Cationic Modulation of Voltage-Gated Sodium Channel (Nav1.5): Neonatal Versus Adult Splice Variants-1. Monovalent (H+) Ions

Sodium channels and the ionic microenvironment of breast tumours

Cancers of epithelial origin such as breast, prostate, cervical, gastric, colon and lung cancer account for a large proportion of deaths worldwide. Better treatment of metastasis, the main cause of cancer deaths, is therefore urgently required. Several of these tumours have been shown to have an abnormally high concentration of Na+ ([Na+ ]) and emerging evidence points to this accumulation being due to elevated intracellular [Na+ ]. This poses intriguing questions about the cellular mechanisms underlying Na+ dysregulation in cancer, and its pathophysiological significance. Elevated intracellular [Na+ ] may be due to alterations in activity of the Na+ /K+ -ATPase, and/or increased influx via Na+ channels and Na+ -linked transporters. Maintenance of the electrochemical Na+ gradient across the plasma membrane is vital to power many cellular processes that are highly active in cancer cells, including glucose and glutamine import. Na+ channels are also upregulated in cancer cells, which in turn promotes tumour growth and metastasis. For example, ENaC and ASICs are overexpressed in cancers, increasing invasion and proliferation. In addition, voltage-gated Na+ channels are also upregulated in a range of tumour types, where they promote metastatic cell behaviours via various mechanisms, including membrane potential depolarisation and altered pH regulation. Together, recent findings relating to elevated Na+ in the tumour microenvironment and how this may be regulated by several classes of Na+ channels provide a link between altered Na+ handling and poor clinical outcome. There are new opportunities to leverage this altered Na+ microenvironment for therapeutic benefit, as exemplified by several ongoing clinical trials.

Discovering the Triad between Nav1.5, Breast Cancer, and the Immune System: A Fundamental Review and Future Perspectives

Nav1.5 is one of the nine voltage-gated sodium channel-alpha subunit (VGSC-α) family members. The Nav1.5 channel typically carries an inward sodium ion current that depolarises the membrane potential during the upstroke of the cardiac action potential. The neonatal isoform of Nav1.5, nNav1.5, is produced via VGSC-α alternative splicing. nNav1.5 is known to potentiate breast cancer metastasis. Despite their well-known biological functions, the immunological perspectives of these channels are poorly explored. The current review has attempted to summarise the triad between Nav1.5 (nNav1.5), breast cancer, and the immune system. To date, there is no such review available that encompasses these three components as most reviews focus on the molecular and pharmacological prospects of Nav1.5. This review is divided into three major subsections: (1) the review highlights the roles of Nav1.5 and nNav1.5 in potentiating the progression of breast cancer, (2) focuses on the general connection between breast cancer and the immune system, and finally (3) the review emphasises the involvements of Nav1.5 and nNav1.5 in the functionality of the immune system and the immunogenicity. Compared to the other subsections, section three is pretty unexploited; it would be interesting to study this subsection as it completes the triad.

Neonatal NaV 1.5: Pharmacological distinctiveness of a cancer-related voltage-gated sodium channel splice variant

BACKGROUND AND PURPOSE

Voltage-gated sodium (Na_V ) channels are expressed de novo in carcinomas where their activity promotes invasiveness. Breast and colon cancer cells express the neonatal splice variant of Na_V 1.5 (nNa_V 1.5) which has several amino acid substitutions in the domain I voltage-sensor compared to its adult counterpart (aNa_V 1.5). This study aimed to determine whether nNa_V 1.5 could be distinguished pharmacologically from aNa_V 1.5.

EXPERIMENTAL APPROACH

Cells expressing either nNa_V 1.5 or aNa_V 1.5 were exposed to small-molecule inhibitors, an antibody or natural toxins, and changes in electrophysiological parameters were measured. Stable expression in EBNA cells and transient expression in Xenopus laevis oocytes were used. Currents were recorded by whole-cell patch clamp and two-electrode voltage-clamp, respectively.

KEY RESULTS

Several clinically-used blockers of Na_v channels (lidocaine, procaine, phenytoin, mexiletine, ranolazine and riluzole) could not distinguish between nNa_V 1.5 or aNa_V 1.5. On the other hand, two tarantula toxins (HaTx and ProTx-II) and a polyclonal antibody (NESOpAb) preferentially inhibited currents elicited by either nNa_V 1.5 or aNa_V 1.5 by binding to the spliced region of the channel. Furthermore, the amino acid residue at position 211 (aspartate in aNa_V 1.5/lysine in nNa_V 1.5), i.e. the charge reversal in the spliced region of the channel, played a key role in the selectivity especially in the antibody binding.

CONCLUSION AND IMPLICATIONS

We conclude that the cancer-related nNa_V 1.5 channel can be distinguished pharmacologically from its nearest neighbour, aNa_V 1.5. Thus, it may be possible to design small molecules as anti-metastatic drugs for non-toxic therapy of nNa_V 1.5-expressing carcinomas.

pH-Channeling in Cancer: How pH-Dependence of Cation Channels Shapes Cancer Pathophysiology

Tissue acidosis plays a pivotal role in tumor progression: in particular, interstitial acidosis promotes tumor cell invasion, and is a major contributor to the dysregulation of tumor immunity and tumor stromal cells. The cell membrane and integral membrane proteins commonly act as important sensors and transducers of altered pH. Cell adhesion molecules and cation channels are prominent membrane proteins, the majority of which is regulated by protons. The pathophysiological consequences of proton-sensitive ion channel function in cancer, however, are scarcely considered in the literature. Thus, the main focus of this review is to highlight possible events in tumor progression and tumor immunity where the pH sensitivity of cation channels could be of great importance.

Cationic Modulation of Voltage-Gated Sodium Channel (Nav1.5): Neonatal Versus Adult Splice Variants-2. Divalent (Cd2+) and Trivalent (Gd3+) Ions

Background: A "neonatal" splice-form of the voltage-gated sodium channel Nav1.5 is functionally expressed in human cancers and potentiates metastatic cell behaviors. Splicing causes the replacement of 7 amino acids, including a negatively charged aspartate211 in the "adult" Nav1.5 (aNav1.5) to a positively charged lysine in the "neonatal" (nNav1.5). These changes occur in the region surrounding the DI:S3-S4 extracellular linker. The splice variants respond differently to changes in extracellular H+ and this could be of pathophysiological significance. However, how the two differentially charged splice variants would react to cations of higher valency is not known. Materials and Methods: We used patch-clamp recording to compare the electrophysiological effects of Cd2+ and Gd3+ on "adult" and "neonatal" Nav1.5 expressed stably in EBNA-293 cells. Several parameters were determined for the two channels and statistically compared. Results: Both cations inhibited peak I Na through reducing G max and induced a positive shift in the voltage range of activation. However, unlike Gd3+, Cd2+ had only a weak effect on voltage dependence of activation, and no effect on voltage dependence of inactivation, recovery from inactivation, or the kinetics of activation/inactivation. Conclusions: The electrophysiological effects of Cd2+ and Gd3+ studied were essentially the same for "neonatal" and "adult" Nav1.5, although these splice variants possess differences in their external charges. In contrast, the effects of H+ were shown earlier to be significantly differential. Taken together, these results suggest that limited adjustment of the charged structure of pharmacological agents could enable selective targeting of neonatal Nav1.5 associated with several cancers.