NaX sodium channel is expressed in non-myelinating Schwann cells and alveolar type II cells in mice

Harmonized cross-species cell atlases of trigeminal and dorsal root ganglia

Peripheral sensory neurons in the dorsal root ganglion (DRG) and trigeminal ganglion (TG) are specialized to detect and transduce diverse environmental stimuli including touch, temperature, and pain to the central nervous system. Recent advances in single-cell RNA-sequencing (scRNA-seq) have provided new insights into the diversity of sensory ganglia cell types in rodents, non-human primates, and humans, but it remains difficult to compare transcriptomically defined cell types across studies and species. Here, we built cross-species harmonized atlases of DRG and TG cell types that describe 18 neuronal and 11 non-neuronal cell types across 6 species and 19 studies. We then demonstrate the utility of this harmonized reference atlas by using it to annotate newly profiled DRG nuclei/cells from both human and the highly regenerative axolotl. We observe that the transcriptomic profiles of sensory neuron subtypes are broadly similar across vertebrates, but the expression of functionally important neuropeptides and channels can vary notably. The new resources and data presented here can guide future studies in comparative transcriptomics, simplify cell type nomenclature differences across studies, and help prioritize targets for future pain therapy development.

Transcriptomic Analysis of the Ocular Posterior Segment Completes a Cell Atlas of the Human Eye

Although the visual system extends through the brain, most vision loss originates from defects in the eye. Its central element is the neural retina, which senses light, processes visual signals, and transmits them to the rest of the brain through the optic nerve (ON). Surrounding the retina are numerous other structures, conventionally divided into anterior and posterior segments. Here we used high-throughput single nucleus RNA sequencing (snRNA-seq) to classify and characterize cells in the extraretinal components of the posterior segment: ON, optic nerve head (ONH), peripheral sclera, peripapillary sclera (PPS), choroid, and retinal pigment epithelium (RPE). Defects in each of these tissues are associated with blinding diseases - for example, glaucoma (ONH and PPS), optic neuritis (ON), retinitis pigmentosa (RPE), and age-related macular degeneration (RPE and choroid). From ∼151,000 single nuclei, we identified 37 transcriptomically distinct cell types, including multiple types of astrocytes, oligodendrocytes, fibroblasts, and vascular endothelial cells. Our analyses revealed a differential distribution of many cell types among distinct structures. Together with our previous analyses of the anterior segment and retina, the new data complete a "Version 1" cell atlas of the human eye. We used this atlas to map the expression of >180 genes associated with the risk of developing glaucoma, which is known to involve ocular tissues in both anterior and posterior segments as well as neural retina. Similar methods can be used to investigate numerous additional ocular diseases, many of which are currently untreatable.

scRNA-seq generates a molecular map of emerging cell subtypes after sciatic nerve injury in rats

Patients with peripheral nerve injury, viral infection or metabolic disorder often suffer neuropathic pain due to inadequate pharmacological options for relief. Developing novel therapies has been challenged by incomplete mechanistic understanding of the cellular microenvironment in sensory nerve that trigger the emergence and persistence of pain. In this study, we report a high resolution transcriptomics map of the cellular heterogeneity of naïve and injured rat sensory nerve covering more than 110,000 individual cells. Annotation reveals distinguishing molecular features of multiple major cell types totaling 45 different subtypes in naïve nerve and an additional 23 subtypes emerging after injury. Ligand-receptor analysis revealed a myriad of potential targets for pharmacological intervention. This work forms a comprehensive resource and unprecedented window into the cellular milieu underlying neuropathic pain and demonstrates that nerve injury is a dynamic process orchestrated by multiple cell types in both the endoneurial and epineurial nerve compartments.

Central regulation of body fluid homeostasis

Extracellular fluids, including blood, lymphatic fluid, and cerebrospinal fluid, are collectively called body fluids. The Na⁺ concentration ([Na⁺]) in body fluids is maintained at 135-145 mM and is broadly conserved among terrestrial animals. Homeostatic osmoregulation by Na⁺ is vital for life because severe hyper- or hypotonicity elicits irreversible organ damage and lethal neurological trauma. To achieve "body fluid homeostasis" or "Na homeostasis", the brain continuously monitors [Na⁺] in body fluids and controls water/salt intake and water/salt excretion by the kidneys. These physiological functions are primarily regulated based on information on [Na⁺] and relevant circulating hormones, such as angiotensin II, aldosterone, and vasopressin. In this review, we discuss sensing mechanisms for [Na⁺] and hormones in the brain that control water/salt intake behaviors, together with the responsible sensors (receptors) and relevant neural pathways. We also describe mechanisms in the brain by which [Na⁺] increases in body fluids activate the sympathetic neural activity leading to hypertension.

Profiling sensory neuron microenvironment after peripheral and central axon injury reveals key pathways for neural repair

Sensory neurons with cell bodies in dorsal root ganglia (DRG) represent a useful model to study axon regeneration. Whereas regeneration and functional recovery occurs after peripheral nerve injury, spinal cord injury or dorsal root injury is not followed by regenerative outcomes. Regeneration of sensory axons in peripheral nerves is not entirely cell autonomous. Whether the DRG microenvironment influences the different regenerative capacities after injury to peripheral or central axons remains largely unknown. To answer this question, we performed a single-cell transcriptional profiling of mouse DRG in response to peripheral (sciatic nerve crush) and central axon injuries (dorsal root crush and spinal cord injury). Each cell type responded differently to the three types of injuries. All injuries increased the proportion of a cell type that shares features of both immune cells and glial cells. A distinct subset of satellite glial cells (SGC) appeared specifically in response to peripheral nerve injury. Activation of the PPARα signaling pathway in SGC, which promotes axon regeneration after peripheral nerve injury, failed to occur after central axon injuries. Treatment with the FDA-approved PPARα agonist fenofibrate increased axon regeneration after dorsal root injury. This study provides a map of the distinct DRG microenvironment responses to peripheral and central injuries at the single-cell level and highlights that manipulating non-neuronal cells could lead to avenues to promote functional recovery after CNS injuries or disease.

Diversity of developing peripheral glia revealed by single-cell RNA sequencing

The peripheral nervous system responds to a wide variety of sensory stimuli, a process that requires great neuronal diversity. These diverse neurons are closely associated with glial cells originating from the neural crest. However, the molecular nature and diversity among peripheral glia are not understood. Here, we used single-cell RNA sequencing to profile developing and mature glia from somatosensory dorsal root ganglia and auditory spiral ganglia. We found that glial precursors (GPs) in these two systems differ in their transcriptional profiles. Despite their unique features, somatosensory and auditory GPs undergo convergent differentiation to generate molecularly uniform myelinating and non-myelinating Schwann cells. By contrast, somatosensory and auditory satellite glial cells retain system-specific features. Lastly, we identified a glial signature gene set, providing new insights into commonalities among glia across the nervous system. This survey of gene expression in peripheral glia constitutes a resource for understanding functions of glia across different sensory modalities.

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.

Corneal nonmyelinating Schwann cells illuminated by single-cell transcriptomics and visualized by protein biomarkers

The cornea is the most innervated tissue in the human body. Myelinated axons upon inserting into the peripheral corneal stroma lose their myelin sheaths and continue into the central cornea wrapped by only nonmyelinating corneal Schwann cells (nm-cSCs). This anatomical organization is believed to be important for central vision. Here we employed single-cell RNA sequencing (scRNA-seq), microscopy, and transgenics to characterize these nm-cSCs of the central cornea. Using principal component analysis, uniform manifold approximation and projection, and unsupervised hierarchal cell clustering of scRNA-seq data derived from central corneal cells of male rabbits, we successfully identified several clusters representing different corneal cell types, including a unique cell cluster representing nm-cSCs. To confirm protein expression of cSC genes, we performed cross-species validation, employing corneal whole-mount immunostaining with confocal microscopy in mouse corneas. The expression of several representative proteins of nm-cSCs were validated. As the proteolipid protein 1 (PLP1) gene was also expressed in nm-cSCs, we explored the Plp1-eGFP transgenic reporter mouse line to visualize cSCs. Specific and efficient eGFP expression was observed in cSCs in adult mice of different ages. Of several putative cornea-specific SC genes identified, Dickkopf-related protein 1 was shown to be present in nm-cSCs. Taken together, our findings, for the first time, identify important insights and tools toward the study nm-cSCs in isolated tissue and adult animals. We expect that our results will advance the future study of nm-cSCs in applications of nerve repair, and provide a resource for the study of corneal sensory function.

Transcriptional and epigenomic landscapes of CNS and non-CNS vascular endothelial cells

Vascular endothelial cell (EC) function depends on appropriate organ-specific molecular and cellular specializations. To explore genomic mechanisms that control this specialization, we have analyzed and compared the transcriptome, accessible chromatin, and DNA methylome landscapes from mouse brain, liver, lung, and kidney ECs. Analysis of transcription factor (TF) gene expression and TF motifs at candidate cis-regulatory elements reveals both shared and organ-specific EC regulatory networks. In the embryo, only those ECs that are adjacent to or within the central nervous system (CNS) exhibit canonical Wnt signaling, which correlates precisely with blood-brain barrier (BBB) differentiation and Zic3 expression. In the early postnatal brain, single-cell RNA-seq of purified ECs reveals (1) close relationships between veins and mitotic cells and between arteries and tip cells, (2) a division of capillary ECs into vein-like and artery-like classes, and (3) new endothelial subtype markers, including new validated tip cell markers.

Innervation of the Human Incisive Papilla: Comparison with Other Oral Regions

Immunohistochemistry for several neurochemical substances was performed on the human incisive papilla and other oral structures. Sodium channel alpha subunit 7 (SCN7A) protein-immunoreactive (IR) Schwann cells and protein gene product 9.5 (PGP 9.5)-IR nerve fibers made nerve plexuses beneath the epithelium of the palate, including the incisive papilla, tongue, and lip. SCN7A immunoreactivity could also be detected in lamellated and nonlamellated capsules of corpuscle endings. Lamellated SCN7A-IR corpuscle endings were mostly restricted to the mucous and cutaneous lips. These endings had thick and spiral-shaped PGP 9.5-IR axons without ramification. Nonlamellated SCN7A-IR corpuscle endings were most numerous in the incisive papilla among the oral regions. On the basis of axonal morphology, the nonlamellated endings were divided into simple and complex types. PGP 9.5-IR terminal axons in the simple type ran straight or meandered with slight ramification, whereas those in the complex type were densely entangled with abundant ramification. Substance P (SP)-, calcitonin gene-related peptide (CGRP)-, and transient receptor potential cation channel subfamily V member 2 (TRPV2)-IR varicose fibers were rarely seen beneath the epithelium of oral structures. The present study indicates that the human incisive papilla has many low-threshold mechanoreceptors with nonlamellated capsules. SP-, CGRP-, and TRPV2-containing nociceptors may be infrequent in the incisive papilla and other oral regions.

Na+ /K+ -ATPase coupled to endothelin receptor type B stimulates peripheral nerve regeneration via lactate signalling

We have recently demonstrated that endothelin (ET) is functionally coupled to Na_x , a Na⁺ concentration-sensitive Na⁺ channel for lactate release via ET receptor type B (ET_B R) and is involved in peripheral nerve regeneration in a sciatic nerve transection-regeneration mouse model. Na_x is known to interact directly with Na⁺ /K⁺ -ATPase, leading to lactate production in the brain. To investigate the role of Na⁺ /K⁺ -ATPase in peripheral nerve regeneration, in this study, we applied ouabain, a Na⁺ /K⁺ -ATPase inhibitor, to the cut site for 4 weeks with an osmotic pump. While functional recovery and nerve reinnervation to the toe started at 5 weeks after axotomy and were completed by 7 weeks, ouabain delayed them by 2 weeks. The delay by ouabain was improved by lactate, and its effect was blocked by α-cyano-4-hydroxy-cinnamic acid (CIN), a broad monocarboxylate transporter (MCT) inhibitor. In primary cultures of dorsal root ganglia, neurite outgrowth of neurons and lactate release into the culture medium was inhibited by ouabain. Conversely, lactate enhanced the neurite outgrowth, which was blocked by CIN, but not by AR-C155858, a MCT1/2-selective inhibitor. ET-1 and ET-3 increased neurite outgrowth of neurons, which was attenuated by an ET_B R antagonist, ouabain and 2 protein kinase C inhibitors. Taken together with the finding that ET_B R was expressed in Schwann cells, these results demonstrate that ET enhanced neurite outgrowth of neurons mediated by Na⁺ /K⁺ -ATPase via ET_B R in Schwann cells. This study suggests that Na⁺ /K⁺ -ATPase coupled to the ET-ET_B R system plays a critical role in peripheral nerve regeneration via lactate signalling.

Association of SCN10A Polymorphisms with the Recurrence of Atrial Fibrillation after Catheter Ablation in a Chinese Han Population

The nonsynonymous SCN10A single nucleotide polymorphism (SNP) rs6795970 has been reported to associate with PR interval and atrial fibrillation (AF) and in strong linkage disequilibrium (LD) with the AF-associated SNP rs6800541. In this study, we investigated whether rs6795970 polymorphisms are associated with AF recurrence after catheter ablation. A total of 502 consecutive patients with AF who underwent catheter ablation were included. AF recurrence was defined as a documented episode of any atrial arrhythmias lasting ≥30 s after a blanking period of 3 months. AF recurrence was observed between 3 and 12 months after catheter ablation in 24.5% of the patients. There was a significant difference in the allele distribution (p = 7.86 × 10⁻⁵) and genotype distribution (p = 1.42 × 10⁻⁵) of rs6795970 between the AF recurrence and no recurrence groups. In a multivariate analysis, we identified the following independent predictors of AF recurrence: the rs6795970 genotypes in an additive model (OR 0.36, 95%CI 0.22~0.60, p = 7.04 × 10⁻⁵), a history of AF ≥36 months (OR 3.57, 95%CI 2.26~5.63, p = 4.33 × 10⁻⁸) and left atrial diameter (LAD) ≥40 mm (OR 1.85, 95%CI 1.08~3.19, p = 0.026). These data suggest that genetic variation in SCN10A may play an important role in predicting AF recurrence after catheter ablation in the Chinese Han population.

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

The concentration-sensitive sodium channel (Nac) is activated by an increase in the extracellular sodium concentration. Although the expression of Nac in alveolar type II epithelial cells (AEC II) has been reported previously, the physiological role of Nac in the lung has not been established. We characterized Nac expression and examined amiloride-insensitive sodium transport mediated by Nac in mouse lung. Immunofluorescence studies revealed that Nac did not colocalize with either aquaporin 5 or cystic fibrosis transmembrane conductance regulator, but partially colocalized with the epithelial sodium channel γ-subunit. Immunoelectron microscopy studies showed that Nac localized at the basolateral membrane of pulmonary microvascular endothelial cells (PMVECs). Nac mRNA and protein were expressed in PMVECs isolated from the lungs of mice. Image analysis indicated that sodium influx into the alveolar wall was dependent on increases in extracellular sodium concentration. We conclude that Nac expressed in PMVECs and AEC II contributes to the reabsorption of sodium via an amiloride-insensitive pathway during alveolar fluid clearance.

Glial Fibrillary Acidic Protein-Expressing Glia in the Mouse Lung

Autonomic nerves regulate important functions in visceral organs, including the lung. The postganglionic portion of these nerves is ensheathed by glial cells known as non-myelinating Schwann cells. In the brain, glia play important functional roles in neurotransmission, neuroinflammation, and maintenance of the blood brain barrier. Similarly, enteric glia are now known to have analogous roles in gastrointestinal neurotransmission, inflammatory response, and barrier formation. In contrast to this, very little is known about the function of glia in other visceral organs. Like the gut, the lung forms a barrier between airborne pathogens and the bloodstream, and autonomic lung innervation is known to affect pulmonary inflammation and lung function. Lung glia are described as non-myelinating Schwann cells but their function is not known, and indeed no transgenic tools have been validated to study them in vivo. The primary goal of this research was, therefore, to investigate the relationship between non-myelinating Schwann cells and pulmonary nerves in the airways and vasculature and to validate existing transgenic mouse tools that would be useful for studying their function. We focused on the glial fibrillary acidic protein promoter, which is a cognate marker of astrocytes that is expressed by enteric glia and non-myelinating Schwann cells. We describe the morphology of non-myelinating Schwann cells in the lung and verify that they express glial fibrillary acidic protein and S100, a classic glial marker. Furthermore, we characterize the relationship of non-myelinating Schwann cells to pulmonary nerves. Finally, we report tools for studying their function, including a commercially available transgenic mouse line.

Channel properties of Nax expressed in neurons

Nax is a sodium-concentration ([Na+])-sensitive Na channel with a gating threshold of ~150 mM for extracellular [Na+] ([Na+]o) in vitro. We previously reported that Nax was preferentially expressed in the glial cells of sensory circumventricular organs including the subfornical organ, and was involved in [Na+] sensing for the control of salt-intake behavior. Although Nax was also suggested to be expressed in the neurons of some brain regions including the amygdala and cerebral cortex, the channel properties of Nax have not yet been adequately characterized in neurons. We herein verified that Nax was expressed in neurons in the lateral amygdala of mice using an antibody that was newly generated against mouse Nax. To investigate the channel properties of Nax expressed in neurons, we established an inducible cell line of Nax using the mouse neuroblastoma cell line, Neuro-2a, which is endogenously devoid of the expression of Nax. Functional analyses of this cell line revealed that the [Na+]-sensitivity of Nax in neuronal cells was similar to that expressed in glial cells. The cation selectivity sequence of the Nax channel in cations was revealed to be Na+ ≈ Li+ > Rb+ > Cs+ for the first time. Furthermore, we demonstrated that Nax bound to postsynaptic density protein 95 (PSD95) through its PSD95/Disc-large/ZO-1 (PDZ)-binding motif at the C-terminus in neurons. The interaction between Nax and PSD95 may be involved in promoting the surface expression of Nax channels because the depletion of endogenous PSD95 resulted in a decrease in Nax at the plasma membrane. These results indicated, for the first time, that Nax functions as a [Na+]-sensitive Na channel in neurons as well as in glial cells.

Sodium sensing in the brain

Sodium (Na) homeostasis is crucial for life, and the Na⁺ level ([Na⁺]) of body fluids is strictly maintained at a range of 135–145 mM. However, the existence of a [Na⁺] sensor in the brain has long been controversial until Na_x was identified as the molecular entity of the sensor. This review provides an overview of the [Na⁺]-sensing mechanism in the brain for the regulation of salt intake by summarizing a series of our studies on Na_x. Na_x is a Na channel expressed in the circumventricular organs (CVOs) in the brain. Among the CVOs, the subfornical organ (SFO) is the principal site for the control of salt intake behavior, where Na_x populates the cellular processes of astrocytes and ependymal cells enveloping neurons. A local expression of endothelin-3 in the SFO modulates the [Na⁺] sensitivity for Na_x activation, and thereby Na_x is likely to be activated in the physiological [Na⁺] range. Na_x stably interacts with Na⁺/K⁺-ATPase whereby Na⁺ influx via Na_x is coupled with activation of Na⁺/K⁺-ATPase associated with the consumption of ATP. The consequent activation of anaerobic glucose metabolism of Na_x-positive glial cells upregulates the cellular release of lactate, and this lactate functions as a gliotransmitter to activate GABAergic neurons in the SFO. The GABAergic neurons presumably regulate hypothetic neurons involved in the control of salt intake behavior. Recently, a patient with essential hypernatremia caused by autoimmunity to Na_x was found. In this case, the hypernatremia was considered to be induced by the complement-mediated cell death in the CVOs, where Na_x specifically populates.

The Nax Channel

Na x, which is preferentially expressed in the glial cells of sensory circumventricular organs in the brain, is a sodium channel that is poorly homologous to voltage-gated sodium channels. We previously reported that Na x is a sodium concentration ([Na+])-sensitive, but not a voltage-sensitive channel that is critically involved in body-fluid homeostasis. Nax-knockout mice do not stop ingesting salt even when dehydrated and transiently develop hypernatremia. [Na+] in body fluids is strictly controlled at 135 to 145 mM in mammals. Although the set point must be within this range, Na x was shown to have a threshold value of ~150 mM for extracellular [Na+] ([Na+]o) for activation in vitro. Therefore, the [Na+]o dependency of Na x in vivo is presumably modified by an as yet unidentified mechanism. We recently demonstrated that the [Na+]o dependency of Na x in the subfornical organ was adjusted to the physiological range by endothelin-3. Pharmacological experiments revealed that endothelin receptor B signaling was involved in this modulation of Na x gating through protein kinase C and ERK1/2 activation. In addition, we identified a case of essential hypernatremia caused by autoimmunity to Na x. Occurrence of a ganglioneuroma composed of Schwann-like cells that robustly expressed Na x was likely to induce the autoimmune response in this patient. An intravenous injection of the immunoglobulin fraction of the patient’s serum, which contained anti-Na x antibodies, into mice reproduced the patient’s symptoms. This review provides an overview of the physiological functions of Na x by summarizing our recent studies.

Endothelin-3 expression in the subfornical organ enhances the sensitivity of Na(x), the brain sodium-level sensor, to suppress salt intake

Salt homeostasis is essential to survival, but brain mechanisms for salt-intake control have not been fully elucidated. Here, we found that the sensitivity of Na(x) channels to [Na(+)](o) is dose-dependently enhanced by endothelin-3 (ET-3). Na(x) channels began to open when [Na(+)](o) exceeded ~150 mM without ET-3, but opened fully at a physiological [Na(+)](o) (135–145 mM) with 1 nM ET-3. Importantly, ET-3 was expressed in the subfornical organ (SFO) along with Nax, and the level was robustly increased by dehydration. Pharmacological experiments revealed that endothelin receptor B (ET(B)R) signaling is involved in this modulation of Na(x) gating through protein kinase C and ERK1/2 activation. ET(B)R agonists increased the firing rate of GABAergic neurons via lactate in the SFO, and an ET(B)R antagonist attenuated salt aversion during dehydration. These results indicate that ET-3 expression in the SFO is tightly coupled with body-fluid homeostasis through modulation of the [Na(+)](o) sensitivity of Na(x).

The sodium-activated sodium channel is expressed in the rat kidney thick ascending limb and collecting duct cells and is upregulated during high salt intake

Increased dietary salt triggers oxidative stress and kidney injury in salt-sensitive hypertension; however, the mechanism for sensing increased extracellular Na(+) concentration ([Na(+)]) remains unclear. A Na(+)-activated Na(+) channel (Na sensor) described in the brain operates as a sensor of extracellular fluid [Na(+)]; nonetheless, its presence in the kidney has not been established. In the present study, we demonstrated the gene expression of the Na sensor by RT-PCR and Western blotting in the Sprague-Dawley rat kidney. Using immunofluorescence, the Na sensor was localized to the luminal side in tubular epithelial cells of collecting ducts colocalizing with aquaporin-2, a marker of principal cells, and in thick ascending limb, colocalizing with the glycoprotein Tamm-Horsfall. To determine the effect of a high-salt diet (HSD) on Na sensor gene expression, we quantified its transcript and protein levels primarily in renal medullas from control rats and rats subjected to 8% NaCl for 7 days (n = 5). HSD increased Na sensor expression levels (mRNA: from 1.2 ± 0.2 to 5.1 ± 1.3 au; protein: from 0.98 ± 0.15 to 1.74 ± 0.28 au P < 0.05) in the kidney medulla, but not in the cortex. These data indicate that rat kidney epithelial cells of the thick ascending limb and principal cells of the collecting duct possess a Na sensor that is upregulated by HSD, suggesting an important role in monitoring changes in tubular fluid [Na(+)].

Lethal arrhythmias in Tbx3-deficient mice reveal extreme dosage sensitivity of cardiac conduction system function and homeostasis

TBX3 is critical for human development: mutations in TBX3 cause congenital anomalies in patients with ulnar-mammary syndrome. Data from mice and humans suggest multiple roles for Tbx3 in development and function of the cardiac conduction system. The mechanisms underlying the functional development, maturation, and maintenance of the conduction system are not well understood. We tested the requirements for Tbx3 in these processes. We generated a unique series of Tbx3 hypomorphic and conditional mouse mutants with varying levels and locations of Tbx3 activity within the heart, and developed techniques for evaluating in vivo embryonic conduction system function. Disruption of Tbx3 function in different regions of the developing heart causes discrete phenotypes and lethal arrhythmias: sinus pauses and bradycardia indicate sinoatrial node dysfunction, whereas preexcitation and atrioventricular block reveal abnormalities in the atrioventricular junction. Surviving Tbx3 mutants are at increased risk for sudden death. Arrhythmias induced by knockdown of Tbx3 in adults reveal its requirement for conduction system homeostasis. Arrhythmias in Tbx3-deficient embryos are accompanied by disrupted expression of multiple ion channels despite preserved expression of previously described conduction system markers. These findings indicate that Tbx3 is required for the conduction system to establish and maintain its correct molecular identity and functional properties. In conclusion, Tbx3 is required for the functional development, maturation, and homeostasis of the conduction system in a highly dosage-sensitive manner. TBX3 and its regulatory targets merit investigation as candidates for human arrhythmias.

Autoimmunity to the sodium-level sensor in the brain causes essential hypernatremia

Na(x) is the sodium-level sensor of body fluids in the brain involved in sodium homeostasis. Na(x)-knockout mice do not stop ingesting salt even when dehydrated. Here we report a case with clinical features of essential hypernatremia without demonstrable hypothalamic structural lesions, who was diagnosed as a paraneoplastic neurologic disorder. The patient had autoantibodies directed against Na(x), along with a ganglioneuroma composed of Schwann-like cells robustly expressing Na(x). The removal of the tumor did not reduce the autoantibody levels or relieve the symptoms. Intravenous injection of the immunoglobulin fraction of the patient's serum into mice induced abnormalities in water/salt intake and diuresis, which led to hypernatremia. In the brains of these mice, cell death was observed along with focal deposits of complement C3 and inflammatory infiltrates in circumventricular organs where Na(x) is specifically expressed. Our findings thus provide new insights into the pathogenesis of hypernatremia relevant to the sodium-level-sensing mechanism in humans.

Identification and functional characterization of the promoter of the mouse sodium-activated sodium channel Na(x) gene (Scn7a)

Na(x) is a sodium channel, thought to be a descendant of the voltage-gated sodium channel family. Nevertheless, Na(x) is not activated by voltage but rather by augmentation of extracellular sodium over 150 mM. In the brain, it is localized to the circumventricular organs, important regions for salt and water homeostasis in mammals, where it operates as a sodium-level sensor of body fluid. Na(x) channel is expressed in lung, uterus, and heart, and it is also found in trigeminal and dorsal root ganglia and in nonmyelinating Schwann cells, where its physiological role remains unclarified. Here we identified the promoter and transcription start sites of Na(x) sodium channel in dorsal root ganglia neurons from mouse. We report a characterization of the basal TATA-less promoter and the sequence requirements for promoter activity in Neuro 2A neuroblastoma cells and in dorsal root ganglia neurons, where basal promoter activity seems to require NGFI-C and Ebox DNA elements. Finally, we provide evidence that a repression mechanism that inhibits Na(x) expression may be present in certain tissues. These findings provide the basis with which to understand tissue-specific regulation of Na(x) sodium channel gene (Scn7a) expression.

Comparative study of the distribution of the alpha-subunits of voltage-gated sodium channels in normal and axotomized rat dorsal root ganglion neurons

We compared the distribution of the alpha-subunit mRNAs of voltage-gated sodium channels Nav1.1-1.3 and Nav1.6-1.9 and a related channel, Nax, in histochemically identified neuronal subpopulations of the rat dorsal root ganglia (DRG). In the naïve DRG, the expression of Nav1.1 and Nav1.6 was restricted to A-fiber neurons, and they were preferentially expressed by TrkC neurons, suggesting that proprioceptive neurons possess these channels. Nav1.7, -1.8, and -1.9 mRNAs were more abundant in C-fiber neurons compared with A-fiber ones. Nax was evenly expressed in both populations. Although Nav1.8 and -1.9 were preferentially expressed by TrkA neurons, other alpha-subunits were expressed independently of TrkA expression. Actually, all IB4(+) neurons expressed both Nav1.8 and -1.9, and relatively limited subpopulations of IB4(+) neurons (3% and 12%, respectively) expressed Nav1.1 and/or Nav1.6. These findings provide useful information in interpreting the electrophysiological characteristics of some neuronal subpopulations of naïve DRG. After L5 spinal nerve ligation, Nav1.3 mRNA was up-regulated mainly in A-fiber neurons in the ipsilateral L5 DRG. Although previous studies demonstrated that nerve growth factor (NGF) and glial cell-derived neurotrophic factor (GDNF) reversed this up-regulation, the Nav1.3 induction was independent of either TrkA or GFRalpha1 expression, suggesting that the induction of Nav1.3 may be one of the common responses of axotomized DRG neurons without a direct relationship to NGF/GDNF supply.

Peripheral neuropathy in the twitcher mouse: accumulation of extracellular matrix in the endoneurium and aberrant expression of ion channels

Globoid cell leukodystrophy (GLD; Krabbe's disease), caused by a genetic galactosylceramidase deficiency, affects both the central and peripheral nervous systems (CNS and PNS). Allogenic hematopoietic stem-cell transplantation (HSCT) has been beneficial for clinical improvement of this disease. However, recent reports by Siddiqi et al. suggested that none of their transplanted patients achieved complete normalization of their peripheral nerve function, despite the well-documented remyelination of the CNS and PNS in the treated patients. We hypothesized that the PNS dysfunction in GLD is due to altered Schwann cell-axon interactions, resulting in structural abnormalities of the node of Ranvier and aberrant expression of ion channels caused by demyelination and that the persistence of this altered interaction is responsible for the dysfunction of the PNS after HSCT. Since there has not been any investigation of the Schwann cell-axonal relationship in twitcher mice, an authentic model of GLD, we first investigated structural abnormalities, focusing on the node of Ranvier in untreated twitcher mice, and compared the results with those obtained after receiving bone marrow transplantation (BMT). As expected, we found numerous supernumerary Schwann cells that formed structurally abnormal nodes of Ranvier. Similar findings, though at somewhat variable extent, were detected in mice treated with BMT. Activated supernumerary Schwann cells expressed GFAP immunoreactivity and generated Alcian blue-positive extracellular matrix (ECM) in the endoneurial space. The processes of these supernumerary Schwann cells often covered and obliterated the nodal regions. Furthermore, the distribution of Na(+) channel immunoreactivity was diffuse without the concentration at the nodes of Ranvier as seen in wild-type mice. Neither K(+) channels nor Neurexin IV/ Caspr/ Paranoidin (NCP-1) were detected in the twi/twi sciatic nerve. The results of our study suggest the importance of normalization of the Schwann cell-axon relationship for the functional recovery of peripheral nerves, when one considers therapeutic strategies for PNS pathology in GLD.

Functional genomic responses to cystic fibrosis transmembrane conductance regulator (CFTR) and CFTR(delta508) in the lung

Cystic fibrosis (CF), a common lethal pulmonary disorder in Caucasians, is caused by mutations in the cystic fibrosis transmembrane conductance regulator gene (CFTR) that disturbs fluid homeostasis and host defense in target organs. The effects of CFTR and delta508-CFTR were assessed in transgenic mice that 1) lack CFTR expression (Cftr-/-); 2) express the human delta508 CFTR (CFTR(delta508)); 3) overexpress the normal human CFTR (CFTR(tg)) in respiratory epithelial cells. Genes were selected from Affymetrix Murine Gene-Chips analysis and subjected to functional classification, k-means clustering, promoter cis-elements/modules searching, literature mining, and pathway exploring. Genomic responses to Cftr-/- were not corrected by expression of CFTR(delta508). Genes regulating host defense, inflammation, fluid and electrolyte transport were similarly altered in Cftr-/- and CFTR(delta508) mice. CFTR(delta508) induced a primary disturbance in expression of genes regulating redox and antioxidant systems. Genomic responses to CFTR(tg) were modest and were not associated with lung pathology. CFTR(tg) and CFTR(delta508) induced genes encoding heat shock proteins and other chaperones but did not activate the endoplasmic reticulum-associated degradation pathway. RNAs encoding proteins that directly interact with CFTR were identified in each of the CFTR mouse models, supporting the hypothesis that CFTR functions within a multiprotein complex whose members interact at the level of protein-protein interactions and gene expression. Promoters of genes influenced by CFTR shared common regulatory elements, suggesting that their co-expression may be mediated by shared regulatory mechanisms. Genes and pathways involved in the response to CFTR may be of interest as modifiers of CF.

Voltage-gated sodium channels: action players with many faces

Voltage-gated sodium channels are responsible for the upstroke of the action potential and thereby play an important role in propagation of the electrical impulse in excitable tissues like muscle, nerve and the heart. Duplication of the sodium channels encoding genes during evolution generated the sodium channel gene family with the different isoforms differing in biophysical properties and tissue distribution. In this review article, mutations in these genes leading to various inherited disorders are discussed.

Sodium-level-sensitive sodium channel Na(x) is expressed in glial laminate processes in the sensory circumventricular organs

Na(x) is an atypical sodium channel that is assumed to be a descendant of the voltage-gated sodium channel family. Our recent studies on the Na(x)-gene-targeting mouse revealed that Na(x) channel is localized to the circumventricular organs (CVOs), the central loci for the salt and water homeostasis in mammals, where the Na(x) channel serves as a sodium-level sensor of the body fluid. To understand the cellular mechanism by which the information sensed by Na(x) channels is transferred to the activity of the organs, we dissected the subcellular localization of Na(x) in the present study. Double-immunostaining and immunoelectron microscopic analyses revealed that Na(x) is exclusively localized to perineuronal lamellate processes extended from ependymal cells and astrocytes in the organs. In addition, glial cells isolated from the subfornical organ, one of the CVOs, were sensitive to an increase in the extracellular sodium level, as analyzed by an ion-imaging method. These results suggest that glial cells bearing the Na(x) channel are the first to sense a physiological increase in the level of sodium in the body fluid, and they regulate the neural activity of the CVOs by enveloping neurons. Close communication between inexcitable glial cells and excitable neural cells thus appears to be the basis of the central control of the salt homeostasis.

The subfornical organ is the primary locus of sodium-level sensing by Na(x) sodium channels for the control of salt-intake behavior

Dehydration causes an increase in the sodium (Na) concentration and osmolarity of body fluid. For Na homeostasis of the body, controls of Na and water intake and excretion are of prime importance. However, the system for sensing the Na level within the brain that is responsible for the control of Na- and water-intake behavior remains to be elucidated. We reported previously that the Na(x) channel is preferentially expressed in the circumventricular organs (CVOs) in the brain and that Na(x) knock-out mice ingest saline in excess under dehydrated conditions. Subsequently, we demonstrated that Na(x) is a Na-level-sensitive Na channel. Here we show that the subfornical organ (SFO) is the principal site for the control of salt-intake behavior, where the Na(x) channel is the Na-level sensor. Infusion of a hypertonic Na solution into the cerebral ventricle induced extensive water intake and aversion to saline in wild-type animals but not in the knock-out mice. Importantly, the aversion to salt was not induced by the infusion of a hyperosmotic mannitol solution with physiological Na concentration in either genotype of mice. When Na(x) cDNA was introduced into the brain of the knock-out mice with an adenoviral expression vector, only animals that received a transduction of the Na(x) gene into the SFO among the CVOs recovered salt-avoiding behavior under dehydrated conditions. These results clearly show that the SFO is the center of the control of salt-intake behavior in the brain, where the Na-level-sensitive Na(x) channel is involved in sensing the physiological increase in the Na level of body fluids.

A comparison of voluntary salt-intake behavior in Nax-gene deficient and wild-type mice with reference to peripheral taste inputs

The Na(x) channel, a subfamily of voltage-gated sodium channels, is thought to be a specific sodium receptor in the central nervous system. Our previous study revealed that Na(x)-gene-deficient mice consumed excessive amounts of NaCl even under water-deprived conditions. In the present study, to investigate whether the peripheral taste inputs are involved in the abnormal intake of salt in Na(x)-deficient mice (homo), voluntary intake of various taste solutions in homo and wild-type mice (wild) was examined under non-deprived conditions. Homo showed a higher preference for 0.15 M NaCl solution than wild. Preference ratios for other basic tastants were identical between groups. Transection of the chorda tympani (CT) or the glossopharyngeal (GP) nerve had little effect on salt-intake behavior in homo and wild. Although combined transection of the superior laryngeal (SL) and GP nerves decreased NaCl intake in homo but not in wild, there were no differences in preference ratios for NaCl in homo before and after SL+GP transection. On the other hand, preference ratios for NaCl in wild tended to increase after combined SL and GP transection. Consequently, preference ratios for NaCl after SL+GP transection were no different between homo and wild. While electrophysiological responses of the CT and the GP to various taste solutions were indistinguishable between homo and wild, those of the SL to NaCl in homo were smaller than those in wild only at lower concentrations (0.01 and 0.03 M). Thus, chemosensory inputs from the oro-pharyngeal regions had little effect on abnormal salt intake in homo, if any. From these results, it is suggested that the higher preference for NaCl in homo is mainly due to the lack of Na(x) channels in the central nervous system.