Voltage-gated ion channels as therapeutics targets
Voltage gated ion channel (VGIC) proteins form aqueous pores across cellular lipid membranes through which charged ions (Na+, Ca2+, K+, and Cl–) can pass. VGICs are sensitive to changes in the electrostatic charge across the membrane, which drives conformational changes in the protein complex, allowing the channels to open and close. VGICs are highly dynamic. Eleven ion channel families have been identified[i] [ii] [iii], for which structural information is increasingly available (e.g.[iv] [v]). The structure and function of the voltage-sensor domain, ion selectivity filter, as well as gating and inactivation mechanisms have been worked out in detail using molecular genetics and structural imaging techniques. Most recently, the development of cryogenic electron microscopy, which freezes the channel protein in its physiological conformation prior to imaging, has successfully resolved the structure of several VGICs at angstrom resolution[vi] [vii] [viii]. The structural elements of VGICs combine to confer a range of biophysical properties that characterise individual channels and channel families. These properties affect the voltage range over which channels open and close, the speed with which this happens, whether or not the channels inactivate after opening, and how quickly they recover from inactivation[ix]. Databases, such as Channelpedia [x] provide a useful resource to navigate amongst the 140+ ion channel proteins that have been cloned.
VGIC channels sculpt the electrical dynamics of excitable cells, and in the case of neurons, orchestrate their characteristic ability to integrate synaptic input, fire action potentials, and release neurotransmitter. Knowledge of the biophysical properties of a specific neuronal channel, coupled with information about its sub-cellular location (somatic, dendritic, or axonal) permits prediction of its physiological role, and how the channel might contribute to human disease[xi]. Given the frequent association between VGICs and cellular excitability, drugs targeting VGICs are most commonly directed towards the treatment of neurological disorders such as epilepsy and neuropathic pain, as well as cardiac disorders such as arrhythmias.
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[vii] Lee CH, MacKinnon R. Structures of the Human HCN1 Hyperpolarization-Activated Channel. Cell. 2017;168(1-2):111-120.
[viii] Henderson R. Overview and future of single particle electron cryomicroscopy. Arch Biochem Biophys. 2015;581:19-24.
[ix] Hille B. Ion channels in excitable membranes. 3rd ed. Sinauer Associates, USA. 2001.
[x] Ranjan R, Khazen G, Gambazzi L, et al. Channelpedia: an integrative and interactive database for ion channels.
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Kv3 ion channels
Kv3 voltage-gated potassium channels (Kv3.1-4) are activated by depolarization of the neuronal plasma membrane to potentials above -20 mV; they open rapidly during the depolarising phase of the neuronal action potential to initiate repolarisation and prevent sodium channel inactivation. As the neuron begins to repolarise, the channels deactivate quickly and so do not contribute significantly to the after-hyperpolarisation [xii] [xiii]. These distinct properties allow the channels to terminate the action potential rapidly without compromising action potential threshold, rise time, or magnitude, and without increasing the duration of the refractory period that follows. Consequently, neurons expressing Kv3 channels can sustain action potential firing at high frequencies [xiv]. Kv3.1-3 subtypes are expressed mainly in the central nervous system, whereas Kv3.4 channels are predominant in skeletal muscle and sympathetic neurons [xv]. Kv3.1-3 channel subtypes are differentially expressed in corticolimbic brain areas [xvi] [xvii] [xviii], thalamus [xix], and cerebellum [xx]. Kv3.1 and Kv3.3 channels are also expressed at high levels in auditory brainstem nuclei [xxi] [xxii].
Autifony has developed compounds that selectively enhance the function of Kv3.1 and Kv3.2 channels [xxiii] [xxiv]. Rosato-Siri et al. showed that one of these, AUT1, caused a leftward shift in the voltage-dependence of activation of human recombinant Kv3.1 and Kv3.2 channels. The compound also restored the ability of somatosensory cortex PV+ interneurons to fire at high frequency [xxv].
[xii] Rudy B, Chow A, Lau D et al. Contributions of Kv3 channels to neuronal excitability. Ann N Y Acad Sci. 1999. 868:304-43.
[xiii] Rudy B, McBain CJ. Kv3 channels: voltage-gated K+ channels designed for high-frequency repetitive firing. Trends Neurosci. 2001. 24(9):517-26.
[xiv] Martina M, Schultz JH, Ehmke H et al. Functional and molecular differences between voltage-gated K+ channels of fast-spiking interneurons and pyramidal neurons of rat hippocampus. J Neurosci. 1998. 18(20):8111-25.
[xv] Weiser M, Vega-Saenz de Miera E, Kentros C et al. Differential expression of Shaw-related K+ channels in the rat central nervous system. J Neurosci. 1994. 14(3 Pt 1):949-72.
[xvi] Chow A, Erisir A, Farb C et al. K(+) channel expression distinguishes subpopulations of parvalbumin- and somatostatin-containing neocortical interneurons. J Neurosci. 1999. 19(21):9332-45.
[xvii] McDonald AJ, Mascagni F. Differential expression of Kv3.1b and Kv3.2 potassium channel subunits in interneurons of the basolateral amygdala. Neuroscience. 2006. 138(2):537-47.
[xviii] Chang SY, Zagha E, Kwon ES et al. Distribution of Kv3.3 potassium channel subunits in distinct neuronal populations of mouse brain. J Comp Neurol. 2007. 502(6):953-72.
[xix] Kasten MR, Rudy B, Anderson MP. Differential regulation of action potential firing in adult murine thalamocortical neurons by Kv3.2, Kv1, and SK potassium and N-type calcium channels. J Physiol. 2007. 584(Pt 2):565-82.
[xx] Sacco T, De Luca A, Tempia F. Properties and expression of Kv3 channels in cerebellar Purkinje cells.
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[xxi] Grigg JJ, Brew HM, Tempel BL. Differential expression of voltage-gated potassium channel genes in auditory nuclei of the mouse brainstem. Hear Res. 2000. 140(1-2):77-90.
[xxii] Li W, Kaczmarek LK, Perney TM. Localization of two high-threshold potassium channel subunits in the rat central auditory system. J Comp Neurol. 2001. 437(2):196-218.
[xxiii] Rosato-Siri MD, Zambello E, Mutinelli C et al. A Novel Modulator of Kv3 Potassium Channels Regulates the Firing of Parvalbumin-Positive Cortical Interneurons. J Pharmacol Exp Ther. 2015. 354(3):251-60.
[xxiv] Brown MR, El-Hassar L, Zhang Y et al. Physiological modulators of Kv3.1 channels adjust firing patterns of auditory brain stem neurons. J Neurophysiol. 2016. 116(1):106-21.
[xxv] Rosato-Siri MD, Zambello E, Mutinelli C et al. A Novel Modulator of Kv3 Potassium Channels Regulates the Firing of Parvalbumin-Positive Cortical Interneurons. J Pharmacol Exp Ther. 2015. 354(3):251-60.