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potency in cellular and in vivo models. By placing an intrinsic ceiling on the
magnitude of PDE4 inhibition, such compounds may better maintain spatial and
temporal patterning of signaling in cAMP microdomains with consequent improved
tolerability. The third is a symmetric PDE4 conformer in which helices from
the C-terminal portion of the catalytic unit cap both active sites (
cis
-capping). We
propose that dual-gating of PDE4 activity may be further fine tuned by accessory
proteins that recognize open or closed conformers of PDE4 regulatory helices.
Keywords Phosphodiesterase 4 (PDE4)
PDE4 Gating
Accessory Proteins
Active Site
Asymmetric Conformer
1
Introduction
Four genes encode mammalian phosphodiesterase 4 (PDE4), all of which contain
multiple upstream exons that undergo complex splicing to generate multiple PDE4
isoforms that differ in N-terminal amino acid sequence (Swinnen et al.
1989
;
Bolger et al.
1993
; Monaco et al.
1994
; Houslay and Adams
2003
). Downstream
exons are common to all splice isoforms and encode the PDE4 catalytic domain,
which is highly conserved across the four genes (PDE4A-D). Three principle types
of PDE4 splice isoforms are found. Long isoforms of the enzyme contain an
N-terminal targeting sequence followed by an Upstream Conserved Regulatory
domain known as UCR1, a Linker Region domain (LR1), a regulatory domain
known as UCR2, another linker domain (LR2), the catalytic domain and, finally, an
extreme C-terminal sequence that is unique to each PDE4 (Conti et al.
1995
;
Houslay
2001
; Houslay et al.
2007
) UCR1 contains a site for protein kinase A
(PKA) phosphorylation (Sette and Conti
1996
; Hoffmann et al.
1998
). Short isoforms
of PDE4 lack UCR1, while supershort isoforms contain an N-terminally truncated
UCR2. The different splice isoforms of PDE4 are illustrated in Fig.
1
.
UCR1 and UCR2 were described by Bolger (Bolger et al.
1993
) as upstream
conserved regions in multiple amino acid sequence alignments of long transcripts
encoding PDE4A-D. UCR1 and UCR2 represent ancestral domains of PDE4 as
these are conserved in
Drosophila
(Davis et al.
1989
) and
Caenorhabditis elegans
(Charlie et al.
2006
) but not in
Dictyostelium
or yeast (Table
1
). Thus, the UCR1
and UCR2 regulatory domains may have developed when eukaryotes made the
evolutionary leap to multicellularity. Unlike mammals, which possess four PDE4
genes,
Drosophila
and
C. elegans
possess a single PDE4 gene (Day et al.
2005
;
Charlie et al.
2006
). Nonetheless, the PKA phosphorylation site on UCR1 is con-
served across organisms, as are important structural features of UCR2 as discussed
below (Table
1
). PKA phosphorylation of UCR1 increases PDE4 activity greater than
two- to threefold in long isoforms from all four PDE4 subfamilies (Sette and Conti
1996
; Hoffmann et al.
1998
), while deletion of UCR2 can increase PDE4 activity as
much as sixfold (Kovala et al.
1997
), implying that it can exert an inhibitory effect
on PDE4 catalytic activity. UCR1 and UCR2 have been proposed to act in concert as
