Biology Reference
In-Depth Information
9 Cyclic Nucleotide Pools in Endothelial Barrier Function
Studies suggest that there may be multiple pools of cAMP and cGMP in endothelial
cells regulated by PDEs (Creighton et al.
2008
; Sayner and Stevens
2006
; Sayner
et al.
2006
). This is not unexpected since it is becoming increasingly clear that
cellular signaling is a highly compartmentalized phenomenon in which there are
many microdomains that are differentially regulated by various proteins and stimuli
(Houslay et al.
2007
; Houslay
2010
).
Even with our recent realization that different amounts of PDE2 and PDE3 may
account for much of the variability in responses reported for cAMP and cGMP on
barrier function, substantial controversy still exists. For example, in contrast to our
results (Surapisitchat et al.
2007
) and that of others (Adamson et al.
1998
; Langeler
and van Hinsbergh
1991
; van Hinsbergh
1997
) indicating that cAMP produced
by transmembrane adenylyl cyclases decreases permeability, Sayner et al.'s (
2004
,
2006
) work suggests that increases in a cytosolic pool of cAMP increase perme-
ability. These authors go on to propose a model in which cAMP made by trans-
membrane adenylyl cyclases is maintained in a microdomain near the plasma
membrane by PDE4. The idea is that if PDE4 is inhibited, cAMP can diffuse into
the cytosolic compartment (Sayner et al.
2006
). Recently, it has been proposed that
PDE4D4 is the PDE4 isoform responsible for maintaining cAMP at the membrane
(Creighton et al.
2008
). This contrast, to previous results by several other groups
and our own data demonstrating a barrier-enhancing effect by forskolin and PDE4
inhibition (Suttorp et al.
1996a
). Our data indicate that forskolin (10
m
M) can
almost completely abolish thrombin-induced permeability and that rolipram can
potentiate the barrier-enhancing effects of lower forskolin concentrations (Fig.
4
).
This discrepancy might, however, be explained by differences in the types of
endothelial cells studied, microvascular versus macrovascular (Creighton et al.
2008
). In particular, microvascular cells are almost always more greatly expanded
and studied at a later passage number. Nevertheless, these findings strongly suggest
that there are different “pools” of cAMP regulating different aspects of barrier
function.
As with cAMP, pools of cGMP regulating various distinct cellular functions
have been described. This was expected with the existence of soluble and particu-
late guanylyl cyclases. In our studies, we found that cGMP generated from either
soluble or particulate guanylyl cyclases had the same biphasic effect on permeabil-
ity. It is possible though that there are still distinct pools of cGMP regulating barrier
function via both PDE2 and PDE3. One possibility is that PDE2 and PDE3 are
regulating the same pool of cAMP that controls permeability and two pools of
cGMP regulate these PDEs, but they cannot be detected by measuring changes in
permeability. For example, cGMP made by soluble guanylyl cyclases may be more
effective than that of cGMP made from particulate cyclases, but this difference, a
shift in the dose-response curves for the agonists, cannot be detected by measuring
permeability. Complicating this further is the time variable in which levels of either
cAMP or cGMP may change within a certain pool and with time. Again, many
