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 Transmembrane protein...

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