IMP Research Report 2002

Midbrain and kidney development
The midbrain and cerebellum develop from an
organizing center at the midbrain-hindbrain boundary
(MHB), which secretes the signaling molecule Fgf8. The
three transcription factors of the Pax2/5/8 family are
co-expressed in this embryonic brain region. Gene
targeting and transgenic analyses have demonstrated
Figure 2: Dual role of Pax5 in B-lymphopoiesis. Pax5 activates
B-lymphoid genes (green) and simultaneously represses lineageinappropriate
genes (red).
genes. To systematically analyze the transcriptional
function of Pax5, we have used cDNA microarray
screening to identify a multitude of novel Pax5-regulated
genes. One of the activated Pax5 target genes codes
for the central adaptor protein BLNK, which couples
signaling from the (pre)B cell receptor to transcriptional
changes in the nucleus (Figure 2). The majority of
identified genes are, however, repressed by Pax5.
These genes are normally expressed during erythroid,
myeloid or T-lymphoid differentiation, demonstrating
that the Pax5-deficient progenitors promiscuously
express genes from different hematopoietic lineages.
We are currently testing the hypothesis that this
promiscuous gene expression is responsible for the
developmental plasticity of early progenitors.
that Pax2, the earliest marker of the MHB region, is
essential not only for Pax5 and Pax8 expression, but
also for activation of the organizer signal Fgf8 itself.
BAC transgenic analysis has identified the transcription
factor Oct3/4 as an upstream regulator of the early
MHB-specific enhancer of Pax2. Consequently, MHB
development is controlled by the following regulatory
cascade: Oct3/4 Pax2 Fgf8. Pax2, together with
Pax8, is also the earliest marker of kidney development.
We have shown that Pax2 and Pax8 together are
essential for the commitment of intermediate mesoderm
cells to the nephric lineage, as Pax2, Pax8 double
mutant embryos fail to form even the pronephros – the
earliest stage of kidney development (Figure 3A, B).
Conversely, retroviral misexpression of Pax2 is sufficient
to induce ectopic nephric structures in the intermediate
mesoderm (Figure 3C). We are currently characterizing
novel Pax target genes involved in the specification of
MHB and kidney development.
We have also investigated the lineage commitment
function of Pax5 by forced expression in hematopoietic
stem cells and early progenitors. Pan-hematopoietic
Pax5 expression strongly promotes B cell development
at the expense of T-lymphopoiesis, whereas myeloid
and erythroid development is only minimally affected.
Pax5 thereby interferes with T-lineage commitment and
early thymocyte development by directly repressing the
transcription of the T cell specification gene Notch1
(Figure 2). This ectopic expression system allows us
now to study the role of Pax5 in controlling V(D)J
rearrangements at the immunoglobulin heavy-chain
locus in heterologous T cells.
Contact: busslinger@imp.univie.ac.at
Figure 3: Nephric lineage specification by Pax2 and Pax8. (A, B)
The expression of -galactosidase from the Pax2 mutant allele was
used as a kidney-specific marker to visualize the development of
the pronephros (pn) and mesonephros (ms) in mouse embryos of
the indicated genotypes. (C) Formation of an ectopic kidney
(asterisks) in chicken embryos that were injected with a Pax2-
expressing retrovirus. Lim1 staining identifies the tubules (tu) and
nephric duct (nd) of the developing kidney.
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