Postsynaptic density-95 scaffolding of Shaker-type K⁺ channels in smooth muscle cells regulates the diameter of cerebral arteries

Physiological functions and pathological involvement of ion channel trafficking in the vasculature

A variety of ion channels regulate membrane potential and calcium influx in arterial smooth muscle and endothelial cells to modify vascular functions, including contractility. The current (I) generated by a population of ion channels is equally dependent upon their number (N), open probability (Po) and single channel current (i), such that I = N.PO.i. A conventional view had been that ion channels traffic to the plasma membrane in a passive manner, resulting in a static surface population. It was also considered that channels assemble with auxiliary subunits prior to anterograde trafficking of the multimeric complex to the plasma membrane. Recent studies have demonstrated that physiological stimuli can regulate the surface abundance (N) of several different ion channels in arterial smooth muscle and endothelial cells to control arterial contractility. Physiological stimuli can also regulate the number of auxiliary subunits present in the plasma membrane to modify the biophysical properties, regulatory mechanisms and physiological functions of some ion channels. Furthermore, ion channel trafficking becomes dysfunctional in the vasculature during hypertension, which negatively impacts the regulation of contractility. The temporal kinetics of ion channel and auxiliary subunit trafficking can also vary depending on the signalling mechanisms and proteins involved. This review will summarize recent work that has uncovered the mechanisms, functions and pathological modifications of ion channel trafficking in arterial smooth muscle and endothelial cells.

Synaptic-like transmission between neural axons and arteriolar smooth muscle cells drives cerebral neurovascular coupling

Neurovascular coupling (NVC) is important for brain function and its dysfunction underlies many neuropathologies. Although cell-type specificity has been implicated in NVC, how active neural information is conveyed to the targeted arterioles in the brain remains poorly understood. Here, using two-photon focal optogenetics in the mouse cerebral cortex, we demonstrate that single glutamatergic axons dilate their innervating arterioles via synaptic-like transmission between neural-arteriolar smooth muscle cell junctions (NsMJs). The presynaptic parental-daughter bouton makes dual innervations on postsynaptic dendrites and on arteriolar smooth muscle cells (aSMCs), which express many types of neuromediator receptors, including a low level of glutamate NMDA receptor subunit 1 (Grin1). Disruption of NsMJ transmission by aSMC-specific knockout of GluN1 diminished optogenetic and whisker stimulation-caused functional hyperemia. Notably, the absence of GluN1 subunit in aSMCs reduced brain atrophy following cerebral ischemia by preventing Ca2+ overload in aSMCs during arteriolar constriction caused by the ischemia-induced spreading depolarization. Our findings reveal that NsMJ transmission drives NVC and open up a new avenue for studying stroke.

Ion channel molecular complexes in vascular smooth muscle

Ion channels that influence membrane potential and intracellular calcium concentration control vascular smooth muscle excitability. Voltage-gated calcium channels (VGCC), transient receptor potential (TRP) channels, voltage (K_V), and Ca²⁺-activated K⁺ (BK) channels are key regulators of vascular smooth muscle excitability and contractility. These channels are regulated by various signaling cues, including protein kinases and phosphatases. The effects of these ubiquitous signaling molecules often depend on the formation of macromolecular complexes that provide a platform for targeting and compartmentalizing signaling events to specific substrates. This manuscript summarizes our current understanding of specific molecular complexes involving VGCC, TRP, and K_V and BK channels and their contribution to regulating vascular physiology.

Bridging the Molecular-Cellular Gap in Understanding Ion Channel Clustering

The clustering of many voltage-dependent ion channel molecules at unique neuronal membrane sites such as axon initial segments, nodes of Ranvier, or the post-synaptic density, is an active process mediated by the interaction of ion channels with scaffold proteins and is of immense importance for electrical signaling. Growing evidence indicates that the density of ion channels at such membrane sites may affect action potential conduction properties and synaptic transmission. However, despite the emerging importance of ion channel density for electrical signaling, how ion channel-scaffold protein molecular interactions lead to cellular ion channel clustering, and how this process is regulated are largely unknown. In this review, we emphasize that voltage-dependent ion channel density at native clustering sites not only affects the density of ionic current fluxes but may also affect the conduction properties of the channel and/or the physical properties of the membrane at such locations, all changes that are expected to affect action potential conduction properties. Using the concrete example of the prototypical Shaker voltage-activated potassium channel (Kv) protein, we demonstrate how insight into the regulation of cellular ion channel clustering can be obtained when the molecular mechanism of ion channel-scaffold protein interaction is known. Our review emphasizes that such mechanistic knowledge is essential, and when combined with super-resolution imaging microscopy, can serve to bridge the molecular-cellular gap in understanding the regulation of ion channel clustering. Pressing questions, challenges and future directions in addressing ion channel clustering and its regulation are discussed.

Molecular determinants of beta-adrenergic signaling to voltage-gated K+ channels in the cerebral circulation

Voltage-gated K⁺ (K_v ) channels are major determinants of membrane potential in vascular smooth muscle cells (VSMCs) and regulate the diameter of small cerebral arteries and arterioles. However, the intracellular structures that govern the expression and function of vascular K_v channels are poorly understood. Scaffolding proteins including postsynaptic density 95 (PSD95) recently were identified in rat cerebral VSMCs. Primarily characterized in neurons, the PSD95 scaffold has more than 50 known binding partners, and it can mediate macromolecular signaling between cell-surface receptors and ion channels. In cerebral arteries, Shaker-type K_v 1 channels appear to associate with the PSD95 molecular scaffold, and PSD95 is required for the normal expression and vasodilator influence of members of this K⁺ channel gene family. Furthermore, recent findings suggest that the β1-subtype adrenergic receptor is expressed in cerebral VSMCs and forms a functional vasodilator complex with K_v 1 channels on the PSD95 scaffold. Activation of β1-subtype adrenergic receptors in VSMCs enables protein kinase A-dependent phosphorylation and opening of K_v 1 channels in the PSD95 complex; the subsequent K⁺ efflux mediates membrane hyperpolarization and vasodilation of small cerebral arteries. Early evidence from other studies suggests that other families of K_v channels and scaffolding proteins are expressed in VSMCs. Future investigations into these macromolecular complexes that modulate the expression and function of K_v channels may reveal unknown signaling cascades that regulate VSMC excitability and provide novel targets for ion channel-based medications to optimize vascular tone.

KV channel trafficking and control of vascular tone

Membrane potential is a principal regulator of arterial contractility. Arterial smooth muscle cells express several different types of ion channel that control membrane potential, including K_V channels. K_V channel activation leads to membrane hyperpolarization, resulting in inhibition of voltage-dependent Ca²⁺ channels, a reduction in [Ca²⁺ ]_i , and vasodilation. In contrast, K_V channel inhibition leads to membrane depolarization and vasoconstriction. The ability of K_V channels to regulate arterial contractility is dependent upon the number of plasma membrane-resident channels and their open probability. Here, we will discuss mechanisms that alter the surface abundance of K_V channel proteins in arterial smooth muscle cells and the functional consequences of such regulation. Cellular processes that will be described include those that modulate K_V channel transcription, retrograde and anterograde trafficking, and protein degradation.

Fundamental role for the KCNE4 ancillary subunit in Kv7.4 regulation of arterial tone

KEY POINTS

KCNE4 alters the biophysical properties and cellular localization of voltage-gated potassium channel Kv7.4. KCNE4 is expressed in a variety of arteries and, in mesenteric arteries, co-localizes with Kv7.4, which is important in the control of vascular contractility. Knockdown of KCNE4 leads to reduced Kv7.4 membrane abundance, a depolarized membrane potential and an augmented response to vasoconstrictors. KCNE4 is a key regulator of the function and expression of Kv7.4 in vascular smooth muscle.

ABSTRACT

The KCNE ancillary subunits (KCNE1-5) significantly alter the expression and function of voltage-gated potassium channels; however, their role in the vasculature has yet to be determined. The present study aimed to investigate the expression and function of the KCNE4 subunit in rat mesenteric arteries and to determine whether it has a functional impact on the regulation of arterial tone by Kv7 channels. In HEK cells expressing Kv7.4, co-expression of KCNE4 increased the membrane expression of Kv7.4 and significantly altered Kv7.4 current properties. Quantitative PCR analysis of different rat arteries found that the KCNE4 isoform predominated and proximity ligation experiments showed that KCNE4 co-localized with Kv7.4 in mesenteric artery myocytes. Morpholino-induced knockdown of KCNE4 depolarized mesenteric artery smooth muscle cells and resulted in their increased sensitivity to methoxamine being attenuated (mean ± SEM EC50 decreased from 5.7 ± 0.63 μm to 1.6 ± 0.23 μm), which coincided with impaired effects of Kv7 modulators. When KCNE4 expression was reduced, less Kv7.4 expression was found in the membrane of the mesenteric artery myocytes. These data show that KCNE4 is consistently expressed in a variety of arteries, and knockdown of the expression product leads to reduced Kv7.4 membrane abundance, a depolarized membrane potential and an augmented response to vasoconstrictors. The present study is the first to demonstrate an integral role of KCNE4 in regulating the function and expression of Kv7.4 in vascular smooth muscle.

Target of HIV-1 Envelope Glycoprotein gp120-Induced Hippocampal Neuron Damage: Role of Voltage-Gated K(+) Channel Kv2.1

Human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein 120 (gp120) has been reported to be toxic to the hippocampal neurons, and to be involved in the pathogenesis of HIV-1-associated neurocognitive disorders (HAND). Accumulating evidence has demonstrated that voltage-gated potassium (Kv) channels, especially the outward delayed-rectifier K(+) (Ik) channels, play a critical role in gp120-induced cortical neuronal death in vitro. However, the potential mechanisms underlying the hippocampal neuronal injury resulted from gp120-mediated neurotoxicity remain poorly understood. Using whole-cell patch clamp recording in cultured hippocampal neurons, this study found that gp120 significantly increased the outward delayed-rectifier K(+) currents (Ik). Meanwhile, Western blot assay revealed that gp120 markedly upregulated Kv2.1 protein levels, which was consistent with the increased Ik density. With Western blot and terminal deoxynucleotidyl transferase dUTP nick end labeling assays, it was discovered that gp120-induced neuronal injury was largely due to activation of Kv2.1 channels and resultant apoptosis mediated by caspase-3 activation, as the pharmacological blockade of Kv2.1 channels largely attenuated gp120-induced cell damage and caspase-3 expression. Moreover, p38 MAPK was demonstrated to participate in gp120-induced hippocampal neural damage, since p38 MAPK antagonist (SB203580) partially abrogated gp120-induced Kv2.1 upregulation and neural apoptosis. Taken together, these results suggest that gp120 induces hippocampal neuron apoptosis by enhancement of the Ik, which might be associated with increased Kv2.1 expression via the p38 MAPK pathway.

Beta1-adrenergic receptor-mediated dilation of rat cerebral artery requires Shaker-type KV1 channels on PSD95 scaffold

Postsynaptic density-95 (PSD95) is a scaffolding protein in cerebral vascular smooth muscle cells (cVSMCs), which binds to Shaker-type K(+) (KV1) channels and facilitates channel opening through phosphorylation by protein kinase A. β1-Adrenergic receptors (β1ARs) also have a binding motif for PSD95. Functional association of β1AR with KV1 channels through PSD95 may represent a novel vasodilator complex in cerebral arteries (CA). We explored whether a β1AR-PSD95-KV1 complex is a determinant of rat CA dilation. RT-PCR and western blots revealed expression of β1AR in CA. Isoproterenol induced a concentration-dependent dilation of isolated, pressurized rat CA that was blocked by the β1AR blocker CGP20712. Cranial window imaging of middle cerebral arterioles in situ showed isoproterenol- and norepinephrine-induced dilation that was blunted by β1AR blockade. Isoproterenol-induced hyperpolarization of cVSMCs in pressurized CA was blocked by CGP20712. Confocal images of cVSMCs immunostained with antibodies against β1AR and PSD95 indicated strong colocalization, and PSD95 co-immunoprecipitated with β1AR in CA lysate. Blockade of KV1 channels, β1AR or disruption of PSD95-KV1 interaction produced similar blunting of isoproterenol-induced dilation in pressurized CA. These findings suggest that PSD95 mediates a vasodilator complex with β1AR and KV1 channels in cVSMCs. This complex may be critical for proper vasodilation in rat CA.

Protein kinase A-phosphorylated KV1 channels in PSD95 signaling complex contribute to the resting membrane potential and diameter of cerebral arteries

RATIONALE

Postsynaptic density-95 (PSD95) is a scaffolding protein that associates with voltage-gated, Shaker-type K(+) (KV1) channels and promotes the expression of KV1 channels in vascular smooth muscle cells of the cerebral (cVSMCs) circulation. However, the physiological role of PSD95 in mediating molecular signaling in cVSMCs is unknown.

OBJECTIVE

We explored whether a specific interaction between PSD95 and KV1 channels enables protein kinase A phosphorylation of KV1 channels in cVSMCs to promote vasodilation.

METHODS AND RESULTS

Rat cerebral arteries were used for analyses. A membrane-permeable peptide (KV1-C peptide) corresponding to the postsynaptic density-95, discs large, zonula occludens-1 binding motif in the C terminus of KV1.2α was designed as a dominant-negative peptide to disrupt the association of KV1 channels with PSD95. Application of KV1-C peptide to cannulated, pressurized cerebral arteries rapidly induced vasoconstriction and depolarized cVSMCs. These events corresponded to reduced coimmunoprecipitation of the PSD95 and KV1 proteins without altering surface expression. Middle cerebral arterioles imaged in situ through cranial window also constricted rapidly in response to local application of KV1-C peptide. Patch-clamp recordings confirmed that KV1-C peptide attenuates KV1 channel blocker (5-(4-phenylalkoxypsoralen))-sensitive current in cVSMCs. Western blots using a phospho-protein kinase A substrate antibody revealed that cerebral arteries exposed to KV1-C peptide showed markedly less phosphorylation of KV1.2α subunits. Finally, phosphatase inhibitors blunted both KV1-C peptide-mediated and protein kinase A inhibitor peptide-mediated vasoconstriction.

CONCLUSIONS

These findings provide initial evidence that protein kinase A phosphorylation of KV1 channels is enabled by a dynamic association with PSD95 in cerebral arteries and suggest that a disruption of such association may compromise cerebral vasodilation and blood flow.

Ion channel remodeling in vascular smooth muscle during hypertension: Implications for novel therapeutic approaches

Ion channels are multimeric, transmembrane proteins that selectively mediate ion flux across the plasma membrane in a variety of cells including vascular smooth muscle cells (VSMCs). The dynamic interplay of Ca(2+) and K(+) channels on the plasma membrane of VSMCs plays a pivotal role in modulating the vascular tone of small arteries and arterioles. The abnormally-elevated arterial tone observed in hypertension thus points to an aberrant expression and function of Ca(2+) and K(+) channels in the VSMCs. In this short review, we focus on the three well-studied ion channels in VSMCs, namely the L-type Ca(2+) (CaV1.2) channels, the voltage-gated K(+) (KV) channels, and the large-conductance Ca(2+)-activated K(+) (BK) channels. First, we provide a brief overview on the physiological role of vascular CaV1.2, KV and BK channels in regulating arterial tone. Second, we discuss the current understanding of the expression changes and regulation of CaV1.2, KV and BK channels in the vasculature during hypertension. Third, based on available proof-of-concept studies, we describe the potential therapeutic approaches targeting these vascular ion channels in order to restore blood pressure to normotensive levels.

Supertertiary structure of the MAGUK core from PSD-95

The family of membrane-associated guanylate kinase (MAGUK) scaffold proteins comprises members that function at neuronal synapses, tight junctions, immunological synapses, and neutrophil membranes. Through their multiple domains, MAGUKs organize events of signal transduction, cell adhesion, and molecular trafficking. Here, we use nuclear magnetic resonance, small-angle X-ray scattering, and Rosetta modeling to reveal the structural preferences and interdomain dynamics of the MAGUK core (PDZ3-SH3-guanylate kinase) from postsynaptic density-95 (PSD-95), the best known MAUGK. PSD-95 is highly abundant in the postsynaptic density of excitatory neurons and is responsible for coupling glutamate receptors with internal postsynaptic structures. These solution-based studies show that the MAGUK core PDZ domain (PDZ3) interacts directly with the SH3 domain via its canonical peptide binding groove, with the connecting linker serving as an adhesive. This weak interaction, however, is dynamic and weakened further by PDZ3 ligands and linker phosphorylation, suggesting that domain dynamics may be central to MAGUK function.

Social networking among voltage-activated potassium channels

Voltage-activated potassium channels (Kv channels) are ubiquitously expressed proteins that subserve a wide range of cellular functions. From their birth in the endoplasmic reticulum, Kv channels assemble from multiple subunits in complex ways that determine where they live in the cell, their biophysical characteristics, and their role in enabling different kinds of cells to respond to specific environmental signals to generate appropriate functional responses. This chapter describes the types of protein-protein interactions among pore-forming channel subunits and their auxiliary protein partners, as well as posttranslational protein modifications that occur in various cell types. This complex oligomerization of channel subunits establishes precise cell type-specific Kv channel localization and function, which in turn drives a diverse range of cellular signal transduction mechanisms uniquely suited to the physiological contexts in which they are found.