Action of Polyamine Spider Toxins on Mammalian Neuroreceptor

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MCB 165, Spring 2007, Professor David Presti





Review
Action of Polyamine Spider Toxins on Mammalian Neuroreceptors and Corresponding Medical Applications
Denis Lankin
Departments of BioEngineering and Statistics, University of California, Berkeley









Abstract

This review summarizes the general facts about polyamine-containing spider toxins, their structure, mechanisms of action, and medical applications. It describes the methods and details of inhibition of mammalian ionotropic channels by the toxins, including various kinds of channel blocks, interactions with other receptor effectors, and selectivity. The features of these drugs and difficulties associated with them are discussed in terms of their potential in scientific, pharmacological, and medical use. The medical applications are elucidated with experimental examples that suggest treatments for serious illnesses such as ischemia, epilepsy and amyotrophic lateral sclerosis.

Keywords: Polyamine toxin; Spider toxin; Ionotropic receptor; Glutamate; AMPA; NMDA; KA; Nicotinic acetylcholine receptor; Channel block; Antagonism; Ischemia; Epilepsy; Amyotrophic lateral sclerosis; Argiope trifasciata; Agelenopsis aperta; Nephila clavata; Parawixia bistriata

Contents
1. Introduction ……………………………………………………………………………….3
2. Polyamine toxin overview ……………………………………………………...............3-7
3. Discussion ………………………………………………………………………………7-9
4. Applications …………………………………………………………………...............9-11
5. Conclusion …………………………………………………………………………...11-12


Introduction


Spider venoms contain a vast variety of biologically active components that exhibit medically interesting effects, including significant neurotransmitter action. It is estimated that the known 39,112 spider species may contain as many as 11 million chemicals (see figure 1a-e) (Estrada et al., 2007). Clearly this plethora of compounds presents a gold mine for pharmacologists and biologists alike, allowing them to pick and choose compounds possessing any given number of parameters, including efficiency, interval of action, delay time, lethal dose, etc. at a given neurotransmitter receptor. Spider toxins include several classes of neurochemcially active compounds, such as polyacylamines, peptides, and more esoteric chemicals. Polyacylamines, generally known as polyamines, represent one of the largest of these classes. They possess a vast variety of specific effects on various ionotropic neuroreceptors and vesicles connected with neurotransmitters. However, polyamine toxins share surprisingly similar structural properties given the diversity of their effects. The discovery, examination, and mechanism deduction of these molecules has been an area of energetic activity among medical scientists and arachnologists alike.


Polyamine toxin overview


Polyamine-containing spider venom components primarily act as blockers of various ionotropic glutamate and nicotinic acetylcholine receptors. For some receptors these antagonists exhibit a simple single-state channel block, but for other receptors, they alternate between open channel and closed channel conformations that transit from one state to another via a complex feedback-oriented mechanism. Most of these mechanisms exhibit complex non-linear voltage-dependence.
Structurally, a polyamine toxin from a spider or a wasp consists of four moieties, the aromatic group at one end (often 2,4-dihydroxyphenyl), connecting asparagine residue, polyamine group, and either guanidine or primary amine on the other end (see figure 2). The large number of primary and secondary amines results in variable valence of these compounds as nitrogen can acquire a net positive charge. This feature seems to play an important role in the open channel block mechanism for this toxin group (Usherwood et al., 2004). Structural similarity between various polyamine compounds from the venoms of different spiders is attributed to the pathway of their biological synthesis, which consists of methodical arrangement of the described groups into the final chemical (Itagaki & Nakajima, 2000).
The mechanism of action of polyamine toxins on N-methyl-D-aspartate (NMDA) receptors is relatively straightforward. A particular subclass of these toxins, argiotoxins from the orb-web spiders Argiope trifasciata (see figure 1a), has been shown to bind to the Mg2+ binding site of the receptor, resulting in strong non-competitive inhibition known as an open channel block. Since the channel transmits Ca2+ ions, the polyvalent properties of polyamines contribute to their effectiveness as non-competitive NMDA receptor antagonists. However, the toxin does compete with other antagonists of NMDA receptor, including MK-801 and Mg2+. Furthermore, high concentration of NMDA in the surroundings prompts the toxin to potentiate currents normally created by NMDA (Usherwood et al., 2004). The potency of the venom depends on the receptor’s subunits due to the difference in an amino acid position, which amino groups of the toxin have to pass for the terminal group to bind to the channel, blocking it. This site is occupied by N in NR2A and NR2B subunits but R in NR3. The latter acid’s basic character precludes the passage of the polyamine groups. Mutation of these positions to Q greatly enhances the potency of the venom (Raditsch et al., 1993).
Polyamine toxins show a similar mechanism of action on α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors, where they bind within the open channel, creating an open channel block. The binding greatly depends on high throughput of Ca2+ ions by the receptor, which is achieved through corresponding amino acid sequence of the receptor’s subunits. Therefore, the potency of the toxins is ameliorated by the presence of subunits such GluR2, whose structure prevents channel rectification and, thus, decreases Ca2+ flow. This is due to the difference in one amino acid position, which can be occupied by either Q or R (see figure 3b for alternative binding). If Q is present, the amine groups can pass, allowing the terminal group to bind with G later on in the pore and block the passage (Tikhonov et al., 2002).
The effect of polyamines on Kainate (KA) receptors has not seen much study, primarily due to the receptors’ rapid desensitization as well as the adjacency of AMPA receptors. However, the toxins are presumed to act by an open channel block mechanism that depends on the subunit specification for the same reasons as AMPA mechanism above does (Savidge and Bristow, 1998; Savidge et al., 1999). A site analogous to the Q/R position is also present in KA receptor and has similar effects on potency.
Polyamines exhibit open channel blocking mechanism in inhibition of nicotinic acetylcholine receptors (nAChRs). Some toxins block the receptors located in neurons while others are more potent on muscle receptors (Liu et al., 1997; Mellor et al., 2003; Brier et al., 2003). The circular inside of a nAchR channel is periodically lined with charged amino acids, which allows for Ca2+ selectivity. The shape of these rings varies between muscular and neuronal receptors, contributing to the different toxin effects.
The exact nature of the inhibitory mechanism is uncertain, but it is supposed to be a predominantly open channel block with a variety of alternatives that may transform into each other, including competitive inhibition, closed channel block, and increased desensitization (see figure 3a). The latter is particular interesting since it does not result in complete block of the nAchR channel but rather attenuates its tendency to dilate when exposed to Ca2+. This inhibition is a composite function of voltage that is further complexified by potentiation properties the toxins exhibit (Strømgaard et al., 2005). These observations have lead researchers to believe that there are two binding sites for at least some spider toxins in the nAChRs, as illustrated in figure 3a (Brier et al., 2002, 2003).
Very specific subclasses of polyamine spider venoms exist that exhibit functions, mechanisms, and properties beyond those described above. For example, Agelenopsis aperta is an American funnel web spider that produces several subclasses of such chemicals, collectively known as agatoxins (see figure 1b). While the general effects of α-agatoxins ω-agatoxins are similar to those of the venoms previously described – inhibiting glutamate receptors and generic Ca2+ channels respectively – the other subclass, μ-agatoxins produce the opposite, excitatory, effect on Na+ channels. It is also interesting to note that while ω-agatoxins do inhibit Ca2+ channels, they only produce a partial inhibition of the channel (Venema et al., 1992). Current research suggests that the partial block may be due to reduction of the current in a channel, although the mechanism of this phenomenon is unknown (McDonough et al., 2002). Furthermore, the toxins’ action sites are different from those of the neurotoxins discussed above since ω-agatoxins’ potency does not depend on the structure of the subunits directly. Indeed, they preserve their inhibitory effect regardless of whether Q, R, or even N occupies the analog of the Q/R site. On the other hand, as long as the channel is capable of high throughput of Ca2+, the potency of the venom remains strong (Yan & Adams, 2000). Such specificity allows for interesting further research and designer drugs of high precision.


¬¬Discussion


The fact that polyamine compounds simultaneously antagonize receptors of excitatory neurotransmitters suggests a variety of useful medical applications. While the blockade of nicotinic acetylcholine results in slowdown of sympathetic nervous system, the obstruction of glutamate channels may depress excitatory impulses in the brain during the same time frame. It is important to note that polyamine venom chemicals exhibit significant action not only on the primary glutamate receptor, NMDA, but the secondary ones as well, such as KA and AMPA receptors. However, these neurotoxins preferentially bind to NMDA receptors probably because NR3 subunits, which decrease the venom’s potency, are not as widespread in the brain as GluR2 subunits, which have a similar role in AMPA receptors (Usherwood et al., 2004).
Since the potency of these compounds is very high in natural state, it may be desirable to lessen their effects for chronic applications that do not require immediate strong response. On the other hand, acute over-activity in a patient can probably be ameliorated by application of the natural drug. Regardless, modification of polyamine containing compounds’ potency is fairly straightforward and can be achieved by decreasing the length of the polyamine moiety as well as interchanging the aromatic groups. Interestingly enough, variation of different parts of the toxins results in inconsistent changes in potency across various areas of the brain, such as hippocampus and cortex (Davies et al., 1992; Mueller et al.,1991). Furthermore, the potency of different toxins on different receptors with different subunits varies as well.
These features present an opportunity to design the drugs that would significantly affect only the desired receptors in the appropriate portions of the human brain. It must be noted, however, that since polyamine venoms primarily act via an open channel block, the channel has to be in fact open for the neurotoxin to take effect. This in turn requires a significant presence of the receptor or its agonists and possibly its essential co-agonists. Additionally, one may question the effectiveness of polyamine containing drugs due to their multiple positive charges since this attribute would normally prevent them from crossing the blood brain barrier. Despite this observation, scientific literature suggests that adding polyamines to compounds that normally do not cross the barrier, such as insulin and albumin, will in fact drastically improve their permeability. The exact process involved in this transaction is uncertain, but it is suggested that the polyamine transporter assists in transportation of these molecules (Poduslo & Curran, 1996).
Unfortunately, the use of polyamine compounds is stifled by the relative difficulty of their obtainment, separation and subsequent study. The process of “milking” spider’s fangs for venom is not straightforward (see figure 4) and the amount of venom received through this process is tiny. Moreover, the structural determination of these chemicals is non-trivial, exacerbating partition and identification of these compounds. However, recent developments in Mass Spectrometry allow relatively stream-lined elucidation of the venom’s components’ structures. Furthermore, newer developments in synthesis of these compounds permit easier if not assembly-line production of desired polyamine toxins (Strømgaard et al., 2005). In particular, solid-phase synthesis provides a strong alternative to traditional methods due to its favorable properties, including easier purification, group protection, and construction of combinatorial libraries (Strømgaard et al., 2001). In fact this technique has seen a lot of recent development allowing specification of selectivity and potency of the synthesized compounds. Examples include first construction of combinatorial library (Strømgaard et al., 2001a) and reductive acetylation technique combined with Fukuyama animation that permits the construction of asymmetric polyamine groups (Wang et al., 2000; Strømgaard et al., 2001b).

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Applications


Let us consider several of proposed applications of spider polyamine containing toxins in medicine. Of particular interest is the antiepileptic effect of the Joro spider toxin (JSTX-3), native to the Nephila clavata species (see figure 1c). It is known for its extremely selective inhibition of glutamate-mediated ion channels, which makes it a useful implement in the neurosciences. It has been shown that induced epileptic seizures are controlled by NMDA receptor activation in the hippocampus (Smolders et al., 1997). An experiment was conducted on a group of rats predisposed to epilepsy. Epileptic seizures were induced in rats through administration of seizure-generating chemicals like pilocarpine. Then the seizures were interrupted, and after some time, rats were anesthetized, their brains excised, sectioned, and analyzed. In particular, hippocampal neurons underwent thorough electrochemical examination. Artificial cerebro-spinal fluid with and without Mg2+ was added to the brain slices in vitro. Since Mg2+ normally obstructs the NMDA channel, its absence results in over-activity of the receptor. However, addition of JSTX-3 immediately stops this activity. Failing to stop NMDA hyper-excitation results in cell death due to overabundance of Ca2+ that flows through the channels. The epileptic over-activity is likely to be also connected with the other Ca2+ channels, namely AMPA and KA receptors. It has been demonstrated that JSTX-3 antagonizes these receptors as well, creating a synergetic anti-convulsant effect (Salamoni et al., 2005).
Interestingly enough, another spider toxin was shown to have anti-convulsant effects in response to administration of chemicals inducing epilepsy. However, this compound, FrPbAII of the social spider Parawixia bistriata, acts by a very different mechanism (see figure 1d). The venom component inhibits reuptake of GABA by affecting its transporters. Thus, FrPbAII prevents the GABAergic blockade that produces epileptic seizures. Experiments on mice in vivo confirm this neurotoxin’s efficacy at providing seizure protection (Liberato et al., 2006). Therefore, spider toxins present a feasible treatment opportunity for the serious medical condition of epilepsy.
Another interesting application of polyamine toxins centers on their protective role in alleviating brain damage caused by ischemia. This condition frequently follows a stroke, resulting in neuronal damage. In this case, spider toxins such as FTX-3,3, isolated from grass spider Agelenopsis aperta, can act as neuron-protective agents (see figure 1b). The toxin binds to several types of voltage sensitive calcium channels, which are believed to be involved in ischemic cell depolarization that ultimately leads to cell death. Synthetic compounds mirroring the effect of FTX-3,3 have been produced. Moreover, other polyamines, such as JSTX-3 mentioned above, neutralize slow excitatory postsynaptic potentials caused by ischemia. If these potentials are not stemmed, they lead to Ca2+ accumulation and, ultimately, cell death. Thus, spider polyamines may provide an effective multi-layer defense for the consequences of stroke. Furthermore, due to their specificity, they are unlikely to cause the side effects exhibited by the medications currently in use (Estrada et al., 2007).
Additionally, JSTX-3’s inhibition of glutamate channels has proven useful in studying the mechanism of allodynia, a type of pain. The toxin selectively blocks AMPA receptors in the spine. The examination of various pain inducing processes on rats, using different channel inhibitors, allowed elucidation of the pain pathways. The subsequent results suggested the respective roles of NMDA, AMPA, and KA receptors with respect to thermal hyperalgesia, oversensitivity to pain, and mechanical allodynia. These experiments allowed distinguishing between central sensitization and hyperalgesia in patients after surgery. Moreover, since JSTX-3 blocks the channel related to pain transmission, it produces an anesthetic effect, that can be harnessed for pharmacological use (Estrada et al., 2007).
Yet another promising application of spider neurotoxins pertains to the treatment of amyotrophic lateral sclerosis (ASL). One of this condition’s negative effects is enhancement of P-channel Ba2+ current by ImmunoglobinG. This increased current leads to cell death and, ultimately, patient’s death from respiratory failure. However, this extremely harmful pathway can be greatly decreased by stemming Ba2+ current with compounds like a polypeptide toxin pFTX from Agelenopsis aperta spider (see figure 1b). The experiments conducted on guinea pig Purkinje cells in vitro confirm that the synthetic analog of pFTX, sFTX, stops inward Ba2+ current in P-channels. Moreover, both versions of FTX toxin block the P-type channel that contributes to overabundance of Ca2+ during ASL, which leads to similar results as magnified Ba2+ current (Llinas et al., 1993).


Conclusion


Thus, polyamine-containing spider toxins form a group of highly selective, specific, and efficient effectors of neuroreceptors. They primarily act as antagonists of ionotropic channels. The plethora of their mechanisms of action and results thereof are tremendous considering the similarity in the polyamines’ structures. These properties make polyamine toxins incredibly useful for study of various receptors, including those in the human brain. Furthermore, their inhibitory action can be employed in medicine through amelioration of such wasteful conditions as ischemia, epilepsy and amyotrophic lateral sclerosis. Moreover, the great variety of arthropods implies that thousands if not millions of such compounds await discovery for the greater good.