W.S. Leal
Department of Entomology, University of
California, Davis
CA 95616, USA
Chemical
communication involves the production and
release of specific chemicals (semiochemicals)
by the emitter, and the detection and
olfactory processing of these signals
leading to appropriate behavioral responses
in the receiver (Roelofs, 1995). Chemical
attraction is the major means of sexual
recruitment in scarab beetles, in
particular, rutelines and melolonthines.
Females are normally the emitters and males
the receivers, and in this case, the
semiochemicals are referred to as sex
pheromones. Male-released aggregation
pheromones have also been reported for a few
Dynastinae. Although a few studies have
reported the chemical ecology of the dung
beetles (Scarabaeinae), most of the emphasis
by research programs on chemical
communication in scarab beetles has focused
on the subfamilies Cetoniinae, Melolonthinae,
Dynastinae, and Rutelinae because of their
economic importance as agricultural and/or
turf pests. Largely, these research projects
are aimed at the development of attractants
(pheromones or food-type lure compounds) for
possible applications in management
programs. In my laboratory, we have taken a
comprehensive approach to chemical
communication in order to gain a better
understanding of both the emitters and
receivers and pave the way for the
development of environmentally sound control
strategies. On the one hand, we focused on
the chemistry of the emitters
(identification and synthesis of pheromones)
and studied the biology, biosynthesis and
physiology of pheromone production. On the
other hand, we investigated the molecular
mechanisms of the olfactory processing in
the receivers.
PHEROMONE
CHEMISTRY
Recent studies
have led to the identification of the sex
pheromones of various species in the
subfamily Rutelinae and Melolonthinae (Leal,
1998). In general, the pheromones of the
former are fatty-acid derived compounds,
whereas the latter utilizes phenolic,
terpenoid, and amino acid derived compounds.
Two interesting exceptions to this general
rule are the pheromones of Heptophylla picea
and Phyllopertha diversa . Although
belonging to the Melolonthinae, H. picea
utilizes (R,Z)-7,15-hexadecadien- 4-olide
(Leal et al., 1996), most likely
biosynthesized from stearic acid. On the
other hand, P. diversa (Rutelinae) produces
an intriguing alkaloid pheromone, which also
has medicinal properties (Leal et al.,
1997). Utilizing pheromone blends that
consist of just a few semiochemicals or even
a single constituent, closely related
species have attained isolated chemical
communication channels and reproductive
isolation (Leal, 1999a; 1999b). Species that
have the same pheromone system are isolated
either temporarily or geographically.
Interestingly, Anomala osakana and Popillia
japonica utilize enantiomers of a chiral
pheromone (japonilure), with one
stereoisomer being an attractant and the
other a behavioral antagonist. P. japonica
and A. osakana produce (R)- and
(S)-japonilure, respectively (Tumlinson et
al. 1977; Leal, 1996). The pheromone of one
species is a behavioral antagonist for the
other. It seems that this
agonist-anatagonist activities of the
enantiomeric pheromones have evolved as part
of the isolation mechanism between these two
species that share the same habitats in
Japan. In general, scarab beetles can detect
only the enantiomer produced by the
conspecific females, but P. japonica and A.
osakana have evolved the ability to detect
both enantiomers, one as an attractant and
the other as a behavioral antagonist (stop
signal).
PHEROMONE
BIOLOGY
Pheromone
gland cells in A. cuprea females were
identified as modified integumental
epithelia of the terminal abdominal
sclerites (Tada and Leal, 1997). The gland
cells are composed of round pheromone
secretory cells with canal structures
bearing an end apparatus. On the other hand,
we determined that in Holotrichia parallela
the pheromone is produced in the posterior
part of a ball-shaped sac exposed during
female calling. Light microscope observation
of the posterior part of the gland revealed
a cuticular epithelium layer composed of
columnar cells, which was assigned as the
tissue involved in the pheromone production
(Kim and Leal, 1999).
PHEROMONE
BIOSYNTHESIS AND PHEROMONE REGULATION
A typical
structure of the sex pheromone of rutelines
is the five-membered gamma-lactones having a
long unsaturated hydrocarbon chain, such as
(R,Z)-5-(—)-(oct-1-enyl)oxacyclopentan-2-one
(buibuilactone) and (R,Z)-5-(—)-(dec-1-enyl)
oxacyclopentan-2-one (japonilure), which are
pheromones for a number of species. Using
deuterated precursors, it has been
demonstrated that the biosynthesis of these
compounds starts from saturated fatty acids
(palmitic and stearic acid), involves their
desaturation followed by stereospecific
8-hydroxylation, chain shortening and
cyclization (Leal et al., 1999). Various
scarab species have developed pathways to
produce unique pheromone molecules by
changing either stereospecificity or
regiospecificity of the hydroxylation step.
Anomala cuprea and Popillia japonica utilize
the (R)-8-hydroxylase, whereas the
hydroxyylase of A. osakana is specific to
the (S)-substrate. It seems that A.
rufocuprea is devoid of the enzyme so it
makes methyl Z-(5)-tetradecenoate (Tamaki et
al., 1985). Pheromone biosynthesis in
scarabs is regulated by a PBAN-like factor.
The pheromone titer in the glands of
decapitated females dramatically decreased
24 hr after surgery, but it resumed after
injection of the brain extracts from virgin
females. The activity of the brain extracts
is lost after treatment with proteinase K.
Because BmPBAN is also active,
characterization of the gene encoding the
peptide was pursued by library screening and
PCR. Hitherto, none of the molecular
approaches led to the identification of the
PBAN gene in scarab beetles. On the other
hand, a bioassay-oriented strategy lead to
isolation of the active peaks by reversed
phase HPLC and ion-exchange chromatography.
The small amount of the isolated peptide
prevented any further characterization.
MOLECULAR
BASIS OF OLFACTION
For their
survival, insects heavily depend on their
ability to detect chemical signals from the
environment, which are buried in complex
mixture of odors from a myriad of sources.
This has been highlighted in the literature
by their highly sensitive and selective
olfactory systems for the detection of sex
pheromones, particularly in Lepidoptera,
which approach the theoretical limit for a
detector. While minimal structural
modifications to pheromone molecules render
them inactive (Kaissling, 1987), a single
molecule of the native ligand is reported to
be sufficient to activate the
pheromone-specific olfactory neurons in the
antennae of the silkworm moth, Bombyx mori
(Kaissling and Priesner, 1970). There is
growing evidence in the literature that this
inordinate sensitivity is achieved by a
combination of the roles of various
olfactory specific proteins, including
odorant receptors, odorant-binding proteins,
and odorant-degrading enzymes. In order to
gain a better understanding of the molecular
basis of olfaction, we aimed at identifying
and characterizing the pheromone-degrading
enzymes, studying the neurophysiological
details of pheromone perception “in vivo,”
and isolating, identifying, and cloning the
genes encoding the pheromone- and
odorant-binding proteins. In order to
elucidate the function(s) of these proteins,
we have been conducting structural studies
in collaboration with Jon Clardy (Cornell
University) and Kurt Wuthrich
(ETH-Switzerland).
PHEROMONE-DEGRADING ENZYMES
Antennal
proteins from the extracts of several
species of scarab beetles can degrade
buibuilactone and japonilure, even those
from species that do not use this group of
compounds as their pheromones. In some cases
there was only one metabolite, identified as
the corresponding hydroxy fatty acid. It
seems that the deactivation of the lactone
signal is obtained by the opening of the
lactone ring. Some species, however,
degraded the pheromone into several more
products. The esterase from A. octiescostata
showed significant preference for
(R)-japonilure over that of the
(S)-enantiomer. This observation is
consistent with the fact that this species
produces only the (R)-enantiomers of the two
pheromone components and it is anosmic to
the (S)-lactones. Analysis of the
degradation products of the unique pheromone
from P. diversa revealed that only the
antennal extract of this species can degrade
the pheromone. The antennal extracts from 10
other scarab species and 4 lepidoptearn
species produced no activity at all.
Separation of the antennal extracts showed
that the enzymatic activity was associated
with the membrane fractions in the absence
of cytosol. Analysis of the degradation
reaction suggested that the major
degradation product was due to a
demethylation at the N-1 position; the
second product was due to hydroxylation of
the aromatic ring. Studies on the
degradation along with potential cofactors
or inhibitors showed that the enzymatic
system requires NADPH and NADH for activity.
On the other hand, the enzymatic activity
was inhibited by proadifen and metyrapone,
two general widely used inhibitors for
cytochrome P450 (Wojtasek and Leal, 1999).
DEGRADATION OF PHEROMONES “IN
VIVO”
The discovery
of the unique pheromone-degrading enzyme in
P. diversa and the identification of
enzymatic inhibitors opened the way to study
pheromone inactivation “in vivo.” When
metyrapone was introduced by diffusion into
the pheromone-specific sensilla in the
antennae of P. diversa, the pheromone
detectors became “silent” to lower
concentrations after application of a large
concentration of the pheromone. The effect
of the inhibitor is remarkably different
from adaptation as will be discussed later.
In addition, metyrapone treatment had no
effect on the sennsila of P. diversa tuned
to (Z)-3-hexenyl acetate nor did it affect
the pheromone-detecting systems in P.
japonica, for which pheromone inactivation
is achieved with a sensillar esterase.
IDENTIFICATION
AND CLONING OF OBPs
We have
identified, cloned, and characterized the
odorant-binding proteins from a number of
scarab species. It is now clear that scarab
beetles possess two families of
odorant-binding proteins, one with 116 and
the other with 133 amino acids, which we
named OBP1 and OBP2, respectively. While
OBP1 is well conserved among all species of
scarab beetles, OBP2 belongs to a more
diverse group and, in contrast to OBP1, it
has not been detected in all species.
Interestingly, OBP2 possesses isoforms,
which can be separated by native gel
electrophoresis. These isoforms have
different binding affinities. For example,
one isoform of OBP2 from P. diversa binds
bombykol, whereas the other conformation
binds japonilure (Wojtasek et al., 1999).
Microheterogeneity of the OBPs in scarab
beetles is not derived from different gene
products, but it is due to the
conformational flexibility of the proteins.
Consistently, we found only one gene
encoding OBP2 in various species..Plenary
Lectures Walter Leal ABSTRACT BOOK I –
XXI-International Congress of Entomology,
Brazil, August 20-26, 2000 XVI
Interestingly, in both A. osakana and P.
japonica, we could detect only one PBP in
the antennal extracts; the proteins from the
two species showed a 96% similarity. Due to
the limited sensitivity of the detection
methods, one cannot rule out the possibility
of the presence of proteins expressed at low
levels. However, electrophysiological
experiments suggest that if two PBPs were
involved in the signal transduction of the
enantiomers of japonilure they would be
expressed at nearly the same level. Single
sensillum recordings from the antennae of
the Japanese and Osaka beetles showed that
enantiospecific receptor neurons respond
equally to (R)- and (S)-japonilure. These
findings and the observation that a single
PBP from A. osakana bound both enantiomers
of japonilure apparently with the same
affinity suggested that in the antennae of
these species, the same PBP may recognize
both the pheromone and the “stop signal”,
i.e., the enantiomers of japonilure
(Wojtasek et al., 1998).
STRUCTURAL
BIOLOGY AND FUNCTION OF PBPs
We envisaged
that in order to determine the molecular
basis of insect olfaction and elucidate the
function of PBPs, we needed to study the
three-dimensional structure of the
pheromone-binding proteins and its
interaction with ligands. We embarked in
collaborations with two groups (Jon Clardy
and Kurt Wuthrich) to determine the 3D
crystal and solution structures of the
pheromone-binding protein from Bombyx mori.
Functional expression of BmPBP was achieved
in E. coli periplasm. The protein appeared
as a single band in gel electrophoresis and
it was homgeneous in most chromatographic
systems. However, NMR experiments conducted
by the Wuthrich group indicated the
existence of at least two conformations at
pH 6.2. Throughout the analysis of both the
native and recombinant proteins, a
remarkable feature of the PBPs appeared.
These proteins have dynamic structures,
altering their conformations in pH-dependent
ways. Studies with model membranes suggested
that upon an interaction with the dendritic
membrane, PBPs undergo a conformational
change that may lead to the release of the
pheromone ligand (Wojtasek and Leal, 1999).
The three-dimensional structure of the BmPBP
with bound bombykol has been determined by
X-ray diffraction (Sandler et al., 2000).
BmPBP has six helices, and bombykol binds in
a completely enclosed hydrophobic cavity
formed by four antiparallel helices.
Bomkykol is bound in this cavity through
numerous hydrophobic interactions. It has
been suggested that a pH drop would result
in protonation of the histidine residues
that form the base of a flexible loop and
protonated histidines could destabilize the
loop covering the binding pocket. Although
the crystal structure did not show clear
evidence for dimers, a comprehensive study
(Western immunoblotting experiments, mass
spectral analysis, gel filtration estimation
of molecular masses, and cross-linking
reactions), showed that BmPBP is a monomer
at acid pH and a dimer at basic, neutral,
and slightly acid pH. This suggests that the
physiologically relevant pH for the early
olfactory processing is not only that of the
sensillar lymph (bulk pH), but also the pH
at the surface of the dendrides (localized
pH) (Leal, 2000).
ACKNOWLEDGMENTS
I gratefully
acknowledge the great contribution that my
past and present collaborators and members
of my research group made to this work. My
research projects in Japan were financially
supported by a special coordination fund for
promoting science and technology by the
Science and Technology Agency of Japan and
by the Programe for Promotion of Basic
Research Activities for Innovative
Biosciences (BRAIN). Work in the US was made
possible through direct financial support
from the department, college, and
Chancellors office at UCD.
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Index terms:
pheromones, pheromone-binding proteins,
pheromone-degrading enzymes, biosynthesis
Copyright:
The copyrights of this work belong to
the author (see right-most box of the
title table). This document also appears
in the Plenary Lectures ABSTRACT BOOK I
– XXI-International Congress of
Entomology, Brazil, August 20-26, 2000
XIV.