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Muscle & Exercise Physiology > Lecture 19 > Flashcards

Lecture 19 Flashcards

1
Q

What are sarcomeres?

A

sarcomeres are the functional unit of muscles and are found on all muscle fibres. They are the units between z lines.

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2
Q

What are costameres?

A

Costamere are specialized sites of transmembrane complexes and are the location where the transmission of force is concentrated.

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3
Q

What are the complexes that link the extracellular matrix to the actin cytoskeleton?

A

dystrophin-glycoprotein complex (dystrophin associated protein complex).

Integrin-Talin-Vinculin complex

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4
Q

What does DGC do?

A

DGC acts to link the basal lamina to the actin skeleton.

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5
Q

What is the function of laminin?

A

laminin resides within the extracellular matrix and binds to alpha dystroglycan.

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6
Q

What is the sequence of structures that bind from the basal lamina to the actin within the dystrophin-glycoprotein complex (DGC)?

A

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.

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7
Q

What causes duchenne’s muscular dystrophy?

A

Dystrophin mutation causes duchenne’s muscular dystrophy

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8
Q

What is dystrophin?

A

Dystrophin is a large rod-like protein.

It is a key component of the DGC complex.

It contains 2 actin-binding domains.

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9
Q

What is the function of dystrophin?

A

Links the actin cytoskeleton to the sarcolemma.

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10
Q

What are the functional domains of the dystrophin protein?

A

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.

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11
Q

What happens when dystrophin is defective or absent?

A

Dissipation of forces throughout the myofiber becomes disrupted and membrane become permeable.

Muscle fiber is constantly broken down and regenerated.

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12
Q

What is the function of dystrophin in relation to syntrophin and dystrobrevin?

A

dystrophin directs syntrophin and dystrobrevin to the sarcolemma.

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13
Q

What happens to muscles in the absence of dystrophin?

A

Absence of dystrophin results in muscles containing varying number of muscle fiber size, inflammatory cell infiltrate and accumulating fibrous tissue and membrane permeability.

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14
Q

What are some signalling roles of dystrophin?

A

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.

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15
Q

What are the components of the dystroglycan complex?

A

Laminin

alpha dystroglycan

beta dystroglycan

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16
Q

What is the function of the dystroglycan complex?

A

Links extracellular matrix to the plasma membrane.

Links to actin cytoskeleton by interacting with dystrophin

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17
Q

How many chains does laminin contain?

A

3 chains. An alpha, beta, and gamma chain.

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18
Q

Which chain of laminin is necessary for interaction with receptors?

A

The alpha chain

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19
Q

What results from absence of laminin?

A

congenital muscular dystrophy (MCD1A)

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20
Q

What are the consequences of congenital muscular dystrophy (MCD1A)?

A

genetic deficiency of laminin alpha 2 chain

Variable muscle fiber size

Presence of immature fibers

Increased connective tissue fibrosis

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21
Q

What is alpha dystroglycan and what is its function?

A

Alpha dystroglycan is an extracellular protein that acts as a receptor for laminin.

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22
Q

What does alpha dystroglycan require for proper function?

A

glycosylation

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23
Q

What is beta dystroglycan and what is its function?

A

beta dystroglycan is transmembrane and links the subsarcolemmal cytoskeleton to the extracellular matrix

24
Q

What are some conditions associated with dystroglycan mutation?

A

No known association between dystroglycan and human disease.

25
Q

What happens when glycosylation of alpha dystroglycan is prevented?

A

Lack of glycosylation does not affect expression of dystroglycan but instead results in an inability to bind laminin.

26
Q

What is the function of phosphorylation in dystroglycan?

A

If not bond to alpha dystroglycan, beta dystroglycan gets phosphorylated and destroyed.

27
Q

What is the sarcoglycan complex composed of?

A

alpha gamma delta, sarcospan and beta sarcoglycan; all are transmembrane.

28
Q

What is the function of the sarcoglycan complex?

A

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.

29
Q

What does the absence of sarcoglycan complex cause?

A

A genetic deletion of sarcoglycan subunits which results in a subset of limb-girdle muscular dystrophy.

30
Q

What happens if there is loss of one sarcoglycan subunit?

A

sarcoglycan complex is lost and this leads to alterations in membrane permeability which results in cell death.

31
Q

What is the position of Vinculin-talin-integrin in a muscle fiber?

A

Vinculin-talin-integrin complex co-localises with DGC at the costamere.

32
Q

What are integrins?

A

Integrins are heterodimers composed of alpha and beta subunits. They interact with laminin and talin and is transmembrane.

33
Q

What is the general function of integrins?

A

To mediate the process of cell adhesion and migration

Regulates the intracellular organization of the actin cytoskeleton.

Important role in many signalling processes.

34
Q

What are the most common isoforms of alpha and beta integrin?

A

alpha7B

beta 1D

35
Q

What is the result of mutations in alpha7 integrin?

A

Mutations in alpha7 integrin result in an early onset CMD

36
Q

What is the function of Talin 1?

A

Talin 1 interacts with actin and integrin beta1, Focal Adhesion Kinase (FAK), and vinculin

37
Q

What is the function of Talin 2?

A

Talin 2 is essential for skeletal muscle development and is essential for skeletal muscle development.

38
Q

What is the pathology of talin mutation?

A

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.

39
Q

What are the domains of vinculin?

A

4 Vh domains; Vh 1 associates with talin and actinin

1 linker domain

Terminal (Vt) domain

40
Q

What part of talin binds to vinculin?

A

At the C terminal site of integrin and talin is an association with vinculin

41
Q

What is the function of vinculin?

A

Vinculin is involved in linkage of integrin to actin cytoskeleton.

42
Q

What is the muscle specific splice variant of vinculin?

A

Metavinculin

43
Q

What condition results from mutations in metavinculin?

A

kliopathic dilated cardiomyopathy (DCM)

44
Q

Why are there 2 costameric protein complexes?

A

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.

45
Q

What is the function of desmin?

A

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.

46
Q

What is the function of desmin?

A

Desmin is required to maintain myofibril integrity.

Desmin is not required for normal muscle development

47
Q

What Is the result of a mutation in the desmin protein?

A

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.

48
Q

What is plectin?

A

A large protein found in almost every mammalian cell and is a structural component of striated muscle.

49
Q

What does plectin do?

A

Acts to link actin microfilaments, microtubules, and intermediate filaments.

localizes desmin and many other proteins.

50
Q

How many isoforms are there of plectin? What are some examples?

A

12 isoforms of plectin;

plectin 1 and 1f are localised to costameres

plectin 1d associated with the z disk

plectin 1b localized to mitochondria

51
Q

What condition results from plectin mutation?

A

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.

52
Q

What is titin?

A

TItin is the largest protein in nature.

53
Q

What is the function of titin?

A

Centers actin filaments within sarcomere during contraction.

Acts as a molecular spring

Mediates signals

Provides structural framework by associating with sarcomeric proteins.

54
Q

What is tibial muscular dystrophy?

to be edited

A

Tibial muscular dystrophy (Limb Girdle muscular dystrophy)

Found in finnish population

Alters the structure and functiuon of titin

May disrupt titin’s interactions

55
Q

How does titin maintain its position?

A

Anchored at the Z and M lines of the sarcomere. It maintains rigid contact with the thick filaments along the A band

56
Q

Where is titin most flexilble?

A

Flexible along the I band

Muscle & Exercise Physiology (33 decks)

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