By Diane Krause, MD, PhD, and Stephanie Halene, MD
2008-09-01
Drs. Krause and Halene indicated no relevant conflicts of interest.
Isern J, Fraser ST, He Z, et al. The fetal liver is a niche for maturation of primitive erythroid cells. Proc Natl Acad Sci USA. 2008;105:6662-7.
Blood formation in development is characterized by two distinct
stages. Primitive hematopoiesis, which is initiated by a population of
primitive hematopoietic stem cells, is progressively replaced by
definitive hematopoiesis, which is derived from definitive
hematopoietic stem cells that maintain hematopoiesis throughout
adulthood. During murine embryogenesis, primitive erythroid cells
(EryP) develop in the yolk sac blood islands on embryonic day 7.5
(E7.5), enter the circulation by E9.5, and remain nucleated until
E12.5. In contrast, definitive erythroid cells (EryD), which appear
later in fetal development, are enucleated prior to entering the blood
stream.
In order to better understand how enucleation of EryP is regulated,
the investigators designed an elegant system to track EryP. In this
system, the promoter elements of the ε-globin gene drive expression of
green fluorescent protein (GFP), such that GFP expression is restricted
to EryP. This allows tracking of EryP throughout development even
during E12 through E16 when progressively greater numbers of EryD
appear. The investigators followed maturation and expansion of EryP as
well as surface markers on the cells before, during, and after
enucleation. They discovered a transient upregulation of particular
integrins from E12.5 to E14.5, with little expression of these
integrins on EryP before or after this time frame. The investigators
hypothesized that upregulation of these integrins promotes adhesion of
EryP to macrophages in the developing fetal liver forming so-called
erythroid blood islands in which developing erythroid cells form
rosettes around central macrophages, which then phagocytose the nuclei
as the erythroid cells enucleate. To test this hypothesis, the
investigators engineered erythroid blood islands in vitro
using isolated fetal liver-derived macrophages from one specific stage
of fetal liver development and EryP from multiple different stages of
development. They found that the EryP from days E12.5 through E14.5
adhered to the macrophages, while those from earlier and later stages
did not. The two-day window during which EryP adhere corresponds to the
time during embryogenesis when EryP upregulate integrins and enucleate.
By E15.5, enucleation of EryP is complete.
As an erythrocyte enucleates, the nucleus is not just expelled from
the cell, but remains surrounded by a thin rim of plasma and plasma
membrane and is pinched off from the remaining erythroblast by
formation of an actin ring. This results in two cells. One is the young
enucleated RBC, which can re-enter the circulation, and the other
contains the membrane-bound nucleus. A key feature of this process is
the asymmetric distribution of plasma membrane components between the
resulting RBC versus the expelled nucleus. To test the hypothesis that
integrins are enriched on the plasma membrane surrounding the expelled
nucleus may be responsible for the retention of the nuclei near the
macrophages, while allowing the RBC to re-enter the circulation, the
investigators took their model a step further. They targeted the GFP
protein expressed under the control of the ε-globin gene to enter the
nucleus, which allowed them to specifically track the "pinched-off"
nucleus. As predicted, expression of the integrins was much higher on
the nuclei than on the enucleated RBCs, and the nuclei were retained in
the fetal liver.
The findings presented in this work not only
represent an astute observation of the distinct development of
primitive versus definitive erythropoiesis, but also employ an elegant
model to elucidate the mechanisms underlying these differences. In an
era when exploration into stem-cell therapies employing more and more
primitive cell types is turning into a realistic hope, a deeper
understanding of developmental mechanisms will certainly contribute to
the field.
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