When present in the cell membrane and following trans-signaling, both Ephs and ephrins are activated and result in the Selleckchem Nutlin-3a phosphorylation of several Rho GEFs, such as Vav2, which, in turn, promote Rac-dependent
actin polymerization required for Eph-ephrin complex endocytosis ( Cowan et al., 2005). Unlike the activated Ephs and ephrins in highly clustered Eph/ephrin trans complexes that are able to elicit downstream signaling, the Eph-ephrin cis complex presumably lacks the high-density clustering and subsequent kinase signaling activity ( Carvalho et al., 2006). Hence, the cis-binding of ephrins by Ephs might not elicit sufficient kinase activity to induce internalization. Alternatively, some proteins, such as the Rho GEF ephexin1, which can bind to unclustered Ephs without being phosphorylated ( Sahin et al., 2005), could potentially be recruited by Eph/ephrin cis-complexes and mediate their internalization. Independent in vitro studies suggest that ephrins in retinal neurons attenuate Eph activity in cis ( Feldheim et al., 2000 and Hornberger et al., 1999) and may also function
as receptors by binding in trans to Ephs in the tectum ( Mann et al., 2002 and Rashid et al., 2005). Our work in LMC neurons supports both ephrin functions, which could act synergistically to control retinal axon trajectory and Dabrafenib supplier thus allow an economical use of the Eph/ephrin system to specify many positional values in the emerging visual topographic map. One fundamental difference between the use of Eph signaling in LMC and retinal axon guidance is that while in the motor system EphA or EphB forward signaling is dominant in nonoverlapping motor neuron populations, in the retina, EphA and EphB forward
signaling can take place in the same neuron, such that interclass interactions appear very limited. In addition to the Ephs and ephrins, multiple modes of interaction between receptors and ligands have been proposed in several other systems. In the Notch/Delta system, Notch and Delta cis-interaction results in a mutual inactivation of Notch and Delta proteins, generating a sensitive switch between mutually Org 27569 exclusive sending (Delta high/Notch low) and receiving (Notch high/Delta low) signaling states ( Jacobsen et al., 1998 and Sprinzak et al., 2010). Our insights into Eph/ephrin signaling contrast these studies by showing that the bidirectional mode of trans-signaling is apparently regulated by ephrin levels, but probably not by Eph receptor levels since increasing EphA4 expression in medial LMC neurons leads to their increased sensitivity to ephrin-As, despite coexpressed ephrin-As ( Eberhart et al., 2002 and Kania and Jessell, 2003). On the other hand, Semaphorin (Sema):neuropilin trans-signaling is modulated by coexpression of Sema in cis with neuropilin in both sensory and motor axons ( Haklai-Topper et al., 2010 and Moret et al., 2007).