1 Extracellular ATP can function as a glial trophic factor as well as a neu
ronal transmitter. In astrocytes, mitogenic signalling by ATP is mediated b
y metabotropic P-2Y receptors that are linked to the extracellular signal r
egulated protein kinase (Erk) cascade, but the types of P-2Y receptors expr
essed in astrocytes have not been defined and it is not known whether all P
-2Y receptor subtypes are coupled to Erk by identical or distinct signallin
g pathways.
2 We found that the P-2Y receptor agonists ATP, ADP: UTP and 2-methylthioAT
P (2MeSATP) activated Erk and its upstream activator MAP/Erk kinase (Mek).
cRaf-1, the first kinase in the Erk cascade, was activated by 2MeSATP, ADP
and UTP but, surprisingly, cRaf-1 was not stimulated by ATP. Furthermore, A
TP did not activate B-Raf, the major isoform of Raf in the brain, nor other
Mek activators such as Mek kinase 1 (MekK1) and MekK2/3.
3 Reverse transcriptase-polymerase chain reaction (RT-PCR) studies using pr
imer pairs for cloned rat P-2Y receptors revealed that rat cortical astrocy
tes express P-2Y1, a receptor subtype stimulated by ATP and ADP and their 2
MeS analogues, as well as P-2Y2 and P-2Y4, subtypes in rats for which ATP a
nd UTP are equipotent. Transcripts for P-2Y6, a pyrimidine-preferring recep
tor, were not detected.
4 ATP did not increase cyclic AMP levels, suggesting that P-2Y11, an ATP-pr
eferring receptor, is not expressed or is not linked to adenylyl cyclase in
rat cortical astrocytes.
5 These signal transduction and RT-PCR experiments reveal differences in th
e activation of cRaf-1 by P-2Y receptor agonists that are inconsistent with
properties of the P-2Y1, P-2Y2 and P-2Y4 receptors shown to be expressed i
n astrocytes, i.e. ATP not equal UTP; ATP not equal 2MeSATP, ADP. This sugg
ests that the properties of the native P-2Y receptors coupled to the Erk ca
scade differ from the recombinant P-2Y receptors or that astrocytes express
novel purine-preferring and pyrimidine-preferring receptors coupled to the
ERK cascade.