Nuclear receptors (L6) Flashcards
What are the different types of receptors?
Cell surface receptors - interact with hydrophilic signals. Needs signal transduction to have an effect within the cell
Intracellular receptors - carried by a protein to the cell and then enter through the membrane and travels to the nucleus. Intracellular signals include gases like NO and CO and nuclear receptors like steroid hormones, thyroid hormones, retinoids and vitamin D
What are the properties of Nitric oxide?
Gaseous, is a free radical (has an unpaired electron in its outer shell, therefore it is very reactive and has a half-life f about 5-10 seconds.
How is NO made?
Made through 2 successive oxidations catalysed by nitric oxide synthase (NOS). The cell starts with L-arginine which goes to N-hydroxy arginine through the addition of an O (from oxygen) and H (from NADPH). The same reaction then happens again to make L-citrulline and NO
What varieties does NOS come in?
Constitutive NOS (cNOS) has an enzymatic activity induced by Ca2+/Calmodulin (CaM). Its expression is constitutive and its activity is short-lived, it makes picomoles of NO. There are 2 types: epithelial NOS which is bound at the cell membrane and is expressed in endothelium, cardiac myocytes, renal mesangial cells, osteoblast/clasts. Neural NOS is cytosolic and is expressed in the CNS, NANC neurones, ENS and retina. Inducible NOS also have constitutive enzymatic activity. Its expression is inducible and it produces nanomoles of NO, its longer lasting. iNOS is transcriptionally induced in response to pathological stimuli e.g. LPN, IFN-g, IL-1. Its expressed in macrophages and Kupffer cells, neutrophils, fibroblasts, vascular smooth muscle and endothelial cells.
What is the effect of NO on vessels?
- Autonomic nerves release ACh which acts on endothelial cells (eNOS)
- endothelial cells release NO which causes surrounding smooth muscle to relax (vasodilation)
- NO inhibits platelet aggregation and vascular smooth muscle proliferation (anti-atherosclerosis)
Explain the signalling by which NO works
ACh activates cNOS in endothelial. NOS generates NO from arginine. NO activates the production of cGMP. cGMP leads to activation of cGMP-dependant protein kinase (PKG). This activates myosin light chain phosphatase which releases myosin light chains.
What are the effects of NO in the brain?
In the CNS, nNOS is tethered close to NMDA-type glutamate receptors so that it can respond to the Ca increases near the open channels. This has implications for synaptic plasticity. Glutamate is produced by a presynaptic terminal and this activates glutamate receptors, especially NMDA receptors, which in turn stimulate the NO pathway. NO produced this way travels to the presynaptic terminal and increases the production of glutamate, thus creating the state of LTP.
What are the effects of NO in the immune system?
NO free radicals act in the immune system by killing bacteria and parasites and inducing programmed cell death.
iNOS in inflammatory cells uses NO as a cytostatic and cytotoxic agent (involves higher levels). It can also trigger inflammatory diseases when overproduced in lungs and intestines, can cause rheumatoid arthritis, chrons and asthma. It plays a role in tumour apoptosis and necrosis.
What is nitroglycerine and how does it work?
Acts as a pharmaceutical but can also be used to make dynamite lol. Can be used for treating angina (pain in the heart resulting from inadequate blood flow) nitroglycerine breaks down in vivo to generate NO which relaxes blood vessels. This decreases the load in the heart and also increases heart blood supply
How does viagra work?
Build up of cGMP in cells is counteracted by phosphodiesterase 5. Viagra is an inhibitor of phosphodiesterase 5. Therefore, causes less breakdown of cGMP, so more builds up. This causes smooth muscle constricting vessels to relax, increasing blood flow.
Give some examples of signalling molecules that bind to nuclear receptors
Steroid hormones, which include cortisol, steroid sew hormones, vitamin D and endysone. These are all made from cholesterol. Thyroid hormones are made from amino acid tyrosine. Retinoids like retinoic acid are made from vitamin A and have important roles e.g. local mediators in vertebral development. All are relatively insoluble in water, so they are made soluble for transport in the bloodstream and other extracellular fluids by binding to specific carrier proteins, from with they dissociate from before entering a target cell.
Explain the topology of nuclear receptors
Inhibitory proteins hold receptors in an inactive state. Binding to the ligand removes the inhibitors leading to a conformational change. the receptors now interact with coactivators. The effects of the ligands are modified in different tissues by the expression of different coactivators. orphan receptors look similar to receptors but have no ligand. The DNA binding domain of each nuclear receptors contains 2 repeats of C4 Zinc fingers (i.e. the zinc is bound by 4 conserved cysteines).
What is the structural organisation of the estrogen receptor?
It has an N terminus domain, DNA binding domain which consists of Zn fingers. - each finger binds to a specific DNA sequence, a hinge region, a ligand binding domain and a C terminal. Structures of many parts of these domains aren’t well known.
Do nuclear receptors act as homodimers or heterodimers?
Both! Nuclear receptors act as homodimers (steroid) which bind to inverted repeats, or heterodimers (vitamin D, thyroid hormones, RA) which bind to direct repeats. For the latter, the spacing between direct repeats in the response element (RE) is important. The nuclear receptor family members which form heterodimers (and bind a direct repeat) all share a partner in RXR e.g. RXR_VDR heterodimer, RXR-RAR heterodimer) RXR is similar in structure to the other nuclear receptors, but in its role as a heterodimer, it isn’t bound to the ligand
Where are the homo/heterodimeric nuclear receptors found? How are they regulated?
The homodimers receptors are found in the cytoplasm in the absence of ligands and then translocate to the nucleus upon hormone binding. In the absence of a hormone, the receptors are anchored in the cytoplasm by inhibitor proteins including Hsp90. Hormone binding causes the release of the inhibitor proteins, allowing them to enter the nucleus. The heterodimeric nuclear receptors are located exclusively in the nucleus. The act as repressors in the absence of ligand (by recruitment of histone deacetylases) and activators in the presence of ligand (by recruitment of histone acetylases)
