Studies on the glycosylation of hydroxylysine residues during collagen biosynthesis and the subcellular localization of collagen galactosyltransferase and collagen glucosyltransferase in tendon and cartilage cells

Glycosylation of FGF/FGFR: An underrated sweet code regulating cellular signaling programs

Fibroblast growth factors (FGFs) and their receptors (FGFRs) constitute plasma-membrane localized signaling hubs that transmit signals from the extracellular environment to the cell interior, governing pivotal cellular processes like motility, metabolism, differentiation, division and death. FGF/FGFR signaling is critical for human body development and homeostasis; dysregulation of FGF/FGFR units is observed in numerous developmental diseases and in about 10% of human cancers. Glycosylation is a highly abundant posttranslational modification that is critical for physiological and pathological functions of the cell. Glycosylation is also very common within FGF/FGFR signaling hubs. Vast majority of FGFs (15 out of 22 members) are N-glycosylated and few FGFs are O-glycosylated. Glycosylation is even more abundant within FGFRs; all FGFRs are heavily N-glycosylated in numerous positions within their extracellular domains. A growing number of studies points on the multiple roles of glycosylation in fine-tuning FGF/FGFR signaling. Glycosylation modifies secretion of FGFs, determines their stability and affects interaction with FGFRs and co-receptors. Glycosylation of FGFRs determines their intracellular sorting, constitutes autoinhibitory mechanism within FGFRs and adjusts FGF and co-receptor recognition. Sugar chains attached to FGFs and FGFRs constitute also a form of code that is differentially decrypted by extracellular lectins, galectins, which transform FGF/FGFR signaling at multiple levels. This review focuses on the identified functions of glycosylation within FGFs and FGFRs and discusses their relevance for the cell physiology in health and disease.

A new matrikine-derived peptide up-regulates longevity genes for improving extracellular matrix architecture and connections of dermal cell with its matrix

OBJECTIVE

Skin extracellular matrix (ECM) is a dense and well-organized structure produced by fibroblasts. This ECM transduces environmental mechano-signals to cell nucleus through the integrin-actin complex, thus triggering ECM protein syntheses. The aim of this study was to discover a novel peptide, structurally related to dermal matrikines, that promotes syntheses of ECM components.

METHODS AND RESULTS

Screening tests with 120 peptides were carried out by using normal dermal human fibroblasts (HF). As a result, one candidate of interest was isolated, the N-Prolyl Palmitoyl Tripeptide-56 Acetate (PP56), which increases collagen and fibronectin productions at gene and/or protein levels. Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), a recent and innovative analytical technology, in addition to more traditional techniques, it was showed that two metabolic pathways were significantly modulated: one for collagen production and one for actin. Moreover, this peptide up-regulated the transcription of Forkhead Box O (FOXO) and sestrin messenger RNAs (mRNA), leading to production of proteins involved into longevity and more recently in collagen production.

RESULTS

Results indicated that this peptide is a potential candidate to improve ECM density and organization in a new way.

Collagen Accumulation in Osteosarcoma Cells lacking GLT25D1 Collagen Galactosyltransferase

Collagen is post-translationally modified by prolyl and lysyl hydroxylation and subsequently by glycosylation of hydroxylysine. Despite the widespread occurrence of the glycan structure Glc(α1-2)Gal linked to hydroxylysine in animals, the functional significance of collagen glycosylation remains elusive. To address the role of glycosylation in collagen expression, folding, and secretion, we used the CRISPR/Cas9 system to inactivate the collagen galactosyltransferase GLT25D1 and GLT25D2 genes in osteosarcoma cells. Loss of GLT25D1 led to increased expression and intracellular accumulation of collagen type I, whereas loss of GLT25D2 had no effect on collagen secretion. Inactivation of the GLT25D1 gene resulted in a compensatory induction of GLT25D2 expression. Loss of GLT25D1 decreased collagen glycosylation by up to 60% but did not alter collagen folding and thermal stability. Whereas cells harboring individually inactivated GLT25D1 and GLT25D2 genes could be recovered and maintained in culture, cell clones with simultaneously inactive GLT25D1 and GLT25D2 genes could be not grown and studied, suggesting that a complete loss of collagen glycosylation impairs osteosarcoma cell proliferation and viability.

The Role of Collagen Organization on the Properties of Bone

Bone is a complex tissue constituted by a collagen matrix filled in with crystal of hydroxyapatite (HAP). Bone mechanical properties are influenced by the collagen matrix which is organized into hierarchical structures from the individual type I collagen heterotrimer flanked by linear telopeptides at each end to the collagen fibrils that are interconnected by enzymatic and non-enzymatic cross-links. Although most studies focused on the role of collagen cross-links in bone strength, other organizational features may also play a role. At the molecular level it has been shown that homotrimer of type I collagen found in bone tissue of some patients with osteogenesis imperfecta (OI) is characterized by decreased mechanical competence compared to the regular heterotrimer. The state of C-telopeptide isomerization-which can be estimated by the measurement in body fluids of the native and isomerized isoforms-has also been shown to be associated with bone strength, particularly the post-yield properties independent of bone size and bone mineral density. Other higher hierarchical features of collagen organization have shown to be associated with changes in bone mechanical behavior in ex vivo models and may also be relevant to explain bone fragility in diseases characterized by collagen abnormalities e.g., OI and Paget's disease. These include the orientation of collagen fibrils in a regular longitudinal direction, the D-spacing period between collagen fibrils and the collagen-HAP interfacial bonding. Preliminary data indicate that some of these organizational features can change during treatment with bisphosphonate, raloxifene, and PTH suggesting that they may contribute to their anti-fracture efficacy. It remains however to be determined which of these parameters play a specific and independent role in bone matrix properties, what is the magnitude of mechanical strength explained by collagen organization, whether they are relevant to explain osteoporosis-induced bone fragility, and how they could be monitored non-invasively to develop efficient bone quality biomarkers.

The role of collagen in bone strength

Bone is a complex tissue of which the principal function is to resist mechanical forces and fractures. Bone strength depends not only on the quantity of bone tissue but also on the quality, which is characterized by the geometry and the shape of bones, the microarchitecture of the trabecular bones, the turnover, the mineral, and the collagen. Different determinants of bone quality are interrelated, especially the mineral and collagen, and analysis of their specific roles in bone strength is difficult. This review describes the interactions of type I collagen with the mineral and the contribution of the orientations of the collagen fibers when the bone is submitted to mechanical forces. Different processes of maturation of collagen occur in bone, which can result either from enzymatic or nonenzymatic processes. The enzymatic process involves activation of lysyl oxidase, which leads to the formation of immature and mature crosslinks that stabilize the collagen fibrils. Two type of nonenzymatic process are described in type I collagen: the formation of advanced glycation end products due to the accumulation of reducible sugars in bone tissue, and the process of racemization and isomerization in the telopeptide of the collagen. These modifications of collagen are age-related and may impair the mechanical properties of bone. To illustrate the role of the crosslinking process of collagen in bone strength, clinical disorders associated with bone collagen abnormalities and bone fragility, such as osteogenesis imperfecta and osteoporosis, are described.

Procollagen trafficking, processing and fibrillogenesis

Collagen fibrils in the extracellular matrix allow connective tissues such as tendon, skin and bone to withstand tensile forces. The fibrils are indeterminate in length, insoluble and form elaborate three-dimensional arrays that extend over numerous cell lengths. Studies of the molecular basis of collagen fibrillogenesis have provided insight into the trafficking of procollagen (the precursor of collagen) through the cellular secretory pathway, the conversion of procollagen to collagen by the procollagen metalloproteinases, and the directional deposition of fibrils involving the plasma membrane and late secretory pathway. Fibril-associated molecules are targeted to the surface of collagen fibrils, and these molecules play an important role in regulating the diameter and interactions between the fibrils.

Impaired secretion of rat mannose-binding protein resulting from mutations in the collagen-like domain

Serum mannose-binding protein (MBP) or mannose-binding lectin initiates the lectin branch of the innate immune response by binding to the surface of potentially pathogenic microorganisms and initiating complement fixation through an N-terminal collagen-like domain. Mutations in this region of human MBP are associated with immunodeficiency resulting from a reduction in the ability of the mutant MBPs to fix complement as well as from reduced serum concentrations. Inefficient secretion of the mutant proteins, which is one possible cause of the reduced serum levels, has been investigated using a mammalian expression system in which each of the naturally occurring human mutations has been recreated in rat serum MBP. The mutations Gly25-->Asp and Gly28-->Glu disrupt the disulfide-bonding arrangement of the protein and cause at least a 5-fold increase in the half-time of secretion of MBP compared with wild-type rat serum MBP. A similar phenotype, including a 3-fold increase in the half-time of secretion, disruption of the disulfide bonding arrangement, and inefficient complement fixation, is observed when nearby glucosylgalactosyl hydroxylysine residues at positions 27 and 30 are replaced with arginine residues. The results suggest that defective secretion resulting from structural changes in the collagen-like domain is likely to be a contributory factor for MBP immunodeficiency.

Biochemical alterations of the tunica albuginea in impotence

PURPOSE

The objectives of this study were to quantify the immunohistochemical stainings of collagen types I, III and IV, and investigate the value of glycohistochemical staining with 3 lectin types specific to a particular glycan structure, Arachis hypogaea, Triticum vulgare and concanavalin A, as a method of defining possible changes in the collagen structure of the tunica albuginea in potent and impotent patients.

MATERIALS AND METHODS

The study involved 4 normal men, 4 with pure venous leakage and 4 with pure arterial disease. Collagen types I, III and IV, and lectins Arachis hypogaea, Triticum vulgare and concanavalin A were studied using a cell image processor. The labeling index relates to the percentage of staining and mean optical density relates to the staining intensity.

RESULTS

Mean labeling index values for the 3 types of collagen and lectins were similar (p > 0.05). Mean optical density value relating to collagen type I was significantly higher in the arteriogenic group than in the other groups (p < 0.05), while mean optical density value of collagen type IV was significantly higher in the venogenic group than in the 2 other groups (p < 0.05). Mean optical density values relating to the 3 lectin types were similar in the 3 clinical groups (p > 0.05).

CONCLUSIONS

An alteration in the distribution and structure of the various collagen types and lectins in the tunica albuginea of impotent patients has been shown that may interfere with normal function and lead to impotence.

Studies on the mechanism of different collagen glucosyltransferase reactions (Golgi apparatus, smooth endoplasmic reticulum, rough endoplasmic reticulum) in chick embryo liver

1. The galactosylhydroxylysylglucosyltransferase (GGT) specific to collagen is located in the RER (rough endoplasmic reticulum), SER (smooth endoplasmic reticulum) and Golgi apparatus for the chick embryo liver. 2. The UDP-glucose collagen glucosyltransferase activities in chick embryo liver were solubilized by Nonidet P-40. 3. The mechanism of collagen glucosyltransferase reaction was studied with enzyme preparation of Golgi apparatus CF2, smooth endoplasmic reticulum CF4 and rough endoplasmic reticulum CF8. 4. For the three fractions, data obtained in experiments were consistent with a sequential ordered mechanism in which the substrates are bound to the enzyme in the following order: Mn2+, collagen and UDP-glucose substrate, with different values for Km and Vmax.

Cell-wall synthesis in Chlamydomonas reinhardtii: an immunological study on the wild type and wall-less mutants cw2 and cw15

Cells of Chlamydomonas reinhardtii Dang. wild type and the cell-wall mutants cw2 and cw15 were grown synchronously. The two mutants secreted copious amounts of cell-wall-like glycoproteins into the culture medium in contrast to the wild type which released only minor quantities. Both the secreted proteins as well as those present in the lumen of the endoplasmic reticulum (ER) and Golgi apparatus (GA) were tested for crossreactivity against a number of monoclonal antibodies (MACs) prepared against the 2BII glycoprotein cell-wall complex of the wild type (E. Smith et al., 1984, Planta 161, 330-338). Of the four monoclonals applied, one, (MAC 6), did not react in either dot blot or Western blots with any of the luminal and medium proteins. By dot blotting, MAC 2 recognized polypeptides only in the wild-type medium. Neither MAC 2 nor MAC 6 were capable of recognizing polypeptides separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, MAC 2 recognized one of the 2BII polypeptides (135 kDa) as well as a large number of other polypeptides in wild-type and mutant media. The 135-kDa polypeptide was also detected in the luminal extracts of ER and GA membranes from the wild type and cw2 mutant. It was also present in the GA fraction of the cw15 mutant. If, as previously claimed, these monoclonal antibodies are indeed directed against the carbohydrate portion of the 2BII complex, our results would indicate that protein O-glycosylation is not restricted to the GA but may start in the ER. They also confirm inferences made by others that the cell-wall mutants cw2 and cw15 possess the capacity to synthesize and secrete the major glycoproteins of the cell wall, but, due to the lack of the W2 wall layer, are unable to assemble these components into a coherent, crystalline wall.

Evidence of partial localization in Golgi apparatus of UDP-glucose collagen glucosyltransferase from chick embryo liver

1. Collagens are the most important components of the connective tissue. 2. Collagen synthesis involves greater than 12 different enzymes whereas three enzymatic systems are involved in the ordered degradation. 3. Some enzymes are found in the rough endoplasmic reticulum (RER). The subcellular localization of disulfur isomerase, alpha D-glucosidase, proteases, galactosyltransferases and glucosyltransferases specific to collagen is unknown. 4. After having determined the best subcellular fractionation conditions for the chick embryo liver, we demonstrate that the galactosylhydroxylysyl glucosyltransferase specific to collagen is located in the RER and in the Golgi apparatus.

Protein secretion in plants

Protein secretion is an ubiquitous but poorly understood process in plants. Secreted proteins are synthesized on the membranes of the rough endoplasmic reticulum and transported to the cell surface by secretary vesicles formed at the Golgi apparatus. Whereas many of the structural details of this process are known the mechanisms underlying secretion are just beginning to be understood, in this article we review some of the recent developments in this field, and we compare the progress made with animal and plant cells. CONTENTS Summary 567 I. Introduction 568 II. Proteins secreted by plants 568 III. Synthesis and post-translational modification of secreted proteins 571 IV. Molecular requirements for secretion 576 V. Vehicles of secretory transport 581 VI. Regulation of secretion 585 VII. Conclusions and Perspective 587 Acknowledgements 588 References 588.

Increased prolylhydroxylase activity in chondrocytes but not synovial cells from adjuvant arthritic rat knees

Prolylhydroxylase and galactosylhydroxylsyl glucosyl (GGH) transferase were assayed in cells cultured from cartilage and synovium (adipose type) removed from normal and adjuvant arthritic rat knees. There was a significant increase in prolylhydroxylase in chondrocytes from arthritic knees compared to the normals but no change in GGH transferase activity was found. In the cells derived from the synovium no variation in either enzyme was observed. No significant changes in DNA or protein content was found in either type of cells from the arthritic joints. Comparison of the prolylhydroxylase activity in the supernatant and pellet fractions prepared from chondrocytes from normal and arthritic joints, demonstrated that the majority of the activity was recovered in the supernatant fraction rather than the pellet in arthritic chondrocytes. The data presented indicate that the degree of hydroxylation of cartilage collagen alters in arthritis.

A collagen:glucosyltransferase at the surface of malignant fibroblasts

3T12 fibroblasts possess glucosyltransferases that catalyze the transfer of glucose from UDP-Glucose to galactosylhydroxylysyl residues on collagenous acceptors. The presence of the enzyme activity at the cell surface is indicated by the following findings: a) suspensions of intact cells, as well as intact cell monolayers, glucosylate gelatinized collagen b) glucose transfer is not due to UDP-Glucose hydrolysis and subsequent intracellular utilization of the free glucose c) experiments using cell suspensions with known proportions of broken cells indicate that the glucosyltransferase activity is attributable to intact cells and not to contamination by intracellular enzymes from broken cells. The Km value for UDP-Glucose is about 20 microM. The enzyme has a pronounced requirement for manganese, and shows highest activity between 2 and 10 mM. The optimal Mn2+ concentration for the intracellular gelatin:glucosyltransferase activity is more restricted (5 to 10 mM). Glucosyltransferase activity is strongly inhibited by diamide and N-ethylmaleimide (5 mM), suggesting that intact sulfhydryl residues present in the enzyme are essential.

Processing of an anglerfish somatostatin precursor to a hydroxylysine-containing somatostatin 28

A novel 28-residue somatostatin (SS) has been isolated from anglerfish pancreatic islets and characterized by complete Edman degradation, peptide mapping, and amino acid analysis. The primary structure of this anglerfish SS-28 (aSS-28) containing hydroxylysine (Hyl) was established to be H-Ser-Val-Asp-Ser-Thr-Asn-Asn-Leu-Pro-Pro-Arg-Glu-Arg-Lys-Ala-Gly-Cys- Lys-Asn-Phe-Tyr-Trp-Hyl-Gly-Phe-Thr-Ser-Cys-OH. This sequence (with the exception of hydroxylysine-23, which is replaced by lysine) is identical to the sequence of the COOH-terminal 28 residues of prepro-SS II predicted on the basis of cDNA analysis [Hobart, P., Crawford, R., Shen, L., Pictet, R. & Rutter, W. J. (1980) Nature (London) 288, 137-141]. This is the first instance in which hydroxylysine (to date characteristically observed in collagen or collagen-like structures) has been found in a potential regulatory peptide. Chromatographic characterization of peptides, radiolabeled in islet culture, revealed that aSS-28 contained 10-12% of the radioactivity incorporated into the 8000- to 1000-dalton SS-like polypeptides, whereas 88-90% of this radioactivity was detected in anglerfish SS-14. It appears probable that aSS-28 represents the predominant primary cleavage product derived from prepro-SS II by cleavage at the COOH-terminal side of a single arginine. Based on knowledge of the collagen biosynthesis, it is speculated that hydroxylation may take place as an early post-translational event.

Biosynthesis of basement membrane by parietal yolk sac cells

The biosynthesis of basement membrane (Reichert's membrane) by parietal yolk sac cells of the mouse embryo was studied using immunohistochemistry and autoradiography with the electron microscope. Fab' fragments of characterized antisera to either a mixture of type IV collagen and laminin, or to type IV collagen alone, were conjugated to horseradish peroxidase for the immunohistochemical study; radiolabeled proline, leucine, and glucosamine were utilized either in vivo or in isolated yolk sac membranes in vitro for autoradiography. Whereas neither of the two antisera employed localized in either the Golgi apparatus or in the cytosol, both localized in the rough endoplasmic reticulum (RER) and in the extracellular basement membrane. Furthermore, no evidence of passage of isotopically labeled precursors of basement membrane from the RER to the Golgi apparatus was observed. Consequently, it is concluded that the basement membrane precursors are synthesized in the RER of parietal yolk sac cells and neither traverse the Golgi apparatus nor diffuse across the cytosol of the cell in the process of secretion. By elimination, secretion must be via direct communications between RER and plasma membrane or via small vesicles observed in the cytosol between RER and plasma membrane. These data are not in accord with the concept of the universality of the Golgi apparatus in the biosynthesis of glycoprotein. Parietal yolk sac cells do not use the Golgi apparatus in the biosynthesis of basement membrane, but do use the Golgi apparatus for the biosynthesis of other proteins.

The subcellular fractionation of embryonic chick tendon and cartilage cells: a re-examination

A re-examination of the subcellular fractions obtained from matrix-free chick tendon and cartilage cells has been made since the discovery that three out of four of the micrographs of chick tendon microsomal fractions published in an earlier paper from this laboratory were not authentic. The present studies demonstrate that by using the procedures previously reported it is possible to isolate microsomal and submicrosomal fractions from tendon and cartilage cells which exhibit typical morphology when examined by electron microscopy. These observations are consistent with our original biochemical characterization of subcellular fractions, which we know to be valid. Other publications from this laboratory in which these fractionation procedures have been applied to studies of collagen biosynthesis are in no way compromised, and indeed, most of our data have been confirmed by several other laboratories.

Studies on the collagen glucosyltransferase activity present in platelets and plasma

1. Collagen glucosyltransferase was demonstrated to be associated with pig platelets by using a specific assay for the synthesis of [(14)C]glucosylgalactosylhydroxylysine. 2. This enzyme from pig platelets required denatured collagen as substrate and the reaction was not inhibited by the presence of triple-helical collagen. These observations indicate that the platelet enzyme cannot form either an enzyme-substrate complex or an enzyme-inhibitor complex with triple-helical collagen. 3. Platelets were fractionated by sucrose-density-gradient centrifugation after either lysis by a glycerol-loading technique or homogenization. Assays of subcellular fractions for collagen glucosyltransferase activity indicated that the enzyme was localized predominantly in the cytosolic fraction and less than 5% of the activity was associated with the membrane fractions. 4. Enzyme assays were carried out on platelet-rich plasma and platelet-poor plasma prepared from pig and human blood. These analyses indicated that most of the collagen glucosyltransferase activity of platelet-rich plasma was in a soluble form and only about 10% was associated with platelets. 5. Comparative studies on the enzyme activity in plasma and platelets of various animal species revealed marked variation, with the guinea pig exhibiting the highest activity. In most cases there was a correlation between the activity found in platelets and plasma, but little species variation was noted in enzyme amounts detected in bone-marrow preparations. 6. The results described here are discussed in the context of the proposal that collagen glucosyltransferase might play a role in mediating collagen-platelet adhesion.

Further characterization of collagen galactosyltransferase from chick embryos

Optimum extraction of collagen galactosyltransferase activity from chick embryos required relatively high concentrations of detergent and salt. The activity was inhibited by concanavalin A, and the enzyme had a high affinity for columns of this lectin coupled to agarose; these results suggest the presence of carbohydrate units in the enzyme molecule. Collagen galactosyltransferase was highly labile, and only 1% of the originally bound enzyme activity could be eluted from the concanavalin A-agarose column with a buffer containing methyl glucoside and ethylene glycol. The purification of the activity over the original supernatant of chick embryo homogenate was 250-300-fold, with the optimum reaction conditions for the purified transferase differing somewhat from those for crude enzyme preparations. The reaction was inhibited by glucose-free basement-membrane collagen, UDP and galactosylhydroxylsine, and also by Co2+ and a number of compounds resembling UDP-galactose. Hydroxylysine was also a weak inhibitor. Immobilized hydroxylysine and UDP-glucuronic acid did not bind the collagen galactosyltransferase, but the enzyme was retarded in a column of UDP-galacturonic acid linked to agarose.

Further studies on the effect of the collagen triple-helix formation on the hydroxylation of lysine and the glycosylations of hydroxylysine in chick-embryo tendon and cartilage cells

The hydroxylation of lysine and glycosylations of hydroxylysine were studied in isolated chick-embryo tendon and cartilage cells under conditions in which collagen triple-helix formation was either inhibited or accelerated. The former situation was obtained by incubating the tendon cells with 0.6mm-dithiothreitol, thus decreasing their proline hydroxylase activity by about 99%. After labelling with [(14)C]proline, the formation of hydroxy[(14)C]proline was found to have declined by about 95%. Since the hydroxylation of a relatively large number of proline residues is required for triple-helix formation at 37 degrees C, the pro-alpha-chains synthesized under these conditions apparently cannot form triple-helical molecules. Labelling experiments with [(14)C]lysine indicated that the degree of hydroxylation of the lysine residues in the collagen synthesized was slightly increased and the degree of the glycosylations of the hydroxylysine residues more than doubled, the largest increase being in the content of glucosylgalactosylhydroxylysine. Recovery of chick-embryo cartilage cells from temporary anoxia was used to obtain accelerated triple-helix formation. A marked decrease was found in the extent of hydroxylation of the lysine residues in the collagen synthesized under these conditions, and an even larger decrease occurred in the glycosylations of the hydroxylysine residues. The results support the previous suggestion that the triple-helix formation of the pro-alpha-chains prevents further hydroxylation of lysine residues and glycosylations of hydroxylysine residues during collagen biosynthesis.

Glycosyltransferases with endogenous acceptor activity in plasma membranes isolated from rat liver

Plasma membrane fractions from rat liver exhibited glycosyltransferase activity with endogenous membrane-associated acceptors and either UDP-galactose, UDPglucose, UDP-N-acetylglucosamine, or GDPmannose donors. Of these, incorporation into non-lipid acceptors was most active with UDP-galactose and only with UDPgalactose and UDPmannose was there incorporation into endogenous lipid acceptors. CMP-N-acetylneuraminic acid was inactive as a donor with the isolated plasma membranes. In order to demonstrate transferase activity, low concentrations of substrate sugar nucleotides and short incubation times were used as well as sulfhydryl protectants and a phosphatase inhibitor (NaF) in the reaction mixtures. The findings support the concept of surface localization of at least a galactosyl transferase in cells of rat liver.

Age-related changes in human skin collagen galactosyltransferase and collagen glucosyltransferase activities

Collagen galactosyltransferase and collagen glucosyltransferase activities were assayed in human skin specimens of about 100 mg wet weight. The assay of the glucosyltransferase activity was found to be highly specific. The assay of the galactosyltransferase activity was somewhat less specific, but there was no difference in specificity between the foetal and adult human skin samples. The activities of the two collagen glycosyltransferases in human skin extract were found to vary with age, being highest in foetal skin, and higher in the skin of young children that in that of adults. The galactosyltransferase and glucosyltransferase activities in foetal skin were respectively about 4 times and 6 times those in adult skin. The magnitudes of the changes with age in the two collagen glycosyltransferase activities were smaller than those occurring in the activities of the two other intracellular enzymes of collagen biosynthesis namely prolyl and lysyl hydroxylase. This difference suggests that the four intracellular enzyme activities of collagen biosynthesis are not regulated in an identical manner.

Effect of L-azetidine-2-carboxylic acid on glycosylations of collagen in chick-embryo tendon cells

The glycosylations of hydroxylysine during collagen biosynthesis in isolated chick-embryo tendon cells were studied by using pulse-chase labelling experiments with [14C]-lysine. The hydroxylation of lysine and the glycosylations of hydroxylysine continued after a 5 min pulse label for up to about 10 min during the chase period. These data differ from those obtained previously in isolated chick-embryo cartilage cells, in which, after a similar 5 min pulse label, these reactions continued during the chase period for up to about 20 min. The collagen synthesized by the isolated chick-embryo tendon cells differed markedly from the type I collagen of adult tissues in its degree of hydroxylation of lysine residues and glycosylations of hydroxylysine residues. When the isolated tendon cells were incubated in the presence of L-azetidine-2-carboxylic acid, the degree of glycosylations of hydroxylysine during the first 10 min of the chase period was identical with that in cells incubated without thcarboxylic acid for at least 60 min, whereas no additional glycosylations took place in the control cells after the 10 min time-point. As a consequence, the collagen synthesized in the presence of this compound contained more carbohydrate than did the collagen synthesized by the control cells. Additional experiments indicated that azetidine-2-carboxylic acid did not increase the collagen glycosyltransferase activities in the tendon cells or the rate of glycosylation reactions when added directly to the enzyme incubation mixture. Control experiments with colchicine indicated that the delay in the rate of collagen secretion, which was observed in the presence of azetidine-2-carboxylic acid, did not in itself affect the degree of glycosylations of collagen. The results thus suggest that the increased glycosylations were due to inhibition of the collagen triple-helix formation, which is known to occur in the presence of azetidine-2-carboxylic acid.

Stimulation of collagen galactosyltransferase and glucosyltransferase activities by lysophosphatidylcholine

Lysophosphatidylcholine stimulated the activities of collagen galactosyl- and glucosyl-transferases in chick-embryo extract and its particulate fractions in vitro, whereas essentially no stimulation was noted in the high-speed supernatant, where the enzymes are soluble and membrane-free. The stimulatory effect of lysophosphatidylcholine was masked by 0.1% Triton X-100. In kinetic experiments lysophosphatidylcholine raised the maximum velocities with respect to the substrates and co-substrates, whereas no changes were observed in the apparant Km values. Phospholipase A preincubation of the chick-embryo extract resulted in stimulation of both transferase activities, probably gy generating lysophosphatides from endogenous phospholipids. No stimulation by lysophosphatidylcholine was found when tested with 500-fold-purified glycosyltransferase. The results suggest that collagen glycosyltransferases must be associated with the membrane structures of the cell in order to be stimulated by lysophosphatidylcholine. Lysophosphatidylcholine could have some regulatory significance in vivo, since its concentration in the cell is comparable with that which produced marked stimulation in vitro.

Intracellular enzymes of collagen biosynthesis in rat liver as a function of age and in hepatic injury induced by dimethylnitrosamine. Changes in prolyl hydroxylase, lysyl hydroxylase, collagen galactosyltransferase and collagen glucosyltransferase activities

The relationship between the changes in the four enzyme activities catalysing intracellular post-translational modifications in collagen biosynthesis were studied in rat liver as a function of age and in experimental hepatic injury induced by the administration of dimethylnitrosamine. During aging, relatively large changes were found in prolyl hydroxylase and lysyl hydroxylase activities, whereas only minor changes took place in collagen galactosyltransferase and collagen glucosyltransferase activities. In hepatic injury, the two hydroxylase activities increased earlier and to a larger extent than did the two glycosyltransferase activities, and the largest was found in lysyl hydroxylase activity. The data support previous suggestions that changes in the rate of collagen biosynthesis in the liver cannot be explained simply by a change in the number of collagen-producing cells, but regulation of the enzyme activities existed, so that the two hydroxylase activities altered considerably more than did the two collagen glycosyltransferase activities.

Hydroxylation of lysine and glycosylation of hydroxylysine during collagen biosynthesis in isolated chick-embryo cartilage cells

Hydroxylation of lysine and glycosylation of hydroxylysine during collagen biosynthesis in isolated chick-embryo cartilage cells were studied by using continuous labelling and pulse-chase labelling experiments with [14C]lysine. Control experiments with [14C]proline indicated that in continuous labelling the hydroxylation of [14C]proline became linear with time after about 4 min and the secretion of collagen after about 35 min, as reported previously. In similar experiments with [14C]lysine the hydroxylation of [14C]lysine and the glycosylations of hydroxy[14C]lysine became linear at about 4 min, suggesting that these reactions were initiated while the polypeptide chains were growing on the ribosomes. Pulse-chase labelling experiments with [14C]lysine indicated that after a 5 min pulse-label the hydroxylation of [14C]lysine and the glycosylations of hydroxyl[14C]lysine continued during the chase period for about 20 min. The data suggest that these reactions are continued after the release of complete polypeptide chains into the cisternae of the endoplasmic reticulum, whereas the reactions are probably not continued after the formation of the triple helix and the movement of the molecules into the Golgi vacuoles.

The route of secretion of procollagen. The influence of alphaalpha'-bipyridyl, colchicine and antimycin A on the secretory process in embryonic-chick tendon and cartilage cells

I. Embryonic-chick tendon cells were pulse-labelled for 4 min with [14C]proline and the 14C-labelled polypeptides were chased with unlabelled proline for up to 30 min. Isolation of subcellular fractions during the chase period and their subsequent analysis for bacterial collagenase-susceptible 14C-labelled peptides demonstrated the transfer of procollagen polypeptides from rough to smooth microsomal fractions and thence to the extracellular medium. Parallel analyses of Golgi-enriched fractions indicated the involvement of this organelle in the secretory pathway of procollagen. Sodium dodecylsulphate/polyacrylamide-gel electrophoresis of the 14C-labelled polypeptides present in the Golgi-enriched fractions demonstrated that the procollagen polypeptides were all present as disulphide-linked pro-gamma components. 2. When similar kinetic studies of the intracellular transport of procollagen were conducted with embryonic-chick cartilage cells almost identical results were obtained, but the rate of translocation of cartilage procollagen was significantly slower than that observed for tendon procollagen. 3. When hydroxylation of procollagen polypeptides was inhibited by alphaalpha'-bipyridyl, the nascent polypeptides accumulated in the rough microsomal fraction. 4. When cells were pulse-labelled for 4min with [14C)proline and the label was chased in the presence of colchicine, secretion of procollagen was inhibited and an intracellular accumulation of procollagen 14C-labelled polypeptides was observed in the Golgi-enriched fractions. 5. The energy-dependence of the intracellular transport of procollagen was demonstrated in experiments in which antimycin A was found to inhibit the transfer of procollagen polypeptides from rough to smooth endoplasmic reticulum. 6. It is concluded that procollagen follows the classical route of secretion taken by other extracellular proteins.