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Nebulin plays a direct role in promoting strong actin-myosin interactions

Marie-Louise Bang^*,,1, Marco Caremani^,, Elisabetta Brunello, Ryan Littlefield^||, Richard L. Lieber^¶,#,, Ju Chen^**, Vincenzo Lombardi and Marco Linari^,,1

^* Dulbecco Telethon Institute at Istituto Tecnologie Biomediche, Consiglio Nazionale delle Ricerche, Segrate, Milan, Italy;

Istituto di Ricovero e Cura a Carattere Scientifico Multimedica, Scientific and Technology Pole, Milan, Italy;

Laboratorio di Fisiologia, Dipartimento di Biologia Evoluzionistica, and

Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Università di Firenze, Florence, Italy;

^|| Center for Cell Dynamics, Friday Harbor Laboratories, University of Washington, Friday Harbor, Washington, USA;

^¶ Department of Bioengineering,

^# Department of Orthopaedic Surgery, and

^** Department of Medicine, University of California–San Diego, La Jolla, California, USA; and

Veterans Affairs Medical Center, La Jolla, California, USA

¹ Correspondence: M.-L.B., Istituto Tecnologie Biomediche-Consiglio Nazionale delle Ricerche (ITB-CNR), Via Fratelli Cervi 93, 20090 Segrate, Milan, Italy. E-mail: m.bang{at}itb.cnr.it; M.L., Laboratorio di Fisiologia, Dipartimento di Biologia Evoluzionistica, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Florence, Italy. E-mail: marco.linari{at}unifi.it

The role of the actin filament-associated protein nebulin on mechanical and kinetic properties of the actomyosin motor was investigated in skeletal muscle of wild-type (wt) and nebulin-deficient (nebulin^–/–) mice that were 1 d old, an age at which sarcomeric structure is still well preserved. In Ca²⁺-activated skinned fibers from psoas muscle, we determined the Ca²⁺ dependence of isometric force and stiffness, the rate of force redevelopment after unloaded shortening (k_TR), the power during isotonic shortening, and the unloaded shortening velocity (V₀). Our results show a 65% reduction in isometric force in nebulin^–/– fibers at saturating [Ca²⁺], whereas neither thin-filament length nor the Ca²⁺ sensitivity of the contractile system is affected. Stiffness measurements indicate that the reduction in isometric force is due to a reduction in the number of actin-attached myosin motors, whereas the force of the motor is unchanged. Furthermore, in nebulin^–/– fibers, k_TR is decreased by 57%, V₀ is increased by 63%, and the maximum power is decreased by 80%. These results indicate that, in the absence of nebulin, the attachment probability of the myosin motors to actin is decreased, revealing a direct role for nebulin in promoting strong actomyosin interactions responsible for force and power production.—Bang, M.-L., Caremani, M., Brunello, E., Littlefield, R., Lieber, R. L., Chen, J., Lombardi, V., Linari, M. Nebulin plays a direct role in promoting strong actin-myosin interactions.

Key Words: muscle performance • muscle force generation • cytoskeletal proteins