Thus, these two regulatory mechanisms
appear to operate in parallel ( Figure 7C). If these two pathways converge on Dscam expression to direct presynaptic arbor growth, the suppression of Dscam function by dFMRP would counteract the enhanced Dscam function in hiw mutants. Indeed, overexpressing dFMRP significantly suppressed 3-MA order the presynaptic arbor overgrowth caused by either hiw mutations ( Figure 7A) or Wnd overexpression ( Figure S5). Having established the importance of Dscam expression regulation for presynaptic arbor growth, we sought to determine the degree of correlation between presynaptic arbor sizes and Dscam protein levels. We plotted relative Dscam expression levels, as assayed by western analysis (Figures 3B and 6B), against relative presynaptic arbor sizes of single C4 da neurons (Figures 1B, 3A, and 6C) in different genetic backgrounds. The statistical analysis showed a striking linear correlation, with a coefficient of determination (R2) of 0.997 between Dscam levels and presynaptic arbor sizes (Figure 7B). This not only suggests that Dscam expression levels are tightly controlled for precise presynaptic arbor growth, but
also emphasizes the function of Dscam expression levels in PF-02341066 research buy determining presynaptic arbor sizes. In this study, we found that in addition to the ectodomain diversity, the expression level of Dscam serves as a code for neuronal development. We identified two regulatory mechanisms, one involving the kinase DLK and another involving the RNA-binding protein FMRP, which control Dscam expression at the level of protein translation. Defects in either of these regulatory pathways lead to aberrant growth of presynaptic arbors. The importance however of this regulation is underscored by the strong correlation between the expression levels of Dscam and the sizes of presynaptic arbors. After reaching their target regions, axons branch and extend to form presynaptic arbors. A presynaptic arbor of a given neuron type typically develops a specific pattern and size, which is critical for establishing appropriate number of synaptic connections with specific targets. How the patterning mechanism relates to the ultimate size that each
presynaptic arbor assumes is unknown. Here, we propose that both the patterning and size control of presynaptic terminals can be instructed by a common regulator, such as Dscam. The isoform diversity of Dscam determines the pattern of presynaptic terminals, whereas the expression levels of Dscam instruct the sizes of these terminals (Figure 7C). Is the function of Dscam in presynaptic arbor size control a consequence of its dendritic functions? Several lines of evidence argue against this possibility. First, while expressing the axon-enriched TM2 isoforms caused dramatic increase of presynaptic arbor growth, expressing the dendrite-enriched TM1 isoforms led to only a minimal increase in presynaptic arbor growth (Figure 1B), suggesting that axonal Dscam regulates presynaptic growth.