unit 16: cell signaling Flashcards
what are the types of signals received by a cell?
Non-chemical signals
- Light (photons)
- Heat, touch
Dissolved gasses
- O2, CO2, NO
Small molecules
- Amino acid and lipid derivatives
- Acetylcholine
- catecholamines (epinephrine, dopamine)
- steroids
Peptides
- Adrenocorticotropic hormone, vasopressin
Proteins
* Protein hormones (e.g. insulin)
* Growth factors (e.g., EGF)
* Cytokines (e.g, interleukins)
types of signaling
- ligands are released/secreted
- May act either on same cell or cells close-by
- May act over long distances when released into blood stream
- ligands remain bound to cell surface
* Contact-dependent signaling
* Affects only cells in direct contact
* Important in immune signaling
how do signaling receptors recognize the signal
they are highly specific for signal
what are hydrophobic ligands/signals
-steroids
- retinoids
- thyroxine
what receptors do hydrophilic and hydrophobic signals use
hydrophilic= transmembrane proteins
small hydrophobic molecules= intracellular receptors
hydrophilic signals
small molecules
peptides
proteins
how is the signal relayed
- by intercellular signal transduction proteins and second messengers
what are the signaling outcomes
- altered protein function (fast),
- then alters cytoplasmic machinery,
- leads to changed cell behavior - nucleus (transcription), altered protein synthesis (slow)
- altered cytoplasmic machinery
- leads to changed cell behavior
types of short distance intracellular signaling
- Autocrine signaling
- target site on the cells that releases the signal
- many growth factors
- paracrine signaling
- Signaling molecules released affect only target cells in close proximity
- many growth factors - contact-dependent
-Signaling molecule remains associated with cell that produced it
- Cell-cell contact required to transmit the signal
- Developmental processes, immune responses
types of long distance intracellular signaling
Synaptic
* Allows highly directed signaling over long distances by neurons
Endocrine
* Signaling molecules synthesized and secreted into the blood affecting distant cells
* Hormones
binding of signals to their receptors depends on
-high specificity of receptors for their ligands
- Binding relies on molecular complementarity and non-covalent interactions
- residues are essential to tight binding with receptor
Kd= [R][L]/[RL]
How is response to cell signals determined
- by Which receptors are expressed on the cell surface
- the Combination of signals the cell is receiving
- the Intracellular signaling pathways present in the cell
- gene expression
what are the cell surface signal transduction pathways
- receptor-associated kinase
- cytosolic kinase
- protein subunit dissociation
- protein cleavage
Activation of Genes depends on?
Gene Accessibility and Presence of Transcription Factors
main types of cell surface receptors
Ion-channel- coupled receptors
* Respond to neurotransmitters
* Open or close ion channels in response to ligand binding
- alter the membrane potential and excitability of the target
cell
- Involved in synaptic signaling between nerve cells or nerve and muscle cells
G-protein- coupled receptor (GPCRs)
* Regulate activity of membrane-bound target protein (enzyme or ion channels)
* Seven-transmembrane-domain receptors (7TM)
* Signaling mediated by G-protein
- Ligand binding to receptor activates a membrane- bound G-protein that, in turn, activates a membrane bound effector protein
Enzyme- coupled receptors
* Intrinsic enzyme activity or coupling with enzyme (usually kinase)
* Single-pass membrane proteins
- Ligand binding to the receptor activates an enzyme, often a kinase, that is an endogenous part of the receptor or coupled to the receptor
what are molecular switches
– Proteins that are turned on or off by other proteins
– G-proteins (trimeric and monomeric)
– Phosphorylation cascades (Tyr, ser/Thr kinases and phosphatases)
what are second messengers
Second messengers
– Low-MW signaling molecules
– Amplify and propagate signal
– Often used to relay signal to distant sites and into organelles – Ca2, cAMP universally used
activated GPCR acts as a what?
guanine exchange factor (GEF)
the effector protein acts as a what?
GTPase activating protein (GAP) signaling second messengers
second messengers in the cytosol are
water soluble such as Ca2+, cAMP
second messengers in membranes are
lipid-soluble second messengers, such as DAG
- cAMP activates
- cGMP
- DAG
- IP3
- protein kinase A (PKA)
- protein kinase G (PKG) and opens cation channels in rod cells
- protein kinase C (PKC)
- open ca2+ channels in the ER
what are the signaling relay mechanisms
Pre-formed signaling complexes
– Grouping of signaling molecules on scaffolds
Induced proximity between signaling molecules
– Receptor activation stimulates transient assemblies of signaling molecules
on cytoplasmic side
Creating phosphoinositide docking sites
– Receptor activation creates a phospholipid modification
– they act as binding sites for signaling molecules
Mediated through interaction domains
– SH2, PTB, etc.
describe the interaction domains
Src homology 2 (SH2), phosphotyrosine- binding (PTB) domains
- Bind specific phospho- tyr on receptors or intracellular signaling domains
(SH3) domains
- Bind to short pro-rich sequences
Pleckstrin homology (PH) domains
- Bind to charged headgroups of certain phosphoinositides
what are the mechanisms of Desensitization to Signals
- Prolonged exposure to stimulus decreases response of the cell
- separating receptor and signal
- receptor down regulation
- receptor inactivation
- inactivating signal protein
- producing inhibitory protein
examples of GPCR
epinephrine receptor
glucagon receptor
describe trimetric g-protein
Heterotrimers(α,β,γsubunits)
- α, γ subunit membrane-anchored through
covalently bound fatty acids
- GTP-binding site in α subunit
- β, γ subunits associated
describe Activation of Trimeric G-Proteins through GPCR
- Ligand binding activates receptor
- Active receptor interacts with α subunit
- Opens nucleotide binding site of α subunit
– Exchange of GDP with GTP
(GEF activity of receptor) - GDP/GTP exchange triggers conformational change and dissociation of α subunit from Gβγ
-Interaction of activated subunits with effector protein leading to downstream signaling
production of cAMP
- produced by activated adenylyl cyclase from ATP
- cAMP activates protein kinase A, cAMP dependent kinase causing a downstream effect
- hydrolysed by phosphodiesterase (PDE) to AMP
GTP-Gas vs GTP-Gai
GTP-Gas stimulates adenylyl cyclase
GTP-Gai inhibits adenylyl cyclase
how does PKA alter gene expression
- activated PKA phosphorylates transcription factor CREB
- Phosporylated-CREB recruits co-factor CBP/P300
- Binds to CRE element (cAMP response element) in DNA, activating transcription
what proteins do the Ca-calmodulin complex active
- Myosin light chain kinase (muscle cells)
- cAMP phosphodiesterase (PDE)
- Protein kinases and phosphatases that regulate the activity of transcription factors
what are the controls of the IP3/DAG pathway
- Low cytoplasmic Ca2+ level restored by Calcium pumps in ER and plasma membrane
- Phosphatase dephosphorylates IP3 to inositol-1,4-bisphosphate
– Cannot bind Ca2+ or IP3-gated channels - Feedback inhibition by reduction of sensitivity of IP3-gated channel to Ca2+ at high Ca2+ levels
- Channels close and cytoplasmic Ca2+ levels drop
what is the regulation of glycogenolysis by Ca2+ and cAMP/PKA
in muscle cells:
- neural stimulation
- Ca2+ activates GPK, increases glycogen degradation
- hormonal stimulation
- cAMP/PKA, decreases glycogen synthesis
in liver cells:
- hormonal stimulation
- DAG/ cAMP activate PKC, decreased glycogen synthesis
- IP3/cAMP, increased glycogen degradation
describe downregulation for GPCR signaling
- Ligand dissociates
- Receptor affinity for ligand reduced after binding of Gas
- Binding to effector proteins (e.g., adenylyl cyclase) stimulates GTPase activity of G-protein inhibitor
- cAMP phosphodiesterase (PDE) activity reduces cAMP levels by hydrolyzing cAMP to AMP
– Reduces activity of PKA
Receptor inactivation by phosphorylation
– Feedback regulation by phosphorylation of cytoplasmic loops by PKA
– Phosphorylation of receptor (ser/thr) sites by GPCR kinases creates sites for inhibitory protein
Regulation of Ion Channels by GPCR
- neurotransmitter binds receptor
- Gβγ subunit of activated G-protein inhibitor binds to and opens K+ channel
clinical correlation of GPCR
- drugs target GPCR
enzyme coupled cell surface receptors? examples
Signal through intrinsic enzyme or closely associated enzyme
– Phosphorylation of tyrosine or serine/threonine)
Receptor tyrosine kinases (RTKs)
– Signal through various downstream signaling cascades
Cytokine receptors
– signal through JAK-STAT pathway
TGFβ family receptors
– signal through Smads
describe Receptor Tyrosine Kinases
- Most abundant enzyme-coupled receptors
- Responds to different types of ligands
– Growth factors (EGF, insulin, PDGF, FGF), hormones
– Both membrane-bound and soluble ligands - Single-pass transmembrane domain
- Ligand binding induces dimerization and activation of intrinsic tyrosine kinase activity
- Control by receptor endocytosis - Binding of EGF forces out a loop region that engages in dimerization
clinical correlation of HER2 (Erb-B2)
HER2 gene amplification and HER2 overexpression in about 25% of breast tumors
- Correlates with poor prognosis
describe Ras
- Small monomeric G-proteins
- Anchored in cell membrane
by covalently attached lipid - Activated indirectly through adaptor proteins downstream of receptors
- Adapter protein GRB2 binds to activated RTK through SH2 domain
- SH3 domain of GRB2 recruits Sos
- Sos binds to and activates Ras
- Hyperactivated Ras associated with 30% of human tumors
describe sos
- acts as GEF for Ras
- sos promotes dissociation of GDP from Ras; GTP binds and active Ras dissociates from sos for signaling
Ras-Raf-MEK-MAPK Pathway
- Ras is activated and binds Raf then activates it
- GTP hydrolysis leads to dissociation of Ras from Raf
- Raf activates MEK (phosphorylation)
- MEK activates MAPK (phosphorylation)
- active MAPk translocates to nucleus and activates transcription factors
- expression of proteins necessary for cell proliferation (Early Response genes)
describe RTK Signaling through Phospatidylinositols
Activated RTKs can activate an isoform of Phospholipase C (PLC-γ)
– Signals through downstream messengers IP3 and DAG
Activated RTKs can recruit phosphoinositide-3 kinase (PI-3 kinase) to the plasma membrane
– Activates pathways promoting cell survival and proliferation or changes in metabolism
explain RTK Activation of Protein Kinase B (PKB) via PI-3 Kinase
- PI-3 kinase is recruited to the plasma membrane by binding of its SH2 domain to the activated RTK.
- PI-3K catalyzes the formation of
PI-3 phosphates on the cytoplasmic face of the plasma membrane.
-PI-3 phosphates serve as docking sites for signaling molecules with PH domain.
- PTEN phosphatase inactivates the signaling pathways by hydrolyzing the 3- phosphate in PI 3-phosphates.
-PKB activates a factor that inhibits apoptosis, thereby promoting cell survival
