August 18, 2005
Researchers Discover New Route to Hemoglobin Synthesis
Researchers studying zebrafish that die from anemia have discovered
a new pathway for the synthesis of heme, the deep red, iron-containing
molecule that is a component of hemoglobin and myoglobin. The research
suggests that defects in this pathway may be an overlooked cause of
anemia in humans.
A research team led by Leonard I. Zon, a Howard Hughes Medical
Institute investigator at Children's Hospital Boston and Harvard
Medical School, published its findings in the August 18, 2005, issue of
the journal Nature. Zon and his colleagues in Boston
collaborated on the studies with researchers from the University of
Rochester Medical Center and the University of Utah School of
Medicine.
“This is a very interesting and unpredicted finding from what we had known before, and our experiments have really defined a new pathway for hemoglobin production.”
Leonard I. Zon
The researchers began their studies hoping to learn why a zebrafish
mutant known as shiraz (sir) failed to produce
hemoglobin. The sir mutant zebrafish, which were first isolated
by Zon and colleagues in the Tübingen Screen Consortium in
Germany, intrigued the researchers because they die from anemia caused
by lack of hemoglobin.
Over the years, Zon and his colleagues have discovered many
zebrafish mutants that fail to make hemoglobin because of defects in
iron metabolism. As they have teased out the causes of these defects,
they have learned that the biochemical pathway involved in hemoglobin
synthesis in zebrafish has been largely conserved over the 300 million
years of evolution between fish and humans. According to Zon, the
easily manipulable fish constitutes an excellent model organism for
studying the regulation of heme formation.
In the current study, the researchers traced the hemoglobin defect
to the gene for an enzyme known as glutaredoxin 5 (grx5). But the
researchers found early on that the enzyme was not directly connected
to hemoglobin production. “Nobody had worked on this gene in
vertebrates before, but we found in the scientific literature that this
gene in yeast was required for the production of iron-sulfur clusters
in the mitochondria,” said Zon. Iron-sulfur clusters are
incorporated into certain proteins to enable their enzymatic functions.
In further experiments, the researchers confirmed that versions of
grx5 in zebrafish, yeast, mice and humans are functionally
equivalent.
“It seemed like the whole process was evolutionarily
conserved,” said Zon. “But the difference is that yeast do
not make hemoglobin. So we needed to figure out a mechanism that would
explain why these fish that have problems making iron-sulfur clusters
could not make hemoglobin.”
Other researchers' studies had indicated that the presence of
iron-sulfur clusters in the cell is important for controlling an enzyme
called iron regulatory protein 1 (IRP1). In turn, IRP1 regulates
another enzyme called ALAS2 that plays a key role in heme synthesis.
Indeed, experiments by Zon and his colleagues demonstrated that the
loss of grx5 in the mutant zebrafish inappropriately activates IRP1,
which blocks the synthesis of ALAS2, and thus heme production. For
example, when they restored ALAS2 by injecting into the sir
mutants a truncated form of ALAS2 that lacked the portion of the
molecule sensitive to IRP1, they complete restored the mutant zebrafish
hemoglobin production.
“People have always thought that hemoglobin synthesis required
only enough iron in the cell for heme production to proceed and then
just the addition of the globin protein to form hemoglobin,” said
Zon. “Now, we've added a fourth component, iron-sulfur clusters,
which are required for heme production. This is a very interesting and
unpredicted finding from what we had known before, and our experiments
have really defined a new pathway for hemoglobin production,” he
said.
Zon said that the findings could apply to developing new treatments
for a rare form of anemia, known as sideroblastic anemia, in which
elevated IRP1 activity causes a deficiency of ALAS2. In most cases, an
increase in IRP1 is likely caused by a mutation in a transporter for
iron-sulfur clusters that traps them in mitochondria, where they cannot
interact with IRP1 to control it.
In a search for possible treatments for the anemia, Zon and his
colleagues are exploring the genetic machinery of hemoglobin production
in zebrafish for targets of drugs that could restore normal levels of
iron-sulfur clusters. “The pathway that we have found is very
sensitive, so our findings might be extended to enable treatments for
other forms of anemia,” said Zon.
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Jennifer Michalowski
