CAMP and protein kinase A

The receptor-mediated activation of adenylyl cyclase (AC) to generate cyclic AMP (cAMP) represents the classical example of a soluble intracellular mediator that regulates a great number of events in nearly all mammalian cells studied to date. The binding of cAMP to the regulatory subunit of protein kinase A (PKA) stimulates the enzyme to phosphorylate a diverse group of substrates involved in the initiation, maintenance and inhibition of the

Table 6.1 Common examples of currents modulated by neurotransmitters

Transmitter

Target receptors

Currents modified

Type of regulation

Selected references

Acetylcholine

muscarinic m2, m4

IGIRK

direct f

(1)

1 K,ATP

direct f

(2)

muscarinic m1, m3

IM

ndirect |

(3,4)

IK,Ca

ndirect |

(5)

ICa,N

ndirect |

(6)

Dopamine

D2,D3

IGIRK

direct f

(7,8)

D2,D3

ICa,N

direct |

(9)

D1

INa

ndirect |

(10)

GABA

GABAb

IGIRK

direct f

(1 .1 .)

Norepinephrine

a1 -AR

IK,leak

ndirect |

(1 2 )

P-AR

IK,Ca

ndirect |

(13)

ICa,L

indirect f

(14)

Serotonin 5-HTs 'girk

indirect j (13)

indirect j (12)

indirect f (18)

indirect j (13)

indirect j (12)

indirect f (18)

K,leak

1.

Logothetis DE etal. (1987) . Nature 325 (6102), 321-6 .

10

Cantrell AAetal. (1997). Neurosci 17(19), 7330-8.

2

Ito H etal. (1992). J Gen Physiol 99(6), 961-83.

11.

Sodickson DL and Bean BP (1996). J Neurosci 16(20),

6374-85.

3

Brown DA et al. (1989). J Physiol 413, 469-88.

12

Talley EM etal. (2000). Neuron 25(2), 399-410.

4.

Marrion NV(1993). Neuron 11(1), 77-84.

13.

Pedarzani Pand Storm JF (1993). Neuron 11(6),

1023-35.

5

Madison DV etal. (1987). J Neurosci 7(3), 733-41.

14.

OsterriederWetal. (1982). Nature 298(5874),

576-8.

6

Beech DJ etal. (1992). Neuron 8(1), 97-106.

15.

AndradeRetal. (1986). Science 234(4781), 1261-5.

7

Pillai G etal. (1998). Neuropharmacology 37(8),

16.

Andrade R and Nicoll RA (1987). J Physiol 394,

983-7.

99-124.

8

Kuzhikandathil EV and Oxford GS (2000). J Gen Physiol

17.

Bayliss, DA et al. (1997). J Neurophysiol 77(3),

115(6), 697-706.

1362-74.

9

Marchetti C etal. (1986). Pflugers Arch 406(2),

18.

Cardenas CG etal. (1999). J Physiol 518(2): 507-23.

104-11.

104-11.

Serotonin 5-HTs 'girk

K,leak

signalling response The effects of cAMP augmentation on channel function have been extensively studied in the heart Sympathetic nerve stimulation increases heart rate and contractive force by releasing norepinephrine onto ^-adrenergic receptors expressed within cardiac tissue (Hartzell 1988; Trautwein and Hescheler 1990, Chapter 12). The activation of these receptors leads to a significant potentiation of the high-threshold activated (L-type) Ca2+ current which causes heart cells to depolarize (Bean 1985). Pharmacological agents that directly activate AC activity such as forskolin are able to mimic the effects of receptor stimulation, and the current increase is also observed upon intracellular perfusion with cAMP in the absence of G protein activation (Tsien etal. 1972; Morad etal. 1981; Fischmeister and Hartzell 1986). In addition, direct injection of the purified catalytic subunit of PKAinto isolated cardiomyocytes produces similar increases in L-type currents (Osterrieder et al. 1982). The enhancement of Ica results from the phosphorylation of L-type Ca2+ channels by PKA on specific sites contained within the a1 and p channel subunits (see Catterall 2000). PKA phosphorylation appears to cause increases in both channel open-probability as well as single-channel mean open time in response to physiological depolarizations (Reuter et al. 1982; Cachelin et al. 1983; Yue et al. 1990).

PKA also phosphorylates the rat brain voltage-gated Na+ channel on a number of residues in the a subunit to cause a suppression of the net current in response to membrane depolarization (Costa etal. 1982; Cantrell etal. 1999; Smith and Goldin 1997). The decrease in sodium current can be reproduced in Xenopus oocytes expressing the cloned channel and injected with cAMP or the regulatory subunit of PKA, thereby obviating the need to activate G proteins (Gershon etal. 1992). In cultures of dissociated hippocampal neurons, Na+ current inhibition caused by receptor activation is mimicked by the isolated addition of specific PKA activators to the cells (Cantrell et al. 1999). Interestingly, PKA-dependent reductions in peak Na+ currents can depend upon convergent activation of PKC and consequent phosphorylation of different sites along the cytoplasmic linkers found between domains I and IV of the sodium channel. Even more fascinating is the finding that such convergence is observed only at relatively hyperpolarized membrane potentials. At more depolarized potentials (-70 mV and above), significant PKA modulation of the channel can occur even in the absence of PKC activation (Cantrell etal. 1999). Such complex regulation enables the sodium channel to act as a coincidence detector which integrates changes in membrane potential with G protein-coupled alterations in PKA and PKC activity (Cantrell and Catterall 2001).

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