MAGI-1 interacts with Slo1 channel proteins and suppresses Slo1 expression on the cell surface

Ion channels and channelopathies in glomeruli

An essential step in renal function entails the formation of an ultrafiltrate that is delivered to the renal tubules for subsequent processing. This process, known as glomerular filtration, is controlled by intrinsic regulatory systems and by paracrine, neuronal, and endocrine signals that converge onto glomerular cells. In addition, the characteristics of glomerular fluid flow, such as the glomerular filtration rate and the glomerular filtration fraction, play an important role in determining blood flow to the rest of the kidney. Consequently, disease processes that initially affect glomeruli are the most likely to lead to end-stage kidney failure. The cells that comprise the glomerular filter, especially podocytes and mesangial cells, express many different types of ion channels that regulate intrinsic aspects of cell function and cellular responses to the local environment, such as changes in glomerular capillary pressure. Dysregulation of glomerular ion channels, such as changes in TRPC6, can lead to devastating glomerular diseases, and a number of channels, including TRPC6, TRPC5, and various ionotropic receptors, are promising targets for drug development. This review discusses glomerular structure and glomerular disease processes. It also describes the types of plasma membrane ion channels that have been identified in glomerular cells, the physiological and pathophysiological contexts in which they operate, and the pathways by which they are regulated and dysregulated. The contributions of these channels to glomerular disease processes, such as focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, as well as the development of drugs that target these channels are also discussed.

Genetic and genomic signatures in ethanol withdrawal seizure-prone and seizure-resistant mice implicate genes involved in epilepsy and neuronal excitability

Alcohol withdrawal is a clinically important consequence and potential driver of Alcohol Use Disorder. However, susceptibility to withdrawal symptoms, ranging from craving and anxiety to seizures and delirium, varies greatly. Selectively bred Withdrawal Seizure-Prone (WSP) and Seizure-Resistant (WSR) mice are an animal model of differential susceptibility to withdrawal and phenotypes with which withdrawal severity correlates. To identify innate drivers of alcohol withdrawal severity, we performed a multi-omic study of the WSP and WSR lines and F2 mice derived from them, using genomic, genetic, and transcriptomic analyses. Genes implicated in seizures and epilepsy were over-represented among those that segregated between WSP and WSR mice and that displayed differential expression in F2 mice high and low in withdrawal. Quantitative trait locus (QTL) analysis of ethanol withdrawal convulsions identified several genome-wide significant loci and pointed to genes that modulate potassium channel function and neural excitability. Perturbations of expression of genes involved in synaptic transmission, including GABAergic and glutamatergic genes, were prominent in prefrontal cortex transcriptome. Expression QTL (eQTL) analysis fine mapped genes within the peak ethanol withdrawal QTL regions. Genetic association analysis in human subjects provided converging evidence for the involvement of those genes in severity of alcohol withdrawal and dependence. Our results reveal a polygenic network and neural signaling pathways contributing to ethanol withdrawal seizures and related phenotypes that overlap with genes modulating epilepsy and neuronal excitability.

The magic of MAGI-1: A scaffolding protein with multi signalosomes and functional plasticity

MAGI-1 is a critical cellular scaffolding protein with over 110 different cellular and microbial protein interactors. Since the discovery of MAGI-1 in 1997, MAGI-1 has been implicated in diverse cellular functions such as polarity, cell-cell communication, neurological processes, kidney function, and a host of diseases including cancer and microbial infection. Additionally, MAGI-1 has undergone nomenclature changes in response to the discovery of an additional PDZ domain, leading to lack of continuity in the literature. We address the nomenclature of MAGI-1 as well as summarize many of the critical functions of the known interactions. Given the importance of many of the interactors, such as human papillomavirus E6, the Coxsackievirus and adenovirus receptor (CAR), and PTEN, the enhancement or disruption of MAGI-based interactions has the potential to affect cellular functions that can potentially be harnessed as a therapeutic strategy for a variety of diseases.

MAGI1, a Scaffold Protein with Tumor Suppressive and Vascular Functions

MAGI1 is a cytoplasmic scaffolding protein initially identified as a component of cell-to-cell contacts stabilizing cadherin-mediated cell–cell adhesion in epithelial and endothelial cells. Clinical-pathological and experimental evidence indicates that MAGI1 expression is decreased in some inflammatory diseases, and also in several cancers, including hepatocellular carcinoma, colorectal, cervical, breast, brain, and gastric cancers and appears to act as a tumor suppressor, modulating the activity of oncogenic pathways such as the PI3K/AKT and the Wnt/β-catenin pathways. Genomic mutations and other mechanisms such as mechanical stress or inflammation have been described to regulate MAGI1 expression. Intriguingly, in breast and colorectal cancers, MAGI1 expression is induced by non-steroidal anti-inflammatory drugs (NSAIDs), suggesting a role in mediating the tumor suppressive activity of NSAIDs. More recently, MAGI1 was found to localize at mature focal adhesion and to regulate integrin-mediated adhesion and signaling in endothelial cells. Here, we review MAGI1′s role as scaffolding protein, recent developments in the understanding of MAGI1 function as tumor suppressor gene, its role in endothelial cells and its implication in cancer and vascular biology. We also discuss outstanding questions about its regulation and potential translational implications in oncology.

MAGI1 Inhibits the Proliferation, Migration and Invasion of Glioma Cells

Background

Membrane-associated guanylate kinase inverted repeat member 1 (MAGI1) acts as a tumor suppressor in a variety of tumors; however, its expression and biological function in glioma are still unknown.

Methods

MAGI1 expression in glioma was examined by immunohistochemistry. In addition, overexpression of MAGI1 in U87 and U373 cells, colony formation and MTT assays were used to evaluate cell proliferation, Transwell assays to determine cell migration and invasion, and a xenograft model established using U87 cells to evaluate the effect of MAGI1 overexpression in vivo. Western blot assays were used to analyze the Akt, MMP2, MMP9 and E-cadherin/N-cadherin/vimentin pathway changes after overexpression of MAGI1.

Results

We demonstrated that MAGI1 was expressed at low levels in glioma. Low MAGI1 expression was positively correlated with the malignant progression of glioma and indicated a poor prognosis. Moreover, we found that overexpressed MAGI1 inhibited the proliferation, migration and invasion of glioma cells by regulating cell growth and EMT through Akt, MMP2, MMP9 and the E-cadherin/N-cadherin/vimentin pathway.

Conclusion

These findings demonstrate a novel function of MAGI1 in glioma progression and suggest that MAGI1 might be a target for the diagnosis and treatment of glioma.

Magi-1 scaffolds NaV1.8 and Slack KNa channels in dorsal root ganglion neurons regulating excitability and pain

Voltage-dependent sodium (Na_V) 1.8 channels regulate action potential generation in nociceptive neurons, identifying them as putative analgesic targets. Here, we show that Na_V1.8 channel plasma membrane localization, retention, and stability occur through a direct interaction with the postsynaptic density-95/discs large/zonula occludens-1-and WW domain-containing scaffold protein called membrane-associated guanylate kinase with inverted orientation (Magi)-1. The neurophysiological roles of Magi-1 are largely unknown, but we found that dorsal root ganglion (DRG)-specific knockdown of Magi-1 attenuated thermal nociception and acute inflammatory pain and produced deficits in Na_V1.8 protein expression. A competing cell-penetrating peptide mimetic derived from the Na_V1.8 WW binding motif decreased sodium currents, reduced Na_V1.8 protein expression, and produced hypoexcitability. Remarkably, a phosphorylated variant of the very same peptide caused an opposing increase in Na_V1.8 surface expression and repetitive firing. Likewise, in vivo, the peptides produced diverging effects on nocifensive behavior. Additionally, we found that Magi-1 bound to sequence like a calcium-activated potassium channel sodium-activated (Slack) potassium channels, demonstrating macrocomplexing with Na_V1.8 channels. Taken together, these findings emphasize Magi-1 as an essential scaffold for ion transport in DRG neurons and a central player in pain.-Pryce, K. D., Powell, R., Agwa, D., Evely, K. M., Sheehan, G. D., Nip, A., Tomasello, D. L., Gururaj, S., Bhattacharjee, A. Magi-1 scaffolds Na_V1.8 and Slack K_Na channels in dorsal root ganglion neurons regulating excitability and pain.

Genome-wide profiling reveals functional diversification of ∆FosB gene targets in the hippocampus of an Alzheimer's disease mouse model

The activity-induced transcription factor ∆FosB has been implicated in Alzheimer’s disease (AD) as a critical regulator of hippocampal function and cognition downstream of seizures and network hyperexcitability. With its long half-life (> 1 week), ∆FosB is well-poised to modulate hippocampal gene expression over extended periods of time, enabling effects to persist even during seizure-free periods. However, the transcriptional mechanisms by which ∆FosB regulates hippocampal function are poorly understood due to lack of identified hippocampal gene targets. To identify putative ∆FosB gene targets, we employed high-throughput sequencing of genomic DNA bound to ∆FosB after chromatin immunoprecipitation (ChIP-sequencing). We compared ChIP-sequencing results from hippocampi of transgenic mice expressing mutant human amyloid precursor protein (APP) and nontransgenic (NTG) wild-type littermates. Surprisingly, only 52 ∆FosB gene targets were shared between NTG and APP mice; the vast majority of targets were unique to one genotype or the other. We also found a functional shift in the repertoire of ∆FosB gene targets between NTG and APP mice. A large number of targets in NTG mice are involved in neurodevelopment and/or cell morphogenesis, whereas in APP mice there is an enrichment of targets involved in regulation of membrane potential and neuronal excitability. RNA-sequencing and quantitative PCR experiments confirmed that expression of putative ∆FosB gene targets were altered in the hippocampus of APP mice. This study provides key insights into functional domains regulated by ∆FosB in the hippocampus, emphasizing remarkably different programs of gene regulation under physiological and pathological conditions.

MAGI1 inhibits migration and invasion via blocking MAPK/ERK signaling pathway in gastric cancer

OBJECTIVE

To explore the association of membrane-associated guanylate kinase inverted 1 (MAGI1) with gastric cancer (GC) and the related molecular mechanisms.

METHODS

The reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry (IHC) were utilized to measure the MAGI1 expression level in GC tissues. Quantitative real-time PCR and Western blotting were used to ensure the MAGI1 expression in GC cell lines. Small hairpin RNA (shRNA) was applied for knockdown of endogenous MAGI1 in GC cells. MTT assay and colony formation assay, scratch wounding migration assay and transwell chamber migration assay, as well as transwell chamber invasion assay were employed respectively to investigate the GC cell proliferation, migration and invasion in MAGI1-knockdown and control GC cells. The potential molecular mechanism mediated by MAGI1 was studied using Western blotting and RT- PCR.

RESULTS

RT-PCR and IHC verified MAGI1 was frequently expressed in matched adjacent noncancerous mucosa compared with GC tissues and the expression of MAGI1 was related to clinical pathological parameters. Functional assays indicated that MAGI1 knockdown significantly promoted GC cell migration and invasion. Further mechanism investigation demonstrated that one pathway of MAGI1 inhibiting migration and invasion was mainly by altering the expression of matrix metalloproteinases (MMPs) and epithelial-mesenchymal transition (EMT)-related molecules via inhibiting MAPK/ERK signaling pathway.

CONCLUSIONS

MAGI1 was associated with GC clinical pathological parameters and acted as a tumor suppressor via inhibiting of MAPK/ERK signaling pathway in GC.

Large-Conductance Calcium-Activated Potassium Channels in Glomerulus: From Cell Signal Integration to Disease

Large-conductance calcium-activated potassium (BK) channels are currently considered as vital players in a variety of renal physiological processes. In podocytes, BK channels become active in response to stimuli that increase local cytosolic Ca²⁺, possibly secondary to activation of slit diaphragm TRPC6 channels by chemical or mechanical stimuli. Insulin increases filtration barrier permeability through mobilization of BK channels. In mesangial cells, BK channels co-expressed with β1 subunits act as a major component of the counteractive response to contraction in order to regulate glomerular filtration. This review aims to highlight recent discoveries on the localization, physiological and pathological roles of BK channels in glomerulus.

Different expressions of large-conductance Ca2+-activated K+ channels in the mouse renal cortex and hippocampus during postnatal development

Large-conductance Ca(+)-activated K(+) (BKCa) channels are widely distributed in a variety of cells and play a pivotal and specific role in many pathophysiological conditions. However, the function of BK(Ca) channels in the kidney cortex and hippocampus during the postnatal development has not received attention. In this study, to elucidate the role of BK(Ca) channels during the development, it is essential to establish the location and quantitation of expression of BK(Ca). The expressions of BK(Ca) were detected in the kidney and hippocampus on postnatal days (P) 1, 3, 5, 7, 14, 21, 28, and 49 by immunohistochemical and Western blot analysis. Our results showed that expressions of BK(Ca) channels were found in tubules and corpuscles at all time points. The expression was also observed at all developmental stages of the renal corpuscles, such as comma-shaped body, S-shaped body, renal corpuscles of stage III, and renal corpuscles of stage IV. During the development, the expression of BK(Ca) channels was decreased and the most prominent change of BK(Ca) protein level appeared between P14 and P21. In contrast, BK(Ca) channels were expressed in all regions of the hippocampus at every time point with the level increasing during the early development (P1 to P14). The findings of the present study suggest that BKCa channels play an important role during the postnatal development in both the renal cortex and hippocampus.

MaxiK channel and cell signalling

The large-conductance Ca2+- and voltage-activated K+ (MaxiK, BK, BKCa, Slo1, KCa1.1) channel role in cell signalling is becoming apparent as we learn how the channel interacts with a multiplicity of proteins not only at the plasma membrane but also in intracellular organelles including the endoplasmic reticulum, nucleus, and mitochondria. In this review, we focus on the interactions of MaxiK channels with seven-transmembrane G protein-coupled receptors and discuss information suggesting that, the channel big C-terminus may act as the nucleus of signalling molecules including kinases relevant for cell death and survival. Increasing evidence indicates that the channel is able to associate with a variety of receptors including β-adrenergic receptors, G protein-coupled estrogen receptors, acetylcholine receptors, thromboxane A2 receptors, and angiotensin II receptors, which highlights the varied functions that the channel has (or may have) not only in regulating contraction/relaxation of muscle cells or neurotransmission in the brain but also in cell metabolism, proliferation, migration, and gene expression. In line with this view, MaxiK channels have been implicated in obesity and in brain, prostate, and mammary cancers. A better understanding on the molecular mechanisms underlying or triggered by MaxiK channel abnormalities like overexpression in certain cancers may lead to new therapeutics to prevent devastating diseases.

Low voltage activation of KCa1.1 current by Cav3-KCa1.1 complexes

Calcium-activated potassium channels of the KCa1.1 class are known to regulate repolarization of action potential discharge through a molecular association with high voltage-activated calcium channels. The current study examined the potential for low voltage-activated Cav3 (T-type) calcium channels to interact with KCa1.1 when expressed in tsA-201 cells and in rat medial vestibular neurons (MVN) in vitro. Expression of the channel α-subunits alone in tsA-201 cells was sufficient to enable Cav3 activation of KCa1.1 current. Cav3 calcium influx induced a 50 mV negative shift in KCa1.1 voltage for activation, an interaction that was blocked by Cav3 or KCa1.1 channel blockers, or high internal EGTA. Cav3 and KCa1.1 channels coimmunoprecipitated from lysates of either tsA-201 cells or rat brain, with Cav3 channels associating with the transmembrane S0 segment of the KCa1.1 N-terminus. KCa1.1 channel activation was closely aligned with Cav3 calcium conductance in that KCa1.1 current shared the same low voltage dependence of Cav3 activation, and was blocked by voltage-dependent inactivation of Cav3 channels or by coexpressing a non calcium-conducting Cav3 channel pore mutant. The Cav3-KCa1.1 interaction was found to function highly effectively in a subset of MVN neurons by activating near –50 mV to contribute to spike repolarization and gain of firing. Modelling data indicate that multiple neighboring Cav3-KCa1.1 complexes must act cooperatively to raise calcium to sufficiently high levels to permit KCa1.1 activation. Together the results identify a novel Cav3-KCa1.1 signaling complex where Cav3-mediated calcium entry enables KCa1.1 activation over a wide range of membrane potentials according to the unique voltage profile of Cav3 calcium channels, greatly extending the roles for KCa1.1 potassium channels in controlling membrane excitability.

Developmental changes of BKCa channels depend on differentiation status in cultured podocytes

The podocyte is a remarkable cell type, which encases the capillaries of the kidney glomerulus. Podocytes are of keen interests because of their key roles in kidney development and disease. Large-conductance Ca(2+)-activated K(+) channels (BKCa channels) are important ion channels located in podocytes and play the essential role in regulating calcium homeostasis cell signaling. In this research, we studied the undergoing developmental changes of BKCa channels and their contribution to functional maturation of podocytes. Our results showed that the distribution of BKCa channels changed with the maturity of differentiation in a conditionally immortalized mouse podocyte cell line. Additionally, the increase of BKCa channel protein expression was detected by immunofluorescence staining with confocal microscopy in podocytes, which was consistent with the increase in the current density of BKCa channels examined by whole-cell patch-clamp technique. Our results suggested that the developmental changes of BKCa channels may help podocytes adapt to changes in pressure gradients occurring in physiological conditions. Those findings may have implications for understanding the physiology and development of kidney and will also serve as a baseline for future studies designed to investigate developmental changes of ion channel expression in podocytes.

Regulation of interferon-β by MAGI-1 and its interaction with influenza A virus NS1 protein with ESEV PBM

The NS1 protein from avian influenza A viruses contains a PDZ binding motif (PBM) at its carboxyl terminus with the consensus sequence ESEV. The ESEV PBM confers binding to several cellular PDZ proteins, including Dlg1, MAGI-1 and Scribble. The interaction between NS1 and Scribble protects infected cells from apoptosis, while the interaction between NS1 and both Dlg1 and Scribble disrupts tight junctions. In this study, we examined the MAGI-1 protein. We made the unexpected observation that siRNA depletion of MAGI-1 activates IRF3 and induces the IFN-β promoter. We found that the ESEV NS1 protein sequesters MAGI-1 away from the plasma membrane in infected cells. Using plasmid vectors to express NS1 proteins, we observed that the ESEV PBM elicits an IFN-β induction signal as indicated by activation of IRF3 and a relative deficiency in NS1 inhibition of induction of the IFN-β promoter by dsRNA or RIG-I. Taken together, our data suggest that disruption of MAGI-1 by the ESEV PBM activates an IFN-β induction signal. During viral infection, however, induction of the IFN-β gene does not occur presumably because other anti-IFN functions dominate over the IFN-activation activity of the ESEV PBM. We postulate that the ESEV PBM's broad binding activity for PDZ proteins may allow NS1 to bind to some PDZ proteins such as MAGI-1 that confer no benefit or may even be detrimental to viral replication. However, the advantage of binding to key PDZ proteins such as Dlg1 and Scribble may dominate and therefore provide an overall benefit for the virus to encode the ESEV PBM.

Coxsackievirus and adenovirus receptor (CAR) mediates trafficking of acid sensing ion channel 3 (ASIC3) via PSD-95

We have previously shown that the Coxsackievirus and adenovirus receptor (CAR) can interact with post-synaptic density 95 (PSD-95) and localize PSD-95 to cell-cell junctions. We have also shown that activity of the acid sensing ion channel (ASIC3), a H(+)-gated cation channel that plays a role in mechanosensation and pain signaling, is negatively modulated by PSD-95 through a PDZ-based interaction. We asked whether CAR and ASIC3 simultaneously interact with PSD-95, and if so, whether co-expression of these proteins alters their cellular distribution and localization. Results indicate that CAR and ASIC3 co-immunoprecipitate only when co-expressed with PSD-95. CAR also brings both PSD-95 and ASIC3 to the junctions of heterologous cells. Moreover, CAR rescues PSD-95-mediated inhibition of ASIC3 currents. These data suggest that, in addition to activity as a viral receptor and adhesion molecule, CAR can play a role in trafficking proteins, including ion channels, in a PDZ-based scaffolding complex.

The PDZ1 and PDZ3 domains of MAGI-1 regulate the eight-exon isoform of the coxsackievirus and adenovirus receptor

Epithelial integrity is essential for homeostasis and poses a formidable barrier to pathogen entry. Major factors for viral entry into epithelial cells are the localization and abundance of the primary receptor. The coxsackievirus and adenovirus receptor (CAR) is a primary receptor for these two pathogenic groups of viruses. In polarized epithelia, a low-abundance, alternatively spliced eight-exon isoform of CAR, CAR(Ex8), is localized apically where it can support viral infection from the air-exposed surface. Using biochemical, cell biology, genetic, and spectroscopic approaches, we show that the levels of apical CAR(Ex8) are negatively regulated by the PDZ domain-containing protein MAGI-1 (membrane-associated guanylate kinase with inverted orientation protein-1) and that two MAGI-1 PDZ domains, PDZ1 and PDZ3, regulate CAR(Ex8) levels in opposing ways. Similar to full-length MAGI-1, expression of the isolated PDZ3 domain significantly reduces cell surface CAR(Ex8) abundance and adenovirus infection. In contrast, the PDZ1 domain is able to rescue CAR(Ex8) and adenovirus infection from MAGI-1-mediated suppression. These data suggest a novel cell-based strategy to either suppress viral infection or augment adenovirus-based gene therapy.

Heparanase-induced GEF-H1 signaling regulates the cytoskeletal dynamics of brain metastatic breast cancer cells

Heparanase is the only mammalian endoglycosidase which has been widely implicated in cancer because of its capability to degrade heparan sulfate chains of heparan sulfate proteoglycans (HSPG). Specifically, the cell surface HSPG syndecan-1 and -4 (SDC1 and SDC4) are modulators of growth factor action, and SDC4 is implicated in cell adhesion as a key member of focal adhesion complexes. We hypothesized that extracellular heparanase modulates brain metastatic breast cancer (BMBC) cell invasiveness by affecting cytoskeletal dynamics, SDC4 carboxy-terminal-associated proteins, and downstream targets. We used two independently derived human BMBC cell systems (MB-231BR and MB-231BR3), which possess distinct cellular morphologies and properties. Highly aggressive spindle-shaped 231BR3 cells changed to a round cell morphology associated with expression of the small GTPase guanine nucleotide exchange factor-H1 (GEF-H1). We showed that GEF-H1 is a new component of the SDC4 signaling complex in BMBC cells. Treatment with heparanase resulted in regulation of the SDC4/protein kinase C α axis while maintaining a constitutive GEF-H1 level. Third, GEF-H1 knockdown followed by cell exposure to heparanase caused a significant regulation of activities of Rac1 and RhoA, which are GEF-H1 targets and fundamental effectors in cell plasticity control. Fourth, L-heparanase augmented expression of β1 integrin in BMBC cells and of vascular cell adhesion molecule 1 (VCAM1; the major β1 integrin receptor) in human brain microvascular endothelial cells. Finally, using a newly developed blood-brain barrier in vitro model, we show that BMBC cell transmigration was significantly reduced in GEF-H1 knockdown cells. These findings implicate heparanase in mechanisms of cytoskeletal dynamics and in the cross-talk between tumor cells and vascular brain endothelium. They are of relevance because they elucidate molecular events in the initial steps leading to BMBC onset and capturing distinct roles of latent and active heparanase in the brain microenvironment.

Effects of insulin and high glucose on mobilization of slo1 BKCa channels in podocytes

Podocytes are dynamic polarized cells that lie on the surface of glomerular capillaries and comprise an essential component of the glomerular filtration barrier. Podocytes are affected in the earliest stages of diabetic nephropathy and insulin signaling to podocytes is essential for normal glomerular function. Large-conductance Ca(2+)-activated K(+) channels (BK(Ca) channels) encoded by the Slo1 gene are expressed in podocytes in a complex with multiple glomerular slit diaphragm proteins including nephrin, TRPC6 channels, and several different actin-binding proteins. Here we show that insulin increases cell surface expression of podocyte BK(Ca) channels, which is accompanied by a corresponding increase in the density of current flowing through these channels. Insulin stimulation of BK(Ca) channels was detectable in 15 min and required activation of both Erk and Akt signaling cascades. Exposure to high glucose (36.1 mM) for 24 h caused a marked reduction in the steady-state surface expression of BK(Ca) channels as well as of the slit diaphragm signaling molecule nephrin. High glucose treatment also abolished the stimulatory effects of insulin on BK(Ca) current density, although insulin continued to increase phosphorylation of Erk and Akt under those conditions. Therefore, in contrast to most other cell types, high glucose abrogates the effects of insulin in podocytes at relatively distal steps in its signaling pathway. Insulin stimulation of BK(Ca) channels in podocytes may prepare podocytes to adapt to changes in pressure gradients that occur during postprandial hyperfiltration.

Identification and quantification of full-length BK channel variants in the developing mouse cochlea

Maxi-K(+) (BK) channel diversity is attributed to alternative splicing in the kcnma1 gene. The resultant variants manifest themselves in different cell types, tissues, and functions, such as excitation, metabolism, and signaling. Immunoelectron microscopy revealed immunogold particle labeling of BK in apical and basal regions of inner and outer hair cells, respectively. Additional labeling occurs in Deiters' cells and the inner mitochondrial membrane. Identification of full-length sequences reveals 27 BK variants from embryonic and postnatal mouse inner ear, per classification by tail motif, VYR, DEC, and ERL, and by exon usage. Three predicted start codons are found encoding MAN, MSS, and MDA, of which MDA shows the greatest expression through all stages in development, whereas MAN is undetectable. Complex splice sites occur between exons 9 and 10 and between 21 and 23. Spliced-in/out exons between 8 and 10 reveal a short fragment composed of exons 8 + 10, detectable on postnatal day (PD) 14 and PD30, and a longer fragment composed of exons 8 + 9 + 10 that is upregulated on embryonic day (ED) 14. Spliced-in exons 22 or 23 are expressed on ED14 but decrease over time; however, exon 22 increases again on PD34. Using tail-specific primers, qRT-PCR from ED14, PD4, -14, and -30 shows that BK-VYR and -ERL dominate expression on ED14, whereas DEC dominates after birth in all cochlear regions. The localization of BK and the changes in expression of its exons and tail types, by alternative splicing during development, may contribute to cochlear organization, acquisition of hearing, and intracellular signaling.

Function and mechanism of axonal targeting of voltage-sensitive potassium channels

Precise localization of various ion channels into proper subcellular compartments is crucial for neuronal excitability and synaptic transmission. Axonal K(+) channels that are activated by depolarization of the membrane potential participate in the repolarizing phase of the action potential, and hence regulate action potential firing patterns, which encode output signals. Moreover, some of these channels can directly control neurotransmitter release at axonal terminals by constraining local membrane excitability and limiting Ca(2+) influx. K(+) channels differ not only in biophysical and pharmacological properties, but in expression and subcellular distribution as well. Importantly, proper targeting of channel proteins is a prerequisite for electrical and chemical functions of axons. In this review, we first highlight recent studies that demonstrate different roles of axonal K(+) channels in the local regulation of axonal excitability. Next, we focus on research progress in identifying axonal targeting motifs and machinery of several different types of K(+) channels present in axons. Regulation of K(+) channel targeting and activity may underlie a novel form of neuronal plasticity. This research field can contribute to generating novel therapeutic strategies through manipulating neuronal excitability in treating neurological diseases, such as multiple sclerosis, neuropathic pain, and Alzheimer's disease.

Regulation of glutamate transporter GLT-1 by MAGI-1

The sodium-dependent glutamate transporter, glutamate transporter subtype 1 (GLT-1) is one of the main glutamate transporters in the brain. GLT-1 contains a COOH-terminal sequence similar to one in an isoform of Slo1 K(+) channel protein previously shown to bind MAGI-1 (membrane-associated guanylate kinase with inverted orientation protein-1). MAGI-1 is a scaffold protein which allows the formation of complexes between certain transmembrane proteins, actin-binding proteins, and other regulatory proteins. The glutathione S-transferase pull-down assay demonstrated that MAGI-1 was a binding partner of GLT-1. The interaction between MAGI-1 and GLT-1 was confirmed by co-immunoprecipitation. Immunofluorescence of MAGI-1 and GLT-1 demonstrated that the distribution of MAGI-1 and GLT-1 overlapped in astrocytes. Co-expression of MAGI-1 with GLT-1 in C6 Glioma cells resulted in a significant reduction in the surface expression of GLT-1, as assessed by cell-surface biotinylation. On the other hand, partial knockdown of endogenous MAGI-1 expression by small interfering RNA in differentiated cultured astrocytes increased glutamate uptake and the surface expression of endogenous GLT-1. Knockdown of MAGI-1 increased dihydrokainate-sensitive, Na(+) -dependent glutamate uptake, indicating that MAGI-1 regulates GLT-1 mediated glutamate uptake. These data suggest that MAGI-1 regulates surface expression of GLT-1 and the level of glutamate in the hippocampus.

Modulation of GEF-H1 induced signaling by heparanase in brain metastatic melanoma cells

Mechanisms of brain metastatic melanoma (BMM) remain largely unknown. Understanding the modulation of signaling pathways that alter BMM cell invasion and metastasis is critical to develop new therapies for BMM. Heparanase has been widely implicated in cancer and is the dominant mammalian endoglycosidase which degrades heparan sulfate chains of proteoglycans (HSPG) including syndecans (SDCs). Recent findings also indicate that heparanase possesses non-enzymatic functions in its latent form. We hypothesized that extracellular heparanase modulates BMM cell signaling by involving SDC1/4 carboxy terminal-associated proteins and downstream targets. We digested BMM cell surface HS with human recombinant active or latent heparanase to delineate their effects on cytoskeletal dynamics and cell invasiveness. We identified the small GTPase guanine nucleotide exchange factor-H1 (GEF-H1) as a new component of a SDC signaling complex that is differentially expressed in BMM cells compared to corresponding non-metastatic counterparts. Second, knockdown of GEF-H1, SDC1, or SDC4 decreased BMM cell invasiveness and GEF-H1 modulated small GTPase activity of Rac1 and RhoA in conjunction with heparanase treatment. Third, both active and latent forms of heparanase affected Rac1 and RhoA activity; notably increasing RhoA activity. Both forms of heparanase were found to mediate the expression and subcellular localization of GEF-H1, and treatment of BMM with latent heparanase modulated SDC1/4 gene expression. Finally, treatment with exogenous heparanase downregulated BMM cell invasion. These studies indicate the relevance of heparanase signaling pathways in BMM progression, and provide insights into the molecular mechanisms regulating HSPG signaling in response to exogenous heparanase.

Heparanase Modulates Shh and Wnt3a Signaling in Human Medulloblastoma Cells

The pathogenesis of medulloblastoma (MB), the most common and aggressive brain tumor in children, is poorly understood. MB tumors respond to factors secreted by cerebellar Purkinje neurons such as Sonic hedgehog (Shh) and Wnt3a. Understanding the modulation of Shh/Wnt signaling is critical to developing new MB treatments. Shh and Wnt3a induce MB cell proliferation, and bind heparan sulfate glycosaminoglycan chains (HS-GAG). HS-GAG are components of syndecans: cell surface HS proteoglycans (HSPG) which act as co-receptors for extracellular matrix based ligands, and are targets of heparanase (HPSE). We hypothesized that extracellular HPSE activity can modulate MB intracellular signaling of Shh/Wnt3a, involving syndecans 1/4 carboxy terminal-associated proteins and downstream targets. We compared the regulation of Shh/Wnt3a signaling subsequent to treatment with exogenous human active HPSE in MB lines possessing increased invasive abilities. We identified GEF-H1, a small GTPase guanine nucleotide exchange factor, as a new component of a syndecan signaling complex. Secondly, we demonstrated that HPSE modulated Shh/Wnt3 dependent expression and intracellular distribution of GEF-H1, β-catenin, and N-Myc. Thirdly, HPSE modulated Shh/Wnt3a - dependent gene expression of HSPG and Gli transcription factors. Fourthly, pretreatment with HPSE, alone or prior to Shh/Wnt3a exposure, altered small GTPase (Rac1/RhoA) activities differentially, and promoted RhoA activation. Finally, the differential regulation of Rac1/RhoA activities by HPSE affected MB cell proliferation and invasion. Our results indicate that the HPSE/HSPG axis is implicated in critical MB cell signaling pathways with potential relevance for MB treatment.

TRPC6 channels and their binding partners in podocytes: role in glomerular filtration and pathophysiology

Loss or dysfunction of podocytes is a major cause of glomerular kidney disease. Several genetic forms of glomerular disease are caused by mutations in genes that encode structural elements of the slit diaphragm or the underlying cytoskeleton of podocyte foot processes. The recent discovery that gain-of-function mutations in Ca(2+)-permeable canonical transient receptor potential-6 channels (TRPC6) underlie a subset of familial forms of focal segmental glomerulosclerosis (FSGS) has focused attention on the basic cellular physiology of podocytes. Several recent studies have examined the role of Ca(2+) dynamics in normal podocyte function and their possible contributions to glomerular disease. This review summarizes the properties of TRPC6 and related channels, focusing on their permeation and gating properties, the nature of mutations associated with familial FSGS, and the role of TRPC channels in podocyte cell biology as well as in glomerular pathophysiology. TRPC6 interacts with several proteins in podocytes, including essential slit diaphragm proteins and mechanosensitive large-conductance Ca(2+)-activated K(+) channels. The signaling dynamics controlling ion channel function and localization in podocytes appear to be quite complex.

Regulation of podocyte BK(Ca) channels by synaptopodin, Rho, and actin microfilaments

Mechanosensitive large-conductance Ca(2+)-activated K(+) channels encoded by the Slo1 gene (BK(Ca) channels) are expressed in podocytes. Here we show that BK(Ca) channels reciprocally coimmunoprecipitate with synaptopodin (Synpo) in mouse glomeruli, in mouse podocytes, and in a heterologous expression system (HEK293T cells) in which these proteins are transiently expressed. Synpo and Slo1 colocalize along the surface of the glomerular basement membrane in mouse glomeruli. Synpo interacts with BK(Ca) channels at COOH-terminal domains that overlap with an actin-binding domain on the channel molecule that is necessary for trafficking of BK(Ca) channels to the cell surface. Moreover, addition of exogenous beta-actin to mouse podocyte lysates reduces BK(Ca)-Synpo interactions. Coexpression of Synpo increases steady-state surface expression of BK(Ca) channels in HEK293T cells. However, Synpo does not affect the stability of cell surface BK(Ca) channels, suggesting a primary effect on the rate of forward trafficking, and Synpo coexpression does not affect BK(Ca) gating. Conversely, stable knockdown of Synpo expression in mouse podocyte cell lines reduces steady-state surface expression of BK(Ca) channels but does not affect total expression of BK(Ca) channels or their gating. The effects of Synpo on surface expression of BK(Ca) are blocked by inhibition of Rho signaling in HEK293T cells and in podocytes. Functional cell surface BK(Ca) channels in podocytes are also reduced by sustained (2 h) but not acute (15 min) depolymerization of actin with cytochalasin D. Synpo may regulate BK(Ca) channels through its effects on actin dynamics and by modulating interactions between BK(Ca) channels and regulatory proteins of the podocyte slit diaphragm.

Therapeutic potential of cystic fibrosis transmembrane conductance regulator (CFTR) inhibitors in polycystic kidney disease

In the common genetic disorder autosomal dominant polycystic kidney disease (ADPKD), kidney function is disrupted by multiple fluid-filled epithelial cysts. Cyst growth in ADPKD involves fluid accumulation within the cyst lumen driven by cystic fibrosis transmembrane conductance regulator (CFTR)-mediated transepithelial Cl- secretion. This suggests that inhibitors of the CFTR Cl- channel might retard cyst growth. This review considers how knowledge of CFTR structure and function and its role in transepithelial salt and water movements provides insight into the mechanism of action of CFTR inhibitors. Some small molecules, termed open-channel blockers, inhibit directly the CFTR Cl- channel by physically obstructing the CFTR pore and preventing Cl- flow. By contrast, other small molecules, termed allosteric inhibitors, bind to CFTR at a site remote from the channel pore and interfere with conformational changes that open the pore. The application of high-throughput screening to CFTR drug discovery has led to the identification of new inhibitors of the CFTR Cl- channel including the thiazolidinone CFTR(inh)-172 and the glycine hydrazide GlyH-101. The demonstration that CFTR inhibitors retard cyst expansion and kidney enlargement in mouse models of ADPKD provides proof of concept for the use of small-molecule CFTR inhibitors in the treatment of ADPKD.

AF17 competes with AF9 for binding to Dot1a to up-regulate transcription of epithelial Na+ channel alpha

We previously reported that Dot1a.AF9 complex represses transcription of the epithelial Na(+) channel subunit alpha (alpha-ENaC) gene in mouse inner medullary collecting duct mIMCD3 cells and mouse kidney. Aldosterone relieves this repression by down-regulating the complex through various mechanisms. Whether these mechanisms are sufficient and conserved in human cells or can be applied to other aldosterone-regulated genes remains largely unknown. Here we demonstrate that human embryonic kidney 293T cells express the three ENaC subunits and all of the ENaC transcriptional regulators examined. These cells respond to aldosterone and display benzamil-sensitive Na(+) currents, as measured by whole-cell patch clamping. We also show that AF17 and AF9 competitively bind to the same domain of Dot1a in multiple assays and have antagonistic effects on expression of an alpha-ENaC promoter-luciferase construct. Overexpression of Dot1a or AF9 decreased mRNA expression of the ENaC subunits and their transcriptional regulators and reduced benzamil-sensitive Na(+) currents. AF17 overexpression caused the opposite effects, accompanied by redirection of Dot1a from the nucleus to the cytoplasm and reduction in histone H3 K79 methylation. The nuclear export inhibitor leptomycin B blocked the effect of AF17 overexpression on H3 K79 hypomethylation. RNAi-mediated knockdown of AF17 yielded nuclear enrichment of Dot1a and histone H3 K79 hypermethylation. As with AF9, AF17 displays nuclear and cytoplasmic co-localization with Sgk1. Therefore, AF17 competes with AF9 to bind Dot1a, decreases Dot1a nuclear expression by possibly facilitating its nuclear export, and relieves Dot1a.AF9-mediated repression of alpha-ENaC and other target genes.

Dominant-negative regulation of cell surface expression by a pentapeptide motif at the extreme COOH terminus of an Slo1 calcium-activated potassium channel splice variant

Large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels regulate the physiology of many cell types. A single vertebrate gene variously known as Slo1, KCa1.1, or KCNMA1 encodes the pore-forming subunits of BK(Ca) channel but is expressed in a potentially very large number of alternative splice variants. Two splice variants of Slo1, Slo1(VEDEC) and Slo1(QEERL), which differ at the extreme COOH terminus, show markedly different steady-state expression levels on the cell surface. Here we show that Slo1(VEDEC) and Slo1(QEERL) can reciprocally coimmunoprecipitate, indicating that they form heteromeric complexes. Moreover, coexpression of even small amounts of Slo1(VEDEC) markedly reduces surface expression of Slo1(QEERL) and total Slo1 as indicated by cell-surface biotinylation assays. The effects of Slo1(VEDEC) on steady-state surface expression can be attributed primarily to the last five residues of the protein based on surface expression of motif-swapped constructs of Slo1 in human embryonic kidney (HEK) 293T cells. In addition, the presence of the VEDEC motif at the COOH terminus of Slo1 channels is sufficient to confer a dominant-negative effect on cell surface expression of itself or other types of Slo1 subunits. Treating cells with short peptides containing the VEDEC motif increased surface expression of Slo1(VEDEC) channels transiently expressed in HEK293T cells and increased current through endogenous BK(Ca) channels in mouse podocytes. Slo1(VEDEC) and Slo1(QEERL) channels are removed from the HEK293T cell surface with similar kinetics and to a similar extent, which suggests that the inhibitory effect of the VEDEC motif is exerted primarily on forward trafficking into the plasma membrane.

Developmental expression of BK channels in chick cochlear hair cells

Background

Cochlear hair cells are high-frequency sensory receptors. At the onset of hearing, hair cells acquire fast, calcium-activated potassium (BK) currents, turning immature spiking cells into functional receptors. In non-mammalian vertebrates, the number and kinetics of BK channels are varied systematically along the frequency-axis of the cochlea giving rise to an intrinsic electrical tuning mechanism. The processes that control the appearance and heterogeneity of hair cell BK currents remain unclear.

Results

Quantitative PCR results showed a non-monotonic increase in BK α subunit expression throughout embryonic development of the chick auditory organ (i.e. basilar papilla). Expression peaked near embryonic day (E) 19 with six times the transcript level of E11 sensory epithelia. The steady increase in gene expression from E11 to E19 could not explain the sudden acquisition of currents at E18-19, implicating post-transcriptional mechanisms. Protein expression also preceded function but progressed in a sequence from diffuse cytoplasmic staining at early ages to punctate membrane-bound clusters at E18. Electrophysiology data confirmed a continued refinement of BK trafficking from E18 to E20, indicating a translocation of BK clusters from supranuclear to subnuclear domains over this critical developmental age.

Conclusions

Gene products encoding BK α subunits are detected up to 8 days before the acquisition of anti-BK clusters and functional BK currents. Therefore, post-transcriptional mechanisms seem to play a key role in the delayed emergence of calcium-sensitive currents. We suggest that regulation of translation and trafficking of functional α subunits, near voltage-gated calcium channels, leads to functional BK currents at the onset of hearing.

The beta1 subunit of Na+/K+-ATPase interacts with BKCa channels and affects their steady-state expression on the cell surface

Large conductance Ca2+-activated K+ channels (BKCa) encoded by the Slo1 gene play a role in the physiological regulation of many cell types. Here, we show that the beta1 subunit of Na+/K+-ATPase (NKbeta1) interacts with the cytoplasmic COOH-terminal region of Slo1 proteins. Reduced expression of endogenous NKbeta1 markedly inhibits evoked BKCa currents with no apparent effect on their gating. In addition, NKbeta1 down-regulated cells show decreased density of Slo1 subunits on the cell surface.

Neph1 regulates steady-state surface expression of Slo1 Ca(2+)-activated K(+) channels: different effects in embryonic neurons and podocytes

Large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels encoded by the Slo1 gene are often components of large multiprotein complexes in excitable and nonexcitable cells. Here we show that Slo1 proteins interact with Neph1, a member of the immunoglobulin superfamily expressed in slit diaphragm domains of podocytes and in vertebrate and invertebrate nervous systems. This interaction was established by reciprocal coimmunoprecipitation of endogenous proteins from differentiated cells of a podocyte cell line, from parasympathetic neurons of the embryonic chick ciliary ganglion, and from HEK293T cells heterologously expressing both proteins. Neph1 can interact with all three extreme COOH-terminal variants of Slo1 (Slo1(VEDEC), Slo1(QEERL), and Slo1(EMVYR)) as ascertained by glutathione S-transferase (GST) pull-down assays and by coimmunoprecipitation. Neph1 is partially colocalized in intracellular compartments with endogenous Slo1 in podocytes and ciliary ganglion neurons. Coexpression in HEK293T cells of Neph1 with any of the Slo1 extreme COOH-terminal splice variants suppresses their steady-state expression on the cell surface, as assessed by cell surface biotinylation assays, confocal microscopy, and whole cell recordings. Consistent with this, small interfering RNA (siRNA) knockdown of endogenous Neph1 in embryonic day 10 ciliary ganglion neurons causes an increase in steady-state surface expression of Slo1 and an increase in whole cell Ca(2+)-dependent K(+) current. Surprisingly, a comparable Neph1 knockdown in podocytes causes a decrease in surface expression of Slo1 and a decrease in whole cell BK(Ca) currents. In podocytes, Neph1 siRNA also caused a decrease in nephrin, even though the Neph1 siRNA had no sequence homology with nephrin. However, we could not detect nephrin in ciliary ganglion neurons.