Black Widow SpiderAlpha Latrotoxin

By:Shireen Abdulrahman

Introduction

• Arachnophobia, an irrational fear of spiders and is considered among the most common of all phobia.

• Of the 30,000 types of spiders, the black widow is probably the one best known and feared.

• Latrodectus mactans, the black widow, is a highly venomous species of spider in the genus Latrodectus.

Introduction

• Members of the “widow” family of spiders are found all over the world and are believed to account for the majority of fatal spider bites.

• The species is native to the United States of America, Canada

and Mexico.

• The female black widow's venom is particularly harmful to humans (males almost never bite humans).

Black widow spider

• The name of the black widow spider, Latrodectus mactans, is a mixture of Latin and Greek, meaning “deadly biting robber” (from latro—robber, bandit; δαγκάνω—to bite; macto—to slay).

• The widow spiders get their name from the females’ supposed eating of the male after mating. In nature, the great majority of males get away.

• The black widow spider is also called“The Hourglass Spider” because of the red hourglass shaped mark on the female’s abdomen.

Fig: The hourglass marking of an adult female black widow. Latrodectus sp.

• The widow spiders have eight eyes arranged in two rows of four and their vision is quite poor.

• Males, generally considered harmless to vertebrates, are generally about half the size of females.

• The bite of a black widow spider is poisonous. It causes severe pain and other serious clinical symptoms in humans, making this spider medically important.

Black widow spider

Fig: Female and male black widow spiders Latrodectus mactans.

Black Widow Spider Venom

• Latrodectus venom is produced by glandular cells in the spider's chelicerae.

• In a process called holocrine secretion, these cells disintegrate and their content is released into the lumen of the gland.

• Although black widow spiders are not large, their venom is extremely potent. It is fifteen times more potent than that of the rattlesnakes and is also reported to be much more potent than the venom of cobras and coral snakes.

• Black widow venom evolved mainly to immobilize and/or kill insects, the spider's natural prey.

• Toxicity against vertebrates is likely to have evolved as a means to protect the species against predation and accidental crushing.

• The actual amount of venom injected, even by a mature female, is very small in physical volume. The males, being much smaller, inject far less venom.

Black Widow Spider Venom

• There are a number of active components in the venom:

1. Latrotoxins2. A number of smaller polypeptides - toxins interacting

with cation channels which display spatial structure homology - which can affect the functioning of calcium, sodium, or potassium channels.

3. Adenosine4. Guanosine5. Inosine6. 2,4,6-trihydroxypurine.

Black Widow Spider Venom

• Latrotoxin is the active component within the venom, and more specifically, alpha-latrotoxin is the toxin that affects vertebrates.

• The venom of the black widow spider is a neurotoxin that

alters the structure and function of nerve terminals without producing any significant local reaction.

Black Widow Spider Venom

Alpha latrotoxin

• Latrotoxins are the main active components of the venom of spiders of the genus Latrodectus (widow spiders).

• They are neurotoxins and are responsible for the symptoms of latrodectism.

• The best-studied latrotoxin is alpha-latrotoxin, which acts presynaptically to release neurotransmitters acetylcholine, norepinephrine, and GABA from vertebrate sensory and motor neurons, as well as on endocrine cells.

• The release of these neurotransmitters leads to the clinical manifestations of envenoming.

• If enough venom is injected into a person's body, initially a severe pain in local muscle groups occurs, and the pain then spreads to regional muscle groups. The spread relates to the toxin initially being carried by the lymphatic system until it reaches the blood stream.

• Once in the blood, the venom is moved by circulation, causing its

toxins to be deposited in nerve ends where nerves insert into muscle.

• The venom acts at nerve endings to prevent relaxation of muscles, causing tetany — constant, strong, painful muscle contractions.

Alpha latrotoxin

Structure

• Alpha-latrotoxin is approximately 130kDa and is a stable dimer that is able to be tetramerized.

• As discovered by the 3D structure from electron microscopy,

there are three distinct domains (head, body wing) of this toxin. The N-terminal wing (36 kDa), the body (76 kDa), and the C-terminal head (18,5 kDa).

• The α-LTX monomer forms a dimer with another α-LTX monomer under normal conditions.

Fig: The domain structure of alpha latrotoxin

• The dimers are able to spontaneously form tetrameters which requires conformational changes.

• These conformational changes are induced by divalent cations, including magnesium or calcium which are also necessary for latrotoxin activity.

• Tetramer formation activates toxicity.

Structure

• The wings of the dimer are facing in opposite directions which makes it unlikely that it will be able to pierce the membrane.

• The tetramer is able to insert itself into the membrane because of its hydrophobic or lipophilic bases and these bases permeate the membrane.

Structure

Fig: 3D reconstruction of latrotoxin oligomers (top). Side view of tetramer inserting itself into lipid (bottom)

Mode of action

• α-LTX in its tetrameric form interacts with receptors (neurexins and latrophilins) on the neuronal membrane, which causes insertion of α-LTX into the membrane.

• Once the tetramer is inserted into the cell membrane it induces exhaustive neurotransmitter exocytosis from vertebrates through two calcium-dependent mechanisms (membrane pore formation and signaling via latrophilin) and a yet to be defined calcium-independent mechanism.

Fig: Example of synaptic cleft and action of neurotransmitter with its receptor.

1) Pore formation

• The pores formed by α-LTX in the membrane are permeable to Ca2+ and therefore allows an influx of Ca2+ into the cell.

• This influx into an excitable cell stimulates exocytosis directly and efficiently.

• The cation influx is proportional to the number of pores formed.

• Also Ca2+ strongly facilitates the forming of the tetramers and so its pore formation.

• The pore is also permeable to neurotransmitters which causes massive leakage of the neurotransmitter pool into the synpatic cleft.

• Alongside the influx of Ca2+, the channel is not very selective, allowing Na+, K+, Ba2+, Sr2+, Mg2+, Li+ and Cs+ to pass the membrane too.

• The pore is not only permeable for cations, but also for water

which causes nerve terminal swelling.

1) Pore formation

2) Signaling via Receptors

• Alpha-LTX protein, is capable of very tight binding with a number of different membrane proteins in several different types of neurons.

• Three receptors for α-latrotoxin have been described:

1. neurexin2. latrophilin 3. Protein tyrosine phosphatase sigma (PTPσ).

• The following mechanism is suggested for receptor-mediated effects:

• The toxin stimulates a receptor, most likely latrophilin.

• Latrophilin in turn activates the downstream effector phospholipase C (PLC) which indirectly induces release of Ca2+ from intracellular stores.

• This rise in cytosolic Ca2+ may increase the probability of release and the rate of spontaneous exocytosis.

• Thus latrophilin with α-LTX induces the effect of exocytosis of transport vesicles. The exact mechanism is yet to be discovered.

2) Signaling via Receptors

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Fig: Alpha latrotoxin mechanism of action

Venom toxicity

• It is notable that the reported LD50 of the best-studied and most stable LTX, α-LTX, ranges widely from 4.3 to 20 to 95 μg/kg.

Latrodectism

• Latrodectism is the clinical syndrome caused by the neurotoxic venom, that can be injected by the bite of any spider that is a member of the spider genus Latrodectus.

Symptoms

Symptoms occur in three main phases: exacerbation phase, dissipation phase and residual phase.

1) Exacerbation phase

During the first 24 hours after a bite:

• Severe pain in muscle groups local to the bite.• Muscle cramping, primarily in the abdomen, back and thighs.• Headache, dizziness, tremors, salivation, diaphoresis (excessive sweating), nausea

and vomiting.• Anxiety, fatigue, insomnia.• Lacrimation (tearing of the eyes).• Migratory arthralgia (joint pain).• Tachycardia (rapid heartbeat), bradycardia (very slow heart beat), restlessness,

hypertension (elevated blood pressure), Tachypnea (hyperventilation).

In some rare and extreme cases, severe complications can arise:

• Spontaneous abortion, preterm labor• Priapism• Acute renal failure (failing of kidney function).• Myocarditis, rhabdomyolysis, paralysis.• Shock, coma, and death.

Symptoms that may be present at or near the wound:

• Rash, slight erythema (redness of skin), Piloerection (goose bumps).• Mild edema (swelling due to excess fluid).• Lesion or mild infection (rare).

Symptoms

2) Dissipation phase

During the first 1 to 3 days after the bite:

• Symptoms start to decline.

3) Residual phase

During the following weeks or months:

• Muscle spasm, tingling, nervousness and weakness. There is a potential risk of paralysis.

Symptoms

Mortality

• Although severe symptoms such as shock and coma are known to have happened, death due to latrodectism is rare. Young children appear to be at highest risk for a lethal bite.

Prognosis

• The vast majority of victims fully recover without significant sequela.

Latrodectism

Treatment

Self-Care at Home

• The options for home care are limited.

• Both cold and warm compresses have been recommended, as have hot baths.

• Over-the-counter pain relievers such as acetaminophen and ibuprofen may be of value in mild cases.

Medical Treatment

• Standard treatments usually involve symptomatic therapy .

• The person bitten by a black widow spider, who has pain severe enough to seek treatment at an Emergency Department, will require narcotic pain relief.

• Muscle relaxants given by injection may also be of value.

• Although calcium gluconate given through an IV has long been advocated, it does not seem to produce much relief of symptoms.

Treatment

• The antivenin available for treatment of black widow spider bites is derived from horse serum.

• The venom produced by various species of black widow spiders is similar, so the antivenin (antivenin) prepared against one venom is effective against the others.

• Some experts recommend that antivenin be used in any severe bite because one vial of the antitoxin produces significant and rapid relief of symptoms.

Treatment

Side effects

• Horse serum-based antivenin carries a significant risk of a severe allergic reaction which can be life-threatening.

• Skin testing before the serum is administered is therefore recommended.

Follow-up

• Follow-up is always necessary in cases where antivenin is used.

• Although serum sickness is uncommon with single-vial doses of horse serum, it may occur 7-12 days after exposure and is characterized by skin lesions, fever, pain in the joints, and swollen lymph glands.

• The process is self-limited, goes away in 2-3 weeks, and may be treated with antihistamines and steroids.

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