DAAM Is Required for Thin Filament Formation and Sarcomerogenesis during Muscle Development in Drosophila

Abstract

During muscle development, myosin and actin containing filaments assemble into the highly organized sarcomeric structure critical for muscle function. Although sarcomerogenesis clearly involves the de novo formation of actin filaments, this process remained poorly understood. Here we show that mouse and Drosophila members of the DAAM formin family are sarcomere-associated actin assembly factors enriched at the Z-disc and M-band. Analysis of dDAAM mutants revealed a pivotal role in myofibrillogenesis of larval somatic muscles, indirect flight muscles and the heart. We found that loss of dDAAM function results in multiple defects in sarcomere development including thin and thick filament disorganization, Z-disc and M-band formation, and a near complete absence of the myofibrillar lattice. Collectively, our data suggest that dDAAM is required for the initial assembly of thin filaments, and subsequently it promotes filament elongation by assembling short actin polymers that anneal to the pointed end of the growing filaments, and by antagonizing the capping protein Tropomodulin.

Loss of Sarcomere-associated Formins Disrupts Z-line Organization, but does not Prevent Thin Filament Assembly in Caenorhabditis elegans Muscle

“…In mice, the FHOD-1-related formin encoded by fhod3 is expressed in heart muscle, and a fhod3 null mutation results in sarcomeres that contain thin filaments, but are sparse and narrow, and have aberrantly small Z-line bodies in the place of normal Z-discs [25]. Similarly, a hypomorphic allele of the mouse formin gene daam1 also results in narrow cardiomyocyte sarcomeres with aberrant Z-discs [26], and disruption of the related Drosophila daam gene has similar effects in somatic and cardiac fly muscles [27]. In all these cases, the mutant sarcomeres are deficient in multiple components, including thin filaments, thick filaments, and Z-line material.…”

Tropomodulins and Leiomodins: Actin Pointed End Caps and Nucleators in Muscles

“…We propose that the effects of Lmod on thin filament length can be explained within the context of dynamic actin subunit exchange at pointed ends controlled by Tmod. In the case of Lmod overexpression (7,41), new filaments would be nucleated whose barbed ends could anneal onto the transiently uncapped pointed ends of preexisting filaments, resulting in longer filaments, as proposed for the Drosophila formin DAAM (95). Alternatively, the pointed ends of the newly formed filaments could compete with preexisting pointed ends for limiting Tmod, resulting in reduced capping frequency, increased actin subunit addition at pointed ends, and net filament elongation, similar to the Tmod inhibition or depletion phenotypes (58,63,68).…”

“…While some formins can nucleate actin polymerization, they are more generally associated with inhibition of barbed end capping and acceleration of processive barbed end elongation from profilin-actin, and several formins have now been shown to team-up with other proteins for nucleation (62,104). Among the sarcomeric formins, the most extensively studied are the FHOD- (96,97,(105)(106)(107) and DAAM-related (95,106,108) formins, both of which have been implicated in myofibrillogenesis and the establishment of sarcomeric ultrastructure. However, FHOD-family formins lack nucleation activity in vitro (96), whereas DAAM is thought to mediate thin filament organization in sarcomeres but not polymerization (106).…”

Unique and Overlapping Functions of Formins Frl and DAAM During Ommatidial Rotation and Neuronal Development in Drosophila

“…Surprisingly, Drosophila DAAM mutants show no PCP defects in the eye or wing. Instead, DAAM is required to pattern the tracheal cuticle via the modification of the actin cytoskeleton, for sarcomeric thin filament formation, and for axon growth, the latter function under control of the Rac GTPase (Matusek et al 2006(Matusek et al , 2008Molnar et al 2014;Gombos et al 2015). Thus, the role of formins and the extent of their cooperation in PCP signaling in fruit flies remained elusive.…”