Human ClC-6 Is a Late Endosomal Glycoprotein that Associates with Detergent-Resistant Lipid Domains

BACKGROUND: The mammalian CLC protein family comprises nine members (ClC-1 to -7 and ClC-Ka, -Kb) that function either as plasma membrane chloride channels or as intracellular chloride/proton antiporters, and that sustain a broad spectrum of cellular processes, such as membrane excitability, transepithelial transport, endocytosis and lysosomal degradation. In this study we focus on human ClC-6, which is structurally most related to the late endosomal/lysomal ClC-7. PRINCIPAL FINDINGS: Using a polyclonal affinity-purified antibody directed against a unique epitope in the ClC-6 COOH-terminal tail, we show that human ClC-6, when transfected in COS-1 cells, is N-glycosylated in a region that is evolutionary poorly conserved between mammalian CLC proteins and that is located between the predicted helices K and M. Three asparagine residues (N410, N422 and N432) have been defined by mutagenesis as acceptor sites for N-glycosylation, but only two of the three sites seem to be simultaneously N-glycosylated. In a differentiated human neuroblastoma cell line (SH-SY5Y), endogenous ClC-6 colocalizes with LAMP-1, a late endosomal/lysosomal marker, but not with early/recycling endosomal markers such as EEA-1 and transferrin receptor. In contrast, when transiently expressed in COS-1 or HeLa cells, human ClC-6 mainly overlaps with markers for early/recycling endosomes (transferrin receptor, EEA-1, Rab5, Rab4) and not with late endosomal/lysosomal markers (LAMP-1, Rab7). Analogously, overexpression of human ClC-6 in SH-SY5Y cells also leads to an early/recycling endosomal localization of the exogenously expressed ClC-6 protein. Finally, in transiently transfected COS-1 cells, ClC-6 copurifies with detergent-resistant membrane fractions, suggesting its partitioning in lipid rafts. Mutating a juxtamembrane string of basic amino acids (amino acids 71-75: KKGRR) disturbs the association with detergent-resistant membrane fractions and also affects the segregation of ClC-6 and ClC-7 when cotransfected in COS-1 cells. CONCLUSIONS: We conclude that human ClC-6 is an endosomal glycoprotein that partitions in detergent resistant lipid domains. The differential sorting of endogenous (late endosomal) versus overexpressed (early and recycling endosomal) ClC-6 is reminiscent of that of other late endosomal/lysosomal membrane proteins (e.g. LIMP II), and is consistent with a rate-limiting sorting step for ClC-6 between early endosomes and its final destination in late endosomes

X-exome sequencing of 405 unresolved families identifies seven novel intellectual disability genes

X-linked intellectual disability (XLID) is a clinically and genetically heterogeneous disorder. During the past two decades in excess of 100 X-chromosome ID genes have been identified. Yet, a large number of families mapping to the X-chromosome remained unresolved suggesting that more XLID genes or loci are yet to be identified. Here, we have investigated 405 unresolved families with XLID. We employed massively parallel sequencing of all X-chromosome exons in the index males. The majority of these males were previously tested negative for copy number variations and for mutations in a subset of known XLID genes by Sanger sequencing. In total, 745 X-chromosomal genes were screened. After stringent filtering, a total of 1297 non-recurrent exonic variants remained for prioritization. Co-segregation analysis of potential clinically relevant changes revealed that 80 families (20%) carried pathogenic variants in established XLID genes. In 19 families, we detected likely causative protein truncating and missense variants in 7 novel and validated XLID genes (CLCN4, CNKSR2, FRMPD4, KLHL15, LAS1L, RLIM and USP27X) and potentially deleterious variants in 2 novel candidate XLID genes (CDK16 and TAF1). We show that the CLCN4 and CNKSR2 variants impair protein functions as indicated by electrophysiological studies and altered differentiation of cultured primary neurons from Clcn4−/− mice or after mRNA knock-down. The newly identified and candidate XLID proteins belong to pathways and networks with established roles in cognitive function and intellectual disability in particular. We suggest that systematic sequencing of all X-chromosomal genes in a cohort of patients with genetic evidence for X-chromosome locus involvement may resolve up to 58% of Fragile X-negative cases

Nucleotide recognition by the cytoplasmic domain of the human chloride transporter ClC-5

The ubiquitous CBS domains, which are found as part of cytoplasmic domains in the ClC family of chloride channels and transporters, have previously been identified as building blocks for regulatory nucleotide-binding sites. Here we report the structures of the cytoplasmic domain of the human transporter ClC-5 in complex with ATP and ADP. The nucleotides bind to a specific site in the protein. As determined by equilibrium dialysis, the affinities for ATP, ADP and AMP are in the high micromolar range. Point mutations that interfere with nucleotide binding change the transport behavior of a ClC-5 mutant expressed in Xenopus laevis oocytes. Our results establish the structural and energetic basis for the interaction of ClC-5 with nucleotides and provide a framework for future investigations

Consensus statement: evaluation of new and existing therapeutics for pediatric multiple sclerosis.

New therapies are being evaluated by clinical trials and, if efficacious, introduced for the treatment of adult MS. The role of these new and existing agents in the management of pediatric MS has yet to be defined. Pediatric investigation plans are now required by the Food and Drug Administration and European Medicines Agency for approval of new biological agents, providing an important opportunity to gather much-needed data for clinicians caring for children and adolescents with MS. However, challenges include the small number of patients, and the need for efficient yet comprehensive study designs incorporating factors necessary to inform the clinical care of children with MS. The elected Steering committee of the International Pediatric MS Study Group (IPMSSG) conducted a structured review of existing data on the disease-modifying therapies in pediatric MS and developed a consensus statement, which was further modified by the IPMSSG general membership, using an online survey tool. Fifty-one IPMSSG members from 21 countries responded to the survey, and 50 approved the final statement. Consensus recommendations regarding use of existing first- and second-line therapies, as well as a proposed definition for inadequate treatment response, are presented. Recommendations for the use and evaluation of emerging therapies (currently in phase III clinical trials or recently approved for adult MS) are discussed. The IPMSSG endorses the inclusion of pediatric MS patients in trials evaluating appropriate new and emerging therapies. Mechanisms for conducting high-impact, multicenter studies, including long-term follow-up in pediatric MS, are required to ensure that all MS patients, irrespective of age, benefit from advances in MS therapeutics