The transport systems of mammalian lysosomes

Complexities of the chemogenetic toolkit: Differential mDAAO activation by d-amino substrates and subcellular targeting

A common approach to investigate oxidant-regulated intracellular pathways is to add exogenous H₂O₂ to living cells or tissues. However, the addition of H₂O₂ to the culture medium of cells or tissues approach does not accurately replicate intracellular redox-mediated cell responses. d-amino acid oxidase (DAAO)-based chemogenetic tools represent informative methodological advances that permit the generation of H₂O₂ on demand with a high spatiotemporal resolution by providing or withdrawing the DAAO substrate d-amino acids. Much has been learned about the intracellular transport of H₂O₂ through studies using DAAO, yet these valuable tools remain incompletely characterized in many cultured cells. In this study, we describe and characterize in detail the features of a new modified variant of DAAO (termed mDAAO) with improved catalytic activities. We tested mDAAO functionality in several cultured cell lines employing live-cell imaging techniques. Our imaging experiments show that mDAAO is suitable for the generation of H₂O₂ under hypoxic conditions imaged with the novel ultrasensitive H₂O₂ sensor (HyPer7). Moreover, this approach was suitable for generating H₂O₂ in a reversible and concentration-dependent manner in subcellular locales. Furthermore, we show that the choice of d-amino acids differentially affects mDAAO-dependent intracellular H₂O₂ generation. When paired with the hydrogen sulfide (H₂S) sensor hsGFP, administration of the sulfur-containing amino acid d-cysteine to cells expressing mDAAO generates robust H₂S signals. We also show that chemogenetic H₂O₂ generation in different cell types yields distinct HyPer7 profiles. These studies fully characterize the new mDAAO as a novel chemogenetic tool and provide multiparametric approaches for cell manipulation that may open new lines of investigations for redox biochemists to dissect the role of ROS signaling pathways with high spatial and temporal precision.

Rapid purification and metabolomic profiling of synaptic vesicles from mammalian brain

Neurons communicate by the activity-dependent release of small-molecule neurotransmitters packaged into synaptic vesicles (SVs). Although many molecules have been identified as neurotransmitters, technical limitations have precluded a full metabolomic analysis of SV content. Here, we present a workflow to rapidly isolate SVs and to interrogate their metabolic contents at high-resolution using mass spectrometry. We validated the enrichment of glutamate in SVs of primary cortical neurons using targeted polar metabolomics. Unbiased and extensive global profiling of SVs isolated from these neurons revealed that the only detectable polar metabolites they contain are the established neurotransmitters glutamate and GABA. In addition, we adapted the approach to enable quick capture of SVs directly from brain tissue and determined the neurotransmitter profiles of diverse brain regions in a cell-type-specific manner. The speed, robustness, and precision of this method to interrogate SV contents will facilitate novel insights into the chemical basis of neurotransmission.

Inherited disorders of lysosomal membrane transporters

Disorders caused by defects in lysosomal membrane transporters form a distinct subgroup of lysosomal storage disorders (LSDs). To date, defects in only 10 lysosomal membrane transporters have been associated with inherited disorders. The clinical presentations of these diseases resemble the phenotypes of other LSDs; they are heterogeneous and often present in children with neurodegenerative manifestations. However, for pathomechanistic and therapeutic studies, lysosomal membrane transport defects should be distinguished from LSDs caused by defective hydrolytic enzymes. The involved proteins differ in function, localization, and lysosomal targeting, and the diseases themselves differ in their stored material and therapeutic approaches. We provide an overview of the small group of disorders of lysosomal membrane transporters, emphasizing discovery, pathomechanism, clinical features, diagnostic methods and therapeutic aspects. We discuss common aspects of lysosomal membrane transporter defects that can provide the basis for preclinical research into these disorders.

Homogentisic acid-derived pigment as a biocompatible label for optoacoustic imaging of macrophages

Macrophages are one of the most functionally-diverse cell types with roles in innate immunity, homeostasis and disease making them attractive targets for diagnostics and therapy. Photo- or optoacoustics could provide non-invasive, deep tissue imaging with high resolution and allow to visualize the spatiotemporal distribution of macrophages in vivo. However, present macrophage labels focus on synthetic nanomaterials, frequently limiting their ability to combine both host cell viability and functionality with strong signal generation. Here, we present a homogentisic acid-derived pigment (HDP) for biocompatible intracellular labeling of macrophages with strong optoacoustic contrast efficient enough to resolve single cells against a strong blood background. We study pigment formation during macrophage differentiation and activation, and utilize this labeling method to track migration of pro-inflammatory macrophages in vivo with whole-body imaging. We expand the sparse palette of macrophage labels for in vivo optoacoustic imaging and facilitate research on macrophage functionality and behavior.

The lysosomal transporter MFSD1 is essential for liver homeostasis and critically depends on its accessory subunit GLMP

Lysosomes are major sites for intracellular, acidic hydrolase-mediated proteolysis and cellular degradation. The export of low-molecular-weight catabolic end-products is facilitated by polytopic transmembrane proteins mediating secondary active or passive transport. A number of these lysosomal transporters, however, remain enigmatic. We present a detailed analysis of MFSD1, a hitherto uncharacterized lysosomal family member of the major facilitator superfamily. MFSD1 is not N-glycosylated. It contains a dileucine-based sorting motif needed for its transport to lysosomes. Mfsd1 knockout mice develop splenomegaly and severe liver disease. Proteomics of isolated lysosomes from Mfsd1 knockout mice revealed GLMP as a critical accessory subunit for MFSD1. MFSD1 and GLMP physically interact. GLMP is essential for the maintenance of normal levels of MFSD1 in lysosomes and vice versa. Glmp knockout mice mimic the phenotype of Mfsd1 knockout mice. Our data reveal a tightly linked MFSD1/GLMP lysosomal membrane protein transporter complex.

Lysosomes are specialized, enclosed compartments within cells with harsh chemical conditions where enzymes break down large molecules into smaller component parts. The products of these reactions are then transported out of the lysosome by transporter proteins so that they can be used to build new molecules that the cell needs.

Despite their importance, only a few lysosomal transporters have been thoroughly studied. A protein called MFSD1 had previously been identified as a potential lysosomal transporter, but its precise role has not been described.

Now, Massa López et al. have characterized the role of MFSD1, by genetically modifying mice so they could no longer make the transporter. These mice developed severe liver damage. In particular, a specific type of cell that is important for lining blood vessels in the liver, seemed to be lost in these mice. Older MFSD1 deficient mice also had more tumors in their livers compared to normal mice.

Massa López et al. next examined what happened to other lysosomal proteins in the MFSD1 deficient mice, and found that these mice had strikingly low levels of a protein called GLMP. To better understand the relationship between GLMP and MFSD1, another strain of genetically modified mice was analyzed, this time missing GLMP. Mice without GLMP were found to have very similar liver problems to those observed in the mice lacking MFSD1. Moreover, the GLMP deficient mice had low levels of the MFSD1 protein.

Further experiments demonstrated that MFSD1 and GLMP physically interact with each other: GLMP seemed to protect MFSD1 from being degraded in the harsh internal environment of the lysosome. Thus both GLMP and MFSD1 were needed to form a stable lysosomal transporter.

Characterizing MFSD1 is important for scientists attempting to understand how the lysosomal membrane and transporters work. Moreover, these findings may shed light on how defects in lysosomal transporters contribute to metabolic disease.

The phagosome and redox control of antigen processing

In addition to debris clearance and antimicrobial function, versatile organelles known as phagosomes play an essential role in the processing of exogenous antigen in antigen presenting cells. While there has been much attention on human leukocyte antigen haplotypes in the determination of antigenic peptide repertoires, the lumenal biochemistries within phagosomes and endosomes are emerging as equally-important determinants of peptide epitope composition and immunodominance. Recently, the lumenal redox microenvironment within these degradative compartments has been shown to impact two key antigenic processing chemistries: proteolysis by lysosomal cysteine proteases and disulfide reduction of protein antigens. Through manipulation of the balance between oxidative and reductive capacities in the phagosome-principally by modulating NADPH oxidase (NOX2) and γ-interferon-inducible lysosomal thiol reductase (GILT) activities-studies have demonstrated changes to antigen processing patterns leading to modified repertoires of antigenic peptides available for presentation, and subsequently, altered disease progression in T cell-driven autoimmunity. This review focuses on the mechanisms and consequences of redox-mediated phagosomal antigen processing, and the potential downstream implications to tolerance and autoimmunity.

Mechanistic Investigation and Multiplexing of Liposome-Assisted Metabolic Glycan Labeling

Metabolic labeling of glycans with bioorthogonal reporters has been widely used for glycan imaging and glycoproteomic profiling. One of the intrinsic limitations of metabolic glycan labeling is the lack of cell-type selectivity. The recently developed liposome-assisted bioorthogonal reporter (LABOR) strategy provides a promising means to overcome this limitation, but the mechanism of LABOR has not been investigated in detail. In this work, we performed a mechanistic study on LABOR and explored its multiplexing capability. Our studies support an endocytosis-salvage mechanism. The ligand-targeted liposomes encapsulating azidosugars are internalized into the endosome via the receptor-mediated endocytosis. Unlike the conventional drug delivery, LABOR does not rely on the endosomal escape pathways. Rather, the liposomes are allowed to enter the lysosome, inside which the azidosugars are released from the liposomes. The released azidosugars then intercept the salvage pathways of monosaccharides and get transported into the cytosol by lysosomal sugar transporters. Based on this mechanism, we expanded the scope of LABOR by evaluating a series of ligand-receptor pairs for targeting sialoglycans in various cell types. Different ligand types including small molecules, antibodies, aptamers, and peptides could be easily implemented into LABOR. Finally, we demonstrated that the sialoglycans in two distinct cell populations in a co-cultured system could be selectively labeled with two distinct chemical reporters by performing a multiplexed LABOR labeling.

SNAT7 is the primary lysosomal glutamine exporter required for extracellular protein-dependent growth of cancer cells

Lysosomes degrade cellular components sequestered by autophagy or extracellular material internalized by endocytosis and phagocytosis. The macromolecule building blocks released by lysosomal hydrolysis are then exported to the cytosol by lysosomal transporters, which remain undercharacterized. In this study, we designed an in situ assay of lysosomal amino acid export based on the transcription factor EB (TFEB), a master regulator of lysosomal biogenesis that detects lysosomal storage. This assay was used to screen candidate lysosomal transporters, leading to the identification of sodium-coupled neutral amino acid transporter 7 (SNAT7), encoded by the SLC38A7 gene, as a lysosomal transporter highly selective for glutamine and asparagine. Cell fractionation confirmed the lysosomal localization of SNAT7, and flux measurements confirmed its substrate selectivity and showed a strong activation by the lysosomal pH gradient. Interestingly, gene silencing or editing experiments revealed that SNAT7 is the primary permeation pathway for glutamine across the lysosomal membrane and it is required for growth of cancer cells in a low free-glutamine environment, when macropinocytosis and lysosomal degradation of extracellular proteins are used as an alternative source of amino acids. SNAT7 may, thus, represent a novel target for glutamine-related anticancer therapies.

How and why intralumenal membrane fragments form during vacuolar lysosome fusion

Lysosomal membrane fusion mediates the last step of the autophagy and endocytosis pathways and supports organelle remodeling and biogenesis. Because fusogenic proteins and lipids concentrate in a ring at the vertex between apposing organelle membranes, the encircled area of membrane can be severed and internalized within the lumen as a fragment upon lipid bilayer fusion. How or why this intralumenal fragment forms during fusion, however, is not entirely clear. To better understand this process, we studied fragment formation during homotypic vacuolar lysosome membrane fusion in Saccharomyces cerevisiae Using cell-free fusion assays and light microscopy, we find that GTPase activation and trans-SNARE complex zippering have opposing effects on fragment formation and verify that this affects the morphology of the fusion product and regulates transporter protein degradation. We show that fragment formwation is limited by stalk expansion, a key intermediate of the lipid bilayer fusion reaction. Using electron microscopy, we present images of hemifusion diaphragms that form as stalks expand and propose a model describing how the fusion machinery regulates fragment formation during lysosome fusion to control morphology and protein lifetimes.

Endogenous and exogenous pathways maintain the reductive capacity of the phagosome

Although endosomes, lysosomes, and phagosomes require a reductive environment for the optimal activity of disulfide reductases and other thiol-dependent enzymes, how these reductive environments are established and maintained remain unknown. Our goal in this study was to begin to elucidate the redox control systems responsible for maintaining redox-sensitive enzymatic activities in the phagolysosome of murine macrophages. Through the use of specific inhibitors and genetic knockdown of known redox enzymes, we identified redox pathways that influence phagosomal disulfide reduction. In particular, known inhibitors of the NADPH-dependent selenoprotein, thioredoxin reductase, were shown to inhibit phagosomal disulfide reduction and phagosomal proteolysis. This was supported by the observation that conditional deletion of the selenocysteine tRNA in macrophages decreased phagosomal disulfide reduction capacity. In addition, pharmacologic inhibition of the pentose phosphate pathway decreased rates of disulfide reduction and proteolysis in the phagosome, implicating NADPH as a source of phagosomal reductive energy. Finally, by analyzing the effect of extracellular redox couples, such as cysteine:cystine on thiol-dependent phagosomal processes, we demonstrated that the extracellular space can additionally supply the phagosome with reductive energy. Collectively, these data demonstrate that defined cytosolic reductive pathways act in concert with the uptake of cysteine from the extracellular space to support thiol-dependent chemistries in the phagosome.

Lysosomal physiology

Lysosomes are acidic compartments filled with more than 60 different types of hydrolases. They mediate the degradation of extracellular particles from endocytosis and of intracellular components from autophagy. The digested products are transported out of the lysosome via specific catabolite exporters or via vesicular membrane trafficking. Lysosomes also contain more than 50 membrane proteins and are equipped with the machinery to sense nutrient availability, which determines the distribution, number, size, and activity of lysosomes to control the specificity of cargo flux and timing (the initiation and termination) of degradation. Defects in degradation, export, or trafficking result in lysosomal dysfunction and lysosomal storage diseases (LSDs). Lysosomal channels and transporters mediate ion flux across perimeter membranes to regulate lysosomal ion homeostasis, membrane potential, catabolite export, membrane trafficking, and nutrient sensing. Dysregulation of lysosomal channels underlies the pathogenesis of many LSDs and possibly that of metabolic and common neurodegenerative diseases.

Role of amino acid transporters in amino acid sensing

Amino acid (AA) transporters may act as sensors, as well as carriers, of tissue nutrient supplies. This review considers recent advances in our understanding of the AA-sensing functions of AA transporters in both epithelial and nonepithelial cells. These transporters mediate AA exchanges between extracellular and intracellular fluid compartments, delivering substrates to intracellular AA sensors. AA transporters on endosomal (eg, lysosomal) membranes may themselves function as intracellular AA sensors. AA transporters at the cell surface, particularly those for large neutral AAs such as leucine, interact functionally with intracellular nutrient-signaling pathways that regulate metabolism: for example, the mammalian target of rapamycin complex 1 (mTORC1) pathway, which promotes cell growth, and the general control non-derepressible (GCN) pathway, which is activated by AA starvation. Under some circumstances, upregulation of AA transporter expression [notably a leucine transporter, solute carrier 7A5 (SLC7A5)] is required to initiate AA-dependent activation of the mTORC1 pathway. Certain AA transporters may have dual receptor-transporter functions, operating as "transceptors" to sense extracellular (or intracellular) AA availability upstream of intracellular signaling pathways. New opportunities for nutritional therapy may include targeting of AA transporters (or mechanisms that upregulate their expression) to promote protein-anabolic signals for retention or recovery of lean tissue mass.

An extended proteome map of the lysosomal membrane reveals novel potential transporters

Lysosomes are membrane-bound endocytic organelles that play a major role in degrading cell macromolecules and recycling their building blocks. A comprehensive knowledge of the lysosome function requires an extensive description of its content, an issue partially addressed by previous proteomic analyses. However, the proteins underlying many lysosomal membrane functions, including numerous membrane transporters, remain unidentified. We performed a comparative, semi-quantitative proteomic analysis of rat liver lysosome-enriched and lysosome-nonenriched membranes and used spectral counts to evaluate the relative abundance of proteins. Among a total of 2,385 identified proteins, 734 proteins were significantly enriched in the lysosomal fraction, including 207 proteins already known or predicted as endo-lysosomal and 94 proteins without any known or predicted subcellular localization. The remaining 433 proteins had been previously assigned to other subcellular compartments but may in fact reside on lysosomes either predominantly or as a secondary location. Many membrane-associated complexes implicated in diverse processes such as degradation, membrane trafficking, lysosome biogenesis, lysosome acidification, signaling, and nutrient sensing were enriched in the lysosomal fraction. They were identified to an unprecedented extent as most, if not all, of their subunits were found and retained by our screen. Numerous transporters were also identified, including 46 novel potentially lysosomal proteins. We expressed 12 candidates in HeLa cells and observed that most of them colocalized with the lysosomal marker LAMP1, thus confirming their lysosomal residency. This list of candidate lysosomal proteins substantially increases our knowledge of the lysosomal membrane and provides a basis for further characterization of lysosomal functions.

Cyclic ADP-Ribose and NAADP in Vascular Regulation and Diseases

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP), two intracellular Ca²⁺ mobilizing second messengers, have been recognized as a fundamental signaling mechanism regulating a variety of cell or organ functions in different biological systems. Here we reviewed the literature regarding these ADP-ribosylcyclase products in vascular cells with a major focus on their production, physiological roles, and related underlying mechanisms mediating their actions. In particular, several hot topics in this area of research are comprehensively discussed, which may help understand some of the controversial evidence provided by different studies. For example, some new models are emerging for the agonist receptor coupling of CD38 or ADP-ribosylcyclase and for the formation of an acidic microenvironment to facilitate the production of NAADP in vascular cells. We also summarized the evidence regarding the NAADP-mediated two-phase Ca²⁺ release with a slow Ca²⁺-induced Ca²⁺ release (CICR) and corresponding physiological relevance. The possibility of a permanent structural space between lysosomes and sarcoplasmic reticulum (SR), as well as the critical role of lysosome trafficking in phase 2 Ca²⁺ release in response to some agonists are also explored. With respect to the molecular targets of NAADP within cells, several possible candidates including SR ryanodine receptors (RyRs), lysosomal transient receptor potential-mucolipin 1 (TRP-ML1) and two pore channels (TPCs) are presented with supporting and opposing evidence. Finally, the possible role of NAADP-mediated regulation of lysosome function in autophagy and atherogenesis is discussed, which may indicate a new direction for further studies on the pathological roles of cADPR and NAADP in the vascular system.

mTOR regulates lysosomal ATP-sensitive two-pore Na(+) channels to adapt to metabolic state

Survival in the wild requires organismal adaptations to the availability of nutrients. Endosomes and lysosomes are key intracellular organelles that couple nutrition and metabolic status to cellular responses, but how they detect cytosolic ATP levels is not well understood. Here, we identify an endolysosomal ATP-sensitive Na(+) channel (lysoNa(ATP)). The channel is a complex formed by two-pore channels (TPC1 and TPC2), ion channels previously thought to be gated by nicotinic acid adenine dinucleotide phosphate (NAADP), and the mammalian target of rapamycin (mTOR). The channel complex detects nutrient status, becomes constitutively open upon nutrient removal and mTOR translocation off the lysosomal membrane, and controls the lysosome's membrane potential, pH stability, and amino acid homeostasis. Mutant mice lacking lysoNa(ATP) have much reduced exercise endurance after fasting. Thus, TPCs make up an ion channel family that couples the cell's metabolic state to endolysosomal function and are crucial for physical endurance during food restriction.

The role of amino acid transporters in nutrition

PURPOSE OF REVIEW

We consider recent advances in epithelial amino acid transport physiology and our understanding of the functioning of amino acid transporters as sensors, as well as carriers, of tissue nutrient supplies.

RECENT FINDINGS

Gut hormones (e.g. leptin) may regulate intestinal amino acid transporter activity by a variety of mechanisms, although the overall functional significance of such regulation is not yet fully understood. Important functional interactions between amino acid transporters and nutrient-signalling pathways which regulate metabolism [e.g. the mammalian target of rapamycin (mTOR)C1 pathway which promotes cell growth] have been revealed in recent studies. Amino acid transporters on endosomal (e.g. lysosomal) membranes may be of unexpected significance as intracellular nutrient sensors. It is also now evident that certain amino acid transporters may have dual receptor-transporter functions and act as 'transceptors' to sense amino acid availability upstream of signal pathways.

SUMMARY

Increased knowledge on the timescale of the amino acid sensor-signal-effector process(es) should help in the optimization of protein-feeding regimes to gain maximum anabolic effect. New opportunities for nutritional therapy include targeting of amino acid transceptors to promote protein-anabolic signals and mechanisms up-regulating amino acid transporter expression to improve absorptive capacity for nutrients.

Chlamydia species-dependent differences in the growth requirement for lysosomes

Genome reduction is a hallmark of obligate intracellular pathogens such as Chlamydia, where adaptation to intracellular growth has resulted in the elimination of genes encoding biosynthetic enzymes. Accordingly, chlamydiae rely heavily on the host cell for nutrients yet their specific source is unclear. Interestingly, chlamydiae grow within a pathogen-defined vacuole that is in close apposition to lysosomes. Metabolically-labeled uninfected host cell proteins were provided as an exogenous nutrient source to chlamydiae-infected cells, and uptake and subsequent labeling of chlamydiae suggested lysosomal degradation as a source of amino acids for the pathogen. Indeed, Bafilomycin A1 (BafA1), an inhibitor of the vacuolar H⁺/ATPase that blocks lysosomal acidification and functions, impairs the growth of C. trachomatis and C. pneumoniae, and these effects are especially profound in C. pneumoniae. BafA1 induced the marked accumulation of material within the lysosomal lumen, which was due to the inhibition of proteolytic activities, and this response inhibits chlamydiae rather than changes in lysosomal acidification per se, as cathepsin inhibitors also inhibit the growth of chlamydiae. Finally, the addition of cycloheximide, an inhibitor of eukaryotic protein synthesis, compromises the ability of lysosomal inhibitors to block chlamydial growth, suggesting chlamydiae directly access free amino acids in the host cytosol as a preferred source of these nutrients. Thus, chlamydiae co-opt the functions of lysosomes to acquire essential amino acids.

Dual control of cytosolic metals by lysosomal transporters in lobster hepatopancreas

This study describes the membrane transport mechanisms used by lobster (Homarus americanus) hepatopancreatic epithelial lysosomes to accumulate and sequester heavy metals from the cytosol, and thereby aid in the regulation of these ions entering the animal from dietary constituents. The present investigation extends previous work describing lysosomal metal uptake by cation exchange with protons and suggests that a second, parallel, lysosomal transport process involving metal—thiol conjugates may work in conjunction with the cation antiporter to control cytoplasmic metal concentrations. Transport of 65Zn2+ by lysosomal membrane vesicles (LMV) incubated in 1 mmol l−1 glutathione (GSH) was not significantly different from metal transport in the absence of the tripeptide. However, preloading LMV with 1 mmol l−1 α-ketoglutarate (AKG), and then incubating in a medium containing 1 mmol l−1 GSH, more than doubled metal uptake, compared with vesicles equilibrated with chloride or possessing an outwardly directed chloride gradient. Kinetic analysis of lysosomal 65Zn2+ influx as a function of zinc concentration, in vesicles containing 1 mmol l−1 AKG and incubated in 1 mmol l−1 GSH, revealed the presence of a sigmoidal, low affinity, high capacity carrier process transporting the metal into the organelle. These data indicated the possible presence of an organic anion exchanger in lobster lysosomal membranes. Western blot analysis of LMV with a rabbit anti-rat OAT1 antibody showed the presence of an orthologous OAT1-like protein (approximate molecular mass of 80 kDa) signal from these membranes. These results, and those published previously, suggest the occurrence of two metal transporters on hepatopancreatic membranes, a high affinity, low capacity cation antiporter and a low affinity, high capacity organic anion exchanger. Together these two systems have the potential to regulate cytoplasmic metals over a wide concentration range.

Molecular physiology and pathophysiology of lysosomal membrane transporters

In contrast to lysosomal hydrolytic enzymes, the lysosomal membrane remains poorly characterized. In particular, although the genetic study of cystinosis and sialic acid storage disorders led to the identification of two lysosomal transporters for cystine and sialic acids, respectively, ten years ago, most transporters responsible for exporting lysosomal hydrolysis products to the cytosol are still unknown at the molecular level. However, two lines of investigation recently started to fill this gap in the knowledge of lysosomal biology. First, novel proteomic approaches are now able to provide a reliable inventory of lysosomal membrane proteins. On the other hand, a novel functional approach based on intracellular trafficking mechanisms allows direct transport measurement in whole cells by redirecting recombinant lysosomal transporters to the cell surface. After surveying the current state of knowledge in this field, the review focuses on the sialic acid transporter sialin and shows how recent functional data using the above whole-cell approach shed new light on the pathogenesis of sialic acid storage disorders by revealing the existence of a residual transport activity associated with Salla disease.

The role of mitochondrial and plasma membrane nucleoside transporters in drug toxicity

Many anticancer and antiviral drugs are nucleoside analogues, which interfere with nucleotide metabolism and DNA replication to produce pharmacological effects. Clinical efficacy and toxicity of nucleoside drugs are closely associated with nucleoside transporters because they mediate the transport of nucleoside drugs across biological membranes. Two families of human nucleoside transporters (equilibrative nucleoside transporters and concentrative nucleoside transporters) have been extensively studied for several decades. They are widely distributed, from the plasma membrane to membranes of organelles such as mitochondria, and the distribution differs in different tissues. In addition, they have different specificities to nucleoside drugs. The characteristics of equilibrative and concentrative nucleoside transporters affect the therapeutic outcomes achieved with anticancer and antiviral nucleoside drugs. In this review, an overview of the role of mitochondrial and plasma membrane nucleoside transporters in nucleoside drug toxicity is provided. Rational design and therapeutic application of nucleoside analogues are also discussed.

Heavy metal detoxification in crustacean epithelial lysosomes: role of anions in the compartmentalization process

Crustacean hepatopancreatic lysosomes are organelles of heavy metal sequestration and detoxification. Previous studies have shown that zinc uptake by lysosomal membrane vesicles (LMV) occurred by a vanadate- and thapsigargin-sensitive ATPase that was stimulated by a transmembrane proton gradient established by a co-localized V-ATPase associated with this organelle. In the present study, hepatopancreatic LMV from the American lobster Homarus americanus were prepared by standard centrifugation methods and 65Zn2+, 36Cl-, 35SO(4)2- and 14C-oxalate2- were used to characterize the interactions between the metal and anions during vesicular detoxification events. Vesicles loaded with SO4(2-) or PO(4)3- led to a threefold greater steady-state accumulation of Zn2+ than similar vesicles loaded with mannitol, Cl- or oxalate2-. The stimulation of 65Zn2+ uptake by intravesicular sulfate was SO(4)2- concentration dependent with a maximal enhancement at 500 micromol l(-1). Zinc uptake in the presence of ATP was proton-gradient enhanced and electrogenic, exhibiting an apparent exchange stoichiometry of 1Zn+/3H+. 35SO4(2-) and 14C-oxalate2- uptakes were both enhanced in vesicles loaded with intravesicular Cl- compared to vesicles containing mannitol, suggesting the presence of anion countertransport. 35SO4(2-) influx was a sigmoidal function of external [SO(4)2-] with 25 mmol l(-1) internal [Cl-], or with several intravesicular pH values (e.g. 7.0, 8.0 and 9.0). In all instances Hill coefficients of approximately 2.0 were obtained, suggesting that 2 sulfate ions exchange with single Cl- or OH- ions. 36Cl- influx was a sigmoidal function of external [Cl-] with intravesicular pH of 7.0 and 9.0. A Hill coefficient of 2.0 was also obtained, suggesting the exchange of 2 Cl- for 1 OH-. 14C-oxalate influx was a hyperbolic function of external [oxalate2-] with 25 mmol l(-1) internal [Cl-], suggesting a 1:1 exchange of oxalate2- for Cl-. As a group, these experiments suggest the presence of an anion exchange mechanism exchanging monovalent for polyvalent anions. Polyvalent inorganic anions (SO4(2-) and PO4(3-)) are known to associate with metals inside vesicles and a detoxification model is presented that suggests how these anions may contribute to concretion formation through precipitation with metals at appropriate vesicular pH.

Zaprinast inhibits hydrogen peroxide-induced lysosomal destabilization and cell death in astrocytes

The lysosomal destabilization that precedes mitochondrial apoptotic changes is an important step in cell death, particularly in oxidative cell death. This study describes the novel pharmacological effects of zaprinast, a cGMP-elevating phosphodiesterase inhibitor, on the inhibition of oxidative cell death in astrocyte cultures. H2O2-induced oxidative cytotoxicity was measured grossly by monitoring lactate dehydrogenase (LDH) release, and was found to be associated with lysosomal acridine orange relocation, lysosomal cathepsin D release into cytosol, and reduced mitochondrial potentials. Moreover, zaprinast (100 microM) inhibited all of these cytotoxic phenomena. In addition, H2O2-induced LDH release was not inhibited by 8-pCPT-cGMP, and the inhibition of this release by zaprinast was unaffected by Rp-8-pCPT-cGMP, a protein kinase G inhibitor. Zaprinast was found to inhibit sphingosine-induced lysosomal acridine orange relocation and the induced decrease in mitochondrial potential, but zaprinast had no effect on rotenone-induced mitochondrial collapse, which was not associated with lysosomal destabilization. However, zaprinast did not inhibit the cellular increase of reactive oxygen species induced by H2O2, which suggests that its protective mechanism differs from that of desferrioxamine, which does inhibit such cellular increase of oxygen free radicals. We suggest that the novel protective effect of zaprinast on H2O2-induced oxidative cell death is primarily associated with its inhibition of lysosomal destabilization.

The role of nucleoside transporters in cancer chemotherapy with nucleoside drugs

Nucleoside analogs are important components of treatment regimens for various malignancies. Nucleoside-specific membrane transporters mediate plasma membrane permeation of physiologic nucleosides and most nucleoside analogs, for which the initial event is cellular conversion of nucleosides to active agents. Understanding of the roles of nucleoside transporters in nucleoside drug toxicity and resistance will provide opportunities for potentiating anticancer efficacy and avoiding resistance. Because transportability is a possible determinant of toxicity and resistance of many nucleoside analogs, nucleoside transporter abundance might be a prognostic marker to assess drug resistance. Elucidation of the structural determinants of nucleoside analogs for interaction with transporter proteins as well as the structural features of transporter proteins required for permeant interaction and translocation will lead to "transportability guidelines" for the rational design and therapeutic application of nucleoside analogs as anticancer drugs. It should eventually be possible to develop clinical assays that predict sensitivity and/or resistance to nucleoside anti-cancer drugs and thus to identify those patient populations that will most likely benefit from optimal nucleoside analog treatments. This review discusses recent results from structure/function studies of human nucleoside transporters, the role of nucleoside transport processes in the cytotoxicity and resistance of several anticancer nucleoside analogs and strategies to improve the nucleoside transporter-related anticancer effects of nucleoside analogs.

Effects of phospholipase A2 on the lysosomal ion permeability and osmotic sensitivity

In this study, we investigated the mechanism of PLA(2)-induced lysosomal destabilization. Through the measurements of lysosomal beta-hexosaminidase free activity, their membrane potential, the intra-lysosomal pH and the lysosomal latency loss in hypotonic sucrose medium, we established that PLA(2) could increase the lysosomal membrane permeability to both potassium ions and protons. The enzyme could also enhance the organelle osmotic sensitivity. The increases in the lysosomal ion permeability promoted influx of potassium ions into the lysosomes via K(+)/H(+) exchange. The resulted osmotic imbalance across the lysosomal membranes osmotically destabilized the lysosomes. In addition, the enhancement of the lysosomal osmotic sensitivity caused the lysosomes to become more liable to destabilization in the osmotic stress. The results explain how PLA(2) destabilized the lysosomes.

Guanosine 5'-[gamma-thio]triphosphate-mediated activation of cytosol phospholipase C caused lysosomal destabilization

Lysosomal disintegration is critical for the organelle functions and cellular viability. In this study, we established that guanosine 5'-[gamma-thio]triphosphate (GTP-gamma-S)-activated cytosol of rat hepatocytes could increase lysosomal permeability to both potassium ions and protons and osmotically destabilize the lysosomes via K(+)/H(+) exchange. These results were obtained through measurements of lysosomal beta-hexosaminidase-free activity, membrane potential and intralysosomal pH. Assays of phospholipase C (PLC) activity show that cytosolic PLC was activated upon addition of GTP-gamma-S to the cytosol. The effects of cytosol on the lysosomes could be abolished by D609, an inhibitor of PLC, but not by the inhibitors of phospholipase A(2). The cytosol-treated lysosomes disintegrated markedly in hypotonic sucrose medium, reflecting that the lysosomal osmotic sensitivity increased. Microscopic observations showed that the lysosomes became more swollen in hypotonic sucrose medium. This indicates that the cytosol treatment induced osmotic shock to the lysosomes and an influx of water into the organelle.

Mechanism of cytosol phospholipase C and sphingomyelinase-induced lysosome destabilization

Lysosomal disintegration may cause apoptosis, necrosis and some diseases. However, mechanisms for these events are still unclear. In this study, we measured lysosomal beta-hexosaminidase free activity, membrane potential and intralysosomal pH. The results revealed that the cytosolic extracts of rat hepatocytes could increase the lysosomal permeability to both potassium ions and protons, and osmotically destabilize lysosomes via K(+)/H(+) exchange. The effects of cytosol on lysosomes could be completely abolished by D609, which inhibited both phospholipase C and sphingomyelinase, and partly prevented by sphingomyelinase inhibitor Ara-AMP, but not by the inhibitors of PLA(2). Moreover, purified phospholipase C could destabilize the lysosomes while phospholipase A(2) and phospholipase D did not produce such effects. The cytosolic phospholipases hydrolyzed lysosomal membrane phospholipids by 50%, which could be prevented by D609. Disintegration of the cytosol-treated lysosomes biphasically depended on the cytosolic [Ca(2+)]. The cytosol did not disintegrate lysosomes below 100 nM or above 10 muM cytosolic [Ca(2+)], but markedly destabilized lysosomes at about 340 nM [Ca(2+)]. The results suggest that cytosolic phospholipase C and sphingomyelinase may be responsible for the alterations in lysosomal stability by increasing the ion permeability.

Glycogen autophagy in glucose homeostasis

Glycogen autophagy, the sequestration and degradation of cell glycogen in the autophagic vacuoles, is a selective, hormonally controlled and highly regulated process, representing a mechanism of glucose homeostasis under conditions of demand for the production of this sugar. In the newborn animals, this process is induced by glucagon secreted during the postnatal hypoglycemia and inhibited by insulin and parenteral glucose, which abolishes glucagon secretion. Hormonal action is mediated by the cAMP/protein kinase A (induction) and phosphoinositides/mTOR (inhibition) pathways that converge on common targets, such as the protein phosphatase 2A to regulate autophgosomal glycogen-hydrolyzing acid glucosidase and glycogen autophagy. Intralysosomal phosphate exchange reactions, which are affected by changes in the calcium levels and acid mannose 6- and acid glucose 6-phosphatase activities, can modify the intralysosomal composition in phosphorylated and nonphosphorylated glucose and promote the exit of free glucose through the lysosomal membrane. Glycogen autophagy-derived nonphosphorylated glucose assists the hyaloplasmic glycogen degradation-derived glucose 6-phosphate to combat postnatal hypoglycemia and participates in other metabolic pathways to secure the fine tuning of glucose homeostasis during the neonatal period.

Intralysosomal iron chelation protects against oxidative stress-induced cellular damage

Oxidant-induced cell damage may be initiated by peroxidative injury to lysosomal membranes, catalyzed by intralysosomal low mass iron that appears to comprise a major part of cellular redox-active iron. Resulting relocation of lytic enzymes and low mass iron would result in secondary harm to various cellular constituents. In an effort to further clarify this still controversial issue, we tested the protective effects of two potent iron chelators--the hydrophilic desferrioxamine (dfo) and the lipophilic salicylaldehyde isonicotinoyl hydrazone (sih), using cultured lysosome-rich macrophage-like J774 cells as targets. dfo slowly enters cells via endocytosis, while the lipophilic sih rapidly distributes throughout the cell. Following dfo treatment, long-term survival of cells cannot be investigated because dfo by itself, by remaining inside the lysosomal compartment, induces apoptosis that probably is due to iron starvation, while sih has no lasting toxic effects if the exposure time is limited. Following preincubation with 1 mM dfo for 3 h or 10 microM sih for a few minutes, both agents provided strong protection against an ensuing approximately LD50 oxidant challenge by preventing lysosomal rupture, ensuing loss of mitochondrial membrane potential, and apoptotic/necrotic cell death. It appears that once significant lysosomal rupture has occurred, the cell is irreversibly committed to death. The results lend strength to the concept that lysosomal membranes, normally exposed to redox-active iron in high concentrations, are initial targets of oxidant damage and support the idea that chelators selectively targeted to the lysosomal compartment may have therapeutic utility in diminishing oxidant-mediated cell injury.

65Zn2+ Transport by lobster hepatopancreatic lysosomal membrane vesicles

In crustaceans, the hepatopancreas is the major organ system responsible for heavy metal detoxification, and within this structure the lysosomes and the endoplasmic reticulum are two organelles that regulate cytoplasmic metal concentrations by selective sequestration processes. This study characterized the transport processes responsible for zinc uptake into hepatopancreatic lysosomal membrane vesicles (LMV) and the interactions between the transport of this metal and those of calcium, copper, and cadmium in the same preparation. Standard centrifugation methods were used to prepare purified hepatopancreatic LMV and a rapid filtration procedure, to quantify 65Zn2+ transfer across this organellar membrane. LMV were osmotically reactive and exhibited a time course of uptake that was linear for 15-30 sec and approached equilibrium by 300 sec. 65Zn2+ influx was a hyperbolic function of external zinc concentration and followed Michaelis-Menten kinetics for carrier transport (Km = 32.3 +/- 10.8 microM; Jmax = 20.7 +/- 2.6 pmol/mg protein x sec). This carrier transport was stimulated by the addition of 1 mM ATP (Km = 35.89 +/- 10.58 microM; Jmax = 31.94+/-3.72 pmol/mg protein/sec) and replaced by an apparent slow diffusional process by the simultaneous presence of 1 mM ATP+250 microM vanadate. Thapsigargin (10 microM) was also a significant inhibitor of zinc influx (Km = 72.87 +/- 42.75 microM; Jmax =22.86 +/- 4.03 pmol/mg protein/sec), but not as effective in this regard as was vanadate. Using Dixon analysis, cadmium and copper were shown to be competitive inhibitors of lysosomal membrane vesicle 65Zn2+ influx by the ATP-dependent transport process (cadmium Ki = 68.1 +/- 3.2 microM; copper Ki = 32.7 +/- 1.9 microM). In the absence of ATP, an outwardly directed H+ gradient stimulated 65Zn2+ uptake, while a proton gradient in the opposite direction inhibited metal influx. The present investigation showed that 65Zn2+ was transported by hepatopancreatic lysosomal vesicles by ATP-dependent, vanadate-, thapsigargin-, and divalent cation-inhibited, carrier processes that illustrated Michaelis-Menten influx kinetics and was stimulated by an outwardly directed proton gradient. These transport properties as a whole suggest that this transporter may be a lysosomal isoform of the ER Sarco-Endoplasmic Reticulum Calcium ATPase.

Lysosomal metabolism of glycoproteins

The lysosomal catabolism of glycoproteins is part of the normal turnover of cellular constituents and the cellular homeostasis of glycosylation. Glycoproteins are delivered to lysosomes for catabolism either by endocytosis from outside the cell or by autophagy within the cell. Once inside the lysosome, glycoproteins are broken down by a combination of proteases and glycosidases, with the characteristic properties of soluble lysosomal hydrolases. The proteases consist of a mixture of endopeptidases and exopeptidases, which act in concert to produce a mixture of amino acids and dipeptides, which are transported across the lysosomal membrane into the cytosol by a combination of diffusion and carrier-mediated transport. Although the glycans of all mature glycoproteins are probably degraded in lysosomes, the breakdown of N-linked glycans has been studied most intensively. The catabolic pathways for high-mannose, hybrid, and complex glycans have been established. They are bidirectional with concurrent sequential removal of monosaccharides from the nonreducing end by exoglycosidases and proteolysis and digestion of the carbohydrate-polypeptide linkage at the reducing end. The process is initiated by the removal of any core and peripheral fucose, which is a prerequisite for the action of the peptide N-glycanase aspartylglucosaminidase, which hydrolyzes the glycan-peptide bond. This enzyme also requires free alpha carboxyl and amino groups on the asparagine residue, implying extensive prior proteolysis. The catabolism of O-linked glycans has not been studied so intensively, but many lysosomal glycosidases appear to act on the same linkages whether they are in N- or O-linked glycans, glycosaminoglycans, or glycolipids. The monosaccharides liberated during the breakdown of N- and O-linked glycans are transported across the lysosomal membrane into the cytosol by a combination of diffusion and carrier-mediated transport. Defects in these pathways lead to lysosomal storage diseases. The structures of some of the oligosaccharides that accumulate in these diseases are not digestion intermediates in the lysosomal catabolic pathways but correspond to intermediates in the biosynthetic pathway for N-linked glycans, suggesting another route of delivery of glycans to the lysosome. Incorrectly folded or glycosylated proteins that are rejected by the quality control mechanism are broken down in the ER and cytoplasm and the end product of the cytosolic degradation of N-glycans is delivered to the lysosomes. This route is enhanced in cells actively secreting glycoproteins or producing increased amounts of aberrant glycoproteins. Thus interaction between the lysosome and proteasome is important for the regulation of the biosynthesis and distribution of N-linked glycoproteins. Another example of the extralysosomal function of lysosomal enzymes is the release of lysosomal proteases into the cytosol to initiate the lysosomal pathway of apoptosis.

Diverse and essential roles of mammalian vacuolar-type proton pump ATPase: toward the physiological understanding of inside acidic compartments

The vacuolar-type H(+)-ATPases (V-ATPase) are a family of multi-subunit ATP-dependent proton pumps involved in a wide variety of physiological processes. They are present in endomembrane organelles such as vacuoles, lysosomes, endosomes, the Golgi apparatus, chromaffin granules and coated vesicles, and acidify the luminal pH of these intracellular compartments. They also pump protons across the plasma membranes of specialized cells including osteoclasts and epithelial cells in kidneys and male genital tracts. Here, we briefly summarize our recent studies on the diverse and essential roles of mammalian V-ATPase.

A growth study of Coxiella burnetii Nine Mile Phase I and Phase II in fibroblasts

Coxiella burnetii, a slow-growing, gram-negative, obligate intracellular bacterium, is the causative agent of Q fever in humans. The avirulent Phase II C. burnetii Nine Mile strain can invade and establish persistent infections in a wide variety of laboratory cell lines, and is generally considered to be easier to grow in culture than the wild-type Phase I organism. Efforts to improve Phase I organism yield in the BHK-21 cell line demonstrated that high CO2 conditions and the use of Dulbecco's modified Eagle's medium (DMEM) with 4.5 g/l glucose supplementation resulted in higher organism yields. Phase II organisms grown in the same cell line and conditions showed lower growth rates. Analysis revealed that increased average numbers of C. burnetii Phase I organisms within fibroblasts was due to higher growth rates within the hosts rather than to increased uptake or to increased cell-to-cell spreading. Addition of the nucleoside cytidine to the growth medium stimulated growth of Phase II but not Phase I organisms.

Varied mechanisms underlie the free sialic acid storage disorders

Salla disease and infantile sialic acid storage disorder are autosomal recessive neurodegenerative diseases characterized by loss of a lysosomal sialic acid transport activity and the resultant accumulation of free sialic acid in lysosomes. Genetic analysis of these diseases has identified several unique mutations in a single gene encoding a protein designated sialin (Verheijen, F. W., Verbeek, E., Aula, N., Beerens, C. E., Havelaar, A. C., Joosse, M., Peltonen, L., Aula, P., Galjaard, H., van der Spek, P. J., and Mancini, G. M. (1999) Nat. Genet. 23, 462-465; Aula, N., Salomaki, P., Timonen, R., Verheijen, F., Mancini, G., Mansson, J. E., Aula, P., and Peltonen, L. (2000) Am. J. Hum. Genet. 67, 832-840). From the biochemical phenotype of the diseases and the predicted polytopic structure of the protein, it has been suggested that sialin functions as a lysosomal sialic acid transporter. Here we directly demonstrate that this activity is mediated by sialin and that the recombinant protein has functional characteristics similar to the native lysosomal sialic acid transport system. Furthermore, we describe the effect of disease-causing mutations on the protein. We find that the majority of the mutations are associated with a complete loss of activity, while the mutations associated with the milder forms of the disease lead to reduced, but residual, function. Thus, there is a direct correlation between sialin function and the disease state. In addition, we find with one mutation that the protein is retained in the endoplasmic reticulum, indicating that altered trafficking of sialin is also associated with disease. This analysis of the molecular mechanism of sialic acid storage disorders is a further step in identifying therapeutic approaches to these diseases.

Glycogen autophagy

Glycogen autophagy, which includes the sequestration and degradation of cell glycogen in the autophagic vacuoles, is a selective process under conditions of demand for the massive hepatic production of glucose, as in the postnatal period. It represents a link between autophagy and glycogen metabolism. The formation of autophagic vacuoles in the hepatocytes of newborn animals is spatially and biochemically related to the degradation of cell glycogen. Many molecular elements and signaling pathways including the cyclic AMP/cyclic AMP-dependent protein kinase and the phosphoinositides/TOR pathways are implicated in the control of this process. These two pathways may converge on the same target to regulate glycogen autophagy.

Lysosome and lysosome-related organelles responsible for specialized functions in higher organisms, with special emphasis on vacuolar-type proton ATPase

Mammals contain various cells differentiated in both morphology and function, which play vital roles in tissue-specific functions. Late endosome/lysosome and lysosomal-related organelles are involved in these specialized functions including antigen presentation, bone remodeling and hormone regulation. To fulfill these diverse roles, lysosomes are present at different levels in different tissues and cell types; however, their morphology within these different tissues varies and the regulation of their activities differs with lysosomal compartments in some cells also functioning as secretory compartments. The luminal acidification of these organelles is closely correlated with their functions. This review will discuss the functions of lysosomes and lysosomal-related organelles, with particular emphasis on the major proton pump, the vacuolar-type proton ATPase (V-ATPase), which is responsible for luminal acidification.

Killing of intraerythrocytic Plasmodium falciparum by lysosomotropic amino acid esters

Esters of amino acids are known to penetrate into cells by simple diffusion. Subsequently, they are hydrolyzed by hydrolases to release the parent amino acid. Due to the abundance of hydrolases in phagolysosomes, amino acids accumulate, there because the rate of influx and hydrolysis exceed the rate of amino acid efflux through specific carriers. The osmotic effect of this accumulation results in the disruption of the organelles. This mechanism has been demonstrated to be responsible for the killing of Leishmania amastigotes by amino acid esters. In this investigation, it is shown that all esters tested, including alcohol esters, N-acetyl esters and the esters of some dipeptides, inhibit the growth of Plasmodium falciparum in culture. Inhibition is time-dependent and, in some cases, ring-stage parasites are more sensitive than trophozoites. Similar to the findings with Leishmania, alcohol esters of Glu, Leu, Met, Phe and Trp are more toxic to Plasmodium whereas Ala, Gly, His and Ile are much less noxious. Esters caused the release of acridine orange that selectively accumulates in the phagolysosome-like food vacuole of the parasite, attesting the ostensible destruction of this organelle by osmotic lysis. The toxicity of the N-acetyl esters is probably associated in part to their ability to inhibit cytosolic proteases. Since excess of amino acids can also inhibit proteolysis, the effect of free amino acids on parasite growth was also tested. Of the 19 odd amino acids tested, only three, namely Cys, His and Trp, were found to be toxic to the parasites at millimolar concentrations and the reasons for their possible specific toxicity are discussed.

The H+-coupled electrogenic lysosomal amino acid transporter LYAAT1 localizes to the axon and plasma membrane of hippocampal neurons

Recent work has identified a lysosomal protein that transports neutral amino acids (LYAAT1). We now show that LYAAT1 mediates H+ cotransport with a stoichiometry of 1 H+/1 amino acid, consistent with a role in the active efflux of amino acids from lysosomes. In neurons, however, LYAAT1 localizes to axonal processes as well as lysosomes. In axons LYAAT1 fails to colocalize with synaptic markers. Rather, axonal LYAAT1 colocalizes with the exocyst, suggesting a role for membranes expressing LYAAT1 in specifying sites for exocytosis. A protease protection assay and measurements of intracellular pH further indicate abundant expression at the plasma membrane, raising the possibility of physiological roles for LYAAT1 on the cell surface as well as in lysosomes.

Influence of the physical states of membrane surface area and center area on lysosomal proton permeability

The physical state of the lysosomal membrane was modulated with the membrane fluidizers n-propanol and n-octanol and with the membrane rigidifiers cholesteryl hemisuccinate and cholesterol. Membrane fluidity was examined by the steady-state fluorescence anisotropy of 2-(9-anthroyloxy) palmitic acid and 16-(9-anthroyloxy) palmitic acid. Fluidizing the membranes at the surface and center areas increased the proton permeability coefficient by 92.8 and 18.0%, respectively. Rigidifying the membranes at the surface and center areas decreased the coefficient by 68.2 and 40.2%, respectively. Proton leakage of the lysosomes increased and decreased similar to the coefficient changes with the treatments. The results indicate that lysosomal proton permeability is affected by its membrane's physical state, and the physical state of the membrane surface area affects the proton permeability more markedly. The proton permeability coefficient of liposomes was similar to that of lysosomes, suggesting that efflux of lysosomal protons might occur through the lipid part of the bilayer but not transmembrane proteins.

Permeability of Coxiella burnetii to ribonucleosides

Knowledge about transport in Coxiella burnetii, an obligate phagolysosomal parasite, is incomplete. The authors investigated the capability of isolated, intact, host-free Coxiella to transport ribonucleosides while incubated at a pH value typical of lysosomes. Because of the low activities and limitations of obtaining experimental quantities of isolated, purified Coxiella, incorporation of substrate into nucleic acid was used as a trap for determination of uptake abilities. Virulent wild-type (phase I) organisms possessed uptake capability for all ribonucleosides. Both phase I and phase II (avirulent) organisms incorporated the purine nucleosides guanosine, adenosine and inosine, and showed a more limited uptake of thymidine and uridine. Both phases were poorly active in cytidine uptake. Neither phase of the organism was capable of transport and incorporation of NTPs, CMP, cytosine or uracil. Water space experiments confirmed that the uptake process concentrated the purine nucleosides within the cytoplasm of both wild-type and phase II Coxiella via a low-pH-dependent mechanism. Comparison of uptake rates in Escherichia coli versus Coxiella verified that the incorporation of ribonucleosides by Coxiella is a slow process. It is concluded that Coxiella possesses some transport pathways consistent with utilization of pools of nucleosides found within its host cell lysosomal pathway.

Cathepsin B stability, but not activity, is affected in cysteine:cystine redox buffers

In order to test the hypothesis that the lysosomal cysteine protease cathepsin B may be redox regulated in vivo, cathepsin B activity and stability were measured in cysteine- and/or cystine-containing buffers. Cathepsin B activity in cysteine-containing buffers was similar at pH 6.0 and pH 7.0, over all thiol concentrations tested. In contrast, the stability of the enzyme was greater at pH 6.0 than at pH 7.0. This suggests that the enzyme's operational pH in vivo may be < pH 7.0. The activity of the enzyme was depressed in glutathione-containing buffers. When assessed in cysteine:cystine redox buffers (pH 6.0-7.0) cathepsin B was active over a broad redox potential range, suggesting that cathepsin B activity may not be redox regulated. However, at pH 7.0, the stability of cathepsin B decreased with increasing reduction potential and ambient cystine concentration. This suggests that the stability of the enzyme at neutral pH is dependent on redox potential, and on the presence of oxidising agents.

Endolysosomal proteolysis and its regulation

The endolysosomal system comprises a unique environment for proteolysis, which is regulated in a manner that apparently does not involve protease inhibitors. The system comprises a series of membrane-bound intracellular compartments, within which endocytosed material and redundant cellular components are hydrolysed. Endocytosed material tends to flow vectorially through the system, proceeding through the early endosome, the endosome carrier vesicle, the late endosome and the lysosome. Phagocytosis and autophagy provide alternative entry points into the system. Late endosomes, lysosome/late endosome hybrid organelles, phagosomes and autophagosomes are the principal sites for proteolysis. In each case, hydrolytic competence is due to components of the endolysosomal system, i.e. proteases, lysosome-associated membrane proteins, H(+)-ATPases and possibly cysteine transporters. The view is emerging that lysosomes are organelles for the storage of hydrolases, perhaps in an inactivated form. Once a substrate has entered a proteolytically competent environment, the rate-limiting proteolytic steps are probably effected by cysteine endoproteinases. As these are affected by pH and possibly redox potential, they may be regulated by the organelle luminal environment. Regulation is probably also affected, among other factors, by organelle fusion reactions, whereby the meeting of enzyme and substrate may be controlled. Such systems would permit simultaneous regulation of a number of unrelated hydrolases.

Life in vacuoles--nutrient acquisition by Leishmania amastigotes

Leishmania have a digenetic life cycle, involving a motile, extracellular stage (promastigote) which parasitises the alimentary tract of a sandfly vector. Bloodfeeding activity by an infected sandfly can result in transmission of infective (metacyclic) promastigotes to mammalian hosts, including humans. Leishmania promastigotes are rapidly phagocytosed but may survive and transform into non-motile amastigote forms which can persist as intracellular parasites. Leishmania amastigotes multiply in an acidic intracellular compartment, the parasitophorous vacuole. pH plays a central role in the developmental switch between promastigote and amastigote stages, and amastigotes are metabolically most active when their environment is acidic, although the cytoplasm of the amastigote is regulated at near-neutral pH by an active process of proton extrusion. A steep proton gradient is thus maintained across the amastigote surface and all membrane processes must be adapted to function under these conditions. Amastigote uptake systems for glucose, amino acids, nucleosides and polyamines are optimally active at acidic pH. Promastigote uptake systems are kinetically distinct and function optimally at more neutral environmental pH, indicating that membrane transport activity is developmentally regulated. The nutrient environment encountered by amastigotes is not well understood but the parasitophorous vacuole can fuse with endosomes, phagosomes and autophagosomes, suggesting that a diverse range of macromolecules will be present. The parasitophorous vacuole is a hydrolytic compartment in which such material will be rapidly degraded to low molecular weight components which are typical substrates for membrane transporters. Amastigote surface transporters must compete for these substrates with equivalent host transporters in the membrane of the parasitophorous vacuole. The elaboration of accumulative transporters with high affinity will be beneficial to amastigotes in this environment. The influence of environmental pH on membrane transporter function is discussed, with emphasis on the potential role of a transmembrane proton gradient in active, high affinity transport.

Apoptosis induced by exposure to a low steady-state concentration of H2O2 is a consequence of lysosomal rupture

We have re-examined the lysosomal hypothesis of oxidative-stress-induced apoptosis using a new technique for exposing cells in culture to a low steady-state concentration of H(2)O(2). This steady-state technique mimics the situation in vivo better than the bolus-administration method. A key aspect of H(2)O(2)-induced apoptosis is that the apoptosis is evident only after several hours, although cells may become committed within a few minutes of exposure to this particular reactive oxygen species. In the present work, we were able to show, for the first time, several correlative links between the triggering effect of H(2)O(2) and the later onset of apoptosis: (i) a short (15 min) exposure to H(2)O(2) caused almost immediate, albeit limited, lysosomal rupture; (ii) early lysosomal damage, and later apoptosis, showed a similar dose-related response to H(2)O(2); (iii) both events were inhibited by pre-treatment with iron chelators, including desferrioxamine. This compound is known to be taken up by endocytosis only and thus to become localized in the lysosomal compartment. After exposure to oxidative stress, when cells were again in standard culture conditions, a time-dependent continuous increase in lysosomal rupture was observed, resulting in a considerably lowered number of intact lysosomes in apoptotic cells, whereas non-apoptotic cells from the same batch of oxidative-stress-exposed cells showed mainly intact lysosomes. Taken together, our results reinforce earlier findings and strongly suggest that lysosomal rupture is an early upstream initiating event, and a consequence of intralysosomal iron-catalysed oxidative processes, when apoptosis is induced by oxidative stress.

A family of yeast proteins mediating bidirectional vacuolar amino acid transport

Seven genes in Saccharomyces cerevisiae are predicted to code for membrane-spanning proteins (designated AVT1-7) that are related to the neuronal gamma-aminobutyric acid-glycine vesicular transporters. We have now demonstrated that four of these proteins mediate amino acid transport in vacuoles. One protein, AVT1, is required for the vacuolar uptake of large neutral amino acids including tyrosine, glutamine, asparagine, isoleucine, and leucine. Three proteins, AVT3, AVT4, and AVT6, are involved in amino acid efflux from the vacuole and, as such, are the first to be shown directly to transport compounds from the lumen of an acidic intracellular organelle. This function is consistent with the role of the vacuole in protein degradation, whereby accumulated amino acids are exported to the cytosol. Protein AVT6 is responsible for the efflux of aspartate and glutamate, an activity that would account for their exclusion from vacuoles in vivo. Transport by AVT1 and AVT6 requires ATP for function and is abolished in the presence of nigericin, indicating that the same pH gradient can drive amino acid transport in opposing directions. Efflux of tyrosine and other large neutral amino acids by the two closely related proteins, AVT3 and AVT4, is similar in terms of substrate specificity to transport system h described in mammalian lysosomes and melanosomes. These findings suggest that yeast AVT transporter function has been conserved to control amino acid flux in vacuolar-like organelles.

Apoptosis induced by exposure to a low steady-state concentration of H2O2 is a consequence of lysosomal rupture

We have re-examined the lysosomal hypothesis of oxidative-stress-induced apoptosis using a new technique for exposing cells in culture to a low steady-state concentration of H(2)O(2). This steady-state technique mimics the situation in vivo better than the bolus-administration method. A key aspect of H(2)O(2)-induced apoptosis is that the apoptosis is evident only after several hours, although cells may become committed within a few minutes of exposure to this particular reactive oxygen species. In the present work, we were able to show, for the first time, several correlative links between the triggering effect of H(2)O(2) and the later onset of apoptosis: (i) a short (15 min) exposure to H(2)O(2) caused almost immediate, albeit limited, lysosomal rupture; (ii) early lysosomal damage, and later apoptosis, showed a similar dose-related response to H(2)O(2); (iii) both events were inhibited by pre-treatment with iron chelators, including desferrioxamine. This compound is known to be taken up by endocytosis only and thus to become localized in the lysosomal compartment. After exposure to oxidative stress, when cells were again in standard culture conditions, a time-dependent continuous increase in lysosomal rupture was observed, resulting in a considerably lowered number of intact lysosomes in apoptotic cells, whereas non-apoptotic cells from the same batch of oxidative-stress-exposed cells showed mainly intact lysosomes. Taken together, our results reinforce earlier findings and strongly suggest that lysosomal rupture is an early upstream initiating event, and a consequence of intralysosomal iron-catalysed oxidative processes, when apoptosis is induced by oxidative stress.

First apyrase splice variants have different enzymatic properties

LALP70 is a novel lysosomal membrane protein belonging to the apyrase protein family. The apyrase protein family comprises enzymes capable of cleaving nucleotide tri- and diphosphates in a calcium- or magnesium-dependent manner, not being altered by P-type, F-type, or V-type NTPase inhibitors. In this study we have cloned and sequenced the human LALP70 gene to determine the genomic structure. The gene is organized in 11 introns and 12 exons covering a genomic region of approximately 16 kilobase pairs. By fluorescence in situ hybridization analysis, the hLALP70 gene was mapped to the human chromosome 8p21.1-p21.3. We further show that there is at least one alternatively spliced variant, hLALP70v, which can be generated via an alternative splice side at the 3'-end of exon 7, leading to a protein variant differing in 8 amino acids (VSFASSQQ). This is the first splice variant that has been described in the apyrase protein family. Reverse transcriptase polymerase chain reaction analysis showed an ubiquitous expression of both variants, with different relative mRNA expression levels in different tissues. Comparison of the enzymatic properties of the splice variants revealed a broader substrate specificity for hLALP70v with CTP, UDP, CDP, GTP, and GDP as preferred substrates, while hLALP70 utilized UTP and TTP preferentially. Furthermore, enzyme activity of hLALP70v was equally dependent on Ca(2+) and Mg(2+), being saturated already at 1 mm concentration. In contrast, hLALP70 enzymatic activity were unsaturated up to 10 mm Ca(2+), while Mg(2+) showed a saturation at already 1 mm concentration with 2-3-fold lower enzymatic activity as observed with Ca(2+). Our data suggest that the presence or absence of the 8-amino acid motif VSFASSQQ provoke differences in substrate specificity and divalent cation dependence of hLALP70/hLALP70v.

A human intracellular apyrase-like protein, LALP70, localizes to lysosomal/autophagic vacuoles

Using antibodies against autophagic vacuole membrane proteins we identified a human cDNA with an open reading frame of 1848 bp, encoding a protein of 70 kDa, which we named lysosomal apyrase-like protein of 70 kDa (LALP70). Sequence analysis revealed that LALP70 belongs to the apyrase or GDA1/CD39 family and is almost identical to a human uridine diphosphatase, with the exception of nine extra amino acids in LALP70. Members of this family were originally described as ectoenzymes, with some intracellular exceptions. Transfected LALP70 fused to the green fluorescent protein localized in the cytoplasm with a punctate pattern in the perinuclear space. These structures colocalized with the autophagic marker monodansylcadaverine and the lysosomal protein lamp1. Hydrophobicity analysis of the encoded protein revealed a transmembrane region at the N and C termini. Most of the sequence is arranged between these transmembrane domains, and contains four apyrase conserved regions. In vitro transcription/translation in the presence of microsomes showed that no signal sequence is cleaved off and that the translation product is protected from trypsin treatment. Our data indicate that LALP70 is a type III lysosomal/autophagic vacuole membrane protein with the apyrase conserved regions facing the luminal space of the vacuoles.

A mammalian lysosomal membrane protein confers multidrug resistance upon expression in Saccharomyces cerevisiae

Mouse transporter protein (MTP) is a highly conserved polytopic membrane protein present in mammalian lysosomes and endosomes. The role of MTP in regulating the in vivo subcellular distribution of numerous structurally distinct small molecules has been examined in this study by its expression in a drug-sensitive strain of the yeast Saccharomyces cerevisiae. Surprisingly, the expression of MTP in membranes of an intracellular compartment resulted in a cellular resistance or hypersensitivity to a range of drugs that included nucleoside and nucleobase analogs, antibiotics, anthracyclines, ionophores, and steroid hormones. The intracellular bioavailability of steroid hormones was altered by MTP, as determined using an in vivo glucocorticoid receptor-driven reporter assay in yeast, suggesting that the MTP-regulated drug sensitivity arose due to a change in the subcellular compartmentalization of steroid hormones and other drugs. MTP-regulated drug sensitivity in yeast was blocked to varying degrees by compounds that inhibit lysosomal function, interfere with intracellular cholesterol transport, or modulate the multidrug resistance phenotype of mammalian cells. These results indicate that MTP is involved in the subcellular compartmentalization of diverse hydrophobic small molecules and contributes to the inherent drug sensitivity or resistance of the mammalian cell.