14.11:
What are Second Messengers?
When messages are received on the cell's surface, secondary messengers relay those messages to target molecules within the cell to create a response. One important intracellular signaling cascade is the inositol triphosphate diacylglycerol or IP3/DAG pathway.
On the cell surface hormones and neurotransmitters bind to G-protein coupled receptors that activate an enzyme, such as phospholipase C. This enzyme hydrolyzes, or breaks down with water, the inner layer of the phospholipid bi-layer yielding two second messengers, DAG and IP3.
DAG remains in the plasma membrane to recruit protein kinase C, while IP3 diffuses through the cytoplasm and binds to IP3 receptors on the endoplasmic reticulum causing calcium ions to be released into the cytoplasm.
These calcium ions bind to and activate the protein kinase C, which phosphorylates other intracellular proteins resulting in a cellular response.
14.11:
What are Second Messengers?
Because many receptor binding ligands are hydrophilic, they do not cross the cell membrane and thus their message must be relayed to a second messenger on the inside. There are several second messenger pathways, each with their own way of relaying information. G-protein coupled receptors can activate both phosphoinositol and cyclic AMP (cAMP) second messenger pathways. The phosphoinositol path is active when the receptor induces phospholipase C to hydrolyze the phospholipid, phosphatidylinositol biphosphate (PIP2), into two second messengers: diacylglycerol (DAG) and inositol triphosphate (IP3). DAG remains near the cell membrane and activates protein kinase C (PKC). IP3 translocates to the endoplasmic reticulum (ER) and becomes the opening ligand for calcium ion channels on the ER membrane- releasing calcium into the cytoplasm.
In the cAMP pathway, the activated receptor induces adenylate cyclase to produce multiple copies of cAMP from nearby adenosine triphosphate (ATP) molecules. cAMP can stimulate protein kinase A (PKA), open calcium ion channels, and initiate the enzyme- Exchange-protein activated by cAMP (Epac).
Similar to cAMP, is cyclic guanosine monophosphate (cGMP). cGMP is synthesized from guanosine triphosphate (GTP) molecules when guanylyl cyclase is activated. As a second messenger, cGMP induces protein kinase G (PKG). PKG has many overlapping functions of PKA, however PKG expression is restricted to vascular tissues, lungs, and the brain.
Phosphatidylinositol triphosphate (PIP3) is a second messenger derived from the phosphorylation of PIP2. This event is triggered when growth factors bind the receptor tyrosine kinase (RTK) receptor. PIP3 recruits Akt (aka. protein kinase B) to the membrane. This kinase is intimately involved in regulating cell survival pathways- including proliferation, apoptosis, and migration.
Suggested Reading
Newton, Alexandra C., Martin D. Bootman, and John D. Scott. “Second Messengers.” Cold Spring Harbor Perspectives in Biology 8, no. 8 (August 2016). [Source]