Genetic control of platelet activation in inbred mouse strains

Progression of Arterial Vasa Vasorum from Regulator of Arterial Homeostasis to Promoter of Atherogenesis

The vasa vasorum (vessels of vessels) are a dynamic microvascular system uniquely distributed to maintain physiological homeostasis of the artery wall by supplying nutrients and oxygen to the outer layers of the artery wall, adventitia, and perivascular adipose tissue, and in large arteries, to the outer portion of the medial layer. Vasa vasorum endothelium and contractile mural cells regulate direct access of bioactive cells and factors present in both the systemic circulation and the arterial perivascular adipose tissue and adventitia to the artery wall. Experimental and human data show that proatherogenic factors and cells gain direct access to the artery wall via the vasa vasorum and may initiate, promote, and destabilize the plaque. Activation and growth of vasa vasorum occur in all blood vessel layers primarily by angiogenesis, producing fragile and permeable new microvessels that may cause plaque hemorrhage and fibrous cap rupture. Ironically, invasive therapies, such as angioplasty and coronary artery bypass grafting, injure the vasa vasorum, leading to treatment failures. The vasa vasorum function both as a master integrator of arterial homeostasis and, once perturbed or injured, as a promotor of atherogenesis. Future studies need to be directed at establishing reliable in vivo and in vitro models to investigate the cellular and molecular regulation of the function and dysfunction of the arterial vasa vasorum.

Quantitative trait loci analysis for peripheral blood parameters in a (BALB/cW × C57BL/6J-Mpl (hlb219)/J) F(2) mice

The genetic basis of the peripheral blood cell parameters is not fully elucidated. Thus, it is essential to research the correlation between blood cell counts levels and the genome in laboratory animals and subsequently in humans. In the present study, we examined 288 F(2) mice from a cross between BALB/cW and C57BL/6J-Mpl(hlb219)/J. The C57BL/6J-Mpl (hlb219)/J strain is a mouse model of thrombocytopenia. We found very strong correlations for PLT counts and revealed some highly significant correlations for RBC counts. On the basis of the obtained results, we presume that genetic control of erythrocyte parameters is divided into two pathways: first, the morphological determinants responsible for the red blood cell count (RBC), hematocrit (HCT), and mean corpuscular volume (MCV), and second, the functional pathway determining the hemoglobin content (HGB). The locus on Chromosome 4 is the only detected quantitative trait locus (QTL) influencing the analyzed platelets parameters. We also detected highly significant correlations for erythrocyte parameters on Chromosome 1 (RBC, MCV, MCH), Chr 7 (HGB), Chr 9 (MCHC), Chr 11 (RBC), and Chr 17 (MCH). Finally, with regards to the given correlations, using the Mouse Genome Database resource, we proposed candidate genes with possible meaning for the level of these parameters: cytokine receptor genes (e.g., Mpl), transcription factor genes (e.g., Xbp1, Ikzf1), hemoglobin chain genes (e.g., Hbb-b1, Hbb-ar), and many others localized in the confidence intervals of found QTLs.

Quantitative trait loci for steady-state platelet count in mice

Platelet count in humans is a strongly genetically regulated trait, with approximately 85% of the interindividual variance in platelet numbers attributable to genetic factors. Inbred mouse strains also have strain-specific platelet count ranges. As part of a project to identify novel factors that regulate platelet count, we identified two inbred mouse strains, CBA/CaH and QSi5, with substantial differences in platelet count (mean values of 581 vs. 1062 x 10(9)/L). An F(2) intercross resource of 1126 animals was bred from these two parental strains for a genomewide scan for quantitative trait loci (QTL) for platelet count. QTL were identified on MMU1 (LOD 6.8, p < 0.0005) and MMU11 (LOD 11.2, p < 0.0005) by selectively genotyping animals from the extremes of the F(2) platelet count distribution. Three other QTL of suggestive statistical significance were also detected on MMU7, 13, and 17. It is noteworthy that no QTL were detected in the vicinity of the genes encoding thrombopoietin ( Thpo), and its receptor ( c-Mpl), both known to influence platelet production. Comparison of gene expression levels between the parental mouse strains by microarrays also showed little difference in the mRNA levels of these known candidate genes. These results represent the first published use of a genetic linkage-based approach in a mouse model toward the identification of genetic factors that regulate platelet count.

Factors that influence platelet recovery after transfusion: resolving donor quality from ABO compatibility

BACKGROUND

A system was established to examine the extent to which the apheresis donor determines platelet recovery after transfusion, to measure the impact of ABO identity, and to predict outcome by evaluating the donor.

STUDY DESIGN AND METHODS

The percentage of platelet recovery was measured after prophylactic transfusion of apheresis units divided from single donors to paired recipients with uncomplicated thrombocytopenia secondary to leukemia chemotherapy. Platelet microaggregation induced by citrate was measured at the time of apheresis.

RESULTS

Platelet recoveries in paired recipients correlated strongly when both transfusions were ABO- identical. When one recipient was ABO-identical and the other was ABO-nonidentical, nonidentical transfusions yielded one-third the recovery of ABO-identical transfusions. In ABO-identical transfusions, platelet recovery in donors having microaggregates in the before-apheresis ACD sample was one-third that in donors without microaggregates. This difference was observed at 1 and 24 hours. Expression of P-selectin in the apheresis units at the time of transfusion correlated well with ACD microaggregates in the before-apheresis sample.

CONCLUSION

When transfusions of platelets are ABO-identical, donor quality dominates recovery in circulation. Donor quality is predicted by a rapid and simple assay of citrate-induced microaggregation performed at the time of apheresis. When donor quality is factored out, ABO identity prevails.

Abnormal platelet function and calcium handling in Dahl salt-hypertensive rats

The effect of dietary salt on platelet function and Ca(2+) homeostasis was studied in Dahl (DS) rats, a genetic model of salt-sensitive hypertension. DS rats were fed a high-salt (DSHS) or a low-salt diet (DSLS) for up to 4 weeks, and the effects of salt loading on systolic blood pressure, platelet P-selectin expression, and platelet Ca(2+) homeostasis were measured. The high-salt diet increased blood pressure and markedly increased the amount of ionomycin (IM)-releasable Ca(2+) in platelet intracellular stores (Ca(2+)/IM). The alteration in Ca(2+) stores was not prevented when the hypertension was prevented by treatment with hydralazine and reserpine. The Ca(2+) store filling during platelet exposure to 1 mmol/L Ca(2+) for 5 minutes and the rate of sarcoplasmic/endoplasmic Ca(2+) ATPase-dependent Ca(45) uptake were higher in DSHS compared with that in DSLS. There was a decrease in thrombin-induced Ca(2+) influx in platelets from DSHS; consistent with this, agonist-induced P-selectin expression was decreased. In DSLS, nitric oxide accelerated reloading of platelet Ca(2+) stores after their emptying by thrombin but failed to do so in DSHS. These results indicate that in DS rats, a high-salt diet increases sarcoplasmic/endoplasmic Ca(2+) ATPase activity and the Ca(2+)/IM but decreases the reuptake of Ca(2+) caused by nitric oxide. Decreases in Ca(2+) influx and platelet P-selectin expression might be explained by changes in intracellular Ca(2+) stores in DSHS rats, which apparently is a heritable response to a high-salt diet.

The clinical utility of flow cytometry in the study of platelets

Flow cytometry has emerged in the past few years as an important technology for the study of platelets. It offers the ability to make measurements on platelets with little or no isolation or manipulation. Most flow cytometric platelet studies can be carried out on whole blood, thus eliminating a host of artifacts. In addition, flow cytometric techniques have been developed that allow the measurement of nearly all of the functional capabilities of platelets, such as activation and aggregation and to identify new functions by permitting observation of platelets interacting with leukocytes and measurement of platelet microparticles. Several of these measurements have already reached the stage of clinical utility and others offer considerable promise for practical applications. This review describes each of the flow cytometric techniques used to study platelets and summarizes their current state of clinical utility. Semin Hematol 38:160-168.

Strain-dependent vascular remodeling phenotypes in inbred mice

We have recently established a mouse model of arterial remodeling in which flow in the left common carotid artery of FVB mice was interrupted by ligation of the vessel near the carotid bifurcation, resulting in a dramatic reduction of the lumen as a consequence of a reduction in vessel diameter and intimal lesion formation. In the present study we applied this model to various inbred strains of mice. Wide variations in the remodeling response with regard to reduction in vessel diameter, intimal lesion formation, lumen area, and medial hypertrophy were found. On carotid artery ligation SJL/J mice revealed the most extensive inward remodeling leading to an approximate 78% decrease in lumen area while lumen narrowing in FVB/NJ mice was largely due to extensive neointima formation as a result of smooth muscle cell (SMC) proliferation. Significant positive remodeling in the contralateral right carotid artery with a >20% increase in lumen area was observed in SM/J and A/J mice. An in vitro comparison of growth properties of SMC isolated from FVB/NJ mice and a strain that exhibited very little SMC proliferation (C3H/HeJ) demonstrated accelerated growth of SMC from FVB/NJ following serum stimulation. In vivo, SMC proliferation in the FVB/NJ strain was preceded by a 37% loss of medial SMC occurring within the 2 days after ligation, however, cell death was not detectable in C3H/HeJ mice. These findings suggest that the mechanisms leading to lumen narrowing in the vascular remodeling process are genetically controlled.

Consequences of GATA-1 Deficiency in Megakaryocytes and Platelets

In the absence of the hematopoietic transcription factor GATA-1, mice develop thrombocytopenia and an increased number of megakaryocytes characterized by marked ultrastructural abnormalities. These observations establish a critical role for GATA-1 in megakaryopoiesis and raise the question as to how GATA-1 influences megakaryocyte maturation and platelet production. To begin to address this, we have performed a more detailed examination of the megakaryocytes and platelets produced in mice that lack GATA-1 in this lineage. Our analysis demonstrates that compared with their normal counterparts, GATA-1–deficient primary megakaryocytes exhibit significant hyperproliferation in liquid culture, suggesting that the megakaryocytosis seen in animals is nonreactive. Morphologically, these mutant megakaryocytes are small and show evidence of retarded nuclear and cytoplasmic development. A significant proportion of these cells do not undergo endomitosis and express markedly lower levels of mRNA of all megakaryocyte-associated genes tested, including GPIb, GPIbβ, platelet factor 4 (PF4), c-mpl, and p45 NF-E2. These results are consistent with regulation of a program of megakaryocytic differentiation by GATA-1. Bleeding times are significantly prolonged in mutant animals. GATA-1–deficient platelets show abnormal ultrastructure, reminiscent of the megakaryocytes from which they are derived, and exhibit modest but selective defects in platelet activation in response to thrombin or to the combination of adenosine diphosphate (ADP) and epinephrine. Our findings indicate that GATA-1 serves multiple functions in megakaryocyte development, influencing both cellular growth and maturation.

Consequences of GATA-1 Deficiency in Megakaryocytes and Platelets

In the absence of the hematopoietic transcription factor GATA-1, mice develop thrombocytopenia and an increased number of megakaryocytes characterized by marked ultrastructural abnormalities. These observations establish a critical role for GATA-1 in megakaryopoiesis and raise the question as to how GATA-1 influences megakaryocyte maturation and platelet production. To begin to address this, we have performed a more detailed examination of the megakaryocytes and platelets produced in mice that lack GATA-1 in this lineage. Our analysis demonstrates that compared with their normal counterparts, GATA-1–deficient primary megakaryocytes exhibit significant hyperproliferation in liquid culture, suggesting that the megakaryocytosis seen in animals is nonreactive. Morphologically, these mutant megakaryocytes are small and show evidence of retarded nuclear and cytoplasmic development. A significant proportion of these cells do not undergo endomitosis and express markedly lower levels of mRNA of all megakaryocyte-associated genes tested, including GPIb, GPIbβ, platelet factor 4 (PF4), c-mpl, and p45 NF-E2. These results are consistent with regulation of a program of megakaryocytic differentiation by GATA-1. Bleeding times are significantly prolonged in mutant animals. GATA-1–deficient platelets show abnormal ultrastructure, reminiscent of the megakaryocytes from which they are derived, and exhibit modest but selective defects in platelet activation in response to thrombin or to the combination of adenosine diphosphate (ADP) and epinephrine. Our findings indicate that GATA-1 serves multiple functions in megakaryocyte development, influencing both cellular growth and maturation.