Topic 11: Cell Signalling Flashcards
1
Q
4 Steps to Cell Signalling
A
- signalling molecules are released (ligands)
- recognition of the signalling molecule by target cell (receptors)
- signal transduction
- Final impact on target cell and subsequent impact on organism as a whole
2
Q
Signal Transduction
A
- the conversion of the extracellular signal into intracellular instructions
3
Q
Modes of communication
A
- direct interaction of a cell with its neighbour
- action of diffusible signalling molecules overs a distance
- some carry signals long distances, others act locally
- cell communication and signal transduction increases in complexity with multicellular organisms
- cell to cell
- paracrine
- autocrine
- endocrine
4
Q
5 Classes of Ligands (signalling molecules)
A
- Steroid hormones
- Eicosanoids
- Neurotransmitters
- Peptide hormones and polypeptide growth factors
- Simple gases
5
Q
Steroid Hormones
A
- small hydrophobic molecule (derived from lipid cholesterol)
- diffuses across cell membrane
- endocrine, paracrine, autocrine modes of action
- bind to intracellular receptors (Nuclear Receptor Superfamily)
6
Q
Eicosanoids
A
- Prostaglandins
- hydrophobic (synthesized from lipids) and are rapidly broken down
- paracrine or autocrine modes of action
7
Q
Neurotransmitters
A
- acetylcholine, dopamine
- hydrophilic molecules that don’t cross the cell membrane
- endocrine, paracrine, autocrine
- bind to cell surface receptors
8
Q
Peptide Hormones and Polypeptide Growth Factors
A
- insulin, epidermal growth fact
- largest and most variable class
- primarily hydrophilic and can’t cross the cell membrane
- endocrine, paracrine, autocrine modes of action
- bind to Cell Surface Receptors
9
Q
Simple Gases
A
- nitric oxide and carbon monoxide
- can move across the cell membrane (passive)
- paracrine mode of action
- bind directly to enzymes, do not use receptors
10
Q
2 Classes of Receptors
A
- Intracellular Receptors
- Nuclear Receptor Superfamily - Cell Surface Receptors
- G-protein coupled receptors
- receptor protein tyrosine kinases
- cytokine receptor superfamily
11
Q
Nuclear Receptor Superfamily
A
- molecules bind these receptors include steroid hormones and thyroid hormones (small, hydrophobic)
- receptors are intracellular proteins (not associated with the membrane)
- these receptors + ligand = transcription factors
- receptors contain both a ligand binding domain and a DNA binding domain
12
Q
Glucocorticoid Action
A
- inactive when bound to a chaperone
- become active when bound to a ligance
- 2 active receptors form a dimer
- dimer translocates to the nucleus
- dimer associates with the co-activator protein HAT
- hormone complex binds to a specific DNA binding site and activates gene transcription
- GLUCOCORTICOID RECEPTOR + LIGAND + HAT COACTIVATOR = ACTIVE GENE TRANSCRIPTION
- nuclear receptors transduce signal from ligand to DNA
- final effect: increase in transcription of a specific gene
13
Q
Gene Regulation by Thyroid Hormone Receptor
A
- thyroid hormone receptor (dimer) is bound to DNA either with or without ligand
- without ligand: receptor binds to corepressor HDAC to repress gene transcription
- hormone present: binds receptor, changing conformation to disassociate from HDAC and associate with HAT allowing gene transcription
14
Q
HDAC
A
- co-repressor
15
Q
HAT
A
- co-activator
16
Q
G-Protein Coupled Receptors
A
- largest family of cell surface receptors
- can bind a variety of ligands
- signals are transmitted to intracellular targets via an intermediary protein (Gprotein)
- contains 3 subunits: alpha, beta, gamma
- transmembrane proteins typically with multiple transmembrane domains
17
Q
Activation of G Protein
A
- extracellular receptor domain binds the ligand
- causes conformational change allowing cytosolic domain to activate a G protein
- alpha subunit dissociate from and carries signal to intracellular target (adenylyl cyclase)
18
Q
Tyrosine Kinase Receptors
A
- CSR linked to intracellular enzymes
- have one transmembrane domain
- enzyme activity by part of intracellular domain of receptor OR separate protein associated with intracellular domain
- receptors dimerize when bound to ligand
19
Q
Tyrosine Kinase Receptors Activation
A
- activate receptors by phosphorylating tyrosine residues on both the receptor and target substrates
- phosphorylated can then associate with downstream targets thereby initiating a signalling cascade
- proteins such as insulin and multiple growth factors recognize these receptors
20
Q
Dimerization and Autophosphorylation of Receptor Protein-Tyrosine Kinases
A
- ligand binds receptor causing dimerization followed by cross phosphorylation of both receptor dimers
- receptors can associate with downstream signalling molecules which begins a signalling cascade
21
Q
Activation of Nonreceptor Tyrosine Kinase
A
- nonreceptor tyrosine kinases are associated with receptors that contain no catalytic activity (cytokine receptors)
- ligand binding induces dimerization and active tyrosine kinases to autophosphorylate themselves and receptor
22
Q
Intracellular Signal Transduction
A
- chain of reactions that transmit chemical signals from cell surface to intracellular targets (signalling cascade)
- frequently, transcription factors are the final targets of signal cascade
23
Q
Signal Transduction from Nuclear Receptors
A
- Glucocorticoid Action
24
Q
Cyclic AMP
A
- a second messenger associated with G protein coupled receptors
- adenosine monophosphate chemical structure has been modified into cyclic structure
- a phosphate group is covalently bound to both 3’ and 5’ carbon by adenylyl cyclase
- important for response of cells to a variety of hormones
25
Second Messenger
- a compound modified as a result of a ligand-receptor interaction
- function to relay the message from the receptor to target
- can be used in multiple pathways
26
Adenylyl Cyclase
- an enzyme that catalyzes the formation of cyclic AMP from ATP
- activated by an activated G protein alpha subunit
27
cAMP Phosphodiesterase
- an enzyme that degrade cyclic AMP
28
Protein Kinase A (PKA)
- a cAMP dependent protein kinase
29
Signal Transduction Initiated through PKA
- 2nd messenger cAMP initiates intracellular transduction (chain of reactions or a signalling cascade)
- cAMP binds to PKA
- causes dissociation of PKA regulatory subunits
- PKA phosphorylates downstream target proteins
30
cAMP Inducible Gene Expression
- cAMP binds PKA
- subunits of PKA dissociate
- PKA activates transcription factor CREB by phosphorylation
- CREB recruits co-activators and initiates transcription of genes at CRE binding sites
31
CRE
- a specific DNA binding element that is in the promotor region of cAMP responsive genes
32
Regulation of Glycogen Metabolism by PKA
- PKA phosphorylates two key downstream target enzymes
- activates phosphorylase kinase
- inhibits glycogen synthase
33
cAMP Signalling Pathways
- can effect both transcription factors and metabolic enzymes
34
Regulation of Protein Phosphorylation
- protein kinases (PKA) don't function in isolation within the cell
- protein phosphatase (PP1) activity counterbalances PKA activity to fine tune this signalling mechanism
35
Amplification of Signal Transduction
1. a single receptor can activate multiple G proteins
- -> stimulates adenylyl cyclase to make cAMP which activates PKA
2. PKA can then phosphorylate multiple targets
- End Result: 1 hormone molecule binding 1 receptor can activate a large # of target proteins
36
MAP Kinase Pathways
- cell surface receptors linked to enzymes to produce intracellular signals
- Mitogen-Activated Protein
- multiple different MAP kinase pathways
- these transduction pathways can be associated with both receptor and non-receptor tyrosine kinases
- "cascade of kinases"
- activated in response to a variety of growth factors and other signalling molecules
- Ras-Raf-MEK-ERK (stereotypical MAP kinase pathway)
37
Raf
- Rapidly Accelerating Fibrosarcoma
- a protein-serine/threonine kinsase
- activated by Ras
- leads to activation of ERK MAP kinase
38
ERK
- Extracellular signal-Regulated Kinase
39
MEK
- Map kinase /ERK kinase
- a dual-specificity protein kinase
- activates members of the ERK family by phosphorylation of both threonine and tyrosine residues separated by 1 amino acid
40
Ras Proteins
- RAt Sarcoma
- guanine nucleotide-binding proteins that function analogously to the alpha subunits of G proteins
- alternate between inactive GDP-bound and active GTP-bound forms
- integral membrane lipoprotein
- one of the first oncogenes identified in human cancers
- could be directly links to growth factor induced cell proliferation
41
Regulation of Ras Proteins
- converted to active GTP-bound state by exchange of GTP for bound GDP, which is stimulated by GEFs
- Was activity is terminated by GTP hydrolysis, which is stimulated by GAPs
42
GEF
- Guanine Exchanger Factor
- activates Ras
- in its mutated form oncogenic Ras is "locked" in its GTP binding form
43
GAP
- Guanine Activating Protein
| - terminates Ras activity
44
Ras Activation Downstream of Receptor Protein-Tyrosine Kinases
- intermediate protein links phosphorylated region of tyrosine kinase with other target molecules
- inities the signalling cascade
- many types of these molecules
45
Activation of Raf Kinase
- initiates a protein kinase cascade
- Raf phosphorylates MEK
- MEK phosphorylates ERK
- ERK phosphorylates other targets
- -> ERK targets include other protein kinases and transcription factors
46
Mammalian and Yeast cells MAP Kinase Pathways
- have multiple MAP K Pathways
- all contain a cascade of 3 kinases
- these pathways are important for the regulation of cell proliferation, differentiation, cell survival
47
Notch Signalling
- cell to cell mode of signalling
- Notch receptor (in p. membrane) receives signal from Delta Ligand (m. protein) on surface of an adjacent cell
- Ligand receptor binding activates γ-secretase enzyme to cleave intracellular domain of Notch
- Notch intracellular domain translocates to the nucleus where it binds and activates transcription factors
- intracellular domain is the vehicle for signal transduction
48
Wnt Pathway
- disruption of "destruction" complex prevents phosphorylation of the transcription factor β- Catenin
- Unphosphorylatedβ- Catenin is stabilized and can translocate to the nucleus to activate gene transcription (transduction)
- binds to a repressor protein and forms a complex that activates transcription
49
Feedback Loops
- regulates the activation of individual pathways
- -> control the extent and duration of signalling activity
- similar in principle to feedback regulation of metabolic pathways
- -> control the activity of signalling pathways
50
Signalling Networks
- signalling pathways don't operate in isolation
- frequent crosstalk bwtn diff. pathways so intracellular signal transduction needs to be understood as an integrated network of connected pathways
- Final impact on cell: multicellular organism depends on signalling pathways intersecting
51
Crosstalk
interaction of one signalling pathway with another
52
Elements of Signalling Networks
1. Negative Feedback
2. Positive Feedback
3. Feedforward Relay
4. Stimulatory Crosstalk
5. Inhibitory Crosstalk
53
Signal Transduction Pathways
- interact to give us the final impact on a target cell
- involve multiple complex interconnected networks formed by the interactions of multiple signalling pathways within a cell
