Cell Communication; GPCR part 1

G protein-coupled receptors, often abbreviated GPCRs, are an abundant superfamily of proteins also known as seven-transmembrane domain receptors, 7TM receptors, 7 pass transmembrane receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors (GPLRs). G protein-coupled receptors are cell surface signalling proteins involved in many physiological functions and in multiple diseases. 

The presence of GPCRs in the genomes of bacteria, yeast, plants, nematodes and other invertebrate groups argues in favor of a relatively early evolutionary origin of this group of molecules. The diversity of GPCRs is dictated both by the multiplicity of stimuli to which they respond, as well as by the variety of intracellular signalling pathways they activate. These include light, neurotransmitters, odorants, biogenic amines, lipids, proteins, amino acids, hormones, nucleotides, chemokines and, undoubtedly, many others. In addition, there are at least 18 different human G-alpha proteins to which GPCRs can be coupled (Hermans, 2003; Wong, 2003). These G-alpha proteins form heterotrimeric complexes with G-beta subunits, of which there are at least 5 types, and G-gamma subunits, of which there are at least 11 types (Hermans, 2003).

How do GPCRs work?

1*The first step in signal transduction is ligand (shown in Golden color in the above fig.) binding.

2*Agonist binding is followed by a change in the conformation of the receptor that may involve disruption of a strong ionic interaction between the third and sixth transmembrane helices (Ballesteros et al., 2001; Shapiro et al., 2002), which facilitates activation of the G-protein heterotrimer.

3*Depending on the type of G protein to which the receptor is coupled, a variety of downstream

signalling pathways can be activated (reviewed by Marinissen and Gutkind, 2001; Neves et al., 2002).

4*Signalling is then attenuated as following:

GPCRs become desensitized when exposed to their ligand for a prolonged period of time. There are two recognized forms of desensitization: 

1) Homologous desensitization, in which the activated GPCR is downregulated

2) Heterologous desensitizationc (cross-desensitisation), wherein the activated GPCR causes downregulation of a different GPCR. The key reaction of this downregulation is the phosphorylation of the intracellular receptor domain by protein kinases.

Ligand + GPCR = conformation change to GPCR = α subunit-GTP + beta-gamma complex = α subunit-GTP activates a specific effector (such as Adenylyl cyclase, PLC, ion channel, phosphodiesteras) = production of second messengers (such as cAMP, DAG, sodium and calcium conc. changes) = physiological response by controlling cellular functions = α subunit-GTP + Regulatory of G protein signalling (RGSs) =[ α subunit-GDP + beta-gamma complex] (GPCR) 

 

-Phosphorylation by cAMP-dependent protein kinases

Cyclic AMP-dependent protein kinases (protein Kinase A) are activated by the signal chain coming from the G protein (that was activated by the receptor) via Adenylyl cyclase and cAMP. In a feedback mechanism, these activated kinases phosphorylate the receptor. The longer the receptor remains active, the more kinases are activated, the more receptors are phosphorylated.

-Phosphorylation by GRKs

GRKs-mediated receptor phosphorylation rapidly initiates profound impairment of receptor signaling and desensitization. Activity of GRKs and subcellular targeting is tightly regulated by interaction with receptor domains, G protein subunits, lipids, anchoring proteins and calcium-sensitive proteins.

G protein:

G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins involved in transmitting chemical signals originating from outside a cell into the inside of the cell. G proteins function as molecular switches. Their activity is regulated by factors that control their ability to bind to and hydrolyze Guanosine triphosphate (GTP) to Guanosine diphosphate (GDP). When they bind GTP, they are 'on', and, when they bind GDP, they are 'off'. G proteins belong to the larger group of enzymes called GTPases.


                        GDP                                                                            GTP

There are two classes of G proteins. The first function as monomeric small GTPases while the second form and function as heterotrimeric G protein complexes. The latter class of complexes are made up of alpha (α), beta (β) and gamma (γ) subunits. In addition, the beta and gamma subunits can form a stable dimeric complex referred to as the beta-gamma complex.

Note: Small GTPases are a family of hydrolase enzymes that can bind and hydrolyze GTP. They are a form of G proteins found in the cytosol which are homologous to the alpha subunit of heterotrimeric G protein complexes, but unlike the alpha subunit of G proteins, a small GTPase can function independently as a hydrolase enzyme to bind to and hydrolyze a GTP to form GDP. The most well-known members are the Ras GTPases and hence they are sometimes called Ras superfamily GTPases.


G proteins are molecular switches that use GDP (colored purple here) to control their signaling cycle. When GDP is bound, as shown here, the G protein is inactive. To activate the protein, the GDP is replaced with GTP, the G protein will deliver its signal. G proteins come in many shapes and sizes. Most are used for cell signaling, but other types play an important role in other tasks, such as powering protein synthesis. The ones described here are termed heterotrimeric G proteins because they are composed of three different chains, denoted as alpha (tan), beta (blue), and gamma (green). The little red piece is a loop on the surface of the alpha subunit that is important in transmitting the signal.

Some Types of Gα Subunits:

#The family consists of the G protein α subunit, which acts as a weak GTPase. G protein classes are defined based on the sequence and the function of their α subunits:

Gα_s:

_{"S" stands for stimulation since it stimulates cAMP-dependent pathway via activating Adenylyl cyclase. It is associated with the receptors for many hormones such as (}Adrenaline, Glucagon, LH, PTH, ACTH). 

Note: Gα_s is the target of the toxin liberated by Vibrio cholerae. Therefore, Binding of cholera toxin to Gα_s keeps it turned "on". The resulting continuous high levels of cAMP causes a massive loss of salts from the cells of the intestinal epithelium. Massive amounts of water follow by osmosis causing a diarrhea that can be fatal if the salts and water are not quickly replaced.

The cAMP Dependent Pathway is used as a signal transduction pathway for many hormones including:

• ADH - Promotes water retention by the kidneys (V2 Cells of Posterior Pituitary)
• GHRH - Stimulates the synthesis and release of GH (Somatotroph Cells of Anterior Pituitary)
• GHIH - Inhibits the synthesis and release of GH (Somatotroph Cells of Anterior Pituitary)
• CRH - Stimulates the synthesis and release of ACTH (Anterior Pituitary)
• ACTH - Stimulates the synthesis and release of Cortisol (zona fasiculata of adrenal cortex in kidneys)
• TSH - Stimulates the synthesis and release of a majority of T4 (Thyroid Gland)
• LH - Stimulates follicular maturation and ovulation in women; Stimulates testosterone production and spermatogenesis in men
• FSH - Stimulates follicular development in women; Stimulates spermatogenesis in men
• PTH - Increases blood calcium levels (PTH1 Receptor: Kidneys and Bone; PTH2 Receptor: Central Nervous system, Bones, Kidneys, Brain)
• Calcitonin - Decreases blood calcium levels (Calcitonin Receptor: Intestines, Bones, Kidneys, Brain)
• Glucagon - Stimulates glycogen breakdown (liver)
• hCG - Promotes cellular differentiation; Potentially involved in apoptosis

Ref:
http://inbehindthedarkness.blogspot.com/p/ce.html

1. http://proteopedia.org/wiki/index.php/G_protein-coupled_receptor

2. http://jcs.biologists.org/content/116/24/4867.full.pdf+html

3. http://en.wikipedia.org/wiki/G_protein-coupled_receptor
4. http://www.wjgnet.com/1007-9327/12/7753.pdf
5. http://www.rcsb.org/pdb/101/motm.do?momID=58
6. http://en.wikipedia.org/wiki/G_protein
7. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellSignaling.html#Introduction

Different food experiences among population

Why we differ in taste perception

Humans use several kinds of information to decide what to eat, and the combination of experience and sensory evaluation helps us to choose whether to consume a particular food. If the sight, smell, and taste of the food are acceptable, and we see others enjoying it, we finish chewing and swallow it. Several senses combine to create the idea of food flavor in the brain. For example, a raw chili pepper has a crisp texture, an odor, a bitter and sour taste, and a chemesthetic ‘burn.’ Each of these sensory modalities is associated with a particular group of receptors: at least three subtypes of somatosensory receptors (touch, pain, and temperature), about 400 human odor receptors, which respond either singly or in combination at least five types of taste receptors [bitter, sour, sweet, salty, and umami (the savory experience associated with monosodium glutamate)], and several families of other receptors tuned to the irritating chemicals in foods, especially herbs and spices (for example, Eugenol found in cloves or Allicin found in garlic). The information from all these receptors are transmitted to the brain, where it is processed and integrated. Experience is a potent modifier of chemosensory perception, and persistent exposure to an odorant is enough to change sensitivity.

The combined senses of taste, smell and the common chemical sense merge to form what we call ‘flavor.’ People show marked differences in their ability to detect many flavors. Most of the genes identified to date encode receptors responsible for detecting tastes or odorants. Although the repertoire of receptors involved in taste perception is relatively small, with 25 bitter and only a few sweet and umami receptors, the number of odorant receptors is much larger, with about 400 functional receptors and another 600 potential odorant receptors predicted to be non-functional. Despite this, to date, there are only a few cases of odorant receptor variants that encode differences in the perception of odors: receptors for androstenone (musky), isovaleric acid (cheesy), cis-3-hexen-1-ol (grassy), and the urinary metabolites of asparagus. A genome-wide study also implicates genes other than olfactory receptors for some individual differences in perception. Although there are only a small number of examples reported to date, there may be many more genetic variants in odor and taste genes yet to be discovered.

Each person lives in a unique flavor world, and part of this difference lies in our genetic composition, especially within our sensory receptors. This idea is illustrated by bitter perception and bitter receptors. The bitter receptor family, TAS2, has approximately 25 receptors, found at three locations in the human genome. We say ‘approximately’ because bitter receptors have copy number variants, and it is currently unclear at what point a recently duplicated gene should be assigned a distinct name. This conundrum is more than a mere matter of record-keeping; the bitter receptor gene copy number is a source of biological variation and may affect perception, although this prospect has not yet been demonstrated empirically. 

The first demonstration that genetic variants contribute to person-to-person differences in human taste perception was for the bitter receptor TAS2R38. It has been known since 1931 that some people are insensitive to the bitter compound phenylthiocarbamide (PTC), a chemical that was synthesized by Arthur Fox for making dyes. While he was working in his laboratory, Fox accidentally tasted the compound and found it bland, yet when his benchmate also accidentally tasted the compound, he found it very bitter. This observation contributed to the formation of a hypothesis, now widely accepted, that there is a family of bitter receptors, at least one of which is sensitive to this compound, but that is inactive in some people. In 2003, this hypothesis was tested using genetic linkage analysis. Relatives such as parents and children were assessed for their ability to taste PTC and for their pattern of DNA sharing. The genomic region most often shared by relatives with similar tasting ability was near the TAS2R38 gene, but this evidence in itself was insufficient to conclude that the TAS2R38 gene was responsible for this sensory trait. Genes encoding bitter taste receptors are physically clustered on chromosomes, and nearby DNA regions tend to be inherited together, so it was not clear whether TAS2R38 or a neighboring receptor was the responsible gene. This issue was resolved later, when individual bitter receptors were introduced into cells without taste receptors. Only those cells that contained the TAS2R38 gene responded to PTC. Moreover, cells containing naturally occurring genetic variants of the TAS2R38 gene from people who could not taste PTC were also unresponsive to this bitter compound. Together, these data showed that TAS2R38 and its variants explained the inability of some people to taste PTC at concentrations at which it is readily detectable to others. The inability to taste PTC as bitter can be considered a categorical trait (either people can taste it or they cannot), and can also be considered a quantitative trait, that is, as a continuum, but with most people falling at either end.

Although the genetics of taste perception has been dominated by the study of PTC and its effects, evidence is gradually accumulating that the ability (or inability) to perceive other bitter tastes is heritable. For example, identical twins, who have identical genetics, are more similar in their perception of bitter compounds (other than PTC) than are fraternal twins, who are no more similar genetically than siblings. A variant in a cluster of bitter receptors on chromosome 12 is associated with quinine perception, and the bitterness of some high-intensity sweeteners is associated with alleles within a cluster of bitter receptors on chromosome 12. These observations suggest that individual differences in bitter perception may be common, and related to genotype.

Bitterness is a part of human life in two ways, in food and in medicine. In general, humans tend to avoid bitter foods; in a study by Mattes, nearly half of people surveyed ate no bitter foods at all. When these subjects were asked to consume a bitter solution, they diluted it with water until the bitterness could no longer be detected. Other common methods to reduce bitterness include cooking, or the addition of salt or flavors, but bitterness is not an inevitable part of life for everyone. To illustrate this point, when we asked 8 people to rate 23 vegetables for bitterness intensity, we found that some people were insensitive to even the most bitter vegetables. Of course,    people who are sensitive to the bitterness of a particular vegetable or other food can avoid eating it.

Beyond the receptor

Most of the known gene variations relating to perceptual differences in taste and smell are specific to a single receptor. It may be that receptor variation affects only the perception of its ligand or it may have broader effects due to brain rewiring (in response to missing input) or to the clustering of receptor variants (LD). Thus, more characterization of human perceptual differences in conjunction with genotype studies is needed. The reduced ability to detect a single compound (such as PTC) might be associated with a reduced ability to detect structurally unrelated bitter compounds or even other taste qualities. 

Food-tasting business (Getting paid to eat)

The only thing that matters here is the food. Behind a door marked "Sensory Staff Only," a dozen or so professional tasters spend their days testing ingredients that end up in thousands of products around the world. They sniff, taste, feel and spit. All day long. 

Painstakingly trained, these tasters can describe the difference between a "vine-y green" and a "fresh-cut grassy green," a roasted peanut note from an over-roasted one. They are 'sensory specialists" — and an increasingly mechanized, sophisticated and complex food industry rests on their taste buds. "You get paid to eat stuff," said Chaquita Johnson-Moore, a chef-turned-sensory panelist, during a recent break from a session tasting a nutritional drink. "People can't believe there are people who do this."

Ref:

1.http://www.flavourjournal.com/content/1/1/9/abstract 

2.http://www.stltoday.com/business/local/article_5fe1a780-f782-5f5f-bd6e-72b4b13f87f8.html