Part V Second Messengers

The first messengers being the extracellular signal molecules and the third messengers being the large protein kinases and phosphatases that are recruited to the plasma membrane. Second messengers are signaling intermediates connecting events taking place at the plasma membrane with the intracellular signaling that eventually converts the signal into a cellular response. Second messengers are small molecules. There are three main kinds; cAMP, lipids and calcium.

I- cAMP & cGMP The first of the second messengers is cyclic adenosine monophosphate (cAMP), is generated from ATP by the enzyme adenylate cyclase embedded in the plasma membrane. The production of cAMP is triggered by signals relayed from the cytosolic surface of plasma membrane bound receptors and other intracellular signaling elements located in its near vicinity.

Stimulative hormone receptor (Rs) is a receptor that can bind with stimulative signal molecules, while inhibitory hormone receptor (Ri) is a receptor that can bind with inhibitory signal molecules. Stimulative regulative G-protein is a G protein-linked to stimulative hormone receptor (Rs) and its α subunit upon activation could stimulate the activity of an enzyme or other intracellular metabolism. On the contrary, inhibitory regulative G-protein is linked to an inhibitory hormone receptor and its α subunit upon activation could inhibit the activity of an enzyme or other intracellular metabolism. The Adenylyl cyclase is a 12-transmembrane glycoprotein that catalyzes ATP to form cAMP with the help of cofactor Mg 2+ or Mn 2+. The cAMP produced is a second messenger in cellular metabolism and is an allosteric activator to Protein kinase A.

Protein kinase A is an important enzyme in cell metabolism due to its ability to regulate cell metabolism by phosphorylating specific committed enzymes in the metabolic pathway. It can also regulate specific gene expression, cellular secretion, and membrane permeability. The protein enzyme contains two catalytic subunits and two regulatory subunits. When there is no cAMP, the complex is inactive. When cAMP binds to the regulatory subunits, their conformation is altered, causing the dissociation of the regulatory subunits, which activates protein kinase A and allows further biological effects. cAMP phosphodiesterase is an enzyme that can degrade cAMP to 5'-AMP, which will terminate the signal.

Cyclic GMP is similarly produced from GTP by the action of guanylyl cyclase. Once the intracellular concentration of cAMP or cGMP is elevated, these nucleotides can bind to two different classes of targets. The target of cGMP is protein kinase G (PKG). In addition, cAMP and cGMP can bind to certain ligand-gated ion channels, thereby influencing neuronal signaling. These cyclic nucleotide-gated channels are particularly important in phototransduction and other sensory transduction processes, such as olfaction. Cyclic nucleotide signals are degraded by phosphodiesterases, enzymes that cleave phosphodiester bonds and convert cAMP into AMP or cGMP into GMP.

Fig.1 Neuronal second messengers. (A) Mechanisms responsible for producing and removing second messengers, as well as the downstream targets of these messengers. (B) Proteins involved in delivering calcium to the cytoplasm and in removing calcium from the cytoplasm. (C) Mechanisms of production and degradation of cyclic nucleotides. (D) Pathways involved in production and removal of diacylglycerol (DAG) and IP3.

II- Diacylglycerol & IP3 Membrane lipids can also be converted into intracellular second messengers. The two most important messengers of this type are produced from phosphatidylinositol bisphosphate (PIP2). This lipid component is cleaved by phospholipase C, an enzyme activated by certain G-proteins and by calcium ions. Phospholipase C splits the PIP2 into two smaller molecules that each act as second messengers. One of these messengers is diacylglycerol (DAG), a molecule that remains within the membrane and activates protein kinase C, which phosphorylates substrate proteins in both the plasma membrane and elsewhere. The other messenger is inositol triphosphate (IP3), a molecule that leaves the cell membrane and diffuses within the cytosol. IP3 binds to IP3 receptors, channels that release calcium from the endoplasmic reticulum. Thus, the action of IP3 is to produce yet another second messenger (perhaps a third messenger, in this case!) that triggers a whole spectrum of reactions in the cytosol. The actions of DAG and IP3 are terminated by enzymes that convert these two molecules into inert forms that can be recycled to produce new molecules of PIP2.

Fig.2 Effector pathways associated with G-protein-coupled receptors. In all three examples shown here, binding of a neurotransmitter to such a receptor leads to activation of a G-protein and subsequent recruitment of second messenger pathways. Gs, Gq, and Gi refer to three different types of heterotrimeric G-protein.

III- Calcium (Ca2+) Ca2+ concentration is usually maintained at a very low level in the cytosol by its reservation in the smooth endoplasmic reticulum and the mitochondria. Ca2+ release from the endoplasmic reticulum into the cytosol results in the binding of the released Ca2+ to signaling proteins that are then activated. It is especially important in neurons and muscle cells. Ca2+ is used in a multitude of processes, among them muscle contraction, release of neurotransmitter from nerve endings, vision in retina cells. Proliferation, secretion, gene expression and metabolism. Cells use Ca2+ as a second messenger in both G-protein pathways and tyrosine-kinase pathways. Various protein pumps transport Ca2+ outside the cell or into the endoplasmic reticulum or other organelles. As a result, the concentration of Ca2+ in the ER is usually much higher than the concentration in the cytosol.

IV- Nitric oxide (NO) The gas nitric oxide is a free radical which diffuses through the plasma membrane and affects nearby cells. NO is made from arginine and oxygen by the enzyme NO synthase, with citrulline as a by-product. NO works mainly through activation of its target receptor, the enzyme soluble guanylate cyclase, which when activated, produces the second messenger cyclic guanosine monophosphate (cGMP). NO can also act through covalent modification of proteins or their metal cofactors. Some of these modifications are reversible and work through a redox mechanism. In high concentrations, NO is toxic, and is thought to be responsible for some damage after a stroke. NO serves multiple functions. These include: Relaxation of blood vessels. Regulation of exocytosis of neurotransmitters. Cellular immune response.

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