438
Cell Signaling During Primitive Hematopoiesis
for
vegfr2
lack all blood and blood ves-
sels, possibly because of failure to recruit
blood and endothelial progenitors to the
correct site where they can receive signals
necessary to commit to an erythroid fate.
VEGF may also be required for progeni-
tor expansion since cultures of
vegfr2
/
ES cells generate reduced total numbers
of hematopoietic progenitors relative to
wild type cells. In zebra±sh,
vegfr2
mutants
produce normal numbers of primitive ery-
throcytes, suggesting that either VEGF sig-
naling is not required for hematopoiesis,
or that another
vegfr2
locus exists. The lat-
ter possibility is feasible as many genes are
duplicated in the zebra±sh genome. In the
chick, VEGFR2 is expressed in prospective
hematopoietic mesoderm, and it has been
demonstrated that VEGFR2 signaling is
required for hematopoietic colony forma-
tion in cultures of prospective yolk sac
mesoderm. However, addition of VEGF
to the culture media does not promote
hematopoietic colony formation, suggest-
ing that a different member of the VEGF
family or a related cytokine may bind to
VEGFR2 and promote hematopoiesis.
A second gene product that is required
for primitive erythropoiesis is
gata2
,a
member of the GATA family of zinc ±nger
transcription factors.
gata2
is expressed in
all vertebrate embryos in a pattern con-
sistent with a role in early hematopoiesis
and
gata2
/
mouse embryos contain two-
to sevenfold fewer circulating primitive
erythrocytes. Culture of mouse
gata2
/
embryonic stem cells (ES cells) and yolk
sac cells in conditions that favor the expan-
sion of hematopoietic progenitors (both
primitive and de±nitive), revealed that
lack of
gata2
caused a decrease in the
total number of all progenitors, includ-
ing erythroid progenitors.
gata2
expression
and/or activity may be regulated by cy-
tokine stimulation, as numerous cytokines
can induce the expansion of HSCs and
progenitors
in vitro
and
in vivo
.A
l
though
gata2
is required for progenitor expansion,
a subsequent reduction in
gata2
expres-
sion in committed erythroid progenitor
cells is required for their differentiation.
Another member of the GATA fam-
ily,
gata1
, is expressed at relatively early
stages of hematopoietic development in
all vertebrates, but its function is only
required after erythroid progenitor cell
commitment as evidenced by differentia-
tion defects in animals that lack functional
gata1
. Numerous genes expressed in de-
veloping and mature erythrocytes such as
scl
, and embryonic
globin
genes contain
GATA sites in their promoters/enhancers.
Mice homozygous for null alleles of
gata1
seemingly lack all circulating primitive
erythrocytes. Closer inspection of periph-
eral blood in these mutants revealed that
nonhemoglobinized proerythroblasts are
present and that they arrest in develop-
ment and undergo apoptosis. Similarly, in
zebra±sh, a point mutation in the
gata1
gene that results in deletion of 79 C-
terminal amino acids has been identi±ed
as the molecular basis of the
vlad tepes
bloodless phenotype. In these mutants,
proerythroblasts do not enter circulation
and hematopoietic cells in the intermedi-
ate cell mass (IM) do not express terminal
erythroid genes.
In the mouse, homozygous mutation of
the friend of GATA gene (
fog1
), a zinc
±nger transcription factor that physically
interacts with
gata1
, arrests primitive ery-
throblast differentiation at the same stage
of differentiation as loss of
gata1
.Fog1and
Gata1 protein interaction has been shown
to be critical for de±nitive erythroid cell
line differentiation and this interaction re-
quires the presence of the N-terminal zinc
±nger of Gata1. Consistent with these ob-
servations, mice in which the
gata1
gene
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