Thrombospondin-1 is required for normal murine pulmonary homeostasis and its absence causes pneumonia

The thrombospondins are a family of extracellular calcium-binding proteins that modulate cellular phenotype. Thrombospondin-1 (TSP-1) reportedly regulates cellular attachment, proliferation, migration, and differentiation in vitro. To explore its function in vivo, we have disrupted the TSP-1 gene by homologous recombination in the mouse genome. Platelets from these mice are completely deficient in TSP-1 protein; however, thrombin-induced platelet aggregation is not diminished. TSP-1-deficient mice display a mild and variable lordotic curvature of the spine that is apparent from birth. These mice also display an increase in the number of circulating white blood cells, with monocytes and eosinophils having the largest percent increases. The brain, heart, kidney, spleen, stomach, intestines, aorta, and liver of TSP-1-deficient mice showed no major abnormalities. However, consistent with high levels of expression of TSP-1 in lung, we observe abnormalities in the lungs of mice that lack the protein. Although normal at birth, histopathological analysis of lungs from 4-wk-old TSP-1-deficient mice reveals extensive acute and organizing pneumonia, with neutrophils and macrophages. The macrophages stain for hemosiderin, indicating that diffuse alveolar hemorrhage is occurring. At later times, the number of neutrophils decreases and a striking increase in the number of hemosiderin-containing macrophages is observed associated with multiple-lineage epithelial hyperplasia and the deposition of collagen and elastin. A thickening and ruffling of the epithelium of the airways results from increasing cell proliferation in TSP-1-deficient mice. These results indicate that TSP-1 is involved in normal lung homeostasis.

Thrombospondin peptides inhibit the secretion-dependent phase of platelet aggregation

Thrombospondin (TSP) is a platelet alpha-granule adhesive glycoprotein (M(r) = 450,000) that is released in large amounts from activated platelets and participates in thrombus formation. The aim of the present study was to assess the effect of peptides corresponding to sequences within the NH2-terminal region and type 1 repeats of TSP on platelet aggregation induced by thrombin in washed platelet suspensions. We found that TSP18 (amino acids 1-174), used at micromolar concentrations, inhibited platelet aggregation by 30-50%, reducing the size of the aggregates formed. Similar results were obtained with the hexapeptide Cys-Ser-Val-Thr-Cys-Gly (amino acids 429-434 and 486-491) used at 1.2 mM. The shorter peptide Val-Thr-Cys-Gly was even more inhibitory whereas the peptide Val-Thr-Lys-Gly, which lacks a cysteine, had no effect. Interestingly, we have constantly found that inhibition of platelet aggregation by these peptides was accompanied by an inhibition of alpha and dense granule secretion, suggesting that the binding of secreted TSP to the plasma membrane may participate in the platelet signaling process. We conclude that peptides of TSP may prove useful in the treatment of thrombosis by impairing both the release of proaggregating substances and platelet macroaggregate formation.

Increased platelet CD36 constitutes a common marker in myeloproliferative disorders

The distribution of the major platelet membrane glycoproteins (GP), Ib, IX, IIb-IIIa and IV (or CD36), which play important roles as receptors for adhesive molecules in haemostasis and thrombosis, was studied in 34 patients with myeloproliferative disorders (MPD): 13 had essential thrombocythaemia (ET), 12 had polycythaemia vera (PV) and nine had chronic myelogenous leukaemia (CML). Only occasionally were modifications of the numbers of GPIb or GPIIb-IIIa measured using the binding of specific radiolabelled antibodies to platelets. In contrast, 2-3-fold increases of the total CD36 content and the surface CD36 expression were measured in almost all patients studied, using a radioimmunoassay and the direct binding of the radiolabelled antibody, FA6-152, to the platelet surface, respectively. These results indicate that the abnormality affected both the external and internal CD36 pools. Therefore platelet CD36 may be a useful tool for the diagnosis and the follow-up of MPD patients. Surface CD36 has been proposed as a platelet receptor for thrombospondin, an adhesive glycoprotein that is released from platelets upon activation and promotes aggregate formation. Despite a 2-fold increase of CD36 molecules, resting and thrombin-activated platelets from ET patients expressed the same amount of thrombospondin as normal platelets, suggesting that there is not a direct correlation between the CD36 expression and thrombospondin binding either spontaneously or after activation.

Quantitation of platelet glycoprotein IV (CD36) in healthy subjects and in patients with essential thrombocythemia using an immunocapture assay

Glycoprotein IV (GPIIb, CD36) is a major platelet membrane glycoprotein which is thought to participate in a number of adhesive reactions and to mediate signal transduction. In order to measure the total content of GPIV in human platelets, we have developed a simple and sensitive solid-phase radioimmunoassay based on the immunocapture of GPIV from Triton X-100-solubilized platelets. FA6-152, a monoclonal antibody to GPIV was coated on microtiter plates and bound antigen was quantified with a radiolabeled polyclonal antibody to GPIV. Using purified GPIV as a standard, the coefficients of variation of the assay were found to be less than 10% at concentrations of GPIV ranging from 0.15 to 0.75 micrograms/ml. The assay was validated by the parallelism obtained between purified GPIV dose-response curves and those obtained with platelet lysates, indicating a similar antigenic activity for GPIV in both samples. The level of GPIV in platelets from healthy donors was 0.23 +/- 0.05 (mean +/- SD, n = 15) micrograms per 100 micrograms of platelet proteins and a mean value of 27,440 +/- 6,200 (SD) molecules per platelet was calculated. The radioimmunoassay could be used to discriminate between the high level of platelet GPIV in patients with essential thrombocythemia (mean +/- SD = 81,850 +/- 27,780 molecules/platelet; n = 8) and the normal GPIV level in patients with secondary thrombocytosis (mean +/- SD = 26,810 +/- 4,030 molecules/platelet; n = 5), thereby demonstrating the clinical usefulness of the assay. The specific increase in platelet GPIV in patients with essential thrombocythemia was confirmed by immunoblot analysis whereas no increase in platelet GPIb or GPIIb-IIIa was observed by this technique.

Molecular requirements for the interaction of thrombospondin with thrombin-activated human platelets: modulation of platelet aggregation

We have investigated the molecular requirements for thrombospondin (TSP) to bind to the platelet surface and to support the subsequent secretion-dependent platelet aggregation. For this, we used two distinct murine monoclonal antibodies (MoAbs), designated MAI and MAII, raised against human platelet TSP, and three polyclonal antibodies, designated R3, R6, and R5, directed against fusion proteins containing the type 1 (Gly 385-Ile 522), type 2 (Pro 559-Ile 669), and type 3 (Asp 784-Val 932) repeating sequences, respectively. Among them, R5 and R6, but not R3, inhibited thrombin-induced aggregation of washed platelets and the concomitant secretion of serotonin. These antibodies, however, did not inhibit the expression of TSP on thrombin-activated platelets, as measured by the binding of a radiolabeled MoAb to TSP, suggesting that they may inhibit platelet aggregation by interfering with a physiologic event subsequent to TSP binding. In contrast, MoAb MAII, which reacts with an epitope located within the heparin-binding domain of TSP, inhibited both TSP surface expression and platelet aggregation/secretion induced by thrombin. In addition, this MoAb inhibited in a dose-dependent manner (IC50 approximately 0.5 mumol/L) the interaction of 125I-TSP with immobilized fibrinogen and platelet glycoprotein IV, both potential physiologic receptors for TSP on thrombin-activated platelets. These results indicate that the interaction of TSP with the surface of activated platelets can be modulated at the level of a specific epitope located within the amino terminal heparin-binding domain of the molecule. Thus, selective inhibition of the platelet/TSP interaction may represent an alternative approach to the inhibition of platelet aggregation.