, 2011), which involves aberrant mossy fiber sprouting (Sutula et al., 1989). Of note, increased occurrence of epileptic seizures Fulvestrant cost is often associated with DS (Musumeci et al., 1999; Stafstrom, 1993) as well as fragile X syndrome (FXS) (Musumeci et al., 1999; Stafstrom, 1993), which is caused by loss of FMRP function (Verkerk et al., 1991). Our study suggests that elevated Dscam levels may contribute to the pathogenesis of these disorders by causing excessive presynaptic arbor growth. It also establishes a functional

link between Dscam and FMRP, raising the intriguing possibility that Dscam might be a mechanistic link between DS and FXS, the two most prevalent genetic causes of mental retardation. Recent studies have shown that axon injury activates the DLK pathway, which is essential for subsequent axon regeneration (Hammarlund et al., 2009; Shin et al., 2012; Watkins et al., 2013; Xiong et al., 2010; Yan et al., 2009). In light of the present study, it will be interesting to determine whether the DLK pathway requires Dscam to instruct axon regeneration. In summary, this study demonstrates

that Dscam expression levels, regulated by the DLK pathway and FMRP, determine presynaptic arbor size. It further shows the functional significance of dysregulated Dscam expression in neuronal development and provides a model for studying the pathogenesis of neurological disorders with dysregulated Dscam expression. hiwΔN, UAS-Hiw::GFP ( Wu et al., 2005); wnd1, wnd3, and UAS-Wnd ( BMN 673 mw Collins et al., 2006); DscamP1 ( Schmucker et al., 2000); Dscam18 ( Wang et al., 2002); UAS-Dscam[TM2]::GFP (3.36.25), UAS-Dscam[TM1]::GFP (3.36.25), and DscamP-Dscam[TM2]::GFP (3.36.25)( Wang et al., 2004); UAS-Dscam[TM2] (11.31.25) ( Zhan et al., 2004); Dscam10.27.25, Dscam3.31.8 and DscamFRT ( Hattori et al., 2007); dFMRP50M, UAS-dFMRP ( Zhang et al., 2001); ppk-Gal4 ( Kuo

et al., 2005); ppk-CD4::tdTomato ( Han et al., 2011); and UAS-Syt::eGFP ( Zhang et al., 2002) were used in this study. cDNA constructs of EGFP expression reporters and dFMRP were subcloned MRIP into the pUAST vector. Dscam cDNA containing variable exons 4.3-6.36-9.25-17.2 ( Wang et al., 2004) were used to generate Dscam[TM2]::GFP constructs with or without the 5′ and/or 3′ UTR of Dscam mRNA in the pUASTattB vector. Using standard methods ( Bateman et al., 2006), UAS-Dscam[TM2]::GFP (3.36.25) transgenic lines were generated using PhiC31 integrase-mediated site-specific insertion at the attP40 landing site. As such, there is no position effect on the transcription of these transgenes. The UAS-EGFP construct containing the Dscam 3′ UTR was used to generate serial deletion constructs of the Dscam 3′ UTR for mapping the required sequence for Wnd regulation. The genomic Dscam transgene used for rescue experiments and the wnd cDNA construct were, respectively, generous gifts from Dr. Tzumin Lee (Howard Hughes Medical Institute) and Dr.