Describe the signalling pathways downstream of the heterotrimeric G proteins Gs, Gi and Gq

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Describe the signalling pathways downstream of the heterotrimeric G proteins Gs, Gi and Gq

Gs, Gi and Gq proteins are part of a family of proteins called heterotrimeric

G-proteins, named due to the binding of such proteins to purine nucleotides, that is, guanosine triphosphate (GTP) and guanosine diphosphate (GDP). G-proteins are divided by the four different isoforms: Gαq, Gαs, Gαi and Gα12/13 and are done so based

on sequence similarityiii, the prior three being the ones that will be discussed in terms of their respective signalling pathways in this essay.

Heterotrimeric G-proteins are formed by a complex of three (tri-) different (hetero-) parts: the Gα subunit, the largest, which contains the binding site that allow the conversion of GTP to GDP to enable a renewal of the G-protein cycle, and the Gβ and Gγ subunits, which form a heterodimer (Gβγ) and are important in the modulation of the α subunit and their own signalling pathwaysiii. Each of these subunits is composed with different amino acid compositions and thus have different structuresiv_.

G-proteins are associated with the inner surface of the plasma membrane and they interact with many cellular proteins including kinases, channels, GPCRs, and arrestins. G-proteins act as signal transducers that communicate signals from many hormones, neurotransmitters, chemokines, and autocrine and paracrine factorsv. The intracellular partner of heterotrimeric G-proteins that will be explored in this essay is the seven transmembrane GPCR (G-protein coupled receptor), which interact differentially with heterotrimeric G-protein isoforms, effectors and second messengersvi. It is interesting to note that receptors from multiple different functions in the human body, that is for adrenergic amines, serotonin, acetylcholine, many peptide hormones, odorants and visual receptors, all belong to the GPCR familyvii - GPCRs play a key role. GPCRs are activated by agonist ligands, causing a conformational change that is transmitted to cytoplasmic loops of the receptors, in turn activating the appropriate G-protein by the replacement of GDP by GTPviii.

G-proteins are a class of membrane-bound regulatory proteinsix_{that control the}

concentrations of second messengers, affected by a series of events called a signalling cascade that include the hydrolysis of GTP and the interactions between subunits,

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which is important in the amplification of transduced signals to reach a final responsex. The process of G-proteins’ success is dependent on the binding of an endogenous ligand to activate a site on the GPCR to allow the binding of the heterotrimeric G-protein. This enables the release of a GDP molecule (ADP equivalent) from the heterotrimer and the binding of GTP (ATP equivalent), which causes the dissociation of the Gα subunit from the Gβγ heterodimer. Moreover, it is important to know that the duration of activation of certain enzymes depends on the longevity of GTP binding to the G-protein rather than the receptor’s affinity for the chemical signalxi. The Gα subunit is then activated to mediate signal transduction through various enzymes, such as adenylyl cyclase and phospholipase C; the βγ complex follows suite, undertaking the modulation of ion channel or enzyme activity; both the Gα subunit and βγ complex migrate laterally to carry out their respective pathways. The cycle of G protein activity is completed by the hydrolysis of GTP to GDP, causing a re-association of the three subunits and restoring them to their inactive statesxii.

G-proteins are essential in the synthesizing of responses to activate specific receptors in transmitting signals to specific enzymes or intercellular receptors from the plasma membrane, such as cyclic AMP (cAMP, a second messenger), regulated by the pathway of Gs and Gi proteins, or inositol 1,4,5-trisphosphate (IP3), regulated by the

pathway of Gq proteins, respectivelyxiii. The pathway of G-proteins is crucial in human

processes, such that mutations in G-proteins or their receptors that modify their interactions can lead to a variety of endocrinopathiesxiv.

The first G-protein to ever be identified was Gαs (Gs), which was found to

activate adenylyl cyclase, subsequently stimulating the production of adenylyl cyclase moleculesxv. Shortly after, the Gi protein was discovered, which was found to inhibit the

actions of the Gs protein - reducing the production of adenylyl cyclase molecules. The

cAMP formed by the activation of Gs binds to other proteins, such as enzymes and ion

channels to alter cell activity, a response that is dependent on the type of enzymes that are expressed within the cellxvi.

An example of the essentiality of the Gs protein is evident in human exercise,

during which the sympathetic nervous system is activated, increasing circulating catecholamines (adrenalin, the chemical signal) secreted by the adrenal medulla, in

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turn increasing metabolism levelsxvii. During the fed state of human beings, that is, when a person is eating or just after eating, skeletal muscle works to convert glucose into large polysaccharide molecules to store energy for when it is required. This glycogen is broken down back into glucose during exercise by use of ATP to give rise to muscle contraction; this is the result of the Gs pathway. This process begins when

the increased levels of adrenalin in the system activate a specific type of adrenergic receptor on the muscle membrane called the β-adrenergic receptor (β-adrenoceptor); these are linked to Gs proteins. Signal transduction is the following step, during which

the α and βγ subunits dissociate and adenylyl cyclase is activated, generating an increase in the level of intracellular cAMP. The amplification of cAMP triggers a cascade of enzyme activations, starting with the protein kinase A (PKA) enzyme by dissociation of its subunits, followed by the enzyme glycogen phosphorylase, which then facilitates the biological response of the breakdown of glycogen into glucosexviii. It is thus easy to see how the activation of these β-adrenoreceptors, or rather the increase of intracellular cAMP, is crucial in allowing humans the ability to increase inotropy and dromotropy, and thus the potential to have mobility.

Due to the key role of this second messenger in human mobility, it is important that it is constantly regulated to ensure muscle does not contract or relax when such an action is not required. This regulation of cAMP molecules is done by an isoform of phosphodiesterases (PDE), also known as cyclic nucleotide phosphodiesterases, which control the rate of cAMP degradation to AMP by breaking a phosphodiester bondxix (adenosine 3’,5’-cyclic phosphate + H2O à adenosine 5’-phosphate)xx ,

inhibiting the amplification signal of cAMP production. Another method of preventing the production of cAMP is to inhibit what precedes it in the signalling cascade, that is, inhibiting adenylyl cyclase. The Gi protein modulates this action; the activation of

receptors coupled to Gi causes a diminishing of intracellular cAMP, therefore

decreasing PKA production.

Similar to Gs proteins, Gq proteins are important in the body’s response to

danger by using catecholamines to induce constriction in blood vessels in the skin, done by acting on Gqα1 receptors; this therefore makes Gq proteins just as important as

Gs proteins. However, G-proteins that are classed under the Gαq class have been

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C isoenzymes (PLC-β)xxi. These isozymes are most commonly activated by GPCRs and heterotrimeric G-proteins either by the release of α-subunits of the Gq family or by

Gβγ dimers from activated Gi family membersxxii, and they act to hydrolyse the

phosphodiester bond of the phosphatidylinositol 4,5-bisphosphate (PIP2) plasma

membrane lipid to generate the second messengers diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3), which function as intracellular mediators and both increase

intracellular concentrations of free calcium ionsxxiii. Gq has also been reported to

activate the transcription factor NFκB through proline-rich tyrosine kinase-2xxiv. The process of stimulation of the second messenger concentration increase is initiated by the release of acetylcholine, which acts on a muscarinic receptor. This receptor activation leads to the dissociation of the G-proteins and the subsequent activation of phospholipase Cxxv_.

DAG generated by the hydrolysis of phosphatidyl inositol is a hydrophobic molecule, it is therefore retained in the membrane when IP3 is producedxxvi. Like many

other membrane lipids DAG is able to diffuse in the plane of the membrane where it progresses to activate the enzyme protein kinase C (PKC) to phosphorylate targeted proteins in generating various physiological responses, such as increasing the rate of DNA transcription or receptor activationxxvii_{, whilst IP}

3 releases calcium from internal

stores, leading to the activation of calcium-dependent events such as secretionxxviii xxix.

In conclusion, it can be seen that G-proteins, in this case Gs, Gi and Gq

proteins, are crucial in the many processes of the human system. These G-proteins allow us to avoid and survive dangers in everyday life and control even smaller ionic processes in the body, such as the regulation of Ca2+ ions.

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Citations

i_{"Pathway Central: GPCR Pathway." Pathway Central: GPCR Pathway. N.p., n.d. Web. 06 Jan. 2013.}

ii_{McCudden, C. R., M. D. Hains, R. J. Kimple, D. P. Siderovski, and F. S. Willard. "G-protein Signaling:}

Back to the Future." CMLS Cellular and Molecular Life Sciences 62.5 (2005): 552. Print.

iii_{Fauci, Anthony S., and Tinsley Randolph Harrison. Harrison's Principles of Internal Medicine. New York}

[etc.: McGraw-Hill, Medical Division, 2012. 2869. Print.

iv_{Pocock, Gillian, and Christopher D. Richards. "Principals of Cell Signaling." The Human Body: An}

Introduction for the Biomedical and Health Sciences. Oxford: Oxford UP, 2009. 55. Print.

v_{Neves, Susana R., Prahlad T. Ram, and Ravi Iyengar. "G Protein Pathways." Science. N.p., 31 May}

2002. Web. 06 Jan. 2013.

vi_{Belcheva, Mariana M., and Carmine J. Coscia. "Diversity of G Protein-Coupled Receptor Signaling}

Pathways to ERK/MAP Kinase." Neurosignals 11.1 (2002): 35. Print.

vii_{Von Zastrow, Mark, MD, PhD, ed. "Drug Receptors & Pharmacodynamics." Basic & Clinical}

Pharmacology. Ed. Bertram G. Katzung, MD, PhD, Susan B. Masters, PhD, and Anthony J. Trevor, PhD.

12th ed. N.p.: McGraw-Hill Medical, 2011. 25. Print. viii_Ibid.

ix_{Pocock, Gillian, and Christopher D. Richards. "Principals of Cell Signaling." The Human Body: An}

x_{Ibid., at 54}

xi_{Von Zastrow, Mark, MD, PhD, ed. "Drug Receptors & Pharmacodynamics." Basic & Clinical}

Pharmacology. Ed. Bertram G. Katzung, MD, PhD, Susan B. Masters, PhD, and Anthony J. Trevor, PhD.

12th ed. N.p.: McGraw-Hill Medical, 2011. 25. Print.

xii_{Fauci, Anthony S., and Tinsley Randolph Harrison. Harrison's Principles of Internal Medicine. New}

York [etc.: McGraw-Hill, Medical Division, 2012. 2869. Print.

xiii_{Pocock, Gillian, and Christopher D. Richards. "Principals of Cell Signaling." The Human Body: An}

xiv_{Fauci, Anthony S., and Tinsley Randolph Harrison. Harrison's Principles of Internal Medicine. New}

York [etc.: McGraw-Hill, Medical Division, 2012. 2869. Print.

xv_{McCudden, C. R., M. D. Hains, R. J. Kimple, D. P. Siderovski, and F. S. Willard. "G-protein Signaling:}

Back to the Future." CMLS Cellular and Molecular Life Sciences 62.5 (2005): 554. Print.

xvi_{Pocock, Gillian, and Christopher D. Richards. "Principals of Cell Signaling." The Human Body: An}

xvii_Ibid. xviii_Ibid.

xix_{Bender, A. T. "Cyclic Nucleotide Phosphodiesterases: Molecular Regulation to Clinical}

Use." Pharmacological Reviews 58.3 (2006): 490. Print.

xx_{"CAMP-specific cyclic Phosphodiesterase 4D - Homo Sapiens (Human)." CAMP-specific}

3',5'-cyclic Phosphodiesterase 4D - Homo Sapiens (Human). N.p., n.d. Web. 30 Dec. 2013.

xxi_{McCudden, C. R., M. D. Hains, R. J. Kimple, D. P. Siderovski, and F. S. Willard. "G-protein Signaling:}

Back to the Future." CMLS Cellular and Molecular Life Sciences 62.5 (2005): 554. Print. xxii_{Ibid., at 556}

xxiii_{"Pathway Central: GPCR Pathway." Pathway Central: GPCR Pathway. N.p., n.d. Web. 06 Jan. 2013.}

xxiv_{Neves, Susana R., Prahlad T. Ram, and Ravi Iyengar. "G Protein Pathways." Science. N.p., 31 May}

2002. Web. 06 Jan. 2013.

xxv_{Pocock, Gillian, and Christopher D. Richards. "Principals of Cell Signaling." The Human Body: An}

xxvi_Ibid. xxvii_Ibid.

xxviii_{Rebecchi, Mario J. "Structure, Function, and Control of Phosphoinositide-Specific Phospholipase} C." Physiological Reviews 80.4 (2000): 1299. Physiological Reviews. 10 Jan. 2000. Web. 3 Jan. 2013.

xxix_{Pocock, Gillian, and Christopher D. Richards. "Principals of Cell Signaling." The Human Body: An}

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Bibliography

Journals

• Belcheva, Mariana M., and Carmine J. Coscia. "Diversity of G Protein-Coupled Receptor Signaling Pathways to ERK/MAP Kinase." Neurosignals 11.1 (2002): 34-44. Print.

• Bender, A. T. "Cyclic Nucleotide Phosphodiesterases: Molecular Regulation to Clinical Use." Pharmacological Reviews 58.3 (2006): 488-520. Print.

• McCudden, C. R., M. D. Hains, R. J. Kimple, D. P. Siderovski, and F. S. Willard. "G-protein Signaling: Back to the Future." CMLS Cellular and Molecular Life

Sciences 62.5 (2005): 551-77. Print.

• Rebecchi, Mario J. "Structure, Function, and Control of Phosphoinositide-Specific Phospholipase C." Physiological Reviews 80.4 (2000): 1291-335. Physiological

Reviews. 10 Jan. 2000. Web. 3 Jan. 2013.

Books

• Fauci, Anthony S., and Tinsley Randolph Harrison. Harrison's Principles of Internal

Medicine. New York [etc.: McGraw-Hill, Medical Division, 2012. Print.

• Klabunde, Richard E. Cardiovascular Physiology Concepts. Philadelphia, PA: Lippincott Williams & Wilkins/Wolters Kluwer, 2012. Print.

• Pocock, Gillian, and Christopher D. Richards. The Human Body: An Introduction for

the Biomedical and Health Sciences. Oxford: Oxford UP, 2009. Print.

• Von Zastrow, Mark, MD, PhD, ed. "Drug Receptors & Pharmacodynamics." Basic &

Clinical Pharmacology. Ed. Bertram G. Katzung, MD, PhD, Susan B. Masters, PhD,

and Anthony J. Trevor, PhD. 12th ed. N.p.: McGraw-Hill Medical, 2011. Print.

Websites

• "CAMP-specific 3',5'-cyclic Phosphodiesterase 4D - Homo Sapiens (Human)."CAMP-specific 3',5'-cyclic Phosphodiesterase 4D - Homo Sapiens

(Human). N.p., n.d. Web. 30 Dec. 2013.

• Neves, Susana R., Prahlad T. Ram, and Ravi Iyengar. "G Protein Pathways." Science. N.p., 31 May 2002. Web. 06 Jan. 2013.