The trochlear projection is unique among the cranial nerves in that it exits the midbrain dorsally to innervate the contralateral superior oblique muscle in all vertebrates. Trochlear as well as oculomotor motoneurons uniquely depend upon Phox2a and Wnt1 , both of which are downstream of Lmx1b , though why trochlear motoneurons display such unusual projections is not fully known. We used Pax2-cre to drive expression of ectopically activated Smoothened ( SmoM2 ) dorsally in the midbrain and anterior hindbrain. We documented the expansion of oculomotor and trochlear motoneurons using Phox2a as a specific marker at E9.5. We show that the initial expansion follows a demise of these neurons by E14.5. Furthermore, SmoM2 expression leads to a ventral exit and ipsilateral projection of trochlear motoneurons. We compare that data with Unc5c mutants that shows a variable ipsilateral number of trochlear fibers that exit dorsal. Our data suggest that Shh signaling is involved in trochlear motoneuron projections and that the deflected trochlear projections after SmoM2 expression is likely due to the dorsal expression of Gli1 , which impedes the normal dorsal trajectory of these neurons.

中文翻译：

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平滑的过度表达导致滑车运动神经元重新路由和支配同侧眼睛

滑车投射在颅神经中是独一无二的，因为它从背侧中脑出来以支配所有脊椎动物的对侧上斜肌。滑车以及动眼神经运动神经元独特地依赖于 Phox2a 和 Wnt1，这两者都位于 Lmx1b 的下游，但为什么滑车运动神经元显示出如此不寻常的预测尚不完全清楚。我们使用 Pax2-cre 来驱动中脑和前后脑中异位激活的 Smoothened (SmoM2) 的背侧表达。我们在 E9.5 记录了使用 Phox2a 作为特定标记物的动眼神经和滑车运动神经元的扩展。我们表明，最初的扩张是在 E14.5 这些神经元死亡之后进行的。此外，SmoM2 表达导致滑车运动神经元的腹侧出口和同侧投影。我们将该数据与 Unc5c 突变体进行比较，后者显示出背侧滑车纤维的同侧数量可变。我们的数据表明 Shh 信号参与滑车运动神经元投射，并且 SmoM2 表达后滑车投射偏转可能是由于 Gli1 的背侧表达，这阻碍了这些神经元的正常背侧轨迹。