Page 259 - The Toxicology of Fishes

Receptor-Mediated Mechanisms of Toxicity 239

A Original gene
H L B with multiple functions
(expression pattern)
H L B H L B Fully redundant duplicates

H B L B Complementary duplicates
Regulatory partitioning

B 1 2
Original protein
1 2 kinase with multiple functions
(substrates)

1 2 kinase 1 2 kinase Fully redundant duplicates

2 kinase 1 kinase Complementary duplicates
Functional partitioning
FIGURE 5.1 The duplication, degeneration, complementation (DDC) model of gene evolution. (A) Regulatory partition-
ing: The original gene is expressed in multiple cell types or tissues, such as heart (H), liver (L), and brain (B), controlled
by tissue-specific transcription factors (colored shapes). After duplication, each gene copy loses regulatory sequences targeted
by different transcription factors, resulting in complementary expression patterns. (B) Functional partitioning: The original
gene encodes a protein with multiple functions, such as an enzyme that can act on different substrates (shapes 1 and 2).
After duplication, residues required for substrate-specific recognition are differentially mutated, resulting in complementary
loss of substrate binding. Schemes are based on the model proposed by Force, Lynch, and colleagues (Force et al., 1999;
Lynch and Force, 2000). (A color version of this figure is available from the first author [mhahn@whoi.edu] upon request.)

from studies of zebrafish paralogs that have distinct patterns of expression that together sum to the
expression pattern of their mammalian ortholog (reviewed by Postlethwait et al., 2004). We refer to this
as regulatory partitioning (Figure 5.1A). For other genes, rather than (or in addition to) the partitioning
of expression patterns, the fish genes may diverge with regard to specific functions—for example,
differential loss of certain functional domains or specialization for subsets of ligands (functional parti-
tioning; Figure 5.1B) (de Souza et al., 2005; Hawkins and Thomas, 2004; Hawkins et al., 2005). In either
case, paralog-specific studies involving knock-down or other approaches applied to fish paralogs may
reveal novel functional aspects of their mammalian ortholog. The possibility that fish paralogs may evolve
new functions (neofunctionalization) must also be considered (Brunet et al., 2006; He and Zhang, 2005).
An understanding of receptor function and whether it is highly conserved, partitioned, or novel requires
the application of quantitative biochemical methods that reveal the toxicologically important properties
of the receptors. In the next two sections, we discuss the general principles governing ligand–receptor
interactions and describe some useful experimental approaches to studying them.

Ligand–Receptor Interactions: General Principles
In physiological terms, a receptor has two properties: It recognizes a change in the environment, and it
produces a response. Most cellular receptors (and nearly all of the receptors of toxicological interest)
are proteins that interact with small molecule ligands. For these receptors, ligand concentration is the
environmental change that must be monitored and responded to. The interaction of the receptor protein
with the ligand leads to a conformational change in the protein that produces a cellular response; however,
ligand binding does not automatically lead to a response, as different ligands for the same receptor may