Model Organisms in Drug Discovery

66 C. ELEGANS FUNCTIONAL GENOMICS IN DRUG DISCOVERY
(Raizen and Avery, 1994; Davis et al., 1995; Franks et al., 2002). Wholeanimal
C. elegans electrophysiology and patch-clamping of target C. elegans
tissue and cells allow functional characterization of the human or insecticidal
ion channel in C. elegans and the pharmacological characterization of hit
compounds. As an example, Devgen has performed high-throughput
compound screens on ligand-gated ion channels. After C. elegans hit filtering,
these compounds have been tested and confirmed in Xenopus (frog) oocyte
voltage-clamp electrophysiology.
Assay development
Assay development is the art of establishing an experimental procedure to
perform hundreds and thousands of tests in a highly reproducible and
quantitative manner. For C. elegans assays, the same principles and goals
applies as for any assay development program, such as the need for
robustness, reproducibility, sensitivity, a procedure with only a few simple
steps, ease of assay validation, reagent supply, up-scaling, assay automation
and cost effectiveness (Bronson et al., 2001). In the following, we shall
highlight two C. elegans-specific challenges in assay development: the
production of C. elegans animals and the automation of phenotypic readouts.
The first challenge for a C. elegans production unit is to deliver millions of
C. elegans animals for each screening day, with all animals in the same
condition. In the case of the ‘drinking assay’, this demands that every animal
is in the same feeding state and has the same feeding activity, because we use
feeding as an indirect measure of the serotonergic tonus. In other words,
millions of animals have to behave in nearly the same way. Scaling up of a
C. elegans population, from a few plates (corresponds to tens of thousands of
animals) sufficient for one experiment to populations of several millions of
animals that must be delivered day by day for several weeks of a screening
campaign, requires sophisticated logistics and the utmost stringency in
adhering to the culture protocol. This can be achieved by establishing and
monitoring every parameter that influences drinking, such as the quantity and
quality of food, the developmental stage of the animal, ambient temperature,
medium components, etc. (Devgen, personal communication).
The second challenge is to enable the analysis of phenotypes in a highthroughput
format. Even a phenotype that is seemingly easy to measure,
such as live versus dead, limits the compound throughput to a few thousand
per day because the examiner must analyze well by well. The assay would
become even more work intensive if the readout had to be quantified and if
the population in each well had to be counted. The drinking assay
incorporates a fluorescent measurement as a readout for pharynx pumping.
This measurement can be used in a plate reader and, as such, is amenable to