Distinct effects of caudalizing factors on regional specification of embryonic stem cell-derived neural precursors

Recent embryological studies have implicated several "caudalizing factors" in the caudal specification of the central nervous system (CNS). In this study, we have examined the effects of three candidate caudalizing factors on neural precursors induced from embryonic stem (ES) cells by the stromal cell-derived inducing activity (SDIA) method. Among retinoic acid (RA), Wnt and FGF signals, RA causes the strongest level of caudalization: inducing suppression of forebrain differentiation and promotion of caudal CNS specification. Obvious suppression of the telencephalic marker Bf1 and that of the forebrain marker Otx2 occur at 2×10 ^-8 and 2×10 ^-7 M, respectively. Activation of the caudal marker genes such as Hoxb9 is observed in a dose-dependent manner over the range of 2×10 ^-9-2×10 ^-6 M. Suppression of the forebrain genes has a narrow critical period of RA response during the early culture phase. In contrast, significant induction of the caudal genes is evoked by a 1-day exposure to RA at any time between days 3 and 8. RA treatment not only induces caudal specification but also inhibits differentiation of ventral CNS tissues, particularly of floor plate cells. FGF4 induces partial caudalization while Wnt-3A exhibits weak caudalizing activities only in the presence of RA. These findings provide useful information on the proper selection of combination of signaling molecules, doses and timing for steering ES cell differentiation by caudalizing factors into caudal neural fates.