cAMP Second Messenger and its Function

cAMP as Second Messenger

• cAMP acts as a second messenger for several regulatory molecules
• Protein Kinase A (PKA) binds to regulatory sites and activates PKA
• Activates enzymes involved in fat mobilization and cortisol synthesis
• cAMP can move into the cell nucleus, which phosphorylates CREB (cAMP response element binding protein)
• Alters the expression of specific genes regulated by [cAMP] in the cell by binding to specific genes on the DNA, either stimulating or inhibiting their transcription
• Calcium ions Ca++ movement

Activation of enzymes involved in fat mobilization

• Glucagon (first messenger) is received by adipocytes and Gs-proteins, this activates AC which increases [cAMP]
• Increases in cAMP stimulate PKA, which activates enzymes involved in fat mobilization (hormone-sensitive lipase)

Activation of enzymes involved in cortisol synthesis

• ACTH (first messenger) is received by the adrenal cortex (target tissue), activating enzymes for cortisol synthesis

Activation of CREB via PKA

• Extracellular signal molecules binding to G-protein-linked receptors leads to the activation of adenylyl cyclase and a rise in cyclic AMP concentration
• Increases in cyclic AMP concentration activate PKA in the cytosol, and the released catalytic subunits that move into the nucleus
• CREB gene regulatory protein becomes phosphorylated
• Once phosphorylated, CREB recruits the coactivator CBP, which stimulates gene transcription
• This signalling pathway controls many processes in cells by ranging from hormone synthesis in endocrine cells to the production of proteins required for long-term memory in the brain
• Intracellular Ca2+ can also phosphorylate and thereby activate CREB

Ca++ Movement

• Calcium movement can be triggered by the cAMP pathway
• Via activation of PKA and cell surface Ca++ channels
• Via activation of PKA and sarcoplasmic reticulum surface Ca++ channels

GPCR Pathways Can Regulate Potassium Ion Channels

• G protein (βy dimer) can open or close ion channels in the plasma membrane
• Acetylcholine (first messenger/ligand) is released by the vagus nerve
• Binds to muscarinic receptors (GPCR)
• Can alter the ion permeability and electrical excitability

Heart Muscle

• Gi inhibits adenylyl cyclase, reducing cAMP and PKA concentrations in the cell
• βy subunits phosphorylate and open K+ channels
• K+ leaves the cell, making depolarization more difficult (intracellular environment becomes more negative)
• Reduces heart rate, slowing the speed of depolarization

Smooth Muscle GI Tract

• In the GIT, reduction in cAMP results in decreased activity of protein kinase A (PKA), leading to a reduction in phosphorylation of proteins that promote muscle relaxation
• Promotes smooth muscle contraction
• βy subunits phosphorylate and close K+ channels
• K+ stay in the cell, making depolarization easier (intracellular environment becomes more positive)
• Stimulates smooth muscle contraction, increasing the speed of depolarization

Message Transduction - Phosphatidylinositol (PIP2) Pathway

• DAG, IP3, and Ca2+ have related roles as second messengers
• Another broad class of GPCRs are coupled to a Gq protein and activates a phospholipase C (PLC)
• The enzyme PLC is specific for a membrane phospholipid: PIP₂ (phosphatidylinositol 4,5-bisphosphate)
• Signal hormone binding to the specific receptor A
• Gq protein is activated
• PLC specific to PIP₂ is activated
• This activation catalyzes the production of two powerful second messengers: diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3)
• IP3 is hydrophilic, diffuses to ER, and binds to specific IP3-gated Ca2+ channels, causing them to open and release stored Ca2+ to the cytosol
• An increase in [Ca²+] in combination with DAG activates Protein Kinase C (PKC)
• Many enzymes activated by PKC affect cytoskeletal proteins, nuclear proteins, and other enzymes (cellular response)

Calcium as Second Messenger

• Ca2+ serves as a second messenger that triggers different intracellular responses
• Exocytosis in neurons (neurotransmitter vesicles), mast cells (histamine granules), endocrine cells (hormones)
• Contraction in muscles
• Cytoskeletal movement in macrophages
• Cytosolic [Ca2+ ] is kept very low by action of Ca2+ pumps (Na+/Ca+2 exchanger, A Ca2+ TPase ) in ER, mitochondria, and the plasma membrane
• Hormonal, neural, or other stimuli cause an influx of Ca2+ into the cell, either through special Ca2+ channels in the membrane or by releasing Ca2+ from ER or mitochondria
• Rises in intracellular [Ca2+ ] will then cause cytosolic responses

Calcium and Calmodulin

• Changes in intracellular [Ca2+ ] are sensed by Calmodulin (CaM)
• Calmodulin (CaM): Ca²⁺-binding proteins that regulate many Ca²⁺-dependent enzymes/proteins
• CaM has 4 high-affinity Ca²⁺-binding sites
• At a certain threshold [Ca2+], binding of Ca2+ to calmodulin causes a conformational change
• Calmodulin bound to Ca2+ can bind to and modulate the activity of different proteins
• i.e., CaM-dependent kinases and other enzymes
• In smooth muscle CaM binds to the myosin light chain kinase, myosin head is activated and stimulates cross-bridge cycling