Lecture 19 Flashcards
What are sarcomeres?
sarcomeres are the functional unit of muscles and are found on all muscle fibres. They are the units between z lines.
What are costameres?
Costamere are specialized sites of transmembrane complexes and are the location where the transmission of force is concentrated.
What are the complexes that link the extracellular matrix to the actin cytoskeleton?
dystrophin-glycoprotein complex (dystrophin associated protein complex).
Integrin-Talin-Vinculin complex
What does DGC do?
DGC acts to link the basal lamina to the actin skeleton.
What is the function of laminin?
laminin resides within the extracellular matrix and binds to alpha dystroglycan.
What is the sequence of structures that bind from the basal lamina to the actin within the dystrophin-glycoprotein complex (DGC)?
Basal lamina bound by laminin.
Laminin connected to alpha dystroglycan which is found in the sarcolemma.
alpha dystroglycan binds to beta dystroglycan which attaches to dystrophin
dystrophin attaches to actin.
What causes duchenne’s muscular dystrophy?
Dystrophin mutation causes duchenne’s muscular dystrophy
What is dystrophin?
Dystrophin is a large rod-like protein.
It is a key component of the DGC complex.
It contains 2 actin-binding domains.
What is the function of dystrophin?
Links the actin cytoskeleton to the sarcolemma.
What are the functional domains of the dystrophin protein?
2 actin binding domains.
24 Spectrin-like repeats which create rod like appreance of dystrophin.
4 hinge regions which bend the dystrophin molecules.
1 Cysteine rich domain which contains a WW domain, 2 EF domains, and a ZZ domain. These domains are required for interaction with dystroglycan.
1 C-terminal domain required for binding to syntrophin and dystrobrevin.
What happens when dystrophin is defective or absent?
Dissipation of forces throughout the myofiber becomes disrupted and membrane become permeable.
Muscle fiber is constantly broken down and regenerated.
What is the function of dystrophin in relation to syntrophin and dystrobrevin?
dystrophin directs syntrophin and dystrobrevin to the sarcolemma.
What happens to muscles in the absence of dystrophin?
Absence of dystrophin results in muscles containing varying number of muscle fiber size, inflammatory cell infiltrate and accumulating fibrous tissue and membrane permeability.
What are some signalling roles of dystrophin?
dystrophin coordinates the assembly of multiple signalling proteins to the DGC (Grb2, calmdulin, SAPK, and nNOS)
nNOS releases NO during muscle contraction to facilitate vasodilation.
What are the components of the dystroglycan complex?
Laminin
alpha dystroglycan
beta dystroglycan
What is the function of the dystroglycan complex?
Links extracellular matrix to the plasma membrane.
Links to actin cytoskeleton by interacting with dystrophin
How many chains does laminin contain?
3 chains. An alpha, beta, and gamma chain.
Which chain of laminin is necessary for interaction with receptors?
The alpha chain
What results from absence of laminin?
congenital muscular dystrophy (MCD1A)
What are the consequences of congenital muscular dystrophy (MCD1A)?
genetic deficiency of laminin alpha 2 chain
Variable muscle fiber size
Presence of immature fibers
Increased connective tissue fibrosis
What is alpha dystroglycan and what is its function?
Alpha dystroglycan is an extracellular protein that acts as a receptor for laminin.
What does alpha dystroglycan require for proper function?
glycosylation
What is beta dystroglycan and what is its function?
beta dystroglycan is transmembrane and links the subsarcolemmal cytoskeleton to the extracellular matrix
What are some conditions associated with dystroglycan mutation?
No known association between dystroglycan and human disease.
What happens when glycosylation of alpha dystroglycan is prevented?
Lack of glycosylation does not affect expression of dystroglycan but instead results in an inability to bind laminin.
What is the function of phosphorylation in dystroglycan?
If not bond to alpha dystroglycan, beta dystroglycan gets phosphorylated and destroyed.
What is the sarcoglycan complex composed of?
alpha gamma delta, sarcospan and beta sarcoglycan; all are transmembrane.
What is the function of the sarcoglycan complex?
Not fully understood; appears to mediate interactions among the extracellular matrix, the sarcolemma, and the cytoskeleton.
Forms a lateral association with the dystroglycan complex and may function to stabilize the dystroglycan complex.
What does the absence of sarcoglycan complex cause?
A genetic deletion of sarcoglycan subunits which results in a subset of limb-girdle muscular dystrophy.
What happens if there is loss of one sarcoglycan subunit?
sarcoglycan complex is lost and this leads to alterations in membrane permeability which results in cell death.
What is the position of Vinculin-talin-integrin in a muscle fiber?
Vinculin-talin-integrin complex co-localises with DGC at the costamere.
What are integrins?
Integrins are heterodimers composed of alpha and beta subunits. They interact with laminin and talin and is transmembrane.
What is the general function of integrins?
To mediate the process of cell adhesion and migration
Regulates the intracellular organization of the actin cytoskeleton.
Important role in many signalling processes.
What are the most common isoforms of alpha and beta integrin?
alpha7B
beta 1D
What is the result of mutations in alpha7 integrin?
Mutations in alpha7 integrin result in an early onset CMD
What is the function of Talin 1?
Talin 1 interacts with actin and integrin beta1, Focal Adhesion Kinase (FAK), and vinculin
What is the function of Talin 2?
Talin 2 is essential for skeletal muscle development and is essential for skeletal muscle development.
What is the pathology of talin mutation?
Mice develop a progressive myopathy after mutation of talin1.
Mice develop a myopathy with centrally located nuclei when talin 2 is mutated.
Loss of both talin 1 and 2 results in severe defects in myoblast fusion and sarcomere assembly.
No mutations reported in humans to date.
What are the domains of vinculin?
4 Vh domains; Vh 1 associates with talin and actinin
1 linker domain
Terminal (Vt) domain
What part of talin binds to vinculin?
At the C terminal site of integrin and talin is an association with vinculin
What is the function of vinculin?
Vinculin is involved in linkage of integrin to actin cytoskeleton.
What is the muscle specific splice variant of vinculin?
Metavinculin
What condition results from mutations in metavinculin?
kliopathic dilated cardiomyopathy (DCM)
Why are there 2 costameric protein complexes?
Loss of expression of proteins in each complex causes various myopathies and dystrophies.
Problems with both complexes accelerates development of a severe muscular dystrophy. (more redundant system)
Loss of dystrophin can be made up for by upregulating more vinculin-talin-integrin complexes.
Signalling functions.
What is the function of desmin?
type 3 muscle filament protein expressed in all muscles. It links adjacent myofibers at the Z discs and binds myofibrils to the sarcolemma at the level of the costameres.
What is the function of desmin?
Desmin is required to maintain myofibril integrity.
Desmin is not required for normal muscle development
What Is the result of a mutation in the desmin protein?
Desminopathy is very rare and is a myofibrillar myopathy.
This mutations prevents desmin filament formation and misalignment of sarcomeres, muscle fibers, and in turn muscle cell death.
What is plectin?
A large protein found in almost every mammalian cell and is a structural component of striated muscle.
What does plectin do?
Acts to link actin microfilaments, microtubules, and intermediate filaments.
localizes desmin and many other proteins.
How many isoforms are there of plectin? What are some examples?
12 isoforms of plectin;
plectin 1 and 1f are localised to costameres
plectin 1d associated with the z disk
plectin 1b localized to mitochondria
What condition results from plectin mutation?
Epidermolysis bullosa simplex
Mutations affect interaction between plasma membrane and cytoskeleton in skin and muscle resulting in blistering of the skin and late-onset muscle weakness.
This condition affects the gut and skin most.
What is titin?
TItin is the largest protein in nature.
What is the function of titin?
Centers actin filaments within sarcomere during contraction.
Acts as a molecular spring
Mediates signals
Provides structural framework by associating with sarcomeric proteins.
What is tibial muscular dystrophy?
to be edited
Tibial muscular dystrophy (Limb Girdle muscular dystrophy)
Found in finnish population
Alters the structure and functiuon of titin
May disrupt titin’s interactions
How does titin maintain its position?
Anchored at the Z and M lines of the sarcomere. It maintains rigid contact with the thick filaments along the A band
Where is titin most flexilble?
Flexible along the I band
