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Cell Signaling


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how do cells in small groups often communicate?
direct cell-cell contact, they have specilized junctions in PM for small molecules to exchange
what do junctions do?
for communication, and determine shape and rigidity
can binding of a protein on one cell trigger differentiation on another cell?
what are the 6 signals of communication by extracellular signals?
synthesis of signal
release of signal
transport of signal
detection of signal
change in metabloism or gene expression
removal of signal for response termination
what are the 4 types of extracellular signaling?
Signaling by PM attached prots
what is endocrine signaling?
hormones travel through the blood and act on target cells distant from their site of synthesis (long distance)
-synthesized in endocrine organs
what is paracrine signaling?
signal molecules released by a cell which only affects target cells in close proximity (a local call)
what is autocrine signaling?
cells respond to substances that they release themselves (they talk to themselves)
what is signaling by PM attahced proteins?
cell to cell contact (touch eachother)
-TNF; Fas-FasL
how are chemical signals classified?
small lipophilic molecules
large lipophilic molecules
how do small lipophilic molecules work in cell signaling?
diffuse through PM and interact with intracellular cytoplasmic receptors
regulate gene expression in target cells
-steroids, NO, thyroid horm
how do hydrophilic and large lipophilic molecules work in cell signaling?
ligand binds to cell surf receptors which lead to short lived changes of intracellular concentrations of 2nd messengers, which leads to alteration in enzyme and nonexzymatic protein activation, controlling cell proliferation, differentation and cell survival
regulates transcription of specific genes
what are examples of some hydrophilic and large lipophilic molecules?
proteins or peptides
insulin, glucagon, vasopressin, NT, prostaglanding, GF, cytokine
what are some 2nd messengers?
cAMP, cGMP, DAG, IP3, Ca++
what are the 4 categories of cell surface receptors?
G-prot linked receptors
Ion-channel receptors
receptors without intrinsic enz activity
receptors with intrinsic enz activity
how do G-prot linked receptors work?
ligand binds and activates G-prot; G-prot activated or inhibits an enzyme that generates a specific 2nd mess of an ion channel; membrane potential changes
what are examples of G-prot linked receptors?
epi, serotonin, glucagon
how do ion-channel receptors work?
ligan binds and changes conformation of the receptor so that specific ions flow through resulting in ion movements; this alters electrical potential across the cell membrane
what are examples of ion-channel receptors?
ACh at the neuromuscular junction
how do receptors WITHOUT intrinsic exzymatic activity work?
directly associated with cytosolic tyrosine kinase; ligand binding causes receptor monomers to dimerize; the dimerized complex interacts with and acticates one or more sytosolic tyrosine kinase
what are examples of receptors WITHOUT intrinsic activity?
cytokines, interferons, GF
how do receptors WITH intrinsic activity work?
ligand binding leads to dimerization of the receptor; these receptors have iether residues of serine/threonine kinases or tyrosine kinase activity; autophosphorylation of these residues create binding sites for several cytosolic enzymes and bring them bear their membrane-bound substrates; in most cases these enzymes generate second messengers
-Receptors with intrinsic tyrosine kinase activity are capable of autophosphorylation as well as phosphorylation of other substrates.
what makes a G-protein linked receptor a G-protein linked receptor?
has an aa sequence with 7 transmembrane alpha helices; the look in the receptor between helices 5&6 and the C-terminal segment is important for G-prot interactions; signal stransducing G prot assiciated with the receptor functions as an on/off molecular switch
what does the G-prot linked receptor look like at rest?
the receptor has a molecule of GDP bound to the G-prot
what does ligand binding cause the G-prot to do?
release it'sbound GDP and bind to GTP and influence either Adenylate cyclase or PL-C
what switches the G-protein back to it's inactive state?
hydrolysis of GTP
what is epinephrine?
aka adrenaline, is a catecholamine that binds to beta-andrenergic receptors located on most tissues; this binding is mediated by a rise in intracellular cAMP resulting from the activation of adenylate cyclase
what does active adenylate cyclase do?
takes ATP and generates cAMP
what does cAMP do?
binds to other enzymes and activates them; PKA depends on cAMP; cAMP, in the presence of phyosphodiesterase turns into AMP which can be built back up to ATP
what are some examples of methyl xanthines?
coffeine, theobromine, theophyline
why does signaling need amplification?
because some cellular responses triggered by cAMP may require tens of thousands or millions of cAMP molecules per cell
what is the kinase cascade?
a series of protein phosphorylations and dephosphorylations that make amplification possible
what does phosphorylation do?
turns things on or off, depending on what we're talking about
what does continuous activation of the G-prot receptor signaling require?
continuous presence of ligand
what does binding of a ligand to rWiea's do?
stimulates the receptor's protein kinase activity which stimulates a signal transduction cascade leading to changes in cellular phys and/or patterns of gene expression; have a wide range of activity (prolif/differentation/survival/metab)
what does PKC do?
phosphorylates its targets
what happens when calmodulin-Ca++ dissociates?
Ca++ is pumped back into ER; dissociates when we have a defecit of IP3
what do methyl xanthines do?
inhibitors of phosphodiesterases
how many beta-andrenergic receptors are on a cell?
only a few thousand... hence the need for amplification
what terminates the cell signal from G-prot linked receptors?
a decrease in the affinity of the receptor that occurs when the inactive G-prot is activated; causes a shift of receptors to the dissociation state causing the removal of the ligand
what are rWiea's?
large and important class of cell-surface receptors whose ligands are soluble or membrane-bound peptides/proteins including insulin and GF
are the number and activity of functional receptors on the surface of cells constant?
no, can be up or down regulated allowing the cells to respond optimally to small changes in ligand level
what does prolonged exposure of a cell to high concentration of a ligand usually result in?
reduction in the number of its functional receptor, thereby desenditizing the cell to the ligand; the # of receptors not reduced but activity is modified
what is produced when PIP2 is cleaved
IP3 and DAG
what does DAG do?
activates PKC
what does IP3 do?
binds to ER and Ca++ channels open and Ca++ flows out, binds to calmodulin, activates target enzymes (PKC)

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