A role for KCC3 in maintaining cell volume of peripheral nerve fibers.

The potassium chloride cotransporter, KCC3, is an electroneutral cotransporter expressed in the peripheral and central nervous system. KCC3 is responsible for the efflux of K+ and Cl- in neurons to help maintain cell volume and intracellular chloride levels. A loss-of-function (LOF) of KCC3 causes Hereditary Motor Sensory Neuropathy with Agenesis of the Corpus Callosum (HMSN/ACC) in a population of individuals in the Charlevoix/Lac-Saint-Jean region of Quebec, Canada. A variety of mouse models have been created to understand the physiological and deleterious effects of a KCC3 LOF. Though this KCC3 LOF in mouse models has recapitulated the peripheral neuropathy phenotype of HMSN/ACC, we still know little about the development of the disease pathophysiology. Interestingly, the most recent KCC3 mouse model that we created recapitulated a peripheral neuropathy-like phenotype originating from a KCC3 gain-of-function (GOF). Despite the past two decades of research in attempting to understand the role of KCC3 in disease, we still do not understand how dysfunction of this cotransporter can lead to the pathophysiology of peripheral neuropathy. This review focuses on the function of KCC3 in neurons and its role in human and health and disease.

Diversification of HP1-like Chromo Domain Proteins in Tetrahymena thermophila.

Proteins that possess a chromo domain are well-known for their roles in heterochromatin assembly and maintenance. The Heterochromatin Protein 1 (HP1) family, with a chromo domain and carboxy-terminal chromo shadow domain, targets heterochromatin through interaction with histone H3 methylated on lysine 9 (H3K9me2/3). The structural and functional diversity of these proteins observed in both fission yeast and metazoans correlate with chromatin specialization. To expand these studies, we examined chromo domain proteins in the ciliate Tetrahymena thermophila, which has functionally diverse and developmentally regulated heterochromatin domains. We identified thirteen proteins similar to HP1. Together they possess only a fraction of the possible chromo domain subtypes and most lack a recognizable chromo shadow domain. Using fluorescence microscopy to track chromatin localization of tagged proteins through the life cycle, we show evidence that in T. thermophila this family has diversified with biological roles in RNAi-directed DNA elimination, germline genome structure, and somatic heterochromatin. Those proteins with H3K27me3 binding sequence characteristics localize to chromatin in mature nuclei, whereas those with H3K9me2/3 binding characteristics localize to developing nuclei undergoing DNA elimination. Findings point to an expanded and diversified family of chromo domain proteins that parallels heterochromatin diversity in ciliates.

A de novo missense mutation of GABRB2 causes early myoclonic encephalopathy.

BACKGROUND: Early myoclonic encephalopathy (EME), a disease with a devastating prognosis, is characterised by neonatal onset of seizures and massive myoclonus accompanied by a continuous suppression-burst EEG pattern. Three genes are associated with EMEs that have metabolic features. Here, we report a pathogenic mutation of an ion channel as a cause of EME for the first time. METHODS: Sequencing was performed for 214 patients with epileptic seizures using a gene panel with 109 genes that are known or suspected to cause epileptic seizures. Functional assessments were demonstrated by using electrophysiological experiments and immunostaining for mutant γ-aminobutyric acid-A (GABAA) receptor subunits in HEK293T cells. RESULTS: We discovered a de novo heterozygous missense mutation (c.859A>C [p.Thr287Pro]) in the GABRB2-encoded ß2 subunit of the GABAA receptor in an infant with EME. No GABRB2 mutations were found in three other EME cases or in 166 patients with infantile spasms. GABAA receptors bearing the mutant ß2 subunit were poorly trafficked to the cell membrane and prevented γ2 subunits from trafficking to the cell surface. The peak amplitudes of currents from GABAA receptors containing only mutant ß2 subunits were smaller than that of those from receptors containing only wild-type ß2 subunits. The decrease in peak current amplitude (96.4% reduction) associated with the mutant GABAA receptor was greater than expected, based on the degree to which cell surface expression was reduced (66% reduction). CONCLUSION: This mutation has complex functional effects on GABAA receptors, including reduction of cell surface expression and attenuation of channel function, which would significantly perturb GABAergic inhibition in the brain.

A patient with multisystem dysfunction carries a truncation mutation in human SLC12A2, the gene encoding the Na-K-2Cl cotransporter, NKCC1.

This study describes a 13-yr-old girl with orthostatic intolerance, respiratory weakness, multiple endocrine abnormalities, pancreatic insufficiency, and multiorgan failure involving the gut and bladder. Exome sequencing revealed a de novo, loss-of-function allele in SLC12A2, the gene encoding the Na-K-2Cl cotransporter-1. The 11-bp deletion in exon 22 results in frameshift (p.Val1026Phefs*2) and truncation of the carboxy-terminal tail of the cotransporter. Preliminary studies in heterologous expression systems demonstrate that the mutation leads to a nonfunctional transporter, which is expressed and trafficked to the plasma membrane alongside wild-type NKCC1. The truncated protein, visible at higher molecular sizes, indicates either enhanced dimerization or misfolded aggregate. No significant dominant-negative effect was observed. K+ transport experiments performed in fibroblasts from the patient showed reduced total and NKCC1-mediated K+ influx. The absence of a bumetanide effect on K+ influx in patient fibroblasts only under hypertonic conditions suggests a deficit in NKCC1 regulation. We propose that disruption in NKCC1 function might affect sensory afferents and/or smooth muscle cells, as their functions depend on NKCC1 creating a Cl- gradient across the plasma membrane. This Cl- gradient allows the γ-aminobutyric acid (GABA) receptor or other Cl- channels to depolarize the membrane affecting processes such as neurotransmission or cell contraction. Under this hypothesis, disrupted sensory and smooth muscle function in a diverse set of tissues could explain the patient's phenotype.