The Drosophila formin Fhos is a primary mediator of sarcomeric thin-filament array assembly.

Arkadi Shwartz,Ben-Zion Shilo,Eyal D Schejter,Nagaraju Dhanyasi

doi:10.7554/elife.16540

Abstract

Actin-based thin filament arrays constitute a fundamental core component of muscle sarcomeres. We have used formation of the Drosophila indirect flight musculature for studying the assembly and maturation of thin-filament arrays in a skeletal muscle model system. Employing GFP-tagged actin monomer incorporation, we identify several distinct phases in the dynamic construction of thin-filament arrays. This sequence includes assembly of nascent arrays after an initial period of intensive microfilament synthesis, followed by array elongation, primarily from filament pointed-ends, radial growth of the arrays via recruitment of peripheral filaments and continuous barbed-end turnover. Using genetic approaches we have identified Fhos, the single Drosophila homolog of the FHOD sub-family of formins, as a primary and versatile mediator of IFM thin-filament organization. Localization of Fhos to the barbed-ends of the arrays, achieved via a novel N-terminal domain, appears to be a critical aspect of its sarcomeric roles.

Highlights

Sarcomeres constitute the basic functional units of muscle fibers, endowing these large and specialized cells with their contractile capacity
Of sarcomeric thin-filament arrays has been traditionally studied by monitoring global changes in sarcomere structure in fixed samples of muscle tissue
We followed the incorporation of GFP-tagged actin monomers into indirect flight muscles (IFMs) sarcomeres, throughout the pupal stages of development, as a means of revealing the assembly and maturation of IFM sarcomeric thin-filament arrays in developing flies

Summary

Introduction

Sarcomeres constitute the basic functional units of muscle fibers, endowing these large and specialized cells with their contractile capacity. While mechanisms relating to size definition and stability of the arrays have been extensively investigated (Fernandes and Schock, 2014; Meyer and Wright, 2013), other key aspects of microfilament array formation and dynamics, including determination of distinct phases of array maturation, the identity and regulation of elements mediating filament nucleation/elongation, and the processes governing incorporation of additional filaments into nascent arrays are not resolved (Ono, 2010) We address these matters in the context of formation and development of the Drosophila indirect flight muscles (IFMs). A major subset of the IFMs, the dorso-longitudinal muscles (DLMs), closely resemble vertebrate skeletal muscles in both their developmental program and in their mature myofibrillar structure (Dutta and VijayRaghavan, 2006; Roy and VijayRaghavan, 1999), making them a attractive model system, in which the powerful

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