Second messengers regulate endosomal acidification in Swiss 3T3 fibroblasts

The host tropism of current zoonotic H7N9 viruses depends mainly on an acid-labile hemagglutinin with a single amino acid mutation in the stalk region

The incidence of human infection by zoonotic avian influenza viruses, especially H5N1 and H7N9 viruses, has increased. Current zoonotic H7N9 avian influenza viruses (identified since 2013) emerged during reassortment of viruses belonging to different subtypes. Despite analyses of their genetic background, we do not know why current H7N9 viruses are zoonotic. Therefore, there is a need to identify the factor(s) responsible for the extended host tropism that enables these viruses to infect humans as well as birds. To identify H7N9-specific amino acids that confer zoonotic properties on H7N9 viruses, we performed multiple alignment of the hemagglutinin (HA) amino acid sequences of A/Shanghai/1/2013 (H7N9) and A/duck/Zhejiang/12/2011(H7N3) (a putative, non- or less zoonotic HA donor to the zoonotic H7N9 virus). We also analyze the function of an H7N9 HA-specific amino acid with respect to HA acid stability, and evaluated the effect of acid stability on viral infectivity and virulence in a mouse model. HA2-116D, preserved in current zoonotic H7N9 viruses, was crucial for loss of HA acid stability. The acid-labile HA protein in H7 viruses played an important role in infection of human airway epithelial cells; HA2-116D contributed to infection and replication of H7 viruses. Finally, HA2-116D served as a H7 virulence factor in mice. These results suggest that acid-labile HA harboring HA2-116D confers zoonotic characteristics on H7N9 virus and that future novel zoonotic avian viruses could emerge from non-zoonotic H7 viruses via acquisition of mutations that remove HA acid stability.

From Pinocytosis to Methuosis-Fluid Consumption as a Risk Factor for Cell Death

The volumes of a cell [cell volume (CV)] and its organelles are adjusted by osmoregulatory processes. During pinocytosis, extracellular fluid volume equivalent to its CV is incorporated within an hour and membrane area equivalent to the cell's surface within 30 min. Since neither fluid uptake nor membrane consumption leads to swelling or shrinkage, cells must be equipped with potent volume regulatory mechanisms. Normally, cells respond to outwardly or inwardly directed osmotic gradients by a volume decrease and increase, respectively, i.e., they shrink or swell but then try to recover their CV. However, when a cell death (CD) pathway is triggered, CV persistently decreases in isotonic conditions in apoptosis and it increases in necrosis. One type of CD associated with cell swelling is due to a dysfunctional pinocytosis. Methuosis, a non-apoptotic CD phenotype, occurs when cells accumulate too much fluid by macropinocytosis. In contrast to functional pinocytosis, in methuosis, macropinosomes neither recycle nor fuse with lysosomes but with each other to form giant vacuoles, which finally cause rupture of the plasma membrane (PM). Understanding methuosis longs for the understanding of the ionic mechanisms of cell volume regulation (CVR) and vesicular volume regulation (VVR). In nascent macropinosomes, ion channels and transporters are derived from the PM. Along trafficking from the PM to the perinuclear area, the equipment of channels and transporters of the vesicle membrane changes by retrieval, addition, and recycling from and back to the PM, causing profound changes in vesicular ion concentrations, acidification, and-most importantly-shrinkage of the macropinosome, which is indispensable for its proper targeting and cargo processing. In this review, we discuss ion and water transport mechanisms with respect to CVR and VVR and with special emphasis on pinocytosis and methuosis. We describe various aspects of the complex mutual interplay between extracellular and intracellular ions and ion gradients, the PM and vesicular membrane, phosphoinositides, monomeric G proteins and their targets, as well as the submembranous cytoskeleton. Our aim is to highlight important cellular mechanisms, components, and processes that may lead to methuotic CD upon their derangement.

HA-Dependent Tropism of H5N1 and H7N9 Influenza Viruses to Human Endothelial Cells Is Determined by Reduced Stability of the HA, Which Allows the Virus To Cope with Inefficient Endosomal Acidification and Constitutively Expressed IFITM3

Previous studies revealed that certain avian influenza A viruses (IAVs), including zoonotic H5N1 and H7N9 IAVs, infect cultured human lung microvascular endothelial cells (HULEC) more efficiently than other IAVs and that tropism to HULEC is determined by viral hemagglutinin (HA). To characterize mechanisms of HA-mediated endotheliotropism, we used 2:6 recombinant IAVs harboring HAs from distinctive avian and human viruses and found that efficient infection of HULEC correlated with low conformational stability of the HA. We next studied effects on viral infectivity of single-point amino acid substitutions in the HA of 2:6 recombinant virus A/Vietnam/1203/2004-PR8 (H5N1). Substitutions H8Q, H103Y, T315I, and K58₂I (K58I in the HA2 subunit), which increased stability of the HA, markedly reduced viral infectivity for HULEC, whereas substitutions K189N and K218Q, which altered typical H5N1 virus-like receptor specificity and reduced binding avidity of the HA, led to only marginal reduction of infectivity. None of these substitutions affected virus infection in MDCK cells. We confirmed the previous observation of elevated basal expression of IFITM3 protein in HULEC and found that endosomal acidification is less efficient in HULEC than in MDCK cells. In accord with these findings, counteraction of IFITM3-mediated restriction by amphotericin B and reduction of endosomal pH by moderate acidification of the extracellular medium enhanced infectivity of viruses with stable HA for HULEC without significant effect on infectivity for MDCK cells. Collectively, our results indicate that relatively high pH optimum of fusion of the HA of zoonotic H5N1 and H7N9 IAVs allows them to overcome antiviral effects of inefficient endosomal acidification and IFITM3 in human endothelial cells.IMPORTANCE Receptor specificity of the HA of IAVs is known to be a critical determinant of viral cell tropism. Here, we show that fusion properties of the HA may also play a key role in the tropism. Thus, we demonstrate that IAVs having a relatively low pH optimum of fusion cannot efficiently infect human endothelial cells owing to their relatively high endosomal pH and increased expression of fusion-inhibiting IFITM3 protein. These restrictions can be overcome by IAVs with elevated pH of fusion, such as zoonotic H5N1 and H7N9. Our results illustrate that the infectivity of IAVs depends on an interplay between HA conformational stability, endosomal acidification and IFITM3 expression in target cells, and the extracellular pH. Given significant variation of levels of HA stability among animal, human, and zoonotic IAVs, our findings prompt further studies on the fusion-dependent tropism of IAVs to different cell types in humans and its role in viral host range and pathogenicity.

Endothelin-converting enzyme-1 regulates glucagon-like peptide-1 receptor signalling and resensitisation

Following nutrient ingestion, glucagon-like peptide 1 (GLP-1) is secreted from intestinal L-cells and mediates anti-diabetic effects, most notably stimulating glucose-dependent insulin release from pancreatic β-cells but also inhibiting glucagon release, promoting satiety and weight reduction and potentially enhancing or preserving β-cell mass. These effects are mediated by the GLP-1 receptor (GLP-1R), which is a therapeutic target in type 2 diabetes. Although agonism at the GLP-1R has been well studied, desensitisation and resensitisation are perhaps less well explored. An understanding of these events is important, particularly in the design and use of novel receptor ligands. Here, using either HEK293 cells expressing the recombinant human GLP-1R or the pancreatic β-cell line, INS-1E with endogenous expressesion of the GLP-1R, we demonstrate GLP-1R desensitisation and subsequent resensitisation following removal of extracellular GLP-1 7-36 amide. Resensitisation is dependent on receptor internalisation, endosomal acidification and receptor recycling. Resensitisation is also regulated by endothelin-converting enzyme-1 (ECE-1) activity, most likely through proteolysis of GLP-1 in endosomes and the facilitation of GLP-1R dephosphorylation and recycling. Inhibition of ECE-1 activity also increases GLP-1-induced activation of extracellular signal-regulated kinase and generation of cAMP, suggesting processes dependent upon the lifetime of the internalised ligand-receptor complex.

Unique Infectious Strategy of H5N1 Avian Influenza Virus Is Governed by the Acid-Destabilized Property of Hemagglutinin

Highly pathogenic avian influenza (HPAI) H5N1 virus emerged in 1997 as a zoonotic disease in Hong Kong. It has since spread to Asia and Europe and is a serious threat to both the poultry industry and human health. For effective surveillance and possible prevention/control of HPAI H5N1 viruses, it is necessary to understand the molecular mechanism underlying HPAI H5N1 pathogenesis. The hemagglutinin (HA) protein of influenza A viruses (IAVs) is one of the major determinants of host adaptation, transmissibility, and viral virulence. The main function of the HA protein is to facilitate viral entry and viral genome release within host cells before infection. To achieve viral infection, IAVs belonging to different subtypes or strains induce viral-cell membrane fusion at different endosomal pH levels after internalization through endocytosis. However, host-specific endosomal pH also affects induction of membrane fusion followed by infection. The HA protein of HPAI H5N1 has a higher pH threshold for membrane fusion than the HA protein of classical avian influenza viruses. Although this particular property of HA (which governs viral infection) is prone to deactivation in the avian intestine or in an ambient environment, it facilitates efficient infection of host cells, resulting in a broad host tropism, regardless of the pH in the host endosome. Accumulated knowledge, together with further research, about the HA-governed mechanism underlying HPAI H5N1 virulence (i.e., receptor tropism and pH-dependent viral-cell membrane fusion) will be helpful for developing effective surveillance strategies and for prevention/control of HPAI H5N1 infection.

Signalome-wide assessment of host cell response to hepatitis C virus

Host cell signalling during infection with intracellular pathogens remains poorly understood. Here we report on the use of antibody microarray technology to detect variations in the expression levels and phosphorylation status of host cell signalling proteins during hepatitis C virus (HCV) replication. Following transfection with HCV RNA, the JNK and NF-κB pathways are suppressed, while the JAK/STAT5 pathway is activated; furthermore, components of the apoptosis and cell cycle control machineries are affected in the expression and/or phosphorylation status. RNAi-based hit validation identifies components of the JAK/STAT, NF-κB, MAPK and calcium-induced pathways as modulators of HCV replication. Selective chemical inhibition of one of the identified targets, the JNK activator kinase MAP4K2, does impair HCV replication. Thus this study provides a comprehensive picture of host cell pathway mobilization by HCV and uncovers potential therapeutic targets. The strategy of identifying targets for anti-infective intervention within the host cell signalome can be applied to any intracellular pathogen.

Development of antiviral strategies depends on an understanding of virus–host interactions. Here, using HCV, Haqshenas et al. show that antibody microarray combined with a targeted siRNA screen can be a powerful tool to identify cellular signalling pathways that are important for virus replication.

Avian Influenza Virus Infection of Immortalized Human Respiratory Epithelial Cells Depends upon a Delicate Balance between Hemagglutinin Acid Stability and Endosomal pH

The highly pathogenic avian influenza (AI) virus, H5N1, is a serious threat to public health worldwide. Both the currently circulating H5N1 and previously circulating AI viruses recognize avian-type receptors; however, only the H5N1 is highly infectious and virulent in humans. The mechanism(s) underlying this difference in infectivity remains unclear. The aim of this study was to clarify the mechanisms responsible for the difference in infectivity between the current and previously circulating strains. Primary human small airway epithelial cells (SAECs) were transformed with the SV40 large T-antigen to establish a series of clones (SAEC-Ts). These clones were then used to test the infectivity of AI strains. Human SAEC-Ts could be broadly categorized into two different types based on their susceptibility (high or low) to the viruses. SAEC-T clones were poorly susceptible to previously circulating AI but were completely susceptible to the currently circulating H5N1. The hemagglutinin (HA) of the current H5N1 virus showed greater membrane fusion activity at higher pH levels than that of previous AI viruses, resulting in broader cell tropism. Moreover, the endosomal pH was lower in high susceptibility SAEC-T clones than that in low susceptibility SAEC-T clones. Taken together, the results of this study suggest that the infectivity of AI viruses, including H5N1, depends upon a delicate balance between the acid sensitivity of the viral HA and the pH within the endosomes of the target cell. Thus, one of the mechanisms underlying H5N1 pathogenesis in humans relies on its ability to fuse efficiently with the endosomes in human airway epithelial cells.

Route, mechanism, and implications of proton import during Na+/K+ exchange by native Na+/K+-ATPase pumps

The Na⁺/K⁺ pump is a hybrid transporter that can also import protons at physiological K⁺ and Na⁺ concentrations.

A single Na⁺/K⁺-ATPase pumps three Na⁺ outwards and two K⁺ inwards by alternately exposing ion-binding sites to opposite sides of the membrane in a conformational sequence coupled to pump autophosphorylation from ATP and auto-dephosphorylation. The larger flow of Na⁺ than K⁺ generates outward current across the cell membrane. Less well understood is the ability of Na⁺/K⁺ pumps to generate an inward current of protons. Originally noted in pumps deprived of external K⁺ and Na⁺ ions, as inward current at negative membrane potentials that becomes amplified when external pH is lowered, this proton current is generally viewed as an artifact of those unnatural conditions. We demonstrate here that this inward current also flows at physiological K⁺ and Na⁺ concentrations. We show that protons exploit ready reversibility of conformational changes associated with extracellular Na⁺ release from phosphorylated Na⁺/K⁺ pumps. Reversal of a subset of these transitions allows an extracellular proton to bind an acidic side chain and to be subsequently released to the cytoplasm. This back-step of phosphorylated Na⁺/K⁺ pumps that enables proton import is not required for completion of the 3 Na⁺/2 K⁺ transport cycle. However, the back-step occurs readily during Na⁺/K⁺ transport when external K⁺ ion binding and occlusion are delayed, and it occurs more frequently when lowered extracellular pH raises the probability of protonation of the externally accessible carboxylate side chain. The proton route passes through the Na⁺-selective binding site III and is distinct from the principal pathway traversed by the majority of transported Na⁺ and K⁺ ions that passes through binding site II. The inferred occurrence of Na⁺/K⁺ exchange and H⁺ import during the same conformational cycle of a single molecule identifies the Na⁺/K⁺ pump as a hybrid transporter. Whether Na⁺/K⁺ pump–mediated proton inflow may have any physiological or pathophysiological significance remains to be clarified.

Genomic editing of the HIV-1 coreceptor CCR5 in adult hematopoietic stem and progenitor cells using zinc finger nucleases

The HIV-1 coreceptor CCR5 is a validated target for HIV/AIDS therapy. The apparent elimination of HIV-1 in a patient treated with an allogeneic stem cell transplant homozygous for a naturally occurring CCR5 deletion mutation (CCR5(Δ32/Δ32)) supports the concept that a single dose of HIV-resistant hematopoietic stem cells can provide disease protection. Given the low frequency of naturally occurring CCR5(Δ32/Δ32) donors, we reasoned that engineered autologous CD34(+) hematopoietic stem/progenitor cells (HSPCs) could be used for AIDS therapy. We evaluated disruption of CCR5 gene expression in HSPCs isolated from granulocyte colony-stimulating factor (CSF)-mobilized adult blood using a recombinant adenoviral vector encoding a CCR5-specific pair of zinc finger nucleases (CCR5-ZFN). Our results demonstrate that CCR5-ZFN RNA and protein expression from the adenoviral vector is enhanced by pretreatment of HSPC with protein kinase C (PKC) activators resulting in >25% CCR5 gene disruption and that activation of the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) signaling pathway is responsible for this activity. Importantly, using an optimized dose of PKC activator and adenoviral vector we could generate CCR5-modified HSPCs which engraft in a humanized mouse model (albeit at a reduced level) and support multilineage differentiation in vitro and in vivo. Together, these data establish the basis for improved approaches exploiting adenoviral vector delivery in the modification of HSPCs.

Retinoic acid induces autophagosome maturation through redistribution of the cation-independent mannose-6-phosphate receptor

Retinoic acids (RAs) have diverse biologic effects and regulate several cellular functions. Here, we investigated the role of RA on autophagy by studying its effects on autophagosome (AUT) maturation, as well as on upstream regulators of autophagosome biogenesis. Our studies, based on the use of pH-sensitive fluorescent reporter markers, suggested that RA promotes AUT acidification and maturation. By using competitive inhibitors and specific agonists, we demonstrated that this effect is not mediated by the classic RAR and RXR receptors. RA did not affect the levels of upstream regulators of autophagy, such as Beclin-1, phospho-mTOR, and phospho-Akt1, but induced redistribution of both endogenous cation-independent mannose-6-phosphate receptor CIMPR and transiently transfected GFP and RFP full-length CIMPR fusion proteins from the trans-Golgi region to acidified AUT structures. Those structures were found to be amphisomes (acidified AUTs) and not autophagolysosomes. The critical role of CIMPR in AUT maturation was further demonstrated by siRNA-mediated silencing of endogenous CIMPR. Transient CIMPR knockdown resulted in remarkable accumulation of nonacidified AUTs, a process that could not be reversed with RA. Our results suggest that RA induces AUT acidification and maturation, a process critical in the cellular autophagic mechanism.

Phagosome dynamics during phagocytosis by neutrophils

The neutrophil is a key player in immunity, and its activities are essential for the resolution of infections. Neutrophil-pathogen interactions usually trigger a large arsenal of antimicrobial measures that leads to the highly efficient killing of pathogens. In neutrophils, the phagocytic process, including the formation and maturation of the phagosome, is in many respects very different from that in other phagocytes. Although the complex mechanisms that coordinate the membrane traffic, oxidative burst, and release of granule contents required for the microbicidal activities of neutrophils are not completely understood, it is evident that they are unique and differ from those in macrophages. Neutrophils exhibit more rapid rates of phagocytosis and higher intensity of oxidative respiratory response than do macrophages. The phagosome maturation pathway in macrophages, which is linked to the endocytic pathway, is replaced in neutrophils by the rapid delivery of preformed granules to nonacidic phagosomes. This review describes the plasticity and dynamics of the phagocytic process with a special focus on neutrophil phagosome maturation.

Dual-fluorophore ratiometric pH nanosensor with tuneable pKa and extended dynamic range

Ratiometric pH nanosensors with tuneable pK(a) were prepared by entrapping combinations of two pH-sensitive fluorophores (fluorescein isothiocyanate dextran (FITC-D) and Oregon Green(®) dextran (OG-D)) and a reference fluorophore (5-(and-6)-carboxytetramethylrhodamine dextran (TAMRA-D)), in a biocompatible polymer matrix. Dual-fluorophore pH nanosensors permit the measurement of an extended dynamic range, from pH 4.0 to 7.5.

Caveolin-1 is ubiquitinated and targeted to intralumenal vesicles in endolysosomes for degradation

Identification of the pathway by which caveolin-1 is degraded when caveolae assembly is compromised suggests that “caveosomes” may be endosomal accumulations of the protein awaiting degradation.

Caveolae are long-lived plasma membrane microdomains composed of caveolins, cavins, and a cholesterol-rich membrane. Little is known about how caveolae disassemble and how their coat components are degraded. We studied the degradation of caveolin-1 (CAV1), a major caveolar protein, in CV1 cells. CAV1 was degraded very slowly, but turnover could be accelerated by compromising caveolae assembly. Now, CAV1 became detectable in late endosomes (LE) and lysosomes where it was degraded. Targeting to the degradative pathway required ubiquitination and the endosomal sorting complex required for transport (ESCRT) machinery for inclusion into intralumenal vesicles in endosomes. A dual-tag strategy allowed us to monitor exposure of CAV1 to the acidic lumen of individual, maturing LE in living cells. Importantly, we found that “caveosomes,” previously described by our group as independent organelles distinct from endosomes, actually correspond to late endosomal compartments modified by the accumulation of overexpressed CAV1 awaiting degradation. The findings led us to a revised model for endocytic trafficking of CAV1.

Metformin increases phagocytosis and acidifies lysosomal/endosomal compartments in AMPK-dependent manner in rat primary microglia

Recent evidence suggests that metformin shows beneficial effects in experimental models of neuroinflammatory diseases. The aim of the present study was to determine the effect of metformin on phagocytosis and acidification of lysosomal/endosomal compartments in rat primary microglia in the presence of lipopolysaccharide (LPS) and/or beta-peptides (25-35), (1-40), and (1-42). Metformin increased the phagocytosis of fluorescent microspheres in the presence or absence of all the beta-peptides. However, the drug had no effect on the phagocytosis in LPS-stimulated microglia regardless of the presence of all the beta-peptides. Metformin acidified the lysosomal/endosomal compartments in the presence or absence of the beta-peptide 1-40 in both resting and activated microglia. To elucidate the mechanism of metformin action, we used 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside as an activator of adenosine monophosphate-activated protein kinase (AMPK) and compound C as a confirmed pharmacological inhibitor of AMPK. We have shown that metformin increased AMPK activity in microglial cells and that all observed effects are AMPK-dependent because the pretreatment of microglia with compound C reversed the effects of the drug. Since degradation of proteins in lysosomal/endosomal compartments depends largely on their phagocytosis and acidification, metformin may be beneficial in proteinopathies affecting the brain.

Chloride intracellular channel protein-4 functions in angiogenesis by supporting acidification of vacuoles along the intracellular tubulogenic pathway

Endothelial cells form capillary tubes through the process of intracellular tubulogenesis. Chloride intracellular channel (CLIC) family proteins have been previously implicated in intracellular tubulogenesis, but their specific role has not been defined. In this study, we show that disruption of the Clic4 gene in mice results in defective angiogenesis in vivo as reflected in a Matrigel plug angiogenesis assay. An angiogenesis defect is also apparent in the retina, both in the decreased spontaneous development of retinal vasculature of unstressed mice and in the dramatically decreased angiogenic response of retinal vessels to an oxygen toxicity challenge. We found that endothelial cells derived from Clic4(-/-) mice demonstrated impaired tubulogenesis in three-dimensional fibrin gels compared with cells derived from wild-type mice. Furthermore, we found that tubulogenesis of wild-type cells in culture was inhibited by both an inhibitor of CLICs and an inhibitor of the vacuolar proton ATPase. Finally, we showed that vacuoles along the endothelial tubulogenesis pathway are acidic in wild-type cells, and that vacuolar acidification is impaired in Clic4(-/-) cells while lysosomal acidification is intact. We conclude that CLIC4 plays a critical role in angiogenesis by supporting acidification of vacuoles along the cell-hollowing tubulogenic pathway.

Unimpaired lysosomal acidification in respiratory epithelial cells in cystic fibrosis

The mechanisms remain uncertain by which mutations in CFTR cause lung disease in cystic fibrosis (CF). Teichgräber et al. recently reported increased ceramide in CF lungs, which was proposed to result from defective lysosomal acidification in airway epithelial cells and consequent impairment of pH-dependent ceramide-metabolizing enzymes (Teichgräber, V., Ulrich, M., Endlich, N., Reithmüller, J., Wilker, B., Conceição Ce Olivereira-Munding, C., van Heeckeren, A. M., Barr, M. L., von Kürthy, G., Schmid, K. W., Weller, M., Tümmler, B., Lang, F., Grassme, H., Döring, G., and Gulbins, E. (2008) Nat. Med. 14, 382-391). Here, we measured lysosomal pH in several CFTR-expressing and -deficient cell lines, freshly isolated airway epithelial cells from non-CF and CF mice and humans, and well-differentiated primary cultures of human non-CF and CF airway epithelial cells. Lysosomal pH was measured by ratio imaging using a fluorescent pH indicator consisting of 40-kDa dextran conjugated to Oregon Green 488 and tetramethylrhodamine. In all cell types, lysosomal pH was approximately 4.5, unaffected by the thiazolidinone CFTR inhibitor CFTR(inh)-172, and increased to approximately 6.5 following bafilomycin inhibition of the vacuolar proton pump. Lysosomal pH did not differ significantly in airway epithelial cells from non-CF versus CF humans or mice. Our results provide direct evidence against the conclusions of Teichgräber et al. that lysosomal acidification is CFTR-dependent, impaired in CF, or responsible for ceramide accumulation. As such, alternative mechanisms are needed to explain increased ceramide in CF airways. Non-CFTR mechanisms, such as ClC-type chloride channels, are likely involved in maintaining electroneutrality during organellar acidification.

Novel interactions of CLN3 protein link Batten disease to dysregulation of fodrin-Na+, K+ ATPase complex

Juvenile neuronal ceroid lipofuscinosis (JNCL, Batten disease) is the most common progressive neurodegenerative disorder of childhood. CLN3, the transmembrane protein underlying JNCL, is proposed to participate in multiple cellular events including membrane trafficking and cytoskeletal functions. We demonstrate here that CLN3 interacts with the plasma membrane-associated cytoskeletal and endocytic fodrin and the associated Na(+), K(+) ATPase. The ion pumping activity of Na(+), K(+) ATPase was unchanged in Cln3(-/-) mouse primary neurons. However, the immunostaining pattern of fodrin appeared abnormal in JNCL fibroblasts and Cln3(-/-) mouse brains suggesting disturbances in the fodrin cytoskeleton. Furthermore, the basal subcellular distribution as well as ouabain-induced endocytosis of neuron-specific Na(+), K(+) ATPase were remarkably affected in Cln3(-/-) mouse primary neurons. These data suggest that CLN3 is involved in the regulation of plasma membrane fodrin cytoskeleton and consequently, the plasma membrane association of Na(+), K(+) ATPase. Most of the processes regulated by multifunctional fodrin and Na(+), K(+) ATPase are also affected in JNCL and Cln3-deficiency implicating that dysregulation of fodrin cytoskeleton and non-pumping functions of Na(+), K(+) ATPase may play a role in the neuronal degeneration in JNCL.

Cell surface levels of organellar Na+/H+ exchanger isoform 6 are regulated by interaction with RACK1

In mammalian cells, four Na(+)/H(+) exchangers (NHE6 - NHE9) are localized to intracellular compartments. NHE6 and NHE9 are predominantly localized to sorting and recycling endosomes, NHE7 to the trans-Golgi network, and NHE8 to the mid-trans-Golgi stacks. The unique localization of NHEs may contribute to establishing organelle-specific pH values and ion homeostasis in cells. Mechanisms underlying the regulation and targeting of organellar NHEs are largely unknown. We identified an interaction between NHE9 and RACK1 (receptor for activated C kinase 1), a cytoplasmic scaffold protein, by yeast two-hybrid screening using the NHE9 C terminus as bait. The NHE9 C terminus is exposed to the cytoplasm, verifying that the interaction is topologically possible. The binding region was further delineated to the central region of the NHE9 C terminus. RACK1 also bound NHE6 and NHE7, but not NHE8, in vitro. Endogenous association between NHE6 and RACK1 was confirmed by co-immunoprecipitation and co-localization in HeLa cells. The luminal pH of the recycling endosome was elevated in RACK1 knockdown cells, accompanied by a decrease in the amount of NHE6 on the cell surface, although the total level of NHE6 was not significantly altered. These results indicate that RACK1 plays a role in regulating the distribution of NHE6 between endosomes and the plasma membrane and contributes to maintaining luminal pH of the endocytic recycling compartments.

Cystic Fibrosis Transmembrane Conductance Regulator-independent Phagosomal Acidification in Macrophages

It was reported recently that the cystic fibrosis transmembrane conductance regulator (CFTR) is required for acidification of phagosomes in alveolar macrophages (Di, A., Brown, M. E., Deriy, L. V., Li, C., Szeto, F. L., Chen, Y., Huang, P., Tong, J., Naren, A. P., Bindokas, V., Palfrey, H. C., and Nelson, D. J. (2006) Nat. Cell Biol. 8, 933-944). Here we determined whether the CFTR chloride channel is a generalized pathway for chloride entry into phagosomes in macrophages and whether mutations in CFTR could contribute to alveolar macrophage dysfunction. The pH of mature phagolysosomes in macrophages was measured by fluorescence ratio imaging using a zymosan conjugate containing Oregon Green(R) 488 and tetramethylrhodamine. Acidification of phagolysosomes in J774A.1 macrophages (pH approximately 5.1 at 45 min), murine alveolar macrophages (pH approximately 5.3), and human alveolar macrophages (pH approximately 5.3) was insensitive to CFTR inhibition by the thiazolidinone CFTR(inh)-172. Acidification of phagolysosomes in alveolar macrophages isolated from mice homozygous for DeltaF508-CFTR, the most common mutation in cystic fibrosis, was not different compared with that in alveolar macrophages isolated from wild-type mice. We also measured the kinetics of phagosomal acidification in J774A.1 and murine alveolar macrophages using a zymosan conjugate containing fluorescein and tetramethylrhodamine. Phagosomal acidification began within 3 min of zymosan binding and was complete within approximately 15 min of internalization. The rate of phagosomal acidification in J774A.1 cells was not slowed by CFTR(inh)-172 and was not different in alveolar macrophages from wild-type versus DeltaF508-CFTR mice. Our data indicate that phagolysosomal acidification in macrophages is not dependent on CFTR channel activity and do not support a proposed mechanism for cystic fibrosis lung disease involving defective phagosomal acidification and bacterial killing in alveolar macrophages.

p62/SQSTM1 binds directly to Atg8/LC3 to facilitate degradation of ubiquitinated protein aggregates by autophagy

Protein degradation by basal constitutive autophagy is important to avoid accumulation of polyubiquitinated protein aggregates and development of neurodegenerative diseases. The polyubiquitin-binding protein p62/SQSTM1 is degraded by autophagy. It is found in cellular inclusion bodies together with polyubiquitinated proteins and in cytosolic protein aggregates that accumulate in various chronic, toxic, and degenerative diseases. Here we show for the first time a direct interaction between p62 and the autophagic effector proteins LC3A and -B and the related gamma-aminobutyrate receptor-associated protein and gamma-aminobutyrate receptor-associated-like proteins. The binding is mediated by a 22-residue sequence of p62 containing an evolutionarily conserved motif. To monitor the autophagic sequestration of p62- and LC3-positive bodies, we developed a novel pH-sensitive fluorescent tag consisting of a tandem fusion of the red, acid-insensitive mCherry and the acid-sensitive green fluorescent proteins. This approach revealed that p62- and LC3-positive bodies are degraded in autolysosomes. Strikingly, even rather large p62-positive inclusion bodies (2 microm diameter) become degraded by autophagy. The specific interaction between p62 and LC3, requiring the motif we have mapped, is instrumental in mediating autophagic degradation of the p62-positive bodies. We also demonstrate that the previously reported aggresome-like induced structures containing ubiquitinated proteins in cytosolic bodies are dependent on p62 for their formation. In fact, p62 bodies and these structures are indistinguishable. Taken together, our results clearly suggest that p62 is required both for the formation and the degradation of polyubiquitin-containing bodies by autophagy.

Interactions between virus proteins and host cell membranes during the viral life cycle

The structure and function of cells are critically dependent on membranes, which not only separate the interior of the cell from its environment but also define the internal compartments. It is therefore not surprising that the major steps of the life cycle of viruses of animals and plants also depend on cellular membranes. Indeed, interactions of viral proteins with host cell membranes are important for viruses to enter into host cells, replicate their genome, and produce progeny particles. To replicate its genome, a virus first needs to cross the plasma membrane. Some viruses can also modify intracellular membranes of host cells to create a compartment in which genome replication will take place. Finally, some viruses acquire an envelope, which is derived either from the plasma membrane or an internal membrane of the host cell. This paper reviews recent findings on the interactions of viral proteins with host cell membranes during the viral life cycle.

Transferrin recycling and dextran transport to lysosomes is differentially affected by bafilomycin, nocodazole, and low temperature

The effects of bafilomycin, nocodazole, and reduced temperature on recycling and the lysosomal pathway have been investigated in various cultured cell lines and have been shown to vary dependent on the cell type examined. However, the way in which these treatments affect recycling and transport to lysosomes within the same cell line has not been analyzed. In the current study, we used fluorophore-labeled transferrin and dextran as typical markers for the recycling and the lysosomal pathways, respectively, to explore the morphology and the intravesicular pH of endocytic compartments in HeLa cells. The V-ATPase inhibitor bafilomycin selectively inhibited the transport of marker destined for lysosomal degradation in early endosomes, whereas the transport of transferrin to the perinuclear recycling compartment (PNRC) still occurred. The kinetics of transferrin acidification was found to be biphasic, indicative of fast and slow recycling pathways via early endosomes (pH 6.0) and PNRC (pH 5.6), respectively. Furthermore, the disruption of microtubules by nocodazole blocked the transport of transferrin to the PNRC in early endosomes and of lysosome-directed marker into endosomal carrier vesicles. In contrast, incubation at 20 degrees C affected the lysosomal pathway by causing retention of internalized dextran in late endosomes and a delay in transferrin recycling. Taken together, these data clearly demonstrate, for the first time, that the transferrin recycling pathway and transport of endocytosed material to lysosomes are differentially affected by bafilomycin, nocodazole, and low temperature in HeLa cells. Consequently, these treatments can be applied to investigate whether internalized macromolecules such as viruses follow a recycling or degradative pathway.

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.

Ocular Melanogenesis: The Role of Antioxidants

Given the propensity of a large number of melanogenic pathways that can be modulated by cellular redox status, a causal role of the deficiency of ocular pigments such as melanin in the pathogenesis of age-related macular degeneration and evidence that melanin production does occur in the adult eye, it seems not improbable that antioxidants (or agents that modify cellular redox status) may have melanin stimulatory (or inhibitory) effects that are superimposible on their effects as mere free radical scavengers. More empirical studies are needed to investigate this phenomenon so that antioxidant therapy may prove more beneficial to patients with ocular degenerative diseases.

Gene transduction and cell entry pathway of fiber-modified adenovirus type 5 vectors carrying novel endocytic peptide ligands selected on human tracheal glandular cells

Monolayers of cystic fibrosis transmembrane conductance regulator (CFTR)-deficient human tracheal glandular cells (CF-KM4) were subjected to phage biopanning, and cell-internalized phages were isolated and sequenced, in order to identify CF-KM4-specific peptide ligands that would confer upon adenovirus type 5 (Ad5) vector a novel cell target specificity and/or higher efficiency of gene delivery into airway cells of patients with cystic fibrosis (CF). Three different ligands, corresponding to prototypes of the most represented families of phagotopes recovered from intracellular phages, were designed and individually inserted into Ad5-green fluorescent protein (GFP) (AdGFP) vectors at the extremities of short fiber shafts (seven repeats [R7]) terminated by scissile knobs. Only one vector, carrying the decapeptide GHPRQMSHVY (abbreviated as QM10), showed an enhanced gene transduction of CF-KM4 cells compared to control nonliganded vector with fibers of the same length (AdGFP-R7-knob). The enhancement in gene transfer efficiency was not specific to CF-KM4 cells but was observed in other mammalian cell lines tested. The QM10-liganded vector was referred to as AdGFP-QM10-knob in its knobbed version and as AdGFP-QM10 in its proteolytically deknobbed version. AdGFP-QM10 was found to transduce cells with a higher efficiency than its knob-bearing version, AdGFP-QM10-knob. Consistent with this, competition experiments indicated that the presence of knob domains was not an absolute requirement for cell attachment of the QM10-liganded vector and that the knobless AdGFP-QM10 used alternative cell-binding domains on its capsid, including penton base capsomer, via a site(s) different from its RGD motifs. The QM10-mediated effect on gene transduction seemed to take place at the step of endocytosis in both quantitative and qualitative manners. Virions of AdGFP-QM10 were endocytosed in higher numbers than virions of the control vector and were directed to a compartment different from the early endosomes targeted by members of species C Ad. AdGFP-QM10 was found to accumulate in late endosomal and low-pH compartments, suggesting that QM10 acted as an endocytic ligand of the lysosomal pathway. These results validated the concept of detargeting and retargeting Ad vectors via our deknobbing system and redirecting Ad vectors to an alternative endocytic pathway via a peptide ligand inserted in the fiber shaft domain.

Intracellular Chloride Channels: Determinants of Function in the Endosomal Pathway

Endosomes, and related subcellular compartments, contain various Cl- channels in the ClC family. In this review, we describe the known roles of intracellular Cl- channels and also explore some of the functional implications of transmembrane Cl- flux in these organelles. Cl- influx acts to control intralumenal pH, both by shunting the effects of the proton pump on membrane potential and, possibly, through direct effects of Cl- on the proton pump. Changes in intralumenal pH likely help regulate membrane trafficking. We propose that changes in intralumenal Cl- concentration ([Cl-]) could theoretically play a direct role in regulating membrane trafficking and organellar function through effects on chloride-sensitive proteins in the vesicular membrane, which could transduce information about intralumenal [Cl-] to the outside of the vesicle and thereby recruit various signaling molecules. We present a model in which regulation of cytosolic [Cl-] and vesicular Cl- conductance could help control the amount or type of neurotransmitter stored in a particular population of synaptic vesicles.

Protein kinase B/Akt acts via glycogen synthase kinase 3 to regulate recycling of alpha v beta 3 and alpha 5 beta 1 integrins

Protein kinase B (PKB)/Akt is known to promote cell migration, and this may contribute to the enhanced invasiveness of malignant cells. To elucidate potential mechanisms by which PKB/Akt promotes the migration phenotype, we have investigated its role in the endosomal transport and recycling of integrins. Whereas the internalization of alpha v beta 3 and alpha 5 beta 1 integrins and their transport to the recycling compartment were independent of PKB/Akt, the return of these integrins (but not internalized transferrin) to the plasma membrane was regulated by phosphatidylinositol 3-kinases and PKB/Akt. The blockade of integrin recycling and cell spreading on integrin ligands effected by inhibition of PKB/Akt was reversed by inhibition of glycogen synthase kinase 3 (GSK-3). Moreover, expression of nonphosphorylatable active GSK-3 beta mutant GSK-3 beta-A9 suppressed recycling of alpha 5 beta 1 and alpha v beta 3 and reduced cell spreading on ligands for these integrins, indicating that PKB/Akt promotes integrin recycling by phosphorylating and inactivating GSK-3. We propose that the ability of PKB/Akt to act via GSK-3 to promote the recycling of matrix receptors represents a key mechanism whereby integrin function and cell migration can be regulated by growth factors.

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.

Fluorescence lifetime-resolved pH imaging of living cells

BACKGROUND

The regulation and maintenance of intracellular pH are critical to diverse metabolic functions of the living cells. Fluorescence time-resolved techniques and instrumentations have advanced rapidly and enabled the imaging of intracellular pH based on the fluorescence lifetimes.

METHODS

The frequency-domain fluorescence lifetime imaging microscopy (FLIM) and fluorophores displaying appropriate pH-dependent lifetime sensitivities were used to determine the temporal and spatial pH distributions in the cytosol and vesicular compartment lysosomes.

RESULTS

We found that cytosolic pH levels are different in 3T3 fibroblasts, Chinese hamster ovary (CHO) cells, and MCF-7 cells when using the pH probe carboxy-SNAFL2. We also tracked the transient cytosolic pH changes in the living CHO cells after treatments with proton pump inhibitors, ion exchanger inhibitors, and weak base and acid. The intracellular lysosomal pH was determined with the acidic lifetime probes DM-NERF dextrans, OG-514 carboxylic acid dextrans, and LysoSensor DND-160. Our results showed that the resting lysosomal pH value obtained from the 3T3 fibroblasts was between 4.5 and 4.9. The increase of lysosomal pH induced by the treatments with proton pump inhibitor and ionophores also were observed in our FLIM measurements.

CONCLUSIONS

Our lifetime-based pH imaging data suggested that FLIM can measure the intracellular pH of the resting cells and follow the pH fluctuations inside the cells after environmental perturbations. To improve the z-axis resolution to the intracellular lifetime-resolved images, we are investigating the implementation of the pseudo-confocal capability to our current FLIM apparatus.

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.

The interaction between the fiber knob domain and the cellular attachment receptor determines the intracellular trafficking route of adenoviruses

Most of the presently used adenovirus (Ad) vectors are based on serotype 5. However, the application of these vectors is limited by the native tropism of Ad5. To address this problem, a series of fiber chimeric vectors were produced to take advantage of the different cellular receptors used by Ad of different subgroups. In this study we utilize an Ad5-based chimeric vector containing sequences encoding the Ad35 fiber knob domain instead of the Ad5 knob (Ad5/35L) to analyze factors responsible for selection of intracellular trafficking routes by Ads. By competition analysis with recombinant Ad5 and Ad35 knobs we showed that the Ad5/35L vector infected cells through a receptor different from the Ad5 receptor. Intracellular trafficking of Ad5 and Ad5/35L viruses was analyzed in HeLa cells by tracking fluorophore-conjugated Ad particles, by immunostaining for capsid hexon protein, by electron microscopy, and by Southern blotting for viral DNA. These studies showed that the interaction with the Ad35 receptor(s) predestines Ad5/35L vector to intracellular trafficking pathways different from those of Ad5. Ad5 efficiently escaped from the endosomes early after infection. In contrast, Ad5/35L remained longer in late endosomal/lysosomal compartments and used them to achieve localization to the nucleus. However, a significant portion of Ad5/35L particles appeared to be recycled back to the cell surface. This phenomenon resulted in significantly less efficient Ad5/35L-mediated gene transfer compared to that of Ad5. We also demonstrated that the selection of intracellular trafficking routes was determined by the fiber knob domain and did not depend on the length of the fiber shaft. This study contributes to a better understanding of the mechanisms that govern the infection of retargeted, capsid-modified vectors which have potential application for hematopoietic stem cell and tumor gene therapy.

Regulation of secretory granule pH in insulin-secreting cells

Luminal acidification is important for the maturation of secretory granules, yet little is known regarding the regulation of pH within them. A pH-sensitive green fluorescent protein (EGFP) was targeted to secretory granules in RIN1046-38 insulinoma cells by using a construct in which the EGFP gene was preceded by the nucleotide sequence for human growth hormone. Stimulatory levels of glucose doubled EGFP secretion from cell cultures, and potentiators of glucose-induced insulin secretion enhanced EGFP release. Thus this targeted EGFP is useful for population measurements of secretion. However, less than ~4% of total cell EGFP was released after 1.5 h of stimulation. Consequently, when analyzed in single cells, fluorescence of the targeted EGFP acts as an indicator of pH within secretory granules. Glucose elicited a decrease in granule pH, whereas inhibitors of the V-type H(+)-ATPase increased pH and blocked the glucose effect. Granule pH also was modified by effectors of the protein kinase A pathway, with activation eliciting granule alkalinization, suggesting that potentiation of peptide release by cAMP may involve regulated changes in secretory granule pH.

Hyperacidification of cellubrevin endocytic compartments and defective endosomal recycling in cystic fibrosis respiratory epithelial cells

The cystic fibrosis transmembrane conductance regulator (CFTR), which is aberrant in patients with cystic fibrosis, normally functions both as a chloride channel and as a pleiotropic regulator of other ion transporters. Here we show, by ratiometric imaging with luminally exposed pH-sensitive green fluorescent protein, that CFTR affects the pH of cellubrevin-labeled endosomal organelles resulting in hyperacidification of these compartments in cystic fibrosis lung epithelial cells. The excessive acidification of intracellular organelles was corrected with low concentrations of weak base. Studies with proton ATPase and sodium channel inhibitors showed that the increased acidification was dependent on proton pump activity and sodium transport. These observations implicate sodium efflux in the pH homeostasis of a subset of endocytic organelles and indicate that a dysfunctional CFTR in cystic fibrosis leads to organellar hyperacidification in lung epithelial cells because of a loss of CFTR inhibitory effects on sodium transport. Furthermore, recycling of transferrin receptor was altered in CFTR mutant cells, suggesting a previously unrecognized cellular defect in cystic fibrosis, which may have functional consequences for the receptors on the plasma membrane or within endosomal compartments.

Chloride concentration in endosomes measured using a ratioable fluorescent Cl- indicator: evidence for chloride accumulation during acidification

A novel long wavelength fluorescent Cl(-) indicator was used to test whether endosomal Cl(-) conductance provides the principal electrical shunt to permit endosomal acidification. The green fluorescent Cl(-)-sensitive chromophore 10,10'-bis[3-carboxypropyl]-9,9'-biacridinium dinitrate (BAC) was conjugated to aminodextran together with the red fluorescent Cl(-)-insensitive chromophore tetramethylrhodamine (TMR). BAC fluorescence is pH-insensitive and quenched by Cl(-) with a Stern-Volmer constant of 36 m(-1). Endosomes in J774 and Chinese hamster ovary (CHO) cells were pulse-labeled with BAC-TMR-dextran by fluid-phase endocytosis. Endosomal [Cl(-)] increased over 45 min from 17 to 53 mm in J774 cells and from 28 to 73 mm in CHO cells, during which time endosomal pH decreased from 6.95 to 5.30 (J774) and 6.92 to 5.60 (CHO). The acidification and increased [Cl(-)] were blocked by bafilomycin. Together with ion substitution and buffer capacity measurements, we conclude that Cl(-) transport accounts quantitatively for the electrical shunt during vacuolar acidification. Measurements of relative endosomal volume by a novel ratio imaging method involving fluorescence self-quenching indicated a 2.5-fold increase in volume during early acidification and Cl(-) accumulation, which was blocked by bafilomycin. These experiments provide the first direct measurement of endosomal [Cl(-)] and indicate that endosomal acidification is accompanied by significant Cl(-) entry and volume increase.

Molecular basis for defective glycosylation and Pseudomonas pathogenesis in cystic fibrosis lung

The CFTR gene encodes a transmembrane conductance regulator, which is dysfunctional in patients with cystic fibrosis (CF). The mechanism by which defective CFTR (CF transmembrane conductance regulator) leads to undersialylation of plasma membrane glycoconjugates, which in turn promote lung pathology and colonization with Pseudomonas aeruginosa causing lethal bacterial infections in CF, is not known. Here we show by ratiometric imaging with lumenally exposed pH-sensitive green fluorescent protein that dysfunctional CFTR leads to hyperacidification of the trans-Golgi network (TGN) in CF lung epithelial cells. The hyperacidification of TGN, glycosylation defect of plasma membrane glycoconjugates, and increased P. aeruginosa adherence were corrected by incubating CF respiratory epithelial cells with weak bases. Studies with pharmacological agents indicated a role for sodium conductance, modulated by CFTR regulatory function, in determining the pH of TGN. These studies demonstrate the molecular basis for defective glycosylation of lung epithelial cells and bacterial pathogenesis in CF, and suggest a cure by normalizing the pH of intracellular compartments.

Lysosomal accumulation of drugs in drug-sensitive MES-SA but not multidrug-resistant MES-SA/Dx5 uterine sarcoma cells

Sequestration of drugs in intracellular vesicles has been associated with multidrug-resistance (MDR), but it is not clear why vesicular drug accumulation, which depends upon intracellular pH gradients, should be associated with MDR. Using a human uterine sarcoma cell line (MES-SA) and a doxorubicin (DOX)-resistant variant cell line (Dx-5), which expresses p-glycoprotein (PGP), we have addressed the relationship between multidrug resistance, vesicular acidification, and vesicular drug accumulation. Consistent with a pH-dependent mechanism of vesicular drug accumulation, studies of living cells vitally labeled with multiple probes indicate that DOX and daunorubicin (DNR) predominately accumulate in lysosomes, whose lumenal pH was measured at < 4.5, but are not detected in endosomes, whose pH was measured at 5.9. However, vesicular DOX accumulation is more pronounced in the drug-sensitive MES-SA cells and minimal in Dx5 cells even when cellular levels of DOX are increased by verapamil treatment. While lysosomal accumulation of DOX correlated well with pharmacologically induced differences in lysosome pH in MES-SA cells, lysosomal accumulation was minimal in Dx5 cells regardless of lysosomal pH. We found no differences in the pH of either endosomes or lysosomes between MES-SA and Dx5 cells, suggesting that, in contrast to other MDR cell systems, the drug-resistant Dx5 cells are refractory to pH-dependent vesicular drug accumulation. These studies demonstrate that altered endomembrane pH regulation is not a necessary consequence of cell transformation, and that vesicular sequestration of drugs is not a necessary characteristic of MDR.

The first step of adenovirus type 2 disassembly occurs at the cell surface, independently of endocytosis and escape to the cytosol

Disassembly is a key event of virus entry into cells. Here, we have investigated cellular requirements for the first step of adenovirus type 2 (Ad2) disassembly, the release of the fibers. Although fiber release coincides temporally with virus uptake, fiber release is not required for Ad2 endocytosis. It is, however, inhibited by actin-disrupting agents or soluble RGD peptides, which interfere with integrin-dependent endocytosis of Ad2. Fiber release occurs at the cell surface. Actin stabilization with jasplakinolide blocks Ad2 entry at extended cell surface invaginations and efficiently promotes fiber release, indicating that fiber release and virus endocytosis are independent events. Fiber release is not sufficient for Ad2 escape from endosomes, since inhibition of protein kinase C (PKC) prevents Ad2 escape from endosomes but does not affect virus internalization or fiber release. PKC-inhibited cells accumulate Ad2 in small vesicles near the cell periphery, indicating that PKC is also required for membrane trafficking of virus. Taken together, our data show that fiber release from incoming Ad2 requires integrins and filamentous actin. Together with correct subcellular transport of Ad2-containing endosomes, fiber release is essential for efficient delivery of virus to the cytosol. We speculate that fiber release at the surface might extend the host range of Ad2 since it is associated with the separation of a small fraction of incoming virus from the target cells.

Activation of melanogenesis by vacuolar type H(+)-ATPase inhibitors in amelanotic, tyrosinase positive human and mouse melanoma cells

In this study, we describe the activation of melanogenesis by selective vacuolar type H(+)-ATPase inhibitors (bafilomycin A1 and concanamycin A) in amelanotic human and mouse melanoma cells which express tyrosinase but show no melanogenesis. Addition of the inhibitors activated tyrosinase within 4 h, and by 24 h the cells contained measurable amounts of melanin. These effects were not inhibited by cycloheximide (2 microgram/ml) which is consistent with a post-translational mechanism of activation. Our findings suggest that melanosomal pH could be an important and dynamic factor in the control of melanogenesis in mammalian cells.

Apical and basolateral endocytic pathways of MDCK cells meet in acidic common endosomes distinct from a nearly-neutral apical recycling endosome

Quantitative confocal microscopic analyses of living, polarized MDCK cells demonstrate different pH profiles for apical and basolateral endocytic pathways, despite a rapid and extensive intersection between the two. Three-dimensional characterizations of ligand trafficking demonstrate that the apical and basolateral endocytic pathways share early, acidic compartments distributed throughout the medial regions of the cell. Polar sorting for both pathways occurs in these common endosomes as IgA is sorted from transferrin to alkaline transcytotic vesicles. While transferrin is directly recycled from the common endosomes, IgA is transported to a downstream apical compartment that is nearly neutral in pH. By several criteria this compartment appears to be equivalent to the previously described apical recycling endosome. The functional significance of the abrupt increase in lumenal pH that accompanies IgA sorting is not clear, as disrupting endosome acidification has no effect on polar sorting. These studies provide the first detailed characterizations of endosome acidification in intact polarized cells and clarify the relationship between the apical and basolateral endocytic itineraries of polarized MDCK cells. The extensive mixing of apical and basolateral pathways underscores the importance of endocytic sorting in maintaining the polarity of the plasma membrane of MDCK cells.

Inhibition of Na+-H+ exchange impairs receptor-mediated albumin endocytosis in renal proximal tubule-derived epithelial cells from opossum

1. Receptor-mediated endocytosis is an important mechanism for transport of macromolecules and regulation of cell-surface receptor expression. In renal proximal tubules, receptor-mediated endocytosis mediates the reabsorption of filtered albumin. Acidification of the endocytic compartments is essential because it interferes with ligand-receptor dissociation, vesicle trafficking, fusion events and coat formation. 2. Here we show that the activity of Na+-H+ exchanger isoform 3 (NHE3) is important for proper receptor-mediated endocytosis of albumin and endosomal pH homeostasis in a renal proximal tubular cell line (opossum kidney cells) which expresses NHE3 only. 3. Depending on their inhibitory potency with respect to NHE3 and their lipophilicity, the NHE inhibitors EIPA, amiloride and HOE694 differentially reduced albumin endocytosis. The hydrophilic inhibitor HOE642 had no effect. 4. Inhibition of NHE3 led to an alkalinization of early endosomes and to an acidification of the cytoplasm, indicating that Na+-H+ exchange contributes to the acidification of the early endosomal compartment due to the existence of a sufficient Na+ gradient across the endosomal membrane. 5. Exclusive acidification of the cytoplasm with propionic acid or by removal of Na+ induced a significantly smaller reduction in endocytosis than that induced by inhibition of Na+-H+ exchange. 6. Analysis of the inhibitory profiles indicates that in early endosomes and endocytic vesicles NHE3 is of major importance, whereas plasma membrane NHE3 plays a minor role. 7. Thus, NHE3-mediated acidification along the first part of the endocytic pathway plays an important role in receptor-mediated endocytosis. Furthermore, the involvement of NHE3 offers new ways to explain the regulation of receptor-mediated endocytosis.

Direct visualization of HIV-1 entry: mechanisms and role of cell surface receptors

Highly fluorescent virions of T- and M-tropic HIV-1 strains were obtained by incorporation of the viral accessory protein Vpr, fused to the green fluorescent protein, in trans. The fluorescent virions displayed normal morphology, were infectious, and could be used for direct visualization of HIV-1 attachment and trafficking in various cell lines. More than 90% of the viral particles were found to enter the cells by direct membrane fusion in T-cells, CD4+ HeLa cells, and macrophages. Visualizing HIV-1 attachment and entry in the absence or presence of CD4 and/or the appropriate coreceptors indicated that CD4 is the major receptor for virus attachment in the case of JR-CSF and NL-4-3 HIV-1 isolates; however, the coreceptors are required for membrane fusion. Internalization of the coreceptor CXCR4 inhibited entry, but did not prevent virus binding suggesting that transient downregulation of the coreceptor(s) may not be the most efficient way of blocking HIV infection in vivo.

Lifetime-based pH sensors: indicators for acidic environments

We characterized the pH-dependent intensity decays of three fluorophores, Oregon green 514 carboxylic acid, Cl-NERF, and DM-NERF, using frequency-domain fluorometry, with the objective of identifying lifetime-based sensors for low pH values. These three probes were originally designed as dual excitation wavelength-ratiometric probes, with high photostability and high quantum yields in aqueous solutions. We found that their fluorescence intensity decays were strongly dependent on pH. Moreover, global intensity decays analysis reveals that these probes have double exponential intensity decays at intermediate pH values and that the decay time amplitudes are greatly dependent on pH. The longer lifetime components originated from the unprotonated forms and the shorter components from the protonated forms. Both forms can emit fluorescence at intermediate pH values. The apparent pKa values were also determined from the titration curves of phase angles and modulations versus pH for the purpose of pH sensing. The apparent pKa values range from pH 3 to 5, a range where lifetime-based sensors are not presently reported. Since these probes show low pKa values and display substantial phase and modulation changes with pH, they are suitable as lifetime-based pH sensors to monitor the pH changes in acidic environments. One potential application of these probes is to trace the pH in different cellular compartments.

Application of magnetic chromatography to the isolation of lysosomes from fibroblasts of patients with lysosomal storage disorders

A method for the purification of lysosomes from fibroblasts has been developed which uses endocytosis of superparamagnetic colloidal iron dextran particles followed by separation of the iron-containing lysosomes in a magnetic field. This permitted isolation of lysosomes from fibroblasts from patients with infantile sialic acid storage disorder and other lysosomal storage diseases in which a shift in lysosomal density induced by the storage material prevents purification by centrifugation in a Percoll gradient. The magnetic lysosomes isolated from these cells are very similar to those from normal cells as judged by lysosomal marker enzyme activity and 2D-PAGE analysis of the enriched proteins.

Calcium uptake via endocytosis with rapid release from acidifying endosomes

A number of specific cellular Ca2+ uptake pathways have been described in many different cell types [1] [2] [3]. The possibility that substantial quantities of Ca2+ could be imported via endocytosis has essentially been ignored, although it has been recognized that endosomes can store Ca2+ [4] [5]. Exocrine cells can release significant amounts of Ca2+ via exocytosis [6], so we have investigated the fate of Ca2+ taken up via endocytosis into endosomes. Ca2+-sensitive and H+-sensitive fluorescent probes were placed in the extracellular solution and subsequently taken up into fibroblasts by endocytosis. Confocal microscopy was used to assess the distribution of fluorescence intensity. Ca2+ taken up by endocytosis was lost from the endosomes within a few minutes, over the same period as endosomal acidification took place. The acidification was inhibited by reducing the extracellular Ca2+ concentration, and Ca2+ loss from the endosomes was blocked by bafilomycin (100 nM), a specific inhibitor of the vacuolar proton ATPase. Quantitative evaluation indicated that endocytosis causes substantial import of Ca2+ because of rapid loss from early endosomes.

Effect of bafilomycin A1 and nocodazole on endocytic transport in HeLa cells: implications for viral uncoating and infection

Bafilomycin A1 (baf), a specific inhibitor of vacuolar proton ATPases, is commonly employed to demonstrate the requirement of low endosomal pH for viral uncoating. However, in certain cell types baf also affects the transport of endocytosed material from early to late endocytic compartments. To characterize the endocytic route in HeLa cells that are frequently used to study early events in viral infection, we used 35S-labeled human rhinovirus serotype 2 (HRV2) together with various fluid-phase markers. These virions are taken up via receptor-mediated endocytosis and undergo a conformational change to C-antigenic particles at a pH of <5.6, resulting in release of the genomic RNA and ultimately in infection (E. Prchla, E. Kuechler, D. Blaas, and R. Fuchs, J. Virol. 68:3713-3723, 1994). As revealed by fluorescence microscopy and subcellular fractionation of microsomes by free-flow electrophoresis (FFE), baf arrests the transport of all markers in early endosomes. In contrast, the microtubule-disrupting agent nocodazole was found to inhibit transport by accumulating marker in endosomal carrier vesicles (ECV), a compartment intermediate between early and late endosomes. Accordingly, lysosomal degradation of HRV2 was suppressed, whereas its conformational change and infectivity remained unaffected by this drug. Analysis of the subcellular distribution of HRV2 and fluid-phase markers in the presence of nocodazole by FFE revealed no difference from the control incubation in the absence of nocodazole. ECV and late endosomes thus have identical electrophoretic mobilities, and intraluminal pHs of <5.6 and allow uncoating of HRV2. As bafilomycin not only dissipates the low endosomal pH but also blocks transport from early to late endosomes in HeLa cells, its inhibitory effect on viral infection could in part also be attributed to trapping of virus in early endosomes which might lack components essential for uncoating. Consequently, inhibition of viral uncoating by bafilomycin cannot be taken to indicate a low pH requirement only.