What are the four basic steps to cell signalling?
- Signalling molecules are released (ligands)
- Recognition of the signalling molecule by target cell (receptors)
- Signal transduction- conversion of the extracellular signal into intracellular instructions (also called signalling cascade)
- Final impact on target cell and impact on organism as a whole
Slide 4
What are the two modes of communication in cell signalling?
- Direct interaction of a cell with its neighbour
- Action of a diffusible signalling molecules over a distance
Cell communication and signal transduction increases in complexity with multicellular organisms
What are the 4 modes of cell signalling?
- cell-cell: embryo development
- Paracrine: neurotransmitters across a synapse
- Endocrine: hormones such as glucocorticoids, thyroid hormone
- Autocrine: T-lymphocytes proliferate with a growth factor
What are the 5 classes of ligands?
- Steroid hormones- hydrophobic derived from lipid cholesterol so it diffuses across membrane
- Eicosanoids- hydrophobic synthesized from lipids and are rapidly broken down
- Neurotransmitters- hydrophilic do not cross membrane
- Peptide hormones and polypeptide growth factors- largest and most variable hydrophilic and don’t cross membrane
- Simple gases- passive diffusion across membrane, bind to enzymes without receptors
What are the 2 classes of receptors?
- Intracellular receptors- nuclear receptor superfamily which primarily function as transcription factors
- Cell surface receptors- G protein coupled receptors, receptor protein tyrosine kinases, cytokine receptor superfamily
What are the characteristics of the nuclear receptor superfamily? (4 steps)
- molecules that bind these receptors include steroid hormones and thyroid hormones (small and hydrophobic)
- receptors and intracellular proteins (not associated with membrane)
- these receptors + ligand= transcription factors
- receptors contain both a ligand binding domain and a DNA binding domain
What is glucocorticoid action?
Inactive when bound to chaperone
Glucocorticoid, active when bound to ligand
receptor+ligand+HAT coactivator= active gene transcription
2 active receptors form a dimer, dimer associates with HAT coactivator, hormone complex bonds to a specific DNA binding site and activated gene transcription
Slide 16
How are genes regulated by the thyroid hormone receptor?
Thyroid hormone receptor binds to DNA with or without ligand, without the ligand the receptors binds the corepressor HDAC to repress gene transcription
When hormone is present, it binds receptor changing it conformation to dissociate from HDAC and associate with the coactivator HAT thereby allowing gene transcription
Slide 17
What are the characteristics of G protein coupled receptors? (5 of them)
- largest family of cell surface receptors
- signals are transmitted to intracellular targets by an intermediary G protein (alpha beta gamma)
- are transmembrane proteins with multiple transmembrane domains
- extracellular receptor domain binds Logan which causes conformation change which allows cytosolic domain to activate G protein
- alpha subunit of G protein dissociates from beta and game subunits and carries signal to intracellular target
Slide 21
What are the characteristics of tyrosine kinase receptors? (4 of them)
- are cell surface receptors linked to intracellular enzymes (have 1 transmembrane domain)
- receptors dimerize when bound to ligand
- activate the receptors by phosphorylating tyrosine residues in receptor and target substrates
- phosphorylated receptors can associate downstream target which initiate signalling cascade
Slide 24
How are nonreceptor tyrosine kinases activated?
They are associated with receptors that contain no catalytic activity
Ligand binding indices dimerizatipn and activate tyrosine kinases to autophosphorylate themselves as well as the receptor, the. The receptor can associate with downstream signalling molecules beginning a signalling cascade
Slide 25
What is intracellular signal transduction?
Chain of reactions that transmits chemical signals from the cell surface to their intracellular targets
Also called signalling cascade
Transcription factors are often the final targets of the signal cascade
What is the Cyclic AMP pathway?
Cyclic AMP is a second messenger which is a compound that is modified as a result of ligand receptor interaction (relays message from receptor to target is signal transduction)
Cyclic AMP is important second messenger associated with G protein receptors
cAMP is important for response of cells to a variety of hormones
What are the two second messengers of cyclic amp? (Adenylyl Cyclase and cAMP phosphodiesterase)
Adenylyl cyclase- enzyme that catalyzes formation of cyclic AMP from ATP (activated G protein alpha subunit stimulates the activity of this)
cAMP phosphodiesterase- enzyme that degrades cyclic AMP
Pictures of them on slide 32 and 33
How is signal transduction initiated through PKA? (Start with second messenger cAMP)
cAMP binds to a cAMP dependent protein kinase called protein kinase A (PKA)
This causes dissociation of PKA regulatory subunits
PKA phosphorylates downstream target proteins
Slide 35
What is the transcription factor CREB?
How is it activated?
Recruits co activators and initiated transcription of genes at CRE binding sites
PKA activates transcription factor CREB by phosphorylation
Slide 36
How does protein kinase A (PKA) regulate glycogen metabolism?
(Activates and inhibits something)
PKA phosphorylates two key downstream target enzymes
- activates phosphorylase kinase
- inhibits glycogen synthase
cAMP signalling pathways can effect both transcription factors and metabolic enzymes
How is protein phosphorylation regulated by protein kinase A (PKA) and protein phosphatase 1 (PP1)
Protein kinases (PKA) do not function in isolation within the cell So protein phosphatase 1 (PP1) activity counterbalances PKA activity to fine tune this signalling mechanism Slide 38
What are MAP kinase pathways?
What does MAP stand for?
What is the stereotypical pathway?
Cell surface receptors linked to enzymes to produce intracellular signals
MAP- mitogen-activated-protein
Can be associated with receptor and nonreceptor tyrosine kinases
Stereotypical pathway- Ras-Raf-MEK-ERK
Ex: Raf phosphorlyates MEK
MEK phosphorylates ERK
Slide 42
What are Ras proteins?
how are they activated and terminated?
Ras (Rat Sarcoma)
Integral membrane lipoprotein
One of first oncogenes found in human cancers
Activated by GEF-guanine exchange factor
Terminated by GAP- guanine activations protein
What are Raf and MEK in MAP kinase pathways?
RAF- (rapidly accelerating fibrosarcoma) is a protein serine/threonine kinase that is activated by Ras and activates ERK
MEK- (map kinase/ERK Kinase) is a 2 specificity protein kinase that activates members of the ERK family
What is Notch signalling?
Example of cell-cell mode of signalling
Notch receptor on plasma membrane receivers signal from delta ligand on surface of another adjacent cell
Ligand receptor binding activates γ-secretase enzyme to cleave intracellular domain of notch
This notch intracellular domain translocates to the nucleus to bind and activate transcription factors
Slide 51
What is the Wnt pathway?
Wnt proteins are family of secreted growth factors in cell fate pathways
Bind to receptor frizzled and LRP
interaction of Wnt and frizzled and LRP recruits protein dishevelled which disrupts construction complex of proteins, this prevents phosphorylation of transcription factor β-catenin, this then translocates to nucleus to activate gene transcription
Slides 53-54
What are feedback loops?
Regulate the activities of individual pathways and control the extent and duration of signalling activity
They are similar in principle to feedback regulation of metabolic pathways in that they control activity of signalling pathways
