BMS 100 Physiology Concepts IIIA PDF

Physiology Concepts IIIA Intracellular Signaling & the Cell Membrane Post-learning – Receptor Tyrosine Kinases and Nitric Oxide Dr. Vargo BMS 100 Week 5 The Enzyme-Coupled Receptors Molecular Biology of the Cell, 6th ed. Alberts et. al. p. 818, fig 15-6 Enzyme-coupled receptors • Transmembran...

Physiology Concepts IIIA Intracellular Signaling & the Cell Membrane Post-learning – Receptor Tyrosine Kinases and Nitric Oxide Dr. Vargo BMS 100 Week 5 The Enzyme-Coupled Receptors Molecular Biology of the Cell, 6th ed. Alberts et. al. p. 818, fig 15-6 Enzyme-coupled receptors • Transmembrane proteins with ligand-binding domain on outer surface of the plasma membrane ▪ Usually only 1 transmembrane domain • Cytosolic domain has either: ▪ Intrinsic enzyme activity • Most common class is receptor tyrosine kinases • We will focus on these for this video ▪ A direct association with an enzyme • For next class Receptor Tyrosine Kinases • Intrinsic kinase activity – i.e. the receptor phosphorylates itself on specific residues of the intracellular face of the receptor • Binding of ligand dimerizes the receptor and activates a tyrosine kinase within the receptor ▪ Phosphorylation by the receptor on its own tyrosine residues activates the receptor → further signaling • Ligand examples ▪ Insulin ▪ Growth factors ▪ Cytokines Receptor Tyrosine Kinases (RTKs) 1. Ligand binds to receptor monomers → 2. Receptor dimerizes and each half phosphorylates the tyrosine residues on the other half 3. Signaling proteins then bind to the phosphorylated receptor and also become activated → signal cascade Molecular Biology of the Cell, 6th ed. Alberts et. al. p. 855, fig 15-44 RTKs – signaling options • Phospholipase C ▪ Same as for Gq • Ras cascade: 1. Ras is a small, intracellular G-protein that is not physically associated with any one receptor – when it encounters an activated RTK, it binds to GTP → activation 2. Ras activates Raf – another small plasma membraneassociated G-protein 3. Activated Raf → activation of MAP kinases • These can phosphorylate transcription factors, enzymes… many effectors 4. Ras inactivates itself by cleaving GTP → GDP (it’s a Gprotein) The RTK → Ras → MAP K cascade Molecular Biology of the Cell, 6th ed. Alberts et. al. p. 855, fig 15-47 The RTK → Ras → MAP K cascade Molecular Biology of the Cell, 6th ed. Alberts et. al. p. 855, fig 15-47 The RTK → Ras → MAP K cascade The Ras-Raf-MAP kinase pathway is the pathway most commonly associated with RTK activation • ** Note – no typical 2nd messengers are produced in this pathway Key pathway for many growth factors Molecular Biology of the Cell, 6th ed. Alberts et. al. p. 855, fig 15-47 RTKs – signaling options PI-3-Kinase (PI3K) → Akt system • Unique signaling mechanism that is key to insulin signaling ▪ Also a wide range of other hormones/growth factors • Basic pathway: 1. RTK is activated, and this causes activation of nearby phosphoinositide-3-kinase (PI3K) • Ras can also directly activate PI3K 2. PI3K attaches an additional phosphate to PIP2 to form PIP3 • Remember – PIP2 is a membrane lipid (about 5% of membrane lipids) 3. PIP3 accumulates and forms “lipid” rafts in the membrane • PIP3 is the 2nd messenger in the system Membrane phospholipids and signaling All of these are variablyphosphorylated phospholipids in the cell membrane • PLC converts PIP2 to IP3 and DAG ▪ Gq activation • PI3K converts many phospholipids to PIP3 RTKs – signaling options • Basic pathway cont… 4. Akt and PDK1 accumulate and cluster together at the site of the PIP3 rafts • Akt and PDK1 are both kinases that are present in the cytosol • PDK1 becomes activated by PIP3 5. When PDK1 is activated, it activates Akt by phosphorylating it • PDK1 = phosphoinositide-dependent kinase 1 6. Akt is the effector – it influences a huge range of intracellular targets • It is also regulated (turned on or off) by many other cellular signals PI3K → PDK1 → Akt • Note how PIP3 brings PDK1 and Akt together at the membrane, as well as acting as a second messenger • https://www.cellsignal.com/contents/science -cst-pathways-pi3k-akt-signalingresources/pi3k-akt-signaling-interactivepathway/pathways-akt-signaling • Interactive diagram that highlights the massive impact of Akt activation… ▪ And also illustrates how Akt is the target of many, many other signals ▪ Take a quick look at the cellular effects (the text not in “bubbles”) • FYI Nitric oxide – a unique messenger • Nitric oxide (NO) is a key mediator that relaxes smooth muscle in a wide variety of blood vessels and visceral organs • Very small, hydrophobic gas that diffuses easily and quickly through cell membranes ▪ Thus can mediate signals within the cell it is produced… ▪ OR can diffuse to another cell • Produced enzymatically by the action of nitric oxide synthase (NOs) on L-arginine ▪ Increases in cytosolic calcium can activate NOs • Nitric oxide is degraded rapidly – less than a minute – since it reacts with oxygen and water ▪ It’s a second messenger – one of the only ones that can diffuse across the cell membrane and impact other cells ▪ Only local effects – it’s a quickly-degraded free radical Nitric oxide-mediated signaling 1. Cytosolic calcium increases ▪ How can we increase cytosolic calcium? 2. Increased intracellular calcium activates NOs 3. NOs produces NO from L-arginine 4. NO binds to and activates guanylyl cyclase (GC) → production of cGMP from GTP ▪ cGMP is also a second messenger 5. Elevations in cytosolic cGMP activates a protein kinase (usually PKG) → changes in cellular activity due to PKG activity ▪ In many cases, results in disengagement of myosin from actin in smooth muscle → smooth muscle relaxation