Structural analysis of sulphated glycoprotein 2 from amino acid sequence. Relationship to clusterin and serum protein 40,40.

Sulphated glycoprotein 2 (SGP-2) is the major secreted protein product of rat Sertoli cells; likewise, clusterin is a major constituent of ram rete testis fluid. Isolation and sequencing of the intact subunits and peptides derived from clusterin show that it is the ram homologue of rat SGP-2. Human serum protein 40,40 (SP-40,40), a component of the SC5b-9 complex of complement, has recently been reported to be the human homologue of rat SGP-2. Analysis of the amino acid sequences of rat SGP-2 and human SP-40,40 show that both of these proteins have a significant relationship to the heavy chain of myosin. The regions of highest sequence similarity correspond to the major amphipathic domains in SGP-2/SP-40,40 and the long alpha-helical-tail domain of myosin, which forms a rod-like structure. SGP-2 has anomalous sedimentation behaviour which indicates that it probably exists in an extended conformation. A putative dinucleotide-binding structure has been identified in the longest stretch of identity between SGP-2 and SP-40,40. Elucidation of these features of SGP-2 and SP-40,40 may help to direct future studies into the role of these proteins in the reproductive and complement systems.     

Studies on the relationship between cell proliferation and cell death: opposite patterns of SGP-2 and ornithine decarboxylase mRNA accumulation in PHA-stimulated human lymphocytes.

To assess the relationship between cell proliferation and cell death, the mRNA accumulation of ornithine decarboxylase (ODC) and sulfated glycoprotein 2 (SGP-2) were measured in human peripheral blood lymphocytes (HPBL) 2-6 hours after stimulation with phytohemagglutinin (PHA). ODC is the rate limiting enzyme of polyamines biosynthesis and its early induction in mitogen-stimulated lymphocytes has been reported. On the other hand, SGP-2, a glycoprotein present in most mammalian tissues, is induced in classical models of apoptosis, such as dexamethasone-treated thymocytes. Indeed, a consistent amount of SGP-2 mRNA in quiescent HPBL, an early and progressive decrease of SGP-2 mRNA and a parallel increase of ODC mRNA accumulation, were observed, in PHA-stimulated HPBL, suggesting that concomitant repression of SGP-2 and induction of ODC genes contribute for the cell entering the cell cycle.     

Biosynthesis and molecular cloning of sulfated glycoprotein 2 secreted by rat Sertoli cells

Sulfated glycoprotein 2 (SGP-2) is the major protein secreted by rat Sertoli cells. Pulse-chase labeling shows that SGP-2 is synthesized as a cotranslationally glycosylated 64-kDa precursor that is modified to a negatively charged 73-kDa form before intracellular cleavage to the mature 47- and 34-kDa subunits. A plasmid cDNA library was constructed from immunopurified mRNA, and a recombinant clone containing the entire protein coding sequence of SGP-2 was isolated. The 1857-nucleotide cDNA consists of a 297-nucleotide 5' noncoding segment, a 1341-nucleotide coding segment, and a 219-nucleotide 3' noncoding sequence. The 5' noncoding region contains five ATG codons followed by four short open reading frames. The derived SGP-2 sequence has a molecular weight of 51,379 and contains six potential N-glycosylation sites. Proteolytic processing sites for the preproprotein were determined by amino-terminal sequencing of the isolated SGP-2 subunits. Northern blots show a wide tissue distribution for the 2.0-kb SGP-2 message, and computer sequence analysis indicates a significant relationship between SGP-2 and human apolipoprotein A-I.     

The gene for SP-40,40, human homolog of rat sulfated glycoprotein 2, rat clusterin, and rat testosterone-repressed prostate message 2, maps to chromosome 8

Sulfated glycoprotein 2 (SGP-2) is a rat glycoprotein that is particularly abundant in seminal fluid, where it is found associated with the acrosome and the tail of mature spermatozoa; for this reason it has been suggested that it has an important role in spermatogenesis. On the basis of nucleotide sequence homology, it has been proposed that the orthologous human gene is that coding for serum protein-40,40 (SP-40,40), a serum protein also called complement lysis inhibitor (CLI), SP-40,40 has been shown to act as a control mechanism of the complement cascade: in fact, it prevents the binding of a C5b-C7 complex to the membrane of the target cell and in this way inhibits complement-mediated cytolysis. SGP-2 and SP-40,40 seem then to be part of different biological systems. Furthermore it has been shown that another protein, testosterone-repressed prostate message 2 (TRPM-2), shares sequence homology with SGP-2 and SP-40,40. TRPM-2 is expressed at high levels and in a temporally precisely defined manner in dying cells, an observation that would suggest its involvement in the cascade of events leading to cell death. We have used a large panel of 24 mouse/human hybrid cell lines and a cDNA for SGP-2, which is also highly homologous to that for rat clusterin, to map the chromosomal location of the orthologous human gene. The mapping data and the Southern analysis presented in this paper, in addition to the data available from the literature, strongly suggest that in the human genome there is a single locus homologous to the probe used and that it codes for the protein which has been called, in different species, SP-40,40, SGP-2, clusterin, and TRPM-2. The chromosomal mapping of the locus for this multiname protein should facilitate its cloning and a better understanding of the apparently many biological functions of its product.     

Biosynthesis and molecular cloning of sulfated glycoprotein 1 secreted by rat Sertoli cells: sequence similarity with the 70-kilodalton precursor to sulfatide/GM1 activator

Sulfated glycoprotein 1 (SGP-1) is one of the abundant proteins secreted by rat Sertoli cells. Pulse-chase labeling shows that SGP-1 is synthesized as a cotranslationally glycosylated 67-kilodalton (kDa) precursor which is posttranslationally modified to a 70-kDa form before secretion to the extracellular space. A plasmid cDNA library was constructed from immunopurified mRNA, and two overlapping clones coding for the entire protein coding sequence were isolated. The cDNAs represent 27 nucleotides of 5' noncoding sequence, 1554 nucleotides of coding sequence, and 594 nucleotides of 3' noncoding sequence. The derived SGP-1 sequence contains 554 amino acids and has a molecular weight of 61,123. Four potential N-glycosylation sites occur within the sequence. An internal region of SGP-1 shows 78% sequence identity with the 67 N-terminal amino acids described for human sulfatide/GM1 activator (SAP-1). Sequence comparisons suggest that SGP-1 is the precursor to sulfatide/GM1 activator; however, the secretion of the protein from Sertoli cells is distinct from the proteolytic processing and lysosomal compartmentalization which have been described for human fibroblasts. The presence of internal sequence similarity suggests that three additional binding sites may occur in SGP-1. Northern blots show similar levels of expression for the 2.6-kilobase SGP-1 mRNA in all tissues examined. The site of SGP-1 synthesis in testis was localized to Sertoli cells by immunofluorescence and in situ hybridization.     

Dynamics of gene expression for a hippocampal glycoprotein elevated in Alzheimer's disease and in response to experimental lesions in rat.

A hippocampal poly(A) RNA, pADHC-9, was cloned by differential screening of a human hippocampal cDNA library. By RNA blot analysis, pADHC-9 was elevated 2-fold in Alzheimer's disease hippocampus. In situ analyses identified pADHC-9 expression in pyramidal and non-pyramidal cells of the hippocampus and entorhinal cortex. Nucleotide sequence analysis identified pADHC-9 as a potential human homolog of rat sulfated glycoprotein 2 (SGP-2). SGP-2 expression increased in rat hippocampus following experimental lesions that mimic intrinsic neuronal loss and/or deafferentation. The function of pADHC-9 in brain has not been defined, but in serum, a similar protein inhibits complement-dependent cytolysis. Increased expression of pADHC-9 in Alzheimer's disease hippocampus may be a compensatory response mounted to retard a complement-driven neurodegenerative cascade.     

Purification and characterization of a sulfated glycoprotein secreted by Sertoli cells

Sulfated glycoprotein 2 (SGP-2), the major secretion product of Sertoli cells, was purified from cell culture medium by reverse-phase high-performance liquid chromatography. The native protein consists of disulfide-linked monomers of 41,000 and 29,000 daltons which have a strong tendency to associate into multimers. The purified SGP-2 was subjected to amino acid analysis and contained high levels of Asx (11.1%), Glx (15.1%), and leucine (11.5%). The oligosaccharides on the purified SGP-2 were analyzed to determine the monosaccharide compositions and the molecular weights of the intact carbohydrate moieties. SGP-2 was shown to be 23.7% carbohydrate and consisted of 1% fucose, 3.5% mannose, 4.1% galactose, 7.1% N-acetylglucosamine, and 8.0% N-acetylneuraminic acid. No N-acetylgalactosamine was detected. When the SGP-2 was digested with proteases, the intact oligosaccharides were chromatographed over a Bio-Gel P-6 column and found to elute in a single symmetrical peak of approximately 3,300 g/mol. On the basis of these results, the oligosaccharides on SGP-2 were proposed to consist of triantennary chains similar to those found on fetuin. When the 35SO4(2-)-labeled SGP-2 was digested with Pronase, the free amino acids could be separated by chromatography from the oligosaccharide. The 35SO4(2-) was shown to be associated with the oligosaccharide portion of SGP-2.     

Molecular cloning of gp 80, a glycoprotein complex secreted by kidney cells in vitro and in vivo. A link to the reproductive system and to the complement cascade

cDNA clones coding for the gp 80 heterodimeric glycoprotein complex secreted constitutively at the apical surface of Madin-Darby canine kidney (MDCK) cells have been isolated from MDCK cDNA libraries in lambda gt11 and lambda gt10. The cloned sequences encode a polypeptide chain of 445 amino acids. The deduced amino acid sequence of the gp 80 protein reveals 80% homology to rat SGP-2, a major secretory protein of the testes epithelium and 83% homology to SP-40,40, a human complement-associated protein. SGP-2 and SP-40,40 have been proposed to be serum and seminal forms of the same protein. The sequence homology as well as the results of Southern and Northern blot analyses and immunological studies suggest that gp 80 is the canine homolog of the rat SGP-2 and the human SP-40,40. The protein is expressed in the embryonic kidney already early during organogenesis. In the adult kidney the protein has been localized along the luminal surfaces of the proximal and distal tubule and the collecting duct cells.     

Hormonal regulation of sulfated glycoprotein-1 synthesis by nonciliated cells of the efferent ducts of adult rats

The objective of this study was to define the factors regulating the endogenous production of sulfated glycoprotein-1 (SGP-1) in nonciliated cells of the efferent ducts. To this end we examined five different groups of animals undergoing the following experimental procedures: (1) hypophysectomized animals at 7, 14, and 28 days, (2) 7-day hypophysectomized rats receiving testosterone implants given at various time intervals thereafter, (3) castration at various time intervals up to 7 days, (4) 7-day castrated rats receiving testosterone implants at various time intervals thereafter, and (5) castrated rats given testosterone implants immediately after castration and sacrificed at different time intervals thereafter. Efferent ducts were fixed by perfusion with 4% paraformaldehyde and 0.5% glutaraldehyde in phosphate buffer for quantitative immunocytochemical analysis at the level of the electron microscope. For each experimental condition and their controls, the number of gold particles/μm² within the endosomal and lysosomal compartments was calculated taking into account the changes in both the volume of the cell and organelles being quantified and expressed as labeling content. The results revealed that hypophysectomy (up to 4 weeks) caused a marked significant decrease in the SGP-1 labeling content of the endosomal and lysosomal compartments. The labeling content of the lysosomal compartment of efferent ducts from rats castrated for up to 1 week did not change significantly. However, there was a significant decrease in the labeling content of endosomes. This decrease is due to SGP-1, which is secreted by Sertoli cells, not being available for uptake in the efferent aucts. These results suggested that testosterone is not required for maintaining the high labeling content of SGP-1 within lysosomes of nonciliated cells, but that a pituitary factor appears to be needed. The administration of testosterone at different intervals to 7-day castrated animals resulted in a significant decrease of lysosomal SGP-1, suggesting that testosterone under these experimental conditions inhibits the production of a pituitary factor that maintains the high labeling content of SGP-1 within lysosomes of the nonciliated cells. Testosterone administered to 7-day hypophysectomized animals over a 24-hr period had no effect on the labeling content of SGP-1 within lysosomes. However, the administration of testosterone to animals immediately following castration showed no differences in the labeling content of SGP-1 within compared to controls. Together these results suggest that the labeling content of SGP-1 within lysosomes of nonciliated cells of the efferent ducts is not dependent on luminal or circulating androgens, nor is it dependent on a testicular factor entering the lumen of the ducts. It does appear, however, that SGP-1 synthesis and targeting to secondary lysosomes is dependent on a pituitary factor that may have a direct or an indirect effect on the nonciliated cells. © 1995 Wiley-Liss, Inc.     

Saccharomyces cerevisiae Npc2p is a functionally conserved homologue of the human Niemann-Pick disease type C 2 protein, hNPC2

Niemann-Pick Disease Type C (NP-C) is a fatal neurodegenerative disease, which is biochemically distinguished by the lysosomal accumulation of exogenously derived cholesterol. Mutation of either the hNPC1 or hNPC2 gene is causative for NP-C. We report the identification of the yeast homologue of human NPC2, Saccharomyces cerevisiae Npc2p. We demonstrate that scNpc2p is evolutionarily related to the mammalian NPC2 family of proteins. We also show, through colocalization, subcellular fractionation, and secretion analyses, that yeast Npc2p is treated similarly to human NPC2 when expressed in mammalian cells. Importantly, we show that yeast Npc2p can efficiently revert the unesterified cholesterol and GM1 accumulation seen in hNPC2-/- patient fibroblasts demonstrating that it is a functional homologue of human NPC2. The present study reveals that the fundamental process of NPC2-mediated lipid transport has been maintained throughout evolution.     

The human homologue of Caenorhabditis elegans CED-6 specifically promotes phagocytosis of apoptotic cells

A key feature of the process of programmed cell death (apoptosis) is the efficiency with which the dying cells are recognized and engulfed by phagocytes [1]. Apoptotic cells are rapidly cleared either by neighbouring cells acting as semi-professional phagocytes or by experts of the macrophage line, so that an inflammatory response is avoided [2]. The Caenorhabditis elegans gene ced-6 is required for efficient engulfment of apoptotic cells [3] and is one of a group of genes that define two partially redundant parallel pathways for the engulfment process [4] [5]. These pathways may be conserved across evolution, as two other engulfment genes have human homologues. A CED-5 homologue is part of a human CrkII-DOCK180-Rac signaling pathway proposed to mediate cytoskeletal reorganization [6] [7] [8] and a CED-7 homologue is similar to the ABC transporters [9] [10]. Here, we report the cloning and characterization of human CED-6, a human homologue of C. elegans CED-6. The 34 kDa hCED-6 protein is expressed in most tissues, some human cancer cells, and in primary human macrophages. We developed an assay that quantitates the phagocytic activity of mammalian macrophages: the number of apoptotic cells that have been internalized is measured by the uptake of lacZ-positive apoptotic cells by adherent transgenic macrophages. The results of this assay demonstrate that overexpression of hCED-6 promotes phagocytosis only of apoptotic cells and suggest that hCED-6 is the mammalian orthologue of C. elegans CED-6 and is a part of a highly conserved pathway that specifically mediates the phagocytosis of apoptotic cells.     

The SGP-2 gene is developmentally regulated in the mouse kidney and abnormally expressed in collecting duct cysts in polycystic kidney disease.

Sulfated glycoprotein-2 (SGP-2) is a secreted, dimeric, glycosylated protein synthesized by a number of different epithelial cell types. Although its function is not yet understood, SGP-2 has been hypothesized to be involved in such diverse processes as the promotion of cell-cell interactions, spermatogenesis, modulation of the complement system, and programmed cell death. We have now found that the SGP-2 gene is developmentally regulated in the mouse kidney. SGP-2 gene expression is first detected in the condensing nephrogenic mesenchyme and is subsequently down-regulated during the maturation of the glomerular epithelia, proximal tubules, and collecting ducts. SGP-2 continues to be expressed in the mature kidney in distal tubules and in the urothelial lining of the calyx and papilla. We have also examined the expression of the SGP-2 gene in polycystic kidneys of the C57BL/6J-cpk mouse, a model of autosomal recessive polycystic kidney disease in which there is development of epithelial-lined cysts arising primarily from the collecting duct system. Abnormally high levels of SGP-2 mRNA were found in the cyst wall epithelium of polycystic kidneys. The expression of the SGP-2 gene in normal development suggests that it plays a role in differentiating epithelial structures; and the abnormally high levels of SGP-2 gene expression in polycystic kidneys suggests that the cells lining cysts are not fully differentiated. It is possible, therefore, that polycystic kidney disease is caused by a defective developmental process in which there is a delay in terminal differentiation.     

Induction of the TRPM-2 gene in cells undergoing programmed death.

RNA and protein products encoded by the testosterone-repressed prostate message-2 gene (TRPM-2) are induced to high levels, coordinate with the onset of cell death, in numerous rodent models of inducible tissue damage. These models include cell death initiated by hormonal stimuli (prostate regression), pressure insult (renal atrophy after ureteral obstruction), developmental stimuli (necrosis of interdigital tissue), and cytotoxic injury (chemotherapeutic regression of a tumor). Sequence analysis of cDNA encoding TRPM-2 revealed its close homology with a product referred to as SGP-2 or clusterin expressed constitutively by Sertoli cells; however, the immunologically related polypeptides expressed in regressing tissues differ in molecular mass from the forms secreted by the testis. Although the function(s) of the products encoded by the TRPM-2 gene remains unclear, their presence provides a remarkable and early indicator of programmed cell death in many types of mammalian cells.     

Differential expression of the TFF-peptides xP1 and xP4 in the gastrointestinal tract of Xenopus laevis

TFF-peptides (formerly P-domain peptides, trefoil factors) represent major secretory products of the mammalian gastrointestinal tract. A molecular cloning approach revealed the existence of two TFF-peptides, xP1 and xP4, also in the stomach of Xenopus laevis. Here, the localization of these two peptides by Western blot analysis as well as immunohistochemistry is presented. xP1 is found predominantly in the surface mucous cells of the stomach, whereas xP4 is mainly localized to a specific population of goblet cells in the esophagus, to mucous neck cells of the stomach, and to closely resembling cells in antral glands. xP4 in the esophagus and in the stomach differ by their N-glycosylation patterns. Compared to mammalian TFF-peptides, xP1 obviously represents the frog homologue of human TFF1 (formerly pS2) and xP4 seems to be the amphibian equivalent of human TFF2 (formerly hSP).     

Cell cycle arrest and apoptosis by expression of a novel TPIP (TPIP-C2) cDNA encoding a C2-domain in HEK-293 cells

The human TPIP (TPTE and PTEN homologous Inositol lipid Phosphatase) belongs to the PTEN (Phosphatase and TENsin homologue deleted on chromosome 10) family of dual-specific phosphatases and is expressed from the human chromosome 13 as multiple splice-variants, e.g., TPIPα, β, γ mRNAs. PTEN is a well characterized tumor suppressor, which controls survival, adhesion, motility and migration of mammalian cells, its C2-domain plays crucial role in controlling these functions. However, role of isolated C2-domain protein in regulation of cell proliferation and apoptosis is not reported. We report sequence analysis and function of a novel human TPIP (TPIP-C2) cDNA encoding a 193 amino acid C2-domain in cell proliferation and apoptosis regulation. In silico analysis and homology modelling revealed that the C2-domain of TPIP-C2 is similar to that of PTEN but with short disorder sequences overlapping or adjacent to the post-translational modification sites. Overexpression of TPIP-C2 cDNA in human embryonic kidney (HEK-293) cells caused cell cycle arrest, inhibition of cell proliferation and induced apoptosis in an activated caspase 3 and PARP-dependent manner in comparison to overexpression of the full length human PTEN cDNA. TPIP-C2 overexpressed cells also showed S-phase cell cycle arrest. We suggest that C2-domain of TPIP-C2 may act as a dominant negative effector, which may bind to and arrest the cell proliferation signalling complex and isolated TPIP-C2-domain-like proteins expressed in mammalian cells/tissues may play important role in regulation of cell proliferation and apoptosis. The TPIP-C2 cDNA may be exploited for inducing cell cycle-inhibition and apoptosis in human cancer cells and tissues.