In Fig. 3a of the original manuscript 1, the passive force at the beginning of the force-time history (approximately from 0 to 1 s) of each of the black and orange traces was interchanged in the ...coloring process. This was an error in the artwork preparation, not the original data. The corrected figure is shown below.
Abstract Residual force enhancement has been observed consistently in skeletal muscles. Despite an abundance of experimental observations, there has been no information about the metabolic cost of ...the force observed after stretch. Our aim was to investigate the energy cost of force production after active stretch in skinned fibres isolated from rabbit psoas muscle, by quantifying the ATPase activity using an enzyme-coupled assay. Fibres were actively stretched from an average sarcomere length of 2.4 μm to average sarcomere lengths of 2.8 and 3.2 μm. Purely isometric reference contractions were performed at average sarcomere lengths of 2.8 and 3.2 μm. Simultaneously with the force measurements, the ATP cost per unit of force produced was measured during the last 40 s of isometric contraction. Results showed that ATPase activity per unit of force was reduced by 17.2±4.1% in the isometric contractions after active stretch, compared to the purely isometric contraction at the corresponding lengths for both stretch magnitudes. Fibres stretched to an average sarcomere length of 3.2 μm showed a higher reduction in ATPase activity per unit of force compared to fibres stretched to an average sarcomere length of 2.8 μm (20.7±4.4 versus 12.4±3.2% respectively). Passive force enhancement was observed in all fibres and was correlated with the decrease in ATPase activity. No difference in stiffness was observed between reference and active stretch contractions. These results suggest that skeletal muscles become more efficient after stretch, either by increasing the amount of force produced per cross bridge or by engaging a passive element.
Muscle-directed gene therapy with adeno-associated viral (AAV) vectors is undergoing clinical development for treating neuromuscular disorders and for systemic delivery of therapeutic proteins. ...Although these approaches show considerable therapeutic benefits, they are also prone to induce potent immune responses against vector or transgene products owing to the immunogenic nature of the intramuscular delivery route, or the high doses required for systemic delivery to muscle. Major immunological concerns include antibody formation against viral capsid, complement activation, and cytotoxic T cell responses against capsid or transgene products. They can negate therapy and even lead to life-threatening immunotoxicities. Herein we review clinical observations and provide an outlook for how the field addresses these problems through a combination of vector engineering and immune modulation.
Due to a high incidence of anterior cruciate ligament (ACL) re‐injury in alpine ski racers, this study aims to assess functional asymmetry in the countermovement jump (CMJ), squat jump (SJ), and leg ...muscle mass in elite ski racers with and without anterior cruciate ligament reconstruction (ACL‐R). Elite alpine skiers with ACL‐R (n = 9; 26.2 ± 11.8 months post‐op) and uninjured skiers (n = 9) participated in neuromuscular screening. Vertical ground reaction force during the CMJ and SJ was assessed using dual force plate methodology to obtain phase‐specific bilateral asymmetry indices (AIs) for kinetic impulse (CMJ and SJ phase‐specific kinetic impulse AI). Dual x‐ray absorptiometry scanning was used to assess asymmetry in lower body muscle mass. Compared with controls, ACL‐R skiers had increased AI in muscle mass (P < 0.001), kinetic impulse AI in the CMJ concentric phase (P < 0.05), and the final phase of the SJ (P < 0.05). Positive associations were observed between muscle mass and AI in the CMJ concentric phase (r = 0.57, P < 0.01) as well as in the late SJ phase (r = 0.66, P < 0.01). Future research is required to assess the role of the CMJ and SJ phase‐specific kinetic impulse AI as a part of a multifaceted approach for improving outcome following ACL‐R in elite ski racers.
Adeno-associated virus (AAV)-mediated CRISPR-Cas9 editing holds promise to treat many diseases. The immune response to bacterial-derived Cas9 has been speculated as a hurdle for AAV-CRISPR therapy. ...However, immunological consequences of AAV-mediated Cas9 expression have thus far not been thoroughly investigated in large mammals. We evaluate Cas9-specific immune responses in canine models of Duchenne muscular dystrophy (DMD) following intramuscular and intravenous AAV-CRISPR therapy. Treatment results initially in robust dystrophin restoration in affected dogs but also induces muscle inflammation, and Cas9-specific humoral and cytotoxic T-lymphocyte (CTL) responses that are not prevented by the muscle-specific promoter and transient prednisolone immune suppression. In normal dogs, AAV-mediated Cas9 expression induces similar, though milder, immune responses. In contrast, other therapeutic (micro-dystrophin and SERCA2a) and reporter (alkaline phosphatase, AP) vectors result in persistent expression without inducing muscle inflammation. Our results suggest Cas9 immunity may represent a critical barrier for AAV-CRISPR therapy in large mammals.
It has been accepted for half a century that, for a given level of activation, the steady-state isometric force of a muscle sarcomere depends exclusively on the amount of overlap between the ...contractile filaments actin and myosin, or equivalently sarcomere length (Gordon AM et al., J Physiol 184: 170-192, 1966). Moreover, according to the generally accepted paradigm of muscle contraction, the cross-bridge theory (Huxley AF, Prog Biophys Biophys Chem 7: 255-318, 1957), this steady-state isometric sarcomere force is independent of the muscle's contractile history (Huxley AF, Prog Biophys Biophys Chem 7: 255-318, 1957; Walcott S and Herzog W, Math Biosci 216: 172-186, 2008); i.e., it is independent of whether a muscle is held at a constant length before and during the contraction or whether the muscle is shortened or lengthened to the same constant length. This, however, is not the case, as muscles and single fibers that are stretched show greatly increased steady-state isometric forces compared with preparations that are held at a constant length (Abbott BC and Aubert XM, J Physiol 117: 77-86, 1952; De Ruiter CJ et al., J Physiol 526.3: 671-681, 2000; Edman KAP et al., J Physiol 281: 139-155, 1978; Edman KAP et al., J Gen Physiol 80: 769-784, 1982; Edman KAP and Tsuchiya T, J Physiol 490.1: 191-205, 1996). This so-called "residual force enhancement" (Edman KAP et al., J Gen Physiol 80: 769-784, 1982) offers a perplexing puzzle for muscle physiologists. Many theories have been advanced to address the discrepancy between prediction and observation with the most popular and accepted being the sarcomere length nonuniformity theory (Morgan DL, Biophys J 57: 209-221, 1990), which explains the residual force enhancement with the development of large nonuniformities in sarcomere lengths during muscle stretching. Here, we performed experiments in mechanically isolated sarcomeres and observed that the residual force enhancement following active stretching is preserved. Since our preparation utilizes a single sarcomere, a redistribution of the length of neighboring sarcomeres to produce the higher force following stretch is, by design, precluded. Furthermore, the enhanced forces in the single sarcomeres always exceed the isometric forces on the plateau of the force-length relationship, thereby eliminating the possibility that our result might have been obtained because of a redistribution of half-sarcomere lengths. Since force enhancement in single myofibrils has been associated with actin-titin interactions (Kulke M et al., Circ Res 89: 874-881, 2001; Li Q et al., Biophys J 69: 1508-1518, 1995) and calcium binding to titin (Joumaa V et al., Am J Physiol Cell Physiol 294: C74-C78, 2008; Labeit D et al., Proc Natl Acad Sci USA 100: 13716-13721, 2003), titin may regulate the sarcomeric force enhancement observed here.
Isometric force after active stretch of muscles is higher than the purely isometric force at the corresponding length. This property is termed residual force enhancement. Active force in skeletal ...muscle depends on calcium attachment characteristics to the regulatory proteins. Passive force has been shown to influence calcium attachment characteristics, specifically the sarcomere length dependence of calcium sensitivity. Since one of the mechanisms proposed to explain residual force enhancement is the increase in passive force that results from engagement of titin upon activation and stretch, our aim was to test if calcium sensitivity of residual force enhancement was different from that of its corresponding purely isometric contraction and if such a difference was related to the molecular spring titin. Force-pCa curves were established in rabbit psoas skinned fibers for reference and residual force-enhanced states at a sarcomere length of 3.0 μm 1) in a titin-intact condition, 2) after treatment with trypsin to partially eliminate titin, and 3) after treatment with trypsin and osmotic compression with dextran T-500 to decrease the lattice spacing in the absence of titin. The force-pCa curves of residual force enhancement were shifted to the left compared with their corresponding controls in titin-intact fibers, indicating increased calcium sensitivity. No difference in calcium sensitivity was observed between reference and residual force-enhanced contractions in trypsin-treated and osmotically compressed trypsin-treated fibers. Furthermore, calcium sensitivity after osmotic compression was lower than that observed for residual force enhancement in titin-intact skinned fibers. These results suggest that titin-based passive force regulates the increase in calcium sensitivity of residual force enhancement by a mechanism other than reduction of the myofilament lattice spacing.
Sarcomere lengths are non-uniform on all structural levels of mammalian skeletal muscle. These non-uniformities have been associated with a variety of mechanical properties, including residual force ...enhancement and depression, creep, increased force capacity, and extension of the plateau of the force-length relationship. However, the nature of sarcomere length non-uniformities has not been explored systematically. The purpose of this study was to determine the properties of sarcomere length non-uniformities in active and passive muscle. Single myofibrils of rabbit psoas (
= 20; with 412 individual sarcomeres) were subjected to three activation/deactivation cycles and individual sarcomere lengths were measured at 4 passive and 3 active points during the activation/deactivation cycles. The myofibrils were divided into three groups based on their initial average sarcomere lengths: short, intermediate, and long average sarcomere lengths of 2.7, 3.2, and 3.6 µm. The primary results were that sarcomere length non-uniformities did not occur randomly but were governed by some structural and/or contractile properties of the sarcomeres and that sarcomere length non-uniformities increased when myofibrils went from the passive to the active state. We propose that the mechanisms that govern the systematic sarcomere lengths non-uniformities observed in active and passive myofibrils may be associated with the variable number of contractile proteins and the variable number and the adjustable stiffness of titin filaments in individual sarcomeres.
Peer-assisted learning (PAL) is a common teaching and learning method in medical education worldwide. In the setting of skills laboratories (skills labs), student tutors are often employed as an ...equivalent alternative to faculty teachers. However, to the best of our knowledge, there is a lack of qualitative studies which explore the reasons for the personal commitment of student tutors. The aim of our study was to examine how undergraduate students experienced and evaluated their roles as skills lab student tutors, what their motivation was, and whether social and cognitive congruence played a role in their teaching experiences.
We conducted in-depth, semi-structured interviews with student tutors who were currently teaching in a skills lab. After the interviews had been transcribed verbatim, two independent investigators performed a qualitative content analysis according to Mayring.
In total, we conducted nine interviews with student tutors. Our results revealed that all student tutors showed great enthusiasm and motivation for their jobs as peer teachers. One of the main motivating factors for student tutors to teach in a skills lab was the possibility to simultaneously share and improve their knowledge and expertise. In general, the participants of our study had high aspirations for their teaching. They found it particularly important to be empathetic with the student learners. At the same time, they thought they would personally benefit from their teaching activities and develop a certain expertise as student tutors.
With the present study we are able to gain some insight into what motivates student tutors to teach in a skills lab and what kind of experiences they have. Our results provide an important input for the future training of highly qualified student tutors.
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