Network topology of NaV1.7 mutations in sodium channel-related painful disorders

Background: Gain-of-function mutations in SCN9A gene that encodes the voltage-gated sodium channel NaV1.7 have been associated with a wide spectrum of painful syndromes in humans including inherited erythromelalgia, paroxysmal extreme pain disorder and small fibre neuropathy. These mutations change the biophysical properties of NaV1.7 channels leading to hyperexcitability of dorsal root ganglion nociceptors and pain symptoms. There is a need for better understanding of how gain-of-function mutations alter the atomic structure of Nav1.7. Results: We used homology modeling to build an atomic model of NaV1.7 and a network-based theoretical approach, which can predict interatomic interactions and connectivity arrangements, to investigate how pain-related NaV1.7 mutations may alter specific interatomic bonds and cause connectivity rearrangement, compared to benign variants and polymorphisms. For each amino acid substitution, we calculated the topological parameters betweenness centrality (Bct), degree (D), clustering coefficient (CCct), closeness (Cct), and eccentricity (Ect), and calculated their variation (value= mutantvalue-WTvalue). Pathogenic NaV1.7 mutations showed significantly higher variation of |Bct| compared to benign variants and polymorphisms. Using the cut-off value \uc2\ub10.26 calculated by receiver operating curve analysis, we found that Bctcorrectly differentiated pathogenic NaV1.7 mutations from variants not causing biophysical abnormalities (nABN) and homologous SNPs (hSNPs) with 76% sensitivity and 83% specificity. Conclusions: Our in-silico analyses predict that pain-related pathogenic NaV1.7 mutations may affect the network topological properties of the protein and suggest |Bct| value as a potential in-silico marker

A Mixed Periodic Paralysis & Myotonia Mutant, P1158S, Imparts pH-Sensitivity in Skeletal Muscle Voltage-gated Sodium Channels

Skeletal muscle channelopathies, many of which are inherited as autosomal dominant mutations, include myotonia and periodic paralysis. Myotonia is defined by a delayed relaxation after muscular contraction, whereas periodic paralysis is defined by episodic attacks of weakness. One sub-type of periodic paralysis, known as hypokalemic periodic paralysis (hypoPP), is associated with low potassium levels. Interestingly, the P1158S missense mutant, located in the third domain S4-S5 linker of the “skeletal muscle”, Nav1.4, has been implicated in causing both myotonia and hypoPP. A common trigger for these conditions is physical activity. We previously reported that Nav1.4 is relatively insensitive to changes in extracellular pH compared to Nav1.2 and Nav1.5. Given that intense exercise is often accompanied by blood acidosis, we decided to test whether changes in pH would push gating in P1158S towards either phenotype. Our results suggest that, unlike in WT-Nav1.4, low pH depolarizes the voltage-dependence of activation and steady-state fast inactivation, decreases current density, and increases late currents in P1185S. Thus, P1185S turns the normally pH-insensitive Nav1.4 into a proton-sensitive channel. Using action potential modeling we predict a pH-to-phenotype correlation in patients with P1158S. We conclude that activities which alter blood pH may trigger the noted phenotypes in P1158S patients

Inhibition of voltage-dependent sodium currents by cannabidiol

Voltage-gated sodium channels initiate action potentials in excitable tissues. Altering these channels’ function can lead to many pathophysiological conditions. The family of voltage-gated sodium channel genes encodes 10 proteins (including Nav2.1) distributed throughout the central and peripheral nervous systems, cardiac and skeletal muscles. The SCN4A gene encodes the Nav1.4 channel, which is primarily responsible for depolarization of the skeletal muscle fibers. Many mutations in SCN4A are found and associated with the myotonic syndromes and periodic paralyses. These conditions are both considered gain-of-function and can be severely life-limiting with respect to performing everyday tasks. From a broader standpoint, hyperexcitability presents as a significant problem in other tissues besides skeletal muscles. Gain-of-function in sodium channels has been linked to a wide-range of pathophysiological conditions such as inherited erythromelalgia, epilepsy, and arrhythmias. Treating these types of pathologies requires an in-depth understanding of their underlying mechanisms. One way to gain this understanding is to investigate physiological triggers. There is also a dire need for novel ways of reducing the hyperexcitability associated with mutant sodium channels. One promising compound is the non-psychotropic component of the Cannabis sativa plant, cannabidiol. This compound has recently been shown to modulate some of the neuronal sodium channels. Although cannabidiol has shown efficacy in clinical trials, the underlying mechanism of action remains unknown. Sodium channels could be among the molecular targets for cannabidiol.In my doctoral research: 1) I studied how a single missense mutation, P1158S, in Nav1.4 causes various degrees of gain-of-function (myotonia and periodic paralysis) by using pH changes to probe P1158S gating modifications; 2) I studied the inhibitory effects of cannabidiol on voltage-dependent sodium currents; 3) I investigated the mechanism through which cannabidiol imparts inhibition. Overall, these data reveal novel insights into sodium channel hyperexcitability and pharmacologically targeting of this hyperexcitability using cannabidiol

Additional file 14: S5. of Network topology of NaV1.7 mutations in sodium channel-related painful disorders

YASARA. Structural modelling of L1267 NaV1.7 variant and their interatomic bonds. (SCE 707 kb

Additional file 13: S4. of Network topology of NaV1.7 mutations in sodium channel-related painful disorders

YASARA. Structural modelling of H858 NaV1.7 variant and their interatomic bonds. (SCE 775 kb

Additional file 6: Figure S2. of Network topology of NaV1.7 mutations in sodium channel-related painful disorders

Degree variation (∆D) in NaV1.7 mutations compared to WT. (DOCX 1691 kb

Additional file 4: Figure S1. of Network topology of NaV1.7 mutations in sodium channel-related painful disorders

Ramachandran plot of NaV1.7 WT. (DOCX 233 kb