Ala-Pro-cresyl violet, a synthetic fluorogenic substrate for the analysis of kinetic parameters of dipeptidyl peptidase IV (CD26) in individual living rat hepatocytes

The cGAS-STING pathway activates transcription factor TFEB to stimulate lysosome biogenesis and pathogen clearance

Induction of autophagy is an ancient function of the cyclic GMP-AMP (cGAMP) synthase (cGAS)-stimulator of interferon genes (STING) pathway through which autophagic cargoes are delivered to lysosomes for degradation. However, whether lysosome function is also modulated by the cGAS-STING pathway remains unknown. Here, we discovered that the cGAS-STING pathway upregulated lysosomal activity by stimulating lysosome biogenesis independently of the downstream protein kinase TANK-binding kinase 1 (TBK1). STING activation enhanced lysosome biogenesis through inducing the nuclear translocation of transcription factor EB (TFEB) as well as its paralogs transcription factor E3 (TFE3) and microphthalmia-associated transcription factor (MITF). STING-induced lipidation of GABA type A receptor-associated protein (GABARAP), an autophagy-related protein, on STING vesicles was responsible for TFEB activation. Membrane-bound GABARAP sequestered the GTPase-activating protein folliculin (FLCN) and FLCN-interacting protein (FNIP) complex to block its function toward the Rag GTPases Ras-related GTP-binding C and D (RagC and RagD), abolishing mechanistic target of rapamycin (mTOR) complex 1 (mTORC1)-dependent phosphorylation and inactivation of TFEB. Functionally, STING-induced lysosome biogenesis within cells facilitated the clearance of cytoplasmic DNA and invading pathogens. Thus, our findings reveal that induction of lysosome biogenesis is another important function of the cGAS-STING pathway.

Synthetic hemozoin as a nanocarrier for cross-presentation

It is known that conventional antigen presentation involves phagocytosis of antigens followed by its internalization in endocytic compartments and presentation of epitopes through MHC class II molecules for CD4 T cells. However, since 1976 a cross-presentation pathway has been studied, in which CD8 T cells are activated via MHC class I with antigens acquired through phagocytosis or endocytosis by dendritic cells (DCs). Among some important molecules involved in the cross-presentation, the C-type lectin receptor of the Dectin-1 cluster (CLECs), particularly the CLEC9A receptor, not only is expressed in dendritic cells but also presents a pivotal role in this context. In special, CLEC12A has been highlighted as a malaria pigment hemozoin (HZ) receptor. During Plasmodium infection, hemozoin crystals defend the parasite against heme toxicity within erythrocytes, as well as the released native HZ elicits pro-inflammatory responses and can induce cross-presentation. Particularly, this crystal can be synthesized from hematin anhydride and mimics the native form, and the gaps generated between the nanocrystal domains during its synthesis allow for substance coupling followed by its coating. Therefore, this study aimed to assess whether synthetic hemozoin (sHz) or hematin anhydride could be a nanocarrier and promote cross-presentation in dendritic cells. Firstly, it was verified that sHz can carry coated and coupled antigens, the compounds can associate to LAMP1-positive vesicles and decrease overall intracellular pH, which can potentially enhance the cross-presentation of ovalbumin and Leishmania infantum antigens. Thus, this study adds important data in the molecular intricacies of antigen presentation by showing not only the sHz immunomodulatory properties but also its potential applications as an antigen carrier.

Exploring lysosomal biology: current approaches and methods

Lysosomes are the degradation centers and signaling hubs in the cell. Lysosomes undergo adaptation to maintain cell homeostasis in response to a wide variety of cues. Dysfunction of lysosomes leads to aging and severe diseases including lysosomal storage diseases (LSDs), neurodegenerative disorders, and cancer. To understand the complexity of lysosome biology, many research approaches and tools have been developed to investigate lysosomal functions and regulatory mechanisms in diverse experimental systems. This review summarizes the current approaches and tools adopted for studying lysosomes, and aims to provide a methodological overview of lysosomal research and related fields.

The HEAT repeat protein HPO-27 is a lysosome fission factor

Lysosomes are degradation and signalling centres crucial for homeostasis, development and ageing1. To meet diverse cellular demands, lysosomes remodel their morphology and function through constant fusion and fission2,3. Little is known about the molecular basis of fission. Here we identify HPO-27, a conserved HEAT repeat protein, as a lysosome scission factor in Caenorhabditis elegans. Loss of HPO-27 impairs lysosome fission and leads to an excessive tubular network that ultimately collapses. HPO-27 and its human homologue MROH1 are recruited to lysosomes by RAB-7 and enriched at scission sites. Super-resolution imaging, negative-staining electron microscopy and in vitro reconstitution assays reveal that HPO-27 and MROH1 self-assemble to mediate the constriction and scission of lysosomal tubules in worms and mammalian cells, respectively, and assemble to sever supported membrane tubes in vitro. Loss of HPO-27 affects lysosomal morphology, integrity and degradation activity, which impairs animal development and longevity. Thus, HPO-27 and MROH1 act as self-assembling scission factors to maintain lysosomal homeostasis and function.

Coxiella burnetii Sterol-Modifying Protein Stmp1 Regulates Cholesterol in the Intracellular Niche

Coxiella burnetii replicates in a phagolysosome-like vacuole called the Coxiella-containing vacuole (CCV). While host cholesterol readily traffics to the CCV, cholesterol accumulation leads to CCV acidification and bacterial death. Thus, bacterial regulation of CCV cholesterol content is essential for Coxiella pathogenesis. Coxiella expresses a sterol-modifying protein, Stmp1, that may function to lower CCV cholesterol through enzymatic modification. Using an Stmp1 knockout (Δstmp1), we determined that Stmp1 is not essential for axenic growth. Inside host cells, however, Δstmp1 mutant bacteria form smaller CCVs which accumulate cholesterol, preferentially fuse with lysosomes, and become more acidic, correlating with a significant growth defect. However, in cholesterol-free cells, Δstmp1 mutant bacteria grow similarly to wild-type bacteria but are hypersensitive to cholesterol supplementation. To better understand the underlying mechanism behind the Δstmp1 mutant phenotype, we performed sterol profiling. Surprisingly, we found that Δstmp1 mutant-infected macrophages accumulated the potent cholesterol homeostasis regulator 25-hydroxycholesterol (25-HC). We next determined whether dysregulated 25-HC alters Coxiella infection by treating wild-type Coxiella-infected cells with 25-HC. Similar to the Δstmp1 mutant phenotype, 25-HC increased CCV proteolytic activity and inhibited bacterial growth. Collectively, these data indicate that Stmp1 alters host cholesterol metabolism and is essential to establish a mature CCV which supports Coxiella growth. IMPORTANCE Coxiella burnetii is the causative agent of human Q fever, an emerging infectious disease and significant cause of culture-negative endocarditis. Acute infections are often undiagnosed, there are no licensed vaccines in the United States, and chronic Q fever requires a prolonged antibiotic treatment. Therefore, new treatment and preventive options are critically needed. Coxiella is an obligate intracellular bacterium that replicates within a large acidic phagolysosome-like compartment, the Coxiella-containing vacuole (CCV). We previously discovered that cholesterol accumulation in the CCV increases its acidification, leading to bacterial death. Therefore, in order to survive in this harsh environment, Coxiella likely regulates CCV cholesterol levels. Here, we found that Coxiella sterol modifying protein (Stmp1) facilitates bacterial growth by reducing CCV cholesterol and host cell 25-hydroxycholesterol (25-HC) levels, which prevents excessive CCV fusion with host lysosomes and CCV acidification. This study establishes that Stmp1-mediated regulation of host cholesterol homeostasis is essential for Coxiella intracellular survival.

CDK4/6 regulate lysosome biogenesis through TFEB/TFE3

Lysosomes are degradation and signaling organelles that adapt their biogenesis to meet many different cellular demands; however, it is unknown how lysosomes change their numbers for cell division. Here, we report that the cyclin-dependent kinases CDK4/6 regulate lysosome biogenesis during the cell cycle. Chemical or genetic inactivation of CDK4/6 increases lysosomal numbers by activating the lysosome and autophagy transcription factors TFEB and TFE3. CDK4/6 interact with and phosphorylate TFEB/TFE3 in the nucleus, thereby inactivating them by promoting their shuttling to the cytoplasm. During the cell cycle, lysosome numbers increase in S and G2/M phases when cyclin D turnover diminishes CDK4/6 activity. These findings not only uncover the molecular events that direct the nuclear export of TFEB/TFE3, but also suggest a mechanism that controls lysosome biogenesis in the cell cycle. CDK4/6 inhibitors promote autophagy and lysosome-dependent degradation, which has important implications for the therapy of cancer and lysosome-related disorders.

Coxiella burnetii Type 4B Secretion System-dependent manipulation of endolysosomal maturation is required for bacterial growth

Upon host cell infection, the obligate intracellular bacterium Coxiella burnetii resides and multiplies within the Coxiella–Containing Vacuole (CCV). The nascent CCV progresses through the endosomal maturation pathway into a phagolysosome, acquiring endosomal and lysosomal markers, as well as acidic pH and active proteases and hydrolases. Approximately 24–48 hours post infection, heterotypic fusion between the CCV and host endosomes/lysosomes leads to CCV expansion and bacterial replication in the mature CCV. Initial CCV acidification is required to activate C. burnetii metabolism and the Type 4B Secretion System (T4BSS), which secretes effector proteins required for CCV maturation. However, we found that the mature CCV is less acidic (pH~5.2) than lysosomes (pH~4.8). Further, inducing CCV acidification to pH~4.8 causes C. burnetii lysis, suggesting C. burnetii actively regulates pH of the mature CCV. Because heterotypic fusion with host endosomes/lysosomes may influence CCV pH, we investigated endosomal maturation in cells infected with wildtype (WT) or T4BSS mutant (ΔdotA) C. burnetii. In WT-infected cells, we observed a significant decrease in proteolytically active, LAMP1-positive endolysosomal vesicles, compared to mock or ΔdotA-infected cells. Using a ratiometric assay to measure endosomal pH, we determined that the average pH of terminal endosomes in WT-infected cells was pH~5.8, compared to pH~4.75 in mock and ΔdotA-infected cells. While endosomes progressively acidified from the periphery (pH~5.5) to the perinuclear area (pH~4.7) in both mock and ΔdotA-infected cells, endosomes did not acidify beyond pH~5.2 in WT-infected cells. Finally, increasing lysosomal biogenesis by overexpressing the transcription factor EB resulted in smaller, more proteolytically active CCVs and a significant decrease in C. burnetii growth, indicating host lysosomes are detrimental to C. burnetii. Overall, our data suggest that C. burnetii inhibits endosomal maturation to reduce the number of proteolytically active lysosomes available for heterotypic fusion with the CCV, possibly as a mechanism to regulate CCV pH.

The obligate intracellular bacterium Coxiella burnetii causes human Q fever, which manifests as a flu-like illness but can develop into a life-threatening and difficult to treat endocarditis. C. burnetii, in contrast to many other intracellular bacteria, thrives within a lysosome-like vacuole in host cells. However, we previously found that the C. burnetii vacuole is not as acidic as lysosomes and increased acidification kills the bacteria, suggesting that C. burnetii regulates the pH of its vacuole. Here, we discovered that C. burnetii blocks endolysosomal maturation and acidification during host cell infection, resulting in fewer lysosomes in the host cell. Moreover, increasing lysosomes in the host cells inhibited C. burnetii growth. Together, our study suggests that C. burnetii regulates vacuole acidity and blocks endosomal maturation in order to produce a permissive intracellular niche.

The immune modulatory peptide FhHDM-1 secreted by the helminth Fasciola hepatica prevents NLRP3 inflammasome activation by inhibiting endolysosomal acidification in macrophages

The NLRP3 inflammasome is a multimeric protein complex that controls the production of IL-1β, a cytokine that influences the development of both innate and adaptive immune responses. Helminth parasites secrete molecules that interact with innate immune cells, modulating their activity to ultimately determine the phenotype of differentiated T cells, thus creating an immune environment that is conducive to sustaining chronic infection. We show that one of these molecules, FhHDM-1, a cathelicidin-like peptide secreted by the helminth parasite, Fasciola hepatica, inhibits the activation of the NLRP3 inflammasome resulting in reduced secretion of IL-1β by macrophages. FhHDM-1 had no effect on the synthesis of pro-IL-1β. Rather, the inhibitory effect was associated with the capacity of the peptide to prevent acidification of the endolysosome. The activation of cathepsin B protease by lysosomal destabilization was prevented in FhHDM-1-treated macrophages. By contrast, peptide derivatives of FhHDM-1 that did not alter the lysosomal pH did not inhibit secretion of IL-1β. We propose a novel immune modulatory strategy used by F. hepatica, whereby secretion of the FhHDM-1 peptide impairs the activation of NLRP3 by lysosomal cathepsin B protease, which prevents the downstream production of IL-1β and the development of protective T helper 1 type immune responses that are detrimental to parasite survival.-Alvarado, R., To, J., Lund, M. E., Pinar, A., Mansell, A., Robinson, M. W., O'Brien, B. A., Dalton, J. P., Donnelly, S. The immune modulatory peptide FhHDM-1 secreted by the helminth Fasciola hepatica prevents NLRP3 inflammasome activation by inhibiting endolysosomal acidification in macrophages.

Recruitment of VPS33A to HOPS by VPS16 Is Required for Lysosome Fusion with Endosomes and Autophagosomes

The mammalian homotypic fusion and vacuole protein sorting (HOPS) complex is comprised of six subunits: VPS11, VPS16, VPS18, VPS39, VPS41 and the Sec1/Munc18 (SM) family member VPS33A. Human HOPS has been predicted to be a tethering complex required for fusion of intracellular compartments with lysosomes, but it remains unclear whether all HOPS subunits are required. We showed that the whole HOPS complex is required for fusion of endosomes with lysosomes by monitoring the delivery of endocytosed fluorescent dextran to lysosomes in cells depleted of individual HOPS proteins. We used the crystal structure of the VPS16/VPS33A complex to design VPS16 and VPS33A mutants that no longer bind each other and showed that, unlike the wild-type proteins, these mutants no longer rescue lysosome fusion with endosomes or autophagosomes in cells depleted of the endogenous proteins. There was no effect of depleting either VIPAR or VPS33B, paralogs of VPS16 and VPS33A, on fusion of lysosomes with either endosomes or autophagosomes and immunoprecipitation showed that they form a complex distinct from HOPS. Our data demonstrate the necessity of recruiting the SM protein VPS33A to HOPS via its interaction with VPS16 and that HOPS proteins, but not VIPAR or VPS33B, are essential for fusion of endosomes or autophagosomes with lysosomes.

Bioresponsive probes for molecular imaging: concepts and in vivo applications

Molecular imaging is a powerful tool to visualize and characterize biological processes at the cellular and molecular level in vivo. In most molecular imaging approaches, probes are used to bind to disease-specific biomarkers highlighting disease target sites. In recent years, a new subset of molecular imaging probes, known as bioresponsive molecular probes, has been developed. These probes generally benefit from signal enhancement at the site of interaction with its target. There are mainly two classes of bioresponsive imaging probes. The first class consists of probes that show direct activation of the imaging label (from "off" to "on" state) and have been applied in optical imaging and magnetic resonance imaging (MRI). The other class consists of probes that show specific retention of the imaging label at the site of target interaction and these probes have found application in all different imaging modalities, including photoacoustic imaging and nuclear imaging. In this review, we present a comprehensive overview of bioresponsive imaging probes in order to discuss the various molecular imaging strategies. The focus of the present article is the rationale behind the design of bioresponsive molecular imaging probes and their potential in vivo application for the detection of endogenous molecular targets in pathologies such as cancer and cardiovascular disease.

Ethanol inhibits activation of NLRP3 and AIM2 inflammasomes in human macrophages--a novel anti-inflammatory action of alcohol

Objective

In the pathogenesis of coronary atherosclerosis, local macrophage-driven inflammation and secretion of proinflammatory cytokines, interleukin-1β (IL-1β) in particular, are recognized as key factors. Moderate alcohol consumption is associated with a reduced risk of coronary artery disease mortality. Here we examined in cultured human macrophages whether ethanol modulates the intracellular processes involved in the secretion of IL-1β.

Results

Ethanol decreased dose-dependently the production of mature IL-1β induced by activators of the NLRP3 inflammasome, i.e. ATP, cholesterol crystals, serum amyloid A and nigericin. Ethanol had no significant effect on the expression of NLRP3 or IL1B mRNA in LPS-primed macrophages. Moreover, secretion of IL-1β was decreased in parallel with reduction of caspase-1 activation, demonstrating that ethanol inhibits inflammasome activation instead of synthesis of pro-IL-1β. Acetaldehyde, a highly reactive metabolite of ethanol, had no effect on the ATP-induced IL-1β secretion. Ethanol also attenuated the secretion of IL-1β triggered by synthetic double-stranded DNA, an activator of the AIM2 inflammasome. Ethanol conferred the inhibitory functions by attenuating the disruption of lysosomal integrity and ensuing leakage of the lysosomal protease cathepsin B and by reducing oligomerization of ASC.

Conclusion

Ethanol-induced inhibition of the NLRP3 inflammasome activation in macrophages may represent a biological pathway underlying the protective effect of moderate alcohol consumption on coronary heart disease.

The chemistry of small-molecule fluorogenic probes

Chemical fluorophores find wide use in biology to detect and visualize different phenomena. A key advantage of small-molecule dyes is the ability to construct compounds where fluorescence is activated by chemical or biochemical processes. Fluorogenic molecules, in which fluorescence is activated by enzymatic activity, light, or environmental changes, enable advanced bioassays and sophisticated imaging experiments. Here, we detail the collection of fluorophores and highlight both general strategies and unique approaches that are employed to control fluorescence using chemistry.

Analyzing lysosomes in live cells

Lysosomes are an important cellular organelle that receive and degrade macromolecules from the secretory, endocytic, autophagic, and phagocytic membrane-trafficking pathways. Defects in lysosome function lead to the development of disease with often-severe consequences to the individual. Since the discovery of lysosomes by Christian de Duve over 50 years ago, research into endocytic and lysosomal biology has allowed for the development of tools to understand further the role of lysosomes in cells. There are now several fluorescent probes that can be used to visualize and assess membrane traffic to the lysosome as well as probes to assess the activity of lysosomal hydrolases in live cells. This chapter describes the current methods used to measure lysosome function in live cells.

Hepatic steatosis inhibits autophagic proteolysis via impairment of autophagosomal acidification and cathepsin expression

Autophagy, one of protein degradation system, contributes to maintain cellular homeostasis and cell defense. Recently, some evidences indicated that autophagy and lipid metabolism are interrelated. Here, we demonstrate that hepatic steatosis impairs autophagic proteolysis. Though accumulation of autophagosome is observed in hepatocytes from ob/ob mice, expression of p62 was augmented in liver from ob/ob mice more than control mice. Moreover, degradation of the long-lived protein leucine was significantly suppressed in hepatocytes isolated from ob/ob mice. More than 80% of autophagosomes were stained by LysoTracker Red (LTR) in hepatocytes from control mice; however, rate of LTR-stained autophagosomes in hepatocytes were suppressed in ob/ob mice. On the other hand, clearance of autolysosomes loaded with LTR was blunted in hepatocytes from ob/ob mice. Although fusion of isolated autophagosome and lysosome was not disturbed, proteinase activity of cathepsin B and L in autolysosomes and cathepsin B and L expression of liver were suppressed in ob/ob mice. These results indicate that lipid accumulation blunts autophagic proteolysis via impairment of autophagosomal acidification and cathepsin expression.

Activatable Optical Probes for the Detection of Enzymes

The early detection of many human diseases is crucial if they are to be treated successfully. Therefore, the development of imaging techniques that can facilitate early detection of disease is of high importance. Changes in the levels of enzyme expression are known to occur in many diseases, making their accurate detection at low concentrations an area of considerable active research. Activatable fluorescent probes show immense promise in this area. If properly designed they should exhibit no signal until they interact with their target enzyme, reducing the level of background fluorescence and potentially endowing them with greater sensitivity. The mechanisms of fluorescence changes in activatable probes vary. This review aims to survey the field of activatable probes, focusing on their mechanisms of action as well as illustrating some of the in vitro and in vivo settings in which they have been employed.

Cholesterol crystals activate the NLRP3 inflammasome in human macrophages: a novel link between cholesterol metabolism and inflammation

Background

Chronic inflammation of the arterial wall is a key element in the pathogenesis of atherosclerosis, yet the factors that trigger and sustain the inflammation remain elusive. Inflammasomes are cytoplasmic caspase-1-activating protein complexes that promote maturation and secretion of the proinflammatory cytokines interleukin(IL)-1β and IL-18. The most intensively studied inflammasome, NLRP3 inflammasome, is activated by diverse substances, including crystalline and particulate materials. As cholesterol crystals are abundant in atherosclerotic lesions, and IL-1β has been linked to atherogenesis, we explored the possibility that cholesterol crystals promote inflammation by activating the inflammasome pathway.

Principal Findings

Here we show that human macrophages avidly phagocytose cholesterol crystals and store the ingested cholesterol as cholesteryl esters. Importantly, cholesterol crystals induced dose-dependent secretion of mature IL-1β from human monocytes and macrophages. The cholesterol crystal-induced secretion of IL-1β was caspase-1-dependent, suggesting the involvement of an inflammasome-mediated pathway. Silencing of the NLRP3 receptor, the crucial component in NLRP3 inflammasome, completely abolished crystal-induced IL-1β secretion, thus identifying NLRP3 inflammasome as the cholesterol crystal-responsive element in macrophages. The crystals were shown to induce leakage of the lysosomal protease cathepsin B into the cytoplasm and inhibition of this enzyme reduced cholesterol crystal-induced IL-1β secretion, suggesting that NLRP3 inflammasome activation occurred via lysosomal destabilization.

Conclusions

The cholesterol crystal-induced inflammasome activation in macrophages may represent an important link between cholesterol metabolism and inflammation in atherosclerotic lesions.

Imaging enzymes at work: metabolic mapping by enzyme histochemistry

For the understanding of functions of proteins in biological and pathological processes, reporter molecules such as fluorescent proteins have become indispensable tools for visualizing the location of these proteins in intact animals, tissues, and cells. For enzymes, imaging their activity also provides information on their function or functions, which does not necessarily correlate with their location. Metabolic mapping enables imaging of activity of enzymes. The enzyme under study forms a reaction product that is fluorescent or colored by conversion of either a fluorogenic or chromogenic substrate or a fluorescent substrate with different spectral characteristics. Most chromogenic staining methods were developed in the latter half of the twentieth century but still find new applications in modern cell biology and pathology. Fluorescence methods have rapidly evolved during the last decade. This review critically evaluates the methods that are available at present for metabolic mapping in living animals, unfixed cryostat sections of tissues, and living cells, and refers to protocols of the methods of choice.

Fluorescein-based amino acids for solid phase synthesis of fluorogenic protease substrates

An efficient synthesis of new type fluorescent amino acids is described. The Fmoc-protected dyes can be prepared in a four-step procedure with approximately 30% overall yield from aminofluoresceins and other inexpensive commercially available precursors. The dyes are much more photostable compared to fluorescein and exhibit constant pH-independent fluorescence that is advantageous in biological applications. The Fmoc-protected fluorescent amino acids are ready for use in solid phase peptide synthesis. As a proof of concept, a fluorogenic papain substrate was synthesized and employed for on-bead detection of the protease activity. By using a novel technique for quantitative analysis of bead fluorescence, a approximately 2.7-fold increase in mean bead brightness was measured and was attributed to substrate cleavage by papain. The new type fluorescent amino acids seem to be a promising tool for the synthesis of fluorescent peptide ligands and fluorogenic protease substrates.

Development of a dual fluorogenic and chromogenic dipeptidyl peptidase IV substrate

A new far-red dual fluorogenic and chromogenic substrate, 5-glycylprolylglycylprolyl-9-di-3-sulfonyl-propylaminobenza[a]phenoxazonium perchlorate (GPGP-2SBPO), was developed for dipeptidyl peptidase IV (DPP-IV) sensing. The glycylprolylglycylprolyl tetrapeptide was chosen as the recognition sequence due to its stability under physiological conditions. In contrast, the truncated substrate, GP-2SBPO, containing only a glycylprolyl peptide, is unstable. Proteolysis of GPGP-2SBPO was assayed by monitoring the absorbance and fluorescence signals from the released fluorochrome, 2SBPO, at 625 and 670nm, respectively.

Fluorescent peptide probes for in vivo diagnostic imaging

Recently, many novel peptide-based near-infrared (NIR) fluorescent molecular probes have been developed for in vivo biomedical imaging. To report specific information of biological targets, the probes were individually designed according to the unique property or functions of their targets. These peptide-based probes can be classified into targeting, crosslinking, and enzyme-activatable probes. Several of them have been tested in various in vitro and in vivo models, and the obtained imaging information has been applied to disease detection, medical diagnosis, and drug evaluations.

Metabolic mapping of proteinase activity with emphasis on in situ zymography of gelatinases: review and protocols

Proteases are essential for protein catabolism, regulation of a wide range of biological processes, and in the pathogenesis of many diseases. Several techniques are available to localize activity of proteases in tissue sections or cell preparations. For localization of the activity of matrix metalloproteinases, in situ zymography was introduced some decades ago. The procedure is based on zymography using SDS polyacrylamide gels containing gelatin, casein, or fibrin as substrate. For in situ zymography, either a photographic emulsion containing gelatin or a fluorescence-labeled proteinaceous macromolecular substrate is brought into contact with a tissue section or cell preparation. After incubation, enzymatic activity is revealed as white spots in a dark background or as black spots in a fluorescent background. However, this approach does not allow precise localization of proteinase activity because of limited sensitivity. A major improvement in sensitivity was achieved with the introduction of dye-quenched (DQ-)gelatin, which is gelatin that is heavily labeled with FITC molecules so that its fluorescence is quenched. After cleavage of DQ-gelatin by gelatinolytic activity, fluorescent peptides are produced that are visible against a weakly fluorescent background. The incubation with DQ-gelatin can be combined with simultaneous immunohistochemical detection of a protein on the same section. To draw valid conclusions from the findings with in situ zymography, specific inhibitors need to be used and the technique has to be combined with immunohistochemistry and zymography. In that case, in situ zymography provides data that extend our understanding of the role of specific proteinases in various physiological and pathological conditions.

CD44 interaction with Na+-H+ exchanger (NHE1) creates acidic microenvironments leading to hyaluronidase-2 and cathepsin B activation and breast tumor cell invasion

We have explored CD44 (a hyaluronan (HA) receptor) interaction with a Na(+)-H(+) exchanger (NHE1) and hyaluronidase-2 (Hyal-2) during HA-induced cellular signaling in human breast tumor cells (MDA-MB-231 cell line). Immunological analyses demonstrate that CD44s (standard form) and two signaling molecules (NHE1 and Hyal-2) are closely associated in a complex in MDA-MB-231 cells. These three proteins are also significantly enriched in cholesterol and ganglioside-containing lipid rafts, characterized as caveolin and flotillin-rich plasma membrane microdomains. The binding of HA to CD44 activates Na(+)-H(+) exchange activity which, in turn, promotes intracellular acidification and creates an acidic extracellular matrix environment. This leads to Hyal-2-mediated HA catabolism, HA modification, and cysteine proteinase (cathepsin B) activation resulting in breast tumor cell invasion. In addition, we have observed the following: (i) HA/CD44-activated Rho kinase (ROK) mediates NHE1 phosphorylation and activity, and (ii) inhibition of ROK or NHE1 activity (by treating cells with a ROK inhibitor, Y27632, or NHE1 blocker, S-(N-ethyl-N-isopropyl) amiloride, respectively) blocks NHE1 phosphorylation/Na(+)-H(+) exchange activity, reduces intracellular acidification, eliminates the acidic environment in the extracellular matrix, and suppresses breast tumor-specific behaviors (e.g. Hyal-2-mediated HA modification, cathepsin B activation, and tumor cell invasion). Finally, down-regulation of CD44 or Hyal-2 expression (by treating cells with CD44 or Hyal-2-specific small interfering RNAs) not only inhibits HA-mediated CD44 signaling (e.g. ROK-mediated Na(+)-H(+) exchanger reaction and cellular pH changes) but also impairs oncogenic events (e.g. Hyal-2 activity, hyaluronan modification, cathepsin B activation, and tumor cell invasion). Taken together, our results suggest that CD44 interaction with a ROK-activated NHE1 (a Na(+)-H(+) exchanger) in cholesterol/ganglioside-containing lipid rafts plays a pivotal role in promoting intracellular/extracellular acidification required for Hyal-2 and cysteine proteinase-mediated matrix degradation and breast cancer progression.

IL-8-mediated cell migration in endothelial cells depends on cathepsin B activity and transactivation of the epidermal growth factor receptor

Microvascular endothelial cells (HMECs) express both the CXCR1 and the CXCR2, but cell migration is almost entirely mediated by the CXCR2. Similarly, NIH 3T3 cells transfected with the CXCR2 migrated toward IL-8, whereas CXCR1-transfected cells failed to do so. This situation differs from that seen in leukocytes, where chemotaxis is primarily a function of the CXCR1. To define signal transduction pathways that explain this difference in behavior, various inhibitors were used to block cell migration. Apart from inhibitors of phosphatidylinositol 3-kinase, which blocked migration in all cases, inhibition of the epidermal growth factor (EGF) receptor blocked IL-8-mediated cell migration in HMECs and in CXCR2-transfected NIH 3T3 cells, but not in RBL2H3 cells, which do not express an EGFR. Blocking Abs against the EGFR or against heparin-binding EGF-like growth factor similarly blocked IL-8-mediated cell migration and in vitro tubulogenesis in HMECs. Furthermore, inhibition of the EGFR also attenuated focus formation in NIH 3T3 expressing the CXCR2. Immunoprecipitations of the EGFR in HMECs and in NIH 3T3 cells expressing the CXCR2 confirmed that the EGFR was phosphorylated following stimulation with IL-8. However, in contrast to previous reports, e.g., for the thrombin receptor, inhibition of matrix metalloproteases blocked IL-8-mediated cell migration only partially, whereas it was ablated by inhibition of cathepsin B. These results indicate that IL-8-induced transactivation of the EGFR is mediated by the CXCR2 and involves cathepsin B, and that this pathway is important for the migratory and tumorigenic effects of IL-8.

Fluorogenic substrate [Ala-Pro]2-cresyl violet but not Ala-Pro-rhodamine 110 is cleaved specifically by DPPIV activity: a study in living Jurkat cells and CD26/DPPIV-transfected Jurkat cells

Fluorogenic substrates [Ala-Pro](2)-cresyl violet and Ala-Pro-rhodamine 110 have been tested for microscopic detection of protease activity of dipeptidyl peptidase IV (DPPIV) in living cells. DPPIV activity is one of the many functions of the multifunctional or moonlighting protein CD26/DPPIV. As a model we used Jurkat cells, which are T-cells that lack CD26/DPPIV expression, and CD26/DPPIV-transfected Jurkat cells. Ala-Pro-rhodamine 110 is not fluorescent, but after proteolytic cleavage rhodamine 110 fluoresces. [Ala-Pro](2)-cresyl violet is fluorescent by itself but proteolytic cleavage into cresyl violet induces a shift to longer wavelengths. This phenomenon enables the simultaneous determination of local (intracellular) substrate and product concentrations, which is important for analysis of kinetics of the cleavage reaction. [Ala-Pro](2)-cresyl violet, but not Ala-Pro-rhodamine 110, appeared to be specific for DPPIV. When microscopic analysis is performed on living cells during the first minutes of the enzyme reaction, DPPIV activity can be precisely localized in cells with the use of [Ala-Pro](2)-cresyl violet. Fluorescent product is rapidly internalized into submembrane granules in transfected Jurkat cells and is redistributed intracellularly via internalization pathways that have been described for CD26/DPPIV. We conclude that [Ala-Pro](2)-cresyl violet is a good fluorogenic substrate to localize DPPIV activity in living cells when the correct wavelengths are used for excitation and emission and images are captured in the early stages of the enzyme reaction.

Transcellular proteolysis demonstrated by novel cell surface-associated substrates of dipeptidyl peptidase IV (CD26)

Proteolytic enzymes contribute to the regulation of cellular functions such as cell proliferation and death, cytokine production, and matrix remodeling. Dipeptidyl peptidase IV (DP IV) catalyzes the cleavage of several cytokines and thereby contributes to the regulation of cytokine production and the proliferation of immune cells. Here we show for the first time that cell surface-bound DP IV catalyzes the cleavage of specific substrates that are associated with the cellular surface of neighboring cells. Rhodamine 110 (R110), a highly fluorescent xanthene dye, was used to synthesize dipeptidyl peptidase IV (DP IV/CD26) substrates Gly(Ala)-Pro-R110-R, thus facilitating a stable binding of the fluorescent moiety on the cell surface. The fixation resulted from the interaction with the reactive anchor rhodamine and allowed the quantification of cellular DP IV activity on single cells. The reactivity, length, and hydrophobicity of rhodamine was characterized as the decisive factor that facilitated the determination of cellular DP IV activity. Using fluorescence microscopy, it was possible to differentiate between different DP IV activities. The hydrolysis of cell-bound substrates Xaa-Pro-R110-R by DP IV of neighboring cells and by soluble DP IV was shown using flow cytometry. These data demonstrate that ectopeptidases such as DP IV may be involved in communication between blood cells via proteolysis of cell-associated substrates.

CD26/DPPIV signal transduction function, but not proteolytic activity, is directly related to its expression level on human Th1 and Th2 cell lines as detected with living cell cytochemistry

CD26/DPPIV is a cell surface glycoprotein that functions both in signal transduction and as a proteolytic enzyme, dipeptidyl peptidase IV (DPPIV). To investigate how two separate functions of one molecule are regulated, we analyzed CD26 protein expression and DPPIV enzyme activity on living human T-helper 1 (Th1) and Th2 cells that express different levels of CD26/DPPIV. DPPIV activity was specifically determined with the synthetic fluorogenic substrate ala-pro-cresyl violet and CD26 protein expression was demonstrated with an FITC-conjugated CD26-specific antibody. Fluorescence of liberated cresyl violet (red) and FITC (green) was detected simultaneously on living T-cells using flow cytometry and spectrofluorometry. Th1 cells expressed three- to sixfold more CD26 protein than Th2 cells. The signal transduction function of the CD26/DPPIV complex, tested by measuring its co-stimulatory potential for proliferation, was directly related to the amount of CD26 protein at the cell surface. However, DPPIV activity was similar in both cell populations at physiological substrate concentrations because of differences in K(m) and V(max) values of DPPIV on Th1 and Th2 cells. Western blotting and zymography of Th1 and Th2 whole-cell lysates demonstrated similar patterns. This study shows that two functions of one molecule can be controlled differentially.

Structural polarity and functional bile canaliculi in rat hepatocyte spheroids

Primary hepatocytes self-assemble into spheroids that possess tight junctions and microvilli-lined channels. We hypothesized that polarity develops gradually and that the channels structurally and functionally resemble bile canaliculi. Immunofluorescence labeling of apical and basolateral proteins demonstrated reorganization of the membrane proteins into a polarized distribution during spheroid culture. By means of fluorescent dextran diffusion and confocal microscopy, an extensive network of channels was revealed in the interior of the spheroids. These channels connected over several planes and opened to pores on the surface. To examine the content of apical proteins in the channel membranes, the bile canalicular enzyme dipeptidyl peptidase IV (DPPIV) was localized using a fluorogenic substrate, Ala-Pro-cresyl violet. The results show that DPPIV activity is heterogeneously distributed in spheroids and localized in part to channels. Bile acid excretion was then investigated to demonstrate functional polarity. A fluorescent bile acid analogue, fluorescein isothiocyanate-labeled glycocholate, was taken up into the spheroids and excreted into bile canalicular channels. Due to the structural polarity of spheroids and their ability to excrete bile into channels, they are a unique three-dimensional model of in vitro liver tissue self-assembly. (Videoanimations of some results are available at http://hugroup.cems.umn.edu/research_movies).

Enzyme cytochemical techniques for metabolic mapping in living cells, with special reference to proteolysis

Specific enzymes play key roles in many pathophysiological processes and therefore are targets for therapeutic strategies. The activity of most enzymes is largely determined by many factors at the post-translational level. Therefore, it is essential to study the activity of target enzymes in living cells and tissues in a quantitative manner in relation to pathophysiological processes to understand its relevance and the potential impact of its targeting by drugs. Proteases, in particular, are crucial in every aspect of life and death of an organism and are therefore important targets. Enzyme activity in living cells can be studied with various tools. These can be endogenous fluorescent metabolites or synthetic chromogenic or fluorogenic substrates. The use of endogenous metabolites is rather limited and nonspecific because they are involved in many biological processes, but novel chromogenic and fluorogenic substrates have been developed to monitor activity of enzymes, and particularly proteases, in living cells and tissues. This review discusses these substrates and the methods in which they are applied, as well as their advantages and disadvantages for metabolic mapping in living cells.

In vivo molecular target assessment of matrix metalloproteinase inhibition

A number of different matrix metalloproteinase (MMP) inhibitors have been developed as cytostatic and anti-angiogenic agents and are currently in clinical testing. One major hurdle in assessing the efficacy of such drugs has been the inability to sense or image anti-proteinase activity directly and non-invasively in vivo. We show here that novel, biocompatible near-infrared fluorogenic MMP substrates can be used as activatable reporter probes to sense MMP activity in intact tumors in nude mice. Moreover, we show for the first time that the effect of MMP inhibition can be directly imaged using this approach within hours after initiation of treatment using the potent MMP inhibitor, prinomastat (AG3340). The developed probes, together with novel near-infrared fluorescence imaging technology will enable the detailed analysis of a number of proteinases critical for advancing the therapeutic use of clinical proteinase inhibitors.

Heterogeneous suppression of experimentally induced colon cancer metastasis in rat liver lobes by inhibition of extracellular cathepsin B

Metastatic rat colon cancer cells but not normal rat hepatocytes showed activity of cathepsin B on their plasma membranes. Activity was visualized in living cells with a new fluorogenic substrate, [Z-Arg]2-cresyl violet, and confocal microscopy. When these cancer cells were injected into the portal vein of rats, the animals developed tumors in the liver in a heterogeneous fashion. Three- to four-fold more tumors were found in the small caudate lobe than in the other three large lobes of the liver. Oral treatment with a selective water-soluble inhibitor of extracellular cathepsin B, Mu-Phe-homoPhe-fluoromethylketone, resulted in 60% reduction of the number of tumors and 80% reduction of the volume of tumors in the three large lobes whereas tumor development was not affected in the small caudate lobe. This study supports the conclusions that (a) extracellular cathepsin B plays a crucial but complex role in liver colonisation by rat colon carcinoma cells in vivo, (b) its selective inhibition suppresses tumor growth heterogeneously in the liver and (c) the caudate lobe of the liver is a relatively large risk factor for tumor development.