A Common Signal Patch Drives AP-1 Protein-dependent Golgi Export of Inwardly Rectifying Potassium Channels

The cellular pathways that maintain the quality control and transport of diverse potassium channels

Potassium channels are multi-subunit transmembrane proteins that permit the selective passage of potassium and play fundamental roles in physiological processes, such as action potentials in the nervous system and organismal salt and water homeostasis, which is mediated by the kidney. Like all ion channels, newly translated potassium channels enter the endoplasmic reticulum (ER) and undergo the error-prone process of acquiring post-translational modifications, folding into their native conformations, assembling with other subunits, and trafficking through the secretory pathway to reach their final destinations, most commonly the plasma membrane. Disruptions in these processes can result in detrimental consequences, including various human diseases. Thus, multiple quality control checkpoints evolved to guide potassium channels through the secretory pathway and clear potentially toxic, aggregation-prone misfolded species. We will summarize current knowledge on the mechanisms underlying potassium channel quality control in the secretory pathway, highlight diseases associated with channel misfolding, and suggest potential therapeutic routes.

DIMPLE: deep insertion, deletion, and missense mutation libraries for exploring protein variation in evolution, disease, and biology

Insertions and deletions (indels) enable evolution and cause disease. Due to technical challenges, indels are left out of most mutational scans, limiting our understanding of them in disease, biology, and evolution. We develop a low cost and bias method, DIMPLE, for systematically generating deletions, insertions, and missense mutations in genes, which we test on a range of targets, including Kir2.1. We use DIMPLE to study how indels impact potassium channel structure, disease, and evolution. We find deletions are most disruptive overall, beta sheets are most sensitive to indels, and flexible loops are sensitive to deletions yet tolerate insertions.

FgAP1σ Is Critical for Vegetative Growth, Conidiation, Virulence, and DON Biosynthesis in Fusarium graminearum

The AP1 complex is a highly conserved clathrin adaptor that plays important roles in regulating cargo protein sorting and intracellular vesicle trafficking in eukaryotes. However, the functions of the AP1 complex in the plant pathogenic fungi including the devastating wheat pathogen Fusarium graminearum are still unclear. In this study, we investigated the biological functions of FgAP1^σ, a subunit of the AP1 complex in F. graminearum. Disruption of FgAP1^σ causes seriously impaired fungal vegetative growth, conidiogenesis, sexual development, pathogenesis, and deoxynivalenol (DON) production. The ΔFgap1^σ mutants were found to be less sensitive to KCl- and sorbitol-induced osmotic stresses but more sensitive to SDS-induced stress than the wild-type PH-1. Although the growth inhibition rate of the ΔFgap1^σ mutants was not significantly changed under calcofluor white (CFW) and Congo red (CR) stresses, the protoplasts released from ΔFgap1^σ hyphae were decreased compared with the wild-type PH-1, suggesting that FgAP1^σ is necessary for cell wall integrity and osmotic stresses in F. graminearum. Subcellular localization assays showed that FgAP1^σ was predominantly localized to endosomes and the Golgi apparatus. In addition, FgAP1^β-GFP, FgAP1^γ-GFP, and FgAP1^μ-GFP also localize to the Golgi apparatus. FgAP1^β interacts with FgAP1^σ, FgAP1^γ, and FgAP1^μ, while FgAP1^σ regulates the expression of FgAP1^β, FgAP1^γ, and FgAP1^μ in F. graminearum. Furthermore, the loss of FgAP1^σ blocks the transportation of the v-SNARE protein FgSnc1 from the Golgi to the plasma membrane and delays the internalization of FM4-64 dye into the vacuole. Taken together, our results demonstrate that FgAP1^σ plays vital roles in vegetative growth, conidiogenesis, sexual reproduction, DON production, pathogenicity, cell wall integrity, osmotic stress, exocytosis, and endocytosis in F. graminearum. These findings unveil the functions of the AP1 complex in filamentous fungi, most notably in F. graminearum, and lay solid foundations for effective prevention and control of Fusarium head blight (FHB).

Ion-ing out the genetic variants of Kir2.1

Deep mutational scanning provides new insights into how mutations alter the expression and activity of the potassium ion channel Kir2.1, which is associated with many diseases.

A trafficking motif alters GEVI activity implicating persistent protein interactions at the membrane

Efficient plasma-membrane expression is critical for genetically encoded voltage indicators (GEVIs). To improve the plasma-membrane expression, we introduced multiple combinations of plasma-membrane trafficking motifs at different positions to members of the Bongwoori family of GEVIs. An improvement from 20% to 27% in the ΔF/F/100 mV depolarization of the plasma membrane was observed when a Golgi transport motif was inserted near the N-terminus in conjunction with an endoplasmic reticulum release motif near the C-terminus of the protein. Unfortunately, this variant was also slower. The weighted tau on of the variant (25 ms) was more than double the original construct (11 ms). The weighted tau off was >20 ms compared with 10 ms for the original GEVI. The voltage range of the GEVI was also shifted to more negative potentials. Insertion of spacer amino acids between the fluorescent-protein domain and the endoplasmic reticulum release motif at the C-terminus rescued the speed of both the tau on and tau off while restoring the voltage range and maintaining the improved voltage-dependent optical signal. These results suggest that while trafficking motifs do improve plasma-membrane expression, they may also mediate persistent associations that affect the functioning of the protein.

Determinants of trafficking, conduction, and disease within a K+ channel revealed through multiparametric deep mutational scanning

A long-standing goal in protein science and clinical genetics is to develop quantitative models of sequence, structure, and function relationships to understand how mutations cause disease. Deep mutational scanning (DMS) is a promising strategy to map how amino acids contribute to protein structure and function and to advance clinical variant interpretation. Here, we introduce 7429 single-residue missense mutations into the inward rectifier K+ channel Kir2.1 and determine how this affects folding, assembly, and trafficking, as well as regulation by allosteric ligands and ion conduction. Our data provide high-resolution information on a cotranslationally folded biogenic unit, trafficking and quality control signals, and segregated roles of different structural elements in fold stability and function. We show that Kir2.1 surface trafficking mutants are underrepresented in variant effect databases, which has implications for clinical practice. By comparing fitness scores with expert-reviewed variant effects, we can predict the pathogenicity of 'variants of unknown significance' and disease mechanisms of known pathogenic mutations. Our study in Kir2.1 provides a blueprint for how multiparametric DMS can help us understand the mechanistic basis of genetic disorders and the structure-function relationships of proteins.

Quantitative Analysis of the Cytoskeleton's Role in Inward Rectifier K IR 2.1 Forward and Backward Trafficking

Alteration of the inward rectifier current I_K1, carried by K_IR2.1 channels, affects action potential duration, impacts resting membrane stability and associates with cardiac arrhythmias. Congenital and acquired K_IR2.1 malfunction frequently associates with aberrant ion channel trafficking. Cellular processes underlying trafficking are intertwined with cytoskeletal function. The extent to which the cytoskeleton is involved in K_IR2.1 trafficking processes is unknown. We aimed to quantify the dependence of K_IR2.1 trafficking on cytoskeleton function. GFP or photoconvertible Dendra2 tagged K_IR2.1 constructs were transfected in HEK293 or HeLa cells. Photoconversion of the Dendra2 probe at the plasma membrane and subsequent live imaging of trafficking processes was performed by confocal laser-scanning microscopy. Time constant of green fluorescent recovery (τg,s) represented recruitment of new K_IR2.1 at the plasma membrane. Red fluorescent decay (τr,s) represented internalization of photoconverted K_IR2.1. Patch clamp electrophysiology was used to quantify I_KIR2.₁. Biochemical methods were used for cytoskeleton isolation and detection of K_IR2.1-cytoskeleton interactions. Cytochalasin B (20 μM), Nocodazole (30 μM) and Dyngo-4a (10 nM) were used to modify the cytoskeleton. Chloroquine (10 μM, 24 h) was used to impair K_IR2.1 breakdown. Cytochalasin B and Nocodazole, inhibitors of actin and tubulin filament formation respectively, strongly inhibited the recovery of green fluorescence at the plasma membrane suggestive for inhibition of K_IR2.1 forward trafficking [τg,s 13 ± 2 vs. 131 ± 31* and 160 ± 40* min, for control, Cytochalasin B and Nocodazole, respectively (*p < 0.05 vs. control)]. Dyngo-4a, an inhibitor of dynamin motor proteins, strongly slowed the rate of photoconverted channel internalization, whereas Nocodazole and Cytochalasin B had less effect [τr,s 20 ± 2 vs. 87 ± 14*, 60 ± 16 and 64 ± 20 min (*p < 0.05 vs. control)]. Cytochalasin B treatment (20 μM, 24 h) inhibited I_KIR2.₁. Chloroquine treatment (10 μM, 24 h) induced intracellular aggregation of K_IR2.1 channels and enhanced interaction with the actin/intermediate filament system (103 ± 90 fold; p < 0.05 vs. control). Functional actin and tubulin cytoskeleton systems are essential for forward trafficking of K_IR2.1 channels, whereas initial backward trafficking relies on a functional dynamin system. Chronic disturbance of the actin system inhibits K_IR2.1 currents. Internalized K_IR2.1 channels become recruited to the cytoskeleton, presumably in lysosomes.

Inwardly Rectifying Potassium Channel Kir2.1 and its "Kir-ious" Regulation by Protein Trafficking and Roles in Development and Disease

Potassium (K+) homeostasis is tightly regulated for optimal cell and organismal health. Failure to control potassium balance results in disease, including cardiac arrythmias and developmental disorders. A family of inwardly rectifying potassium (Kir) channels helps cells maintain K+ levels. Encoded by KCNJ genes, Kir channels are comprised of a tetramer of Kir subunits, each of which contains two-transmembrane domains. The assembled Kir channel generates an ion selectivity filter for K+ at the monomer interface, which allows for K+ transit. Kir channels are found in many cell types and influence K+ homeostasis across the organism, impacting muscle, nerve and immune function. Kir2.1 is one of the best studied family members with well-defined roles in regulating heart rhythm, muscle contraction and bone development. Due to their expansive roles, it is not surprising that Kir mutations lead to disease, including cardiomyopathies, and neurological and metabolic disorders. Kir malfunction is linked to developmental defects, including underdeveloped skeletal systems and cerebellar abnormalities. Mutations in Kir2.1 cause the periodic paralysis, cardiac arrythmia, and developmental deficits associated with Andersen-Tawil Syndrome. Here we review the roles of Kir family member Kir2.1 in maintaining K+ balance with a specific focus on our understanding of Kir2.1 channel trafficking and emerging roles in development and disease. We provide a synopsis of the vital work focused on understanding the trafficking of Kir2.1 and its role in development.

Improving Protein Subcellular Location Classification by Incorporating Three-Dimensional Structure Information

The subcellular locations of proteins are closely related to their functions. In the past few decades, the application of machine learning algorithms to predict protein subcellular locations has been an important topic in proteomics. However, most studies in this field used only amino acid sequences as the data source. Only a few works focused on other protein data types. For example, three-dimensional structures, which contain far more functional protein information than sequences, remain to be explored. In this work, we extracted various handcrafted features to describe the protein structures from physical, chemical, and topological aspects, as well as the learned features obtained by deep neural networks. We then used these features to classify the protein subcellular locations. Our experimental results demonstrated that some of these structural features have a certain effect on the protein location classification, and can help improve the performance of sequence-based location predictors. Our method provides a new view for the analysis of protein spatial distribution, and is anticipated to be used in revealing the relationships between protein structures and functions.

De novo and bi-allelic variants in AP1G1 cause neurodevelopmental disorder with developmental delay, intellectual disability, and epilepsy

Adaptor protein (AP) complexes mediate selective intracellular vesicular trafficking and polarized localization of somatodendritic proteins in neurons. Disease-causing alleles of various subunits of AP complexes have been implicated in several heritable human disorders, including intellectual disabilities (IDs). Here, we report two bi-allelic (c.737C>A [p.Pro246His] and c.1105A>G [p.Met369Val]) and eight de novo heterozygous variants (c.44G>A [p.Arg15Gln], c.103C>T [p.Arg35Trp], c.104G>A [p.Arg35Gln], c.229delC [p.Gln77Lys^∗11], c.399_400del [p.Glu133Aspfs^∗37], c.747G>T [p.Gln249His], c.928-2A>C [p.?], and c.2459C>G [p.Pro820Arg]) in AP1G1, encoding gamma-1 subunit of adaptor-related protein complex 1 (AP1γ1), associated with a neurodevelopmental disorder (NDD) characterized by mild to severe ID, epilepsy, and developmental delay in eleven families from different ethnicities. The AP1γ1-mediated adaptor complex is essential for the formation of clathrin-coated intracellular vesicles. In silico analysis and 3D protein modeling simulation predicted alteration of AP1γ1 protein folding for missense variants, which was consistent with the observed altered AP1γ1 levels in heterologous cells. Functional studies of the recessively inherited missense variants revealed no apparent impact on the interaction of AP1γ1 with other subunits of the AP-1 complex but rather showed to affect the endosome recycling pathway. Knocking out ap1g1 in zebrafish leads to severe morphological defect and lethality, which was significantly rescued by injection of wild-type AP1G1 mRNA and not by transcripts encoding the missense variants. Furthermore, microinjection of mRNAs with de novo missense variants in wild-type zebrafish resulted in severe developmental abnormalities and increased lethality. We conclude that de novo and bi-allelic variants in AP1G1 are associated with neurodevelopmental disorder in diverse populations.

Cardiac potassium inward rectifier Kir2: Review of structure, regulation, pharmacology, and arrhythmogenesis

Potassium inward rectifier channel Kir2 is an important component of terminal cardiac repolarization and resting membrane stability. This functionality is part of balanced cardiac excitability and is a defining feature of excitable cardiac membranes. “Gain-of-function” or “loss-of-function” mutations in KCNJ2, the gene encoding Kir2.1, cause genetic sudden cardiac death syndromes, and loss of the Kir2 current I_K1 is a major contributing factor to arrhythmogenesis in failing human hearts. Here we provide a contemporary review of the functional structure, physiology, and pharmacology of Kir2 channels. Beyond the structure and functional relationships, we will focus on the elements of clinically used drugs that block the channel and the implications for treatment of atrial fibrillation with I_K1-blocking agents. We will also review the clinical disease entities associated with KCNJ2 mutations and the growing area of research into associated arrhythmia mechanisms. Lastly, the presence of Kir2 channels has become a tipping point for electrical maturity in induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) and highlights the significance of understanding why Kir2 in iPS-CMs is important to consider for Comprehensive In Vitro Proarrhythmia Assay and drug safety testing.

Life Cycle of the Cardiac Voltage-Gated Sodium Channel NaV1.5

Cardiac voltage-gated sodium channel Na_V1.5, encoded by SCN5A, is crucial for the upstroke of action potential and excitation of cardiomyocytes. Na_V1.5 undergoes complex processes before it reaches the target membrane microdomains and performs normal functions. A variety of protein partners are needed to achieve the balance between SCN5A transcription and mRNA decay, endoplasmic reticulum retention and export, Golgi apparatus retention and export, selective anchoring and degradation, activation, and inactivation of sodium currents. Subtle alterations can impair Na_V1.5 in terms of expression or function, eventually leading to Na_V1.5-associated diseases such as lethal arrhythmias and cardiomyopathy.

Disease Associated Mutations in KIR Proteins Linked to Aberrant Inward Rectifier Channel Trafficking

The ubiquitously expressed family of inward rectifier potassium (K_IR) channels, encoded by KCNJ genes, is primarily involved in cell excitability and potassium homeostasis. Channel mutations associate with a variety of severe human diseases and syndromes, affecting many organ systems including the central and peripheral neural system, heart, kidney, pancreas, and skeletal muscle. A number of mutations associate with altered ion channel expression at the plasma membrane, which might result from defective channel trafficking. Trafficking involves cellular processes that transport ion channels to and from their place of function. By alignment of all K_IR channels, and depicting the trafficking associated mutations, three mutational hotspots were identified. One localized in the transmembrane-domain 1 and immediately adjacent sequences, one was found in the G-loop and Golgi-export domain, and the third one was detected at the immunoglobulin-like domain. Surprisingly, only few mutations were observed in experimentally determined Endoplasmic Reticulum (ER)exit-, export-, or ER-retention motifs. Structural mapping of the trafficking defect causing mutations provided a 3D framework, which indicates that trafficking deficient mutations form clusters. These “mutation clusters” affect trafficking by different mechanisms, including protein stability.

Identification and functional characterization of two novel mutations in KCNJ10 and PI4KB in SeSAME syndrome without electrolyte imbalance

Background

Dysfunction in inwardly rectifying potassium channel Kir4.1 has been implicated in SeSAME syndrome, an autosomal-recessive (AR), rare, multi-systemic disorder. However, not all neurological, intellectual disability, and comorbid phenotypes in SeSAME syndrome can be mechanistically linked solely to Kir4.1 dysfunction.

Methods

We therefore performed whole-exome sequencing and identified additional genetic risk-elements that might exert causative effects either alone or in concert with Kir4.1 in a family diagnosed with SeSAME syndrome.

Results

Two variant prioritization pipelines based on AR inheritance and runs of homozygosity (ROH), identified two novel homozygous variants in KCNJ10 and PI4KB and five rare homozygous variants in PVRL4, RORC, FLG2, FCRL1, NIT1 and one common homozygous variant in HSPA6 segregating in all four patients. The novel mutation in KCNJ10 resides in the cytoplasmic domain of Kir4.1, a seat of phosphatidylinositol bisphosphate (PIP2) binding. The mutation altered the subcellular localization and stability of Kir4.1 in patient-specific lymphoblastoid cells (LCLs) compared to parental controls. Barium-sensitive endogenous K⁺ currents in patient-specific LCLs using whole-cell patch-clamp electrophysiology revealed membrane depolarization and defects in inward K⁺ ion conductance across the membrane, thereby suggesting a loss-of-function effect of KCNJ10 variant.

Conclusion

Altogether, our findings implicate the role of new genes in SeSAME syndrome without electrolyte imbalance and thereby speculate the regulation of Kir4.1 channel activity by PIP2 and integrin-mediated adhesion signaling mechanisms.

Electronic supplementary material

The online version of this article (10.1186/s40246-019-0236-0) contains supplementary material, which is available to authorized users.

Inward Rectifier Potassium Channels (Kir2.x) and Caveolin-3 Domain-Specific Interaction: Implications for Purkinje Cell-Dependent Ventricular Arrhythmias

BACKGROUND

In human cardiac ventricle, I_K1 is mainly comprised Kir2.1, but Kir2.2 and Kir2.3 heterotetramers occur and modulate I_K1. Long-QT syndrome-9-associated CAV3 mutations cause decreased Kir2.1 current density, but Kir2.x heterotetramers have not been studied. Here, we determine the effect of long-QT syndrome-9-CAV3 mutation F97C on Kir2.x homo- and heterotetramers and model-associated arrhythmia mechanisms.

METHODS AND RESULTS

Super-resolution microscopy, co-immunoprecipitation, cellular electrophysiology, on-cell Western blotting, and simulation of Purkinje and ventricular myocyte mathematical models were used. Kir2.x isoforms have unique subcellular colocalization in human cardiomyocytes and coimmunoprecipitate with Cav3. F97C-Cav3 decreased peak inward Kir2.2 current density by 50% (-120 mV; P=0.019) and peak outward by 75% (-40 mV; P<0.05) but did not affect Kir2.3 current density. FRET (Förster resonance energy transfer) efficiency for Kir2.2 with Cav3 is high, and on-cell Western blotting demonstrates decreased Kir2.2 membrane expression with F97C-Cav3. Cav3-F97C reduced peak inward and outward current density of Kir2.2/Kir2.1 or Kir2.2/Kir2.3 heterotetramers (P<0.05). Only Cav3 scaffolding and membrane domains co-immunoprecipitation with Kir2.1 and Kir2.2 and Kir2.x-N-terminal Cav3 binding motifs are required for interaction. Mathematical Purkinje, but not ventricular, myocyte model incorporating simulated current reductions, predicts spontaneous delayed after-depolarization-mediated triggered activity.

CONCLUSIONS

Kir2.x isoforms have a unique intracellular pattern of distribution in association with specific Cav3 domains and that critically depends on interaction with N-terminal Kir2.x Cav3-binding motifs. Long-QT syndrome-9-CAV3 mutation differentially regulates current density and cell surface expression of Kir2.x homomeric and heteromeric channels. Mathematical Purkinje cell model incorporating experimental findings suggests delayed after-depolarization-type triggered activity as a possible arrhythmia mechanism.

The Potassium Channel Odyssey: Mechanisms of Traffic and Membrane Arrangement

Ion channels are transmembrane proteins that conduct specific ions across biological membranes. Ion channels are present at the onset of many cellular processes, and their malfunction triggers severe pathologies. Potassium channels (KChs) share a highly conserved signature that is necessary to conduct K⁺ through the pore region. To be functional, KChs require an exquisite regulation of their subcellular location and abundance. A wide repertoire of signatures facilitates the proper targeting of the channel, fine-tuning the balance that determines traffic and location. These signature motifs can be part of the secondary or tertiary structure of the protein and are spread throughout the entire sequence. Furthermore, the association of the pore-forming subunits with different ancillary proteins forms functional complexes. These partners can modulate traffic and activity by adding their own signatures as well as by exposing or masking the existing ones. Post-translational modifications (PTMs) add a further dimension to traffic regulation. Therefore, the fate of a KCh is not fully dependent on a gene sequence but on the balance of many other factors regulating traffic. In this review, we assemble recent evidence contributing to our understanding of the spatial expression of KChs in mammalian cells. We compile specific signatures, PTMs, and associations that govern the destination of a functional channel.

G protein-coupled receptors differentially regulate glycosylation and activity of the inwardly rectifying potassium channel Kir7.1

Kir7.1 is an inwardly rectifying potassium channel with important roles in the regulation of the membrane potential in retinal pigment epithelium, uterine smooth muscle, and hypothalamic neurons. Regulation of G protein-coupled inwardly rectifying potassium (GIRK) channels by G protein-coupled receptors (GPCRs) via the G protein βγ subunits has been well characterized. However, how Kir channels are regulated is incompletely understood. We report here that Kir7.1 is also regulated by GPCRs, but through a different mechanism. Using Western blotting analysis, we observed that multiple GPCRs tested caused a striking reduction in the complex glycosylation of Kir7.1. Further, GPCR-mediated reduction of Kir7.1 glycosylation in HEK293T cells did not alter its expression at the cell surface but decreased channel activity. Of note, mutagenesis of the sole Kir7.1 glycosylation site reduced conductance and open probability, as indicated by single-channel recording. Additionally, we report that the L241P mutation of Kir7.1 associated with Lebers congenital amaurosis (LCA), an inherited retinal degenerative disease, has significantly reduced complex glycosylation. Collectively, these results suggest that Kir7.1 channel glycosylation is essential for function, and this activity within cells is suppressed by most GPCRs. The melanocortin-4 receptor (MC4R), a GPCR previously reported to induce ligand-regulated activity of this channel, is the only GPCR tested that does not have this effect on Kir7.1.

Golgin-97 Targets Ectopically Expressed Inward Rectifying Potassium Channel, Kir2.1, to the trans-Golgi Network in COS-7 Cells

The inward rectifying potassium channel, Kir2.1, is selected as cargo at the trans-Golgi network (TGN) for export to the cell surface through a unique signal-dependent interaction with the AP1 clathrin-adaptor, but it is unknown how the channel is targeted at earlier stages in the secretory pathway for traffic to the TGN. Here we explore a mechanism. A systematic screen of Golgi tethers identified Golgin-97 as a Kir2.1 binding partner. In vitro protein-interaction studies revealed the interaction is direct, occurring between the GRIP domain of Golgin-97 and the cytoplasmic domain of Kir2.1. Imaging and interaction studies in COS-7 cells suggest that Golgi-97 binds to the channel en route through the Golgi. RNA interference-mediated knockdown of Golgin-97 prevented exit of Kir2.1 from the Golgi. These observations identify Golgin-97 as a Kir2.1 binding partner that is required for targeting the channel to the TGN. Based on our studies in COS-7 cells, we propose Golgi-97 facilitates formation of AP1-dependent export carriers for Kir2.1 by coupling anterograde delivery of Kir2.1 with retrograde recycling of AP-1 containing endosomes to the TGN.

Hypoxic stress upregulates Kir2.1 expression by a pathway including hypoxic-inducible factor-1α and dynamin2 in brain capillary endothelial cells

Brain capillary endothelial cells (BCECs) play a central role in maintenance of blood-brain barrier (BBB) function and, therefore, are essential for central nervous system homeostasis and integrity. Although brain ischemia damages BCECs and causes disruption of BBB, the related influence of hypoxia on BCECs is not well understood. Hypoxic stress can upregulate functional expression of specific K+ currents in endothelial cells, e.g., Kir2.1 channels without any alterations in the mRNA level, in t-BBEC117, a cell line derived from bovine BCECs. The hyperpolarization of membrane potential due to Kir2.1 channel upregulation significantly facilitates cell proliferation. In the present study, the mechanisms underlying the hypoxia-induced Kir2.1 upregulation was examined. We emphasize the involvement of dynamin2, a protein known to be involved in a number of surface expression pathways. Hypoxic culture upregulated dynamin2 expression in t-BBEC117 cells. The inhibition of dynamin2 by Dynasore canceled hypoxia-induced upregulation of Kir2.1 currents by reducing surface expression. On the contrary, Kir2.1 currents and proteins in t-BBEC117 cultured under normoxia were increased by overexpression of dynamin2, but not by dominant-negative dynamin2. Molecular imaging based on bimolecular fluorescence complementation, double-immunostaining, and coimmunoprecipitation assays revealed that dynamin2 can directly bind to the Kir2.1 channel. Moreover, hypoxic culture downregulated hypoxic-inducible factor-1α (HIF-1α) expression. Knockdown of HIF-1α increased dynamin2 expression in t-BBEC117 cells, in both normoxic and hypoxic culture conditions. In summary, our results demonstrated that hypoxia downregulates HIF-1α, increases dynamin2 expression, and facilitates Kir2.1 surface expression, resulting in hyperpolarization of membrane potential and subsequent increase in Ca2+ influx in BCECs.

AP1 mediates uPA/uPAR induced FUT4 expression and trophoblast invasion

Trophoblast invasion is crucial for embryo implantation and successful pregnancy. Urokinase-type plasminogen activator (uPA)/urokinase-type plasminogen activator receptor (uPAR) are expressed on trophoblasts and involved in trophoblast invasion. The transcription factor activator protein 1 (AP1) (c-Fos and cJun) and fucosyltransferase IV (FUT4) have been found to be involved in this process. However, the relationship of uPA/uPAR, AP1 and FUT4 is unclear. The current study aimed to investigate the role of AP1 in uPA/uPAR induced FUT4 expression and trophoblast invasion. We found that p-c-Fos and p-c-Jun were decreased in abortion patients compared to that in normal pregnant women. Employing human trophoblastic cells, we then demonstrated that uPA/uPAR induced the expression of p-c-Fos and p-c-Jun. Applying an electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP), we further proved that transcription factor AP1 bound to FUT4 promoter that could increase FUT4 transcriptional activity, further promoting trophoblast cell migration and invasion through JNK MAPK signaling pathway. Taken together, these results suggest that uPA/uPAR induces FUT4 expression, and trophoblast cell invasion mediated by AP1 transcription factor (c-Fos and c-Jun). Our findings provide novel insights into the relationship between AP1 and abortion.

Investigating Potassium Channels in Budding Yeast: A Genetic Sandbox

Like all species, the model eukaryote Saccharomyces cerevisiae, or Bakers' yeast, concentrates potassium in the cytosol as an electrogenic osmolyte and enzyme cofactor. Yeast are capable of robust growth on a wide variety of potassium concentrations, ranging from 10 µM to 2.5 M, due to the presence of a high-affinity potassium uptake system and a battery of cation exchange transporters. Genetic perturbation of either of these systems retards yeast growth on low or high potassium, respectively. However, these potassium-sensitized yeast are a powerful genetic tool, which has been leveraged for diverse studies. Notably, the potassium-sensitive cells can be transformed with plasmids encoding potassium channels from bacteria, plants, or mammals, and subsequent changes in growth rate have been found to correlate with the activity of the introduced potassium channel. Discoveries arising from the use of this assay over the past three decades have increased our understanding of the structure-function relationships of various potassium channels, the mechanisms underlying the regulation of potassium channel function and trafficking, and the chemical basis of potassium channel modulation. In this article, we provide an overview of the major genetic tools used to study potassium channels in S. cerevisiae, a survey of seminal studies utilizing these tools, and a prospective for the future use of this elegant genetic approach.

DUSP1 and KCNJ2 mRNA upregulation can serve as a biomarker of mechanical asphyxia-induced death in cardiac tissue

The incidence of death by asphyxia is second to the incidence of death by mechanical injury; however, death by mechanical asphyxia may be difficult to prove in court, particularly in cases in which corpses do not exhibit obvious signs of asphyxia. To identify a credible biomarker of asphyxia, we first examined the expression levels of 47,000 mRNAs in human cardiac tissue specimens from individuals who died of mechanical asphyxia and compared the expression levels with the levels of the corresponding mRNAs in specimens from individuals who died of craniocerebral injury using microarray. We selected 119 differentially expressed mRNAs, examined the expression levels of these mRNAs in 44 human cardiac tissue specimens of individuals who died of mechanical asphyxia, craniocerebral injury, hemorrhagic shock, or other causes. That the expression of dual-specificity phosphatase 1 (DUSP1) and potassium voltage-gated channel subfamily J member 2 (KCNJ2) was upregulated in human cardiac tissues from the mechanical asphyxia group compared with control tissues, regardless of age, environmental temperature, and postmortem interval (PMI), indicating that DUSP1 and KCNJ2 may be associated with mechanical asphyxia-induced death and can thus serve as useful biomarkers of death by mechanical asphyxia.