The loss of nuclear pSMAD and Tbx3 signals reflects reduced expression levels, not the loss of neurons, since neuronal number, as measured by Isl1 staining, is unaffected ( Figure 8B). These data indicate that target-derived BDNF is critical for the acquisition of positional identity markers in the trigeminal ganglia. We considered the possibility that the induction of the positional

identity markers was delayed in BDNF−/− embryos. However, Tbx3 expression was also absent at E13.5 ( Figures 8B). As an additional control, we examined whether BMP4 expression was affected in BDNF−/− embryos. Immunostaining GSK1210151A solubility dmso of BMP4 in the face was essentially identical in BDNF+/− and BDNF−/− embryos ( Figure S8F), indicating that impaired expression of positional identity

markers in the trigeminal ganglia cannot be attributed to reduced expression of BMP4 in BDNF−/− embryos. Lastly, we addressed whether SMAD transcript expression levels are affected in BDNF−/− embryos. In situ hybridization shows that SMAD1, 5, and 8 expression was unaffected in BDNF−/− embryos ( Figure S8G), suggesting SCH727965 cost that reduced pSMAD in neuronal nuclei and SMAD in axons is not due to impaired SMAD transcription. Together, these data indicate that BDNF is a target-derived factor that physiologically regulates axonal SMAD1/5/8 levels and is required for patterning the trigeminal ganglia. Although combinations of distinct inductive cues are critical for the specification of early neuronal types from neuronal progenitors, it has not previously been known if multiple inductive cues also act on axons to specify neuronal identity. In this case, distal axons will require mechanisms to generate retrograde signals that reflect the simultaneous detection of multiple cues. We find that retrograde patterning of the trigeminal ganglia requires both BMP4 and BDNF and that both signals act on distal axons to generate a retrograde

signal. Our results identify local translation as a mechanism that allows the axon to elicit a retrograde signal only upon detection of both target-derived factors. We find that SMAD1, 5, and 8 transcripts are localized to below trigeminal ganglia axons and that these mRNAs are selectively translated in response to BDNF. Axonal SMAD1/5/8 is then retrogradely trafficked to the cell body, along with BMP4 signaling endosomes, to regulate target gene transcription in the maxillary and ophthalmic neuron subsets of the trigeminal ganglia ( Figure S8H). In this manner, intra-axonal translation provides a mechanism by which spatially and temporally coincident target-derived signals are processed to obtain specific responses, by limiting retrograde BMP4 signaling to regions that also contain BDNF. Our results also identify a new mechanism by which the transcription factor repertoire of neuronal subtypes is determined.