September 15, 1999
Learning How Organs Tell Left From Right
From all outward appearances, the human
body is symmetrical. If one were to divide the body into two halves, for
example, each side would have a single arm and leg. But a look inside at the
internal organs shows that there is not perfect symmetry throughout the
body. The heart and spleen normally reside on the left side and the liver
and gallbladder are on the right. Furthermore, the organs themselves are
asymmetrical.
Understanding how genes control the shape and spatial orientation of
organs is one of the goals of developmental biologists. In the mouse, as in
other vertebrates, cells begin receiving and responding to instructions
about their positional fate during the earliest stages of development, when
the embryo resembles little more than a flat sheet of cells. Two proteins in
particular, called Nodal and Pitx2, are produced predominantly on the left
side of the mouse embryo and they appear to direct the growth of organs in a
way that is appropriately "left-sided." Understanding how these two proteins
interact may allow scientists to understand a great deal more about how
organs are segregated into left-side and right-side.
“Cell proliferation stops. There is a failure to progress past the initial determination of organ identity.”
Michael G. Rosenfeld
In attempting to unravel the genetic program that determines left/right
orientation, Howard Hughes Medical Institute (HHMI) investigator
Michael
Rosenfeld
and colleagues at the University of California, San Diego, and
The Salk Institute, engineered knockout mice that lacked the
Pitx2
gene. The mice exhibited a number of developmental abnormalities, including
inappropriate position of the heart and lungs.
Humans with one defective copy of the
Pitx2
gene have a
constellation of problems called Rieger syndrome, the signs of which include
irregularly shaped eyes, lack of tooth growth, craniofacial deformities,
and, more rarely, problems with growth hormone production.
Rosenfeld's team found that
Pitx2
knockout mice had defective
growth in their teeth and pituitary glands, the source of growth hormone.
"Cell proliferation stops. There is a failure to progress past the initial
determination of organ identity," Rosenfeld said.
Summarizing the results of these studies, which are published in the
September 16, 1999, issue of the journal
Nature
, Rosenfeld said, "the
work shows that the control of the left/right orientation of organs is more
complicated than we thought."
Researchers knew that Nodal is a signal-carrying protein, and Pitx2 is a
transcription factora protein that turns on other genes. Once the Nodal
signal arrives, it induces Pitx2 to help implement its directives. One
testable issue in the current study, says Rosenfeld, "was to determine
whether Pitx2 implements the entire Nodal program or whether Nodal acts via
additional targets."
The results of earlier experiments that attempted to untangle the
functions of Nodal and Pitx2 were intriguing, but did not produce a clear
answer. When researchers added Nodal or Pitx2 to the right side of a mouse
embryo, for example, the right side developed as a mirror image of the left.
In other experiments, researchers added extra Nodal or Pitx2 to embryos only
to find that the two proteins had similar effects. And knocking out the
Nodal
gene alone resulted in either mirror-image or randomized
organization of multiple organs.
Given these results, Rosenfeld says he would have expected the knockout
of Pitx2 to cause the same sort of organ abnormalities reported by
scientists who performed the Nodal knockout experiments.
The actual result was a mixed bag, but it appears that Pitx2 is crucial
in determining the "leftness of the lung," say Rosenfeld and his colleagues.
Without
Pitx2,
both sides of the lungs of the knockout mice looked
like the right side of a normal lung. The left side of a normal pair of
lungs is usually smaller to accommodate the position of the heart.
Studies of the heart orientation in the Pitx2 knockout mice proved less
conclusive. In the
Pitx2
knockout mice, the heart was located on the
right side of the body, not in its normal location on the left side. Despite
its improper location, however, the direction of cardiac looping was normal.
"Everyone might well have predicted the result would be the same as after
removal of
Nodal
," said Rosenfeld. "But now we know that this single
transcription factor in the pathway does not account for the whole of
left/right asymmetry."
Rosenfeld is now searching for other transcription factors that respond
to Nodal signals. These transcription factors may be involved in a condition
called situs invertus, which affects approximately 1 in 10,000 humans and
gives rise to a mirror-image organization of the internal organs. In many
cases the disorder has no adverse consequences, though it can be associated
with heart abnormalities.
Rosenfeld also hopes to begin identifying genes that are switched on by
Pitx2 when organs grow. Such genes are likely to provide additional insight
into understanding left/right determination, he says.
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