Lecture 15. Conservation of signaling pathways between organisms Flashcards
What is juxtacrine signaling ?
Signalling by plasma membrane attached proteins. Signalling cell is adjacent to target cell
What is an example of juxtacrine signaling ?
Notch signalling pathway
What is the function of the Notch signaling pathway ?
Regulates cellular identity, proliferation, differentiation and apoptosis in animals by developmental processes such as lateral inhibition
What happens in the process of lateral inhibition ?
Adjacent and developmentally equivalent cells assume completely different fates
What must happen for the activation of the notch signaling pathway to occur ?
The ligand (Delta type) and the receptor (notch) must be be located in membranes of adjacent cells
What does the notch signaling pathway involve ?
Ligand induced protein cleavage
What are the key players in the basic operation of the notch pathway ?
- Delta type ligand
- Receptor notch
- Proteases
- CSL transcription factor
What is the receptor family in the notch pathway ?
Notch/lin-12/glp-1
What is the ligand in the notch pathway ?
Delta, serrate, delta like, jagged
What are the processing proteases in the notch pathway ?
Furins, ADAMs and secretases
What are the downstream effectors in the notch pathway ?
CBF2, Su(hairless), Lag 1
What is the notch protein synthesised as ?
A monomeric membrane protein in the endoplasmic reticulum
What happens to the notch protein once it is synthesised ?
Transferred to the golgi
What happens once the notch protein is in the golgi ?
Undergoes proteolytic cleavage by a furin protease (S1 cleavage) to generate an extracellular subunit and a transmembrane cytosolic cleavage
What do the two subunits of the notch receptor do ?
Remain non-covalently associated with each other in the absence of interaction with delta like ligand residing on an adjacent cell
Once the notch receptor undergoes proteolytic processing in the the golgi what happens ?
Transported to cell surface membrane
What binds with the notch ligands expressed by adjacent sending cell ?
The extracellular domain of the notch receptor in the signaling receiving cell
What induces the second proteolytic step ?
Binding of the receiving cell to the sending cell
What is the second proteolytic step ?
S2 cleavage by ADAM metalloproteases
What does S2 cleavage by ADAM metalloproteases lead to ?
The endocytosis of the notch extracellular domain into the ligand expressing cell
What happens after S2 cleavage step ?
S3-S4 cleavage by gamma-secretases protease to release the notch intracellular domain as a soluble protein into the cytosol
Where does the resulting active Notch ICD soluble protein translocate to ?
The nucleus and interacts with the CSL protein
What happens in the absence of notch-ICD ?
The CSL is bound to co-repressor which actively represses the transcription of notch target genes
What does binding of notch-ICD with CSL displace ?
The co-repressor
What does displacing the co-repressor result in ?
The formation of an active complex with MAML and other co-activators and leads to the transcription of notch target genes
What are some examples of notch target genes ?
HES, cyclin D1 and c-MYC
What happens once target genes have been expressed ?
Notch-ICD is downregulated
What targets Notch-ICD to a proteasome ?
An E3 ubiquitin ligase attaches multiple ubiquitin molecules to notch-ICD
What is a proteasome ?
A large multifunctional protease complex in the cytosol that degrades intracellular proteins marked for destruction by ubiquitin
Where is de-regulated notch signalling found ?
in T-cell leukaemia, breast cancer, prostrate cancer, colorectal cancer and lung cancer as well as CNS malignancies
What are two major classes of notch-inhibitors which may be an emerging target for cancer treatment ?
- Y-secretase inhibitors
2. Monoclonal antibodies
What do pathways such as nutrient and energy sensing pathways in multicellular eukaryotes converge on ?
TOR
What is TOR ?
A large protein kinase
What does Tor kinase do ?
Regulates several cellular processes in response to nutritional status and signals from cell surface receptors
What can translation initiation factors and ribosomal proteins be regulated by ?
Modification such as phosphorylation
What are the two distinct mTOR containing complexes in mammalian cells ?
- mTORC1
2. mTORC2
In addition to mTOR kinase what are the core components for mTORC1 ?
- Raptor
2. mLST8
What controls protein synthesis in nutrient and energy saving pathways ?
Active mTORC1
How does active mTORC1 control protein synthesis ?
By phosphorylating kinase S6K1/2 and 4E-BP1
What do kinase S6K1/2 and 4E-BP1 do ?
Regulate translation
What do kinase S6K1/2 and 4E-BP1 lead to ?
An increase in protein synthesis
What is the function of S6K1/2 ?
Phosphorylates ribosomal proteins leading to an increase in the rate of protein synthesis
What is the function of 4E-BP1 ?
Inhibits interaction of a key translation initiation factor with mRNA so inhibits protein synthesis.
When phosphorylated by mTORC1, 4E-BP1 releases the initiation factor thus stimulation translation initiation
What is mTOR activity regulated by ?
G-protein RHEB
What is Rheb activity controlled ?
TSC1/TCS2
What does TSC1/TSC2 do ?
A Rheb GTPase activating protein which causes hydrolysis of bound GTP to GDP, thereby inactivatinf mTOR
What happens to TCS2 in response to insulin ?
AKT inhibits TSC2 through phosphorylation, allowing Rheb-GTP to accumulate and activate mTORC1
How does AMPK effect mTORC1 ?
Inhibits it through activation of TSC2 and inhibition of Raptor through phosphorylation of key serine residue
What are intracellular amino acids sensed by ?
mTORC1 through an undefined pathway affecting Rag GTPase
What is at the heart of life ?
Coupling the availability of nutrients and energy with growth factors to drive cell proliferation
What are crucial components regulating the perception and the responses to nutrients and energy levels ?
mTOR and AMPK/SnF1
What sort of relationship exists between the mTOR and AMPK pathway ?
An antagonistic
What are AMPKs activated by ?
A decrease in energy levels sensed by an increase in the AMP/ATP ratio for AMPK or by nutrient starvation
What is TOR kinase globally activated by ?
Favourable and nutrient replete conditions
What can be found in plants ?
TORC1 complex
What did TOR silencing lead to ?
Yellowing of leaves linked to chlorophyll breakdown and premature senescence
What is SnRK1 ?
A AMPK ortholog found in plants
What is SnRK1 implicated in ?
The regulation of development, growth and defence
What is SnRK1 a central integrator of ?
Stress and energy modulating the expression of more than 1000 genes through the phosphorylation of various transcription factors
What can SnRK1 phosphorylate ?
An inhibitory conserved serine residue in the raptor protein allowing direct control of TORC1 activity
What represents an important regulatory pathway connecting nutrient sensing to the cell growth machinery ?
The functional conservation of AMPK/SNF1/SnRK1 and TOR in eukaryotes together with inhibitory phosphorylation in raptor
What interconnects and control plant growth ?
Sugar signaling pathways
What is an important factor in regulating plant growth ?
The cellular metabolic status
What activates SnRK1 and what does it result in ?
- Nutrient stress
2. Inhibition of growth
What is plant SnRK1 regulated by ?
Sugar phosphates
What is SnRK1 activity repressed by ?
Glucose 6-phosphate and trelahose-6-phosphate
What happens under a high metabolic status ?
Trelahose-6-phosphate and glucose-6-phosphate inhibit SnRK1 and the active TOR kinase then stimulates translation and growth
