Therefore, PlexB-mediated Sema-2b signaling solidifies specific projection positioning originally established by the Robo code. Together, these two distinct Robo and plexin guidance cue signaling modules function in a sequential and complementary fashion to specify both long range medial-to-lateral positioning (Robo) and short-range local fasciculation (PlexB). PlexA, the other Drosophila plexin receptor, and its ligand Sema-1a are specifically required for the proper formation of the 1D4-l pathway ( Winberg et al., 1998b and Yu et al., 1998). However, Sema-1a Ibrutinib concentration does not show
restricted expression within the medio-lateral axis of the nerve cord analogous to that observed for Sema-2b ( Yu et al., 1998), suggesting a different mechanism may underlie Sema-1a–PlexA regulation
of fasciculation in the most lateral CNS longitudinal region. Following medio-lateral specification by Slit-Robo Trametinib in vitro signaling and general organization of longitudinal regions by Sema-plexin signaling, additional cues are likely to mediate local interactions among neural processes already restricted to defined regions in the neuropile. Several cell surface proteins may serve such functions; for example, the cell adhesion molecule (CAM) connectin, like Sema-2b, shows exquisitely restricted expression along a subset of longitudinal projections (Nose et al., 1992). More widely expressed CAMs also play important roles in maintaining the fasciculated state of longitudinally projecting processes that are part of the same connective; indeed, in the absence of the Drosophila Ig super family member FasII, axons that contribute to the MP1 pathway show reduced association when examined at high resolution ( Lin et al., 1994). Therefore, an ensemble of short-range cues expressed in distinct subsets of longitudinally projecting neurons allows for individual pathways to be established following more global restriction to appropriate locations, and as we demonstrate here, this process is critical for the neural circuit function. It seems likely that similar mechanisms underlie the segregation of complex trajectories, the establishment
of laminar organization, and the formation of discrete neural maps in other regions of invertebrate below and vertebrate nervous systems ( Matsuoka et al., 2011 and Sanes and Yamagata, 2009). Our analyses allow for a comparison between the effects of the secreted semaphorins Sema-2a and Sema-2b on both CNS interneuron trajectories and sensory afferent targeting within the CNS. We observed in both LOF and GOF genetic paradigms that Sema-2a acts as a repellent, consistent with previous observations (Ayoob et al., 2006, Bates and Whitington, 2007, Carrillo et al., 2010, Matthes et al., 1995, Winberg et al., 1998a and Zlatic et al., 2009). Sema-2b, in contrast, serves an opposite guidance function and promotes neurite fasciculation.