Contribution of concentration-sensitive sodium channels to the absorption of alveolar fluid in mice

The Nax (SCN7A) channel: an atypical regulator of tissue homeostasis and disease

Within an articulately characterized family of ion channels, the voltage-gated sodium channels, exists a black sheep, SCN7A (Nax). Nax, in contrast to members of its molecular family, has lost its voltage-gated character and instead rapidly evolved a new function as a concentration-dependent sensor of extracellular sodium ions and subsequent signal transducer. As it deviates fundamentally in function from the rest of its family, and since the bulk of the impressive body of literature elucidating the pathology and biochemistry of voltage-gated sodium channels has been performed in nervous tissue, reports of Nax expression and function have been sparse. Here, we investigate available reports surrounding expression and potential roles for Nax activity outside of nervous tissue. With these studies as justification, we propose that Nax likely acts as an early sensor that detects loss of tissue homeostasis through the pathological accumulation of extracellular sodium and/or through endothelin signaling. Sensation of homeostatic aberration via Nax then proceeds to induce pathological tissue phenotypes via promotion of pro-inflammatory and pro-fibrotic responses, induced through direct regulation of gene expression or through the generation of secondary signaling molecules, such as lactate, that can operate in an autocrine or paracrine fashion. We hope that our synthesis of much of the literature investigating this understudied protein will inspire more research into Nax not simply as a biochemical oddity, but also as a potential pathophysiological regulator and therapeutic target.

Knockout of sodium channel Nax delays re-epithelializathion of splinted murine excisional wounds

Mammalian wound healing is a carefully orchestrated process in which many cellular and molecular effectors respond in concert to perturbed tissue homeostasis in order to close the wound and re-establish the skin barrier. The roles of many of these molecular effectors, however, are not entirely understood. Our lab previously demonstrated that the atypical sodium channel Na_x (encoded by Scn7a) responds to wound-induced epidermal dehydration, resulting in molecular cascades that drive pro-inflammatory signaling. Acute inhibition of Na_x was sufficient to attenuate dermatopathological symptoms in models of hypertrophic scar and dermatitis. To date, however, the role of Na_x in excisional wound healing has not been demonstrated. Here we report development of a knockout mouse that lacks expression of functional Na_x , and we demonstrate that lack of functional Na_x results in deficient wound healing in a murine splinted excisional wound healing model. This deficiency in wound healing was reflected in impaired re-epithelialization and decreased keratinocyte proliferation, a finding which was further supported by decreased proliferation upon Na_x knockdown in HaCaT cells in vitro. Defective wound healing was observed alongside increased expression of inflammatory genes in the wound epidermis of Na_x ^-/- mice, suggesting that mice lacking functional Na_x retain the ability to undergo skin inflammation. Our observations here motivate further investigation into the roles of Na_x in wound healing and other skin processes.

Application of cell-based assays for toxicity characterization of complex wastewater matrices: Possible applications in wastewater recycle and reuse

Exposure to pre-concentrated inlet or outlet STP wastewater extracts at different concentrations (0.001% to 1%) induced dose-dependent toxicity in MCF-7 cells, whereas drinking water extracts did not induce cytotoxicity in cells treated. GC-MS analysis revealed the occurrence of xenobiotic compounds (Benzene, Phthalate, etc.) in inlet/outlet wastewater extracts. Cells exposed to inlet/outlet extract showed elevated levels of reactive oxygen species (ROS: inlet: 186.58%, p<0.05, outlet, 147.8%, p<0.01) and loss of mitochondrial membrane potential (Δψm: inlet, 74.91%, p<0.01; outlet, 86.70%, p<0.05) compared to the control. These concentrations induced DNA damage (Tail length: inlet: 34.4%, p<0.05, outlet, 26.7%, p<0.05) in treated cells compared to the control (Tail length: 7.5%). Cell cycle analysis displayed drastic reduction in the G1 phase in treated cells (inlet, G1:45.0%; outlet, G1:58.3%) compared to the control (G1:67.3%). Treated cells showed 45.18% and 28.0% apoptosis compared to the control (1.2%). Drinking water extracts did not show any significant alterations with respect to ROS, Δψm, DNA damage, cell cycle and apoptosis compared to the control. Genes involved in cell cycle and apoptosis were found to be differentially expressed in cells exposed to inlet/outlet extracts. Herein, we propose cell-based toxicity assays to evaluate the efficacies of wastewater treatment and recycling processes.

Gene expression of the concentration-sensitive sodium channel is suppressed in lipopolysaccharide-induced acute lung injury in mice

PURPOSE

The concentration-sensitive sodium channel (Na_C) is expressed in alveolar type II epithelial cells and pulmonary microvascular endothelial cells in mouse lungs. We recently reported that Na_C contributes to amiloride-insensitive sodium transport in mouse lungs (Respiratory Physiology & Neurobiology, 2016). However, details regarding its physiological role in the lung remain unknown. To examine whether Na_C is involved in alveolar fluid clearance during an acute lung injury (ALI), we analyzed the relationship between Na_C gene expression in the lung and the development of pulmonary edema in lipopolysaccharide (LPS)-induced ALI mice.

METHODS

LPS-induced ALI mice were prepared by the intratracheal administration of LPS. Bronchoalveolar lavage (BAL) neutrophils and lung water content (LWCs) were used as a marker of ALI and pulmonary edema, respectively. Na_C protein production in the lung was detected by immunoblotting and immunofluorescence. The gene expressions of Na_C and the epithelial sodium channel (ENaC) of LPS-induced ALI mice were examined by quantitative RT-PCR over a time course of 14 days.

RESULTS

The BAL neutrophil count increased until day 2 after LPS administration and had nearly recovered by day 6. LWCs in LPS-induced mice gradually increased until day 8 and had recovered by day 14. The expression of the Na_C protein in the lungs of LPS-induced mice dramatically decreased from day 2 to day 6, but recovered by day 8. The mRNA expression of Na_C decreased in the lung, as well as those for α-, β-, and γ-ENaC during ALI. Thus, Na_C expression is suppressed during the development stage of pulmonary edema and then recovers in the convalescent phase.

CONCLUSION

Our results suggest that suppression of the gene expression of Na_C is involved in the development of pulmonary edema in ALI.