Organization of the gene for human platelet/endothelial cell adhesion molecule-1 shows alternatively spliced isoforms and a functionally complex cytoplasmic domain

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is a cell-cell adhesion molecule that is expressed on circulating platelets, on leukocytes, and at the intercellular junctions of vascular endothelial cells and mediates the interactions of these cells during the process of transendothelial cell migration. The cDNA for PECAM-1 encodes an open reading frame of 738 amino acids (aa) that is organized into a 27- aa signal peptide, a 574-aa extracellular domain composed of 6 Ig homology units, and a relatively long cytoplasmic tail of 118 aa containing multiple sites for posttranslational modification and postreceptor signal transduction. To provide a molecular basis for the precise evaluation of the structure and function of this transmembrane glycoprotein, we have determined the organization of the human PECAM-1 gene. The PECAM-1 gene, which has been localized to human chromosome 17, is a single-copy gene of approximately 65 kb in length and is broken into 16 exons by introns ranging in size from 86 to greater than 12,000 bp in length. Typical of other members of the Ig superfamily, each of the extracellular Ig homology domains is encoded by a separate exon, consistent with PECAM-1 having arisen by gene duplication and exon shuffling of ancestral Ig superfamily genes. However, the cytoplasmic domain was found to be surprisingly complex, being encoded by seven short exons that may represent discrete functional entities. Alternative splicing of the cytoplasmic tail appears to generate multiple PECAM-1 isoforms that may regulate phosphorylation, cytoskeletal association, and affinity modulation of the mature protein. Finally, a processed pseudogene having 76% identity with PECAM- 1 cDNA was identified and localized to human chromosome 3. These findings should have important implications for structure/function analysis of PECAM-1 and its role in vascular adhesive interactions.     

Role of the B domain for factor VIII and factor V expression and function

Factor V and factor VIII are homologous cofactors in the blood coagulation cascade that have the domain structure A1-A2-B-A3-C1-C2, of which the B domain has extensively diverged. In transfected COS-1 monkey cells, expression of factor VIII is approximately 10-fold less efficient than that of factor V, primarily because of inefficient protein secretion and, to a lesser extent, reduced mRNA expression. To study the functional significance and effect of the B domain on expression and activity, chimeric cDNAs were constructed in which the B domains of factor V and factor VIII were exchanged. Expression of a factor VIII chimera harboring the B-domain of factor V yielded a fully functional factor VIII molecule that was expressed twofold more efficiently than wild-type factor VIII because of increased mRNA expression. Thus, sequences within the factor VIII B domain were not responsible for the inefficient secretion of factor VIII compared with factor V. Expression of a factor V chimera harboring the B domain of factor VIII was slightly reduced compared with wild-type factor V, although the secreted molecule had significantly reduced procoagulant activity correlating with dissociated heavy and light chains and resistance to thrombin activation. Interestingly, the factor V chimera containing the factor VIII B domain was efficiently activated by Russell's viper venum (RVV). A factor V B domain deletion (residues 710- 1545) molecule also exhibited significantly reduced procoagulant activity caused by resistance to thrombin cleavage and activation, although this molecule was activatable by RVV. These results show that, in contrast to factor VIII, thrombin activation of factor V requires sequences within the B domain. In addition, thrombin activation of factor V occurs through a different mechanism than activation by RVV.     

Expression and function of CD40 on Hodgkin and Reed-Sternberg cells and the possible relevance for Hodgkin's disease

CD40 was originally described as a B-cell-restricted antigen and was subsequently found to be a member of the tumor necrosis factor (TNF) receptor superfamily. CD40 is also expressed on dendritic cells, thymic epithelium, monocytes, and some carcinoma cell lines, and plays a critical role in cell contact-dependent activation. Primary and cultured Hodgkin and Reed-Sternberg (H-RS) cells, the presumed malignant cells of Hodgkin's disease (HD); were found to express high levels of cell surface CD40. We found that recombinant CD40 ligand (CD40L) induced interleukin-8 (IL-8) secretion and enhanced IL-6, TNF, and lymphotoxin-alpha (LT-alpha/TNF-beta) release from cultured H-RS cells. These cytokines play a significant role in the clinical presentation and pathology of HD, a tumor of cytokine-producing cells. CD40L had no mitogenic activity for HD-derived cell lines. In contrast, CD40L enhanced expression of costimulatory molecules intracellular adhesion molecule-T and B7-1 on cultured H-RS cells, both of which are overexpressed on primary H-RS cells. In addition, CD40L induced a 40% to 60% reduction of the expression of the HD-associated CD30 antigen, another member of the TNF receptor superfamily. Primary and cultured H- RS cells express not only CD30, but also CD40. CD40L has pleiotropic biologic activities on H-RS cells, and the CD40-CD40L interaction might be a critical element in the deregulated cytokine network and cell contact-dependent activation cascade typical for HD.     

A constitutively active mutant beta 2-adrenergic receptor is constitutively desensitized and phosphorylated.

The beta 2-adrenergic receptor (beta 2AR) can be constitutively activated by mutations in the third intracellular loop. Whereas the wild-type receptor exists predominantly in an inactive conformation (R) in the absence of agonist, the mutant receptor appears to spontaneously adopt an active conformation (R*). We now demonstrate that not only is the mutant beta 2AR constitutively active, it is also constitutively desensitized and down-regulated. To assess whether the mutant receptor can constitutively engage a known element of the cellular desensitization machinery, the receptor was purified and reconstituted into phospholipid vesicles. These preparations retained the essential properties of the constitutively active mutant receptor: agonist-independent activity [to stimulate guanine nucleotide-binding protein (Gs)-GTPase] and agonist-specific increase in binding affinity. Moreover, the purified mutant receptor, in the absence of agonist, was phosphorylated by recombinant beta AR-specific kinase (beta ARK) in a fashion comparable to the agonist-occupied wild-type receptor. Thus, the conformation of the mutated receptor is equivalent to the active conformation (R*), which stimulates Gs protein and is identical to the beta ARK substrate.     

Functionally active targeting domain of the beta-adrenergic receptor kinase: an inhibitor of G beta gamma-mediated stimulation of type II adenylyl cyclase.

The beta-adrenergic receptor kinase (beta ARK) phosphorylates its membrane-associated receptor substrates, such as the beta-adrenergic receptor, triggering events leading to receptor desensitization. beta ARK activity is markedly stimulated by the isoprenylated beta gamma subunit complex of heterotrimeric guanine nucleotide-binding proteins (G beta gamma), which translocates the kinase to the plasma membrane and thereby targets it to its receptor substrate. The amino-terminal two-thirds of beta ARK1 composes the receptor recognition and catalytic domains, while the carboxyl third contains the G beta gamma binding sequences, the targeting domain. We prepared this domain as a recombinant His6 fusion protein from Escherichia coli and found that it had both independent secondary structure and functional activity. We demonstrated the inhibitory properties of this domain against G beta gamma activation of type II adenylyl cyclase both in a reconstituted system utilizing Sf9 insect cell membranes and in a permeabilized 293 human embryonic kidney cell system. Gi alpha-mediated inhibition of adenylyl cyclase was not affected. These data suggest that this His6 fusion protein derived from the carboxyl terminus of beta ARK1 provides a specific probe for defining G beta gamma-mediated processes and for studying the structural features of a G beta gamma-binding domain.     

Engraftment of dogs with Ia-positive marrow cells isolated by avidin- biotin immunoadsorption

Previous work has shown failure of engraftment in lethally irradiated dogs when autologous marrow was depleted of Ia-positive cells with an anti-Ia antibody and complement before infusion. In the current study, we have utilized an avidin-biotin immunoadsorption procedure to obtain a population of highly enriched Ia-positive cells for autologous bone marrow transplantation in dogs given lethal irradiation. Dog marrow cells (2.4 to 7.0 X 10(9) cells) that contained 8.6% to 19.9% Ia- positive cells were treated successively with monoclonal antibody 7.2, which reacts with a framework determinant of Ia-antigen, and biotin- conjugated goat antimouse immunoglobulin. These treated cells were passed over a column of avidin-Biogel (polyacrylamide) and the adherent cells removed by mechanical agitation. Seven lethally irradiated dogs were transplanted with 5.9 to 33.4 X 10(6) recovered adherent cells per kilogram of which 69.0% to 88.0% were Ia-positive. All dogs had hematologic recovery; six are alive and well with durable engraftment and one died on day 15 posttransplant. They are immunologically normal as determined by lymph node and bone marrow biopsies, lymphocyte function, and immunophenotyping of peripheral blood and bone marrow cells. These data provide further evidence that canine hematopoietic stem cells express Ia-like antigens and that these cells are capable of complete hematopoietic and immunologic reconstitution in an autologous model.     

Immunohistochemical characterization of a 183 KD myeloid-specific-DNA- binding protein in B5 fixed, paraffin-embedded tissues, and bone marrow aspirates by monoclonal antibody BM-1

A monoclonal antibody, designated BM-1, which is reactive in B5 formalin-fixed, paraffin-embedded tissues, has been generated against a cytoplasmic and nuclear antigen expressed in human myeloid precursor cells and derived leukemias. Using the avidin-biotin-complex immunoperoxidase procedure, BM-1 was found to stain selectively myeloid precursor cells in normal bone marrow and mature granulocytes in the blood. In a screen of 26 normal adult and fetal human organs fixed in B5 formalin, BM-1 was negative in all nonhematopoietic tissues with the exception of tissue granulocytes and scattered cells in the peripheral cortex of the thymus. Likewise a screen of 30 solid tumor cell lines including a spectrum of carcinomas, sarcomas, and neural-derived tumors was negative. BM-1 was also negative with 21 T and B cell lymphomas and 11 Hodgkin's disease tumors. A preliminary study of tumors of the hematopoietic system revealed that BM-1 was reactive with M2 and M3 acute myelogenous leukemias (AML), chronic myelogenous leukemias (CML) and myelomonocytic leukemias, and granulocytic sarcomas. M1, M4, M5, and M6 AML clot preparations were negative in this study, indicating that BM-1 may have a role in the histopathologic diagnosis of myelogenous leukemia. Myeloid leukemic cell lines HL-60, ML-2, KG1, and TPH-1-O showed BM-1 nuclear and/or cytoplasmic reactivity in a subpopulation of cells, but erythroid and lymphoid leukemias and all lymphoma cell lines were negative. Immunoperoxidase studies of a panel of fetal tissues showed BM-1 positive cells in the peripheral cortex of the thymus and portal myelopoietic regions of the liver at 18 weeks gestation. Finally, DNA-cellulose and solid phase radioimmunoassay (RIA) techniques developed in our laboratory demonstrate that the BM-1 antigenic domain is reactive only after binding to eukaryotic but not prokaryotic single- or double-stranded DNA. Immunoblot techniques using a DNA-cellulose purified protein sample revealed that BM-1 recognizes a 183 kD protein. These studies indicate that BM-1 is recognizing a myeloid-specific antigen that, because of its DNA binding characteristics, may have an important role in the differentiation of myeloid cells at the molecular level.     

Coupling of histamine H3 receptors to neuronal Na+/H+ exchange: a novel protective mechanism in myocardial ischemia

In myocardial ischemia, adrenergic nerves release excessive amounts of norepinephrine (NE), causing dysfunction and arrhythmias. With anoxia and the concomitant ATP depletion, vesicular storage of NE is impaired, resulting in accumulation of free NE in the axoplasm of sympathetic nerves. Intraneuronal acidosis activates the Na(+)/H(+) exchanger (NHE), leading to increased Na(+) entry in the nerve terminals. These conditions favor availability of the NE transporter to the axoplasmic side of the membrane, causing massive carrier-mediated efflux of free NE. Neuronal NHE activation is pivotal in this process; NHE inhibitors attenuate carrier-mediated NE release. We previously reported that activation of histamine H(3) receptors (H(3)R) on cardiac sympathetic nerves also reduces carrier-mediated NE release and alleviates arrhythmias. Thus, H(3)R activation may be negatively coupled to NHE. We tested this hypothesis in individual human SKNMC neuroblastoma cells stably transfected with H(3)R cDNA, loaded with the intracellular pH (pH(i)) indicator BCECF. These cells possess amiloride-sensitive NHE. NHE activity was measured as the rate of Na(+)-dependent pH(i) recovery in response to an acute acid pulse (NH(4)Cl). We found that the selective H(3)R-agonist imetit markedly diminished NHE activity, and so did the amiloride derivative EIPA. The selective H(3)R antagonist thioperamide abolished the imetit-induced NHE attenuation. Thus, our results provide a link between H(3)R and NHE, which may limit the excessive release of NE during protracted myocardial ischemia. Our previous and present findings uncover a novel mechanism of cardioprotection: NHE inhibition in cardiac adrenergic neurons as a means to prevent ischemic arrhythmias associated with carrier-mediated NE release.     

Fibrinogen Longmont: a dysfibrinogenemia that causes prolonged clot-based test results only when using an optical detection method

A new fibrinogen variant was discovered as a result of discrepancies found in routine laboratory screening. The patient, a healthy 37-year-old woman, had a mild bleeding history. Initial coagulation studies on the patient revealed a prolonged prothrombin time (PT) and a normal activated partial thromboplastin time (APTT). Further investigation on the patient and her mother demonstrated both had a PT with no end point using an optical detection method (ACL3000+) and a normal PT using an electromechanical detection method (ST4 Clot Detection System). The APTT for both the patient and her mother were essentially normal with both optical and mechanical detection methods. The patient and her mother also had markedly prolonged thrombin time and reptilase time results on the ACL3000+, but they were normal on the ST4. Coagulation test results on the patient's father were all normal. We believe the fibrinogen defect in this family may affect fibrin polymerization only enough to effect light scatter interpretation, while there is enough polymerization to increase plasma viscosity and yield an end point using an electromechanical analyzer. This report should alert pathologists and clinicians to possible discrepancies between mechanical and spectrophotometric clot testing methods.     

Daytime sleepiness predicts mortality and cardiovascular disease in older adults. The Cardiovascular Health Study Research Group

INTRODUCTION: As part of the baseline examination in the Cardiovascular Health Study, sleep disturbance symptoms including snoring and daytime sleepiness, were assessed as potential risk factors or precipitants of cardiovascular disease (CVD). Because of the association of sleep disturbance with poorer health and the possible associations of sleep apnea with CVD, we hypothesized that those with poorer sleep or daytime sleepiness may be at increased risk of mortality or incident CVD. SETTING: Participants (n = 5888) were recruited in 1989, with an additional minority cohort recruited in 1993, in four US communities for a cohort study designed to evaluate risk factors for cardiovascular disease. METHODS: An interview-administered questionnaire regarding health and sleep habits with ongoing ascertainment of total mortality and cardiovascular disease morbidity and mortality, including total CVD morbidity and mortality, incident myocardial infarction, and congestive heart failure. RESULTS: Daytime sleepiness was the only sleep symptom that was significantly associated with mortality in both men and women. The unadjusted hazard ratio was 2.12 (1.66, 2.72) in women and 1.40 (1.12, 1.73) in men. Men who reported difficulty falling asleep also had an increased mortality rate (HR = 1.43 (1.14, 1.80)) which was not seen in women. The risks were attenuated with adjustment for age but remained significant for daytime sleepiness in women (HR = 1.82 (1.42, 2.34)) and for difficulty falling asleep in men. (HR = 1.29 (1.03, 1.63)). Frequent awakenings, early morning awakening, and snoring were not associated with a significantly increased risk of mortality in these older men and women. Crude event rates were evaluated for total incident cardiovascular morbidity and mortality, incident myocardial infarction, and incident congestive heart failure (CHF). Incident CVD rates were higher in both men and women with daytime sleepiness. The aged adjusted HR was 1.35 (95% CI = 1.03, 1.76) in men and was 1.66 (95% CI = 1.28, 2.16) in women. Incident CVD was not higher in those with any other sleep disturbance including snoring. The risk of CVD events associated with daytime sleepiness was attenuated but remained significant in women after adjustment for age. Incident myocardial infarction (MI) rates were also higher in women with daytime sleepiness but were not significantly higher in men. Incident CHF rates were increased in both men and women with daytime sleepiness. In men, the age adjusted HR was 1.49 (95% CI, 1.12- 1.98) and in women, was 2.21 (95% CI, 1.64-2.98). Women reporting both daytime sleepiness and frequent awakening had a hazard ratio of 2.34 (95% CI, 1.66-3.29) for incident CHF compared with those with daytime sleepiness but without frequent awakening. This interaction was not found in men. CONCLUSIONS: In this study, daytime sleepiness was the only sleep disturbance symptom that was associated with mortality, incident CVD morbidity and mortality, MI, and CHF. These findings were stronger in women than men, i.e., the associations persisted for mortality, CVD, and CHF in women after adjustment for age and other factors. Thus, a report of daytime sleepiness identifies older adults at increased risk for total and cardiovascular mortality, and is an independent risk factor in women.