Second Messenger:Cyclic AMP Pathway

Continuation of Cell Communication

ARIANE RUBY B. SOGO-AN

Objectives :

• Explain the role of the Primary and Secondary Messengers in the cAMP Pathway

• Determine the steps during the cAMP pathway.

What do you think these people are doing in this picture?

Communication

• How important is communication in our everyday life? – Achieving productivity– Maintaining strong relationships– Understand each other– Make the proper response

Cell Communication/Signalling

• Critical for the function and survival of cells that compose a multicellular animal.

– Ways/modes:• Adjacent Cells – Gap junctions• Specific contact between cells – Specific molecules on

cell surface• Through intercellular chemical messengers

The Hormone System

Hormones are secreted by glands into the blood stream. There are two kinds of glands:

Exocrine glands secrete chemicals to the outside, or to body cavities, usually through ducts (tubes). E.g. sweat glands, mammary glands, digestive glands.

Endocrine glands do not have ducts but secrete chemicals directly into the tissue fluid whence they diffuse into the blood stream. E.g. thyroid gland, pituitary gland, adrenal gland. The hormone-secreting glands are all endocrine glands.

• Once a hormone has diffused into the blood stream it is carried all round the body to all organs. However, it only affects certain target organs, which can respond to it.

• These target organs have specific receptor molecules in their cells to which the hormone binds. These receptors are protein molecules, and they form specific hormone-receptor complexes, very much like enzyme-substrate complexes. Cells without the specific receptor will just ignore a hormone.

• The hormone-receptor complex can affect almost any aspect of a cell’s function, including metabolism, transport, protein synthesis, cell division or cell death.

Second Messengers

• Second messengers are intracellular signalling molecules released by the cell to trigger physiological changes such as proliferation, differentiation, migration, survival, and apoptosis.

• Secondary messengers are therefore one of the initiating components of intracellular signal transduction cascades.

• Second messengers are molecules that relay signals from receptors on the cell surface in accordance to the type of first messenger to produce biochemical signal to target molecules inside the cell.

• They greatly amplify the strength of the signal, cause some kind of change in the activity of the cell.

• They are a component of cell signaling pathways.

Second messengers

• Short lived intracellular signaling molecules

• Elevated concentration of second messenger leads to rapid alteration in the activity of one or more cellular enzymes

• Removal or degradation of second messenger terminate the cellular response

• The cell releases second messenger molecules in response to exposure to extracellular signals - the First messengers.

• Because hormones and neurotransmitters typically comprise biochemically hydrophilic molecules, first messengers may not physically cross the phospholipid bilayer cell membrane to initiate changes within the cell directly.

• The cell releases second messenger molecules in response to exposure to extracellular signals - the First messengers.

• Because hormones and neurotransmitters typically comprise biochemically hydrophilic molecules, first messengers may not physically cross the phospholipid bilayer cell membrane to initiate changes within the cell directly.

• This functional limitation necessitates the cell to devise signal transduction mechanisms to transduce first into second messengers, so that the extracellular signal may be propagated intracellularly.

• An important feature of the second messenger signaling system is that second messengers may be coupled downstream to multi-cyclic kinase cascades to greatly amplify the strength of the original first messenger signal.

Signalling sequence in the Target Cell

• Reception– Binding of a signal molecule with a specific

receptor of the target cells. • Transduction– Process of changing the signal into the form

necessary to cause the cellular response. May or may not include cascade of reaction that includes several different molecules.

Signalling sequence in the Target Cell

• Response– Transduced signal causes a specific cellular

response.

Steps in Communication Via Extracellular signals

1. Synthesis2. Release of the signalling molecule by the

signalling cell3. Transport of the signal to the target cell 4. Binding of the signal by a specific receptor

protein leading to its activation5. Initiation of one or more intracellular signal

transduction pathways by the acticvated receptor

6. Specific changes in the cellular function, metabolism and development7. Removal of the signal

Cell Communication/Signalling

• Cell communication systems based on surface receptors have three (3) components: – The extracellular signal molecules released by

controlling cells– The surface receptors on target cells that recieves

the signals– The internal response pathways triggered when

the receptors binds a signal.

Intercellular chemical messengers

• One cell “Controlling Cell” synthesizes specific molecule that acts a signaling molecule to affect the activity of another cell called the target cell.

• Example: – In response to stress, cells of mammal’s adrenal

gland secrets hormones ephinephrine into the bloodstream. Epinephrine acts on target cells to increase the amount of glucose in the blood.

• The specificity of a receptor refers to its ability to distinguish closely related substances. The insulin receptor, for example, binds insulin and a related hormone called insulin-like growth factor 1, but no other peptide hormones.

Ligand Binding to it complementary cell Receptor

The signaling molecule acts as a ligand, which binds to a structurally complementary site on the extracellular or membrane-spanning domains of the receptor.

• Addition of two methyl groups to epinephrine generates isoproterenol, an agonist that binds to epinephrine receptors on bronchial smooth muscle cells about tenfold more strongly than does epinephrine.

• The antagonist alprenolol and related compounds, referred to as beta-blockers, have a very high affinity for these epinephrine receptors. Such antagonists are used to slow heart contractions in the treatment of cardiac arrhythmias and angina.

Overview of seven major classes of cellsurface receptors

GASES

NO

H2S 

CO

HYDROPHOBIC

Diacylglycerol

Phosphatidylinositols

HYDROPHILIC

cAMP

cGMP

IP3

Ca2+.

TYPES OF SECOND MESSENGERS

• The binding of ligands (“first messengers”) to many cellsurface receptors leads to a short-lived increase (or decrease) in the concentration of certain low-molecular-weight intracellular signaling molecules termed second messengers.

• Other important second messengers are Ca2 and various inositol phospholipids, also called phosphoinositides, which are embedded in cellular membranes.

Four common intracellular second messengers.

More receptors using the same second messenger system

cGMP – Atrial Natreuretic Factor – Guanylate Cyclase

Dicylglycerol – Insulin, Thyroid stimulating hormone

IP3- Leutenizing Hormone, Parathyroid Hormone, Thyrotropin releasing hormone

• In liver cells, for instance, the hormones epinephrine, glucagon, and ACTH bind to different members of the G protein–coupled receptor family, bureceptors activate the same signal-transduction pathway, one that promotes synthesis of cyclic AMP (cAMP).

ITS VIDEO TIME!!!!

The G Protein Coupled Receptor

The G Protein Coupled Receptor

The G Protein Coupled Receptor

• The human genome, for instance, encodes several thousand G protein–coupled receptors.

• These include receptors in the visual, olfactory (smell), and gustatory (taste) systems, many neurotransmitter receptors, and most of the receptors for hormones that control carbohydrate, amino acid, and fat metabolism.

ALL (GPCRs) contain seven membrane-spanning regions with their N-terminal segment on the exoplasmic face and their C-terminal segment on the cytosolic face of the plasma membrane

Schematic diagram of the general structure of G protein–coupled receptors.

• All receptors of this type have the same orientation in the membrane and contain seven transmembrane -helical regions (H1–H7), four extracellular segments (E1–E4), and four cytosolic segments (C1–C4). The carboxyl-terminal segment (C4), the C3 loop, and, in some receptors, also the C2 loop are involved in interactions with a coupled trimeric G protein.

• The signal-transducing G proteins contain three subunits designated , , and . During intracellular signaling the and subunits remain bound together and are usually referred to as the G subunit.

• The G subunit is a GTPase switch protein that alternates between an active (on) state with bound GTP and an inactive (off) state with bound GDP

Ligand Binding Specificity

• The response of a cell or tissue to specific external signals is dictated by the particular receptors it possesses, by the signal-transduction pathways they activate, and by the intracellular processes ultimately affected.

• Each receptor protein is characterized by binding

specificity for a particular ligand, and the resulting receptor-ligand complex exhibits effector specificity

The ability of a G protein to interact with other proteins and thus transduce a signal differs in the GTP-bound “on” state and GDP-bound “off” state.

• These guanine nucleotide–binding proteins are turned “on” when bound to GTP and turned “off” when bound to GDP. Signal-induced conversion of the inactive to active state is mediated by a guanine nucleotide–exchange factor (GEF), which causes release of GDP from the switch protein.

• All G proteins contain regions like switch I and switch II that modulate the activity of specific effector proteins by direct protein-protein interactions when the

G protein is bound to GTP.

• Subsequent binding of GTP, favored by its high intracellular concentration, induces a conformational change in two segments of the protein, termed switch I and switch II, allowing the protein to bind to and activate other downstream signaling proteins.

• The intrinsic GTPase activity of the switch proteins then hydrolyzes the bound GTP to GDP and Pi, thus changing the conformation of switch I and switch II from the active form back to the inactive form. The rate of GTP hydrolysis frequently is enhanced by a GTPase-accelerating protein (GAP)

Application

• Some bacterial toxins contain a subunit that penetrates the plasma membrane of cells and catalyzes a chemical modification of Gs·GTP that prevents hydrolysis of bound GTP to GDP. As a result, Gs remains in the active state, continuously activating adenylyl cyclase in the absence of hormonal stimulation.

Hormone-induced activation and inhibition of adenylyl cyclase in adipose cells.

Conversion of ATP to cAMP

Protein Kinase A

Synthesis and degradation of glycogen.

Regulation of glycogen metabolism by cAMP in liver and muscle cells.

Mechanisms Regulate Signalingfrom G Protein–Coupled Receptors

• 1. The affinity of the receptor for hormone decreases when the GDP bound to Gs is replaced with a GTP following hormone binding.

• 2. The GTP bound to Gs is quickly hydrolyzed, reversing the activation of adenylyl cyclase and production of cAMP.

• 3. cAMP phosphodiesterase acts to hydrolyze cAMP to 5-AMP, terminating the cellular response.

• The intracellular levels of cAMP are regulated by the balance between the activities of two enzymes: adenylyl cyclase (AC) and cyclic nucleotide phosphodiesterase (PDE).

• When a Gs protein–coupled receptor is exposed to hormonal stimulation for several hours, several serine and threonine residues in the cytosolic domain of the receptor become phosphorylated by protein kinase A (PKA).

• The phosphorylated receptor can bind its ligand, but ligand binding leads to reduced activation of adenylyl cyclase; thus the receptor is desensitized.

• This is an example of feedback suppression, in which the end product of a pathway (here activated PKA) blocks an early step in the pathway (here, receptor activation).

Role of -arrestin in GPCR desensitization and signal transduction.

END OF REPORT