Skeletal Muscle Diseases: Techniques: Functional assessments to evaluate therapeutic efficacy. Flashcards
(24 cards)
What causes duchenne’s muscular dystrophy?
Mutation in dystrophin gene on Xp21 resulting in deficiency in dystrophin expression.
What is dystrophin?
427 kDa cytoskeletal protein associated with several structures nearby. Dystrophin associated proteins and DGC.
What proteins is dystrophin closely associated with?
sacroglycan complex
dystroglycan complex
laminin
F-actin
How are animal models used to test muscular dystrophy?
Various animal models are used to test several aspects of muscular dystrophy
Why do several therapies fail when clinical trials are performed?
A more rigorous pre-clinical testing period is needed when evaluating therapeutic potential.
Several therapies are not as functional in humans as they are in animals due to differing physiology.
Many other unknown reasons are also contributing.
What is a physiologists goal in treating muscle disorders?
Attenuating loss of muscle preserves function because increasing muscle mass optimises potential for improving muscle function.
Improving muscle function could improve quality of life for patients as well as quantity for people with dystrophic disorders.
What are the factors that should be looked into when evaluating interventions for preclinical testing?
Good animal models of human diseases/conditions.
Rigorous assessment of muscle function.
Complementary physiological, histological, and molecular biochemical assessments.
*This is especially relevant for pharmocological, gene, cell, and bioengineering based therapies.
How are muscles from mdx mice different to muscles of wild type mice?
Muscles from mdx mice are larger than wild type mice. But they produce less force per CSA than the wild type. This muscle is also more fragile.
What factors are important during pre-clinical evaluation?
Non- or minimally invasive tests.
Overall health and functional capacity of the animal.
Track performance.
What are cellular functional assays?
Mechanically skinned fibers
Chemically permeabilized fibers
What do mechanically skinned fibers allow us to study?
Excitation contraction coupling.
SR Ca2+ release
Ca2+ uptake rates SR function
What do chemically permeabilized fibers allow us to study?
Contractile apparatus
Force-pCa Force-pSr
Vmax
Contraction induced injury
What are the pros and cons of using cellular functional assays?
Can study cellular level for mechanistic understanding.
Cons: Technically difficult and requires expensive equipment.
What muscle is typically used when studying muscular dystrophy? Why?
In muscular dystrophy diaphragm muscle strips can be used because diaphragm is the most important muscle for survival.
What other analyses are performed to evaluate muscle effectiveness?
Structure-function assessments (need to perform complementary analyses of histology, immunohistochemical, and molecular biochemical analyses).
Creatine Kinase levels
Assessment of muscle fiber size (CSA)
Biochemical pathway analyses
What are the features of dystrophic muscles?
Dystrophic muscles are not only weaker but also more susceptible to contraction-induced injury.
Sarcolemmal fragility of fibers from dystrophic mice is worse than wild type.
Greater susceptibility to rupture following osmotic shock.
Greater susceptibility to injury following stretch.
Dystrophic models are deficient in force production following stretch
What happens to dystrophic muscle relative to normal muscle?
Dystrophic muscle exhibts a small deficit until 30% after which the deficit is so much that the muscle can’t function adequately.
What happens to calcium release in muscle cells during muscular dystrophy?
Normally control of calcium is strictly regulated by muscles. In dystrophic muscle the calcium intracellular levels are often leaking out of the SR and this is especially a problem during exercise.
What happens to contraction strength over time in dystrophic muscle?
Damage to muscle causes decrease in contractility and as a result muscle strength diminishes.
How are muscles damaged when DGC components are missing?
No universal agreement on which mechanism predominates.
What does the mechanical hypothesis state about muscle damage during DGC malfunctions?
Mechanical hypothesis states that loss of DGC leads to contraction-induced rupture of muscle cell membrane.
What evidence supports the mechanical hypothesis?
Exercise in DMD patients and mdx causes greater muscle damage that in unaffected controls.
What does the calcium hypothesis state?
Influx of calcium into the cell overwhelms muscle cell’s ability to maintain physiologic Ca2+ levels.
Elevated Ca2+ levels causes programmed cell death via activation of proteases such as calpains.
*overexpression of calpastatin, an endogenous inhibitor of calpains, reduces necrosis.
Why do people tend to die young with muscular dystrophy?
Initially the regeneration of muscle is adequate but it gets worse and worse with time. For this reason people die young with this condition.
