Publications

Hereditary elliptocytosis (HE) is a clinically and biochemically heterogenous group of diseases characterized by elliptically shaped erythrocytes and an autosomal dominant mode of inheritance. Whereas the self-association of spectrin heterodimers to tetramers is defective in a subpopulation of HE patients, designated HE[SpD-SpD], it is normal in others. We have examined the peptide pattern produced by limited tryptic digestion of spectrin extracts from patients with HE[SpD-SpD] to determine if the functional defects in spectrin self-association could be correlated with structural changes in the spectrin molecule. Although the peptide pattern produced by limited tryptic digestion of spectrin extracts from those HE patients with normal spectrin self-association was indistinguishable from the pattern from control normal volunteers, digestion of the spectrin extracts from the HE[SpD-SpD] patients showed a reproducible diminution in the 80,000-D domain of the alpha-subunit, which is involved in spectrin dimer self-association. The decrease in the 80,000-D fragment was associated with an increase in a 74,000-D fragment in eight of nine families, or, in one family, with an increase of fragments at 46,000 and 17,000 D. These atypical peptide patterns were similar to those previously reported in two variants of hereditary pyropoikilocytosis (HPP), which also had defective self-association of spectrin. These data indicate that two distinct structural variants of spectrin alpha-subunit are associated with the defective spectrin heterodimer self-association in a subpopulation of HE patients.

Removal of calcium from thrombospondin with EDTA results in a decrease in the intensity of the negative CD peak between 200 and 250 nm. Quantitative analysis of the CD spectrum of thrombospondin indicates that thrombospondin contains approximately 11% alpha-helix, 43% beta-sheet, and 46% random coil in the presence of calcium and that a small change in secondary structure may occur upon removal of calcium with EDTA. When the change in the CD at 220 nm is measured as a function of calcium concentration, a sigmoidal curve with a transition midpoint of 120 microM is obtained, indicating that the binding is cooperative. Analysis of Hill plots of the data revealed a Hill coefficient of 12.3. Calcium was also found to affect the peptide pattern produced by limited tryptic digestion of thrombospondin, with some portions of the molecule being resistant to trypsin in the presence of calcium. When the change in quantity of a 65,000-dalton tryptic fragment was measured as a function of calcium concentration, a sigmoidal curve was again obtained. The midpoint of this transition is achieved at a free calcium concentration of 45 microM at 0 degrees C and 103 microM at 25 degrees C. These data indicate that thrombospondin contains at least 12 binding sites for calcium and that cooperative interactions between sites are associated with a conformational change in the thrombospondin molecule.

To investigate serum requirements for optimal erythropoiesis in vitro, we studied the response of erythroid progenitor cell proliferation in culture to platelet-derived growth factor (PDGF). Human bone marrow cells cultured with platelet-poor plasma-derived serum (PDS) form fewer erythroid colonies than do cells cultured with human whole blood serum or fetal calf serum (P less than 0.05). Treatment of washed platelets with thrombin releases a low molecular weight (less than 100,000) factor that enhances colony growth. This secreted factor appears to be PDGF, based upon the ability of partially purified and electrophoretically pure PDGF to restore colony-forming capacity of PDS-containing cultures to 70-96% of the level found in control cultures with whole blood serum or fetal calf serum. Enhancement of colony growth by PDGF was noted only in marrow cultures supplemented with erythropoietin and PDS. Presence of bioactive erythropoietin in PDGF preparations was excluded by assay in hypertransfused, polycythemic mice, and in fasted rats. Although PDGF stimulates erythroid burst formation in marrow cultures containing optimal concentrations of burst-promoting activity (BPA), it does not influence proliferation of circulating erythroid bursts, regardless of BPA concentration added to culture. We conclude that PDGF is a serum determinant of optimal erythroid progenitor cell proliferation in marrow culture. The activity of PDGF is distinct from that of the apparent erythroid specific growth factors erythropoietin and BPA.

In hereditary pyropoikilocytosis (HPP) the erythrocyte membrane skeleton exhibits mechanical instability that can be correlated to defective self-association of spectrin heterodimers. To detect structural changes in the functional domains of HPP spectrin we have examined the peptide pattern produced by limited tryptic digestion of spectrin extracts from two families that contain three HPP patients. Limited tryptic digestion of all three HPP patients revealed a similar and reproducible decrease in the staining intensity of an 80,000-, and 22,000-, and an 88,000-dalton polypeptide with a concomitant increase in a 74,000- and a 90,000-dalton polypeptide as compared with controls. Only changes in the 80,000-, and 74,000-, and 22,000-dalton polypeptides could be correlated to defective spectrin self-association and the amount of spectrin dimers in 0 degrees C extracts of the HPP patients and their affected kindred. Similar results were obtained when the tryptic digests were analyzed by two-dimensional isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis with the affected 74,000- and 80,000-dalton polypeptides focusing into multiple spots ranging in isoelectric point from 5.3-5.4. When HPP spectrin dimers and tetramers were separated and subjected to trypsin digestion, changes in the 80,000-, 74,000-, and 22,000-dalton polypeptides were found predominantly in the spectrin dimer pool. Similar results were obtained for spectrin from two of the probands' mother, whom we have identified as an HPP carrier. We conclude that these HPP patients contain a population of normal, (principally tetrameric) and mutant (principally dimeric) spectrin. The latter is characterized by a defective spectrin dimer self-association due to conformational changes that affect the 80,000-dalton domain.

A hydrodynamic model of the high molecular weight platelet glycoprotein thrombospondin is formulated from physical solution measurements of molecular weight, partial specific volume, sedimentation coefficient, and intrinsic viscosity. Simultaneous sedimentation equilibrium analysis of thrombospondin in buffered saline prepared in H2O and D2O yielded values of 420,000 and 0.714 ml/g for the molecular weight and partial specific volume, respectively. A sedimentation coefficient of 8.6 S was found to be independent of the protein concentration over the range 0-6 mg/ml. The intrinsic viscosity was determined to be 40 ml/g at 20 degrees C for native thrombospondin and 52 ml/g for thrombospondin in the presence of 6 M guanidine-HCl. Based on these values thrombospondin is best described as a prolate ellipsoid with an axial ratio of 9.3. This model agrees well with the electron microscopic image of thrombospondin as a nodular rod (7 X 65 nm)-shaped molecule with an axial ratio of 10. Sedimentation equilibrium analysis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that thrombospondin is comprised of three 140,000-dalton polypeptide chains. The percentage of residues in alpha-helix was calculated to be only 3% from the circular dichroism spectrum.