Sarcoplasmic Reticulum Function in Smooth Muscle - Physiological ...

126 SUSAN WRAY AND THEODOR BURDYGA
SERCA is in direct contact with several SR lipids but
in a cholesterol-poor environment, and this is essential for
its function, as a highly fluid lipid membrane is needed for
the large conformational changes required for SERCA
function. Maximal activity of SERCA is affected by the
thickness and head group composition of the SR phospholipids;
phosphocholine containing 18C fatty acids
yielded maximal ATPase activity, while longer or shorter
fatty acids lowered its activity (261, 499). Dietary fatty
acid changes have been shown to change the composition
of the SR membrane (674), and its cholesterol content
may increase with age (373) and contribute to the decreased
SERCA function found in older animals (198) as
the SR membrane becomes increasingly rigid.
Variation in the lipid composition of the SR from
different skeletal muscles has been described and related
to differences in SERCA activity (73), but nothing is
known about how composition may differ between
smooth muscles. In a study of macrophage SERCA, Li
et al. (397) showed that enrichment with cholesterol inhibited
SERCA2b activity. This loss of fluidity may explain
SERCA decreased activity, as the large conformational
changes required for Ca transport are limited. In terms of
macrophage cell function, the authors noted that dysfunction
of SERCA could lead to protein unfolding in the ER
and contribute to atherosclerotic lesions in foam cells
(cholesterol-loaded macrophages).
C. Phospholamban
1. Introduction
Phospholamban is a 52-amino acid homopentameric
protein, which, depending on its phosphorylation state,
reversibly binds to SERCA and affects its activity (431,
639, 680). The phosphorylation of phospholamban occurs
by both cAMP-dependent protein kinase (PKA) at Ser-16
and Ca/calmodulin-dependent protein kinase II (CaM kinase
II) at Thr-17, and the name phospholamban was
given to mean “phosphate receptor” (679; see also Katz,
1998 and associated articles in the same volume). Phospholamban
inhibits SERCA by lowering its apparent affinity
for Ca through direct protein-protein interactions,
probably of a single phospholamban molecule associated
with two SERCA molecules, and thus phospholamban
may restrict the large domain movements needed for
SERCA activity (21, 790). Upon phosphorylation there is a
large change in phospholamban charge, and this greatly
decreases its inhibitory effect on SERCA. It is clear in
cardiac muscle that phosphorylation of phospholamban is
a highly important part of the mechanism whereby -agonists
via cAMP increase cardiac contractility; by relieving
the inhibition of phospholamban on SERCA, more Ca can
enter the SR and contribute to an increased Ca release on
the next heart beat, and relaxation occurs at a faster rate.
Physiol Rev VOL 90 JANUARY 2010 www.prv.org
In heart, chronic inhibition of SERCA, as can occur with
some human phospholamban null mutations (447, 621), is
associated with dilated cardiomyopathy, as contractility
is diminished.
2. Distribution in smooth muscle
Phospholamban is expressed in smooth muscle, although
at lower levels than occur in cardiac myocytes
(378, 643). Raeymaekers and Jones (584) first reported the
presence of phospholamban in smooth muscle. They
found it in pig stomach and rabbit and dog aorta but not
pig aorta. This group then used pig stomach to cDNA
clone and sequence phospholamban (725) and concluded
it was 100% sequence identical to cardiac phospholamban
and was the product of the same gene. Thus any differences
in effects of phospholamban between cardiac and
smooth muscle cannot be explained by isoform diversity.
An immunogold EM study of phospholamban showed a
patchy SR distribution of it in a variety of smooth muscles
(ileum, iliac artery, and aorta), again suggesting that its
density was lower than that of cardiac SR (172). Interestingly,
phospholamban labeling of the outer nuclear envelope
was also seen, although this observation does not
appear to have been further investigated. These early
studies concluded that whereas SERCA expression was
approximately comparable between smooth muscles,
phospholamban mRNA levels varied 12-fold, with aorta
expressing low levels compared with ileum and stomach
(162).
Given the importance of PKA and CaM kinase II to
smooth muscle force modulation, phosphorylation of
phospholamban would be predicted to be one of their
targets. It has also been suggested that in smooth muscle
cGMP is an important mediator of phospholamban phosphorylation
(129, 583) acting at Ser-16. A recent report
(363) has found that in tracheal smooth muscle phospholamban
is associated with a PKA signaling complex, in
addition to its expected association with SERCA. If this
were also to be the case in other smooth muscles, then
this may lead to facilitated IP3-induced Ca release, rather
than effects via SERCA Ca pumping, i.e., phospholamban
may have multiple functions and effects in smooth muscles.
As the target for several second messenger-dependent
kinases, phospholamban may also play a role in Ca
signaling events associated with vascular smooth muscle
migration and hyperplasia, or influence endothelial Ca
signals, but a discussion of this is beyond the scope of this
review (117, 563, 672).
3. Functional effects
The earliest studies performed in smooth muscle to
investigate phospholamban’s role involved vesicle studies
of Ca uptake (582, 745). Sarcevic et al. (615), using cultured
aortic smooth muscle cells, suggested that the