Research Interests
Monday, 2004 July 05 09:30'11 PDT (GMT-0700) |
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Research in my laboratory uses Drosophila as a model system to focus on two
problems that are relevant to understanding the developmental mechanisms that
underlie congenital defects. One major interest is in determining how nuclear
position and migration within a cell is regulated, and the consequences of disrupting
this process. A second question we are pursuing is how extracellular proteoglycans
affect the response of cells to growth factor signaling.
Regulation of nuclear position and migration: D-nudC and D-Lis1
 Most
differentiated cells have an inherent polarity that is apparent in the uneven
distribution of a subset of macromolecules and organelles. This asymmetry extends
to the nucleus that is usually present at a defined subcellular position and
maintained there through an active process. In differentiated cells the asymmetric
position of the nucleus is likely to facilitate cellular communication, while
intracellular nuclear migration is known to be essential for developmental events
that range from the union of the male and female pronuclei during fertilization
to later stages of organ and tissue differentiation. Although nuclear migration
is ubiquitous, relatively little is known about its mechanistic basis in higher
eukaryotes. Classical studies using specific inhibitors and antibodies have
indicated that nuclear movement is microtubule-dependent. However, the particular
motors involved have not been unambiguously identified, and the genetic and
cellular means by which nuclear migration is regulated are still unknown.
 Work
in my laboratory centers on two genes, Drosophila nudC (DnudC), and Drosophila
Lissencephaly1 (DLis1), that are critical components of an evolutionarily conserved
pathway regulating nuclear migration. These genes act by affecting the activity
of cytoplasmic dynein, a minus-end directed microtubule motor. Consistent with
this, we have recently been able to directly implicate dynein in nuclear migration
by disrupting motor function using a dominant negative approach. These experiments
lead to two unexpected findings - that dynein activity is essential to anchor
the nucleus after migration and that activity of the plus-end directed kinesin
motor is also necessary for nuclear migration and anchoring. Work on nuclear
migration is likely to have clinical significance since mutations in the human
homolog of DLis1 cause Miller-Dieker Lissencephaly, an inherited human birth
defect associated with severe mental retardation. This devastating syndrome
results from a failure of neuronal migration during embryogenesis, suggesting
a link between nuclear and neuronal movement. Questions we are currently focusing
on include, (i) understanding the requirement for DLis1 in coupling the motor
complex with nuclear migration, (ii) using genetic interaction screens with
DLis1 and DnudC to identify other genes in the nuclear migration pathway that
may play regulatory rather than structural roles, and (iii) determining the
extent to which the functional relationship between the NudC and Lis1 proteins
has been conserved.
Proteoglycan-mediated effects on Growth-Factor Signalling: D-EXT2
A second area of research is based on our isolation of mutations
in the Drosophila homolog of Ext2, a gene that causes Hereditary multiple exostoses
(HME), a bone overgrowth syndrome in humans. Biochemical and cell culture studies
suggest that human Ext2 and the closely related Ext1 gene encode co-polymerases
that synthesize glycosaminoglycans, polysaccharide chains attached to extracellular
proteins such as heparan sulfate. Thus it would be predicted that mutations
in Ext1 and Ext2 would have similar phenotypes. Surprisingly, our studies indicate
that ext2 mutants show defects in multiple growth factor signaling pathways
in contrast to the Ext1 ortholog tout velou (ttv) that is known to only affect
diffusion of the growth factor Hedgehog (Hh).
Our initial objectives are to characterize the phenotype of mutations
in ext2, to establish the role of ext2 in signaling by growth factors of the
Wnt, Hh and TGFß family, and to investigate the basis for the differential
requirement for ttv and ext2. This work is relevant to birth defects at two
levels. In a direct sense, it is pertinent since mutations in the human Ext2
gene cause HME, an inherited autosomal dominant disorder that results from bone
overgrowth initiated during childhood. Affected individuals have limbs that
may be shorter than normal or develop unevenly in addition to showing characteristic
bony projections (exostoses). At another level, the fact that Ext1 and Ext2
are likely to modulate multiple growth factor pathways emphasizes the need to
understand their roles in patterning and development during embryogenesis.
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