Fig. 4. Proposed mechanism of SFRP1 activity as an axon guidance cue. SFRP1 can directly modify and reorient the growth of retinal ganglion cell growth cones. This activity does not require Wnt inhibition and is mediated by Fz2. It requires pertussis-toxin-sensitive activation of Ga protein, involves protein synthesis and degradation, and is modulated by different levels of cAMP and cGMP (Rodriguez et al., 2005).