DAAM family members leading a novel path into formin research

Abstract

Formins are an important and evolutionarily well conserved class of actin binding proteins with essential biological functions. Although their molecular roles in actin regulation have been clearly demonstrated in vitro, their functions at the cellular or organism levels are still poorly understood. To illustrate this problem, but also to demonstrate potential ways forward, we focus here on the DAAM group of formins. In vertebrates, DAAM group members have been demonstrated to be important regulators of cellular and tissue morphogenesis but, as for all formins, the molecular mechanisms underlying these morphogenetic functions remain to be uncovered. The genome of the fruitfly Drosophila encodes a single DAAM gene which is evolutionarily highly conserved. Recent work on dDAAM has already provided a unique combination of observations and experimental opportunities unrivalled by any other Drosophila formin. These comprise in vitro actin polymerisation assays, subcellular studies in culture and in vivo, and a range of developmental phenotypes revealing a role in tracheal morphogenesis, axonal growth and muscle organization. At all these levels, future work on dDAAM will capitalise on the power of fly genetics, raising unique opportunities to advance our understanding of dDAAM at the systems level, with obvious implications for other formins.  

Acknowledgments

We are grateful to Tom Millard for comments and critical reading. Work in the laboratory of J.M. is supported by OTKA grant K82039 and a Pfizer Hungary Award. A.P. and N.S.S. are supported by grants from the Wellcome Trust (077748/Z/05/Z and 092403/Z/10/Z) and the BBSRC (BB/I002448/1), as well as a studentship of the Fundacao para a Ciencia e a Tecnologia to C.G.P.

Figures and Tables

Figure 1 Properties and functions of Drosophila DAAM. (A) Mouse and Drosophila DAAM are of similar length and display the same functional domains: GTPase binding (G), diaphanous inhibitory (DID), N-terminal dimerisation (DD), coiled-coil (CC), formin homology 1 and 2 (FH1, FH2), diaphanous autoregulatory domain (DAD); residues demarcating the functional domains are shown below. (B) In embryos, dDAAM is strongly expressed in main (white curved arrow) and side branches (black curved arrow) of tracheal trees. (C) At high magnification, tracheae show parallel lines of F-actin enrichment (magenta) that run across cellular junctions (green, stained for De-Cadherin) in the main airways. (D) In tracheae of dDAAM loss-of-function mutant embryos, ordered F-actin patterns are abolished. (E) dDAAM is strongly expressed in the ladder-shaped neuropile within the embryonic CNS (white curved arrow). (F) In wild-type embryos, the axonal marker BP102 labels the ladder-like arrangement of the neuropile. (G) In dDAAM loss-of-function mutant embryos, the neuropile is severely disrupted indicating strong axonal growth defects. (H) In cultured primary embryonic neurons, dDAAM displays a punctate pattern along the axon (black curved arrow), but also at the growth cone (white arrow head) and in filopodia (white curved arrow). (I) In wild-type neurons, growth cones frequently display a hand-shaped broad appearance. (J) In dDAAM loss-of-function mutant neurons, growth cones tend to be narrow and display significantly reduced numbers of filopodia. (K–M) Potential functions of dDAAM during filopodia formation in Drosophila neurons. (K) dDAAM acts as an actin nucleator in parallel to Scar complex/Arp2/3 complex activity. (L) Like enabled, dDAAM promotes processive elongation of actin filaments, expected to collaborate with profilin and potentially antagonising the capping activities of the CapA/B complex. (M) The ability to bind and bundle actin filaments in vitro25 suggests potential roles for dDAAM in clustering of actin filament barbed ends (together with enabled; left) or the stabilisation of F-actin bundles (together with other bundlers, such as fascin; right).