Sodium/Proton Exchanger 3 in the Medulla Oblongata and Set Point of Breathing Control

Revisiting the association of sudden infant death syndrome (SIDS) with polymorphisms of NHE3 and IL13

OBJECTIVES

Disturbances of the central nervous system and immune system are thought to play a role in sudden infant death syndrome (SIDS). Dysregulated expression of sodium (Na+)/hydrogen (H+) exchanger 3 (NHE3) in the brainstem and of interleukin 13 (IL13) in the lungs has been observed in SIDS. An association of single-nucleotide polymorphisms (SNPs) in NHE3 and IL13 with SIDS has been proposed, but controversial results were reported. Therefore, there is a need to revisit the association of SNPs in NHE3 and IL13 with SIDS.

METHODS

Genotyping of rs71597645 (G1131A) and rs2247114 (C2405T) in NHE3 and rs20541 (+ 4464A/G) in IL13 was performed in 201 SIDS cases and 338 controls. A meta-analysis was performed after merging our data with previously published data (all from European populations).

RESULTS

Polymorphisms rs2247114 (NHE3) and rs20541 (IL13) were significantly associated with SIDS overall and in multiple subgroups, but no association was found for rs71597645 (NHE3). After combining our data with previously published data, a fixed-effect meta-analysis showed that rs2247114 in NHE3 retained a significant association with SIDS under a recessive model (OR 2.78, 95%CI 1.53 to 5.06; p = 0.0008).

CONCLUSION

Our findings suggest an association of NHE3 variant rs2247114 (C2405T), though not rs71597645 (NHE3), with SIDS. A potential role of rs20541 (IL13) still has to be elucidated. Especially NHE3 seems to be an interesting topic for future SIDS research.

Decreased Brain pH and Pathophysiology in Schizophrenia

Postmortem studies reveal that the brain pH in schizophrenia patients is lower than normal. The exact cause of this low pH is unclear, but increased lactate levels due to abnormal energy metabolism appear to be involved. Schizophrenia patients display distinct changes in mitochondria number, morphology, and function, and such changes promote anaerobic glycolysis, elevating lactate levels. pH can affect neuronal activity as H⁺ binds to numerous proteins in the nervous system and alters the structure and function of the bound proteins. There is growing evidence of pH change associated with cognition, emotion, and psychotic behaviors. Brain has delicate pH regulatory mechanisms to maintain normal pH in neurons/glia and extracellular fluid, and a change in these mechanisms can affect, or be affected by, neuronal activities associated with schizophrenia. In this review, we discuss the current understanding of the cause and effect of decreased brain pH in schizophrenia based on postmortem human brains, animal models, and cellular studies. The topic includes the factors causing decreased brain pH in schizophrenia, mitochondria dysfunction leading to altered energy metabolism, and pH effects on the pathophysiology of schizophrenia. We also review the acid/base transporters regulating pH in the nervous system and discuss the potential contribution of the major transporters, sodium hydrogen exchangers (NHEs), and sodium-coupled bicarbonate transporters (NCBTs), to schizophrenia.

Sodium/proton exchanger 3 (NHE3) and sudden infant death syndrome (SIDS)

The sodium/proton exchanger protein 3 (NHE3) is located in chemosensitive areas of the medulla oblongata and plays an important role in the central control of respiration. Overexpression of NHE3 is correlated with lower respiration and might therefore contribute to the vulnerability of infants dying suddenly and unexpected (sudden infant death syndrome, SIDS). Our aim in this study was to verify already reported genetic variations in the NHE3 gene in an independent SIDS cohort from Switzerland. Two single nucleotide polymorphisms (SNPs) in the promoter region (G1131A and C1197T) and one variation in the coding sequence of exon 16 (C2405T) in the NHE3 gene were analyzed in 160 Caucasian SIDS infants and 192 Swiss adult controls by using a single base extension method (SNaPshot multiplex). No significant differences were detected in the allelic frequencies of the three NHE3 polymorphisms between SIDS cases and controls. We conclude that the three investigated NHE3 SNPs are unlikely to play a major role in the pathogenesis of SIDS in Caucasian infants. However, further genetic investigations in different ethnicities are required to determine whether variations in NHE3 are associated with an increased SIDS risk.

Intracellular pH regulation by acid-base transporters in mammalian neurons

Intracellular pH (pH_i) regulation in the brain is important in both physiological and physiopathological conditions because changes in pH_i generally result in altered neuronal excitability. In this review, we will cover 4 major areas: (1) The effect of pH_i on cellular processes in the brain, including channel activity and neuronal excitability. (2) pH_i homeostasis and how it is determined by the balance between rates of acid loading (J_L) and extrusion (J_E). The balance between J_E and J_L determine steady-state pH_i, as well as the ability of the cell to defend pH_i in the face of extracellular acid-base disturbances (e.g., metabolic acidosis). (3) The properties and importance of members of the SLC4 and SLC9 families of acid-base transporters expressed in the brain that contribute to J_L (namely the Cl-HCO₃ exchanger AE3) and J_E (the Na-H exchangers NHE1, NHE3, and NHE5 as well as the Na⁺- coupled HCO₃⁻ transporters NBCe1, NBCn1, NDCBE, and NBCn2). (4) The effect of acid-base disturbances on neuronal function and the roles of acid-base transporters in defending neuronal pH_i under physiopathologic conditions.

The Na(+)/H (+) exchanger NHE5 is sorted to discrete intracellular vesicles in the central and peripheral nervous systems

The pH milieu of the central and peripheral nervous systems is an important determinant of neuronal excitability, function, and survival. In mammals, neural acid-base homeostasis is coordinately regulated by ion transporters belonging to the Na(+)/H(+) exchanger (NHE) and bicarbonate transporter gene families. However, the relative contributions of individual isoforms within the respective families are not fully understood. This report focuses on the NHE family, specifically the plasma membrane-type NHE5 which is preferentially transcribed in brain, but the distribution of the native protein has not been extensively characterized. To this end, we generated a rabbit polyclonal antibody that specifically recognizes NHE5. In both central (cortex, hippocampus) and peripheral (superior cervical ganglia, SCG) nervous tissue of mice, NHE5 immunostaining was punctate and highly concentrated in the somas and to lesser amounts in the dendrites of neurons. Very little signal was detected in axons. Similarly, in primary cultures of differentiated SCG neurons, NHE5 localized predominantly to vesicles in the somatodendritic compartment, though some immunostaining was also evident in punctate vesicles along the axons. NHE5 was also detected predominantly in intracellular vesicles of cultured SCG glial cells. Dual immunolabeling of SCG neurons showed that NHE5 did not colocalize with markers for early endosomes (EEA1) or synaptic vesicles (synaptophysin), but did partially colocalize with the transferrin receptor, a marker of recycling endosomes. Collectively, these data suggest that NHE5 partitions into a unique vesicular pool in neurons that shares some characteristics of recycling endosomes where it may serve as an important regulated store of functional transporters required to maintain cytoplasmic pH homeostasis.

Impact of sodium/proton exchanger 3 gene variants on sudden infant death syndrome

OBJECTIVE

To determine the contribution of variations in the sodium/proton exchanger 3 (NHE3) gene in sudden infant death syndrome (SIDS).

STUDY DESIGN

Variations in the exons and promoter of the NHE3 gene were analyzed with direct sequencing analysis and mini sequencing (SNaPshot analysis) in 251 cases of SIDS, plus 50 infant control subjects who had died of other causes, and 170 healthy adults.

RESULTS

The C2405T variant (exon 16) and 2 polymorphisms in the promoter (G1131A and C1197T) were encountered significantly more frequently in cases of SIDS than in control subjects. At least 1 of these 3 variants was detected in 49% of SIDS cases, but only in 30% of control subjects.

CONCLUSIONS

Our findings suggest the involvement of polymorphisms in the NHE3 gene and promoter in cases of SIDS, which may result in an overexpression of NHE3 in the medulla oblongata and which possibly leads to a disturbance in breathing control. Furthermore, our results underline the heterogeneous character of SIDS.

pH regulating transporters in neurons from various chemosensitive brainstem regions in neonatal rats

We studied the membrane transporters that mediate intracellular pH (pH(i)) recovery from acidification in brainstem neurons from chemosensitive regions of neonatal rats. Individual neurons within brainstem slices from the retrotrapezoid nucleus (RTN), the nucleus tractus solitarii (NTS), and the locus coeruleus (LC) were studied using a pH-sensitive fluorescent dye and fluorescence imaging microscopy. The rate of pH(i) recovery from an NH(4)Cl-induced acidification was measured, and the effects of inhibitors of various pH-regulating transporters determined. Hypercapnia (15% CO(2)) resulted in a maintained acidification in neurons from all three regions. Recovery in RTN neurons was nearly entirely eliminated by amiloride, an inhibitor of Na(+)/H(+) exchange (NHE). Recovery in RTN neurons was blocked approximately 50% by inhibitors of isoform 1 of NHE (NHE-1) but very little by an inhibitor of NHE-3 or by DIDS (an inhibitor of HCO(3)-dependent transport). In NTS neurons, amiloride blocked over 80% of the recovery, which was also blocked approximately 65% by inhibitors of NHE-1 and 26% blocked by an inhibitor of NHE-3. Recovery in LC neurons, in contrast, was unaffected by amiloride or blockers of NHE isoforms but was dependent on Na(+) and increased by external HCO(3)(-). On the basis of these findings, pH(i) recovery from acidification appears to be largely mediated by NHE-1 in RTN neurons, by NHE-1 and NHE-3 in NTS neurons, and by a Na- and HCO(3)-dependent transporter in LC neurons. Thus, pH(i) recovery is mediated by different pH-regulating transporters in neurons from different chemosensitive regions, but recovery is suppressed by hypercapnia in all of the neurons.

Systemic inhibition of the Na(+)/H (+) exchanger type 3 in intact rats activates brainstem respiratory regions

Selective inhibition of the Na(+)/H(+) exchanger type 3 (NHE3) increases the firing rate of brainstem ventrolateral CO(2)/H(+) sensitive neurons, resembling the responses evoked by hypercapnic stimuli. In anesthetized animals, NHE3 inhibition has also been shown to stimulate the central chemosensitive drive. We aimed to analyze the respiratory-related brainstem regions affected by NHE3 inhibition in anaesthetized spontaneously-breathing rats with intact peripheral afferents. For that, c-Fos immunopositive cells were counted along the brainstem in rats intravenously infused with the selective NHE3 inhibitor AVE1599. A rostral extension of the ventral respiratory column which includes the pre-Bötzinger complex was activated by the NHE3 inhibitor. In addition, the number of c-Fos positive cells resulted significantly increased in the most rostral extension of the retrotrapezoid nucleus/parapyramidal region. In the pons, the intravenous infusion of AVE1599 activated the lateral parabrachial and Kölliker-Fuse nuclei. Thus, selective NHE3 inhibition in anaesthetized rats activates the respiratory network and evokes a pattern of c-Fos expressing cells similar to that induced by hypercapnia.

Mini-review: Cell surface receptor for thyroid hormone and nongenomic regulation of ion fluxes in excitable cells

Thyroid hormone has been shown experimentally to affect cellular ion fluxes. For example, thyroid hormone-induced modulation has been described of cellular sodium current (I(Na)), inward rectifying potassium current (IKir) and sodium pump (Na, K-ATPase) and of calcium pump (Ca(2+)-ATPase) activities. Certain of these actions appear to reflect nongenomic mechanisms of hormone action that are initiated at the plasma membrane receptor for iodothyronines described on integrin alphavbeta3. One such action is the recent demonstration of enhancement by the hormone of I(Na) in neurons. Nongenomic actions of thyroid hormone initiated at the plasma membrane may be specifically inhibited by tetraiodothyroacetic acid (tetrac), a deaminated thyroid hormone analogue. Important behavioral changes are associated with clinical states of excessive or deficient thyroid function. The molecular basis for these changes has not been established. It is proposed that nongenomic actions of thyroid hormone in neurons-such as that on sodium current-underlie certain of these behaviors. The contribution of such nongenomic actions of the hormone to animal behavioral paradigms possibly relevant to thyroid hormone actions in human subjects may be tested in vivo with tetrac.

Respiratory response to systemic inhibition of the Na+/H+ exchanger type 3 in intact rats

The Na+/H+ exchangers (NHEs) are a family of antiporters involved in the maintenance of neural steady-state intracellular pH. The NHE3 seems to be the predominant subtype in central chemosensitive cells. We aimed to analyze the effect of a selective NHE3 inhibition on the respiratory pattern in spontaneously breathing rats with intact vagi. Rats were intravenously infused for 10 min with the selective NHE3 inhibitor AVE1599 (Aventis Pharma Deustchland, 0.5 and 2 mg/kg) or with phosphate buffer. Whole-body plethysmography was used to monitor breathing pattern before, during, and up to 30 min after the drug infusion. Immunohistochemistry for the c-Fos protein was performed in the animal brains and c-Fos-positive cells were counted along the brainstem. Selective NHE3 inhibition induced a significant increase in the respiratory frequency and in the number of c-Fos immunopositive cells in the lateral parabrachial nucleus, the pre-Bötzinger complex and a rostral extension of the retrotrapezoid nucleus/parapyramidal region (p<0.05, ANOVA). We conclude that systemic administration of AVE1599 increases respiratory frequency and activates ponto-medullary areas implicated in the central control of breathing and chemoreception.

Role of Brainstem Sodium/Proton Exchanger 3 for Breathing Control during Chronic Acid–Base Imbalance

RATIONALE

The sodium/proton exchanger (NHE) 3 is expressed in brainstem areas with prevalence for central chemosensitivity. Selective NHE3 inhibitors can evoke CO(2) mimetic responses both in vitro and in vivo, demonstrating the functional significance of this pH-regulating protein. Moreover, levels of NHE3 expression are inversely correlated to interindividual differences of baseline ventilation in conscious rabbits.

OBJECTIVES

We explored the influence of chronic acid-base disturbances on mRNA levels of brainstem NHE3 in relation to breathing control.

METHODS

Alveolar ventilation (Va), blood gases, systemic base excess (BE), and metabolic Vco(2) were determined in rabbits shortly after exposure to either CO(2)-enriched air for 3 days (n = 5) or to ammonium chloride with drinking water for 2 days (n = 6). Untreated animals served as controls (n = 24). NHE3 mRNA within the obex region was quantified by real-time reverse transcription-polymerase chain reaction.

MEASUREMENTS AND MAIN RESULTS

After chronic hypercapnia, we found a compensatory rise of BE (mean +/- SEM) to 5.3 +/- 0.5 mmol x L(-1) with slightly elevated Pa(CO(2)). Brainstem NHE3 mRNA as well as Va were not significantly different from control levels. In the NH(4)Cl group, arterial pH was approximately 0.09 units lower than control, and BE decreased to -6.5 +/- 1.6 mmol x L(-1) with slightly decreased Pa(CO(2)), but considerably reduced Va (by approximately 25%; P < 0.05) and Vco(2). Concomitantly, brainstem NHE3 mRNA had increased from control level of 1.45 +/- 0.19 to 3.64 +/- 0.37 fg cDNA/mug RNA; P < 0.01.

CONCLUSIONS

Expression of brainstem NHE3 is up-regulated by chronic metabolic acidosis but not by prolonged hypercapnia. It is proposed that elevated brainstem NHE3 expression contributes to limit maladaptive hyperventilation during metabolic acidosis.

Enhanced c-Fos expression in the rostral ventral respiratory complex and rostral parapyramidal region by inhibition of the Na+/H+ exchanger type 3

Previous studies have shown that selective inhibition of Na+/H+ exchanger type 3 (NHE3) induces intracellular acidification and activates CO2/H+-sensitive medullary neurons, mimicking the responses evoked by hypercapnic stimuli. In addition, NHE3 blockers administration decreases the duration of apnoea induced by laryngeal stimulation, presumably by means of central chemoreceptor activation. To test the hypothesis that the central chemoreceptor network may be affected by NHE3 inhibition, brainstem c-Fos immunoreactive cell counting was performed after systemic administration of the NHE3 blocker AVE1599 (Aventis Pharma Deutschland GmbH) (2 mg/kg). The rostro-caudal quantitative c-Fos analysis showed a significant increase in the number of c-Fos positive cells in the rostral part of the ventral respiratory complex (VRC) as well as in the rostral part of the parapyramidal (Ppy) region. The VRC activated region (-4.2 to -3.2mm interaural) included the pre-Bötzinger complex, the rostral ventral respiratory group and the rostral ventrolateral medulla, all of them involved in cardiorespiratory control. The activated Ppy region corresponded with the rostral chemosensitive area, which elicits the strongest ventilatory response upon ventral medullary surface stimulation with H+/CO2. Most cells activated in Ppy after NHE3 inhibition were serotonergic. Hence, systemic application of NHE3 blockers may induce central chemoreceptors activation and an increase in the respiratory network activity in a similar way to known physiological stimuli such as hypercapnia. On the other hand, selective NHE3 blockers could be excellent tools for treatment of pathological states where central chemoreceptor function is diminished or absent, such as central hypoventilation syndrome or sudden infant death syndrome.