PERFORIN • Proteins containing a domain highly homologue to membrane attack

complex/perforin (MACPF)

– MACPF domains play important roles in vertebrate immunity, embryonic development, and neural-cell migration

– Ubiquitary in immune cells of vertebrate systems (identified in the complement system produced by human T-cells and NK-cells)

– Normally, the membrane attack complex (MAC) interact with perforins (PF) forming pores in the plasma membranes in response to pathogen infection

– It is delivered by T-cells and NK-cells and forms oligomeric pores in the plasma membrane of pathogens

• It is believed that the same mechanism occur for cnidarian MACPF as a defense mechanism

MEMBRANE ATTACK COMPLEX-PERFORIN

NEUROTOXINS • Are among the best characterized toxins in terms of the mechanism

of action.

• Produced by sea anemones (Anthozoa): fundamental role in immobilizing the prey rapidly and to defend against predators.

• Essentially, they are voltage-gated ion channel toxins:

– Prolonging the action potential of the excitable membranes (sensory neurons, cardiac and skeletal muscle cells)

– Modifying the Na+ channel or blocking the K+ channel gating during the repolarization stage

– Cells become initially hyperactive and subsequently release massive amounts of neurotransmitters (initial spastic stage followed by flaccid paralysis)

NEUROTOXINS

• The most important subclasses are:

– Voltage-gated sodium channel toxins (NaTxs)

– Voltage-gated potassium channel toxins (KTxs) and Kunitz peptides

– Small Cysteine-Rich Peptides (SCRiPs)

– Inhibitors of acid-sensing ion channels (ASIC)

– Inhibitors of transient receptor potential cation channel subfamily V member 1 (TRPV1), also known as Vanilloid receptor 1

VOLTAGE-GATED SODIUM CHANNELS TOXINS (NaTxs)

• Voltage-gated sodium channels (Nav) have a pivotal role in excitability of most animals

– Trigger the initiation and propagation of action potentials

• Transmembrane complexes composed of two subunits (α and β).

– The highly conserved α-subunit consists of 4 homologous domains (D1–D4)

– Each domain contains 6 hydrophobic transmembrane regions (S1–S6)

– Auxiliary β subunit has a regulatory role


• Anthozoan NaTxs bind to site 3 (loop S3–S4 in D4) of the Nav channels

– The open state of the channel is prolonged during the depolarization phase

– Slow down inactivation of the sodium channels

– Positive inotropic effects

– Arrhythmia (high doses)

– Muscular damages


• 3 classes basing on their aa sequences:

• Type 1 and Type 2

– Polypeptides of 46–49 aa with a high sequence similarity

– Toxins from the Actiniidae family are Type 1, while both types of toxins can be found in the family Stichodactylidae

• Type 3

– Shorter peptides (27-32 aa) not sharing homologous sequences

– Identified in the family Actiniidae


• The main toxins are:

– ATX II from Anemonia sulcata

– ATX III:

• only 27 residues

• target crustacean, insect and a lesser extent, other animal sodium channels

• no action on vertebrate channels has been reported

– Anthopleurin A and Anthopleurin B from Anthopleura xanthogrammica

VOLTAGE-GATED POTASSIUM CHANNEL TOXINS (KTxs)

• Voltage-gated potassium channels (Kv) are transmembrane channels specific for K+ and sensitive to voltage changes of membrane potential

• Transmembrane complexes composed of two subunits (α and β).

– α-subunit: the conductance pore

– 12 different subclasses of α subunits = Kv1.x – Kv12.x

– Auxiliary β subunit has a regulatory role


• At least 11 KTxs that have been identified in various sea anemones

• These toxins fall into three classes based on structural and functional differences:

• Type 1 toxins

– potently block Kv1 channels (brain)

– contain 35-37 amino acids

– Stichodactyla toxin from Stichodactyla helianthus


• Stichodactyla toxin (Shk toxin)

– Blocks Kv1.1, Kv1.3, Kv1.6, Kv3.2

– The peptide binds to all four subunits

– Blocks the Kv1.3 channel in T cells

– Blocks the neuronal Kv1.1 and Kv1.6 channels

– acetylcholine release (neuromuscular junctions)


• Type 2 toxins

– Called also Kunitz peptides (homology with Kunitz domain = protease inhibitor)

– Act against trypsin proteases to inhibit the rapid degradation of the venom by endogenous enzymes of the prey

– Kalicludines 1-3 from Anemonia sulcata

– contain 58-59 amino acids

– bind competitively to Kv1.2 channels to paralyze preys

– block Kv1 channels (less potency than Type 1 toxins)


• Type 3 toxins

– Blood-depressing substance (BDS 1-2 toxins) from Anemonia sulcata

– Contain 43 aa

– selective blockers of the fast-inactivating Kv3.4 channel

– Rapid depolarization that makes the channel more difficult to open, and slows both the activation and inactivation kinetics

– Additionally, BDS-1 affects the inactivation of Nav

NON-PROTEIN BIOACTIVE COMPOUNDS

• Different non-protein pharmacologically active compounds

• Identified mainly in Anthozoa

• Compounds with different nature:

– Histamine (Hydra)

– Serotonin (A. sulcata and A. equina)

– Both cause instant pain in predators (defensive properties)

– Vasodilation: enhancement of other toxins effects

cnidaria - units.it

Documents