Red blood cell - production and survival Flashcards
RBC production
2x10power11 a day
Erythropoiesis starts of in the bone marrow with a stem cell (hemocytoblast) and it goes through multiple stages under the influence of cytokines.
We start off with the stem cell, which becomes the proerythroblast. This differentiates into an early (basophilic) erythroblast.
At this stage, Phase 1 is occurring in the cell which is ribosome synthesis. The early erythroblast then differentiates into the late (polychromatophil) erythroblast and haemoglobin starts to accumulate in the cell as it is synthesised (phase 2).
The erythroblast then differentiates into a normoblast. Here, the nucleus gets extruded to make the reticulocyte, which has no nucleus (Hb is still being formed from left over mRNA).
This eventually forms the erythrocyte.
Regulation of RBC production
Number of rbc is inversely related to ambient O2 pressure -> hence if o2 levels low in blood means there is hypoxia. the blood will pas through kidneys and sense the low o2 and begin to produce epo which will be released to circulation and stimulate rbc production in bone marrow
Key regulator of feedback loop is EPO
what is epo?
Epo is a glycosylated polypeptide hormone.
90% produced by the kidneys as a result of low oxygen tension in the kidneys
It stimulates marrow production of rbc
EPO and HIF
Studies of EPO regulation led to the discovery of hypoxia-inducible transcription factors (HIFs) which is produced either in liver or kidneys
HIF determines Epo gene expression in kidneys;
it also enhances expression of iron-absorbing genes
Iron absorbing proteins is ferroportin
While EPO is a growth factor for erythroid precursors, iron is their most important substrate, being essential for the synthesis of haemoglobin, the molecular oxygen carrier.
What regulates Iron?
Hepcidin, which is predominately produced by the liver, serves as a master regulator of iron homeostasis.
Hepcidin inhibits intestinal iron reabsorption and iron release from macrophages, thereby reducing iron availability by binding to ferroportin and induces it internalisation and degradation.
The hormoneerythroferrone, produced by erythroblasts, acts on hepatocytes to suppress hepcidin production.
HIF-α regulation
In the presence of sufficient oxygen, HIF-α is hydroxylated by PHD (prolyl hydroxylase domain). Hydroxylated HIF-α is recognized by vHL, which results in proteasomal degradation
In hypoxic conditions or PHD inhibition, HIF-α accumulates in the cytosol and forms a heterodimer with HIF-β, the hypoxia-insensitive unit. The heterodimer translocates to the nucleus and acts as a transcriptional factor that binds to HRE (hypoxia related element) and acts as a transciption factor to stimulate gene transcription for production of the agent needed for RBC production
oxygen-sensing mechanism
The oxygen-dependency of prolyl-hydroxylase domain enzymes (PHDs), which target HIFα for proteasomal degradation, provides the basis of a widespread oxygen-sensing mechanism
Iron soruces and absoprtion
Iron
Sources:
meat, eggs, vegetables, dairy foods
Absorption:
Normal Western diet provides 15mg daily.
5-10% absorbed (1mg) principally in duodenum and jejunum.
gastric secretion (HCl) and ascorbic acid help absorption.
iron absorption regulation
Iron absorption is regulated by DMT-1 and ferroportin;
Controlled by: total body iron status; intracellular iron levels; Erythropoiesis
DMT-1 at the brush border of the enterocyte transporting iron into cells and ferroportin at the basal membrane then transport iron from enterocytes into circulation.
Causes of Iron deficiency
Causes of Iron deficiency
• Decreased uptake due to inadequate intake in diet or malabsorption in GI tract
• Increased demand by our body e.g. in pregnancy or if having a growth spurt
• Increased loss of iron due to GI bleed or during menstruation
Iron deficiency leads to microcytic anaemia, where RBCs are smaller and have low levels of haemoglobin
Vitamin B12 and Folic acid
Both essential for RBC maturation & DNA synthesis
Both needed for formation of thymidine triphosphate.
B12 is coenzyme for methionine synthase in methylation of homocysteine to methionine.
Deficiency leads to failure of nuclear maturation hence RBC may not be the only cell affected
Causes of vitamin B12 deficiency
Causes of vitamin B12 deficiency include:
• Inadequate uptake e.g. vegans
• An absorption defect like blind loop syndrome or tropical sprue
• IF deficiency (B12 usually binds to intrinsic factor (IF) so if you have a deficiency you cannot transport B12 to the bone marrow) may be caused by pernicious anaemia, gastrectomy (as IF found in small intestine).
where is folate abd b12 absorbed?
Folate is absorbed in proximal SI
B12 is absorbed in distal SI by the help of IF
Causes of folate deficiency
Causes of folate deficiency include:
• Inadequate intake, a folate free diet causes deficiency within a few weeks
• Malabsorption e.g. coeliac disease
• Excess utilisation of folate e.g. pregnancy, haemolysis, cancer
• Drugs e.g. anticonvulsants may induce malabsorption of folate
Effects of folate & B12 deficiencies
If we have folate and or B12 deficiency we end up with macrocytic (large RBC) anaemia.
We may also have reduced WBC and platelet count, megaloblastic (larger nuclei) change in bone marrow, megaloblastic anaemia, sore tongue, abnormal gut mucosa.
only b12 defiency
In B12 deficiency you also may get demyelination of CNS.
What else can affect RBC prod?
Renal dx - ineffective erythropoiesis (low epo)
Reduced BM erythroid cells
Aplastic anaemia (serious condition affecting the blood, where the bone marrow and stem cells do not produce enough blood cells)
Marrow infiltration by leukaemia or other malignancies
Classification of Haemolytic Anaemia
Haemolytic Anaemia can be classified as:
Hereditary (congenital) or Acquired)
Intrinsic or extrinsic
Intravascular or extravascular
RBC survival
RBC circulate for approx. 120 days without nuclei or cytoplasmic organelles
300 miles travelled through microcirculation
7.8 microns diameter
capillaries as small as 3.5 microns
RBC metabolism and survival
Components needed for function and survival are present in matured rbc.
Function does not require the consumption of much energy.
rbc therefore are capable of limited metabolic activity.
Metabolic processes include : Embden-Meyerhof pathway Hexose monophosphate shunt (or PPP) Luebering Rapaport Shunt Methaemoglobin reductase pathway
RBC metabolic pathways
Two main enzymes are involved in red cell metabolism that allow the red cell to stay alive during its 120-day life span.
These are:
- Glucose-6-Phosphate Dehydrogenase (G-6-PD)
- Pyruvate Kinase (PK)
These two enzymes support two main metabolic pathways; the pentose phosphate pathway and glycolytic pathway.
- G6PD helps to generate NADPH, NADPH is used for glutathione (GSSG) reduction.
- Reduced glutathione (GSH) maintains Hb solubility and membrane, it also protects red cell proteins from oxidative stress (i.e. it is an antioxidant).
RBC metabolic pathways in and out overview
1.Glycolysis – energy- ATP
Na/K pump
3 Na+ out 2 K+ in
ATP -> ADP+Pi
2.HMS – reducing power - NADPH/GSH
3.Rapoport Luebering shunt
2,3 Bi-PhosphoGlycerate (2,3 BPG) – modulates O2 binding to Hb
Glycolytic pathway
Generates energy in ATP;
to maintain red cell shape and deformability
regulates intracellular cation conc. via cation pumps (Na/K pump),
ADP is catalysed by pyruvate kinase to produce ATP and pyruvate
Pyruvate Kinase (PK) deficiency
PK deficiency will lead to ATP depletion, ATP depleted cells lose a large amount of potassium and water, this makes them dehydrated and rigid
As a result
• The cation pumps fail to function
o This causes chronic non-spherocytic haemolytic anaemia -> shape of RBC changes
o Leads to excess haemolysis leads to jaundice and gallstones.
Pentose Phosphate pathway
Oxidants cause RBC damage and cause Hb to precipitate out (producing Heinz bodies).
GSH acts as an antioxidant (prevents oxidative stress) by removing these oxidants to convert to GSSG.
- Glucose is converted to glucose-6-phosphate (G6P) which in turn is converted to 6 phosphoglycerate (6PG), by the enzyme G6PD (glucose-6-phospate dehydrogenase).
- This produces NADPH, which is used to reduce glutathione (GSSH) to reduced glutathione (GSH) which acts as the antioxidant as mentioned.
The pentose phosphate “shunt” pathway provides NADPH to maintain GSH
G6PD deficiency
NADPH and GSH generation impaired
Acute haemolysis on exposure to oxidant stress: oxidative drugs, fava (broad) beans or infections
Hb precipitation – Heinz bodies
G6PD deficiency most common known enzymopathy, estimated to affect 400 million people worldwide.
Luebering Rapoport Shunt
2,3 DPG binds to deoxyHb to stabilise at lower O2 affinity state
this makes it harder for O2 to bind to Hb; favours O2 release
Methaemoglobin reductase pathway
it reduces cytochrome b5(cb5), which in turn reduces oxidized ferric ion of haemoglobin.
without this reaction, haem iron would be oxidized to methaemoglobin, which is not a functional oxygen transporter
Red Cell membrane disorders
Hereditary spherocytosis
Loss of membrane integrity, the RBCs become spherical
Common hereditary haemolytic anaemia in N. Europ.
deficiency in proteins with vertical interactions between the membrane skeleton and the lipid bilayer: e.g. ankyrin def
Hereditary elliptocytosis
mutations in horizontal interactions e.g. spectrin, ankyrin; actin, protein 4.1 deficiency.
Haemoglobinopathies
Globin disorders
The genes for globin chains occur in clusters on chromosomes:
•On chromosome 11 (epsilon, gamma, delta, beta globins)
•On chromosome 16 (zeta, alpha 1, alpha 2 globins)
Normal adult blood type has three types of Hb –
•HbA (made from α2β2 chains) – Forms about 90% of Hb
•HbF (α2γ2)
•HbA2 (α2δ2)
Mutations or deletions may lead to abnormal synthesis of globin chain as in the Sickle Cell Diseases.
•Or it could lead to a reduced rate of synthesis of normal alpha or beta globin chains as in the Thalassaemia.
Sickle cell disease
Sickle Cell Disease is a group of Hb disorders with an inherited sickle β-globin gene. The most common is homozygous Sickle Cell Anaemia (Hb SS). But there are also heterozygote conditions (HbS/ βthal, HbSC, HbSD)
Point mutation in the β globin gene: e.g. glutamic acid → valine (HbS)
Insoluble Hb tetramer when deoxygenated → polymerisation
“Sickle” shaped cells
Features of SCD
clinical Painful crises Aplastic crises Infections Acute sickling: Chest syndrome Splenic sequestration Stroke Chronic sickling effects: Renal failure Avascular necrosis bone
lab Anaemia Hb often 65-85 Reticulocytosis Increased NRBC Raised bilirubin Low creatinine
globin disorders
Thalassaemia
Beta-thalassaemia
Loss of 1 B-chain causes mild microcytic anaemia (thalassaemia trait)
Loss of both (B0) causes thalassaemia major
Excess α-chains precipitate in erythroblasts causing haemolysis and ineffective erythropoiesis.
Alpha-thalassaemia
There can be loss of 1, 2, 3 or 4 alpha chains.
diagnosis of thalassaemia
Asymptomatic
Microcytic hypochromic anaemia
Low Hb, MCV, MCH
Increased RBC
Alpha-thalassaemia
Alpha Thal:
Loss of 1 or 2 causes mild microcytic anaemia
Loss of 3 causes moderate anaemia - Hb H disease
Loss of 4 causes death in utero (hydrops fetalis)
Beta Thalassaemia Major
Transfusion dependent in 1st year of life
If not transfused:
Failure to thrive
Progressive hepatosplenomegaly
Bone marrow expansion – skeletal abnormalities
Death in 1st 5 years of life from anaemia
Side effects of transfusion: Iron overload Endocrinopathies Heart failure Liver cirrhosis
Haemolysis/ RBC breakdown
Normal RBC breakdown is extravascular, in macrophages in the spleen which break it down into amino acids (recycled), Iron (recycled, binds to transferrin) and unconjugated bilirubin which travels to the liver, gets conjugated and then is then passed out in urine or faeces. (if too much breakdown, liver overwhelmed so can’t conjugate bilirubin hence increase conjugated bilirubin )
Abnormal breakdown of RBC is intravascular and leads to blood being found in urine.
Low Hb level
LOw red count
Low MCV
High RDW
Microcytic, hypochromic anaemia
