Venomous drugs from spiders

Spiders are nature’s pest controllers. These eight-legged, web-forming arachnid predators are equipped with two venom glands full of valuable chemicals designed to kill insect prey. Such compounds, from small organic molecules to complex structures such as acylpolyamines, neuropeptides and enzymes, are precious ligands that target several biological receptors. Since insect receptors are not substantially different from those of humans and other mammals, the majority of the molecules contained in spider venom could also target human receptors.

The potential medical uses of spider venoms are largely due to their selectivity and affinity for ion channels [proteins that allow ions to cross cell membranes] and other receptors. This makes them suitable for studying cell function and for designing therapeutic drugs. As an example, the venom of the theraphosid Grammostola spatulata from South America contains a peptide, GsMtx-4, that blocks stretch-activated ion channels. These channels are sensitive to muscle contraction and blood pressure and play an important role in coordinating a heartbeat. Potentially, GsMtx-4 could be used to prevent atrial fibrillation after a heart attack and to treat cardiac patients.

LIPID BOUND Like related peptide toxins, GsMTx4 has a highly hydrophobic face (green). The cysteines that make up the protein’s core are colored yellow, and positive and negative residues are shown in blue and red, respectively.
NATURE ©2004

Peptides make up a substantial part of spider venom, and modulate ionic currents across Ca²⁺, Na⁺, or K⁺ ion channels. Some spider peptides can discriminate between ion channel subtypes and several will inhibit peripheral neurons, the nerve cells that are associated with supplying sensation to the skin and skeletal muscles. Spider toxins that block the neuronal Ca²⁺ ion channel could prove important for the treatment of chronic pain.

A special group of the spider peptides have a mixed hydrophilic-hydrophobic nature – they are amphipathic. These form -helical structures that insert into cell membranes to form pores, resulting in loss of cell function. Although most of these peptides will destroy red blood cells, they could potentially be used in topical applications, such as antibacterial coatings for medical implants, in inhibiting the growth of oral bacteria associated with tooth decay and early plaque formation and in treating skin infections.

Venom peptides contain a common basic structure called a ‘cysteine knot,’ a tangle of protein chains and disulfide bridges that gives them an excellent molecular stability. Also, the small organic components of spider venom, such as organic acids, amines, nucleic acids and amino acids, are thought to stabilise the mixture and enhance the delivery and effectiveness of the peptides.

Of all the venom components, the acylpolyamines represent the vast majority of the molecules in the mixture. These have been shown to suppress epileptic activity in brain tissue. They can also act as pain-killers, by blocking capsaicin receptor channels, non-selective cation channels in sensory neurons that respond to pain-causing stimuli. Moreover, brain damage caused by restricted blood flow, for example during a stroke, can be prevented with acylpolyamines. The compounds work by blocking Ca²⁺ voltage-gated ion channels or preventing glutamate release, both of which are implicated in neuronal death.

Finally, enzymes and large protein components of spider venoms are of special medical importance. For example, the neurotoxic protein -latrotoxin, from the black widow spider, causes massive neurotransmitter release. Similarly, an active enzyme in the venom of the brown recluse spider is sphingomyelinase D, which degrades cell membranes and causes painful lesions to develop. Another component of brown recluse spider venom, hyaluronidase, belongs to a family of compounds that have shown medical potential as tumour treatments.

Most spider species are harmless to humans, so peptides or drug molecules from these spiders are likely to be safe. By modifying the molecular surfaces and active sites of peptides and enzymes from spiders, whilst keeping the spider scaffold, it is possible to gain specificity and/or affinity for a given receptor. Therefore, acylpolyamines, peptides and enzymes from spider venoms represent an interesting source of molecules for the design of novel pharmaceutical drugs.

References