ClC-3 chloride channels facilitate endosomal acidification and chloride accumulation

We investigated the involvement of ClC-3 chloride channels in endosomal acidification by measurement of endosomal pH and chloride concentration [Cl-] in control versus ClC-3-deficient hepatocytes and in control versus ClC-3-transfected Chinese hamster ovary cells. Endosomes were labeled with pH or [Cl-]-sensing fluorescent transferrin (Tf), which targets to early/recycling endosomes, or alpha2-macroglobulin (alpha2M), which targets to late endosomes. In pulse label-chase experiments, [Cl-] was 19 mM just after internalization in alpha2M-labeled endosomes in primary cultures of hepatocytes from wild-type mice, increasing to 58 mM over 45 min, whereas pH decreased from 7.1 to 5.4. Endosomal acidification and [Cl-] accumulation were significantly impaired in hepatocytes from ClC-3 knock-out mice, with [Cl-] increasing from 16 to 43 mM and pH decreasing from 7.1 to 6.0. Acidification and Cl- accumulation were blocked by bafilomycin. In Tf-labeled endosomes, [Cl-] was 46 mM in wild-type versus 35 mM in ClC-3-deficient hepatocytes at 15 min after internalization, with corresponding pH of 6.1 versus 6.5. Approximately 4-fold increased Cl- conductance was found in alpha2M-labeled endosomes isolated from hepatocytes of wild-type versus ClC-3 null mice. In contrast, Golgi acidification was not impaired in ClC-3-deficient hepatocytes. In transfected Chinese hamster ovary cells expressing ClC-3A, endosomal acidification and [Cl-] accumulation were enhanced. [Cl-] in alpha2M-labeled endosomes was 42 mM (control) versus 53 mM (ClC-3A) at 45 min, with corresponding pH 5.8 versus 5.2; [Cl-] in Tf-labeled endosomes at 15 min was 37 mM (control) versus 49 mM (ClC-3A) with pH 6.3 versus 5.9. Our results provide direct evidence for involvement of ClC-3 in endosomal acidification by Cl- shunting of the interior-positive membrane potential created by the vacuolar H+ pump.     

Intra-endosomal pH-sensitive recruitment of the Arf-nucleotide exchange factor ARNO and Arf6 from cytoplasm to proximal tubule endosomes

Kidney proximal tubule epithelial cells have an extensive apical endocytotic apparatus that is critical for the reabsorption and degradation of proteins that traverse the glomerular filtration barrier and that is also involved in the extensive recycling of functionally important apical plasma membrane transporters. We show here that an Arf-nucleotide exchange factor, ARNO (ADP-ribosylation factor nucleotide site opener) as well as Arf6 and Arf1 small GTPases are located in the kidney proximal tubule receptor-mediated endocytosis pathway, and that ARNO and Arf6 recruitment from cytosol to endosomes is pH-dependent. In proximal tubules in situ, ARNO and Arf6 partially co-localized with the V-ATPase in apical endosomes in proximal tubules. Arf1 was localized both at the apical pole of proximal tubule epithelial cells, but also in the Golgi. By Western blot analysis ARNO, Arf6, and Arf1 were detected both in purified endosomes and in proximal tubule cytosol. A translocation assay showed that ATP-driven endosomal acidification triggered the recruitment of ARNO and Arf6 from proximal tubule cytosol to endosomal membranes. The translocation of both ARNO and Arf6 was reversed by V-type ATPase inhibitors and by uncouplers of endosomal intralumenal pH, and was correlated with the magnitude of intra-endosomal acidification. Our data suggest that V-type ATPase-dependent acidification stimulates the selective recruitment of ARNO and Arf6 to proximal tubule early endosomes. This mechanism may play an important role in the pH-dependent regulation of receptor-mediated endocytosis in proximal tubules in situ.     

Determinants of [Cl-] in recycling and late endosomes and Golgi complex measured using fluorescent ligands

Chloride concentration ([Cl-]) was measured in defined organellar compartments using fluorescently labeled transferrin, alpha2-macroglobulin, and cholera toxin B-subunit conjugated with Cl--sensitive and -insensitive dyes. In pulse-chase experiments, [Cl-] in Tf-labeled early/recycling endosomes in J774 cells was 20 mM just after internalization, increasing to 41 mM over approximately 10 min in parallel to a drop in pH from 6.91 to 6.05. The low [Cl-] just after internalization (compared with 137 mM solution [Cl-]) was prevented by reducing the interior-negative Donnan potential. [Cl-] in alpha2-macroglobulin-labeled endosomes, which enter a late compartment, increased from 28 to 58 mM at 1-45 min after internalization, whereas pH decreased from 6.85 to 5.20. Cl- accumulation was prevented by bafilomycin but restored by valinomycin. A Cl- channel inhibitor slowed endosomal acidification and Cl- accumulation by approximately 2.5-fold. [Cl-] was 49 mM and pH was 6.42 in cholera toxin B subunit-labeled Golgi complex in Vero cells; Golgi compartment Cl- accumulation and acidification were reversed by bafilomycin. Our experiments provide evidence that Cl- is the principal counter ion accompanying endosomal and Golgi compartment acidification, and that an interior-negative Donnan potential is responsible for low endosomal [Cl-] early after internalization. We propose that reduced [Cl-] and volume in early endosomes permits endosomal acidification and [Cl-] accumulation without lysis.     

ClC-5: a chloride channel with multiple roles in renal tubular albumin uptake

ClC-5 is a chloride (Cl(-)) channel expressed in renal tubules and is critical for normal tubular function. Loss of function nonsense or missense mutations in ClC-5 are associated with Dent's disease, a condition in which patients present with low molecular weight (LMW) proteinuria (including albuminuria), hypercalciuria and nephrolithiasis. Several key studies in ClC-5 knockout mice have shown that the proteinuria results from defective tubular reabsorption of proteins. ClC-5 is typically regarded as an intracellular Cl(-) channel and thus the defect in this receptor-mediated uptake pathway was initially attributed to the failure of the early endosomes to acidify correctly. ClC-5 was postulated to play a key role in transporting the Cl(-) ions required to compensate for the movement of H(+) during endosomal acidification. However, more recent studies suggest additional roles for ClC-5 in the endocytosis of albumin. ClC-5 is now known to be expressed at low levels at the cell surface and appears to be a key component in the assembly of the macromolecular complex involved in protein endocytosis. Furthermore, mutations in ClC-5 affect the trafficking of v-H(+)-ATPase and result in decreased expression of the albumin receptor megalin/cubulin. Thus, the expression of ClC-5 at the cell surface as well as its presence in endosomes appears to be essential for normal protein uptake by the renal proximal tubule.     

大鼠肾近球小管细胞胞饮体膜氢泵活性和水的渗透性转运的特征

本实验采用异硫氰酸-葡聚糖荧光素(fluorescein isothiocyanate-dextran,FITC-dextran)体内标记法,研究大鼠肾近球小管细胞胞饮体(endosome)膜上 H~+-ATP 酶的活性及水的渗透性转运。通过观察在胞饮体外加入一定量 ATP 后,胞饮体内 pH 值的时间反应曲线,从而测定 ATP-依赖的 H~+在胞饮体膜上的转运情况。胞饮体内的酸化速度及 pH 的最低值与加入的 ATP 浓度有关。在加入 ATP 前,胞饮体内的 pH 值为7.4,加入不同浓度的 ATP 后,即[ATP]为0.005,0.05,0.5,5和10mmol/L,胞饮体内 pH 最低值分别为7.30,6.99,6.68,6.38和6.39。此种由 ATP 引起的酸化反应,被0.5mmol/L N-ethylmaleimide(NEM)抑制97%,但不被钒酸盐和 oligomycin 所抑制。实验还同时观察了此种胞饮体水的渗透性转运机制。通过在胞饮体膜内外建立一个蔗糖浓度梯度。观察 FITC-dextran 荧光信号的快速动力学变化过程,从而测定由于渗透压梯度引起的水在胞饮体膜上转运的特征。在230℃时,水的渗透性通透系数(osmotic water permeability coefficient,P_f)为0.03cm/s;加入0.5mmol/L HgCl_2后,水的转运被抑制70%。此抑制反应可被5mmol/L 巯基乙醇(β-Mcrcaptoethanol)完全逆转。上述结果提示:大鼠肾近球小管胞饮体膜含有H~+-ATP 酶和水的转运通道。胞饮     

Acidification of endosome subpopulations in wild-type Chinese hamster ovary cells and temperature-sensitive acidification-defective mutants

During endocytosis in Chinese hamster ovary (CHO) cells, Semliki Forest virus (SFV) passes through two distinct subpopulations of endosomes before reaching lysosomes. One subpopulation, defined by cell fractionation using free flow electrophoresis as "early endosomes," constitutes the major site of membrane and receptor recycling; while "late endosomes," an electrophoretically distinct endosome subpopulation, are involved in the delivery of endosomal content to lysosomes. In this paper, the pH-sensitive conformational changes of the SFV E1 spike glycoprotein were used to study the acidification of these defined endosome subpopulations in intact wild-type and acidification-defective CHO cells. Different virus strains were used to measure the kinetics at which internalized SFV was delivered to endosomes of pH less than or equal to 6.2 (the pH at which wild-type E1 becomes resistant to trypsin digestion) vs. endosomes of pH less than or equal to 5.3 (the threshold pH for E1 of the SFV mutant fus-1). By correlating the kinetics of acquisition of E1 trypsin resistance with the transfer of SFV among distinct endosome subpopulations defined by cell fractionation, we found that after a brief residence in vesicles of relatively neutral pH, internalized virus encountered pH less than or equal to 6.2 in early endosomes with a t1/2 of 5 min. Although a fraction of the virus reached a pH of less than or equal to 5.3 in early endosomes, most fus-1 SFV did not exhibit the acid-induced conformational change until arrival in late endosomes (t1/2 = 8-10 min). Thus, acidification of both endosome subpopulations was heterogeneous. However, passage of SFV through a less acidic early endosome subpopulation always preceded arrival in the more acidic late endosome subpopulation. In mutant CHO cells with temperature-sensitive defects in endosome acidification in vitro, acidification of both early and late endosomes was found to be impaired at the restrictive temperature (41 degrees C). The acidification defect was also found to be partially penetrant at the permissive temperature, resulting in the inability of any early endosomes in these cells to attain pH less than or equal to 5.3. In vitro studies of endosomes isolated from mutant cells suggested that the acidification defect is most likely in the proton pump itself. In one mutant, this defect resulted in increased sensitivity of the electrogenic H+ pump to fluctuations in the endosomal membrane potential.     

The chloride channel ClC-4 contributes to endosomal acidification and trafficking

Mutations in the gene coding for the chloride channel ClC-5 cause Dent's disease, a disease associated with proteinuria and renal stones. Studies in ClC-5 knockout mice suggest that this phenotype is related to defective endocytosis of low molecular weight proteins and membrane proteins by the renal proximal tubule. In this study, confocal micrographs of proximal tubules and cultured epithelial cells revealed that the related protein ClC-4 is expressed in endosomal membranes suggesting that this channel may also contribute to the function of this organelle. In support of this hypothesis, specific disruption of endogenous ClC-4 expression by transfection of ClC-4 antisense cDNA acidified endosomal pH and altered transferrin trafficking in cultured epithelial cells to the same extent as the specific disruption of ClC-5. Both channels can be co-immunoprecipitated, arguing that they may partially contribute to endosomal function as a channel complex. These studies prompt future investigation of the role of ClC-4 in renal function in health and in Dent's disease. Future studies will assess whether the severity of Dent's disease relates not only to the impact of particular mutations on ClC-5 but also on the consequences of those mutations on the functional expression of ClC-4.     

Intracellular regulation of human ClC‐5 by adenine nucleotides

ClC-5, an endosomal Cl⁻/H⁺ antiporter that is mutated in Dent disease, is essential for endosomal acidification and re-uptake of small molecular weight proteins in the renal proximal tubule. Eukaryotic chloride channels (CLCs) contain two cytoplasmic CBS domains, motifs present in different proteins, the function of which is still poorly understood. Structural studies have shown that ClC-5 can bind to ATP at the interface between the CBS domains, but so far the potential functional consequences of nucleotide binding to ClC-5 have not been investigated. Here, we show that the direct application of ATP, ADP and AMP in inside-out patch experiments potentiates the current mediated by ClC-5 with similar affinities. The nucleotides increase the probability of ClC-5 to be in an active, transporting state. The residues Tyr 617 and Asp 727, but not Ser 618, are crucial for the potentiation. These results provide a mechanistic and structural framework for the interpretation of nucleotide regulation of a CLC transporter.     

Theoretical considerations on the role of membrane potential in the regulation of endosomal pH.

Na+,K(+)-ATPase has been observed to partially inhibit acidification of early endosomes by increasing membrane potential, whereas chloride channels have been observed to enhance acidification in endosomes and lysosomes. However, little theoretical analysis of the ways in which different pumps and channels may interact has been carried out. We therefore developed quantitative models of endosomal pH regulation based on thermodynamic considerations. We conclude that 1) both size and shape of endosomes will influence steady-state endosomal pH whenever membrane potential due to the pH gradient limits proton pumping, 2) steady-state pH values similar to those observed in early endosomes of living cells can occur in endosomes containing just H(+)-ATPases and Na+,K(+)-ATPases when low endosomal buffering capacities are present, and 3) inclusion of active chloride channels results in predicted pH values well below those observed in vivo. The results support the separation of endocytic compartments into two classes, those (such as early endosomes) whose acidification is limited by attainment of a certain membrane potential, and those (such as lysosomes) whose acidification is limited by the attainment of a certain pH. The theoretical framework and conclusions described are potentially applicable to other membrane-enclosed compartments that are acidified, such as elements of the Golgi apparatus.     

A cation counterflux supports lysosomal acidification

The profound luminal acidification essential for the degradative function of lysosomes requires a counter-ion flux to dissipate an opposing voltage that would prohibit proton accumulation. It has generally been assumed that a parallel anion influx is the main or only counter-ion transport that enables acidification. Indeed, defective anion conductance has been suggested as the mechanism underlying attenuated lysosome acidification in cells deficient in CFTR or ClC-7. To assess the individual contribution of counter-ions to acidification, we devised means of reversibly and separately permeabilizing the plasma and lysosomal membranes to dialyze the cytosol and lysosome lumen in intact cells, while ratiometrically monitoring lysosomal pH. Replacement of cytosolic Cl⁻ with impermeant anions did not significantly alter proton pumping, while the presence of permeant cations in the lysosomal lumen supported acidification. Accordingly, the lysosomes were found to acidify to the same pH in both CFTR- and ClC-7–deficient cells. We conclude that cations, in addition to chloride, can support lysosomal acidification and defects in lysosomal anion conductance cannot explain the impaired microbicidal capacity of CF phagocytes.     

Heterogeneity in ATP-dependent acidification in endocytic vesicles from kidney proximal tubule. Measurement of pH in individual endocytic vesicles in a cell-free system.

Measurement of membrane transport in suspensions of isolated membrane vesicles provides averaged information over a potentially very heterogeneous vesicle population. To examine the regulatory mechanisms for ATP-dependent acidification, methodology was developed to measure pH in individual endocytic vesicles. Endocytic vesicles from proximal tubule apical membrane of rat kidney were labeled in vivo by intravenous infusion of FITC-dextran (9 kD); a microsomal fraction was obtained from dissected renal cortex by homogenization and differential centrifugation. Vesicles were immobilized on a polylysine coated coverglass and imaged at high magnification by a silicon intensified target camera. ATP-dependent acidification was not influenced by endosome immobilization. Endosome pH was determined from the integrated fluorescence intensity of individual labeled vesicles after background subtraction. Calibration studies with high K and nigericin showed nearly identical fluorescence vs. pH curves for different endosomes with a standard deviation for a single pH measurement in a single endosome of approximately 0.2 pH units. In response to addition of 1 mM MgATP in the presence of K and valinomycin, endosome pH decreased from 7.2 to a mean of 6.4 with a unimodal distribution with width at half-maximum of approximately 1 pH unit. The drop in endosome pH increased and the shape of the distribution changed when the time between FITC-dextran infusion and kidney removal was increased from 5 to 20 min. Differences in ATP-dependent acidification could not be attributed to heterogeneity in passive proton conductance. These results establish a direct method to measure pH in single endocytic vesicles and demonstrate remarkable heterogeneity in ATP-dependent acidification which was interpreted in terms of heterogeneity in the number and/or activity of proton pumps at serial stages of endocytosis.     

On the mechanism of gating charge movement of ClC-5, a human Cl(-)/H(+) antiporter

ClC-5 is a Cl(-)/H(+) antiporter that functions in endosomes and is important for endocytosis in the proximal tubule. The mechanism of transport coupling and voltage dependence in ClC-5 is unclear. Recently, a transport-deficient ClC-5 mutant (E268A) was shown to exhibit transient capacitive currents. Here, we studied the external and internal Cl(-) and pH dependence of the currents of E268A. Transient currents were almost completely independent of the intracellular pH. Even though the transient currents are modulated by extracellular pH, we could exclude that they are generated by proton-binding/unbinding reactions. In contrast, the charge movement showed a nontrivial dependence on external chloride, strongly supporting a model in which the movement of an intrinsic gating charge is followed by the voltage-dependent low-affinity binding of extracellular chloride ions. Mutation of the external Glu-211 (a residue implicated in the coupling of Cl(-) and proton transport) to aspartate abolished steady-state transport, but revealed transient currents that were shifted by ~150 mV to negative voltages compared to E268A. This identifies Glu(ext) as a major component of the gating charge underlying the transient currents of the electrogenic ClC-5 transporter. The molecular events underlying the transient currents of ClC-5 emerging from these results can be explained by an inward movement of the side chain of Glu(ext), followed by the binding of extracellular Cl(-) ions.     

Chloride accumulation and swelling in endosomes enhances DNA transfer by polyamine-DNA polyplexes

The "proton sponge hypothesis" postulates enhanced transgene delivery by cationic polymer-DNA complexes (polyplexes) containing H+ buffering polyamines by enhanced endosomal Cl- accumulation and osmotic swelling/lysis. To test this hypothesis, we measured endosomal Cl- concentration, pH, and volume after internalization of polyplexes composed of plasmid DNA and polylysine (POL), a non-buffering polyamine, or the strongly buffering polyamines polyethylenimine (PEI) or polyamidoamine (PAM). [Cl-] and pH were measured by ratio imaging of fluorescently labeled polyplexes containing Cl- or pH indicators. [Cl-] increased from 41 to 80 mM over 60 min in endosomes-contained POL-polyplexes, whereas pH decreased from 6.8 to 5.3. Endosomal Cl- accumulation was enhanced (115 mM at 60 min) and acidification was slowed (pH 5.9 at 60 min) for PEI and PAM-polyplexes. Relative endosome volume increased 20% over 75 min for POL-polyplexes versus 140% for PEI-polyplexes. Endosome lysis was seen at >45 min for PEI but not POL-containing endosomes, and PEI-containing endosomes showed increased osmotic fragility in vitro. The slowed endosomal acidification and enhanced Cl- accumulation and swelling/lysis were accounted for by the greater H+ buffering capacity of endosomes containing PEI or PAM versus POL (>90 mM versus 46 H+/pH unit). Our results provide direct support for the proton sponge hypothesis and thus a rational basis for the design of improved non-viral vectors for gene delivery.     

Molecular cloning and characterization of an intracellular chloride channel in the proximal tubule cell line, LLC-PK1

CLC5 is an intracellular chloride channel of unknown function, expressed in the renal proximal tubule. The subcellular localization and function of CLC5 were investigated in the LLC-PK1 porcine proximal tubule cell line. We cloned a cDNA for the porcine CLC5 ortholog (pCLC5) that is predicted to encode an 83-kDa protein with 97% amino acid sequence identity to rat and human CLC5. By immunofluorescence, pCLC5 was localized to early endosomes of the apical membrane fluid-phase endocytotic pathway and to the Golgi complex. Xenopus oocytes injected with pCLC5 cRNA exhibited outwardly rectifying whole cell currents with a relative conductance profile (nitrate Cl(-) approximately Br(-) > I(-) > acetate > gluconate) different from that of control oocytes. Acidification of the extracellular medium reversibly inhibited this outward current with a pK(a) of 6.0 and a Hill coefficient of 1. Overexpression of CLC5 in LLC-PK1 cells resulted in morphological changes, including loss of cell-cell contacts and the appearance of multiple prominent vesicles. These findings are consistent with a potential role for CLC5 in the acidification of membrane compartments of both the endocytic and the exocytic pathway and suggest that its function may be important for normal intercellular adhesion and vesicular trafficking.     

Second messengers regulate endosomal acidification in Swiss 3T3 fibroblasts.

Acidification of the endosomal pathway is important for ligand and receptor sorting, toxin activation, and protein degradation by lysosomal acid hydrolases. Fluorescent probes and imaging methods were developed to measure pH to better than 0.2 U accuracy in individual endocytic vesicles in Swiss 3T3 fibroblasts. Endosomes were pulse labeled with transferrin (Tf), alpha 2-macroglobulin (alpha 2M), or dextran, each conjugated with tetramethylrhodamine and carboxyfluorescein (for pH 5-8) or dichlorocarboxyfluorescein (for pH 4-6); pH in individual labeled vesicles was measured by ratio imaging using a cooled CCD camera and novel image analysis software. Tf-labeled endosomes acidified to pH 6.2 +/- 0.1 with a t1/2 of 4 min at 37 degrees C, and remained small and near the cell periphery. Dextran- and alpha 2M-labeled endosomes acidified to pH 4.7 +/- 0.2, becoming larger and moving toward the nucleus over 30 min; approximately 15% of alpha 2M-labeled endosomes were strongly acidic (pH less than 5.5) at only 1 min after labeling. Replacement of external Cl by NO3 or isethionate strongly and reversibly inhibited acidification. Addition of ouabain (1 mM) at the time of labeling strongly enhanced acidification in the first 5 min; Tf-labeled endosomes acidified to pH 5.3 without a change in morphology. Activation of phospholipase C by vasopressin (50 nM) enhanced acidification of early endosomes; activation of protein kinase C by PMA (100 nM) enhanced acidification strongly, whereas elevation of intracellular Ca by A23187 (1 microM) had no effect on acidification. Activation of protein kinase A by CPT-cAMP (0.5 mM) or forskolin (50 microM) inhibited acidification. Lysosomal pH was not affected by ouabain or the protein kinase activators. These results establish a methodology for quantitative measurement of pH in individual endocytic vesicles, and demonstrate that acidification of endosomes labeled with Tf and alpha 2M (receptor-mediated endocytosis) and dextran (fluid-phase endocytosis) is sensitive to intracellular anion composition, Na/K pump inhibition, and multiple intracellular second messengers.     

Chloride channel (Clc)-5 is necessary for exocytic trafficking of Na+/H+ exchanger 3 (NHE3)

ClC-5, a chloride/proton exchanger, is predominantly expressed and localized in subapical endosomes of the renal proximal tubule. Mutations of the CLCN5 gene cause Dent disease. The symptoms of Dent disease are replicated in Clcn5 knock-out mice. Absence of ClC-5 in mice is associated with reduced surface expression of NHE3 in proximal tubules. The molecular basis for this change is not fully understood. In this study, we investigated the mechanisms by which ClC-5 regulates trafficking of NHE3. Whether ClC-5-dependent endocytosis, exocytosis, or both contributed to the altered distribution of NHE3 was examined. First, NHE3 activity in proximal tubules of wild type (WT) and Clcn5 KO mice was determined by two-photon microscopy. Basal and dexamethasone-stimulated NHE3 activity of Clcn5 KO mice was decreased compared with that seen in WT mice, whereas the degree of inhibition of NHE3 activity by increasing cellular concentration of cAMP (forskolin) or Ca(2+) (A23187) was not different in WT and Clcn5 KO mice. Second, NHE3-dependent absorption of HCO(3)(-), measured by single tubule perfusion, was reduced in proximal tubules of Clcn5 KO mice. Third, by cell surface biotinylation, trafficking of NHE3 was examined in short hairpin RNA (shRNA) plasmid-transfected opossum kidney cells. Surface NHE3 was reduced in opossum kidney cells with reduced expression of ClC-5, whereas the total protein level of NHE3 did not change. Parathyroid hormone decreased NHE3 surface expression, but the extent of decrease and the rate of endocytosis observed in both scrambled and ClC-5 knockdown cells were not significantly different. However, the rates of basal and dexamethasone-stimulated exocytosis of NHE3 were attenuated in ClC-5 knockdown cells. These results show that ClC-5 plays an essential role in exocytosis of NHE3.     

Loss of the ClC-7 chloride channel leads to osteopetrosis in mice and man

Chloride channels play important roles in the plasma membrane and in intracellular organelles. Mice deficient for the ubiquitously expressed ClC-7 Cl(-) channel show severe osteopetrosis and retinal degeneration. Although osteoclasts are present in normal numbers, they fail to resorb bone because they cannot acidify the extracellular resorption lacuna. ClC-7 resides in late endosomal and lysosomal compartments. In osteoclasts, it is highly expressed in the ruffled membrane, formed by the fusion of H(+)-ATPase-containing vesicles, that secretes protons into the lacuna. We also identified CLCN7 mutations in a patient with human infantile malignant osteopetrosis. We conclude that ClC-7 provides the chloride conductance required for an efficient proton pumping by the H(+)-ATPase of the osteoclast ruffled membrane.     

Cofilin interacts with ClC-5 and regulates albumin uptake in proximal tubule cell lines

Receptor-mediated endocytosis is a constitutive high capacity pathway for the reabsorption of proteins from the glomerular filtrate by the renal proximal tubule. ClC-5 is a voltage-gated chloride channel found in the proximal tubule where it has been shown to be essential for protein uptake, based on evidence from patients with Dent's disease and studies in ClC-5 knockout mice. To further delineate the role of ClC-5 in albumin uptake, we performed a yeast two-hybrid screen with the C-terminal tail of ClC-5 to identify any interactions of the channel with proteins involved in endocytosis. We found that the C-terminal tail of ClC-5 bound the actin depolymerizing protein, cofilin, a result that was confirmed by GST-fusion pulldown assays. In cultured proximal tubule cells, cofilin was distributed in nuclear, cytoplasmic, and microsomal fractions and co-localized with ClC-5. Phosphorylation of cofilin by overexpressing LIM kinase 1 resulted in a stabilization of the actin cytoskeleton. Phosphorylation of cofilin in two proximal tubule cell models (porcine renal proximal tubule and opossum kidney) was also accompanied by a pronounced inhibition of albumin uptake. This study identifies a novel interaction between the C-terminal tail of ClC-5 and cofilin, an actin-associated protein that is crucial in the regulation of albumin uptake by the proximal tubule.     

Endosomal Maturation,Rab7 GTPase and Phosphoinositides in African Swine Fever Virus Entry

Here we analyzed the dependence of African swine fever virus (ASFV) infection on the integrity of the endosomal pathway. Using confocal immunofluorescence with antibodies against viral capsid proteins, we found colocalization of incoming viral particles with early endosomes (EE) during the first minutes of infection. Conversely, viral capsid protein was not detected in acidic late endosomal compartments, multivesicular bodies (MVBs), late endosomes (LEs) or lysosomes (LY). Using an antibody against a viral inner core protein, we found colocalization of viral cores with late compartments from 30 to 60 minutes postinfection. The absence of capsid protein staining in LEs and LYs suggested that virus desencapsidation would take place at the acid pH of these organelles. In fact, inhibitors of intraluminal acidification of endosomes caused retention of viral capsid staining virions in Rab7 expressing endosomes and more importantly, severely impaired subsequent viral protein production. Endosomal acidification in the first hour after virus entry was essential for successful infection but not thereafter. In addition, altering the balance of phosphoinositides (PIs) which are responsible of the maintenance of the endocytic pathway impaired ASFV infection. Early infection steps were dependent on the production of phosphatidylinositol 3-phosphate (PtdIns3P) which is involved in EE maturation and multivesicular body (MVB) biogenesis and on the interconversion of PtdIns3P to phosphatidylinositol 3, 5-biphosphate (PtdIns(3,5)P₂). Likewise, GTPase Rab7 activity should remain intact, as well as processes related to LE compartment physiology, which are crucial during early infection. Our data demonstrate that the EE and LE compartments and the integrity of the endosomal maturation pathway orchestrated by Rab proteins and PIs play a central role during early stages of ASFV infection.     

pH-Controlled Two-Step Uncoating of Influenza Virus

Upon endocytosis in its cellular host, influenza A virus transits via early to late endosomes. To efficiently release its genome, the composite viral shell must undergo significant structural rearrangement, but the exact sequence of events leading to viral uncoating remains largely speculative. In addition, no change in viral structure has ever been identified at the level of early endosomes, raising a question about their role. We performed AFM indentation on single viruses in conjunction with cellular assays under conditions that mimicked gradual acidification from early to late endosomes. We found that the release of the influenza genome requires sequential exposure to the pH of both early and late endosomes, with each step corresponding to changes in the virus mechanical response. Step 1 (pH 7.5–6) involves a modification of both hemagglutinin and the viral lumen and is reversible, whereas Step 2 (pH <6.0) involves M1 dissociation and major hemagglutinin conformational changes and is irreversible. Bypassing the early-endosomal pH step or blocking the envelope proton channel M2 precludes proper genome release and efficient infection, illustrating the importance of viral lumen acidification during the early endosomal residence for influenza virus infection.