G protein-coupled/enzyme linked receptor signalling

what is signal transduction

• when signals are converted into a cellular response

describe the structure of GPCR receptor structure and the function

• 7 transmembrane alpha helixes

• e.g - olfaction, regulation of immune responses and inflammation

what are some of the functions of GPCRs

• signal transducing molecules - transfer extracellular signals into the cell, leading to a cellular response

• inactive G proteins are trimers consisting of 3 subunits (alpha, beta and y) with a GDP molecule bound to the alpha subunit

• G proteins are tethered to the cell membrane by the a and y subunits, freely diffusible in the plane of the membrane

process of GPCR activation and inactivation

• ligand bind to GPCR and causes conformational shape change of receptor

• resulting in coupling of the receptor with a G protein trimer, coupling causes a conformational shape change in the G protein

• GDP on the alpha subunit of the G protein dissociates and is replaced by intracellular GTP

• the alpha subunit GTP complex and the by-complex dissociate from the receptor and from eachother, both subunits are free to diffuse in the membrane and interact with enzymes or ion channels leading to signalling within the cell

• GTP is hydrolysed to GDP by GTPase activity of the alpha subunit

• resulting alpha subunit-GDP complex reunites with the by complex and signalling stops

example of G- protein coupled receptor in practise

• apomorphine is a drug used to induce vomiting

what are the 4 main classes of G protein

• Ga_s, Ga_i/o, Ga_q/11, Ga_12/13

• Ga_s and Ga_i/o produce stimulation and inhibition of adenylyl cylase which produce cyclic AMP

example of GPCR linked to Ga_s

• B1 adrenorceptor

• stimulation of the B1 adrenoreceptor by adrenaline increases heart rate

example of GPCR linked to Ga_i/o

• opioid mu-receptor

• opioid analgesics - both Ga_i and GBy components work to reduce pain signals

function of Ga_q/11 receptors

• catalyses phospholipase C to produce inositol triphosphate (IP3) and diaglycerol (DAG)

• IP3 is a secondary messenger which leads to the release of intracellular calcium (important role in the action of hormones - DAG activates protein kinase C, role in cell function)

example of GPCR linked to Ga_q

• histamine (H1) receptor

• allergic disease

• stimulation of histamine receptors on mast cells leads to the release of inflammatory mediators and increased nasal secretion

function of Ga_12/13

• subunits interact with guanine nucleotide exchange factors for Rho family small GTPases, play a key role in signal transduction pathways that regulate various cellular processes, including cytoskeletal dynamics, cell migration, proliferation and survival

• targeting this pathway is a therapeutic interest for conditions like metastatic cancers and inflammatory diseases

• primary effectors of GBY complexes are ion channels, adenylyl cyclase, phospholipase C and PI3 kinase

• no currently approved drugs target this type of GPCR

describe how enzyme linked receptors are activated and the main types

• activated by a wide variety of mediators including growth factors, cytokines and hormones

• major role in growth and cell division, inflammation, immune responses

• main types are receptor tyrosine kinases (RTKs), receptor serine/threonine kinases and cytokine receptors

• effect takes minutes to hours

describe the enzyme linked receptor structure

• large extraceullar binding domain connected via a single membrane spanning a-helix to the intracellular domain (tyrosine kinase activity)

• activation of the receptor polypeptide chains usually involved dimerization followed by autophosphorylation of tyrosine residues, relay proteins bind to the phosphorylated tyrosines and activate downstream signalling

describe the process of kinase-linked receptor activation and inactivation

• ligand binds to receptor and leads to dimerization

• association between the 2 intracellular domains creates and active kinase enzyme

• tyrosine residues are phosphorylated

• phosphorylated tyrosine residues act as docking sites for other intracellular relay proteins which are then activated themselves

• a cascade of events ultimately leads to biological effect

• activity of the receptor is terminated by protein tyrosine phosphatases

describe how RTKs act through Ras and PI3-kinase pathways

• Ras and PI3-kinase pathways play a key role in cell division, growth and differentiation

• Ras and PI3-kinase pathways are important targets for anti cancer therapy

describe the Jak-Stat pathway

• the Jak-Stat pathway is activated by many cytokines and is important in inflammation

• oclacitinib in a janus kinase inhibitor - target cytokines are those that are pro inflammatory or have a role in allergic responses (also those involved in host defence or haematopoiesis)