Neuronal inwardly rectifying K(+) channels differentially couple to PDZ proteins of the PSD-95/SAP90 family

Roles of neuroligins in central nervous system development: focus on glial neuroligins and neuron neuroligins

Neuroligins are postsynaptic cell adhesion molecules that are relevant to many neurodevelopmental disorders. They are differentially enriched at the postsynapse and interact with their presynaptic ligands, neurexins, whose differential binding to neuroligins has been shown to regulate synaptogenesis, transmission, and other synaptic properties. The proper functioning of functional networks in the brain depends on the proper connection between neuronal synapses. Impaired synaptogenesis or synaptic transmission results in synaptic dysfunction, and these synaptic pathologies are the basis for many neurodevelopmental disorders. Deletions or mutations in the neuroligins genes have been found in patients with both autism and schizophrenia. It is because of the important role of neuroligins in synaptic connectivity and synaptic dysfunction that studies on neuroligins in the past have mainly focused on their expression in neurons. As studies on the expression of genes specific to various cells of the central nervous system deepened, neuroligins were found to be expressed in non-neuronal cells as well. In the central nervous system, glial cells are the most representative non-neuronal cells, which can also express neuroligins in large amounts, especially astrocytes and oligodendrocytes, and they are involved in the regulation of synaptic function, as are neuronal neuroligins. This review examines the mechanisms of neuron neuroligins and non-neuronal neuroligins in the central nervous system and also discusses the important role of neuroligins in the development of the central nervous system and neurodevelopmental disorders from the perspective of neuronal neuroligins and glial neuroligins.

Kir Channel Molecular Physiology, Pharmacology, and Therapeutic Implications

For the past two decades several scholarly reviews have appeared on the inwardly rectifying potassium (Kir) channels. We would like to highlight two efforts in particular, which have provided comprehensive reviews of the literature up to 2010 (Hibino et al., Physiol Rev 90(1):291-366, 2010; Stanfield et al., Rev Physiol Biochem Pharmacol 145:47-179, 2002). In the past decade, great insights into the 3-D atomic resolution structures of Kir channels have begun to provide the molecular basis for their functional properties. More recently, computational studies are beginning to close the time domain gap between in silico dynamic and patch-clamp functional studies. The pharmacology of these channels has also been expanding and the dynamic structural studies provide hope that we are heading toward successful structure-based drug design for this family of K⁺ channels. In the present review we focus on placing the physiology and pharmacology of this K⁺ channel family in the context of atomic resolution structures and in providing a glimpse of the promising future of therapeutic opportunities.

The Potassium Channel Odyssey: Mechanisms of Traffic and Membrane Arrangement

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

Domain insertion permissibility-guided engineering of allostery in ion channels

Allostery is a fundamental principle of protein regulation that remains hard to engineer, particularly in membrane proteins such as ion channels. Here we use human Inward Rectifier K⁺ Channel Kir2.1 to map site-specific permissibility to the insertion of domains with different biophysical properties. We find that permissibility is best explained by dynamic protein properties, such as conformational flexibility. Several regions in Kir2.1 that are equivalent to those regulated in homologs, such as G-protein-gated inward rectifier K⁺ channels (GIRK), have differential permissibility; that is, for these sites permissibility depends on the structural properties of the inserted domain. Our data and the well-established link between protein dynamics and allostery led us to propose that differential permissibility is a metric of latent allosteric capacity in Kir2.1. In support of this notion, inserting light-switchable domains into sites with predicted latent allosteric capacity renders Kir2.1 activity sensitive to light.

Allostery is a fundamental principle of protein regulation that remains challenging to engineer. Here authors screen human Inward Rectifier K + Channel Kir2.1 for permissibility to domain insertions and propose that differential permissibility is a metric of latent allosteric capacity in Kir2.1.

Lipopolysaccharides increase Kir2.1 expression in lung endothelial cells

Kir2.1 is an inwardly rectifying K⁺ channel that modulates membrane potential. It is expressed widely in smooth muscle, neurons, and endothelial cells. Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are severe clinical syndromes, often causing the damage of epithelial and endothelial cells. Lipopolysaccharides (LPS) usually cause ALI/ARDS, directly or indirectly, and are used to reproduce the model in vivo. Here, we used differentially expressed gene analysis to find increasing Kir2.1 channel expression in human pulmonary microvascular endothelial cells cultured with LPS. Primary cultured mice pulmonary microvascular endothelial cells were verified by immunofluorescence. LPS incubation increased Kir2.1 channel expression in cultured mice pulmonary microvascular endothelial cells. A whole-cell voltage clamp was used to record the K⁺ current in cultured endothelial cells, showing increased whole-cell current in LPS treatment compared with controls. Additionally, the application of Ba²⁺, as an inhibitor of Kir2.1 channel, inhibited K⁺ current in both groups. We demonstrated that LPS may increase Kir2.1 channel expression in mice pulmonary microvascular endothelial cells to increase K⁺ flux, maintain hyperpolarization, and cause vasodilation, which may increase blood flow in pulmonary vessel bed, leading to pulmonary congestion contributing pneumonemia and ALI/ARDS.

Boosting the signal: Endothelial inward rectifier K+ channels

Endothelial cells express a diverse array of ion channels including members of the strong inward rectifier family composed of K_IR 2 subunits. These two-membrane spanning domain channels are modulated by their lipid environment, and exist in macromolecular signaling complexes with receptors, protein kinases and other ion channels. Inward rectifier K⁺ channel (K_IR ) currents display a region of negative slope conductance at membrane potentials positive to the K⁺ equilibrium potential that allows outward current through the channels to be activated by membrane hyperpolarization, permitting K_IR to amplify hyperpolarization induced by other K⁺ channels and ion transporters. Increases in extracellular K⁺ concentration activate K_IR allowing them to sense extracellular K⁺ concentration and transduce this change into membrane hyperpolarization. These properties position K_IR to participate in the mechanism of action of hyperpolarizing vasodilators and contribute to cell-cell conduction of hyperpolarization along the wall of microvessels. The expression of K_IR in capillaries in electrically active tissues may allow K_IR to sense extracellular K⁺ , contributing to functional hyperemia. Understanding the regulation of expression and function of microvascular endothelial K_IR will improve our understanding of the control of blood flow in the microcirculation in health and disease and may provide new targets for the development of therapeutics in the future.

Protein crowding and lipid complexity influence the nanoscale dynamic organization of ion channels in cell membranes

Cell membranes are crowded and complex environments. To investigate the effect of protein-lipid interactions on dynamic organization in mammalian cell membranes, we have performed coarse-grained molecular dynamics simulations containing >100 copies of an inwardly rectifying potassium (Kir) channel which forms specific interactions with the regulatory lipid phosphatidylinositol 4,5-bisphosphate (PIP2). The tendency of protein molecules to cluster has the effect of organizing the membrane into dynamic compartments. At the same time, the diversity of lipids present has a marked effect on the clustering behavior of ion channels. Sub-diffusion of proteins and lipids is observed. Protein crowding alters the sub-diffusive behavior of proteins and lipids such as PIP2 which interact tightly with Kir channels. Protein crowding also affects bilayer properties, such as membrane undulations and bending rigidity, in a PIP2-dependent manner. This interplay between the diffusion and the dynamic organization of Kir channels may have important implications for channel function.

GIRK2 splice variants and neuronal G protein-gated K+ channels: implications for channel function and behavior

Many neurotransmitters directly inhibit neurons by activating G protein-gated inwardly rectifying K⁺ (GIRK) channels, thereby moderating the influence of excitatory input on neuronal excitability. While most neuronal GIRK channels are formed by GIRK1 and GIRK2 subunits, distinct GIRK2 isoforms generated by alternative splicing have been identified. Here, we compared the trafficking and function of two isoforms (GIRK2a and GIRK2c) expressed individually in hippocampal pyramidal neurons lacking GIRK2. GIRK2a and GIRK2c supported comparable somato-dendritic GIRK currents in Girk2^−/− pyramidal neurons, although GIRK2c achieved a more uniform subcellular distribution in pyramidal neurons and supported inhibitory postsynaptic currents in distal dendrites better than GIRK2a. While over-expression of either isoform in dorsal CA1 pyramidal neurons restored contextual fear learning in a conditional Girk2^−/− mouse line, GIRK2a also enhanced cue fear learning. Collectively, these data indicate that GIRK2 isoform balance within a neuron can impact the processing of afferent inhibitory input and associated behavior.

Expression of Kir4.1 and Kir5.1 inwardly rectifying potassium channels in oligodendrocytes, the myelinating cells of the CNS

The inwardly rectifying K+ channel subtype Kir5.1 is only functional as a heteromeric channel with Kir4.1. In the CNS, Kir4.1 is localised to astrocytes and is the molecular basis of their strongly negative membrane potential. Oligodendrocytes are the specialised myelinating glia of the CNS and their resting membrane potential provides the driving force for ion and water transport that is essential for myelination. However, little is known about the ion channel profile of mature myelinating oligodendrocytes. Here, we identify for the first time colocalization of Kir5.1 with Kir4.1 in oligodendrocytes in white matter. Immunolocalization with membrane-bound Na+/K+-ATPase and western blot of the plasma membrane fraction of the optic nerve, a typical CNS white matter tract containing axons and the oligodendrocytes that myelinate them, demonstrates that Kir4.1 and Kir5.1 are colocalized on oligodendrocyte cell membranes. Co-immunoprecipitation provides evidence that oligodendrocytes and astrocytes express a combination of homomeric Kir4.1 and heteromeric Kir4.1/Kir5.1 channels. Genetic knock-out and shRNA to ablate Kir4.1 indicates plasmalemmal expression of Kir5.1 in glia is largely dependent on Kir4.1 and the plasmalemmal anchoring protein PSD-95. The results demonstrate that, in addition to astrocytes, oligodendrocytes express both homomeric Kir4.1 and heteromeric Kir4.1/Kir5.1 channels. In astrocytes, these channels are essential to their key functions of K+ uptake and CO2/H+ chemosensation. We propose Kir4.1/Kir5.1 channels have equivalent functions in oligodendrocytes, maintaining myelin integrity in the face of large ionic shifts associated with action potential propagation along myelinated axons.

Aquaporins: Multiple Roles in the Central Nervous System

Aquaporins (AQPs) represent a diverse family of membrane proteins found in prokaryotes and eukaryotes. The primary aquaporins expressed in the mammalian brain are AQP1, which is densely packed in choroid plexus cells lining the ventricles, and AQP4, which is abundant in astrocytes and concentrated especially in the end-feet structures that surround capillaries throughout the brain and are present in glia limitans structures, notably in osmosensory areas such the supraoptic nucleus. Water movement in brain tissues is carefully regulated from the micro- to macroscopic levels, with aquaporins serving key roles as multifunctional elements of complex signaling assemblies. Intriguing possibilities suggest links for AQP1 in Alzheimer's disease, AQP4 as a target for therapy in brain edema, and a possible contribution of AQP9 in Parkinson's disease. For all the aquaporins, new contributions to physiological functions are likely to continue to be discovered with ongoing work in this rapidly expanding field of research. NEUROSCIENTIST 13(5):470—485, 2007.

Synapse Loss and Dendrite Remodeling in a Mouse Model of Glaucoma

It has been hypothesized that synaptic pruning precedes retinal ganglion cell degeneration in glaucoma, causing early dysfunction to retinal ganglion cells. To begin to assess this, we studied the excitatory synaptic inputs to individual ganglion cells in normal mouse retinas and in retinas with ganglion cell degeneration from glaucoma (DBA/2J), or following an optic nerve crush. Excitatory synapses were labeled by AAV2-mediated transfection of ganglion cells with PSD-95-GFP. After both insults the linear density of synaptic inputs to ganglion cells decreased. In parallel, the dendritic arbors lost complexity. We did not observe any cells that had lost dendritic synaptic input while preserving a normal or near-normal morphology. Within the temporal limits of these observations, dendritic remodeling and synapse pruning thus appear to occur near-simultaneously.

Channel properties of Nax expressed in neurons

Nax is a sodium-concentration ([Na+])-sensitive Na channel with a gating threshold of ~150 mM for extracellular [Na+] ([Na+]o) in vitro. We previously reported that Nax was preferentially expressed in the glial cells of sensory circumventricular organs including the subfornical organ, and was involved in [Na+] sensing for the control of salt-intake behavior. Although Nax was also suggested to be expressed in the neurons of some brain regions including the amygdala and cerebral cortex, the channel properties of Nax have not yet been adequately characterized in neurons. We herein verified that Nax was expressed in neurons in the lateral amygdala of mice using an antibody that was newly generated against mouse Nax. To investigate the channel properties of Nax expressed in neurons, we established an inducible cell line of Nax using the mouse neuroblastoma cell line, Neuro-2a, which is endogenously devoid of the expression of Nax. Functional analyses of this cell line revealed that the [Na+]-sensitivity of Nax in neuronal cells was similar to that expressed in glial cells. The cation selectivity sequence of the Nax channel in cations was revealed to be Na+ ≈ Li+ > Rb+ > Cs+ for the first time. Furthermore, we demonstrated that Nax bound to postsynaptic density protein 95 (PSD95) through its PSD95/Disc-large/ZO-1 (PDZ)-binding motif at the C-terminus in neurons. The interaction between Nax and PSD95 may be involved in promoting the surface expression of Nax channels because the depletion of endogenous PSD95 resulted in a decrease in Nax at the plasma membrane. These results indicated, for the first time, that Nax functions as a [Na+]-sensitive Na channel in neurons as well as in glial cells.

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.

Wnt-5a increases NO and modulates NMDA receptor in rat hippocampal neurons

Wnt signaling has a crucial role in synaptic function at the central nervous system. Here we evaluate whether Wnts affect nitric oxide (NO) generation in hippocampal neurons. We found that non-canonical Wnt-5a triggers NO production; however, Wnt-3a a canonical ligand did not exert the same effect. Co-administration of Wnt-5a with the soluble Frizzled related protein-2 (sFRP-2) a Wnt antagonist blocked the NO production. Wnt-5a activates the non-canonical Wnt/Ca(2+) signaling through a mechanism that depends on Ca(2+) release from Ryanodine-sensitive internal stores. The increase in NO levels evoked by Wnt-5a promotes the insertion of the GluN2B subunit of the NMDA receptor (NMDAR) into the neuronal cell surface. To the best of our knowledge, this is the first time that Wnt-5a signaling is related to NO production, which in turn increases NMDARs trafficking to the cell surface.

Stabilization of the angiotensin-(1-7) receptor Mas through interaction with PSD95

The functions and signalling mechanisms of the Ang-(1-7) [angiotensin-(1-7)] receptor Mas have been studied extensively. However, less attention has been paid to the intracellular regulation of Mas protein. In the present study, PSD95 (postsynaptic density 95), a novel binding protein of Mas receptor, was identified, and their association was characterized further. Mas specifically interacts with PDZ1-2, but not the PDZ3, domain of PSD95 via Mas-CT (Mas C-terminus), and the last four amino acids [ETVV (Glu-Thr-Val-Val)] of Mas-CT were determined to be essential for this interaction, as shown by GST pull-down, co-immunoprecipitation and confocal co-localization experiments. Gain-of-function and loss-of-function studies indicated that PSD95 enhanced Mas protein expression by increasing the stabilization of the receptor. Mas degradation was robustly inhibited by the proteasome inhibitor MG132 in time- and dose-dependent manners, and the expression of PSD95 impaired Mas ubiquitination, indicating that the PSD95-Mas association inhibits Mas receptor degradation via the ubiquitin-proteasome proteolytic pathway. These findings reveal a novel mechanism of Mas receptor regulation by which its expression is modulated at the post-translational level by ubiquitination, and clarify the role of PSD95, which binds directly to Mas, blocking the ubiquitination and subsequent degradation of the receptor via the ubiquitin-proteasome proteolytic pathway.

The degradation of the inwardly rectifying potassium channel, Kir2.1, depends on the expression level: examination with fluorescent proteins

The expression of ion channels is regulated by their synthesis as well as degradation, and some ion channels are degraded in an expression level-dependent way. Recently, new techniques of fluorescent proteins have been developed and seem to be useful to study protein degradation. To examine the regulation of the degradation of strongly inwardly rectifying potassium channel (Kir2.1) and the usefulness of the fluorescent proteins, we constructed Kir2.1 fusion proteins with SNAP tag and fluorescent timer (FT). The SNAP tag, which covalently binds to a specific membrane-permeable fluorescent dye, enables a pulse-chase experiment with fluorescence. When the SNAP-Kir2.1 proteins were expressed in 293T cells by low and high expression plasmids, the half-life of the fusion protein expressed by a high-expression plasmid was shorter (18.2±1.9 h) than that expressed by a low-expression plasmid (35.1+2.3h). The addition of Ba(2+), a selective blocker of Kir2.1, slowed the degradation, suggesting a current-dependency of degradation. Consistently, patch-clamp recording showed that cultivation in the presence of Ba(2+) increased the whole cell conductance of SNAP-Kir2.1. Since the fluorescence of FT changes gradually changes from green to red, the green/red ratio should allow us to monitor the changes in the degradation rate of FT-Kir2.1. Using this method, we confirmed the slower degradation by Ba(2+). The results suggest a homeostatic regulation of the degradation of Kir2.1 in the 293T cells, and the usefulness of fluorescence-based methods for examining the degradation of ion channels.

Phenotype variability in patients carrying KCNJ2 mutations

BACKGROUND

Mutations of KCNJ2, the gene encoding the human inward rectifier potassium channel Kir2.1, cause Andersen-Tawil syndrome (ATS), a disease exhibiting ventricular arrhythmia, periodic paralysis, and dysmorphic features. However, some KCNJ2 mutation carriers lack the ATS triad and sometimes share the phenotype of catecholaminergic polymorphic ventricular tachycardia (CPVT). We investigated clinical and biophysical characteristics of KCNJ2 mutation carriers with "atypical ATS."

METHODS AND RESULTS

Mutational analyses of KCNJ2 were performed in 57 unrelated probands showing typical (≥2 ATS features) and atypical (only 1 of the ATS features or CPVT) ATS. We identified 24 mutation carriers. Mutation-positive rates were 75% (15/20) in typical ATS, 71% (5/7) in cardiac phenotype alone, 100% (2/2) in periodic paralysis, and 7% (2/28) in CPVT. We divided all carriers (n=45, including family members) into 2 groups: typical ATS (A) (n=21, 47%) and atypical phenotype (B) (n=24, 53%). Patients in (A) had a longer QUc interval [(A): 695 ± 52 versus (B): 643 ± 35 ms] and higher U-wave amplitude (0.24 ± 0.07 versus 0.18 ± 0.08 mV). C-terminal mutations were more frequent in (A) (85% versus 38%, P<0.05). There were no significant differences in incidences of ventricular tachyarrhythmias. Functional analyses of 4 mutations found in (B) revealed that R82Q, R82W, and G144D exerted strong dominant negative suppression (current reduction by 95%, 97%, and 96%, respectively, versus WT at -50 mV) and T305S moderate suppression (reduction by 89%).

CONCLUSIONS

KCNJ2 gene screening in atypical ATS phenotypes is of clinical importance because more than half of mutation carriers express atypical phenotypes, despite their arrhythmia severity.

Dynamic reciprocity of sodium and potassium channel expression in a macromolecular complex controls cardiac excitability and arrhythmia

The cardiac electrical impulse depends on an orchestrated interplay of transmembrane ionic currents in myocardial cells. Two critical ionic current mechanisms are the inwardly rectifying potassium current (I(K1)), which is important for maintenance of the cell resting membrane potential, and the sodium current (I(Na)), which provides a rapid depolarizing current during the upstroke of the action potential. By controlling the resting membrane potential, I(K1) modifies sodium channel availability and therefore, cell excitability, action potential duration, and velocity of impulse propagation. Additionally, I(K1)-I(Na) interactions are key determinants of electrical rotor frequency responsible for abnormal, often lethal, cardiac reentrant activity. Here, we have used a multidisciplinary approach based on molecular and biochemical techniques, acute gene transfer or silencing, and electrophysiology to show that I(K1)-I(Na) interactions involve a reciprocal modulation of expression of their respective channel proteins (Kir2.1 and Na(V)1.5) within a macromolecular complex. Thus, an increase in functional expression of one channel reciprocally modulates the other to enhance cardiac excitability. The modulation is model-independent; it is demonstrable in myocytes isolated from mouse and rat hearts and with transgenic and adenoviral-mediated overexpression/silencing. We also show that the post synaptic density, discs large, and zonula occludens-1 (PDZ) domain protein SAP97 is a component of this macromolecular complex. We show that the interplay between Na(v)1.5 and Kir2.1 has electrophysiological consequences on the myocardium and that SAP97 may affect the integrity of this complex or the nature of Na(v)1.5-Kir2.1 interactions. The reciprocal modulation between Na(v)1.5 and Kir2.1 and the respective ionic currents should be important in the ability of the heart to undergo self-sustaining cardiac rhythm disturbances.

Developmental regulation of G protein-gated inwardly-rectifying K+ (GIRK/Kir3) channel subunits in the brain

G protein-gated inwardly-rectifying K(+) (GIRK/family 3 of inwardly-rectifying K(+) ) channels are coupled to neurotransmitter action and can play important roles in modulating neuronal excitability. We investigated the temporal and spatial expression of GIRK1, GIRK2 and GIRK3 subunits in the developing and adult brain of mice and rats using biochemical, immunohistochemical and immunoelectron microscopic techniques. At all ages analysed, the overall distribution patterns of GIRK1-3 were very similar, with high expression levels in the neocortex, cerebellum, hippocampus and thalamus. Focusing on the hippocampus, histoblotting and immunohistochemistry showed that GIRK1-3 protein levels increased with age, and this was accompanied by a shift in the subcellular localization of the subunits. Early in development (postnatal day 5), GIRK subunits were predominantly localized to the endoplasmic reticulum in the pyramidal cells, but by postnatal day 60 they were mostly found along the plasma membrane. During development, GIRK1 and GIRK2 were found primarily at postsynaptic sites, whereas GIRK3 was predominantly detected at presynaptic sites. In addition, GIRK1 and GIRK2 expression on the spine plasma membrane showed identical proximal-to-distal gradients that differed from GIRK3 distribution. Furthermore, although GIRK1 was never found within the postsynaptic density (PSD), the level of GIRK2 in the PSD progressively increased and GIRK3 did not change in the PSD during development. Together, these findings shed new light on the developmental regulation and subcellular diversity of neuronal GIRK channels, and support the contention that distinct subpopulations of GIRK channels exert separable influences on neuronal excitability. The ability to selectively target specific subpopulations of GIRK channels may prove effective in the treatment of disorders of excitability.

Altered stress stimulation of inward rectifier potassium channels in Andersen-Tawil syndrome

Inward rectifier potassium channels of the Kir2 subfamily are important determinants of the electrical activity of brain and muscle cells. Genetic mutations in Kir2.1 associate with Andersen-Tawil syndrome (ATS), a familial disorder leading to stress-triggered periodic paralysis and ventricular arrhythmia. To identify the molecular mechanisms of this stress trigger, we analyze Kir channel function and localization electrophysiologically and by time-resolved confocal microscopy. Furthermore, we employ a mathematical model of muscular membrane potential. We identify a novel corticoid signaling pathway that, when activated by glucocorticoids, leads to enrichment of Kir2 channels in the plasma membranes of mammalian cell lines and isolated cardiac and skeletal muscle cells. We further demonstrate that activation of this pathway can either partly restore (40% of cases) or further impair (20% of cases) the function of mutant ATS channels, depending on the particular Kir2.1 mutation. This means that glucocorticoid treatment might either alleviate or deteriorate symptoms of ATS depending on the patient's individual Kir2.1 genotype. Thus, our findings provide a possible explanation for the contradictory effects of glucocorticoid treatment on symptoms in patients with ATS and may open new pathways for the design of personalized medicines in ATS therapy.

Ion channelopathies in endocrinology: recent genetic findings and pathophysiological insights

Ion channels serve diverse cellular functions, mainly in cell signal transduction. In endocrine cells, these channels play a major role in hormonal secretion, Ca(2+)-mediated cell signaling, transepithelial transport, cell motility and growth, volume regulation and cellular ionic content and acidification of lysosomal compartments. Ion channel dysfunction can cause endocrine disorders or endocrine-related manifestations, such as pseudohypoaldosteronism type 1, Liddle syndrome, Bartter syndrome, persistent hyperinsulinemic hypoglycemia of infancy, neonatal diabetes mellitus, cystic fibrosis, Dent's disease, hypomagnesemia with secondary hypocalcemia, nephrogenic diabetes insipidus and, the most recently genetically identified channelopathy, thyrotoxic hypokalemic periodic paralysis. This review briefly recapitulates the membrane action potential in endocrine cells and offers a short overview of known endocrine channelopathies with focus on recent progress regarding the pathophysiological mechanisms and functional genetic defects.

Novel etiopathophysiological aspects of thyrotoxic periodic paralysis

Thyrotoxicosis can lead to thyrotoxic periodic paralysis (TPP), an endocrine channelopathy, and is the most common cause of acquired periodic paralysis. Typically, paralytic attacks cease when hyperthyroidism is abolished, and recur if hyperthyroidism returns. TPP is often underdiagnosed, as it has diverse periodicity, duration and intensity. The age at which patients develop TPP closely follows the age at which thyrotoxicosis occurs. All ethnicities can be affected, but TPP is most prevalent in people of Asian and, secondly, Latin American descent. TPP is characterized by hypokalemia, suppressed TSH levels and increased levels of thyroid hormones. Nonselective β adrenergic blockers, such as propranolol, are an efficient adjuvant to antithyroid drugs to prevent paralysis; however, an early and definitive treatment should always be pursued. Evidence indicates that TPP results from the combination of genetic susceptibility, thyrotoxicosis and environmental factors (such as a high-carbohydrate diet). We believe that excess T(3) modifies the insulin sensitivity of skeletal muscle and pancreatic β cells and thus alters potassium homeostasis, but only leads to a depolarization-induced acute loss of muscle excitability in patients with inherited ion channel mutations. An integrated etiopathophysiological model is proposed based on molecular findings and knowledge gained from long-term follow-up of patients with TPP.

A bicistronic, Ubiquitin-10 promoter-based vector cassette for transient transformation and functional analysis of membrane transport demonstrates the utility of quantitative voltage clamp studies on intact Arabidopsis root epidermis

To date the use of fluorescent reporter constructs in analysing membrane transport has been limited primarily to cell lines expressing stably either the tagged transporter protein(s) or markers to identify lineages of interest. Strategies for transient expression have yet to be exploited in transport analysis, despite their wide application in cellular imaging studies. Here we describe a Gateway-compatible, bicistronic vector, incorporating the constitutive Ubiqutin-10 gene promoter of Arabidopsis that gives prolonged expression after transient transformation and enables fluorescence marking of cells without a fusion construct. We show that Arabidopsis root epidermal cells are readily transformed by co-cultivation with Agrobacterium and are tractable for quantitative electrophysiological analysis. As a proof of principle, we transiently transformed Arabidopsis with the bicistronic vector carrying GFP as the fluorescent marker and, separately, the integral plasma membrane protein SYP121 essential for the inward K+ channel current. We demonstrate that transient expression of SYP121 in syp121 mutant plants is sufficient to rescue the K+ current in vivo. The combination of transient expression and use of the bicistronic vector promises significant advantages for studies of membrane transport and nutrient acquisition in roots.

Mechanism underlying selective regulation of G protein-gated inwardly rectifying potassium channels by the psychostimulant-sensitive sorting nexin 27

G protein-gated inwardly rectifying potassium (GIRK) channels are important gatekeepers of neuronal excitability. The surface expression of neuronal GIRK channels is regulated by the psychostimulant-sensitive sorting nexin 27 (SNX27) protein through a class I (-X-Ser/Thr-X-Φ, where X is any residue and Φ is a hydrophobic amino acid) PDZ-binding interaction. The G protein-insensitive inward rectifier channel (IRK1) contains the same class I PDZ-binding motif but associates with a different synaptic PDZ protein, postsynaptic density protein 95 (PSD95). The mechanism by which SNX27 and PSD95 discriminate these channels was previously unclear. Using high-resolution structures coupled with biochemical and functional analyses, we identified key amino acids upstream of the channel's canonical PDZ-binding motif that associate electrostatically with a unique structural pocket in the SNX27-PDZ domain. Changing specific charged residues in the channel's carboxyl terminus or in the PDZ domain converts the selective association and functional regulation by SNX27. Elucidation of this unique interaction site between ion channels and PDZ-containing proteins could provide a therapeutic target for treating brain diseases.

Müller cells upregulate the expression of SAP97 in light-injured rat retina

SAP97 is thought to play key roles in synapse assembly and synaptic plasticity. This study was carried out to determine whether it is involved in the Müller cell response to blue light injury. In light-injured rats, obvious intracellular edema in the outer retina was observed by transmission electron microscopy. The immunostaining of SAP97 was upregulated and concentrated in the Müller cell processes after photic injury, which was similar to the changes of AQP4 and the inwardly rectifying potassium channel, Kir4.1. Western blots showed that SAP97 and AQP4 protein levels were both increased on the third day after light exposure when compared with the control group (P<0.05). The upregulation of SAP97 coincides with the redistribution of AQP4 and Kir4.1 in blue light-injured rat retina.

Kir2.6 regulates the surface expression of Kir2.x inward rectifier potassium channels

Precise trafficking, localization, and activity of inward rectifier potassium Kir2 channels are important for shaping the electrical response of skeletal muscle. However, how coordinated trafficking occurs to target sites remains unclear. Kir2 channels are tetrameric assemblies of Kir2.x subunits. By immunocytochemistry we show that endogenous Kir2.1 and Kir2.2 are localized at the plasma membrane and T-tubules in rodent skeletal muscle. Recently, a new subunit, Kir2.6, present in human skeletal muscle, was identified as a gene in which mutations confer susceptibility to thyrotoxic hypokalemic periodic paralysis. Here we characterize the trafficking and interaction of wild type Kir2.6 with other Kir2.x in COS-1 cells and skeletal muscle in vivo. Immunocytochemical and electrophysiological data demonstrate that Kir2.6 is largely retained in the endoplasmic reticulum, despite high sequence identity with Kir2.2 and conserved endoplasmic reticulum and Golgi trafficking motifs shared with Kir2.1 and Kir2.2. We identify amino acids responsible for the trafficking differences of Kir2.6. Significantly, we show that Kir2.6 subunits can coassemble with Kir2.1 and Kir2.2 in vitro and in vivo. Notably, this interaction limits the surface expression of both Kir2.1 and Kir2.2. We provide evidence that Kir2.6 functions as a dominant negative, in which incorporation of Kir2.6 as a subunit in a Kir2 channel heterotetramer reduces the abundance of Kir2 channels on the plasma membrane.

The role of G proteins in assembly and function of Kir3 inwardly rectifying potassium channels

Kir3 channels (also known as GIRK channels) are important regulators of electrical excitability in both cardiomyocytes and neurons. Much is known regarding the assembly and function of these channels and the roles that their interacting proteins play in controlling these events. Further, they are one of the best studied effectors of heterotrimeric G proteins in general and Gβγ subunits in particular. However, our understanding of the roles of multiple Gβγ binding sites on Kir3 channels is still rudimentary. We discuss potential roles for Gβγ in channel assembly and trafficking in addition to their known role in cellular signaling.

Regulation of cardiac inward rectifier potassium current (I(K1)) by synapse-associated protein-97

Synapse-associated protein-97 (SAP97) is a membrane-associated guanylate kinase scaffolding protein expressed in cardiomyocytes. SAP97 has been shown to associate and modulate voltage-gated potassium (Kv) channel function. In contrast to Kv channels, little information is available on interactions involving SAP97 and inward rectifier potassium (Kir2.x) channels that underlie the classical inward rectifier current, I(K1). To investigate the functional effects of silencing SAP97 on I(K1) in adult rat ventricular myocytes, SAP97 was silenced using an adenoviral short hairpin RNA vector. Western blot analysis showed that SAP97 was silenced by approximately 85% on day 3 post-infection. Immunostaining showed that Kir2.1 and Kir2.2 co-localize with SAP97. Co-immunoprecipitation (co-IP) results demonstrated that Kir2.x channels associate with SAP97. Voltage clamp experiments showed that silencing SAP97 reduced I(K1) whole cell density by approximately 55%. I(K1) density at -100 mV was -1.45 +/- 0.15 pA/picofarads (n = 6) in SAP97-silenced cells as compared with -3.03 +/- 0.37 pA/picofarads (n = 5) in control cells. Unitary conductance properties of I(K1) were unaffected by SAP97 silencing. The major mechanism for the reduction of I(K1) density appears to be a decrease in Kir2.x channel abundance. Furthermore, SAP97 silencing impaired I(K1) regulation by beta(1)-adrenergic receptor (beta1-AR) stimulation. In control, isoproterenol reduced I(K1) amplitude by approximately 75%, an effect that was blunted following SAP97 silencing. Our co-IP data show that beta1-AR associates with SAP97 and Kir2.1 and also that Kir2.1 co-IPs with protein kinase A and beta1-AR. SAP97 immunolocalizes with protein kinase A and beta1-AR in the cardiac myocytes. Our results suggest that in cardiac myocytes SAP97 regulates surface expression of channels underlying I(K1), as well as assembles a signaling complex involved in beta1-AR regulation of I(K1).

Emerging roles for G protein-gated inwardly rectifying potassium (GIRK) channels in health and disease

G protein-gated inwardly rectifying potassium (GIRK) channels hyperpolarize neurons in response to activation of many different G protein-coupled receptors and thus control the excitability of neurons through GIRK-mediated self-inhibition, slow synaptic potentials and volume transmission. GIRK channel function and trafficking are highly dependent on the channel subunit composition. Pharmacological investigations of GIRK channels and studies in animal models suggest that GIRK activity has an important role in physiological responses, including pain perception and memory modulation. Moreover, abnormal GIRK function has been implicated in altering neuronal excitability and cell death, which may be important in the pathophysiology of diseases such as epilepsy, Down's syndrome, Parkinson's disease and drug addiction. GIRK channels may therefore prove to be a valuable new therapeutic target.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

SAP97 regulates Kir2.3 channels by multiple mechanisms

We examined the impact of coexpressing the inwardly rectifying potassium channel, Kir2.3, with the scaffolding protein, synapse-associated protein (SAP) 97, and determined that coexpression of these proteins caused an approximately twofold increase in current density. A combination of techniques was used to determine if the SAP97-induced increase in Kir2.3 whole cell currents resulted from changes in the number of channels in the cell membrane, unitary channel conductance, or channel open probability. In the absence of SAP97, Kir2.3 was found predominantly in a cytoplasmic, vesicular compartment with relatively little Kir2.3 localized to the plasma membrane. The introduction of SAP97 caused a redistribution of Kir2.3, leading to prominent colocalization of Kir2.3 and SAP97 and a modest increase in cell surface Kir2.3. The median Kir2.3 single channel conductance in the absence of SAP97 was approximately 13 pS, whereas coexpression of SAP97 led to a wide distribution of channel events with three distinct peaks centered at 16, 29, and 42 pS. These changes occurred without altering channel open probability, current rectification properties, or pH sensitivity. Thus association of Kir2.3 with SAP97 in HEK293 cells increased channel cell surface expression and unitary channel conductance. However, changes in single channel conductance play the major role in determining whole cell currents in this model system. We further suggest that the SAP97 effect results from SAP97 binding to the Kir2.3 COOH-terminal domain and altering channel conformation.

Subcellular compartment-specific molecular diversity of pre- and post-synaptic GABA-activated GIRK channels in Purkinje cells

Activation of G protein-gated inwardly-rectifying K(+) (GIRK or Kir3) channels by metabotropic gamma-aminobutyric acid (B) (GABA(B)) receptors is an essential signalling pathway controlling neuronal excitability and synaptic transmission in the brain. To investigate the relationship between GIRK channel subunits and GABA(B) receptors in cerebellar Purkinje cells at post- and pre-synaptic sites, we used biochemical, functional and immunohistochemical techniques. Co-immunoprecipitation analysis demonstrated that GIRK subunits are co-assembled with GABA(B) receptors in the cerebellum. Immunoelectron microscopy showed that the subunit composition of GIRK channels in Purkinje cell spines is compartment-dependent. Thus, at extrasynaptic sites GIRK channels are formed by GIRK1/GIRK2/GIRK3, post-synaptic densities contain GIRK2/GIRK3 and dendritic shafts contain GIRK1/GIRK3. The post-synaptic association of GIRK subunits with GABA(B) receptors in Purkinje cells is supported by the subcellular regulation of the ion channel and the receptor in mutant mice. At pre-synaptic sites, GIRK channels localized to parallel fibre terminals are formed by GIRK1/GIRK2/GIRK3 and co-localize with GABA(B) receptors. Consistent with this morphological evidence we demonstrate their functional interaction at axon terminals in the cerebellum by showing that GIRK channels play a role in the inhibition of glutamate release by GABA(B) receptors. The association of GIRK channels and GABA(B) receptors with excitatory synapses at both post- and pre-synaptic sites indicates their intimate involvement in the modulation of glutamatergic neurotransmission in the cerebellum.

Targeted tandem affinity purification of PSD-95 recovers core postsynaptic complexes and schizophrenia susceptibility proteins

The molecular complexity of mammalian proteomes demands new methods for mapping the organization of multiprotein complexes. Here, we combine mouse genetics and proteomics to characterize synapse protein complexes and interaction networks. New tandem affinity purification (TAP) tags were fused to the carboxyl terminus of PSD-95 using gene targeting in mice. Homozygous mice showed no detectable abnormalities in PSD-95 expression, subcellular localization or synaptic electrophysiological function. Analysis of multiprotein complexes purified under native conditions by mass spectrometry defined known and new interactors: 118 proteins comprising crucial functional components of synapses, including glutamate receptors, K⁺ channels, scaffolding and signaling proteins, were recovered. Network clustering of protein interactions generated five connected clusters, with two clusters containing all the major ionotropic glutamate receptors and one cluster with voltage-dependent K⁺ channels. Annotation of clusters with human disease associations revealed that multiple disorders map to the network, with a significant correlation of schizophrenia within the glutamate receptor clusters. This targeted TAP tagging strategy is generally applicable to mammalian proteomics and systems biology approaches to disease.

Charge-to-substrate ratio during organic cation uptake by rat OCT2 is voltage dependent and altered by exchange of glutamate 448 with glutamine

Uptake of substrate and electric charge was measured simultaneously in voltage-clamped Xenopus laevis oocytes expressing rat organic cation transporter 2 (rOCT2). At 0 mV, saturating substrate concentrations induced uptake of more positive elementary charges than monovalent organic cations, with charge-to-substrate ratios of 1.5 for guanidinium+, 3.5 for tetraethylammonium+, and 4.0 for 1-methyl-4-phenylpyridinium+. At negative holding potentials, the charge-to-substrate ratios decreased toward unity. At 0 mV, charge-to-substrate ratios higher than unity were observed at different extracellular pH and after replacement of extracellular Na+, K+, Ca2+, Mg2+, and/or Cl−. Charge-to-substrate ratios were not influenced by intracellular succinate2− or glutarate2−. The effects of membrane potential and ion substitution strongly suggest that the surplus of transported positive charge is not generated by passive ion permeabilities. Rather, we hypothetize that small cations are taken up together with organic cation substrates whereas the outward reorientation of the empty transporter is electroneutral. Nonselective cotransport of small cations was supported by the three-dimensional structures of rOCT2 in its inward-facing and outward-facing conformations, which we determined by homology modeling based on known corresponding structures of H+-lactose permease of E. coli, and by functional analysis of OCT mutants. In our model, the innermost cavity of the outward-open binding cleft is negatively charged by Glu448 and Asp475, whereas the inward-open innermost cavity is electroneutral, containing Asp379, Asp475, Lys215, and Arg440. Substitution of Glu448 by glutamine reduced the charge-to-TEA+ ratio at 0 mV to unity. The observed charge excess associated with organic cation uptake into depolarized cells may contribute to tubular damage in renal failure.

GPCR-Kir channel signaling complexes: defining rules of engagement

Ion channels and G protein-coupled receptors (GPCRs) are integral transmembrane proteins vital to a multitude of cell signaling and physiological functions. Members of these large protein families are known to interact directly with various intracellular protein partners in a dynamic and isoform-dependent manner, ultimately shaping their life cycle and signal output. The family of G protein-gated inwardly rectifying potassium channels (Kir3 or GIRK) expressed in brain, heart, and endocrine tissues were recently shown to stably associate with several different GPCRs, forming the basis of a macromolecular ion channel-GPCR signaling complex. The molecular determinants that mediate and maintain GPCR-Kir3 channel complexes are currently not well understood. Recent findings and emerging hypotheses on the assembly and stability of multiprotein GPCR-Kir channel signaling complexes are discussed, highlighting distinct mechanisms used by different Kir channel families. These protein-protein interaction processes are crucial in determining both the synaptic response times and the extent of GPCR "cross-talk" in Kir3-mediated inhibitory synaptic transmission.

Synaptic adhesion molecules and PSD-95

Synaptic adhesion molecules are known to participate in various steps of synapse development including initial contacts between dendrites and axons, formation of early synapses, and their maturation and plastic changes. Notably, a significant subset of synaptic adhesion molecules associates with synaptic scaffolding proteins, suggesting that they may act in concert to couple trans-synaptic adhesion to molecular organization of synaptic proteins. Here, we describe an emerging group of synaptic adhesion molecules that directly interact with the abundant postsynaptic scaffold PSD-95, which include neuroligins, NGLs, SALMs, and ADAM22, and discuss how these proteins and PSD-95 act together to regulate synaptic development. PSD-95 may be one of the central organizers of synaptic adhesion that recruits diverse proteins to sites of synaptic adhesion, promotes trans-synaptic signaling, and couples neuronal activity with changes in synaptic adhesion.

Small-angle X-ray scattering and NMR studies of the conformation of the PDZ region of SAP97 and its interactions with Kir2.1

The functional localization of potassium inward rectifiers is regulated by SAP97, a PDZ membrane-associated guanylate kinase protein. We describe here an investigation of the conformation of the PDZ domain region of SAP97 PDZ1-3. The NMR and SAXS data reveal conformational dynamics. The NMR data show minimal interdomain contacts, with the U3 linker region between PDZ2 and PDZ3 being largely unstructured. Shape analysis of the SAXS profiles revealed a dumbbell for the PDZ12 double domain. An overall elongated, asymmetric shape comprised of two to three distinct components characterizes the triple domain PDZ1-3. In addition, rigid body modeling shows that the representative average shape does not provide the full picture and that the data for the triple domain are consistent with large variations, suggesting significant conformational flexibility. However, the dynamics appears to be restricted as PDZ3 is located essentially within approximately 40 A from PDZ12. We also show that the Kir2.1 cytoplasmic domain interacts with all three PDZ domains but with a clear preference for PDZ2 even in the presence of the U3 region. We speculate that the restricted dynamics and preferential Kir2.1 binding to PDZ2 are features that enable SAP97 to function as a scaffold protein, allowing other proteins each to bind to the other two PDZ domains in sufficient proximity to yield productive channelosomes.

A specific role of AGS3 in the surface expression of plasma membrane proteins

Activator of G protein signaling 3 (AGS3), originally identified in a functional screen for mammalian proteins that activate heterotrimeric G protein signaling, is known to be involved in drug-seeking behavior and is up-regulated during cocaine withdrawal in animal models. These observations indicate a potential role for AGS3 in the formation or maintenance of neural plasticity. We have found that the overexpression of AGS3 alters the surface-to-total ratios of a subset of heterologously expressed plasma membrane receptors and channels. Further analysis of the endocytic trafficking of one such protein by a biotin-based internalization assay suggests that overexpression of AGS3 moderately affects the internalization or recycling of surface proteins. Moreover, AGS3 overexpression and siRNA-mediated knockdown of AGS3 both result in the dispersal of two endogenously expressed trans-Golgi network (TGN)-associated cargo proteins without influencing those in the cis- or medial-Golgi compartments. Finally, adding a TGN-localization signal to a CD4-derived reporter renders the trafficking of fusion protein sensitive to AGS3. Taken together, our data support a model wherein AGS3 modulates the protein trafficking along the TGN/plasma membrane/endosome loop.

A unique sorting nexin regulates trafficking of potassium channels via a PDZ domain interaction

G protein-gated potassium (Kir3) channels are important for controlling neuronal excitability in the brain. Using a proteomics approach, we have identified a unique rodent intracellular protein, sorting nexin 27 (SNX27), which regulates the trafficking of Kir3 channels. Like most sorting nexins, SNX27 possesses a functional PX domain that selectively binds the membrane phospholipid phosphatidylinositol-3-phosphate (PI3P) and is important for trafficking to the early endosome. SNX27, however, is the only sorting nexin to contain a PDZ domain. This PDZ domain discriminates between channels with similar class I PDZ-binding motifs, associating with the C-terminal end of Kir3.3 and Kir3.2c (-ESKV), but not with that of Kir2.1 (-ESEI) or Kv1.4 (-ETDV). SNX27 promotes the endosomal movement of Kir3 channels, leading to reduced surface expression, increased degradation and smaller Kir3 potassium currents. The regulation of endosomal trafficking via sorting nexins reveals a previously unknown mechanism for controlling potassium channel surface expression.

Analysis of a quantitative trait locus for seizure susceptibility in mice using bacterial artificial chromosome-mediated gene transfer

PURPOSE

Previous quantitative trait loci (QTL) mapping studies from our laboratory identified a 6.6 Mb segment of distal chromosome 1 that contains a gene (or genes) having a strong influence on the difference in seizure susceptibility between C57BL/6 (B6) and DBA/2 (D2) mice. A gene transfer strategy involving a bacterial artificial chromosome (BAC) DNA construct that contains several candidate genes from the critical interval was used to test the hypothesis that a strain-specific variation in one (or more) of the genes is responsible for the QTL effect.

METHODS

Fertilized oocytes from a seizure-sensitive congenic strain (B6.D2-Mtv7a/Ty-27d) were injected with BAC DNA and three independent founder lines of BAC-transgenic mice were generated. Seizure susceptibility was quantified by measuring maximal electroshock seizure threshold (MEST) in transgenic mice and nontransgenic littermates.

RESULTS

Seizure testing documented significant MEST elevation in all three transgenic lines compared to littermate controls. Allele-specific RT-PCR analysis confirmed gene transcription from genome-integrated BAC DNA and copy-number-dependent phenotypic effects were observed.

CONCLUSIONS

Results of this study suggest that the gene(s) responsible for the major chromosome 1 seizure QTL is found on BAC RPCI23-157J4 and demonstrate the utility of in vivo gene transfer for studying quantitative trait genes in mice. Further characterization of this transgenic model will provide new insight into mechanisms of seizure susceptibility.

NMR studies of interactions between C-terminal tail of Kir2.1 channel and PDZ1,2 domains of PSD95

Control of surface expression of inwardly rectifying potassium (Kir) channels is important for regulating membrane excitability. Kir2 channels have been shown to interact directly with PDZ-containing proteins in the postsynaptic density (PSD). These scaffold proteins, such as PSD95, bind to Kir2.1 channels via a PDZ-binding motif (T/S-x-Phi) in the C-terminal tail (SEI428). By utilizing a multidimensional solution NMR approach, we show that the previously unresolved structure of Kir2.1 tail (residues 372-428) is highly flexible. Using in vitro binding assays, we determined that shortening the flexible tail of Kir2.1 preceding the C-terminal region (residues 414-428) does not significantly disrupt PDZ binding. We also investigated which amino acids in the Kir2.1 tail associated with PSD95 PDZ1,2 by NMR spectroscopy, revealing that a stretch of 12 C-terminal amino acids is involved in interaction with both PDZ domains (residues 417-428). Deletion of the 11 amino acids preceding the C-terminal tail, Delta414-424, completely disrupts binding to PSD95 PDZ1,2. Therefore, the molecular interfaces formed between PDZ domains and Kir2.1 tail involve regions outside the previously identified binding motif (SEI428) and may be important for additional channel-specific interactions with associating PDZ-containing proteins.

Molecular organization and assembly of the postsynaptic density of excitatory brain synapses

The postsynaptic density (PSD) is a postsynaptic membrane specialization at excitatory synapses. The PSD is made of macromolecular multiprotein complexes, which contain a variety of synaptic proteins including membrane, scaffolding, and signaling proteins. By coaggregating with postsynaptic cell adhesion molecules, PSD proteins promote the formation and maturation of excitatory synapses. PSD proteins organize signaling pathways to coordinate structural and functional changes in synapses, and they regulate trafficking and recycling of glutamate receptors, which determines synaptic strength and plasticity. Synaptic activity dynamically regulates the assembly of the PSD through mechanisms including protein phosphorylation, palmitoylation, and protein degradation. PSD proteins associate with diverse motor proteins, suggesting that they function as adaptors linking motors to their specific cargoes.

Identification of gamma-aminobutyric acid receptor-interacting factor 1 (TRAK2) as a trafficking factor for the K+ channel Kir2.1

To identify proteins that regulate potassium channel activity and expression, we performed functional screening of mammalian cDNA libraries in yeast that express the mammalian K(+) channel Kir2.1. Growth of Kir2.1-expressing yeast in media with low K(+) concentration is a function of K(+) uptake via Kir2.1 channels. Therefore, the host strain was transformed with a human cDNA library, and cDNA clones that rescued growth at low K(+) concentration were selected. One of these clones was identical to the protein of unknown function isolated previously as gamma-aminobutyric acid receptor-interacting factor 1 (GRIF-1) (Beck, M., Brickley, K., Wilkinson, H., Sharma, S., Smith, M., Chazot, P., Pollard, S., and Stephenson, F. (2002) J. Biol. Chem. 277, 30079-30090). GRIF-1 specifically enhanced Kir2.1-dependent growth in yeast and Kir2.1-mediated (86)Rb(+) efflux in HEK293 cells. Quantitative microscopy and flow cytometry analysis of immunolabeled surface Kir2.1 channel showed that GRIF-1 significantly increased the number of Kir2.1 channels in the plasma membrane of COS and HEK293 cells. Physical interaction of Kir2.1 channel and GRIF-1 was demonstrated by co-immunoprecipitation from HEK293 lysates and yeast two-hybrid assay. In vivo association of Kir2.1 and GRIF-1 was demonstrated by co-immunoprecipitation from brain lysate. Yeast two-hybrid assays showed that an N-terminal region of GRIF-1 interacts with a C-terminal region of Kir2.1. These results indicate that GRIF-1 binds to Kir2.1 and facilitates trafficking of this channel to the cell surface.

Abnormal expression of the G-protein-activated inwardly rectifying potassium channel 2 (GIRK2) in hippocampus, frontal cortex, and substantia nigra of Ts65Dn mouse: a model of Down syndrome

Ts65Dn, a mouse model of Down syndrome (DS), demonstrates abnormal hippocampal synaptic plasticity and behavioral abnormalities related to spatial learning and memory. The molecular mechanisms leading to these impairments have not been identified. In this study, we focused on the G-protein-activated inwardly rectifying potassium channel 2 (GIRK2) gene that is highly expressed in the hippocampus region. We studied the expression pattern of GIRK subunits in Ts65Dn and found that GIRK2 was overexpressed in all analyzed Ts65Dn brain regions. Interestingly, elevated levels of GIRK2 protein in the Ts65Dn hippocampus and frontal cortex correlated with elevated levels of GIRK1 protein. This suggests that heteromeric GIRK1-GIRK2 channels are overexpressed in Ts65Dn hippocampus and frontal cortex, which could impair excitatory input and modulate spike frequency and synaptic kinetics in the affected regions. All GIRK2 splicing isoforms examined were expressed at higher levels in the Ts65Dn in comparison to the diploid hippocampus. The pattern of GIRK2 expression in the Ts65Dn mouse brain revealed by in situ hybridization and immunohistochemistry was similar to that previously reported in the rodent brain. However, in the Ts65Dn mouse a strong immunofluorescent staining of GIRK2 was detected in the lacunosum molecular layer of the CA3 area of the hippocampus. In addition, tyrosine hydroxylase containing dopaminergic neurons that coexpress GIRK2 were more numerous in the substantia nigra compacta and ventral tegmental area in the Ts65Dn compared to diploid controls. In summary, the regional localization and the increased brain levels coupled with known function of the GIRK channel may suggest an important contribution of GIRK2 containing channels to Ts65Dn and thus to DS neurophysiological phenotypes.