DROSOPHILA DEVELOPMENTAL BIOLOGY

The regulation of growth cone actin dynamics is a critical aspect of axonal growth control. However, the proteins that are directly involved in the regulation of actin dynamics in developing neurons have so far not been clearly identified. Recently, we have shown that the Drosophila formin DAAM plays a critical role in axonal morphogenesis by promoting filopodia formation at the growth cone periphery. Moreover, we have collected several lines of evidence suggesting that the function of DAAM in developing neurons has been conserved during evolution. Currently, we are using genetical, cell biological and biochemical approaches to understand the molecular mechanisms of how formin proteins of the DAAM family contribute to axonal growth regulation in Drosophila and mouse.

Formin proteins of the DAAM subfamily play a role in axonal growth

In the developing nervous system, axons are guided to their targets by highly motile growth cones located at their distal tips. Directed growth cone motility in response to extracellular cues is produced by the coordinated regulation of peripheral F-actin and central microtubule networks. The peripheral F-actin is organized into long bundled actin filaments underlying the finger-like filopodia and diffuse networks of shorter actin filaments contained in the veil-like lamellipodia. Key regulators of actin dynamics are the so-called nucleation factors, such as the Arp2/3 complex and formins, which use different mechanisms to seed new actin filaments. These types of actin assembly factors are well-defined in migrating cells; in growth cones, however, prior to our work the essential nucleators have not been unambiguously identified.

By using a number of different model systems, our group has recently provided compelling evidence that formin proteins of the DAAM subfamily play a pivotal role during axonal growth regulation. We found that the Drosophila DAAM protein is highly enriched in the embryonic neurites, in particular, the protein localizes in dots throughout the F-actin rich growth cones including the filopodia. The loss of dDAAM function in embryonic neurons results in reduced neurite densities and abnormal axonal pathfinding. In addition, we have shown that filopodia are shorter and reduced in number in dDAAM mutant primary neurons. The constitutively activated versions of dDAAM promote filopodia and neurite formation, i.e. cause phenotypes opposite to the loss-of-function analyses in all assays we used. Because the FH2 domain of dDAAM is a potent actin nucleation factor in vitro, these observations together strongly suggest that dDAAM plays a major role in the regulation of actin assembly during axonal growth. Consistent with the fact that axonal growth regulation is an evolutionary highly conserved process, we have shown that dDAAM promotes the formation of neurite-like protrusions when expressed in mouse P19 cells, whereas murine Daam1 can functionally replace dDAAM in Drosophila. Thus, our data strongly suggest that the regulation of actin assembly during axon growth is very likely to signify an evolutionary conserved DAAM function.

Our main goal in the future is to gain deeper insights into the molecular mechanism whereby DAAM family formins control axonal growth regulation. We want to understand how dDAAM activity is regulated and how it is connected to the axon guidance cues, and we aim to study the molecular mechanism of dDAAM-dependent filopodia formation. Given that certain developmental disorders, accidental injuries and neurodegenerative diseases often result in severe axonal growth defects or axonal injuries, our studies on a key regulator of axonal growth may help the development of more efficient therapeutical tools.