Heiner Westphal,
M.D., Principal Investigator
Yangu Zhao, Staff
Scientist
Lan Chen, Ph.D., Postdoctoral
Fellow
Marat Gorivodsky, Postdoctoral
Fellow
Shiga Hasuike, M.D., Postdoctoral
Fellow
Woon Kyu Lee, Postdoctoral
Fellow
Chang Ma, Ph.D., Postdoctoral
Fellow
Nasir Malik, Ph.D., Postdoctoral
Fellow
Toshinobu Miyamoto, M.D., Ph.D., Postdoctoral
Fellow
Mahua Mukhopadhyay, Ph.D., Postdoctoral
Fellow
Andreas Teufel, M.D., Postdoctoral
Fellow
Alexander Grinberg, D.V.M.,
Senior Research Assistant
Sing-Ping Huang, Senior
Research Assistant
Eric Lee, D.V.M., Senior
Research Assistant
Aaron Powell, Predoctoral
Fellow
Alice Schindler, Predoctoral Fellow
Eric Wong, Guest
Investigator
Victoria Crumity, Research
Assistant
Lisa Williams-Simons, Research
Assistant
For More
Information
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Current work in the Section on Mammalian Molecular Genetics is directed
toward characterizing the function of genes that play pivotal roles in
mouse embryo patterning, axis formation, and organogenesis.
Dkk1 is Required for Embryonic Head Induction and Limb Morphogenesis
Mukhopadhyay, Shtrom, Chen, Grinberg, Huang, Westphal in collaboration
with NIEHRS (German Cancer Research Center, Heidelberg) and J.C. Izpisúa-Belmonte
(Salk Institute)
During the past 12 months, we have focused on studying the function of
Dkk1, a member of the Dickkopf family of secreted proteins. The Dkk1
gene product has previously been described as a secreted Wnt inhibitor
of Xenopus and is thought to be involved in inductive signaling.
We performed a phenotypic analysis of mouse embryos that carry a null
mutation in the Dkk1 gene.
![The photograph shows two embryos at a late stage of gestation, a Dkk1 null mutant (right) and a control littermate (left).](graphics/westphal_fig32.jpg)
FIGURE 32
The photograph shows two embryos at a late stage of gestation, a Dkk1
null mutant (right) and a control littermate (left). As
seen here, the mutant lacks head structures anterior of the midbrain.
An analysis of chimeric mice showed that Dkk1 is required in anterior
axial mesendoderm, but not in anterior visceral endoderm, to promote head
formation. Dkk1 is unique among several growth factor antagonists
linked to Spemann organizer function (e.g., noggin, chordin, follistatin,
cerberus, frzb) because it is the first one whose ablation results
in an organizer-related phenotype, i.e., an axial defect. This implies
that the function of the ligand is irreplaceable during a critical time
of early embryonic development. As such, Dkk1 null mutants represent
an important model for studies of head induction in vertebrates.
In addition, Dkk1 null mutant embryos display duplications and
fusions of forelimb digits. Molecular characterization of this phenotype
together with misexpression analysis in chick limbs indicates a role for
Dkk1 in the apical ectodermal ridge and in programmed cell death.
Our results reveal a requirement for inhibition of Wnt signaling during
mouse axis formation and limb morphogenesis. The observed phenotype identifies
Dkk1 as a modulator of Wnt activity involved in regulating the
balance between cell proliferation and cell death during limb outgrowth.
To the best of our knowledge, our effort represents the first instance
in which a gene product has been directly implicated in this still poorly
understood process. We find that lack of Dkk1 activity (and, hence, an
increase of Wnt activity ) correlates with an increase of FGF activity
in the apical ectodermal ridge. This illuminates an important relationship
between FGF and Wnt signaling during limb outgrowth.
Functions of LIM-Homeobox Genes during Organogenesis
Zhao, Powell, Malik, Wong, Westphal
A long-term project of the laboratory concerns the function of LIM-homeobox
(Lhx) genes. The Lhx genes encode transcription factors that exert
crucial control functions during the development of invertebrate and vertebrate
organisms. We have used a loss-of-function approach to analyze functions
of various LIM-homeobox (Lhx) genes in the developing mouse embryo.
From our observation of Lhx gene action in the prospective brain,
pituitary gland, spinal chord, gonad, and other fields of the developing
embryo, we have noted common themes. The Lhx genes become active
as cells begin to express determinants that convey specific identities.
The individual or combined activities of individual members of the Lhx
gene family in different types of precursor cells suggest common mechanisms
leading to cell proliferation and initial differentiation, a prerequisite
for correct arrangement of nascent tissues in the developing embryo. An
ongoing study is addressing the individual and combined functions of two
closely related Lhx genes, Lhx2 and Lhx9, during
brain formation. The two genes display distinct and partially overlapping
expression patterns in different regions of the developing forebrain.
In an effort to distinguish between the genes' individual and redundant
functions in neuronal precursor cells, we have generated mutants that
carry null alleles of both genes. From the analysis of the mutant embryos,
we expect to derive detailed knowledge concerning pathways that control
cell differentiation and migration patterns in the nascent forebrain.
Ldb1, a Cofactor of LIM-Homeobox Gene Action
Mukhopadhyay, Teufel, Chen, Fujii, Westphal
While our knowledge of Lhx gene function has been considerably
advanced through mutant analysis, the mechanism of action of the Lhx
gene family has remained enigmatic. Lhx proteins can engage in complex
formation with the family of LIM binding proteins as well as with other
proteins. Such a complex formation appears to be a necessary functional
requirement in different cellular contexts. Therefore, we hope that knowledge
of the composition of such complexes and of interference with their assembly
in specific cells and tissues will bring us closer to establishing a framework
for understanding the mechanism of Lhx action. Studies with CHIP, the
Drosophila homolog of Ldb, suggest that the LIM binding proteins
play a central role in the assembly of polypeptide complexes. CHIP is
maternally supplied to the egg and can interact with a diverse array of
homeodomain proteins to mediate key developmental regulatory events. If
the same holds for Ldb proteins, their ablation should have negative effects
on vertebrate development very early on, when homeodomain factors begin
to exert their patterning activities. This is indeed the case. We have
ablated the functions of the LIM binding proteins Ldb1 and Ldb2 by targeted
mutagenesis of the respective genes. While we observed no phenotype in
Ldb2 KO mutants, our preliminary findings indicate that knockout
of Ldb1 results in an early and lethal embryonic phenotype. Severe
truncation of anterior structures, frequent duplication of the primitive
streak, lack of a heart, and defective extraembryonic structures all indicate
a breakdown of multiple regulatory circuits, suggesting a simultaneous
functional requirement of Ldb1 in the context of different key regulators
of development.
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PUBLICATIONS
- Fiorenza
MT, Mukhopadhyay M, Westphal H. Expression screening for Lhx3 downstream
genes identifies Thg-1pit as a novel gene involved in pituitary development.
Gene 2001;278:125-130.
- Mukhopadhyay
M, Shtrom S, Rodriguez-Esteban C, Chen L, Tsukui T, Gomer L, Dorward
DW, Glinka A, Grinberg A, Huang SP, Niehrs C, Izpisúa-Belmonte
JC, Westphal H. Dickkopf 1 is required for embryonic head induction
and limb morphogenesis in the mouse. Dev Cell 2001;1:423-434.
- Paylor
R, Zhao Y, Libbey M, Westphal H, Crawley JN. Learning impairments
and motor dysfunctions in adult Lhx5-deficient mice displaying hippocampal
disorganization. Physiol Behav 2001;73:781-792.
- Westphal H, Hobert O. LIM homeodomain proteins. In: Creighton T,
ed. Encyclopedia of molecular medicine. New York: John Wiley & Sons,
2001;3:1922-1925.
- Zhang
Z, Kundu GC, Zheng F, Yuan CJ, Lee E, Westphal H, Ward J, DeMayo F,
Mukherjee AB. Insight into the physiological function(s) of uteroglobin
by gene-knockout and antisense-transgenic approaches. Ann NY Acad Sci
2000;923:210-233.
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