C. elegans: Model Organism in the Discovery of PKD

Model organisms have always played an important role in genetics research. For example, during the mid-nineteenth century, Gregor Mendel experimented with pea plants in his monastery garden in order to address a big question: Why didn't a hybrid of two plant strains look like a blend of the two? Mendel opted to use pea plants in his experiments because they possessed a number of traits well suited to his research situation and his area of interest; for example, they could be grown easily in large numbers, their reproduction could be manipulated, and they exhibited several traits that occurred in only two forms. Furthermore, Mendel knew that his findings in pea plants would have implications for the genetics of many organisms, including humans.

Today, some 150 years after Mendel's groundbreaking work, geneticists continue to rely on model organisms, from single-celled bacteria and yeast, to fruit flies and nematodes, all the way up to mammals such as mice and rats. Of course, just as the diversity of model organisms has increased, so has the range of questions researchers seek to answer through their work with these species. For instance, modern geneticists frequently use model organisms to determine the function of certain genes, as well as the disease-related effects of various mutations. Similar genes and mutations can then be identified and localized on the human genome, thereby giving researchers a clearer picture of the molecular basis of many human disorders. Surprisingly enough, some of the most successful efforts in this area have involved use of the simple roundworm Caenorhabditis elegans.

One example of the value of model organisms-and C. elegans in particular-involves studies of the condition known as polycystic kidney disease (PKD). PKD is a genetic disorder in humans in which multiple fluid-filled sacs (cysts) grow in the kidney (Figure 1). Not only do the cysts make the kidney abnormally large, but they also can take the place of functioning tissue. Normally, the function of the kidney is to filter waste from the body's fluids and regulate electrolyte balance in the body. In the case of PKD, these functions are compromised, and ultimately, the kidney can fail. In fact, half of all people with PKD need dialysis or a kidney transplant by the age of 60.

PKD results from gene mutations, which appear to localize at more than one place on the genome (Reeders, 1992). The disease occurs in two major forms-autosomal dominant and autosomal recessive-both of which are inherited. The autosomal dominant form is more common and affects over 500,000 people in the U.S. alone. Currently, there is no effective therapy for PKD.

Thankfully, animal models of PKD provide a convenient way for researchers to test hypotheses about the biochemical pathways that lead to cysts and to test experimental therapies. For example, in 1999, Maureen Barr sought to learn more about the function of two proteins-polycystin-1 (PKD1) and polycystin-2 (PKD2)-that are defective in human polycystic kidney disease (Barr & Sternberg, 1999). Using C. elegans as a model organism (Figure 2), Barr showed that worm proteins related to human polycystins (i.e., homologues) mediate signaling in sensory neurons. Of course, in Barr's experiments, a gene mutation in the PKD1 homologue didn't affect kidney function, because nematodes (including C. elegans) don't have kidneys. Rather, this mutation affected a specific step in the mating behavior in male worms. In fact, in worms, the PKD1 homologue is called LOV1 (short for "location of vulva") to reflect its function.

In humans, mutations in either PKD1 or PKD2 cause almost indistinguishable clinical symptoms. Previous studies had suggested that the proteins encoded by these genes (polycystin-1 and polycystin-2, respectively) work together at the cell membrane. Barr's results in the worm model revealed a similar interaction between two proteins. Specifically, she found that the worm homologue of polycystin-2 was expressed in the same neurons as the gene product of LOV1.

In addition to C. elegans, numerous mouse models have been used in the study of PKD, and many of these models have shared phenotypic characteristics of the human pathology. Indeed, once the human genes were identified, mouse homologues of PKD1 and PKD2 were found and targeted for experimental mutation (Guay-Woodford, 2003). These experiments revealed that heterozygous mice with mutations in either gene develop cysts in the kidney, as well as in other organs, similar to the human disease. Homozygous mice with mutations also have cysts, but they usually die before birth.

Interestingly, dysfunctional cilia are a feature common to both the mutant worm and the mutant mouse models used in the study of PKD. The worm homologues, LOV1 and PKD2, are found on sensory neuron cilia, and if one or both of the related genes are mutated, males show abnormal mating behavior as a result of sensory defect. In addition, studies in mice have found polycystin-1 and polycystin-2 localized in the cilia of kidney cells; further, overexpression of polycystin-2 has been demonstrated (Igarashi & Somlo, 2002). Because the function of renal cilia is not clear, researchers have more work to do to understand how abnormal cilia function would produce kidney cysts (Figure 1).

At first glance, it may seem odd that simple organisms like C. elegans are used so extensively in the study of PKD and other human disorders. In fact, in a review article published in 1999, Maureen Barr herself comments on the seeming gulf between worm sex and human kidney disease: "At first glance, worms and kidneys have as little to do with each other as do Caenorhabditis elegans geneticists and practicing medical nephrologists." However, Barr goes on to make a strong case for the lessons learned from this model organism: "Forward or classical genetics is aimed at understanding a biologic process and moves from mutant phenotype to gene/protein identification. Reverse genetics starts with the knowledge of gene/protein sequence and moves to function/phenotype." She continues, "C. elegans is amenable to both approaches, making the worm an attractive model system in which to study your favorite gene."

Mice are effective model organisms as well, and much closer to humans in terms of genome homology than are invertebrates and plants. In some cases, however, less complex organisms, such as C. elegans, do offer certain advantages. For instance, in her review, Barr recounts all the reasons why the worm has served so well, including its small size, short life cycle, large brood size, genetic amenability, and simple cellular makeup (Barr, 2005; Figure 2). These same qualities are also true of a variety of other model organisms, from yeast to peas.

Model organisms have long played a prominent role in genetic research, beginning with Gregor Mendel's experimentation with pea plants in the mid-1800s. Today, geneticists continue to rely these organisms, especially when investigating questions of gene expression, function, and mutation. Once researchers identify particular genes and gene variants in model organisms, they can frequently find similar pieces of DNA within the human genome; in this way, they can develop a better understanding of the molecular basis of many human traits, including disorders such as PKD. Interestingly (and perhaps surprisingly to some people), even simple organisms like C. elegans can reveal much about the molecular basis of human disease; in fact, the simple structure, short life span, and easy manipulability of this species make it particularly amenable to ongoing use in the research environment.