The role of Sns during the formation of dendrites

Based on its mutant phenotype, Sns is required for the first steps of myoblast fusion in Drosophila (Bour et al., 2000). Through its interaction with Duf or Rst, Sns mediates the initial attraction (Ruiz-Gomez et al., 2000; Strunkelnberg et al., 2001) and adherence (Galletta et al., 2004) between the two different myoblast types at the onset of muscle formation during the embryonic development. The interaction between Sns and Duf (Galletta et al., 2004; Kesper et al., 2007) recruits molecules to the site of the myoblast fusion in FCMs (Kim et al., 2007; Massarwa et al., 2007) and muscle founder cells (Chen et al., 2006; Chen and Olson, 2001; Menon and Chia, 2001) that are involved in the modulation of the actin-cytoskeleton. In FCMs, Sns binds to the SH2-SH3 adaptor protein D-Crk that in turn recruits Solitary, a Wasp interaction protein, and the Wasp/Arp2-3 complex (Kim et al., 2007;

Massarwa et al., 2007). Solitary and Wasp are needed for the localised induction of actin polymerisation at the sites of myoblast fusion (Kim et al., 2007). Moreover, this actin-polymerisation is essential for the directed transport of exocytotic vesicles towards the area between aligned myoblasts that is defined by the interaction of Sns and Duf (Kim et al., 2007). Hence, Sns provides a positional cue for actin polymerisation and consequently the transport of secretory vesicles. Similarly, the vertebrate homologue of Sns, named Nephrin (Bour et al., 2000; Putaala et al., 2001), binds the SH2-SH3 adaptor protein Nck that is known to modulate the actin cytoskeleton through its association with Wasp (Buday et al., 2002; Jones et al., 2006). Nephrin is expressed in kidney, pancreas and the brain, where it is localised in the cerebellum and the mesencephalon (Putaala et al., 2001). The polymerisation of the actin cytoskeleton upon interaction of Nephrin and Nck is crucial for the maintenance of the podocyte cellular junction, and has been shown to induce process formation in HEK-293T cells (Jones et al., 2006; Li et al., 2006).

Furthermore, the binding of Nck to Nephrin and the resulting actin polymerisation depends on a Tyrosin phosphorylation of the cytoplasmatic tail of Nephrin through the Scr-family kinase Fyn (Jones et al., 2006; Li et al., 2006). Interestingly, Fyn is required for different aspects of neuronal development, like the semaphorin dependent formation of dendritic branches and spine maturation in hippocampal cells (Morita et al., 2006) or axon guidance through phosphorylation of the Netrin receptor DCC (Meriane et al., 2004). Although an interaction of Sns with a member of the Src kinase family has not been shown so far, the presence of two putative kinase recognition sites (Artero et al., 2001) suggest that the Sns function might be also controlled by phosphorylation of its cytoplasmatic tail, which could be provided by Src kinases.

During myoblast fusion in Drosophila, the function of Sns depends on its interaction with two other transmembrane molecules of the Ig-superfamily, which are called Duf and Rst (Galletta et al., 2004; Ruiz-Gomez et al., 2000; Strunkelnberg et al., 2001). A broad functional diversity has been reported for Rst in Drosophila, where it is essential for axon guidance in the visual system

Discussion

(Reiter et al., 1996). Could the function of Sns during dendrite morphogenesis be also affected by these two molecules? In vertebrates, Nephrin interacts with Neph1 and Neph2, which represent homologues of roughest (Gerke et al., 2003). Both Neph proteins are expressed in kidney and the brain, specifically in dendrites and synapses of Purkinje cells (Gerke et al., 2006). A direct interaction between Nephrin and Nephs has not been shown so far, but appears very likely due to a similar expression pattern. What could be the function of an interaction between Sns and Rst? As mentioned above, Rst is involved in the guidance of axons in the visual system of Drosophila (Schneider et al., 1995). Moreover, the homologue of Rst in C.elegans, SYG- 1, is expressed in specific motorneurons and controls the positioning of synapses through its interaction with an unknown ligand expressed by epidermal guidepost cells (Shen and Bargmann, 2003). Hence, Rst seems to be involved in targeting events within the nervous system and could serve as an extrinsic cue for Sns that might control the spatiotemporal formation of dendritic branches.

It seems that the members of the Nephrin subfamily of transmembrane adhesion molecules organise the actin polymerisation machinery to specific sites, through interaction with SH2-SH3 adaptor proteins that in turn activate asp A possible function of Sns in the nervous system is supported by the expression pattern of vertebrate Nephrins. The ability of the Nephrin-Nck complex to induce “spikey” protrusion in HEK-293T cells shows that the rearrangement of the actin cytoskeleton through Nephrin can cause morphological changes of cells (Li et al., 2006). Thus, Sns might regulate the branching of dendrites through a targeted polymerisation of the actin cytoskeleton. It could serve as a positional cue that determines where new branches would be added and guide the delivery of exocytotic vesicles to the new branch. In addition, the activity of Sns might be regulated through phosphorylation of its intracellular tail or interaction with other adhesion molecules from the extracellular environment.

Accordingly, this idea would imply that the action of Sns promotes the formation of new branches on the dendritic arbour of md-da neurons. How does this fit to the phenotypes seen in the vpda and ddaE neurons? If Sns

promotes the branch formation in dendrites, the dendrites of md-da neurons in sns mutants should consequently create less branches. This is partially seen in ddaE neurons of mutant line snsS660. In contrast, the vpda neuron produced a reliable overbranching phenotype in sns mutants, which means that Sns limits the formation of high order branches in this neuron. This could indicate that Sns is also involved in a different signal event that limits the activity of an independed cellular machinery that is responsible for the creation of high order branches.