Lecture 18 - Key functional proteins in skeletal muscle Flashcards
What is a sarcomere/
Basic functional unit of striated muscle
repeated units
run between z-discs along the length of the myofibril
What is a costamere?
Rib-like structures on the surface of the sarcolemma
specialised sites of transmembrane complexes, occuring over the entire sarcolemma, where the transmission of force is concentrated
What is the role of the costamere?
Force transmission
What are the protein complexes that transmit the force at the costamere
The Dystrophin-glycoprotein complex (DCG)
The Integrin-Talin-Vinculin complex
Describe the DCG complex
The DGC is a mutimeric portein complex that acts to link the basal lamina to the actin cytoskeleton in skeletal muscle
Dystrophin binds to ___ within the cytosol
actin
Describe Dystrophin and its Function
Large rod-like protein enriched at skeletal muscle sarcomeres
Key intracellular component of the DGC
Contains 2 actin-binding domains
Fucntions:
Acts to link the Sarcolemma to the actin cytoskeleton by interactions with F-actin and Beta-dystroglycan
May also act to modulate signalling pathways through interactions with syntrophin and dystrobrevin
If Dystrophin is missing, the connection between what is missing
basal lamina and actin cytoskeleton
What are the different domains/regions of the dystrophin protein?
2 actin-binding domains:
One N-terminal composed of calponin homology domains
One centrally located composed of spectrin-like repeats
24 spectrin-like repeats (which create the rod-like structure)
4 hinge regions (for flexibility)
1 cysteine rich domains (site of protein-protein interactions)
- composed of a WW domain, two EF hand domains and a ZZ domain
1 C-terminal domain (site of interaction with syntropin and dystrobrevin
What happens when dystrophin is defective or absent?
dissipation of forces throughout the myofiber become disrupted
membranes become permeable
high rate of muscle fiber breakdown and regeneration
DMD/ Becker’s MD (milder)
In the absense of dystrophin, other DGC members fail to accumulate on the _______ indicating a role for dystrophin in ______ these proteins
In the absense of dystrophin, other DGC members fail to accumulate on the sarcolemma indicating a role for dystrophin in stabilising these proteins
True or False
The absense of dystrophin results in muscles containing varying muscle fiber size, inflammatory cell infiltrate, accumulating fibrous tissue and membrane permeability
True
True or False
Dystrophin also regulates multiple signalling pathways
True
via interactions with
Syntrophin/Dystrobrevin - coordinate the assembly of multiple signalling proteins to the DGC
nNOS - produces NO during muscle contraction to facilitate vasodilation
What composes the Dystroglycan complex?
Laminin, a-dystroglycan, b-dystroglycan
What is the function of the Dystroglycan complex?
Acts to link the ECM to the plsama membrane
Links the actin cytoskeleton of the muscle fibre by interaction with dystrophin
The laminin protein family is trimeric, meaning?
It is comprosed of 3 chains needed for its functionality
a chain- necessary for interaction with receptors
y chain
B chain
What is the function of the laminin protein
Forms a direct link between the ECM and dystroglycan to maintain skeletal muscle structure
What happens when laminin is defective?
Congenital Musclular Dystrophy (MCD1A)
genetic deficiency of laminin a2 chain
variable muscle fiber size
presence of immature fibers (central nucleation)
Increased connective tissue (fibrosis)
What is post-translational modification?
A step in protein biosynthesis
INcreases the ofunctional diversity of the proteome
Includes: phosphorylation, glycosylation, ubiquitination, nitrosylation, methylation, acetylation, lipidation, proteolysis
Regulates the activity, localisation and interaction of proteins with other cellular molecules
known to regulate the function of many of the key muscle proteins - only focusing on its role in dystroglycan
True or False
the extra-cellular component of the Dystroglycan complex (a-dystroglycan) contains muscle sites for glycosylation
True
requires glycosylation for proper function (the receptor for laminin in skeletal muscle)
True or False
Beta-dystroglycan is a transmembrane protein that binds a-dystroglycan and actin
False
Beta-dystroglycan is a transmembrane protein that binds a-dystroglycan and dystrophin
Function: provides a link between the subsarcolemmal cytoskeleton and ECM
Does Dystroglycan become dysfunctional?
No known association betwen mutations in dystroglycan and human disease
However, mutations in some glycosyltransferaess result in dystroglycan dysfunction as a result of a lack of glycosylation
Lack of glycosylation does not affect dystroglycan expression at the sarcolemma but is required for …
proper ligand binding (laminin)
Phosphorylation of beta-dystroglycn has recently been shown to …
target the b-dystrolgycan protein for destruction of not bound to dystrophin
Phosphorylation of beta-dystroglycn has recently been shown to …
target the b-dystroglycan protein for destruction of not bound to dystrophin
What composes the Sarcoglycan complex?
alpha-sarcoglycan
gamma-sarcoglycan
sigma-sarcoglycan
beta- sarcoglycan
sarcospan
What is the function of the sarcoglycan complex?
Not fully understood
appear to play both mechanical and non-mechanical roles that mediate interactions among the ACM, the sarcolemma and the cytoskeleton
forms a lateral associated with the dystroglycan complex
may function to stabilise the DGC at the sarcolemma
True or false
Absense of sarcoglycans leads to alterations in membrane permeability, ultimately resulting in cell death
true
genetic deletion of the sarcoglycan subunits of Limb-girdle muscular dystrophy reffered to as
“sarcoglycanopathies’
What is the function of the Vinculin-talin-Integrin complex?
Focal adhesion complex of skeletal msucle
Co-localises with the DGC at the costamere
composed of integrin, talin and viculin
force transducer
Integrin has to form a _____ to be functional
heterodimers
composed of a and b subunits
22 heterodimers have been identified
the a7B and Beta1D isoforms are the most common integrins
Which isoforms are the most common integrins found in adult skeletal muscle?
the aplha7B and Beta1D isoforms are the most common integrins
What is the function of integrin?
To mediate the processses of cell adhesion and migration
Regulate the intracellular organisation of the actin cytoskeleton
Important role in many signalling processes
Mutations in alpha7 integrin result in…
early onset CMD
What is Talin and how many isoforms exist?
Two isoforms (talin1 and talin2)
Talin 1 is central for integrin signalling
- interacts with the cytoplsmic domain of beta-1 integrin and with focal adhesion kinase and vinculin
Binds to F-actin to establish a link between Beta1-integrins and the cytoskeleton
True or False
Talin 2 is expressed at higher levels in skeletal muscle that Talin1
True
but both Talin 1 and 2 are essential for skeletal muscle development
What does loss of the talin 1 gene in skeletal muscle lead to…
A progressive myopathy caused by mechanical failure of MTJs
Mice lacking talin 2 are viable and fertile but develop…
a myopathy with centrally located nuclei that is assocaited with defects in the maintenance of MTJs
Loss of both Talin 1 and 2 causes…
severe defects in myoblast fusion and sarcomere assembly
True or False
Mutations in Talin have been identified
False
What is Vinculin?
membrane - cytoskeletal protein involved in linkage of integrin to the actin cytoskeleton
What is the Muscle specific splice variant of Vinculin?
metavinculin
Mutations in metavinculin are associated with idiopathic dilated cardiomyopathy (DCM) in the human population
Why do we need two costameric protein complexes?
Loss of expression of proteins in each complex causes disease
compromosed integrity of both complexes accelerates the develpoment of a severe muscular dystrophy
Loss of dystrophin reults in a compensatory increase of integrin in dystrophic mouse models and DMD patients
Therefore, the DGC and integrin complexes have both overlapping and compensatory functions
What is Desmin?
Where is it expressed and what is its function
intermediate filament protein - muscle specific type III
Expressed in skeletal, cardiac and smooth muscle
Function: links adjacent myofibres at the level of the Z disc and binds myofibrils to the sarcolemma at the level of the costameres
True or False
Desmin is required for normal muscle development
False
Desmin is essential for the maintenance of the myofibril integrity following ____
stress
Is Desminopathy common and what causes it?
Very rare (only 60 patients to date)
Mutation in desmin gene preventing the formation of desmin filaments. This results in misalignment of sarcomeres, disorganisation of muscle fibers, and muscle cell death
What is Plectin?
A large 500kDa protein located in nearly all mammalian cells
Structural component of striated muscle
Acts to link actin microfilaments, microtubules and intermediate filaments
Approx. 12 different isoforms of plectin - 1, 1b, 1d and 1f most prominent
Where are the following isoforms of plectin localised:
1 and If
1d
1b
1 and If - localised to costameres
1d - associated with the z-disc
1b - localised to mitochondria
Plectin mutations in Epidermolysis bullosa simplex (EBS) and with MD affect the interaction between…
The plasma membrane and the cytoskeleton in skin and muscle resulting in blistering of the skin and late-onset muscle weakness
What is titin?
Largest known protein in nature
Anchored at the Z and M lines of the sarcomere
Maintains rigid contact with the thick filament along the A band
Flexible along the I band
'’Molecular bungee cord’of skeletal msucle - functions to protect muscle fibres from damage due to overs stretching
What are the 4 main roles of Titin in skeletal muscle?
- Provide a structural framework through association with sarcomeric proteins
- Centreing the thick filament within the sarcomere during contraction
- Act as a molecular spring
- Signalling mediator
What is the effect of Tininopathy?
Altered structure and function of Titin
May disrupt Titin’s interactions with other sarcomeric proteins
Disrupts normal muscle contraction, resulting in msucle weakness and wasting over time
Not known why disease is confined to lower leg muscles
