The forkhead transcription factors, Foxp1 and Foxp2, identify different subpopulations of projection neurons in the mouse cerebral cortex.

Foxp1 and Foxp2, which belong to the forkhead transcription factor family, are expressed in the developing and adult mouse brain, including the striatum, thalamus, and cerebral cortex. Recent reports suggest that FOXP1 and FOXP2 are involved in the development of speech and language in humans. Although both Foxp1 and Foxp2 are expressed in the neural circuits that mediate speech and language, including the corticostriatal circuit, the functions of Foxp1 and Foxp2 in the cerebral cortex remain unclear. To gain insight into the functions of Foxp1 and Foxp2 in the cerebral cortex, we characterized Foxp1- and Foxp2-expressing cells in postnatal and adult mice using immunohistochemistry. In adult mice, Foxp1 was expressed in neurons of layers III-VIa in the neocortex, whereas the expression of Foxp2 was restricted to dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein, 32 kDa (DARPP-32)(+) neurons of layer VI. In addition, Foxp2 was weakly expressed in the neurons of layer V of the motor cortex and hindlimb and forelimb regions of the primary somatosensory cortex. Both Foxp1 and Foxp2 were expressed in the ionotropic glutamate receptor (GluR) 2/3(+) neurons, and colocalized with none of GluR1, gamma-aminobutyric acid, calbindin, and parvalbumin, indicating that expression of Foxp1 and Foxp2 is restricted to projection neurons. During the postnatal stages, Foxp1 was predominantly expressed in Satb2(+)/Ctip2(-) corticocortical projection neurons of layers III-V and in Tbr1(+) corticothalamic projection neurons of layer VIa. Although Foxp2 was also expressed in Tbr1(+) corticothalamic projection neurons of layer VI, no colocalization of Foxp1 with Foxp2 was observed from postnatal day (P) 0 to P7. These findings suggest that Foxp1 and Foxp2 may be involved in the development of different cortical projection neurons during the early postnatal stages in addition to the establishment and maintenance of different cortical circuits from the late postnatal stage to adulthood.