Insect Control: Biological and Synthetic Agents - Index of

Today, photoaffinity labeling (e.g., with optimized
5-azido-6-chloropyridin-3-ylmethyl photoaffinity
probes) combined with mass spectrometry
(MS) provides a direct and physiologically relevant
chemical biology method for 3D structural investigations
of ligand–receptor interactions (Tomizawa
et al., 2007a, b). This is an important approach for
studying subtype-selective agonists (Tomizawa et al.,
2008) and mapping the elusive neonicotinoid binding
site (Tomizawa et al., 2007b). The results were
used to establish structural models of the two
AChBP subtypes. In A-AChBP, neonicotinoids and
nicotinoids are nestled in similar bound conformations
(Tomizawa et al., 2008). Surprisingly, for
L-AChBP, the neonicotinoid insecticides have two
bound conformations that are inversely relative to
each other. Accordingly, the subtype selectivity is
based on two disparate bound conformations of
nicotinic agonists (Tomizawa et al., 2008). Recently,
the nicotinic agonist interactions with A-AChBP
have been precisely defined by scanning 17 Met
and Tyr mutants within the binding site by photoaffinity
labeling with 5-azido-6-chloropyridin-3ylmethyl
probes that have similar affinities to their
non-azido counterparts (Tomizawa et al., 2009).
On the other hand, agonist actions of commercial
neonicotinoid insecticides were studied by single
electrode voltage-clamp electrophysiology on
nAChRs expressed by neurons isolated from thoracic
ganglia of the American cockroach, Periplaneta
americana (Ihara et al., 2006; Tan et al., 2007).
Based on maximal inward currents, neonicotinoid
insecticides could be divided into the two subgroups
defined above: (1) ring systems containing neonicotinoids
and (S)-nicotine were relatively weak partial
agonists causing only 20–25% of the maximal ACh
current and (2) neonicotinoids having non-cyclic
structures were much more effective agonists producing
60–100% of the maximal ACh current
(Miyagi et al., 2006; Ihara et al., 2006; Tan et al.,
2007). Thiamethoxam, even at 100 mM, failed to
cause an inward current and showed no competitive
interaction with other neonicotinoids on nAChRs,
indicating that it is not a direct-acting agonist or
antagonist (Tan et al., 2007).
In addition, in most insect non-a subunits, lysine
(Lys) or arginine (Arg) moieties are found. These
basic residues may interact with the N-nitro group
of neonicotinoid insecticides through electrostatic
forces and H-bonding, strengthening the nAChR–
insecticide interaction (Shimomura et al., 2006;
Wang et al., 2007). On the other hand, highresolution
crystal structures of L-AChBP with neonicotinoid
insecticides such as imidacloprid and
clothianidin suggested that the guanidine moiety in
A3: Addendum 115
both stacks with Tyr 185, while the N-nitro group
of imidacloprid but not of clothianidin makes
a H-bond with Gln 55. The H-bond of NH at
position 1 with the backbone carbonyl group of
Trp 143, offers for clothianidin an explanation for
the diverse actions of neonicotinoids on insect
nAChRs (Ihara et al., 2008).
Since 2004, several research groups have published
further evidence related to the submolecular
basis for the mechanism of target-site selectivity of
commercial neoncotinoids for insect nAChRs (e.g.,
structural features of agonist binding loops C and D;
Toshima et al., 2009) over vertebrate nAChRs, based
on the nAChR subunit composition (Tomizawa and
Casida, 2005; Matsuda et al., 2005). The activity of
neonicotinoids on wild-type and mutant a7 nicotinic
receptors was also investigated using voltage-clamp
electrophysiology. It was found, that when neonicotinoids
bind to the receptor, the N-nitro group is
located close to loops D and F, which was supported
by the models of the agonist binding domain of the
a7 nicotinic receptor (Shimomura et al., 2006). Recently,
similar electrophysiological studies on native
nAChRs and on wild-type and mutagenized recombinant
nAChRs have shown that basic residues particular
to loop D of insect nAChRs are likely to
interact electrostatically with the N-nitro group of
neonicotinoid insecticides (Matsuda et al., 2009).
Ongoing design of active novel ingredients and
their optimization has to consider in its process
conformational transitions of nAChRs (Jeschke,
2007b). Molecular interactions of neonicotinoid
insecticides containing different pharmacophore
variants with nAChRs have been mapped by chemical
and structural neurobiological approaches,
thereby encouraging the biorational and receptor
structure-guided design of novel nicotinic ligands
(Ohno et al., 2009). Recently, replacement of
the nitromethylene pharmacophore with nitroconjugated
systems was described (Jeschke, 2007b;
Shao et al., 2009). The methyl [1-(2-trifluoromethylpyridin-5-yl)ethyl]-N-cyano-sulfoximine(Sulfoxaflor;
common name ISO-proposed) was prepared
by Dow AgroSciences, which has a new N-cyanosulfoximine
[–S(O)=N–CN] pharmacophore variant
(Loso et al., 2007).
After 16 years of use, insects pests such as whiteflies
Bemisia tabaci (Gennadius) and Trialeurodes
vaporariorum (Westwood) (Gorman et al., 2007),
the brown planthopper Nilaparvata lugens (Sta˚l)
(Nauen and Denholm, 2005; Gorman et al., 2008),
the Colorado potato beetle Leptinotarsa decemlineata
(Say) (Nauen and Denholm, 2005), and a
few others like the mango leafhopper Idioscopus
clypealis (Lethierry) have developed resistance to