Slick, R. A. | bioRxiv Science

bioRxiv2024

Two operational modes of atomic force microscopy reveal similar mechanical properties for homologous regions of dystrophin and utrophin

Duchenne muscular dystrophy (DMD) is a lethal muscle disease caused by the absence of the protein dystrophin. Dystrophin is hypothesized to work as a molecular shock absorber that limits myofiber membrane damage when undergoing reversible unfolding upon muscle stretching and contraction. Utrophin is a dystrophin homologue that is under investigation as a protein replacement therapy for DMD. However, it remains uncertain whether utrophin can mechanically substitute for dystrophin. Here, we compared the mechanical properties of homologous utrophin and dystrophin fragments encoding the N terminus through spectrin repeat 3 (UtrN-R3, DysN-R3) using two operational modes of atomic force microscopy (AFM), constant speed and constant force. Our comprehensive data, including the statistics of force magnitude at which the folded domains unfold in constant speed mode and the time of unfolding statistics in constant force mode, show consistent results. We recover parameters of the energy landscape of the domains and conducted Monte Carlo simulations which corroborate the conclusions drawn from experimental data. Our results confirm that UtrN-R3 expressed in bacteria exhibits significantly lower mechanical stiffness compared to insect UtrN-R3, while the mechanical stiffness of the homologous region of dystrophin (DysN-R3) is intermediate between bacterial and insect UtrN-R3, showing greater similarity to bacterial UtrN-R3. SignificanceDuchenne muscular dystrophy (DMD) is a severe muscle wasting disorder caused by mutations in DMD gene encoding dystrophin. Utrophin, a fetal homologue of dystrophin, is under active investigation as a dystrophin replacement therapy for DMD. However, it is still unknown if it can substitute dystrophin mechanically. Here, we report mechanical properties of both utrophin and dystrophin fragments encoding the N terminus through spectrin repeat 3 (UtrN-R3, DysN-R3) using atomic force microscope (AFM) through two operational modes, constant speed and constant force. Our data, consistent across both modes, confirm that UtrN-R3 expressed in bacteria exhibits significantly lower mechanical stiffness than insect UtrN-R3. Additionally, bacterial DysN-R3 lies between bacterial and insect UtrN-R3 in terms of mechanical properties, leaning closer to bacterial UtrN-R3.

biophysics↗

bioRxiv2020

Contributions of Implicit and Explicit Memories to Sensorimotor Adaptation of Movement Extent During Goal-Directed Reaching

We examined how explicit memory of kinematic performance contributes to motor adaptation during goal-directed reaching. Twenty subjects grasped the handle of a horizontal planar robot that rendered spring-like resistance to movement. Subjects made rapid "out-and-back" reaches to capture a remembered visual target at the point of maximal reach extent. The robots resistance changed unpredictably between reaches, inducing target capture errors that subjects attempted to correct from one trial to the next. The subjects performed four sets of trials. Three were performed without concurrent visual cursor feedback of hand motion. Two of these required self-assessment of performance between trials, whereby subjects reported peak reach extent on the most recent trial. This was done by either moving a cursor on a horizontal display (visual self-assessment), or by moving the robots handle back to the recalled location (proprioceptive self-assessment). Two control conditions were performed either without or with concurrent visual cursor feedback of hand motion. We analyzed movement kinematics and used regression analyses to quantify the extent to which prior reach errors and explicit memories of prior performance contribute to subsequent reach performance. Consistent with prior reports, providing concurrent visual feedback of hand motion increased reach accuracy and reduced the impact of past performance errors on future performance, relative to the no-vision control condition. By contrast, we found no impact of interposed self-assessment on reach kinematics or on how prior target capture errors influence subsequent reach performance. Self-assessments were biased toward the remembered target location, differing markedly from actual reach performances. Including self-assessments as explicit memory of performance in the regression analyses did not improve model predictive accuracy. Therefore, we conclude that explicit memories of prior kinematic performance do not contribute to implicit sensorimotor adaptation of movement extent.

neuroscience↗