Download Sensory control of longevity by the ODR-3 and GPA

Lans et al.: Sensory plasticity and longevity
Signaling proteins that regulate NaCl chemotaxis responses modulate longevity in C.
elegans
Hannes Lans1, Martijn P.J. Dekkers1, Renate K. Hukema1, Nathan J. Bialas2, Michel R.
Leroux2 & Gert Jansen1,3
1
MGC Department of Cell Biology and Genetics, and Center for Biomedical Genetics,
Erasmus MC, PO Box 2040, 3000 CA Rotterdam, The Netherlands
2
Dept. of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, BC,
Canada
3
Corresponding author, E-mail: [email protected]
Abstract
The life span of the nematode Caenorhabditis elegans is regulated by sensory signals
detected by the amphid neurons. In these neurons, C. elegans expresses at least 14 G
subunits and a G subunit. We have identified 7 sensory Gα subunits that modulate life span.
Genetic experiments suggest that multiple sensory signaling pathways exist that modulate
life span and that some G proteins function in multiple pathways, most of which, but probably
not all, involve insulin/IGF-1 like signaling. Interestingly, of the sensory G proteins involved in
regulating life span, only one Gα probably functions directly in the detection of sensory cues.
The other G proteins seem to function in modulating the sensitivity of the sensory neurons.
We hypothesize that in addition to the mere detection of sensory cues regulation of the
sensitivity of sensory neurons also plays a role in the regulation of life span.
keywords:
C. elegans, G proteins, sensory signaling, longevity, aging
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G protein signal transduction
Signal transduction via heterotrimeric G proteins is one of the most widely used mechanisms
of eukaryotic cells to deal with signals from the environment1,2. Many signals, including
hormones, light, odorants and tastants, activate G protein coupled receptors (GPCRs), which
signal to one or more G proteins to activate a downstream cascade of effectors.
Heterotrimeric G proteins consist of a Gα, a Gβ and a Gγ subunit. In the inactive form, the
three subunits are tightly associated, and a GDP molecule is bound to the Gα subunit.
Activation of a G protein mediated by the exchange of GDP for GTP results in activation of
multiple downstream effectors, like ion channels and enzymes that generate second
messengers, by both the Gα subunit and the Gβγ dimeric complex. Eventually, signaling
ceases when the GTP is hydrolyzed to GDP, due to the slow intrinsic GTPase activity of the
Gα subunit, and this subunit reassociates with the Gβγ complex.
C. elegans heterotrimeric G proteins
The genome of nematode Caenorhabditis elegans encodes 21 Gα, 2 Gβ and 2 Gγ
subunits3,4. Each of the mammalian classes of Gα subunits is represented by one member in
C. elegans: gsa-1 for Gs, goa-1 for Gi/o, egl-30 for Gq and gpa-12 for G12. The conserved
Gα subunits, the two Gβ subunits and one Gγ, gpc-2, are expressed widely in muscle and
neuronal cells and regulate many different processes, including early development, egg
laying and locomotion5.
In addition to the conserved Gα subunits, C. elegans has 17 Gα subunits that do not
clearly belong to any of the mammalian classes of Gα subunits: gpa-1 to gpa-11, gpa-13 to
gpa-17 and odr-33-5. 14 of these show specific expression in subsets of chemosensory
neurons, suggesting that they have specific roles in sensory signaling3 (Fig.1). Also, one Gγ
subunit, gpc-1, is expressed in a subset of chemosensory neurons6. Characterization of
mutants for almost all G proteins revealed behavioral defects for a small subset of loss-offunction alleles, whereas several gain-of-function alleles produced phenotypes. These initial
analyses showed that many of the sensory G proteins are involved in sensory signaling or
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the development of the sensory system, and suggested extensive functional redundancy or
subtle modulating functions6.
C. elegans perceives its environment primarily through 12 bilateral pairs of ciliated
amphid neurons, each of which is involved in the detection of a certain class of compounds7
(Fig. 1). Extensive genetic and behavioral analyses by us and others have confirmed the
idea of both extensive functional redundancy and subtle regulatory roles for the sensory Gα
subunits6,8-11. For example, olfaction in C. elegans is mediated by five Gα subunits, which
constitute at least two redundant stimulatory and one inhibitory signaling pathways per
cell11,12.
Sensory signaling modulates life span
Genetic analysis of life span in C. elegans has greatly aided our understanding of the
mechanisms that regulate aging. An intriguing finding is that the nematode’s life span is
under control of sensory perception13,14. Laser ablation of some of the neurons that detect
environmental cues, either increase (ASI, ASG, AWA, AWC) or decrease life span (ASJ,
ASK), suggesting that sensory information provides both life span promoting and inhibitory
signals14. A recent study in Drosophila melanogaster showed that aging in flies is also
influenced by sensory cues15.
Sensory perception is thought to influence longevity in part by controlling the
insulin/IGF-1 pathway13,14, which regulates life span in C. elegans as well as in many other
organisms16. Mutations in insulin signaling genes, such as the insulin/IGF-1 receptor
homologue daf-2, greatly extend life span. This life span extension depends on the activity of
the FOXO family transcription factor daf-16, which translocates to the nucleus to promote
longevity in daf-2 mutants16.
Multiple sensory G protein pathways modulate life span
Since the sensory cues and precise sensory signaling mechanisms that regulate longevity
are not known, we set out to characterize the functions of all sensory G proteins in the
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regulation of longevity17. These experiments showed that loss-of-function of gpa-1, gpa-5,
gpa-9, odr-3 and gpc-1 and overexpression of gpa-2, gpa-9, gpa-11 and gpc-1 significantly
extended life span (e.g. Fig. 2). Mutation of odr-3 had been previously reported to increase
lifespan15. Most of the other mutations in G subunits had either no effect or seemed to
decrease life span17.
Thus far, six sensory amphid neurons have been shown to regulate longevity: AWA,
AWC, ASG, ASI, ASJ and ASK14. The involvement of gpa-11 implies that also the ASH
and/or ADL neurons regulate longevity, since gpa-11 is expressed exclusively in these two
neuron pairs3. Interestingly, also odr-3, gpa-1 and gpc-1 are expressed in the ASH and/or
ADL neurons.
The analysis of life span in double mutants between gpa-1, gpa-11, odr-3, gpc-1 and
gpa-11XS revealed complex genetic interactions17 (e.g. Fig. 2). The results suggest that
multiple sensory G protein signaling pathways exist that affect life span, and that some of
these G proteins function in multiple pathways. We found that the regulation of longevity by
gpa-1, gpa-11, odr-3 and gpc-1 involves insulin/IGF-1 signaling17. Surprisingly, however, life
span extension in some mutants was only partially dependent on daf-16 or daf-2.
Sensory functions of G proteins that affect life span
Previous studies have identified some functions of the sensory G proteins involved in the
regulation of life span. Interestingly, only odr-3 animals show clear defects in the detection of
environmental cues. odr-3 is important in the AWA and AWC neurons for the detection of
olfactory stimuli and in the ASH neurons for the detection of nociceptive stimuli9,10,12.
Because laser ablation of the AWA and AWC neurons was found to increase life span15, it is
very well possible that odr-3 functions in these neurons to regulate longevity.
The other Gα subunits seem not to mediate the initial detection of environmental
cues, but rather to modulate the behavior of the animal. gpa-11 mediates the transduction of
a food/serotonin signal in the ASH neurons, which modulates the avoidance response to the
odorant octanol8. Therefore, gpa-11 might regulate longevity by mediating a food/serotonin
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signal, although we found no effect of exogenous serotonin, or reduction of serotonin levels
in bas-1 animals on life span17. However, a recent report showed that mutation of the
serotonin receptor ser-1 increased life span, suggesting that serotonin does play a role in
regulating life span18.
In addition to gpa-11, gpa-1, gpc-1 and odr-3 also play important roles in modulating
the behavior of C. elegans. These genes regulate the response of C. elegans to NaCl, a
behavior called gustatory plasticity10.
Gustatory plasticity
Naïve C. elegans are attracted to 0.1 – 200 mM NaCl, but avoid higher NaCl
concentrations10,19-21. Chemo-attraction to NaCl is mainly mediated by the ASE neurons,
while avoidance of high concentrations is mediated by the ASH neurons22. However, after
prolonged exposure to 100 mM NaCl, in the absence of food, the animals show reduced
attraction or even avoidance of NaCl at all concentrations (Fig. 3). We call this behavioral
switch gustatory plasticity6,10,23. This behavior involves signals from at least four pairs of
sensory neurons, the ASE, ADF, ASH and ASI neurons10. We have identified many genes
that function in gustatory plasticity10, including the G protein subunits gpa-1, gpc-1 and odr-3
(Fig. 3).
We have used behavioral assays and single cell Ca2+ imaging to determine the
contributions of the gustatory and nociceptive neurons to both the naïve behavior and
gustatory plasticity24. In the naïve situation the ASE sensory neurons not only respond to low
but also to high NaCl concentrations. The ASH nociceptive neurons only show Ca2+
responses to NaCl concentrations above 100 mM. After prolonged exposure to NaCl, in the
absence of food, the ASE neurons are desensitized, while the ASH nociceptive neurons are
sensitized and respond to normally attractive NaCl concentrations24. It is not yet clear what
the functions of the various sensory G proteins are in these processes. We hypothesize that
they mediate neurotransmitter or endocrine signals that affect the sensitivity of the gustatory
and nociceptive neurons.
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Concluding remarks
It is becoming increasingly clear that environmental cues play a role in regulating life span,
not only in C. elegans but also in Drosophila13-15. Not much is known about the sensory
signals that affect life span, although the nature of the sensory neurons involved suggests
that both odorants and water-soluble cues might be important13,14,17. The Kenyon lab has
identified 2 putative odorant receptors that play a role in regulating longevity, but the ligands
of the receptors are not known14,25. We identified several sensory G proteins. Interestingly,
only one of these G proteins, odr-3, seems directly involved in the detection of environmental
cues and might couple directly with odorant or gustatory receptors11,12,14,17. All G proteins,
including odr-3, play a role in behaviors that involve changes of the sensitivity of the sensory
neurons: modulation of octanol avoidance and gustatory plasticity6,8,10,17. We have recently
identified another gene that when mutated affects both longevity and gustatory plasticity, the
Parkin coregulated gene pcrg-1. This gene is specifically expressed in a small subset of
sensory neurons, where it localizes to cilia, and interacts genetically with sensory G proteins
in both regulation of longevity and in gustatory plasticity (NJB, MPJD, GJ and MRL,
unpublished results). We hypothesize that in addition to the mere detection of sensory cues,
regulation of the sensitivity of sensory neurons also plays a role in modulating life span.
In future studies, we would like to address in which neurons and circuitries these
sensory proteins function to regulate both life span and gustatory plasticity. A subsequent
step would be to identify the neurotransmitters or endocrine signals involved. In this respect it
is interesting to note that serotonin plays a role both in regulating gustatory plasticity and life
span18,23. In addition, recent studies have shown that insulin signaling, which is well known to
play a role in regulating life span16, affects gustatory plasticity (RH & GJ, unpublished results)
and a related behavior called salt chemotaxis learning26. We expect that a better
understanding of the sensory signaling pathways involved, will eventually contribute to the
identification of environmental cues that affect life span.
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Acknowledgements
This work was financially supported by the Netherlands Organization for Scientific Research
(NWO/ALW), the Royal Netherlands Academy of Sciences (KNAW), and the Center for
Biomedical Genetics.
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Figure legends
Fig. 1. Amphid neurons in the head of C. elegans.
Image of C. elegans larva, with the approximate positions of the chemosensory neurons
schematically depicted in the head. Indicated are the cell bodies of the 11 chemosensory
neurons on one side of the animal, in colors, and the dendrite (d) and axon (a) of one of
these cells. The respective neurons in which Gα subunits are expressed and the sensory
processes in which they function are indicated. Anterior is to the left.
Fig. 2. Sensory G proteins regulate life span.
Loss-of-function mutations in odr-3 and overexpression of gpa-11 (gpa-11XS) extend life
span. odr-3; gpa-11XS animals show synergistic life span extension. Shown are the results
of single representative life span assays at 20°C.
Fig. 3. Sensory G proteins that modulate life span regulate the response to NaCl.
Responses of wild type and sensory G protein mutants to 25 mM NaCl, after 15 min wash in
buffer without (open bars) or with (black bars) 100 mM NaCl, without food. gpc-1, gpa-1 and
odr-3 mutants showed defects in gustatory plasticity. Indicated are the averages of at least 4
assays ± s.e.m. Chemotaxis index: (A-C)/(A+C), where A is the number of animals at the
quadrants with NaCl, and C is the number of animals at the quadrants without attractant.
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