Biomedical Engineering Reference
In-Depth Information
of.hormones,.neurotransmitter,.ions,.photons,.tastants,.and.odorants.(Wettschureck.and.
Offermanns. 2005,. Lagerström. and. Schiöth. 2008).. Since. GPCRs. are. involved. in. many.
pathophysiological.processes,.it.is.not.surprising.that.over.50%.of.pharmaceuticals.on.the.
current.market.target.these.receptors.(Lagerström.and.Schiöth.2008)..What.makes.GPCRs.
ideal.for.use.in.biosensor.technology.is.their.sensitivity.and.afinity.for.a.wide.range.of.
chemical.ligands..In.addition,.the.ield.can.take.advantage.of.the.breadth.of.knowledge.we.
have.acquired.over.recent.decades.into.the.structure-function.properties.of.these.proteins.
(Sakmar.2010)..The.majority.of.interest.in.GPCR.sensor.development.has.focused.on.their.
use. for. drug. discovery.. These. assays. often. take. advantage. of. measuring. intracellular.
second. messengers. such. as. Calcium. (Ca
2+
). and. 3′-5′-cyclic. adenosine. monophosphate.
(cAMP),. and. may. use. luorescently. labeled. dyes. to. target. these. downstream. signaling.
molecules..While.cell-based.GPCR.based.biosensor.assays.will.not.be.discussed.here,.the.
reader.is.referred.to.a.number.of.reviews.describing.these.(Thomsen.et.al..2005,.Siehler.
2008,.Scott.and.Peters.2010).
GPCRs. comprise. seven. hydrophobic. α. helices. that. span. the. plasma. membrane.. The.
intracellular.C-terminus.and.cytoplasmic.loops.are.involved.in.binding.the.heterotrimeric.
G-protein.complex.(α,.β,.and.γ)..The.N-terminus.and.extracellular.loops.have.been.asso-
ciated.with.larger.ligand.binding.such.as.peptides,.while.smaller.chemical.ligands.bind.
on.sites.within.the.transmembrane.α.helical.core.(Gether.and.Kobilka.1998)..Upon.ligand.
binding,. the. GPCR. coupled. to. the. heterotrimeric. G-protein. complex. undergoes. confor-
mational. changes. that. result. in. the. activation. of. the. α. subunit. by. guanosine. nucleotide.
exchange. of. GDP. for. GTP.. The. α. subunit. dissociates. from. the. remaining. protein. com-
plex. to. bind. an. effector. protein,. which. regulates. further. downstream. signaling. events..
The.effector.protein.target.is.dependent.on.the.particular.G-protein.subunit.involved..In.
olfactory.signaling,.this.Gα.olf.protein.interacts.with.and.activates.adenylate.cyclase.(AC).
(Spehr.and.Munger.2009)..The.cAMP.levels.thus.rise.and.act.on.cAMP-gated.cation.chan-
nels,.resulting.in.an.inlux.of.Ca
2+
.and.Na
2+
..Ca
2+
.in.turn.regulates.chloride.(Cl
−
).channels,.
causing. an. eflux. of. Cl
−
. and. membrane. depolarization.. This. leads. to. propagation. of. an.
action.potential.down.the.axon.of.the.olfactory.neuron.to.transmit.the.signal.to.the.brain.
(Figure.10.1).
10.3.2 Olfactory receptors
Olfactory.receptors.comprise.the.largest.number.of.GPCR.proteins.known.(Buck.and.Axel.
1991)..These.receptors.belong.to.the.Rhodopsin.or.Class.A.family.of.GPCRs..It.is.believed.
that.rodents.express.around.1000.functional.ORs,.whereas.humans.have.evolved.to.only.
have.approximately.350.ORs.(Spehr.and.Munger.2009)..This.is.likely.due.to.the.ability.of.
higher.primates.to.effectively.coordinate.their.senses,.such.as.vision.and.smell.
ORs. are. found. in. olfactory. sensory. neurons. in. the. olfactory. epithelium. of. the. nose..
A.unique.aspect.to.the.olfactory.system.is.that.one.olfactory.neuron.expresses.a.distinct.
type. of. odorant. receptor. (Serizawa. et. al.. 2003).. ORs. are. concentrated. on. the. cilia. of. the.
neuronal.dendrite.that.faces.the.nasal.lumen,.where.they.come.in.contact.with.the.odorant/
ligand.either.directly.or.after.the.odorant.has.been.captured.by.a.binding.protein.located.
in.the.olfactory.mucosa..Upon.OR-ligand.engagement,.signaling.is.initiated.as.explained.
earlier,.culminating.in.transmission.of.an.electrical.signal.down.the.axon.to.the.olfactory.
bulb.and.further.into.the.olfactory.cortex.of.the.brain.(Glatz.and.Bailey-Hill.2011)..Given.
their. physiological. role,. olfactory. receptors. are. not. presently. targets. for. drug. discovery..
However,.their.unique.ability.to.identify.a.large.variety.of.chemicals.with.great.precision.
makes.this.group.of.GPCRs.very.attractive.for.biosensor.development.
