The AE4 transporter mediates kidney acid-base sensing

The kidney plays a key role in the correction of systemic acid-base imbalances. Central for this regulation are the intercalated cells in the distal nephron, which secrete acid or base into the urine. How these cells sense acid-base disturbances is a long-standing question. Intercalated cells exclusively express the Na⁺-dependent Cl^-/HCO₃^- exchanger AE4 (Slc4a9). Here we show that AE4-deficient mice exhibit a major dysregulation of acid-base balance. By combining molecular, imaging, biochemical and integrative approaches, we demonstrate that AE4-deficient mice are unable to sense and appropriately correct metabolic alkalosis and acidosis. Mechanistically, a lack of adaptive base secretion via the Cl^-/HCO₃^- exchanger pendrin (Slc26a4) is the key cellular cause of this derailment. Our findings identify AE4 as an essential part of the renal sensing mechanism for changes in acid-base status.

Large deletions play a minor but essential role in congenital coagulation factor VII and X deficiencies

Congenital factor VII (FVII) and factor X (FX) deficiencies belong to the group of rare bleeding disorders which may occur in separate or combined forms since both the F7 and F10 genes are located in close proximity on the distal long arm of chromosome 13 (13q34). We here present data of 192 consecutive index cases with FVII and/or FX deficiency. 10 novel and 53 recurrent sequence alterations were identified in the F7 gene and 5 novel as well as 11 recurrent in the F10 gene including one homozygous 4.35 kb deletion within F7 (c.64+430_131–6delins - TCGTAA) and three large heterozygous deletions involving both the F7 and F10 genes. One of the latter proved to be cytogenetically visible as a chromosome 13q34 deletion and associated with agenesis of the corpus callosum and psychomotor retardation.

Conclusions

Large deletions play a minor but essential role in the mutational spectrum of the F7 and F10 genes. Copy number analyses (e. g. MLPA) should be considered if sequencing cannot clarify the underlying reason of an observed coagulopathy. Of note, in cases of combined FVII/FX deficiency, a deletion of the two contiguous genes might be part of a larger chromosomal rearrangement.

Mutations in CRLF1 cause familial achalasia

We here report a family from Libya with three siblings suffering from early onset achalasia born to healthy parents. We analyzed roughly 5000 disease-associated genes by a next-generation sequencing (NGS) approach. In the analyzed sibling we identified two heterozygous variants in CRLF1 (cytokine receptor-like factor 1). Mutations in CRLF1 have been associated with autosomal recessive Crisponi or cold-induced sweating syndrome type 1 (CS/CISS1), which among other symptoms also manifests with early onset feeding difficulties. Segregation analysis revealed compound heterozygosity for all affected siblings, while the unaffected mother carried the c.713dupC (p.Pro239Alafs*91) and the unaffected father carried the c.178T>A (p.Cys60Ser) variant. The c.713dupC variant has already been reported in affected CS/CISS1 patients, the pathogenicity of the c.178T>A variant was unclear. As reported previously for pathogenic CRLF1 variants, cytokine receptor-like factor 1 protein secretion from cells transfected with the c.178T>A variant was severely impaired. From these results we conclude that one should consider a CRLF1-related disorder in early onset achalasia even if other CS/CISS1 related symptoms are missing.

Large deletions play a minor but essential role in congenital coagulation factor VII and X deficiencies

Congenital factor VII (FVII) and factor X (FX) deficiencies belong to the group of rare bleeding disorders which may occur in separate or combined forms since both the F7 and F10 genes are located in close proximity on the distal long arm of chromosome 13 (13q34). We here present data of 192 consecutive index cases with FVII and/or FX deficiency. 10 novel and 53 recurrent sequence alterations were identified in the F7 gene and 5 novel as well as 11 recurrent in the F10 gene including one homozygous 4.35 kb deletion within F7 (c.64+430_131-6delinsTCGTAA) and three large heterozygous deletions involving both the F7 and F10 genes. One of the latter proved to be cytogenetically visible as a chromosome 13q34 deletion and associated with agenesis of the corpus callosum and psychomotor retardation.

CONCLUSIONS

Large deletions play a minor but essential role in the mutational spectrum of the F7 and F10 genes. Copy number analyses (e. g. MLPA) should be considered if sequencing cannot clarify the underlying reason of an observed coagulopathy. Of note, in cases of combined FVII/FX deficiency, a deletion of the two contiguous genes might be part of a larger chromosomal rearrangement.

Mutation and phenotypic spectrum in patients with cardio-facio-cutaneous and Costello syndrome

Cardio-facio-cutaneous (CFC) and Costello syndrome (CS) are congenital disorders with a significant clinical overlap. The recent discovery of heterozygous mutations in genes encoding components of the RAS-RAF-MAPK pathway in both CFC and CS suggested a similar underlying pathogenesis of these two disorders. While CFC is heterogeneous with mutations in BRAF, MAP2K1, MAP2K2 and KRAS, HRAS alterations are almost exclusively associated with CS. We carried out a comprehensive mutation analysis in 51 CFC-affected patients and 31 individuals with CS. Twelve different BRAF alterations were found in twenty-four patients with CFC (47.0%), two MAP2K1 mutations in five (9.8%) and two MAP2K2 sequence variations in three CFC-affected individuals (5.9%), whereas three patients had a KRAS alteration (5.9%). We identified four different missense mutations of HRAS in twenty-eight cases with CS (90.3%), while KRAS mutations were detected in two infants with a phenotype meeting criteria for CS (6.5%). In 14 informative families, we traced the parental origin of HRAS alterations and demonstrated inheritance of the mutated allele exclusively from the father, further confirming a paternal bias in the parental origin of HRAS mutations in CS. Careful clinical evaluation of patients with BRAF and MAP2K1/2 alterations revealed the presence of slight phenotypic differences regarding craniofacial features in MAP2K1- and MAP2K2-mutation positive individuals, suggesting possible genotype-phenotype correlations.

Expression of anion exchanger 3 influences respiratory rate in awake and isoflurane anesthetized mice

The anion exchanger 3 (AE3) is involved in neuronal pH regulation of which may include chemosensitive neurons. Here we examined the effect of AE3 expression on respiratory rate (RR) in vivo. AE3 knockout (KO, n=5) and wild type (WT, n=6) mice were subjected to body plethysmography, both while awake and during isoflurane anesthesia. RR was significantly lower in awake AE3 KO (162+/-7SE min(-1)) than in WT mice (212+/-20 min(-1), P=0.036). The same was found during isoflurane anesthesia at 0.5 MAC (KO: 123+/-9 min(-1), WT: 168+/-15 min(-1), P=0.026) and 1.0 MAC (KO: 51+/-6 min(-1), WT: 94+/-6 min(-1), P=0.001). Hypercapnia (5% CO2) increased RR in awake and decreased RR in nesthetized (1.0 MAC) mice, whereby relative changes were larger in AE3 KO mice. Recovery from isoflurane anesthesia in respect to RR regaining baseline values was more pronounced in AE3 KO. Results show that AE3 expression profoundly influences control of breathing in mice.

Mechanisms of hypotonic inhibition of the sodium, proton exchanger type 1 (NHE1) in a biliary epithelial cell line (Mz-Cha-1)

AIM

To elucidate the cellular events that results in inhibition of Na(+), H(+) exchanger type 1 (NHE1) by hypotonicity.

METHODS

Intracellular pH (pH(i)) was measured in biliary epithelial cells, with the pH-sensitive fluorochrome 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) using a spectrophotometer. Regulatory volume decrease (RVD) was analysed from confocal images. Changes in NHE1 membrane content were visualized by confocal laser scanning microscopy after transfection of Mz-Cha-1 cells with a NHE1-cMyc fusion protein.

RESULTS

In Mz-Cha-1 cells hypotonicity (-80 mmol L(-1) NaCl) inhibited endogenous Na(+), H(+) exchange. Tyrosine and serine kinase inhibitors were incapable to prevent inhibition. As several signalling pathways influence Na(+), H(+) exchange, we tested the effect of the Ca(++), Calmodulin, protein kinase C or the cAMP, protein kinase A system on inhibition of Na(+), H(+) exchange by hypotonic challenge, but neither system was involved. In contrast, cytoskeleton did influence the effect of hypotonicity. Inhibition of microtubule polymerization by colchicine prevented inhibition of NHE1, and also restored Na(+), H(+) exchange kinetics. Specific inhibition of Src kinases with PP2, attenuated pH(i) recovery rate from 1.93 +/- 0.16 pH units min(-1) (normotonic environment) to 1.02 +/- 0.50 pH units min(-1) (hypotonic environment). Membrane staining of NHE1-cMyc fusion protein was maintained after hypotonic exposure in colchicine pre-treated cells as was RVD. Microfilament inhibition by cytochalasin preserved NHE1 activity. Inhibition of phosphatidylinositol-3'-kinase was unable to restore Na(+), H(+) exchange activity.

CONCLUSION

We conclude that regulation of Na(+), H(+) exchange during RVD is mediated by cytoskeletal elements. This receptor independent pathway is regulated by Src.

A new type of leukoencephalopathy with metaphyseal chondrodysplasia maps to Xq25-q27

BACKGROUND

The authors report a three-generation family with four male patients presenting with a novel type of X-chromosomal leukoencephalopathy associated with skeletal abnormalities.

METHODS

The index patient and his brother reached their early motor milestones in due time and had normal language development. Between the ages of 2 and 3 years, first signs of spastic paraplegia were noticed. Furthermore, the patients developed tremor, ataxia, optic atrophy, and spastic tetraparesis. Both boys had broad wrists and knees without significant contractures. A maternal uncle and a granduncle had the same disease.

RESULTS

Leukoencephalopathy (MRI, MRS) and metaphyseal chondrodysplasia (X-ray, MRI) were diagnosed. MRS showed a reduction of choline-containing compounds in the white matter. An autopsy on one of the patients, who died at age 37 years, revealed an orthochromatic type of leukoencephalopathy. In bone and cartilage tissue, unspecific signs of a mild chondrodysplasia were found. At the PLP gene locus an obligate recombination was observed, which excludes the Pelizaeus-Merzbacher locus on Xq21-22. However, affected males share a fragment of the long arm of chromosome X.

CONCLUSION

The authors report a new type of leukoencephalopathy associated with metaphyseal chondrodysplasia located on Xq25-q27.

Mild Pelizaeus-Merzbacher disease caused by a point mutation affecting correct splicing of PLP1 mRNA

We describe a 28-year-old male patient with a mild course of Pelizaeus-Merzbacher disease (PMD) who presented with developmental delay in his second year of life and was able to walk until 12 years of age. Several computed tomography scans in infancy and youth were normal, the diagnosis of PMD was eventually suggested by magnetic resonance imaging at the age of 24 years. Analysis of the proteolipid protein gene (PLP1) revealed a nucleotide exchange (c.762G>T) at the 3' border of exon 6, which did not entail an amino acid exchange but adversely affected splicing. PCR analysis of fibroblast cDNA showed that c.762G>T resulted in partial skipping of exon 6 in the PLP1 mRNA. Exclusion of exon 6 does not alter the reading frame but leads to absence of amino acids 232-253 that constitute a main part of the fourth transmembrane helix of the PLP protein. Remarkably, residual wild-type splicing was also detected in the patient's cultured fibroblasts. This might explain the mild phenotype in this case, as exon 6 skipping mutations resulted in a severe course of disease in other patients.

Expression of the sodium-driven chloride bicarbonate exchanger NCBE during prenatal mouse development

In the immature central nervous system (CNS) GABA-mediated excitation is thought to be an important developmental signal. It depends on a high intracellular chloride concentration ([Cl(-)](i)) of the particular neuron. [Cl(-)](i) is a consequence of chloride transport processes across the plasma membrane. The ongoing expression of the KCl-co-transporter KCC2 eventually lowers [Cl(-)](i) in most CNS neurons and thus renders GABA hyperpolarizing. As NCBE, a sodium-dependent chloride-bicarbonate exchanger, also lowers [Cl(-)](i) and may thus modulate the GABA-response, we analyzed its expression during prenatal mouse development before establishment of the mature KCC2 expression. Indeed, NCBE is expressed very early in CNS neurons and precedes the expression of KCC2. Unlike KCC2, NCBE is expressed in the peripheral nervous system and in non-neuronal tissues as the choroid plexus, the dura, and some epithelia including the acid secreting epithelium of the stomach and the duodenal epithelium.

IRP1 activation by extracellular oxidative stress in the perfused rat liver

The expression of several proteins with critical functions in iron metabolism is regulated post-transcriptionally by the binding of iron regulatory proteins, IRP1 and IRP2, to mRNA iron responsive elements (IREs). In iron-deficient tissues and cultured cells, both IRP1 and IRP2 are activated for high affinity IRE binding. Previous work showed that IRP1 is also activated when cultured cells are exposed to H(2)O(2). The well established role of iron and H(2)O(2) in tissue injury (based on Fenton chemistry) suggests that this response may have important pathophysiological implications. This is particularly relevant in inflammation, where cytotoxic immune cells release large amounts of reactive oxygen species. Here, we describe a rat liver perfusion model to study IRP1 activation under H(2)O(2) generation conditions that mimic a physiological inflammatory response, using steady-state concentrations of H(2)O(2) produced by a glucose/ glucose oxidase/catalase system. We show first that stimulated neutrophils are able to increase serum levels of H(2)O(2) by a factor of 10, even in the presence of H(2)O(2)-degrading erythrocytes. We further show that perfusion of rat liver with glucose oxidase leads to a rapid activation of IRE binding activity in the intact organ. Mobility shift assays with liver extracts and IRP1 or IRP2-specific probes indicate that only IRP1 responds to H(2)O(2). Our study demonstrates a principal existence of iron regulation by oxidative stress at the intact organ level. It also provides a link between iron metabolism and the inflammatory response, as H(2)O(2) is a major product of the oxidative burst of neutrophils and macrophages.

Disruption of KCC2 reveals an essential role of K-Cl cotransport already in early synaptic inhibition

Synaptic inhibition by GABA(A) and glycine receptors, which are ligand-gated anion channels, depends on the electrochemical potential for chloride. Several potassium-chloride cotransporters can lower the intracellular chloride concentration [Cl(-)](i), including the neuronal isoform KCC2. We show that KCC2 knockout mice died immediately after birth due to severe motor deficits that also abolished respiration. Sciatic nerve recordings revealed abnormal spontaneous electrical activity and altered spinal cord responses to peripheral electrical stimuli. In the spinal cord of wild-type animals, the KCC2 protein was found at inhibitory synapses. Patch-clamp measurements of embryonic day 18.5 spinal cord motoneurons demonstrated an excitatory GABA and glycine action in the absence, but not in the presence, of KCC2, revealing a crucial role of KCC2 for synaptic inhibition.

Expression of the Na-K-2Cl-cotransporter NKCC1 during mouse development

Here we describe the expression pattern of the Na-K-2Cl-cotransporter NKKC1 during embryonal and early postnatal mouse development. During early stages hybridization signals were detected over single cells of the developing neuroepithelia, whereas the neuroepithelium of the basal telencephalon was labeled continuously. With ongoing differentiation a distinct pattern of hybridization became apparent, which switched from a neuronal to a more glial pattern in the adult. Outside the nervous system NKCC1 transcripts were present in many organs and were mostly confined to epithelia.