the Miller Worm Farm
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.
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
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.
of a good model
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.
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
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 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.
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.
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 College
of Medicine. 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.
the 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.”
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
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
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.
key insights to cell death, axon guidance
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.
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
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.
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 DNA
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.
the molecular signaling pathways that guide nerve cell growth and
connections could lead to strategies for repairing nervous system
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.
The Miller lab
is supported by grants from the National Institutes of Health.