| Regulation
of cell growth and differentiation by post-translational modification
of transcription factors
Research in the Gill
laboratory is directed towards understanding the molecular mechanisms
that regulate transcription. Failure to properly regulate transcription,
that is to turn the right genes on or off at the right place and
time, contributes to diverse pathological conditions including developmental
abnormalities, degenerative diseases and cancer.
In the Gill laboratory,
we are investigating how post-translational modification of the
Sp family transcription factors Sp3 and Sp4 by SUMO and other modifications
contributes to complex programs of gene expression important for
cell proliferation, differentiation, and viability, particularly
in the mammalian nervous system.
The major efforts in the laboratory are directed towards:
(1) understanding how SUMO and SUMO-specific proteases regulate
gene expression
(2) understanding how regulation of Sp3 and Sp4 contribute to cell
type-specific and activity-dependent
gene expression in post-mitotic neurons.
These studies
have broad implications for understanding how signal-dependent regulation
of transcription factor activity by post-translational modifications
contributes to the proliferation, differentiation, and apoptosis
pathways that are disregulated in human diseases including diabetes,
cancer and neurodegenerative disease.
Regulation of gene expression by SUMO and SUMO-specific
proteases
The Small ubiquitin
related modifier, SUMO, has been shown to covalently modify
a large number of proteins with important roles in many cellular
processes including gene expression, chromatin structure, signal
transduction and maintenance of the genome. Sp3 is a dual function
transcription factor that can activate or repress transcription
dependent on context. We have reported that SUMO modification is
required for the repressor activity of Sp3 (Ross et al., 2002b).
Polymorphisms in binding sites for the related Sp1, Sp3, and Sp4
transcription factors have been correlated with human disease risk
for osteoporesis, diabetes, and cancer. Our findings suggest that
SUMO modification contributes to the complexity of gene expression
programs dependent on the widely used Sp1/Sp3 binding site. Furthermore,
our studies were the first to show that SUMO has an intrinsic repression
function which suggests a general mechanism for SUMO-dependent repression
of many transcription factors. Using a proteomics approach, we have
identified several chromatin modifying complexes that associate
non-covalently with SUMO (Rosendorff et al., 2006; and unpublished
data). We are investigating how these SUMO-associated proteins contribute
to SUMO-dependent regulation of transcription.
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| SUMO-modification
regulates Sp3-dependent gene expression. According
to this model, SUMO-modified Sp3 recruits a co-repressor (CoR)
containing a SUMO-binding domain (SBD) to repress transcription.
SUMOylation is reversible by the action of SUMO-specific proteases
(SENPs) and non-SUMOylated Sp3 activates transcription. |
Post-translational modification
by SUMO is reversible and we are investigating the biochemical and
biological activities of the mammalian SUMO-specific proteases. We
have found that SENP5 is a nucleolar SUMO-specific protease that is
required for cell proliferation (Di Bacco et al. 2006). Our findings
support the hypothesis that the different mammalian SUMO-specific
proteases have unique biological functions dependent on distinct substrate
specificities which are regulated by both the catalytic and non-catalytic
domains of these enzymes (Hemelaar et al., 2004; Di Bacco et al.,
2006). Further characterization of the SUMO-specific proteases will
provide new insights into the molecular mechanisms that regulate SUMOylation
of Sp3 and other substrates important for cell proliferation, differentiation,
and survival.
Regulation and function of Sp3 and Sp4 in post-mitotic
neurons
Precise temporal, cell
type-specific, and quantitative control of gene expression is critical
for the normal development and function of the mammalian brain.
The Sp transcription factors regulate expression of genes implicated
in neurodegenerative disease onset or progression. We have reported
that binding sites for Sp1/Sp3/Sp4 are required for neuron-specific
expression of the cdk5 activator p35 (Ross et al., 2002a). Improper
activity of the cdk5/p35 kinase has been implicated in the pathogenesis
of Alzheimer's disease. We are investigating the mechanisms that
regulate the activity of the Sp1, Sp3 and Sp4 transcription factors
in post-mitotic neurons during differentiation and in response to
synaptic activity.
| Morphology
of a cerebellar granule neuron. Cerebellar granule
neurons were visualized after transfection with GFP in cerebellar
slices. Evident features are: the cell body (asterisk), the
“T” shaped axon (arrowhead), and several mature
dendrites with “claw” structures at their ends that
make synapses with neighboring neurons. (Image from Belen Ramos). |
The pattern of dendrites
extended by a neuron is important for the formation of proper connections
in the nervous system. Many mental retardation disorders are associated
with defects in dendritic patterning. The genetic programs that
regulate dendritic patterning in mammals remain incompletely understood.
We have found that the transcription factor Sp4 regulates dendritic
patterning in the developing cerebellum (Belen Ramos, unpublished
data). We are currently investigating target genes that act downstream
of Sp4 to regulate dendritic development. Interestingly, reduced
Sp4 levels in mice have been associated with deficits in learning
and memory. Our ongoing studies of Sp transcription factor function
and regulation in neurons will provide insight into the fundamental
mechanisms that regulate the development and function of the mammalian
nervous system. This knowledge will provide a foundation to understanding
how these mechanisms are subverted in neurodevelopmental and neurodegenerative
diseases.
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