The dystrophin-associated protein complex
Jeffrey Ehmsen,
Ellen Poon and
Kay Davies*
Functional Genetics Unit, Department of Human Anatomy and Genetics,
University of Oxford, South Parks Road, Oxford, UK
*
Author for correspondence (e-mail:
kay.davies{at}anat.ox.ac.uk
)
 |
Introduction
|
---|
The lethal muscle-wasting disorder, Duchenne muscular dystrophy, is caused
by mutations or deletions in the dystrophin gene. In skeletal and cardiac
muscle, dystrophin associates with various proteins to form the
dystrophin-associated protein complex (DAPC). The DAPC is thought to play a
structural role in linking the actin cytoskeleton to the extracellular matrix,
stabilizing the sarcolemma during repeated cycles of contraction and
relaxation, and transmitting force generated in the muscle sarcomeres to the
extracellular matrix (Petrof et al.,
1993
). There is also evidence that the DAPC is involved in cell
signalling via its interactions with calmodulin, Grb2 and nNOS
(Rando, 2001
). Various members
of the DAPC, such as the sarcoglycans, have already been implicated in a
number of muscle diseases, illustrating the vital role this complex plays in
the maintenance of muscle integrity. This brief review offers a glimpse of the
major known proteins that constitute the DAPC and the defects caused by their
absence.
 |
Dystrophin
|
---|
The muscle isoform of dystrophin is a 427 kDa protein consisting of an
N-terminal actin-binding domain, a central rod-like domain comprising 24
spectrin-like triple helical coiled coils, and a cysteine-rich C-terminus that
allows assembly of the DAPC. Dystrophin stretches laterally along F-actin
filaments, binding primarily via three sites within the N-terminal region
(Norwood et al., 2000
) and
electrostatically through a cluster of basic repeats (11-17) within the rod
domain (Amann et al., 1998
).
Although remarkable evolutionary conservation exists, with even the C.
elegans homologue possessing the same number of spectrin repeats, much of
the rod domain appears dispensable and a dystrophin molecule comprising at
least eight integral repeats remains relatively functional
(Harper et al., 2002
).
Dystrophin deficiency results in loss of the associated protein complex and
severe muscular dystrophy, underscoring the central role that dystrophin plays
in assembling and maintaining the link between cytoskeletal actin and the
extracellular matrix.
 |
Dystroglycans
|
---|
The widely expressed
and ß dystroglycans make up the core of
the DAPC, establishing the transmembrane link between laminin-2 and
dystrophin. Both proteins are produced from a single post-translationally
modified polypeptide, and are heavily glycosylated prior to being sorted to
their respective extracellular and transmembrane locations. These
glycosylation patterns are developmentally regulated and largely correlate
with the diversity of binding partners in different tissues
(Winder, 2001
). Total deletion
of dystroglycan in mouse is embryonic lethal owing to disruption of formation
of the extra-embryonic basement membrane, and the void of naturally occurring
mutations or other dystroglycan-associated diseases suggests that its function
is indispensable for survival (Williamson
et al., 1997
). Disrupting the dystroglycan-dystrophin link causes
a Duchenne-like phenotype, whereas disruption of the laminin-dystroglycan link
causes congenital muscular dystrophy. Defects in post-translational
modification of dystroglyan may also be pathogenic, as a nonsense mutation in
the glycosyltransferase, Large, is the primary defect in the myodystrophy
mouse (Grewal et al.,
2001
).
 |
Sarcoglycans
|
---|
Five transmembrane proteins, all expressed primarily in skeletal muscle,
constitute the sarcoglycan family:
(50 kDa, also called adhalin),
ß (43 kDa),
(35 kDa),
(35 kDa) and
(50 kDa). The
ß,
and
sarcoglycans co-purify, with ß and
forming an especially tight link, whereas
sarcoglycan may be spatially
separated. Dystrophin and
sarcoglycan can interact directly, and
sarcoglycan appears to be coordinated to the dystroglycan complex
(Chan et al., 1998
). Mutations
abolishing the expression of any one of the sarcoglycans cause loss of the
others from the sarcolemma; the four recessive limb girdle muscular
dystrophies 2D, 2E, 2C and 2F are caused by absence of the
, ß,
or
sarcoglycans, respectively
(Bushby, 1999
).
 |
Sarcospan
|
---|
Sarcospan is a 25 kDa membrane protein with four transmembrane domains and
intracellular N- and C-termini, a unique feature for transmembrane proteins of
the DAPC (Crosbie et al.,
1997
). Expression is seen predominantly in skeletal and cardiac
muscle, but shorter isoforms exist in other tissues. No human disease is
currently known to be associated with sarcospan deficiency, and sarcospan-null
mice maintain expression of all sarcoglycans at the sarcolemma and do not
develop muscular dystrophy (Lebakken et
al., 2000
).
 |
-Dystrobrevins
|
---|
Alternative splicing produces five
-dystrobrevin isoforms, differing
as C-terminal truncations. Three of these are found in muscle, but only
-dystrobrevin-2 is abundantly expressed at the sarcolemma. This isoform
contains two tandem
-helical syntrophin-binding sites, which may be
alternatively spliced to modulate the stoichiometry of syntrophin association
with the DAPC (Newey et al.,
2000
). As dystrobrevin contains several tyrosine kinase consensus
sites, protein-protein interactions may also be regulated by tyrosine
phosphorylation. Dystrobrevin associates with dystrophin via coiled-coil
interactions, but an independent link with the sarcoglycan/sarcospan complex
might also exist as dystrobrevin and syntrophin can bind to the DAPC in the
absence of the C-terminal region of dystrophin
(Crawford et al., 2000
).
Intriguingly, dystrobrevin-knockout mice present with a DMD-like phenotype
while retaining the DAPC and sarcolemmal integrity. Loss of dystrobrevin is
expected to disrupt the syncoilin-mediated link between the DAPC and desmin
intermediate filaments (see below), perhaps contributing to dystrophy.
Alternatively, the coordinate reduction of sarcolemmal nNOS that is observed
in these knockouts suggests that signalling defects may be at fault
(Grady et al., 1999
).
 |
Syntrophins
|
---|
All three of the 58 kDa syntrophin isoforms are found at the neuromuscular
junction in skeletal muscle, but only the
1 and ß1 isoforms are
present along the sarcolemma. Each contains two pleckstrin homology (PH)
domains, which are modules of
100 amino acids found in many signalling
proteins. Within each first syntrophin PH domain is a PDZ domain capable of
facilitating homo- and hetero-dimerization with other PDZ-containing proteins.
Indeed, through these types of interactions, the syntrophins may function as
modular adaptors in recruiting signalling proteins to the sarcolemma and DAPC:
binding interactions exist with skeletal muscle sodium channels, nNOS,
serine/threonine kinases, MAST205 and stress-activated protein kinase-3
(Rando, 2001
). The highly
conserved C-terminal 57 amino acids [constituting the syntrophin-unique (SU)
domain] probably contain binding sites for dystrophin family members. No human
disease has been correlated with syntrophin mutations. Mice lacking
1
syntrophin (the predominant muscle isoform) display no overt phenotype, but
nNOS is absent from the sarcolemma and the postsynaptic membrane is grossly
abnormal (Kameya et al., 1999
;
Adams et al., 2000
).
 |
Syncoilin
|
---|
Syncoilin was first identified via its interaction with
-dystrobrevin in muscle (Newey et
al., 2001
). Sequence analysis revealed the presence of a unique
N-terminus domain and a coiled-coil domain that is typical of those found in
intermediate filament proteins. Syncoilin is highly expressed in skeletal,
cardiac and smooth muscle at the sarcolemma, Z-lines and neuromuscular
junction. Through its interaction with desmin, syncoilin is thought to provide
a link between the DAPC at the sarcolemma and the intermediate filament
protein network (Poon et al.,
2002
). Its upregulation in a range of muscular dystrophies may be
a compensatory mechanism against muscle damage.
 |
nNOS
|
---|
The production of nitric oxide (NO) by nNOS is important for increasing
local blood flow to match the increased metabolic load of contracting muscles,
such as during exercise. The presence of nNOS at the sarcolemma is mediated
through PDZ domain interactions with syntrophin, and it is lost in a number of
muscular dystrophies including DMD. Indeed, patients with DMD show abnormal
blood vessel constriction presumably due to lack of nNOS at the sarcolemma,
and it is the only other protein known to correlate so closely with the
fiber-type-specific onset of dystrophy. This observation is in accordance with
the supposed role of the DAPC in signalling and the contribution of ischemic
stress to muscle degeneration. However, abolishing nNOS expression alone in
mice does not cause overt dystrophy
(Crosbie et al., 1998
;
Chao et al., 1998
).
 |
Laminin-2
|
---|
Laminin-2 is composed of
2, ß1 and
1 chains and binds to
-dystroglycan and the
7ß1 integrin complex. Laminins are
thought to form the structural part of the basement membranes along with
collagen IV, nidogen and perlecan. Mutations of the laminin
2 gene
cause severe congenital muscular dystrophy but do not appear to cause damage
to the sarcolemma (Patton,
2000
).
 |
Caveolin-3
|
---|
Caveolae are vesicular invaginations of the plasma membrane and are found
in most cell types. The primary scaffolding protein of skeletal muscle
caveolae is caveolin-3, and although traditionally considered distinct from
the DAPC, a number of studies imply an important relationship. Caveolin-3
interacts directly with the C-terminus of ß-dystroglycan, co-fractionates
with dystrophin and
-sarcoglycan, and shows elevated expression in
Duchenne muscular dystrophy. Transgenic mice overexpressing caveolin-3 present
with DMD-like pathology, suggesting that competitive downregulation of
dystrophin at the sarcolemma may occur. Mutations in caveolin-3 are associated
with autosomal dominant limb-girdle muscular dystrophy (LGMD-1C), hyperCKemia
and rippling muscle disease (Galbiati et
al., 2001
).
 |
Sodium channels
|
---|
Voltage-gated sodium channels bind to syntrophin PDZ domains, an
interaction that may influence ion conduction properties or association with
other channels (Gee et al.,
1998
). However, even in the absence of syntrophin, normal sodium
channel distribution is seen along the sarcolemma, suggesting that this
interaction is not essential for channel localization
(Adams et al., 2001
;
Ribaux et al., 2001
).
 |
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