Expression of human cathepsin D in Xenopus oocytes: phosphorylation and intracellular targeting

Long-Term Assessment of AAV-Mediated Zinc Finger Nuclease Expression in the Mouse Brain

Gene editing tools like TALENs, ZFNs and Crispr/Cas now offer unprecedented opportunities for targeted genetic manipulations in virtually all species. Most of the recent research in this area has concentrated on manipulation of the genome in isolated cells, which then give rise to transgenic animals or modified stem cell lines. Much less is known about applicability of genetic scissors in terminally differentiated, non-dividing cells like neurons of the adult brain. We addressed this question by expression of a pair of ZFNs targeting the murine cathepsin D gene in CNS neurons by means of an optimized AAV viral vector. We show that ZFN expression resulted in substantial depletion of cathepsin D from neuronal lysosomes, demonstrating a robust gene deletion. Importantly, long-term ZFN expression in CNS neurons did not impair essential neuronal functionality and did not cause inflammation or neurodegeneration, suggesting that potent genetic scissors can be expressed safely in the mouse brain. This finding opens up new venues to create novel research models for neurodegenerative disorders.

"Late" macroendosomes and acidic endosomes in vertebrate motor nerve terminals

Activity at the vertebrate nerve-muscle synapse creates large macroendosomes (MEs) via bulk membrane infolding. Visualized with the endocytic probe FM1-43, most (94%) of the ∼25 MEs/terminal created by brief (30-Hz, 18-second) stimulation dissipate rapidly (∼1 minute) into vesicles. Others, however, remain for hours. Here we study these "late" MEs by using 4D live imaging over a period of ∼1 hour after stimulation. We find that some (51/398 or 13%) disappear spontaneously via exocytosis, releasing their contents into the extracellular milieu. Others (at least 15/1,960 or 1%) fuse or closely associate with a second class of endosomes that take up acidophilic dyes (acidic endosomes [AEs]). AEs are plentiful (∼47/terminal) and exist independent of stimulation. Unlike MEs, which exhibit Brownian motion, AEs exhibit directed motion (average, 83 nm/sec) on microtubules within and among terminal boutons. AEs populate the axon as well, where movement is predominantly retrograde. They share biochemical and immunohistochemical markers (e.g., lysosomal-associated membrane protein [LAMP-1]) with lysosomes. Fusion/association of MEs with AEs suggests a sorting/degradation pathway in nerve terminals wherein the role of AEs is similar to that of lysosomes. Based on our data, we propose that MEs serve as sorting endosomes. Thus their contents, which include plasma membrane proteins, vesicle proteins, and extracellular levels of Ca(2+) , can be targeted either toward the reformation and budding of synaptic vesicles, toward secretion via exocytosis, or toward a degradation process that utilizes AEs either for lysis within the terminal or for transport toward the cell body.

Comparison of functional proteomic analyses of human breast cancer cell lines T47D and MCF7

T47D and MCF7 are two human hormone-dependent breast cancer cell lines which are widely used as experimental models for in vitro and in vivo (tumor xenografts) breast cancer studies. Several proteins involved in cancer development were identified in these cell lines by proteomic analyses. Although these studies reported the proteomic profiles of each cell line, until now, their differential protein expression profiles have not been established. Here, we used two-dimensional gel and mass spectrometry analyses to compare the proteomic profiles of the two cell lines, T47D and MCF7. Our data revealed that more than 164 proteins are differentially expressed between them. According to their biological functions, the results showed that proteins involved in cell growth stimulation, anti-apoptosis mechanisms and cancerogenesis are more strongly expressed in T47D than in MCF7. These proteins include G1/S-specific cyclin-D3 and prohibitin. Proteins implicated in transcription repression and apoptosis regulation, including transcriptional repressor NF-X1, nitrilase homolog 2 and interleukin-10, are, on the contrary, more strongly expressed in MCF7 as compared to T47D. Five proteins that were previously described as breast cancer biomarkers, namely cathepsin D, cathepsin B, protein S100-A14, heat shock protein beta-1 (HSP27) and proliferating cell nuclear antigen (PCNA), are found to be differentially expressed in the two cell lines. A list of differentially expressed proteins between T47D and MCF7 was generated, providing useful information for further studies of breast cancer mechanisms with these cell lines as models.

Hepatic Endosome Protein Profiling in Apolipoprotein E Deficient Mice Expressing Apolipoprotein B48 but not B100

Liver cells absorb apolipoprotein (Apo) B48-carrying lipoproteins in ApoE's absence, albeit not as efficiently as the ApoE-mediated process. Our objective was to identify differentially expressed hepatic endosome proteins in mice expressing ApoB48 but lacking ApoE and ApoB100 expression (ApoE-/-/B48/48). We purified early and late endosomes from ApoE-/-/B48/48 and wild-type mouse's livers. In ApoE-/-/B48/48 mouse's hepatic endosomes, proteomic analysis revealed elevated protein levels of major urinary protein 6 (MUP), calreticulin, protein disulfide isomerases (PDI) A1, and A3. These proteins are capable of interacting with lipids/lipoproteins and triggering receptor-mediated endocytosis. In addition, hepatic endosomes from ApoE-/- /B48/48 mice exhibited significantly reduced protein levels of haptoglobin, hemopexin, late endosome/lysosome interacting protein, cell division control protein 2 homolog, γ-soluble Nethylmaleimide- sensitive factor attachment protein, vacuolar ATP synthase catalytic subunit A1, dipeptidyl peptidases II, cathepsin B, D, H, and Z. These proteins participate in plasma heme clearance, receptor-mediated signaling, membrane fusion, endosomal/lysosomal acidification, and protein degradation. The significance of increasing endosomal MUP, calreticulin and PDIs in ApoE-/-/B48/48 mouse liver cells is not clear; however, reducing endosomal/ lysosomal membrane proteins and hydrolases might be, at least partially, responsible for the retarded clearance of plasma ApoB-carrying lipoproteins in ApoE-/-/B48/48 mice.

Posttranslational cleavage and adaptor protein complex-dependent trafficking of mucolipin-1

Mucolipin-1 (ML1) is a member of the transient receptor potential ion channel superfamily that is thought to function in the biogenesis of lysosomes. Mutations in ML1 result in mucolipidosis type IV, a lysosomal storage disease characterized by the intracellular accumulation of enlarged vacuolar structures containing phospholipids, sphingolipids, and mucopolysaccharides. Little is known about how ML1 trafficking or activity is regulated. Here we have examined the processing and trafficking of ML1 in a variety of cell types. We find that a significant fraction of ML1 undergoes cell type-independent cleavage within the first extracellular loop of the protein during a late step in its biosynthetic delivery. To determine the trafficking route of ML1, we systematically examined the effect of ablating adaptor protein complexes on the localization of this protein. Whereas ML1 trafficking was not apparently affected in fibroblasts from mocha mice that lack functional adaptor protein complex (AP)-3, small interfering RNA-mediated knockdown revealed a requirement for AP-1 in Golgi export of ML1. Knockdown of functional AP-2 had no effect on ML1 localization. Interestingly, cleavage of ML1 was not compromised in AP-1-deficient cells, suggesting that proteolysis occurs in a prelysosomal compartment, possibly the trans-Golgi network. Our results suggest that posttranslational processing of ML1 is more complex than previously described and that this protein is delivered to lysosomes primarily via an AP-1-dependent route that does not involve passage via the cell surface.

Increased expression of the mannose 6-phosphate/insulin-like growth factor-II receptor in breast cancer cells alters tumorigenic properties in vitro and in vivo

The mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGF-IIR) is thought to act as a suppressor of tumor growth by binding the mitogenic peptide IGF-II and modulating its extracellular levels via degradation. This receptor has been found to be absent or nonfunctional in a high proportion of breast tumors as a result of LOH and mutation of the gene. In our study, we have examined the effect of increasing expression of M6P/IGF-IIR on breast cancer cell tumorigenicity. MDA-MB-231 breast cancer cells stably transfected with M6P/IGF-IIR cDNA exhibited not only a greatly reduced ability to form tumors but also a markedly reduced growth rate in nude mice. In vitro, increased M6P/IGF-IIR expression resulted in 2-fold reduced uptake of IGF-II and was associated with reduced cellular invasiness and motility. Cells with increased M6P/IGF-IIR expression exhibited reduced phosphorylation of IGF-I receptor and p44/42 MAPK compared to vector transfectants, or wild-type MDA-MB-231 cells. These results therefore suggest that M6P/IGF-IIR levels can modulate breast cancer cell tumorigenicity by a mechanism that may involve altered IGF-I receptor signaling.

Plasma Membrane Disruption: Repair, Prevention, Adaptation

Many metazoan cells inhabit mechanically stressful environments and, consequently, their plasma membranes are frequently disrupted. Survival requires that the cell rapidly repair or reseal the disruption. Rapid resealing is an active and complex structural modification that employs endomembrane as its primary building block, and cytoskeletal and membrane fusion proteins as its catalysts. Endomembrane is delivered to the damaged plasma membrane through exocytosis, a ubiquitous Ca2+-triggered response to disruption. Tissue and cell level architecture prevent disruptions from occurring, either by shielding cells from damaging levels of force, or, when this is not possible, by promoting safe force transmission through the plasma membrane via protein-based cables and linkages. Prevention of disruption also can be a dynamic cell or tissue level adaptation triggered when a damaging level of mechanical stress is imposed. Disease results from failure of either the preventive or resealing mechanisms.

Prorenin accumulation and activation in human endothelial cells: importance of mannose 6-phosphate receptors

ACE inhibitors improve endothelial dysfunction, possibly by blocking endothelial angiotensin production. Prorenin, through its binding and activation by endothelial mannose 6-phosphate (M6P) receptors, may contribute to this production. Here, we investigated this possibility as well as prorenin activation kinetics, the nature of the prorenin-activating enzyme, and M6P receptor-independent prorenin binding. Human umbilical vein endothelial cells (HUVECs) were incubated with wild-type prorenin, K/A-2 prorenin (in which Lys42 is mutated to Ala, thereby preventing cleavage by known proteases), M6P-free prorenin, and nonglycosylated prorenin, with or without M6P, protease inhibitors, or angiotensinogen. HUVECs bound only M6P-containing prorenin (K(d) 0.9+/-0.1 nmol/L, maximum number of binding sites [B(max)] 1010+/-50 receptors/cell). At 37 degrees C, because of M6P receptor recycling, the amount of prorenin internalized via M6P receptors was >25 times B(max). Inside the cells, wild-type and K/A-2 prorenin were proteolytically activated to renin. Renin was subsequently degraded. Protease inhibitors interfered with the latter but not with prorenin activation, thereby indicating that the activating enzyme is different from any of the known prorenin-activating enzymes. Incubation with angiotensinogen did not lead to endothelial angiotensin generation, inasmuch as HUVECs were unable to internalize angiotensinogen. Most likely, therefore, in the absence of angiotensinogen synthesis or endocytosis, M6P receptor-mediated prorenin internalization by endothelial cells represents prorenin clearance.

High-affinity prorenin binding to cardiac man-6-P/IGF-II receptors precedes proteolytic activation to renin

Mannose-6-phosphate (man-6-P)/insulin-like growth factor-II (man-6-P/IgF-II) receptors are involved in the activation of recombinant human prorenin by cardiomyocytes. To investigate the kinetics of this process, the nature of activation, the existence of other prorenin receptors, and binding of native prorenin, neonatal rat cardiomyocytes were incubated with recombinant, renal, or amniotic fluid prorenin with or without man-6-P. Intact and activated prorenin were measured in cell lysates with prosegment- and renin-specific antibodies, respectively. The dissociation constant (K(d)) and maximum number of binding sites (B(max)) for prorenin binding to man-6-P/IGF-II receptors were 0.6 +/- 0.1 nM and 3,840 +/- 510 receptors/myocyte, respectively. The capacity for prorenin internalization was greater than 10 times B(max). Levels of internalized intact prorenin decreased rapidly (half-life = 5 +/- 3 min) indicating proteolytic prosegment removal. Prorenin subdivision into man-6-P-free and man-6-P-containing fractions revealed that only the latter was bound. Cells also bound and activated renal but not amniotic fluid prorenin. We concluded that cardiomyocytes display high-affinity binding of renal but not extrarenal prorenin exclusively via man-6-P/IGF-II receptors. Binding precedes internalization and proteolytic activation to renin thereby supporting the concept of cardiac angiotensin formation by renal prorenin.

The cellular trafficking and zinc dependence of secretory and lysosomal sphingomyelinase, two products of the acid sphingomyelinase gene

The acid sphingomyelinase (ASM) gene, which has been implicated in ceramide-mediated cell signaling and atherogenesis, gives rise to both lysosomal SMase (L-SMase), which is reportedly cation-independent, and secretory SMase (S-SMase), which is fully or partially dependent on Zn2+ for enzymatic activity. Herein we present evidence for a model to explain how a single mRNA gives rise to two forms of SMase with different cellular trafficking and apparent differences in Zn2+ dependence. First, we show that both S-SMase and L-SMase, which contain several highly conserved zinc-binding motifs, are directly activated by zinc. In addition, SMase assayed from a lysosome-rich fraction of Chinese hamster ovary cells was found to be partially zinc-dependent, suggesting that intact lysosomes from these cells contain subsaturating levels of Zn2+. Analysis of Asn-linked oligosaccharides and of N-terminal amino acid sequence indicated that S-SMase arises by trafficking through the Golgi secretory pathway, not by cellular release of L-SMase during trafficking to lysosomes or after delivery to lysosomes. Most importantly, when Zn2+-dependent S-SMase was incubated with SMase-negative cells, the enzyme was internalized, trafficked to lysosomes, and became zinc-independent. We conclude that L-SMase is exposed to cellular Zn2+ during trafficking to lysosomes, in lysosomes, and/or during cell homogenization. In contrast, the pathway targeting S-SMase to secretion appears to be relatively sequestered from cellular pools of Zn2+; thus S-SMase requires exogeneous Zn2+ for full activity. This model provides important information for understanding the enzymology and regulation of L- and S-SMase and for exploring possible roles of ASM gene products in cell signaling and atherogenesis.

Mutant Rab7 causes the accumulation of cathepsin D and cation-independent mannose 6-phosphate receptor in an early endocytic compartment

Stable BHK cell lines inducibly expressing wild-type or dominant negative mutant forms of the rab7 GTPase were isolated and used to analyze the role of a rab7-regulated pathway in lysosome biogenesis. Expression of mutant rab7N125I protein induced a dramatic redistribution of cation-independent mannose 6-phosphate receptor (CI-MPR) from its normal perinuclear localization to large peripheral endosomes. Under these circumstances approximately 50% of the total receptor and several lysosomal hydrolases cofractionated with light membranes containing early endosome and Golgi markers. Late endosomes and lysosomes were contained exclusively in well-separated, denser gradient fractions. Newly synthesized CI-MPR and cathepsin D were shown to traverse through an early endocytic compartment, and functional rab7 was crucial for delivery to later compartments. This observation was evidenced by the fact that 2 h after synthesis, both markers were more prevalent in fractions containing light membranes. In addition, both were sensitive to HRP-DAB- mediated cross-linking of early endosomal proteins, and the late endosomal processing of cathepsin D was impaired. Using similar criteria, the lysosomal membrane glycoprotein 120 was not found accumulated in an early endocytic compartment. The data are indicative of a post-Golgi divergence in the routes followed by different lysosome-directed molecules.

Human vascular endothelial cells are a rich and regulatable source of secretory sphingomyelinase. Implications for early atherogenesis and ceramide-mediated cell signaling

We recently reported that macrophages and fibroblasts secrete a Zn2+-dependent sphingomyelinase (S-SMase), which, like lysosomal SMase, is a product of the acid SMase gene. S-SMase may cause subendothelial retention and aggregation of lipoproteins during atherogenesis, and the acid SMase gene has been implicated in ceramide-mediated cell signaling, especially involving apoptosis of endothelial cells. Because of the central importance of the endothelium in each of these processes, we now sought to examine the secretion and regulation of S-SMase by vascular endothelial cells. Herein we show that cultured human coronary artery and umbilical vein endothelial cells secrete massive amounts of S-SMase (up to 20-fold more than macrophages). Moreover, whereas S-SMase secreted by macrophages and fibroblasts is almost totally dependent on the addition of exogenous Zn2+, endothelium-derived S-SMase was partially active even in the absence of added Zn2+. Secretion of S-SMase by endothelial cells occurred both apically and basolaterally, suggesting an endothelial contribution to both serum and arterial wall SMase. When endothelial cells were incubated with inflammatory cytokines, such as interleukin-1beta and interferon-gamma, S-SMase secretion by endothelial cells was increased 2-3-fold above the already high level of basal secretion, whereas lysosomal SMase activity was decreased. The mechanism of interleukin-1beta-stimulated secretion appears to be through increased routing of a SMase precursor protein through the secretory pathway. In summary, endothelial cells are a rich and regulatable source of enzymatically active S-SMase, suggesting physiologic and pathophysiologic roles for this enzyme.

Maintenance of calcium homeostasis in the endoplasmic reticulum by Bcl-2

The oncogene bcl-2 encodes a 26-kD protein localized to intracellular membranes, including the ER, mitochondria, and perinuclear membrane, but its mechanism of action is unknown. We have been investigating the hypothesis that Bcl-2 regulates the movement of calcium ions (Ca2+) through the ER membrane. Earlier findings in this laboratory indicated that Bcl-2 reduces Ca2+ efflux from the ER lumen in WEHI7.2 lymphoma cells treated with the Ca2+-ATPase inhibitor thapsigargin (TG) but does not prevent capacitative entry of extracellular calcium. In this report, we show that sustained elevation of cytosolic Ca2+ due to capacitative entry is not required for induction of apoptosis by TG, suggesting that ER calcium pool depletion may trigger apoptosis. Bcl-2 overexpression maintains Ca2+ uptake in the ER of TG-treated cells and prevents a TG-imposed delay in intralumenal processing of the endogenous glycoprotein cathepsin D. Also, Bcl-2 overexpression preserves the ER Ca2+ pool in untreated cells when extracellular Ca2+ is low. However, low extracellular Ca2+ reduces the antiapoptotic action of Bcl-2, suggesting that cytosolic Ca2+ elevation due to capacitative entry may be required for optimal ER pool filling and apoptosis inhibition by Bcl-2. In summary, the findings suggest that Bcl-2 maintains Ca2+ homeostasis within the ER, thereby inhibiting apoptosis induction by TG.

Expression of tyrosine-sulfated secretory proteins in Xenopus laevis oocytes. Differential export of constitutive and regulated proteins

Xenopus laevis oocytes were used to study the tyrosine sulfation and secretion of exogenous proteins. Secretogranin II (SgII), a tyrosine-sulfated protein found in secretory granules of a wide variety of endocrine cells and neurons, became tyrosine-sulfated by the oocytes when expressed by injection of poly(A)-rich RNA isolated from the neuroendocrine cell line PC12. The same result was observed when SgII was expressed from cloned SgII cRNA, showing that its tyrosine sulfation did not require the coexpression of exogenous tyrosylprotein sulfotransferase (TPST) but occurred by means of the endogenous oocyte TPST. Sulfophilin, an artificial protein consisting of 12 repeats of a heptapeptide tyrosine-sulfation site, was highly sulfated upon injection of its RNA, indicating the presence of TPST levels sufficient for stoichiometric sulfation of appropriate reporter proteins. Comparison of the secretion of [35S]sulfate-labelled SgII with that of sulfophilin and an exogenous heparan sulfate proteoglycan (HSPG), two proteins delivered to the cell surface by the constitutive pathway of secretion, revealed striking differences. The majority of sulfophilin and the HSPG was found in the medium, whereas that of SgII was found intracellularly. Prolactin, another secretory granule protein, showed the same secretion behaviour as SgII. These results show that oocytes express TPST and that these cells secrete constitutive and regulated secretory proteins in a differential manner.

Cysteine34 of the cytoplasmic tail of the cation-dependent mannose 6-phosphate receptor is reversibly palmitoylated and required for normal trafficking and lysosomal enzyme sorting

We have examined whether the two cysteine residues (Cys30 and Cys34) in the cytoplasmic tail of the cation-dependent mannose 6-phosphate receptor are palmitoylated via thioesters and whether these residues influence the biologic function of the receptor. To do this, mouse L cells expressing wild-type and mutant receptors were analyzed by metabolic labeling with [3H]palmitate, immunoprecipitation, and SDS-PAGE. Both Cys30 and Cys34 were found to be sites of palmitoylation and together they accounted for the total palmitoylation of the receptor. The palmitate rapidly turned over with a half-life of approximately 2 h compared to a half-life of greater than 40 h for the protein. Mutation of Cys34 to Ala resulted in the gradual accumulation of the receptor in dense lysosomes and the total loss of cathepsin D sorting function in the Golgi. A Cys30 to Ala mutation had no biologic consequences, showing the importance of Cys34. Mutation of amino acids 35-39 to alanines impaired palmitoylation of Cys30 and Cys34 and resulted in abnormal receptor trafficking to lysosomes and loss of cathepsin D sorting. These data suggest that palmitoylation of Cys30 and Cys34 leads to anchoring of this region of the cytoplasmic tail to the lipid bilayer. Anchoring via Cys34 is essential for the normal trafficking and lysosomal enzyme sorting function of the receptor.

Lysine residues in the C-terminal lobe and lysosomal targeting of procathepsin D

A major pathway to the lysosome for soluble hydrolases involves the 6-phosphorylation of mannose residues. The initial step in this reaction is catalyzed by a phosphotransferase which recognizes lysosomal precursors. We constructed mutants of human procathepsin D whose targeting to the lysosome could be assayed directly in intact cells. Eight lysine residues were individually converted to glutamic acid on the surface of the carboxyl terminal lobe of the protein. Mutants with as many as four Lys to Glu mutations were normally targeted to the lysosome and processed to the mature form of the enzyme in transfected cells. We conclude that the C-terminal lobe of procathepsin D may not carry a determinant essential for lysosomal targeting in intact fibroblasts.

The propeptide is nonessential for the expression of human cathepsin D

When the 44-amino acid propeptide of human procathepsin D was deleted by mutagenesis in vitro, the mature protein was stably expressed and secreted from transfected mammalian cells. The secreted protein was correctly folded as judged by its binding to pepstatinylagarose. We were unable to detect lysosomal targeting of the propeptide-deleted protein, and targeting was not restored by the substitution of the propeptides from pepsin or renin. We conclude that its propeptide is not essential for the folding of nascent cathepsin D. Efficient lysosomal targeting in mammalian cells appears to require the precursor form of the molecule.

Serial killing by cytotoxic T lymphocytes: T cell receptor triggers degranulation, re-filling of the lytic granules and secretion of lytic proteins via a non-granule pathway

CD8+ cytotoxic T lymphocyte (CTL) clones begin to synthesize the lytic proteins granzyme A, granzyme B and perforin after stimulation with allogeneic target cells. The lytic proteins are stored in the secretory granules which are released after cross-linking of the T cell receptor (TcR) upon target cell recognition. During lytic granule biogenesis granzyme A protein synthesis can be detected between 2 and 10 days after allogeneic stimulation of the CTL. Although granzyme A is stored in the lytic granules over this period, the majority of granzyme A synthesized is secreted directly from the CTL. TcR triggering of degranulation also results in new synthesis of the lytic proteins, which can be inhibited by cycloheximide (CHX). Some of the newly synthesized lytic proteins can be stored in the cell and refill the granules. But up to one third of granzymes A and B can be secreted directly from the CTL via the constitutive secretory pathway as shown by granzyme A enzymatic activity and immunoblots of secreted granzyme B, where one third of the protein fails to acquire the granule targeting signal. Perforin is also secreted via the constitutive pathway, both from the natural killer cell line, YT, and from CTL clones after TcR cross-linking. Constitutive secretion of the lytic proteins can be blocked by both CHX and brefeldin A (BFA). While BFA does not affect the directional killing of recognized targets, it abrogates bystander killing, indicating that bystander killing arises from newly synthesized lytic proteins delivered via a non-granule route. These results demonstrate that the perforin/granzyme-mediated lytic pathway can be maintained while CTL kill multiple targets. We show that CTL not only re-fill their granules during killing, but also secrete lytic proteins via a non-granule-mediated pathway.

Molecular analysis of wild-type and mutant alleles at the Opaque-2 regulatory locus of maize reveals different mutations and types of O2 products

The expression of the various members of the zein multigene family in maize endosperm is controlled by different regulatory loci. One of these loci, Opaque-2, coding for a bZIP transcriptional factor, controls the expression of a subset of zein genes. Analysis of genomic DNA from plants carrying wild-type (O2) or mutant o2 alleles shows specific DNA restriction patterns that correlate with transcript types and their various gene products. Northern and western analyses show the presence in different wild types of a 1.7 kb transcript coding for different sizes of normal O2 proteins that migrate as doublets in the 68-72 kDa range. Among the various o2 mutants analysed we showed the occurrence of various null-transcript alleles, the presence of alleles with a normal size transcript which, however, produce a different-sized o2 protein, and a mutant producing both a normal size transcript and a longer transcript, but generating only a single o2 product migrating around 40 kDa. Analysis of other mutations (o7, fl2) known to affect zein polypeptide synthesis shows no interference of these mutations in the expression of the O2 gene products. The overall results indicate the occurrence of micro heterogeneity in the O2 wild-type genes and a broad spectrum of o2 mutations, both producing different sizes of O2 or o2 proteins. A nomenclature of the O2 and o2 genes based on the RFLP, transcripts and products of the various alleles is presented.

Mannose 6-phosphate-independent targeting of lysosomal enzymes in I-cell disease B lymphoblasts

B lymphocytes from patients with I-cell disease (ICD) maintain normal cellular levels of lysosomal enzymes despite a deficiency of the enzyme UDP-N-acetylglucosamine: lysosomal enzyme N-acetylglucosamine-1-phosphotransferase. We find that an ICD B lymphoblastoid cell line targets about 45% of the lysosomal protease cathepsin D to dense lysosomes. This targeting occurs in the absence of detectable mannose 6-phosphate residues on the cathepsin D and is not observed in ICD fibroblasts. The secretory protein pepsinogen, which is closely related to cathepsin D in both amino acid sequence and three-dimensional structure, is mostly excluded from dense lysosomes, indicating that the lymphoblast targeting pathway is specific. Carbohydrate residues are not required for lysosomal targeting, since a non-glycosylated mutant cathepsin D is sorted with comparable efficiency to the wild type protein. Analysis of a number of cathepsin D/pepsinogen chimeric proteins indicates that an extensive polypeptide determinant in the cathepsin D carboxyl lobe can confer efficient lysosomal sorting when introduced into the pepsinogen sequence. This determinant overlaps but is not identical to the recognition marker for phosphotransferase. These results indicate that a specific protein recognition event underlies Man-6-P-independent lysosomal sorting in ICD lymphoblasts.

Molecular cloning and functional characterization of chicken cathepsin D, a key enzyme for yolk formation

Upon receptor-mediated endocytosis of very-low-density lipoprotein (VLDL) and vitellogenin into growing chicken oocytes, the protein moieties of these lipoproteins are proteolytically cleaved. Unlike the complete lysosomal degradation in somatic cells, enzymatic ligand breakdown in oocytes generates a characteristic set of polypeptides, which enter yolk storage compartments for subsequent utilization by the embryo. Here, we demonstrate directly that the catalyst for the intraoocytic processing of both apolipoprotein B and vitellogenin is cathepsin D. The enzyme was purified from oocytic yolk, preovulatory follicle homogenates, and liver by affinity chromatography. When plasma VLDL and vitellogenin were incubated with the purified enzyme, fragments indistinguishable from those found in yolk were generated from both precursors under identical, mildly acidic conditions. Amino-terminal sequencing of the pure enzyme demonstrated 88% identity with mammalian cathepsin Ds over 34 residues. On the basis of this information, a full-length clone specifying chicken preprocathepsin D was isolated from a chicken follicle cDNA library by screening with a human cathepsin D probe. Whereas previous studies have demonstrated that the receptors for lipoproteins in somatic cells and oocytes, respectively, of the chicken are the products of different genes, Southern and Northern blot hybridization experiments showed that the enzymes expressed in oocytes and liver are the product of a single gene, giving rise to a 3.3-kb transcript. The primary structure of the 335-residue mature protein suggests a high degree of conservation of known crucial features of aspartyl proteases; however, the absence of the so-called processing region and of an aromatic residue in a region thought to partake in catalysis raise questions with possible evolutionary implications.

The cytoplasmic tail of the mannose 6-phosphate/insulin-like growth factor-II receptor has two signals for lysosomal enzyme sorting in the Golgi

The mannose 6-phosphate/insulin-like growth factor-II (Man-6-P/IGF-II) receptor is known to cycle between the Golgi, endosomes, and the plasma membrane. In the Golgi the receptor binds newly synthesized lysosomal enzymes and transports them directly to an endosomal (prelysosomal) compartment without traversing the plasma membrane. Deletion of the carboxyl-terminal Leu-Leu-His-Val residues of the 163 amino acid cytoplasmic tail of the bovine Man-6-P/IGF-II receptor partially impaired this function, resulting in the diversion of a portion of the receptor-ligand complexes to the cell surface, where they were endocytosed. The same phenotype was observed when 134 residues of the cytoplasmic tail were deleted from the carboxyl terminus. Disruption of the Tyr24-Lys-Tyr-Ser-Lys-Val29 plasma membrane internalization signal alone had little effect on Golgi sorting, but when combined with either deletion resulted in a complete loss of this function. The mutant receptors retained the ability to recycle to the Golgi and bind cathepsin D. These results indicate that the cytoplasmic tail of the Man-6-P/IGF-II receptor contains two signals that contribute to Golgi sorting, presumably by interacting with the Golgi clathrin-coated pit adaptor proteins. The Leu-Leu-containing sequence represents a novel motif for mediating interaction with Golgi adaptor proteins.

A method for [3H]mannose labeling of Asn-linked oligosaccharides on recombinant glycoproteins synthesized in Xenopus oocytes

We have developed an efficient method for labeling the Asn-linked oligosaccharides of recombinant glycoproteins synthesized in Xenopus laevis oocytes. By coinjecting GDP-[3,4-(3)H]mannose with mRNA for human cathepsin D, it was possible to incorporate as much as 1800 cpm per oocyte into each of the two Asn-linked oligosaccharides of this glycoprotein. Overall, about 50% of the microinjected GDP-[3,4-(3)H]mannose was incorporated into Asn-linked oligosaccharides, a 10-fold greater value than that obtained when [2-(3)H]mannose was microinjected. Less than 10% of the injected GDP-[3,4-(3)H]mannose was metabolized to water or converted to amino acids. This technique should facilitate studies of Asn-linked oligosaccharide biosynthesis, processing, and structure in recombinant proteins synthesized in Xenopus oocytes.

Expression of the chicken hepatic glycoprotein receptor in Xenopus oocytes: conservation of ligand and receptor targeting signals

We have obtained expression of the beta-N-acetylglucosamine-binding receptor from chicken hepatocytes in Xenopus oocytes by injecting mRNA synthesized in vitro from a full length cDNA cloned into an expression vector (Mellow et al: J. Biol Chem 263: 5468-5473, 1988). Immunoprecipitation of the receptor after labeling of oocytes with [35S]-methionine for times ranging from 6 to 72 h revealed 4-5 closely spaced bands of 25-30 kDa after SDS-PAGE. Although these bands were largely resistant to endoglycosidase H cleavage, endoglycosidase F reduced the size of all bands to a single species at 23-24 kDa, indicating that they resulted from heterogeneity in glycosylation of a single polypeptide. Incubation of oocytes expressing this receptor with [125I]-GlcNAc-BSA resulted in 1.8 to 10 x higher levels of cell-associated ligand in mRNA-injected vs. water-injected control oocytes, 2-35% of cell-associated counts was removed by EGTA rinse at 20 degrees C, suggesting that most ligand was inaccessible (presumably intracellular). Immunoprecipitation of sucrose gradient fractions detected receptor molecules predominantly in a light organelle at 1.09-1.12 g/cc (the density of early endosomes and plasma membrane vesicles), with no evidence of the receptor in much heavier yolk platelet fractions even in the presence of ligand. In contrast, internalized [125I]-GlcNAc-BSA was found either at the top of the gradients or in organelles at 1.09-1.17 g/cc and in yolk platelets. TCA precipitation indicated that much intracellular ligand was degraded to acid-soluble fragments. Addition of vitellogenin (the yolk protein precursor) to the medium together with the [125I]-GlcNAc-BSA shifted much of the ligand into yolk platelets. These data indicate that the chicken glycoprotein receptor expressed in oocytes mediates binding and internalization of this ligand into an organelle in which ligand-receptor dissociation occurs, allowing for separation of these two molecules into different compartments. The behavior of ligand in Xenopus oocytes expressing the chicken receptor closely resembles its behavior in hepatocytes.

A highly efficient, cell-free translation/translocation system prepared from Xenopus eggs

We describe the use of a Xenopus laevis egg extract for the in vitro translation and post translational modification of membrane and secretory proteins. This extract is capable of the translation and segregation into membranes of microgram per millilitre levels of protein from added mRNAs. Signal sequences of segregated proteins are efficiently cleaved and appropriate N-linked glycosylation patterns are produced. The extract also supports the quantitative assembly of murine immunoglobulin heavy and light chains into tetramers, and two events which take place beyond the endoplasmic reticulum, mannose 6 phosphorylation of murine cathepsin D and O-linked glycosylation of coronavirus E1 protein, also occur, but at reduced efficiency. The stability of the membranes allows protease protection studies and quantitative centrifugal fractionation of segregated and unsegregated proteins to be performed. Conditions for the use of stored extract have also been determined.

Heterologous in vivo processing of human preproendothelin 1 into bioactive peptides

Endothelin (ET) is an extremely potent vasoconstrictor peptide of 21 amino acids, originally found in the supernatant of cultured vascular endothelial cells. To gain insights into its biosynthetic pathway, we expressed a synthetic RNA coding for the 212-amino acid precursor of human ET-1 (preproET-1) in Xenopus oocytes. Cell homogenates and oocyte incubation medium were tested by RIA using an anti-ET-1 serum. ET-1-like immunoreactivity was detected in oocytes injected with preproET-1 synthetic RNA but not in control oocytes and was much higher in medium than in cell homogenates. When preproET-1 was expressed in oocytes treated with monensin, a dramatic decrease in secretion of immunoreactive material was observed, indicating that secretion is mediated by the Golgi complex. ET-1-like immunoreactive material present in oocyte incubation medium was fractionated by reverse-phase HPLC into two main peaks, corresponding to the retention times of human big ET-1 and ET-1. Incubation medium of oocytes expressing the synthetic preproET-1 RNA elicited a characteristic vasoconstrictor response on rabbit vena cava, consistent with the biological activity that would be predicted from the amount of ET-1-like immunoreactivity measured. These results suggest that common pathways of ET maturation exist in widely different cells and that Xenopus oocytes may represent a useful tool in studying the cell biology of ET-1 synthesis.

Differential segregation of human and hamster cathepsin D in transfected baby-hamster kidney cells

The segregation of human cathepsin D, studied in baby-hamster kidney cells (BHK) transfected with human cathepsin D cDNA and compared with that of hamster cathepsin D in the same cells, showed that, in cells that expressed human cathepsin D at a low rate, most of the enzyme remained intracellular. In contrast, when the enzyme was expressed at a high rate, most was secreted. The segregation was examined with an anti-(human cathepsin D) antibody that reacted with the human enzyme exclusively and an anti-(rat cathepsin D) antibody that reacted with both enzymes. In one protocol the cells were metabolically labelled and the two antibodies were used in sequence to precipitate the enzymes from extracts of cells and medium. High expression of the human enzyme did not interfere with the segregation of hamster cathepsin D. In another protocol the activity of cathepsin D in cells and medium was measured before and after titration with anti-(human cathepsin D) antiserum. Human cathepsin D was found predominantly in the medium, and hamster cathepsin D mainly in the cells. In the presence of 10 mM-NH4Cl the intracellular segregation of hamster cathepsin D was strongly inhibited, while the segregation of human cathepsin D was only slightly diminished. In BHK cells, at least two systems participate in the sorting of the two cathepsins, one of them being rather insensitive to NH4Cl.

Expression and maturation of human cathepsin D in baby-hamster kidney cells

In medium and in homogenates from baby-hamster kidney cells (BHK) transfected with human cathepsin D cDNA, an elevated activity of cathepsin D was found as compared to non-transfected cells. The elevated activity was removed by titrating the homogenates with an anti-(human cathepsin D) antibody. Metabolic labelling and immunoprecipitation revealed that, in the transfected cells, human cathepsin D was synthesized as a 53-kDa precursor indistinguishable from that found in human cells. A portion of the precursor was secreted and the remainder was processed to intermediate and mature chains within a few hours of synthesis. The precursor that was released from the transfected cells had a slightly smaller apparent size than that from cultured human fibroblasts. This difference was abrogated when the precursors were treated with glycopeptidase F. In the intracellular small chain a difference was observed in the size of carbohydrate chains that were cleavable with endo-beta-N-acetylglucosaminidase H. Sequence analysis of the N-termini of mature intracellular cathepsin D indicated a N-terminal trimming in both large and small chains from both human and transfected hamster cells. The proteolytic maturation of human cathepsin D in BHK cells closely resembles that in human cells, whereas a portion of the carbohydrate side chains is processed differently. The trimming of the N-termini in mature cathepsin D is proposed to be a part of the maturation and aging of this protein.

The vacuolar protein-targeting signal of yeast carboxypeptidase is functional in oocytes from Xenopus laevis

Carboxypeptidase Y, a yeast vacuolar glycoprotein was expressed in oocytes from Xenopus laevis and its biosynthesis and sorting were examined. In yeast, targeting to the vacuole, the functional equivalent of the lysosome, is not mannose-6-phosphate-receptor dependent. It was found that carboxypeptidase enters the secretory pathway of the oocyte and is there glycosylated, phosphorylated in the carbohydrate part and delivered to the lysosome. Deletion of an amino acid sequence, previously shown to determine intracellular targeting of this enzyme in yeast, caused a loss of phosphorylation and mislocalization of carboxypeptidase Y into the oocyte medium. Inhibition of glycosylation of carboxypeptidase by tunicamycin did not lead to its secretion. In-frame fusion of the targeting domain to a secretory yeast glycoprotein, invertase, did not prevent its secretion. However, a hybrid containing 80% carboxypeptidase abolished invertase secretion. The results indicate that the vacuolar protein-targeting signal from yeast carboxypeptidase can, in principal, function in a higher eukaryote.

Generation of a lysosomal enzyme targeting signal in the secretory protein pepsinogen

Lysosomal enzymes contain a common protein determinant that is recognized by UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase, the initial enzyme in the formation of mannose 6-phosphate residues. To identify this protein determinant, we constructed chimeric molecules between two aspartyl proteases: cathepsin D, a lysosomal enzyme, and pepsinogen, a secretory protein. When expressed in Xenopus oocytes, the oligosaccharides of cathepsin D were efficiently phosphorylated, whereas the oligosaccharides of a glycosylated form of pepsinogen were not phosphorylated. The combined substitution of two noncontinuous sequences of cathepsin D (lysine 203 and amino acids 265-292) into the analogous positions of glycopepsinogen resulted in phosphorylation of the oligosaccharides of the expressed chimeric molecule. These two sequences are in direct apposition on the surface of the molecule, indicating that amino acids from different regions come together in three-dimensional space to form this recognition domain. Other regions of cathepsin D were identified that may be components of a more extensive recognition marker.

The influence of glycosylation on the fate of renin expressed in Xenopus oocytes

It has been recently reported that, in Xenopus oocytes injected with the mRNA for human renin, this secretory renal glycoprotein acquires phosphomannosyl residues on its asparagine-linked oligosaccharide chains, remains intracellular and undergoes a proteolytic cleavage which removes the prosegment. To understand the influence of glycosylation on the fate of renin in Xenopus oocytes and whether it is specific for human renin, we have expressed human renin and mouse Ren1 renin, which are glycosylated at two and three selected asparagine residues, respectively, and mouse Ren2 renin, which is not glycosylated, in Xenopus oocytes. The majority of human and Ren1 renins remained intracellular and underwent proteolytic cleavage, whereas mouse Ren2 renin was secreted efficiently. When human and Ren1 renins were expressed in oocytes treated with tunicamycin, both were secreted efficiently. A mutant of human renin, which had amino-acid substitutions at both glycosylation sites, was also secreted efficiently, whereas that mutated at one of the two sites was not. These results indicate that the majority of all of the glycosylated renin molecules remain intracellular and undergo proteolytic cleavage, probably due to the acquisition of phosphomannosyl residues, and the human renin remains intracellular if it is only glycosylated at one of the two sites.

Restoration of normal lysosomal function in mucopolysaccharidosis type VII cells by retroviral vector-mediated gene transfer

Retroviral vectors were constructed containing a rat beta-glucuronidase cDNA driven by heterologous promoters. Vector-mediated gene transfer into human and canine beta-glucuronidase-deficient mucopolysaccharidosis type VII fibroblasts completely corrected the deficiency in beta-glucuronidase enzymatic activity. In primary cultures of canine mucopolysaccharidosis type VII retinal pigment epithelial cells, which contain large amounts of undegraded glycosaminoglycan substrates, vector correction restored normal processing of specific glycosaminoglycans in the lysosomal compartment. In canine mucopolysaccharidosis type VII bone marrow cells, beta-glucuronidase was expressed at high levels in transduced cells. Thus, the vector-encoded beta-glucuronidase was expressed at therapeutic levels in the appropriate organelle and corrected the metabolic defect in cells exhibiting the characteristic pathology of this lysosomal storage disorder.

Cathepsin D and prognosis in breast cancer

We investigated the possibility that cathepsin D, an estrogen-induced lysosomal protease, might have value as a prognostic factor in breast cancer by studying frozen tissue specimens from 397 patients. We measured the 34-kd mature form of the enzyme by Western blot assay and densitometry. Among 199 patients with node-negative disease, but not among 198 with node-positive disease, high levels of cathepsin D proved to be a significant predictor of reduced disease-free survival (median follow-up, 64 months), either as a continuous variable (log cathepsin D; P = 0.018) or as a dichotomous variable with an optimized cutoff point (P = 0.0001). Results were similar for overall survival (P = 0.009 and 0.0001, respectively). Relating the level of cathepsin D to other prognostic factors in the patients with node-negative disease, we found an association with aneuploidy but none with estrogen or progesterone receptors, tumor size, or the age of the patient. In multivariate analyses, a high level of cathepsin D was the most important independent factor in predicting shorter disease-free and overall survival in patients with node-negative disease. As compared with the risk in women with low levels of cathepsin D, the relative risk of tumor recurrence was 2.6 (95 percent confidence interval, 1.6 to 4.4) and the relative risk of death was 3.9 (95 percent confidence interval, 2.1 to 7.3) among those with high levels of cathepsin D. For disease-free survival, cathepsin D status was predictive of outcome primarily among those with aneuploid tumors; the actuarial five-year recurrence rates of aneuploid tumors were 60 percent among women with high levels of cathepsin D and 29 percent among those with low levels, as compared with 22 percent for all diploid tumors. We conclude that cathepsin D may be an independent predictor of early recurrence and death in node-negative breast cancer.

Mutations in the cytoplasmic domain of the 275 kd mannose 6-phosphate receptor differentially alter lysosomal enzyme sorting and endocytosis

The cation-independent mannose 6-phosphate receptor (Cl-MPR) sorts newly synthesized lysosomal enzymes in the Golgi and endocytoses extracellular lysosomal enzymes. To determine the role of the 163 amino acid cytoplasmic domain of the Cl-MPR in these functions, receptor-deficient mouse L cells were transfected with normal bovine Cl-MPR cDNA or cDNAs mutated in the cytoplasmic domain. The normal Cl-MPR functioned in sorting and endocytosis. Mutant receptors with 40 and 89 residues deleted from the carboxyl terminus of the cytoplasmic tail functioned normally in endocytosis, but were partially impaired in sorting. Mutant receptors with larger deletions leaving only 7 and 20 residues of the cytoplasmic tail were defective in endocytosis and sorting. A mutant receptor containing alanine instead of tyrosine residues at positions 24 and 26 was defective in endocytosis, and partially impaired in sorting. Receptors deficient in endocytosis accumulated at the cell surface. These results indicate that the cytoplasmic domain of the Cl-MPR contains different signals for rapid endocytosis and efficient lysosomal enzyme sorting.

Synthesis of a truncated Mr 46,000 mannose 6-phosphate receptor that is secreted and retains ligand binding

The Mr 46,000 mannose 6-phosphate receptor is an integral membrane protein with its ligand-binding site in the ectoplasmic domain. By site-directed mutagenesis, a stop codon was introduced in the receptor cDNA at the border between the ectoplasmic and membrane-spanning domain. The truncated receptor was expressed in three different systems, Xenopus oocytes, COS cells and BHK-21 cells. In all three systems the truncated receptor behaved as a soluble protein. In oocytes only small amounts of the truncated receptor were secreted within 48 h after synthesis. Accumulation of endoglucosaminidase H-sensitive forms of the truncated receptor in oocytes suggested that exit from the endoplasmic reticulum was slowed down. In COS and BHK-21 cells, the truncated receptor was secreted and, as for wild-type receptor, most of the N-linked oligosaccharides were processed to complex forms. Both the intracellularly-retained (oocytes) and the secreted (COS and BHK-21 cells) truncated receptors bound to phosphomannan-Sepharose in a mannose-6-phosphate-dependent manner. Using chemical cross-linking, the truncated receptor was shown to be secreted as a homodimer.

Nonhuman cells correctly sort and process the human lysosomal enzyme cathepsin D

Cathepsin D, like most lysosomal enzymes, undergoes multiple proteolytic cleavages during its lifetime. Although the significance of the earliest cleavages of cathepsin D is apparent (loss of the NH2-terminal signal peptide and activation peptide), functions of the two later cleavages are not understood and do not occur in all species. To examine these later events, a cDNA coding for human cathepsin D, which is normally processed to a two-chain form, was isolated and then expressed in mammalian cells from species which do not process the enzyme to the two-chain form. Analysis of the expressed human cathepsin D demonstrated proteolytic processing identical with that seen in normal human fibroblasts. Since processing to the two-chain form of the enzyme occurs in the lysosome, these studies revealed that the human cathepsin D was correctly sorted. The data also indicated that the sorting mechanism was conserved between diverse species and that late proteolytic processing in a variety of species was not determined by the presence or absence of the processing enzymes in the cell.

Specific postendocytic proteolysis of apolipoprotein B in oocytes does not abolish receptor recognition

Upon receptor-mediated transfer of plasma very low density lipoprotein (VLDL) particles into growing chicken oocytes, their major apolipoprotein (apo) component, apoB, is proteolytically cleaved. apoB fragmentation appears to be catalyzed by cathepsin D or a similar pepstatin A-sensitive protease and results in the presence of a characteristic set of polypeptides on yolk VLDL particles. The nicks introduced into the apoB backbone during postendocytic processing occur in yolk platelets and appear to prepare internalized VLDL for storage in yolk. Since yolk VLDL binds to chicken receptors specific for apoB-containing lipoproteins in identical fashion to plasma VLDL, the possibility exists that the developing embryo utilizes yolk VLDL as a nutrient by way of receptor-mediated endocytosis.

Isolation of plasma membrane complexes from Xenopus oocytes

A method for the isolation of plasma membrane fractions from Xenopus oocytes has been developed, and the membranes have been characterized biochemically and morphologically. Plasma membrane complexes prepared by this procedure consisted of large sheets of the membrane, with associated vitelline envelope (a nonmembranous meshwork of fibers) and cortical (secretory) granules still attached. The morphology of cell surface microvilli and coated pits was well preserved. Cortical granules were removed by gentle homogenization in a low ionic strength medium, and integral and peripheral membrane proteins were then separated from vitelline envelopes by detergent extraction and phase separation in Triton-X-114. Biochemical characterization of the plasma membrane fractions indicated substantial levels of 5'-nucleotidase and alkaline phosphodiesterase activity associated with the oocyte cell surface, with 44-66% recovery of these markers in the final membrane preparations. Lectin blotting and lectin affinity chromatography with Concanavalin A and wheat germ agglutinin were used to characterize the major glycoprotein species associated with the plasma membrane complexes. Plasma membrane fractions prepared by this procedure should be very useful in both biochemical and morphological studies of membrane protein sorting in the Xenopus oocyte system.