NT-3 regulates expression of Brn3a but not Brn3b in developing mouse trigeminal sensory neurons.

We have used a quantitative RT-PCR approach to determine the levels of Brn3a and Brn3b POU domain transcription factor mRNAs in the developing mouse trigeminal ganglion from E10 to E18. Using low density neuronal cultures, we have shown that NT-3 can regulate the expression of Brn3a mRNA in trigeminal neurons during the periods that they are differentiating and innervating their peripheral and central targets. In contrast to Brn3a, Brn3b mRNA is expressed at extremely low levels in the early trigeminal ganglion. Trigeminal neurons from early ganglia express low levels of Brn3b mRNA in culture and do not up-regulate Brn3b mRNA in response to a number of growth factors and experimental conditions. However, at later ages, when in vivo levels of Brn3b mRNA are high, FGF2, TGFbeta1 and retinoic acid all up-regulate Brn3b mRNA expression in cultured trigeminal neurons. Since NT-3 regulates the developmental expression of Brn3a, Brn3a may mediate some of the effects that NT-3 exerts on sensory neurons and their progenitors. Similarly, Brn3b may mediate some of the effects that FGF2, TGFbeta1 and retinoic acid have on neurons.