Transgenic mice overexpressing erythropoietin adapt to excessive erythrocytosis by regulating blood viscosity

Abstract

Severe elevation of red blood cell number is often associated with hypertension and thromboembolism resulting in severe cardiovascular complications. However, some individuals such as high altitude dwellers cope well with an increased hematocrit level. We analyzed adaptive mechanisms to excessive erythrocytosis in our transgenic (tg) mice that, due to hypoxia-independent erythropoietin (Epo) overexpression, reached hematocrit values of 0.8 to 0.9 without alteration of blood pressure, heart rate, or cardiac output. Extramedullar erythropoiesis occurred in the tg spleen, leading to splenomegaly. Upon splenectomy, hematocrit values in tg mice decreased from 0.89 to 0.62. Tg mice showed doubled reticulocyte counts and an increased mean corpuscular volume. In tg mice, plasma volume was not elevated whereas blood volume was up to 25% of the body weight compared with 8% in wild-type (wt) siblings. Although plasma viscosity did not differ between tg and wt mice, tg wholeblood viscosity increased to a lower degree (4-fold) than expected from corresponding hemoconcentrated wt blood (8-fold). This moderate increase in viscosity is explicable by the up to 3-fold higher elongation of tg erythrocytes at physiologic shear rates. Apart from the nitric oxide-mediated vasodilation we reported earlier, adaptation to high hematocrit levels in tg mice involves regulated ele- IntroductionHigh hematocrit levels are observed in patients suffering from erythrocytoses such as polycythemia vera and chronic mountain sickness, as well as in lowlanders at high altitude and erythropoietin (Epo)-abusing athletes. Severe elevation of the hematocrit level is often associated with hypertension and thromboembolism, leading to severe clinical complications and frequently to death. 1 However, several reports demonstrate that some individuals can cope with excessive erythrocytosis. One case report describes a Chilean miner working at 5950 m above sea level who reached an hematocrit level of 0.75 without showing impaired health conditions or reduced physical activity. 2 In keeping with this, Peruvian miners living and working at extreme altitude and exposed to cobalt (known to induce Epo expression) have been found to reach hematocrit levels of 0.75 to 0.91. 3 One should keep in mind, however, that adaptive mechanisms of high altitude dwellers might be population dependent as recently shown by differences in nitric oxide (NO) metabolism of the lung in Tibetans and Bolivian Aymara. 4 Excessive erythrocytosis is also found in sports medicine: an endurance athlete with an autosomal dominant erythrocytosis 5 resulting in hematocrit levels up to 0.68 has won several Olympic gold medals in the past. 6 These and other reports 7 indicate that adaptive mechanisms to excessive erythrocytosis exist. Because the blood's flow resistance is regulated mainly by the radius of the vessel and the whole-blood viscosity (reviewed in Pearson and Path 8 ) it is conceivable to expect that adaptation to highly increased hematocrit levels involves vasodilation and reduced vi...

Non-erythroid activities of erythropoietin: Functional effects on murine dendritic cells

“…Preparation of splenocytes -Mice were sacrificed, and their spleens were incubated with 1 mg/ml fresh collagenase D (Sigma) in RPMI for 30 min at 37 o C. Note that the tg6 spleen was enlarged (Vogel et al, 2003). The spleens were then immersed and forced through 200-m pore-size wire mesh, using the plunger from a 5 ml syringe to obtain a 7 single cell suspension.…”

“…More recently we noted EPO effects also on the humoral immune response of antigen-injected mice in both EPO-treated mice and tg6 mice (Ruschitzka et al, 2000;Vogel et al, 2003) -transgenic mice over-expressing human EPO (HuEPO) (Katz et al, 2007). Whereas these findings strongly suggest that EPO is an immunomodulator, the cellular mediators and underlying mechanisms are not understood, especially since EPO-Rs were not detected on lymphocytes ((Prutchi-Sagiv et al, 2006) and unpublished data).…”

Constitutive excessive erythrocytosis causes inflammation and increased vascular permeability in aged mouse brain

“…Excessive erythrocytosis compromises vascular homeostasis as a result of significantly increased hematocrit and decreased plasma levels (Neunteufl et al, 2001). We have previously shown that NOmediated vasodilatation and increased blood viscosity are two ways by which tg6 mice adapt to the extremely high hematocrit levels (Vogel et al, 2003). However elevated blood viscosity significantly reduces blood velocity causing a concomitant lowering of shear stress to levels that are probably detrimental in the long term.…”

“…In addition to its well known role in erythropoiesis however a diverse array of cells have now been identified that produce Epo and/or express the Epo receptor including endothelial cells, smooth muscle cells and cells of the central nervous system (Digicaylioglu et al, 1995;Masuda et al, 1994). In vitro Epo has been shown to regulate a variety of neural functions such as calcium flux (Korbel et al, 2004) neurotransmitter synthesis and cell survival (Velly et al, 2010;Vogel et al, 2003). Furthermore Epo has neurotrophic effects (Chen et al, 2007;Grimm et al, 2002;Junk et al, 2002), can induce an angiogenic phenotype in cultured endothelial cells, is a potent angiogenic factor in vivo (Ribatti et al, 2003) and enhances ventilation in hypoxic conditions (Soliz et al, 2005).…”

“…Generation and characterization of this line showed these mice have a hematocrit of 0.8-0.9 after the first 8-9 weeks without increased blood pressure or altered cardiac output as well as reduced exercise performance and a significantly reduced life span of 9-12 months (Heinicke et al, 2006;Ogunshola et al, 2006;Ruschitzka et al, 2000;Vogel et al, 2003;Wagner et al, 2001). Interestingly both NO mediated relaxation and circulating NO levels are markedly elevated leading to increased vasodilation and protection from cardiac complications (Ruschitzka et al, 2000).…”