Browsing by Author "Leonard, Timothy R."

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    Are titin properties reflected in single myofibrils?
    (Journal of Biomechanics, 2012-07-26) Herzog, Jens A.; Leonard, Timothy R.; Jinha, Azim; Herzog, Walter
    Titin is a structural protein in muscle that spans the half sarcomere from Z-band to M-line. Although there are selected studies on titin's mechanical properties from tests on isolated molecules or titin fragments, little is known about its behavior within the structural confines of a sarcomere. Here, we tested the hypothesis that titin properties might be reflected well in single myofibrils. Single myofibrils from rabbit psoas were prepared for measurement of passive stretch-shortening cycles at lengths where passive titin forces occur. Three repeat stretch-shortening cycles with magnitudes between 1.0 and 3.0μm/sarcomere were performed at a speed of 0.1μm/s·sarcomere and repeated after a ten minute rest at zero force. These tests were performed in a relaxation solution (passive) and an activation solution (active) where cross-bridge attachment was inhibited with 2,3 butanedionemonoxime. Myofibrils behaved viscoelastically producing an increased efficiency with repeat stretch-shortening cycles, but a decreased efficiency with increasing stretch magnitudes. Furthermore, we observed a first distinct inflection point in the force-elongation curve at an average sarcomere length of 3.5μm that was associated with an average force of 68±5nN/mm. This inflection point was thought to reflect the onset of Ig domain unfolding and was missing after a ten minute rest at zero force, suggesting a lack of spontaneous Ig domain refolding. These passive myofibrillar properties observed here are consistent with those observed in isolated titin molecules, suggesting that the mechanics of titin are well preserved in isolated myofibrils, and thus, can be studied readily in myofibrils, rather than in the extremely difficult and labile single titin preparations.
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    Chronic uphill and downhill exercise protocols do not lead to sarcomerogenesis in mouse skeletal muscle
    (Journal of Biomechanics, 2019-11-05) Morais, Gustavo Paroschi; da Rocha, Alisson Luiz; Neave, Louise M.; de Araújo Lucas, Guilherme; Leonard, Timothy R.; Carvalho, Andrea; Silva, Adelino S. R.; Herzog, Walter
    It has been suggested that eccentric contraction (EC) is associated with increases in serially-arranged sarcomeres (sarcomerogenesis), while concentric contraction (CC) has been associated with serial sarcomeres decrease. Sarcomerogenesis following EC is thought to be a protective muscle adaptation, preventing muscle injury in future eccentric exercise bouts (repeated bout effect). However, the mechanisms underlying sarcomerogenesis in EC remain unknown, and the sarcomerogenic responses observed in response to EC and CC are contradictory. We measured sarcomere length, sarcomere length uniformity, serial sarcomere number, and fascicle length in gastrocnemius medialis, tibialis anterior, vastus medialis and vastus lateralis in sedentary (SED) mice, and in mice following protocols of moderate uphill (TRU) and downhill (TRD) training and uphill (OTU) and downhill (OTD) overtraining. We found pain sensitivity after the first bout of EC exercise on TRD and OTD followed by a normalized sensory response after four weeks of training, indicating a repeated bout effect. However, these findings were not associated with sarcomerogenesis, as serial sarcomere numbers did not increase in TRD and OTD skeletal muscle samples compared to controls (SED). However, we found a decrease in serial sarcomere number in VL and TA in OTU group mice, which was associated with a decrease in fascicle length and no change of sarcomere length at the tested joint configuration. We conclude that excessive concentric muscle contraction (OTU group mice), leads to a decrease in serial sarcomere number, while moderate or excessive eccentric training, did not result in sarcomerogenesis, as reported in the literature.
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    Molecular mechanisms of muscle force regulation: a role for titin?
    (Exercise Sport Science Reviews, 2012-01) Herzog, Walter; DuVall, Michael M.; Leonard, Timothy R.
    Muscle contraction and force regulation is thought to occur exclusively through the interaction of the contractile proteins actin and myosin and in accordance with the assumptions underlying the cross-bridge theory. Here, we demonstrate that a third protein, titin, plays a major role in muscle force regulation, particularly for eccentric contractions and at long muscle and sarcomere lengths.
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    The natural initiation and progression of osteoarthritis in the anterior cruciate ligament deficient feline knee
    (2019-04) Leumann, Andrè G.; Leonard, Timothy R.; Nüesch, Corina; Horisberger, Monika; Mündermann, Annegret; Herzog, Walter
    The aim of this study was to document the natural history of development and long-term progression of osteoarthritis (OA) in the feline knee after minimally-invasive anterior cruciate ligament (ACL) transection.
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    A novel three-filament model of force generation in eccentric contraction of skeletal muscles
    (PLOS One, 2015-01) Schappacher-Tilp, Gudrun; Leonard, Timothy R.; Desch, Gertrud Wolfgang; Herzog, Walter
    We propose and examine a three filament model of skeletal muscle force generation, thereby extending classical cross-bridge models by involving titin-actin interaction upon active force production. In regions with optimal actin-myosin overlap, the model does not alter energy and force predictions of cross-bridge models for isometric contractions. However, in contrast to cross-bridge models, the three filament model accurately predicts history-dependent force generation in half sarcomeres for eccentric and concentric contractions, and predicts the activation-dependent forces for stretches beyond actin-myosin filament overlap.
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    Residual Force Enhancement Following Eccentric Contractions: A New Mechanism Involving Titin
    (American Physiology Society, 2016-01) Herzog, Walter; Schappacher, G.; DuVall, Michael M.; Leonard, Timothy R.; Herzog, Jens A.
    Eccentric muscle properties are not well characterized by the current paradigm of the molecular mechanism of contraction: the cross-bridge theory. Findings of force contributions by passive structural elements a decade ago paved the way for a new theory. Here, we present experimental evidence and theoretical support for the idea that the structural protein titin contributes to active force production, thereby explaining many of the unresolved properties of eccentric muscle contraction.
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    The sarcomere force-length relationship in an intact muscle-tendon unit
    (The Company of Biologists, 2020-03-25) Moo, Engkuan; Leonard, Timothy R.; Herzog, Walter
    The periodic striation pattern in skeletal muscle reflects the length of the basic contractile unit: the sarcomere. More than half a century ago, Gordon, Huxley and Julian provided strong support for the 'sliding filament' theory through experiments with single muscle fibres. The sarcomere force-length (FL) relationship has since been extrapolated to whole muscles in an attempt to unravel in vivo muscle function. However, these extrapolations were frequently associated with non-trivial assumptions, such as muscle length changes corresponding linearly to SL changes. Here, we determined the in situ sarcomere FL relationship in a whole muscle preparation by simultaneously measuring muscle force and individual SLs in an intact muscle-tendon unit (MTU) using state-of-the-art multi-photon excitation microscopy. We found that despite great SL non-uniformity, the mean value of SLs measured from a minute volume of the mid-belly, equivalent to about 5×10-6% of the total muscle volume, agrees well with the theoretically predicted FL relationship, but only if the precise contractile filament lengths are known, and if passive forces from parallel elastic components and activation-associated sarcomere shortening are considered properly. As SLs are not uniformly distributed across the whole muscle and changes in SL with muscle length are location dependent, our results may not be valid for the proximal or distal parts of the muscle. The approach described here, and our findings, may encourage future studies to determine the role of SL non-uniformity in influencing sarcomere FL properties in different muscles and for different locations within single muscles.
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    Stiffness of Hip Adductor Myofibrils is decreased in Children with Spastic Cerebral Palsy
    (Elsevier, 2019-02-23) Leonard, Timothy R.; Howard, Jason J.; Larkin-Kaiser, Kelly A.; Joumaa, Venus; Logan, Karl J.; Orlik, Benjamin; El-Hawary, Ron; Gauthier, Luke E.; Herzog, Walter
    Cerebral palsy (CP) is the result of a static brain lesion which causes spasticity and muscle contracture. The source of the increased passive stiffness in patients is not understood and while whole muscle down to single muscle fibres have been investigated, the smallest functional unit of muscle (the sarcomere) has not been. Muscle biopsies (adductor longus and gracilis) from pediatric patients were obtained (CP n=9 and control n=2) and analyzed for mechanical stiffness, in-vivo sarcomere length and titin isoforms. Adductor longus muscle was the focus of this study and the results for sarcomere length showed a significant increase in length for CP (3.6µm) compared to controls (2.6µm). Passive stress at the same sarcomere length for CP compared to control was significantly lower in CP and the elastic modulus for the physiological range of muscle was lower in CP compared to control (98.2kPa and 166.1kPa, respectively). Our results show that CP muscle at its most reduced level (the myofibril) is more compliant compared to normal , which is completely opposite to what is observed at higher structural levels (single fibres, muscle fibre bundles and whole muscle). It is noteworthy that at the in vivo sarcomere length in CP, the passive forces are greater than normal, purely as a functional of these more compliant sarcomeres operating at long lengths. Titin isoforms were not different between CP and non-CP adductor longus but titin:nebulin was reduced in CP muscle, which may be due to titin loss or an over-expression of nebulin in CP muscles.
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    The three filament model of skeletal muscle stability and force production
    (Tech Science Press, 2012-01) Herzog, Walter; Leonard, Timothy R.; Joumaa, Venus; DuVall, Michael M.; Panchangam, Appaji
    Ever since the 1950s, muscle force regulation has been associated with the cross-bridge interactions between the two contractile filaments, actin and myosin. This gave rise to what is referred to as the "two-filament sarcomere model". This model does not predict eccentric muscle contractions well, produces instability of myosin alignment and force production on the descending limb of the force-length relationship, and cannot account for the vastly decreased ATP requirements of actively stretched muscles. Over the past decade, we and others, identified that a third myofilament, titin, plays an important role in stabilizing the sarcomere and the myosin filament. Here, we demonstrate additionally how titin is an active participant in muscle force regulation by changing its stiffness in an activation/force dependent manner and by binding to actin, thereby adjusting its free spring length. Therefore, we propose that skeletal muscle force regulation is based on a three filament model that includes titin, rather than a two filament model consisting only of actin and myosin filaments consisting only of actin and myosin filaments