fundamental molecular receptor pharmacology two Flashcards
homologous desensitisation of GPCR ?
Homologous Desensitisation of the G protein coupled receptor occurs as phosphorylation of the activated receptor by a specific kinase (GPCR kinase, GRK) happens. The phosphorylated receptor then binds to beta arrestin, causing the receptor to lose its ability to associate with a G-protein, and to undergo endocytosis, which removes the receptor from the membrane and hinders subsequent binding of a ligand to the receptor. The beta gamma subunits lose their ability to bind to the G protein and activate it. The GTP, GDP cycle does not occur, and the effector enzymes are not activated and as a result the secondary messengers are not produced
heterologous desensitisation ?
Heterologous (cross-) desensitisation of the GPCR occurs as a result of phosphorylation of one type of receptor as a result of activation of kinases by another. PKA and PKC, protein kinase A and C, respectively
howis cAMP removed ?
cyclic AMP/cGMP the action of phosphodiesterase’s can terminate the cyclic AMP/cGMP signal. Cyclic AMP is hydrolysed to inactive AMP by a family of cyclic AMP phosphodiesterases (there are 11 family members).
PDE4 inhibitors work ?
Disease processes can be characterised by inappropriate desensitisation. For example the processes haven’t been deactivated and there is persistent stimulation occuring by Gi , Gs and Gq. PDE4 is involved in the activation of inflammatory cells. Therefore PDE4 inhibitors are viable anti-inflammatory drugs (Roflumilast-COPD, Apremilast-psoriasis).
male incompitence ?
in male incompetence the cyclic GMP is an important molecule in the male erection disfunction. In erectile dysfunction the phosphodiesterase that breaks down cyclic GMP is elevated so cyclic GMP is not made. Inhibitors can work around this issue , as this raises the cyclic GMP levels. PDE5 is the Viagra target. Viagra (sildenafil) was developed by structure-focussed design.
loss of function mutations ?
Loss-of-Function Mutations involving the G protein and GPCR is when there is a block in the signalling response to the corresponding agonist(s) which binds to the receptor
gain of function mutations ?
Gain-of-Function mutations is a lead to constitutive, agonist-independent activation of signalling. The responses are independent of agonist as they are always turned on. This mutation can be in the Gs protein.
polymorphisms ?
Polymorphisms occur in the genes that encode for GPCR and G proteins , they occur in the promoter region and high levels or very low levels of these receptors and G proteins are expressed in disease states.
germline mutations ?
For germline mutations, the phenotype will be pleiotropic for widely expressed genes and more focal for genes expressed more narrowly. For example, in the sperm and egg and passed to the developing foetus.
somatic mutations ?
In contrast, somatic mutation of even a ubiquitously expressed gene can cause focal manifestations. Occur in an adult and the result is a localised response.
2 types of kinase liked receptors ?
There are 2 types of kinase linked receptors, these are receptors that contain their own intrinsic kinases and others that need to recruit kinases to the receptors.
These kinases modulate phosphorylation events that are within the cellular signalling pathway. This leads to the production of transcription factors that promote gene expression , which are converted to proteins that modulate the pathways.
insulin receptor ?
One of the best characterised is the insulin receptor. Like the EGF and PDGF receptor it contains a tyrosine specific kinase activity. This is an enzyme which catalyses the transfer of the terminal phosphate of ATP into various important biological proteins. However, it does so by phosphorylating tyrosine residues in these proteins. Other kinases such as protein kinase A, G and C do so specifically on serine/threonine residues. This is of interest because several oncogenes (cancer genes) which promote cell growth and differentiation are also constitutively active tyrosyl kinases and these appear to be the primary signal proteins for promoting the development of cancers. Kinase linked receptor involves a process where binding of a ligand to the receptor causes the recruitment of a tyrosine kinase to the receptor.
structure of kinase linked receptor ?
There’s a single polypeptide chain and the N terminus on the outside of the cell which binds the ligand. The catalytic domain extends in the cytosol and can influence the enzymes located in the cytosol of a cell. The binding of a ligand to the N terminal domain causes a conformational change in the receptor and this is transmitted to the cytoplasmic domain.
These receptors are different than the GPCR as they only contain one transmembrane domain instead of 7 TM seen in the GPCR.
explain growth factor pathway ?
Growth factor could be insulin ( exists as a dimer) receptors which bind to growth factor and causes a intermolecular tyrosyl phosphorylation of the receptor.
This causes the recruitment of a protein from the cytoplasm, termed Grb-2, which interacts with the receptor by binding close to the tyrosine phosphorylation site via an SH-2 domain. Grb-2 in turn binds a protein called msos (son of sevenless) via an SH-3 domain.
msos causes activation of a proto-oncogene termed p21ras by promoting the GDP-GTP exchange in this protein. Activated GTP-bound p21ras in turn localises a protein called raf-1-kinase to the cell membrane. This sets in motion a kinase cascade that results in the activation of Mec and also mitogen-activated kinase (MAPK) and which catalyses the phosphorylation and activation of p90rsk . Activated MAPK and p90rsk translocates to the nucleus where it activates transcription factors, which in turn induces immediate early genes, e.g. c-fos, which provoke DNA synthesis and commit the cells to divide. There’s an intrinsic amplification that occurs at each step, one Mec can activate a 100 MAPK. In cancer there is an increase in the activation of the proteins, to promote cell growth. These are called onco-genes.
cytokine receptors ?
Cytokine receptors are also important in cell signalling , an example is IL-6 receptors which are involved in inflammatory response. Binding of a ligand to the receptor ( dimer) causes the recruitment of a non-receptor tyrosine kinase called JAK (Janus kinase). This phosphorylates the receptor on tyrosine residues-which can establish signalling similar to receptor tyrosine kinase. JAK also phosphorylates itself and this causes binding of the transcription factor STAT (Signal Transducer Activator of Transcription). The JAK-STAT complex which is now a dimer moves to the nucleus to induce gene expression.
