Protein isoform-function relationships of single skeletal muscle fibers from weight-bearing and hindlimb suspended mice Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9306t147k

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  • The goals of this research were to a) characterize the protein-function relationships of skeletal muscle single fibers from the mouse hindlimb b) examine mouse-strain related differences in myosin heavy chain composition (MHC) and single fiber contractile function, and c) quantify changes in fiber size and contractile function in response to 7 days of non-weight bearing. This research is significant because mechanistic approaches to understanding relationships between muscle protein expression, contractile function, and mechanical loading will likely benefit from a transition from the traditional laboratory rat to genetically modified mouse models. The methods used in this research feature an in vitro skinned-fiber preparation and single-fiber gel electrophoresis. Hindlimb muscles of mice were excised, and dissected into smaller bundles from which single muscle fibers were isolated. Single fibers were placed in skinning solution that permeabilized the fiber's membrane. The ends of skinned single muscle segments were attached to stainless steel troughs, which were connected to an isometric force transducer and a direct-current position motor. This system allowed the measurement of the fiber's cross-sectional area (CSA), peak isometric force (P₀), and unloaded maximal shortening velocity (V₀) during maximal Ca²⁺-activating. The identification of the fiber's MHC content was subsequently achieved by electrophoresis of a sample of each fiber segment. The results showed that the C57BL/6 mouse soleus muscle contains a MHC composition (20% type I) that is dramatically different than the ICR and CBA/J mouse strains (50% type I, respectively). Type I fibers from the C57BL/6 mouse had V₀ that was 25% lower than type I fibers from ICR and CBA/J mice. Following 7 days of hindlimb suspension (HS) all strains experienced significant soleus muscle and single-fiber atrophy and decreases in the absolute and specific (force/fiber CSA) of type I and II fibers. However, type I fibers from C57BL/6 mice showed no change in V₀ whereas type I fibers from ICR and CBA/J showed increased V₀. In conclusion, this research demonstrates that unlike the rat and human models of non-weight bearing, mouse soleus type I and II fibers are equally affected by HS with respect to decreases in fiber CSA and force. However, type I fiber V₀ was elevated only in mouse strains with solei containing at least 50% type I MHC. These findings challenge the current view that non-weight bearing affects slow fibers more than fast fibers, and suggests that changes in single fiber contractile function with HS may be influenced in part by the MHC distribution of the muscle.
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