Lecture 17: Cell Communication - Enzyme-Linked Receptors PDF

Lecture 17: Cell Communication Enzyme-linked receptors  Learning Objectives Understand the differences between fast and slow acting signaling pathways Be able to explain the different mechanisms cells can signal via direct communication, paracrine, and endocrine signaling Ho...

Lecture 17: Cell Communication Enzyme-linked receptors  Learning Objectives Understand the differences between fast and slow acting signaling pathways Be able to explain the different mechanisms cells can signal via direct communication, paracrine, and endocrine signaling How are membrane receptors different from intracellular receptors with respect to their ligands and their mode of actin? Understand the similarities and differences between the three classes of membrane receptors Know the downstream signal transduction mechanisms of GPCRS and RTKs  Functions of signaling pathways Transduce the signal into a molecular form that can carry out the response Relay the signal from point of reception to point of action in the cell Amplify the received signal Integrate multiple signals that in combination can cause distinct responses Distribute the signal to coordinate several responses in parallel  Extracellular signals can act slowly or rapidly Rapid on/off response Sustained or long-term response  Example of slow signaling: steroid hormones signal through intracellular receptors Cell surface receptors fall into 3 main classes G-protein-coupled receptors (GPCRs) G proteins dissociate into two signaling complexes when activated Heterotrimeric G proteins: 3 subunits: α, β, γ Gα binds to GTP or GDP GDP bound = OFF GTP bound = ON GTP causes dissociation When active, Gα and Gβ γ dissociate and interact with downstream signaling molecules Reverse reaction: The Gα subunit inactivates itself by hydrolyzing its GTP (but also RGS proteins)  Some G proteins regulate ion channels Acetylcholine slows the heart Receptor activation ==> dissociation of Gα and Gβγ Gβγ opens K+ channels to decrease the amplitude of contraction Some G proteins regulate membrane-bound enzymes to make second messengers The two most common enzymes activated by G proteins to make second messengers Adenylyl cyclase - converts ATP into cyclic AMP (cAMP) Phospholipase C - cleaves a lipid (isositol phospholipid) into isositol-1,4,5- trisphosphate (IP3, a hydrophilic sugar) and diacylglycerol (DAG, a lipid in the membrane) Cyclic AMP (cAMP) is a common second messenger Adrenaline stimulates glycogen breakdown in skeletal muscle cells (fast signaling) Structure of glycogen  Increased cAMP levels can activate gene transcription through PKA (slow signaling) Physiological functions of PKA Smooth muscle cell contraction Digestion (peristalisis) Vision (pupillary diameter) Circulatory (vasoconstriction) Respiratory (bronchoconstriction) Nervous system Memory formation Neuronal excitation Adipose tissue Increases fat utilization  Physiological functions of PKA Smooth muscle cell contraction Physiological functions of PKA Nervous system: Memory formation The two most common enzymes activated by G proteins to make second messengers Adenylyl cyclase - converts ATP into cyclic AMP (cAMP) Phospholipase C - cleaves a lipid (inositol phospholipid) into inositol-1,4,5- trisphosphate (IP3, a hydrophilic sugar) and diacylglycerol (DAG, a lipid in the membrane)  Calcium pumps maintain low Ca++ ion concentrations in the cytosol to establish “baseline” Phospholipase C creates two second messengers  Physiological functions of PKC Smooth muscle cell contraction Stimulates glucose production from stored glycogen Sequesters Ca+2 in ER Respiratory (bronchoconstriction) Nervous system Memory formation Dopaminergic (reward) system Neuronal development Kidneys Regulates ion and water balance Cell cycle progression and cell polarity  Physiological functions of PKC Nervous system Neuronal development  Physiological functions of PKC Cancer progression Overexpression of PKC in cancers leads to disease Physiological functions of PKC PKC! Cell polarity  Calcium transients trigger many cellular processes (not just PKC) Many signals trigger Ca+2 release (not just GPCRs) Skeletal muscle contracts in response to calcium release (we will talk about this more later in the course) Ca+2 triggers regulated secretion (e.g. in neurons) Sperm entry triggers a calcium wave during fertilization  Fertilization induces a rise in Ca+2 that starts embryogenesis Starfish egg loaded with a calcium-sensitive fluorescent dye Fertilized in vitro and monitored by fluorescence microscopy Fertilization induces a rise in Ca+2 that starts embryogenesis Cell surface receptors fall into 3 main classes  Receptor tyrosine kinases (RTKs) Ligands are soluble or membrane-bound peptide or protein hormones (such as insulin, growth factors) Some RTKs have been identified in studies of human cancers – constitutively active mutant forms send proliferative signals to cells in the absence of the normal signal  Receptor tyrosine kinases dimerize and phosphorylate each other Phosphorylate tyrosine residues on each other Phosphorylated tyrosines are recognized by other proteins that then carry out downstream signaling Terminated by tyrosine phosphatases  Tyrosine receptor signaling complexes As many as 10 or 20 downstream signaling molecules - differ among receptors Examples: phospholipases, lipid kinases, other protein kinases, Ras Protein tyrosine phosphatases turn the pathway back off Most receptor tyrosine kinases activate the G protein Ras  Ras activates a cascade of mitogen- activated protein kinases (MAP-kinases) A mutation that activates Ras causes excess cell division and can contribute to cancer (30%)  Ras activates a cascade of mitogen- activated protein kinases (MAP-kinases) Scaffolding protein RTKs can activate the PI 3-kinase-Akt pathway to make lipid docking sites on membrane Promotes cell growth and survival by inhibiting apoptosis  Activated Akt can promote cell survival and/or stimulate cell growth Inactive Bcl2 Promotes programmed cell death (apoptosis) Receptor signaling may be inactivated by multiple mechanisms GPCRs and RTKs activate multiple pathways