Model Organisms in Drug Discovery

aspirin, which has potent inhibitory effects on the aggregation of human
platelets (reviewed by Patrono et al., 2001). The role of zebrafish COXs during
embryonic development was also analyzed. In mammals, zygotic transcription
of both COX genes appears to be dispensable during embryonic development,
although postnatal renal dysplasia develops in COX-2-deficient mice
(Langenbach et al., 1995; Morham et al., 1995). Knock-down of zebrafish
COX-2 protein also had no discernable effect on embryonic development.
However, knock-down of zebrafish COX-1 caused a significant delay in
epiboly, a developmental process dependent upon cell proliferation and cell
migration. The discordant embryonic phenotypes produced by inhibition of
teleost versus mammalian COX-1 may be explained by the fact that antisense
morpholinos are capable of inhibiting the translation of both maternal and
zygotic COX transcripts in zebrafish, whereas gene targeting in mammals
perturbs only zygotic gene expression.
8.5 Future directions
FUTURE DIRECTIONS 217
Elucidation of the regulatory mechanisms that control prostanoid production
and bioactivity remains an active area of research. Given the high degree of
structural and functional conservation between zebrafish and humans COX
genes, studies directed toward these questions seem feasible using this model
system. High-throughput genetic analyses are particularly attractive to
questions of gene regulation. For example, mutagenesis strategies that assay
COX protein levels immunohistochemically, or via reporter genes in
transgenic fish, may identify mutations that perturb COX RNA or protein
expression and/or stabilization. Such mutants could lead to the identification
of novel COX-1 regulators, which to date have largely eluded detection.
Similarly, such screens may also define motifs within either COX protein that
are pharmacologically relevant. The COX-deficient mutants recovered in this
manner, which would be predicted to be fully viable, could be used to generate
compound mutants by matings with fish that carry established mutations.
Such compound mutants then could be assayed for a variety of prostanoidrelated
biochemical or physiological defects.
Biochemical-based mutagenesis screens are also feasible using the zebrafish.
High-throughput assays of prostanoid production using mass spectrometry is
one example. A physiological mutagenesis screen such as this would identify
not only mutations that perturb COX activity directly but also mutations that
perturb the function of upstream and downstream COX regulators, such as
the genes predicted to couple COXs to PLA2s or PG synthases. The zebrafish
also provides a convenient means to assay the role of known genes in
prostanoid biosynthesis using the aforementioned antisense techniques.
Finally, recently devised techniques for directly identifying specific gene