Sarcomere Dysfunction in Nemaline Myopathy

Abstract

Nemaline myopathy (NM) is among the most common non-dystrophic congenital myopathies (incidence 1:50.000). Hallmark features of NM are skeletal muscle weakness and the presence of nemaline bodies in the muscle fiber. The clinical phenotype of NM patients is quite diverse, ranging from neonatal death to normal lifespan with almost normal motor function. As the respiratory muscles are involved as well, severely affected patients are ventilator-dependent. The mechanisms underlying muscle weakness in NM are currently poorly understood. Therefore, no therapeutic treatment is available yet.Eleven implicated genes have been identified: ten genes encode proteins that are either components of thin filament, or are thought to contribute to stability or turnover of thin filament proteins. The thin filament is a major constituent of the sarcomere, the smallest contractile unit in muscle. It is at this level of contraction - thin-thick filament interaction - where muscle weakness originates in NM patients.This review focusses on how sarcomeric gene mutations directly compromise sarcomere function in NM. Insight into the contribution of sarcomeric dysfunction to muscle weakness in NM, across the genes involved, will direct towards the development of targeted therapeutic strategies.

Keywords: Nemaline myopathy; contractility; thin filament.

Fig.1

Schematic of skeletal muscle, from the level of the whole muscle to the sarcomeric thin filament. Note that the schematic of the thin filament illustrates the protein products of all thin filament genes implicated in nemaline myopathy.

Fig.2

Overview of experimental techniques employed on muscle fibers isolated from patients’ biopsies. A) Single muscle fibers are isolated from a muscle biopsy and treated with Triton-X, a detergent that permeabilizes the membraneous structures and leaves the sarcomeres within the muscle fibers intact. The permeabilized muscle fiber is mounted between a force transducer and a length motor, activated by exogenous calcium, and the generated force is measured. B) By activating a muscle fiber at incremental sarcomere lengths, the sarcomere length-dependence of force can be determined. The shape of the force-sarcomere length relation provides important information on the length of thin and thick filaments; SL_opt: sarcomere length at which maximal force is generated. For instance, in patients with nebulin deficiency SL_opt is shifted leftwards. C) By rapidly releasing and restretching the muscle fiber, while activated, the rate constant of force development is determined (left panel shows the imposed change in fiber length; right panel the force response). This rate constant, K_tr, reflects the kinetics of cross bridge attachment and detachment. D) By exposing the muscle fiber to incremental calcium solutions (pCa = the negative logarithm of [Ca^2 +]), the calcium sensitivity of force is determined. A rightward shift of the force-pCa relation, i.e. a decreased pCa₅₀, indicates a reduced calcium sensitivity of force.

Fig.3

Schematic overview of the mechanisms by which mutations in the genes implicated in nemaline myopathy cause muscle weakness. Only mechanisms involving sarcomere contractility are shown.