Term
| What are some different things cell signaling pathways can regulate? |
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Definition
cell cycle- proliferation
cytoskeleton- migration
transcription- differentiation
membrane traffic- exocytosis
cell survival and death- apoptosis
development- body plan |
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Term
| What are some different types of signal molecules? |
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Definition
Amino acid derivatives (Histamine)
Peptides and Proteins (Insulin, PDGF)
Steroids (Estradiol, Testosterone)
Dissolved gases (NO, CO)
Others, such as nucleotides, retinoids, fatty acid derivatives |
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Term
| What are the four major pathways a signal can enter the cell? |
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Definition
1. Hydrophobic molecules - across the plasma membrane
2. Ion channels.
3. G-protein-coupled receptors.
4. Enzymes |
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Term
| What is endocrine signaling? |
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Definition
Endocrine: ligands (hormones) released by endocrine cell and carried in bloodstream to distal target cells
Example: release of insulin by cells in the pancreas, travels in the blood stream and acts on distal liver, muscle and fat cells |
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Term
| What is paracrine signaling? |
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Definition
Paracrine: signaling molecules only affect target
cells in close proximity to secreting cells
Examples: somatostatin release by pancreas cells acts locally. Neurotransmission can also be considered to be a type of paracrine signaling |
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Term
| What is autocrine signaling? |
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Definition
Autocrine: cells respond to substances that they themselves release
Example: Some neurotransmitters and growth factors bind to the cells that release them |
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Term
| How does membrane-bound signaling work? |
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Definition
A signaling cell will display signal molecules on its surface and the adjacent target cell will bind the signal molecules using receptors on its surface
Example: signaling by T cells in the immune system |
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Term
| What are the basics of a ligand? |
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Definition
It is the signaling molecule (e.g., hormone, pheromone, ion, neurotransmitter, drug).
It binds to or “fits,” a site on a receptor molecule on the target cell. |
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Term
| What type of ligands bind to cell surface receptors on the plasma membrane and what is their principal mechanism of action? |
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Definition
Proteins and peptides
Mechanism - generation of second messengers which alter the activity of other molecules, usually enzymes, within the cell |
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Term
| What type of ligands bind to intracellular receptors in the cytoplasm and/or nucleus and what is their mechanism of action? |
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Definition
Steroids and thyroid hormones
They alter transcription activity of responsive genes |
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Term
| What are the general features of the group 1 of ligands (Steriods, iodothyronines, calcitriol, retinoids)? |
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Definition
Lipophilic (hydrophobic)
Use transport proteins
Plasma half-life of hours to days
Use intracellular receptor
Signaling mediator is receptor-hormone complex |
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Term
| What are the general features of the group 2 of ligands (Polypeptides, proteins, glycoproteins, catecholamies)? |
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Definition
Hydrophilic
Use a plasma membrane receptor
Short plasma half-life of minutes
Signaling mediators are cAMP, cGMP, Ca2+, metabolites |
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Term
| What are the seven steps in cell-to-cell signaling? |
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Definition
(1) synthesis of the signal molecule by the signaling cell
(2) release of the signal molecule by the signaling cell
(3) transport of the signal molecule to the target cell
(4) detection of the signal by a specific receptor protein
(5) initiation of intracellular signal-transduction pathways
(6) a change in cellular metabolism, function, or
development of the target cell
(7) removal of the signal, which usually terminates the cellular response |
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Term
| Explain how nuclear signal receptors work |
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Definition
Small hydrophobic molecules like steroid hormones, thyroid hormones, retinoids and vitamin D can diffuse across the plasma membrane and bind to intracellular receptor proteins
Signal moleclue binding to the receptor proteins cause the receptors to bind to the promoter regions of the DNA and activate transcription
Regardless of exact location of the receptor, ligand binding alters the conformation of the receptor, causing it to dissociate from the inhibitor & to associate with co-activator proteins that induce gene transcription.
ex's - thyroid receptors are already bound to DNA even in the absence of ligand, cortisol binds to receptors in the cytosol and then the complex enters the nucleus |
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Term
| Explain the mechanism of lipid soluble ligand signaling |
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Definition
1)Hormone diffuses through phospholipid bilayer & into cell
2)Formation of a receptor/ligand complex can occur in the cytosol or the nucleus and typically leads to dimerization of the occupied receptor.
3)Dimerized receptor binds to the hormone regulatory element (HRE, 8-15 base pairs) on the target DNA molecule
4)The receptor complex turn on/off specific genes
5)New mRNA is formed & directs synthesis of new proteins
6)New protein alters cell’s activity |
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Term
| Explain how the different regions of the intracellular receptor are important for signaling |
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Definition
The C-terminus provides unique ligand binding site and sites for protein dimerization.
The N-terminus defines the region essential for transcriptional activation
Middle domain (~80 residues) contains the DNA binding site; exhibits as much as 50% homology with other receptors; may associate with proteins that modulate their function. |
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Term
| What are the characteristics of hydrophilic ligands? |
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Definition
Hydrophilic ligands (Peptide hormones) are comprised of chains on amino acids.
Like most proteins, hydrophilic ligands are synthesized on ribosomes of the (rough) endoplasmic reticulum of endocrine cells.
Hydrophilic ligands can be stored in vesicles in endocrine cells until they are needed at some later point
Hydrophilic ligands do not readily pass through cell membranes (lipid bilayers) and they are referred to as water soluble.
Receptors for hydrophilic ligands are found on the cell surface of their target cells |
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Term
| What are the four second messenger systems? |
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Definition
1) cAMP - used by glucagon, thyroid stimulating hormone, calcitonin
2) protein kinase activity - used by insulin, growth hormone, prolactin, oxytocin, several growth factors
3) Ca2+ and/or phosphoinositides - used by Angiotensin II, antidiuretic hormone, gonadotropin-releasing hormone, thyroid-releasing hormone
4) cGMP - used by atrial natriuretic hormone, nitric oxide |
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Term
| How does the cAMP second messenger system work? |
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Definition
Ligands can not diffuse through plasma membrane
Ligand receptors are integral membrane proteins
act as first messenger
Receptor protein activates G-protein in membrane
G-protein activates adenylate cyclase to convert ATP to cAMP in the cytosol
Cyclic AMP is the 2nd messenger
Activates kinases in the cytosol to speed up/slow down physiological responses
Phosphodiesterase inactivates cAMP quickly
Cell response is turned off unless new hormone molecules arrive |
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Term
| Explain the connection between cAMP and PKA |
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Definition
Cyclic AMP activates cyclic AMP dependent protein kinase (PKA).
In the inactive state, PKA consists of a complex of two catalytic subunits and two regulatory subunits.
The binding of four cyclic AMP molecules to the two regulatory subunits alters their conformation, causing them to dissociate from the complex.
The released catalytic subunits are thereby activated to phosphorylate specific substrate protein molecules. |
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Term
| What are the basics of the G-proteins? |
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Definition
The full name of "G protein" is GTP-binding protein because in the active state it binds to GTP (guanosine triphosphate).
There are two types of G proteins:
heterotrimeric G proteins and
monomeric G proteins (or small G proteins).
G-protein-coupled receptors are coupled to heterotrimeric G proteins.
The structure of heterotrimeric G protein, consisting of three subunits: alpha, beta and gamma. Note that in the inactive state, the a subunit binds to GDP (guanosine diphosphate). |
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Term
| Relate the structure of the G-protein coupled receptors to function |
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Definition
| The amino-terminal end is in contact with the extracellular space, where it can bind to the appropriate hormones. The carboxyl-terminal end is within the cytoplasm, where it can elicit an intracellular response |
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Term
| Explain the steps in G protein activation (and the inactivation that follows) |
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Definition
1) binding of a hormone induces a conformational change in the receptor
2) activated receptor binds to the G-alpha subunit
3) binding induces a conformational change in G-alpha, bound GDP dissociates and is replaced by GTP, G-alpha dissociates from G-beta-gamma
4) hormone dissociates from receptor; G-alpha binds to the effector and activates it
5) hydrolysis of GTP to GDP causes G-alpha to dissociate from effector and re-associate with G-beta-gamma |
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Term
| What are the different types of G proteins? |
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Definition
Gs: stimulate the enzyme adenylate cyclase,
Gi: inhibits the enzyme adenylate cyclase, and
Gq : the stimulate the enzyme phospholipase C.
G12: regulate ion channels. |
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Term
| What are the basics of ion channels? |
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Definition
Ion channels are membrane proteins that allow ions to pass through.
In terms of ion selectivity, they are classified as calcium channels, sodium channels, potassium channels, etc.
In terms of gating (how channels are opened), they may be classified as voltage-gated channels, ligand-gated channels, etc. |
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Term
| What are the two types of Ca2+ channels? |
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Definition
(1) The voltage-activated Ca2+ channels located on the plasma membrane. The opening probability depends on the membrane potential.
(2) The IP3-sensitive Ca2+ channels located on the membrane of endoplasmic reticulum. The opening probability is regulated by IP3 (inositol trisphosphate) |
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Term
| What is an example of a G-protein coupled receptor that can affect an ion channel? |
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Definition
Muscarinic ACh receptor
ACh binding to the receptor activates the G protein.
G-beta-gamma dissociates from G-alpha and it binds to a K+ channel and opens it
This is used to slow heart rate contraction |
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Term
| What is the relation between cAMP, G-proteins, and adenyl cyclase? |
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Definition
Hormones bind receptors, and membrane-bound adenylyl cyclase is activated via G protein binding
Adenylyl cyclase catalyzes the conversion of ATP to cAMP
It is important to realize that they are G-proteins (inhibitory G protein complexes) that inhibit adenyl cyclase as well - ex of signal molecules are PGE1 and adenosine |
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Term
| What are the basics behind protein kinase A's? |
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Definition
Protein kinases phosphorylate proteins – i.e. they add one or more phosphates
The change in charge at that site makes major changes in the conformation, affects activity
Activated by cAMP
cAMP is inactivated by phosphodiesterases, which quickly act to downregulate its activity thus the activity of the protein kinases |
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Term
| What are the basics of the cGMP second messenger pathway? |
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Definition
Cyclic GMP is made from GTP by the enzyme guanylyl cyclase, which exist in soluble and membrane bound forms.
Cell signaling can increase the amount of cGMP through both forms of guanylyl cyclase
Inhibitors of cGMP phosphodiesterase enhance and prolong these responses.
The increased cGMP activates cGMP-dependent protein kinase (PKG), which in turn phosphorylates a number of smooth muscle proteins. |
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Term
| How are the two forms of guanylyl cyclase activated? |
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Definition
The membrane-bound form of guanylate cyclase is acti-
vated directly by hormone binding to its receptor, e.g.
atrial natriuretic peptides.
The cytosolic form is a different protein and is activated by nitric oxide (NO). NO can diffuse across the plasma membrane |
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Term
| What is the purpose of nitric oxide activating cGMP production? |
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Definition
NO produced in endothelial cells lining the lumen of small arterial blood vessels diffuses into smooth muscle cells in the vessel wall, and activates cGMP production.
cGMP-Dependent Protein Kinase then phosphorylates various proteins, including Ca++ channels and Ca++-ATPase pumps, which may result in decreased cytosolic [Ca++].Smooth muscles relax and arterioles dilate.
Nitroglycerin, used clinically to dilate blood vessels in the heart, acts by releasing ·NO as it breaks down. |
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Term
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Definition
| Viagra inhibits PED-5. PED-5 degrades the activity of cGMP. Since cGMP causes dilation and increased blood flow, inhibition of PED-5 will lead to more dilation and increased blood flow (through relaxation of smooth muscle cells) |
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Term
| What is the relation between Gq proteins, phospholipase C and second messengers? |
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Definition
Gq protein when activated, it stimulates phospholipase C.
Phospholipase C is a membrane-bound enzyme that hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2), which is a membrane phospholipids.
Hydrolysed product also include inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG), both of which are second messengers. |
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Term
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Definition
They are 2nd messengers that work together to activate protein kinase C
IP3, a water-soluble molecule, diffuses into the cytosol and causes the release of calcium ions from intracellular stores. IP3 is rapidly inactivated by dephosphorylation.
DAG, a lipid-soluble molecule, diffuses laterally in the membrane and activates protein kinase C, which is calcium-dependent. DAG is rapidly inactivated by hydrolysis. |
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Term
| Explain how IP3 and DAG work together to activate protein kinase C |
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Definition
1) hydrolysis of PIP2 by phospholipase C releases IP3 into the cytosol
2) IP3 binds to the IP3-gated Ca2+ channel on the ER
3) Ca2+ is released
4) Ca2+ binds to protein kinase C (not activated yet but a required step)
5) protein kinase C moves towards the plasma membrane and is activated by DAG
6) protein kinase C phosphorylates the appropriate substrates |
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Term
| What are the different ways to turn off signaling? |
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Definition
Receptor sequestration
Receptor downregulation
Receptor inactivation by its modification
Inactivation of signaling proteins
Production of inhibitory proteins including decoy proteins
Cross-inhibition of different signaling pathways |
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Term
| What is the clinical relevance of G protein coupled receptors? |
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Definition
Half of all known drugs work through G-protein coupled receptors
The genome project has revealed vast numbers of new GPCRs (possible new drug targets?) |
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