Structure and mechanism of the cation-chloride cotransporter NKCC1.

Cation-chloride cotransporters (CCCs) mediate the electroneutral transport of chloride, potassium and/or sodium across the membrane. They have critical roles in regulating cell volume, controlling ion absorption and secretion across epithelia, and maintaining intracellular chloride homeostasis. These transporters are primary targets for some of the most commonly prescribed drugs. Here we determined the cryo-electron microscopy structure of the Na-K-Cl cotransporter NKCC1, an extensively studied member of the CCC family, from Danio rerio. The structure defines the architecture of this protein family and reveals how cytosolic and transmembrane domains are strategically positioned for communication. Structural analyses, functional characterizations and computational studies reveal the ion-translocation pathway, ion-binding sites and key residues for transport activity. These results provide insights into ion selectivity, coupling and translocation, and establish a framework for understanding the physiological functions of CCCs and interpreting disease-related mutations.

Advanced-Stage Colorectal Cancer in Persons Younger Than 50 Years Not Associated With Longer Duration of Symptoms or Time to Diagnosis.

BACKGROUND & AIMS: The incidence of colorectal cancer (CRC) is increasing in the United States among adults younger than the age of 50 years. Studies of young-onset CRC have focused on outcomes and treatment patterns. We examined patient presentation, provider evaluation, and time to diagnosis, which can affect stage and prognosis. METHODS: In a retrospective study, we collected data from patients with a diagnosis of colorectal adenocarcinoma, confirmed by pathologists, seen at the Stanford Cancer Institute from January 1, 2008, through December 31, 2014. We compared symptoms, clinical features, time to diagnosis, and cancer stage in patients with young-onset CRC (diagnosed at an age younger than 50 years; n = 253) with patients diagnosed with CRC at an age of 50 years or older (n = 232). RESULTS: A higher proportion of patients with young-onset CRC were diagnosed with advanced-stage tumors (72%) compared with older patients (63%) (P = .03). Larger proportions of patients with young-onset CRC also had a family history of CRC (25% vs 17% in older patients; P = .03), confirmed or probable hereditary cancer syndromes (7% vs 1% in older patients; P < .01), and left-sided disease (distal colon cancer in 41% vs 34% in older patients; P = .01; and rectal cancer in 40% vs 35% in older patients; P = .29). Patients with young-onset CRC had a significantly longer median time to diagnosis (128 vs 79 days for older patients; P < .05), symptom duration (60 vs 30 days for older patients; P < .01), and time of evaluation (31 vs 22 days; P < .05). In multivariable analyses, time to diagnosis was 1.4-fold longer for younger than for older patients (P < .01). Among younger patients, those with stage III or IV CRC had shorter durations of symptoms and evaluations than those with stage I or II CRC. CONCLUSIONS: In a retrospective analysis of patients with CRC, we found that greater proportions of patients younger than 50 years were diagnosed with advanced-stage tumors than older patients; this difference could not be explained simply by delays from symptom onset to diagnosis. Although tumor biology may be an important determinant of stage at diagnosis, clinicians should be aware of CRC alarm symptoms, family history, and genetic syndromes, to speed evaluation and diagnosis of younger patients and potentially improve outcomes. It remains to be determined whether subgroups of persons at risk for young-onset CRC who benefit from early screening can be identified.

13C NMR detects conformational change in the 100-kD membrane transporter ClC-ec1.

CLC transporters catalyze the exchange of Cl(-) for H(+) across cellular membranes. To do so, they must couple Cl(-) and H(+) binding and unbinding to protein conformational change. However, the sole conformational changes distinguished crystallographically are small movements of a glutamate side chain that locally gates the ion-transport pathways. Therefore, our understanding of whether and how global protein dynamics contribute to the exchange mechanism has been severely limited. To overcome the limitations of crystallography, we used solution-state (13)C-methyl NMR with labels on methionine, lysine, and engineered cysteine residues to investigate substrate (H(+)) dependent conformational change outside the restraints of crystallization. We show that methyl labels in several regions report H(+)-dependent spectral changes. We identify one of these regions as Helix R, a helix that extends from the center of the protein, where it forms the part of the inner gate to the Cl(-)-permeation pathway, to the extracellular solution. The H(+)-dependent spectral change does not occur when a label is positioned just beyond Helix R, on the unstructured C-terminus of the protein. Together, the results suggest that H(+) binding is mechanistically coupled to closing of the intracellular access-pathway for Cl(-).

High-Resolution Views and Transport Mechanisms of the NKCC1 and KCC Transporters.

Cation-chloride cotransporters (CCCs) are responsible for the coupled co-transport of Cl- with K+ and/or Na+ in an electroneutral manner. They play important roles in myriad fundamental physiological processes--from cell volume regulation to transepithelial solute transport and intracellular ion homeostasis--and are targeted by medicines commonly prescribed to treat hypertension and edema. After several decades of studies into the functions and pharmacology of these transporters, there have been several breakthroughs in the structural determination of CCC transporters. The insights provided by these new structures for the Na+/K+/Cl- cotransporter NKCC1 and the K+/Cl- cotransporters KCC1, KCC2, KCC3 and KCC4 have deepened our understanding of their molecular basis and transport function. This focused review discusses recent advances in the structural and mechanistic understanding of CCC transporters, including architecture, dimerization, functional roles of regulatory domains, ion binding sites, and coupled ion transport.

A designed inhibitor of a CLC antiporter blocks function through a unique binding mode.

The lack of small-molecule inhibitors for anion-selective transporters and channels has impeded our understanding of the complex mechanisms that underlie ion passage. The ubiquitous CLC "Chloride Channel" family represents a unique target for biophysical and biochemical studies because its distinctive protein fold supports both passive chloride channels and secondary-active chloride-proton transporters. Here, we describe the synthesis and characterization of a specific small-molecule inhibitor directed against a CLC antiporter (ClC-ec1). This compound, 4,4'-octanamidostilbene-2,2'-disulfonate (OADS), inhibits ClC-ec1 with low micromolar affinity and has no specific effect on a CLC channel (ClC-1). Inhibition of ClC-ec1 occurs by binding to two distinct intracellular sites. The location of these sites and the lipid dependence of inhibition suggest potential mechanisms of action. This compound will empower research to elucidate differences between antiporter and channel mechanisms and to develop treatments for CLC-mediated disorders.