Adrenaline responses via alpha1 receptors Flashcards
Different GPCRs are linked to?
Different G-proteins and regulate different effector proteins
The different types of G protein alpha subunit in particular gives?
Different downstream responses
α1-Adrenergic signalling (overview) ?
- Adrenaline binding to the alpha1 receptor in the liver activates Gαq
- Gαq activates phospholipase C beta (PLCβ), which produces two second messengers: 1,2-diacylglycerol (DAG) and inositol-1,4,5-trisphosphate (IP3)
- IP3 induces Ca2+ release from the ER, which in turn activates protein kinase C (PKC) with DAG
Explain Phospholipase C beta hydrolysing PIP2 ?
- Gαq-GTP associates with PLCβ and activates it
2. PLCbeta cleaves PIP2 to IP3 and DAG, which are second messengers: they activate further downstream signalling
Explain the domain structure of Protein Kinase C ?
- Comprised of hydrophobic regulatory and hydrophilic catalytic domains
- Regulatory domain binds Ca2+ and anionic phospholipids e.g. phosphatidyl serine (C2 domain) and diacylglycerol (C1 domain)
- C3 domain is ATP binding domain
- C4 is catalytic domain with pseudosubstrate binding site
What is Protein Kinase C activated by ?
Calcium and DAG
- Once activated, PKC phosphorylates target proteins on S/T residues
Give brief points about TPA ?
- 12-O-Tetradecanoylphorbol-13-acetate (TPA)
- Phorbol Ester
- Analogue of Diacylglycerol
- Tumour Promoter
Explain how TPA continuously activates PKC ?
- TPA keeps PKC unfolded for a long period of time, allowing calpain to cleave in the hinge region
- This releases the catalytic domain into the cytoplasm where it is degraded
- Ultimately, you completely deplete the cell of PKC
Explain Calcium released by IP3 binds calmodulin to activate calcium kinases?
- 2 globular heads at either end linked together by a flexible alpha helical region
- Globular head groups have two EF hand regions
- These are helix-loop-helix motifs that bind Ca2+(so 4 binding sites in total)
- Calcium binding unfurls the protein, exposing hydrophobic patches that allow it to bind target proteins, e.g. CaM kinase (calmodulin-dependent kinase), myosin light chain kinase
Give a summary of alpha1 downstream effects ?
- Activated receptor results in Gαq G protein exchanging GDP for GTP and dissociating from receptor
- Gαq GTP associates with and activates PLCβ
- PLCβ cleaves PIP2 to IP3 and DAG
- DAG binds to and stimulates the activity of PKC, which phosphorylates target proteins. - IP3 activates Ca2+ channels on the endoplasmic reticulum and causes an increase in cytoplasmic Ca2+ levels
- The activities of PKC and calmodulin are both regulated by Ca2+ binding
Give statements about signalling through GPCRs that are true ?
- Some G proteins inhibit the production of cyclic AMP
- All GPCR signal transduction pathways involve amplification
- Some GPCR signalling pathways can alter gene expression
Give a summary of PLCbeta downstream effects ?
- Alpha1 adrenergic receptors have a number of different tissue-specific effects (common feature of cell signalling pathways)
- However PLCbeta itself can have a number of different responses depending on what PKC and the calmodulin kinases are acting on
- In the liver they stimulate glycogen breakdown in response to adrenaline and vasopressin hormone
Explain how Adrenaline activation of the α1 receptor in liver leads to glycogen breakdown ?
- Activation of phospholipase C signaling by adrenaline leads to a net increase in glucose export as a result of increased glycogen degradation and decreased glycogen synthesis
- PKC can phosphorylate glycogen enzymes
- Calcium will stimulate phosphorylase kinase
Explain how Adrenaline activation leads to glycogen breakdown by two pathways in the liver ?
- Adrenaline (epinephrine) can activate glycogen breakdown via both the beta2 and alpha1 receptors (and also via glucagon, which acts via cAMP)
- Parallel pathways allows for finer control. Note that some of the other target pathways for alpha1 and beta2 will be different
