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A Christianson syndrome-linked deletion mutation (?(287)ES(288)) in SLC9A6 disrupts recycling endosomal function and elicits neurodegeneration and cell death.

ABSTRACT: Christianson Syndrome, a recently identified X-linked neurodevelopmental disorder, is caused by mutations in the human gene SLC9A6 encoding the recycling endosomal alkali cation/proton exchanger NHE6. The patients have pronounced limitations in cognitive ability, motor skills and adaptive behaviour. However, the mechanistic basis for this disorder is poorly understood as few of the more than 20 mutations identified thus far have been studied in detail.Here, we examined the molecular and cellular consequences of a 6 base-pair deletion of amino acids Glu(287) and Ser(288) (?ES) in the predicted seventh transmembrane helix of human NHE6 expressed in established cell lines (CHO/AP-1, HeLa and neuroblastoma SH-SY5Y) and primary cultures of mouse hippocampal neurons by measuring levels of protein expression, stability, membrane trafficking, endosomal function and cell viability.In the cell lines, immunoblot analyses showed that the nascent mutant protein was properly synthesized and assembled as a homodimer, but its oligosaccharide maturation and half-life were markedly reduced compared to wild-type (WT) and correlated with enhanced ubiquitination leading to both proteasomal and lysosomal degradation. Despite this instability, a measurable fraction of the transporter was correctly sorted to the plasma membrane. However, the rates of clathrin-mediated endocytosis of the ?ES mutant as well as uptake of companion vesicular cargo, such as the ligand-bound transferrin receptor, were significantly reduced and correlated with excessive endosomal acidification. Notably, ectopic expression of ?ES but not WT induced apoptosis when examined in AP-1 cells. Similarly, in transfected primary cultures of mouse hippocampal neurons, membrane trafficking of the ?ES mutant was impaired and elicited marked reductions in total dendritic length, area and arborization, and triggered apoptotic cell death.These results suggest that loss-of-function mutations in NHE6 disrupt recycling endosomal function and trafficking of cargo which ultimately leads to neuronal degeneration and cell death in Christianson Syndrome.

SUBMITTER: Ilie A

PROVIDER: S-EPMC5010692 | biostudies-literature | 2016 Sep

REPOSITORIES: biostudies-literature

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A Christianson syndrome-linked deletion mutation (∆(287)ES(288)) in SLC9A6 disrupts recycling endosomal function and elicits neurodegeneration and cell death.

Ilie Alina A Gao Andy Y L AY Reid Jonathan J Boucher Annie A McEwan Cassandra C Barrière Hervé H Lukacs Gergely L GL McKinney R Anne RA Orlowski John J

Molecular neurodegeneration 20160902 1

<h4>Background</h4>Christianson Syndrome, a recently identified X-linked neurodevelopmental disorder, is caused by mutations in the human gene SLC9A6 encoding the recycling endosomal alkali cation/proton exchanger NHE6. The patients have pronounced limitations in cognitive ability, motor skills and adaptive behaviour. However, the mechanistic basis for this disorder is poorly understood as few of the more than 20 mutations identified thus far have been studied in detail.<h4>Methods</h4>Here, we ...[more]

PMID: 27590723

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Project description:Christianson syndrome (CS) is an X-linked neurodevelopmental and neurological disorder characterized in males by core symptoms that include non-verbal status, intellectual disability, epilepsy, truncal ataxia, postnatal microcephaly and hyperkinesis. CS is caused by mutations in the SLC9A6 gene, which encodes a multipass transmembrane sodium (potassium)-hydrogen exchanger 6 (NHE6) protein, functional in early recycling endosomes. The extent and variability of the CS phenotype in female heterozygotes, who presumably express the wild-type and mutant SLC9A6 alleles mosaically as a result of X-chromosome inactivation (XCI), have not yet been systematically characterized. Slc9a6 knockout mice (Slc9a6 KO) were generated by insertion of the bacterial lacZ/?-galactosidase (?-Gal) reporter into exon 6 of the X-linked gene. Mutant Slc9a6 KO male mice have been shown to develop late endosomal/lysosomal dysfunction associated with glycolipid accumulation in selected neuronal populations and patterned degeneration of Purkinje cells (PCs). In heterozygous female Slc9a6 KO mice, ?-Gal serves as a transcriptional/XCI reporter and thus facilitates testing of effects of mosaic expression of the mutant allele on penetrance of the abnormal phenotype. Using ?-Gal, we demonstrated mosaic expression of the mutant Slc9a6 allele and mosaically distributed lysosomal glycolipid accumulation and PC pathology in the brains of heterozygous Slc9a6 KO female mice. At the behavioral level, we showed that heterozygous female mice suffer from visuospatial memory and motor coordination deficits similar to but less severe than those observed in X-chromosome hemizygous mutant males. Our studies in heterozygous Slc9a6 KO female mice provide important clues for understanding the likely phenotypic range of Christianson syndrome among females heterozygous for SLC9A6 mutations and might improve diagnostic practice and genetic counseling by helping to characterize this presumably underappreciated patient/carrier group.

Project description:Through substitution mapping studies, we previously identified that a <330kb region from a rat strain with no renal pathology (the Lewis rat), which when introgressed onto the genetic background of a rat with renal disease (the Dahl Salt-sensitive (S) rat), caused an increase rather than the expected decrease in proteinuria. The purpose of this study was to prioritize a candidate gene and further delineate the mechanism underlying the observed increased in proteinuria. A higher level of proteinuria independent of dietary salt was observed in the congenic rat at a very young age (50-52 day old). The critical congenic segment was further mapped to <42.5kb containing a single candidate gene, rififylin. Rififylin was expressed 1.59 fold higher in the congenic strain compared with S. Overexpression of rififylin is known to delay recycling of endosomes. Renal transcriptome analysis indicated that Atp1a1 one of the most highly differentially expressed genes. Atp1a1 was 5.33 fold higher in the congenic strain compared with S. The protein product of Atp1a1, the alpha subunit of Na+K+ATPase, was also significantly higher in the endosomes of proximal tubules from the congenic strain compared with S. To determine whether the higher amounts of this protein in the endosomes is due to a delay in recycling of endosomes caused by the overexpression of rififylin in the congenic strain, recycling of exogenously labeled-transferrin by single cell cultures of proximal tubules was monitored by confocal microscopy. Recycling of transferrin was significantly delayed in the congenic strain compared with S. These results suggest that impaired endosomal recycling in the proximal tubules from the congenic strain caused by the overexpression of rififylin is a novel molecular mechanism linked to the observed increase in proteinuria of the congenic strain. Three male S control and 3 male congenic S.LEW(10)x12x2x3x5 rats born on the same day were selected, weaned at 30 days of age, and caged with 1 congenic and 1 S rat per cage. They were raised on a low-salt (0.3%) diet (Harlan Teklad diet TD 7034; Harlan–Sprague-Dawley) and sacrificed at 53 days of age and total RNAs were isolated from the kidney. The isolated RNA from each animal was used for the cRNA preparation. cRNA was prepared and fragmented as suggested by Affymetrix technical manual, and simultaneously hybridized (15 µg adjusted cRNA for each chip) to Rat Expression Array 230 2.0 (3' IVT Expression Analysis). Statistical analyses of the microarray data were performed with BH adjustment using R statistical package (version 2.8.1).

Project description:Mutations in solute carrier family 9 isoform 6 on chromosome Xq26.3 encoding sodium-hydrogen exchanger 6, a protein mainly expressed in early and recycling endosomes are known to cause a complex and slowly progressive degenerative human neurological disease. Three resulting phenotypes have so far been reported: an X-linked Angelman syndrome-like condition, Christianson syndrome and corticobasal degeneration with tau deposition, with each characterized by severe intellectual disability, epilepsy, autistic behaviour and ataxia. Hypothesizing that a sodium-hydrogen exchanger 6 deficiency would most likely disrupt the endosomal-lysosomal system of neurons, we examined Slc9a6 knockout mice with tissue staining and related techniques commonly used to study lysosomal storage disorders. As a result, we found that sodium-hydrogen exchanger 6 depletion leads to abnormal accumulation of GM2 ganglioside and unesterified cholesterol within late endosomes and lysosomes of neurons in selective brain regions, most notably the basolateral nuclei of the amygdala, the CA3 and CA4 regions and dentate gyrus of the hippocampus and some areas of cerebral cortex. In these select neuronal populations, histochemical staining for ?-hexosaminidase activity, a lysosomal enzyme involved in the degradation of GM2 ganglioside, was undetectable. Neuroaxonal dystrophy similar to that observed in lysosomal disease was observed in the cerebellum and was accompanied by a marked and progressive loss of Purkinje cells, particularly in those lacking the expression of Zebrin II. On behavioural testing, Slc9a6 knockout mice displayed a discrete clinical phenotype attributable to motor hyperactivity and cerebellar dysfunction. Importantly, these findings show that sodium-hydrogen exchanger 6 loss of function in the Slc9a6-targeted mouse model leads to compromise of endosomal-lysosomal function similar to lysosomal disease and to conspicuous neuronal abnormalities in specific brain regions, which in concert could provide a unified explanation for the cellular and clinical phenotypes in humans with SLC9A6 mutations.

Dataset Information

A Christianson syndrome-linked deletion mutation (?(287)ES(288)) in SLC9A6 disrupts recycling endosomal function and elicits neurodegeneration and cell death.

Publications

A Christianson syndrome-linked deletion mutation (∆(287)ES(288)) in SLC9A6 disrupts recycling endosomal function and elicits neurodegeneration and cell death.

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