Mary Jane Spiro

Effect of Diabetes on the Biosynthesis of the Renal Glomerular Basement Membrane: Studies on the Glucosyltransferase

Kidney glucosyltransferase (UDP-glucose: galactosylhydroxylysine-basement membrane glucosyltransferase), which is involved in the synthesis of the hydroxylsine-linked disaccharide units of glomerular basement membrane, has been measured in the renal cortices of normal and alloxan diabetic rats. The level of this enzyme in diabetic kidneys was found to be significantly elevated over that of age-matched controls, expressed either as specific or total activity, at all times studied (one to five months). However the difference between the normal and diabetic animalsincreased with the duration of the disease. When short-term diabetics were treated with insulin, the level of the glucosyltransferase could be restored to normal. Insulin treatment of long-term diabetic animals, which were more difficult to control, brought the enzyme level close to normal but did not completely restore it. The glucosyltransferase activity in several other tissues of the alloxan diabetic rat, including lung, liver, testes, spleen, and uterus, did not show any significant elevation over that of the normal. Measurement in kidney of a glycosyltransferase (UDP-galactose :N-acetylglucosamine-glycoprotein galactosyltransferase) involved in the synthesis of a different type of carbohydrate unit did not show the marked elevation noted for the glucosyltransferase. It is believed that the insulin-reversible glucosyltransferase elevation of the diabetic renal cortex reflects the increased basement membrane synthesis occurring in this tissue and indicates that this process is directly or indirectly under the control of this hormone.

Effect of High Glucose on Type IV Collagen Production by Cultured Glomerular Epithelial, Endothelial, and Mesangial Cells

Immunochemical and metabolic radiolabeling procedures revealed that homogeneous cultures of calf glomerular epithelial, endothelial, and mesangial cells actively synthesize type IV collagen (primarily as α1(Iv)3) which is secreted into the medium and incorporated into the extracellular matrix. Exposure of confluent cultures of the three cell types to a high glucose concentration (30 mM) for 60 h resulted in a pronounced increase (two- to threefold) in type IV collagen production over that observed at a physiological level (5 mM) of this sugar, as determined by either immunoblotting or fluorography of electrophoretically separated media or cell-matrix components. The elevated glucose did not bring about a change in the rate of cell proliferation or fibronectin production. Moreover, studies with mannitol indicated that the stimulation of type IV collagen synthesis was not a function of hyperosmolarity. In contrast to the glomerular cells, glucose-induced enhancement of formation of this collagen was not observed in 3T3 cells despite a substantial acceleration in the consumption of this sugar. Time studies indicated that the response of the glomerular cells to high glucose occurs over an extended period (maximal at ∼78 h) and, furthermore, that the stimulatory effect on type IV collagen production is only slowly reversed after restoration of the glucose to a normal level. We believe that these findings are relevant to an understanding of the sequence of events that lead to the development of diabetic glomerular lesions.

Synthesis of Type VI Collagen by Cultured Glomerular Cells and Comparison of Its Regulation by Glucose and Other Factors With That of Type IV Collagen

Homogeneous cultures of calf glomerular mesangial and endothelial cells were found to be active in the synthesis of type VI as well as type IV collagen in contrast to the epithelial cells that were devoted primarily to the production of the latter collagen. Studies with rat mesangial cells indicated that they responded to high glucose (20 mM) in the medium by a significant (P < 0.001) increase in type VI collagen synthesis as measured by the production of the protein and its mRNA level, both of which were closely correlated to each other and to glucose consumption. Similar observations were made with type IV collagen, but the enhanced formation of this protein was not as rapidly apparent as that of type VI and, moreover, could not be as readily reversed on restoration of the glucose to a physiological level (5 mM). Evaluation of a number of other agents indicated that although mannitol had no effect, L-glucose and NaCI significantly stimulated synthesis of both type VI and IV collagens and glucose consumption. Insulin-like growth factor I and aldosterone, on the other hand, also increased glucose consumption but brought about an enhancement of only type IV collagen production, suggesting that the two collagens are independently regulated. This possibility was supported by our observation that pyruvate, which was actively taken up by the cells, selectively stimulated type IV collagen production. Although the details of the regulation of these two major glomerular extracellular matrix components have not yet been defined, it is apparent that mesangial cells in culture reflect the enhanced formation of type VI and type IV collagen, which is evident in the diabetic glomerular lesions.

Identification of a Glycoprotein from Rat Liver Mitochondrial Inner Membrane and Demonstration of Its Origin in the Endoplasmic Reticulum

Employing antisera against various subfractions of rat liver mitochondria (mitoplast, inner membrane, intermembrane, and matrix) as well as metabolically radiolabeled BRL-3A rat liver cells, we undertook a search for the presence of glycoproteins in this major cellular compartment for which little information in regard to glycoconjugates was available. Subsequent to [35S]methionine labeling of BRL-3A cells, a peptide:N-glycosidase-sensitive protein (45 kDa) was observed by SDS-polyacrylamide gel electrophoresis of the inner membrane immunoprecipitate, which was reduced to a molecular mass of 42 kDa by this enzyme. The 45-kDa protein was readily labeled with [2-3H]mannose, and indeed the radioactivity of the inner membrane immunoprecipitate was almost exclusively present in this component. Moreover, antisera directed against mitochondrial NADH-ubiquinone oxidoreductase (complex I) or F1F0-ATPase (complex V) also precipitated a 45-kDa protein from BRL-3A cell lysates as the predominant mannose-radiolabeled constituent. Endo-β-N-acetylglucosaminidase completely removed the radiolabel from this glycoprotein, and the released oligosaccharides were of the partially trimmed polymannose type (Glc1Man9GlcNAc to Man8GlcNAc). Cycloheximide as well as tunicamycin resulted in total inhibition of radiolabeling of the inner membrane glycoprotein, and moreover, pulse-chase studies employing metrizamide density gradient centrifugation demonstrated that the glycoprotein was initially present in the endoplasmic reticulum (ER) and subsequently appeared in a mitochondrial location. Early movement of the glycoprotein to the mitochondria after synthesis in the ER was also evident from the limited processing undergone by its N-linked oligosaccharides; this stood in contrast to lysosomal glycoproteins in which we noted extensive conversion to complex oligosaccharides. Our findings suggest that the 45-kDa glycoprotein migrates from ER to mitochondria by the previously observed contact sites between the two organelles. Furthermore, the presence of this glycoprotein in at least two major mitochondrial multienzyme complexes would be consistent with a role in mitochondrial translocations.

Glucose entry into rat mesangial cells is mediated by both Na+-coupled and facilitative transporters

SummarySince previous studies from our laboratory have demonstrated that increased glucose consumption by cultured rat mesangial cells is accompanied by an accelerated production of type IV and type VI collagen, we have now examined the manner by which glucose is transported into these cells. A progressive stimulation of glucose uptake by the mesangial cells was observed with increasing concentrations of NaCl so that at 145 mmol/l about twice as much glucose entered the cells as in its absence (substituted by choline chloride). Moreover, since phlorizin inhibited the NaCl-promoted uptake of glucose and this salt was found to increase the accumulation of α-methylglucoside in a manner which could not be duplicated by KCl or mannitol, both Na+-coupled and facilitative glucose transporters appeared to be present in the cells. Km values of 1.93 mmol/l and 1.36 mmol/l were determined for the co-transport and facilitated transport pathways, respectively, with their Vmax being 29.5 and 18.0 nmol·mg protein−1· h−1. Both uptake activities were found to be down-regulated by exposure of the cells to high glucose and furthermore the Na+-dependent transport could no longer be detected after about 12 passages of the cells. Hybridization of mesangial cell mRNA with cDNA probes revealed transcripts for the Na+/glucose co-transporter as well as GLUT1 and to a lesser extent GLUT4. The identification of the co-transporter in these non-polarized cells is pertinent to an understanding of the intracellular signals which can lead to the development of the diabetic glomerular lesions; in the hyperglycaemic state this carrier provides an additional route for accelerated glucose entry and furthermore by the attendant increase in Na+ flux may bring about an alteration in the ionic composition of the cell.

Molecular cloning and expression of rat liver endo-alpha-mannosidase, an N-linked oligosaccharide processing enzyme.

A clone containing the open reading frame of endo-α-d-mannosidase, an enzyme involved in earlyN-linked oligosaccharide processing, has been isolated from a rat liver λgt11 cDNA library. This was accomplished by a strategy that involved purification of the endomannosidase from rat liver Golgi by ligand affinity chromatography (Hiraizumi, S., Spohr, U., and Spiro, R. G. (1994) J. Biol. Chem. 269, 4697–4700) and preparative electrophoresis, followed by sequence determinations of tryptic peptides. Using degenerate primers based on these sequences, the polymerase chain reaction with rat liver cDNA as a template yielded a 470-base pair product suitable for library screening as well as Northern blot hybridization. EcoRI digestion of the purified λ DNA released a 5.4-kilobase fragment that was amplified in Bluescript II SK(−) vector. Sequence analysis indicated that the deduced open reading frame of the endomannosidase extended from nucleotides 89 to 1441, encoding a protein of 451 amino acids and corresponding to a molecular mass of 52 kDa. Data base searches revealed no homology with any other known protein. When a vector coding for this protein fused to an NH2-terminal peptide containing a polyhistidine region was introduced intoEscherichia coli, high levels of the enzyme were expressed upon induction with isopropyl-β-d-thiogalactoside. Purification of the endomannosidase to electrophoretic homogeneity fromE. coli lysates was accomplished by Ni2+-chelate and Glcα1→3Man-O-(CH2)8CONH-Affi-Gel ligand chromatographies. Polyclonal antibodies raised against this protein reacted with Golgi endomannosidase. By both immunoblotting and silver staining, the purified E. coli-expressed enzyme was approximately 8 kDa smaller than anticipated from the open reading frame; timed induction studies indicated that this was due to scission of the enzyme’s COOH-terminal end by host cell proteases. All rat tissues examined demonstrated mRNA levels (4.9-kilobase message) for the endomannosidase that correlated well with their enzyme activity.

Monosaccharide determination of glycoconjugates by reverse-phase high-performance liquid chromatography of their phenylthiocarbamyl derivatives☆

A method for the determination of neutral sugars and hexosamines present in glycoconjugates by reverse-phase high-performance liquid chromatography (HPLC) of their phenylthiocarbamyl (PTC) derivatives has been developed. After acid hydrolysis, neutral sugars are converted to glycamines by reaction with ammonium acetate in the presence of sodium cyanoborohydride and are subsequently derivatized with phenylisothiocyanate, while the hexosamines present in the same hydrolysate, after separation on Dowex 50, are treated directly with this reagent. HPLC of the PTC-glycamines of the neutral sugars is performed on Microsorb C18 in an isocratic manner while chromatography of the PTC-hexosamines employs a Pico-Tag column with gradient elution to achieve separation from the PTC-amino acids. The procedure has proven to be highly sensitive, requiring as little as picomole amounts for the chromatographic step; monosaccharide compositions determined on glycoproteins and glycopeptides by this method were found to compare favorably to those previously obtained by other techniques.

Uronic acid analysis by automated anion exchange chromatography.

Abstract A rapid and sensitive assay for individual uronic acids has been developed based on their separation on a 0.5 × 22-cm column of Aminex A-25 in 0.12 m Tris-acetate buffer, pH 7.4. Quantification of these sugars is accomplished by coupling the column to the analytical portion of a Technicon sugar analyzer. Each determination is complete in 3 hr, and as little as 25 nmol of uronic acid can be measured with accuracy.

Effect of high glucose on formation of extracellular matrix components by cultured rat heart endothelial cells.

SummaryIn an attempt to define the basis for the microvascular changes observed in diabetic myocardium, a study was undertaken on the effect of elevated glucose on the synthesis by rat heart endothelial cells of the extracellular matrix components, types VI, IV and I collagen, as well as fibronectin. Confluent cultures of these cells, isolated by fluorescence-activated cell sorting after treatment with rhodamine-labelled acetylated low density lipoprotein, showed a three to fivefold enhancement in the synthesis of type VI collagen after exposure for 48 h to high glucose (20 to 30 mmol/l), as determined by immunoblot analysis. Increased production of type IV collagen and fibronectin was also observed, but the change was smaller and no effect on type I collagen was found. Measurement of mRNA levels by hybridization with cDNA probes indicated that 48-h exposure to high glucose significantly increased the level of transcripts for type VI and IV collagens but not for type I collagen. While glucose consumption by endothelial cells in high glucose doubled in the initial 24-h period, utilization returned to normal by 48 h, concomitant with a reduction in GLUT1 transcript levels, suggesting that signals for stimulation of collagen synthesis must be active during the initial period of exposure to elevated glucose levels.

Myocardial glycoproteins in diabetes: Type VI collagen is a major PAS-reactive extracellular matrix protein

An investigation of myocardial glycoproteins was undertaken to elucidate the molecules responsible for the periodic acid-Schiff (PAS) reactivity of the increased extracellular matrix of diabetic cardiomyopathy. Perfusion with radiolabeled mannose indicated an enhanced formation of matrix components in the diabetic compared to the normal rat heart. Electrophoretic separation of radiolabeled extracts demonstrated the presence of glycoproteins with Mr values of 205, 142 and 90 kDa which could be separated by Bio-Gel A-5 m filtration. Fractionation of non-perfused hearts resulted in the isolation of only the 205 and 142 kDa components, which were shown by amino acid analyses and collagenase digestion to belong to the collagen family of proteins and by immunoblotting to represent type VI collagen. The carbohydrate content of these rat myocardial type VI collagen subunits, determined from monosaccharide analyses, was 11 and 12%, respectively, and N-glycanase digestion of the 142 kDa chain resulted in a decrease in size of approximately 14 kDa, indicating the presence of asparagine-linked units. Examination of normal and diabetic rat heart sections indicated that the latter contained abundant PAS-positive strands and nodules which corresponded to the distribution of anti type VI collagen reactivity. Moreover, immunoblots showed higher levels of Type VI collagen in diabetic than in normal heart extracts. Type VI collagen therefore appears to represent a major glycoprotein of myocardial extracellular matrix and to be implicated in diabetic cardiomyopathy.