Biology Reference
In-Depth Information
upon the presence of regulatory UCR domains (UCR1 and UCR2), with long forms
having both UCRs, short forms having UCR2, super-short forms having a truncated
UCR2 and the catalytically inactive, dead-short isoforms having neither (Bolger
et al.
2006
; Conti and Beavo
2007
; Houslay
2001
,
2010
).
While the structure of the catalytic domains from all four PDE4 subfamilies is
known (Ke and Wang
2007a
), until very recently nothing has been known about the
structure of the UCR regulatory domains or interactions of UCRs with the catalytic
domain. A recent report documents the X-ray crystal structure of UCR2, a helical
structure that can bind adjacent to and over the PDE4 catalytic pocket, thereby
gating cN substrate access (Burgin et al.
2010
). This gives a structural basis to the
previous biochemical evidence and proposal that UCR2 provides an autoinhibitory
domain (Lim et al.
1999
) and for previous observations that PDE4 enzymes can
adopt distinct conformational states that vary in sensitivity to certain inhibitors
(Houslay
2001
; Houslay and Adams
2003
; Houslay et al.
2005
). Burgin et al. used
information derived from a collection of X-ray crystal structures of PDE4 isoen-
zymes, molecular modeling, site-directed mutagenesis, and systematic synthetic
chemistry to produce the novel small molecule inhibitors of PDE4D. These are
bifunctional compounds that form contacts with both the catalytic site and the
UCR2, thereby locking the enzyme in the inactive state; the double set of contacts
increases points of interaction with PDE4, thereby enhancing inhibitory potency
(Burgin et al.
2010
; Houslay and Adams
2010
).
The diverse interactions of various inhibitors with PDE4 are exemplified by
RS25344 and roflumilast. RS25344 interacts with the gating sequence, which
stabilizes the UCR2-capped state (Burgin et al.
2010
), whereas roflumilast, which
has recently been approved in Europe for treatment of chronic obstructive pulmo-
nary disease (COPD) (see Tenor et al.
2011
), occupies the uncapped catalytic pocket
and interacts minimally with residues in the UCR. These discoveries provide the
beginnings of insight into the complex inhibition kinetics of compounds, such as
rolipram, that may have different affinity for the catalytic pocket in the UCR2-
uncapped and UCR2-capped states. This discovery also offers a molecular explana-
tion (Burgin et al.
2010
) for the reported observations that PKA phosphorylation
alters the inhibitor sensitivity (Hoffmann et al.
1998
; Sette and Conti
1996
a), for
example, by stabilizing an uncapped, activated state. Some of the new compounds
produce only partial inhibition (80-90%) of catalytic activity, which may contribute
to their actions in the biological setting. The proximity and arrangement of regu-
latory domains of other PDEs with respect to their catalytic sites as well as their
mechanisms of activation/autoinhibition will likely dictate whether this innovative
approach to drug design is useful for other PDEs or restricted to the PDE4 family.
The recent structural insights for UCR2 interaction with the PDE4 catalytic unit
(Burgin et al.
2010
) will undoubtedly stimulate new lines of research and drug
discovery relating to PDE4. However, it is also likely to provide a stimulus to
understanding of the PDE superfamily as a whole, where paired regulatory regions,
for example, GAFs, located amino-terminal to the catalytic domain are common.
Solution of the X-ray crystallographic structure of the PDE2 holoenzyme has
suggested a different mechanism of regulation of catalytic activity (Pandit et al.
