Chapter 5 - Chemical Messengers

Intercellular Communication
Direct Communication
Gap Junctions are channels between cell membranes that allow ions and small molecules to pass directly from one cell to another. These channels are formed by membrane proteins called connexins. Six connexins in the cell membrane form a channel called a connexon. Connexons from two cells form a gap junction. Gap junctions permit electrical signals to pass directly from one cell to another. Gap junctions are found in heart and smooth muscle cells and between some neurons.
Indirect Communication via Chemical Messengers
   Most cells communicate by secreting a chemical (ligand) that reversibly binds to a receptor on a target cell. The binding of the ligand to the receptor produces a response in the cell by mechanisms called signal transduction.
   The strength of the response is proportional to the number of bound receptors. The number of bound receptors increases with an increase in the concentration of the ligands and the number of receptors in the membrane. 
Chemical Messengers
Functional Classification
   Chemicals that communicate with neighboring cells. Target cells must be close enough to permit paracrines to reach target cells by diffusion. e.g. Histamine released by mast cells triggers a local inflammatory response.
   Chemicals that act upon the cell that secreted them. In other words, these cells stimulate themselves. Many autocrines also act as paracrines, and visa versa. Histamine, again, is an example (see page 139). 
   Communication by neurotransmitters is also called synaptic signaling. This is because the communication occurs over a juncture (a place where two cells come into very close contact) called a synapse. The cell releasing the neurotransmitter is called the presynaptic cell and the neurotransmitter is concentrated in an axon terminal of a presynaptic nerve cell. The neurotransmitter binds with receptors on the postsynaptic cell. An example is acetylcholine released by motor neurons at the motor end plate which triggers muscle contraction. 
   These chemicals are released by endocrine glands. Hormones are secreted into the interstitial fluid but then diffuse into the blood and travel to target cells throughout the body. An example is insulin which regulates energy metabolism. 
   Neurohormones are a special class of hormones that are released from modified neurons by the same mechanism involved in neurotransmitter release. An example is ADH produced by neurons located in the hypothalamus and released at their axon terminals in the posterior pituitary. 
   Cytokines are released mainly by white blood cells and function in the immune system. Cytokines usually travel only a short distance by diffusion but may travel further in the body through the blood stream. Examples include interleukins and interferons.
Chemical Classification
   Chemical messengers can be classified by their chemical characteristics including their solubility in water and chemical structure. 
Water Solubility
Hydrophobic (lipophilic) - Molecules are lipid soluble and can easily cross the plasma membrane.
Hydrophilic (lipophobic) - Molecules that are water soluble and do not readily cross the plasma membrane.
Chemical Structure
Amino Acids
   Four amino acids are neurotransmitters in the brain and spinal cord. They include glutamate, aspartate, glycine and gamma amino butyric acid (GABA). They are lipophobic.
   Amines are derived from amino acids and contain an amine group (-NH2). An important group of these are the catecholamines which contain a 6 carbon ring (catechol) and are derived from tyrosine. These include dopamine, norepinephrine and epinephrine. Others include serotonin a neurotransmitter derived from tryptophan, thyroid hormones derived from tyrosine, and histamine  derived from histidine. All except thyroid hormones are hydrophilic (lipophobic). 
   Peptides are polypeptides or proteins and include many neurotransmitters and hormones and all cytokines. The size of the peptides vary considerably. Generally, peptides consisting of fewer than 50 amino acids are peptides and those with more are called proteins. Peptides are hydrophilic (lipophobic). 
   Steroids are derived from cholesterol and function as hormones. All are lipophilic.
   Eicosanoids are derived from arachidonic acid (20 carbon fatty acid) and include paracrines produced by almost every cell in the body. They are lipophilic and include prostaglandins, leukotrienes and thromboxanes. 
   Acetylcholine and nitrous oxide (NO) are two important chemical messengers that do not fit in the above classification scheme. They will be discussed in future notes. 

Synthesis and Release of Chemical Messengers
Amino Acids
   The four amino acids that function as neurotransmitters are synthesized within the neuron that secrete them. Glutamate and aspartate are converted from intermediates of the tricarboxylic acid cycle. Glycine is synthesized from an intermediate of the glycolytic pathway. GABA is synthesized from glutamate. 
   Following synthesis, the amino acid neurotransmitters are transported in vesicles where they are stored until released by exocytosis.
   All amines except thyroid hormones are derived from amino acids in the cytosol by enzyme-catalyzed reactions. All the catecholamines are derived from tyrosine. First, dopamine is derived from tyrosine; norepinephrine is derived from dopamine; and epinephrine is finally derived from norepinephrine. The amines are stored in cytosolic vesicles until their release is triggered. 
   Peptides are synthesized as described earlier. Peptides that serve as hormones undergo further processing as follows (Fig. 5.4):
1. A prepropeptide is synthesized and released into the rough ER.
2. Proteolytic enzymes in the RER cleave off some amino acids to yield propeptides.
3. In the smooth ER propeptides are packaged into transport vesicles.
4. The vesicles are transported to Golgi complexes.
5. Golgi complexes package the propeptide into secretory vesicles. Either in the GA or secretory vesicles more amino acids are cleaved to yield the final peptide.
6. The peptides are released by exocytosis. 
   Steroids are synthesized and released as needed by enzymes in smooth ER and mitochondria from cholesterol. All readily diffuse through cell membranes because of their lipophilic character. Steroids cannot be stored in membrane bound vesicles and are synthesized as needed and are released immediately. 
   Eicosanoids are lipophilic and are synthesized from arachidonic acid and released on demand. The first step in eicosanoid synthesis involves the release of arachidonic acid from phospholipids in the cell membrane by phospholipase A2
   Eicosanoids are derived from arachidonic acid by one of two pathways. The cyclooxygenase pathway results in the synthesis of prostaglandins, prostacyclins, and thromboxanes. Aspirin interferes with synthesis by this pathway. The lipoxygenase leads to synthesis of leukotrienes. 
   Prostacyclins and thromboxanes are important in blood clotting while prostaglandins and leukotrienes contribute to the inflammatory response.
Transport of Messengers
   Chemicals may reach their target cells by simple diffusion (autocrines, paracrines, neurotransmitters and most cytokines) or be transported in the blood (some cytokines, hormones and neurohormones).
   Hydrophilic messengers that travel in blood may do so in dissolved form although catecholamines can be transported bound to carrier proteins. Hydrophobic messengers such as steroids and thyroid hormones are transported bound to carrier proteins. 
   Blood-borne messengers are degraded by the liver or are secreted by the kidneys. The persistence of a messenger in the blood is measured by their half-life. Dissolved messengers have a shorter half-life than messengers bound to carrier proteins.
Signal Transduction Mechanisms
Properties of Receptors
   Receptors bind to only one messenger or class of messengers. This property called specificity. The strength of the binding between a messenger and its receptor is called affinity. 
   A single messenger can bind to more than one receptor and have different affinities for each one. For example, both epinephrine and norepinephrine bind to adrenergic receptors. Different types of adrenergic receptors exist including a1 and a2, and b1, b2 and b3. The a receptors have a greater affinity for norepinephrine and b2 receptors have a greater affinity for epinephrine. b1 and b3 have equal affinities for both epinephrine and norepinephrine. 
   Cells vary in the receptors they have and a single target cell may have receptors for different chemical messengers.
Receptor Binding and Target Cell Response
   Three factors influence the degree of the target cell's response to a messenger:
1. Messenger Concentration
   The greater the concentration of the messenger the greater the response.
2. Number of Receptors
   The more receptors the target cell has the greater the response. Cell can up-regulate or increase the number of receptors when messenger concentration is low or down-regulate or decrease receptors when messenger concentration is high.
3. Affinity of Receptor
   The greater the affinity of the receptor for the messenger the greater the response.
   Ligands that bind to receptors to cause a biological response are called agonists. When a ligand binds to a receptor and does not produce a response it is called an antagonist. Artificial receptor agonists and antagonists often have therapeutic and experimental usefulness.
Signal Transduction Mechanisms by Intracellular Receptor-Mediated Responses (Fig. 5.11)
   Intracellular receptors bind to lipophilic messengers that can readily cross cell membranes. The receptor may be located in the cytosol or in the nucleus. The messenger binds to the receptor to form a hormone-receptor complex that binds to a certain region of DNA called the hormone response element. This binding may then either activate or deactivate a gene resulting in either the synthesis or nonsynthesis of protein. Because activating or deactivating protein synthesis involves some time, the hormones have actions that are slow to develop and persist for some time.
Signal Transduction Mechanisms by Membrane Bound Receptors
   Lipophobic messengers depend upon receptors in the plasma membrane facing the extracellular environment. We will place these membrane-bound receptors into three categories:
1. Channel-Linked Receptors
   Ion-channels that open or close in response to the binding of chemical messengers are called ligand-gated channels. Ligand-gated channels are proteins that function as both receptor and ion channels and are fast. The binding of the messenger to the protein opens the channel and ions either enter or leave the cell. The movement of ions can have two different effects:
1. Change of membrane potential.  
   Acetylcholine, for example, binds to a ligand-gated channel in muscle cells. The binding increases the permeability of the membrane to Na+ and causes it to rush in and depolarize the membrane. The response is brief and does not last very long.
2. Influx of Ca++
   Some fast ligand-gated channels allow calcium ion to enter the cell increasing intracellular calcium ion concentration. Calcium ion may trigger a muscle contraction, secretion by exocytosis or change the activity of intracellular proteins. In the last case, calcium ion acts as a secondary messenger. Calcium does this by first binding with a cytosolic protein called calmodulin. The calcium-calmodulin complex activates a protein kinase that affects protein activity by phosphorylation.
   The cell is sensitive to changes in Ca concentration because of the steep concentration gradient between the [Ca++] in the cytosol compared to that in the extracellular fluid and that sequestered in the smooth ER and mitochondria.
2. Enzyme-Linked Receptors
   Enzyme-linked receptors are transmembrane proteins with a receptor side facing the interstitial fluid and the enzyme side facing the cytosol. Examples include tyrosine kinases and guanylate cyclases.
   We will model the action of tyrosine kinase in the laboratory. An important hormone that exerts its effects through tyrosine kinase is insulin. 
3. G Protein-Linked Receptors
   G-protein-linked receptors respond to their ligands by activating a special protein called G proteins. G proteins are located on the intracellular side of the cell membrane. 
   The G protein has three subunits  a, b and g. The a unit binds to guanosine dinucleotide (hence G protein) in its inactive state. When a messenger binds the receptor the G protein releases GDP and binds GTP and becomes activated. The a subunit is released and then moves to a target protein.  The a subunit deactivates itself by hydrolyzing GTP into GDP.
   There are three basic types of G proteins:
1. Affecters of channel proteins
2. Stimulatory G proteins that activate amplifier proteins
3. Inhibitory G proteins that inhibit amplifier proteins
Slow ligand-gated ion channel
   The G protein may regulate a slow ligand-gated ion channel by either opening or closing the channel. This is in contrast to the more direct ligand-gate channels that only open in response to messenger binding. G protein-linked ion channels also take longer to open and stay open longer.
G protein-regulated enzymes
   These G proteins act by activating or inhibiting an amplifier protein through the GTP-a subunit. The amplifier protein catalyzes the synthesis of a second messenger. The second messenger activates a protein kinase that catalyzes the phosphorylation of a protein. This leads to the response in the cell (Fig. 5.16).
   The only exception to this scheme is Ca++. Ca++ appears as a second messenger not as the result of an amplifier enzyme but as the result of an influx of  Ca++ when Ca++ channels are opened.
   There are five major second messengers:
1. cAMP
2. cGMP
3. inositol triphosphate
4. diacylglycerol
5. calcium ion
cAMP second messenger system (Fig. 5.17a):
1. Messenger binds to receptor and activates a Gs protein. (The Gi protein that inhibits adenylate cyclase is also possible in this step.)
2. a subunit is released and activates the enzyme adenylate cyclase.
3. ATP    cAMP by adenylate cyclase.
4. cAMP activates protein kinase A (cAMP-dependent protein kinase).
5. A protein is phosphorylated by protein kinase A.
6. Phosphorylated proteins' activity is now altered. Cellular response results.
   Response terminates as:
1. cAMP is degraded by cAMP phosphodiesterase. (The effects of caffeine are associated with its inhibition of cAMP phosphodiesterase and the resulting decrease in levels of cAMP.)
2. Protein is dephosporylated by phosphoprotein phosphatases.
cGMP second messenger system
cGMP is similar to cAMP except that protein kinase G (cGMP-dependent protein kinase) is activated.
Phosphatidylinositol second messenger system (Fig. 5.17b):
1. Messenger binds to receptor and activates G protein. 
2. GTP-a subunit is released and activates phospholipase C.
3. Phospholipase C catalyzes conversion of phosphatidylinositol-4,5-biphosphate (PIP2) to diacylglycerol (DAG) and inositoltriphosphate (IP3) each of which serves as a second messenger:
DAG as a second messenger
4a. DAG remains in the membrane and activates protein kinase C.
5a. Protein kinase C catalyzes the phosphorylation of a protein.
6b. Phosphorylated protein causes an effect in the cell. 
IP3 as a second messenger (at the same time)
4b. IP3 moves into cytosol.
5b. IP3 triggers release of Ca++ from the endoplasmic reticulum. 
Calcium as a second messenger
1. act on proteins directly to stimulate contraction or secretion.
2. binds to calmodulin to activate a protein kinase.
Signal Amplification in Chemical Messenger Systems
   Second messenger systems are complicated but create a phenomenon called signal amplification. Signal amplification enables one molecule to activate an enormous number of proteins (see Fig. 5.18). This enables the cell to be very sensitive in response to the presence of messenger even at very low concentrations. This is an example of a cascade in which there are sequential steps that progressively increase in magnitude.