Uses for Recombinant Protein Drugs - Biotechnology and Your Health: Pharmaceutical Applications - page 70

The Production of New Blood Cells
Millions of our blood cells die and are replaced every day. The production system for these
critical cells is an orchestrated process of growth and development of specialized cells from
immature precursors, driven by protein factors. This only happens once we are adults. The
general process is named for the differentiation—the change from an unspecialized
cell to a specialized cell able to do a job because of the presence of particular proteins.
A single type of precursor cell, the hematopoietic stem cell , gives rise to red blood
cells, white blood cells to fight infections and populate our immune system, and platelets
to help heal wounds and plug up leaks in blood vessels.
Except for lymphocytes, each specialized blood cell is a workhorse at the end of its road;
each remains in the circulatory system or in a tissue and then dies, perhaps as a result of
doing its job. Red cells ( erythrocytes ) have lost their nucleus, and platelets are fragments
of precursor cells. Because they have no nuclei, human red cells and platelets cannot divide.
The drivers of the cell division and differentiation of each cell are signal proteins that
dock onto the membrane of a less specialized cell and trigger that cell to go to the next
step to make the particular required proteins; for example, erythropoietin triggers the
developing red cell to manufacture hemoglobin . The cells in the intermediate stage,
those moving from hematopoietic stem cells to more committed cells, have on their
membranes the docking proteins needed to allow the cell to respond to the protein
trigger for the next transition.
Neutrophils , eosinophils , and basophils are infection-fighting cells that develop from
a common precursor cell. Each has distinctive granules in its cytoplasm and is named for
the way those granules take up certain dyes. A protein called granulocyte-stimulating factor
(G-CSF) causes the final step (Figure 5.2). The process can be traced further back to the pro-
duction of the shared granulocyte precursor cell, which is triggered by granulocyte-monocyte
stimulating factor (GM-CSF). These factors were named “colony stimulating factors” because
the scientists who discovered them relied on the ability of these factors to cause colonies,
or groups, of the particular cells to grow in laboratory cultures. Erythropoietin drives develop-
ment of a process through which precursor cells eventually give rise to red cells through
several steps. The same precursor cell, if triggered by different protein factors—interleukin-
11 and thrombopoietin—develops into a cell that clips off bits of itself to make platelets. The
names may seem complicated, but they make sense if you know their sources. Interleukin
is the general name for proteins produced by one white cell that influences another;
11 indicates that it was the 11 th such white cell signal protein to be discovered; poietin at
the end of the names comes from a Greek word that means “make more;” erythro means
“red;” thrombo means “clot;” and “leuko” comes for the Latin word for white.