Regulation of gene expression in a multicellular organism occurs at
several levels. Most of this regulation occurs at the level of
transcription. The impact of this regulation on the cell impacts
cell growth, development and death. All three of these cellular
fates have profound impacts on a multicelled organism, from proper
development to cancer.
Development
In 1995 the Nobel
Prize in Medicine was awarded for the experiments that led to our
understanding of development in Drosophila.
Development in a multicellular organism requires the
activation of specific genes at specific times. This is controlled
by maternal factors and by cascades of transcription factors.
Maternal factors, such as the Drosophila transcription
factor bicoid, are present in a gradient in a developing
embryo.
The transcription factor Bicoid stimulates the
production of another transcription factor, Hunchback (Hb). Bicoid
concentrations are highest near the head of the embryo.
Translation of Hb mRNA is blocked by another maternal
factor called Nanos that is foud at the tail of the embryo.
Hunchback in turn is a transcription factor that turns
on the expression of other transcription factors. This cascade of
transcription factors initiates the synthesis of specific genes in
specific locations and times during development.
Cell Cycle
The 2001 Nobel
Prize in Medicine was awarded for work on the regulation of the cell
cycle.
Researchers mutagenized yeast and discovered several
mutants that arrested in specific stages of the cell cycle. They
cloned the genes that had been mutated in each different cell line and
identified the genes by DNA sequencing. One of the proteins (cdc2)
was a kinase.
Antibodies were made against cdc2 and it was found that the
concentrations of these kinases were present throughout the cell cycle. However,
adding radioactive ATP showed that the activity of the kinase peaked
during mitosis
When researchers labeled proteins with 35S
amino acids in dividing sea urchin eggs, they found that most proteins
incorporated increasing label over time.
However, the radioactivity in some proteins increased and then
disappeared, only to reappear on the next cell cycle.
These proteins were named cyclins.
We now know that cyclins bind to and activate kinases.
These activated kinases then phosphorylate proteins, allowing a stage of
the cell cycle to proceed. The next stage of the cell cycle will not
start until the cyclins are degraded.
For a much more in-depth exploration of the regulation of
the cell cycle, visit this link
by Dr. Forsburg at the Salk Institute.
Apoptosis
The 2002 Nobel
Prize in Medicine was awarded for research on programmed cell death,
or apoptosis.
The nematode C. elegans grows from a single
fertilized egg into an adult worm with 959 cells. In the process 131
cells are programmed to die. This programmed cell death, or
apoptosis is important to the proper development of most multi-celled
organisms, including humans.
Scientists were able to generate mutants in apoptosis and
isolate four genes which were necessary for apoptosis.
CED-3 and CED-4 mutations resulted in all 1,090 cells
appearing in the adult. CED-3 is a cysteine protease which cuts
after aspartate residues. These enzymes were termed Caspases.
Several proteins were discovered to be involved in this
apoptosis cascade, including cell surface receptors, mitochondrial proteins and the
Caspases .
Caspases also function in a cascade, where Caspase 9 cleaves
and activates Caspase 3, which in turn cleaves and activates Caspase 6,
etc.
Cancer
To fully understand cancer, we need to understand the
regulation of the cell cycle and apoptosis. Cancer occurs when
mutations occur in genes that regulate the cell cycle. Two groups of
genes need to be mutated for a tumor to form.
Oncogenes
are genes that can lead to uncontrolled growth if they are activated
at the wrong time or constitutively.
Tumor suppressors
are genes that normally inhibit the cell cycle.
If these activities are lost it can also lead to uncontrolled
growth.
This is analogous to stepping on the gas pedal
(activating oncogenes) and cutting the brakes (losing tumor suppressors).
Mutations
in oncogenes tend to be dominant, you only need one copy
stuck in the “on” position to increase cell division.
Mutations
in tumor suppressors tend to be recessive, you need to
delete both copies to lose activity.
Oncogenes
myc
is a transcription factor. Increased
gene expression leads to increased cell division.
Erb is a mutated EGF receptor.
The mutated receptor lacks an extracellular domain and is always
on.
Trk is a tyrosine kinase receptor
fused with a dimeric protein called tropomyosin through a chromosomal
translocation. This causes
the tyrosine kinase to form a permanent dimer and as a result is always
“on”.
Ras
is a G-protein. Can’t
hydrolyze GTP and the protein is stuck in the “on” position.
Tumor suppressors
One of the most important
tumor suppressors is p53. Mutations in p53 are found in 50% of all
tumors. p53 regulates a branch point between the normal cell
cycle, arrest of the cell cycle and triggering apoptosis.
p53 normally increases the
transcription of mdm2, which in turn increases the rate of destruction
of p53 in the cell.
If the DNA in a cell is
damaged mildly, p53 binds to the DNA, and mdm2 falls off of p53.
As a result, p53 levels increase and an increase in p21 transcription
occurs. p21 triggers arrest of the cell cycle until the DNA damage
can be repaired.
If the DNA is damaged
severely, p53 becomes phosphorylated, mdm2 falls off of p53, and the
phosphorylated p53 triggers apoptosis.
|
