How Do Red Blood Cells Help Animals?

Production

Erythrocytes are produced from hematopoietic stem cells in the bone marrow under the influence of the cytokine, erythropoietin. There is controversy as to the exact origin of the erythropoietin cell in the kidney. Withal, the main erythropoietin-producing cell in the kidney, specially under states of hypoxia, is the peritubular interstitial cell, which is generally institute in the cortex about the junction of the cortex and medulla (Haase 2013, Shih et al 2018). There is also prove in rats that erythropoietin is produced in renal tubular epithelial cells, particularly in intercalated cells in the collecting ducts, under steady land or normal weather condition (Nagai et al 2014). In addition, low amounts of erythropoietin are produced in other sites including the liver (likely explaining secondary erythrocytosis due to some hepatic tumors, like hepatoblastoma [Lennox et al 2000, Axon et al 2008, Golden et al 2008]), testes, brain and spleen, withal these sites cannot substitute for lack of renal production. The master stimulus for erythropoiesis is hypoxia or decreased tissue oxygen tension. This results in stabilization of the transcription factors, hypoxia-inducible factor (HIF), which normally are degraded under normoxic conditions (Shih et al 2018). HIF consists of two subunits, α and β and at that place are three different α variants (HIF-1α, HIF-2α, HIF-3α), with HIF-1α and HIF-2α having the principal roles in erythropoiesis (Shih et al 2018). HIF bind to HIF-responsive elements (HRE) in the erythropoietin promoter, stimulating transcription (Souma et al 2015) and increasing erythropoietin production. It is currently idea that oxygen sensing and erythropoietin production occur in the same jail cell, ie. the peritubular or interstitial cell in states of hypoxia (and to a lesser extent tubular epithelial cells in rats (Nagai et al 2014)), but this has not been fully elucidated. Note that HIF expression is non restricted to the above described cells in the kidney and occurs in many jail cell types (especially HIF-1α) and HIF binds to other promoters than that on the erythropoietin gene and regulate changes in these cells in response to hypoxia (Schödel and Ratcliffe 2019).

Erythropoietin binds to the erythropoietin receptor on developing erythroblasts, which triggers a signaling cascade mediated through the not-receptor tyrosine kinase (phosphorylates tyrosine residues on other proteins), Janus-activated kinase-ii (JAK2), resulting in the differentiation and survival (by upregulating anti-apoptotic genes, such as BcL-xL) of erythroid progenitors. Mutations in JAK2, primarily at amino acid 617 (in which a valine [V] is exchanged for phenylalanine [F]) cause constitutive activation of the erythropoietin receptor, so signaling occurs in the absence of erythropoietin, driving erythropoiesis in the absence of hypoxia. This causes the neoplastic condition, polycythemia vera or chronic erythrocyte leukemia, a chronic myeloproliferative disorder of erythrocytes. A homozygous JAK2 mutation (V617F) has been identified in up to ninety% of human patients with polycythemia vera, indicating it is the well-nigh common causative mutation of this hematopoietic tumour (Stein et al 2015). A JAK2 mutation has been detected in 1 of five dogs with suspected polycythemia vera, indicating conservation of this pathophysiologic machinery across species (Beurlet et al 2011). Conversely, the absence of erythropoietin results in a RBC production defect, with subsequent anemia. Decreased erythropoietin production in the kidney is thought to be 1 mechanism responsible for anemia of chronic renal affliction (although other factors, such as inflammatory cytokines, tin contribute and concurrent blood loss tin can worsen the anemia). Interestingly, the renal fibroblast-like cells are thought to contribute to fibrosis (through secretion of fibrogenic cytokines) that occurs in (and likely worsens) some renal diseases (Souma et al 2015). Notation, that JAK2 is besides downstream of the thrombopoietin receptor. Heterozygous mutations in JAK2 (same amino acrid – V617F) result in essential thrombocythemia or chronic platelet leukemia (megakaryocytes are more sensitive to JAK2 than erythroid cells).

Erythroid maturation

Erythropoiesis pyramid

The earliest erythroid progenitor is the blast forming unit-erythroid (BFU-E), followed by the colony forming unit-erythroid (CFU-E). These give rise to recognizable erythroid precursors, proerythroblasts (rubriblasts, prorubricytes), basophilic erythroblast (basophilic rubricyte), polychromatophilic erythroblast (polychromatophilic metarubricyte), orthrochromic erythroblast (metarubricyte), reticulocyte and mature RBC. Note the unlike terminology is that used in human medicine with terms used in veterinarian medicine in parentheses. It is the belatedly CFU-E that take erythropoietin receptors, which are gradually shed equally the precursors differentiate, such that they are absent on reticulocytes and mature scarlet blood cells. Unlike general hematopoiesis, erythroid maturation is considered to be tree-like in hierarchy, with i progenitor giving rise to two girl cells, which are smaller. The most immature cells accept the most protein constructed machinery and have larger nuclei (for Dna synthesis and transcription) and bluer cytoplasm (RNA/ribosomes for translation). Equally the cells carve up, the nucleus becomes smaller and pyknotic and is finally expelled as it is obsolete. Hemoglobin starts being produced and equally it accumulates, the need for protein synthesis declines, leading to a decrease in organelles, including ribosomes, in the cell resulting in less blue cytoplasm (that is kickoff by the color of hemoglobin, and then y'all get a mixture of red and blue, depending on which dominates). The terminal mature blood-red blood cell just contains large amounts of hemoglobin.

Erythroblastic island

Central to erythropoiesis is the need for cellular coordination of iron acquisition (needed for hemoglobin production) and cell proliferation. Macrophages in the bone marrow provide erythroid precursors with iron, via releasing iron from intracellular stores (ferritin) through a carrier poly peptide, ferroportin. The liberated fe binds to transferrin, which is and then taken upwardly by transferrin receptors on the developing erythroid cells via clathrin-mediated endocytosis. The iron-costless transferrin is then recycled to the surface of the jail cell for more than iron uptake. Additional scavenger receptors have been identified that likewise allow erythroblasts to have-up iron that is protein bound. An alternative route for iron uptake is pinocytosis, which has been shown in vitro, but is of unknown relevance in vivo. To acquire the iron, developing erythroid progenitors are clustered around macrophages in the marrow, in then-called "erythroblastic" islands. The interaction betwixt erythroid precursors and macrophages is more than than just about iron uptake; macrophages also provide adhesive interactions, such as via erythroblast macrophage protein, CD163 (the hemoglobin-haptoglobin receptor) and VCAM-i and VLA-4 (on macrophages and erythroblasts, respectively), which facilitate erythropoiesis, and take up extruded nuclei (de Back et al 2014). The process of nuclear extrusion is fascinating, seeming to be a highly co-ordinated event, requiring a process akin to cell segmentation (cytokinesis), differential protein sorting, vesicle motion and autophagy (east.g. of mitochrondria), which results in the formation of a cherry-red claret cell fragment containing the nucleus, called a pyrenocyte. During nuclear extrusion, vimentin is downregulated; this intermediate filament is thought to exist essential for retentiveness of the nucleus in the cell. Indeed, retention of vimentin may explain the presence of nuclei in red blood cells from birds, reptiles, fish and amphibians. In addition, specific proteins are compartmentalized to the anucleated reticulocyte portion or the nucleated pyrenocyte portion (eastward.g. VLA-four). Partitioning of VLA-4 to the pyrenocyte makes sense, considering this integrin will bind to VCAM-1 on macrophages within the erythroblastic island, facilitating uptake. Uptake of the pyrenocyte by macrophages appears to involve additional mechanisms, including phosphatidylserine exteriorization (with recognition by phosphatidylserine receptors on macrophage) and erythroblast macrophage poly peptide. The phagocytized pyrenocyte and its nucleus are degraded within the macrophage (Keerthivasan et al 2011).

The anucleated erythrocyte that nevertheless contains RNA is chosen a reticulocyte and nosotros measure out reticulocytes (in dogs and cats, mostly) to assess the regenerative response by the marrow to an anemia (of hemolytic or hemorrhagic origin). One time a reticulocyte is formed, it stays in the bone marrow for approximately 24 hours (in people, we do not know precisely how long this stage takes in our common domestic species) and then the cell is released into circulation. The plasma membrane limerick of the RBC changes to facilitate this release, largely by decreasing the expression of, and cleaving off, adhesion molecules. As reticulocytes mature to erythrocytes inside their apportionment, they degrade their RNA into nucleotides (which are and so broken down to nucleosides by pyrimidine 5′-nucleotidase, which is inhibited by lead, resulting in basophilic stippling or nucleotide retention in erythrocytes), eliminate their mitochondria (potentially through autophagia and exosomes), remodel (losing surface and volume area) to become more flexible, and driblet off receptors (such every bit the transferrin receptor, which is required for iron uptake and hemoglobin synthesis in nucleated erythroid progenitors) (Ney 2011).

Construction

The structure of the RBC is amazingly well-tailored to the complex function these cells accept. For the duration of the RBC lifespan, these cells must traverse the blood stream at relatively high speeds and maintain the perfect degree of fluidity to permit them to motion through the vessel without breaking and meantime allow for efficient gas exchange across the cell membrane. This fluidity is accomplished past the complex cytoskeletal structure of RBC, which is enclosed by aphospholipid bilayer, containing numerous transmembrane proteins. The cytoskeleton of the RBC has a high tensile strength, but it also highly deformable. The mechanical properties and shape of the RBC allow the RBC to become through very minor capillaries without being damaged, considering the diameter of the RBC is larger than the bore of the smallest capillaries information technology must traverse. Alterations in the RBC cytoskeleton can lead to RBC fragility (osmotic and mechanical) and aberrant RBC shapes.

RBC cytoskeleton

The prison cell membrane is composed of a lipid bilayer with embedded carbohydrates and proteins, and an underlying cytoskeleton. The membrane lipids are important for maintaining prison cell shape and surface surface area. The membrane carbohydrates form claret groups, which are predominantly defined by the types of carbohydrates in the cherry jail cell membrane. The membrane proteins course various membrane receptors and send channels. A number of the proteins are linked, both mechanically and functionally, to the metabolism of the RBC. The location of these proteins and affinity for other proteins can exist contradistinct by pH within the prison cell, oxygenation of hemoglobin, and oxidation. Investigators have postulated that some of the "loosening" on the linkages that occurs with deoxygenation of hemoglobin helps to increment the flexibility of the RBC thus potentially improving blood menses in hypoxic tissues. This could too pb to membrane damage and "crumbling" of the RBC.

The RBC cytoskeleton consists primarily of the protein spectrin, which forms a sub-membraneous cytoskeletal framework (reviewed by Lux 2016). Spectrin consists of two elongated chains (α and β) which are intertwined and can be stretched or coiled. The spectrin network is associated with other proteins, such every bit ankyrin and band 4.1. Together, these proteins form a circuitous lattice-like meshwork that allows the RBC to accept incredibly high tensile strength besides as deformability. The spectrin bondage are linked to actin and to the RBC membrane via adaptor proteins, such every bit ankyrin, to ring 3, a transmembrane protein that is involved in anion and carbon dioxide exchange.Actin also binds to the membrane bilayer in a complex with protein 4.1, p55 and glycophorins C and D. Defects in the cytoskeleton proteins tin can lead to a loss of structural integrity of the prison cell, which can shorten its lifespan or issue in visibly-recognizable defects in shape. Defects in spectrin are responsible for the condition of hereditary spherocytosis in human beings. Spectrin deficiency has been identified in asymptomatic Dutch Golden Retrievers , which displayed increased osmotic fragility, simply no spherocytes were evident in blood smears (Slappendal et al 2005). A mutant β-spectrin was identified in an asymptomatic canis familiaris with elliptocytosis (de Terlizzi et al 2009). A deficiency of poly peptide 4.i was reported in a domestic dog (Smith et al 1983). As for other reported membrane defects in dogs, RBCs in the domestic dog of this instance report were also osmotically fragile. Oxidative injury, which accumulates in RBCs as they circulate over time, alters band 3, rendering it antigenic. Naturally occurring antibodies recognize the altered ring 3 and facilitate phagocytosis by mononuclear cells (primarily in the spleen) resulting in the removal of effete RBCs.

Function

The main functional part of the RBC is to transport oxygen from the lungs to the tissues and carbon dioxide from tissues to the lungs for expulsion. This functional role is served by hemoglobin (Hgb). Each Hgb molecule is composed of four iron-containing heme units and 4 polypeptide globin units, consisting of ii α and 2 β chains. The heme unit is a porphyrin ring and is the portion of the hemoglobin molecule that binds iron. The globin unit consists of polypeptides and functions in part to forestall the fe-containing heme molecules from coming in close contact with each other, which would interfere with part. Oxygenbinds reversibly to the iron within the heme molecule and is released in tissues (promoted by multiple factors including two,iii-DPG). Carbon dioxideis transported from the tissues back to the lungs in 3 ways: Some (about 20-30%) is transported reversibly bound to amine groups in hemoglobin (not to iron), a small amount (about 7%) is transported every bit dissolved CO2 in the blood, and about (well-nigh 70%) is transported within the red blood cells as carbonic acid produced from COii and water by carbonic anhydrase. The hemoglobin also binds hydrogen and intracellular Hgb serves as a body buffer or base, protecting against changes in pH.

Hemoglobin also provides viscosity to the RBC cytoplasm. This is crucial in maintenance of cell shape and membrane stability. In states of iron deficiency, the decrease in cytoplasmic hemoglobin is thought to contribute to a subtract in cytoplasmic viscosity, and this is followed by membrane instability (hence we can often come across cellular fragmentation, including acanthocytes and keratocytes, in astringent iron deficiency).

Considering RBCs acquit so much oxygen, they are decumbent to oxidative injury. Unremarkably, equally oxygen binds to and is released from the fe in Hgb, superoxide radicals can be formed in small amounts. RBCs have antioxidant mechanisms to deal with this as long every bit information technology is not excessive (more on this below). However, exposure to certain exogenous oxidative compounds can overwhelm the RBC antioxidant capability and lead to oxidative harm. Some of the more clinically important oxidative compounds include onions, wilted ruby-red maple leaves (horses), acetaminophen, and various other drugs/toxic plants. Endogenous oxidants are produced in disease states include inflammation, diabetes and neoplasia. Lipids in the cell membrane and thiol (SH, sulfhydryl) groups are especially sensitive to oxidation. If membrane lipids/proteins are oxidatively-injured they may stick together and lead to the formation of eccentrocytes. Similarly, if hemoglobin is oxidized and precipitates, this can lead to the germination of Heinz bodies.

Metabolism

Since RBCs lack mitochondria, they can just undergo anaerobic glycolysis (Embden-Meyerhof pathway). The generation of ATP and metabolic intermediates through anaerobic glycolysis allows the RBC to maintain membrane integrity, cell size, and redox status. ATP is required to maintain jail cell shape and deformability, since it is needed to regulate water and electrolyte content (which affects size and shape of the cell) and to maintain the cytoskeleton (which is needed for proper deformability). As a result, RBCs can lyse when depleted of ATP. The generation of ATP occurs at two master steps of the glycolytic pathway: a) Conversion of ane,iii DPG to 3 PG (this ATP is non produced if the 1,iii DPG is shunted off for 2,3-DPG production, encounter below) and b) Conversion of phosphoenolpyruvate to pyruvate via pyruvate kinase.

There are three main co-operative points off the glycolytic pathway that produce various other compounds that are necessary inside the RBCs: the hexose monophosphate shut (likewise called the pentose phosphate pathway), the methemoglobin reductase pathway, and the Luebering-Rapaport pathway (which is unremarkably referred to equally the ii,3- DPG shunt).

Hexose monophosphate shunt: This helps protect RBCs against oxidative injury. The enzyme, glucose-6-phosphate dehydrogenase (G6PD)produces NADPH from glucose 6-phosphate in the glycolytic pathway, which is utilized to aid maintain glutathione (GSH) in a reduced land. Reduced glutathione is an important antioxidant peptide and can protect against oxidation of iron, Hgb sulfhydryl groups, or cell membrane proteins. This pathway is incredibly important in RBC as they lack the organelles other cells use to assist them in dealing with oxidative stress (nucleated cells tin alter gene transcription and increase synthesis of proteins that are involved in anti-oxidant and protective pathways, such every bit the heat shock proteins). Inherited defects in this pathway atomic number 82 to oxidant injury in RBCs, manifesting as eccentrocytes and Heinz bodies. A presumed built defect in G6PD has been reported in a horse (Stockham et al 1994, Harvey 2006 review).
Methemoglobin reductase pathway: This too helps protect RBCs against oxidative injury, but is focused on hemoglobin oxidation, specifically the iron portion of the heme ring. Methemoglobin (Hgb-Atomic number 26³⁺) is hemoglobin that contains non-functional oxidized ferric fe (Fe³⁺) rather than the functional form of ferrous atomic number 26 (Fe²⁺). Methemoglobin is considered non-functional because it cannot demark oxygen; still, this oxidative alter to iron in hemoglobin is reversible. NADH produced in the glycolytic pathway is a cofactor for the enzyme methemoglobin reductase, which reverses the oxidation of iron and returns information technology to the normal reduced (Fe²⁺) state. Methemoglobin is now known to consist of 2 enzymes, cytochrome B5 and cytochrome B5 reductase. Congenital deficiencies in the methemoglobin reductase complex or a cofactor for the enzyme circuitous, flavine adenine dinucleotide (FAD), have been reported in animals (Harvey et al 2003, Harvey 2006 review. McKenna et al 2014). In one affected mustang mare, supportive laboratory findings were eccentrocytes and methemoglobinemia (manifesting as brown-tinged blood) (Harvey et al 2003). In a domestic dog with a congenital defect in methemoglobin reductase, due to a deletion in the promoter region of the cytochrome B5 factor, the domestic dog presented with astringent cyanosis and marked methemoglobinemia (38% methemoglobin, normal <3%) (McKenna et al 2014). Similar clinical signs were seen in an affected kitten with methemoglobin reductase deficiency (unknown genetic defect) (Harvey et al 1994).
Luebering-Rapaport pathway: This produces ii,3-DPG from an intermediate within the glycolytic pathway. two,3-DPG is an organic phosphate compound that functions to promote release of oxygen from hemoglobin into tissue. It binds with greater analogousness to partially deoxygenated hemoglobin and promotes release of the remaining oxygen molecules bound to the hemoglobin. When 2,3- DPG is bound to Hgb, the dissociation curve of Hgb is shifted to the right. A decrease in pH (acidosis in the tissue, which occurs with increased production of lactate) and an increase in temperature have the same effect on the Hgb saturation curve. Dogs with phosphofructokinase (PFK) deficiency have decreased amounts of 2,3-DPG, because the synthesis of this chemical compound is below the PFK pace of the glycolytic pathway (Harvey 2006 review).

Senescence

RBC life spans (the length of time that RBCs broadcast in the blood) vary depending on species. The RBC life span in routine domestic species is about 2 (cats) to four (dogs) to vi (horses, cattle) months, but is much shorter is some small mammals (~10 days in gerbils) and much longer in exotics (~600-800 days in eastern box turtles).

In healthy animals, nigh 1% of RBCs are removed from circulation each mean solar day due to normal aging and damage. As the RBCs accumulate damage, they get denser and less deformable, largely every bit the result of repeated rounds of oxidative stress (remember mature RBC are unable to synthesize any new proteins). This leads to accumulation of oxidant injury to hemoglobin, which promotes accumulation of surface-bound immunoglobulins. These immunoglobulins serve as opsonins and are one of the main signals for splenic macrophages to remove older RBCs from apportionment. Aged RBC also invert their lipid membrane, much like nucleated cells undergoing apoptosis, and expose phosphatidylserine on their surface membranes. In RBCs, this procedure is called eryptosis and it leads to uptake of RBC by macrophages, similarly to uptake of extruded nuclei in pyrenocytes. Another important aging change in RBCs is a decreased ability to produce ATP. This tin can lead to decreased membrane deformability. If RBCs cannot traverse splenic sinusoids well due to decreased deformability, this signals splenic macrophages to remove them from the apportionment. Within the macrophages, the RBCs are degraded. The amino acids from the globin portion of Hgb are recycled and tin can be used again. The heme portion is broken downward into fe and porphyrin, with the latter being converted to unconjugated bilirubin (in those species that accept biliverdin reductase) or biliverdin (due east.thousand. birds). The iron is either released to transferrin for transport to marrow or stored as ferritin in the macrophage. Thus, the atomic number 26 released from degraded effete red blood cells can be used again in the bone marrow for production of new RBCs. Thus, removal of effecte RBCs past macrophages is a form of extravascular hemolysis, but it is physiologic and non pathologic (RBCs are existence removed once they have completed their lifespan and not prematurely, since the latter would result in anemia). Usually, a very small amount of intravascular hemolysis (popping of the cells in the blood stream) does occur, when aged or damaged RBCs rupture while still in circulation. This tin can occur when the RBC are ATP-depleted. The free hemoglobin is then rapidly cleared from circulation by binding to carrier proteins such as haptoglobin. The hemoglobin:haptoglobin complexes are taken up via scavenger receptors, such as CD163, into the macrophages. Nosotros would never see such a small-scale amount of liberated hemoglobin as hemoglobinemia, thus any hemoglobinemia occurring in vivo is pathologic, indicating red claret cells are lysing (popping or rupturing) in the circulation due to a red blood cell defect (due east.g. phosphofructokinase deficiency), oxidative injury (eastward.1000. red maple leaf poisoning) or the presence of complement or complement fixing antibodies on the RBC membrane (intravascular variants of IMHA).

Source: https://eclinpath.com/hematology/physiology/erythrocytes/

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