Sandra Larkin

Patterns of arginine and nitric oxide in patients with sickle cell disease with vaso-occlusive crisis and acute chest syndrome.

Purpose Our objective was to evaluate l-arginine and nitric oxide metabolite (NOx) levels in children with sickle cell disease (SCD) at steady-state and during vaso-occlusive crisis (VOC). Because alterations in nitric oxide production may have an important role in the pathophysiology of SCD, our second aim was to determine if a relationship exists between these levels and vaso-occlusive crisis (VOC). Patients and Methods Plasma l-arginine and serum NOx levels were examined in 36 patients with SCD with 39 episodes of VOC and 10 children with SCD at steady-state. Daily levels were obtained in children requiring hospitalization. Results Steady-state l-arginine levels were normal in children with SCD. l-arginine levels were low, however, in children with VOC (37.4 ± 2.7 vs. 53.6 ± 4.6 &mgr;mol/L;P = 0.008) but returned to baseline during hospitalization. In contrast, NOx levels were normal at presentation but decreased during hospitalization for both patients with VOC and patients with acute chest syndrome (ACS) (21.1 ± 2.0, 17.4 ± 2.4, and 12.3 ± 1.6 &mgr;mol/L, respectively;P < 0.05). In the patients with VOC who had ACS develop, l-arginine decreased to the lowest levels at the time of the ACS diagnosis, correlating with decreasing NOx levels. Conclusion These data suggest that there may be a relationship between the l-arginine–nitric oxide pathway and vaso-occlusion in SCD. Low arginine levels during VOC could reflect a state of acute substrate depletion that results in a decrease in nitric oxide production.

Hemolysis-associated pulmonary hypertension in thalassemia.

Abstract: Accumulating evidence supports the existence of a condition involving hemolysis‐associated pulmonary hypertension (PHT). Hemolysis‐induced release of cell‐free hemoglobin and red blood cell arginase, resulting in impaired nitric oxide bioavailability, endothelial dysfunction, and PHT, has been reported in sickle cell disease. Since thalassemia is also a condition of chronic hemolysis, these patients are at risk. The data demonstrate that hemolysis‐induced dysregulation of arginine metabolism and PHT also occurs in thalassemia. Erythrocyte release of arginase during hemolysis contributes to the development of PHT. Therapies that maximize arginine and nitric oxide bioavailability may benefit patients with thalassemia.

Hydrolysis of Phosphatidylserine-exposing Red Blood Cells by Secretory Phospholipase A2 Generates Lysophosphatidic Acid and Results in Vascular Dysfunction

Secretory phospholipase A2 (sPLA2) type IIa, elevated in inflammation, breaks down membrane phospholipids and generates arachidonic acid. We hypothesized that sPLA2 will hydrolyze red blood cells that expose phosphatidylserine (PS) and generate lysophosphatidic acid (LPA) from phosphatidic acid that is elevated in PS-exposing red blood cells. In turn, LPA, a powerful lipid mediator, could affect vascular endothelial cell function. Although normal red blood cells were not affected by sPLA2, at levels of sPLA2 observed under inflammatory conditions (100 ng/ml) PS-exposing red blood cells hemolyzed and generated LPA (1.2 nm/108 RBC). When endothelial cell monolayers were incubated in vitro with LPA, a loss of confluence was noted. Moreover, a dose-dependent increase in hydraulic conductivity was identified in rat mesenteric venules in vivo with 5 μm LPA, and the combination of PS-exposing red blood cells with PLA2 caused a similar increase in permeability. In the presence of N-palmitoyl l-serine phosphoric acid, a competitive inhibitor for the endothelial LPA receptor, loss of confluence in vitro and the hydraulic permeability caused by 5 μm LPA in vivo were abolished. The present study demonstrates that increased sPLA2 activity in inflammation in the presence of cells that have lost their membrane phospholipid asymmetry can lead to LPA-mediated endothelial dysfunction and loss of vascular integrity.

Tropomodulin 1-null mice have a mild spherocytic elliptocytosis with appearance of Tropomodulin 3 in red blood cells and disruption of the membrane skeleton

The short actin filaments in the red blood cell (RBC) membrane skeleton are capped at their pointed ends by tropomodulin 1 (Tmod1) and coated with tropomyosin (TM) along their length. Tmod1-TM control of actin filament length is hypothesized to regulate spectrin-actin lattice organization and membrane stability. We used a Tmod1 knockout mouse to investigate the in vivo role of Tmod1 in the RBC membrane skeleton. Western blots of Tmod1-null RBCs confirm the absence of Tmod1 and show the presence of Tmod3, which is normally not present in RBCs. Tmod3 is present at only one-fifth levels of Tmod1 present on wild-type membranes, but levels of actin, TMs, adducins, and other membrane skeleton proteins remain unchanged. Electron microscopy shows that actin filament lengths are more variable with spectrin-actin lattices displaying abnormally large and more variable pore sizes. Tmod1-null mice display a mild anemia with features resembling hereditary spherocytic elliptocytosis, including decreased RBC mean corpuscular volume, cellular dehydration, increased osmotic fragility, reduced deformability, and heterogeneity in osmotic ektacytometry. Insufficient capping of actin filaments by Tmod3 may allow greater actin dynamics at pointed ends, resulting in filament length redistribution, leading to irregular and attenuated spectrin-actin lattice connectivity, and concomitant RBC membrane instability.

Transfusion prevents acute chest syndrome predicted by elevated secretory phospholipase A2

Acute pulmonary injury is known as acute chest syndrome (ACS) in patients with sickle cell disease (SCD). Secretory phospholipase A2 (sPLA2) was found to predict those at risk for ACS and a trial was designed to determine if red blood cell transfusion can be used to prevent ACS. Patients with an elevated sPLA2 were randomised to either receive a single transfusion or standard care. Five of the eight patients (63%) randomised to standard care developed ACS versus none of the seven patients randomised to the transfusion arm (P = 0·026, Odds ratio = 23·6, 95% confidence interval 1, 557). This study suggests that transfusion may prevent ACS.

Interaction of an annexin V homodimer (Diannexin) with phosphatidylserine on cell surfaces and consequent antithrombotic activity

Annexin V (AV), a protein with anticoagulant activity, exerts antithrombotic activity by binding to phosphatidylserine (PS), inhibiting activation of serine proteases important in blood coagulation. The potential use of this protein as an anticoagulant is limited as it rapidly passes from the blood into the kidneys due to its relatively small size (36 kDa). We used recombinant DNA technology to produce a homodimer of human AV (DAV, 73 kDa), which exceeds the renal filtration threshold, and has a 6.5-hour half-life in the rat circulation. Human red blood cells with externalized PS were used to show that DAV had a higher affinity for PS-exposing cells than AV. DAV labeling sensitively identifies PS-exposing cells, was found to be a potent inhibitor of the activity of the prothombinase complexes and inhibits the ability of secretory phospholipaseA(2) to hydrolyze phospholipids of PS-exposing cells, reducing the formation of mediators of blood coagulation and reperfusion injury. DAV exerts dose-dependent antithrombotic activity in rat veins. This combination of activities suggests that DAV is a valuable probe to measure PS exposure and may be efficacious as a novel drug in a wide range of clinical situations.

A pilot study of the short-term use of simvastatin in sickle cell disease: effects on markers of vascular dysfunction

Sickle cell disease (SCD) is characterized by progressive vascular injury and its pathophysiology is strikingly similar to that of atherosclerosis. Statins decrease inflammation and improve endothelial function in cardiovascular disease, but their effect in SCD is not known. In this pilot study, we examined the safety and effect of short‐term simvastatin on biomarkers of vascular dysfunction in SCD. We treated 26 SCD patients with simvastatin, 20 or 40 mg/d, for 21 d. Plasma nitric oxide metabolites (NOx), C‐reactive protein (CRP), interleukin‐6 (IL‐6), vascular cell adhesion molecule‐1 (VCAM‐1), tissue factor (TF) and vascular endothelial growth factor (VEGF) were analyzed and responses to simvastatin were compared between the two treatment groups. Simvastatin increased NOx levels by 23% in the low‐dose (P = 0·01) and 106% in the moderate‐dose (P = 0·01) groups, and by 52% overall (P = 0·0008). CRP decreased similarly in both dose groups and by 68% overall (P = 0·02). Levels of IL‐6 decreased by 50% (P = 0·04) and 71% (P < 0·05) in the low‐ and moderate‐dose groups, respectively. Simvastatin had no effect on VEGF, VCAM1 or TF. Simvastatin was well‐tolerated and safe. Our preliminary findings showing a dose‐related effect of simvastatin on levels of NOx, CRP and IL‐6 suggest a potential therapeutic role for simvastatin in SCD.

Human term placenta as a source of hematopoietic cells.

The main barrier to a broader clinical application of umbilical cord blood (UCB) transplantation is its limiting cellular content. Thus, the discovery of hematopoietic progenitor cells in murine placental tissue led us investigate whether the human placenta contains hematopoietic cells, sites of hematopoiesis, and to develop a procedure of processing and storing placental hematopoietic cells for transplantation. Here we show that the human placenta contains large numbers of CD34-expressing hematopoietic cells, with the potential to provide a cellular yield several-fold greater than that of a typical UCB harvest. Cells from fresh or cryopreserved placental tissue generated erythroid and myeloid colonies in culture, and also produced lymphoid cells after transplantation in immunodeficient mice. These results suggest that human placenta could become an important new source of hematopoietic cells for allogeneic transplantation.The main barrier to a broader clinical application of umbilical cord blood (UCB) transplantation is its limiting cellular content. Thus, the discovery of hematopoietic progenitor cells in murine placental tissue led us investigate whether the human placenta contains hematopoietic cells, sites of hematopoiesis, and to develop a procedure of processing and storing placental hematopoietic cells for transplantation. Here we show that the human placenta contains large numbers of CD34-expressing hematopoietic cells, with the potential to provide a cellular yield several-fold greater than that of a typical UCB harvest. Cells from fresh or cryopreserved placental tissue generated erythroid and myeloid colonies in culture, and also produced lymphoid cells after transplantation in immunodeficient mice. These results suggest that human placenta could become an important new source of hematopoietic cells for allogeneic transplantation.

A BRIEF COMMUNICATION

The main barrier to a broader clinical application of umbilical cord blood (UCB) transplantation is its limiting cellular content. Thus, the discovery of hematopoietic progenitor cells in murine placental tissue led us investigate whether the human placenta contains hematopoietic cells, sites of hematopoiesis, and to develop a procedure of processing and storing placental hematopoietic cells for transplantation. Here we show that the human placenta contains large numbers of CD34-expressing hematopoietic cells, with the potential to provide a cellular yield several-fold greater than that of a typical UCB harvest. Cells from fresh or cryopreserved placental tissue generated erythroid and myeloid colonies in culture, and also produced lymphoid cells after transplantation in immunodeficient mice. These results suggest that human placenta could become an important new source of hematopoietic cells for allogeneic transplantation.The main barrier to a broader clinical application of umbilical cord blood (UCB) transplantation is its limiting cellular content. Thus, the discovery of hematopoietic progenitor cells in murine placental tissue led us investigate whether the human placenta contains hematopoietic cells, sites of hematopoiesis, and to develop a procedure of processing and storing placental hematopoietic cells for transplantation. Here we show that the human placenta contains large numbers of CD34-expressing hematopoietic cells, with the potential to provide a cellular yield several-fold greater than that of a typical UCB harvest. Cells from fresh or cryopreserved placental tissue generated erythroid and myeloid colonies in culture, and also produced lymphoid cells after transplantation in immunodeficient mice. These results suggest that human placenta could become an important new source of hematopoietic cells for allogeneic transplantation.

Sildenafil therapy in thalassemia patients with Doppler-defined risk of pulmonary hypertension

Pulmonary hypertension is a common but often overlooked complication associated with thalassemia syndromes. There are limited data on the safety and efficacy of selective pulmonary vasodilators in this at-risk population. We, therefore, designed a 12-week, open-label, phase 1/2, pilot-scale, proof-of-principle trial of sildenafil therapy in 10 patients with β-thalassemia and at increased risk of pulmonary hypertension based on an elevated tricuspid regurgitant jet velocity >2.5 m/s on Doppler-echocardiography. Variables compared at baseline and after 12 weeks of sildenafil treatment included Doppler-echocardiographic parameters, 6-minute walked distance, Borg Dyspnea Score, New York Heart Association functional class, pulmonary function, and laboratory parameters. Treatment with sildenafil resulted in a significant decrease in tricuspid regurgitant jet velocity by 13.3% (3.0±0.7 versus 2.6±0.5 m/s, P=0.04), improved left ventricular end systolic/diastolic volume, and a trend towards a improved New York Heart Association functional class. No significant change in 6-minute walked distance was noted. Sildenafil was well tolerated, although minor expected adverse events were commonly reported. The total dose of sildenafil (mg) was strongly correlated with percent change in nitric oxide metabolite concentration in the plasma (ρ=0.80, P=0.01). There were also significant increases in plasma and erythrocyte arginine concentrations. Our study suggests that sildenafil is safe and may improve pulmonary hemodynamics in patients at risk of pulmonary hypertension; however, it was not demonstrated to improve the distance walked in 6 minutes. Clinical trials are needed to identify the best treatment strategy for pulmonary hypertension in patients with β-thalassemia. (clinicaltrials.gov identifier: NCT00872170)