March 19, 1999
New Class of Molecular Cues Guides Nervous System Wiring
Repulsion of spinal motor axons by Slit2:
Pieces of embryonic rat ventral spinal cord were cultured with cells engineered to secrete recombinant Slit2 (left and middle panels) or control cells (right panel). Motor axons that emerge from these pieces of tissue, labelled in red with a fluorescent antibody marker, are repelled by the Slit2-expressing cells but not the control cells.
In the developing nervous system, a group of molecular road signs
directs growing nerve cells toward their correct destination. Now,
collaborating groups of Howard Hughes Medical Institute (HHMI)
investigators have discovered a new type of road sign
that—depending upon the conditions—either repels growing
neurons or triggers neurons to sprout new connections.
As reported in three articles in the March 19, 1999, issue of the
journal Cell, the seemingly contradictory "Stop-and-Go" behavior
of the protein Slit opens an intriguing new chapter in understanding
how the nervous system is wired, say the scientists. If the researchers
can understand the machinery controlling such wiring decisions, their
insights may lead to new approaches to regrow or repair damaged or
severed spinal cord nerves.
The Cell articles describe how the two HHMI laboratories,
headed by long-time friends and scientific collaborators Corey Goodman
and Marc Tessier-Lavigne worked together closely to study the Slit
protein in two very different organisms—fruit flies and
mammals.
A repellent signal for axons, Slit (in red), and its
receptor, Robo (in green), help determine whether an axon crosses the
center or "midline" of the fruit fly nervous system.
In a first set of studies, Goodman, an HHMI investigator at the
University of California at Berkeley, and HHMI associate Tom Kidd and
colleague Kim Bland used genetic mutant screening studies of the fruit
fly Drosophila to pinpoint the Slit protein as a key repellent
molecule in developing fly embryos. The group had previously shown that
the roundabout, or Robo, protein is a repulsive receptor on the surface
of growing axons. They also demonstrated that Robo controlled whether
axons crossed or recrossed the center, or "midline," of the fruit fly
nervous system—to properly connect the brain's two halves—by
responding to an unknown midline repellent signal.
In the current study, Goodman and his colleagues were searching for
that midline repellent signal and found that the Slit protein prevented
neurons that had crossed the midline from crossing back over. Once
axons cross the midline, they turn up their levels of Slit receptor
(Robo) and this prevents them from crossing again, says Goodman. Thanks
to the Slit protein, which Goodman and his colleagues in the first of
the three Cell articles call the "midline repellent," the
developing brain avoids a tangle of catastrophic miswiring.
These investigators then collaborated with HHMI investigator Marc
Tessier-Lavigne and colleagues at the University of California San
Francisco to explore Slit's function in mammals, extending the work in
Drosophila. In the second Cell article, predoctoral
fellow Katja Brose and others in Tessier-Lavigne's and Goodman's
laboratory reported finding mammalian versions of Slit, which also act
as repellents for growing mammalian neurons.
But that was not the end of the Slit story. In a another set of
studies, Kuan Hong Wang, an HHMI predoctoral fellow in
Tessier-Lavigne's laboratory, had been working independently at a lab
bench next to Brose's for several years on what they thought was a
separate project. While Brose was isolating the mammalian versions of
Slit in collaboration with the Goodman laboratory, Wang was attempting
to isolate an unknown protein that induced branching of neuronal axons,
the cable-like structures that neurons grow to establish contacts with
other neurons.
When Wang finally isolated the branch-inducing protein and
determined its structure, he and Brose realized that their two
molecules were basically identical. "At that point, we fell off our
chairs," said Tessier-Lavigne. "It's one of those amazing moments in
the laboratory when you think you're working on two different things,
but in fact you're working on two different faces of the same
thing."
Both Goodman and Tessier-Lavigne view the serendipitous convergence
of these two lines of study, reported in the third Cell article,
as opening a promising new pathway to understanding the intricacies of
how the brain and nervous system is wired.
"We know that neurons grow through a series of choice points, like
driving a car to a destination," said Goodman. "You don't just
dead-reckon straight to your destination. You turn onto one road, then
turn onto another, make turns at a series of intersections, and finally
arrive at your destination."
Neurons, like automobile drivers, depend on road signs, adds
Tessier-Lavigne, and for neurons the Slit protein is clearly an
important neural road sign. "You can think of guidance molecules such
the Slit protein as a sign or an arrow pointing in a particular
direction," he said. "A neuron can respond to that arrow in one of
three ways: be attracted into going in the direction of the arrow, be
repelled into going in the other direction, or just ignore it
altogether."
The growing neurons likely decide on their response to these
guidance proteins through sensors or receptors on their tips, called
growth cones. These receptors allow the growing neuron to "read" the
sign posts in the developing brain. In the case of Slit, the
researchers already have an inroad into understanding how the signal is
read by the growth cone because they had previously identified its
repulsive receptor, Robo.
The concept of multiple responses to the same guidance molecule has
already been confirmed in other axon-guiding molecules, including
families known as netrins, ephrins and semaphorins, say Goodman and
Tessier-Lavigne. The Slit proteins, however, constitute a new family of
such proteins, offering yet another pathway for exploring the
intricacies of neuronal wiring.
The HHMI scientists will devote future research efforts to exploring
the control machinery of the Robo receptor and Slit, attempting to
understand in detail how Slit acts as a repellent in some cases and an
axon-branching promoter in others.
Their hope is that such knowledge will enable them to use one set of
chemicals to persuade severed neurons, as in damaged spinal cords, to
produce new growth cones capable of reacting to Slit as a growth
promoter. If they succeed, such neurons may be able to form new
functional connections, and thus, restore lost nervous system
function.
Image: Slit2 images: Tessier-Lavigne Laboratory/HHMI at UCSF
Robo image: Corey Goodman/HHMI at UC, Berkeley
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