Dr. Han uses Caenorhabditis
elegans and other model systems to study problems
related to cell signaling, developmental pattern formation and
morphogenesis,
genetic redundancy, and lipid functions.
Cell differentiation, organ morphogenesis, and lipid metabolism in C. elegans.
Due to its
genetic accessibility and other advantages, the nematode
Caenorhabditis elegans has been an enormously powerful model
system for studying many mainstream biological problems including those
closely
related to human diseases. In recent years, research in our laboratory
has
addressed important questions in the following areas.
Spatial and Temporal Control of Cell
Differentiation
Cell-cell communication
plays a key role in directing cells to differentiate during animal
development.
Vulval induction in C. elegans
hermaphrodites is controlled by multiple cell signaling and regulatory
pathways, including a conserved RTK/RAS/MAPK signal transduction
pathway that
induces three epidermal cells to differentiate into vulval
tissues. By screening mutations that either
enhance or suppress the mutant phenotypes of existing mutations, we
have in the
past identified many genes that encode key factors relaying the signals
from
Ras, factors positively or negatively modifying the activity of the
pathway,
and factors involved in regulating the timing of vulval development.
Vulval
induction is also inhibited by functions of the so-called Synthetic
Multivulva
(SynMuv) genes that encode transcription and chromatin-associated
factors,
including the ortholog of the mammalian tumor suppressor RB. Collaborating with Iva Greenwald at HHMI and
Columbia and Paul Sternberg at HHMI and Caltech, we have shown that the
SynMuv
A and SynMuv B gene classes are functionally redundant for
transcriptional repression
of the key target gene lin-3/EGF in
the large epidermal syncytial cell hyp7. This result provided key
insights into
the mechanism of the SynMuv phenotype and underscored the importance of
repressing gene expression in generating specific cell signaling events
for
developmental pattern formation.
Using a
genome-wide screen, we have identified more than thirty genes that
encode
potential chromatin-modifying proteins that antagonize the functions of
class B
SynMuv genes in a variety of cellular and developmental processes
including:
vulval induction, germline-soma distinction, RNA interference, and
somatic
transgene silencing. These results
suggest that multiple chromatin remodeling complexes are involved in
regulating
the expression of specific genes for proper developmental decisions.
Besides the
positional cues provided by several signaling pathways, a developmental
timing
control mechanism mediated by genes in the so-called heterochronic
pathway
provides temporal regulation of vulval development.
In the suppressor screens for factors
involved in vulval differentiation, we have identified mutations in
five genes
that regulate the timing of postembryonic development. We have analyzed
two of
these genes, ain-1 and lin-66,
extensively and revealed their
roles in timing regulation. Our studies of lin-66
indicate that it represses the expression of the master timing
regulator lin-28. We also provide evidence that
the stage specific expression of lin-28
is regulated by multiple independent mechanisms including inhibitory
regulations by lin-66, nuclear
receptor nhr-12, and multiple miRNAs.
Analysis of genes defined by additional suppressors is underway.
Nuclear
Positioning, Cell Migration and Morphogenesis During Development
The
position of the nucleus within a cell is important to the proper
function of a
wide variety of cell types. Collaborating with Robert Horvitz at HHMI
and MIT, we have
previously established the function of UNC-84 and UNC-83 in nuclear
positioning
at the nuclear envelope and defined the SUN domain protein family by
identifying the mammalian SUN1 and SUN2 proteins as the homologs of
UNC-84. Our studies on UNC-83 and ANC-1
established the role of a KASH domain-containing protein in nuclear
migration
and anchorage and the role of the SUN protein in anchoring the KASH
protein at
the nuclear envelope. Collaborating with
researchers at the Institute of Developmental Biology and Molecular
Medicine
(IDM) of Fudan University and Mark Grady/Joshua Sanes at Washington
University,
we also explored the physiological functions of the KASH-SUN
interaction at the
nuclear envelope in mice and Drosophila.
We found that
KASH-containing Syne proteins play critical roles in anchoring both
synaptic
and non-synaptic nuclei in each skeleton muscle cell that contains many
nuclei. In Drosophila,
the homolog is essential in anchoring the nurse cell
nuclei during cytoplasmic transfer of oogenesis. Furthermore, the
student at
IDM determined that SUN1 is required for telomere attachment to the
nuclear
envelope and gametogenesis in mice.
We
have been carrying out genetic screens for genes involved in cell
migration
during morphogenesis and C. elegans
development. We have identified a number of genes that interact with
cell
adhesion molecules, cell migration functions in embryos, and that regulate cell
migration and cytokinesis in larval epidermal cells.
Genetic (or
functional) redundancy by structurally unrelated genes is an extremely
common
biological phenomenon and an impediment for biologists seeking to
determine gene
functions through straightforward genetic approaches. We have designed
and
performed screens to isolate mutations that synthetically interact with
null
mutations in the C. elegans orthologs
of two human tumor suppressor genes RB and Pten. From
analyzing genes acting in concert with
RB, we and David Fay at the
Key
words:
Signal
transduction, cell differentiation, pattern formation, developmental
timing,
morphogenesis, cell migration, nuclear migration and anchorage, Ras,
microRNA,
Rb, Pten, SUN protein, KASH proteins, genetic redundancy, synthetic
phenotype,
tumor suppressor, fatty acid