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.

TRPC6 interacted with KCa1.1 channels to regulate the proliferation and apoptosis of glioma cells

Malignant glioma is the most aggressive and deadliest brain malignancy. TRPC6 and K_Ca1.1, two ion channels, have been considered as potential therapeutic targets for malignant glioma treatment. TRPC6, a Ca²⁺-permeable channel, plays a vital role in promoting tumorigenesis and the progression of glioma. K_Ca1.1, a large-conductance Ca²⁺-activated channel, is also involved in growth and migration of glioma. However, the underlying mechanism by which these two ion channels promote glioma progression was unclear. In our study, we found that TRPC6 upregulated the expression of K_Ca1.1, while the immunoprecipitation analysis also showed that TRPC6 interacts with K_Ca1.1 channels in glioma cells. The currents of K_Ca1.1 recorded by the whole-cell patch clamp technique were increased by TRPC6 in glioma cells, suggesting that TRPC6 can provide a Ca²⁺ source for the activation of K_Ca1.1 channels. It was also suggested that TRPC6 regulates the proliferation and apoptosis of glioma cells through K_Ca1.1 channels in vitro. Therefore, C6-bearing glioma rats were established to validate the results in vitro. After the administration of paxilline (a specific inhibitor of K_Ca1.1 channels), TRPC6-dependent growth of glioma was inhibited in vivo. We also found that TRPC6 enhanced co-expression with K_Ca1.1 in glioma. These all suggested that TRPC6/K_Ca1.1 signal plays a role in promoting the growth of glioma. Our results provided new evidence for TRPC6 and K_Ca1.1 as potential targets for glioma treatment.

Protein partners of the calcium channel β subunit highlight new cellular functions

Calcium plays a key role in cell signalling by its intervention in a wide range of physiological processes. Its entry into cells occurs mainly via voltage-gated calcium channels (VGCC), which are found not only in the plasma membrane of excitable cells but also in cells insensitive to electrical signals. VGCC are composed of different subunits, α1, β, α2δ and γ, among which the cytosolic β subunit (Cavβ) controls the trafficking of the channel to the plasma membrane, its regulation and its gating properties. For many years, these were the main functions associated with Cavβ. However, a growing number of proteins have been found to interact with Cavβ, emphasizing the multifunctional role of this versatile protein. Interestingly, some of the newly assigned functions of Cavβ are independent of its role in the regulation of VGCC, and thus further increase its functional roles. Based on the identity of Cavβ protein partners, this review emphasizes the diverse cellular functions of Cavβ and summarizes both past findings as well as recent progress in the understanding of VGCC.

BK Channels in the Central Nervous System

Large conductance Ca(2+)- and voltage-activated K(+) (BK) channels are widely distributed in the postnatal central nervous system (CNS). BK channels play a pleiotropic role in regulating the activity of brain and spinal cord neural circuits by providing a negative feedback mechanism for local increases in intracellular Ca(2+) concentrations. In neurons, they regulate the timing and duration of K(+) influx such that they can either increase or decrease firing depending on the cellular context, and they can suppress neurotransmitter release from presynaptic terminals. In addition, BK channels located in astrocytes and arterial myocytes modulate cerebral blood flow. Not surprisingly, both loss and gain of BK channel function have been associated with CNS disorders such as epilepsy, ataxia, mental retardation, and chronic pain. On the other hand, the neuroprotective role played by BK channels in a number of pathological situations could potentially be leveraged to correct neurological dysfunction.

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.

Regulation of high-voltage-activated Ca2+ channel function, trafficking, and membrane stability by auxiliary subunits

Voltage-gated Ca²⁺ (Ca_V) channels mediate Ca²⁺ ions influx into cells in response to depolarization of the plasma membrane. They are responsible for initiation of excitation-contraction and excitation-secretion coupling, and the Ca²⁺ that enters cells through this pathway is also important in the regulation of protein phosphorylation, gene transcription, and many other intracellular events. Initial electrophysiological studies divided Ca_V channels into low-voltage-activated (LVA) and high-voltage-activated (HVA) channels. The HVA Ca_V channels were further subdivided into L, N, P/Q, and R-types which are oligomeric protein complexes composed of an ion-conducting Ca_Vα₁ subunit and auxiliary Ca_Vα₂δ, Ca_Vβ, and Ca_Vγ subunits. The functional consequences of the auxiliary subunits include altered functional and pharmacological properties of the channels as well as increased current densities. The latter observation suggests an important role of the auxiliary subunits in membrane trafficking of the Ca_Vα₁ subunit. This includes the mechanisms by which Ca_V channels are targeted to the plasma membrane and to appropriate regions within a given cell. Likewise, the auxiliary subunits seem to participate in the mechanisms that remove Ca_V channels from the plasma membrane for recycling and/or degradation. Diverse studies have provided important clues to the molecular mechanisms involved in the regulation of Ca_V channels by the auxiliary subunits, and the roles that these proteins could possibly play in channel targeting and membrane Stabilization.

Structure and function of the β subunit of voltage-gated Ca²⁺ channels

The voltage-gated Ca²⁺ channel β subunit (Ca(v)β) is a cytosolic auxiliary subunit that plays an essential role in regulating the surface expression and gating properties of high-voltage activated (HVA) Ca²⁺ channels. It is also crucial for the modulation of HVA Ca²⁺ channels by G proteins, kinases, Ras-related RGK GTPases, and other proteins. There are indications that Ca(v)β may carry out Ca²⁺ channel-independent functions. Ca(v)β knockouts are either non-viable or result in a severe pathophysiology, and mutations in Ca(v)β have been implicated in disease. In this article, we review the structure and various biological functions of Ca(v)β, as well as recent advances. This article is part of a Special Issue entitled: Calcium channels.

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.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

Neuronal fast activating and meningeal silent modulatory BK channel splice variants cloned from rat

The big conductance calcium-activated K(+) channel (BK) is involved in regulating neuron and smooth muscle cell excitability. Functional diversity of BK is generated by alpha-subunit splice variation and co-expression with beta subunits. Here, we present six different splice combinations cloned from rat brain or cerebral vascular/meningeal tissues, of which at least three variants were previously uncharacterized (X1, X2(92), and X2(188)). An additional variant was identified by polymerase chain reaction but not cloned. Expression in Xenopus oocytes showed that the brain-specific X1 variant displays reduced current, faster activation, and less voltage sensitivity than the insert-less Zero variant. Other cloned variants Strex and Slo27,3 showed slower activation than Zero. The X1 variant contains sequence inserts in the S1-S2 extracellular loop (8 aa), between intracellular domains RCK1 and RCK2 (4 aa at SS1) and upstream of the calcium "bowl" (27 aa at SS4). Two other truncated variants, X2(92) and X2(188), lacking the intracellular C-terminal (stop downstream of S6), were cloned from cerebral vascular/meningeal tissue. They appear non-functional as no current expression was observed, but the X2(92) appeared to slow the activation of the Zero variant when co-expressed. Positive protein expression of X2(92) was observed in oocytes by immunoblotting and fluorescence using a yellow fluorescent protein-tagged construct. The functional characteristics of the X1 variant may be important for a subpopulation of BK channels in the brain, while the "silent" truncated variants X2(92) and X2(188) may play a role as modulators of other BK channel variants in a way similar to well known beta subunits.

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.

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.

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

Large conductance Ca(2+)-activated K(+) (BK(Ca)) channels encoded by the Slo1 gene (also known as KCNMA1) are physiologically important in a wide range of cell types and form complexes with a number of other proteins that affect their function. We performed a yeast two-hybrid screen to identify proteins that interact with BK(Ca) channels using a bait construct derived from domains in the extreme COOH-terminus of Slo1. A protein known as membrane-associated guanylate kinase with inverted orientation protein-1 (MAGI-1) was identified in this screen. MAGI-1 is a scaffolding protein that allows formation of complexes between certain transmembrane proteins, actin-binding proteins, and other regulatory proteins. MAGI-1 is expressed in a number of tissues, including podocytes and the brain. The interaction between MAGI-1 and BK(Ca) channels was confirmed by coimmunoprecipitation and glutathione S-transferase pull-down assays in differentiated cells of a podocyte cell line and in human embryonic kidneys (HEK)293T cells transiently coexpressing MAGI-1a and three different COOH-terminal Slo1 variants. Coexpression of MAGI-1 with Slo1 channels in HEK-293T cells results in a significant reduction in the surface expression of Slo1, as assessed by cell-surface biotinylation assays, confocal microscopy, and whole cell recordings. Partial knockdown of endogenous MAGI-1 expression by small interfering RNA (siRNA) in differentiated podocytes increased the surface expression of endogenous Slo1 as assessed by electrophysiology and cell-surface biotinylation assays, whereas overexpression of MAGI-1a reduced steady-state voltage-evoked outward current through podocyte BK(Ca) channels. These data suggest that MAGI-1 plays a role in regulation of surface expression of BK(Ca) channels in the kidney and possibly in other tissues.

Canonical transient receptor potential channel (TRPC)3 and TRPC6 associate with large-conductance Ca2+-activated K+ (BKCa) channels: role in BKCa trafficking to the surface of cultured podocytes

Large-conductance (BK(Ca) type) Ca(2+)-activated K(+) channels encoded by the Slo1 gene and various canonical transient receptor potential channels (TRPCs) are coexpressed in many cell types, including podocytes (visceral epithelial cells) of the renal glomerulus. In this study, we show by coimmunoprecipitation and GST pull-down assays that BK(Ca) channels can associate with endogenous TRPC3 and TRPC6 channels in differentiated cells of a podocyte cell line. Both types of TRPC channels colocalize with Slo1 in podocytes and in human embryonic kidney (HEK) 293T cells transiently coexpressing the TRPC channels with Slo1. In HEK293T cells, coexpression of TRPC6 increased surface expression of a Slo1 subunit splice variant (Slo1(VEDEC)) that is typically retained in intracellular compartments, as assessed by cell-surface biotinylation assays and confocal microscopy. Corresponding currents through BK(Ca) channels were also increased with TRPC6 coexpression, as assessed by whole-cell and excised inside-out patch recordings. By contrast, coexpression of TRPC3 had no effect on the surface expression of BK(Ca) channels in HEK293T cells or on the amplitudes of currents in whole cells or excised patches. In podocytes, small interfering RNA knockdown of endogenous TRPC6 reduced steady-state surface expression of endogenous Slo1 channels, but knockdown of TRPC3 had no effect. TRPC6, but not TRPC3 knockdown also reduced voltage-evoked outward current through podocyte BK(Ca) channels. These data indicate that TRPC6 and TRPC3 channels can bind to Slo1, and this colocalization may allow them to serve as a source of Ca(2+) for the activation of BK(Ca) channels. TRPC6 channels also play a role in the regulation of surface expression of a subset of podocyte BK(Ca) channels.

The guanylate kinase domain of the beta-subunit of voltage-gated calcium channels suffices to modulate gating

Inactivation of voltage-gated calcium channels is crucial for the spatiotemporal coordination of calcium signals and prevention of toxic calcium buildup. Only one member of the highly conserved family of calcium channel beta-subunits--Ca(V)beta--inhibits inactivation. This unique property has been attributed to short variable regions of the protein; however, here we report that this inhibition actually is conferred by a conserved guanylate kinase (GK) domain and, moreover, that this domain alone recapitulates Ca(V)beta-mediated modulation of channel activation. We expressed and refolded the GK domain of Ca(V)beta(2a), the unique variant that inhibits inactivation, and of Ca(V)beta(1b), an isoform that facilitates it. The refolded domains of both Ca(V)beta variants were found to inhibit inactivation of Ca(V)2.3 channels expressed in Xenopus laevis oocytes. These findings suggest that the GK domain endows calcium channels with a brake restraining voltage-dependent inactivation, and thus facilitation of inactivation by full-length Ca(V)beta requires additional structural determinants to antagonize the GK effect. We found that Ca(V)beta can switch the inactivation phenotype conferred to Ca(V)2.3 from slow to fast after posttranslational modifications during channel biogenesis. Our findings provide a framework within which to understand the modulation of inactivation and a new functional map of Ca(V)beta in which the GK domain regulates channel gating and the other conserved domain (Src homology 3) may couple calcium channels to other signaling pathways.