Blood consists of red and white cells which, along with platelets, are all suspended in plasma. All peripheral blood cells are derived from a single cell type: the stem cell (also known as a pluripotential, pluripotent or haemopoietic stem cell). These stem cells reside in the bone marrow, alongside additional cell types, including (marrow) stromal cells. Pluripotential stem cells have the capacity to undergo prolonged or indefinite self-renewal.
The details of haemopoiesis presented thus far prompt two very important questions. How is the correct balance between stem cell self-renewal and differentiation maintained? And what forces exist that regulate the process of differentiation? The answer to both questions, in particular the latter, is beginning to emerge in the form of a group of cytokines termed ‘haemopoietic growth factors’ (Figure 1). This group includes:
- several (of the previously described) interleukins (ILs) that primarily affect production and differentiation of lymphocytes;
- colony stimulating factors (CSFs), which play a major role in the differentiation of stemderived cells into neutrophils, macrophages, megakaryocytes (from which platelets are derived), eosinophils and basophils;
- erythropoietin, which is essential in the production of red blood cells;
- thrombopoietin, which is essential in the production of platelets.
Figure 1: Major haemopoietic growth factors described to date
GRANULOCYTE-COLONY STIMULATING FACTOR (G-CSF)
G-CSF is also known as pluripoietin and CSF-β. Two slight variants are known, one consisting of 174 amino acids, the other of 177. These are products of alternative splicing. The smaller polypeptide predominates, and also displays significantly greater biological activity than the larger variant. It is synthesized by various cell types, and functions as a growth and differentiation factor for neutrophils and their precursor cells. It also appears to activate mature neutrophils (which are leukocytes capables of ingesting and killing bacteria). G-CSF also appears to act in synergy with additional haemopoietic growth factors to stimulate growth/differentiation of various other haemopoietic progenitor cells. In addition, this cytokine promotes the proliferation and migration of endothelial cells.
MACROPHAGE COLONY-STIMULATING FACTOR (M-CSF)
M-CSF serves as a growth, differentiation and activation factor for macrophages and their precursor cells. It is also known as CSF-1. This cytokine is produced by various cell types. The mature form is a glycoprotein (containing three potential N-linked glycosylation sites), and its 3-D structure is stabilized by multiple disulphide linkages. Three related forms of human M-CSF have been characterized. All are ultimately derived from the same gene and share common C- and N-termini. The largest consists of 522 amino acids with the 406 and 224 amino acid forms lacking different lengths of the internal sequence of the 522 form. The molecular masses of these mature M-CSFs are in the range 45-90 kDa.
GRANULOCYTE-MACROPHAGE COLONY STIMULATING FACTOR (GM-CSF)
GM-CSF is also known as CSF-a or pluripoietin-a. It is a 127 amino acid, single chain, glycosylated polypeptide, exhibiting a molecular mass in the region of 22 kDa. Its 3-D structure exhibits a bundle of four a-helices and a two-stranded antiparallel b-sheet. It thus resembles the other CSFs. GM-CSF is produced by various cells and studies have indicated that its biological activities include:
- Proliferation/differentiation factor of haemopoietic progenitor cells, particularly those yielding neutrophils (a variety of granulocyte) and macrophages, but also eosinophils, erythrocytes and megakaryocytes. In vivo studies also demonstrate this cytokine’s ability to promote haemopoiesis.
- Activation of mature haemopoietic cells, resulting in: enhanced phagocytic activity; enhanced microbiocidal activity; augmented anti-tumour activity; enhanced leukocyte chemotaxis
LEUKAEMIA INHIBITORY FACTOR (LIF)
LIF is an additional haemopoietic growth factor. It is also known as HILDA (human interleukin for DA cells) and hepatocyte stimulatory factor III. It is produced by a range of cell types, including T cells, liver cells and fibroblasts. LIF is a 180 amino acid, heavily glycosylated, 45 kDa protein. It affects both haemopoietic and non haemopoietic tissue, often acting in synergy with other cytokines, particularly IL-3. It stimulates the differentiation of macrophages and promotes enhanced platelet formation. It also prompts synthesis of acute phase proteins by the liver and promotes increased bone resorption. The greatest concentrations of LIF receptors are associated with monocytes, embryonic stem cells, liver cells and the placenta. The receptor complex is composed of two transmembrane glycoproteins: the 190kDa LIFRa-chain which displays affinity for LIF, and a b-chain (gp130), which also forms part of the IL-6 receptor.
ERYTHROPOIETIN (EPO)
EPO is an atypical cytokine in that it acts as a true (endocrine) hormone and is not synthesized by any type of white blood cell. It is encoded by a single copy gene, located on (human) chromosome 7. The gene consists of four introns and five exons. The mature EPO gene product contains 166 amino acids and exhibits a molecular mass in the region of 36 kDa EPO is a glycoprotein, almost 40% of which is carbohydrate. Three N-linked and one O-linked glycosylation site are evident. The O linked carbohydrate moiety apears to play no essential role in the (in vitro or in vivo) biological activity of EPO. Interestingly, removal of the N-linked sugars, while having little effect on EPO’s in vitro activity, destroys its in vivo activity. The sugar components of EPO are likely to contribute to the molecule’s: solubility; cellular processing and secretion; in vivo metabolism. EPO stimulates erythropoiesis by:
- increasing the number of committed cells capable of differentiating into erythrocytes;
- accelerating the rate of differentiation of such precursors;
- increasing the rate of haemoglobin synthesis in developing cells.
THROMBOPOIETIN
Thrombopoietin (TPO) is the haemopoietic growth factor now shown to be the primary physiological regulator of platelet production. Although its existence had been inferred for several decades, its purification from blood proved an almost impossible task, due to its low production levels and the availability of only an extremely cumbersome TPO bioassay. Its existence was finally proved in the mid-1990s when thrombopoietin cDNA was cloned. This molecule is likely to represent an important future therapeutic agent in combating depressed plasma platelet levels, although this remains to be proved by clinical trials. A number of disorders have been identified that are primarily caused by the presence of abnormal platelet levels in the blood. Thrombocythaemia is a disease characterized by abnormal megakaryocyte proliferation, leading to elevated blood platelet levels. In many instances, this results in an elevated risk of spontaneous clot formation within blood vessels. In other instances, the platelets produced are defective, which can increase the risk of spontaneous or prolonged bleeding events.
Reference:
Walsh, G. (2013). Biopharmaceuticals: biochemistry and biotechnology. John Wiley & Sons.