of the Max - MDC

Molecular Mechanisms
in Embryonic Forebrain
Development
Annette Hammes
(Helmholtz Fellow)
Defects in early forebrain development can lead to a fatal disorder, defined as holoprosencephaly
(HPE), in which the cerebral hemispheres fail to separate along the midline. HPE is the most
common brain malformation in human embryos. Megalin, a member of the low density lipoprotein
receptor gene family, is a candidate gene for holoprosencephaly and recent studies have indeed
identified megalin as a novel genetic factor, that affects dorsal midline separation and ventral neuronal
cell fate specification in the developing forebrain. Our group is studying the mechanisms whereby
megalin regulates molecular pathways important for early embryonic forebrain development.
The role of megalin/LRP2 in forebrain development:
Identification of defects in the molecular
pathways underlying holoprosencephaly
(collaboration with Thomas Willnow)
Megalin/Lrp2 is expressed in the neuroepithelium and the
yolk sac of the early embryo. Loss of megalin expression in
mutant mice results in forebrain defects and in holoprosencephaly
(HPE), indicating an essential function in brain
development.
In detailed studies we could show that megalin-deficiency in
the neuroepithelium leads to changes in the expression and
activity of the key morphogens bone morphogenic protein 4
(BMP4), sonic hedgehog (SHH), and fibroblast growth factor
8 (FGF8) that regulate dorso-ventral patterning of the
neural tube. An intricate balance of dorsal (BMPs) and ventral
(SHH) morphogen gradients is crucial for specifying
dorso-ventral forebrain patterning. We could show that
Bmp4 expression and signalling in the rostral and dorsal
neuroepithelium is increased in megalin -/- embryos, Shh
expression is lost in the ventral telencephalon, and Fgf8 is
aberrantly expressed in the rostral midline of the forebrain.
As a consequence of absent SHH activity, ventrally derived
oligodendroglial and interneuronal cell populations are lost
in the forebrain of megalin -/- embryos.
These findings identified megalin as a novel genetic factor
that affects ventral neuronal cell fate specification in the
forebrain and show that lack of megalin is central to the
axial patterning defects and to the pathophysiology of
holoprosencephaly.
Identification of the mechanisms underlying megalindependent
holoprosencephaly and identification of novel
candidate genes for HPE will substantially extend our understanding
of neurodevelopmental disorders such as holoprosencephaly
and the signals that are crucial for normal
forebrain development.
Phenotypic changes in the
subventricular zone (SVZ) of
the lateral ventricles in the
brain of megalin-deficient
mice
Immunofluorescence analysis
shows an altered expression pattern
for glial fibrillary acidic protein
(GFAP) in the SVZ of megalin
-/- mice (B) with a much less
complex staining as compared to
wildtype controls (A). GFAP is a
marker for astrocytes, which are
the neuronal precursor cells in
the SVZ. In addition, the signal
for doublecortin (DCX), a marker
for immature neurons, is reduced
in the SVZ of megalin-deficient
mice. Megalin staining can be
detected in ependymal cells
(arrow) lining the ventricle of
the forebrain in wildtype mice
(A). GFAP (green), DCX (blue),
megalin (red)
One major research goal is to identify the position of megalin
in the hierarchy of known molecular and genetic signalling
pathways and to identify novel megalin-dependent
genes or genetic pathways important for forebrain patterning
and its pathology.
Cardiovascular and Metabolic Disease Research 9