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cAMP Signalling - Coggle Diagram

- - - - Caveolae are prominent in endothelial cells and are marked by caveolin and all Ca2+ sensitive ACs (all but 2, 4 & 7) are enriched within these and other lipid rafts (Fagan, 2000, J Biol Chem)
    - - TM domains traffic to non-raft PM domains if cytosolic domains are not kept, these are key to raft targeting
    - - Ordered and disordered parts of the PM are seen in PC/cholesterol mixtures (Risselada)
      - FRAP experiments have shown that caveolin at the PM (where it accumulates in lipid rafts) is much less mobile than in the cytoplasm, suggesting that they are not artefacts
        
        'Picket fences' can also constrain membrane-bound proteins which have significant cytosolic domains
        
        Proteins and molecules in lipid rafts are still moving, just slower than molecules outside of the raft
  - - - Association was shown not to require store-depletion or catalytic activity of AC8 but did require the NTD of AC8
      - AC8, Orai1, STIM1 and caveolin can all localise to lipid rafts
        As can palmitoylated AKAP79 (palmitoylation is a common modification to target proteins to lipid rafts and relies on cysteine residues)
        
        Mutation of cys36 and cys129 excludes AKAP79 and PKA from rafts and PKA no longer phosphorylates raft proteins (Delint-Martinez, 2011)
- - - - Ca2+ inhibition ensures that the heart beats are short by inhibition of b-AR signals through AC5/6 that stimulate L-type Ca2+ channels (Cooper, 1995, Nature)
        
        In non-excitable cells Ca2+ homeostasis ensures that any Ca2+ peak is merely transient through SERCA and PMCA despite the rapid onset ensured by SOCE and IP3R
    - - Three genes encode the identical protein sequence and it cooperatively binds Ca2+ through four E-F hands
      - Central helical strand of CaM mediates binding to other proteins (Wilson, 2000, J Mol Biol)
      - In AC8 CaM is bound to the NTD and Ca2+ to the CTD at rest
        Upon an increase in [Ca2+] Ca2+ binds the EF hands and CaM binds the CTD, relieving the autoinhibition, similar to CaM's function in L-type channels
        
        Both AC1 and AC8 have one amphipathic helix (in the C1b and Ntm domains respectively) but AC8 also has the IQ-like motif within the C2b domain
        
        AC1 activation is direct, AC8 activation is the removal of inhibition
        This allows AC1 to be regulated faster than AC8
    - - Mg2+ is key to formation of cAMP and hence exclusion of it from the active site is highly inhibitory
      - Additionally binding of Ca2+ stabilises PPi binding within the catalytic site because it has a higher coordination number (Mou, 2009, Biochemistry)
- - - - The variants are physiologically relevant though:
        PDE4A5 signalling impairs long-term memory (Havekes, 2016, J Neurosci)
        PDE4D7 can be used as a biomarker for prostate cancer outcomes (van Strijp, 2018, Prostate Cancer)
  - - - Further experiments showed that oscillations required AC8 and that AC1 was insufficient to drive oscillatory behaviour
- - - - AKAPs are key to PKA targeting and ~50 species have been described
        
        Contain a PKA binding region, an anchor and binding regions for other signalling components
        
        They force PKAII to have specific effects and directs the modulation of various proteins
        This ensures only localised signalling
        
        Mutations in yotiao mislocalise the PKA which regulates KCNQ1 resulting in long-QT syndrome (Chen, 2007, PNAS)
        
        AKAP79 binds multiple proteins to ensure precise regulation in the cell: PKA, PDE4, CaM, Calcineurin, PKC, AMPAR, AC2, AC8 etc
        
        AC/AKAP signalling complexes can be very important in regulating signals and ensuring the correct localisation of them (Dessauer, 2009, Mol Pharm)
        
        AKAP79 -
        
        SOCE channels appear to bind to a protein which binds AKAP79 (which binds PP2A and, through another protein, AC)
    - - Recent evidence suggests that full dissociation may not always occur (when low [cAMP] are used and that the C-subunits retain some activity in this form (Smith, 2017, Science)
- - - - It is thought that this is energy efficient
    - - The first of these that was used was a PKA sensor (tethered to Z-bands with AKAPs) by Zaccolo & Pozan (Science, 2002)
        Issue with this paper is that because the sensor was localised to the microdomains it didn't actually show that cAMP increased here relative to the rest of the cell
      - CNGC sensors were used by WIlloughby (2006, EMBO J) and showed that PDE4D and gravin were key - it observed PDE4, PKA and an AKAP regulating near-membrane cAMP dynamics
- - - - This interactions between C domains is promoted by Gs and opposed by Gi
      - Relatively little diversity for TM and C domains but a reasonable amount at the NTD and CTD
    - - G proteins differentiate between AC subtypes in their action
        
        As stimulates all ACs but 1 & 8 to a lower extent
        Ai has no effect on AC2, 4 and 7
        Bg stimulates AC2, 4 and 7 and inhibits AC1, 3 and 8 (do not have to be from Gs or Gi)
      - Calcium stimulates AC isoforms found in the hippocampus (I & VIII) and inhibits those in the heart (V & VI) - this shows a greater degree of control in the AC isoforms that are found in sophisticated areas of the body
    - - Actin cytoskeleton & NHE1 (indirectly)
        SERCA, Dynein, Integrin, PP2A etc (directly)
- - - - The N and C lobes of CaM have different Ca2+ affinities
        CaM switching between the NTD and CTD of AC8 is associated with a change in affinities for Ca2+
        Both of these are potential mechanisms to detect the time derivative of [Ca2+]