Date Published: April 9, 2019
Publisher: Public Library of Science
Author(s): Jong-Hee Kim, Ted G. Graber, Haiming Liu, Atsushi Asakura, LaDora V. Thompson, Ashok Kumar.
Disuse induces adaptations in skeletal muscle, which lead to muscle deterioration. Hindlimb-unloading (HU) is a well-established model to investigate cellular mechanisms responsible for disuse-induced skeletal muscle dysfunction. In myosin heavy chain (MHC) type IIB fibers HU induces a reduction in contraction speed (Vo) and a reduction in the relative myosin light chain 3f (MLC3f) protein content compared with myosin light chain 1f (MLC1f) protein. This study tested the hypothesis that increasing the relative MLC3f protein content via rAd-MLC3f vector delivery would attenuate the HU-induced decline in Vo in single MHC type IIB fibers. Fischer-344 rats were randomly assigned to one of three groups: control, HU for 7 days, and HU for 7 days plus rAd-MLC3f. The semimembranosus muscles were injected with rAd-MLC3f (3.75 x 1011–5 x 1011 ifu/ml) at four days after the initiation of HU. In single MHC type IIB fibers the relative MLC3f content decreased by 25% (12.00±0.60% to 9.06±0.66%) and Vo was reduced by 29% (3.22±0.14fl/s vs. 2.27±0.08fl/s) with HU compared to the control group. The rAd-MLC3f injection resulted in an increase in the relative MLC3f content (12.26±1.19%) and a concomitant increase in Vo (2.90±0.15fl/s) of MHC type IIB fibers. A positive relationship was observed between the percent of MLC3f content and Vo. Maximal isometric force and specific tension were reduced with HU by 49% (741.45±44.24μN to 379.09±23.77μN) and 33% (97.58±4.25kN/m2 to 65.05±2.71kN/m2), respectively compared to the control group. The rAd-MLC3f injection did not change the HU-induced decline in force or specific tension. Collectively, these results indicate that rAd-MLC3f injection rescues hindlimb unloading-induced decline in Vo in MHC type IIB single muscle fibers.
Contractile velocity (Vo, unloaded shortening velocity) in single skeletal muscle fibers is dependent on the type of myosin heavy chain (MHCI, MHCIIA, MHCIIB) and the composition of the essential myosin light chain isoforms (MLC1f, MLC3f) [1,2]. The MHC isoform influences contractile velocity by its ability to hydrolyze ATP [3,4], whereas the essential myosin light chains fine-tune the speed of contraction. This fine-tuning role is demonstrated in single fiber contractility experimentation in which single MHC type IIB fibers with more relative MLC3f content contract faster compared to fibers with more relative MLC1f content, in which both essential MLC isoform genes are generated from a single gene locus via alternative and different modes of splicing [2,5]. This fine-tuning control of contractile velocity by the essential myosin light chains is linked to the interaction of the N-terminus of the MLC with actin during the myosin-actin cross-bridge cycle .
One-way ANOVA was used to determine the mean differences in fiber size, force generation, MLC protein content and Vo among CON, HU, and HUM, followed by a Tukey’s post-hoc test when appropriate. To examine the relationship between the %MLC3f protein content and Vo in MHC type IIB fibers we performed a simple linear regression analysis. SPSS software (Version 24.0) was used for statistical analysis and statistics were expressed as mean ± SEM with significance at p-value < 0.05. Because MLC3f has the potential to regulate shortening velocity in MHC type IIB fibers, we hypothesized that increasing the relative MLC3f protein content via rAd-MLC3f injection would attenuate HU-induced decline in Vo. The major findings of this study include that the relative MLC3f content and Vo in MHC type IIB fibers are significantly reduced following one-week of HU, the administration of the rAd-MLC3f significantly attenuates the HU-induced decline in MLC3f content and Vo in MHC type IIB fibers, and there is a positive relationship between percent of MLC3f content and Vo. Collectively, increased MLC3f protein content, via rAd-MLC3f, may provide a potential protection against HU-induced decline in Vo in MHC type IIB single muscle fibers. Source: http://doi.org/10.1371/journal.pone.0214982