Efficient differentiation of mouse embryonic stem cells into motor neurons

Direct differentiation of embryonic stem (ES) cells into functional motor neurons represents a promising resource to study disease mechanisms, to screen new drug compounds, and to develop new therapies for motor neuron diseases such as spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS). Many current protocols use a combination of retinoic acid (RA) and sonic hedgehog (Shh) to differentiate mouse embryonic stem (mES) cells into motor neurons. However, the differentiation efficiency of mES cells into motor neurons has only met with moderate success. We have developed a two-step differentiation protocol that significantly improves the differentiation efficiency compared with currently established protocols. The first step is to enhance the neuralization process by adding Noggin and fibroblast growth factors (FGFs). Noggin is a bone morphogenetic protein (BMP) antagonist and is implicated in neural induction according to the default model of neurogenesis and results in the formation of anterior neural patterning. FGF signaling acts synergistically with Noggin in inducing neural tissue formation by promoting a posterior neural identity. In this step, mES cells were primed with Noggin, bFGF, and FGF-8 for two days to promote differentiation towards neural lineages. The second step is to induce motor neuron specification. Noggin/FGFs exposed mES cells were incubated with RA and a Shh agonist, Smoothened agonist (SAG), for another 5 days to facilitate motor neuron generation. To monitor the differentiation of mESs into motor neurons, we used an ES cell line derived from a transgenic mouse expressing eGFP under the control of the motor neuron specific promoter Hb9. Using this robust protocol, we achieved 51 ± 0.8% of differentiation efficiency (n = 3; p < 0.01, Student's t-test). Results from immunofluorescent staining showed that GFP+ cells express the motor neuron specific markers, Islet-1 and choline acetyltransferase (ChAT). Our two-step differentiation protocol provides an efficient way to differentiate mES cells into spinal motor neurons.

Inhibitors of cathepsins B and L induce autophagy and cell death in neuroblastoma cells

This study was designed to test the hypothesis that specific inhibition of cathepsins B and L will cause death of neuroblastoma cells. Five compounds that differ in mode and rate of inhibition of these two enzymes were all shown to cause neuroblastoma cell death. Efficacy of the different compounds was related to their ability to inhibit the activity of the isolated enzymes. A dose- and time-response for induction of cell death was demonstrated for each compound. A proteomic study showed that inhibitor treatment caused an increase of markers of cell stress, including induction of levels of the autophagy marker, LC-3-II. Levels of this marker protein were highest at cytotoxic inhibitor concentrations, implicating autophagy in the cell death process. An in vivo mouse model showed that one of these inhibitors markedly impaired tumor growth. It is concluded that development of drugs to target these two proteases may provide a novel approach to treating neuroblastoma.

Proteomic assessment of a cell model of spinal muscular atrophy

Background

Deletion or mutation(s) of the survival motor neuron 1 (SMN1) gene causes spinal muscular atrophy (SMA), a neuromuscular disease characterized by spinal motor neuron death and muscle paralysis. Complete loss of the SMN protein is embryonically lethal, yet reduced levels of this protein result in selective death of motor neurons. Why motor neurons are specifically targeted by SMN deficiency remains to be determined. In this study, embryonic stem (ES) cells derived from a severe SMA mouse model were differentiated into motor neurons in vitro by addition of retinoic acid and sonic hedgehog agonist. Proteomic and western blot analyses were used to probe protein expression alterations in this cell-culture model of SMA that could be relevant to the disease.

Results

When ES cells were primed with Noggin/fibroblast growth factors (bFGF and FGF-8) in a more robust neural differentiation medium for 2 days before differentiation induction, the efficiency of in vitro motor neuron differentiation was improved from ~25% to ~50%. The differentiated ES cells expressed a pan-neuronal marker (neurofilament) and motor neuron markers (Hb9, Islet-1, and ChAT). Even though SMN-deficient ES cells had marked reduced levels of SMN (~20% of that in control ES cells), the morphology and differentiation efficiency for these cells are comparable to those for control samples. However, proteomics in conjunction with western blot analyses revealed 6 down-regulated and 14 up-regulated proteins with most of them involved in energy metabolism, cell stress-response, protein degradation, and cytoskeleton stability. Some of these activated cellular pathways showed specificity for either undifferentiated or differentiated cells. Increased p21 protein expression indicated that SMA ES cells were responding to cellular stress. Up-regulation of p21 was confirmed in spinal cord tissues from the same SMA mouse model from which the ES cells were derived.

Conclusion

SMN-deficient ES cells provide a cell-culture model for SMA. SMN deficiency activates cellular stress pathways, causing a dysregulation of energy metabolism, protein degradation, and cytoskeleton stability.

Abstract 666: Regulation of IGF-1 signaling by lysosomal cathepsins in neuroblastoma

Neuroblastoma is the most common extracranial solid tumor of childhood. Despite intensive combination therapies, the survival rate for children diagnosed with high-risk neuroblastoma remains below 40%, with surviving children suffering from severe side-effects of aggressive surgery, cytotoxic chemotherapy and radiation. We show that specific inhibition of a group of lysosomal proteases results in accumulation of autophagic vacuoles and eventual apoptosis of neuroblastoma cell lines. Cell death is not caused in breast and prostate cancer cell lines or mouse primary fibroblasts, indicating specificity of the apoptotic response in neuroblastoma. In this study we have examined mechanisms by which inhibition of these proteases can lead to the specific death of these tumors. IGF-1 signaling is essential for cellular proliferation, survival, and apoptotic resistance. We found that in neuroblastoma cells IGF-1 signaling is impaired by inhibition of cathepsins B and L. Western blotting shows that lower molecular weight fragments of the IGF-1 receptor beta subunit accumulate in inhibitor-treated cells, indicating that lysosomal proteases are essential for the complete the degradation of the IGF-1 receptor. Phosphorylation and activation of full-length IGF-1 receptor was not modified by inhibitor treatment, but phosphorylation and activation of MAPK and Akt (downstream effectors of IGF-1 receptor signaling) was greatly diminished. Fractionation of cellular lysates showed that the IGF-1R β fragments accumulate in compartments that contain a cleaved form of LC-3, a marker of autophagic vacuoles. Intermediates of the IGF-1 signaling cascade were also sequestered within these compartments, and significantly depleted in the cytoplasm. We conclude that inhibition of lysosomal proteases results in sequestration of critical components of the cell signaling transduction pathway, causing cell death by uncoupling of cell surface and endosomal signaling from intracellular effector signals.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 666.

Protein processing by the placental protease, cathepsin P

Cathepsin P is a member of a family of placentally expressed cathepsins (PECs). The closest human homolog of cathepsin P is cathepsin L, a broad specificity enzyme that has functions in many tissues in addition to placenta. The gene duplications that gave rise to the PECs provide a rare opportunity to define proteolytic functions in placenta, a transient organ unique to mammals. Peptidyl substrate and inhibitor libraries have shown that cathepsin P has evolved an unusually restricted preference for substrates containing hydrophobic amino acids. Proteomic techniques were used to probe for substrates of this enzyme. Recombinant cathepsin P was incubated with rat choriocarcinoma (Rcho-1) cell proteins to identify substrates using two-dimensional difference gel electrophoresis. Substrate proteins were excised from gels and characterized by trypsin digestion and MALDI MS/MS. Two endoplasmic reticulum (ER) proteins, gp96 and calreticulin, emerged as potential substrates, and western blotting showed that these proteins are processed by cathepsin P from their C-terminus, removing the KDEL ER retention signal. Immunohistochemistry showed that a portion of cathepsin P co-localizes with calreticulin in Rcho-1 cells. Extracellular calreticulin induces differentiation of Rcho-1 cells, indicating a potential role of cathepsin P in processing and secretion of calreticulin during differentiation of trophoblast giant cells.

Emerging functions of placental cathepsins

A series of tandem duplications of an ancestral cathepsin L gene has given rise to a family of eight placenta-specific cathepsins in mice. These genes are differentially regulated both spatially and temporally and thus each can perform unique placental functions. Analysis of the function and expression of these genes is yielding new insights into gene regulation and proteolytic processes in placenta, and may dissect critical placental roles of the single human functional ortholog, cathepsin L.

Development of a specific inhibitor for the placental protease, cathepsin P

Gene duplications in rodents have given rise to a family of proteases that are expressed exclusively in placenta. To define the biological role of these enzymes specific inhibitors are needed to differentiate their activities from other more ubiquitously expressed proteases, such as cathepsins B and L. Libraries of peptidyl inhibitors based upon a 4-cyclohexanone pharmacophore were screened for inhibition of cathepsins P, L, and B. The tightest binding dipeptidyl inhibitor for cathepsin P contained Tyr in P(2) and Trp in P(2)('), consistent with the specificity of this enzyme for hydrophobic amino acids at these sites in synthetic substrates. An inhibitor containing Trp in both P(2) and P(2)(') provided better discrimination between cathepsin P and cathepsins B and L. Extension of the inhibitors to include P(3), and P(3)(') amino acids identified an inhibitor with Trp in P(2), P(2)('), and P(3), and Phe in P(3)(') that bound to cathepsin P with a K(i) of 32 nM. This specificity for inhibitors with hydrophobic aromatic amino acids in these four positions is unique among the lysosomal cysteine proteases. This inhibitor bound to cathepsin P an order of magnitude tighter than to mouse and human cathepsin L and two orders of magnitude tighter than to human cathepsin B. Cbz-Trp-Trp-4-cyclohexanone-Trp-Phe-OMe can discriminate cathepsin P from cathepsins B and L and consequently can be used to specifically inhibit and identify cathepsin P in cellular systems.

Expression of cathepsin P mRNA, protein and activity in the rat choriocarcinoma cell line, Rcho-1, during giant cell transformation

Lysosomal proteases perform critical functions in protein turnover and are essential for normal growth and development. Cathepsin P is a member of a newly discovered family of lysosomal cysteine proteases uniquely expressed in rodent placenta (PECs), and is closely related to human cathepsin L. Using the rat choriocarcinoma cell line model, Rcho-1, mRNA for the PECs cathepsins P, M, Q, R, 1, 2 was found to increase in expression during differentiation into a trophoblast giant cell phenotype. By contrast, expression of cathepsin L was not regulated. A specific enzyme assay was developed to show that activity of cathepsin P mirrored mRNA expression during differentiation. Cathepsin P protein co-localizes with cathepsin B, indicating that the enzyme probably functions in the endosomal-lysosomal compartment. This study demonstrates that the PEC genes produce functional proteases that can perform specific placental roles that are probably performed by broader specificity proteases in human placenta.

Cathepsin L stabilizes the histone modification landscape on the Y chromosome and pericentromeric heterochromatin

Posttranslational histone modifications and histone variants form a unique epigenetic landscape on mammalian chromosomes where the principal epigenetic heterochromatin markers, trimethylated histone H3(K9) and the histone H2A.Z, are inversely localized in relation to each other. Trimethylated H3(K9) marks pericentromeric constitutive heterochromatin and the male Y chromosome, while H2A.Z is dramatically reduced at these chromosomal locations. Inactivation of a lysosomal and nuclear protease, cathepsin L, causes a global redistribution of epigenetic markers. In cathepsin L knockout cells, the levels of trimethylated H3(K9) decrease dramatically, concomitant with its relocation away from heterochromatin, and H2A.Z becomes enriched at pericentromeric heterochromatin and the Y chromosome. This change is also associated with global relocation of heterochromatin protein HP1 and histone H3 methyltransferase Suv39h1 away from constitutive heterochromatin; however, it does not affect DNA methylation or chromosome segregation, phenotypes commonly associated with impaired histone H3(K9) methylation. Therefore, the key constitutive heterochromatin determinants can dynamically redistribute depending on physiological context but still maintain the essential function(s) of chromosomes. Thus, our data show that cathepsin L stabilizes epigenetic heterochromatin markers on pericentromeric heterochromatin and the Y chromosome through a novel mechanism that does not involve DNA methylation or affect heterochromatin structure and operates on both somatic and sex chromosomes.

Defining the substrate specificity of mouse cathepsin P

Cathepsin P is a recently discovered placental cysteine protease that is structurally related to the more ubiquitously expressed, broad-specificity enzyme, cathepsin L. We studied the substrate specificity requirements of recombinant mouse cathepsin P using fluorescence resonance energy transfer (FRET) peptides derived from the lead sequence Abz-KLRSSKQ-EDDnp (Abz, ortho-aminobenzoic acid and EDDnp, N-[2,4-dinitrophenyl]ethylenediamine). Systematic modifications were introduced resulting in five series of peptides to map the S(3) to S(2)(') subsites of the enzyme. The results indicate that the subsites S(1), S(2), S(1)('), and S(2)('), present a clear preference for hydrophobic residues. The specificity requirements of the S(2) subsite were found to be more restricted, preferring hydrophobic aliphatic amino acids. The S(3) subsite of the enzyme presents a broad specificity, accepting negatively charged (Glu), positively charged (Lys, Arg), and hydrophobic aliphatic or aromatic residues (Val, Phe). For several substrates, the activity of cathepsin P was markedly regulated by kosmotropic salts, particularly Na(2)SO(4). No significant effect on secondary or tertiary structure could be detected by either circular dichroism or size exclusion chromatography, indicating that the salts most probably disrupt unfavorable ionic interactions between the substrate and enzyme active site. A substrate based upon the preferred P(3) to P(2)(') defined by the screening study, ortho-aminobenzoic-Glu-Ile-Phe-Val-Phe-Lys-Gln-N-(2,4-dinitrophenyl)ethylenediamine (cleaved at the Phe-Val bond) was efficiently hydrolyzed in the absence of high salt. The k(cat)/K(m) for this substrate was almost two orders of magnitude higher than that of the original parent compound. These results show that cathepsin P, in contrast to other mammalian cathepsins, has a restricted catalytic specificity.

Expression and characterization of cathepsin P

The mouse genome contains a family of clan C1A proteases that appear to be restricted to rodents within Eutherian (placental) mammals. mRNA analysis has shown that these genes are expressed exclusively in placenta. Sequence analysis predicts that the expressed proteins will be functional and consequently it was proposed that this family of proteases may have evolved to perform subspecialized functions of the closely related protease, cathepsin L, that is expressed in placental tissues of all mammalian species. In the present study, it was shown that cathepsin P can be expressed in Pichia pastoris as an inactive zymogen that can be activated with proteinase K, chymotrypsin or pancreatic elastase at neutral pH. Unlike other mammalian cathepsins, cathepsin P could also be autoactivated at neutral pH, but not at acidic pH. The activated enzyme was capable of hydrolysing peptidyl substrates and the protein substrates azocasein and transferrin, with optimal activity at pH 6.5-7.5. Little activity could be detected at pH 5.0 and below. Salts such as Na2SO4 and hyaluronate stimulated the activity of the protease against peptidyl substrates. The properties of cathepsin P appear to be quite distinct from those of cathepsin L, indicating that the duplication that gave rise to cathepsin P has probably not yielded an enzyme that provides a subfunction of cathepsin L in rodents. It seems probable that cathepsin P has evolved to perform a function that is performed by an evolutionarily unrelated protease in other mammalian species.

Nitric oxide does not mediate promotion of cellular potassium release by phenolphthalein in COS-7 cells

1. It has been proposed that phenolphthalein exerts its laxative effect via an intracellular cascade that begins with the activation of nitric oxide synthase (NOS) and ends with an inhibition of NaCl and water reabsorption from the colon. Phenolphthalein also promotes the release of potassium from cells, but it is not known how this is related to its effect on sodium and water uptake. 2. An established in vitro system was used to examine the role of nitric oxide (NO) in phenolphthalein-induced release of (86)Rb(+) from COS-7 cells. 3. Sodium nitroprusside, an NOS-independent NO source, was unable to mimic the effects of phenolphthalein and N(G)-nitro-L-arginine methyl ester, an NOS inhibitor, was unable to block the effect of phenolphthalein. 4. It is concluded that NO generation is not required for phenolphthalein-stimulated potassium release. It is proposed that the effect of phenolphthalein on cellular potassium release is mechanistically distinct from the effect on NaCl and water uptake by colonocytes.

Regulation of 86Rb+ ion transport across polarized human colonocytes by bis-phenolic compounds

1. Phenolphthalein, a well-known laxative, stimulates the secretion of Na+ and Cl- ions and accompanying water into the intestinal tract. Measurement of 86Rb+ efflux from several, but not all, cell types indicates that phenolphthalein also results in release of cellular K+ ions. 2. In the present study, the transport of 86Rb+ across human colonocyte cells (T84) cultured on trans-well inserts was examined. The T84 cells were cultured until they developed tight junctions and a high trans-epithelial resistance. 3. Results show that phenolphthalein applied to the apical, but not the basolateral, surface of cells causes the release of 86Rb+ from the apical surface. Basolateral treatment of cells with phenolphthalein had no effect on the release of 86Rb+. 4. Simultaneously with the increased 86Rb+ efflux, indirect evidence of enhanced Na+/K+-ATPase activity was also observed. 5. Although ouabain inhibited the increased Na+ pump activity, it did not affect apical 86Rb+ release. 6. As evidenced by near steady state 86Rb+ uptake data, the increased Na+/K+-ATPase activity was insufficient to restore intracellular concentrations of K+ in the presence of phenolphthalein. 7. 4,4(9-Fluorenylidene)diphenol, a homologue of phenolphthalein, had a similar effect on 86Rb+ transport by T84 cells. 8. These results indicate a primary stimulation of 86Rb+ efflux from the apical surface of polarized T84 cells by apically applied bis-phenolic compounds. 9. A secondary stimulation of the basolateral Na+/K+-ATPase is thought to result from intracellular Na+ increase, as documented in several other cell types exposed to bis-phenolic compounds, although not directly measured in these experiments. 10. The results also indicate that bis-phenolic compounds interact specifically with some apical but not basolateral membrane structures in regulating 86Rb+ efflux from polarized T84 cells.

Evolution of placental proteases

The placenta is a critical organ in mammals required for the transport of nutrients from the mother to the fetus during gestation. Other critical functions of the placenta include hormone regulation and immune regulation. The origin of the mammals and early placenta is relatively recent in evolutionary terms, and consequently there are few placenta-specific genes. In two separate branches of mammalian evolution, gene duplications have given rise to two large families of protease genes that are expressed only by placental tissues. A family of aspartic protease genes is expressed only in artiodactyls, and a family of cysteine protease genes is expressed only in rodents. These genes have probably evolved to perform specific functions in the placenta that are carried out by broader specificity proteases in mammalian species that do not express these proteases.

Evolution of placentally expressed cathepsins

Species and strain variants of a family of placentally expressed cathepsins (PECs) were cloned and sequenced in order to identify evolutionary conserved structural characteristics of this large family of cysteine proteases. Cathepsins M, P, Q, and R, are conserved in mice and rats but homologs of these genes are not found in human or rabbit placenta, showing that this family of proteases are probably restricted to rodents. Species-specific gene duplications have given rise to variants of cathepsin M in mice, and cathepsin Q in rats. Although the PECs have diverged at a greater rate than the other lysosomal cathepsins, residues around the specificity sub-sites of the individual enzymes are conserved. Strain-specific polymorphisms show that the evolutionary rate of divergence of cathepsins M and 3, the most recently duplicated pair of mouse genes, is even higher than the other PECs. In human placenta, critical functions of the PECs are probably performed by broader specificity proteases such as cathepsins B and L.

Characterization of mouse cathepsin R, a new member of a family of placentally expressed cysteine proteases

A new mouse cysteine protease, termed cathepsin R, has been identified. The complete nucleotide sequence of this gene was derived from a set of cDNAs generated from 15.5-day mouse placenta. Sequence analysis revealed an open reading frame encoding a 334 amino acid long polypeptide closely related to placentally expressed cathepsins P, Q, and M. RT-PCR analysis indicated that cathepsin R is only expressed in placenta and thus is a new member of the emerging family of cathepsins whose expression is regulated during mouse embryonic development. Modeling and structural analysis suggests that cathepsin R will have a restricted substrate specificity when compared to that of cathepsin L.

Mouse cathepsin M, a placenta-specific lysosomal cysteine protease related to cathepsins L and P

The complete nucleotide sequence of a novel cathepsin cDNA derived from mouse placenta was determined and is termed cathepsin M. The predicted protein of 333 amino acid is a member of the family C1A proteases and is related to mouse cathepsins L and P. Mouse cathepsin M is highly expressed in placenta, whereas no detectable levels were found in lung, spleen, heart, brain, kidney, thymus, testicle, liver, or embryo. Phylogenic analyses of the sequences of human and mouse cathepsins show that cathepsin M is most closely related to cathepsins P and L. However, the differences are sufficiently large to indicate that the enzymes will be found in other species. This is in contrast to human cathepsins L and V, which probably resulted from a gene duplication after divergence of mammalian species.

Cathepsin Q, a novel lysosomal cysteine protease highly expressed in placenta

The complete nucleotide sequence of a novel cathepsin cDNA derived from rat placenta was determined and is termed cathepsin Q. The predicted protein of 343 amino acid is a member of the family C1A protease related to cathepsin L. Rat cathepsin Q and its mouse counterpart were found highly expressed in placenta, whereas no detectable levels were found in lung, spleen, heart, brain, kidney, thymus, testicle, liver, or embryonic tissues. It is predicted that cathepsin Q will differ in catalytic specificity to another placental-specific protease, cathepsin P, indicating that these enzymes will have unique proteolytic functions in extra-embryonic tissues.

Expression of cysteine proteases in extraembryonic tissues during mouse embryogenesis

The expression of cathepsin B- and L-specific mRNAs as well as active forms of the enzymes was determined in mouse placenta and visceral yolk sac from 7.5 through 17.5 days postconception, a period marked by major anatomic transitions in the mouse conceptus. The level of specific mRNA was determined relative to the 28S ribosomal RNA in a series of multiprobe ribonuclease protection assays using high-specific-activity antisense cathepsin B and L riboprobes. The molecular forms of active cysteine proteases present in the tissues at the time of extraction were detected using a membrane-permeant radiolabeled active site-specific inhibitor, Fmoc-[(125)I(2)]Tyr-Ala-CHN(2). The results of this study show that the expression of active cathepsin L relative to active cathepsin B is significantly higher in visceral yolk sac than in placenta, consistent with a higher proteolytic requirement for the former tissue. Active cathepsin L was highest at Day 9.5 in visceral yolk sac, a stage at which it has been shown that proteolysis in this organ is required for production of amino acids for embryonic protein synthesis. Cathepsin L mRNA was also elevated in the Day 9.5 placenta, but paradoxically this did not result in an increase in cellular active enzyme. An unknown protein, termed p14, highly expressed in placenta, also reacted with the inhibitor. Expression of this protein was highest early during gestation in the ectoplacental cone, suggesting that p14 may be important in the implantation process.

Cathepsin P, a novel protease in mouse placenta

The complete cDNA nucleotide sequence of a novel cathepsin derived from mouse placenta, termed cathepsin P, was determined. mRNA for cathepsin P was expressed in placenta and at lower levels in visceral yolk sac, but could not be detected in a range of adult tissues. The expression pattern of this protease indicates that it probably plays an important role during implantation and fetal development.

Design of a transferrin-proteinase inhibitor conjugate to probe for active cysteine proteinases in endosomes

A new technique has been developed to identify active proteinases in endosomes that does not require prior isolation of organelles and extraction of the active enzymes. [125I]Iodotyrosylalanyldiazomethane was reversibly conjugated to transferrin to selectively deliver it to endosomes. The protein was conjugated to the inhibitor via a disulphide bond using N-succinimidyl 3-(2-pyridyldithio)propionate. The inhibitor portion of the conjugate bound irreversibly to active cathepsins B and L, and subsequently the reacted enzymes were separated from the transferrin after SDS/PAGE under reducing conditions. Uptake of the protein-inhibitor conjugate and incorporation of inhibitor into cathepsins was blocked at 4 degreesC, demonstrating that the conjugate enters cells by receptor-mediated endocytosis. Furthermore, endocytosed transferrin-inhibitor conjugate could be recycled back to the extracellular medium and binding to the transferrin receptor could be blocked by native transferrin. Labelling of the enzymes was not blocked by incubating cells at 16 degreesC, consistent with the majority of the reagent being targeted to endosomes. The inhibited enzymes remained conjugated to transferrin, showing that the disulphide bond between the transferrin and inhibitor was not reduced in the endosome. Results from these studies show that endosomes contain both intermediate and late biosynthetic forms of active cathepsin B, which are indistinguishable from those found in mature lysosomes. These results indicate that the active enzymes in endosomes are not early biosynthetic forms in transit to lysosomes but most probably enter the endosome via retrograde traffic from the lysosome.

Quantification of cathepsins B and L in cells

A method for quantifying active cysteine proteinases in mammalian cells has been developed using an active-site-directed inhibitor. Fluoren-9-ylmethoxycarbonyl(di-iodotyrosylalanyl)-diaz omethane (Fmoc-[I2]Tyr-Ala-CHN2) was prepared and shown to react irreversibly with cathepsins B and L, but not with cathepsin S. The non- and mono-iodo forms of the inhibitor reacted with all three enzymes. These results demonstrate that, unlike cathepsins B and L, cathepsin S has a restricted S2-binding site that cannot accommodate the bulky di-iodotyrosine. Fmoc-[I2]Tyr-Ala-CHN2 was able to penetrate cells and react with active enzymes within the cells. A radiolabelled form of the inhibitor was synthesized and the concentration of functional inhibitor was established by titration with papain. This inhibitor was used to quantify active cysteine proteinases in cultured cells. Active cathepsin B was found to be expressed by all of the cells studied, consistently with a housekeeping role for this enzyme. Active forms of cathepsin L were also expressed by all of the cells, but in different quantities. Two additional proteins were labelled in some of the cells, and these may represent other non-characterized proteinases. Higher levels of active cathepsins B and L, and an unidentified protein of Mr 39000, were found in breast tumour cells that are invasive, compared with those that are not invasive. From the data obtained, it can be calculated that the concentrations of both active cathepsins B and L in lysosomes can be as high as 1 mM, each constituting up to 20% of total protein in the organelle. This new technique provides a more direct procedure for determining the proteolytic potential of cellular lysosomes.

Structural basis for different inhibitory specificities of human cystatins C and D

Human cystatins C and D share almost identical primary structures of two out of the three segments proposed to be of importance for enzyme interactions but have markedly different profiles for inhibition of the target cysteine peptidases, cathepsins B, H, L, and S. To investigate if the N-terminal binding regions of the inhibitors are responsible for the different inhibition profiles, and thereby confer biological selectivity, two hybrid cystatins were produced in Escherichia coli expression systems. In one hybrid, the N-terminal segment of cystatin C was placed on the framework of cystatin D, and the second was engineered with the N-terminal segment of cystatin D on the cystatin C scaffold. Truncated cystatin C and D variants, devoid of their N-terminal segments, were obtained by incubation with glycyl endopeptidase and isolated, in a second approach to assess the importance of the N-terminal binding regions for cystatin function and specificity. The affinities of the four cystatin variants for cathepsins B, H, L, and S were measured. By comparison with corresponding results for wild-type cystatins C and D, it was concluded (1) that both the N-terminal and framework part of the molecules significantly contribute to the observed differences in inhibitory activities of cystatins C and D and (2) that the N-terminal segment of cystatin C increases the inhibitory activity of cystatin D against cathepsin S and cathepsin L but results in decreased activity against cathepsin H. These differences in specificity were explained by the residues interacting with the S2 subsite of peptidases (Val- and Ala-10 in cystatin C and D, respectively). Also, removal of the N-terminal segment results in total loss of enzyme affinity for cystatin D but not for cystatin C. Therefore, structural differences in the framework parts, as well as in the N-terminal segments, are critical for both inhibitory specificity and potency. Homology modeling was used to identify residues likely responsible for the generally reduced inhibitory potency of cystatin D.