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

Brain sodium sensing for regulation of thirst, salt appetite, and blood pressure

The brain possesses intricate mechanisms for monitoring sodium (Na) levels in body fluids. During prolonged dehydration, the brain detects variations in body fluids and produces sensations of thirst and aversions to salty tastes. At the core of these processes Nax , the brain's Na sensor, exists. Specialized neural nuclei, namely the subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT), which lack the blood-brain barrier, play pivotal roles. Within the glia enveloping the neurons in these regions, Nax collaborates with Na+ /K+ -ATPase and glycolytic enzymes to drive glycolysis in response to elevated Na levels. Lactate released from these glia cells activates nearby inhibitory neurons. The SFO hosts distinct types of angiotensin II-sensitive neurons encoding thirst and salt appetite, respectively. During dehydration, Nax -activated inhibitory neurons suppress salt-appetite neuron's activity, whereas salt deficiency reduces thirst neuron's activity through cholecystokinin. Prolonged dehydration increases the Na sensitivity of Nax via increased endothelin expression in the SFO. So far, patients with essential hypernatremia have been reported to lose thirst and antidiuretic hormone release due to Nax -targeting autoantibodies. Inflammation in the SFO underlies the symptoms. Furthermore, Nax activation in the OVLT, driven by Na retention, stimulates the sympathetic nervous system via acid-sensing ion channels, contributing to a blood pressure elevation.

Iridium Acylnitrenoid-Initiated Biomimetic Cascade Cyclizations: Stereodefined Access to Polycyclic δ-Lactams

Ring-fused azacyclic compounds are important building units in the synthesis of biorelevant natural products, pharmaceutical agents, and molecular materials. Herein, we present a new approach to these condensed azacycles by a biomimetic cascade cyclization of arylalkenyl dioxazolones. This cascade reaction was found to proceed with excellent stereoselectivity and a high functional group tolerance. The substrate scope of arylalkenyl dioxazolones turned out to be highly flexible and extendable to additional terminating subunits, such as heteroaryl and alkynyl moieties. This biomimetic cyclization was elucidated to be initiated by an intramolecular transfer of the in situ generated electrophilic Ir-acylnitrenoid to the tethered olefinic double bond, leading to a key N-acylaziridine intermediate, which is in turn reacted with pendant (hetero)arenes or alkynes in a highly regio- and stereoselective manner to produce ring-fused azacyclic compounds.

The Emergence of TRP Channels Interactome as a Potential Therapeutic Target in Pancreatic Ductal Adenocarcinoma

Integral membrane proteins, known as Transient Receptor Potential (TRP) channels, are cellular sensors for various physical and chemical stimuli in the nervous system, respiratory airways, colon, pancreas, bladder, skin, cardiovascular system, and eyes. TRP channels with nine subfamilies are classified by sequence similarity, resulting in this superfamily's tremendous physiological functional diversity. Pancreatic Ductal Adenocarcinoma (PDAC) is the most common and aggressive form of pancreatic cancer. Moreover, the development of effective treatment methods for pancreatic cancer has been hindered by the lack of understanding of the pathogenesis, partly due to the difficulty in studying human tissue samples. However, scientific research on this topic has witnessed steady development in the past few years in understanding the molecular mechanisms that underlie TRP channel disturbance. This brief review summarizes current knowledge of the molecular role of TRP channels in the development and progression of pancreatic ductal carcinoma to identify potential therapeutic interventions.

Structural Understanding of Peptide-Bound G Protein-Coupled Receptors: Peptide-Target Interactions

The activation of G protein-coupled receptors (GPCRs) is triggered by ligand binding to their orthosteric sites, which induces ligand-specific conformational changes. Agonists and antagonists bound to GPCR orthosteric sites provide detailed information on ligand-binding modes. Among these, peptide ligands play an instrumental role in GPCR pharmacology and have attracted increased attention as therapeutic drugs. The recent breakthrough in GPCR structural biology has resulted in the remarkable availability of peptide-bound GPCR complexes. Despite the several structural similarities shared by these receptors, they exhibit distinct features in terms of peptide recognition and receptor activation. From this perspective, we have summarized the current status of peptide-bound GPCR structural complexes, largely focusing on the interactions between the receptor and its peptide ligand at the orthosteric site. In-depth structural investigations have yielded valuable insights into the molecular mechanisms underlying peptide recognition. This study would contribute to the discovery of GPCR peptide drugs with improved therapeutic effects.

Glial functions in the blood-brain communication at the circumventricular organs

The circumventricular organs (CVOs) are located around the brain ventricles, lack a blood-brain barrier (BBB) and sense blood-derived molecules. This review discusses recent advances in the importance of CVO functions, especially glial cells transferring periphery inflammation signals to the brain. The CVOs show size-limited vascular permeability, allowing the passage of molecules with molecular weight <10,000. This indicates that the lack of an endothelial cell barrier does not mean the free movement of blood-derived molecules into the CVO parenchyma. Astrocytes and tanycytes constitute a dense barrier at the distal CVO subdivision, preventing the free diffusion of blood-derived molecules into neighboring brain regions. Tanycytes in the CVOs mediate communication between cerebrospinal fluid and brain parenchyma via transcytosis. Microglia and macrophages of the CVOs are essential for transmitting peripheral information to other brain regions via toll-like receptor 2 (TLR2). Inhibition of TLR2 signaling or depletion of microglia and macrophages in the brain eliminates TLR2-dependent inflammatory responses. In contrast to TLR2, astrocytes and tanycytes in the CVOs of the brain are crucial for initiating lipopolysaccharide (LPS)-induced inflammatory responses via TLR4. Depletion of microglia and macrophages augments LPS-induced fever and chronic sickness responses. Microglia and macrophages in the CVOs are continuously activated, even under normal physiological conditions, as they exhibit activated morphology and express the M1/M2 marker proteins. Moreover, the microglial proliferation occurs in various regions, such as the hypothalamus, medulla oblongata, and telencephalon, with a marked increase in the CVOs, due to low-dose LPS administration, and after high-dose LPS administration, proliferation is seen in most brain regions, except for the cerebral cortex and hippocampus. A transient increase in the microglial population is beneficial during LPS-induced inflammation for attenuating sickness response. Transient receptor potential receptor vanilloid 1 expressed in astrocytes and tanycytes of the CVOs is responsible for thermoregulation upon exposure to a warm environment less than 37°C. Alternatively, Nax expressed in astrocytes and tanycytes of the CVOs is crucial for maintaining body fluid homeostasis. Thus, recent findings indicate that glial cells in the brain CVOs are essential for initiating neuroinflammatory responses and maintaining body fluid and thermal homeostasis.

Lactate inhibited sodium intake in dehydrated rats

Recent studies have suggested that glial cells, especially astrocytes, are involved in balanced hydromineral modulation. In response to increased extracellular Na+ concentration, astrocytic Nax channels are activated, promoting lactate production and release. Furthermore, previous in vitro studies have suggested that lactate and hypertonic Na + solution activate SFO GABAergic neurons involved in the salt-appetite central pathways. Here, we evaluated the role of lactate in dehydration-induced sodium and water intake. To this end, intracerebroventricular microinjection (icv) of l-lactate or α-cyano-4-hydroxycinnamic acid (α-CHCA, MCT lactate transporter inhibitor) was performed in rats subjected to 48 h of water deprivation (WD) and 1 h of partial rehydration after 48 h of WD (WD-PR). The rehydration protocol was used to distinguish the mechanisms of thirst and sodium appetite induced by WD. Then, water and sodium (0.3 M NaCl) intake were evaluated for 2 h. Our results showed that central α-CHCA induced an increase in sodium preference in WD rats. Furthermore, central lactate increased water intake but reduced sodium intake in WD-PR animals. In contrast, central lactate transporter inhibition did not change water or sodium intake in WD-PR rats. Our results suggest that lactate is involved in inhibitory mechanisms that induce sodium intake avoidance in dehydrated rats.

Post-Ganglionic Sympathetic Neurons can Directly Sense Raised Extracellular Na+ via SCN7a/Nax

The relationship between dietary NaCl intake and high blood pressure is well-established, and occurs primarily through activation of the sympathetic nervous system. Nax, a Na+-sensitive Na+ channel, plays a pivotal role in driving sympathetic excitability, which is thought to originate from central regions controlling neural outflow. We investigated whether post-ganglionic sympathetic neurons from different ganglia innervating cardiac and vasculature tissue can also directly sense extracellular Na+. Using whole-cell patch clamp recordings we demonstrate that sympathetic neurons from three sympathetic ganglia (superior cervical, stellate and superior mesenteric/coeliac) respond to elevated extracellular NaCl concentration. In sympathetic stellate ganglia neurons, we established that the effect of NaCl was dose-dependent and independent of osmolarity, Cl- and membrane Ca2+ flux, and critically dependent on extracellular Na+ concentration. We show that Nax is expressed in sympathetic stellate ganglia neurons at a transcript and protein level using single-cell RNA-sequencing and immunohistochemistry respectively. Additionally, the response to NaCl was prevented by siRNA-mediated knockdown of Nax, but not by inhibition of other membrane Na+ pathways. Together, these results demonstrate that post-ganglionic sympathetic neurons are direct sensors of extracellular Na+ via Nax, which could contribute to sympathetic driven hypertension.

Structure-guided unlocking of NaX reveals a non-selective tetrodotoxin-sensitive cation channel

Unlike classical voltage-gated sodium (Na_V) channels, Na_X has been characterized as a voltage-insensitive, tetrodotoxin-resistant, sodium (Na⁺)-activated channel involved in regulating Na⁺ homeostasis. However, Na_X remains refractory to functional characterization in traditional heterologous systems. Here, to gain insight into its atypical physiology, we determine structures of the human Na_X channel in complex with the auxiliary β3-subunit. Na_X reveals structural alterations within the selectivity filter, voltage sensor-like domains, and pore module. We do not identify an extracellular Na⁺-sensor or any evidence for a Na⁺-based activation mechanism in Na_X. Instead, the S6-gate remains closed, membrane lipids fill the central cavity, and the domain III-IV linker restricts S6-dilation. We use protein engineering to identify three pore-wetting mutations targeting the hydrophobic S6-gate that unlock a robust voltage-insensitive leak conductance. This constitutively active Na_X-QTT channel construct is non-selective among monovalent cations, inhibited by extracellular calcium, and sensitive to classical Na_V channel blockers, including tetrodotoxin. Our findings highlight a functional diversity across the Na_V channel scaffold, reshape our understanding of Na_X physiology, and provide a template to demystify recalcitrant ion channels.

Aprocitentan and the endothelin system in resistant hypertension

Endothelin has emerged as a target for therapeutic intervention in systemic hypertension. As a vasoconstrictor, co-mitogenic agent, linking pulse pressure and vascular remodeling, and mediator of aldosterone and catecholamine release, endothelin is a key player in hypertension and end-organ damage. In 10-20% of the hypertensive population, the high blood pressure is resistant to administration of antihypertensive drugs of different classes in combination. Because endothelin is not targeted by the current antihypertensive drugs this may suggest that this resistance is due, in part at least, to a dependence on endothelin. This hypothesis is supported by the observation that this form of hypertension is often salt-sensitive, and that the endothelin system is stimulated by salt. In addition, the endothelin system is activated in subjects at risk of developing resistant hypertension, such as African-Americans or patients with obesity or obstructive sleep apnea. Aprocitentan is a novel, potent, dual endothelin receptor antagonist (ERA) currently in phase 3 development for the treatment of difficult-to-treat hypertension. This article discusses the research which underpinned the discovery of this ERA and the choice of its first clinical indication for patients with forms of hypertension which cannot be well controlled with classical antihypertensive drugs.

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.

Astrocytes: The Housekeepers and Guardians of the CNS

Astroglia are a diverse group of cells in the central nervous system. They are of the ectodermal, neuroepithelial origin and vary in morphology and function, yet, they can be collectively defined as cells having principle function to maintain homeostasis of the central nervous system at all levels of organisation, including homeostasis of ions, pH and neurotransmitters; supplying neurones with metabolic substrates; supporting oligodendrocytes and axons; regulating synaptogenesis, neurogenesis, and formation and maintenance of the blood-brain barrier; contributing to operation of the glymphatic system; and regulation of systemic homeostasis being central chemosensors for oxygen, CO₂ and Na⁺. Their basic physiological features show a lack of electrical excitability (inapt to produce action potentials), but display instead a rather active excitability based on variations in cytosolic concentrations of Ca²⁺ and Na⁺. It is expression of neurotransmitter receptors, pumps and transporters at their plasmalemma, along with transports on the endoplasmic reticulum and mitochondria that exquisitely regulate the cytosolic levels of these ions, the fluctuation of which underlies most, if not all, astroglial homeostatic functions.

Analysis of water and electrolyte imbalance in a patient with adipsic hypernatremia associated with subfornical organ-targeting antibody

Patients with adipsic hypernatremia present with chronic hypernatremia because of defects in thirst sensation and dysregulated salt appetite, without demonstrable hypothalamic structural lesions. The involvement of autoantibodies directed against the sodium channel, Na_x in the subfornical organ (SFO) has recently been reported. However, the pathophysiology of water and electrolyte imbalance underlying the disease has yet to be elucidated. We describe the case of a 5-year-old boy who complained of headaches and vomiting that gradually worsened. Brain magnetic resonance imaging detected no abnormal lesions. Blood laboratory testing revealed a serum sodium (Na) concentration of 152 mmol/L and a serum osmolarity of 312 mOsm/L. His body weight had slightly decreased, and his thirst sensation was absent. His plasma vasopressin concentration was 0.9 pg/mL, despite the high serum osmolarity. He was encouraged to drink water, and oral 1-deamino-8-D-arginine-vasopressin was administered. When serum sodium concentrations were normalized, plasma vasopressin concentrations were apparently normal and ranged from 0.8 to 2.0 pg/mL. He did not present with polyuria at any time. Immunohistochemical study using mouse brain sections and the patient's serum revealed the deposition of human immunoglobulin G (IgG) antibody in the mouse SFO. In conclusion, our observations suggested that water and electrolyte imbalance in adipsic hypernatremia is characterized by a certain amount of vasopressin release regardless of serum sodium concentrations with no response to hyperosmolarity.

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.

Investigation of the Molecular Mechanisms by Which Endothelin-3 Stimulates Preadipocyte Growth

Endothelins induce many biological responses, and they are composed of three peptides: ET-1, ET-2, and ET-3. Reports have indicated that ET-1 regulates cell proliferation, adipogenesis, and other cell responses and that ET-3 stimulates the growth of gastrointestinal epithelial cells and melanocytes. However, the signalling pathways of ET3 that mediate the growth of fat cells are still unclear. Using 3T3-L1 white preadipocytes, we found that ET-3 induced increases in both cell number and BrdU incorporation. Pretreatment with an ETAR antagonist (but not an ETBR antagonist) blocked the ET-3-induced increases in both cell number and BrdU incorporation. Additionally, BQ610 suppressed the ET-3-induced increases in phosphorylation of AMPK, c-JUN, and STAT3 proteins, and pretreatment with specific inhibitors of AMPK, JNK/c-JUN, or JAK/STAT3 prevented the ET-3-induced increases in phosphorylation of AMPK, c-JUN, and STAT3, respectively. Neither p38 MAPK inhibitor nor PKC inhibitor altered the effects of ET-3 on cell growth. These data suggest that ET-3 stimulates preadipocyte growth through the ETAR, AMPK, JNK/c-JUN, and STAT3 pathways. Moreover, ET-3 did not alter HIB1B brown preadipocyte and D12 beige preadipocyte growth, suggesting a preadipocyte type-dependent effect. The results of this study may help explain how endothelin mediates fat cell activity and fat cell-associated diseases.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

[Na+] Increases in Body Fluids Sensed by Central Nax Induce Sympathetically Mediated Blood Pressure Elevations via H+-Dependent Activation of ASIC1a

Increases in sodium concentrations ([Na⁺]) in body fluids elevate blood pressure (BP) by enhancing sympathetic nerve activity (SNA). However, the mechanisms by which information on increased [Na⁺] is translated to SNA have not yet been elucidated. We herein reveal that sympathetic activation leading to BP increases is not induced by mandatory high salt intakes or the intraperitoneal/intracerebroventricular infusions of hypertonic NaCl solutions in Na_x-knockout mice in contrast to wild-type mice. We identify Na_x channels expressed in specific glial cells in the organum vasculosum lamina terminalis (OVLT) as the sensors detecting increases in [Na⁺] in body fluids and show that OVLT neurons projecting to the paraventricular nucleus (PVN) are activated via acid-sensing ion channel 1a (ASIC1a) by H⁺ ions exported from Na_x-positive glial cells. The present results provide an insight into the neurogenic mechanisms responsible for salt-induced BP elevations.

Crystal structures of human ETB receptor provide mechanistic insight into receptor activation and partial activation

Endothelin receptors (ET_A and ET_B) are class A GPCRs activated by vasoactive peptide endothelins, and are involved in blood pressure regulation. ET_B-selective signalling induces vasorelaxation, and thus selective ET_B agonists are expected to be utilized for improved anti-tumour drug delivery and neuroprotection. Here, we report the crystal structures of human ET_B receptor in complex with ET_B-selective agonist, endothelin-3 and an ET_B-selective endothelin analogue IRL1620. The structure of the endothelin-3-bound receptor reveals that the disruption of water-mediated interactions between W6.48 and D2.50 is critical for receptor activation, while these hydrogen-bonding interactions are partially preserved in the IRL1620-bound structure. Consistently, functional analysis reveals the partial agonistic effect of IRL1620. The current findings clarify the detailed molecular mechanism for the coupling between the orthosteric pocket and the G-protein binding, and the partial agonistic effect of IRL1620, thus paving the way for the design of improved agonistic drugs targeting ET_B.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Hypothalamic Signaling in Body Fluid Homeostasis and Hypertension

PURPOSE OF REVIEW

The central nervous system plays a pivotal role in the regulation of extracellular fluid volume and consequently arterial blood pressure. Key hypothalamic regions sense and integrate neurohumoral signals to subsequently alter intake (thirst and salt appetite) and output (renal excretion via neuroendocrine and autonomic function). Here, we review recent findings that provide new insight into such mechanisms that may represent new therapeutic targets.

RECENT FINDINGS

Implementation of cutting edge neuroscience approaches such as opto- and chemogenetics highlight pivotal roles of circumventricular organs to impact body fluid homeostasis. Key signaling mechanisms within these areas include the N-terminal variant of transient receptor potential vannilloid type-1, NaX, epithelial sodium channel, brain electroneutral transporters, and non-classical actions of vasopressin. Despite the identification of several new mechanisms, future studies need to better define the neurochemical phenotype and molecular profiles of neurons within circumventricular organs for future therapeutic potential.

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.

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.

Distinct neural mechanisms for the control of thirst and salt appetite in the subfornical organ

Body fluid conditions are continuously monitored in the brain to regulate thirst and salt-appetite sensations. Angiotensin II drives both thirst and salt appetite; however, the neural mechanisms underlying selective water- and/or salt-intake behaviors remain unknown. Using optogenetics, we show that thirst and salt appetite are driven by distinct groups of angiotensin II receptor type 1a-positive excitatory neurons in the subfornical organ. Neurons projecting to the organum vasculosum lamina terminalis control water intake, while those projecting to the ventral part of the bed nucleus of the stria terminalis control salt intake. Thirst-driving neurons are suppressed under sodium-depleted conditions through cholecystokinin-mediated activation of GABAergic neurons. In contrast, the salt appetite-driving neurons were suppressed under dehydrated conditions through activation of another population of GABAergic neurons by Na_x signals. These distinct mechanisms in the subfornical organ may underlie the selective intakes of water and/or salt and may contribute to body fluid homeostasis.

Sodium sensing in the subfornical organ and body-fluid homeostasis

The brain monitors conditions of body fluids and levels of circulating neuroactive factors to maintain the systemic homeostasis. Unlike most regions in the brain, circumventricular organs (CVOs) lack the blood-brain barrier, and serve as the sensing center. Among the CVOs, the subfornical organ (SFO) is the sensing site of Na⁺ levels in body fluids to control water and salt intake. The SFO harbors neuronal cell bodies with a variety of hormone receptors and innervates many brain loci. In addition, the SFO harbors specialized glial cells (astrocytes and ependymal cells) expressing Na_x, a Na⁺-level-sensitive sodium channel. These glial cells wrap a specific population of neurons with their processes, and control the firing activities of the neurons by gliotransmitters, such as lactate and epoxyeicosatrienoic acids (EETs), relevant to water/salt-intake behaviors. Recent advances in the understanding of physiological functions of the SFO are reviewed herein with a focus on the Na⁺-sensing mechanism by Na_x.

Nax signaling evoked by an increase in [Na+] in CSF induces water intake via EET-mediated TRPV4 activation

Water-intake behavior is under the control of brain systems that sense body fluid conditions at sensory circumventricular organs (sCVOs); however, the underlying mechanisms have not yet been elucidated in detail. Nax is a sodium (Na+) level sensor in the brain, and the transient receptor potential vanilloid (TRPV) channels TRPV1 and TRPV4 have been proposed to function as osmosensors. We herein investigated voluntary water intake immediately induced after an intracerebroventricular administration of a hypertonic NaCl solution in TRPV1-, TRPV4-, Na x-, and their double-gene knockout (KO) mice. The induction of water intake by TRPV1-KO mice was normal, whereas intake by TRPV4-KO and Na x-KO mice was significantly less than that by WT mice. Water intake by Na x /TRPV4-double KO mice was similar to that by the respective single KO mice. When TRPV4 activity was blocked with a specific antagonist HC-067047, water intake by WT mice was significantly reduced, whereas intake by TRPV4-KO and Na x-KO mice was not. Similar results were obtained with the administration of miconazole, which inhibits the biosynthesis of epoxyeicosatrienoic acids (EETs), endogenous agonists for TRPV4, from arachidonic acid (AA). Intracerebroventricular injection of hypertonic NaCl with AA or 5,6-EET restored water intake by Na x-KO mice to the wild-type level but not that by TRPV4-KO mice. These results suggest that the Na+ signal generated in Nax-positive glial cells leads to the activation of TRPV4-positive neurons in sCVOs to stimulate water intake by using EETs as gliotransmitters. Intracerebroventricular injection of equiosmolar hypertonic sorbitol solution induced small but significant water intake equally in all the genotypes, suggesting the presence of an unknown osmosensor in the brain.

Effects of Controlled Cortical Impact on the Mouse Brain Vasculome

Perturbations in blood vessels play a critical role in the pathophysiology of brain injury and neurodegeneration. Here, we use a systematic genome-wide transcriptome screening approach to investigate the vasculome after brain trauma in mice. Mice were subjected to controlled cortical impact and brains were extracted for analysis at 24 h post-injury. The core of the traumatic lesion was removed and then cortical microvesels were isolated from nondirectly damaged ipsilateral cortex. Compared to contralateral cortex and normal cortex from sham-operated mice, we identified a wide spectrum of responses in the vasculome after trauma. Up-regulated pathways included those involved in regulation of inflammation and extracellular matrix processes. Decreased pathways included those involved in regulation of metabolism, mitochondrial function, and transport systems. These findings suggest that microvascular perturbations can be widespread and not necessarily localized to core areas of direct injury per se and may further provide a broader gene network context for existing knowledge regarding inflammation, metabolism, and blood-brain barrier alterations after brain trauma. Further efforts are warranted to map the vasculome with higher spatial and temporal resolution from acute to delayed phase post-trauma. Investigating the widespread network responses in the vasculome may reveal potential mechanisms, therapeutic targets, and biomarkers for traumatic brain injury.

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.

Selective blocking effects of 4,9-anhydrotetrodotoxin, purified from a crude mixture of tetrodotoxin analogues, on NaV1.6 channels and its chemical aspects

Tetrodotoxin (TTX) is a potent neurotoxin found in a number of marine creatures including the pufferfish, where it is synthesized by bacteria and accumulated through the food chain. It is a potent and selective blocker of some types of voltage-gated Na⁺ channel (Na_V channel). 4,9-Anhydrotetrodotoxin (4,9-anhydroTTX) was purified from a crude mixture of TTX analogues (such as TTX, 4-epiTTX, 6-epiTTX, 11-oxoTTX and 11-deoxyTTX) by the use of liquid chromatography-fluorescence detection (LC-FLD) techniques. Recently, it has been reported that 4,9-anhydroTTX selectively blocks the activity of Na_V1.6 channels with a blocking efficacy 40–160 times higher than that for other TTX-sensitive Na_V1.x channel isoforms. However, little attention has been paid to the molecular properties of the α-subunit in Na_V1.6 channels and the characteristics of binding of 4,9-anhydroTTX. From a functional point of view, it is important to determine the relative expression of Na_V1.6 channels in a wide variety of tissues. The aim of this review is to discuss briefly current knowledge about the pharmacology of 4,9-anhydroTTX, and provide an analysis of the molecular structure of native Na_V1.6 channels. In addition, chemical aspects of 4,9-anhydroTTX are briefly covered.

Mechanisms of brain renin angiotensin system-induced drinking and blood pressure: importance of the subfornical organ

It is critical for cells to maintain a homeostatic balance of water and electrolytes because disturbances can disrupt cellular function, which can lead to profound effects on the physiology of an organism. Dehydration can be classified as either intra- or extracellular, and different mechanisms have developed to restore homeostasis in response to each. Whereas the renin-angiotensin system (RAS) is important for restoring homeostasis after dehydration, the pathways mediating the responses to intra- and extracellular dehydration may differ. Thirst responses mediated through the angiotensin type 1 receptor (AT1R) and angiotensin type 2 receptors (AT2R) respond to extracellular dehydration and intracellular dehydration, respectively. Intracellular signaling factors, such as protein kinase C (PKC), reactive oxygen species (ROS), and the mitogen-activated protein (MAP) kinase pathway, mediate the effects of central angiotensin II (ANG II). Experimental evidence also demonstrates the importance of the subfornical organ (SFO) in mediating some of the fluid intake effects of central ANG II. The purpose of this review is to highlight the importance of the SFO in mediating fluid intake responses to dehydration and ANG II.

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.

Activity of protein kinase C-α within the subfornical organ is necessary for fluid intake in response to brain angiotensin

Angiotensin-II production in the subfornical organ acting through angiotensin-II type-1 receptors is necessary for polydipsia, resulting from elevated renin-angiotensin system activity. Protein kinase C and mitogen-activated protein kinase pathways have been shown to mediate effects of angiotensin-II in the brain. We investigated mechanisms that mediate brain angiotensin-II-induced polydipsia. We used double-transgenic sRA mice, consisting of human renin controlled by the neuron-specific synapsin promoter crossed with human angiotensinogen controlled by its endogenous promoter, which results in brain-specific overexpression of angiotensin-II, particularly in the subfornical organ. We also used the deoxycorticosterone acetate-salt model of hypertension, which exhibits polydipsia. Inhibition of protein kinase C, but not extracellular signal-regulated kinases, protein kinase A, or vasopressin V₁A and V₂ receptors, corrected the elevated water intake of sRA mice. Using an isoform selective inhibitor and an adenovirus expressing dominant negative protein kinase C-α revealed that protein kinase C-α in the subfornical organ was necessary to mediate elevated fluid and sodium intake in sRA mice. Inhibition of protein kinase C activity also attenuated polydipsia in the deoxycorticosterone acetate-salt model. We provide evidence that inducing protein kinase C activity centrally is sufficient to induce water intake in water-replete wild-type mice, and that cell surface localization of protein kinase C-α can be induced in cultured cells from the subfornical organ. These experimental findings demonstrate a role for central protein kinase C activity in fluid balance, and further mechanistically demonstrate the importance of protein kinase C-α signaling in the subfornical organ in fluid intake stimulated by angiotensin-II in the brain.