Disorders of the central nervous system (CNS), such as pain, epilepsy and stroke, remain some of the world’s most serious and debilitating health problems. Chronic pain is the largest unresolved medical condition in the world – symptoms can be treated but there is currently no cure. Globally, stroke is the second leading cause of death and the leading cause of morbidity. Epilepsy cannot be effectively treated in more than one-third of all cases.
Pain, stroke and epilepsy have three things in common. First, they are all influenced by mutated protein channels in the body. Second, there are currently few treatments to address them. Third, they all have the potential to be successfully treated with the venoms of spiders, centipedes and scorpions. Research groups, such as the Peptide Kings group at my university, the University of Queensland, are undertaking research for potential cures for these diseases.
Epilepsy and Dravet syndrome
Epilepsy, a disorder characterised by recurrent seizures, occurs because of uncontrolled electrical impulses from neurons in the brain’s cerebrum, or outer shell. There are many different types of epileptic seizures, and the underlying cause of most attacks is unknown. However, it has been shown that 30–40% have a genetic origin. The seizures that characterise Dravet syndrome are one such example, presenting largely in childhood and frequently having catastrophic outcomes.
Individuals with Dravet syndrome experience polymorphic seizures. Each seizure is characterised by alternation of two or more different types of epileptic attacks. Clinically, Dravet syndrome presents as one of the most pharmaco-resistant types of epilepsy. As such, patients with Dravet syndrome commonly develop multiple neuropsychiatric comorbidities and have an increased risk of premature death.
Loss-of-function mutations in one version of the gene SCN1A causes Dravet syndrome. SCN1A encodes for a specific variant of voltage-gated sodium (NaV) channel, NaV1.1.
NaV channels are cell-membrane proteins responsible for moving sodium ions from one side of the membrane to the other. This movement is of particular importance in neurological function because sodium ion influxes lead to initiation and propagation of neuronal impulses. In the brain, NaV1.1 plays a key role in sending inhibitory impulses from inhibitory neurons to excitatory neurons. Inactivation of these channels through mutation can therefore cause uncontrolled hyperexcitation, leading to epileptic seizures.
Why scorpion venom?
Many animal venoms have evolved to target the important Nav channels, often as prey capture or defence mechanisms. This means venoms are key targets in potential drug therapies.
It has previously been demonstrated that inhibition of NaV1.1 inactivation by a peptide isolated from spider venom eliminated seizures and premature death in a mouse model of Dravet syndrome. However, whether venoms from other animals also contain compounds that target NaV1.1 was unknown – until recently. Recent work by the Peptide Kings group, led by Professor Glenn King at the University of Queensland’s Institute for Molecular Bioscience, identified two scorpion-venom peptides that have agonistic actions on NaV1.1: Hj1a and Hj2a (ACS Pharmacol. Transl. Sci. 2020, vol. 3, pp. 119–34). In fact, these peptides were shown to be some of the most potent activators of this channel ever discovered.
Isolating Hj1a and Hj2a
In order to isolate peptides of interest, venom was first extracted from a species of previously unstudied scorpion, Hottentotta jayakari, by electrical stimulation, and then separated by reverse-phase HPLC.
This technique resulted in large groups of toxins with similar hydrophobicities. These groups were then put through electrophysiological screening, a method used to determine NaV channel activation. Groups that elicited high NaV1.1 activation were sent back through the same isolation steps. Eventually, two ‘hit’ peptides were isolated: Hj1a and Hj2a.
Hj1a and Hj2a as Dravet syndrome therapeutics
When developing pharmaceutical drugs from NaV channel activators, two characteristics are of particular importance: potency (strength of molecule function) and selectivity (accuracy of function). These characteristics are extremely important because pharmaco-modulation of any CNS-dominant NaV channels, such as those in heart and muscle cells, can cause life-threatening side-effects such as seizure, cardiac arrest and respiratory failure.
Despite their high potency towards NaV1.1, Hj1a and Hj2a lack the subtype selectivity needed for an antiepileptic drug. However, the researchers believe it is possible that this could be resolved through route of administration and dosage. As they say in their research paper:
For example, Hj1a and Hj2a have agonistic activity on NaV1.4, which is found in smooth and skeletal muscle, and NaV1.5, which is localised to the heart ... Since these subtypes are restricted to the peripheral nervous system, activity at these channels may not be a serious concern for a CNS-restricted antiepileptic drug.
The Peptide Kings paper concluded that additional research is needed, and that exploring these peptides further may very well lead to a cure for Dravet syndrome. The group noted that modelling:
structures of Hj1a and/or Hj2a in complex with a human NaV1 channel would provide important insights into the molecular mechanism by which these peptides impede channel activation, and they would facilitate rational design of analogues with enhanced selectivity for NaV1.1.
More recently, King said:
We have developed a more selective and stable activator of NaV1.1 that we plan to test in rodent models of DS [Dravet syndrome] while simultaneously examining methods for specifically targeting the peptide to the brain.