Supplementary Material for: Detailed Expression Pattern of Foxp1 and Its Possible Roles in Neurons of the Spinal Cord during Embryogenesis

A member of winged-helix/forkhead transcription factors, Foxp1, is expressed in the developing spinal cord during mouse embryogenesis. To shed light on the potential role of Foxp1 in neurons of the developing spinal cord, we investigated the detailed expression pattern of Foxp1 between embryonic day (E) 9.5 and E17.5. At E10.25, some postmitotic neurons with strong expression of Foxp1 (Foxp1<sup>high</sup>) were first detected in the ventral half of the brachial spinal cord. By E11.5, Foxp1<sup>high</sup> neurons increased in the ventral spinal cord at the limb levels. All of Foxp1<sup>high</sup> neurons at the limb levels were Islet2<sup>+</sup>/Lhx3<sup>–</sup> motor neurons (MNs) of the lateral motor column and some neurons that expressed Foxp1 weakly (Foxp1<sup>low</sup>) at the thoracic level were MNs of the preganglionic motor column. Between E12.5 and E17.5, Foxp1<sup>low</sup> neurons were also observed in the intermediate zone throughout the ventral spinal cord, all of which were Pax2<sup>+</sup>, En1<sup>+</sup>, Evx1<sup>–</sup>, Chx10<sup>–</sup>, Gata3<sup>–</sup>, and Lhx3<sup>–</sup> V1 interneurons. Interestingly, no colocalization of Foxp1 with Lhx3 was observed in the developing spinal cord. In addition, overexpression of Foxp1 markedly attenuated the endogenous expression of Lhx3 in a neuroendocrine cell line. Chromatin immunoprecipitation assays in a neuronal cell line and E13.5 spinal cords revealed an interaction between Foxp1 and the consensus motif in the Lhx3 promoter. These results suggest that Foxp1 may play some important roles in the determination of neuronal fates of the ventral spinal cord, possibly through the suppression of Lhx3 expression.