The MyoRobot technology discloses a premature biomechanical decay of skeletal muscle fiber bundles derived from R349P desminopathy mice

Abstract

Mutations in the Des gene coding for the muscle-specific intermediate filament protein desmin lead to myopathies and cardiomyopathies. We previously generated a R349P desmin knock-in mouse strain as a patient-mimicking model for the corresponding most frequent human desmin mutation R350P. Since nothing is known about the age-dependent changes in the biomechanics of affected muscles, we investigated the passive and active biomechanics of small fiber bundles from young (17–23 wks), adult (25–45 wks) and aged (>60 wks) heterozygous and homozygous R349P desmin knock-in mice in comparison to wild-type littermates. We used a novel automated biomechatronics platform, the MyoRobot , to perform coherent quantitative recordings of passive (resting length-tension curves, visco-elasticity) and active (caffeine-induced force transients, pCa-force, ‘slack-tests’) parameters to determine age-dependent effects of the R349P desmin mutation in slow-twitch soleus and fast-twitch extensor digitorum longus small fiber bundles. We demonstrate that active force properties are not affected by this mutation while passive steady-state elasticity is vastly altered in R349P desmin fiber bundles compatible with a pre-aged phenotype exhibiting stiffer muscle preparations. Visco-elasticity on the other hand, was not altered. Our study represents the first systematic age-related characterization of small muscle fiber bundle preparation biomechanics in conjunction with inherited desminopathy.

Growing old too early, automated assessment of skeletal muscle single fiber biomechanics in ageing R349P desmin knock-in mice using the MyoRobot technology

“…Particularly mutation-bearing EDL fibers displayed a myofibrillar Ca 2+ -sensitization already within the aged age group, while in wt littermates, this became only apparent in the senile group. This corroborates well with results from our very recent age-related biomechanical assessment of R349P desmin small fiber bundles, where a very similar desensitization of about 0.2-0.3 pCa units was seen from the adult to the aged age group in EDL bundles, both the het and the hom Haug et al (2019) . For SOL, the single fiber data here do not seem to show a firm consistent trend among the genotypes with age, apart from a large scattering between individual SOL fibers.…”

“…Similar to our previous assessment in small fiber bundles Clemen et al (2015); Diermeier et al (2017a); Haug et al (2019) , single fibers showed a marked increase in passive restoration forces in RLT experiments, which was at least three-times larger in aged animals with R349P background. Restoration forces were significantly increased and axial compliance accordingly decreased, already in single EDL fibers from aged R349P desmin knock-in animals compared to the equivalent observations found in wt animals in the senile group, only.…”

“…3C). Since these results indicate an increased axial stiffness or elasticity, the 10% strain-wise compliance was computed by linear fits to the RLT curve at the indicated strain bins, as described in Haug et al (2019) , and plotted for all genotypes, ages and strains in Fig.3D. The compliance plots confirm similar mechanical axial compliance for all genotypes in the adult age group while compliance was significantly reduced in mutant fibers of the aged age group.…”

“…Our previous work in small fiber bundles from SOL muscles demonstrated an increased axial stiffness in R349P desmin knock-in mice Diermeier et al (2017a); Haug et al (2019) . However, since we also documented increased fibrosis in these muscles Diermeier et al (2017a) , it cannot be ruled out to which extent the observed fibrosis would impact on the increased axial stiffness.…”

“…Since robust biomechanical experiments in small muscle fiber bundle preparations are difficult to carry out and require precise actuation to record steady-state resting length tension curves at slow strain speeds, we engineered a novel automated biomechatronics system that also contains a sensitive force transducer technology for recordings of active and passive axial muscle forces Haug et al (2018) . Alongside with the ongoing engineering progress, our system was also systematically validated extending our previous recordings in SOL bundles from R349P desmin mice not only to the fast-twitch extensor digitorum longus (EDL) muscle, but also to a wide age range from young (17-23 weeks) to aged (60-80 weeks) animals Haug et al (2019) . Again, our findings in young animals of increased tissue stiffness were confirmed in both muscle entities with a pre-aged phenotype in the desminopathy model.…”