The Organism
The nematode Caenorhabditis elegans (C. elegans) has been the subject of intense analysis by biologists around the world. Nematodes, or roundworms, are simple metazoan animals that have cells specialized to form tissues and organs such as nerve tissue and digestive tissue. Analysis of genetic control of the events that lead to the formation of the tissues in C. elegans has revealed biological mechanisms that also control the differentiation of tissues and organs in more complex organisms such as humans.
Caenorhabditis elegans is a microscopic, 1-millimeter-long roundworm that lives in soils and eats bacteria from decaying materials. It belongs to the phylum Nematoda (the roundworms), which includes many significant plant and animal parasites. Caenorhabditis elegans, however, is free-living (nonparasitic) and does not cause any human diseases. It exists as two sexes, males (containing a single X chromosome) and hermaphrodites
(containing two X chromosomes). Both male and hermaphrodite worms have five pairs of autosomal (non-sex) chromosomes. The hermaphrodites are self-fertile. They produce sperm first, which they store, and later “switch” gonads to begin producing eggs. These eggs may be fertilized by the hermaphrodite’s own sperm, or if the hermaphrodite mates with a male, sperm from the male will fertilize the eggs. A hermaphrodite that is not mated will lay approximately three hundred fertilized eggs in the first four days of adulthood; hermaphrodites that mate with males will continue to lay eggs as long as sperm are present.
Caenorhabditis elegans eggs begin development within the uterus. They hatch as small L1 larvae and molt four times as they proceed through the easily recognizable larval stages of L2, L3, L4, and adult. The adult hermaphrodite is a little larger than the adult male and can be distinguished by the presence of fertilized eggs lined up in the uterus. The smaller males have specialized tails that contain structures for mating called copulatory spicules.
A Model Organism
Because of its small size and simple diet (bacteria), C. elegans is easily adapted to laboratory culture conditions. The worms are grown on small agar-filled petri plates that are seeded with E. coli. The worms live comfortably at room temperature, but elevating or lowering the temperature can speed up or slow down development, and changes in temperatures can even reveal conditional phenotypes of some genetic mutations.
One unmated hermaphrodite will produce three hundred progeny over the first four days of adulthood. Additionally, C. elegans has a short generation time of approximately three weeks. Obtaining large numbers of progeny allows thorough statistical analysis of the way a mutation is segregated within a population. Because researchers can screen large numbers of worms in a short period of time, extremely rare mutations are likely to be revealed. Genetically “pure” strains are also quickly produced.
Hermaphrodite genetics also provides advantages. Because hermaphrodites are self-fertile, getting homozygous mutations is not difficult. A hermaphrodite that is heterozygous for a given mutation (has one wild-type copy of a gene and one mutated copy of a gene) will produce progeny, one-fourth containing two mutated copies of the gene (homozygotes). Additionally, for researchers studying mutations that affect reproduction or mating behavior, having self-fertile hermaphrodites allows them to maintain mutations that affect processes such as sperm production. A hermaphrodite that cannot make its own sperm can be mated to a wild-type male, and the mutation causing the defect can be maintained. This is not possible in organisms that are strictly male/female or that are strictly hermaphroditic.
Another strength of C. elegans is that the genetic strains can be frozen in liquid nitrogen and maintained indefinitely. Even fruit flies have to be constantly mated or “passaged” to maintain the genetic stocks for a laboratory. Caenorhabditis elegans strains are maintained in a central location, giving all scientists access to the same well-characterized genetic stocks.
Caenorhabditis elegans is a transparent worm, ideally suited for microscopic analysis. The origin and ultimate fate of every cell in the worm (the cell lineage) has been mapped and traced microscopically. Adult hermaphrodites have 959 somatic (non-sex) cell nuclei, and males have 1,021. Because the entire cell lineage for the worm is known and the worm is transparent, researchers can use a laser to destroy a single, specific cell and observe how loss of one cell affects development of the worm. These kinds of studies have contributed to the understanding of how neurons find target cells and how one cell can direct the fate of another.
Embryonic Development: Asymmetric Divisions
Research on C. elegans has revealed how programmed genetic factors (autonomous development) and cell-cell interactions guide development of an organism from egg to adult. The very first division of the fertilized egg (zygote) in C. elegans is asymmetric (uneven) and creates the first difference in the cells of the organism that is reflected in the adult. This division produces two daughter cells called P and AB. AB is a large cell that gives rise to tissues such as muscle and digestive tract. P is a much smaller cell that ultimately produces the cells that become the gonads (sex cell-producing tissues). The difference in P and AB is determined by the segregation of small P granules in the cell. The location of these granules and the asymmetry of this initial division are determined by the point of entry of the sperm. Until the eight-cell stage, there is no genetic activity by the embryo; the first few divisions are directed by the maternal gene products. This is one example of how maternal gene products can influence the early development of an embryo.
Neural Development
One of the areas of later development that is particularly well understood in C. elegans is the development of the nervous system. The nervous system has been completely reconstructed with serial electron micrographs that reveal precisely how one neuron connects to another. Some neurons migrate to assume their final cell fate and function. These migrations are easily studied in the worm because of its transparency, and a single neuron can be visualized by marking it with green fluorescent protein. Many genes and their encoded proteins that have been identified as important for directing the growth, connectivity, and migration of C. elegans neurons are highly conserved in evolution and control axon guidance in the vertebrate spinal cord.
Apoptosis: Programmed Cell Death
The 2002 Nobel Prize in Physiology or Medicine was awarded to Sydney Brenner,
H. Robert Horvitz, and John E. Sulston for research describing the regulation of organ development and programmed cell death. Cell death is an important part of development in plants and animals. For instance, human embryos have webbing between fingers and toes. This webbing is composed of cells that die in the course of normal development before a human baby is born. The death of these cells occurs because of a genetic program in the cells, apoptosis. The genes that control apoptosis are highly conserved throughout evolution. Apoptosis also plays a role in cancer. Often cancer is thought of as resulting from uncontrolled proliferation of cells, but it can also result when cells that should die during development fail to die. Scientists are looking at ways to specifically activate apoptosis in tumor cells in order to kill tumors. The clues for what genes to target for such treatments come, in part, from studies of the apoptosis pathway in organisms such as C. elegans.
A Molecular Tool
The first metazoan genome
that was sequenced was C. elegans. Many of the technologies (automated machines, chemistries for isolating and preparing DNA) that were developed in the course of the C. elegans genome-sequencing project were directly applied to the human genome sequencing project, and many of the scientists involved in sequencing the C. elegans genome contributed expertise to the Human Genome Project as well.
In 2008, Osamu Shimomura,
Martin Chalfie, and Roger Y. Tsien were awarded the Nobel Prize in Chemistry for the discovery and development of the green fluorescent protein (GFP). GFP is a protein originally isolated from jellyfish that glows bright green under ultraviolet light. Once the gene for GPP was isolated and cloned, researchers began using it as a “marker” to trace specific cell types. Chalfie first used this protein to identify six specific cells in C. elegans. GFP is now used in experiments to follow specific cells, such as migrating neurons during development, and in experiments that trace the transport or localization of proteins within cells. Researchers also have used GFP to “mark” tumor cells to trace their spread within an organism.
RNA interference (RNAi)
allows eukaryotic cells to degrade foreign RNA molecules, such as double-stranded RNA molecules from infecting viruses. The RNA molecule is cleaved into small fragments (approximately 23 nucleotide pairs), which can then bind to complementary RNA sequences within the cell and disrupt their expression. In 2006, C. elegans researchers Andrew Fire and Craig Mello were awarded the Nobel Prize in Physiology or Medicine for their work describing the mechanism of RNAi and showing that cleavage of the foreign double-stranded RNA could lead to specific suppression of gene expression. In the research laboratory, RNAi is used to specifically knock out expression of a target gene. This technique is useful for researchers working with human or other mammal cell culture systems because it does not require laborious cloning work. RNAi may also have therapeutic uses in knocking out expression of specific cancer-related genes in tumor cells.
Caenorhabditis elegans research identified the first presenilin, a class of proteins later implicated in Alzheimer’s disease. Research on the worm has led to a greater understanding of certain proteins that are involved in cellular aging. Studies in C. elegans are even contributing to a better understanding of learning and behavior. Most C. elegans scientists are studying the worm because it provides a tool for answering many of the hows and whys of biology that cannot be answered easily in more complex systems. The answers to seemingly esoteric questions, such as how C. elegans sperm move, will shed light on fundamental biological processes shared by all organisms.
Key Terms
apoptosis
:
a genetically programmed series of events that results in the death of a cell without affecting or damaging the surrounding cells and tissue; apoptosis can be triggered by events such as DNA damage or can be part of the normal development of an organ or tissue
cell differentiation
:
a process during which a cell specifically expresses certain genes, ultimately adopting its final cell fate to become a specific type of cell, such as a neuron, or undergoing programmed cell death (apoptosis)
model organism
:
an organism well suited for genetic research because it has a well-known genetic history, a short life cycle, and genetic variation between individuals in the population
RNA interference
:
a specialized type of RNA degradation in which foreign double-stranded RNA molecules stimulate the activity of an enzyme complex containing RNAse, which cleaves the RNA molecule into small fragments that can then bind to complementary RNA sequences and disrupt expression of specific genes
Bibliography
Alberts, B., et al. Molecular Biology of the Cell. 5th ed. New York: Garland Science, 2007. Print.
Bernards, R. “Exploring Uses of RNAi–Gene Knockdown and the Nobel Prize.” New England Journal of Medicine 355.23 (2006): 2391–3. Print.
Chang, K. “Three Chemists Win Nobel Prize.” New York Times. New York Times, 8 Oct. 2008. Web. 16 July 2014.
Lewin, Benjamin. Genes VII. New York: Oxford UP, 2001. Print.
Rothman, Joel H., and Andrew Singson. Caenorhabditis elegans: Cell Biology and Physiology. 2nd ed. Waltham: Academic, 2012. Print.
Schedl, Tim. Germ Cell Development in C. elegans. New York: Springer, 2013. Print.
Topper, Stephen, et al. "Alcohol Disinhibition of Behaviors in C. elegans." PLoS ONE 9.3 (2014): 1–9. Print.
Wood, W. B., et al. The Nematode “Caenorhabditis elegans.” Cold Spring Harbor: Cold Spring Harbor Laboratory, 1988. Print.
No comments:
Post a Comment