Exploring the Miller Worm Farm
By Leigh MacMillan
March 11, 2002
It seems like any other laboratory door sign, until the words on it make you look twice.
“Miller Worm Farm,” announces the small blue placard next to David Miller’s laboratory. Worm
Farm?, you wonder, expecting to glimpse dirt-filled aquariums boasting intricate tunnels of
wriggling worms. Instead, a standard lab scene greets your inquiring eyes—microscopes and
computers are as exotic as it gets.
Miller’s laboratory is indeed a place dedicated to cultivating worms, but not the juicy kind
that nourish gardens. These worms are tiny, about the thickness of an eyelash and only a
millimeter long. And Miller doesn’t raise them just for kicks—he’s in the business of scientific
discovery, tackling in particular the tough questions of how nerve cells choose partners.
“The worm,” as it is fondly known, is an increasingly popular organism for biomedical
research. Many of the fundamental cellular processes found in the worm are shared across
millions of years of evolution with human beings. By studying a simpler “model” organism like the
worm, investigators hope to learn about the processes at work in human cells.
The making of a good model
Scientifically, the worm is called Caenorhabditis elegans, C. elegans for short. It is a
nematode, a member of a large phylum of worms that live in soil or water, or as parasites in
plants or animals.
C. elegans got its start as a model system for biological research in 1965. Sydney
Brenner, an investigator at the Medical Research Council in Cambridge, England, selected the
worm as a promising animal for investigating the development and function of a simple nervous
system.
Brenner had already participated, with Francis Crick, in the identification of messenger
RNA and the elucidation of the triplet genetic code. In the early 1960’s, he and Crick felt that the
future of molecular biology “lay in tackling more complex biological problems,” especially in the
fields of development and the nervous system, Brenner wrote in a Foreword to the 1988 book
The Nematode Caenorhabditis elegans.
Brenner originally selected a C. elegans cousin, C. briggsiae, as a promising candidate
for rapid genetic and biochemical analysis of the mechanisms of cellular development. He was
looking for a multicellular organism that had a short life cycle, could be easily cultivated, was
small enough to be handled in large numbers, had relatively few cells so that cell lineage and
patterns could be studied, and was amenable to genetic analysis. Both nematodes fit the bill, and
C. elegans was eventually selected in preference.
In the nearly four decades since Brenner first proposed studying a nematode, hundreds
of investigators have joined the cause. The development of every cell, starting with the single egg
and progressing to the 959 body cells of the adult worm, has been mapped. “We have in essence
a very large family tree diagram that maps out the parents of every single cell,” Miller explains.
The worm field also benefits from a complete “wiring diagram” of the animal’s 302-celled
nervous system. Investigators spent more than 10 years “slicing up the worm—like a salami—into
thousands of thin sections, looking at each one with an electron microscope, tracing out all of the
neurons, and reconstructing the whole circuitry,” Miller says. He shows off the tattered volume of
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Exploring the Miller Worm Farm
the journal that published the work. “In here are catalogued the morphology and connections of
every single worm neuron,” he says. “It’s the only organism for which this kind of information
exists.”
The fully charted cell lineage and nervous system wiring maps are invaluable tools for
investigators, who can easily compare cell patterns in genetically manipulated and normal worms.
Most recently, the worm became one of the first multicellular organisms to have its entire
genome sequenced. Since that development in 1998, even more investigators have joined the
ranks of those interested in studying fundamental biological processes in a simple animal.
“The completion of the genome really jumpstarted worm research,” Miller says. “It has
made the worm accessible to focused molecular approaches that weren’t necessarily available
before then.”
Uncoordinated worms shed light on nerve cell connections
Miller was a graduate student at Rice University in Houston when he heard that Sydney
Brenner was giving a presentation at nearby Baylor University. It was 1978, about five years after
Brenner’s first paper using the worm. The seminar charted Miller’s future course, prompting him
to pursue postdoctoral research in the worm field—eventually with Brenner himself.
“He presented the whole idea of using this organism to understand biology, of using
genetic approaches to figure out the key molecules in biochemical pathways,” Miller recalls. “I
was really swept away by the notion that you could use genetics to dissect a complex process
without making any assumptions about what the players are.”
Using chemicals to induce genetic mutations, Brenner had isolated worms with unusual
or uncoordinated movements. His collection of “unc” mutants is still being studied today. Miller’s
group, in fact, studies the unc-4 mutant, a worm unable to crawl backward. In unc-4 mutant
worms, the pattern of connections between nerve cells is changed—specific motor neurons are
incorrectly “wired” with the wrong synaptic inputs.
Miller and colleagues cloned and characterized the unc-4 gene and the protein it encodes
(UNC-4). UNC-4 is a protein called a transcription factor; it works in the cell nucleus to turn other
genes “off.” “UNC-4 turns off the ‘wrong’ genes so that neurons make the ‘right’ connections,”
Miller says. In fact, this negative mechanism of gene regulation to specify motor neuron
connections appears to be evolutionarily conserved in the vertebrate spinal cord.
In unc-4 mutant worms, Miller says, the “wrong” genes that would normally be turned off
by UNC-4 are being left “on.” And somehow this is changing nerve cell connections. Miller hopes
that figuring out which genes UNC-4 regulates will answer “fundamental questions about how
neurons choose synaptic partners.”
To try to find the genes UNC-4 controls, Miller and colleagues will take advantage of an
advanced technology called DNA microarrays, which are tiny glass “chips” with thousands of
DNA spots dotting their surfaces. Scientists can use them to probe which genes are on or off in a
certain kind of cell, for example the cells of an unc-4 mutant worm.
Miller and collaborators including Kevin Strange recently developed a method for
culturing worm cells in a dish and separating the unc-4-expressing neurons from other cells on
the basis of a fluorescent protein (GFP). Comparing the pattern of gene expression in unc-4
mutant neurons and normal control neurons should reveal the genes regulated by UNC-4, Miller
says.
Worms provide key insights to cell death, axon guidance
Other investigators have pursued studies of uncoordinated worms or have carried out
genetic screens for other kinds of defects. For at least two areas, cell death and axon guidance,
“genetic approaches in the worm have yielded key insights into biological pathways that are
highly conserved between worms and human beings,” Miller says.
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Exploring the Miller Worm Farm
During the course of worm development, approximately 300 extra cells are generated
and then self-destruct. Because the worm is transparent, investigators can actually watch worm
cells as they divide and as they die. Programs of cell suicide are normal and happen on a grand
scale in the developing human body. The classic example, Miller says, is the programmed death
of cells that, in the fetus, web together the fingers and toes.
Robert Horvitz at the Massachusetts Institute of Technology pioneered the search for
genes controlling cell death. Like Brenner, he used chemicals to cause genetic mutations in the
worm. Instead of looking for uncoordinated movements though, Horvitz picked out mutant worms
with abnormal patterns of cell death. The cell death—“ced”—genes he and colleagues identified
have defined a core cell death pathway and accelerated understanding of cell death in
mammalian systems.
These insights have direct bearing on the molecular understanding of cancer, which can
result from abnormal control of cell death pathways.
Similarly, the worm has accelerated studies of axon guidance—how the communicating
processes of nerve cells find their way to the right target cells during development.
In just one example, a series of the “unc” mutants led to the identification of a
biochemical signaling pathway required for certain neurons to send their axons in the correct
direction. The conserved molecules were cloned more quickly from vertebrates by using the
known genetic sequences of the worm molecules. “This is one of many cases where the genetics
of C. elegans predicted a paired signal-receptor set important for steering axons,” Miller says.
Understanding the molecular signaling pathways that guide nerve cell growth and
connections could lead to strategies for repairing nervous system damage.
Studies using the worm as a model system will undoubtedly continue to illuminate the
molecular participants in biological pathways that are simply too cumbersome to tackle in a
mammalian system. Miller and other Vanderbilt investigators are along for the exciting ride, eager
to probe fundamental questions of cell biology that will inform understanding of human health and
disease.
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