Argiotoxin, a glutamate-activated postsynaptic ion channel blocking polyamine from the spider Argiope lobata.
Polyamines
Despite the vast majority of spiders being harmless to humans,
many spider venoms contain in their venoms potent small molecules
called polyamines. Polyamines are often very effective and specific
blockers of ion channels or of receptors. Their target receptors
are those which recognize excitatory amino acids in the mammalian
central nervous system and are classified into three major subtypes,
ones which prefer N-methyl-D-aspartate (NMDA), quisqualate (QA),
or kainate (KA) as type agonists respectively .
Spider toxins have been isolated which are selective and reversible
noncompetitive antagonists of NMDA (N-methyl-D-aspartate) receptors
in the mammalian brain. By way of example, Dolomedes okefinokensii
(Fishing spider) bites rarely cause anything but the mildest of
edema and possibly minor necrosis. The venom, however, has yielded
a compound that may have use as an analgesic, a polyamine that
reversibly blocks L-and R-type voltage-sensitive calcium channels.
Other examples of the usefulness of spider polyamines are the alpha-agatoxins Aga-GI and Aga-G2 from the American funnel web spider Agelenopsis aperta. These low molecular weight molecules cause rapid, reversible paralysis correlated with use-dependent postsynaptic block of excitatory postsynaptic potential and ionophoretic glutamate potentials. Aga-GI is a toxin capable of inhibiting an elusive neuronal calcium channel that is resistant to blockage by either omega-conotoxin GVIA or nifedipine making this component a novel tool with which to probe this channel. Intravenous or intracerebroventricular administration of Aga-G2 produces dose-dependent suppression of behavioral convulsions induced in rats by kainic acid, picrotoxin, or bicuculline. The combination of their small size but potent actions makes these toxins a notable class of compound worthy of in-depth study.
Another interesting toxin quite useful in studying receptors for excitatory amino acids in the mammalian central nervous system is jorotoxin from the venom of Nephila clavata (Joro spider). Jorotoxin has been shown to be a specific blocker of the postsynaptic glutamate receptors, in contrast with bacterial pertuissus toxin that blocks the presynaptic glutamate receptors. These low molecular weight toxins may provide novel tools for anticonvulsant research and therapy, demonstrating yet again the potential use of venom components to be used as investigational ligands or even therapeutics in their own right.
Spider venom peptides
Mu-Agatoxins
The nomenclature of spider peptidic neurotoxins is, unlike
that of scorpions, based on the same convention as that of cone-snails
and snakes: alpha-toxins inhibit the acetylcholine receptor; mu-toxins
directly abolish muscle action potentials through the inhibition
of muscle sodium channels; and omega-toxins prevent voltage-activated
entry of calcium into the nerve terminal and the release of acetylcholine.
Spider mu-toxins are cysteine-rich polypeptides, which cause irreversible paralysis and repetitive action potentials originating in presynaptic axons or nerve terminals. Clinical effects of a bite from one of the North American funnel web-spider Agelenopsis aperta are negligible. The venom, however, has yielded several interesting peptides that have pharmacological properties that may prove to be quite useful. Two specific mu-agatoxins have been identified from A. aperta, mu-agatoxin-I and mu-agatoxin-IV, both of which contain 36 amino acids and four internal disulfide bonds. Although these toxins specifically modify voltage-sensitive sodium channel activity, they have structural similarities with a plethora of other peptide toxins targeting a myriad of channel types. Analysis of these variations may shed light not just on functional residues of the particular scaffold but also provide data as to the structure and specificity of the binding sites of the channel being affected. This adds to the emerging wealth of data of a common scaffold being modified for divergent use.
The omega-agatoxins from A. aperta consist of two subtypes
of neuronal calcium channel toxins with different structural characteristics
and binding specificities. Type I agatoxins, such as omega-Aga-I,
may define a binding site on neuronal calcium channels that is
common to both vertebrates and invertebrates. Type II omega-toxins,
such omega-Aga-II, III and IV, share limited amino acid sequence
similarity with Type I toxins. However, omega-Aga-IVA is able
to block omega-CTX-GVIA (Conus geographus) resistant calcium
channels. This use of structurally distinct toxins from common
as well as divergent sources is an excellent manner in which to
study a channel.
The peptidic neurotoxins from A. aperta have similar locations of cysteine residues with neurotoxins from the venom of another North American funnel web spider Hololena curta as well as neurotoxins from the more distantly related Brazilian spider Phoneutria nigriventer (Wandering spider). This conservation of structure among these three species is interesting to view from a taxonomic standpoint in that venom peptide sequences may be useful as chemotaxonomic tools. Despite being the only lethal species amongst the three spiders, the toxins of the Phoneutria nigriventer share strong structural similarity, particularly in the location of the cysteine residues with neurotoxins from these other two spiders.
While the lethal neurotoxin PhTx1 from Phoneutria nigriventer
venom has a primary structure shows no homology to any other identified
spider toxin, cDNA libraries constructed from the venom glands
revealed that the structure of the preprotoxin initially synthesized
by the Tx1 gene shows similarity in sequence and also in processing
with the synthesis and processing of omega-agatoxin IA from A.
aperta. Thus, this toxin may not be quite so unrelated but
rather may represent a case of a spider that is taxonomically
divergent, as P. nigriventer is from the more closely related
two N. American species, with the venom diverging along with it.
Delta Atracotoxin
Unlike the American funnel web spiders, the venoms of Australian
funnel web spiders are quite lethal, consisting of a large number
of acute acting neurotoxins. These venoms slow the inactivation
of primate sodium channels causing envenomation symptoms involving
pain at the bite site, salivation, lachrymation, piloerection,
generalized skeletal muscle fasciculation, sweating, nausea, vomiting,
diarrhea, pulmonary edema, dyspnoea followed by respiratory failure,
tachycardia and hypertension followed by hypotension and circulatory
failure.
The primary toxic components of the venoms of two species have
been isolated and characterized. The lethal toxins from A.
robustus and H. versuta venoms are the 42 amino acid
peptide components robustoxin (atracotoxin) and versutoxin respectively.
Versutoxin differs from robustoxin by only 8 amino acid residues.
Disulfide-bridged cysteine residues at both the amino- and carboxy-termini
and a triplet of cysteines at residues 14-16 makes these components
unprecedented amongst neurotoxins.
These presynaptic neurotoxins produce spontaneous, repetitive
firing of autonomic and motor neuron action potentials. Characteristic
of this autonomic storm is a wave of endogenous acetylcholine,
noradrenaline and adrenaline. These two toxins bind to tetrodotoxin-sensitive
sodium channels, competing with alpha-scorpion toxins. These toxins
cause a voltage dependent slowing of sodium channel inactivation
by binding to the outer surface of the channel, thus interfering
with the conformational changes necessary for gating of the channel.
Latrotoxin
In contrast to the mostly channel binding small peptide toxins
from the Australian funnel web spiders and that from most other
spiders, the major bioactive component (latrotoxin) from the venom
of the lethal American black widow spider (Latrodectus mactans)
is a much larger protein. Latrotoxin is responsible for many of
the neurological symptoms clinically seen, causing a massive release
of acetylcholine that is stimulated by the presence of Ca2+ ions.
It is interesting to compare the activity of latrotoxin with the
smaller peptidic venom component omega-conotoxin. Latrotoxin is
an activator of synaptosomal calcium uptake, while omega-conotoxin
GVIA is an inhibitor of voltage-sensitive calcium channels of
the N-type, yet both toxins ultimately produce cramping or rigid
paralysis.
