The Most Prevalent Freeman-Sheldon Syndrome Mutations in the Embryonic Myosin Motor Share Functional Defects

Jonathan Walklate,Carlos Vera,Marieke J Bloemink,Michael A Geeves,Leslie Leinwand

doi:10.1074/jbc.m115.707489

Jonathan Walklate, Carlos Vera + Show 3 more

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https://doi.org/10.1074/jbc.m115.707489

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Abstract

The embryonic myosin isoform is expressed during fetal development and rapidly down-regulated after birth. Freeman-Sheldon syndrome (FSS) is a disease associated with missense mutations in the motor domain of this myosin. It is the most severe form of distal arthrogryposis, leading to overcontraction of the hands, feet, and orofacial muscles and other joints of the body. Availability of human embryonic muscle tissue has been a limiting factor in investigating the properties of this isoform and its mutations. Using a recombinant expression system, we have studied homogeneous samples of human motors for the WT and three of the most common FSS mutants: R672H, R672C, and T178I. Our data suggest that the WT embryonic myosin motor is similar in contractile speed to the slow type I/β cardiac based on the rate constant for ADP release and ADP affinity for actin-myosin. All three FSS mutations show dramatic changes in kinetic properties, most notably the slowing of the apparent ATP hydrolysis step (reduced 5–9-fold), leading to a longer lived detached state and a slowed Vmax of the ATPase (2–35-fold), indicating a slower cycling time. These mutations therefore seriously disrupt myosin function.

Highlights

The embryonic myosin isoform is expressed during fetal development and rapidly down-regulated after birth
Adult myosins and tertiary fibers gradually increase toward the end of gestation [9, 10] as both myosin heavy chain (MyHC)-emb and MyHC-peri are down-regulated, with MyHC-emb and MyHC-peri only being faintly detectable in a few fibers in 1-month-old infants
Adult muscle fibers from individuals with wild type MyHC-emb and those with the R672C mutation do not differ in expression levels of MyHC-emb protein [13]

Summary

Introduction

The embryonic myosin isoform is expressed during fetal development and rapidly down-regulated after birth. FreemanSheldon syndrome (FSS) is a disease associated with missense mutations in the motor domain of this myosin It is the most severe form of distal arthrogryposis, leading to overcontraction of the hands, feet, and orofacial muscles and other joints of the body. All three FSS mutations show dramatic changes in kinetic properties, most notably the slowing of the apparent ATP hydrolysis step (reduced 5–9-fold), leading to a longer lived detached state and a slowed Vmax of the ATPase (2–35-fold), indicating a slower cycling time Samples taken from the same patient at 5 years showed a predominance of type-1/MyHC-␤ expression with small, scattered type-1 fibers This suggests that there could be a compensatory effect or a developmental defect that causes MyHC-peri to be expressed later or longer in postnatal development. Because distal muscles are more affected in FSS, it is interesting to note that distal muscles appear to be formed earlier during development

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