Guido A. Zampighi

A method for determining the unitary functional capacity of cloned channels and transporters expressed in Xenopus laevis oocytes

The Xenopus laevis oocyte is widely used to express exogenous channels and transporters and is well suited for functional measurements including currents, electrolyte and nonelectrolyte fluxes, water permeability and even enzymatic activity. It is difficult, however, to transform functional measurements recorded in whole oocytes into the capacity of a single channel or transporter because their number often cannot be estimated accurately. We describe here a method of estimating the number of exogenously expressed channels and transporters inserted in the plasma membrane of oocytes. The method is based on the facts that the P (protoplasmic) face in water-injected control oocytes exhibit an extremely low density of endogenous particles (212±48 particles/μm2, mean, sd) and that exogenously expressed channels and transporters increased the density of particles (up to 5,000/μm2) only on the P face. The utility and generality of the method were demonstrated by estimating the “gating charge” per particle of the Na+/ glucose cotransporter (SGLT1) and a nonconducting mutant of the Shaker K+ channel proteins, and the single molecule water permeability of CHIP (Channel-like Intramembrane Protein) and MIP (Major Intrinsic Protein). We estimated a “gating charge” of ∼3.5 electronic charges for SGLT1 and ∼9 for the mutant Shaker K+ channel from the ratio of Qmax to density of particles measured on the same oocytes. The “gating charges” were 3-fold larger than the “effective valences” calculated by fitting a Boltzmann equation to the same charge transfer data suggesting that the charge movement in the channel and cotransporter occur in several steps. Single molecule water permeabilities (pfs) of 1.4 × 10−14 cm3/ sec for CHIP and of 1.5 × 10−16 cm3/sec for MIP were estimated from the ratio of the whole-oocyte water permeability (Pf) to the density of particles. Therefore, MIP is a water transporter in oocytes, albeit ∼100-fold less effective than CHIP.

Comparison of the Water Transporting Properties of MIP and AQP1

AbstractIn this paper we compare the water-transport properties of Aquaporin (AQP1), a known water channel, and those of the 28 kD Major Intrinsic Protein of Lens (MIP), a protein with an undefined physiological role. To make the comparison as direct as possible we measured functional properties in Xenopus laevis oocytes injected with cRNAs coding for the appropriate protein. We measured the osmotic permeability, Pf, (using rate of swelling) and the surface density of plasma membrane proteins (using freeze-fracture electron microscopy) in the same oocytes. Knowing both Pf and the number of exogenously expressed proteins in the membrane, we estimated the single-molecule permeability to be 2.8 × 10−16 cm3/sec for MIP and 1.2 × 10−14 cm3/sec for AQP1. As a negative control, a mutant MIP, truncated at the carboxyl-terminal, was shown by western blotting to be expressed, but this protein resulted in no increase in either water permeability or particle density. (Interestingly, the truncated protein was glycosylated, while the complete MIP transcript was not.) Water transport by MIP had a higher activation energy (∼7 Kcal/mole) than water transport by AQP1 (∼2.5 Kcal/Mole) but a substantially lower activation energy than water flux across bare oolemma (∼20 Kcal/mole). Though the water-transport properties of MIP and AQP1 differ quantitatively, they are qualitatively quite similar. We conclude that MIP, like AQP1, forms water channels when expressed in oocytes. Thus water transport in the lens seems a plausible physiological role for MIP.

Inhibition of gap junction hemichannels by chloride channel blockers

Electrophysiological methods were used to assess the effect of chloride-channel blockers on the macroscopic and microscopic currents of mouse connexin50 (Cx50) and rat connexin46 (Cx46) hemichannels expressed in Xenopus laevis oocytes. Oocytes were voltage-clamped at -50 mV and hemichannel currents (ICx50 or ICx46) were activated by lowering the extracellular Ca2+ concentration ([Ca2+]o) from 5 mM to 10 microM. Ion-replacement experiments suggested that ICx50 is carried primarily (>95%) by monovalent cations (PK : PNa : PCl = 1.0 : 0.74 : 0.05). ICx50 was inhibited by 18beta-glycyrrhetinic acid (apparent Ki, 2 microM), gadolinium (3 microM), flufenamic acid (3 microM), niflumic acid (11 microM), NPPB (15 microM), diphenyl-2-carboxylate (26 microM), and octanol (177 microM). With the exception of octanol, niflumic acid, and diphenyl-2-carboxylate, the above agents also inhibited ICx46. Anthracene-9-carboxylate, furosemide, DIDS, SITS, IAA-94, and tamoxifen had no inhibitory effect on either ICx50 or ICx46. The kinetics of ICx50 inhibition were not altered at widely different [Ca2+]o (10-500 microM), suggesting that drug-hemichannel interaction does not involve the Ca2+ binding site. In excised membrane patches, application of flufenamic acid or octanol to the extracellular surface of Cx50 hemichannels reduced single channel-open probability without altering the single-channel conductance, but application to the cytoplasmic surface had no effect on the channels. We conclude that some chloride-channel blockers inhibit lens-connexin hemichannels by acting on a site accessible only from the extracellular space, and that drug-hemichannel interaction involves a high-affinity site other than the Ca2+ binding site.

Two forms of isolated gap junctions

Abstract Gap junctions, containing regular hexagonal arrays of connexons, have been isolated from rat liver. The projected structures of these gap junctions have been studied to a resolution of 18 A by electron microscopy of negatively stained samples. Two closely related forms of junction were produced that have different structures for the connexon, but the same hexagonal lattice constant. In one form the connexon is seen as a weakly contrasted annulus, which is broadest at the locations where other connexons come closest; in the other, the connexon is seen as a strongly contrasted annulus, which is broadest midway between the locations where other connexons come closest. The forms appear to reflect two configurations of the connexon subunits.

Molecular characterization of Vibrio parahaemolyticus vSGLT: a model for sodium-coupled sugar cotransporters.

The Na+/galactose cotransporter (vSGLT) of Vibrio parahaemolyticus, tagged with C-terminal hexahistidine, has been purified to apparent homogeneity by Ni2+ affinity chromatography and gel filtration. Resequencing the vSGLT gene identified an important correction: the N terminus constitutes an additional 13 functionally essential residues. The mass of His-tagged vSGLT expressed under its native promoter, as determined by electrospray ionization-mass spectrometry (ESI-MS), verifies these 13 residues in wild-type vSGLT. A fusion protein of vSGLT and green fluorescent protein, comprising a mass of over 90 kDa, was also successfully analyzed by ESI-MS. Reconstitution of purified vSGLT yields proteoliposomes active in Na+-dependent galactose uptake, with sugar preferences (galactose > glucose > fucose) reflecting those of wild-type vSGLT in vivo. Substrates are transported with apparent 1:1 stoichiometry and apparent K m values of 129 mm (Na+) and 158 μm(galactose). Freeze-fracture electron microscopy of functional proteoliposomes shows intramembrane particles of a size consistent with vSGLT existing as a monomer. We conclude that vSGLT is a suitable model for the study of sugar cotransporter mechanisms and structure, with potential applicability to the larger SGLT family of important sodium:solute cotransporters. It is further demonstrated that ESI-MS is a powerful tool for the study of proteomics of membrane transporters.

Phosphorylation modulates the voltage dependence of channels reconstituted from the major intrinsic protein of lens fiber membranes

SummaryMajor intrinsic polypeptide (MIP), a 28-kDa protein isolated from lens fiber cell membranes, forms large, nonselective channels when reconstituted into lipid bilayers. MIP channels are regulated by voltage, such that these channels close when the potential across the membrane is greater than 30 mV. We have investigated the modulation of the voltage-dependent closure of MIP channels by phosphorylation. In this report, we describe the isolation of two isomers of MIP from lens fiber cell membranes. These isomers differ by a single phosphate at a protein kinase A phosphorylation site. The phosphorylated isomer produces channels that close in response to applied voltages when reconstituted into bilayers. The nonphosphorylated isomer produces voltage-independent hannels. Direct phosphorylation with protein kinase A converts voltage-independent channels to voltage-dependent channels in situ. Analyses of macroscopic and single channel currents suggest that phosphorylation increases the voltage-dependent closure of MIP channels by increasing closed channel lifetimes and the rate of channel closure following the application of voltage.

The specialized junctions of the lens.

Publisher Summary This chapter presents a comparison between the structure and chemical composition of different types of junctions in the lens. It also distinguishes between the possible functional roles of these junctions in ion transport and the maintenance of lens-fluid balance. Lens is a transparent and moderately deformable organ whose principal function is to focus light on the retina. Electrolytes and water move throughout the lens, either by crossing plasma membranes of lenticular cells (transcellular pathway) or by diffusing through the extracellular clefts between cells (paracellular pathway). Cells in the lens are connected by four types of plasma membrane contacts: (1) tight junctions, (2) gap junctions, (3) 18–20 nm junctions, and (4) 11–13 nm junctions. These junctions mediate and control different pathways for the diffusion of ions and small nonelectrolytes within and out of the lens and play fundamental roles in lens-fluid balance and transparency. The most important plasma membrane specialization regulating the permeability of the paracellular pathway to ions and nonelectrolytes is the tight junction. In the lens, tight junctions are found only in the epithelium. Such an asymmetric localization of tight junctions transforms the extracellular clefts in the posterior pole into the pathway of least resistance to the diffusion of molecules within the lens.

Nerve Growth Factor Signals via Preexisting TrkA Receptor Oligomers

Nerve growth factor (NGF) promotes neuronal survival and differentiation by activating TrkA receptors. Similar to other receptor tyrosine kinases, ligand-induced dimerization is thought to be required for TrkA receptor activation. To study this process, we expressed TrkA receptors in Xenopus laevis oocytes and analyzed their response to NGF by using a combination of functional, biochemical, and structural approaches. TrkA receptor protein was detected in the membrane fraction of oocytes injected with TrkA receptor cRNA, but not in uninjected or mock-injected oocytes. Application of NGF to TrkA receptor-expressing oocytes promoted tyrosine phosphorylation and activated an oscillating transmembrane inward current, indicating that the TrkA receptors were functional. Freeze-fracture electron microscopic analysis demonstrated novel transmembrane particles in the P-face (protoplasmic face) of oocytes injected with TrkA cRNA, but not in uninjected or mock injected oocytes. Incubating TrkA cRNA-injected oocytes with the transcriptional inhibitor actinomycin D did not prevent the appearance of these P-face particles or electrophysiological responses to NGF, demonstrating that they did not arise from de novo transcription of an endogenous Xenopus oocyte gene. The appearance of these particles in the plasma membrane correlated with responsiveness to NGF as detected by electrophysiological analysis and receptor phosphorylation, indicating that these novel P-face particles were TrkA receptors. The dimensions of these particles (8.6 x 10 nm) were too large to be accounted for by TrkA monomers, suggesting the formation of TrkA receptor oligomers. Application of NGF did not lead to a discernible change in the size or shape of these TrkA receptor particles during an active response. These results indicate that in Xenopus oocytes, NGF activates signaling via pre-formed TrkA receptor oligomers.

Conical Tomography of a Ribbon Synapse: Structural Evidence for Vesicle Fusion

To characterize the sites of synaptic vesicle fusion in photoreceptors, we evaluated the three-dimensional structure of rod spherules from mice exposed to steady bright light or dark-adapted for periods ranging from 3 to 180 minutes using conical electron tomography. Conical tilt series from mice retinas were reconstructed using the weighted back projection algorithm, refined by projection matching and analyzed using semiautomatic density segmentation. In the light, rod spherules contained ∼470 vesicles that were hemi-fused and ∼187 vesicles that were fully fused (omega figures) with the plasma membrane. Active zones, defined by the presence of fully fused vesicles, extended along the entire area of contact between the rod spherule and the horizontal cell ending, and included the base of the ribbon, the slope of the synaptic ridge and ribbon-free regions apposed to horizontal cell axonal endings. There were transient changes of the rod spherules during dark adaptation. At early periods in the dark (3–15 minutes), there was a) an increase in the number of fully fused synaptic vesicles, b) a decrease in rod spherule volume, and c) an increase in the surface area of the contact between the rod spherule and horizontal cell endings. These changes partially compensate for the increase in the rod spherule plasma membrane following vesicle fusion. After 30 minutes of dark-adaptation, the rod spherules returned to dimensions similar to those measured in the light. These findings show that vesicle fusion occurs at both ribbon-associated and ribbon-free regions, and that transient changes in rod spherules and horizontal cell endings occur shortly after dark onset.

TIGHT JUNCTIONS AND THE EXPERIMENTAL MODIFICATIONS OF LIPID CONTENT

Abstract. Tight junctions (TJs) are cell-to-cell contacts made of strands, which appear as ridges on P faces and complementary furrows on E faces on freeze fracture replicas. Evidences and opinions on whether these strands are composed of either membrane-bound proteins or lipid micelles are somewhat varied. In the present work we alter the lipid composition of Madin-Darby canine kidney monolayers using a novel approach, while studying (i) their transepithelial electrical resistance, a parameter that depends on the degree of sealing of the TJs; (ii) the apical-to-basolateral flux of 4 kD fluorescent dextran (JDEX), that reflects the permeability of the intercellular spaces; (iii) the ability of TJs to restrict apical-to-basolateral diffusion of membrane lipids; and (iv) the pattern of distribution of endogenous and transfected occludin, the sole membrane protein presently known to form part of the TJs. We show that changing the total composition of phospholipids, sphingolipids, cholesterol and the content of fatty acids, does not alter TER nor the structure of the strands. Interestingly, enrichment with linoleic acid increases the JDEX by 631%. The fact that this increase is not reflected in a decrease of TER, suggests that junctional strands do not act as simple resistive elements but may contain mobile translocating mechanisms.