SIGNAL TRANSDUCTION AND SECOND MESSENGERS

Dominic S. O. AlwalaMaseno university

INTRODUCTION• The ability of cells to receive and act on signals

from beyond the plasma membrane is fundamental to life. Bacterial cells receive constant input from membrane proteins that act as information receptors, sampling the surrounding medium for pH, the availability of food, oxygen, presence of noxious chemicals or competitors for food.

• These signals elicit appropriate responses, such as motion toward food or away from toxic substances or the formation of dormant spores in a nutrient-depleted medium.

• Animal cells on the other hand exchange information about the concentrations of ions and glucose in extracellular fluids, the interdependent metabolic activities taking place in different tissues, etc.

• In all these cases, the signal represents information that is detected by specific receptors and converted to a cellular response, which always involves a chemical process. This conversion of information into a chemical change, signal transduction, is a universal property of living cells.

• A basic process involving the conversion of a signal from outside the cell to a functional change within the cell.

• Ligand - a molecule e.g. hormone, drug, functional group etc. that binds specifically and reversibly to a receptor to form a larger complex.

• As the result of binding the receptor, other molecules or second messengers are produced within the target cell. Second messengers relay the signal from one location to another (such as from plasma membrane to nucleus).

• Considering the molecular details of several representative signal-transduction systems; the trigger for each system is different, but the general features of signal transduction are common to all:

1. A signal interacts with a receptor2. The activated receptor interacts with cellular

machinery producing a second signal or a change in the activity of a cellular protein

3. The metabolic activity of the target cell undergoes a change

4. Finally the transduction event ends and the cell returns to its prestimulus state.

• As the result of binding the receptor, other molecules or second messengers are produced within the target cell. Second messengers relay the signal from one location to another (such as from plasma membrane to nucleus).

• Often a cascade of changes occur within the cell which results in a change in the cell’s function or identity. The signal transduction pathway can act to amplify the cellular response to an external signal.

• Messenger molecules (ligands) may be amino acids, peptides, proteins, fatty acids, lipids, nucleosides or nucleotides.

Second messengers

• Second messengers are molecules that relay signals received at receptors on the cell surface such as the arrival of protein hormones, growth factors, etc. to target molecules in the cytosol and/or nucleus. In addition to their job as relay molecules, second messengers serve to greatly amplify the strength of the signal. Binding of a ligand to a single receptor at the cell surface may end up causing massive changes in the biochemical activities within the cell.

G Proteins• G proteins are so-called because they bind the guanine

nucleotides GDP and GTP. They are heterotrimers (made of three different subunits) associated with the inner surface of the plasma membrane and transmembrane receptors of hormones, etc. These are called G protein-coupled receptors (GPCRs).

• The three subunits are: 1. Gα, which carries the binding site for the nucleotide.

At least 20 different kinds of Gα molecules are found in mammalian cells.

2. Gβ 3. Gγ

• In the inactive state, Gα has GDP in its binding site. • When a hormone or other ligand binds to the associated

GPCR, an allosteric (change in shape) change takes place in the receptor (that is, its tertiary structure changes).

• This triggers an allosteric change in Gα causing GDP to leave and be replaced by GTP.

• GTP activates Gα causing it to dissociate from GβGγ (which remain linked as a dimer).

• Activated Gα in turn activates an effector molecule. In a common example, the effector molecule is adenylyl cyclase - an enzyme in the inner face of the plasma membrane which catalyzes the conversion of ATP into the "second messenger" cyclic AMP (cAMP)

• Activated Gα is a GTPase so it quickly converts its GTP to GDP. This conversion, coupled with the return of the Gβ and Gγ subunits, restores the G protein to its inactive state.

• In a common example (shown above), the effector molecule is adenylyl cyclase - an enzyme in the inner face of the plasma membrane which catalyzes the conversion of ATP into the "second messenger" cyclic AMP (cAMP)

• Activated Gα is a GTPase so it quickly converts its GTP to GDP. This conversion, coupled with the return of the Gβ and Gγ subunits, restores the G protein to its inactive state.

Second messengers cont……..

There are 3 classes of second messengers: 1. Cyclic nucleotides (e.g. cAMP and cGMP) 2. Inositol trisphosphate (IP3) and diacylglycerol

(DAG) 3. Calcium ions (Ca2+)

1. CYCLIC NUCLEOTIDES

Cyclic AMP (cAMP)• Some of the hormones that achieve their effects

through cAMP as a second messenger: – Adrenaline – Glucagon– Luteinizing hormone (LH)

• Cyclic AMP is synthesized from ATP by the action of the enzyme adenylyl cyclase. Binding of the hormone to its receptor activates a G protein which, in turn, activates adenylyl cyclase.

Cyclic AMP (cAMP) cont…..

• The resulting rise in cAMP turns on the appropriate response in the cell by either (or both): –Changing the molecular activities in the

cytosol, often using Protein Kinase A (PKA) a cAMP-dependent protein kinase that phosphorylates target proteins; – Turning on a new pattern of gene

transcription.

Cyclic GMP (cGMP)• Cyclic GMP is synthesized from the nucleotide GTP

using the enzyme guanylyl cyclase. Cyclic GMP serves as the second messenger for – Atrial natriuretic peptide (ANP) – Nitric oxide (NO) – The response of the rods of the retina to light.

• Some of the effects of cGMP are mediated through Protein Kinase G (PKG) — a cGMP-dependent protein kinase that phosphorylates target proteins in the cell.

2. Inositol trisphosphate (IP3) and diacylglycerol (DAG)

• Peptide and protein hormones like vasopressin, thyroid-stimulating hormone (TSH), and angiotensin; and neurotransmitters like GABA(gamma aminobutyric acid) bind to G protein-coupled receptors (GPCRs) that activate the intracellular enzyme phospholipase C (PLC).

• As its name suggests (phospholipase C), it hydrolyzes phospholipids specifically phosphatidylinositol-4,5-bisphosphate (PIP2) which is found in the inner layer of the plasma membrane. Hydrolysis of PIP2 yields two products:

1. Diacylglycerol (DAG)2. Inositol-1,4,5-trisphosphate (IP3)

• Diacylglycerol DAG remains in the inner layer of the plasma membrane. It recruits Protein Kinase C (PKC) — a calcium-dependent kinase that phosphorylates many other proteins that bring about the changes in the cell.

• As its name suggests, activation of PKC requires calcium ions. These are made available by the action of the other second messenger — IP3.

Inositol-1,4,5-trisphosphate (IP3) • This soluble molecule diffuses through the cytosol

and binds to receptors on the endoplasmic reticulum causing the release of calcium ions (Ca2+) into the cytosol.

• The rise in intracellular calcium triggers the response.

• Example: the calcium rise is needed for NF-AT (the "nuclear factor of activated T cells") to turn on the appropriate genes in the nucleus.

• The binding of an antigen to its receptor on a B cell (the BCR) also generates the second messengers DAG and IP3.

3. Calcium ions (Ca2+)

• As the functions of IP3 and DAG indicate, calcium ions are also important intracellular messengers. In fact, calcium ions are probably the most widely used intracellular messengers.

• In response to many different signals, a rise in the concentration of Ca2+ in the cytosol triggers many types of events such as :-

• Muscle contraction- Ca2+ diffuses among the thick and thin filaments where it binds to troponin on the thin filaments. This turns on the interaction between actin and myosin and the sarcomere contracts. When the process is over, the calcium is pumped back into the sarcoplasmic reticulum using a Ca2+ ATPase.

• Exocytosis- Arrival of an action potential at the synaptic knob opens Ca2+ channels in the plasma membrane. The influx of Ca2+ triggers the exocytosis of some of the vesicles. Their neurotransmitter is released into the synaptic cleft. The neurotransmitter molecules bind to receptors on the postsynaptic membrane. These receptors are ligand-gated ion channels.

• Activation of T cells and B cells when they bind antigen with their antigen receptors -Activation of the TCR (when aided by costimulator molecules also present in the plasma membrane) causes a rise in intracellular Ca2+ which activates calcineurin, a phosphatase which removes phosphate from NF-AT ("Nuclear Factor of Activated T cells"). Dephosphorylated NF-AT enters the nucleus, and with the help of accessory transcription factors, binds to the promoters of some 100 genes expressed in activated T cells.

• Adhesion of cells to the extracellular matrix (ECM) • Apoptosis• A variety of biochemical changes mediated by Protein

Kinase C (PKC).

Getting Ca2+ into/out of the cytosol

• Voltage-gated channels open in response to a change in membrane potential, e.g. the depolarization of an action potential; are found in excitable cells like: – Skeletal muscle – Smooth muscle (These are the channels blocked by

drugs, such as felodipine [Plendil®], used to treat high blood pressure. The influx of Ca2+ contracts the smooth muscle walls of the arterioles, raising blood pressure. The drugs block this.)

– Neurons. When the action potential reaches the presynaptic terminal, the influx of Ca2+ triggers the release of the neurotransmitter.

• Receptor-operated channels These are found in the post-synaptic membrane and open when they bind the neurotransmitter.

• G-protein-coupled receptors (GPCRs). These are not channels but they trigger a release of Ca2+ from the endoplasmic reticulum (as discussed above)

………….end